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Into the Tissues: Extracellular Matrix and Its Artificial Substitutes: Cell Signalling Mechanisms

by 

Aleksandra Bandzerewicz

 and

Agnieszka Gadomska-Gajadhur

 *

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3 Street, 00-664 Warsaw, Poland

*

Author to whom correspondence should be addressed.

Cells 2022, 11(5), 914; https://doi.org/10.3390/cells11050914

Submission received: 17 February 2022 / Revised: 2 March 2022 / Accepted: 4 March 2022 / Published: 7 March 2022

(This article belongs to the Topic Cell Signaling Pathways)

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Abstract

The existence of orderly structures, such as tissues and organs is made possible by cell adhesion, i.e., the process by which cells attach to neighbouring cells and a supporting substance in the form of the extracellular matrix. The extracellular matrix is a three-dimensional structure composed of collagens, elastin, and various proteoglycans and glycoproteins. It is a storehouse for multiple signalling factors. Cells are informed of their correct connection to the matrix via receptors. Tissue disruption often prevents the natural reconstitution of the matrix. The use of appropriate implants is then required. This review is a compilation of crucial information on the structural and functional features of the extracellular matrix and the complex mechanisms of cell–cell connectivity. The possibilities of regenerating damaged tissues using an artificial matrix substitute are described, detailing the host response to the implant. An important issue is the surface properties of such an implant and the possibilities of their modification.

요약

조직과 기관과 같은 질서정연한 구조의 존재는 

세포 부착, 즉 

세포가 인접한 세포와 

세포 외 기질 형태의 지지 물질에 부착하는 과정에 의해 가능해집니다. 

orderly structures

cell adhesion

세포 외 기질(세포사이 기질)은 

콜라겐, 엘라스틴, 

다양한 프로테오글리칸과 

당단백질로 구성된 3차원 구조입니다. 

이것은 

여러 가지 신호 전달 인자를 저장하는 창고입니다. 

세포는 

수용체를 통해 기질에 올바르게 연결되어 있다는 

정보를 받습니다. 

조직의 파괴는 

종종 세포 외 기질의 자연적인 재구성을 방해합니다. 

--> 손상(염증) --> 섬유화..

그러면 적절한 임플란트를 사용해야 합니다. 

이 리뷰는 

세포 외 기질의 구조적, 기능적 특징과 

세포-세포 연결의 복잡한 메커니즘에 대한 중요한 정보를 모아놓은 것입니다. 

인공 기질 대체물을 사용하여 

손상된 조직을 재생할 수 있는 가능성에 대해 설명하고, 

임플란트에 대한 숙주 반응을 자세히 설명합니다. 

중요한 문제는 

이러한 임플란트의 표면 특성과 그 변형 가능성입니다.

Keywords: 

extracellular matrix; cellular receptors; cell adhesion; cell signalling; scaffolds; biomaterials

Graphical Abstract

1. Introduction

A cell is the smallest structural and functional unit of a living organism, capable of carrying out all basic life processes. Cells show remarkable morphological and biochemical diversity. They may constitute an independent organism or hierarchically build multicellular organisms. Appropriate structural support makes the orderly organisation of cells into tissues and organs possible. The substance filling the spaces between cells is called the extracellular matrix (ECM). It is a network of proteins and polysaccharides secreted locally by the cells. The composition of the extracellular matrix determines the properties of the tissue it builds [1].

The correct development of tissue and the maintenance of its functionality result from a controlled flow of information between cells. Receiving and responding to signals from the surrounding environment is made possible by receptors. Receptors process and amplify signals and transduce them into the cell via a series of signalling molecules. Coordinated interaction between cytoskeleton, receptor and matrix is essential. Therefore, cell adhesion is a dynamic process and not just a passive anchoring to the matrix [2,3,4].

As a result of mechanical damage or lesions, the structural integrity of the tissue can be damaged. The body is often incapable of naturally repairing such defects. Therefore, the cells need an artificial scaffold that mimics the natural extracellular matrix and the properties of the regenerated tissue. Such an implant should accelerate the healing process and, above all, not provoke a negative response from the body. It depends mainly on the surface properties of the implant. Both new materials and methods of their modification are constantly being sought to improve patient treatment [5].

1. 소개

세포는

모든 기본적인 생명 과정을 수행할 수 있는 살아있는 유기체의

가장 작은 구조적, 기능적 단위입니다.

세포는

형태학적으로나 생화학적으로 매우 다양합니다.

세포는

독립적인 유기체를 구성하거나

다세포 유기체를 계층적으로 구축할 수 있습니다.

적절한 구조적 지원이 있으면

세포가 조직과 기관으로 질서 있게 조직될 수 있습니다.

세포 사이의 공간을 채우는 물질을

세포외기질(ECM)이라고 합니다.

이것은

세포에 의해 국소적으로 분비되는 단백질과

다당류의 네트워크입니다.

세포 외 기질의 구성은

그것이 형성하는 조직의 특성을 결정합니다 [1].

조직의 올바른 발달과 기능의 유지는

세포들 사이의 통제된 정보 흐름에 의해 결정됩니다.

수용체는

주변 환경으로부터 신호를 받아들이고

이에 반응하는 것을 가능하게 합니다.

수용체는

신호를 처리하고 증폭한 다음,

일련의 신호 전달 분자를 통해 세포로 전달합니다.

세포 골격, 수용체, 그리고 기질 사이의 협응력 있는 상호 작용은 필수적입니다.

따라서

세포 부착은 기질에 대한 수동적인 고정만이 아니라

역동적인 과정입니다 [2,3,4].

기계적 손상이나 병변의 결과로

조직의 구조적 완전성이 손상될 수 있습니다.

신체는 종종 이러한 결함을 자연적으로 복구할 수 없습니다.

--> 건생병사로 복구할 수 있다!!

따라서 세포는

자연적인 세포 외 기질과 재생 조직의 특성을 모방하는

인공 지지대가 필요합니다.

이러한 임플란트는

치유 과정을 가속화하고

무엇보다도 신체에서 부정적인 반응을 일으키지 않아야 합니다.

이는 주로 임플란트의 표면 특성에 달려 있습니다.

환자 치료를 개선하기 위해 새로운 재료와 그 변형 방법이 끊임없이 연구되고 있습니다 [5].

2. The Extracellular Matrix—Composition, Structure, Functions

The extracellular matrix is often referred to as the natural scaffold of tissues and organs. Still, the functions of this structure go far beyond being mere physical support for the cells. The extracellular matrix regulates cell life processes from adhesion, differentiation, proliferation, migration to apoptosis because of the extensive network of matrix components, their ability to interact with each other, with signalling factors and with membrane receptors [2,6,7,8].

The ECM is an essential component primarily of connective tissue, one of the four main tissue types in the human body (along with epithelial, muscle, and nerve tissue). It is a complex mixture of water, proteins, and polysaccharides. The balance of these three components is determined mainly by the tissue type (cartilage, bone, fat, connective tissue that builds tendons, etc.) and by its development stage and pathophysiological state [2,9,10,11,12]. The ECM components are locally synthesised and secreted by cells, mainly fibroblasts, the most numerous, although least specialised, of the connective tissue cells [8,13,14]. The organisation of the matrix structure is influenced by the arrangement and orientation of the intracellular cytoskeleton [8].

2. 세포 외 기질 - 구성, 구조, 기능

세포 외 기질은

종종 조직과 기관의 자연적인 지지대로 불립니다.

그러나 이 구조의 기능은

단순한 세포의 물리적 지지 그 이상입니다.

세포 외 기질은

기질 구성 요소의 광범위한 네트워크,

서로 상호 작용하는 능력,

신호 전달 인자 및 막 수용체와의 상호 작용 능력으로 인해

세포의 부착, 분화, 증식, 이동, 세포 사멸에 이르는 세포 생활 과정을 조절합니다 [2,6,7,8].

ECM은

인체 내의 네 가지 주요 조직 유형(상피 조직, 근육 조직, 신경 조직과 함께) 중 하나인

결합 조직의 주요 구성 요소입니다.

그것은

물,

단백질,

다당류의 복합 혼합물입니다.

이 세 가지 구성 요소의 균형은

주로 조직 유형(연골, 뼈, 지방, 힘줄을 만드는 결합 조직 등)과

발달 단계 및 병리 생리학적 상태에 의해 결정됩니다 [2,9,10,11,12].

ECM 구성 요소는

주로 섬유 아세포(섬유 아세포는 결합 조직 세포 중 가장 많지만 가장 덜 전문화된 세포)에 의해

세포 내에서 합성 및 분비됩니다 [8,13,14].

매트릭스 구조의 조직은

세포 내 세포 골격의 배열과 방향에 영향을 받습니다 [8].

2.1. Two Types of the Extracellular Matrix

Although the basic organisation of the ECM structure is the same throughout, two basic types of the matrix are distinguished by their location and composition: the interstitial matrix, which forms a three-dimensional porous network surrounding the cells (especially connective tissues), and the pericellular matrix, which is more compact and forms a layer adjacent to the cells [15,16] (see Figure 1).

2.1. 두 가지 유형의 세포외기질

세포외기질 구조의 기본 조직은

전체적으로 동일하지만,

두 가지 기본 유형의 기질은 위치와 구성에 따라 구분됩니다.

세포(특히 결합 조직)를 둘러싸는 3차원 다공성 네트워크를 형성하는 간질 기질과

세포에 인접한 층을 형성하는 더 조밀한 세포내 기질입니다 [15,16] (그림 1 참조).

Figure 1. Simplified extracellular matrix structure: three-dimensional macromolecular network composed of various proteins and polysaccharides. The pericellular matrix forms a layer adjacent to the cells: integrins bind to polymerised laminin, which, in turn, is connected via nidogen to the type IV collagen. Interstitial matrix forms porous network of fibrillar collagens, elastic fibres, and proteoglycans.

그림 1. 단순화된 세포 외 기질 구조:

다양한 단백질과 다당류로 구성된 3차원 거대 분자 네트워크.

세포 주변 기질은 세포에 인접한 층을 형성합니다: 인테그린은 중합된 라미닌에 결합하고, 라미닌은 니도겐을 통해 IV형 콜라겐에 연결됩니다. 간질 기질은 섬유상 콜라겐, 탄성 섬유, 프로테오글리칸으로 구성된 다공성 네트워크를 형성합니다.

The interstitial matrix can be equated with the “proper” matrix, as it forms the structural scaffolding for the cells. Its basic components are heterotypic fibrils, composed mainly of type I collagen with small amounts of type III and V collagens in variable proportions, both playing an important role in fibrillogenesis [16]. The collagens of the interstitial matrix are mostly secreted by fibroblasts [17]. Important components of this “amorphous three-dimensional gel” also include fibronectin and elastin, involved in the organisation of the structure [18,19].

A typical example of the pericellular matrix is the basement membrane, a delicate and flexible nanostructure that separates the epithelium from the deeper layers of connective tissues. It ensheathes smooth, skeletal, and cardiac muscle fibres, Schwann cells, and adipocytes. The basement membrane forms a specific boundary of many organs in mature tissues, often surrounding their functional units [16,20,21,22]. It is mainly composed of type IV collagen, laminins, nidogens and heparan sulfate proteoglycans (HSPGs): perlecan and agrin [21]. The basement membrane contains so-called matricellular proteins that do not contribute to its physical stability or structural integrity, although they may be connected to building components. Instead, they have regulatory functions and interact with surface receptors, proteases, hormones or other biologically active molecules. They may be tissue-specific in terms of function and structure [23,24,25,26]. Matricellular proteins include SPARC (secreted protein acidic and rich in cysteine, or osteonectin; characteristic of mineralising tissues, mainly bone), thrombospondin-1 (which is rich in platelet α-granules; when secreted, it causes, among other things, activation of TGF-β1, i.e., transforming growth factor-beta 1), and tenascin-C (the gene of this protein is expressed during embryonic life, while in adult tissues, tenascin-C is very poorly detectable, being present rather in the course of pathological processes [27,28,29]. The tasks of the basement membrane include regulation of tissue development, function, and regeneration by controlling the cellular response. It is a storehouse of growth factors and modulates their activity and concentration. It serves to maintain the phenotype of the cells it surrounds [30]. The interstitial matrix and the basement membrane are closely interconnected, ensuring the integrity of the tissue [16].

The functional equivalent of the basement membrane described above is a type of pericellular matrix that surrounds chondrocytes in articular cartilage [31]. It acts as a physical barrier that filters molecules entering and leaving the cells. Together with an adjacent thin layer of matrix, each chondrocyte forms a structural unit called a chondron [32]. The morphology of chondrons varies. They can take a discoid/ellipsoid/rounded shape and a variable orientation, which depends on the position, i.e., the depth of location in the cartilage. In some cases, a chondron comprises more than one cell (up to four) [33]. In this case, an essential component of the pericellular matrix is type VI collagen, although it generally constitutes a negligible percentage of the collagens of cartilage tissue [34]. However, because of its specific presence in the chondrocyte environment of articular cartilage, it often serves as a marker of chondrons [34,35]. A characteristic feature of articular cartilage is the small number of chondrocytes compared to the extensive extracellular (interstitial) matrix for which synthesis, organisation, and maintenance they are responsible [34].

When describing the types of the ECM, the term ‘pericellular matrix’ is sometimes omitted and replaced by the basement membrane itself, leading to the mistaken assumption that it is the same structure [17,18,19]. However, the basement membrane should be considered a more specialised form of the pericellular matrix.

interstitial matrix은

셀의 구조적 기초를 형성하기 때문에 “

적절한” matrix와 동일시될 수 있습니다.

그 기본 구성 요소는

주로 제1형 콜라겐으로 구성되어 있고,

제3형과 제5형 콜라겐이 소량씩 다양한 비율로 혼합되어 있는 이형성 섬유소입니다.

이 두 가지 유형의 콜라겐은

모두 섬유소형성에 중요한 역할을 합니다 [16].

간질 기질의 콜라겐은

대부분 섬유모세포에 의해 분비됩니다 [17].

이 “무정형 3차원 젤”의 중요한 구성 요소에는

구조의 조직화에 관여하는

피브로넥틴과 엘라스틴도 포함됩니다 [18,19].

amorphous three-dimensional gel

세포 외 기질의 전형적인 예로는

상피와 결합 조직의 더 깊은 층을 분리하는 섬세하고 유연한 나노 구조인 기저막이 있습니다.

기저막은

평활근, 골격근, 심장근 섬유, 슈반 세포, 지방세포를 감싸고 있습니다.

기저막은

성숙한 조직의 많은 기관에 특정한 경계를 형성하며,

종종 기능 단위를 둘러싸고 있습니다 [16,20,21,22].

주로 유형 IV 콜라겐, 라미닌, 니도겐, 헤파란 설페이트 프로테오글리칸(HSPGs)으로 구성되어 있습니다:

펄레칸과 아그린 [21].

기저막은

물리적 안정성이나 구조적 완전성에 기여하지 않는

소위 매트릭셀 단백질을 포함하고 있지만,

구성 요소와 연결되어 있을 수 있습니다.

대신,

그것들은 조절 기능을 가지고 있으며

표면 수용체, 프로테아제, 호르몬 또는 기타 생물학적 활성 분자와 상호 작용합니다.

이들은 기능과 구조 면에서 조직 특이적일 수 있습니다 [23,24,25,26].

매트릭셀 단백질에는

SPARC(분비 단백질 산성 및 시스테인 풍부, 또는 오스테오네틴; 주로 뼈와 같은 광물화 조직의 특징),

혈소판 α-과립이 풍부한 트롬보스폰딘-1(thrombospondin-1; 분비될 때, 무엇보다도 TGF-β1의 활성화를 유발함) 등이

포함됩니다. (즉, 변형 성장 인자-베타 1)을 활성화시킵니다),

그리고

테나신-C(이 단백질의 유전자는

배아 생애 동안 발현되는 반면,

성인 조직에서는 테나신-C가 매우 잘 검출되지 않고,

병리학적 과정의 과정에서 존재합니다 [27,28,29].

전신성 경화증(SSc)에서 지속적인 장기 섬유화를 유지하는 요인은 알려져 있지 않지만, 최근 증거에 따르면 SSc의 발병 기전에 Toll-like receptors(TLRs)가 관여하고 있는 것으로 보입니다. 여기에서는 SSc 환자의 피부, 피부 섬유 아세포, 그리고 장기 섬유증의 마우스 모델에서 내인성 TLR 활성화제의 발현, 작용 기전, 그리고 병원성 역할을 보여줍니다. 테나신-C의 수준은 SSc 피부 생검 샘플, 혈청, SSc 섬유 아세포, 그리고 마우스의 섬유증 피부 조직에서 상승합니다. 외인성 테나신-C는 TLR4 신호를 통해 콜라겐 유전자 발현과 근섬유 아세포의 변형을 촉진합니다. 테나신-C가 결핍된 생쥐는 피부와 폐의 섬유화가 약화되고, 섬유화 해결이 가속화되는 것으로 나타났습니다. 이러한 결과는 테나신-C가 SSc에서 상향 조절되는 내인성 위험 신호로서, TLR4 의존성 섬유세포 활성화를 유도하고, 그 지속성으로 인해 섬유화 해결을 방해한다는 것을 확인시켜 줍니다. 이 섬유화 증폭 고리를 방해하는 것은 SSc 치료를 위한 실행 가능한 전략이 될 수 있습니다.

기저막의 역할은

세포 반응을 조절함으로써 조직의 발달, 기능, 재생 등을 조절하는 것입니다.

기저막은

성장 인자를 저장하는 창고 역할을 하며,

성장 인자의 활동과 농도를 조절합니다.

기저막은 주변 세포의 표현형을 유지하는 역할을 합니다 [30].

간질 기질과 기저막은

밀접하게 연결되어 조직의 완전성을 보장합니다 [16].

위에서 설명한

기저막과 기능적으로 동등한 것은

관절 연골의 연골 세포를 둘러싸고 있는 세포질 주변 기질입니다 [31].

이것은

세포 안으로 들어오고 나가는 분자를 걸러내는

물리적 장벽 역할을 합니다.

인접한 얇은 기질층과 함께,

각 연골 세포는

연골이라는 구조 단위를 형성합니다 [32].

연골의 형태는

다양합니다.

이것들은 원반형/타원형/둥근 모양과 가변적인 방향을 가질 수 있는데,

이는 연골의 위치,

즉 연골 내 위치의 깊이에 따라 달라집니다.

어떤 경우에는

연골이 하나 이상의 세포(최대 4개)로 구성되기도 합니다[33].

이 경우, 세포 외 기질의 필수 구성 요소는

제6형 콜라겐이지만,

일반적으로 연골 조직의 콜라겐 중 무시할 만한 비율을 차지합니다[34].

그러나

관절 연골의 연골세포 환경에 특이적으로 존재하기 때문에,

종종 연골의 표지자 역할을 합니다 [34,35].

관절 연골의 특징은

합성, 조직화, 유지 관리를 담당하는 광범위한 세포 외(간질) 매트릭스에 비해

적은 수의 연골세포입니다 [34].

ECM의 유형을 설명할 때, '세포 외 기질'이라는 용어가 생략되고

기저막 자체로 대체되는 경우가 있는데,

이로 인해 기저막이 동일한 구조라고 잘못 가정하는 경우가 발생합니다 [17,18,19].

그러나

기저막은 세포 외 기질의 보다

특수한 형태로 간주되어야 합니다.

2.2. Major Components of the Extracellular Matrix and Their Functions

2.2.1. Collagens

Collagen proteins account for up to 30% of all proteins in vertebrates and are major extracellular matrix components. The basic collagen macromolecules are composed of three same (homotrimers) or different (heterotrimers) polypeptide chains. They are characterised by the repetitive Gly-X-Y sequences, where X usually stands for proline and Y for 4-hydroxyproline. The intertwined chains form a specific triple helix structure [17,36,37].

Due to their supramolecular organisation, fibrillar (types I, II, III, V, XI, XXIV, and XXVII) and non-fibrillar collagens are distinguished. Characteristic for the non-fibrillar collagens is a disrupted continuity of the typical structure. Compared to fibrillar collagens, they contain shorter (although more numerous) helical (collagenous) domains interspersed with so-called telopeptides, i.e., non-helical domains. As a result, they may occur in various forms, forming, e.g., network systems (types IV, VIII, X), anchor fibres (type VII), beaded filaments (type VI), or belong to the FACIT group (i.e., fibril-associated collagen with interrupted triple helix, types XI, XII, XIV, XVI, XIX-XXII). The terminology and affiliation are not fully systematised. Collectively, collagens form a family of 28 proteins [38,39,40,41,42,43,44].

Historically, collagens were thought to have only a supportive function. Although their main function is indeed to form the structural scaffolding of cells (especially for types I, II, and III), it is known that their role is much broader [15,18,37,45]. Collagens are involved in regulating the course of cell adhesion (as ligands of cell receptors) [46,47,48,49], cell migration (contact guidance) [50,51,52] and tissue reconstruction and remodelling [37,40,45,53,54]. Not only is the physical deposition or movement of cells itself important, but also the processes conditioned by this, e.g., wound healing, immune response, etc. Although collagens are present in most body tissues and affect their mechanical properties, their distribution varies, e.g., type I collagen is characteristic of bone, skin and tendon, and type II collagen of cartilage tissue [55,56].

2.2. 세포 외 기질의 주요 구성 요소와 그 기능

2.2.1. 콜라겐

콜라겐 단백질은

척추동물에서 모든 단백질의 30%를 차지하며,

세포 외 기질의 주요 구성 요소입니다.

기본 콜라겐 거대 분자는

3개의 동일한(호모트리머) 또는 다른(헤테로트리머) 폴리펩티드 사슬로 구성되어 있습니다.

이들은 반복적인 Gly-X-Y 서열을 특징으로 하는데,

여기서 X는 보통 프롤린을, Y는 4-하이드록시프롤린을 나타냅니다.

서로 얽힌 사슬들은 특정한 삼중 나선 구조를 형성합니다 [17,36,37].

그들의 초분자 조직 때문에,

섬유상(유형 I, II, III, V, XI, XXIV, XXVII)과 비섬유상 콜라겐이 구분됩니다.

fibrillar (types I, II, III, V, XI, XXIV, and XXVII) and

non-fibrillar collagens

비섬유질 콜라겐의 특징은

전형적인 구조의 연속성이 깨졌다는 것입니다.

Characteristic for the non-fibrillar collagens is a disrupted continuity of the typical structure.

섬유질 콜라겐과 비교했을 때,

비섬유질 콜라겐은 더 짧지만(더 많지만)

소위 텔로펩티드라고 불리는 비나선성 도메인,

즉 비나선성 콜라겐 도메인이 산재되어 있습니다.

그 결과, 다양한 형태로 존재할 수 있으며,

예를 들어, 네트워크 시스템(유형 IV, VIII, X), 앵커 섬유(유형 VII), 비드 필라멘트(유형 VI)를 형성하거나, FACIT 그룹(즉, 삼중 나선 구조가 중단된 피브릴 관련 콜라겐, 유형 XI, XII, XIV, XVI, XIX-XXII)에 속할 수 있습니다. 용어와 소속은 완전히 체계화되어 있지 않습니다. 콜라겐은 총 28개의 단백질 군을 형성합니다 [38,39,40,41,42,43,44].

역사적으로 콜라겐은

단지 보조적인 기능만을 가지고 있다고 여겨졌습니다.

그들의 주요 기능은

실제로 세포의 구조적 골격(특히 유형 I, II, III)을 형성하는 것이지만,

그들의 역할은 훨씬 더 광범위하다는 것이 알려져 있습니다 [15,18,37,45].

콜라겐은

세포 부착 과정(세포 수용체의 리간드로써) [46,47,48,49],

세포 이동(접촉 유도) [50,51,52] 및 조직 재건과 리모델링 [37,40,45,53,54]을 조절하는 데 관여합니다.

세포의 물리적 침착이나 이동 자체뿐만 아니라,

이로 인해 조절되는 과정,

예를 들어 상처 치유, 면역 반응 등도 중요합니다.

콜라겐은

대부분의 신체 조직에 존재하며

기계적 특성에 영향을 미치지만,

그 분포는 다양합니다.

예를 들어,

제1형 콜라겐은 뼈, 피부, 힘줄의 특징이고,

제2형 콜라겐은 연골 조직의 특징입니다 [55,56].

2.2.2. Elastin

Elastin is a hydrophobic fibrillar protein, which owes its characteristic elastic properties to extensive covalent cross-linking of the structure [57]. The monomer from which the mature insoluble protein is formed is tropoelastin, secreted by fibroblasts, smooth muscle cells, endothelial cells, respiratory epithelial cells, chondrocytes, and keratinocytes [58,59,60,61,62]. After secretion into the intercellular space, tropoelastin spontaneously associates into larger particles through interactions between hydrophobic domains in a process called coacervation [63]. Such precursors undergo oxidative deamination of lysine residues in tropoelastin. The process is catalysed by LOX family enzymes (lysyl oxidases). The result is the formation of allysine from lysine. Cross-linking occurs via the reaction between lysine and allysine residues (Schiff base reaction) or by aldol condensation of two allysine residues [64,65,66,67]. The final fibres are not composed of elastin alone. Elastin forms a core (about 90% of the whole structure), covered by an envelope of microfibrils composed mainly of glycoproteins from the fibrillin group (fibrillin-1 and -2) [58,68,69]. In this way, elastic fibres are formed, giving tissues susceptibility to stretching. They are a particularly important component of blood vessel walls, skin, lungs, heart, tendons, ligaments, bladder, elastic cartilage tissue (e.g., auricle, larynx, epiglottis), etc. [15,68,70].

The gene expression‎ and formation of elastic fibres occur at early development stages —prenatal and early childhood. De novo production of elastin in adult organisms is unlikely to occur, which is quite uncommon among the ECM components [58,71,72,73,74]. However, elastin has high metabolic stability and a half-life of approximately 70 years, making the limited synthesis time sufficient (by comparison, the half-life of type VII collagen is estimated to be approximately one month [75,76]. The adult organism cannot reconstitute elastic fibres that become damaged or degrade progressively with age. They are then repaired incorrectly and consequently do not perform their normal functions. The tissues become too stiff, leading to cardiovascular disease, lung disease or typical signs of ageing, such as loss of skin elasticity [15,67,77,78].

2.2.2. 엘라스틴

엘라스틴은

소수성 섬유 단백질로서,

그 특징적인 탄성 특성은

구조의 광범위한 공유 결합에 기인합니다 [57].

hydrophobic fibrillar protein

성숙한 불용성 단백질이 형성되는 단량체는

트로포엘라스틴으로,

섬유 아세포,

평활근 세포,

내피 세포,

호흡기 상피 세포,

연골 세포, 각질 세포에 의해 분비됩니다 [58,59,60,61,62].

세포 간 공간으로 분비된 후,

트로포엘라스틴은 코아세르베이션(coacervation)이라고 불리는 과정에서

소수성 영역 간의 상호작용을 통해

자발적으로 더 큰 입자로 결합합니다[63].

이러한 전구체는

트로포엘라스틴의 리신 잔기에서

산화 탈아미노화를 겪습니다.

이 과정은 LOX 계열 효소(lysyl oxidases)에 의해 촉매됩니다. 그

결과, 라이신으로부터 알리신이 형성됩니다.

교차결합은

라이신과 알리신 잔기 사이의 반응(쉬프 염기 반응) 또는

두 개의 알리신 잔기의 알돌 축합을 통해 발생합니다 [64,65,66,67].

최종 섬유는

엘라스틴만으로 구성되지 않습니다.

엘라스틴은

전체 구조의 약 90%를 차지하는 코어를 형성하고,

그 코어는 주로 피브릴린 그룹(피브릴린-1과 -2)의 당단백으로 구성된

미세섬유 외피로 덮여 있습니다[58,68,69].

이런 식으로

탄성 섬유가 형성되어

조직이 늘어나는 성질을 갖게 됩니다.

이들은 특히

혈관벽, 피부, 폐, 심장, 힘줄, 인대, 방광, 탄력성 연골 조직(예: 귓바퀴, 후두, 후두개) 등의

중요한 구성 요소입니다 [15,68,70].

탄력성 섬유의 유전자 발현과 형성은

태아기 및 유아기 등 초기 발달 단계에서 발생합니다.

성인 유기체에서 엘라스틴의 새로운 생성은 일어날 가능성이 낮으며,

이는 ECM 구성 요소들 사이에서 매우 드문 현상입니다 [58,71,72,73,74].

그러나

엘라스틴은

대사 안정성이 높고 반감기가 약 70년으로

제한된 합성 시간으로 충분합니다(비교해 보면, 제7형 콜라겐의 반감기는 약 1개월로 추정됩니다 [75,76].

elastin has high metabolic stability and a half-life of approximately 70 years

성인 유기체는

나이가 들면서 손상되거나 점진적으로 분해되는

탄성 섬유를 재구성할 수 없습니다.

그러면 제대로 복구되지 않아

정상적인 기능을 수행하지 못하게 됩니다.

조직이 너무 뻣뻣해져서

심혈관 질환, 폐 질환 또는 피부 탄력 상실과 같은

전형적인 노화 징후가 나타납니다 [15,67,77,78].

2.2.3. Proteoglycans

Proteoglycans are macromolecules of a complex three-dimensional structure. They are composed of a protein core covalently linked to one or more chains of glycosaminoglycans (GAGs), a type of linear, unbranched heteropolysaccharides. The glycosaminoglycan chains may belong to one or different types. Based on localisation, four basic groups of proteoglycans can be distinguished: intracellular and those occurring on the cell surface, in the pericellular space (basement membrane) or intercellular space [15,79,80,81].

GAG chains are built by repeating disaccharide units, where one residue is an amino sugar (N-acetylated hexosamine), and the other is uronic acid (D-glucuronic or L-iduronic acid). GAGs differ in the type of monosaccharide residues and the geometry of the linkages between the constituent units (α- and β-glycosidic linkages) and the degree of sulfation of the polysaccharide backbone and the position of this substitution. Based on the chemical structure of the chain, four basic groups of glycosaminoglycans are distinguished: heparan/heparan sulfate, keratan sulfate, chondroitin sulfate/dermatan sulfate, and hyaluronic acid [15,82,83,84,85,86]. Hyaluronic acid represents the simplest type of structure. It is the only one that does not contain sulfate groups (hydroxyl groups are not esterified with sulfate groups) and does not undergo complex modifications in the Golgi apparatus [84,87,88]. Unlike other GAGs, it does not form covalent bonds with proteins and, therefore, is not part of typical proteoglycans. Instead, it can exist in the form of non-covalent complexes with other protein components of the ECM [85,86,89,90,91].

Hyaluronan has excellent water retention ability. It is abundant in the skin, cartilage, brain, vitreous body, umbilical cord, and synovial fluid. Its physical and physiological properties depend on molecular weight and concentration in the tissue. When highly concentrated, hyaluronan molecules form a three-dimensional meshwork structure exhibiting remarkable viscoelasticity. The organised structure acts as a molecular sieve of proteins and other macromolecules. Hyaluronan is reported to modulate cellular behaviours via the reprogramming of cellular metabolism coupled to its production [92]. Hyaluronan activates signalling cascade by interacting with CD44 receptor. CD44 was originally identified as a hyaluronan and hyaluronic acid receptor but can bind to various other ligands. It also serves as a marker for stem cells of several types [93].

Glycosaminoglycan chains (and, therefore, the proteoglycans) are negatively charged. It is the result of carboxyl and sulfate residues in their structure [88,94]. Due to the strong negative charge, these molecules tend to elongate in solution under physiological conditions. This allows them to bind large amounts of water and form a gel. Such properties provide tissues with resistance to deformation by high physical forces, as exemplified by aggrecan, the most important cartilage proteoglycan [79,86,95,96]. The proteoglycan family also includes compounds, such as syndecans (trans-membrane receptors; they bind numerous ligands present in the ECM, mediate signal transduction, cell adhesion, migration et al.) [97,98], serglycin (the only known intracellular proteoglycan; found in leukocyte granules, regulates granulopoiesis) [99,100,101], perlecan and agrin (characteristic of the basement membrane, regulators of many cellular processes; agrin is involved in the formation of neuromuscular synapses) [102,103,104,105,106,107] and fibromodulin (involved in the collagen fibrillogenesis) [108,109].

2.2.3. 프로테오글리칸

프로테오글리칸은

복잡한 3차원 구조의 거대 분자입니다.

이들은 선형, 비분지 헤테로다당류의 일종인 글리코사미노글리칸(GAG)의 하나 이상의 사슬에 공유결합으로 연결된 단백질 코어로 구성되어 있습니다. 글리코사미노글리칸 사슬은 하나 또는 여러 가지 유형에 속할 수 있습니다. 위치에 따라, 세포 내 및 세포 표면, 세포 주변 공간(기저막) 또는 세포 간 공간에서 발생하는 네 가지 기본 프로테오글리칸 그룹을 구분할 수 있습니다 [15,79,80,81].

GAG 사슬은 이당류 단위가 반복적으로 결합되어 만들어지는데,

한쪽은 아미노당(N-아세틸화 헥소사민)이고,

다른 한쪽은 우론산(D-글루쿠론산 또는 L-이두론산)입니다.

GAG는

단당류 잔기의 유형과 구성 단위(α- 및 β-글리코시드 결합) 사이의 결합의 기하학적 구조,

다당류 골격의 황화 정도,

그리고 이 치환의 위치가 다릅니다.

사슬의 화학 구조에 따라

네 가지 기본 글리코사미노글리칸 그룹이 구분됩니다:

헤파란/헤파란 설페이트,

케라탄 설페이트,

콘드로이틴 설페이트/더마탄 설페이트,

히알루론산 [15,82,83,84,85,86].

히알루론산은 가장 단순한 형태의 구조를 나타냅니다.

히알루론산은

황산염기를 포함하지 않으며(히드록실기가 황산염기와 에스테르화되지 않음),

골지체에서 복잡한 변형을 거치지 않습니다[84,87,88].

다른 GAG와는 달리,

히알루론산은 단백질과 공유 결합을 형성하지 않으므로,

전형적인 프로테오글리칸의 일부가 아닙니다.

대신,

ECM의 다른 단백질 구성 요소와 비공유 복합체의 형태로 존재할 수 있습니다 [85,86,89,90,91].

히알루론산은

우수한 수분 보유 능력을 가지고 있습니다.

히알루론산은

피부, 연골, 뇌, 유리체, 탯줄, 활액 등에 풍부하게 존재합니다.

히알루론산의 물리적, 생리적 특성은

분자량과 조직 내 농도에 따라 달라집니다.

고농축 상태의 히알루론산 분자는

놀라운 점탄성을 나타내는 3차원 망상 구조를 형성합니다.

이 조직화된 구조는 단백질과 다른 거대 분자의 분자 체 역할을 합니다.

히알루론산은

세포 대사의 재프로그래밍과 결합된 생산을 통해

세포의 행동을 조절하는 것으로 보고되었습니다 [92].

히알루론산은

CD44 수용체와 상호 작용하여

신호 전달 과정을 활성화합니다.

CD44는 원래 히알루론산과 히알루론산 수용체로 확인되었지만,

다양한 다른 리간드에 결합할 수 있습니다.

또한

여러 유형의 줄기세포에 대한

표지자 역할을 합니다 [93].

글리코사미노글리칸 사슬(그리고, 따라서, 프로테오글리칸)은

음전하를 띱니다.

이것은

그 구조에 있는 카르복실기와 황산염 잔기 때문에

생기는 현상입니다 [88,94].

강한 음전하 때문에,

이 분자들은 생리학적 조건 하에서 용액에서 길어지려는 경향이 있습니다.

이 때문에 다량의 물을 결합하여 젤을 형성할 수 있습니다.

이러한 특성 덕분에 가장 중요한 연골 프로테오글리칸인 아그레칸(aggrecan)이 보여주는 것처럼, 조직이 높은 물리적 힘에 의한 변형에 저항할 수 있습니다 [79,86,95,96]. 프로테오글리칸 계열에는 신데칸(syndecans)과 같은 화합물도 포함됩니다(신체막 수용체; ECM에 존재하는 수많은 리간드에 결합하고, 신호 전달, 세포 부착, 이동 등을 매개합니다). [97,98], 세르글리신(단백질-글리칸으로 알려진 유일한 세포 내 단백질-글리칸; 백혈구 과립에서 발견되며, 과립구 생성을 조절함) [99,100,101], 펄레칸과 아그린(기저막의 특징, 많은 세포 과정의 조절자; 아그린은 신경근육 시냅스 형성에 관여함) [102,103,104,105,106,107] 및 피브로모둘린(콜라겐 섬유형성에 관여) [108,109].

2.2.4. Glycoproteins

Like proteoglycans, glycoproteins are composed of covalently linked protein and carbohydrate parts. However, the saccharide chains are much shorter, contain no (or few) repeating units, and are usually branched [2,110,111]. Glycoproteins often act as connectors in the ECM, as they have functional groups capable of binding other proteins, growth factors, or receptors [2,112,113]. Their participation is essential for many biological processes: fertilisation, immune and inflammatory response, blood coagulation, wound healing, etc. [112,114,115,116,117,118,119,120]. The two most important glycoproteins are fibronectin and laminin. The glycoprotein family also includes fibulins [121], tenascin [122], fibrinogen [123], vitronectin [124], osteonectin [27], bone sialoprotein [125], and reelin [126].

The basic structural unit of fibronectin is a dimer composed of two nearly identical polypeptide chains linked by a pair of disulfide bonds. Each such chain is built by irregularly repeating amino acid units (types I, II, and III), forming a mosaic structure of the protein. The molecules consist of domains, i.e., differently structured sections with different functions [127,128,129]. Fibronectin contains domains capable of interacting with the ECM proteins (e.g., collagen), glycosaminoglycans, surface receptors and other fibronectin molecules. Due to these properties, fibronectin can simultaneously bind to cells and components of the surrounding matrix [128,130,131,132,133,134]. In the body, fibronectin exists in two forms: soluble plasma fibronectin (synthesised by hepatocytes and secreted into the blood) and insoluble cellular fibronectin (produced by fibroblasts, endothelial cells, chondrocytes, myocytes, and others). The insoluble form is a fibrillar cross-linked structure on the cell surface and in the ECM. It is responsible for cell adhesion, proliferation, migration, and the ECM protein deposition [128,135,136,137,138,139]. Both forms of fibronectin are encoded by one gene, while structural differences result from alternative mRNA splicing [140,141].

Laminins are a group of large, multi-domain glycoproteins of a heterotrimeric structure. The three subunits (α, β, and γ chains) connected by a pair of disulfide bonds form a characteristic Latin cross-shaped structure (a Y-shape/rod shape form is also possible [142,143,144]). The three shorter arms (their globular N-terminal domains) are mainly involved in laminin polymerisation and network self-assembly. At the same time, the longer one mediates cell–cell interactions by binding to receptors [145,146,147,148,149]. Proteins of the laminin family are an integral part of the basement membrane and play an essential role in forming and maintaining its structure. A critical step in developing the basement membrane is the polymerisation of laminin [149,150,151]. This process is initiated by binding laminin molecules to the cell surface. A connection is formed between the long arm of the protein and the receptors—cognate integrin and dystroglycan. As a result, there is a local increase in the concentration of laminin, and after exceeding a critical value, polymerisation occurs. The structure, thus, formed binds to nidogens and HSPGs (perlecan). The entire network is further stabilised by polymerising type IV collagen [151,152,153,154,155,156,157,158]. The basement membrane layer built up by the complex network of the described components is called lamina densa (the middle layer between the lamina lucida and the lamina fibroreticularis [159].

2.2.4. 당단백질

프로테오글리칸과 마찬가지로,

당단백질은 공유결합으로 연결된

단백질과 탄수화물 부분으로 구성되어 있습니다.

그러나 당쇄는 훨씬 짧고, 반복 단위가 없거나(또는 거의 없음) 일반적으로 분지되어 있습니다 [2,110,111]. 당단백질은 다른 단백질, 성장 인자 또는 수용체를 결합할 수 있는 작용기를 가지고 있기 때문에 ECM에서 연결자 역할을 하는 경우가 많습니다 [2,112,113]. 이러한 단백질의 참여는 수정, 면역 및 염증 반응, 혈액 응고, 상처 치유 등 많은 생물학적 과정에 필수적입니다 [112,114,115,116,117,118,119,120]. 가장 중요한 두 가지 당단백질은 피브로넥틴과 라미닌입니다. 당단백질 계열에는 피불린[121], 테나신[122], 피브리노겐[123], 비트로넥틴[124], 오스테오넥틴[27], 뼈 시알로프로테인[125], 그리고 릴린[126]도 포함됩니다.

피브로넥틴의 기본 구조 단위는 두 개의 거의 동일한 폴리펩티드 사슬이 한 쌍의 이황화 결합으로 연결된 이량체로 구성되어 있습니다. 이러한 각 사슬은 불규칙적으로 반복되는 아미노산 단위(유형 I, II, III)로 구성되어 단백질의 모자이크 구조를 형성합니다. 분자는 도메인, 즉 서로 다른 기능을 가진 서로 다른 구조의 부분으로 구성되어 있습니다[127,128,129]. 피브로넥틴은 ECM 단백질(예: 콜라겐), 글리코사미노글리칸, 표면 수용체 및 다른 피브로넥틴 분자와 상호작용할 수 있는 도메인을 포함하고 있습니다. 이러한 특성 때문에 피브로넥틴은 주변 매트릭스의 세포와 구성 요소에 동시에 결합할 수 있습니다 [128,130,131,132,133,134]. 인체에서 피브로넥틴은 수용성 혈장 피브로넥틴(간세포에 의해 합성되어 혈액으로 분비됨)과 불용성 세포 내 피브로넥틴(섬유 아세포, 내피 세포, 연골 세포, 근세포 등에 의해 생성됨)의 두 가지 형태로 존재합니다. 불용성 형태는 세포 표면과 ECM에 있는 섬유상 교차 결합 구조입니다. 그것은 세포 부착, 증식, 이동, 그리고 ECM 단백질 침착을 담당합니다 [128,135,136,137,138,139]. 두 가지 형태의 피브로넥틴은 모두 하나의 유전자에 의해 암호화되지만, 구조적 차이는 대체 mRNA 스플라이싱에 의해 발생합니다 [140,141].

라미닌은 이종 삼량체 구조의 큰 다중 도메인 당단백의 그룹입니다. 이황화 결합으로 연결된 세 개의 서브유닛(α, β, γ 사슬)은 특징적인 라틴 십자가 모양의 구조를 형성합니다(Y자형/막대 모양도 가능합니다 [142,143,144]). 세 개의 짧은 팔(구형 N-말단 영역)은 주로 라미닌 중합과 네트워크 자기 조립에 관여합니다. 동시에, 더 긴 쪽은 수용체에 결합함으로써 세포-세포 상호작용을 매개합니다 [145,146,147,148,149]. 라미닌 계열의 단백질은 기저막의 필수적인 부분이며, 기저막의 구조를 형성하고 유지하는 데 중요한 역할을 합니다. 기저막을 형성하는 데 중요한 단계는 라미닌의 중합입니다 [149,150,151]. 이 과정은 라미닌 분자를 세포 표면에 결합시킴으로써 시작됩니다. 단백질의 긴 팔과 수용체(동종 인테그린과 디스트로글리칸) 사이에 연결이 형성됩니다. 그 결과, 라미닌의 농도가 국소적으로 증가하고, 임계값을 초과하면 중합이 일어납니다. 이렇게 형성된 구조는 니도겐과 HSPG(펄레칸)에 결합합니다. 전체 네트워크는 IV형 콜라겐의 중합에 의해 더욱 안정화됩니다 [151,152,153,154,155,156,157,158]. 설명된 구성 요소들의 복잡한 네트워크에 의해 구축된 기저막층은 라미나 덴사(lamina densa)라고 불립니다(라미나 루시다와 라미나 피브로레티큘라리스[lamina lucida and the lamina fibroreticularis] 사이의 중간층 [159]).

2.3. The Dynamic Structure of the Extracellular Matrix

The structure of the extracellular matrix undergoes continuous remodelling, during which changes in its composition and overall architecture occur. Cells embedded in the ECM are actively involved in its reorganisation. In addition to synthesising and secreting building components, they are also the source of enzymes that degrade these components. Remodelling processes are complex and must be tightly regulated to maintain environmental homeostasis [19,160,161,162].

Protein-degrading enzymes belong to the class of hydrolases and are called proteases (proteinases). Depending on the mechanism of catalysis, they can be divided into several families, including serine proteases (serine residue in the enzyme active site), cysteine proteases (cysteine residue) or metalloproteases (they require the presence of a metal cation in the active centre). These enzymes can be secreted by the cell into its external environment or remain anchored in the cell membrane [163,164].

The main group of enzymes involved in ECM degradation are the zinc-dependent matrix metalloproteinases (MMPs). More than 20 representatives of this group are known, capable of degrading different types of collagen, gelatin, elastin, laminin, fibronectin and many others [165,166,167]. The sources of MMPs are mainly connective tissue cells (fibroblasts, osteoblasts), inflammatory cells (macrophages, neutrophils, mast cells), and endothelial cells [165,168]. MMPs are secreted in the form of zymogens, inactive precursors that must undergo biochemical modifications to be activated [19,165,168]. Through controlled degradation of ECM proteins, metalloproteinases facilitate cell migration and trigger the release of growth factors [169,170,171]. They participate in tissue remodelling, an interesting example of which is postpartum uterine involution. In addition, they regulate angiogenesis (blood vessel formation), wound healing, embryonic development, etc. [165,172,173]. In pathological states, their abnormal and/or increased activity contributes to the course of cardiovascular, cancer, autoimmune diseases, etc. [165,174,175,176].

The proteolysis occurring in tissues relates not only to the extracellular matrix per se but also concerns the so-called ectodomain shedding, i.e., proteolytic cleavage of cell surface proteins. Modification, degradation, and changes in the activity of these proteins are one of the mechanisms of the cell’s response to changes in microenvironment conditions [177,178]. Enzymes of the ADAM (a disintegrin and metalloproteases) family, also known as adamalysins, are mainly involved in this process. They have various functions, primarily engaged in intercellular interactions and signal transduction [19,179,180]. The release of biologically active extracellular domains of multiple proteins (cytokines, adhesion molecules, growth factors) from the cell membrane can contribute, e.g., to inflammation (physiological and pathological), as occurs as a result of ADAM17 enzyme activity. The pro-inflammatory action of this sheddase consists of a modification of the cell surface and enrichment of its environment with active soluble molecules [181,182,183,184]. The structure and function of ADAM group proteins are similar to the metalloproteinases found in snake venom, responsible for the typical effects of snakebites (haemorrhage, tissue necrosis) [185].

2.3. 세포 외 기질의 동적 구조

세포 외 기질의 구조는 지속적인 재구성을 거치며, 이 과정에서 구성과 전체 구조의 변화가 발생합니다. ECM에 내장된 세포는 이 구조의 재구성에 적극적으로 관여합니다. 세포는 구성 요소를 합성하고 분비하는 것 외에도, 이러한 구성 요소를 분해하는 효소의 원천이기도 합니다. 재구성 과정은 복잡하며 환경 항상성을 유지하기 위해 엄격하게 규제되어야 합니다 [19,160,161,162].

단백질 분해 효소는 가수분해 효소의 한 종류이며 프로테아제(단백질 분해 효소)라고 불립니다. 촉매 작용의 메커니즘에 따라 세린 프로테아제(효소 활성 부위의 세린 잔기), 시스테인 프로테아제(시스테인 잔기) 또는 메탈로프로테아제(활성 중심에 금속 양이온이 있어야 함)를 포함한 여러 가지 종류로 나눌 수 있습니다. 이러한 효소는 세포에 의해 외부 환경으로 분비되거나 세포막에 고정되어 있을 수 있습니다[163,164].

ECM 분해에 관여하는 주요 효소 그룹은 아연 의존성 매트릭스 메탈로프로테이나제(MMP)입니다. 이 그룹의 대표적 효소로는 20가지가 넘으며, 이들은 다양한 유형의 콜라겐, 젤라틴, 엘라스틴, 라미닌, 피브로넥틴 등을 분해할 수 있는 것으로 알려져 있습니다 [165,166,167]. MMP의 공급원은 주로 결합 조직 세포(섬유아세포, 골아세포), 염증 세포(대식세포, 호중구, 비만 세포), 내피 세포입니다[165,168]. MMP는 활성화되기 위해서는 생화학적인 변형을 거쳐야 하는 비활성 전구체인 지모겐의 형태로 분비됩니다[19,165,168]. ECM 단백질의 분해를 조절함으로써, 메탈로프로테이나제는 세포 이동을 촉진하고 성장 인자의 방출을 유발합니다 [169,170,171]. 그들은 조직 재형성에 관여하는데, 흥미로운 예로 산후 자궁 퇴축이 있습니다. 또한, 그들은 혈관 신생(혈관 형성), 상처 치유, 배아 발달 등을 조절합니다 [165,172,173]. 병리학적 상태에서는 이들의 비정상적 및/또는 증가된 활동이 심혈관 질환, 암, 자가면역 질환 등의 진행에 기여합니다 [165,174,175,176].

조직에서 일어나는 단백질 분해는 세포 외 기질 자체와 관련이 있을 뿐만 아니라, 세포 표면 단백질의 단백질 분해적 절단인 소위 말단부(ectodomain)의 분해와도 관련이 있습니다. 이러한 단백질의 변형, 분해, 그리고 활동의 변화는 미세환경 조건의 변화에 대한 세포의 반응 메커니즘 중 하나입니다 [177,178]. 아다말리신(adamalysin)으로도 알려진 ADAM(a disintegrin and metalloproteases) 계열의 효소가 주로 이 과정에 관여합니다. 이들은 다양한 기능을 가지고 있으며, 주로 세포 간 상호작용과 신호 전달에 관여합니다 [19,179,180]. 세포막으로부터 여러 단백질(사이토카인, 접착 분자, 성장 인자)의 생물학적 활성 세포외 영역이 방출되면, 예를 들어, ADAM17 효소 활동의 결과로 발생하는 염증(생리적 및 병리학적)에 기여할 수 있습니다. 이 탈락효소의 전염성 작용은 세포 표면의 변형과 활성 가용성 분자로 환경의 풍부화로 구성됩니다 [181,182,183,184]. ADAM 그룹 단백질의 구조와 기능은 뱀독에서 발견되는 메탈로프로테이나제(metalloproteinase)와 유사하며, 뱀에 물렸을 때 나타나는 전형적인 증상(출혈, 조직 괴사)을 일으키는 원인이 됩니다[185].

2.4. The Extracellular Matrix as a Storehouse of Growth Factors

The ECM significantly influences the cell’s most important natural biological processes: growth, proliferation, and programmed death [186]. In addition to mediating interactions and activating relevant mechanisms by contact with its building proteins, the ECM serves as a storehouse of growth factors (and proteases and protease inhibitors). These molecules can be released by proteolytic degradation of the matrix, and the degradation itself regulates the rate, site and intensity of such activation. The fact that growth factors are stored in the vicinity of cells favours increased specificity of their action [19,187,188,189].

Growth factors are generally not freely dispersed in the extracellular space but bind, for example, to heparan sulphate proteoglycans. HSPGs then participate in the matrix storage function by preventing the movement and proteolysis of growth factors. They allow their controlled release when necessary. However, another role of HSPGs is also to bind to such molecules to activate them. Then, they act as a coreceptor in ligand–receptor interactions [187,190,191,192]. The type of interaction of HSPGs with growth factors depends on the localisation of these proteoglycans. They may remain anchored to the cell membrane or form a structural component of the ECM [193].

A well-studied group is the fibroblast growth factors (FGF), which include 22 proteins with key functions in cell development, morphogenesis, tissue repair processes, and angiogenesis. They are among the neurotrophic factors, i.e., those that stimulate and regulate neurogenesis. Some are being investigated for involvement in the development of depression [192,194]. FGF molecules are mainly bound by heparan sulfate and heparin chains [195,196]. Proteolytic release of FGF allows subsequent binding of FGF ligands to receptors on the cell surface. This stimulates cell signalling [2].

Another example is transforming growth factor-beta (TGF-β), specifically its three isoforms, responsible for stimulating and inhibiting cellular proliferation. Among other things, the TGF-β cytokine controls the course of wound healing by interacting with different cell types. For example, TGF-β1 is released in large amounts at the wounding site by platelets and stimulates chemotaxis of monocytes and fibroblasts [197]. Cells secrete biologically inactive TGF-β molecules, which, in the latent form, are bound by matrix proteins (via glycoproteins of the LTBP family bound to ECM proteins, mainly fibronectin and fibrillins) in the form of complexes [188,198,199]. TGF-β activation (in vivo and in vitro) can occur in several ways: through enzymatic proteolysis of the complex (matrix metalloproteinases, serine proteases) [200,201], interaction with integrins [202,203,204] or other proteins [205,206] and in response to physicochemical conditions (radiation [207,208], low/high pH [209,210], temperature [211], and reactive oxygen species [212]).

2.4. 성장 인자의 저장소로서의 세포 외 기질(ECM)

ECM은 세포의 가장 중요한 자연적 생물학적 과정인 성장, 증식, 그리고 프로그램된 죽음에 상당한 영향을 미칩니다 [186]. ECM은 그 구성 단백질과의 접촉을 통해 상호 작용을 매개하고 관련 메커니즘을 활성화하는 것 외에도 성장 인자(및 프로테아제와 프로테아제 억제제)의 저장소 역할을 합니다. 이러한 분자들은 매트릭스의 단백질 분해 작용에 의해 방출될 수 있으며, 분해 작용 자체가 활성화의 속도, 위치, 강도를 조절합니다. 성장 인자가 세포 근처에 저장된다는 사실은 그 작용의 특이성을 증가시키는 데 유리합니다 [19,187,188,189].

성장 인자는 일반적으로 세포 외 공간에 자유롭게 분산되지 않고, 예를 들어 헤파란 설페이트 프로테오글리칸에 결합합니다. HSPG는 성장 인자의 이동과 단백질 분해를 방지함으로써 기질 저장 기능에 관여합니다. 필요할 때 조절된 방출을 가능하게 합니다. 그러나 HSPG의 또 다른 역할은 이러한 분자를 결합하여 활성화하는 것입니다. 그런 다음, 그들은 리간드-수용체 상호작용에서 코어셉터 역할을 합니다 [187,190,191,192]. HSPG와 성장 인자의 상호작용 유형은 이러한 프로테오글리칸의 국소화에 달려 있습니다. 이들은 세포막에 고정되어 있거나, ECM의 구조적 구성 요소를 형성할 수 있습니다 [193].

잘 연구된 그룹은 섬유 아세포 성장 인자(FGF)로, 세포 발달, 형태 형성, 조직 복구 과정, 혈관 신생에 핵심적인 기능을 하는 22개의 단백질을 포함합니다. 이들은 신경 영양 인자, 즉 신경 발생을 자극하고 조절하는 인자 중 하나입니다. 일부 FGF는 우울증의 발달과 관련이 있는 것으로 조사되고 있습니다 [192,194]. FGF 분자는 주로 헤파란 설페이트와 헤파린 사슬에 의해 결합됩니다 [195,196]. FGF의 단백질 분해 방출은 FGF 리간드가 세포 표면의 수용체에 결합할 수 있도록 합니다. 이것은 세포 신호를 자극합니다 [2].

또 다른 예로, 세포 증식을 자극하고 억제하는 역할을 하는 변형 성장 인자-베타(TGF-β), 특히 그 세 가지 이소형이 있습니다. 무엇보다도, TGF-β 사이토카인은 다양한 세포 유형과 상호작용함으로써 상처 치유의 과정을 조절합니다. 예를 들어, TGF-β1은 혈소판에 의해 상처 부위에 대량으로 방출되어 단핵구와 섬유 아세포의 화학 주성을 자극합니다 [197]. 세포는 생물학적으로 비활성인 TGF-β 분자를 분비하는데, 이 분자는 잠재 형태로 매트릭스 단백질에 결합되어 복합체의 형태로 존재합니다 (주로 피브로넥틴과 피브릴린과 같은 ECM 단백질에 결합된 LTBP 계열의 당단백질을 통해) [188,198,199]. TGF-β 활성화(생체 내 및 생체 외)는 여러 가지 방법으로 일어날 수 있습니다: 복합체의 효소적 단백질 분해(매트릭스 메탈로프로테이나제, 세린 프로테아제) [200,201], 인테그린과의 상호작용 [202,203,204 ] 또는 다른 단백질 [205,206]과 상호작용하고 물리화학적 조건(방사선 [207,208], 낮은/높은 pH [209,210], 온도 [211], 그리고 활성 산소 [212])에 반응하여 활성화될 수 있습니다.

2.5. Anoikis—Programmed Death

The normal functioning of most cells depends on their proper connection to the matrix. The cell is informed of this connection mainly by integrins (one type of membrane protein), acting as the ECM signal transducers [3,8,213,214]. However, if a cell detaches from the surface of the matrix, there is a risk that it will move and become embedded elsewhere. To prevent the abnormal proliferation of cells away from their parent tissue, a defence mechanism called anoikis (Greek for homeless), a type of apoptosis, is induced in the adherent cell if contact with the matrix is lost [215,216,217,218].

The term anoikis was introduced in 1994 by Frisch and Francis, who conducted studies on Madin–Darby Canine Kidney (MDCK) [217]. The relationship between a cell’s ability to anchor to a substrate and proliferate by affecting cell cycle progression was already known [219]. In subsequent years, this form of apoptosis was studied and described in a number of different types of adherent cells, such as fibroblasts [220,221], endothelial cells [222,223], keratinocytes [224,225], oligodendrocytes (glial cells) [226], and neurons (dopaminergic) [227], as well as bronchial [228,229], intestinal [230,231,232], or mammary gland epithelial cells [233]. The mechanism of anoikis can be induced by various signalling pathways, all of which ultimately lead to the activation of proteolytic enzymes of the caspase family and the degradation of cellular proteins [224,233,234,235,236]. Apoptotic cell death comes to an end with removing the cell’s genetic material, i.e., DNA fragmentation controlled by endonuclease enzymes [236,237].

The acquisition of resistance to anoikis is a characteristic feature of circulating tumour cells, enabling them to survive in non-adherent conditions. After detaching from the primary tumour, they are transported with the peripheral blood and are responsible for forming metastases [236]. Studies suggest that there are different mechanisms for the development of such immunity [238,239,240,241].

2.5. 아노이키스(Anoikis) — 프로그램화된 죽음

대부분의 세포가 정상적으로 기능하기 위해서는 기질과의 적절한 연결이 필요합니다. 세포는 주로 ECM 신호 변환기 역할을 하는 인테그린(integrin, 막 단백질의 일종)을 통해 이러한 연결을 인지합니다 [3,8,213,214]. 그러나 세포가 기질 표면에서 분리되면 이동하여 다른 곳에 박힐 위험이 있습니다. 모세포 조직으로부터 떨어져 비정상적으로 증식하는 것을 방지하기 위해, 매트릭스와의 접촉이 끊어지면 부착된 세포에서 아노이키스(그리스어로 노숙자라는 뜻)라는 일종의 세포자살 방어 메커니즘이 유도됩니다 [215,216,217,218].

아노이키스라는 용어는 1994년 프리쉬(Frisch)와 프랜시스(Francis)가 마딘-다비 개 신장(MDCK)에 대한 연구를 수행하면서 처음 사용되었습니다 [217]. 기질에 고정되어 세포주기의 진행에 영향을 미치면서 증식하는 세포의 능력 사이의 관계는 이미 알려져 있었습니다 [219]. 이후 몇 년 동안, 이러한 형태의 세포 사멸이 섬유 아세포[220,221], 내피 세포[222,223], 각질 형성 세포[224,225], 올리고교세포(oligodendrocytes)와 같은 다양한 유형의 부착 세포에서 연구되고 설명되었습니다. 신경교세포[226], 그리고 신경세포(도파민성)[227], 기관지[228,229], 장[230,231,232], 또는 유선 상피세포[233]와 같은 다양한 유형의 부착 세포에서 이러한 형태의 세포 사멸이 연구되고 기술되었습니다. 아노이키스의 메커니즘은 다양한 신호 전달 경로에 의해 유도될 수 있으며, 이 모든 경로는 궁극적으로 카스파제 계열의 단백질 분해 효소의 활성화와 세포 단백질의 분해를 유도합니다 [224,233,234,235,236]. 세포의 유전 물질, 즉 엔도뉴클레아제 효소에 의해 제어되는 DNA 단편화가 제거되면 세포의 아폽토시스 세포 사멸이 종료됩니다 [236,237].

아노이키스에 대한 저항성을 획득하는 것은 순환하는 종양 세포의 특징적인 특징으로, 비접착성 조건에서 생존할 수 있게 해줍니다. 원발성 종양에서 분리된 후, 그들은 말초혈액과 함께 운반되어 전이를 형성하는 역할을 합니다 [236]. 연구에 따르면 이러한 면역의 발달에는 다양한 메커니즘이 존재한다고 합니다 [238,239,240,241].

2.6. Genetic Mutations of Matrix Components and Their Consequences

To emphasise how important the extracellular matrix components are for the proper functioning of the organism, one should look at the consequences of abnormalities in their synthesis. The consequence of mutations in genes encoding the ECM proteins is a wide group of genetic disorders of connective tissue, with better or worse understood pathogenesis and often varied course. Dysfunction of the matrix may occur due to two different mechanisms of mutations. The first one involves a violation of the structural integrity of the matrix by quantitative reduction in its components as a result of nonsense mutations (formation of a premature stop codon) and/or frameshift mutations (insertion/deletion of a number of nucleotides indivisible by three). In the second type, the secretion of mutant proteins qualitatively affects the matrix structure, as it disrupts the stability of their interactions with normal, genetically unaltered components [242,243].

One of the most important is osteogenesis imperfecta (OI), a group of inherited disorders characterised by low bone mass leading to increased bone susceptibility to fracture [242,243]. The incidence rate is estimated to be around 1/10,000 births [244], so it is a relatively rare condition. In most cases, OI is caused by mutations in the genes encoding the α1(I) and α2(I) chains of type I collagen, manifesting as reduced production of this protein or its structural deformities [244,245]. Several clinical types of OI have been identified. According to Sillence’s 1979 classification, four are distinguished based on clinical and radiological symptoms and mode of inheritance [246]. New OI types have been described in recent years, resulting from mutations in so-called non-collagenous genes [247]. Clinically, however, they do not differ from the classical forms of this disease and are, therefore, included in them [242]. A rather peculiar symptom of OI is the blue sclera. Collagen fibres are one of the main building components of the sclera. A reduction in their thickness causes the deeper-laying choroid to become visible [244,248,249].

Another group of diseases mainly associated with abnormalities in the synthesis of fibrillar collagens or enzymes responsible for their post-translational processing is Ehlers-Danlos syndrome (EDS). Thirteen subtypes of EDS have been recognised (six according to the older Villefranche classification), manifested by a range of symptoms. The most characteristic is joint hypermobility, skin hyperelasticity, and general tissue tenderness [250,251,252]. The changes seen in classical EDS include loosely and irregularly packed collagen fibres and fibres called “cauliflowers” because of their characteristic cross-sectional shape. In the normal fibres, the cross-section is circular [251,253]. People affected by the vascular type of Ehlers–Danlos syndrome have translucent skin, a distinctive facial appearance (thin lips and nose, small chin, large eyes), and are prone to spontaneous bruising and a rupture of the arteries, intestine, and, in the case of pregnancy, the uterus [254,255]. It is estimated that between 1/2500 and 1/5000 people suffer from EDS. These numbers may be underestimated because patients with mild symptoms often go undiagnosed [252,255,256]. Depending on the type, EDS is inherited in an autosomal dominant or recessive manner, but a de novo mutation may also occur [252]. Like OI, Ehlers–Danlos syndrome is an incurable disease. Current therapies are aimed only at improving the quality of the patient life [255].

Genetic diseases of connective tissue do not only include abnormalities in collagen synthesis. One of the representatives of fibrilinopathies is Marfan syndrome (MFS), caused by a mutation in the FBN1 gene located in chromosome 15, which encodes fibrillin-1 [257]. Structural defects of this protein result in a violation of the stability of elastin fibres and their disorganisation in the connective tissue of various organs [258,259]. In addition, fibrillin-1 can bind TGF-β, so its dysfunctions result in increased levels of free TGF-β, activating abnormal degradation mechanisms [260]. The greatest threat to the lives of patients diagnosed with MFS is related to cardiovascular dysfunction. Progressive aortic root dilatation associated with the disintegration of elastin fibres causes aortic dissection, dangerous especially in the ascending part, i.e., closest to the heart. The formation of aneurysms is also possible. If undiagnosed and untreated, such abnormalities can be fatal at an early age [261,262,263]. Nevertheless, the most characteristic changes in the MFS course are those in the osteoarticular system. These include disproportionately long limbs and arachnodactyly (“spider fingers”), deformities of the thorax and spine (scoliosis, pathological kyphosis), protrusion acetabula (medial displacement of the acetabulum into the true pelvis), and overly flexible joints. These changes are often accompanied by ocular abnormalities, such as ectopia lentis [261,264,265]. Due to medical advancement, especially the possibility of preventive aortic aneurysm surgery, the life expectancy of people affected by Marfan syndrome has nearly doubled over the years [261,265,266].

Disorders caused by mutations in genes encoding elements of the extracellular matrix are, of course, far more numerous than those mentioned. They include several other collagenopathies, such as Stickler syndrome [267,268], Bethlem myopathy [269,270], Ullrich congenital muscular dystrophy [271,272], or the dystrophic epidermolysis bullosa [273,274]. Conditions may also result, for example, from abnormalities in the structure and function of perlecan (Schwartz–Jampel syndrome [275,276]), laminin (Pierson syndrome [277,278]), fibulin (age-related macular degeneration [279,280]). Many of those mentioned are rare disorders, still poorly understood.

3. Interactions between Cells and Their Environment

Receiving signals from the surrounding environment is essential for normal cell development and function. The environment includes other cells, the extracellular matrix and various soluble factors. Information arriving from outside as a ligand (chemical substance) or physical stimulus is received by a membrane receptor or intracellular receptor. The information is transduced (carried and appropriately transformed) into the cell. It initiates a series of reactions and changes the cell’s physiological behaviour or maintenance of appropriate activity. A complex communication system is the basis for regulating cellular processes and, as a result, the functioning of the whole organism [2,281,282].

3.1. Forms of Signalling

The basic type of communication between cells is the transfer of information in the form of a chemical compound, i.e., a protein, peptide, amino acid, lipid or their derivatives. These are various hormones, cytokines, growth factors, neurotransmitters, etc., synthesised by signalling cells. Once released into the intercellular space, they can be bound as ligands by receptors capable of recognising them [283,284,285,286,287,288,289,290]. Depending on the distance the ligand must travel between the signalling and target cell, endocrine, paracrine and autocrine signalling can be distinguished. A fourth type is signalling by direct contact [4,8] (see Figure 2).

Figure 2. Forms of cell signalling: indirect (endocrine, paracrine, autocrine) and direct (juxtacrine).

Endocrine signalling occurs via hormones. Hormones are produced by specialised cells, secreted into the bloodstream, and distributed throughout the body. With the bloodstream, they reach distant target cells. It is, therefore, long-distance signalling, but it occurs relatively slowly because it depends on the speed of blood flow [4,8,291]. The distance covered by the signalling molecule is shorter in paracrine signalling. It acts as a local carrier and affects cells in the immediate vicinity. A specific form of this type of communication is synaptic signalling. Two nerve cells (or a nerve cell and a target cell) are bound by a connection called a synapse, usually a chemical synapse. There are also electrical synapses, which work by the direct flow of ions. Stimulated by a nerve impulse, the presynaptic (transmitting) neuron releases a neurotransmitter, carrying the information to the postsynaptic (receiving) neuron [4,8,284,292,293,294,295,296]. In autocrine signalling, the cell responds to substances secreted by itself, i.e., it both produces a ligand and has a receptor that binds it. Such a mechanism is fundamental in the early stages of organism development or inflammatory processes. It is also characteristic of cancer cells [4,8,293,297,298,299]. Direct interaction between cells, i.e., juxtacrine signalling, does not use molecules secreted into the extracellular space. Two cells connect via complementary surface proteins, one acting as a signalling agent and the other as a receptor. An example is pathogen recognition by immune cells. Cells can also form gap junctions, water-filled protein channels made up of two connexons, hexameric assembly of connexin proteins. The gap junction is the contact site between the cytoplasm of neighbouring cells. It allows substances, rather small in size, such as calcium ions, to flow between them [4,284,300,301,302,303,304,305].

An important issue is the effect of the ligand binding to the receptor, as not every ligand causes receptor stimulation. A signalling substance that, when bound, changes the receptor’s conformation (i.e., has intrinsic activity) and causes a programmed change in cellular activity is called an agonist. The opposite of an agonist is an antagonist, which has no intrinsic activity despite its ability to bind to a receptor. An antagonist blocks the receptor without eliciting a biological response from the cell and prevents activation by the agonist [306,307].

3.2. Receptors

The activity of receptors mainly involves converting one form of signal into another, which usually initiates a multi-step chain of information transfer through several signalling molecules. Often the signal is also amplified. In this way, signalling cascades are formed, leading to an effector response, i.e., various changes in cell activity [308,309,310,311,312,313].

The vast majority of signalling molecules do not enter the cell. The classical model is based on ligand binding by a specific receptor protein. Exceptions include small lipophilic molecules that can cross the barrier formed by the cell membrane, the lipid bilayer. The external signal is then received by intracellular receptors, located mainly in the cell nucleus or cytoplasm [4,8]. Steroid hormones belong to this type of signalling molecules. For years, it has been a common belief that they enter the cell by passive diffusion. However, they may require the involvement of transporter proteins [314,315,316,317,318].

A cell is exposed to hundreds of signalling molecules, so it must respond selectively, i.e., have the right set of receptors. It makes the cell capable of completely bypassing some of the signals. Different cells may respond differently to the same signalling molecule depending on the signalling pathway initiated [319,320,321].

Receptors can be divided into two main groups: intracellular and cell-surface (membrane) receptors. Intracellular receptors have already been described as interacting with steroid hormones. They also bind thyroid hormone, retinoic acid (a derivative of vitamin A) and vitamin D. The receptors for these compounds belong to the nuclear hormone receptor superfamily, formed by structurally homologous proteins. The name can be misleading, as the subcellular location of the unliganded receptors varies. However, after binding to a ligand, they are mainly translocated to the cell nucleus, where they act as transcription factors, i.e., regulate gene transcription. A specific group of orphan intracellular receptors is included in this superfamily. The existence of endogenous ligands binding to them has not been confirmed [322,323,324,325,326].

Cell-surface receptors are anchored to the cell membrane. Depending on the type of information transfer, three subtypes of cell-surface receptors are distinguished (see Figure 3). The first is enzyme-linked receptors (catalytic receptors). The binding of a ligand from the extracellular side causes conformational changes (phosphorylation/autophosphorylation) of the receptor, stimulating enzymatic activity of its cytoplasmic domain. Most commonly, the intracellular domain is responsible for a tyrosine kinase (insulin receptor, growth factor receptors) or serine/threonine protein kinase (TGF-β superfamily receptors) activity. The receptor’s conformation in an inactive form prevents the attachment to the active site (enzymatic domain) of a substrate molecule, i.e., various types of cytosolic proteins that modulate intracellular reactions [327,328,329,330,331,332].

Figure 3. Cell surface receptors embedded in the cell membrane. They act by ligand binding to the extracellular domain of the receptor. The intracellular (cytoplasmic) domain of the receptor communicates via interactions with effector proteins.

The second group are ligand-gated ion channels, called ionotropic receptors. Ion channels are protein structures that pierce the lipid bilayer and control the flow of ions into or from the cell. A conformational change in the receptor protein caused by the binding of a signalling molecule causes the channel to open. Flow occurs according to a concentration gradient (by diffusion). Ion channels are selective, which means that they distinguish between positive and negative ions. It is determined by the charge accumulated on the side chains of the amino acids. Ion channels are permeable (mainly, but not exclusively) to ions of a given type, e.g., sodium ions [333,334,335,336,337]. The function of channels can be stopped by binding of so-called blockers [338]. In addition to ligand-gated ion channels, there are also voltage-gated ion channels (activated by changes in electrical membrane potential) and stretch-activated ion channels (responding to membrane stress) [339,340,341].

G protein-coupled receptors (GPCRs) are the most numerous and highly diverse cell-surface receptors. All GPCRs are composed of a single polypeptide chain, piercing the cell membrane seven times; hence, they are also called seven-transmembrane receptors (7TM). The hydrophobic transmembrane domains are α-helical regions of the chain, connected by hydrophilic intracellular and extracellular loops. The carboxyl end of the polypeptide (C-terminus) is located on the cytosolic side of the cell membrane, while the amino end (N-terminus) is located in the extracellular region. Together with the extracellular and intracellular loops, they are involved in ligand binding and G-protein interactions, respectively [342,343,344]. G-proteins are proteins binding GTP/GDP likewise. They have a heterotrimeric structure and belong to one of four distinguished families depending on the amino acid sequence in the α subunit [345]. In the inactive state, the α subunit binds GDP and forms a complex with the Gβγ dimer. Activation induced by ligand attachment to the extracellular domain of the GPCR receptor results in the release of GDP, the binding of GTP in its place and the dissociation of Gα-GTP from the βγ subunits. Both structures (Gα-GTP and Gβγ) can participate in further signal transduction to effector proteins. Hydrolysis of GTP to GDP due to the intrinsic GTPase activity of the Gα subunit leads to the re-formation of a Gα-GTP complex with Gβγ, ready for subsequent activation [345,346,347,348,349].

Cell Adhesion Molecules (CAMs)

A specific group of proteins are molecules involved in cell–matrix and cell–cell adhesion. The known representatives of this group are classified into four families: integrins, selectins, cadherins, and immunoglobulin superfamily (IgSF) (see Figure 4). This division is due to differences in molecular structure and is strongly related to the heterogeneity of the types of cellular connections formed by these cell-surface receptors [350].

Figure 4. Four families of cell adhesion molecules (CAMs). They are involved in the binding of cells with other cells or with the extracellular matrix in a process called cell adhesion.

Many CAMs are integrins, heterodimers composed of non-covalently linked α and β units. The large extracellular domain provides the binding site for their ligands, while the much shorter cytoplasmic domain binds to cytoskeletal proteins. Integrins mediate bidirectional signal transmission (environment-cell and cell-environment) as they can be activated by proteins that bind to extracellular and intracellular domains [351,352,353]. Cell-to-cell adhesion is enabled by the formation of connections between integrins and the ECM components: collagens, fibronectin, vitronectin, laminin et al. [354]. In the focal adhesion type connection, the cytoplasmic domain of the integrin is bound to actin filaments through adapter proteins, such as vinculin, paxillin, or talin [355,356,357,358,359]. Focal adhesion kinase, a cytoplasmic tyrosine kinase that, when activated, initiates signalling pathways that regulate various cellular functions, plays an important role in the transmission of the signal received by integrin receptors from the ECM [355,360,361]. Another type of integrin adhesion junction are hemidesmosomes. These specialised multiprotein complexes are responsible for anchoring epithelial cells to the basement membrane by binding to cytoskeletal filaments (keratin intermediate filaments) via plectin. The stability of such connections is vital in maintaining the integrity of the skin, where integrins are involved in the formation of the structure: basal keratinocytes—basement membrane—dermis [354,362,363].

Selectins have a multi-domain structure and are characterised by a lectin domain in their extracellular (N-terminal) part. It allows them to mediate cell–cell interactions by recognising and binding carbohydrates present on cell surfaces. The mechanism is calcium ion-dependent [364,365]. Due to differences in structure and pattern of cell-type expression‎, three types of selectins are distinguished: leukocyte (L)-selectin, platelet (P)-selectin, and endothelial (E)-selectin. L-selectin is found on the surface of leukocytes. P-selectin is stored in membranes of α granules of platelets and Weibel–Palade bodies of endothelial cells. After activation, P-selectin is incorporated into the membrane of these cells. E-selectin resides in vascular endothelial cells, but a noticeable increase in its surface expression‎ occurs only after stimulation with the appropriate cytokines. Therefore, E-selectins act as a sensitive indicator of the inflammatory process [366,367,368]. The basic function of selectins is to mediate heterotypic interactions between leukocytes and endothelial cells in the initial stages of the inflammatory reaction. The effect of such binding is the so-called leukocyte rolling along the endothelium and their migration to the site of damage/inflammation. It also contributes to the activation of relevant combinations of signalling factors. In the inflammatory process, selectins interact with integrins [367,369,370,371,372,373].

Cadherins are a significant group of calcium ion-dependent CAMs. Primarily, they are mediators of homotypic adhesion, i.e., interaction of two cadherins of the same type. An increase in calcium ion concentration causes a stiffening of the cadherin molecule, which can bind to the cadherin of a neighbouring cell [374,375]. The cadherin family includes several subfamilies. The best known are classical or type I cadherins and atypical or type II cadherins. Representatives of each subfamily differ in the structure of the extracellular part, i.e., in the number of ectodomain (EC) modules with a repetitive amino acid sequence [375,376,377,378]. Classical cadherins are bound to actin filaments of the cytoskeleton via -β-catenin-α-catenin- linkage [379]. It is a dynamic rather than a stable structure, where α-catenin additionally acts as a regulator of the organisation of actin filaments [378,380]. Desmosomal cadherins (desmoglein and desmocollin) have a similar extracellular domain structure to classical cadherins. They participate in cell-to-cell adhesion through structures called desmosomes. The cytoplasmic domain of these cadherins binds to other intracellular anchor proteins (desmoplakin, plakoglobin, plakophilin) and, consequently, to intermediate filaments. One of the hallmarks of the desmosome is the outer dense plaque, consisting of mediatory proteins. Desmosomes provide strong intercellular adhesion. Thus, they are particularly abundant in tissues such as the epidermis and myocardium continually assailed by mechanical forces [378,381,382].

The immunoglobulin superfamily (IgSF) is one of the most numerous and diverse proteins described in the body. The structural feature that determines membership of this superfamily is the presence of one or more characteristic immunoglobulin folds. It is a sandwich structure composed of two opposing antiparallel β-pleated sheets, stabilised by a disulphide bridge [383,384,385]. Ig domains can interact with many types of ligands (integrins, carbohydrates). They readily bind to other Ig domains of the same kind [386]. Due to their properties, many IgSF molecules act as surface receptors (e.g., antigen receptors found on the surface of T cells) or as CAMs [384,386,387,388]. IgSF adhesion molecules often contain some extracellular domains other than Ig, e.g., fibronectin (Fn) type II or III. These are thought to act as ‘fillers’ of the structure, elongating the chain and shifting the position of the Ig-binding domain. Size exclusion mechanism determines the selectivity of the interactions. Fn domains may also be involved in cis interactions (between ligand and receptor of the same cell) of Ig molecules, the formation of their clusters on the cell surface and the stabilisation of adhesion [389,390,391]. An important issue related to IgSF molecules is their role in the development and function of the nervous system, where they are involved, among other things, in the processes of axon growth and guidance [392,393,394].

4. Artificial Substitutes of the Extracellular Matrix

Cells may be deprived of the necessary connection with the matrix as a result of various types of mechanical or pathological damage. The reconstruction of the structure and restoration of the tissue’s ability to function properly requires replacing the natural extracellular matrix with an artificial substitute. The substitute should support biological regeneration processes. The search for suitable materials and the manufacture of cellular scaffolds for tissue reconstruction is a fundamental goal of tissue engineering. The process is demanding because of the need to match the scaffold’s properties to the tissue’s characteristics. It is essential to confirm‎ that the material is safe for the body and does not cause adverse acute or long-term reactions. Similar requirements are placed on various other implants, not only artificial cellular scaffolds [395,396,397,398,399,400,401,402].

4.1. Host Response to Implantation

The body’s first biological reaction to an implant is forming a layer of water on its surface. This happens in just a few nanoseconds. Water molecules form a mono- or bilayer, and the way they are ordered is strongly dependent on surface properties at the atomic level. Water molecules can dissociate on a highly reactive substrate, resulting in the hydroxylation of the implant surface, i.e., it becomes covered with -OH groups. Water molecules can also be strongly bound but not dissociate. Both of these cases occur as a result of contact with a hydrophilic surface. If the surface is hydrophobic, its interactions with water are much weaker. Therefore, the strength of water-binding determines hydrophobicity or hydrophilicity to the surface. It influences the value of the wetting angle formed between the solid and the plane tangent to the droplet deposited on it. Hydrated ions, such as Cl−, Na+, Ca2+, enter the formed water layer [403,404].

Once the aqueous layer covers the material’s surface, proteins from body fluids (extravasated blood/tissue fluid) reach it. In the first stage, mainly smaller proteins with the highest mobility are adsorbed, resulting from faster diffusion of small than large molecules. It is a transient state. A dynamic adsorption–desorption equilibrium is established at the contact surface, as proteins with larger size and a stronger affinity for the implanted material, arriving late, can force the desorption of smaller, weak-bound molecules. This phenomenon is called the Vroman effect. It should be kept in mind that fluids in contact with the implant, such as plasma, contain hundreds of different proteins competing for access to the surface. Therefore, the adsorption–desorption process is much more complex and depends on factors, such as the protein concentration in the fluid. The higher the concentration, the greater the primary surface dominance [404,405].

Proteins usually have an asymmetric structure in which domains of different chemical nature can be distinguished. They have a more or less ellipsoidal shape (globular proteins) [406]. As a result of adsorption, conformational changes of the molecule can occur if it is sufficiently susceptible. It is the effect of binding to the substrate with a privileged side in a given case. As a result, the molecule adopts a certain orientation where part of it invariably contacts the body fluid [407,408,409]. Structurally stable proteins do not readily undergo conformational changes. Their adsorption may occur along the longest axis (“side-on”). Otherwise, this axis is perpendicular to the implant surface (“end-on”) [407]. The issue is not insignificant in the context of establishing a dynamic adsorption–desorption equilibrium, as the ability to structurally reorient increases the possibility of contact with the substrate [407,410].

A major problem with implantation is the foreign body response (FBR), a complex process involving different cell types. Neutrophils are the first to reach the implant site and adhere (via proteins) to the protein-coated surface of the material. Activated neutrophils attempt to degrade the implant by secreting factors, such as proteolytic enzymes or reactive oxygen species. They release chemokines that attract other immune cells, mainly monocytes [411,412,413]. These, in turn, reaching their target, differentiate into macrophages [414]. The number of macrophages at the implantation site increases due to their progressive proliferation. They replace the initial wave of nucleophiles and release further pro-inflammatory factors. It may lead to implant damage and/or the release of toxic substances into the surrounding tissue environment [415,416]. Macrophages may fuse into foreign body giant cells (FBGCs) due to chronic cytokine activity. FGBCs can adhere to the material’s surface for an extended time, leading to collagen deposition and fibrous encapsulation (approximately 3–4 weeks after implantation). As a result, the implant is isolated from the surrounding tissues. It prevents integration and vascularisation and ultimately leads to implant loss [412,417]. The fibrous layer is usually thinner on porous than on solid materials [418,419]. The presence of mast cells, degranulating upon activation, is also characteristic at the implant site. Among other things, histamine is released from the granules. Histamine dilates blood vessels, improves their permeability and facilitates the arrival of other immune cells. Pro- and anti-inflammatory cytokines and angiogenic or profibrotic factors are also secreted [420,421].

Immunosuppressive drugs are used to weaken the body’s immune response and prevent implant rejection. A more recent solution is to incorporate anti-inflammatory agents into the implanted material. They must be released in a controlled manner and at an appropriate rate. An additional requirement is to promote angiogenesis [422,423]. For years, biomaterials engineering has been focused on obtaining biologically inert materials, i.e., minimising the interaction with the organism and reducing the immune response. The contemporary trend is the generation of biomimetic materials, i.e., mimicking the natural solutions of the organism and stimulating the desired responses. These include enhancing or inhibiting the normal functioning of immune cells [424,425,426].

4.2. Influence of Material Properties on Cell Adhesion

Cells do not experience direct contact with the implanted material but are only ‘informed’ of its physicochemical properties via proteins deposited on the surface. One of the more important characteristics of the material is the wettability of its surface, which, in the case of an aqueous environment, can be equated with hydrophilicity. It is assumed that the ability of cells to adhere increases on hydrophilic surfaces and decreases on hydrophobic surfaces, even though it is hydrophobic surfaces that are generally considered to be more protein-adsorbent [427].

The surface protein layer that forms shortly after implantation consists mainly of albumin, fibrinogen, immunoglobulin G, fibronectin, vitronectin et al. The first interactions are usually dominated by albumin due to its relatively small size (66 kDa) and high-concentration in plasma [411,428,429]. It binds much more readily to hydrophobic than hydrophilic surfaces but does not promote cell adhesion. The strong adsorption of albumin reduces the likelihood of being replaced by larger adhesion-promoting proteins, such as fibronectin and vitronectin [430,431,432]. The ability of fibronectin to displace surface-bound albumin is limited on hydrophobic surfaces. As a result of the strong binding of albumin molecules, changes in their secondary structure occur and the degree of denaturation increases [433]. Proteins tend to denature as the contact time with the material increases, which occurs when albumin adsorbs onto a hydrophobic material. The binding energy of the adsorbed phase then increases, and, as a result, the probability of desorption decreases [406].

Adsorption occurs more readily if there is a charge difference between the protein molecules and the material surface [434]. Furthermore, the affinity of the protein for the material may show greater specificity than the distinction between hydrophobicity/hydrophilicity and be based on the recognition of specific functional groups [429,433]. Additionally, the cells themselves, depending on the type, show a different preference for the functionality of the surface groups [435,436,437,438].

Adhesion of cells to the implant surface is made possible by integrins recognising and binding to specific amino acid sequences in the polypeptide chain of the adsorbed protein. It mimics the formation of integrin connections with the ECM proteins under natural conditions. The best known among the pro-adhesive sequences is the tripeptide RGD (arginine-glycine-aspartic acid), present, e.g., in the structure of fibronectin [439,440]. One way to modify the material to increase biocompatibility is the coating of tripeptide RGD on its surface in the form of immobilised proteins or short synthetic polypeptide ligands. In addition to RGD, the collagen peptide GFOGER (glycine-phenylalanine-hydroxyproline-glycine-glutamate-arginine) and the laminin-specific sequences IKVAV (isoleucine-lysine-valine-alanine-valine) and YIGSR (tyrosine-isoleucine-glycine-serine-arginine), among others, have been identified [441,442,443,444,445].

Functionalisation of the implant surface with peptides containing the RGD sequence has drawbacks. Integrins that recognise RGD may require the presence of other peptides (synergistic effect) to form a bond. The biological activity of short synthetic peptides is less than that of a whole protein. In turn, modification of these peptides (e.g., by chain elongation) can also result in an undesirable change (increase/reduction) in their activity. Another problem is that cells adhere too strongly to the surface, reducing their movement ability [446,447,448].

An interesting conclusion is provided by the study of cell adhesion on materials exhibiting extreme wettability types. Superhydrophobic surfaces are characterised by a water contact angle value higher than 150°, while superhydrophilic surfaces are around 0°. Although the type of cell determines the contact behaviour, only a few show good adhesion to a surface if the material is superhydrophobic. If the surface has highly hydrophilic and hydrophobic regions, cells will usually selectively attach to the superhydrophilic areas [449].

A significant feature of an implant is the topography of its surface, which, like the chemical composition, influences the interactions with integrins and ultimately stimulates the cellular response [450]. The shape of the natural matrix at the micro- and nanoscale is understood to be the structure formed by the ECM proteins and the neighbouring cells. For synthetic materials, it is the degree of roughness, the type and size of patterns on the surface. Modifications of these features at the nanoscale affect the activity of the adsorbing proteins by forcing specific changes in their conformation. However, the detailed investigation of such relationships is complicated because the cellular response is always a resultant of the influence of different stimuli. In addition, modifications of topography may be accompanied by changes in surface chemistry [451,452,453,454,455].

Techniques to create micropatterns on substrates can be divided into two main types: (1) coating portions of the material with an agent that promotes selective adhesion or (2) applying a layer that blocks adhesion and subsequently removing it without harming the cells embedded around it [456]. The resulting pattern geometry influences the subsequent formation of cells, e.g., it promotes cell elongation. Furthermore, it supports/inhibits the spreading of cells on the surface. It is related to facilitating/hindering their movement, respectively, depending on the continuity of the pattern [457]. The size of the contact area between cells can influence their differentiation, i.e., result in different types of daughter cells [458]. Discontinuities in topography are the cause of local differences in surface free energy. If the cell can detect it, it will modify the contact orientation by reorganising its cytoskeleton. Mechanical signals transmitted to the cell nucleus affect changes at the level of gene transcription and consequently determine cell behaviour. However, the mechanisms underlying the cellular response are still poorly understood [459,460].

In addition to patterns characterised by uniformity of shape and size, cell adhesion is influenced by the surface roughness, understood as the overall three-dimensional topography of the substrate, regardless of its regularity. The surfaces of the used materials are rarely smooth at the molecular level, while roughness is not uniformly describable in this case. Cells must be able to recognise a rough surface to react in a certain way, which is dependent on the cell type, as the primary determining factor is the size of the cell. It means that a cell will recognise a surface as smooth if the peak-to-peak distance is greater than the size of the cell [461,462,463].

Experimental results on the relationship between material roughness and cell behaviour are often contradictory because of different cell types and materials, making it difficult to compare results. However, it is generally accepted that rough surfaces materials promote cell adhesion because they have a larger specific surface area than smooth surface materials. On a smooth surface, the cell needs more connection points to hold on [464,465,466,467,468,469].

5. Conclusions

The extracellular matrix is a complex dynamic network structure. Its components are synthesised, secreted, and degraded in a manner controlled by the cells. The matrix fills the spaces between cells, provides structural support, and binds tissues together, providing them with proper mechanical properties. It is an essential component of connective tissues. The intercellular matrix controls cells’ behaviour and vital functions, thus regulating the normal development of tissues and maintaining their homeostasis. Mutations in the genes encoding matrix components cause many serious diseases.

Cells can receive, process, and respond to signals from the external environment because they are equipped with a set of appropriate receptors. The information reaching the receptor, usually in the form of a chemical carrier, may come from the immediate vicinity of the cell and distant parts of the body. The binding of the ligand to the receptor initiates the signalling pathway. The effector response depends on the cell type and the receptor type. A specific group of receptors are adhesion molecules involved in forming cell–cell and cell–matrix connections.

Controlled processes of degradation and secretion of intercellular matrix components mean that tissues are constantly remodelled. However, the body cannot repair certain structural defects on its own. It then needs support in the form of an artificial scaffold on which the cells can settle, multiply and differentiate, and begin to produce the building blocks of the matrix. Over time, the implanted substitute degrades and gives way to a reconstituted protein–polysaccharide network. When designing materials for such implants, the influence of hydrophilicity, topography, roughness, and surface functional groups on cell growth processes must be considered. The results of studies on biomaterials do not give conclusive results, but it should be kept in mind that they are highly dependent on the cell type.

Author Contributions

Conceptualization, A.B. and A.G.-G.; validation, formal analysis, A.B. and A.G.-G.; investigation, A.B. and A.G.-G.; writing—original draft preparation, A.B.; writing—review and editing, A.B. and A.G.-G.; figures designing, A.B; supervision, A.G.-G.; funding acquisition, A.G.-G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by (POB Biotechnology and Biomedical Engineering) of Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) programme as a research project titled “Biomimetic biodegradable cell scaffolds for differentiation of stem cells towards osteoblasts and chondrocytes” (504/04496/1020/45010418).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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