혈액-뇌장벽 .. 중요한 의문들.. 2020 영향력지수 12점 논문

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Review|March 25 2020

The blood–brain barrier in health and disease: Important unanswered questions

In Special Collection: Neurodegeneration and Neuroinflammation 2020

Caterina P. Profaci 

 ,

Roeben N. Munji 

 ,

Robert S. Pulido,

Richard Daneman 

Author and Article Information

J Exp Med (2020) 217 (4): e20190062.

https://doi.org/10.1084/jem.20190062

Article history

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The blood vessels vascularizing the central nervous system exhibit a series of distinct properties that tightly control the movement of ions, molecules, and cells between the blood and the parenchyma. This “blood–brain barrier” is initiated during angiogenesis via signals from the surrounding neural environment, and its integrity remains vital for homeostasis and neural protection throughout life. Blood–brain barrier dysfunction contributes to pathology in a range of neurological conditions including multiple sclerosis, stroke, and epilepsy, and has also been implicated in neurodegenerative diseases such as Alzheimer’s disease. This review will discuss current knowledge and key unanswered questions regarding the blood–brain barrier in health and disease.

중추 신경계에 혈관을 공급하는 혈관은 

혈액과 실질 사이의 이온, 

분자 및 세포의 이동을 엄격하게 제어하는 일련의 뚜렷한 특성을 나타냅니다. 

이 “혈액-뇌 장벽”은 

주변 신경 환경의 신호를 통해 혈관 신생 중에 시작되며, 

평생 동안 항상성과 신경 보호에 필수적인 기능을 유지합니다. 

혈액뇌장벽 기능 장애는 

다발성 경화증, 뇌졸중, 간질 등 

다양한 신경 질환의 병리에 기여하며, 

알츠하이머병과 같은 신경 퇴행성 질환과도 관련이 있는 것으로 밝혀졌습니다. 

이 리뷰에서는 

건강과 질병에서 

혈액뇌장벽에 관한 최신 지식과 아직 해결되지 않은 주요 질문에 대해 논의합니다.

Subjects:

Neuroinflammation, Neuroscience

Blood vessels provide the vital infrastructure for delivery of oxygen and essential nutrients throughout the body, and the term “blood–brain barrier” (BBB) is used to describe the unique characteristics of the blood vessels that vascularize the central nervous system (CNS; Saunders et al., 2008; Zlokovic, 2008; Obermeier et al., 2013). The BBB is not a single physical entity but rather the combined function of a series of physiological properties possessed by endothelial cells (ECs) that limit vessel permeability. The BBB tightly regulates the movement of ions, molecules, and cells between the blood and the parenchyma and is thus critical for neuronal function and protection. The interaction of ECs with different neural and immune cells is commonly referred to as the neurovascular unit (NVU; Fig. 1 A). The complex properties that define the BBB are often altered in disease states, and BBB dysfunction has been identified as a critical component in several neurological conditions. This review will discuss BBB development, regulation, and dysfunction, emphasizing important unanswered questions.

혈관은 

몸 전체에 산소와 필수 영양소를 전달하는 중요한 인프라를 제공하며, 

중추 신경계를 혈관화하는 혈관의 고유한 특성을 설명하기 위해 

“혈액 뇌 장벽”(BBB)이라는 용어가 사용됩니다

(CNS; 손더스 외, 2008; 즐로코비치, 2008; 오베르마이어 외, 2013). 

BBB는 

하나의 물리적 실체가 아니라 

혈관 투과성을 제한하는 내피 세포(EC)가 가진 

일련의 생리적 특성이 결합된 기능입니다. 

BBB는 

혈액과 실질 사이의 이온, 분자, 세포의 이동을 엄격하게 조절하므로 

신경세포의 기능과 보호에 매우 중요합니다. 

다른 신경 세포 및 면역 세포와 EC의 상호 작용을 

일반적으로 신경혈관 단위(NVU, 그림 1 A)라고 합니다. 

BBB를 정의하는 복잡한 특성은 

질병 상태에서 종종 변경되며, 

BBB 기능 장애는 여러 신경 질환에서 중요한 구성 요소로 확인되었습니다. 

이 리뷰에서는 

BBB의 발달, 조절 및 기능 장애에 대해 논의하며, 

아직 답이 나오지 않은 중요한 질문들을 강조합니다.

The NVUECs

A cross-section of an artery or vein might contain dozens of ECs, while in the smallest capillaries, a single EC forms the vessel circumference (Aird, 2007). In all tissues, adherens junctions, composed of vascular endothelial cadherin and catenins, comprise the basic cellular adhesions between ECs, supporting the integrity of the vascular tube and regulating tensile forces. PECAM1 is a critical regulator of EC adhesion, promoting adherens junction formation (Biswas et al., 2006; Privratsky and Newman, 2014). CNS ECs are further specialized to restrict paracellular and transcellular movement of solutes.

NVUEC

동맥이나 정맥의 단면에는 

수십 개의 EC가 포함될 수 있지만,

 가장 작은 모세혈관에서는 하나의 EC가 혈관 둘레를 형성합니다(Aird, 2007). 

모든 조직에서 혈관 내피 카데린과 카테닌으로 구성된 접착 접합부는 

EC 사이의 기본적인 세포 접착을 구성하여 

혈관관의 완전성을 유지하고 인장력을 조절합니다. 

PECAM1은

 EC 접착의 중요한 조절자로서 접착 접합부 형성을 촉진합니다(Biswas et al., 2006; Privratsky and Newman, 2014). 

CNS EC는 

용질의 세포 내 및 세포 간 이동을 제한하도록 

더욱 특화되어 있습니다.

https://www.nature.com/articles/s41380-022-01511-z

혈액-뇌 장벽(BBB)은 혈액과 뇌 실질 사이의 화합물 교환을 정교하게 제어하여 뇌 항상성을 유지하는 데 필수적인 역할을 합니다. 또한 BBB는 원치 않는 독소나 병원균이 뇌로 유입되는 것을 방지합니다. 그러나 이 장벽은 나이가 들면서 무너져 많은 노화 관련 장애의 특징이 됩니다. 지금까지 BBB 기능을 보호하거나 회복하기 위해 여러 가지 약물이 연구되었습니다. 최근 BBB와 장내 미생물의 연관성이 밝혀지면서 미생물 유래 대사산물이 BBB 생리를 보호하고 회복하는 기능이 있는지에 대해 연구되고 있습니다. 이 리뷰에서는 BBB를 구성하는 중요한 구성 요소에 초점을 맞추고 장벽의 파괴 수준을 분석하며 장벽 무결성을 유지하는 현재 약물 및 치료법과 미생물 유래 대사산물이 BBB 생리학에 미치는 최근의 발견에 대해 논의합니다.

건강하고 온전한 BBB 구조와 주변 세포 및 주요 구성 요소. 내피 세포는 뇌의 혈관을 감싸고 있는 주요 물리적 장벽을 형성하며, 그 사이에는 단단한 접합 단백질이 있습니다. 백혈구는 지속적으로 순환합니다. 내피 세포는 내피 세포와 밀접하게 접촉하는 성상세포를 포함하는 기저막으로 둘러싸여 있습니다. 성상세포 말단은 내피 세포 및 주변 세포와 밀접하게 상호 작용하며 BBB 무결성을 유지하는 데 도움을 줍니다. 비활성화된 미세아교세포와 기능성 뉴런은 건강한 신경혈관 단위에서 존재합니다. B BBB가 파괴되면 다양한 수준에서 무결성이 손상될 수 있습니다. BBB의 파괴 특성에는 타이트 접합 단백질의 손실, 내피 세포 수축, 세포 주위 수준 및 일부 경우 세포 간 수준에서의 분자 수송의 변화, 백혈구 침윤 증가와 같은 내피 세포의 변화가 포함됩니다. 일부 장애 모델에서는 말단이 부어 오르거나 분리되는 등의 성상세포 변화뿐만 아니라 말단세포 기능 장애 또는 손실이 명백합니다. 또한 미세아교세포가 활성화되어 신경세포가 탈수초화되거나 손상될 수 있습니다.

Tight junctions (TJs)

TJs are cell adhesions consisting of multiple transmembrane proteins that directly interact via their extracellular components, linking two cells’ membranes together (Furuse, 2010; Fig. 1 B). CNS TJs are specialized in their molecular and structural P-face composition to form a high-resistance electrical barrier, and the specific combination of TJ proteins at the BBB determines its paracellular permeability.

The composition of claudins, a family of 27 four-pass transmembrane proteins, within a TJ is thought to determine the size and charge selectivity of paracellular permeability (Amasheh et al., 2005; Hou et al., 2006; Furuse et al., 1999). Claudin 5 (CLDN5) is the most abundant claudin at the BBB, and Cldn5 knockout mice exhibit size-selective leakage of the BBB and die at birth (Morita et al., 1999; Nitta et al., 2003). ECs in peripheral vascular beds also express CLDN5, and thus its expression‎ alone is not sufficient for barrier formation. Other key components of TJs include claudin 12, occludin, and junctional adhesion molecules. Cytoplasmic proteins including ZO-1, ZO-2, ZO-3, cingulin, JACOP, MAG1, and MUPP1 aid TJ formation, binding TJs to the cytoskeleton, adherens junctions, and polarity complexes (Umeda et al., 2004; Tietz and Engelhardt, 2015; Sawada, 2013). It is still unknown why CLDN5 and ZO-1 expression‎ does not confer the same low paracellular permeability in peripheral vessels as in the CNS. Expression‎ data suggest that the answer might lie in the CNS-specific enrichment of certain cytoplasmic adaptors (e.g., JACOP, MPP7) and tricellular TJ molecules such as LSR and MARVELD2 (Daneman et al., 2010a; Sohet et al., 2015).

타이트 접합(TJ)

TJ는 

세포 외 성분을 통해 직접 상호 작용하는 여러 세포막 통과 단백질로 구성된 세포 접착으로, 

두 세포막을 서로 연결합니다(Furuse, 2010; 그림 1 B). 

CNS TJ는 

고저항 전기 장벽을 형성하기 위해 분자적, 구조적 P면 구성에 특화되어 있으며, 

BBB에서 TJ 단백질의 특정 조합에 따라 세포막 투과성이 결정됩니다.

27개의 4통과 막단백질로 구성된 클라우딘의 구성은 

세포막 투과성의 크기와 전하 선택성을 결정하는 것으로 생각됩니다(Amasheh 등, 2005; Hou 등, 2006; Furuse 등, 1999). 

클라우딘 5(CLDN5)는 

BBB에서 가장 풍부한 클라우딘이며, 

Cldn5 녹아웃 마우스는 BBB의 크기 선택적 누출을 나타내며 출생 시 사망합니다(Morita et al., 1999; Nitta et al., 2003). 

말초 혈관 층의 EC도 CLDN5를 발현하므로 이 유전자 발현만으로는 장벽 형성에 충분하지 않습니다. 

TJ의 다른 주요 구성 요소로는 

클라우딘 12, 오클루딘, 접합부 접착 분자가 있습니다. 

ZO-1, ZO-2, ZO-3, 싱귤린, JACOP, MAG1, MUPP1을 포함한 세포질 단백질은 

TJ를 세포 골격에 결합하고 접합부 및 극성 복합체를 접착하여 

TJ 형성을 돕습니다

(Umeda et al., 2004; Tietz and Engelhardt, 2015; Sawada, 2013). 

CLDN5와 ZO-1의 발현이 말초 혈관에서 

CNS에서와 같이 낮은 세포 주변 투과성을 보이지 않는 이유는 

아직 밝혀지지 않았습니다. 

발현 데이터에 따르면 그 해답은 특정 세포질 어댑터(예: JACOP, MPP7)와 LSR 및 MARVELD2와 같은 삼세포 TJ 분자의 CNS 특이적 농축에 있을 수 있습니다(Daneman et al., 2010a; Sohet et al., 2015).

Transcellular permeability

Peripheral ECs possess properties that confer transcellular permeability, including high rates of caveolin-mediated transcytosis, diaphragm-containing pores termed fenestrae, or large discontinuities or gaps in the endothelial layer (Aird, 2007; Fig. 1 A). In contrast, CNS ECs form a continuous lining that lacks fenestrations and has low levels of transcytosis, properties that greatly limit transcellular permeability (Fig. 1, A–H). MFSD2A, enriched in CNS ECs, limits caveolin-dependent transcytosis by regulating EC lipid composition (Ben-Zvi et al., 2014; Nguyen et al., 2014; Andreone et al., 2017). Plasmalemma vesicle-associated protein (PLVAP) is important both for vesicle formation and fenestrations. Its down-regulation in CNS ECs, along with up-regulation of MFSD2A, coincides with BBB formation during embryogenesis (Hallmann et al., 1995; Hnasko et al., 2002; Chow and Gu, 2017).

세포 간 투과성

말초 EC는 

높은 비율의 카베올린 매개 형질 전환, 

횡경막을 포함하는 기공(페네스트라고 함), 

내피층의 큰 불연속성 또는 틈새 등 세포 투과성을 부여하는 특성을 가지고 있습니다(Aird, 2007; 그림 1 A). 

이와는 대조적으로, 

CNS EC는 

기공이 없고 세포 

간 투과성을 크게 제한하는 특성인 

낮은 수준의 세포 이동을 갖는 

연속적인 내벽을 형성합니다(그림 1, A-H). 

CNS EC에 풍부한 MFSD2A는 

EC 지질 조성을 조절하여 카베올린 의존성 세포증식을 제한합니다(Ben-Zvi et al., 2014; Nguyen et al., 2014; Andreone et al., 2017). 

플라스말렘마 소포 관련 단백질(PLVAP)은 소포 형성과 세포막 형성 모두에 중요합니다. 

CNS EC에서 

이 단백질의 하향 조절은 

MFSD2A의 상향 조절과 함께 배아 발생 중 

BBB 형성과 일치합니다(Hallmann et al., 1995; Hnasko et al., 2002; Chow and Gu, 2017).

Transporters

Numerous transporters are enriched in brain ECs, which generally fall into two categories: efflux and solute transporters (Miller, 2015; Nałęcz, 2017; Strazielle and Ghersi-Egea, 2015; Fig. 1 E).

Efflux transporters, concentrated on the luminal side of the membrane, use ATP hydrolysis to transport a wide range of small molecules up their concentration gradients back into the blood (Shen and Zhang, 2010). MDR1/P-glycoprotein (PGP) and breast cancer resistance protein are the most abundant BBB efflux proteins and limit entry of many xenobiotics and endogenous molecules, including steroids such as aldosterone (Hindle et al., 2017).

Solute transporters carry specific substrates down their concentration gradients, ensuring barrier passage to specific nutrients, such as glucose, that are vital for energy and homeostasis (Simpson et al., 2007). Transport of glucose, lactate, amino acids, and fatty acids occurs via GLUT1 (Slc2a1), MCT1 (Slc16a1), LAT1 (Slc7a5), and MFSD2A, respectively (Boado et al., 1999; Cornford et al., 1994; Kido et al., 2000; Nguyen et al., 2014). Other transporters provide receptor-mediated vesicular transport, including the transferrin receptor (TFR1) and low-density lipoprotein receptors (Jefferies et al., 1984; Méresse et al., 1989). Substrate-specific solute transporters can also be important for removing molecules from the CNS; lipoprotein receptor-related protein-1 (LRP1) is a critical transporter for eliminating β-amyloid (Shibata et al., 2000; Storck et al., 2016).

수송체

뇌 EC에는 

수많은 수송체가 풍부하게 존재하며, 

일반적으로 유출 수송체와 용질 수송체의 두 가지 범주로 분류됩니다(Miller, 2015; Nałęcz, 2017; Strazielle and Ghersi-Egea, 2015; 그림 1 E).

유출 수송체는 

막의 내강 쪽에 집중되어 있으며, 

ATP 가수분해를 통해 다양한 저분자를 농도 구배를 따라 

혈액으로 다시 운반합니다(Shen and Zhang, 2010). 

MDR1/P-당단백질(PGP)과 유방암 저항성 단백질은 

가장 풍부한 BBB 유출 단백질이며 

알도스테론과 같은 스테로이드를 포함한 

많은 외인성 및 내인성 분자의 유입을 제한합니다(Hindle et al., 2017).

용질 수송체는 농도 구배에 따라 특정 기질을 운반하여 

에너지와 항상성에 필수적인 포도당과 같은 

특정 영양소에 대한 장벽 통과를 보장합니다(Simpson et al., 2007). 

포도당, 젖산염, 아미노산 및 지방산의 수송은 

각각 GLUT1(Slc2a1), MCT1(Slc16a1), LAT1(Slc7a5) 및 MFSD2A를 통해 발생합니다(Boado et al., 1999; Cornford et al., 1994; Kido et al., 2000; Nguyen et al., 2014). 

트랜스페린 수용체(TFR1) 및 저밀도 지단백질 수용체를 포함한 다른 수송체는 

수용체 매개 소포 수송을 제공합니다(Jefferies et al., 1984; Méresse et al., 1989). 

기질 특이적 용질 수송체는 또한 

CNS에서 분자를 제거하는 데 중요할 수 있으며, 

지단백질 수용체 관련 단백질-1(LRP1)은 

β-아밀로이드 제거에 중요한 수송체입니다(Shibata et al., 2000; Storck et al., 2016).

Leukocyte adhesion molecules

Leukocyte adhesion molecules on EC surfaces initiate binding of leukocytes, the first step of their entrance into tissues (Bevilacqua, 1993). Healthy CNS ECs exhibit lower leukocyte adhesion molecule expression‎ compared with peripheral ECs (Daneman et al., 2010a), and thus there is minimal leukocyte crossing of the BBB in health (Fig. 1, E and F; and Fig. 2 A). Instead, CNS immune surveillance by lymphocytes in health occurs primarily at the blood–CSF interfaces of the meninges and choroid plexus (Ransohoff and Engelhardt, 2012; Kipnis et al., 2012; Shechter et al., 2013; Box 1).

백혈구 접착 분자

EC 표면의 백혈구 부착 분자는

백혈구가 조직으로 진입하는 첫 단계인

백혈구 결합을 시작합니다(베빌라쿠아, 1993).

건강한 CNS EC는

말초 EC에 비해 백혈구 부착 분자 발현이 낮으며(Daneman et al., 2010a),

따라서 건강한 상태에서는 BBB의 백혈구 교차가 최소화됩니다(그림 1, E 및 F; 및 그림 2 A).

대신, 건강한 상태의 림프구에 의한 CNS 면역 감시는

주로 수막과 맥락막 신경총의 혈액-CSF 인터페이스에서 발생합니다

(Ransohoff and Engelhardt, 2012; Kipnis 등, 2012; Shechter 등, 2013; 박스 1).

Box 1.

Barriers of the CNS

There are small regions of the brain that lack an endothelial BBB and are instead vascularized by permeable fenestrated capillaries. In these regions, a specialized glial barrier takes on the role of the endothelial BBB. (A) Among these regions is the choroid plexus, the structure that generates most of the cerebrospinal fluid. The specialized ependymal epithelial cells of the choroid plexus surround its fenestrated capillaries and filter the fluid that enters through fenestrae to generate the cerebrospinal fluid. The choroid plexus epithelial cells possess similar properties as the ECs of the BBB. (B and C) These properties include (compare panel B to C): (1) a dense formation of junctional complexes that restrict paracellular diffusion of hydrophilic solutes; (2) expression‎ of efflux transporters and low rates of transcytosis that limit transcellular movement of molecules; and (3) expression‎ of selective transporters that import necessary nutrients or export wastes (Marques et al., 2017). CVOs are vascularized by fenestrated capillaries and allow a small subset of neurons and glia to sense blood-derived signals or secrete hormones into the blood to regulate peripheral processes such as fluid homeostasis, osmoregulation, body temperature, energy balance, and inflammation. The subfornical organ, area postrema, and organum vasculosum of the lamina terminalis are the sensory CVOs, while the median eminence and neurohypophysis are the secretory CVOs. Each of these CVOs possess a glia-derived, cellular barrier generated by tanycytes or tanycyte-like cells that limit further diffusion of blood-derived solutes into neighboring regions or the cerebrospinal fluid (Ganong, 2000; Miyata, 2015). (D) In the median eminence, β1 tanycytes limit diffusion of solutes originating from the ventrally localized fenestrated capillaries into the arcuate nucleus while β2 tanycytes restrict chemical exchange between the median eminence and CSF (Miyata, 2015; Langlet et al., 2013).

뇌에는 내피성 BBB가 없고

대신 투과성 모세혈관으로 혈관이 형성된 작은 영역이 있습니다.

이 영역에서는

특수 신경교 교세포 장벽이 내피 BBB의 역할을 합니다.

(A) 이 영역 중에는 대부분의 뇌척수액을 생성하는 구조인 맥락막 신경총이 있습니다. 맥락막총의 특수한 내피 상피 세포는 구멍이 뚫린 모세혈관을 둘러싸고 있으며, 구멍을 통해 들어오는 액체를 여과하여 뇌척수액을 생성합니다. 맥락막총 상피 세포는 BBB의 EC와 유사한 특성을 가지고 있습니다. (B와 C) 이러한 특성에는 다음이 포함됩니다(패널 B와 C 비교): (1) 친수성 용질의 세포 내 확산을 제한하는 접합 복합체의 조밀한 형성, (2) 분자의 세포 간 이동을 제한하는 유출 수송체의 발현 및 낮은 세포 이동률, (3) 필요한 영양분을 수입하거나 노폐물을 배출하는 선택적 수송체의 발현(Marques et al., 2017). CVO는 모세혈관에 의해 혈관이 형성되어 있으며 뉴런과 신경교세포의 일부가 혈액 유래 신호를 감지하거나 호르몬을 혈액으로 분비하여 체액 항상성, 삼투압 조절, 체온, 에너지 균형 및 염증과 같은 말초 과정을 조절할 수 있도록 합니다. 말초의 말초 하부 기관, 말초 후부 및 혈관 기관은 감각 CVO이며, 중추 및 신경 저형성은 분비 CVO입니다. 이러한 각 CVO는 신경교세포 또는 신경교세포 유사 세포에 의해 생성된 신경교 유래 세포 장벽을 가지고 있어 혈액 유래 용질이 주변 영역이나 뇌척수액으로 더 이상 확산되는 것을 제한합니다(Ganong, 2000; Miyata, 2015). (D) 정중엽에서 β1 타니세포는 복부에 국한된 모세혈관에서 아치형 핵으로 용질이 확산되는 것을 제한하고, β2 타니세포는 정중엽과 CSF 사이의 화학적 교환을 제한합니다(Miyata, 2015; Langlet et al., 2013).

The NVU

The luminal surface of the capillary endothelium is covered by the EC glycocalyx (Ausprunk et al., 1981a, b; Pillinger and Kam, 2017). Brain ECs have a denser glycocalyx than peripheral vasculature; average glycocalyx coverage is 40.1% in brain vessels compared with 15.1% and 3.2% in cardiac and pulmonary vessels, respectively (Ando et al., 2018). This dense network of luminal glycoproteins prevents larger molecules from interacting with the EC. While small dyes such as fluorescein (376 daltons) and Alexa Fluor (643 daltons) permeate the glycocalyx, dextrans (40–150 kD) penetrate <60% of its volume (Kutuzov et al., 2018). In disease, glycocalyx degradation is associated with more severe BBB leakage in models of multiple sclerosis (MS) and cardiac arrest (DellaValle et al., 2018; Zhu et al., 2018).

The abluminal surface of the EC is covered by the basal lamina (Fig. 1 A), a structural matrix of laminins, fibronectin, collagens, tenascin, and proteoglycans. This basement membrane (BM) surrounds ECs and pericytes, acting as an interface for the binding of molecules and migration of cells, while also limiting passage of macromolecules (Del Zoppo et al., 2006). The BM consists of two layers: the inner vascular BM secreted by ECs and pericytes, and the outer glial BM secreted by astrocytes (Sorokin, 2010). These BMs are merged surrounding capillaries but separate at post-capillary venules, creating a CSF-drained perivascular space for immune surveillance (Engelhardt and Ransohoff, 2012).

Mural cells—vascular smooth muscle cells (VSMCs) and pericytes—are found on the abluminal side of blood vessels in all tissues. VSMCs line all larger vessels but are more abundant on arteries and arterioles, forming a complete layer around them (Smyth et al., 2018; Vanlandewijck et al., 2018; Armulik et al., 2011). VSMC myosin fibers regulate blood flow via vasoconstriction and vasodilation (Aird, 2007). Pericytes are embedded in the BM and form an incomplete layer on the surface of CNS micro-vessels (Fig. 1 A). Pericytes play a key role in the regulation of angiogenesis, vascular remodeling, vascular tone, and BBB formation (Daneman et al., 2010a; Armulik et al., 2005, 2010; Winkler et al., 2011). Perivascular fibroblasts are found in the walls of large vessels (Vanlandewijck et al., 2018); however, their role in cerebrovascular function remains unexplored.

Astrocytes extend cellular processes terminating in endfeet that ensheath synapses, nodes of Ranvier, and ECs, contacting the BM around parenchymal vessels (Fig. 1 A). This astrocyte–endothelial interaction is critical in regulating blood flow (Mishra et al., 2016). Several groups have shown that CSF flows between the BM and astrocyte endfeet of arteries and capillaries, with arteriole pulsations driving bulk fluid flow through the parenchyma, although others have argued about the extent of bulk flow (Abbott et al., 2018; Hladky and Barrand, 2019). This “glymphatic” system helps to clear interstitial solutes such as amyloid via paravenous drainage pathways (Iliff et al., 2012; Xie et al., 2013; Mestre et al., 2018) and has been visualized in human patients via magnetic resonance imaging (MRI; Meng et al., 2019; Fultz et al., 2019). Expression‎ of water channel aquaporin-4 in astrocyte endfeet has been reported to play a critical role in the movement of CSF into the parenchyma (Haj-Yasein et al., 2011; Iliff et al., 2012; Mestre et al., 2018).

CNS-associated macrophages, which express a gene signature of Mrc1 (CD206), Pf4, Cbr2, Ms4a7, and Stab1, include choroid plexus, dural, leptomeningeal, and perivascular macrophages (Kierdorf et al., 2019; Jordão et al., 2019). Perivascular macrophages are elongated cells residing between the astrocytic endfeet and parenchymal vessels (primarily arteries and veins). While nonmotile, they extend processes along the perivascular space, providing the first line of defense by collecting debris (Hickey and Kimura, 1988; Prinz et al., 2017). Microglia, derived from yolk-sac progenitor cells (Takahashi et al., 1989; Alliot et al., 1999), reside within the CNS parenchyma. They possess a highly ramified morphology and perform immune surveillance, phagocytosing infectious agents that evade the barrier (Streit et al., 2005; Prinz et al., 2011). Microglia have also been shown to regulate BBB resealing following vascular injury and disease (Fernández-López et al., 2016; Lou et al., 2016). In disease states, leukocytes such as neutrophils and T cells can interact with the BBB, increasing permeability via release of cytokines, reactive oxygen species, and other mediators of barrier dysfunction (Hudson et al., 2005; Persidsky et al., 1999).

Thus, the BBB is a series of structural, transport, and metabolic barriers that together limit CNS entry of nonspecific molecules while ensuring the delivery of specific nutrients, thereby controlling the extracellular environment. Several important questions remain. What exactly gets through the barrier, how much, and by which route(s)? The barrier is not absolute. Small, nonpolar molecules enter unrestricted through passive diffusion unless they are substrates of efflux transporters. In contrast, large or polar molecules are greatly restricted in access unless they are substrates of specific nutrient transporters. However, even large molecules enter the CNS parenchyma at 0.1% of their blood concentration through an unsaturable mechanism (Yu and Watts, 2013; Poduslo et al., 1994), likely via nonspecific transcytosis, which occurs at low rates. Future work fully characterizing the substrate specificity of BBB transporters and their dynamic response to various stimuli may enable manipulation of these transporters for CNS drug delivery.

There is heterogeneity of gene expression‎ among different branches of the vascular tree (Macdonald et al., 2010; Vanlandewijck et al., 2018; Murugesan et al., 2011). It is thought that this heterogeneity enables capillaries, arterioles, and venules to be specialized for regulation of solute transport, blood flow, and inflammation, respectively. But what is the relevance of this arteriovenous zonation in terms of barrier function? How is this phenotypic continuum programmed during development?

It is also currently unknown whether there is regional heterogeneity of the BBB. Several regions of the CNS termed circumventricular organs (CVOs)—the area postrema, subfornical organ, pineal gland, and median eminence of the hypothalamus—have fenestrated capillaries that lack BBB properties (Box 1; Gross, 1992). This vascular permeability allows for the exchange of sensory or secretory signaling molecules between the brain and blood, enabling CVO-mediated regulation of body homeostasis. Much less is known about whether there are region-specific differences among areas with a functional BBB, including the cortex, hippocampus, cerebellum, and white matter tracks, and whether BBB heterogeneity might contribute to the specialized function of a particular brain region or render that region more vulnerable to disease.

NVU

모세혈관 내피의 내강 표면은 EC 글리코칼릭스로 덮여 있습니다(Ausprunk et al., 1981a, b; Pillinger and Kam, 2017). 뇌 EC는 말초 혈관보다 글리코칼릭스 밀도가 더 높으며, 평균 글리코칼릭스 커버리지가 뇌 혈관의 경우 40.1%인데 비해 심장 및 폐 혈관은 각각 15.1%와 3.2%입니다(Ando et al., 2018). 이 조밀한 내강 당단백질 네트워크는 더 큰 분자가 EC와 상호 작용하는 것을 방지합니다. 플루오레세인(376 달톤) 및 알렉사 플루오르(643 달톤)와 같은 작은 염료는 당소체에 침투하지만 덱스트란(40-150 kD)은 부피의 60 % 미만을 투과합니다(Kutuzov et al., 2018). 질병에서 글리코칼릭스 분해는 다발성 경화증(MS) 및 심정지 모델에서 더 심각한 BBB 누출과 관련이 있습니다(DellaValle et al., 2018; Zhu et al., 2018).

EC의 복부 표면은 라미닌, 피브로넥틴, 콜라겐, 테나신, 프로테오글리칸으로 구성된 구조 매트릭스인 기저층(그림 1 A)으로 덮여 있습니다. 이 기저막(BM)은 EC와 주변 세포를 둘러싸고 있으며, 분자의 결합과 세포의 이동을 위한 인터페이스 역할을 하는 동시에 거대 분자의 통과를 제한합니다(Del Zoppo et al., 2006). BM은 두 개의 층으로 구성됩니다: EC와 주변세포에서 분비되는 내부 혈관 BM과 성상교세포에서 분비되는 외부 신경교세포 BM(Sorokin, 2010). 이러한 BM은 모세혈관을 둘러싸고 합쳐져 있지만 모세혈관 후 정맥에서 분리되어 면역 감시를 위한 CSF가 배출되는 혈관 주위 공간을 만듭니다(Engelhardt and Ransohoff, 2012).

혈관평활근세포(VSMC)와 혈관주위세포는 모든 조직에서 혈관의 복부 쪽에서 발견됩니다. VSMC는 모든 큰 혈관을 감싸고 있지만 동맥과 세동맥에 더 많이 존재하여 혈관 주위에 완전한 층을 형성합니다(Smyth et al., 2018; Vanlandewijck et al., 2018; Armulik et al., 2011). VSMC 미오신 섬유는 혈관 수축과 혈관 확장을 통해 혈류를 조절합니다(Aird, 2007). 혈관내피세포는 BM에 내장되어 있으며 CNS 미세혈관 표면에 불완전한 층을 형성합니다(그림 1 A). 혈관주위세포는 혈관 신생, 혈관 리모델링, 혈관 긴장도 및 BBB 형성의 조절에 중요한 역할을 합니다(Daneman et al., 2010a; Armulik et al., 2005, 2010; Winkler et al., 2011). 혈관 주위 섬유아세포는 큰 혈관 벽에서 발견되지만(Vanlandewijck et al., 2018), 뇌혈관 기능에서 섬유아세포의 역할은 아직 밝혀지지 않았습니다.

성상세포는 시냅스, 랜비어 노드 및 EC를 감싸는 말단에서 종결되는 세포 프로세스를 확장하여 실질 혈관 주변의 BM과 접촉합니다(그림 1 A). 이러한 성상세포와 내피세포의 상호작용은 혈류 조절에 매우 중요합니다(Mishra et al., 2016). 몇몇 그룹은 CSF가 동맥과 모세혈관의 BM과 성상세포 말단부 사이를 흐르며 동맥 맥동이 실질로 대량 유체 흐름을 유도한다는 것을 보여주었지만, 대량 흐름의 정도에 대해서는 다른 그룹이 논쟁을 벌였습니다(Abbott et al., 2018; Hladky and Barrand, 2019). 이 “글리프” 시스템은 모세혈관 배수 경로를 통해 아밀로이드와 같은 간질성 용질을 제거하는 데 도움이 되며(일리프 등, 2012; 시에 등, 2013; 메스트레 등, 2018) 자기공명영상(MRI; 멩 등, 2019; 풀츠 등, 2019)을 통해 인간 환자에서 시각화되었습니다. 성상세포 말단에서 수로 아쿠아포린-4의 발현은 CSF가 실질로 이동하는 데 중요한 역할을 하는 것으로 보고되었습니다(Haj-Yasein et al., 2011; Iliff et al., 2012; Mestre et al., 2018).

Mrc1(CD206), Pf4, Cbr2, Ms4a7, Stab1의 유전자 서명을 발현하는 CNS 관련 대식세포에는 맥락막 신경총, 경막, 연수막 및 혈관 주위 대식세포가 포함됩니다(Kierdorf et al., 2019; Jordão et al., 2019). 혈관 주위 대식세포는 성상세포 말단과 실질 혈관(주로 동맥과 정맥) 사이에 존재하는 길쭉한 세포입니다. 운동성이 없지만 혈관 주변 공간을 따라 이동하며 이물질을 수집하여 1차 방어선을 제공합니다(Hickey and Kimura, 1988; Prinz et al., 2017). 노른자낭 전구세포에서 유래한 미세아교세포(Takahashi et al., 1989; Alliot et al., 1999)는 CNS 실질 내에 존재합니다. 미세아교세포는 고도로 파급된 형태를 가지고 있으며 면역 감시를 수행하고 장벽을 회피하는 감염원을 포식합니다(Streit et al., 2005; Prinz et al., 2011). 미세아교세포는 또한 혈관 손상 및 질병에 따른 BBB 재봉합을 조절하는 것으로 나타났습니다(Fernández-López et al., 2016; Lou et al., 2016). 질병 상태에서 호중구 및 T 세포와 같은 백혈구는 BBB와 상호 작용하여 사이토카인, 활성 산소 종 및 기타 장벽 기능 장애의 매개체를 방출하여 투과성을 증가시킬 수 있습니다(Hudson et al., 2005; Persidsky et al., 1999).

따라서

BBB는

일련의 구조적, 수송 및 대사 장벽으로, 특정 영양소의 전달을 보장하면서

비특이적 분자의 CNS 진입을 제한하여 세포 외 환경을 제어합니다.

몇 가지 중요한 질문이 남아 있습니다.

정확히 무엇이, 얼마나, 어떤 경로를 통해 장벽을 통과할까요?

장벽은 절대적인 것이 아닙니다. 작고 극성이 없는 분자는 유출 수송체의 기질이 아닌 한 수동 확산을 통해 제한 없이 들어갑니다. 반대로, 크거나 극성인 분자는 특정 영양소 수송체의 기질이 아닌 한 접근이 크게 제한됩니다. 그러나 큰 분자라도 낮은 비율로 발생하는 비특이적 세포 이동을 통해 불포화 메커니즘을 통해 혈중 농도의 0.1%로 CNS 실질에 들어갈 수 있습니다(Yu and Watts, 2013; Poduslo et al., 1994). BBB 수송체의 기질 특이성과 다양한 자극에 대한 동적 반응을 완전히 특성화하는 향후 연구를 통해 CNS 약물 전달을 위한 이러한 수송체의 조작이 가능할 수 있습니다.

혈관 나무의 여러 가지 가지 사이에는 유전자 발현의 이질성이 존재합니다(Macdonald et al., 2010; Vanlandewijck et al., 2018; Murugesan et al., 2011). 이러한 이질성 덕분에 모세혈관, 세동맥 및 정맥이 각각 용질 수송, 혈류 및 염증 조절에 특화될 수 있는 것으로 생각됩니다.

하지만

장벽 기능 측면에서 이러한 동정맥 구역화는 어떤 관련이 있을까요?

이 표현형 연속체는 발달 과정에서 어떻게 프로그래밍될까요?

또한 현재 BBB의 지역적 이질성이 있는지 여부도 알려져 있지 않습니다. 뇌신경계의 여러 부위, 즉 뇌실후부, 뇌실하부 기관, 송과선, 시상하부 중앙부에는 BBB 특성이 없는 모세혈관이 있습니다(상자 1; Gross, 1992). 이러한 혈관 투과성은 뇌와 혈액 사이의 감각 또는 분비 신호 분자의 교환을 허용하여 CVO를 매개로 한 신체 항상성 조절을 가능하게 합니다. 피질, 해마, 소뇌, 백질 트랙 등 기능적 BBB가 있는 영역 간에 영역별 차이가 있는지, BBB 이질성이 특정 뇌 영역의 특수 기능에 기여할 수 있는지 또는 해당 영역을 질병에 더 취약하게 만들 수 있는지에 대해서는 알려진 바가 많지 않습니다.

BBB formation and regulation

How BBB properties are regulated in development and maintained in adulthood remains a fundamental field of study (Blanchette and Daneman, 2015). Transplanted CNS tissue is sufficient to induce BBB-like properties in the gut endothelium in vivo (Stewart and Wiley, 1981), suggesting a role for the neural microenvironment in BBB formation. Transplantation of astrocytes into nonneural tissues of adult rats induces barrier properties in local ECs (Janzer and Raff, 1987), and several astrocyte-secreted proteins are sufficient to induce EC barrier properties in vitro and in vivo, including Sonic hedgehog, angiotensin, and basic fibroblast growth factor (Alvarez et al., 2011; Sobue et al., 1999; Wosik et al., 2007). However, barrier properties arise during development before astrogliogenesis takes place (Ben-Zvi et al., 2014; Daneman et al., 2010a; Sohet et al., 2015; Sauvageot and Stiles, 2002), delaying astrocytic contact with ECs does not affect barrier formation (Saunders et al., 2016), and laser ablation of astrocyte endfeet in adult mice does not induce BBB leakage (Kubotera et al., 2019). These data suggest that astrocytes are not necessary for BBB formation, but perhaps provide dynamic BBB regulation in response to specific stimuli. For instance, reactive astrocytes have been shown to be critical for BBB repair following neurological disease (Bush et al., 1999).

Neural progenitor–derived Wnt signaling induces BBB properties during the angiogenic program (Daneman et al., 2009; Liebner et al., 2008; Stenman et al., 2008; Ye et al., 2009; Wang et al., 2012; Zhou and Nathans, 2014; Cho et al., 2017). Loss of Wnt signaling disrupts angiogenesis specifically in the CNS, reducing the expression‎ of TJ proteins and solute transporters while increasing PLVAP (Daneman et al., 2009; Liebner et al., 2008; Stenman et al., 2008). Interestingly, β-catenin activation in the more permeable CVO vessels is sufficient to induce BBB properties (Benz et al., 2019; Wang et al., 2019). These data suggest that the same signal that drives angiogenic invasion of the CNS also induces initial BBB properties within the endothelium.

Pericytes are also essential in BBB development, and EC recruitment of pericytes is concomitant with development of barrier properties. The BBB fails to completely seal in mice lacking CNS pericytes, as they inhibit nonspecific transcytosis and leukocyte adhesion molecule expression‎ (Daneman et al., 2010b; Armulik et al., 2010).

Thus, the BBB is regulated by a series of different cellular interactions: BBB “tight” properties are induced during the angiogenic program by Wnt signaling, “leaky” properties are inhibited by pericytes, and the overall phenotype of the BBB can be influenced by astrocytes, pericytes, and other cell types throughout life.

Important questions still remain. How is the induction of different BBB properties coordinated? Interestingly, Wnt signaling induces endothelial secretion of platelet-derived growth factor B, the key ligand for pericyte recruitment (Reis et al., 2012), suggesting that induction of different BBB properties is tightly coordinated via Wnt-mediated pericyte recruitment. Are the same signals required for induction also responsible for regulating BBB maintenance in adulthood? Although Wnt signaling decreases in ECs after angiogenesis, this pathway is critical for BBB maintenance; disruption of Wnt signaling in adulthood leads to cell-autonomous loss of TJ integrity and an increase in PLVAP in the retina and cerebellum (Wang et al., 2012). Additionally, pericytes are important for BBB function throughout life (Armulik et al., 2010), suggesting that similar signals are required for BBB formation and maintenance. Do region-specific differences in signaling influence BBB heterogeneity? Different Wnt ligands and receptor complexes have been shown to promote BBB formation in different regions of the CNS (Daneman et al., 2009; Wang et al., 2012, 2018; Zhou et al., 2014); however, it is not clear whether this induces regional heterogeneity or is merely a remnant of dorsal–ventral and rostral–caudal axis specification.

How dynamic is each BBB property in a healthy CNS? Are properties modulated by neural activity or environmental stimuli such as exercise and diet? Single-cell sequencing has revealed vascular changes in response to neural activity (Hrvatin et al., 2018), and neuronal activity has been shown to modulate BBB insulin-like growth factor 1 (Nishijima et al., 2010). However, whether neural activity dynamically regulates specific properties of the BBB to modulate circuit function remains unknown. While exercise might help to protect against BBB dysfunction in aging or disease, solid evidence is still forthcoming (Małkiewicz et al., 2019). A high-fat diet can increase BBB permeability (de Aquino et al., 2019; Salameh et al., 2019; Yamamoto et al., 2019), but the specific BBB properties affected have not been thoroughly characterized. Not only can diet affect the BBB, but the BBB can in turn dynamically regulate nutrient availability; animals entering hibernation up-regulate ketone transporters at the BBB to modulate energy utilization during inactivity (Andrews et al., 2009). How dynamic are BBB properties over the course of 24 h, and how might these fluctuations influence brain microenvironment and waste clearance? PGP expression‎ levels follow a diurnal pattern (Savolainen et al., 2016; Kervezee et al., 2014), and a circadian clock in glial cells of the Drosophila melanogaster BBB regulates xenobiotic efflux (Cuddapah et al., 2019; Zhang et al., 2018), but the extent and functional implications of circadian oscillations at the BBB remain unclear.

Are there differences in the BBB across individuals? Are there sex differences in BBB properties? There is evidence for variation in male and female patient CSF/serum albumin ratio (Parrado-Fernández et al., 2018), and BBB sexual dimorphism has been proposed to underlie differences in response to traumatic brain injury and infection and in proclivity to autoimmune disease (Cruz-Orengo et al., 2014; Jullienne et al., 2018; Maggioli et al., 2016).

How do BBB properties change in age? Several studies have reported age-related decline in BBB function (Mooradian, 1988; Montagne et al., 2015; Erdő et al., 2017), and age-related pericyte dysfunction contributes to BBB permeability (Bell et al., 2010). VCAM1 up-regulation at the BBB is a crucial step in age-related cognitive deficits and increased inflammatory tone (Yousef et al., 2019), highlighting VCAM1 as a potential therapeutic target for age-related neurodegeneration.

BBB 형성 및 조절
BBB 특성이 발달 과정에서 어떻게 조절되고 성인이 되어서도 유지되는지는 여전히 기본적인 연구 분야로 남아 있습니다(Blanchette and Daneman, 2015). 이식된 CNS 조직은 생체 내 장 내피에서 BBB와 유사한 특성을 유도하기에 충분하며(Stewart and Wiley, 1981), 이는 BBB 형성에 있어 신경 미세 환경의 역할을 시사합니다. 성상교세포를 성체 쥐의 비신경 조직에 이식하면 국소 EC에서 장벽 특성이 유도되며(Janzer and Raff, 1987), 소닉 헤지혹, 안지오텐신, 기본 섬유아세포 성장 인자 등 여러 성상교세포 분비 단백질이 시험관 및 생체 내에서 EC 장벽 특성을 유도하기에 충분합니다(Alvarez 등, 2011; Sobue 등, 1999; Wosik et al., 2007). 

그러나 성상교세포 형성이 일어나기 전 발달 과정에서 장벽 특성이 발생하고(Ben-Zvi 외, 2014; Daneman 외, 2010a; Sohet 외, 2015; Sauvageot and Stiles, 2002), 성상세포와 EC의 접촉 지연은 장벽 형성에 영향을 주지 않으며(Saunders 외, 2016), 성체 생쥐에서 성상세포 말단의 레이저 제거는 BBB 누출을 유도하지 않습니다(Kubotera 외., 2019). 이러한 데이터는 성상교세포가 BBB 형성에 필수적인 것은 아니지만 특정 자극에 대한 반응으로 동적 BBB 조절을 제공할 수 있음을 시사합니다. 예를 들어, 반응성 성상교세포는 신경 질환 후 BBB 복구에 중요한 역할을 하는 것으로 나타났습니다(Bush et al., 1999).

신경 전구세포 유래 Wnt 신호는 혈관 신생 프로그램 동안 BBB 특성을 유도합니다(Daneman et al., 2009; Liebner et al., 2008; Stenman et al., 2008; Ye et al., 2009; Wang et al., 2012; Zhou and Nathans, 2014; Cho et al., 2017). Wnt 신호의 손실은 특히 CNS에서 혈관 신생을 방해하여 TJ 단백질과 용질 수송체의 발현을 감소시키는 동시에 PLVAP를 증가시킵니다(Daneman et al., 2009; Liebner et al., 2008; Stenman et al., 2008). 흥미롭게도, 투과성이 높은 CVO 혈관에서의 β-카테닌 활성화는 BBB 특성을 유도하기에 충분합니다(Benz et al., 2019; Wang et al., 2019). 이러한 데이터는 CNS의 혈관 신생성 침입을 유도하는 동일한 신호가 내피 내에서도 초기 BBB 특성을 유도한다는 것을 시사합니다.
또한 혈관내피세포는 BBB 발달에 필수적이며, 혈관내피세포의 EC 모집은 장벽 특성의 발달과 수반됩니다. BBB는 비특이적 전이세포증과 백혈구 부착 분자 발현을 억제하기 때문에 CNS 주변세포가 없는 마우스에서는 완전히 밀봉되지 않습니다(Daneman 외., 2010b; Armulik 외., 2010).

따라서 BBB는 일련의 다른 세포 상호 작용에 의해 조절됩니다: BBB의 “단단한” 특성은 혈관 신생 프로그램 동안 Wnt 신호에 의해 유도되고, “새는” 특성은 주변 세포에 의해 억제되며, BBB의 전반적인 표현형은 성상 세포, 주변 세포 및 기타 세포 유형에 의해 일생 동안 영향을 받을 수 있습니다.

중요한 질문은 여전히 남아 있습니다. 

서로 다른 BBB 특성의 유도는 어떻게 조정될까요? 흥미롭게도 Wnt 신호는 혈관내피세포 모집의 핵심 리간드인 혈소판 유래 성장인자 B의 내피 분비를 유도하는데(Reis et al., 2012), 이는 서로 다른 BBB 특성의 유도가 Wnt 매개 혈관내피세포 모집을 통해 긴밀하게 조정된다는 것을 시사합니다. 유도에 필요한 동일한 신호가 성체기의 BBB 유지 조절에도 관여할까요? 혈관 신생 후 EC에서 Wnt 신호가 감소하지만, 이 경로는 BBB 유지에 매우 중요하며, 성체에서 Wnt 신호가 중단되면 세포 자율적으로 TJ 완전성이 손실되고 망막과 소뇌에서 PLVAP가 증가합니다(Wang et al., 2012). 또한, 신경세포는 일생 동안 BBB 기능에 중요하며(Armulik et al., 2010), 이는 BBB 형성 및 유지에 유사한 신호가 필요하다는 것을 시사합니다. 신호의 지역적 차이가 BBB 이질성에 영향을 미칠까요? 서로 다른 Wnt 리간드와 수용체 복합체가 CNS의 다른 영역에서 BBB 형성을 촉진하는 것으로 나타났습니다(Daneman et al., 2009; Wang et al., 2012, 2018; Zhou et al., 2014); 그러나 이것이 지역 이질성을 유도하는지 아니면 단순히 등-배축 및 등-꼬리축 사양의 잔재에 불과한지는 명확하지 않습니다.

건강한 CNS에서 각 BBB 속성은 얼마나 역동적일까요? 

신경 활동이나 운동과 식이 같은 환경 자극에 의해 특성이 조절될까요? 

단일 세포 시퀀싱을 통해 신경 활동에 따른 혈관 변화가 밝혀졌고(Hrvatin 등, 2018), 신경 활동이 BBB 인슐린 유사 성장 인자 1을 조절하는 것으로 나타났습니다(Nishijima 등, 2010). 그러나 신경 활동이 BBB의 특정 특성을 동적으로 조절하여 회로 기능을 조절하는지는 아직 밝혀지지 않았습니다. 운동이 노화나 질병으로 인한 BBB 기능 장애를 예방하는 데 도움이 될 수 있지만, 확실한 증거는 아직 나오지 않았습니다(Małkiewicz 등, 2019). 고지방 식단은 BBB 투과성을 증가시킬 수 있지만(드 아퀴노 외, 2019; 살라메 외, 2019; 야마모토 외, 2019), 영향을 받는 특정 BBB 특성은 완전히 규명되지 않았습니다. 식이 요법은 BBB에 영향을 미칠 수 있을 뿐만 아니라, 동면에 들어간 동물은 비활동 기간 동안 에너지 이용을 조절하기 위해 BBB에서 케톤 수송체를 상향 조절하여 영양소 가용성을 동적으로 조절할 수 있습니다(Andrews et al., 2009). 24시간 동안 BBB 특성은 얼마나 역동적이며, 이러한 변동이 뇌 미세 환경과 노폐물 제거에 어떤 영향을 미칠 수 있을까요? PGP 발현 수준은 일주기적 패턴을 따르고(Savolainen 등, 2016; Kervezee 등, 2014), 초파리 멜라노가스터 BBB의 신경교세포에 있는 일주기 시계는 외래생물 유출을 조절하지만(Cuddapah 등, 2019; Zhang 등, 2018), BBB에서 일주기 진동의 범위와 기능적 의미는 아직 명확하지 않은 상태입니다.

개인마다 BBB에 차이가 있을까요? 

BBB 특성에 성별 차이가 있을까요? 

남성과 여성 환자의 CSF/혈청 알부민 비율에 차이가 있다는 증거가 있으며(Parrado-Fernández 외, 2018), 외상성 뇌 손상 및 감염에 대한 반응과 자가 면역 질환에 대한 성향의 차이의 근거로 BBB 성적 이형성이 제안되었습니다(Cruz-Orengo 외, 2014; Jullienne 외, 2018; Maggioli 외, 2016).

연령에 따라 BBB 특성은 어떻게 변화하나요? 

여러 연구에서 노화와 관련된 BBB 기능 저하를 보고했으며(Mooradian, 1988; Montagne 등, 2015; Erdő 등, 2017), 노화와 관련된 혈관내피세포 기능 장애가 BBB 투과성에 기여한다고 합니다(Bell 등, 2010). BBB에서의 VCAM1 상향 조절은 노화 관련 인지 결손과 염증성 톤 증가의 중요한 단계이며(Yousef et al., 2019), 노화 관련 신경 퇴행의 잠재적 치료 표적으로서 VCAM1을 강조하고 있습니다.

BBB dysfunction

BBB dysfunction occurs in a number of diseases, including MS, epilepsy, and stroke. In these conditions, BBB dysfunction is a central element of the pathology, whereas in others, such as Alzheimer’s disease (AD), the incidence and extent of breakdown are more controversial and an area of burgeoning research. BBB disruption causes ion dysregulation, edema, and neuroinflammation, which can lead to neuronal dysfunction, increased intracranial pressure, and neuronal degeneration. However, the mechanisms underlying BBB dysfunction and its role in the onset and progression of disease or recovery are not fully understood.

The phrase “BBB breakdown” conjures images of the destruction of a physical wall, allowing an unabated flow of molecules from the blood into the brain. However, the BBB is not a wall but a series of physiological properties, and a change in just one property (transcytosis, transport) can significantly alter the neural environment (Fig. 2). For instance, dysfunction of GLUT1 glucose transport, LAT1 amino acid transport, and MCT8 thyroid hormone transport across the BBB leads to seizure, autism spectrum, and psychomotor retardation syndromes, respectively (Seidner et al., 1998; Tărlungeanu et al., 2016; Vatine et al., 2017).

Importantly, leakage of nonspecific molecules is distinct from leukocyte extravasation, which occurs via an active trafficking process. Single-cell sequencing has identified many subsets of immune cells with distinct roles in neuroinflammation that likely interact with the BBB in disease (Mrdjen et al., 2018; Jordão et al., 2019; Kierdorf et al., 2019; Masuda et al., 2019; Mundt et al., 2019). Parenchymal ECs up-regulate leukocyte adhesion molecules, thus increasing leukocyte trafficking. P-selectin and E-selectin mediate the rolling of leukocytes along the endothelium, ICAM1 and VCAM1 mediate firm adhesion, and proteins like PLVAP—also up-regulated in disease—aid in transmigration across ECs (Engelhardt and Ransohoff, 2012; Ioannidou et al., 2006). Leukocyte extravasation across the BBB can be either transcellular or paracellular (Carman et al., 2007; Winger et al., 2014). Levels of ICAM1 and PECAM1 can influence T cell diapedesis route (Abadier et al., 2015; Wimmer et al., 2019), and specific subsets of T cells prefer different routes (Lutz et al., 2017).

Thus, the BBB is not an on–off switch, and it is critical to understand the specificities and consequences underlying each instance of dysfunction.

BBB 기능 장애
BBB 기능 장애는 다발성 경화증, 간질, 뇌졸중을 비롯한 여러 질환에서 발생합니다. 이러한 질환에서 BBB 기능 장애는 병리의 핵심 요소인 반면, 알츠하이머병(AD)과 같은 다른 질환에서는 발생률과 파괴 정도에 대해 논란이 많으며 연구가 활발히 진행되고 있는 분야입니다. BBB가 파괴되면 이온 조절 장애, 부종, 신경 염증이 발생하여 신경 기능 장애, 두개 내압 증가, 신경 퇴행으로 이어질 수 있습니다. 그러나 BBB 기능 장애의 근본적인 메커니즘과 질병의 발병과 진행 또는 회복에 있어서의 역할은 완전히 이해되지 않았습니다.

“BBB 파괴"라는 문구는 물리적 벽이 파괴되어 혈액에서 뇌로 분자의 흐름이 차단되는 이미지를 떠올리게 합니다. 그러나 BBB는 벽이 아니라 일련의 생리적 특성으로, 한 가지 특성(세포 이동, 수송)만 변화해도 신경 환경이 크게 달라질 수 있습니다(그림 2). 예를 들어, GLUT1 포도당 수송, LAT1 아미노산 수송, MCT8 갑상선 호르몬 수송의 기능 장애는 각각 발작, 자폐 스펙트럼, 정신 운동 지체 증후군으로 이어집니다(Seidner 등, 1998; Tărlungeanu 등, 2016; Vatine 등, 2017).

중요한 것은 비특이적 분자의 누출은 활성 트래피킹 과정을 통해 발생하는 백혈구 외부 유출과 구별된다는 점입니다. 단일 세포 시퀀싱을 통해 신경염증에서 뚜렷한 역할을 하는 면역 세포의 많은 하위 집합이 질병에서 BBB와 상호 작용할 가능성이 있는 것으로 확인되었습니다(Mrdjen 외, 2018; Jordão 외, 2019; Kierdorf 외, 2019; Masuda 외, 2019; Mundt 외, 2019). 실질 EC는 백혈구 부착 분자를 상향 조절하여 백혈구 이동을 증가시킵니다. P-셀렉틴과 E-셀렉틴은 내피를 따라 백혈구가 구르는 것을 매개하고, ICAM1과 VCAM1은 견고한 접착을 매개하며, 질병에서 상향 조절되는 PLVAP와 같은 단백질은 EC 간 이동을 돕습니다(Engelhardt and Ransohoff, 2012; Ioannidou et al., 2006). BBB를 가로지르는 백혈구의 혈관 외 유출은 세포 간 또는 세포 주변으로 이루어질 수 있습니다(Carman et al., 2007; Winger et al., 2014). ICAM1과 PECAM1의 수준은 T세포의 탈피 경로에 영향을 미칠 수 있으며(Abadier 등, 2015; Wimmer 등, 2019), T세포의 특정 하위 집합은 서로 다른 경로를 선호합니다(Lutz 등, 2017).
따라서 BBB는 온오프 스위치가 아니며, 각 기능 장애 사례의 기저에 있는 특이성과 결과를 이해하는 것이 중요합니다.

BBB dysfunction in CNS disorders MS

BBB dysfunction is a central feature of MS, and the time course of leakage has been studied with dynamic contrast-enhanced MRI (Bastianello et al., 1990; Harris et al., 1991; Guttmann et al., 2016; Gaitán et al., 2011; Fig. 2 G). While barrier leakage is almost always present in new lesions, it is rarely observed in older lesions (Bastianello et al., 1990, Harris et al., 1991). Interestingly, MRI evidence suggests that BBB permeability is the initial event in the formation of a subset of lesions, but in others, lesion formation occurs before barrier dysfunction (Guttmann et al., 2016).

CNS immune infiltration is a critical step in MS pathophysiology, and the dynamics of this process have been primarily studied in experimental autoimmune encephalomyelitis (EAE), a rodent model of MS. The primary sites of CNS immune surveillance in health are the blood–CSF barriers of the choroid plexus and meninges, and both are important sites of initial lymphocyte activation in EAE (Bartholomäus et al., 2009; Schläger et al., 2016; Mundt et al., 2019; Engelhardt et al., 2001, 2017; Carrithers et al., 2000; Reboldi et al., 2009). These immune cells first enter the perivascular space surrounding post-capillary venules (Greter et al., 2005) and gain parenchymal access after breaking down the BM (Song et al., 2017; Wu et al., 2009). Leukocyte-derived cytokines activate CNS ECs, inducing expression‎ of leukocyte adhesion molecules (Carrithers et al., 2000; Barkalow et al., 1996; Lou et al., 1996), which leads to massive parenchymal infiltration of immune cells. Limiting immune cell trafficking across the BBB has proven effective in treating MS. Natalizumab, which targets the α4 integrin on immune cells, preventing their interaction with endothelial VCAM1, greatly reduces new lesion formation (Miller et al., 2003).

It is critical to note that while leukocyte invasion is often assumed to be detrimental, leukocyte trafficking is required at low levels in order to limit infections. Of great interest is the identification of leukocyte adhesion molecules that facilitate the extravasation of only certain subsets of immune cells (Steinman, 2015). This could enable targeting pathological inflammation without rendering patients more vulnerable to infection. Indeed, ninjurin1 (NINJ1; monocytes), activated leukocyte cell adhesion molecule (ALCAM; CD4+ T cells, monocytes), junction adhesion molecule–like (JAML; monocytes, CD8+ T cells), and melanoma cell adhesion molecule (MCAM; CD8, T helper cell 17) regulate the entry of specific immune cell populations into the CNS (Alvarez et al., 2015; Cayrol et al., 2008; Flanagan et al., 2012; Ifergan et al., 2011; Larochelle et al., 2015). It will be necessary to ensure that targeting these molecules does not produce secondary effects; Alcam knockout mice develop more severe EAE as ALCAM also enforces TJ integrity (Lécuyer et al., 2017).

Many questions remain unanswered. How much of MS pathophysiology directly results from BBB dysfunction? Is there a subset of lesions caused by leakage while others have a different etiology? If these lesion subsets exist, do they vary with respect to severity and repair processes? Does the BBB interact with the lymphatic system to regulate leukocyte efflux during remission?

Ischemia/stroke

BBB dysfunction during stroke follows a biphasic time course. Leakage is evident within hours of the primary insult, is subsequently reduced, and then reappears the day after (Huang et al., 1999; Kuroiwa et al., 1985; Fig. 2, E, F, and H). An increase in transcytosis of nonspecific molecules is the first stage of dysfunction, followed by structural alteration of TJs (Knowland et al., 2014). Questions still remain regarding the importance of leukocyte infiltration in pathogenesis. Several reports have shown that leukocyte adhesion molecule knockouts or antibodies directed against leukocyte adhesion molecules minimize infarct volume (Bowes et al., 1993; Connolly et al., 1996; Mayadas et al., 1993), whereas others have not been able to replicate this effect (Enzmann et al., 2018).

Much of the cell death that leads to neurological deficits occurs in the days following a stroke; thus, the second phase of BBB leakage may be an important therapeutic target. Major outstanding questions in stroke research surround the relevance of this biphasic BBB dysfunction. It is unknown whether the first and second openings are mechanistically different; perhaps the first opening is due to dynamic signaling while the second results from changes in BBB gene expression‎.

중추신경계 장애 다발성 경화증에서의 BBB 기능 장애
BBB 기능 장애는 다발성 경화증의 핵심 특징이며, 누출의 시간 경과가 동적 조영증강 MRI로 연구되었습니다(Bastianello 등, 1990; Harris 등, 1991; Guttmann 등, 2016; Gaitán 등, 2011; 그림 2 G). 장벽 누출은 거의 항상 새로운 병변에서 나타나지만 오래된 병변에서는 거의 관찰되지 않습니다 (Bastianello 등, 1990, Harris 등, 1991). 흥미롭게도 MRI 증거는 BBB 투과성이 병변의 하위 집합 형성의 초기 사건이지만 다른 경우에는 병변 형성이 장벽 기능 장애 전에 발생한다는 것을 시사합니다 (Guttmann et al., 2016).
CNS 면역 침윤은 MS 병리 생리학에서 중요한 단계이며, 이 과정의 역학은 주로 설치류 모델인 실험적 자가면역성 뇌척수염(EAE)에서 연구되었습니다. 건강에서 CNS 면역 감시의 주요 부위는 맥락막 신경총과 수막의 혈액-CSF 장벽이며, 이 두 부위는 EAE에서 초기 림프구 활성화의 중요한 부위입니다

(Bartholomäus 외, 2009; Schläger 외, 2016; Mundt 외, 2019; Engelhardt 외, 2001, 2017; Carrithers 외, 2000; Reboldi 외, 2009). 이러한 면역 세포는 먼저 모세혈관 후 정맥을 둘러싼 혈관 주위 공간으로 들어가서(Greter et al., 2005) BM을 분해한 후 실질에 접근합니다(Song et al., 2017; Wu et al., 2009). 백혈구 유래 사이토카인은 CNS EC를 활성화하여 백혈구 접착 분자의 발현을 유도하고(Carrithers 등, 2000; Barkalow 등, 1996; Lou 등, 1996), 이는 면역 세포의 대규모 실질 침윤으로 이어집니다. BBB를 통한 면역 세포 이동을 제한하는 것은 다발성 경화증 치료에 효과적인 것으로 입증되었습니다. 면역 세포의 α4 인테그린을 표적으로 하여 내피 VCAM1과의 상호 작용을 막는 나탈리주맙은 새로운 병변 형성을 크게 감소시킵니다(Miller et al., 2003).

백혈구 침입은 종종 해로운 것으로 간주되지만 감염을 제한하기 위해 백혈구 이동은 낮은 수준에서 필요하다는 점에 유의하는 것이 중요합니다. 면역 세포의 특정 하위 집합만 침입하도록 촉진하는 백혈구 부착 분자의 확인이 큰 관심을 끌고 있습니다(Steinman, 2015). 이를 통해 환자를 감염에 더 취약하게 만들지 않고 병적 염증을 표적으로 삼을 수 있습니다. 실제로 닌주린1(NINJ1; 단핵구), 활성화 백혈구 세포 부착 분자(ALCAM; CD4+ T 세포, 단핵구), 접합부 부착 분자 유사(JAML; 단핵구, CD8+ T 세포), 흑색종 세포 부착 분자(MCAM; CD8, T 헬퍼 세포 17)는 특정 면역 세포 집단이 CNS로 진입하는 것을 조절합니다(Alvarez et al, 2015; Cayrol 외., 2008; Flanagan 외., 2012; Ifergan 외., 2011; Larochelle 외., 2015). 이러한 분자를 표적으로 삼는 것이 이차적인 효과를 일으키지 않도록 해야 할 것입니다. 알캠 녹아웃 마우스는 알캠이 TJ 무결성을 강화하기 때문에 더 심각한 EAE가 발생합니다(Lécuyer et al., 2017).

아직 많은 의문이 풀리지 않았습니다. 

다발성 경화증의 병리 생리학 중 얼마나 많은 부분이 BBB 기능 장애로 인해 직접적으로 발생합니까? 

누출로 인한 병변의 하위 집합이 있는 반면 다른 병인은 다른 원인을 가지고 있나요? 

이러한 병변 하위 집합이 존재한다면 중증도 및 회복 과정에 따라 차이가 있나요? 

BBB가 림프계와 상호 작용하여 관해 중에 백혈구 유출을 조절합니까?

허혈/뇌졸중

뇌졸중 중 BBB 기능 장애는 2단계의 시간 경과를 따릅니다. 누출은 일차적 손상 후 몇 시간 내에 분명하게 나타나고, 이후 감소했다가 다음 날 다시 나타납니다(Huang et al., 1999; Kuroiwa et al., 1985; 그림 2, E, F, H). 비특이적 분자의 전이세포증 증가는 기능 장애의 첫 번째 단계이며, 그 다음에는 TJ의 구조적 변화가 뒤따릅니다(Knowland et al., 2014). 발병 기전에서 백혈구 침윤의 중요성에 대한 의문은 여전히 남아 있습니다. 여러 보고에 따르면 백혈구 부착 분자 녹아웃 또는 백혈구 부착 분자에 대한 항체가 경색 부피를 최소화하는 것으로 나타난 반면(Bowes 등, 1993; Connolly 등, 1996; Mayadas 등, 1993), 다른 연구에서는 이 효과를 재현하지 못했습니다(Enzmann 등, 2018).

신경학적 결손을 초래하는 세포 사멸의 대부분은 뇌졸중 발생 후 며칠 동안 발생하므로 BBB 누출의 두 번째 단계는 중요한 치료 표적이 될 수 있습니다. 뇌졸중 연구의 주요 미해결 과제는 이 2단계 BBB 기능 장애의 관련성에 관한 것입니다. 첫 번째와 두 번째 개방이 기계적으로 다른지, 첫 번째 개방은 동적 신호로 인한 것이고 두 번째 개방은 BBB 유전자 발현의 변화로 인한 것인지는 알려지지 않았습니다.

Epilepsy

There is a clear association between epilepsy and BBB dysfunction. BBB leakage in epilepsy patients is visible with contrast-enhanced MRI (Horowitz et al., 1992; Alvarez et al., 2010; Rüber et al., 2018; Fig. 2 I), and analysis of brain tissue from epileptic patients shows increased parenchymal albumin (Cornford et al., 1998a; Mihály and Bozóky, 1984), implicating blood-to-brain extravasation of large molecules. Furthermore, patient samples exhibit regional reduction in GLUT1 (Cornford et al., 1998b), and positron emission tomography scans demonstrate decreased uptake and metabolism in seizure foci (Cornford et al., 1998a; Janigro, 1999).

BBB dysfunction itself may be epileptogenic or may help propagate seizures. Experimental disruption of the BBB with osmotic shock leads to seizures in patients (Marchi et al., 2007), and diseases in which the BBB is compromised such as infection, inflammation, stroke, and traumatic brain injury can lead to seizures and epilepsy (Oby and Janigro, 2006; van Vliet et al., 2007). Furthermore, neuroinflammation has been hypothesized to be involved in seizure etiology; blockage of leukocyte–vascular interactions either pharmacologically or by genetic knockout inhibits both induction and recurrence of seizures (Fabene et al., 2008). Interestingly, patients with a BBB-GLUT1 deficiency develop epilepsy (De Vivo et al., 1991; De Vivo et al., 2002), demonstrating a critical role for BBB transport in normal brain function.

AD

The extent of BBB dysfunction in AD and its role in etiology are an important ongoing focus of research. Several techniques have been used to examine BBB function in AD patients, including staining postmortem brain tissue for serum components, measurement of blood/CSF albumin concentrations, and various imaging modalities. Histological analyses have shown increased albumin and immunoglobulins in areas of heavy plaque burden (Wisniewski et al., 1997) as well as increased levels of fibrinogen (Ryu and McLarnon, 2009). A three-dimensional in vitro AD model has shown evidence of BBB dysfunction, phenocopying vascular changes reported in patients (Shin et al., 2019). Additionally, several imaging studies have found evidence of a leakier BBB in AD patients and propose BBB dysfunction as an early biomarker of AD (Starr et al., 2009; Montagne et al., 2015; van de Haar et al., 2016; Nation et al., 2019). While many older reports found no change in CSF albumin levels or contrast-enhanced imaging (Alafuzoff et al., 1987; Frölich et al., 1991; Kay et al., 1987; Mecocci et al., 1991; Bronge and Wahlund, 2000; Dysken et al., 1990; Schlageter et al., 1987), several of these studies did find evidence of BBB leakage in patients with vascular disease, suggesting that even in the absence of widespread leakage, there is a crucial vascular component to pathology (Erickson and Banks, 2013; Farrall and Wardlaw, 2009; Mecocci et al., 1991; Alafuzoff et al., 1983). As new imaging technology with greater resolution has gained wider use, BBB dysfunction has been further implicated in the pathogenesis of AD (Montagne et al., 2015; van de Haar et al., 2016; Nation et al., 2019). With these new tools, it will be vital to perform a more detailed analysis to determine at what stage and in which brain regions BBB dysfunction occurs, whether leakage is transient or chronic, and which cellular BBB properties are affected.

Regardless of the extent of widespread BBB leakage, there are several links between BBB dysfunction and AD pathology (Petersen et al., 2018). Fibrin accumulates in amyloid-positive vessels in AD patients and mouse models, and fibrin depletion protects against cognitive deficits in mice (Paul et al., 2007; Cortes-Canteli et al., 2010). Perhaps small amounts of BBB leakage related to injury, infection, or aging increase fibrin deposition, setting in motion an inflammatory cascade that plays an important role in AD pathology (Petersen et al., 2018; Kumar et al., 2016; Kinney et al., 2018).

In addition to nonspecific leakage, dysfunction of BBB Aβ transport may drive AD pathology (Bell and Zlokovic, 2009; Erickson and Banks, 2013). LRP1, a cell-surface receptor expressed on ECs, regulates Aβ clearance from the parenchyma (Shibata et al., 2000). EC-specific Lrp1 knockout increases levels of soluble brain Aβ and the severity of learning and memory deficits in an AD mouse model (Storck et al., 2016). A phosphatidylinositol binding clathrin assembly protein (PICALM)/PGP-dependent mechanism also aids in the clearance of Aβ across the BBB. PICALM regulates clathrin-dependent internalization of Aβ, guiding receptor-mediated transcytosis and clearance of Aβ, potentially presenting Aβ to efflux transporters (Zhao et al., 2015). PGP deficiency in an AD mouse model cuts Aβ clearance rate in half and increases CNS Aβ deposition (Cirrito et al., 2005), and Aβ40 triggers ubiquitination and internalization of PGP (Hartz et al., 2016), suggesting a dangerous feedback cycle. Conversely, receptor for advanced glycation endproducts (RAGE) imports Aβ into the CNS (Deane et al., 2003), and alterations in LRP:RAGE activity are hypothesized to drive CNS amyloid deposition in AD patients (Jeynes and Provias, 2008).

Another factor that might contribute to BBB dysfunction in AD is apolipoprotein E (APOE) genotype. Transgenic mice expressing human APOE4, the AD risk allele, exhibit cerebral vasculature with a thinner BM and BBB dysfunction due to cyclophillin/MMP9 signaling in pericytes (Bell et al., 2012; Alata et al., 2015). Further, postmortem AD tissue has revealed decreased TJ proteins and MMP9 elevation along with pericyte degeneration in APOE4 carriers (Bell et al., 2012; Halliday et al., 2016; Nishitsuji et al., 2011). However, there are conflicting reports; others show no changes in BBB function in Apoe4 knockout or APOE4 transgenic mice (Bien-Ly et al., 2015). One possible explanation is that APOE4 might cause minor, highly localized BBB leakage while not disrupting global BBB integrity (Ulrich et al., 2015).

To address the outstanding questions in the field, a deeper understanding of the association between vascular damage and AD pathology is necessary. This will require a focus on finding causal rather than correlational information linking BBB leakage, inflammation, and AD pathology. For instance, a recent study found that BBB dysfunction is an early marker of cognitive decline independent of Aβ or tau accumulation (Nation et al., 2019), but more details are needed regarding the extent of BBB dysfunction at various points during the AD time course. Furthermore, it is critical to understand how the BBB, glymphatics, and lymphatics cooperate to remove Aβ and other waste products from the CNS parenchyma, and what role this plays in AD pathophysiology (Stower, 2018; Rasmussen et al., 2018; Sweeney and Zlokovic, 2018; Da Mesquita et al., 2018).

Looking forward

Several important questions remain regarding the BBB in the context of disease. How is each BBB property altered in neurological diseases, and how do these changes affect the extracellular environment of the CNS? One problem is that different studies in humans or mouse models often use a single modality to detect BBB breakdown, whether sampling postmortem tissue, measuring markers in the CSF or blood, quantifying leakage of an exogenous tracer, or performing live imaging with a contrast agent. The BBB is not a single entity that is “open” or “shut,” and moving forward, it is imperative to understand exactly how the complex physiology of the BBB changes in each disease. It is especially important to consider whether alterations are induced by the same or different signals across neurological conditions. If mechanistic similarities exist, it might be possible to design a therapeutic strategy applicable to a wide range of disorders (Munji et al., 2019). Indeed, several molecular factors regulate BBB dysfunction in multiple diseases, including vascular endothelial growth factor (Argaw et al., 2009, 2012), inflammatory cytokines (tumor necrosis factor α [Nishioku et al., 2010], interleukins 1 and 6 [Chiaretti et al., 2005; Paré et al., 2018; Wang et al., 2014]), reactive oxygen species (Maier et al., 2006; Pun et al., 2009; Relton et al., 1997), and matrix metalloproteinases (Gidday et al., 2005; Ugarte-Berzal et al., 2018). However, there is also evidence that barrier dysfunction is due not only to “breakdown signals” but also to disrupted maintenance signals. Disruption of Wnt signaling can lead to vascular permeability and worse disease outcomes (Wang et al., 2012; Chang et al., 2017); thus, increasing CNS EC Wnt signaling might have therapeutic potential.

Can subtle changes in different BBB properties cause specific neurological symptoms? Dysfunction in several BBB transporters causes specific developmental disorders (Seidner et al., 1998; Tărlungeanu et al., 2016; Vatine et al., 2017), and there may be more undiscovered instances of this pattern. It is possible that regional heterogeneity at the BBB renders particular brain regions vulnerable to certain disease pathologies. For instance, if the BBB is indeed specialized to cater to the distinct nutrient and signaling needs of individual brain regions, loss of one of those BBB specializations might lead to deficits in local circuit function.

It is also important to also think beyond ECs. Disruption of pericyte coverage leads to an increase in EC nonspecific transcytosis and leukocyte adhesion molecules expression‎, and it is unclear to what extent this drives neurological disease. Furthermore, disruption of astrocyte endfeet at the NVU would decrease glymphatic clearance, potentially contributing to pathological accumulation of proteins including Aβ. Future work analyzing how each cell type of the NVU, and the glycocalyx and BMs, is altered will be critical to understand the pathophysiology of different neurological diseases.

Another fundamental question is how the BBB is repaired. While the BBB becomes less permeable to molecular tracers at chronic phases of disease models, it is unclear whether there are functional or structural compromises made in the process of reversing leakiness. More work is needed to fully characterize the repaired BBB at the levels of physical integrity and transcriptomics. It is also unknown what endogenous signals induce BBB repair, and whether repair occurs cell-autonomously within ECs or with mediation from other cell types. Interestingly, both microglia and reactive astrocytes regulate repair of the BBB in response to injury, highlighting the importance of the interactions of cells within the NVU (Lou et al., 2016, Bush et al., 1999, Fernández-López et al., 2016).

Concluding remarks

The BBB is not a single entity, but rather a complex series of physiological properties allowing CNS ECs to tightly regulate the extracellular environment of the parenchyma. These properties are vital for proper neural function, and dysfunction of the BBB can lead to critical pathology in many neurological diseases. However, more work is needed in order to understand exactly what crosses the healthy BBB, the degree to which the BBB dynamically responds to environmental stimuli, the extent of its regional heterogeneity, and the signaling mechanisms underlying its maintenance, disruption, and repair (Box 2). As future research answers these questions and further reveals the cellular and molecular intricacies underlying the BBB, the clinical advantages will be twofold: a deeper knowledge of the BBB will provide therapeutic targets for BBB repair in a range of neurological conditions and will also enable more effective strategies for delivering drugs to the CNS.

Box 2.

Important unanswered questions

Acknowledgments

We would like to thank Dr. John Hesselink (University of California, San Diego, San Diego, CA) for generously providing MRI and FLAIR images for Fig. 2.

Author contributions: C.P. Profaci, R.N. Munji, R.S. Pulido, and R. Daneman contributed to the conceptualization, research, and writing of the manuscript. C.P. Profaci and R. Daneman edited the manuscript. C.P. Profaci and R.N. Munji created the figures and boxes.

Competing Interests

Disclosures: The authors declare no competing interests exist.

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