도파민 뉴런 --＞ 뉴로멜라닌 탐구!!

[문형철]

Neural Regen Res. 2017 Mar; 12(3): 372–375.

doi: 10.4103/1673-5374.202928

PMCID: PMC5399705

PMID: 28469642

Neuromelanin, one of the most overlooked molecules in modern medicine, is not a spectator

Robert L. Haining, Ph.D.* and Cindy Achat-Mendes

Author information Article notes Copyright and License information PMC Disclaimer

Abstract

The loss of pigmented neurons from the human brain has long been the hallmark of Parkinson's disease (PD). Neuromelanin (NM) in the pre-synaptic terminal of dopamine neurons is emerging as a primary player in the etiology of neurodegenerative disorders including PD. This mini-review discusses the interactions between neuromelanin and different molecules in the synaptic terminal and describes how these interactions might affect neurodegenerative disorders including PD. Neuromelanin can reversibly bind and interact with amine containing neurotoxins, e.g., MPTP, to augment their actions in the terminal, eventually leading to the instability and degeneration of melanin-containing neurons due to oxidative stress and mitochondrial dysfunction. In particular, neuromelanin appears to confer susceptibility to chemical toxicity by providing a large sink of iron-bound, heme-like structures in a pi-conjugated system, a system seemingly purposed to allow for stabilizing interactions including pi-stacking as well as ligand binding to iron. Given the progressive accumulation of NM with age corresponding with an apparent decrease in dopamine synthetic pathways, the immediate question of whether NM is also capable of binding dopamine, the primary functional monoamine utilized in this cell, should be raised. Despite the rather glaring implications of this finding, this idea appears not to have been adequately addressed. As such, we postulate on potential mechanisms by which dopamine might dissociate from neuromelanin and the implications of such a reversible relationship. Intriguingly, if neuromelanin is able to sequester and release dopamine in membrane bound vesicles, this intracellular pre-synaptic mechanism could be the basis for a form of chemical memory in dopamine neurons.

인간의 뇌에서

색소 뉴런이 소실되는 것은

오랫동안 파킨슨병(PD)의 특징이었습니다.

도파민 뉴런의 시냅스 전 말단에 있는 뉴로멜라닌(NM)은

PD를 비롯한 신경 퇴행성 질환의

주요 원인으로 떠오르고 있습니다.

이 미니 리뷰에서는

뉴로멜라닌과 시냅스 말단의 여러 분자 간의 상호작용에 대해 설명하고

이러한 상호작용이 PD를 포함한 신경 퇴행성 질환에 어떤 영향을 미치는지 설명합니다.

뉴로멜라닌은

가역적으로 아민을 함유한 신경독소( 예: MPTP)와

결합하고

상호작용하여

말단에서 그 작용을 강화함으로써

결국 산화 스트레스와 미토콘드리아 기능 장애로 인해

멜라닌 함유 뉴런의 불안정성과 퇴행을 유발할 수 있습니다.

특히

뉴로멜라닌은

파이 접합 시스템에서

철과 결합된 헴 유사 구조의 큰 싱크대를 제공함으로써

화학 독성에 대한 감수성을 부여하는 것으로 보이며,

이 시스템은 파이 스택과 철에 대한

리간드 결합을 포함한 상호작용을 안정화하기 위한 것으로 보입니다.

나이가 들면서

도파민 합성 경로의 명백한 감소와 함께

NM이 점진적으로 축적되는 것을 고려할 때,

NM이 이 세포에서 사용되는 주요 기능성 모노아민인 도파민과도

결합할 수 있는지에 대한 즉각적인 의문이 제기되어야 합니다.

이 발견이 시사하는 바가 매우 큼에도 불구하고

이 아이디어는 적절히 다루어지지 않은 것으로 보입니다.

따라서

우리는

도파민이 뉴로멜라닌에서 분리될 수 있는 잠재적 메커니즘과

이러한 가역적 관계의 의미에 대해 가정합니다.

흥미롭게도

뉴로멜라닌이

막 결합 소포에서 도파민을 격리하고 방출할 수 있다면,

이 세포 내 시냅스 전 메커니즘은

도파민 뉴런의 화학적 기억의 한 형태가 될 수 있습니다.

Keywords: dopamine, dopamine storage, Parkinson's disease, neurodegeneration, neuromelanin, neurotoxicity

Neuromelanin: What Is It?

For over 300 years, Parkinsonism has been associated with a loss of pigmentation from the human brain. Termed neuromelanin (NM) to distinguish it from the numerous melanin sources found throughout the body, it is absent at birth and naturally increases throughout a person's lifetime, accelerating in synthesis and accumulation in adolescence. This pigmentation is most pronounced in catecholaminergic neurons of the substantia nigra pars compacta (SNpc) and locus coeruleus, leading to a blackened appearance in these regions of aged brains. Loss of NM and subsequent depigmentation of these brain regions is a hallmark feature of Parkinson's disease (PD). This is primarily due to immune-mediated death of pigmented cells. There is evidence to suggest that DA neurons with high amounts of NM are more susceptible to degeneration. For example, NM-rich SNpc neurons undergo cell death compared to non-pigmented ventral tegmental area neurons in PD (Zecca et al., 2003; Zucca et al., 2015).

Dopamine is normally thought to be protected from auto-oxidation in secretory vesicles through co-localization of ascorbic acid and other antioxidants. Sulzer et al. (2000) showed that neuromelanin forms in cultured dopaminergic neurons when cytoplasmic concentrations of dopamine are artificially raised. In their experiment, PC-12 cells were exposed to high levels of dopamine through addition of L-3,4-dihydroxyphenylalanine (L-DOPA) to the cell culture media, which is rapidly taken up and subsequently converted to dopamine. Cells which had been artificially enhanced in their ability to sequester dopamine into vesicles showed significantly less accumulation of pigment. This critical result suggests that under the normal range of living conditions, times exist when a firing neuron becomes over-burdened and is unable to sequester dopamine into vesicles before overloading the neuron's antioxidant defenses. Such antioxidants often take the form of thiols including cysteine and glutathione; as such, it is perhaps not surprising that natural NM has cysteine in its pheomelanin core, while being surrounded by a eumelanin component lacking cysteine.

NM is eventually found sequestered into double membrane granules inside neurons and contains lipids and peptides bound to its structure in vivo (Zecca et al., 2000). The finding that this structure is carefully packaged and retained is evidence in favor of an important function for this molecule, yet we still know little about its implications for the cell. That this pigmentation is not an enzymatically driven process in dopaminergic neurons may be highly relevant given the lack of a central genetic basis for most cases of PD.

뉴로멜라닌: 뉴로멜라닌이란?

300년 이상 

파킨슨병은 

인간의 뇌에서 색소가 손실되는 것과 관련이 있는 것으로 

알려져 있습니다. 

몸 전체에서 발견되는 수많은 멜라닌과 구별하기 위해 

뉴로멜라닌(NM)이라고 불리는 이 색소는 

출생 시에는 없으며 

일생 동안 자연적으로 증가하여 

청소년기에 합성 및 축적이 가속화됩니다. 

이 색소 침착은 

흑질 흑질(SNpc)의 카테콜아민성 뉴런과 흑질 흑질 유전자좌에서 

가장 두드러지게 나타나며, 

노화된 뇌의 이러한 영역이 검게 보입니다. 

이러한 

뇌 영역의 NM 소실과 

그에 따른 탈색은 

파킨슨병(PD)의 특징적인 특징입니다. 

이는 주로 

색소 세포의 면역 매개 사멸 때문입니다. 

NM이 많은 DA 뉴런은 

퇴행에 더 취약하다는 증거가 있습니다. 

예를 들어, 

NM이 풍부한 SNpc 뉴런은 

PD에서 색소가 없는 복측 피질 영역 뉴런에 비해 

세포 사멸을 겪습니다

(Zecca et al., 2003; Zucca et al., 2015).

도파민은 

일반적으로 

아스코르브 산과 다른 항산화 물질의 공동 국소화를 통해 

분비 소포의자가 산화로부터 보호되는 것으로 생각됩니다. 

Sulzer 등(2000)은 

도파민의 세포질 농도를 인위적으로 높이면 

배양된 도파민성 뉴런에서 

뉴로멜라닌이 형성된다는 것을 보여주었습니다. 

이들의 실험에서 PC-12 세포는 세포 배양액에 L-3,4-디하이드록시페닐알라닌(L-DOPA)을 첨가하여 높은 수준의 도파민에 노출시켰고, 이는 빠르게 흡수되어 이후 도파민으로 전환되었습니다. 

도파민을 소포에 격리하는 능력이 인위적으로 강화된 세포는 

색소 축적이 현저히 줄어든 것으로 나타났습니다. 

이 중요한 결과는 

정상적인 생활 조건에서 뉴런이 과부하가 걸려 

도파민을 소포에 격리하지 못하면 

뉴런의 항산화 방어 기능이 과부하가 걸리는 시기가 존재한다는 것을 시사합니다. 

이러한 항산화제는 

시스테인과 글루타치온을 포함한 티올의 형태를 띠는 경우가 많으므로 

천연 NM이 페오멜라닌 코어에 시스테인이 있고 

시스테인이 없는 유멜라닌 성분으로 둘러싸인 것은 어쩌면 당연한 일인지도 모릅니다.

NM은 결국 

뉴런 내부의 이중막 과립으로 격리되어 발견되며 

생체 내에서 그 구조에 결합된 지질과 펩타이드를 포함하고 있습니다(Zecca et al., 2000). 

이 구조가 조심스럽게 포장되어 유지된다는 사실은 

이 분자의 중요한 기능을 뒷받침하는 증거이지만, 

세포에 미치는 영향에 대해서는 아직 알려진 바가 거의 없습니다. 

이러한 색소 침착이 

도파민 신경세포에서 효소에 의한 과정이 아니라는 것은 

대부분의 PD 사례에 대한 중심 유전적 근거가 부족하다는 점을 고려할 때 매우 관련이 있을 수 있습니다.

Structure of Endogenous NM

NM has been historically poorly characterized largely due to the intrinsic nature of this spontaneously occurring polymer. It is structurally similar to non-neural melanins, with a predicted 3-D lattice structure and the ability to chelate and neutralize transition metals, ions and lipids. However, due to the spontaneous nature of the free radical reaction that occurs in neurons, this process is likely to be characterized by a certain degree of lack of control. In contrast, other natural melanins found in humans are enzymatically driven, not spontaneous. As such, the assortment of chemical precursors of ill-defined proportion which can incorporate in addition to dopamine and cysteine during the polymerization cascade could also result in the lack of a consistent, regularly ordered structure, particularly in the case of toxic chemical insult such as that imposed by 6-hydroxydopamine. In addition, catechol polymers are not easily de-polymerized, unlike other macromolecules utilized in biological systems such as polysaccharides, proteins, and nucleic acids which can by analyzed by relatively simple hydrolysis. Nevertheless, progress has been made in understanding the subunit makeup of natural NM. In one study, the ratio of hydrogen peroxide (H2O2) degradation products indicated that NM is derived mostly from dopamine, with 25% incorporation of cysteine in the form of a benzothiazine structure. In the same study, hydriodic acid reductive hydrolysis of NM revealed a 21% incorporation of Cys-DA-derived units into NM, while DOPA was found incorporated at a level near 6% for that observed with dopamine (Wakamatsu et al., 2003).

It is now known that NM isolated from human brain is comprised of granules with a diameter of 30 nm, similar to that observed for Sepia cuttlefish, bovine eye, and human eye and hair melanosomes. NM is a mixture of pheomelanin and eumelanin, the two major forms corresponding to the widely known light-haired and dark-haired phenotypes, respectively. Kinetic studies suggest that in such mixed pigments, pheomelanin formation occurs first with eumelanin formation occurring only after cysteine levels have been depleted. This leads to a predicted structural motif with pheomelanin at the core and eumelanin at the surface (Bush et al., 2006). The major lipid component of neuromelanin pigment derived from human SN was found to be the polyisoprenoid dolichol, accounting for 14% of the mass of the isolated pigment (Fedorow et al., 2005).

내인성 NM의 구조

NM은 자연적으로 발생하는 폴리머의 본질적인 특성으로 인해 역사적으로 제대로 특성화되지 못했습니다. 비신경 멜라닌과 구조적으로 유사하며, 예상되는 3D 격자 구조와 전이 금속, 이온 및 지질을 킬레이트화 및 중화할 수 있는 능력을 가지고 있습니다. 그러나 뉴런에서 발생하는 자유 라디칼 반응의 자발적인 특성으로 인해 이 과정은 어느 정도 통제력이 부족하다는 특징이 있을 수 있습니다. 

이와 대조적으로 

인간에게서 발견되는 

다른 천연 멜라닌은 자발적인 것이 아니라 

효소에 의해 생성됩니다. 

따라서 중합 과정에서 도파민과 시스테인 외에 포함될 수 있는 정의되지 않은 비율의 화학적 전구체가 다양하게 존재하면 특히 6-하이드록시도파민과 같은 독성 화학적 모욕의 경우 일관되고 규칙적인 구조가 부족할 수 있습니다. 또한 카테콜 중합체는 비교적 간단한 가수분해로 분석할 수 있는 다당류, 단백질, 핵산 등 생물학적 시스템에서 사용되는 다른 거대 분자와 달리 쉽게 해중합되지 않습니다. 그럼에도 불구하고 천연 NM의 하위 단위 구성을 이해하는 데는 진전이 있었습니다. 한 연구에서 과산화수소(H2O2) 분해 산물의 비율을 보면 NM은 대부분 도파민에서 파생되며, 벤조티아진 구조의 시스테인이 25% 포함되어 있는 것으로 나타났습니다. 같은 연구에서 NM의 염산 환원 가수분해 결과, NM에 Cys-DA 유래 단위가 21% 통합된 것으로 나타난 반면, 도파민에서 관찰된 6%에 가까운 수준에서 DOPA가 통합된 것으로 밝혀졌습니다(Wakamatsu et al., 2003).

현재 인간의 뇌에서 분리된 NM은 세피아 오징어, 소의 눈, 인간의 눈과 머리카락 멜라노좀에서 관찰되는 것과 유사한 직경 30nm의 과립으로 구성되어 있는 것으로 알려져 있습니다. NM은 페오멜라닌과 유멜라닌의 혼합물로, 각각 널리 알려진 밝은 머리와 어두운 머리 표현형에 해당하는 두 가지 주요 형태입니다. 동역학 연구에 따르면 이러한 혼합 색소에서는 페오멜라닌 형성이 먼저 일어나고 시스테인 수치가 고갈된 후에야 유멜라닌 형성이 일어납니다. 이것은 중심부에 페오멜라닌이 있고 표면에 유멜라닌이 있는 예상 구조 모티프로 이어집니다(Bush et al., 2006). 인간 SN에서 추출한 뉴로멜라닌 색소의 주요 지질 성분은 분리된 색소 질량의 14%를 차지하는 폴리이소프레노이드 돌리콜인 것으로 밝혀졌습니다(Fedorow et al., 2005).

Chemical Binding to NM as a Causative Factor in PD

It has also been known for several decades that a curious feature of natural melanins is their ability to bind and retain organic compounds for long periods of times. Not surprisingly, abundant evidence suggests that the massive accumulation of pigment within the aged dopaminergic neuron also corresponds with the accumulation of numerous potentially toxic chemical entities. In particular, organic amines and metal ions such as manganese have been shown to exhibit high binding affinities with melanins (Karlsson and Lindquist, 2013). MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) for example is believed to induce its Parkinsonian-inducing effects after first being converted to methyl-phenyl-pyridinium ion (MPP+) by the enzyme monoamine oxidase B in nearby glial cells (Singer et al., 2006). MPP+ appears to accumulate in pigmented neurons due to the chelating nature of the pigmentation itself (D’Amato et al., 1986). However, it is also thought to eventually kill the cell by disrupting mitochondrial electron transport. Sequestration of MPP+ then appears to induce cellular apoptosis leading to cell death and a downstream cascading immune response due to the release of previously unseen antigens (Sulzer et al., 2008). Blocking the uptake of MPP+ blocks the toxic effects of MPTP administration, yet it is unclear whether MPTP is concentrated into pigmented tissue before or after conversion to MPP+. MPP+, apparently binds to NM in a reversible equilibria in order to exert its known toxic effects on mitochondria.

PD의 원인 인자로서의 NM에 대한 화학적 결합

천연 멜라닌의 흥미로운 특징은 유기 화합물과 오랫동안 결합하고 유지하는 능력이라는 사실도 수십 년 동안 알려져 왔습니다. 당연히 노화된 도파민 신경세포 내에 색소가 대량으로 축적되는 것은 잠재적으로 독성이 있는 수많은 화학 물질의 축적과도 일치한다는 풍부한 증거가 있습니다. 특히 유기 아민과 망간과 같은 금속 이온은 멜라닌과 높은 결합 친화력을 보이는 것으로 나타났습니다(Karlsson and Lindquist, 2013). 예를 들어 MPTP(1-메틸-4-페닐-1,2,3,6-테트라하이드로피리딘)는 인근 신경교세포에서 효소 모노아민 산화효소 B에 의해 메틸-페닐-피리디늄 이온(MPP+)으로 먼저 전환된 후 파킨슨병 유발 효과를 유도하는 것으로 추정됩니다(Singer et al., 2006). MPP+는 색소 자체의 킬레이트화 특성으로 인해 색소 뉴런에 축적되는 것으로 보입니다(D'Amato et al., 1986). 그러나 미토콘드리아 전자 수송을 방해하여 결국 세포를 죽이는 것으로 생각되기도 합니다. MPP+의 격리는 세포 사멸을 유도하여 이전에 볼 수 없었던 항원의 방출로 인해 세포 사멸과 연쇄적인 면역 반응을 유도하는 것으로 보입니다(Sulzer et al., 2008). MPP+의 흡수를 차단하면 MPTP 투여의 독성 효과를 차단할 수 있지만, MPTP가 MPP+로 전환되기 전 또는 후에 색소 조직에 집중되는지는 불분명합니다. MPP+는 미토콘드리아에 알려진 독성 효과를 발휘하기 위해 가역적 평형 상태에서 NM에 결합하는 것으로 보입니다.

Possible Beneficial Functions for Chemical Binding to NM: Is There a Natural Substrate?

Many investigators have assumed that the accumulation of NM pigment is simply an unavoidable by-product of aging. Merely a form of molecular garbage that the cell is unable to dispose of, they argue, thus dismissing its presence as uninteresting and non-essential in the process. In this view, the binding of toxic chemicals to NM may simply be a by-product of the existence of the garbage itself, eventually leading to the instability and death of melanin-containing neurons due to oxidative stress. Another prevailing hypothesis for NM's protective role in dopaminergic neurons lies in the prevention of neurotoxic pathways of quinones that are formed during dopamine oxidation. When dopamine is oxidized to dopamine o-quinone, aminochrome and 5,6-indolequinone are formed and typically undergo polymerization to form the dark pigment, neuromelanin (Munoz et al., 2012). In the absence of polymerization and neuromelanin, aminochrome can form adducts with alpha synuclein generating neurotoxic oligomers that can trigger mitochondrial dysfunction (Wang et al., 2012), and could induce oxidative and endoplasmic reticulum stress thereby affecting protein synthesis and degradation dysfunction. Alternatively, it has been proposed that NM serves a dual function, protecting the cell from oxidative stress through iron chelation in the early stages, yet contributing to the autoimmune aggravation of PD through release of novel antigens upon eventual cell death (Zecca et al., 2006). Ultrafast laser spectroscopy studies suggest that the integrity of the pigmented granules in tissue may play an important role in the balance between the photo-protective and photo-damaging behaviors attributed to melanins (Liu and Simon, 2003).

What cannot be ignored in this discussion however is the oxidative stress necessitated by the handling of high concentrations of catecholamines. In other words, in order to function as an active dopaminergic or adrenergic cell, the cell is necessarily at risk for toxicity from the catechols themselves, molecules known primarily for their high reactivity and redox potential in forming quinones. As has been pointed out, sequestration into secretory vesicles apparently alleviates this toxicity and stress on the cell. What few seem to have suggested to date however is that the accumulation of NM itself could serves as a storage, protection, and re-release mechanism for dopamine (in the substantia nigra) or epinephrine (in the locus coeruleus), possibly acting as a very real molecular memory loop. This seems at first glance counter-intuitive, that the product of oxidative stress (NM) could then function to alleviate the very thing that created itself (excess dopamine), yet in reality would be quite an elegant solution to a problem.

Since the structure of NM granules has not been established at sub-μm resolutions, it is not known exactly how the polymer is arranged. However, modelling studies suggest the granules adopt a layered structure containing a semi-regularly ordered array highly suitable for trapping monoamines (Charkoudian and Franz, 2006). In the case of such naturally occurring, aromatic organic catecholamines, the coordination of a lone-pair of electrons from the de-protonated amine nitrogen to NM-chelated iron combined with potential pi-stacking interactions through NM indole subunits provides a plausible mechanism for observed binding interactions. The putative ability of pigmented granules to concentrate catecholamines such as dopamine from surrounding tissue could supplant some of the function of vesicular monoamine transporters, which serve to sequester potentially toxic dopamine into vesicles for synaptic transmission. Such an extraordinary ability could even supplant the function of tyrosine hydroxylase (TH), the enzyme primarily used to locally synthesize dopamine and which is normally used as a marker for dopaminergic neurons. In one intriguing study, the authors compared the number of dopaminergic neurons in young, middle aged, and older squirrel monkeys. The absolute number of these neurons appeared to decrease over the lifespan of the animals when anti-TH antibodies were used alone to detect their presence, yet remained constant when pigmented neurons were counted along with TH-positive neurons (McCormack et al., 2004). This suggests that as pigmentation accumulates in a given neuron, TH expression‎ is down regulated and dopamine synthesis presumably slowed.

NM에 대한 화학적 결합의 가능한 유익한 기능: 천연 기질이 있을까요?
많은 연구자들은 NM 색소의 축적을 단순히 노화의 피할 수 없는 부산물이라고 가정해 왔습니다. 세포가 처리할 수 없는 분자 쓰레기의 한 형태일 뿐이며, 따라서 그 존재는 흥미롭지 않고 필수적이지 않다고 무시한다고 주장합니다. 이러한 관점에서 독성 화학물질이 NM에 결합하는 것은 단순히 쓰레기 자체의 부산물일 수 있으며, 결국 산화 스트레스로 인해 멜라닌을 함유한 뉴런이 불안정해지고 죽게 됩니다. 도파민성 뉴런에서 NM의 보호 역할에 대한 또 다른 일반적인 가설은 도파민 산화 중에 형성되는 퀴논의 신경 독성 경로를 예방하는 데 있습니다. 도파민이 도파민 o-퀴논으로 산화되면 아미노크롬과 5,6-인돌퀴논이 형성되고 일반적으로 중합을 거쳐 어두운 색소인 뉴로멜라닌을 형성합니다(Munoz et al., 2012). 중합과 뉴로멜라닌이 없는 경우 아미노크롬은 알파 시뉴클레인과 부가체를 형성하여 미토콘드리아 기능 장애를 유발할 수 있는 신경 독성 올리고머를 생성할 수 있으며(Wang et al., 2012), 산화 및 소포체 스트레스를 유발하여 단백질 합성 및 분해 기능 장애에 영향을 미칠 수 있습니다. 또는 NM은 초기 단계에서 철 킬레이션을 통해 산화 스트레스로부터 세포를 보호하는 동시에 최종 세포 사멸 시 새로운 항원을 방출하여 PD의 자가 면역 악화에 기여하는 이중 기능을 수행한다고 제안되었습니다(Zecca et al., 2006). 초고속 레이저 분광학 연구에 따르면 조직 내 색소 과립의 무결성이 멜라닌에 의한 광 보호 작용과 광 손상 작용 사이의 균형에 중요한 역할을 할 수 있다고 합니다(Liu and Simon, 2003).

그러나 이 논의에서 무시할 수 없는 것은 고농도의 카테콜아민을 처리하는 데 필요한 산화 스트레스입니다. 즉, 활성 도파민 또는 아드레날린 세포로 기능하기 위해서는 세포가 퀴논을 형성할 때 주로 높은 반응성과 산화 환원 잠재력으로 알려진 분자인 카테콜 자체의 독성 위험에 노출될 수밖에 없습니다. 이미 지적한 바와 같이, 분비 소포에 격리하면 이러한 독성과 세포에 대한 스트레스가 완화되는 것으로 보입니다. 그러나 현재까지 제안된 바에 따르면 NM 자체의 축적이 도파민(흑질)이나 에피네프린(흑색질)의 저장, 보호 및 재방출 메커니즘으로 작용하여 실제 분자 기억 루프 역할을 할 수 있다는 가능성이 제기되고 있습니다. 산화 스트레스(NM)의 산물이 그 자체를 생성한 바로 그 물질(과잉 도파민)을 완화하는 기능을 할 수 있다는 것은 언뜻 직관적이지 않은 것처럼 보이지만, 실제로는 문제에 대한 매우 우아한 해결책이 될 수 있습니다.

NM 과립의 구조는 μm 이하의 해상도에서 밝혀지지 않았기 때문에 폴리머가 어떻게 배열되어 있는지는 정확히 알려져 있지 않습니다. 그러나 모델링 연구에 따르면 과립은 모노아민을 포획하는 데 매우 적합한 반규칙적으로 정렬된 배열을 포함하는 층상 구조를 채택하고 있습니다(Charkoudian and Franz, 2006). 이러한 자연 발생 방향족 유기 카테콜아민의 경우, 탈양성자화된 아민 질소에서 NM 킬레이트 철에 이르는 외쌍 전자의 조정과 NM 인돌 서브 유닛을 통한 잠재적 파이 스택 상호작용이 결합하여 관찰된 결합 상호작용에 대한 그럴듯한 메커니즘을 제공합니다. 주변 조직에서 도파민과 같은 카테콜아민을 농축하는 색소 과립의 추정 능력은 시냅스 전달을 위해 잠재적으로 독성이 있는 도파민을 소포에 격리하는 역할을 하는 소포 모노아민 수송체의 일부 기능을 대체할 수 있습니다. 이러한 특별한 능력은 도파민을 국소적으로 합성하는 데 주로 사용되며 일반적으로 도파민 신경세포의 마커로 사용되는 효소인 티로신 하이드 록실 라제 (TH)의 기능을 대체 할 수도 있습니다. 한 흥미로운 연구에서 저자들은 젊은 다람쥐원숭이, 중년 다람쥐원숭이, 노년 다람쥐원숭이의 도파민 신경세포 수를 비교했습니다. 항-TH 항체만 사용하여 그 존재를 감지했을 때 이러한 뉴런의 절대적인 수는 동물의 수명에 따라 감소하는 것으로 보였지만, 색소성 뉴런을 TH 양성 뉴런과 함께 세었을 때는 일정하게 유지되었습니다(McCormack et al., 2004). 이는 특정 뉴런에 색소 침착이 축적되면 TH 발현이 하향 조절되고 도파민 합성이 느려진다는 것을 시사합니다.

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Possible Role of NM in Neurons: Sequestration and Release of Dopamine as a Chemical Memory System?

Though highly speculative in nature at the current time, such a system (illustrated in Figure 1) presents an elegant mechanism whereby the very subunits that need protection from oxidation result in a polymer that is capable of protecting and furthering an identical signal. If dopamine that becomes NM is packaged into double-membrane bound vesicles in a reducing environment (as illustrated for example by its susceptibility to Fontana-Masson silver staining) has the ability to bind, store, protect, and release free dopamine, the chemical loop is complete. It could explain in part the reinforcement of addictive memories and associated behavior that comes with dopamine-releasing drugs, since excess cytoplasmic dopamine creates oxidative stress which leads to self-polymerization up to a certain point.

Figure 1

Hypothetical role of Neuromelanin (NM) in the storage and release of dopamine.

A dopaminergic axon terminal is shown, with dopamine (DA) being synthesized and packed into vesicles via vesicular monoamine transporters (VMAT) for release into the synapse. Released dopamine is taken up through a dopamine transporter (DAT). Excess cytosolic dopamine is thought to co-polymerize with cysteine into a polymer of ill-defined composition which is eventually packaged into a larger double-membrane bound vesicle (NM granule). It is known that this pigmented granule sequesters toxins such as methyl phenyl pyridinium ion. We hypothesize that molecular sequestration is in fact the natural role of this pigmented granule, and that dopamine is the intended beneficiary of this uniquely reducing environment. Should a mechanism be discovered (question mark) whereby sequestered dopamine could be re-released upon specific triggering, this seems to provide a complete self-reinforcing molecular memory loop. Although such a re-release mechanism is purely speculative at this time, a number of interesting plausible scenarios are apparent, including optical and electrical stimulation of the granule, changes in cellular pH or oxidation state, or displacement of NM-bound dopamine by alternate catecholamines or metal ions. We suggest that interruption of this putative sequestration and release mechanism may underlie the molecular basis of Parkinson's disease initiation.

One of the strongest proposals for memory in human brains involves synaptic plasticity, that is, the extensive pattern of connections between the neurons can be altered through strengthening (by repeated stimulation) or weakening (by lack of stimulation) of particular pathways. Dopamine-binding NM structures would seem to retain a semi-permanence despite being made of components that are themselves susceptible to turnover. Thus it's possible that pigmentation in neurons could play a significant and as yet unappreciated role in the strengthening of neuronal connections through localized catecholamine sequestration. Concentration of neurotransmitter into pigmented granules would localize a pool of transmissible signal in locations that had previously seen high dopamine release and uptake activity. If there is then a mechanism for re-release of bound dopamine to synaptic vesicles or to the synapse itself, the pigmented granule could replace or assist in neurotransmission. Such a mechanism would appear to constitute a form of molecular memory. Therefore, we believe NM represents an obvious, if overlooked, mechanism for implicit (nondeclarative) memory which should be on the radar of many more investigators than it is today.

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Disruption of NM Function as a Predictor of PD Inducers

If membrane bound, granular NM indeed proves to bind dopamine in a reversible manner, we can predict that any subsequent disruption of this function would result in symptoms of PD, analogous to what is seen in cases including MPP+, haloperidol (HPP+), and quite possibly, 6-OH dopamine, all potent chemical inducers of Parkinson's disease. Given its structure, it is not surprising for example that the 6-hydroxy analogue of dopamine is taken up through dopaminergic transport mechanisms. Owing to the extra hydroxyl group on the aromatic ring, it is also much more susceptible to quinone formation than is dopamine itself, a process believed underlying its toxic effects. To our knowledge, the possible interaction between NM and 6-hydroxy dopamine has not been previously explored. Manganism, which exhibits symptoms very similar to Parkinson's disease (Lucchini et al., 2009), also displays a clearly plausible mechanism in this scenario through replacement of the iron normally found bound to NM. Utilizing such a mechanism appears to have the unfortunate consequence of binding toxic agents that resemble neurotransmitter monoamines. Such sequestration could lead to several possible outcomes including abnormal polymerization of NM, abnormal packaging of the resulting NM polymer, or abnormal functioning of NM granules following successful formation. Given the commonalities between these agents and the unique environment afforded by pigmented neurons, we believe that chemical binding to and/or incorporation into the NM polymer itself may represent a general mechanism for early stage chemical induction of PD.

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Conclusions

Though most often associated with toxin binding, the uniquely reducing environment of the pigmented granule may instead provide safe harbor for endogenous catecholamines such as dopamine itself which are otherwise highly susceptible to the oxidative stress of the cytoplasm in an actively firing neuron. Such stabilization could supplant or assist in the role of synaptic vesicles to form a semi-permanent connection in an elegant form of molecular memory. Dopamine and adrenaline should be thoroughly investigated as candidates for an endogenous NM substrate with profound implications. Preliminary evidence from our laboratory suggests that dopamine does in fact bind to a synthetic NM polymer in a saturable manner at micromolar concentrations (Haining et al., 2016). At a minimum, the possible influence of catecholamine binding and release by NM must be ruled out as a contributory mechanism to dopamine storage and release if we are to understand this phenomenon fully. Following the discovery of the role of DNA in heredity, scientific focus has inevitably shifted toward genes and gene products as being the most important players in the process of life. What cannot be forgotten during this transition however is that life is driven by biochemistry at its core, not by genes and gene products. In any case, it is becoming abundantly clear that neuromelanin is not a mere spectator in the brain but in fact serves a very important active function.

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Footnotes

Conflicts of interest: None declared.

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References

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