The role of multivesicular endosomes and OA1 in melanosome biogenesis

黑色素在皮肤中的主要功能之一是充当防晒霜，吸收紫外线辐射，防止DNA损伤和皮肤癌。黑色素合成并储存在被称为黑素体的膜结合细胞器中。黑素体存在于皮肤的黑素细胞中，也存在于眼睛的细胞中，包括视网膜色素上皮（RPE）细胞中。视网膜色素上皮细胞直接位于视杆细胞和视锥细胞（眼睛中检测和反射光线的细胞）之下。黑色素缺乏，如在各种类型的白化病中发现的，导致严重的视力障碍。某些类型的白化病是由合成黑色素的能力缺陷引起的，而其他类型的白化病是由制造黑素体的能力缺陷引起的。黑素体形成的机制尚未完全表征，但它们涉及内吞途径，这是细胞外分子如病毒和营养素被吸收到细胞中的途径。细胞外分子被传递到内体，在那里它们遇到细胞新合成的蛋白质。蛋白质可以从内体递送到许多不同的地方，例如溶酶体进行降解或返回细胞表面。我们已经表明，这一关键决定是在多泡内体/体（MVB）。待递送至溶酶体进行降解的蛋白质被分选到MVB的内部囊泡上，而待返回至质膜的蛋白质停留在MVB的周边膜上。在黑色素合成细胞中，MVB也可能参与将黑素体蛋白递送至黑素体，或者MVB的亚群可能发育成黑素体。在产生黑色素的细胞中，一种叫做pmel 17的蛋白质是黑色素沉积在其上的黑素小体内部条纹产生所必需的。在黑素体形成的非常早期阶段，在MVB的内部囊泡上发现pmel 17，当足够的蛋白质在MVB的内部囊泡上积累时，条纹开始形成。如果对通常不产生黑色素的细胞进行基因修饰以产生pmel 17，则这些细胞的MVB内甚至会开始形成条纹。这意味着MVB内的内向囊泡化可能在内部条纹的形成和黑素体生物发生中起关键作用。最近已经鉴定了参与MVB内蛋白质分选和MVB形成的分子机制的组分。我们一直在使用电子显微镜（EM）来确定其中一些蛋白质的功能，因为EM是可视化MVB非常小（直径50 nm）的内部囊泡的唯一方法。到目前为止，我们已经确定了其中一些蛋白质在不产生黑素体的细胞中的功能。我们现在建议确定这些蛋白质在黑素细胞和RPE细胞中黑素体形成中的作用。除了确定MVB机制的核心成分的作用外，我们还将确定仅在制造黑素体的细胞中表达的蛋白质的作用。OA 1是在最常见类型的眼部白化病中有缺陷的蛋白质，患有这种疾病的患者可以制造黑色素，但制造数量减少的异常大的黑色素体。已发现该蛋白与MVB相关，并已提出在黑素体形成中在MVB水平上发挥作用。我们将使用EM检测来确定OA 1在MVB中的作用。这些研究将共同阐明调节细胞器形成的基本机制，细胞器是人体抵御阳光影响的自然防御机制，在视觉中也起着重要作用。这将有助于我们了解这些机制在人类疾病中是如何失效的。

One of the main functions of melanin pigment in the skin is to act as a sunscreen to absorb ultraviolet radiation and prevent DNA damage and skin cancer. Melanin is synthesised and stored in membrane bound organelles called melanosomes. Melanosomes are found in melanocytes of the skin and also in cells of the eye, including in retinal pigment epithelial (RPE) cells. RPE cells lie immediately beneath the rods and cones (the cells of the eye that detect and transduce light). Deficit of melanin pigment, as is found in the various types of albinism, leads to severe visual impairment. Some types of albinsm are caused by defects in the ability to synthesise melanin while others are caused by defects in the ability to make melanosomes. The mechanisms whereby melanosomes are formed are not fully characterised but they involve the endocytic pathway, which is the route where by extracellular molecules like viruses and nutrients are taken up into the cell. Extracellular molecules are delivered to endosomes where they meet proteins that have been newly synthesised by the cell. From the endosome proteins may be delivered to many different places, such as the lysosome for degradation or back to the cell surface. We have shown that this critical decision is taken in multivesicular endosomes/bodies (MVB). Proteins that are to be delivered to the lysosome for degradation are sorted onto the internal vesicles of MVB while proteins that are to be returned to the plasma membrane stay on the perimeter membrane of the MVB. In melanin synthesising cells the MVB may also be involved in delivering melanosomal proteins to the melanosome or a subpopulation of MVBs may develop into melanosomes. In melanin producing cells a protein called pmel 17 is required for the generation of internal striations within the forming melanosome upon which melanin is deposited. In the very early stages of melanosome formation pmel17 is found on the internal vesicles of MVBs and when sufficient protein has accumulated on the internal vesicles of MVB striations begin to form. Striations will even begin to form within the MVBs of cells which do not normally make melanin if they are genetically modified so that they make pmel17. This implies that inward vesiculation within MVB may play a critical role in formation of internal striations and in melanosome biogenesis. Recently components of the molecular machinery that are involved in sorting of proteins within MVB and in MVB formation have been identified. We have been determining the function of some of these proteins using electron microscopy (EM), as EM is the only way to visualise the very small (50nm diameter) internal vesicles of MVB. So far we have determined the functions of some of these proteins in cells which do not make melanosomes. We now propose to determine the role of these proteins in the formation of melanosomes in melanocytes and RPE cells. In addition to determining the role of the core components of the MVB machinery we will determine the role of a protein that is only expressed in cells that make melanosomes. OA1 is the protein which is defective in the most common type of ocular albinism and patients with this disease can make melanin but make a reduced number of unusually large melanosomes. This protein has been found associated with MVBs and has been proposed to play a role at the level of the MVB in melanosome formation. We will use our EM assays to determine what OA1 does within the MVB. Together these studies will elucidate the fundamental mechanisms regulating the formation of an organelle which is the body's natural defence mechanism against the effects of the sun and which also plays an important role in vision. This will help us to understand how these mechanisms fail in human disease.