MECHANISMS OF DENDRITE FORMATION & MELANOSOME TRANSFER

黑色素小体（专门用于黑色素生成的细胞器）从黑色素细胞到角质形成细胞的有效转移是皮肤光保护的绝对要求。黑素细胞是一种产生色素的细胞，位于表皮的基底层，形成多个长树突，将黑素小体从黑素细胞细胞体转运到树突尖端，然后再转运到角质形成细胞。包括α -黑色素细胞刺激激素、内皮素-1和神经生长因子在内的激素以及紫外线都能刺激树突的形成和黑色素小体向角化细胞的转移，但在哺乳动物模型中，树突形成和黑色素小体转移的第二信使途径尚不清楚。黑素细胞树突的形成与肌动蛋白细胞骨架的重组密切相关，这导致树突通过板足的形成而延伸，并将黑素转运到树突尖端。我们的长期研究目标是确定1)参与树突形成的上游信号中间体和2)参与黑素小体转移的分子介质和调节因子。Rac1是一种单体GTP结合蛋白，在其他细胞类型中协调肌动蛋白重组，形成板足和树突样结构，并被几种不同的激素调节的第二信使系统激活。基于我们的初步研究，我们假设rac1是蛋白激酶a （PKA）、蛋白激酶C （PKC）和Ca2+第二信使系统的中心汇聚点，并且黑素小体向角质形成细胞的转移与rac1的激活密切相关。此外，我们假设黑素小体的转移是通过PKA依赖性的黑素小体相关蛋白磷酸化来调节的，rab3a和rabphilin是其他细胞类型中参与调节胞外分泌的蛋白，是黑素小体向角质形成细胞转移的调节分子。本提案中提出的研究是第一个使用最先进的分子和细胞生物学技术在哺乳动物模型中关注和瞄准这两个过程的研究，这项工作的结果将代表一个主要局限于形态分析领域的重大进步。这些研究的结果可能对新疗法的设计具有重要意义，以增强黑素细胞为皮肤提供光保护的能力，从而降低皮肤癌的发病率。

The effective transfer of melanosomes, specialized organelles for melanin production, from melanocytes to keratinocytes, is an absolute requirement for photoprotection in the skin. Melanocytes are pigment producing cells that reside in the basal layer of the epidermis, and form multiple long dendritic processes that transport melanosomes from the melanocyte cell body to the dendritic tips, and then to keratinocytes. Dendrite formation and melanosome transfer to keratinocytes is stimulated by hormones including alpha-melanocyte stimulating hormone, endothelin-1 and nerve growth factor, as well as ultraviolet light, but the second messenger pathways involved in dendrite formation and in melanosome transfer are poorly understood in mammalian models. The formation of dendritic processes in melanocytes is closely linked to reorganization of the actin cytoskeleton, which results in extension of dendrites through the formation of lamellipodia, and transport of melanoses to the dendrite tip. The long term goal of our research efforts is to determine 1) the upstream signaling intermediates involved in dendrite formation and 2) the molecular mediators and regulatory factors involved in melanosome transfer. Rac1 is a monomeric GTP- binding protein that orchestrates actin reorganization with the formation of lamellipodia and dendrite-like structures in other cell types and is activated by several different hormonally regulated second messenger systems. Based on our initial studies, we hypothesize that rac1 is a central convergence point for the protein kinase A (PKA), protein kinase C (PKC) and Ca2+ second messenger systems, and that melanosome transfer to keratinocytes is closely linked to activation of rac1. Further, we hypothesize that melanosome transfer is regulated through PKA- dependent phosphorylation of melanosomal associated proteins and that rab3a and rabphilin, proteins involved in regulated exocytosis in other cell types, are regulatory molecules for melanosome transfer to keratinocytes. The studies proposed in this proposal are the first to focus on and target these two processes in a mammalian model using state of the art molecular and cell biology technqiues, and results from this work would represent a major advance in a field which has been largely confined to morphologic analyses. Results from these studies could have significant implications for the design of new therapies to enhance the ability of melanocytes to provide photoprotection to the skin, and thus decrease the incidence of cutaneous cancer.