MECHANISMS OF DENDRITE FORMATION & MELANOSOME TRANSFER

黑素小体是产生黑色素的特殊细胞器，有效地从黑素细胞转移到角质形成细胞，是皮肤光保护的绝对要求。黑素细胞是一种色素产生细胞，位于表皮的基底层，形成多个长的树突，将黑素小体从黑素细胞体输送到树突尖端，然后输送到角质形成细胞。树突状细胞的形成和黑素小体向角质形成细胞的转移受到包括α-黑素细胞刺激素、内皮素-1和神经生长因子在内的激素以及紫外线的刺激，但在哺乳动物模型中，参与树突形成和黑素小体转移的第二信使途径知之甚少。黑素细胞中树突状突起的形成与肌动蛋白细胞骨架的重组密切相关，肌动蛋白细胞骨架的重组导致树突通过片状脂膜的形成延伸，并将黑色素输送到树突尖端。我们研究工作的长期目标是确定1)参与树突形成的上游信号中间体，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，在其他类型的细胞中协调肌动蛋白重组，形成片状脂膜和树突样结构，并被几种不同的激素调节的第二信使系统激活。根据我们的初步研究，我们假设rac1是蛋白激酶A(PKA)、蛋白激酶C(PKC)和钙离子第二信使系统的中心汇聚点，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依赖于PKA的黑素小体相关蛋白的磷酸化来调节的，而在其他细胞类型中参与调节胞吐的蛋白Rab3a和RabPhilin是黑素小体向角质形成细胞转移的调节分子。这项建议中提出的研究是第一次利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并针对这两个过程，这项工作的结果将代表着这一领域的重大进步，该领域在很大程度上局限于形态分析。这些研究的结果可能对设计新的治疗方法具有重要意义，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生率。

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.