Regulation of apoptosis in fish skin pigment cells during long-term chromatic adaptation

许多硬骨鱼可以通过改变其皮肤色素细胞的形态和密度来使其身体颜色适应背景颜色。许多硬骨鱼在长期适应白色背景的过程中，皮肤色素细胞的密度会降低。我们以前的报告，细胞凋亡是负责黑色素细胞的减少和交感神经递质，去甲肾上腺素（NE），诱导青鳉，青鳉的细胞凋亡。由于cAMP依赖性信号通路可能参与促进黑色素细胞的存活，NE似乎通过减弱存活信号诱导细胞凋亡。在本研究中，我们试图阐明长期色适应过程中色素细胞凋亡的调控机制。添加10%胎牛血清可防止NE诱导的皮肤外植体和鳞屑皮肤培养中黑色素细胞凋亡。我们研究了PKA、PI 3-K和MAPK抑制剂的作用。PKA和MAPK依赖性信号的缺失被认为是诱导凋亡，而PI 3-K抑制剂阻止凋亡。因此，我们比较了青鳉在长期适应白色背景的过程中，幼虫和成年青鳉的黑素细胞凋亡。在这些幼虫中，黑色素细胞并没有因凋亡而减少，NE也没有诱导来自胚胎细胞的培养黑色素细胞的凋亡。这些结果表明，斑马鱼幼鱼和成鱼色素细胞的凋亡调控系统不同，同时还表明，从青鳉皮肤中发现的一种新的RGS蛋白MeRGS 3L和垂体激素生长抑素（somatolactin）可能参与了色素细胞的凋亡调控。我们发现，表层色素细胞的凋亡有助于背部皮肤的有效适应，而长期适应也诱导了侧腹皮下色素细胞的凋亡，从而改变了条纹状色素模式。

Many teleost fish can adapt their body color to a background color by changing the morphology and density of their skin pigment cells. The density of skin pigment cells in many teleost fish decreases during long term adaptation to a white background. We previously reported that apoptosis is responsible for the decrease in melanophores and that a sympathetic neurotransmitter, norepinephrine(NE), induces their apoptosis in medaka, Oryzias latipes. Because cAMP-dependent signaling pathways could be involved in promoting the survival of melanophores, NE seems to induce apoptosis by attenuating survival signals. In this study, we sought to elucidate the regulation mechanisms of pigment cell apoptosis during long-term chromatic adaptation. Addition of 10% fetal bovine serum prevented apoptotic melanophore death induced by NE in skin explant and scale skin culture. We examined effects of inhibitors of PKA, PI3-K, and MAPK. The depletion of PKA- and MAPK-dependent signals was suggested to induce apoptosis, while PI3-K inhibitors prevented apoptosis.It is known that fish larvae can adapt to a background color immediately after hatching. So we compared melanophore apoptosis in larvae with that in adult medaka during long-term adaptation to a white background. Melanophores did not decrease by apoptosis in these larvae, and NE did not induce apoptosis in cultured melanophores derived from embryo cells. These results suggested that the regulation system of apoptosis is different between larvae and adult fish.We also showed that MeRGS3L, a new RGS protein identified from medaka skin, and somatolactin, one of pituitary hormone have a possibility to be involved in the regulation of apoptosis in fish pigment cells.Using zebra fish, Danio rerio, we showed that apoptosis in superficial chromatophores contribute to the effective adaptation of dorsal skin and that prolonged adaptation also induced apoptosis in hypodermal chromatophores in the flank to alter the striped pigment patterns.