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

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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.