Effects of constitutive expression of somatolactin alpha on skin pigmentation in medaka

Somatolactin alpha (SLa) is a fish-specific peptide hormone secreted from the pituitary. Its functions have been investigated for the last decade but are still under debate. We previously reported a frame-shift mutation on SLa in a medaka mutant, color interfere (ci), which shows defects in body-color regulation, lipid storage, and cortisol secretion. In this study, we examined the effects of introducing a DNA fragment which constitutively expresses wild-type SLa under regulation of the beta-actin (Actb) promoter into the ci genome. We successfully visualized the transgene expression by taking advantage of an internal ribosome entry site (IRES) and the green fluorescent protein (GFP). The transgenic medaka, Actb-SLa:GFP, exhibited a reversed body-color phenotype of ci; more orange xanthophores and less white leucophores. We also detected more black melanophores and less silver iridophores, which indicates SLa's comprehensive role in regulating all types of pigment cells in the skin. Unexpectedly, the defects in lipid/cortisol contents remained in Actb-SLa:GFP. Therefore, the causal relationship between SLa and lipid/cortisol metabolisms relapses to an open question which needs to be reassessed by other types of experiments. Both the Actb-SLa:GFP and ci fish grow and mature similar to wild type, indicating SLa contributes little to growth regulation in spite of the fact that it binds to a teleost-specific paralog of growth-hormone receptor in vitro. The present study provides definitive evidence for SLa's principal and indispensable role in body-color regulation in medaka.