Reassessment of the function of somatolactin alpha in lipid metabolism using medaka mutant and transgenic strains

Yuko Sasano,Shoji Fukamachi,Asami Yoshimura

doi:10.1186/1471-2156-13-64

Abstract

BackgroundSomatolactin alpha (SLa) is a fish-specific peptide hormone secreted from the pituitary. In medaka, SLa functions to darken the skin color and lack of SLa makes it pale. Transcription of SLa is enhanced or suppressed when fish are kept in dark or bright conditions, respectively, indicating SLa’s important role in background acclimation of the skin color. Bizarrely, however, the lack of SLa seems to cause the additional defect of increased triglycerides in organs, which could not be rescued (decreased) by its overexpression.ResultsTo assess this enigmatic result, we investigated genetic (the SLa, Slc45a2, r, and Y genes) and nongenetic (age, fasting, water temperature, and background color) effects on hepatic triglycerides. These experiments found that percent hepatic triglycerides quickly change in response to external/internal environments. Effects of SLa seemed to be much less obvious, although it may increase the proportion of hepatic triglycerides at least during certain breeding conditions or under certain genetic backgrounds.ConclusionsThe present results do not exclude the possibility that SLa takes part in lipid metabolism or other physiological processes. However, we suggest that skin-color regulation is the only definite role of SLa so far demonstrated in this species.

Highlights

Somatolactin alpha (SLa) is a fish-specific peptide hormone secreted from the pituitary
We previously identified that ci has a frame-shift mutation on the SLa gene [5] and could rescue the ci phenotype by transgenic overexpression of SLa [2]; i.e., the pale gray skin of ci became dark brown in the SLatransgenic strain (Actb-SLa:GFP)
SLa, b, r, and Y loci on hepatic triglycerides We further investigated the potential role of SLa to regulate hepatic triglycerides

Summary

Introduction

Somatolactin alpha (SLa) is a fish-specific peptide hormone secreted from the pituitary. We expected that the pale ci and the dark Actb-SLa:GFP should have additional defects most likely in these organs. From this point of view, phenotypes of the cobalt variant of rainbow trout looked suggestive, because the fish lacks most of the pars intermedia of the pituitary, decreases SLa in the plasma, has pale skin (as ci), and is obese due to excessive fat accumulation in the abdominal cavity [14]. We detected more triglycerides in the liver and muscles of ci than we did in those of wild-type fish [13] This fat accumulation in ci could not be rescued in Actb-SLa:GFP, unlike the case for skin coloration [2]. The causal relationship between SLa and lipid metabolism remains an open question that should be carefully reassessed

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Conclusion