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

Abstract

Background: Somatolactin 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.

Results: To 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.

Conclusions: The 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.

Figure 1

Effects of aging, rearing density, or other breeding conditions on hepatic triglycerides in O. celebensis. Mean and SEM of body weight (a), HSI (b), and percent hepatic triglycerides (c) are shown; n = 6, 6, 8, 8, and 5 for 10, 12, 14, 15, and 17 months after hatch, respectively. Letters in the bars indicate significant differences between the groups (P < 0.05, one-way ANOVA and Tukey’s post hoc HSD test).

Figure 2

Effects of background colors, fasting, and SLa expression on hepatic triglycerides. Four pairs of three strains (ci, Actb-SLa:GFP, and HNI) were kept under four breeding conditions (control, white background, black background, or fasting). a and b, environmental effects on the HSI (a) and percent hepatic triglycerides (b). c and d, genetic effects on the HSI (c) and percent hepatic triglycerides (d). Letters in the bars indicate significant differences between the groups according to a two-way ANOVA and a Dunnett post hoc (a and b) or Tukey’s post hoc HSD (c and d) test.

Figure 3

Effects of theY, ci, SLa, b, and r genes on hepatic triglycerides. a, HSI of the Cross I siblings. Asterisks indicate significant differences (*P < 0.05; two-tailed multiple t test with Bonferroni correction). b, percent hepatic triglycerides of the Cross I siblings. c, percent hepatic triglycerides of the Cross II siblings.