RODGERS, Buel D; WEBER, Gregory M; KELLEY, Kevin M; LEVINE, Michael A: Nutritional regulation of myostatin mRNA in the tilapia

In mammals, myostatin initiates myoblast differentiation by cell-cycle withdrawal and G1 growth arrest. Mice and cattle possessing null alleles for myostatin display a double muscled phenotype characterized by extreme skeletal muscle hypertrophy and hyperplasia. Our lab has recently characterized myostatin cDNA clones from several commercially important fish including the tilapia Oreochromis mossambicus, which expresses myostatin in skeletal muscle as well as in other tissues. To determine whether catabolic insult influences myostatin expression, mRNA levels were measured in skeletal muscle biopsies of fed and fasted adult tilapia and in whole larvae. In adults, fasting for 28 days reduced some indices of fish growth, although myostatin mRNA levels were unchanged. By contrast, fasting for 3 days similarly reduced both whole larval body weights and myostatin mRNA levels by 25% and 50%, respectively. After 6 days of fasting, body weights were further reduced by 60% while myostatin mRNA was not detected. Myostatin expression was equally reduced in heads and gutted bodies suggesting that these suppressive effects were systemically regulated. As nutritional stress is accompanied by increased glucocorticoid production, myostatin mRNA levels were measured in larvae immersed in 0.01, 0.1 and 1 ppm cortisol for 3 and 6 h. Neither of the lower doses altered whole larvae myostatin mRNA. By contrast, 1 ppm cortisol reduced these levels by 65% after 3 h and by 75% after 6 h. These results suggest that chronic nutritional stress significantly and similarly reduces somatic growth and myostatin mRNA levels in larval tilapia and that hypercortisolemia could be partly responsible.