A comparative examination of cortisol effects on muscle myostatin and HSP90 gene expression in salmonids

Cortisol, the primary corticosteroid in teleost fishes, is released in response to stressors to elicit local
functions, however little is understood regarding muscle-specific responses to cortisol in these fishes.
In mammals, glucocorticoids strongly regulate the muscle growth inhibitor, myostatin, via glucocorticoid
response elements (GREs) leading to muscle atrophy. Bioinformatics methods suggest that this regulatory
mechanism is conserved among vertebrates, however recent evidence suggests some fishes exhibit divergent
regulation. Therefore, the aim of this study was to evaluate the conserved actions of cortisol on myostatin
and hsp90 expression to determine if variations in cortisol interactions have emerged in salmonid
species. Representative salmonids; Chinook salmon (Oncorhynchus tshawytscha), cutthroat trout
(Oncorhynchus clarki), brook trout (Salvelinus fontinalis), and Atlantic salmon (Salmo salar); were injected
intraperitoneally with a cortisol implant (50 lg/g body weight) and muscle gene expression was quantified
after 48 h. Plasma glucose and cortisol levels were significantly elevated by cortisol in all species,
demonstrating physiological effectiveness of the treatment. HSP90 mRNA levels were elevated by cortisol
in brook trout, Chinook salmon, and Atlantic salmon, but were decreased in cutthroat trout. Myostatin
mRNA levels were affected in a species, tissue (muscle type), and paralog specific manner. Cortisol treatment
increased myostatin expression in brook trout (Salvelinus) and Atlantic salmon (Salmo), but not in
Chinook salmon (Oncorhynchus) or cutthroat trout (Oncorhynchus). Interestingly, the VC alone increased
myostatin mRNA expression in Chinook and Atlantic salmon, while the addition of cortisol blocked the
response. Taken together, these results suggest that cortisol affects muscle-specific gene expression in
species-specific manners, with unique Oncorhynchus-specific divergence observed, that are not predictive
solely based upon mammalian stress responses.