Amaral, I.P., and Johnston, I.A. (2012) Circadian expression of clock and putative clock-controlled genes in skeletal muscle of the zebrafish. American journal of physiology. Regulatory, integrative and comparative physiology. 302(1):R193-206.
To identify circadian patterns of gene expression in skeletal muscle, adult male zebrafish were acclimated for two weeks to a 12:12h light:dark photoperiod and then exposed to continuous darkness for 86h with ad libitum feeding. The increase in gut food content associated with the subjective light period (SLP) was much diminished by the third cycle enabling feeding and circadian rhythms to be distinguished. Expression of zebrafish paralogues of mammalian transcriptional activators of the circadian mechanism (bmal1, clock1 and rora) followed a rhythmic pattern with a ~24h periodicity. Peak expression of rora paralogues occurred at the beginning of the SLP [Zeitgeber time (ZT)07 and ZT02 for roraa and rorab] whereas the highest expression of bmal1 and clock paralogues occurred 12h later (ZT13-15 and ZT16 for bmal and clock paralogues). Expression of the transcriptional repressors cry1a, per1a/1b, per2, per3, nr1d2a/2b, and nr1d1 also followed a circadian pattern with peak expression at ZT0-02. Expression of the two paralogues of cry2 occurred in phase with clock1a/1b. Duplicated genes had a high correlation of expression except for paralogues of clock1, nr1d2 and per1, with cry1b showing no circadian pattern. The highest expression difference was 9.2-fold for the activator bmal1b and 51.7-fold for the repressor per1a. Out of 32 candidate clock-controlled genes, only myf6, igfbp3, igfbp5b and hsf2 showed circadian expression patterns. Igfbp3, igfbp5b and myf6 were expressed in phase with clock1a/1b and had an average of 2-fold change in expression from peak to trough whereas hsf2 transcripts were expressed in phase with cry1a and had a 7.2 fold-change in expression. The changes in expression of clock and clock-controlled genes observed during continuous darkness were also observed at similar ZTs in fish exposed to a normal photoperiod in a separate control experiment. The role of circadian clocks in regulating muscle maintenance and growth are discussed.