| Summary: | A microRNA, termed miR-In300 or miR-3906, suppresses the transcription of myf5 through silencing dickkopf-related protein 3 (dkk3r/dkk3a) during early development when myf5 is highly transcribed, but not at late stages when myf5 transcription is reduced. Moreover, after 24 hpf, when muscle cells are starting to differentiate, Dkk3a could not be detected in muscle tissue at 20 hpf. To explain these reversals, we collected embryos at 32 hpf, performed assays, and identified homer-1b, which regulates calcium release from sarcoplasmic reticulum, as the target gene of miR-3906. We further found that either miR-3906 knockdown or homer-1b overexpression increased expressions of fmhc4 and atp2a1 of calcium-dependent fast muscle fibrils, but not slow muscle fibrils, and caused a severe disruption of sarcomeric actin and Z-disc structure. Additionally, compared to control embryos, the intracellular calcium concentration ([Ca(2+)]i) of these treated embryos was increased as high as 83.9-97.3% in fast muscle. In contrast, either miR-3906 overexpression or homer-1b knockdown caused decreases of [Ca(2+)]i and, correspondingly, defective phenotypes in fast muscle. These defects could be rescued by inducing homer-1b expression at later stage. These results indicate that miR-3906 controls [Ca(2+)]i homeostasis in fast muscle through fine tuning homer-1b expression during differentiation to maintain normal muscle development.
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