Effect of the sdc4 gene knockdown on muscle development in zebrafish

The growth and regeneration of skeletal muscle are closely related to syndecan-4 (Sdc4), which is a type I transmembrane heparan sulfate proteoglycan belonging to the syndecan family. However, it remains unclear how the sdc4 gene affects fish muscle development. Therefore, an sdc4 knockdown zebrafish line (sdc4^-/-) was generated by CRISPR/Cas9 technology in this study, and its phenotypes were analyzed. The results revealed that sdc4^-/- zebrafish exhibited reduced body length and weight compared to the wild-type (WT) at 90 days post fertilization (dpf). Furthemore, sdc4^-/- zebrafish also showed a significantly larger number of muscle fibers, and significantly reduced individual muscle fiber cross-sectional area. The mRNA expression levels of genes associated with myogenic regulatory factors (MRFs) and the wnt/β-catenin pathway were all significantly downregulated. Based on dual luciferase reporter assays, sdc4 gene expression was regulated by the transcription factor myocyte enhancer factor 2aa (Mef2aa) and miR-141-3p, which bind to its promoter and 3' untranslated region (UTR), respectively. Additionally, the reduced average swimming speed and distance observed in sdc4^-/- zebrafish at 90 dpf were concomitant with a significant downregulation of mitochondrial respiratory chain complex-related genes and a reduction in ATP concentration. This research aids understanding of sdc4 function in fish and may provide a new perspective for studying the molecular mechanisms of muscle growth and development.