Rtn4-Supplementary data from <i>microRNA-206</i> modulates an Rtn4a/Cxcr4a/Thbs3a axis in newly forming somite to maintain and stabilize the somite boundary formation of zebrafish embryos

Although microRNA-206 (<i>miR-206</i>) is known to regulate proliferation and differentiation of muscle fibroblasts, the role of <i>miR-206</i> in early-stage somite development is still unknown. During somitogenesis of zebrafish embryos, <i>reticulon4a</i> (<i>rtn4a</i>) is specifically repressed by <i>miR-206.</i> Somite boundary was defective, and actin filaments were crossing over boundary in either <i>miR-206</i>-knockdown or <i>rtn4a</i>-overexpressed embryos. In these treated embryos, C–X–C motif chemokine receptor 4a (<i>cxcr4a</i>) was reduced, while thrombospondin 3a (<i>thbs3a</i>) was increased. The defective boundary was phenocopied in either <i>cxcr4a</i>-knockdown or <i>thbs3a-</i>overexpressed embryos. Repression of <i>thbs3a</i> expression by <i>cxcr4a</i> reduced the occurrence of boundary defect. We demonstrated that <i>cxcr4a</i> is an upstream regulator of <i>thbs3a</i> and that defective boundary cells could not process epithelialization in the absence of intracellular accumulation of the phosphorylated focal adhesion kinase (p-FAK) in boundary cells. Therefore, in the newly forming somites, <i>miR-206-</i>mediated downregulation of <i>rtn4a</i> increases <i>cxcr4a.</i> This activity largely decreases <i>thbs3a</i> expression in the epithelial cells of somite boundary, which causes epithelialization of boundary cells through mesenchymal–epithelial transition (MET) and eventually leads to somite boundary formation. Collectively, we suggest that <i>miR-206</i> mediates novel pathway Rtn4a/Cxcr4a/Thbs3a axis that allows boundary cells to undergo MET and form somite boundaries in the newly forming somites of zebrafish embryos.