The Isl1/Shh/Wnt5a cascade controls the invasion of myoblasts by tuning the CXCL12/CXCR4 axis during tongue morphogenesis

The migration and invasion of myoblast cells derive from the occipital somites is critical for tongue morphogenesis. However, the molecular mechanisms underlying myoblast migration and invasion remain elusive. In this study, we report that transcription factor Isl1 is required for tongue development. Loss of Isl1 in the distal mandibular epithelium results in aglossia due to the defect of myoblast cells invasion. In the absence of Isl1 expression, myoblast cells in the first branchial arch fail to migrate into the presumptive tongue primordium. Down-regulation of Shh was detected in the distal mandibular epithelium after Isl1 deletion. As a downstream target gene of Shh, Wnt5a attracts the migration of mesenchymal cells expressing chemokine ligand 12 (CXCL12). CXCL12+ mesenchymal cells in the Isl1 mutant embryo fail to migrate to the distal region but accumulate in a relatively small proximal domain of the first branchial arch. CXCL12 serves as a bidirectional cue for myoblasts that express its receptor CXCR4, attracting CXCR4+ myoblasts invasion at low concentration but repelling at high concentration. The defect in the distal migration of CXCL12+ mesenchymal cells leads to a high local concentration of CXCL12, thereby preventing the invasion of CXCR4+ myoblasts. In addition, transgenic activation of Ihh alleviates the defect in tongue development and rescues the invasion of myoblasts, corroborating the functional involvement of Hedgehog signaling in tongue development. In summary, this study provides the first line of genetic evidence for the Isl1/Shh/Wnt5a cascade to regulate myoblasts invasion by tuning the CXCL12/CXCR4 axis.