Sonic hedgehog signaling and skeletal patterning during zebrafish fin regeneration - PROJECT SUMMARY This project provides the applicant with Ph.D. training in cell and developmental biology. Thesis research will further our understanding how cell signaling mediates cell-cell interactions to facilitate intricate skeletal patterning. Additionally, the applicant will pursue activities supporting professional development, advanced imaging, mentorship, and science communication skills. Zebrafish rapidly and robustly regenerate their caudal fins, including their characteristic bony ray skeleton. During fin development and regeneration, most rays branch to form distinct daughter rays. Ray branching occurs through the splitting of progenitor osteoblast (pOb) pools adjacent to sonic hedgehog a (shha)-expressing basal epidermal cells (bEps). The shha-expressing bEp domains themselves split prior to overt branching. Small molecule inhibition of Sonic hedgehog/Smoothened (Shh/Smo) signaling specifically prevents ray branching. The unbranched rays grow to their normal length and the shha-expressing bEp domains still split. However, the Shh/Smo target genes and how they influence cell behaviors that divide pOb pools are unknown. This proposal aims to identify relevant Shh/Smo target genes and how they promote interactions between shha-expressing bEps and pObs for ray branching morphogenesis. Previous studies and the applicant’s preliminary data using a new long-term live imaging approach indicate that collectively migrating shha-expressing bEps contact individual pObs and progressively escort them into split pools during ray outgrowth. Further, the applicant has identified a Shh-dependent gene, fibrillin 2b (fbn2b), that when germline mutated causes unbranched rays in regenerated fins. Together, these results support the applicant’s hypothesis that Shh/Smo promotes transient Fbn2b-dependent heterotypic cell associations underlying co-movements of shha-expressing bEps and pObs. The applicant will explore this hypothesis with two aims. Aim 1 will identify Shh/Smo target genes that mediate ray branching during fin regeneration. Aim 2 will define Shh/Smo driven associations of migrating Shh+ basal epidermal cells and underlying progenitor osteoblasts throughout the duration of branching morphogenesis. Completing these aims will reveal Shh/Smo- dependent, coordinated cell behaviors and new mediators of skeletal ray branching morphogenesis. These insights will advance our mechanistic understanding of skeletal outgrowth and patterning with potential biomedical significance for skeletal regenerative medicine.
