The Role of 5’ hox13 Genes in Danio rerio (Zebrafish) Caudal Fin Ray/Joint Development and Regeneration

Abstract

Zebrafish are part of the teleost infraclass (bony fish) of the ray-finned fish. Like other teleosts, zebrafish possess the ability to regenerate most tissues, including their fins. Zebrafish fins contain segmented bony fin rays that longitudinally span the fin. The segments of fin ray are separated by fibrous joints at regularly spaced intervals providing segmentation and flexibility for the fin. Based on gene expression and changes in cell morphology, joint cell differentiation during development and regeneration proceeds through three stages: presumptive joint, joint-forming, and mature joint cells. Our lab has shown that new joint formation correlates with the upregulation of 5’ hoxa gene, hoxa13a. The hox genes encode transcription factors important for patterning in development. In mice, phenotypes resulting from loss- and gain-of-function mutations in Hox genes have revealed that the spatiotemporal expression of these genes is critical for the correct morphogenesis of the limb, a homologous structure to the fin. The first experiments in this thesis use the NTR/MTZ mechanism to partially ablate hoxa13a-expressing cells in the joints and blastema of the regenerating caudal fin. Partial ablation of the hoxa13a-expressing cells results in shorter bone segments following regeneration of the fin. This experiment draws the conclusion that hoxa13a-expressing cells are involved in the regulation of segment length. To examine the function of the 5’ hoxa/d genes in zebrafish, our lab created CRISPR/Cas9 mutations that inactivate hoxa13a, hoxa13b, and hoxd13a. The triple mutants created through serial breeding, show fin-specific defects in the formation and patterning of joints, as well as general defects in the morphology of the ray and in the actinotrichia, collagenous fibres found at the distal edge of the fin. Overall, our data suggest that hox13 genes are necessary for joint formation and proper fin ray growth. With further phenotypic and genotypic analyses our lab proposes that the dosage of hox13 alleles is responsible for anomalies in fin ray formation found in hox13 mutants.