Characterization of Actinodin 1 and Actinodin 2 Loss-of-function Mutations in Zebrafish

Abstract

Zebrafish (Danio rerio) are ray-finned fish of the teleost class, whose fins consist of an exoskeletal domain and an endoskeletal domain. The exoskeletal domain of the fins contains the fin rays and originate from embryonic fin folds that elongate as the fins are growing. The elongation of the fin fold is supported by two parallel sets of rigid fibrils oriented along the proximal-distal axis, called actinotrichia. Actinotrichia fibrils are composed of two primary components, a collagenous component and actinodin proteins. The actinodin proteins are encoded for by the actinodin (and) family of genes which are found in the genomes of finned fish while absent in limbed tetrapods. CRISPR/Cas9 was used to create loss-of-function deletions in the and1 and and2 genes, resulting in the absence of actinotrichia in the zebrafish double mutants. We hypothesised that the loss of actinotrichia during zebrafish development would result in developmental defects leading to fin ray defects in the adult zebrafish. The and1/2 mutants that lack actinotrichia presented with fin fold and cell migration defects during development that persisted into adulthood and resulted in shorter fins, disturbed fin ray patterning, and a decrease in ray number. In addition, an unexpected fusion between the hypurals of the caudal fin endoskeleton revealed an additional function of the actinotrichia fibrils in caudal fin endoskeletal patterning. During zebrafish development, actinotrichia fibrils play a vital role in ensuring normal fin development, normal patterning and formation of the fin rays, and the normal development of the caudal fin endoskeleton.