Fndc3a (Fibronectin Domain Containing Protein 3A) influences median fin fold development and caudal fin regeneration in zebrafish by ECM alteration

Summary statement We investigated potential functions of Fndc3a during caudal fin development and regeneration in zebrafish. Reduced function interferes with correct epidermal cells structure and implies a role during vertebrate extremity development. Inherited genetic alterations are often found to be disease-causing factors of patient phenotypes. To unravel the molecular consequences of newly identified factors functional investigations in vivo are eminent. We investigated molecular functions of FNDC3A (Fibronectin Domain Containing Protein 3A; HUGO), a novel candidate gene for split-hand/foot malformations (SHFM) in humans, by utilizing zebrafish ( Danio rerio ) as a vertebrate model. Patients with congenital SHFM display prominent limb malformations, which are caused by disturbance of limb development due to defects in apical ectodermal ridge (AER) establishment and maintenance. Initial gene expression and protein localization studies clarified the presence of fndc3a in developing and regenerating fins of zebrafish. For functional studies we established a hypomorphic fndc3a mutant line ( fndc3a wue1/wue1 ) via CRISPR/Cas9, exhibiting phenotypic malformations and changed gene expression patterns during early stages of median fin fold development. Furthermore, fndc3a wue1/wue1 mutants display abnormal collagen localization, actinotrichia breakup and cellular defects in epidermal cells during caudal fin development. The observed effects are only temporary and later result in rather normal fin development in adults. In accordance with early fin development, proper caudal fin regeneration in adult fndc3a wue1/wue1 mutants is hampered by interference with actinotrichia formation and epidermal cell abnormalities. Investigation of cellular matrix formation implied that loss of ECM structure is a common cause for both phenotypes. Our results thereby provide a molecular link between Fndc3a function during both developmental processes in zebrafish and foreshadow Fndc3a as a novel temporal regulator of epidermal cell properties during extremity development in vertebrates.