Zebrafish Otolith Biomineralization Requires Polyketide Synthase

Deflecting biomineralized crystals attached to vestibular hair cells are necessary for maintaining balance. Zebrafish ( Danio rerio ) are useful organisms to study these biomineralized crystals called otoliths, as many required genes are homologous to human otoconial development. We sought to identify and characterize the causative gene in a trio of mutants, no content (nco) and corkscrew (csr) , and vanished (vns) , which fail to develop otoliths during early ear development. We show that nco, csr , and vns have potentially deleterious mutations in polyketide synthase ( pks1 ), a multi-modular protein that has been previously implicated in biomineralization events in chordates and echinoderms. We found that Otoconin-90 (Oc90) expression within the otocyst is normal in nco and csr ; therefore, it is not sufficient for otolith biomineralization in zebrafish. Similarly, normal localization of Otogelin, a protein required for otolith tethering in the otolithic membrane, is not sufficient for Oc90 attachment. Furthermore, eNOS signaling and Endothelin-1 signaling were the most up- and down-regulated pathways during otolith agenesis in nco , respectively. Our results demonstrate distinct processes for otolith nucleation and biomineralization in vertebrates and will be a starting point for models that are independent of Oc90-mediated seeding. This study will serve as a basis for investigating the role of eNOS signaling and Endothelin-1 signaling during otolith formation.