Radial spokes are structurally conserved, macromolecular complexes that are essential for the motility of 9 + 2 motile cilia. In Chlamydomonas species, mutations in radial spoke proteins result in ciliary motility defects. However, little is known about the function of radial spoke proteins during embryonic development. Here, we investigated the role of a novel radial spoke protein, leucine-rich repeat containing protein 23 (Lrrc23), during zebrafish embryonic development. Mutations in lrrc23 resulted in a selective otolith formation defect during early ear development. Similar otolith defects were also present in the radial spoke head 3 homolog ( rsph3) and radial spoke head 4 homolog A ( rsph4a) radial spoke mutants. Notably, the radial spoke protein mutations specifically affected ciliary motility in the otic vesicle (OV), whereas motile cilia in other organs functioned normally. Via high-speed video microscopy, we found that motile cilia formation was stochastic and transient in the OV. Importantly, all the motile cilia in the OV beat circularly, in contrast to the planar beating pattern of typical 9 + 2 motile cilia. We identified the key time frame for motile cilia formation during OV development. Finally, we showed that the functions of radial spoke proteins were conserved between zebrafish and Tetrahymena. Together, our results suggest that radial spoke proteins are essential for ciliary motility in the OV and that radial spoke-regulated OV motile cilia represent a unique type of cilia during early zebrafish embryonic development.-Han, X., Xie, H., Wang, Y., Zhao, C. Radial spoke proteins regulate otolith formation during early zebrafish development.
Keywords: cilia; lrrc23; otic vesicle; rsph3; rsph4a.