A Novel Zebrafish Mutant Reveals New Insight into the Regulation of Cilia Motility and Body Axis Formation

Motile cilia are responsible for critical functions in development, including left-right patterning and cerebrospinal fluid flow. Their motility depends on the assembly of outer dynein arms: ATPases which power ciliary beating. Defects in dynein arm function occur in Primary Ciliary Dyskinesia, a disorder affecting 1:15,000–30,000 human births. Daw1 is a cytoplasmic protein thought to be required for cilia beating by controlling import of dynein arms into cilia. Here, I use zebrafish as a model to understand Daw1 function during development and growth. I characterize daw1b1403 mutants, a new daw1 mutant line harboring a 2-amino acid deletion in a conserved region of the protein generated by CRISPR mutagenesis. Defects associated with motile cilia dysfunction in daw1b1403 mutants, including otolith abnormalities, left-right patterning defects, and abnormal body axis curvature are observed. Surprisingly, daw1b1403 mutants exhibit recovery of body curve defects later in development. Consequently, we hypothesize that Daw1 is not essential for cilia motility per se, but only for timely onset of beating over developmental timescales. To support this, live imaging of the central canal showed that the beating of motile cilia is abrogated on the first day post-fertilization (dpf) in daw1b1403 mutants but recovered to an indistinguishable level from sibling controls by the second dpf. Lastly, collaborators based in the United Kingdom have identified patients from two families with homozygous Daw1 mutations that present with situs defects. Here, I show that upon injection of Daw1 mRNA containing the human mutations, daw1b1403 mutants fail to rescue, suggesting that these mutations encode a null protein, and that the defects present in human patients, like our mutants, are the result of Daw1 abrogation. Importantly, this Daw1 model of delayed cilia motility and body straightening provides an opportunity to study how early embryos can sense, or correct, shape deformations, which is an exciting and relatively unknown aspect of developmental morphogenesis. Ultimately, understanding these processes may help inform our treatments of congenital disorders