Cell cycle-related kinase regulates mammalian eye development through positive and negative regulation of the Hedgehog pathway

Cell cycle-related kinase (CCRK) is a conserved regulator of ciliogenesis whose loss in mice leads to a wide range of developmental defects, including exencephaly, preaxial polydactyly, skeletal abnormalities, and microphthalmia. Here, we investigate the role of CCRK in mouse eye development. Ccrk mutants show dramatic patterning defects, with an expansion of the optic stalk domain into the optic cup, as well as an expansion of the retinal pigment epithelium (RPE) into neural retina (NR) territory. In addition, Ccrk mutants display a shortened optic stalk. These defects are associated with bimodal changes in Hedgehog (Hh) pathway activity within the eye, including the loss of proximal, high level responses but a gain in distal, low level responses. We simultaneously removed the Hh activator GLI2 in Ccrk mutants (Ccrk-/-;Gli2-/-), which resulted in rescue of optic cup patterning and exacerbation of optic stalk length defects. Next, we disrupted the Hh pathway antagonist GLI3 in mutants lacking CCRK (Ccrk-/-;Gli3-/-), which lead to even greater expansion of the RPE markers into the NR domain and a complete loss of NR specification within the optic cup. These results indicate that CCRK functions in eye development by both positively and negatively regulating the Hh pathway, and they reveal distinct requirements for Hh signaling in patterning and morphogenesis of the eyes.