Ciliary ARL13B Is Essential for Vision and Morphogenesis of Cone Outer Segments.

Mutations in ARL13B lead to Joubert syndrome, a ciliopathy with neurological and retinal abnormalities. In photoreceptors, ARL13B localizes to the connecting cilia and outer segments. However, the specific function and the need for ARL13B in photoreceptor cilia remain unclear. We used a knock-in mouse model with the Arl13b V358A mutation, which disrupts ciliary localization while preserving guanine exchange factor (GEF) activity, to investigate the role of ARL13B in the photoreceptor cilia. Using female and male littermates, we show by electroretinogram (ERG) that the exclusion of ARL13B from photoreceptor cilia leads to an early loss of cone-mediated vision followed by a decline in rod-mediated vision. This phenotype was unique to the cilia-excluded V358A model, as analysis using the GEF-impaired R79Q model did not show similar changes in photoreceptor function. Morphological analyses using immunohistochemistry (IHC) and transmission electron microscopy revealed shortened cone axonemes and structural abnormalities in cone outer segments. IHC staining further demonstrated that loss of ciliary ARL13B disrupts the localization of intraflagellar transport protein 88 (IFT88) in photoreceptors. In addition, the phosphoinositol-4,5-bisphosphate (PIP2) binding protein, Tubby-like protein 1 (TULP1), associated with inherited retinal diseases, was mislocalized to cone outer segments in the V358A model. These findings establish an essential role for ciliaryARL13B in maintaining cone photoreceptor axoneme length, outer segment organization, and proper protein localization. They also suggest that phosphoinositide gradients are critical for cone photoreceptor function and morphology. Together, our findings provide new insights into the molecular mechanisms regulating photoreceptor cilia and the pathogenesis of ciliopathies.