Ciliary ARL13B Drives Cystogenesis in the Kidney via Its GEF Activity

BACKGROUND: Polycystic kidney disease (PKD) is the leading genetic cause of kidney failure, resulting in the accumulation of fluid filled cysts and gross enlargement of the kidney. Mutations in PKD1 or PKD2, which encode ciliary polycystin proteins, are the most common cause of PKD. These proteins function in a cilia-dependent cyst activation (CDCA) pathway-one that requires cilia for its pro-cystic function-yet the molecular driver(s) of this pathway are unknown. ARL13B is a regulatory GTPase enriched in cilia and links to cystogenesis in the kidneys. ARL13B possesses guanine nucleotide exchange factor (GEF) activity for ARL3, another ARL with links to cilia. METHODS: We used two distinct Arl13b mouse alleles to investigate whether ARL13B is a component of the CDCA pathway: Arl13bV358A encodes for enzymatically normal ARL13B that is undetectable in cilia, and Arl13bR79Q encodes for cilia-localized ARL13B lacking a residue critical for its ARL3 GEF activity. We used these alleles in a Pkd1-deficient adult mouse model and investigated kidney morphology (H&E and cystic index analysis), physiology (blood urea nitrogen measurements), kidney fibrosis (picrosirius staining and α-smooth muscle actin levels), kidney injury (SOX9 immunofluorescent staining and quantification), and Wnt signaling (β-catenin and cyclin D1 protein levels). RESULTS: We found that loss of ciliary ARL13B or mutation of a single residue critical for its ARL3 GEF activity suppressed Pkd1-dependent cysts. We observed reductions in kidney size, cystic index, and blood urea nitrogen. We also observed suppression of kidney fibrosis, kidney injury, and β-catenin and cyclin D1 protein levels. CONCLUSIONS: Our results identified a subcellular location and mechanism driving Pkd1-dependent cystogenesis in the kidneys. We demonstrated that expression of a critical residue for ARL13B's GEF activity specifically in cilia is a key mechanism of the CDCA pathway driving cystogenesis in the kidneys.