Ciliary ARLs drive renal cystogenesis

Polycystic kidney disease (PKD) is the leading genetic cause of renal 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 with guanine nucleotide exchange factor (GEF) activity and links to renal cystogenesis.

We use two distinct Arl13b mouse alleles to investigate whether ARL13B is a component of the CDCA pathway: Arl13b^V358A encodes for enzymatically normal ARL13B that is undetectable in cilia, and Arl13b^R79Q encodes for cilia-localized ARL13B lacking a residue critical for its GEF activity. We used these alleles in a Pkd1-deficient adult mouse model, and investigated renal morphology (H&E and cystic index analysis), physiology (blood urea nitrogen measurements), renal fibrosis (picrosirius staining and α-smooth muscle actin levels), renal injury (SOX9 immunofluorescent staining and quantification), and Wnt signaling (β-catenin and cyclin D1 protein levels).

We found that loss of ciliary ARL13B or mutating a single residue critical for its GEF activity suppressed Pkd1-dependent cysts. We observed a reduction in kidney size, cystic index, and blood urea nitrogen. We also observed suppression of renal fibrosis, renal injury, and β-catenin and cyclin D1 protein levels.

Our results identify a subcellular location and mechanism driving Pkd1-dependent renal cystogenesis. We demonstrate that expression of a critical residue for ARL13B's GEF activity specifically in cilia is a key mechanism of the CDCA pathway driving renal cystogenesis.