Genetic dissection of ciliary ARL13B in kidney cystogenesis

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The objective of this proposal is to refine the understanding of the signaling mechanism(s) within the primary cilium in relation to polycystic kidney disease (PKD). Renal cysts interfere with kidney function and lead to major, long-term health complications. PKD and its related health complications result in a $7 billion per year economic burden in the United States so identifying molecular targets for treatment is a pressing unmet clinical need. Cilia, the slender protrusions on all renal cells, play key roles in cell signaling and cystogenesis but the details and underlying mechanism are ill-defined. Autosomal dominant polycystic kidney disease (ADPKD) is primarily due to mutations in the cilia-associated polycystin genes, PKD1 and PKD2. Normally, the polycystins function in cilia to suppress a pathway that promotes cysts. Identification of the molecular components of this pathway, termed the cilia-dependent cyst activator (CDCA), represents a clear molecular target for therapeutic strategies to counter ADPKD. The major challenge in working out the CDCA pathway, and in understanding ciliary signaling more generally, is that the tools in the field are too blunt. Mouse models typically delete genes, resulting in complete loss of both the ciliary and cellular pools of protein, making any interpretation of cilia-specific function impossible. We circumvented this challenge with the ciliary GTPase ARL13B, a proposed mediator of the CDCA. We removed ARL13B specifically from cilia by engineering a mouse expressing a cilia-excluded ARL13BV358A variant that retains all known biochemical activities. Arl13bV358A/V358A mice are viable and fertile yet display cystic kidneys indicating that it is the absence of ciliary ARL13B that is specifically critical to kidney cyst formation. Our goal in this proposal is to test our central hypothesis that ciliary ARL13B plays key roles in regulating kidney cystogenesis. The proposed work is important in forging fundamental knowledge of the identity and the timing of critical signaling mechanisms from within the primary cilium that could lead to therapeutic intervention for PKD.