Investigate the Novel Role of a Cilium-Specific Phosphoinositide Pathway in the Context of Polycystic Kidney Disease

The proposed research project will address one of the Fiscal Year 2019 Peer Reviewed Medical Research Program Topic Areas, polycystic kidney disease (PKD). The autosomal dominant PKD (ADPKD) is the most common form of PKD leading to end-stage renal disease in adults. It is also the most common inherited kidney disease that affects ~1 in 1,000 live births. ADPKD causes progressive formation of kidney cysts, and 50% of ADPKD patients by age 60 need regular dialysis or renal transplantation to survive. To date, there is no cure for ADPKD and the only U.S. Food and Drug Administration (FDA)-approved ADPKD drug, Tolvaptan, shows limited beneficial effects but significant side effects. Thus, finding novel drug targets for ADPKD treatment is an urgent medical need. More than 90% of ADPKD cases associate with mutations in PKD1 and PKD2 genes, which encode polycystin proteins (PCs). PCs form a complex and function in the sensory organelle primary cilium. However, why mutated PCs drive the continuous growth of kidney cysts is not fully understood. This limits the development of effective therapies against ADPKD.

Recent studies indicate that the severity and progression of ADPKD is correlated with the reduced functional PCs in cilia. More than one-third of ADPKD pathogenic mutations cause amino acid substitution in PCs, which often interrupts their transport in cilia and/or weakens their functions. Nevertheless, one normal PKD1 (or PKD2) allele remains in ADPKD patients at least at the early stage of the disease. This suggests that increasing the ciliary dosage of PCs may compensate for their functional loss and suppress ADPKD progression. Yet, how the ciliary trafficking of PCs is regulated and how to elevate the ciliary level of PCs remain unclear. We found that a cilium-specific phospholipid pathway participates in controlling the level of PCs in cilia. Activating the lipid kinase PIPKIgamma or inhibiting its counteracting phosphatase INPP5E specifically in cilia can increase the ciliary level of PCs and suppress cyst formation in vitro and ex vivo. Our results suggest that this cilium-specific phospholipid pathway controls the level of PCs in cilia and can serve as a novel target for PKD treatment.

The current research project is designed to test this exciting hypothesis. We will first determine the molecular mechanisms by which PIPKIgamma and INPP5E regulate the ciliary level of PCs. The goals of this part of study are to confirm the roles of PIPKIgamma, INPP5E, and their lipid substrates and products in regulating the ciliary level of PCs and to resolve the molecular network that coordinates the ciliary trafficking of PCs downstream of PIPKIgamma and INPP5E. We expect to identify proteins that can be targeted to increase the level of PCs in cilia.

Second, we will determine how PIPKIgamma and INPP5E can be specifically manipulated in the context of cilia to increase the ciliary level of PCs. Our preliminary studies have identified some proteins that regulate the activity of PIPKIgamma and INPP5E in cilia. We will clarify the functional connections between PIPKIgamma/INPP5E and these proteins. Because PIPKIgamma activation and INPP5E inhibition raise the ciliary level of PCs, their regulators could serve the same purpose and potentially be targets for increasing PCs in cilia and compensation for functional loss of PCs in kidney.

Lastly, we will evaluate the therapeutic potential of restoring the ciliary dosage of PCs for ADPKD treatment. We found that an INPP5E inhibitor increased the ciliary level of PCs and suppressed kidney cyst formation in vitro and ex vivo. Results from our preliminary studies identified other proteins that can be inhibited to elevate the ciliary level of PCs in vitro. We will administrate these inhibitors to preclinical ADPKD mouse model to testify whether increasing the ciliary dosage of PCs can suppress the disease initiation and/or progression in vivo. We will use a mouse model that mimics the adult-onsite ADPKD condition.

The impact of this research project will apply to both basic research and translational research of ADPKD. We will dissect and illustrate the cilium-specific phospholipid-signaling network that regulates the homeostasis of PCs in cilia. Our studies will provide proof-of-principle data for an exciting, novel concept that increasing the dosage of PCs in cilia can compensate for the functional loss caused by ADPKD pathogenic mutations and subsequently slow down PKD progression. Knowledge derived from this project will reveal novel molecules and/or pathways that can be targeted for PKD treatment in the future. Importantly, our study will inspire future research enthusiasm on finding new approaches that can elevate the functional dosage of PCs in cilia to counteract the pathogenic defects. Considering the high incidence, high cost of medical care, poor outcome, and lack of treatment of ADPKD, our study is highly significant for providing a noteworthy and promising direction for therapeutic development. Since ADPKD is an inherited genetic disorder, outcomes from this research project will benefit the military and civilian populations equally.