Primary cilia play counterregulatory roles in cystogenesis—they inhibit cyst formation in the normal renal tubule but promote cyst growth when the function of polycystins is impaired. Key upstream cilia-specific signals and components involved in driving cystogenesis have remained elusive. Recent studies of the tubby family protein, Tubby-like protein 3 (TULP3), have provided new insights into the cilia-localized mechanisms that determine cyst growth. TULP3 is a key adapter of the intraflagellar transport complex A (IFT-A) in the trafficking of multiple proteins specifically into the ciliary membrane. Loss of TULP3 results in the selective exclusion of its cargoes from cilia without affecting their extraciliary pools and without disrupting cilia or IFT-A complex integrity. Epistasis analyses have indicated that TULP3 inhibits cystogenesis independently of the polycystins during kidney development but promotes cystogenesis in adults when polycystins are lacking. In this review, we discuss the current model of the cilia-dependent cyst activation (CDCA) mechanism in autosomal dominant polycystic kidney disease (ADPKD) and consider the possible roles of ciliary and extraciliary polycystins in regulating CDCA. We then describe the limitations of this model in not fully accounting for how cilia single knockouts cause significant cystic changes either in the presence or absence of polycystins. Based on available data from TULP3/IFT-A-mediated differential regulation of cystogenesis in kidneys with deletion of polycystins either during development or in adulthood, we hypothesize the existence of cilia-localized components of CDCA (cCDCA) and cilia-localized cyst inhibition (CLCI) signals. We develop the criteria for cCDCA/CLCI signals and discuss potential TULP3 cargoes as possible cilia-localized components that determine cystogenesis in kidneys during development and in adult mice.

Fibrocystin/Polyductin (FPC) is encoded by PKHD1 which, when mutated, causes autosomal recessive polycystic kidney disease (ARPKD). FPC’s function and its role in cystogenesis are unknown. We describe that endogenous FPC undergoes complex proteolytic processing, generating three small C-terminal fragments (ICDs), one of which (ICD15) contains a novel mitochondrial targeting sequence that directs its translocation into mitochondria. Pkhd1 inactivation leads to aberrant ultrastructural morphology of mitochondria in proximal tubules of developing kidneys. FPC inactivation enhances renal cystogenesis and causes severe pancreatic cystogenesis in a Pkd1 mouse mutant with blocked G protein‐coupled receptor proteolysis site (GPS) cleavage of the Pkd1-encoded Polycystin-1 protein. Deletion of the ICD15 region, encoded by exon 67, is sufficient to enhance renal cystogenesis but does not cause pancreatic cystogenesis. Our results connect FPC, a ciliary protein, directly to a mitochondrial signaling pathway whose perturbation is implicated in PKD pathogenesis.

Background and Aims Autosomal recessive polycystic kidney disease is caused by mutations in PKHD1 encoding FPC, and is characterized by severe renal cystogenesis in neonates, yet mouse models do not fully recapitulate the human phenotype. Indeed, even the Pkhd1 null allele does not cause renal cystogenesis in the mouse. Several cleavage products of FPC are reported yet their function remains unknown. The aim of this study was to determine the function of the FPC cleavage products and their effects on cyst development in ARPKD. Method Three Pkhd1 mutant mouse lines and the cystic Pkd1V mouse were crossed to produce digenic mice with which to study renal cystogenesis. Biochemical analysis was used to investigate FPC cleavage patterns using a panel of new antibodies. Cell models and electron microscopy revealed underlying mitochondrial defects in Pkhd1 Knockout mice. Results Pkhd1 mutation modifies a Pkd1 uncleavable mutant (Pkd1V), enhancing the cystic phenotype in both the kidney and pancreas. The hypomorphic Pkhd1 mutant and Pkhd1 KO both enhance the Pkd1V kidney phenotype, making distal tubule cysts more severe and initiating cystogenesis in the proximal tubules. FPC displays differential cleavage to produce fragments of unknown function. New antibodies were generated to interrogate FPC cleavage products. Three small C terminal cleavage fragments were identified which contain a mitochondrial targeting sequence and are recruited to mitochondria. Mitochondrial ultrastructural changes were evident after deletion of Pkhd1 including mitochondrial fragmentation and dilated cristae, suggesting disrupted mitochondrial function. Deletion of just the C-terminal fragment of FPC (ΔCT), the portion that directly corresponds to the portion that cleaves and localises to the mitochondria, is sufficient to enhance the renal cystic phenotype of the PC1 cleavage mutant. Unlike the other Pkhd1 mutants however, FPC (ΔCT) does not result in the pancreatic cystogenesis when combined with Pkd1V, suggesting that the FPC C terminus in not required to prevent pancreatic cyst development. Conclusion Our results suggest that the C-terminus of FPC plays an important role in preventing renal cystogenesis via a newly discovered mitochondria specific function. Our work reveal some aspects of FPC's function, in particular a previously unrecognised mitochondria function that is mediated through FPC cleavage products and is essential in the kidney to prevent the enhancement of cystogenesis in the digenic model.