#4181 the autosomal recessive polycystic kidney disease protein, fpc, releases mitochondria localising fragments involved in preventing cystogenesis

Abstract 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.