Endoplasmic Reticulum (ER)-Mitochondrion Connection Is Important in the Pathogenesis of Autosomal Dominant Polycystic Kidney Disease

Background: Mutations on PKD1 and PKD2 coding for PC1 and PC2 cause ADPKD. Metabolic reprogramming/mitochondrial dysfunction is an important modifier of ADPKD cystogenesis. The cause of mitochondrial dysfunction in ADPKD remains elusive. PC2 is a cation channel that conducts monovalent cation K+ and Na+ more than Ca2+. We have recently shown that ER-localized PC2 functions as a K+-permeable channel to mediate K+-Ca2+ exchange to facilitate ER Ca2+ release. TricB is an ER-restricted K+ channel. We showed that TricB reverses ER Ca2+ release defect in PC2-deficient cell line and cystogenesis in Pkd2-cKO mice. ER and mitochondrion each occupies a large share of cell volume and form close inter-organelle contact known as mitochondria-associated ER membranes (MAMs). We examined the role of ER-mitochondria connection in ADPKD pathogenesis. Methods: Mitochondria morphology in pre-cystic (4 weeks) and cystic (16 weeks) kidneys in Pax8-LC1 driven Pkd2-cKO mice were examined by transmission (TEM) and scanning electron microscopy (SEM). Mitochondrial respiration in isolated proximal tubules were studied by Seahorse oxygen consumption rate (OCR) assays. Mitochondrial DNA mass and regulator gene expression were examined by real-time PCR. Results: TEM showed that Pkd2-cKO kidneys vs control have altered mitochondria morphology including decreased area and cristae density, and increased roundness. The distance between ER-mitochondria contact was increased and the length of contact was decreased in Pkd2-cKO. These changes occurred in pre-cystic kidneys, and no further progression from precystic to cystic kidneys. SEM with 3D rendering showed decreased individual mitochondria size. Proximal tubules isolated from cKO kidneys showed decreased OCR. Mitochondria master regulators PGC1α and PPARα, DNA mass, and mitofusin-2 (a mitochondria profusion factor and MAM regulator) were decreased in cKO kidneys. All above changes in cKO, in precystic as well cystic stages, were reversed by TricB. Conclusion: Mitochondria defects occur in early precystic stages. ER and mitochondria are in close contact with 60% mitochondria surface in contact with ER. TricB transgene reverses cystogenesis and mitochondria defects in Pkd2-cKO mice. The results support that ER Ca2+ homeostasis defect is important in mitochondrial dysfunction and the pathogenesis of ADPKD. Funding: NIDDK Support