Mitochondria Permeability Transition versus Necroptosis in Oxalate-Induced AKI
Authors: Mulay Shrikant Ramesh , Honarpisheh Mohsen M. , Foresto-Neto Orestes , Shi Chongxu , Desai Jyaysi , Zhao Zhi Bo , Marschner Julian A. , Popper Bastian , Buhl Ewa Miriam , Boor Peter , Linkermann Andreas , Liapis Helen , Bilyy Rostyslav , Herrmann Martin , Romagnani Paola , Belevich Ilya , Jokitalo Eija , Becker Jan U. , Anders Hans-Joachim 
Published: July 2019

Significance Statement
Sudden increases in serum oxalate levels occurring with certain dietary exposures or ethylene glycol poisoning are a well known cause of AKI. The authors recently reported that intrarenal precipitation of calcium oxalate crystals activates NACHT, LRR, and PYD domains-containing protein-3 (NLRP3)–dependent inflammation and mixed lineage kinase domain-like (MLKL) protein-dependent tubule necroptosis, i.e., renal necroinflammation. In this study, they show that calcium oxalate crystals and other microparticles activate another route of regulated cell necrosis, peptidylprolyl isomerase F (PPIF)–dependent mitochondrial permeability transition, a process involving crystal phagocytosis and lysosomal destabilization. Mice deficient in Ppif or treated with an inhibitor of mitochondrial permeability transition were protected from oxalate-induced AKI. These results point to a previously unknown pathomechanism of type 2 crystal nephropathies and identify a potential molecular target for renoprotective intervention.

Background
Serum oxalate levels suddenly increase with certain dietary exposures or ethylene glycol poisoning and are a well known cause of AKI. Established contributors to oxalate crystal–induced renal necroinflammation include the NACHT, LRR and PYD domains-containing protein-3 (NLRP3) inflammasome and mixed lineage kinase domain-like (MLKL) protein–dependent tubule necroptosis. These studies examined the role of a novel form of necrosis triggered by altered mitochondrial function.

Methods
To better understand the molecular pathophysiology of oxalate-induced AIK, we conducted in vitro studies in mouse and human kidney cells and in vivo studies in mice, including wild-type mice and knockout mice deficient in peptidylprolyl isomerase F (Ppif) or deficient in both Ppif and Mlkl.

Results
Crystals of calcium oxalate, monosodium urate, or calcium pyrophosphate dihydrate, as well as silica microparticles, triggered cell necrosis involving PPIF–dependent mitochondrial permeability transition. This process involves crystal phagocytosis, lysosomal cathepsin leakage, and increased release of reactive oxygen species. Mice with acute oxalosis displayed calcium oxalate crystals inside distal tubular epithelial cells associated with mitochondrial changes characteristic of mitochondrial permeability transition. Mice lacking Ppif or Mlkl or given an inhibitor of mitochondrial permeability transition displayed attenuated oxalate-induced AKI. Dual genetic deletion of Ppif and Mlkl or pharmaceutical inhibition of necroptosis was partially redundant, implying interlinked roles of these two pathways of regulated necrosis in acute oxalosis. Similarly, inhibition of mitochondrial permeability transition suppressed crystal-induced cell death in primary human tubular epithelial cells. PPIF and phosphorylated MLKL localized to injured tubules in diagnostic human kidney biopsies of oxalosis-related AKI.

Conclusions
Mitochondrial permeability transition–related regulated necrosis and necroptosis both contribute to oxalate-induced AKI, identifying PPIF as a potential molecular target for renoprotective intervention.