BackgroundThe primary site of damage during AKI, proximal tubular epithelial cells, are highly metabolically active, relying on fatty acids to meet their energy demands. These cells are rich in mitochondria and peroxisomes, the two organelles that mediate fatty acid oxidation. Emerging evidence shows that both fatty acid pathways are regulated by reversible posttranslational modifications, particularly by lysine acylation. Sirtuin 5 (Sirt5), which localizes to both mitochondria and peroxisomes, reverses post-translational lysine acylation on several enzymes involved in fatty acid oxidation. However, the role of the Sirt5 in regulating kidney energy metabolism has yet to be determined.MethodsWe subjected male Sirt5-deficient mice (either +/− or −/−) and wild-type controls, as well as isolated proximal tubule cells, to two different AKI models (ischemia-induced or cisplatin-induced AKI). We assessed kidney function and injury with standard techniques and measured fatty acid oxidation by the catabolism of 14C-labeled palmitate to 14CO2.ResultsSirt5 was highly expressed in proximal tubular epithelial cells. At baseline, Sirt5 knockout (Sirt5−/−) mice had modestly decreased mitochondrial function but significantly increased fatty acid oxidation, which was localized to the peroxisome. Although no overt kidney phenotype was observed in Sirt5−/− mice, Sirt5−/− mice had significantly improved kidney function and less tissue damage compared with controls after either ischemia-induced or cisplatin-induced AKI. This coincided with higher peroxisomal fatty acid oxidation compared with mitochondria fatty acid oxidation in the Sirt5−/− proximal tubular epithelial cells.ConclusionsOur findings indicate that Sirt5 regulates the balance of mitochondrial versus peroxisomal fatty acid oxidation in proximal tubular epithelial cells to protect against injury in AKI. This novel mechanism might be leveraged for developing AKI therapies.

Edited by George M. Carman SIRT5 is a lysine desuccinylase known to regulate mitochondrial fatty acid oxidation and the urea cycle. Here, SIRT5 was observed to bind to cardiolipin via an amphipathic helix on its N terminus. In vitro, succinyl-CoA was used to succinylate liver mitochondrial membrane proteins. SIRT5 largely reversed the succinyl-CoA-driven lysine succinylation. Quantitative mass spectrometry of SIRT5-treated membrane proteins pointed to the electron transport chain, particularly Complex I, as being highly targeted for desuccinylation by SIRT5. Mitochondrial oxidative energy metabolism is fundamental to human health, and changes in mitochondrial function are now recognized as playing an etiological role in diseases such as cancer, diabetes, and neurodegeneration. Of particular importance to mitochondrial function are the ϳ200 proteins that reside on the inner mitochondrial membrane (IMM).3 IMM proteins include a large family of solute transporters, several ion channels, four electron transport chain (ETC) complexes, and ATP synthase. Additionally, the IMM has many peripherally associated proteins that bind to the membrane electrostatically rather than via transmembrane domains. This group includes enzymes such as carnitine palmitoyltransferase-2, mitochondrial trifunctional protein (MTP), and very-longchain acyl-CoA dehydrogenase (VLCAD), which together catalyze long-chain fatty acid oxidation (FAO). All three show membrane localization through electrostatic binding to cardiolipin on the IMM (1-3). Cardiolipin also appears to facilitate assembly of higher-order ETC "supercomplexes" that mediate maximal respiratory efficiency (4). In an additional layer of complexity, there is increasing evidence that metabolic pathways that directly produce reducing equivalents, such as FAO and the tricarboxylic acid cycle (TCA), can physically interact with the ETC and potentially with each other (5).To date, the factors that regulate metabolic complex assembly and facilitate protein-protein and protein-lipid interactions on the IMM are poorly understood. Reversible post-translational protein modifications such as lysine acylation are widespread in the mitochondria and represent a potential regulatory mechanism for the formation and function of IMM protein complexes. For example, we recently showed that succinylation of three lysine residues in the VLCAD C terminus leads to a complete loss of membrane binding (1). Succinylation converts the positively charged lysine residues to negative charges, thereby disrupting the electrostatic interaction between the amphipathic helix of VLCAD and cardiolipin. Incubation with the mitochondrial desuccinylase SIRT5 restores membrane binding of VLCAD. Mass spectrometry studies have identified SIRT5-targeted lysines on other IMM proteins such as MTP, ADP/ATP translocase, isocitrate dehydrogenase, and hydroxymethylglutaryl-CoA synthase-2 (6 -10), but its role in maintaining the integrity of the IMM proteome has not been directly

Juvenile kidneys displayed renal papillary hypoplasia and a paucity of collecting ducts. Twenty-four-hour urine collections revealed that postnatal mice had urinary-concentrating defects. Thus, while lineage-tracing revealed that the renal cortical stroma gave rise to a small subset of endothelial progenitors, these Flk1-expressing stromal cells are critical for patterning the peritubular capillaries. Also, loss of Flk1 in the renal stroma leads to nonautonomous-patterning defects in ureteric lineages.