Aging is associated with a decline in mitochondrial function which may contribute to age-related diseases such as neurodegeneration, cancer, and cardiovascular diseases. Recently, mitochondrial Complex II has emerged as an important player in the aging process. Mitochondrial Complex II converts succinate to fumarate and plays an essential role in both the tricarboxylic acid (TCA) cycle and the electron transport chain (ETC). The dysfunction of Complex II not only limits mitochondrial energy production; it may also promote oxidative stress, contributing, over time, to cellular damage, aging, and disease. Intriguingly, succinate, the substrate for Complex II which accumulates during mitochondrial dysfunction, has been shown to have widespread effects as a signaling molecule. Here, we review recent advances related to understanding the function of Complex II, succinate signaling, and their combined roles in aging and aging-related diseases.

Karyomegalic interstitial nephritis (KIN) is a genetic adult-onset chronic kidney disease (CKD) characterized by genomic instability and mitotic abnormalities in the tubular epithelial cells. KIN is caused by recessive mutations in the FAN1 DNA repair enzyme. However, the endogenous source of DNA damage in FAN1/KIN kidneys has not been identified. Here we show, using FAN1-deficient human renal tubular epithelial cells (hRTECs) and FAN1-null mice as a model of KIN, that FAN1 kidney pathophysiology is triggered by hypersensitivity to endogenous reactive oxygen species (ROS), which cause chronic oxidative and double-strand DNA damage in the kidney tubular epithelial cells, accompanied by an intrinsic failure to repair DNA damage. Furthermore, persistent oxidative stress in FAN1-deficient RTECs and FAN1 kidneys caused mitochondrial deficiencies in oxidative phosphorylation and fatty acid oxidation. The administration of subclinical, low-dose cisplatin increased oxidative stress and aggravated mitochondrial dysfunction in FAN1-deficient kidneys, thereby exacerbating KIN pathophysiology. In contrast, treatment of FAN1 mice with a mitochondria-targeted ROS scavenger, JP4-039, attenuated oxidative stress and accumulation of DNA damage, mitigated tubular injury, and preserved kidney function in cisplatin-treated FAN1-null mice, demonstrating that endogenous oxygen stress is an important source of DNA damage in FAN1-deficient kidneys and a driver of KIN pathogenesis. Our findings indicate that therapeutic modulation of kidney oxidative stress may be a promising avenue to mitigate FAN1/KIN kidney pathophysiology and disease progression in patients.

Lysine succinylation, and its reversal by sirtuin-5 (SIRT5), is known to modulate mitochondrial fatty acid β-oxidation (FAO). We recently showed that feeding mice dodecanedioic acid, a 12-carbon dicarboxylic acid (DC₁₂) that can be chain-shortened four rounds to succinyl-CoA, drives high-level protein hypersuccinylation in the peroxisome, particularly on peroxisomal FAO enzymes. However, the ability of SIRT5 to reverse DC₁₂-induced peroxisomal succinylation, or to regulate peroxisomal FAO in this context, remained unexplored. Here, we showed that feeding DC₁₂ strongly recruits SIRT5 into hepatic peroxisomes. Knocking out SIRT5 impaired peroxisomal FAO as evidenced by reduced ¹⁴C-DC₁₂ flux in liver homogenates and elevated levels of partially shortened DC₁₂ catabolites in urine. Further, mass spectrometry revealed a trend toward less peroxisomal protein succinylation in SIRT5 knockout liver. This is consistent with a reduced flux of DC₁₂ through the peroxisomal FAO pathway, thereby reducing the production of the succinyl-CoA that chemically reacts with lysine residues to produce protein succinylation. Mass spectrometry comparisons of site-level succinylation in wildtype and SIRT5 knockout liver did not reveal any clear pattern of SIRT5 target sites in the peroxisome after DC₁₂ feeding. However, SIRT5 co-immunoprecipitated with 15 peroxisomal proteins, including the key peroxisomal FAO enzymes acyl-CoA oxidase-1 and enoyl-CoA/3-hydroxyacyl-CoA dehydrogenase (EHHADH). In vitro, recombinant SIRT5 partially desuccinylated chemically modified recombinants ACOX1a, ACOX1b, and EHHADH. Desuccinylation by SIRT5 had no effect on enzyme activity for ACOX1a and EHHADH. For ACOX1b, SIRT5-mediated desuccinylation decreased activity by ~15%. Possible interpretations of these data are discussed.

Lysine succinylation, and its reversal by sirtuin-5 (SIRT5), is known to modulate mitochondrial fatty acid β-oxidation (FAO). We recently showed that feeding mice dodecanedioic acid, a 12-carbon dicarboxylic acid (DC12) that can be chain-shortened four rounds to succinyl-CoA, drives high-level protein hypersuccinylation in the peroxisome, particularly on peroxisomal FAO enzymes. But the ability of SIRT5 to reverse DC12-induced peroxisomal succinylation, or to regulate peroxisomal FAO in this context, remained unexplored. Here, we showed that feeding DC12 strongly recruits SIRT5 into hepatic peroxisomes. Knocking out SIRT5 impaired peroxisomal FAO as evidenced by reduced 14C-DC12 flux in liver homogenates and elevated levels of partially shortened DC12 catabolites in urine. Further, mass spectrometry revealed a trend toward less peroxisomal protein succinylation in SIRT5 knockout liver. This is consistent with reduced flux of DC12 through the peroxisomal FAO pathway, thereby reducing production of the succinyl-CoA that chemically reacts with lysine residues to produce protein succinylation. Mass spectrometry comparisons of site-level succinylation in wildtype and SIRT5 knockout liver did not reveal any clear pattern of SIRT5 target sites in the peroxisome after DC12 feeding. However, SIRT5 co-immunoprecipitated with 15 peroxisomal proteins including the key peroxisomal FAO enzymes acyl-CoA oxidase-1 and enoyl-CoA/3-hydroxyacyl-CoA dehydrogenase (EHHADH). In vitro, recombinant SIRT5 partially desuccinylated chemically modified recombinant ACOX1a, ACOX1b, and EHHADH. Desuccinylation by SIRT5 had no effect on enzyme activity for ACOX1a and EHHADH. For ACOX1b, SIRT5-mediated desuccinylation decreased activity by ~15%. Possible interpretations of this data are discussed.