Accumulation of Succinate in Cardiac Ischemia Primarily Occurs via Canonical Krebs Cycle Activity

Abstract

Succinate accumulates during ischemia, and its oxidation at reperfusion drives injury. The mechanism of ischemic succinate accumulation is controversial and is proposed to involve reversal of mitochondrial complex II. Herein, using stable-isotope-resolved metabolomics, we demonstrate that complex II reversal is possible in hypoxic mitochondria but is not the primary succinate source in hypoxic cardiomyocytes or ischemic hearts. Rather, in these intact systems succinate primarily originates from canonical Krebs cycle activity, partly supported by aminotransferase anaplerosis and glycolysis from glycogen. Augmentation of canonical Krebs cycle activity with dimethyl-α-ketoglutarate both increases ischemic succinate accumulation and drives substrate-level phosphorylation by succinyl-CoA synthetase, improving ischemic energetics. Although two-thirds of ischemic succinate accumulation is extracellular, the remaining one-third is metabolized during early reperfusion, wherein acute complex II inhibition is protective. These results highlight a bifunctional role for succinate: its complex-II-independent accumulation being beneficial in ischemia and its complex-II-dependent oxidation being detrimental at reperfusion.

Keywords: complex II; heart; ischemia; mitochondria; reperfusion; succinate.

Figure 1. Metabolic Pathways Investigated

Upper panel: normoxic metabolism. The mitochondrial respiratory chain is shown center left, positioned in the mitochondrial inner membrane, with dashed lines denoting electron flow. Krebs cycle and other metabolites are shown using common abbreviations (e.g., α-KG, α-ketoglutarate; Fum, fumarate). Metabolite transporters (e.g., malate/aspartate shuttle) are shown at right. Dotted lines denote multi-step metabolite interconversions. Inset panels (boxed) show pathways relevant to Figure S1E. Enzyme or transporter inhibitors are shown in red. Lower panels: proposed models for ischemic succinate generation. Model A: Cx II reversal/aspartate model. Model B: Cx II inhibition/canonical Krebs cycle/aminotransferase anaplerosis model. Species shown in color (glucose, orange; palmitate, green; aspartate, blue; glutamine, pink) denote metabolites employed in stable isotope labeling experiments. Dimethyl-α-ketoglutarate (DM-α-KG) is shown in yellow.

Figure 2. Cx II Reversal Is Not the Primary Mechanism of Ischemic Succinate Generation In Situ

(A) Schematic of isolated mitochondrial experiments. Pyruvate (Pyr) and carnitine were present to generate NADH in a manner that did not engage the Krebs cycle (i.e., acetyl-CoA exits mitochondria as acetyl-carnitine [Ac-Carn]). Rot, rotenone; AA5, atpenin A5; PDH, pyruvate dehydrogenase; Fum, fumarate; Succ, succinate. (B) Isolated mouse heart mitochondria respired to hypoxia, and then 1 mM fumarate was added, with 1 μM Rot or 1 μM AA5, followed by hypoxic incubation for 20 min. Succinate was measured using HPLC. n = 4–6. *p < 0.05 versus hypoxia with substrates. nd, not detectable. (C) Perfused hearts were treated with rotenone (1 μM/5 min), AA5 (1 μM/5 min), dimethyl malonate (DMM) (5 mM/10 min), malonate (5 mM/10 min), or thenoyltrifluorocetone (TTFA) (1 mM/5 min) and then immediately subjected to 25 min of ischemia. Hearts were snap-frozen and succinate measured using HPLC. n = 3–8. *p < 0.05 versus untreated baseline (BL). ns, no significant difference between indicated groups. (D) Isolated primary cardiomyocytes were treated with 1 μM Rot or 1 μM AA5 and subjected to 60 min of hypoxia and then succinate measured using HPLC. n = 3–5. *p < 0.05 versus baseline (BL). All data are means ± SEM. See also Figure S1.

Figure 3. Minor Contribution of Aspartate to Ischemic Succinate Accumulation

(A) Left: perfused hearts were treated with 5 mM aminooxyacetate (AOA) for 5 min and then subjected to 25 min ischemia. Succinate was measured using LC-MS/MS. n = 3–4. *p < 0.05 versus baseline (BL), †p < 0.05 versus ischemia alone. Pyruvate (middle) and α-KG (right) were measured in normoxic perfused hearts treated with 5 mM AOA for 5 min, n = 3–4. (B) Schematic of SIRM experiments with [U-¹³C]aspartate delivery for 10 min, followed by sampling immediately or after 25 min of ischemia. Succinate was measured using LC-MS/MS. See also Figure S2 for predicted labeling resulting from [U¹³-C]aspartate delivery. (C) Left: abundances of each ¹³C-labeled succinate species in hearts from (B) are shown, with log-transformed data in the inset. Pictograms below represent labeling status of carbons in succinate (white, unlabeled; blue, labeled). Right: fractional labeling of succinate isotopologues. Pictograms below represent calculated fractions. n = 3. *p < 0.05 versus baseline (BL). ns, no significant difference between indicated groups. All data are means ± SEM.

Figure 4. Glycolysis and Glycogenolysis Facilitate Ischemic Succinate Accumulation

(A) Left: hearts were equilibrated for 20 min with either 5 mM glucose and 100 μM palmitate (Glu+Fat) or palmitate only (Fat only) and sampled either at baseline (BL) or after 25 min of ischemia (Isch’). Succinate was measured using HPLC. n = 3–4. *p < 0.05 versus Glu+Fat baseline (BL), †p < 0.05 versus Glu+Fat ischemia. ND, not detectable. Right: the glycogen content was assayed in the same hearts. n = 3–4. *p < 0.05 versus Glu+Fat baseline (BL), †p < 0.05 versus Fat only ischemia. (B) Schematic of SIRM experiments with [U-¹³C]glucose delivery for 5 min, followed by sampling immediately or after 25 min of ischemia. (C) Left: lactate was measured in hearts from (B) using LC-MS/MS. Abundances of each ¹³C-labeled species are shown. Pictograms below represent labeling status of carbons in lactate (white, unlabeled; orange, labeled). Right: fractional labeling of all lactate isotopologues. Pictogram below represents fraction calculated. n = 5. *p < 0.05 versus baseline (BL). (D) Left: succinate was measured in the same hearts as (C) using LC-MS/MS. Abundances of each ¹³C-labeled succinate species are shown, with log-transformed data in the inset. Pictograms below represent labeling status of carbons in succinate. Right: total fractional labeling of succinate. Pictogram below represents calculated fraction. n = 5. *p < 0.05 versus baseline (BL). (E) Hearts were perfused with 10 μM of the mitochondrial pyruvate carrier inhibitor UK5099 for 5 min and then subjected to 25 min of ischemia. Succinate was measured using HPLC. n = 3. ns, no significant difference between indicated groups. All data are means ± SEM.

Figure 5. Canonical Krebs cycle Activity, Supported in Part by Aminotransferase Anaplerosis, Is the Primary Source of Ischemic Succinate

(A) Schematic of SIRM experiments with [U-¹³C,¹⁵N]glutamine delivery for 10 min, followed by sampling immediately or after 25 min of ischemia. α-KG and succinate were measured using LC-MS/MS. (B) Left: abundances of each ¹³C-labeled α-KG species in hearts from (A) are shown, with log-transformed data in the inset. Pictograms below represent labeling status of carbons in α-KG (white, unlabeled; pink, labeled) Right: total fractional labeling of α-KG. Pictogram below represents fraction calculated. n = 6. *p < 0.05 versus baseline (BL). (C) Perfused hearts were sampled either at normoxic baseline (BL) or following 25 min of ischemia (Isch’), and total succinyl-CoA levels were measured using LC-MS/MS. n = 3. *p < 0.05 versus baseline. (D) Left: abundances of each ¹³C-labeled succinate species in hearts from (A) are shown, with log-transformed data in the inset. Pictograms below represent labeling status of carbons in succinate. Right: total fractional labeling of succinate. Pictogram below represents fraction calculated. n = 6. *p < 0.05 versus baseline (BL). (E) Schematic of steady-state SIRM experiments, with [U-¹³C]palmitate delivery for 5 min, achieving steady-state labeling of Krebs cycle metabolites. Hearts were sampled immediately or after 25 min of ischemia. Succinate was measured using LC-MS/MS. (F) Left: abundances of each ¹³C-labeled succinate species in hearts from (E) are shown. Pictograms below represent the labeling status of carbons in succinate (white, unlabeled; green, labeled). Right: total fractional labeling of succinate. Pictogram below represents fraction calculated. n = 3. ns, no significant difference between indicated groups. All data are means ± SEM.

Figure 6. Ischemic Succinate Generation Enhances Cardiac Energetics

(A) Hearts were perfused with 5 mM dimethyl-α-KG (DM-α-KG) for 10 min and sampled immediately or following 25 min of ischemia. Succinate was measured using LC-MS/MS. n = 3–5. *p < 0.05 versus untreated baseline (BL), †p < 0.05 versus untreated ischemia. (B) Hearts were perfused with 5 mM DM-α-KG for 10 min and/or 5 mM AOA for 5 min and sampled either at baseline (BL) or after 25 min of ischemia. Succinate was measured using LC-MS/MS. n = 3–6. *p < 0.05 versus untreated baseline, †p < 0.05 versus untreated ischemia, §p < 0.05 versus ischemia +AOA. (C) Schematic of experiments investigating mid-point ischemic energetics. Hearts were perfused with 5 mM DM-α-KG for 10 min and sampled after 12 min of ischemia. Metabolites were detected using LC-MS/MS. (D) Metabolites of hearts from (C). n = 5–6. *p < 0.05 versus untreated (with Bonferroni correction for multiple testing). (E) Energy charge was calculated from (D). n = 5–6. *p < 0.05 versus untreated. (F) Time to peak ischemic contracture was measured in hearts perfused with 5 mM DM-α-KG for 10 min and then subjected to ischemia. n = 6. *p < 0.05 versus untreated. All data are means ± SEM.

Figure 7. Rapid Succinate Oxidation in Early Reperfusion Mediates IR Injury

(A) Schematic of experiments measuring succinate consumption during early reperfusion (Rpf’). (B) Hearts were subjected to 25 min ischemia then reperfused for 0, 1, 2, and 5 min. Succinate was measured in hearts (black bars). Effluent was collected during reperfusion, binned into 0–1 min (red), 1–2 min (orange), and 2–5 min (yellow). No succinate was detected in the effluent after 5 min reperfusion. Succinate was measured using HPLC. n = 3–4. Brackets to the right of graph show the fraction of total cardiac succinate at the end of ischemia (i.e., leftmost black bar) that was released into effluent, with the remainder presumed to be metabolized. (C) Hearts were subjected to 25 min ischemia and 60 min reperfusion. 100 nM AA5 was infused for 5 min at the onset of reperfusion where indicated. Cardiac functional recovery (rate x pressure product) was monitored. n = 6–8. *p < 0.05 versus control (ctrl) (D) Infarct sizes as percent of area-at-risk (i.e., total heart area) in hearts from (C). n = 6–8. All data are means ± SEM.