Ester Prodrugs of Malonate with Enhanced Intracellular Delivery Protect Against Cardiac Ischemia-Reperfusion Injury In Vivo

Long-Term Protective Effects of Succinate Dehydrogenase Inhibition during Reperfusion with Malonate on Post-Infarction Left Ventricular Scar and Remodeling in Mice

Succinate dehydrogenase inhibition with malonate during initial reperfusion reduces myocardial infarct size in both isolated mouse hearts subjected to global ischemia and in in situ pig hearts subjected to transient coronary ligature. However, the long-term effects of acute malonate treatment are unknown. Here, we investigated whether the protective effects of succinate dehydrogenase inhibition extend to a reduction in scar size and adverse left ventricular remodeling 28 days after myocardial infarction. Initially, ten wild-type mice were subjected to 45 min of left anterior descending coronary artery (LAD) occlusion, followed by 24 h of reperfusion, and were infused during the first 15 min of reperfusion with saline with or without disodium malonate (10 mg/kg/min, 120 μL/kg/min). Malonate-treated mice depicted a significant reduction in infarct size (15.47 ± 3.40% of area at risk vs. 29.34 ± 4.44% in control animals, p < 0.05), assessed using triphenyltetrazolium chloride. Additional animals were then subjected to a 45 min LAD ligature, followed by 28 days of reperfusion. Treatment with a single dose of malonate during the first 15 min of reperfusion induced a significant reduction in scar area, measured using Picrosirius Red staining (11.94 ± 1.70% of left ventricular area (n = 5) vs. 23.25 ± 2.67% (n = 9), p < 0.05), an effect associated with improved ejection fraction 28 days after infarction, as determined using echocardiography, and an attenuated enhancement in expression of the pro-inflammatory and fibrotic markers NF-κB and Smad2/3 in remote myocardium. In conclusion, a reversible inhibition of succinate dehydrogenase with a single dose of malonate at the onset of reperfusion has long-term protective effects in mice subjected to transient coronary occlusion.

The Development of a Regulator of Human Serine Racemase for N-Methyl-D-aspartate Function

It is crucial to regulate N-methyl-D-aspartate (NMDA) function bivalently depending on the central nervous system (CNS) conditions. CNS disorders with NMDA hyperfunction are involved in the pathogenesis of neurotoxic and/or neurodegenerative disorders with elevated D-serine, one of the NMDA receptor co-agonists. On the contrary, NMDA-enhancing agents have been demonstrated to improve psychotic symptoms and cognition in CNS disorders with NMDA hypofunction. Serine racemase (SR), the enzyme regulating both D- and L-serine levels through both racemization (catalysis from L-serine to D-serine) and β-elimination (degradation of both D- and L-serine), emerges as a promising target for bidirectional regulation of NMDA function. In this study, we explored using dimethyl malonate (DMM), a pro-drug of the SR inhibitor malonate, to modulate NMDA activity in C57BL/6J male mice via intravenous administration. Unexpectedly, 400 mg/kg DMM significantly elevated, rather than decreased (as a racemization inhibitor), D-serine levels in the cerebral cortex and plasma. This outcome prompted us to investigate the regulatory effects of dodecagalloyl-α-D-xylose (α12G), a synthesized tannic acid analog, on SR activity. Our findings showed that α12G enhanced the racemization activity of human SR by about 8-fold. The simulated and fluorescent assay of binding affinity suggested a noncooperative binding close to the catalytic residues, Lys56 and Ser84. Moreover, α12G treatment can improve behaviors associated with major CNS disorders with NMDA hypofunction including hyperactivity, prepulse inhibition deficit, and memory impairment in animal models of positive symptoms and cognitive impairment of psychosis. In sum, our findings suggested α12G is a potential therapeutic for treating CNS disorders with NMDA hypofunction.

Mitochondrial complex I activity in microglia sustains neuroinflammation

Sustained smouldering, or low-grade activation, of myeloid cells is a common hallmark of several chronic neurological diseases, including multiple sclerosis1. Distinct metabolic and mitochondrial features guide the activation and the diverse functional states of myeloid cells2. However, how these metabolic features act to perpetuate inflammation of the central nervous system is unclear. Here, using a multiomics approach, we identify a molecular signature that sustains the activation of microglia through mitochondrial complex I activity driving reverse electron transport and the production of reactive oxygen species. Mechanistically, blocking complex I in pro-inflammatory microglia protects the central nervous system against neurotoxic damage and improves functional outcomes in an animal disease model in vivo. Complex I activity in microglia is a potential therapeutic target to foster neuroprotection in chronic inflammatory disorders of the central nervous system3.

Dimethyl malonate preserves renal and mitochondrial functions following ischemia-reperfusion via inhibition of succinate dehydrogenase

BACKGROUND

Acute kidney injury (AKI), often experienced at the intensive care units, is associated with high morbidity/mortality where ischemia-reperfusion injury is a main causative factor. Succinate accumulation during ischemia contributes to the excessive generation of reactive oxygen species at reperfusion. Inhibition of succinate dehydrogenase has been associated with protective outcome in cardiac ischemia-reperfusion after 24h, but the effects on kidney and mitochondrial functions are less well studied.

AIM

To investigate the therapeutic potential of succinate dehydrogenase inhibition, by using dimethyl malonate (DMM), on kidney and mitochondria functions in a mouse model of AKI.

METHODS

Male C57BL/6J mice were pre-treated with DMM or placebo, i.p. 30min prior to bilateral renal ischemia (20min). After 3-days of reperfusion, glomerular filtration rate (GFR) was calculated from plasma clearance of FITC-inulin. Kidney mitochondria was isolated and mass specific and intrinsic mitochondrial function were evaluated by high resolution respirometry. Kidney sections were stained (i.e., hematoxylin-eosin and TUNEL) and analyzed for histopathological evaluation of injuries and apotosis, respectively. NADPH oxidase activity in kidney and human proximal tubular cell-line (HK2) were measured luminometrically.

RESULTS

DMM treatment improved GFR (p < 0.05) and reduced levels of blood urea nitrogen (p < 0.01) compared to untreated animals, which was associated with lower degree of ischemia-reperfusion-induced tubular injuries (P < 0.001) and apoptosis (P < 0.01). These therapeutic renal effects were linked with improved mitochondrial function, both mass-specific and intrinsic. Finally, DMM treatment prevented ischemia-reperfusion-induced NADPH oxidase activity in the kidney (p < 0.001), which was showed also in HK2 cells exposed to hypoxia and reoxygenation (P < 0.01).

CONCLUSION

Inhibition of succinate dehydrogenase with DMM, in conjunction with the ischemia-reperfusion phase, significantly improved both renal and mitochondrial functions. These findings may have clinical implications for future therapeutic strategies to prevent development of AKI and associated adverse complications, especially in high risk hospitalized patients.

Mitochondrial reverse electron transport in myeloid cells perpetuates neuroinflammation

Sustained smouldering, or low grade, activation of myeloid cells is a common hallmark of several chronic neurological diseases, including multiple sclerosis (MS) 1 . Distinct metabolic and mitochondrial features guide the activation and the diverse functional states of myeloid cells 2 . However, how these metabolic features act to perpetuate neuroinflammation is currently unknown. Using a multiomics approach, we identified a new molecular signature that perpetuates the activation of myeloid cells through mitochondrial complex II (CII) and I (CI) activity driving reverse electron transport (RET) and the production of reactive oxygen species (ROS). Blocking RET in pro-inflammatory myeloid cells protected the central nervous system (CNS) against neurotoxic damage and improved functional outcomes in animal disease models in vivo . Our data show that RET in myeloid cells is a potential new therapeutic target to foster neuroprotection in smouldering inflammatory CNS disorders 3 .

Malonate differentially affects cell survival and confers chemoresistance in cancer cells via the induction of p53-dependent autophagy

Metabolic network intertwines with cancerous signaling and drug responses. Malonate is a prevailing metabolite in cancer and a competitive inhibitor of succinate dehydrogenase (SDH). Recent studies showed that malonate induced reactive oxygen species (ROS)-dependent apoptosis in neuroblastoma cells, but protected cells from ischemia-reperfusion injury. We here revealed that malonate differentially regulated cell death and survival in cancer cells. While high-dose malonate triggered ROS-dependent apoptosis, the low-dose malonate induced autophagy and conferred resistance to multiple chemotherapeutic agents. Mechanistically, our results showed that malonate increased p53 stability and transcriptionally up-regulated autophagy modulator DRAM (damage-regulated autophagy modulator), thus promoting autophagy. We further proved that autophagy is required for malonate-associated chemoresistance. Collectively, our findings suggest that malonate plays a double-edge function in cancer response to stressors, and highlights a pro-cancer impact of p53-induced autophagy in response to malonate.

Preventing mitochondrial reverse electron transport as a strategy for cardioprotection

In the context of myocardial infarction, the burst of superoxide generated by reverse electron transport (RET) at complex I in mitochondria is a crucial trigger for damage during ischaemia/reperfusion (I/R) injury. Here we outline the necessary conditions for superoxide production by RET at complex I and how it can occur during reperfusion. In addition, we explore various pathways that are implicated in generating the conditions for RET to occur and suggest potential therapeutic strategies to target RET, aiming to achieve cardioprotection.

Complex II Biology in Aging, Health, and Disease

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.

Spontaneous reperfusion enhances succinate concentration in peripheral blood from stemi patients but its levels does not correlate with myocardial infarct size or area at risk

Succinate is enhanced during initial reperfusion in blood from the coronary sinus in ST-segment elevation myocardial infarction (STEMI) patients and in pigs submitted to transient coronary occlusion. Succinate levels might have a prognostic value, as they may correlate with edema volume or myocardial infarct size. However, blood from the coronary sinus is not routinely obtained in the CathLab. As succinate might be also increased in peripheral blood, we aimed to investigate whether peripheral plasma concentrations of succinate and other metabolites obtained during coronary revascularization correlate with edema volume or infarct size in STEMI patients. Plasma samples were obtained from peripheral blood within the first 10 min of revascularization in 102 STEMI patients included in the COMBAT-MI trial (initial TIMI 1) and from 9 additional patients with restituted coronary blood flow (TIMI 2). Metabolite concentrations were analyzed by ¹H-NMR. Succinate concentration averaged 0.069 ± 0.0073 mmol/L in patients with TIMI flow ≤ 1 and was significantly increased in those with TIMI 2 at admission (0.141 ± 0.058 mmol/L, p < 0.05). However, regression analysis did not detect any significant correlation between most metabolite concentrations and infarct size, extent of edema or other cardiac magnetic resonance (CMR) variables. In conclusion, spontaneous reperfusion in TIMI 2 patients associates with enhanced succinate levels in peripheral blood, suggesting that succinate release increases overtime following reperfusion. However, early plasma levels of succinate and other metabolites obtained from peripheral blood does not correlate with the degree of irreversible injury or area at risk in STEMI patients, and cannot be considered as predictors of CMR variables.Trial registration: Registered at www.clinicaltrials.gov (NCT02404376) on 31/03/2015. EudraCT number: 2015-001000-58.

Targeting succinate metabolism to decrease brain injury upon mechanical thrombectomy treatment of ischemic stroke

Current treatments for acute ischemic stroke aim to reinstate a normal perfusion in the ischemic territory but can also cause significant ischemia-reperfusion (IR) injury. Previous data in experimental models of stroke show that ischemia leads to the accumulation of succinate, and, upon reperfusion, the accumulated succinate is rapidly oxidized by succinate dehydrogenase (SDH) to drive superoxide production at mitochondrial complex I. Despite this process initiating IR injury and causing further tissue damage, the potential of targeting succinate metabolism to minimize IR injury remains unexplored. Using both quantitative and untargeted high-resolution metabolomics, we show a time-dependent accumulation of succinate in both human and mouse brain exposed to ischemia ex vivo. In a mouse model of ischemic stroke/mechanical thrombectomy mass spectrometry imaging (MSI) shows that succinate accumulation is confined to the ischemic region, and that the accumulated succinate is rapidly oxidized upon reperfusion. Targeting succinate oxidation by systemic infusion of the SDH inhibitor malonate upon reperfusion leads to a dose-dependent decrease in acute brain injury. Together these findings support targeting succinate metabolism upon reperfusion to decrease IR injury as a valuable adjunct to mechanical thrombectomy in ischemic stroke.

Short-chain fatty acid metabolism and multiple effects on cardiovascular diseases

Cardiovascular diseases (CVDs) are the leading cause of mortality worldwide, and fatty acid metabolism has been well studied. Short-chain fatty acids (SCFAs) have been less discussed than long-chain fatty acids (LCFAs) in CVDs. However, increasing evidence indicates the importance of SCFAs in regulating cardiac function. Here, we summarize the current understanding of SCFAs in hypertension, ischaemic reperfusion, myocardial infarction, atherosclerosis and heart failure. Most SCFAs exert positive effects in regulating related diseases. Butyrate and propionate can reduce blood pressure, improve I/R injury and decrease the risk of coronary artery disease (CAD) and atherosclerosis. Acetate can also play a positive role in regulating hypertension and preventing atherosclerosis, and malonate can improve cardiac function after MI. They affect these diseases by regulating inflammation, the immune system and related G protein-coupled receptors, with multiple neurohumoural regulation participation. In contrast, succinate can accelerate IR injury, increasing mitochondrial ROS production. SCFAs ultimately affect the regulation of different pathophysiological processes in heart failure. Here, we clarified the importance of short-chain fatty acids in the cardiovascular system and their multiple effects in various pathophysiological processes, providing new insights into their promising clinical application. More research should be conducted to further elucidate the underlying mechanism and different effects of single or multiple SCFA supplementation on the cardiovascular system.

Ischemia-Selective Cardioprotection by Malonate for Ischemia/Reperfusion Injury

BACKGROUND

Inhibiting SDH (succinate dehydrogenase), with the competitive inhibitor malonate, has shown promise in ameliorating ischemia/reperfusion injury. However, key for translation to the clinic is understanding the mechanism of malonate entry into cells to enable inhibition of SDH, its mitochondrial target, as malonate itself poorly permeates cellular membranes. The possibility of malonate selectively entering the at-risk heart tissue on reperfusion, however, remains unexplored.

METHODS

C57BL/6J mice, C2C12 and H9c2 myoblasts, and HeLa cells were used to elucidate the mechanism of selective malonate uptake into the ischemic heart upon reperfusion. Cells were treated with malonate while varying pH or together with transport inhibitors. Mouse hearts were either perfused ex vivo (Langendorff) or subjected to in vivo left anterior descending coronary artery ligation as models of ischemia/reperfusion injury. Succinate and malonate levels were assessed by liquid chromatography-tandem mass spectrometry LC-MS/MS, in vivo by mass spectrometry imaging, and infarct size by TTC (2,3,5-triphenyl-2H-tetrazolium chloride) staining.

RESULTS

Malonate was robustly protective against cardiac ischemia/reperfusion injury, but only if administered at reperfusion and not when infused before ischemia. The extent of malonate uptake into the heart was proportional to the duration of ischemia. Malonate entry into cardiomyocytes in vivo and in vitro was dramatically increased at the low pH (≈6.5) associated with ischemia. This increased uptake of malonate was blocked by selective inhibition of MCT1 (monocarboxylate transporter 1). Reperfusion of the ischemic heart region with malonate led to selective SDH inhibition in the at-risk region. Acid-formulation greatly enhances the cardioprotective potency of malonate.

CONCLUSIONS

Cardioprotection by malonate is dependent on its entry into cardiomyocytes. This is facilitated by the local decrease in pH that occurs during ischemia, leading to its selective uptake upon reperfusion into the at-risk tissue, via MCT1. Thus, malonate's preferential uptake in reperfused tissue means it is an at-risk tissue-selective drug that protects against cardiac ischemia/reperfusion injury.

Dimethyl malonate slows succinate accumulation and preserves cardiac function in a swine model of hemorrhagic shock

BACKGROUND

Succinate (SI) is a citric acid cycle metabolite that accumulates in tissues during hemorrhagic shock (HS) due to electron transport chain uncoupling. Dimethyl malonate (DMM) is a competitive inhibitor of SI dehydrogenase, which has been shown to reduce SI accumulation and protect against reperfusion injury. Whether DMM can be therapeutic after severe HS is unknown. We hypothesized that DMM would prevent SI buildup during resuscitation (RES) in a swine model of HS, leading to better physiological recovery after RES.

METHODS

The carotid arteries of Yorkshire pigs were cannulated with a 5-Fr catheter. After placement of a Swan-Ganz catheter and femoral arterial line, the carotid catheters were opened and the animals were exsanguinated to a mean arterial pressure (MAP) of 45 mm. After 30 minutes in the shock state, the animals were resuscitated to a MAP of 60 mm using lactated ringers. A MAP above 60 mm was maintained throughout RES. One group received 10 mg/kg of DMM (n = 6), while the control received sham injections (n = 6). The primary end-point was SI levels. Secondary end-points included cardiac function and lactate.

RESULTS

Succinate levels increased from baseline to the 20-minute RES point in control, while the DMM cohort remained unchanged. The DMM group required less intravenous fluid to maintain a MAP above 60 (450.0 vs. 229.0 mL; p = 0.01). The DMM group had higher pulmonary capillary wedge pressure at the 20-minute and 40-minute RES points. The DMM group had better recovery of cardiac output and index during RES, while the control had no improvement. While lactate levels were similar, DMM may lead to increased ionized calcium levels.

DISCUSSION

Dimethyl malonate slows SI accumulation during HS and helps preserve cardiac filling pressures and function during RES. In addition, DMM may protect against depletion of ionized calcium. Dimethyl malonate may have therapeutic potential during HS.

Gpx3 and Egr1 Are Involved in Regulating the Differentiation Fate of Cardiac Fibroblasts under Pressure Overload

Objectives

Although myocardial fibrosis is a common pathophysiological process associated with many heart diseases, the molecular mechanisms regulating the development of fibrosis have not been fully determined. Recently, single cell RNA sequencing (scRNA-seq) analysis has been used to examine cellular fate and function during cellular differentiation and has contributed to elucidating the mechanisms of various diseases. The main purpose of this study was to characterize the fate of cardiac fibroblasts (CFs) and the dynamic gene expression patterns in a model of cardiac pressure overload using scRNA-seq analysis.

Methods

The public scRNA-seq dataset of the transverse aortic coarctation (TAC) model in mice was downloaded from the GEO database, GSE155882. First, we performed quality control, dimensionality reduction, clustering, and annotation of the data through the Seurat R package (v4.0.5). Then, we constructed the pseudotime trajectory of cell development and identified key regulatory genes using the Monocle R package (v2.22.0). Different cell fates and groups were fully characterized by Gene Set Enrichment Analysis (GSEA) analysis and Transcription factor (TF) activity analysis. Finally, we used Cytoscape (3.9.1) to extensively examine the gene regulatory network related to cell fate.

Results

Pseudotime analysis showed that CFs differentiated into two distinct cell fates, one of which produced activated myofibroblasts, and the other which produced protective cells that were associated with reduced fibrosis levels, increased antioxidative stress responses, and the ability to promote angiogenesis. In the TAC model, activated CFs were significantly upregulated, while protective cells were downregulated. Treatment with the bromodomain inhibitor JQ1 reversed this change and improved fibrosis. Analysis of dynamic gene expression revealed that Gpx3 was significantly upregulated during cell differentiation into protective cells. Gpx3 expression was affected by JQ1 treatment. Furthermore, Gpx3 expression levels were negatively correlated with the different levels of fibrosis observed in the various treatment groups. Finally, we found that transcription factors Jun, Fos, Atf3, and Egr1 were upregulated in protective cells, especially Egr1 was predicted to be involved in the regulation of genes related to antioxidant stress and angiogenesis, suggesting a role in promoting differentiation into this cell phenotype.

Conclusions

The scRNA-seq analysis was used to characterize the dynamic changes associated with fibroblast differentiation and identified Gpx3 as a factor that might be involved in the regulation of myocardial fibrosis under cardiac pressure overload. These findings will help to further understanding of the mechanism of fibrosis and provide potential intervention targets.

Tumor necrosis factor induces pathogenic mitochondrial ROS in tuberculosis through reverse electron transport

Tumor necrosis factor (TNF) is a critical host resistance factor against tuberculosis. However, excess TNF produces susceptibility by increasing mitochondrial reactive oxygen species (mROS), which initiate a signaling cascade to cause pathogenic necrosis of mycobacterium-infected macrophages. In zebrafish, we identified the mechanism of TNF-induced mROS in tuberculosis. Excess TNF in mycobacterium-infected macrophages elevates mROS production by reverse electron transport (RET) through complex I. TNF-activated cellular glutamine uptake leads to an increased concentration of succinate, a Krebs cycle intermediate. Oxidation of this elevated succinate by complex II drives RET, thereby generating the mROS superoxide at complex I. The complex I inhibitor metformin, a widely used antidiabetic drug, prevents TNF-induced mROS and necrosis of Mycobacterium tuberculosis-infected zebrafish and human macrophages; metformin may therefore be useful in tuberculosis therapy.

Mitochondrial metabolism and bioenergetic function in an anoxic isolated adult mouse cardiomyocyte model of in vivo cardiac ischemia-reperfusion injury

Cell models of cardiac ischemia-reperfusion (IR) injury are essential to facilitate understanding, but current monolayer cell models poorly replicate the in vivo IR injury that occurs within a three-dimensional tissue. Here we show that this is for two reasons: the residual oxygen present in many cellular hypoxia models sustains mitochondrial oxidative phosphorylation; and the loss of lactate from cells into the incubation medium during ischemia enables cells to sustain glycolysis. To overcome these limitations, we incubated isolated adult mouse cardiomyocytes anoxically while inhibiting lactate efflux. These interventions recapitulated key markers of in vivo ischemia, notably the accumulation of succinate and the loss of adenine nucleotides. Upon reoxygenation after anoxia the succinate that had accumulated during anoxia was rapidly oxidized in association with extensive mitochondrial superoxide/hydrogen peroxide production and cell injury, mimicking reperfusion injury. This cell model will enable key aspects of cardiac IR injury to be assessed in vitro.

Paeoniflorin alleviates CFA-induced inflammatory pain by inhibiting TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway

BACKGROUND

Treatment of chronic inflammatory pain remains a major goal in the clinic. It is thus of prime importance to characterize inherent pathophysiological pathways to design new therapeutic strategies and analgesics for pain management. Paeoniflorin (PF), a monoterpenoid glycoside from Paeonia lactiflora Pallas plants, possesses promising anti-nociceptive property. However, therapeutic effect and underlying mechanism of action of PF on inflammatory pain have not yet been fully elucidated. In this study, we aim to investigate the analgesic effect further and clarify its mechanism of action of PF on complete freund's adjuvant (CFA)-evoked inflammatory pain.

METHODS

Twenty-four male mice were divided into 3 groups: sham, CFA, and CFA + PF groups (n = 8/group). Mice were treated with normal saline or PF (30 mg/kg) for 11 days. Footpad swelling (n = 8/group), mechanical (n = 8/group) and thermal hypersensitivity (n = 8/group) were measured to evaluate the analgesic effect of PF on CFA-injected mice. At the end of the animal experiment, blood and L4-L6 dorsal root ganglion neurons were collected to assess the therapeutic effect of PF on CFA-induced inflammatory pain. Next, hematoxylin and eosin, quantitative realtime PCR, ELISA, capsaicin and dimethyl succinate induced pain test (n = 8/group), motor coordination test (n = 8/group), tail flicking test (n = 8/group), pyruvate and succinate dehydrogenase assay (n = 6/group), immunohistochemical staining, were performed to clarify the action mechanism of PF on CFA-evoked inflammatory pain. Besides, the effect of PF on TRPV1 was evaluated by whole-cell patch clamp recording on primary neurons (n = 7). Finally, molecular docking further performed to evaluate the binding ability of PF to TRPV1.

RESULTS

PF significantly relieved inflammatory pain (P < 0.001) and paw edema (P < 0.001) on a complete Freund adjuvant (CFA)-induced peripheral inflammatory pain model. Furthermore, PF inhibited neutrophil infiltration (P < 0.01), IL-1β increase (P < 0.01), and pain-related peptide substance P release (P < 0.001). Intriguingly, CFA-induced succinate aggregation was notably reversed by PF via modulating pyruvate and SDH activity (P < 0.01). In addition, PF dampened the high expression of subsequent succinate receptor SUCNR1 (P < 0.01), HIF-1α (P < 0.05), as well as the activation of NLPR3 inflammasome (P < 0.05) and TRPV1 (P < 0.05). More importantly, both capsaicin and dimethyl succinate supplementation obviously counteracted the pain-relieving effect of PF and TRPV1 (P < 0.01 or P < 0.001).

CONCLUSION

Our findings suggest that PF can significantly relieve CFA-induced paw swelling, as well as mechanical and thermal hyperalgesia. PF alleviated inflammatory pain partly through inhibiting the activation of TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway. Furthermore, we found that PF exerted its analgesic effect without affecting motor coordination and pain-related cold ion-channels. In summary, this study may provide valuable evidence for the potential application of PF as therapeutic strategy for inflammatory pain treatment.

Mitochondrial respiratory chain and Krebs cycle enzyme function in human donor livers subjected to end-ischaemic hypothermic machine perfusion

INTRODUCTION

Marginal human donor livers are highly susceptible to ischaemia reperfusion injury and mitochondrial dysfunction. Oxygenation during hypothermic machine perfusion (HMP) was proposed to protect the mitochondria but the mechanism is unclear. Additionally, the distribution and uptake of perfusate oxygen during HMP are unknown. This study aimed to examine the feasibility of mitochondrial function analysis during end-ischaemic HMP, assess potential mitochondrial viability biomarkers, and record oxygenation kinetics.

METHODS

This was a randomised pilot study using human livers retrieved for transplant but not utilised. Livers (n = 38) were randomised at stage 1 into static cold storage (n = 6), hepatic artery HMP (n = 7), and non-oxygen supplemented portal vein HMP (n = 7) and at stage 2 into oxygen supplemented and non-oxygen supplemented portal vein HMP (n = 11 and 7, respectively). Mitochondrial parameters were compared between the groups and between low- and high-risk marginal livers based on donor history, organ steatosis and preservation period. The oxygen delivery efficiency was assessed in additional 6 livers using real-time measurements of perfusate and parenchymal oxygen.

RESULTS

The change in mitochondrial respiratory chain (complex I, II, III, IV) and Krebs cycle enzyme activity (aconitase, citrate synthase) before and after 4-hour preservation was not different between groups in both study stages (p > 0.05). Low-risk livers that could have been used clinically (n = 8) had lower complex II-III activities after 4-hour perfusion, compared with high-risk livers (73 nmol/mg/min vs. 113 nmol/mg/min, p = 0.01). Parenchymal pO2 was consistently lower than perfusate pO2 (p ≤ 0.001), stabilised in 28 minutes compared to 3 minutes in perfusate (p = 0.003), and decreased faster upon oxygen cessation (75 vs. 36 minutes, p = 0.003).

CONCLUSIONS

Actively oxygenated and air-equilibrated end-ischaemic HMP did not induce oxidative damage of aconitase, and respiratory chain complexes remained intact. Mitochondria likely respond to variable perfusate oxygen levels by adapting their respiratory function during end-ischaemic HMP. Complex II-III activities should be further investigated as viability biomarkers.

Hetero-Trifunctional Malonate-Based Nanotheranostic System for Targeted Breast Cancer Therapy

Designing multifunctional linkers is crucial for tricomponent theranostic targeted nanomedicine development as they are essential to enrich polymeric systems with different functional moieties. Herein, we have obtained a hetero-trifunctional linker from malonic acid and demonstrated its implication as an amphiphilic targeted nanotheranostic system (CB DX UN PG FL). We synthesized it with varying hydrophilic segment to fine-tune the hydrophobic/hydrophilic ratio to optimize its self-assembly. pH-responsive hydrazone-linked doxorubicin was conjugated to the backbone (UN PG FL) containing folate as a targeting ligand. Cobalt carbonyl complex was used for T₂-weighted magnetic resonance imaging (MRI). Electron micrographs of optimized molecule CB DX UN PG_(4 kDa) FL in an aqueous system have demonstrated about 50-60 nm-sized uniform micelles. The relaxivity study and the one-dimensional (1D) imaging experiments clearly revealed the effect of the nanotheranostics system on transverse relaxation (T₂) of water molecules, which validated the system as a T₂-weighted MRI contrast agent. The detailed in vitro biological studies validated the targeted delivery and anticancer potential of CB DX UN PG_(4 kDa) FL. Combining the data on transverse relaxation, folate mediated uptake, and anticancer activity, the designed molecule will have a significant impact on the development of targeted theranostic.

Cellular crosstalk in cardioprotection: Where and when do reactive oxygen species play a role?

A well-balanced intercellular communication between the different cells within the heart is vital for the maintenance of cardiac homeostasis and function. Despite remarkable advances on disease management and treatment, acute myocardial infarction remains the major cause of morbidity and mortality worldwide. Gold standard reperfusion strategies, namely primary percutaneous coronary intervention, are crucial to preserve heart function. However, reestablishment of blood flow and oxygen levels to the infarcted area are also associated with an accumulation of reactive oxygen species (ROS), leading to oxidative damage and cardiomyocyte death, a phenomenon termed myocardial reperfusion injury. In addition, ROS signaling has been demonstrated to regulate multiple biological pathways, including cell differentiation and intercellular communication. Given the importance of cell-cell crosstalk in the coordinated response after cell injury, in this review, we will discuss the impact of ROS in the different forms of inter- and intracellular communication, as well as the role of gap junctions, tunneling nanotubes and extracellular vesicles in the propagation of oxidative damage in cardiac diseases, particularly in the context of ischemia/reperfusion injury.

Citric Acid Cycle Metabolites Predict Infarct Size in Pigs Submitted to Transient Coronary Artery Occlusion and Treated with Succinate Dehydrogenase Inhibitors or Remote Ischemic Perconditioning

Succinate dehydrogenase (SDH) inhibition with malonate during reperfusion reduced myocardial infarction in animals, whereas its endogenous substrate, succinate, is detected in plasma from STEMI patients. We investigated whether protection by SDH inhibition is additive to that of remote ischemic perconditioning (RIC) in pigs submitted to transient coronary artery occlusion, and whether protective maneuvers influence plasma levels of citric acid cycle metabolites. Forty pigs were submitted to 40 min coronary occlusion and reperfusion, and allocated to four groups (controls, sodium malonate 10 mmol/L, RIC, and malonate + RIC). Plasma was obtained from femoral and great cardiac veins and analyzed by LC-MS/MS. Malonate, RIC, and malonate + RIC reduced infarct size (24.67 ± 5.98, 25.29 ± 3.92 and 29.83 ± 4.62% vs. 46.47 ± 4.49% in controls, p < 0.05), but no additive effects were detected. Enhanced concentrations of succinate, fumarate, malate and citrate were observed in controls during initial reperfusion in the great cardiac vein, and most were reduced by cardioprotective maneuvers. Concentrations of succinate, fumarate, and malate significantly correlated with infarct size. In conclusion, despite the combination of SDH inhibition during reperfusion and RIC did not result in additive protection, plasma concentrations of selected citric acid cycle metabolites are attenuated by protective maneuvers, correlate with irreversible injury, and might become a prognosis tool in STEMI patients.

Mitochondrial redox and TCA cycle metabolite signaling in the heart

Mitochondria are essential signaling organelles that regulate a broad range of cellular processes and thereby heart function. Multiple mechanisms participate in the communication between mitochondria and the nucleus that maintain cardiomyocyte homeostasis, including mitochondrial reactive oxygen species (ROS) and metabolic shifts in TCA cycle metabolite availability. An increased rate of ROS generation can cause irreversible damage to the cell and proposed to be a leading cause of many pathologies, including accelerated aging and heart disease. Myocardial impairments are also characterised by specific coordinated metabolic changes and dysregulated inflammatory responses. Hence, the mitochondrial respiratory chain is an important mediator between health and disease in the heart. This review will first outline the sources of ROS in the heart, mitochondrial metabolite dynamics, and provide an overview of their implications for heart disease. In addition, we will concentrate our discussion around current cardioprotective strategies relevant to mitochondrial ROS. Thorough understanding of mitochondrial signaling and the complex interplay with vital signaling pathways in the heart might allow us to develop novel therapeutic approaches to cardiovascular disease.

Structural basis for a complex I mutation that blocks pathological ROS production

Mitochondrial complex I is central to the pathological reactive oxygen species (ROS) production that underlies cardiac ischemia-reperfusion (IR) injury. ND6-P25L mice are homoplasmic for a disease-causing mtDNA point mutation encoding the P25L substitution in the ND6 subunit of complex I. The cryo-EM structure of ND6-P25L complex I revealed subtle structural changes that facilitate rapid conversion to the "deactive" state, usually formed only after prolonged inactivity. Despite its tendency to adopt the "deactive" state, the mutant complex is fully active for NADH oxidation, but cannot generate ROS by reverse electron transfer (RET). ND6-P25L mitochondria function normally, except for their lack of RET ROS production, and ND6-P25L mice are protected against cardiac IR injury in vivo. Thus, this single point mutation in complex I, which does not affect oxidative phosphorylation but renders the complex unable to catalyse RET, demonstrates the pathological role of ROS production by RET during IR injury.

NLR in eXile: Emerging roles of NLRX1 in immunity and human disease

NLRX1 is a member of the NOD-like receptor family, a set of pattern recognition receptors associated with innate immunity. Interestingly, NLRX1 exists in somewhat of an exile from its NLR counterparts with unique features that mediate atypical functions compared with traditional NOD-like receptors (NLRs). Aside from a mitochondrial targeting sequence, the N-terminal region is yet to be characterized. Mitochondrially located, NLRX1 sits within a subgroup of regulatory NLRs responsible for negatively regulating cellular inflammatory signalling. As well as modulating pathogen response, emerging evidence is implicating NLRX1 as a central homeostatic gatekeeper between mitochondrial biology and immunological response. More recently, NLRX1 has been implicated in a wide range of disease, both pathogen-driven and otherwise. Emerging links of NLRX1 in cancer biology, autoimmunity and other inflammatory conditions are raising the potential of targeting NLRX1 therapeutically, with recent studies in inflammatory bowel disease showing great promise. Within this review, we address the unique features of NLRX1, its roles in innate immune signalling and its involvement in a range of inflammatory, metabolic and oncology disease indications with a focus on areas that could benefit from therapeutic targeting of NLRX1.

Acid enhancement of ROS generation by complex-I reverse electron transport is balanced by acid inhibition of complex-II: Relevance for tissue reperfusion injury

Generation of mitochondrial reactive oxygen species (ROS) is an important process in triggering cellular necrosis and tissue infarction during ischemia-reperfusion (IR) injury. Ischemia results in accumulation of the metabolite succinate. Rapid oxidation of this succinate by mitochondrial complex II (Cx-II) during reperfusion reduces the co-enzyme Q (Co-Q) pool, thereby driving electrons backward into complex-I (Cx-I), a process known as reverse electron transport (RET), which is thought to be a major source of ROS. During ischemia, enhanced glycolysis results in an acidic cellular pH at the onset of reperfusion. While the process of RsET within Cx-I is known to be enhanced by a high mitochondrial trans-membrane ΔpH, the impact of pH itself on the integrated process of Cx-II to Cx-I RET has not been fully studied. Using isolated mouse heart and liver mitochondria under conditions which mimic the onset of reperfusion (i.e., high [ADP]), we show that mitochondrial respiration (state 2 and state 3) as well as isolated Cx-II activity are impaired at acidic pH, whereas the overall generation of ROS by Cx-II to Cx-I RET was insensitive to pH. Together these data indicate that the acceleration of Cx-I RET ROS by ΔpH appears to be cancelled out by the impact of pH on the source of electrons, i.e. Cx-II. Implications for the role of Cx-II to Cx-I RET derived ROS in IR injury are discussed.

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ROS from complex I (Cx-I) reverse electron transport (RET) is enhanced at acidic pH.

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Mitochondrial complex II (Cx-II) activity is inhibited at acidic pH.

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These effects cancel out, yielding no net pH response of Cx-II to Cx-I RET ROS.

Selective Delivery of Dicarboxylates to Mitochondria by Conjugation to a Lipophilic Cation via a Cleavable Linker

Many mitochondrial metabolites and bioactive molecules contain two carboxylic acid moieties that make them unable to cross biological membranes. Hence, there is considerable interest in facilitating the uptake of these molecules into cells and mitochondria to modify or report on their function. Conjugation to the triphenylphosphonium (TPP) lipophilic cation is widely used to deliver molecules selectively to mitochondria in response to the membrane potential. However, permanent attachment to the cation can disrupt the biological function of small dicarboxylates. Here, we have developed a strategy using TPP to release dicarboxylates selectively within mitochondria. For this, the dicarboxylate is attached to a TPP compound via a single ester bond, which is then cleaved by intramitochondrial esterase activity, releasing the dicarboxylate within the organelle. Leaving the second carboxylic acid free also means mitochondrial uptake is dependent on the pH gradient across the inner membrane. To assess this strategy, we synthesized a range of TPP monoesters of the model dicarboxylate, malonate. We then tested their mitochondrial accumulation and ability to deliver malonate to isolated mitochondria and to cells, in vitro and in vivo. A TPP–malonate monoester compound, TPP₁₁–malonate, in which the dicarboxylate group was attached to the TPP compound via a hydrophobic undecyl link, was most effective at releasing malonate within mitochondria in cells and in vivo. Therefore, we have developed a TPP–monoester platform that enables the selective release of bioactive dicarboxylates within mitochondria.