TMEM65 regulates NCLX-dependent mitochondrial calcium efflux.

The balance between mitochondrial calcium (mCa2+) uptake and efflux regulates ATP production, but if perturbed causes energy starvation or mCa2+ overload and cell death. The mitochondrial sodium-calcium exchanger, NCLX, is a critical route of mCa2+ efflux in excitable tissues, such as the heart and brain, and animal models support NCLX as a promising therapeutic target to limit pathogenic mCa2+ overload. However, the mechanisms that regulate NCLX activity remain largely unknown. We used proximity biotinylation proteomic screening to identify the NCLX interactome and define novel regulators of NCLX function. Here, we discover the mitochondrial inner membrane protein, TMEM65, as an NCLX-proximal protein that potently enhances sodium (Na+)-dependent mCa2+ efflux. Mechanistically, acute pharmacologic NCLX inhibition or genetic deletion of NCLX ablates the TMEM65-dependent increase in mCa2+ efflux. Further, loss-of-function studies show that TMEM65 is required for Na+-dependent mCa2+ efflux. Co-fractionation and in silico structural modeling of TMEM65 and NCLX suggest these two proteins exist in a common macromolecular complex in which TMEM65 directly stimulates NCLX function. In line with these findings, knockdown of Tmem65 in mice promotes mCa2+ overload in the heart and skeletal muscle and impairs both cardiac and neuromuscular function. We further demonstrate that TMEM65 deletion causes excessive mitochondrial permeability transition, whereas TMEM65 overexpression protects against necrotic cell death during cellular Ca2+ stress. Collectively, our results show that loss of TMEM65 function in excitable tissue disrupts NCLX-dependent mCa2+ efflux, causing pathogenic mCa2+ overload, cell death and organ-level dysfunction, and that gain of TMEM65 function mitigates these effects. These findings demonstrate the essential role of TMEM65 in regulating NCLX-dependent mCa2+ efflux and suggest modulation of TMEM65 as a novel strategy for the therapeutic control of mCa2+ homeostasis.

MCU gain- and loss-of-function models define the duality of mitochondrial calcium uptake in heart failure.

Background: Mitochondrial calcium (mCa2+) uptake through the mitochondrial calcium uniporter channel (mtCU) stimulates metabolism to meet acute increases in cardiac energy demand. However, excessive mCa2+ uptake during stress, as in ischemia-reperfusion, initiates permeability transition and cell death. Despite these often-reported acute physiological and pathological effects, a major unresolved controversy is whether mtCU-dependent mCa2+ uptake and long-term elevation of cardiomyocyte mCa2+ contributes to the heart's adaptation during sustained increases in workload. Objective: We tested the hypothesis that mtCU-dependent mCa2+ uptake contributes to cardiac adaptation and ventricular remodeling during sustained catecholaminergic stress. Methods: Mice with tamoxifen-inducible, cardiomyocyte-specific gain (αMHC-MCM × flox-stop-MCU; MCU-Tg) or loss (αMHC-MCM × Mcufl/fl; Mcu-cKO) of mtCU function received 2-wk catecholamine infusion. Results: Cardiac contractility increased after 2d of isoproterenol in control, but not Mcu-cKO mice. Contractility declined and cardiac hypertrophy increased after 1-2-wk of isoproterenol in MCU-Tg mice. MCU-Tg cardiomyocytes displayed increased sensitivity to Ca2+- and isoproterenol-induced necrosis. However, loss of the mitochondrial permeability transition pore (mPTP) regulator cyclophilin D failed to attenuate contractile dysfunction and hypertrophic remodeling, and increased isoproterenol-induced cardiomyocyte death in MCU-Tg mice. Conclusions: mtCU mCa2+ uptake is required for early contractile responses to adrenergic signaling, even those occurring over several days. Under sustained adrenergic load excessive MCU-dependent mCa2+ uptake drives cardiomyocyte dropout, perhaps independent of classical mitochondrial permeability transition pore opening, and compromises contractile function. These findings suggest divergent consequences for acute versus sustained mCa2+ loading, and support distinct functional roles for the mPTP in settings of acute mCa2+ overload versus persistent mCa2+ stress.

Cortical bone stem cell-derived exosomes' therapeutic effect on myocardial ischemia-reperfusion and cardiac remodeling.

Heart failure is the one of the leading causes of death in the United States. Heart failure is a complex syndrome caused by numerous diseases, including severe myocardial infarction (MI). MI occurs after an occlusion of a cardiac artery causing downstream ischemia. MI is followed by cardiac remodeling involving extensive remodeling and fibrosis, which, if the original insult is severe or prolonged, can ultimately progress into heart failure. There is no "cure" for heart failure because therapies to regenerate dead tissue are not yet available. Previous studies have shown that in both post-MI and post-ischemia-reperfusion (I/R) models of heart failure, administration of cortical bone stem cell (CBSC) treatment leads to a reduction in scar size and improved cardiac function. Our first study investigated the ability of mouse CBSC-derived exosomes (mCBSC-dEXO) to recapitulate mouse CBSCs (mCBSC) therapeutic effects in a 24-h post-I/R model. This study showed that injection of mCBSCs and mCBSC-dEXOs into the ischemic region of an infarct had a protective effect against I/R injury. mCBSC-dEXOs recapitulated the effects of CBSC treatment post-I/R, indicating exosomes are partly responsible for CBSC's beneficial effects. To examine if exosomes decrease fibrotic activation, adult rat ventricular fibroblasts (ARVFs) and adult human cardiac fibroblasts (NHCFs) were treated with transforming growth factor ß (TGFß) to activate fibrotic signaling before treatment with mCBSC- and human CBSC (hCBSC)-dEXOs. hCBSC-dEXOs caused a 100-fold decrease in human fibroblast activation. To further understand the signaling mechanisms regulating the protective decrease in fibrosis, we performed RNA sequencing on the NHCFs after hCBSC-dEXO treatment. The group treated with both TGFß and exosomes showed a decrease in small nucleolar RNA (snoRNA), known to be involved with ribosome stability.NEW & NOTEWORTHY Our work is noteworthy due to the identification of factors within stem cell-derived exosomes (dEXOs) that alter fibroblast activation through the hereto-unknown mechanism of decreasing small nucleolar RNA (snoRNA) signaling within cardiac fibroblasts. The study also shows that the injection of stem cells or a stem-cell-derived exosome therapy at the onset of reperfusion elicits cardioprotection, emphasizing the importance of early treatment in the post-ischemia-reperfusion (I/R) wounded heart.

Molecular Signature of HFpEF: Systems Biology in a Cardiac-Centric Large Animal Model.

In this study the authors used systems biology to define progressive changes in metabolism and transcription in a large animal model of heart failure with preserved ejection fraction (HFpEF). Transcriptomic analysis of cardiac tissue, 1-month post-banding, revealed loss of electron transport chain components, and this was supported by changes in metabolism and mitochondrial function, altogether signifying alterations in oxidative metabolism. Established HFpEF, 4 months post-banding, resulted in changes in intermediary metabolism with normalized mitochondrial function. Mitochondrial dysfunction and energetic deficiencies were noted in skeletal muscle at early and late phases of disease, suggesting cardiac-derived signaling contributes to peripheral tissue maladaptation in HFpEF. Collectively, these results provide insights into the cellular biology underlying HFpEF progression.

Mitochondrial calcium exchange links metabolism with the epigenome to control cellular differentiation.

Fibroblast to myofibroblast differentiation is crucial for the initial healing response but excessive myofibroblast activation leads to pathological fibrosis. Therefore, it is imperative to understand the mechanisms underlying myofibroblast formation. Here we report that mitochondrial calcium (mCa2+) signaling is a regulatory mechanism in myofibroblast differentiation and fibrosis. We demonstrate that fibrotic signaling alters gating of the mitochondrial calcium uniporter (mtCU) in a MICU1-dependent fashion to reduce mCa2+ uptake and induce coordinated changes in metabolism, i.e., increased glycolysis feeding anabolic pathways and glutaminolysis yielding increased α-ketoglutarate (αKG) bioavailability. mCa2+-dependent metabolic reprogramming leads to the activation of αKG-dependent histone demethylases, enhancing chromatin accessibility in loci specific to the myofibroblast gene program, resulting in differentiation. Our results uncover an important role for the mtCU beyond metabolic regulation and cell death and demonstrate that mCa2+ signaling regulates the epigenome to influence cellular differentiation.