ER-mitochondria distance is a critical parameter for efficient mitochondrial Ca2+ uptake and oxidative metabolism.

IP3 receptor (IP3R)-mediated Ca2+ transfer at the mitochondria-endoplasmic reticulum (ER) contact sites (MERCS) drives mitochondrial Ca2+ uptake and oxidative metabolism and is linked to different pathologies, including Parkinson's disease (PD). The dependence of Ca2+ transfer efficiency on the ER-mitochondria distance remains unexplored. Employing molecular rulers that stabilize ER-mitochondrial distances at 5 nm resolution, and using genetically encoded Ca2+ indicators targeting the ER lumen and the sub-mitochondrial compartments, we now show that a distance of ~20 nm is optimal for Ca2+ transfer and mitochondrial oxidative metabolism due to enrichment of IP3R at MERCS. In human iPSC-derived astrocytes from PD patients, 20 nm MERCS were specifically reduced, which correlated with a reduction of mitochondrial Ca2+ uptake. Stabilization of the ER-mitochondrial interaction at 20 nm, but not at 10 nm, fully rescued mitochondrial Ca2+ uptake in PD astrocytes. Our work determines with precision the optimal distance for Ca2+ flux between ER and mitochondria and suggests a new paradigm for fine control over mitochondrial function.

Imeglimin Is Neuroprotective Against Ischemic Brain Injury in Rats-a Study Evaluating Neuroinflammation and Mitochondrial Functions.

Imeglimin is a novel oral antidiabetic drug modulating mitochondrial functions. However, neuroprotective effects of this drug have not been investigated. The aim of this study was to investigate effects of imeglimin against ischemia-induced brain damage and neurological deficits and whether it acted via inhibition of mitochondrial permeability transition pore (mPTP) and suppression of microglial activation. Ischemia in rats was induced by permanent middle cerebral artery occlusion (pMCAO) for 48 h. Imeglimin (135 µg/kg/day) was injected intraperitoneally immediately after pMCAO and repeated after 24 h. Immunohistochemical staining was used to evaluate total numbers of neurons, astrocytes, and microglia as well as interleukin-10 (IL-10) producing cells in brain slices. Respiration of isolated brain mitochondria was assessed using high-resolution respirometry. Assessment of ionomycin-induced mPTP opening in intact cultured primary rat neuronal, astrocytic, and microglial cells was performed using fluorescence microscopy. Treatment with imeglimin significantly decreased infarct size, brain edema, and neurological deficits after pMCAO. Moreover, imeglimin protected against pMCAO-induced neuronal loss as well as microglial proliferation and activation, and increased the number of astrocytes and the number of cells producing anti-inflammatory cytokine IL-10 in the ischemic hemisphere. Imeglimin in vitro acutely prevented mPTP opening in cultured neurons and astrocytes but not in microglial cells; however, treatment with imeglimin did not prevent ischemia-induced mitochondrial respiratory dysfunction after pMCAO. This study demonstrates that post-stroke treatment with imeglimin exerts neuroprotective effects by reducing infarct size and neuronal loss possibly via the resolution of neuroinflammation and partly via inhibition of mPTP opening in neurons and astrocytes.

Neuroprotective Effect of a Novel ATP-Synthase Inhibitor Bedaquiline in Cerebral Ischemia-Reperfusion Injury.

Mitochondrial dysfunction during ischemic stroke ultimately manifests as ATP depletion. Mitochondrial ATP synthase upon loss of mitochondrial membrane potential during ischemia rapidly hydrolyses ATP and thus contributes to ATP depletion. Increasing evidence suggests that inhibition of ATP synthase limits ATP depletion and is protective against ischemic tissue damage. Bedaquiline (BDQ) is an anti-microbial agent, approved for clinical use, that inhibits ATP synthase of Mycobacteria; however recently it has been shown to act on mitochondrial ATP synthase, inhibiting both ATP synthesis and hydrolysis in low micromolar concentrations. In this study, we investigated whether preconditioning with BDQ can alleviate ischemia/reperfusion-induced brain injury in Wistar rats after middle cerebral artery occlusion-reperfusion and whether it affects mitochondrial functions. We found that BDQ was effective in limiting necrosis and neurological dysfunction during ischemia-reperfusion. BDQ also caused inhibition of ATPase activity, mild uncoupling of respiration, and stimulated mitochondrial respiration both in healthy and ischemic mitochondria. Mitochondrial calcium retention capacity was unaffected by BDQ preconditioning. We concluded that BDQ has neuroprotective properties associated with its action on mitochondrial respiration and ATPase activity.

Effects of itaconic acid on neuronal viability and brain mitochondrial functions.

Recent studies have identified that under stimulation by bacterial lipopolysaccharide mammalian macrophages produce itaconic acid. Yet, it is unknown whether itaconate has any effect on viability of brain cells. Here we used extracellularly added itaconate to investigate its effects on viability of cerebellar granule cells (CGC) in cultures and respiratory functions of these cells and isolated brain mitochondria. We found that 3-5 mM itaconate had no effect on the viability of neurons, but 10 mM itaconate was toxic and induced neuronal apoptosis. Removal of itaconate after 24 h incubation resulted in further decrease in viability and number of neurons. Respiration of intact neurons was not affected by itaconate, but permeabilized cells as well as isolated brain mitochondria demonstrated decreased rates of respiration in the presence of itaconate. Using isolated adult rat brain mitochondria we found that itaconate decreased mitochondrial phosphorylating respiration, mitochondrial calcium retention capacity, production of reactive oxygen species with Complex I and Complex II substrates as well as inhibition of Complex I, Complex IV and ATP synthase. In conclusion, the results suggest that itaconic acid at millimolar concentrations affects mitochondrial functions and viability of neurons.

Nitric Oxide Donor NOC-18-Induced Changes of Mitochondrial Phosphoproteome in Rat Cardiac Ischemia Model.

Background and objective: Nitric oxide (NO) is known to exert cardioprotective effects against heart ischemic damage and may be involved in ischemic pre- and postconditioning. NO-triggered cardioprotective mechanisms are not well understood but may involve regulation of mitochondrial permeability transition pore (mPTP). In this study, we aimed to identify differentially phosphorylated mitochondrial proteins possibly involved in the NO/protein kinase G (PKG)/mPTP signaling pathway that can increase the resistance of cardiomyocytes to ischemic damage. Materials and methods: Isolated hearts from Wistar rats were perfused with NO donor NOC-18 prior to induction of stop-flow ischemia. To quantify and characterize the phosphoproteins, mitochondrial proteins were resolved and analyzed by two-dimensional gel electrophoresis followed by Pro-Q Diamond phosphoprotein gel staining, excision, trypsin digestions, and mass spectrometry. Quantitative proteomic analysis coupled with liquid chromatography-tandem mass spectrometry was also performed. Results: Mitochondrial protein phosphorylation patterns in NOC-18-pretreated ischemic hearts versus ischemic hearts were compared. Pretreatment of hearts with NOC-18 caused changes in mitochondrial phosphoproteome after ischemia which involved modifications of 10 mitochondrial membrane-bound and 10 matrix proteins. Among them, α-subunit of ATP synthase and adenine nucleotide (ADP/ATP) translocase 1, both of which are considered as potential structural components of mPTP, were identified. We also found that treatment of isolated non-ischemic mitochondria with recombinant PKG did not cause the same protein phosphorylation as pretreatment of hearts with NOC-18. Conclusions: Our study suggests that pretreatment of hearts with NOC-18 causes changes in mitochondrial phosphoproteome after ischemia which involves modifications of certain proteins thought to be involved in the regulation of mPTP opening and intracellular redox state. These proteins may be potential targets for pharmacological preconditioning of the heart.