The mitochondrial permeability transition pore was originally described in the 1970’s as a Ca2+ activated pore and has since been attributed to the pathogenesis of many diseases. Here we evaluate how each of the current models of the pore complex fit to what is known about how Ca2+ regulates the pore, and any insight that provides into the molecular identity of the pore complex. We also discuss the central role of Ca2+ in modulating the pore’s open probability by directly regulating processes, such as ATP/ADP balance through the tricarboxylic acid cycle, electron transport chain, and mitochondrial membrane potential. We review how Ca2+ influences second messengers such as reactive oxygen/nitrogen species production and polyphosphate formation. We discuss the evidence for how Ca2+ regulates post-translational modification of cyclophilin D including phosphorylation by glycogen synthase kinase 3 beta, deacetylation by sirtuins, and oxidation/nitrosylation of key residues. Lastly we introduce a novel view into how Ca2+ activated proteolysis through calpains in the mitochondria may be a driver of sustained pore opening during pathologies such as ischemia reperfusion injury.

Aims: Mitochondrial Ca 2+ homeostasis is crucial for balancing cell survival and death. The recent discovery of the molecular identity of the mitochondrial Ca 2+ uniporter pore (MCU) opens new possibilities for applying genetic approaches to study mitochondrial Ca 2+ regulation in various cell types, including cardiac myocytes. Basal tyrosine phosphorylation of MCU was reported from mass spectroscopy of human and mouse tissues, but the signaling pathways that regulate mitochondrial Ca 2+ entry through posttranslational modifications of MCU are completely unknown. Therefore, we investigated a 1 -adrenergic-mediated signal transduction of MCU posttranslational modification and function in cardiac cells. Results: a 1 -adrenoceptor (a 1 -AR) signaling translocated activated proline-rich tyrosine kinase 2 (Pyk2) from the cytosol to mitochondrial matrix and accelerates mitochondrial Ca 2+ uptake via Pyk2-dependent MCU phosphorylation and tetrametric MCU channel pore formation. Moreover, we found that a 1 -AR stimulation increases reactive oxygen species production at mitochondria, mitochondrial permeability transition pore activity, and initiates apoptotic signaling via Pyk2-dependent MCU activation and mitochondrial Ca 2+ overload. Innovation: Our data indicate that inhibition of a 1 -AR-Pyk2-MCU signaling represents a potential novel therapeutic target to limit or prevent mitochondrial Ca 2+ overload, oxidative stress, mitochondrial injury, and myocardial death during pathophysiological conditions, where chronic adrenergic stimulation is present. Conclusion: The a 1 -AR-Pyk2-dependent tyrosine phosphorylation of the MCU regulates mitochondrial Ca 2+ entry and apoptosis in cardiac cells. Antioxid. Redox Signal. 21, 863-879.

Engagement of the T-cell receptor (TCR) initiates

ϩ influx to mitochondria is an important trigger for both mitochondrial dynamics and ATP generation in various cell types, including cardiac cells. Mitochondrial Ca 2ϩ influx is mainly mediated by the mitochondrial Ca 2ϩ uniporter (MCU). Growing evidence also indicates that mitochondrial Ca 2ϩ influx mechanisms are regulated not solely by MCU but also by multiple channels/transporters. We have previously reported that skeletal muscle-type ryanodine receptor (RyR) type 1 (RyR1), which expressed at the mitochondrial inner membrane, serves as an additional Ca 2ϩ uptake pathway in cardiomyocytes. However, it is still unclear which mitochondrial Ca 2ϩ influx mechanism is the dominant regulator of mitochondrial morphology/dynamics and energetics in cardiomyocytes. To investigate the role of mitochondrial RyR1 in the regulation of mitochondrial morphology/function in cardiac cells, RyR1 was transiently or stably overexpressed in cardiac H9c2 myoblasts. We found that overexpressed RyR1 was partially localized in mitochondria as observed using both immunoblots of mitochondrial fractionation and confocal microscopy, whereas RyR2, the main RyR isoform in the cardiac sarcoplasmic reticulum, did not show any expression at mitochondria. Interestingly, overexpression of RyR1 but not MCU or RyR2 resulted in mitochondrial fragmentation. These fragmented mitochondria showed bigger and sustained mitochondrial Ca 2ϩ transients compared with basal tubular mitochondria. In addition, RyR1-overexpressing cells had a higher mitochondrial ATP concentration under basal conditions and showed more ATP production in response to cytosolic Ca 2ϩ elevation compared with nontransfected cells as observed by a matrix-targeted ATP biosensor. These results indicate that RyR1 possesses a mitochondrial targeting/retention signal and modulates mitochondrial morphology and Ca 2ϩ -induced ATP production in cardiac H9c2 myoblasts. fluorescence resonance energy transfer; mitochondrial Ca 2ϩ uniporter; mitochondria; mitochondrial morphology; ryanodine receptor type 1 MITOCHONDRIAL Ca 2ϩ is critical for the regulation of various cellular functions, including energy metabolism, ROS generation, spatiotemporal dynamics of Ca 2ϩ signaling, and cell growth/development and death (11,18,23). Historically, Ca 2ϩ was found to be accumulated by mitochondria over 5 decades ago (for reviews, see Refs. 24, 64, and 69), and, shortly thereafter, it was also recognized that Ca 2ϩ stimulates the oxidative phosphorylation [tricarboxylic acid (TCA) cycle] and electron transport chain activity, which results in the stimulation of ATP synthesis (for a review, see Ref. 23). Additionally, the coexistence of mitochondrial dysfunction and loss of cellular Ca 2ϩ homeostasis are frequently observed in various cardiovascular diseases, but it is still not clear how altered mitochondrial Ca 2ϩ handing and/or mitochondrial dysfunction are involved in the pathogenesis of each different disease setting (23,33,56).Although the basic functional and pharmacological properties of mitochondrial ...

Significance: Mitochondrial ion channels/transporters and the electron transport chain (ETC) serve as key sensors and regulators for cellular redox signaling, the production of reactive oxygen species (ROS) and nitrogen species (RNS) in mitochondria, and balancing cell survival and death. Although the functional and pharmacological characteristics of mitochondrial ion transport mechanisms have been extensively studied for several decades, the majority of the molecular identities that are responsible for these channels/transporters have remained a mystery until very recently. Recent Advances: Recent breakthrough studies uncovered the molecular identities of the diverse array of major mitochondrial ion channels/transporters, including the mitochondrial Ca 2+ uniporter pore, mitochondrial permeability transition pore, and mitochondrial ATP-sensitive K + channel. This new information enables us to form detailed molecular and functional characterizations of mitochondrial ion channels/transporters and their roles in mitochondrial redox signaling. Critical Issues: Redox-mediated post-translational modifications of mitochondrial ion channels/transporters and ETC serve as key mechanisms for the spatiotemporal control of mitochondrial ROS/RNS generation. Future Directions: Identification of detailed molecular mechanisms for redox-mediated regulation of mitochondrial ion channels will enable us to find novel therapeutic targets for many diseases that are associated with cellular redox signaling and mitochondrial ion channels/transporters. Antioxid. Redox Signal. 21, 987-1006.