Why F-ATP Synthase Remains a Strong Candidate as the Mitochondrial Permeability Transition Pore

The multifaceted role of mitochondria in autism spectrum disorder

Normal brain functioning relies on high aerobic energy production provided by mitochondria. Failure to supply a sufficient amount of energy, seen in different brain disorders, including autism spectrum disorder (ASD), may have a significant negative impact on brain development and support of different brain functions. Mitochondrial dysfunction, manifested in the abnormal activities of the electron transport chain and impaired energy metabolism, greatly contributes to ASD. The aberrant functioning of this organelle is of such high importance that ASD has been proposed as a mitochondrial disease. It should be noted that aerobic energy production is not the only function of the mitochondria. In particular, these organelles are involved in the regulation of Ca2+ homeostasis, different mechanisms of programmed cell death, autophagy, and reactive oxygen and nitrogen species (ROS and RNS) production. Several syndromes originated from mitochondria-related mutations display ASD phenotype. Abnormalities in Ca2+ handling and ATP production in the brain mitochondria affect synaptic transmission, plasticity, and synaptic development, contributing to ASD. ROS and Ca2+ regulate the activity of the mitochondrial permeability transition pore (mPTP). The prolonged opening of this pore affects the redox state of the mitochondria, impairs oxidative phosphorylation, and activates apoptosis, ultimately leading to cell death. A dysregulation between the enhanced mitochondria-related processes of apoptosis and the inhibited autophagy leads to the accumulation of toxic products in the brains of individuals with ASD. Although many mitochondria-related mechanisms still have to be investigated, and whether they are the cause or consequence of this disorder is still unknown, the accumulating data show that the breakdown of any of the mitochondrial functions may contribute to abnormal brain development leading to ASD. In this review, we discuss the multifaceted role of mitochondria in ASD from the various aspects of neuroscience.

Cardiac troponin I directly binds and inhibits mitochondrial ATP synthase with a noncanonical role in the post-ischemic heart

Cardiac troponin I (cTnI) is a key regulator of cardiomyocyte contraction. However, its role in mitochondria is unknown. Here we show that cTnI localized to mitochondria in the heart, inhibited mitochondrial functions when stably expressed in noncardiac cells and increased the opening of the mitochondrial permeability transition pore under oxidative stress. Direct, specific and saturable binding of cTnI to F1FO-ATP synthase was demonstrated in vitro using immune-captured ATP synthase and in cells using proximity ligation assay. cTnI binding doubled ATPase activity, whereas skeletal troponin I and several human pathogenic cTnI variants associated with familial hypertrophic cardiomyopathy did not. A rationally designed peptide, P888, inhibited cTnI binding to ATP synthase, inhibited cTnI-induced increase in ATPase activity in vitro and reduced cardiac injury following transient ischemia in vivo. We suggest that cTnI-bound ATP synthase results in lower ATP levels, and releasing this interaction during cardiac ischemia-reperfusion may increase the reservoir of functional mitochondria to reduce cardiac injury.

STING in tumors: a focus on non-innate immune pathways

Cyclic GMP-AMP synthase (cGAS) and downstream stimulator of interferon genes (STING) are involved in mediating innate immunity by promoting the release of interferon and other inflammatory factors. Mitochondrial DNA (mtDNA) with a double-stranded structure has greater efficiency and sensitivity in being detected by DNA sensors and thus has an important role in the activation of the cGAS-STING pathway. Many previous findings suggest that the cGAS-STING pathway-mediated innate immune regulation is the most important aspect affecting tumor survival, not only in its anti-tumor role but also in shaping the immunosuppressive tumor microenvironment (TME) through a variety of pathways. However, recent studies have shown that STING regulation of non-immune pathways is equally profound and also involved in tumor cell progression. In this paper, we will focus on the non-innate immune system pathways, in which the cGAS-STING pathway also plays an important role in cancer.

mtDNA Maintenance and Alterations in the Pathogenesis of Neurodegenerative Diseases

Considerable evidence indicates that the semiautonomous organelles mitochondria play key roles in the progression of many neurodegenerative disorders. Mitochondrial DNA (mtDNA) encodes components of the OXPHOS complex but mutated mtDNA accumulates in cells with aging, which mirrors the increased prevalence of neurodegenerative diseases. This accumulation stems not only from the misreplication of mtDNA and the highly oxidative environment but also from defective mitophagy after fission. In this review, we focus on several pivotal mitochondrial proteins related to mtDNA maintenance (such as ATAD3A and TFAM), mtDNA alterations including mtDNA mutations, mtDNA elimination, and mtDNA release-activated inflammation to understand the crucial role played by mtDNA in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. Our work outlines novel therapeutic strategies for targeting mtDNA.

Linking the Amyloid, Tau, and Mitochondrial Hypotheses of Alzheimer's Disease and Identifying Promising Drug Targets

Damage or loss of brain cells and impaired neurochemistry, neurogenesis, and synaptic and nonsynaptic plasticity of the brain lead to dementia in neurodegenerative diseases, such as Alzheimer's disease (AD). Injury to synapses and neurons and accumulation of extracellular amyloid plaques and intracellular neurofibrillary tangles are considered the main morphological and neuropathological features of AD. Age, genetic and epigenetic factors, environmental stressors, and lifestyle contribute to the risk of AD onset and progression. These risk factors are associated with structural and functional changes in the brain, leading to cognitive decline. Biomarkers of AD reflect or cause specific changes in brain function, especially changes in pathways associated with neurotransmission, neuroinflammation, bioenergetics, apoptosis, and oxidative and nitrosative stress. Even in the initial stages, AD is associated with Aβ neurotoxicity, mitochondrial dysfunction, and tau neurotoxicity. The integrative amyloid-tau-mitochondrial hypothesis assumes that the primary cause of AD is the neurotoxicity of Aβ oligomers and tau oligomers, mitochondrial dysfunction, and their mutual synergy. For the development of new efficient AD drugs, targeting the elimination of neurotoxicity, mutual potentiation of effects, and unwanted protein interactions of risk factors and biomarkers (mainly Aβ oligomers, tau oligomers, and mitochondrial dysfunction) in the early stage of the disease seems promising.

PM2.5-Induced Programmed Myocardial Cell Death via mPTP Opening Results in Deteriorated Cardiac Function in HFpEF Mice

PM2.5 exposure can induce or exacerbate heart failure and is associated with an increased risk of heart failure hospitalization and mortality; however, the underlying mechanisms remain unclear. This study focuses on the potential mechanisms underlying PM2.5 induction of cardiomyocyte programmed necrosis as well as its promotion of cardiac function impairment in a mouse model of heart failure with preserved ejection fraction (HFpEF). HFpEF mice were exposed to concentrated ambient PM2.5 (CAP) (CAP group) or filtered air (FA) (FA group) for 6 weeks. Changes in myocardial pathology and cardiac function were documented for comparisons between the two groups. In vitro experiments were performed to measure oxidative stress and mitochondrial permeability transition pore (mPTP) dynamics in H9C2 cells following 24 h exposure to PM2.5. Additionally, co-immunoprecipitation was conducted to detect p53 and cyclophilin D (CypD) interactions. The results showed exposure to CAP promoted cardiac function impairment in HFpEF mice. Myocardial pathology analysis and in vitro experiments demonstrated that PM2.5 led to mitochondrial damage in cardiomyocytes and, eventually, their necrosis. Moreover, our experiments also suggested that PM2.5 increases mitochondrial reactive oxygen species (ROS), induces DNA oxidative damage, and decreases the inner mitochondrial membrane potential (ΔΨm). This indicates the presence of mPTP opening. Co-immunoprecipitation results showed a p53/CypD interaction in the myocardial tissue of HFpEF mice in the CAP group. Inhibition of CypD by cyclosporin A was found to reverse PM2.5-induced mPTP opening and H9C2 cell death. In conclusion, PM2.5 induces mPTP opening to stimulate mitochondria-mediated programmed necrosis of cardiomyocytes, and it might exacerbate cardiac function impairment in HFpEF mice.

Targeting the mitochondrial permeability transition pore for drug discovery: Challenges and opportunities

Several drug targets have been amenable to drug discovery pursuit not until the characterization of the mitochondrial permeability transition pore (MPTP), a pore with an undefined molecular identity that forms on the inner mitochondrial membrane upon mitochondrial permeability transition (MPT) under the influence of calcium overload and oxidative stress. The opening of the pore which is presumed to cause cell death in certain human diseases also has implications under physiological parlance. Different models for this pore have been postulated following its first identification in the last six decades. The mitochondrial community has witnessed many protein candidates such as; voltage-dependent anion channel (VDAC), adenine nucleotide translocase (ANT), Mitochondrial phosphate carrier (PiC), Spastic Paralegin (SPG7), disordered proteins, and F1Fo ATPase. However, genetic studies have cast out most of these candidates with only F1Fo ATPase currently under intense argument. Cyclophilin D (CyPD) remains the widely accepted positive regulator of the MPTP known to date, but no drug candidate has emerged as its inhibitor, raising concern issues for therapeutics. Thus, in this review, we discuss various models of MPTP reported with the hope of stimulating further research in this field. We went beyond the classical description of the MPTP to ascribe a 'two-edged sword property' to the pore for therapeutic function in human disease because its inhibition and activation have pharmacological relevance. We suggested putative proteins upstream to CyPD that can regulate its activity and prevent cell deaths in neurodegenerative disease and ischemia-reperfusion injury.

Terpene-rich fractions of Ficus mucoso (Welw) modulate lipopolysaccharide-induced inflammatory mediators and aberrant permeability of the inner mitochondrial membrane in murine animal model

Ficus mucoso is traditionally used to treat bronchial infections. This study compared the efficacy of terpene-rich fractions of F. mucoso root bark on lipopolysaccharide(LPS)-induced inflammation, liver mitochondrial permeability transition (mPT), an index of mitochondrial health, and associated pathological alterations. Terpene-Rich Fractions of Dichloromethane (TRDF) and Ethylacetate Fractions of F. mucoso (TREF) were obtained according to standard procedures. To induce systemic inflammation, a single intraperitoneal injection of 1mgLPS/kgbw was given to mice. Spectrophotometric techniques were used to evaluate the effects of the oral administration of TRDF and TREF (3 days) on levels of pro-inflammatory mediators (TNF-α, IL-1β, IL-6) using ELSA techniques as well as antioxidant indices in normal and LPS-treated mice. The mPT pore opening, mitochondrial ATPase activity and lipid peroxidation were monitored spectrophotometrically. Our results revealed that treatment with LPS caused significant elevation in serum cytokine levels while administration of 50 and 100 mg/kg TRDF and TREF significantly reduced elevated serum levels of cytokines (TNF-α, IL-1β, IL-6) in LPS-challenged mice. In addition, activitities of superoxide dismutase, catalase and liver marker enzymes (ALT and AST) as well as levels of mitochondrial lipid peroxides were significantly reduced in mice treated with TRDF and TREF relative to LPS-fed mice. Furthermore, LPS caused induction of opening of the liver mPT pore which was significantly inhibited by TRDF at 100 and 200 mg/kg bw by 71% and 88%, respectively, but only at 100 mg/kg TREF. Furthermore, mitochondrial ATPase activity was inhibited largely by TRDF. UPLC-ESI-MS analysis revealed the presence of terpenoid derivatives and a few aromatic metabolites in TRDF. The terpene dominance of TRDF metabolites was further justified on the ¹H NMR fingerprint. Overall, TRDF is more effective as a cocktail of anti-inflammatory compounds than TREF against LPS-induced acute systemic inflammation.

ATP synthase: Evolution, energetics, and membrane interactions

The synthesis of ATP, life’s “universal energy currency,” is the most prevalent chemical reaction in biological systems and is responsible for fueling nearly all cellular processes, from nerve impulse propagation to DNA synthesis. ATP synthases, the family of enzymes that carry out this endless task, are nearly as ubiquitous as the energy-laden molecule they are responsible for making. The F-type ATP synthase (F-ATPase) is found in every domain of life and has facilitated the survival of organisms in a wide range of habitats, ranging from the deep-sea thermal vents to the human intestine. Accordingly, there has been a large amount of work dedicated toward understanding the structural and functional details of ATP synthases in a wide range of species. Less attention, however, has been paid toward integrating these advances in ATP synthase molecular biology within the context of its evolutionary history. In this review, we present an overview of several structural and functional features of the F-type ATPases that vary across taxa and are purported to be adaptive or otherwise evolutionarily significant: ion channel selectivity, rotor ring size and stoichiometry, ATPase dimeric structure and localization in the mitochondrial inner membrane, and interactions with membrane lipids. We emphasize the importance of studying these features within the context of the enzyme’s particular lipid environment. Just as the interactions between an organism and its physical environment shape its evolutionary trajectory, ATPases are impacted by the membranes within which they reside. We argue that a comprehensive understanding of the structure, function, and evolution of membrane proteins—including ATP synthase—requires such an integrative approach.

Mitochondrial DNA may act as a biomarker to predict donor-kidney quality

Kidney transplantation is the best therapy for end-stage renal disease. Demand for kidney transplantation rises year-on-year, and the gap between kidney supply and demand remains large. To meet this clinical need, a gradual expansion in the supply of donors is required. However, clinics lack appropriate tools capable of quickly and accurately predicting post-transplant renal allograft function, and thus assess donor-kidney quality before transplantation. Mitochondrial DNA (mtDNA) is a key component of damage-associated molecular patterns (DAMPs) and plays an important part in ischemia-reperfusion injury (IRI), accelerating the progression of IRI by inducing inflammation and type I interferon responses. mtDNA is known to be closely involved in delayed graft function (DGF) and acute kidney injury (AKI) after transplantation. Thus, mtDNA is a potential biomarker able to predict post-transplant renal allograft function. This review summarizes mtDNA biology, the role mtDNA plays in renal transplantation, outlines advances in detecting mtDNA, and details mtDNA's able to predict post-transplant renal allograft function. We aim to elucidate the potential value of mtDNA as a biomarker in the prediction of IRI, and eventually provide help for predicting donor-kidney quality.

Plumbagin induces testicular damage via mitochondrial-dependent cell death

Different aspects of reproductive functions are regulated by mitochondrial-controlled events. This study investigated the effect of plumbagin (PL) on testicular mitochondria with a view to unravelling the mechanism of the antifertility potential of plumbagin in testis of healthy rats. Thirty-two male Wistar strain albino rats were randomly allocated into four groups of eight animals each. The control or healthy group received orally 0.1 % DMSO while animals in the remaining three groups received 2.5 mg PL/kg bdwt, 5.0 mg PL/kg bdwt and 10 mg PL/kg bdwt, respectively, for 14 days. In study two, twenty-four male Wistar rats were randomly divided into three (3) groups and were orally administered 0.1% DMSO (control), 30 and 100 mg/kg PL, respectively once daily for 72 h. Rat testis mitochondria were isolated using differential centrifugation. The mitochondrial Permeability Transition (mPT) pore, mitochondrial ATPase (mATPase) activity and mitochondrial lipid peroxidation were assessed spectrophotometrically. Expression of apoptotic proteins (p53, Bax, Bcl-2) and the release of cytochrome c were determined by immunochemical technique. Reproductive receptors (FSH, PR), the expression of aromatase, Testis Specific Kinase-1 {TESK-1} were quantified by RT-PCR. The various doses of plumbagin (2.5, 5.0 and 10 mg/kg bdwt) induced opening of the testicular mPT pore by 2, 5 and 8 folds, respectively, after 14 days of oral administration. These doses of plumbagin also caused enhancement of mATPase activity, elevated generation of mLPO as well as increases in the concentrations of caspases 9 and 3. Sperm analysis revealed that these doses of PL also caused significant decreases in sperm count and motility and increased sperm abnormalities compared to control. Interestingly, these effects were accompanied by dose-dependent expressions of the Bak, p53 and cytochrome c release. Conversely, the abundance of anti-apoptotic Bcl-2 protein decreased relative to control. The levels of transcripts of FSH and progesterone receptors as well as TESK-1 and aromatase decreased significantly relative to control. Furthermore, PL strongly inhibited p53-MDM2 compared to control. Altogether, these findings show that plumbagin damages testicular cells through the activation of mitochondrial pathway involving the p53 protein network.

Evidence for the Role of Mitochondrial DNA Release in the Inflammatory Response in Neurological Disorders

Mitochondria are regarded as the metabolic centers of cells and are integral in many other cell processes, including the immune response. Each mitochondrion contains numerous copies of mitochondrial DNA (mtDNA), a small, circular, and bacterial-like DNA. In response to cellular damage or stress, mtDNA can be released from the mitochondrion and trigger immune and inflammatory responses. mtDNA release into the cytosol or bloodstream can occur as a response to hypoxia, sepsis, traumatic injury, excitatory cytotoxicity, or drastic mitochondrial membrane potential changes, some of which are hallmarks of neurodegenerative and mood disorders. Released mtDNA can mediate inflammatory responses observed in many neurological and mood disorders by driving the expression of inflammatory cytokines and the interferon response system. The current understanding of the role of mtDNA release in affective mood disorders and neurodegenerative diseases will be discussed.

Pharmacological Inhibition of S-Nitrosoglutathione Reductase Reduces Cardiac Damage Induced by Ischemia-Reperfusion

The cardioprotective effects of nitric oxide (NO) have been described through S-nitrosylation of several important proteins in the mitochondria of the cardiomyocyte. S-nitrosoglutathione reductase (GSNOR) is an enzyme involved in the metabolism of S-nitrosothiols by producing denitrosylation, thus limiting the cardioprotective effect of NO. The effect of GSNOR inhibition on the damage by cardiac ischemia–reperfusion is still unclear. We tested the hypothesis that pharmacological inhibition of GSNOR promotes cardioprotection by increasing the levels of protein S-nitrosylation. In a model of ischemia–reperfusion in isolated rat heart, the effect of a GSNOR inhibitor, 5-chloro-3-(2-[4-ethoxyphenyl) (ethyl) amino]-2-oxoethyl)-1H-indole-2-carboxylic acid (C2), was investigated. Ventricular function and hemodynamics were determined, in addition to tissue damage and S-nitrosylation of mitochondrial proteins. Hearts treated with C2 showed a lower release of myocardial damage marker creatine kinase and a reduction in the infarcted area. It also improved post-ischemia ventricular function compared to controls. These results were associated with increasing protein S-nitrosylation, specifically of the mitochondrial complexes III and V. The pharmacological inhibition of GSNOR showed a concentration-dependent cardioprotective effect, being observed in functional parameters and myocardial damage, which was maximal at 1 µmol/L, associated with increased S-nitrosylation of mitochondrial proteins. These data suggest that GSNOR is an interesting pharmacological target for cardiac reperfusion injury.

Mitochondrial Permeability Transition: A Pore Intertwines Brain Aging and Alzheimer's Disease

Advanced age is the greatest risk factor for aging-related brain disorders including Alzheimer's disease (AD). However, the detailed mechanisms that mechanistically link aging and AD remain elusive. In recent years, a mitochondrial hypothesis of brain aging and AD has been accentuated. Mitochondrial permeability transition pore (mPTP) is a mitochondrial response to intramitochondrial and intracellular stresses. mPTP overactivation has been implicated in mitochondrial dysfunction in aging and AD brains. This review summarizes the up-to-date progress in the study of mPTP in aging and AD and attempts to establish a link between brain aging and AD from a perspective of mPTP-mediated mitochondrial dysfunction.

Mitochondrial Calcium Signaling in Pancreatic β-Cell

Accumulation of calcium in energized mitochondria of pancreatic β-cells is emerging as a crucial process for pancreatic β-cell function. β-cell mitochondria sense and shape calcium signals, linking the metabolism of glucose and other secretagogues to the generation of signals that promote insulin secretion during nutrient stimulation. Here, we describe the role of mitochondrial calcium signaling in pancreatic β-cell function. We report the latest pharmacological and genetic findings, including the first mitochondrial calcium-targeted intervention strategies developed to modulate pancreatic β-cell function and their potential relevance in the context of diabetes.

Targeting the Mitochondrial Permeability Transition Pore to Prevent Age-Associated Cell Damage and Neurodegeneration

The aging process is associated with significant alterations in mitochondrial function. These changes in mitochondrial function are thought to involve increased production of reactive oxygen species (ROS), which over time contribute to cell death, senescence, tissue degeneration, and impaired tissue repair. The mitochondrial permeability transition pore (mPTP) is likely to play a critical role in these processes, as increased ROS activates mPTP opening, which further increases ROS production. Injury and inflammation are also thought to increase mPTP opening, and chronic, low-grade inflammation is a hallmark of aging. Nicotinamide adenine dinucleotide (NAD+) can suppress the frequency and duration of mPTP opening; however, NAD+ levels are known to decline with age, further stimulating mPTP opening and increasing ROS release. Research on neurodegenerative diseases, particularly on Parkinson's disease (PD) and Alzheimer's disease (AD), has uncovered significant findings regarding mPTP openings and aging. Parkinson's disease is associated with a reduction in mitochondrial complex I activity and increased oxidative damage of DNA, both of which are linked to mPTP opening and subsequent ROS release. Similarly, AD is associated with increased mPTP openings, as evidenced by amyloid-beta (Aβ) interaction with the pore regulator cyclophilin D (CypD). Targeted therapies that can reduce the frequency and duration of mPTP opening may therefore have the potential to prevent age-related declines in cell and tissue function in various systems including the central nervous system.

Dysfunction of Mitochondrial Ca2+ Regulatory Machineries in Brain Aging and Neurodegenerative Diseases

Calcium ions (Ca²⁺) play critical roles in neuronal processes, such as signaling pathway activation, transcriptional regulation, and synaptic transmission initiation. Therefore, the regulation of Ca²⁺ homeostasis is one of the most important processes underlying the basic cellular viability and function of the neuron. Multiple components, including intracellular organelles and plasma membrane Ca²⁺-ATPase, are involved in neuronal Ca²⁺ control, and recent studies have focused on investigating the roles of mitochondria in synaptic function. Numerous mitochondrial Ca²⁺ regulatory proteins have been identified in the past decade, with studies demonstrating the tissue- or cell-type-specific function of each component. The mitochondrial calcium uniporter and its binding subunits are major inner mitochondrial membrane proteins contributing to mitochondrial Ca²⁺ uptake, whereas the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) and mitochondrial permeability transition pore (mPTP) are well-studied proteins involved in Ca²⁺ extrusion. The level of cytosolic Ca²⁺ and the resulting characteristics of synaptic vesicle release properties are controlled via mitochondrial Ca²⁺ uptake and release at presynaptic sites, while in dendrites, mitochondrial Ca²⁺ regulation affects synaptic plasticity. During brain aging and the progress of neurodegenerative disease, mitochondrial Ca²⁺ mishandling has been observed using various techniques, including live imaging of Ca²⁺ dynamics. Furthermore, Ca²⁺ dysregulation not only disrupts synaptic transmission but also causes neuronal cell death. Therefore, understanding the detailed pathophysiological mechanisms affecting the recently discovered mitochondrial Ca²⁺ regulatory machineries will help to identify novel therapeutic targets. Here, we discuss current research into mitochondrial Ca²⁺ regulatory machineries and how mitochondrial Ca²⁺ dysregulation contributes to brain aging and neurodegenerative disease.

A systematic review of post-translational modifications in the mitochondrial permeability transition pore complex associated with cardiac diseases

The mitochondrial permeability transition pore (mPTP) opening is involved in the pathophysiology of multiple cardiac diseases, such as ischemia/reperfusion injury and heart failure. A growing number of evidence provided by proteomic screening techniques has demonstrated the role of post-translational modifications (PTMs) in several key components of the pore in response to changes in the extra/intracellular environment and bioenergetic demand. This could lead to a fine, complex regulatory mechanism that, under pathological conditions, can shift the state of mitochondrial functions and, thus, the cell's fate. Understanding the complex relationship between these PTMs is still under investigation and can provide new, promising therapeutic targets and treatment approaches. This review, using a systematic review of the literature, presents the current knowledge on PTMs of the mPTP and their role in health and cardiac disease.

SAM50, a side door to the mitochondria: The case of cytotoxic proteases

SAM50, a 7-8 nm diameter β-barrel channel of the mitochondrial outer membrane, is the central channel of the sorting and assembly machinery (SAM) complex involved in the biogenesis of β-barrel proteins. Interestingly, SAM50 is not known to have channel translocase activity; however, we have recently found that this channel is necessary and sufficient for mitochondrial entry of cytotoxic proteases. Cytotoxic lymphocytes eliminate cells that pose potential hazards, such as virus- and bacteria-infected cells as well as cancer cells. They induce cell death following the delivery of granzyme cytotoxic proteases into the cytosol of the target cell. Although granzyme A and granzyme B (GA and GB), the best characterized of the five human granzymes, trigger very distinct apoptotic cascades, they share the ability to directly target the mitochondria. GA and GB do not have a mitochondrial targeting signal, yet they enter the target cell mitochondria to disrupt respiratory chain complex I and induce mitochondrial reactive oxygen species (ROS)-dependent cell death. We found that granzyme mitochondrial entry requires SAM50 and the translocase of the inner membrane 22 (TIM22). Preventing granzymes' mitochondrial entry compromises their cytotoxicity, indicating that this event is unexpectedly an important step for cell death. Although mitochondria are best known for their roles in cell metabolism and energy conversion, these double-membrane organelles are also involved in Ca²⁺ homeostasis, metabolite transport, cell cycle regulation, cell signaling, differentiation, stress response, redox homeostasis, aging, and cell death. This multiplicity of functions is matched with the complexity and plasticity of the mitochondrial proteome as well as the organelle's morphological and structural versatility. Indeed, mitochondria are extremely dynamic and undergo fusion and fission events in response to diverse cellular cues. In humans, there are 1500 different mitochondrial proteins, the vast majority of which are encoded in the nuclear genome and translated by cytosolic ribosomes, after which they must be imported and properly addressed to the right mitochondrial compartment. To this end, mitochondria are equipped with a very sophisticated and highly specific protein import machinery. The latter is centered on translocase complexes embedded in the outer and inner mitochondrial membranes working along five different import pathways. We will briefly describe these import pathways to put into perspective our finding regarding the ability of granzymes to enter the mitochondria.

Mitochondrial Dysfunction, Oxidative Stress, and Neuroinflammation: Intertwined Roads to Neurodegeneration

Oxidative stress develops as a response to injury and reflects a breach in the cell’s antioxidant capacity. Therefore, the fine-tuning of reactive oxygen species (ROS) generation is crucial for preserving cell’s homeostasis. Mitochondria are a major source and an immediate target of ROS. Under different stimuli, including oxidative stress and impaired quality control, mitochondrial constituents (e.g., mitochondrial DNA, mtDNA) are displaced toward intra- or extracellular compartments. However, the mechanisms responsible for mtDNA unloading remain largely unclear. While shuttling freely within the cell, mtDNA can be delivered into the extracellular compartment via either extrusion of entire nucleoids or the generation and release of extracellular vesicles. Once discarded, mtDNA may act as a damage-associated molecular pattern (DAMP) and trigger an innate immune inflammatory response by binding to danger-signal receptors. Neuroinflammation is associated with a large array of neurological disorders for which mitochondrial DAMPs could represent a common thread supporting disease progression. The exploration of non-canonical pathways involved in mitochondrial quality control and neurodegeneration may unveil novel targets for the development of therapeutic agents. Here, we discuss these processes in the setting of two common neurodegenerative diseases (Alzheimer’s and Parkinson’s disease) and Down syndrome, the most frequent progeroid syndrome.

Regulation of the mitochondrial permeability transition pore and its effects on aging

Aging is an evolutionarily conserved process and is tightly connected to mitochondria. To uncover the aging molecular mechanisms related to mitochondria, different organisms have been extensively used as model systems. Among these, the budding yeast Saccharomyces cerevisiae has been reported multiple times as a model of choice when studying cellular aging. In particular, yeast provides a quick and trustworthy system to identify shared aging genes and pathway patterns. In this viewpoint on aging and mitochondria, I will focus on the mitochondrial permeability transition pore (mPTP), which has been reported and proposed as a main player in cellular aging. I will make several parallelisms with yeast to highlight how this unicellular organism can be used as a guidance system to understand conserved cellular and molecular events in multicellular organisms such as humans. Overall, a thread connecting the preservation of mitochondrial functionality with the activity of the mPTP emerges in the regulation of cell survival and cell death, which in turn could potentially affect aging and aging-related diseases.

A 20/20 view of ANT function in mitochondrial biology and necrotic cell death

The adenosine nucleotide translocase (ANT) family of proteins are inner mitochondrial membrane proteins involved in energy homeostasis and cell death. The primary function of ANT proteins is to exchange cytosolic ADP with matrix ATP, facilitating the export of newly synthesized ATP to the cell while providing new ADP substrate to the mitochondria. As such, the ANT proteins are central to maintaining energy homeostasis in all eukaryotic cells. Evidence also suggests that the ANTs constitute a pore-forming component of the mitochondrial permeability transition pore (MPTP), a structure that forms in the inner mitochondrial membrane that is thought to underlie regulated necrotic cell death. Additionally, emerging studies suggest that ANT proteins are also critical for mitochondrial uncoupling and for promoting mitophagy. Thus, the ANTs are multifunctional proteins that are poised to participate in several aspects of mitochondrial biology and the greater regulation of cell death, which will be discussed here.

mPTP Proteins Regulated by Streptozotocin-Induced Diabetes Mellitus Are Effectively Involved in the Processes of Maintaining Myocardial Metabolic Adaptation

Mitochondrial permeability transition pores (mPTPs) have become an important topic in investigating the initiation and signaling pathways involved in cardioprotection. Experimental streptozotocin-induced diabetes mellitus (D) was shown to provide sufficient protection to the myocardium via compensatory mechanisms enabling mitochondria to produce energy under pathological conditions during the acute phase. The hypothesized involvement of mPTPs in these processes prompted us to use liquid chromatography and mass spectrometry-based proteomic analysis to investigate the effects of the acute-phase D condition on the structural and regulatory components of this multienzyme complex and the changes caused by compensation events. We detected ADT1, ATP5H, ATPA, and ATPB as the most abundant mPTP proteins. The between-group differences in protein abundance of the mPTP complex as a whole were significantly upregulated in the D group when compared with the control (C) group (p = 0.0106), but fold changes in individual protein expression levels were not significantly altered except for ATP5H, ATP5J, and KCRS. However, none of them passed the criterion of a 1.5-fold change in differential expression for biologically meaningful change. Visualization of the (dis-)similarity between the C and D groups and pairwise correlations revealed different patterns of protein interactions under the C and D conditions which may be linked to endogenous protective processes, of which beneficial effects on myocardial function were previously confirmed.

Mitochondrial ion channels in pancreatic β-cells: Novel pharmacological targets for the treatment of Type 2 diabetes

Pancreatic beta‐cells are central regulators of glucose homeostasis. By tightly coupling nutrient sensing and granule exocytosis, beta‐cells adjust the secretion of insulin to the circulating blood glucose levels. Failure of beta‐cells to augment insulin secretion in insulin‐resistant individuals leads progressively to impaired glucose tolerance, Type 2 diabetes, and diabetes‐related diseases. Mitochondria play a crucial role in β‐cells during nutrient stimulation, linking the metabolism of glucose and other secretagogues to the generation of signals that promote insulin secretion. Mitochondria are double‐membrane organelles containing numerous channels allowing the transport of ions across both membranes. These channels regulate mitochondrial energy production, signalling, and cell death. The mitochondria of β‐cells express ion channels whose physio/pathological role is underappreciated. Here, we describe the mitochondrial ion channels identified in pancreatic β‐cells, we further discuss the possibility of targeting specific β‐cell mitochondrial channels for the treatment of Type 2 diabetes, and we finally highlight the evidence from clinical studies.

LINKED ARTICLES

This article is part of a themed issue on Cellular metabolism and diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.10/issuetoc

ATP Synthase C-Subunit-Deficient Mitochondria Have a Small Cyclosporine A-Sensitive Channel, but Lack the Permeability Transition Pore

Permeability transition (PT) is an increase in mitochondrial inner membrane permeability that can lead to a disruption of mitochondrial function and cell death. PT is responsible for tissue damage in stroke and myocardial infarction. It is caused by the opening of a large conductance (~1.5 nS) channel, the mitochondrial PT pore (mPTP). We directly tested the role of the c-subunit of ATP synthase in mPTP formation by measuring channel activity in c-subunit knockout mitochondria. We found that the classic mPTP conductance was lacking in c-subunit knockout mitochondria, but channels sensitive to the PT inhibitor cyclosporine A could be recorded. These channels had a significantly lower conductance compared with the cyclosporine A-sensitive channels detected in parental cells and were sensitive to the ATP/ADP translocase inhibitor bongkrekic acid. We propose that, in the absence of the c-subunit, mPTP cannot be formed, and a distinct cyclosporine A-sensitive low-conductance channel emerges.

Neginskaya et al. report that c-subunit-deficient mitochondria contain a CSA-sensitive channel. This channel is much smaller compared with the wild-type permeability transition pore and is sensitive to inhibitors of adenine nucleotide translocase. This work highlights the importance of the c-subunit in forming the permeability transition pore.

Mechanisms of mitochondrial DNA escape and its relationship with different metabolic diseases

It is well-known that mitochondrial DNA (mtDNA) can escape to intracellular or extracellular compartments under different stress conditions, yet understanding their escape mechanisms remains a challenge. Although Bax/Bak pores and VDAC oligomers are the strongest possibilities, other mechanisms may be involved. For example, mitochondria permeability transition, altered mitophagy, and mitochondrial dynamics are associated with intracellular mtDNA escape, while extracellular traps and extracellular vesicles can participate in extracellular mtDNA escape. The evidence suggests that mtDNA escape is a complex event with more than one mechanism involved. In addition, once the mtDNA is outside the mitochondria, the effects can be complex. Different danger signal sensors recognize the mtDNA as a damage-associated molecular pattern, triggering an innate immune inflammatory response that can be observed in multiple metabolic diseases characterized by chronic inflammation, including autoimmune diseases, diabetes, cancer, and cardiovascular disorders. For these reasons, we will review the most recent evidence regarding mtDNA escape mechanisms and their impact on different metabolic diseases.

Delineating a role for the mitochondrial permeability transition pore in diabetic kidney disease by targeting cyclophilin D

Mitochondrial stress has been widely observed in diabetic kidney disease (DKD). Cyclophilin D (CypD) is a functional component of the mitochondrial permeability transition pore (mPTP) which allows the exchange of ions and solutes between the mitochondrial matrix to induce mitochondrial swelling and activation of cell death pathways. CypD has been successfully targeted in other disease contexts to improve mitochondrial function and reduced pathology. Two approaches were used to elucidate the role of CypD and the mPTP in DKD. Firstly, mice with a deletion of the gene encoding CypD (Ppif-/-) were rendered diabetic with streptozotocin (STZ) and followed for 24 weeks. Secondly, Alisporivir, a CypD inhibitor was administered to the db/db mouse model (5 mg/kg/day oral gavage for 16 weeks). Ppif-/- mice were not protected against diabetes-induced albuminuria and had greater glomerulosclerosis than their WT diabetic littermates. Renal hyperfiltration was lower in diabetic Ppif-/- as compared with WT mice. Similarly, Alisporivir did not improve renal function nor pathology in db/db mice as assessed by no change in albuminuria, KIM-1 excretion and glomerulosclerosis. Db/db mice exhibited changes in mitochondrial function, including elevated respiratory control ratio (RCR), reduced mitochondrial H2O2 generation and increased proximal tubular mitochondrial volume, but these were unaffected by Alisporivir treatment. Taken together, these studies indicate that CypD has a complex role in DKD and direct targeting of this component of the mPTP will likely not improve renal outcomes.

Role of Mitochondrial Calcium and the Permeability Transition Pore in Regulating Cell Death

Adult cardiomyocytes are postmitotic cells that undergo very limited cell division. Thus, cardiomyocyte death as occurs during myocardial infarction has very detrimental consequences for the heart. Mitochondria have emerged as an important regulator of cardiovascular health and disease. Mitochondria are well established as bioenergetic hubs for generating ATP but have also been shown to regulate cell death pathways. Indeed many of the same signals used to regulate metabolism and ATP production, such as calcium and reactive oxygen species, are also key regulators of mitochondrial cell death pathways. It is widely hypothesized that an increase in calcium and reactive oxygen species activate a large conductance channel in the inner mitochondrial membrane known as the PTP (permeability transition pore) and that opening of this pore leads to necroptosis, a regulated form of necrotic cell death. Strategies to reduce PTP opening either by inhibition of PTP or inhibiting the rise in mitochondrial calcium or reactive oxygen species that activate PTP have been proposed. A major limitation of inhibiting the PTP is the lack of knowledge about the identity of the protein(s) that form the PTP and how they are activated by calcium and reactive oxygen species. This review will critically evaluate the candidates for the pore-forming unit of the PTP and discuss recent data suggesting that assumption that the PTP is formed by a single molecular identity may need to be reconsidered.

Myocardial Adaptation in Pseudohypoxia: Signaling and Regulation of mPTP via Mitochondrial Connexin 43 and Cardiolipin

Therapies intended to mitigate cardiovascular complications cannot be applied in practice without detailed knowledge of molecular mechanisms. Mitochondria, as the end-effector of cardioprotection, represent one of the possible therapeutic approaches. The present review provides an overview of factors affecting the regulation processes of mitochondria at the level of mitochondrial permeability transition pores (mPTP) resulting in comprehensive myocardial protection. The regulation of mPTP seems to be an important part of the mechanisms for maintaining the energy equilibrium of the heart under pathological conditions. Mitochondrial connexin 43 is involved in the regulation process by inhibition of mPTP opening. These individual cardioprotective mechanisms can be interconnected in the process of mitochondrial oxidative phosphorylation resulting in the maintenance of adenosine triphosphate (ATP) production. In this context, the degree of mitochondrial membrane fluidity appears to be a key factor in the preservation of ATP synthase rotation required for ATP formation. Moreover, changes in the composition of the cardiolipin’s structure in the mitochondrial membrane can significantly affect the energy system under unfavorable conditions. This review aims to elucidate functional and structural changes of cardiac mitochondria subjected to preconditioning, with an emphasis on signaling pathways leading to mitochondrial energy maintenance during partial oxygen deprivation.

Fundamental Mechanisms of Regulated Cell Death and Implications for Heart Disease

Twelve regulated cell death programs have been described. We review in detail the basic biology of nine including death receptor-mediated apoptosis, death receptor-mediated necrosis (necroptosis), mitochondrial-mediated apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, ferroptosis, pyroptosis, parthanatos, and immunogenic cell death. This is followed by a dissection of the roles of these cell death programs in the major cardiac syndromes: myocardial infarction and heart failure. The most important conclusion relevant to heart disease is that regulated forms of cardiomyocyte death play important roles in both myocardial infarction with reperfusion (ischemia/reperfusion) and heart failure. While a role for apoptosis in ischemia/reperfusion cannot be excluded, regulated forms of necrosis, through both death receptor and mitochondrial pathways, are critical. Ferroptosis and parthanatos are also likely important in ischemia/reperfusion, although it is unclear if these entities are functioning as independent death programs or as amplification mechanisms for necrotic cell death. Pyroptosis may also contribute to ischemia/reperfusion injury, but potentially through effects in non-cardiomyocytes. Cardiomyocyte loss through apoptosis and necrosis is also an important component in the pathogenesis of heart failure and is mediated by both death receptor and mitochondrial signaling. Roles for immunogenic cell death in cardiac disease remain to be defined but merit study in this era of immune checkpoint cancer therapy. Biology-based approaches to inhibit cell death in the various cardiac syndromes are also discussed.

Purified F-ATP synthase forms a Ca2+-dependent high-conductance channel matching the mitochondrial permeability transition pore

The molecular identity of the mitochondrial megachannel (MMC)/permeability transition pore (PTP), a key effector of cell death, remains controversial. By combining highly purified, fully active bovine F-ATP synthase with preformed liposomes we show that Ca²⁺ dissipates the H⁺ gradient generated by ATP hydrolysis. After incorporation of the same preparation into planar lipid bilayers Ca²⁺ elicits currents matching those of the MMC/PTP. Currents were fully reversible, were stabilized by benzodiazepine 423, a ligand of the OSCP subunit of F-ATP synthase that activates the MMC/PTP, and were inhibited by Mg²⁺ and adenine nucleotides, which also inhibit the PTP. Channel activity was insensitive to inhibitors of the adenine nucleotide translocase (ANT) and of the voltage-dependent anion channel (VDAC). Native gel-purified oligomers and dimers, but not monomers, gave rise to channel activity. These findings resolve the long-standing mystery of the MMC/PTP and demonstrate that Ca²⁺ can transform the energy-conserving F-ATP synthase into an energy-dissipating device.

The molecular identity of the mitochondrial megachannel (MMC)/permeability transition pore (PTP), a key effector of cell death, remains controversial. Here authors demonstrate that the membrane embedded bovine F-ATP synthase elicits Ca2 + -dependent currents matching those of the MMC/PTP.

Persistence of the permeability transition pore in human mitochondria devoid of an assembled ATP synthase

The opening of the permeability transition pore, a nonspecific channel in inner mitochondrial membranes, is triggered by an elevated total concentration of calcium ions in the mitochondrial matrix, leading to disruption of the inner membrane and necrotic cell death. Cyclosporin A inhibits pore opening by binding to cyclophilin D, which interacts with the pore. It has been proposed that the pore is associated with the ATP synthase complex. Previously, we confirmed an earlier observation that the pore survives in cells lacking membrane subunits ATP6 and ATP8 of ATP synthase, and in other cells lacking the enzyme's c₈ rotor ring or, separately, its peripheral stalk subunits b and oligomycin sensitive conferral protein. Here, we investigated whether the pore is associated with the remaining membrane subunits of the enzyme. Individual deletion of subunits e, f, g, and 6.8-kDa proteolipid disrupts dimerization of the complex, and deletion of DAPIT (diabetes-associated protein in insulin sensitive tissue) possibly influences oligomerization of dimers, but removal of each subunit had no effect on the pore. Also, we removed together the enzyme's membrane bound c₈ ring and the δ-subunit from the catalytic domain. The resulting cells assemble only a subcomplex derived from the peripheral stalk and membrane-associated proteins. Despite diminished levels of respiratory complexes, these cells generate a membrane potential to support uptake of calcium into the mitochondria, leading to pore opening, and retention of its characteristic properties. It is most unlikely that the ATP synthase, dimer or monomer, or any component, provides the permeability transition pore.

Arg-8 of yeast subunit e contributes to the stability of F-ATP synthase dimers and to the generation of the full-conductance mitochondrial megachannel

The mitochondrial F-ATP synthase is a complex molecular motor arranged in V-shaped dimers that is responsible for most cellular ATP synthesis in aerobic conditions. In the yeast F-ATP synthase, subunits e and g of the F_O sector constitute a lateral domain, which is required for dimer stability and cristae formation. Here, by using site-directed mutagenesis, we identified Arg-8 of subunit e as a critical residue in mediating interactions between subunits e and g, most likely through an interaction with Glu-83 of subunit g. Consistent with this hypothesis, (i) the substitution of Arg-8 in subunit e (eArg-8) with Ala or Glu or of Glu-83 in subunit g (gGlu-83) with Ala or Lys destabilized the digitonin-extracted F-ATP synthase, resulting in decreased dimer formation as revealed by blue-native electrophoresis; and (ii) simultaneous substitution of eArg-8 with Glu and of gGlu-83 with Lys rescued digitonin-stable F-ATP synthase dimers. When tested in lipid bilayers for generation of Ca²⁺-dependent channels, WT dimers displayed the high-conductance channel activity expected for the mitochondrial megachannel/permeability transition pore, whereas dimers obtained at low digitonin concentrations from the Arg-8 variants displayed currents of strikingly small conductance. Remarkably, double replacement of eArg-8 with Glu and of gGlu-83 with Lys restored high-conductance channels indistinguishable from those seen in WT enzymes. These findings suggest that the interaction of subunit e with subunit g is important for generation of the full-conductance megachannel from F-ATP synthase.

Mitochondrial Entry of Cytotoxic Proteases: A New Insight into the Granzyme B Cell Death Pathway

The mitochondria represent an integration and amplification hub for various death pathways including that mediated by granzyme B (GB), a granule enzyme expressed by cytotoxic lymphocytes. GB activates the proapoptotic B cell CLL/lymphoma 2 (Bcl-2) family member BH3-interacting domain death agonist (BID) to switch on the intrinsic mitochondrial death pathway, leading to Bcl-2-associated X protein (Bax)/Bcl-2 homologous antagonist/killer- (Bak-) dependent mitochondrial outer membrane permeabilization (MOMP), the dissipation of mitochondrial transmembrane potential (ΔΨm), and the production of reactive oxygen species (ROS). GB can also induce mitochondrial damage in the absence of BID, Bax, and Bak, critical for MOMP, indicating that GB targets the mitochondria in other ways. Interestingly, granzyme A (GA), GB, and caspase 3 can all directly target the mitochondrial respiratory chain complex I for ROS-dependent cell death. Studies of ROS biogenesis have revealed that GB must enter the mitochondria for ROS production, making the mitochondrial entry of cytotoxic proteases (MECP) an unexpected critical step in the granzyme death pathway. MECP requires an intact ΔΨm and is mediated though Sam50 and Tim22 channels in a mtHSP70-dependent manner. Preventing MECP severely compromises GB cytotoxicity. In this review, we provide a brief overview of the canonical mitochondrial death pathway in order to put into perspective this new insight into the GB action on the mitochondria to trigger ROS-dependent cell death.

Dynamics of the mitochondrial permeability transition pore: Transient and permanent opening events

A gentle optical examination of the mitochondrial permeability transition pore (mPTP) opening events was carried out in isolated quiescent ventricular myocytes by tracking the inner membrane potential (ΔΨM) using TMRM (tetramethylrhodamine methyl ester). Zeiss Airyscan 880 ″super-resolution" or "high-resolution" imaging was done with very low levels of illumination (0.009% laser power). In cellular areas imaged every 9 s (ROI or regions of interest), transient depolarizations of variable amplitudes occurred at increasing rates for the first 30 min. The time to first depolarization events was 8.4 min (±1.1 SEM n = 21 cells). At longer times, essentially permanent and irreversible depolarizations occurred at an increasing fraction of all events. In other cellular areas surrounding the ROI, mitochondria were rarely illuminated (once per 5 min) and virtually no permanent depolarization events occurred for over 1 h of imaging. These findings suggest that photon stress due to the imaging itself plays an important role in the generation of both the transient mPTP opening events as well as the permanent mPTP opening events. Consistent with the evidence that photon "stress" in mitochondria loaded with virtually any photon absorbing substance, generates reactive oxygen species (ROS) [1-5], we show that cyclosporine-A (CsA, 10 μM) and the antioxidant n-acetyl cysteine (NAC, 10 mM), reduced the number of events by 80% and 93% respectively. Furthermore, CsA and NAC treatment led to the virtual disappearance of permanent depolarization events. Nevertheless, all transient depolarization events in any condition (control, CsA and NAC) appeared to repolarize with a similar half-time of 30 ± 6 s (n = 478) at 37 °C. Further experiments showed quantitatively similar results in cerebral vascular smooth muscle cells, using a different confocal system, and different photon absorbing reagent (TMRE; tetramethylrhodamine ethyl ester). In these experiments, using modest power (1% laser power) transient depolarization events were seen in only 8 out of 23 cells while with higher power (8%), all cells showed transient events, which align with the level of photon stress being the driver of the effect. Together, our findings suggest that photon-induced ROS is sufficient to cause depolarization events of individual mitochondria in quiescent cells; without electrical or mechanical activity to stimulates mitochondrial metabolism, and without raising the mitochondrial matrix Ca2+. In a broad context, these findings neither support nor deny the relevance or occurrence of ΔΨM depolarization events in specific putatively physiologic mitochondrial behaviors such as MitoFlashes [6,7] or MitoWinks [8]. Instead, our findings raise a caution with regards to the physiological and pathophysiological functions attributed to singular ΔΨM depolarization events when those functions are investigated using photon absorbing substances. Nevertheless, using photon stress as a tool ("Optical Stress-Probe"), we can extract information on the activation, reversibility, permanency and kinetics of mitochondrial depolarization. These data may provide new information on mPTP, help identify the mPTP protein complex, and establish the physiological function of the mPTP protein complex and their links to MitoFlashes and MitoWinks.

A New Current for the Mitochondrial Permeability Transition

Mitochondrial F1/FO ATP synthase participation in the mitochondrial permeability transition pore complex (PTPC) remains controversial. Neginskaya et al. (Cell Rep. 2019;26:11-17) reported an unexpected current with PTPC-like properties in F1/FO ATP synthase C subunit knockout cells that could explain part of the conflictual literature.

The Mitochondrial Permeability Transition in Mitochondrial Disorders

Mitochondrial permeability transition pore (PTP), a (patho)physiological phenomenon discovered over 40 years ago, is still not completely understood. PTP activation results in a formation of a nonspecific channel within the inner mitochondrial membrane with an exclusion size of 1.5 kDa. PTP openings can be transient and are thought to serve a physiological role to allow quick Ca2+ release and/or metabolite exchange between mitochondrial matrix and cytosol or long-lasting openings that are associated with pathological conditions. While matrix Ca2+ and oxidative stress are crucial in its activation, the consequence of prolonged PTP opening is dissipation of the inner mitochondrial membrane potential, cessation of ATP synthesis, bioenergetic crisis, and cell death-a primary characteristic of mitochondrial disorders. PTP involvement in mitochondrial and cellular demise in a variety of disease paradigms has been long appreciated, yet the exact molecular entity of the PTP and the development of potent and specific PTP inhibitors remain areas of active investigation. In this review, we will (i) summarize recent advances made in elucidating the molecular nature of the PTP focusing on evidence pointing to mitochondrial FoF1-ATP synthase, (ii) summarize studies aimed at discovering novel PTP inhibitors, and (iii) review data supporting compromised PTP activity in specific mitochondrial diseases.