The roles of phosphate and the phosphate carrier in the mitochondrial permeability transition pore

Hydrogen Sulfide Delivery to Enhance Bone Tissue Engineering Cell Survival

Though crucial for natural bone healing, local calcium ion (Ca2+) and phosphate ion (Pi) concentrations can exceed the cytotoxic limit leading to mitochondrial overload, oxidative stress, and cell death. For bone tissue engineering applications, H2S can be employed as a cytoprotective molecule to enhance mesenchymal stem cell (MSC) tolerance to cytotoxic Ca2+/Pi concentrations. Varied concentrations of sodium hydrogen sulfide (NaSH), a fast-releasing H2S donor, were applied to assess the influence of H2S on MSC proliferation. The results suggested a toxicity limit of 4 mM for NaSH and that 1 mM of NaSH could improve cell proliferation and differentiation in the presence of cytotoxic levels of Ca2+ (32 mM) and/or Pi (16 mM). To controllably deliver H2S over time, a novel donor molecule (thioglutamic acid-GluSH) was synthesized and evaluated for its H2S release profile. Excitingly, GluSH successfully maintained cytoprotective level of H2S over 7 days. Furthermore, MSCs exposed to cytotoxic Ca2+/Pi concentrations in the presence of GluSH were able to thrive and differentiate into osteoblasts. These findings suggest that the incorporation of a sustained H2S donor such as GluSH into CaP-based bone graft substitutes can facilitate considerable cytoprotection, making it an attractive option for complex bone regenerative engineering applications.

Quantitative Assessment of Intracellular Effectors and Cellular Response in RAGE Activation

The review delves into the methods for the quantitative assessment of intracellular effectors and cellular response of Receptor for Advanced Glycation End products (RAGE), a vital transmembrane receptor involved in a range of physiological and pathological processes. RAGE bind to Advanced Glycation End products (AGEs) and other ligands, which in turn activate diverse downstream signaling pathways that impact cellular responses such as inflammation, oxidative stress, and immune reactions. The review article discusses the intracellular signaling pathways activated by RAGE followed by differential activation of RAGE signaling across various diseases. This will ultimately guide researchers in developing targeted and effective interventions for diseases associated with RAGE activation. Further, we have discussed how PCR, western blotting, and microscopic examination of various molecules involved in downstream signaling can be leveraged to monitor, diagnose, and explore diseases involving proteins with unique post-translational modifications. This review article underscores the pressing need for advancements in molecular approaches for disease detection and management involving RAGE.

Mitophagy in intracerebral hemorrhage: a new target for therapeutic intervention

Intracerebral hemorrhage is a life-threatening condition with a high fatality rate and severe sequelae. However, there is currently no treatment available for intracerebral hemorrhage, unlike for other stroke subtypes. Recent studies have indicated that mitochondrial dysfunction and mitophagy likely relate to the pathophysiology of intracerebral hemorrhage. Mitophagy, or selective autophagy of mitochondria, is an essential pathway to preserve mitochondrial homeostasis by clearing up damaged mitochondria. Mitophagy markedly contributes to the reduction of secondary brain injury caused by mitochondrial dysfunction after intracerebral hemorrhage. This review provides an overview of the mitochondrial dysfunction that occurs after intracerebral hemorrhage and the underlying mechanisms regarding how mitophagy regulates it, and discusses the new direction of therapeutic strategies targeting mitophagy for intracerebral hemorrhage, aiming to determine the close connection between mitophagy and intracerebral hemorrhage and identify new therapies to modulate mitophagy after intracerebral hemorrhage. In conclusion, although only a small number of drugs modulating mitophagy in intracerebral hemorrhage have been found thus far, most of which are in the preclinical stage and require further investigation, mitophagy is still a very valid and promising therapeutic target for intracerebral hemorrhage in the long run.

Mitochondria in health, disease, and aging

Mitochondria are well known as organelles responsible for the maintenance of cellular bioenergetics through the production of ATP. Although oxidative phosphorylation may be their most important function, mitochondria are also integral for the synthesis of metabolic precursors, calcium regulation, the production of reactive oxygen species, immune signaling, and apoptosis. Considering the breadth of their responsibilities, mitochondria are fundamental for cellular metabolism and homeostasis. Appreciating this significance, translational medicine has begun to investigate how mitochondrial dysfunction can represent a harbinger of disease. In this review, we provide a detailed overview of mitochondrial metabolism, cellular bioenergetics, mitochondrial dynamics, autophagy, mitochondrial damage-associated molecular patterns, mitochondria-mediated cell death pathways, and how mitochondrial dysfunction at any of these levels is associated with disease pathogenesis. Mitochondria-dependent pathways may thereby represent an attractive therapeutic target for ameliorating human disease.

The Interplay between Mitochondrial Dysfunction and Ferroptosis during Ischemia-Associated Central Nervous System Diseases

Cerebral ischemia, a leading cause of disability and mortality worldwide, triggers a cascade of molecular and cellular pathologies linked to several central nervous system (CNS) disorders. These disorders primarily encompass ischemic stroke, Alzheimer's disease (AD), Parkinson's disease (PD), epilepsy, and other CNS conditions. Despite substantial progress in understanding and treating the underlying pathological processes in various neurological diseases, there is still a notable absence of effective therapeutic approaches aimed specifically at mitigating the damage caused by these illnesses. Remarkably, ischemia causes severe damage to cells in ischemia-associated CNS diseases. Cerebral ischemia initiates oxygen and glucose deprivation, which subsequently promotes mitochondrial dysfunction, including mitochondrial permeability transition pore (MPTP) opening, mitophagy dysfunction, and excessive mitochondrial fission, triggering various forms of cell death such as autophagy, apoptosis, as well as ferroptosis. Ferroptosis, a novel type of regulated cell death (RCD), is characterized by iron-dependent accumulation of lethal reactive oxygen species (ROS) and lipid peroxidation. Mitochondrial dysfunction and ferroptosis both play critical roles in the pathogenic progression of ischemia-associated CNS diseases. In recent years, growing evidence has indicated that mitochondrial dysfunction interplays with ferroptosis to aggravate cerebral ischemia injury. However, the potential connections between mitochondrial dysfunction and ferroptosis in cerebral ischemia have not yet been clarified. Thus, we analyzed the underlying mechanism between mitochondrial dysfunction and ferroptosis in ischemia-associated CNS diseases. We also discovered that GSH depletion and GPX4 inactivation cause lipoxygenase activation and calcium influx following cerebral ischemia injury, resulting in MPTP opening and mitochondrial dysfunction. Additionally, dysfunction in mitochondrial electron transport and an imbalanced fusion-to-fission ratio can lead to the accumulation of ROS and iron overload, which further contribute to the occurrence of ferroptosis. This creates a vicious cycle that continuously worsens cerebral ischemia injury. In this study, our focus is on exploring the interplay between mitochondrial dysfunction and ferroptosis, which may offer new insights into potential therapeutic approaches for the treatment of ischemia-associated CNS diseases.

Mechanistic correlation between mitochondrial permeability transition pores and mitochondrial ATP dependent potassium channels in ischemia reperfusion

Mitochondrial dysfunction is one of the fundamental causes of ischemia reperfusion (I/R) damage. I/R refers to the paradoxical progression of cellular dysfunction and death that occurs when blood flow is restored to previously ischemic tissues. I/R causes a significant rise in mitochondrial permeability resulting in the opening of mitochondrial permeability transition pores (MPTP). The MPTP are broad, nonspecific channels present in the inner mitochondrial membrane (IMM), and are known to mediate the deadly permeability alterations that trigger mitochondrial driven cell death. Protection from reperfusion injury occurs when long-term ischemia is accompanied by short-term ischemic episodes or inhibition of MPTP from opening via mitochondrial ATP dependent potassium (mitoK_ATP) channels. These channels located in the IMM, play an essential role in ischemia preconditioning (PC) and protect against cell death by blocking MPTP opening. This review primarily focuses on the interaction between the MPTP and mitoK_ATP along with their role in the I/R injury. This article also describes the molecular composition of the MPTP and mitoK_ATP in order to promote future knowledge and treatment of diverse I/R injuries in various organs.

Inhibition of mitochondrial phosphate carrier prevents high phosphate-induced superoxide generation and vascular calcification

Vascular calcification is a serious complication of hyperphosphatemia that causes cardiovascular morbidity and mortality. Previous studies have reported that plasmalemmal phosphate (Pi) transporters, such as PiT-1/2, mediate depolarization, Ca²⁺ influx, oxidative stress, and calcific changes in vascular smooth muscle cells (VSMCs). However, the pathogenic mechanism of mitochondrial Pi uptake in vascular calcification associated with hyperphosphatemia has not been elucidated. We demonstrated that the phosphate carrier (PiC) is the dominant mitochondrial Pi transporter responsible for high Pi-induced superoxide generation, osteogenic gene upregulation, and calcific changes in primary VSMCs isolated from rat aortas. Notably, acute incubation with high Pi markedly increased the protein abundance of PiC via ERK1/2- and mTOR-dependent translational upregulation. Genetic suppression of PiC prevented Pi-induced ERK1/2 activation, superoxide production, osteogenic differentiation, and vascular calcification of VSMCs in vitro and aortic rings ex vivo. Pharmacological inhibition of mitochondrial Pi transport using butyl malonate (BMA) or mersalyl abolished all pathologic changes involved in high Pi-induced vascular calcification. BMA or mersalyl also effectively prevented osteogenic gene upregulation and calcification of aortas from 5/6 subtotal nephrectomized mice fed a high-Pi diet. Our results suggest that mitochondrial Pi uptake via PiC is a critical molecular mechanism mediating mitochondrial superoxide generation and pathogenic calcific changes, which could be a novel therapeutic target for treating vascular calcification associated with hyperphosphatemia.

Anastrozole-mediated modulation of mitochondrial activity by inhibition of mitochondrial permeability transition pore opening: an initial perspective

The mitochondrial permeability transition pore (mtPTP) plays a vital role in altering the structure and function of mitochondria. Cyclophilin D (CypD) is a mitochondrial protein that regulates mtPTP function and a known drug target for therapeutic studies involving mitochondria. While the effect of aromatase inhibition on the mtPTP has been studied previously, the effect of anastrozole on the mtPTP has not been completely elucidated. The role of anastrozole in modulating the mtPTP was evaluated by docking, molecular dynamics and network-guided studies using human CypD data. The peripheral blood mononuclear cells (PBMCs) of patients with mitochondrial disorders and healthy controls were treated with anastrozole and evaluated for mitochondrial permeability transition pore (mtPTP) function and apoptosis using a flow cytometer. Spectrophotometry was employed for estimating total ATP levels. The anastrozole-CypD complex is more stable than cyclosporin A (CsA)-CypD. Anastrozole performed better than cyclosporine in inhibiting mtPTP. Additional effects included inducing mitochondrial membrane depolarization and a reduction in mitochondrial swelling and superoxide generation, intrinsic caspase-3 activity and cellular apoptosis, along with an increase in ATP levels. Anastrozole may serve as a potential therapeutic agent for mitochondrial disorders and ameliorate the clinical phenotype by regulating the activity of mtPTP. However, further studies are required to substantiate our preliminary findings.Communicated by Ramaswamy H. Sarma.

Non-conventional mitochondrial permeability transition: Its regulation by mitochondrial dynamics

Mitochondrial permeability transition (MPT) is a phenomenon that the inner mitochondrial membrane (IMM) loses its selective permeability, leading to mitochondrial dysfunction and cell injury. Electrophysiological evidence indicates the presence of a mega-channel commonly called permeability transition pore (PTP) whose opening is responsible for MPT. However, the molecular identity of the PTP is still under intensive investigations and debates, although cyclophilin D that is inhibited by cyclosporine A (CsA) is the established regulatory component of the PTP. PTP can also open transiently and functions as a rapid mitochondrial Ca2+ releasing mechanism. Mitochondrial fission and fusion, the main components of mitochondrial dynamics, control the number and size of mitochondria, and have been shown to play a role in regulating MPT directly or indirectly. Studies by us and others have indicated the potential existence of a form of transient MPT that is insensitive to CsA. This "non-conventional" MPT is regulated by mitochondrial dynamics and may serve a protective role possibly by decreasing the susceptibility for a frequent or sustained PTP opening; hence, it may have a therapeutic value in many disease conditions involving MPT.

Calcium Overload and Mitochondrial Metabolism

Mitochondria calcium is a double-edged sword. While low levels of calcium are essential to maintain optimal rates of ATP production, extreme levels of calcium overcoming the mitochondrial calcium retention capacity leads to loss of mitochondrial function. In moderate amounts, however, ATP synthesis rates are inhibited in a calcium-titratable manner. While the consequences of extreme calcium overload are well-known, the effects on mitochondrial function in the moderately loaded range remain enigmatic. These observations are associated with changes in the mitochondria ultrastructure and cristae network. The present mini review/perspective follows up on previous studies using well-established cryo-electron microscopy and poses an explanation for the observable depressed ATP synthesis rates in mitochondria during calcium-overloaded states. The results presented herein suggest that the inhibition of oxidative phosphorylation is not caused by a direct decoupling of energy metabolism via the opening of a calcium-sensitive, proteinaceous pore but rather a separate but related calcium-dependent phenomenon. Such inhibition during calcium-overloaded states points towards mitochondrial ultrastructural modifications, enzyme activity changes, or an interplay between both events.

SR-Mitochondria Crosstalk Shapes Ca Signalling to Impact Pathophenotype in Disease Models Marked by Dysregulated Intracellular Ca Release

AIMS

Diastolic Ca release (DCR) from sarcoplasmic reticulum (SR) Ca release channel ryanodine receptor (RyR2) has been linked to multiple cardiac pathologies, but its exact role in shaping divergent cardiac pathologies remains unclear. We hypothesize that the SR-mitochondria interplay contributes to disease phenotypes by shaping Ca signalling.

METHODS AND RESULTS

A genetic model of catecholaminergic polymorphic ventricular tachycardia (CPVT2 model of CASQ2 knockout) and a pre-diabetic cardiomyopathy model of fructose-fed mice (FFD), both marked by DCR, are employed in this study. Mitochondria Ca (mCa) is modulated by pharmacologically targeting mitochondria Ca uniporter (MCU) or permeability transition pore (mPTP), mCa uptake, and extrusion mechanisms, respectively. An MCU activator abolished Ca waves in CPVT2 but exacerbated waves in FFD cells. Mechanistically this is ascribed to mitochondria's function as a Ca buffer or source of reactive oxygen species (mtROS) to exacerbate RyR2 functionality, respectively. Enhancing mCa uptake reduced and elevated mtROS production in CPVT2 and FFD, respectively. In CPVT2, mitochondria took up more Ca in permeabilized cells, and had higher level of mCa content in intact cells vs. FFD. Conditional ablation of MCU in the CPVT2 model caused lethality and cardiac remodelling, but reduced arrhythmias in the FFD model. In parallel, CPVT2 mitochondria also employ up-regulated mPTP-mediated Ca efflux to avoid mCa overload, as seen by elevated incidence of MitoWinks (an indicator of mPTP-mediated Ca efflux) vs. FFD. Both pharmacological and genetic inhibition of mPTP promoted mtROS production and exacerbation of myocyte Ca handling in CPVT2. Further, genetic inhibition of mPTP exacerbated arrhythmias in CPVT2.

CONCLUSION

In contrast to FFD, which is more susceptible to mtROS-dependent RyR2 leak, in CPVT2 mitochondria buffer SR-derived DCR to mitigate Ca-dependent pathological remodelling and rely on mPTP-mediated Ca efflux to avoid mCa overload. SR-mitochondria interplay contributes to the divergent pathologies by disparately shaping intracellular Ca signalling.

Parkinson’s Disease Etiology: Insights and Associations with Phosphate Toxicity

The present paper investigated the association of Parkinson’s disease etiology with phosphate toxicity, a pathophysiological condition in which dysregulated phosphate metabolism causes excessive inorganic phosphate sequestration in body tissue that damages organ systems. Excessive phosphate is proposed to reduce Complex I function of the mitochondrial electron transport chain in Parkinson’s disease and is linked to opening of the mitochondrial permeability transition pore, resulting in increased reactive oxygen species, inflammation, DNA damage, mitochondrial membrane depolarization, and ATP depletion causing cell death. Parkinson’s disease is associated with α-synuclein and Lewy body dementia, a secondary tauopathy related to hyperphosphorylation of tau protein, and tauopathy is among several pathophysiological pathways shared between Parkinson’s disease and diabetes. Excessive phosphate is also associated with ectopic calcification, bone mineral disorders, and low levels of serum vitamin D in patients with Parkinson’s disease. Sarcopenia and cancer in Parkinson’s disease patients are also associated with phosphate toxicity. Additionally, Parkinson’s disease benefits are related to low dietary phosphate intake. More studies are needed to investigate the potential mediating role of phosphate toxicity in the etiology of Parkinson’s disease.

Oxidative Stress Related to Plasmalemmal and Mitochondrial Phosphate Transporters in Vascular Calcification

Inorganic phosphate (Pi) is essential for maintaining cellular function but excess of Pi leads to serious complications, including vascular calcification. Accumulating evidence suggests that oxidative stress contributes to the pathogenic progression of calcific changes. However, the molecular mechanism underlying Pi-induced reactive oxygen species (ROS) generation and its detrimental consequences remain unclear. Type III Na⁺-dependent Pi cotransporter, PiT-1/-2, play a significant role in Pi uptake of vascular smooth muscle cells. Pi influx via PiT-1/-2 increases the abundance of PiT-1/-2 and depolarization-activated Ca²⁺ entry due to its electrogenic properties, which may lead to Ca²⁺ and Pi overload and oxidative stress. At least four mitochondrial Pi transporters are suggested, among which the phosphate carrier (PiC) is known to be mainly involved in mitochondrial Pi uptake. Pi transport via PiC may induce hyperpolarization and superoxide generation, which may lead to mitochondrial dysfunction and endoplasmic reticulum stress, together with generation of cytosolic ROS. Increase in net influx of Ca²⁺ and Pi and their accumulation in the cytosol and mitochondrial matrix synergistically increases oxidative stress and osteogenic differentiation, which could be prevented by suppressing either Ca²⁺ or Pi overload. Therapeutic strategies targeting plasmalemmal and mitochondrial Pi transports can protect against Pi-induced oxidative stress and vascular calcification.

Effects of Tl+ on the inner membrane thiol groups, respiration, and swelling in succinate-energized rat liver mitochondria were modified by thiol reagents

The effects of both Tl⁺ and thiol reagents were studied on the content of the inner membrane free SH-groups, detected with Ellman reagent, and the inner membrane potential as well as swelling and respiration of succinate-energized rat liver mitochondria in medium containing TlNO₃ and KNO₃. These effects resulted in a rise in swelling and a decrease in the content, the potential, and mitochondrial respiration in 3 and 2,4-dinitrophenol-uncoupled states. A maximal effect was seen when phenylarsine oxide reacting with thiol groups recessed into the hydrophobic regions of the membrane. Compared with phenylarsine oxide, the effective concentrations of other reagents were approximately one order of magnitude higher in experiments with mersalyl and 4,4'-diisothiocyanostilbene-2,2'-disulfonate, and two orders of magnitude higher in experiments with tert-butyl hydroperoxide and diamide. The above effects of Tl⁺ and the thiol reagents became even more pronounced with calcium overload of mitochondria. However, the effects were suppressed by inhibitors of the mitochondrial permeability transition pore (cyclosporine A, ADP, and n-ethylmaleimide). These findings suggest that opening of the pore induced by Tl⁺ in the inner membrane can be dependent on the conformation state of the adenine nucleotide translocase, which depends on the activity of its thiol groups.

The mitochondrial permeability transition pore: an evolving concept critical for cell life and death

In this review, we summarize current knowledge of perhaps one of the most intriguing phenomena in cell biology: the mitochondrial permeability transition pore (mPTP). This phenomenon, which was initially observed as a sudden loss of inner mitochondrial membrane impermeability caused by excessive calcium, has been studied for almost 50 years, and still no definitive answer has been provided regarding its mechanisms. From its initial consideration as an in vitro artifact to the current notion that the mPTP is a phenomenon with physiological and pathological implications, a long road has been travelled. We here summarize the role of mitochondria in cytosolic calcium control and the evolving concepts regarding the mitochondrial permeability transition (mPT) and the mPTP. We show how the evolving mPTP models and mechanisms, which involve many proposed mitochondrial protein components, have arisen from methodological advances and more complex biological models. We describe how scientific progress and methodological advances have allowed milestone discoveries on mPTP regulation and composition and its recognition as a valid target for drug development and a critical component of mitochondrial biology.

Mitochondria and the permeability transition pore in cancer metabolic reprogramming

Mitochondria are a major source of ATP provision as well as cellular suicidal weapon store. Accumulating evidences demonstrate that mitochondrial bioenergetics, biosynthesis and signaling are important mediators of tumorigenesis. Metabolic plasticity enables cancer cell reprogramming to cope with cellular and environmental alterations, a process requires mitochondria biology. Mitochondrial metabolism emerges to be a promising arena for cancer therapeutic targets. The permeability transition pore (PTP) participates in physiological Ca²⁺ and ROS homeostasis as well as cell death depending on the open state. The hypothesis that PTP forms from F-ATP synthase provides clues to the potential collaborative role of mitochondrial respiration and PTP in regulating cancer cell fate and metabolic reprogramming.

Regulated cell death in cisplatin-induced AKI: relevance of myo-inositol metabolism

Regulated cell death (RCD), distinct from accidental cell death, refers to a process of well-controlled programmed cell death with well-defined pathological mechanisms. In the past few decades, various terms for RCDs were coined, and some of them have been implicated in the pathogenesis of various types of acute kidney injury (AKI). Cisplatin is widely used as a chemotherapeutic drug for a broad spectrum of cancers, but its usage was hampered because of being highly nephrotoxic. Cisplatin-induced AKI is commonly seen clinically, and it also serves as a well-established prototypic model for laboratory investigations relevant to acute nephropathy affecting especially the tubular compartment. Literature reports over a period of three decades have indicated that there are multiple types of RCDs, including apoptosis, necroptosis, pyroptosis, ferroptosis, and mitochondrial permeability transition-mediated necrosis, and some of them are pertinent to the pathogenesis of cisplatin-induced AKI. Interestingly, myo-inositol metabolism, a vital biological process that is largely restricted to the kidney, seems to be relevant to the pathogenesis of certain forms of RCDs. A comprehensive understanding of RCDs in cisplatin-induced AKI and their relevance to myo-inositol homeostasis may yield novel therapeutic targets for the amelioration of cisplatin-related nephropathy.

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.

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.

Interaction of Agaric Acid with the Adenine Nucleotide Translocase Induces Mitochondrial Oxidative Stress

Mitochondrial permeability transition is characterized by the opening of a transmembranal pore that switches membrane permeability from specific to nonspecific. This structure allows the free traffic of ions, metabolites, and water across the mitochondrial inner membrane. The opening of the permeability transition pore is triggered by oxidative stress along with calcium overload. In this work, we explored if oxidative stress is a consequence, rather than an effector of the pore opening, by evaluating the interaction of agaric acid with the adenine nucleotide translocase, a structural component of the permeability transition pore. We found that agaric acid induces transition pore opening, increases the generation of oxygen-derived reactive species, augments the oxidation of unsaturated fatty acids in the membrane, and promotes the detachment of cytochrome c from the inner membrane. The effect of agaric acid was inhibited by the antioxidant tamoxifen in association with decreased binding of the thiol reagent eosin-3 maleimide to the adenine nucleotide translocase. We conclude that agaric acid promotes the opening of the pore, increasing ROS production that exerts oxidative modification of critical thiols in the adenine nucleotide translocase.

Maternal hypothyroidism during pregnancy alters the function of the retinol-binding protein 4-mediated mitochondrial permeability conversion pore in the kidneys of offspring rats

OBJECTIVES

To determine the role of the RBP4/PiC/SIRT3 signaling pathway in the opening of the mitochondria permeability transition pore (mPTP) in offspring rats with hypothyroidism during pregnancy.

METHODS

Sixty Sprague-Dawley (SD) rats were employed in this study. Pregnancy was deemed successful when a sperm was found in the uterus. After one week of pregnancy, offspring rats were divided into the following groups: overall hypothyroidism group (OH group), subclinical hypothyroidism group (SCH group), and normal control group (CON group). The establishment of the hypothyroidism model was confirmed when the serum thyroid stimulating hormone (TSH) levels were higher than normal value and TT4 level was within the normal range. The renal mitochondria of offspring rats were extracted on the 14th postnatal day (P14) and 35th postnatal day (P35).

RESULTS

At P14, no significant differences in the degree of mPTP opening and expression of phosphoric acid carrier vector (PiC) were detected between the rats in the OH group and the SCH group. However, the expression level of silent mating-type information regulation 3 homolog (SIRT3) was markedly reduced. Retinol-binding protein 4 (RBP4) expression increased in the rats from the OH group, relative to that in those from the SCH group. At P35, the degree of mPTP opening and the expression levels of PiC and RBP4 in the OH group were higher than those in the SCH group. However, SIRT3 expression in the OH group was lower than that observed in the SCH group.

CONCLUSION

RBP4 plays an important role in early renal mitochondrial damage and renal impairment in rats suffering from hypothyroidism during pregnancy. The RBP4/PiC/SIRT3 pathway is thus involved in the opening of the renal mPTP in offspring rats with hyperthyroidism.

Recent progress in the use of mitochondrial membrane permeability transition pore in mitochondrial dysfunction-related disease therapies

Mitochondria have various cellular functions, including ATP synthesis, calcium homeostasis, cell senescence, and death. Mitochondrial dysfunction has been identified in a variety of disorders correlated with human health. Among the many underlying mechanisms of mitochondrial dysfunction, the opening up of the mitochondrial permeability transition pore (mPTP) is one that has drawn increasing interest in recent years. It plays an important role in apoptosis and necrosis; however, the molecular structure and function of the mPTP have still not been fully elucidated. In recent years, the abnormal opening up of the mPTP has been implicated in the development and pathogenesis of diverse diseases including ischemia/reperfusion injury (IRI), neurodegenerative disorders, tumors, and chronic obstructive pulmonary disease (COPD). This review provides a systematic introduction to the possible molecular makeup of the mPTP and summarizes the mitochondrial dysfunction-correlated diseases and highlights possible underlying mechanisms. Since the mPTP is an important target in mitochondrial dysfunction, this review also summarizes potential treatments, which may be used to inhibit pore opening up via the molecules composing mPTP complexes, thus suppressing the progression of mitochondrial dysfunction-related diseases.

Mitochondrial calcium transport and permeability transition as rational targets for plant protection

The mitochondrial permeability transition (MPT) is a death-inducing mechanism that collapses electrochemical gradients across inner mitochondrial membranes. Several studies in model plants have detailed potential MPT-dependent cell death upon abiotic stress in response to heat shock, ultraviolet radiation, heavy metal toxicity and waterlogging. However, the molecular specifics of the MPT and its possible role on plant cell death remain controversial. This review addresses previous and recent developments on the role(s) of the MPT in plants. Considering these advances, MPT targeting can constitute a plausible strategy to ameliorate cell death in plants upon abiotic stress.

Physiological Ca2+ Transients Versus Pathological Steady-State Ca2+ Elevation, Who Flips the ROS Coin in Skeletal Muscle Mitochondria

Mitochondria are both the primary provider of ATP and the pivotal regulator of cell death, which are essential for physiological muscle activities. Ca²⁺ plays a multifaceted role in mitochondrial function. During muscle contraction, Ca²⁺ influx into mitochondria activates multiple enzymes related to tricarboxylic acid (TCA) cycle and oxidative phosphorylation, resulting in increased ATP synthesis to meet the energy demand. Pathophysiological conditions such as skeletal muscle denervation or unloading also lead to elevated Ca²⁺ levels inside mitochondria. However, the outcomes of this steady-state elevation of mitochondrial Ca²⁺ level include exacerbated reactive oxygen species (ROS) generation, sensitized opening of mitochondrial permeability transition pore (mPTP), induction of programmed cell death, and ultimately muscle atrophy. Previously, both acute and long-term endurance exercises have been reported to activate certain signaling pathways to counteract ROS production. Meanwhile, electrical stimulation is known to help prevent apoptosis and alleviate muscle atrophy in denervated animal models and patients with motor impairment. There are various mechanistic studies that focus on the excitation-transcription coupling framework to understand the beneficial role of exercise and electrical stimulation. Interestingly, a recent study has revealed an unexpected role of rapid mitochondrial Ca²⁺ transients in keeping mPTP at a closed state with reduced mitochondrial ROS production. This discovery motivated us to contribute this review article to inspire further discussion about the potential mechanisms underlying differential outcomes of physiological mitochondrial Ca²⁺ transients and pathological mitochondrial Ca²⁺ elevation in skeletal muscle ROS production.

Regulation of Cell Death by Mitochondrial Transport Systems of Calcium and Bcl-2 Proteins

Mitochondria represent the fundamental system for cellular energy metabolism, by not only supplying energy in the form of ATP, but also by affecting physiology and cell death via the regulation of calcium homeostasis and the activity of Bcl-2 proteins. A lot of research has recently been devoted to understanding the interplay between Bcl-2 proteins, the regulation of these interactions within the cell, and how these interactions lead to the changes in calcium homeostasis. However, the role of Bcl-2 proteins in the mediation of mitochondrial calcium homeostasis, and therefore the induction of cell death pathways, remain underestimated and are still not well understood. In this review, we first summarize our knowledge about calcium transport systems in mitochondria, which, when miss-regulated, can induce necrosis. We continue by reviewing and analyzing the functions of Bcl-2 proteins in apoptosis. Finally, we link these two regulatory mechanisms together, exploring the interactions between the mitochondrial Ca2+ transport systems and Bcl-2 proteins, both capable of inducing cell death, with the potential to determine the cell death pathway-either the apoptotic or the necrotic one.

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.

Mitochondria as a promising target for developing novel agents for treating Alzheimer's disease

The mitochondria-targeting drugs can be conventionally divided into the following groups: those compensating for the energy deficit involved in neurodegeneration, including stimulants of mitochondrial bioenergetics and activators of mitochondrial biogenesis; and neuroprotectors, that are compounds increasing the resistance of mitochondria to opening of mitochondrial permeability transition (MPT) pores. Although compensating for the energy deficit and inhibition of MPT are obvious targets for drugs used in the very early stages of Alzheimer-like pathology, but their use as the monotherapy for patients with severe symptoms is unlikely to be sufficiently effective. It would be optimal to combine targets that would provide the cognitive-stimulating, the neuroprotective effects and the ability to affect specific disease-forming mechanisms. In the design of such drugs, assessment of their potential mitochondrial-targeted effects is of particular importance. The possibility of targeted drug design for simultaneous action on mitochondrial and neurotransmitter's receptors targets is, in particularly, based on the known interplay of various cellular pathways and the presence of common structural components. Of particular interest is directed search for multitarget drugs that would act simultaneously on mitochondrial calcium-dependent functions, the targets (receptors, enzymes, etc.) facilitating neurotransmission, and the molecular targets related to the action of so-called disease-modifying factors, in particular, the formation and overcoming of the toxicity of β-amyloid or hyperphosphorylated tau protein. The examples of such approaches realized on the level of preclinical and clinical trials are presented below.

Mitochondrial Transfer as a Therapeutic Strategy Against Ischemic Stroke

Stroke is a debilitating disease that remains the second leading cause of death and disability worldwide. Despite accumulating knowledge of the disease pathology, treatments for stroke are limited, and clinical translation of the neuroprotective agents has not been a complete success. Accumulating evidence links mitochondrial dysfunction to brain impairments after stroke. Recent studies have implicated the important roles of healthy mitochondria in neuroprotection and neural recovery following ischemic stroke. New and convincing studies have shown that mitochondrial transfer to the damaged cells can help revive cells energetic in the recipient cells. Hence, mitochondrial transplantation has shown to replace impaired or dysfunctional mitochondria with exogenous healthy mitochondria after ischemic stroke. We highlight the potential of mitochondrial transfer by stem cells as a therapeutic strategy for the treatment of ischemic stroke. This review captures the recent advances in the mitochondrial transfer as a novel and promising treatment for ischemic stroke.

Mitochondrial SLC25 Carriers: Novel Targets for Cancer Therapy

The transfer of metabolites through the mitochondrial membranes is a vital process that is highly controlled and regulated by the inner membrane. A variety of metabolites, nucleotides, and cofactors are transported across the inner mitochondrial membrane (IMM) by a superfamily of membrane transporters which are known as the mitochondrial carrier family (MCF) or the solute carrier family 25 (SLC25 protein family). In humans, the MCF has 53 members encoded by nuclear genes. Members of the SLC25 family of transporters, which is the largest group of solute carriers, are also known as mitochondrial carriers (MCs). Because MCs are nuclear-coded proteins, they must be imported into the IMM. When compared with normal cells, the mitochondria of cancer cells exhibit significantly increased transmembrane potentials and a number of their transporters are altered. SLC25 members were identified as potential biomarkers for various cancers. The objective of this review is to summarize what is currently known about the involvement of mitochondrial SLC25 carriers in associated diseases. This review suggests that the SLC25 family could be used for the development of novel points of attack for targeted cancer therapy.

Autophagy protects kidney from phosphate-induced mitochondrial injury

Hyperphosphatemia is a common complication in patients with advanced chronic kidney disease (CKD) as well as an increased risk of cardiovascular mortality; however, the molecular mechanisms of phosphate-mediated kidney injury are largely unknown. Autophagy is a lysosomal degradation system, which plays protective roles against kidney diseases. Here, we studied the role of autophagy in kidney proximal tubular cells (PTECs) during phosphate overload. Temporal cessation of autophagy in drug-induced PTEC-specific autophagy-deficient mice that were fed high phosphate diet induced mild cytosolic swelling and an accumulation of SQSTM1/p62-and ubiquitin-positive protein aggregates in PTECs, indicating that phosphate overload requires enhanced autophagic activity for the degradation of increasing substrate. Morphological and biochemical analysis demonstrated that high phosphate activates mitophagy in PTECs in response to oxidative stress. PTEC-specific autophagy-deficient mice receiving heminephrectomy and autophagy-deficient cultured PTECs exhibited mitochondrial dysfunction, increased reactive oxygen species production, and reduced ATP production in response to phosphate overload, suggesting that high phosphate-induced autophagy counteracts mitochondrial injury and maintains cellular bioenergetics in PTECs. Thus, potentiating autophagic activity could be a therapeutic option for suppressing CKD progression during phosphate overload.

Mitochondria as a therapeutic target for ischemic stroke

Stroke is the leading cause of death and physical disability worldwide. Mitochondrial dysfunction has been considered as one of the hallmarks of ischemic stroke and contributes to the pathology of ischemia and reperfusion. Mitochondria is essential in promoting neural survival and neurological improvement following ischemic stroke. Therefore, mitochondria represent an important drug target for stroke treatment. This review discusses the mitochondrial molecular mechanisms underlying cerebral ischemia and involved in reactive oxygen species generation, mitochondrial electron transport dysfunction, mitochondria-mediated regulation of inflammasome activation, mitochondrial dynamics and biogenesis, and apoptotic cell death. We highlight the potential of mitochondrial transfer by stem cells as a therapeutic target for stroke treatment and provide valuable insights for clinical strategies. A better understanding of the roles of mitochondria in ischemia-induced cell death and protection may provide a rationale design of novel therapeutic interventions in the ischemic stroke.

Cyclosporin A Increases Mitochondrial Buffering of Calcium: An Additional Mechanism in Delaying Mitochondrial Permeability Transition Pore Opening

Regulation of mitochondrial free Ca²⁺ is critically important for cellular homeostasis. An increase in mitochondrial matrix free Ca²⁺ concentration ([Ca²⁺]_m) predisposes mitochondria to opening of the permeability transition pore (mPTP). Opening of the pore can be delayed by cyclosporin A (CsA), possibly by inhibiting cyclophilin D (Cyp D), a key regulator of mPTP. Here, we report on a novel mechanism by which CsA delays mPTP opening by enhanced sequestration of matrix free Ca²⁺. Cardiac-isolated mitochondria were challenged with repetitive CaCl₂ boluses under Na⁺-free buffer conditions with and without CsA. CsA significantly delayed mPTP opening primarily by promoting matrix Ca²⁺ sequestration, leading to sustained basal [Ca²⁺]_m levels for an extended period. The preservation of basal [Ca²⁺]_m during the CaCl₂ pulse challenge was associated with normalized NADH, matrix pH (pH_m), and mitochondrial membrane potential (ΔΨ_m). Notably, we found that in PO₄³⁻ (P_i)-free buffer condition, the CsA-mediated buffering of [Ca²⁺]_m was abrogated, and mitochondrial bioenergetics variables were concurrently compromised. In the presence of CsA, addition of P_i just before pore opening in the P_i-depleted condition reinstated the Ca²⁺ buffering system and rescued mitochondria from mPTP opening. This study shows that CsA promotes P_i-dependent mitochondrial Ca²⁺ sequestration to delay mPTP opening and, concomitantly, maintains mitochondrial function.

Inhibition of mitochondrial permeability transition by deletion of the ANT family and CypD

The mitochondrial permeability transition pore (MPTP) has resisted molecular identification. The original model of the MPTP that proposed the adenine nucleotide translocator (ANT) as the inner membrane pore-forming component was challenged when mitochondria from Ant1/2 double null mouse liver still had MPTP activity. Because mice express three Ant genes, we reinvestigated whether the ANTs comprise the MPTP. Liver mitochondria from Ant1, Ant2, and Ant4 deficient mice were highly refractory to Ca2+-induced MPTP formation, and when also given cyclosporine A (CsA), the MPTP was completely inhibited. Moreover, liver mitochondria from mice with quadruple deletion of Ant1, Ant2, Ant4, and Ppif (cyclophilin D, target of CsA) lacked Ca2+-induced MPTP formation. Inner-membrane patch clamping in mitochondria from Ant1, Ant2, and Ant4 triple null mouse embryonic fibroblasts showed a loss of MPTP activity. Our findings suggest a model for the MPTP consisting of two distinct molecular components: The ANTs and an unknown species requiring CypD.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

Mitophagy, a potential therapeutic target for stroke

Mitochondria autophagy, termed as mitophagy, is a mechanism of specific autophagic elimination of mitochondria. Mitophagy controls the quality and the number of mitochondria, eliminating dysfunctional or excessive mitochondria that can generate reactive oxygen species (ROS) and cause cell death. Mitochondria are centrally implicated in neuron and tissue injury after stroke, due to the function of supplying adenosine triphosphate (ATP) to the tissue, regulating oxidative metabolism during the pathologic process, and contribution to apoptotic cell death after stroke. As a catabolic mechanism, mitophagy links numbers of a complex network of mitochondria, and affects mitochondrial dynamic process, fusion and fission, reducing mitochondrial production of ROS, mediated by the mitochondrial permeability transition pore (MPTP). The precise nature of mitophagy’s involvement in stroke, and its underlying molecular mechanisms, have yet to be fully clarified. This review aims to provide a comprehensive overview of the integration of mitochondria with mitophagy, also to introduce and discuss recent advances in the understanding of the potential role, and possible signaling pathway, of mitophagy in the pathological processes of both hemorrhagic and ischemic stroke. The author also provides evidence to explain the dual role of mitophagy in stroke.

The still uncertain identity of the channel-forming unit(s) of the mitochondrial permeability transition pore

Mitochondria from different organisms can undergo a sudden process of inner membrane unselective leakiness to molecules known as the mitochondrial permeability transition (MPT). This process has been studied for nearly four decades and several proteins have been claimed to constitute, or at least regulate the usually inactive pore responsible for this transition. However, no protein candidate proposed as the actual pore-forming unit has passed rigorous gain- or loss-of-function genetic tests. Here we review evidence for -and against- putative channel-forming components of the MPT pore. We conclude that the structure of the MPT pore still remains largely undefined and suggest that future studies should follow established technical considerations to unambiguously consolidate the channel forming constituent(s) of the MPT pore.

Mersalyl prevents the Tl+-induced permeability transition pore opening in the inner membrane of Ca2+-loaded rat liver mitochondria

It was earlier shown that the calcium load of rat liver mitochondria in medium containing TlNO₃ and KNO₃ resulted in the Tl⁺-induced mitochondrial permeability transition pore (MPTP) opening in the inner membrane. This opening was accompanied by an increase in swelling and membrane potential dissipation and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration. This respiratory decrease was markedly leveled by mersalyl (MSL), the phosphate symporter (PiC) inhibitor which poorly stimulated the calcium-induced swelling, but further increased the potential dissipation. All of these effects of Ca²⁺ and MSL were visibly reduced in the presence of the MPTP inhibitors (ADP, N-ethylmaleimide, and cyclosporine A). High MSL concentrations attenuated the ability of ADP to inhibit the MPTP. Our data suggest that the PiC can participate in the Tl⁺-induced MPTP opening in the inner membrane of Ca²⁺-loaded rat liver mitochondria.

Toxicity of Pb2+ on rat liver mitochondria induced by oxidative stress and mitochondrial permeability transition

Pb2+ exposure in humans occurs mainly through air inhalation, food and water uptake which has been shown to be generally associated with numerous body functions such as the central and peripheral nervous systems, the red blood cells, the kidneys and the liver. It has been reported that the liver is the storage site and an important primary target in Pb2+ toxicity, and the hepatotoxicity of Pb2+ could be resulted from the impairment of the liver mitochondria. In this study, several mitochondrial dysfunctions following the addition of Pb2+ (10-160 μM) were investigated. We found that Pb2+ inhibited the enzyme activities of mitochondrial respiratory complexes and complex III was the major source of Pb2+-induced significant reactive oxygen species (ROS) formation. As a consequence, our results showed that Pb2+ induced significant progress in mitochondrial lipid peroxidation, adenosine triphosphate (ATP) consumption and glutathione (GSH) oxidation. On the other hand, Pb2+ induced marked changes in mitochondrial permeability transition (MPT) accompanied by mitochondrial swelling, mitochondrial membrane potential collapse, mitochondrial membrane fluidity decrease and cytochrome c (Cyt c) release. Additionally, several mitochondrial MPT inhibitors and chelators were utilized to determine the possible interaction sites of Pb2+ on mitochondria. In general, our data supported that the Pb2+-induced liver toxicity was a result of the disruptive effect on the mitochondrial respiratory complexes. This disruptive effect caused oxidative stress and MPT, which led to mitochondrial dysfunctions and even cell death signalling via mitochondrial permeability transition pore (MPTP) opening and Cyt c release.

SIPP, a Novel Mitochondrial Phosphate Carrier, Mediates in Self-Incompatibility

In Solanaceae, the S-specific interaction between the pistil S-RNase and the pollen S-Locus F-box protein controls self-incompatibility (SI). Although this interaction defines the specificity of the pollen rejection response, the identification of three pistil essential modifier genes unlinked to the S-locus (HT-B, 120K, and NaStEP) unveils a higher degree of complexity in the pollen rejection pathway. We showed previously that NaStEP, a stigma protein with homology with Kunitz-type protease inhibitors, is essential to SI in Nicotiana spp. During pollination, NaStEP is taken up by pollen tubes, where potential interactions with pollen tube proteins might underlie its function. Here, we identified NaSIPP, a mitochondrial protein with phosphate transporter activity, as a novel NaStEP-interacting protein. Coexpression of NaStEP and NaSIPP in pollen tubes showed interaction in the mitochondria, although when expressed alone, NaStEP remains mostly cytosolic, implicating NaSIPP-mediated translocation of NaStEP into the organelle. The NaSIPP transcript is detected specifically in mature pollen of Nicotiana spp.; however, in self-compatible plants, this gene has accumulated mutations, so its coding region is unlikely to produce a functional protein. RNA interference suppression of NaSIPP in Nicotiana spp. pollen grains disrupts the SI by preventing pollen tube inhibition. Taken together, our results are consistent with a model whereby the NaStEP and NaSIPP interaction, in incompatible pollen tubes, might destabilize the mitochondria and contribute to arrest pollen tube growth.

Mechanistic Role of mPTP in Ischemia-Reperfusion Injury

Acute myocardial infarction (MI) is a major cause of death and disability worldwide. The treatment of choice for reducing ischemic injury and limiting infarct size (IS) in patients with ST-segment elevation MI (STEMI) is timely and effective myocardial reperfusion via primary percutaneous coronary intervention (PCI). However, myocardial reperfusion itself may induce further cardiomyocyte death, a phenomenon known as reperfusion injury (RI). The opening of a large pore in the mitochondrial membrane, namely, the mitochondrial permeability transition pore (mPTP), is widely recognized as the final step of RI and is responsible for mitochondrial and cardiomyocyte death. Although myocardial reperfusion interventions continue to improve, there remain no effective therapies for preventing RI due to incomplete knowledge regarding RI components and mechanisms and to premature translations of findings from animals to humans. In the last year, increasing amounts of data describing mPTP components, structure, regulation and function have surfaced. These data may be crucial for gaining a better understanding of RI genesis and for planning future trials evaluating new cardioprotective strategies. In this chapter, we review the role of the mPTP in RI pathophysiology and report on recent studies investigating its structure and components. Finally, we provide a brief overview of principal cardioprotective strategies and their pitfalls.

Mitochondrial permeability transition in cardiac ischemia-reperfusion: whether cyclophilin D is a viable target for cardioprotection?

Growing number of studies provide strong evidence that the mitochondrial permeability transition pore (PTP), a non-selective channel in the inner mitochondrial membrane, is involved in the pathogenesis of cardiac ischemia-reperfusion and can be targeted to attenuate reperfusion-induced damage to the myocardium. The molecular identity of the PTP remains unknown and cyclophilin D is the only protein commonly accepted as a major regulator of the PTP opening. Therefore, cyclophilin D is an attractive target for pharmacological or genetic therapies to reduce ischemia-reperfusion injury in various animal models and humans. Most animal studies demonstrated cardioprotective effects of PTP inhibition; however, a recent large clinical trial conducted by international groups demonstrated that cyclosporine A, a cyclophilin D inhibitor, failed to protect the heart in patients with myocardial infarction. These studies, among others, raise the question of whether cyclophilin D, which plays an important physiological role in the regulation of cell metabolism and mitochondrial bioenergetics, is a viable target for cardioprotection. This review discusses previous studies to provide comprehensive information on the physiological role of cyclophilin D as well as PTP opening in the cell that can be taken into consideration for the development of new PTP inhibitors.

Mitochondrial VDAC1: A Key Gatekeeper as Potential Therapeutic Target

Mitochondria are the key source of ATP that fuels cellular functions, and they are also central in cellular signaling, cell division and apoptosis. Dysfunction of mitochondria has been implicated in a wide range of diseases, including neurodegenerative and cardiac diseases, and various types of cancer. One of the key proteins that regulate mitochondrial function is the voltage-dependent anion channel 1 (VDAC1), the most abundant protein on the outer membrane of mitochondria. VDAC1 is the gatekeeper for the passages of metabolites, nucleotides, and ions; it plays a crucial role in regulating apoptosis due to its interaction with apoptotic and anti-apoptotic proteins, namely members of the Bcl-2 family of proteins and hexokinase. Therefore, regulation of VDAC1 is crucial not only for metabolic functions of mitochondria, but also for cell survival. In fact, multiple lines of evidence have confirmed the involvement of VDAC1 in several diseases. Consequently, modulation or dysregulation of VDAC1 function can potentially attenuate or exacerbate pathophysiological conditions. Understanding the role of VDAC1 in health and disease could lead to selective protection of cells in different tissues and diverse diseases. The purpose of this review is to discuss the role of VDAC1 in the pathogenesis of diseases and as a potentially effective target for therapeutic management of various pathologies.

Mitochondrial permeability transition pore: Back to the drawing board

Current models theorizing on what the mitochondrial permeability transition (mPT) pore is made of, implicate the c-subunit rings of ATP synthase complex. However, two very recent studies, one on atomistic simulations and in the other disrupting all genes coding for the c subunit disproved those models. As a consequence of this, the structural elements of the pore remain unknown. The purpose of the present short-review is to (i) briefly review the latest findings, (ii) serve as an index for more comprehensive reviews regarding mPT specifics, (iii) reiterate on the potential pitfalls while investigating mPT in conjunction to bioenergetics, and most importantly (iv) suggest to those in search of mPT pore identity, to also look elsewhere.

Mitochondrial dysfunction - Silent killer in cerebral ischemia

Mitochondrial dysfunction aggravates ischemic neuronal injury through activation of various pathophysiological and molecular mechanisms. Ischemic neuronal injury is particularly intensified during reperfusion due to impairment of mitochondrial function. Mitochondrial mutilation instigates alterations in calcium homeostasis in neurons, which plays a pivotal role in the maintenance of normal neuronal function. Increase in intracellular calcium level in mitochondria triggers the opening of mitochondrial transition pore and over production of reactive oxygen species (ROS). Several investigations have concluded that ROS not only contribute to lipids and proteins damage, but also transduce apoptotic signals leading to neuronal death. In addition to the above mentioned reasons, endoplasmic reticulum (ER) stress due to excitotoxicity also leads to neuronal death. Recently, some newer proteins have been claimed to induce "mitophagy" by triggering the receptors on autophagic membranes leading to neurodegeneration. This review summarizes the mechanisms underlying neuronal death involving mitochondrial dysfunction and mitophagy.

Mitochondrial Ca2+ and regulation of the permeability transition pore

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.