The mitochondrial permeability transition pore in Ca2+ homeostasis

Point of No Return-What Is the Threshold of Mitochondria With Permeability Transition in Cells to Trigger Cell Death

Programmed cell death (apoptosis) is essential part of the process of tissue regeneration that also plays role in the mechanism of pathology. The phenomenon of fast and transient permeability of mitochondrial membranes by various triggers, known as permeability transition pore (mPTP) leads to the release of proapoptotic proteins and acts as an initial step in initiation of apoptosis. However, a role for mPTP was also suggested for physiology and it is unclear if there is a threshold in number of mitochondria with mPTP which induces cell death and how this mechanism is regulated in different tissues. Using simultaneous measurements of mitochondrial membrane potential and a fluorescent marker for caspase-3 activation we studied the number of mitochondria with calcium-induced mPTP opening necessary for induction of apoptosis in rat primary cortical neurons, astrocytes, fibroblasts, and cancer (BT-474) cells. The induction of apoptosis was correlated with 80%-90% mitochondrial signal loss in neural cells but only 35% in fibroblasts, and in BT-474 cancer cells over 90% of mitochondria opens mPTP before apoptosis becomes obvious. The number of mitochondria with mPTP which induce cell death did not correlate with total expression levels of proapoptotic proteins but was consistent with the Bax/Bcl-2 ratio in these cells. Calcium-induced mPTP opening increased levels of necrosis which was higher in fibroblasts compared to neurons, astrocytes and BT-474 cells. Thus, different tissues require specific numbers of mitochondria with PTP opening to induce apoptosis and it correlates to the proapoptotic/antiapoptotic proteins expression ratio in them.

TRPC5 Promotes Intermittent Hypoxia-Induced Cardiomyocyte Injury Through Oxidative Stress

Purpose

Intermittent hypoxia (IH), a defining feature of obstructive sleep apnea (OSA), is associated with heart damage and linked to transient receptor potential canonical channel 5 (TRPC5). Nonetheless, the function of TRPC5 in OSA-induced cardiac injury remains uncertain. For this research, we aimed to explore the role and potential mechanism of TRPC5 in cardiomyocyte injury induced by intermittent hypoxia.

Methods

30 patients with newly diagnosed OSA and 30 patients with primary snoring(PS) were included in this study. Participants were subjected to polysomnography (PSG) for OSA diagnosis. Echocardiography was used to evaluate the structure and function of the heart, while peripheral blood samples were obtained. Additionally, RT-qPCR was utilized to quantify the relative expression level of TRPC5 mRNA in peripheral blood. H9c2 cells experienced IH or normoxia. TRPC5 levels in H9c2 cells were determined via RT-qPCR and Western blotting (WB) methods. H9c2 cells overexpressing TRPC5 were subjected to either normoxic or intermittent hypoxia conditions. Cell viability was determined by CCK8, the apoptosis rate, reactive oxygen species(ROS) levels, and Ca2+ concentration were assessed by flow cytometry, and the protein levels of TRPC5, Bcl-2, Bax, and Caspase-3 were analyzed by WB. Mitochondrial membrane potential(MMP), mitochondrial membrane permeability transition pore(mPTP), and transmission electron microscopy(TEM) were employed to observe mitochondrial function and structure. After inhibiting ROS with N-acetylcysteine (NAC), apoptosis, mitochondrial function and structure, and the concentration of Ca2+ were further detected.

Results

TRPC5 and left atrial diameter (LAD) were higher in OSA individuals, while the E/A ratio was lower(all P<0.05). IH impaired cell viability, triggered cell apoptosis, and enhanced TRPC5 expression in H9c2 cells(all P<0.05). The effects of IH on apoptosis, cell viability, mitochondrial function and structure damage, and oxidative stress (OxS) in H9c2 cells were accelerated by the overexpression of TRPC5(all P<0.05). Furthermore, cell apoptosis and mitochondrial structural and functional damage caused by overexpression of TRPC5 were attenuated by ROS inhibition.

Conclusion

TRPC5 is associated with structural and functional cardiac damage in patients with OSA, and TRPC5 promotes IH-induced apoptosis and mitochondrial damage in cardiomyocytes through OxS. TRPC5 may be a novel target for the diagnosis and treatment of OSA-induced myocardial injury.

PTH1R Suppressed Apoptosis of Mesenchymal Progenitors in Mandibular Growth

Genetic abnormalities of the parathyroid hormone 1 receptor (PTH1R) lead to profound craniomaxillofacial bone and dentition defects on account of inappropriate tissue metabolism and cellular differentiation. The coordinated activity of differentiation and viability in bone cells is indispensable for bone metabolism. Recent research demonstrates mesenchymal progenitors are responsive to PTH1R signaling for osteogenic differentiation, whereas the effect of PTH1R on cellular survival remains incompletely understood. Here, we report that mice with deletion of PTH1R in Prx1-positive mesenchymal cells (Prx1Cre;PTH1Rfl/fl) exhibit decreased alveolar bone mass due in part to apoptotic response activation. The exploration of oral bone-derived mesenchymal stem cells (OMSCs) with PTH1R deficiency suggests PTH1R signaling modulates OMSCs' apoptosis by interfering mitochondrial function and morphology. The underlying molecular mechanisms are studied by transcriptome sequencing analysis, finding that inositol trisphosphate receptor-3 (IP3R-3), an endoplasmic reticulum calcium channel protein, serves as a modulator of pro-apoptosis in OMSCs. Furthermore, we find PTH1R and its downstream protein kinase A (PKA) pathway dampen IP3R-3's expression. Of note, OMSCs with IP3R-3 overexpression recapitulate the PTH1R-deletion phenotypes, while IP3R-3 silence rescues mitochondrial dysfunction. Altogether, our study uncovers the anti-apoptotic function of PTH1R signaling in OMSCs and proves that excess apoptosis partly contributes to a weakening potential of osteogenic differentiation and aberrant mandibular development.

Human F-ATP synthase as a drug target

Practical and conceptual barriers have kept human F-ATP synthase out of reach as a target for the treatment of human diseases. Although this situation has persisted for decades, it may change in the near future. In this review the principal functionalities of human F-ATP synthase--proton motive force / ATP interconversion, membrane bending and mitochondrial permeability transition--are surveyed in the context of their respective potential for pharmaceutical intervention. Further, the technical requirements necessary to allow drug designs that are effective at the multiple levels of functionality and modality of human F-ATP synthase are discussed. The structure-based development of gastric proton pump inhibitors is used to exemplify what might be feasible for human F-ATP synthase. And finally, four structural regions of the human F-ATP synthase are examined as potential sites for the development of structure based drug development.

Oxidative Stress and Inflammation in Myocardial Ischemia-Reperfusion Injury: Protective Effects of Plant-Derived Natural Active Compounds

Acute myocardial infarction (AMI) remains a leading cause of death among patients with cardiovascular diseases. Percutaneous coronary intervention (PCI) has been the preferred clinical treatment for AMI due to its safety and efficiency. However, research indicates that the rapid restoration of myocardial oxygen supply following PCI can lead to secondary myocardial injury, termed myocardial ischemia-reperfusion injury (MIRI), posing a grave threat to patient survival. Despite ongoing efforts, the mechanisms underlying MIRI are not yet fully elucidated. Among them, oxidative stress and inflammation stand out as critical pathophysiological mechanisms, playing significant roles in MIRI. Natural compounds have shown strong clinical therapeutic potential due to their high efficacy, availability, and low side effects. Many current studies indicate that natural compounds can mitigate MIRI by reducing oxidative stress and inflammatory responses. Therefore, this paper reviews the mechanisms of oxidative stress and inflammation during MIRI and the role of natural compounds in intervening in these processes, aiming to provide a basis and reference for future research and development of drugs for treating MIRI.

ANT-Mediated Inhibition of the Permeability Transition Pore Alleviates Palmitate-Induced Mitochondrial Dysfunction and Lipotoxicity

Hyperlipidemia is a major risk factor for vascular lesions in diabetes mellitus and other metabolic disorders, although its basis remains poorly understood. One of the key pathogenetic events in this condition is mitochondrial dysfunction associated with the opening of the mitochondrial permeability transition (MPT) pore, a drop in the membrane potential, and ROS overproduction. Here, we investigated the effects of bongkrekic acid and carboxyatractyloside, a potent blocker and activator of the MPT pore opening, respectively, acting through direct interaction with the adenine nucleotide translocator, on the progression of mitochondrial dysfunction in mouse primary lung endothelial cells exposed to elevated levels of palmitic acid. Palmitate treatment (0.75 mM palmitate/BSA for 6 days) resulted in an 80% decrease in the viability index of endothelial cells, which was accompanied by mitochondrial depolarization, ROS hyperproduction, and increased colocalization of mitochondria with lysosomes. Bongkrekic acid (25 µM) attenuated palmitate-induced lipotoxicity and all the signs of mitochondrial damage, including increased spontaneous formation of the MPT pore. In contrast, carboxyatractyloside (10 μM) stimulated cell death and failed to prevent the progression of mitochondrial dysfunction under hyperlipidemic stress conditions. Silencing of gene expression of the predominate isoform ANT2, similar to the action of carboxyatractyloside, led to increased ROS generation and cell death under conditions of palmitate-induced lipotoxicity in a stably transfected HEK293T cell line. Altogether, these results suggest that targeted manipulation of the permeability transition pore through inhibition of ANT may represent an alternative approach to alleviate mitochondrial dysfunction and cell death in cell culture models of fatty acid overload.

Interplay of mitochondrial calcium signalling and reactive oxygen species production in the brain

Intracellular communication and regulation in brain cells is controlled by the ubiquitous Ca2+ and by redox signalling. Both of these independent signalling systems regulate most of the processes in cells including the cell surviving mechanism or cell death. In physiology Ca2+ can regulate and trigger reactive oxygen species (ROS) production by various enzymes and in mitochondria but ROS could also transmit redox signal to calcium levels via modification of calcium channels or phospholipase activity. Changes in calcium or redox signalling could lead to severe pathology resulting in excitotoxicity or oxidative stress. Interaction of the calcium and ROS is essential to trigger opening of mitochondrial permeability transition pore - the initial step of apoptosis, Ca2+ and ROS-induced oxidative stress involved in necrosis and ferroptosis. Here we review the role of redox signalling and Ca2+ in cytosol and mitochondria in the physiology of brain cells - neurons and astrocytes and how this integration can lead to pathology, including ischaemia injury and neurodegeneration.

Elevations in the Mitochondrial Matrix Protein Cyclophilin D Correlate With Reduced Parvalbumin Expression in the Prefrontal Cortex of Patients With Schizophrenia

BACKGROUND AND HYPOTHESIS

Cognitive deficits in schizophrenia are linked to dysfunctions of the dorsolateral prefrontal cortex (DLPFC), including alterations in parvalbumin (PV)-expressing interneurons (PVIs). Redox dysregulation and oxidative stress may represent convergence points in the pathology of schizophrenia, causing dysfunction of GABAergic interneurons and loss of PV. Here, we show that the mitochondrial matrix protein cyclophilin D (CypD), a critical initiator of the mitochondrial permeability transition pore (mPTP) and modulator of the intracellular redox state, is altered in PVIs in schizophrenia.

STUDY DESIGN

Western blotting was used to measure CypD protein levels in postmortem DLPFC specimens of schizophrenic patients (n = 27) and matched comparison subjects with no known history of psychiatric or neurological disorders (n = 26). In a subset of this cohort, multilabel immunofluorescent confocal microscopy with unbiased stereological sampling methods were used to quantify (1) numbers of PVI across the cortical mantle (20 unaffected comparison, 14 schizophrenia) and (2) PV and CypD protein levels from PVIs in the cortical layers 2-4 (23 unaffected comparison, 18 schizophrenia).

STUDY RESULTS

In schizophrenic patients, the overall number of PVIs in the DLPFC was not significantly altered, but in individual PVIs of layers 2-4 PV protein levels decreased along a superficial-to-deep gradient when compared to unaffected comparison subjects. These laminar-specific PVI alterations were reciprocally linked to significant CypD elevations both in PVIs and total DLPFC gray matter.

CONCLUSIONS

Our findings support previously reported PVI anomalies in schizophrenia and suggest that CypD-mediated mPTP formation could be a potential contributor to PVI dysfunction in schizophrenia.

Pore-Forming VDAC Proteins of the Outer Mitochondrial Membrane: Regulation and Pathophysiological Role

Voltage-dependent anion channels (VDAC1-3) of the outer mitochondrial membrane are a family of pore-forming β-barrel proteins that carry out controlled "filtration" of small molecules and ions between the cytoplasm and mitochondria. Due to the conformational transitions between the closed and open states and interaction with cytoplasmic and mitochondrial proteins, VDACs not only regulate the mitochondrial membrane permeability for major metabolites and ions, but also participate in the control of essential intracellular processes and pathological conditions. This review discusses novel data on the molecular structure, regulatory mechanisms, and pathophysiological role of VDAC proteins, as well as future directions in this area of research.

Understanding mitochondrial potassium channels: 33 years after discovery

Mitochondrial investigations have extended beyond their traditional functions, covering areas such as ATP synthesis and metabolism. Mitochondria are now implicated in new functional areas such as cytoprotection, cellular senescence, tumor function and inflammation. The basis of these new areas still relies on fundamental biochemical/biophysical mitochondrial functions such as synthesis of reactive oxygen species, mitochondrial membrane potential, and the integrity of the inner mitochondrial membrane i.e., the passage of various molecules through the mitochondrial membranes. In this view transport of potassium cations, known as the potassium cycle, plays an important role. It is believed that K+ influx is mediated by various potassium channels present in the inner mitochondrial membrane. In this article, we present an overview of the key findings and characteristics of mitochondrial potassium channels derived from research of many groups conducted over the past 33 years. We propose a list of six fundamental observations and most important ideas dealing with mitochondrial potassium channels. We also discuss the contemporary challenges and future prospects associated with research on mitochondrial potassium channels.

Mitochondrial Permeability Transition, Cell Death and Neurodegeneration

Neurodegenerative diseases are chronic conditions occurring when neurons die in specific brain regions that lead to loss of movement or cognitive functions. Despite the progress in understanding the mechanisms of this pathology, currently no cure exists to treat these types of diseases: for some of them the only help is alleviating the associated symptoms. Mitochondrial dysfunction has been shown to be involved in the pathogenesis of most the neurodegenerative disorders. The fast and transient permeability of mitochondria (the mitochondrial permeability transition, mPT) has been shown to be an initial step in the mechanism of apoptotic and necrotic cell death, which acts as a regulator of tissue regeneration for postmitotic neurons as it leads to the irreparable loss of cells and cell function. In this study, we review the role of the mitochondrial permeability transition in neuronal death in major neurodegenerative diseases, covering the inductors of mPTP opening in neurons, including the major ones-free radicals and calcium-and we discuss perspectives and difficulties in the development of a neuroprotective strategy based on the inhibition of mPTP in neurodegenerative disorders.

Acetaminophen Hepatotoxicity: Paradigm for Understanding Mechanisms of Drug-Induced Liver Injury

Acetaminophen (APAP) overdose is the clinically most relevant drug hepatotoxicity in western countries, and, because of translational relevance of animal models, APAP is mechanistically the most studied drug. This review covers intracellular signaling events starting with drug metabolism and the central role of mitochondrial dysfunction involving oxidant stress and peroxynitrite. Mitochondria-derived endonucleases trigger nuclear DNA fragmentation, the point of no return for cell death. In addition, adaptive mechanisms that limit cell death are discussed including autophagy, mitochondrial morphology changes, and biogenesis. Extensive evidence supports oncotic necrosis as the mode of cell death; however, a partial overlap with signaling events of apoptosis, ferroptosis, and pyroptosis is the basis for controversial discussions. Furthermore, an update on sterile inflammation in injury and repair with activation of Kupffer cells, monocyte-derived macrophages, and neutrophils is provided. Understanding these mechanisms of cell death led to discovery of N-acetylcysteine and recently fomepizole as effective antidotes against APAP toxicity.

Neither too much nor too little: mitochondrial calcium concentration as a balance between physiological and pathological conditions

Ca2+ ions serve as pleiotropic second messengers in the cell, regulating several cellular processes. Mitochondria play a fundamental role in Ca2+ homeostasis since mitochondrial Ca2+ (mitCa2+) is a key regulator of oxidative metabolism and cell death. MitCa2+ uptake is mediated by the mitochondrial Ca2+ uniporter complex (MCUc) localized in the inner mitochondrial membrane (IMM). MitCa2+ uptake stimulates the activity of three key enzymes of the Krebs cycle, thereby modulating ATP production and promoting oxidative metabolism. As Paracelsus stated, "Dosis sola facit venenum,"in pathological conditions, mitCa2+ overload triggers the opening of the mitochondrial permeability transition pore (mPTP), enabling the release of apoptotic factors and ultimately leading to cell death. Excessive mitCa2+ accumulation is also associated with a pathological increase of reactive oxygen species (ROS). In this article, we review the precise regulation and the effectors of mitCa2+ in physiopathological processes.

Antimicrobial peptide LL-37 disrupts plasma membrane and calcium homeostasis in Candida albicans via the Rim101 pathway

IMPORTANCE

Candida albicans is a major human fungal pathogen, and antimicrobial peptides are key components of innate immunity. Studying the interplay between C. albicans and human antimicrobial peptides would enhance a better understanding of pathogen-host interactions. Moreover, potential applications of antimicrobial peptides in antifungal therapy have aroused great interest. This work explores new mechanisms of LL-37 against C. albicans and reveals the complex connection among calcium homeostasis, oxidative stress, signaling, and possibly organelle interaction. Notably, these findings support the possible use of antimicrobial peptides to prevent and treat fungal infections.

Warm Cells, Hot Mitochondria: Achievements and Problems of Ultralocal Thermometry

Temperature is a crucial regulator of the rate and direction of biochemical reactions and cell processes. The recent data indicating the presence of local thermal gradients associated with the sites of high-rate thermogenesis, on the one hand, demonstrate the possibility for the existence of "thermal signaling" in a cell and, on the other, are criticized on the basis of thermodynamic calculations and models. Here, we review the main thermometric techniques and sensors developed for the determination of temperature inside living cells and diverse intracellular compartments. A comparative analysis is conducted of the results obtained using these methods for the cytosol, nucleus, endo-/sarcoplasmic reticulum, and mitochondria, as well as their biological consistency. Special attention is given to the limitations, possible sources of errors and ambiguities of the sensor's responses. The issue of biological temperature limits in cells and organelles is considered. It is concluded that the elaboration of experimental protocols for ultralocal temperature measurements that take into account both the characteristics of biological systems, as well as the properties and limitations of each type of sensor is of critical importance for the generation of reliable results and further progress in this field.

Blocking Mitochondrial Pyruvate Transport Alters Corneal Myofibroblast Phenotype: A New Target for Treating Fibrosis

Purpose

The purpose of this study was to critically test the hypothesis that mitochondrial pyruvate carrier (MPC) function is essential for maintenance of the corneal myofibroblast phenotype in vitro and in vivo.

Methods

Protein and mRNA for canonical profibrotic markers were assessed in cultured cat corneal myofibroblasts generated via transforming growth factor (TGF)-β1 stimulation and treated with either the thiazolidinedione (TZD) troglitazone or the MPC inhibitor alpha-cyano-beta-(1-phenylindol-3-yl) acrylate (UK-5099). RNA sequencing was used to gain insight into signaling modules related to instructive, permissive, or corollary changes in gene expression following treatment. A feline photorefractive keratectomy (PRK) model of corneal wounding was used to test the efficacy of topical troglitazone at reducing α-smooth muscle actin (SMA)-positive staining when applied 2 to 4 weeks postoperatively, during peak fibrosis.

Results

Troglitazone caused cultured myofibroblasts to adopt a fibroblast-like phenotype through a noncanonical, peroxisome proliferator-activated receptor (PPAR)-γ-independent mechanism. Direct MPC inhibition using UK-5099 recapitulated this effect, but classic inhibitors of oxidative phosphorylation (OXPHOS) did not. Gene Set Enrichment Analysis (GSEA) of RNA sequencing data converged on energy substrate utilization and the Mitochondrial Permeability Transition pore as key players in myofibroblast maintenance. Finally, troglitazone applied onto an established zone of active fibrosis post-PRK significantly reduced stromal α-SMA expression.

Conclusions

Our results provide empirical evidence that metabolic remodeling in myofibroblasts creates selective vulnerabilities beyond simply mitochondrial energy production, and that these are critical for maintenance of the myofibroblast phenotype. For the first time, we provide proof-of-concept data showing that this remodeling can be exploited to treat existing corneal fibrosis via inhibition of the MPC.

Patch-clamp technique to study mitochondrial membrane biophysics

Mitochondria are double-membrane organelles crucial for oxidative phosphorylation, enabling efficient ATP synthesis by eukaryotic cells. Both of the membranes, the highly selective inner mitochondrial membrane (IMM) and a relatively porous outer membrane (OMM), harbor a number of integral membrane proteins that help in the transport of biological molecules. These transporters are especially enriched in the IMM, where they help maintain transmembrane gradients for H+, K+, Ca2+, PO43-, and metabolites like ADP/ATP, citrate, etc. Impaired activity of these transporters can affect the efficiency of energy-transducing processes and can alter cellular redox state, leading to activation of cell-death pathways or metabolic syndromes in vivo. Although several methodologies are available to study ion flux through membrane proteins, the patch-clamp technique remains the gold standard for quantitatively analyzing electrogenic ion exchange across membranes. Direct patch-clamp recordings of mitoplasts (mitochondria devoid of outer membrane) in different modes, such as whole-mitoplast or excised-patch mode, allow researchers the opportunity to study the biophysics of mitochondrial transporters in the native membrane, in real time, in isolation from other fluxes or confounding factors due to changes in ion gradients, pH, or mitochondrial potential (ΔΨ). Here, we summarize the use of patch clamp to investigate several membrane proteins of mitochondria. We demonstrate how this technique can be reliably applied to record whole-mitoplast Ca2+ currents mediated via mitochondrial calcium uniporter or H+ currents mediated by uncoupling protein 1 and discuss critical considerations while recording currents from these small vesicles of the IMM (mitoplast diameter = 2-5 µm).

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.