From calcium signaling to cell death: two conformations for the mitochondrial permeability transition pore. Switching from low- to high-conductance state

Ferroptosis in Glaucoma: A Promising Avenue for Therapy

Glaucoma, a blind-leading disease largely since chronic pathological intraocular high pressure (ph-IOP). Hitherto, it is reckoned incurable for irreversible neural damage and challenges in managing IOP. Thus, it is significant to develop neuroprotective strategies. Ferroptosis, initially identified as an iron-dependent regulated death that triggers Fenton reactions and culminates in lipid peroxidation (LPO), has emerged as a focal point in multiple tumors and neurodegenerative diseases. Researches show that iron homeostasis play critical roles in the optic nerve (ON) and retinal ganglion cells (RGCs), suggesting targeted treatments could be effective. In glaucoma, apart from neural lesions, disrupted metal balance and increased oxidative stress in trabecular meshwork (TM) are observed. These disturbances lead to extracellular matrix excretion disorders, known as sclerotic mechanisms, resulting in refractory blockages. Importantly, oxidative stress, a significant downstream effect of ferroptosis, is also a key factor in cell senescence. It plays a crucial role in both the etiology and risk of glaucoma. Moreover, ferroptosis also induces non-infectious inflammation, which exacerbate glaucomatous injury. Therefore, the relevance of ferroptosis in glaucoma is extensive and multifaceted. In this review, the study delves into the current understanding of ferroptosis mechanisms in glaucoma, aiming to provide clues to inform clinical therapeutic practices.

The role of mitochondrial uncoupling in the regulation of mitostasis after traumatic brain injury

Mitostasis, the maintenance of healthy mitochondria, plays a critical role in brain health. The brain's high energy demands and reliance on mitochondria for energy production make mitostasis vital for neuronal function. Traumatic brain injury (TBI) disrupts mitochondrial homeostasis, leading to secondary cellular damage, neuronal degeneration, and cognitive deficits. Mild mitochondrial uncoupling, which dissociates ATP production from oxygen consumption, offers a promising avenue for TBI treatment. Accumulating evidence, from endogenous and exogenous mitochondrial uncoupling, suggests that mitostasis is closely regulating by mitochondrial uncoupling and cellular injury environments may be more sensitive to uncoupling. Mitochondrial uncoupling can mitigate calcium overload, reduce oxidative stress, and induce mitochondrial proteostasis and mitophagy, a process that eliminates damaged mitochondria. The interplay between mitochondrial uncoupling and mitostasis is ripe for further investigation in the context of TBI. These multi-faceted mechanisms of action for mitochondrial uncoupling hold promise for TBI therapy, with the potential to restore mitochondrial health, improve neurological outcomes, and prevent long-term TBI-related pathology.

Molecular mechanisms underlying mitochondrial damage, endoplasmic reticulum stress, and oxidative stress induced by environmental pollutants

Mitochondria and endoplasmic reticulum (ER) are essential organelles playing pivotal roles in the regulation of cellular metabolism, energy production, and protein synthesis. In addition, these organelles are important targets susceptible to external stimuli, such as environmental pollutants. Exposure to environmental pollutants can cause the mitochondrial damage, endoplasmic reticulum stress (ERS), and oxidative stress, leading to cellular dysfunction and death. Therefore, understanding the toxic effects and molecular mechanisms of environmental pollution underlying these processes is crucial for developing effective strategies to mitigate the adverse effects of environmental pollutants on human health. In the present study, we summarized and reviewed the toxic effects and molecular mechanisms of mitochondrial damage, ERS, and oxidative stress caused by exposure to environmental pollutants as well as interactions inducing the cell apoptosis and the roles in exposure to environmental pollutants.

Mitochondrial Dynamics and Cristae Shape Changes During Metabolic Reprogramming

Significance: The architecture of the mitochondrial network and cristae critically impact cell differentiation and identity. Cells undergoing metabolic reprogramming to aerobic glycolysis (Warburg effect), such as immune cells, stem cells, and cancer cells, go through controlled modifications in mitochondrial architecture, which is critical for achieving the resulting cellular phenotype. Recent Advances: Recent studies in immunometabolism have shown that the manipulation of mitochondrial network dynamics and cristae shape directly affects T cell phenotype and macrophage polarization through altering energy metabolism. Similar manipulations also alter the specific metabolic phenotypes that accompany somatic reprogramming, stem cell differentiation, and cancer cells. The modulation of oxidative phosphorylation activity, accompanied by changes in metabolite signaling, reactive oxygen species generation, and adenosine triphosphate levels, is the shared underlying mechanism. Critical Issues: The plasticity of mitochondrial architecture is particularly vital for metabolic reprogramming. Consequently, failure to adapt the appropriate mitochondrial morphology often compromises the differentiation and identity of the cell. Immune, stem, and tumor cells exhibit striking similarities in their coordination of mitochondrial morphology with metabolic pathways. However, although many general unifying principles can be observed, their validity is not absolute, and the mechanistic links thus need to be further explored. Future Directions: Better knowledge of the molecular mechanisms involved and their relationships to both mitochondrial network and cristae morphology will not only further deepen our understanding of energy metabolism but may also contribute to improved therapeutic manipulation of cell viability, differentiation, proliferation, and identity in many different cell types. Antioxid. Redox Signal. 39, 684-707.

Characterization of the Intracellular Acidity Regulation of Brain Tumor Cells and Consequences for Therapeutic Optimization of Temozolomide

A well-known feature of tumor cells is high glycolytic activity, leading to acidification of the tumor microenvironment through extensive lactate production. This acidosis promotes processes such as metastasis, aggressiveness, and invasiveness, which have been associated with a worse clinical prognosis. Moreover, the function and expression of transporters involved in regulation of intracellular pH might be altered. In this study, the capacity of tumor cells to regulate their intracellular pH when exposed to a range of pH from very acidic to basic was characterized in two glioma cell lines (F98 and U87) using a new recently published method of fluorescence imaging. Our results show that the regulation of acidity in tumors is not the same for the two investigated cell lines; U87 cells are able to reduce their intracellular acidity, whereas F98 cells do not exhibit this property. On the other hand, F98 cells show a higher level of resistance to acidity than U87 cells. Intracellular regulation of acidity appears to be highly cell-dependent, with different mechanisms activated to preserve cell integrity and function. This characterization was performed on 2D monolayer cultures and 3D spheroids. Spatial heterogeneities were exhibited in 3D, suggesting a spatially modulated regulation in this context. Based on the corpus of knowledge available in the literature, we propose plausible mechanisms to interpret our results, together with some new lines of investigation to validate our hypotheses. Our results might have implications on therapy, since the activity of temozolomide is highly pH-dependent. We show that the drug efficiency can be enhanced, depending on the cell type, by manipulating the extracellular pH. Therefore, personalized treatment involving a combination of temozolomide and pH-regulating agents can be considered.

A mechanistically approached review upon assorted cell lines stimulated by athermal electromagnetic irradiation

The probable influence of electromagnetic irradiation on cancer treatment has been deduced from the interaction of artificial electromagnetic emissions with biological organisms. Nonetheless, the suspected health effects induced by electromagnetic-based technology imply that such a treatment may contaminate the adjacent healthy cells. Thus, gaining mechanistic insights into the problem is required to avoid athermal health hazards. To tackle that, the current review, based upon in vitro studies into assorted cell lines, depicts the alterations in physiological processes triggered by electromagnetic irradiation via addressing gene regulatory cascades. Furthermore, decisive factors in the hypothesized cause-effect linkage in terms of the cell line-associated, exposure-associated, or endpoint-associated parameters are highlighted. As a result, subcellular structures such as aberrant Ca2+ channels, rich glycocalyx charge, or high water content in cancerous cells, which have attracted a great deal of attention, can explain their higher susceptibility compared with healthy cells under irradiation. Affected by cell components or geometry, the cellular biological window correlates with the metabolic or cell cycle status and determines the irradiation that causes the maximum influence. For instance, correlations between the frequency (or intensity) of irradiation and cell excitability or between the duration of irradiation and cell doubling time are observed. There are unspecified signaling pathways such as the pathway of PPAR-γ or MAPKs, and also proteins devoid of any investigation such as p14, or S phase-related and G2 phase-related proteins. Other chains, such as the cAMP connection with mitochondrial ATP or ERK signaling, the association of Hsps releases with signaling pathways of MAPKs, or the role of different ion channels in regulating various cell processes, require further investigation.

Serotonin type-3 receptor antagonists selectively kill melanoma cells through classical apoptosis, microtubule depolymerisation, ERK activation, and NF-κB downregulation

Malignant melanoma is a highly metastatic tumour, resistant to treatment. Serotonin type-3 (5-HT3) receptor antagonists, such as tropisetron and ondansetron, are well-tolerated antiemetic drugs commonly used to prevent nausea caused by chemotherapy or radiotherapy. We investigated the anticancer effects of these drugs on melanoma cancer cell lines WM-266-4 and B16F10 with or without paclitaxel. We constructed IC50 curves and performed Chou-Talalay analysis, using data obtained with the MTT assay. Flow cytometry and fluorescent microscopy were used to examine characteristics of the cell cycle, cell death and cytoskeleton changes. Protein levels and activation were analysed by western blotting and molecular docking studies carried out. Data were analysed by one way ANOVA and post hoc testing. Ondansetron and tropisetron showed selective concentration-dependent cytotoxicity in melanoma cell lines WM-266-4 and B16F10. The effect in combination with paclitaxel was synergistic. The drugs did not cause cell cycle arrest but did promote characteristics of classical apoptosis, including accumulation of subG1 DNA, cleaved caspase-3, mitochondrial membrane permeability and phosphatidylserine exposure. As well, the cytosolic calcium level in the melanoma cells was enhanced, phosphorylated ERK1/2 induced and NF-κB inhibited. Finally, the formation of microtubules was shown to be impaired in melanoma cells treated with ondansetron or tropisetron. Docking studies were used to predict that these drugs could bind to the colchicine binding site on the tubulin molecule. Antiemetic drugs, already given in combination with chemotherapy, may enhance the cytotoxic effect of chemotherapy, following successful delivery to the tumour site.

Methods to Evaluate Changes in Mitochondrial Structure and Function in Cancer

Mitochondria are regulators of key cellular processes, including energy production and redox homeostasis. Mitochondrial dysfunction is associated with various human diseases, including cancer. Importantly, both structural and functional changes can alter mitochondrial function. Morphologic and quantifiable changes in mitochondria can affect their function and contribute to disease. Structural mitochondrial changes include alterations in cristae morphology, mitochondrial DNA integrity and quantity, and dynamics, such as fission and fusion. Functional parameters related to mitochondrial biology include the production of reactive oxygen species, bioenergetic capacity, calcium retention, and membrane potential. Although these parameters can occur independently of one another, changes in mitochondrial structure and function are often interrelated. Thus, evaluating changes in both mitochondrial structure and function is crucial to understanding the molecular events involved in disease onset and progression. This review focuses on the relationship between alterations in mitochondrial structure and function and cancer, with a particular emphasis on gynecologic malignancies. Selecting methods with tractable parameters may be critical to identifying and targeting mitochondria-related therapeutic options. Methods to measure changes in mitochondrial structure and function, with the associated benefits and limitations, are summarized.

The Mitochondrial Permeability Transition Pore-Current Knowledge of Its Structure, Function, and Regulation, and Optimized Methods for Evaluating Its Functional State

The mitochondrial permeability transition pore (MPTP) is a calcium-dependent, ion non-selective membrane pore with a wide range of functions. Although the MPTP has been studied for more than 50 years, its molecular structure remains unclear. Short-term (reversible) opening of the MPTP protects cells from oxidative damage and enables the efflux of Ca²⁺ ions from the mitochondrial matrix and cell signaling. However, long-term (irreversible) opening induces processes leading to cell death. Ca²⁺ ions, reactive oxygen species, and changes in mitochondrial membrane potential regulate pore opening. The sensitivity of the pore to Ca²⁺ ions changes as an organism ages, and MPTP opening plays a key role in the pathogenesis of many diseases. Most studies of the MPTP have focused on elucidating its molecular structure. However, understanding the mechanisms that will inhibit the MPTP may improve the treatment of diseases associated with its opening. To evaluate the functional state of the MPTP and its inhibitors, it is therefore necessary to use appropriate methods that provide reproducible results across laboratories. This review summarizes our current knowledge of the function and regulation of the MPTP. The latter part of the review introduces two optimized methods for evaluating the functional state of the pore under standardized conditions.

The mitochondrial permeability transition pore in Ca2+ homeostasis

The mitochondrial Permeability Transition Pore (PTP) can be defined as a Ca²⁺ activated mega-channel involved in mitochondrial damage and cell death, making its inhibition a hallmark for therapeutic purposes in many PTP-related paradigms. Although long-lasting PTP openings have been widely studied, the physiological implications of transient openings (also called "flickering" behavior) are still poorly understood. The flickering activity was suggested to play a role in the regulation of Ca²⁺ and ROS homeostasis, and yet this hypothesis did not reach general consensus. This state of affairs might arise from the lack of unquestionable experimental evidence, due to limitations of the available techniques for capturing transient PTP activity and to a still partial understanding of its molecular identity. In this review we will focus on possible implications of the PTP in physiology, in particular its role as a Ca²⁺ release pathway, discussing the consequences of its forced inhibition. We will also consider the recent hypothesis of the existence of more permeability pathways and their potential involvement in mitochondrial physiology.

ATP synthase interactome analysis identifies a new subunit l as a modulator of permeability transition pore in yeast

The mitochondrial ATP synthase, an enzyme that synthesizes ATP and is involved in the formation of the mitochondrial mega-channel and permeability transition, is a multi-subunit complex. In S. cerevisiae, the uncharacterized protein Mco10 was previously found to be associated with ATP synthase and referred as a new 'subunit l'. However, recent cryo-EM structures could not ascertain Mco10 with the enzyme making questionable its role as a structural subunit. The N-terminal part of Mco10 is very similar to k/Atp19 subunit, which along with subunits g/Atp20 and e/Atp21 plays a major role in stabilization of the ATP synthase dimers. In our effort to confidently define the small protein interactome of ATP synthase we found Mco10. We herein investigate the impact of Mco10 on ATP synthase functioning. Biochemical analysis reveal in spite of similarity in sequence and evolutionary lineage, that Mco10 and Atp19 differ significantly in function. The Mco10 is an auxiliary ATP synthase subunit that only functions in permeability transition.

Targeted for destruction: degradation of singlet oxygen-damaged chloroplasts

Photosynthesis is an essential process that plants must regulate to survive in dynamic environments. Thus, chloroplasts (the sites of photosynthesis in plant and algae cells) use multiple signaling mechanisms to report their health to the cell. Such signals are poorly understood but often involve reactive oxygen species (ROS) produced from the photosynthetic light reactions. One ROS, singlet oxygen (¹O₂), can signal to initiate chloroplast degradation, but the cellular machinery involved in identifying and degrading damaged chloroplasts (i.e., chloroplast quality control pathways) is unknown. To provide mechanistic insight into these pathways, two recent studies have investigated degrading chloroplasts in the Arabidopsis thaliana¹O₂ over-producing plastid ferrochelatase two (fc2) mutant. First, a structural analysis of degrading chloroplasts was performed with electron microscopy, which demonstrated that damaged chloroplasts can protrude into the central vacuole compartment with structures reminiscent of fission-type microautophagy. ¹O₂-stressed chloroplasts swelled before these interactions, which may be a mechanism for their selective degradation. Second, the roles of autophagosomes and canonical autophagy (macroautophagy) were shown to be dispensable for ¹O₂-initiated chloroplast degradation. Instead, putative fission-type microautophagy genes were induced by chloroplast ¹O₂. Here, we discuss how these studies implicate this poorly understood cellular degradation pathway in the dismantling of ¹O₂-damaged chloroplasts.

The Joint Influence of Tl+ and Thiol-Modifying Agents on Rat Liver Mitochondrial Parameters In Vitro

Recent data have shown that the mitochondrial permeability transition pore (MPTP) is the complex of the Ca²⁺-modified adenine nucleotide translocase (ANT) and the Ca²⁺-modified ATP synthase. We found in a previous study that ANT conformational changes may be involved in Tl⁺-induced MPTP opening in the inner membrane of Ca²⁺-loaded rat liver mitochondria. In this study, the effects of thiol-modifying agents (eosin-5-maleimide (EMA), fluorescein isothiocyanate (FITC), Cu(o-phenanthroline)₂ (Cu(OP)₂), and embelin (Emb)), and MPTP inhibitors (ADP, cyclosporine A (CsA), n-ethylmaleimide (NEM), and trifluoperazine (TFP)) on MPTP opening were tested simultaneously with increases in swelling, membrane potential (ΔΨ_mito) decline, decreases in state 3, 4, and 3U_DNP (2,4-dinitrophenol-uncoupled) respiration, and changes in the inner membrane free thiol group content. The effects of these thiol-modifying agents on the studied mitochondrial characteristics were multidirectional and showed a clear dependence on their concentration. This research suggests that Tl⁺-induced MPTP opening in the inner membrane of calcium-loaded mitochondria may be caused by the interaction of used reagents (EMA, FITC, Emb, Cu(OP)₂) with active groups of ANT, the mitochondrial phosphate carrier (PiC) and the mitochondrial respiratory chain complexes. This study provides further insight into the causes of thallium toxicity and may be useful in the development of new treatments for thallium poisoning.

The Many Roles Mitochondria Play in Mammalian Aging

Significance: Aging is a natural process that affects most living organisms, resulting in increased mortality. As the world population ages, the prevalence of age-associated diseases, and their associated health care costs, has increased sharply. A better understanding of the molecular mechanisms that lead to cellular dysfunction may provide important targets for interventions to prevent or treat these diseases. Recent Advances: Although the mitochondrial theory of aging had been proposed more than 40 years ago, recent new data have given stronger support for a central role for mitochondrial dysfunction in several pathways that are deregulated during normal aging and age-associated disease. Critical Issues: Several of the experimental evidence linking mitochondrial alterations to age-associated loss of function are correlative and mechanistic insights are still elusive. Here, we review how mitochondrial dysfunction may be involved in many of the known hallmarks of aging, and how these pathways interact in an intricate net of molecular relationships. Future Directions: As it has become clear that mitochondrial dysfunction plays causative roles in normal aging and age-associated diseases, it is necessary to better define the molecular interactions and the temporal and causal relationship between these changes and the relevant phenotypes seen during the aging process. Antioxid. Redox Signal. 36, 824-843.

Molecular mechanisms and consequences of mitochondrial permeability transition

Mitochondrial permeability transition (mPT) is a phenomenon that abruptly causes the flux of low molecular weight solutes (molecular weight up to 1,500) across the generally impermeable inner mitochondrial membrane. The mPT is mediated by the so-called mitochondrial permeability transition pore (mPTP), a supramolecular entity assembled at the interface of the inner and outer mitochondrial membranes. In contrast to mitochondrial outer membrane permeabilization, which mostly activates apoptosis, mPT can trigger different cellular responses, from the physiological regulation of mitophagy to the activation of apoptosis or necrosis. Although there are several molecular candidates for the mPTP, its molecular nature remains contentious. This lack of molecular data was a significant setback that prevented mechanistic insight into the mPTP, pharmacological targeting and the generation of informative animal models. In recent years, experimental evidence has highlighted mitochondrial F₁F_o ATP synthase as a participant in mPTP formation, although a molecular model for its transition to the mPTP is still lacking. Recently, the resolution of the F₁F_o ATP synthase structure by cryogenic electron microscopy led to a model for mPTP gating. The elusive molecular nature of the mPTP is now being clarified, marking a turning point for understanding mitochondrial biology and its pathophysiological ramifications. This Review provides an up-to-date reference for the understanding of the mammalian mPTP and its cellular functions. We review current insights into the molecular mechanisms of mPT and validated observations - from studies in vivo or in artificial membranes - on mPTP activity and functions. We end with a discussion of the contribution of the mPTP to human disease. Throughout the Review, we highlight the multiple unanswered questions and, when applicable, we also provide alternative interpretations of the recent discoveries.

Shaping of Hepatic Ischemia/Reperfusion Events: The Crucial Role of Mitochondria

Hepatic ischemia reperfusion injury (HIRI) is a major hurdle in many clinical scenarios, including liver resection and transplantation. Various studies and countless surgical events have led to the observation of a strong correlation between HIRI induced by liver transplantation and early allograft-dysfunction development. The detrimental impact of HIRI has driven the pursuit of new ways to alleviate its adverse effects. At the core of HIRI lies mitochondrial dysfunction. Various studies, from both animal models and in clinical settings, have clearly shown that mitochondrial function is severely hampered by HIRI and that its preservation or restoration is a key indicator of successful organ recovery. Several strategies have been thus implemented throughout the years, targeting mitochondrial function. This work briefly discusses some the most utilized approaches, ranging from surgical practices to pharmacological interventions and highlights how novel strategies can be investigated and implemented by intricately discussing the way mitochondrial function is affected by HIRI.

Analysis of Mitochondrial Calcium Retention Capacity in Cultured Cells: Permeabilized Cells Versus Isolated Mitochondria

In response to various pathological stimuli, such as oxidative and energy stress accompanied by high Ca²⁺, mitochondria undergo permeability transition (PT) leading to the opening of the non-selective PT pores (PTP) in the inner mitochondrial membrane. Opening of the pores at high conductance allows the passage of ions and solutes <1.5 kD across the membrane, that increases colloid osmotic pressure in the matrix leading to excessive mitochondrial swelling. Calcium retention capacity (CRC) reflects maximum Ca²⁺ overload of mitochondria that occurs just before PTP opening. Quantification of CRC is important for elucidating the effects of different pathological stimuli and the efficacy of pharmacological agents on the mitochondria. Here, we performed a comparative analysis of CRC in mitochondria isolated from H9c2 cardioblasts, and in permeabilized H9c2 cells in situ to highlight the strengths and weaknesses of the CRC technique in isolated cell mitochondria vs. permeabilized cells. The cells were permeabilized by digitonin or saponin, and the Ca²⁺-sensitive fluorescence probe Calcium Green-5N was used in both preparations. Results demonstrated the interference of dye-associated fluorescence signals with saponin and the adverse effects of digitonin on mitochondria at high concentrations. Analysis of the CRC in permeabilized cells revealed a higher CRC in the saponin-permeabilized cells in comparison with the digitonin-permeabilized cells. In addition, the mitochondrial CRC in saponin-permeabilized cells was higher than in isolated mitochondria. Altogether, these data demonstrate that the quantification of the mitochondrial CRC in cultured cells permeabilized by saponin has more advantages compared to the isolated mitochondria.

Mitochondrial Phenotypes in Parkinson's Diseases-A Focus on Human iPSC-Derived Dopaminergic Neurons

Established disease models have helped unravel the mechanistic underpinnings of pathological phenotypes in Parkinson’s disease (PD), the second most common neurodegenerative disorder. However, these discoveries have been limited to relatively simple cellular systems and animal models, which typically manifest with incomplete or imperfect recapitulation of disease phenotypes. The advent of induced pluripotent stem cells (iPSCs) has provided a powerful scientific tool for investigating the underlying molecular mechanisms of both familial and sporadic PD within disease-relevant cell types and patient-specific genetic backgrounds. Overwhelming evidence supports mitochondrial dysfunction as a central feature in PD pathophysiology, and iPSC-based neuronal models have expanded our understanding of mitochondrial dynamics in the development and progression of this devastating disorder. The present review provides a comprehensive assessment of mitochondrial phenotypes reported in iPSC-derived neurons generated from PD patients’ somatic cells, with an emphasis on the role of mitochondrial respiration, morphology, and trafficking, as well as mitophagy and calcium handling in health and disease. Furthermore, we summarize the distinguishing characteristics of vulnerable midbrain dopaminergic neurons in PD and report the unique advantages and challenges of iPSC disease modeling at present, and for future mechanistic and therapeutic applications.

Comparative toxicities of BPA, BPS, BPF, and TMBPF in the nematode Caenorhabditis elegans and mammalian fibroblast cells

Bisphenol A (BPA) is a chemical compound commonly used in the production of plastics for daily lives and industry. As BPA is well known for its adverse health effects, several alternative materials have been developed. This study comprehensively analyzed the toxicity of BPA and its three substitutes including bisphenol S (BPS), bisphenol F (BPF), and tetramethyl bisphenol F (TMBPF) on aging, healthspan, and mitochondria using an in vivo Caenorhabditis elegans (C. elegans) model animal and cultured mammalian fibroblast cells. C. elegans treated with 1 mM BPA exhibited abnormalities in the four tested parameters related to development and growth, including delayed development, decreased body growth, reduced reproduction, and abnormal tissue morphology. Exposure to the same concentration of each alternative including TMBPF, which has been proposed as a relatively safe BPA alternative, detrimentally affected at least three of these events. Moreover, all bisphenols (except BPS) remarkably shortened the organismal lifespan and increased age-related changes in neurons. Exposure to BPA and BPF resulted in mitochondrial abnormalities, such as reduced oxygen consumption and mitochondrial membrane potential. In contrast, the ATP levels were noticeably higher after treatment with all bisphenols. In mammalian fibroblast cells, exposure to increasing concentrations of all bisphenols (ranging from 50 μM to 500 μM) caused a severe decrease in cell viability in a dose-dependent manner. BPA increased ATP levels and decreased ROS but did not affect mitochondrial permeability transition pores (mPTP). Notably, TMBPF was the only bisphenol that caused a significant increase in mitochondrial ROS and mPTP opening. These results suggest that the potentially harmful physiological effects of BPA alternatives should be considered.

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.

Protectors of the Mitochondrial Permeability Transition Pore Activated by Iron and Doxorubicin

AIM

The study is aimed at examining of action of iron, DOX, and their complex on the Mitochondrial Permeability Transition Pore (MPTP) opening and detecting of possible protectors of MPTP in the conditions close to mitochondria-dependent ferroptosis.

BACKGROUND

The Toxicity of Doxorubicin (DOX) is mainly associated with free iron accumulation and mitochondrial dysfunction. DOX can provoke ferroptosis, iron-dependent cell death driven by membrane damage. The Mitochondrial Permeability Transition Pore (MPTP) is considered as a common pathway leading to the development of apoptosis, necrosis, and, possibly, ferroptosis. The influence of DOX on the Ca²⁺ -induced MPTP opening in the presence of iron has not yet been studied.

OBJECTIVE

The study was conducted on isolated liver and heart mitochondria. MPTP and succinate- ubiquinone oxidoreductase were studied as targets of DOX in mitochondria-dependent ferroptosis. The iron chelator deferoxamine (DFO), the lipid radical scavenger butyl-hydroxytoluene (BHT), and rutenium red (Rr), as a possible inhibitor of ferrous ions uptake in mitochondria, were tested as MPTP protectors. The role of medium alkalization was also examined.

METHODS

Changes of threshold calcium concentrations required for MPTP opening were measured by a Ca²⁺ selective electrode, mitochondrial membrane potential was registered by tetraphenylphosphonium (TPP+)-selective electrode, and mitochondrial swelling was recorded as a decrease in absorbance at 540 nm. The activity of Succinate Dehydrogenase (SDH) was determined by the reduction of the electron acceptor DCPIP.

CONCLUSION

MPTP and the respiratory complex II are identified as the main targets of the iron-dependent action of DOX on the isolated mitochondria. All MPTP protectors tested abolished or weakened the effect of iron and a complex of iron with DOX on Ca²⁺ -induced MPTP opening, acting in different stages of MPTP activation. These data open new approaches to the modulation of the toxic influence of DOX on mitochondria with the aim to reduce their dysfunction.

Advances in Intracellular Calcium Signaling Reveal Untapped Targets for Cancer Therapy

Intracellular Ca2+ distribution is a tightly regulated process. Numerous Ca2+ chelating, storage, and transport mechanisms are required to maintain normal cellular physiology. Ca2+-binding proteins, mainly calmodulin and calbindins, sequester free intracellular Ca2+ ions and apportion or transport them to signaling hubs needing the cations. Ca2+ channels, ATP-driven pumps, and exchangers assist the binding proteins in transferring the ions to and from appropriate cellular compartments. Some, such as the endoplasmic reticulum, mitochondria, and lysosomes, act as Ca2+ repositories. Cellular Ca2+ homeostasis is inefficient without the active contribution of these organelles. Moreover, certain key cellular processes also rely on inter-organellar Ca2+ signaling. This review attempts to encapsulate the structure, function, and regulation of major intracellular Ca2+ buffers, sensors, channels, and signaling molecules before highlighting how cancer cells manipulate them to survive and thrive. The spotlight is then shifted to the slow pace of translating such research findings into anticancer therapeutics. We use the PubMed database to highlight current clinical studies that target intracellular Ca2+ signaling. Drug repurposing and improving the delivery of small molecule therapeutics are further discussed as promising strategies for speeding therapeutic development in this area.

Ca2+ handling at the mitochondria-ER contact sites in neurodegeneration

Mitochondria-endoplasmic reticulum (ER) contact sites (MERCS) are morpho-functional units, formed at the loci of close apposition of the ER-forming endomembrane and outer mitochondrial membrane (OMM). These sites contribute to fundamental cellular processes including lipid biosynthesis, autophagy, apoptosis, ER-stress and calcium (Ca²⁺) signalling. At MERCS, Ca²⁺ ions are transferred from the ER directly to mitochondria through a core protein complex composed of inositol-1,4,5 trisphosphate receptor (InsP₃R), voltage-gated anion channel 1 (VDAC1), mitochondrial calcium uniporter (MCU) and adaptor protein glucose-regulated protein 75 (Grp75); this complex is regulated by several associated proteins. Deregulation of ER-mitochondria Ca²⁺ transfer contributes to pathogenesis of neurodegenerative and other diseases. The efficacy of Ca²⁺ transfer between ER and mitochondria depends on the protein composition of MERCS, which controls ER-mitochondria interaction regulating, for example, the transversal distance between ER membrane and OMM and the extension of the longitudinal interface between ER and mitochondria. These parameters are altered in neurodegeneration. Here we overview the ER and mitochondrial Ca²⁺ homeostasis, the composition of ER-mitochondrial Ca²⁺ transfer machinery and alterations of the ER-mitochondria Ca²⁺ transfer in three major neurodegenerative diseases: motor neurone diseases, Parkinson disease and Alzheimer's disease.

Stretching tension effects in permeability transition pores of inner mitochondrial membrane

Presently a mechanism of permeability transition pore (PTP) opening was proposed and discussed. This mechanism is based on mechanical stretching of inner mitochondrial membrane (IMM) caused by mitochondrial swelling (MS). The latter is induced by osmotic pressure generated by solute imbalance between the matrix and the surrounding cyto(sarco)plasm. Modelled by the Monte-Carlo method, an IMM fragment of 350 simulated biological molecules exhibited formation of micro-domains containing two protein and seven phospholipid molecules. The energies (-0.191 eV per molecule) in these micro-domains were significantly larger than those (-0.375 eV per molecule) of other parts of the IMM fragment. Stretching forces applied to such domains expanded them much more than other parts of the IMM fragment. We identify these micro-domains as the PTPs. Both linear and nonlinear functions were used for the strain-stress relation of the IMM fragment, with nonlinear effects more important at large IMM stretching strains. Thus, two main factors are incorporated into the PTP opening mechanism: (1) presence of micro-domains in the IMM structure and (2) IMM stretching stress caused by MS. Taking into account both of these factors, the equation for the probability of PTP opening was deduced, with matrix Ca²⁺ and H⁺ ionic concentrations as its parameters. Note that the equation deduced was similar to an earlier reported empirical equation describing PTP opening dynamics. This correspondence provides support to the presently proposed mechanism. Thus, a new look at the PTP opening mechanism is provided, of interest to various research areas related to mitochondrial biophysics.

Butyrate Feeding Reverses CypD-Related Mitoflash Phenotypes in Mouse Myofibers

Mitoflashes are spontaneous transients of the biosensor mt-cpYFP. In cardiomyocytes, mitoflashes are associated with the cyclophilin D (CypD) mediated opening of mitochondrial permeability transition pore (mPTP), while in skeletal muscle they are considered hallmarks of mitochondrial respiration burst under physiological conditions. Here, we evaluated the potential association between mitoflashes and the mPTP opening at different CypD levels and phosphorylation status by generating three CypD derived fusion constructs with a red shifted, pH stable Ca2+ sensor jRCaMP1b. We observed perinuclear mitochondrial Ca2+ efflux accompanying mitoflashes in CypD and CypDS42A (a phosphor-resistant mutation at Serine 42) overexpressed myofibers but not the control myofibers expressing the mitochondria-targeting sequence of CypD (CypDN30). Assisted by a newly developed analysis program, we identified shorter, more frequent mitoflash activities occurring over larger areas in CypD and CypDS42A overexpressed myofibers than the control CypDN30 myofibers. These observations provide an association between the elevated CypD expression and increased mitoflash activities in hindlimb muscles in an amyotrophic lateral sclerosis (ALS) mouse model previously observed. More importantly, feeding the mice with sodium butyrate reversed the CypD-associated mitoflash phenotypes and protected against ectopic upregulation of CypD, unveiling a novel molecular mechanism underlying butyrate mediated alleviation of ALS progression in the mouse model.

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.

The Multifaceted Roles of Zinc in Neuronal Mitochondrial Dysfunction

Zinc is a highly abundant cation in the brain, essential for cellular functions, including transcription, enzymatic activity, and cell signaling. However, zinc can also trigger injurious cascades in neurons, contributing to the pathology of neurodegenerative diseases. Mitochondria, critical for meeting the high energy demands of the central nervous system (CNS), are a principal target of the deleterious actions of zinc. An increasing body of work suggests that intracellular zinc can, under certain circumstances, contribute to neuronal damage by inhibiting mitochondrial energy processes, including dissipation of the mitochondrial membrane potential (MMP), leading to ATP depletion. Additional consequences of zinc-mediated mitochondrial damage include reactive oxygen species (ROS) generation, mitochondrial permeability transition, and excitotoxic calcium deregulation. Zinc can also induce mitochondrial fission, resulting in mitochondrial fragmentation, as well as inhibition of mitochondrial motility. Here, we review the known mechanisms responsible for the deleterious actions of zinc on the organelle, within the context of neuronal injury associated with neurodegenerative processes. Elucidating the critical contributions of zinc-induced mitochondrial defects to neurotoxicity and neurodegeneration may provide insight into novel therapeutic targets in the clinical setting.

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 Dysfunction and Permeability Transition in Neonatal Brain and Lung Injuries

This review discusses the potential mechanistic role of abnormally elevated mitochondrial proton leak and mitochondrial bioenergetic dysfunction in the pathogenesis of neonatal brain and lung injuries associated with premature birth. Providing supporting evidence, we hypothesized that mitochondrial dysfunction contributes to postnatal alveolar developmental arrest in bronchopulmonary dysplasia (BPD) and cerebral myelination failure in diffuse white matter injury (WMI). This review also analyzes data on mitochondrial dysfunction triggered by activation of mitochondrial permeability transition pore(s) (mPTP) during the evolution of perinatal hypoxic-ischemic encephalopathy. While the still cryptic molecular identity of mPTP continues to be a subject for extensive basic science research efforts, the translational significance of mitochondrial proton leak received less scientific attention, especially in diseases of the developing organs. This review is focused on the potential mechanistic relevance of mPTP and mitochondrial dysfunction to neonatal diseases driven by developmental failure of organ maturation or by acute ischemia-reperfusion insult during development.

Pharmacological inhibition of the mitochondrial Ca2+ uniporter: Relevance for pathophysiology and human therapy

Mitochondrial Ca2+ uptake has long been considered crucial for meeting the fluctuating energy demands of cells in the heart and other tissues. Increases in mitochondrial matrix [Ca2+] drive mitochondrial ATP production via stimulation of Ca2+-sensitive dehydrogenases. Mitochondria-targeted sensors have revealed mitochondrial matrix [Ca2+] rises that closely follow the cytoplasmic [Ca2+] signals in many paradigms. Mitochondrial Ca2+ uptake is mediated by the Ca2+ uniporter (mtCU). Pharmacological manipulation of the mtCU is potentially key to understanding its physiological significance, but no specific, cell-permeable inhibitors were identified. In the past decade, as the molecular identity of the mtCU was brought to light, efforts have focused on genetic targeting. However, in the cells/animals that are able to survive impaired mtCU function, robust compensatory changes were found in the mtCU as well as other mechanisms. Thus, the discovery, through chemical library screens on normal and mtCU-deficient cells, of new small-molecule inhibitors with improved cell permeability and specificity might offer a better chance to test the relevance of mitochondrial Ca2+ uptake. Success with the development of small molecule mtCU inhibitors is also expected to have clinical impact, considering the growing evidence for the role of mitochondrial Ca2+ uptake in a variety of diseases, including heart attack, stroke and various neurodegenerative disorders. Here, we review the progress in pharmacological targeting of mtCU and illustrate the challenges in this field using data obtained with MCU-i11, a new small molecule inhibitor.

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.

Impaired flickering of the permeability transition pore causes SPG7 spastic paraplegia

Background

Mutations of the mitochondrial protein paraplegin cause hereditary spastic paraplegia type 7 (SPG7), a so-far untreatable degenerative disease of the upper motoneuron with still undefined pathomechanism.

The intermittent mitochondrial permeability transition pore (mPTP) opening, called flickering, is an essential process that operates to maintain mitochondrial homeostasis by reducing intra-matrix Ca²⁺ and reactive oxygen species (ROS) concentration, and is critical for efficient synaptic function.

Methods

We use a fluorescence-based approach to measure mPTP flickering in living cells and biochemical and molecular biology techniques to dissect the pathogenic mechanism of SPG7. In the SPG7 animal model we evaluate the potential improvement of the motor defect, neuroinflammation and neurodegeneration by means of an mPTP inducer, the benzodiazepine Bz-423.

Findings

We demonstrate that paraplegin is required for efficient transient opening of the mPTP, that is impaired in both SPG7 patients-derived fibroblasts and primary neurons from Spg7^−/− mice. We show that dysregulation of mPTP opening at the pre-synaptic terminal impairs neurotransmitter release leading to ineffective synaptic transmission. Lack of paraplegin impairs mPTP flickering by a mechanism involving increased expression and activity of sirtuin3, which promotes deacetylation of cyclophilin D, thus hampering mPTP opening. Pharmacological treatment with Bz-423, which bypasses the activity of CypD, normalizes synaptic transmission and rescues the motor impairment of the SPG7 mouse model.

Interpretation

mPTP targeting opens a new avenue for the potential therapy of this form of spastic paraplegia.

Funding

Telethon Foundation grant (TGMGCSBX16TT); Dept. of Defense, US Army, grant W81XWH-18–1–0001

The very low number of calcium-induced permeability transition pores in the single mitochondrion

Mitochondrial permeability transition (PT) is a phenomenon of stress-induced increase in nonspecific permeability of the mitochondrial inner membrane that leads to disruption of oxidative phosphorylation and cell death. Quantitative measurement of the membrane permeability increase during PT is critically important for understanding the PT's impact on mitochondrial function. The elementary unit of PT is a PT pore (PTP), a single channel presumably formed by either ATP synthase or adenine nucleotide translocator (ANT). It is not known how many channels are open in a single mitochondrion during PT, which makes it difficult to quantitatively estimate the overall degree of membrane permeability. Here, we used wide-field microscopy to record mitochondrial swelling and quantitatively measure rates of single-mitochondrion volume increase during PT-induced high-amplitude swelling. PT was quantified by calculating the rates of water flux responsible for measured volume changes. The total water flux through the mitochondrial membrane of a single mitochondrion during PT was in the range of (2.5 ± 0.4) × 10-17 kg/s for swelling in 2 mM Ca2+ and (1.1 ± 0.2) × 10-17 kg/s for swelling in 200 µM Ca2+. Under these experimental conditions, a single PTP channel with ionic conductance of 1.5 nS could allow passage of water at the rate of 0.65 × 10-17 kg/s. Thus, we estimate the integral ionic conductance of the whole mitochondrion during PT to be 5.9 ± 0.9 nS for 2 mM concentration of Ca2+ and 2.6 ± 0.4 nS for 200 µM of Ca2+. The number of PTPs per mitochondrion ranged from one to nine. Due to the uncertainties in PTP structure and model parameters, PTP count results may be slightly underestimated. However, taking into account that each mitochondrion has ∼15,000 copies of ATP synthases and ANTs, our data imply that PTP activation is a rare event that occurs only in a small subpopulation of these proteins.

Cyclophilin D-dependent oligodendrocyte mitochondrial ion leak contributes to neonatal white matter injury

Postnatal failure of oligodendrocyte maturation has been proposed as a cellular mechanism of diffuse white matter injury (WMI) in premature infants. However, the molecular mechanisms for oligodendrocyte maturational failure remain unclear. In neonatal mice and cultured differentiating oligodendrocytes, sublethal intermittent hypoxic (IH) stress activated cyclophilin D-dependent mitochondrial proton leak and uncoupled mitochondrial respiration, leading to transient bioenergetic stress. This was associated with development of diffuse WMI: poor oligodendrocyte maturation, diffuse axonal hypomyelination, and permanent sensorimotor deficit. In normoxic mice and oligodendrocytes, exposure to a mitochondrial uncoupler recapitulated the phenotype of WMI, supporting the detrimental role of mitochondrial uncoupling in the pathogenesis of WMI. Compared with WT mice, cyclophilin D-knockout littermates did not develop bioenergetic stress in response to IH challenge and fully preserved oligodendrocyte maturation, axonal myelination, and neurofunction. Our study identified the cyclophilin D-dependent mitochondrial proton leak and uncoupling as a potentially novel subcellular mechanism for the maturational failure of oligodendrocytes and offers a potential therapeutic target for prevention of diffuse WMI in premature infants experiencing chronic IH stress.

Mitochondrial calcium uniporter complex activation protects against calcium alternans in atrial myocytes

Cardiac alternans, defined as beat-to-beat alternations in action potential duration, cytosolic Ca transient (CaT) amplitude, and cardiac contraction is associated with atrial fibrillation (AF) and sudden cardiac death. At the cellular level, cardiac alternans is linked to abnormal intracellular calcium handling during excitation-contraction coupling. We investigated how pharmacological activation or inhibition of cytosolic Ca sequestration via mitochondrial Ca uptake and mitochondrial Ca retention affects the occurrence of pacing-induced CaT alternans in isolated rabbit atrial myocytes. Cytosolic CaTs were recorded using Fluo-4 fluorescence microscopy. Alternans was quantified as the alternans ratio (AR = 1 - CaT_small/CaT_large, where CaT_small and CaT_large are the amplitudes of the small and large CaTs of a pair of alternating CaTs). Inhibition of mitochondrial Ca sequestration via mitochondrial Ca uniporter complex (MCUC) with Ru360 enhanced the severity of CaT alternans (AR increase) and lowered the pacing frequency threshold for alternans. In contrast, stimulation of MCUC mediated mitochondrial Ca uptake with spermine-rescued alternans (AR decrease) and increased the alternans pacing threshold. Direct measurement of mitochondrial [Ca] in membrane permeabilized myocytes with Fluo-4 loaded mitochondria revealed that spermine enhanced and accelerated mitochondrial Ca uptake. Stimulation of mitochondrial Ca retention by preventing mitochondrial Ca efflux through the mitochondrial permeability transition pore with cyclosporin A also protected from alternans and increased the alternans pacing threshold. Pharmacological manipulation of MCUC activity did not affect sarcoplasmic reticulum Ca load. Our results suggest that activation of Ca sequestration by mitochondria protects from CaT alternans and could be a potential therapeutic target for cardiac alternans and AF prevention.NEW & NOTEWORTHY This study provides conclusive evidence that mitochondrial Ca uptake and retention protects from Ca alternans, whereas uptake inhibition enhances Ca alternans. The data suggest pharmacological mitochondrial Ca cycling modulation as a potential therapeutic strategy for alternans-related cardiac arrhythmia prevention.

ATP synthase c-subunit ring as the channel of mitochondrial permeability transition: Regulator of metabolism in development and degeneration

The mitochondrial permeability transition pore (mPTP) or mitochondrial megachannel is arguably one of the most mysterious phenomena in biology today. mPTP has been at the center of ongoing extensive scientific research for the last several decades. In this review we will discuss recent advances in the field that enhance our understanding of the molecular composition of mPTP, its regulatory mechanisms and its pathophysiological role. We will describe our recent findings on the role of ATP synthase c-subunit ring as a central player in mitochondrial permeability transition and as an important metabolic regulator during development and in degenerative 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.

The short-chain perfluorinated compounds PFBS, PFHxS, PFBA and PFHxA, disrupt human mesenchymal stem cell self-renewal and adipogenic differentiation

Per- and polyfluorinated alkyl substances (PFASs) are commonly used in industrial processes and daily life products. Because they are persistent, they accumulate in the environment, wildlife and humans. Although many studies have focused on two of the most representative PFASs, PFOS and PFOA, the potential toxicity of short-chain PFASs has not yet been given sufficient attention. We used a battery of assays to evaluate the toxicity of several four-carbon and six-carbon perfluorinated sulfonates and carboxyl acids (PFBS, PFHxS, PFBA and PFHxA), with a human mesenchymal stem cell (hMSC) system. Our results demonstrate significant cyto- and potential developmental toxicity for all the compounds analyzed, with shared but also distinct mechanisms of toxicity. Moreover, the effects of PFBS and PFHxS were stronger than those of PFBA and PFHxA, but occurred at higher doses compared to PFOS or PFOA.

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.

Crosstalk between Mitochondria and Cytoskeleton in Cardiac Cells

Elucidation of the mitochondrial regulatory mechanisms for the understanding of muscle bioenergetics and the role of mitochondria is a fundamental problem in cellular physiology and pathophysiology. The cytoskeleton (microtubules, intermediate filaments, microfilaments) plays a central role in the maintenance of mitochondrial shape, location, and motility. In addition, numerous interactions between cytoskeletal proteins and mitochondria can actively participate in the regulation of mitochondrial respiration and oxidative phosphorylation. In cardiac and skeletal muscles, mitochondrial positions are tightly fixed, providing their regular arrangement and numerous interactions with other cellular structures such as sarcoplasmic reticulum and cytoskeleton. This can involve association of cytoskeletal proteins with voltage-dependent anion channel (VDAC), thereby, governing the permeability of the outer mitochondrial membrane (OMM) to metabolites, and regulating cell energy metabolism. Cardiomyocytes and myocardial fibers demonstrate regular arrangement of tubulin beta-II isoform entirely co-localized with mitochondria, in contrast to other isoforms of tubulin. This observation suggests the participation of tubulin beta-II in the regulation of OMM permeability through interaction with VDAC. The OMM permeability is also regulated by the specific isoform of cytolinker protein plectin. This review summarizes and discusses previous studies on the role of cytoskeletal proteins in the regulation of energy metabolism and mitochondrial function, adenosine triphosphate (ATP) production, and energy transfer.

The Role of Mitochondrial Calcium Signaling in the Pathophysiology of Cancer Cells

The pioneering work of Richard Altman on the presence of mitochondria in cells set in motion a field of research dedicated to uncovering the secrets of the mitochondria. Despite limitations in studying the structure and function of the mitochondria, advances in our understanding of this organelle prompted the development of potential treatments for various diseases, from neurodegenerative conditions to muscular dystrophy and cancer. As the powerhouses of the cell, the mitochondria represent the essence of cellular life and as such, a selective advantage for cancer cells. Much of the function of the mitochondria relies on Ca²⁺ homeostasis and the presence of effective Ca²⁺ signaling to maintain the balance between mitochondrial function and dysfunction and subsequently, cell survival. Ca²⁺ regulates the mitochondrial respiration rate which in turn increases ATP synthesis, but too much Ca²⁺ can also trigger the mitochondrial apoptosis pathway; however, cancer cells have evolved mechanisms to modulate mitochondrial Ca²⁺ influx and efflux in order to sustain their metabolic demand and ensure their survival. Therefore, targeting the mitochondrial Ca²⁺ signaling involved in the bioenergetic and apoptotic pathways could serve as potential approaches to treat cancer patients. This chapter will review the role of Ca²⁺ signaling in mediating the function of the mitochondria and its involvement in health and disease with special focus on the pathophysiology of cancer.

Mitochondrial bioenergetics and redox dysfunctions in hypercholesterolemia and atherosclerosis

In the first part of this review, we summarize basic mitochondrial bioenergetics concepts showing that mitochondria are critical regulators of cell life and death. Until a few decades ago, mitochondria were considered to play essential roles only in respiration, ATP formation, non-shivering thermogenesis and a variety of metabolic pathways. However, the concept presented by Peter Mitchell regarding coupling between electron flow and ATP synthesis through the intermediary of a H⁺ electrochemical potential leads to the recognition that the proton-motive force also regulates a series of relevant cell signalling processes, such as superoxide generation, redox balance and Ca²⁺ handling. Alterations in these processes lead to cell death and disease states. In the second part of this review, we discuss the role of mitochondrial dysfunctions in the specific context of hypercholesterolemia-induced atherosclerosis. We provide a literature analysis that indicates a decisive role of mitochondrial redox dysfunction in the development of atherosclerosis and discuss the underlying molecular mechanisms. Finally, we highlight the potential mitochondrial-targeted therapeutic strategies that are relevant for atherosclerosis.

In silico simulation of reversible and irreversible swelling of mitochondria: The role of membrane rigidity

Mitochondria have been widely accepted as the main source of ATP in the cell. The inner mitochondrial membrane (IMM) is important for the maintenance of ATP production and other functions of mitochondria. The electron transport chain (ETC) generates an electrochemical gradient of protons known as the proton-motive force across the IMM and thus produces the mitochondrial membrane potential that is critical to ATP synthesis. One of the main factors regulating the structural and functional integrity of the IMM is the changes in the matrix volume. Mild (reversible) swelling regulates mitochondrial metabolism and function; however, excessive (irreversible) swelling causes mitochondrial dysfunction and cell death. The central mechanism of mitochondrial swelling includes the opening of non-selective channels known as permeability transition pores (PTPs) in the IMM by high mitochondrial Ca²⁺ and reactive oxygen species (ROS). The mechanisms of reversible and irreversible mitochondrial swelling and transition between these two states are still unknown. The present study elucidates an upgraded biophysical model of reversible and irreversible mitochondrial swelling dynamics. The model provides a description of the PTP regulation dynamics using an additional differential equation. The rigidity tensor was used in numerical simulations of the mitochondrial parameter dynamics with different initial conditions defined by Ca²⁺ concentration in the sarco/endoplasmic reticulum. We were able to estimate the values of the IMM rigidity tensor components by fitting the model to the previously reported experimental data. Overall, the model provides a better description of the reversible and irreversible mitochondrial swelling dynamics.