Discovery of Novel 1,3,8-Triazaspiro[4.5]decane Derivatives That Target the c Subunit of F1/FO-Adenosine Triphosphate (ATP) Synthase for the Treatment of Reperfusion Damage in Myocardial Infarction

Spiropiperidine-Based Oligomycin-Analog Ligands To Counteract the Ischemia-Reperfusion Injury in a Renal Cell Model

Finding a therapy for ischemia-reperfusion injury, which consists of cell death following restoration of blood flowing into the artery affected by ischemia, is a strong medical need. Nowadays, only the use of broad-spectrum molecular therapies has demonstrated a partial efficacy in protecting the organs following reperfusion, while randomized clinical trials focused on more specific drug targets have failed. In order to overcome this problem, we applied a combination of molecular modeling and chemical synthesis to identify novel spiropiperidine-based structures active in mitochondrial permeability transition pore opening inhibition as a key process to enhance cell survival after blood flow restoration. Our results were confirmed by biological assay on an in vitro cell model on HeLa and human renal proximal tubular epithelial cells and pave the way to further investigation on an in vivo model system.

Hydrolytic Activity of Mitochondrial F1FO-ATP Synthase as a Target for Myocardial Ischemia-Reperfusion Injury: Discovery and In Vitro and In Vivo Evaluation of Novel Inhibitors

F1FO-ATP synthase is the mitochondrial complex responsible for ATP production. During myocardial ischemia, it reverses its activity, hydrolyzing ATP and leading to energetic deficit and cardiac injury. We aimed to discover novel inhibitors of ATP hydrolysis, accessing the druggability of the target within ischemia(I)/reperfusion(R) injury. New molecular scaffolds were revealed using ligand-based virtual screening methods. Fifty-five compounds were tested on isolated murine heart mitochondria and H9c2 cells for their inhibitory activity. A pyrazolo[3,4-c]pyridine hit structure was identified and optimized in a hit-to-lead process synthesizing nine novel derivatives. Three derivatives significantly inhibited ATP hydrolysis in vitro, while in vivo, they reduced myocardial infarct size (IS). The novel compound 31 was the most effective in reducing IS, validating that inhibition of F1FO-ATP hydrolytic activity can serve as a target for cardioprotection during ischemia. Further examination of signaling pathways revealed that the cardioprotection mechanism is related to the increased ATP content in the ischemic myocardium and increased phosphorylation of PKA and phospholamban, leading to the reduction of apoptosis.

Development of novel cytoprotective small compounds inhibiting mitochondria-dependent cell death

We identified cytoprotective small molecules (CSMs) by a cell-based high-throughput screening of Bax inhibitors. Through a medicinal chemistry program, M109S was developed, which is orally bioactive and penetrates the blood-brain/retina barriers. M109S protected retinal cells in ocular disease mouse models. M109S directly interacted with Bax and inhibited the conformational change and mitochondrial translocation of Bax. M109S inhibited ABT-737-induced apoptosis both in Bax-only and Bak-only mouse embryonic fibroblasts. M109S also inhibited apoptosis induced by staurosporine, etoposide, and obatoclax. M109S decreased maximal mitochondrial oxygen consumption rate and reactive oxygen species production, whereas it increased glycolysis. These effects on cellular metabolism may contribute to the cytoprotective activity of M109S. M109S is a novel small molecule protecting cells from mitochondria-dependent apoptosis both in vitro and in vivo. M109S has the potential to become a research tool for studying cell death mechanisms and to develop therapeutics targeting mitochondria-dependent cell death pathway.

Identity, structure, and function of the mitochondrial permeability transition pore: controversies, consensus, recent advances, and future directions

The mitochondrial permeability transition (mPT) describes a Ca2+-dependent and cyclophilin D (CypD)-facilitated increase of inner mitochondrial membrane permeability that allows diffusion of molecules up to 1.5 kDa in size. It is mediated by a non-selective channel, the mitochondrial permeability transition pore (mPTP). Sustained mPTP opening causes mitochondrial swelling, which ruptures the outer mitochondrial membrane leading to subsequent apoptotic and necrotic cell death, and is implicated in a range of pathologies. However, transient mPTP opening at various sub-conductance states may contribute several physiological roles such as alterations in mitochondrial bioenergetics and rapid Ca2+ efflux. Since its discovery decades ago, intensive efforts have been made to identify the exact pore-forming structure of the mPT. Both the adenine nucleotide translocase (ANT) and, more recently, the mitochondrial F1FO (F)-ATP synthase dimers, monomers or c-subunit ring alone have been implicated. Here we share the insights of several key investigators with different perspectives who have pioneered mPT research. We critically assess proposed models for the molecular identity of the mPTP and the mechanisms underlying its opposing roles in the life and death of cells. We provide in-depth insights into current controversies, seeking to achieve a degree of consensus that will stimulate future innovative research into the nature and role of the mPTP.

The RISK pathway leading to mitochondria and cardioprotection: how everything started

Ischaemic heart disease, which often manifests clinically as myocardial infarction (MI), remains a major cause of mortality worldwide. Despite the development of effective pre-clinical cardioprotective therapies, clinical translation has been disappointing. Nevertheless, the 'reperfusion injury salvage kinase' (RISK) pathway appears to be a promising target for cardioprotection. This pathway is crucial for the induction of cardioprotection by numerous pharmacological and non-pharmacological interventions, such as ischaemic conditioning. An important component of the cardioprotective effects of the RISK pathway involves the prevention of mitochondrial permeability transition pore (MPTP) opening and subsequent cardiac cell death. Here, we will review the historical perspective of the RISK pathway and focus on its interaction with mitochondria in the setting of cardioprotection.

Synthetic Routes to Approved Drugs Containing a Spirocycle

The use of spirocycles in drug discovery and medicinal chemistry has been booming in the last two decades. This has clearly translated into the landscape of approved drugs. Among two dozen clinically used medicines containing a spirocycle, 50% have been approved in the 21st century. The present review focuses on the notable synthetic routes to such drugs invented in industry and academia, and is intended to serve as a useful reference source of synthetic as well as general drug information for researchers engaging in the design of new spirocyclic scaffolds for medicinal use or embarking upon analog syntheses inspired by the existing approved drugs.

1,3,8-Triazaspiro[4.5]decane Derivatives Inhibit Permeability Transition Pores through a FO-ATP Synthase c Subunit Glu119-Independent Mechanism That Prevents Oligomycin A-Related Side Effects

Permeability transition pore (PTP) molecular composition and activity modulation have been a matter of research for several years, especially due to their importance in ischemia reperfusion injury (IRI). Notably, c subunit of ATP synthase (Csub) has been identified as one of the PTP-forming proteins and as a target for cardioprotection. Oligomycin A is a well-known Csub interactor that has been chemically modified in-depth for proposed new pharmacological approaches against cardiac reperfusion injury. Indeed, by taking advantage of its scaffold and through focused chemical improvements, innovative Csub-dependent PTP inhibitors (1,3,8-Triazaspiro[4.5]decane) have been synthetized in the past. Interestingly, four critical amino acids have been found to be involved in Oligomycin A-Csub binding in yeast. However, their position on the human sequence is unknown, as is their function in PTP inhibition. The aims of this study are to (i) identify for the first time the topologically equivalent residues in the human Csub sequence; (ii) provide their in vitro validation in Oligomycin A-mediated PTP inhibition and (iii) understand their relevance in the binding of 1,3,8-Triazaspiro[4.5]decane small molecules, as Oligomycin A derivatives, in order to provide insights into Csub interactions. Notably, in this study we demonstrated that 1,3,8-Triazaspiro[4.5]decane derivatives inhibit permeability transition pores through a FO-ATP synthase c subunit Glu119-independent mechanism that prevents Oligomycin A-related side effects.

1,5-disubstituted-1,2,3-triazoles counteract mitochondrial dysfunction acting on F1FO-ATPase in models of cardiovascular diseases

The compromised viability and function of cardiovascular cells are rescued by small molecules of triazole derivatives (Tzs), identified as 3a and 3b, by preventing mitochondrial dysfunction. The oxidative phosphorylation improves the respiratory control rate in the presence of Tzs independently of the substrates that energize the mitochondria. The F₁F_O-ATPase, the main candidate in mitochondrial permeability transition pore (mPTP) formation, is the biological target of Tzs and hydrophilic F₁ domain of the enzyme is depicted as the binding region of Tzs. The protective effect of Tz molecules on isolated mitochondria was corroborated by immortalized cardiomyocytes results. Indeed, mPTP opening was attenuated in response to ionomycin. Consequently, increased mitochondrial roundness and reduction of both length and interconnections between mitochondria. In in-vitro and ex-vivo models of cardiovascular pathologies (i.e., hypoxia-reoxygenation and hypertension) were used to evaluate the Tzs cardioprotective action. Key parameters of porcine aortic endothelial cells (pAECs) oxidative metabolism and cell viability were not affected by Tzs. However, in the presence of either 1 μM 3a or 0.5 μM 3b the impaired cell metabolism of pAECs injured by hypoxia-reoxygenation was restored to control respiratory profile. Moreover, endothelial cells isolated from SHRSP exposed to high-salt treatment rescued the Complex I activity and the endothelial capability to form vessel-like tubes and vascular function in presence of Tzs. As a result, the specific biochemical mechanism of Tzs to block Ca²⁺-activated F₁F_O-ATPase protected cell viability and preserved the pAECs bioenergetic metabolism upon hypoxia-reoxygenation injury. Moreover, SHRSP improved vascular dysfunction in response to a high-salt treatment.

Perspectives on mitochondrial relevance in cardiac ischemia/reperfusion injury

Cardiovascular disease is the most common cause of death worldwide and in particular, ischemic heart disease holds the most considerable position. Even if it has been deeply studied, myocardial ischemia-reperfusion injury (IRI) is still a side-effect of the clinical treatment for several heart diseases: ischemia process itself leads to temporary damage to heart tissue and obviously the recovery of blood flow is promptly required even if it worsens the ischemic injury. There is no doubt that mitochondria play a key role in pathogenesis of IRI: dysfunctions of these important organelles alter cell homeostasis and survival. It has been demonstrated that during IRI the system of mitochondrial quality control undergoes alterations with the disruption of the complex balance between the processes of mitochondrial fusion, fission, biogenesis and mitophagy. The fundamental role of mitochondria is carried out thanks to the finely regulated connection to other organelles such as plasma membrane, endoplasmic reticulum and nucleus, therefore impairments of these inter-organelle communications exacerbate IRI. This review pointed to enhance the importance of the mitochondrial network in the pathogenesis of IRI with the aim to focus on potential mitochondria-targeting therapies as new approach to control heart tissue damage after ischemia and reperfusion process.

Identification of small-molecule urea derivatives as PTPC modulators targeting the c subunit of F1/Fo-ATP synthase

Maintaining a high percentage of living and functional cells in those pathologies in which excessive cell death occurs, such as neurodegenerative disorders and cardiovascular diseases, is one of the most intriguing challenges in the field of biochemical research for drug discovery. Here, mitochondrial permeability transition-driven regulated cell death is the main mechanism of mitochondrial impairment and cell fate; this pathway is still lacking of satisfying pharmacological treatments to counteract its becoming; for this reason, it needs continuous and intense research to find new compounds as modulator of the permeability transition pore complex (PTPC) activity. In this study, we report the identification of small-molecule urea derivatives able to inhibit PTPC opening following calcium overload and selected for future use in cytoprotection.

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.

Discovery of novel KRAS‒PDEδ inhibitors with potent activity in patient-derived human pancreatic tumor xenograft models

KRAS‒PDEδ interaction is revealed as a promising target for suppressing the function of mutant KRAS. The bottleneck in clinical development of PDEδ inhibitors is the poor antitumor activity of known chemotypes. Here, we identified novel spiro-cyclic PDEδ inhibitors with potent antitumor activity both in vitro and in vivo. In particular, compound 36l (K _D = 127 ± 16 nmol/L) effectively bound to PDEδ and interfered with KRAS-PDEδ interaction. It influenced the distribution of KRAS in Mia PaCa-2 cells, downregulated the phosphorylation of t-ERK and t-AKT and promoted apoptosis of the cells. The novel inhibitor 36l exhibited significant in vivo antitumor potency in pancreatic cancer patient-derived xenograft (PDX) models. It represents a promising lead compound for investigating the druggability of KRAS‒PDEδ interaction.

Modulation and Pharmacology of the Mitochondrial Permeability Transition: A Journey from F-ATP Synthase to ANT

The permeability transition (PT) is an increased permeation of the inner mitochondrial membrane due to the opening of the PT pore (PTP), a Ca²⁺-activated high conductance channel involved in Ca²⁺ homeostasis and cell death. Alterations of the PTP have been associated with many pathological conditions and its targeting represents an incessant challenge in the field. Although the modulation of the PTP has been extensively explored, the lack of a clear picture of its molecular nature increases the degree of complexity for any target-based approach. Recent advances suggest the existence of at least two mitochondrial permeability pathways mediated by the F-ATP synthase and the ANT, although the exact molecular mechanism leading to channel formation remains elusive for both. A full comprehension of this to-pore conversion will help to assist in drug design and to develop pharmacological treatments for a fine-tuned PT regulation. Here, we will focus on regulatory mechanisms that impinge on the PTP and discuss the relevant literature of PTP targeting compounds with particular attention to F-ATP synthase and ANT.

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.

Mitochondrial Ca2+ Signaling in Health, Disease and Therapy

The divalent cation calcium (Ca²⁺) is considered one of the main second messengers inside cells and acts as the most prominent signal in a plethora of biological processes. Its homeostasis is guaranteed by an intricate and complex system of channels, pumps, and exchangers. In this context, by regulating cellular Ca²⁺ levels, mitochondria control both the uptake and release of Ca²⁺. Therefore, at the mitochondrial level, Ca²⁺ plays a dual role, participating in both vital physiological processes (ATP production and regulation of mitochondrial metabolism) and pathophysiological processes (cell death, cancer progression and metastasis). Hence, it is not surprising that alterations in mitochondrial Ca²⁺ (mCa²⁺) pathways or mutations in Ca²⁺ transporters affect the activities and functions of the entire cell. Indeed, it is widely recognized that dysregulation of mCa²⁺ signaling leads to various pathological scenarios, including cancer, neurological defects and cardiovascular diseases (CVDs). This review summarizes the current knowledge on the regulation of mCa²⁺ homeostasis, the related mechanisms and the significance of this regulation in physiology and human diseases. We also highlight strategies aimed at remedying mCa²⁺ dysregulation as promising therapeutical approaches.

A naturally occurring mutation in ATP synthase subunit c is associated with increased damage following hypoxia/reoxygenation in STEMI patients

Preclinical models of ischemia/reperfusion injury (RI) demonstrate the deleterious effects of permeability transition pore complex (PTPC) opening in the first minutes upon revascularization of the occluded vessel. The ATP synthase c subunit (Csub) influences PTPC activity in cells, thus impacting tissue injury. A conserved glycine-rich domain in Csub is classified as critical because, when mutated, it modifies ATP synthase properties, protein interaction with the mitochondrial calcium (Ca²⁺) uniporter complex, and the conductance of the PTPC. Here, we document the role of a naturally occurring mutation in the Csub-encoding ATP5G1 gene at the G87 position found in two ST-segment elevation myocardial infarction (STEMI) patients and how PTPC opening is related to RI in patients affected by the same disease. We report a link between the expression of ATP5G1^G87E and the response to hypoxia/reoxygenation of human cardiomyocytes, which worsen when compared to those expressing the wild-type protein, and a positive correlation between PTPC and RI.

Molecular and Supramolecular Structure of the Mitochondrial Oxidative Phosphorylation System: Implications for Pathology

Under aerobic conditions, mitochondrial oxidative phosphorylation (OXPHOS) converts the energy released by nutrient oxidation into ATP, the currency of living organisms. The whole biochemical machinery is hosted by the inner mitochondrial membrane (mtIM) where the protonmotive force built by respiratory complexes, dynamically assembled as super-complexes, allows the F1FO-ATP synthase to make ATP from ADP + Pi. Recently mitochondria emerged not only as cell powerhouses, but also as signaling hubs by way of reactive oxygen species (ROS) production. However, when ROS removal systems and/or OXPHOS constituents are defective, the physiological ROS generation can cause ROS imbalance and oxidative stress, which in turn damages cell components. Moreover, the morphology of mitochondria rules cell fate and the formation of the mitochondrial permeability transition pore in the mtIM, which, most likely with the F1FO-ATP synthase contribution, permeabilizes mitochondria and leads to cell death. As the multiple mitochondrial functions are mutually interconnected, changes in protein composition by mutations or in supercomplex assembly and/or in membrane structures often generate a dysfunctional cascade and lead to life-incompatible diseases or severe syndromes. The known structural/functional changes in mitochondrial proteins and structures, which impact mitochondrial bioenergetics because of an impaired or defective energy transduction system, here reviewed, constitute the main biochemical damage in a variety of genetic and age-related diseases.

Ca2+ as cofactor of the mitochondrial H+ -translocating F1 FO -ATP(hydrol)ase

The mitochondrial F₁ F_O -ATPase in the presence of the natural cofactor Mg²⁺ acts as the enzyme of life by synthesizing ATP, but it can also hydrolyze ATP to pump H⁺ . Interestingly, Mg²⁺ can be replaced by Ca²⁺ , but only to sustain ATP hydrolysis and not ATP synthesis. When Ca²⁺ inserts in F₁ , the torque generation built by the chemomechanical coupling between F₁ and the rotating central stalk was reported as unable to drive the transmembrane H⁺ flux within F_O . However, the failed H⁺ translocation is not consistent with the oligomycin-sensitivity of the Ca²⁺ -dependent F₁ F_O -ATP(hydrol)ase. New enzyme roles in mitochondrial energy transduction are suggested by recent advances. Accordingly, the structural F₁ F_O -ATPase distortion driven by ATP hydrolysis sustained by Ca²⁺ is consistent with the permeability transition pore signal propagation pathway. The Ca²⁺ -activated F₁ F_O -ATPase, by forming the pore, may contribute to dissipate the transmembrane H⁺ gradient created by the same enzyme complex.

Spirocyclic Scaffolds in Medicinal Chemistry

Spirocyclic scaffolds are incorporated in various approved drugs and drug candidates. The increasing interest in less planar bioactive compounds has given rise to the development of synthetic methodologies for the preparation of spirocyclic scaffolds. In this Perspective, we summarize the diverse synthetic routes to obtain spirocyclic systems. The impact of spirocycles on potency and selectivity, including the aspect of stereochemistry, is discussed. Furthermore, we examine the changes in physicochemical properties as well as in in vitro and in vivo ADME using selected studies that compare spirocyclic compounds to their nonspirocyclic counterparts. In conclusion, the value of spirocyclic scaffolds in medicinal chemistry is discussed.

Natural products and other inhibitors of F1FO ATP synthase

F₁F_O ATP synthase is responsible for the production of >95% of all ATP synthesis within the cell. Dysregulation of its expression, activity or localization is linked to various human diseases including cancer, diabetes, and Alzheimer's and Parkinson's disease. In addition, ATP synthase is a novel and viable drug target for the development of antimicrobials as evidenced by bedaquiline, which was approved in 2012 for the treatment of tuberculosis. Historically, natural products have been a rich source of ATP synthase inhibitors that help unravel the role of F₁F_O ATP synthase in cellular bioenergetics. During the last decade, new modulators of ATP synthase have been discovered through the isolation of novel natural products as well as through a ligand-based drug design process. In addition, new data has been obtained with regards to the structure and function of ATP synthase under physiological and pathological conditions. Crystal structure studies have provided a significant insight into the rotary function of the enzyme and may provide additional opportunities to design a new generation of inhibitors. This review provides an update on recently discovered ATP synthase modulators as well as an update on existing scaffolds.

A Novel Conceptual Model for the Dual Role of FOF1-ATP Synthase in Cell Life and Cell Death

The mitochondrial permeability transition (MPT) has been one of the longstanding enigmas in biology. Its cause is currently at the center of an extensive scientific debate, and several hypotheses on its molecular nature have been put forward. The present view holds that the transition arises from the opening of a high-conductance channel in the energy-transducing membrane, the permeability transition pore (PTP), also called the mitochondrial megachannel or the multiconductance channel (MMC). Here, the novel hypothesis is proposed that the aqueous access channels at the interface of the c-ring and the a-subunit of FO in the FOF1-ATP synthase are repurposed during induction of apoptosis and constitute the elusive PTP/ MMC. A unifying principle based on regulation by local potentials is advanced to rationalize the action of the myriad structurally and chemically diverse inducers and inhibitors of PTP/MMC. Experimental evidence in favor of the hypothesis and its differences from current models of PTP/MMC are summarized. The hypothesis explains in considerable detail how the binding of Ca2+ to a β-catalytic site (site 3) in the F1 portion of ATP synthase triggers the opening of the PTP/MMC. It is also shown to connect to longstanding proposals within Nath's torsional mechanism of energy transduction and ATP synthesis as to how the binding of MgADP to site 3 does not induce PTP/MMC, but instead catalyzes physiological ATP synthesis in cell life. In the author's knowledge, this is the first model that explains how Ca2+ transforms the FOF1-ATP synthase from an exquisite energy-conserving enzyme in cell life into an energy-dissipating structure that promotes cell death. This has major implications for basic as well as for clinical research, such as for the development of drugs that target the MPT, given the established role of PTP/MMC dysregulation in cancer, ischemia, cardiac hypertrophy, and various neurodegenerative diseases.

The new role of F1Fo ATP synthase in mitochondria-mediated neurodegeneration and neuroprotection

The mitochondrial F₁F_o ATP synthase is one of the most abundant proteins of the mitochondrial inner membrane, which catalyzes the final step of oxidative phosphorylation to synthesize ATP from ADP and Pi. ATP synthase uses the electrochemical gradient of protons (Δμ_H⁺) across the mitochondrial inner membrane to synthesize ATP. Under certain pathophysiological conditions, ATP synthase can run in reverse to hydrolyze ATP and build the necessary Δμ_H⁺ across the mitochondrial inner membrane. Tight coupling between these two processes, proton translocation and ATP synthesis, is achieved by the unique rotational mechanism of ATP synthase and is necessary for efficient cellular metabolism and cell survival. The uncoupling of these processes, dissipation of mitochondrial inner membrane potential, elevated levels of ROS, low matrix content of ATP in combination with other cellular malfunction trigger the opening of the mitochondrial permeability transition pore in the mitochondrial inner membrane. In this review we will discuss the new role of ATP synthase beyond oxidative phosphorylation. We will highlight its function as a unique regulator of cell life and death and as a key target in mitochondria-mediated neurodegeneration and neuroprotection.

Physiopathology of the Permeability Transition Pore: Molecular Mechanisms in Human Pathology

Mitochondrial permeability transition (MPT) is the sudden loss in the permeability of the inner mitochondrial membrane (IMM) to low-molecular-weight solutes. Due to osmotic forces, MPT is paralleled by a massive influx of water into the mitochondrial matrix, eventually leading to the structural collapse of the organelle. Thus, MPT can initiate outer-mitochondrial-membrane permeabilization (MOMP), promoting the activation of the apoptotic caspase cascade and caspase-independent cell-death mechanisms. The induction of MPT is mostly dependent on mitochondrial reactive oxygen species (ROS) and Ca²⁺, but is also dependent on the metabolic stage of the affected cell and signaling events. Therefore, since its discovery in the late 1970s, the role of MPT in human pathology has been heavily investigated. Here, we summarize the most significant findings corroborating a role for MPT in the etiology of a spectrum of human diseases, including diseases characterized by acute or chronic loss of adult cells and those characterized by neoplastic initiation.

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.

Mitochondrial F-type ATP synthase: multiple enzyme functions revealed by the membrane-embedded FO structure

Of the two main sectors of the F-type ATP synthase, the membrane-intrinsic F_O domain is the one which, during evolution, has undergone the highest structural variations and changes in subunit composition. The F_O complexity in mitochondria is apparently related to additional enzyme functions that lack in bacterial and thylakoid complexes. Indeed, the F-type ATP synthase has the main bioenergetic role to synthesize ATP by exploiting the electrochemical gradient built by respiratory complexes. The F_O membrane domain, essential in the enzyme machinery, also participates in the bioenergetic cost of synthesizing ATP and in the formation of the cristae, thus contributing to mitochondrial morphology. The recent enzyme involvement in a high-conductance channel, which forms in the inner mitochondrial membrane and promotes the mitochondrial permeability transition, highlights a new F-type ATP synthase role. Point mutations which cause amino acid substitutions in F_O subunits produce mitochondrial dysfunctions and lead to severe pathologies. The F_O variability in different species, pointed out by cryo-EM analysis, mirrors the multiple enzyme functions and opens a new scenario in mitochondrial biology.

The mystery of mitochondria-ER contact sites in physiology and pathology: A cancer perspective

Mitochondria-associated membranes (MAM), physical platforms that enable communication between mitochondria and the endoplasmic reticulum (ER), are enriched with many proteins and enzymes involved in several crucial cellular processes, such as calcium (Ca²⁺) homeostasis, lipid synthesis and trafficking, autophagy and reactive oxygen species (ROS) production. Accumulating studies indicate that tumor suppressors and oncogenes are present at these intimate contacts between mitochondria and the ER, where they influence Ca²⁺ flux between mitochondria and the ER or affect lipid homeostasis at MAM, consequently impacting cell metabolism and cell fate. Understanding these fundamental roles of mitochondria-ER contact sites as important domains for tumor suppressors and oncogenes can support the search for new and more precise anticancer therapies. In the present review, we summarize the current understanding of basic MAM biology, composition and function and discuss the possible role of MAM-resident oncogenes and tumor suppressors.

A mitochondrial megachannel resides in monomeric F1FO ATP synthase

Purified mitochondrial ATP synthase has been shown to form Ca2+-activated, large conductance channel activity similar to that of mitochondrial megachannel (MMC) or mitochondrial permeability transition pore (mPTP) but the oligomeric state required for channel formation is being debated. We reconstitute purified monomeric ATP synthase from porcine heart mitochondria into small unilamellar vesicles (SUVs) with the lipid composition of mitochondrial inner membrane and analyze its oligomeric state by electron cryomicroscopy. The cryo-EM density map reveals the presence of a single ATP synthase monomer with no density seen for a second molecule tilted at an 86o angle relative to the first. We show that this preparation of SUV-reconstituted ATP synthase monomers, when fused into giant unilamellar vesicles (GUVs), forms voltage-gated and Ca2+-activated channels with the key features of mPTP. Based on our findings we conclude that the ATP synthase monomer is sufficient, and dimer formation is not required, for mPTP activity.

The mitochondrial permeability transition pore in cell death: A promising drug binding bioarchitecture

Bioenergetic failure often features programmed cell death involved in some severe pathologies. When the cell is fated to die, the inner mitochondrial membrane becomes permeable to ions and solutes, due to the formation and opening of a channel known as mitochondrial permeability transition pore (mPTP). Up to now, the still-elusive mPTP structure and mechanism prevented any attempt to identify/design drugs to rule its formation and limit cell death. Latest advances, which strongly suggest that the F₁ F_O -ATPase can coincide with the mPTP, open new perspectives in therapy. Compounds targeting and inhibiting cyclophilin D, a known mPTP promoter, could be exploited to block mPTP formation. Moreover, if the mPTP-F₁ F_O -ATPase connection will be consolidated, selected F₁ F_O -ATPase inhibitors could represent novel therapeutic options to attenuate mPTP-related diseases by directly acting on mPTP molecular mechanism. This intriguing perspective, which raises new hopes to counteract mPTP-related diseases, stimulates further studies to clarify the mPTP architecture and mechanism.

Cytoplasmic and Mitochondrial Calcium Signaling: A Two-Way Relationship

Intracellular Ca²⁺ signals are well organized in all cell types, and trigger a variety of vital physiological processes. The temporal and spatial characteristics of cytosolic Ca²⁺ increases are mainly governed by the fluxes of this ion across the membrane of the endoplasmic/sarcoplasmic reticulum and the plasma membrane. However, various Ca²⁺ transporters also allow for Ca²⁺ exchanges between the cytoplasm and mitochondria. Increases in mitochondrial Ca²⁺ stimulate the production of ATP, which allows the cells to cope with the increased energy demand created by the stimulus. Less widely appreciated is the fact that Ca²⁺ handling by mitochondria also shapes cytosolic Ca²⁺ signals. Indeed, the frequency, amplitude, and duration of cytosolic Ca²⁺ increases can be altered by modifying the rates of Ca²⁺ transport into, or from, mitochondria. In this review, we focus on the interplay between mitochondria and Ca²⁺ signaling, highlighting not only the consequences of cytosolic Ca²⁺ changes on mitochondrial Ca²⁺, but also how cytosolic Ca²⁺ dynamics is controlled by modifications of the Ca²⁺-handling properties and the metabolism of mitochondria.

A Therapeutic Role for the F1FO-ATP Synthase

Recently, the F₁F_O-ATP synthase, due to its dual role of life enzyme as main adenosine triphosphate (ATP) maker and of death enzyme, as ATP dissipator and putative structural component of the mitochondrial permeability transition pore (mPTP), which triggers cell death, has been increasingly considered as a drug target. Accordingly, the enzyme offers new strategies to counteract the increased antibiotic resistance. The challenge is to find or synthesize compounds able to discriminate between prokaryotic and mitochondrial F₁F_O-ATP synthase, exploiting subtle structural differences to kill pathogens without affecting the host. From this perspective, the eukaryotic enzyme could also be made refractory to macrolide antibiotics by chemically produced posttranslational modifications. Moreover, because the mitochondrial F₁F_O-ATPase activity stimulated by Ca²⁺ instead of by the natural modulator Mg²⁺ is most likely involved in mPTP formation, effectors preferentially targeting the Ca²⁺-activated enzyme may modulate the mPTP. If the enzyme involvement in the mPTP is confirmed, Ca²⁺-ATPase inhibitors may counteract conditions featured by an increased mPTP activity, such as neurodegenerative and cardiovascular diseases and physiological aging. Conversely, mPTP opening could be pharmacologically stimulated to selectively kill unwanted cells. On the basis of recent literature and promising lab findings, the action mechanism of F₁ and F_O inhibitors is considered. These molecules may act as enzyme modifiers and constitute new drugs to kill pathogens, improve compromised enzyme functions, and limit the deathly enzyme role in pathologies. The enzyme offers a wide spectrum of therapeutic strategies to fight at the molecular level diseases whose treatment is still insufficient or merely symptomatic.

The greater effect of high-intensity interval training versus moderate-intensity continuous training on cardioprotection against ischemia-reperfusion injury through Klotho levels and attenuate of myocardial TRPC6 expression

BACKGROUND

Myocardial ischemia-reperfusion (IR) injury is a leading cause of death all over the world, so developing practical approaches to promote cardioprotection against IR injury is essential. Exercise training is an effective strategy to improve cardioprotection. Hence, the purpose of this study was to investigate the effect of short-term preconditioning with two types of high-intensity interval training (HIIT) and moderate intensity continuous training (MICT) on klotho and TRPC6 mechanisms in cardioprotection.

METHODS

Eighty Male Wistar rats (250-300 g) were randomly divided into 7 groups, including Control, HIIT, MICT, Sham, IR, HIIT+IR, and MICT+IR. Training was performed in 5 consecutive days. HIIT protocol consisted of running on the treadmill at intervals 85-90% vo₂max that separated by slow intensity periods at 50-60% vo₂max. MICT program was performed at 70% VO₂max at the same running distance with HIIT groups. The cardiac IR injury was induced by LAD occlusion followed by reperfusion. ELISA kit was used in order to measure the plasma levels of klotho, LDH and CK-MB, and TRPC6 expression was determined using the western blot technique. Data were analyzed using one way ANOVA and Tukey's post hoc tests.

RESULTS

The results of this study showed that both types of exercise training programs significantly increase plasma levels of klotho and reduce the infarct size and heart injury. In addition, the exercise training decreased the amount of TRPC6 channels expression during IR. However, the effect of HIIT on increasing the klotho and cardioprotection was greater compared to MICT.

CONCLUSIONS

Based on the results, even a short-term of aerobic exercise training, especially HIIT, promotes cardioprotection against IR injury and decreases infarct size via an increase in klotho and attenuate of protein expression of myocardial TRPC6 during IR.

A New Current for the Mitochondrial Permeability Transition

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

The Mitochondrial Permeability Transition in Mitochondrial Disorders

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

Melatonin as a master regulator of cell death and inflammation: molecular mechanisms and clinical implications for newborn care

Melatonin, more commonly known as the sleep hormone, is mainly secreted by the pineal gland in dark conditions and regulates the circadian rhythm of the organism. Its intrinsic properties, including high cell permeability, the ability to easily cross both the blood–brain and placenta barriers, and its role as an endogenous reservoir of free radical scavengers (with indirect extra activities), confer it beneficial uses as an adjuvant in the biomedical field. Melatonin can exert its effects by acting through specific cellular receptors on the plasma membrane, similar to other hormones, or through receptor-independent mechanisms that involve complex molecular cross talk with other players. There is increasing evidence regarding the extraordinary beneficial effects of melatonin, also via exogenous administration. Here, we summarize molecular pathways in which melatonin is considered a master regulator, with attention to cell death and inflammation mechanisms from basic, translational and clinical points of view in the context of newborn care.

Mitochondrial Permeability Transition: A Molecular Lesion with Multiple Drug Targets

Mitochondrial permeability transition, as the consequence of opening of a mitochondrial permeability transition pore (mPTP), is a cellular catastrophe. Initiating bioenergetic collapse and cell death, it has been implicated in the pathophysiology of major human diseases, including neuromuscular diseases of childhood, ischaemia-reperfusion injury, and age-related neurodegenerative disease. Opening of the mPTP represents a major therapeutic target, as it can be mitigated by a number of compounds. However, clinical studies have so far been disappointing. We therefore address the prospects and challenges faced in translating in vitro findings to clinical benefit. We review the role of mPTP opening in disease, discuss recent findings defining the putative structure of the mPTP, and explore strategies to identify novel, clinically useful mPTP inhibitors, highlighting key considerations in the drug discovery process.