Second-Generation Inhibitors of the Mitochondrial Permeability Transition Pore with Improved Plasma Stability

Adenine nucleotide translocator and ATP synthase cooperate in mediating the mitochondrial permeability transition

The permeability transition (PT) is a permeability increase of the mitochondrial inner membrane causing mitochondrial swelling in response to matrix Ca2+. The PT is mediated by regulated channel(s), the PT pore(s) (PTP), which can be generated by at least two components, adenine nucleotide translocator (ANT) and ATP synthase. Whether these provide independent permeation pathways remains to be established. Here, we assessed the contribution of ANT to the PT based on the effects of the selective ANT inhibitors atractylate (ATR) and bongkrekate (BKA), which trigger and inhibit channel formation by ANT, respectively. BKA partially inhibited Ca2+-dependent PT and did not prevent the inducing effect of phenylarsine oxide, which was still present in mouse embryonic fibroblasts deleted for all ANT isoforms. The contribution of ANT to the PT emerged at pH 6.5 (a condition that inhibits ATP synthase channel opening) in the presence of ATR, which triggered mitochondrial swelling and elicited currents in patch-clamped mitoplasts. Unexpectedly, ANT-dependent PT at pH 6.5 could also be stimulated by benzodiazepine-423 [a selective ligand of the oligomycin sensitivity conferral protein (OSCP) subunit of ATP synthase], suggesting that the ANT channel is regulated by the peripheral stalk of ATP synthase. In keeping with docking simulations, ANT could be co-immunoprecipitated with ATP synthase subunits c and g, and oligomycin (which binds adjacent c subunits) decreased the association of ANT with subunit c. These results reveal a close cooperation between ANT and ATP synthase in the PT and open new perspectives in the study of this process. KEY POINTS: We have assessed the relative role of adenine nucleotide translocator (ANT) and ATP synthase in generating the mitochondrial permeability transition (PT). At pH 7.4, bongkrekate had little effect on Ca2+-dependent PT, and did not prevent the inducing effect of phenylarsine oxide, which was still present in mouse embryonic fibroblasts deleted for all ANT isoforms. The contribution of ANT emerged at pH 6.5 (which inhibits ATP synthase channel opening) in the presence of atractylate, which triggered mitochondrial swelling and elicited currents in patch-clamped mitoplasts. Benzodiazepine-423, a selective ligand of the oligomycin sensitivity conferral protein subunit of ATP synthase, stimulated ANT-dependent PT at pH 6.5, suggesting that the ANT channel is regulated by the peripheral stalk of ATP synthase. ANT could be co-immunoprecipitated with ATP synthase subunits c and g; oligomycin, which binds adjacent c subunits, decreased the association with subunit c, in keeping with docking simulations.

Modulation of the Adjuvant Potential of Imidazoquinoline-Based TLR7/8 Agonists via Alum Adsorption

Toll-like receptor (TLR)-7/8 agonists are promising candidates for the development of new-generation vaccine adjuvants. Adsorption of TLR7/8 agonists on aluminum salts (alum) may further enhance vaccine immunogenicity. Evaluation of the adjuvanticity of the most active dual TLR7/8 agonists, 1-(3-(aminomethyl)benzyl)-2-butyl-1H-imidazo[4,5-c]quinolin-4-amine (m-AM-BBIQ, 10) and its para derivative p-AM-BBIQ (11), along with their gallic acid and protocatechuic acid amides in a recombinant-protein-based COVID-19 vaccine platform confirmed the importance of vic-polyphenolic functionality in TLR7/8 agonists for the alum adsorption, thereby resulting in a balanced Th1/Th2 immune response. A novel 7,8-dihydroxy-IMDQ derivative (dh-p-AM-BBIQ, 21) was designed wherein the vic-diphenolic functionality was introduced in the quinoline ring of the imidazo[4,5-c]quinoline scaffold. Compound 21 not only retained the TLR7 agonistic activity (EC50 = 3.72 μM) but also showed high adsorption to alum and induced a potent antibody response to SARS-CoV-2 spike protein and hepatitis B surface antigen immunized mice. The combination adjuvant comprising compound 21 adsorbed to alum represents a promising candidate for further development as a human and veterinary vaccine adjuvant.

Design and Synthesis of Polyphenolic Imidazo[4,5-c]quinoline Derivatives to Modulate Toll Like Receptor-7 Agonistic Activity and Adjuvanticity

TLR-7/8 agonists are a well-known class of vaccine adjuvants, with a leading example now included in Covaxin, a licensed human COVID-19 vaccine. This thereby provides the opportunity to develop newer, more potent adjuvants based on structure-function studies of these classes of compounds. Imidazoquinoline-based TLR7/8 agonists are the most potent, but when used as a vaccine adjuvant side effects can arise due to diffusion from the injection site into a systemic circulation. In this work, we sought to address this issue through structural modifications in the agonists to enhance their adsorption capacity to the classic adjuvant alum. We selected a potent TLR7-selective agonist, BBIQ (EC50 = 0.85 μM), and synthesized polyphenolic derivatives to assess their TLR7 agonistic activity and adjuvant potential alone or in combination with alum. Most of the phenolic derivatives were more active than BBIQ and, except for 12b, all were TLR7 specific. Although the synthesized compounds were less active than resiquimod, the immunization data on combination with alum, specifically the IgG1, IgG2b and IgG2c responses, were superior in comparison to BBIQ as well as the reference standard resiquimod. Compound 12b was 5-fold more potent (EC50 = 0.15 μM in TLR7) than BBIQ and induced double the IgG response to SARS-CoV-2 and hepatitis antigens. Similarly, compound 12c (EC50 = 0.31 μM in TLR7) was about 3-fold more potent than BBIQ and doubled the IgG levels. Even though compound 12d exhibited low TLR7 activity (EC50 = 5.13 μM in TLR7), it demonstrated superior adjuvant results, which may be attributed to its enhanced alum adsorption capability as compared with BBIQ and resiquimod. Alum-adsorbed polyphenolic TLR7 agonists thereby represent promising combination adjuvants resulting in a balanced Th1/Th2 immune response.

An ex vivo screening using mouse brain mitochondria identified seco-cycline D as an inhibitor of mitochondrial permeability transition pore

Mitochondrial dysfunction is implicated in neuropsychiatric disorders. Inhibition of mitochondrial permeability transition pore (mPTP) and thereby enhancement of mitochondrial Ca2+ retention capacity (CRC) is a promising treatment strategy. Here, we screened 1718 compounds to search for drug candidates inhibiting mPTP by measuring their effects on CRC in mitochondria isolated from mouse brains. We identified seco-cycline D (SCD) as an active compound. SCD and its derivative were more potent than a known mPTP inhibitor, cyclosporine A (CsA). The mechanism of action of SCD was suggested likely to be different from CsA that acts on cyclophilin D. Repeated administration of SCD decreased ischemic area in a middle cerebral artery occlusion model in mice. These results suggest that SCD is a useful probe to explore mPTP function.

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.

Modeling Human Muscular Dystrophies in Zebrafish: Mutant Lines, Transgenic Fluorescent Biosensors, and Phenotyping Assays

Muscular dystrophies (MDs) are a heterogeneous group of myopathies characterized by progressive muscle weakness leading to death from heart or respiratory failure. MDs are caused by mutations in genes involved in both the development and organization of muscle fibers. Several animal models harboring mutations in MD-associated genes have been developed so far. Together with rodents, the zebrafish is one of the most popular animal models used to reproduce MDs because of the high level of sequence homology with the human genome and its genetic manipulability. This review describes the most important zebrafish mutant models of MD and the most advanced tools used to generate and characterize all these valuable transgenic lines. Zebrafish models of MDs have been generated by introducing mutations to muscle-specific genes with different genetic techniques, such as (i) N-ethyl-N-nitrosourea (ENU) treatment, (ii) the injection of specific morpholino, (iii) tol2-based transgenesis, (iv) TALEN, (v) and CRISPR/Cas9 technology. All these models are extensively used either to study muscle development and function or understand the pathogenetic mechanisms of MDs. Several tools have also been developed to characterize these zebrafish models by checking (i) motor behavior, (ii) muscle fiber structure, (iii) oxidative stress, and (iv) mitochondrial function and dynamics. Further, living biosensor models, based on the expression of fluorescent reporter proteins under the control of muscle-specific promoters or responsive elements, have been revealed to be powerful tools to follow molecular dynamics at the level of a single muscle fiber. Thus, zebrafish models of MDs can also be a powerful tool to search for new drugs or gene therapies able to block or slow down disease progression.

Novel Regioselective Synthesis of 1,3,4,5-Tetrasubstituted Pyrazoles and Biochemical Valuation on F1FO-ATPase and Mitochondrial Permeability Transition Pore Formation

An efficient, eco-compatible, and very cheap method for the construction of fully substituted pyrazoles (Pzs) via eliminative nitrilimine-alkene 1,3-dipolar cycloaddition (ENAC) reaction was developed in excellent yield and high regioselectivity. Enaminones and nitrilimines generated in situ were selected as dipolarophiles and dipoles, respectively. A deep screening of the employed base, solvent, and temperature was carried out to optimize reaction conditions. Recycling tests of ionic liquid were performed, furnishing efficient performance until six cycles. Finally, a plausible mechanism of cycloaddition was proposed. Then, the effect of three different structures of Pzs was evaluated on the F1FO-ATPase activity and mitochondrial permeability transition pore (mPTP) opening. The Pz derivatives' titration curves of 6a, 6h, and 6o on the F1FO-ATPase showed a reduced activity of 86%, 35%, and 31%, respectively. Enzyme inhibition analysis depicted an uncompetitive mechanism with the typical formation of the tertiary complex enzyme-substrate-inhibitor (ESI). The dissociation constant of the ESI complex (Ki') in the presence of the 6a had a lower order of magnitude than other Pzs. The pyrazole core might set the specific mechanism of inhibition with the F1FO-ATPase, whereas specific functional groups of Pzs might modulate the binding affinity. The mPTP opening decreased in Pz-treated mitochondria and the Pzs' inhibitory effect on the mPTP was concentration-dependent with 6a and 6o. Indeed, the mPTP was more efficiently blocked with 0.1 mM 6a than with 1 mM 6a. On the contrary, 1 mM 6o had stronger desensitization of mPTP formation than 0.1 mM 6o. The F1FO-ATPase is a target of Pzs blocking mPTP formation.

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 F1FO-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 F1FO-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.

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.

Skeletal Muscle Mitochondria Dysfunction in Genetic Neuromuscular Disorders with Cardiac Phenotype

Mitochondrial dysfunction is considered the major contributor to skeletal muscle wasting in different conditions. Genetically determined neuromuscular disorders occur as a result of mutations in the structural proteins of striated muscle cells and therefore are often combined with cardiac phenotype, which most often manifests as a cardiomyopathy. The specific roles played by mitochondria and mitochondrial energetic metabolism in skeletal muscle under muscle-wasting conditions in cardiomyopathies have not yet been investigated in detail, and this aspect of genetic muscle diseases remains poorly characterized. This review will highlight dysregulation of mitochondrial representation and bioenergetics in specific skeletal muscle disorders caused by mutations that disrupt the structural and functional integrity of muscle cells.

Targeting mitochondrial ion channels for cancer therapy

Pharmacological targeting of mitochondrial ion channels is emerging as a promising approach to eliminate cancer cells; as most of these channels are differentially expressed and/or regulated in cancer cells in comparison to healthy ones, this strategy may selectively eliminate the former. Perturbation of ion fluxes across the outer and inner membranes is linked to alterations of redox state, membrane potential and bioenergetic efficiency. This leads to indirect modulation of oxidative phosphorylation, which is/may be fundamental for both cancer and cancer stem cell survival. Furthermore, given the crucial contribution of mitochondria to intrinsic apoptosis, modulation of their ion channels leading to cytochrome c release may be of great advantage in case of resistance to drugs triggering apoptotic events upstream of the mitochondrial phase. In the present review, we give an overview of the known mitochondrial ion channels and of their modulators capable of killing cancer cells. In addition, we discuss state-of-the-art strategies using mitochondriotropic drugs or peptide-based approaches allowing a more efficient and selective targeting of mitochondrial ion channel-linked events.

Translational research on Myology and Mobility Medicine: 2021 semi-virtual PDM3 from Thermae of Euganean Hills, May 26 - 29, 2021

On 19-21 November 2020, the meeting of the 30 years of the Padova Muscle Days was virtually held while the SARS-CoV-2 epidemic was hitting the world after a seemingly quiet summer. During the 2020-2021 winter, the epidemic is still active, despite the start of vaccinations. The organizers hope to hold the 2021 Padua Days on Myology and Mobility Medicine in a semi-virtual form (2021 S-V PDM3) from May 26 to May 29 at the Thermae of Euganean Hills, Padova, Italy. Here the program and the Collection of Abstracts are presented. Despite numerous world problems, the number of submitted/selected presentations (lectures and oral presentations) has increased, prompting the organizers to extend the program to four dense days.

Treatment with a triazole inhibitor of the mitochondrial permeability transition pore fully corrects the pathology of sapje zebrafish lacking dystrophin

High-throughput screening identified isoxazoles as potent but metabolically unstable inhibitors of the mitochondrial permeability transition pore (PTP). Here we have studied the effects of a metabolically stable triazole analog, TR001, which maintains the PTP inhibitory properties with an in vitro potency in the nanomolar range. We show that TR001 leads to recovery of muscle structure and function of sapje zebrafish, a severe model of Duchenne muscular dystrophy (DMD). PTP inhibition fully restores the otherwise defective respiration in vivo, allowing normal development of sapje individuals in spite of lack of dystrophin. About 80 % sapje zebrafish treated with TR001 are alive and normal at 18 days post fertilization (dpf), a point in time when not a single untreated sapje individual survives. Time to 50 % death of treated zebrafish increases from 5 to 28 dpf, a sizeable number of individuals becoming young adults in spite of the persistent lack of dystrophin expression. TR001 improves respiration of myoblasts and myotubes from DMD patients, suggesting that PTP-dependent dysfunction also occurs in the human disease and that mitochondrial therapy of DMD with PTP-inhibiting triazoles is a viable treatment option.

Modelling Neuromuscular Diseases in the Age of Precision Medicine

Advances in knowledge resulting from the sequencing of the human genome, coupled with technological developments and a deeper understanding of disease mechanisms of pathogenesis are paving the way for a growing role of precision medicine in the treatment of a number of human conditions. The goal of precision medicine is to identify and deliver effective therapeutic approaches based on patients’ genetic, environmental, and lifestyle factors. With the exception of cancer, neurological diseases provide the most promising opportunity to achieve treatment personalisation, mainly because of accelerated progress in gene discovery, deep clinical phenotyping, and biomarker availability. Developing reproducible, predictable and reliable disease models will be key to the rapid delivery of the anticipated benefits of precision medicine. Here we summarize the current state of the art of preclinical models for neuromuscular diseases, with particular focus on their use and limitations to predict safety and efficacy treatment outcomes in clinical trials.

1,5-Disubstituted-1,2,3-triazoles as inhibitors of the mitochondrial Ca2+ -activated F1 FO -ATP(hydrol)ase and the permeability transition pore

The mitochondrial permeability transition pore (mPTP), a high-conductance channel triggered by a sudden Ca²⁺ concentration increase, is composed of the F₁ F_O -ATPase. Since mPTP opening leads to mitochondrial dysfunction, which is a feature of many diseases, a great pharmacological challenge is to find mPTP modulators. In our study, the effects of two 1,5-disubstituted 1,2,3-triazole derivatives, five-membered heterocycles with three nitrogen atoms in the ring and capable of forming secondary interactions with proteins, were investigated. Compounds 3a and 3b were selected among a wide range of structurally related compounds because of their chemical properties and effectiveness in preliminary studies. In swine heart mitochondria, both compounds inhibit Ca²⁺ -activated F₁ F_O -ATPase without affecting F-ATPase activity sustained by the natural cofactor Mg²⁺ . The inhibition is mutually exclusive, probably because of their shared enzyme site, and uncompetitive with respect to the ATP substrate, since they only bind to the enzyme-ATP complex. Both compounds show the same inhibition constant (K'_i ), but compound 3a has a doubled inactivation rate constant compared with compound 3b. Moreover, both compounds desensitize mPTP opening without altering mitochondrial respiration. The results strengthen the link between Ca²⁺ -activated F₁ F_O -ATPase and mPTP and suggest that these inhibitors can be pharmacologically exploited to counteract mPTP-related diseases.

TRAP1 attenuates H9C2 myocardial cell injury induced by extracellular acidification via the inhibition of MPTP opening

Extracellular acidification leads to cardiac dysfunction in numerous diseases. Mitochondrial dysfunction plays an important role in this process. However, the mechanisms through which extracellular acidification induces mitochondrial dysfunction remain unclear. Tumor necrosis factor receptor-associated protein 1 (TRAP1) maintains mitochondrial function and cell viability in tumor and non-tumor cells. In the present study, extracellular acidification was found to induce H9C2 cell apoptosis, mitochondrial dysfunction and TRAP1 expression. The overexpression of TRAP1 attenuated H9C2 cell injury, while the silencing of TRAP1 exacerbated it. Moreover, mitochondrial permeability transition pore (MPTP) opening, which is associated with the mitochondrial apoptotic pathway and cell death, was also increased in acidic medium. The overexpression of TRAP1 inhibited MPTP opening, while the silencing of TRAP1 promoted it. The protective effect of TRAP1 on cardiomyocytes was abolished by the addition of a specific MPTP opening promoter. Similarly, a specific MPTP opening inhibitor reversed cell injury by silencing TRAP1. Taken together, the findings of the present study demonstrate that TRAP1 attenuates H9C2 cell injury induced by extracellular acidification by inhibiting MPTP opening.

A novel class of cardioprotective small-molecule PTP inhibitors

Ischemia/reperfusion (I/R) injury is mediated in large part by opening of the mitochondrial permeability transition pore (PTP). Consequently, inhibitors of the PTP hold great promise for the treatment of a variety of cardiovascular disorders. At present, PTP inhibition is obtained only through the use of drugs (e.g. cyclosporine A, CsA) targeting cyclophilin D (CyPD) which is a key modulator, but not a structural component of the PTP. This limitation might explain controversial findings in clinical studies. Therefore, we investigated the protective effects against I/R injury of small-molecule inhibitors of the PTP (63 and TR002) that do not target CyPD. Both compounds exhibited a dose-dependent inhibition of PTP opening in isolated mitochondria and were more potent than CsA. Notably, PTP inhibition was observed also in mitochondria devoid of CyPD. Compounds 63 and TR002 prevented PTP opening and mitochondrial depolarization induced by Ca²⁺ overload and by reactive oxygen species in neonatal rat ventricular myocytes (NRVMs). Remarkably, both compounds prevented cell death, contractile dysfunction and sarcomeric derangement induced by anoxia/reoxygenation injury in NRVMs at sub-micromolar concentrations, and were more potent than CsA. Cardioprotection was observed also in adult mouse ventricular myocytes and human iPSc-derived cardiomyocytes, as well as ex vivo in perfused hearts. Thus, this study demonstrates that 63 and TR002 represent novel cardioprotective agents that inhibit PTP opening independent of CyPD targeting.