On the opening of an insensitive cyclosporin A non-specific pore by phenylarsine plus mersalyl

Crosstalk between adenine nucleotide transporter and mitochondrial swelling: experimental and computational approaches

Mitochondrial metabolism and function are modulated by changes in matrix Ca²⁺. Small increases in the matrix Ca²⁺ stimulate mitochondrial bioenergetics, whereas excessive Ca²⁺ leads to cell death by causing massive matrix swelling and impairing the structural and functional integrity of mitochondria. Sustained opening of the non-selective mitochondrial permeability transition pores (PTP) is the main mechanism responsible for mitochondrial Ca²⁺ overload that leads to mitochondrial dysfunction and cell death. Recent studies suggest the existence of two or more types of PTP, and adenine nucleotide translocator (ANT) and F_OF₁-ATP synthase were proposed to form the PTP independent of each other. Here, we elucidated the role of ANT in PTP opening by applying both experimental and computational approaches. We first developed and corroborated a detailed model of the ANT transport mechanism including the matrix (ANT_M), cytosolic (ANT_C), and pore (ANT_P) states of the transporter. Then, the ANT model was incorporated into a simple, yet effective, empirical model of mitochondrial bioenergetics to ascertain the point when Ca²⁺ overload initiates PTP opening via an ANT switch-like mechanism activated by matrix Ca²⁺ and is inhibited by extra-mitochondrial ADP. We found that encoding a heterogeneous Ca²⁺ response of at least three types of PTPs, weakly, moderately, and strongly sensitive to Ca²⁺, enabled the model to simulate Ca²⁺ release dynamics observed after large boluses were administered to a population of energized cardiac mitochondria. Thus, this study demonstrates the potential role of ANT in PTP gating and proposes a novel mechanism governing the cryptic nature of the PTP phenomenon.

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 Regulation of Non-Specific Membrane Permeability Transition in Yeast Mitochondria under Oxidative Stress

In this study, the mechanism of non-specific membrane permeability (yPTP) in the Endomyces magnusii yeast mitochondria under oxidative stress due to blocking the key antioxidant enzymes has been investigated. We used monitoring the membrane potential at the cellular (potential-dependent staining) and mitochondrial levels and mitochondria ultra-structural images with transmission electron microscopy (TEM) to demonstrate the mitochondrial permeability transition induction due to the pore opening. Analysis of the yPTP opening upon respiring different substrates showed that NAD(P)H completely blocked the development of the yPTP. The yPTP opening was inhibited by 5–20 mM Pi, 5 mM Mg2+, adenine nucleotides (AN), 5 mM GSH, the inhibitor of the Pi transporter (PiC), 100 μM mersalyl, the blockers of the adenine nucleotide transporter (ANT) carboxyatractyloside (CATR), and bongkrekic acid (BA). We concluded that the non-specific membrane permeability pore opens in the E. magnusii mitochondria under oxidative stress, and the ANT and PiC are involved in its formation. The crucial role of the Ca2+ ions in the process has not been confirmed. We showed that the Ca2+ ions affected the yPTP both with and without the Ca2+ ionophore ETH129 application insignificantly. This phenomenon in the E. magnusii yeast unites both mitochondrial unselective channel (ScMUC) features in the Saccharomyces cerevisiae mitochondria and the classical membrane pore in the mammalian ones (mPTP).

A systematic review of post-translational modifications in the mitochondrial permeability transition pore complex associated with cardiac diseases

The mitochondrial permeability transition pore (mPTP) opening is involved in the pathophysiology of multiple cardiac diseases, such as ischemia/reperfusion injury and heart failure. A growing number of evidence provided by proteomic screening techniques has demonstrated the role of post-translational modifications (PTMs) in several key components of the pore in response to changes in the extra/intracellular environment and bioenergetic demand. This could lead to a fine, complex regulatory mechanism that, under pathological conditions, can shift the state of mitochondrial functions and, thus, the cell's fate. Understanding the complex relationship between these PTMs is still under investigation and can provide new, promising therapeutic targets and treatment approaches. This review, using a systematic review of the literature, presents the current knowledge on PTMs of the mPTP and their role in health and cardiac disease.

Amorphous SiO2 nanoparticles promote cardiac dysfunction via the opening of the mitochondrial permeability transition pore in rat heart and human cardiomyocytes

Background

Silica nanoparticles (nanoSiO₂) are promising systems that can deliver biologically active compounds to tissues such as the heart in a controllable manner. However, cardiac toxicity induced by nanoSiO₂ has been recently related to abnormal calcium handling and energetic failure in cardiomyocytes. Moreover, the precise mechanisms underlying this energetic debacle remain unclear. In order to elucidate these mechanisms, this article explores the ex vivo heart function and mitochondria after exposure to nanoSiO₂.

Results

The cumulative administration of nanoSiO₂ reduced the mechanical performance index of the rat heart with a half-maximal inhibitory concentration (IC₅₀) of 93 μg/mL, affecting the relaxation rate. In isolated mitochondria nanoSiO₂ was found to be internalized, inhibiting oxidative phosphorylation and significantly reducing the mitochondrial membrane potential (ΔΨ_m). The mitochondrial permeability transition pore (mPTP) was also induced with an increasing dose of nanoSiO₂ and partially recovered with, a potent blocker of the mPTP, Cyclosporine A (CsA). The activity of aconitase and thiol oxidation, in the adenine nucleotide translocase, were found to be reduced due to nanoSiO₂ exposure, suggesting that nanoSiO₂ induces the mPTP via thiol modification and ROS generation. In cardiac cells exposed to nanoSiO₂, enhanced viability and reduction of H₂O₂ were observed after application of a specific mitochondrial antioxidant, MitoTEMPO. Concomitantly, CsA treatment in adult rat cardiac cells reduced the nanoSiO₂-triggered cell death and recovered ATP production (from 32.4 to 65.4%). Additionally, we performed evaluation of the mitochondrial effect of nanoSiO₂ in human cardiomyocytes. We observed a 40% inhibition of maximal oxygen consumption rate in mitochondria at 500 μg/mL. Under this condition we identified a remarkable diminution in the spare respiratory capacity. This data indicates that a reduction in the amount of extra ATP that can be produced by mitochondria during a sudden increase in energy demand. In human cardiomyocytes, increased LDH release and necrosis were found at increased doses of nanoSiO₂, reaching 85 and 48%, respectively. Such deleterious effects were partially prevented by the application of CsA. Therefore, exposure to nanoSiO₂ affects cardiac function via mitochondrial dysfunction through the opening of the mPTP.

Conclusion

The aforementioned effects can be partially avoided reducing ROS or retarding the opening of the mPTP. These novel strategies which resulted in cardioprotection could be considered as potential therapies to decrease the side effects of nanoSiO₂ exposure.

Changes in the Stoichiometry of Uniplex Decrease Mitochondrial Calcium Overload and Contribute to Tolerance of Cardiac Ischemia/Reperfusion Injury in Hypothyroidism

Background: Thyroid hormone status in hypothyroidism (HT) downregulates key elements in Ca²⁺ handling within the heart, reducing contractility, impairing the basal energetic balance, and increasing the risk of cardiovascular disease. Mitochondrial Ca²⁺ transport is reduced in HT, and tolerance to reperfusion damage has been documented, but the precise mechanism is not well understood. Therefore, we aimed to determine the stoichiometry and activity of the mitochondrial Ca²⁺ uniporter or uniplex in an HT model and the relevance to the opening of the mitochondrial permeability transition pores (mPTP) during ischemia/reperfusion (I/R) injury. Methods: An HT model was established in Wistar rats by treatment with 6-propylthiouracil for 28 days. Uniplex composition and activity were determined in cardiac mitochondria. Hearts were perfused ex vivo to induce I/R injury, and functional parameters related to contractility and tissue viability were evaluated. Results: The cardiac stoichiometry between two subunits of the uniplex (MICU1/MCU) increased by 25% in animals with HT. The intramitochondrial Ca²⁺ content was reduced by 40% and was less prone to the mPTP opening. After I/R injury, ischemic contracture and the onset of ventricular fibrillation were delayed in animals with HT, concomitant with a reduction in oxidative damage and mitochondrial dysfunction. Conclusions: Our results suggest that HT is associated with an increase in the cardiac MICU1/MCU ratio, thereby changing the stoichiometry between these subunits to increase the threshold to cytosolic Ca²⁺ and reduce mitochondrial Ca²⁺ overload. Our results also demonstrate that this HT model can be used to explore the role of mitochondrial Ca²⁺ transport in cardiac diseases due to its induced tolerance to cardiac damage.

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

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

Continuous Infusion of Phenelzine, Cyclosporine A, or Their Combination: Evaluation of Mitochondrial Bioenergetics, Oxidative Damage, and Cytoskeletal Degradation following Severe Controlled Cortical Impact Traumatic Brain Injury in Rats

To date, all monotherapy clinical traumatic brain injury (TBI) trials have failed, and there are currently no Food and Drug Administration (FDA)-approved pharmacotherapies for the acute treatment of severe TBI. Due to the complex secondary injury cascade following injury, there is a need to develop multi-mechanistic combinational neuroprotective approaches for the treatment of acute TBI. As central mediators of the TBI secondary injury cascade, both mitochondria and lipid peroxidation-derived aldehydes make promising therapeutic targets. Cyclosporine A (CsA), an FDA-approved immunosuppressant capable of inhibiting the mitochondrial permeability transition pore, and phenelzine (PZ), an FDA-approved monoamine oxidase inhibitor capable of scavenging neurotoxic lipid peroxidation-derived aldehydes, have both been shown to be partially neuroprotective following experimental TBI. Therefore, it follows that the combination of PZ and CsA may enhance neuroprotection over either agent alone through the combining of distinct but complementary mechanisms of action. Additionally, as the first 72 h represents a critical time period following injury, it follows that continuous drug infusion over the first 72 h following injury may also lead to optimal neuroprotective effects. This is the first study to examine the effects of a 72 h subcutaneous continuous infusion of PZ, CsA, and the combination of these two agents on mitochondrial respiration, mitochondrial bound 4-hydroxynonenal (4-HNE), and acrolein, and α-spectrin degradation 72 h following a severe controlled cortical impact injury in rats. Our results indicate that individually, both CsA and PZ are able to attenuate mitochondrial 4-HNE and acrolein, PZ is able to maintain mitochondrial respiratory control ratio and cytoskeletal integrity but together, PZ and CsA are unable to maintain neuroprotective effects.

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

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

To involvement the conformation of the adenine nucleotide translocase in opening the Tl(+)-induced permeability transition pore in Ca(2+)-loaded rat liver mitochondria

The conformation of adenine nucleotide translocase (ANT) has a profound impact in opening the mitochondrial permeability transition pore (MPTP) in the inner membrane. Fixing the ANT in 'c' conformation by phenylarsine oxide (PAO), tert-butylhydroperoxide (tBHP), and carboxyatractyloside as well as the interaction of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) with mitochondrial thiols markedly attenuated the ability of ADP to inhibit the MPTP opening. We earlier found (Korotkov and Saris, 2011) that calcium load of rat liver mitochondria in medium containing TlNO3 and KNO3 stimulated the Tl(+)-induced MPTP opening in the inner mitochondrial membrane. The MPTP opening as well as followed increase in swelling, a drop in membrane potential (ΔΨmito), and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration were visibly enhanced in the presence of PAO, tBHP, DIDS, and carboxyatractyloside. However, these effects were markedly inhibited by ADP and membrane-penetrant hydrophobic thiol reagent, N-ethylmaleimide (NEM) which fix the ANT in 'm' conformation. Cyclosporine A additionally potentiated these effects of ADP and NEM. Our data suggest that conformational changes of the ANT may be directly involved in the opening of the Tl(+)-induced MPTP in the inner membrane of Ca(2+)-loaded rat liver mitochondria. Using the Tl(+)-induced MPTP model is discussed in terms finding new transition pore inhibitors and inducers among different chemical and natural compounds.

Cyclosporin A inhibits UV-radiation-induced membrane damage but is unable to inhibit carboxyatractyloside-induced permeability transition

This work was undertaken to gain further information on the chemical characteristics of the membrane entity involved in the formation of the nonspecific pore. Mitochondria were subjected to oxidative stress by exposure to UV radiation. The results indicate that ultraviolet C radiation induces structural modifications in the adenine nucleotide translocase that lead to membrane permeability transition. Membrane leakage was assessed by measuring mitochondrial Ca2+ transport, the transmembrane electric gradient, and mitochondrial swelling. UV-irradiated mitochondria were unable to retain matrix Ca2+ or to maintain a high level of membrane potential when Ca2+ was added; furthermore, UV-irradiated mitochondria underwent large amplitude swelling. Release of cytochrome c and formation of malondialdehyde, owing to lipid peroxidation, were also seen. Structural modifications of the translocase were revealed by an increase in the binding of the fluorescent probe eosin-5-maleimide to thiol residues of the ADP/ATP carrier. These modifications, taken together with findings indicating that cyclosporin resulted unable to inhibit carboxyatractyloside-induced permeability transition, prompted us to conclude that the translocase could constitute the nonspecific pore or at least be an important modulator of it.

Transient opening of mitochondrial permeability transition pore by reactive oxygen species protects myocardium from ischemia-reperfusion injury

Reactive oxygen species (ROS) production during ischemia-reperfusion (I/R) is thought to be a critical factor for myocardial injury. However, a small amount of ROS during the ischemic preconditioning (IPC) may provide a signal for cardioprotection. We have previously reported that the repetitive pretreatment of a small amount of ROS [hydrogen peroxide (H(2)O(2)), 2 microM] mimicked the IPC-induced cardioprotection in the Langendorff-perfused rat hearts. We further investigated the mechanisms of the ROS-induced cardioprotection against I/R injury and tested the hypothesis whether it could mediate the mitochondrial permeability transition pore (mPTP) opening. The Langendorff-perfused rat hearts were subjected to 35 min ischemia and 40 min reperfusion, and the pretreatment of H(2)O(2) (2 microM) significantly improved the postischemic recoveries in left ventricular developed pressure, intracellular phosphocreatine, and ATP levels. A specific mPTP inhibitor cyclosporin A (CsA; 0.2 microM) canceled these H(2)O(2)-induced effects. In isolated permeabilized myocytes, H(2)O(2) (1 microM) accelerated the calcein leakage from mitochondria in a CsA-sensitive manner, indicating the opening of mPTP by H(2)O(2). However, H(2)O(2) did not depolarize mitochondrial membrane potential (DeltaPsi(m)) even in the presence of oligomycin (F(1)/F(0) ATPase inhibitor; 1 microM) and decreased mitochondrial Ca(2+) concentration ([Ca(2+)](m)) by accelerating the mitochondrial Ca(2+) extrusion via an mPTP. We conclude that the transient mPTP opening could be involved in the H(2)O(2)-induced cardioprotection against reperfusion injury, and the reduction of [Ca(2+)](m) without the change in DeltaPsi(m) might be a possible mechanism for the protection.

The effect of N-ethylmaleimide on permeability transition as induced by carboxyatractyloside, agaric acid, and oleate

In this work, we studied the effect of N-ethylmaleimide on permeability transition. The findings indicate that the amine inhibited the effects of carboxyatractyloside and agaric acid. It is known that these reagents interact with the adenine nucleotide carrier through the cytosolic side. When oleate, which interacts through the matrix side, was used it was found that the amine amplified the effects of oleate on permeability transition. The results also show that N-ethylmaleimide strengthened the inhibition induced by carboxyatractyloside, agaric acid, and oleate on ADP exchange. Furthermore, it was also found that oleate improved the binding of eosin-5-maleimide on the adenine nucleotide translocase.