Molecular mechanisms of naringenin modulation of mitochondrial permeability transition acting on F1FO-ATPase and counteracting saline load-induced injury in SHRSP cerebral endothelial cells

Naringenin (NRG) was characterized for its ability to counteract mitochondrial dysfunction which is linked to cardiovascular diseases. The F1FO-ATPase can act as a molecular target of NRG. The interaction of NRG with this enzyme can avoid the energy transmission mechanism of ATP hydrolysis, especially in the presence of Ca2+ cation used as cofactor. Indeed, NRG was a selective inhibitor of the hydrophilic F1 domain displaying a binding site overlapped with quercetin in the inside surface of an annulus made by the three α and the three β subunits arranged alternatively in a hexamer. The kinetic constant of inhibition suggested that NRG preferred the enzyme activated by Ca2+ rather than the F1FO-ATPase activated by the natural cofactor Mg2+. From the inhibition type mechanism of NRG stemmed the possibility to speculate that NRG can prevent the activation of F1FO-ATPase by Ca2+. The event correlated to the protective role in the mitochondrial permeability transition pore opening by NRG as well as to the reduction of ROS production probably linked to the NRG chemical structure with antioxidant action. Moreover, in primary cerebral endothelial cells (ECs) obtained from stroke prone spontaneously hypertensive rats NRG had a protective effect on salt-induced injury by restoring cell viability and endothelial cell tube formation while also rescuing complex I activity.

Selenite ameliorates the ATP hydrolysis of mitochondrial F1FO-ATPase by changing the redox state of thiol groups and impairs the ADP phosphorylation

Selenite as an inorganic form of selenium can affect the redox state of mitochondria by modifying the thiol groups of cysteines. The F1FO-ATPase has been identified as a mitochondrial target of this compound. Indeed, the bifunctional mechanism of ATP turnover of F1FO-ATPase was differently modified by selenite. The activity of ATP hydrolysis was stimulated, whereas the ADP phosphorylation was inhibited. We ascertain that a possible new protein adduct identified as seleno-dithiol (-S-Se-S-) mercaptoethanol-sensitive caused the activation of F-ATPase activity and the oxidation of free -SH groups in mitochondria. Conversely, the inhibition of ATP synthesis by selenite might be irreversible. The kinetic analysis of the activation mechanism was an uncompetitive mixed type with respect to the ATP substrate. Selenite bound more selectively to the F1FO-ATPase loaded with the substrate by preferentially forming a tertiary (enzyme-ATP-selenite) complex. Otherwise, the selenite was a competitive mixed-type activator with respect to the Mg2+ cofactor. Thus, selenite more specifically bound to the free enzyme forming the complex enzyme-selenite. However, even if the selenite impaired the catalysis of F1FO-ATPase, the mitochondrial permeability transition pore phenomenon was unaffected. Therefore, the reversible energy transduction mechanism of F1FO-ATPase can be oppositely regulated by selenite.

'Rotor free-wheeling' in impaired F1FO-ATPase induces congenital hypermetabolism

A de novo heterozygous variant in the catalytic subunit of mitochondrial F₁F_O-ATPase has been recently discovered by Ganetzky et al. to be the main cause of an autosomal dominant syndrome of hypermetabolism associated with defective ATP production. We describe how the 'rotor free-wheeling' causes this F₁F_O-ATPase dysfunction in primary congenital hypothyroidism.

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.

What happens when the mitochondrial H+-translocating F1FO-ATP(hydrol)ase becomes a molecular target of calcium? The pore opens

The F₁F_O-ATPase has Mg²⁺ cofactor as the natural divalent cation to support the bifunctional activity of ATP synthesis and hydrolysis. Different physio(patho)logical conditions permit the molecular interaction of Ca²⁺ with the enzyme and the modification of the biological role. Three distinct binding regions of Ca²⁺ have been localized on the enzyme complex: one in the F₁ catalytic sites and the other two sites in the membrane-embedded domain F_O. In all likelihood, Ca²⁺-activated enzyme most frequently works as an H⁺-translocating F₁F_O-ATP(hydrol)ase with a monofunctional activity that triggers the formation of mitochondrial permeability transition pore (mPTP) phenomenon. The protein(s) component of the mPTP is considered an arcane mystery. However, the F₁F_O-ATPase could reveal the molecular mechanism of pore opening when Ca²⁺ is bound to the enzyme. In this regard, the role of Ca²⁺-dependent function of the F₁F_O-ATPase in the formation of the mPTP is discussed.

The mitochondrial F1FO-ATPase exploits the dithiol redox state to modulate the permeability transition pore

The dithiol reagents phenylarsine oxide (PAO) and dibromobimane (DBrB) have opposite effects on the F₁F_O-ATPase activity. PAO 20% increases ATP hydrolysis at 50 μM when the enzyme activity is activated by the natural cofactor Mg²⁺ and at 150 μM when it is activated by Ca²⁺. The PAO-driven F₁F_O-ATPase activation is reverted to the basal activity by 50 μM dithiothreitol (DTE). Conversely, 300 μM DBrB decreases the F₁F_O-ATPase activity by 25% when activated by Mg²⁺ and by 50% when activated by Ca²⁺. In both cases, the F₁F_O-ATPase inhibition by DBrB is insensitive to DTE. The mitochondrial permeability transition pore (mPTP) formation, related to the Ca²⁺-dependent F₁F_O-ATPase activity, is stimulated by PAO and desensitized by DBrB. Since PAO and DBrB apparently form adducts with different cysteine couples, the results highlight the crucial role of cross-linking of vicinal dithiols on the F₁F_O-ATPase, with (ir)reversible redox states, in the mPTP modulation.

The inhibition of gadolinium ion (Gd3+) on the mitochondrial F1FO-ATPase is linked to the modulation of the mitochondrial permeability transition pore

The mitochondrial permeability transition pore (PTP), which drives regulated cell death when Ca²⁺ concentration suddenly increases in mitochondria, was related to changes in the Ca²⁺-activated F₁F_O-ATPase. The effects of the gadolinium cation (Gd³⁺), widely used for diagnosis and therapy, and reported as PTP blocker, were evaluated on the F₁F_O-ATPase activated by Mg²⁺ or Ca²⁺ and on the PTP. Gd³⁺ more effectively inhibits the Ca²⁺-activated F₁F_O-ATPase than the Mg²⁺-activated F₁F_O-ATPase by a mixed-type inhibition on the former and by uncompetitive mechanism on the latter. Most likely Gd³⁺ binding to F₁, is favoured by Ca²⁺ insertion. The maximal inactivation rates (k_inact) of pseudo-first order inactivation are similar either when the F₁F_O-ATPase is activated by Ca²⁺ or by Mg²⁺. The half-maximal inactivator concentrations (K_I) are 2.35 ± 0.35 mM and 0.72 ± 0.11 mM, respectively. The potency of a mechanism-based inhibitor (k_inact/K_I) also highlights a higher inhibition efficiency of Gd³⁺ on the Ca²⁺-activated F₁F_O-ATPase (0.59 ± 0.09 mM^-1∙s^-1) than on the Mg²⁺-activated F₁F_O-ATPase (0.13 ± 0.02 mM^-1∙s^-1). Consistently, the PTP is desensitized in presence of Gd³⁺. The Gd³⁺ inhibition on both the mitochondrial Ca²⁺-activated F₁F_O-ATPase and the PTP strengthens the link between the PTP and the F₁F_O-ATPase when activated by Ca²⁺ and provides insights on the biological effects of Gd³⁺.

Sulfide affects the mitochondrial respiration, the Ca2+-activated F1FO-ATPase activity and the permeability transition pore but does not change the Mg2+-activated F1FO-ATPase activity in swine heart mitochondria

In mammalian cells enzymatic and non-enzymatic pathways produce H₂S, a gaseous transmitter which recently emerged as promising therapeutic agent and modulator of mitochondrial bioenergetics. To explore this topic, the H₂S donor NaHS, at micromolar concentrations, was tested on swine heart mitochondria. NaHS did not affect the F₁F_O-ATPase activated by the natural cofactor Mg², but, when Mg²⁺ was replaced by Ca²⁺, a slight 15% enzyme inhibition at 100 µM NaHS was shown. Conversely, both the NADH-O₂ and succinate-O₂ oxidoreductase activities were totally inhibited by 200 μM NaHS with IC₅₀ values of 61.6 ± 4.1 and 16.5 ± 4.6 μM NaHS, respectively. Since the mitochondrial respiration was equally inhibited by NaHS at both first or second respiratory substrates sites, the H₂S generation may prevent the electron transfer from complexes I and II to downhill respiratory chain complexes, probably because H₂S competes with O₂ in complex IV, thus reducing membrane potential as a consequence of the cytochrome c oxidase activity inhibition. The Complex IV blockage by H₂S was consistent with the linear concentration-dependent NADH-O₂ oxidoreductase inhibition and exponential succinate-O₂ oxidoreductase inhibition by NaHS, whereas the coupling between substrate oxidation and phosphorylation was unaffected by NaHS. Even if H₂S is known to cause sulfhydration of cysteine residues, thiol oxidizing (GSSG) or reducing (DTE) agents, did not affect the F₁F_O-ATPase activities and mitochondrial respiration, thus ruling out any involvement of post-translational modifications of thiols. The permeability transition pore, the lethal channel which forms when the F₁F_O-ATPase is stimulated by Ca²⁺, did not open in the presence of NaHS, which showed a similar effect to ruthenium red, thus suggesting a putative Ca²⁺ transport cycle inhibition.

Mitochondrial F1FO-ATPase and permeability transition pore response to sulfide in the midgut gland of Mytilus galloprovincialis

The molecular mechanisms which rule the formation and opening of the mitochondrial permeability transition pore (mPTP), the lethal mechanism which permeabilizes mitochondria to water and solutes and drives the cell to death, are still unclear and particularly little investigated in invertebrates. Since Ca²⁺ increase in mitochondria is accompanied by mPTP opening and the participation of the mitochondrial F₁F_O-ATPase in the mPTP is increasingly sustained, the substitution of the natural cofactor Mg²⁺ by Ca²⁺ in the F₁F_O-ATPase activation has been involved in the mPTP mechanism. In mussel midgut gland mitochondria the similar kinetic properties of the Mg²⁺- or Ca²⁺-dependent F₁F_O-ATPase activities, namely the same affinity for ATP and bi-site activation kinetics by the ATP substrate, in spite of the higher enzyme activity and coupling efficiency of the Mg²⁺-dependent F₁F_O-ATPase, suggest that both enzyme activities are involved in the bioenergetic machinery. Other than being a mitochondrial poison and environmental contaminant, sulfide at low concentrations acts as gaseous mediator and can induce post-translational modifications of proteins. The sulfide donor NaHS, at micromolar concentrations, does not alter the two F₁F_O-ATPase activities, but desensitizes the mPTP to Ca²⁺ input. Unexpectedly, NaHS, under the conditions tested, points out a chemical refractoriness of both F₁F_O-ATPase activities and a failed relationship between the Ca²⁺-dependent F₁F_O-ATPase and the mPTP in mussels. The findings suggest that mPTP role and regulation may be different in different taxa and that the F₁F_O-ATPase insensitivity to NaHS may allow mussels to cope with environmental sulfide.

Phenylglyoxal inhibition of the mitochondrial F1FO-ATPase activated by Mg2+ or by Ca2+ provides clues on the mitochondrial permeability transition pore

Phenylglyoxal (PGO), known to cause post-translational modifications of Arg residues, was used to highlight the role of arginine residues of the F₁F_O-ATPase, which may be crucial to yield the mitochondrial permeability transition pore (mPTP). In swine heart mitochondria PGO inhibits ATP hydrolysis by the F₁F_O-ATPase either sustained by the natural cofactor Mg²⁺ or by Ca²⁺ by a similar uncompetitive inhibition mechanism, namely the tertiary complex (ESI) only forms when the ATP substrate is already bound to the enzyme, and with similar strength, as shown by the similar K'i values (0.82 ± 0.07 mM in presence of Mg²⁺ and 0.64 ± 0.05 mM in the presence of Ca²⁺). Multiple inhibitor analysis indicates that features of the F₁ catalytic sites and/or the F_O proton binding sites are apparently unaffected by PGO. However, PGO and F₁ or F_O inhibitors can bind the enzyme combine simultaneously. However they mutually hinder to bind the Mg²⁺-activated F₁F_O-ATPase, whereas they do not mutually exclude to bind the Ca²⁺-activated F₁F_O-ATPase. The putative formation of PGO-arginine adducts, and the consequent spatial rearrangement in the enzyme structure, inhibits the F₁F_O-ATPase activity but, as shown by the calcium retention capacity evaluation in intact mitochondria, apparently favours the mPTP formation.

The carboxyl-terminal helical domain of the ATP synthase γ subunit is involved in ε subunit conformation and energy coupling

The γ subunit located at the center of ATP synthase (F_OF₁) plays critical roles in catalysis. Escherichia coli mutant with Pro substitution of the γ subunit residue γLeu218, which are located the rotor shaft near the c subunit ring, decreased NADH-driven ATP synthesis activity and ATP hydrolysis-dependent H⁺ transport of membranes to ~60% and ~40% of the wild type, respectively, without affecting F_OF₁ assembly. Consistently, the mutant was defective in growth by oxidative phosphorylation, indicating that energy coupling is impaired by the mutation. The ε subunit conformations in the γLeu218Pro mutant enzyme were investigated by cross-linking between cysteine residues introduced into both the ε subunit (εCys118 and εCys134, in the second helix and the hook segment, respectively) and the γ subunit (γCys99 and γCys260, located in the globular domain and the carboxyl-terminal helix, respectively). In the presence of ADP, the two γ260 and ε134 cysteine residues formed a disulfide bond in both the γLeu218Pro mutant and the wild type, indicating that the hook segment of ε subunit penetrates into the α₃β₃-ring along with the γ subunits in both enzymes. However, γ260/ε134 cross-linking in the γLeu218Pro mutant decreased significantly in the presence of ATP, whereas this effect was small in the wild type. These results suggested that the γ subunit carboxyl-terminal helix containing γLeu218 is involved in the conformation of the ε subunit hook region during ATP hydrolysis and, therefore, is required for energy coupling in F_OF₁.

From the Ca2+-activated F1FO-ATPase to the mitochondrial permeability transition pore: an overview

Based on recent advances on the Ca²⁺-activated F₁F_O-ATPase features, a novel multistep mechanism involving the mitochondrial F₁F_O complex in the formation and opening of the still enigmatic mitochondrial permeability transition pore (MPTP), is proposed. MPTP opening makes the inner mitochondrial membrane (IMM) permeable to ions and solutes and, through cascade events, addresses cell fate to death. Since MPTP forms when matrix Ca²⁺ concentration rises and ATP is hydrolyzed by the F₁F_O-ATPase, conformational changes, triggered by Ca²⁺ insertion in F₁, may be transmitted to F_O and locally modify the IMM curvature. These events would cause F₁F_O-ATPase dimer dissociation and MPTP opening.

Mercury and protein thiols: Stimulation of mitochondrial F1FO-ATPase and inhibition of respiration

In spite of the known widespread toxicity of mercury, its impact on mitochondrial bioenergetics is a still poorly explored topic. Even if many studies have dealt with mercury poisoning of mitochondrial respiration, as far as we are aware Hg²⁺ effects on individual complexes are not so clear. In the present study changes in swine heart mitochondrial respiration and F₁F_O-ATPase (F-ATPase) activity promoted by micromolar Hg²⁺ concentrations were investigated. Hg²⁺ was found to inhibit the respiration of NADH-energized mitochondria, whereas it was ineffective when the substrate was succinate. Interestingly, the same micromolar Hg²⁺ doses which inhibited the NADH-O₂ activity stimulated the F-ATPase, most likely by interacting with adjacent thiol residues. Accordingly, Hg²⁺ dose-dependently decreased protein thiols and all the elicited effects on mitochondrial complexes were reversed by the thiol reducing agent DTE. These findings clearly indicate that Hg²⁺ interacts with Cys residues of these complexes and differently modulate their functionality by modifying the redox state of thiol groups. The results, which cast light on some implications of metal-thiol interactions up to now not fully explored, may contribute to clarify the molecular mechanisms of mercury toxicity to mitochondria.