From the Consiglio Nazionale delle Ricerche Unit for the Study of Biomembranes and the Laboratory of Biophysics and Membrane Biology, Department of Biomedical Sciences, University of Padova Medical School, Viale Giuseppe Colombo 3, I-35121 Padova, Italy
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ABSTRACT |
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We have investigated the regulation of the permeability transition pore (PTP), a cyclosporin A-sensitive channel, in rat skeletal muscle mitochondria. As is the case with mitochondria isolated from a variety of sources, skeletal muscle mitochondria can undergo a permeability transition following Ca2+ uptake in the presence of Pi. We find that the PTP opening is dramatically affected by the substrates used for energization, in that much lower Ca2+ loads are required when electrons are provided to complex I rather than to complex II or IV. This increased sensitivity of PTP opening does not depend on differences in membrane potential, matrix pH, Ca2+ uptake, oxidation-reduction status of pyridine nucleotides, or production of H2O2, but is directly related to the rate of electron flow through complex I. Indeed, and with complex I substrates only, pore opening can be observed when depolarization is induced with uncoupler (increased electron flow) but not with cyanide (decreased electron flow). Consistent with pore regulation by electron flow, we find that PTP opening is inhibited by ubiquinone 0 at concentrations that partially inhibit respiration and do not depolarize the inner membrane. These data allow identification of a novel site of regulation of the PTP, suggest that complex I may be part of the pore complex, and open new perspectives for its pharmacological modulation in living cells.
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INTRODUCTION |
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The permeability transition is an in vitro increase of the inner mitochondrial membrane permeability to solutes, which is favored by Ca2+ uptake. This phenomenon is now interpreted as due to the opening of a proteinaceous but yet unidentified large conductance channel, the PTP.1 The PTP open-closed transitions are modulated by the transmembrane electrical potential, by matrix pH, by redox potential, by adenine nucleotides, and by Me2+. Finally, the permeability transition can be induced or inhibited by a variety of drugs (1-4). It has been proposed that the PTP may provide mitochondria with a fast Ca2+ release channel (4), and recent evidence indeed indicates that transient PTP opening in a low conductance mode (i.e. a state permeable to ions but not to larger molecules such as sucrose) may contribute to physiological Ca2+ signaling through amplification of inositol 1,4,5-trisphosphate-dependent Ca2+ release (5).
PTP opening in vitro leads to collapse of the proton-motive force, disruption of ionic homeostasis, mitochondrial swelling, and massive ATP hydrolysis by the F0F1 ATPase. This sequence of events has drawn considerable attention to the permeability transition as a potential player in the cellular pathways to cell death through at least four mechanisms, i.e. decreased levels of cellular ATP (6, 7), increase of cytosolic Ca2+ (6, 8, 9), release of apoptosis-inducing factor, a mitochondrial caspase (10), and release of cytochrome c (11, 12).
With the long term goal of defining the role of mitochondria in the pathways to muscle cell death, we carried out a characterization of PTP regulation in isolated skeletal muscle mitochondria. Unexpectedly, we found that the Ca2+ sensitivity of the permeability transition was strongly affected by the respiratory substrate used for energization, in that PTP opening was much easier with electron donors at complex I than with electron donors at complex II or IV. This increased Ca2+ sensitivity of PTP opening did not depend on differences in proton electrochemical gradient, Ca2+ uptake rates, oxidation-reduction status of PN, or production of H2O2, but rather on the rate of electron flux through complex I. Consistently, we find that PTP opening is inhibited by Ub0. These findings suggest that complex I may be part of the PTP and open new perspectives for its pharmacological modulation in living cells.
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MATERIALS AND METHODS |
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Rat skeletal muscle mitochondria were prepared according to
Madsen et al. (13) with slight modifications. Albino Wistars rats weighing 250-350 g were killed by decapitation, and the
gastrocnemius muscles were rapidly excised and transferred into
isolation medium (150 mM sucrose, 75 mM KCl, 50 mM Tris-HCl, 1 mM
KH2PO4, 5 mM MgCl2, 1 mM EGTA, pH 7.4). Muscles were minced with scissors and trimmed clean of visible fat and connective tissues. Muscle pieces were
transferred to 30 ml of isolation medium supplemented with 0.2% bovine
serum albumin and 0.2 mg·ml1 Nagarse (Fluka, Buchs).
After 1 min, muscles were homogenized using a motor-driven
Plexiglas/Plexiglas potter, transferred to 120 ml of isolation medium
supplemented with 0.2% bovine serum albumin, and centrifuged at
700 × g for 10 min. The supernatant was decanted and
centrifuged at 10,000 × g for 10 min. The resulting pellet was resuspended in a medium containing 250 mM
sucrose, 0.1 mM EGTA-Tris, 10 mM Tris-HCl, pH
7.4, and centrifuged at 7,000 × g for 6 min. The final
mitochondrial pellet was resuspended in 0.5 ml of the same medium at a
final protein concentration of about 20 mg·ml
1. All
procedures were carried out at 0-4 °C. Rat liver mitochondria were
prepared according to standard procedures.
Mitochondrial oxygen consumption was measured polarographically at
25 °C using a Clark-type electrode. Measurements of membrane potential, extramitochondrial Ca2+, matrix pH, and PN
oxidation-reduction status were carried out fluorimetrically with a
Perkin-Elmer 650-40 spectrofluorimeter equipped with magnetic stirring
and thermostatic control. Membrane potential was measured in the
presence of 0.2 µM rhodamine 123 as described in Emaus
et al. (14) (excitation-emission, 503-525 nm). The resting
potential was determined through parallel determinations with a
TPMP+-selective electrode as described in Zoratti and
Petronilli (15). The residual Donnan potential after full
depolarization with FCCP was estimated to be 80 mV (in this
range the electrode sensitivity is too low for a precise
determination). Extramitochondrial Ca2+ concentration was
measured in the presence of 1 µM Calcium Green-5N as
described in Ichas et al. (5) (excitation-emission, 505-535 nm). Calibration of the signal was achieved by the addition of known
amounts of Ca2+. Mitochondrial loading with BCECF and
pHi determinations were carried out exactly as described in
Bernardi et al. (16). The PN oxidation-reduction status was
evaluated based on endogenous NAD(P)H fluorescence
(excitation-emission, 340-460 nm). Calibration of the signal was
achieved by the addition of glutamate, pyruvate, and malate (levels
taken as 100% reduction) or of 0.8 µM FCCP (level taken
as 100% oxidation). All chemicals were of the highest purity
commercially available, and cyclosporin A was a gift of Novartis
(Basel, Switzerland).
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RESULTS |
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Effect of Respiratory Substrates on PTP Opening Induced by
Ca2+ Loading--
Since no specific characterization of
the PTP in skeletal muscle mitochondria was available, the first aim of
this work was to assess the basic features of the permeability
transition in these mitochondria. In the experiments of Fig.
1, we assessed the occurrence of PTP
opening induced by Ca2+ addition as measured from the
change of membrane potential of mitochondria incubated in an isotonic
sucrose medium supplemented with 10 mM
Pi.2 Addition of
100 nmol of Ca2+·mg protein1 to
mitochondria energized with glutamate, malate, and pyruvate induced a
full depolarization (trace a). Identical results were obtained using different associations of complex I substrates such as
glutamate plus malate, malate plus pyruvate, or 2-oxoglutarate (not
shown). When the experiment was performed in the presence of 1 µM CsA, the same Ca2+ addition induced only a
moderate and transient depolarization linked to Ca2+ uptake
(trace b, see also Fig. 2),
indicating that the full depolarization of trace a was dependent on PTP
opening. Surprisingly, when mitochondria were energized with succinate
in the presence of rotenone, the same amount of Ca2+ was
not able to induce a permeability transition, and only the moderate and
transient depolarization accompanying Ca2+ uptake could be
observed (trace c, see also Fig. 2).
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Effect of Respiratory Substrates on PTP Opening Induced by Depolarization-- It has been shown that in liver mitochondria the PTP behaves like a voltage-dependent channel, in that the open probability increases at decreasing membrane potentials (19). To study the voltage dependence of the PTP in skeletal muscle mitochondria, we compared the depolarization induced by increasing concentration of FCCP in the presence of CsA (i.e. with a closed PTP) to that obtained after uptake of a small Ca2+ load, which was unable to open the pore per se. As shown in Fig. 4, panel A, when mitochondria were energized with malate, glutamate, and pyruvate, the FCCP-induced depolarization was much more pronounced after Ca2+ loading (open circles) than in the presence of CsA (closed squares), revealing a major contribution of the PTP to depolarization under this particular condition. Fig. 4, panel B, shows that under identical condition of Ca2+ loading, but with succinate (in the presence of rotenone) as the substrate, contribution of the pore to depolarization was much less pronounced.3 To determine whether the promoting effect of complex I substrates on FCCP-induced pore opening only depended on a change of the threshold voltage, or also on the rate of electron flux through complex I, we performed similar experiments where depolarization was rather achieved through inhibition of electron flow with cyanide. Fig. 4, panel C, shows that the major sensitization to PTP opening with complex I substrates disappeared in this protocol, the PTP contribution to depolarization becoming indistinguishable from that observed with succinate plus rotenone (panel D). These results underline the importance of electron flux through complex I as a major determinant of pore modulation, with an effect which can be clearly discriminated from a change in the threshold voltage for PTP opening.
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Mechanism of PTP Regulation by Complex I; Inhibition by Ub0-- We next addressed the questions of whether the inducing effect of complex I substrates on pore opening could be accounted for by an overproduction of H2O2, or by differences in the oxidation-reduction levels of PN. Fig. 5 shows that with complex I substrates PTP opening by Ca2+ was only slightly delayed in the presence of excess catalase (compare traces a and b), suggesting that overproduction of H2O2 is not essential for induction of PTP opening.
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DISCUSSION |
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In this study we have shown that isolated rat skeletal muscle mitochondria can undergo a CsA-sensitive permeability transition as a result of Ca2+ accumulation. Of more general interest is the observation that the probability of PTP opening is modulated by electron flow through complex I of the respiratory chain, with an effect that can be dissociated from the oxidation-reduction state of the mitochondrial PN pool.
Modulation by Complex I Substrates Is a General Feature of the Permeability Transition-- A first question raised by our findings on the effects of complex I substrates and Ub0 in skeletal muscle mitochondria was whether these defined a tissue-specific modulation of the pore, or rather depended on specific incubation conditions. Most studies on the permeability transition have been performed in mitochondria isolated from liver, which are therefore the standard for the mechanistic aspects of modulation (see Refs. 1-4 for reviews). Relative to the conditions commonly employed with liver mitochondria, in the present study we used higher Pi concentrations (10 mM) because this allowed attainment of the highest respiratory control ratios (i.e. the maximum efficiency of energy coupling), and minimized matrix pH changes following Ca2+ uptake. When the Pi concentration was 5 mM we could easily observe PTP regulation by electron flux within complex I in liver mitochondria as well, provided that Mg2+ was added (Fig. 9) in agreement with a previous report (21). Furthermore, PTP inhibition by Ub0 was also detected in liver mitochondria even at lower Pi concentrations (not shown). On the other hand, we would like to stress that modulation of PTP opening by complex I substrates could still be observed in skeletal muscle at 1 mM Pi (not shown), i.e. a condition where the effect was undetectable in liver mitochondria (Fig. 9).
Taken together, these data suggest that (i) inhibition by Ub0 and regulation by electron transfer within complex I are general properties of the permeability transition and (ii) that quantitative differences exist reflecting the variable control strength exerted by this process on the overall probability of pore opening in different tissues and/or under different conditions.Mechanism of PTP Modulation by Electron Flux at Complex I-- The role of the oxidation-reduction status of PN in regulation of the PTP has been recognized for many years (22, 23). The novel contribution of the present work is the demonstration that electron transfer can independently modulate the probability of pore opening. Our results show that the relevant regulatory event(s) occur within complex I, that they are related to the rate of electron flow, and that they are independent of changes of the PN oxidation-reduction levels as such. Indeed, first, upon addition of Ca2+ (in the absence of rotenone) the PTP opens when complex I, but not complex IV substrates are being oxidized, yet PN undergo oxidation in both cases (Fig. 6). These observations demonstrate that the inducing effects of increased electron flow are site-specific and suggest regulation by a sensing element located within complex I.
Second, PTP opening with complex I substrates is observed after the addition of FCCP but not of cyanide, despite identical levels of depolarization (Fig. 4). This regulatory effect is therefore distinct from that exerted by the transmembrane voltage since the latter can be observed irrespective of the respiratory substrate, of whether depolarization is induced with a H+ or a K+ current, and of the presence of respiratory inhibitors (19, 24, 25). Thus, the PTP is directly modulated by both electron flux (the primary event of energy transduction, which leads to charge separation through H+ extrusion at the energy-conserving sites), and by the transmembrane proton electrochemical difference (the output force of substrate oxidation). Third, Ub0 inhibits Ca2+-dependent pore opening at concentrations that both cause oxidation of PN per se, and maximize Ca2+-dependent depolarization and PN oxidation during Ca2+ uptake (Fig. 7). Ub0 oxidizes NADH via two different electron pathways within complex I (26, 27). The first pathway goes from NADH to the physiological hydrophobic ubiquinone-binding site, and couples electron flux to proton extrusion in a rotenone-sensitive manner. The second pathway goes from NADH to a hydrophilic binding site (28), is rotenone-insensitive, and is not coupled to proton translocation (26-28). Thus, Ub0 may be able to oxidize NADH through the rotenone-insensitive site without increasing electron flux through the rotenone-sensitive pathway, consistent with the results of Fig. 7. The mechanism of inhibition of complex I by Ub0 remains unclear, but we note that a similar effect has been reported for Ub3 (29). Based on the triggering effect of electron flux through the rotenone-sensitive pathway, and on the inhibitory effect of Ub0 on PTP opening we suggest that endogenous ubiquinone may bind to a regulatory site (possibly the hydrophilic one) in complex I stabilizing the pore in the closed conformation. Increasing electron flux at complex I might displace ubiquinone from this binding site, increasing in turn the pore open probability. Conversely, addition of Ub0 might stabilize the pore in the closed conformation either through direct binding to this site, or through a decrease of electron flux through complex I. Irrespective of the detailed mechanism, the demonstration that the PTP can be inhibited by Ub0 is an original observation which may have far reaching implications for the pharmacological control of the pore. The idea that the oxidation-reduction status of ubiquinone indirectly modulates the PTP through changes in the rate of production of H2O2 has been put forward in several studies (30, 31). It must be stressed that this mechanism is unlikely to play a role under our experimental conditions since pore opening by complex I substrates was only marginally affected by the addition of catalase (Fig. 5).Consequences of PTP Opening on Respiration-- An early consequence of PTP opening in vitro is loss of PN (21, 32, 33). Although it is almost invariably assumed that PTP opening is accompanied by uncoupling, the most likely result of a permeability transition is in fact respiratory inhibition due to loss of PN. This is clearly shown in the experiments of Fig. 3, which document that uncoupling can be observed exclusively with electron donors of complex II like succinate, while the permeability transition results in a complete inhibition of respiration with complex I electron donors. The concentration of PN is significantly higher in mitochondria than in the cytosol, and the gradient of free PN between mitochondria and the cytosol in the liver of fed rats is above 14 (34). If a permeability transition ever occurs in vivo, we predict that it will result in respiratory inhibition because of mitochondrial PN and substrate depletion, blocking in turn the Krebs cycle and oxidative metabolism.
It should be noted that respiratory inhibition following pore opening cannot be accounted for entirely by PN depletion. The maximal rate of respiration indeed remained about 50% inhibited even in the presence of millimolar concentrations of added NADH (Fig. 3). It should be mentioned that under these conditions the pore remained open, since addition of succinate plus rotenone did not repolarize mitochondria unless EGTA was also added to close the pore (not shown). This result suggests that PTP opening might also have direct consequences on electron transfer within complex I, since the inhibitory effect is not observed with succinate (Fig. 3). We cannot exclude, however, that under these particular conditions the rate-limiting step in respiration is represented by diffusion of NADH through the pore.Nature of the PTP-- Despite recent progress in our understanding of PTP modulation, its molecular nature has remained elusive. The findings of the present study offer a hitherto unrecognized possibility, i.e. that the pore may comprise, or be in close contact with, proteins of the mitochondrial complex I. It is striking that Saccharomyces cerevisiae mitochondria, which lack a classical Ca2+- and CsA-sensitive permeability transition (35) also lack complex I and possess two NADH dehydrogenases, which are insensitive to rotenone and, most importantly, do not pump protons (36). Expression of mammalian complex I proteins in yeast may provide an answer to this unresolved question of mitochondrial pathophysiology.
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ACKNOWLEDGEMENTS |
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We thank Prof. Giovanni Felice Azzone and Fabio Di Lisa for helpful discussions.
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FOOTNOTES |
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* This work was supported in part by Grants from the Consiglio Nazionale delle Ricerche, Dotazione Centro and Grant 96.03009.CTO4 (to P. B), the Ministero dell'Università e della Ricerca Scientifica e Tecnologica (Progetto "Bioenergetica e Trasporto di Membrana"), Telethon-Italy Grant 847 (to P. B.), and the EEC Fellowship ERBFMBICT961385 (to E. F.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Permanent address: Laboratoire de Bioénergétique
Fondamentale et Appliquée, Université J. Fourier, F-38041
Grenoble, France.
§ On leave from the Institute of Biomedicine, University of Helsinki, Helsinki Fin00014, Finland.
¶ To whom reprint requests should be addressed: Dipartimento di Scienze Biomediche Sperimentali, Viale Giuseppe Colombo 3, I-35121 Padova, Italy.
1
The abbreviations used are: PTP, permeability
transition pore; CsA, cyclosporin A; , transmembrane electric
potential difference; FCCP, carbonyl cyanide
p-trifluoromethoxyphenylhydrazone; PN, pyridine
nucleotide(s); Ub0, ubiquinone 0; TMPD,
N,N,N',N'-tetramethyl-1,4-phenylenediamine dihydrochloride; TPMP+, triphenylmethylphosphonium; BCECF,
2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; Mops,
4-morpholinepropanesulfonic acid.
2 Due to the low yield of mitochondria, this method was chosen for its higher sensitivity. Comparable results could be obtained in standard swelling assays based on permeabilization to sucrose, and from membrane potential measurements based on TPMP+ distribution.
3 It must be stressed that PTP contribution to depolarization with succinate (plus rotenone) as the substrate became more apparent at higher Ca2+ loads, consistent with earlier reports on liver mitochondria.
4 Note that rotenone was omitted in the experiments with succinate, so that the degree of PN oxidation was comparable irrespective of whether complex I or II substrates were being oxidized.
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REFERENCES |
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