|Following the initial studies by Hunter and Haworth on the “Ca2+-
induced membrane transition” [1-3], a plethora of studies addressing
the pharmacology, bioenergetics and structure of the Mitochondrial
Permeability Transition (MPT) pore have allowed us to slowly decipher
the pathophysiological relevance of this mitochondrial entity [4-6].
Although most of the MPT pore regulatory aspects were envisaged
a few years after its discovery, it has taken decades and different
approaches to reveal some of its structural aspects (see below).
|Pharmacological inhibition of the MPT pore with Adenine
Nucleotide Translocator (ANT) and Cyclophilin D (CypD) ligands led
to the long lasting and widespread notion that the MPT pore was formed
by the ANT acting as a channel component and CypD as a regulatory
factor. In this model, ADP, ATP and the ANT ligand bonkrekate
would inhibit pore opening by directly binding to ANT whereas CypD
inhibition with Cyclosporin A would inhibit a conformational change
mediated by CypD on a proline due to its peptidyl-prolylcis-trans
isomerase activity. Although this hypothesis was widely accepted and
a plethora of evidence indeed pointed towards a central role of this
mitochondrial translocator as a MPT pore constituent , experiments
with mouse lacking the two major isoforms of ANT demonstrated its
dispensability for MPT to ensue . It is important to mention that the
pore detected in these knockout animals was insensitive to ANT ligands
and threefold more resistant to Ca2+, whereas CsA further inhibited its
opening. On the other hand, mitochondria from CypD knockout mice
were also resistant to Ca2+ (twofold), oxidative stress and desensitized
to CsA [8-10]. Overall, these results strongly suggested that ANT and
CypD were MPT pore regulatory factors.
|In a recent set of studies, two different groups addressed the
possibility that ATP-synthase may play an important role on the
MPT pore, possibly being the pore itself [11,12]. This finding per se
would put an end to a long-lasting search and consequently redirect
research efforts to elucidate pore formation mechanisms and potential
molecular-selective therapies aiming to dissipate MPT-pore dependent
cell pathology while bypassing ATP synthase normal functioning. A
note of caution should be stated as more studies in this direction are
warranted before precipitous conclusions can be drawn (see above).
Although both studies strongly suggest the MPT pore may be composed
of ATP synthase subunit(s), the main question still to be addressed is:
What is the MPT pore?
|In the work by Bonora et al.  ATP-synthase subunit c, located
in the membrane (F0) sector of the enzyme was shown to be necessary
for MPT to ensue. In this study, the authors decreased the expression
levels of subunit c and showed a concomitant resistance to Ca2+ and
H2O2- induced MPT pore opening. Antithetical overexpression of
subunit c rendered the cells susceptible to MPT pore dependent
depolarization and cell death. While these experiments are indeed
promising, it is important to mention that ATP-synthase oligomers
are thought to confer the particular cristae architecture of the inner
mitochondrial membrane and depletion of this oligomers results in
mitochondria with onion-like multiple inner membranes . It is thus possible to speculate that genetic manipulation of the c subunit would
consequently alter the overall mitochondrial architecture potentially
affecting MPT onset indirectly.
|In the work by Giorgio et al. , the authors used a more
direct approach and detected a Multiple Conductance Channel-like
activity when purified dimers of ATP synthase were reconstituted for
electrophysiological measurements. In these experiments, channel
opening was achieved by adding an excess of Ca2+ and was preventable
with Mg2+, ADP and AMP-PNP but not with CsA or bongkrekate
consistent with preparations lacking both CypD and ANT. It is
noteworthy to mention that the potent MPT pore inducer phenylarsine
oxide did not activate this channel. Furthermore, modulation by other
effectors such as diamide or ubiquinone analogues was not tested
. In this work, CypD was shown to selectively bind to oligomycin
sensitivity conferral protein (OSCP) and consequently modulate the
(still undefined) MPT pore through the lateral stalk of ATP synthase.
Knockdown of the alleged CypD target (i.e. OSCP) sensitized
mitochondria to pore opening. This result requires more attention as
OSCP depletion results in the assembly failure of stalk subunits such as
a, b and c [15,16] and the consequent ATP synthase dimer disruption.
As noted above, this condition considerably impacts mitochondrial
ultrastructure potentially affecting the Ca2+threshold of the pore. One
interesting finding however was that ATP synthase working as an
ATPase increases twofold the Ca2+ threshold of the pore. This finding
suggests a direct and physiological relationship between ATP synthase
and the MPT pore. Finally, the authors did not solve what is the MPT
pore and just mentioned that it could be formed at the interface between
dimers potentially in the membrane. Although this could explain the
sometimes-unruly nature of the pore and its sensitivity to molecules
affecting membrane fluidity such as local anesthetics or fatty acids, it
could well mean that a still unidentified channel closely interacts with
ATP synthase but is not ATP synthase per se.
|Hopefully these studies will pave the beginning of a renewed search
for answers to an old question. The suggestion that the MPT pore may
be formed by discrete subunits of ATP synthase or at least closely
interacting with this enzymeis indeed appealing. Nevertheless, more
studies are still warranted to truly address what is the MPT pore.
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- Haworth RA, Hunter DR (1979) The Ca2+-induced membrane transition in mitochondria. II. Nature of the Ca2+ trigger site. Arch Biochem Biophys 195: 460-467.
- Hunter DR, Haworth RA (1979) The Ca2+-induced membrane transition in mitochondria. III. Transitional Ca2+ release. Arch Biochem Biophys 195: 468-477.
- Rasola A, Bernardi P (2011) Mitochondrial permeability transition in Ca2+-dependent apoptosis and necrosis. Cell Calcium 50: 222-33.
- Halestrap AP (2010) A pore way to die: the role of mitochondria in reperfusion injury and cardioprotection. Biochem Soc Trans 38: 841-860.
- Baines CP (2010) The cardiac mitochondrion: nexus of stress. Annu Rev Physiol 72: 61-80.
- Kokoszka JE, Waymire KG, Levy SE, Sligh JE, Cai J, et al. (2004) The ADP/ATP translocator is not essential for the mitochondrial permeability transition pore. Nature 427: 461-465
- Basso E, Fante L, Fowlkes J, Petronilli V, Forte MA, et al. (2005) Properties of the permeability transition pore in mitochondria devoid of Cyclophilin D. J Biol Chem 280: 18558-18561.
- Baines CP, Kaiser RA, Purcell NH, Blair NS, Osinska H, et al. (2005) Loss of cyclophilin D reveals a critical role for mitochondrial permeability transition in cell death. Nature 434: 658-662.
- Nakagawa T, Shimizu S, Watanabe T, Yamaguchi O, Otsu K, et al. (2005) Cyclophilin D-dependent mitochondrial permeability transition regulates some necrotic but not apoptotic cell death. Nature 434:652-658.
- Bonora M, Bononi A, De Marchi E, Giorgi C, Lebiedzinska M, et al. (2013) Role of the c subunit of the FO ATP synthase in mitochondrial permeability transition. Cell Cycle 12: 674-683.
- Giorgio V, von Stockum S, Antoniel M, Fabbro A, Fogolari F, et al. (2013) Dimers of mitochondrial ATP synthase form the permeability transition pore. Proc Natl Acad Sci 110: 5887-92
- Paumard P, Vaillier J, Coulary B, Schaeffer J, Soubannier V, et al. (2002) The ATP synthase is involved in generating mitochondrial cristae morphology. EMBO J 21: 221-230.
- Martinucci S, Szabo I, Tombola F, Zoratti M (2000) Ca2+-reversible inhibition of the mitochondrial megachannel by ubiquinone analogues. FEBS Lett 480: 89-94.
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