Mitochondria – the kraken wakes!

One of the major ways in which mitochondria appear to be able to participate in cellular pathology is through activation of the MPT. As discussed earlier, when the channel opens transiently it can mediate normal signaling events. However, opening of the MPT in the high-conductance mode appears to be irreversible and has some profound consequences for cell function. For example, MPT activation of this type is associated with osmotic changes and mitochondrial swelling. The mitochondrial cristae straighten out and, as the inner membrane is larger than the outer, this leads to rupturing of the outer membrane and release of a miasma of destructive molecules from the intermembrane space into the cytoplasm. Included in these are apoptosis inducing factor (AIF) and, strange to tell, cytochrome c ([13xKroemer, G., Zamzami, N., and Susin, S.A. Immunol. Today. 1997; 18: 44–51Abstract | Full Text PDF | PubMed | Scopus (1279)See all References, 14xZamzami, N. et al. J. Bioenerg. Biomembr. 1997; 29: 185–193Crossref | PubMed | Scopus (271)See all References, 15xEllenby, H.M. et al. J. Neurosci. 1997; 17: 6165–6178PubMedSee all References]). The movement of both of these molecules from the mitochondria into the cytoplasm appears to be an early event in the triggering of cell death in many instances. What kind of things can trigger MPT activation of this type and its catastrophic sequelae[11xBernardi, P. and Petronelli, U.C. J. Bioenerg. Biomembr. 1996; 28: 131–138Crossref | PubMed | Scopus (332)See all References, 12xZoratti, M. and Szabo, I. Biochim. Biophys. Acta. 1995; 1241: 139–176Crossref | PubMed | Scopus (1917)See all References, 13xKroemer, G., Zamzami, N., and Susin, S.A. Immunol. Today. 1997; 18: 44–51Abstract | Full Text PDF | PubMed | Scopus (1279)See all References, 14xZamzami, N. et al. J. Bioenerg. Biomembr. 1997; 29: 185–193Crossref | PubMed | Scopus (271)See all References]? There is currently a very large literature on this subject. Certain things do stand out however. It is clear, as discussed earlier, that accumulation of Ca2+ in the matrix favors MPT activation. But when does the reversible low conductance mode of MPT operation, associated with normal Ca2+ signaling, switch over to the irreversible, high-conductance state associated with cell death? One clue might be found in another important property of the mitochondria: their propensity to leak reactive oxygen species (ROS)[16xJohnson, J.E. et al. See all References, 17xBindokas, V.P. et al. J. Neurosci. 1996; 16: 1324–1336PubMedSee all References]. One of the unfortunate byproducts of aerobic metabolism is that the mitochondrial electron-transport chain constantly leaks superoxide and hydroxyl free radicals, primarily from the ubiquinone site[16xJohnson, J.E. et al. See all References[16]. These molecules are powerful oxidizing agents either on their own or following reaction with other molecules. For example, reaction of superoxide with nitric oxide (NO) (a product of the Ca2+-activated enzyme NOS) yields peroxynitrite, a highly toxic species capable of damaging protein, lipid and nucleic acid molecules[18xIadecola, C. Trends Neurosci. 1997; 20: 132–139Abstract | Full Text | Full Text PDF | PubMed | Scopus (855)See all References[18]. Among other things these ROS are powerful activators of the MPT ([19xChen, Q.X. et al. J. Neurosci. 1997; 17: 4032–4036PubMedSee all References[19]). Cells possess a battery of enzymatic defenses against ROS (for example, superoxide dismutase and catalase). However, their slowly accumulating toxic effects might be responsible for many things, including senescence[16xJohnson, J.E. et al. See all References[16]. Let us consider a situation where neuronal Ca2+ uptake is unusually large. During excitotoxic death, for example, glutamate receptor activation might trigger the opening of novel Ca2+-permeable channels in the cell membrane, thus allowing uncontrolled Ca2+ entry[19xChen, Q.X. et al. J. Neurosci. 1997; 17: 4032–4036PubMedSee all References[19]. Much of this Ca2+ will enter the mitochondria, which will pump out protons in an effort to keep pace, burning ATP and generating increased amounts of ROS at the same time. Faced with such a constant barrage of Ca2+ uptake and ROS production, at some point the cell will decide to call it quits, MPT opening will become irreversible and death will follow. Support for a mechanism of this type comes from observations that inhibitors of the MPT, such as cyclosporin A, block excitotoxic damage to neurons and seem to ameliorate damage in experimental stroke models[20xSchindler, A.F. et al. J. Neurosci. 1996; 16: 6125–6133PubMedSee all References, 21xUchino, H. et al. Acta Physiol. Scand. 1995; 155: 469–471Crossref | PubMedSee all References].Given the central role that the MPT seems to play in neuropathology and the likelihood that drugs that inhibit it might be neuroprotective, we are entitled to ask what it actually looks like from the molecular point of view. There is no clear answer to this question. However, there are several important clues provided by pharmacological studies of the MPT. It is thought that the MPT might be a multiprotein complex that is located at contact sites where the inner and outer mitochondrial membranes come into close juxtaposition to one another[14xZamzami, N. et al. J. Bioenerg. Biomembr. 1997; 29: 185–193Crossref | PubMed | Scopus (271)See all References[14]. In particular, it is thought that the inner membrane adenine-nucleotide translocator might be part of the complex (Fig. 2Fig. 2). Drugs that bind to the translocator such as atractyloside and bongkrekic acid are also powerful regulators of MPT function. In addition, other molecules including the mitochondrial peripheral benzodiazepine-binding protein and mitochondrial cyclophilin might also participate in the MPT complex. In addition there is potentially one other intriguing cellular regulator. It is widely known that proteins of the extended bcl-2 family are potent inhibitors of both apoptotic and necrotic cell-death[22xReed, J.C. Nature. 1997; 387: 773–776Crossref | PubMed | Scopus (1288)See all References[22]. Expression of bcl-2 has been shown to inhibit the opening of the MPT under stressful conditions, thus stabilizing the mitochondrial membrane potential, Ca2+ homeostasis and ROS production and inhibiting the release of cytochrome c ([22xReed, J.C. Nature. 1997; 387: 773–776Crossref | PubMed | Scopus (1288)See all References, 23xKharbanda, S. et al. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 6939–6942Crossref | PubMed | Scopus (325)See all References, 24xPrehn, J.H.M. et al. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 12599–12603Crossref | PubMed | Scopus (168)See all References]). Bcl-2 resides in the outer mitochondrial membrane and at the contact sites between the inner and outer membranes in particular. Thus, it is possible that bcl-2 can somehow directly interact with, and influence, the activity of the MPT. Such an interaction could certainly underlie many of the known effects of bcl-2.Fig. 2Speculative model of the mitochondrial permeability transition pore (MPT). Abbreviations: ANT, adenine nucleotide; CK, creatine kinase; HK, hexokinase; PBR, peripheral benzodiazepine receptor; SH, sulphydryl groups; VDAC, voltage-dependent anion channel. (Modified from Zamzami et al.[14xZamzami, N. et al. J. Bioenerg. Biomembr. 1997; 29: 185–193Crossref | PubMed | Scopus (271)See all References[14])View Large Image | Download PowerPoint SlideOn reflection, then, how should we consider mitochondria – friends or foes? When left alone in their deep evolutionary slumber they are certainly our friends. But, please – don't disturb them!Below the thunders of the upper deep; Far, far beneath in the abysmal sea, His ancient, dreamless, uninvaded sleep The Kraken sleepeth: faintest sunlights flee About his shadowy sides: above him swell Huge sponges of millennial growth and height;And far away into the sickly light, From many a wondrous grot and secret cell Unnumbered and enormous polypi Winnow with giant arms the slumbering green.There hath he lain for ages and will lie Battening upon huge seaworms in his sleep, Until the latter fire shall heat the deep; Then once by man and angels to be seen, In roaring he shall rise and on the surface die.[Alfred, Lord Tennyson (1832)]