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Mitochondrial Control of Acute Glutamate Excitotoxicity in Cultured Cerebellar Granule Cells

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Mitochondrial Control of Acute Glutamate Excitotoxicity in Cultured Cerebellar Granule Cells The Journal of Neuroscience, December 15, 1998, 18(24):10277–10286 Mitochondrial Control of Acute Glutamate Excitotoxicity in Cultured Cerebellar Granule Cells Roger F. Castilho, Oskar Hansson, Manus W. Ward, Samantha L. Budd, and David G. Nicholls Neurosciences Institute, Department of Pharmacology and Neuroscience, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom Mitochondria within cultured rat cerebellar granule cells have a synthase inhibition, respiratory chain inhibitors produced an 21 21 21 complex influence on cytoplasmic free Ca ([Ca ]c )re- immediate Ca deregulation (ICD), ascribed to an ATP deficit. 21 sponses to glutamate. A decreased initial [Ca ]c elevation in In contrast to DCD, ICD could be reversed by subsequent ATP cells whose mitochondria are depolarized by inhibition of the synthase inhibition with or without additional NMDA receptor ATP synthase and respiratory chain (conditions which avoid blockade. DCD could not be ascribed to the failure of an ATP ATP depletion) was attributed to enhanced Ca 21 extrusion from yielding metabolic pathway. It is concluded that mitochondria the cell rather than inhibited Ca 21 entry via the NMDA receptor. can control Ca 21 extrusion from glutamate-exposed granule Even in the presence of elevated extracellular Ca 21, when cells by the plasma membrane in three ways: by competing 21 21 [Ca ]c responses were restored to control values, such cells with efflux pathways for Ca , by restricting ATP supply, and by showed resistance to acute excitotoxicity, defined as a delayed inducing a delayed failure of Ca 21 extrusion. Inhibitors of the cytoplasmic Ca 21 deregulation (DCD) during glutamate expo- mitochondrial permeability transition only marginally delayed sure. DCD was a function of the duration of mitochondrial the onset of DCD. polarization in the presence of glutamate rather than the total period of glutamate exposure. Once initiated, DCD could not be Key words: glutamate; excitotoxicity; mitochondria; calcium; reversed by NMDA receptor inhibition. In the absence of ATP granule cell; NMDA In common with many other cultured neurons, rat cerebellar 1997; Kroemer et al., 1998; but see Kluck et al., 1997; Bossy- granule cells die after extended NMDA receptor activation Wetzel et al., 1998). (Garthwaite, 1986; Novelli et al., 1988; Manev et al., 1989). There Glutamate-induced neuronal necrosis is preceded by a rapid 21 21 is increasing evidence that mitochondrial dysfunction plays a uncontrolled increase in cytoplasmic free Ca ([Ca ]c ) con- primary role in the initiation of both necrotic and apoptotic centration, which occurs stochastically within a field of neurons 1 neuronal cell death (for review, see Beal, 1996; Petit et al., 1996; (Dubinsky et al., 1995) and has been termed delayed Ca 2 Richter et al., 1996; Henneberry, 1997; Nicotera and Leist, 1997; deregulation (DCD) (Tymianski et al., 1993). Mitochondrial de- Nicholls and Budd, 1998). However, unraveling the multiple roles polarization after glutamate exposure has been reported to be an 1 of the organelle in this context is not trivial, because each mito- early event associated with neuronal Ca 2 loading (Ankarcrona chondrial function can in turn influence, and be influenced by, the et al., 1996; Isaev et al., 1996; Khodorov et al., 1996b; Schinder et other parameters. In particular, the mitochondrial membrane al., 1996; White and Reynolds, 1996; Keelan et al., 1998). How- DC D potential ( m ) or the full proton electrochemical potential ( p) ever, the distinction is not always drawn between a reversible, controls ATP generation by the ATP synthase (Nicholls and physiological response to increased energy demand and a patho- 21 Ferguson, 1992), Ca sequestration within the mitochondrial logical depolarization after mitochondrial damage. Because we matrix (Nicholls and Åkerman, 1982; Gunter and Gunter, 1994), (Budd and Nicholls, 1996b) and others (Tan et al., 1997; Stout et and superoxide generation by the respiratory chain (Beal et al., al., 1998) have reported that complete depolarization of mito- 1997; Korshunov et al., 1997; Ne`gre-Salvayre et al., 1997), and chondria before glutamate exposure is acutely neuroprotective, may additionally affect the mitochondrial permeability transition the implication is that polarized mitochondria, perhaps in re- 1 (Halestrap et al., 1997; Scorrano et al., 1997), outer membrane sponse to excessive Ca 2 loading, generate a factor or create a rupture, and cytochrome c release (Liu et al., 1996; Yang et al., condition triggering subsequent necrotic cell death. However, the 21 Received June 22, 1998; revised Sept. 25, 1998; accepted Sept. 25, 1998. nature of this factor, which could be a high cytoplasmic Ca per This work was supported by Wellcome Trust Grant 040237 and the Medical se, the mitochondrial permeability transition, reactive oxygen- Research Council (research studentships to S.L.B. and M.W.W.). We thank Hazel induced damage, metabolic inhibition, etc., remains unclear. Leith for expert technical assistance. The existence of specific mitochondrial inhibitors allows each Correspondence should be addressed to D. G. Nicholls, Neurosciences Institute, Department of Pharmacology and Neuroscience, University of Dundee, Ninewells function of the organelle to be investigated separately. Thus, it is Medical School, Dundee DD1 9SY, Scotland, United Kingdom. possible to eliminate a failure of oxidative phosphorylation as a Dr. Hansson’s present address: Wallenberg Neuroscience Center, Department of Physiology and Neuroscience, Lund University, Soelvegatan 17, Lund S-223 62, cause of DCD in cerebellar granule cells, because the delay and Sweden. extent of glutamate-induced deregulation are unaffected in cells Dr. Budd’s present address: Cerebrovascular and Neuroscience Institute, Harvard whose mitochondrial ATP synthase has been inhibited by oligo- Medical School, 221 Longwood Avenue, Brigham and Women’s Hospital, Boston, MA 02115. mycin (Budd and Nicholls, 1996b) and thus rely on glycolysis to Copyright © 1998 Society for Neuroscience 0270-6474/98/1810277-10$05.00/0 maintain ATP/ADP ratios (Budd and Nicholls, 1996a). In the 10278 J. Neurosci., December 15, 1998, 18(24):10277–10286 Castilho et al. • Mitochondria and Excitotoxicity present report, we investigate the significance of other mitochon- drial parameters in the induction of DCD, as well as the basis of 1 the counter-intuitive finding that cytoplasmic Ca 2 transients in these cells are consistently decreased when mitochondria are depolarized, without depleting ATP (Budd and Nicholls, 1996b), an observation which has recently been challenged (Khodorov et 1 al., 1996c). It is concluded that mitochondrial Ca 2 accumulation 1 influences Ca 2 efflux from the granule cells both acutely, ac- 1 counting for the effect on the initial Ca 2 transients, and pro- gressively, culminating in DCD. MATERIALS AND METHODS Materials. Fura-2 acetoxymethyl ester (fura-2 AM) was obtained from Molecular Probes (Leiden, The Netherlands). (5R,10S)-(1)-5-methyl- 10,11-dihydro[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) was obtained from Research Biochemicals (SEMAT, St. Albans, Hert- fordshire, UK). Fetal calf serum and minimal essential medium were obtained from Life Technologies (Paisley, Strathclyde, UK). Bongkrekic acid and N-methylval-4-cyclosporin (SDZ 220–384) were kindly donated by Dr. M. Klingenberg (University of Munich, Mu¨nchen, Germany) and Novartis Pharma Inc. (Basel, Switzerland), respectively. Cyclosporin A, ionomycin, ketamine, oligomycin, rotenone, and all other reagents were obtained from Sigma (Poole, Dorset, UK). Preparation of cerebellar granule cells. Granule cells were prepared as described previously (Courtney et al., 1990) from 7-d-old Wistar rats. Cells were plated on poly-D-lysine-coated glass coverslips (13 mm circu- lar for nonperfusion experiments and 22 mm square for use with the perfusion chamber in Fig. 1A–C) at a density of 280,000 cells per 1 coverslip for imaging studies and 750,000 per coverslip for 45Ca2 Figure 1. Mitochondrial depolarization depresses the cytoplasmic free 1 determination. Cells were cultured in minimal essential medium contain- Ca2 response to glutamate. A–C, Granule cells were perfused with ing Earle’s salts (Life Technologies) plus 10% (v/v) fetal calf serum, 25 incubation medium at 25°C and exposed to three sequential 60 sec pulses m m m 21 mM KCl, 30 mM glucose, 2 mM glutamine, 100 g/ml streptomycin, and of 100 M glutamate–10 M glycine ( glut). [Ca ]c was monitored in a 100 U/ml penicillin. After 24 hr, 10 mM cytosine arabinoside was added minimum of 10 randomly selected somata. A, Individual somata show 21 to inhibit non-neuronal cell proliferation. Cells were maintained at highly reproducible [Ca ]c transients in response to three pulses. B, m 21 37°C in a humidified atmosphere of 5% CO2–95% air and were used Oligomycin (2.5 g/ml) (oligo) did not affect the second [Ca ]c transient, after 6–8 d in vitro. but the addition of 2 mM rotenone (rot) depressed the third transient. C, Incubation conditions. The incubation medium contained 120 mM Antimycin A (0.5 mM)(AA) caused a similar depression of the response NaCl, 3.1 mM KCl, 0.4 mM KH2PO4 , and 20 mM N-tris in the presence of oligomycin. Under energy-deficient conditions, when in (hydroxymethyl)methyl-2-aminoethanesulfonic acid, pH adjusted to 7.4 the presence of rotenone minus oligomycin (D) or in the absence of at the temperature of the subsequent experiment (37°C, except for Fig. glucose (E), glutamate–glycine produces a large transient, followed by 1), with NaOH, 5 mM NaHCO3 , 1.2 mM Na2SO4 , and 1.3 mM CaCl2. immediate deregulation. Data are representative of at least four indepen- Unless otherwise stated, all experiments were performed at 37°C in dent experiments. 21 Mg -free incubation medium in the presence of 15 mM glucose. Exper- iments shown in Figure 1A–C were designed to correspond to the conditions used by Khodorov et al. (1996c) and were performed at 25°C Manganese quenching of cytoplasmic fura-2 was monitored by alter- in the presence of 5 mM glucose.
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