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Neurochemical Research, Vol. 28, No. 11, November 2003 (© 2003), pp. 1621–1624
Glutamate, Cell Death, and Hats Off to Carl Cotman* Dennis W. Choi1 It is a pleasure to contribute to this special issue of Neurochemistry Research honoring Carl Cotman, as I count myself amongst the many neuroscientists who have benefited from Carl’s long and distinguished career to date. My own debt is clear. Twenty years ago, emerging from long sequestration in neurology clinical training, I joined the faculty at Stanford University and began to set up my laboratory. As a graduate student, I had worked previously on GABAergic neurotransmission and benzodiazepines, but during my clinical training this field had moved on. So I decided to switch over to the synaptic signaling mediated by GABA’s excitatory amino acid partner, glutamate: equally widespread, accessible to similar electrophysiological approaches, and, at the time, attractively enigmatic. Why was the postsynaptic response to applied glutamate so complex, at times even suggestive of conductance closure, rather than the expected conductance gating? And, was it possible that a ubiquitous signaling molecule could become a killing agent under certain pathological conditions, destroying central neurons in a process John Olney called “excitotoxicity.” The former puzzle was quickly solved when the voltage-dependent block of NMDA receptor–gated channels by magnesium was identified (1,2). The latter puzzle yielded more slowly, and Carl’s extensive studies in the glutamate field provided boost and guidance over many years. He was a pioneer in studying central glutamatergic neurotransmission (3,4) and the phenomenon of excitotoxicity (5). His work supporting the idea of a glutamine–glutamate cycle (6) helped to highlight the importance of astrocytes in regulating transmitter glutamate synthesis and set the stage for appreciation of an equally prominent role of astrocytes in the pathogenesis of excitotoxicity after acute insults. Under depolarizing conditions, later compounded by acid stress and energy failure, astrocytes fail to clear extracellular glutamate;
rather, they export glutamate via reverse transport and thus contribute to a toxic buildup of extracellular glutamate in the ischemic brain (7). Carl’s lab also produced beautiful maps of the central distribution of major glutamate receptor families, providing insights into the complexity and spatial diversity of brain glutamatergic neurotransmission (8). The significance of these regional differences in glutamate receptor family expression still remains largely to be elucidated, but these point to the possibility that selective antagonists or potentiators might gain useful leverage against disease targets. In early days, antiexcitotoxic treatments seemed implausible practical treatments: how could one hope to block a large fraction of glutamate receptors without disrupting vital brain functions? A path forward seemed to appear when the NMDA subtype of glutamate receptors was identified as bearing prominent responsibility for the neurotoxicity of glutamate (9), a consequence of high calcium permeability. But NMDA antagonists have been disappointing in stroke clinical trials. Is the approach finished? Some have suggested that NMDA antagonists might still find application as a treatment for stroke if carried to a higher level of selectivity. In particular, subsubtype selective NMDA antagonists targeting NR2B receptors might achieve a better therapeutic index than pan NMDA receptor antagonists, preferentially attenuating excitotoxic damage in forebrain with reduced motor impairment (10) and perhaps reduced tendency to promote apoptosis because of an excessive block of NMDA receptor– mediated calcium influx (11). Partial, use-dependent antagonism may be another route to improving therapeutic index in neuroprotection; the low-affinity, usedependent NMDA antagonist memantine has recently been demonstrated to reduce the progression of Alzheimer’s disease (12). And AMPA/kainate antagonists may have their own neuroprotective uses, for example, in protecting CA1 neurons after transient global ischemia. These selectively vulnerable neurons prominently express Ca2- and Zn2-permeable AMPA receptors, as a result
* Special issue dedicated to Dr. Carl Cotman. 1 Merck Research Labs 14-2500, West Point, Pennsylvania. Tel: 215652-1411; E-mail:
[email protected]
1621 0364-3190/03/1100–1621/0 © 2003 Plenum Publishing Corporation
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1622 of ischemia-induced expression of neuronal repressor element-1 silencing transcription factor (REST) and consequent suppression of GluR2 expression (13); furthermore, AMPA/kainate antagonists reduce excitotoxicity on oligodendrocytes (14) and both oligodendrocyte death and axonal damage in experimental allergic encephalomyelitis, a model of multiple sclerosis (15). Beyond neuroprotection, NMDA, AMPA, or kainate antagonists may be useful in the treatment of disease states such as pain, movement disorders, seizures, and bipolar disorder. On the flip side, pharmacological enhancement of NMDA or AMPA receptor function may also be therapeutically useful, for example, against schizophrenia or memory loss, respectively. Another currently promising way to attenuate excitotoxicity or other manifestations of overactive ion channel–linked glutamate receptors is via an indirect approach, not blocking those receptors but rather by manipulating metabotropic glutamate receptors. Carl and his colleagues demonstrated the feasibility of such an approach when they discovered that the selective mGluR agonist trans-ACPD was not excitotoxic but rather could reduce the excitotoxicity of concurrently applied glutamate (16). Multiple members of the mGluR family have been now been delineated, some typically excitatory (mGluR1, mGluR5) and others typically inhibitory (e.g., mGluR2 and mGluR3). The coupling between the mGluR system and ion channel–linked glutamate receptors appears to be protean and may encompass modulation of endogenous agonist release from presynaptic terminals, membrane-delimited inhibition or phosphorylation of ion channel–linked glutamate receptors, and modulation of postsynaptic signaling cascades. Marga Behrens in my laboratory at Washington University delineated a specific positive link between mGluR1 and NMDA receptors, that likely contributes to the proexcitotoxic influence of the mGluR system described by Carl. (mediated by Pyk2/Src family kinase–induced phosphorylation of NR2 (17)) The neuroprotective power of manipulating mGluRs is unlikely to equal that of direct glutamate receptor blockade in tackling the fulminant excitotoxicity associated with acute brain ischemia, but may prove to be a useful method for limiting more indolent forms of excitotoxicity in chronic diseases states. Furthermore, subtype selective mGluR agonists and antagonists have considerable potential to produce therapeutic alterations in nervous system function; efforts are underway in many academic and industry labs to determine if these drugs might be useful in relieving the symptoms of diseases as diverse as Parkinson’s disease, schizophrenia, anxiety, and addiction (18).
Choi In other important studies, Carl’s lab explored intricate links between axonal sprouting, synaptic plasticity, and the pathological changes in neural pathways associated with Alzheimer’s disease. He suggested that regenerative or plastic changes in excitatory pathways were likely helpful in compensating for neurodegenerative damage, but might also contribute to plaque formation (19) or excitotoxicity (20), another possible example of normal brain mechanisms being transformed by disease into pathogenetic factors. These ideas helped influence my lab to explore a potential dangerous side of neurotrophins, NGF, BDNF, and CNTF. We found that the same concentrations of neurotrophins that promote neuronal survival under many conditions can also enhance excitotoxic necrosis (21). Although paradoxical in the setting of conventional perspective, this potentially harmful side makes mechanistic sense: promoting anabolic growth might divert crucial resources and energy at a time of stress and energy failure. As I heard Robert Sapolsky note during a memorable lecture on the injury-promoting potential of glucocorticoids, a house fire is not a great time to begin building an addition to the front porch! The themes most prominently advanced by Carl and his colleagues in recent years have been the roles of oxidative stress and apoptosis in chronic neurodegenerative diseases, in particular Alzheimer’s disease, themes that have also been of interest to my lab over the years. They have elegantly dissected candidate biochemical pathways, using both postmortem human tissue and model systems, providing in sum important evidence for the engagement of these mechanisms in driving neuronal cell loss. Their work, in concert with other studies in particular some carried out by the editor of this issue, Mark Mattson, raises the following two questions that I find especially provocative: 1. How important is diet in influencing these mechanisms? It is potentially of substantial medical or public health import to know if a diet enriched in antioxidants and mitochondrial factors (22) or restricted in caloric content (23,24) can protect the human brain from the ravages of aging or neurodegenerative disease. 2. Can the molecular mechanisms of cellular apoptosis can be harnessed to mediate neuritic degeneration (“neuritic apoptosis”) (25,26)? This is a powerful, far-reaching extension of the classic concept of programmed cell death. One current reservation about utilizing antiapoptotic interventions to ameliorate neurodegenerative processes is the concern that apoptosis cascades may be triggered at a point when nerve cells are already damaged beyond a possibility of useful
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Glutamate, Cell Death, and Hats Off to Carl Cotman salvage. But if some of the biochemical pathways of apoptosis are responsible for damaging the dendritic arbors of viable neurons, antiapoptotic interventions may produce benefits beyond improving cell survival. On the other hand, if apoptotic mechanisms are also engaged in the normal pruning of unwanted synaptic contacts, a normal “day job” additional to function as defense against viral invasion or neoplastic transformation, another type of downside adverse effect might result from therapeutic blockade. In addition to his specific scientific contributions, Carl has been an influential leader in advocating the mining of neuroscience for medical benefit. His interest in glutamate receptors, synaptic transmission, and excitotoxicity led him a number of years ago to the study of spinal cord injury. While serving as the chairman of the Scientific Advisory Board of the American Paralysis Association (APA, now the Christopher Reeve Paralysis Foundation), he collaborated with the organization’s Scientific Director, Susan Howley, to lead a systematic enhancement of that organization’s scientific grants program. This resulted in dramatic growth in the scale and impact of the funded research portfolio. He recruited me to that Advisory Board, and, over time, the opportunity to work with Carl and others in the APA/Reeve Foundation influenced me to initiate studies of spinal cord injury in my own lab. I consider the opportunity to participate in this Reeve Foundation Research Consortium to be one of the highlights of my career. With his trademark pithy comments and gruff voice, Carl was a highly effective instigator and organizer, exhorting his own lab and his consortium colleagues to make progress against this terrible disorder. Carl’s interest in translational research of course extended to Alzheimer’s disease. Not content to limit his study of oxidative stress to model systems and transgenic mice, he participated in a key clinical study of selegiline or high-dose vitamin E in Alzheimer’s disease patients (27). He was ahead of the curve: a distinguished basic neuro-scientist pushing the translational envelope before doing so became popular, and, in doing so, demonstrating to many the ability of fundamental molecular and cellular research to lead directly to the identification of treatment approaches. His lab directly trained a distinguished cadre of investigators, many of whom are now in leading positions in academic or industry labs. I note with personal gratitude that this list includes one of my former graduate students, Jae Koh, now Associate Professor of Neurology at Ulsan College of Medicine in Seoul, Korea. Cheers, Carl.
1623 REFERENCES 1. Mayer, M. L., Westbrook, G. L., and Guthrie, P. B. 1984. Voltagedependent block by Mg2 of NMDA responses in spinal cord neurones. Nature 309:261–263. 2. Nowak, L., Bregestovski, P., Ascher, P., Herbert, A., and Prochiantz, A. 1984. Magnesium gates glutamate-activated channels in mouse central neurons. Nature 307:462–465. 3. Nadler, J. V., Vaca, K. W., White, W. F., Lynch, G. S., and Cotman, C. W. 1976. Aspartate and glutamate as possible transmitters of excitatory hippocampal afferents. Nature 260:538–540. 4. Cotman, C. W., Foster, A., and Lanthorn, T. 1981. An overview of glutamate as a neurotransmitter. Adv. Biochem. Psychopharmacol. 27:1–27. 5. Nadler, J. V., Perry B. W., and Cotman, C. W. 1978. Intraventricular kainic acid preferentially destroys hippocampal pyramidal cells. Nature. 271:676–677. 6. Hamberger, A. C., Chiang, G. H., Nylen, E. S., Scheff, S. W., and Cotman, C. W. 1979. Glutamate as a CNS transmitter: I. Evaluation of glucose and glutamine as precursors for the synthesis of preferentially released glutamate. Brain Res. 168:513–530. 7. Cervos-Navarro, J. and Diemer, N. H. 1991. Selective vulnerability in brain hypoxia. Crit. Rev. Neurobiol. 6:149–182. 8. Monaghan, D. T., Yao, D., and Cotman, C. W. 1985. L-[3H]Glutamate binds to kainate-, NMDA- and AMPA-sensitive binding sites: An autoradiographic analysis. Brain Res. 340:378–383. 9. Choi, D. W., Koh, J. Y., and Peters, S. 1988. Pharmacology of glutamate neurotoxicity in cortical cell culture: Attenuation by NMDA antagonists. J. Neurosci. 8:185–196. 10. Gill, R., Alanine, A., Bourson, A., Buttelmann, B., Fischer, G., Heitz, M. P., Kew, J. N., Levet-Trafit, B., Lorez, H. P., Malherbe, P., Miss, M. T., Mutel, V., Pinard, E., Roever, S., Schmitt, M., Trube, G., Wybrecht, R., Wyler, R., and Kemp, J. A. 2002. Pharmacological characterization of Ro 63-1908 (1-[2(4-hydroxy-phenoxy)-ethyl]-4-(4-methyl-benzyl)-piperidin-4-o1), a novel subtype-selective N-methyl-D-aspartate antagonist. J. Pharmacol. Exp. Ther. 302:940–948. 11. Lee, J. M., Zipfel, G. J., and Choi, D. W. 1999. The changing landscape of ischaemic brain injury mechanisms. Nature 399 (suppl.): A7–A14. 12. Reisberg, B., Doody, R., Stoffler, A., Schmitt, F., Ferris, S., and Mobius, H. J. 2003. Memantine in moderate-to-severe Alzheimer’s disease. N. Engl. J. Med. 348:1333–1341. 13. Calderone, A., Jover, T., Noh, K. M., Tanaka, H., Yokota, H., Lin, Y., Grooms, S. Y., Regis, R., Bennett, M. V., Zukin, R. S. 2003. Ischemic insults derepress the gene silencer REST in neurons destined to die. J. Neurosci. 23:2112–2121. 14. McDonald, J. W., Althomsons, S. P., Hyrc, K. L., Choi, D. W., and Goldberg, M. P. 1998. Oligodendrocytes from forebrain are highly vulnerable to AMPA/kainate receptor-mediated excitotoxicity. Nat. Med. 4:291–297. 15. Pitt, D., Werner, P., and Raine, C. S. 2000. Glutamate excitotoxicity in a model of multiple sclerosis. Nat. Med. 6:67–70. 16. Koh, J. Y., Palmer, E., and Cotman, C. W. 1991. Activation of the metabotropic glutamate receptor attenuates N-methyl-D-aspartate neurotoxicity in cortical cultures. Proc. Natl. Acad. Sci. USA 88:9431–9435. 17. Heidinger, V., Manzerra, P., Wang, X. Q., Strasser, U., Yu, S.-P., Choi, D. W., and Behrens, M. M. 2002. Metabotropic glutamate receptor 1-induced upregulation of NMDA receptor current: Mediation through the Pyk2/Src-family kinase pathway in cortical neurons. J. Neurosci. 22:5452–5461. 18. Schoepp, D. D. and Conn, P. J. 2002. Metabotropic glutamate receptors. Pharmacol. Biochem. Behav. 74:255–256. 19. Geddes, J. W., Anderson, K. J., and Cotman, C. W. 1986. Senile plaques as aberrant sprout-stimulating structures. Exp. Neurol. 94:767–776.
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1624 20. Geddes, J. W. and Cotman, C. W. 1986. Plasticity in hippocampal excitatory amino acid receptors in Alzheimer’s disease. Neurosci. Res. 3:672–678. 21. Koh, J., Gwag, B. J., Lobner, D., and Choi, D. W. 1995. Potentiated necrosis of cultured cortical neurons by neurotrophins. Science 268:573–575. 22. Milgram, N. W., Head, E., Muggenburg, B., Holowachuk, D., Murphey, H., Estrada, J., Ikeda-Douglas, C. J., Zicker, S. C., and Cotman, C. W. 2002. Landmark discrimination learning in the dog: Effects of age, an antioxidant fortified food, and cognitive strategy. Neurosci. Biobehav. Rev. 26:679–695. 23. Major, D. E., Kesslak, J. P., Cotman, C. W., Finch, C. E., and Day, J. R. 1997. Life-long dietary restriction attenuates agerelated increases in hippocampal glial fibrillary acidic protein mRNA. Neurobiol. Aging 18:523–526.
Choi 24. Mattson, M. P., Chan, S. L., and Duan, W. 2002. Modification of brain aging and neurodegenerative disorders by genes, diet, and behavior. Physiol. Rev. 82:637–672. 25. Ivins, K. J., Bui, E. T., and Cotman, C. W. 1998. Beta-amyloid induces local neurite degeneration in cultured hippocampal neurons: Evidence for neuritic apoptosis. Neurobiol. Dis. 5: 365–378. 26. Mattson, M. P., Keller, J. N., and Begley, J. G. 1998. Evidence for synaptic apoptosis. Exp. Neurol. 153:35–48. 27. Sano, M., Ernesto, C., Thomas, R. G., Klauber, M. R., Schafer, K., Grundman, M., Woodbury, P., Growdon, J., Cotman, C. W., Pfeiffer, E., Schneider, L. S., and Thal, L. J. 1997. A controlled trial of selegiline, alpha-tocopherol, or both as treatment for Alzheimer’s disease. The Alzheimer’s Disease Cooperative Study. N. Engl. J. Med. 336:1216–1222.
