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Trophic factors and Alzheimer's disease
Neurohi.logy of Aging. Vol.
10, pp. 584-586.
Pergamort f:~re.-,s pie, 1989. Printed i, the U . S . A
i i i ' ~ ' 4580/~9 53.t~i) ,
~!
Trophic Factors and Alzheimer's Disease
CREIGHTON H. PHELPS 1
Neuroscience and Neuropsychology of Aging Program National Institute on Aging, Building 31, Room 5C35, 9000 Rockville Pike, Bethesda, MD 20892 The paper by Butcher and Woolf presents a troublinghypothesis, that neurotrophicfactors may be contributoryto the pathology of Alzheimer's disease rather than potentiallyservingas therapeutic agents. Much more experimentalwork, especiallyin primates, will be required to tease out the positive and/or negative influencesthese factors may have on the pathogenesisof the neurodegenerative diseases causing dementias. WHILE the cause(s) of Alzheimer's disease ~AD) remains unknown, dysfunction and eventual death of specific neurons in the cerebral cortex, hippocampus, and basal forebrain are associated with the cognitive decline seen in the disease. Butcher and Woolf (3) have presented a thought-provoking hypothesis for the role of trophic factors in the etiology of AD based on stimulation of abnormal growth leading to cytoskeletal defects. While their thesis appears to run counter to the ideas of many other workers in the field, their premises should be examined very carefully along with the evidence they use to bolster their arguments. Trophic factors are associated with the development, long-term maintenance, and function of different classes of neurons in the brain. This has led to speculation that the loss of specific neurons m various neurodegenerative diseases such as Parkinson's disease. Alzheimer's disease, and amyotrophic lateral sclerosis may result from a loss of trophic support; therefore, strategies for early therapeutic intervention might include replacement of a specific trophic factor to prevent further neuron loss (1,10). The discovery of nerve growth factor (NGF) and its receptor in the CNS has stimulated research on its role in brain function. It is now clear that NGF affects survival and function of certain classes of cholinerglc neurons in the basal forebrain of experimental animals (8,28), an area implicated in the pathology of Alzheimer's disease in man (26). Trophic factors other than NGF appear to have broader ranges of actions on neurons and also influence neuroglia and vascular elements in the brain (9). One of the basic assumptions stated in the Butcher and Wolfe hypothesis (3) is that AD is triggered by alterations m mechanisms regulating the expression of cytoskeletal proteins. Much attention has been paid to the cytoskeletal abnormalities present in the brains of AD patients examined at autopsy, i.e.. the neurofibrillary tangles (NFrs). NFrs represent markers for end-stage degeneration of neurons and are seen in increasing frequency with advancing age in the brains of disease-free individuals as welt as in a variety of brain diseases (29). While NFTs have been considered pathognomonic for Alzheimer's disease, it is now clear that some patients with AD do not demonstrate NFTs at autopsy, even though they were cognitively impaired (24). Therefore. NFTs could be interpreted as tombstones for dying neurons and may not relate directly to the etiology of AD. A hallmark of AD is amyloid protein deposition in the brain parenchyma. Amyloid deposition occurs in the brains of both the cognitively normal as well as AD patients. Massive localized deposits are often present in AD, while the deposits are smaller
and more diffuse in normal aging brain (22). It is not clear what function amyloid serves in normal individuals, hut disordered processing leads to the massive deposits seen in A D Neuritic plaques may develop as a result of amytoid deposition and are associated with neurites and glial processes surromading an amyloid core. Although it had earlier been reported that the number of neuritic plaques correlates well with the degree of dementia in AD (17), more contemporary studies show that the brains of nondemented elderly subjects also can contain numerous plaques (12). Furthermore. some demented patients show little evidence of this pathology. Since amyloid is derived from a transmembrane protein, it would logically follow that changes in membrane properties might lead to the release of the protein and, after cleavage, extraceUular deposition of amyloid and other abnormal cleavage products. Recent evidence indicates that changes in the phospholipid composition of membranes are the earliest manifestations yet identified in the pathogenesis of AD (18,19). Elevation in levels of phosphomonoesters, precursors of membrane phospholipids, occurs early before neuritic plaques or neurofibrillary tangles appear. Later, when plaques begin to appear, degeneration of neural membranes is well-advanced, as indicated by the presence of higher levels of phosphodiesters, breakdown products of membrane phospholipids (18,19). Many of the subsequent biochemical and pathological changes associated with AD. including neuroflbrillary tangles, neuritic plaques, and amyloid deposition, may stern from earlier modifications of protein and lipid processing in cells, but it is unlikely these changes are due to trophic factors. The role of neurotrophic factors in the normal aging brain and in Alzheimer's disease Is a topic of great interest. Barde (2) suggests that a factor can be called neurotrophic only if it prevents neuron death. By this definition NGF and brain-derived neurotrophic factor (BDNF) are the only factors demonstrated to protect specific classes of neurons in the CNS. the basal forebrain choliuergic neurons by NGF (6, 16.27). and retinal ganglion cells by BDNF (2). Other trophic factors appear to be less specific in their actions and influence, not only neurons, but other cells in the nervous system such as neuroglia and cells associated with the brain vasculature (9). NGF exerts its influence by binding to specific receptors on the surface of the cell and after internalization is retrogradely transported to the perikaryon where it influences gene expression and stimulates both cell maintenance and neuritic outgrowth. The latter depends on the assembly of cytoskeletal elements such as microtubules and filaments. Withdrawal or
~Current address: Division of Medical and Scientific Affairs, Alzheimer's Association, 70 E. Lake Street, Suite 600, Chicago, IL 60601.
COMMENTARY
585
absence of NGF inhibits neurite outgrowth and can lead to neuronal cell death (15). In the mature nervous system, cell maintenance requires a balance of synthesis and degradation of molecular constituents of the cell. This constant turnover leads eventually to the accumulation of breakdown products in the cytoplasm of aging neurons, which in itself may make neurons more vulnerable. There is accumulating evidence that NGF prevents cell destruction and allows the cell to function within normal limits of turnover. When NGF is removed from the system an active death program is initiated, which results in cell death (15). An appropriate animal model for Alzheimer's disease, demonstrating the full spectrum of behavioral and pathological symptoms would provide a means to confirm or deny the role of trophic factors. The published observations on the effects of trophic factors on rodent neurons both in vivo and in vitro must be examined carefully and not assumed to apply to the primate brain without further confirmation. Nonhuman primate research has only recently included behavioral testing in old monkeys followed by pathological evaluation of their brains (25). The pathological changes reported in behaviorally compromised old monkeys suggests that this may be the best animal model for changes occurring in older humans and Alzheimer patients.
Before using either neurotrophic factors (10,20) or substances which repress trophic factor action (3) to treat patients with Alzheimer's disease, much more experimental work is needed, particularly in nonhuman primates. A report from a recent workshop held at the National Institute on Aging concluded that there is sufficient evidence to pursue studies on NGF as a possible treatment for Alzheimer's disease (20). The evidence included the following: 1) memory loss associated with AD is correlated with cholinergic dysfunction in the basal forebrain (5, 11, 17~; 2) markers of cholinergic activity colocalize with nerve growth factor receptors on basal forebrain neurons in all species examined (7, 13, 21, 23); and 3) rodent studies demonstrate that NGF rescues basal forebrain cholinergic neurons from death in normal aging (4) and following experimental lesions of the fimbria-fornix (8, 14, 28). Only if efficacy and safety are demonstrated in further animal studies, including nonhuman primates, should clinical trials of NGF for treatment of AD be considered. Much more basic research is necessary before a rationale can be developed to use trophic factors other than NGF in clinical trials for AD. If factors are identified which prevent the death of specific neurons implicated in cognitive functioning in man, these could be considered for clinical trials after appropriate animal studies have demonstrated efficacy and safety.
REFERENCES 1. Appel, S. H. A unifying hypothesis for the cause of amyotrophic lateral sclerosis, parkinsonism, and Alzheimer's disease. Ann. Neurol. 10:499-505; 1981. 2. Barde, Y-A. What, if anything, is a neurotrophic factor? Trends Neurosci. 11:343-346; 1988. 3. Butcher, L. L.; Woolf, N. J. Neurotrophic agents exacerbate the pathologic cascade of Alzheimer's disease. Neurobiol. Aging 10: 557-570; 1989. 4, Fischer, W.; Wictorin, K.; Bjorklund, A.; Williams, L. R.; Varon, S.; Gage, F. H. Amelioration of cholinergic neuron atrophy and spatial memory impairment in aged rats by nerve growth factor. Nature 329:65-68; 1987. 5, Francis, P. T.; Palmer, A. M.; Sims, N. R.; Bowen, D. M.; Davison, A. N.; Esiri, M. M.; Neary, D.; Snowden, J. S.; Wilcock, G. C. Neurochemical studies of early-onset Alzheimer's disease: possible influence on treatment. N. Engl. J. Med. 313:7-11; 1985. 6. Gnahn, H.; Hefti, F.; Heumann, R.; Schwab, M. E.; Thoenen, H. NGF-mediated increase of choline acetyltransferase (CHAT) in the neonatal rat forebrain: evidence for a physiological role of NGF in the brain. Dev. Brain Res. 9:45-52; 1989. 7. Goedert, M.; Fine, A.; Dawbarn, D.; Wilcock, G. K.; Chao, M. V. Nerve growth factor receptor mRNA distribution in human brain: normal levels in basal forebrain in Alzheimer's disease. Mol. Brain Res. 5:1-7; 1989. 8. Hefti, F. Nerve growth factor promotes survival of septal cholinergic neurons after fimbria transection. J. Neurosci. 6:2155-2162; 1986. 9. Hefti, F.; Knusel, B.; Michel, P. P. Selective and non-selective trophic actions on central cholinergic and dopaminergic neurons in vitro. In: Coleman, P. D.; Higgins, G. A.; Phelps, C. H., eds. Molecular and cellular mechanisms of neuronal plasticity in aging and Alzheimer's disease. Amsterdam: Elsevier; 1989: in press. 10. Hefti, F.; Weiner, W. J. Nerve growth factor and Alzheimer's disease. Ann. Neurol. 20:275-28l; 1986. 11. Katzman, R.; Brown, T.; Field, P.; Thai, L.; Davies, P.; Terry, R. Significance of neurotransmitter abnormalities in Alzheimer's disease?. In: Martin, J. B.; Barchas, J. D., eds. Neuropeptides in neurologic and psychiatric disease. New York: Raven Press; 1986: 279-280. 12. Katzman, R.; Terry, R.; DeTeresa, R.; Brown, T.; Davies, P.; Fuld, P.; Renbing, X.; Peck, A. Clinical, pathological, and neurochemical changes in dementia: A subgroup with preserved mental status and numerous neocortical plaques. Ann. Neurol. 23:138-144; 1988. 13. Korsching, S.; Auburger, G.; Heumann, R.; Scott, J. Thoenen, H. Levels of nerve growth factor receptor and its mRNA in the central
14. 15.
16.
17.
18.
19.
20.
21. 22. 23. 24.
25.
nervous system of the rat correlate with cholinergic innervation. EMBO J. 4:1389-1393; 1985. Kromer, L. F. Nerve growth factor treatment after brain injury prevents neuronal death. Science 235:214-217; 1989. Martin, D. P.; Schmidt, R. E.; DiStefano, P. S.; Lowry, O. H.; Carter, J. G.; Johnson, E. M., Jr. Inhibitors of protein synthesis and RNA synthesis prevent neuronal death caused by nerve growth factor deprivation. J. Cell. Biol. 106:829-844; 1988. Mobley, W. C.; Rutkowski, J. L.; Tennekoon, G. I.; Gemski, J.; Buchanan, K.; Johnston, M. V. Nerve growth factor increases choline acetyltransferase activity in developing basal forebrain neurons. Mol. Brain Res. 1:53-62; 1986. Perry, E. K.; Tomlinson, B. E.; Blessed, G.; Bergman, K.; Gibson, P. H.; Perry, R. H. Correlation of cholinergic abnormalities with senile plaques and mental test scores in senile dementia. Br. Med. J. 2:1457-1459; 1978. Pettegrew, J. W.; Moossy, J.; Withers, G.; McKeag, D.; Panchalingam, K. 31P nuclear magnetic resonance study of the brain in Alzheimer's disease. J. Neuropathol. Exp. Neurol. 47:235-248; 1988. Pettegrew, J. W.; Panchalingam, K.', Moossy, J.; Martinez, J.: Ran, G.; Boiler, F. Correlation of phosphorus-31 magnetic resonance spectroscopy and morphologic findings in Alzheimer's disease. Arch. Neurol. 45:1093-1096; 1988. Phelps, C. H.; Gage, F. H.; Growden, J. H.; Hefti, F.; Harbaugh, R.; Johnston, M. V.: Khachaturian, Z. S.; Mobtey, W. C.; Price, D. L.; Raskind, M.; Simpkins, J.; Thai, L. J.; Woodcock, J. Potential use of nerve growth factor to treat Alzheimer's disease. Neurobiol. Aging 10:205-207; 1989. Richardson, P. M.; Verga Issa, V. M. K.: Riopelle, R. J. Distribution of neuronal receptors for nerve growth factor in the rat. J. Neurosci. 6:2312-2321; 1986. Selkoe, D. J. Biochemistry of altered brain proteins in Alzheimer's disease. Annu. Rev. Neurosci. 12:463490; 1989. Taniuchi, M.; Schweitzer, J. B.; Johnson, E. M., Jr. Nerve growth factor receptor molecules in rat brain. Proc. Natl. Acad. Sci. USA 83:1950-1954; 1986. Terry, R.; Hansen, L. A.; DeTeresa, R.; Davies, P.; Tobias, H.; Katzman, R. Senile dementia of the Alzheimer type without neocortical neurofibrillary tangles. J. Neuropathol. Exp. Neurol. 46:262268; 1987. Walker, L. C.; Kitt, C. A.; Struble, R. G.; Wagster, M. V.; Price, D. L.; Cork, L. C. The neural basis of memory decline in aged monkeys. Neurobiol. Aging 9:667-683: 1988.
Neurobiology ~t'Aging. Vol.
10. pp. 586-587. ~;' Pergamon Press plc. 1989. Printed m the U . S , A
26. Whitehouse, P. J.; Price, D. L.; Struble, R. G.; Clark, A. W.; Coyle, J. T.; DeLong, M. R. Alzheimer's disease and senile dementia: loss of neurons in the basal forebrain. Science 215:1237-1239; 1982. 27. Will, B.; Hefti, F. Behavioral and neurochemicat effects of chronic intraventricular injections of nerve growth factor in adult rats with fimbria lesions. Behav. Brain Res. 17:17-24; 1985. 28. Williams, L. R.; Varon, S.; Peterson, G. M.; Wictorin, K.; Fischer.
~il~J; s580/89 33 oii - iii~
W.; Bjorklund, A.; Gage, F. H. Continuous intusion of nerve grov~th factor prevents basal forebraln neuronal death after fimhria fomix transection. Proc. Natl. Acad. Sci. USA 83:9231-9235: 1986. 29. Wisniewski, K,; Jervis, G. A.; Moretz, R. C.: Wisniewski, H. M. Alzheimer neurofibrillary tangles in diseases other than senile and presenile dementia. Ann. Neurol. 5:288-294: t979.
Epidermal Growth Factor Receptor ¢ Nerve Growth Factor
L A W R E N C E R. W I L L I A M S
CNS Diseases Research, Unit 7251-209-5, The Upjohn Company 301 Henrietta St., Kalamazoo, MI 49001
I am perplexed by the authors' complete lack of def'mition of neurotrophic factors. The agents Butcher and Wootf want to blame are neurite promoting factors, not neurotrophic factors. Treatment of Alzheimer's disease with NGF antagonists might instead exacerbate the death of both basal forebrain neurons and their cortical target neurons, accelerating the progress of dementia.
BUTCHER and Woolf present a scenario for the etiology of neuritic plaque formation and neuronal death in Alzheimer's disease (AD). In their hypothesis, abnormal neuronal sprouting is the primary cause of AD; age-related alterations in the CNS milieu result in expression of aberrant neuronal proteins that destabilize the cytoskeleton and induce neurite sprouting. The aberrant proteins accumulate in the neurite terminals, cause the terminals to rupture, and kill the parent neuron. The culprits to blame for triggering this pathologic cascade, according to Butcher and Woolf, are the "greater than normal" expression of neurotrophic factors, nerve growth factor (NGF) in particular. The authors recommend treating AD with drugs that "diminish synthesis of NGF, block NGF receptors, decrease saliency of allied neurotrophic agents," etc. Although some of the authors' speculations are justified by sound scientific evidence, 1 am perplexed by their complete lack of definition of neurotrophic factors, and their inconsistent use of poorly defined data to justify the conclusion that neurotrophic factors cause AD. Butcher and Woolf group neurotrophic factors, neurotrophic agents, neuronal growth factors, growth-promoting substances, and related neurochemical entities into one family; NGF, triiodothyronine (T3), thyroid releasing hormone, the somatomedins, and even epidermal growth factor receptor are equated to have similar, if not identical, neurotrophic properties. Varon et al. (12) defined neurotrophic factor (a.k,a. neuronotrophic factor) as a molecule that promotes the survival and maintenance of particular populations of neurons. Other molecules may specify, modify, or influence the metabolic status of a neuronal population such as the hormone. T 3. Neurite promoting factors comprise one subset of specifying molecules; neurite promoting factors induce neurite sprouting and outgrowth from otherwise alive neurons. It is essential to distinguish the functional identity of these groups of molecules in order to maintain an accurate, logical argument for or against the role of a particular molecule in the etiology of AD. Butcher and Woolf reference the work of Goedert et al. who
reported no change in NGF or NGF receptor mRNA in a small sampling of human brains with histologically confmned diagnosis of AD; no correlation was made with the degree of basal forebrain dystrophy or dementia. Still, the authors implicate elevations of NGF as a causative agent in their speculative cascade based on a preliminary report from Atterwill and Bowen suggesting that AD brain extract contains elevated levels of a molecule(s) affecting ChAT activity in aggregate cultures of whole brain. This apparent increase in ChAT stimulating activity may actually be due to a loss of an inhibitory molecule (11). Butcher and Woolf attempt to formulate a mechanism by which depositions of aberrant cytoskeletal material from ruptured terminal sprouts will form plaques that involve other adjacent neurons. They envision "trails of neuronal growth factors" produced by hypertrophic astrocytes migrating away from the center of amyloid plaques as inducing adjacent neuronal sprouting, and perpetuating the cascade of neuronal death. The agents Butcher and Woolf want to blame are neurite promoting factors, not neurotrophic factors. Guenther et al. ~4) have clearly demonstrated the neurite promoting properties of a glial-derived protease inhibitor. Intriguingly, a protease inhibitor has recently been found to be a consistent component of amyloid plaques (1). In the context of the Butcher and Woolf hypothesis. alterations in the environmental homeostasis of neurite promoting molecules could be the primary cause of AD and initiate the aberrant sprouting that begins the pathological scenario described in their review. NGF was classically described as being neurotrophic for peripheral, neural crest-derived neurons, and only recently has been found to be produced in the CNS and affect CNS neurons. particularly the cholinergic neurons in the basal forebrain (14). Much of the experimental evidence for the effects of neurotrophic and neurite promoting factors has come from tissue culture studies using embryonic neurons. Although culture experiments have provided many insights into the metabolic effects of such molecules, tissue culture experiments are not globally predictive of the in vivo effect of a given molecule, particularly in the mature adult
10, pp. 584-586.
Pergamort f:~re.-,s pie, 1989. Printed i, the U . S . A
i i i ' ~ ' 4580/~9 53.t~i) ,
~!
Trophic Factors and Alzheimer's Disease
CREIGHTON H. PHELPS 1
Neuroscience and Neuropsychology of Aging Program National Institute on Aging, Building 31, Room 5C35, 9000 Rockville Pike, Bethesda, MD 20892 The paper by Butcher and Woolf presents a troublinghypothesis, that neurotrophicfactors may be contributoryto the pathology of Alzheimer's disease rather than potentiallyservingas therapeutic agents. Much more experimentalwork, especiallyin primates, will be required to tease out the positive and/or negative influencesthese factors may have on the pathogenesisof the neurodegenerative diseases causing dementias. WHILE the cause(s) of Alzheimer's disease ~AD) remains unknown, dysfunction and eventual death of specific neurons in the cerebral cortex, hippocampus, and basal forebrain are associated with the cognitive decline seen in the disease. Butcher and Woolf (3) have presented a thought-provoking hypothesis for the role of trophic factors in the etiology of AD based on stimulation of abnormal growth leading to cytoskeletal defects. While their thesis appears to run counter to the ideas of many other workers in the field, their premises should be examined very carefully along with the evidence they use to bolster their arguments. Trophic factors are associated with the development, long-term maintenance, and function of different classes of neurons in the brain. This has led to speculation that the loss of specific neurons m various neurodegenerative diseases such as Parkinson's disease. Alzheimer's disease, and amyotrophic lateral sclerosis may result from a loss of trophic support; therefore, strategies for early therapeutic intervention might include replacement of a specific trophic factor to prevent further neuron loss (1,10). The discovery of nerve growth factor (NGF) and its receptor in the CNS has stimulated research on its role in brain function. It is now clear that NGF affects survival and function of certain classes of cholinerglc neurons in the basal forebrain of experimental animals (8,28), an area implicated in the pathology of Alzheimer's disease in man (26). Trophic factors other than NGF appear to have broader ranges of actions on neurons and also influence neuroglia and vascular elements in the brain (9). One of the basic assumptions stated in the Butcher and Wolfe hypothesis (3) is that AD is triggered by alterations m mechanisms regulating the expression of cytoskeletal proteins. Much attention has been paid to the cytoskeletal abnormalities present in the brains of AD patients examined at autopsy, i.e.. the neurofibrillary tangles (NFrs). NFrs represent markers for end-stage degeneration of neurons and are seen in increasing frequency with advancing age in the brains of disease-free individuals as welt as in a variety of brain diseases (29). While NFTs have been considered pathognomonic for Alzheimer's disease, it is now clear that some patients with AD do not demonstrate NFTs at autopsy, even though they were cognitively impaired (24). Therefore. NFTs could be interpreted as tombstones for dying neurons and may not relate directly to the etiology of AD. A hallmark of AD is amyloid protein deposition in the brain parenchyma. Amyloid deposition occurs in the brains of both the cognitively normal as well as AD patients. Massive localized deposits are often present in AD, while the deposits are smaller
and more diffuse in normal aging brain (22). It is not clear what function amyloid serves in normal individuals, hut disordered processing leads to the massive deposits seen in A D Neuritic plaques may develop as a result of amytoid deposition and are associated with neurites and glial processes surromading an amyloid core. Although it had earlier been reported that the number of neuritic plaques correlates well with the degree of dementia in AD (17), more contemporary studies show that the brains of nondemented elderly subjects also can contain numerous plaques (12). Furthermore. some demented patients show little evidence of this pathology. Since amyloid is derived from a transmembrane protein, it would logically follow that changes in membrane properties might lead to the release of the protein and, after cleavage, extraceUular deposition of amyloid and other abnormal cleavage products. Recent evidence indicates that changes in the phospholipid composition of membranes are the earliest manifestations yet identified in the pathogenesis of AD (18,19). Elevation in levels of phosphomonoesters, precursors of membrane phospholipids, occurs early before neuritic plaques or neurofibrillary tangles appear. Later, when plaques begin to appear, degeneration of neural membranes is well-advanced, as indicated by the presence of higher levels of phosphodiesters, breakdown products of membrane phospholipids (18,19). Many of the subsequent biochemical and pathological changes associated with AD. including neuroflbrillary tangles, neuritic plaques, and amyloid deposition, may stern from earlier modifications of protein and lipid processing in cells, but it is unlikely these changes are due to trophic factors. The role of neurotrophic factors in the normal aging brain and in Alzheimer's disease Is a topic of great interest. Barde (2) suggests that a factor can be called neurotrophic only if it prevents neuron death. By this definition NGF and brain-derived neurotrophic factor (BDNF) are the only factors demonstrated to protect specific classes of neurons in the CNS. the basal forebrain choliuergic neurons by NGF (6, 16.27). and retinal ganglion cells by BDNF (2). Other trophic factors appear to be less specific in their actions and influence, not only neurons, but other cells in the nervous system such as neuroglia and cells associated with the brain vasculature (9). NGF exerts its influence by binding to specific receptors on the surface of the cell and after internalization is retrogradely transported to the perikaryon where it influences gene expression and stimulates both cell maintenance and neuritic outgrowth. The latter depends on the assembly of cytoskeletal elements such as microtubules and filaments. Withdrawal or
~Current address: Division of Medical and Scientific Affairs, Alzheimer's Association, 70 E. Lake Street, Suite 600, Chicago, IL 60601.
COMMENTARY
585
absence of NGF inhibits neurite outgrowth and can lead to neuronal cell death (15). In the mature nervous system, cell maintenance requires a balance of synthesis and degradation of molecular constituents of the cell. This constant turnover leads eventually to the accumulation of breakdown products in the cytoplasm of aging neurons, which in itself may make neurons more vulnerable. There is accumulating evidence that NGF prevents cell destruction and allows the cell to function within normal limits of turnover. When NGF is removed from the system an active death program is initiated, which results in cell death (15). An appropriate animal model for Alzheimer's disease, demonstrating the full spectrum of behavioral and pathological symptoms would provide a means to confirm or deny the role of trophic factors. The published observations on the effects of trophic factors on rodent neurons both in vivo and in vitro must be examined carefully and not assumed to apply to the primate brain without further confirmation. Nonhuman primate research has only recently included behavioral testing in old monkeys followed by pathological evaluation of their brains (25). The pathological changes reported in behaviorally compromised old monkeys suggests that this may be the best animal model for changes occurring in older humans and Alzheimer patients.
Before using either neurotrophic factors (10,20) or substances which repress trophic factor action (3) to treat patients with Alzheimer's disease, much more experimental work is needed, particularly in nonhuman primates. A report from a recent workshop held at the National Institute on Aging concluded that there is sufficient evidence to pursue studies on NGF as a possible treatment for Alzheimer's disease (20). The evidence included the following: 1) memory loss associated with AD is correlated with cholinergic dysfunction in the basal forebrain (5, 11, 17~; 2) markers of cholinergic activity colocalize with nerve growth factor receptors on basal forebrain neurons in all species examined (7, 13, 21, 23); and 3) rodent studies demonstrate that NGF rescues basal forebrain cholinergic neurons from death in normal aging (4) and following experimental lesions of the fimbria-fornix (8, 14, 28). Only if efficacy and safety are demonstrated in further animal studies, including nonhuman primates, should clinical trials of NGF for treatment of AD be considered. Much more basic research is necessary before a rationale can be developed to use trophic factors other than NGF in clinical trials for AD. If factors are identified which prevent the death of specific neurons implicated in cognitive functioning in man, these could be considered for clinical trials after appropriate animal studies have demonstrated efficacy and safety.
REFERENCES 1. Appel, S. H. A unifying hypothesis for the cause of amyotrophic lateral sclerosis, parkinsonism, and Alzheimer's disease. Ann. Neurol. 10:499-505; 1981. 2. Barde, Y-A. What, if anything, is a neurotrophic factor? Trends Neurosci. 11:343-346; 1988. 3. Butcher, L. L.; Woolf, N. J. Neurotrophic agents exacerbate the pathologic cascade of Alzheimer's disease. Neurobiol. Aging 10: 557-570; 1989. 4, Fischer, W.; Wictorin, K.; Bjorklund, A.; Williams, L. R.; Varon, S.; Gage, F. H. Amelioration of cholinergic neuron atrophy and spatial memory impairment in aged rats by nerve growth factor. Nature 329:65-68; 1987. 5, Francis, P. T.; Palmer, A. M.; Sims, N. R.; Bowen, D. M.; Davison, A. N.; Esiri, M. M.; Neary, D.; Snowden, J. S.; Wilcock, G. C. Neurochemical studies of early-onset Alzheimer's disease: possible influence on treatment. N. Engl. J. Med. 313:7-11; 1985. 6. Gnahn, H.; Hefti, F.; Heumann, R.; Schwab, M. E.; Thoenen, H. NGF-mediated increase of choline acetyltransferase (CHAT) in the neonatal rat forebrain: evidence for a physiological role of NGF in the brain. Dev. Brain Res. 9:45-52; 1989. 7. Goedert, M.; Fine, A.; Dawbarn, D.; Wilcock, G. K.; Chao, M. V. Nerve growth factor receptor mRNA distribution in human brain: normal levels in basal forebrain in Alzheimer's disease. Mol. Brain Res. 5:1-7; 1989. 8. Hefti, F. Nerve growth factor promotes survival of septal cholinergic neurons after fimbria transection. J. Neurosci. 6:2155-2162; 1986. 9. Hefti, F.; Knusel, B.; Michel, P. P. Selective and non-selective trophic actions on central cholinergic and dopaminergic neurons in vitro. In: Coleman, P. D.; Higgins, G. A.; Phelps, C. H., eds. Molecular and cellular mechanisms of neuronal plasticity in aging and Alzheimer's disease. Amsterdam: Elsevier; 1989: in press. 10. Hefti, F.; Weiner, W. J. Nerve growth factor and Alzheimer's disease. Ann. Neurol. 20:275-28l; 1986. 11. Katzman, R.; Brown, T.; Field, P.; Thai, L.; Davies, P.; Terry, R. Significance of neurotransmitter abnormalities in Alzheimer's disease?. In: Martin, J. B.; Barchas, J. D., eds. Neuropeptides in neurologic and psychiatric disease. New York: Raven Press; 1986: 279-280. 12. Katzman, R.; Terry, R.; DeTeresa, R.; Brown, T.; Davies, P.; Fuld, P.; Renbing, X.; Peck, A. Clinical, pathological, and neurochemical changes in dementia: A subgroup with preserved mental status and numerous neocortical plaques. Ann. Neurol. 23:138-144; 1988. 13. Korsching, S.; Auburger, G.; Heumann, R.; Scott, J. Thoenen, H. Levels of nerve growth factor receptor and its mRNA in the central
14. 15.
16.
17.
18.
19.
20.
21. 22. 23. 24.
25.
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Epidermal Growth Factor Receptor ¢ Nerve Growth Factor
L A W R E N C E R. W I L L I A M S
CNS Diseases Research, Unit 7251-209-5, The Upjohn Company 301 Henrietta St., Kalamazoo, MI 49001
I am perplexed by the authors' complete lack of def'mition of neurotrophic factors. The agents Butcher and Wootf want to blame are neurite promoting factors, not neurotrophic factors. Treatment of Alzheimer's disease with NGF antagonists might instead exacerbate the death of both basal forebrain neurons and their cortical target neurons, accelerating the progress of dementia.
BUTCHER and Woolf present a scenario for the etiology of neuritic plaque formation and neuronal death in Alzheimer's disease (AD). In their hypothesis, abnormal neuronal sprouting is the primary cause of AD; age-related alterations in the CNS milieu result in expression of aberrant neuronal proteins that destabilize the cytoskeleton and induce neurite sprouting. The aberrant proteins accumulate in the neurite terminals, cause the terminals to rupture, and kill the parent neuron. The culprits to blame for triggering this pathologic cascade, according to Butcher and Woolf, are the "greater than normal" expression of neurotrophic factors, nerve growth factor (NGF) in particular. The authors recommend treating AD with drugs that "diminish synthesis of NGF, block NGF receptors, decrease saliency of allied neurotrophic agents," etc. Although some of the authors' speculations are justified by sound scientific evidence, 1 am perplexed by their complete lack of definition of neurotrophic factors, and their inconsistent use of poorly defined data to justify the conclusion that neurotrophic factors cause AD. Butcher and Woolf group neurotrophic factors, neurotrophic agents, neuronal growth factors, growth-promoting substances, and related neurochemical entities into one family; NGF, triiodothyronine (T3), thyroid releasing hormone, the somatomedins, and even epidermal growth factor receptor are equated to have similar, if not identical, neurotrophic properties. Varon et al. (12) defined neurotrophic factor (a.k,a. neuronotrophic factor) as a molecule that promotes the survival and maintenance of particular populations of neurons. Other molecules may specify, modify, or influence the metabolic status of a neuronal population such as the hormone. T 3. Neurite promoting factors comprise one subset of specifying molecules; neurite promoting factors induce neurite sprouting and outgrowth from otherwise alive neurons. It is essential to distinguish the functional identity of these groups of molecules in order to maintain an accurate, logical argument for or against the role of a particular molecule in the etiology of AD. Butcher and Woolf reference the work of Goedert et al. who
reported no change in NGF or NGF receptor mRNA in a small sampling of human brains with histologically confmned diagnosis of AD; no correlation was made with the degree of basal forebrain dystrophy or dementia. Still, the authors implicate elevations of NGF as a causative agent in their speculative cascade based on a preliminary report from Atterwill and Bowen suggesting that AD brain extract contains elevated levels of a molecule(s) affecting ChAT activity in aggregate cultures of whole brain. This apparent increase in ChAT stimulating activity may actually be due to a loss of an inhibitory molecule (11). Butcher and Woolf attempt to formulate a mechanism by which depositions of aberrant cytoskeletal material from ruptured terminal sprouts will form plaques that involve other adjacent neurons. They envision "trails of neuronal growth factors" produced by hypertrophic astrocytes migrating away from the center of amyloid plaques as inducing adjacent neuronal sprouting, and perpetuating the cascade of neuronal death. The agents Butcher and Woolf want to blame are neurite promoting factors, not neurotrophic factors. Guenther et al. ~4) have clearly demonstrated the neurite promoting properties of a glial-derived protease inhibitor. Intriguingly, a protease inhibitor has recently been found to be a consistent component of amyloid plaques (1). In the context of the Butcher and Woolf hypothesis. alterations in the environmental homeostasis of neurite promoting molecules could be the primary cause of AD and initiate the aberrant sprouting that begins the pathological scenario described in their review. NGF was classically described as being neurotrophic for peripheral, neural crest-derived neurons, and only recently has been found to be produced in the CNS and affect CNS neurons. particularly the cholinergic neurons in the basal forebrain (14). Much of the experimental evidence for the effects of neurotrophic and neurite promoting factors has come from tissue culture studies using embryonic neurons. Although culture experiments have provided many insights into the metabolic effects of such molecules, tissue culture experiments are not globally predictive of the in vivo effect of a given molecule, particularly in the mature adult