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Plaques and tangles in Alzheimer's Disease: Cause, consequence or epiphenomenon?
45()
I)R A ( H M A N
Plaques and Tangles in Alzheimer's Disease: Cause, Consequence or Epiphenomenon? D A V I D A. D R A C H M A N
Professor and Chairman, D e p a r t m e n t of Neurology University o f M a s s a c h u s e t t s Medical Center. 55 Lake Ave. North, Worcester, M A 01605
Plaques, tangles and amyloid are characteristic of Alzheimer's Disease. Study of the composition and origin of these proteins will be of value in determining the etiology and pathogenesis of AD if they are causally related to the neural damage, but of uncertain usefulness if they are merely secondary epiphenornena. D E N N I S Selkoe's thoughtful and comprehensive review of " A l t e r e d structural proteins in plaques and tangles" provides a state of the art summary of this facet of Alzheimer's Disease research. Equally importantly, it presents a reasonable perspective on the probable significance of these findings in the search for an understanding of Alzheimer's Disease. Two key questions must be asked about investigations into the composition and origin of plaques, tangles and amyloid: (1) Are these structural intruders responsible for the disruption of neural function that leads to dementia?; and (2) How .does information regarding the structure and composition of plaques and tangles illuminate our understanding of the etiology of AD? At present, we do not know whether plaques, tangles or amyloid are, in fact, harmful in themseh, es to the function of otherwise normal nervous tissue. They appear obtrusive; are found in association with AD and other disease states; and, in the case of "ghost tangles," indicate that actual neuronal death took place where they remain. The famous paper of Blessed, Tomlinson and Roth is often quoted (and misquoted) as showing that senile plaques are highly correlated with the degree of dementia [1]. Yet these authors wisely noted that, "Plaques may merely provide evidence of neuronal degeneration which might arise from a variety of c a u s e s . " The issue is causality: if plaques, tangles or amyloid cause neuronal death or dysfunction, they become of great interest in themselves; if they only mark the site of neuronal damage, dysfunction or demise from some other cause, then they may be reduced to lesser significance as the castoff detritus of injured or dying neurons. If altered structural proteins in AD are causally related to the damage of neurons, then the value of determining their origin, analyzing their structure, and establishing the genetic instructions that code for and regulate the synthesis of these proteins is obvious. With this knowledge, one could hope to interrupt the destructive process at a molecular level, and to identify individuals genetically at risk for AD. And, if the process is genetically determined, but regulated by epigenetic influences, those regulating factors could be sought and tested using a definable instrument of measurement: the alteration of m R N A transcription in response to suspected regulatory factors. What if plaques, tangles and amyloid are merely harmless epiphenomena, however, marking the trail of dead and dying neurons killed by a variety of age-associated causes, while producing no dysfunction in themselves? This is far from a trivial possibility, even though Alzheimer's Disease has been
inseparably connected with these structures through eighty years of neuropathologic thought [4]. The fact that neurofib, rillary tangles occur in multiple contexts [5], and even result from such a non-specific mechanical triggering event as head trauma (in dementia pugilistica [3]) provides support for this possibility. If these altered protein structures are epiphenomena, then they may be of interest primarily as possible diagnostic markers. Even in this context, their value is uncertain; if they are present in normal aging, and often overlap even quantitatively the findings in Alzheimer's Disease [2], they would be of only modest value as a test for the presence of AD. Rather, their absence might be useful in patients with known dementia, to indicate that another cause of dementia should be sought. Can analysis of plaques, tangles and amyloid provide useful clues to the etiology of Alzheimer's Disease'? This is a much more complex question; and it depends on whether the specific cause of AD produces these altered structural proteins by direct damage to their immediate precursors. If, for example, neurofibrillary tangles were the result of a small number of changes induced in the instructions for synthesis of neurofilaments by a viral agent, this discovery could pinpoint the site of viral attack. No longer a needle in a haystack, the agent could be sought in a specific locus, making its eventual recognition more probable. This would be equally true even if the mode of neuronal damage and death were unrelated to the formation of tangles, which might be only an inadvertent byproduct. On the other hand, it is also quite possible that plaques, tangles and amyloid are quite indirect results of the neuronal damage that underlies AD: the nonspecific end product of neural tissue undergoing degeneration and death. If that is so, then these structures represent red herrings for the neuroscientist, dragged across the trail and obscuring a proximate cause of the disease. Dr. Selkoe has wisely and correctly noted that, "Tangles, plaques and other morphological changes in AD brain such as amyloid angiopathy and granulovacuolar degeneration of pyramidal neurons are qualitatively indistinguishable from the lesions accompanying normal aging of the human brain but are quantitatively much increased in AD.'" If indeed the cause of these changes is identical in normal aging and AD, the quantitative differences may reflect a pathologic acceleration of the normal aging process, or a biologic variation, representing an extreme end of a Gaussian curve: In either case, discovery of the cause of neural decline with age would present the possibility of finding a means of arresting the process; the long-sought elixir of youth. At present, the plaques, tangles and amyloid found in
COMMENTARY
451
Alzheimer's Disease remain the most visible evidence of the degenerative changes occurring in neural tissue, and as such their analysis is among the most interesting current approaches to the study of AD. Whether the studies currently being so elegantly pursued will establish these structures as
responsible for neuronal damage, lead to discovery of the etiology of AD, or turn out to be high-tech analyses of mere epiphenomena in the pathogenesis of AD remains clouded for the present.
REFERENCES I. Blessed, G., B. E. Tomlinson and M. Roth. The association between quantitative measures of dementia and of senile change in the cerebral grey matter of elderly subjects. Br J Psychiato' 114" 797-811. 1968. 2. Miller, F. DEW., S. P. Hicks, C. J. D'Amato and J. R. Landis. A descriptive study of neuritic plaques and neurofibrillary tangles in an autopsy population. J Epidemiol 120: 331-341, 1984. 3. Strich, S. J. Cerebral trauma. In. Greet~[h,ld's Neuropathology. third edition, edited by W. Blackwood and J. A. N. Corsellis. Edinburgh: Edward Arnold, 1976.
4. Terry, R. and R. Katzman. Senile dementia of the Alzheimer type: defining a disease. In: The Neurology ~fA~,im,,. edited by R. Katzman and R. D. Terry. Philadelphia: F.A. Davis Co., 1983. 5. Wisniewski, K., G. A. Jervis, R. C. Moretz and H. M. Wisniewski. AIzheimer neurofibrillary tangles in diseases other than senile and presenile dementia. Ann Nettrol 5: 288-294, 1979.
Paired Helical Filaments: Do They Contain Neurofilament Epitopes? P. G A M B E T T I , G. P E R R Y A N D L. A U T I L I O - G A M B E T T I
Division o f Neuropathology, D e p a r t m e n t o f Pathology Case Western R e s e r v e University, Cleveland, O H 44106
The presence of neurofilament epitopes in paired helical filaments is discussed in view of a recent study showing no cross-reaction between at least one monoclonal antibody to neurofilament and tau protein.
O N E of the many puzzling features of Alzheimer disease (AD) is the complexity and multiplicity of the pathologic changes. Selective degeneration of certain neuronal populations, presence of neuritic plaques (NP) and neurofibrillary tangles (NFT), deposition of amyloid in the vascular wall and senile plaque cores, presence of neuronal degenerative changes such as granulo-vascular degeneration and Hirano bodies are all concurrently present in AD. A challenge for the investigators of AD is to determine whether and how these changes are correlated. The approach of most investigators has been to study these changes individually in search of clues as to their reciprocal correlations. In an elegant and perceptive article, Dennis Selkoe has reviewed the studies on the nature of, and possible correlations between, neurofibrillary tangles (NFT) and neuritic plaques. One of the issues reviewed in detail is the current view of the composition of PHF, as determined by their reactivity with specific antibodies. Over the last few years, we have tried to characterize and locate specific epitopes present in NFT. Using a variety of antibodies we have consistently obtained evidence that paired helical filaments (PHF), the main component of NFT, share antigenic determinants with normal neurofilament (NF) proteins as well as with the microtubule associated proteins, MAP2 and tau, and possibly other undetermined proteins [10,11]. The presence of N F epitopes in N F T has been viewed with skepticism in several articles and this review is no exception.
Thus, in this commentary we will briefly consider the issue of the presence and possible role of N F epitopes in PHF. Most of past and present skepticism on the presence of N F epitopes in PHF stems from two observations: (a) only very few antibodies to N F immunoreact with N F T and (b) these antibodies, as claimed in Dr. Selkoe's review, crossreact with tau proteins. When we obtained monoclonal antibodies from mice immunized with N F preparations only two of these antibodies to N F cross-reacted with N F T [ I ]. Similar results have been reported by others. Moreover, we observed that adsorption of polyclonal antibodies to N F with N F proteins blocked immunostaining of N F T at lower concentrations of the adsorbent that were needed to block immunostaining of N F [6]. These findings indicate that the shared N F sequence is poorly represented in PHF. Therefore the first observation is undoubtedly correct but does not rule out the presence of N F epitopes in PHF. The second observation still must be confirmed. Our monoclonal antibody (Mab) l.l.1. [1] is directed to a phosphorylated epitope of the 200 kDa NF subunit and it has been shown to immunoreact consistently with PHF in intact tissue as well as with PHF isolated and extracted with ionic detergents I I0] and guanidine-HCI (unpublished results). Thus, this Mab recognizes an epitope that is an intrinsic constituent of the PHF. We have assessed whether immunoreaction of Mab 1.1.1. with P H F was due to cross-reactivity with MAP2 or tau proteins, a possibility strongly suggested in Dr. Selkoe's
I)R A ( H M A N
Plaques and Tangles in Alzheimer's Disease: Cause, Consequence or Epiphenomenon? D A V I D A. D R A C H M A N
Professor and Chairman, D e p a r t m e n t of Neurology University o f M a s s a c h u s e t t s Medical Center. 55 Lake Ave. North, Worcester, M A 01605
Plaques, tangles and amyloid are characteristic of Alzheimer's Disease. Study of the composition and origin of these proteins will be of value in determining the etiology and pathogenesis of AD if they are causally related to the neural damage, but of uncertain usefulness if they are merely secondary epiphenornena. D E N N I S Selkoe's thoughtful and comprehensive review of " A l t e r e d structural proteins in plaques and tangles" provides a state of the art summary of this facet of Alzheimer's Disease research. Equally importantly, it presents a reasonable perspective on the probable significance of these findings in the search for an understanding of Alzheimer's Disease. Two key questions must be asked about investigations into the composition and origin of plaques, tangles and amyloid: (1) Are these structural intruders responsible for the disruption of neural function that leads to dementia?; and (2) How .does information regarding the structure and composition of plaques and tangles illuminate our understanding of the etiology of AD? At present, we do not know whether plaques, tangles or amyloid are, in fact, harmful in themseh, es to the function of otherwise normal nervous tissue. They appear obtrusive; are found in association with AD and other disease states; and, in the case of "ghost tangles," indicate that actual neuronal death took place where they remain. The famous paper of Blessed, Tomlinson and Roth is often quoted (and misquoted) as showing that senile plaques are highly correlated with the degree of dementia [1]. Yet these authors wisely noted that, "Plaques may merely provide evidence of neuronal degeneration which might arise from a variety of c a u s e s . " The issue is causality: if plaques, tangles or amyloid cause neuronal death or dysfunction, they become of great interest in themselves; if they only mark the site of neuronal damage, dysfunction or demise from some other cause, then they may be reduced to lesser significance as the castoff detritus of injured or dying neurons. If altered structural proteins in AD are causally related to the damage of neurons, then the value of determining their origin, analyzing their structure, and establishing the genetic instructions that code for and regulate the synthesis of these proteins is obvious. With this knowledge, one could hope to interrupt the destructive process at a molecular level, and to identify individuals genetically at risk for AD. And, if the process is genetically determined, but regulated by epigenetic influences, those regulating factors could be sought and tested using a definable instrument of measurement: the alteration of m R N A transcription in response to suspected regulatory factors. What if plaques, tangles and amyloid are merely harmless epiphenomena, however, marking the trail of dead and dying neurons killed by a variety of age-associated causes, while producing no dysfunction in themselves? This is far from a trivial possibility, even though Alzheimer's Disease has been
inseparably connected with these structures through eighty years of neuropathologic thought [4]. The fact that neurofib, rillary tangles occur in multiple contexts [5], and even result from such a non-specific mechanical triggering event as head trauma (in dementia pugilistica [3]) provides support for this possibility. If these altered protein structures are epiphenomena, then they may be of interest primarily as possible diagnostic markers. Even in this context, their value is uncertain; if they are present in normal aging, and often overlap even quantitatively the findings in Alzheimer's Disease [2], they would be of only modest value as a test for the presence of AD. Rather, their absence might be useful in patients with known dementia, to indicate that another cause of dementia should be sought. Can analysis of plaques, tangles and amyloid provide useful clues to the etiology of Alzheimer's Disease'? This is a much more complex question; and it depends on whether the specific cause of AD produces these altered structural proteins by direct damage to their immediate precursors. If, for example, neurofibrillary tangles were the result of a small number of changes induced in the instructions for synthesis of neurofilaments by a viral agent, this discovery could pinpoint the site of viral attack. No longer a needle in a haystack, the agent could be sought in a specific locus, making its eventual recognition more probable. This would be equally true even if the mode of neuronal damage and death were unrelated to the formation of tangles, which might be only an inadvertent byproduct. On the other hand, it is also quite possible that plaques, tangles and amyloid are quite indirect results of the neuronal damage that underlies AD: the nonspecific end product of neural tissue undergoing degeneration and death. If that is so, then these structures represent red herrings for the neuroscientist, dragged across the trail and obscuring a proximate cause of the disease. Dr. Selkoe has wisely and correctly noted that, "Tangles, plaques and other morphological changes in AD brain such as amyloid angiopathy and granulovacuolar degeneration of pyramidal neurons are qualitatively indistinguishable from the lesions accompanying normal aging of the human brain but are quantitatively much increased in AD.'" If indeed the cause of these changes is identical in normal aging and AD, the quantitative differences may reflect a pathologic acceleration of the normal aging process, or a biologic variation, representing an extreme end of a Gaussian curve: In either case, discovery of the cause of neural decline with age would present the possibility of finding a means of arresting the process; the long-sought elixir of youth. At present, the plaques, tangles and amyloid found in
COMMENTARY
451
Alzheimer's Disease remain the most visible evidence of the degenerative changes occurring in neural tissue, and as such their analysis is among the most interesting current approaches to the study of AD. Whether the studies currently being so elegantly pursued will establish these structures as
responsible for neuronal damage, lead to discovery of the etiology of AD, or turn out to be high-tech analyses of mere epiphenomena in the pathogenesis of AD remains clouded for the present.
REFERENCES I. Blessed, G., B. E. Tomlinson and M. Roth. The association between quantitative measures of dementia and of senile change in the cerebral grey matter of elderly subjects. Br J Psychiato' 114" 797-811. 1968. 2. Miller, F. DEW., S. P. Hicks, C. J. D'Amato and J. R. Landis. A descriptive study of neuritic plaques and neurofibrillary tangles in an autopsy population. J Epidemiol 120: 331-341, 1984. 3. Strich, S. J. Cerebral trauma. In. Greet~[h,ld's Neuropathology. third edition, edited by W. Blackwood and J. A. N. Corsellis. Edinburgh: Edward Arnold, 1976.
4. Terry, R. and R. Katzman. Senile dementia of the Alzheimer type: defining a disease. In: The Neurology ~fA~,im,,. edited by R. Katzman and R. D. Terry. Philadelphia: F.A. Davis Co., 1983. 5. Wisniewski, K., G. A. Jervis, R. C. Moretz and H. M. Wisniewski. AIzheimer neurofibrillary tangles in diseases other than senile and presenile dementia. Ann Nettrol 5: 288-294, 1979.
Paired Helical Filaments: Do They Contain Neurofilament Epitopes? P. G A M B E T T I , G. P E R R Y A N D L. A U T I L I O - G A M B E T T I
Division o f Neuropathology, D e p a r t m e n t o f Pathology Case Western R e s e r v e University, Cleveland, O H 44106
The presence of neurofilament epitopes in paired helical filaments is discussed in view of a recent study showing no cross-reaction between at least one monoclonal antibody to neurofilament and tau protein.
O N E of the many puzzling features of Alzheimer disease (AD) is the complexity and multiplicity of the pathologic changes. Selective degeneration of certain neuronal populations, presence of neuritic plaques (NP) and neurofibrillary tangles (NFT), deposition of amyloid in the vascular wall and senile plaque cores, presence of neuronal degenerative changes such as granulo-vascular degeneration and Hirano bodies are all concurrently present in AD. A challenge for the investigators of AD is to determine whether and how these changes are correlated. The approach of most investigators has been to study these changes individually in search of clues as to their reciprocal correlations. In an elegant and perceptive article, Dennis Selkoe has reviewed the studies on the nature of, and possible correlations between, neurofibrillary tangles (NFT) and neuritic plaques. One of the issues reviewed in detail is the current view of the composition of PHF, as determined by their reactivity with specific antibodies. Over the last few years, we have tried to characterize and locate specific epitopes present in NFT. Using a variety of antibodies we have consistently obtained evidence that paired helical filaments (PHF), the main component of NFT, share antigenic determinants with normal neurofilament (NF) proteins as well as with the microtubule associated proteins, MAP2 and tau, and possibly other undetermined proteins [10,11]. The presence of N F epitopes in N F T has been viewed with skepticism in several articles and this review is no exception.
Thus, in this commentary we will briefly consider the issue of the presence and possible role of N F epitopes in PHF. Most of past and present skepticism on the presence of N F epitopes in PHF stems from two observations: (a) only very few antibodies to N F immunoreact with N F T and (b) these antibodies, as claimed in Dr. Selkoe's review, crossreact with tau proteins. When we obtained monoclonal antibodies from mice immunized with N F preparations only two of these antibodies to N F cross-reacted with N F T [ I ]. Similar results have been reported by others. Moreover, we observed that adsorption of polyclonal antibodies to N F with N F proteins blocked immunostaining of N F T at lower concentrations of the adsorbent that were needed to block immunostaining of N F [6]. These findings indicate that the shared N F sequence is poorly represented in PHF. Therefore the first observation is undoubtedly correct but does not rule out the presence of N F epitopes in PHF. The second observation still must be confirmed. Our monoclonal antibody (Mab) l.l.1. [1] is directed to a phosphorylated epitope of the 200 kDa NF subunit and it has been shown to immunoreact consistently with PHF in intact tissue as well as with PHF isolated and extracted with ionic detergents I I0] and guanidine-HCI (unpublished results). Thus, this Mab recognizes an epitope that is an intrinsic constituent of the PHF. We have assessed whether immunoreaction of Mab 1.1.1. with P H F was due to cross-reactivity with MAP2 or tau proteins, a possibility strongly suggested in Dr. Selkoe's