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Pyramidal Nerve Cell Loss in Alzheimer's Disease
NEURODEGENERATION, Vol. 5, pp 423–427 (1996)
Pyramidal Nerve Cell Loss in Alzheimer’s Disease David M.A. Mann Department of Pathological Sciences, University of Manchester, Oxford Road, Manchester M13 9PT Loss of the large pyramidal cells of the association neocortex and hippocampus, along with plaques and tangles, is fundamental to the neuropathology of Alzheimer’s disease. The extent of Alzheimer-specific cell loss, relative to controls, is age-dependent with maximal losses in younger subjects though, because of the (additive) effects of ‘normal’ ageing on such cells, the absolute loss remains constant at all ages. The cause of the cell loss remains unknown but probably relates to neurofibrillary degeneration through a crowding out of organelles and a disruption of intracellular transport; oxidative stress may also contribute. The degree of clinical dementia correlates well with the extent of pyramidal cell loss. © 1996 Academic Press Limited
Key words: pyramidal cells, cerebral cortex, hippocampus, dementia, Alzheimer’s disease
Over the past decades, many studies have addressed the question of nerve cell loss from the cerebral cortex and hippocampus, yet a consensus of opinion has still to be reached even today.
Introduction THE DISTRIBUTION of the classical histopathological lesions of Alzheimer’s disease (AD), viz. the senile (or neuritic) plaques (SP) and the neurofibrillary tangles (NFT), principally involves the association areas of the neocortex and the archicortical regions of the hippocampus and amygdala. As such, it implies that AD is essentially a cortical dementia which is caused by damage to the superficial grey matter regions of the cerebral hemispheres. The key effector cells of the cerebral cortex and hippocampus are the pyramidal nerve cells that are located in layers III and V and in area CA1 of these regions, respectively. Because it has long been known that such cell types are particularly prone to NFT formation in AD and as they provide the major input and output pathways to the association cortex it is only natural to assume that damage to the cerebral cortex in AD should be reflected by changes in these in terms of cell number, or cell structure or both, and that such alterations should explain the cerebral atrophy of the disorder (Najlerahim & Bowen, 1989; Mann, 1991) and the progressive disturbances of memory and intellect that accompany the pathological process (Neary et al., 1986a).
The extent of pyramidal nerve cell loss in Alzheimer’s disease Early studies, based on only a few patients (Shefer, 1973; Colon, 1973), suggested that a loss of pyramidal nerve cells from the cerebral cortex might indeed occur in AD. With automatic and semi-automatic cell counting techniques firmer quantitative data have now emerged. In 1981, Terry et al. reported an overall loss of 22% of neurones from the superior temporal cortex and a 26% reduction in the mid-frontal gyrus in a group of 18 elderly patients with AD, aged between 70 and 90 years. The major proportion of cell loss was amongst those cells with a perikaryal size of more than 90 µm2 (assumed to be pyramidal cells) where reductions in number of 46% and 40% respectively were determined. Mountjoy et al. (1983) also reported substantial decreases in the number of large (presumably pyramidal) cells in frontal, cingulate and temporal gyri, but not in parietal or occipital regions, again in a group of 25 elderly subjects, such losses being particularly prominent in those patients under 80 years of age. Mann et al. (1985) estimated an average 60% loss
Correspondence to: D. M. A. Mann © 1996 Academic Press Limited 1055-8330/96/040423 1 5 $25.00/0
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424 of pyramidal cells from layers III and V of the temporal cortex in 32 patients aged between 48 and 92 years of age, compared to 48% in those over this age. In a later study (Neary et al., 1986b) of 17 younger patients (aged 52–69 years) examined at cerebral biopsy, Mann reported an average loss of 58% of pyramidal cells again from the middle temporal gyrus. Hubbard & Anderson (1985) observed a 33% loss of large nerve cells from the temporal cortex and a 26% loss from the frontal cortex in 21 patients of 64–92 years. Likewise, these authors noted that greatest loss of such cells usually occurred in those patients under 80 years of age. Both in this latter study and that by Terry et al. (1981) the number of smaller neurones was unchanged. By contrast, both Tomlinson & Henderson (1976) and Braak & Braak (1986) found no evidence of overall nerve cell loss in the pre- and post-central gyrus, and temporal and frontal cortex in AD. However, in both these studies different size categories of neurones were not considered and it remained possible that some loss of the larger pyramidal cells might indeed have occurred, this being ‘masked’ by a lack of change in the number of smaller neurones. Finally, in 11 patients, all over 79 years of age, Regeur et al. (1994) were also unable to detect any overall loss of nerve cells from the neocortex. While differences in the quantitative procedures used may have contributed to some of this variation in findings, clear principles nonetheless emerge. Firstly, it seems likely that in AD nerve cell loss from the neocortex is confined essentially to the larger pyramidal cells; indeed this is the very nerve cell type most prone to neurofibrillary degeneration. Secondly, the extent of pyramidal cell loss in AD is age-dependent; younger patients (i.e. those under 80 years of age) show a greater loss of cells, compared to non-dements of that age, than do older subjects (i.e. those over 80 years) where little or no additional cell loss may occur. These data equate with observations of cerebral atrophy and brain weight (Terry et al., 1981; Hubbard & Anderson, 1981, 1985; Mann et al., 1985; Hansen et al., 1988; De La Monte, 1989; Mann, 1991) showing that major changes in such features usually occur only in younger patients; in older subjects with AD brain weight is little reduced and often unchanged, compared to non-dements, and cerebral atrophy, if present, is confined to the temporal lobe. Hence, because of the increasing fall-out of larger nerve cells from the cerebral cortex with ageing (Mann et al., 1985) proportionately more of these neurones will be lost in younger patients with AD, due to the process of AD itself, than will be in older subjects where most of the required cell loss will have already
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occurred due to ageing alone with only little additional loss due to AD being superimposed. In both younger and older subjects however the total cell loss remains the same. The question of nerve cell loss from the hippocampus in AD is less controversial. Several studies have examined this, concentrating usually on areas CA1 or subiculum, and in all a severe loss of pyramidal cells occurs, irrespective of the age of the patient (Ball, 1977; Mann et al., 1985; Shefer, 1977; Anderson et al., 1983; Davies et al., 1992; Doebler et al., 1987).
Dendritic and axonal changes While perikaryal loss is perhaps the most visible aspect of cellular degeneration in AD it remains possible that in those elderly cases where no obvious cell loss occurs, nerve cells may have suffered axonal and dendritic changes which compromise their effectiveness, and produce an atrophy of the cortex, while retaining the actual cell body. Indeed, both quantitative electron microscopic (Davies et al., 1987, DeKosky & Scheff, 1990; Scheff et al., 1990; Scheff & Price, 1993) and numerous immunohistochemical (Hamos et al., 1989; Masliah et al., 1991; Terry et al., 1991; Honer et al., 1992; Lippa et al., 1992; Zhan et al., 1993; Brun et al., 1995; Heinonen et al., 1995) studies have all shown that considerable loss of synapses occurs at all ages in AD. Further, the extent of this synapse loss may outweigh the actual loss of neuronal perikarya (Davies et al., 1987) though this latter observation may ‘simply’ reflect the preferential loss of the larger pyramidal cells which will have more synaptic contacts than the smaller neighbouring neurones.
Mechanisms underlying nerve cell death Because the accumulation of NFT is the most visible cytological change in pyramidal neurones in AD it is tempting to assume that this reflects directly the neurodegenerative process that affects such cells and ultimately leads to their demise. Indeed, even a cursory microscopical observation suggests that such cells are hardly likely to be normally functioning cells, especially when the amount of tangle within them is high. Furthermore, the presence of similar tangle material lying apparently freely within the neuropil (as extracellular tangle or ghost tangle) marks the location of a once functional neurone. Nonetheless, it remains possible that nerve cells might die in AD through routes
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other than that involving NFT formation and accumulation and that, despite its conspicuousness within neurones, the NFT may simply represent an ‘innocent’ cellular marker of a more insidious neurotoxic process. Certainly, studies of the amount of ribonucleic acid within nerve cells or the size of the nucleolus (both being measures of protein synthetic capacity) show these to be reduced in tangled cells compared with their non-tangled neighbours (Dayan & Ball, 1973; Mann & Yates, 1981; Mann et al., 1981a, 1981b; Doebler et al., 1987). At ultrastructural level (Sumpter et al., 1986) decreases in the amount of rough endoplasmic reticulum, the number of ribosomes and the surface area of mitochondria occur in line with an increasing tangle mass. Such data clearly indicate an ongoing failure of metabolism in such cells. However, it is possible that these changes represent a gradual failure to supply (and therefore a requirement to produce) proteins to the parts of the neurone where they are needed because of the collapse of the microtubular network of the cell consequent to the phosphorylation of tau and its incorporation into NFT. Furthermore NFT, and the abnormal tau especially, become increasingly resistant to proteolysis because of participation in Maillard-type condensation reactions which result in an extensive crosslinking of the fibrils (Smith et al., 1994a, 1995a; Yan et al., 1994, 1995). Such reactions may impose oxidative stress upon affected cells; the presence of ‘heat-shock’ proteins like HSP27 (Renkawek et al., 1993), HSP70 (Hamos et al., 1991) and haem-oxygenase-1 (HSP32) (Smith et al., 1994b, 1995b) would accord with such a stress mechanism. In a recent study (Cras et al., 1995) the total number of intracellular tangle bearing cells, extracellular tangles and non-tangled cells in the hippocampus in AD matched the total number of unaffected neurones in control subjects. Hence, alternative mechanisms of nerve cell loss, not involving NFT formation, may be unlikely.
Relationship to plaques and β amyloid Although it has been widely proposed that amyloid β protein is neurotoxic, and even perhaps responsible for inducing intracellular changes that result in NFT formation (e.g. tau hyperphosphorylation) and accumulation, possibly involving oxidative stress (Behl et al., 1994), it is quite clear that the extent of nerve cell loss or even the density of neurofibrillary fails to correlate with the number of neuritic plaques (Neary
et al., 1986b). However, levels of precursor protein for β amyloid (amyloid precursor protein, APP) are reduced in AD, in line with the remaining number of functional neurones (Francis et al., 1993), emphasizing the principal neuronal localization of this particular protein.
Relationship to clinical dementia Although the early studies of Blessed and Roth (Blessed et al., 1968) suggested an overall correlation between neuritic plaque density and the degree of dementia in a cohort of elderly demented and nondemented subjects, later studies (Neary et al., 1986b; Terry et al., 1991; Arriagada et al., 1992) have failed to substantiate this only within individuals suffering from AD. Indices of NFT formation correlate better than plaques (Wilcock & Esiri, 1982; Terry et al., 1991; Arriagada et al., 1992; Neary et al., 1986b; McKee et al., 1991) yet here again the correlation is generally still weak. Since the numbers of SP and NFT continually change during the course of the illness with old plaques and tangles being removed and replaced by newer ones (Hyman et al., 1993; Mann et al., 1986) such lack of correlation should be anticipated. Much better correlations have been achieved between the degree of dementia and reductions in the number of surviving pyramidal cells (Neary et al., 1986b) or synapses (Terry et al., 1991; DeKosky & Scheff, 1990; Davies et al., 1987). This should perhaps be expected since not only do these latter measures represent more accurately an accumulative tissue deficit over the course of the illness, but also because they are key markers of the integrative capabilities of the neocortex. As if to emphasize the importance of an intact complement of pyramidal neurones and their synapses to the proper functioning of the cerebral cortex, similar losses of neurones have been reported in other cortical dementias such as Down’s syndrome (Mann et al., 1985, 1987) or frontal lobe dementia (Mann et al., 1993). Neocortical synapse loss is also seen in frontal lobe dementia (Brun et al., 1995) and, as in AD, a reduction in cortical APP levels accompanies the loss of pyramidal cells (Francis et al., 1993).
Conclusion A loss of large pyramidal cells of layers III and V of the cerebral cortex, and those of areas CA1 and subiculum of the hippocampus, is a fundamental aspect of
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the neuropathology of AD. Death of these nerve cell types is probably mediated through neurofibrillary degeneration though the precise mechanism underlying neurotoxicity is still uncertain. The loss of pyramidal cells and their connections is likely to form the critical pathological substrate of the dementia of AD, and also that of other cortical dementias like Down’s syndrome and frontal lobe dementia.
References Arriagada PV, Growdon JH, Hedley-White ET, Hyman BT (1992) Neurofibrillary tangles but not senile plaques parallel duration and severity of Alzheimer’s disease. Neurology 42:631–639 Ball MJ (1977) Neuronal loss, neurofibrillary tangles and granulovacuolar degeneration in the hippocampus with ageing and dementia. A quantitative study. Acta Neuropathol 37:111–118 Behl C, Davis JB, Lesley R, Schubert D (1994) Hydrogen peroxide mediates amyloid β protein toxicity. Cell 77:1–20 Blessed G, Tomlinson BE, Roth M (1968) The association between quantitative measures of dementia and of senile change in the cerebral grey matter of elderly subjects. Brit J Psychiat 114:797–811 Braak H, Braak E (1986) Ratio of pyramidal cells versus non-pyramidal cells in the human frontal isocortex and changes in ratio with ageing and Alzheimer’s disease. In: Swaab DF, Fliers E, Mirmiran M, Van Gool WA, Van Haaren F. Progress in Brain Research. Vol. 70. Elsevier Science Publishers BV (Biomedical Division), 185–212 Brun A, Liu X, Erikson C (1995) Synapse loss and gliosis in the molecular layer of the cerebral cortex in Alzheimer’s disease and in frontal lobe degeneration. Neurodegeneration 4:171–177 Colon EJ (1973) The cerebral cortex in presenile dementia. A quantitative analysis. Acta Neuropathol 23:281–290 Cras P, Smith MA, Richey PL, Siedlak SL, Mulvihill P, Perry G (1995) Extracellular neurofibrillary tangles reflect neuronal loss and provide further evidence of extensive protein cross-linking in Alzheimer disease. Acta Neuropathol 89:291–295 Davies CA, Mann DMA, Sumpter PQ, Yates P (1987) A quantitative analysis of the neuronal and synaptic content of the frontal and temporal cortex in patients with Alzheimer’s disease. J Neurol Sci 78:151–164 Davies DC, Horwood N, Isaacs SL, Mann DMA (1992) The effect of age and Alzheimer’s disease in pyramidal neurone density in the individual fields of the hippocampal formation. Acta Neuropathol 83:510–517 Dayan AD, Ball MJ (1973) Histometric observations on the metabolism of tangle-bearing neurons. J Neurol Sci 19:433–436 DeKosky ST, Sheff SW (1990) Synapse loss in frontal cortex biopsies in Alzheimer’s disease: Correlation with cognitive severity. Ann Neurol 27:457–564 De La Monte S (1989) Quantitation of cerebral atrophy in preclinical and end-stage Alzheimer’s disease. Ann Neurol 25:450–459. Doebler JA, Markesbery WR, Anthony A, Rhoads RE (1987) Neuronal RNA in relation to neuronal loss and neurofibrillary pathology in the hippocampus in Alzheimer’s disease. J Neuropathol Exp Neurol 46:28–39 Francis PT, Webster M-T, Procter AW, Clarke NA, Holmes C, Bowen D, Doshi B, Mann DMA, Snowden J, Neary D (1993) Soluble βamyloid precursor protein and pyramidal neurone loss are not diagnostic of Alzheimer’s disease. Lancet 341, 431 Gravina SA, Ho L, Eckman CB, Long KE, Otvos L, Younkin LH, Suzuki N, Younkin SG (1995) Amyloid β protein (Aβ) in Alzheimer’s disease brain. J Biol Chem 270:7013–7016
Hamos JE, DeGennaro LJ, Drachman DA (1989) Synaptic loss in Alzheimer ’s disease and other dementias. Neurology 39:355–361 Hamos JE, Oblas B, Pulaski-Salo D, Welch WJ, Bole DG, Drachman DA (1991) Expression of heat shock proteins in Alzheimer’s disease. Neurology 41:345–350 Hansen LA, Teresa R, Davies P, Terry RD (1988) Neocortical morphometry, lesion count and choline acetyl transferase levels in the age spectrum of Alzheimer’s disease. Neurology 38:48–54 Heinonen O, Lehtovirta M, Soininen H, Helisalmi S, Mannermaa A, Sorvari H, Kosunen O, Paljarvi L, Ryynanen M, Riekkinen PJ (1995) Alzheimer pathology of patients carrying apolipoprotein E E4 allele. Neurobiol Aging 16:505–513 Honer WG, Dickson DW, Gleeson J, Davies P (1992) Regional synaptic pathology in Alzheimer’s pathology. Neurobiol Aging 13:375–382 Hubbard BM, Anderson JM (1981) A quantitative study of cerebral atrophy in old age and senile dementia. J Neurol Sci 50:135–145 Hubbard BM, Anderson JM (1985) Age-related variations in the neurone content of the cerebral cortex in senile dementia of Alzheimer type. Neuropath Appl Neurobiol 11:369–382 Hyman BT, Marzloff K, Arriagada PV (1993) The lack of accumulation of senile plaques or amyloid burden in Alzheimer’s disease suggests a dynamic balance between amyloid deposition and resolution. J Neuropathol Exp Neurol 52:594–600 Lippa CF, Hamos JE, Pulaski-Salo D, Degennaro LJ, Drachman DA (1992) Alzheimer’s disease and aging: Effects on perforant pathway perikarya and synapses. Neurobiol Aging 13:405–411 Mann DMA, Yates PO (1981) The relationship between formation of senile plaques and neurofibrillary tangles and changes in nerve cell metabolism in Alzheimer-type dementia. Mech Ageing Dev 17:395–401 Mann DMA, Neary D, Yates PO, Lincoln J, Snowden JS, Stanworth P (1981a) Neurofibrillary pathology and protein synthetic capability in nerve cells in Alzheimer’s disease. Neuropath Appl Neurobiol 7:37–47 Mann DMA, Neary D, Yates PO, Lincoln J, Snowden JS, Stanworth P (1981b) Alterations in protein synthetic capability in nerve cells in Alzheimer’s disease. J Neurol Neurosurg Psychiatry 44:97–102 Mann DMA, Yates PO, Marcyniuk B (1985) Some morphometric observation on the cerebral cortex and hippocampus in presenile Alzheimer’s disease, senile dementia of Alzheimer-type and Down’s syndrome in middle age. J Neurol Sci 69:139–159 Mann DMA, Marcyniuk B, Yates PO, Neary D, Snowden JS (1988) The progression of the pathological changes of Alzheimer’s disease in frontal and temporal neocortex examined both at biopsy and at autopsy. Neuropath Appl Neurobiol 14:177–195 Mann DMA, Yates PO, Marcyniuk B, Ravindra CR (1987) Loss of nerve cells from cortical and subcortical areas in Down’s syndrome patients at middle age: quantitative comparisons with younger Down’s patients and patients with Alzheimer’s disease. J Neurol Sci 80:79–89 Mann DMA, South PW, Snowden JS, Neary D (1993) Dementia of frontal lobe type:neuropathology and immunohistochemistry. J Neurol Neurosurg Psychiatry 56:605–614 Masliah E, Hansen T, Mallory M, Terry RD (1991) Immunoelectron microscopic study of synaptic pathology in Alzheimer’s disease. Acta Neuropathol 81:428–433 McKee AC, Kosik KS, Kowall NW (1991) Neuritic pathology and dementia in Alzheimer’s disease. Ann Neurol 30:156–165 Mountjoy CQ, Roth M, Evans NJR, Evans HM (1983) Cortical neuronal counts in normal elderly controls and demented patients. Neurobiol Aging 4:1–11 Najlerahim A, Bowen DM (1989) Regional weight loss in the cerebral cortex and some subcortical nuclei in senile dementia of the Alzheimer type. Acta Neuropathol 75:509–512 Neary D, Snowden JS, Bowen DM, Sims NR, Mann DMA, Benton JS, Northen B, Yates PO, Davison AN (1986) Neuropsychological
Pyramidal nerve cell loss in AD syndromes in presenile dementia due to cerebral atrophy. J Neurol Neurosurg Psychiatry 49:163–174 Neary D, Snowden JS, Mann DMA, Bowen DM, Sims NR, Northen B, Yates PO, Davison AN (1986) Alzheimer’s disease: a correlative study. J Neurol Neurosurg Psychiatry 49:229–237 Regeur L, Jensen BJ, Pakkenberg H, Evans SM, Pakkenberg B (1994) No global neocortical nerve cell loss in brains from patients with senile dementia of Alzheimer’s type. Neurobiol Aging 15:347–352 Renkawek K, Basman GJCGM, de Jong WW (1994) Expression of small heat-shock protein hsp27 in reactive gliosis in Alzheimer disease and other types of dementia. Acta Neuropathol 87:511–519 Scheff SW, Price DA (1993) Synapse loss in the temporal lobe in Alzheimer’s disease. Ann Neurol 33:190–199 Scheff SW, DeKosky ST, Price DA (1990) A quantitative assessment of cortical synaptic density in Alzheimer’s disease. Neurobiol Aging 11:29–37 Shefer VF (1973) Absolute numbers of neurones and thickness of the cerebral cortex during ageing, senile and vascular dementia, and Pick’s and Alzheimer’s diseases. Neurosci Behav Physiol 6:319–324 Shefer VF (1977) Hippocampal pathology as a possible factor in the pathogenesis of senile dementias. Neurosci Behav Physiol 8:236–239 Smith MA, Taneda S, Richey PL, Miyata S, Yan S-D, Stern D, Sayre LM, Monnier VM, Perry G (1994) Advanced Maillard reaction end products are associated with Alzheimer disease pathology. Proc Natl Acad Sci (USA) 91:5710–5714 Smith MA, Kutty RK, Richey PL, Yan S-D, Stern D, Chader GJ, Wiggert B, Petersen RB, Perry G (1994) Heme Oxygenase-1 associated with the neurofibrillary pathology of Alzheimer’s disease. Am J Pathol 145:42–47 Smith MA, Rudnicka-Nawrot M, Richey PL, Praprotnik D, Mulvihill P, Miller CA, Sayre LM, Perry G (1995) Carbonyl-related posttranslational modification of neurofilament protein in the neurofibrillary pathology of Alzheimer’s disease. J Neurochem 64:2660–2666
427 Smith MA, Richey PL, Kutty RK, Wiggert B, Perry G (1995) Ultrastructural localization of Heme Oxygenase-1 to the neurofibrillary pathology of Alzheimer disease. Molec Chem Neuropathol 24:227–230 Sumpter PQ, Mann DMA, Davies CA, Yates PO, Snowden JS, Neary D (1986) An ultrastructural analysis of the effects of accumulation of neurofibrillary tangle in pyramidal cells of the cerebral cortex in Alzheimer’s disease. Neuropath Appl Neurobiol 12:305–319 Terry RD, De Teresa R, Hansen LA (1987) Neocortical cell counts in normal human adult ageing. Ann Neurol 21:530–539 Terry RD, Masliah E, Salmon P, Butters N, De Teresa R, Hill R, Hansen LA, Katzman R (1991) Physical basis of cognitive alterations in Alzheimer’s disease: synapse loss is the major correlate of cognitive impairment. Ann Neurol 30:572–580 Tomlinson BE, Henderson G (1976) Some quantitative cerebral findings in normal and demented old people. In Terry RD, Gerhon S (eds) Neurobiology of Aging. Raven Press, New York, 183–204 Wilcock GK, Esiri MM (1982) Plaques, tangles and dementia. J Neurol Sci 56:343–356 Yan S-D, Chen X, Schmidt A-M, Brett J, Godman G, Zou Y-S, Scott CW, Caputo C, Frappier T, Smith MA, Perry G, Yen S-H, Stern D (1994) Glycated tau protein in Alzheimer disease: A mechanism for induction of oxidant stress. Proc Natl Acad Sci 91:7787–7791 Yan SD, Yan SF, Chen X, Fu J, Chen M, Kuppusamy P, Smith MA, Perry G, Godman GC, Nawroth P, Zweier JL, Stern D (1995) Non-enzymatically glycated tau in Alzheimer’s disease induces neuronal oxidant stress resulting in cytokine gene expression and release of amyloid β-peptide. Nature Medicine 1:693–699 Zhan S-S, Beyreuther K, Schmitt HP (1993) Quantitative assessment of the synaptophysin immuno-reactivity of the cortical neuropil in various neurodegenerative disorders with dementia. Dementia 4:66–74
Pyramidal Nerve Cell Loss in Alzheimer’s Disease David M.A. Mann Department of Pathological Sciences, University of Manchester, Oxford Road, Manchester M13 9PT Loss of the large pyramidal cells of the association neocortex and hippocampus, along with plaques and tangles, is fundamental to the neuropathology of Alzheimer’s disease. The extent of Alzheimer-specific cell loss, relative to controls, is age-dependent with maximal losses in younger subjects though, because of the (additive) effects of ‘normal’ ageing on such cells, the absolute loss remains constant at all ages. The cause of the cell loss remains unknown but probably relates to neurofibrillary degeneration through a crowding out of organelles and a disruption of intracellular transport; oxidative stress may also contribute. The degree of clinical dementia correlates well with the extent of pyramidal cell loss. © 1996 Academic Press Limited
Key words: pyramidal cells, cerebral cortex, hippocampus, dementia, Alzheimer’s disease
Over the past decades, many studies have addressed the question of nerve cell loss from the cerebral cortex and hippocampus, yet a consensus of opinion has still to be reached even today.
Introduction THE DISTRIBUTION of the classical histopathological lesions of Alzheimer’s disease (AD), viz. the senile (or neuritic) plaques (SP) and the neurofibrillary tangles (NFT), principally involves the association areas of the neocortex and the archicortical regions of the hippocampus and amygdala. As such, it implies that AD is essentially a cortical dementia which is caused by damage to the superficial grey matter regions of the cerebral hemispheres. The key effector cells of the cerebral cortex and hippocampus are the pyramidal nerve cells that are located in layers III and V and in area CA1 of these regions, respectively. Because it has long been known that such cell types are particularly prone to NFT formation in AD and as they provide the major input and output pathways to the association cortex it is only natural to assume that damage to the cerebral cortex in AD should be reflected by changes in these in terms of cell number, or cell structure or both, and that such alterations should explain the cerebral atrophy of the disorder (Najlerahim & Bowen, 1989; Mann, 1991) and the progressive disturbances of memory and intellect that accompany the pathological process (Neary et al., 1986a).
The extent of pyramidal nerve cell loss in Alzheimer’s disease Early studies, based on only a few patients (Shefer, 1973; Colon, 1973), suggested that a loss of pyramidal nerve cells from the cerebral cortex might indeed occur in AD. With automatic and semi-automatic cell counting techniques firmer quantitative data have now emerged. In 1981, Terry et al. reported an overall loss of 22% of neurones from the superior temporal cortex and a 26% reduction in the mid-frontal gyrus in a group of 18 elderly patients with AD, aged between 70 and 90 years. The major proportion of cell loss was amongst those cells with a perikaryal size of more than 90 µm2 (assumed to be pyramidal cells) where reductions in number of 46% and 40% respectively were determined. Mountjoy et al. (1983) also reported substantial decreases in the number of large (presumably pyramidal) cells in frontal, cingulate and temporal gyri, but not in parietal or occipital regions, again in a group of 25 elderly subjects, such losses being particularly prominent in those patients under 80 years of age. Mann et al. (1985) estimated an average 60% loss
Correspondence to: D. M. A. Mann © 1996 Academic Press Limited 1055-8330/96/040423 1 5 $25.00/0
423
424 of pyramidal cells from layers III and V of the temporal cortex in 32 patients aged between 48 and 92 years of age, compared to 48% in those over this age. In a later study (Neary et al., 1986b) of 17 younger patients (aged 52–69 years) examined at cerebral biopsy, Mann reported an average loss of 58% of pyramidal cells again from the middle temporal gyrus. Hubbard & Anderson (1985) observed a 33% loss of large nerve cells from the temporal cortex and a 26% loss from the frontal cortex in 21 patients of 64–92 years. Likewise, these authors noted that greatest loss of such cells usually occurred in those patients under 80 years of age. Both in this latter study and that by Terry et al. (1981) the number of smaller neurones was unchanged. By contrast, both Tomlinson & Henderson (1976) and Braak & Braak (1986) found no evidence of overall nerve cell loss in the pre- and post-central gyrus, and temporal and frontal cortex in AD. However, in both these studies different size categories of neurones were not considered and it remained possible that some loss of the larger pyramidal cells might indeed have occurred, this being ‘masked’ by a lack of change in the number of smaller neurones. Finally, in 11 patients, all over 79 years of age, Regeur et al. (1994) were also unable to detect any overall loss of nerve cells from the neocortex. While differences in the quantitative procedures used may have contributed to some of this variation in findings, clear principles nonetheless emerge. Firstly, it seems likely that in AD nerve cell loss from the neocortex is confined essentially to the larger pyramidal cells; indeed this is the very nerve cell type most prone to neurofibrillary degeneration. Secondly, the extent of pyramidal cell loss in AD is age-dependent; younger patients (i.e. those under 80 years of age) show a greater loss of cells, compared to non-dements of that age, than do older subjects (i.e. those over 80 years) where little or no additional cell loss may occur. These data equate with observations of cerebral atrophy and brain weight (Terry et al., 1981; Hubbard & Anderson, 1981, 1985; Mann et al., 1985; Hansen et al., 1988; De La Monte, 1989; Mann, 1991) showing that major changes in such features usually occur only in younger patients; in older subjects with AD brain weight is little reduced and often unchanged, compared to non-dements, and cerebral atrophy, if present, is confined to the temporal lobe. Hence, because of the increasing fall-out of larger nerve cells from the cerebral cortex with ageing (Mann et al., 1985) proportionately more of these neurones will be lost in younger patients with AD, due to the process of AD itself, than will be in older subjects where most of the required cell loss will have already
D.M.A. Mann
occurred due to ageing alone with only little additional loss due to AD being superimposed. In both younger and older subjects however the total cell loss remains the same. The question of nerve cell loss from the hippocampus in AD is less controversial. Several studies have examined this, concentrating usually on areas CA1 or subiculum, and in all a severe loss of pyramidal cells occurs, irrespective of the age of the patient (Ball, 1977; Mann et al., 1985; Shefer, 1977; Anderson et al., 1983; Davies et al., 1992; Doebler et al., 1987).
Dendritic and axonal changes While perikaryal loss is perhaps the most visible aspect of cellular degeneration in AD it remains possible that in those elderly cases where no obvious cell loss occurs, nerve cells may have suffered axonal and dendritic changes which compromise their effectiveness, and produce an atrophy of the cortex, while retaining the actual cell body. Indeed, both quantitative electron microscopic (Davies et al., 1987, DeKosky & Scheff, 1990; Scheff et al., 1990; Scheff & Price, 1993) and numerous immunohistochemical (Hamos et al., 1989; Masliah et al., 1991; Terry et al., 1991; Honer et al., 1992; Lippa et al., 1992; Zhan et al., 1993; Brun et al., 1995; Heinonen et al., 1995) studies have all shown that considerable loss of synapses occurs at all ages in AD. Further, the extent of this synapse loss may outweigh the actual loss of neuronal perikarya (Davies et al., 1987) though this latter observation may ‘simply’ reflect the preferential loss of the larger pyramidal cells which will have more synaptic contacts than the smaller neighbouring neurones.
Mechanisms underlying nerve cell death Because the accumulation of NFT is the most visible cytological change in pyramidal neurones in AD it is tempting to assume that this reflects directly the neurodegenerative process that affects such cells and ultimately leads to their demise. Indeed, even a cursory microscopical observation suggests that such cells are hardly likely to be normally functioning cells, especially when the amount of tangle within them is high. Furthermore, the presence of similar tangle material lying apparently freely within the neuropil (as extracellular tangle or ghost tangle) marks the location of a once functional neurone. Nonetheless, it remains possible that nerve cells might die in AD through routes
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other than that involving NFT formation and accumulation and that, despite its conspicuousness within neurones, the NFT may simply represent an ‘innocent’ cellular marker of a more insidious neurotoxic process. Certainly, studies of the amount of ribonucleic acid within nerve cells or the size of the nucleolus (both being measures of protein synthetic capacity) show these to be reduced in tangled cells compared with their non-tangled neighbours (Dayan & Ball, 1973; Mann & Yates, 1981; Mann et al., 1981a, 1981b; Doebler et al., 1987). At ultrastructural level (Sumpter et al., 1986) decreases in the amount of rough endoplasmic reticulum, the number of ribosomes and the surface area of mitochondria occur in line with an increasing tangle mass. Such data clearly indicate an ongoing failure of metabolism in such cells. However, it is possible that these changes represent a gradual failure to supply (and therefore a requirement to produce) proteins to the parts of the neurone where they are needed because of the collapse of the microtubular network of the cell consequent to the phosphorylation of tau and its incorporation into NFT. Furthermore NFT, and the abnormal tau especially, become increasingly resistant to proteolysis because of participation in Maillard-type condensation reactions which result in an extensive crosslinking of the fibrils (Smith et al., 1994a, 1995a; Yan et al., 1994, 1995). Such reactions may impose oxidative stress upon affected cells; the presence of ‘heat-shock’ proteins like HSP27 (Renkawek et al., 1993), HSP70 (Hamos et al., 1991) and haem-oxygenase-1 (HSP32) (Smith et al., 1994b, 1995b) would accord with such a stress mechanism. In a recent study (Cras et al., 1995) the total number of intracellular tangle bearing cells, extracellular tangles and non-tangled cells in the hippocampus in AD matched the total number of unaffected neurones in control subjects. Hence, alternative mechanisms of nerve cell loss, not involving NFT formation, may be unlikely.
Relationship to plaques and β amyloid Although it has been widely proposed that amyloid β protein is neurotoxic, and even perhaps responsible for inducing intracellular changes that result in NFT formation (e.g. tau hyperphosphorylation) and accumulation, possibly involving oxidative stress (Behl et al., 1994), it is quite clear that the extent of nerve cell loss or even the density of neurofibrillary fails to correlate with the number of neuritic plaques (Neary
et al., 1986b). However, levels of precursor protein for β amyloid (amyloid precursor protein, APP) are reduced in AD, in line with the remaining number of functional neurones (Francis et al., 1993), emphasizing the principal neuronal localization of this particular protein.
Relationship to clinical dementia Although the early studies of Blessed and Roth (Blessed et al., 1968) suggested an overall correlation between neuritic plaque density and the degree of dementia in a cohort of elderly demented and nondemented subjects, later studies (Neary et al., 1986b; Terry et al., 1991; Arriagada et al., 1992) have failed to substantiate this only within individuals suffering from AD. Indices of NFT formation correlate better than plaques (Wilcock & Esiri, 1982; Terry et al., 1991; Arriagada et al., 1992; Neary et al., 1986b; McKee et al., 1991) yet here again the correlation is generally still weak. Since the numbers of SP and NFT continually change during the course of the illness with old plaques and tangles being removed and replaced by newer ones (Hyman et al., 1993; Mann et al., 1986) such lack of correlation should be anticipated. Much better correlations have been achieved between the degree of dementia and reductions in the number of surviving pyramidal cells (Neary et al., 1986b) or synapses (Terry et al., 1991; DeKosky & Scheff, 1990; Davies et al., 1987). This should perhaps be expected since not only do these latter measures represent more accurately an accumulative tissue deficit over the course of the illness, but also because they are key markers of the integrative capabilities of the neocortex. As if to emphasize the importance of an intact complement of pyramidal neurones and their synapses to the proper functioning of the cerebral cortex, similar losses of neurones have been reported in other cortical dementias such as Down’s syndrome (Mann et al., 1985, 1987) or frontal lobe dementia (Mann et al., 1993). Neocortical synapse loss is also seen in frontal lobe dementia (Brun et al., 1995) and, as in AD, a reduction in cortical APP levels accompanies the loss of pyramidal cells (Francis et al., 1993).
Conclusion A loss of large pyramidal cells of layers III and V of the cerebral cortex, and those of areas CA1 and subiculum of the hippocampus, is a fundamental aspect of
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the neuropathology of AD. Death of these nerve cell types is probably mediated through neurofibrillary degeneration though the precise mechanism underlying neurotoxicity is still uncertain. The loss of pyramidal cells and their connections is likely to form the critical pathological substrate of the dementia of AD, and also that of other cortical dementias like Down’s syndrome and frontal lobe dementia.
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