E. Masliah

Quantitative synaptic alterations in the human neocortex during normal aging

We quantified the synaptic population density in the frontal cortex of 25 individuals without dementia 16 to 98 years old, using sections double-immunolabeled for β/A4 amyloid and for synaptophysin, and found a significant inverse correlation between the presynaptic terminal (PT) counts and age (r = −0.7, p < 0.001). Individuals older than 60 years had an average 20% decrease in PT density compared with individuals younger than 60 years. There were no significant correlations between the age and the number of (β/A4 amyloid-positive plaques or between synaptic density and the number of amyloid plaques. Further analysis of the digitized serial optical images showed focal areas of synapse loss and distended synaptophysin-containing boutons in the mature plaques of the normal aged cases. However, we found no microscopic changes in the synaptic content inside and outside the diffuse plaques. We suggest that a loss of synaptic input in the neocortex is an age-dependent factor that contributes to the overall synaptic loss in Alzheimers disease, but that this might be largely independent of the (β/A4-amyloid deposition.

Genetic association of the low-density lipoprotein receptor-related protein gene (LRP), and apolipoprotein E receptor, with late-onset Alzheimer's disease

The presence of the APOE ϵ4 allele encoding apolipoprotein E4 (apoE4) is the major genetic risk factor for late-onset Alzheimers disease (AD). However, the molecular and cellular mechanisms by which APOE ϵ4 renders AD risk are unclear. In this report, we present genetic evidence that an apoE receptor, LRP, may be associated with the expression of late-onset AD. Using a biallelic genetic marker in exon 3 LRP, late-onset AD cases markedly differed from the control subjects in the distribution of LRP genotypes, and this difference was highly accentuated among AD cases with positive family history of senile dementia. Furthermore, the numbers of neuritic plaques were significantly altered as a consequence of different LRP genotypes in postmortem AD cases. Taken together, our results implicate the pathophysiology of LRP in the expression of late-onset AD.

Selective neuronal vulnerability in HIV encephalitis.

Recent studies of human immunodeficiency virus type 1 (HIV-1) encephalitis have shown that in addition to well established white matter damage, the neocortex shows thinning, loss of large neurons and dendritic damage. In order to identify neuronal populations affected in HIV encephalitis and to determine how neuronal damage relates to the severity of HIV infection within the nervous system, we quantified parvalbumin (PV+) and neurofilament (NF+) immunoreactive neurons in the frontal cortex and hippocampus. We found that in the neocortex, the density of NF+ and PV+ neurons was independent of severity of HIV encephalitis, and therefore changes in these neuronal subsets did not account for previously reported neuronal loss. However, neuritic processes of PV+ neurons were fragmented, atrophic and in some cases distended. In contrast to the frontal cortex, there was a trend toward decreased density of PV+ neurons in the hippocampus which only reached significance in the CA3 layer where there was a 50–90% decrease in PV+ neurons. This decrease was closely correlated with the severity of HI V encephalitis. Double-label immunocytochemical analysis confirmed neuritic damage to intemeurons. These results suggest that HIV encephalitis differentially involves specific subpopulations of neurons. Since direct HIV infection of neuronal cells was not detected, damage to PV+ cells and fibers may be indirectly mediated by cytokines released by HIV-infected microglia.

Defective insulin signaling pathway and increased glycogen synthase kinase-3 activity in the brain of diabetic mice: parallels with Alzheimer's disease and correction by insulin.

We have evaluated the effect of peripheral insulin deficiency on brain insulin pathway activity in a mouse model of type 1 diabetes, the parallels with Alzheimers disease (AD), and the effect of treatment with insulin. Nine weeks of insulin‐deficient diabetes significantly impaired the learning capacity of mice, significantly reduced insulin‐degrading enzyme protein expression, and significantly reduced phosphorylation of the insulin‐receptor and AKT. Phosphorylation of glycogen synthase kinase‐3 (GSK3) was also significantly decreased, indicating increased GSK3 activity. This evidence of reduced insulin signaling was associated with a concomitant increase in tau phosphorylation and amyloid β protein levels. Changes in phosphorylation levels of insulin receptor, GSK3, and tau were not observed in the brain of db/db mice, a model of type 2 diabetes, after a similar duration (8 weeks) of diabetes. Treatment with insulin from onset of diabetes partially restored the phosphorylation of insulin receptor and of GSK3, partially reduced the level of phosphorylated tau in the brain, and partially improved learning ability in insulin‐deficient diabetic mice. Our data indicate that mice with systemic insulin deficiency display evidence of reduced insulin signaling pathway activity in the brain that is associated with biochemical and behavioral features of AD and that it can be corrected by insulin treatment.

Three-Dimensional Analysis of the Relationship Between Synaptic Pathology and Neuropil Threads in Alzheimer Disease

Recent studies have shown that the Alzheimer disease (AD) neocortex is characterized by a loss of large neurons, the presence of dilated terminal axons, widespread loss of synapses, and a disruption of the dendritic cytoskeleton which is manifested as Tau immunoreactivc threads. In the present study we have investigated the relationship between synaptic and dendritic abnormalities in the neocortex of Alzheimer patients and examined the extent to which these structural alterations correlate with the severity of cognitive impairment in AD. Quantitative ncuroanatomical data were obtained from immunofluorescence- labeled specimens using a laser-scanning confocal microscope, computer-assisted image processing and serial section reconstruction techniques. We found that the AD cases showed a 34% loss in the number of prcsynaptic terminals per 100 square (sq) fim, many of which showed structural abnormalities. The AD neuropil had an average of 10 ± 7 dendritic threads per 1,000 sq fim, with the average thread measuring 2 sq nm. Severe AD cases had thicker threads compared with mild to moderate AD cases. Three-dimensional analysis showed clustering of synapses around threads, as well as presynaptic boutons apposcd to dendritic neuropil threads. Statistical analysis showed that the strongest correlation was between synapse density and Blessed score of cognitive impairment. Thread counts did not correlate with either but were correlated with tangle counts. Stepwise multiple regression analysis showed that tangle counts, but not threads, strengthened the correlation between Blessed score and synapses. We conclude that synaptic damage may precede dendritic thread and tangle formation, and that threads do not necessarily induce synaptic pathology. Instead, dendrite sprouting in the denervated regions could be associated with increased accumulation of cytoskeletal proteins observed in the dendritic threads.

Immunoelectron microscopic study of synaptic pathology in Alzheimer's disease*

SummaryAlzheimers disease (AD) is characterized by an extensive loss of neurons and synapses in the neocortex which correlates strongly with psychometric tests of dementia. To characterize the ultrastructural changes in presynaptic terminals in AD, we studied biopsy material from the frontal cortex. We also examined, at the ultrastructural level, abnormal neurites scattered in the AD neuropil and in the plaque region using sections from autopsy material immunolabeled with anti-synaptophysin. We found that, regardless of amyloid deposits, some presynaptic terminals were distended and contained swollen vesicles and dense bodies. These altered synaptic organelles were similar to those found in dystrophic neurites. The latter structures displayed synaptophysin immunoreactivity, mostly localized to outer membranes of synaptic vesicles and dense bodies. The present study supports the hypothesis of progressive synaptic pathology in AD neocortex and favors the notion that the dystrophic process originates from presynaptic terminals.

The decline in synapses and cholinergic activity is asynchronous in Alzheimer's disease.

Objective: To determine the timing of cholinergic loss and reduction of synapses in AD. Background: Decrements in neocortical synapses and cholinergic function occur in AD and correlate with cognitive decline. However, how early in the disease process these changes appear remains unclear. Methods: An autopsy series of 89 demented patients with pathologically confirmed AD (National Institute on Aging and Consortium to Establish a Registry for Alzheimer’s Disease criteria) and 18 normal control subjects (NC). The AD cases were stratified according to their last Mini-Mental State Examination (MMSE) score prior to death as mild (MMSE = 20; n = 14), moderate (MMSE = 10 to 19; n = 20), severe (MMSE = 1 to 9; n = 29), and very severe (MMSE = 0; n = 26). Midfrontal (MF) synapse density was assessed by dot-immunobinding assay for synaptophysin (Syn), and MF choline acetyltransferase (ChAT) activity was determined using standard protocols. Results: Compared with those in NC, neither Syn nor ChAT was appreciably reduced in patients with mild AD at death. Decline of ChAT was significant only in AD patients who died in the late stages of the disease and was maximal in those who had more severely deteriorated. In contrast, decline of Syn was significant and almost maximal in patients in intermediate or moderate stages. Consequently, the last MMSE score prior to death correlated more strongly with ChAT than Syn when the AD cohort included more impaired patients (r = 0.46 versus 0.40). The reverse occurred when only less impaired patients (MMSE = 10) were included in the analyses (r = 0.28 versus 0.64). There was only a modest correlation between Syn and ChAT activity. Conclusions: The results imply an asynchronous pattern of decline of synapses and cholinergic activity, with Syn loss preceding ChAT decrements. However, neither MF synapse reduction nor cholinergic dysfunction appears to be an early event in AD.

A neuropathological subset of Alzheimer's disease with concomitant Lewy body disease and spongiform change

SummaryThe neuropathological heterogeneity of Alzheimers disease (AD) is increasingly recognized. Diffuse Lewy body disease, for example, most frequently occurs in cases fulfilling histopathological criteria for AD, and these patients usually present with dementia rather than parkinsonism. We report five cases of concomitant AD and diffuse Lewy body disease with still another coexistent neuropathological feature: localized and stereotyped spongiform change in the neuropil. This spongiform change was most striking in the superior and inferior temporal, entorhinal, and insular cortex and the amygdala and was virtually indistinguishable from that seen in Creutzfeldt-Jakob disease. Electron microscopic study on one case revealed membrane-containing vacuoles in close association with neuritic plaques and plaired helical filament-filled processes. Immunocytochemistry using antibodies to prion proteins (PrPsc or PrP27–30) failed to label plaque or vascular amyloid in the five cases. Four primates inoculated with brain tissue from one case have not evidenced neurological disease in the 3 years since the transmission experiment. We conclude that these cases represent a neuropathological subset of AD with relatively widespread Lewy bodies and a localized spongiform change, predominantly involving the medial temporal region. Despite the light and electron microscopic commonality with Creutzfeldt-Jakob disease, there is no clear evidence that these cases represent a form of transmissible spongiform encephalopathy.

Hippocampal connectivity and Alzheimer's dementia: Effects of synapse loss and tangle frequency in a two‐component model

Our prior research on patients with Alzheimers disease (AD) found a high correspondence between premortem dementia and accumulation of neurofibrillary tangles (NFTs) with concurrent loss of synapse density in several brain regions. In the present study, we examined these same clinicopathologic relationships in the context of seven subregions of the hippocampal formation using a sample of 16 AD patients who had been administered three well-known mental status tests antemortem. We found NFT counts to be most strongly correlated with degree of dementia when they were seen in CA1, the subiculum, and CA4; NFTs in these subregions appeared significantly clustered on factor analysis. Synapse loss was most strongly correlated with dementia when it occurred in the molecular layers of the dentate fasciculus and stratum lacunosum, CA2/3, and CA4; synapse loss in these subregions appeared significantly clustered on factor analysis. In general, these results were compatible with a two-component model of hippocampal connectivity and function in the context of AD. The first component consists of subregions preceding CA1 in a hypothesized input-processing sequence intrinsic to the hippocampus that summates neuronal excitation and that influences cognition primarily through synapse density. The second component consists of an “output module,” mainly CA1 and the subiculum, that receives the processed signal, passes it on to extrahippocampal cortical and subcortical targets, and affects cognition primarily by NFT accumulation in output neurons. A “net pathology” score combining standardized z-scores for synapse density and NFTs was significantly correlated with all three mental status measures in all hippocampal subregions except the entorhinal cortex, and stepwise regressions on these data found net pathology in CA4 to be the most independent significant predictor of premortem dementia.

Immunoreactivity of CD45, a protein phosphotyrosine phosphatase, in Alzheimer's disease.

SummaryBoth protein kinases and phosphoprotein phosphatases are important components of signal transduction systems in cells. Recent studies in Alzheimers disease (AD) have shown abnormal protein phosphorylation in the cortex suggesting an alteration in these enzymes. In the present study, an antibody against CD45 was used to analyze the status of this protein phosphotyrosine phosphatase in AD. We studied and quantified the immunohistochemical and immunochemical distribution of this integral membrane protein in control and AD brain. We found that anti-CD45 immunostained the great majority of microglia, both resting and activated. These cells were Ricinus communis agglutinin I positive and glial fibrillary acidic protein and neurofilament negative. The AD frontal cortex showed a 35% (P<0.01) increase in the number of anti-CD45 immunoreactive microglia as compared with controls. These results were consistent with the immunoblot quantification of CD45 immunoreactivity following native gel electrophoresis. In AD, 30% of the CD45-immunostained microglia were clustered in the neuritic plaques (about six per plaque) while the remaining 70% were scattered in the neuropil. The AD hippocampus showed an increase in CD45-immunoreactive microglia in the molecular layer of the dentte gyrus. At the ultrastructural level, CD45 immunoreactivity was localized exclusively to the plasma membrane of the microglia. The presence of the anti-CD45 immunoreactivity in microglia suggests the possibility that they may require the presence of CD45 as a cell surface receptor which may regulate cell function through modulation of intracellular signaling.