Isolated senile plaque cores in Alzheimer's disease and Down's syndrome show differences in morphology

A History of Senile Plaques: From Alzheimer to Amyloid Imaging

Senile plaques have been studied in postmortem brains for more than 120 years and the resultant knowledge has not only helped us understand the etiology and pathogenesis of Alzheimer disease (AD), but has also pointed to possible modes of prevention and treatment. Within the last 15 years, it has become possible to image plaques in living subjects. This is arguably the single greatest advance in AD research since the identification of the Aβ peptide as the major plaque constituent. The limitations and potentialities of amyloid imaging are still not completely clear but are perhaps best glimpsed through the perspective gained from the accumulated postmortem histological studies. The basic morphological classification of plaques into neuritic, cored and diffuse has been supplemented by sophisticated immunohistochemical and biochemical analyses and increasingly detailed mapping of plaque brain distribution. Changes in plaque classification and staging have in turn contributed to changes in the definition and diagnostic criteria for AD. All of this information continues to be tested by clinicopathological correlations and it is through the insights thereby gained that we will best be able to employ the powerful tool of amyloid imaging.

Postmortem Neocortical 3H-PiB Binding and Levels of Unmodified and Pyroglutamate Aβ in Down Syndrome and Sporadic Alzheimer's Disease

Individuals with Down syndrome (DS) have a genetic predisposition for amyloid-β (Aβ) overproduction and earlier onset of Aβ deposits compared to patients with sporadic late-onset Alzheimer’s disease (AD). Positron emission tomography (PET) with Pittsburgh Compound-B (PiB) detects fibrillar Aβ pathology in living people with DS and AD, but its relationship with heterogeneous Aβ forms aggregated within amyloid deposits is not well understood. We performed quantitative in vitro³H-PiB binding assays and enzyme-linked immunosorbent assays of fibrillar (insoluble) unmodified Aβ40 and Aβ42 forms and N-terminus truncated and pyroglutamate-modified AβNpE3-40 and AβNpE3-42 forms in postmortem frontal cortex and precuneus samples from 18 DS cases aged 43–63 years and 17 late-onset AD cases aged 62–99 years. Both diagnostic groups had frequent neocortical neuritic plaques, while the DS group had more severe vascular amyloid pathology (cerebral amyloid angiopathy, CAA). Compared to the AD group, the DS group had higher levels of Aβ40 and AβNpE3-40, while the two groups did not differ by Aβ42 and AβNpE3-42 levels. This resulted in lower ratios of Aβ42/Aβ40 and AβNpE3-42/AβNpE3-40 in the DS group compared to the AD group. Correlations of Aβ42/Aβ40 and AβNpE3-42/AβNpE3-40 ratios with CAA severity were strong in DS cases and weak in AD cases. Pyroglutamate-modified Aβ levels were lower than unmodified Aβ levels in both diagnostic groups, but within group proportions of both pyroglutamate-modified Aβ forms relative to both unmodified Aβ forms were lower in the DS group but not in the AD group. The two diagnostic groups did not differ by ³H-PiB binding levels. These results demonstrate that compared to late-onset AD cases, adult DS individuals with similar severity of neocortical neuritic plaques and greater CAA pathology have a preponderance of both pyroglutamate-modified AβNpE3-40 and unmodified Aβ40 forms. Despite the distinct molecular profile of Aβ forms and greater vascular amyloidosis in DS cases, cortical ³H-PiB binding does not distinguish between diagnostic groups that are at an advanced level of amyloid plaque pathology. This underscores the need for the development of CAA-selective PET radiopharmaceuticals to detect and track the progression of cerebral vascular amyloid deposits in relation to Aβ plaques in individuals with DS.

Using mouse models to understand Alzheimer's disease mechanisms in the context of trisomy of chromosome 21

People who have Down syndrome are at significantly elevated risk of developing early onset Alzheimer's disease that causes dementia (AD-DS). Here we review recent progress in modeling the development of AD-DS in mouse models. These studies provide insight into mechanisms underlying Alzheimer's disease and generate new clinical research questions. In addition, they suggest potential new targets for disease prevention therapies.

Neuropathological correlates of amyloid PET imaging in Down syndrome

Down syndrome (DS) results in an overproduction of amyloid-β (Aβ) peptide associated with early onset of Alzheimer's disease (AD). DS cases have Aβ deposits detectable histologically as young as 12-30 years of age, primarily in the form of diffuse plaques, the type of early amyloid pathology also seen at pre-clinical (i.e., pathological aging) and prodromal stages of sporadic late onset AD. In DS subjects aged >40 years, levels of cortical Aβ deposition are similar to those observed in late onset AD and in addition to diffuse plaques involve cored plaques associated with dystrophic neurites (neuritic plaques), which are of neuropathological diagnostic significance in AD. The purpose of this review is to summarize and discuss findings from amyloid PET imaging studies of DS in reference to postmortem amyloid-based neuropathology. PET neuroimaging applied to subjects with DS has the potential to (a) track the natural progression of brain pathology, including the earliest stages of amyloid accumulation, and (b) determine whether amyloid PET biomarkers predict the onset of dementia. In addition, the question that is still incompletely understood and relevant to both applications is the ability of amyloid PET to detect Aβ deposits in their earliest form.

A genetic cause of Alzheimer disease: mechanistic insights from Down syndrome

Down syndrome, which arises in individuals carrying an extra copy of chromosome 21, is associated with a greatly increased risk of early-onset Alzheimer disease. It is thought that this risk is conferred by the presence of three copies of the gene encoding amyloid precursor protein (APP)--an Alzheimer disease risk factor--although the possession of extra copies of other chromosome 21 genes may also play a part. Further study of the mechanisms underlying the development of Alzheimer disease in people with Down syndrome could provide insights into the mechanisms that cause dementia in the general population.

Personality and behaviour changes mark the early stages of Alzheimer's disease in adults with Down's syndrome: findings from a prospective population-based study

BACKGROUND

Research based on retrospective reports by carers suggests that the presentation of dementia in people with Down's syndrome may differ from that typical of Alzheimer's disease (AD) in the general population, with the earliest changes tending to be in personality or behaviour rather than in memory. This is the first long-term prospective study to test the hypothesis that such changes, which are more typical of dementia of frontal type (DFT) in the general population, mark the preclinical stage of AD in DS.

METHODS

A previously identified population sample of older people with DS, first assessed in 1994 and followed-up 18 months later, were reassessed after a further 5 years. This study focuses on the 55 individuals who took part in the second follow-up. Dementia diagnosis was made using the modified CAMDEX informant interview and neuropsychological assessment was undertaken using the CAMCOG. Progression in clinical presentation was examined and degree of cognitive decline over time (on the CAMCOG and derived measures of executive function (EF) and memory) was compared across groups based on diagnosis and age: AD, DFT, personality/behaviour changes insufficient for a diagnosis of DFT (PBC), no diagnosis <50 years and no diagnosis 50 + years.

RESULTS

Progression was observed from early changes in personality and behaviour to an increase in characteristics associated with frontal lobe dysfunction and/or a deterioration in memory, prior to the development of full AD. Individuals who met criteria for DFT were significantly more likely to progress to a diagnosis of AD over the following 5 years than those who did not and those with PBC were significantly more likely to progress to a more severe diagnosis (DFT or AD) than those without. In the 5 years prior to diagnosis, participants with PBC and DFT had shown a degree of global cognitive decline intermediate between those with no dementia and those with AD. Both these groups had shown a significant decline in EF but not in memory, while the AD group had shown significant decline on both measures, with a significantly greater degree of decline in memory. Older participants without informant reported changes showed a more generalised pattern of decline.

CONCLUSIONS

These findings confirm that the early presentation of AD in DS is characterized by prominent personality and behaviour changes, associated with executive dysfunction, providing support for the notion that the functions of the frontal lobes may be compromised early in the course of the disease in this population. This has important implications for the diagnosis, treatment and management of dementia in people with DS.

Imaging linear birefringence and dichroism in cerebral amyloid pathologies

New advances in polarized light microscopy were used to image Congo red-stained cerebral amyloidosis in sharp relief. The rotating-polarizer method was used to separate the optical effects of transmission, linear birefringence, extinction, linear dichroism, and orientation of the electric dipole transition moments and to display them as false-color maps. These effects are typically convolved in an ordinary polarized light microscope. In this way, we show that the amyloid deposits in Alzheimer's disease plaques contain structurally disordered centers, providing clues to mechanisms of crystallization of amyloid in vivo. Comparisons are made with plaques from tissues of subjects having Down's syndrome and a prion disease. In plaques characteristic of each disease, the Congo red molecules are oriented radially. The optical orientation in amyloid deposited in blood vessels from subjects having cerebral amyloid angiopathy was 90 degrees out of phase from that in the plaques, suggesting that the fibrils run tangentially with respect to the circumference of the blood vessels. Our result supports an early model in which Congo red molecules are aligned along the long fiber axis and is in contrast to the most recent binding models that are based on computation. This investigation illustrates that the latest methods for the optical analysis of heterogeneous substances are useful for in situ study of amyloid.

Population-based study of the prevalence and presentation of dementia in adults with Down's syndrome

BACKGROUND

The reported prevalence rates of dementia in people with Down's syndrome have varied considerably across studies. The aim of this study was to investigate the extent of clinical change with age using an established diagnostic instrument in an unbiased, population-based sample of older people with Down's syndrome.

METHOD

Changes in memory, personality, general mental functioning and daily living skills were assessed using a modified version of the informant interview of the Cambridge Examination for Mental Disorders of the Elderly (CAMDEX).

RESULTS

Age-specific prevalence rates of dementia varied according to the diagnostic criteria used. Using CAMDEX criteria for Alzheimer's disease, prevalence rates increased from 3.4 to 10.3 to 40% in the 30-39, 40-49 and 50-59 age group, respectively.

CONCLUSIONS

Overall, the age-related pattern of presentation and dementia diagnoses differs from that seen in the general elderly population. However, age-specific prevalence rates of Alzheimer's disease were similar but 30-40 years earlier in life.

Normal ageing in adults with Down's syndrome: a longitudinal study

The ubiquitous presence of the neuropathology of Alzheimer disease (AD) in individuals with Down's syndrome (DS) over 40 years of age suggests that this group of people will exhibit a high prevalence of dementia of the Alzheimer type (DAT) as they age. The present study indicates that there is a clear discrepancy between the presumed presence of AD neuropathology and the clinical expression of DAT among older people with DS. In the first 6 years of a longitudinal study, the present authors compared 91 adults (31-63 years of age) with DS and mild or moderate mental retardation to 64 adults (31-76 years of age) with other forms of mental retardation (MR) on yearly measures of mental status, short- and long-term memory, speeded psychomotor function, and visuospatial organization. The results indicated that, over repeated testing on the verbal long-term memory test, younger participants with DS showed small increases in their scores, while older participants with DS showed very slight decreases. Overall performance scores on this test and a speeded psychomotor task were poorer for both diagnostic groups in individuals aged 50 years and older. The magnitude and type of these selective changes in performance were consistent with performance profiles observed in older healthy adults without mental retardation on tests measuring similar cognitive functions. Only four out of the 91 people with DS in the present sample showed changes in functioning that have led to a diagnosis of possible DAT, and in these individuals, alternative causes of performance declines were concurrently present (e.g. thyroid dysfunction). These findings indicate that some age-associated changes in functioning are related to "normal' but probably precocious ageing among adults with DS. Furthermore, these findings suggest that adults with DS and mild or moderate mental retardation may be at lower risk for dementia during their fourth and fifth decades of life than previous studies have suggested.

Chemical and immunological heterogeneity of fibrillar amyloid in plaques of Alzheimer's disease and Down's syndrome brains revealed by confocal microscopy

Amyloid beta peptides (A beta) are deposited in the brains of Alzheimer's disease (AD) and elderly Down's syndrome (DS) patients in a variety of amyloid plaques. Among these are classical plaques composed of a spherical core and corona. Analyzing AD tissue sections single and double stained with anti-A beta antibodies and thioflavin S (thioS) by bright field, fluorescence, and confocal microscopy revealed that spherical plaque cores consist of a thioS-positive center and an anti-A beta antibody immunoreactive rim. This indicates that there is a fibrillar form of amyloid that is thioS positive but not immunoreactive with anti-A beta antibodies. In contrast, classical plaques in DS patients have irregular cores that are thioS positive as well as anti-A beta immunoreactive. In addition, a subset of plaques in both DS and AD patients have a distinct "fibrous" appearance when stained with thioS, but are amorphous when immunostained. These findings suggest that anti-A beta antibodies and thioS stain similar; as well as different forms of fibrillar amyloid. A beta may become thioS positive by interacting with one or more of its known molecular chaperons, and this may be important for the pathogenesis of AD, given that thioS-positive A beta deposits are associated with neuritic and synaptic damage.

Down's syndrome and the links with Alzheimer's disease

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Assessment of cognitive decline associated with aging: a comparison of individuals with Down syndrome and other etiologies

Cognitive processes and their decline with aging were studied in individuals with Down Syndrome (DS) and individuals of comparable mental handicap without Down Syndrome (NonDS). The cognitive processes were measured by tests of Planning, Attention, Simultaneous, and Successive processing. The DS and NonDS samples were divided into age groups of 26-40 years (DS = 23, NonDS = 23) and 41-60 years (DS = 8, NonDS = 18). Analyses of variance using factor scores demonstrated articulation to be significantly poorer in the DS sample at and above 40 years. Specifically, the tests that showed the interaction effects between DS/NonDS and the two age groups were Number Finding, Expressive Attention, and Speech Rate. When the cutoff age was raised to 50 years, an additional Attention and Planning task (Receptive Attention and Matching Numbers) also showed the interaction effect. These tests hold the promise for diagnosing early signs of dementia of Alzheimer type. Implications for rehabilitation are described.

Differences in beta-amyloid (beta/A4) deposition in human patients with Down's syndrome and sporadic Alzheimer's disease

The density of diffuse, primitive, classic and compact beta-amyloid (beta/A4) deposits was estimated in the hippocampus and adjacent gyri in human patients with Down's syndrome (DS) and sporadic Alzheimer's disease (AD). The objective of the study was to determine whether there were differences in beta/A4 deposition in DS and sporadic AD and whether these differences could be attributed to overexpression of the amyloid precursor gene (APP) in DS. Total beta/A4 deposit density was greater in DS than AD in all brain regions studied but the DS/AD density ratios varied between brain regions. In the majority of brain regions, the ratio of primitive to diffuse beta/A4 deposits was greater in DS but the ratio of classic to diffuse deposits was greater in AD. The data were consistent with the hypothesis that overexpression of the APP gene in DS may lead to increased beta/A4 deposition. However, local brain factors also appear to be important in beta/A4 deposition in DS. Overexpression of the APP gene may also be responsible for increased production of paired helical filaments (PHF) and result in enhanced formation of primitive beta/A4 deposits in DS. In addition, increased formation of classic deposits in AD suggests that factors necessary for the production of a compact amyloid core are enhanced in AD compared with DS.

Early senile plaques in Down's syndrome brains show a close relationship with cell bodies of neurons

A sensitive methenamine silver/Nissl stain was used to study the morphology and relationship of pre-plaques (presumed early senile plaques) in Down's syndrome brains to glial nuclei, capillaries and neuronal perikarya. The larger pre-plaques (greater than 50 microns) usually encompassed all of these tissue elements. However, the smaller pre-plaques (less than or equal to 50 microns) were almost always found immediately adjacent to, or around the cell bodies of neurons (often with associated satellite cells), and they failed to show any consistent, close spatial relationship to the other tissue components. Thus we consider an early stage of pre-plaque formation to be the deposition of amyloid adjacent to the cell body of a morphologically normal neuron. Based on the study of transitional forms, we suggest that the amyloid progressively accumulates around the cell body until the enclosed neuron degenerates. How these pre-plaque lesions might eventually develop into the typical plaque structure is uncertain. Our observations support the theory of a neuronal origin for plaque amyloid.

Age-related changes in the density and morphology of plaques and neurofibrillary tangles in Down syndrome brain

Fifteen cases of Down syndrome between age 25-59 years were examined neuropathologically. A variety of histological methods were used to identify plaques and neurofibrillary tangles (NFT). All cases had some plaques or NFT, but their density was generally not high before the age of 40 years. Plaques and NFT tended to appear at about the same time although in somewhat different cortical areas. Changes appeared first in the dentate gyrus, subiculum, entorhinal and association neocortex. The stages in the evolution of plaque morphology were quantitated in the dentate gyrus. The earliest change was the extracellular accumulation of fibrillar material with the histological characteristics of amyloid. In the second stage there was an exuberant neuritic reaction with swollen processes that contained little or no paired helical filaments (PHF). Stage 1 and 2 plaques were seen predominantly between ages 25-38 years, and were not obviously associated with blood vessels or glial cells. In the third stage of plaque formation neurites appeared to degenerate, contained more PHF, and surrounded a compact core of amyloid. Stage 3 plaques were never very numerous, and were seen only between ages 48-55 years. Stage 4 plaques consisted of a cloud of silver-positive debris. They appeared to be the final stage and were the predominant morphological type in the dentate gyrus after age 48 years. Amyloid angiopathy was present only after age 48, and was a prominent finding in only three cases.

Alzheimer's disease and Down's syndrome

The neuropathology of Down's syndrome at middle age is compared with that of Alzheimer's disease at that age, through a review of the published literature and from the author's personal observations. The pathological changes of Down's syndrome at middle age, i.e. the form and distribution of senile plaques and neurofibrillary tangles, and the pattern of involvement (atrophy) of neuronal systems are qualitatively the same as those of Alzheimer's disease at that age. Quantitative differences do occur and these may relate to biological or sociological variations inherent to the two parent populations. It is concluded that, in pathological terms, patients with Down's syndrome at middle age do indeed have Alzheimer's disease. Some ways in which a study of patients with Down's syndrome can give insight into the nature and development of the pathological changes of Alzheimer's disease are put forward and discussed.

The pathological association between Down syndrome and Alzheimer disease

The neuropathology of Down syndrome (DS) at middle age is compared with that of Alzheimer disease (AD) at that age, through a review of the published literature and from the author's personal observations on brains from a series of patients of different ages with DS. It is noted that the pathological changes of DS at middle age (i.e. the form and distribution of senile plaques and neurofibrillary tangles, the pattern of involvement (atrophy) of neuronal systems) are qualitatively the same as those of AD at that age, though quantitative differences do occur and these may relate to biological or sociological variations inherent to the two parent populations. It is concluded that in pathological terms patients with DS at middle age do indeed have AD. Some ways in which a study of patients with DS can give insight into the nature and development of the pathological changes of AD are put forward and discussed.

Molecular genetics of human chromosome 21

Chromosome 21 is the smallest autosome, comprising only about 1.9% of human DNA, but represents one of the most intensively studied regions of the genome. Much of the interest in chromosome 21 can be attributed to its association with Down's syndrome, a genetic disorder that afflicts one in every 700 to 1000 newborns. Although only 17 genes have been assigned to chromosome 21, a very large number of cloned DNA segments of unknown function have been isolated and regionally mapped. The majority of these segments detect restriction fragment length polymorphisms (RFLPs) and therefore represent useful genetic markers. Continued molecular genetic investigation of chromosome 21 will be central to elucidating molecular events leading to meiotic non-disjunction and consequent trisomy, the contribution of specific genes to the pathology of Down's syndrome, and the possible role of chromosome 21 in Alzheimer's disease and other as yet unmapped genetic defects.