Michael C. Irizarry

Impaired synaptic plasticity and learning in aged amyloid precursor protein transgenic mice

We investigated synaptic communication and plasticity in hippocampal slices from mice overexpressing mutated 695-amino-acid human amyloid precursor protein (APP695SWE), which show behavioral and histopathological abnormalities simulating Alzheimers disease. Although aged APP transgenic mice exhibit normal fast synaptic transmission and short term plasticity, they are severely impaired in in-vitro and in-vivo long-term potentiation (LTP) in both the CA1 and dentate gyrus regions of the hippocampus. The LTP deficit was correlated with impaired performance in a spatial working memory task in aged transgenics. These deficits are accompanied by minimal or no loss of presynaptic or postsynaptic elementary structural elements in the hippocampus, suggesting that impairments in functional synaptic plasticity may underlie some of the cognitive deficits in these mice and, possibly, in Alzheimers patients.

APPsw Transgenic Mice Develop Age-related Aβ Deposits and Neuropil Abnormalities, but no Neuronal Loss in CA1

The recent availability of transgenic mouse models of Alzheimer disease has allowed direct in vivo assessment of the molecular and neuropathological effects of cerebral amyloid deposition. We examined 16-month-old Tg(HuAPP695. K670N-M671L)2576 mice expressing human APP K670N-M671L (APPSw), which have amyloid deposition and behavioral deficits by 11 months of age. Transgene expression is predominantly neuronal, and results in amyloid deposits, comparable to human senile plaques, at terminal zones of transgene positive neurons in cortical and limbic regions. Amyloid deposits were associated with prominent gliosis and neuritic dystrophy, without neuronal loss in CAI, loss of synaptophysin immunoreactivity in the hippocampal dentate gyrus, or loss of messenger RNA for neuronal synaptic, cytoskeletal, or metabolic proteins. We conclude that Aβ is not acutely neurotoxic, but can disrupt neuronal processes and provoke an inflammatory response.

Early Aβ accumulation and progressive synaptic loss, gliosis, and tangle formation in AD brain

Background: Pathologic changes in the Alzheimer disease (AD) brain occur in a hierarchical neuroanatomical pattern affecting cortical, subcortical, and limbic regions. Objective: To define the time course of pathologic and biochemical changes—amyloid deposition, amyloid β-peptide (Aβ) accumulation, neurofibrillary tangle (NFT) formation, synaptic loss, and gliosis—within the temporal association cortex of AD cases of varying disease duration, relative to control brains. Methods: Stereologic assessments of amyloid burden and tangle density as well as ELISA-based measurements of Aβ, synaptophysin, and glial fibrillary acidic protein (GFAP) were performed in the superior temporal sulcus from a cohort of 83 AD and 26 nondemented control brains. Results: Relative to control cases, AD brains were characterized by accumulation of NFT and amyloid plaques, increase of tris- and formic acid–extractable Aβ species, reduced levels of synaptophysin, and elevated levels of GFAP. In AD cases, the duration of dementia correlated with the degree of tangle formation, gliosis, and synaptic loss but not with any Aβ measures. Accumulation of Aβ, measured both neuropathologically and biochemically, was markedly increased in AD brains independent of disease duration, even in cases of short duration. Conclusions: These data support distinct processes in the initiation and progression of AD pathology within the temporal cortex: Deposition of Aβ reaches a “ceiling” early in the disease process, whereas NFT formation, synaptic loss, and gliosis continue throughout the course of the illness.

Nigral and cortical Lewy bodies and dystrophic nigral neurites in Parkinson's disease and cortical Lewy body disease contain alpha-synuclein immunoreactivity.

A mutation in the α-synuclein gene has recently been linked to some cases of familial Parkinsons disease (PD). We characterized the expression of this presynaptic protein in the midbrain, striatum, and temporal cortex of control, PD, and dementia with Lewy bodies (DLB) brain. Control brain showed punctate pericellular immunostaining. PD brain demonstrated α-synuclein immunoreactivity in nigral Lewy bodies, pale bodies and abnormal aeurites. Rare neuronal soma in PD brain were immunoreactive for α-synuclein. DLB cases demonstrated these findings as well as α-synuclein immunoreactivity in cortical Lewy bodies and CA2-3 neurites. These results suggest that, even in sporadic cases, there is an early and direct role for α-synuclein in the pathogenesis of PD and the neuropathologically related disorder DLB.

Apolipoprotein E facilitates neuritic and cerebrovascular plaque formation in an Alzheimer's disease model.

The ε4 allele of apolipoprotein E (ApoE) is an important genetic risk factor for Alzheimers disease (AD). Increasing evidence suggests that this association may be linked to the ability of ApoE to interact with the amyloid‐β (Aβ) peptide and influence its concentration and structure. To determine the effect of ApoE on Aβ and other AD pathology in vivo, we used APPsw transgenic mice and ApoE knockout (−/−) mice to generate APPsw animals that carried two (ApoE +/+), one (ApoE +/−), or no copies (ApoE −/−) of the normal mouse ApoE gene. At 12 months of age, Aβ deposition was present in the cortex and hippocampus and was also prominent within leptomeningeal and cortical blood vessels of all APPsw ApoE +/+ mice. Importantly, although Aβ deposition still occurred in APPsw ApoE −/− mice, no fibrillar Aβ deposits were detected in the brain parenchyma or cerebrovasculature. There was also no neuritic degeneration associated with Aβ deposition in the absence of ApoE. These data demonstrate that ApoE facilitates the formation of both neuritic and cerebrovascular plaques, which are pathological hallmarks of AD and cerebral amyloid angiopathy. Ann Neurol 2000;47:739–747

Plasma β-amyloid and white matter lesions in AD, MCI, and cerebral amyloid angiopathy

Background: Microvascular brain injury, typically measured by extent of white matter hyperintensity (WMH) on MRI, is an important contributor to cognitive impairment in the elderly. Recent studies suggest a role for circulating β-amyloid peptide in microvascular dysfunction and white matter disease. Methods: The authors performed a cross-sectional study of clinical, biochemical, and genetic factors associated with WMH in 54 subjects with Alzheimer disease (AD) or mild cognitive impairment (AD/MCI) and an independent group of 42 subjects with cerebral amyloid angiopathy (CAA). Extent of WMH was determined by computer-assisted volumetric measurement normalized to intracranial size (nWMH). Biochemical measurements included plasma concentrations of the 40- and 42-amino acid species of β-amyloid (Aβ40 and Aβ42) detected by specific enzyme-linked immunosorbent assays. Results: Plasma Aβ40 concentrations were associated with nWMH in both groups (correlation coefficient = 0.48 in AD/MCI, 0.42 in CAA, p ≤ 0.005). Plasma Aβ40 remained independently associated with nWMH after adjustment for potential confounders among age, hypertension, diabetes, homocysteine, creatinine, folate, vitamin B12, and APOE genotype. The presence of lacunar infarctions was also associated with increased Aβ40 in both groups. nWMH was greater in CAA (19.8 cm3) than AD (11.1 cm3) or MCI (10.0 cm3; p < 0.05 for both comparisons). Conclusions: Plasma β-amyloid 40 concentration is independently associated with extent of white matter hyperintensity in subjects with Alzheimer disease, mild cognitive impairment, or cerebral amyloid angiopathy. If confirmed in longitudinal studies, these data would suggest circulating β-amyloid peptide as a novel biomarker or risk factor for microvascular damage in these common diseases of the elderly.

β-Secretase Activity Increases with Aging in Human, Monkey, and Mouse Brain

Amyloid beta protein (A beta) accumulates in the brains of aging humans, amyloid precursor protein (APP) transgenic mouse lines, and rhesus monkeys. We tested the hypothesis that aging was associated with increased activity of the beta-site amyloid precursor protein cleaving enzyme (beta-secretase, BACE) in brain. We evaluated BACE activity, BACE protein, and formic acid-extractable A beta levels in cohorts of young (4 months old) and old (14 to 18 months old) nontransgenic mice (n = 16) and Tg2576 APP transgenic mice (n = 17), young (4.4 to 12.7 years old) and old (20.9 to 30.4 years old) rhesus monkeys (n = 17), and a wide age range (18 to 92 years old) of nondemented human brains (n = 25). Aging was associated with increased brain A beta levels in each cohort. Furthermore BACE activity increased significantly with age in mouse, monkey, and human brains, independent of brain region. BACE protein levels, however, were unchanged with age. BACE activity correlated with formic acid-extractable A beta levels in transgenic mouse, nontransgenic mouse, and human cortex, but not in monkey brain. These data suggest that an age-related increase of BACE activity contributes to the increased production and accumulation of brain A beta, and potentially predisposes to Alzheimers disease in humans.

Biomarkers of Alzheimer Disease in Plasma

SummaryPlasma and serum biochemical markers proposed for Alzheimer disease (AD) are based on pathophysiologic processes such as amyloid plaque formation [amyloid β-protein (Aβ), Aβ autoantibodies, platelet amyloid precursor protein (APP) isoforms], inflammation (cytokines), oxidative stress (vitamin E, isoprostanes), lipid metabolism (apolipoprotein E, 24S-hydroxycholesterol), and vascular disease [homocysteine, lipoprotein (a)]. Most proteins or metabolites evaluated in plasma or serum thus far are, at best, biological correlates of AD: levels are statistically different in AD versus controls in some cohorts, but they lack sensitivity or specificity for diagnosis or for tracking response to therapy. Approaches combining panels of existing biomarkers or surveying the range of proteins in plasma (proteomics) show promise for discovering biomarker profiles that are characteristic of AD, yet distinct from nondemented patients or patients with other forms of dementia.

Age-Related Amyloid β Deposition in Transgenic Mice Overexpressing Both Alzheimer Mutant Presenilin 1 and Amyloid β Precursor Protein Swedish Mutant Is Not Associated with Global Neuronal Loss

To analyze the relationship between the deposition of amyloid beta peptides (Abeta) and neuronal loss in transgenic models of Alzheimers disease (AD), we examined the frontal neocortex (Fc) and CA1 portion of hippocampus (CA1) in PSAPP mice doubly expressing AD-associated mutant presenilin 1 (PS1) and Swedish-type mutant beta amyloid precursor protein (APPsw) by morphometry of Abeta burden and neuronal counts. Deposition of Abeta was detected as early as 3 months of age in the Fc and CA1 of PSAPP mice and progressed to cover 28.3% of the superior frontal cortex and 18.4% of CA1 at 12 months: approximately 20- (Fc) and approximately 40- (CA1) fold greater deposition than in APPsw mice. There was no significant difference in neuronal counts in either CA1 or the frontal cortex between nontransgenic (non-tg), PS1 transgenic, APPsw, and PSAPP mice at 3 to 12 months of age. In the PSAPP mice, there was disorganization of the neuronal architecture by compact amyloid plaques, and the average number of neurons was 8 to 10% fewer than the other groups (NS, P > 0.10) in CA1 and 2 to 20% fewer in frontal cortex (NS, P = 0.31). There was no loss of total synaptophysin immunoreactivity in the Fc or dentate gyrus molecular layer of the 12-month-old PSAPP mice. Thus, although co-expression of mutant PS1 with Swedish mutant betaAPP leads to marked cortical and limbic Abeta deposition in an age-dependent manner, it does not result in the dramatic neuronal loss in hippocampus and association cortex characteristic of AD.

Demonstration by FRET of BACE interaction with the amyloid precursor protein at the cell surface and in early endosomes

Amyloid-β peptide, which accumulates in senile plaques in Alzheimers disease, is derived from the amyloid precursor protein (APP) by proteolytic processing. β-secretase (Asp2), which cleaves APP at the N-terminus of amyloid-β, has recently been identified to be the protease BACE. In the present study, we examined the subcellular localization of interactions between APP and BACE by using both double immunofluorescence and a fluorescence resonance energy transfer (FRET) approach. Cell surface APP and BACE, studied by using antibodies directed against their ectodomains in living H4 neuroglioma cells co-transfected with APP and BACE, showed exquisite co-localization and demonstrated a very close interaction by FRET analysis. The majority of cell surface APP and BACE were internalized after 15 minutes, but they remained strongly co-localized together in the early endosomal compartment, where FRET analysis demonstrated a continued close interaction. By contrast, at later timepoints, almost no co-localization or FRET was observed in lysosomal compartments. To determine whether the APP-BACE interaction on cell surface and endosomes contributed to amyloid-β synthesis, we labeled cell surface APP and demonstrated detectable levels of labeled amyloid-β within 30 minutes. APP-Swedish mutant protein enhanced amyloid-β synthesis from cell surface APP, consistent with the observation that it is a better BACE substrate than wild-type APP. Taken together, these data confirm a close APP-BACE interaction in early endosomes, and highlight the cell surface as an additional potential site of APP-BACE interaction.