Modeling regional vulnerability to Alzheimer pathology

The Role of Apolipoprotein E ε4 in Early and Late Mild Cognitive Impairment

BACKGROUND

Apolipoprotein E (APOE) ε4 is highly associated with mild cognitive impairment (MCI). However, the specific influence of APOE ε4 status on tau pathology and cognitive decline in early MCI (EMCI) and late MCI (LMCI) is poorly understood. Our goal was to evaluate the association of APOE ε4 with cerebrospinal fluid (CSF) tau levels and cognition in EMCI and LMCI patients in the Alzheimer's Disease Neuroimaging Initiative database, and whether this association was mediated by amyloid-β (Aβ).

METHODS

Participants were 269 cognitively normal (CN), 262 EMCI, and 344 LMCI patients. They underwent CSF Aβ42 and tau detection, APOE ε4 genotyping, Mini-Mental State Examination, (MMSE), and Alzheimer's disease assessment scale (ADAS)-cog assessments. Linear regressions were used to examine the relation of APOE ε4 and CSF tau levels and cognitive scores in persons with and without Aβ deposition (Aβ+ and Aβ-).

RESULTS

The prevalence of APOE ε4 is higher in EMCI and LMCI than in CN (p < 0.001 for both), and in LMCI than in EMCI (p = 0.001). APOE ε4 allele was significantly higher in Aβ+ subjects than in Aβ- subjects (p < 0.001). Subjects who had a lower CSF Aβ42 level and were APOE ε4-positive experienced higher levels of CSF tau and cognitive scores in EMCI and/or LMCI.

CONCLUSIONS

An APOE ε4 allele is associated with increased CSF tau and worse cognition in both EMCI and LMCI, and this association may be mediated by Aβ. We conclude that APOE ε4 may be an important mediator of tau pathology and cognition in the early stages of AD.

What Is T+? A Gordian Knot of Tracers, Thresholds, and Topographies

In this review we examine, in the context of the amyloid, tau, and neurodegeneration framework, the available evidence and potential alternatives on how to establish tau positivity (T+) for multiple tau-imaging tracers in order to reach a consensus on normal and abnormal tau imaging values that can be universally implemented in clinical research and therapeutic trials.

Association of Hypercholesterolemia with Alzheimer's Disease Pathology and Cerebral Amyloid Angiopathy

BACKGROUND

Animal studies have shown that diet-induced hypercholesterolemia (HC) increases amyloid-β (Aβ) accumulation and accelerates Alzheimer's disease (AD) pathology. However, the association of HC with AD in human studies has not been consistently established.

OBJECTIVE

We aimed to investigate the relationship between HC and risk of AD neuropathology in a large national sample with autopsies.

METHODS

This study used neuropathological and clinical data from 3,508 subjects from the National Alzheimer's Coordinating Center (NACC) who underwent autopsies from 2005 to 2017. Demographic and clinical characteristics, as well as neuropathological outcomes were compared between subjects with and without HC. Associations between HC and AD neuropathology were examined by multivariate ordinal logistic regressions adjusting for potential confounders.

RESULTS

HC was not associated with any AD neuropathology in a model only adjusting for demographic variables. However, HC was significantly associated with higher CERAD neuritic and diffuse plaque burden, higher Braak stage, and more severe cerebral amyloid angiopathy when analyzed in a multivariate model controlling for comorbidities. Additional adjusting for cerebrovascular conditions did not diminish these associations. The association between HC and increased risk of neuritic plaques weakened but remained significant even after controlling for ApoE genotype.

CONCLUSION

This study suggested that HC was associated with increased severity of AD pathology, which could only be partially accounted for by ApoE genotype. The associations were not mediated by cerebrovascular conditions.

Cortical amyloid accumulation is associated with alterations of structural integrity in older people with subjective memory complaints

We determined the effect of cortical amyloid load using ¹⁸F-florbetapir PET on cognitive performance and gray matter structural integrity derived from MRI in 318 cognitively normally performing older people with subjective memory impairment from the INSIGHT-preAD cohort using multivariate partial least squares regression. Amyloid uptake was associated with reduced gray matter structural integrity in hippocampus, entorhinal and cingulate cortex, middle temporal gyrus, prefrontal cortex, and lentiform nucleus (p < 0.01, permutation test). Higher amyloid load was associated with poorer global cognitive performance, delayed recall and attention (p < 0.05), independently of its effects on gray matter connectivity. These findings agree with the assumption of a two-stage effect of amyloid on cognition, (1) an early direct effect in the preclinical stages of Alzheimer's disease and (2) a delayed effect mediated by downstream effects of amyloid accumulation, such as gray matter connectivity decline.

Association of APOE with tau-tangle pathology with and without β-amyloid

This study tested the hypothesis that the association of apolipoprotein E (APOE) with paired helical filament tau (PHF-tau) tangle pathology differs in brains with and without β-amyloid. Participants were 1056 autopsied individuals from 2 clinical-pathologic cohort studies of aging and Alzheimer's disease (AD), the Religious Orders Study, and the Rush Memory and Aging Project. Neuropathologic measures were obtained using immunohistochemistry targeting β-amyloid and PHF-tau tangles in 8 brain regions. Linear regression was used to compare the relation of APOE ε4 and ε2 to PHF-tau-tangle density in persons with β-amyloid relative to persons without β-amyloid. We found an interaction between APOE ε4 carriers and presence of β-amyloid (β = -0.968, p = 0.013) such that the association of APOE ε4 with PHF-tau tangles was much stronger in brains with β-amyloid. Stratified analysis shows that the association of APOE ε4 with PHF-tau tangles was considerably stronger among those with β-amyloid (β = 0.757, p = 1.1 × 10(-15)) compared to those without β-amyloid which was not significant (β = -0.201, p = 0.424). Separately, APOE ε2 was associated with fewer tangles in brains with β-amyloid (β = -0.425, p = 7.6 × 10(-4)) compared to those without β-amyloid which was not significant (β = -0.102, p = 0.506). Thus, the presence of APOE ε4 and ε2 alleles was not associated with PHF-tau tangles in the absence of β-amyloid. The data provide additional evidence that PHF-tau tangles in the absence of β-amyloid may reflect a pathologic process distinct from Alzheimer's disease.

FDG-PET and amyloid-PET imaging: the diverging paths

PURPOSE OF REVIEW

The availability of PET neuroimaging tools for the in-vivo assessment of metabolic dysfunction and amyloid burden in Alzheimer's disease has opened important methodological and practical issues in the diagnostic design and the conduct of new clinical trials. This review, addressing the different molecular information that the amyloid-PET and fluorodeoxyglucose-PET (FDG-PET) tools can provide, highlights their diverging paths in Alzheimer's disease and possible new perspectives in research and clinical applications.

RECENT FINDINGS

Senile plaques and neurofibrillary tangles are prominent neuropathological hallmarks in Alzheimer's disease and are considered to be targets for therapeutic intervention and biomarkers for diagnostic in-vivo imaging agents. Alzheimer's disease is a slowly progressing disorder, in which pathophysiological abnormalities, detectable in vivo by PET biomarkers, precede clinical symptoms by many years to decades. The unitary view of Alzheimer's disease as a sequential pathological pathway, with beta-amyloid (Aβ) as the only initial and causal event (the 'amyloid cascade hypothesis'), is likely to be progressively replaced by a more complex picture, also on the basis of recent PET imaging findings showing that neuronal injury biomarkers and tau pathology can be independent of β-amyloid deposition.

SUMMARY

The different molecular paths that PET in-vivo biomarkers can reveal in the timeframe of Alzheimer's disease progression reflect the events leading to deposition of Aβ and phosphorylated tau, neuronal injury and neurodegeneration, which can run in parallel instead of in a sequential manner. The amyloid and neuronal injury paths may diverge along the Alzheimer's disease cascade and bear separate relationships with Alzheimer's disease symptoms and clinical phenotypes. All these evidences are crucial for the diagnosis and the development of new drugs aimed at slowing or preventing dementia.

The temporospatial evolution of neuritic plaque-related and independent tauopathies: implications for dementia staging

Neuritic plaque (NP) formation can be dated in vivo. This analysis attempts to "date" the progression of neurofibrillary tangles (NFT) using the spatial distribution of NP as a reference. Autopsy data from 471 participants in the Honolulu-Asia Aging Study (HAAS) were combined into latent factor measures of NFT and NP counts. The variance in "early" and "late" NP pathology was used to estimate the spatial distribution of "early" and "late" NFT formation. A third latent factor representing "non-NP-related NFT" was also constructed. "Early" NP and "late" NP correlated significantly with objectively early and later cognitive performance, respectively. In contrast to our expectations, neocortical NFT correlated best with "early" NP pathology, while NFT in allocortical structures correlated best with "late" NP pathology. Therefore, the NP-related fraction of NFT appears to be co-localized spatially with NP. However, since the latter evolve corticofugally in time, this suggests that NP-related NFT do so as well. Corticotropic NFT formation must therefore be either unrelated to NP formation, a temporally distinct process, or both.

Cholinergic basal forebrain atrophy predicts amyloid burden in Alzheimer's disease

We compared accuracy of hippocampus and basal forebrain cholinergic system (BFCS) atrophy to predict cortical amyloid burden in 179 cognitively normal subjects (CN), 269 subjects with early stages of mild cognitive impairment (MCI), 136 subjects with late stages of MCI, and 86 subjects with Alzheimer's disease (AD) dementia retrieved from the Alzheimer's Disease Neuroimaging Initiative database. Hippocampus and BFCS volumes were determined from structural magnetic resonance imaging scans at 3 Tesla, and cortical amyloid load from AV45 (florbetapir) positron emission tomography scans. In receiver operating characteristics analyses, BFCS volume provided significantly more accurate classification into amyloid-negative and -positive categories than hippocampus volume. In contrast, hippocampus volume more accurately identified the diagnostic categories of AD, late and early MCI, and CN compared with whole and anterior BFCS volume, whereas posterior BFCS and hippocampus volumes yielded similar diagnostic accuracy. In logistic regression analysis, hippocampus and posterior BFCS volumes contributed significantly to discriminate MCI and AD from CN, but only BFCS volume predicted amyloid status. Our findings suggest that BFCS atrophy is more closely associated with cortical amyloid burden than hippocampus atrophy in predementia AD.

Alzheimer's disease patients not carrying the apolipoprotein E ε4 allele show more severe slowing of oscillatory brain activity

The objective of this study was to quantitatively assess the relationship between apolipoprotein (APOE) genotype and electroencephalographic oscillatory brain dynamics in Alzheimer's disease (AD) patients and control subjects and its regional distribution. We obtained resting-state electroencephalographs of 320 AD patients and 246 control subjects, categorized into APOE ε4 carriers and noncarriers. Peak frequency and relative power in 4 different frequency bands were calculated. We tested the associations between APOE genotype and relative power in 4 brain regions. Peak frequency was comparable in APOE ε4 carrying and noncarrying control subjects, but lower in APOE ε4 noncarrying AD patients. In control subjects, APOE ε4 carriers had a different regional distribution of alpha power than noncarriers. We found no APOE effect in beta, delta, and theta bands. In AD, APOE ε4 noncarriers had lower alpha and higher delta power than carriers. This difference was most pronounced in the parieto-occipital region. In the theta band, APOE ε4 noncarriers had a different regional distribution of power compared with carriers. In conclusion, the most pronounced effect of genotype was seen in AD patients, and APOE ε4 noncarriers showed slower activity, especially in parieto-occipital regions.

Estimating the temporal evolution of Alzheimer's disease pathology with autopsy data

The temporal growth of Alzheimer's disease (AD) neuropathology cannot be easily determined because autopsy data are available only after death. We combined autopsy data from 471 participants in the Honolulu-Asia Aging Study (HAAS) into latent factor measures of neurofibrillary tangle and neuritic plaque counts. These were associated with intercept and slope parameters from a latent growth curve (LGC) model of 9-year change in cognitive test performance in 3244 autopsied and non-autopsied HAAS participants. Change in cognition fully mediated the association between baseline cognitive performance and AD lesions counts. The mediation effect of cognitive change on both AD lesion models effectively dates them within the period of cognitive surveillance. Additional analyses could lead to an improved understanding of lesion propagation in AD.

Fortress brain

Neurodegenerative diseases are associated with neuronal inclusions, comprised of protein aggregates. In Alzheimer's Disease (AD) and Lewy Body Disease (LBD) such lesions are distributed in a hierarchical retrograde transynaptic spatial pattern. This implies a retrograde transynaptic temporal propagation as well. There can be few explanations for this other than infectious agents (prions and viruses). This suggests that AD and LBD (at least) may have infectious origins. Transynaptic infiltration of the CNS along cranial nerve or other major projections, by one or more infectious agents has important implications. The clinical syndrome and natural history of each neurodegenerative disorder will reflect its portal of entry. There may be a different neurodegenerative syndrome for each cranial nerve or other portal of entry, and not all may manifest as "dementia". Each syndrome may be associated with more than one pathological lesion. Each pathology may be associated with several clinical syndromes. Host-parasite interactions are species specific. This may explain the rarity of AD-like pathology in most other older mammals. Over evolutionary timescales, the human brain should be adapted to predation by neurotropic agents. Viewed from this perspective, the prion-like pro-inflammatory and pro-apoptotic properties of β-amyloid and other proteins may be adaptive, and anti-microbial. Reductions in synaptic density may slow the progress of invading pathogens, while perineuronal nets and other structures may guard the gates. This suggests a defense in depth of a structure, the brain, that is inherently vulnerable to invasion along its neural networks.