Linares, Cynthia

AbstractBackgroundHomozygosity for the rare APOE3‐Christchurch (APOE3Ch) variant, encoding for apoE3‐R136S (apoE3‐Ch), was linked to resistance against an aggressive form of familial Alzheimer’s disease (AD). Carrying two copies of APOE3Ch was sufficient to delay autosomal AD onset by 30 years. This remarkable protective effect makes it a strong candidate for uncovering new therapies against AD. Thus, we aim to explore the protective mechanisms of APOE3Ch against AD onset, to inform therapy.MethodsWe used both amyloid mouse models and human induced‐pluripotent stem cells (iPSC) models to address this aim. We examined whether astrocytic expression of apoE3‐Ch through an AAV‐mediated approach can mitigate AD‐related pathology and toxicity in early (3.5 months old) and late (8 months old) amyloid pathology in 5xFAD mice. We also used isogenic iPSC lines carrying the APOE3Ch variant that were generated from APOE3 parental lines using CRISPR/Cas9 technology. Biochemical and biophysical properties of both recombinant and native apoE3‐Ch lipoprotein particles secreted by iPSC‐derived astrocytes have been investigated using different techniques including heparin affinity chromatography and size‐exclusion chromatography.ResultsWe investigated the impact of astrocytic apoE3‐Ch on amyloid pathology and toxicity in 3.5‐month‐old and 8‐month‐old 5xFAD mice. We confirmed successful AAV‐mediated overexpression of apoE3 and apoE3‐Ch in astrocytes. We found apoE3‐Ch specific changes in protein levels and solubility. Astrocytic expression of apoE3‐Ch reduced soluble Aβ oligomer levels and ameliorated ER stress response compared to apoE3 in the 8‐month‐old 5xFAD mice. Furthermore, our findings showed altered biochemical properties of HEK cell‐derived apoE3‐Ch protein, including decreased heparin binding, altered receptor binding, and enhanced lipid accepting capacity. We also characterized APOE3Ch iPSC‐derived cell models, including iPSC‐derived astrocytes, neurons, and cerebral organoids, and found differences between APOE3 and APOE3Ch at the functional level in these iPSC models.ConclusionsThe proposed work and the developed methods to study apoE3‐Ch promises to provide new insights into the possible roles of this rare, mutated protein in protecting against AD, offering new therapeutic avenues for AD treatment.

ObjectiveRecent evidence supports a link between increased TDP‐43 burden and the presence of an APOE4 gene allele in Alzheimer's disease (AD); however, it is difficult to conclude the direct effect of APOE on TDP‐43 pathology due to the presence of mixed AD pathologies. The goal of this study is to address how APOE isoforms impact TDP‐43 pathology and related neurodegeneration in the absence of typical AD pathologies.MethodsWe overexpressed human TDP‐43 via viral transduction in humanized APOE2, APOE3, APOE4 mice, and murine Apoe‐knockout (Apoe‐KO) mice. Behavior tests were performed across ages. Animals were harvested at 11 months of age and TDP‐43 overexpression‐related neurodegeneration and gliosis were assessed. To further address the human relevance, we analyzed the association of APOE with TDP‐43 pathology in 160 postmortem brains from autopsy‐confirmed amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with motor neuron disease (FTLD‐MND) in the Mayo Clinic Brain Bank.ResultsWe found that TDP‐43 overexpression induced motor function deficits, neuronal loss, and gliosis in the motor cortex, especially in APOE2 mice, with much milder or absent effects in APOE3, APOE4, or Apoe‐KO mice. In the motor cortex of the ALS and FTLD‐MND postmortem human brains, we found that the APOE2 allele was associated with more severe TDP‐43‐positive dystrophic neurites.InterpretationOur data suggest a genotype‐specific effect of APOE on TDP‐43 proteinopathy and neurodegeneration in the absence of AD pathology, with the strongest association seen with APOE2. ANN NEUROL 2023;93:830–843

TREM2 is exclusively expressed by microglia in the brain and is strongly linked to the risk for Alzheimer’s disease (AD). As microglial responses modulated by TREM2 are central to AD pathogenesis, enhancing TREM2 signaling has been explored as an AD therapeutic strategy. However, the effective therapeutic window targeting TREM2 is unclear. Here, by using microglia-specific inducible mouse models overexpressing human wild-type TREM2 (TREM2-WT) or R47H risk variant (TREM2-R47H), we show that TREM2-WT expression reduces amyloid deposition and neuritic dystrophy only during the early amyloid seeding stage, whereas TREM2-R47H exacerbates amyloid burden during the middle amyloid rapid growth stage. Single-cell RNA sequencing reveals suppressed disease-associated microglia (DAM) signature and reduced DAM population upon TREM2-WT expression in the early stage, whereas upregulated antigen presentation pathway is detected with TREM2-R47H expression in the middle stage. Together, our findings highlight the dynamic effects of TREM2 in modulating AD pathogenesis and emphasize the beneficial effect of enhancing TREM2 function in the early stage of AD development.

A rare APOE variant APOE3 -Jac reduces self-aggregation and promotes healthy brain aging.

AbstractIntroductionCerebrovascular pathologies including cerebral amyloid angiopathy (CAA) and blood‐brain barrier (BBB) dysregulation are prominent features in the majority of Alzheimer's disease (AD) cases.MethodsWe performed neuropathologic and biochemical studies on a large, neuropathologically confirmed human AD cohort (N = 469). Amounts of endothelial tight junction proteins claudin‐5 (CLDN5) and occludin (OCLN), and major AD‐related molecules (amyloid beta [Aβ40], Aβ42, tau, p‐tau, and apolipoprotein E) in the temporal cortex were assessed by ELISA.ResultsHigher levels of soluble tau, insoluble p‐tau, and apolipoprotein E (apoE) were independently correlated with lower levels of endothelial tight junction proteins CLDN5 and OCLN in AD brains. Although high Aβ40 levels, APOE ε4, and male sex were predominantly associated with exacerbated CAA severity, those factors did not influence tight junction protein levels.DiscussionRefining the molecular mechanisms connecting tau, Aβ, and apoE with cerebrovascular pathologies is critical for greater understanding of AD pathogenesis and establishing effective therapeutic interventions for the disease.

APOE4 exacerbates α-synuclein pathology in a synucleinopathy mouse model, and APOE4 genotype associates with increased α-synuclein pathology in humans.