Astrocytic expression of the Alzheimer's disease risk allele, ApoEε4, potentiates neuronal tau pathology in multiple preclinical models.

ApoEε4 is a major genetic risk factor for Alzheimer's disease (AD), a disease hallmarked by extracellular amyloid-beta (Aß) plaques and intracellular neurofibrillary tangles (NFTs). The presence of the ApoEε4 allele is associated with increased Aß deposition and a role for ApoEε4 in the potentiation of tau pathology has recently emerged. This study focused on comparing the effects of adeno-associated virus (AAV)-mediated overexpression of the three predominant human ApoE isoforms within astrocytes. The isoform-specific effects of human ApoE were evaluated within in vitro models of tau pathology within neuron/astrocyte co-cultures, as well as in a transgenic tau mouse model. Tau aggregation, accumulation, and phosphorylation were measured to determine if the three isoforms of human ApoE had differential effects on tau. Astrocytic overexpression of the human ApoEε4 allele increased phosphorylation and misfolding of overexpressed neuronal tau in multiple models, including the aggregation and accumulation of added tau oligomers, in an isoform-specific manner. The ability of ApoEε4 to increase tau aggregation could be inhibited by an ApoEε4-specific antibody. This study indicates that astrocytic expression of ApoEε4 can potentiate tau aggregation and phosphorylation within neurons and supports a gain of toxic function hypothesis for the effect of hApoEε4 on tau.

Apolipoprotein E4 genotype compromises brain exosome production.

In addition to being the greatest genetic risk factor for Alzheimer's disease, expression of the ɛ4 allele of apolipoprotein E can lead to cognitive decline during ageing that is independent of Alzheimer's amyloid-ß and tau pathology. In human post-mortem tissue and mouse models humanized for apolipoprotein E, we examined the impact of apolipoprotein E4 expression on brain exosomes, vesicles that are produced within and secreted from late-endocytic multivesicular bodies. Compared to humans or mice homozygous for the risk-neutral ɛ3 allele we show that the ɛ4 allele, whether homozygous or heterozygous with an ɛ3 allele, drives lower exosome levels in the brain extracellular space. In mice, we show that the apolipoprotein E4-driven change in brain exosome levels is age-dependent: while not present at age 6 months, it is detectable at 12 months of age. Expression levels of the exosome pathway regulators tumor susceptibility gene 101 (TSG101) and Ras-related protein Rab35 (RAB35) were found to be reduced in the brain at the protein and mRNA levels, arguing that apolipoprotein E4 genotype leads to a downregulation of exosome biosynthesis and release. Compromised exosome production is likely to have adverse effects, including diminishing a cell's ability to eliminate materials from the endosomal-lysosomal system. This reduction in brain exosome levels in 12-month-old apolipoprotein E4 mice occurs earlier than our previously reported brain endosomal pathway changes, arguing that an apolipoprotein E4-driven failure in exosome production plays a primary role in endosomal and lysosomal deficits that occur in apolipoprotein E4 mouse and human brains. Disruption of these interdependent endosomal-exosomal-lysosomal systems in apolipoprotein E4-expressing individuals may contribute to amyloidogenic amyloid-ß precursor protein processing, compromise trophic signalling and synaptic function, and interfere with a neuron's ability to degrade material, all of which are events that lead to neuronal vulnerability and higher risk of Alzheimer's disease development. Together, these data suggest that exosome pathway dysfunction is a previously unappreciated component of the brain pathologies that occur as a result of apolipoprotein E4 expression.

Proteomic analysis of mice expressing human ApoE demonstrates no differences in global protein solubility between APOE 3 and APOE 4 young mice.

Apolipoprotein E (ApoE) is a major lipid carrier protein. In humans, ApoE is expressed in three polymorphic isoforms, which are encoded by three different alleles APOE2, APOE3, and APOE4. In the brains of Alzheimer's disease (AD) patients, each one of these three allelic isoforms is found in several "isoelectric" protein isoforms (qPI), i.e. protein isoforms resulting from PTMs altering the net charge (q) of the polypeptide. AD is a complex disease in which multiple causes and several risk factors affect the onset and disease outcome. A major risk factor for AD is ApoE4; therefore, it is important to characterize the different ApoE qPIs. We have implemented a detergent-based method for isolation and quantitation of protein isoforms, and we found differences in the solubility of protein isoforms depending on the type of solvent used. In this manuscript, we describe these methods and applied them to young human-ApoE targeted replacement mice. Our results indicate that there are no significant differences in the hippocampus proteome of these mice as a function of the APOE genotype.

Domains of apoE4 required for the biogenesis of apoE-containing HDL.

INTRODUCTION. We have studied the functions of truncated apoE4 forms in vitro and in vivo in order to identify the domains of apoE4 required for the biogenesis of apoE-containing high-density lipoprotein (HDL). RESULTS. We have found that apoE4-185, -202, -229, or -259 could promote ATP-binding cassette transporter A1 (ABCA1)-dependent cholesterol efflux in vitro, although less efficiently than Full-length apoE4, and had diminished capacity to activate lecithin cholesterol acyltransferase (LCAT). Formation of HDL in vivo was assessed by various methods following gene transfer in apolipoprotein A-I(-/-) × apoE(-/-) mice. Fast protein liquid chromatography of plasma showed that the truncated apoE forms, except apoE4-185, generated an apoE-containing HDL peak. Two-dimensional gel electrophoresis of plasma and electron microscopy showed that truncated apoE forms generated distinct HDL subpopulations and formed discoidal HDL particles which could be converted to spherical by co-administration of truncated apoE4-202 and LCAT. CONCLUSION. Overall, the in-vivo and in-vitro data are consistent and indicate that apoE4-185 is the shortest truncated form that supports formation of discoidal apoE4-containing HDL particles.

Effect of truncated-ApoE4 overexpression on tau phosphorylation in cultured N2a cells.

The carboxyl-terminal amino acids 272-299-truncated apoE4 (delta272-299) is the main fragments of apoE4 hydrolysate in neurons. The effects of truncated-ApoE4 (delta272-299) overexpression on tau phosphorylation in cultured N2a cells were investigated. The truncated-apoE4 (delta272-299) cDNA was subcloned into pEGFP-c3 to form recombinant pEGFP-T-apoE4. pEGFP-c3, pEGFP-T-apoE4 and pEGFP-apoE4 were transfected into N2a cells respectively by lipofectamine 2000 method. After 24--48 h, tau phosphorylation was detected by Western blot assay and glycogen synthase kinase-3 (GSK-3) activity by using GSK-3 activity assay. The results showed that the overexpression of both full length-apoE4 and truncated apoE4 fragments in N2a cells induced a dramatic increase in phosphorylation of tau at Ser202 sites and the activation of GSK-3 as compared with untransfected cells, most significantly in the cells transfected with pEGFP-T-apoE4 (P < 0.05). It was concluded that in vitro overexpression of truncated-ApoE4 (delta272-299) can result in tau hyperphosphorylation in N2a cells by activating GSK-3, suggesting truncated-ApoE4 (delta272-299) might contribute the pathogenesis of Alzheimer disease.