Astrocytic APOE4 removal confers cerebrovascular protection despite increased cerebral amyloid angiopathy.

BACKGROUND: Alzheimer Disease (AD) and cerebral amyloid angiopathy (CAA) are both characterized by amyloid-ß (Aß) accumulation in the brain, although Aß deposits mostly in the brain parenchyma in AD and in the cerebrovasculature in CAA. The presence of CAA can exacerbate clinical outcomes of AD patients by promoting spontaneous intracerebral hemorrhage and ischemia leading to CAA-associated cognitive decline. Genetically, AD and CAA share the ε4 allele of the apolipoprotein E (APOE) gene as the strongest genetic risk factor. Although tremendous efforts have focused on uncovering the role of APOE4 on parenchymal plaque pathogenesis in AD, mechanistic studies investigating the role of APOE4 on CAA are still lacking. Here, we addressed whether abolishing APOE4 generated by astrocytes, the major producers of APOE, is sufficient to ameliorate CAA and CAA-associated vessel damage. METHODS: We generated transgenic mice that deposited both CAA and plaques in which APOE4 expression can be selectively suppressed in astrocytes. At 2-months-of-age, a timepoint preceding CAA and plaque formation, APOE4 was removed from astrocytes of 5XFAD APOE4 knock-in mice. Mice were assessed at 10-months-of-age for Aß plaque and CAA pathology, gliosis, and vascular integrity. RESULTS: Reducing the levels of APOE4 in astrocytes shifted the deposition of fibrillar Aß from the brain parenchyma to the cerebrovasculature. However, despite increased CAA, astrocytic APOE4 removal reduced overall Aß-mediated gliosis and also led to increased cerebrovascular integrity and function in vessels containing CAA. CONCLUSION: In a mouse model of CAA, the reduction of  APOE4 derived specifically from astrocytes, despite increased fibrillar Aß deposition in the vasculature, is sufficient to reduce Aß-mediated gliosis and cerebrovascular dysfunction.

Murine roseolovirus does not accelerate amyloid-ß pathology and human roseoloviruses are not over-represented in Alzheimer disease brains.

BACKGROUND: The role of viral infection in Alzheimer Disease (AD) pathogenesis is an area of great interest in recent years. Several studies have suggested an association between the human roseoloviruses, HHV-6 and HHV-7, and AD. Amyloid-ß (Aß) plaques are a hallmark neuropathological finding of AD and were recently proposed to have an antimicrobial function in response to infection. Identifying a causative and mechanistic role of human roseoloviruses in AD has been confounded by limitations in performing in vivo studies. Recent -omics based approaches have demonstrated conflicting associations between human roseoloviruses and AD. Murine roseolovirus (MRV) is a natural murine pathogen that is highly-related to the human roseoloviruses, providing an opportunity to perform well-controlled studies of the impact of roseolovirus on Aß deposition. METHODS: We utilized the 5XFAD mouse model to test whether MRV induces Aß deposition in vivo. We also evaluated viral load and neuropathogenesis of MRV infection. To evaluate Aß interaction with MRV, we performed electron microscopy. RNA-sequencing of a cohort of AD brains compared to control was used to investigate the association between human roseolovirus and AD. RESULTS: We found that 5XFAD mice were susceptible to MRV infection and developed neuroinflammation. Moreover, we demonstrated that Aß interacts with viral particles in vitro and, subsequent to this interaction, can disrupt infection. Despite this, neither peripheral nor brain infection with MRV increased or accelerated Aß plaque formation. Moreover, -omics based approaches have demonstrated conflicting associations between human roseoloviruses and AD. Our RNA-sequencing analysis of a cohort of AD brains compared to controls did not show an association between roseolovirus infection and AD. CONCLUSION: Although MRV does infect the brain and cause transient neuroinflammation, our data do not support a role for murine or human roseoloviruses in the development of Aß plaque formation and AD.

Overexpressing low-density lipoprotein receptor reduces tau-associated neurodegeneration in relation to apoE-linked mechanisms.

APOE is the strongest genetic risk factor for late-onset Alzheimer's disease. ApoE exacerbates tau-associated neurodegeneration by driving microglial activation. However, how apoE regulates microglial activation and whether targeting apoE is therapeutically beneficial in tauopathy is unclear. Here, we show that overexpressing an apoE metabolic receptor, LDLR (low-density lipoprotein receptor), in P301S tauopathy mice markedly reduces brain apoE and ameliorates tau pathology and neurodegeneration. LDLR overexpression (OX) in microglia cell-autonomously downregulates microglial Apoe expression and is associated with suppressed microglial activation as in apoE-deficient microglia. ApoE deficiency and LDLR OX strongly drive microglial immunometabolism toward enhanced catabolism over anabolism, whereas LDLR-overexpressing microglia also uniquely upregulate specific ion channels and neurotransmitter receptors upon activation. ApoE-deficient and LDLR-overexpressing mice harbor enlarged pools of oligodendrocyte progenitor cells (OPCs) and show greater preservation of myelin integrity under neurodegenerative conditions. They also show less reactive astrocyte activation in the setting of tauopathy.

Activated microglia mitigate Aß-associated tau seeding and spreading.

In Alzheimer's disease (AD) models, AD risk variants in the microglial-expressed TREM2 gene decrease Aß plaque-associated microgliosis and increase neuritic dystrophy as well as plaque-associated seeding and spreading of tau aggregates. Whether this Aß-enhanced tau seeding/spreading is due to loss of microglial function or a toxic gain of function in TREM2-deficient microglia is unclear. Depletion of microglia in mice with established brain amyloid has no effect on amyloid but results in less spine and neuronal loss. Microglial repopulation in aged mice improved cognitive and neuronal deficits. In the context of AD pathology, we asked whether microglial removal and repopulation decreased Aß-driven tau seeding and spreading. We show that both TREM2KO and microglial ablation dramatically enhance tau seeding and spreading around plaques. Interestingly, although repopulated microglia clustered around plaques, they had a reduction in disease-associated microglia (DAM) gene expression and elevated tau seeding/spreading. Together, these data suggest that TREM2-dependent activation of the DAM phenotype is essential in delaying Aß-induced pathological tau propagation.

APOE immunotherapy reduces cerebral amyloid angiopathy and amyloid plaques while improving cerebrovascular function.

The ε4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD) and greatly influences the development of amyloid-ß (Aß) pathology. Our current study investigated the potential therapeutic effects of the anti-human APOE antibody HAE-4, which selectively recognizes human APOE that is co-deposited with Aß in cerebral amyloid angiopathy (CAA) and parenchymal amyloid pathology. In addition, we tested whether HAE-4 provoked brain hemorrhages, a component of amyloid-related imaging abnormalities (ARIA). ARIA is an adverse effect secondary to treatment with anti-Aß antibodies that can occur in blood vessels with CAA. We used 5XFAD mice expressing human APOE4 +/+ (5XE4) that have prominent CAA and parenchymal plaque pathology to assess the efficacy of HAE-4 compared to an Aß antibody that removes parenchymal Aß but increases ARIA in humans. In chronically treated 5XE4 mice, HAE-4 reduced Aß deposition including CAA compared to a control antibody, whereas the anti-Aß antibody had no effect on CAA. Furthermore, the anti-Aß antibody exacerbated microhemorrhage severity, which highly correlated with reactive astrocytes surrounding CAA. In contrast, HAE-4 did not stimulate microhemorrhages and instead rescued CAA-induced cerebrovascular dysfunction in leptomeningeal arteries in vivo. HAE-4 not only reduced amyloid but also dampened reactive microglial, astrocytic, and proinflammatory-associated genes in the cortex. These results suggest that targeting APOE in the core of both CAA and plaques could ameliorate amyloid pathology while protecting cerebrovascular integrity and function.

Impact of TREM2R47H variant on tau pathology-induced gliosis and neurodegeneration.

Alzheimer's disease (AD) is characterized by plaques containing amyloid-ß (Aß) and neurofibrillary tangles composed of aggregated, hyperphosphorylated tau. Beyond tau and Aß, evidence suggests that microglia play an important role in AD pathogenesis. Rare variants in the microglia-expressed triggering receptor expressed on myeloid cells 2 (TREM2) gene increase AD risk 2- to 4-fold. It is likely that these TREM2 variants increase AD risk by decreasing the response of microglia to Aß and its local toxicity. However, neocortical Aß pathology occurs many years before neocortical tau pathology in AD. Thus, it will be important to understand the role of TREM2 in the context of tauopathy. We investigated the impact of the AD-associated TREM2 variant (R47H) on tau-mediated neuropathology in the PS19 mouse model of tauopathy. We assessed PS19 mice expressing human TREM2CV (common variant) or human TREM2R47H. PS19-TREM2R47H mice had significantly attenuated brain atrophy and synapse loss versus PS19-TREM2CV mice. Gene expression analyses and CD68 immunostaining revealed attenuated microglial reactivity in PS19-TREM2R47H versus PS19-TREM2CV mice. There was also a decrease in phagocytosis of postsynaptic elements by microglia expressing TREM2R47H in the PS19 mice and in human AD brains. These findings suggest that impaired TREM2 signaling reduces microglia-mediated neurodegeneration in the setting of tauopathy.

Targeting of nonlipidated, aggregated apoE with antibodies inhibits amyloid accumulation.

The apolipoprotein E E4 allele of the APOE gene is the strongest genetic factor for late-onset Alzheimer disease (LOAD). There is compelling evidence that apoE influences Alzheimer disease (AD) in large part by affecting amyloid ß (Aß) aggregation and clearance; however, the molecular mechanism underlying these findings remains largely unknown. Herein, we tested whether anti-human apoE antibodies can decrease Aß pathology in mice producing both human Aß and apoE4, and investigated the mechanism underlying these effects. We utilized APPPS1-21 mice crossed to apoE4-knockin mice expressing human apoE4 (APPPS1-21/APOE4). We discovered an anti-human apoE antibody, anti-human apoE 4 (HAE-4), that specifically recognizes human apoE4 and apoE3 and preferentially binds nonlipidated, aggregated apoE over the lipidated apoE found in circulation. HAE-4 also binds to apoE in amyloid plaques in unfixed brain sections and in living APPPS1-21/APOE4 mice. When delivered centrally or by peripheral injection, HAE-4 reduced Aß deposition in APPPS1-21/APOE4 mice. Using adeno-associated virus to express 2 different full-length anti-apoE antibodies in the brain, we found that HAE antibodies decreased amyloid accumulation, which was dependent on Fcγ receptor function. These data support the hypothesis that a primary mechanism for apoE-mediated plaque formation may be a result of apoE aggregation, as preferentially targeting apoE aggregates with therapeutic antibodies reduces Aß pathology and may represent a selective approach to treat AD.

Age-Dependent Effects of apoE Reduction Using Antisense Oligonucleotides in a Model of ß-amyloidosis.

The apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset Alzheimer disease. Previous studies suggest that reduction of apoE levels through genetic manipulation can reduce Aß pathology. However, it is not clear how reduction of apoE levels after birth would affect amyloid deposition. We utilize an antisense oligonucleotide (ASO) to reduce apoE expression in the brains of APP/PS1-21 mice homozygous for the APOE-ε4 or APOE-ε3 allele. ASO treatment starting after birth led to a significant decrease in Aß pathology when assessed at 4 months. Interestingly, ASO treatment starting at the onset of amyloid deposition led to an increase in Aß plaque size and a reduction in plaque-associated neuritic dystrophy with no change in overall plaque load. These results suggest that lowering apoE levels prior to plaque deposition can strongly affect the initiation of Aß pathology while lowering apoE after Aß seeding modulates plaque size and toxicity.