Competition between distinct ApoE alleles and mCRP for the endothelial receptor CD31 differentially regulates neurovascular inflammation and Alzheimer’s disease pathology

Abstract

BACKGROUND C-reactive protein (CRP) in peripheral inflammation is associated with increased Alzheimer’s disease (AD) risk in Apolipoprotein E4 (ApoE4), but not ApoE3 or E2, humans. It remains unknown whether peripheral monomeric CRP (mCRP) induces AD pathogenesis through some receptor of blood-facing endothelia in the brain in an ApoE genotype dependent fashion. METHODS We used human samples, ApoE knock-in and deficient mouse models, and primary brain endothelia. Different ApoE mice were intraperitoneally (i.p.) injected with mCRP. The characterizations by immunostaining, proximity ligation assay (PLA) and siRNA were conducted to identify the receptor for mCRP. Brain microvessel and endothelia were isolated for RNA sequencing to explore the molecular pathway. RESULTS We demonstrate that CD31 (PECAM-1), a blood-facing endothelial receptor in brain, is a competitive target of both mCRP and ApoE protein. ApoE2 competes more strongly with mCRP for CD31 than ApoE4 does, and expressing ApoE4 or knocking out ApoE gene results in higher levels of mCRP-CD31 binding, leading to a decrease of CD31 expression but an increase in CD31 phosphorylation, along with greater cerebrovascular damage and AD pathology. This competitive binding mediates differential endothelial molecular responses depending on ApoE genotype, increasing cerebrovascular inflammation and mitochondria impairment in ApoE4 mice, while inducing vasculogenesis and protective changes in the presence of ApoE2. CONCLUSIONS Our study reveals a novel and dynamic endothelial ApoE-mCRP-CD31 pathway for AD pathogenesis during chronic inflammation and provides some insight into the opposing ApoE4-neurodegenerative and ApoE2-neuroprotective effects in AD. Clinical Perspective WHAT IS NEW? CD31 is a competitive target of both mCRP and ApoE in brain endothelia in an ApoE-allele dependent pattern mCRP increases CD31 phosphorylation in the brain endothelia and damages cerebrovasculature in ApoE4 carriers and AD brains mCRP expression results in neuroprotective or neurodegenerative pathway activation in an ApoE-dependent manner WHAT ARE THE CLINICAL IMPLICATIONS? Although ApoE4 is a major genetic risk factor of AD, some ApoE4 carriers do not develop AD by the age of 90. Elderly people often experience peripheral inflammatory attacks and develop chronic low-grade inflammation, which results in the formation and release of mCRP. Because CRP is routine clinical laboratory test, clinicians can use blood CRP level to predict AD risk in ApoE4 carriers. Evidence of Apoe4 genotype and chronic low-grade inflammation stages marked by elevated CRP levels should be targeted in personalized treatment and clinical trials for AD.