Rescuing the Secretion and Lipidation Deficits of APOE4 Using HDL Mimetic Peptides in Primary Glial Cells

Abstract

Alzheimer’s disease (AD) is one of the most prevalent neurodegenerative diseases with age and genetics significantly contributing to its development. Genome-wide association studies (GWAS) have identified apolipoprotein E4 (APOE4) as the greatest genetic risk factor for developing sporadic AD; however, the exact underlying mechanism remains undeciphered. Several clinical studies have established a strong correlation between low levels of high-density lipoproteins (HDL) and an increased risk and severity of AD. The human APOE, encoded by three alleles—ε2, ε3, ε4, is a key constituent of HDL-like particles in the brain. APOE and HDL in the brain regulate cognition through multiple processes including cholesterol homeostasis, synaptic growth, anti-inflammation, and protein clearance. Lipidation of APOE is isoform dependent (ε2>ε3>ε4) and is crucial for HDL formation and function. APOE4 exhibits altered physiological functions because of its decreased propensity to form HDL in the brain, which may be a potential mechanism driving APOE4-linked AD pathology. We hypothesize that reversing APOE4’s lipidation deficit corrects its functions and ameliorates APOE4 associated AD pathology. Previously, Chernick et al demonstrated that an HDL mimetic peptide 4F mitigates amyloid-β (Aβ)-induced inhibition of APOE secretion and lipidation in astrocytes, a major source of APOE and HDL in the brain. Expanding upon this work, we investigated the isoform dependent effects of 4F and a modified 4F (X-4F) in the presence or absence of aggregated Aβ. To this end, we cultured primary murine astrocytes derived from homozygous human APOE3 and APOE4 knock in (KI) mice. We also investigated the effects of 4F and X-4F on primary APOE4 KI astrocytes overexpressing mutant forms of human amyloid-β precursor protein (APP) and presenilin-1 (PS1) to model endogenous Aβ production in vitro. In this study, we confirmed the well-established lipidation deficit of APOE4. We further demonstrated that HDL-mimetic peptides 4F and X-4F increase the secretion and lipidation of both APOE4 and APOE3. Importantly, 4F and X-4F improve lipidation of APOE4 to a greater extent than APOE3, reversing the lipidation deficit of APOE4. In addition, 4F and X-4F mediated enhancement of APOE secretion and lipidation persist in APOE4 KI and APOE3 KI astrocytes treated with aggregated Aβ42 as well as in APOE4 KI astrocytes overexpressing APP/PS1. In conclusion, the present study provides additional in vitro evidence of HDL-mimetic peptides as potential APOE4 modulating agents. Future studies are warranted to assess the efficacy of HDL-mimetic peptides to restore the beneficial functions of HDL and APOE in the brain and to mitigate the pathogenic process of AD.