Endothelial Cell APOE3 Regulates Neurovascular, Neuronal, and Behavioral Function

BACKGROUND

Specialized brain endothelial cells and human APOE3 are independently important for neurovascular function, yet whether APOE3 expression by endothelial cells contributes to brain function is currently unknown. In the present study, we determined whether the loss of endothelial cell APOE3 impacts brain vascular and neural function.

METHODS

We developed APOE3fl/fl/Cdh5(PAC)-CreERT2+/- (APOE3Cre+/-) and APOE3fl/fl/Cdh5(PAC)-CreERT2-/- (APOE3Cre-/-, control) mice and induced endothelial cell APOE3 knockdown with tamoxifen at ≈4 to 5 weeks of age. Neurovascular and neuronal function were evaluated by biochemistry, immunohistochemistry, behavioral testing, and electrophysiology at 9 months of age.

RESULTS

We found that the loss of endothelial APOE3 expression was sufficient to cause neurovascular dysfunction including higher permeability and lower vessel coverage in tandem with deficits in spatial memory and fear memory extinction and a disruption of cortical excitatory/inhibitory balance.

CONCLUSIONS

Our data collectively support the novel concept that endothelial APOE3 plays a critical role in the regulation of the neurovasculature, neural circuit function, and behavior.

Biophysical, Biochemical, and Behavioral Implications of ApoE3 Conjugated Donepezil Nanomedicine in a Aβ1-42 Induced Alzheimer's Disease Rat Model

Alzheimer's disease (AD) is a progressive neurological disorder and is the most common type of dementia. Amyloid β (Aβ) plaques play an important role in the pathophysiology of AD. However, the existing therapeutic strategies are not effective for the management of both Aβ-induced neurotoxicity and Aβ fibrils clearance in biological conditions. Herein, we have developed lipoprotein conjugated polymeric nanoparticles that can boost the clearance rate of Aβ fibrils and mitigate Aβ-induced neurotoxicity in AD rat. These nanoparticles were designed by loading donepezil in an amphiphilic polymer with a lipoprotein (ApoE3) integrated over the surface. Polymeric nanoparticles were prepared by a nanoprecipitation method, and ApoE3 was conjugated to the polymer layer by polysorbate 80. In the present study, we intended to examine the protective effect of ApoE3 nanoparticles against Aβ-induced neurotoxicity both in vitro and in vivo to evaluate if these can reduce the Aβ fibril formation and cognitive and behavioral deficits observed in AD induced rats. In the in vitro study, neurotoxicity induced by Aβ_1-42 in human neuroblastoma (SH-SY5Y) cells was found to be significantly reduced upon treatment with ApoE3 donepezil nanoparticles. The presence of the ApoE3 significantly modified the morphology of Aβ fibrils and also inhibited the formation Aβ oligomers. Moreover, in the in vivo study, following treatment, AD induced rats were tested on Morris water maze (MWM) and passive avoidance task for their cognitive ability and sacrificed for biochemical estimations. From our observations, ApoE3 donepezil nanoparticles exhibited neuroprotection in the Aβ_1-42 induced model by mitigating the pathological features and cognitive impairments. Thus, we anticipate that the nanosized lipoprotein carriers will possibly offer a rational therapeutic strategy in the formulation development of AD.

Cellular Uptake and Clearance of Oxidatively-modified Apolipoprotein E3 by Cerebral Cortex Endothelial Cells

Apolipoprotein E3 (apoE3) plays a critical role in the metabolism of lipoproteins and lowers plasma lipid levels by serving as a ligand for the low-density lipoprotein receptor (LDLr) family of proteins and by promoting macrophage cholesterol efflux. The current study examines the effect of acrolein (an endogenously generated metabolite and an environmental pollutant) modification on the structure and function of apoE3. Acrolein modification was confirmed in Western blots by reactivity with acrolein–lysine-specific antibody and by the presence of oligomeric species due to cross-linking. LC-MS/MS analysis revealed modification of 10 out of 12 lysines in apoE3, with N^ε-(3-methylpyridinium)-lysine being the predominant form of modification, and Lys75 being a ‘hot spot’ in terms of susceptibility to oxidation. Circular dichroism spectroscopy showed no major change in overall secondary structure compared to unmodified apoE3. Reconstituted high density lipoprotein (HDL) bearing acrolein modified apoE3 showed loss of binding to soluble LDLr; however, incubation with mouse endothelioma bEnd.3 cells showed that it was internalized. Incubation with excess LDL did not abolish cellular uptake of acrolein modified apoE3, suggesting alternative mechanism(s) not involving LDLr. Incubation with anti-CD36 antibody did not show a decrease in internalization while incubation with anti- lectin-like oxidized LDL receptor 1 (LOX1) showed partial internalization. However, incubation with anti-scavenger receptor class B type I (SRB1) antibody abolished internalization of acrolein modified apoE3. Taken together, our studies suggest that acrolein modification of apoE3 at lysine residues leads to increase in net negative charge, and as a consequence, results in clearance by LOX1 and SRB1 on endothelial cells. Overall, oxidative modification of apoE3 likely impairs its role in regulating plasma cholesterol homeostasis, eventually leading to lipid disorders.

Conformational analysis of apolipoprotein E3/E4 heteromerization

Apolipoprotein E (apoE) is a 299 residue, exchangeable apolipoprotein that has essential roles in cholesterol homeostasis and reverse cholesterol transport. It is a two-domain protein with the C-terminal (CT) domain mediating protein self-association via helix-helix interactions. In humans, the APOE gene is polymorphic with three common alleles, ε2, ε3, and ε4, occurring in frequencies of ~ 5%, 77%, and 18%, respectively. Heterozygotes expressing apoE3 and apoE4 isoforms, which differ in residue at position 112 in the N-terminal domain (C112 in apoE3 and R112 in apoE4), represent the highest population of ε4 carriers, an allele highly associated with Alzheimer's disease. The objective of this study was to determine if apoE3 and apoE4 have the ability to hybridize to form a heteromer in lipid-free state. Refolding an equimolar mixture of His-apoE3 and FLAG-apoE4 (or vice versa) followed by pull-down and immunoblotting indicated formation of apoE3/apoE4 heteromers. Förster resonance energy transfer between donor fluorophore on one isoform and acceptor on the other, both located in the respective CT domains, revealed a distance of separation of ~ 46 Å between the donor/acceptor pair. Similarly, a quencher placed on one was able to mediate significant quenching of fluorescence emission on the other, indicative of spatial proximity within collisional distance between the two. ApoE3/apoE4 heteromer association was also noted in lipid-associated state in reconstituted lipoprotein particles. The possibility of heteromerization of apoE3/apoE4 bears implications in the potential mitigating role of apoE3 on the folding and physiological behavior of apoE4 and its role in maintaining cholesterol homeostasis.

Human ApoE Isoforms Differentially Modulate Glucose and Amyloid Metabolic Pathways in Female Brain: Evidence of the Mechanism of Neuroprotection by ApoE2 and Implications for Alzheimer's Disease Prevention and Early Intervention

Three major genetic isoforms of apolipoprotein E (ApoE), ApoE2, ApoE3, and ApoE4, exist in humans and lead to differences in susceptibility to Alzheimer's disease (AD). This study investigated the impact of human ApoE isoforms on brain metabolic pathways involved in glucose utilization and amyloid-β (Aβ) degradation, two major areas that are significantly perturbed in preclinical AD. Hippocampal RNA samples from middle-aged female mice with targeted human ApoE2, ApoE3, and ApoE4 gene replacement were comparatively analyzed with a qRT-PCR custom array for the expression of 85 genes involved in insulin/insulin-like growth factor (Igf) signaling. Consistent with its protective role against AD, ApoE2 brain exhibited the most metabolically robust profile among the three ApoE genotypes. When compared to ApoE2 brain, both ApoE3 and ApoE4 brains exhibited markedly reduced levels of Igf1, insulin receptor substrates (Irs), and facilitated glucose transporter 4 (Glut4), indicating reduced glucose uptake. Additionally, ApoE4 brain exhibited significantly decreased Pparg and insulin-degrading enzyme (Ide), indicating further compromised glucose metabolism and Aβ dysregulation associated with ApoE4. Protein analysis showed significantly decreased Igf1, Irs, and Glut4 in ApoE3 brain, and Igf1, Irs, Glut4, Pparg, and Ide in ApoE4 brain compared to ApoE2 brain. These data provide the first documented evidence that human ApoE isoforms differentially affect brain insulin/Igf signaling and downstream glucose and amyloid metabolic pathways, illustrating a potential mechanism for their differential risk in AD. A therapeutic strategy that enhances brain insulin/Igf1 signaling activity to a more robust ApoE2-like phenotype favoring both energy production and amyloid homeostasis holds promise for AD prevention and early intervention.

apoE3[K146N/R147W] acts as a dominant negative apoE form that prevents remnant clearance and inhibits the biogenesis of HDL

The K146N/R147W substitutions in apoE3 were described in patients with a dominant form of type III hyperlipoproteinemia. The effects of these mutations on the in vivo functions of apoE were studied by adenovirus-mediated gene transfer in different mouse models. Expression of the apoE3[K146N/R147W] mutant in apoE-deficient (apoE(-/-)) or apoA-I-deficient (apoA-I(-/-))×apoE(-/-) mice exacerbated the hypercholesterolemia and increased plasma apoE and triglyceride levels. In apoE(-/-) mice, the apoE3[K146N/R147W] mutant displaced apoA-I from the VLDL/LDL/HDL region and caused the accumulation of discoidal apoE-containing HDL. The WT apoE3 cleared the cholesterol of apoE(-/-) mice without induction of hypertriglyceridemia and promoted formation of spherical HDL. A unique property of the truncated apoE3[K146N/R147W]202 mutant, compared with similarly truncated apoE forms, is that it did not correct the hypercholesterolemia. The contribution of LPL and LCAT in the induction of the dyslipidemia was studied. Treatment of apoE(-/-) mice with apoE3[K146N/R147W] and LPL corrected the hypertriglyceridemia, but did not prevent the formation of discoidal HDL. Treatment with LCAT corrected hypertriglyceridemia and generated spherical HDL. The combined data indicate that the K146N/R147W substitutions convert the full-length and the truncated apoE3[K146N/R147W] mutant into a dominant negative ligand that prevents receptor-mediated remnant clearance, exacerbates the dyslipidemia, and inhibits the biogenesis of HDL.