ApoE2-Mediated Neuroprotection Against Alzheimer’s Disease: from Mechanism to Translation
Alzheimer’s disease (AD) is the most common form of age-related dementia. Despite decades of research, the etiology and pathogenesis of AD are not well understood. Brain glucose hypometabolism has long been recognized as a prominent anomaly that occurs in the preclinical stage of AD. Recent studies suggest that glycolytic metabolism, the cytoplasmic pathway of the breakdown of glucose, may play a critical role in the development of AD. Glycolysis is essential for a variety of neural activities in the brain, including energy production, synaptic transmission, and redox homeostasis. Decreased glycolytic flux has been shown to correlate with the severity of amyloid and tau pathology in both preclinical and clinical AD patients. Moreover, increased glucose accumulation found in the brains of AD patients supports the hypothesis that glycolytic deficit may be a contributor to the development of this phenotype. In addition to AD, glycolic dysfunction has been observed in other neurodegenerative diseases, including Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis, strengthening the concept of glycolytic dysfunction as a common pathway leading to neurodegeneration. Humans possess three primary variants of the apolipoprotein E (ApoE) gene – ApoE*ԑ2, ApoE*ԑ3, and ApoE*ԑ4 – that confer differential susceptibility to AD. Recent findings indicate that neuronal glycolysis is significantly affected by human ApoE isoforms and glycolytic robustness may serve as a major mechanism that renders an ApoE2-bearing brain more resistant against the neurodegenerative risks for AD. In particular, these advances highlight a promising translational opportunity that involves targeting glycolysis to bolster brain metabolic resilience, and by such to alter the course of brain aging in fighting against the risks for not only AD but also other neurodegenerative diseases. In addition, AD has a multifactoral etiology, with age, sex, and ApoE genotype established as the top three risk factors. Women have a higher risk of developing AD, and they represent approximately 68% of all AD patients. Findings from the majority of studies indicate that the ԑ4 allele of ApoE gene confers a greater negative impact in women than in men. Sex difference has also been identified in lipid profile during aging, with women displaying by a wider range of metabolites. Further clarifications of age-sex-ApoE interactions will provide important improvements for population-targeted AD diagnoses and interventions. Therefore, this dissertation investigated the neuroprotective role of ApoE2 in ApoE4 models and explored the underlying mechanisms by which ApoE isoforms differentially regulate glycolysis.In the first chapter, ApoE2 neuron-like cells were found to exhibit significantly enhanced glycolytic function through upregulation of hexokinase 1 (HK1) protein expression and enzymatic activity as compared to ApoE4 neuron-like cells. Introduction of the ApoE2 gene, but not the ApoE3 gene, into the ApoE4 genotype significantly ameliorated ApoE4-associated glycolytic deficits. Furthermore, a dose-dependent decrease in endogenous ApoE4 protein levels was observed upon expression of ApoE2. Additionally, we found that ApoE-dependent PI3K/Akt signaling activity modulated hexokinase 2 (HK2) protein expression but not HK1; HK2 is known to play a crucial role in cell survival, proliferation, and development. In the second chapter we aimed to investigate the ApoE isoform specific regulation on synaptic function in human ApoE knock-in (hApoEKI) mouse models through characterization of synaptosomal release activity with consideration given to sex, age, and stress induced by hyperglycemia. Our findings showed that hApoE3KI mice possess enhanced synaptosomal activity in mice aged 15-18 months old under hyperglycemic conditions compared to hApoE4KI mice. Of note, female hApoE4KI mice performed more poorly than male mice in response to hyperglycemia. In the third chapter, based on our observations of the role of ApoE2 in improving glycolysis in the ApoE4 genotype, we conducted a series of experiments to evaluate the impact of administering recombinant human ApoE2 protein (rhApoE2) in hApoE4KI primary neuron cultures under both normal and neurodegenerative conditions, as well as in aged hApoE4KI mouse models facilitated by E-cadherin peptides, a blood- brain barrier (BBB) modulator. Our translational studies yielded promising results, suggesting the potential of delivering rhApoE2 as a therapeutic disease-modifying strategy in ameliorating neuronal endogenous ApoE4 and peripheral inflammatory markers, while promoting brain glycolysis, synaptosomal activity, spatial memory function, and improving serum lipid profiles in the ApoE4 genotype. In summary, our studies identified HK as the key modulator in ApoE-mediated regulation of glycolysis in neuronal cells. Furthermore, we demonstrated that under hyperglycemia condition, relatively aged hApoE4KI mice exhibited impaired synaptosomal release activity compared to hApoE3KI mice with the same age, particularly, female hApoE4KI displayed the lowest. Finally, we generated proof-of-concept preliminary evidence for developing rhApoE2-based protein therapy as a disease-modifying strategy to enhance brain resilience against AD.
Pharmacology, Neurosciences, Alzheimer's disease, apolipoprotein E, glycolysis, hexokinase, neuroprotection
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