Apolipoprotein E4: A causative factor and therapeutic target in neuropathology, including Alzheimer’s disease

Abstract

The premise of this review is that apolipoprotein (apo) E4 is much more than a contributing factor to neurodegeneration. ApoE has critical functions in redistributing lipids among CNS cells for normal lipid homeostasis, repairing injured neurons, maintaining synaptodendritic connections, and scavenging toxins. In multiple pathways affecting neuropathology, including Alzheimer's disease, apoE acts directly or in concert with age, head injury, oxidative stress, ischemia, inflammation, and excess amyloid ␤ peptide production to cause neurological disorders, accelerating progression, altering prognosis, or lowering age of onset. We envision that unique structural features of apoE4 are responsible for apoE4-associated neuropathology. Although the structures of apoE2, apoE3, and apoE4 are in dynamic equilibrium, apoE4, which is detrimental in a variety of neurological disorders, is more likely to assume a pathological conformation. Importantly, apoE4 displays domain interaction (an interaction between the N-and C-terminal domains of the protein that results in a compact structure) and molten globule formation (the formation of stable, reactive intermediates with potentially pathological activities). In response to CNS stress or injury, neurons can synthesize apoE. ApoE4 uniquely undergoes neuron-specific proteolysis, resulting in bioactive toxic fragments that enter the cytosol, alter the cytoskeleton, disrupt mitochondrial energy balance, and cause cell death. Our findings suggest potential therapeutic strategies, including the use of ''structure correctors'' to convert apoE4 to an ''apoE3-like'' molecule, protease inhibitors to prevent the generation of toxic apoE4 fragments, and ''mitochondrial protectors'' to prevent cellular energy disruption. mitochondria ͉ neurodegeneration ͉ cytoskeleton ͉ protein folding A polipoprotein (apo) E plays a fundamental role in the maintenance and repair of neurons, but its three isoforms differ in their abilities to accomplish these critical tasks (1-3). ApoE4 is associated with a wide variety of neuropathological processes. We hypothesize that different insults associated with a variety of disorders, in concert with apoE4, can lead to neuropathology. We believe that those processes are mediated by the cellular origin of apoE (astrocytes, neurons, or microglia), the nature of various injurious factors (''second hits''), and the structure of apoE4. Thus, understanding the structure and function of the apoE isoforms will yield new strategies for treating neuropathologies.Although apoE is involved in many neuropathologies, we will focus on its role in Alzheimer's disease (AD). We will consider the unique structural features that distinguish apoE4 from apoE3 and apoE2, the sites of synthesis and normal roles of apoE in the nervous system, and the pathological roles of apoE4 with or without amyloid ␤ (A␤) peptide. The evidence suggests that apoE4 is considerably more than a simple contributing factor in AD pathogenesis. ApoE and NeuropathologyApoE4's involvement in neuropathology is we...

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