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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...

Apolipoprotein E (apoE), a plasma apolipoprotein that plays a central role in lipoprotein metabolism, is localized in the senile plaques, congophilic angiopathy, and neurofibrillary tangles of Alzheimer disease. Late-onset familial and sporadic Alzheimer disease patients have an increased frequency of one of the three common apoE alleles, e4, suggesting apoE4 is associated with increased susceptibility to disease. To follow up on this suggestion, we compared the binding of synthetic amyloid .8 (.3/A4) peptide to purified apoE4 and apoE3, the most common isoform. Both isoforms bound synthetic P/A4 peptide, the primary constituent of the plaque and angiopathy, forming a complex that resisted dissociation by boiling in SDS. Oxygen-mediated complex formation was implicated because binding was increased in oxygenated buffer, reduced in nitrogen-purged buffer, and prevented by reduction with dithiothreitol or 2-mercaptoethanol. Binding of fI/A4 peptide was saturable at 10-4 M peptide and required residues 12-28. Examination of apoE fragments revealed that residues 244-272 are critical for complex formation. Both oxidized apoE4 and apoE3 bound fi/A4 peptide; however, binding to apoE4 was observed in minutes, whereas binding to apoE3 required hours. In addition, apoE4 did not bind (8/A4 peptide at pH < 6.6, whereas apoE3 bound P/A4 peptide from pH 7.6 to 4.6. Together these results indicate differences in the two isoforms in complexing with the f/A4 peptide. Binding of P/A4 peptide by oxidized apoE may determine the sequestration or targeting of either apoE or ./A4 peptide, and isoformspecific differences in apoE binding or oxidation may be involved in the pathogenesis of the intra-and extracellular lesions of Alzheimer disease.

Apolipoprotein (apo) E has roles beyond lipoprotein metabolism. The detrimental effects of apoE4 in cardiovascular, neurological, and infectious diseases correlate with its structural features (e.g., domain interaction) that distinguish it from apoE3 and apoE2. Structure/function studies revealed that apoE2 is severely defective in LDL receptor binding because of a structural difference that alters the receptor binding region and helped unravel the mechanism of type III hyperlipoproteinemia. ApoE4 is the major genetic risk factor for Alzheimerʼs disease and sets the stage for neuropathological disorders precipitated by genetic, metabolic, and environmental stressors. ApoE also influences susceptibility to parasitic, bacterial, and viral infections. In HIV-positive patients, apoE4 homozygosity hastens progression to AIDS and death and increases susceptibility to opportunistic infections. The next phase in our understanding of apoE will be characterized by clinical intervention to prevent or reverse the detrimental effects of apoE4 by modulating its structure or blocking the pathological processes it mediates.-Mahley, R. W., K. H. Weisgraber, and Y. Huang. Apolipoprotein E: structure determines function, from atherosclerosis to Alzheimerʼs disease to AIDS. J. Lipid Res. 2009. 50: S183-S188. Supplementary key words cholesterolStructural differences in apolipoprotein (apo) E isoforms impact cardiovascular, neurological, and infectious diseases (1-4). Discovered in the 1970s, this 34-kDa, 299-amino-acid protein was identified in triglyceride-rich lipoproteins and induced by cholesterol feeding in animal models and humans (1,3,5,6). The three common isoforms (apoE2, apoE3, and apoE4) are encoded by a gene on chromosome 19. The three alleles differ in their frequencies: ε4 (15-20%), ε3 (65-70%), and ε2 (5-10%) and give rise to three homozygous and three heterozygous phenotypes. The nomenclature arose by consensus among key investigators (7).Utermann, Hees, and Steinmetz (8) recognized differences in the apoE isoform patterns that distinguished normolipidemic subjects from patients with the genetic lipid disorder type III hyperlipoproteinemia (HLP) (dysbetalipoproteinemia). Studies by Zannis, Breslow, Havel, and others (9, 10) helped unravel the isoform pattern. Ultimately, apoE2 was associated with type III HLP. Gladstone investigators elucidated the structural basis for the polymorphism of apoE and showed that apoE isoforms differ at two sites: apoE3 has Cys-112 and Arg-158, whereas apoE4 has arginines at both sites, and apoE2 has cysteines (11, 12). They also determined the structure of apoE mRNA and the 3.6-kb gene encoding apoE and elucidated the regulatory elements that control its expression (13-15).Plasma apoE (?40-70 mg/ml) arises primarily from hepatic synthesis (.75%). The second most common site of synthesis is the brain (16). Although astrocytes produce a large proportion of cerebrospinal fluid apoE (?3-5 mg/ml), neurons synthesize apoE when stressed (17). Macrophages and other cell types also synt...

Human apolipoprotein E, a blood plasma protein, mediates the transport and uptake of cholesterol and lipid by way of its high affinity interaction with different cellular receptors, including the low-density lipoprotein (LDL) receptor. The three-dimensional structure of the LDL receptor-binding domain of apoE has been determined at 2.5 angstrom resolution by x-ray crystallography. The protein forms an unusually elongated (65 angstroms) four-helix bundle, with the helices apparently stabilized by a tightly packed hydrophobic core that includes leucine zipper-type interactions and by numerous salt bridges on the mostly charged surface. Basic amino acids important for LDL receptor binding are clustered into a surface patch on one long helix. This structure provides the basis for understanding the behavior of naturally occurring mutants that can lead to atherosclerosis.

No abstract

Apolipoprotein E4 (apoE4), one of the three common isoforms of apoE, has been implicated in Alzheimer's disease. The effects of apoE on neuronal growth were determined in cultures of dorsal root ganglion neurons. In the presence of beta-migrating very low density lipoproteins (beta-VLDL), apoE3 increased neurite outgrowth, whereas apoE4 decreased outgrowth. The effects of apoE3 or apoE4 in the presence of beta-VLDL were prevented by incubation with a monoclonal antibody to apoE or by reductive methylation of apoE, both of which block the ability of apoE to interact with lipoprotein receptors. The data suggest that receptor-mediated binding or internalization (or both) of apoE-enriched beta-VLDL leads to isoform-specific differences in interactions with cellular proteins that affect neurite outgrowth.

Apolipoprotein (apo) E4 increases the risk and accelerates the onset of Alzheimer's disease (AD). However, the underlying mechanisms remain to be determined. We previously found that apoE undergoes proteolytic cleavage in AD brains and in cultured neuronal cells, resulting in the accumulation of carboxyl-terminaltruncated fragments of apoE that are neurotoxic. Here we show that this fragmentation is caused by proteolysis of apoE by a chymotrypsin-like serine protease that cleaves apoE4 more efficiently than apoE3. Transgenic mice expressing the carboxylterminal-cleaved product, apoE4(⌬272-299), at high levels in the brain died at 2-4 months of age. The cortex and hippocampus of these mice displayed AD-like neurodegenerative alterations, including abnormally phosphorylated tau (p-tau) and Gallyas silverpositive neurons that contained cytosolic straight filaments with diameters of 15-20 nm, resembling preneurofibrillary tangles. Transgenic mice expressing lower levels of the truncated apoE4 survived longer but showed impaired learning and memory at 6 -7 months of age. Thus, carboxyl-terminal-truncated fragments of apoE4, which occur in AD brains, are sufficient to elicit AD-like neurodegeneration and behavioral deficits in vivo. Inhibiting their formation might inhibit apoE4-associated neuronal deficits.H uman apolipoprotein (apo) E, a 34-kDa protein composed of 299 amino acids, occurs as three major isoforms, apoE2, apoE3, and apoE4 (1-4). ApoE4 is a major risk or susceptibility factor for Alzheimer's disease (AD) (5-7). The apoE4 allele, which is found in 40-65% of cases of sporadic and familial AD, increases the occurrence and lowers the age of onset of the disease (7,8).Biochemical, cell biological, and transgenic animal studies have suggested several potential mechanisms to explain apoE4's contribution to the pathogenesis of AD. These include the modulation of the deposition and clearance of amyloid ␤ peptides and the formation of plaques (9-15), impairment of the antioxidative defense system (16), dysregulation of neuronal signaling pathways (17), disruption of cytoskeletal structure and function (18,19), and altered phosphorylation of tau and the formation of neurofibrillary tangles (NFTs) (20-23). However, the mechanisms of these apoE4-mediated detrimental effects are still largely unknown, and it is not known which are the primary effects and which are subsequent or downstream effects.The neuropathological hallmarks of AD include extracellular amyloid plaques and intracellular NFTs in the brain (24-27). The plaques consist primarily of amyloid ␤ peptides (24-26). The NFTs are composed largely of the highly phosphorylated microtubule-associated protein tau (p-tau) (25) and, to a lesser extent, of phosphorylated neurofilaments (28, 29). Both amyloid plaques and NFTs contain apoE (5, 30, 31); however, the role of apoE in the pathogenesis of these two lesions is uncertain. Histopathological and behavioral analyses of transgenic mice expressing different human apoE isoforms in the brain have revealed clear evi...