Allele-dependent thermodynamic and structural perturbations in ApoE variants associated with the correction of dyslipidemia and formation of spherical ApoE-containing HDL particles

Apolipoprotein E4 heterologous expression, purification under non-denaturing conditions, and effects on neuronal clonal cell lines

The ε4 allele of the apolipoprotein E gene (APOE4) constitutes the main genetic risk factor for late-onset Alzheimer disease (AD). High amounts of pure apolipoprotein E4 (ApoE4), in a rapid and reproducible fashion, could be of value for studying its pathophysiological roles in AD. The aim of the present work was to optimize a preparative method to obtain highly purified recombinant ApoE4 (rApoE4) with full biological activity. rApoE4 was expressed in the E. Coli BL21(D3) strain and a soluble form of the protein was purified by a combination of affinity and size-exclusion chromatography that precluded a denaturation step. The structural integrity and the biochemical activity of the purified rApoE4 were confirmed by circular dichroism and a lipid-binding assay. Several biological parameters affected by rApoE4, such as mitochondrial morphology, mitochondrial membrane potential and reactive oxygen species production were studied in CNh cells, a neuronal cell line, and neurodifferentiation and dendritogenesis were analyzed in the SH-SY5Y neuroblastoma cell line. The improved rApoE4 purification technique reported here enables the production of highly purified protein that retain the structural properties and functional activity of the native protein, as confirmed by tests in two different neuronal cell lines in culture.

Mercury Ion Binding to Apolipoprotein E Variants ApoE2, ApoE3, and ApoE4: Similar Binding Affinities but Different Structure Induction Effects

Mercury intoxication typically produces more severe outcomes in people with the APOE-ε4 gene, which codes for the ApoE4 variant of apolipoprotein E, compared to individuals with the APOE-ε2 and APOE-ε3 genes. Why the APOE-ε4 allele is a risk factor in mercury exposure remains unknown. One proposed possibility is that the ApoE protein could be involved in clearing of heavy metals, where the ApoE4 protein might perform this task worse than the ApoE2 and ApoE3 variants. Here, we used fluorescence and circular dichroism spectroscopies to characterize the in vitro interactions of the three different ApoE variants with Hg(I) and Hg(II) ions. Hg(I) ions displayed weak binding to all ApoE variants and induced virtually no structural changes. Thus, Hg(I) ions appear to have no biologically relevant interactions with the ApoE protein. Hg(II) ions displayed stronger and very similar binding affinities for all three ApoE isoforms, with K _D values of 4.6 μM for ApoE2, 4.9 μM for ApoE3, and 4.3 μM for ApoE4. Binding of Hg(II) ions also induced changes in ApoE superhelicity, that is, altered coil-coil interactions, which might modify the protein function. As these structural changes were most pronounced in the ApoE4 protein, they could be related to the APOE-ε4 gene being a risk factor in mercury toxicity.

Apolipoprotein E - A Multifunctional Protein with Implications in Various Pathologies as a Result of Its Structural Features

Apolipoprotein E (apoE), a 34 kDa glycoprotein, mediates hepatic and extrahepatic uptake of plasma lipoproteins and cholesterol efflux from lipid-laden macrophages. In humans, three structural different apoE isoforms occur, with subsequent functional changes and pathological consequences. Here, we review data supporting the involvement of apoE structural domains and isoforms in normal and altered lipid metabolism, cardiovascular and neurodegenerative diseases, as well as stress-related pathological states. Studies using truncated apoE forms provided valuable information regarding the regions and residues responsible for its properties. ApoE3 renders protection against cardiovascular diseases by maintaining lipid homeostasis, while apoE2 is associated with dysbetalipoproteinemia. ApoE4 is a recognized risk factor for Alzheimer's disease, although the exact mechanism of the disease initiation and progression is not entirely elucidated. ApoE is also implicated in infections with herpes simplex type-1, hepatitis C and human immunodeficiency viruses. Interacting with both viral and host molecules, apoE isoforms differently interfere with the viral life cycle. ApoE exerts anti-inflammatory effects, switching macrophage phenotype from the proinflammatory M1 to the anti-inflammatory M2, suppressing CD4+ and CD8+ lymphocytes, and reducing IL-2 production. The anti-oxidative properties of apoE are isoform-dependent, modulating the levels of various molecules (Nrf2 target genes, metallothioneins, paraoxonase). Mimetic peptides were designed to exploit apoE beneficial properties. The "structure correctors" which convert apoE4 into apoE3-like molecules have pharmacological potential. Despite no successful strategy is yet available for apoE-related disorders, several promising candidates deserve further improvement and exploitation.