👤 Angeliki Chroni

Apolipoprotein E (apoE), a major protein for lipid transport in circulation and the brain, has three common isoforms, apoE2, apoE3 and apoE4. APOE4 is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). Recently identified rare apoE variants, the apoE3(R136S)-Christchurch, apoE3(V236E)-Jacksonville and apoE4(R251G), appear to exert protective functions against AD and reduce the disease risk, but the molecular basis behind these effects is unknown. ApoE is a structurally dynamic protein, undergoing significant rearrangements that are important for its biological function. To examine the structural basis behind the properties of the protective apoE variants, we analyzed their structural and thermodynamic integrity both in APOE3 and APOE4 allelic backgrounds compared to their wild-type counterparts. Circular dichroism spectroscopy showed that only the V236E variation significantly alters the secondary structure of apoE3 and apoE4 in lipid-free form. This variant was also less prone to oligomerization. Chemical denaturation analysis indicated changes in the unfolding profile of V236E and R251G apoE variants in lipid-free form. Thermal unfolding analysis revealed small thermodynamic alterations in each variant compared to their wild-type apoE counterparts in lipid-free form, but a thermodynamic stabilization in lipoprotein-associated form. Additionally, following lipidation, all protective apoE variants were found to enhance the viability of SK-N-SH neuroblastoma cells and reduce the production of TNFα from BV2 microglia cells. Overall, these findings suggest that the specific amino acid substitutions found in AD-protective apoE variants can induce changes in the molecule's stability and conformation that may underlie common functional consequences, which are independent of the apoE background. Show less

High-Density Lipoprotein cholesterol (HDL-C) levels do not correlate well with Coronary Artery Disease (CAD) risk, while HDL functionality affects atherogenesis and is a better prognostic marker for CAD. Often, the extreme HDL-C levels have a multigenic origin. Here, we searched for single-nucleotide polymorphisms (SNPs) in ten genes of HDL metabolism in a Greek cohort with very low (<10th percentile, n = 13) or very high (>90th percentile, n = 21) HDL-C. We also evaluated the association between HDL-C levels, HDL functionality (anti-oxidant capacity) and CAD in the subjects of this cohort. Individuals with low HDL-C levels had higher triglyceride levels, lower apoA-I levels, decreased HDL anti-oxidant capacity and higher incidence of CAD compared with individuals with control or high HDL-C levels. With next generation sequencing we identified 18 exonic SNPs in 6 genes of HDL metabolism and for selected amino acid changes we performed computer-aided structural analysis and modeling. A previously uncharacterized rare apolipoprotein A-IV variant, apoA-IV [V336M], present in a subject with low HDL-C (14 mg/dL) and CAD, was expressed in recombinant form and structurally and functionally characterized. ApoA-IV [V336M] had similar α-helical content to WT apoA-IV but displayed a small thermodynamic stabilization by chemical unfolding analysis. ApoA-IV [V336M] was able to associate with phospholipids but presented reduced kinetics compared to WT apoA-IV. Overall, we identified a rare apoA-IV variant in a subject with low HDL levels and CAD with altered biophysical and phospholipid binding properties and showed that subjects with very low HDL-C presented with HDL dysfunction and higher incidence of CAD in a Greek cohort. Show less

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research. Show less