Enhanced binding of apolipoprotein A-I variants associated with hypertriglyceridemia to triglyceride-rich particles

Human apoA-I[Lys107del] mutation affects lipid surface behavior of apoA-I and its ability to form large nascent HDL

Population studies have found that a natural human apoA-I variant, apoA-I[K107del], is strongly associated with low HDL-C but normal plasma apoA-I levels. We aimed to reveal properties of this variant that contribute to its unusual phenotype associated with atherosclerosis. Our oil-drop tensiometry studies revealed that compared to WT, recombinant apoA-I[K107del] adsorbed to surfaces of POPC-coated triolein drops at faster rates, remodeled the surfaces to a greater extent, and was ejected from the surfaces at higher surface pressures on compression of the lipid drops. These properties may drive increased binding of apoA-I[K107del] to and its better retention on large triglyceride-rich lipoproteins, thereby increasing the variant's content on these lipoproteins. While K107del did not affect apoA-I capacity to promote ABCA1-mediated cholesterol efflux from J774 cells, it impaired the biogenesis of large nascent HDL particles resulting in the formation of predominantly smaller nascent HDL. Size-exclusion chromatography of spontaneously reconstituted 1,2-dimyristoylphosphatidylcholine-apoA-I complexes showed that apoA-I[K107del] had a hampered ability to form larger complexes but formed efficiently smaller-sized complexes. CD analysis revealed a reduced ability of apoA-I[K107del] to increase α-helical structure on binding to 1,2-dimyristoylphosphatidylcholine or in the presence of trifluoroethanol. This property may hinder the formation of large apoA-I[K107del]-containing discoidal and spherical HDL but not smaller HDL. Both factors, the increased content of apoA-I[K107del] on triglyceride-rich lipoproteins and the impaired ability of the variant to stabilize large HDL particles resulting in reduced lipid:protein ratios in HDL, may contribute to normal plasma apoA-I levels along with low HDL-C and increased risk for CVD.

Role of apolipoproteins, ABCA1 and LCAT in the biogenesis of normal and aberrant high density lipoproteins

In this review, we focus on the pathway of biogenesis of HDL, the essential role of apoA-I, ATP binding cassette transporter A1 (ABCA1), and lecithin: cholesterol acyltransferase (LCAT) in the formation of plasma HDL; the generation of aberrant forms of HDL containing mutant apoA-I forms and the role of apoA-IV and apoE in the formation of distinct HDL subpopulations. The biogenesis of HDL requires functional interactions of the ABCA1 with apoA-I (and to a lesser extent with apoE and apoA-IV) and subsequent interactions of the nascent HDL species thus formed with LCAT. Mutations in apoA-I, ABCA1 and LCAT either prevent or impair the formation of HDL and may also affect the functionality of the HDL species formed. Emphasis is placed on three categories of apoA-I mutations. The first category describes a unique bio-engineered apoA-I mutation that disrupts interactions between apoA-I and ABCA1 and generates aberrant preβ HDL subpopulations that cannot be converted efficiently to α subpopulations by LCAT. The second category describes natural and bio-engineered apoA-I mutations that generate preβ and small size α4 HDL subpopulations, and are associated with low plasma HDL levels. These phenotypes can be corrected by excess LCAT. The third category describes bio-engineered apoA-I mutations that induce hypertriglyceridemia that can be corrected by excess lipoprotein lipase and also have defective maturation of HDL. The HDL phenotypes described here may serve in the future for diagnosis, prognoses and potential treatment of abnormalities that affect the biogenesis and functionality of HDL.

Increased Binding of Apolipoproteins A-I and E4 to Triglyceride-Rich Lipoproteins is linked to Induction of Hypertriglyceridemia

Hypertriglyceridemia (HTG) is an independent factor of atherosclerotic cardiovascular disease and a hallmark of many metabolic disorders. However, the molecular etiology of HTG is still largely unknown. In mice, severe HTG may be induced by expression of specific mutants of apolipoprotein (apo) A-I or wild type (WT) apoE4. Expression of a certain apoE4 mutant results in mild HTG, while expression of another apoE4 mutant or WT apoA-I results in normal plasma triglyceride (TG) levels. Biophysical studies of the apoA-I and apoE4 forms associated with HTG help better understand the molecular mechanisms of induction of HTG by these proteins. The studies show that the apoA-I and apoE4 forms that induce HTG have a destabilized and more loosely folded conformation in solution than their counterparts not associated with HTG. Disruption of the protein salt bridge networks by the mutations is likely responsible for the observed structural changes. Each apoA-I and apoE4 form that induced HTG show enhanced binding to model TG-rich particles. HTG appeared to positively correlate with the apolipoprotein ability to bind to TG-rich particles. This implies that in vivo, the conformational changes in the apolipoproteins that induce HTG facilitate their binding to plasma TG-rich lipoproteins. We discuss metabolic pathways leading to the development of HTG that may result from enhanced binding of the apolipoproteins to TG-rich lipoproteins in circulation. While various factors may be involved in the development of HTG in humans, it is possible that structural alterations that increase affinity of apolipoproteins to TG-rich lipoproteins may contribute to some cases of this disorder.

HDL biogenesis, remodeling, and catabolism

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.

Binding of human apoA-I[K107del] variant to TG-rich particles: implications for mechanisms underlying hypertriglyceridemia

We found earlier that apoA-I variants that induced hypertriglyceridemia (HTG) in mice had increased affinity to TG-rich lipoproteins and thereby impaired their catabolism. Here, we tested whether a naturally occurring human apoA-I mutation, Lys107del, associated with HTG also promotes apoA-I binding to TG-rich particles. We expressed apoA-I[Lys107del] variant in Escherichia coli, studied its binding to TG-rich emulsion particles, and performed a physicochemical characterization of the protein. Compared with WT apoA-I, apoA-I[Lys107del] showed enhanced binding to TG-rich particles, lower stability, and greater exposure of hydrophobic surfaces. The crystal structure of truncated, Δ(185-243), apoA-I suggests that deletion of Lys107 disrupts helix registration and disturbs a stabilizing salt bridge network in the N-terminal helical bundle. To elucidate the structural changes responsible for the altered function of apoA-I[Lys107del], we studied another mutant, apoA-I [Lys107Ala]. Our findings suggest that the registry shift and ensuing disruption of the inter-helical salt bridges in apoA-I[Lys107del] result in destabilization of the helical bundle structure and greater exposure of hydrophobic surfaces. We conclude that the structural changes in the apoA-I[Lys107del] variant facilitate its binding to TG-rich lipoproteins and thus, may reduce their lipolysis and contribute to the development of HTG in carriers of the mutation.

Amyloidogenic mutations in human apolipoprotein A-I are not necessarily destabilizing - a common mechanism of apolipoprotein A-I misfolding in familial amyloidosis and atherosclerosis

High-density lipoproteins and their major protein, apolipoprotein A-I (apoA-I), remove excess cellular cholesterol and protect against atherosclerosis. However, in acquired amyloidosis, nonvariant full-length apoA-I deposits as fibrils in atherosclerotic plaques; in familial amyloidosis, N-terminal fragments of variant apoA-I deposit in vital organs, damaging them. Recently, we used the crystal structure of Δ(185-243)apoA-I to show that amyloidogenic mutations destabilize apoA-I and increase solvent exposure of the extended strand 44-55 that initiates β-aggregation. In the present study, we test this hypothesis by exploring naturally occurring human amyloidogenic mutations, W50R and G26R, within or close to this strand. The mutations caused small changes in the protein's α-helical content, stability, proteolytic pattern and protein-lipid interactions. These changes alone were unlikely to account for amyloidosis, suggesting the importance of other factors. Sequence analysis predicted several amyloid-prone segments that can initiate apoA-I misfolding. Aggregation studies using N-terminal fragments verified this prediction experimentally. Three predicted N-terminal amyloid-prone segments, mapped on the crystal structure, formed an α-helical cluster. Structural analysis indicates that amyloidogenic mutations or Met86 oxidation perturb native packing in this cluster. Taken together, the results suggest that structural perturbations in the amyloid-prone segments trigger α-helix to β-sheet conversion in the N-terminal ~ 75 residues forming the amyloid core. Polypeptide outside this core can be proteolysed to form 9-11 kDa N-terminal fragments found in familial amyloidosis. Our results imply that apoA-I misfolding in familial and acquired amyloidosis follows a similar mechanism that does not require significant structural destabilization or proteolysis. This novel mechanism suggests potential therapeutic interventions for apoA-I amyloidosis.

Alteration of negatively charged residues in the 89 to 99 domain of apoA-I affects lipid homeostasis and maturation of HDL

In this study, we investigated the role of positively and negatively charged amino acids within the 89-99 region of apolipoprotein A-I (apoA-I), which are highly conserved in mammals, on plasma lipid homeostasis and the biogenesis of HDL. We previously showed that deletion of the 89-99 region of apoA-I increased plasma cholesterol and phospholipids, but it did not affect plasma triglycerides. Functional studies using adenovirus-mediated gene transfer of two apoA-I mutants in apoA-I-deficient mice showed that apoA-I[D89A/E91A/E92A] increased plasma cholesterol and caused severe hypertriglyceridemia. HDL levels were reduced, and approximately 40% of the apoA-I was distributed in VLDL/IDL. The HDL consisted of mostly spherical and a few discoidal particles and contained preβ1 and α4-HDL subpopulations. The lipid, lipoprotein, and HDL profiles generated by the apoA-I[K94A/K96A] mutant were similar to those of wild-type (WT) apoA-I. Coexpression of apoA-I[D89A/E91A/E92A] and human lipoprotein lipase abolished hypertriglyceridemia, restored in part the α1,2,3,4 HDL subpopulations, and redistributed apoA-I in the HDL2/HDL3 regions, but it did not prevent the formation of discoidal HDL particles. Physicochemical studies showed that the apoA-I[D89A/E91A/E92A] mutant had reduced α-helical content and effective enthalpy of thermal denaturation, increased exposure of hydrophobic surfaces, and increased affinity for triglyceride-rich emulsions. We conclude that residues D89, E91, and E92 of apoA-I are important for plasma cholesterol and triglyceride homeostasis as well as for the maturation of HDL.