Identification of functional domains in the plasma apolipoproteins by analysis of inter-species sequence variability

Autosomal dominant ApoA4 mutations present as tubulointerstitial kidney disease with medullary amyloidosis

Sporadic cases of apolipoprotein A-IV medullary amyloidosis have been reported. Here we describe five families found to have autosomal dominant medullary amyloidosis due to two different pathogenic APOA4 variants. A large family with autosomal dominant chronic kidney disease (CKD) and bland urinary sediment underwent whole genome sequencing with identification of a chr11:116692578 G>C (hg19) variant encoding the missense mutation p.L66V of the ApoA4 protein. We identified two other distantly related families from our registry with the same variant and two other distantly related families with a chr11:116693454 C>T (hg19) variant encoding the missense mutation p.D33N. Both mutations are unique to affected families, evolutionarily conserved and predicted to expand the amyloidogenic hotspot in the ApoA4 structure. Clinically affected individuals suffered from CKD with a bland urinary sediment and a mean age for kidney failure of 64.5 years. Genotyping identified 48 genetically affected individuals; 44 individuals had an estimated glomerular filtration rate (eGFR) under 60 ml/min/1.73 m2, including all 25 individuals with kidney failure. Significantly, 11 of 14 genetically unaffected individuals had an eGFR over 60 ml/min/1.73 m2. Fifteen genetically affected individuals presented with higher plasma ApoA4 concentrations. Kidney pathologic specimens from four individuals revealed amyloid deposits limited to the medulla, with the mutated ApoA4 identified by mass-spectrometry as the predominant amyloid constituent in all three available biopsies. Thus, ApoA4 mutations can cause autosomal dominant medullary amyloidosis, with marked amyloid deposition limited to the kidney medulla and presenting with autosomal dominant CKD with a bland urinary sediment. Diagnosis relies on a careful family history, APOA4 sequencing and pathologic studies.

The Lipocalin Apolipoprotein D Functional Portrait: A Systematic Review

Apolipoprotein D is a chordate gene early originated in the Lipocalin protein family. Among other features, regulation of its expression in a wide variety of disease conditions in humans, as apparently unrelated as neurodegeneration or breast cancer, have called for attention on this gene. Also, its presence in different tissues, from blood to brain, and different subcellular locations, from HDL lipoparticles to the interior of lysosomes or the surface of extracellular vesicles, poses an interesting challenge in deciphering its physiological function: Is ApoD a moonlighting protein, serving different roles in different cellular compartments, tissues, or organisms? Or does it have a unique biochemical mechanism of action that accounts for such apparently diverse roles in different physiological situations? To answer these questions, we have performed a systematic review of all primary publications where ApoD properties have been investigated in chordates. We conclude that ApoD ligand binding in the Lipocalin pocket, combined with an antioxidant activity performed at the rim of the pocket are properties sufficient to explain ApoD association with different lipid-based structures, where its physiological function is better described as lipid-management than by long-range lipid-transport. Controlling the redox state of these lipid structures in particular subcellular locations or extracellular structures, ApoD is able to modulate an enormous array of apparently diverse processes in the organism, both in health and disease. The new picture emerging from these data should help to put the physiological role of ApoD in new contexts and to inspire well-focused future research.

Apolipoprotein A-IV: A Multifunctional Protein Involved in Protection against Atherosclerosis and Diabetes

Apolipoprotein A-IV (apoA-IV) is a lipid-binding protein, which is primarily synthesized in the small intestine, packaged into chylomicrons, and secreted into intestinal lymph during fat absorption. In the circulation, apoA-IV is present on chylomicron remnants, high-density lipoproteins, and also in lipid-free form. ApoA-IV is involved in a myriad of physiological processes such as lipid absorption and metabolism, anti-atherosclerosis, platelet aggregation and thrombosis, glucose homeostasis, and food intake. ApoA-IV deficiency is associated with atherosclerosis and diabetes, which renders it as a potential therapeutic target for treatment of these diseases. While much has been learned about the physiological functions of apoA-IV using rodent models, the action of apoA-IV at the cellular and molecular levels is less understood, let alone apoA-IV-interacting partners. In this review, we will summarize the findings on the molecular function of apoA-IV and apoA-IV-interacting proteins. The information will shed light on the discovery of apoA-IV receptors and the understanding of the molecular mechanism underlying its mode of action.

The structure of human apolipoprotein A-IV as revealed by stable isotope-assisted cross-linking, molecular dynamics, and small angle x-ray scattering

Apolipoprotein (apo)A-IV plays important roles in dietary lipid and glucose metabolism, and knowledge of its structure is required to fully understand the molecular basis of these functions. However, typical of the entire class of exchangeable apolipoproteins, its dynamic nature and affinity for lipid has posed challenges to traditional high resolution structural approaches. We previously reported an x-ray crystal structure of a dimeric truncation mutant of apoA-IV, which showed a unique helix-swapping molecular interface. Unfortunately, the structures of the N and C termini that are important for lipid binding were not visualized. To build a more complete model, we used chemical cross-linking to derive distance constraints across the full-length protein. The approach was enhanced with stable isotope labeling to overcome ambiguities in determining molecular span of the cross-links given the remarkable similarities in the monomeric and dimeric apoA-IV structures. Using 51 distance constraints, we created a starting model for full-length monomeric apoA-IV and then subjected it to two modeling approaches: (i) molecular dynamics simulations and (ii) fitting to small angle x-ray scattering data. This resulted in the most detailed models yet for lipid-free monomeric or dimeric apoA-IV. Importantly, these models were of sufficient detail to direct the experimental identification of new functional residues that participate in a "clasp" mechanism to modulate apoA-IV lipid affinity. The isotope-assisted cross-linking approach should prove useful for further study of this family of apolipoproteins in both the lipid-free and -bound states.

Helical domains that mediate lipid solubilization and ABCA1-specific cholesterol efflux in apolipoproteins C-I and A-II

Many of the apolipoproteins in HDL can elicit cholesterol efflux via ABCA1, a critical initial step in HDL formation. Recent work has indicated that omnipresent amphipathic helices play a critical role, and these have been studied intensively in the most common HDL protein, apolipoprotein (apo)A-I. However, little information exists about helical domain arrangement in other apolipoproteins. We studied two of the smallest apolipoproteins known to interact with ABCA1, human apoA-II and apoC-I, in terms of ability to reorganize phospholipid (PL) bilayers and to promote ABCA1-mediated cholesterol. We found that both proteins contained helical domains that were fast and slow with respect to solubilizing PL. ABCA1-medated efflux required a minimum of a bihelical polypeptide comprised of at least one each of a slow and fast lipid reorganizing domain. In both proteins, the fast helix was located at the C terminus preceded by a slow helix. Helical placement in apoC-I was not critical for ABCA1 activity, but helix swaps in apoA-II dramatically disrupted cholesterol efflux, indicating that the tertiary structure of the longer apolipoprotein is important for the pathway. This work has implications for a more complete molecular understanding of apolipoprotein-mediated cholesterol efflux.

ApoA-IV modulates the secretory trafficking of apoB and the size of triglyceride-rich lipoproteins

Although the evidence linking apoA-IV expression and triglyceride (TG)-rich lipoprotein assembly and secretion is compelling, the intracellular mechanisms by which apoA-IV could modulate these processes remain poorly understood. We therefore examined the functional impact of apoA-IV expression on endogenous apoB, TG, and VLDL secretion in stably transfected McA-RH7777 rat hepatoma cells. Expression of apoA-IV modified with the endoplasmic reticulum (ER) retention signal KDEL (apoA-IV-KDEL) dramatically decreased both the rate and efficiency of endogenous apoB secretion, suggesting a presecretory interaction between apoA-IV-KDEL and apoB or apoB-containing lipoproteins. Expression of native apoA-IV using either a constitutive or tetracycline-inducible promoter delayed the initial rate of apoB secretion and reduced the final secretion efficiency by ∼40%. However, whereas apoA-IV-KDEL reduced TG secretion by 75%, expression of native apoA-IV caused a 20-35% increase in TG secretion, accompanied by a ∼55% increase in VLDL-associated apoB, an increase in the TG:phospholipid ratio of secreted d < 1.006 lipoproteins, and a 10.1 nm increase in peak VLDL(1) particle diameter. Native apoA-IV expression had a negligible impact on expression of the MTP gene. These data suggest that by interacting with apoB in the secretory pathway, apoA-IV alters the trafficking kinetics of apoB-containing TG-rich lipoproteins through cellular lipidation compartments, which in turn, enhances particle expansion and increases TG secretion.

Distinctive structure and interfacial activity of the human apolipoprotein A-IV 347S isoprotein

The T347S polymorphism in the human apolipoprotein (apo) A-IV gene is present at high frequencies among all the world's populations. Carriers of a 347S allele exhibit faster clearance of triglyceride-rich lipoproteins, greater adiposity, and increased risk for developing atherosclerosis, which suggests that this conservative amino acid substitution alters the structure of apo A-IV. Herein we have used spectroscopic and surface chemistry techniques to examine the structure, stability, and interfacial properties of the apo A-IV 347S isoprotein. Circular dichroism spectroscopy revealed that the 347S isoprotein has similar alpha-helical structure but lower thermodynamic stability than the 347T isoprotein. Fluorescence spectroscopy found that the 347S isoprotein exhibits an enhanced tyrosine emission and reduced tyrosine-->tryptophan energy transfer, and second derivative UV absorption spectra noted increased tyrosine exposure, suggesting that the 347S isoprotein adopts a looser tertiary conformation. Surface chemistry studies found that although the 347S isoprotein bound rapidly to the lipid interface, it has a lower interfacial exclusion pressure and lower elastic modulus than the 347T isoprotein. Together, these observations establish that the T347S substitution alters the conformation of apo A-IV and lowers its interfacial activity-changes that could account for the effect of this polymorphism on postprandial lipid metabolism.

A three-dimensional homology model of lipid-free apolipoprotein A-IV using cross-linking and mass spectrometry

Human apolipoprotein A-IV (apoA-IV) is a 46-kDa exchangeable plasma protein with many proposed functions. It is involved in chylomicron assembly and secretion, protection from atherosclerosis through a variety of mechanisms, and inhibition of food intake. There is little structural basis for these proposed functions due to the lack of a solved three-dimensional structure of the protein by x-ray crystallography or NMR. Based on previous studies, we hypothesized that lipid-free apoA-IV exists in a helical bundle, like other apolipoprotein family members and that regions near the N and C termini may interact. Utilizing a homobifunctional lysine cross-linking agent, we identified 21 intramolecular cross-links by mass spectrometry. These cross-links were used to constrain the building of a sequence threaded homology model using the I-TASSER server. Our results indicate that lipid-free apoA-IV does indeed exist as a complex helical bundle with the N and C termini in close proximity. This first structural model of lipid-free apoA-IV should prove useful for designing studies aimed at understanding how apoA-IV interacts with lipids and possibly with unknown protein partners.

Genotype-by-diet effects on co-variation in Lp-PLA2 activity and LDL-cholesterol concentration in baboons fed an atherogenic diet

Both lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) activity, a biomarker of inflammation, and concentration of its primary associated lipoprotein, LDL, are correlated with adverse coronary outcomes. We previously reported a quantitative trait locus (QTL) corresponding to HSA2p24.3-p23.2 with pleiotropic effects on Lp-PLA(2) activity and LDL-cholesterol (LDL-C) concentration in baboons fed a basal diet. Here, our goal was to locate pleiotropic QTLs influencing both traits in the same baboons fed a high-cholesterol, high-fat (HCHF) diet, and to assess whether shared genetic effects on these traits differ between diets. We assayed Lp-PLA(2) activity and LDL-C concentration in 683 baboons fed the HCHF diet. We used a bivariate maximum likelihood-based variance components approach in whole-genome linkage screens to locate a QTL [logarithm of odds (LOD) = 3.13, genome-wide P = 0.019] corresponding to HSA19q12-q13.2 with pleiotropic effects on Lp-PLA(2) activity and LDL-C levels in the HCHF diet. We additionally found significant evidence of genetic variance in response to diet for Lp-PLA(2) activity (P = 0.0017) and for LDL-C concentration (P = 0.00001), revealing a contribution of genotype-by-diet interaction to covariation in these two traits. We conclude that the pleiotropic QTLs detected at 2p24.3-p23.2 and 19q12-q13.2 on the basal and HCHF diets, respectively, exert diet-specific effects on covariation in Lp-PLA(2) activity and LDL-C concentration.

Apolipoprotein CI stimulates the response to lipopolysaccharide and reduces mortality in Gram‐negative sepsis

Gram-negative sepsis is a major death cause in intensive care units. Accumulating evidence indicates the protective role of plasma lipoproteins such as high-density lipoprotein (HDL) in sepsis. It has recently been shown that septic HDL is almost depleted from apolipoprotein CI (apoCI), suggesting that apoCI may be a protective factor in sepsis. Sequence analysis revealed that apoCI possesses a highly conserved consensus KVKEKLK binding motif for lipopolysaccharide (LPS), an outer-membrane component of gram-negative bacteria. Through avid binding to LPS involving this motif, apoCI improved the presentation of LPS to macrophages in vitro and in mice, thereby stimulating the inflammatory response to LPS. Moreover, apoCI dose-dependently increased the early inflammatory response to Klebsiella pneumoniae-induced pneumonia, reduced the number of circulating bacteria, and protected mice against fatal sepsis. Our data support the hypothesis that apoCI is a physiological protector against infection by enhancing the early inflammatory response to LPS and suggest that timely increase of apoCI levels could be used to efficiently prevent and treat early sepsis.

Specific Sequences in the N and C Termini of Apolipoprotein A-IV Modulate Its Conformation and Lipid Association

Apolipoprotein (apoA-IV) is a 376-residue exchangeable apolipoprotein that may play a number of important roles in lipid metabolism, including chylomicron assembly, reverse cholesterol transport, and appetite regulation. In vivo, apoA-IV exists in both lipid-poor and lipid-associated forms, and the balance between these states may determine its function. We examined the structural elements that modulate apoA-IV lipid binding by producing a series of deletion mutants and determining their ability to interact with phospholipid liposomes. We found that the deletion of residues 333-343 strongly increased the lipid association rate versus native apoA-IV. Additional mutagenesis revealed that two phenylalanine residues at positions 334 and 335 mediated this lipid binding inhibitory effect. We also observed that residues 11-20 in the N terminus were required for the enhanced lipid affinity induced by deletion of the C-terminal sequence. We propose a structural model in which these sequences can modulate the conformation and lipid affinity of apoA-IV.

Structure and interfacial properties of human apolipoprotein A-V

Apolipoprotein A-V (apoA-V), the newest member of the plasma apolipoprotein family, was recently discovered by comparison of the mouse and human genomes. Studies in rodents and population surveys of human apoA-V polymorphisms have noted a strong effect of apoA-V on plasma triglyceride levels. Toward the elucidation of the biologic function of apoA-V, we used spectroscopic and surface chemistry techniques to probe its structure and interfacial activity. Computer-assisted sequence analysis of apoA-V predicts that it is very hydrophobic, contains a significant amount of alpha-helical secondary structure, and probably is composed of discrete structural regions with varying degrees of lipid affinity. Fluorescence spectroscopy of recombinant human apoA-V provided evidence of tertiary folding, and light scattering studies indicated that apoA-V transforms dimyristoylphosphatidylcholine vesicles into discoidal complexes with an efficiency similar to that of apoA-I. Surface chemistry techniques revealed that apoA-V displays high affinity, low elasticity, and slow binding kinetics at hydrophobic interfaces, properties we propose may retard triglyceride-rich particle assembly. Metabolic labeling and immunofluorescence studies of COS-1 cells transfected with human apoA-V demonstrated that apoA-V is poorly secreted, remains associated with the endoplasmic reticulum, and does not traffic to the Golgi. Given that overexpression of the apoA-V gene lowers plasma triglycerides in mice, these data together suggest that apoA-V may function intracellularly to modulate hepatic VLDL synthesis and/or secretion.

Apolipoprotein A-IV polymorphisms and diet-gene interactions

Apolipoprotein A-IV is a 46kDa glycoprotein that is synthesized by intestinal enterocytes and is incorporated into the surface of nascent chylomicrons. Considerable evidence suggests that apolipoprotein A-IV plays a role in intestinal lipid absorption and chylomicron assembly. We have proposed that polymorphisms that alter the interfacial behavior of apolipoprotein A-IV may modulate the physical properties and metabolic fate of plasma chylomicrons. Of the reported genetic polymorphisms of apolipoprotein A-IV, two, Q360H and T347S, are known to occur at high frequencies among the world populations. Biophysical studies have established that the Q360H isoprotein displays higher lipid affinity; conversely the T347S isoprotein is predicted to be less lipid avid. Recent studies have shown that the Q360H polymorphism is associated with increased postprandial hypertriglyceridemia, a reduced low-density lipoprotein response to dietary cholesterol in the setting of a moderate fat intake, an increased high-density lipoprotein response to changes in total dietary fat content, and lower body mass and adiposity; the T347S polymorphism appears to confer the opposite effects. Studies on the diet-gene interactions of other apolipoprotein A-IV alleles are needed, as are studies on the interactions between apolipoprotein A-IV alleles and other apolipoprotein polymorphisms.

Expression and conservation of apolipoprotein AIV in an avian species

In birds, intestinally derived lipoproteins are thought to be secreted directly into the portal vein rather than to enter the circulation via the lymphatic system as in mammals. Hepatic clearance of these so-called portomicrons must be rapid, but the protein(s) mediating their catabolism, presumably analogues of the 36-kDa mammalian apolipoprotein E, have not been identified. In searching for such a mediator(s), we have isolated a hitherto unknown 38-kDa protein from chicken serum, which we identified by microsequencing and molecular cloning as a counterpart to mammalian apolipoprotein AIV (apoAIV). Mature chicken apoAIV consists of 347 amino acids, lacks cysteine residues, and displays 57% sequence identity with human apoAIV and, to a significantly lesser extent, with apoAIVs of rodents. This first nonmammalian apoAIV characterized is the smallest homologue reported so far, because of the lack of repeated motifs at the carboxyl terminus with the consensus sequence Glu-Gln-Glu/Ala-Gln, a hallmark of mammalian apoAIVs. Chicken apoAIV (isoelectric point, 4.65) is also considerably more acidic than its human counterpart. Agarose gel electrophoresis revealed that unlike human apoAIV, which migrates to a pre-alpha-position, chicken apoAIV shows fast alpha migration. Functional characterization demonstrated that the avian protein is able to activate the enzyme lecithin:cholesterol acyltransferase. Roosters and hens express apoAIV predominantly in the gut, one-fifth as much in the liver, and no other sites of expression are identifiable by Northern blot analysis. Although pronounced intestinal synthesis is common to apoAIVs, the features of the avian protein support the notion that it represents a prototype of an apoprotein that evolved to acquire possibly distinct functions in mammals and birds.

Identification of a sequence of apolipoprotein A-I associated with the efflux of intracellular cholesterol to human serum and apolipoprotein A-I containing particles

The effect of monoclonal antibodies against apolipoprotein A-I (apoA-I) on the efflux of intracellular and plasma membrane cholesterol from HepG2 cells to human serum, high-density lipoprotein (HDL), apoA-I, and apoA-I/phosphatidylcholine complex (apoA-I/PC) was studied. Fab fragments of two monoclonal antibodies, AI-3 (residues 140-147) and Al-4.2 (residues 149-150), inhibited the efflux of intracellular cholesterol to serum in a dose-dependent manner. In combination, these antibodies were twice as effective than when used alone. None of the antibodies tested inhibited efflux of the plasma membrane cholesterol. When different types of acceptors were compared for their ability to promote intracellular cholesterol efflux, they were effective in the following order: serum > HDL > apoA-I/PC > pure apoA-I. Antibody AI-3 inhibited efflux of intracellular cholesterol to serum, HDL, and pure apoA-I, but not to apoA-I/PC. Antibody AI-4.2 inhibited efflux to serum, apoA-I/PC, and pure apoA-I, but not to HDL. An explanation for this is that antibody AI-4.2 reacts poorly with isolated alpha-HDL in an immunoprecipitation assay and has higher affinity for pre beta 2-HDL and pre beta 3-HDL particles than antibody AI-3 in nondenaturing two-dimensional electrophoresis. In conclusion, we have demonstrated that a region of apoA-I within or adjacent to residues 140-150 determines the ability of apoA-I to promote intracellular cholesterol efflux.