Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency

Τhe Antioxidant Function of HDL in Atherosclerosis

Atherosclerosis is a multistep process that progresses over a long period of time and displays a broad range of severity. In its final form, it manifests as a lesion of the intimal layer of the arterial wall. There is strong evidence supporting that oxidative stress contributes to coronary heart disease morbidity and mortality and antioxidant high-density lipoprotein (HDL) could have a beneficial role in the prevention and prognosis of the disease. Indeed, certain subspecies of HDL may act as natural antioxidants preventing oxidation of lipids on low-density lipoprotein (LDL) and biological membranes. The antioxidant function may be attributed to inhibition of synthesis or neutralization of free radicals and reactive oxygen species by HDL lipids and associated enzymes or transfer of oxidation prone lipids from LDL and biological membranes to HDL for catabolism. A limited number of clinical trials suggest that the increased antioxidant potential of HDL correlates with decreased risk for atherosclerosis. Some nutritional interventions to increase HDL antioxidant activity have been proposed with limited success so far. The limitations in measuring and understanding HDL antioxidant function in vivo are also discussed.

Targeted Deletion of Hepatocyte Abca1 Increases Plasma HDL (High-Density Lipoprotein) Reverse Cholesterol Transport via the LDL (Low-Density Lipoprotein) Receptor

OBJECTIVE

The role of hepatocyte Abca1 (ATP binding cassette transporter A1) in trafficking hepatic free cholesterol (FC) into plasma versus bile for reverse cholesterol transport (RCT) is poorly understood. We hypothesized that hepatocyte Abca1 recycles plasma HDL-C (high-density lipoprotein cholesterol) taken up by the liver back into plasma, maintaining the plasma HDL-C pool, and decreasing HDL-mediated RCT into feces. Approach and Results: Chow-fed hepatocyte-specific Abca1 knockout (HSKO) and control mice were injected with human HDL radiolabeled with ¹²⁵I-tyramine cellobiose (¹²⁵I-TC; protein) and ³H-cholesteryl oleate (³H-CO). ¹²⁵I-TC and ³H-CO plasma decay, plasma HDL ³H-CO selective clearance (ie, ³H-¹²⁵I fractional catabolic rate), liver radiolabel uptake, and fecal ³H-sterol were significantly greater in HSKO versus control mice, supporting increased plasma HDL RCT. Twenty-four hours after ³H-CO-HDL injection, HSKO mice had reduced total hepatic ³H-FC (ie, ³H-CO hydrolyzed to ³H-FC in liver) resecretion into plasma, demonstrating Abca1 recycled HDL-derived hepatic ³H-FC back into plasma. Despite similar liver LDLr (low-density lipoprotein receptor) expression between genotypes, HSKO mice treated with LDLr-targeting versus control antisense oligonucleotide had slower plasma ³H-CO-HDL decay, reduced selective ³H-CO clearance, and decreased fecal ³H-sterol excretion that was indistinguishable from control mice. Increased RCT in HSKO mice was selective for ³H-CO-HDL, since macrophage RCT was similar between genotypes.

CONCLUSIONS

Hepatocyte Abca1 deletion unmasks a novel and selective FC trafficking pathway that requires LDLr expression, accelerating plasma HDL-selective CE uptake by the liver and promoting HDL RCT into feces, consequently reducing HDL-derived hepatic FC recycling into plasma.

Triplet Male Lambs Are More Susceptible than Twins to Dietary Soybean Oil-Induced Fatty Liver

BACKGROUND

Litter size affects fetal development but its relation to diet-induced fatty liver later in life is unknown.

OBJECTIVES

This aim of this study was to test the hypothesis that litter size influences postweaning fatty liver development in response to soybean oil-supplemented diet.

METHODS

Weanling twin (TW) or triplet (TP) male lambs (n = 16) were fed a control diet or 2% soybean oil-supplemented diet (SO) for 90 d. Liver tissue morphology, biochemical parameters, and lipid metabolic enzymes were determined. Hepatic gene expression was analyzed by RNA sequencing (n = 3), followed by enrichment analysis according to Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes. Differentially expressed genes involved in lipid metabolism were further verified by quantitative reverse transcriptase-polymerase chain reaction (n = 4). All data were analyzed by a 2-factor ANOVA, apart from differentially expressed genes, which were identified by the Benjamini-Hochberg approach (q value ≤0.05).

RESULTS

SO increased liver triglyceride (by 55%) and nonesterified fatty acid (by 54%) concentrations in TPs (P ≤ 0.05) but not in TWs (P > 0.05). SO also induced a 2.3- and 2.1-fold increase in the liver steatosis score of TPs and TWs, respectively (P ≤ 0.05). Moreover, SO reduced the activity of lipolytic enzymes including hepatic lipase and total lipase in TPs by 47% and 25%, respectively (P ≤ 0.05). In contrast, activities of lipogenic enzymes, including malic enzyme and acetyl coenzyme A carboxylase, were significantly higher in TPs (P ≤ 0.05). Moreover, TPs had higher expression of lipogenic genes, such as FASN (by 45%) and APOB (by 72%), and lower expression of lipolytic genes, such as PRKAA2 (by 28%) and CPT1A (by 43%), compared with TWs (P ≤ 0.05).

CONCLUSIONS

TPs have a gene expression profile that is more susceptible to SO-induced fatty liver than that of TWs, which indicates that insufficient maternal nutrient supply at fetal and neonatal stages may increase the risk of nonalcoholic fatty liver disease.

HDL from an Alzheimer's disease perspective

PURPOSE OF REVIEW

We review current knowledge regarding HDL and Alzheimer's disease, focusing on HDL's vasoprotective functions and potential as a biomarker and therapeutic target for the vascular contributions of Alzheimer's disease.

RECENT FINDINGS

Many epidemiological studies have observed that circulating HDL levels associate with decreased Alzheimer's disease risk. However, it is now understood that the functions of HDL may be more informative than levels of HDL cholesterol (HDL-C). Animal model studies demonstrate that HDL protects against memory deficits, neuroinflammation, and cerebral amyloid angiopathy (CAA). In-vitro studies using state-of-the-art 3D models of the human blood-brain barrier (BBB) confirm that HDL reduces vascular Aβ accumulation and attenuates Aβ-induced endothelial inflammation. Although HDL-based therapeutics have not been tested in clinical trials for Alzheimer's disease , several HDL formulations are in advanced phase clinical trials for coronary artery disease and atherosclerosis and could be leveraged toward Alzheimer's disease .

SUMMARY

Evidence from human studies, animal models, and bioengineered arteries supports the hypothesis that HDL protects against cerebrovascular dysfunction in Alzheimer's disease. Assays of HDL functions relevant to Alzheimer's disease may be desirable biomarkers of cerebrovascular health. HDL-based therapeutics may also be of interest for Alzheimer's disease, using stand-alone or combination therapy approaches.

Reactions at Biomembrane Interfaces

Membranes surrounding the biological cell and its internal compartments host proteins that catalyze chemical reactions essential for the functioning of the cell. Rather than being a passive structural matrix that holds membrane-embedded proteins in place, the membrane can largely shape the conformational energy landscape of membrane proteins and impact the energetics of their chemical reaction. Here, we highlight the challenges in understanding how lipids impact the conformational energy landscape of macromolecular membrane complexes whose functioning involves chemical reactions including proton transfer. We review here advances in our understanding of how chemical reactions occur at membrane interfaces gleaned with both theoretical and experimental advances using simple protein systems as guides. Our perspective is that of bridging experiments with theory to understand general physicochemical principles of membrane reactions, with a long term goal of furthering our understanding of the role of the lipids on the functioning of complex macromolecular assemblies at the membrane interface.

Identification of pathogenic genes of pterygium based on the Gene Expression Omnibus database

AIM

To identify the pathogenic genes in pterygium.

METHODS

We obtained mRNA expression profiles from the Gene Expression Omnibus database (GEO) to identify differentially expressed genes (DEGs) between pterygium tissues and normal conjunctiva tissues. The Gene Ontology, Kyoto Encyclopedia of Genes and Genomes pathway analysis, protein-protein interaction (PPI) network and transcription factors (TFs)-target gene regulatory network was performed to understand the function of DEGs. The expression of selected DEGs were validated by the quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

A total of 557 DEGs were identified between pterygium and normal individual. In PPI network, several genes were with high degrees such as FN1, KPNB1, DDB1, NF2 and BUB3. SSH1, PRSS23, LRP5L, MEOX1, RBM14, ABCA1, JOSD1, KRT6A and UPK1B were the most downstream genes regulated by TFs. qRT-PCR results showed that FN1, PRSS23, ABCA1, KRT6A, ECT2 and SPARC were significantly up-regulated in pterygium and MEOX1 and MMP3 were also up-regulated with no significance, which was consistent with the our integrated analysis.

CONCLUSION

The deregulated genes might be involved in the pathology of pterygium and could be used as treatment targets for pterygium.

Evacetrapib reduces preβ-1 HDL in patients with atherosclerotic cardiovascular disease or diabetes

BACKGROUND AND AIMS

Cholesteryl ester transfer protein (CETP) inhibitor-mediated induction of HDL-cholesterol has no effect on the protection from cardiovascular disease (CVD). However, the mechanism is still unknown. Data on the effects of this class of drugs on subclasses of HDL are either limited or insufficient. In this study, we investigated the effect of evacetrapib, a CETP inhibitor, on subclasses of HDL in patients with atherosclerotic cardiovascular disease or diabetes.

METHODS

Baseline and 3-month post-treatment samples from atorvastatin 40 mg plus evacetrapib 130 mg (n = 70) and atorvastatin 40 mg plus placebo (n = 30) arms were used for this purpose. Four subclasses of HDL (large HDL, medium HDL, small HDL, and preβ-1 HDL) were separated according to their size and quantified by densitometry using a recently developed native polyacrylamide gel electrophoresis (PAGE) system.

RESULTS

Relative to placebo, while evacetrapib treatment dramatically increased large HDL and medium HDL subclasses, it significantly reduced small HDL (27%) as well as preβ-1 HDL (36%) particles. Evacetrapib treatment reduced total LDL, but also resulted in polydisperse LDL with LDL particles larger and smaller than the LDL subclasses of the placebo group.

CONCLUSION

Evacetrapib reduced preβ-1 HDL and small HDL in patients with ASCVD or diabetes on statin. Preβ-1 HDL and medium HDL are negatively interrelated. The results could give a clue to understand the effect of CETP inhibitors on cardiovascular outcomes.

Extracellular vesicles in atherosclerosis

Extracellular vesicles (EVs), which exist in human blood, are increased in some inflammation-related cardiovascular diseases. EVs are involved in inflammation, immunity, signal transduction, cell survival and apoptosis, angiogenesis, thrombosis, and autophagy, all of which are highly significant for maintaining homeostasis and disease progression. Therefore, EVs are also associated with key steps in atherosclerosis, including cellular lipid metabolism, endothelial dysfunction and vascular wall inflammation, ultimately resulting in vascular remodelling. In this review, we summarize recent studies on EV contents and biological function, focusing on their potential effect in atherosclerosis, including cholesterol metabolism, vascular inflammation, angiogenesis, coagulation and the development of atherosclerotic lesions. EVs may represent potential biomarkers and pharmacological targets for atherosclerotic diseases.

Progress in finding pathogenic DNA copy number variations in dyslipidemia

PURPOSE OF REVIEW

DNA copy number variations (CNVs) are large-scale mutations that include deletions and duplications larger than 50 bp in size. In the era when single-nucleotide variations were the major focus of genetic technology and research, CNVs were largely overlooked. However, CNVs clearly underlie a substantial proportion of clinical disorders. Here, we update recent progress in identifying CNVs in dyslipidemias.

RECENT FINDINGS

Until last year, only the LDLR and LPA genes were appreciated as loci within which clinically relevant CNVs contributed to familial hypercholesterolemia and variation in Lp(a) levels, respectively. Since 2017, next-generation sequencing panels have identified pathogenic CNVs in at least five more genes underlying dyslipidemias, including a PCSK9 whole-gene duplication in familial hypercholesterolemia; LPL, GPIHBP1, and APOC2 deletions in hypertriglyceridemia; and ABCA1 deletions in hypoalphalipoproteinemia.

SUMMARY

CNVs are an important class of mutation that contribute to the molecular genetic heterogeneity underlying dyslipidemias. Clinical applications of next-generation sequencing technologies need to consider CNVs concurrently with familiar small-scale genetic variation, given the likely implications for improved diagnosis and treatment.

Changes in the asymmetric distribution of cholesterol in the plasma membrane influence streptolysin O pore formation

ATP-binding cassette A1 (ABCA1) plays a key role in generating high-density lipoprotein (HDL) and preventing atherosclerosis. ABCA1 exports cholesterol and phospholipid to apolipoprotein A-I (apoA-I) in serum to generate HDL. We found that streptolysin O (SLO), a cholesterol-dependent pore-forming toxin, barely formed pores in ABCA1-expressing cells, even in the absence of apoA-I. Neither cholesterol content in cell membranes nor the amount of SLO bound to cells was affected by ABCA1. On the other hand, binding of the D4 domain of perfringolysin O (PFO) to ABCA1-expressing cells increased, suggesting that the amount of cholesterol in the outer leaflet of the plasma membrane (PM) increased and that the cholesterol dependences of these two toxins differ. Addition of cholesterol to the PM by the MβCD-cholesterol complex dramatically restored SLO pore formation in ABCA1-expressing cells. Therefore, exogenous expression of ABCA1 causes reduction in the cholesterol level in the inner leaflet, thereby suppressing SLO pore formation.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Apolipoprotein A-I directly interacts with extracellular domain 1 of human ABCA1

ATP-binding cassette transporter A1 (ABCA1) is critical for the generation of nascent high-density lipoprotein (HDL) and plays important roles in cholesterol homeostasis. ABCA1 has two large extracellular domains (ECDs), which may interact directly with apolipoprotein A-I (apoA-I). However, the molecular mechanisms underlying HDL formation and the importance of ABCA1–apoA-I interactions in HDL formation remain unclear. We investigated the ABCA1–apoA-I interaction in photo-activated crosslinking experiments using sulfo-SBED–labeled apoA-I. ApoA-I bound to cells expressing ABCA1, but not to untransfected cells or cells expressing non-functional ABCA1. Binding was inhibited by sulfo-SBED–labeled apoA-I, and crosslinking of sulfo-SBED–labeled apoA-I with ABCA1 was inhibited by non-labeled apoA-I, suggesting that sulfo-SBED–labeled apoA-I specifically binds and crosslinks with functional ABCA1. Proteolytic digestion of crosslinked ABCA1 revealed that apoA-I bound the N-terminal half of ABCA1, and that the first ECD of ABCA1 is an apoA-I binding site.

Abbreviations: ABC: ATP-binding cassette; apoA-I: apolipoprotein A-I; ATP: adenosine triphosphate; CHAPS: 3-(3-cholamidepropyl)dimethylammonio-1- propanesulphonate; DTT: dithiothreitol; ECD: extra cellular domain; EDTA: ethylenediaminetetraacetic acid; GFP: green fluorescent protein; HA: hemagglutinin; HDL: high density lipoprotein; HEK: human embryonic kidney; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; sulfo-SBED: (sulfosuccinimidyl-2-[6-(biotinamido)-2-(p-azidobenzamido)hexanoamido] ethyl-1,3ʹ-dithiopropionate; NHS-ester, N-hydroxysuccinimide-ester

Protective Variants in Alzheimer's Disease

PURPOSE OF REVIEW

Over the last decade over 40 loci have been associated with risk of Alzheimer's disease (AD). However, most studies have either focused on identifying risk loci or performing unbiased screens without a focus on protective variation in AD. Here, we provide a review of known protective variants in AD and their putative mechanisms of action. Additionally, we recommend strategies for finding new protective variants.

RECENT FINDINGS

Recent Genome-Wide Association Studies have identified both common and rare protective variants associated with AD. These include variants in or near APP, APOE, PLCG2, MS4A, MAPT-KANSL1, RAB10, ABCA1, CCL11, SORL1, NOCT, SCL24A4-RIN3, CASS4, EPHA1, SPPL2A, and NFIC.

SUMMARY

There are very few protective variants with functional evidence and a derived allele with a frequency below 20%. Additional fine mapping and multi-omic studies are needed to further validate and characterize known variants as well as specialized genome-wide scans to identify novel variants.

A review on coronary artery disease, its risk factors, and therapeutics

Coronary artery disease (CAD) is one of the major cardiovascular diseases affecting the global human population. This disease has been proved to be the major cause of death in both the developed and developing countries. Lifestyle, environmental factors, and genetic factors pose as risk factors for the development of cardiovascular disease. The prevalence of risk factors among healthy individuals elucidates the probable occurrence of CAD in near future. Genome-wide association studies have suggested the association of chromosome 9p21.3 in the premature onset of CAD. The risk factors of CAD include diabetes mellitus, hypertension, smoking, hyperlipidemia, obesity, homocystinuria, and psychosocial stress. The eradication and management of CAD has been established through extensive studies and trials. Antiplatelet agents, nitrates, β-blockers, calcium antagonists, and ranolazine are some of the few therapeutic agents used for the relief of symptomatic angina associated with CAD.

Genome-wide association study (GWAS) of human host factors influencing viral severity of herpes simplex virus type 2 (HSV-2)

Herpes simplex virus type 2 (HSV-2) is an incurable viral infection with severity ranging from asymptomatic to frequent recurrences. The viral shedding rate has been shown as a reproducible HSV-2 severity end point that correlates with lesion rates. We used a genome-wide association study (GWAS) to investigate the role of common human genetic variation in HSV-2 severity. We performed a GWAS on 223 HSV-2-positive participants of European ancestry. Severity was measured by viral shedding rate, as defined by the percent of days PCR+ for HSV-2 DNA over at least 30 days. Analyses were performed under linear regression models, adjusted for age, sex, and ancestry. There were no genome-wide significant (p < 5E-08) associations with HSV-2 viral shedding rate. The top nonsignificant SNP (rs75932292, p = 6.77E-08) associated with HSV-2 viral shedding was intergenic, with the nearest known biologically interesting gene (ABCA1) ~130 kbp downstream. Several other SNPs approaching significance were in or near genes with viral or neurological associations, including four SNPs in KIF1B. The current study is the first comprehensive genome-wide investigation of human genetic variation in virologic severity of established HSV-2 infection. However, no significant associations were observed with HSV-2 virologic severity, leaving the exact role of human variation in HSV-2 severity unclear.

Genetic control of the mouse HDL proteome defines HDL traits, function, and heterogeneity

HDLs are nanoparticles with more than 80 associated proteins, phospholipids, cholesterol, and cholesteryl esters. The potential inverse relation of HDL to coronary artery disease (CAD) and the effects of HDL on myriad other inflammatory conditions warrant a better understanding of the genetic basis of the HDL proteome. We conducted a comprehensive genetic analysis of the regulation of the proteome of HDL isolated from a panel of 100 diverse inbred strains of mice (the hybrid mouse diversity panel) and examined protein composition and efflux capacity to identify novel factors that affect the HDL proteome. Genetic analysis revealed widely varied HDL protein levels across the strains. Some of this variation was explained by local cis-acting regulation, termed cis-protein quantitative trait loci (QTLs). Variations in apoA-II and apoC-3 affected the abundance of multiple HDL proteins, indicating a coordinated regulation. We identified modules of covarying proteins and defined a protein-protein interaction network that describes the protein composition of the naturally occurring subspecies of HDL in mice. Sterol efflux capacity varied up to 3-fold across the strains, and HDL proteins displayed distinct correlation patterns with macrophage and ABCA1-specific cholesterol efflux capacity and cholesterol exchange, suggesting that subspecies of HDL participate in discrete functions. The baseline and stimulated sterol efflux capacity phenotypes were associated with distinct QTLs with smaller effect size, suggesting a multigenetic regulation. Our results highlight the complexity of HDL particles by revealing the high degree of heterogeneity and intercorrelation, some of which is associated with functional variation, and support the concept that HDL-cholesterol alone is not an accurate measure of HDL’s properties, such as protection against CAD.

Vitamin E Metabolic Effects and Genetic Variants: A Challenge for Precision Nutrition in Obesity and Associated Disturbances

Vitamin E (VE) has a recognized leading role as a contributor to the protection of cell constituents from oxidative damage. However, evidence suggests that the health benefits of VE go far beyond that of an antioxidant acting in lipophilic environments. In humans, VE is channeled toward pathways dealing with lipoproteins and cholesterol, underlining its relevance in lipid handling and metabolism. In this context, both VE intake and status may be relevant in physiopathological conditions associated with disturbances in lipid metabolism or concomitant with oxidative stress, such as obesity. However, dietary reference values for VE in obese populations have not yet been defined, and VE supplementation trials show contradictory results. Therefore, a better understanding of the role of genetic variants in genes involved in VE metabolism may be crucial to exert dietary recommendations with a higher degree of precision. In particular, genetic variability should be taken into account in targets concerning VE bioavailability per se or concomitant with impaired lipoprotein transport. Genetic variants associated with impaired VE liver balance, and the handling/resolution of oxidative stress might also be relevant, but the core information that exists at present is insufficient to deliver precise recommendations.

Rare genetic variation implicated in non-Hispanic white families with Alzheimer disease

OBJECTIVE

To identify genetic variation influencing late-onset Alzheimer disease (LOAD), we used a large data set of non-Hispanic white (NHW) extended families multiply-affected by LOAD by performing whole genome sequencing (WGS).

METHODS

As part of the Alzheimer Disease Sequencing Project, WGS data were generated for 197 NHW participants from 42 families (affected individuals and unaffected, elderly relatives). A two-pronged approach was taken. First, variants were prioritized using heterogeneity logarithm of the odds (HLOD) and family-specific LOD scores as well as annotations based on function, frequency, and segregation with disease. Second, known Alzheimer disease (AD) candidate genes were assessed for rare variation using a family-based association test.

RESULTS

We identified 41 rare, predicted-damaging variants that segregated with disease in the families that contributed to the HLOD or family-specific LOD regions. These included a variant in nitric oxide synthase 1 adaptor protein that segregates with disease in a family with 7 individuals with AD, as well as variants in RP11-433J8, ABCA1, and FISP2. Rare-variant association identified 2 LOAD candidate genes associated with disease in these families: FERMT2 (p-values = 0.001) and SLC24A4 (p-value = 0.009). These genes still showed association while controlling for common index variants, indicating the rare-variant signal is distinct from common variation that initially identified the genes as candidates.

CONCLUSIONS

We identified multiple genes with putative damaging rare variants that segregate with disease in multiplex AD families and showed that rare variation may influence AD risk at AD candidate genes. These results identify novel AD candidate genes and show a role for rare variation in LOAD etiology, even at genes previously identified by common variation.

A novel splicing mutation in the ABCA1 gene, causing Tangier disease and familial HDL deficiency in a large family

Tangier disease is a rare disorder of lipoprotein metabolism that presents with extremely low levels of HDL cholesterol and apoprotein A-I. It is caused by mutations in the ATP-binding cassette transporter A1 (ABCA1) gene. Clinical heterogeneity and mutational pattern of Tangier disease are poorly characterized. Moreover, also familial HDL deficiency may be caused by mutations in ABCA1 gene. ATP-binding cassette transporter A1 (ABCA1) gene mutations in a patient with Tangier disease, who presented an uncommon clinical history, and in his family were found and characterized. He was found to be compound heterozygous for two intronic mutations of ABCA1 gene, causing abnormal pre-mRNAs splicing. The novel c.1510-1G > A mutation was located in intron 12 and caused the activation of a cryptic splice site in exon 13, which determined the loss of 22 amino acids of exon 13 with the introduction of a premature stop codon. Five heterozygous carriers of this mutation were also found in proband's family, all presenting reduced HDL cholesterol and ApoAI (0.86 ± 0.16 mmol/L and 92.2 ± 10.9 mg/dL respectively), but not the typical features of Tangier disease, a phenotype compatible with the diagnosis of familial HDL deficiency. The other known mutation c.1195-27G > A was confirmed to cause aberrant retention of 25 nucleotides of intron 10 leading to the insertion of a stop codon after 20 amino acids of exon 11. Heterozygous carriers of this mutation also showed the clinical phenotype of familial HDL deficiency. Our study extends the catalog of pathogenic intronic mutations affecting ABCA1 pre-mRNA splicing. In a large family, a clear demonstration that the same mutations may cause Tangier disease (if in compound heterozygosis) or familial HDL deficiency (if in heterozygosis) is provided.

Reconstituted Discoidal High-Density Lipoproteins: Bioinspired Nanodiscs with Many Unexpected Applications

PURPOSE OF REVIEW

Summarize the initial discovery of discoidal high-density lipoprotein (HDL) in human plasma and review more recent innovations that span the use of reconstituted nanodisc HDL for membrane protein characterization to its use as a drug carrier and a novel therapeutic agent for cardiovascular disease.

RECENT FINDINGS

Using a wide variety of biophysical techniques, the structure and composition of endogenous discoidal HDL have now largely been solved. This has led to the development of new methods for the in vitro reconstitution of nanodisc HDL, which have proven to have a wide variety of biomedical applications. Nanodisc HDL has been used as a platform for mimicking the plasma membrane for the reconstitution and investigation of the structures of several plasma membrane proteins, such as cytochrome P450s and ABC transporters. Nanodisc HDL has also been designed as drug carriers to transport amphipathic, as well as hydrophobic small molecules, and has potential therapeutic applications for several diseases. Finally, nanodisc HDL itself like native discoidal HDL can mediate cholesterol efflux from cells and are currently being tested in late-stage clinical trials for cardiovascular disease. The discovery of the characterization of native discoidal HDL has inspired a new field of synthetic nanodisc HDL, which has offered a growing number of unanticipated biomedical applications.

Administration of Downstream ApoE Attenuates the Adverse Effect of Brain ABCA1 Deficiency on Stroke

The ATP-binding cassette transporter member A1 (ABCA1) and apolipoprotein E (ApoE) are major cholesterol transporters that play important roles in cholesterol homeostasis in the brain. Previous research demonstrated that specific deletion of brain-ABCA1 (ABCA1^-B/-B) reduced brain grey matter (GM) and white matter (WM) density in the ischemic brain and decreased functional outcomes after stroke. However, the downstream molecular mechanism underlying brain ABCA1-deficiency-induced deficits after stroke is not fully understood. Adult male ABCA1^-B/-B and ABCA1-floxed control mice were subjected to distal middle-cerebral artery occlusion and were intraventricularly infused with artificial mouse cerebrospinal fluid as vehicle control or recombinant human ApoE2 into the ischemic brain starting 24 h after stroke for 14 days. The ApoE/apolipoprotein E receptor 2 (ApoER2)/high-density lipoprotein (HDL) levels and GM/WM remodeling and functional outcome were measured. Although ApoE2 increased brain ApoE/HDL levels and GM/WM density, negligible functional improvement was observed in ABCA1-floxed-stroke mice. ApoE2-administered ABCA1^-B/-B stroke mice exhibited elevated levels of brain ApoE/ApoER2/HDL, increased GM/WM density, and neurogenesis in both the ischemic ipsilateral and contralateral brain, as well as improved neurological function compared with the vehicle-control ABCA1^-B/-B stroke mice 14 days after stroke. Ischemic lesion volume was not significantly different between the two groups. In vitro supplementation of ApoE2 into primary cortical neurons and primary oligodendrocyte-progenitor cells (OPCs) significantly increased ApoER2 expression and enhanced cholesterol uptake. ApoE2 promoted neurite outgrowth after oxygen-glucose deprivation and axonal outgrowth of neurons, and increased proliferation/survival of OPCs derived from ABCA1^-B/-B mice. Our data indicate that administration of ApoE2 minimizes the adverse effects of ABCA1 deficiency after stroke, at least partially by promoting cholesterol traffic/redistribution and GM/WM remodeling via increasing the ApoE/HDL/ApoER2 signaling pathway.

Genetic and secondary causes of severe HDL deficiency and cardiovascular disease

We assessed secondary and genetic causes of severe HDL deficiency in 258,252 subjects, of whom 370 men (0.33%) and 144 women (0.099%) had HDL cholesterol levels <20 mg/dl. We excluded 206 subjects (40.1%) with significant elevations of triglycerides, C-reactive protein, glycosylated hemoglobin, myeloperoxidase, or liver enzymes and men receiving testosterone. We sequenced 23 lipid-related genes in 201 (65.3%) of 308 eligible subjects. Mutations (23 novel) and selected variants were found at the following gene loci: 1) ABCA1 (26.9%): 2 homozygotes, 7 compound or double heterozygotes, 30 heterozygotes, and 2 homozygotes and 13 heterozygotes with variants rs9282541/p.R230C or rs111292742/c.-279C>G; 2) LCAT (12.4%): 1 homozygote, 3 compound heterozygotes, 13 heterozygotes, and 8 heterozygotes with variant rs4986970/p.S232T; 3) APOA1 (5.0%): 1 homozygote and 9 heterozygotes; and 4) LPL (4.5%): 1 heterozygote and 8 heterozygotes with variant rs268/p.N318S. In addition, 4.5% had other mutations, and 46.8% had no mutations. Atherosclerotic cardiovascular disease (ASCVD) prevalence rates in the ABCA1, LCAT, APOA1, LPL, and mutation-negative groups were 37.0%, 4.0%, 40.0%, 11.1%, and 6.4%, respectively. Severe HDL deficiency is uncommon, with 40.1% having secondary causes and 48.8% of the subjects sequenced having ABCA1, LCAT, APOA1, or LPL mutations or variants, with the highest ASCVD prevalence rates being observed in the ABCA1 and APOA1 groups.

ATP binding cassette family A protein 1 determines hexosylceramide and sphingomyelin levels in human and mouse plasma

Sphingolipids, including ceramide, SM, and hexosylceramide (HxCer), are carried in the plasma by lipoproteins. They are possible markers of metabolic diseases, but little is known about their control. We previously showed that microsomal triglyceride transfer protein (MTP) is critical to determine plasma ceramide and SM, but not HxCer, levels. In human plasma and mouse models, we examined possible HxCer-modulating pathways, including the role of ABCA1 in determining sphingolipid plasma concentrations. Compared with control samples, plasma from patients with Tangier disease (deficient in ABCA1) had significantly lower HxCer (-69%) and SM (-40%) levels. Similarly, mice deficient in hepatic and intestinal ABCA1 had significantly reduced HxCer (-79%) and SM (-85%) levels. Tissue-specific ablation studies revealed that hepatic ABCA1 determines plasma HxCer and SM levels; that ablation of MTP and ABCA1 in the liver and intestine reduces plasma HxCer, SM, and ceramide levels; and that hepatic and intestinal MTP contribute to plasma ceramide levels, whereas only hepatic MTP modulates plasma SM levels. These results identify the contribution of ABCA1 to plasma SM and HxCer levels and suggest that MTP and ABCA1 are critical determinants of plasma sphingolipid levels.

Identification of 26 novel loci that confer susceptibility to early-onset coronary artery disease in a Japanese population

Early-onset coronary artery disease (CAD) has a strong genetic component. Although genome-wide association studies have identified various genes and loci significantly associated with CAD mainly in European populations, genetic variants that contribute toward susceptibility to this condition in Japanese patients remain to be definitively identified. In the present study, exome-wide association studies (EWASs) were performed to identify genetic variants that confer susceptibility to early-onset CAD in Japanese. A total of 7,256 individuals aged ≤65 years were enrolled in the present study. EWAS were conducted on 1,482 patients with CAD and 5,774 healthy controls. Genotyping of single nucleotide polymorphisms (SNPs) was performed using Illumina Human Exome-12 DNA Analysis BeadChip or Infinium Exome-24 BeadChip arrays. The association between allele frequencies for 31,465 SNPs that passed quality control and CAD was examined using Fisher's exact test. To compensate for multiple comparisons of allele frequencies with CAD, a false discovery rate (FDR) of <0.05 was applied for statistically significant associations. The association between allele frequencies for 31,465 SNPs and CAD, as determined by Fisher's exact test, demonstrated that 170 SNPs were significantly (FDR <0.05) associated with CAD. Multivariable logistic regression analysis with adjustment for age, sex, and the prevalence of hypertension, diabetes mellitus and dyslipidemia revealed that 162 SNPs were significantly (P<0.05) associated with CAD. A stepwise forward selection procedure was performed to examine the effects of genotypes for the 162 SNPs on CAD. The 54 SNPs were significant (P<0.05) and independent [coefficient of determination (R²), 0.0008 to 0.0297] determinants of CAD. These SNPs together accounted for 15.5% of the cause of CAD. Following examination of results from previous genome-wide association studies and linkage disequilibrium of the identified SNPs, 21 genes (RNF2, YEATS2, USP45, ITGB8, TNS3, FAM170B-AS1, PRKG1, BTRC, MKI67, STIM1, OR52E4, KIAA1551, MON2, PLUT, LINC00354, TRPM1, ADAT1, KRT27, LIPE, GFY and EIF3L) and five chromosomal regions (2p13, 4q31.2, 5q12, 13q34 and 20q13.2) that were significantly associated with CAD were newly identified in the present study. Gene ontology analysis demonstrated that various biological functions were predicted in the 18 genes identified in the present study. The network analysis revealed that the 18 genes had potential direct or indirect interactions with the 30 genes previously revealed to be associated with CAD or with the 228 genes identified in previous genome-wide association studies. The present study newly identified 26 loci that confer susceptibility to CAD. Determination of genotypes for the SNPs at these loci may prove informative for assessment of the genetic risk for CAD in Japanese patients.

Primary genetic disorders affecting high density lipoprotein (HDL)

There is extensive evidence demonstrating that there is a clear inverse correlation between plasma high density lipoprotein cholesterol (HDL-C) concentration and cardiovascular disease (CVD). On the other hand, there is also extensive evidence that HDL functionality plays a very important role in atheroprotection. Thus, genetic disorders altering certain enzymes, lipid transfer proteins, or specific receptors crucial for the metabolism and adequate function of HDL, may positively or negatively affect the HDL-C levels and/or HDL functionality and subsequently either provide protection or predispose to atherosclerotic disease. This review aims to describe certain genetic disorders associated with either low or high plasma HDL-C and discuss their clinical features, associated risk for cardiovascular events, and treatment options.

Tangier disease may cause early onset of atherosclerotic cerebral infarction: A case report

RATIONALE

The present study explored the relationship between the adenosine triphosphate (ATP)-binding cassette A1 (ABCA1) gene, atherosclerosis, and cerebral infarction. The diagnosis and treatment ideas of stroke caused by Tangier disease via the summary of the diagnosis and treatment process of one case with juvenile stroke were explored. The relevant literature on the clinical manifestations, laboratory examinations, and treatment of Tangier disease was reviewed.

PATIENT CONCERNS

The brain magnetic resonance imaging (MRI) of a juvenile man with acute onset of sudden right limb weakness and speechlessness revealed infarct lesions. The laboratory tests found low serum high-density lipoprotein (HDL), while further genetic testing identified ABCD1 gene mutation. The mother also carried the mutant gene.

DIAGNOSES

Tangier disease was diagnosed.

INTERVENTIONS

Statin treatment was administered for platelet aggregation.

OUTCOMES

After 3 years of follow-up, the patient was declared to be in a stable condition.

LESSONS

ABCA1 gene mutation caused early onset of atherosclerosis, leading to the occurrence of cerebral infarction. The cerebral infarction associated with reduced high-density lipoprotein (HDL), was under intensive focus with respect to ABCA1 gene. Child and Juvenile stroke patients with low HDL should not be excluded from the possibility of Tangier disease.

Loss of sterol metabolic homeostasis triggers inflammasomes - how and why

Proper regulation of sterol biosynthesis is critical for eukaryotic cellular homeostasis. Cholesterol and isoprenoids serve key roles in eukaryotic cells by regulating membrane fluidity and correct localization of proteins. It is becoming increasingly appreciated that dysregulated sterol metabolism engages pathways that lead to inflammation. Of particular importance are inflammasomes, which are multiplatform protein complexes that activate caspase-1 in order to process the pro-inflammatory and pyrogenic cytokines IL-1β and IL-18. In this review, we highlight recent research that links altered sterol biosynthetic pathway activity to inflammasome activation. We discuss how clues from human genetics have led to new insights into how alterations in isoprenoid biosynthesis connect to inflammation. We also discuss new mechanisms that show how macrophage cholesterol buildup can lead to inflammasome activation.

ABCA1 haplodeficiency affects the brain transcriptome following traumatic brain injury in mice expressing human APOE isoforms

Expression of human Apolipoprotein E (APOE) modulates the inflammatory response in an isoform specific manner, with APOE4 isoform eliciting a stronger pro-inflammatory response, suggesting a possible mechanism for worse outcome following traumatic brain injury (TBI). APOE lipidation and stability is modulated by ATP-binding cassette transporter A1 (ABCA1), a transmembrane protein that transports lipids and cholesterol onto APOE. We examined the impact of Abca1 deficiency and APOE isoform expression on the response to TBI using 3-months-old, human APOE3+/+ (E3/Abca1+/+) and APOE4+/+ (E4/Abca1+/+) targeted replacement mice, and APOE3+/+ and APOE4+/+ mice with only one functional copy of the Abca1 gene (E3/Abca1+/-; E4/Abca1+/-). TBI-treated mice received a craniotomy followed by a controlled cortical impact (CCI) brain injury in the left hemisphere; sham-treated mice received the same surgical procedure without the impact. We performed RNA-seq using samples from cortices and hippocampi followed by genome-wide differential gene expression analysis. We found that TBI significantly impacted unique transcripts within each group, however, the proportion of unique transcripts was highest in E4/Abca1+/- mice. Additionally, we found that Abca1 haplodeficiency increased the expression of microglia sensome genes among only APOE4 injured mice, a response not seen in injured APOE3 mice, nor in either group of sham-treated mice. To identify gene networks, or modules, correlated to TBI, APOE isoform and Abca1 haplodeficiency, we used weighted gene co-expression network analysis (WGCNA). The module that positively correlated to TBI groups was associated with immune response and featured hub genes that were microglia-specific, including Trem2, Tyrobp, Cd68 and Hexb. The modules positively correlated with APOE4 isoform and negatively to Abca1 haplodeficient mice represented "protein translation" and "oxidation-reduction process", respectively. Our results reveal E4/Abca1+/- TBI mice have a distinct response to injury, and unique gene networks are associated with APOE isoform, Abca1 insufficiency and injury.

Cholesterol and bile acid-mediated regulation of autophagy in fatty liver diseases and atherosclerosis

Liver is the major organ that regulates whole body cholesterol metabolism. Disrupted hepatic cholesterol homeostasis contributes to the pathogenesis of nonalcoholic steatohepatitis, dyslipidemia, atherosclerosis, and cardiovascular diseases. Hepatic bile acid synthesis is the major catabolic mechanism for cholesterol elimination from the body. Furthermore, bile acids are signaling molecules that regulate liver metabolism and inflammation. Autophagy is a highly-conserved lysosomal degradation mechanism, which plays an essential role in maintaining cellular integrity and energy homeostasis. In this review, we discuss emerging evidence linking hepatic cholesterol and bile acid metabolism to cellular autophagy activity in hepatocytes and macrophages, and how these interactions may be implicated in the pathogenesis and treatment of fatty liver disease and atherosclerosis.

ABCA1 Is Coordinated with ABCB1 in the Arsenic-Resistance of Human Cells

Arsenic is one of the most widespread global environmental toxicants associated with endemic poisoning. ATP-binding cassette (ABC) proteins are transmembrane channels that transport and dispose of lipids and metabolic products across the plasma membrane. The majority of ABC family members (including ABCB1 and ABCC1) are reported to play a role in the development of arsenic and drug resistance in mammals. Previously, we established a human arsenic-resistant ECV-304 (AsRE) cell line and identified ABCA1 as a novel arsenic resistance gene. In the current study, we further investigated the potential contribution of ABCA1, ABCB1, and ABCC1 to arsenic resistance through measurement of survival rates and arsenic accumulation in AsRE cells with RNA interference. The arsenic resistance capacity of ABCC1 was the strongest among the three genes, while those of ABCA1 and ABCB1 were similar. Double or triple gene knockdown of ABCA1, ABCB1, and ABCC1 via RNA interference led to a decrease significant in arsenic resistance when ABCA1/ABCB1 or ABCB1/ABCC1 were simultaneously silenced. Interestingly, no differences were evident between cells with ABCA1/ABCC1 and ABCC1 only knockdown. Our findings suggest that ABCA1 and ABCB1 proteins display similar arsenic resistance capabilities and possibly coordinate to promote arsenic resistance in AsRE cells.

A critical role for ABC transporters in persistent lung inflammation in the development of emphysema after smoke exposure

Macrophage infiltration is common to both emphysema and atherosclerosis, and cigarette smoke down-regulates the macrophage cholesterol efflux transporter ATP binding cassette (ABC)A1. This decreased cholesterol efflux results in lipid-laden macrophages. We hypothesize that cigarette smoke adversely affects cholesterol transport via an ABCA1-dependent mechanism in macrophages, enhancing TLR4/myeloid differentiation primary response gene 88 (Myd88) signaling and resulting in matrix metalloproteinase (MMP) up-regulation and exacerbation of pulmonary inflammation. ABCA1 is significantly down-regulated in the lung upon smoke exposure conditions. Macrophages exposed to cigarette smoke in vivo and in vitro exhibit impaired cholesterol efflux correlating with significantly decreased ABCA1 expression, up-regulation of the TLR4/Myd88 pathway, and downstream MMP-9 and MMP-13 expression. Treatment with liver X receptor (LXR) agonist restores ABCA1 expression after short-term smoke exposure and attenuates the inflammatory response; after long-term smoke exposure, there is also attenuated physiologic and morphologic changes of emphysema. In vitro, treatment with LXR agonist decreases macrophage inflammatory activation in wild-type but not ABCA1 knockout mice, suggesting an ABCA1-dependent mechanism of action. These studies demonstrate an important association between cigarette smoke exposure and cholesterol-mediated pathways in the macrophage inflammatory response. Modulation of these pathways through manipulation of ABCA1 activity effectively blocks cigarette smoke-induced inflammation and provides a potential novel therapeutic approach for the treatment of chronic obstructive pulmonary disease.-Sonett, J., Goldklang, M., Sklepkiewicz, P., Gerber, A., Trischler, J., Zelonina, T., Westerterp, M., Lemaître, V., Okada, V., D'Armiento, J. A critical role for ABC transporters in persistent lung inflammation in the development of emphysema after smoke exposure.

Large-scale deletions of the ABCA1 gene in patients with hypoalphalipoproteinemia

Copy-number variations (CNVs) have been studied in the context of familial hypercholesterolemia but have not yet been evaluated in patients with extreme levels of HDL cholesterol. We evaluated targeted, next-generation sequencing data from patients with very low levels of HDL cholesterol (i.e., hypoalphalipoproteinemia) with the VarSeq-CNV^® caller algorithm to screen for CNVs that disrupted the ABCA1, LCAT, or APOA1 genes. In four individuals, we found three unique deletions in ABCA1: a heterozygous deletion of exon 4, a heterozygous deletion that spanned exons 8 to 31, and a heterozygous deletion of the entire ABCA1 gene. Breakpoints were identified with Sanger sequencing, and the full-gene deletion was confirmed by using exome sequencing and the Affymetrix CytoScan HD array. Previously, large-scale deletions in candidate HDL genes had not been associated with hypoalphalipoproteinemia; our findings indicate that CNVs in ABCA1 may be a previously unappreciated genetic determinant of low levels of HDL cholesterol. By coupling bioinformatic analyses with next-generation sequencing data, we can successfully assess the spectrum of genetic determinants of many dyslipidemias, including hypoalphalipoproteinemia.

Peripheral neuropathy in Tangier disease: A literature review and assessment

Tangier disease (TD) (OMIM#205400) is a rare cause of inherited metabolic neuropathies characterized by marked deficiency of high-density lipoproteins and accumulation of cholesterol esters in various tissue resulting from reverse cholesterol transport deficiency. We report a case of a patient with TD with multifocal demyelinating neuropathy with conduction block who presents with winging scapula, tongue, and asymmetric extremity weakness. We also present a review of all studies published from 1960 to 2017 regarding peripheral neuropathy in TD. Our search identified 54 patients with TD with peripheral neuropathy. Syringomyelia-like neuropathy subtype (52.4%) was more frequent than multifocal sensorial and motor neuropathy subtype (26.2%), focal neuropathy subtype (19.1%), and distal symmetric polyneuropathy subtype (2.4%). Splenomegaly was the most common (40.7%) clinical manifestation in these patients. The pattern of electrodiagnostic abnormalities are: (1) demyelinating abnormalities were more predominant in the upper extremities than in the lower extremities and (2) slowing of motor nerve conduction was more prominent in the intermediate segment than in distal nerve segments. The sural-sparing pattern was present in 34.6% and conduction block was present in 11.5% of the patients. Our literature review and our case showed the clinical spectrum of TD neuropathy is quite wide and that it should be considered in the differential diagnosis of non-uniform demyelinating neuropathies.

Temporary sequestration of cholesterol and phosphatidylcholine within extracellular domains of ABCA1 during nascent HDL generation

The quality and quantity of high-density lipoprotein (HDL) in blood plasma are important for preventing coronary artery disease. ATP-binding cassette protein A1 (ABCA1) and apolipoprotein A-I (apoA-I) play essential roles in nascent HDL formation, but controversy persists regarding the mechanism by which nascent HDL is generated. In the “direct loading model”, apoA-I acquires lipids directly from ABCA1 while it is bound to the transporter. By contrast, in the “indirect model”, apoA-I acquires lipids from the specific membrane domains created by ABCA1. In this study, we found that trypsin treatment causes rapid release of phosphatidylcholine (PC) and cholesterol from BHK/ABCA1 cells, and that the time course of lipid release coincides with those of trypsin digestion of extracellular domains (ECDs) of surface ABCA1 and of release of ECD fragments into the medium. This trypsin-dependent lipid release was dependent on ABCA1 ATPase activity, and did not occur in cells that express ABCG1, which exports lipids like ABCA1 but does not have large ECDs. These results suggest that the trypsin-sensitive sites on the cell surface are the large ECDs of ABCA1, and that lipids transported by ABCA1 are temporarily sequestered within the ECDs during nascent HDL formation.

Rethinking reverse cholesterol transport and dysfunctional high-density lipoproteins

Human plasma high-density lipoprotein cholesterol concentrations are a negative risk factor for atherosclerosis-linked cardiovascular disease. Pharmacological attempts to reduce atherosclerotic cardiovascular disease by increasing plasma high-density lipoprotein cholesterol have been disappointing so that recent research has shifted from HDL quantity to HDL quality, that is, functional vs dysfunctional HDL. HDL has varying degrees of dysfunction reflected in impaired reverse cholesterol transport (RCT). In the context of atheroprotection, RCT occurs by 2 mechanisms: one is the well-known trans-hepatic pathway comprising macrophage free cholesterol (FC) efflux, which produces early forms of FC-rich nascent HDL (nHDL). Lecithin:cholesterol acyltransferase converts HDL-FC to HDL-cholesteryl ester while converting nHDL from a disc to a mature spherical HDL, which transfers its cholesteryl ester to the hepatic HDL receptor, scavenger receptor B1 for uptake, conversion to bile salts, or transfer to the intestine for excretion. Although widely cited, current evidence suggests that this is a minor pathway and that most HDL-FC and nHDL-FC rapidly transfer directly to the liver independent of lecithin:cholesterol acyltransferase activity. A small fraction of plasma HDL-FC enters the trans-intestinal efflux pathway comprising direct FC transfer to the intestine. SR-B1^-/- mice, which have impaired trans-hepatic FC transport, are characterized by high plasma levels of a dysfunctional FC-rich HDL that increases plasma FC bioavailability in a way that produces whole-body hypercholesterolemia and multiple pathologies. The design of future therapeutic strategies to improve RCT will have to be formulated in the context of these dual RCT mechanisms and the role of FC bioavailability.

Apoptotic cells trigger the ABCA1/STAT6 pathway leading to PPAR-γ expression and activation in macrophages

The signal transducer and activator of transcription 6 (STAT6) transcription factor activates peroxisome proliferator-activated receptor gamma (PPAR-γ)-regulated gene expression in immune cells. We investigated proximal membrane signaling that was initiated in macrophages after exposure to apoptotic cells that led to enhanced PPAR-γ expression and activity, using specific siRNAs for ABCA1, STAT6, and PPAR-γ, or their antagonists. The interactions between mouse bone marrow-derived macrophages or RAW 264.7 cells and apoptotic Jurkat cells, but not viable cells, resulted in the induction of STAT6 phosphorylation as well as PPAR-γ expression and activation. Knockdown of ATP-binding cassette transporter A1 (ABCA1) after the transfection of macrophages with ABCA1-specific siRNAs reduced apoptotic cell-induced STAT6 phosphorylation as well as PPAR-γ mRNA and protein expression. ABCA1 knockdown also reduced apoptotic cell-induced liver X receptor α (LXR-α) mRNA and protein expression. Moreover, inhibition of STAT6 with specific siRNAs or the pharmacological inhibitor AS1517499AS reversed the induction of PPAR-γ, LXR-α, and ABCA1 by apoptotic Jurkat cells. PPAR-γ-specific siRNAs or the PPAR-γ antagonist GW9662 inhibited apoptotic cell-induced increases in LXR-α and ABCA1 mRNA and protein levels. Thus, these results indicate that apoptotic cells trigger the ABCA1/STAT6 pathway, leading to the activation of the PPAR-γ/LXR-α/ABCA1 pathway in macrophages.

Accelerated Atherogenicity in Tangier Disease

We report a case of Tangier disease with Leriche syndrome and bleeding tendency. In this male patient, nasal hemorrhage had been observed frequently throughout childhood. At 46 years old, he experienced effort angina, and coronary angiography demonstrated 75% stenosis in the right coronary artery. Orange-colored tonsils, mild hepatosplenomegaly and very low levels of serum high-density lipoprotein cholesterol (HDL-C) were observed, and the patient was diagnosed with Tangier disease. At 52 years old, effort angina recurred. Coronary angiography revealed 75% stenosis of the left main trunk, left anterior descending, and right coronary arteries. Stenosis of the brachiocephalic and right common iliac arteries was also recorded. Stents were implanted, and coronary artery bypass surgery was performed. At 53 years old, 15 months after surgery, the patient reported intermittent claudication, coldness of feet, and impotence. Aortic angiography showed progression of the stenosis at the bifurcation of the common iliac artery. The patient was diagnosed with Leriche syndrome, and aorta–left external iliac artery graft bypass surgery was performed. After surgery, oozing from subcutaneous tissue and leaking from the anastomotic region were observed. Additional analysis revealed two single-nucleotide polymorphisms (V825I and N935T) in the ATP-binding cassette transporter A1 (ABCA1) gene, and accumulation of small dense low-density lipoprotein together with low levels of HDL-C. In Tangier disease, HDL-C is markedly decreased because of ABCA1 deficiency. However, this is the first reported case to exhibit extensive atherosclerosis and bleeding tendency. This patient had atypical extensive and multiple atherosclerotic lesions, accompanied by Leriche syndrome and uncontrollable bleeding.

Methods for Monitoring ABCA1-Dependent Sterol Release

Releasing sterols to the extracellular milieu is an important part of sterol homeostasis in cells and in the body. ATP-binding cassette transporter A1 (ABCA1) plays an essential role in cellular phospholipid and sterol release to lipid-free or lipid-poor apolipoprotein A-I (apoA-I), the major apolipoprotein in high-density lipoprotein (HDL), and constitutes the first step in the formation of nascent HDL. Loss-of-function mutations in the ABCA1 gene lead to a rare disease known as Tangier disease that causes severe deficiency in plasma HDL level. Mammalian cells receive exogenous cholesterol mainly from low-density lipoprotein. In addition, they synthesize cholesterol endogenously, as well as multiple precursor sterols that are sterol intermediates en route to be converted to cholesterol. HDL contains phospholipids, cholesterol, and precursor sterols, and ABCA1 has an ability to release phospholipids and various sterol molecules. Recent studies using model cell lines showed that ABCA1 prefers to use sterols newly synthesized endogenously as its preferred substrate, rather than cholesterol derived from LDL or cholesterol being recycled within the cells. Here, we describe several methods at the cell culture level to monitor ABCA1-dependent release of sterol molecules to apoA-I present at the cell exterior. Sterol release can be assessed by using a simple colorimetric enzymatic assay, and/or by monitoring the radioactivities of radiolabeled cholesterol incorporated into the cells, and/or of sterols biosynthesized from radioactive acetate, and/or by using gas chromatography-mass spectrometry analysis of various sterols present in medium and in cells. We also discuss the pros and cons of these methods. Together, these methods allow researchers to detect the release not only of cholesterol but also of other sterols present in minor quantities.

Membrane microdomains and the regulation of HDL biogenesis

PURPOSE OF REVIEW

The major cardio-protective function of HDL is to remove excess cellular cholesterol in the process of HDL particle formation and maturation. The HDL biogenic procedure requiring protein-lipid interactions has been incompletely understood, and here we discuss recent progress and insights into the mechanism of HDL biogenesis.

RECENT FINDINGS

The initial and rate-limiting step of HDL biogenesis is the interaction between apoA-I and plasma membrane microdomains created by ATP-binding cassette transporter A1 (ABCA1) transporter. Computer simulation of molecular dynamics suggests that ABCA1 translocates phospholipids from the inner to the outer leaflet of the plasma membrane to create a transbilayer density gradient leading to the formation of an exovesiculated plasma membrane microdomain. The cryo-electron microscopy structure of ABCA1 suggests that an elongated hydrophobic tunnel formed by the extracellular domain of ABCA1 may function as a passageway to deliver lipids to apoA-I. In contrast to ABCA1-created plasma membrane microdomains, desmocollin 1 (DSC1) contained in a cholesterol-rich plasma membrane microdomain binds apoA-I to prevent HDL biogenesis. The identification of DSC1-containing plasma membrane microdomains as a negative regulator of HDL biogenesis may offer potential therapeutic avenues.

SUMMARY

Isolation and characterization of plasma membrane microdomains involved in HDL biogenesis may lead to a better understanding of the molecular mechanism of HDL biogenesis.

ABCA7 Mediates Phagocytic Clearance of Amyloid-β in the Brain

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by dementia and abnormal deposits of aggregated amyloid-β in the brain. Recent genome-wide association studies have revealed that ABCA7 is strongly associated with AD. In vitro evidence suggests that the role of ABCA7 is related to phagocytic activity. Deletion of ABCA7 in a mouse model of AD exacerbates cerebral amyloid-β plaque load. However, the biological role of ABCA7 in AD brain pathogenesis is unknown. We show that ABCA7 is highly expressed in microglia and when monocytes are differentiated into macrophages. We hypothesized that ABCA7 plays a protective role in the brain that is related to phagocytic clearance of amyloid-β. We isolated microglia and macrophages from Abca7-/- and wild type mice and tested them for their capacity to phagocytose amyloid-β oligomers. We found that the phagocytic clearance of amyloid-β was substantially reduced in both microglia and macrophages from Abca7-/- mice compared to wild type mice. Consistent with these results, in vivo phagocytic clearance of amyloid-β oligomers in the hippocampus was reduced in Abca7-/- mice. Furthermore, ABCA7 transcription was upregulated in AD brains and in amyloidogenic mouse brains specifically in the hippocampus as a response to the amyloid-β pathogenic state. Together these results indicate that ABCA7 mediates phagocytic clearance of amyloid-β in the brain, and reveal a mechanism by which loss of function of ABCA7 increases the susceptibility to AD.

Sterol Metabolism and Transport in Atherosclerosis and Cancer

Cholesterol is a vital lipid molecule for mammalian cells, regulating fluidity of biological membranes, and serving as an essential constituent of lipid rafts. Mammalian cells acquire cholesterol from extracellular lipoproteins and from de novo synthesis. Cholesterol biosynthesis generates various precursor sterols. Cholesterol undergoes metabolic conversion into oxygenated sterols (oxysterols), bile acids, and steroid hormones. Cholesterol intermediates and metabolites have diverse and important cellular functions. A network of molecular machineries including transcription factors, protein modifiers, sterol transporters/carriers, and sterol sensors regulate sterol homeostasis in mammalian cells and tissues. Dysfunction in metabolism and transport of cholesterol, sterol intermediates, and oxysterols occurs in various pathophysiological settings such as atherosclerosis, cancers, and neurodegenerative diseases. Here we review the cholesterol, intermediate sterol, and oxysterol regulatory mechanisms and intracellular transport machineries, and discuss the roles of sterols and sterol metabolism in human diseases.

Possible Insulinotropic Action of Apolipoprotein A-I Through the ABCA1/Cdc42/cAMP/PKA Pathway in MIN6 Cells

Aims/Introduction: We studied the mechanisms for the possible insulinotropic action of apolipoprotein (Apo) A-I in mouse insulinoma (MIN6) cells. Materials and Methods: The effects of ApoA-I on cAMP production and glucose-stimulated insulin secretion (GSIS), and the dose dependency (ApoA-I at 5, 10, 25, and 50 μg/ml) were determined using MIN6 cells. The effects of the small-interference ribonucleic acid (siRNA) of ATP-binding cassette transporter A1(ABCA1) and Cell division control protein 42 homolog (Cdc42) on the insulinotropic action of ApoA-I was studied, as well as mRNA and protein levels of ABCA1 and Cdc42. Then, the influence of cAMP inhibitor SQ22536, and the cAMP-dependent protein kinase inhibitor Rp-cAMPS on ApoA-I action were studied. Results: Addition of ApoA-I produced cAMP and increased insulin secretion, dose-dependently in high glucose concentration (25 mmmol/l). and ABCA1 protein and Cdc42 mRNA and protein were also enhanced. Specific ABCA1 and Cdc42 siRNA significantly decreased the effects of ApoA-I on insulin secretion compared with negative controls. Manifestations of ABCA1 and Cdc42 mRNA and protein were less than that of the negative control group. Both cAMP inhibiror (SQ22536) and protein kinases inhibitor (Rp-cAMPS) strongly inhibited the effects of ApoA-I on insulin secretion. Conclusions: We demonstrated that ApoA-I enhances glucose-stimulated insulin release in high glucose at least partially through the ABCA1/Cdc42/cAMP/ Protein kinase A (PKA) pathway.

Can Tangier disease cause male infertility? A case report and an overview on genetic causes of male infertility and hormonal axis involved

Tangier disease is an autosomal recessive disorder caused by mutations in the ABCA1 gene and characterized by the accumulation of cholesteryl ester in various tissues and a near absence of high-density lipoprotein. The subject in this investigation was a 36-year-old Italian man with Tangier disease. He and his wife had come to the In Vitro Fertilization Unit, Pesaro Hospital (Azienda Ospedaliera Ospedali Riuniti Marche Nord) seeking help regarding fertility issues. The man was diagnosed with severe oligoasthenoteratozoospermia. Testosterone is the sex hormone necessary for spermatogenesis and cholesterol is its precursor; hence, we hypothesized that the characteristic cholesterol deficiency in Tangier disease patients could compromise their fertility. The aim of the study was to therefore to determine if there is an association between Tangier disease and male infertility. After excluding viral, infectious, genetic and anatomical causes of the subject's oligoasthenoteratozoospermia, we performed a hormonal analysis to verify our hypothesis. The patient was found to be negative for frequent bacteria and viruses. The subject showed a normal male karyotype and tested negative for Yq microdeletions and Cystic Fibrosis Transmembrane Conductance Regulator gene mutations. A complete urological examination was performed, and primary hypogonadism was also excluded. Conversely, hormonal analyses showed that the subject had a high level of follicle stimulating hormone and luteinizing hormone, low total testosterone and a significant decline in inhibin B. We believe that the abnormally low cholesterol levels typically found in subjects with Tangier disease may result in a reduced testosterone production which in turn could affect the hormonal axis responsible for spermatogenesis leading to a defective maturation of spermatozoa.

AGE-albumin enhances ABCA1 degradation by ubiquitin-proteasome and lysosomal pathways in macrophages

BACKGROUND AND AIMS

Advanced glycation end products (AGEs) induce cellular oxidative/endoplasmic reticulum stress and inflammation. We investigated its underlying mechanisms for atherogenesis focusing on regulation of ABCA1 protein decay in macrophages.

METHODS

The ABCA1 decay rate was evaluated in macrophages after treatment with LXR agonist and by incubation with control (C) or AGE-albumin concomitant or not with cycloheximide, MG-132, ammonium chloride and calpain inhibitors were utilized to inhibit, respectively, proteasome, lysosome and ABCA1 proteolysis at cell surface. ABCA1 was determined by immunoblot and the protein decay rate calculated along time by the slope of the linear regression. Ubiquitination level was determined in ABCA1 immunoprecipitated from whole cell lysate or bulk cell membrane. AGE effect was also analyzed in THP-1 cells transfected with siRNA-RAGE. Carboxymethyllysine (CML) and pyrraline (PYR) were determined by LC/MS. One-way ANOVA and Student t test were utilized to compare results.

RESULTS

CML and PYR-albumin were higher in AGE-albumin as compared to C. AGE-albumin reduced ABCA1 in J774 and THP-1 macrophages (20-30%) and induced a higher ABCA1 ubiquitination and a faster protein decay rate that was dependent on the presence of AGE during the kinetics of measurement in the presence of cycloheximide. Proteasomal inhibition restored and lysosomal inhibition partially recovered ABCA1 in cells treated with AGE-albumin. Calpain inhibition was not able to rescue ABCA1. RAGE knockdown prevented the reduction in ABCA1 elicited by AGE.

CONCLUSIONS

AGE-albumin diminishes ABCA1 by accelerating its degradation through the proteasomal and lysosomal systems. This may increase lipid accumulation in macrophages by diminishing cholesterol efflux via RAGE signaling contributing to atherosclerosis in diabetes mellitus.