ABCG1 and ABCG4 are coexpressed in neurons and astrocytes of the CNS and regulate cholesterol homeostasis through SREBP-2

Behavioral and Metabolic Effects of ABCG4 KO in the APPswe,Ind (J9) Mouse Model of Alzheimer's Disease

The pathogenesis of Alzheimer's disease (AD) is complex and involves an imbalance between production and clearance of amyloid-ß peptides (Aß), resulting in accumulation of Aß in senile plaques. Hypercholesterolemia is a major risk factor for developing AD, with cholesterol shown to accumulate in senile plaques and increase production of Aß. ABCG4 is a member of the ATP-binding cassette transporters predominantly expressed in the CNS and has been suggested to play a role in cholesterol and Aß efflux from the brain. In this study, we bred Abcg4 knockout (KO) with the APPSwe,Ind (J9) mouse model of AD to test the hypothesis that loss of Abcg4 would exacerbate the AD phenotype. Unexpectedly, no differences were observed in novel object recognition (NOR) and novel object placement (NOP) behavioral tests, or on histologic examinations of brain tissues for senile plaque numbers. Furthermore, clearance of radiolabeled Aß from the brains did not differ between Abcg4 KO and control mice. Metabolic testing by indirect calorimetry, glucose tolerance test (GTT), and insulin tolerance test (ITT) were also mostly similar between groups with only a few mild metabolic differences noted. Overall, these data suggest that the loss of ABCG4 did not exacerbate the AD phenotype.

Adrenal Abcg1 Controls Cholesterol Flux and Steroidogenesis

Cholesterol is the precursor of all steroids, but how cholesterol flux is controlled in steroidogenic tissues is poorly understood. The cholesterol exporter ABCG1 is an essential component of the reverse cholesterol pathway and its global inactivation results in neutral lipid redistribution to tissue macrophages. The function of ABCG1 in steroidogenic tissues, however, has not been explored. To model this, we inactivated Abcg1 in the mouse adrenal cortex, which led to an adrenal-specific increase in transcripts involved in cholesterol uptake and de novo synthesis. Abcg1 inactivation did not affect adrenal cholesterol content, zonation, or serum lipid profile. Instead, we observed a moderate increase in corticosterone production that was not recapitulated by the inactivation of the functionally similar cholesterol exporter Abca1. Altogether, our data imply that Abcg1 controls cholesterol uptake and biosynthesis and regulates glucocorticoid production in the adrenal cortex, introducing the possibility that ABCG1 variants may account for physiological or subclinical variation in stress response.

Brain cholesterol homeostasis and its association with neurodegenerative diseases

The brain is the most cholesterol-rich organ in mammals. However, cholesterol metabolism in the brain is completely independent of other tissues due to the presence of the blood-brain barrier (BBB). Neurons, astrocytes and oligodendrocytes are the main cells responsible for cholesterol synthesis in the brain. The cholesterol content in the brain is maintained at a relatively constant level under strict regulation of synthesis, transport, and turnover, that is, brain cholesterol homeostasis. Once this balance is disrupted, neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD) ensue. This review summarizes the processes controlling cholesterol homeostasis with respect to the synthesis, transport and turnover of cholesterol in the brain. We further focus on how cholesterol imbalance contributes to neurodegenerative diseases to explore the possibilities to modulate the key steps involved, which will provide clues for the development of therapies for the treatment of central nervous system (CNS) diseases.

The role of the ATP-Binding Cassette A1 (ABCA1) in neurological disorders: a mechanistic review

INTRODUCTION

Cholesterol homeostasis is critical for normal brain function. It is tightly controlled by various biological elements. ATP-binding cassette transporter A1 (ABCA1) is a membrane transporter that effluxes cholesterol from cells, particularly astrocytes, into the extracellular space. The recent studies pertaining to ABCA1's role in CNS disorders were included in this study.

AREAS COVERED

In this comprehensive literature review, preclinical and human studies showed that ABCA1 has a significant role in the following diseases or disorders: Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, neuropathy, anxiety, depression, psychosis, epilepsy, stroke, and brain ischemia and trauma.

EXPERT OPINION

ABCA1 via modulating normal and aberrant brain functions such as apoptosis, phagocytosis, BBB leakage, neuroinflammation, amyloid β efflux, myelination, synaptogenesis, neurite outgrowth, and neurotransmission promotes beneficial effects in aforementioned diseases. ABCA1 is a key molecule in the CNS. By boosting its expression or function, some CNS disorders may be resolved. In preclinical studies, liver X receptor agonists have shown promise in treating CNS disorders via ABCA1 and apoE enhancement.

Behavioral and metabolic and effects of ABCG4 KO in the APPswe,Ind (J9) mouse model of Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is complex and involves an imbalance between production and clearance of amyloid-ß peptides (Aß), resulting in accumulation of Aß in senile plaques. Hypercholesterolemia is a major risk factor for developing AD, with cholesterol shown to accumulate in senile plaques and increase production of Aß. ABCG4 is a member of the ATP-binding cassette transporters predominantly expressed in the CNS, and has been suggested to play a role in cholesterol and Aß efflux from the brain. In this study, we bred Abcg4 knockout (KO) with the APPSwe,Ind (J9) mouse model of AD to test the hypothesis that loss of Abcg4 would exacerbate the AD phenotype. Unexpectedly, no differences were observed in Novel object recognition (NOR) and Novel object placement (NOP) behavioral tests, or on histologic examinations of brain tissues for senile plaque numbers. Furthermore, clearance of radiolabeled Aß from the brains did not differ between Abcg4 KO and control mice. Metabolic testing by indirect calorimetry, glucose tolerance test (GTT) and insulin tolerance test (ITT), were also mostly similar between groups with only a few mild metabolic differences noted. Overall these data suggest that the loss of ABCG4 did not exacerbate the AD phenotype.

Is reverse cholesterol transport regulated by active cholesterol?

This review considers the hypothesis that a small portion of plasma membrane cholesterol regulates reverse cholesterol transport in coordination with overall cellular homeostasis. It appears that almost all of the plasma membrane cholesterol is held in stoichiometric complexes with bilayer phospholipids. The minor fraction of cholesterol that exceeds the complexation capacity of the phospholipids is called active cholesterol. It has an elevated chemical activity and circulates among the organelles. It also moves down its chemical activity gradient to plasma HDL, facilitated by the activity of ABCA1, ABCG1, and SR-BI. ABCA1 initiates this process by perturbing the organization of the plasma membrane bilayer, thereby priming its phospholipids for translocation to apoA-I to form nascent HDL. The active excess sterol and that activated by ABCA1 itself follow the phospholipids to the nascent HDL. ABCG1 similarly rearranges the bilayer and sends additional active cholesterol to nascent HDL, while SR-BI simply facilitates the equilibration of the active sterol between plasma membranes and plasma proteins. Active cholesterol also flows downhill to cytoplasmic membranes where it serves both as a feedback signal to homeostatic ER proteins and as the substrate for the synthesis of mitochondrial 27-hydroxycholesterol (27HC). 27HC binds the LXR and promotes the expression of the aforementioned transport proteins. 27HC-LXR also activates ABCA1 by competitively displacing its inhibitor, unliganded LXR. § Considerable indirect evidence suggests that active cholesterol serves as both a substrate and a feedback signal for reverse cholesterol transport. Direct tests of this novel hypothesis are proposed.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Lipid metabolism and retinal diseases

PURPOSE

The retina has enormous lipids demands and must meet those needs. Retinal lipid homeostasis appears to be based on the symbiosis between neurons, Müller glial cells (MGC), and retinal pigment epithelium (RPE) cells, which can be impacted in several retinal diseases. The current research challenge is to better understand lipid-related mechanisms involved in retinal diseases, such as age-related macular degeneration (AMD) and glaucoma.

RESULTS

In a first axis, in vitro and focus on Müller glial cell, we aimed to characterize whether the 24S-hydroxycholesterol (24S-OHC), an overexpressed end-product of cholesterol elimination pathway in neural tissue and likely produced by suffering retinal ganglion cells in glaucoma, may modulate MGC membrane organization, such as lipid rafts, to trigger cellular signalling pathways related to retinal gliosis. We have found that lipid composition appears to be a key factor of membrane architecture, especially for lipid raft microdomain formation, in MGC. However, 24S-OHC did not appear to trigger retinal gliosis via the modulation of lipid or protein composition within lipid rafts microdomains. This study provided a better understanding of the complex mechanisms involved in the pathophysiology of glaucoma. On a second clinical ax, we focused on the lipid-related mechanisms involved in the dysfunction of aging RPE and the appearance of drusenoid deposits in AMD. Using the Montrachet population-based study, we intend to report the frequency of reticular pseudodrusen (RPD) and its ocular and systemic risk factors, particularly related to lipid metabolisms, such as plasma lipoprotein levels, carotenoids levels, and lipid-lowering drug intake. Our study showed that RPD was less common in subjects taking lipid-lowering drugs. Lipid-lowering drugs, such as statins, may reduce the risk of RPD through their effect on the production and function of lipoproteins. This observation highlights the potential role of retinal lipid trafficking via lipoproteins between photoreceptors and retinal pigment epithelium cells in RPD formation. Those findings have been complemented with preliminary results on the analysis of plasma fatty acid (FA) profile, a surrogate marker of short-term dietary lipid intake, according to the type of predominant drusenoid deposit, soft drusen or RPD, in age-related maculopathy.

CONCLUSION

Further research on lipid metabolism in retinal diseases is warranted to better understand the pathophysiology of retinal diseases and develop new promising diagnostic, prognostic, and therapeutic tools for our patients.

HDL-like-Mediated Cell Cholesterol Trafficking in the Central Nervous System and Alzheimer's Disease Pathogenesis

The main aim of this work is to review the mechanisms via which high-density lipoprotein (HDL)-mediated cholesterol trafficking through the central nervous system (CNS) occurs in the context of Alzheimer’s disease (AD). Alzheimer’s disease is characterized by the accumulation of extracellular amyloid beta (Aβ) and abnormally hyperphosphorylated intracellular tau filaments in neurons. Cholesterol metabolism has been extensively implicated in the pathogenesis of AD through biological, epidemiological, and genetic studies, with the APOE gene being the most reproducible genetic risk factor for the development of AD. This manuscript explores how HDL-mediated cholesterol is transported in the CNS, with a special emphasis on its relationship to Aβ peptide accumulation and apolipoprotein E (ApoE)-mediated cholesterol transport. Indeed, we reviewed all existing works exploring HDL-like-mediated cholesterol efflux and cholesterol uptake in the context of AD pathogenesis. Existing data seem to point in the direction of decreased cholesterol efflux and the impaired entry of cholesterol into neurons among patients with AD, which could be related to impaired Aβ clearance and tau protein accumulation. However, most of the reviewed studies have been performed in cells that are not physiologically relevant for CNS pathology, representing a major flaw in this field. The ApoE4 genotype seems to be a disruptive element in HDL-like-mediated cholesterol transport through the brain. Overall, further investigations are needed to clarify the role of cholesterol trafficking in AD pathogenesis.

Lipoproteins in the Central Nervous System: From Biology to Pathobiology

The brain, as one of the most lipid-rich organs, heavily relies on lipid transport and distribution to maintain homeostasis and neuronal function. Lipid transport mediated by lipoprotein particles, which are complex structures composed of apolipoproteins and lipids, has been thoroughly characterized in the periphery. Although lipoproteins in the central nervous system (CNS) were reported over half a century ago, the identification of APOE4 as the strongest genetic risk factor for Alzheimer's disease has accelerated investigation of the biology and pathobiology of lipoproteins in the CNS. This review provides an overview of the different components of lipoprotein particles, in particular apolipoproteins, and their involvements in both physiological functions and pathological mechanisms in the CNS. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Mutants of the white ABCG Transporter in Drosophila melanogaster Have Deficient Olfactory Learning and Cholesterol Homeostasis

Drosophila's white gene encodes an ATP-binding cassette G-subfamily (ABCG) half-transporter. White is closely related to mammalian ABCG family members that function in cholesterol efflux. Mutants of white have several behavioral phenotypes that are independent of visual defects. This study characterizes a novel defect of white mutants in the acquisition of olfactory memory using the aversive olfactory conditioning paradigm. The w¹¹¹⁸ mutants learned slower than wildtype controls, yet with additional training, they reached wildtype levels of performance. The w¹¹¹⁸ learning phenotype is also found in the w^apricot and w^coral alleles, is dominant, and is rescued by genomic white and mini-white transgenes. Reducing dietary cholesterol strongly impaired olfactory learning for wildtype controls, while w¹¹¹⁸ mutants were resistant to this deficit. The w¹¹¹⁸ mutants displayed higher levels of cholesterol and cholesterol esters than wildtype under this low-cholesterol diet. Increasing levels of serotonin, dopamine, or both in the white mutants significantly improved w¹¹¹⁸ learning. However, serotonin levels were not lower in the heads of the w¹¹¹⁸ mutants than in wildtype controls. There were also no significant differences found in synapse numbers within the w¹¹¹⁸ brain. We propose that the w¹¹¹⁸ learning defect may be due to inefficient biogenic amine signaling brought about by altered cholesterol homeostasis.

The Role of ABC Transporters in Lipid Metabolism and the Comorbid Course of Chronic Obstructive Pulmonary Disease and Atherosclerosis

Chronic obstructive pulmonary disease (COPD) ranks among the leading causes of morbidity and mortality worldwide. COPD rarely occurs in isolation and is often combined with various diseases. It is considered that systemic inflammation underlies the comorbid course of COPD. The data obtained in recent years have shown the importance of violations of the cross-links of lipid metabolism and the immune response, which are links in the pathogenesis of both COPD and atherosclerosis. The role of lipid metabolism disorders in the pathogenesis of the comorbid course of COPD and atherosclerosis and the participation of ATP-binding cassette (ABC) transporters in these processes is discussed in this article. It is known that about 20 representatives of a large family of ABC transporters provide lipid homeostasis of cells by moving lipids inside the cell and in its plasma membrane, as well as removing lipids from the cell. It was shown that some representatives of the ABC-transporter family are involved in various links of the pathogenesis of COPD and atherosclerosis, which can determine their comorbid course.

Analysis of Sequence Divergence in Mammalian ABCGs Predicts a Structural Network of Residues That Underlies Functional Divergence

The five members of the mammalian G subfamily of ATP-binding cassette transporters differ greatly in their substrate specificity. Four members of the subfamily are important in lipid transport and the wide substrate specificity of one of the members, ABCG2, is of significance due to its role in multidrug resistance. To explore the origin of substrate selectivity in members 1, 2, 4, 5 and 8 of this subfamily, we have analysed the differences in conservation between members in a multiple sequence alignment of ABCG sequences from mammals. Mapping sets of residues with similar patterns of conservation onto the resolved 3D structure of ABCG2 reveals possible explanations for differences in function, via a connected network of residues from the cytoplasmic to transmembrane domains. In ABCG2, this network of residues may confer extra conformational flexibility, enabling it to transport a wider array of substrates.

Mammalian ABCG-transporters, sterols and lipids: To bind perchance to transport?

Members of the ATP binding cassette (ABC) transporter family perform a critical function in maintaining lipid homeostasis in cells as well as the transport of drugs. In this review, we provide an update on the ABCG-transporter subfamily member proteins, which include the homodimers ABCG1, ABCG2 and ABCG4 as well as the heterodimeric complex formed between ABCG5 and ABCG8. This review focusses on progress made in this field of research with respect to their function in health and disease and the recognised transporter substrates. We also provide an update on post-translational regulation, including by transporter substrates, and well as the involvement of microRNA as regulators of transporter expression and activity. In addition, we describe progress made in identifying structural elements that have been recognised as important for transport activity. We furthermore discuss the role of lipids such as cholesterol on the transport function of ABCG2, traditionally thought of as a drug transporter, and provide a model of potential cholesterol binding sites for ABCG2.

Maturation of the Olfactory Sensory Neuron and Its Cilia

Olfactory sensory neurons (OSNs) are bipolar neurons, unusual because they turn over continuously and have a multiciliated dendrite. The extensive changes in gene expression accompanying OSN differentiation in mice are largely known, especially the transcriptional regulators responsible for altering gene expression, revealing much about how differentiation proceeds. Basal progenitor cells of the olfactory epithelium transition into nascent OSNs marked by Cxcr4 expression and the initial extension of basal and apical neurites. Nascent OSNs become immature OSNs within 24-48 h. Immature OSN differentiation requires about a week and at least 2 stages. Early-stage immature OSNs initiate expression of genes encoding key transcriptional regulators and structural proteins necessary for further neuritogenesis. Late-stage immature OSNs begin expressing genes encoding proteins important for energy production and neuronal homeostasis that carry over into mature OSNs. The transition to maturity depends on massive expression of one allele of one odorant receptor gene, and this results in expression of the last 8% of genes expressed by mature OSNs. Many of these genes encode proteins necessary for mature function of axons and synapses or for completing the elaboration of non-motile cilia, which began extending from the newly formed dendritic knobs of immature OSNs. The cilia from adjoining OSNs form a meshwork in the olfactory mucus and are the site of olfactory transduction. Immature OSNs also have a primary cilium, but its role is unknown, unlike the critical role in proliferation and differentiation played by the primary cilium of the olfactory epithelium's horizontal basal cell.

The Interplay of ABC Transporters in Aβ Translocation and Cholesterol Metabolism: Implicating Their Roles in Alzheimer's Disease

The occurrence of Alzheimer's disease (AD) worldwide has been progressively accelerating at an alarming rate, without any successful therapeutic strategy for the disease mitigation. The complexity of AD pathogenesis needs to be targeted with an alternative approach, as provided by the superfamily of ATP-binding cassette (ABC) transporters, which constitutes an extensive range of proteins, capable of transporting molecular entities across biological membranes. These protein moieties have been implicated in AD, based upon their potential in lipid transportation, resulting in maintenance of cholesterol homeostasis. These transporters have been reported to target the primary hallmark of AD pathogenesis, namely, beta-amyloid hypothesis, which is associated with accumulation of beta-amyloid (Aβ) plaques in AD patients. The ABC transporters have been observed to be localized to the capillary endothelial cells of the blood-brain barrier and neural parenchymal cells, where they exhibit different roles, consequently influencing the neuronal expression of Aβ peptides. The review highlights different families of ABC transporters, ABCB1 (P-glycoprotein), ABCA (ABCA1, ABCA2, and ABCA7), ABCG2 (BCRP; breast cancer resistance protein), ABCG1 and ABCG4, as well as ABCC1 (MRP; multidrug resistance protein) in the CNS, and their interplay in regulating cholesterol metabolism and Aβ peptide load in the brain, simultaneously exerting protective effects against neurotoxic substrates and xenobiotics. The authors aim to establish the significance of this alternative approach as a novel therapeutic target in AD, to provide the researchers an opportunity to evaluate the potential aspects of ABC transporters in AD treatment.

The ABCs of Sterol Transport

Cholesterol homeostasis and trafficking are critical to the maintenance of the asymmetric plasma membrane of eukaryotic cells. Disruption or dysfunction of cholesterol trafficking leads to numerous human diseases. ATP-binding cassette (ABC) transporters play several critical roles in this process, and mutations in these sterol transporters lead to disorders such as Tangier disease and sitosterolemia. Biochemical and structural information on ABC sterol transporters is beginning to emerge, with published structures of ABCA1 and ABCG5/G8; these two proteins function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Although both of these transporters belong to the ABC family and mediate the efflux of a sterol substrate, they have many distinct differences. Here, we summarize the current understanding of sterol transport driven by ABC transporters, with an emphasis on these two extensively characterized transporters.

Regulation of ABCG4 transporter expression by sterols and LXR ligands

BACKGROUND

Oxysterols, which are derivatives of cholesterol produced by enzymic or non-enzymic pathways, are potent regulators of cellular lipid homeostasis. Sterol homeostasis in the brain is an important area of interest with regards to neurodegenerative conditions like Alzheimer's disease (AD). Brain cells including neurons and astrocytes express sterol transporters belonging to the ABC transporter family of proteins, including ABCA1, ABCG1 and ABCG4, and these transporters are considered of interest as therapeutic targets. Although regulation of ABCA1 and ABCG1 is well established, regulation of ABCG4 is still controversial, in particular whether the transporter is an Liver X receptor (LXR) target. ABCG4 is thought to transport cholesterol, oxysterols and cholesterol synthesis intermediates, and was recently found on the blood brain barrier (BBB), implicated in amyloid-beta export. In this study, we investigate the regulation of ABCG4 by oxysterols, cholesterol-synthesis intermediates and cholesterol itself.

METHODS

ABC transporter expression was measured in neuroblastoma and gliablastoma cell lines and cells overexpressing ABCG4 in response to synthetic LXR ligands, oxysterols and cholesterol-synthesis intermediates.

RESULTS

In contrast to previous reports, ABCG4 expression was induced by a synthetic LXR ligand in U87-MG astrocytes but not in neuroblastoma and BBB endothelial cell lines. In addition, ABCG4 protein was stabilized by cholesterol as was previously shown for ABCG1. ABCG4 protein was furthermore stabilized by cholesterol-synthesis intermediates, desmosterol, lathosterol and lanosterol.

CONCLUSIONS

These results identify new aspects of the post-translational control of ABCG4 that warrant further exploration into the role of this transporter in the maintenance of sterol homeostasis in the brain.

ApoE Lipidation as a Therapeutic Target in Alzheimer's Disease

Apolipoprotein E (APOE) is the major cholesterol carrier in the brain, affecting various normal cellular processes including neuronal growth, repair and remodeling of membranes, synaptogenesis, clearance and degradation of amyloid β (Aβ) and neuroinflammation. In humans, the APOE gene has three common allelic variants, termed E2, E3, and E4. APOE4 is considered the strongest genetic risk factor for Alzheimer’s disease (AD), whereas APOE2 is neuroprotective. To perform its normal functions, apoE must be secreted and properly lipidated, a process influenced by the structural differences associated with apoE isoforms. Here we highlight the importance of lipidated apoE as well as the APOE-lipidation targeted therapeutic approaches that have the potential to correct or prevent neurodegeneration. Many of these approaches have been validated using diverse cellular and animal models. Overall, there is great potential to improve the lipidated state of apoE with the goal of ameliorating APOE-associated central nervous system impairments.

Role of Microglia in Regulating Cholesterol and Tau Pathology in Alzheimer's Disease

Cholesterol, a principal constituent of the cell membrane, plays a crucial role in the brain by regulating the synaptic transmission, neuronal signaling, as well as neurodegenerative diseases. Defects in the cholesterol trafficking are associated with enhanced generation of hyperphosphorylated Tau and Amyloid-β protein. Tau, a major microtubule-associated protein in the brain, is the key regulator of the mature neuron. Abnormally hyperphosphorylated Tau hampers the major functions related to microtubule assembly by promoting neurofibrillary tangles of paired helical filaments, twisted ribbons, and straight filaments. The observed pathological changes due to impaired cholesterol and Tau protein accumulation cause Alzheimer's disease. Thus, in order to regulate the pathogenesis of Alzheimer's disease, regulation of cholesterol metabolism, as well as Tau phosphorylation, is essential. The current review provides an overview of (1) cholesterol synthesis in the brain, neurons, astrocytes, and microglia; (2) the mechanism involved in modulating cholesterol concentration between the astrocytes and brain; (3) major mechanisms involved in the hyperphosphorylation of Tau and amyloid-β protein; and (4) microglial involvement in its regulation. Thus, the answering key questions will provide an in-depth information on microglia involvement in managing the pathogenesis of cholesterol-modulated hyperphosphorylated Tau protein.

Alteration in the Function and Expression of SLC and ABC Transporters in the Neurovascular Unit in Alzheimer's Disease and the Clinical Significance

The neurovascular unit (NVU) plays an important role in maintaining the function of the central nervous system (CNS). Emerging evidence has indicated that the NVU changes function and molecules at the early stage of Alzheimer’s disease (AD), which initiates multiple pathways of neurodegeneration. Cell types in the NVU have become attractive targets in the interventional treatment of AD. The NVU transportation system contains a variety of proteins involved in compound transport and neurotransmission. Brain transporters can be classified as members of the solute carrier (SLC) and ATP-binding cassette (ABC) families in the NVU. Moreover, the transporters can regulate both endogenous toxins, including amyloid-beta (Aβ) and xenobiotic homeostasis, in the brains of AD patients. Genome-wide association studies (GWAS) have identified some transporter gene variants as susceptibility loci for late-onset AD. Therefore, the present study summarizes changes in blood-brain barrier (BBB) permeability in AD, identifies the location of SLC and ABC transporters in the brain and focuses on major SLC and ABC transporters that contribute to AD pathology.

Quercetin inhibition of SREBPs and ChREBP expression results in reduced cholesterol and fatty acid synthesis in C6 glioma cells

Quercetin (Que), a widely distributed flavonoid in the human diet, exerts neuroprotective action because of its property to antagonize oxidative stress. Here, we investigated the effects of Que on lipid synthesis in C6 glioma cells. A rapid Que-induced inhibition of cholesterol and, to a lesser extent, of fatty acid synthesis from [1-¹⁴C]acetate was observed. The maximum decrease was detected at the level of palmitate, the end product of de novo fatty acid synthesis. The effect of Que on the enzyme activities of acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS), the two enzymes of this pathway, was investigated directly in situ in permeabilized C6 cells. An inhibitory effect on ACC1 was observed after 4 h of 25 μM Que treatment, while FAS activity was not affected. A reduction of polar lipid biosynthesis was also detected. A remarkable decrease of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) activity, regulatory enzyme of cholesterol synthesis, was evidenced. Expression studies demonstrated that Que acts at transcriptional level, by reducing the mRNA abundance and protein amount of ACC1 and HMGCR. Deepening the molecular mechanism, we found that Que decreased the expression of SREBP-1 and SREBP-2, transcriptional factors representing the main regulators of de novo fatty acid and cholesterol synthesis, respectively. Que also reduced the nuclear content of ChREBP, a glucose-induced transcription factor involved in the regulation of lipogenic genes. Our results represent the first evidence that a direct and rapid downregulatory effect of Que on cholesterol and de novo fatty acid synthesis is elicited in C6 cells.

ABCA1- and ABCG1-mediated cholesterol efflux capacity of cerebrospinal fluid is impaired in Alzheimer's disease

HDL-like particles in human cerebrospinal fluid (CSF) promote the efflux of cholesterol from astrocytes toward the neurons that rely on this supply for their functions. We evaluated whether cell cholesterol efflux capacity of CSF (CSF-CEC) is impaired in Alzheimer's disease (AD) by analyzing AD (n = 37) patients, non-AD dementia (non-AD DEM; n = 16) patients, and control subjects (n = 39). As expected, AD patients showed reduced CSF Aβ 1-42, increased total and phosphorylated tau, and a higher frequency of the apoε4 genotype. ABCA1- and ABCG1-mediated CSF-CEC was markedly reduced in AD (-73% and -33%, respectively) but not in non-AD DEM patients, in which a reduced passive diffusion CEC (-40%) was observed. Non-AD DEM patients displayed lower CSF apoE concentrations (-24%) compared with controls, while apoA-I levels were similar among groups. No differences in CSF-CEC were found by stratifying subjects for apoε4 status. ABCG1 CSF-CEC positively correlated with Aβ 1-42 (r = 0.305, P = 0.025), while ABCA1 CSF-CEC inversely correlated with total and phosphorylated tau (r = -0.348, P = 0.018 and r = -0.294, P = 0.048, respectively). The CSF-CEC impairment and the correlation with the neurobiochemical markers suggest a pathophysiological link between CSF HDL-like particle dysfunction and neurodegeneration in AD.

Exposure to carbon black nanoparticles during pregnancy persistently damages the cerebrovascular function in female mice

Maternal exposure to carbon black nanoparticles (CBNPs) during pregnancy have been well documented to induce harmful outcomes of offspring on brain function. However, it remains largely unknown whether females exposed to CBNPs during sensitive period of pregnancy can cause the neurotoxic effects on their own body after parturition. In this study, our results showed that pregnancy CBNPs exposure induced the persistent pathological changes in the cerebral cortex tissues and impaired cerebrovascular function of mice manifested by significant alterations of endothelin-1, endothelial nitric oxide synthase, vascular endothelial growth factor-A and ATP-binding cassette transporter G1. Intriguingly, we observed that these deleterious effects on brain and cerebrovascular functions in mice could persist for 49 days after delivery of pups. By using in vitro human umbilical vein endothelial cells, we further verified the potential vascular dysfunction after CBNPs exposure. In summary, our results provide the first evidence that pregnancy CBNPs exposure-induced brain pathological changes and cerebrovascular dysfunction can persist for a relative long time. These finding suggest exposure to CBNPs during sensitive stages of pregnancy may not only show the harmful effects on offspring neurodevelopment, but also result in the irreversible brain damage on mother body.

A critical appraisal of the measurement of serum 'cholesterol efflux capacity' and its use as surrogate marker of risk of cardiovascular disease

The 'cholesterol efflux capacity (CEC)' assay is a simple in vitro measure of the capacities of individual sera to promote the first step of the reverse cholesterol transport pathway, the delivery of cellular cholesterol to plasma HDL. This review describes the cell biology of this model and critically assesses its application as a marker of cardiovascular risk. We describe the pathways for cell cholesterol export, current cell models used in the CEC assay with their limitations and consider the contribution that measurement of serum CEC provides to our understanding of HDL function in vivo.

The effects of cigarette smoking and smoking cessation on high-density lipoprotein functions: implications for coronary artery disease

BACKGROUND

Smoking cessation was associated with improved prognosis of coronary artery disease. This study was designed to investigate the effect of smoking cessation on high-density lipoprotein functionality in coronary artery disease patients.

METHODS

In this prospective, randomized and parallel controlled study, coronary artery disease smokers ( n = 28) and healthy smokers ( n = 30) were divided into smoking cessation group and continuous smoking group, respectively. Blood samples were collected before and after three-month smoking cessation. Plasma high-density lipoprotein was isolated by density gradient centrifugation. The ability of high-density lipoprotein against copper-induced oxidation of lipoprotein was determined to evaluate the antioxidative property of high-density lipoprotein, and the macrophage migration inhibited by high-density lipoprotein was tested to identify the antichemotactic property of high-density lipoprotein. High-density lipoprotein-induced macrophage cholesterol efflux was measured by fluorescence spectrometry using NBD cholesterol analogue. Healthy non-smoking volunteers were enrolled as the baseline control.

RESULTS

The baseline antioxidative, antichemotactic ability of high-density lipoprotein and high-density lipoprotein-induced cellular cholesterol efflux in coronary artery disease smokers and healthy smokers were significantly attenuated when compared with those in healthy non-smokers. After three-month smoking cessation, both the antioxidative ability and antichemotactic ability of high-density lipoprotein were improved significantly in coronary artery disease smokers. However, high-density lipoprotein-induced cellular cholesterol efflux was not increased by smoking cessation. In in vitro experiments, carbon monoxide reduced the antioxidative ability and nicotine enhanced the antichemotactic ability of high-density lipoprotein.

CONCLUSIONS

Smoking cessation is an effective measure to improve high-density lipoprotein functions in coronary artery disease smokers. Our study re-emphasizes the importance of smoking cessation in the secondary prevention of coronary artery disease.

LCCC 1025: a phase II study of everolimus, trastuzumab, and vinorelbine to treat progressive HER2-positive breast cancer brain metastases

PURPOSE

HER2 + breast cancer (BC) is an aggressive subtype with high rates of brain metastases (BCBM). Two-thirds of HER2 + BCBM demonstrate activation of the PI3K/mTOR pathway driving resistance to anti-HER2 therapy. This phase II study evaluated everolimus (E), a brain-permeable mTOR inhibitor, trastuzumab (T), and vinorelbine (V) in patients with HER2 + BCBM.

PATIENTS AND METHODS

Eligible patients had progressive HER2 + BCBM. The primary endpoint was intracranial response rate (RR); secondary objectives were CNS clinical benefit rate (CBR), extracranial RR, time to progression (TTP), overall survival (OS), and targeted sequencing of tumors from enrolled patients. A two-stage design distinguished intracranial RR of 5% versus 20%.

RESULTS

32 patients were evaluable for toxicity, 26 for efficacy. Intracranial RR was 4% (1 PR). CNS CBR at 6 mos was 27%; at 3 mos 65%. Median intracranial TTP was 3.9 mos (95% CI 2.2-5). OS was 12.2 mos (95% CI 0.6-20.2). Grade 3-4 toxicities included neutropenia (41%), anemia (16%), and stomatitis (16%). Mutations in TP53 and PIK3CA were common in BCBM. Mutations in the PI3K/mTOR pathway were not associated with response. ERBB2 amplification was higher in BCBM compared to primary BC; ERBB2 amplification in the primary BC trended toward worse OS.

CONCLUSION

While intracranial RR to ETV was low in HER2 + BCBM patients, one-third achieved CNS CBR; TTP/OS was similar to historical control. No new toxicity signals were observed. Further analysis of the genomic underpinnings of BCBM to identify tractable prognostic and/or predictive biomarkers is warranted.

CLINICAL TRIAL

(NCT01305941).

ApoE isoforms and carboxyl-terminal-truncated apoE4 forms affect neuronal BACE1 levels and Aβ production independently of their cholesterol efflux capacity

The β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) initiates the production of amyloid-β peptide (Aβ), which is central to the pathogenesis of Alzheimer's disease (AD). Changes in brain cholesterol homeostasis have been suggested to affect Aβ metabolism. Cholesterol homeostasis is maintained in the brain by apolipoprotein E (apoE). The apoE4 isoform constitutes the major risk factor for AD. Here, we investigated the effect of apoE forms on Aβ generation and on BACE1 levels. We also examined the potential involvement in these processes of cholesterol transporters ABCG1 and ABCG4 or the lipoprotein receptor SR-BI, which are implicated in cholesterol efflux to apoE. It was found that reconstituted lipoprotein-associated apoE isoforms promoted the increase of Aβ production and oligomerization and of BACE1 levels in human neuroblastoma SK-N-SH cells, with an apoE4 ≥ apoE3 > apoE2 potency rank order. Progressive carboxyl-terminal apoE4 deletions between residues 230–299 decreased the protein's ability to increase BACE1, while further truncations up to residue 166 prevented apoE4 from increasing BACE1 and Aβ levels in SK-N-SH and primary mouse neuronal cells. ABCG1, but not ABCG4 or SR-BI, moderately increased Aβ production and BACE1 levels in SK-N-SH cells. All apoE forms affected Aβ production/oligomerization and BACE1 levels in a pattern that did not follow that of their capacity to promote ABCG1, ABCG4 or SR-BI-mediated cholesterol efflux. Overall, our data indicate that apoE-containing lipoprotein particles can have a direct effect on BACE1 levels and Aβ secretion and possibly contribute to AD pathogenetic processes, independently of their capacity to promote cholesterol efflux.

Omega-3 fatty acids modulate the lipid profile, membrane architecture, and gene expression of leiomyoma cells

Uterine leiomyomas (fibroids or myomas) are the most common benign tumors of premenopausal women and new medical treatments are needed. This study aimed to determine the effects of omega-3 fatty acids on the lipid profile, membrane architecture and gene expression patterns of extracellular matrix components (collagen1A1, fibronectin, versican, or activin A), mechanical signaling (integrin β1, FAK, and AKAP13), sterol regulatory molecules (ABCG1, ABCA1, CAV1, and SREBF2), and mitochondrial enzyme (CYP11A1) in myometrial and leiomyoma cells. Myometrial tissues had a higher amount of arachidonic acid than leiomyoma tissues while leiomyoma tissues had a higher level of linoleic acid than myometrial tissues. Treatment of primary myometrial and leiomyoma cells with eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) reduced the monounsaturated fatty acid (MUFA) content and increased the polyunsaturated fatty acid (PUFA) content in both cell types. Myometrial and leiomyoma cell membranes were in the liquid-crystalline phase, but EPA- and DHA-treated cells had decreased membrane fluidity. While we found no changes in the mRNA expression of ECM components, EPA and DHA treatment reduced levels of ABCG1, ABCA1, and AKAP13 in both cell types. EPA and DHA also reduced FAK and CYP11A1 expression in myometrial cells. The ability of omega-3 fatty acids to remodel membrane architecture and downregulate the expression of genes involved in mechanical signaling and lipid accumulation in leiomyoma cells offers to further investigate this compound as preventive and/or therapeutic option.

Expression and function of Abcg4 in the mouse blood-brain barrier: role in restricting the brain entry of amyloid-β peptide

ABCG4 is an ATP-binding cassette transmembrane protein which has been shown, in vitro, to participate in the cellular efflux of desmosterol and amyloid-β peptide (Aβ). ABCG4 is highly expressed in the brain, but its localization and function at the blood-brain barrier (BBB) level remain unknown. We demonstrate by qRT-PCR and confocal imaging that mouse Abcg4 is expressed in the brain capillary endothelial cells. Modelling studies of the Abcg4 dimer suggested that desmosterol showed thermodynamically favorable binding at the putative sterol-binding site, and this was greater than for cholesterol. Additionally, unbiased docking also showed Aβ binding at this site. Using a novel Abcg4-deficient mouse model, we show that Abcg4 was able to export Aβ and desmosterol at the BBB level and these processes could be inhibited by probucol and L-thyroxine. Our assay also showed that desmosterol antagonized the export of Aβ, presumably as both bind at the sterol-binding site on Abcg4. We show for the first time that Abcg4 may function in vivo to export Aβ at the BBB, in a process that can be antagonized by its putative natural ligand, desmosterol (and possibly cholesterol).

Critical Role of the Human ATP-Binding Cassette G1 Transporter in Cardiometabolic Diseases

ATP-binding cassette G1 (ABCG1) is a member of the large family of ABC transporters which are involved in the active transport of many amphiphilic and lipophilic molecules including lipids, drugs or endogenous metabolites. It is now well established that ABCG1 promotes the export of lipids, including cholesterol, phospholipids, sphingomyelin and oxysterols, and plays a key role in the maintenance of tissue lipid homeostasis. Although ABCG1 was initially proposed to mediate cholesterol efflux from macrophages and then to protect against atherosclerosis and cardiovascular diseases (CVD), it becomes now clear that ABCG1 exerts a larger spectrum of actions which are of major importance in cardiometabolic diseases (CMD). Beyond a role in cellular lipid homeostasis, ABCG1 equally participates to glucose and lipid metabolism by controlling the secretion and activity of insulin and lipoprotein lipase. Moreover, there is now a growing body of evidence suggesting that modulation of ABCG1 expression might contribute to the development of diabetes and obesity, which are major risk factors of CVD. In order to provide the current understanding of the action of ABCG1 in CMD, we here reviewed major findings obtained from studies in mice together with data from the genetic and epigenetic analysis of ABCG1 in the context of CMD.

EFAD transgenic mice as a human APOE relevant preclinical model of Alzheimer's disease

Identified in 1993, APOE4 is the greatest genetic risk factor for sporadic Alzheimer's disease (AD), increasing risk up to 15-fold compared with APOE3, with APOE2 decreasing AD risk. However, the functional effects of APOE4 on AD pathology remain unclear and, in some cases, controversial. In vivo progress to understand how the human (h)-APOE genotypes affect AD pathology has been limited by the lack of a tractable familial AD-transgenic (FAD-Tg) mouse model expressing h-APOE rather than mouse (m)-APOE. The disparity between m- and h-apoE is relevant for virtually every AD-relevant pathway, including amyloid-β (Aβ) deposition and clearance, neuroinflammation, tau pathology, neural plasticity and cerebrovascular deficits. EFAD mice were designed as a temporally useful preclinical FAD-Tg-mouse model expressing the h-APOE genotypes for identifying mechanisms underlying APOE-modulated symptoms of AD pathology. From their first description in 2012, EFAD mice have enabled critical basic and therapeutic research. Here we review insights gleaned from the EFAD mice and summarize future directions.

Implications of cerebrovascular ATP-binding cassette transporter G1 (ABCG1) and apolipoprotein M in cholesterol transport at the blood-brain barrier

Impaired cholesterol/lipoprotein metabolism is linked to neurodegenerative diseases such as Alzheimer's disease (AD). Cerebral cholesterol homeostasis is maintained by the highly efficient blood-brain barrier (BBB) and flux of the oxysterols 24(S)-hydroxycholesterol and 27-hydroxycholesterol, potent liver-X-receptor (LXR) activators. HDL and their apolipoproteins are crucial for cerebral lipid transfer, and loss of ATP binding cassette transporters (ABC)G1 and G4 results in toxic accumulation of oxysterols in the brain. The HDL-associated apolipoprotein (apo)M is positively correlated with pre-β HDL formation in plasma; its presence and function in the brain was thus far unknown. Using an in vitro model of the BBB, we examined expression, regulation, and functions of ABCG1, ABCG4, and apoM in primary porcine brain capillary endothelial cells (pBCEC). RT Q-PCR analyses and immunoblotting revealed that in addition to ABCA1 and scavenger receptor, class B, type I (SR-BI), pBCEC express high levels of ABCG1, which was up-regulated by LXR activation. Immunofluorescent staining, site-specific biotinylation and immunoprecipitation revealed that ABCG1 is localized both to early and late endosomes and on apical and basolateral plasma membranes. Using siRNA interference to silence ABCG1 (by 50%) reduced HDL-mediated [³H]-cholesterol efflux (by 50%) but did not reduce [³H]-24(S)-hydroxycholesterol efflux. In addition to apoA-I, pBCEC express and secrete apoM mainly to the basolateral (brain) compartment. HDL enhanced expression and secretion of apoM by pBCEC, apoM-enriched HDL promoted cellular cholesterol efflux more efficiently than apoM-free HDL, while apoM-silencing diminished cellular cholesterol release. We suggest that ABCG1 and apoM are centrally involved in regulation of cholesterol metabolism/turnover at the BBB.

Structure of the human multidrug transporter ABCG2

ABCG2 is a constitutively expressed ATP-binding cassette (ABC) transporter that protects many tissues against xenobiotic molecules. Its activity affects the pharmacokinetics of commonly used drugs and limits the delivery of therapeutics into tumour cells, thus contributing to multidrug resistance. Here we present the structure of human ABCG2 determined by cryo-electron microscopy, providing the first high-resolution insight into a human multidrug transporter. We visualize ABCG2 in complex with two antigen-binding fragments of the human-specific, inhibitory antibody 5D3 that recognizes extracellular loops of the transporter. We observe two cholesterol molecules bound in the multidrug-binding pocket that is located in a central, hydrophobic, inward-facing translocation pathway between the transmembrane domains. Combined with functional in vitro analyses, our results suggest a multidrug recognition and transport mechanism of ABCG2, rationalize disease-causing single nucleotide polymorphisms and the allosteric inhibition by the 5D3 antibody, and provide the structural basis of cholesterol recognition by other G-subfamily ABC transporters.

ABCG1 regulates pulmonary surfactant metabolism in mice and men

Idiopathic pulmonary alveolar proteinosis (PAP) is a rare lung disease characterized by accumulation of surfactant. Surfactant synthesis and secretion are restricted to epithelial type 2 (T2) pneumocytes (also called T2 cells). Clearance of surfactant is dependent upon T2 cells and macrophages. ABCG1 is highly expressed in both T2 cells and macrophages. ABCG1-deficient mice accumulate surfactant, lamellar body-loaded T2 cells, lipid-loaded macrophages, B-1 lymphocytes, and immunoglobulins, clearly demonstrating that ABCG1 has a critical role in pulmonary homeostasis. We identify a variant in the ABCG1 promoter in patients with PAP that results in impaired activation of ABCG1 by the liver X receptor α, suggesting that ABCG1 basal expression and/or induction in response to sterol/lipid loading is essential for normal lung function. We generated mice lacking ABCG1 specifically in either T2 cells or macrophages to determine the relative contribution of these cell types on surfactant lipid homeostasis. These results establish a critical role for T2 cell ABCG1 in controlling surfactant and overall lipid homeostasis in the lung and in the pathogenesis of human lung disease.

The ATP binding cassette transporter, ABCG1, localizes to cortical actin filaments

The ATP-binding cassette sub-family G member 1 (ABCG1) exports cellular cholesterol to high-density lipoproteins (HDL). However, a number of recent studies have suggested ABCG1 is predominantly localised to intracellular membranes. In this study, we found that ABCG1 was organized into two distinct cellular pools: one at the plasma membrane and the other associated with the endoplasmic reticulum (ER). The plasma membrane fraction was organized into filamentous structures that were associated with cortical actin filaments. Inhibition of actin polymerization resulted in complete disruption of ABCG1 filaments. Cholesterol loading of the cells increased the formation of the filamentous ABCG1, the proximity of filamentous ABCG1 to actin filaments and the diffusion rate of membrane associated ABCG1. Our findings suggest that the actin cytoskeleton plays a critical role in the plasma membrane localization of ABCG1.

Components of the SNARE-containing regulon are co-regulated in root cells undergoing defense

The term regulon has been coined in the genetic model plant Arabidopsis thaliana, denoting a structural and physiological defense apparatus defined genetically through the identification of the penetration (pen) mutants. The regulon is composed partially by the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) syntaxin PEN1. PEN1 has homology to a Saccharomyces cerevisae gene that regulates a Secretion (Sec) protein, Suppressor of Sec 1 (Sso1p). The regulon is also composed of the β-glucosidase (PEN2) and an ATP binding cassette (ABC) transporter (PEN3). While important in inhibiting pathogen infection, limited observations have been made regarding the transcriptional regulation of regulon genes until now. Experiments made using the model agricultural Glycine max (soybean) have identified co-regulated gene expression of regulon components. The results explain the observation of hundreds of genes expressed specifically in the root cells undergoing the natural process of defense. Data regarding additional G. max genes functioning within the context of the regulon are presented here, including Sec 14, Sec 4 and Sec 23. Other examined G. max homologs of membrane fusion genes include an endosomal bromo domain-containing protein1 (Bro1), syntaxin6 (SYP6), SYP131, SYP71, SYP8, Bet1, coatomer epsilon (ϵ-COP), a coatomer zeta (ζ-COP) paralog and an ER to Golgi component (ERGIC) protein. Furthermore, the effectiveness of biochemical pathways that would function within the context of the regulon ave been examined, including xyloglucan xylosyltransferase (XXT), reticuline oxidase (RO) and galactinol synthase (GS). The experiments have unveiled the importance of the regulon during defense in the root and show how the deposition of callose relates to the process.

ABC-Transporter Mediated Sterol Export from Cells Using Radiolabeled Sterols

Cholesterol export from cells to extracellular acceptors represents the first step of the reverse cholesterol transport process and is an essential part of the multifaceted pathway for cells to control their cholesterol levels. Malfunction of this pathway leads to cholesterol accumulation in cells such as macrophages, which can form the basis of conditions like atherosclerosis. A number of ATP-binding cassette (ABC) transporters, namely ABCA1, ABCA7, ABCG1, and ABCG4, play an essential role in this process. In this chapter, we describe methods utilizing radiolabeled sterols for measuring ABC-transporter mediated sterol export, utilizing endogenously expressed transporters as well as overexpression systems.

Proliferation rates and gene expression profiles in human lymphoblastoid cell lines from patients with depression characterized in response to antidepressant drug therapy

The current therapy success of depressive disorders remains in need of improvement due to low response rates and a delay in symptomatic improvement. Reliable functional biomarkers would be necessary to predict the individual treatment outcome. On the basis of the neurotrophic hypothesis of antidepressant's action, effects of antidepressant drugs on proliferation may serve as tentative individual markers for treatment efficacy. We studied individual differences in antidepressant drug effects on cell proliferation and gene expression in lymphoblastoid cell lines (LCLs) derived from patients treated for depression with documented clinical treatment outcome. Cell proliferation was characterized by EdU (5-ethynyl-2'-deoxyuridine) incorporation assays following a 3-week incubation with therapeutic concentrations of fluoxetine. Genome-wide expression profiling was conducted by microarrays, and candidate genes such as betacellulin-a gene involved in neuronal stem cell regeneration-were validated by quantitative real-time PCR. Ex vivo assessment of proliferation revealed large differences in fluoxetine-induced proliferation inhibition between donor LCLs, but no association with clinical response was observed. Genome-wide expression analyses followed by pathway and gene ontology analyses identified genes with different expression before vs after 21-day incubation with fluoxetine. Significant correlations between proliferation and gene expression of WNT2B, FZD7, TCF7L2, SULT4A1 and ABCB1 (all involved in neurogenesis or brain protection) were also found. Basal gene expression of SULT4A1 (P=0.029), and gene expression fold changes of WNT2B by ex vivo fluoxetine (P=0.025) correlated with clinical response and clinical remission, respectively. Thus, we identified potential gene expression biomarkers eventually being useful as baseline predictors or as longitudinal targets in antidepressant therapy.

MicroRNA-463-3p/ABCG4: A new axis in glucose-stimulated insulin secretion

OBJECTIVE

Glucose-stimulated insulin secretion (GSIS) is known to be essential in the control of metabolic fuel homeostasis, though the molecular mechanisms involved remain unclear.

METHODS

MicroRNA (miRNA)-463-3p and ATP-binding cassette A4 (ABCG4) expression was analyzed by real-time PCR, and the potential role of miRNA-463-3p or ABCG4 was evaluated by overexpressing or silencing such miRNA or genes.

RESULTS

The miRNA-463-3p inhibited GSIS without affecting cell viability. Further, mechanistic studies demonstrated that ABCG4 was a direct target of microRNA-463-3p and, to this effect, that ABCG4 played an important role in GSIS. The targeting was relevant in pancreatic islet β-cells, where GSIS through the miRNA-463-3p/ABCG4 axis was observed. Interestingly, in type 2 diabetes human pancreatic islets, expression of miRNA-463-3p and insulin was upregulated and ABCG4 downregulated compared with nondiabetic controls, and their expression levels were closely correlated.

CONCLUSIONS

The findings collectively establish a link between GSIS and the miRNA-463-3p/ABCG4 axis and represent a promising target for future diabetes mellitus treatments.

Age-Dependent Regulation of the Blood-Brain Barrier Influx/Efflux Equilibrium of Amyloid-β Peptide in a Mouse Model of Alzheimer's Disease (3xTg-AD)

The involvement of transporters located at the blood-brain barrier (BBB) has been suggested in the control of cerebral Aβ levels, and thereby in Alzheimer's disease (AD). However, little is known about the regulation of these transporters at the BBB in animal models of AD. In this study, we investigated the BBB expression of Aβ influx (Rage) and efflux (Abcb1-Abcg2-Abcg4-Lrp-1) transporters and cholesterol transporter (Abca1) in 3-18-month-old 3xTg-AD and control mice. The age-dependent effect of BBB transporters regulation on the brain uptake clearance (Clup) of [3H]cholesterol and [3H]Aβ1 - 40 was then evaluated in these mice, using the in situ brain perfusion technique. Our data suggest that transgenes expression led to the BBB increase in Aβ influx receptor (Rage) and decrease in efflux receptor (Lrp-1). Our data also indicate that mice have mechanisms counteracting this increased net influx. Indeed, Abcg4 and Abca1 are up regulated in 3- and 3/6-month-old 3xTg-AD mice, respectively. Our data show that the balance between the BBB influx and efflux of Aβ is maintained in 3 and 6-month-old 3xTg-AD mice, suggesting that Abcg4 and Abca1 control the efflux of Aβ through the BBB by a direct (Abcg4) or indirect (Abca1) mechanism. At 18 months, the BBB Aβ efflux is significantly increased in 3xTg-AD mice compared to controls. This could result from the significant up-regulation of both Abcg2 and Abcb1 in 3xTg-AD mice compared to control mice. Thus, age-dependent regulation of several Aβ and cholesterol transporters at the BBB could ultimately limit the brain accumulation of Aβ.

Lipid somersaults: Uncovering the mechanisms of protein-mediated lipid flipping

Membrane lipids diffuse rapidly in the plane of the membrane but their ability to flip spontaneously across a membrane bilayer is hampered by a significant energy barrier. Thus spontaneous flip-flop of polar lipids across membranes is very slow, even though it must occur rapidly to support diverse aspects of cellular life. Here we discuss the mechanisms by which rapid flip-flop occurs, and what role lipid flipping plays in membrane homeostasis and cell growth. We focus on conceptual aspects, highlighting mechanistic insights from biochemical and in silico experiments, and the recent, ground-breaking identification of a number of lipid scramblases.

9-cis β-Carotene Increased Cholesterol Efflux to HDL in Macrophages

Cholesterol efflux from macrophages is a key process in reverse cholesterol transport and, therefore, might inhibit atherogenesis. 9-cis-β-carotene (9-cis-βc) is a precursor for 9-cis-retinoic-acid (9-cis-RA), which regulates macrophage cholesterol efflux. Our objective was to assess whether 9-cis-βc increases macrophage cholesterol efflux and induces the expression of cholesterol transporters. Enrichment of a mouse diet with βc from the alga Dunaliella led to βc accumulation in peritoneal macrophages. 9-cis-βc increased the mRNA levels of CYP26B1, an enzyme that regulates RA cellular levels, indicating the formation of RA from βc in RAW264.7 macrophages. Furthermore, 9-cis-βc, as well as all-trans-βc, significantly increased cholesterol efflux to high-density lipoprotein (HDL) by 50% in RAW264.7 macrophages. Likewise, food fortification with 9-cis-βc augmented cholesterol efflux from macrophages ex vivo. 9-cis-βc increased both the mRNA and protein levels of ABCA1 and apolipoprotein E (APOE) and the mRNA level of ABCG1. Our study shows, for the first time, that 9-cis-βc from the diet accumulates in peritoneal macrophages and increases cholesterol efflux to HDL. These effects might be ascribed to transcriptional induction of ABCA1, ABCG1, and APOE. These results highlight the beneficial effect of βc in inhibition of atherosclerosis by improving cholesterol efflux from macrophages.

Intracellular Localization of Endogenous Mouse ABCG1 Is Mimicked by Both ABCG1-L550 and ABCG1-P550-Brief Report

OBJECTIVE

In a recent article in Arteriosclerosis, Thrombosis, and Vascular Biology, it was reported that ATP-binding cassette transporter G1 (ABCG1) containing leucine at position 550 (ABCG1-L550) was localized to the plasma membrane, whereas ABCG1-P550 (proline at position 550) was intracellular. Because the published data on the subcellular localization of ABCG1 are controversial, we performed additional experiments to determine the importance of leucine or proline at amino acid 550.

APPROACH AND RESULTS

We transfected multiple cell lines (CHO-K1, Cos-7, and HEK293 [human embryonic kidney]) with untagged or FLAG-tagged ABCG1 containing either leucine or proline at position 550. Immunofluorescence studies demonstrated that in all cases, ABCG1 localized to intracellular endosomal vesicles. We also show that both ABCG1-L550 and ABCG1-P550 are equally active in both promoting the efflux of cellular cholesterol to exogenous high-density lipoprotein and in inducing the activity of sterol regulatory element-binding protein-2, presumably as a result of redistributing intracellular sterols away from the endoplasmic reticulum. Importantly, we treated nontransfected primary peritoneal macrophages with a liver X receptor agonist and demonstrate, using immunofluorescence, that although endogenous ABCG1 localizes to intracellular endosomes, none was detectable at the cell surface/plasma membrane.

CONCLUSIONS

ABCG1, irrespective of either a leucine or proline at position 550, is an intracellular protein that localizes to vesicles of the endosomal pathway where it functions to mobilize sterols away from the endoplasmic reticulum and out of the cell.

Functional Cooperativity between ABCG4 and ABCG1 Isoforms

ABCG4 belongs to the ABCG subfamily, the members of which are half transporters composed of a single transmembrane and a single nucleotide-binding domain. ABCG proteins have a reverse domain topology as compared to other mammalian ABC transporters, and have to form functional dimers, since the catalytic sites for ATP binding and hydrolysis, as well as the transmembrane domains are composed of distinct parts of the monomers. Here we demonstrate that ABCG4 can form homodimers, but also heterodimers with its closest relative, ABCG1. Both the full-length and the short isoforms of ABCG1 can dimerize with ABCG4, whereas the ABCG2 multidrug transporter is unable to form a heterodimer with ABCG4. We also show that contrary to that reported in some previous studies, ABCG4 is predominantly localized to the plasma membrane. While both ABCG1 and ABCG4 have been suggested to be involved in lipid transport or regulation, in accordance with our previous results regarding the long version of ABCG1, here we document that the expression of both the short isoform of ABCG1 as well as ABCG4 induce apoptosis in various cell types. This apoptotic effect, as a functional read-out, allowed us to demonstrate that the dimerization between these half transporters is not only a physical interaction but functional cooperativity. Given that ABCG4 is predominantly expressed in microglial-like cells and endothelial cells in the brain, our finding of ABCG4-induced apoptosis may implicate a new role for this protein in the clearance mechanisms within the central nervous system.

ABCG1 and ABCG4 Suppress γ-Secretase Activity and Amyloid β Production

ATP-binding cassette G1 (ABCG1) and ABCG4, expressed in neurons and glia in the central nervous system, mediate cholesterol efflux to lipid acceptors. The relationship between cholesterol level in the central nervous system and Alzheimer's disease has been reported. In this study, we examined the effects of ABCG1 and ABCG4 on amyloid precursor protein (APP) processing, the product of which, amyloid β (Aβ), is involved in the pathogenesis of Alzheimer's disease. Expression of ABCG1 or ABCG4 in human embryonic kidney 293 cells that stably expressed Swedish-type mutant APP increased cellular and cell surface APP levels. Products of cleavage from APP by α-secretase and by β-secretase also increased. The levels of secreted Aβ, however, decreased in the presence of ABCG1 and ABCG4, but not ABCG4-KM, a nonfunctional Walker-A lysine mutant. In contrast, secreted Aβ levels increased in differentiated SH-SY5Y neuron-like cells in which ABCG1 and ABCG4 were suppressed. Furthermore, Aβ42 peptide in the cerebrospinal fluid from Abcg1 null mice significantly increased compared to the wild type mice. To examine the underlying mechanism, we analyzed the activity and distribution of γ-secretase. ABCG1 and ABCG4 suppressed γ-secretase activity and disturbed γ-secretase localization in the raft domains where γ-secretase functions. These results suggest that ABCG1 and ABCG4 alter the distribution of γ-secretase on the plasma membrane, leading to the decreased γ-secretase activity and suppressed Aβ secretion. ABCG1 and ABCG4 may inhibit the development of Alzheimer's disease and can be targets for the treatment of Alzheimer's disease.

Convergent Signaling Pathways Controlled by LRP1 (Receptor-related Protein 1) Cytoplasmic and Extracellular Domains Limit Cellular Cholesterol Accumulation

The low density lipoprotein receptor-related protein 1 (LRP1) is a ubiquitously expressed cell surface receptor that protects from intracellular cholesterol accumulation. However, the underlying mechanisms are unknown. Here we show that the extracellular (α) chain of LRP1 mediates TGFβ-induced enhancement of Wnt5a, which limits intracellular cholesterol accumulation by inhibiting cholesterol biosynthesis and by promoting cholesterol export. Moreover, we demonstrate that the cytoplasmic (β) chain of LRP1 suffices to limit cholesterol accumulation in LRP1(-/-) cells. Through binding of Erk2 to the second of its carboxyl-terminal NPXY motifs, LRP1 β-chain positively regulates the expression of ATP binding cassette transporter A1 (ABCA1) and of neutral cholesterol ester hydrolase (NCEH1). These results highlight the unexpected functions of LRP1 and the canonical Wnt5a pathway and new therapeutic potential in cholesterol-associated disorders including cardiovascular diseases.