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

Rebuilding insight into the pathophysiology of Alzheimer's disease through new blood-brain barrier models

The blood-brain barrier is a unique function of the microvasculature in the brain parenchyma that maintains homeostasis in the central nervous system. Blood-brain barrier breakdown is a common pathology in various neurological diseases, such as Alzheimer's disease, stroke, multiple sclerosis, and Parkinson's disease. Traditionally, it has been considered a consequence of neuroinflammation or neurodegeneration, but recent advanced imaging techniques and detailed studies in animal models show that blood-brain barrier breakdown occurs early in the disease process and may precede neuronal loss. Thus, the blood-brain barrier is attractive as a potential therapeutic target for neurological diseases that lack effective therapeutics. To elucidate the molecular mechanism underlying blood-brain barrier breakdown and translate them into therapeutic strategies for neurological diseases, there is a growing demand for experimental models of human origin that allow for functional assessments. Recently, several human induced pluripotent stem cell-derived blood-brain barrier models have been established and various in vitro blood-brain barrier models using microdevices have been proposed. Especially in the Alzheimer's disease field, the human evidence for blood-brain barrier dysfunction has been demonstrated and human induced pluripotent stem cell-derived blood-brain barrier models have suggested the putative molecular mechanisms of pathological blood-brain barrier. In this review, we summarize recent evidence of blood-brain barrier dysfunction in Alzheimer's disease from pathological analyses, imaging studies, animal models, and stem cell sources. Additionally, we discuss the potential future directions for blood-brain barrier research.

Enhancing of cerebral Abeta clearance by modulation of ABC transporter expression: a review of experimental approaches

Clearance of amyloid-beta (Aβ) from the brain is impaired in both early-onset and late-onset Alzheimer's disease (AD). Mechanisms for clearing cerebral Aβ include proteolytic degradation, antibody-mediated clearance, blood brain barrier and blood cerebrospinal fluid barrier efflux, glymphatic drainage, and perivascular drainage. ATP-binding cassette (ABC) transporters are membrane efflux pumps driven by ATP hydrolysis. Their functions include maintenance of brain homeostasis by removing toxic peptides and compounds, and transport of bioactive molecules including cholesterol. Some ABC transporters contribute to lowering of cerebral Aβ. Mechanisms suggested for ABC transporter-mediated lowering of brain Aβ, in addition to exporting of Aβ across the blood brain and blood cerebrospinal fluid barriers, include apolipoprotein E lipidation, microglial activation, decreased amyloidogenic processing of amyloid precursor protein, and restricting the entrance of Aβ into the brain. The ABC transporter superfamily in humans includes 49 proteins, eight of which have been suggested to reduce cerebral Aβ levels. This review discusses experimental approaches for increasing the expression of these ABC transporters, clinical applications of these approaches, changes in the expression and/or activity of these transporters in AD and transgenic mouse models of AD, and findings in the few clinical trials which have examined the effects of these approaches in patients with AD or mild cognitive impairment. The possibility that therapeutic upregulation of ABC transporters which promote clearance of cerebral Aβ may slow the clinical progression of AD merits further consideration.

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.

Blood-brain barrier transporters: An overview of function, dysfunction in Alzheimer's disease and strategies for treatment

The blood-brain-barrier (BBB) has a major function in maintaining brain homeostasis by regulating the entry of molecules from the blood to the brain. Key players in BBB function are BBB transporters which are highly expressed in brain endothelial cells (BECs) and critical in mediating the exchange of nutrients and waste products. BBB transporters can also influence drug delivery into the brain by inhibiting or facilitating the entry of brain targeting therapeutics for the treatment of brain disorders, such as Alzheimer's disease (AD). Recent studies have shown that AD is associated with a disrupted BBB and transporter dysfunction, although their roles in the development in AD are not fully understand. Modulation of BBB transporter activity may pose a novel approach to enhance the delivery of drugs to the brain for enhanced treatment of AD. In this review, we will give an overview of key functions of BBB transporters and known changes in AD. In addition, we will discuss current strategies for transporter modulation for enhanced drug delivery into the brain.

Alzheimer's disease brain endothelial-like cells reveal differential drug transporter expression and modulation by potentially therapeutic focused ultrasound

The blood-brain barrier (BBB) has a key function in maintaining homeostasis in the brain, partly modulated by transporters, which are highly expressed in brain endothelial cells (BECs). Transporters mediate the uptake or efflux of compounds to and from the brain and they can also challenge the delivery of drugs for the treatment of Alzheimer's disease (AD). Currently there is a limited understanding of changes in BBB transporters in AD. To investigate this, we generated brain endothelial-like cells (iBECs) from induced pluripotent stem cells (iPSCs) with familial AD (FAD) Presenilin 1 (PSEN1) mutation and identified AD-specific differences in transporter expression compared to control (ctrl) iBECs. We first characterized the expression levels of 12 BBB transporters in AD-, Ctrl-, and isogenic (PSEN1 corrected) iBECs to identify any AD specific differences. We then exposed the cells to focused ultrasound (FUS) in the absence (FUSonly) or presence of microbubbles (MB) (FUS+MB), which is a novel therapeutic method that can be used to transiently open the BBB to increase drug delivery into the brain, however its effects on BBB transporter expression are largely unknown. Following FUSonly and FUS+MB, we investigated whether the expression or activity of key transporters could be modulated. Our findings demonstrate that PSEN1 mutant FAD (PSEN1AD) possess phenotypical differences compared to control iBECs in BBB transporter expression and function. Additionally, we show that FUSonly and FUS+MB can modulate BBB transporter expression and functional activity in iBECs, having potential implications on drug penetration and amyloid clearance. These findings highlight the differential responses of patient cells to FUS treatment, with patient-derived models likely providing an important tool for modelling therapeutic effects of FUS.

Conjoint hepatobiliary-enterohepatic cycles for amyloid excretion and enhancing its drug-induced clearance: a systems biology approach to Alzheimer's disease

The liver is the major organ responsible for metabolism of amyloid-beta, the primary toxic misfolded protein responsible for Alzheimer's disease (AD). The present study focuses on the crucial role of liver in AD. We have developed a framework that formulates and integrates two reciprocal transport processes of amyloid, via hepato-biliary and enterohepatic circulations (EHC). Our system analysis approach shows that activating the liver X-receptor (LXR) can reduce amyloid-beta formation by increasing expression of the genes: ATP-binding-cassette-transporter (ABCA1) and Stearoyl-CoA-desaturase (SCD). Besides, activating the pregnane-X-receptor (PXR) can enhance the clearance of amyloid-beta by increasing the expression of the genes: ATP-Binding-Cassette-Superfamily-G-member-2 (ABCG2) and multidrug-resistance protein-1 (MDR1). We also identified receptor-like apical sodium-dependent bile-acid transporter (ASBT) of intestinal enterocyte, showing affinity towards amyloid-beta, suggesting amyloid-beta's possible reuptake from intestinal contents to the systemic circulation through this receptor. Further, we have performed protein-protein interaction to evaluate the binding affinity of amyloid-beta to these receptors. Moreover, we undertook molecular docking and molecular dynamic simulation of some repurposed drugs (rifampicin, 24-hydroxycholesterols, resveratrol, cilostazol) which can target the aforesaid receptors to enhance amyloid-beta's fecal clearance, reduce amyloid-beta formation, and prevent the reuptake of amyloid-beta from intestinal feces. Additionally, network pharmacology and synergism analysis were utilized to validate our hypothesis and identify the drug combinations, respectively. Gene-ontology investigation, network pharmacology, and consolidated pathway analysis validate the alteration of the above-mentioned gene expression profiles. Furthermore, our neuropharmacological synergism study identifies the optimal combination of the repurposed drugs. Finally, our findings on candidate drugs are substantiated by clinical-trial outcomes.Communicated by Ramaswamy H. Sarma.

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.

White matter injury, cholesterol dysmetabolism, and APP/Abeta dysmetabolism interact to produce Alzheimer's disease (AD) neuropathology: A hypothesis and review

We postulate that myelin injury contributes to cholesterol release from myelin and cholesterol dysmetabolism which contributes to Abeta dysmetabolism, and combined with genetic and AD risk factors, leads to increased Abeta and amyloid plaques. Increased Abeta damages myelin to form a vicious injury cycle. Thus, white matter injury, cholesterol dysmetabolism and Abeta dysmetabolism interact to produce or worsen AD neuropathology. The amyloid cascade is the leading hypothesis for the cause of Alzheimer's disease (AD). The failure of clinical trials based on this hypothesis has raised other possibilities. Even with a possible new success (Lecanemab), it is not clear whether this is a cause or a result of the disease. With the discovery in 1993 that the apolipoprotein E type 4 allele (APOE4) was the major risk factor for sporadic, late-onset AD (LOAD), there has been increasing interest in cholesterol in AD since APOE is a major cholesterol transporter. Recent studies show that cholesterol metabolism is intricately involved with Abeta (Aβ)/amyloid transport and metabolism, with cholesterol down-regulating the Aβ LRP1 transporter and upregulating the Aβ RAGE receptor, both of which would increase brain Aβ. Moreover, manipulating cholesterol transport and metabolism in rodent AD models can ameliorate pathology and cognitive deficits, or worsen them depending upon the manipulation. Though white matter (WM) injury has been noted in AD brain since Alzheimer's initial observations, recent studies have shown abnormal white matter in every AD brain. Moreover, there is age-related WM injury in normal individuals that occurs earlier and is worse with the APOE4 genotype. Moreover, WM injury precedes formation of plaques and tangles in human Familial Alzheimer's disease (FAD) and precedes plaque formation in rodent AD models. Restoring WM in rodent AD models improves cognition without affecting AD pathology. Thus, we postulate that the amyloid cascade, cholesterol dysmetabolism and white matter injury interact to produce and/or worsen AD pathology. We further postulate that the primary initiating event could be related to any of the three, with age a major factor for WM injury, diet and APOE4 and other genes a factor for cholesterol dysmetabolism, and FAD and other genes for Abeta dysmetabolism.

ATP-binding cassette transporters and neurodegenerative diseases

ATP-binding cassette (ABC) transporters are one of the largest groups of transporter families in humans. ABC transporters mediate the translocation of a diverse range of substrates across cellular membranes, including amino acids, nucleosides, lipids, sugars and xenobiotics. Neurodegenerative diseases are a group of brain diseases that detrimentally affect neurons and other brain cells and are usually associated with deposits of pathogenic proteins in the brain. Major neurodegenerative diseases include Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. ABC transporters are highly expressed in the brain and have been implicated in a number of pathological processes underlying neurodegenerative diseases. This review outlines the current understanding of the role of ABC transporters in neurodegenerative diseases, focusing on some of the most important pathways, and also suggests future directions for research in this field.

Alix: A Candidate Serum Biomarker of Alzheimer's Disease

Alzheimer’s disease (AD) is the most common fatal neurodegenerative disease of the elderly worldwide. The identification of AD biomarkers will allow for earlier diagnosis and thus earlier intervention. The aim of this study was to find such biomarkers. It was observed that the expression of Alix was significantly decreased in brain tissues and serum samples from AD patients compared to the controls. A significant correlation between Alix levels and cognitive decline was observed (r = 0.80; p < 0.001) as well as a significant negative correlation between Alix and Aβ₄₀ in serum levels (r =−0.60, p < 0.001). The receiver operating characteristic curve (ROC) analysis showed the area under the curve (AUC) of Alix was 0.80, and the optimal cut-off point of 199.5 pg/ml was selected with the highest sum of sensitivity and specificity. The diagnostic accuracy for serum Alix was 74%, with 76% sensitivity and 71% specificity respectively, which could differentiate AD from controls. In addition, the expression of Alix was found to be significantly decreased in AD compared to vascular dementia (VaD). ROC analysis between AD and VaD showed that the AUC was 0.777, which could be indicative of the role of serum Alix as a biomarker in the differential diagnosis between AD and VaD. Most surprisingly, the decreased expression of Alix was attenuated after the treatment of Memantine in different AD animal models. In conclusion, our results indicate the possibility of serum Alix as a novel and non-invasive biomarker for AD for the first time.

Role of the gut microbiome in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, affecting millions of individuals each year and this number is expected to significantly increase. The complicated microorganisms residing in human gut are closely associated with our health. Emerging evidence has suggested possible involvement of human gut microbiome in AD. Symbiotic gut microbiomes are known to maintain brain health by modulating host's barriers integrity, metabolic system, immune system, nervous system and endocrine system. However, in the event of gut dysbiosis and barriers disruption, gut pathobionts disrupt homeostasis of the metabolic system, immune system, nervous system, and endocrine system, resulting in deterioration of neurological functions and subsequently promoting development of AD. Multiple therapeutic approaches, such as fecal microbiome transplant, antibiotics, prebiotics, probiotics, symbiotic, and diet are discussed as potential treatment options for AD by manipulating the gut microbiome to reverse pathological alteration in the systems above.

The role of brain barriers in the neurokinetics and pharmacodynamics of lithium

Lithium (Li) is the most widely used mood stabilizer in treating patients with bipolar disorder. However, more than half of the patients do not or partially respond to Li therapy, despite serum Li concentrations in the serum therapeutic range. The exact mechanisms underlying the pharmacokinetic-pharmacodynamic (PK-PD) relationships of lithium are still poorly understood and alteration in the brain pharmacokinetics of lithium may be one of the mechanisms explaining the variability in the clinical response to Li. Brain barriers such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) play a crucial role in controlling blood-to-brain and brain-to-blood exchanges of various molecules including central nervous system (CNS) drugs. Recent in vivo studies by nuclear resonance spectroscopy revealed heterogenous brain distribution of Li in human that were not always correlated with serum concentrations, suggesting regional and variable transport mechanisms of Li through the brain barriers. Moreover, alteration in the functionality and integrity of brain barriers is reported in various CNS diseases, as a cause or a consequence and in this regard, Li by itself is known to modulate BBB properties such as the expression and activity of various transporters, metabolizing enzymes, and the specialized tight junction proteins on BBB. In this review, we will focus on recent knowledge into the role of the brain barriers as key-element in the Li neuropharmacokinetics which might improve the understanding of PK-PD of Li and its interindividual variability in drug response.

Brain Distribution of Drugs: Brain Morphology, Delivery Routes, and Species Differences

Neuropharmacokinetics considers cerebral drug distribution as a critical process for central nervous system drug action as well as for drug penetration through the CNS barriers. Brain distribution of small molecules obeys classical rules of drug partition, permeability, binding to fluid proteins or tissue components, and tissue perfusion. The biodistribution of all drugs, including both small molecules and biologics, may also be influenced by specific brain properties related to brain anatomy and physiological barriers, fluid dynamics, and cellular and biochemical composition, each of which can exhibit significant interspecies differences. All of these properties contribute to select optimal dosing paradigms and routes of drug delivery to reach brain targets for classical small molecule drugs as well as for biologics. The importance of these properties for brain delivery and exposure also highlights the need for efficient new analytical technologies to more comprehensively investigate drug distribution in the CNS, a complex multi-compartmentalized organ system.

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.

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.

Do ABC transporters regulate plasma membrane organization?

The plasma membrane (PM) spatiotemporal organization is one of the major factors controlling cell signaling and whole-cell homeostasis. The PM lipids, including cholesterol, determine the physicochemical properties of the membrane bilayer and thus play a crucial role in all membrane-dependent cellular processes. It is known that lipid content and distribution in the PM are not random, and their transversal and lateral organization is highly controlled. Mainly sphingolipid- and cholesterol-rich lipid nanodomains, historically referred to as rafts, are extremely dynamic “hot spots” of the PM controlling the function of many cell surface proteins and receptors. In the first part of this review, we will focus on the recent advances of PM investigation and the current PM concept. In the second part, we will discuss the importance of several classes of ABC transporters whose substrates are lipids for the PM organization and dynamics. Finally, we will briefly present the significance of lipid ABC transporters for immune responses.

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.

ABC Transporters at the Blood-Brain Interfaces, Their Study Models, and Drug Delivery Implications in Gliomas

Drug delivery into the brain is regulated by the blood-brain interfaces. The blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the blood-arachnoid barrier (BAB) regulate the exchange of substances between the blood and brain parenchyma. These selective barriers present a high impermeability to most substances, with the selective transport of nutrients and transporters preventing the entry and accumulation of possibly toxic molecules, comprising many therapeutic drugs. Transporters of the ATP-binding cassette (ABC) superfamily have an important role in drug delivery, because they extrude a broad molecular diversity of xenobiotics, including several anticancer drugs, preventing their entry into the brain. Gliomas are the most common primary tumors diagnosed in adults, which are often characterized by a poor prognosis, notably in the case of high-grade gliomas. Therapeutic treatments frequently fail due to the difficulty of delivering drugs through the brain barriers, adding to diverse mechanisms developed by the cancer, including the overexpression or expression de novo of ABC transporters in tumoral cells and/or in the endothelial cells forming the blood-brain tumor barrier (BBTB). Many models have been developed to study the phenotype, molecular characteristics, and function of the blood-brain interfaces as well as to evaluate drug permeability into the brain. These include in vitro, in vivo, and in silico models, which together can help us to better understand their implication in drug resistance and to develop new therapeutics or delivery strategies to improve the treatment of pathologies of the central nervous system (CNS). In this review, we present the principal characteristics of the blood-brain interfaces; then, we focus on the ABC transporters present on them and their implication in drug delivery; next, we present some of the most important models used for the study of drug transport; finally, we summarize the implication of ABC transporters in glioma and the BBTB in drug resistance and the strategies to improve the delivery of CNS anticancer drugs.

Potential microRNA-related targets in clearance pathways of amyloid-β: novel therapeutic approach for the treatment of Alzheimer's disease

Imbalance between amyloid-beta (Aβ) peptide synthesis and clearance results in Aβ deregulation. Failure to clear these peptides appears to cause the development of Alzheimer's disease (AD). In recent years, microRNAs have become established key regulators of biological processes that relate among others to the development and progression of neurodegenerative diseases, such as AD. This review article gives an overview on microRNAs that are involved in the Aβ cascade and discusses their inhibitory impact on their target mRNAs whose products participate in Aβ clearance. Understanding of the mechanism of microRNA in the associated signal pathways could identify novel therapeutic targets for the treatment of AD.

ABC Transporters in Neurological Disorders: An Important Gateway for Botanical Compounds Mediated Neuro-Therapeutics

Neurodegeneration is a distinguishing feature of many age related disorders and other vector borne neuroinflammatory diseases. There are a number of factors that can modulate the pathology of these disorders. ATP-binding cassette (ABC) transporters are primarily involved in the maintenance of normal brain homeostasis by eliminating toxic peptides and compounds from the brain. Also, ABC transporters protect the brain from the unwanted effects of endogenous and exogenous toxins that can enter the brain parenchyma. Therefore, these transporters have the ability to determine the pathological outcomes of several neurological disorders. For instance, ABC transporters like P-glycoprotein (ABCB1), and BCRP (ABCG2) have been reported to facilitate the clearance of peptides such as amyloid-β (Aβ) that accumulate in the brain during Alzheimer's disease (AD) progression. Other members such as ABCA1, ABCA2, ABCC8, ABCC9, ABCG1 and ABCG4 also have been reported to be involved in the progression of various brain disorders such as HIV-associated dementia, Multiple sclerosis (MS), Ischemic stroke, Japanese encephalitis (JE) and Epilepsy. However, these defective transporters can be targeted by numerous botanical compounds such as Verapamil, Berberine and Fascalpsyn as a therapeutic target to treat these neurological outcomes. These compounds are already reported to modulate ABC transporter activity in the CNS. Nonetheless, the exact mechanisms involving the ABC transporters role in normal brain functioning, their role in neuronal dysfunction and how these botanical compounds ensure and facilitate their therapeutic action in association with defective transporters still remain elusive. This review therefore, summarizes the role of ABC transporters in neurological disorders, with a special emphasis on its role in AD brains. The prospect of using botanical/natural compounds as modulators of ABC transporters in neurological disorders is discussed in the latter half of the article.

The Adaptor Protein Alix is Involved in the Interaction Between the Ubiquitin Ligase NEDD4-1 and its Targets, ABCG1 and ABCG4

Several ATP-Binding Cassette (ABC) transporters, including ABCG1 and the related ABCG4, are essential regulators of cellular lipid homeostasis. ABCG1 is expressed ubiquitously and is functional in the context of atherosclerosis. However, ABCG4 is expressed almost exclusively in brain and has been linked to Alzheimer's disease (AD). These transporters are highly regulated post-translationally by E3 ubiquitin ligases, with the ligase NEDD4-1 (Neural precursor cell-expressed developmentally downregulated gene 4) implicated in their protein stability. In this study, we investigated interacting partners of ABCG1 using peptide-mass spectrometry and identified the potential adaptor protein, Alix (apoptosis-linked gene 2-interacting protein X). In this paper, we hypothesized and investigated whether Alix could facilitate the interaction between NEDD4-1 and the ABC transporters. We showed that Alix and NEDD4-1 proteins were co-expressed in several commonly used cell lines. Knockdown of Alix in cells overexpressing ABCG1 or ABCG4 increased transporter protein expression while co-immunoprecipitation experiments showed interaction between NEDD4-1, Alix, and ABC transporters. In summary, we provide evidence that Alix serves as a co-factor for the interaction between the E3-ubiquitin ligase NEDD4-1 and the ABC transporter targets, ABCG1 and ABCG4.

The concerted amyloid-beta clearance of LRP1 and ABCB1/P-gp across the blood-brain barrier is linked by PICALM

The accumulation of neurotoxic amyloid-beta (Aβ) in the brain is a characteristic hallmark of Alzheimer's disease (AD). The blood-brain barrier (BBB) provides a large surface area and has been shown to be an important mediator for removal of brain Aβ. Both, the ABC transporter P-glycoprotein (ABCB1/P-gp) and the receptor low-density lipoprotein receptor-related protein 1 (LRP1) have been implicated to play crucial roles in Aβ efflux from brain. Here, with immunoprecipitation experiments, co-immunostainings and dual inhibition of ABCB1/P-gp and LRP1, we show that both proteins are functionally linked, mediating a concerted transcytosis of Aβ through endothelial cells. Late-onset AD risk factor Phosphatidylinositol binding clathrin assembly protein (PICALM) is associated with both ABCB1/P-gp and LRP1 representing a functional link and guiding both proteins through the brain endothelium. Together, our results give more mechanistic insight on Aβ transport across the BBB and show that the functional interplay of different clearance proteins is needed for the rapid removal of Aβ from the brain.

ABC transporters as cancer drivers: Potential functions in cancer development

BACKGROUND

ABC transporters have attracted considerable attention for their function as drug transporters in a broad range of tumours and are therefore considered as major players in cancer chemoresistance. However, less attention has been focused on their potential role as active players in cancer development and progression.

SCOPE OF REVIEW

This review presents the evidence suggesting that ABC transporters might have a more active role in cancer other than the well known involvement in multidrug resistance and discusses the potential strategies to target each ABC transporter for a specific tumour setting.

MAJOR CONCLUSIONS

Emerging evidence suggests that ABC transporters are able to transport bioactive molecules capable of playing key roles in tumour development. Characterization of the effects of these transporters in specific cancer settings opens the possibility for the development of personalized treatments.

GENERAL SIGNIFICANCE

A more targeted approach of ABC transporters should be implemented that considers which specific transporter is playing a major role in a particular tumour setting in order to achieve a more successful outcome for ABC transporters inhibitors in cancer therapy.

Structural snapshot of the cholesterol-transport ATP-binding cassette proteins 1

The ATP-binding cassette (ABC) proteins play critical roles in maintaining lipid and sterol homeostasis in higher eukaryotes. In humans, several subfamily-A and -G members function as cholesterol transporters across the cellular membranes. Deficiencies of these ABC proteins can cause dyslipidemia that is associated with health conditions such as atherosclerosis, diabetes, fatty liver disease, and neurodegeneration. The physiological roles of ABC cholesterol transporters have been implicated in mediating cholesterol efflux for reverse cholesterol transport and in maintaining membrane integrity for cell survival. The precise role of these ABC transporters in cells remains elusive, and little is known about the sterol-transport mechanism. The membrane constituents of ABC transporters have been postulated to play a key role in determining the transport substrates and the translocation mechanisms via the transmembrane domains. Recent breakthroughs in determining high-resolution structures of the human sterol transporter ABCG5/G8 and its functional homologs have shed light on new structural features of ABC transporters, providing a more relevant framework for mechanistic analysis of cholesterol-transport ABC proteins. This minireview outlines what is known about ABCG cholesterol transporters, addresses key structural features in the putative sterol translocation pathway on the transmembrane domains, and concludes by proposing a mechanistic model of ABC cholesterol transporters.

ABCG5 and ABCG8: more than a defense against xenosterols

The elucidation of the molecular basis of the rare disease, sitosterolemia, has revolutionized our mechanistic understanding of how dietary sterols are excreted and how cholesterol is eliminated from the body. Two proteins, ABCG5 and ABCG8, encoded by the sitosterolemia locus, work as obligate dimers to pump sterols out of hepatocytes and enterocytes. ABCG5/ABCG8 are key in regulating whole-body sterol trafficking, by eliminating sterols via the biliary tree as well as the intestinal tract. Importantly, these transporters keep xenosterols from accumulating in the body. The sitosterolemia locus has been genetically associated with lipid levels and downstream atherosclerotic disease, as well as formation of gallstones and the risk of gallbladder cancer. While polymorphic variants raise or lower the risks of these phenotypes, loss of function of this locus leads to more dramatic phenotypes, such as premature atherosclerosis, platelet dysfunction, and thrombocytopenia, and, perhaps, increased endocrine disruption and liver dysfunction. Whether small amounts of xenosterol exposure over a lifetime cause pathology in normal humans with polymorphic variants at the sitosterolemia locus remains largely unexplored. The purpose of this review will be to summarize the current state of knowledge, but also highlight key conceptual and mechanistic issues that remain to be explored.

Computational simulation and modeling of the blood-brain barrier pathology

In silico methods and models in the pathology of the blood-brain barrier (BBB) or also called BBB "computational pathology", are based on using mathematical approaches together with complex, high-dimensional experimental data to evaluate and predict disease-related impacts on the CNS. These computational methods and tools have been designed to deal with BBB-linked neuropathology at the molecular, cellular, tissue, and organ levels. The molecular and cellular levels mainly include molecular docking and molecular dynamics simulations (atomistic and coarse-grain) of mutated or misfolded tight junction proteins, receptors, and various BBB transporters. The tissue and organ levels encompass the mechanistic and pharmacokinetic models as well as finite-element method and pathway analyses enriched with multiple sources of raw data (e.g., in vitro and in vivo, histopathological records, "-omics", and imaging data). Overall, this review discusses comprehensive computational techniques and strategies at different levels of complexity, providing new insights and future directions for diagnosis, treatment improvement, and a deeper understanding of BBB-related neuropathological events.