A direct interaction of cholesterol with the dopamine transporter prevents its out-to-inward transition

The efficacy of the analgesic GlyT2 inhibitor, ORG25543, is determined by two connected allosteric sites

Glycine Transporter 2 (GlyT2) inhibitors have shown considerable potential as analgesics for the treatment of neuropathic pain but also display considerable side effects. One potential source of side effects is irreversible inhibition. In this study, we have characterized the mechanism of ORG25543 inhibition of GlyT2 by first considering three potential ligand binding sites on GlyT2-the substrate site, the vestibule allosteric site and the lipid allosteric site. The three sites were tested using a combination of molecular dynamics simulations and analysis of the inhibition of glycine transport of a series point mutated GlyT2 using electrophysiological methods. We demonstrate that the lipid allosteric site on GlyT2 is the most likely binding site for ORG25543. We also demonstrate that cholesterol derived from the cell membrane can form specific interactions with inhibitor-bound transporters to form an allosteric network of regulatory sites. These observations will guide the future design of GlyT2 inhibitors with the objective of minimising on-target side effects and improving the therapeutic window for the treatment of patients suffering from neuropathic pain.

Pitavastatin attenuates hypercholesterolemia-induced decline in serotonin transporter availability

INTRODUCTION

Hypercholesterolemia is associated with increased inflammation and impaired serotonin neurotransmission, potentially contributing to depressive symptoms. However, the role of statins, particularly pitavastatin, in modulating serotonin transporter (SERT) function within this context remains underexplored. This study aimed to investigate whether pitavastatin counteracts the neurobiological effects of hypercholesterolemia.

METHODS

Low-density lipoprotein receptor knockout (LDLR-/-) mice on a C57BL/6 background were assigned to three groups: a control group fed a standard chow diet, a group fed a high-fat diet (HFD), and a third group fed a high-fat diet supplemented with pitavastatin (HFD + Pita). We evaluated the effects of HFD with or without pitavastatin on lipid profiles, inflammatory markers, and SERT availability using small-animal positron emission tomography (PET) scans with the radioligand 4-[18F]-ADAM over a 20-week period.

RESULTS

Pitavastatin treatment in HFD-fed mice significantly reduced both total cholesterol and LDL cholesterol levels in HFD-fed mice compared to those on HFD alone. Elevated inflammatory markers such as IL-1α, MCP-1/CCL2, and TNF-α in HFD mice were notably decreased in the HFD + Pita group. PET scans showed reduced SERT availability in the brains of HFD mice; however, pitavastatin improved this in brain regions associated with mood regulation, suggesting enhanced serotonin neurotransmission. Additionally, the sucrose preference test showed a trend towards increased preference in the HFD + Pita group compared to the HFD group, indicating a potential reduction in depressive-like behavior.

CONCLUSION

Our findings demonstrate that pitavastatin not only lowers cholesterol and reduces inflammation but also enhances SERT availability, suggesting a potential role in alleviating depressive symptoms associated with hypercholesterolemia. These results highlight the multifaceted benefits of pitavastatin, extending beyond its lipid-lowering effects to potentially improving mood regulation and neurotransmitter function.

Cholesterol Allosterically Modulates the Structure and Dynamics of the Taurocholate Export Pump (ABCB11)

The BSEP/ABCB11 transmembrane protein translocates taurine- and glycine-conjugated bile salts across the hepatocyte bilayer driven by ATP-hydrolysis. Direct inhibition of BSEP/ABCB11 leads to idiosyncratic drug-induced liver injury. ABCB11 is localized within the cholesterol-enriched lipid raft, and membrane cholesterol depletion leads to impaired taurocholate transport. However, structural insight into the mechanism of the cholesterol-mediated regulation of ABCB11 activity remains elusive. We used extensive molecular dynamics simulation coupled with well-tempered metadynamics to elucidate the role of membrane cholesterol in the structure and dynamics of ABCB11. We identified specific high-residence binding sites for cholesterol within the transmembrane domain. The free-energy simulations have elucidated that the bound cholesterol stabilizes the "inward-open" conformation of the protein. Cholesterol-ABCB11 interactions trigger allosteric communications between the transmembrane and nucleotide-binding domains through the linker region. Cholesterol depletion destabilizes the allosteric network of the protein. As a result, it adopts a more collapsed form with a reduced volume of the taurocholate-binding pocket.

Structure of the human dopamine transporter in complex with cocaine

The dopamine transporter (DAT) is crucial for regulating dopamine signalling and is the prime mediator for the rewarding and addictive effects of cocaine1. As part of the neurotransmitter sodium symporter family, DAT uses the Na+ gradient across cell membranes to transport dopamine against its chemical gradient2. The transport mechanism involves both intra- and extracellular gates that control substrate access to a central site. However, the molecular intricacies of this process and the inhibitory mechanism of cocaine have remained unclear. Here, we present the molecular structure of human DAT in complex with cocaine at a resolution of 2.66 Å. Our findings reveal that DAT adopts the expected LeuT-fold, posing in an outward-open conformation with cocaine bound at the central (S1) site. Notably, while an Na+ occupies the second Na+ site (Na2), the Na1 site seems to be vacant, with the side chain of Asn82 occupying the presumed Na+ space. This structural insight elucidates the mechanism for the cocaine inhibition of human DAT and deepens our understanding of neurotransmitter transport. By shedding light on the molecular underpinnings of how cocaine acts, our study lays a foundation for the development of targeted medications to combat addiction.

Structure of the human dopamine transporter and mechanisms of inhibition

The neurotransmitter dopamine has central roles in mood, appetite, arousal and movement1. Despite its importance in brain physiology and function, and as a target for illicit and therapeutic drugs, the human dopamine transporter (hDAT) and mechanisms by which it is inhibited by small molecules and Zn2+ are without a high-resolution structural context. Here we determine the structure of hDAT in a tripartite complex with the competitive inhibitor and cocaine analogue, (-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane2 (β-CFT), the non-competitive inhibitor MRS72923 and Zn2+ (ref. 4). We show how β-CFT occupies the central site, approximately halfway across the membrane, stabilizing the transporter in an outward-open conformation. MRS7292 binds to a structurally uncharacterized allosteric site, adjacent to the extracellular vestibule, sequestered underneath the extracellular loop 4 (EL4) and adjacent to transmembrane helix 1b (TM1b), acting as a wedge, precluding movement of TM1b and closure of the extracellular gate. A Zn2+ ion further stabilizes the outward-facing conformation by coupling EL4 to EL2, TM7 and TM8, thus providing specific insights into how Zn2+ restrains the movement of EL4 relative to EL2 and inhibits transport activity.

Transport and inhibition mechanisms of the human noradrenaline transporter

The noradrenaline transporter (also known as norepinephrine transporter) (NET) has a critical role in terminating noradrenergic transmission by utilizing sodium and chloride gradients to drive the reuptake of noradrenaline (also known as norepinephrine) into presynaptic neurons1-3. It is a pharmacological target for various antidepressants and analgesic drugs4,5. Despite decades of research, its structure and the molecular mechanisms underpinning noradrenaline transport, coupling to ion gradients and non-competitive inhibition remain unknown. Here we present high-resolution complex structures of NET in two fundamental conformations: in the apo state, and bound to the substrate noradrenaline, an analogue of the χ-conotoxin MrlA (χ-MrlAEM), bupropion or ziprasidone. The noradrenaline-bound structure clearly demonstrates the binding modes of noradrenaline. The coordination of Na+ and Cl- undergoes notable alterations during conformational changes. Analysis of the structure of NET bound to χ-MrlAEM provides insight into how conotoxin binds allosterically and inhibits NET. Additionally, bupropion and ziprasidone stabilize NET in its inward-facing state, but they have distinct binding pockets. These structures define the mechanisms governing neurotransmitter transport and non-competitive inhibition in NET, providing a blueprint for future drug design.

Assessing the Martini 3 protein model: A review of its path and potential

Coarse-grained (CG) protein models have become indispensable tools for studying many biological protein details, from conformational dynamics to the organization of protein macro-complexes, and even the interaction of proteins with other molecules. The Martini force field is one of the most widely used CG models for bio-molecular simulations, partly because of the enormous success of its protein model. With the recent release of a new and improved version of the Martini force field - Martini 3 - a new iteration of its protein model was also made available. The Martini 3 protein force field is an evolution of its Martini 2 counterpart, aimed at improving many of the shortcomings that had been previously identified. In this mini-review, we first provide a general overview of the model and then focus on the successful advances made in the short time since its release, many of which would not have been possible before. Furthermore, we discuss reported limitations, potential directions for model improvement and comment on what the likely future development and application avenues are.

Dimerization and antidepressant recognition at noradrenaline transporter

The noradrenaline transporter has a pivotal role in regulating neurotransmitter balance and is crucial for normal physiology and neurobiology1. Dysfunction of noradrenaline transporter has been implicated in numerous neuropsychiatric diseases, including depression and attention deficit hyperactivity disorder2. Here we report cryo-electron microscopy structures of noradrenaline transporter in apo and substrate-bound forms, and as complexes with six antidepressants. The structures reveal a noradrenaline transporter dimer interface that is mediated predominantly by cholesterol and lipid molecules. The substrate noradrenaline binds deep in the central binding pocket, and its amine group interacts with a conserved aspartate residue. Our structures also provide insight into antidepressant recognition and monoamine transporter selectivity. Together, these findings advance our understanding of noradrenaline transporter regulation and inhibition, and provide templates for designing improved antidepressants to treat neuropsychiatric disorders.

Molecular mechanisms of Na+-driven bile acid transport in human NTCP

Human Na+ taurocholate co-transporting protein (hNTCP) is a key bile salt transporter to maintain enterohepatic circulation and is responsible for the recognition of hepatitis B and D viruses. Despite landmark cryoelectron microscopy studies revealing open-pore and inward-facing states of hNTCP stabilized by antibodies, the transport mechanism remains largely unknown. To address this knowledge gap, we used molecular dynamics and enhanced sampling metadynamics simulations to elucidate the intrinsic mechanism of hNTCP-mediated taurocholate acid (TCA) transport driven by Na+ binding. We uncovered three TCA-binding modes, including one that closely matched the limited cryoelectron microscopy density observed in the open-pore hNTCP. We also captured several key hNTCP conformations in the substrate transport cycle, particularly including an outward-facing, substrate-bound state. Furthermore, we provided thermodynamic evidence supporting that changes in the Na+-binding state drive the TCA transport by exploiting the amphiphilic nature of the substrate and modulating the protein environment, thereby enabling the TCA molecule to flip through. Understanding these mechanistic details of Na+-driven bile acid transport may aid in the development of hNTCP-targeted therapies for liver diseases.

Comprehensive Characterization of LAT1 Cholesterol-Binding Sites

The human L-type amino acid transporter 1 (LAT1; SLC7A5), is an amino acid exchanger protein, primarily found in the blood-brain barrier, placenta, and testis, where it plays a key role in amino acid homeostasis. Cholesterol is an essential lipid that has been highlighted to play a role in regulating the activity of membrane transporters, such as LAT1, yet little is known about the molecular mechanisms driving this phenomenon. Here we perform a comprehensive computational analysis to investigate cholesterol's role in LAT1 structure and function, focusing on four cholesterol-binding sites (CHOL1-4) identified in a recent LAT1-apo inward-open conformation cryo-EM structure. Through a series of independent molecular dynamics (MD) simulations, molecular docking, MM/GBSA free energy calculations, and other analysis tools, we explored the interactions between LAT1 and cholesterol. Our findings suggest that CHOL3 forms the most stable and favorable interactions with LAT1. Principal component analysis (PCA) and center of mass (COM) distance assessments show that CHOL3 binding stabilizes the inward-open state of LAT1 by preserving the spatial arrangement of the hash and bundle domains. Additionally, we propose an alternative cholesterol-binding site for originally assigned CHOL1. Overall, this study improves the understanding of cholesterol's modulatory effect on LAT1 and proposes candidate sites for the discovery of future allosteric ligands with rational design.

Future opportunities in solute carrier structural biology

Solute carriers (SLCs) control the flow of small molecules and ions across biological membranes. Over the last 20 years, the pace of research in SLC biology has accelerated markedly, opening new opportunities to treat metabolic diseases, cancer and neurological disorders. Recently, new families of atypical SLCs, with roles in organelle biology, metabolite signaling and trafficking, have expanded their roles in the cell. This Perspective discusses work leading to current advances and the emerging opportunities to target and modulate SLCs to uncover new biology and treat human disease.

Computational Chemistry in Structure-Based Solute Carrier Transporter Drug Design: Recent Advances and Future Perspectives

Solute carrier transporters (SLCs) are a class of important transmembrane proteins that are involved in the transportation of diverse solute ions and small molecules into cells. There are approximately 450 SLCs within the human body, and more than a quarter of them are emerging as attractive therapeutic targets for multiple complex diseases, e.g., depression, cancer, and diabetes. However, only 44 unique transporters (∼9.8% of the SLC superfamily) with 3D structures and specific binding sites have been reported. To design innovative and effective drugs targeting diverse SLCs, there are a number of obstacles that need to be overcome. However, computational chemistry, including physics-based molecular modeling and machine learning- and deep learning-based artificial intelligence (AI), provides an alternative and complementary way to the classical drug discovery approach. Here, we present a comprehensive overview on recent advances and existing challenges of the computational techniques in structure-based drug design of SLCs from three main aspects: (i) characterizing multiple conformations of the proteins during the functional process of transportation, (ii) identifying druggability sites especially the cryptic allosteric ones on the transporters for substrates and drugs binding, and (iii) discovering diverse small molecules or synthetic protein binders targeting the binding sites. This work is expected to provide guidelines for a deep understanding of the structure and function of the SLC superfamily to facilitate rational design of novel modulators of the transporters with the aid of state-of-the-art computational chemistry technologies including artificial intelligence.

Post-translational mechanisms in psychostimulant-induced neurotransmitter efflux

The availability of monoamine neurotransmitters in the brain is under the control of dopamine, norepinephrine, and serotonin transporters expressed on the plasma membrane of monoaminergic neurons. By regulating transmitter levels these proteins mediate crucial functions including cognition, attention, and reward, and dysregulation of their activity is linked to mood and psychiatric disorders of these systems. Amphetamine-based transporter substrates stimulate non-exocytotic transmitter efflux that induces psychomotor stimulation, addiction, altered mood, hallucinations, and psychosis, thus constituting a major component of drug neurochemical and behavioral outcomes. Efflux is under the control of transporter post-translational modifications that synergize with other regulatory events, and this review will summarize our knowledge of these processes and their role in drug mechanisms.

Cholesterol modulation of interactions between psychostimulants and dopamine transporters

The dopamine transporter (DAT) is a key site of action for cocaine and amphetamines. Dysfunctional DAT is associated with aberrant synaptic dopamine transmission and enhanced drug-seeking and taking behavior. Studies in cultured cells and ex vivo suggest that DAT function is sensitive to membrane cholesterol content. Although it is largely unknown whether psychostimulants alter cholesterol metabolism in the brain, emerging evidence indicates that peripheral cholesterol metabolism is altered in patients with psychostimulant use disorder and circulating cholesterol levels are associated with vulnerability to relapse. Cholesterol interacts with sphingolipids forming lipid raft microdomains on the membrane. These cholesterol-rich lipid raft microdomains serve to recruit and assemble other lipids and proteins to initiate signal transduction. There are two spatially and functionally distinct populations of the DAT segregated by cholesterol-rich lipid raft microdomains and cholesterol-scarce non-raft microdomains on the plasma membrane. These two DAT populations are differentially regulated by DAT blockers (e.g. cocaine), substrates (e.g. amphetamine), and protein kinase C providing distinct cholesterol-dependent modulation of dopamine uptake and efflux. In this chapter, we summarize the impact of depletion and addition of membrane cholesterol on DAT conformational changes between the outward-facing and the inward-facing states, lipid raft-associated DAT localization, basal and induced DAT internalization, and DAT function. In particular, we focus on how the interactions of the DAT with cocaine and amphetamine are influenced by membrane cholesterol. Lastly, we discuss the therapeutic potential of cholesterol-modifying drugs as a new avenue to normalize DAT function and dopamine transmission in patients with psychostimulant use disorder.

Dopamine transporter membrane mobility is bidirectionally regulated by phosphorylation and palmitoylation

The primary regulator of dopamine availability in the brain is the dopamine transporter (DAT), a plasma membrane protein that drives reuptake of released dopamine from the extracellular space into the presynaptic neuron. DAT activity is regulated by post-translational modifications that establish clearance capacity through impacts on transport kinetics, and dysregulation of these events may underlie dopaminergic imbalances in mood and psychiatric disorders. Here, using fluorescence recovery after photobleaching, we show that phosphorylation and palmitoylation induce opposing effects on DAT lateral membrane mobility, which may influence functional outcomes by regulating subcellular localization and binding partner interactions. Membrane mobility was also impacted by amphetamine and in polymorphic variant A559V in directions consistent with enhanced phosphorylation. These findings grow the list of DAT properties controlled by these post-translational modifications and highlight their role in establishment of dopaminergic tone in physiological and pathophysiological states.

Targeting SLC transporters: small molecules as modulators and therapeutic opportunities

Solute carrier (SLCs) transporters mediate the transport of a broad range of solutes across biological membranes. Dysregulation of SLCs has been associated with various pathologies, including metabolic and neurological disorders, as well as cancer and rare diseases. SLCs are therefore emerging as key targets for therapeutic intervention with several recently approved drugs targeting these proteins. Unlocking this large and complex group of proteins is essential to identifying unknown SLC targets and developing next-generation SLC therapeutics. Recent progress in experimental and computational techniques has significantly advanced SLC research, including drug discovery. Here, we review emerging topics in therapeutic discovery of SLCs, focusing on state-of-the-art approaches in structural, chemical, and computational biology, and discuss current challenges in transporter drug discovery.

Membrane cholesterol regulates inhibition and substrate transport by the glycine transporter, GlyT2

Membrane cholesterol binds to and modulates the function of various SLC6 neurotransmitter transporters, including stabilizing the outward-facing conformation of the dopamine and serotonin transporters. Here, we investigate how cholesterol binds to GlyT2 (SLC6A5), modulates glycine transport rate, and influences bioactive lipid inhibition of GlyT2. Bioactive lipid inhibitors are analgesics that bind to an allosteric site accessible from the extracellular solution when GlyT2 adopts an outward-facing conformation. Using molecular dynamics simulations, mutagenesis, and cholesterol depletion experiments, we show that bioactive lipid inhibition of glycine transport is modulated by the recruitment of membrane cholesterol to a binding site formed by transmembrane helices 1, 5, and 7. Recruitment involves cholesterol flipping from its membrane orientation, and insertion of the 3' hydroxyl group into the cholesterol binding cavity, close to the allosteric site. The synergy between cholesterol and allosteric inhibitors provides a novel mechanism of inhibition and a potential avenue for the development of potent GlyT2 inhibitors as alternative therapeutics for the treatment of neuropathic pain and therapeutics that target other SLC6 transporters.

Atorvastatin differentially regulates the interactions of cocaine and amphetamine with dopamine transporters

The function of the dopamine transporter (DAT) is regulated by membrane cholesterol content. A direct, acute removal of membrane cholesterol by methyl-β-cyclodextrin (MβCD) has been shown to reduce dopamine (DA) uptake and release mediated by the DAT. This is of particular interest because a few widely prescribed statins that lower peripheral cholesterol levels are blood-brain barrier (BBB) penetrants, and therefore could alter DAT function through brain cholesterol modulation. The goal of this study was to investigate the effects of prolonged atorvastatin treatment (24 h) on DAT function in neuroblastoma 2A cells stably expressing DAT. We found that atorvastatin treatment effectively lowered membrane cholesterol content in a concentration-dependent manner. Moreover, atorvastatin treatment markedly reduced DA uptake and abolished cocaine inhibition of DA uptake, independent of surface DAT levels. These deficits induced by atorvastatin treatment were reversed by cholesterol replenishment. However, atorvastatin treatment did not change amphetamine (AMPH)-induced DA efflux. This is in contrast to a small but significant reduction in DA efflux induced by acute depletion of membrane cholesterol using MβCD. This discrepancy may involve differential changes in membrane lipid composition resulting from chronic and acute cholesterol depletion. Our data suggest that the outward-facing conformation of DAT, which favors the binding of DAT blockers such as cocaine, is more sensitive to atorvastatin-induced cholesterol depletion than the inward-facing conformation, which favors the binding of DAT substrates such as AMPH. Our study on statin-DAT interactions may have clinical implications in our understanding of neurological side effects associated with chronic use of BBB penetrant statins.

Association of cholesterol level with dopamine loss and motor deficits in Parkinson disease: A cross-sectional study

BACKGROUND AND PURPOSE

Cholesterol is vital in neuronal function; however, the influence of cholesterol levels on parkinsonism is unclear. This study investigated the relationship between baseline total cholesterol (TC) levels, dopamine loss, and motor symptoms in drug-naïve Parkinson disease (PD).

METHODS

This cross-sectional study enrolled 447 drug-naïve patients with PD who underwent dopamine transporter (DAT) imaging. Multivariate linear regression was used to investigate the effect of cholesterol levels on Unified Parkinson's Disease Rating Scale Part III (UPDRS-III) total score and each subscore after adjusting for the covariates. An interaction analysis was performed to examine the interaction between TC levels and statin use on the UPDRS-III scores.

RESULTS

No significant correlation was found between TC levels and DAT availability after adjusting for potential confounders. Multivariate linear regression showed that TC levels were significantly and negatively associated with the UPDRS-III total score (β = -0.116, p = 0.013) and bradykinesia subscore (β = -0.145, p = 0.011). Dichotomized analysis according to TC levels showed that TC levels were significantly associated with UPDRS-III total score, and rigidity, bradykinesia, and axial subscores only in the low TC group. There was an interaction effect between TC levels and statin use for the axial subscores (β = -0.523, p = 0.025). Subgroup analysis showed that TC levels were significantly and negatively associated with the axial subscore in statin users; however, no association was found in statin nonusers.

CONCLUSIONS

This study suggests that TC levels affect parkinsonian motor symptoms, especially in subjects with low cholesterol status, whereas the severity of axial motor symptoms is negatively associated with TC levels only in statin users.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Structural and functional investigation of ABC transporter STE6-2p from Pichia pastoris reveals unexpected interaction with sterol molecules

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are multidomain transmembrane proteins, which facilitate the transport of various substances across cell membranes using energy derived from ATP hydrolysis. They are important drug targets since they mediate decreased drug susceptibility during pharmacological treatments. For the methylotrophic yeast Pichia pastoris, a model organism that is a widely used host for protein expression, the role and function of its ABC transporters is unexplored. In this work, we investigated the Pichia ABC-B transporter STE6-2p. Functional investigations revealed that STE6-2p is capable of transporting rhodamines in vivo and is active in the presence of verapamil and triazoles in vitro. A phylogenetic analysis displays homology among multidrug resistance (MDR) transporters from pathogenic fungi to human ABC-B transporters. Further, we present high-resolution single-particle electron cryomicroscopy structures of an ABC transporter from P. pastoris in the apo conformation (3.1 Å) and in complex with verapamil and adenylyl imidodiphosphate (AMP-PNP) (3.2 Å). An unknown density between transmembrane helices 4, 5, and 6 in both structures suggests the presence of a sterol-binding site of unknown function.

Higher Total Cholesterol Concentration May Be Associated with Better Cognitive Performance among Elderly Females

BACKGROUND

The brain contains the highest level of cholesterol in the body, and the total amount of serum cholesterol in the blood has a huge impact on brain aging and cognitive performance. However, the association of total serum cholesterol with cognitive function remains uncertain. This study determines whether there is an association between the total amount of cholesterol in the blood and cognitive performance in elderly females without a history of stroke.

METHODS

This population-based cross-sectional study was conducted on elderly (over 60 years old) females and males without a history of stroke from 2011 to 2014 in the US National Health and Nutrition Examination Survey (NHANES). The primary exposure was total blood cholesterol, and the main outcome was cognitive performance; this association was assessed with logistic regression analysis and restricted cubic splines.

RESULTS

1309 female and 1272 male participants were included. In females, higher total cholesterol was significantly associated with higher cognitive scores, particularly in the digit symbol substitution test (OR 0.51, 95% CI (0.36-0.72)) and the animal fluency test (OR 0.64, 95% CI (0.45-0.91)). This association remained significant in models adjusted for age, race, smoking status, education level, and chronic conditions (OR 0.40, 95% CI (0.25-0.63)). This association was not significant in males, however.

CONCLUSIONS

A higher concentration of total cholesterol measured in later life may be a protective factor for cognitive performance among females over 60 years old without a history of stroke. Further, this association was more pronounced among women with higher levels of education than women with lower or no education.

Structural Rearrangement of the Serotonin Transporter Intracellular Gate Induced by Thr276 Phosphorylation

The reuptake of the neurotransmitter serotonin from the synaptic cleft by the serotonin transporter, SERT, is essential for proper neurological signaling. Biochemical studies have shown that Thr276 of transmembrane helix 5 is a site of PKG-mediated SERT phosphorylation, which has been proposed to shift the SERT conformational equilibria to promote inward-facing states, thus enhancing 5-HT transport. Recent structural and simulation studies have provided insights into the conformation transitions during substrate transport but have not shed light on SERT regulation via post-translational modifications. Using molecular dynamics simulations and Markov state models, we investigate how Thr276 phosphorylation impacts the SERT mechanism and its role in enhancing transporter stability and function. Our simulations show that Thr276 phosphorylation alters the hydrogen-bonding network involving residues on transmembrane helix 5. This in turn decreases the free energy barriers for SERT to transition to the inward-facing state, thus facilitating 5-HT import. The results provide atomistic insights into in vivo SERT regulation and can be extended to other pharmacologically important transporters in the solute carrier family.

The substrate import mechanism of the human serotonin transporter

The serotonin transporter (SERT) initiates the reuptake of extracellular serotonin in the synapse to terminate neurotransmission. The cryogenic electron microscopy structures of SERT bound to ibogaine and the physiological substrate serotonin resolved in different states have provided a glimpse of the functional conformations at atomistic resolution. However, the conformational dynamics and structural transitions to intermediate states are not fully understood. Furthermore, the molecular basis of how serotonin is recognized and transported remains unclear. In this study, we performed unbiased microsecond-long simulations of the human SERT to investigate the structural dynamics to various intermediate states and elucidated the complete substrate import pathway. Using Markov state models, we characterized a sequential order of conformational-driven ion-coupled substrate binding and transport events and calculated the free energy barriers of conformation transitions associated with the import mechanism. We find that the transition from the occluded to inward-facing state is the rate-limiting step for substrate import and that the substrate decreases the free energy barriers to achieve the inward-facing state. Our study provides insights on the molecular basis of dynamics-driven ion-substrate recognition and transport of SERT that can serve as a model for other closely related neurotransmitter transporters.

Bile Acids Gate Dopamine Transporter Mediated Currents

Bile acids (BAs) are molecules derived from cholesterol that are involved in dietary fat absorption. New evidence supports an additional role for BAs as regulators of brain function. Sterols such as cholesterol interact with monoamine transporters, including the dopamine (DA) transporter (DAT) which plays a key role in DA neurotransmission and reward. This study explores the interactions of the BA, obeticholic acid (OCA), with DAT and characterizes the regulation of DAT activity via both electrophysiology and molecular modeling. We expressed murine DAT (mDAT) in Xenopus laevis oocytes and confirmed its functionality. Next, we showed that OCA promotes a DAT-mediated inward current that is Na+-dependent and not regulated by intracellular calcium. The current induced by OCA was transient in nature, returning to baseline in the continued presence of the BA. OCA also transiently blocked the DAT-mediated Li+-leak current, a feature that parallels DA action and indicates direct binding to the transporter in the absence of Na+. Interestingly, OCA did not alter DA affinity nor the ability of DA to promote a DAT-mediated inward current, suggesting that the interaction of OCA with the transporter is non-competitive, regarding DA. Docking simulations performed for investigating the molecular mechanism of OCA action on DAT activity revealed two potential binding sites. First, in the absence of DA, OCA binds DAT through interactions with D421, a residue normally involved in coordinating the binding of the Na+ ion to the Na2 binding site (Borre et al., J. Biol. Chem., 2014, 289, 25764-25773; Cheng and Bahar, Structure, 2015, 23, 2171-2181). Furthermore, we uncover a separate binding site for OCA on DAT, of equal potential functional impact, that is coordinated by the DAT residues R445 and D436. Binding to that site may stabilize the inward-facing (IF) open state by preventing the re-formation of the IF-gating salt bridges, R60-D436 and R445-E428, that are required for DA transport. This study suggests that BAs may represent novel pharmacological tools to regulate DAT function, and possibly, associated behaviors.

Ultrasensitive electrochemical biosensors for dopamine and cholesterol: recent advances, challenges and strategies

The rapid and accurate determination of the dopamine (neurotransmitter) and cholesterol level in bio-fluids is significant because they are crucial bioanalytes for several lethal diseases, which require early diagnosis. The level of DA in the brain is modulated by the dopamine active transporter (DAT), and is influenced by cholesterol levels in the lipid membrane environment. Accordingly, electrochemical biosensors offer rapid and accurate detection and exhibit unique features such as low detection limits even with reduced volumes of analyte, affordability, simple handling, portability and versatility, making them appropriate to deal with augmented challenges in current clinical and point-of-care diagnostics for the determination of dopamine (DA) and cholesterol. This feature article focuses on the development of ultrasensitive electrochemical biosensors for the detection of cholesterol and DA for real-time and onsite applications that can detect targeted analytes with reduced volumes and sub-picomolar concentrations with quick response times. Furthermore, the development of ultrasensitive biosensors via cost-effective, simple fabrication procedures, displaying high sensitivity, selectivity, reliability and good stability is significant in the impending era of electrochemical biosensing. Herein, we emphasize on recent advanced nanomaterials used for the ultrasensitive detection of DA and cholesterol and discuss in depth their electrochemical activities towards ultrasensitive responses. Key points describing future perspectives and the challenges during detection with their probable solutions are discussed, and the current market is also surveyed. Further, a comprehensive review of the literature indicates that there is room for improvement in the miniaturization of cholesterol and dopamine biosensors for lab-on-chip devices and overcoming the current technical limitations to facilitate full utilization by patients at home.

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.

Glycine Transporter 2: Mechanism and Allosteric Modulation

Neurotransmitter sodium symporters (NSS) are a subfamily of SLC6 transporters responsible for regulating neurotransmitter signalling. They are a major target for psychoactive substances including antidepressants and drugs of abuse, prompting substantial research into their modulation and structure-function dynamics. Recently, a series of allosteric transport inhibitors have been identified, which may reduce side effect profiles, compared to orthosteric inhibitors. Allosteric inhibitors are also likely to provide different clearance kinetics compared to competitive inhibitors and potentially better clinical outcomes. Crystal structures and homology models have identified several allosteric modulatory sites on NSS including the vestibule allosteric site (VAS), lipid allosteric site (LAS) and cholesterol binding site (CHOL1). Whilst the architecture of eukaryotic NSS is generally well conserved there are differences in regions that form the VAS, LAS, and CHOL1. Here, we describe ligand-protein interactions that stabilize binding in each allosteric site and explore how differences between transporters could be exploited to generate NSS specific compounds with an emphasis on GlyT2 modulation.

New insights on the mechanism of polyethylenimine transfection and their implications on gene therapy and DNA vaccines

Polyethylenimine (PEI) has been demonstrated as an efficient DNA delivery vehicle both in vitro and in vivo. There is a consensus that PEI-DNA complexes enter the cells by endocytosis and escape from endosomes by the so-called "proton sponge" effect. However, little is known on how and where the polyplexes are de-complexed for DNA transcription and replication to occur inside the cell nucleus. To better understand this issue, we (i) tracked the cell internalization of PEI upon transfection to human epithelial cells and (ii) studied the interaction of PEI with phospholipidic layers mimicking nuclear membranes. Both the biological and physicochemical experiments provided evidence of a strong binding affinity between PEI and the lipidic bilayer. Firstly, confocal microscopy revealed that PEI alone could not penetrate the cell nucleus; instead, it arranged throughout the cytoplasm and formed a sort of aureole surrounding the nuclei periphery. Secondly, surface tension measurements, fluorescence dye leakage assays, and differential scanning calorimetry demonstrated that a combination of hydrophobic and electrostatic interactions between PEI and the phospholipidic monolayers/bilayers led to the formation of stable defects along the model membranes, allowing the intercalation of PEI through the monolayer/bilayer structure. Results are also supported by molecular dynamics simulation of the pore formation in PEI-lipidic bilayers. As discussed throughout the text, these results might shed light on a the mechanism in which the interaction between PEI and the nucleus membrane might play an active role on the DNA release: on the one hand, the PEI-membrane interaction is anticipated to facilitate the DNA disassembly from the polyplex by establishing a competition with DNA for the PEI binding and on the other hand, the forming defects are expected to serve as channels for the entrance of de-complexed DNA into the cell nucleus. A better understanding of the mechanism of transfection of cationic polymers opens paths to development of more efficiency vectors to improve gene therapy treatment and the new generation of DNA vaccines.

Interaction of Dopamine with Zwitterionic DMPC and Anionic DMPS Multilamellar Vesicle Membranes

Dopamine (DA), a naturally occurring neurotransmitter, plays a crucial role in the function of the mammalian nervous system. DA-lipid-membrane interaction is inevitable during the neurotransmission process. In this report, we have studied the interaction of DA with anionic 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS), neutral (zwitterionic) 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and synaptic membrane-mimicking mixed DMPC/DMPS (3:1 molar ratio) model multilamellar vesicle (MLV) membranes. Differential scanning calorimetry (DSC) studies suggest a strong specific interaction of DA with the anionic DMPS membrane, a weak interaction with the zwitterionic DMPC membrane, and a moderate interaction with the mixed DMPC/DMPS (3:1) membrane. The intrinsic fluorescence of DA was used as a new approach to gain a molecular-level understanding of DA-lipid-membrane interaction. Toward this end, a detailed photophysical study of DA, including its steady-state fluorescence anisotropy and fluorescence lifetime, was undertaken for the first time. The partition coefficient, location, and distribution of DA in the DMPS and DMPC model membranes were studied by employing intrinsic fluorescence. The effect of DA on the phase transition of the model membranes was also examined using the intrinsic fluorescence of DA. Zeta potential studies suggest a strong electrostatic interaction of DA with the anionic DMPS membrane and a nonspecific, relatively weak interaction of DA with the zwitterionic DMPC membrane. In addition, we observed cholesterol-induced DA expulsion from both DMPS and DMPC membranes. We believe that this work will provide a more in-depth understanding of DA-membrane interaction at a molecular level.

The Role of the Membrane in Transporter Folding and Activity

The synthesis, folding, and function of membrane transport proteins are critical factors for defining cellular physiology. Since the stability of these proteins evolved amidst the lipid bilayer, it is no surprise that we are finding that many of these membrane proteins demonstrate coupling of their structure or activity in some way to the membrane. More and more transporter structures are being determined with some information about the surrounding membrane, and computational modeling is providing further molecular details about these solvation structures. Thus, the field is moving towards identifying which molecular mechanisms - lipid interactions, membrane perturbations, differential solvation, and bulk membrane effects - are involved in linking membrane energetics to transporter stability and function. In this review, we present an overview of these mechanisms and the growing evidence that the lipid bilayer is a major determinant of the fold, form, and function of membrane transport proteins in membranes.

Novel therapeutic approaches for Parkinson's disease by targeting brain cholesterol homeostasis

OBJECTIVES

Human brain is composed of 25% of the cholesterol & any dysfunction in brain cholesterol homeostasis contributes to neurodegenerative disorders such as Parkinson, Alzheimer's, Huntington's disease, etc. A growing literature indicates that alteration in neurotransmission & brain cholesterol metabolism takes place in the early stage of the disease. The current paper summarizes the role of cholesterol & its homeostasis in the pathophysiology of Parkinson's disease.

KEY FINDINGS

Literature findings suggest the possible role of lipids such as oxysterols, lipoproteins, etc. in Parkinson's disease pathophysiology. Cholesterol performs a diverse role in the brain but any deviation in its levels leads to neurodegeneration. Dysregulation of lipid caused by oxidative stress & inflammation leads to α-synuclein trafficking which contributes to Parkinson's disease progression. Also, α-synuclein by binding to membrane lipid forms lipid-protein complex & results in its aggregation. Different targets such as Phospholipase A2, Stearoyl-CoA desaturase enzyme, proprotein convertase subtilisin/kexin type 9, etc. have been identified as a potential novel approach for Parkinson's disease treatment.

SUMMARY

In the current review, we have discussed the possible molecular role of cholesterol homeostasis in Parkinson's disease progression. We also identified potential therapeutic targets that need to be evaluated clinically for the development of Parkinson's treatment.

The Effects of Statins on Neurotransmission and Their Neuroprotective Role in Neurological and Psychiatric Disorders

Statins are among the most widely used drug classes in the world. Apart from their basic mechanism of action, which is lowering cholesterol levels, many pleiotropic effects have been described so far, such as anti-inflammatory and antiatherosclerotic effects. A growing number of scientific reports have proven that these drugs have a beneficial effect on the functioning of the nervous system. The first reports proving that lipid-lowering therapy can influence the development of neurological and psychiatric diseases appeared in the 1990s. Despite numerous studies about the mechanisms by which statins may affect the functioning of the central nervous system (CNS), there are still no clear data explaining this effect. Most studies have focused on the metabolic effects of this group of drugs, however authors have also described the pleiotropic effects of statins, pointing to their probable impact on the neurotransmitter system and neuroprotective effects. The aim of this paper was to review the literature describing the impacts of statins on dopamine, serotonin, acetylcholine, and glutamate neurotransmission, as well as their neuroprotective role. This paper focuses on the mechanisms by which statins affect neurotransmission, as well as on their impacts on neurological and psychiatric diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), vascular dementia (VD), stroke, and depression. The pleiotropic effects of statin usage could potentially open floodgates for research in these treatment domains, catching the attention of researchers and clinicians across the globe.

FRET detects lateral interaction between transmembrane domain of EGF receptor and ganglioside GM3 in lipid bilayers

Ganglioside GM3 in the plasma membranes suppresses cell growth by preventing the autophosphorylation of the epidermal growth factor receptor (EGFR). Biological studies have suggested that GM3 interacts with the transmembrane segment of EGFR. Further biophysical experiments are particularly important for quantitative evaluation of the peptide-glycolipid interplay in bilayer membranes using a simple reconstituted system. To examine these interactions in this way, we synthesized the transmembrane segment of EGFR bearing a nitrobenzoxadiazole fluorophore (NBD-TM) at the N-terminus. The affinity between EGFR and GM3 was evaluated based on Förster resonance energy transfer (FRET) between NBD-TM and ATTO594-labeled GM3 in bilayers where their non-specific interaction due to lateral proximity was subtracted by using NBD-labeled phospholipid. This method for selectively detecting the specific lipid-peptide interactions in model lipid bilayers disclosed that the lateral interaction between GM3 and the transmembrane segment of EGFR plays a certain role in disturbing the formation of active EGFR dimers.

The Effect of Cholesterol on Membrane-Bound Islet Amyloid Polypeptide

Islet amyloid polypeptide (IAPP) is a proposed cause of the decreased beta-cell mass in patients with type-II diabetes. The molecular composition of the cell-membrane is important for regulating IAPP cytotoxicity and aggregation. Cholesterol is present at high concentrations in the pancreatic beta-cells, and in-vitro experiments have indicated that it affects the amyloid formation of IAPP either by direct interactions or by changing the properties of the membrane. In this study we apply atomistic, unbiased molecular dynamics simulations at a microsecond timescale to investigate the effect of cholesterol on membrane bound IAPP. Simulations were performed with various combinations of cholesterol, phosphatidylcholine (PC) and phosphatidylserine (PS) lipids. In all simulations, the helical structure of monomer IAPP was stabilized by the membrane. We found that cholesterol decreased the insertion depth of IAPP compared to pure phospholipid membranes, while PS lipids counteract the effect of cholesterol. The aggregation propensity has previously been proposed to correlate with the insertion depth of IAPP, which we found to decrease with the increased ordering of the lipids induced by cholesterol. Cholesterol is depleted in the vicinity of IAPP, and thus our results suggest that the effect of cholesterol is indirect.

Effect of palmitoylation on the dimer formation of the human dopamine transporter

The human dopamine transporter (hDAT) is one in three members of the monoamine transporter family (MAT). hDAT is essential for regulating the dopamine concentration in the synaptic cleft through dopamine reuptake into the presynaptic neuron; thereby controlling hDAT dopamine signaling. Dysfunction of the transporter is linked to several psychiatric disorders. hDAT and the other MATs have been shown to form oligomers in the plasma membrane, but only limited data exists on which dimeric and higher order oligomeric states are accessible and energetically favorable. In this work, we present several probable dimer conformations using computational coarse-grained self-assembly simulations and assess the relative stability of the different dimer conformations using umbrella sampling replica exchange molecular dynamics. Overall, the dimer conformations primarily involve TM9 and/or TM11 and/or TM12 at the interface. Furthermore, we show that a palmitoyl group (palm) attached to hDAT on TM12 modifies the free energy of separation for interfaces involving TM12, suggesting that S-palmitoylation may change the relative abundance of dimers involving TM12 in a biological context. Finally, a comparison of the identified interfaces of hDAT and palmitoylated hDAT to the human serotonin transporter interfaces and the leucine transporter interface, suggests similar dimer conformations across these protein family.

The allosteric inhibition of glycine transporter 2 by bioactive lipid analgesics is controlled by penetration into a deep lipid cavity

The role of lipids in modulating membrane protein function is an emerging and rapidly growing area of research. The rational design of lipids that target membrane proteins for the treatment of pathological conditions is a novel extension in this field and provides a step forward in our understanding of membrane transporters. Bioactive lipids show considerable promise as analgesics for the treatment of chronic pain and bind to a high-affinity allosteric-binding site on the human glycine transporter 2 (GlyT2 or SLC6A5). Here, we use a combination of medicinal chemistry, electrophysiology, and computational modeling to develop a rational structure-activity relationship for lipid inhibitors and demonstrate the key role of the lipid tail interactions for GlyT2 inhibition. Specifically, we examine how lipid inhibitor head group stereochemistry, tail length, and double-bond position promote enhanced inhibition. Overall, the l-stereoisomer is generally a better inhibitor than the d-stereoisomer, longer tail length correlates with greater potency, and the position of the double bond influences the activity of the inhibitor. We propose that the binding of the lipid inhibitor deep into the allosteric-binding pocket is critical for inhibition. Furthermore, this provides insight into the mechanism of inhibition of GlyT2 and highlights how lipids can modulate the activity of membrane proteins by binding to cavities between helices. The principles identified in this work have broader implications for the development of a larger class of compounds that could target SLC6 transporters for disease treatment.

Cholesterol Metabolism in Neurodegenerative Diseases: Molecular Mechanisms and Therapeutic Targets

Cholesterol is an indispensable component of the cell membrane and plays vital roles in critical physiological processes. Brain cholesterol accounts for a large portion of total cholesterol in the human body, and its content must be tightly regulated to ensure normal brain function. Disorders of cholesterol metabolism in the brain are linked to neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and other atypical cognitive deficits that arise at old age. However, the specific role of cholesterol metabolism disorder in the pathogenesis of neurodegenerative diseases has not been fully elucidated. Statins that are a class of lipid-lowering drugs have been reported to have a positive effect on neurodegenerative diseases. Herein, we reviewed the physiological and pathological conditions of cholesterol metabolism and discussed the possible mechanisms of cholesterol metabolism and statin therapy in neurodegenerative diseases.

Investigating the lipid fingerprint of SLC6 neurotransmitter transporters: a comparison of dDAT, hDAT, hSERT, and GlyT2

The local lipid annulus, or "fingerprint", of four SLC6 transporters (dDAT, hDAT, hSERT, and GlyT2) embedded in a complex neuronal membrane were compared and characterised using molecular dynamics. Our analysis included the development of new tools to improve membrane leaflet detection and the analysis of leaflet-dependent properties. Overall, the lipid fingerprints of the four transporters are comprised of similar lipids when grouped by headgroup or tail saturation. The enrichment and depletion of specific lipids, including sites of cholesterol contacts, varies between transporters. The subtle differences in lipid fingerprints results in varying membrane biophysical properties near the transporter. Our results highlight that the lipid-fingerprint of SLC6 transporters in complex membranes is highly dependent on membrane composition. Our results further characterize how the presence and identity of membrane proteins affects the complex interplay of lipid-protein interactions, influencing the local lipid environment and membrane biophysical properties.

ATP modulates SLC7A5 (LAT1) synergistically with cholesterol

The plasma membrane transporter hLAT1 is responsible for providing cells with essential amino acids. hLAT1 is over-expressed in virtually all human cancers making the protein a hot-spot in the fields of cancer and pharmacology research. However, regulatory aspects of hLAT1 biology are still poorly understood. A remarkable stimulation of transport activity was observed in the presence of physiological levels of cholesterol together with a selective increase of the affinity for the substrate on the internal site, suggesting a stabilization of the inward open conformation of hLAT1. A synergistic effect by ATP was also observed only in the presence of cholesterol. The same phenomenon was detected with the native protein. Altogether, the biochemical assays suggested that cholesterol and ATP binding sites are close to each other. The computational analysis identified two neighboring regions, one hydrophobic and one hydrophilic, to which cholesterol and ATP were docked, respectively. The computational data predicted interaction of the ϒ-phosphate of ATP with Lys 204, which was confirmed by site-directed mutagenesis. The hLAT1-K204Q mutant showed an impaired function and response to ATP. Interestingly, this residue is conserved in several members of the SLC7 family.

SLC6 transporter oligomerization

Transporters of the solute carrier 6 (SLC6) family mediate the reuptake of neurotransmitters such as dopamine, norepinephrine, serotonin, GABA, and glycine. SLC6 family members are 12 transmembrane helix‐spanning proteins that operate using the transmembrane sodium gradient for transport. These transporters assume various quaternary arrangements ranging from monomers to complex stoichiometries with multiple subunits. Dopamine and serotonin transporter oligomerization has been implicated in trafficking of newly formed proteins from the endoplasmic reticulum to the plasma membrane with a pre‐fixed assembly. Once at the plasma membrane, oligomers are kept fixed in their quaternary assembly by interaction with phosphoinositides. While it remains unclear how oligomer formation precisely affects physiological transporter function, it has been shown that oligomerization supports the activity of release‐type psychostimulants. Most recently, single molecule microscopy experiments unveiled that the stoichiometry differs between individual members of the SLC6 family. The present overview summarizes our understanding of the influence of plasma membrane constituents on transporter oligomerization, describes the known interfaces between protomers and discusses open questions.

Toward a Systematic Structural and Functional Annotation of Solute Carriers Transporters-Example of the SLC6 and SLC7 Families

SLC transporters are emerging key drug targets. One important step for drug development is the profound understanding of the structural determinants defining the substrate selectivity of each transporter. Recently, the improvement of computational power and experimental methods such as X-ray and cryo-EM crystallography permitted to conduct structure-based studies on specific transporters having important pharmacological impact. However, a lot remains to be discovered regarding their dynamics, transport modulation and ligand recognition. A detailed functional characterization of transporters would provide opportunities to develop new compounds targeting these key drug targets. Here, we are giving an overview of two major human LeuT-fold families, SLC6 and SLC7, with an emphasis on the most relevant members of each family for drug development. We gather the most recent understanding on the structural determinants of selectivity within and across the two families. We then use this information to discuss the benefits of a more generalized structural and functional annotation of the LeuT fold and the implications of such mapping for drug discovery.

Computational Dissection of Membrane Transport at a Microscopic Level

Membrane transporters are key gatekeeper proteins at cellular membranes that closely control the traffic of materials. Their function relies on structural rearrangements of varying degrees that facilitate substrate translocation across the membrane. Characterizing these functionally important molecular events at a microscopic level is key to our understanding of membrane transport, yet challenging to achieve experimentally. Recent advances in simulation technology and computing power have rendered molecular dynamics (MD) simulation a powerful biophysical tool to investigate a wide range of dynamical events spanning multiple spatial and temporal scales. Here, we review recent studies of diverse membrane transporters using computational methods, with an emphasis on highlighting the technical challenges, key lessons learned, and new opportunities to illuminate transporter structure and function.

Effect of Cholesterol on the Organic Cation Transporter OCTN1 (SLC22A4)

The effect of cholesterol was investigated on the OCTN1 transport activity measured as [¹⁴C]-tetraethylamonium or [³H]-acetylcholine uptake in proteoliposomes reconstituted with native transporter extracted from HeLa cells or the human recombinant OCTN1 over-expressed in E. coli. Removal of cholesterol from the native transporter by MβCD before reconstitution led to impairment of transport activity. A similar activity impairment was observed after treatment of proteoliposomes harboring the recombinant (cholesterol-free) protein by MβCD, suggesting that the lipid mixture used for reconstitution contained some cholesterol. An enzymatic assay revealed the presence of 10 µg cholesterol/mg total lipids corresponding to 1% cholesterol in the phospholipid mixture used for the proteoliposome preparation. On the other way around, the activity of the recombinant OCTN1 was stimulated by adding the cholesterol analogue, CHS to the proteoliposome preparation. Optimal transport activity was detected in the presence of 83 µg CHS/ mg total lipids for both [¹⁴C]-tetraethylamonium or [³H]-acetylcholine uptake. Kinetic analysis of transport demonstrated that the stimulation of transport activity by CHS consisted in an increase of the Vmax of transport with no changes of the Km. Altogether, the data suggests a direct interaction of cholesterol with the protein. A further support to this interpretation was given by a docking analysis indicating the interaction of cholesterol with some protein sites corresponding to CARC-CRAC motifs. The observed direct interaction of cholesterol with OCTN1 points to a possible direct influence of cholesterol on tumor cells or on acetylcholine transport in neuronal and non-neuronal cells via OCTN1.

The Fats of Life: Using Computational Chemistry to Characterise the Eukaryotic Cell Membrane

Our current knowledge of the structural dynamics and complexity of lipid bilayers is still developing. Computational techniques, especially molecular dynamics simulations, have increased our understanding significantly as they allow us to model functions that cannot currently be experimentally resolved. Here we review available computational tools and techniques, the role of the major lipid species, insights gained into lipid bilayer structure and function from molecular dynamics simulations, and recent progress towards the computational modelling of the physiological complexity of eukaryotic lipid bilayers.

Quantum dots reveal heterogeneous membrane diffusivity and dynamic surface density polarization of dopamine transporter

The presynaptic dopamine transporter mediates rapid reuptake of synaptic dopamine. Although cell surface DAT trafficking recently emerged as an important component of DAT regulation, it has not been systematically investigated. Here, we apply our single quantum dot (Qdot) tracking approach to monitor DAT plasma membrane dynamics in several heterologous expression cell hosts with nanometer localization accuracy. We demonstrate that Qdot-tagged DAT proteins exhibited highly heterogeneous membrane diffusivity dependent on the local membrane topography. We also show that Qdot-tagged DATs were localized away from the flat membrane regions and were dynamically retained in the membrane protrusions and cell edges for the duration of imaging. Single quantum dot tracking of wildtype DAT and its conformation-defective coding variants (R60A and W63A) revealed a significantly accelerated rate of dysfunctional DAT membrane diffusion. We believe our results warrant an in-depth investigation as to whether compromised membrane dynamics is a common feature of brain disorder-derived DAT mutants.

Integrating hydrogen-deuterium exchange mass spectrometry with molecular dynamics simulations to probe lipid-modulated conformational changes in membrane proteins

Biological membranes define the boundaries of cells and are composed primarily of phospholipids and membrane proteins. It has become increasingly evident that direct interactions of membrane proteins with their surrounding lipids play key roles in regulating both protein conformations and function. However, the exact nature and structural consequences of these interactions remain difficult to track at the molecular level. Here, we present a protocol that specifically addresses this challenge. First, hydrogen-deuterium exchange mass spectrometry (HDX-MS) of membrane proteins incorporated into nanodiscs of controlled lipid composition is used to obtain information on the lipid species that are involved in modulating the conformational changes in the membrane protein. Then molecular dynamics (MD) simulations in lipid bilayers are used to pinpoint likely lipid-protein interactions, which can be tested experimentally using HDX-MS. By bringing together the MD predictions with the conformational readouts from HDX-MS, we have uncovered key lipid-protein interactions implicated in stabilizing important functional conformations. This protocol can be applied to virtually any integral membrane protein amenable to classic biophysical studies and for which a near-atomic-resolution structure or homology model is available. This protocol takes ~4 d to complete, excluding the time for data analysis and MD simulations, which depends on the size of the protein under investigation.

Identification of an allosteric binding site on the human glycine transporter, GlyT2, for bioactive lipid analgesics

The treatment of chronic pain is poorly managed by current analgesics, and there is a need for new classes of drugs. We recently developed a series of bioactive lipids that inhibit the human glycine transporter GlyT2 (SLC6A5) and provide analgesia in animal models of pain. Here, we have used functional analysis of mutant transporters combined with molecular dynamics simulations of lipid-transporter interactions to understand how these bioactive lipids interact with GlyT2. This study identifies a novel extracellular allosteric modulator site formed by a crevice between transmembrane domains 5, 7, and 8, and extracellular loop 4 of GlyT2. Knowledge of this site could be exploited further in the development of drugs to treat pain, and to identify other allosteric modulators of the SLC6 family of transporters.