Membrane topology of human ASBT (SLC10A2) determined by dual label epitope insertion scanning mutagenesis. New evidence for seven transmembrane domains

Toward a Topology-Based Therapeutic Design of Membrane Proteins: Validation of NaPi2b Topology in Live Ovarian Cancer Cells

NaPi2b is a sodium-dependent phosphate transporter that belongs to the SLC34 family of transporters which is mainly responsible for phosphate homeostasis in humans. Although NaPi2b is widely expressed in normal tissues, its overexpression has been demonstrated in ovarian, lung, and other cancers. A valuable set of antibodies, including L2 (20/3) and MX35, and its humanized versions react strongly with an antigen on the surface of ovarian and other carcinoma cells. Although the topology of NaPi2b was predicted in silico, no direct experimental data are available for the orientation of NaPi2b extracellular domains in cancer cells. The presented results of antibody mapping of untagged NaPi2b in live ovarian carcinoma cells OVCAR-4 provide a platform for current and future epitope-based cancer therapies and serological diagnostics.

Partial proteolysis improves the identification of the extracellular segments of transmembrane proteins by surface biotinylation

Transmembrane proteins (TMP) play a crucial role in several physiological processes. Despite their importance and diversity, only a few TMP structures have been determined by high-resolution protein structure characterization methods so far. Due to the low number of determined TMP structures, the parallel development of various bioinformatics and experimental methods was necessary for their topological characterization. The combination of these methods is a powerful approach in the determination of TMP topology as in the Constrained Consensus TOPology prediction. To support the prediction, we previously developed a high-throughput topology characterization method based on primary amino group-labelling that is still limited in identifying all TMPs and their extracellular segments on the surface of a particular cell type. In order to generate more topology information, a new step, a partial proteolysis of the cell surface has been introduced to our method. This step results in new primary amino groups in the proteins that can be biotinylated with a membrane-impermeable agent while the cells still remain intact. Pre-digestion also promotes the emergence of modified peptides that are more suitable for MS/MS analysis. The modified sites can be utilized as extracellular constraints in topology predictions and may contribute to the refined topology of these proteins.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Human bile acid transporter ASBT (SLC10A2) forms functional non-covalent homodimers and higher order oligomers

The human apical sodium-dependent bile acid transporter, hASBT/SLC10A2, plays a central role in cholesterol homeostasis via the efficient reabsorption of bile acids from the distal ileum. hASBT has been shown to self-associate in higher order complexes, but while the functional role of endogenous cysteines has been reported, their implication in the oligomerization of hASBT remains unresolved. Here, we determined the self-association architecture of hASBT by site-directed mutagenesis combined with biochemical, immunological and functional approaches. We generated a cysteine-less form of hASBT by creating point mutations at all 13 endogenous cysteines in a stepwise manner. Although Cysless hASBT had significantly reduced function correlated with lowered surface expression, it featured an extra glycosylation site that facilitated its differentiation from wt-hASBT on immunoblots. Decreased protein expression was associated with instability and subsequent proteasome-dependent degradation of Cysless hASBT protein. Chemical cross-linking of wild-type and Cysless species revealed that hASBT exists as an active dimer and/or higher order oligomer with apparently no requirement for endogenous cysteine residues. This was further corroborated by co-immunoprecipitation of differentially tagged (HA-, Flag-) wild-type and Cysless hASBT. Finally, Cysless hASBT exhibited a dominant-negative effect when co-expressed with wild-type hASBT which validated heterodimerization/oligomerization at the functional level. Combined, our data conclusively demonstrate the functional existence of hASBT dimers and higher order oligomers irrespective of cysteine-mediated covalent bonds, thereby providing greater understanding of its topological assembly at the membrane surface.

(-)-Epigallocatechin-3-gallate inhibits entry of hepatitis B virus into hepatocytes

Hepatitis B virus (HBV) is a major cause of liver disease and hepatocellular carcinoma. Chronic HBV infection is currently managed with either nucleoside/nucleotide-based or interferon-based therapies, but fails to clear infection in a substantial proportion of cases, and antiviral strategies targeting the early stages of infection are therefore required for the prevention of HBV infection. In this study, we examined some common phytochemicals and identified epigallocatechin-3-gallate (EGCG) as a new inhibitor of HBV entry. EGCG, a flavonoid present in green tea extract, belongs to the subclass of catechins. We demonstrated that EGCG at a concentration of 50μM inhibited HBV entry into immortalized human primary hepatocytes by more than 80%, whereas the other four catechins tested had much weaker inhibitory effects. DMSO-differentiated HuS-E/2 cells expressed sodium taurocholate cotransporting polypeptide (NTCP), which is a receptor for HBV. Application of EGCG during HBV inoculation markedly inhibited infection in both DMSO-differentiated HuS-E/2 cells and HA-NTCP-expressing Huh7 cells. Interestingly, EGCG induced clathrin-dependent endocytosis of NTCP from the plasma membrane followed by protein degradation. In addition, EGCG inhibited the clathrin-mediated endocytosis of transferrin. Treatment of cells with EGCG had no effect on HBV genome replication or virion secretion. Moreover, the characteristic of HBV virion and the expression of known HBV entry factors were unaltered by EGCG. Finally, the antiviral activity of EGCG on HBV entry was observed using four different genotypes, A to D. These results show that the green tea-derived molecule EGCG potently inhibits HBV entry and could be used in prevention of HBV reinfection.

Homologue gene of bile acid transporters ntcp, asbt, and ost-alpha in rainbow trout Oncorhynchus mykiss: tissue expression, effect of fasting, and response to bile acid administration

Bile acid transporters belonging to the SLC10A protein family, Na+ taurocholate cotransporting polypeptide (NTCP or SLC10A1), apical sodium-dependent bile salt transporter (ASBT or SLC10A2), and organic solute transporter alpha (Ost-alpha) have been known to play critical roles in the enterohepatic circulation of bile acids in mammals. In this study, ntcp, asbt, and ost-alpha-1/-2 cDNA were cloned, their tissue distributions were characterized, and the effects of fasting and bile acid administration on their expression were examined in rainbow trout Oncorhynchus mykiss. The structural characteristics of Ntcp, Asbt, and Ost-alpha were well conserved in trout, and three-dimensional structure analysis showed that Ntcp and Asbt were similar to each other. Tissue distribution analysis revealed that trout asbt was primarily expressed in the hindgut, while ntcp expression occurred in the brain, and ost-alpha-1/-2 was mainly expressed in the liver or ovary. Although asbt and ost-alpha-1 mRNA levels in the gut increased in response to fasting for 4 days, ost-alpha-1 expression in the liver decreased. Similarly, bile acid administration increased asbt and ost-alpha-1 expression levels in the gut, while those of ntcp and ost-alpha-2 in the liver decreased. These results suggested that the genes asbt, ntcp, and ost-alpha are involved in bile acid transport in rainbow trout.

Oligomeric bile acid-mediated oral delivery of low molecular weight heparin

Intestinal transporters are limited to the transport of small molecular substrates. Here, we describe the development of apical sodium-dependent bile acid transporter (ASBT)-targeted high-affinity oligomeric bile acid substrates that mediate the transmembrane transport of low molecular weight heparin (LMWH). Several oligomers of deoxycholic acid (oligoDOCA) were synthesized to investigate the substrate specificity of ASBT. To see the binding of oligoDOCA on the substrate-binding pocket of ASBT, molecular docking was used and the dissociation rate constants (KD) were measured using surface plasmon resonance. The KD for tetrameric DOCA (tetraDOCA) was 50-fold lower than that for monomeric DOCA, because tetraDOCA interacted with several hydrophobic grooves in the substrate-binding pocket of ASBT. The synthesized oligoDOCA compounds were subsequently chemically conjugated to macromolecular LMWH. In vitro, tetraDOCA-conjugated LMWH (LHe-tetraD) had highest selectivity for ASBT during its transport. Orally administered LHe-tetraD showed remarkable systemic anticoagulation activity and high oral bioavailability of 33.5±3.2% and 19.9±2.5% in rats and monkeys, respectively. Notably, LHe-tetraD successfully prevented thrombosis in a rat model of deep vein thrombosis. These results represent a major advancement in ASBT-mediated LMWH delivery and may facilitate administration of many important therapeutic macromolecules through a non-invasive oral route.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Transmembrane domain II of the human bile acid transporter SLC10A2 coordinates sodium translocation

Human apical sodium-dependent bile acid transporter (hASBT, SLC10A2) is responsible for intestinal reabsorption of bile acids and plays a key role in cholesterol homeostasis. We used a targeted and systematic approach to delineate the role of highly conserved transmembrane helix 2 on the expression and function of hASBT. Cysteine mutation significantly depressed transport activity for >60% of mutants without affecting cell surface localization of the transporter. All mutants were inaccessible toward chemical modification by membrane-impermeant MTSET reagent, strongly suggesting that transmembrane 2 (TM2) plays an indirect role in bile acid substrate translocation. Both bile acid uptake and sodium dependence of TM2 mutants revealed a distinct α-helical periodicity. Kinetic studies with conservative and non-conservative mutants of sodium sensitive residues further underscored the importance of Gln(75), Phe(76), Met(79), Gly(83), Leu(86), Phe(90), and Asp(91) in hASBT function. Computational analysis indicated that Asp(91) may coordinate with sodium during the transport cycle. Combined, our data propose that a consortium of sodium-sensitive residues along with previously reported residues (Thr(134), Leu(138), and Thr(149)) from TM3 may form the sodium binding and translocation pathway. Notably, residues Gln(75), Met(79), Thr(82), and Leu(86) from TM2 are highly conserved in TM3 of a putative remote bacterial homologue (ASBTNM), suggesting a universal mechanism for the SLC10A transporter family.

Transmembrane domain V plays a stabilizing role in the function of human bile acid transporter SLC10A2

The human apical sodium-dependent bile acid transporter (hASBT, SLC10A2), primarily expressed in the ileum, is involved in both the recycling of bile acids and cholesterol homeostasis. In this study, the structure-function relationship of transmembrane domain 5 (TM5) residues involved in transport is elucidated. Cysteine scanning mutagenesis of each consecutive residue on TM5 resulted in 96% of mutants having a significantly decreased transport activity, although each was expressed at the cell surface. Specifically, G197 and I208 were no longer functional, and G201 and G212 functioned at a level of <10% upon cysteine mutation. Interestingly, each of these exists along one face of the helix. Studies suggest that neither G201 nor G212 is on the substrate pathway. Conservative alanine mutations of the four residues displayed a higher activity in all but G197A, indicating its functional importance. G197 and G201 form a GxxxG motif, which has been found to be important in helix-helix interactions. According to our model, G197 and G201 face transmembrane domain 4 (TM4) residues G179 and P175, respectively. Similarly, G212 faces G237, which forms part of a GxxxG domain in transmembrane domain 6 (TM6). It is possible that these GxxxG domains and their interacting partners are responsible for maintaining the structure of the helices and their interactions with one another. I205 and I208 are both in positions to anchor the GxxxG domains and direct the change in interaction of TM5 from TM4 to TM6. Combined, the results suggest that residues along TM5 are critical for ASBT function but are not directly involved in substrate translocation.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

Importance of uncharged polar residues and proline in the proximal two-thirds (Pro107-Ser128) of the highly conserved region of mouse ileal Na+-dependent bile acid transporter, Slc10a2, in transport activity and cellular expression

Background

SLC10A2-mediated reabsorption of bile acids at the distal end of the ileum is the first step in enterohepatic circulation. Because bile acids act not only as detergents but also as signaling molecules in lipid metabolism and energy production, SLC10A2 is important as the key transporter for understanding the in vivo kinetics of bile acids. SLC10A family members and the homologous genes of various species share a highly conserved region corresponding to Gly¹⁰⁴–Pro¹⁴² of SLC10A2. The functional importance of this region has not been fully elucidated.

Results

To elucidate the functional importance of this region, we previously performed mutational analysis of the uncharged polar residues and proline in the distal one-third (Thr¹³⁰–Pro¹⁴²) of the highly conserved region in mouse Slc10a2. In this study, proline and uncharged polar residues in the remaining two-thirds of this region in mouse Slc10a2 were subjected to mutational analysis, and taurocholic acid uptake and cell surface localization were examined. Cell surface localization of Slc10a2 is necessary for bile acid absorption. Mutants in which Asp or Leu were substituted for Pro¹⁰⁷ (P107N or P107L) were abundantly expressed, but their cell surface localization was impaired. The S126A mutant was completely impaired in cellular expression. The T110A and S128A mutants exhibited remarkably enhanced membrane expression. The S112A mutant was properly expressed at the cell surface but transport activity was completely lost. Replacement of Tyr¹¹⁷ with various amino acids resulted in reduced transport activity. The degree of reduction roughly depended on the van der Waals volume of the side chains.

Conclusions

The functional importance of proline and uncharged polar residues in the highly conserved region of mouse Slc10a2 was determined. This information will contribute to the design of bile acid-conjugated prodrugs for efficient drug delivery or SLC10A2 inhibitors for hypercholesterolemia treatment.

Structural requirements of bile acid transporters: C-3 and C-7 modifications of steroidal hydroxyl groups

The apical sodium dependent bile acid transporter (ASBT) and sodium-taurocholate cotransporting polypeptide (NTCP) are potential prodrug targets, but the structural requirements for these transporters are incompletely defined. The objective of this study was to evaluate the effect of C-3 and C-7 substitution on bile acid interaction with these bile acid transporters. Nineteen bile acid analogs were tested against ASBT and NTCP for binding, as well as translocation. Results indicated that ASBT and NTCP accommodated a wide range of substituents for binding, but all major C-7 modifications resulted in analogs that did not demonstrate active uptake by either ASBT or NTCP. A C-3 modification that was not tolerated at C-7 still afforded translocation via ASBT and NTCP, confirming the relative unacceptability of C-7 modification. Both ASBT and NTCP demonstrated a generally similar binding potency. Results suggest that drug conjugation to the C-3 hydroxyl group, rather than C-7, has potential to lead to a successful prodrug targeting ASBT and NTCP.

Putative irreversible inhibitors of the human sodium-dependent bile acid transporter (hASBT; SLC10A2) support the role of transmembrane domain 7 in substrate binding/translocation

PURPOSE

To explore the involvement of transmembrane domain (TM) 7 of the human apical sodium-dependent bile acid transporter (hASBT) on bile acid (BA) binding/translocation, using two electrophilic BA derivatives as molecular probes.

METHODS

Two electrophilic derivatives of chenodeoxycholic acid (CDCA) were designed, synthesized and evaluated for their ability to inactivate hASBT, and the human organic cation/carnitine transporter (hOCTN2) as a control (i.e. a non-BA transporting model). The ability of electrophilic derivatives to interact with hASBT was evaluated by 2-aminoethyl-methanethiosulfonate (MTSEA)-biotin labeling of thiol groups in TM7 cysteine mutants.

RESULTS

Unlike native BAs, the electrophilic CDCA derivatives specifically inactivated hASBT, but not hOCTN2, and inhibited hASBT in a time- and concentration-dependent fashion. Preincubation of hASBT Cys-mutants in the exofacial half of TM7 with reactive electrophilic probes blocked transporter biotinylation by MTSEA-biotin, similar to 2-(trimethylammonium)ethyl-methanethiosulfonate (MTSET) blocking. This blocking pattern differed from that produced by native BAs, which exposed exofacial TM7 residues, thereby increasing staining.

CONCLUSION

Kinetic and biochemical data indicate these novel electrophilic BAs are potent and specific irreversible inhibitors of hASBT and offer new evidence about the role of TM7 in binding/translocation of bile acids.

Crystal structure of a bacterial homologue of the bile acid sodium symporter ASBT

High cholesterol levels greatly increase the risk of cardiovascular disease. By its conversion into bile acids, about 50% of cholesterol is eliminated from the body. However bile acids released from the bile duct are constantly recycled, being reabsorbed in the intestine via the Apical Sodium dependent Bile acid Transporter (ASBT). It has been shown in animal models that plasma cholesterol levels are significantly lowered by specific inhibitors of ASBT^1,2, thus ASBT is a target for hypercholesterolemia drugs. Here, we describe the crystal structure of a bacterial homologue of ASBT from Neisseria meningitidis (ASBT_NM) at 2.2Å. ASBT_NM contains two inverted structural repeats of five transmembrane helices. A Core domain of six helices harbours two sodium ions while the remaining helices form a Panel-like domain. Overall the architecture of the protein is remarkably similar to the sodium-proton antiporter NhaA³ despite no detectable sequence homology. A bile acid molecule is situated between the Core and Panel domains in a large hydrophobic cavity. Residues near to this cavity have been shown to affect the binding of specific inhibitors of human ASBT⁴. The position of the bile acid together with the molecular architecture suggests the rudiments of a possible transport mechanism.

Transmembrane helix 1 contributes to substrate translocation and protein stability of bile acid transporter SLC10A2

The human apical sodium-dependent bile acid transporter (hASBT, SLC10A2) plays a critical role in the enterohepatic circulation of bile acids, as well as in cholesterol homeostasis. ASBT reclaims bile acids from the distal ileum via active sodium co-transport, in a multistep process, orchestrated by key residues in exofacial loop regions, as well as in membrane-spanning helices. Here, we unravel the functional contribution of highly conserved transmembrane helix 1 (TM1) on the hASBT transport cycle. Consecutive cysteine substitution of individual residues along the TM1 helix (Ile(29)-Gly(50)), as well as exofacial Asn(27) and Asn(28), resulted in functional impairment of ∼70% of mutants, despite appreciable cell surface expression for all but G50C. Cell surface expression of G50C and G50A was rescued upon MG132 treatment as well as cyclosporine A, but not by FK506 or bile acids, suggesting that Gly(50) is involved in hASBT folding. TM1 accessibility to membrane-impermeant MTSET remains confined to the exofacial half of the helix along a single, discrete face. Substrate protection from MTSET labeling was temperature-dependent for L34C, T36C, and L38C, consistent with conformational changes playing a role in solvent accessibility for these mutants. Residue Leu(30) was shown to be critical for both bile acid and sodium affinity, while Asn(27), Leu(38), Thr(39), and Met(46) participate in sodium co-transport. Combined, our data demonstrate that TM1 plays a pivotal role in ASBT function and stability, thereby providing further insight in its dynamic transport mechanism.

Role of the intestinal bile acid transporters in bile acid and drug disposition

Membrane transporters expressed by the hepatocyte and enterocyte play critical roles in maintaining the enterohepatic circulation of bile acids, an effective recycling and conservation mechanism that largely restricts these potentially cytotoxic detergents to the intestinal and hepatobiliary compartments. In doing so, the hepatic and enterocyte transport systems ensure a continuous supply of bile acids to be used repeatedly during the digestion of multiple meals throughout the day. Absorption of bile acids from the intestinal lumen and export into the portal circulation is mediated by a series of transporters expressed on the enterocyte apical and basolateral membranes. The ileal apical sodium-dependent bile acid cotransporter (abbreviated ASBT; gene symbol, SLC10A2) is responsible for the initial uptake of bile acids across the enterocyte brush border membrane. The bile acids are then efficiently shuttled across the cell and exported across the basolateral membrane by the heteromeric Organic Solute Transporter, OSTα-OSTβ. This chapter briefly reviews the tissue expression, physiology, genetics, pathophysiology, and transport properties of the ASBT and OSTα-OSTβ. In addition, the chapter discusses the relationship between the intestinal bile acid transporters and drug metabolism, including development of ASBT inhibitors as novel hypocholesterolemic or hepatoprotective agents, prodrug targeting of the ASBT to increase oral bioavailability, and involvement of the intestinal bile acid transporters in drug absorption and drug-drug interactions.

Targeting drug transporters - combining in silico and in vitro approaches to predict in vivo

Transporter proteins are expressed throughout the human body in different vital organs. They play an important role to various extents in determining absorption, distribution, metabolism, excretion, and toxicity (ADME/Tox) properties of therapeutic molecules. Over the past decade, numerous drug transporters have been cloned and considerable progress has been made toward understanding the molecular characteristics of individual transporters. In this chapter several in vitro and in silico techniques are described with applications to understand transporter behavior. These include employing new techniques to rapidly identify novel ligands for transporters. Ultimately these methods should lead to a greater overall appreciation of the role of transporters in vivo.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Identifying assembly-inhibiting and assembly-tolerant sites in the SbsB S-layer protein from Geobacillus stearothermophilus

Surface layer (S-layer) proteins self-assemble into two-dimensional crystalline lattices that cover the cell wall of all archaea and many bacteria. We have generated assembly-negative protein variants of high solubility that will facilitate high-resolution structure determination. Assembly-negative versions of the S-layer protein SbsB from Geobacillus stearothermophilus PV72/p2 were obtained using an insertion mutagenesis screen. The haemagglutinin epitope tag was inserted at 23 amino acid positions known to be located on the monomer protein surface from a previous cysteine accessibility screen. Limited proteolysis, circular dichroism, and fluorescence were used to probe whether the epitope insertion affected the secondary and tertiary structures of the monomer, while electron microscopy and size-exclusion chromatography were employed to examine proteins' ability to self-assemble. The screen not only identified assembly-compromised mutants with native fold but also yielded correctly folded, self-assembling mutants suitable for displaying epitopes for biomedical and biophysical applications, as well as cryo-electron microscopy imaging. Our study marks an important step in the analysis of the S-layer structure. In addition, the approach of concerted insertion and cysteine mutagenesis can likely be applied for other supramolecular assemblies.

The cytosolic half of helix III forms the substrate exit route during permeation events of the sodium/bile acid cotransporter ASBT

Site-directed alkylation of consecutively introduced cysteines was employed to probe the solvent-accessible profile of highly conserved transmembrane helix 3 (TM3), spanning residues V127-T149 of the apical sodium-dependent bile acid transporter (ASBT), a key membrane protein involved in cholesterol homeostasis. Sequence alignment of SLC10 family members has previously identified a signature motif (ALGMMPL) localized to TM3 of ASBT with as yet undetermined function. Cysteine mutagenesis of this motif resulted in severe decreases in uptake activity only for mutants M141C and P142C. Additional conservative and nonconservative replacement of P142 suggests its structural and functional importance during the ASBT transport cycle. Significant decreases in transport activity were also observed for three cysteine mutants clustered along the exofacial half of the helix (M129C, T130C, S133C) and five mutants consecutively lining the cytosolic half of TM3 (L145C-T149C). Measurable surface expression was detected for all TM3 mutants. Using physicochemically different alkylating reagents, sites predominantly lining the cytosolic half of the TM3 helix were found to be solvent accessible (i.e., S128C, L143C-T149C). Analysis of substrate kinetics for select TM3 mutants demonstrates significant loss of taurocholic acid affinity for mutants S128C and L145C-T149C. Overall, we conclude (i) the functional and structural importance of P142 during the transport cycle and (ii) the presence of a large hydrophilic cleft region lining the cytosolic half of TM3 that may form portions of the substrate exit route during permeation. Our studies provide unique insight into molecular mechanisms guiding the ASBT transport cycle with respect to substrate binding and translocation events.

Mutational analysis of uncharged polar residues and proline in the distal one-third (Thr130-Pro142) of the highly conserved region of mouse Slc10a2

Solute carrier family member 2 (SLC10A2) reabsorbs bile acids at the distal terminus of the ileum in an Na(+)-dependent manner. Alignment of deduced amino acid sequences of SLC10 family members and homologous genes in various species revealed a highly conserved region that corresponds to Gly(104)-Pro(142) of SLC10A2. To elucidate the functional importance of this region, uncharged polar residues and Pro in the distal one-third of this region in mouse Slc10a2 (mSlc10a2) were submitted to mutational analysis, and taurocholic acid uptake and cell surface localization were evaluated. In addition to mutations that abolished almost all of the transport activity with and without cellular localization failure (P142V and T130A respectively), a mutation that perhaps affected affinity for taurocholic acid was identified (T134A). These results suggest that the highly conserved region contains residues involved in the substrate interaction, function, and cellular localization of mSlc10a2.

Investigation of the structure and function of a Shewanella oneidensis arsenical-resistance family transporter

The toxic metalloid arsenic is an abundant element and most organisms possess transport systems involved in its detoxification. One such family of arsenite transporters, the ACR3 family, is widespread in fungi and bacteria. To gain a better understanding of the molecular mechanism of arsenic transport, we report here the expression and characterization of a family member, So_ACR3, from the bacterium Shewanella oneidensis MR-1. Surprisingly, expression of this transporter in the arsenic-hypersensitive Escherichia coli strain AW3110 conferred resistance to arsenate, but not to arsenite. Purification of a C-terminally His-tagged form of the protein allowed the binding of putative permeants to be directly tested: arsenate but not arsenite quenched its intrinsic fluorescence in a concentration-dependent fashion. Fourier transform infrared spectroscopy showed that the purified protein was predominantly alpha-helical. A mutant bearing a single cysteine residue at position 3 retained the ability to confer arsenate resistance, and was accessible to membrane impermeant thiol reagents in intact cells. In conjunction with successful C-terminal tagging with oligohistidine, this finding is consistent with the experimentally-determined topology of the homologous human apical sodium-dependent bile acid transporter, namely 7 transmembrane helices and a periplasmic N-terminus, although the presence of additional transmembrane segments cannot be excluded. Mutation to alanine of the conserved residue proline 190, in the fourth putative transmembrane region, abrogated the ability of the transporter to confer arsenic resistance, but did not prevent arsenate binding. An apparently increased thermal stability is consistent with the mutant being unable to undergo the conformational transitions required for permeant translocation.

Membrane topology of the human breast cancer resistance protein (BCRP/ABCG2) determined by epitope insertion and immunofluorescence

The human breast cancer resistance protein (BCRP/ABCG2) mediates efflux of drugs and organic anions across the plasma membrane. Hydropathy analysis suggests that BCRP consists of a nucleotide-binding domain (residues approximately 1-395) and a membrane-spanning domain (MSD) (residues approximately 396-655); however, its exact topology structure remains unknown. In this study, we determined the topology structure of BCRP by inserting hemagglutinin (HA) tags in its predicted hydrophilic regions of the MSD. HA-tagged BCRP mutants were expressed in HEK cells and tested for their ability to efflux mitoxantrone and BODIPY-prazosin. Polarity of the inserted tags with respect to the plasma membrane was determined by immunofluorescence. All of the mutants were expressed at levels comparable to wild-type BCRP as revealed by immunoblotting with specific antibodies against BCRP and the HA tag. Insertions at residues 423, 454, 462, 499, 529, 532, and 651 produced functional mutants, whereas insertions at residues 560, 594, and 623 resulted in mutants with significantly reduced activity and insertions at residues 387, 420, 474, and 502 completely abrogated the activity. HA tags inserted at residues 387, 474, 529, 532, 560, and 651 were localized intracellularly, whereas those inserted at residues 420, 423, 454, 499, 502, 594, and 623 revealed an extracellular location. Residue 462 was localized in a transmembrane (TM) segment. These results provide the first direct experimental evidence in support of a 6-TM model for BCRP with the amino and carboxyl termini of the MSD located intracellularly. These data may have important implications for understanding the transport mechanism of BCRP.

Bile acid transporters in health and disease

In recent years the discovery of a number of major transporter proteins expressed in the liver and intestine specifically involved in bile acid transport has led to improved understanding of bile acid homeostasis and the enterohepatic circulation. Sodium (Na(+))-dependent bile acid uptake from portal blood into the liver is mediated primarily by the Na(+) taurocholate co-transporting polypeptide (NTCP), while secretion across the canalicular membrane into the bile is carried out by the bile salt export pump (BSEP). In the ileum, absorption of bile acids from the lumen into epithelial cells is mediated by the apical Na(+) bile salt transporter (ASBT), whereas exit into portal blood across the basolateral membrane is mediated by the organic solute transporter alpha/beta (OSTalpha/beta) heterodimer. Regulation of transporter gene expression and function occurs at several different levels: in the nucleus, members of the nuclear receptor superfamily, regulated by bile acids and other ligands are primarily involved in controlling gene expression, while cell signalling events directly affect transporter function, and subcellular localization. Polymorphisms, dysfunction, and impaired adaptive responses of several of the bile acid transporters, e.g. BSEP and ASBT, results in liver and intestinal disease. Bile acid transporters are now understood to play central roles in driving bile flow, as well as adaptation to various pathological conditions, with complex regulation of activity and function in the nucleus, cytoplasm, and membrane.

Conserved aspartic acid residues lining the extracellular loop 1 of sodium-coupled bile acid transporter ASBT Interact with Na+ and 7alpha-OH moieties on the ligand cholestane skeleton

Functional contributions of residues Val-99-Ser-126 lining extracellular loop (EL) 1 of the apical sodium-dependent bile acid transporter were determined via cysteine-scanning mutagenesis, thiol modification, and in silico interpretation. Despite membrane expression for all but three constructs (S112C, Y117C, S126C), most EL1 mutants (64%) were inactivated by cysteine mutation, suggesting a functional role during sodium/bile acid co-transport. A negative charge at conserved residues Asp-120 and Asp-122 is required for transport function, whereas neutralization of charge at Asp-124 yields a functionally active transporter. D124A exerts low affinity for common bile acids except deoxycholic acid, which uniquely lacks a 7alpha-hydroxyl (OH) group. Overall, we conclude that (i) Asp-122 functions as a Na(+) sensor, binding one of two co-transported Na(+) ions, (ii) Asp-124 interacts with 7alpha-OH groups of bile acids, and (iii) apolar EL1 residues map to hydrophobic ligand pharmacophore features. Based on these data, we propose a comprehensive mechanistic model involving dynamic salt bridge pairs and hydrogen bonding involving multiple residues to describe sodium-dependent bile acid transporter-mediated bile acid and cation translocation.

Electrostatic and potential cation-pi forces may guide the interaction of extracellular loop III with Na+ and bile acids for human apical Na+-dependent bile acid transporter

The hASBT (human apical Na(+)-dependent bile acid transporter) constitutes a key target of anti-hypercholesterolaemic therapies and pro-drug approaches; physiologically, hASBT actively reclaims bile acids along the terminal ileum via Na(+) co-transport. Previously, TM (transmembrane segment) 7 was identified as part of the putative substrate permeation pathway using SCAM (substitute cysteine accessibility mutagenesis). In the present study, SCAM was extended through EL3 (extracellular loop 3; residues Arg(254)-Val(286)) that leads into TM7 from the exofacial matrix. Activity of most EL3 mutants was significantly hampered upon cysteine substitution, whereas ten (out of 31) were functionally inactive (<10% activity). Since only E282C lacked plasma membrane expression, EL3 amino acids predominantly fulfill critical functional roles during transport. Oppositely charged membrane-impermeant MTS (methanethiosulfonate) reagents {MTSET [(2-trimethylammonium) ethyl MTS] and MTSES [(2-sulfonatoethyl) MTS]} produced mostly similar inhibition profiles wherein only middle and descending loop segments (residues Thr(267)-Val(286)) displayed significant MTS sensitivity. The presence of bile acid substrate significantly reduced the rates of MTS modification for all MTS-sensitive mutants, suggesting a functional association between EL3 residues and bile acids. Activity assessments at equilibrative [Na(+)] revealed numerous Na(+)-sensitive residues, possibly performing auxiliary functions during transport such as transduction of protein conformational changes during translocation. Integration of these data suggests ligand interaction points along EL3 via electrostatic interactions with Arg(256), Glu(261) and probably Glu(282) and a potential cation-pi interaction with Phe(278). We conclude that EL3 amino acids are essential for hASBT activity, probably as primary substrate interaction points using long-range electrostatic attractive forces.

Cytosolic half of transmembrane domain IV of the human bile acid transporter hASBT (SLC10A2) forms part of the substrate translocation pathway

We report the involvement of transmembrane domain 4 (TM4) of hASBT in forming the putative translocation pathway, using cysteine-scanning mutagenesis in conjunction with solvent-accessibility studies using the membrane-impermeant, sulfhydryl-specific methanethiosulfonate reagents. We individually mutated each of the 21 amino acids in TM4 to cysteine on a fully functional, MTS-resistant C270A-hASBT template. The single-cysteine mutants were expressed in COS-1 cells, and their cell surface expression levels, transport activities [uptake of the prototypical hASBT substrate taurocholic acid (TCA)], and sensitivities to MTS exposure were determined. Only P161 lacked cell-surface expression. Overall, cysteine replacement was tolerated at charged and polar residues, except for mutants I160C, Y162C, I165C, and G179C (<or=20% TCA uptake versus the control). TCA uptake was significantly inhibited by MTSES and MTSET for N164C, T167C, S168C, A171C, V173C, and P175C. Interestingly, all of these residues were clustered along one face of the putative alpha helix. TM4 mutants were not sensitive to equilibrative (12 mM) sodium concentrations, thereby ruling out a direct role of TM4 in sodium translocation. Our results demonstrate that primarily the cytosolic half of TM4 is highly solvent-accessible and plays an important role in ASBT function and substrate translocation. Consistent with the existing experimental data, a three-dimensional model for the orientation of TM4 is proposed.

Conformational flexibility of helix VI is essential for substrate permeation of the human apical sodium-dependent bile acid transporter

The present study characterizes the methanethiosulfonate (MTS) inhibition profiles of 26 consecutive cysteine-substituted mutants comprising transmembrane (TM) helix 6 of the human apical Na(+)-dependent bile acid transporter (SLC10A2). TM6 is linked exofacially to TM7 via extracellular loop 3. TM7 was identified previously as lining part of the substrate permeation path ( Mol Pharmacol 70: 1565, 2006 ). Most TM6 cysteine replacements were well tolerated, except for five residues with either severely hampered (I229C, G249C) or abolished (P234C, G237C, G241C) activity. Disruption of protein synthesis or folding and stability may account for lack of activity for mutant P234C. Subsequent Pro234 amino acid replacement reveals its participation in both structural and functional aspects of the transport cycle. Application of polar MTS reagents (1 mM) significantly inhibited the activity of six mutants (V235C, S239C, F242C, R246C, A248C, and Y253C), for which rates of modification were almost fully reversed (except Y253C) upon inclusion of bile acid substrates or removal of Na(+) from the MTS preincubation medium. Activity assessments at equilibrative [Na(+)] revealed numerous Na(+)-sensitive residues, suggesting their proximity in or around Na(+) interaction sites. In silico modeling reveals the intimate and potentially cooperative orientation of MTS-accessible TM6 residues toward functionally important TM7 amino acids, substantiating TM6 participation during the transport cycle. We conclude a functional requirement for helical flexibility imparted by Pro234, Gly237, and Gly241, probably forming a "conformational switch" requisite for substrate turnover; meanwhile, MTS-accessible residues, which line a helical face spatially distinct from this switch, may participate during substrate permeation.

Effects of Ala substitution for conserved Cys residues in mouse ileal and hepatic Na+-dependent bile acid transporters

Although ileal and hepatic Na(+)-dependent bile acid transporters (SLC10A2 and SLC10A1 respectively) share structural similarities, the mutation of conserved amino acids often has distinct effects on them. We have identified two Cys residues in mouse Slc10a2 (Cys(51) and Cys(106)) the replacement of which by Ala remarkably reduces taurocholic acid (TCA) transport. Although Cys(51) is conserved in Slc10a1 as Cys(44), Ala substitution gave no apparent difference in TCA uptake. Here, we further analyzed the kinetics of TCA uptake and cell surface localization of these mutants. The C51A and C106A mutants of Slc10a2 showed significantly reduced TCA uptake, while no apparent difference in TCA uptake was observed for the Slc10a1-C44A mutant. The K(m) values for TCA uptake by these mutants were comparable, suggesting that these residues are not involved in the interaction with TCA.

Molecular and phylogenetic characterization of a novel putative membrane transporter (SLC10A7), conserved in vertebrates and bacteria

The 'Solute Carrier Family SLC10' consists of six annotated members in humans, comprising two bile acid carriers (SLC10A1 and SLC10A2), one steroid sulfate transporter (SLC10A6), and three orphan carriers (SLC10A3 to SLC10A5). In this study we report molecular characterization and expression analysis of a novel member of the SLC10 family, SLC10A7, previously known as C4orf13. SLC10A7 proteins consist of 340-343 amino acids in humans, mice, rats, and frogs and show an overall amino acid sequence identity of >85%. SLC10A7 genes comprise 12 coding exons and show broad tissue expression pattern. When expressed in Xenopus laevis oocytes and HEK293 cells, SLC10A7 was detected in the plasma membrane but revealed no transport activity for bile acids and steroid sulfates. By immunofluorescence analysis of dual hemagglutinin (HA)- and FLAG-labeled SLC10A7 proteins in HEK293 cells, we established a topology of 10 transmembrane domains with an intracellular cis orientation of the N-terminal and C-terminal ends. This topology pattern is clearly different from the seven-transmembrane domain topology of the other SLC10 members but similar to hitherto uncharacterized non-vertebrate SLC10A7-related proteins. In contrast to the established SLC10 members, which are restricted to the taxonomic branch of vertebrates, SLC10A7-related proteins exist also in yeasts, plants, and bacteria, making SLC10A7 taxonomically the most widespread member of this carrier family. Vertebrate and bacterial SLC10A7 proteins exhibit >20% sequence identity, which is higher than the sequence identity of SLC10A7 to any other member of the SLC10 carrier family.

Cloning and Functional Characterization of Human Sodium-dependent Organic Anion Transporter (SLC10A6)

We have cloned human sodium-dependent organic anion transporter (SOAT) cDNA, which consists of 1502 bp and encodes a 377-amino acid protein. SOAT shows 42% sequence identity to the ileal apical sodium-dependent bile acid transporter ASBT and 33% sequence identity to the hepatic Na(+)/taurocholate-cotransporting polypeptide NTCP. Immunoprecipitation of a SOAT-FLAG-tagged protein revealed a glycosylated form at 46 kDa that decreased to 42 kDa after PNGase F treatment. SOAT exhibits a seven-transmembrane domain topology with an outside-to-inside orientation of the N-terminal and C-terminal ends. SOAT mRNA is most highly expressed in testis. Relatively high SOAT expression was also detected in placenta and pancreas. We established a stable SOAT-HEK293 cell line that showed sodium-dependent transport of dehydroepiandrosterone sulfate, estrone-3-sulfate, and pregnenolone sulfate with apparent K(m) values of 28.7, 12.0, and 11.3 microm, respectively. Although bile acids, such as taurocholic acid, cholic acid, and chenodeoxycholic acid, were not substrates of SOAT, the sulfoconjugated bile acid taurolithocholic acid-3-sulfate was transported by SOAT-HEK293 cells in a sodium-dependent manner and showed competitive inhibition of SOAT transport with an apparent K(i) value of 0.24 mum. Several nonsteroidal organosulfates also strongly inhibited SOAT, including 1-(omega-sulfooxyethyl)pyrene, bromosulfophthalein, 2- and 4-sulfooxymethylpyrene, and alpha-naphthylsulfate. Among these inhibitors, 2- and 4-sulfooxymethylpyrene were competitive inhibitors of SOAT, with apparent K(i) values of 4.3 and 5.5 microm, respectively, and they were also transported by SOAT-HEK293 cells.

Bile acid transporters: structure, function, regulation and pathophysiological implications

Specific transporters expressed in the liver and the intestine, play a critical role in driving the enterohepatic circulation of bile acids. By preserving a circulating pool of bile acids, an important factor influencing bile flow, these transporters are involved in maintaining bile acid and cholesterol homeostasis. Enterohepatic circulation of bile acids is fundamentally composed of two major processes: secretion from the liver and absorption from the intestine. In the hepatocytes, the vectorial transport of bile acids from blood to bile is ensured by Na+ taurocholate co-transporting peptide (NTCP) and organic anion transport polypeptides (OATPs). After binding to a cytosolic bile acid binding protein, bile acids are secreted into the canaliculus via ATP-dependent bile salt excretory pump (BSEP) and multi drug resistant proteins (MRPs). Bile acids are then delivered to the intestinal lumen through bile ducts where they emulsify dietary lipids and cholesterol to facilitate their absorption. Intestinal epithelial cells reabsorb the majority of the secreted bile acids through the apical sodium dependent bile acid transporter (ASBT) and sodium independent organic anion transporting peptide (OATPs). Cytosolic ileal bile acid binding protein (IBABP) mediates the transcellular movement of bile acids to the basolateral membrane across which they exit the cells via organic solute transporters (OST). An essential role of bile acid transporters is evident from the pathology associated with their genetic disruption or dysregulation of their function. Malfunctioning of hepatic and intestinal bile acid transporters is implicated in the pathophysiology of cholestatic liver disease and the depletion of circulating pool of bile acids, respectively. Extensive efforts have been recently made to enhance our understanding of the structure, function and regulation of the bile acid transporters and exploring new potential therapeutics to treat bile acid or cholesterol related diseases. This review will highlight current knowledge about structure, function and molecular characterization of bile acid transporters and discuss the implications of their defects in various hepatic and intestinal disorders.

Differential inhibition of rat and human Na+-dependent taurocholate cotransporting polypeptide (NTCP/SLC10A1)by bosentan: a mechanism for species differences in hepatotoxicity

Bile acid accumulation in hepatocytes due to inhibition of the canalicular bile salt export pump (BSEP/ABCB11) has been proposed as a mechanism for bosentan-induced hepatotoxicity. The observation that bosentan does not induce hepatotoxicity in rats, although bosentan has been reported to inhibit rat Bsep and cause elevated serum bile acids, challenges this mechanism. The lack of hepatotoxicity could be explained if bosentan inhibited hepatocyte uptake as well as canalicular efflux of bile acids. In the current study, bosentan was found to be a more potent inhibitor of Na(+)-dependent taurocholate uptake in rat (IC(50) 5.4 microM) than human (IC(50) 30 microM) suspended hepatocytes. In addition, bosentan was a more potent inhibitor of taurocholate uptake by rat Na(+)-dependent taurocholate co-transporting polypeptide (Ntcp/Slc10a1) (IC(50) 0.71 microM) than human NTCP (SLC10A1) (IC(50) 24 microM) expressed in HEK293 cells. Thus, bosentan is a more potent inhibitor of Ntcp than NTCP, and this should result in less intrahepatocyte accumulation of bile acids in rats during bosentan treatment. To begin characterization of this species difference, two chimeric molecules were generated and expressed in HEK293 cells; NTCP(1-140)/Ntcp(141-362) and Ntcp(1-140)/NTCP(141-349). The mode of bosentan inhibition was noncompetitive for Ntcp, and competitive for NTCP (K(i) 18 microM) and NTCP(1-140)/Ntcp(141-362) (K(i) 1.7 microM); bosentan affected both the K(m) and V(max) of Ntcp(1-140)/NTCP(141-349) (K(i) 7.0 microM). The carboxyl portions of NTCP and Ntcp were found to confer species differences in basal taurocholate transport V(max). In conclusion, differential inhibition of Ntcp and NTCP may represent a novel mechanism for species differences in bosentan-induced hepatotoxicity.

Transmembrane domain VII of the human apical sodium-dependent bile acid transporter ASBT (SLC10A2) lines the substrate translocation pathway

Recent evidence implicating transmembrane (TM) segment 7 of the apical sodium-dependent bile acid transporter (ASBT) in substrate interaction warranted examination of its aqueous accessibility. Therefore, cysteine substitution of 22 consecutive amino acids was performed against a methanethiosulfonate (MTS)-resistant background (C270A). Activity and susceptibility to polar MTS derivatives [(2-aminoethyl)-methanethiosulfonate (MTSEA), [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET), and methanethiosulfonate ethylsulfonate (MTSES)] of mutants were evaluated in COS-1 cells. Thr289, Tyr293, Gln297, Ala301, Phe307, and Tyr308 represented loss-of-function mutants; furthermore, the measurable residual activities for T289C, Y293C, and A301C (<or=20% control) proved insensitive to MTS treatment. MTSES and MTSET inhibition was confined to residues lining the extracellular half of TM7; amino acids situated deeper within the membrane were unaffected. In contrast, the entire length of TM7 was susceptible to the relatively smaller MTSEA; moreover, MTSEA sensitivity was significantly amended by coapplication with substrates. This selective pattern of modification suggests that the highly conserved lower half of TM7 lies within a water-filled cavity easily accessible from the extracellular milieu, whereas residues approaching the cytosolic/membrane interface reside in pores for which accessibility is modulated by molecular volume. Functionally inactive and MTS-inaccessible residues (T289C, Y293C, Q297C, and A301C) within TM7 may play a structural role critical to transporter function; conversely, MTS-sensitive residues are spatially distinct and may demarcate a face of the TM involved in substrate translocation. In addition, computational analysis of solvent-accessible domains identified five key solvent pockets that predominantly line the hydrophilic face of TM7. Combined, our data suggest that TM7 plays a dominant role in the hASBT translocation process.

Apical sodium dependent bile acid transporter (ASBT, SLC10A2): a potential prodrug target

A major hurdle impeding the successful clinical development of drug candidates can be poor intestinal permeability. Low intestinal permeability may be enhanced by a prodrug approach targeting membrane transporters in the small intestine. Transporter specificity, affinity, and capacity are three factors in targeted prodrug design. The human apical sodium dependent bile acid transporter (SLC10A2) belongs to the solute carrier family (SLC) of transporters and is an important carrier protein expressed in the small intestine. In spite of its appearing to be an excellent target for prodrug design, few studies have targeted human apical sodium dependent bile acid transporter (hASBT) to improve oral bioavailability. This review discusses bile acids including their chemistry and their absorptive disposition. Additionally, hASBT-mediated prodrug targeting is discussed, including QSAR, in vitro models for hASBT assay, and the current progress in utilizing hASBT as a drug delivery target.