Effects of cellular, chemical, and pharmacological chaperones on the rescue of a trafficking-defective mutant of the ATP-binding cassette transporter proteins ABCB1/ABCB4

ABC transporters involved in respiratory and cholestatic diseases: From rare to very rare monogenic diseases

ATP-binding cassette (ABC) transporters constitute a 49-member superfamily in humans. These proteins, most of them being transmembrane, allow the active transport of an important variety of substrates across biological membranes, using ATP hydrolysis as an energy source. For an important proportion of these ABC transporters, genetic variations of the loci encoding them have been correlated with rare genetic diseases, including cystic fibrosis and interstitial lung disease (variations in CFTR/ABCC7 and ABCA3) as well as cholestatic liver diseases (variations in ABCB4 and ABCB11). In this review, we first describe these ABC transporters and how their molecular dysfunction may lead to human diseases. Then, we propose a classification of the genetic variants according to their molecular defect (expression, traffic, function and/or stability), which may be considered as a general guideline for all ABC transporters' variants. Finally, we discuss recent progress in the field of targeted pharmacotherapy, which aim to correct specific molecular defects using small molecules. In conclusion, we are opening the path to treatment repurposing for diseases involving similar deficiencies in other ABC transporters.

Identification of new correctors for traffic-defective ABCB4 variants by a high-content screening approach

ABCB4 is located at the canalicular membrane of hepatocytes and is responsible for the secretion of phosphatidylcholine into bile. Genetic variations of this transporter are correlated with rare cholestatic liver diseases, the most severe being progressive familial intrahepatic cholestasis type 3 (PFIC3). PFIC3 patients most often require liver transplantation. In this context of unmet medical need, we developed a high-content screening approach to identify small molecules able to correct ABCB4 molecular defects. Intracellularly-retained variants of ABCB4 were expressed in cell models and their maturation, cellular localization and function were analyzed after treatment with the molecules identified by high-content screening. In total, six hits were identified by high-content screening. Three of them were able to correct the maturation and canalicular localization of two distinct intracellularly-retained ABCB4 variants; one molecule was able to significantly restore the function of two ABCB4 variants. In addition, in silico molecular docking calculations suggest that the identified hits may interact with wild type ABCB4 residues involved in ATP binding/hydrolysis. Our results pave the way for their optimization in order to provide new drug candidates as potential alternative to liver transplantation for patients with severe forms of ABCB4-related diseases, including PFIC3.

Oligomerization of drug transporters: Forms, functions, and mechanisms

Drug transporters are essential players in the transmembrane transport of a wide variety of clinical drugs. The broad substrate spectra and versatile distribution pattern of these membrane proteins infer their pharmacological and clinical significance. With our accumulating knowledge on the three-dimensional structure of drug transporters, their oligomerization status has become a topic of intense study due to the possible functional roles carried out by such kind of post-translational modification (PTM). In-depth studies of oligomeric complexes formed among drug transporters as well as their interactions with other regulatory proteins can help us better understand the regulatory mechanisms of these membrane proteins, provide clues for the development of novel drugs, and improve the therapeutic efficacy. In this review, we describe different oligomerization forms as well as their structural basis of major drug transporters in the ATP-binding cassette and solute carrier superfamilies, summarize our current knowledge on the influence of oligomerization for protein expression level and transport function of these membrane proteins, and discuss the regulatory mechanisms of oligomerization. Finally, we highlight the challenges associated with the current oligomerization studies and propose some thoughts on the pharmaceutical application of this important drug transporter PTM.

High-content Screen Identifies Cyclosporin A as a Novel ABCA3-specific Molecular Corrector

ATP-binding cassette (ABC) subfamily A member 3 (ABCA3) is a lipid transporter expressed in alveolar type II cells and localized in the limiting membrane of lamellar bodies. It is crucial for pulmonary surfactant storage and homeostasis. Mutations in the ABCA3 gene are the most common genetic cause of respiratory distress syndrome in mature newborns and interstitial lung disease in children. Apart from lung transplantation, there is no cure available. To address the lack of causal therapeutic options for ABCA3 deficiency, a rapid and reliable approach is needed to investigate variant-specific molecular mechanisms and to identify pharmacological modulators for mono- or combination therapies. To this end, we developed a phenotypic cell-based assay to autonomously identify ABCA3 wild-type-like or mutant-like cells by using machine-learning algorithms aimed at identifying morphological differences in WT and mutant cells. The assay was subsequently used to identify new drug candidates for ABCA3 specific molecular correction by high-content screening of 1,280 food and drug administration-approved small molecules. Cyclosporin A (CsA) was identified as a potent corrector, specific for some, but not all ABCA3 variants. Results were validated by our previously established functional small format assays. Hence, CsA may be selected for orphan drug evaluation in controlled repurposing trials in patients.

ATP-binding cassette transporters mediate differential biosynthesis of glycosphingolipid species

The cytosolic-oriented glucosylceramide (GlcCer) synthase is enigmatic, requiring nascent GlcCer translocation to the luminal Golgi membrane to access glycosphingolipid (GSL) anabolic glycosyltransferases. The mechanism by which GlcCer is flipped remains unclear. To investigate the role of GlcCer-binding partners in this process, we previously made cleavable, biotinylated, photoreactive GlcCer analogs in which the reactive nitrene was closely apposed to the GlcCer head group, while maintaining a C16-acyl chain. GlcCer-binding protein specificity was validated for both photoprobes. Using one probe, XLB, here we identified ATP-binding cassette (ABC) transporters ABCA3, ABCB4, and ABCB10 as unfractionated microsomal GlcCer-binding proteins in DU-145 prostate tumor cells. siRNA knockdown (KD) of these transporters differentially blocked GSL synthesis assessed in toto and via metabolic labeling. KD of ABCA3 reduced acid/neutral GSL levels, but increased those of LacCer, while KD of ABCB4 preferentially reduced neutral GSL levels, and KD of ABCB10 reduced levels of both neutral and acidic GSLs. Depletion of ABCA12, implicated in GlcCer transport, preferentially decreased neutral GSL levels, while ABCB1 KD preferentially reduced gangliosides, but increased neutral GSL Gb3. These results imply that multiple ABC transporters may provide distinct but overlapping GlcCer and LacCer pools within the Golgi lumen for anabolism of different GSL series by metabolic channeling. Differential ABC family member usage may fine-tune GSL biosynthesis depending on cell/tissue type. We conclude that ABC transporters provide a new tool for the regulation of GSL biosynthesis and serve as potential targets to reduce selected GSL species/subsets in diseases in which GSLs are dysregulated.

RAB10 Interacts with ABCB4 and Regulates Its Intracellular Traffic

ABCB4 (ATP-binding cassette subfamily B member 4) is an ABC transporter expressed at the canalicular membrane of hepatocytes where it ensures phosphatidylcholine secretion into bile. Genetic variations of ABCB4 are associated with several rare cholestatic diseases. The available treatments are not efficient for a significant proportion of patients with ABCB4-related diseases and liver transplantation is often required. The development of novel therapies requires a deep understanding of the molecular mechanisms regulating ABCB4 expression, intracellular traffic, and function. Using an immunoprecipitation approach combined with mass spectrometry analyses, we have identified the small GTPase RAB10 as a novel molecular partner of ABCB4. Our results indicate that the overexpression of wild type RAB10 or its dominant-active mutant significantly increases the amount of ABCB4 at the plasma membrane expression and its phosphatidylcholine floppase function. Contrariwise, RAB10 silencing induces the intracellular retention of ABCB4 and then indirectly diminishes its secretory function. Taken together, our findings suggest that RAB10 regulates the plasma membrane targeting of ABCB4 and consequently its capacity to mediate phosphatidylcholine secretion.

Effect of CFTR correctors on the traffic and the function of intracellularly retained ABCB4 variants

BACKGROUND & AIM

ABCB4 is expressed at the canalicular membrane of hepatocytes. This ATP-binding cassette (ABC) transporter is responsible for the secretion of phosphatidylcholine into bile canaliculi. Missense genetic variations of ABCB4 are correlated with several rare cholestatic liver diseases, the most severe being progressive familial intrahepatic cholestasis type 3 (PFIC3). In a repurposing strategy to correct intracellularly retained ABCB4 variants, we tested 16 compounds previously validated as cystic fibrosis transmembrane conductance regulator (CFTR) correctors.

METHODS

The maturation, intracellular localization and activity of intracellularly retained ABCB4 variants were analyzed in cell models after treatment with CFTR correctors. In addition, in silico molecular docking calculations were performed to test the potential interaction of CFTR correctors with ABCB4.

RESULTS

We observed that the correctors C10, C13, and C17, as well as the combinations of C3 + C18 and C4 + C18, allowed the rescue of maturation and canalicular localization of four distinct traffic-defective ABCB4 variants. However, such treatments did not permit a rescue of the phosphatidylcholine secretion activity of these defective variants and were also inhibitory of the activity of wild type ABCB4. In silico molecular docking analyses suggest that these CFTR correctors might directly interact with transmembrane domains and/or ATP-binding sites of the transporter.

CONCLUSION

Our results illustrate the uncoupling between the traffic and the activity of ABCB4 because the same molecules can rescue the traffic of defective variants while they inhibit the secretion activity of the transporter. We expect that this study will help to design new pharmacological tools with potential clinical interest.

Medically Important Alterations in Transport Function and Trafficking of ABCG2

Several polymorphisms and mutations in the human ABCG2 multidrug transporter result in reduced plasma membrane expression and/or diminished transport function. Since ABCG2 plays a pivotal role in uric acid clearance, its malfunction may lead to hyperuricemia and gout. On the other hand, ABCG2 residing in various barrier tissues is involved in the innate defense mechanisms of the body; thus, genetic alterations in ABCG2 may modify the absorption, distribution, excretion of potentially toxic endo- and exogenous substances. In turn, this can lead either to altered therapy responses or to drug-related toxic reactions. This paper reviews the various types of mutations and polymorphisms in ABCG2, as well as the ways how altered cellular processing, trafficking, and transport activity of the protein can contribute to phenotypic manifestations. In addition, the various methods used for the identification of the impairments in ABCG2 variants and the different approaches to correct these defects are overviewed.

Molecular Regulation of Canalicular ABC Transporters

The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients with these diseases requires a deep understanding of the biology of these transporters. In this review, we report the current knowledge regarding the regulation of canalicular ABC transporters' folding, trafficking, membrane stability and function, and we highlight the role of molecular partners in these regulating mechanisms.

The ABCG2/BCRP transporter and its variants - from structure to pathology

The ABCG2 protein has a key role in the transport of a wide range of structurally dissimilar endo- and xenobiotics in the human body, especially in the tissue barriers and the metabolizing or secreting organs. The human ABCG2 gene harbors a high number of polymorphisms and mutations, which may significantly modulate its expression and function. Recent high-resolution structural data, complemented with molecular dynamic simulations, may significantly help to understand intramolecular movements and substrate handling, as well as the effects of mutations on the membrane transporter function of ABCG2. As reviewed here, structural alterations may result not only in direct alterations in drug binding and transporter activity, but also in improper folding or problems in the carefully regulated process of trafficking, including vesicular transport, endocytosis, recycling, and degradation. Here, we also review the clinical importance of altered ABCG2 expression and function in general drug metabolism, cancer multidrug resistance, and impaired uric acid excretion, leading to gout.

Evaluation of a Novel Missense Mutation in ABCB4 Gene Causing Progressive Familial Intrahepatic Cholestasis Type 3

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a hepatic disorder occurring predominantly in childhood and is difficult to diagnose. PFIC3, being a rare autosomal recessive disease, is caused by genetic mutations in both alleles of ABCB4, resulting in the disruption of the bile secretory pathway. The identification of pathogenic effects resulting from different mutations in ABCB4 is the key to revealing the internal cause of disease. These mutations cause truncation, instability, misfolding, and impaired trafficking of the MDR3 protein. Here, we reported a girl, with a history of intrahepatic cholestasis and progressive liver cirrhosis, with an elevated gamma-glutamyltransferase level. Genetic screening via whole exome sequencing found a novel homozygous missense mutation ABCB4:c.1195G>C:p.V399L, and the patient was diagnosed with PFIC3. Various computational tools predicted the variant to be deleterious and evolutionary conserved. For functional characterization studies, plasmids, encoding ABCB4 wild-type and selected established mutant constructs, were expressed in human embryonic kidney (HEK-293T) and hepatocellular carcinoma (HepG2) cells. In vitro expression analysis observed a reduced expression of mutant protein compared to wild-type protein. We found that ABCB4 wild type was localized at the apical canalicular membrane, while mutant p.V399L showed intracellular retention. Intracellular mistrafficking proteins usually undergo proteasomal or lysosomal degradation. We found that after treatment with proteasomal inhibitor MG132 and lysosomal inhibitor bafilomycin A1, MDR3 expression of V399L was significantly increased. A decrease in MDR3 expression of mutant V399L protein may be a result of proteasomal or lysosomal degradation. Pharmacological modulator cyclosporin A and intracellular low temperature (30°C) treatment significantly rescued both the folding defect and the active maturation of the mutant protein. Our study identified a novel pathogenic mutation which expanded the mutational spectrum of the ABCB4 gene and may contribute to understanding the molecular basis of PFIC3. Therefore, genetic screening plays a conclusive role in the diagnosis of rare heterogenic disorders like PFIC3.

Disorders of FZ-CRD; insights towards FZ-CRD folding and therapeutic landscape

The ER is hub for protein folding. Proteins that harbor a Frizzled cysteine-rich domain (FZ-CRD) possess 10 conserved cysteine motifs held by a unique disulfide bridge pattern which attains a correct fold in the ER. Little is known about implications of disease-causing missense mutations within FZ-CRD families. Mutations in FZ-CRD of Frizzled class receptor 4 (FZD4) and Muscle, skeletal, receptor tyrosine kinase (MuSK) and Receptor tyrosine kinase-like orphan receptor 2 (ROR2) cause Familial Exudative Vitreoretinopathy (FEVR), Congenital Myasthenic Syndrome (CMS), and Robinow Syndrome (RS) respectively. We highlight reported pathogenic inherited missense mutations in FZ-CRD of FZD4, MuSK and ROR2 which misfold, and traffic abnormally in the ER, with ER-associated degradation (ERAD) as a common pathogenic mechanism for disease. Our review shows that all studied FZ-CRD mutants of RS, FEVR and CMS result in misfolded proteins and/or partially misfolded proteins with an ERAD fate, thus we coin them as "disorders of FZ-CRD". Abnormal trafficking was demonstrated in 17 of 29 mutants studied; 16 mutants were within and/or surrounding the FZ-CRD with two mutants distant from FZ-CRD. These ER-retained mutants were improperly N-glycosylated confirming ER-localization. FZD4 and MuSK mutants were tagged with polyubiquitin chains confirming targeting for proteasomal degradation. Investigating the cellular and molecular mechanisms of these mutations is important since misfolded protein and ER-targeted therapies are in development. The P344R-MuSK kinase mutant showed around 50% of its in-vitro autophosphorylation activity and P344R-MuSK increased two-fold on proteasome inhibition. M105T-FZD4, C204Y-FZD4, and P344R-MuSK mutants are thermosensitive and therefore, might benefit from extending the investigation to a larger number of chemical chaperones and/or proteasome inhibitors. Nonetheless, FZ-CRD ER-lipidation it less characterized in the literature and recent structural data sheds light on the importance of lipidation in protein glycosylation, proper folding, and ER trafficking. Current treatment strategies in-place for the conformational disease landscape is highlighted. From this review, we envision that disorders of FZ-CRD might be receptive to therapies that target FZ-CRD misfolding, regulation of fatty acids, and/or ER therapies; thus paving the way for a newly explored paradigm to treat different diseases with common defects.

ABCB4/MDR3 in health and disease – at the crossroads of biochemistry and medicine

Several ABC transporters of the human liver are responsible for the secretion of bile salts, lipids and cholesterol. Their interplay protects the biliary tree from the harsh detergent activity of bile salts. Among these transporters, ABCB4 is essential for the translocation of phosphatidylcholine (PC) lipids from the inner to the outer leaflet of the canalicular membrane of hepatocytes. ABCB4 deficiency can result in altered PC to bile salt ratios, which led to intrahepatic cholestasis of pregnancy, low phospholipid associated cholelithiasis, drug induced liver injury or even progressive familial intrahepatic cholestasis type 3. Although PC lipids only account for 30–40% of the lipids in the canalicular membrane, 95% of all phospholipids in bile are PC lipids. We discuss this discrepancy in the light of PC synthesis and bile salts favoring certain lipids. Nevertheless, the in vivo extraction of PC lipids from the outer leaflet of the canalicular membrane by bile salts should be considered as a separate step in bile formation. Therefore, methods to characterize disease causing ABCB4 mutations should be considered carefully, but such an analysis represents a crucial point in understanding the currently unknown transport mechanism of this ABC transporter.

Chaperoning Endoplasmic Reticulum-Associated Degradation (ERAD) and Protein Conformational Diseases

Misfolded proteins compromise cellular homeostasis. This is especially problematic in the endoplasmic reticulum (ER), which is a high-capacity protein-folding compartment and whose function requires stringent protein quality-control systems. Multiprotein complexes in the ER are able to identify, remove, ubiquitinate, and deliver misfolded proteins to the 26S proteasome for degradation in the cytosol, and these events are collectively termed ER-associated degradation, or ERAD. Several steps in the ERAD pathway are facilitated by molecular chaperone networks, and the importance of ERAD is highlighted by the fact that this pathway is linked to numerous protein conformational diseases. In this review, we discuss the factors that constitute the ERAD machinery and detail how each step in the pathway occurs. We then highlight the underlying pathophysiology of protein conformational diseases associated with ERAD.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Structural analogues of roscovitine rescue the intracellular traffic and the function of ER-retained ABCB4 variants in cell models

Adenosine triphosphate binding cassette transporter, subfamily B member 4 (ABCB4) is the transporter of phosphatidylcholine at the canalicular membrane of hepatocytes. ABCB4 deficiency, due to genetic variations, is responsible for progressive familial intrahepatic cholestasis type 3 (PFIC3) and other rare biliary diseases. Roscovitine is a molecule in clinical trial that was shown to correct the F508del variant of cystic fibrosis transmembrane conductance regulator (CFTR), another ABC transporter. In the present study, we hypothesized that roscovitine could act as a corrector of ABCB4 traffic-defective variants. Using HEK and HepG2 cells, we showed that roscovitine corrected the traffic and localisation at the plasma membrane of ABCB4-I541F, a prototypical intracellularly retained variant. However, roscovitine caused cytotoxicity, which urged us to synthesize non-toxic structural analogues. Roscovitine analogues were able to correct the intracellular traffic of ABCB4-I541F in HepG2 cells. Importantly, the phospholipid secretion activity of this variant was substantially rescued by three analogues (MRT2-235, MRT2-237 and MRT2-243) in HEK cells. We showed that these analogues also triggered the rescue of intracellular traffic and function of two other intracellularly retained ABCB4 variants, i.e. I490T and L556R. Our results indicate that structural analogues of roscovitine can rescue genetic variations altering the intracellular traffic of ABCB4 and should be considered as therapeutic means for severe biliary diseases caused by this class of variations.

Modulation of proteostasis and protein trafficking: a therapeutic avenue for misfolded G protein-coupled receptors causing disease in humans

Proteostasis refers to the process whereby the cell maintains in equilibrium the protein content of different compartments. This system consists of a highly interconnected network intended to efficiently regulate the synthesis, folding, trafficking, and degradation of newly synthesized proteins. Molecular chaperones are key players of the proteostasis network. These proteins assist in the assembly and folding processes of newly synthesized proteins in a concerted manner to achieve a three-dimensional structure compatible with export from the endoplasmic reticulum to other cell compartments. Pharmacologic interventions intended to modulate the proteostasis network and tackle the devastating effects of conformational diseases caused by protein misfolding are under development. These include small molecules called pharmacoperones, which are highly specific toward the target protein serving as a molecular framework to cause misfolded mutant proteins to fold and adopt a stable conformation suitable for passing the scrutiny of the quality control system and reach its correct location within the cell. Here, we review the main components of the proteostasis network and how pharmacoperones may be employed to correct misfolding of two G protein-coupled receptors, the vasopressin 2 receptor and the gonadotropin-releasing hormone receptor, whose mutations lead to X-linked nephrogenic diabetes insipidus and congenital hypogonadotropic hypogonadism in humans respectively.

Functional rescue of misfolding ABCA3 mutations by small molecular correctors

Adenosine triphosphate (ATP)-binding cassette subfamily A member 3 (ABCA3), a phospholipid transporter in lung lamellar bodies (LBs), is essential for the assembly of pulmonary surfactant and LB biogenesis. Mutations in the ABCA3 gene are an important genetic cause for respiratory distress syndrome in neonates and interstitial lung disease in children and adults, for which there is currently no cure. The aim of this study was to prove that disease causing misfolding ABCA3 mutations can be corrected in vitro and to investigate available options for correction. We stably expressed hemagglutinin (HA)-tagged wild-type ABCA3 or variants p.Q215K, p.M760R, p.A1046E, p.K1388N or p.G1421R in A549 cells and assessed correction by quantitation of ABCA3 processing products, their intracellular localization, resembling LB morphological integrity and analysis of functional transport activity. We showed that all mutant proteins except for M760R ABCA3 were rescued by the bithiazole correctors C13 and C17. These variants were also corrected by the chemical chaperone trimethylamine N-oxide and by low temperature. The identification of lead molecules C13 and C17 is an important step toward pharmacotherapy of ABCA3 misfolding-induced lung disease.

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.

Folding correction of ABC-transporter ABCB1 by pharmacological chaperones: a mechanistic concept

Point mutations of ATP‐binding cassette (ABC) proteins are a common cause of human diseases. Available crystal structures indicate a similarity in the architecture of several members of this protein family. Their molecular architecture makes these proteins vulnerable to mutation, when critical structural elements are affected. The latter preferentially involve the two transmembrane domain (TMD)/nucleotide‐binding domain (NBD) interfaces (transmission interfaces), formation of which requires engagement of coupling helices of intracellular loops with NBDs. Both, formation of the active sites and engagement of the coupling helices, are contingent on correct positioning of ICLs 2 and 4 and thus an important prerequisite for proper folding. Here, we show that active site compounds are capable of rescuing P‐glycoprotein (P‐gp) mutants ∆Y490 and ∆Y1133 in a concentration‐dependent manner. These trafficking deficient mutations are located at the transmission interface in pseudosymmetric position to each other. In addition, the ability of propafenone analogs to correct folding correlates with their ability to inhibit transport of model substrates. This finding indicates that folding correction and transport inhibition by propafenone analogs are brought about by binding to the active sites. Furthermore, this study demonstrates an asymmetry in folding correction with cis‐flupentixol, which reflects the asymmetric binding properties of this modulator to P‐gp. Our results suggest a mechanistic model for corrector action in a model ABC transporter based on insights into the molecular architecture of these transporters.

Current and future therapies for inherited cholestatic liver diseases

Familial intrahepatic cholestasis (FIC) comprises a group of rare cholestatic liver diseases associated with canalicular transport defects resulting predominantly from mutations in ATP8B1, ABCB11 and ABCB4. Phenotypes range from benign recurrent intrahepatic cholestasis (BRIC), associated with recurrent cholestatic attacks, to progressive FIC (PFIC). Patients often suffer from severe pruritus and eventually progressive cholestasis results in liver failure. Currently, first-line treatment includes ursodeoxycholic acid in patients with ABCB4 deficiency (PFIC3) and partial biliary diversion in patients with ATP8B1 or ABCB11 deficiency (PFIC1 and PFIC2). When treatment fails, liver transplantation is needed which is associated with complications like rejection, post-transplant hepatic steatosis and recurrence of disease. Therefore, the need for more and better therapies for this group of chronic diseases remains. Here, we discuss new symptomatic treatment options like total biliary diversion, pharmacological diversion of bile acids and hepatocyte transplantation. Furthermore, we focus on emerging mutation-targeted therapeutic strategies, providing an outlook for future personalized treatment for inherited cholestatic liver diseases.

Functional defect of variants in the adenosine triphosphate-binding sites of ABCB4 and their rescue by the cystic fibrosis transmembrane conductance regulator potentiator, ivacaftor (VX-770)

ABCB4 (MDR3) is an adenosine triphosphate (ATP)-binding cassette (ABC) transporter expressed at the canalicular membrane of hepatocytes, where it mediates phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene are responsible for several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ABCB4 missense variations that reside in the highly conserved motifs of ABC transporters, involved in ATP binding. Five disease-causing variations in these motifs have been identified in ABCB4 (G535D, G536R, S1076C, S1176L, and G1178S), three of which are homologous to the gating mutations of cystic fibrosis transmembrane conductance regulator (CFTR or ABCC7; i.e., G551D, S1251N, and G1349D), that were previously shown to be function defective and corrected by ivacaftor (VX-770; Kalydeco), a clinically approved CFTR potentiator. Three-dimensional structural modeling predicted that all five ABCB4 variants would disrupt critical interactions in the binding of ATP and thereby impair ATP-induced nucleotide-binding domain dimerization and ABCB4 function. This prediction was confirmed by expression in cell models, which showed that the ABCB4 mutants were normally processed and targeted to the plasma membrane, whereas their PC secretion activity was dramatically decreased. As also hypothesized on the basis of molecular modeling, PC secretion activity of the mutants was rescued by the CFTR potentiator, ivacaftor (VX-770).

CONCLUSION

Disease-causing variations in the ATP-binding sites of ABCB4 cause defects in PC secretion, which can be rescued by ivacaftor. These results provide the first experimental evidence that ivacaftor is a potential therapy for selected patients who harbor mutations in the ATP-binding sites of ABCB4. (Hepatology 2017;65:560-570).

Functional characterization of ABCB4 mutations found in progressive familial intrahepatic cholestasis type 3

Multidrug resistance 3 (MDR3), encoded by the ATP-binding cassette, subfamily B, member 4 gene (ABCB4), localizes to the canalicular membrane of hepatocytes and translocates phosphatidylcholine from the inner leaflet to the outer leaflet of the canalicular membrane. Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a rare hepatic disease caused by genetic mutations of ABCB4. In this study, we characterized 8 ABCB4 mutations found in PFIC3 patients, using in vitro molecular assays. First, we examined the transport activity of each mutant by measuring its ATPase activity using paclitaxel or phosphatidylcholine. Then, the pathogenic mechanisms by which these mutations affect MDR3 were examined through immunoblotting, cell surface biotinylation, and immunofluorescence. As a result, three ABCB4 mutants showed significantly reduced transport activity. Among these mutants, one mutation A364V, located in intracellular domains, markedly decreased MDR3 expression on the plasma membrane, while the others did not affect the expression. The expression of MDR3 on the plasma membrane and transport activity of A364V was rescued by a pharmacological chaperone, cyclosporin A. Our study provides the molecular mechanisms of ABCB4 mutations and may contribute to the understanding of PFIC3 pathogenesis and the development of a mutation-specific targeted treatment for PFIC3.

A functional classification of ABCB4 variations causing progressive familial intrahepatic cholestasis type 3

Progressive familial intrahepatic cholestasis type 3 is caused by biallelic variations of ABCB4, most often (≥70%) missense. In this study, we examined the effects of 12 missense variations identified in progressive familial intrahepatic cholestasis type 3 patients. We classified these variations on the basis of the defects thus identified and explored potential rescue of trafficking-defective mutants by pharmacological means. Variations were reproduced in the ABCB4 complementary DNA and the mutants, thus obtained, expressed in HepG2 and HEK293 cells. Three mutants were either fully (I541F and L556R) or largely (Q855L) retained in the endoplasmic reticulum, in an immature form. Rescue of the defect, i.e., increase in the mature form at the bile canaliculi, was obtained by cell treatments with cyclosporin A or C and, to a lesser extent, B, D, or H. Five mutations with little or no effect on ABCB4 expression at the bile canaliculi caused a decrease (F357L, T775M, and G954S) or almost absence (S346I and P726L) of phosphatidylcholine secretion. Two mutants (T424A and N510S) were normally processed and expressed at the bile canaliculi, but their stability was reduced. We found no defect of the T175A mutant or of R652G, previously described as a polymorphism. In patients, the most severe phenotypes appreciated by the duration of transplant-free survival were caused by ABCB4 variants that were markedly retained in the endoplasmic reticulum and expressed in a homozygous status.

CONCLUSION

ABCB4 variations can be classified as follows: nonsense variations (I) and, on the basis of current findings, missense variations that primarily affect the maturation (II), activity (III), or stability (IV) of the protein or have no detectable effect (V); this classification provides a strong basis for the development of genotype-based therapies.

Recent advances in understanding and managing cholestasis

Cholestatic liver diseases are hereditary or acquired disorders with impaired hepatic excretion and enterohepatic circulation of bile acids and other cholephiles. The distinct pathological mechanisms, particularly for the acquired forms of cholestasis, are not fully revealed, but advances in the understanding of the molecular mechanisms and identification of key regulatory mechanisms of the enterohepatic circulation of bile acids have unraveled common and central mechanisms, which can be pharmacologically targeted. This overview focuses on the central roles of farnesoid X receptor, fibroblast growth factor 19, and apical sodium-dependent bile acid transporter for the enterohepatic circulation of bile acids and their potential as new drug targets for the treatment of cholestatic liver disease.

Functional Rescue of Trafficking-Impaired ABCB4 Mutants by Chemical Chaperones

Multidrug resistance protein 3 (MDR3, ABCB4) is a hepatocellular membrane protein that mediates biliary secretion of phosphatidylcholine. Null mutations in ABCB4 gene give rise to severe early-onset cholestatic liver disease. We have previously shown that the disease-associated mutations p.G68R, p.G228R, p.D459H, and p.A934T resulted in retention of ABCB4 in the endoplasmic reticulum, thus failing to target the plasma membrane. In the present study, we tested the ability of two compounds with chaperone-like activity, 4-phenylbutyrate and curcumin, to rescue these ABCB4 mutants by assessing their effects on subcellular localization, protein maturation, and phospholipid efflux capability. Incubation of transfected cells at a reduced temperature (30°C) or exposure to pharmacological doses of either 4-PBA or curcumin restored cell surface expression of mutants G228R and A934T. The delivery of these mutants to the plasma membrane was accompanied by a switch in the ratio of mature to inmature protein forms, leading to a predominant expression of the mature protein. This effect was due to an improvement in the maturation rate and not to the stabilization of the mature forms. Both mutants were also functionally rescued, displaying bile salt-dependent phospholipid efflux activity after addition of 4-PBA or curcumin. Drug-induced rescue was mutant specific, given neither 4-PBA nor curcumin had an effect on the ABCB4 mutants G68R and A934T. Collectively, these data indicate that the functionality of selected trafficking-defective ABCB4 mutants can be recovered by chemical chaperones through restoration of membrane localization, suggesting a potential treatment for patients carrying such mutations.

Heterozygous ABCB4 mutations in children with cholestatic liver disease

BACKGROUND & AIMS

Monoallelic defects in ABCB4, which encodes the canalicular floppase for phosphatidylcholine MDR3, have been encountered in association with a variety of hepatobiliary disorders, particularly in adult subjects. In this study, we examined the presence of heterozygous ABCB4 variants in a cohort of children with chronic cholestasis and assessed the pathogenicity of the missense changes identified.

METHODS

Sixty-seven children with chronic liver dysfunction were studied by the sequencing of ABCB4 and multiplex ligation-dependent probe amplification analysis. The molecular defects arising from missense variants were analysed in MDCK-II and AD-293 cells.

RESULTS

Defects in a single allele of ABCB4 were identified in nine subjects. They included one small insertion (p.I1242Nfs), one nonsense mutation (p.R144X) and six missense changes (p.T175A, p.G228R, p.A250T, p.S320F, p.P352L and p.A934T). In four children, these defects in ABCB4 co-existed with various medical conditions. In vitro phenotyping of the six missense variants revealed that four (T175A, G228R, S320F and A934T) led to reduced MDR3 protein levels. Two mutations (G228R and A934T) resulted in trapping of the protein in the endoplasmic reticulum. Phosphatidylcholine efflux activity was decreased to 56-18% of reference levels for MDR3 mutants T175A, A250T and S320F. The G228R, P352L and A934T mutants were found to be non-functional.

CONCLUSIONS

These results illustrate the varying effects of ABCB4 missense mutations and suggest that even a modest reduction in MDR3 activity may contribute or predispose to the onset of cholestatic liver disease in the paediatric age.

A PDZ-Like Motif in the Biliary Transporter ABCB4 Interacts with the Scaffold Protein EBP50 and Regulates ABCB4 Cell Surface Expression

ABCB4/MDR3, a member of the ABC superfamily, is an ATP-dependent phosphatidylcholine translocator expressed at the canalicular membrane of hepatocytes. Defects in the ABCB4 gene are associated with rare biliary diseases. It is essential to understand the mechanisms of its canalicular membrane expression in particular for the development of new therapies. The stability of several ABC transporters is regulated through their binding to PDZ (PSD95/DglA/ZO-1) domain-containing proteins. ABCB4 protein ends by the sequence glutamine-asparagine-leucine (QNL), which shows some similarity to PDZ-binding motifs. The aim of our study was to assess the potential role of the QNL motif on the surface expression of ABCB4 and to determine if PDZ domain-containing proteins are involved. We found that truncation of the QNL motif decreased the stability of ABCB4 in HepG2-transfected cells. The deleted mutant ABCB4-ΔQNL also displayed accelerated endocytosis. EBP50, a PDZ protein highly expressed in the liver, strongly colocalized and coimmunoprecipitated with ABCB4, and this interaction required the QNL motif. Down-regulation of EBP50 by siRNA or by expression of an EBP50 dominant-negative mutant caused a significant decrease in the level of ABCB4 protein expression, and in the amount of ABCB4 localized at the canalicular membrane. Interaction of ABCB4 with EBP50 through its PDZ-like motif plays a critical role in the regulation of ABCB4 expression and stability at the canalicular plasma membrane.

ABCB4: Insights from pathobiology into therapy

Adenosine triphosphate (ATP)-binding cassette, sub-family B, member 4 (ABCB4), also called multidrug resistance 3 (MDR3), is a member of the ATP-binding cassette transporter superfamily, which is localized at the canalicular membrane of hepatocytes, and mediates the translocation of phosphatidylcholine into bile. Phosphatidylcholine secretion is crucial to ensure solubilization of cholesterol into mixed micelles and to prevent bile acid toxicity towards hepatobiliary epithelia. Genetic defects of ABCB4 may cause progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare autosomic recessive disease occurring early in childhood that may be lethal in the absence of liver transplantation, and other cholestatic or cholelithiasic diseases in heterozygous adults. Development of therapies for these conditions requires understanding of the biology of this transporter and how gene variations may cause disease. This review focuses on our current knowledge on the regulation of ABCB4 expression, trafficking and function, and presents recent advances in fundamental research with promising therapeutic perspectives.

Phosphorylation of ABCB4 impacts its function: insights from disease-causing mutations

The ABCB4 transporter mediates phosphatidylcholine (PC) secretion at the canalicular membrane of hepatocytes and its genetic defects cause biliary diseases. Whereas ABCB4 shares high sequence identity with the multidrug transporter, ABCB1, its N-terminal domain is poorly conserved, leading us to hypothesize a functional specificity of this domain. A database of ABCB4 genotyping in a large series of patients was screened for variations altering residues of the N-terminal domain. Identified variants were then expressed in cell models to investigate their biological consequences. Two missense variations, T34M and R47G, were identified in patients with low-phospholipid-associated cholelithiasis or intrahepatic cholestasis of pregnancy. The T34M and R47G mutated proteins showed no or minor defect, respectively, in maturation and targeting to the apical membrane, in polarized Madin-Darby Canine Kidney and HepG2 cells, whereas their stability was similar to that of wild-type (WT) ABCB4. By contrast, the PC secretion activity of both mutants was markedly decreased. In silico analysis indicated that the identified variants were likely to affect ABCB4 phosphorylation. Mass spectrometry analyses confirmed that the N-terminal domain of WT ABCB4 could undergo phosphorylation in vitro and revealed that the T34M and R47G mutations impaired such phosphorylation. ABCB4-mediated PC secretion was also increased by pharmacological activation of protein kinases A or C and decreased by inhibition of these kinases. Furthermore, secretion activity of the T34M and R47G mutants was less responsive than that of WT ABCB4 to protein kinase modulators.

CONCLUSION

We identified disease-associated variants of ABCB4 involved in the phosphorylation of its N-terminal domain and leading to decreased PC secretion. Our results also indicate that ABCB4 activity is regulated by phosphorylation, in particular, of N-terminal residues.

Molecular mechanisms for biliary phospholipid and drug efflux mediated by ABCB4 and bile salts

On the canalicular membranes of hepatocytes, several ABC transporters are responsible for the secretion of bile lipids. Among them, ABCB4, also called MDR3, is essential for the secretion of phospholipids from hepatocytes into bile. The biliary phospholipids are associated with bile salts and cholesterol in mixed micelles, thereby reducing the detergent activity and cytotoxicity of bile salts and preventing cholesterol crystallization. Mutations in the ABCB4 gene result in progressive familial intrahepatic cholestasis type 3, intrahepatic cholestasis of pregnancy, low-phospholipid-associated cholelithiasis, primary biliary cirrhosis, and cholangiocarcinoma. In vivo and cell culture studies have demonstrated that the secretion of biliary phospholipids depends on both ABCB4 expression and bile salts. In the presence of bile salts, ABCB4 located in nonraft membranes mediates the efflux of phospholipids, preferentially phosphatidylcholine. Despite high homology with ABCB1, ABCB4 expression cannot confer multidrug resistance. This review summarizes our current understanding of ABCB4 functions and physiological relevance, and discusses the molecular mechanism for the ABCB4-mediated efflux of phospholipids.

Pharmacological chaperoning: a primer on mechanism and pharmacology

Approximately forty percent of diseases are attributable to protein misfolding, including those for which genetic mutation produces misfolding mutants. Intriguingly, many of these mutants are not terminally misfolded since native-like folding, and subsequent trafficking to functional locations, can be induced by target-specific, small molecules variably termed pharmacological chaperones, pharmacoperones, or pharmacochaperones (PCs). PC targets include enzymes, receptors, transporters, and ion channels, revealing the breadth of proteins that can be engaged by ligand-assisted folding. The purpose of this review is to provide an integrated primer of the diverse mechanisms and pharmacology of PCs. In this regard, we examine the structural mechanisms that underlie PC rescue of misfolding mutants, including the ability of PCs to act as surrogates for defective intramolecular interactions and, at the intermolecular level, overcome oligomerization deficiencies and dominant negative effects, as well as influence the subunit stoichiometry of heteropentameric receptors. Not surprisingly, PC-mediated structural correction of misfolding mutants normalizes interactions with molecular chaperones that participate in protein quality control and forward-trafficking. A variety of small molecules have proven to be efficacious PCs and the advantages and disadvantages of employing orthostatic antagonists, active-site inhibitors, orthostatic agonists, and allosteric modulator PCs are considered. Also examined is the possibility that several therapeutic agents may have unrecognized activity as PCs, and this chaperoning activity may mediate/contribute to therapeutic action and/or account for adverse effects. Lastly, we explore evidence that pharmacological chaperoning exploits intrinsic ligand-assisted folding mechanisms. Given the widespread applicability of PC rescue of mutants associated with protein folding disorders, both in vitro and in vivo, the therapeutic potential of PCs is vast. This is most evident in the treatment of lysosomal storage disorders, cystic fibrosis, and nephrogenic diabetes insipidus, for which proof of principle in humans has been demonstrated.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Functional Characterization of ABCB4 Mutations Found in Low Phospholipid-Associated Cholelithiasis (LPAC)

Multidrug resistance 3 (MDR3) is expressed on the canalicular membrane of the hepatocytes and plays an important role in protecting the liver from bile acids. Altered ABCB4 gene expression can lead to a rare hepatic disease, low phospholipid-associated cholelithiasis (LPAC). In this study, we characterized 3 ABCB4 mutations in LPAC patients using various in vitro assay systems. We first measured the ability of each mutant to transport paclitaxel and then the mechanisms by which these mutations might change MDR3 transport activity were determined using immunoblotting, cell surface protein biotinylation, and immunofluorescence. Through a membrane vesicular transport assay, we observed that the uptake of paclitaxel was significantly reduced in membrane vesicles expressing 2 ABCB4 mutations, F165I and S320F. Both mutants showed significantly decreased total and cell surface MDR3 expression. These data suggest two missense mutations of ABCB4 may alter function of MDR3 and ultimately can be determined as LPAC-causing mutations.

Defining the blanks--pharmacochaperoning of SLC6 transporters and ABC transporters

SLC6 family members and ABC transporters represent two extremes: SLC6 transporters are confined to the membrane proper and only expose small segments to the hydrophilic milieu. In ABC transporters the hydrophobic core is connected to a large intracellular (eponymous) ATP binding domain that is comprised of two discontiguous repeats. Accordingly, their folding problem is fundamentally different. This can be gauged from mutations that impair the folding of the encoded protein and give rise to clinically relevant disease phenotypes: in SLC6 transporters, these cluster at the protein–lipid interface on the membrane exposed surface. Mutations in ABC-transporters map to the interface between nucleotide binding domains and the coupling helices, which provide the connection to the hydrophobic core. Folding of these mutated ABC-transporters can be corrected with ligands/substrates that bind to the hydrophobic core. This highlights a pivotal role of the coupling helices in the folding trajectory. In contrast, insights into pharmacochaperoning of SLC6 transporters are limited to monoamine transporters – in particular the serotonin transporter (SERT) – because of their rich pharmacology. Only ligands that stabilize the inward facing conformation act as effective pharmacochaperones. This indicates that the folding trajectory of SERT proceeds via the inward facing conformation. Mutations that impair folding of SLC6 family members can be transmitted as dominant or recessive alleles. The dominant phenotype of the mutation can be rationalized, because SLC6 transporters are exported in oligomeric form from the endoplasmic reticulum (ER). Recessive transmission requires shielding of the unaffected gene product from the mutated transporter in the ER. This can be accounted for by a chaperone-COPII (coatomer protein II) exchange model, where proteinaceous ER-resident chaperones engage various intermediates prior to formation of the oligomeric state and subsequent export from the ER. It is likely that the action of pharmacochaperones is contingent on and modulated by these chaperones.

Mutations in intracellular loops 1 and 3 lead to misfolding of human P-glycoprotein (ABCB1) that can be rescued by cyclosporine A, which reduces its association with chaperone Hsp70

P-glycoprotein (P-gp) is an ATP binding cassette transporter that effluxes a variety of structurally diverse compounds including anticancer drugs. Computational models of human P-gp in the apo- and nucleotide-bound conformation show that the adenine group of ATP forms hydrogen bonds with the conserved Asp-164 and Asp-805 in intracellular loops 1 and 3, respectively, which are located at the interface between the nucleotide binding domains and transmembrane domains. We investigated the role of Asp-164 and Asp-805 residues by substituting them with cysteine in a cysteine-less background. It was observed that the D164C/D805C mutant, when expressed in HeLa cells, led to misprocessing of P-gp, which thus failed to transport the drug substrates. The misfolded protein could be rescued to the cell surface by growing the cells at a lower temperature (27 °C) or by treatment with substrates (cyclosporine A, FK506), modulators (tariquidar), or small corrector molecules. We also show that short term (4-6 h) treatment with 15 μM cyclosporine A or FK506 rescues the pre-formed immature protein trapped in the endoplasmic reticulum in an immunophilin-independent pathway. The intracellularly trapped misprocessed protein associates more with chaperone Hsp70, and the treatment with cyclosporine A reduces the association of mutant P-gp, thus allowing it to be trafficked to the cell surface. The function of rescued cell surface mutant P-gp is similar to that of wild-type protein. These data demonstrate that the Asp-164 and Asp-805 residues are not important for ATP binding, as proposed earlier, but are critical for proper folding and maturation of a functional transporter.

Inside job: ligand-receptor pharmacology beneath the plasma membrane

Most drugs acting on the cell surface receptors are membrane permeable and thus able to engage their target proteins in different subcellular compartments. However, these drugs' effects on cell surface receptors have historically been studied on the plasma membrane alone. Increasing evidence suggests that small molecules may also modulate their targeted receptors through membrane trafficking or organelle-localized signaling inside the cell. These additional modes of interaction have been reported for functionally diverse ligands of GPCRs, ion channels, and transporters. Such intracellular drug-target engagements affect cell surface expression. Concurrent intracellular and cell surface signaling may also increase the complexity and therapeutic opportunities of small molecule modulation. Here we discuss examples of ligand-receptor interactions that are present in both intra- and extracellular sites, and the potential therapeutic opportunities presented by this phenomenon.

P-glycoprotein is fully active after multiple tryptophan substitutions

P-glycoprotein (Pgp) is an important contributor to multidrug resistance of cancer. Pgp contains eleven native tryptophans (Trps) that are highly conserved among orthologs. We replaced each Trp by a conservative substitution to determine which Trps are important for function. Individual Trp mutants W44R, W208Y, W132Y, W704Y and W851Y, situated at the membrane surface, revealed significantly reduced Pgp induced drug resistance against one or more fungicides and/or reduced mating efficiencies in Saccharomyces cerevisiae. W158F and W799F, located in the intracellular coupling helices, abolished mating but retained resistance against most drugs. In contrast, W228F and W311Y, located within the membrane, W694L, at the cytoplasmic membrane interface, and W1104Y in NBD2 retained high levels of drug resistance and mating efficiencies similar to wild-type Pgp. Those were combined into pair (W228F/W311Y and W694L/W1104Y) and quadruple (W228F/W311Y/W694L/W1104Y) mutants that were fully active in yeast, and could be purified to homogeneity. Purified pair and quad mutants exhibited drug-stimulated ATPase activity with binding affinities very similar to wild-type Pgp. The combined mutations reduced Trp fluorescence by 35%, but drug induced fluorescence quenching was unchanged from wild-type Pgp suggesting that several membrane-bound Trps are sensitive to drug binding. Overall, we conclude that Trps at the membrane surface are critical for maintaining the integrity of the drug binding sites, while Trps in the coupling helices are important for proper interdomain communication. We also demonstrate that functional single Trp mutants can be combined to form a fully active Pgp that maintains drug polyspecificity, while significantly reducing intrinsic fluorescence.

Reversing ABCB1-mediated multi-drug resistance from within cells using translocating immune conjugates

Multi-drug resistance (MDR) is still a major cause of the eventual failure of chemotherapy in cancer treatment. Different approaches have been taken to render these cells drug sensitive. Here, we attempted sensitizing drug-resistant cells from within, using a translocating immune conjugate approach. To that effect, a monoclonal antibody, C219, directed against the intracellular ATP-binding site of the membrane-anchored MDR transporter ABCB1 [P-glycoprotein (P-gp), MDR1], was conjugated to human immunodeficiency virus [HIV(37-72)Tat] translocator peptide through a disulfide bridge. Fluorescence-labelled IgG-Tat conjugates accumulated in drug resistant Chinese hamster ovary (CHO) cells within less than 20 min. Preincubation with C219-S-S-(37-72)Tat conjugate augmented calcein accumulation in drug-resistant CHO and mouse lymphoma cells, indicating reduction in ABCB1 transporter activity. A thioether conjugate C219-S-(37-72)Tat was ineffective, as were disulfide and thioether conjugates of an irrelevant antibody. Furthermore, in the presence of C219-S-S-(37-72)Tat, drug resistant cells were sensitized to colchicine and doxorubicin. Taken together, these findings demonstrate, as proof of principle, a novel approach for the reversal of MDR from within cells, by delivery of translocating immune conjugates as sensitizing agents towards chemotherapy.

Phenylalanine hydroxylase misfolding and pharmacological chaperones

Phenylketonuria (PKU) is a loss-of-function inborn error of metabolism. As many other inherited diseases the main pathologic mechanism in PKU is an enhanced tendency of the mutant phenylalanine hydroxylase (PAH) to misfold and undergo ubiquitin-dependent degradation. Recent alternative approaches with therapeutic potential for PKU aim at correcting the PAH misfolding, and in this respect pharmacological chaperones are the focus of increasing interest. These compounds, which often resemble the natural ligands and show mild competitive inhibition, can rescue the misfolded proteins by stimulating their renaturation in vivo. For PKU, a few studies have proven the stabilization of PKU-mutants in vitro, in cells, and in mice by pharmacological chaperones, which have been found either by using the tetrahydrobiopterin (BH(4)) cofactor as query structure for shape-focused virtual screening or by high-throughput screening of small compound libraries. Both approaches have revealed a number of compounds, most of which bind at the iron-binding site, competitively with respect to BH(4). Furthermore, PAH shares a number of ligands, such as BH(4), amino acid substrates and inhibitors, with the other aromatic amino acid hydroxylases: the neuronal/neuroendocrine enzymes tyrosine hydroxylase (TH) and the tryptophan hydroxylases (TPHs). Recent results indicate that the PAH-targeted pharmacological chaperones should also be tested on TH and the TPHs, and eventually be derivatized to avoid unwanted interactions with these other enzymes. After derivatization and validation in animal models, the PAH-chaperoning compounds represent novel possibilities in the treatment of PKU.