Rescue of Folding Defects in ABC Transporters Using Pharmacological Chaperones

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.

Ligand-based virtual-screening identified a novel CFTR ligand which improves the defective cell surface expression of misfolded ABC transporters

Cystic fibrosis (CF) is a monogenetic disease caused by the mutation of CFTR, a cAMP-regulated Cl- channel expressing at the apical plasma membrane (PM) of epithelia. ∆F508-CFTR, the most common mutant in CF, fails to reach the PM due to its misfolding and premature degradation at the endoplasmic reticulum (ER). Recently, CFTR modulators have been developed to correct CFTR abnormalities, with some being used as therapeutic agents for CF treatment. One notable example is Trikafta, a triple combination of CFTR modulators (TEZ/ELX/IVA), which significantly enhances the functionality of ΔF508-CFTR on the PM. However, there's room for improvement in its therapeutic effectiveness since TEZ/ELX/IVA doesn't fully stabilize ΔF508-CFTR on the PM. To discover new CFTR modulators, we conducted a virtual screening of approximately 4.3 million compounds based on the chemical structures of existing CFTR modulators. This effort led us to identify a novel CFTR ligand named FR3. Unlike clinically available CFTR modulators, FR3 appears to operate through a distinct mechanism of action. FR3 enhances the functional expression of ΔF508-CFTR on the apical PM in airway epithelial cell lines by stabilizing NBD1. Notably, FR3 counteracted the degradation of mature ΔF508-CFTR, which still occurs despite the presence of TEZ/ELX/IVA. Furthermore, FR3 corrected the defective PM expression of a misfolded ABCB1 mutant. Therefore, FR3 may be a potential lead compound for addressing diseases resulting from the misfolding of ABC transporters.

Molecular Mechanisms and Determinants of Innovative Correction Approaches in Coagulation Factor Deficiencies

Molecular strategies tailored to promote/correct the expression and/or processing of defective coagulation factors would represent innovative therapeutic approaches beyond standard substitutive therapy. Here, we focus on the molecular mechanisms and determinants underlying innovative approaches acting at DNA, mRNA and protein levels in inherited coagulation factor deficiencies, and in particular on: (i) gene editing approaches, which have permitted intervention at the DNA level through the specific recognition, cleavage, repair/correction or activation of target sequences, even in mutated gene contexts; (ii) the rescue of altered pre-mRNA processing through the engineering of key spliceosome components able to promote correct exon recognition and, in turn, the synthesis and secretion of functional factors, as well as the effects on the splicing of missense changes affecting exonic splicing elements; this section includes antisense oligonucleotide- or siRNA-mediated approaches to down-regulate target genes; (iii) the rescue of protein synthesis/function through the induction of ribosome readthrough targeting nonsense variants or the correction of folding defects caused by amino acid substitutions. Overall, these approaches have shown the ability to rescue the expression and/or function of potentially therapeutic levels of coagulation factors in different disease models, thus supporting further studies in the future aimed at evaluating the clinical translatability of these new strategies.

Stability of the ABCD1 Protein with a Missense Mutation: A Novel Approach to Finding Therapeutic Compounds for X-Linked Adrenoleukodystrophy

Mutations in the ABCD1 gene that encodes peroxisomal ABCD1 protein cause X-linked adrenoleukodystrophy (X-ALD), a rare neurodegenerative disorder. More than 70% of the patient fibroblasts with this missense mutation display either a lack or reduction of the ABCD1 protein because of posttranslational degradation. In this study, we analyzed the stability of the missense mutant ABCD1 proteins (p.A616T, p.R617H, and p.R660W) in X-ALD fibroblasts and found that the mutant ABCD1 protein p.A616T has the capacity to recover its function by incubating at low temperature. In the case of such a mutation, chemical compounds that stabilize mutant ABCD1 proteins could be therapeutic candidates. Here, we prepared CHO cell lines stably expressing ABCD1 proteins with a missense mutation in fusion with green fluorescent protein (GFP) at the C-terminal. The stability of each mutant ABCD1-GFP in CHO cells was similar to the corresponding mutant ABCD1 protein in X-ALD fibroblasts. Furthermore, it is of interest that the GFP at the C-terminal was degraded together with the mutant ABCD1 protein. These findings prompted us to use CHO cells expressing mutant ABCD1-GFP for a screening of chemical compounds that can stabilize the mutant ABCD1 protein. We established a fluorescence-based assay method for the screening of chemical libraries in an effort to find compounds that stabilize mutant ABCD1 proteins. The work presented here provides a novel approach to finding therapeutic compounds for X-ALD patients with missense mutations.

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.

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.

Non-native Conformers of Cystic Fibrosis Transmembrane Conductance Regulator NBD1 Are Recognized by Hsp27 and Conjugated to SUMO-2 for Degradation

A newly identified pathway for selective degradation of the common mutant of the cystic fibrosis transmembrane conductance regulator (CFTR), F508del, is initiated by binding of the small heat shock protein, Hsp27. Hsp27 collaborates with Ubc9, the E2 enzyme for protein SUMOylation, to selectively degrade F508del CFTR via the SUMO-targeted ubiquitin E3 ligase, RNF4 (RING finger protein 4) (1). Here, we ask what properties of CFTR are sensed by the Hsp27-Ubc9 pathway by examining the ability of NBD1 (locus of the F508del mutation) to mimic the disposal of full-length (FL) CFTR. Similar to FL CFTR, F508del NBD1 expression was reduced 50-60% by Hsp27; it interacted preferentially with the mutant and was modified primarily by SUMO-2. Mutation of the consensus SUMOylation site, Lys(447), obviated Hsp27-mediated F508del NBD1 SUMOylation and degradation. As for FL CFTR and NBD1 in vivo, SUMO modification using purified components in vitro was greater for F508del NBD1 versus WT and for the SUMO-2 paralog. Several findings indicated that Hsp27-Ubc9 targets the SUMOylation of a transitional, non-native conformation of F508del NBD1: (a) its modification decreased as [ATP] increased, reflecting stabilization of the nucleotide-binding domain by ligand binding; (b) a temperature-induced increase in intrinsic fluorescence, which reflects formation of a transitional NBD1 conformation, was followed by its SUMO modification; and (c) introduction of solubilizing or revertant mutations to stabilize F508del NBD1 reduced its SUMO modification. These findings indicate that the Hsp27-Ubc9 pathway recognizes a non-native conformation of mutant NBD1, which leads to its SUMO-2 conjugation and degradation by the ubiquitin-proteasome system.

Functional polymorphisms of the ABCG2 gene are associated with gout disease in the Chinese Han male population

Background

Gout is a common type of arthritis that is characterized by hyperuricemia, tophi and joint inflammation. Genetic variations in the ABCG2 gene have been reported to influence serum uric acid levels and to participate in the pathogenesis of gout, but no further data have been reported in the Han Chinese population.

Methods

Peripheral blood DNA was isolated from 352 male patients with gout and 350 gout-free normal male controls. High-resolution melting analysis and Sanger sequencing were performed to identify the genetic polymorphisms V12M, Q141K and Q126X in the ABCG2 gene. Genotype and haplotype analyses were utilized to determine the disease odds ratios (ORs). A prediction model for gout risk using ABCG2 protein function was established based on the genotype combination of Q126X and Q141K.

Results

For Q141K, the A allele frequency was 49.6% in the gout patients and 30.9% in the controls (OR 2.20, 95% confidence interval (CI): 1.77–2.74, p = 8.99 × 10⁻¹³). Regarding Q126X, the T allele frequency was 4.7% in the gout patients and 1.7% in the controls (OR 2.91, 95% CI: 1.49–5.68, p = 1.57 × 10⁻³). The A allele frequency for V12M was lower (18.3%) in the gout patients than in the controls (29%) (OR 0.55, 95% CI 0.43–0.71, p = 2.55 × 10⁻⁶). In the order of V12M, Q126X and Q141K, the GCA and GTC haplotypes indicated increased disease risk (OR = 2.30 and 2.71, respectively). Patients with mild to severe ABCG2 dysfunction accounted for 78.4% of gout cases.

Conclusion

The ABCG2 126X and 141K alleles are associated with an increased risk of gout, whereas 12M has a protective effect on gout susceptibility in the Han Chinese population. ABCG2 dysfunction can be used to evaluate gout risk.

Using pharmacological chaperones to restore proteostasis

Normal organismal physiology depends on the maintenance of proteostasis in each cellular compartment to achieve a delicate balance between protein synthesis, folding, trafficking, and degradation while minimizing misfolding and aggregation. Defective proteostasis leads to numerous protein misfolding diseases. Pharmacological chaperones are cell-permeant small molecules that promote the proper folding and trafficking of a protein via direct binding to that protein. They stabilize their target protein in a protein-pharmacological chaperone state, increasing the natively folded protein population that can effectively engage trafficking machinery for transport to the final destination for function. Here, as regards the application of pharmacological chaperones, we focus on their capability to promote the folding and trafficking of lysosomal enzymes, G protein coupled receptors (GPCRs), and ion channels, each of which is presently an important drug target. Pharmacological chaperones hold great promise as potential therapeutics to ameliorate a variety of protein misfolding diseases.

Transitioning pharmacoperones to therapeutic use: in vivo proof-of-principle and design of high throughput screens

A pharmacoperone (from "pharmacological chaperone") is a small molecule that enters cells and serves as molecular scaffolding in order to cause otherwise-misfolded mutant proteins to fold and route correctly within the cell. Pharmacoperones have broad therapeutic applicability since a large number of diseases have their genesis in the misfolding of proteins and resultant misrouting within the cell. Misrouting may result in loss-of-function and, potentially, the accumulation of defective mutants in cellular compartments. Most known pharmacoperones were initially derived from receptor antagonist screens and, for this reason, present a complex pharmacology, although these are highly target specific. In this summary, we describe efforts to produce high throughput screens that identify these molecules from chemical libraries as well as a mouse model which provides proof-of-principle for in vivo protein rescue using existing pharmacoperones.

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.

Genetics of hyperuricemia and gout: implications for the present and future

Gout is the most common inflammatory arthropathy and occurs in the setting of elevated serum urate levels. Gout is also known to be associated with multiple comorbidities including cardiovascular disease and the metabolic syndrome. Recent advances in research have increased our understanding and improved our knowledge of the pathophysiology of gout. Genome-wide association studies have permitted the identification of several new and common genetic factors that contribute to hyperuricemia and gout. Most of these are involved with the renal urate transport system (the uric acid transportasome), generally considered the most influential regulator of serum urate homeostasis. Thus far, SCL22A12, SCL2A9, and GLUT9 have been found to have the greatest variation and most influence on serum urate levels. However, genetics are only a part of the explanation in the development of hyperuricemia and gout. As results have been mixed, the role of known urate influential genes in gout's associated comorbidities remains unclear. Regardless, GWAS findings have expanded our understanding of the pathophysiology of hyperuricemia and gout, and will likely play a role in the development of future therapies and treatment of this ancient disease.

Histone deacetylase inhibitors influence chemotherapy transport by modulating expression and trafficking of a common polymorphic variant of the ABCG2 efflux transporter

Histone deacetylase inhibitors (HDI) have exhibited some efficacy in clinical trials, but it is clear that their most effective applications have yet to be fully determined. In this study, we show that HDIs influence the expression of a common polymorphic variant of the chemotherapy drug efflux transporter ABCG2, which contributes to normal tissue protection. As one of the most frequent variants in human ABCG2, the polymorphism Q141K impairs expression, localization, and function, thereby reducing drug clearance and increasing chemotherapy toxicity. Mechanistic investigations revealed that the ABCG2 Q141K variant was fully processed but retained in the aggresome, a perinuclear structure, where misfolded proteins aggregate. In screening for compounds that could correct its expression, localization, and function, we found that the microtubule-disrupting agent colchicine could induce relocalization of the variant from the aggresome to the cell surface. More strikingly, we found that HDIs could produce a similar effect but also restore protein expression to wild-type levels, yielding a restoration of ABCG2-mediated specific drug efflux activity. Notably, HDIs did not modify aggresome structures but instead rescued newly synthesized protein and prevented aggresome targeting, suggesting that HDIs disturbed trafficking along microtubules by eliciting changes in motor protein expression. Together, these results showed how HDIs are able to restore wild-type functions of the common Q141K polymorphic isoform of ABCG2. More broadly, our findings expand the potential uses of HDIs in the clinic.

Repair of CFTR folding defects with correctors that function as pharmacological chaperones

The major cause of cystic fibrosis is the presence of processing mutations in CFTR (such as deletion of Phe-508 (F508del-CFTR)) that disrupt folding of the protein and trafficking to the cell surface. Processing mutations appear to inhibit folding of CFTR so that it accumulates in the endoplasmic reticulum as a partially folded protein. Expressing the proteins in the presence of small molecules called correctors can repair CFTR folding defects. Some correctors appear to function as pharmacological chaperones that specifically bind to the CFTR processing mutants and induce them to complete the folding process. In this chapter, we describe techniques to examine the effects of correctors on folding of CFTR processing mutants.

Signaling pathways of proteostasis network unraveled by proteomic approaches on the understanding of misfolded protein rescue

Attempts to promote normal processing and function of F508del-CFTR, the most common mutant in cystic fibrosis (CF), have been described as potential therapeutic strategies in the management of this disease. Here we described the proteomic approaches, namely two-dimensional electrophoresis (2DE), mass spectrometry (MS), and bioinformatics tools used in our recent studies to gain insight into the proteins potentially involved in low-temperature or mutagenic treatment-induced rescue process of F508del-CFTR. The proteins identified are part of the proteostasis network, such as the unfolded protein response (UPR) signaling pathways that may regulate the processing of CF transmembrane conductance regulator (CFTR) through the folding and trafficking progression. The complete characterization of these signaling pathways and their regulators in CF will certainly contribute to the development of novel therapeutic strategies against CF.

Proteomics uncovering possible key players in F508del-CFTR processing and trafficking

The achievement and maintenance of a protein native conformation is a very complex cellular process involving a multitude of key factors whose contribution to a successful folding remains to be elucidated. On top of this, it is known that correct folding is crucial for proteins to play their normal role and, consequently, for the maintenance of cellular homeostasis or proteostasis. If the folding process is affected, the protein is unable to achieve its native conformation, compromising its life and function, and a pathological condition may arise. Protein-misfolding diseases are characterized by either formation of protein aggregates that are toxic to the cell (gain-of-toxic-function diseases) or by an incorrect processing of proteins, which leads to a deficiency in protein activity (loss-of-function diseases). In this article we have focused on proteomics advances in the molecular knowledge of protein-misfolding diseases with direct impact on possible key players in F508del-CFTR processing and trafficking.

Salicylate-induced translocation of prestin having mutation in the GTSRH sequence to the plasma membrane

Prestin is a key molecule for mammalian hearing. The present study investigated changes in characteristics of prestin by culturing prestin-transfected cells with salicylate, an antagonist of prestin. As a result, the plasma membrane localization of prestin bearing a mutation in the GTSRH sequence, which normally accumulates in the cytoplasm, was recovered. Moreover, the nonlinear capacitance of the majority of the mutants, which is a signature of prestin activity, was also recovered. Thus, the present study discovered a new effect of salicylate on prestin, namely, the promotion of the plasma membrane expression of prestin mutants in an active state.

Mutational analysis of threonine 402 adjacent to the GXXXG dimerization motif in transmembrane segment 1 of ABCG2

ABCG2 is an ATP-binding cassette half-transporter important in normal tissue protection, drug distribution, and excretion. ABCG2 requires homodimerization for function, though the mechanism for dimerization has not been elucidated. We conducted mutational analysis of threonine 402, three residues from the GXXXG motif in TM1, to study its potential role in ABCG2 dimerization (TXXXGXXXG). Single mutations to leucine (T402L) or arginine (T402R) did not have a significant impact on the ABCG2 protein. On the other hand, combining the T402 mutations with the GXXXG glycine to leucine mutations (T402L/G406L/G410L and T402R/G406L/G410L) resulted in a substantially reduced level of expression, altered glycosylation, degradation by a proteosome-independent pathway, and partial retention in the endoplasmic reticulum as suggested by immunostaining, Endo H sensitivity, and MG132 and bafilomycin failed effect. The T402L/G406L/G410L mutant when incubated with the ABCG2 substrate MX showed a shift on immunoblot analysis to the band representing the fully mature glycoprotein. The T402R/G406L/G410L mutant carrying the more drastic substitution was found to primarily localize intracellularly. The same set of mutations also displayed impaired dimerization in the TOXCAT assay for TM1 compared to that of the wild type. Homology modeling of ABCG2 places the TXXXGXXXG motif at the dimer interface. These studies are consistent with a role for the extended TXXXGXXXG motif in ABCG2 folding, processing, and/or dimerization.

Identifying protein stabilizing ligands using GroEL

Over the past 5 years, it has become increasingly apparent to researchers that the initial promise and excitement of using gene replacement therapies to ameliorate folding diseases are still far from being broadly or easily applicable. Because a large number of human diseases are protein folding diseases (approximately 30 to 50%), many researchers now realize that more directed approaches to target and reverse the fundamental misfolding reactions preceding disease are highly feasible and offer the potential of developing more targeted drug therapies. This is also true with a large number of so called orphan protein folding diseases. The development of a broad-based general screening array method using the chaperonin as a detection platform will enable us to screen large chemical combinatorial libraries for specific ligands against the elusive transient, primary reactions that often lead to protein misfolding. This development will provide a highly desirable tool for the pharmaceutical, academic, and medical professions.

Identification of the minimal functional unit of the homo-oligomeric human reduced folate carrier

The reduced folate carrier (RFC) is the major transport system for folates in mammals. We previously demonstrated the existence of human RFC (hRFC) homo-oligomers and established the importance of these higher order structures to intracellular trafficking and carrier function. In this report, we examined the operational significance of hRFC oligomerization and the minimal functional unit for transport. In negative dominance experiments, multimeric transporters composed of different ratios of active (either wild type (WT) or cysteine-less (CLFL)) and inactive (either inherently inactive (Y281L and R373A) due to mutation, or resulting from inactivation of the Y126C mutant by (2-sulfonatoethyl) methanethiosulfonate (MTSES)) hRFC monomers were expressed in hRFC-null HeLa (R5) cells, and residual WT or CLFL activity was measured. In either case, residual transport activity with increasing levels of inactive mutant correlated linearly with the fraction of WT or CLFL hRFC in plasma membranes. When active covalent hRFC dimers, generated by fusing CLFL and Y126C monomers, were expressed in R5 cells and treated with MTSES, transport activity of the CLFL-CLFL dimer was unaffected, whereas Y126C-Y126C was potently (64%) inhibited; heterodimeric CLFL-Y126C and Y126C-CLFL were only partly (27 and 23%, respectively) inhibited by MTSES. In contrast to Y126C-Y126C, trans-stimulation of methotrexate uptake by intracellular folates for Y126C-CLFL and CLFL-Y126C was nominally affected by MTSES. Collectively, these results strongly support the notion that each hRFC monomer comprises a single translocation pathway for anionic folate substrates and functions independently of other monomers (i.e. despite an oligomeric structure, hRFC functions as a monomer).

Low temperature restoring effect on F508del-CFTR misprocessing: A proteomic approach

To gain insight into the proteins potentially involved in the low temperature-induced F508del-CFTR rescue process, we have explored by two-dimensional electrophoresis (2DE) the proteome of BHK cell lines expressing wt or F508del-CFTR, grown at 37 degrees C or 26 degrees C/24h or 26 degrees C/48h followed by 3h of metabolic labelling with [(35)S]-methionine. A set of 139 protein spots (yielding 125 mass spectrometry identifications) was identified as differentially expressed (p ANOVA<0.05) among the six phenotypic groups analysed. The data analysis suggests that the unfolded protein response (UPR) induction and some cell-metabolism repression are the major cold-shock responses that may generate a favourable cellular environment to promote F508del-CFTR rescue. Down-regulation of proteasome regulatory PA28 and/or COP9 signalosome subunit, both involved in CFTR degradation, could also be a relevant cold-shock-induced condition for F508de-CFTR rescue. Moreover, cold-shock may promote the reestablishment of some proteostasis imbalance associated with over-expression of F508del-CFTR. In BHK-F508del cells, the deregulation of RACK1, a protein described to be important for stable expression of CFTR in the plasma membrane, is partially repaired after low temperature treatment. Together these findings give new insights about F508del-CFTR rescue by low temperature treatment and the proteins involved could ultimately constitute potential therapeutic targets in CF disease.

Arginine 383 is a crucial residue in ABCG2 biogenesis

ABCG2 is an ATP-binding cassette half-transporter initially identified in multidrug-resistant cancer cell lines and recently suggested to play an important role in pharmacokinetics. Here we report studies of a conserved arginine predicted to localize near the cytoplasmic side of TM1. First, we determined the effect of losing charge and bulk at this position via substitutions with glycine and alanine. The R383G mutant when transfected into HEK cells was not detectable on immunoblot or by functional assay, while the R383A mutant exhibited detectable but significantly decreased levels compared to wild-type, partial retention in the ER and altered glycosylation. Efflux of the ABCG2-substrates mitoxantrone and pheophorbide a was observed. Our experiments suggested rapid degradation of the R383A mutant by the proteasome via a kifunensine-insensitive pathway. Interestingly, overnight treatment of the R383A mutant with mitoxantrone assisted in protein maturation as evidenced by a shift to the N-glycosylated form. The R383A mutant when expressed in insect cells, though detected on the surface, had no measurable ATPase activity. In addition, substitution with the positively charged lysine resulted in significantly decreased protein expression levels in HEK cells, while retaining function. In conclusion, arginine 383 is a crucial residue for ABCG2 biogenesis, where even the most conservative mutations have a large impact.

Trafficking and quality control of the gonadotropin releasing hormone receptor in health and disease

In order to serve as enzymes, receptors and ion channels, proteins require structural precision. This is monitored by a cellular quality control system (QCS) that rejects misfolded proteins and thereby protects the cell against aberrant activity. Misfolding can result in protein molecules that retain intrinsic function, yet become misrouted within the cell; these cease to perform normally and result in disease. A therapeutic opportunity exists to correct misrouting and rescue mutants using "pharmacoperones" (small molecular folding templates, often peptidomimetics, which promote correct folding and rescue) thereby restoring function and potentially curing the underlying disease. Because of its small size, the GnRH (gonadotropin-releasing hormone) receptor (GnRHR) is an excellent model for GPCR (G protein-coupled receptor) and has allowed elucidation of the precise biochemical mechanism of pharmacoperone action necessary for rational design of new therapeutic agents. This review summarizes what has been learned about the structural requirements of the GnRHR that govern its interaction with the QCS and now presents the potential for the rational design of pharmacoperones. Because of the role of protein processing, this approach is likely to be applicable to other GCPCs and other proteins in general.

Mutations in the Arabidopsis peroxisomal ABC transporter COMATOSE allow differentiation between multiple functions in planta: insights from an allelic series

COMATOSE (CTS), the Arabidopsis homologue of human Adrenoleukodystrophy protein (ALDP), is required for import of substrates for peroxisomal beta-oxidation. A new allelic series and a homology model based on the bacterial ABC transporter, Sav1866, provide novel insights into structure-function relations of ABC subfamily D proteins. In contrast to ALDP, where the majority of mutations result in protein absence from the peroxisomal membrane, all CTS mutants produced stable protein. Mutation of conserved residues in the Walker A and B motifs in CTS nucleotide-binding domain (NBD) 1 resulted in a null phenotype but had little effect in NBD2, indicating that the NBDs are functionally distinct in vivo. Two alleles containing mutations in NBD1 outside the Walker motifs (E617K and C631Y) exhibited resistance to auxin precursors 2,4-dichlorophenoxybutyric acid (2,4-DB) and indole butyric acid (IBA) but were wild type in all other tests. The homology model predicted that the transmission interfaces are domain-swapped in CTS, and the differential effects of mutations in the conserved "EAA motif" of coupling helix 2 supported this prediction, consistent with distinct roles for each NBD. Our findings demonstrate that CTS functions can be separated by mutagenesis and the structural model provides a framework for interpretation of phenotypic data.

Role of N-linked oligosaccharides in the biosynthetic processing of the cystic fibrosis membrane conductance regulator

The epithelial chloride channel CFTR is a glycoprotein that is modified by two N-linked oligosaccharides. The most common mutant CFTR protein in patients with cystic fibrosis, DeltaF508, is misfolded and retained by ER quality control. As oligosaccharide moieties of glycoproteins are known to mediate interactions with ER lectin chaperones, we investigated the role of N-linked glycosylation in the processing of wild-type and DeltaF508 CFTR. We found that N-glycosylation and ER lectin interactions are not major determinants of trafficking of wild-type and DeltaF508 from the ER to the plasma membrane. Unglycosylated CFTR, generated by removal of glycosylation sites or treatment of cells with the N-glycosylation inhibitor tunicamycin, did not bind calnexin, but did traffic to the cell surface and exhibited chloride channel activity. Most importantly, unglycosylated DeltaF508 CFTR still could not escape quality control in the early secretory pathway and remained associated with the ER. However, the absence of N-linked oligosaccharides did reduce the stability of wild-type CFTR, causing significantly more-rapid turnover in post-ER compartments. Surprisingly, the individual N-linked carbohydrates do not play equivalent roles and modulate the fate of the wild-type protein in different ways in its early biosynthetic pathway.

F413C and A531V but not R894X myotonia congenita mutations cause defective endoplasmic reticulum export of the muscle-specific chloride channel CLC-1

In northern Finland myotonia congenita is caused by three main mutations in the ClC-1 chloride channel. We studied the molecular basis of these mutations (1238T>G/F413C, 1592C>T/A531V, and 2680C>T/R894X). The mutated cDNAs were expressed either in L6 myotubes or in isolated rat myofibers using recombinant Semliki Forest virus. Experiments in L6 cells indicated that A531V and R894X proteins suffered from stability problems in these cells. Analysis in myofibers indicated that the A531V protein was totally retained in the endoplasmic reticulum (ER), whereas the export of the F413C protein was severely reduced. The C-terminal nonsense mutant (R894X), however, was normally transported to the Golgi elements in the myofibers. Defective export or reduced stability of the mutated proteins may thus be reasons for the myotonic symptoms.

Enhanced cell-surface stability of rescued DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) by pharmacological chaperones

Misfolded proteins destined for the cell surface are recognized and degraded by the ERAD [ER (endoplasmic reticulum) associated degradation] pathway. TS (temperature-sensitive) mutants at the permissive temperature escape ERAD and reach the cell surface. In this present paper, we examined a TS mutant of the CFTR [CF (cystic fibrosis) transmembrane conductance regulator], CFTR DeltaF508, and analysed its cell-surface trafficking after rescue [rDeltaF508 (rescued DeltaF508) CFTR]. We show that rDeltaF508 CFTR endocytosis is 6-fold more rapid (approximately 30% per 2.5 min) than WT (wild-type, approximately 5% per 2.5 min) CFTR at 37 degrees C in polarized airway epithelial cells (CFBE41o-). We also investigated rDeltaF508 CFTR endocytosis under two further conditions: in culture at the permissive temperature (27 degrees C) and following treatment with pharmacological chaperones. At low temperature, rDeltaF508 CFTR endocytosis slowed to WT rates (20% per 10 min), indicating that the cell-surface trafficking defect of rDeltaF508 CFTR is TS. Furthermore, rDeltaF508 CFTR is stabilized at the lower temperature; its half-life increases from <2 h at 37 degrees C to >8 h at 27 degrees C. Pharmacological chaperone treatment at 37 degrees C corrected the rDeltaF508 CFTR internalization defect, slowing endocytosis from approximately 30% per 2.5 min to approximately 5% per 2.5 min, and doubled DeltaF508 surface half-life from 2 to 4 h. These effects are DeltaF508 CFTR-specific, as pharmacological chaperones did not affect WT CFTR or transferrin receptor internalization rates. The results indicate that small molecular correctors may reproduce the effect of incubation at the permissive temperature, not only by rescuing DeltaF508 CFTR from ERAD, but also by enhancing its cell-surface stability.

Endoplasmic reticulum-associated degradation of mutant CFTR requires a guanine nucleotide-sensitive step

Proteasome degradation of endoplasmic reticulum (ER)-misfolded proteins requires retrograde transport from ER to the cytosol. To date, it is not clear whether this event constitutes the exclusive ER degradation process for non-native membrane proteins. Here we describe the role of GTP in the degradation of DeltaF508-CFTR and the alpha subunit of the T-cell receptor (TCRalpha), representative misfolded ER membrane proteins. Selective intracellular GTP depletion extended the DeltaF508-CFTR half-life sixfold, whereas ATP depletion accelerated its turnover and inhibited only 80% of the proteasome activity that was not affected by GTP depletion. AlF(4)(-), a well-known inhibitor of heterotrimeric G proteins, but not of AlF(3), delayed the mutant CFTR turnover in vivo, in semi-intact cells and in ER-enriched microsomes, without affecting ER to Golgi cargo transport. DeltaF508-CFTR degradation was also inhibited by alkaline stripping of ER-associated membrane proteins. We propose that at the ER, GTP may participate in the disposal of misfolded membrane proteins through activation of heterotrimeric G proteins.

Phenotypic differences in PFIC2 and BRIC2 correlate with protein stability of mutant Bsep and impaired taurocholate secretion in MDCK II cells

Progressive familial cholestasis (PFIC) 2 and benign recurrent intrahepatic cholestasis (BRIC) 2 are caused by mutations in the bile salt export pump (BSEP, ABCB11) gene; however, their prognosis differs. PFIC2 progresses to cirrhosis and requires liver transplantation, whereas BRIC2 is clinically benign. To identify the molecular mechanism(s) responsible for the phenotypic differences, eight PFIC2 and two BRIC2 mutations were introduced in rat Bsep, which was transfected in MDCK II cells. Taurocholate transport activity, protein expression, and subcellular distribution of these mutant proteins were studied in a polarized MDCK II monolayer. The taurocholate transport activity was approximately half of the wild-type (WT) in BRIC2 mutants (A570T and R1050C), was substantially less in two PFIC2 mutants (D482G and E297G), and was almost abolished in six other PFIC2 mutants (K461E, G982R, R1153C, R1268Q, 3767-3768insC, and R1057X). Bsep protein expression levels correlated closely with transport activity, except for R1057X. The half-life of the D482G mutant was shorter than that of the WT (1.35 h vs. 3.49 h in the mature form). BRIC2 mutants and three PFIC mutants (D482G, E297G, and R1057X) were predominantly distributed in the apical membrane. The other PFIC2 mutants remained intracellular. The R1057X mutant protein was stably expressed and trafficked to the apical membrane, suggesting that the COOH-terminal tail is required for transport activity but not for correct targeting. In conclusion, taurocholate transport function was impaired in proportion to rapid degradation of Bsep protein in the mutants, which were aligned in the following order: A570T and R1050C > D482G > E297G > K461E, G982R, R1153C, R1268Q, 3767-3768insC, and R1057X. These results may explain the phenotypic difference between BRIC2 and PFIC2.

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Altered biogenesis of deltaF508-CFTR following treatment with doxorubicin

Cystic fibrosis (CF) is caused by mutations to the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The most common of these mutations is deletion of a phenylalanine residue at position 508 (Delta F508), which accounts for approximately 70% of all CF alleles. This mutation interferes with the biogenesis and maturation of Delta F508-CFTR to the plasma membrane. However, Delta F508-CFTR can partially function upon proper localization. Thus, pharmacological correction of Delta F508-CFTR maturation holds promise in CF therapy. Our previous studies indicate that a single non-cytotoxic dose of the anthracycline doxorubicin (Dox) significantly increase Delta F508-CFTR-associated chloride secretion in MDCK cells by increasing the expression of this protein at the apical plasma membrane. We report here that Dox alters the biogenesis of Delta F508-CFTR. Treatment with Dox increases the resistance of Delta F508-CFTR to trypsin digestion, possibly by expediting protein folding. Further, treatment with Dox reduces the amount of polyubiquitinated Delta F508-CFTR in cells and prolongs the half-life of this protein. Concomitantly, treatment with Dox decreases the association of Delta F508-CFTR with HSP70 but does not alter the expression of major HSP70 family members. Based on these results, we propose that Dox expedites the folding and maturation of Delta F508-CFTR by acting as a pharmacological chaperone, which consequently promotes the functional expression of this protein in MDCK cells.

Additive effect of multiple pharmacological chaperones on maturation of CFTR processing mutants

The most common cause of CF (cystic fibrosis) is the deletion of Phe(508) (DeltaF508) in the CFTR [CF TM (transmembrane) conductance regulator] chloride channel. One major problem with DeltaF508 CFTR is that the protein is defective in folding so that little mature protein is delivered to the cell surface. Expression of DeltaF508 CFTR in the presence of small molecules known as correctors or pharmacological chaperones can increase the level of mature protein. Unfortunately, the efficiency of corrector-induced maturation of DeltaF508 CFTR is probably too low to have therapeutic value and approaches are needed to increase maturation efficiency. We postulated that expression of DeltaF508 CFTR in the presence of multiple correctors that bound to different sites may have an additive effect on maturation. In support of this mechanism, we found that expression of P-glycoprotein (CFTR's sister protein) processing mutants in the presence of two compounds that bind to different sites (rhodamine B and Hoechst 33342) had an additive effect on maturation. Therefore we tested whether expression of DeltaF508 CFTR in the presence of combinations of three different classes of corrector molecules would increase its maturation efficiency. It was found that the combination of the quinazoline VRT-325 together with the thiazole corr-2b or bisaminomethylbithiazole corr-4a doubled the steady-state maturation efficiency of DeltaF508 CFTR (approx. 40% of total CFTR was mature protein) compared with expression in the presence of a single compound. The additive effect of the correctors on DeltaF508 CFTR maturation suggests that they directly interact at different sites of the protein.

Defects in the Leptin Axis Reduce Abundance of the ABCG5-ABCG8 Sterol Transporter in Liver

ABGG5 (G5) and ABCG8 (G8) are ABC half-transporters that dimerize within the endoplasmic reticulum, traffic to the cell surface, and mediate cholesterol excretion into bile. Mice harboring defects in the leptin axis (db/db and ob/ob) have reduced biliary cholesterol concentrations. Rapid weight loss brought about by administration of leptin or dietary restriction increases biliary cholesterol excretion. We hypothesized that the reduction in biliary cholesterol in mice harboring defects in the leptin axis is associated with a reduction in G5G8 transporters and that levels of the transporter would increase with leptin administration and dietary restriction. We examined mRNA and protein levels for G5 and G8 in db/db and ob/ob mice. In both models G5 and G8 protein levels were reduced. In ob/ob mice, both leptin administration and dietary restriction increased G5 and G8 protein and biliary cholesterol concentrations. Finally, we examined the effects of tauroursodeoxycholate, which has been shown to increase biliary cholesterol excretion and function as a molecular chaperone. Tauroursodeoxycholate increased G5 and G8 protein and biliary cholesterol concentrations in both wild-type and db/db mice. Our results indicate that the mechanism for reduced biliary cholesterol excretion in db/db and ob/ob mice involves reductions in G5 and G8 protein levels and that this may occur at the level of G5G8 heterodimer assembly within the endoplasmic reticulum.

Autocrine growth hormone: effects on growth hormone receptor trafficking and signaling

GH and GH receptor are expressed in many extrapituitary tissues, permitting autocrine/paracrine activity. Autocrine GH has regulatory functions in embryonic development and cellular differentiation and proliferation and is reported to be involved in the development and metastasis of tumor cells. To understand the principles of transport and signaling of autocrine GH and GH receptor, we used a model system to express both proteins in the same cell. Our experiments show that GH binds the GH receptor immediately after synthesis in the endoplasmic reticulum and facilitates maturation of GH receptor. The hormone-receptor complexes arrive at the cell surface where exogenously added GH is unable to bind these receptors. Autocrine GH activates the GH receptors, but signal transduction occurs only after exiting the endoplasmic reticulum. This model study explains why autocrine GH-producing cells may be insensitive for GH (antagonist) treatment and clarifies autocrine signaling events.

Evidence for a Sav1866-like architecture for the human multidrug transporter P-glycoprotein

The recently reported structures of the bacterial multidrug exporter Sav1866 suggest a domain architecture in which both nucleotide-binding domains (NBDs) of this ATP binding cassette (ABC) transporter contact both transmembrane domains (TMDs). Such a domain arrangement is particularly unexpected because it is not found in the structures of three solute importers BtuCD, HI1470/1, and ModBC from the same protein family. There is also no precedent for such an arrangement from biochemical studies with any ABC transporter. Sav1866 is homologous with the clinically relevant human P-glycoprotein (ABCB1). If the structure proposed for Sav1866 is physiologically relevant, the long intracellular loops of P-glycoprotein TMD2 should contact NBD1. We have tested this by using cysteine mutagenesis and chemical cross-linking to verify proximal relationships of the introduced sulfhydryls across the proposed interdomain interface. We report the first biochemical evidence in support of the domain arrangement proposed for the multidrug resistance class of ABC transporters. With a domain arrangement distinctly different from the three solute importers it seems likely that the TMDs of ABC importers and exporters have evolved different mechanisms to couple to common conformational changes at conserved NBDs.

Molecular targeting of CFTR as a therapeutic approach to cystic fibrosis

One of the major challenges facing the pharmaceutical field is the identification of novel, 'druggable' targets common to distinct diseases that, despite their clinical diversity, share the same basic molecular defect(s) - thus, being termed 'horizontal diseases'. Membrane proteins constitute one of the largest families in the human genome and, given their major roles in cells and organisms, they are relevant to common human disorders such as cardiovascular disease and cancer, but also to rare genetic conditions such as cystic fibrosis (CF). Here, we review therapeutic approaches to correcting the basic defect in CF, which is caused mainly by the intracellular retention of a misfolded protein, and focus on various recent drug-discovery strategies for this important and paradigmatic disease. These strategies have possible applications in many membrane protein disorders, including other channelopathies. The mechanisms of action of potent and specific compounds, representing promising drug leads for CF pharmacotherapy, are explained and discussed.

N-glycan structure dictates extension of protein folding or onset of disposal

The endoplasmic reticulum (ER) is the site of folding for proteins that are resident in the ER or that are destined for the Golgi, endosomes, lysosomes, the plasma membrane, or secretion. Cotranslational addition of preassembled glucose(3)-mannose(9)-N-acetylglucosamine(2) core oligosaccharides (N-glycosylation) is a common event for polypeptides synthesized in this compartment. Protein-bound oligosaccharides are exposed to several ER glycanases that sequentially remove terminal glucose or mannose residues. Their activity must be tightly regulated because the N-glycan composition determines whether the associated protein is subjected to folding attempts in the ER lumen or whether it is retrotranslocated into the cytosol and degraded.

Membrane chaperone Shr3 assists in folding amino acid permeases preventing precocious ERAD

The yeast endoplasmic reticulum (ER) membrane-localized chaperone Shr3 plays a critical role in enabling amino acid permeases (AAPs) to fold and attain proper structures required for functional expression at the plasma membrane. In the absence of Shr3, AAPs specifically accumulate in the ER, where despite the correct insertion of their 12 transmembrane segments (TMSs), they aggregate forming large molecular weight complexes. We show that Shr3 prevents aggregation and facilitates the functional assembly of independently coexpressed N- and C-terminal fragments of the general AAP Gap1. Shr3 interacts with and maintains the first five TMSs in a conformation that can posttranslationally assemble with the remaining seven TMSs. We also show that Doa10- and Hrd1-dependent ER-associated degradation (ERAD) pathways redundantly degrade AAP aggregates. In combination, doa10Delta hrd1Delta mutations stabilize AAP aggregates and partially suppress amino acid uptake defects of shr3 mutants. Consequently, in cells with impaired ERAD, AAPs are able to attain functional conformations independent of Shr3. These findings illustrate that folding and degradation are tightly coupled processes during membrane protein biogenesis.

Specific Rescue of Cystic Fibrosis Transmembrane Conductance Regulator Processing Mutants Using Pharmacological Chaperones

Most mutants of the cystic fibrosis transmembrane conductance regulator (CFTR) that cause severe symptoms of cystic fibrosis do not reach the cell surface because they are defective in folding. Many CFTR folding mutants, however, including the DeltaF508 mutant found in more than 90% of cystic fibrosis patients, are potentially functional at the cell surface if they can be induced to fold correctly. In a previous study (Mol Pharm 2:407-413, 2005), we reported that a quinazoline derivative (CFcor-325) could rescue CFTR processing mutants. The corrector was not specific however, as it could also rescue a processing mutant of CFTR's sister protein, the multidrug resistance P-glycoprotein. The goal of this study was to test whether it was possible to specifically rescue CFTR processing mutants using a pharmacological chaperone. In this article, we report that two compounds, 4-methyl-2-(5-phenyl-1H-pyrazol-3-yl)-phenol (CFpot-532) and 2-phenylamino-4-(4-ethylene-phenyl)-thiazole (corr-2b) could rescue CFTR processing mutants such as DeltaF508 CFTR but not a P-glycoprotein processing mutant. The compound CFpot-532 also acts as a potentiator of DeltaF508 CFTR channel activity. Therefore, the results suggest that the mechanism whereby CFpot-532 and corr-2b promote folding of CFTR processing mutants is through direct interaction with the CFTR mutant proteins. The compound CFpot-532 could be a particularly useful lead compound for treatment of cystic fibrosis because it is both a CFTR channel potentiator as well as a specific pharmacological chaperone.