Reduced Transport Function Research Articles

Putative purine nucleoside interacting residues in the malaria parasite purine uptake transporter PfENT1 are critical for transporter function.

Malaria remains a major public health threat for billions of people worldwide. Infection with obligate intracellular, unicellular parasites from the genus Plasmodium causes malaria. Plasmodium falciparum causes the deadliest form of human malaria. Plasmodium parasites are purine auxotrophic. They rely on purine import from the host red blood cell cytoplasm via equilibrative nucleoside transporters to supply substrates to the purine salvage pathway. We previously developed a high throughput screening assay to identify inhibitors of the P. falciparum Equilibrative Nucleoside Transporter Type 1 (PfENT1). Screening a small molecule library identified PfENT1 inhibitors that blocked proliferation of P. falciparum parasites in in vitro culture. The goal of the current work was to validate a high-resolution model of PfENT1 predicted by the AlphaFold protein structure prediction program. We superimposed the predicted PfENT1 structure on the human homologue structure, hENT1, and developed a structure-based sequence alignment. We mutated the residues in PfENT1 aligned with and flanking the residues in hENT1 that interact with the purine analog, nitrobenzylthioinosine (NBMPR). Mutation of the PfENT1 residues Q135, D287, and R291 that are predicted to form hydrogen bonds to purine nucleosides eliminated purine and pyrimidine transport function in various yeast-based growth and radiolabeled substrate uptake assays. Mutation of two flanking residues, W53 and S290, also resulted in inactive protein. Mutation of L50 that forms hydrophobic interactions with the purine nucleobase reduced transport function. Based on our results the AlphaFold predicted structure for PfENT1 may be useful in guiding medicinal chemistry efforts to improve the potency of our PfENT1 inhibitors.

Exaggerated trans-membrane charge of ammonium transporters in nutrient-poor marine environments.

Transporter proteins are a vital interface between cells and their environment. In nutrient-limited environments, microbes with transporters that are effective at bringing substrates into their cells will gain a competitive advantage over variants with reduced transport function. Microbial ammonium transporters (Amt) bring ammonium into the cytoplasm from the surrounding periplasm space, but diagnosing Amt adaptations to low nutrient environments solely from sequence data has been elusive. Here, we report altered Amt sequence amino acid distribution from deep marine samples compared to variants sampled from shallow water in two important microbial lineages of the marine water column community-Marine Group I Archaea (Thermoproteota) and the uncultivated gammaproteobacterial lineage SAR86. This pattern indicates an evolutionary pressure towards an increasing dipole in Amt for these clades in deep ocean environments and is predicted to generate stronger electric fields facilitating ammonium acquisition. This pattern of increasing dipole charge with depth was not observed in lineages capable of accessing alternative nitrogen sources, including the abundant alphaproteobacterial clade SAR11. We speculate that competition for ammonium in the deep ocean drives transporter sequence evolution. The low concentration of ammonium in the deep ocean is therefore likely due to rapid uptake by Amts concurrent with decreasing nutrient flux.

Reduced iron export associated with hepcidin resistance can explain the iron overload spectrum in ferroportin disease.

Background & AimsFerroportin disease (FD) and hemochromatosis type 4 (HH4) are associated with variants in the ferroportin‐encoding gene SLC40A1. Both phenotypes are characterized by iron overload despite being caused by distinct variants that either mediate reduced cellular iron export in FD or resistance against hepcidin‐induced inactivation of ferroportin in HH4. The aim of this study was to assess if reduced iron export also confers hepcidin resistance and causes iron overload in FD associated with the R178Q variant.MethodsThe ferroportin disease variants R178Q andA77D and the HH4‐variant C326Y were overexpressed in HEK‐293T cells and subcellular localization was characterized by confocal microscopy and flow cytometry. Iron export and cytosolic ferritin were measured as markers of iron transport and radioligand binding studies were performed. The hepcidin‐ferroportin axis was assessed by ferritin/hepcidin correlation in patients with different iron storage diseases.ResultsIn the absence of hepcidin, the R178Q and A77D variants exported less iron when compared to normal and C326Y ferroportin. In the presence of hepcidin, the R178Q and C326Y, but not the A77D‐variant, exported more iron than cells expressing normal ferroportin. Regression analysis of serum hepcidin and ferritin in patients with iron overload are compatible with hepcidin deficiency in HFE hemochromatosis and hepcidin resistance in R178Q FD.ConclusionsThese results support a novel concept that in certain FD variants reduced iron export and hepcidin resistance could be interlinked. Evasion of mutant ferroportin from hepcidin‐mediated regulation could result in uncontrolled iron absorption and iron overload despite reduced transport function.

Dysfunctional LAT2 Amino Acid Transporter Is Associated With Cataract in Mouse and Humans.

Cataract, the loss of ocular lens transparency, accounts for ∼50% of worldwide blindness and has been associated with water and solute transport dysfunction across lens cellular barriers. We show that neutral amino acid antiporter LAT2 (Slc7a8) and uniporter TAT1 (Slc16a10) are expressed on mouse ciliary epithelium and LAT2 also in lens epithelium. Correspondingly, deletion of LAT2 induced a dramatic decrease in lens essential amino acid levels that was modulated by TAT1 defect. Interestingly, the absence of LAT2 led to increased incidence of cataract in mice, in particular in older females, and a synergistic effect was observed with simultaneous lack of TAT1. Screening SLC7A8 in patients diagnosed with congenital or age-related cataract yielded one homozygous single nucleotide deletion segregating in a family with congenital cataract. Expressed in HeLa cells, this LAT2 mutation did not support amino acid uptake. Heterozygous LAT2 variants were also found in patients with cataract some of which showed a reduced transport function when expressed in HeLa cells. Whether heterozygous LAT2 variants may contribute to the pathology of cataract needs to be further investigated. Overall, our results suggest that defects of amino acid transporter LAT2 are implicated in cataract formation, a situation that may be aggravated by TAT1 defects.

Characterization of Plasma Membrane Localization and Phosphorylation Status of Organic Anion Transporting Polypeptide (OATP) 1B1 c.521 T>C Nonsynonymous Single-Nucleotide Polymorphism.

Membrane transport protein organic anion transporting polypeptide (OATP) 1B1 mediates hepatic uptake of many drugs (e.g. statins). The OATP1B1 c.521T > C (p. V174A) polymorphism has reduced transport activity. Conflicting in vitro results exist regarding whether V174A-OATP1B1 has reduced plasma membrane localization; no such data has been reported in physiologically relevant human liver tissue. Other potential changes, such as phosphorylation, of the V174A-OATP1B1 protein have not been explored. Current studies characterized the plasma membrane localization of V174A-OATP1B1 in genotyped human liver tissue and cell culture and compared the phosphorylation status of V174A- and wild-type (WT)-OATP1B1. Localization of V174A- and WT-OATP1B1 were determined in OATP1B1 c.521T > C genotyped human liver tissue (n = 79) by immunohistochemistry and in transporter-overexpressing human embryonic kidney (HEK) 293 and HeLa cells by surface biotinylation and confocal microscopy. Phosphorylation and transport of OATP1B1 was determined using 32P-orthophosphate labeling and [3H]estradiol-17β-glucuronide accumulation, respectively. All three methods demonstrated predominant plasma membrane localization of both V174A- and WT-OATP1B1 in human liver tissue and in cell culture. Compared to WT-OATP1B1, the V174A-OATP1B1 has significantly increased phosphorylation and reduced transport. We report novel findings of increased phosphorylation, but not impaired membrane localization, in association with the reduced transport function of the V174A-OATP1B1.

Regulation of Organic Anion Transporting Polypeptides (OATP) 1B1- and OATP1B3-Mediated Transport: An Updated Review in the Context of OATP-Mediated Drug-Drug Interactions.

Organic anion transporting polypeptides (OATP) 1B1 and OATP1B3 are important hepatic transporters that mediate the uptake of many clinically important drugs, including statins from the blood into the liver. Reduced transport function of OATP1B1 and OATP1B3 can lead to clinically relevant drug-drug interactions (DDIs). Considering the importance of OATP1B1 and OATP1B3 in hepatic drug disposition, substantial efforts have been given on evaluating OATP1B1/1B3-mediated DDIs in order to avoid unwanted adverse effects of drugs that are OATP substrates due to their altered pharmacokinetics. Growing evidences suggest that the transport function of OATP1B1 and OATP1B3 can be regulated at various levels such as genetic variation, transcriptional and post-translational regulation. The present review summarizes the up to date information on the regulation of OATP1B1 and OATP1B3 transport function at different levels with a focus on potential impact on OATP-mediated DDIs.

The polymorphism L204F affects transport and membrane expression of the sodium-dependent organic anion transporter SOAT (SLC10A6)

Sodium-dependent organic anion transporter (SOAT) represents a membrane transporter specific for sulfated steroid hormones, which are supposed to participate in the regulation of reproductive processes. In man, SOAT shows predominant mRNA expression in the testis and here was localized to primary spermatocytes. SOAT mRNA expression is significantly downregulated in different disorders of spermatogenesis, including hypospermatogenesis. The resulting decline of SOAT-mediated transport of sulfated steroids may participate in the impairment of functional spermatogenesis. Apart from downregulation of SOAT mRNA expression, genetic polymorphisms affecting the transport function of SOAT may have the same negative effect on spermatogenesis. Therefore, in the present study we searched for functionally relevant SOAT polymorphisms, aiming to comparatively analyze their occurrence in patients with impaired spermatogenesis vs. patients with intact spermatogenesis. We found that the SOAT polymorphism L204F showed a significantly reduced transport function for DHEAS when expressed in HEK293 cells. Although the Km value was identical with that of the SOAT wildtype, the Vmax value dramatically declined for the SOAT-L204F variant (942.5 vs. 313.6pmol×mg protein−1×min−1). Although the same amount of total SOAT-L204F protein was detected in transfected HEK293 cells compared to the SOAT wildtype, plasma membrane expression was significantly reduced, which points to a plasma membrane sorting defect of the SOAT-L204F variant. Groups of 20 subjects with normal spermatogenesis and 26 subjects with hypospermatogenesis were genotyped for this polymorphism. Both groups showed nearly identical distributions of the SOAT-L204F polymorphism (∼10% heterozygous and ∼5% homozygous), indicating that this polymorphism seems not be causative for hypospermatogenesis.

Functional characterization of common protein variants in the efflux transporter ABCC11 and identification of T546M as functionally damaging variant.

Multidrug resistance protein 8 (ABCC11) is an efflux transporter for anionic lipophilic compounds, conferring resistance to antiviral and anticancer agents like 5-fluorouracil (5-FU). ABCC11 missense variants may contribute to variability in drug response but functional consequences, except for the 'earwax variant' c.538G>A, are unknown. Using the 'Screen and Insert' technology, we generated human embryonic kidney 293 cells stably expressing ABCC11 missense variants frequently occurring in different ethnic populations: c.57G>A, c.538G>A, c.950C>A, c.1637C>T, c.1942G>A, c.4032A>G. A series of in silico prediction analyses and in vitro plasma membrane vesicle uptake, immunoblotting and immunolocalization experiments were undertaken to investigate functional consequences. We identified c.1637C>T (T546M), previously associated with 5-FU-related toxicity, as a novel functionally damaging ABCC11 variant exhibiting markedly reduced transport function of 5-FdUMP, the active cytotoxic metabolite of 5-FU. Detailed analysis of 14 subpopulations revealed highest allele frequencies of c.1637C>T in Europeans and Americans (up to 11%) compared with Africans and Asians (up to 3%).

Single-nucleotide polymorphisms of ABCG2 increase the efficacy of tyrosine kinase inhibitors in the K562 chronic myeloid leukemia cell line

The tyrosine kinase inhibitors (TKIs) used in the treatment of chronic myeloid leukemia are substrates for the efflux transport protein ATP-binding cassette subfamily G member 2 (ABCG2). Variations in ABCG2 activity might influence pharmacokinetics and therapeutic outcome of TKIs. The role of ABCG2 single-nucleotide polymorphisms (SNPs) in TKI treatment is not clear and functional in-vitro studies are lacking. The aim of this study was to investigate the consequences of ABCG2 SNPs for transport and efficacy of TKIs [imatinib, N-desmethyl imatinib (CGP74588), dasatinib, nilotinib, and bosutinib]. ABCG2 SNPs 34G>A, 421C>A, 623T>C, 886G>C, 1574T>G, and 1582G>A were constructed from ABCG2 wild-type cDNA and transduced to K562 cells by retroviral gene transfer. Variant ABCG2 expression in cell membranes was evaluated and the effects of ABCG2 SNPs on transport and efficacy of TKIs were measured as the ability of ABCG2 variants to protect against TKI cytotoxicity. Wild-type ABCG2 had a protective effect against the cytotoxicity of all investigated compounds except bosutinib. It was found that ABCG2 expression provided better protection against CGP74588 than its parent compound, imatinib. ABCG2 421C>A, 623T>C, 886G>C, and 1574T>G reduced cell membrane expression of ABCG2 and the protective effect of ABCG2 against imatinib, CGP74588, dasatinib, and nilotinib cytotoxicity. These findings show that the ABCG2 SNPs 421C>A, 623T>C, 886G>C, and 1574T>G increase the efficacy of investigated TKIs, indicating a reduced transport function that might influence TKI pharmacokinetics in vivo. Furthermore, the active imatinib metabolite CGP74588 is influenced by ABCG2 expression to a greater extent than the parent compound.

Impact of Severe Hypothyroidism on Cyclophosphamide Disposition and Routes of Metabolism and Transport in a Patient with Treatment-Resistant Lupus Nephritis

To report what we believe to be the first case of severe hypothyroidism with reduced drug metabolism and transport activity. A 32-year-old African American woman with a history of treatment-resistant lupus nephritis and concurrent hypothyroidism was participating in a clinical study to evaluate cyclophosphamide pharmacokinetics in patients with glomerulonephritis due to lupus nephritis and small-vessel vasculitis. Thyroid-stimulating hormone levels ranged from 60 to 300 μIU/mL, despite high doses of thyroid replacement hormone (levothyroxine 400 μg twice weekly). The pharmacokinetics of the probe drug cocktail (flurbiprofen/fexofenadine) were altered, with formation clearance of flurbiprofen (CYP2C9 function) lower in our patient versus the average value in our study cohort, suggesting a reduction in activity. The area under the concentration-time curve from 0 to 24 hours for fexofenadine (transporter function) was 2-fold higher in our patient compared to that of other study patients. Pharmacokinetic data showed markedly decreased cyclophosphamide clearance and exposure to 4-hydroxycyclophosphamide, as well as a reduced metabolic ratio of 4-hydroxycyclophosphamide to cyclophosphamide. Previous cases of altered pharmacokinetics and toxicity of medications in patients with mild to moderate thyroid dysfunction have been published. Our case evaluated the impact of a severe form of hypothyroidism on cyclophosphamide pharmacokinetics and probe drug metabolism and transport. If changes were not demonstrated at the extreme spectrum of hypothyroidism, there would be little concern for changes in patients with less severe disease. Profound hypothyroidism likely contributed to the patient's poor response to cyclophosphamide treatment through its influence on CYP isoenzymes responsible for the activation to 4-hydroxycyclophosphamide and possibly through reduced transport function. Clinicians should monitor for significant hypothyroidism in patients who are prescribed drugs (eg, cyclophosphamide) that require metabolic conversion to form active therapeutic moieties.

Dependence of Heavy Soil Transport Function on Soil Profile Depth

Dependence of Heavy Soil Transport Function on Soil Profile DepthThe aim of this work was to quantify the effect of soil profile depth on the transport function of heavy soils. Treatments were carried out between 2006 and 2009 in Milhostov. Two variants were examined : the (conventional soil tillage and long-time no-tilled variant). Soil samples were taken in spring and autumn from soil profile depth of 0.00-0.60 m from each 0.10 m. For further evaluation the average values were used. Particle size composition, bulk density, total porosity and maximum capillary capacity were determined. Content of clay particles in soil profile was in interval 59.64-68.53% and such soils are characterised in the range from clay-loamy soil to clayey soil. The bulk density increased with the depth of soil profile and its values reached 1 184-1 646 kg m-3. The total porosity was in range 37.68-55.17% and it decreased with the depth of soil profile. The values of maximum capillary capacity were characterised for heavy soils with high content of clay particles. The depth had statistically significant effect on all observed parameters. In average, on both variants the bulk density was higher than 1 400 kg m-3, the total porosity was lower than 47% and the content of clay was higher than 30%, pointing to the possibility of soil compaction, which will result in reduced transport function of heavy soils.

Rare mutations in the human Na-K-Cl cotransporter (NKCC2) associated with lower blood pressure exhibit impaired processing and transport function

The Na-K-Cl cotransporter (NKCC2) is the major salt transport pathway in the thick ascending limb of Henle's loop and is part of the molecular mechanism for blood pressure regulation. Recent screening of ∼3,000 members of the Framingham Heart Study identified nine rare independent mutations in the gene encoding NKCC2 (SLC12A1) associated with clinically reduced blood pressure and protection from hypertension (Ji WZ, Foo JN, O'Roak BJ, Zhao H, Larson MG, Simon DB, Newton-Cheh C, State M, Levy D, Lifton RP. Nat Genet 40: 592-599, 2008). To investigate their functional consequences, we introduced the nine mutations in human NKCC2A and examined protein function, expression, localization, regulation, and ion transport kinetics using heterologous expression in Xenopus laevis oocytes and HEK-293 cells. When expressed in oocytes, four of the mutants (T235M, R302W, L505V, and P569H) exhibited reduced transport function compared with wild-type. In HEK-293 cells, the same four mutants exhibited reduced function, and in addition N399S and P1083A had significantly lower activity than wild-type. The two most functionally impaired mutants (R302W and L505V) exhibited dramatically diminished production of complex-glycosylated protein and a decrease in or absence of plasma membrane localization, indicative of a processing defect. All of the functional human (h) NKCC2A variants were regulated by changes in oocyte volume and intracellular chloride in HEK cells, but P254A and N399S exhibited a lower constitutive activity in HEK cells. The P569H mutant exhibited a 50% reduction in sodium affinity compared with wild-type, predicting lower transport activity at lower intratubular salt concentrations, while the P254A mutant exhibited a 35% increase in rubidium affinity. We conclude that defects in NKCC2 processing, transport turnover rate, regulation, and ion affinity contribute to impaired transport function in six of the nine identified mutants, providing support for the predictive approach of Ji et al. to identify functionally important residues by sequence conservation. Such mutations in hNKCC2A are likely to reduce renal salt reabsorption, providing a mechanism for lower blood pressure.

Entry to “ [formula omitted] Tunnel” Revealed by SLC4A4 Human Mutation and Structural Model

Glaucoma, cataracts, and proximal renal tubular acidosis are diseases caused by point mutations in the human electrogenic Na(+) bicarbonate cotransporter (NBCe1/SLC4A4) (1, 2). One such mutation, R298S, is located in the cytoplasmic N-terminal domain of NBCe1 and has only moderate (75%) function. As SLC transporters have high similarity in their membrane and N-terminal primary sequences, we homology-modeled NBCe1 onto the crystal structure coordinates of Band 3(AE1) (3). Arg-298 is predicted to be located in a solvent-inaccessible subsurface pocket and to associate with Glu-91 or Glu-295 via H-bonding and charge-charge interactions. We perturbed these putative interactions between Glu-91 and Arg-298 by site-directed mutagenesis and used expression in Xenopus oocyte to test our structural model. Mutagenesis of either residue resulted in reduced transport function. Function was "repaired" by charge reversal (E91R/R298E), implying that these two residues are interchangeable and interdependent. These results contrast the current understanding of the AE1 N terminus as protein-binding sites and propose that hkNBCe1 (and other SLC4) cytoplasmic N termini play roles in controlling HCO(3)(-) permeation.

Rosuvastatin pharmacokinetics and pharmacogenetics in white and Asian subjects residing in the same environment

Systemic exposure to rosuvastatin had been observed to be approximately 2-fold higher in Japanese subjects living in Japan compared with white subjects in Western Europe or the United States. The organic anion transporting polypeptide 1B1 contributes to the hepatic uptake of rosuvastatin. Polymorphisms in the SLCO1B1 gene can lead to reduced transport function in vitro (T 521>C). This study was conducted to determine whether the pharmacokinetic differences between Japanese and white subjects extended to other Asian ethnic groups and to determine whether polymorphisms in the SLCO1B1 gene contribute to any pharmacokinetic differences observed. Rosuvastatin pharmacokinetics was studied in an open-label, parallel-group, single-oral dose (40 mg) study in 36 white, 36 Chinese, 35 Malay, and 35 Asian-Indian subjects living in Singapore, Singapore. Plasma concentrations of rosuvastatin and metabolites were determined by HPLC-mass spectrophotometry. Two SLCO1B1 polymorphisms (A 388>G and T 521>C) were genotyped. Ratios for rosuvastatin area under the plasma concentration-time curve from time 0 to the time of the last quantifiable concentration were 2.31, 1.91, and 1.63 and ratios for maximum plasma concentration were 2.36, 2.00, and 1.68 in Chinese, Malay, and Asian-Indian subjects, respectively, compared with white subjects. Similar increases in exposure to N-desmethyl rosuvastatin and rosuvastatin-lactone were observed. SLCO1B1 genotypes did not account for the observed pharmacokinetic differences between Asians and white subjects. Plasma exposure to rosuvastatin and its metabolites was significantly higher in Chinese, Malay, and Asian-Indian subjects compared with white subjects living in the same environment.

The Fourth Transmembrane Segment of the Na,K-ATPase α Subunit: A SYSTEMATIC MUTAGENESIS STUDY

The Na,K-ATPase is a major ion-motive ATPase of the P-type family responsible for many aspects of cellular homeostasis. To determine the structure of the pathway for cations across the transmembrane portion of the Na,K-ATPase, we mutated 24 residues of the fourth transmembrane segment into cysteine and studied their function and accessibility by exposure to the sulfhydryl reagent 2-aminoethyl-methanethiosulfonate. Accessibility was also examined after treatment with palytoxin, which transforms the Na,K-pump into a cation channel. Of the 24 tested cysteine mutants, seven had no or a much reduced transport function. In particular cysteine mutants of the highly conserved "PEG" motif had a strongly reduced activity. However, most of the non-functional mutants could still be transformed by palytoxin as well as all of the functional mutants. Accessibility, determined as a 2-aminoethyl-methanethiosulfonate-induced reduction of the transport activity or as inhibition of the membrane conductance after palytoxin treatment, was observed for the following positions: Phe(323), Ile(322), Gly(326), Ala(330), Pro(333), Glu(334), and Gly(335). In accordance with a structural model of the Na,K-ATPase obtained by homology modeling with the two published structures of sarcoplasmic and endoplasmic reticulum calcium ATPase (Protein Data Bank codes 1EUL and 1IWO), the results suggest the presence of a cation pathway along the side of the fourth transmembrane segment that faces the space between transmembrane segments 5 and 6. The phenylalanine residue in position 323 has a critical position at the outer mouth of the cation pathway. The residues thought to form the cation binding site II ((333)PEGL) are also part of the accessible wall of the cation pathway opened by palytoxin through the Na,K-pump.

Na-K-ATPase activity decreases with aging in female rat brain synaptosomes.

To understand why elderly females are better able to tolerate hyponatremia, we measured brain Na-K-ATPase activity to determine whether this adaptive mechanism was affected by age. Using synaptosomes from 2-, 12-, and 19-mo-old female rats, we show in our results that Na-K-ATPase activity changes with age in female rats. Enzyme activity was significantly (P = 0.0026) reduced (17%) from 0.416 +/- 0.01 at 2 mo to 0.345 +/- 0.01 at 12 mo and reduced (P = 0.0001) (34%) to 0.274 +/- 0.02 micromol. min(-1). mg protein(-1) at 19 mo. To determine whether this decrease was due to reduced transport function of the Na-K-ATPase pump, we performed potassium transport using rubidium ((86)Rb+) as tracer. Ouabain-sensitive potassium uptake at 2 mo was 16.18 +/- 1.31 nmol/mg protein, was significantly (P = 0.0063) reduced (39%) to 9.79 +/- 1.44 nmol/mg at 12 mo, and was significantly (P = 0.0003) reduced (62%) to 6.12 +/- 1.05 nmol/mg protein at 19 mo. On the contrary, Na-K-ATPase activity remained elevated in males during aging. These data suggest that the Na-K-ATPase pump in female rat brain synaptosomes is decreased with increased age, and that this decrease is probably due in part to decreased potassium transport by the Na-K-ATPase pump.