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Hyperkalemia contributes to significant mortality and limits the use of cardioprotective and renoprotective renin–angiotensin–aldosterone blockers. Current therapies are poorly tolerated and not always effective. Here we conducted a phase 2 randomized, double-blind, placebo-controlled dose-escalation study to assess safety and efficacy of ZS-9. This oral selective cation exchanger that preferentially entraps potassium in the gastrointestinal tract was given to patients with stable Stage 3 chronic kidney disease and hyperkalemia (5.0 to 6.0 mEq/l) during a 2-day period. Of 90 eligible patients with mean baseline serum potassium of 5.1 mEq/l, 30 were randomized to placebo, 12–0.3 g, 24–3 g, or 24 to 10 g of ZS-9 three times daily for 2 days with regular meals. None withdrew and ZS-9 dose-dependently reduced serum potassium. The primary efficacy end point (rate of serum potassium decline in the first 48 h) was met with significance in the 3- and 10-g cohorts. From baseline, mean serum potassium was significantly decreased by 0.92±0.52 mEq/l at 38 h. Urinary potassium excretion significantly decreased with 10-g ZS-9 as compared to placebo at day 2 (+15.8 +/- 21.8 vs. +8.9 +/- 22.9 mEq per 24h) from placebo at day 2. In this short-term study, no serious adverse events were reported; only mild constipation in the 3-g dose group was possibly related to treatment. Thus, ZS-9 was well-tolerated in patients with stable chronic kidney disease and hyperkalemia leading to a rapid, sustained reduction in serum potassium.

High-capacity tonoplast cation/H+ antiport in plants is partially mediated by a family of CAX transporters. Previous studies have reported that CAX activity is affected by an N-terminal autoinhibitory region. CAXs may be present as heterodimers in plant cells, and this phenomenon necessitates further study. In this study, we demonstrate that there is an interaction between CAX4 and CAX1 as determined by the use of a yeast two-hybrid system and a bimolecular fluorescence complementation assay. More specifically, the N-terminal of CAX4 interacts with CAX1. We further observed the over-expression and either a single or double mutant of CAX1 and CAX4 in response to abiotic stress in Arabidopsis. These results suggest that CAX1 and CAX4 can interact to form a heterodimer, and the N-terminal regions of CAX4 play important roles in vivo; this may provide a foundation for a deep study of CAX4 function in the future.

Cysteine plays an essential role in cellular redox homoeostasis as a key constituent of the tripeptide glutathione (GSH). A rate limiting step in cellular GSH synthesis is the availability of cysteine. However, circulating cysteine exists in the blood as the oxidised di-peptide cystine, requiring specialised transport systems for its import into the cell. System xc − is a dedicated cystine transporter, importing cystine in exchange for intracellular glutamate. To counteract elevated levels of reactive oxygen species in cancerous cells system xc − is frequently upregulated, making it an attractive target for anticancer therapies. However, the molecular basis for ligand recognition remains elusive, hampering efforts to specifically target this transport system. Here we present the cryo-EM structure of system xc − in both the apo and glutamate bound states. Structural comparisons reveal an allosteric mechanism for ligand discrimination, supported by molecular dynamics and cell-based assays, establishing a mechanism for cystine transport in human cells. System xc- is a cystine transporter that is expressed in the plasma membrane and imports cystine in exchange for intracellular glutamate. Here, the authors present the cryo-EM structure of human system xc- both in the apo form and the glutamate bound state, and further supported by molecular dynamics and cell-based assays they discuss its cystine transport mechanism.

Arabidopsis thaliana contains the putative K + efflux transporters KEA1-KEA6, similar to KefB and KefC of Escherichia coli . KEA1-KEA3 are involved in the regulation of photosynthetic electron transport and chloroplast development. KEA4-KEA6 mediate pH regulation of the endomembrane network during salinity stress. However, the ion transport activities of KEA1-KEA6 have not been directly characterized. In this study, we used an E . coli expression system to examine KEA activity. KEA1-KEA3 and KEA5 showed bi-directional K + transport activity, whereas KEA4 and KEA6 functioned as a K + uptake system. The thylakoid membrane-localized Na + /H + antiporter NhaS3 from the model cyanobacterium Synechocystis is the closest homolog of KEA3. Changing the putative Na + /H + selective site of KEA3 (Gln-Asp) to that of NhaS3 (Asp-Asp) did not alter the ion selectivity without loss of K + transport activity. The first residue in the conserved motif was not a determinant for K + or Na + selectivity. Deletion of the possible nucleotide-binding KTN domain from KEA3 lowered K + transport activity, indicating that the KTN domain was important for this function. The KEA3-G422R mutation discovered in the Arabidopsis dpgr mutant increased K + transport activity, consistent with the mutant phenotype. These results indicate that Arabidopsis KEA1-KEA6 act as K + transport systems, and support the interpretation that KEA3 promotes dissipation of ΔpH in the thylakoid membrane.

Plants remodel their cells through the dynamic endomembrane system. Intracellular pH is important for membrane trafficking, but the determinants of pH homeostasis are poorly defined in plants. Electrogenic proton (H+) pumps depend on counter-ion fluxes to establish transmembrane pH gradients at the plasma membrane and endomembranes. Vacuolar-type H+-ATPase-mediated acidification of the trans-Golgi network is crucial for secretion and membrane recycling. Pump and counter-ion fluxes are unlikely to fine-tune pH; rather, alkali cation/H+ antiporters, which can alter pH and/or cation homeostasis locally and transiently, are prime candidates. Plants have a large family of predicted cation/H+ exchangers (CHX) of obscure function, in addition to the well-studied K+(Na+)/H+ exchangers (NHX). Here, we review the regulation of cytosolic and vacuolar pH, highlighting the similarities and distinctions of NHX and CHX members. In planta, alkalinization of the trans-Golgi network or vacuole by NHXs promotes membrane trafficking, endocytosis, cell expansion, and growth. CHXs localize to endomembranes and/or the plasma membrane and contribute to male fertility, pollen tube guidance, pollen wall construction, stomatal opening, and, in soybean (Glycine max), tolerance to salt stress. Three-dimensional structural models and mutagenesis of Arabidopsis (Arabidopsis thaliana) genes have allowed us to infer that AtCHX17 and AtNHX1 share a global architecture and a translocation core like bacterial Na+/H+ antiporters. Yet, the presence of distinct residues suggests that some CHXs differ from NHXs in pH sensing and electrogenicity. How H+ pumps, counter-ion fluxes, and cation/H+ antiporters are linked with signaling and membrane trafficking to remodel membranes and cell walls awaits further investigation.

Drug-resistance dissemination by horizontal gene transfer remains poorly understood at the cellular scale. Using live-cell microscopy, we reveal the dynamics of resistance acquisition by transfer of the Escherichia coli fertility factor–conjugation plasmid encoding the tetracycline-efflux pump TetA. The entry of the single-stranded DNA plasmid into the recipient cell is rapidly followed by complementary-strand synthesis, plasmid-gene expression, and production of TetA. In the presence of translation-inhibiting antibiotics, resistance acquisition depends on the AcrAB-TolC multidrug efflux pump, because it reduces tetracycline concentrations in the cell. Protein synthesis can thus persist and TetA expression can be initiated immediately after plasmid acquisition. AcrAB-TolC efflux activity can also preserve resistance acquisition by plasmid transfer in the presence of antibiotics with other modes of action.

Here, the function of the tomato (Solanum lycopersicon) K⁺/H⁺ antiporter LeNHX2 was studied using 35S-driven gene overexpression of a histagged LeNHX2 protein in Arabidopsis thaliana and LeNHX2 gene silencing in tomato. Transgenic A. thaliana plants expressed the histagged LeNHX2 both in shoots and in roots, as assayed by western blotting. Transitory expression of a green fluorescent protein (GFP) tagged protein showed that the antiporter is present in small vesicles. Internal membrane vesicles from transgenic plants displayed enhanced K⁺/H⁺ exchange activity, confirming the K⁺/H⁺ antiporter function of this enzyme. Transgenic A. thaliana plants overexpressing the histagged tomato antiporter LeNHX2 exhibited inhibited growth in the absence of K⁺ in the growth medium, but were more tolerant to high concentrations of Na⁺ than untransformed controls. When grown in the presence of NaCl, transgenic plants contained lower concentrations of intracellular Na⁺, but more K⁺, as compared with untransformed controls. Silencing of LeNHX2 in S. lycopersicon plants produced significant inhibition of plant growth and fruit and seed production as well as increased sensitivity to NaCl. The data indicate that regulation of K⁺ homeostasis by LeNHX2 is essential for normal plant growth and development, and plays an important role in the response to salt stress by improving K⁺ accumulation.

Virulent enteric pathogens such as Escherichia coli strain O157:H7 rely on acid-resistance (AR) systems to survive the acidic environment in the stomach. A major component of AR is an arginine-dependent arginine:agmatine antiporter that expels intracellular protons. Here, we report the crystal structure of AdiC, the arginine:agmatine antiporter from E. coli O157:H7 and a member of the amino acid/polyamine/organocation (APC) superfamily of transporters at 3.6 Å resolution. The overall fold is similar to that of several Na⁺-coupled symporters. AdiC contains 12 transmembrane segments, forms a homodimer, and exists in an outward-facing, open conformation in the crystals. A conserved, acidic pocket opens to the periplasm. Structural and biochemical analysis reveals the essential ligand-binding residues, defines the transport route, and suggests a conserved mechanism for the antiporter activity.

Abstract Background Down-Regulated in Adenoma (DRA) plays a critical role in intestinal chloride absorption and a decrease in its expression is a key event in diarrheal disorders. Recently, DRA has emerged as an Inflammatory Bowel Disease (IBD) susceptibility gene. Therefore, the strategies to upregulate DRA expression are potentially novel approaches to not only treat IBD-associated diarrhea but also gut inflammation. In this study, the effect of dexamethasone (DEX), an anti-inflammatory corticosteroid on DRA expression was investigated. Methods GR (glucocorticoid receptor) overexpressed Caco-2 cells and C57BL/6/J mice and anti-αIL-10R mAb model of IBD were used. Protein expression was assessed by immunoblotting and immunofluorescence. Transcript levels were assessed by quantative-real-time polymerase chain reaction (qRT-PCR) and promoter activity was measured by luciferase assays. Results Our results showed that DEX significantly increased DRA mRNA and protein expression in GR overexpressing Caco-2 cells. DEX-induced upregulation of DRA was GR dependent and appeared at least in part to occur via a transcriptional mechanism, as promoter activity of the DRA construct (−1183/+114 bp) was significantly increased in response to DEX. The increase in DRA mRNA was abrogated in the presence of MKP-1 inhibitor, triptolide. Administration of DEX (2 mg/kg body weight) to mice for 24 and 48 hours significantly increased the DRA expression in mouse colon. DEX treatment to mice for 7 days in the αIL-10R mAb model of colitis was able to significantly attenuate the gut inflammation and associated decrease in DRA expression. Conclusions We demonstrate that DEX stimulates DRA expression via transcriptional mechanisms and suggest that upregulation of DRA may contribute to both anti-inflammatory and pro-absorptive effects of DEX. Lay Summary DRA is critical for intestinal chloride absorption, and a decreased DRA expression is a key event in IBD-associated diarrhea. In our study, DEX upregulated DRA expression via transcriptional mechanisms, which could contribute to the dual benefits of DEX in anti-inflammatory and pro-absorptive processes.

The human glucose-6-phosphate transporter (G6PT) moves glucose-6-phosphate (G6P) into the lumen of endoplasmic reticulum, playing a vital role in glucose homeostasis. Dysregulation of G6PT causes glycogen storage disease 1b. Despite its functional importance, the structure, G6P recognition, and inhibition mechanism of G6PT remain unclear. Here, we report the cryo-EM structures of human G6PT in apo, G6P-bound, and the specific inhibitor chlorogenic acid (CHA)-bound forms, elucidating the structural basis for G6PT transport and inhibition. The G6P pocket comprises subsite A for phosphate and subsite B for glucose. The CHA occupies the G6P site and locks G6PT in a partly-occluded state. Functional assays demonstrate that G6PT activity is enhanced by co-expression of glucose-6-phosphatase (G6PC), but G6PT does not form a complex with G6PC. Together, this study provides a solid foundation for understanding the structure‒function relationships and pathology of G6PT and sheds light on the future development of potential therapeutics targeting G6PT. G6PT plays a vital role in glucose homeostasis by transporting glucose-6-phosphate to the lumen of ER. Here, authors present the cryo-EM structures of human G6PT in distinct forms, revealing the structural basis for transport and inhibition of G6PT.