Activation of Ca(2+)-activated K+ (maxi-K+) channel by angiotensin II in myocytes of the guinea pig ileum

The role of Na+-coupled bicarbonate transporters (NCBT) in health and disease

Cellular and organism survival depends upon the regulation of pH, which is regulated by highly specialized cell membrane transporters, the solute carriers (SLC) (For a comprehensive list of the solute carrier family members, see: https://www.bioparadigms.org/slc/ ). The SLC4 family of bicarbonate (HCO3-) transporters consists of ten members, sorted by their coupling to either sodium (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE), chloride (AE1, AE2, AE3), or borate (BTR1). The ionic coupling of SLC4A9 (AE4) remains controversial. These SLC4 bicarbonate transporters may be controlled by cellular ionic gradients, cellular membrane voltage, and signaling molecules to maintain critical cellular and systemic pH (acid-base) balance. There are profound consequences when blood pH deviates even a small amount outside the normal range (7.35-7.45). Chiefly, Na+-coupled bicarbonate transporters (NCBT) control intracellular pH in nearly every living cell, maintaining the biological pH required for life. Additionally, NCBTs have important roles to regulate cell volume and maintain salt balance as well as absorption and secretion of acid-base equivalents. Due to their varied tissue expression, NCBTs have roles in pathophysiology, which become apparent in physiologic responses when their expression is reduced or genetically deleted. Variations in physiological pH are seen in a wide variety of conditions, from canonically acid-base related conditions to pathologies not necessarily associated with acid-base dysfunction such as cancer, glaucoma, or various neurological diseases. The membranous location of the SLC4 transporters as well as recent advances in discovering their structural biology makes them accessible and attractive as a druggable target in a disease context. The role of sodium-coupled bicarbonate transporters in such a large array of conditions illustrates the potential of treating a wide range of disease states by modifying function of these transporters, whether that be through inhibition or enhancement.

Upregulated LRRC55 promotes BK channel activation and aggravates cell injury in podocytes

Podocyte injury is a common hallmark in various glomerular diseases. The level of LRRC55 was increased in podocytes of patients with focal segmental glomerulosclerosis (FSGS), diabetic nephropathy (DN), and membranous nephropathy (MN). Upregulated LRRC55 and increased intracellular Ca2+ led to BK channel activation and the loss of intracellular potassium, resulting in apoptosome formation and caspase-3 activation in angiotensin II (Ang II)-treated podocytes. Knockout of Lrrc55 or the BK channel prevented the BK current and ameliorated podocyte injury in Ang II-treated mice. Upstream, NFATc3 regulated the expression of LRRC55. Increased LRRC55 expression in podocytes was also evident in animal models of FSGS, DN, and MN. Treatment with losartan or LRRC55 siRNA suppressed LRRC55 expression, prevented BK channel activation, and attenuated podocyte injury in animal models of FSGS, DN, and MN. In conclusion, upregulated LRRC55 promotes BK channel activation and aggravates cell injury in podocytes in FSGS, DN, and MN. LRRC55 inhibition may represent a new therapeutic approach for podocyte injury.

Metabolic acidosis and hyperkalemia differentially regulate cation HCN3 channel in the rat nephron

The kidney controls body fluids, electrolyte and acid-base balance. Previously, we demonstrated that hyperpolarization-activated and cyclic nucleotide-gated (HCN) cation channels participate in ammonium excretion in the rat kidney. Since acid-base balance is closely linked to potassium metabolism, in the present work we aim to determine the effect of chronic metabolic acidosis (CMA) and hyperkalemia (HK) on protein abundance and localization of HCN3 in the rat kidney. CMA increased HCN3 protein level only in the outer medulla (2.74 ± 0.31) according to immunoblot analysis. However, immunofluorescence assays showed that HCN3 augmented in cortical proximal tubules (1.45 ± 0.11) and medullary thick ascending limb of Henle's loop (4.48 ± 0.45) from the inner stripe of outer medulla. HCN3 was detected in brush border membranes (BBM) and mitochondria of the proximal tubule by immunogold electron and confocal microscopy in control conditions. Acidosis did not alter HCN3 levels in BBM and mitochondria but augmented them in lysosomes. HCN3 was also immuno-detected in mitoautophagosomes. In the distal nephron, HCN3 was expressed in principal and intercalated cells from cortical to medullary collecting ducts. CMA did not change HCN3 abundance in these nephron segments. In contrast, HK doubled HCN3 level in cortical collecting ducts and favored its basolateral localization in principal cells from the inner medullary collecting ducts. These findings further support HCN channels contribution to renal acid-base and potassium balance.

Expression, Localization, and Regulation of the Sodium Bicarbonate Cotransporter NBCe1 in the Thyroid

BACKGROUND

The intrafollicular space of thyroid follicles is the storage compartment for thyroid hormones. Its pH has been established at around 7.6 at least after thyrotropin (TSH) stimulation. This alkaline intrafollicular pH is thought to be critical for iodide coupling to thyroglobulin and internalization of iodinated thyroglobulin. At least in mice, this alkalinization requires the expression of pendrin (Slc26a4) within the apical membrane, and a lack of pendrin results in acidic follicular lumen pH. Yet, the mechanism importing HCO₃^- into the cytoplasm is unknown. This study investigated whether the rather ubiquitous sodium bicarbonate cotransporter NBCe1 (SLC4A4) might play this role. It also examined which variant was expressed and where it was localized in both rat and human thyroid tissue. Lastly, the dependence of its expression on TSH was studied.

METHODS

Reverse transcription polymerase chain reaction, immunofluorescence, and Western blotting were used to test whether TSH stimulated NBCe1 protein expression in vivo. Subcellular localization of NBCe1 was performed using immunofluorescence in both rat and human thyroid. Cultured thyroid cells were also used to attempt to define how TSH affects NBCe1 expression.

RESULTS

Only transcripts of the NBCe1-B variant were detected in both rat and human thyroid. Of interest, NBCe1-C was not detected in human tissues, not even in the brain. On immunofluorescence microscopy, the immunostaining of NBCe1 mainly appeared in the basolateral membrane upon stimulation with TSH. This TSH induction of basolateral membrane expression of NBCe1 protein was confirmed in vivo in rat thyroid and in vitro on human thyroid slices.

CONCLUSIONS

This study demonstrates the expression of the sodium bicarbonate cotransporter NBCe1-B in rat and human thyroid. Additionally, the data suggest that TSH blocks the degradation of NBCe1 protein by trafficking it to the basolateral membrane. Hence, TSH increases NBCe1 half-life without increasing its synthesis.

Development, regeneration, and evolution of feathers

The feather is a complex ectodermal organ with hierarchical branching patterns. It provides functions in endothermy, communication, and flight. Studies of feather growth, cycling, and health are of fundamental importance to avian biology and poultry science. In addition, feathers are an excellent model for morphogenesis studies because of their accessibility, and their distinct patterns can be used to assay the roles of specific molecular pathways. Here we review the progress in aspects of development, regeneration, and evolution during the past three decades. We cover the development of feather buds in chicken embryos, regenerative cycling of feather follicle stem cells, formation of barb branching patterns, emergence of intrafeather pigmentation patterns, interplay of hormones and feather growth, and the genetic identification of several feather variants. The discovery of feathered dinosaurs redefines the relationship between feathers and birds. Inspiration from biomaterials and flight research further fuels biomimetic potential of feathers as a multidisciplinary research focal point.

Covariation in life-history traits: differential effects of diet on condition, hormones, behavior, and reproduction in genetic finch morphs

The relative contribution of genetic and environmental factors in determining variation in life-history traits is of central interest to evolutionary biologists, but the physiological mechanisms underlying these traits are still poorly understood. Here we experimentally demonstrate opposing effects of nutritional stress on immune function, endocrine physiology, parental care, and reproduction between red and black head-color morphs of the Gouldian finch (Erythrura gouldiae). Although the body condition of black morphs was largely unaffected by diet manipulation, red birds were highly sensitive to dietary changes, exhibiting considerable within-individual changes in condition and immune function. Consequently, nutritionally stressed red birds delayed breeding, produced smaller broods, and reared fewer and lower-quality foster offspring than black morphs. Differences in offspring quality were largely due to morph-specific differences in parental effort: red morphs reduced parental provisioning, whereas black morphs adaptively elevated their provisioning effort to meet the increased nutritional demands of their foster brood. Nutritionally stressed genetic morphs also exhibited divergent glucocorticoid responses. Black morphs showed reduced corticosterone-binding globulin (CBG) concentrations and increased levels of free corticosterone, whereas red morphs exhibited reduced free corticosterone levels and elevated CBG concentrations. These opposing glucocorticoid responses highlight intrinsic differences in endocrine sensitivities and plasticity between genetic morphs, which may underlie the morph-specific differences in condition, behavior, and reproduction and thus ultimately contribute to the evolution and maintenance of color polymorphism.

Slc26a9 is inhibited by the R-region of the cystic fibrosis transmembrane conductance regulator via the STAS domain

SLC26 proteins function as anion exchangers, channels, and sensors. Previous cellular studies have shown that Slc26a3 and Slc26a6 interact with the R-region of the cystic fibrosis transmembrane conductance regulator (CFTR), (R)CFTR, via the Slc26-STAS (sulfate transporter anti-sigma) domain, resulting in mutual transport activation. We recently showed that Slc26a9 has both nCl(-)-HCO(3)(-) exchanger and Cl(-) channel function. In this study, we show that the purified STAS domain of Slc26a9 (a9STAS) binds purified (R)CFTR. When Slc26a9 and (R)CFTR fragments are co-expressed in Xenopus oocytes, both Slc26a9-mediated nCl(-)-HCO(3)(-) exchange and Cl(-) currents are almost fully inhibited. Deletion of the Slc26a9 STAS domain (a9-DeltaSTAS) virtually eliminated the Cl(-) currents with only a modest affect on nCl(-)-HCO(3)(-) exchange activity. Co-expression of a9-DeltaSTAS and the (R)CFTR fragment did not alter the residual a9-DeltaSTAS function. Replacing the Slc26a9 STAS domain with the Slc26a6 STAS domain (a6-a9-a6) does not change Slc26a9 function and is no longer inhibited by (R)CFTR. These data indicate that the Slc26a9-STAS domain, like other Slc26-STAS domains, binds CFTR in the R-region. However, unlike previously reported data, this binding interaction inhibits Slc26a9 ion transport activity. These results imply that Slc26-STAS domains may all interact with (R)CFTR but that the physiological outcome is specific to differing Slc26 proteins, allowing for dynamic and acute fine tuning of ion transport for various epithelia.

Structural and functional characterization of the human NBC3 sodium/bicarbonate co-transporter carboxyl-terminal cytoplasmic domain

The sodium bicarbonate co-transporter, NBC3, is expressed in a range of tissues including heart, skeletal muscle and kidney, where it modulates intracellular pH and bicarbonate levels. NBC3 has a three-domain structure: 67 kDa N-terminal cytoplasmic domain, 57 kDa membrane domain and an 11 kDa C-terminal cytoplasmic domain (NBC3Ct). The role of C-terminal domains as important regulatory regions is an emerging theme in bicarbonate transporter physiology. This study determined the functional role of human NBC3Ct and characterized its structure using biochemical techniques. The NBC3 C-terminal domain deletion mutant (NBC3DeltaCt) had only 12 +/- 5% of wild-type transport activity. This low activity is attributable to low steady-state levels of NBC3DeltaCt and almost complete retention inside the cell, as assessed by immunoblots and confocal microscopy, suggesting a role of NBC3Ct in cell surface processing. To characterize the structure of NBC3Ct, amino acids 1127-1214 of NBC3 were expressed as a GST fusion protein (GST.NBC3Ct). GST.NBC3Ct was cleaved with PreScission Protease and native NBC3Ct could be purified to 94% homogeneity. Gel permeation chromatography and sedimentation velocity ultracentrifugation of NBC3Ct indicated a Stokes radius of 26 and 30 angstroms, respectively. Shape modelling revealed NBC3Ct as a prolate shape with long and short axes of 19 and 2 nm, respectively. The circular dichroism spectra of NBC3Ct did not change over the pH 6.2-7.8 range, which rules out a large change of secondary structure as a component of pH sensor function. Proteolysis with trypsin and chymotrypsin identified two proteolytically sensitive regions, R1129 and K1183-K1186, which could form protein interaction sites.

Expression of the electrogenic Na+-HCO3--cotransporters NBCe1-A and NBCe1-B in rat pancreatic islet cells

It was recently proposed that, in rat pancreatic islets, the production of bicarbonate accounts for the major fraction of the carbon dioxide generated by the oxidative catabolism of nutrient insulin secretagogues. In search of the mechanism(s) supporting the membrane transport of bicarbonate, the possible role of the electrogenic Na(+)-HCO(3) (-)-cotransporters NBCe1-A and NBCe1-B in rat pancreatic islet cells was investigated. Expression of NBCe1-A and NBCe1-B in rat pancreatic islet cells was documented by RT-PCR, western blotting, and immunocytochemistry. The latter procedure suggested a preferential localization of NBCe1-B in insulin-producing cells. Tenidap (3-100 microM), previously proposed as an inhibitor of NBCe1-A-mediated cotransport in proximal tubule kidney cells, caused a concentration-related inhibition of glucose-stimulated insulin secretion. It also inhibited 2-ketoisocaproate-induced insulin release and to a relatively lesser extent, the secretory response to L: -leucine. Tenidap (50-100 microM) also inhibited the metabolism of D: -glucose in isolated islets, increased (22)Na net uptake by dispersed islet cells, lowered intracellular pH and provoked hyperpolarization of plasma membrane in insulin-producing cells. This study thus reveals the expression of the electrogenic Na(+)-HCO(3) (-)-cotransporters NBCe1-A and NBCe1-B in rat pancreatic islet cells, and is consistent with the participation of such transporters in the process of nutrient-stimulated insulin secretion.

Slc26a9--anion exchanger, channel and Na+ transporter

The SLC26 gene family encodes anion transporters with diverse functional attributes: (a) anion exchanger, (b) anion sensor, and (c) anion conductance (likely channel). We have cloned and studied Slc26a9, a paralogue expressed mostly in lung and stomach. Immunohistochemistry shows that Slc26a9 is present at apical and intracellular membranes of lung and stomach epithelia. Using expression in Xenopus laevis oocytes and ion-sensitive microelectrodes, we discovered that Slc26a9 has a novel function not found in any other Slc26 proteins: cation coupling. Intracellular pH and voltage measurements show that Slc26a9 is a nCl(-)-HCO(3)(-) exchanger, suggesting roles in gastric HCl secretion or pulmonary HCO(3)(-) secretion; Na(+) electrodes and uptakes reveal that Slc26a9 has a cation dependence. Single-channel measurements indicate that Slc26a9 displays discrete open and closed states. These experiments show that Slc26a9 has three discrete physiological modes: nCl(-)-HCO(3)(-) exchanger, Cl(-) channel, and Na(+)-anion cotransporter. Thus, the Slc26a9 transporter channel is uniquely suited for dynamic and tissue-specific physiology or regulation in epithelial tissues.

Concentration-dependent effects on intracellular and surface pH of exposing Xenopus oocytes to solutions containing NH3/NH4(+)

Others have shown that exposing oocytes to high levels of NH(3)/NH(4)(+) (10-20 mM) causes a paradoxical fall in intracellular pH (pH(i)), whereas low levels (e.g., 0.5 mM) cause little pH(i) change. Here we monitored pH(i) and extracellular surface pH (pH(S)) while exposing oocytes to 5 or 0.5 mM NH(3)/NH(4)(+). We confirm that 5 mM NH(3)/NH(4)(+) causes a paradoxical pH(i) fall (-DeltapH(i) approximately equal 0.2), but also observe an abrupt pH(S) fall (-DeltapH(S) approximately equal 0.2)-indicative of NH(3) influx-followed by a slow decay. Reducing [NH(3)/NH(4)(+)] to 0.5 mM minimizes pH(i) changes but maintains pH(S) changes at a reduced magnitude. Expressing AmtB (bacterial Rh homologue) exaggerates -DeltapH(S) at both NH(3)/NH(4)(+) levels. During removal of 0.5 or 5 mM NH(3)/NH(4)(+), failure of pH(S) to markedly overshoot bulk extracellular pH implies little NH(3) efflux and, thus, little free cytosolic NH(3)/NH(4)(+). A new analysis of the effects of NH(3) vs. NH(4)(+) fluxes on pH(S) and pH(i) indicates that (a) NH(3) rather than NH(4)(+) fluxes dominate pH(i) and pH(S) changes and (b) oocytes dispose of most incoming NH(3). NMR studies of oocytes exposed to (15)N-labeled NH(3)/NH(4)(+) show no significant formation of glutamine but substantial NH(3)/NH(4)(+) accumulation in what is likely an acid intracellular compartment. In conclusion, parallel measurements of pH(i) and pH(S) demonstrate that NH(3) flows across the plasma membrane and provide new insights into how a protein molecule in the plasma membrane-AmtB-enhances the flux of a gas across a biological membrane.

Impaired cardiac contractility in mice lacking both the AE3 Cl-/HCO3- exchanger and the NKCC1 Na+-K+-2Cl- cotransporter: effects on Ca2+ handling and protein phosphatases

To analyze the cardiac functions of AE3, we disrupted its gene (Slc4a3) in mice. Cl(-)/HCO3(-) exchange coupled with Na+-dependent acid extrusion can mediate pH-neutral Na+ uptake, potentially affecting Ca2+ handling via effects on Na+/Ca2+ exchange. AE3 null mice appeared normal, however, and AE3 ablation had no effect on ischemia-reperfusion injury in isolated hearts or cardiac performance in vivo. The NKCC1 Na+-K+-2Cl(-) cotransporter also mediates Na+ uptake, and loss of NKCC1 alone does not impair contractility. To further stress the AE3-deficient myocardium, we combined the AE3 and NKCC1 knock-outs. Double knock-outs had impaired contraction and relaxation both in vivo and in isolated ventricular myocytes. Ca2+ transients revealed an apparent increase in Ca2+ clearance in double null cells. This was unlikely to result from increased Ca2+ sequestration, since the ratio of phosphorylated phospholamban to total phospholamban was sharply reduced in all three mutant hearts. Instead, Na+/Ca2+ exchanger activity was found to be enhanced in double null cells. Systolic Ca2+ was unaltered, however, suggesting more direct effects on the contractile apparatus of double null myocytes. Expression of the catalytic subunit of protein phosphatase 1 was increased in all mutant hearts. There was also a dramatic reversal, between single null and double null hearts, in the carboxymethylation and localization to the myofibrillar fraction, of the catalytic subunit of protein phosphatase 2A, which corresponded to the loss of normal contractility in double null hearts. These data show that AE3 and NKCC1 affect Ca2+ handling, PLN regulation, and expression and localization of major cardiac phosphatases and that their combined loss impairs cardiac function.

The IRBIT domain adds new functions to the AHCY family

During the past few years, the IRBIT domain has emerged as an important add-on of S-adenosyl-L-homocystein hydrolase (AHCY), thereby creating the new family of AHCY-like proteins. In this review, we discuss the currently available data on this new family of proteins. We describe the IRBIT domain as a unique part of these proteins and give an overview of its regulation via (de)phosphorylation and proteolysis. The second part of this review is focused on the potential functions of the AHCY-like proteins. We propose that the IRBIT domain serves as an anchor for targeting AHCY-like proteins towards cytoplasmic targets. This leads to regulation of (i) intracellular Ca2+ via the inositol 1,4,5-trisphosphate receptor (IP3R), (ii) intracellular pH via the Na+/HCO3 - cotransporters (NBCs); whereas inactivation of the IRBIT domain induces (iii) nuclear translocation and regulation of AHCY activity. Dysfunction of AHCY-like proteins will disturb these three important functions, with various biological implications.

Entry to "formula 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.

Characterization of human SLC4A10 as an electroneutral Na/HCO3 cotransporter (NBCn2) with Cl- self-exchange activity

The SLC4A10 gene product, commonly known as NCBE, is highly expressed in rodent brain and has been characterized by others as a Na(+)-driven Cl-HCO(3) exchanger. However, some of the earlier data are not consistent with Na(+)-driven Cl-HCO(3) exchange activity. In the present study, northern blot analysis showed that, also in humans, NCBE transcripts are predominantly expressed in brain. In some human NCBE transcripts, splice cassettes A and/or B, originally reported in rats and mice, are spliced out. In brain cDNA, we found evidence of a unique partial splice of cassette B that is predicted to produce an NCBE protein with a novel C terminus containing a protein kinase C phosphorylation site. We used pH-sensitive microelectrodes to study the molecular physiology of human NCBE expressed in Xenopus oocytes. In agreement with others we found that NCBE mediates the 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid-sensitive, Na(+)-dependent transport of HCO(3)(-). For the first time, we demonstrated that this transport process is electroneutral. Using Cl(-)-sensitive microelectrodes positioned at the oocyte surface, we found that, unlike both human and squid Na(+)-driven Cl-HCO(3) exchangers, human NCBE does not normally couple the net influx of HCO(3)(-) to a net efflux of Cl(-). Moreover we found that that the (36)Cl efflux from NCBE-expressing oocytes, interpreted by others to be coupled to the influx of Na(+) and HCO(3)(-), actually represents a CO(2)/HCO(3)(-)-stimulated Cl(-) self-exchange not coupled to either Na(+) or net HCO(3)(-) transport. We propose to rename NCBE as the second electroneutral Na/HCO(3) cotransporter, NBCn2.

Zebrafish Slc5a12 Encodes an Electroneutral Sodium Monocarboxylate Transporter (SMCTn)

We have identified and characterized two different sodium-coupled monocarboxylate cotransporters (SMCT) from zebrafish (Danio rerio), electrogenic (zSMCTe) and electroneutral (zSMCTn). zSMCTn is the 12th member of the zebrafish Slc5 gene family (zSlc5a12). Both zSMCT sequences have approximately 50% homology to human SLC5A8 (hSMCT). Transport function and kinetics were measured in Xenopus oocytes injected with zSMCT cRNAs by measurement of intracellular Na(+) concentration ([Na(+)](i)) and membrane potential. Both zSMCTs oocytes increased [Na(+)](i) with addition of monocarboxylates (MC) such as lactate, pyruvate, nicotinate, and butyrate. By using two electrode voltage clamp experiments, we measured currents elicited from zSMCTe after MC addition. MC-elicited currents from zSMCTe were similar to hSMCT currents. In contrast, we found no significant MC-elicited current in either zSMCTn or control oocytes. Kinetic data show that zSMCTe has a higher affinity for lactate, nicotinate, and pyruvate (K(m)(L-lactate) = 0.17 +/- 0.02 mM, K(m)(nicotinate) = 0.54 +/- 0.12 mM at -150 mV) than zSMCTn (K(m)(L-lactate) = 1.81 +/- 0.19 mM, K(m)(nicotinate) = 23.68 +/- 4.88 mM). In situ hybridization showed that 1-, 3-, and 5-day-old zebrafish embryos abundantly express both zSMCTs in the brain, eyes, intestine, and kidney. Within the kidney, zSMCTn mRNA is expressed in pronephric tubules, whereas zSMCTe mRNA is more distal in pronephric ducts. zSMCTn is expressed in exocrine pancreas, but zSMCTe is not. Roles for Na(+)-coupled monocarboxylate cotransporters have not been described for the brain or eye. In summary, zSMCTe is the zebrafish SLC5A8 ortholog, and zSMCTn is a novel, electroneutral SMCT (zSlc5a12). Slc5a12 in higher vertebrates is likely responsible for the electroneutral Na(+)/lactate cotransport reported in mammalian and amphibian kidneys.

A Cysteine-scanning Mutagenesis Study of Transmembrane Domain 8 of the Electrogenic Sodium/Bicarbonate Cotransporter NBCe1

Na/HCO(3) cotransporters (NBCs) such as NBCe1 are members of a superfamily of bicarbonate transporters that includes anion exchangers. Residues within putative transmembrane domain 8 (TMD8) of anion exchanger 1 are involved in ion translocation (Tang, X. B., Kovacs, M., Sterling, D., and Casey, J. R. (1999) J. Biol. Chem. 274, 3557-3564), and the corresponding domain in NBCe1 variants is highly homologous. We performed cysteine-scanning mutagenesis to examine the role of TMD8 residues in ion translocation by rat NBCe1-A. We accessed function and/or sulfhydryl sensitivity and p-chloromercuribenzene sulfonate (pCMBS) accessibility of 21 cysteine-substituted NBC mutants expressed in Xenopus oocytes using the two-electrode, voltage clamp technique. Five NBC mutants displayed <10% wild-type activity: P743C, A744C, L746C, D754C, and T758C. For the remaining 16 mutants, we compared transporter-mediated inward currents elicited by removing external Na(+) before and after exposing oocytes to either 2-aminoethylmethane thiosulfonate (MTSEA) or pCMBS. MTSEA inhibited NBC mutants T748C, I749C, I751C, F752C, M753C, and Q756C by 9-19% and stimulated mutants A739C, A741C, L745C, V747C, Q755C, and I757C by 11-21%. pCMBS mildly inhibited mutants A739C, A740, V747C, and Q756C by 5 or 8%, and stimulated I749C by 10%. However, both sulfhydryl reagents strongly inhibited the L750C mutant by > or =85%. Using the substituted cysteine accessibility method, we examined the accessibility of the NBC mutant L750C under different transporter conditions. pCMBS accessibility is (i) reduced when the transporter is active in the presence of both Na(+) and HCO(3)(-), likely due to substrate competition with pCMBS; (ii) reduced in the presence of a stilbene inhibitor; and (iii) stimulated at more positive membrane potentials. In summary, TMD8 residues of NBCe1, particularly L750, are involved in ion translocation, and accessibility is influenced by the state of transporter activity.

A novel missense mutation in the sodium bicarbonate cotransporter (NBCe1/SLC4A4) causes proximal tubular acidosis and glaucoma through ion transport defects

In humans and terrestrial vertebrates, the kidney controls systemic pH in part by absorbing filtered bicarbonate in the proximal tubule via an electrogenic Na+/HCO3- cotransporter (NBCe1/SLC4A4). Recently, human genetics revealed that NBCe1 is the major renal contributor to this process. Homozygous point mutations in NBCe1 cause proximal renal tubular acidosis (pRTA), glaucoma, and cataracts (Igarashi, T., Inatomi, J., Sekine, T., Cha, S. H., Kanai, Y., Kunimi, M., Tsukamoto, K., Satoh, H., Shimadzu, M., Tozawa, F., Mori, T., Shiobara, M., Seki, G., and Endou, H. (1999) Nat. Genet. 23, 264-266). We have identified and functionally characterized a novel, homozygous, missense mutation (S427L) in NBCe1, also resulting in pRTA and similar eye defects without mental retardation. To understand the pathophysiology of the syndrome, we expressed wild-type (WT) NBCe1 and S427L-NBCe1 in Xenopus oocytes. Function was evaluated by measuring intracellular pH (HCO3- transport) and membrane currents using microelectrodes. HCO3- -elicited currents for S427L were approximately 10% of WT NBCe1, and CO2-induced acidification was approximately 4-fold faster. Na+ -dependent HCO3- transport (currents and acidification) was also approximately 10% of WT. Current-voltage (I-V) analysis reveals that S427L has no reversal potential in HCO3-, indicating that under physiological ion gradient conditions, NaHCO3 could not move out of cells as is needed for renal HCO3- absorption and ocular pressure homeostasis. I-V analysis without Na+ further shows that the S427L-mediated NaHCO3 efflux mode is depressed or absent. These experiments reveal that voltage- and Na+ -dependent transport by S427L-hkNBCe1 is unfavorably altered, thereby causing both insufficient HCO3- absorption by the kidney (proximal RTA) and inappropriate anterior chamber fluid transport (glaucoma).

Bicarbonate transport proteins

Bicarbonate is not freely permeable to membranes. Yet, bicarbonate must be moved across membranes, as part of CO2 metabolism and to regulate cell pH. Mammalian cells ubiquitously express bicarbonate transport proteins to facilitate the transmembrane bicarbonate flux. These bicarbonate transporters, which function by different transport mechanisms, together catalyse transmembrane bicarbonate movement. Recent advances have allowed the identification of several new bicarbonate transporter genes. Bicarbonate transporters cluster into two separate families: (i) the anion exachanger (AE) family of Cl-/HCO3- exchangers is related in sequence to the NBC family of Na+/HCO3- cotransporters and the Na(+)-dependent Cl/HCO3- exchangers and (ii) some members of the SLC26a family of sulfate transporters will also transport bicarbonate but are not related in sequence to the AE/NBC family of transporters. This review summarizes our understanding of the mammalian bicarbonate transporter superfamily.

Cloning and functional characterization of a novel aquaporin from Xenopus laevis oocytes

We have cloned a novel aquaporin (AQP) from Xenopus laevis oocytes, which we have provisionally named AQPxlo. The predicted protein showed highest homology (39-50%) to aquaglyceroporins. Northern blot analysis showed strong hybridization to an approximately 1.4-kb transcript in X. laevis fat body and oocytes, whereas a weaker signal was obtained in kidney. We injected in vitro transcribed cRNA encoding AQPxlo into Xenopus oocytes for functional characterization. AQPxlo expression increased osmotic water permeability (P(f)), as well as the uptake of glycerol and urea. However, AQPxlo excluded larger polyols and thiourea. An alkaline extracellular pH (pH(o)) increased P(f) and to a lesser extent urea uptake but not glycerol uptake. Remarkably, low HgCl(2) concentrations (0.3-10 microm) reduced P(f) and urea uptake, whereas high concentrations (300-1000 microm) reversed the inhibition. We propose that AQPxlo is a new AQP paralogue unknown in mammals.

Renin-angiotensin system expression and secretory function in cultured human ciliary body non-pigmented epithelium

BACKGROUND

Renin-angiotensin system (RAS) components have been identified in human ciliary body and aqueous humour, pointing to a role for the RAS in the regulation of aqueous humour dynamics. Here, the authors examine the functional expression of a RAS and the effects of angiotensin II (AII) on a signal transduction pathway and ion secretion mechanism in cultured human ciliary body non-pigmented epithelium (HNPE).

METHODS

RAS expression was examined in cultured HNPE cells using polymerase chain reaction (PCR) analysis. Secretory function was determined using spectrofluorescence imaging microscopy to measure cell calcium (Ca(2+)(I)) and volume responses. Single channel patch clamp techniques were employed to investigate ion channel activity.

RESULTS

PCR analysis demonstrated the expression of angiotensinogen and the AT(1b) receptor in HNPE cells. A large conductance potassium (BK) channel (mean 190 (SEM 5.6) pS, n = 22 cells), was observed in plasma membrane patches. This channel was calcium sensitive with channel open probability (Po) increasing with increasing Ca(2+)(I) (K(0.5) 10.79 (0.44) microM Ca(2+), Hill coefficient of 1.04 (0.04)). AII (100 nM) increased the number (N) of active BK channels in HNPE cells and also the probability of channel opening (Po). N.P(o) increased from 0.008 (0.002) to 1.38 (0.4) following the addition of AII (p=0.0064). AII also induced a rapid rise in Ca(2+)(I) from resting values of 112 (14) nM to a peak of 992 (106) nM (p<10(-4)). A simultaneous cell volume reduction of 24.70% (3.34%) (p<10(-4)) occurred during this calcium signal. Losartan (1 microM) significantly blocked the AII induced BK channel activation (p=0.0131), the Ca(2+)(I) response (p<10(-4)), and the AII induced volume effect (p=0.0046).

CONCLUSION

It was demonstrated that AII activates a Ca(2+)(I) signalling system which subsequently increases potassium ion channel activity. These effects are accompanied simultaneously by cell volume loss, indicating that AII acts as receptor operated secretagogue in HNPE cells. The ability of an AT(1) receptor antagonist to inhibit these processes may thus offer a new family of pharmaceutical agents to the current armamentarium in the treatment of glaucoma.

Protein kinase C modulators enhance angiotensin II desensitization of guinea pig ileum via maxi-K+ channels

We investigated the role of protein kinase C in the desensitization of the angiotensin II-induced contraction of guinea pig ileum. In contrast to their antagonistic effects on enzymatic activity, both activator and blockers accelerated the dissipation of the 10(-7) M angiotensin II isometric contractile response. These agents indirectly activated maxi-K+ channels in cell-attached membrane patches from freshly dispersed myocytes bathed in high-K+ solution and clamped at -40 mV. In parallel with the contractile responses, fura 2-loaded myocytes bathed in Tyrode solution showed additive increases in [Ca2+]i in response to both angiotensin II and phorbol dibutyrate (PDB). The PDB-promoted increase of the rate of angiotensin II desensitization was completely abolished by pretreatment of the tissue strips with 93 nM iberiotoxin or 8 mM KCl. Thus, we conclude that protein kinase C modulators promote faster angiotensin II desensitization by recruiting maxi-K+ channels and inducing membrane repolarization rather than by affecting the protein kinase C activity.

Cloning, characterization, and chromosomal mapping of a human electroneutral Na(+)-driven Cl-HCO3 exchanger

The electroneutral Na(+)-driven Cl-HCO3 exchanger is a key mechanism for regulating intracellular pH (pH(i)) in neurons, glia, and other cells. Here we report the cloning, tissue distribution, chromosomal location, and functional characterization of the cDNA of such a transporter (NDCBE1) from human brain (GenBank accession number AF069512). NDCBE1, which encodes 1044 amino acids, is 34% identical to the mammalian anion exchanger (AE2); approximately 50% to the electrogenic Na/HCO3 cotransporter (NBCe1) from salamander, rat, and humans; approximately 73% to mammalian electroneutral Na/HCO3 cotransporters (NBCn1); 71% to mouse NCBE; and 47% to a Na(+)-driven anion exchanger (NDAE1) from Drosophila. Northern blot analysis of NDCBE1 shows a robust approximately 12-kilobase signal in all major regions of human brain and in testis, and weaker signals in kidney and ovary. This human gene (SLC4A8) maps to chromosome 12q13. When expressed in Xenopus oocytes and running in the forward direction, NDCBE1 is electroneutral and mediates increases in both pH(i) and [Na(+)](i) (monitored with microelectrodes) that require HCO3(-) and are blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The pH(i) increase also requires extracellular Na(+). The Na(+):HCO3(-) stoichiometry is 1:2. Forward-running NDCBE1 mediates a 36Cl efflux that requires extracellular Na(+) and HCO3(-) and is blocked by DIDS. Running in reverse, NDCBE1 requires extracellular Cl(-). Thus, NDCBE1 encodes a human, electroneutral Na(+)-driven Cl-HCO3 exchanger.

Cloning and characterization of a Na+-driven anion exchanger (NDAE1). A new bicarbonate transporter

Regulation of intra- and extracellular ion activities (e.g. H(+), Cl(-), Na(+)) is key to normal function of the central nervous system, digestive tract, respiratory tract, and urinary system. With our cloning of an electrogenic Na(+)/HCO(3)(-) cotransporter (NBC), we found that NBC and the anion exchangers form a bicarbonate transporter superfamily. Functionally three other HCO(3)(-) transporters are known: a neutral Na(+)/ HCO(3)(-) cotransporter, a K(+)/ HCO(3)(-) cotransporter, and a Na(+)-dependent Cl(-)-HCO(3)(-) exchanger. We report the cloning and characterization of a Na(+)-coupled Cl(-)-HCO(3)(-) exchanger and a physiologically unique bicarbonate transporter superfamily member. This Drosophila cDNA encodes a 1030-amino acid membrane protein with both sequence homology and predicted topology similar to the anion exchangers and NBCs. The mRNA is expressed throughout Drosophila development and is prominent in the central nervous system. When expressed in Xenopus oocytes, this membrane protein mediates the transport of Cl(-), Na(+), H(+), and HCO(3)(-) but does not require HCO(3)(-). Transport is blocked by the stilbene 4,4'-diisothiocyanodihydrostilbene- 2, 2'-disulfonates and may not be strictly electroneutral. Our functional data suggest this Na(+) driven anion exchanger (NDAE1) is responsible for the Na(+)-dependent Cl(-)-HCO(3)(-) exchange activity characterized in neurons, kidney, and fibroblasts. NDAE1 may be generally important for fly development, because disruption of this gene is apparently lethal to the Drosophila larva.

An electroneutral sodium/bicarbonate cotransporter NBCn1 and associated sodium channel

Two electroneutral, Na+-driven HCO3- transporters, the Na+-driven Cl-/HCO3- exchanger and the electroneutral Na+/HCO3- cotransporter, have crucial roles in regulating intracellular pH in a variety of cells, including cardiac myocytes, vascular smooth-muscle, neurons and fibroblasts; however, it is difficult to distinguish their Cl- dependence in mammalian cells. Here we report the cloning of three variants of an electroneutral Na+/HCO3- cotransporter, NBCn1, from rat smooth muscle. They are 89-92% identical to a human skeletal muscle clone, 55-57% identical to the electrogenic NBCs and 33-43% identical to the anion exchangers. When expressed in Xenopus oocytes, NBCn1-B (which encodes 1,218 amino acids) is electroneutral, Na+-dependent and HCO3(-)-dependent, but not Cl(-)-dependent. Oocytes injected with low levels of NBCn1-B complementary RNA exhibit a Na+ conductance that 4,4-diisothiocyanatostilbene-2,2'-disulphonate stimulates slowly and irreversibly.

Mode of action of ANG II on ion transport in guinea pig distal colon

The effect of ANG II on mucosal ion transport and localization of ANG type 1 receptor (AT(1)R) in the guinea pig distal colon was investigated. Submucosal/mucosal segments were mounted in Ussing flux chambers, and short-circuit current (I(sc)) was measured as an index of ion transport. Serosal addition of ANG II produced a concentration-dependent (10(-9)-10(-5) M) increase in I(sc). The maximal response was observed at 10(-6) M; the increase in I(sc) was 164.4 +/- 11.8 microA/cm(2). The ANG II (10(-6) M)-evoked response was mainly due to Cl(-) secretion. Tetrodotoxin, atropine, the neurokinin type 1 receptor antagonist FK-888, and piroxicam significantly reduced the ANG II (10(-6) M)-evoked response to 28, 45, 58, and 16% of control, respectively. Pretreatment with prostaglandin E(2) (10(-5) M) resulted in a threefold increase in the ANG II-evoked response. The AT(1)R antagonist FR-130739 completely blocked ANG II (10(-6) M)-evoked responses, whereas the ANG type 2 receptor antagonist PD-123319 had no effect. Localization of AT(1)R was determined by immunohistochemistry. In the immunohistochemical study, AT(1)R-immunopositive cells were distributed clearly in enteric nerves and moderately in surface epithelial cells. These results suggest that ANG II-evoked electrogenic Cl(-) secretion may involve submucosal cholinergic and tachykinergic neurons and prostanoid synthesis pathways through AT(1)R on the submucosal plexus and surface epithelial cells in guinea pig distal colon.

A novel H(+)-coupled oligopeptide transporter (OPT3) from Caenorhabditis elegans with a predominant function as a H(+) channel and an exclusive expression in neurons

We have cloned and functionally characterized a novel, neuron-specific, H(+)-coupled oligopeptide transporter (OPT3) from Caenorhabditis elegans that functions predominantly as a H(+) channel. The opt3 gene is approximately 4.4 kilobases long and consists of 13 exons. The cDNA codes for a protein of 701 amino acids with 11 putative transmembrane domains. When expressed in mammalian cells and in Xenopus laevis oocytes, OPT3 cDNA induces H(+)-coupled transport of the dipeptide glycylsarcosine. Electrophysiological studies of the transport function of OPT3 in Xenopus oocytes show that this transporter, although capable of mediating H(+)-coupled peptide transport, functions predominantly as a H(+) channel. The H(+) channel activity of OPT3 is approximately 3-4-fold greater than the H(+)/peptide cotransport activity as determined by measurements of H(+) gradient-induced inward currents in the absence and presence of the dipeptide using the two-microelectrode voltage clamp technique. A downhill influx of H(+) was accompanied by a large intracellular acidification as evidenced from the changes in intracellular pH using an ion-selective microelectrode. The H(+) channel activity exhibits a K(0.5)(H) of 1.0 microM at a membrane potential of -50 mV. At the level of primary structure, OPT3 has moderate homology with OPT1 and OPT2, two other H(+)-coupled oligopeptide transporters previously cloned from C. elegans. Expression studies using the opt3::gfp fusion constructs in transgenic C. elegans demonstrate that opt3 gene is exclusively expressed in neurons. OPT3 may play an important physiological role as a pH balancer in the maintenance of H(+) homeostasis in C. elegans.

Cloning and immunolocalization of a rat pancreatic Na(+) bicarbonate cotransporter

In the rat, pancreatic HCO(-)(3) secretion is believed to be mediated by duct cells with an apical Cl(-)/HCO(-)(3) exchanger acting in parallel with a cAMP-activated Cl(-) channel and protons being extruded through a basolateral Na(+)/H(+) exchanger. However, this may not be the only mechanism for HCO(-)(3) secretion by the rat pancreas. Recently, several members of electrogenic Na(+)/HCO(-)(3) cotransporters (NBC) have been cloned. Here we report the cloning of a NBC from rat pancreas (rpNBC). This rpNBC is 99% identical to the longer, more common form of NBC [pNBC; 1079 amino acids (aa); 122 kDa in human heart, pancreas, prostate, and a minor clone in kidney]. The longer NBC isoforms are identical to the rat and human kidney-specific forms (kNBC; 1035 aa; 116 kDa) at the approximately 980 C-terminal aa's and are unique (with different lengths) at the initial N-terminus. Using polyclonal antibodies to the common N- and C-termini of rat kidney NBC, a approximately 130-kDa protein band was labeled by immunoblotting of rat pancreas homogenate and was enriched in the plasma membrane fraction. Immunofluorescence and immunoperoxidase light microscopy of rat pancreatic tissue with both antibodies revealed basolateral labeling of acinar cells. Labeling of both apical and basolateral membranes was found in centroacinar cells, intra- and extralobular duct, and main duct cells. The specificity of the antibody labeling was confirmed by antibody preabsorption experiments with the fusion protein used for immunization. The data suggest that rpNBC likely plays a more important role in the transport of HCO(-)(3) by rat pancreatic acinar and duct cells than previously believed.

Desensitization to ANG II in guinea pig ileum depends on membrane repolarization: role of maxi-K(+) channel

Desensitization of ANG II tonic contractile response of the guinea pig ileum is related to membrane repolarization determined by Ca(2+)-activated K(+) (maxi-K(+)) channel opening. ANG II-stimulated depolarized myocytes presented sustained activation of maxi-K(+) channels, characterized by reduction from 415 to 12 ms of the closed time constant. ANG II desensitization was prevented by 100 nM iberiotoxin, being reversible within 30 min. Depolarization by KCl, higher than 4 mM, impaired desensitization, suggesting that the membrane potential must attain a threshold to counteract the repolarization induced by maxi-K(+) channel opening. Once this value is attained, there is no time dependency because the desensitization process was shut off by addition of KCl along the time course of the tonic response. In contrast, the sustained ACh tonic component was not altered by these maneuvers. We conclude that desensitization of the ANG II tonic component is foremost due to the opening of maxi-K(+) channels, leading to membrane repolarization, thus closing the voltage-dependent Ca(2+) channels responsible for the Ca(2+) influx that sustains the tonic component in this muscle.

Cloning, tissue distribution, genomic organization, and functional characterization of NBC3, a new member of the sodium bicarbonate cotransporter family

Previous functional studies have demonstrated that muscle intracellular pH regulation is mediated by sodium-coupled bicarbonate transport, Na+/H+ exchange, and Cl-/bicarbonate exchange. We report the cloning, sequence analysis, tissue distribution, genomic organization, and functional analysis of a new member of the sodium bicarbonate cotransporter (NBC) family, NBC3, from human skeletal muscle. mNBC3 encodes a 1214-residue polypeptide with 12 putative membrane-spanning domains. The approximately 7.8-kilobase transcript is expressed uniquely in skeletal muscle and heart. The NBC3 gene (SLC4A7) spans approximately 80 kb and is composed of 25 coding exons and 24 introns that are flanked by typical splice donor and acceptor sequences. Expression of mNBC3 cRNA in Xenopus laevis oocytes demonstrated that the protein encodes a novel stilbene-insensitive 5-(N-ethyl-N-isopropyl)-amiloride-inhibitable sodium bicarbonate cotransporter.

Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Kidney proximal tubule cells take up Krebs cycle intermediates for metabolic purposes and for secretion of organic anions through dicarboxylate/organic anion exchange. Alteration in reabsorption of citrate is closely related to renal stone formation. The presence of distinct types of sodium-coupled dicarboxylate transporters has been postulated on either side of the polarized epithelial membrane in the kidney proximal tubule. Using a PCR-based approach, we isolated a novel member of the sodium-dependent dicarboxylate/sulfate transporter called SDCT2. SDCT2 is a 600-amino acid residue protein that has 47-48% amino acid identity to SDCT1 and NaDC-1, previously identified in kidney and intestine. Northern analysis gave a single band of 3.3 kb for SDCT2 in kidney, liver, and brain. In situ hybridization revealed that SDCT2 is prominently expressed in kidney proximal tubule S3 segments and in perivenous hepatocytes, consistent with the sites of high-affinity dicarboxylate transport identified based on vesicle studies. A signal was also detected in the meningeal layers of the brain. SDCT2 expressed in Xenopus oocytes mediated sodium-dependent transport of di- and tricarboxylates with substrate preference for succinate rather than citrate, but excluding monocarboxylates. SDCT2, unlike SDCT1, displayed a unique pH dependence for succinate transport (optimal pH 7.5-8.5) and showed a high affinity for dimethylsuccinate, two features characteristic of basolateral transport. These data help to interpret the mechanisms of renal citrate transport, their alteration in pathophysiological conditions, and their role in the elimination of organic anions and therapeutic drugs.

Electrogenic Na+/HCO3- cotransporters: cloning and physiology

Bicarbonate and CO2 comprise the major pH buffer of biological fluids. In the renal proximal tubule most of the filtered HCO3- is reabsorbed by an electrogenic Na/HCO3 cotransporter located at the basolateral membrane. This Na+ bicarbonate cotransporter (NBC) was recently cloned. This review highlights the recent developments leading to and since the cloning of NBC: NBC expression cloning, protein features, clone physiology, isoforms and genes, mRNA distribution, and protein distribution. With the NBC amino acid sequence 30-35% identical to the anion exchangers (AE1-3), a superfamily of HCO3- transporters is emerging. Physiologically, NBC is electrogenic, Na+ dependent, HCO3- dependent, Cl- independent, and inhibited by stilbenes (DIDS and SITS). NBC clones and proteins have been isolated from several tissues (other than kidney) thought to have physiologically distinct HCO3- transporters. For example, NBC occurs in pancreas, prostate, brain, heart, small and large intestine, stomach, and epididymis. Finally, there are at least two genes that encode NBC proteins. Possible future directions of research are discussed.

Molecular cloning of a new sodium bicarbonate cotransporter cDNA from human retina

Sodium bicarbonate cotransport is an electrogenic process and a principal regulator of pH. A recently cloned sodium bicarbonate cotransporter (NBC1) is predominantly expressed in kidney. The presence of several isoforms is suggested from functional studies. We have cloned a new member of this family from human retina, which was named hNBC2 (for human sodium bicarbonate cotransporter 2). The hNBC2 has 1018 amino acid residues with 10 hydrophobic transmembrane domains. A cluster of five N-glycosylation sites is present at the second extracellular loop. It has 53% amino acids identity with hNBC1 and 38% identity with anion exchanger 1. Thus, NBC2 is a new member of the anion exchanger superfamily. Northern blot of human tissues revealed its expression in many tissues such as testis, spleen, ovary, small intestine, colon, thymus, heart, and muscle (8.5 kb). The predominant expression at retina was documented with Northern blot of rat tissues. NBC2 may be a major regulator of extracellular pH of retina where light stimulation produces an extracellular alkalization and may contribute as a solute transporter to the prevention of retinal detachment.