Exchanger AE1 Research Articles

Distinguishing among HCO 3- , CO 3= , and H + as Substrates of Proteins That Appear To Be "Bicarbonate" Transporters.

Differentiating among HCO 3- , CO 3= , and H + movements across membranes has long seemed impossible. We now seek to discriminate unambiguously among three alternate mechanisms: the inward flux of 2 HCO 3- (mechanism 1), the inward flux of 1 CO 3= (mechanism 2), and the CO 2 /HCO 3- -stimulated outward flux of 2 H + (mechanism 3). As a test case, we use electrophysiology and heterologous expression in Xenopus oocytes to examine SLC4 family members that appear to transport "bicarbonate" ("HCO 3- "). First, we note that cell-surface carbonic anhydrase should catalyze the forward reaction CO 2 +OH - →HCO 3- if HCO 3- is the substrate; if it is not, the reverse reaction should occur. Monitoring changes in cell-surface pH ( Δ pH S ) with or without cell-surface carbonic anhydrase, we find that the presumed Cl-"HCO 3 " exchanger AE1 (SLC4A1) does indeed transport HCO 3- (mechanism 1) as long supposed, whereas the electrogenic Na/"HCO 3 " cotransporter NBCe1 (SLC4A4) and the electroneutral Na + -driven Cl-"HCO 3 " exchanger NDCBE (SLC4A8) do not. Second, we use mathematical simulations to show that each of the three mechanisms generates unique quantities of H + at the cell surface (measured as Δ pH S ) per charge transported (measured as change in membrane current, ΔIm ). Calibrating ΔpH S /Δ Im in oocytes expressing the H + channel H V 1, we find that our NBCe1 data align closely with predictions of CO 3= transport (mechanism 2), while ruling out HCO 3- (mechanism 1) and CO 2 /HCO 3- -stimulated H + transport (mechanism 3). Our surface chemistry approach makes it possible for the first time to distinguish among HCO 3- , CO 3= , and H + fluxes, thereby providing insight into molecular actions of clinically relevant acid-base transporters and carbonic-anhydrase inhibitors.

SLC26A9 deficiency causes gastric intraepithelial neoplasia in mice and aggressive gastric cancer in humans

BackgroundSolute carrier family 26 member (SLC26A9) is a Cl− uniporter with very high expression levels in the gastric mucosa. Here, we describe morphological and molecular alterations in gastric mucosa of slc26a9−/− mice and in selective parietal cell-deleted slc26a9fl/fl/Atp4b-Cre mice and correlate SLC26A9 expression levels with morphological and clinical parameters in a cohort of gastric cancer (GC) patients.MethodsThe expression patterns of genes related to transport and enzymatic function, proliferation, apoptosis, inflammation, barrier integrity, metaplasia and neoplasia development were studied by immunohistochemistry (IHC), quantitative RT-PCR, in situ hybridization and RNA microarray analysis. SLC26A9 expression and cellular/clinical phenotypes were studied in primary human GC tissues and GC cell lines.ResultsWe found that both complete and parietal cell-selective Slc26a9 deletion in mice caused spontaneous development of gastric premalignant and malignant lesions. Dysregulated differentiation of gastric stem cells in an inflammatory environment, activated Wnt signaling, cellular hyperproliferation, apoptosis inhibition and metaplasia were observed. Analysis of human gastric precancerous and cancerous tissues revealed that SLC26A9 expression progressively decreased from atrophic gastritis to GC, and that downregulation of SLC26A9 was correlated with patient survival. Exogenous expression of SLC26A9 in GC cells induced upregulation of the Cl−/HCO3− exchanger AE2, G2/M cell cycle arrest and apoptosis and suppressed their proliferation, migration and invasion.ConclusionsOur data indicate that SLC26A9 deletion in parietal cells is sufficient to trigger gastric metaplasia and the development of neoplastic lesions. In addition, we found that SLC26A9 expression decreases during human gastric carcinogenesis, and that exogenous SLC26A9 expression in GC cells reduces their malignant behavior.Graphical abstract

Rescue of chloride and bicarbonate transport by elexacaftor-ivacaftor-tezacaftor in organoid-derived CF intestinal and cholangiocyte monolayers

BackgroundIn cystic fibrosis (CF), loss of CF transmembrane conductance regulator (CFTR)-dependent bicarbonate secretion precipitates the accumulation of viscous mucus in the lumen of respiratory and gastrointestinal epithelial tissues. We investigated whether the combination of elexacaftor (ELX), ivacaftor (IVA) and tezacaftor (TEZ), apart from its well-documented effect on chloride transport, also restores Phe508del-CFTR-mediated bicarbonate transport. MethodsEpithelial monolayers were cultured from intestinal and biliary (cholangiocyte) organoids of homozygous Phe508del-CFTR patients and controls. Transcriptome sequencing was performed, and bicarbonate and chloride transport were assessed in the presence or absence of ELX/IVA/TEZ, using the intestinal current measurement technique. ResultsELX/IVA/TEZ markedly enhanced bicarbonate and chloride transport across intestinal epithelium. In biliary epithelium, it failed to enhance CFTR-mediated bicarbonate transport but effectively rescued CFTR-mediated chloride transport, known to be requisite for bicarbonate secretion through the chloride-bicarbonate exchanger AE2 (SLC4A2), which was highly expressed by cholangiocytes. Biliary but not intestinal epithelial cells expressed an alternative anion channel, anoctamin-1/TMEM16A (ANO1), and secreted bicarbonate and chloride upon purinergic receptor stimulation. ConclusionsELX/IVA/TEZ has the potential to restore both chloride and bicarbonate secretion across CF intestinal and biliary epithelia and may counter luminal hyper-acidification in these tissues.

Role of the IgG4-related cholangitis autoantigen annexin A11 in cholangiocyte protection

Annexin A11 was identified as autoantigen in IgG4-related cholangitis (IRC), a B-cell driven disease. Annexin A11 modulates calcium-dependent exocytosis, a crucial mechanism for insertion of proteins into their target membranes. Human cholangiocytes form an apical 'biliary bicarbonate umbrella' regarded as defense against harmful hydrophobic bile acid influx. The bicarbonate secretory machinery comprises the chloride/bicarbonate exchanger AE2 and the chloride channel ANO1. We aimed to investigate the expression and function of annexin A11 in human cholangiocytes and a potential role of IgG1/IgG4-mediated autoreactivity against annexin A11 in the pathogenesis of IRC. Expression of annexin A11 in human liver was studied by immunohistochemistry and immunofluorescence. In human control and ANXA11 knockdown H69 cholangiocytes, intracellular pH, AE2 and ANO1 surface expression, and bile acid influx were examined using ratio microspectrofluorometry, cell surface biotinylation, and 22,23-3H-glycochenodeoxycholic acid permeation, respectively. The localization of annexin A11-mEmerald and ANO1-mCherry was investigated by live-cell microscopy in H69 cholangiocytes after incubation with IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies or disease control serum. Annexin A11 was strongly expressed in human cholangiocytes, but not hepatocytes. Knockdown of ANXA11 led to reduced plasma membrane expression of ANO1, but not AE2, alkalization of intracellular pH and uncontrolled bile acid influx. High intracellular calcium conditions led to annexin A11 membrane shift and colocalization with ANO1. Incubation with IRC patient serum inhibited annexin A11 membrane shift and reduced ANO1 surface expression. Cholangiocellular annexin A11 mediates apical membrane abundance of the chloride channel ANO1, thereby supporting biliary bicarbonate secretion. Insertion is inhibited by IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies. Anti-annexin A11 autoantibodies may contribute to the pathogenesis of IRC by weakening the 'biliary bicarbonate umbrella'. We previously identified annexin A11 as a specific autoantigen in immunoglobulin G4-related cholangitis (IRC), a B-cell driven disease affecting the bile ducts. Human cholangiocytes are protected against harmful hydrophobic bile acid influx by a defense mechanism referred to as the 'biliary bicarbonate umbrella'. We found that annexin A11 is required for the formation of a robust bicarbonate umbrella. Binding of patient-derived annexin A11 autoantibodies inhibits annexin A11 function, possibly contributing to bile duct damage by weakening the biliary bicarbonate umbrella in patients with IRC.

Primary biliary cholangitis: pathogenic mechanisms.

Purpose of reviewPrimary biliary cholangitis (PBC) is characterized by autoimmune damage of intrahepatic bile ducts associated with a loss of tolerance to mitochondrial antigens. PBC etiopathogenesis is intriguing because of different perplexing features, namely: a) although mitochondria are present in all cell types and tissues, the damage is mainly restricted to biliary epithelial cells (BECs); b) despite being an autoimmune disorder, it does not respond to immunosuppressive drugs but rather to ursodeoxycholic acid, a bile salt that induces HCO3− rich choleresis; c) the overwhelming female preponderance of the disease remains unexplained. Here we present an etiopathogenic view of PBC which sheds light on these puzzling facts of the disease.Recent findingsPBC develops in patients with genetic predisposition to autoimmunity in whom epigenetic mechanisms silence the Cl−/HCO3− exchanger AE2 in both cholangiocytes and lymphoid cells. Defective AE2 function can produce BECs damage as a result of decreased biliary HCO3− secretion with disruption of the protective alkaline umbrella that normally prevents the penetration of toxic apolar bile salts into cholangiocytes. AE2 dysfunction also causes increased intracellular pH (pHi) in cholangiocytes, leading to the activation of soluble adenylyl cyclase, which sensitizes BECs to bile salt-induced apoptosis. Recently, mitophagy was found to be inhibited by cytosolic alkalization and stimulated by acidification. Accordingly, we propose that AE2 deficiency may disturb mitophagy in BECs, thus, promoting the accumulation of defective mitochondria, oxidative stress and presentation of mitochondrial antigens to the immune cells. As women possess a more acidic endolysosomal milieu than men, mitophagy might be more affected in women in an AE2-defective background. Apart from affecting BECs function, AE2 downregulation in lymphocytes may also contribute to alter immunoregulation facilitating autoreactive T-cell responses.SummaryPBC can be considered as a disorder of Cl−/HCO3− exchange in individuals with genetic predisposition to autoimmunity.

Severe impairment of salivation in Na+/K+/2Cl- cotransporter (NKCC1)-deficient mice.

The salivary fluid secretory mechanism is thought to require Na(+)/K(+)/2Cl(-) cotransporter-mediated Cl(-) uptake. To directly test this possibility we studied the in vivo and in vitro functioning of acinar cells from the parotid glands of mice with targeted disruption of Na(+)/K(+)/2Cl(-) cotransporter isoform 1 (Nkcc1), the gene encoding the salivary Na(+)/K(+)/2Cl(-) cotransporter. In wild-type mice NKCC1 was localized to the basolateral membranes of parotid acinar cells, whereas expression was not detected in duct cells. The lack of functional NKCC1 resulted in a dramatic reduction (>60%) in the volume of saliva secreted in response to a muscarinic agonist, the primary in situ salivation signal. Consistent with defective Cl(-) uptake, a loss of bumetanide-sensitive Cl(-) influx was observed in parotid acinar cells from mice lacking NKCC1. Cl(-)/ HCO(3)(-) exchanger activity was increased in parotid acinar cells isolated from knockout mice suggesting that the residual saliva secreted by mice lacking NKCC1 is associated with anion exchanger-dependent Cl(-) uptake. Indeed, expression of the Cl(-)/ HCO(3)(-) exchanger AE2 was enhanced suggesting that this transporter compensates for the loss of functional Na(+)/K(+)/2Cl(-) cotransporter. Furthermore, the ability of the parotid gland to conserve NaCl was abolished in NKCC1-deficient mice. This deficit was not associated with changes in the morphology of the ducts, but transcript levels for the alpha-, beta-, and gamma-subunits of the epithelial Na(+) channel were reduced. These data directly demonstrate that NKCC1 is the major Cl(-) uptake mechanism across the basolateral membrane of acinar cells and is critical for driving saliva secretion in vivo.

Intracellular Ca2+-Mediated AE2 Is Involved in the Vectorial Movement of HaCaT Keratinocyte.

Keratinocyte migration is initiated toward the wound skin barrier as a crucial process in wound healing. However, the migratory machinery used by keratinocytes is relatively unknown. Histamine signaling, including an increase in the Ca2+ signal, mediated the enhanced protein expression and chloride/bicarbonate exchange activity of anion exchanger AE2 in keratinocytes. In this study, we applied an agarose spot assay to induce a vectorial motion. The vectorial stimulation of the histamine-containing agarose spot enhanced the HaCaT keratinocyte migration, compared to non-directional stimulation. AE2 is associated with the vectorial movement of HaCaT keratinocytes. Enhanced expression of AE2 was mainly associated with an increase in Ca2+ and was abolished by the treatment with the Ca2+ chelating agent BAPTA-AM. These findings revealed that the directionality of Ca2+-exerted stimulation can play a prominent role in facilitating migration through the involvement of AE2 as a migratory machinery in HaCaT keratinocytes.

The devil is in the C-tail: a NBCe1-carbonic anhydrase bicarbonate transport metabolon?

The concept of metabolons is well established in biochemistry as structural–metabolic complexes in cells that facilitate substrate channelling through the complex and is best exemplified by the citric acid cycle enzymes. Similarly, an arrangement of enzymes was suggested to promote the transport of acid/base equivalents across the plasma membrane by the Reithmeier group. The concept was supported by direct evidence of physical interaction of an acid/base transporter and carbonic anhydrase (Vince & Reithmeier, 1998) and emboldened by the demonstration of a functional advantage for such an arrangement called a ‘transport metabolon’ (Sterling et al. 2001). The direct protein interaction was first reported for the C-terminal of anion exchanger AE1 (band 3) and the cytosolic ubiquitous carbonic anhydrase II (CAII). Since then, the field has evolved to other acid/base transporters of the SLC4 gene family of HCO3− transporters, the Na+/H+ exchangers (NHE) and monocarboxylate transporters interacting with soluble or membrane-bound CA isozymes in various expression systems and native tissues. Discrepancies have since evolved both regarding the direct physical interaction of acid/base transporters and CA enzymes, and regarding the functional interaction between the individual molecules of such complexes. This ‘Perspectives’ article only addresses the case of the functional interaction of the human renal form of the electrogenic Na+:HCO3− cotransporter NBCe1-A (kNBCe1) and CAII when expressed in Xenopus laevis oocytes, but may prove highly relevant for the metabolon field as such. In one study, the NBCe1 function determined as the current slope conductance (Gm INBC) was not affected by injection of CAII protein or the expression of either NBCe1-CAII or eGFP-NBCe1-CAII fusion proteins (Lu et al. 2006). No effect of the cell-permeant CA inhibitor ethoxyzolamide (EZA) on the Gm INBC was observed. In a separate and opposing study, NBCe1 function assessed as the changes in membrane current (Im), the Na+ flux (d[Na+]i/dt), as well as the membrane conductance (Gm) was augmented by bovine CA protein injection or CAII co-expression (Becker & Deitmer 2007). The induced changes in current and Na+ fluxes were EZA-sensitive in this study. Thus, although the two studies applied similar methodological set-ups at first glance, the results were contrasting. Each paper is compelling to the reader, and both studies were carefully and elegantly performed. Moss and Boron (2020) first carefully replicate the findings by Lu et al. (2006) and then elegantly rule out differences in the amount of injected cRNA, CAII protein and eGFP tag as contributing factors to conflicting results by the two groups. Furthermore, they show that prolonged EZA exposure or the high EZA concentration by Lu et al. were not causing the inconsistency between the previous studies. It also appears that the variations in voltage-clamping protocols cannot explain the contradicting results per se (Moss & Boron 2020). Remarkably, the authors are able to closely replicate the fundamental observations by Becker & Deitmer (2007) that injection of bovine CA in oocytes expressing untagged NBCe1 increases the Na+ flux (d[Na+]i/dt)max as well as the membrane current Im, effects that are sensitive to CA inhibition by EZA. This is apparently only possible 1) in the absence of eGFP tag, 2) by bovine CA injection instead of human CAII, and 3) applying the prolonged -40mV voltage clamp and 10 s step duration of I-V ramps. Surprisingly, Moss and Boron demonstrate that injection of either human CAII or bovine CA alone in oocytes induces an electroneutral process that increases Na+ uptake and acid extrusion upon CO2-induced acid loading. The authors ascribe the effects to the induction of endogenous NHE activity, as this process is inhibited by EIPA. Interestingly, EIPA also prevents the EZA-sensitive change in Im and (d[Na+]i/dt)max induced by injection of bovine CA using the prolonged -40 mV voltage clamp protocol. Thus, Moss and Boron explain the discrepancy between the previous studies (Lu et al. 2006 vs. Becker & Deitmer 2007) by the induction of native NHE activity by exogenous CA in Xenopus oocytes. With the study by Moss and Boron (2020), the specific controversy regarding enhanced NBCe1 activity by CA in oocytes has most likely been fully resolved. The authors accomplished a systematic comparison of experimental approaches resulting in a study with high internal consistency presented in a respectful and transparent manner. The take-home message is that even the most careful studies by world-renowned laboratories can reach conflicting conclusions and that unexpected sources of discrepancies can be identified by hard and meticulous work. The findings by Moss and Boron (2020) should impact the broader transport metabolon field; however, the conclusions can probably not be extrapolated to all other cases of acid/base transporters, CA forms and cell types for which transport metabolons have been suggested. Hopefully, time will tell. None. Sole author. None.

P0004REDUCED EXPRESSION OF PROXIMAL ACID-BASE TRANSPORT PROTEINS IN KIDNEY TRANSPLANT PATIENTS WITH METABOLIC ACIDOSIS

Abstract Background and Aims Metabolic acidosis (MA) is a frequent complication of chronic kidney disease and an independent risk factor for kidney disease progression and mortality. MA is highly prevalent after kidney transplantation (12%-58%)(1). However, there are scarcely any data available on the underlying pathomechanisms and in particular molecular mechanisms involved in metabolic acidosis after kidney transplantation. Thus, we wanted to investigate the expression of key acid base transport proteins in kidney biopsies of kidney transplant recipients with and without metabolic acidosis. Method We evaluated 22 kidney transplant biopsies including 9 biopsies from kidney transplant recipients (KTR) with MA, nine biopsies from KTRs without MA (control) and four biopsies from KTRs with MA that were consequently subjected to alkali therapy (Alkali therapy). Immunofluorescence staining was used to identify key renal acid-base transport proteins. Additionally, six control kidneys were analyzed. Immunofluorescence staining was used to identify key renal acid-base transport proteins along the nephron. In addition, RNA extraction and full RNA sequencing analysis of all biopsies –where available- was performed. Results In the proximal tubule, we observed reduced immunostaining for the sodium bicarbonate cotransporter NBCe1 (SLC4A4) in the MA group compared to the control and alkali group, whereas the alkali group demonstrated the strongest staining of all three groups. In the distal nephron, expression of the chloride/bicarbonate exchanger Pendrin (SLC26A4) and the B1 subunit of the V-ATPase (ATP6V1B1) were markedly stronger in the alkali and control group compared to the MA group. Expression of other acid base proteins such as Renal ammonia transporter RhCG (SLC42A3), Carbonic Anhydrase II, Glutamate dehydrogenase, anion exchanger AE1 (SLC4A1) and the B2 subunit of the V-ATPase (ATP6V1B2) showed no difference among all groups. Interestingly, the B2 subunit was absent in the proximal tubule in transplant biopsies of all groups. In kidney biopsies of transplant recipients with metabolic acidosis RNA abundance of NBCe1, CAII and Pendrin was lower while RhCG and B1 RNA counts were not different when compared to recipients without metabolic acidosis. Conclusion Our data demonstrate altered protein and mRNA expression of several key acid base transporters in kidney biopsies of transplant recipients with metabolic acidosis. Treatment with alkali may have the potential to reverse or prevent these changes in renal allografts after transplantation.

Multiple acid-base and electrolyte disturbances upregulate NBCn1, NBCn2, IRBIT and L-IRBIT in the mTAL.

The roles of the Na+ /HCO3- cotransporters NBCn1 and NBCn2 as well as their activators IRBIT and L-IRBIT in the regulation of the mTAL transport of NH4+ , HCO3- , and NaCl are investigated. Dietary challenges of NH4 Cl, NaHCO3 or NaCl all increase the abundance of NBCn1 and NBCn2 in the outer medulla. The three challenges generally produce parallel increases in the abundance of IRBIT and L-IRBIT in the outer medulla. Both IRBIT and L-IRBIT powerfully stimulate the activities of the mTAL isoforms of NBCn1 and NBCn2 as expressed in Xenopus oocytes. Our findings support the hypothesis that NBCn1, NBCn2, IRBIT and L-IRBIT appropriately promote NH4+ shunting but oppose HCO3- and NaCl reabsorption in the mTAL, and thus are at the nexus of the regulation pathways for multiple renal transport processes. The medullary thick ascending limb (mTAL) plays a key role in urinary acid and NaCl excretion. NBCn1 and NBCn2 are present in the basolateral mTAL, where NBCn1 promotes NH4+ shunting. IRBIT and L-IRBIT (the IRBITs) are two powerful activators of certain acid-base transporters. Here we use western blotting and immunofluorescence to examine the effects of multiple acid-base and electrolyte disturbances on expression of NBCn1, NBCn2 and the IRBITs in rat kidney. We also use electrophysiology to examine the functional effects of IRBITs on NBCn1 and NBCn2 in Xenopus oocytes. NH4 Cl-induced metabolic acidosis (MAc) substantially increases protein expression of NBCn1 and NBCn2 in the outer medulla (OM) of rat kidney. Surprisingly, NaHCO3 -induced metabolic alkalosis (MAlk) and high-salt diet (HSD) also increase expression of NBCn1 and NBCn2 (effect of NaHCO3 >HSD). Moreover, all three challenges generally increase OM expression of the IRBITs. In Xenopus oocytes, the IRBITs substantially increase the activities of NBCn1 and NBCn2. We propose that upregulation of basolateral NBCn1 and NBCn2 plus the IRBITs in the mTAL: (1) promotes NH4+ shunting by increasing basolateral HCO3- uptake to neutralize apical NH4+ uptake during MAc; (2) inhibits HCO3- reabsorption during MAlk by opposing HCO3- efflux via the basolateral anion exchanger AE2; and (3) inhibits NaCl reabsorption by mediating (with AE2) net NaCl backflux into the mTAL cell during HSD. Thus, NBCn1, NBCn2 and the IRBITs are at the nexus of the regulatory pathways for multiple renal transport processes.

Drug Repurposing as an Antitumor Agent: Disulfiram-Mediated Carbonic Anhydrase 12 and Anion Exchanger 2 Modulation to Inhibit Cancer Cell Migration.

Disulfiram has been used in the treatment of alcoholism and exhibits an anti-tumor effect. However, the intracellular mechanism of anti-tumor activity of Disulfiram remains unclear. In this study, we focused on the modulatory role of Disulfiram via oncogenic factor carbonic anhydrase CA12 and its associated transporter anion exchanger AE2 in lung cancer cell line A549. The surface expression of CA12 and AE2 were decreased by Disulfiram treatment with a time-dependent manner. Disulfiram treatment did not alter the expression of Na+-bicarbonate cotransporters, nor did it affect autophagy regulation. The chloride bicarbonate exchanger activity of A549 cells was reduced by Disulfiram treatment in a time-dependent manner without change in the resting pH level. The expression and activity of AE2 and the expression of CA12 were also reduced by Disulfiram treatment in the breast cancer cell line. An invasion assay and cell migration assay revealed that Disulfiram attenuated the invasion and migration of A549 cells. In conclusion, the attenuation of AE2 and its supportive enzyme CA12, and the inhibitory effect on cell migration by Disulfiram treatment in cancer cells provided the molecular evidence supporting the potential of Disulfiram as an anticancer agent.

Exploring the Transport Mechanism of the Human AE4 (SLC4A9) CL-/HCO3- Exchanger

The SLC4 family includes Na+-independent Cl-/HCO3- exchangers and Na+-coupled HCO3- transporters. SLC4 proteins are crucial for important physiological processes, including CO2 transport, intracellular and extracellular pH regulation and HCO3- transport in epithelia. The AE4 (SLC4A9) protein plays a role in NaCl reabsorption and Cl--dependent fluid secretion in distal nephron and salivary glands respectively; however, its transport mechanism remains controversial. It has been proposed that AE4 mediates Cl-/HCO3- exchange, Na+-HCO3- cotransport or Cl-/cation-HCO3- exchange suggesting that shares functional features with both the Cl-/HCO3- exchanger AE1(SLC4A1) and the Na+-HCO3- co-transporter NBCe1 (SLC4A4). Our sequence alignments show that AE1, NBCe1 and AE4 share high amino-acidic identity in the transmembrane domain. Functionally relevant residues in AE1 and NBCe1 are conserved in AE4, suggesting that the ion transport mechanism should be encoded by the same set of residues. We explored the transport mechanism of the human version of AE4 using molecular modeling, site-directed mutagenesis and functional assays. Our molecular simulations show that the proposed anion-binding site in AE1, between the helical ends of TM 3 and TM10, is also present in AE4. Three residues at this region have been postulated to be crucial for anion transport in AE1 and NBCe1. We mutated homologous D709, T448 and I758 in AE4 and found that only T448 is functionally important. Additional mutations of S754 and T756 in this region dońt show functional changes compared to WT AE4. These results suggest that the predicted ion-binding site in AE4 requires T448 to support the transport cycle but, in contrast to AE1 and NBCe1, residues at equivalent positions 709 and 758 are not important for function.

A new model for an old friend.

JGP study suggests the anion exchanger AE1 operates via an elevator-like mechanism.

Abscisic acid influx into human nucleated cells occurs through the anion exchanger AE2

Abscisic acid (ABA) is a hormone conserved from cyanobacteria to higher plants, where it regulates responses to environmental stimuli. ABA also plays a role in mammalian physiology, pointedly in inflammatory responses and in glycemic control. As the animal ABA receptor is on the intracellular side of the plasma membrane, a transporter is required for the hormone’s action. Here we demonstrate that ABA transport in human nucleated cells occurs via the anion exchanger AE2. Together with the recent demonstration that ABA influx into human erythrocytes occurs via Band 3, this result identifies the AE family members as the mammalian ABA transporters.

Essential role of carbonic anhydrase XII in secretory gland fluid and HCO3 (-) secretion revealed by disease causing human mutation.

Fluid and HCO3 (-) secretion is essential for all epithelia; aberrant secretion is associated with several diseases. Carbonic anhydrase XII (CA12) is the key carbonic anhydrase in epithelial fluid and HCO3 (-) secretion and works by activating the ductal Cl(-) -HCO3 (-) exchanger AE2. Delivery of CA12 to salivary glands increases salivation in mice and of the human mutation CA12(E143K) markedly inhibits it. The human mutation CA12(E143K) causes disease due to aberrant CA12 glycosylation, and misfolding resulting in loss of AE2 activity. Aberrant epithelial fluid and HCO3 (-) secretion is associated with many diseases. The activity of HCO3 (-) transporters depends of HCO3 (-) availability that is determined by carbonic anhydrases (CAs). Which CAs are essential for epithelial function is unknown. CA12 stands out since the CA12(E143K) mutation causes salt wasting in sweat and dehydration in humans. Here, we report that expression of CA12 and of CA12(E143K) in mice salivary glands respectively increased and prominently inhibited ductal fluid secretion and salivation in vivo. CA12 markedly increases the activity and is the major HCO3 (-) supplier of ductal Cl(-) -HCO3 (-) exchanger AE2, but not of NBCe1-B. The E143K mutation alters CA12 glycosylation at N28 and N80, resulting in retention of the basolateral CA12 in the ER. Knockdown of AE2 and of CA12 inhibited pancreatic and salivary gland ductal AE2 activity and fluid secretion. Accordingly, patients homozygous for the CA12(E143K) mutation have a dry mouth, dry tongue phenotype. These findings reveal an unsuspected prominent role of CA12 in epithelial function, explain the disease and call for caution in the use of CA12 inhibitors in cancer treatment.

Structure and Function of SLC4 Family [Formula: see text] Transporters.

The solute carrier SLC4 family consists of 10 members, nine of which are transporters, including three Na+-independent Cl−/ exchangers AE1, AE2, and AE3, five Na+-coupled transporters NBCe1, NBCe2, NBCn1, NBCn2, and NDCBE, as well as “AE4” whose Na+-dependence remains controversial. The SLC4 transporters play critical roles in pH regulation and transepithelial movement of electrolytes with a broad range of demonstrated physiological relevances. Dysfunctions of these transporters are associated with a series of human diseases. During the past decades, tremendous amount of effort has been undertaken to investigate the topological organization of the SLC4 transporters in the plasma membrane. Based upon the proposed topology models, mutational and functional studies have identified important structural elements likely involved in the ion translocation by the SLC4 transporters. In the present article, we review the advances during the past decades in understanding the structure and function of the SLC4 transporters.

NBCe1 as a model carrier for understanding the structure–function properties of Na+-coupled SLC4 transporters in health and disease

SLC4 transporters are membrane proteins that in general mediate the coupled transport of bicarbonate (carbonate) and share amino acid sequence homology. These proteins differ as to whether they also transport Na(+) and/or Cl(-), in addition to their charge transport stoichiometry, membrane targeting, substrate affinities, developmental expression, regulatory motifs, and protein-protein interactions. These differences account in part for the fact that functionally, SLC4 transporters have various physiological roles in mammals including transepithelial bicarbonate transport, intracellular pH regulation, transport of Na(+) and/or Cl(-), and possibly water. Bicarbonate transport is not unique to the SLC4 family since the structurally unrelated SLC26 family has at least three proteins that mediate anion exchange. The present review focuses on the first of the sodium-dependent SLC4 transporters that was identified whose structure has been most extensively studied: the electrogenic Na(+)-base cotransporter NBCe1. Mutations in NBCe1 cause proximal renal tubular acidosis (pRTA) with neurologic and ophthalmologic extrarenal manifestations. Recent studies have characterized the important structure-function properties of the transporter and how they are perturbed as a result of mutations that cause pRTA. It has become increasingly apparent that the structure of NBCe1 differs in several key features from the SLC4 Cl(-)-HCO3 (-) exchanger AE1 whose structural properties have been well-studied. In this review, the structure-function properties and regulation of NBCe1 will be highlighted, and its role in health and disease will be reviewed in detail.

Na+/H+ exchange is inactivated during mouse oocyte meiosis, facilitating glycine accumulation that maintains embryo cell volume

The coupled action of the Na(+)/H(+) exchanger NHE1 and the HCO3(-)/Cl(-) exchanger AE2 constitutes the principal mechanism for acute correction of decreased cell volume in mammalian somatic cells, while, when acting separately, they regulate intracellular pH. It was previously found that AE2 becomes inactivated during meiosis in mouse oocytes. Similarly, NHE1 activity stimulated by intracellular acidosis was present in preovulatory germinal vesicle stage (GV) mouse oocytes and then decreased during meiotic maturation. In contrast, NHE1 activity stimulated by decreased cell volume was low in GV oocytes but became active during meiotic maturation as the oocyte detached from the zona pellucida. It then decreased again in mature eggs similar to activity stimulated by acidosis. The subcellular localization of NHE1 was investigated with YFP-tagged NHE1. Exogenous NHE1 expressed in GV oocytes localized to the plasma membrane and resulted in increased Na(+)/H(+) exchanger activity, but only when co-expressed with calcineurin homologous protein 1 (CHP1). When oocytes expressing functional NHE1 were matured to eggs, however, membrane localization of NHE1 and Na(+)/H(+) exchanger activity were lost. It was unknown why NHE1 and AE2 activities are suppressed during meiotic maturation. Maintenance of cell volume in preimplantation embryos requires glycine accumulation via the GLYT1 transporter, a process unique to eggs and early embryos that is initiated during meiotic maturation. When NHE1 and AE2 activities were maintained in GV oocytes by exogenous expression, glycine accumulation was inhibited. We propose that NHE1-mediated acute cell volume regulation is inactivated during meiotic maturation to allow preferential accumulation of glycine in eggs.

Therapeutic Equivalence of Ursodeoxycholic Acid Tablets and Ursodeoxycholic Acid Capsules for the Treatment of Primary Biliary Cirrhosis.

Primary biliary cirrhosis (PBC) is a chronic cholestatic, progressive inflammatory liver disease with a suspected autoimmunological pathogenesis. The disease is typically diagnosed in women at an age between 30 and 65 years and has a prevalence of up to 1 in 1000 women over the age of 40. The only approved medical therapy for PBC is ursodeoxycholic acid (UDCA) in a recommended dose of 13–15mg/kg/day. UDCA is a hydrophilic dihydroxy bile acid (3a,7bdihydroxy-5b-cholanoic acid). In humans, UDCA accounts for up to 4% of the bile acid pool. Following oral administration, approximately 30–60% of UDCA is absorbed in the gut. After intestinal absorption, UDCA enters the portal circulation and is then taken up by the hepatocytes via specific bile acid transporters. Within the hepatocyte, UDCA is conjugated to glycine or taurine and is subsequently transported into the bile ducts by the bile salt export pump (BSEP). In bile, UDCA concentration peaks 1 3 h following oral administration. In humans, the biological half-life of UDCA is 3.5–5.8 days and the predominant route of elimination is by feces. UDCA exerts its beneficial effects at the level of hepatocytes and cholangiocytes. It stimulates hepatocyte secretion by mostly post-transcriptional mechanisms, including insertion of transporters like BSEP and the anion exchanger AE2. In addition, UDCA exerts antiapoptotic effects in hepatocytes and protects cholangiocytes against endogenous toxic bile acids by modifying micelle formation. UDCA consistently improves the biochemical parameters of cholestasis, delays the histological progression and the time to liver transplantation in patients suffering from PBC. Both, UDCA 250mg capsules and UDCA 500mg tablets (“Ursofalk1”), are registered in Germany and other European countries for the treatment of PBC. UDCA 500mg tablets were developed to facilitate medication intake compared to the standard UDCA 250mg capsule preparations. Tablets are considerably smaller, but contain twice as much active pharmaceutical ingredient compared to the capsule preparation, which might improve the compliance of patients and thereby increase its therapeutic efficacy. Ursofalk tablets were registered in Germany on the basis of bioequivalence studies comparing tmax, Cmax, and AUC values of UDCA between capsules and tablets. Cmax values of UDCA in plasma are probably of only limited clinical relevance for the efficacy and safety of UDCA. Aim of this trial was to assess if the therapeutic efficacy of UDCA 500mg tablets is equivalent to that of UDCA 250mg capsules (both at 14 2mg/kg BW/day once daily) in patients with PBC. Therapeutic efficacy was measured by comparing the liver enzyme parameters alkaline phosphatase (AP), g-glutamyl transferase (gGT), and alanine aminotransferase (ALT) under both treatments. The secondary objectives were to assess the safety Clinical Pharmacology in Drug Development XX(XX) 1–6 © The Author(s) 2013 DOI: 10.1002/cpdd.24

Killing me softly – Suicidal erythrocyte death

Similar to nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is characterized by cell shrinkage, cell membrane blebbing and cell membrane phospholipid scrambling. Eryptotic cells are removed and thus prevented from undergoing hemolysis. Eryptosis is stimulated by Ca2+ following Ca2+ entry through unspecific cation channels. Ca2+ sensitivity is enhanced by ceramide, a product of acid sphingomyelinase. Eryptosis is triggered by hyperosmolarity, oxidative stress, energy depletion, hyperthermia and a wide variety of xenobiotics and endogenous substances. Eryptosis is inhibited by nitric oxide, catecholamines and a variety of further small molecules. Erythropoietin counteracts eryptosis in part by inhibiting the Ca2+-permeable cation channels but by the same token may foster formation of erythrocytes, which are particularly sensitive to eryptotic stimuli. Eryptosis is triggered in several clinical conditions such as iron deficiency, diabetes, renal insufficiency, myelodysplastic syndrome, phosphate depletion, sepsis, haemolytic uremic syndrome, mycoplasma infection, malaria, sickle-cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase-(G6PD)-deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, and Wilson's disease. Enhanced eryptosis is observed in mice with deficient annexin 7, cGMP-dependent protein kinase type I (cGKI), AMP-activated protein kinase AMPK, anion exchanger AE1, adenomatous polyposis coli APC and Klotho as well as in mouse models of sickle cell anemia and thalassemia. Eryptosis is decreased in mice with deficient phosphoinositide dependent kinase PDK1, platelet activating factor receptor, transient receptor potential channel TRPC6, janus kinase JAK3 or taurine transporter TAUT. If accelerated eryptosis is not compensated by enhanced erythropoiesis, clinically relevant anemia develops. Eryptotic erythrocytes may further bind to endothelial cells and thus impede microcirculation.