Identification of the chloride channel, leucine-rich repeat-containing protein 8, subfamily a (LRRC8A), in mouse cholangiocytes.

Background And Aims: Chloride (Cl ) channels in the apical membrane of biliary epithelial cells (BECs), also known as cholangiocytes, provide the driving force for biliary secretion. Although two Cl channels have been identified on a molecular basis, the Cystic Fibrosis Transmembrane Conductance Regulator and Transmembrane Member 16A, a third Cl channel with unique biophysical properties has been described. Leucine-Rich Repeat-Containing Protein 8, subfamily A (LRRC8A) is a newly identified protein capable of transporting Cl in other epithelium in response to cell swelling.

Systemic Succinate Homeostasis and Local Succinate Signaling Affect Blood Pressure and Modify Risks for Calcium Oxalate Lithogenesis.

Background: In the kidney, low urinary citrate increases the risk for developing kidney stones, and elevation of luminal succinate in the juxtaglomerular apparatus increases renin secretion, causing hypertension. Although the association between stone formation and hypertension is well established, the molecular mechanism linking these pathophysiologies has been elusive.

Methods: To investigate the relationship between succinate and citrate/oxalate levels, we assessed blood and urine levels of metabolites, renal protein expression, and BP (using 24-hour telemetric monitoring) in male mice lacking slc26a6 (a transporter that inhibits the succinate transporter NaDC-1 to control citrate absorption from the urinary lumen).

Modulation of Cl signaling and ion transport by recruitment of kinases and phosphatases mediated by the regulatory protein IRBIT.

IRBIT is a multifunctional protein that controls the activity of various epithelial ion transporters including NBCe1-B. Interaction with IRBIT increases NBCe1-B activity and exposes two cryptic Cl-sensing GXXXP sites that enable regulation of NBCe1-B by intracellular Cl (Cl ). Here, phosphoproteomic analysis revealed that IRBIT controlled five phosphorylation sites in NBCe1-B that determined both the active conformation of the transporter and its regulation by Cl Mutational analysis suggested that the phosphorylation status of Ser, Ser, and Ser was regulated by IRBIT and determined whether NBCe1 transporters are in active or inactive conformations.

Functional characteristics of L1156F-CFTR associated with alcoholic chronic pancreatitis in Japanese.

Although cystic fibrosis is rare in Japanese, measurement of sweat Cl(-) has suggested mild dysfunction of cystic fibrosis transmembrane conductance regulator (CFTR) in some patients with chronic pancreatitis. In the present study, we have investigated the association of CFTR variants and chronic pancreatitis in Japanese and the functional characteristics of a Japanese- and pancreatitis-specific CFTR variant, L1156F. Seventy patients with alcoholic chronic pancreatitis, 18 patients with idiopathic chronic pancreatitis, and 180 normal subjects participated.

Intracellular Cl- as a signaling ion that potently regulates Na+/HCO3- transporters.

Cl(-) is a major anion in mammalian cells involved in transport processes that determines the intracellular activity of many ions and plasma membrane potential. Surprisingly, a role of intracellular Cl(-) (Cl(-) in) as a signaling ion has not been previously evaluated. Here we report that Cl(-) in functions as a regulator of cellular Na(+) and HCO3 (-) concentrations and transepithelial transport through modulating the activity of several electrogenic Na(+)-HCO3 (-) transporters.

Mechanism and synergism in epithelial fluid and electrolyte secretion.

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-).

SLC26A6 and NaDC-1 transporters interact to regulate oxalate and citrate homeostasis.

The combination of hyperoxaluria and hypocitraturia can trigger Ca(2+)-oxalate stone formation, even in the absence of hypercalciuria, but the molecular mechanisms that control urinary oxalate and citrate levels are not understood completely. Here, we examined the relationship between the oxalate transporter SLC26A6 and the citrate transporter NaDC-1 in citrate and oxalate homeostasis. Compared with wild-type mice, Slc26a6-null mice exhibited increased renal and intestinal sodium-dependent succinate uptake, as well as urinary hyperoxaluria and hypocitraturia, but no change in urinary pH, indicating enhanced transport activity of NaDC-1.

Irbit mediates synergy between ca(2+) and cAMP signaling pathways during epithelial transport in mice.

Background & Aims: The cyclic adenosine monophosphate (cAMP) and Ca(2+) signaling pathways synergize to regulate many physiological functions. However, little is known about the mechanisms by which these pathways interact. We investigated the synergy between these signaling pathways in mouse pancreatic and salivary gland ducts.

Convergence of IRBIT, phosphatidylinositol (4,5) bisphosphate, and WNK/SPAK kinases in regulation of the Na+-HCO3- cotransporters family.

Fluid and HCO3(-) secretion is a vital function of secretory epithelia, involving basolateral HCO3(-) entry through the Na(+)-HCO3(-) cotransporter (NBC) NBCe1-B, and luminal HCO3(-) exit mediated by cystic fibrosis transmembrane conductance regulator (CFTR) and solute carrier family 26 (SLC26) Cl(-)/HCO3(-) exchangers. HCO3(-) secretion is highly regulated, with the WNK/SPAK kinase pathway setting the resting state and the IRBIT/PP1 pathway setting the stimulated state. However, we know little about the relationships between the WNK/SPAK and IRBIT/PP1 sites in the regulation of the transporters.

Solute carrier family 26 member a2 (Slc26a2) protein functions as an electroneutral SOFormula/OH-/Cl- exchanger regulated by extracellular Cl-.

Slc26a2 is a ubiquitously expressed SO(4)(2-) transporter with high expression levels in cartilage and several epithelia. Mutations in SLC26A2 are associated with diastrophic dysplasia. The mechanism by which Slc26a2 transports SO(4)(2-) and the ion gradients that mediate SO(4)(2-) uptake are poorly understood.

Determinants of coupled transport and uncoupled current by the electrogenic SLC26 transporters.

Members of the SLC26 family of anion transporters mediate the transport of diverse molecules ranging from halides to carboxylic acids and can function as coupled transporters or as channels. A unique feature of the two members of the family, Slc26a3 and Slc26a6, is that they can function as both obligate coupled and mediate an uncoupled current, in a channel-like mode, depending on the transported anion. To identify potential features that control the two modes of transport, we performed in silico modeling of Slc26a6, which suggested that the closest potential fold similarity of the Slc26a6 transmembrane domains is to the CLC transporters, despite their minimal sequence identity.

IRBIT: it is everywhere.

IRBIT (IP(3)Rs binding protein released with IP(3)) is a protein originally identified by the Mikoshiba group as an inhibitor of IP(3) receptors function. Subsequently it was found to have multiple functions and regulate the activity of diverse proteins, including regulation of HCO(3)(-) transporters to coordinate epithelial HCO(3)(-) secretion and to determine localization of the Fip1 subunit of the CPSF complex to regulate mRNA processing. This review highlights the remarkably divers functions of IRBIT that are likely only a fraction of all the potential functions of this protein.

IRBIT coordinates epithelial fluid and HCO3- secretion by stimulating the transporters pNBC1 and CFTR in the murine pancreatic duct.

Fluid and HCO3- secretion are vital functions of secretory epithelia. In most epithelia, this entails HCO3- entry at the basolateral membrane, mediated by the Na+-HCO3- cotransporter, pNBC1, and exit at the luminal membrane, mediated by a CFTR-SLC26 transporters complex. Here we report that the protein IRBIT (inositol-1,4,5-trisphosphate [IP3] receptors binding protein released with IP3), a previously identified activator of pNBC1, activates both the basolateral pNBC1 and the luminal CFTR to coordinate fluid and HCO3- secretion by the pancreatic duct.

Diverse transport modes by the solute carrier 26 family of anion transporters.

The solute carrier 26 (SLC26) transporters are anion transporters with diverse substrate specificity. Several members are ubiquitous while others show limited tissue distribution. They are expressed in many epithelia and to the extent known, play a central role in anion secretion and absorption.

The Slc26a4 transporter functions as an electroneutral Cl-/I-/HCO3- exchanger: role of Slc26a4 and Slc26a6 in I- and HCO3- secretion and in regulation of CFTR in the parotid duct.

Transcellular Cl(-) and HCO(3)(-) transport is a vital function of secretory epithelia and exit across the luminal membrane is mediated by members of the SLC26 transporters in conjunction with cystic fibrosis transmembrane conductance regulator (CFTR) channel. Typically, secretory epithelia express several SLC26 transporters in the same tissue; however, how their specific function is determined in vivo is not known. In the present work we used the parotid gland duct which expressed Slc26a4 and Slc26a6 and the model systems of Slc26a4(-/-) and Slc26a6(-/-) mice to study the role and regulation of these SLC26 transporters.

The solute carrier 26 family of proteins in epithelial ion transport.

Transepithelial Cl(-) and HCO(3)(-) transport is critically important for the function of all epithelia and, when altered or ablated, leads to a number of diseases, including cystic fibrosis, congenital chloride diarrhea, deafness, and hypotension (78, 111, 119, 126). HCO(3)(-) is the biological buffer that maintains acid-base balance, thereby preventing metabolic and respiratory acidosis (48). HCO(3)(-) also buffers the pH of the mucosal layers that line all epithelia, protecting them from injury (2).

Congenital chloride-losing diarrhea causing mutations in the STAS domain result in misfolding and mistrafficking of SLC26A3.

Congenital chloride-losing diarrhea (CLD) is a genetic disorder causing watery stool and dehydration. Mutations in SLC26A3 (solute carrier 26 family member 3), which functions as a coupled Cl(-)/HCO(3)(-) exchanger, cause CLD. SLC26A3 is a membrane protein predicted to contain 12 transmembrane-spanning alpha-helices and a C-terminal STAS (sulfate transporters and anti-sigma-factor) domain homologous to the bacterial anti-sigma-factor antagonists.

SLC26A9 is a Cl(-) channel regulated by the WNK kinases.

SLC26A9 is a member of the SLC26 family of anion transporters, which is expressed at high levels in airway and gastric surface epithelial cells. The transport properties and regulation of SLC26A9, and thus its physiological function, are not known. Here we report that SLC26A9 is a highly selective Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability that is regulated by the WNK kinases.

SLC26A7 can function as a chloride-loading mechanism in parietal cells.

To date three potential candidates for parietal cell basolateral Cl(-) entry have been described: the highly 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS)-sensitive Cl(-)/HCO(3)(-) exchanger AE2, the HCO(3)(-) and lowly DIDS-sensitive SLC26A7 protein, and the Na(+)-2Cl(-)K(+) cotransporter (NKCC1). In this study we investigate the contribution of these pathways to secretagogue stimulated acid secretion. Individually hand-dissected rat gastric glands were microfluorimetrically monitored for Cl(-) influx and pH(i) changes.

Regulatory interaction between CFTR and the SLC26 transporters.

Most epithelia that express CFTR secrete fluid rich in HCO3- and poor in Cl- that is generated by a CFTR-dependent Cl- absorption and HCO3- secretion process that when aberrant leads to human diseases such as cystic fibrosis and congenital chloride diarrhoea. Epithelial Cl- absorption and HCO3- secretion require expression of CFTR and other Cl- and HCO3- transporters in the luminal membrane of the secreting cells. Recent advances in understanding this critical epithelial function revealed that the luminal Cl- and HCO3- transporters are members of the SLC26 family.

Slc26a6 regulates CFTR activity in vivo to determine pancreatic duct HCO3- secretion: relevance to cystic fibrosis.

Fluid and HCO(3)(-) secretion are vital functions of the pancreatic duct and other secretory epithelia. CFTR and Cl(-)/HCO(3)(-) exchange activity at the luminal membrane are required for these functions. The molecular identity of the Cl(-)/HCO(3)(-) exchangers and their relationship with CFTR in determining fluid and HCO(3)(-) secretion are not known.

Coupling modes and stoichiometry of Cl-/HCO3- exchange by slc26a3 and slc26a6.

The SLC26 transporters are a family of mostly luminal Cl- and HCO3- transporters. The transport mechanism and the Cl-/HCO3- stoichiometry are not known for any member of the family. To address these questions, we simultaneously measured the HCO3- and Cl- fluxes and the current or membrane potential of slc26a3 and slc26a6 expressed in Xenopus laevis oocytes and the current of the transporters expressed in human embryonic kidney 293 cells.

TRP-ML1 is a lysosomal monovalent cation channel that undergoes proteolytic cleavage.

Mutations in the gene MCOLN1 coding for the TRP (transient receptor potential) family ion channel TRP-ML1 lead to the lipid storage disorder mucolipidosis type IV (MLIV). The function and role of TRP-ML1 are not well understood. We report here that TRP-ML1 is a lysosomal monovalent cation channel.

Borate transport and cell growth and proliferation. Not only in plants.

Boron is an abundant mineral essential for the life cycle of plants and may play a role in animal development and growth. Very little is known about boron homeostasis in plant and animal cells and the physiological roles of boron in animals. The recent identification of boron transporters, BOR1 in plants and NaBC1 in mammals, and that NaBC1 functions as an electrogenic Na+-coupled borate transporter essential for cell growth and proliferation open the way to probe the roles of boron in cellular function and physiology.

SLC26A7 is a Cl- channel regulated by intracellular pH.

Members of the SLC26 transporter family play an essential role in several epithelial functions, as revealed by diseases associated with mutations in members of the family. Several members were shown to function as Cl(-) and HCO(3)(-) transporters that likely play an important role in epithelial Cl(-) absorption and HCO(3)(-) secretion. However, the mechanism of most transporters is not well understood.