Cation channel regulation by COOH-terminal cytoplasmic tail of polycystin-1: mutational and functional analysis.

Polycystin-1 (PKD1) mutations account for approximately 85% of autosomal dominant polycystic kidney disease (ADPKD). We have shown previously that oocyte surface expression of a transmembrane fusion protein encoding part of the cytoplasmic COOH terminus of PKD1 increases activity of a Ca2+-permeable cation channel. We show here that human ADPKD mutations incorporated into this fusion protein attenuated or abolished encoded cation currents. Point mutations and truncations showed that cation current expression requires integrity of a region encompassing the putative coiled coil domain of the PKD1 cytoplasmic tail. Whereas these loss-of-function mutants did not exhibit dominant negative phenotypes, coexpression of a fusion protein expressing the interacting COOH-terminal cytoplasmic tail of PKD2 did suppress cation current. Liganding of the ectodomain of the PKD1 fusion protein moderately activated cation current. The divalent cation permeability and pharmacological profile of the current has been extended. Inducible expression of the PKD1 fusion in EcR-293 cells was also associated with activation of cation channels and increased Ca2+ entry.

Mouse strain-specific coding polymorphism in the Slc4a2/Ae2 gene encodes Ae2c2 variants differing in isoform-specific dominant negative activity.

The Slc4a2/Ae2 gene encodes multiple polypeptides arising from alternate promoter usage. The Ae2c promoter gives rise to only one Ae2c transcript from the human Ae2 gene, but to two, alternatively spliced, Ae2c1 and Ae2c2 transcripts from the mouse and rat genes. Unlike in the rat, the mouse Ae2c2 transcript encodes a novel Ae2c2 polypeptide. Here we report that the Ae2c2 residue 9 can be either proline or serine in a mouse strain-specific manner. Both Ae2c2 polypeptides express low function in Xenopus oocytes secondary to reduced or absent surface expression. Ae2c2S, but not Ae2c2P, exerts a dominant negative effect when coexpressed with Ae2a polypeptide, has a less prominent effect when coexpressed with Ae2b1 or Ae2c1 polypeptides, but has no effect on the function of coexpressed Ae2b2 polypeptide. Coexpression of Ae2c2P does not reduce activity of any Ae2 polypeptide variant. Ae2c2S and Ae2c2P also express low functional activity in HEK-293 cells. Knowledge of strain-specific coding polymorphisms with potential functional consequences such as that of Ae2c2 should aid in interpretation of strain-specific phenotypes investigated in the mouse phenome project.

Human ADPKD primary cyst epithelial cells with a novel, single codon deletion in the PKD1 gene exhibit defective ciliary polycystin localization and loss of flow-induced Ca2+ signaling.

Autosomal dominant polycystic kidney disease (ADPKD) gene products polycystin-1 (PC1) and polycystin-2 (PC2) colocalize in the apical monocilia of renal epithelial cells. Mouse and human renal cells without PC1 protein show impaired ciliary mechanosensation, and this impairment has been proposed to promote cystogenesis. However, most cyst epithelia of human ADPKD kidneys appear to express full-length PC1 and PC2 in normal or increased abundance. We show that confluent primary ADPKD cyst cells with the novel PC1 mutation DeltaL2433 and with normal abundance of PC1 and PC2 polypeptides lack ciliary PC1 and often lack ciliary PC2, whereas PC1 and PC2 are both present in cilia of confluent normal human kidney (NK) epithelial cells in primary culture. Confluent NK cells respond to shear stress with transient increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), dependent on both extracellular Ca(2+) and release from intracellular stores. In contrast, ADPKD cyst cells lack flow-sensitive [Ca(2+)](i) signaling and exhibit reduced endoplasmic reticulum Ca(2+) stores and store-depletion-operated Ca(2+) entry but retain near-normal [Ca(2+)](i) responses to ANG II and to vasopressin. Expression of wild-type and mutant CD16.7-PKD1(115-226) fusion proteins reveals within the COOH-terminal 112 amino acids of PC1 a coiled-coil domain-independent ciliary localization signal. However, the coiled-coil domain is required for CD16.7-PKD1(115-226) expression to accelerate decay of the flow-induced Ca(2+) signal in NK cells. These data provide evidence for ciliary dysfunction and polycystin mislocalization in human ADPKD cells with normal levels of PC1.

Alkaline-shifted pHo sensitivity of AE2c1-mediated anion exchange reveals novel regulatory determinants in the AE2 N-terminal cytoplasmic domain.

The mouse anion exchanger AE2/SLC4A2 Cl(-)/HCO(-)(3) exchanger is essential to post-weaning life. AE2 polypeptides regulate pH(i), chloride concentration, cell volume, and transepithelial ion transport in many tissues. Although the AE2a isoform has been extensively studied, the function and regulation of the other AE2 N-terminal variant mRNAs of mouse (AE2b1, AE2b2, AE2c1, and AE2c2) have not been examined. We now present an extended analysis of AE2 variant mRNA tissue distribution and function. We show in Xenopus oocytes that all AE2 variant polypeptides except AE2c2 mediated Cl(-) transport are subject to inhibition by acidic pH(i) and to activation by hypertonicity and NH(+)(4). However, AE2c1 differs from AE2a, AE2b1, and AE2b2 in its alkaline-shifted pH(o)((50)) (7.70 +/- 0.11 versus 6.80 +/- 0.05), suggesting the presence of a novel AE2a pH-sensitive regulatory site between amino acids 99 and 198. Initial N-terminal deletion mutagenesis restricted this site to the region between amino acids 120 and 150. Further analysis identified AE2a residues 127-129, 130-134, and 145-149 as jointly responsible for the difference in pH(o)((50)) between AE2c1 and the longer AE2a, AE2b1, and AE2b2 polypeptides. Thus, AE2c1 exhibits a unique pH(o) sensitivity among the murine AE2 variant polypeptides, in addition to a unique tissue distribution. Physiological coexpression of AE2c1 with other AE2 variant polypeptides in the same cell should extend the range over which changing pH(o) can regulate AE2 transport activity.

The abts and sulp families of anion transporters from Caenorhabditis elegans.

The slc4 and slc26 gene families encode two distinct groups of gene products that transport HCO(3)(-) and other anions in mammalian cells. The SLC4 and SLC26 proteins are important contributors to transepithelial movement of fluids and electrolytes and to cellular pH and volume regulation. Herein we describe the cDNA cloning from the nematode Caenorhabditis elegans of four anion bicarbonate transporter (abts) homologs of slc4 cDNA and eight sulfate permease (sulp) homologs of slc26 cDNA. Analysis of transgenic nematode strains carrying promoter::GFP fusions suggests relatively restricted expression patterns for many of these genes. At least three genes are expressed primarily in the intestine, three are expressed primarily in the excretory cell, and one is expressed in both of these polarized cell types. One of the genes is also expressed exclusively in the myoepithelium-like cells of the pharynx. Many of the sulp gene products localize to the basolateral membrane rather than to the apical membrane. Several ABTS and SULP proteins exhibited anion transport function in Xenopus oocytes. The strongest Cl(-) transporter among these also mediated Cl(-)/HCO(3)(-) exchange. These findings encourage exploitation of the genetic strengths of the nematode model system in the study of the physiological roles of anion transport by the proteins of these two highly conserved gene families.

Apparent receptor-mediated activation of Ca2+-dependent conductive Cl- transport by shark-derived polyaminosterols.

The shark liver antimicrobial polyaminosterol squalamine is an angiogenesis inhibitor under clinical investigation as an anti-cancer agent and as a treatment for the choroidal neovascularization associated with macular degeneration of the retina. The related polyaminosterol MSI-1436 is an appetite suppressant that decreases systemic insulin resistance. However, the mechanisms of action of these polyaminosterols are unknown. We report effects of MSI-1436 on Xenopus oocytes consistent with the existence of a receptor for polyaminosterols. MSI-1436 activates bidirectional, trans-chloride-independent Cl- flux in Xenopus oocytes. At least part of this DIDS-sensitive Cl- flux is conductive, as measured using two-electrode voltage-clamp and on-cell patch-clamp techniques. MSI-1436 also elevates cytosolic Ca2+ concentration ([Ca2+]) and increases bidirectional 45Ca2+ flux. Activation of Cl- flux and elevation of cytosolic [Ca2+] by MSI-1436 both are accelerated by lowering bath Ca2+ and are not acutely inhibited by extracellular EGTA. Elevation of cytosolic [Ca2+] by MSI-1436 requires heparin-sensitive intracellular Ca2+ stores. Although injected EGTA abolishes the increased conductive Cl- flux, that Cl- flux is not dependent on heparin-sensitive stores. In low-bath Ca2+ conditions, several structurally related polyaminosterols act as strong agonists or weak agonists of conductive Cl- flux in oocytes. Weak agonist polyaminosterols antagonize the strong agonist, MSI-1436, but upon addition of the conductive Cl- transport inhibitor DIDS, they are converted into strong agonists. Together, these properties operationally define a polyaminosterol receptor at or near the surface of the Xenopus oocyte, provide an initial description of receptor signaling, and suggest routes toward further understanding of a novel class of appetite suppressants and angiogenesis inhibitors.

Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants: differences in anion selectivity, regulation, and electrogenicity.

The unusually low 78% amino acid identity between the orthologous human SLC26A6 and mouse slc26a6 polypeptides prompted systematic comparison of their anion transport functions in Xenopus oocytes. Multiple human SLC26A6 variant polypeptides were also functionally compared. Transport was studied as unidirectional fluxes of (36)Cl(-), [(14)C]oxalate, and [(35)S]sulfate; as net fluxes of HCO(3)(-) by fluorescence ratio measurement of intracellular pH; as current by two-electrode voltage clamp; and as net Cl(-) flux by fluorescence intensity measurement of relative changes in extracellular and intracellular [Cl(-)]. Four human SLC26A6 polypeptide variants each exhibited rates of bidirectional [(14)C]oxalate flux, Cl(-)/HCO(3)(-) exchange, and Cl(-)/OH(-) exchange nearly equivalent to those of mouse slc26a6. Cl(-)/HCO(3)(-) exchange by both orthologs was cAMP-sensitive, further enhanced by coexpressed wild type cystic fibrosis transmembrane regulator but inhibited by cystic fibrosis transmembrane regulator DeltaF508. However, the very low rates of (36)Cl(-) and [(35)S]sulfate transport by all active human SLC26A6 isoforms contrasted with the high rates of the mouse ortholog. Human and mouse orthologs also differed in patterns of acute regulation. Studies of human-mouse chimeras revealed cosegregation of the high (36)Cl(-) transport phenotype with the transmembrane domain of mouse slc26a6. Mouse slc26a6 and human SLC26A6 each mediated electroneutral Cl(-)/HCO(3)(-) and Cl(-)/OH(-) exchange. In contrast, whereas Cl(-)/oxalate exchange by mouse slc26a6 was electrogenic, that mediated by human SLC26A6 appeared electroneutral. The increased currents observed in oocytes expressing either mouse or human ortholog were pharmacologically distinct from the accompanying monovalent anion exchange activities. The human SLC26A6 polypeptide variants SLC26A6c and SLC26A6d were inactive as transporters of oxalate, sulfate, and chloride. Thus, the orthologous mouse and human SLC26A6 proteins differ in anion selectivity, transport mechanism, and acute regulation, but both mediate electroneutral Cl(-)/HCO(3)(-) exchange.

Zebrafish slc4a2/ae2 anion exchanger: cDNA cloning, mapping, functional characterization, and localization.

Although the zebrafish has been used increasingly for the study of pronephric kidney development, studies of renal ion transporters and channels of the zebrafish remain few. We report the cDNA cloning and characterization of the AE2 anion exchanger ortholog from zebrafish kidney, slc4a2/ae2. The ae2 gene in linkage group 2 encodes a polypeptide of 1,228 aa exhibiting 64% aa identity with mouse AE2a. The exon-intron boundaries of the zebrafish ae2 gene are nearly identical to those of the rodent and human genes. Whole-mount in situ hybridization detects ae2 mRNA in prospective midbrain as early as the five-somite stage, then later in the pronephric primordia and the forming pronephric duct, where it persists through 72 h postfertilization (hpf). Zebrafish Ae2 expressed in Xenopus laevis oocytes mediates Na(+)-independent, electroneutral (36)Cl(-)/Cl(-) exchange moderately sensitive to inhibition by DIDS, is inhibited by acidic intracellular pH and by acidic extracellular pH, but activated by (acidifying) ammonium and by hypertonicity. Zebrafish Ae2 also mediates Cl(-)/HCO(3)(-) exchange in X. laevis oocytes and accumulates in or near the plasma membrane in transfected HEK-293 cells. In 24-48 hpf zebrafish embryos, the predominant but not exclusive localization of Ae2 polypeptide is the apical membrane of pronephric duct epithelial cells. Thus Ae2 resembles its mammalian orthologs in function, mechanism, and acute regulation but differs in its preferentially apical expression in kidney. These results will inform tests of the role of Ae2 in zebrafish kidney development and function.

Structure-function relationships of AE2 regulation by Ca(i)(2+)-sensitive stimulators NH(4+) and hypertonicity.

We showed previously that the nonerythroid anion exchanger AE2 and the erythroid anion exchanger AE1 differ greatly in their regulation by acute changes in intracellular pH (pH(i)) and extracellular pH (pH(o)). We have now examined how AE2, but not AE1, is activated by two stimuli with opposing effects on oocyte pH(i): an alkalinizing stimulus, hypertonicity, and an acidifying stimulus, NH(4)(+). We find that both NH(2)-terminal cytoplasmic and COOH-terminal transmembrane domains of AE2 are required for activation by either stimulus. Directed by initial deletion mutagenesis studies of the NH(2)-terminal cytoplasmic domain, an alanine scan of AE2 amino acids 336-347 identified residues whose individual mutation abolished or severely attenuated sensitivity to both or only one activating stimulus. Chelation of cytoplasmic Ca(2+) (Ca(i)(2+)) diminished or abolished AE2 stimulation by NH(4)(+) and by hypertonicity. Calmidazolium inhibited AE2 activity, but not that of AE1. AE2 was insensitive to many other modifiers of Ca(2+) signaling. Unlike AE2 stimulation by NH(4)(+) and by hypertonicity, AE2 inhibition by calmidazolium required only AE2's COOH-terminal transmembrane domain.

Deficient HCO3- transport in an AE1 mutant with normal Cl- transport can be rescued by carbonic anhydrase II presented on an adjacent AE1 protomer.

Cl-/HCO3- exchange activity mediated by the AE1 anion exchanger is reduced by carbonic anhydrase II (CA2) inhibition or by prevention of CA2 binding to the AE1 C-terminal cytoplasmic tail. This type of AE1 inhibition is thought to represent reduced metabolic channeling of HCO3- to the intracellular HCO3- binding site of AE1. To test the hypothesis that CA2 binding might itself allosterically activate AE1 in Xenopus oocytes, we compared Cl-/Cl- and Cl-/HCO3- exchange activities of AE1 polypeptides with truncation and missense mutations in the C-terminal tail. The distal renal tubular acidosis-associated AE1 901X mutant exhibited both Cl-/Cl- and Cl-/HCO3- exchange activities. In contrast, AE1 896X, 891X, and AE1 missense mutants in the CA2 binding site were inactive as Cl-/HCO3- exchangers despite exhibiting normal Cl-/Cl- exchange activities. Co-expression of CA2 enhanced wild-type AE1-mediated Cl-/HCO3- exchange, but not Cl-/Cl- exchange. CA2 co-expression could not rescue Cl-/HCO3- exchange activity in AE1 mutants selectively impaired in Cl-/HCO3- exchange. However, co-expression of transport-incompetent AE1 mutants with intact CA2 binding sites completely rescued Cl-/HCO3- exchange by an AE1 missense mutant devoid of CA2 binding, with activity further enhanced by CA2 co-expression. The same transport-incompetent AE1 mutants failed to rescue Cl-/HCO3- exchange by the AE1 truncation mutant 896X, despite preservation of the latter's core CA2 binding site. These data increase the minimal extent of a functionally defined CA2 binding site in AE1. The inter-protomeric rescue of HCO3- transport within the AE1 dimer shows functional proximity of the C-terminal cytoplasmic tail of one protomer to the anion translocation pathway in the adjacent protomer within the AE1 heterodimer. The data strongly support the hypothesis that an intact transbilayer anion translocation pathway is completely contained within an AE1 monomer.

Diarrhea-associated HIV-1 APIs potentiate muscarinic activation of Cl- secretion by T84 cells via prolongation of cytosolic Ca2+ signaling.

Aspartyl protease inhibitors (APIs) effectively extend the length and quality of life in human immunodeficiency virus (HIV)-infected patients, but dose-limiting side effects such as lipodystrophy, insulin resistance, and diarrhea have limited their clinical utility. Here, we show that the API nelfinavir induces a secretory form of diarrhea in HIV-infected patients. In vitro studies demonstrate that nelfinavir potentiates muscarinic stimulation of Cl(-) secretion by T84 human intestinal cell monolayers through amplification and prolongation of an apical membrane Ca(2+)-dependent Cl(-) conductance. This stimulated ion secretion is associated with increased magnitude and duration of muscarinically induced intracellular Ca(2+) transients via activation of a long-lived, store-operated Ca(2+) entry pathway. The enhanced intracellular Ca(2+) signal is associated with uncoupling of the Cl(-) conductance from downregulatory intracellular mediators generated normally by muscarinic activation. These data show that APIs modulate Ca(2+) signaling in secretory epithelial cells and identify a novel target for treatment of clinically important API side effects.

Acute regulation of the SLC26A3 congenital chloride diarrhoea anion exchanger (DRA) expressed in Xenopus oocytes.

Mutations in the human SLC26A3 gene, also known as down-regulated in adenoma (hDRA), cause autosomal recessive congenital chloride-losing diarrhoea (CLD). hDRA expressed in Xenopus oocytes mediated bidirectional Cl--Cl- and Cl--HCO3- exchange. In contrast, transport of oxalate was low, and transport of sulfate and of butyrate was undetectable. Two CLD missense disease mutants of hDRA were nonfunctional in oocytes. Truncation of up to 44 C-terminal amino acids from the putatively cytoplasmic C-terminal hydrophilic domain left transport function unimpaired, but deletion of the adjacent STAS (sulfate transporter anti-sigma factor antagonist) domain abolished function. hDRA-mediated Cl- transport was insensitive to changing extracellular pH, but was inhibited by intracellular acidification and activated by NH4+ at acidifying concentrations. These regulatory responses did not require the presence of either hDRA's N-terminal cytoplasmic tail or its 44 C-terminal amino acids, but they did require more proximate residues of the C-terminal cytoplasmic domain. Although only weakly sensitive to inhibition by stilbenes, hDRA was inhibited with two orders of magnitude greater potency by the anti-inflammatory drugs niflumate and tenidap. cAMP-insensitive Cl--HCO3- exchange mediated by hDRA gained modest cAMP sensitivity when co-expressed with cystic fibrosis transmembrane conductance regulator (CFTR). Despite the absence of hDRA transcripts in human cell lines derived from CFTR patients, DRA mRNA was present at wild-type levels in proximal colon and nearly so in the distal ileum of CFTR(-/-) mice. Thus, pharmacological modulation of DRA might be a useful adjunct treatment of cystic fibrosis.