Debra Newman

Molecular Cloning, Chromosomal Localization, Tissue Distribution, and Functional Expression of the Human Pancreatic Sodium Bicarbonate Cotransporter

We report the cloning, sequence analysis, tissue distribution, functional expression, and chromosomal localization of the human pancreatic sodium bicarbonate cotransport protein (pancreatic NBC (pNBC)). The transporter was identified by searching the human expressed sequence tag data base. An I.M.A.G.E. clone W39298 was identified, and a polymerase chain reaction probe was generated to screen a human pancreas cDNA library. pNBC encodes a 1079-residue polypeptide that differs at the N terminus from the recently cloned human sodium bicarbonate cotransporter isolated from kidney (kNBC) (Burnham, C. E., Amlal, H., Wang, Z., Shull, G. E., and Soleimani, M. (1997) J. Biol. Chem. 272, 19111–19114). Northern blot analysis using a probe specific for the N terminus of pNBC revealed an ∼7.7-kilobase transcript expressed predominantly in pancreas, with less expression in kidney, brain, liver, prostate, colon, stomach, thyroid, and spinal chord. In contrast, a probe to the unique 5′ region of kNBC detected an ∼7.6-kilobase transcript only in the kidney. In situhybridization studies in pancreas revealed expression in the acini and ductal cells. The gene was mapped to chromosome 4q21 using fluorescentin situ hybridization. Expression of pNBC in Xenopus laevis oocytes induced sodium bicarbonate cotransport. These data demonstrate that pNBC encodes the sodium bicarbonate cotransporter in the mammalian pancreas. pNBC is also expressed at a lower level in several other organs, whereas kNBC is expressed uniquely in kidney.

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 ∼ 7.8-kilobase transcript is expressed uniquely in skeletal muscle and heart. The NBC3 gene (SLC4A7) spans ∼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 inXenopus laevis oocytes demonstrated that the protein encodes a novel stilbene-insensitive 5-(N-ethyl-N-isopropyl)-amiloride-inhibitable sodium bicarbonate cotransporter.

Cloning, characterization and chromosomal assignment of NBC4, a new member of the sodium bicarbonate cotransporter family

We report the cloning, characterization and chromosomal assignment of a new member of the sodium bicarbonate cotransporter (NBC) family, NBC4, from human heart. NBC4 maps to chromosome 2p13 and is a new candidate gene for Alstrom syndrome. NBC4 encodes a 1074-residue polypeptide with 12 putative membrane-spanning domains. Unlike other members of the NBC family, NBC4 has a unique glycine-rich region (amino acids 438-485). In addition, NBC4 lacks the lysine-rich C-terminus of NBC1 with which it is most homologous. The first of two putative stilbene binding motifs (K(M/L)(X)K) is lacking in NBC4 (amino acids 655-658). The approximately 6 kb NBC4 transcript is moderately expressed in heart, with the highest expression in liver, testes and spleen.

Molecular mechanism of kNBC1–carbonic anhydrase II interaction in proximal tubule cells

We have recently shown that carbonic anhydrase II (CAII) binds in vitro to the C‐terminus of the electrogenic sodium bicarbonate cotransporter kNBC1 (kNBC1‐ct). In the present study we determined the molecular mechanisms for the interaction between the two proteins and whether kNBC1 and CAII form a transport metabolon in vivo wherein bicarbonate is transferred from CAII directly to the cotransporter. Various residues in the C‐terminus of kNBC1 were mutated and the effect of these mutations on both the magnitude of CAII binding and the function of kNBC1 expressed in mPCT cells was determined. Two clusters of acidic amino acids, L958DDV and D986NDD in the wild‐type kNBC1‐ct involved in CAII binding were identified. In both acidic clusters, the first aspartate residue played a more important role in CAII binding than others. A significant correlation between the magnitude of CAII binding and kNBC1‐mediated flux was shown. The results indicated that CAII activity enhances flux through the cotransporter when the enzyme is bound to kNBC1. These data are the first direct evidence that a complex of an electrogenic sodium bicarbonate cotransporter with CAII functions as a transport metabolon.

Slc4a11 gene disruption in mice: Cellular targets of sensorineuronal abnormalities

NaBC1 (the SLC4A11 gene) belongs to the SLC4 family of sodium-coupled bicarbonate (carbonate) transporter proteins and functions as an electrogenic sodium borate cotransporter. Mutations in SLC4A11 cause either corneal abnormalities (corneal hereditary dystrophy type 2) or a combined auditory and visual impairment (Harboyan syndrome). The role of NaBC1 in sensory systems is poorly understood, given the difficulty of studying patients with NaBC1 mutations. We report our findings in Slc4a11−/− mice generated to investigate the role of NaBC1 in sensorineural systems. In wild-type mice, specific NaBC1 immunoreactivity was detected in fibrocytes of the spiral ligament, from the basal to the apical portion of the cochlea. NaBC1 immunoreactivity was present in the vestibular labyrinth, in stromal cells underneath the non-immunoreactive sensory epithelia of the macula utricle, sacule, and crista ampullaris, and the membranous vestibular labyrinth was collapsed. Both auditory brain response and vestibular evoked potential waveforms were significantly abnormal in Slc4a11−/− mice. In the cornea, NaBC1 was highly expressed in the endothelial cell layer with less staining in epithelial cells. However, unlike humans, the corneal phenotype was mild with a normal slit lamp evaluation. Corneal endothelial cells were morphologically normal; however, both the absolute height of the corneal basal epithelial cells and the relative basal epithelial cell/total corneal thickness were significantly increased in Slc4a11−/− mice. Our results demonstrate for the first time the importance of NaBC1 in the audio-vestibular system and provide support for the hypothesis that SLC4A11 should be considered a potential candidate gene in patients with isolated sensorineural vestibular hearing abnormalities.

Phosphorylation-induced modulation of pNBC1 function: distinct roles for the amino- and carboxy-termini

The human NBC1 (SLC4A4) gene encodes the electrogenic sodium bicarbonate cotransporters kNBC1 and pNBC1, which are highly expressed in the kidney and pancreas, respectively. The HCO3−:Na+ stoichiometry of these cotransporters is an important determinant of the direction of ion flux. Recently we showed in a mouse proximal tubule (mPCT) cell line expressing kNBC1, that 8‐Br‐cAMP shifts the stoichiometry of the cotransporter from 3:1 to 2:1 via protein kinase A (PKA)‐dependent phosphorylation of Ser982. pNBC1 has the identical carboxy‐terminal consensus phosphorylation PKA site (KKGS1026), and an additional site in its amino‐terminus (KRKT49). In this study we determined the potential role of these sites in regulating the function of pNBC1. The results demonstrated that in mPCT cells expressing pNBC1, PKA‐dependent phosphorylation of Ser1026 following 8‐Br‐cAMP treatment shifted the stoichiometry from 3:1 to 2:1. The effect was electrostatic in nature as replacing Ser1026 with Asp resulted in a similar stoichiometry shift. In addition to shifting the stoichiometry, 8‐Br‐cAMP caused a significant increase in the 4,4′‐dinitrostilbene‐2,2′‐disulfonic acid (DNDS)‐sensitive basolateral membrane conductance (GDS) of cells expressing pNBC1, but not kNBC1. Although, the effect did not involve phosphorylation of Thr49, which was endogenously phosphorylated, replacing this residue with Asp or Ala abolished the 8‐Br‐cAMP‐induced increase in GDS. In the mPEC pancreatic duct cell line, where endogenous pNBC1 functions with a HCO3−:Na+ stoichiometry of 2:1, 8‐Br‐cAMP increased GDS by ∼90 % without altering the stoichiometry or inducing phosphorylation of the cotransporter. The results demonstrate that phosphorylation of Ser1026 mediates the cAMP‐dependent shift in the stoichiometry of pNBC1, whereas Thr49 plays an essential role in the cAMP‐induced increase in GDS.

Two C-Terminal Variants of NBC4, a New Member of the Sodium Bicarbonate Cotransporter Family: Cloning, Characterization, and Localization

We report the cloning, characterization, and chromosomal assignment of a new member of the sodium bicarbonate cotransporter (NBC) family, NBC4. The NBC4 gene was mapped to chromosome 2p13 and is a new candidate gene for Alstrom syndrome. Two variants of the transporter have been isolated from human testis and heart, which differ in their C termini. NBC4a encodes a 1137‐residue polypeptide and is widely expressed in various tissues, including liver, testis, and spleen. NBC4b is identical to NBC4a except that it has a 16‐nucleotide insert, creating a C‐terminal frame shift. NBC4b encodes a 1074‐residue polypeptide and is highly expressed in heart. Amino acids 1‐1046 are common to both NBC4 variants. NBC4a has two protein‐interacting domains that are lacking in NBC4b: a proline‐rich sequence, PPPSVIKIP (amino acids 1102‐1110), and a consensus PDZ‐interacting domain, SYSL (1134‐1137). NBC4b lacks the stretch of charged residues present in the C terminus of NBC4a and other members of the NBC family.Unlike other members of the NBC family, both NBC4a and NBC4b have a unique glycine‐rich region (amino acids 440‐ 469). In comparison with other members of the bicarbonate transport superfamily, NBC4a and NBC4b are most similar structurally to the electrogenic sodium bicarbonate cotransporters (NBC1).

Critical amino acid residues involved in the electrogenic sodium-bicarbonate cotransporter kNBC1-mediated transport.

We have previously reported a topological model of the electrogenic Na+–HCO3− cotransporter (NBC1) in which the cotransporter spans the plasma membrane 10 times with N‐ and C‐termini localized intracellularly. An analysis of conserved amino acid residues among members of the SLC4 superfamily in both the transmembrane segments (TMs) and intracellular/extracellular loops (ILs/ELs) provided the basis for the mutagenesis approach taken in the present study to determine amino acids involved in NBC1‐mediated ion transport. Using large‐scale mutagenesis, acidic and basic amino acids putatively involved in ion transport mediated by the predominant variant of NBC1 expressed in the kidney (kNBC1) were mutated to neutral and/or oppositely charged amino acids. All mutant kNBC1 cotransporters were expressed in HEK‐293T cells and the Na+‐dependent base flux of the mutants was determined using intracellular pH measurements with 2′,7′‐bis‐(carboxyethyl)‐5(6)‐carboxyfluorescein (BCECF). Critical glutamate, aspartate, lysine, arginine and histidine residues in ILs/ELs and TMs were detected that were essential for kNBC1‐mediated Na+‐dependent base transport. In addition, critical phenylalanine, serine, tyrosine, threonine and alanine residues in TMs and ILs/ELs were detected. Furthermore, several amino acid residues in ILs/ELs and TMs were shown to be essential for membrane targeting. The data demonstrate asymmetry of distribution of kNBC1 charged amino acids involved in ion recognition in putative outward‐facing and inward‐facing conformations. A model summarizing key amino acid residues involved in kNBC1‐mediated ion transport is presented.

Specificity of aminoacylase III-mediated deacetylation of mercapturic acids

Trichloroethylene (TCE) and other halogenated alkenes are known environmental contaminants with cytotoxic and nephrotoxic effects, and are potential carcinogens. Their metabolism via the mercapturate metabolic pathway was shown to lead to their detoxification. The final products of this pathway, mercapturic acids or N-acetyl-l-cysteine S-conjugates, are secreted into the lumen in the renal proximal tubule. The proximal tubule may also deacetylate mercapturic acids, and the resulting cysteine S-conjugates are transformed by cysteine S-conjugate β-lyases to nephrotoxic reactive thiols. The specificity and rate of mercapturic acid deacetylation may determine the toxicity of certain mercapturic acids; however, the exact enzymologic processes involved are not known in detail. In the present study we characterized the kinetics of the recently cloned mouse aminoacylase III (AAIII) toward a wide spectrum of halogenated mercapturic acids and N-acetylated amino acids. In general, the Vmax value of AAIII was significantly larger with chlorinated and brominated mercapturic acids, whereas fluorination significantly decreased it. The enzyme deacetylated mercapturic acids derived from the TCE metabolism including N-acetyl-S-(1,2-dichlorovinyl)-l-cysteine (NA-1,2-DCVC) and N-acetyl-S-(2,2-dichlorovinyl)-l-cysteine (NA-2,2-DCVC). Both mercapturic acids induced cytotoxicity in mouse proximal tubule mPCT cells expressing AAIII, which was decreased by an inhibitor of β-lyase, aminooxyacetate. The toxic effect of NA-2,2-DCVC was smaller than that of NA-1,2-DCVC, indicating that factors other than the intracellular activity of AAIII mediate the cytotoxicity of these mercapturic acids. Our results indicate that in proximal tubule cells, AAIII plays an important role in deacetylating several halogenated mercapturic acids, and this process may be involved in their cyto- and nephrotoxicity.

Missense mutation T485S alters NBCe1-A electrogenicity causing proximal renal tubular acidosis.

Mutations in SLC4A4, the gene encoding the electrogenic Na(+)-HCO3(-) cotransporter NBCe1, cause severe proximal renal tubular acidosis (pRTA), growth retardation, decreased IQ, and eye and teeth abnormalities. Among the known NBCe1 mutations, the disease-causing mechanism of the T485S (NBCe1-A numbering) mutation is intriguing because the substituted amino acid, serine, is structurally and chemically similar to threonine. In this study, we performed intracellular pH and whole cell patch-clamp measurements to investigate the base transport and electrogenic properties of NBCe1-A-T485S in mammalian HEK 293 cells. Our results demonstrated that Ser substitution of Thr485 decreased base transport by ~50%, and importantly, converted NBCe1-A from an electrogenic to an electroneutral transporter. Aqueous accessibility analysis using sulfhydryl reactive reagents indicated that Thr485 likely resides in an NBCe1-A ion interaction site. This critical location is also supported by the finding that G486R (a pRTA causing mutation) alters the position of Thr485 in NBCe1-A thereby impairing its transport function. By using NO3(-) as a surrogate ion for CO3(2-), our result indicated that NBCe1-A mediates electrogenic Na(+)-CO3(2-) cotransport when functioning with a 1:2 charge transport stoichiometry. In contrast, electroneutral NBCe1-T485S is unable to transport NO3(-), compatible with the hypothesis that it mediates Na(+)-HCO3(-) cotransport. In patients, NBCe1-A-T485S is predicted to transport Na(+)-HCO3(-) in the reverse direction from blood into proximal tubule cells thereby impairing transepithelial HCO3(-) absorption, possibly representing a new pathogenic mechanism for generating human pRTA.