Identification and Purification of a Renal Amiloride-binding Protein with Properties of the Na+-H+ Exchanger

NHERF and regulation of the renal sodium-hydrogen exchanger NHE3

The sodium-hydrogen exchanger 3 (NHE3) isoform is the major regulated sodium transporter in the proximal convoluted tubule of the kidney. Study of the regulation of NHE3 by hormonal stimuli has identified a number of PDZ adaptor proteins that form an apical/subapical membrane scaffold that binds NHE3 and facilitates down-regulation of its activity in response to cAMP and activation of protein kinase A. The precise relation of proximal tubule adaptor proteins such as sodium-hydrogen exchanger regulatory factor-1 (NHERF-1), NHERF-2, and PDZ domain-containing-protein-1 (PDZK1) with each other and with protein targets such as NHE3 has been evolving with the development of specific reagents and genetically altered animals. In this review, we trace the discovery of NHERF-1 and NHERF-2, and update our current understanding of the relation between these proteins and the regulation and trafficking of NHE3.

Molecular cloning of complementary DNA encoding mouse seminal vesicle-secreted protein SVS I and demonstration of homology with copper amine oxidases

The primary structure of mouse SVS I was determined by peptide sequencing and nucleotide sequencing of cloned cDNA. The precursor molecule consists of 820 amino acid residues, including a signal peptide of 24 residues, and the mature polypeptide chain of 91 kDa has one site for potential N-linked glycosylation. The SVS I is homologous with amiloride-binding protein 1 (ABP1), a diamine oxidase. However, it probably lacks enzymatic activity, because the cDNA codes for His instead of Tyr at the position of the active-site topaquinon. The SVS I monomer probably binds one molecule of copper, because the His residues coordinated by Cu(II) are conserved. The SVS I gene consists of five exons and is situated on mouse chromosome 6,B2.3. It is located in a region of 100 kilobases (kb) containing several genes with homology to SVS I, including the gene of ABP1 and two other proteins with homology to diamine oxidase. The locus is conserved on rat chromosome 4q24, but the homologous region on human chromosome 7q34-q36 solely contains ABP1. The other genes with homology to diamine oxidase were probably present in a progenitor of primates and rodents but were lost in the evolutionary lineage leading to humans-presumably during recombination between chromosomes. The estimated molecular mass of rat SVS I is 102 kDa (excluding glycosylation). The species difference in size of SVS I is caused by tandem repeats of 18 amino acid residues in the central part of the molecule: The mouse has seven repeats, and the rat has 12 repeats.

Identification and expression of the Na+/H+ exchanger in mammalian cerebrovascular and choroidal tissues: characterization by amiloride-sensitive [3H]MIA binding and RT-PCR analysis

We report the initial characterization of [3H]5-(N-methyl-N-isobutyl)amiloride (MIA) binding to the Na+/H+ exchanger (NHE) and expression of its gene in mammalian cerebrovascular, choroidal and neocortical tissues. [3H]MIA bound reversibly to particulate fractions of rat, pig and human cerebral microvessels, choroid plexus and cerebral cortex. Scatchard analyses revealed binding to a single amiloride-sensitive site with dissociation constants (Kd) ranging from 20 to 90 nM for the various tissue preparations. The maximal binding capacities (Bmax) were between 2 to 17 pmol/mg protein and were several-fold greater in cerebral microvessels compared to the cerebral cortex. Amiloride, MIA, 5-(N, N-hexamethylene)amiloride (HMA), 5-(N, N-dimethyl)amiloride (DMA) and 5-(N-methyl-N-isopropyl)amiloride (IPA) variably displaced [3H]MIA binding to the microvessels in the following rank order: MIA>HMA>/=IPA>DMA>amiloride. Benzamil, a potent ligand of the Na+/Ca+ transporter was the least sensitive. These binding results were most compatible with the existence of the amiloride-sensitive NHE type 1 in the brain vascular and choroidal tissues. To substantiate this, we utilized reverse transcription polymerase chain reaction (RT-PCR) techniques to search for NHE-1 mRNA. Using primers corresponding to conserved sequences of the human growth factor-activatable NHE gene, RT-PCR revealed strong expression of NHE-1 mRNA in cerebral microvessels, choroid plexus, pial vessels and vascular smooth muscle cells relative to neocortical tissues from several species including rat, pig, cow, monkey and human subjects. Further confirmation of NHE-1 isoform mRNA expression in the cerebrovascular tissues was obtained by HpaII restriction digestion analysis and by subcloning and sequencing of the PCR amplified products. Our study suggests that mammalian cerebrovascular and choroidal tissues contain high amounts of the ubiquitous amiloride-sensitive [3H]MIA binding proteins consistent with the expression of NHE type 1 mRNA.

Coimmunoprecipitation of a 24-kDa protein with NHE1, the ubiquitous isoform of the Na+/H+ exchanger

Ancillary proteins have been proposed to account for phosphorylation-independent regulation of the Na+/H+ exchanger (NHE), but such putative proteins have not been identified. Here we describe the specific association of NHE1 with a protein of approximately 24 kDa (p24). Immunoprecipitation of NHE1 from lysates of [35S] cysteine- and/or methionine-labeled cells with the use of an anti-NHE1 antibody demonstrated specific coimmunoprecipitation of p24 with NHE1. The stoichiometry of p24 relative to NHE1, assessed by their radiolabel content, was consistent between experiments and among cell types. Immunoblotting demonstrated that p24 is not a proteolytic product of NHE1. Internal deletion mutants and chimeras of NHE1/NHE3 suggest that p24 binds to residues 515-566 or 695-815 of NHE1 or to the transmembrane region of both NHE1 and NHE3. Protein p24 is not constitutively phosphorylated nor could phosphorylation be induced by serum or phorbol ester treatment. Binding of p24 to NHE1 is Ca2+ independent. Protein p24 failed to bind [gamma-32P]GTP in a blot-overlay assay, suggesting that it is not a low-molecular-weight GTP-binding protein. Identification of the p24:NHE1 interaction may contribute to our understanding of antiporter regulation.

Structure-activity relations of amiloride derivatives, acting as antagonists of cation binding on Na+/K(+)-ATPase

In a search for an organic analogue of K+ or Na+ ions that binds to the cation binding sites of Na+/K(+)-ATPase with high affinity, the potency of the diuretic amiloride and its derivatives in blocking Rb+ occlusion has been tested. Although amiloride itself has a low affinity (> 200 microM), insertion of short alkyl chains in position 5 of the pyrazine ring of the molecule dramatically increased the affinity of the compound. For example, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) competes with a Ki approximately 10 microM. In derivatives lacking a halogen in position 6 of the ring, a 6-fold decrease in affinity was found. Substitutions in the guanidinium moiety did not produce high affinity inhibitors of Rb+ occlusion. Several derivatives at positions 5 and 6 of the pyrazine ring were found to be strictly competitive inhibitors with respect to Rb+ ions. The highest affinity was observed around pH 8.0-8.2, and low temperature. EIPA and 5-(N-methyl-N-isobutyl)amiloride (MIBA) stabilized the E1 form of FITC1-labelled Na+/K(+)-ATPase, behaving as Na+ analogues. The present findings are similar to our previous results, showing that alkyl- and arylguanidinium derivatives are competitive Na(+)-like antagonists in cation sites. Conclusions concerning the structural features of amiloride derivatives which are necessary to produce the highest binding affinity, are being exploited in synthesis of competitive cation analogues. Derivatives with sufficiently high affinity (0.1-1 microM) will be converted to affinity and photoaffinity reagents.

Purification of a synaptic membrane Na+/Ca2+ antiporter and immunoextraction with antibodies to a 36-kDa protein

The conditions for optimal solubilization and reconstitution of bovine brain synaptic plasma membrane Na+/Ca2+ exchange activity were examined and a series of chromatographic procedures were used for the isolation of a protein involved in this transport activity. The zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate in the presence of 20% (vol/vol) glycerol led to optimal solubilization, and soybean phospholipids in low-pH medium were found to produce optimal reconstitution of activity after dialysis to remove the detergent. Sequential chromatography steps involving the use of gel filtration on Sephacryl S-400 HR, ion exchange on diethylaminoethyl-Sephacel, and metal chelate chromatography on tris-(carboxymethyl)ethylenediamine loaded with LaCl3 led to the isolation of a fraction highly enriched in both Na+/Ca2+ exchange activity and two protein bands identified by denaturing electrophoresis. The estimated molecular masses of the two proteins were 50 and 36 kDa. Development of polyclonal antibodies to the 36-kDa protein permitted immunoextraction of greater than 95% of the antiporter activity from solubilized synaptic plasma membranes. These antibodies cross-reacted with the electroeluted 50-kDa protein on enzyme-linked immunosorbent assays, suggesting a close relationship between the two proteins. These results indicate that the 36-kDa protein is at least a component of the brain membrane Na+/Ca2+ antiporter.

Identification of the protein and cDNA of the cardiac Na+/H+ exchanger

We examined the myocardial form of the Na+/H+ exchanger. A partial length cDNA clone was isolated from a rabbit cardiac library and it encoded for a Na+/H+ exchange protein. In comparison with the human Na+/H+ exchanger, the sequence of the 5' end of the cDNA was highly conserved, much more than the 3' region, while the deduced amino acid sequence was also highly conserved. To further characterize the myocardial Na+/H+ exchange protein, we examined Western blots of isolated sarcolemma with antibody produced against a fusion protein of the Na+/H+ exchanger. The antibodies reacted with a sarcolemma protein of 50 kDa and with a protein of 70 kDa. The results show that the rabbit myocardium does possess a Na+/H+ exchanger protein homologous to the known human Na+/H+ exchanger.

Inhibition by aminoacyl-chloromethane protease inhibitors of superoxide anion production by phorbol-ester-stimulated human neutrophils. The labeled target is a membrane protein

In a previous paper, we described the kinetic characteristics of the inhibition exerted by the protease inhibitors tosylphenylalanyl and tosyllysyl chloromethanes on superoxide production by human polymorphonuclear leukocytes when stimulated by phorbol esters [E. C. Conseiller & F. Lederer (1989) Eur. J. Biochem. 183, 107-114]. The results suggested the existence of a specific target which was affinity labeled by the inhibitors. The target appeared to be neither a protease, nor intracellular enzymes which can be inhibited in vitro by the chloromethanes (protein kinase C, hexokinase and enzymes of the hexose monophosphate shunt). In the present work, using the cell-free reconstitution assay for superoxide production, we substantiate the hypothesis that the chloromethanes, target is on the plasma membrane. We have radiolabeled the membranes of cells inactivated before or after phorbol ester stimulation, using either [3H]KBH4 reduction after reaction with unlabeled inactivator, or tritiated tosylphenylalanyl chloromethane. In all cases, besides a certain background of non-specific labeling, a radioactive band of Mr 15,000 can be observed upon SDS/PAGE of radiolabeled membranes. We suggest that it is the chemical modification of this protein which is responsible for inactivation of superoxide production. Its identity and its role in the oxidative burst remain to be determined.

Cooperative regulation of the Na+/H(+)-antiporter in Halobacterium halobium by delta pH and delta phi

The contributions of the transmembrane pH gradient (delta pH) and electrical potential (delta phi) to the delta mu H(+)-driven Na+ efflux (mediated by the N,N'-dicyclohexylcarbodiimide-sensitive Na+/H(+)-antiporter) were investigated in membrane vesicles of Halobacterium halobium. Kinetic analysis in the dark revealed that two different Na(+)-binding sites are located asymmetrically across the membrane: One, accessible from the external medium, has a Kd (half-maximal stimulation of Na+ efflux) of about less than 50 mM, and the Na+ binding to the site is a prerequisite for the antiporter activation by delta mu H+. The other cytoplasmic site is the Na+ transport site. The Km for the cytoplasmic Na+ decreased as the delta pH increased, while the Vmax remained essentially constant in the presence of defined delta phi (140 mV). On the other hand, delta phi elevation above the gating potential (approximately 100 mV) increased the Vmax without changes in the Km in the presence of a fixed delta pH. It was also noted that the Km value in the absence of delta phi was completely different from and far higher than that observed in the presence of delta phi (greater than 100 mV), indicating the existence of two distinct conformations in the antiporter, resting and delta phi gated; the latter state may be reactive only to delta pH. On the basis of the present data and the previous data on the pH effect (N. Murakami and T. Konishi, 1989 Arch. Biochem. Biophys. 271, 515-523), a model for the delta pH-delta phi regulation of the antiporter activation is proposed.

Physiology and molecular biology of the renal Na/H antiporter

The renal Na/H antiporter is involved in cell pH regulation and, predominantly in the proximal tubule, also mediates transcellular NaCl and NaHCO3 transport. The transporter is regulated by systemic acid-base factors and hormones such as angiotensin II, PTH, glucocorticoids and alpha2-adrenergics. The human gene of the Na/H antiporter has been cloned and sequenced recently.