Sodium-hydrogen exchange isoform expression in blood cells: implications for studies in diabetes mellitus

Multiomic analysis of the Arabian camel (Camelus dromedarius) kidney reveals a role for cholesterol in water conservation

The Arabian camel (Camelus dromedarius) is the most important livestock animal in arid and semi-arid regions and provides basic necessities to millions of people. In the current context of climate change, there is renewed interest in the mechanisms that enable camelids to survive in arid conditions. Recent investigations described genomic signatures revealing evolutionary adaptations to desert environments. We now present a comprehensive catalogue of the transcriptomes and proteomes of the dromedary kidney and describe how gene expression is modulated as a consequence of chronic dehydration and acute rehydration. Our analyses suggested an enrichment of the cholesterol biosynthetic process and an overrepresentation of categories related to ion transport. Thus, we further validated differentially expressed genes with known roles in water conservation which are affected by changes in cholesterol levels. Our datasets suggest that suppression of cholesterol biosynthesis may facilitate water retention in the kidney by indirectly facilitating the AQP2-mediated water reabsorption.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Focal adhesion kinase controls pH-dependent epidermal barrier homeostasis by regulating actin-directed Na+/H+ exchanger 1 plasma membrane localization

Ubiquitously expressed focal adhesion kinase (FAK), linked to multiple intracellular signaling pathways, has previously been shown to control cell motility, invasion, proliferation, and survival. Using mice with a keratinocyte-restricted deletion of fak (FAK(K5 KO)), we report here a novel role for FAK: maintenance of adult epidermal permeability barrier homeostasis. Abundant lacunae of unprocessed lipids in stratum corneum (SC) of FAK(K5 KO) mice and delayed barrier recovery pointed to malfunction of pH-dependent enzymes active in extracellular space of SC. Measuring the SC pH gradient showed significantly more neutral pH values in FAK(K5 KO) mice, suggesting the importance of FAK for acidification. Moreover, normal functions were restored when FAK(K5 KO) mice were exposed to a surface pH typical of mouse SC (pH = 5.5). Baseline levels and response to barrier disruption of secretory phospholipase A2 isoforms, enzymes that mediate generation of free fatty acids in epidermis, appeared similar in both FAK(K5 KO) and control littermates. We found that the critical SC acidification regulator Na(+)/H(+) exchanger 1 failed to localize to the plasma membrane in FAK-deficient keratinocytes both in vivo and in vitro. Thus, for plasma membrane localization in terminally differentiated keratinocytes, Na(+)/H(+) exchanger 1 requires an intact actin cytoskeleton, which is impaired in FAK-deficient cells.

Loss of calcineurin homologous protein-1 in chicken B lymphoma DT40 cells destabilizes Na+/H+ exchanger isoform-1 protein

NHE1/SLC9A1 is a ubiquitous isoform of vertebrate Na+/H+ exchangers (NHEs) functioning in maintaining intracellular concentrations of Na+ and H+ ions. Calcineurin homologous protein-1 (CHP1) binds to the hydrophilic region of NHE1 and regulates NHE1 activity but reportedly does not play a role in translocating NHE1 from the endoplasmic reticulum to the plasma membrane. However, an antiport function of NHE1 requiring CHP1 remains to be clarified. Here we established CHP1-deficient chicken B lymphoma DT40 cells by gene targeting to address CHP1 function. CHP1-deficient cells showed extensive decreases in Na+/H+ activities in intact cells. Although NHE1 mRNA levels were not affected, NHE1 protein levels were significantly reduced not only in the plasma membrane but in whole cells. The expression of a CHP1 transgene in CHP1-deficient cells rescued NHE1 protein expression. Expression of mutant forms of CHP1 defective in Ca2+ binding or myristoylation also partially decreased NHE1 protein levels. Knockdown of CHP1 also caused a moderate decrease in NHE1 protein in HeLa cells. These data indicate that CHP1 primarily plays an essential role in stabilization of NHE1 for reaching of NHE1 to the plasma membrane and its exchange activity.

The preischemic combination of the sodium–hydrogen exchanger inhibitor cariporide and the adenosine agonist AMP579 acts additively to reduce porcine myocardial infarct size

BACKGROUND

We tested whether the combination of two known cardioprotective agents, the type-1 sodium-hydrogen exchanger inhibitor cariporide plus the adenosine A(1)/A(2a) receptor agonist AMP579 ([1S-[1a,2b,3b, 4a(S*)]]-4-[7-[[2-(3-chloro-2-thienyl)-1-methylpropyl]amino]-[(3)H]-imidazo[4,5-b]pyridyl-3-yl]cyclopentane carboxamide), acted additively to reduce myocardial infarct size.

STUDY DESIGN

Pigs underwent 1 hour of coronary artery occlusion and 3 hours reperfusion. Vehicle-treated controls were compared with animals treated before ischemia with low-dose and high-dose cariporide and AMP579, and low-dose cariporide plus AMP579. The effects of both agents, alone and in combination, were also tested in isolated porcine polymorphonuclear neutrophils (PMNs). The PMN respiratory burst was induced with phorbol 12-myristate 13-acetate and quantified by the increase in 2',7'-dichlorofluorescein fluorescence, measured by flow cytometry.

RESULTS

Infarct size in the control pigs was 65 +/- 1% of the area at risk. Cariporide dose-dependently reduced infarct size to 39 +/- 2% and 24 +/- 3% in the low- and high-dose groups, respectively. Infarct size was 54 +/- 3% in the low-dose AMP579 group and 47 +/- 3% with high dose. The combination of low doses of cariporide and AMP579 reduced infarction to 25 +/- 6% of the area at risk. In the PMN studies, cariporide and AMP579 alone had no effect on 2',7'-dichlorofluorescein fluorescence, but the combination of the two agents reduced the PMN 2',7'-dichlorofluorescein increase to 79 +/- 5% of the vehicle control response.

CONCLUSIONS

The preischemic combination of low doses of cariporide and AMP579 decreased myocardial infarct size more than either agent used alone in low concentration, indicating an additive effect of the two agents. Given the effects that cariporide plus AMP579 combination exerted on PMN activity, it appears that this combination has the potential to reduce PMN-mediated effects during myocardial reperfusion.

Immunological detection of Na(+)/H(+) exchangers in the gills of a hagfish, Myxine glutinosa, an elasmobranch, Raja erinacea, and a teleost, Fundulus heteroclitus

Na(+)/H(+) exchangers (NHE) are a family of ion exchangers with diverse functions that are well defined in mammals. NHE-1 is expressed in the plasma membrane of most mammalian cells where it regulates intracellular pH, and usually in the basolateral membrane of epithelial cells. It has also been detected in teleost gills where it may participate in systemic pH regulation. NHE-3 is usually expressed in the apical membrane of mammalian epithelial cells where it helps reabsorb Na(+) and HCO(3)(-); it has also been detected in teleost gills. We used Western blotting and heterologous antibodies to screen for expression of NHE-1 and NHE-3 in gills of an agnathan (Myxine glutinosa) and an elasmobranch (Raja erinacea), and NHE-3 in gills of a teleost (Fundulus heteroclitus). Positive NHE-1 bands were detected in gills from the agnathan and elasmobranch. Using the NHE-3 antibody, bands were detected in the gills of the elasmobranch and teleost. These data are some of the first direct evidence of NHEs in the gills of an agnathan and elasmobranch, and confirm the presence of NHEs in the gills of teleosts.

Negative regulation of the platelet Na+/H+ exchanger by trimeric G-proteins

Human platelets contain a Na+/H+ exchanger (NHE) that regulates the cytosolic pH. The role of trimeric G-proteins in NHE control was investigated in plasma membrane vesicles by measuring exchange of intravesicular protons for extravesicular Na+. Exchange was saturable, independent of membrane potential and inhibited by ethylisopropyl amiloride (Ki 0.05 micromol.L-1), demonstrating the involvement of NHE-1. The G-protein activators AlF4- and GMP-P(NH)P reduced exchange by increasing the Km for Na+ from 11.3 +/- 2.1 mM to 21.6 +/- 1.4 mM (AlF4-) and 19.8 +/- 1.1 mM (GMP-P(NH)P), leaving Vmax and the Hill coefficient unchanged. This effect was abolished by inhibitors of Gi-proteins (N-ethylmaleimide, holoenzyme- and A-protomer of pertussis toxin) and by an anti-Galpha Ig and GDP(beta)S. Activation of Gi-proteins by mastoparan and its synthetic analogue Mas7 also strongly reduced NHE activity. These data show that in platelets NHE-1 is under negative control of the Gi-family of trimeric G-proteins.

Structural study of the porcine Na+/H+ exchanger NHE1 gene and its 5'-flanking region

The Na+/H+ exchanger is an ubiquitous mammalian plasma membrane protein that is important for the regulation of intracellular pH and cell volume. In order to provide some insight into the molecular basis of NHE1 expression we have isolated and characterized genomic DNA clones containing the coding region and 5'-flanking region of the porcine NHE1 gene. The gene spans more than 30 kb in length and consists of twelve exons that are flanked by typical splice donor and acceptor sequences at the exon-intron boundaries. The positions of the splicing sites are conserved in relation to the human NHE1 gene. The 5' distal transcription initiation site, identified by primer extension analysis, is positioned 766 bp upstream of the translation initiation codon and 36 bp downstream of a TATA box. A 5'-flanking region of 1.62 kb in length contains a number of potential regulatory elements, and exhibits several features that distinguish the pig gene from those of rabbit, mouse and human. The NHE1 gene is located in a CpG island. The promoter sequence of 500 bp is compared with that for NHE1 genes from different species. The homology between the porcine and the human, rabbit and mouse genes is 78, 76 and 75%, respectively. Several consensus elements for transcription factors, including AP-1, C/EBP, and Sp1 are phylogenetically conserved between pig and human, while AP3 and PEA3 are found only in pig. Some conserved elements are found in the pig in multiple copies. These results suggest broadly similar regulatory mechanisms for NHE1 transcription among the different mammalian species but show some species or tissue-specific differences.

High glucose induces the activity and expression of Na(+)/H(+) exchange in glomerular mesangial cells

Changes in activity or expression of transporters may account for alterations in cell behavior in diabetes. We sought to ascertain if mesangial cells (MC) grown in different glucose concentrations exhibit changes in activity and expression of acid-extruding transporters, the Na(+)/H(+) and Na(+)-dependent Cl(-)/HCO(-)(3) exchanger. pH(i) was determined by the use of the fluorescent pH-sensitive dye BCECF. In MCs grown in 5 mM glucose (control), the Na(+)/H(+) exchanger was responsible for 31.8 +/- 5.1% of steady-state pH(i), whereas Na(+)-dependent Cl(-)/HCO(-)(3) contributed 62.9 +/- 4.0% (n = 11). In MCs grown in high glucose for 2 wk, Na(+)/H(+) exchange contribution to acid-extrusion increased as follows: 42.3 +/- 4.6% [n = 8, 10 mM, not significant (NS)], 51.1 +/- 5.1% (n = 8, 20 mM, P < 0.01), and 64.8 +/- 5.5% (n = 7, 30 mM, P < 0.001). The Na(+)-dependent Cl(-)/HCO(-)(3) exchanger contributed less [47.0 +/- 4.6, 38.6 +/- 5.8, and 21.1 +/- 3.8%, for 10, 20, and 30 mM glucose, respectively (n > 7)]. We sought to ascertain if the magnitude of the acute stimulated response to ANG II by the Na(+)/H(+) and Na(+)-dependent Cl(-)/HCO(-)(3) exchanger is changed. Na(+)/H(+) exchanger (1.89-fold increase in 30 vs. 5 mM, P < 0.002), but not Na(+)-dependent Cl(-)/HCO(-)(3) exchange (0. 17-fold, NS), exhibited an enhanced response to ANG II (1 microM). Na(+)/H(+) exchange (NHE1) expression was significantly different (1. 72-fold) after prolonged exposure to high glucose. These results suggest that the Na(+)/H(+) exchanger, but not Na(+)-dependent Cl(-)/HCO(-)(3) exchanger, may play an early role in the response to hyperglycemia in the diabetic state.

Human Na+/H+ exchanger genes : identification of polymorphisms by radiation hybrid mapping and analysis of linkage in end-stage renal disease

The Na+/H+ exchangers (NHEs) are membrane-bound transporters that catalyze the electro-neutral movement of extracellular Na+ for intracellular H+. NHE genes play a critical role in pH homeostasis and cellular volume regulation and can be considered candidate genes for essential hypertension and renal disease. This study was performed to determine whether the NHE genes contributed to genetic susceptibility in end-stage renal disease (ESRD). To date, 5 isoforms of NHE have been cloned in mammals (NHE1 to NHE5). The complementary DNA (cDNA) sequences of NHE1 to NHE3 and NHE5 are known in humans. Because the chromosomal structure of the NHE genes is unknown, we used cDNA sequences to design polymerase chain reaction primers for use in radiation hybrid mapping. Radiation hybrid mapping of NHE genes identified nearby polymorphic markers for NHE1 to NHE3 (NHE1: D1S197, D1S2677; NHE2: D2S373, D2S1789; and NHE3: D5S678, D5S2005). We used these markers, and other previously identified polymorphic markers for NHE5, in linkage and association analyses of ESRD. The NHE1 to NHE3 and NHE5 loci did not demonstrate evidence for linkage to ESRD. However, NHE5 showed significant evidence for association (P</=1.0x10(-4)). The strongest evidence for association was observed with allele 6 of NHE5 (P</=0.001 to 0. 01). Allele 6 appeared to have a renoprotective effect, with a frequency of 0.15 in the control population and 0.06 to 0.09 in patients with ESRD. The combined approach of designing primers from cDNA and radiation hybrid mapping has proven successful in identifying polymorphisms for human genes of which only cDNA sequences were previously available. The NHE primers and associated polymorphic loci identified in this study can be used in genomic, linkage, and association analysis of NHE genes in future genetic studies of hypertension and renal failure. Given the allelic association, further evaluation of the role of NHE5 in ESRD susceptibility appears warranted.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.