Donor screening for human T-cell lymphotrophic virus 1/2: changing paradigms for changing testing capacity

Organ Procurement and Transplant Network (OPTN) policy currently requires the testing of all potential organ donors for human T-cell lymphotrophic virus (HTLV)-1/2. Most Organ Procurement Organizations (OPO) use the Abbott HTLV-I/HTLV-II Enzyme Immunoassay (EIA). This assay will no longer be manufactured after December 31, 2009; the only commercially available FDA-licensed assay will be the Abbott PRISM HTLV-I/II assay which poses many challenges to OPO use for organ donor screening. As a result, screening donors for HTLV-1/2 in a timely manner pretransplant after December 31, 2009 will be challenging. The true incidence of HTLV-1 in United States (U.S.) organ donors is not well described but appears to be low ( approximately 0.03-0.5%). HTLV-1 is associated with malignancy and neurological disease; HTLV-2 has not been convincingly associated with disease in humans. Donors that are HTLV-1/2 seropositive are infrequently used despite most results being either false positive or resulting from HTLV-2 infection. There is urgent need to encourage the development of assays, instruments and platforms optimized for organ donors that can be used to screen for transmissible disease in donors; these must have appropriate sensitivity and specificity to identify all infections while minimizing organ loss through false positive testing.

Diffusion-weighted imaging and status epilepticus during vagus nerve stimulation

PURPOSE

Transient abnormalities have been reported on diffusion-weighted imaging (DWI) during status epilepticus. Vagus nerve stimulation (VNS) is a therapy for epilepsy that has previously demonstrated alteration in regional cerebral blood flow on functional neuroimaging. We describe the peri-ictal DWI abnormalities in a patient with status epilepticus.

METHODS

A 21-year-old woman with pharmacoresistant localization-related epilepsy was treated with VNS and underwent brain magnetic resonance imaging (MRI) with DWI for clinical purposes.

RESULTS

Transient and reversible hyperintense signal abnormalities were noted on DWI at the site of seizure onset, in addition to the thalamus and midbrain bilaterally. A concomitant decrease in the apparent diffusion coefficient mimicked ischemia, yet complete clinical, and electrographic resolution occurred following successful termination of status.

CONCLUSIONS

High-energy brain MRI sequences using DWI were safely performed in our epilepsy patient with a vagus nerve stimulator who experienced status epilepticus. This case highlights the bilateral and robust involvement of subcortical structures present immediately following status epilepticus. Additionally, bilateral abnormalities in the thalamus and midbrain in addition to the region of seizure origin, were observed in our patient implanted with a vagus nerve stimulator. Modulation of regional cerebral blood flow is one potential mechanism of action for VNS in humans; therefore, these regions of involvement could reflect the effects of status epilepticus, activation or facilitation by VNS, or both.

Identification of sites in the second exomembrane loop and ninth transmembrane helix of the mammalian Na+/H+ exchanger important for drug recognition and cation translocation

Mammalian Na(+)/H(+) exchanger (NHE) isoforms are differentially sensitive to inhibition by several distinct classes of pharmacological agents, including amiloride- and benzoyl guanidinium-based derivatives. The determinants of drug sensitivity, however, are only partially understood. Earlier studies of the drug-sensitive NHE1 isoform have shown that residues within the fourth membrane-spanning helix (M4) (Phe(165), Phe(166), Leu(167), and Gly(178)) and a 66-amino acid segment encompassing M9 contribute significantly to drug recognition. In this report, we have identified two residues within M9, one highly conserved (Glu(350)) and the other non-conserved (Gly(356)), that are major determinants of drug sensitivity. In addition, residues in the second exomembrane loop between M3 and M4 (Gly(152), Phe(157), and Pro(158)) were also found to modestly influence drug sensitivity. A double substitution of crucial sites within M4 and M9 of NHE1 with the corresponding residues present in the drug-resistant NHE3 isoform (i.e. L167F/G356A) greatly reduced drug sensitivity in a cooperative manner to levels nearing that of wild type NHE3. The above mutations did not appreciably affect Na(o)(+) affinity but did markedly decrease the catalytic turnover of the transporter. These data suggest that specific sites encompassing M4 and M9 are critical determinants of both drug recognition and cation translocation.

Role of the cytoskeleton in mediating cAMP-dependent protein kinase inhibition of the epithelial Na+/H+ exchanger NHE3

The Na(+)/H(+) exchanger NHE3 isoform mediates the entry of Na(+) into epithelial cells of the kidney and gastrointestinal tract. Hormones and pharmacological agents that activate cAMP-dependent protein kinase A (PKA) are potent inhibitors of native and ectopically expressed NHE3 in epithelial and Chinese hamster ovary AP-1 cells, respectively. Previous studies have shown that acute inhibition is coupled to direct phosphorylation of the exchanger, but this only partly accounts for the observed effects. In this report, we show that inhibition of NHE3 activity by forskolin, an activator of adenylate cyclase, occurs without changes in surface expression of the exchanger but is associated with altered cytoskeletal structure. This effect resembles that obtained with cytochalasin D or latrunculin B, actin disrupting agents that also inhibit NHE3. Such similarities prompted us to further investigate the relationship between PKA-induced inhibition of the exchanger and changes in the actin cytoskeleton. Inhibition of NHE3 by cytochalasin D does not require PKA, because the inhibitory effect is preserved in a mutant NHE3 that is not phosphorylated by PKA and in cells pretreated with the PKA inhibitor H89. In contrast, involvement of actin in the effect of cAMP on the exchanger is supported by the following observations: (i) jasplakinolide, an F-actin stabilizer, prevents the inhibition caused by forskolin, and (ii) constitutively active forms of RhoA and Rho kinase interfere with actin disruption by forskolin and also decrease inhibition of the transporter. These results suggest that reorganization of the cytoskeleton by PKA is involved in mediating inhibition of NHE3.

Modulation of Na+/H+ exchange activity by Cl-

Na+/H+ exchanger (NHE) activity is exquisitely dependent on the intra- and extracellular concentrations of Na+ and H+. In addition, Cl- ions have been suggested to modulate NHE activity, but little is known about the underlying mechanism, and the Cl- sensitivity of the individual isoforms has not been established. To explore their Cl- sensitivity, types 1, 2, and 3 Na+/H+ exchangers (NHE1, NHE2, and NHE3) were heterologously expressed in antiport-deficient cells. Bilateral replacement of Cl- with nitrate or thiocyanate inhibited the activity of all isoforms. Cl- depletion did not affect cell volume or the cellular ATP content, which could have indirectly altered NHE activity. The number of plasmalemmal exchangers was unaffected by Cl- removal, implying that inhibition was due to a decrease in the intrinsic activity of individual exchangers. Analysis of truncated mutants of NHE1 revealed that the anion sensitivity resides, at least in part, in the COOH-terminal domain of the exchanger. Moreover, readdition of Cl- into the extracellular medium failed to restore normal transport, suggesting that intracellular Cl- is critical for activity. Thus interaction of intracellular Cl- with the COOH terminus of NHE1 or with an associated protein is essential for optimal activity.

Molecular cloning and characterization of a novel (Na+,K+)/H+ exchanger localized to the trans-Golgi network

The luminal pH of organelles along the secretory and endocytic pathways of mammalian cells is acidic and tightly regulated, with the [H+] varying up to 100-fold between compartments. Steady-state organellar pH is thought to reflect a balance between the rates of H+ pumping by the vacuolar-type H+-ATPase and H+ efflux through ill-defined pathways. Here, we describe the cloning of a novel gene (NHE7) in humans that is homologous to Na+/H+ exchangers, is ubiquitously expressed, and localizes predominantly to the trans-Golgi network. Significantly, NHE7 mediates the influx of Na+ or K+ in exchange for H+. The activity of NHE7 was also found to be relatively insensitive to inhibition by amiloride but could be antagonized by the analogue benzamil and the unrelated compound quinine. Thus, NHE7 displays unique functional and pharmacological properties and may play an important role in maintaining cation homeostasis of this important organelle.

Regulation of the epithelial Na(+) /H(+) exchanger isoform by the cytoskeleton

Members of the Na(+)/H(+) exchanger (NHE) family mediate electroneutral countertransport of H(+) for Na(+) across cellular membranes. The six known isoforms mediate transepithelial Na(+) transport processes and housekeeping functions such as the regulation of cellular and organellar pH and volume. NHE3 is found primarily in the apical membrane of epithelial cells of the kidney and gastrointestinal tract, where it mediates Na(+) (re)absorption. Its fine regulation, whether by hormones that utilize cAMP as a signalling mechanism, or by physical parameters such as the cell volume, provides the adjustments necessary for the maintenance of systemic salt and fluid balance. Although the exact molecular mechanism of this control is unknown, two major modes of regulation have been invoked: 1) alteration of the number of cell surface transporters by changes in the rate of endocytosis and/or exocytosis and 2) regulation of the intrinsic activity of the individual exchangers. NHE3 requires an intact cytoskeleton for its optimal function. Pharmacological interference with actin polymerization or myosin phosphorylation markedly inhibits the exchanger, without altering the number of transporters exposed at the surface. This effect is isoform specific and is mediated by the cytoplasmic tail of the transporter. The small GTP-binding protein, RhoA and its downstream effector, Rho kinase regulate NHE3, possibly by controlling the level of myosin phosphorylation, that in turn determines the organization of actin. The cytoskeleton may not only be involved in the maintenance of the basal rate of transport, but is also likely to sense physical alterations and transmit signals to modulate NHE3 activity, thus providing fast and effective control of the exchanger.

Direct binding of the Na--H exchanger NHE1 to ERM proteins regulates the cortical cytoskeleton and cell shape independently of H(+) translocation

The association of actin filaments with the plasma membrane maintains cell shape and adhesion. Here, we show that the plasma membrane ion exchanger NHE1 acts as an anchor for actin filaments to control the integrity of the cortical cytoskeleton. This occurs through a previously unrecognized structural link between NHE1 and the actin binding proteins ezrin, radixin, and moesin (ERM). NHE1 and ERM proteins associate directly and colocalize in lamellipodia. Fibroblasts expressing NHE1 with mutations that disrupt ERM binding, but not ion translocation, have impaired organization of focal adhesions and actin stress fibers, and an irregular cell shape. We propose a structural role for NHE1 in regulating the cortical cytoskeleton that is independent of its function as an ion exchanger.

RhoA and rho kinase regulate the epithelial Na+/H+ exchanger NHE3. Role of myosin light chain phosphorylation

The activity of the Na(+)/H(+) exchanger NHE3 isoform, which is found primarily in epithelial cells, is sensitive to the state of actin polymerization. Actin assembly, in turn, is controlled by members of the small GTPase Rho family, namely Rac1, Cdc42, and RhoA. We therefore investigated the possible role of these GTPases in modulating NHE3 activity. Cells stably expressing NHE3 were transiently transfected with inhibitory forms of Rac1, Cdc42, or RhoA and transport activity was assessed using microfluorimetry. NHE3 activity was not adversely affected by either dominant-negative Rac1 or Cdc42. By contrast, the inhibitory form of RhoA greatly depressed NHE3 activity, without noticeably altering its subcellular distribution. NHE3 activity was equally reduced by inhibiting p160 Rho-associated kinase I (ROK), a downstream effector of RhoA, with the selective antagonist Y-27632 and a dominant-negative form of ROK. Furthermore, inhibition of ROK reduced the phosphorylation of myosin light chain. A comparable net dephosphorylation was achieved by the myosin light chain kinase inhibitor ML9, which similarly inhibited NHE3. These data suggest that optimal NHE3 activity requires a functional RhoA-ROK signaling pathway which acts, at least partly, by controlling the phosphorylation of myosin light chain and, ultimately, the organization of the actin cytoskeleton.

Vancomycin anaphylaxis in a patient with vancomycin-induced red man syndrome

Vancomycin is a powerful glycopeptide antibiotic that is increasingly being used owing to the emergence of highly resistant organisms such as methicillin-resistant Staphylococcus aureus. Although a generally safe medication, administration of vancomycin is not benign, and there have been a number of adverse reactions reported. We present the case of a patient with vancomycin-induced red man syndrome who developed vancomycin anaphylaxis. Our case illustrates that red man syndrome may be a marker for true vancomycin allergy, although it was generally not thought of as so in the past.

Regulation of NHE3 activity by G protein subunits in renal brush-border membranes

NHE3 activity is regulated by phosphorylation/dephosphorylation processes and membrane recycling in intact cells. However, the Na(+)/H(+) exchanger (NHE) can also be regulated by G proteins independent of cytoplasmic second messengers, but the G protein subunits involved in this regulation are not known. Therefore, we studied G protein subunit regulation of NHE3 activity in renal brush-border membrane vesicles (BBMV) in a system devoid of cytoplasmic components and second messengers. Basal NHE3 activity was not regulated by G(s)alpha or G(i)alpha, because antibodies to these G proteins by themselves were without effect. The inhibitory effect of D(1)-like agonists on NHE3 activity was mediated, in part, by G(s)alpha, because it was partially reversed by anti-G(s)alpha antibodies. Moreover, the amount of G(s)alpha that coimmunoprecipitated with NHE3 was increased by fenoldopam in both brush-border membranes and renal proximal tubule cells. Furthermore, guanosine 5'-O-(3-thiotriphosphate) but not guanosine 5'-O-(2-thiodiphosphate), the inactive analog of GDP, increased the amount of G(s)alpha that coimmunoprecipitated with NHE3. The alpha(2)-adrenergic agonist, UK-14304 or pertussis toxin (PTX) alone had no effect on NHE3 activity, but UK-14304 and PTX treatment attenuated the D(1)-like receptor-mediated NHE3 inhibition. The ability of UK-14304 to attenuate the D(1)-like agonist effect was not due to G(i)alpha, because the attenuation was not blocked by anti-G(i)alpha antibodies or by PTX. Anti-Gbeta(common) antibodies, by themselves, slightly inhibited NHE3 activity but had little effect on D(1)-like receptor-mediated NHE3 inhibition. However, anti-Gbeta(common) antibodies reversed the effects of UK-14304 and PTX on D(1)-like agonist-mediated NHE3 inhibition. These studies provide concrete evidence of a direct regulatory role for G(s)alpha, independent of second messengers, in the D(1)-like-mediated inhibition of NHE3 activity in rat renal BBMV. In addition, beta/gamma dimers of heterotrimeric G proteins appear to have a stimulatory effect on NHE3 activity in BBMV.

Kinetic and pharmacological properties of human brain Na(+)/H(+) exchanger isoform 5 stably expressed in Chinese hamster ovary cells

The recently cloned Na(+)/H(+) exchanger isoform 5 (NHE5) is expressed predominantly in brain, yet little is known about its functional properties. To facilitate its characterization, a full-length cDNA encoding human NHE5 was stably transfected into NHE-deficient Chinese hamster ovary AP-1 cells. Pharmacological analyses revealed that H(+)(i)-activated (22)Na(+) influx mediated by NHE5 was inhibited by several classes of drugs (amiloride compounds, 3-methylsulfonyl-4-piperidinobenzoyl guanidine methanesulfonate, cimetidine, and harmaline) at half-maximal concentrations that were intermediate to those determined for the high affinity NHE1 and the low affinity NHE3 isoforms, but closer to the latter. Kinetic analyses showed that the extracellular Na(+) dependence of NHE5 activity followed a simple hyperbolic relationship with an apparent affinity constant (K(Na)) of 18.6 +/- 1.6 mM. By contrast to other NHE isoforms, NHE5 also exhibited a first-order dependence on the intracellular H(+) concentration, achieving half-maximal activation at pH 6.43 +/- 0.08. Extracellular monovalent cations, such as H(+) and Li(+), but not K(+), acted as effective competitive inhibitors of (22)Na(+) influx by NHE5. In addition, the transport activity of NHE5 was highly dependent on cellular ATP levels. Overall, these functional features distinguish NHE5 from other family members and closely resemble those of an amiloride-resistant NHE isoform identified in hippocampal neurons.

The epithelial Na(+)/H(+) exchanger, NHE3, is internalized through a clathrin-mediated pathway

Trafficking of the Na(+)/H(+) exchanger isoform 3 (NHE3) between sub-apical vesicles and apical membrane of epithelial cells is a suggested mechanism of regulation of NHE3 activity. When epitope-tagged NHE3 was stably expressed in NHE-deficient Chinese hamster ovary cells, a sizable fraction was found in recycling endosomes. This system was used to analyze the mechanism of endocytosis of NHE3. Immunofluorescence and radiolabeling experiments showed that inhibition of clathrin-mediated endocytosis using hypertonicity, acid treatment, or K(+) depletion inhibited internalization of NHE3. Moreover, transient transfection of an inhibitory mutant of dynamin (DynS45N) blocked the clathrin-mediated uptake of transferrin, as well as the endocytosis of NHE3. In ileal villus cells, endogenous NHE3 was also found to co-purify with isolated clathrin-coated vesicles, thereby confirming their association in native tissues. The role of COP-I subunits in the intracellular traffic of NHE3 was evaluated using ldlF cells, which bear a temperature-sensitive mutation in the epsilon-COP subunit. At the permissive temperature, NHE3 distributed normally, whereas at the restrictive temperature, which induces rapid degradation of epsilon-COP, NHE3 was still internalized, but its subcellular distribution was altered. These results indicate that endocytosis of NHE3 occurs primarily via clathrin-coated pits and vesicles and that normal intracellular trafficking of NHE3 involves an epsilon-COP-dependent step.

The apical Na(+)/H(+) exchanger isoform NHE3 is regulated by the actin cytoskeleton

The epithelial isoform of the Na(+)/H(+) exchanger, NHE3, associates with at least two related regulatory factors called NHERF1/EBP50 and NHERF2/TKA-1/E3KARP. These factors in addition interact with the cytoskeletal protein ezrin, which in turn binds to actin. The possible linkage of NHE3 with the cytoskeleton prompted us to test the effect of actin-modifying agents on NHE3 activity. Cytochalasins B and D and latrunculin B, which interfere with actin polymerization, induced a profound inhibition of NHE3 activity. The effect was isoform-specific inasmuch as the "housekeeping" exchanger NHE1 was virtually unaffected. Cytoskeletal disorganization was associated with a subcellular redistribution of NHE3, which accumulated at sites where actin aggregated, suggesting a physical interaction of exchangers with the cytoskeleton. An interaction was further suggested by the co-sedimentation of a detergent-insoluble fraction of NHE3 with the actin cytoskeleton. Inhibition of transport was not due to diminution in the number of transporters at the plasmalemma. Functional analyses of NHE1/NHE3 chimeras revealed that the cytoplasmic domain of NHE3 conferred sensitivity to cytochalasin B. Progressive carboxyl-terminal and internal deletions of the cytoplasmic region of NHE3 indicated that the region between residues 650 and 684 is critical for this response. This region overlaps with the domain reported to interact with NHERF and also contains a putative ezrin-binding site; hence, it likely plays a role in interactions with the cytoskeleton.

Na+/H+ exchangers. Molecular diversity and relevance to heart

During the last several years, significant advances have been made in our understanding of the molecular, cellular, and physiological diversity of mammalian Na+/H+ exchangers. This transporter forms a multigene family of at least six members (NHE1-NHE6) that share approximately 20-60% amino acid identity. NHE1 is the most predominant isoform expressed in heart and it contributes significantly to myocardial pHi homeostasis, which is important for maintaining contractility. However, hyperactivation of NHE1 during episodes of cardiac ischemia and reperfusion disrupts the intracellular ion balance, leading to cardiac dysfunction and damage in several animal models, but which can be prevented by pharmacological antagonists of NHE1. Molecular studies have indicated that the predicted transmembrane segments M4 and M9 contain several residues involved in drug sensitivity. Molecular dissection of the drug binding region should facilitate the rational design of more potent and isoform-specific drugs that may provide therapeutic benefit in the prevention of cardiac ischemia and reperfusion injuries.

Proline-rich motifs of the Na+/H+ exchanger 2 isoform. Binding of Src homology domain 3 and role in apical targeting in epithelia

The NHE2 isoform of the Na+/H+ exchanger (NHE) displays two proline-rich sequences in its C-terminal region that resemble SH3 (Src homology 3)-binding domains. We investigated whether these regions (743PPSVTPAP750, termed Pro-1, and 786VPPKPPP792, termed Pro-2) can bind to SH3 domains and whether they are essential for NHE2 function and targeting. A fusion protein containing the Pro-1 region showed promiscuous binding to SH3 domains of several proteins in vitro, whereas a Pro-2 fusion bound preferentially to domains derived from kinases. In contrast, cytoplasmic regions of NHE1, NHE3, or NHE4 failed to interact. When expressed in antiporter-deficient cells, truncated NHE2 lacking both Pro-rich regions catalyzed Na+/H+ exchange, retained sensitivity to intracellular ATP, and was activated by hyperosmolarity, resembling full-length NHE2. The role of the Pro-rich regions in subcellular targeting was examined by transfection of epitope-tagged forms of NHE2 in porcine renal epithelial LLC-PK1 cells. Both full-length and Pro-2-truncated NHE2 localized almost exclusively to the apical membrane. By contrast, a mutant devoid of both Pro-1 and Pro-2 was preferentially sorted to the basolateral surface but also accumulated intracellularly. These observations indicate that the region encompassing Pro-1 is essential for appropriate subcellular targeting of NHE2.

Molecular cloning, genomic organization, and functional expression of Na+/H+ exchanger isoform 5 (NHE5) from human brain

To isolate a cDNA encoding Na+/H+ exchanger isoform 5 (NHE5), we screened a human spleen library using exon sequences of the NHE5 gene. Clones spanning 2.9 kilobase pairs were isolated; however, they contained several introns and were missing coding sequences at both the 5' and 3' ends. The missing 5' sequences were obtained by 5'-rapid amplification of cDNA ends and by analysis of an NHE5 genomic clone, and the missing 3' sequences were obtained by 3'-rapid amplification of cDNA ends. Polymerase chain reaction amplification of brain cDNA yielded products in which each of the introns had been correctly excised, whereas the introns were retained in products from spleen and testis, suggesting that the NHE5 transcripts expressed in these organs do not encode a functional transporter. The intron/exon organization of the NHE5 gene was analyzed and found to be very similar to that of the NHE3 gene. The NHE5 cDNA, which encodes an 896-amino acid protein that is most closely related to NHE3, was expressed in Na+/H+ exchanger-deficient fibroblasts and shown to mediate Na+/H+ exchange activity. Northern blot analysis demonstrated that the mRNA encoding NHE5 is expressed in multiple regions of the brain, including hippocampus, consistent with the possibility that it regulates intracellular pH in hippocampal and other neurons.

Subcellular localization of the Na+/H+ exchanger NHE1 in rat myocardium

The Na+/H+ exchanger NHE1 isoform is an integral component of cardiac intracellular pH homeostasis that is critically important for myocardial contractility. To gain further insight into its physiological significance, we determined its cellular distribution in adult rat heart by using immunohistochemistry and confocal microscopy. NHE1 was localized predominantly at the intercalated disk regions in close proximity to the gap junction protein connexin 43 of atrial and ventricular muscle cells. Significant labeling of NHE1 was also observed along the transverse tubular systems, but not the lateral sarcolemmal membranes, of both cell types. In contrast, the Na+-K+-ATPase alpha1-subunit was readily labeled by a specific mouse monoclonal antibody (McK1) along the entire ventricular sarcolemma and intercalated disks and, to a lesser extent, in the transverse tubules. These results indicate that NHE1 has a distinct distribution in heart and may fulfill specialized roles by selectively regulating the pH microenvironment of pH-sensitive proteins at the intercalated disks (e.g., connexin 43) and near the cytosolic surface of sarcoplasmic reticulum cisternae (e.g., ryanodine receptor), thereby influencing impulse conduction and excitation-contraction coupling.

Endosomal recycling of the Na+/H+ exchanger NHE3 isoform is regulated by the phosphatidylinositol 3-kinase pathway

The NHE3 isoform of the Na+/H+ exchanger localizes to both the plasmalemmal and endosomal compartments in polarized epithelial and transfected Chinese hamster ovary (AP-1) cells. It is unclear how the distribution of NHE3 between these compartments is regulated. In this study, we examined the potential involvement of phosphatidylinositol 3'-kinase (PI3-K) in regulating the activity and distribution of NHE3, as this lipid kinase has been implicated in modulating vesicular traffic in the endosomal recycling pathway. Wortmannin and LY294002, both potent inhibitors of PI3-K, markedly inhibited NHE3-mediated H+ extrusion across the plasma membrane in a concentration- and time-dependent manner. The subcellular distribution of the antiporters was monitored by transfecting epitope-tagged NHE3 into AP-1 cells. In parallel with the inhibition of transport, PI3-K antagonists induced a pronounced loss of NHE3 from the cell surface and its accumulation in an intracellular compartment, as assessed by immunofluorescence microscopy and enzyme-linked immunosorbent assays. Further analysis using cells transfected with antiporters bearing an external epitope tag revealed that the redistribution reflected primarily a decrease in the rate of recycling of intracellular NHE3 to the cell surface. The wortmannin-induced inhibition and redistribution of NHE3 were prevented when cells were incubated at 4 degreesC, consistent with the known temperature dependence of the endocytic process. These observations demonstrate that NHE3 activity is controlled by dynamic endocytic and recycling events that are modulated by PI3-K.

Topological analysis of NHE1, the ubiquitous Na+/H+ exchanger using chymotryptic cleavage

Proteases, glycosidases, and impermeant biotin derivatives were used in combination with antibodies to analyze the subcellular distribution and transmembrane disposition of the Na+/H+ exchanger NHE1. Both native human NHE1 in platelets and epitope-tagged rat NHE1 transfected into antiport-deficient cells were used for these studies. The results indicated that 1) the entire population of exchangers is present on the surface membrane of unstimulated platelets, ruling out regulation by recruitment of internal stores of NHE1; 2) the putative extracellular loops near the NH2 terminus are exposed to the medium and contain all the N- and O-linked carbohydrates; 3) by contrast, the putative extracellular loops between transmembrane domains 9-10 and 11-12 are not readily accessible from the outside and may be folded within the protein, perhaps contributing to an aqueous ion transport pathway; 4) the extreme COOH terminus of the protein was found to be inaccessible to extracellular proteases, antibodies, and other impermeant reagents, consistent with a cytosolic localization; and 5) detachment of approximately 150 amino acids from the NH2-terminal end of the protein had little effect on the transport activity of NHE1.

Contributions of Na+/H+ exchanger isoforms to preimplantation development of the mouse

Previous work provided evidence of Na+/H+ exchanger activity in the apical domain of mouse trophectodermal plasma membranes that provides a route for entry of extracellular Na+ (Manejwala et al., 1989). This activity was hypothesized to contribute to the trans-trophectodermal Na+ flux that is required for blastocoel expansion. In the present work, we have used reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry to identify members of the Na+/H+ exchanger (NHE) family that are likely to participate in this process. When cDNA preparations from ovulated oocytes and several stages of preimplantation development were tested with PCR primers specific for the NHE-1, -2, -3, and -4 isoforms of the exchanger, only amplicons representing the NHE-1 and NHE-3 isoforms were detected. The identity of these amplicons was confirmed by direct sequencing. NHE-1 mRNA is present in oocytes and in all preimplantation stages, increasing threefold on a per embryo basis between the 4-cell and blastocyst stages. NHE-3 mRNA, on the other hand, was only detected in oocytes. Immunocytochemical analysis of blastocysts revealed that NHE-1 is localized in the basolateral domain of the trophectoderm, whereas NHE-3 is localized in the apical domain, a situation like that in epithelia of adult organs. We conclude that NHE-3, an oogenetic product that persists into the blastocyst stage, is the Na+/H+ exchanger isoform most likely to be involved in blastocoel expansion.

Identification of a mitochondrial Na+/H+ exchanger

The electroneutral exchange of protons for Na+ and K+ across the mitochondrial inner membrane contributes to organellar volume and Ca2+ homeostasis. The molecular nature of these transporters remains unknown. In this report, we characterize a novel gene (YDR456w; renamed NHA2) in Saccharomyces cerevisiae whose deduced protein sequence is homologous to members of the mammalian Na+/H+ exchanger gene family. Fluorescence microscopy showed that a Nha2-green fluorescent protein chimera colocalizes with 4',6-diamidino-2-phenylindole staining of mitochondrial DNA. To assess the function of Nha2, we deleted the NHA2 gene by homologous disruption and found that benzamil-inhibitable, acid-activated 22Na+ uptake into mitochondria was abolished in the mutant strain. It also showed retarded growth on nonfermentable carbon sources and severely reduced survival during the stationary phase of the cell cycle compared with the parental strain, consistent with a defect in aerobic metabolism. Sequence comparisons revealed that Nha2 has highest identity to a putative Na+/H+ exchanger homologue (KIAA0267; renamed NHE6) in humans. Northern blot analysis demonstrated that NHE6 is ubiquitously expressed but is most abundant in mitochondrion-rich tissues such as brain, skeletal muscle, and heart. Fluorescence microscopy showed that a NHE6-green fluorescent protein chimera also accumulates in mitochondria of transfected HeLa cells. These data indicate that NHA2 and NHE6 encode homologous Na+/H+ exchangers and suggest they may be important for mitochondrial function in lower and higher eukaryotes, respectively.

The epithelial sodium-hydrogen antiporter Na+/H+ exchanger 3 accumulates and is functional in recycling endosomes

Na+/H+ exchangers (NHEs) mediate electroneutral exchange of Na+ for H+ and thereby play a central role in pH regulation and Na+ homeostasis. NHE3, the predominant epithelial isoform, is found in apical membranes of renal and intestinal epithelial cells, where it contributes to NaCl (re)absorption. NHE activity has been detected in endomembrane vesicles of epithelial cells, but the precise compartment involved and its functional role have not been defined. Many aspects of the targeting machinery that defines the compartmentation and polarity of epithelia are also functional in nonepithelial cells. We therefore compared the targeting of NHE1, the basolateral isoform, with that of NHE3 in Chinese hamster ovary cells. To circumvent the confounding effects of endogenous exchangers, epitope-tagged constructs of NHE1 and NHE3 were stably expressed in antiport-deficient (AP-1) cells. While NHE1 was found almost exclusively in the surface membrane, NHE3 was also found intracellularly, accumulating in a juxtanuclear compartment. Confocal microscopy showed this compartment to be distinct from the Golgi, trans-Golgi network, and lysosomes. Instead, NHE3 colocalized with transferrin receptors and with cellubrevin, markers of recycling endosomes. The activity of NHE3 in endomembranes was assessed by targeting pH-sensitive probes to the recycling endosomes using a chimeric cellubrevin construct with an accessible extracellular epitope. Fluorescence ratio imaging indicated that cellubrevin resides intracellularly in an acidic compartment. In AP-1 cells, endosomal acidification was unaffected by omission of Na+ but was dissipated entirely by concanamycin, a blocker of H(+)-ATPases. In contrast, the cellubrevin compartment was more acidic in NHE3 transfectants, and the acidification was only partially reduced by concanamycin. Moreover, removal of extracellular Na+ resulted in a significant alkalization of the endocytic compartment. These results indicate that NHE3 is present and active in recycling endosomes. By recruiting NHE3 to the plasma membrane, modulation of vesicular traffic could contribute to the regulation of Na+ reabsorption across epithelia.

Identification of sites required for down-regulation of Na+/H+ exchanger NHE3 activity by cAMP-dependent protein kinase. phosphorylation-dependent and -independent mechanisms

We recently identified a region within the cytoplasmic C-terminal tail of the Na+/H+ exchanger NHE3 isoform (residues 579 to 684) which is essential for inhibition of transport activity by cAMP-dependent protein kinase (PKA) (Cabado, A. G., Yu, F. H., Kapus, A., Gergely, L., Grinstein, S., and Orlowski, J. (1996) J. Biol. Chem. 271, 3590-3599). To further define determinants of PKA regulation, six serine residues located in potential recognition sequences for PKA within, or adjacent to, this region (positions 552, 605, 634, 661, 690, and 691) were altered either independently or in various combinations using site-directed mutagenesis. Wild type and mutant NHE3s tagged with the influenza virus hemagglutinin epitope were stably expressed in exchanger-deficient Chinese hamster ovary cells (AP-1) for functional studies. Of the individual mutations examined, only substitutions at Ser605 or Ser634 affected sensitivity to forskolin, an activator of adenylate cyclase, although partial inhibition of NHE3 activity by forskolin remained. By contrast, simultaneous mutation of both these serines completely abolished cAMP-mediated inhibition of NHE3 without greatly affecting basal transport activity. Two-dimensional analysis of tryptic digests of immunoprecipitated NHE3 labeled in vivo with [32P]orthophosphate revealed several phosphopeptides under basal conditions. Phosphorylation was increased approximately 3-fold in one of these peptides following forskolin treatment, and this change was eliminated by mutation of residue Ser605. Thus, phosphorylation of Ser605 is essential for cAMP-mediated inhibition of NHE3. In addition, Ser634 is also required for the effect of cAMP, even though this residue does not become phosphorylated upon activation of PKA.

ATP dependence of Na+/H+ exchange. Nucleotide specificity and assessment of the role of phospholipids

We studied the ATP dependence of NHE-1, the ubiquitous isoform of the Na+/H+ antiporter, using the whole-cell configuration of the patch-clamp technique to apply nucleotides intracellularly while measuring cytosolic pH (pHi) by microfluorimetry. Na+/H+ exchange activity was measured as the Na(+)-driven pHi recovery from an acid load, which was imposed via the patch pipette. In Chinese hamster ovary (CHO) fibroblasts stably transfected with NHE-1, omission of ATP from the pipette solution inhibited Na+/H+ exchange. Conversely, ATP perfusion restored exchange activity in cells that had been metabolically depleted by 2-deoxy-D-glucose and oligomycin. In cells dialyzed in the presence of ATP, no "run-down" was observed even after extended periods, suggesting that the nucleotide is the only diffusible factor required for optimal NHE-1 activity. Half-maximal activation of the antiporter was obtained at approximately 5 mM Mg-ATP. Submillimolar concentrations failed to sustain Na+/H+ exchange even when an ATP regenerating system was included in the pipette solution. High ATP concentrations are also known to be required for the optimal function of other cation exchangers. In the case of the Na/Ca2+ exchanger, this requirement has been attributed to an aminophospholipid translocase, or "flippase.". The involvement of this enzyme in Na+/H+ exchange was examined using fluorescent phosphatidylserine, which is actively translocated by the flippase. ATP depletion decreased the transmembrane uptake of NBD-labeled phosphatidylserine (NBD-PS), indicating that the flippase was inhibited. Diamide, an agent reported to block the flippase, was as potent as ATP depletion in reducing NBD-PS uptake. However, diamide had no effect on Na+/H+ exchange, implying that the effect of ATP is not mediated by changes in lipid distribution across the plasma membrane. K-ATP and ATP gamma S were as efficient as Mg-ATP in sustaining NHE-1 activity, while AMP-PNP and AMP-PCP only partially substituted for ATP. In contrast, GTP gamma S was ineffective. We conclude that ATP is the only soluble factor necessary for optimal activity of the NHE-1 isoform of the antiporter. Mg2+ does not appear to be essential for the stimulatory effect of ATP. We propose that two mechanisms mediate the activation of the antiporter by ATP: one requires hydrolysis and is likely an energy-dependent event. The second process does not involve hydrolysis of the gamma-phosphate, excluding mediation by protein or lipid kinases. We suggest that this effect is due to binding of ATP to an as yet unidentified, nondiffusible effector that activates the antiporter.

Muscarinic agonists induce phosphorylation-independent activation of the NHE-1 isoform of the Na+/H+ antiporter in salivary acinar cells

Cholinergic agonists stimulate isotonic fluid secretion in the parotid gland. This process is driven by the apical exit of Cl-, which enters the cells partly via Cl-/HCO-3 exchange across the basolateral membrane. Acidification of the cytosol by the extrusion of HCO-3 is prevented by the concomitant activation of the Na+/H+ exchanger (NHE), which is directly activated by cholinergic stimulation. Multiple isoforms of the NHE have been described in mammalian cells, but the particular isoform(s) present in salivary glands and their mechanism of activation have not been defined. Reverse transcriptase-polymerase chain reaction with isoform-specific primers was used to establish that NHE-1 and NHE-2, but not NHE-3 or NHE-4, are expressed in parotid glands. The presence of NHE-1 was confirmed by immunoblotting and immunofluorescence, which additionally demonstrated that this isoform is abundant in the basolateral membrane of acinar cells. The predominant role of NHE-1 in carbachol-induced Na+/H+ exchange was established pharmacologically using HOE694, an inhibitor with differential potency toward the individual isoforms. Because muscarinic agonists induce stimulation of protein kinases in acinar cells, we assessed the role of phosphorylation in the activation of the antiport. Immunoprecipitation experiments revealed that, although NHE-1 was phosphorylated in the resting state, no further phosphorylation occurred upon treatment with carbachol. Similar phosphopeptide patterns were observed in control and carbachol-treated samples. Together, these findings indicate that NHE-1, the predominant isoform of the antiporter in the basolateral membrane of acinar cells, is activated during muscarinic stimulation by a phosphorylation-independent event. Other processes, such as association of Ca2+-calmodulin complexes to the cytosolic domain of the antiporter, may be responsible for the activation of Na+/H+ exchange.

Angiotensin II and the maturation of renal cortical Na+/H+ exchanger activity during fetal life in sheep

The postnatal rise in renal Na+ reabsorption is associated with an increase in proximal tubule apical membrane Na+/H+ exchanger (NHE) activity in sheep. Inasmuch as circulating angiotensin II (ANG II) levels increase immediately after birth and ANG II is known to upregulate NHE activity in the adult proximal tubule, we postulated that ANG II plays a role in mediating maturational changes in NHE activity. We therefore studied the effects of ANG II infusion (10 micrograms/h) for 24 h on renal cortical NHE activity in chronically instrumented, twin ovine fetuses (129 +/- 2 days gestation, term is 145 days, n = 10 pairs); one twin of each pair served as a control. After 24 h, the fetuses were killed and brush-border membrane vesicles (BBMV) were prepared from the renal cortices. Postinfusion plasma ANG II levels were significantly higher and plasma renin activities were significantly lower in treated fetuses compared with controls. Kinetic analysis revealed an increase in NHE activity after ANG II treatment; however, the difference was not statistically significant: maximal velocity (in nmol.s-1.mg protein-1) control 1.65 +/- 0.50, treated 2.31 +/- 0.66 (P = 0.11, n = 9 pairs); Michaelis constant control 8.29 +/- 1.17 mM, treated 9.84 +/- 1.26 mM (P = 0.11). Northern blots of total RNA from the cortices of these animals were hybridized to a D-[32P]UTP-labeled antisense RNA probe prepared from a 1.3-kb rat NHE3 cDNA fragment. There were no differences between the groups in NHE3 mRNA levels (32P counts were control 413 +/- 54, treated 340 +/- 46). ANG II does not appear to play an important role in the regulation of NHE activity in the proximal tubule of the near-term sheep fetus.

Galpha12 differentially regulates Na+-H+ exchanger isoforms

Activation of several GTPases stimulates Na+-H+ exchange, resulting in an increased efflux of intracellular H+. These GTPases include alpha subunits of the heterotrimeric G proteins Gq and G13, as well as the low molecular weight GTP-binding proteins Ras, Cdc42, and Rho (Hooley, R., Yu, C.-Y., Simon, M., and Barber, D. L. (1996) J. Biol. Chem. 271, 6152-6158). GTPases coupled to the inhibition of Na+-H+ exchange, however, have not been identified. Several neurotransmitters, including somatostatin and dopamine, inhibit Na+-H+ exchange through a guanine-nucleotide-dependent mechanism, suggesting the involvement of a GTPase. In this study we determined that mutational activation of the alpha subunit of G12 inhibits the ubiquitously expressed Na+-H+ exchanger isoform, NHE1. Transient expression of mutationally activated Galpha12 inhibited serum- and Galpha13-stimulated NHE1 activity in HEK293 cells and CCL39 fibroblasts. In addition, in NHE-deficient AP1 cells stably expressing specific NHE isoforms, mutationally activated Galpha12 inhibited NHE1 activity but stimulated activities of the Na+-H+ exchanger (NHE) isoforms NHE2 and NHE3. In contrast, mutationally activated Galpha13, another member of the Galpha12/13 family, stimulated all three NHE isoforms. Although previous studies have identified a parallel action of Galpha12 and Galpha13 in regulating MAP (mitogen-activated protein) kinases and cell growth, these GTPases have opposing effects on NHE1 activity.

Delineation of transmembrane domains of the Na+/H+ exchanger that confer sensitivity to pharmacological antagonists

Plasma membrane Na+/H+ exchanger (NHE) isoforms NHE1 and NHE3 exhibit very different sensitivities to amiloride and its 5-amino-substituted analogues, benzoyl guanidinium derivatives (e.g. (3-methylsulfonyl-4-piperidinobenzoyl)guanidine methanesulfonate (HOE694)), and cimetidine. To define structural domains that confer differential sensitivity to these antagonists, unique restriction endonuclease sites were engineered into cDNAs for each isoform near the regions that encode the putative membrane-spanning domains. These new sites did not modify their pharmacological properties and allowed several chimeric Na+/H+ exchangers to be constructed by exchanging homologous segments. The modified parental (E1' and E3') and chimeric molecules were stably expressed in exchanger-deficient Chinese hamster ovary AP-1 cells and assayed for their sensitivities to amiloride, ethylisopropylamiloride, HOE694, and cimetidine. Most chimeras showed drug sensitivities corresponding to the dominant parental segment. However, interchanging a 66-amino acid segment containing the putative ninth transmembrane (M9) domain and its adjacent loops caused reciprocal alterations in the sensitivities of E1' and E3' to all antagonists. In addition, substituting the first five putative membrane-spanning domains of E3' with the corresponding region of E1' modestly reduced the transporter's sensitivity to cimetidine but not the other compounds. These data indicate that the protein segment between M8 and M10 may be a major site of interaction with these antagonists, although other regions modestly influence sensitivity to certain drugs.

Structurally diverse N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) inhibit the Na+/H+ exchanger NHE3 isoform by binding to the PTH/PTHRP receptor type I and activating distinct signaling pathways

N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) elicit a wide variety of biological responses in target cells, including the inhibition of Na+/H+ exchanger NHE3 activity in renal cells. This response is believed to be mediated by ligand binding to a common receptor (i.e. PTH/PTHRP receptor type I) and activation of cAMP-dependent and/or Ca2+/phospholipid-dependent protein kinases (PKA and PKC, respectively). However, the mechanism of action of these N-terminal peptides is now unclear because of recent data reporting the existence of additional receptor isoforms. Therefore, to directly examine the ligand binding and signaling characteristics of the PTH/PTHRP receptor type I and its ability to elicit a biological response, cDNAs encoding the rat type I receptor and the rat NHE3 isoform were transfected into Chinese hamster ovary (AP-1) cells that lack endogenous expression of these proteins. Competition binding assays using [125I-Tyr36]PTHRP-(1-36)-NH2 radioligand indicated that several biologically active human N-terminal PTH and PTHRP fragments (PTH-(1-34), PTH-(3-34), PTH-(28-42), PTH-(28-48), and PTHRP-(1-34)) were capable of binding to the type I receptor. Both PTH-(1-34) and PTHRP-(1-34) stimulated adenylate cyclase and PKC activities in these cells, whereas PTH-(3-34), PTH-(28-42), and PTH-(28-48) selectively enhanced only PKC activity. PTHRP-(1-16), a biologically inert fragment, was incapable of binding to this receptor and influencing either the PKA or PKC pathway. Furthermore, all the analogues with the exception of PTHRP-(1-16) inhibited NHE3 activity. Inhibition of PKC by the potent antagonist chelerythrine chloride abolished the depression of NHE3 activity by PTH-(3-34), PTH-(28-42), and PTH-(28-48) but did not alleviate the effects of PTH-(1-34). Likewise, antagonism of PKA by H-89 was unable to prevent the inhibition caused by PTH-(1-34). However, inhibition of both PKA and PKC by the nonselective protein kinase antagonist H-7 abolished the reduction of NHE3 activity by PTH-(1-34). These data indicate that discrete N-terminal analogues of PTH and PTHRP can interact with the classical PTH/PTHRP receptor type I and activate PKA and/or PKC. Activation of either signaling pathway independently leads to inhibition of NHE3.

Transient expression of Na+/H+ exchanger isoform NHE-2 in LLC-PK1 cells: inhibition of endogenous NHE-3 and regulation by hypertonicity

Na+/H+ exchanger isoforms NHE-2 and NHE-3 demonstrate distinct tissue expression patterns in renal epithelial cells. NHE-2 is predominantly expressed in the inner medulla whereas NHE-3 is highly expressed in the proximal tubule cells. The purpose of the current experiments was to study the characteristics of NHE-2 upon its own expression in cultured proximal tubule cells, LLC-PK1. Toward this end, LLC-PK1 cells were subjected to six cycles of proton suicide. The mutant cells, when grown to confluence and assayed for Na+/H+ exchanger by 22Na+ influx, showed significant reduction in NHE activity as compared to the parent cells (10.4 nmole/mg prot/4 min in parent cells vs. 1.8 in mutant cells, P < 0.001, n = 4). This remaining exchanger activity was mostly mediated via NHE-3 as shown by inhibition of the Na influx following PKC stimulation (65% with PMA vs. 100% without PMA. P < 0.05, n = 4). The mutant cells were transiently transfected with a pCMV/NHE-2 expression vector using calcium phosphate precipitation method. Northern blot analysis showed the expression of a 3.4 kb transcript only in the transfected cells. The expression peaked at 48 hr and diminished by 96 hr. The exchanger activity at 48 hr after transfection was mostly due to NHE-3 (as shown by inhibition in the presence of PMA) but was significantly lower than in sham transfected cells (1.2 nmoles/mg prot. in NHE-2-transfected and 2.1 in sham-transfected, P < 0.05, n = 4). At 60 hr after transfection, the cells exhibited PMA-stimulated Na influx (>28%) indicating functional expression of NHE-2. Increasing the osmolality of the media to 510 mOsm/l stimulated the Na+/H+ exchanger in NHE-2 transfected cells but inhibited the exchanger activity in sham transfected cells. In conclusion, NHE-2 appears as a 3.4 kb transcript in transfected LLC-PK1 cells and functional expression of NHE-2 is preceded by inhibition of endogenous NHE-3 activity. The NHE-2 is stimulated by hypertonicity, indicating a likely role for this isoform in cell volume regulation.

Genomic organization and glucocorticoid transcriptional activation of the rat Na+/H+ exchanger Nhe3 gene

The activity of the apical membrane Na+/H+ exchanger NHE3 isoform of renal or intestinal epithelial cells is chronically regulated by a wide variety of stimuli, including acidosis, cAMP, glucocorticoids, and thyroid hormone. To understand the molecular mechanisms responsible for long term regulation of this cation transporter, we have isolated and determined the structure of this gene from a rat genomic library. The Nh3 gene spans > 40 kilobases and contains 17 exons that are flanked by typical splice donor and acceptor sequences at the exon-intron boundaries. The transcription initiation site was mapped by S1 nuclease protection analyses of mRNA from rat kidney and intestine. Multiple start sites were clustered between nucleotides -100 and -96 relative to the translation initiation codon. An atypical TATA-box and CCAAT-box are centered 30 and 147 nucleotides, respectively, upstream of the predominant transcription initiation site. Sequence analysis of approximately 1.4 kilobases of the 5'-flanking promoter region also revealed the presence of other putative cis-acting elements recognized by various transcription factors (e.g. AP-1, AP-2, C/EBP, NF-I, OCT-1/OTF-1, PEA3, Sp1, glucocorticoid, and thyroid hormone receptors), some of which may participate in the chronic regulation of this gene. The glucocorticoid responsiveness of the Nhe3 gene was assessed by fusing its 5' regulatory region to the firefly luciferase reporter gene and then by measuring the expression of the chimeric gene in transiently transfected renal epithelial OK and LLC-PK1 cells. Glucocorticoid treatment significantly increased the luciferase activity of the chimeric gene in both cell lines, thereby indicating that glucocorticoid regulation of Nhe3 is mediated primarily by a transcriptional mechanism.

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.

Na+/H+ exchange activity during phagocytosis in human neutrophils: role of Fcgamma receptors and tyrosine kinases

In neutrophils, binding and phagocytosis facilitate subsequent intracellular killing of microorganisms. Activity of Na+/H+ exchangers (NHEs) participates in these events, especially in regulation of intracellular pH (pHi) by compensating for the H+ load generated by the respiratory burst. Despite the importance of these functions, comparatively little is known regarding the nature and regulation of NHE(s) in neutrophils. The purpose of this study was to identify which NHE(s) are expressed in neutrophils and to elucidate the mechanisms regulating their activity during phagocytosis. Exposure of cells to the phagocytic stimulus opsonized zymosan (OpZ) induced a transient cytosolic acidification followed by a prolonged alkalinization. The latter was inhibited in Na+-free medium and by amiloride analogues and therefore was due to activation of Na+/H+ exchange. Reverse transcriptase PCR and cDNA sequencing demonstrated that mRNA for the NHE-1 but not for NHE-2, 3, or 4 isoforms of the exchanger was expressed. Immunoblotting of purified plasma membranes with isoform-specific antibodies confirmed the presence of NHE-1 protein in neutrophils. Since phagocytosis involves Fcgamma (FcgammaR) and complement receptors such as CR3 (a beta2 integrin) which are linked to pathways involving alterations in intracellular [Ca2+]i and tyrosine phosphorylation, we studied these pathways in relation to activation of NHE-1. Cross-linking of surface bound antibodies (mAb) directed against FcgammaRs (FcgammaRII > FcgammaRIII) but not beta2 integrins induced an amiloride-sensitive cytosolic alkalinization. However, anti-beta2 integrin mAb diminished OpZ-induced alkalinization suggesting that NHE-1 activation involved cooperation between integrins and FcgammaRs. The tyrosine kinase inhibitors genistein and herbimycin blocked cytosolic alkalinization after OpZ or FcgammaR cross-linking suggesting that tyrosine phosphorylation was involved in NHE-I activation. An increase in [Ca2+]i was not required for NHE-1 activation because neither removal of extracellular Ca2+ nor buffering of changes in [Ca2+]i inhibited alkalinization after OpZ or Fc-gammaR cross-linking. In summary, Fc-gammaRs and beta2 integrins cooperate in activation of NHE-1 in neutrophils during phagocytosis by a signaling pathway involving tyrosine phosphorylation.

Distinct structural domains confer cAMP sensitivity and ATP dependence to the Na+/H+ exchanger NHE3 isoform

Agents known to increase cAMP levels in renal and intestinal epithelia decrease sodium absorption by inhibiting NHE3, an isoform of the Na+/H+ exchanger expressed at high levels in apical membranes of these cells. In contrast, the ubiquitous, housekeeping isoform of the exchanger (NHE1) is stimulated by cAMP in some cell types. Optimal activity of NHE3 as well as NHE1 requires the presence of ATP. To gain insight into the molecular mechanisms of ATP dependence and cAMP regulation of NHE3, a series of mutations were constructed by progressively truncating segments of the C-terminal cytoplasmic domain of the transporter at amino acid positions 684, 638, and 579 (named NHE3delta684, NHE3delta638, and NHE3delta579). In addition, chimeric antiporters were constructed with the N-terminal transmembrane domain of NHE3 linked to the entire cytoplasmic region of NHE1 (chimera NHE3/1) or vice versa (chimera NHE1/3). These constructs were heterologously expressed in antiport-deficient Chinese hamster ovary cells, and their activities were assessed by fluorimetric measurements of intracellular pH and by radioisotope determinations of Na+ influx. Forskolin, which directly stimulates adenylate cyclase, inhibited NHE3 as well as NHE1/3, but not NHE3/1, suggesting that the cytoplasmic domain of NHE3 was sufficient to confer sensitivity to inhibition by cAMP. Forskolin also inhibited the truncated mutant NHE3delta684 to an extent similar to that for wild type NHE3. However, the inhibitory effect was greatly reduced in NHE3delta638 and more profound truncations (NHE3delta579 obliterated the effect of forskolin. These findings suggest that a region found between amino acids 579 and 684 is essential for the cAMP response of NHE3. In contrast, comparable ATP dependence was observed in all exchanger constructs examined. These observations indicate that ATP dependence is conferred by a region of the molecule in or adjacent to the transmembrane domain, which is most conserved between isoforms. It is concluded that different sites, and therefore different mechanisms, underlie inhibition of NHE3 by cAMP and by depletion of ATP.

Plasma membrane Na+/H+ exchanger isoforms (NHE-1, -2, and -3) are differentially responsive to second messenger agonists of the protein kinase A and C pathways

Na+/H+ exchanger (NHE) activity is regulated by several types of receptors directly coupled to distinct classes (i.e. Gs, Gi, Gq, and G12) of heterotrimeric (alpha beta gamma) GTP-binding proteins (G proteins), which, upon activation, modulate production of various second messengers (e.g. cAMP, cGMP, diacylglycerol, inositol trisphosphate, and Ca2+). Recently, four isoforms of the rat Na+/H+ exchanger were identified by molecular cloning. To examine their intrinsic responsiveness to G protein and second messenger stimulation, three of these isoforms, NHE-1, -2, and -3, were stably expressed in mutant Chinese hamster ovary cells devoid of endogenous NHE activity (AP-1 cells). Incubation of cells with either AIF4-, a general agonist of G proteins, or cholera toxin, a selective activator of G alpha s that stimulates adenylate cyclase, accelerated the rates of amiloride-inhibitable 22Na+ influx mediated by NHE-1 and -2, whereas they inhibited that by NHE-3. Similarly, short term treatment with phorbol 12-myristate 13-acetate, which mimics diacylglycerol activation of protein kinase C (PKC), or with agents (i.e. forskolin, 8-(4-chlorophenylthio)-cAMP, and isobutylmethylxanthine) that lead to activation of cAMP-dependent protein kinase (PKA) also stimulated transport by NHE-1 and NHE-2 but depressed that by NHE-3. The effects of phorbol 12-myristate 13-acetate were blocked by depleting cells of PKC or by inhibiting PKC using chelerythrine chloride, confirming a role for PKC in modulating NHE isoform activities. Likewise, the PKA antagonist, H-89, attenuated the effects of elevated cAMPi on NHE-1, -2, and -3, further demonstrating the regulation by PKA. Unlike cAMPi, elevation of cGMPi by treatment with dibutyryl-cGMP or 8-bromo-cGMP had no influence on NHE isoform activities, thereby excluding the possibility of a role for cGMP-dependent protein kinase in these cells. These data support the concept that the NHE isoforms are differentially responsive to agonists of the PKA and PKC pathways.

Cytoplasmic domain of the ubiquitous Na+/H+ exchanger NHE1 can confer Ca2+ responsiveness to the apical isoform NHE3

The Na+/H+ exchanger isoforms NHE1 and NHE3 are regulated differently by various stimuli. Calcium has been recognized as one of the major second messengers in such exchanger regulation. We previously proposed that Ca(2+)-induced activation of NHE1 occurs via displacement of its autoinhibitory domain from the H+ modifier site due to direct binding of Ca2+/calmodulin. To further validate this hypothesis, the functional role of the cytoplasmic domain was studied in both wild-type and chimeric exchangers, i.e. NHE1, NHE3, NHE1 with the cytoplasmic domain of NHE3 (N1N3), and NHE3 with the cytoplasmic domain of NHE1 (N3N1). After expression in exchanger-deficient fibroblasts (PS120), early response (< 80 s) to external stimuli was assessed as 5-(N-ethyl-N-isopropyl)amiloride-sensitive 22Na+ uptake. Among stimuli tested (ionomycin, alpha-thrombin, phorbol ester, hyperosmotic stress, and platelet-derived growth factor) that are all known to activate NHE1, only ionomycin and thrombin induced a significant intracellular Ca2+ mobilization and early activation of 22Na+ uptake, implying that Ca2+ is a main regulator of NHE1 in the early phase of the agonist response. However, all the stimuli did not activate NHE3 or N1N3. In contrast, a significant stimulation of 22Na+ uptake in response to ionomycin and thrombin was observed in N3N1, accompanied by an alkaline shift of pHi sensitivity (approximately 0.2 pH units). Deletion of the cytoplasmic calmodulin-binding domain within N3N1 resulted in a constitutive alkaline shift of pHi sensitivity and abolished the activation by ionomycin and thrombin. Together, these data reinforce our concept of Ca(2+)-induced activation of NHE1. Furthermore, they provide evidence for a functional interaction of the autoinhibitory domain of NHE1 with the H(+)-modifier site of a different isoform, NHE3.

Responsiveness of mutants of NHE1 isoform of Na+/H+ antiport to osmotic stress

Hypertonic activation of NHE1, the ubiquitous Na+/H+ exchanger, plays a central role in cell volume regulation, yet little is known about the underlying mechanism. We probed the osmotic responsiveness of full-length and truncated constructs of NHE1 transfected into cells lacking endogenous antiport activity. The hypertonic stimulation of NHE1 was preserved after heterologous transfection of the full-length NHE1 or of constructs truncated at positions 698 or 703. In contrast, mutants truncated at position 635 (delta 635) failed to respond to osmotic challenge. Transfectants (delta 635) behaved as if constitutively activated, having a permanently elevated cytosolic pH (pHi) under isotonic, unstimulated conditions. The delta 635 mutant displayed H+ binding with high affinity and low cooperativity. Constructs delta 582 or delta 566 had a reduced H+ sensitivity and were therefore inactive at resting pHi. Such cells were unresponsive to osmotic stress near physiological pHi but could be activated by shrinking after an acid load. Jointly, these results suggest that the H+ affinity and high cooperativity of the antiporter, earlier attributed to a single "modifier site," can be varied independently and are probably controlled by different regions of the molecule. The data indicate that volume or osmolarity-sensitive site(s) exist between the NH2-terminus and residue 566. This putative volume-sensitive site is therefore different from the site(s) postulated to mediate the stimulatory effects of calcium and growth factors.

Parathyroid hormone and parathyroid hormone-related peptide activate the Na+/H+ exchanger NHE-1 isoform in osteoblastic cells (UMR-106) via a cAMP-dependent pathway

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHRP) regulate Na+/H+ exchanger activity in osteoblastic cells, although the signaling components involved are not precisely defined. Since these peptide hormones can stimulate production of diverse second messengers (i.e. cAMP and diacylglycerol) that activate protein kinase A (PKA) and protein kinase C (PKC) in target cells, it is conceivable that either one or both of these pathways can participate in modulating exchanger activity. To discriminate among these possibilities, a series of synthetic PTH and PTHRP fragments were used that stimulate adenylate cyclase and/or PKC. In the osteoblastic cell line UMR-106, human PTH(1-34) and PTHRP(1-34) augmented adenylate cyclase activity, whereas PTH(3-34), PTH(28-42), and PTH(28-48) had no effect. Nevertheless, all these peptide fragments were found to enhance PKC translocation from the cytosol to the membrane in a dose-dependent (10(-11) to 10(-7) M) manner. PTHRP(1-16), a biologically inert fragment, was incapable of influencing either the PKA or PKC pathway. PTH(1-34) and PTHRP(1-34), but not PTH(3-34), PTH(28-42), PTH(28-48), or PTHRP(1-16), elevated Na+/H+ exchanger activity, implicating cAMP as the transducing signal. In accordance with this observation, forskolin (10 microM), which directly stimulates adenylate cyclase, also activated Na+/H+ exchanger activity. The involvement of PKA was verified when the highly specific PKA inhibitor, H-89, completely abolished the stimulatory effect of PTH(1-34) and forskolin on Na+/H+ exchange. In addition, Northern blot analysis revealed the presence of only the NHE-1 isoform of the Na+/H+ exchanger in UMR-106 cells. In summary, these results indicated that PTH and PTHRP activate the Na+/H+ exchanger NHE-1 isoform in osteoblastic UMR-106 cells exclusively via a cAMP-dependent pathway.

Parathyroid hormone and parathyroid hormone-related peptide inhibit the apical Na+/H+ exchanger NHE-3 isoform in renal cells (OK) via a dual signaling cascade involving protein kinase A and C

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHRP) interact with a common G protein-coupled receptor and stimulate production of diverse second messengers (i.e. cAMP, diacylglycerol, and inositol 1,4,5-trisphosphate) that varies depending on the target cell. In renal proximal tubule OK cells, PTH inhibits the activity of the apical membrane Na+/H+ exchanger, although it is unclear whether the signal is transmitted through protein kinase A (PKA) and/or protein kinase C (PKC). To delineate the signaling circuitry, a series of synthetic PTH and PTHRP fragments were used that stimulate the adenylate cyclase-cAMP-PKA and/or phospholipase C-diacylglycerol-PKC pathways. Human PTH-(1-34) and PTHRP-(1-34) stimulated adenylate cyclase and PKC activity, whereas the PTH analogues, PTH-(3-34), PTH-(28-42), and PTH-(28-48), selectively enhanced only PKC activity. However, each peptide fragment inhibited Na+/H+ exchanger activity by 40-50%, suggesting that PKC and possibly PKA were capable of transducing the PTH/PTHRP signal to the transporter. This was corroborated when forskolin and phorbol 12-myristate 13-acetate (PMA), direct agonists of adenylate cyclase and PKC, respectively, both inhibited the Na+/H+ exchanger. The specific PKA antagonist, H-89, abolished the forskolin-mediated suppression of Na+/H+ exchanger activity, but did not prevent the inhibitory effects of PTH-(1-34) or PMA. In comparison, the potent PKC inhibitor, chelerythrine chloride, prevented the inhibition of Na+/H+ exchanger activity mediated by PTH-(28-48) and PMA but did not avert the negative regulation caused by PTH-(1-34) or forskolin. However, inhibition of both PKA and PKC prevented PTH-(1-34)-mediated suppression of Na+/H+ exchanger activity, indicating that PTH-(1-34) acted through both signaling pathways. In addition, Northern blot analysis revealed the presence of only the NHE-3 isoform of the Na+/H+ exchanger in OK cells. In summary, these results demonstrated that NHE-3 is expressed in OK cells and that activation of the PTH receptor can stimulate both the PKA and PKC pathways, each of which can independently lead to inhibition of NHE-3 activity.

The mammalian Na+/H+ antiporters NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent, but can couple to an H+ conductance

Na+/H+ exchange in vertebrates is thought to be electroneutral and insensitive to the membrane voltage. This basic concept has been challenged by recent reports of antiport-associated currents in the turtle colon epithelium (Post and Dawson, 1992, 1994). To determine the electrogenicity of mammalian antiporters, we used the whole-cell patch clamp technique combined with microfluorimetric measurements of intracellular pH (pHi). In murine macrophages, which were found by RT-PCR to express the NHE-1 isoform of the antiporter, reverse (intracellular Na(+)-driven) Na+/H+ exchange caused a cytosolic acidification and activated an outward current, whereas forward (extracellular Na(+)-driven) exchange produced a cytosolic alkalinization and reduced a basal outward current. The currents mirrored the changes in pHi, were strictly dependent on the presence of a Na+ gradient and were reversibly blocked by amiloride. However, the currents were seemingly not carried by the Na+/H+ exchanger itself, but were instead due to a shift in the voltage dependence of a preexisting H+ conductance. This was supported by measurements of the reversal potential (Erev) of tail currents, which identified H+ (equivalents) as the charge carrier. During Na+/H+ exchange, Erev changed along with the measured changes in pHi (by 60-69 mV/pH). Moreover, the current and Na+/H+ exchange could be dissociated. Zn2+, which inhibits the H+ conductance, reversibly blocked the currents without altering Na+/H+ exchange. In Chinese hamster ovary (CHO) cells, which lack the H+ conductance, Na+/H+ exchange produced pHi changes that were not accompanied by transmembrane currents. Similar results were obtained in CHO cells transfected with either the NHE-1, NHE-2, or NHE-3 isoforms of the antiporter, indicating that exchange through these isoforms is electroneutral. In all the isoforms tested, the amplitude and time-course of the antiport-induced pHi changes were independent of the holding voltage. We conclude that mammalian NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent. In cells endowed with a pH-sensitive H+ conductance, such as macrophages, activation of Na(+)-H+ exchange can modulate a transmembrane H+ current. The currents reported in turtle colon might be due to a similar "cross-talk" between the antiporter and a H+ conductance.

Hemodialysis removal of acyclovir

A 59-y-old with a history of chronic renal failure on hemodialysis was diagnosed with herpes zoster and begun on 800 mg acyclovir 5 times daily. Two days later the patient developed visual hallucinations, ataxia, confusion and memory loss along with focal myoclonus, nausea and vomiting. No fever, elevated WBC count or significant electrolyte imbalance was found. CT scan of the brain was unremarkable. The patient was then dialyzed for presumed acyclovir toxicity. Her acyclovir level was later found to have been 3.4 micrograms/ml (normal peak range 0.4-2 micrograms/ml) prior to dialysis. After 3 h of hemodialysis, her post-dialysis acyclovir level was 1.9 micrograms/ml. After a second course of hemodialysis the next day the patient's mental status improved, and she was discharged 5 d later. Due to its low volume of distribution (0.6 L/kg), low protein binding (about 15%) and water solubility, acyclovir is an example of the ideal drug that can be removed by hemodialysis. About 45% of the total body amount can be extracted through a 3-h course of hemodialysis with resultant improvement in symptoms.