doi: 10.1161/CIRCRESAHA.108.186445
© 2008 American Heart Association, Inc.
Editorials |
Pushing and Pulling the Cardiac Sodium/Hydrogen Exchanger
From the Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, Oxford, United Kingdom.
Correspondence to Richard D. Vaughan-Jones, Department of Physiology, Anatomy and Genetics, Burdon Sanderson Cardiac Science Center, Parks Rd, Oxford OX1 3PT, United Kingdom. E-mail richard.vaughan-jones{at}dpag.ox.ac.uk
See related article, pages 881–890
Key Words: pHi • NHE1 • PKB/Akt • hypertrophy • ischemia
The strangest animal in Dr Dolittle’s zoo was a Pushmi-pullyu (pronounced push-me-pull-you), a mythical chimera of 2 antelopes, with a head at each end. Sometimes it looked and moved forwards, sometimes backwards. The direction could be coaxed by Dr Dolittle (played originally by Rex Harrison in the Hollywood movie), who spoke to the animal in its own language. A report from the laboratory of Avkiran, published in this issue of Circulation Research,1 shows that there is something strangely reminiscent of the Pushmi-pullyu in the cardiac sodium–hydrogen exchanger (NHE1), a ubiquitous, dimeric glycoprotein, intimately involved with the regulation of intracellular pH (pHi) (reviewed elsewhere2). In cardiac myocytes, the NHE1 protein product is expressed at the sarcolemma. It is a secondary active transporter expelling one H+ ion in exchange for the entry of one Na+ ion, and its activity is governed acutely by the intracellular H+ ion concentration (ie, pHi). Activity increases steeply when pHi falls from its resting level of 
7.2. This is why the protein is so effective as a pHi regulator.
But why the resemblance to a Pushmi-pullyu? It is becoming apparent that NHE1 in the cardiac cell can be both stimulated and inhibited directly by intracellular signaling cascades. Like Dr Dolittle, these talk to the transporter, and the chorus determines whether cellular H+ extrusion during intracellular acidosis is stimulated or inhibited. It has long been known that the pHi sensitivity of NHE1 can be enhanced through a coupling of the transport protein to surface membrane receptors, operating via endogenous intracellular ligands.2 Many of the pathways involved have been highlighted in other tissues, but, in the cardiac myocyte, typical extracellular receptor agonists that stimulate NHE1 activity are angiotensin II (acting on AT1 receptor), catecholamines (acting on 
1 adrenoceptor), endothelin 1, and acetylcholine (reviewed elsewhere3). Common endogenous agents include intracellular Ca2+ (Ca2+i) and a variety of excitatory signaling proteins downstream of the sarcolemmal receptors. Key among these are mitogen-activated protein kinases, including extracellular signal-regulated kinase (ERK),4 which can regulate RSK (p90 ribosomal S6 kinase).5 The stimulant effect of Ca2+ is mediated through interaction with calmodulin (CaM), which activates calmodulin kinase II (CaMKII) but can also bind directly to the NHE1 protein.2,6,7 Various relevant ligands and receptors, and their relationship with the target NHE1 protein, are summarized schematically in the Figure (right). Although the details of action of many ligands are unknown, the stimulatory effect of some results in phosphorylation of the transporter.3,5 This occurs at sites on its cytoplasmic carboxyl-terminal domain (CD). Interaction of the CD with the transmembrane domain (TMD) then upregulates H+ efflux acutely during acidosis. This is the "pushmi" side of the NHE protein. The article by Snabaitis et al now focuses on the "pullyu" side. In an elegant and carefully executed series of molecular and functional studies, they have shown, for the first time, that direct CD phosphorylation (principally at serine 648) by protein kinase B (PKB) (also known as Akt1/2) acutely decreases pHi sensitivity of NHE1. This decreases NHE-mediated H+ extrusion from a mammalian ventricular myocyte (elements of this scheme are illustrated in the Figure [left]). Thus, NHE1 can be both stimulated and inhibited by phosphorylation, making it a true Pushmi-pullyu protein.
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Inhibitory effects of other endogenous and exogenous ligands on NHE1 activity have been documented previously (these are highlighted for the cardiac myocyte in the Figure [left]), but, in many cases, the mechanisms of action, at least in the wild-type cardiac cell, are either unknown or only partly elucidated. Thus, β1 adrenoceptor activation by catecholamines, or maneuvers that elevate intracellular cAMP, result in a slowing of cardiac NHE1 activity.2 Similarly, the nitric oxide donor sodium nitroprusside8 or elevation of intracellular cGMP9 also slows cardiac NHE1 activity. However, whether the effect involves direct phosphorylation of the transport protein by a canonical kinase such as PKA or PKG has not been established. Interestingly, extracellular adenosine can inhibit NHE1, but this is via the action of a phosphatase enzyme (PPA2)10 that dephosphorylates the CD, possibly at the stimulatory site targeted by extracellular signal-regulated kinase/RSK, an example of deactivation rather than inactivation of the transporter (Figure). The phosphorylation of serine 648 by PKB/Akt is, therefore, the first evidence of a direct link between an endogenous regulator kinase and cardiac NHE1 inhibition.
The involvement of PKB/Akt in the regulation of cardiac NHE1 activity is of particular interest, because isoforms Akt1 and -2 are intimately involved in the control of cell survival, glucose uptake, and glucose metabolism (reviewed elsewhere11). The coinvolvement of a transport protein that helps to discharge the acidic waste of metabolism is therefore not likely to be merely fortuitous. PKB/Akt is also intimately involved in the downstream effects of insulin receptor activation in the cardiac myocyte. Indeed, Snabaitis et al show in their work that a novel Akt1/2 inhibitor (Akti-1/2), which enhances NHE activation by intracellular H+ ions, also blocks insulin-induced phosphorylation of another Akt target protein, glycogen synthase kinase 3β. Because insulin activates Akt, one may ask whether it is also an acute inhibitory controller of cardiac NHE1 activity. It is intriguing, for example, that the Goto–Kakizaki rat model of insulin-resistant type 2 diabetes appears to be associated with powerful upregulation of cardiac NHE1 expression and activity.12 Surprisingly, the possible effect of exogenous insulin on cardiac NHE is not particularly well documented in the literature, although insulin appears to stimulate NHE activity in other cell types, including skeletal muscle and vascular smooth muscle. In fact, application of insulin can alkalinize resting pHi in cardiac myocytes,13 an effect that is inhibited by cariporide (a selective NHE inhibitor), suggesting a possible stimulation rather than inhibition of cardiac NHE1 activity. Postreceptor insulin signaling, however, is complex, and may involve activation of both Akt and mitogen-activated protein kinase–mediated pathways.11 Because the latter can stimulate NHE1 (Figure), a possibility is that inhibitory and excitatory phosphorylation of NHE1 results, on balance, in a modest stimulation. Snabaitis et al may, in the future, be able to tell us whether this is correct.
An exciting clinical possibility emerging from the work of Snabaitis et al is the possibility of manipulating the pushmi and pullyu effects on NHE1 of endogenous ligands, in favor of inhibition. Selective stimulation of PKB/Akt may offer a novel means for achieving this. Despite disappointing clinical trials,14 there is ample evidence from animal models of the cardioprotective effects of NHE1-selective inhibitors in pathophysiological circumstances, such as postischemic reperfusion of the heart. Similarly, in various animal models of maladaptive cardiac hypertrophy, chronic application of pharmacological NHE1 inhibitors, such as cariporide, may ameliorate or even reverse the hypertrophic phenotype and slow its progression toward heart failure.15 The pathophysiological effects of NHE1 activity are believed to be secondary to the associated rise of intracellular Na+.16 By acting on sarcolemmal Na+-Ca2+ exchange, this raises Ca2+i, which may then underpin mechanisms of myocardial injury and hypertrophy. It is perhaps pertinent that many of the ligands and receptors arranged on the right-hand side of the Figure have been invoked in these pathological events, whereas some of the agents on the left-hand side, such as NO, cGMP, and now PKB/Akt may, under the right circumstances, be cardioprotective. It is therefore satisfying that Snabaitis et al have discovered that phosphorylation of NHE1 by PKB/Akt is associated with a reduced binding of Ca2+–calmodulin. PKB activation may thus abrogate one of the key possible mechanisms for the hypertrophic influence of Ca2+i, its stimulation of NHE1.
Speculation on the possible clinical benefits of PKB activation should be tempered by a warning. Although it slows NHE1 activity, the effects of PKB on other key pHi regulatory transport proteins expressed at the cardiac sarcolemma,17,18 such as Na+-HCO3– cotransport (NBC), Cl–-HCO3– exchange, Cl–-OH– exchange, and H+-lactate cotransport, remain to be assessed. It is noteworthy, for example, that acute inhibition of cardiac NHE1 activity by a β-adrenoceptor agonist can be paralleled by a coordinate stimulation of acid extrusion (and hence Na+ influx) on NBC.19 The challenge now, perhaps, will be to assess possible pushmi-pullyu effects of PKB on parallel acid and base extrusion pathways. The far-sighted work of Snabaitis et al and the laboratory of Avkiran have robustly laid down this challenge. It would appear that Dr Dolittle has more than a little to do.
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Acknowledgments |
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Sources of Funding
This work was supported by a Programme Grant from the British Heart Foundation.
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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 Top References |
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1. Snabaitis AK, Cuello F, Avkiran M. Protein kinase B/Akt phosphorylates and inhibits the cardiac Na+/H+ exchanger NHE1. Circ Res. 2008; 103: 881–890.
2. Wakabayashi S, Shigekawa M, Pouyssegur J. Molecular physiology of vertebrate Na+/H+ exchangers. Physiol Rev. 1997; 77: 51–74.
3. Avkiran M, Haworth RS. Regulatory effects of G protein-coupled receptors on cardiac sarcolemmal Na+/H+ exchanger activity: signalling and significance. Cardiovasc Res. 2003; 57: 942–952.
4. Malo ME, Li L, Fliegel L. Mitogen-activated protein kinase-dependent activation of the Na+/H+ exchanger is mediated through phosphorylation of amino acids Ser770 and Ser771. J Biol Chem. 2007; 282: 6292–6299.
5. Cuello F, Snabaitis AK, Cohen MS, Taunton J, Avkiran M. Evidence for direct regulation of myocardial Na+/H+ exchanger isoform 1 phosphorylation and activity by 90-kDa ribosomal S6 kinase (RSK): effects of the novel and specific RSK inhibitor fmk on responses to alpha1-adrenergic stimulation. Mol Pharmacol. 2007; 71: 799–806.
6. Wakabayashi S, Bertrand B, Ikeda T, Pouyssegur J, Shigekawa M. Mutation of calmodulin-binding site renders the Na+/H+ exchanger (NHE1) highly H+-sensitive and Ca2+ regulation-defective. J Biol Chem. 1994; 269: 13710–13715.
7. Fliegel L, Walsh MP, Singh D, Wong C, Barr A. Phosphorylation of the C-terminal domain of the Na+/H+ exchanger by Ca2+/calmodulin-dependent protein kinase II. Biochem J. 1992; 282 (pt 1): 139–145.[Medline] [Order article via Infotrieve]
8. Ito N, Bartunek J, Spitzer KW, Lorell BH. Effects of the nitric oxide donor sodium nitroprusside on intracellular pH and contraction in hypertrophied myocytes. Circulation. 1997; 95: 2303–2311.
9. Perez NG, Piaggio MR, Ennis IL, Garciarena CD, Morales C, Escudero EM, Cingolani OH, Chiappe de Cingolani G, Yang XP, Cingolani HE. Phosphodiesterase 5A inhibition induces Na+/H+ exchanger blockade and protection against myocardial infarction. Hypertension. 2007; 49: 1095–1103.
10. Snabaitis AK, D'Mello R, Dashnyam S, Avkiran M. A novel role for protein phosphatase 2A in receptor-mediated regulation of the cardiac sarcolemmal Na+/H+ exchanger NHE1. J Biol Chem. 2006; 281: 20252–20262.
11. Bertrand L, Horman S, Beauloye C, Vanoverschelde JL. Insulin signalling in the heart. Cardiovasc Res. 2008; 79: 238–248.
12. Darmellah A, Baetz D, Prunier F, Tamareille S, Rucker-Martin C, Feuvray D. Enhanced activity of the myocardial Na+/H+ exchanger contributes to left ventricular hypertrophy in the Goto-Kakizaki rat model of type 2 diabetes: critical role of Akt. Diabetologia. 2007; 50: 1335–1344.[CrossRef][Medline] [Order article via Infotrieve]
13. Yang J, Gillingham AK, Hodel A, Koumanov F, Woodward B, Holman GD. Insulin-stimulated cytosol alkalinization facilitates optimal activation of glucose transport in cardiomyocytes. Am J Physiol Endocrinol Metab. 2002; 283: E1299–E1307.
14. Avkiran M, Marber MS. Na+/H+ exchange inhibitors for cardioprotective therapy: progress, problems and prospects. J Am Coll Cardiol. 2002; 39: 747–753.
15. Karmazyn M, Kilic A, Javadov S. The role of NHE-1 in myocardial hypertrophy and remodelling. J Mol Cell Cardiol. 2008; 44: 647–653.[CrossRef][Medline] [Order article via Infotrieve]
16. Nakamura TY, Iwata Y, Arai Y, Komamura K, Wakabayashi S. Activation of Na+/H+ exchanger 1 is sufficient to generate Ca2+ signals that induce cardiac hypertrophy and heart failure. Circ Res. 2008; 103.
17. Leem CH, Lagadic-Gossmann D, Vaughan-Jones RD. Characterization of intracellular pH regulation in the guinea-pig ventricular myocyte. J Physiol. 1999; 517 (pt 1): 159–180.
18. Poole RC, Halestrap AP, Price SJ, Levi AJ. The kinetics of transport of lactate and pyruvate into isolated cardiac myocytes from guinea pig. Kinetic evidence for the presence of a carrier distinct from that in erythrocytes and hepatocytes. Biochem J. 1989; 264: 409–418.[Medline] [Order article via Infotrieve]
19. Lagadic-Gossmann D, Vaughan-Jones RD. Coupling of dual acid extrusion in the guinea-pig isolated ventricular myocyte to alpha 1- and beta-adrenoceptors. J Physiol. 1993; 464: 49–73.
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