Regulation of the cardiac Na⁺/H⁺ exchanger in health and disease

Cardiomyocyte-specific NHE1 overexpression confers protection against myocardial infarction during hyperglycemia

BACKGROUND

Acute hyperglycemia on admission is frequently observed during the early phase after acute myocardial infarction (MI), even without the history of diabetes mellitus. We previously reported that inhibiting Na+/H+ exchanger 1 (NHE1) activity post-MI may improve outcomes, but not in the setting of MI with acute hyperglycemia. However, the precise role of NHE1 in the pathophysiology of MI with acute hyperglycemia remains to be elucidated, and there are no effective strategies for its prevention or treatment.

METHODS AND RESULTS

We analyzed 85 post-MI patients, identifying acute hyperglycemia (glucose > 7 mM) in non-diabetic individuals, linked to elevated BNP, CK-MB, and reduced plasma Na+. Using retrospective cohort studies and MI with acute hyperglycemia mouse models, we demonstrated that hyperglycemia exacerbates myocardial injury by reducing extracellular Na+, increasing intracellular Na+, and elevating pH, suggesting NHE1 activation as inferred from the observed intracellular pH (pHi) shift. Cardiomyocyte-specific NHE1 ablation or pharmacological inhibition worsened cardiac dysfunction and fibrosis in MI with acute hyperglycemia, while NHE1 overexpression conferred protection. RNA sequencing and drug screening identified accelerated NHE1 activation via 3% NaCl and lithospermic acid (LA) as a novel strategy to mitigate cardiomyocyte necroptosis, alleviating ischemic injury in MI and ischemia reperfusion models. Hypoxia-hyperglycemia and necroptosis induction models in NHE1-knockout, NHE1-overexpressing, and MLKL-overexpressing cardiomyocytes revealed that NHE1 activation, unlike its protective role in oxygen-glucose deprivation, promotes MLKL degradation via autophagosome-lysosomal pathways, reducing cardiomyocyte death. MLKL knockout and MLKL-NHE1 double knockout mice confirmed that MLKL ablation counteracts NHE1 inhibition's detrimental effects.

CONCLUSIONS

Activation of myocardial NHE1 promotes MLKL autophagic degradation, mitigating cardiomyocyte necroptosis and acute hyperglycemia-exacerbated MI, highlighting NHE1 as a hyperglycemia-dependent cardioprotective target. Moderate NHE1 activation may represent a novel therapeutic strategy for MI with acute hyperglycemia.

Sodium-glucose cotransporter 2 inhibitor empagliflozin enhances autophagy and reverses remodeling in hearts with large, old myocardial infarctions

Large clinical trials recently showed that sodium-glucose cotransporter 2 (SGLT2) inhibitors improve the prognosis of heart failure patients with or without diabetes. Using a mouse model of large myocardial infarction, we investigated the therapeutic effects and underlying molecular mechanisms of the highly selective SGLT2 inhibitor empagliflozin in heart failure. Four weeks after myocardial infarction induced by left coronary artery ligation, the surviving mice were assigned to vehicle or empagliflozin groups and treated for 8 weeks. Empagliflozin did not alter body weight, blood pressure, glycohemoglobin, blood glucose or beta-hydroxybutyrate levels but significantly attenuated cardiac dysfunction and left ventricular dilatation (remodeling). Hearts from empagliflozin-treated mice showed less fibrosis, less cardiomyocyte hypertrophy, and lower myocardial ANP levels than those from vehicle-treated mice. Autophagy was augmented in cardiomyocytes from empagliflozin-treated mice, as indicated by increased myocardial microtubule-associated protein-1 LC3 (light chain 3)-II levels and LC-3-II/I ratio as well as increased levels of cathepsin D and ATP. Additionally, numerous autophagic vacuoles and lysosomes were observed, accompanied by increased AMP-activated protein kinase (AMPK) phosphorylation and suppression of mammalian target of rapamycin phosphorylation. Myocardial sodium-hydrogen antiporter (NHE)-1 expression was increased in infarcted mice, and that effect was unchanged by empagliflozin. In vitro, empagliflozin increased autophagic flux and induced an intracellular pH drop, AMPK activation and ATP production in cardiomyocytes. These effects were similar to those of the NHE-1 inhibitor cariporide, suggesting a possibility that they both act on the same pathway. Empagliflozin is a beneficial pharmacological tool that enhances autophagy to reverse remodeling in the postinfarction heart.

Cardioprotection and neurobehavioral impact of swimming training in ovariectomized rats

Cardiovascular (CV) disease is the major cause of mortality. Estrogens (E) exert multiple CV and neuroprotective effects. During menopause, CV and cognitive pathologies increase dramatically. At present, it is known that E exert many of their beneficial effects through the G protein-coupled estrogen receptor (GPER). Exercise reduces the risk of developing CV diseases. Sodium/proton exchanger (NHE-1) is overexpressed in ovariectomized (OVX) rats, probably due to the increase in reactive oxidative species (ROS). Insulin-like growth factor 1 (IGF-1), the main humoral mediator of exercise, inhibits the NHE-1. We aim to explore the subcellular mechanisms involved in the heart and brain impact of physiological exercise in OVX rats. We speculate that physical training, via IGF-1, prevents the increase in ROS, improving heart and brain physiological functions during menopause. Exercise diminished cardiac ROS production and increased catalase (CAT) activity in OVX rats. In concordance, IGF-1 treatment reduces brain ROS, surely contributing to the improvement in brain behavior. Moreover, the aerobic routine was able to prevent, and IGF-1 therapy to revert, NHE-1 hyperactivity in OVX rats. Finally, our results confirm the proposed signaling pathway as IGF-1/PI3K-AKT/NO. Surprisingly, GPER inhibitor (G36) was able to abolish the IGF-1 effect, suggesting that directly or indirectly GPER is part of the IGF-1 pathway. We propose that IGF-1 is the main responsible for the protective effect of aerobic training both in the heart and brain in OVX rats. Moreover, we showed that not only it is possible to prevent but also to revert the menopause-induced NHE-1 hyperactivity by exercise/IGF-1 cascade.

Suppression of B-type natriuretic peptide gene expression in cardiomyocytes under anoxic conditions

Several cell biology studies have focused on the effects of hypoxic environments on cardiomyocytes. However, the effect of anoxic conditions on cardiomyocytes remains largely unexplored. In the present study, we investigated the direct effects of anoxia on B-type natriuretic peptide (BNP) gene expression in cardiomyocytes. Neonatal rat cardiomyocytes (NRCMs) were exposed to anoxia using an airtight chamber saturated with 95 % N2/5 % CO2. BNP mRNA levels in NRCM were substantially reduced after more than 8 h of anoxia exposure, whereas after reoxygenation, BNP gene expression levels recovered in a time-dependent manner and significantly increased after 24 h of reoxygenation. BNP mRNA levels suppressed under anoxic conditions were significantly increased by aldosterone-induced activation of sodium-proton exchanger 1 (NHE1), which was canceled by an NHE1 inhibitor, suggesting that anoxia reduces BNP gene expression, at least in part, in an NHE1-dependent manner. In summary, we found that BNP gene expression in cardiomyocytes decreases under anoxic conditions, in contrast to previous research findings that BNP expression increases under hypoxic conditions. These findings reveal a new insight that, within a single heart tissue in various cardiovascular diseases, such as myocardial infarction, the biological responses of cardiomyocytes are fundamentally different in regions of anoxia and hypoxia.

ATP-mediated increase in H+ efflux from retinal Müller cells of the axolotl

Previous work has shown that activation of tiger salamander retinal radial glial cells by extracellular ATP induces a pronounced extracellular acidification, which has been proposed to be a potent modulator of neurotransmitter release. This study demonstrates that low micromolar concentrations of extracellular ATP similarly induce significant H+ effluxes from Müller cells isolated from the axolotl retina. Müller cells were enzymatically isolated from axolotl retina and H+ fluxes were measured from individual cells using self-referencing H+-selective microelectrodes. The increased H+ efflux from axolotl Müller cells induced by extracellular ATP required activation of metabotropic purinergic receptors and was dependent upon calcium released from internal stores. We further found that the ATP-evoked increase in H+ efflux from Müller cells of both tiger salamander and axolotl were sensitive to pharmacological agents known to interrupt calmodulin and protein kinase C (PKC) activity: chlorpromazine (CLP), trifluoperazine (TFP), and W-7 (all calmodulin inhibitors) and chelerythrine, a PKC inhibitor, all attenuated ATP-elicited increases in H+ efflux. ATP-initiated H+ fluxes of axolotl Müller cells were also significantly reduced by amiloride, suggesting a significant contribution by sodium-hydrogen exchangers (NHEs). In addition, α-cyano-4-hydroxycinnamate (4-cin), a monocarboxylate transport (MCT) inhibitor, also reduced the ATP-induced increase in H+ efflux in both axolotl and tiger salamander Müller cells, and when combined with amiloride, abolished ATP-evoked increase in H+ efflux. These data suggest that axolotl Müller cells are likely to be an excellent model system to understand the cell-signaling pathways regulating H+ release from glia and the role this may play in modulating neuronal signaling.NEW & NOTEWORTHY Glial cells are a key structural part of the tripartite synapse and have been suggested to regulate synaptic transmission, but the regulatory mechanisms remain unclear. We show that extracellular ATP, a potent glial cell activator, induces H+ efflux from axolotl retinal Müller (glial) cells through a calcium-dependent pathway that is likely to involve calmodulin, PKC, Na+/H+ exchange, and monocarboxylate transport, and suggest that such H+ release may play a key role in modulating neuronal transmission.

Comparison of SGLT2 inhibitors with DPP-4 inhibitors combined with metformin in patients with acute myocardial infarction and diabetes mellitus

BACKGROUND

Although sodium-glucose cotransporter 2 inhibitors (SGLT2i) have demonstrated cardiovascular benefits in patients with type 2 diabetes mellitus, real-world evidence regarding their benefits to diabetic patients with acute myocardial infarction (AMI) is insufficient. This study evaluated cardiovascular outcomes by comparing SGLT2i with dipeptidyl peptidase-4 inhibitors (DPP-4i) in combination with metformin in diabetic patients with AMI.

METHODS

This study involved 779 diabetic participants with AMI from a Korean nationwide multicenter observational cohort, who were divided into two groups: (1) metformin plus SGLT2i group (SGLT2i group, n = 186) and (2) metformin plus DPP-4i (DPP-4i group, n = 593). The primary endpoint was one year of major adverse composite events (MACEs), a composite outcome of all-cause mortality, non-fatal myocardial infarction, any revascularization, cerebrovascular accident, and stent thrombosis. To balance the baseline differences, inverse probability of treatment weighting (IPTW) was performed.

RESULTS

After IPTW, the rate of MACEs in the SGLT2i group was not significantly lower than that in the DPP-4i group (hazard ratio [HR], 0.99; 95% confidence interval [Cl], 0.46 to 2.14, p = 0.983). In the unadjusted and adjusted analyses, all items for clinical outcomes were comparable between the two groups. In our exploratory analysis, the left ventricular ejection fraction showed a significant improvement in the SGLT2i group than in the DPP-4i group before achieving statistical balancing (6.10 ± 8.30 versus 2.95 ± 10.34, p = 0.007) and after IPTW adjustment (6.91 ± 8.91 versus 3.13 ± 10.41, p = 0.027).

CONCLUSIONS

Our findings demonstrated that SGLT2i did not influence the rate of MACEs compared with DPP-4i in combination with metformin in diabetic patients with AMI but did improve left ventricular ejection fraction.

TRIAL REGISTRATION

Not applicable (retrospectively registered).

Cardiac metabolism in HFpEF: from fuel to signalling

Heart failure (HF) is marked by distinctive changes in myocardial uptake and utilization of energy substrates. Among the different types of HF, HF with preserved ejection fraction (HFpEF) is a highly prevalent, complex, and heterogeneous condition for which metabolic derangements seem to dictate disease progression. Changes in intermediate metabolism in cardiometabolic HFpEF-among the most prevalent forms of HFpEF-have a large impact both on energy provision and on a number of signalling pathways in the heart. This dual, metabolic vs. signalling, role is played in particular by long-chain fatty acids (LCFAs) and short-chain carbon sources [namely, short-chain fatty acids (SCFAs) and ketone bodies (KBs)]. LCFAs are key fuels for the heart, but their excess can be harmful, as in the case of toxic accumulation of lipid by-products (i.e. lipotoxicity). SCFAs and KBs have been proposed as a potential major, alternative source of energy in HFpEF. At the same time, both LCFAs and short-chain carbon sources are substrate for protein post-translational modifications and other forms of direct and indirect signalling of pivotal importance in HFpEF pathogenesis. An in-depth molecular understanding of the biological functions of energy substrates and their signalling role will be instrumental in the development of novel therapeutic approaches to HFpEF. Here, we summarize the current evidence on changes in energy metabolism in HFpEF, discuss the signalling role of intermediate metabolites through, at least in part, their fate as substrates for post-translational modifications, and highlight clinical and translational challenges around metabolic therapy in HFpEF.

Clinical pharmacology of SGLT-2 inhibitors in heart failure

INTRODUCTION

Sodium-glucose cotransporter 2 (SGLT2) inhibitors constitute a class of oral antiglycemic agents that have emerged as a new therapeutic strategy for heart failure (HF) with reduced ejection fraction (HFrEF) and, potentially, for HF with preserved ejection fraction (HFpEF).

AREAS COVERED

Ongoing efforts to clarify the exact mechanisms of action of SGLT2 inhibitors (SGLT2i) reveal that glycosuria and osmotic diuresis, resulting from the blockade of renal receptors, is not the sole pathophysiological mechanism. Nevertheless, the underlying mechanisms, accounting for their cardiovascular beneficial effects which have been clearly demonstrated in clinical trials, remain unclear. The aim of this review is to summarize the primary outcomes of large-scale studies regarding the use of SGLT2i in HF and provide an overview of the potential pathways involved in the SGLT2i-mediated cardioprotection.

EXPERT OPINION

SGLT2i exhibit favorable pleiotropic effects, which extend beyond their primary indication as pharmaceutical agents intended for glycemic control. Given their unique pathophysiological profile, these agents have revolutionized the management of HF, while in the near future, it is possible that evolving research in the field may unfold further perspectives on their potential use in the treatment of other chronic conditions.

Polyunsaturated ω3 fatty acids prevent the cardiac hypertrophy in hypertensive rats

It has been demonstrated that supplementation with the two main omega 3 polyunsaturated fatty acids (ω3 FAs), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), leads to modifications in the cardiac physiology. ω3 FAs can affect the membrane's lipid composition, as well as proteins' location and/or function. The Na⁺/H⁺ exchanger (NHE1) is an integral membrane protein involved in the maintenance of intracellular pH and its hyperactivity has been associated with the development of various cardiovascular diseases such as cardiac hypertrophy. Our aim was to determine the effect of ω3 FAs on systolic blood pressure (SBP), lipid profiles, NHE1 activity, and cardiac function in spontaneously hypertensive rats (SHR) using Wistar rats (W) as normotensive control. After weaning, the rats received orally ω3 FAs (200 mg/kg body mass/day/ 4 months). We measured SBP, lipid profiles, and different echocardiography parameters, which were used to calculate cardiac hypertrophy index, systolic function, and ventricular geometry. The rats were sacrificed, and ventricular cardiomyocytes were obtained to measure NHE1 activity. While the treatment with ω3 FAs did not affect the SBP, lipid analysis of plasma revealed a significant decrease in omega-6/omega-3 ratio, correlated with a significant reduction in left ventricular mass index in SHR. The NHE1 activity was significantly higher in SHR compared with W. While in W the NHE1 activity was similar in both groups, a significant decrease in NHE1 activity was detected in SHRs supplemented with ω3 FAs, reaching values comparable with W. Altogether, these findings revealed that diet supplementation with ω3 FAs since early age prevents the development of cardiac hypertrophy in SHR, perhaps by decreasing NHE1 activity, without altering hemodynamic overload.

Sodium-glucose Cotransporter 2 Inhibitors and Pathological Myocardial Hypertrophy

Sodium-glucose cotransporter 2 (SGLT2) inhibitors are a new type of oral hypoglycemic drugs that exert a hypoglycemic effect by blocking the reabsorption of glucose in the proximal renal tubules, thus promoting the excretion of glucose from urine. Their hypoglycemic effect is not dependent on insulin. Increasing data shows that SGLT2 inhibitors improve cardiovascular outcomes in patients with type 2 diabetes. Previous studies have demonstrated that SGLT2 inhibitors can reduce pathological myocardial hypertrophy with or without diabetes, but the exact mechanism remains to be elucidated. To clarify the relationship between SGLT2 inhibitors and pathological myocardial hypertrophy, with a view to providing a reference for the future treatment thereof, this study reviewed the possible mechanisms of SGLT2 inhibitors in attenuating pathological myocardial hypertrophy. We focused specifically on the mechanisms in terms of inflammation, oxidative stress, myocardial fibrosis, mitochondrial function, epicardial lipids, endothelial function, insulin resistance, cardiac hydrogen and sodium exchange, and autophagy.

Na+/H+ Exchanger 1, a Potential Therapeutic Drug Target for Cardiac Hypertrophy and Heart Failure

Cardiac hypertrophy is defined as increased heart mass in response to increased hemodynamic requirements. Long-term cardiac hypertrophy, if not counteracted, will ultimately lead to heart failure. The incidence of heart failure is related to myocardial infarction, which could be salvaged by reperfusion and ultimately invites unfavorable myocardial ischemia-reperfusion injury. The Na⁺/H⁺ exchangers (NHEs) are membrane transporters that exchange one intracellular proton for one extracellular Na⁺. The first discovered NHE isoform, NHE1, is expressed almost ubiquitously in all tissues, especially in the myocardium. During myocardial ischemia-reperfusion, NHE1 catalyzes increased uptake of intracellular Na⁺, which in turn leads to Ca²⁺ overload and subsequently myocardial injury. Numerous preclinical research has shown that NHE1 is involved in cardiac hypertrophy and heart failure, but the exact molecular mechanisms remain elusive. The objective of this review is to demonstrate the potential role of NHE1 in cardiac hypertrophy and heart failure and investigate the underlying mechanisms.

Regulation of proton partitioning in kinase-activating acute myeloid leukemia and its therapeutic implication

Gain-of-function kinase mutations are common in AML and usually portend an inferior prognosis. We reported a novel mechanism whereby kinase mutants induced intracellular alkalization characteristic in oncogenesis. Thirteen kinases were found to activate sodium/hydrogen exchanger (NHE1) in normal hematopoietic progenitors, of which FLT3-ITD, KRAS^G12D, and BTK phosphorylated NHE1 maintained alkaline intracellular pH (pHi) and supported survival of AML cells. Primary AML samples with kinase mutations also showed increased NHE1 phosphorylation and evidence of NHE1 addiction. Amiloride enhanced anti-leukemic effects and intracellular distribution of kinase inhibitors and chemotherapy. Co-inhibition of NHE1 and kinase synergistically acidified pHi in leukemia and inhibited its growth in vivo. Plasma from patients taking amiloride for diuresis reduced pHi of leukemia and enhanced cytotoxic effects of kinase inhibitors and chemotherapy in vitro. NHE1-mediated intracellular alkalization played a key pathogenetic role in transmitting the proliferative signal from mutated-kinase and could be exploited for therapeutic intervention in AML.

Structural basis of autoinhibition of the human NHE3-CHP1 complex

Sodium-proton exchanger 3 (NHE3/SLC9A3) located in the apical membrane of renal and gastrointestinal epithelia mediates salt and fluid absorption and regulates pH homeostasis. As an auxiliary regulatory factor of NHE proteins, calcineurin B homologous protein 1 (CHP1) facilitates NHE3 maturation, plasmalemmal expression, and pH sensitivity. Dysfunctions of NHE3 are associated with renal and digestive system disorders. Here, we report the cryo-electron microscopy structure of the human NHE3-CHP1 complex in its inward-facing conformation. We found that a cytosolic helix-loop-helix motif in NHE3 blocks the intracellular cavity formed between the core and dimerization domains, functioning as an autoinhibitory element and hindering substrate transport. Furthermore, two phosphatidylinositol molecules are found to bind to the peripheric juxtamembrane sides of the complex, function as anchors to stabilize the complex, and may thus enhance its transport activity.

Luseogliflozin inhibits high glucose-induced TGF-2 expression in mouse cardiomyocytes by suppressing NHE-1 activity

Objective

Sodium-glucose cotransporter-2 (SGLT2) inhibitors exhibit cardioprotective properties in patients with diabetes. However, SGLT2 is not expressed in the heart, and the underlying molecular mechanisms are not fully understood. We investigated whether the SGLT2 inhibitor luseogliflozin exerts beneficial effects on high glucose-exposed cardiomyocytes via the suppression of sodium-hydrogen exchanger-1 (NHE-1) activity.

Methods

Mouse cardiomyocytes were incubated under normal or high glucose conditions with vehicle, luseogliflozin, or the NHE-1 inhibitor cariporide. NHE-1 activity and gene expression were evaluated by the SNARF assay and real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis, respectively. Six-week-old male db/db mice were treated with vehicle or luseogliflozin for 6 weeks, and the hearts were collected for histological, RT-PCR, and western blot analyses.

Results

High glucose increased NHE-1 activity and transforming growth factor (Tgf)-β2 mRNA levels in cardiomyocytes, both of which were inhibited by luseogliflozin or cariporide, whereas their combination showed no additive suppression of Tgf-β2 mRNA levels. Luseogliflozin attenuated cardiac hypertrophy and fibrosis in db/db mice in association with decreased mRNA and protein levels of TGF-β2.

Conclusions

Luseogliflozin may suppress cardiac hypertrophy in diabetes by reducing Tgf-β2 expression in cardiomyocytes via the suppression of NHE-1 activity.

SGLT2 inhibitors: a focus on cardiac benefits and potential mechanisms

This paper highlights the cardioprotective potential of sodium-glucose cotransporter 2 inhibitors (SLGT2i), as well as several most discussed mechanisms responsible for their cardioprotection. Cardiovascular diseases are considered a primary cause of death in nearly 80% of type 2 diabetes mellitus (T2DM) patients, with a 2-4-fold greater incidence of heart failure (HF) among diabetics. As novel hypoglycemics, SGLT2i showed exceptional cardiovascular benefits, reflected through robust reductions of cardiovascular mortality and hospitalization for HF in T2DM patients. Recently, those effects have been reported even in patients with HF and reduced ejection fraction irrespectively of diabetic status, suggesting that cardioprotective effects of SGLT2i are driven independently of their hypoglycemic actions. SGLT2i exerted hemodynamic and metabolic effects, partially driven by natriuresis and osmotic diuresis. However, those systemic effects are modest, and therefore cannot be completely related to the cardiac benefits of these agents in T2DM patients. Hence, increased circulating ketone levels during SGLT2i administration have brought out another hypothesis of a cardiac metabolic switch. Moreover, SGLT2i influence ion homeostasis and exert anti-inflammatory and antifibrotic effects. Their enviable influence on oxidative stress markers, as well as anti- and pro-apoptotic factors, have also been reported. However, since the main mechanistical contributor of their cardioprotection has not been elucidated yet, a joint action of systemic and molecular mechanisms has been suggested. In the light of ongoing trials evaluating the effects of SGLT2i in patients with HF and preserved ejection fraction, a new chapter of beneficial SGLT2i mechanisms is expected, which might resolve their main underlying action.

The HVCN1 voltage-gated proton channel contributes to pH regulation in canine ventricular myocytes

KEY POINTS

Intracellular pH (pH_i ) regulation is crucial for cardiac function, as acidification depresses contractility and causes arrhythmias. H⁺ ions are generated in cardiomyocytes from metabolic processes and particularly from CO₂ hydration, which has been shown to facilitate CO₂ -venting from mitochondria. Currently, the NHE1 Na⁺ /H⁺ exchanger is viewed as the dominant H⁺ -extrusion mechanism in cardiac muscle. We show that the HVCN1 voltage-gated proton channel is present and functional in canine ventricular myocytes, and that HVCN1 and NHE1 both contribute to pH_i regulation. HVCN1 provides an energetically-efficient mechanism of H⁺ -extrusion that would not cause Na⁺ -loading, which can cause pathology, and that could contribute to transport-mediated CO₂ disposal. These results provide a major advance in our understanding of pH_i regulation in cardiac muscle.

ABSTRACT

Regulation of intracellular pH (pH_i ) in cardiomyocytes is crucial for cardiac function; however, currently known mechanisms for direct or indirect extrusion of acid from cardiomyocytes seem insufficient for energetically-efficient extrusion of the massive H⁺ loads generated under in vivo conditions. In cardiomyocytes, voltage-sensitive H⁺ channel activity mediated by the HVCN1 proton channel would be a highly efficient means of disposing of H⁺ , while avoiding Na⁺ -loading, as occurs during direct acid extrusion via Na⁺ /H⁺ exchange or indirect acid extrusion via Na⁺ -HCO₃ ^- cotransport. PCR and immunoblotting demonstrated expression of HVCN1 mRNA and protein in canine heart. Patch clamp analysis of canine ventricular myocytes revealed a voltage-gated H⁺ current that was highly H⁺ -selective. The current was blocked by external Zn²⁺ and the HVCN1 blocker 5-chloro-2-guanidinobenzimidazole (ClGBI). Both the gating and Zn²⁺ blockade of the current were strongly influenced by the pH gradient across the membrane. All characteristics of the observed current were consistent with the known hallmarks of HVCN1-mediated H⁺ current. Inhibition of HVCN1 and the NHE1 Na⁺ /H⁺ exchanger, singly and in combination, showed that either mechanism is largely sufficient to maintain pH_i in beating cardiomyocytes, but that inhibition of both activities causes rapid acidification. These results show that HVCN1 is expressed in canine ventricular myocytes and provides a major H⁺ -extrusion activity, with a capacity similar to that of NHE1. In the beating heart in vivo, this activity would allow Na⁺ -independent extrusion of H⁺ during each action potential and, when functionally coupled with anion transport mechanisms, could facilitate transport-mediated CO₂ disposal. Abstract figure legend The HVCN1 proton channel is expressed in canine ventricular myocytes and contributes to H⁺ extrusion. This article is protected by copyright. All rights reserved.

Alterations of sodium-hydrogen exchanger 1 function in response to SGLT2 inhibitors: what is the evidence?

This review summarizes and describes the current evidence addressing how sodium-glucose cotransporter 2 (SGLT2) inhibitors alter the function of sodium-hydrogen exchanger 1 (NHE-1), in association with their protective effects against adverse cardiovascular events. In the heart, SGLT2 inhibitors modulate the function of NHE-1 (either by direct inhibition or indirect attenuation of protein expression), which promotes cardiac contraction and an enhanced energy supply, in association with improved mitochondrial function, reduced inflammation/oxidative/endoplasmic reticulum stress, and attenuated fibrosis and apoptotic/autophagic cell death. The vasodilating effect of SGLT2 inhibitors has also been proposed due to NHE-1 inhibition. Moreover, platelet-expressed NHE-1 might serve as a target for SGLT2 inhibitors, since these drugs and selective NHE-1 inhibitors produce comparable activity against adenosine diphosphate-stimulated platelet activation. Overall, it is promising that the modulation of the functions of NHE-1 on the heart, blood vessels, and platelets may act as a contributing pathway for the cardiovascular benefits of SGLT2 inhibitors in diabetes and heart failure.

Hypotension in hereditary cardiomyopathy

It is well accepted that hypertension may lead to the development of heart failure (HF). However, little is known about the development of hypotension that may contribute to the onset of hereditary cardiomyopathy (HCM), thus promoting heart failure and early death. The purpose of this study is to verify whether a decrease in blood pressure takes place during different phases of HCM (asymptomatic, necrosis, hypertrophy, and heart failure). Using the well-known animal model, the UM-X7.1 hamster strain of HCM (HCMH), our results showed the absence of a change in mean arterial pressure (MAP) during the asymptomatic phase preceding the development of necrosis in HCMHs when compared to age-matched normal hamster (NH). However, there was a progressive decrease in MAP that reached its lowest level during the heart failure phase. The MAP during the development of the necrosis phase of HCM was accompanied by a significant increase in the level of the sodium-hydrogen exchanger, NHE1. Treatments with the potent NHE1 inhibitor, EMD 87580 (rimeporide), did not affect MAP of NH. However, treatments with EMD 87580 during the three phases of the development of HCM significantly reversed the hypotension associated with HCM.Our results showed that the development of HCM is associated with hypotension. These results suggest that a decrease in blood pressure could be a biomarker signal for HCM leading to HF and early death. Since the blockade of NHE1 significantly but partially prevented the reduction in MAP, this suggests that other mechanisms can contribute to the development of hypotension in HCM.

Diabetes, Heart Failure and Beyond: Elucidating the Cardioprotective Mechanisms of Sodium Glucose Cotransporter 2 (SGLT2) Inhibitors

Approximately 5 million individuals in the US are living with congestive heart failure (CHF), with 650,000 new cases being diagnosed every year. CHF has a multifactorial etiology, ranging from coronary artery disease, hypertension, valvular abnormalities and diabetes mellitus. Currently, guidelines by the American College of Cardiology advocate the use of angiotensin-converting enzyme (ACE) inhibitors, angiotensin II receptor blockers, β-blockers, diuretics, aldosterone antagonists, and inotropes for the medical management of heart failure. The sodium glucose cotransporter 2 (SGLT2) inhibitors are a class of drug that have been widely used in the management of type 2 diabetes mellitus that work by inhibiting the reabsorption of glucose in the proximal convoluted tubule. Since the EMPA-REG OUTCOME trial, several studies have demonstrated the benefits of SGLT2 inhibitors in reducing cardiovascular risk related to heart failure. While the cardiovascular benefits could be explained by their ability to reduce weight, improve glycemic index and lower blood pressure, several recent trials have suggested that SGLT2 inhibitors exhibit pleiotropic effects that underlie their cardioprotective properties. These findings have led to an expansion in preclinical and clinical research aiming to understand the mechanisms by which SGLT2 inhibitors improve heart failure outcomes.

Crosstalk between Sodium-Glucose Cotransporter Inhibitors and Sodium-Hydrogen Exchanger 1 and 3 in Cardiometabolic Diseases

Abnormality in glucose homeostasis due to hyperglycemia or insulin resistance is the hallmark of type 2 diabetes mellitus (T2DM). These metabolic abnormalities in T2DM lead to cellular dysfunction and the development of diabetic cardiomyopathy leading to heart failure. New antihyperglycemic agents including glucagon-like peptide-1 receptor agonists and the sodium-glucose cotransporter-2 inhibitors (SGLT2i) have been shown to attenuate endothelial dysfunction at the cellular level. In addition, they improved cardiovascular safety by exhibiting cardioprotective effects. The mechanism by which these drugs exert their cardioprotective effects is unknown, although recent studies have shown that cardiovascular homeostasis occurs through the interplay of the sodium-hydrogen exchangers (NHE), specifically NHE1 and NHE3, with SGLT2i. Another theoretical explanation for the cardioprotective effects of SGLT2i is through natriuresis by the kidney. This theory highlights the possible involvement of renal NHE transporters in the management of heart failure. This review outlines the possible mechanisms responsible for causing diabetic cardiomyopathy and discusses the interaction between NHE and SGLT2i in cardiovascular diseases.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Proanthocyanidins Maintain Cardiac Ionic Homeostasis in Aldosterone-Induced Hypertension and Heart Failure

Excess aldosterone promotes pathological remodeling of the heart and imbalance in cardiac ion homeostasis of sodium, potassium and calcium. Novel treatment with proanthocyanidins in aldosterone-treated rats has resulted in downregulation of cardiac SGK1, the main genomic aldosterone-induced intracellular mediator of ion handling. It therefore follows that proanthocyanidins could be modulating cardiac ion homeostasis in aldosterone-treated rats. Male Wistar rats received aldosterone (1 mg kg⁻¹ day⁻¹) +1% NaCl for three weeks. Half of the animals in each group were simultaneously treated with the proanthocyanidins-rich extract (80% w/w) (PRO80, 5 mg kg⁻¹ day⁻¹). PRO80 prevented cardiac hypertrophy and decreased calcium content. Expression of ion channels (ROMK, NHE1, NKA and NCX1) and calcium transient mediators (CAV1.2, pCaMKII and oxCaMKII) were reduced by PRO80 treatment in aldosterone-treated rats. To conclude, our data indicate that PRO80 may offer an alternative treatment to conventional MR-blockade in the prevention of aldosterone-induced cardiac pathology.

The Na/K-ATPase Signaling and SGLT2 Inhibitor-Mediated Cardiorenal Protection: A Crossed Road?

In different large-scale clinic outcome trials, sodium (Na⁺)/glucose co-transporter 2 (SGLT2) inhibitors showed profound cardiac- and renal-protective effects, making them revolutionary treatments for heart failure and kidney disease. Different theories are proposed according to the emerging protective effects other than the original purpose of glucose-lowering in diabetic patients. As the ATP-dependent primary ion transporter providing the Na⁺ gradient to drive other Na⁺-dependent transporters, the possible role of the sodium–potassium adenosine triphosphatase (Na/K-ATPase) as the primary ion transporter and its signaling function is not explored.

Cardiac Late Sodium Channel Current Is a Molecular Target for the Sodium/Glucose Cotransporter 2 Inhibitor Empagliflozin

BACKGROUND

SGLT2 (sodium/glucose cotransporter 2) inhibitors exert robust cardioprotective effects against heart failure in patients with diabetes, and there is intense interest to identify the underlying molecular mechanisms that afford this protection. Because the induction of the late component of the cardiac sodium channel current (late-INa) is involved in the etiology of heart failure, we investigated whether these drugs inhibit late-INa.

METHODS

Electrophysiological, in silico molecular docking, molecular, calcium imaging, and whole heart perfusion techniques were used to address this question.

RESULTS

The SGLT2 inhibitor empagliflozin reduced late-INa in cardiomyocytes from mice with heart failure and in cardiac Nav1.5 sodium channels containing the long QT syndrome 3 mutations R1623Q or ΔKPQ. Empagliflozin, dapagliflozin, and canagliflozin are all potent and selective inhibitors of H2O2-induced late-INa (half maximal inhibitory concentration = 0.79, 0.58, and 1.26 µM, respectively) with little effect on peak sodium current. In mouse cardiomyocytes, empagliflozin reduced the incidence of spontaneous calcium transients induced by the late-INa activator veratridine in a similar manner to tetrodotoxin, ranolazine, and lidocaine. The putative binding sites for empagliflozin within Nav1.5 were investigated by simulations of empagliflozin docking to a three-dimensional homology model of human Nav1.5 and point mutagenic approaches. Our results indicate that empagliflozin binds to Nav1.5 in the same region as local anesthetics and ranolazine. In an acute model of myocardial injury, perfusion of isolated mouse hearts with empagliflozin or tetrodotoxin prevented activation of the cardiac NLRP3 (nuclear-binding domain-like receptor 3) inflammasome and improved functional recovery after ischemia.

CONCLUSIONS

Our results provide evidence that late-INa may be an important molecular target in the heart for the SGLT2 inhibitors, contributing to their unexpected cardioprotective effects.

Shift of conventional paradigm of heart failure treatment: from angiotensin receptor neprilysin inhibitor to sodium-glucose co-transporter 2 inhibitors?

Current clinical guidelines for heart failure (HF) contain a brand new therapeutic strategy for HF with reduced ejection fraction (HFrEF), which is based on neurohumoral modulation through the use of angiotensin receptor neprilysin inhibitors. There is a large body of evidence for the fact that sodium-glucose co-transporter 2 inhibitors may significantly improve all-cause mortality, cardiovascular mortality and hospitalization for HF in patients with HFrEF who received renin-angiotensin system blockers including angiotensin receptor neprilysin inhibitors, β-blockers and mineralocorticoid receptor antagonists. The review discusses that sodium-glucose co-transporter 2 inhibitors have a wide spectrum of favorable molecular effects and contribute to tissue protection, improving survival in HFrEF patients.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Thioredoxin 1 (TRX1) Overexpression Cancels the Slow Force Response (SFR) Development

The stretch of cardiac muscle increases developed force in two phases. The first phase occurs immediately after stretch and is the expression of the Frank–Starling mechanism, while the second one or slow force response (SFR) occurs gradually and is due to an increase in the calcium transient amplitude. An important step in the chain of events leading to the SFR generation is the increased production of reactive oxygen species (ROS) leading to redox sensitive ERK1/2, p90RSK, and NHE1 phosphorylation/activation. Conversely, suppression of ROS production blunts the SFR. The purpose of this study was to explore whether overexpression of the ubiquitously expressed antioxidant molecule thioredoxin-1 (TRX1) affects the SFR development and NHE1 phosphorylation. We did not detect any change in basal phopho-ERK1/2, phopho-p90RSK, and NHE1 expression in mice with TRX1 overexpression compared to wild type (WT). Isolated papillary muscles from WT or TRX1-overexpressing mice were stretched from 92 to 98% of its maximal length. A prominent SFR was observed in WT mice that was completely canceled in TRX1 animals. Interestingly, myocardial stretch induced a significant increase in NHE1 phosphorylation in WT mice that was not detected in TRX1-overexpressing mice. These novel results suggest that magnification of cardiac antioxidant defense power by overexpression of TRX1 precludes NHE1 phosphorylation/activation after stretch, consequently blunting the SFR development.

Cardiac Mineralocorticoid Receptor and the Na+/H+ Exchanger: Spilling the Beans

Current evidence reveals that cardiac mineralocorticoid receptor (MR) activation following myocardial stretch plays an important physiological role in adapting developed force to sudden changes in hemodynamic conditions. Its underlying mechanism involves a previously unknown nongenomic effect of the MR that triggers redox-mediated Na⁺/H⁺ exchanger (NHE1) activation, intracellular Na⁺ accumulation, and a consequent increase in Ca²⁺ transient amplitude through reverse Na⁺/Ca²⁺ exchange. However, clinical evidence assigns a detrimental role to MR activation in the pathogenesis of severe cardiac diseases such as congestive heart failure. This mini review is meant to present and briefly discuss some recent discoveries about locally triggered cardiac MR signals with the objective of shedding some light on its physiological but potentially pathological consequences in the heart.

Cardiac Complications of Propionic and Other Inherited Organic Acidemias

Clinical observations and experimental studies have determined that systemic acid-base disturbances can profoundly affect the heart. A wealth of information is available on the effects of altered pH on cardiac function but, by comparison, much less is known about the actions of the organic anions that accumulate alongside H⁺ ions in acidosis. In the blood and other body fluids, these organic chemical species can collectively reach concentrations of several millimolar in severe metabolic acidoses, as in the case of inherited organic acidemias, and exert powerful biological actions on the heart that are not intuitive to predict. Indeed, cardiac pathologies, such as cardiomyopathy and arrhythmia, are frequently reported in organic acidemia patients, but the underlying pathophysiological mechanisms are not well established. Research efforts in the area of organic anion physiology have increased dramatically in recent years, particularly for propionate, which accumulates in propionic acidemia, one of the commonest organic acidemias characterized by a high incidence of cardiac disease. This Review provides a comprehensive historical overview of all known organic acidemias that feature cardiac complications and a state-of-the-art overview of the cardiac sequelae observed in propionic acidemia. The article identifies the most promising candidates for molecular mechanisms that become aberrantly engaged by propionate anions (and its metabolites), and discusses how these may result in cardiac derangements in propionic acidemia. Key clinical and experimental findings are considered in the context of potential therapies in the near future.

Role of Genetic Mutations of the Na+/H+ Exchanger Isoform 1, in Human Disease and Protein Targeting and Activity

The mammalian Na⁺/H⁺ exchanger isoform one (NHE1) is a plasma membrane protein that is ubiquitously present in human cells. It functions to regulate intracellular pH removing an intracellular proton in exchange for one extracellular sodium and is involved in heart disease and in promoting metastasis in cancer. It is made of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. The membrane domain is thought to have 12 transmembrane segments and a large membrane-associated extracellular loop. Early studies demonstrated that in mice, disruption of the NHE1 gene results in locomotor ataxia and a phenotype of slow-wave epilepsy. Defects included a progressive neuronal degeneration. Growth and reproductive ability were also reduced. Recent studies have identified human autosomal homozygous recessive mutations in the NHE1 gene (SLC9A1) that result in impaired development, ataxia and other severe defects, and explain the cause of the human disease Lichtenstein-Knorr syndrome. Other human mutations have been identified that are stop codon polymorphisms. These cause short non-functional NHE1 proteins, while other genetic polymorphisms in the NHE1 gene cause impaired expression of the NHE1 protein, reduced activity, enhanced protein degradation or altered kinetic activation of the protein. Since NHE1 plays a key role in many human physiological functions and in human disease, genetic polymorphisms of the protein that significantly alter its function and are likely play significant roles in varying human phenotypes and be involved in disease.

Cardiac up-regulation of NBCe1 emerges as a beneficial consequence of voluntary wheel running in mice

Physical training stimulates the development of physiologic cardiac hypertrophy (CH), being a key event in this process the inhibition of the Na⁺/H⁺ exchanger. However, the role of the sodium bicarbonate cotransporter (NBC) has not been explored yet under this circumstance. C57/Bl6 mice were allowed to voluntary exercise (wheel running) for five weeks. Cardiac mass was evaluated by echocardiography and histomorphometry detecting that training promoted the development of physiological CH (heart weight/tibia length ratio, mg/mm: 6.54 ± 0.20 vs 8.81 ± 0.24; interstitial collagen content, %: 3.14 ± 0.63 vs. 1.57 ± 0.27; and cross-sectional area of cardiomyocytes, μm²: 200.6 ± 8.92 vs. 281.9 ± 24.05; sedentary (Sed) and exercised (Ex) mice, respectively). The activity of the electrogenic isoform of the cardiac NBC (NBCe1) was estimated by recording intracellular pH under high potassium concentration and by measuring action potential duration (APD). NBCe1 activity was significantly increased in isolated cardiomyocytes of trained mice. Additionally, the APD was shorter and the alkalization due to high extracellular potassium-induced depolarization was greater in this group, indicating that the NBCe1 was hyperactive. These results are online with the observed myocardial up-regulation of the NBCe1 (Western Blot, %: 100 ± 13.86 vs. 202 ± 29.98; Sed vs. Ex, n = 6 each group). In addition, we detected a reduction in H₂O₂ production in the myocardium of trained mice. These results support that voluntary training induces the development of physiologic CH with up-regulation of the cardiac NBCe1 in mice. Furthermore, the improvement in the antioxidant capacity contributes to the beneficial cardiovascular consequences of physical training.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Protective Mechanism and Clinical Application of Hydrogen in Myocardial Ischemia-reperfusion Injury

Cardiovascular disease accounts for one-third of all deaths, with ischemic heart disease as the main cause of death. Under pathological conditions, ischemia-reperfusion injury (IRI) often occurs in tissues. Ischemic injury is mainly caused by anaerobic cell death and reperfusion which results in a wide range of inflammatory responses. These responses are able to increase tissue damage and even damage to the whole body. IRI can also aggravate the original cardiovascular disease during the treatment of cardiovascular disease. Therefore, it is particularly important to understand the mechanism of myocardial ischemia-reperfusion injury (MIRI) for clinical treatment and application. At the same time, it is necessary to find a safe, reliable and feasible method for treating MIRI to reduce the incidence of complications and mortality as well as improve the prognosis and quality of life of patients. As a selective antioxidant, hydrogen can neutralize excessive free radicals, has certain anti-apoptotic and anti-inflammatory effects and it has gradually become a focus and hotspot of preclinical and clinical research. Hydrogen has been shown to have a certain therapeutic effect on MIRI, which can provide a new therapeutic direction for the clinical treatment of myocardial ischemia-reperfusion injury. In this review, the protective mechanism and clinical application of hydrogen in myocardial ischemia-reperfusion injury is discussed.

Calcineurin

The serine/threonine phosphatase calcineurin acts as a crucial connection between calcium signaling the phosphorylation states of numerous important substrates. These substrates include, but are not limited to, transcription factors, receptors and channels, proteins associated with mitochondria, and proteins associated with microtubules. Calcineurin is activated by increases in intracellular calcium concentrations, a process that requires the calcium sensing protein calmodulin binding to an intrinsically disordered regulatory domain in the phosphatase. Despite having been studied for around four decades, the activation of calcineurin is not fully understood. This review largely focuses on what is known about the activation process and highlights aspects that are currently not understood. Video abstract.

Pathological role of ion channels and transporters in the development and progression of triple-negative breast cancer

Breast cancer is a common malignancy in women. Among breast cancer types, triple-negative breast cancer (TNBC) tends to affect younger women, is prone to axillary lymph node, lung, and bone metastases; and has a high recurrence rate. Due to a lack of classic biomarkers, the currently available treatments are surgery and chemotherapy; no targeted standard treatment options are available. Therefore, it is urgent to find a novel and effective therapeutic target. As alteration of ion channels and transporters in normal mammary cells may affect cell growth, resulting in the development and progression of TNBC, ion channels and transporters may be promising new therapeutic targets for TNBC. This review summarizes ion channels and transporters related to TNBC and may provide new tumor biomarkers and help in the development of novel targeted therapies.

Mechanisms of Cardiovascular Benefits of Sodium Glucose Co-Transporter 2 (SGLT2) Inhibitors: A State-of-the-Art Review

Recent clinical trials have shown that sodium glucose co-transport 2 (SGLT2) inhibitors have dramatic beneficial cardiovascular outcomes. These include a reduced incidence of cardiovascular death and heart failure hospitalization in people with and without diabetes, and those with and without prevalent heart failure. The actual mechanism(s) responsible for these beneficial effects are not completely clear. Several potential theses have been proposed to explain the cardioprotective effects of SGLT2 inhibition, which include diuresis/natriuresis, blood pressure reduction, erythropoiesis, improved cardiac energy metabolism, inflammation reduction, inhibition of the sympathetic nervous system, prevention of adverse cardiac remodeling, prevention of ischemia/reperfusion injury, inhibition of the Na+/H+-exchanger, inhibition of SGLT1, reduction in hyperuricemia, increasing autophagy and lysosomal degradation, decreasing epicardial fat mass, increasing erythropoietin levels, increasing circulating pro-vascular progenitor cells, decreasing oxidative stress, and improving vascular function. The strengths and weaknesses of these proposed mechanisms are reviewed in an effort to try to synthesize and prioritize the mechanisms as they relate to clinical event reduction.

Overexpression of Na+/H+ exchanger 1 specifically induces cell death in human iPS cells via sustained activation of the Rho kinase ROCK

Understanding the specific properties of human induced pluripotent stem cells (iPSCs) is important for quality control of iPSCs. Having incidentally discovered that overexpression of plasma membrane Na⁺/H⁺ exchanger 1 (NHE1) induces cell death in iPSCs, we investigated the mechanism of NHE1-induced cell death. Doxycycline-induced NHE1 overexpression arrested cell growth, and nearly all cells were killed by a necrotic process within 72 h. NHE1 overexpression led to sustained activation of Rho-associated coiled-coil kinase (ROCK), accompanied by dramatic changes in cell shape, cell elongation, and swelling of peripheral cells in iPSC colonies, as well as marked stress fiber formation. The ROCK inhibitor Y27632 reduced NHE1-induced cell death. ROCK-dependent phenotypes were suppressed by a loss-of-function mutation of NHE1 and inhibited by an inhibitor of NHE1 activity, indicating that NHE1-mediated transport activity is required. Moreover, ROCK was activated by trimethylamine treatment-mediated cytosolic alkalinization and accumulated in the plasma membrane near NHE1 in peripheral iPSCs of cell colonies. By contrast, cell death did not occur in mesendoderm-like cells that had differentiated from iPSCs, indicating that the NHE1-mediated effects were specific for iPSCs. These results suggest that NHE1 overexpression specifically induces death of iPSCs via sustained ROCK activation, probably caused by an increase in local pH near NHE1. Finally, monensin, a Na⁺/H⁺ exchange ionophore, selectively killed iPSCs, suggesting that monensin could help eliminate iPSCs that remain after differentiation, a strategy that might be useful for improving regenerative medicine.

Na+/H+ exchanger and cardiac hypertrophy

Reactive cardiac hypertrophy (CH) is an increase in heart mass in response to hemodynamic overload. Exercise-induced CH emerges as an adaptive response with improved cardiac function, in contrast to pathological CH that represents a risk factor for cardiovascular health. The Na⁺/H⁺ exchanger (NHE-1) is a membrane transporter that not only regulates intracellular pH but also intracellular Na⁺ concentration. In the scenario of cardiovascular diseases, myocardial NHE-1 is activated by a variety of stimuli, such as neurohumoral factors and mechanical stress, leading to intracellular Na⁺ overload and activation of prohypertrophic cascades. NHE-1 hyperactivity is intimately linked to heart diseases, including ischemia-reperfusion injury, maladaptive CH and heart failure. In this review, we will present evidence to support that the NHE-1 hyperactivity constitutes a "switch on/off" for the pathological phenotype during CH development. We will also discuss some classical and novel strategies to avoid NHE-1 hyperactivity, and that are therefore worthwhile to improve cardiovascular health.

Structural and Functional Changes in the Na+/H+ Exchanger Isoform 1, Induced by Erk1/2 Phosphorylation

The human Na⁺/H⁺ exchanger isoform 1 (NHE1) is a plasma membrane transport protein that plays an important role in pH regulation in mammalian cells. Because of the generation of protons by intermediary metabolism as well as the negative membrane potential, protons accumulate within the cytosol. Extracellular signal-regulated kinase (ERK)-mediated regulation of NHE1 is important in several human pathologies including in the myocardium in heart disease, as well as in breast cancer as a trigger for growth and metastasis. NHE1 has a N-terminal, a 500 amino acid membrane domain, and a C-terminal 315 amino acid cytosolic domain. The C-terminal domain regulates the membrane domain and its effects on transport are modified by protein binding and phosphorylation. Here, we discuss the physiological regulation of NHE1 by ERK, with an emphasis on the critical effects on structure and function. ERK binds directly to the cytosolic domain at specific binding domains. ERK also phosphorylates NHE1 directly at multiple sites, which enhance NHE1 activity with subsequent downstream physiological effects. The NHE1 cytosolic regulatory tail possesses both ordered and disordered regions, and the disordered regions are stabilized by ERK-mediated phosphorylation at a phosphorylation motif. Overall, ERK pathway mediated phosphorylation modulates the NHE1 tail, and affects the activity, structure, and function of this membrane protein.

Calcineurin B homologous protein 3 binds with high affinity to the CHP binding domain of the human sodium/proton exchanger NHE1

The Na+/H+ exchanger NHE1 is critical for cell vitality as it controls intracellular pH and cell volume. Its functionality is influenced by calcineurin B homologous proteins (CHPs). The human isoform CHP3 is important for transport of NHE1 to the plasma membrane and for its activity. Here, we characterized the binding interaction of human CHP3 with the regulatory domain of NHE1. The exact binding site of CHP3 was previously debated. CHP3 as well as both regions of NHE1 in question were produced and purified. CHP3 specifically formed stable complexes with the CHP-binding region (CBD) of NHE1 (residues 503-545) in size-exclusion chromatography (SEC), but not with the C-terminal region (CTD, residues 633-815). CTD was functional as shown by Ca2+-dependent binding of calmodulin in SEC analysis. CHP3 bound with high affinity to CBD with an equilibrium dissociation constant (KD) of 56 nM determined by microscale thermophoresis. The high affinity was substantiated by isothermal calorimetry analysis (KD = 3 nM), which also revealed that the interaction with CBD is strongly exothermic (ΔG° = -48.6 kJ/mol, ΔH = -75.3 kJ/mol, -TΔS° = 26.7 kJ/mol). The data provide insights in the molecular mechanisms that underlie the regulatory interaction of CHP3 and NHE1 and more general of calcineurin homologous proteins with their target proteins.

Cardioprotective role of IGF-1 in the hypertrophied myocardium of the spontaneously hypertensive rats: A key effect on NHE-1 activity

AIM

Myocardial Na⁺/H⁺ exchanger-1 (NHE-1) hyperactivity and oxidative stress are interrelated phenomena playing pivotal roles in the development of pathological cardiac hypertrophy and heart failure. Exercise training is effective to convert pathological into physiological hypertrophy in the spontaneously hypertensive rats (SHR), and IGF-1-key humoral mediator of exercise training-inhibits myocardial NHE-1, at least in normotensive rats. Therefore, we hypothesize that IGF-1 by hampering NHE-1 hyperactivity and oxidative stress should exert a cardioprotective effect in the SHR.

METHODS

NHE-1 activity [proton efflux ( J H + ) mmol L^-1 min^-1], expression and phosphorylation; H₂O₂ production; superoxide dismutase (SOD) activity; contractility and calcium transients were measured in SHR hearts in the presence/absence of IGF-1.

RESULTS

IGF-1 significantly decreased NHE-1 activity ( J H + at pH_i 6.95: 1.39 ± 0.32, n = 9 vs C 3.27 ± 0.3, n = 20, P < .05); effect prevented by AG1024, an antagonist of IGF-1 receptor (2.7 ± 0.4, n = 7); by the PI3K inhibitor wortmannin (3.14 ± 0.41, n = 7); and the AKT inhibitor MK2206 (3.37 ± 0.43, n = 14). Moreover, IGF-1 exerted an antioxidant effect revealed by a significant reduction in H₂O₂ production accompanied by an increase in SOD activity. In addition, IGF-1 improved cardiomyocyte contractility as evidenced by an increase in sarcomere shortening and a decrease in the relaxation constant, underlined by an increase in the amplitude and rate of decay of the calcium transients.

CONCLUSION

IGF-1 exerts a cardioprotective role on the hypertrophied hearts of the SHR, in which the inhibition of NHE-1 hyperactivity, as well as the positive inotropic and antioxidant effects, emerges as key players.

Exploring miRNA-mRNA regulatory network in cardiac pathology in Na+/H+ exchanger isoform 1 transgenic mice

Numerous studies have demonstrated that Na⁺/H⁺ exchanger isoform 1 (NHE1) is elevated in myocardial diseases and its effect is detrimental. To better understand the involvement of NHE1, we have previously studied cardiac-specific NHE1 transgenic mice and shown that these mice develop cardiac hypertrophy, interstitial fibrosis, and cardiac dysfunction. The purpose of current study was to identify microRNAs and their mRNA targets involved in NHE1-mediated cardiac injury. An unbiased high-throughput sequencing study was performed on both microRNAs and mRNAs. RNA sequencing showed that differentially expressed genes were enriched in hypertrophic cardiomyopathy pathway by Kyoto Encyclopedia of Genes and Genomes annotation in NHE1 transgenic hearts. These genes were classified as contraction defects (e.g., Myl2, Myh6, Mybpc3, and Actb), impaired intracellular Ca²⁺ homeostasis (e.g., SERCA2a, Ryr2, Rcan1, and CaMKII delta), and signaling molecules for hypertrophic cardiomyopathy (e.g., Itga/b, IGF-1, Tgfb2/3, and Prkaa1/2). microRNA sequencing revealed that 15 microRNAs were differentially expressed (2-fold, P < 0.05). Six of them (miR-1, miR-208a-3p, miR-199a-5p, miR-21-5p, miR-146a-5p, and miR-30c-5p) were reported to be related to cardiac pathological functions. The integrative analysis of microRNA and RNA sequencing data identified several crucial microRNAs including miR-30c-5p, miR-199a-5p, miR-21-5p, and miR-34a-5p as well as 10 of their mRNA targets that may affect the heart via NFAT hypertrophy and cardiac hypertrophy signaling. Furthermore, important microRNAs and mRNA targets were validated by quantitative PCR. Our study comprehensively characterizes the expression patterns of microRNAs and mRNAs, establishes functional microRNA-mRNA pairs, elucidates the potential signaling pathways, and provides novel insights on the mechanisms underlying NHE1-medicated cardiac injury.

Angiotensin II-induced podocyte apoptosis is mediated by endoplasmic reticulum stress/PKC-δ/p38 MAPK pathway activation and trough increased Na+/H+ exchanger isoform 1 activity

Background

Angiotensin II (Ang II) contributes to the progression of renal diseases associated with proteinuria and glomerulosclerosis mainly by inducing podocyte apoptosis. In the present study, we investigated whether the chronic effects of Ang II via AT1 receptor (AT1R) would result in endoplasmic reticulum (ER) stress/PKC-delta/p38 MAPK stimulation, and consequently podocyte apoptosis.

Methods

Wistar rats were treated with Ang II (200 ng·kg⁻¹·min⁻¹, 42 days) and or losartan (10 mg·kg⁻¹·day⁻¹, 14 days). Immortalized mouse podocyte were treated with 1 μM Ang II and/or losartan (1 μM) or SB203580 (0.1 μM) (AT1 receptor antagonist and p38 MAPK inhibitor) for 24 h. Kidney sections and cultured podocytes were used to evaluate protein expression by immunofluorescence and immunoblotting. Apoptosis was evaluated by flow cytometry and intracellular pH (pHi) was analyzed using microscopy combined with the fluorescent probe BCECF/AM.

Results

Compared with controls, Ang II via AT1R increased chaperone GRP 78/Bip protein expression in rat glomeruli (p < 0.001) as well as in podocyte culture (p < 0.01); increased phosphorylated eIf2-α (p < 0.05), PKC-delta (p < 0.01) and p38 MAPK (p < 0.001) protein expression. Furthermore, Ang II induced p38 MAPK-mediated late apoptosis and increased the Bax/Bcl-2 ratio (p < 0.001). Simultaneously, Ang II via AT1R induced p38 MAPK-NHE1-mediated increase of pHi recovery rate after acid loading.

Conclusion

Together, our results indicate that Ang II-induced podocyte apoptosis is associated with AT1R/ER stress/PKC-delta/p38 MAPK axis and enhanced NHE1-mediated pHi recovery rate.

A polyclonal antibody against extracellular loops 1 of chNHE1 blocks avian leukosis virus subgroup J infection

Avian leukosis virus subgroup J (ALV-J), an oncogenic retrovirus, induces myelocytomas and other various tumors, leading to great economical losses in poultry industry. It is a great challenge to develop effective preventive methods for ALV-J control due to its antigenic variations in the variable regions of envelope. In present study, we generated a mouse polyclonal antibody targeting the first extracellular loop (ECL1) of chicken Na⁺/H⁺ exchanger isoform 1 (chNHE1), the receptor of ALV-J, to block ALV-J infection in vitro and in vivo. In ALV-J infected DF-1 cells, chNHE1 expression and the intracellular pH (pHi) were up-regulated with "wave" pattern, indicating that the disequilibrium of ALV-J infected cells associated with chNHE1. Next, we validated that ALV-J infection was significantly blocked with time dependent after treating with anti-ECL1 antibody and accordingly the pHi value were recovered, indicating the blockage of ALV-J infection did not affect Na⁺/H⁺ exchange. Furthermore, in anti-ECL1 antibody treatment chickens that infected by ALV-J, weight gain and immune organs were recovered, and viral loads were significantly decreased, and the tissue injury and inflammation were reduced significantly from 21 to 35 days of age. The study demonstrated that anti-ECL1 antibody effectively blocks ALV-J infection without affecting Na⁺/H⁺ exchange, and sheds light on a novel strategy for retroviruses control.

The role of mitochondria in cardiac development and protection

Mitochondria are essential for the development as well as maintenance of the myocardium, the most energy consuming tissue in the human body. Mitochondria are not only a source of ATP energy but also generators of reactive oxygen species (ROS), that cause oxidative damage, but also regulate physiological processes such as the switch from hyperplastic to hypertrophic growth after birth. As excess ROS production and oxidative damage are associated with cardiac pathology, it is not surprising that much of the research focused on the deleterious aspects of free radicals. However, cardiomyocytes are naturally highly adapted against repeating oxidative insults, with evidence suggesting that moderate and acute ROS exposure has beneficial consequences for mitochondrial maintenance and cardiac health. Antioxidant defenses, mitochondrial quality control, mtDNA maintenance mechanisms as well as mitochondrial fusion and fission improve mitochondrial function and cardiomyocyte survival under stress conditions. As these adaptive processes can be induced, promoting mitohormesis or mitochondrial biogenesis using controlled ROS exposure could provide a promising strategy to increase cardiomyocyte survival and prevent pathological remodeling of the myocardium.