Deciphering the mechanisms of the Na+/H+ exchanger-3 regulation in organ dysfunction

The potential anti-arrhythmic effect of SGLT2 inhibitors

Sodium-glucose cotransporter type 2 inhibitors (SGLT2i) were initially recommended as oral anti-diabetic drugs to treat type 2 diabetes (T2D), by inhibiting SGLT2 in proximal tubule and reduce renal reabsorption of sodium and glucose. While many clinical trials demonstrated the tremendous potential of SGLT2i for cardiovascular diseases. 2022 AHA/ACC/HFSA guideline first emphasized that SGLT2i were the only drug class that can cover the entire management of heart failure (HF) from prevention to treatment. Subsequently, the antiarrhythmic properties of SGLT2i have also attracted attention. Although there are currently no prospective studies specifically on the anti-arrhythmic effects of SGLT2i. We provide clues from clinical and fundamental researches to identify its antiarrhythmic effects, reviewing the evidences and mechanism for the SGLT2i antiarrhythmic effects and establishing a novel paradigm involving intracellular sodium, metabolism and autophagy to investigate the potential mechanisms of SGLT2i in mitigating arrhythmias.

SGLT2 inhibitors: from glucose-lowering to cardiovascular benefits

An increasing number of individuals are at high risk of type 2 diabetes (T2D) and its cardiovascular complications, including heart failure (HF), chronic kidney disease (CKD), and eventually premature death. The sodium-glucose co-transporter-2 (SGLT2) protein sits in the proximal tubule of human nephrons to regulate glucose reabsorption and its inhibition by gliflozins represents the cornerstone of contemporary T2D and HF management. Herein, we aim to provide an updated overview of the pleiotropy of gliflozins, provide mechanistic insights and delineate related cardiovascular (CV) benefits. By discussing contemporary evidence obtained in preclinical models and landmark randomized controlled trials, we move from bench to bedside across the broad spectrum of cardio- and cerebrovascular diseases. With landmark randomized controlled trials confirming a reduction in major adverse CV events (MACE; composite endpoint of CV death, non-fatal myocardial infarction, and non-fatal stroke), SGLT2 inhibitors strongly mitigate the risk for heart failure hospitalization in diabetics and non-diabetics alike while conferring renoprotection in specific patient populations. Along four major pathophysiological axes (i.e. at systemic, vascular, cardiac, and renal levels), we provide insights into the key mechanisms that may underlie their beneficial effects, including gliflozins' role in the modulation of inflammation, oxidative stress, cellular energy metabolism, and housekeeping mechanisms. We also discuss how this drug class controls hyperglycaemia, ketogenesis, natriuresis, and hyperuricaemia, collectively contributing to their pleiotropic effects. Finally, evolving data in the setting of cerebrovascular diseases and arrhythmias are presented and potential implications for future research and clinical practice are comprehensively reviewed.

GLP-1 Receptor Agonist Improves Mitochondrial Energy Status and Attenuates Nephrotoxicity In Vivo and In Vitro

High-sugar and high-fat diets cause significant harm to health, especially via metabolic diseases. In this study, the protective effects of the antidiabetic drug exenatide (synthetic exendin-4), a glucagon-like peptide 1 (GLP-1) receptor agonist, on high-fat and high-glucose (HFHG)-induced renal injuries were investigated in vivo and in vitro. In vivo and in vitro renal injury models were established. Metabolomic analysis based on 1H-nuclear magnetic resonance was performed to examine whether exenatide treatment exerts a protective effect against kidney injury in diabetic rats and to explore its potential molecular mechanism. In vivo, 8 weeks of exenatide treatment resulted in the regulation of most metabolites in the diabetes mellitus group. In vitro results showed that exendin-4 restored the mitochondrial functions of mesangial cells, which were perturbed by HFHG. The effects of exendin-4 included the improved antioxidant capacity of mesangial cells, increased the Bcl-2/Bax ratio, and reduced protein expression of cyt-c and caspase-3 activation. In addition, exendin-4 restored mesangial cell energy metabolism by increasing succinate dehydrogenase and phosphofructokinase activities and glucose consumption while inhibiting pyruvate dehydrogenase E1 activity. In conclusion, GLP-1 agonists improve renal injury in diabetic rats by ameliorating metabolic disorders. This mechanism could be partially related to mitochondrial functions and energy metabolism.

Empagliflozin and Renal Sodium-Hydrogen Exchange in Healthy Subjects

CONTEXT

Sodium glucose co-transporter-2 inhibitors exert clinically relevant cardiorenal protection. Among several mechanisms, inhibition of sodium-hydrogen exchanger-3 (NHE3) in proximal renal tubules has been proposed in rodents. Demonstration of this mechanism with the associated electrolyte and metabolic changes in humans is lacking.

OBJECTIVE

The present proof-of-concept study was designed to explore the involvement of NHE3 in modulating the response to sodium glucose co-transporter-2 inhibitors in humans.

METHODS

Twenty healthy male volunteers received 2 tablets of empagliflozin 25 mg during a standardized hydration scheme; freshly voided urines and blood samples were collected at timed intervals for 8 hours. Protein expression of relevant transporters was examined in exfoliated tubular cells.

RESULTS

Urine pH levels increased after empagliflozin (from 5.81 ± 0.5 to 6.16 ± 0.6 at 6 hours, P = .008) as did urinary output (from median, 1.7; interquartile range [IQR, 0.6; 2.5] to 2.5 [IQR, 1.7; 3.5] mL/min-1, P = .008) and glucose (from median, 0.03 [IQR, 0.02; 0.04] to 34.8 [IQR, 31.6; 40.2] %, P < .0001), and sodium fractional excretion rates (from median, 0.48 [IQR, 0.34; 0.65] to 0.71 [IQR, 0.55; 0.85] %, P = .0001), whereas plasma glucose and insulin concentrations decreased and plasma and urinary ketones increased. Nonsignificant changes in NHE3, phosphorylated NHE3, and membrane-associated protein 17 protein expression were detected in urinary exfoliated tubular cells. In a time-control study in 6 participants, neither urine pH nor plasma and urinary parameters changed.

CONCLUSIONS

In healthy young volunteers, empagliflozin acutely increases urinary pH while inducing a substrate shift toward lipid utilization and ketogenesis, without significant changes in renal NHE3 protein expression.

Fluxomics reveals cellular and molecular basis of increased renal ammoniagenesis

The kidney plays a critical role in excreting ammonia during metabolic acidosis and liver failure. The mechanisms behind this process have been poorly explored. The present study combines results of in vivo experiments of increased total ammoniagenesis with systems biology modeling, in which eight rats were fed an amino acid-rich diet (HD group) and eight a normal chow diet (AL group). We developed a method based on elementary mode analysis to study changes in amino acid flux occurring across the kidney in increased ammoniagenesis. Elementary modes represent minimal feasible metabolic paths in steady state. The model was used to predict amino acid fluxes in healthy and pre-hyperammonemic conditions, which were compared to experimental fluxes in rats. First, we found that total renal ammoniagenesis increased from 264 ± 68 to 612 ± 87 nmol (100 g body weight)^-1 min^-1 in the HD group (P = 0.021) and a concomitated upregulation of NKCC2 ammonia and other transporters in the kidney. In the kidney metabolic model, the best predictions were obtained with ammonia transport as an objective. Other objectives resulting in a fair correlation with the measured fluxes (correlation coefficient >0.5) were growth, protein uptake, urea excretion, and lysine and phenylalanine transport. These predictions were improved when specific gene expression data were considered in HD conditions, suggesting a role for the mitochondrial glycine pathway. Further studies are needed to determine if regulation through the mitochondrial glycine pathway and ammonia transporters can be modulated and how to use the kidney as a therapeutic target in hyperammonemia.

Intestinal Gastrin/CCKBR (Cholecystokinin B Receptor) Ameliorates Salt-Sensitive Hypertension by Inhibiting Intestinal Na+/H+ Exchanger 3 Activity Through a PKC (Protein Kinase C)-Mediated NHERF1 and NHERF2 Pathway

Background:

The present study directly tested the crucial role of intestinal gastrin/CCKBR (cholecystokinin B receptor) in the treatment of salt-sensitive hypertension.

Methods:

Adult intestine-specific Cckbr-knockout mice (Cckbr^fl/flvillin-Cre) and Dahl salt-sensitive rats were studied on the effect of high salt intake (8% NaCl, 6–7 weeks) on intestinal Na^+/H⁺ exchanger 3 expression, urine sodium concentration, and blood pressure. High-salt diet increased urine sodium concentration and systolic blood pressure to a greater extent in Cckbr^fl/flvillin-Cre mice and Dahl salt-sensitive rats than their respective controls, Cckbr^fl/flvillin mice and SS13^BN rats. We constructed gastrin-SiO₂ microspheres to enable gastrin to stimulate specifically and selectively intestinal CCKBR without its absorption into the circulation.

Results:

Gastrin-SiO₂ microspheres treatment prevented the high salt-induced hypertension and increase in urine Na concentration by inhibiting intestinal Na^+/H⁺ exchanger 3 trafficking and activity, increasing stool sodium without inducing diarrhea. Gastrin-mediated inhibition of intestinal Na^+/H⁺ exchanger 3 activity, related to a PKC (protein kinase C)-mediated activation of NHERF1 and NHERF2.

Conclusions:

These results support a crucial role of intestinal gastrin/CCKBR in decreasing intestinal sodium absorption and keeping the blood pressure in the normal range. The gastrointestinal administration of gastrin-SiO₂ microspheres is a promising and safe strategy to treat salt-sensitive hypertension without side effects.

Beneficial cardiovascular and remodeling effects of SGLT2 inhibitors: pathophysiologic mechanisms

INTRODUCTION

The intent of this paper is to review the data regarding the multipotential effects of the sodium-glucose cotransporter 2 (SGLT 2) inhibitors, their cardiovascular protective effects, and their mechanism of action.

AREAS COVERED

The SGLT2 inhibitors exert their beneficial antidiabetic and cardioprotective effects through increased glucose excretion from the kidneys, blood pressure and weight lowering, vasodilation and other potential beneficial effects. They have been used for the treatment of patients with type 2 diabetes mellitus (T2DM) as well as in patients with cardiovascular disease (CVD), coronary artery disease (CAD),and heart failure with reduced ejection fraction (HFrEF) and heart failure with preserved ejection fraction (HFpEF). In order to get a better understanding of their mechanism of action for their multiple cardiovascular protective effects, a Medline search of the English language literature was conducted between 2015 and February 2022 and 46 pertinent papers were selected.

EXPERT OPINION

The analysis of data clearly demonstrated that the use of the SGLT2 inhibitors besides their antidiabetic effects, provide additional protection against CVD, CAD, and HFrEF and HFpEF, and death, but not stroke, in both diabetic and non-diabetic patients. Therefore, they should be preferably used for the treatment of patients with T2DM with preexisting CVD, CAD, and HFrEF and HFpEF.

Sodium-Glucose Cotransporter 2 Inhibitors and Heart Failure: A Bedside-to-Bench Journey

Type 2 diabetes mellitus (T2DM) and heart failure (HF) are multifactorial diseases sharing common risk factors, such as obesity, hyperinsulinemia, and inflammation, with underlying mechanisms including endothelial dysfunction, inflammation, oxidative stress, and metabolic alterations. Cardiovascular benefits of sodium-glucose cotransporter 2 (SGLT2) inhibitors observed in diabetic and non-diabetic patients are also related to their cardiac-specific, SGLT-independent mechanisms, in addition to the metabolic and hemodynamic effects. In search of the possible underlying mechanisms, a research campaign has been launched proposing varied mechanisms of action that include intracellular ion homeostasis, autophagy, cell death, and inflammatory processes. Moreover, the research focus was widened toward cellular targets other than cardiomyocytes. At the moment, intracellular sodium level reduction is the most explored mechanism of direct cardiac effects of SGLT2 inhibitors that mediate the benefits in heart failure in addition to glucose excretion and diuresis. The restoration of cardiac Na⁺ levels with consequent positive effects on Ca²⁺ handling can directly translate into improved contractility and relaxation of cardiomyocytes and have antiarrhythmic effects. In this review, we summarize clinical trials, studies on human cells, and animal models, that provide a vast array of data in support of repurposing this class of antidiabetic drugs.

Activation of the intestinal tissue renin-angiotensin system by transient sodium loading in salt-sensitive rats

BACKGROUND

The renal tissue renin-angiotensin system is known to be activated by salt loading in salt-sensitive rats; however, the response in other organs remains unclear.

METHOD

Spontaneously hypertensive rats were subjected to normal tap water or transient high-salt-concentration water from 6 to 14 weeks of age and were thereafter given normal tap water. From 18 to 20 weeks of age, rats given water with a high salt concentration were treated with an angiotensin II type 1 receptor blocker, valsartan.

RESULTS

Sustained blood pressure elevation by transient salt loading coincided with a persistent decrease in the fecal sodium content and sustained excess of the circulating volume in spontaneously hypertensive rats. Administration of valsartan sustainably reduced the blood pressure and normalized the fecal sodium levels. Notably, transient salt loading persistently induced the intestinal tissue renin-angiotensin system and enhanced sodium transporter expression exclusively in the small intestine of salt-sensitive rats, suggesting the potential connection of intestinal sodium absorption to salt sensitivity.

CONCLUSION

These results reveal the previously unappreciated contribution of the intestinal tissue renin-angiotensin system to sodium homeostasis and blood pressure regulation in the pathophysiology of salt-sensitive hypertension.

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.

The Interplay between Histamine H4 Receptor and the Kidney Function: The Lesson from H4 Receptor Knockout Mice

Previous studies implicated the histamine H₄ receptor in renal pathophysiology. The aim here is to elucidate the role of this receptor on renal function using H₄ receptor knockout mice (H₄R^-/-). Healthy and diabetic H₄R^-/- mice compared to their C57BL/6J wild-type counterpart for renal function and the expression of crucial tubular proteins. H₄R^-/- and wild-type mice, matched for ages, showed comparable weight gain curves reaching similar median weight at the end of the study. However, H₄R^-/- mice displayed a higher basal glycemia. H₄R^-/- mice showed a lower urine 24 h outflow, and albumin-to-creatinine ratio (ACR) compared to wild-type mice. Consistently, H₄R^-/- mice presented a higher expression of megalin and a lower basal expression of the sodium-hydrogen exchanger (NHE)3 and aquaporin (AQP)2. According to these basal differences, diabetic H₄R^-/- mice developed more severe hyperglycemia and a higher 24 h urine volume, but a lower increase in ACR and decrease in urine pH were observed. These events were paralleled by a reduced NHE3 over-expression and megalin loss in diabetic H₄R^-/- mice. The AQP1 and AQP7 patterns were also different between H₄R^-/- and wild-type diabetic mice. The collected results highlight the role of the histamine H₄ receptor in the control of renal reabsorption processes, particularly albumin uptake.

Differential Pathophysiological Mechanisms in Heart Failure With a Reduced or Preserved Ejection Fraction in Diabetes

Diabetes promotes the development of both heart failure with a reduced ejection fraction and heart failure with a preserved ejection fraction through diverse mechanisms, which are likely mediated through hyperinsulinemia rather than hyperglycemia. Diabetes promotes nutrient surplus signaling (through Akt and mammalian target of rapamycin complex 1) and inhibits nutrient deprivation signaling (through sirtuin-1 and its downstream effectors); this suppresses autophagy and promotes endoplasmic reticulum and oxidative stress and mitochondrial dysfunction, thereby undermining the health of diabetic cardiomyocytes. The hyperinsulinemia of diabetes may also activate sodium-hydrogen exchangers in cardiomyocytes (leading to injury and loss) and in the proximal renal tubules (leading to sodium retention). Diabetes may cause epicardial adipose tissue expansion, and the resulting secretion of proinflammatory adipocytokines onto the adjoining myocardium can lead to coronary microcirculatory dysfunction and myocardial inflammation and fibrosis. Interestingly, sodium-glucose cotransporter 2 (SGLT2) inhibitors-the only class of antidiabetic medication that reduces serious heart failure events-may act to mitigate each of these mechanisms. SGLT2 inhibitors up-regulate sirtuin-1 and its downstream effectors and autophagic flux, thus explaining the actions of these drugs to reduce oxidative stress, normalize mitochondrial structure and function, and mute proinflammatory pathways in the stressed myocardium. Inhibition of SGLT2 may also lead to a reduction in the activity of sodium-hydrogen exchangers in the kidney (leading to diuresis) and in the heart (attenuating the development of cardiac hypertrophy and systolic dysfunction). Finally, SGLT2 inhibitors reduce the mass and mute the adverse biology of epicardial adipose tissue (and reduce the secretion of leptin), thus explaining the capacity of these drugs to mitigate myocardial inflammation, microcirculatory dysfunction, and fibrosis, and improve ventricular filling dynamics. The pathophysiological mechanisms by which SGLT2 inhibitors may benefit heart failure likely differ depending on ejection fraction, but each represents interference with distinct pathways by which hyperinsulinemia may adversely affect cardiac structure and function.

Sodium-Glucose Cotransporter 2 Inhibition: Rationale and Mechanisms for Kidney and Cardiovascular Protection in People With and Without Diabetes

Large-scale randomized trials have demonstrated the remarkable capacity of sodium-glucose cotransporter 2 inhibitors to reduce the risk of cardiovascular outcomes and kidney disease progression, irrespective of the presence or absence of type 2 diabetes mellitus. Although the results of these trials have transformed clinical practice guidelines, the mechanisms underpinning the wide-ranging benefits of this class of agents remain incompletely understood and subject to ongoing investigation. Improvements in cardiometabolic risk factors such as glucose, blood pressure, body weight, and albuminuria likely contribute. However, other direct effects on physiological and cellular function, such as restoration of tubuloglomerular feedback, improvements in kidney and cardiac oxygenation and energy efficiency, as well as restoration of normal autophagy are also likely to be important. This review summarizes the rationale and potential mechanisms for cardiorenal protection with sodium-glucose cotransporter 2 inhibitors in people with and without diabetes, their relative importance, and the experimental and clinical lines of evidence supporting these hypotheses.

Long-Term Angiotensin II Infusion Induces Oxidative and Endoplasmic Reticulum Stress and Modulates Na+ Transporters Through the Nephron

High plasma angiotensin II (Ang II) levels are related to many diseases, including hypertension, and chronic kidney diseases (CKDs). Here, we investigated the relationship among prolonged Ang II infusion/AT1 receptor (AT1R) activation, oxidative stress, and endoplasmic reticulum (ER) stress in kidney tissue. In addition, we explored the chronic effects of Ang II on tubular Na⁺ transport mechanisms. Male Wistar rats were subjected to sham surgery as a control or prolonged Ang II treatment (200 ng⋅kg^–1⋅min^–1, 42 days) with or without losartan (10 mg⋅kg^–1⋅day^–1) for 14 days. Ang II/AT1R induced hypertension with a systolic blood pressure of 173.0 ± 20 mmHg (mmHg, n = 9) compared with 108.0 ± 7 mmHg (mmHg, n = 7) in sham animals. Under these conditions, gene and protein expression levels were evaluated. Prolonged Ang II administration/AT1R activation induced oxidative stress and ER stress with increased Nox2, Nox4, Cyba and Ncf1 mRNA expression, phosphorylated PERK and eIF2α protein expression as well as Atf4 mRNA expression. Ang II/AT1R also raised Il1b, Nfkb1 and Acta2 mRNA expression, suggesting proinflammatory, and profibrotic effects. Regarding Na⁺ tubular handling, Ang II/AT1R enhanced cortical non-phosphorylated and phospho/S552/NHE3, NHE1, ENaC β, NKCC2, and NCC protein expression. Our results also highlight the therapeutic potential of losartan, which goes beyond the antihypertensive effect, playing an important role in kidney tissue. This treatment reduced oxidative stress and ER stress signals and recovered relevant parameters of the maintenance of renal function, preventing the progression of Ang II-induced CKD.

Implication of cation-proton antiporters (CPA) in human health and diseases causing microorganisms

Cation and protons perform a substantial role in all the organism and its homeostasis within the cells are maintained by the cation-proton antiporters (CPAs). CPA is the huge family of the membrane transporter protein throughout the plant and animal kingdom including microorganism. In human, any malfunctioning of these proteins may lead to severe diseases like hypertension, heart diseases etc and CPAs are recently proposed to be responsible for the virulent property of various pathogens including Vibrio cholerae, Yersinia pestis etc. Human Sodium-Proton exchangers (Na⁺/H⁺ exchangers, NHEs) are crucial in ion homeostasis whereas Ec-NhaA, Na + -H + Antiporters maintain a balance of Na+ and proton in E. coli, regulating pH and cell volume within the cell. These Sodium-Proton antiporters are found to be responsible for the virulence in various pathogens causing human diseases. Understanding of these CPAs may assist investigators to target such human diseases, that further may lead to establishing the effective path for therapeutics or drug designing against associated human disease. Here we have compiled all such information on CPAs and provide a systematic approach to unravel the mechanism and role of antiporter proteins in a wide range of organisms. Being involved throughout all the species, this review on cation-proton antiporters may attract the attention of many investigators and concerned researchers and will be provided with the recent detailed information on the role of CPA in human health.

High blood pressure induced by vitamin D deficiency is associated with renal overexpression and hyperphosphorylation of Na+-K+-2Cl- cotransporter type 2

OBJECTIVES

Clinical and epidemiological studies have suggested a correlation between vitamin D deficiency (VDD) and high blood pressure (BP). This study aimed to test the hypothesis that high BP induced by VDD is associated with altered expression and covalent modification of apical sodium transporters along the nephron. The contributions of the intrarenal renin-angiotensin system (RAS) and oxidative stress were also investigated.

METHODS

Male Wistar rats were fed a vitamin D-free (n = 26) or standard diet (n = 25) for 30 days. BP was recorded using noninvasive and invasive procedures. The expression levels of total and phosphorylated apical sodium transporters in rat renal cortex and medulla were evaluated by immunoblotting. Intrarenal RAS components were assessed by immunoblotting and ELISA. Renal oxidative stress was analyzed by measuring the concentrations of thiobarbituric acid reactive substances and reduced glutathione.

RESULTS

Higher BP levels in VDD rats than controls were accompanied by overexpression and hyperphosphorylation of renal cortical and medullary Na+-K+-2Cl- cotransporter type 2, enhanced levels of phosphorylated Na+/H+ exchanger type 3, and reduced expression levels of total and phosphorylated Na+/Cl- cotransporter. Changes in intrarenal RAS induced by VDD vs. controls included the marked elevation of medullary renin expression, higher expression of cortical angiotensinogen, higher urinary angiotensinogen excretion, and higher cortical and medullary angiotensin II content. VDD rats displayed higher thiobarbituric acid reactive substances/glutathione ratios in the renal cortex and medulla than controls.

CONCLUSION

These results suggest that the molecular mechanisms underlying the effects of VDD on BP may include the upregulation of Na+-K+-2Cl- cotransporter type 2 and activation of intrarenal RAS and oxidative stress.

Sodium-Glucose Co-transporter 2 Inhibitors in the Failing Heart: a Growing Potential

Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are a new drug class designed to treat patients with type 2 diabetes (T2D). However, cardiovascular outcome trials showed that SGLT2i also offer protection against heart failure (HF)-related events and cardiovascular mortality. These benefits appear to be independent of glycaemic control and have recently been demonstrated in the HF population with reduced ejection fraction (HFrEF), with or without T2D. This comprehensive, evidence-based review focuses on the published studies concerning HF outcomes with SGLT2i, discussing issues that may underlie the different results, along with the impact of these new drugs in clinical practice. The potential translational mechanisms behind SGLT2i cardio-renal benefits and the information that ongoing studies may add to the already existing body of evidence are also reviewed. Finally, we focus on practical management issues regarding SGLT2i use in association with other T2D and HFrEF common pharmacological therapies. Safety considerations are also highlighted. Considering the paradigm shift in T2D management, from a focus on glycaemic control to a broader approach on cardiovascular protection and event reduction, including the potential for wide SGLT2i implementation in HF patients, with or without T2D, we are facing a promising time for major changes in the global management of cardiovascular disease.

A role for tubular Na+/H+ exchanger NHE3 in the natriuretic effect of the SGLT2 inhibitor empagliflozin

Inhibitors of proximal tubular Na⁺-glucose cotransporter 2 (SGLT2) are natriuretic, and they lower blood pressure. There are reports that the activities of SGLT2 and Na⁺-H⁺ exchanger 3 (NHE3) are coordinated. If so, then part of the natriuretic response to an SGLT2 inhibitor is mediated by suppressing NHE3. To examine this further, we compared the effects of an SGLT2 inhibitor, empagliflozin, on urine composition and systolic blood pressure (SBP) in nondiabetic mice with tubule-specific NHE3 knockdown (NHE3-ko) and wild-type (WT) littermates. A single dose of empagliflozin, titrated to cause minimal glucosuria, increased urinary excretion of Na⁺ and bicarbonate and raised urine pH in WT mice but not in NHE3-ko mice. Chronic empagliflozin treatment tended to lower SBP despite higher renal renin mRNA expression and lowered the ratio of SBP to renin mRNA, indicating volume loss. This effect of empagliflozin depended on tubular NHE3. In diabetic Akita mice, chronic empagliflozin enhanced phosphorylation of NHE3 (S552/S605), changes previously linked to lesser NHE3-mediated reabsorption. Chronic empagliflozin also increased expression of genes involved with renal gluconeogenesis, bicarbonate regeneration, and ammonium formation. While this could reflect compensatory responses to acidification of proximal tubular cells resulting from reduced NHE3 activity, these effects were at least in part independent of tubular NHE3 and potentially indicated metabolic adaptations to urinary glucose loss. Moreover, empagliflozin increased luminal α-ketoglutarate, which may serve to stimulate compensatory distal NaCl reabsorption, while cogenerated and excreted ammonium balances urine losses of this "potential bicarbonate." The data implicate NHE3 as a determinant of the natriuretic effect of empagliflozin.

Autophagy stimulation and intracellular sodium reduction as mediators of the cardioprotective effect of sodium-glucose cotransporter 2 inhibitors

In five large-scale trials involving >40 000 patients, sodium-glucose cotransporter 2 (SGLT2) inhibitors decreased the risk of serious heart failure events by 25-40%. This effect cannot be explained by control of hyperglycaemia, since it is not observed with antidiabetic drugs with greater glucose-lowering effects. It cannot be attributed to ketogenesis, since it is not causally linked to ketone body production, and the benefit is not enhanced in patients with diabetes. The effect cannot be ascribed to a natriuretic action, since SGLT2 inhibitors decrease natriuretic peptides only modestly, and they reduce cardiovascular death, a benefit that diuretics do not possess. Although SGLT2 inhibitors increase red blood cell mass, enhanced erythropoiesis does not favourably influence the course of heart failure. By contrast, experimental studies suggest that SGLT2 inhibitors may reduce intracellular sodium, thereby preventing oxidative stress and cardiomyocyte death. Additionally, SGLT2 inhibitors induce a transcriptional paradigm that mimics nutrient and oxygen deprivation, which includes activation of adenosine monophosphate-activated protein kinase, sirtuin-1, and/or hypoxia-inducible factors-1α/2α. The interplay of these mediators stimulates autophagy, a lysosomally-mediated degradative pathway that maintains cellular homeostasis. Autophagy-mediated clearance of damaged organelles reduces inflammasome activation, thus mitigating cardiomyocyte dysfunction and coronary microvascular injury. Interestingly, the action of hypoxia-inducible factors-1α/2α to both stimulate erythropoietin and induce autophagy may explain why erythrocytosis is strongly correlated with the reduction in heart failure events. Therefore, the benefits of SGLT2 inhibitors on heart failure may be mediated by a direct cardioprotective action related to modulation of pathways responsible for cardiomyocyte homeostasis.

Cardioprotection conferred by sodium-glucose cotransporter 2 inhibitors: a renal proximal tubule perspective

Sodium-glucose cotransporter 2 (SGLT2) inhibitors, also known as gliflozins, improve glycemia by suppressing glucose reuptake in the renal proximal tubule. Currently, SGLT2 inhibitors are primarily indicated as antidiabetic agents; however, their benefits extend far beyond glucose control. Cardiovascular outcome trials indicated that all studied SGLT2 inhibitors remarkably and consistently reduce cardiovascular mortality and hospitalization for heart failure (HF) in type 2 diabetes (T2D) patients. Nevertheless, the mechanisms underlying the unprecedented cardiovascular benefits of gliflozins remain elusive. Multiple processes that directly or indirectly improve myocardial performance may be involved, including the amelioration of proximal tubular dysfunction. Therefore, this paper provides a perspective on the potential cellular and molecular mechanisms of the proximal tubule that may, at least in part, mediate the cardioprotection conferred by SGLT2 inhibitors. Specifically, we focus on the effects of SGLT2 on extracellular volume homeostasis, including its plausible functional and physical association with the apical Na⁺/H⁺ exchanger isoform 3 as well as its complex and its possible bidirectional interactions with the intrarenal angiotensin system and renal sympathetic nervous system. We also discuss evidence supporting a potential benefit of gliflozins in reducing cardiovascular risk, attributable to their effect on proximal tubule handling of uric acid and albumin as well as in erythropoietin production. Unraveling the mechanisms behind the beneficial actions of SGLT2 inhibitors may not only contribute to a better understanding of the pathophysiology of cardiovascular diseases but also enable repurposing of gliflozins to improve the routine management of HF patients with or without T2D.

Effect of renal tubule-specific knockdown of the Na+/H+ exchanger NHE3 in Akita diabetic mice

Na+/H+ exchanger isoform 3 (NHE3) contributes to Na+/bicarbonate reabsorption and ammonium secretion in early proximal tubules. To determine its role in the diabetic kidney, type 1 diabetic Akita mice with tubular NHE3 knockdown [Pax8-Cre; NHE3-knockout (KO) mice] were generated. NHE3-KO mice had higher urine pH, more bicarbonaturia, and compensating increases in renal mRNA expression for genes associated with generation of ammonium, bicarbonate, and glucose (phosphoenolpyruvate carboxykinase) in proximal tubules and H+ and ammonia secretion and glycolysis in distal tubules. This left blood pH and bicarbonate unaffected in nondiabetic and diabetic NHE3-KO versus wild-type mice but was associated with renal upregulation of proinflammatory markers. Higher renal phosphoenolpyruvate carboxykinase expression in NHE3-KO mice was associated with lower Na+-glucose cotransporter (SGLT)2 and higher SGLT1 expression, indicating a downward tubular shift in Na+ and glucose reabsorption. NHE3-KO was associated with lesser kidney weight and glomerular filtration rate (GFR) independent of diabetes and prevented diabetes-associated albuminuria. NHE3-KO, however, did not attenuate hyperglycemia or prevent diabetes from increasing kidney weight and GFR. Higher renal gluconeogenesis may explain similar hyperglycemia despite lower SGLT2 expression and higher glucosuria in diabetic NHE3-KO versus wild-type mice; stronger SGLT1 engagement could have affected kidney weight and GFR responses. Chronic kidney disease in humans is associated with reduced urinary excretion of metabolites of branched-chain amino acids and the tricarboxylic acid cycle, a pattern mimicked in diabetic wild-type mice. This pattern was reversed in nondiabetic NHE3-KO mice, possibly reflecting branched-chain amino acids use for ammoniagenesis and tricarboxylic acid cycle upregulation to support formation of ammonia, bicarbonate, and glucose in proximal tubule. NHE3-KO, however, did not prevent the diabetes-induced urinary downregulation in these metabolites.

Effect of Bilastine on Diabetic Nephropathy in DBA2/J Mice

Diabetic nephropathy is an unmet therapeutic need, and the search for new therapeutic strategies is warranted. Previous data point to histamine H₁ receptor as a possible target for glomerular dysfunction associated with long term hyperglycaemia. Therefore, this study investigated the effects of the H₁ receptor antagonist bilastine on renal morphology and function in a murine model of streptozotocin-induced diabetes. Diabetes was induced in DBA2/J male mice and, from diabetes onset (glycaemia ≥200 mg/dL), mice received bilastine (1–30 mg/kg/day) by oral gavage for 14 consecutive weeks. At the end of the experimental protocol, diabetic mice showed polyuria (+195.5%), increase in Albumin-to-Creatine Ratio (ACR, +284.7%), and a significant drop in creatinine clearance (p < 0.05). Bilastine prevented ACR increase and restored creatinine clearance in a dose-dependent manner, suggesting a positive effect on glomerular filtration. The ultrastructural analysis showed a preserved junctional integrity. Preservation of the basal nephrin, P-cadherin, and synaptopodin expression could explain this effect. In conclusion, the H₁ receptor could contribute to the glomerular damage occurring in diabetic nephropathy. Bilastine preserved the glomerular junctional integrity, leading to the hypothesis of anti-H₁ antihistamines as a possible add-on therapy for diabetic nephropathy.

The potential role of myosin motor proteins in mediating the subcellular distribution of NHE3 in the renal proximal tubule

Isoform 3 of the Na⁺/H⁺ exchanger (NHE3) is responsible for the majority of the reabsorption of NaCl, NaHCO₃, and, consequently, water in the renal proximal tubule. As such, this transporter plays an essential role in acid-base balance and extracellular fluid volume homeostasis and determining systemic arterial blood pressure levels. NHE3 activity is modulated by a number of mechanisms, including the redistribution of the transporter between the body of the microvilli (where NHE3 is active) and the base of the microvilli (where NHE3 is less active). Although the physiological, pathophysiological, and pharmacological importance of the subcellular distribution of NHE3 has been well established, the exact mechanism whereby NHE3 is translocated along microvilli microdomains of the proximal tubule apical membrane is unknown. Nonmuscle myosin IIA and unconventional myosin VI move cargoes in anterograde and retrograde directions, respectively, and are known to redistribute along with NHE3 in the proximal tubule in response to a variety of natriuretic and antinatriuretic stimuli, including stimulation or inhibition of the renin-angiotensin system, high dietary Na⁺ intake, and high blood pressure. Therefore, this review aims to discuss the current evidence that suggests a potential role of myosin IIA and myosin VI in mediating the subcellular distribution of NHE3 along the kidney proximal tubule microvilli and their possible contribution in modifying NHE3-mediated Na⁺ reabsorption under both physiological and pathophysiological conditions.

Sodium-Glucose Cotransporter 2 Inhibition and Diabetic Kidney Disease

Diabetic kidney disease (DKD) is now the principal cause of chronic kidney disease leading to end-stage kidney disease worldwide. As a primary contributor to the excess risk of all-cause and cardiovascular death in diabetes, DKD is a major contributor to the progressively expanding global burden of diabetes-associated morbidity and mortality. Sodium-glucose cotransporter 2 (SGLT2) inhibitors are a newer class of antihyperglycemic agents that exert glucose-lowering effects via glycosuric actions. Preclinical studies and clinical trials of SGLT2 inhibitors have consistently demonstrated reduction of albuminuria and preservation of kidney function. In particular, SGLT2 inhibitors lower risk of congestive heart failure, a major cardiovascular complication in DKD. This Perspective summarizes proposed mechanisms of action for SGLT2 inhibitors, integrates these data with results of recent cardiovascular outcomes trials, and discusses clinical applications for patients with DKD. The American Diabetes Association/European Association for the Study of Diabetes Consensus Report published online in October 2018 recommends SGLT inhibitors as preferred add-on therapy for patients with type 2 diabetes and established cardiovascular disease or chronic kidney disease, if kidney function is adequate. Results of the ongoing and just completed clinical trials conducted in patients with established DKD will facilitate further refinement of current guidelines.

Adenosine A2A receptor blocks the A 1 receptor inhibition of renal Na + transport and oxygen consumption

A high renal oxygen (O₂ ) need is primarily associated with the renal tubular O₂ consumption (VO₂ ) necessary for a high rate of sodium (Na⁺ ) transport. Limited O₂ availability leads to increased levels of adenosine, which regulates the kidney via activation of both A₁ and A_2A adenosine receptors (A1R and A2AR, respectively). The relative contributions of A1R and A2AR to the regulation of renal Na⁺ transport and VO₂ have not been determined. We demonstrated that A1R activation has a dose-dependent biphasic effect on both renal Na⁺ /H⁺ exchanger-3 (NHE3), a major player in Na⁺ transport, and VO₂ . Here, we report concentration-dependent effects of adenosine: less than 5 × 10^-7  M adenosine-stimulated NHE3 activity; between 5 × 10^-7  M and 10^-5  M adenosine-inhibited NHE3 activity; and greater than 10^-5  M adenosine reversed the change in NHE3 activity (returned to baseline). A1R activation mediated the activation and inhibition of NHE3 activity, whereas 10^-4  M adenosine had no effect on the NHE3 activity due to A2AR activation. The following occurred when A1R and A2AR were activated: (a) Blockade of the A2AR receptor restored the NHE3 inhibition mediated by A1R activation, (b) the NHE-dependent effect on VO₂ mediated by A1R activation became NHE independent, and (c) A2AR bound to A1R. In summary, A1R affects VO₂ via NHE-dependent mechanisms, whereas A2AR acts via NHE-independent mechanisms. When both A1R and A2AR are activated, the A2AR effect on NHE3 and VO₂ predominates, possibly via an A1R-A2AR protein interaction. A2AR-A1R heterodimerization is proposed as the molecular mechanism enabling the NHE-independent control of renal VO₂ .

Dipeptidyl Peptidase 4 Inhibition Ameliorates Chronic Kidney Disease in a Model of Salt-Dependent Hypertension

Cardiovascular diseases frequently coexist with chronic kidney disease that constitutes a major determinant of outcome in patients with heart failure. Dysfunction of both organs is related to chronic inflammation, endothelial dysfunction, oxidative stress, and fibrosis. Widespread expression of serine protease DPP4 that degrades varieties of substrates suggests its involvement in numerous physiological processes. In this study, we tested the effects of selective DPP4 inhibition on the progression of renal disease in a nondiabetic model of hypertensive heart disease using Dahl salt-sensitive rats. Chronic DPP4 inhibition positively affected renal function with a significant reduction in albuminuria and serum creatinine. DPP4 inhibition attenuated the inflammatory component by reducing the expression of NF-κB, TNFα, IL-1β, IL-6, and MCP-1. Kidney macrophages expressed GLP-1R, and DPP4 inhibition promoted macrophage polarization toward the anti-inflammatory M2 phenotype. Finally, high degrees of NADPH oxidase 4 expression and oxidation of nucleic acids, lipids, and proteins were reduced upon DPP4 inhibition. Our study provides evidence of renoprotection by DPP4 inhibition in a nondiabetic hypertension-induced model of chronic cardiorenal syndrome, indicating that DPP4 pathway remains a valid object to study in the context of chronic multiorgan diseases.

Histamine and diabetic nephropathy: an up-to-date overview

The classification of diabetic nephropathy (DN) as a vascular complication of diabetes makes the possible involvement of histamine, an endogenous amine that is well known for its vasoactive properties, an interesting topic for study. The aim of the present review is to provide an extensive overview of the possible involvement of histamine in the onset and progression of DN. The evidence collected on the role of histamine in kidney function together with its well-known pleiotropic action suggest that this amine may act simultaneously on glomerular hyperfiltration, tubular inflammation, fibrosis development and tubular hypertrophy.

Contribution of NHE3 and dietary phosphate to lithium pharmacokinetics

Lithium is one of the mainstays for the treatment of bipolar disorder despite its side effects on the endocrine, neurological, and renal systems. Experimentally, lithium has been used as a measure to determine proximal tubule reabsorption based on the assumption that lithium and sodium transport go in parallel in the proximal tubule. However, the exact mechanism by which lithium is reabsorbed remains elusive. The majority of proximal tubule sodium reabsorption is directly or indirectly mediated by the sodium-hydrogen exchanger 3 (NHE3). In addition, sodium-phosphate cotransporters have been implicated in renal lithium reabsorption. In order to better understand the role of sodium-phosphate cotransporters involved in lithium (re)absorption, we studied lithium pharmacokinetics in: i) tubule-specific NHE3 knockout mice (NHE3^{loxloxPax8Cre}), and ii) mice challenged with low or high phosphate diets. Intravenous or oral administration of lithium did not result in differences in lithium bioavailability, half-life, maximum plasma concentrations, area under the curve, lithium clearance, or urinary lithium/creatinine ratios between control and NHE3^{loxloxPax8Cre} mice. After one week of dietary phosphate challenges, lithium bioavailability was ~30% lower on low versus high dietary phosphate, possibly the consequence of a smaller area under the curve after oral administration. This was associated with higher apparent lithium clearance after oral administration and lower urinary lithium/creatinine ratios on low versus high dietary phosphate. Collectively, renal NHE3 does not play a role in lithium pharmacokinetics; however, dietary phosphate could have an indirect effect on lithium bioavailability and lithium disposition.

Expression of lysophosphatidic acid receptor 5 is necessary for the regulation of intestinal Na+/H+ exchanger 3 by lysophosphatidic acid in vivo

Lysophosphatidic acid (LPA) is a bioactive lipid molecule, which regulates a broad range of pathophysiological processes. Recent studies have demonstrated that LPA modulates electrolyte flux in the intestine, and its potential as an antidiarrheal agent has been suggested. Of six LPA receptors, LPA₅ is highly expressed in the intestine. Recent studies by our group have demonstrated activation of Na⁺/H⁺ exchanger 3 (NHE3) by LPA₅. However, much of what has been elucidated was achieved using colonic cell lines that were transfected to express LPA₅. In the current study, we engineered a mouse that lacks LPA₅ in intestinal epithelial cells, Lpar5^ΔIEC, and investigated the role of LPA₅ in NHE3 regulation and fluid absorption in vivo. The intestine of Lpar5^ΔIEC mice appeared morphologically normal, and the stool frequency and fecal water content were unchanged compared with wild-type mice. Basal rates of NHE3 activity and fluid absorption and total NHE3 expression were not changed in Lpar5^ΔIEC mice. However, LPA did not activate NHE3 activity or fluid absorption in Lpar5^ΔIEC mice, providing direct evidence for the regulatory role of LPA₅. NHE3 activation involves trafficking of NHE3 from the terminal web to microvilli, and this mobilization of NHE3 by LPA was abolished in Lpar5^ΔIEC mice. Dysregulation of NHE3 was specific to LPA, and insulin and cholera toxin were able to stimulate and inhibit NHE3, respectively, in both wild-type and Lpar5^ΔIEC mice. The current study for the first time demonstrates the necessity of LPA₅ in LPA-mediated stimulation of NHE3 in vivo. NEW & NOTEWORTHY This study is the first to assess the role of LPA₅ in NHE3 regulation and fluid absorption in vivo using a mouse that lacks LPA₅ in intestinal epithelial cells, Lpar5^ΔIEC. Basal rates of NHE3 activity and fluid absorption, and total NHE3 expression were not changed in Lpar5^ΔIEC mice. However, LPA did not activate NHE3 activity or fluid absorption in Lpar5^ΔIEC mice, providing direct evidence for the regulatory role of LPA₅.

Pharmacodynamics, Safety, and Tolerability of the NHE3 Inhibitor Tenapanor: Two Trials in Healthy Volunteers

Background

Tenapanor, a small molecule with minimal systemic availability, is a first-in-class sodium/hydrogen exchanger 3 (NHE3) inhibitor that acts in the gut. Here, we evaluate the pharmacodynamics and safety of tenapanor in healthy adults.

Methods

Two phase I, single-center, randomized, double-blind, placebo-controlled studies were performed. The first study assessed single-ascending oral tenapanor doses of 10, 50, 150, 450, and 900 mg (n = 8 per group; six tenapanor, two placebo) and multiple ascending doses over 7 days of 3, 10, 30, and 100 mg q.d. (n = 10 per group; eight tenapanor, two placebo). In the second study, different tenapanor regimens were evaluated over 7 days (n = 15 per group; 12 tenapanor, three placebo): 15 mg twice daily (b.i.d.), 30 mg once daily (q.d.), 30 mg b.i.d., 30 mg three times daily (t.i.d.), 60 mg b.i.d., escalating b.i.d. dose (daily total 30–90 mg), 30 mg b.i.d. with psyllium.

Results

Tenapanor produced generally dose-dependent increases in stool sodium excretion and decreases in urinary sodium excretion versus placebo; in addition, twice-daily dosing appeared to have a greater effect on sodium absorption than once-daily dosing with an equivalent daily dose. Tenapanor softened stool consistency and increased stool frequency and weight from baseline versus placebo. Tenapanor concentrations were below the quantification limit (0.5 ng/ml) in 98.5% of 895 plasma samples. Adverse events were mild or moderate in severity, and were typically gastrointestinal in nature. There were no clinically relevant changes in serum electrolytes.

Conclusions

Tenapanor was well tolerated and resulted in reduced intestinal sodium absorption and softer stool consistency versus placebo. Systemic exposure to tenapanor was minimal. These results support potential use of tenapanor in patients who could benefit from modification of gastrointestinal sodium balance.

ClinicalTrials.gov identifiers

NCT02819687, NCT02796131.

Electronic supplementary material

The online version of this article (10.1007/s40261-017-0614-0) contains supplementary material, which is available to authorized users.

Na+ /H+ exchanger 3 blockade ameliorates type 2 diabetes mellitus via inhibition of sodium-glucose co-transporter 1-mediated glucose absorption in the small intestine

AIM

To elucidate the role of Na⁺ /H⁺ exchanger 3 (NHE3) in sodium-glucose co-transporter 1 (SGLT1)-mediated small intestinal brush border membrane (BBM) glucose absorption and its functional implications in type 2 diabetes mellitus (T2DM).

MATERIALS AND METHODS

Human jejunal samples were obtained from patients undergoing gastrectomy. ¹⁴ C-glucose absorption was measured by liquid scintillation counting. NHE3 expression was suppressed by siRNA-mediated knockdown or augmented in Caco2 cells. Glucose and insulin tolerance in db/db and m+/db mice was assessed with oral and intraperitoneal glucose tolerance tests, and an intraperitoneal insulin tolerance test. Insulin resistance and β-cell function were assessed using homeostatic model assessment of insulin resistance and β-cell function.

RESULTS

NHE3 expression was upregulated in db/db mouse jejunal BBM and high-glucose-treated Caco2 cells. NHE3 blockade impaired SGLT1-mediated glucose absorption in human jejunum, m+/db and db/db mouse jejunums, and Caco2 cells, via serum/glucocorticoid-regulated kinase 1 (SGK1). NHE3 knockdown suppressed SGLT1-mediated glucose uptake and reduced mRNA and protein levels of SGK1 and SGLT1, which were conversely enhanced by NHE3 overexpression. Chronic S3226 treatment diminished postprandial glucose levels and ameliorated glucose intolerance in db/db mice.

CONCLUSION

NHE3 is essential in the modulation of small intestinal BBM glucose absorption. Our findings provide a rationale for future possible clinical application of NHE3 for treatment of T2DM through reducing intestinal glucose uptake and counteracting postprandial hyperglycaemia.

Application of Electrophysiology Measurement to Study the Activity of Electro-Neutral Transporters

The transport of ions through cell membranes ensures the fine control of ion content within and outside the cell that is indispensable for cell survival. These transport mechanisms are mediated by the activities of specialized transporter proteins. Specifically,pH dynamics are finely controlled by plasma membrane proton (H⁺) extrusion systems, such as the Na⁺/H⁺ exchanger (NHE) protein family. Despite extensive efforts to study the mechanisms underlying NHE regulation, our current understanding of the biophysical and molecular properties of the NHE family is inadequate because of the limited availability of methods to effectively measure NHE activity. In this manuscript, we used H⁺-selective electrodes during whole-cell patch clamping recording to measure NHE-induced H⁺ flux. We proposed this approach to overcome some limitations of typically used methods to measure NHE activity, such as radioactive uptake and fluorescent membrane permeants. Measurement of NHE activity using the described method enables high sensitivity and time resolution and more efficient control of intracellular H⁺ concentrations. H⁺-selective electrodes are based on the fact that transporter activity creates an ion gradient in close proximity to the cell membrane. An H⁺-selective electrode moving up to and away from the cell membrane in a repetitive, oscillatory fashion records a voltage difference that is dependent on H⁺ flux. While H⁺-selective electrodes are used to detect H⁺ flux moving out of the cell, the patch clamp method in the whole-cell configuration is used to control the intracellular ion composition. Moreover, application of the giant patch clamp technique allows modification of the intracellular composition of not only ions but also lipids. The transporter activity of NHE isoform 3 (NHE3) was measured using this technical approach to study the molecular basis of NHE3 regulation by phosphoinositides.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

Renal Proteomic Responses to Severe Sepsis and Surgical Trauma: Dynamic Analysis of Porcine Tissue Biopsies

Although the burden of septic acute kidney injury continues to increase, the molecular pathogenesis remains largely obscure. The aim of this exploratory study was a discovery-driven analysis of dynamic kidney tissue protein expression changes applied for the first time in a classic large mammal model of sepsis. To achieve this goal, analyses of protein expression alterations were performed in serial samples of kidney cortical biopsies (before, 12 and 22 h of sepsis) in mechanically ventilated pigs challenged with continuous infusion of pseudomonas aeruginosa and compared with sham-operated control data. Global protein expression was analyzed using two-dimensional gel electrophoresis and mass spectrometry-based proteomics. Normodynamic sepsis was associated with 43% reduction in glomerular filtration. The exposure to surgical stress per se altered the renal protein expression profile, while sepsis induced distinct and highly dynamic proteome evolution shifting the balance toward cellular distress phenotype. We identified 20 proteins whose expression changes discriminated effects of sepsis from those induced by surgery. The data implicate endoplasmic reticulum stress, oxidative stress, mitochondrial energy metabolism, immune/inflammatory signaling, and tubular transport as major activated pathways. Thus, by coupling the power of sequential tissue proteomics with whole-animal physiological studies, our study helped to establish a first global overview of critical renal proteomic events occurring during surgical trauma and early sepsis in a porcine model. The study supports the notion that multiple potentially subtle and even transient changes in several proteins which are members of key functional interrelated systems appear to play a role in septic acute kidney injury.

Histamine H4 receptor antagonism prevents the progression of diabetic nephropathy in male DBA2/J mice

Due to the incidence of diabetes and the related morbidity of diabetic nephropathy, identification of new therapeutic strategies represents a priority. In the last few decades new and growing evidence on the possible role of histamine in diabetes has been provided. In particular, the histamine receptor H₄R is emerging as a new promising pharmacological target for diabetic nephropathy. The aim of this study was to evaluate the efficacy of selective H₄R antagonism by JNJ39758979 on the prevention of diabetic nephropathy progression in a murine model of diabetes induced by streptozotocin injection. JNJ39758979 (25, 50, 100 mg/kg/day p.o.) was administered for 15 weeks starting from the onset of diabetes. Functional parameters were monitored throughout the experimental period. JNJ39758979 did not significantly affect glycaemic status or body weight. The urine analysis indicated a dose-dependent inhibitory effect of JNJ39758979 on Albumin-Creatinine-Ratio, the Creatinine Clearance, the 24 h urine volume, and pH urine acidification (P < 0.05). The beneficial effects of JNJ39758979 on renal function paralleled comparable effects on renal morphological integrity. These effects were sustained by a significant immune infiltration and fibrosis reduction. Notably, megalin and sodium-hydrogen-exchanger 3 expression levels were preserved. Our data suggest that the H₄R participates in diabetic nephropathy progression through both a direct effect on tubular reabsorption and an indirect action on renal tissue architecture via inflammatory cell recruitment. Therefore, H₄R antagonism emerges as a possible new multi-mechanism therapeutic approach to counteract development of diabetic nephropathy development.

Activation and Inhibition of Sodium-Hydrogen Exchanger Is a Mechanism That Links the Pathophysiology and Treatment of Diabetes Mellitus With That of Heart Failure

The mechanisms underlying the progression of diabetes mellitus and heart failure are closely intertwined, such that worsening of one condition is frequently accompanied by worsening of the other; the degree of clinical acceleration is marked when the 2 coexist. Activation of the sodium-hydrogen exchanger in the heart and vasculature (NHE1 isoform) and the kidneys (NHE3 isoform) may serve as a common mechanism that links both disorders and may underlie their interplay. Insulin insensitivity and adipokine abnormalities (the hallmarks of type 2 diabetes mellitus) are characteristic features of heart failure; conversely, neurohormonal systems activated in heart failure (norepinephrine, angiotensin II, aldosterone, and neprilysin) impair insulin sensitivity and contribute to microvascular disease in diabetes mellitus. Each of these neurohormonal derangements may act through increased activity of both NHE1 and NHE3. Drugs used to treat diabetes mellitus may favorably affect the pathophysiological mechanisms of heart failure by inhibiting either or both NHE isoforms, and drugs used to treat heart failure may have beneficial effects on glucose tolerance and the complications of diabetes mellitus by interfering with the actions of NHE1 and NHE3. The efficacy of NHE inhibitors on the risk of cardiovascular events may be enhanced when heart failure and glucose intolerance coexist and may be attenuated when drugs with NHE inhibitory actions are given concomitantly. Therefore, the sodium-hydrogen exchanger may play a central role in the interplay of diabetes mellitus and heart failure, contribute to the physiological and clinical progression of both diseases, and explain certain drug-drug and drug-disease interactions that have been reported in large-scale randomized clinical trials.

Phosphorylation of NHE3-S719 regulates NHE3 activity through the formation of multiple signaling complexes

CK2 regulates NHE3 by phosphorylating a single C-terminal amino acid, which, when mutated, reduces basal NHE3 activity and its acute stimulation and inhibition. It also is necessary for binding of proteins throughout the C-terminus, which means that it determines the C-terminal structure.

Casein kinase 2 (CK2) binds to the NHE3 C-terminus and constitutively phosphorylates a downstream site (S719) that accounts for 40% of basal NHE3 activity. The role of CK2 in regulation of NHE3 activity in polarized Caco-2/bbe cells was further examined by mutation of NHE3-S⁷¹⁹ to A (not phosphorylated) or D (phosphomimetic). NHE3-S719A but not -S719D had multiple changes in NHE3 activity: 1) reduced basal NHE3 activity—specifically, inhibition of the PI3K/AKT-dependent component; 2) reduced acute stimulation of NHE3 activity by LPA/LPA₅R stimulation; and 3) reduced acute inhibition of NHE3 activity—specifically, elevated Ca²⁺ related (carbachol/Ca²⁺ ionophore), but there was normal inhibition by forskolin and hyperosmolarity. The S719A mutant had reduced NHE3 complex size, reduced expression in lipid rafts, increased BB mobile fraction, and reduced binding to multiple proteins that bind throughout the NHE3 intracellular C-terminus, including calcineurin homologous protein, the NHERF family and SNX27 (related PDZ domains). These studies show that phosphorylation of the NHE3 at a single amino acid in the distal part of the C-terminus affects multiple aspects of NHE3 complex formation and changes the NHE3 lipid raft distribution, which cause changes in specific aspects of basal as well as acutely stimulated and inhibited Na⁺/H⁺ exchange activity.

Effect of Food Intake on the Pharmacodynamics of Tenapanor: A Phase 1 Study

Tenapanor (RDX5791/AZD1722) is a minimally systemic small-molecule inhibitor of the sodium/hydrogen exchanger NHE3. Tenapanor acts in the gut to reduce absorption of sodium and phosphate. This phase 1 open-label, 3-way crossover study (NCT02226783) evaluated the effect of food on the pharmacodynamics of tenapanor. Eighteen volunteers completed a randomized sequence of three 4-day treatments with tenapanor hydrochloride 15 mg twice daily: before food, after food, and while fasting. Participants received a diet standardized for sodium content. Stool sodium was significantly higher with tenapanor administration before versus after food (difference, +8.8 mmol/day, P = .006) or while fasting (+11.8 mmol/day, P = .0004). Differences in urinary sodium were not significant. Stool phosphorus was not significantly different with tenapanor before versus after food and significantly higher before food versus while fasting (+4.9 mmol/day, P = .006). Urinary phosphorus was significantly lower when tenapanor was administered before (-3.9 mmol/day, P = .0005) or after food (-3.7 mmol/day, P = .0009) versus while fasting. No serious adverse events were reported. These data suggest the effect of tenapanor on sodium absorption is most pronounced when administered before meals, whereas the effect on phosphate is similar whether administered before or after meals. This may support different timings of tenapanor administration with respect to food for sodium- and phosphate-related indications.

Novel developments in differentiating the role of renal and intestinal sodium hydrogen exchanger 3

The Na⁺/H⁺ exchanger isoform 3 (NHE3) facilitates Na⁺ absorption and H⁺ secretion and is expressed in the intestine, proximal tubule, and thick ascending limb of the kidney. While the function of NHE3 for Na⁺ and [Formula: see text](re)absorption has been defined using conventional NHE3 knockout mice (NHE3^-/-), the recent generation of conditional NHE3 knockout mice started to give critical new insight into the role of this protein by allowing for temporal and spatial control of NHE3 expression. For example, in contrast to NHE3^-/- mice, knockout of NHE3 in the S1 and S2 segments of the proximal tubule or along the entire tubule/collecting duct does not cause any lethality. Nonabsorbable NHE3 inhibitors have been developed, and preclinical as well as clinical trials indicate possible pharmacological use in fluid overload, hypertension, chronic kidney disease, hyperphosphatemia, and constipation. Some of the therapeutic considerations seem to be directly related to the pharmacodynamic properties of these drugs; however, little is known about the effects of these nonabsorbable NHE3 inhibitors on intestinal phosphate transport and the mechanisms so far remain elusive. This review focuses on novel findings of NHE3 in the intestine and the kidney as well as novel drug developments targeting NHE3.

Angiotensin II counteracts the effects of cAMP/PKA on NHE3 activity and phosphorylation in proximal tubule cells

Binding of angiotensin II (ANG II) to the AT₁ receptor (AT₁R) in the proximal tubule stimulates Na⁺/H⁺ exchanger isoform 3 (NHE3) activity through multiple signaling pathways. However, the effects of ANG II/AT₁R-induced inihibitory G protein (G_i) activation and subsequent decrease in cAMP accumulation on NHE3 regulation are not well established. We therefore tested the hypothesis that ANG II reduces cAMP/PKA-mediated phosphorylation of NHE3 on serine 552 and, in doing so, stimulates NHE3 activity. Under basal conditions, ANG II stimulated NHE3 activity but did not affect PKA-mediated NHE3 phosphorylation at serine 552 in opossum kidney (OKP) cells. However, in the presence of the cAMP-elevating agent forskolin (FSK), ANG II blocked FSK-induced NHE3 inhibition, reduced intracellular cAMP concentrations, lowered PKA activity, and prevented the FSK-mediated increase in NHE3 serine 552 phosphorylation. All effects of ANG II were blocked by pretreating OKP cells with the AT₁R antagonist losartan, highlighting the contribution of the AT₁R/G_i pathway in ANG II-mediated NHE3 upregulation under cAMP-elevating conditions. Accordingly, G_i inhibition by pertussis toxin treatment decreased NHE3 activity both in vitro and in vivo and, more importantly, prevented the stimulatory effect of ANG II on NHE3 activity in rat proximal tubules. Collectively, our results suggest that ANG II counteracts the effects of cAMP/PKA on NHE3 phosphorylation and inhibition by activating the AT₁R/G_i pathway. Moreover, these findings support the notion that NHE3 dephosphorylation at serine 552 may represent a key event in the regulation of renal proximal tubule sodium handling by ANG II in the presence of natriuretic hormones that promote cAMP accumulation and transporter phosphorylation.

Dipeptidyl Peptidase IV Inhibition Exerts Renoprotective Effects in Rats with Established Heart Failure

Circulating dipeptidyl peptidase IV (DPPIV) activity is associated with worse cardiovascular outcomes in humans and experimental heart failure (HF) models, suggesting that DPPIV may play a role in the pathophysiology of this syndrome. Renal dysfunction is one of the key features of HF, but it remains to be determined whether DPPIV inhibitors are capable of improving cardiorenal function after the onset of HF. Therefore, the present study aimed to test the hypothesis that DPPIV inhibition by vildagliptin improves renal water and salt handling and exerts anti-proteinuric effects in rats with established HF. To this end, male Wistar rats were subjected to left ventricle (LV) radiofrequency ablation or sham operation. Six weeks after surgery, radiofrequency-ablated rats who developed HF were randomly divided into two groups and treated for 4 weeks with vildagliptin (120 mg/kg/day) or vehicle by oral gavage. Echocardiography was performed before (pretreatment) and at the end of treatment (post-treatment) to evaluate cardiac function. The fractional area change (FAC) increased (34 ± 5 vs. 45 ± 3%, p < 0.05), and the isovolumic relaxation time decreased (33 ± 2 vs. 27 ± 1 ms; p < 0.05) in HF rats treated with vildagliptin (post-treatment vs. pretreatment). On the other hand, cardiac dysfunction deteriorated further in vehicle-treated HF rats. Renal function was impaired in vehicle-treated HF rats as evidenced by fluid retention, low glomerular filtration rate (GFR) and high levels of urinary protein excretion. Vildagliptin treatment restored urinary flow, GFR, urinary sodium and urinary protein excretion to sham levels. Restoration of renal function in HF rats by DPPIV inhibition was associated with increased active glucagon-like peptide-1 (GLP-1) serum concentration, reduced DPPIV activity and increased activity of protein kinase A in the renal cortex. Furthermore, the anti-proteinuric effect of vildagliptin treatment in rats with established HF was associated with upregulation of the apical proximal tubule endocytic receptor megalin and of the podocyte main slit diaphragm proteins nephrin and podocin. Collectively, these findings demonstrate that DPPIV inhibition exerts renoprotective effects and ameliorates cardiorenal function in rats with established HF. Long-term studies with DPPIV inhibitors are needed to ascertain whether these effects ultimately translate into improved clinical outcomes.

AT2 Receptor Activation Prevents Sodium Retention and Reduces Blood Pressure in Angiotensin II-Dependent Hypertension

RATIONALE

Compound 21 (C-21) is a highly selective nonpeptide angiotensin AT2 receptor (AT2R) agonist.

OBJECTIVE

To test the hypothesis that chronic AT2R activation with C-21 induces natriuresis via an action at the renal proximal tubule (RPT) and lowers blood pressure (BP) in experimental angiotensin II (Ang II)-dependent hypertension.

METHODS AND RESULTS

In rats, Ang II infusion increased both sodium (Na(+)) retention and BP on day 1, and BP remained elevated throughout the 7-day infusion period. Either intrarenal or systemic administration of C-21 prevented Ang II-mediated Na(+) retention on day 1, induced continuously negative cumulative Na(+) balance compared with Ang II alone, and reduced BP chronically. The effects of C-21 are likely to be mediated by action on the RPT as acute systemic C-21-induced natriuresis was additive to that induced by chlorothiazide and amiloride. At 24 hours of Ang II infusion, AT2R activation with C-21, both intrarenally and systemically, translocated AT2Rs from intracellular sites to the apical plasma membranes of RPT cells without altering the total cellular pool of AT2Rs and internalized/inactivated major RPT Na(+) transporters Na(+)-H(+)-exchanger-3 and Na(+)/K(+)ATPase. C-21 lowered BP to a similar degree whether administered before or subsequent to the establishment of Ang II-dependent hypertension.

CONCLUSIONS

Chronic AT2R activation initiates and sustains receptor translocation to RPT apical plasma membranes, internalizes/inactivates Na(+)-H(+)-exchanger-3 and Na(+)/K(+)ATPase, prevents Na(+) retention resulting in negative cumulative Na(+) balance, and lowers BP in experimental Ang II-induced hypertension. Acting uniquely at the RPT, C-21 is a promising candidate for the treatment of hypertension and Na(+)-retaining states in humans.

Emerging roles of Na⁺/H⁺ exchangers in epilepsy and developmental brain disorders

Epilepsy is a common central nervous system (CNS) disease characterized by recurrent transient neurological events occurring due to abnormally excessive or synchronous neuronal activity in the brain. The CNS is affected by systemic acid-base disorders, and epileptic seizures are sensitive indicators of underlying imbalances in cellular pH regulation. Na(+)/H(+) exchangers (NHEs) are a family of membrane transporter proteins actively involved in regulating intracellular and organellar pH by extruding H(+) in exchange for Na(+) influx. Altering NHE function significantly influences neuronal excitability and plays a role in epilepsy. This review gives an overview of pH regulatory mechanisms in the brain with a special focus on the NHE family and the relationship between epilepsy and dysfunction of NHE isoforms. We first discuss how cells translocate acids and bases across the membrane and establish pH homeostasis as a result of the concerted effort of enzymes and ion transporters. We focus on the specific roles of the NHE family by detailing how the loss of NHE1 in two NHE mutant mice results in enhanced neuronal excitability in these animals. Furthermore, we highlight new findings on the link between mutations of NHE6 and NHE9 and developmental brain disorders including epilepsy, autism, and attention deficit hyperactivity disorder (ADHD). These studies demonstrate the importance of NHE proteins in maintaining H(+) homeostasis and their intricate roles in the regulation of neuronal function. A better understanding of the mechanisms underlying NHE1, 6, and 9 dysfunctions in epilepsy formation may advance the development of new epilepsy treatment strategies.

Crosstalk between the renal sympathetic nerve and intrarenal angiotensin II modulates proximal tubular sodium reabsorption

NEW FINDINGS

What is the topic of this review? The sympathetic control of renal sodium tubular reabsorption is dependent on activation of the intrarenal renin-angiotensin system and activation of the angiotensin II type 1 (AT1 ) receptor by angiotensin II. What advances does it highlight? Despite the fact that the interaction between the sympathetic nervous system and angiotensin II regarding salt reabsorption is a well-known classical mechanism for the maintenance of extracellular volume homeostasis, the underlying molecular signalling is not clearly understood. It has been shown recently that renal nerve stimulation increases intrarenal angiotensin II and activates the AT1 receptor, triggering a signalling cascade that leads to elevations of Na(+) -H(+) exchanger isoform 3-mediated tubular transport. In this short review, the crosstalk between intrarenal angiotensin II and renal nerve activity and its effect on sodium reabsorption is addressed. In this review, we address the importance of the interaction between the sympathetic nervous system and intrarenal renin-angiotensin system in modulating renal tubular handling of sodium and water. We have recently shown that increased Na(+) -H(+) exchanger isoform 3 (NHE3) activity induced by renal nerve stimulation (RNS) depends on the activation of the angiotensin II type 1 (AT1 ) receptor by angiotensin II (Ang II). Low-frequency RNS resulted in higher levels of intrarenal angiotensinogen and Ang II independent of changes in blood pressure, the glomerular filtration rate and systemic angiotensinogen. Angiotensin II, via the AT1 receptor, triggered an intracellular pathway activating NHE3 in the renal cortex, leading to antinatriuresis and antidiuresis. Pharmacological blockade of the AT1 receptor with losartan prior to RNS abolished both the functional and the molecular responses, suggesting that intrarenal Ang II acting via the AT1 receptor is a major factor for NHE3-mediated sodium and water reabsorption induced by RNS.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

Potential role of dipeptidyl peptidase IV in the pathophysiology of heart failure

Dipeptidyl peptidase IV (DPPIV) is a widely expressed multifunctional serine peptidase that exists as a membrane-anchored cell surface protein or in a soluble form in the plasma and other body fluids. Numerous substrates are cleaved at the penultimate amino acid by DPPIV, including glucagon-like peptide-1 (GLP-1), brain natriuretic peptide (BNP) and stromal cell-derived factor-1 (SDF-α), all of which play important roles in the cardiovascular system. In this regard, recent reports have documented that circulating DPPIV activity correlates with poorer cardiovascular outcomes in human and experimental heart failure (HF). Moreover, emerging evidence indicates that DPPIV inhibitors exert cardioprotective and renoprotective actions in a variety of experimental models of cardiac dysfunction. On the other hand, conflicting results have been found when translating these promising findings from preclinical animal models to clinical therapy. In this review, we discuss how DPPIV might be involved in the cardio-renal axis in HF. In addition, the potential role for DPPIV inhibitors in ameliorating heart disease is revised, focusing on the effects of the main DPPIV substrates on cardiac remodeling and renal handling of salt and water.