Role of intestinal Na(+)/H(+) exchanger inhibition in the prevention of cardiovascular and kidney disease

Phosphate Control: The Next Frontier in Dialysis Cardiovascular Mortality

BACKGROUND

Cardiovascular disease (CVD) is a major cause of death in patients with chronic kidney disease (CKD) on dialysis. Mortality rates are still unacceptably high even though they have fallen in the past 2 decades. Hyperphosphatemia (elevated serum phosphate levels) is seen in almost all patients with advanced CKD and is by far the largest remaining modifiable contributor to CKD mortality.

SUMMARY

Phosphate retention drives multiple physiological mechanisms linked to increased risk of CVD. Fibroblast growth factor 23 and parathyroid hormone (PTH) levels, both of which have been suggested to have direct pathogenic CV effects, increase in response to phosphate retention. Phosphate, calcium, and PTH levels are linked in a progressively worsening cycle. Maladaptive upregulation of phosphate absorption is also likely to occur further exacerbating hyperphosphatemia. Even higher phosphate levels within the normal range may be a risk factor for vascular calcification and, thus, CV morbidity and mortality. A greater degree of phosphate control is important to reduce the risk of CV morbidity and mortality. Improved phosphate control and regular monitoring of phosphate levels are guideline-recommended, established clinical practices. There are several challenges with the current phosphate management approaches in patients with CKD on dialysis. Dietary restriction of phosphate and thrice-weekly dialysis alone are insufficient/unreliable to reduce phosphate to <5.5 mg/dL. Even with the addition of phosphate binders, the only pharmacological treatment currently indicated for hyperphosphatemia, the majority of patients are unable to achieve and maintain phosphate levels <5.5 mg/dL (or more normal levels) [PhosLo® gelcaps (calcium acetate): 667 mg (prescribing information), 2011, VELPHORO®: (Sucroferric oxyhydroxide) (prescribing information), 2013, FOSRENAL®: (Lanthanum carbonate) (prescribing information), 2016, AURYXIA®: (Ferric citrate) tablets (prescribing information), 2017, RENVELA®: (Sevelamer carbonate) (prescribing information), 2020, RealWorld dynamix. Dialysis US: Spherix Global Insights, 2019]. Phosphate binders do not target the primary pathway of phosphate absorption (paracellular), have limited binding capacity, and bind nonspecifically [PhosLo® gelcaps (calcium acetate): 667 mg (prescribing information). 2013, VELPHORO®: (Sucroferric oxyhydroxide) (prescribing information), 2013, FOSRENAL®: (Lanthanum carbonate) (prescribing information), 2016, AURYXIA®: (Ferric citrate) tablets (prescribing information), 2017, RENVELA®: (Sevelamer carbonate) (prescribing information) 2020]. Key Messages: Despite current phosphate management strategies, most patients on dialysis are unable to consistently achieve target phosphate levels, indicating a need for therapeutic innovations [RealWorld dynamix. Dialysis US: Spherix Global Insights, 2019]. Given a growing evidence base that the dominant mechanism of phosphate absorption is the intestinal paracellular pathway, new therapies are investigating ways to reduce phosphate levels by blocking absorption through the paracellular pathway.

Inhibition of sodium-proton-exchanger subtype 3-mediated sodium absorption in the gut: A new antihypertensive concept

Arterial hypertension is one of the main contributors to cardiovascular diseases, including stroke, heart failure, and coronary heart disease. Salt plays a major role in the regulation of blood pressure and is one of the most critical factors for hypertension and stroke. At the individual level, effective salt reduction is difficult to achieve and available methods for managing sodium balance are lacking for many patients. As part of the ingested food, salt is absorbed in the gastrointestinal tract by the sodium proton exchanger subtype 3 (NHE3 also known as Slc9a3), influencing extracellular fluid volume and blood pressure. In this review, we discuss the beneficial effects of pharmacological inhibition of NHE3-mediated sodium absorption in the gut and focus on the effect on blood pressure and end-organ damage.

Emerging Therapy in Hypertension

PURPOSE OF THE REVIEW

Pharmacology remains the mainstay of treatment for hypertension across the globe. In what may seem like a well-trodden field, there are actually an exciting array of new pathways for the treatment of hypertension on the horizon. This review seeks to discuss the most recent research in ongoing areas of drug development in the field of hypertension.

RECENT FINDINGS

Novel areas of research in the field of hypertension pharmacology include central nervous system regulators, peripheral noradrenergic inhibitors, gastrointestinal sodium modulators, and a counter-regulatory arm of the renin-angiotensin-aldosterone system. This review discusses these pathways in a look into the current status of emerging pharmacological therapies for hypertension.

Simulating the Function of the MjNhaP1 Transporter

The structures of transport proteins have been steadily revealed in the last few decades, and yet the conversion of this information into molecular-level understanding of their function is still lagging behind. In this study, we try to elucidate how the action of the archaeal sodium/proton antiporter MjNhaP1 depends on its structure-energy relationship. To this end, we calculate the binding energies of its substrates and evaluate the conformational change barrier, focusing on the rotation of the catalytic residue D161. We find that sodium ions and protons compete against a common binding site and that the accessibility of this binding site is restricted to either the inside or outside of the cell. We suggest that the rotation of D161 χ1 angle correlates with the conformational change and is energetically unfavorable when D161 does not bind any substrate. This restriction ensures coupling between the sodium ions and the protons, allowing MjNhaP1 and probably other similar transporters to exchange substrates with minimal leak. Using Monte Carlo simulations we demonstrate the feasibility of our model. Overall we present a complete picture that reproduces the electroneutral (at 1:1 substrate ratio) and coupled transport activity of MjNhaP1 including the energetic basis for the criteria provided by Jardetzky half a century ago.

The Ec-NhaA antiporter switches from antagonistic to synergistic antiport upon a single point mutation

The Na⁺, Li⁺/H⁺ antiporter of Escherichia coli (Ec-NhaA) maintains pH, Na⁺ homeostasis in enterobacteria. We used isothermal titration calorimetry to perform a detailed thermodynamic analysis of Li⁺ binding to Ec-NhaA and several of its mutants. We found that, in line with the canonical alternative access mechanistic model of secondary transporters, Li⁺/H⁺ binding to the antiporter is antagonistically coupled. Binding of Li⁺ displaces 2 H⁺ from the binding site. The process is enthalpically driven, the enthalpic gain just compensating for an entropic loss and the buffer-associated enthalpic changes dominate the overall free-energy change. Li⁺ binding, H⁺ release and antiporter activity were all affected to the same extent by mutations in the Li⁺ binding site (D163E, D163N, D164N, D164E), while D133C changed the H⁺/Li⁺ stoichiometry to 4. Most striking, however, was the mutation, A167P, which converted the Ec-NhaA antagonistic binding into synergistic binding which is only known to occur in Cl⁻/H⁺ antiporter.

Simulating the function of sodium/proton antiporters

The molecular basis of the function of transporters is a problem of significant importance, and the emerging structural information has not yet been converted to a full understanding of the corresponding function. This work explores the molecular origin of the function of the bacterial Na+/H+ antiporter NhaA by evaluating the energetics of the Na+ and H+ movement and then using the resulting landscape in Monte Carlo simulations that examine two transport models and explore which model can reproduce the relevant experimental results. The simulations reproduce the observed transport features by a relatively simple model that relates the protein structure to its transporting function. Focusing on the two key aspartic acid residues of NhaA, D163 and D164, shows that the fully charged state acts as an Na+ trap and that the fully protonated one poses an energetic barrier that blocks the transport of Na+. By alternating between the former and latter states, mediated by the partially protonated protein, protons, and Na+ can be exchanged across the membrane at 2:1 stoichiometry. Our study provides a numerical validation of the need of large conformational changes for effective transport. Furthermore, we also yield a reasonable explanation for the observation that some mammalian transporters have 1:1 stoichiometry. The present coarse-grained model can provide a general way for exploring the function of transporters on a molecular level.