Localization and functional characterization of Na+/H+ exchanger isoform NHE4 in rat thick ascending limbs

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

State of knowledge on ammonia handling by the kidney

The disposal of ammonia, the main proton buffer in the urine, is important for acid-base homeostasis. Renal ammonia excretion is the predominant contributor to renal net acid excretion, both under basal condition and in response to acidosis. New insights into the mechanisms of renal ammonia production and transport have been gained in the past decades. Ammonia is the only urinary solute known to be produced in the kidney and selectively transported through the different parts of the nephron. Both molecular forms of total ammonia, NH3 and NH4+, are transported by specific proteins. Proximal tubular ammoniagenesis and the activity of these transport processes determine the eventual fate of total ammonia produced and excreted by the kidney. In this review, we summarized the state of the art of ammonia handling by the kidney and highlighted the newest processes described in the last decade.

The Na+/H+ Exchanger 3 in the Intestines and the Proximal Tubule of the Kidney: Localization, Physiological Function, and Key Roles in Angiotensin II-Induced Hypertension

The sodium (Na⁺)/hydrogen (H⁺) exchanger 3 (NHE3) is one of the most important Na⁺/H⁺ antiporters in the small intestines of the gastrointestinal tract and the proximal tubules of the kidney. The roles of NHE3 in the regulation of intracellular pH and acid-base balance have been well established in cellular physiology using in vitro techniques. Localized primarily on the apical membranes in small intestines and proximal tubules, the key action of NHE3 is to facilitate the entry of luminal Na⁺ and the extrusion of intracellular H⁺ from intestinal and proximal tubule tubular epithelial cells. NHE3 is, directly and indirectly, responsible for absorbing the majority of ingested Na⁺ from small and large intestines and reabsorbing >50% of filtered Na⁺ in the proximal tubules of the kidney. However, the roles of NHE3 in the regulation of proximal tubular Na⁺ transport in the integrative physiological settings and its contributions to the basal blood pressure regulation and angiotensin II (Ang II)-induced hypertension have not been well studied previously due to the lack of suitable animal models. Recently, novel genetically modified mouse models with whole-body, kidney-specific, or proximal tubule-specific deletion of NHE3 have been generated by us and others to determine the critical roles and underlying mechanisms of NHE3 in maintaining basal body salt and fluid balance, blood pressure homeostasis, and the development of Ang II-induced hypertension at the whole-body, kidney, or proximal tubule levels. The objective of this invited article is to review, update, and discuss recent findings on the critical roles of intestinal and proximal tubule NHE3 in maintaining basal blood pressure homeostasis and their potential therapeutic implications in the development of angiotensin II (Ang II)-dependent hypertension.

Role of the Basolateral Na+/H+ Exchanger-2 (NHE2) in Ionocytes of Seawater- Acclimated Medaka (Oryzias latipes)

Ionocytes in the skin and gills of seawater (SW) fishes are responsible for acid-base regulation and salt secretion. Na⁺/H⁺ exchangers (NHEs) are considered the major acid (H⁺)-secreting transporters in ionocytes of SW fishes. However, the subcellular localization and function of a specific NHE isoform (NHE2) have never clearly been revealed. In this study, we cloned and sequenced NHE2 from an SW-acclimated medaka (Oryzias latipes) and examined its functions in medaka embryos. A phylogenetic analysis showed that the evolutionary relationships of mammalian NHE2 and NHE4 are close to those of fish NHE2. A gene structure analysis showed that tetrapod NHE4 might be a tandem duplication of fish NHE2. Immunohistochemistry with a medaka-specific antibody localized NHE2 to the basolateral membrane of ionocytes. Lost-of-function experiments with photo-activated morpholino oligonucleotides showed that both H⁺ and Cl^– secretion by ionocytes were suppressed in NHE2-knockdown embryos, suggesting that the basolateral NHE2 facilitates acid and salt secretion by ionocytes of medaka in seawater.

Multiple acid-base and electrolyte disturbances upregulate NBCn1, NBCn2, IRBIT and L-IRBIT in the mTAL

KEY POINTS

The roles of the Na⁺ /HCO₃ ^- cotransporters NBCn1 and NBCn2 as well as their activators IRBIT and L-IRBIT in the regulation of the mTAL transport of NH₄ ⁺ , HCO₃ ^- , and NaCl are investigated. Dietary challenges of NH₄ Cl, NaHCO₃ or NaCl all increase the abundance of NBCn1 and NBCn2 in the outer medulla. The three challenges generally produce parallel increases in the abundance of IRBIT and L-IRBIT in the outer medulla. Both IRBIT and L-IRBIT powerfully stimulate the activities of the mTAL isoforms of NBCn1 and NBCn2 as expressed in Xenopus oocytes. Our findings support the hypothesis that NBCn1, NBCn2, IRBIT and L-IRBIT appropriately promote NH₄ ⁺ shunting but oppose HCO₃ ^- and NaCl reabsorption in the mTAL, and thus are at the nexus of the regulation pathways for multiple renal transport processes.

ABSTRACT

The medullary thick ascending limb (mTAL) plays a key role in urinary acid and NaCl excretion. NBCn1 and NBCn2 are present in the basolateral mTAL, where NBCn1 promotes NH₄ ⁺ shunting. IRBIT and L-IRBIT (the IRBITs) are two powerful activators of certain acid-base transporters. Here we use western blotting and immunofluorescence to examine the effects of multiple acid-base and electrolyte disturbances on expression of NBCn1, NBCn2 and the IRBITs in rat kidney. We also use electrophysiology to examine the functional effects of IRBITs on NBCn1 and NBCn2 in Xenopus oocytes. NH₄ Cl-induced metabolic acidosis (MAc) substantially increases protein expression of NBCn1 and NBCn2 in the outer medulla (OM) of rat kidney. Surprisingly, NaHCO₃ -induced metabolic alkalosis (MAlk) and high-salt diet (HSD) also increase expression of NBCn1 and NBCn2 (effect of NaHCO₃  > HSD). Moreover, all three challenges generally increase OM expression of the IRBITs. In Xenopus oocytes, the IRBITs substantially increase the activities of NBCn1 and NBCn2. We propose that upregulation of basolateral NBCn1 and NBCn2 plus the IRBITs in the mTAL: (1) promotes NH₄ ⁺ shunting by increasing basolateral HCO₃ ^- uptake to neutralize apical NH₄ ⁺ uptake during MAc; (2) inhibits HCO₃ ^- reabsorption during MAlk by opposing HCO₃ ^- efflux via the basolateral anion exchanger AE2; and (3) inhibits NaCl reabsorption by mediating (with AE2) net NaCl backflux into the mTAL cell during HSD. Thus, NBCn1, NBCn2 and the IRBITs are at the nexus of the regulatory pathways for multiple renal transport processes.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Ion channels and transporters in diabetic kidney disease

Type 1 and 2 diabetes mellitus are major medical epidemics affecting millions of patients worldwide. Diabetes mellitus is the leading cause of diabetic kidney disease (DKD), which is the most common cause of end-stage renal disease (ESRD). DKD is associated with significant changes in renal hemodynamics and electrolyte transport. Alterations in renal ion transport triggered by pathophysiological conditions in diabetes can exacerbate hypertension, accelerate renal injury, and are integral to the development of DKD. Renal ion transporters and electrolyte homeostasis play a fundamental role in functional changes and injury to the kidney during DKD. With the large number of ion transporters involved in DKD, understanding the roles of individual transporters as well as the complex cascades through which they interact is essential in the development of effective treatments for patients suffering from this disease. This chapter aims to gather current knowledge of the major renal ion transporters with altered expression and activity under diabetic conditions, and provide a comprehensive overview of their interactions and collective functions in DKD.

Ammonia Transporters and Their Role in Acid-Base Balance

Acid-base homeostasis is critical to maintenance of normal health. Renal ammonia excretion is the quantitatively predominant component of renal net acid excretion, both under basal conditions and in response to acid-base disturbances. Although titratable acid excretion also contributes to renal net acid excretion, the quantitative contribution of titratable acid excretion is less than that of ammonia under basal conditions and is only a minor component of the adaptive response to acid-base disturbances. In contrast to other urinary solutes, ammonia is produced in the kidney and then is selectively transported either into the urine or the renal vein. The proportion of ammonia that the kidney produces that is excreted in the urine varies dramatically in response to physiological stimuli, and only urinary ammonia excretion contributes to acid-base homeostasis. As a result, selective and regulated renal ammonia transport by renal epithelial cells is central to acid-base homeostasis. Both molecular forms of ammonia, NH₃ and NH₄⁺, are transported by specific proteins, and regulation of these transport processes determines the eventual fate of the ammonia produced. In this review, we discuss these issues, and then discuss in detail the specific proteins involved in renal epithelial cell ammonia transport.

Na+-H+ exchanger-1 (NHE1) regulation in kidney proximal tubule

The ubiquitously expressed plasma membrane Na(+)-H(+) exchanger NHE1 is a 12 transmembrane-spanning protein that directs important cell functions such as homeostatic intracellular volume and pH control. The 315 amino acid cytosolic tail of NHE1 binds plasma membrane phospholipids and multiple proteins that regulate additional, ion-translocation independent functions. This review focuses on NHE1 structure/function relationships, as well as the role of NHE1 in kidney proximal tubule functions, including pH regulation, vectorial Na(+) transport, cell volume control and cell survival. The implications of these functions are particularly critical in the setting of progressive, albuminuric kidney diseases, where the accumulation of reabsorbed fatty acids leads to disruption of NHE1-membrane phospholipid interactions and tubular atrophy, which is a poor prognostic factor for progression to end stage renal disease. This review amplifies the vital role of the proximal tubule NHE1 Na(+)-H(+) exchanger as a kidney cell survival factor.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

Structural dynamics and regulation of the mammalian SLC9A family of Na⁺/H⁺ exchangers

Mammalian Na⁺/H⁺ exchangers of the SLC9A family are widely expressed and involved in numerous essential physiological processes. Their primary function is to mediate the 1:1 exchange of Na⁺ for H⁺ across the membrane in which they reside, and they play central roles in regulation of body, cellular, and organellar pH. Their function is tightly regulated through mechanisms involving interactions with multiple protein and lipid-binding partners, phosphorylations, and other posttranslational modifications. Biochemical and mutational analyses indicate that the SLC9As have a short intracellular N-terminus, 12 transmembrane (TM) helices necessary and sufficient for ion transport, and a C-terminal cytoplasmic tail region with essential regulatory roles. No high-resolution structures of the SLC9As exist; however, models based on crystal structures of the bacterial NhaAs support the 12 TM organization and suggest that TMIV and XI may form a central part of the ion-translocation pathway, whereas pH sensing may involve TMII, TMIX, and several intracellular loops. Similar to most ion transporters studied, SLC9As likely exist as coupled dimers in the membrane, and this appears to be important for the well-studied cooperativity of H⁺ binding. The aim of this work is to summarize and critically discuss the currently available evidence on the structural dynamics, regulation, and binding partner interactions of SLC9As, focusing in particular on the most widely studied isoform, SLC9A1/NHE1. Further, novel bioinformatic and structural analyses are provided that to some extent challenge the existing paradigm on how ions are transported by mammalian SLC9As.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

The SLC9 gene family encodes Na(+)/H(+) exchangers (NHEs). These transmembrane proteins transport ions across lipid bilayers in a diverse array of species from prokaryotes to eukaryotes, including plants, fungi, and animals. They utilize the electrochemical gradient of one ion to transport another ion against its electrochemical gradient. Currently, 13 evolutionarily conserved NHE isoforms are known in mammals [22, 46, 128]. The SLC9 gene family (solute carrier classification of transporters: www.bioparadigms.org) is divided into three subgroups [46]. The SLC9A subgroup encompasses plasmalemmal isoforms NHE1-5 (SLC9A1-5) and the predominantly intracellular isoforms NHE6-9 (SLC9A6-9). The SLC9B subgroup consists of two recently cloned isoforms, NHA1 and NHA2 (SLC9B1 and SLC9B2, respectively). The SLC9C subgroup consist of a sperm specific plasmalemmal NHE (SLC9C1) and a putative NHE, SLC9C2, for which there is currently no functional data [46]. NHEs participate in the regulation of cytosolic and organellar pH as well as cell volume. In the intestine and kidney, NHEs are critical for transepithelial movement of Na(+) and HCO3(-) and thus for whole body volume and acid-base homeostasis [46]. Mutations in the NHE6 or NHE9 genes cause neurological disease in humans and are currently the only NHEs directly linked to human disease. However, it is becoming increasingly apparent that members of this gene family contribute to the pathophysiology of multiple human diseases.

The regulation of NHE₁ and NHE₃ activity by angiotensin II is mediated by the activation of the angiotensin II type I receptor/phospholipase C/calcium/calmodulin pathway in distal nephron cells

Angiotensin II (Ang II), acting via the AT1 receptor, induces an increase in intracellular calcium [Ca(2+)]i that then interacts with calmodulin (CaM). The Ca(2+)/CaM complex directly or indirectly activates sodium hydrogen exchanger 1 (NHE1) and phosphorylates calmodulin kinase II (CaMKII), which then regulates sodium hydrogen exchanger 3 (NHE3) activity. In this study, we investigated the cellular signaling pathways responsible for Ang II-mediated regulation of NHE1 and NHE3 in Madin-Darby canine kidney (MDCK) cells. The NHE1- and NHE3-dependent pHi recovery rates were evaluated by fluorescence microscopy using the fluorescent probe BCECF/AM, messenger RNA was evaluated with the reverse transcription polymerase chain reaction (RT-PCR), and protein expression was evaluated by immunoblot. We demonstrated that treatment with Ang II (1pM or 1 nM) for 30 min induced, via the AT1 but not the AT2 receptor, an equal increase in NHE1 and NHE3 activity that was reduced by the specific inhibitors HOE 694 and S3226, respectively. Ang II (1 nM) did not change the total expression of NHE1, NHE3 or calmodulin, but it induced CaMKII, cRaf-1, Erk1/2 and p90(RSK) phosphorylation. The stimulatory effects of Ang II (1 nM) on NHE1 or NHE3 activity or protein abundance was reduced by ophiobolin-A (CaM inhibitor), KN93 (CaMKII inhibitor) or PD98059 (Mek inhibitor). These results indicate that after 30 min, Ang II treatment may activate G protein-dependent pathways, including the AT1/PLC/Ca(2+)/CaM pathway, which induces CaMKII phosphorylation to stimulate NHE3 and induces cRaf-1/Mek/Erk1/2/p90(RSK) activity to stimulate NHE1.

Renal ammonia metabolism and transport

Renal ammonia metabolism and transport mediates a central role in acid-base homeostasis. In contrast to most renal solutes, the majority of renal ammonia excretion derives from intrarenal production, not from glomerular filtration. Renal ammoniagenesis predominantly results from glutamine metabolism, which produces 2 NH4(+) and 2 HCO3(-) for each glutamine metabolized. The proximal tubule is the primary site for ammoniagenesis, but there is evidence for ammoniagenesis by most renal epithelial cells. Ammonia produced in the kidney is either excreted into the urine or returned to the systemic circulation through the renal veins. Ammonia excreted in the urine promotes acid excretion; ammonia returned to the systemic circulation is metabolized in the liver in a HCO3(-)-consuming process, resulting in no net benefit to acid-base homeostasis. Highly regulated ammonia transport by renal epithelial cells determines the proportion of ammonia excreted in the urine versus returned to the systemic circulation. The traditional paradigm of ammonia transport involving passive NH3 diffusion, protonation in the lumen and NH4(+) trapping due to an inability to cross plasma membranes is being replaced by the recognition of limited plasma membrane NH3 permeability in combination with the presence of specific NH3-transporting and NH4(+)-transporting proteins in specific renal epithelial cells. Ammonia production and transport are regulated by a variety of factors, including extracellular pH and K(+), and by several hormones, such as mineralocorticoids, glucocorticoids and angiotensin II. This coordinated process of regulated ammonia production and transport is critical for the effective maintenance of acid-base homeostasis.

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

More actors in ammonia absorption by the thick ascending limb

This review will briefly summarize current knowledge on the basolateral ammonia transport mechanisms in the thick ascending limb (TAL) of the loop of Henle. This segment transports ammonia against a concentration gradient and is responsible for the accumulation of ammonia in the medullary interstitium, which, in turn, favors ammonia secretion across the collecting duct. Experimental data indicate that the sodium/hydrogen ion exchanger isoform 4 (NHE4; Scl9a4) is a sodium/ammonia exchanger and plays a major role in this process. Disruption of murine NHE4 leads to metabolic acidosis with inappropriate urinary ammonia excretion and decreases the ability of the TAL to absorb ammonia and to build the corticopapillary ammonia gradient. However, NHE4 does not account for the entirety of ammonia absorption by the TAL, indicating that, at least, one more transporter is involved.

Functional role of NHE4 as a pH regulator in rat and human colonic crypts

To regulate ionic and fluid homeostasis, the colon relies upon a series of Na(+)-dependent transport proteins. Recent studies have identified a sodium/hydrogen exchanger (NHE) 4 (NHE4) protein in the gastrointestinal tract but to date there has been little description of its function. Additionally, we have previously shown that aldosterone can rapidly modulate Na(+)-dependent proton excretion via NHE proteins. In this study we examined the role of NHE4 in rat and human colonic crypts, determined the effect of aldosterone on NHE4 specifically, and explored the intracellular pathways leading to activation. Colonic samples were dissected from Sprague-Dawley rats. Human specimens were obtained from patients undergoing elective colon resections. Crypts were isolated using ethylenediaminetetraacetic acid and intracellular pH (pH(i)) changes were monitored using 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Crypts were exposed to 7 μM ethylisopropylamiloride or 400 μM amiloride, doses previously shown to inhibit NHE1 and NHE3 but allow NHE4 to remain active. Functional NHE4 activity was demonstrated in both rat and human colonic crypts. NHE4 activity was increased in the presence of 1 μM aldosterone. In the rat model, crypts were exposed to 100 μM 3-isobutyl-1-methylxanthine/1 μM forskolin and demonstrated a decrease in NHE4 activity with increased cAMP levels. No significant change in NHE4 activity was seen by increasing osmolarity. These results demonstrate functional NHE4 activity in the rat and human colon and an increase in activity by aldosterone. This novel exchanger is capable of modulating intracellular pH over a wide pH spectrum and may play an important role in maintaining cellular pH homeostasis.

Role of NH3 and NH4+ transporters in renal acid-base transport

Renal ammonia excretion is the predominant component of renal net acid excretion. The majority of ammonia excretion is produced in the kidney and then undergoes regulated transport in a number of renal epithelial segments. Recent findings have substantially altered our understanding of renal ammonia transport. In particular, the classic model of passive, diffusive NH3 movement coupled with NH4+ "trapping" is being replaced by a model in which specific proteins mediate regulated transport of NH3 and NH4+ across plasma membranes. In the proximal tubule, the apical Na+/H+ exchanger, NHE-3, is a major mechanism of preferential NH4+ secretion. In the thick ascending limb of Henle's loop, the apical Na+-K+-2Cl- cotransporter, NKCC2, is a major contributor to ammonia reabsorption and the basolateral Na+/H+ exchanger, NHE-4, appears to be important for basolateral NH4+ exit. The collecting duct is a major site for renal ammonia secretion, involving parallel H+ secretion and NH3 secretion. The Rhesus glycoproteins, Rh B Glycoprotein (Rhbg) and Rh C Glycoprotein (Rhcg), are recently recognized ammonia transporters in the distal tubule and collecting duct. Rhcg is present in both the apical and basolateral plasma membrane, is expressed in parallel with renal ammonia excretion, and mediates a critical role in renal ammonia excretion and collecting duct ammonia transport. Rhbg is expressed specifically in the basolateral plasma membrane, and its role in renal acid-base homeostasis is controversial. In the inner medullary collecting duct (IMCD), basolateral Na+-K+-ATPase enables active basolateral NH4+ uptake. In addition to these proteins, several other proteins also contribute to renal NH3/NH4+ transport. The role and mechanisms of these proteins are discussed in depth in this review.

NHE4 is critical for the renal handling of ammonia in rodents

Ammonia absorption by the medullary thick ascending limb of Henle's loop (MTALH) is thought to be a critical step in renal ammonia handling and excretion in urine, in which it is the main acid component. Basolateral Na+/H+ exchangers have been proposed to play a role in ammonia efflux out of MTALH cells, which express 2 exchanger isoforms: Na+/H+ exchanger 1 (NHE1) and NHE4. Here, we investigated the role of NHE4 in urinary acid excretion and found that NHE4-/- mice exhibited compensated hyperchloremic metabolic acidosis, together with inappropriate urinary net acid excretion. When challenged with a 7-day HCl load, NHE4-/- mice were unable to increase their urinary ammonium and net acid excretion and displayed reduced ammonium medulla content compared with wild-type littermates. Both pharmacologic inhibition and genetic disruption of NHE4 caused a marked decrease in ammonia absorption by the MTALH. Finally, dietary induction of metabolic acidosis increased NHE4 mRNA expression in mouse MTALH cells and enhanced renal NHE4 activity in rats, as measured by in vitro microperfusion of MTALH. We therefore conclude that ammonia absorption by the MTALH requires the presence of NHE4 and that lack of NHE4 reduces the ability of MTALH epithelial cells to create the cortico-papillary gradient of NH3/NH4+ needed to excrete an acid load, contributing to systemic metabolic acidosis.

A mathematical model of rat ascending Henle limb. II. Epithelial function

A mathematical model of ascending Henle limb (AHL) epithelium has been fashioned using kinetic representations of Na+-K+-2Cl- cotransporter (NKCC2), KCC4, and type 3 Na+/H+ exchanger (NHE3), with transporter densities selected to yield the reabsorptive Na+ flux expected for rat tubules in vivo. Of necessity, this model predicts fluxes that are higher than those measured in vitro. The kinetics of the NKCC and KCC are such that Na+ reabsorption by the model tubule is responsive to variation in luminal NaCl concentration over the range of 30 to 130 mM, with only minor changes in cell volume. Peritubular KCC accounts for about half the reabsorptive Cl- flux, with the remainder via peritubular Cl- channels. Transcellular Na+ flux is turned off by increasing peritubular KCl, which produces increased cytosolic Cl- and thus inhibits NKCC2 transport. In the presence of physiological concentrations of ammonia, there is a large acid challenge to the cell, due primarily to NH4+ entry via NKCC2, with diffusive NH3 exit to both lumen and peritubular solutions. When NHE3 density is adjusted to compensate this acid challenge, the model predicts luminal membrane proton secretion that is greater than the HCO3(-)-reabsorptive fluxes measured in vitro. The model also predicts luminal membrane ammonia cycling, with uptake via NKCC2 or K+ channel, and secretion either as NH4+ by NHE3 or as diffusive NH3 flux in parallel with a secreted proton. If such luminal ammonia cycling occurs in vivo, it could act in concert with luminal K+ cycling to facilitate AHL Na+ reabsorption via NKCC2. With physiological ammonia, peritubular KCl also blunts NHE3 activity by inhibiting NH4+ uptake on the Na-K-ATPase, and alkalinizing the cell.

Luminal Na(+)/H (+) exchange in the proximal tubule

The proximal tubule is critical for whole-organism volume and acid-base homeostasis by reabsorbing filtered water, NaCl, bicarbonate, and citrate, as well as by excreting acid in the form of hydrogen and ammonium ions and producing new bicarbonate in the process. Filtered organic solutes such as amino acids, oligopeptides, and proteins are also retrieved by the proximal tubule. Luminal membrane Na(+)/H(+) exchangers either directly mediate or indirectly contribute to each of these processes. Na(+)/H(+) exchangers are a family of secondary active transporters with diverse tissue and subcellular distributions. Two isoforms, NHE3 and NHE8, are expressed at the luminal membrane of the proximal tubule. NHE3 is the prevalent isoform in adults, is the most extensively studied, and is tightly regulated by a large number of agonists and physiological conditions acting via partially defined molecular mechanisms. Comparatively little is known about NHE8, which is highly expressed at the lumen of the neonatal proximal tubule and is mostly intracellular in adults. This article discusses the physiology of proximal Na(+)/H(+) exchange, the multiple mechanisms of NHE3 regulation, and the reciprocal relationship between NHE3 and NHE8 at the lumen of the proximal tubule.

The Na+/H+ exchanger modulates long-term potentiation in rat hippocampal slices

Although present in great variety in the brain, the role of Na(+)/H(+) exchangers (NHEs) in hippocampal plasticity is still unknown and the effect of NHE inhibition on long-term potentiation (LTP) has not been studied yet. As it is conceivable that NHE inhibitors may severely affect mechanisms that are considered to underlie learning and memory we investigated whether the broad-spectrum NHE inhibitor 5'-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 microM) influences LTP induced by different stimuli based on a theta burst in interface hippocampus slices from 7-8-week-old Wistar and 30-month-old Fischer 344/Brown-Norway F1 hybrid (F344/BN) rats. EIPA did not affect basal synaptic transmission, paired pulse inhibition, or LTP induced by a weak stimulus, but improved the maintenance of the LTP of the population spike induced by a strong tetanus. Our data suggest that NHE activity serves as a negative feedback mechanism to control neuronal excitability and plasticity in both young and senescent animals.

Calcium-sensing receptor stimulates luminal K+-dependent H+ excretion in medullary thick ascending limbs of Henle's loop of mouse kidney

The calcium-sensing receptor (CaSR) is known well as a sensor of extracellular calcium for regulating parathyroid hormone secretion. CaSR is located along all nephron segments in the kidney. While hypercalcemia strongly enhances urinary acidification, the relationship between CaSR and acid-base metabolism in the kidney is still uncertain. In the present study, we examined whether CaSR activation caused acid secretion in the medullary thick ascending limb (mTAL), which is one of the major nephron segments involved in both mineral and acid-base regulation. The effects of a potent calcimimetic neomycin (Neo) on intracellular pH (pHi) were analyzed in the in vitro miroperfused mouse mTALs. The mTALs were incubated with 2,7-bis-(2-carboxyethyl)-5(6)-carboxyfluoresceine-acetoxymethylester (BCECF-AM) for microfluorescent pHi measurements. In HCO(3)(-)/CO(2)-buffered solution, the steady-state pHi was 7.17 +/- 0.01 (n = 19). Basolateral Neo at 0.4 mM in basolateral side significantly alkalinized the mTAL cells to 7.28 +/- 0.02 (n = 19), while Neo in the lumen had no effect on pHi. Neo in the basolateral side alkalinized the mTALs in the absence of ambient Na(+) and the presence of H(+)-ATPase inhibitor bafilomycin in the lumen, indicating that the effect of Neo is unrelated to Na(+)-dependent acid-base transporters such as Na(+)-H(+) exchangers and Na(+)-HCO(3)(-) cotransporter, or to luminal H(+)-ATPase. In contrast, the effect of Neo on pHi was inhibited by K(+) removal or treatment with specific H(+)-K(+)-ATPase (HKa) inhibitors, ouabain and Sch-28080, in the lumen. Our results suggest that hypercalcemia induces urinary acidification partly by stimulating luminal K(+)-dependent H(+)-excretion via CaSR in mouse mTALs.

HCO3(-) secretion by murine nasal submucosal gland serous acinar cells during Ca2+-stimulated fluid secretion

Airway submucosal glands contribute to airway surface liquid (ASL) composition and volume, both important for lung mucociliary clearance. Serous acini generate most of the fluid secreted by glands, but the molecular mechanisms remain poorly characterized. We previously described cholinergic-regulated fluid secretion driven by Ca²⁺-activated Cl⁻ secretion in primary murine serous acinar cells revealed by simultaneous differential interference contrast (DIC) and fluorescence microscopy. Here, we evaluated whether Ca²⁺-activated Cl⁻ secretion was accompanied by secretion of HCO₃⁻, possibly a critical ASL component, by simultaneous measurements of intracellular pH (pH_i) and cell volume. Resting pH_i was 7.17 ± 0.01 in physiological medium (5% CO₂–25 mM HCO₃⁻). During carbachol (CCh) stimulation, pH_i fell transiently by 0.08 ± 0.01 U concomitantly with a fall in Cl⁻ content revealed by cell shrinkage, reflecting Cl⁻ secretion. A subsequent alkalinization elevated pH_i to above resting levels until agonist removal, whereupon it returned to prestimulation values. In nominally CO₂–HCO₃⁻-free media, the CCh-induced acidification was reduced, whereas the alkalinization remained intact. Elimination of driving forces for conductive HCO₃⁻ efflux by ion substitution or exposure to the Cl⁻ channel inhibitor niflumic acid (100 μM) strongly inhibited agonist-induced acidification by >80% and >70%, respectively. The Na⁺/H⁺ exchanger (NHE) inhibitor dimethylamiloride (DMA) increased the magnitude (greater than twofold) and duration of the CCh-induced acidification. Gene expression profiling suggested that serous cells express NHE isoforms 1–4 and 6–9, but pharmacological sensitivities demonstrated that alkalinization observed during both CCh stimulation and pH_i recovery from agonist-induced acidification was primarily due to NHE1, localized to the basolateral membrane. These results suggest that serous acinar cells secrete HCO₃⁻ during Ca²⁺-evoked fluid secretion by a mechanism that involves the apical membrane secretory Cl⁻ channel, with HCO₃⁻ secretion sustained by activation of NHE1 in the basolateral membrane. In addition, other Na⁺-dependent pH_i regulatory mechanisms exist, as evidenced by stronger inhibition of alkalinization in Na⁺-free media.

Molecular detection and immunological localization of gill Na+/H+ exchanger in the dogfish (Squalus acanthias)

The dogfish (Squalus acanthias) can make rapid adjustments to gill acid-base transfers to compensate for internal acidosis/alkalosis. Branchial Na+/H+ exchange (NHE) has been postulated as one mechanism driving the excretion of H+ following acidosis. We have cloned gill cDNA that includes an open reading frame coding for a 770-residue protein most homologous (approximately 71%) to mammalian NHE2. RT-PCR revealed NHE2 transcripts predominantly in gill, stomach, rectal gland, intestine, and kidney. In situ hybridization with an antisense probe against NHE2 in gill sections revealed a strong mRNA signal from a subset of interlamellar and lamellae cells. We developed dogfish-specific polyclonal antibodies against NHE2 that detected a approximately 70-kDa protein in Western blots and immunologically recognized branchial cells having two patterns of protein expression. Cytoplasmic and apical NHE2 immunoreactivity were observed in cells coexpressing basolateral Na+-K+-ATPase. Other large ovoid cells more generally staining for NHE2 also were strongly positive for basolateral H+-ATPase. Gill mRNA levels for NHE2 and H+-ATPase did not change following systemic acidosis (as measured by quantitative PCR 2 h after a 1- or 2-meq/kg acid infusion). These data indicate that posttranslational adjustments of NHE2 and other transport systems (e.g., NHE3) following acidosis may be of importance in the short-term pH adjustment and net branchial H+ efflux observed in vivo. NHE2 may play multiple roles in the gills, involved with H+ efflux from acid-secreting cells, basolateral H+ reabsorption for pHi regulation, and in parallel with H+-ATPase for the generation of HCO3(-) in base-secreting cells.

Na+/H+ exchangers in renal regulation of acid-base balance

The kidney plays key roles in extracellular fluid pH homeostasis by reclaiming bicarbonate (HCO(3)(-)) filtered at the glomerulus and generating the consumed HCO(3)(-) by secreting protons (H(+)) into the urine (renal acidification). Sodium-proton exchangers (NHEs) are ubiquitous transmembrane proteins mediating the countertransport of Na(+) and H(+) across lipid bilayers. In mammals, NHEs participate in the regulation of cell pH, volume, and intracellular sodium concentration, as well as in transepithelial ion transport. Five of the 10 isoforms (NHE1-4 and NHE8) are expressed at the plasma membrane of renal epithelial cells. The best-studied isoform for acid-base homeostasis is NHE3, which mediates both HCO(3)(-) absorption and H(+) excretion in the renal tubule. This article reviews some important aspects of NHEs in the kidney, with special emphasis on the role of renal NHE3 in the maintenance of acid-base balance.

Na+/H+ exchangers: physiology and link to hypertension and organ ischemia

PURPOSE OF REVIEW

Na/H exchangers (NHEs) are ubiquitous proteins with a very wide array of physiological functions, and they are summarized in this paper in view of the most recent advances. Hypertension and organ ischemia are two disease states of paramount importance in which NHEs have been implicated. The involvement of NHEs in the pathophysiology of these disorders is incompletely understood. This paper reviews the principal findings and current hypotheses linking NHE dysfunction to hypertension and ischemia.

RECENT FINDINGS

With the advent of large-scale sequencing projects and powerful in-silico analyses, we have come to know what is most likely the entire mammalian NHE gene family. Recent advances have detailed the roles of NHE proteins, exploring new functions such as anchoring, scaffolding and pH regulation of intracellular compartments. Studies of NHEs in disease models, even though not conclusive to date, have contributed new evidence on the interplay of ion transporters and the delicate ion balances that may become disrupted.

SUMMARY

This paper provides the interested reader with a concise overview of NHE physiology, and aims to address the implication of NHEs in the pathophysiology of hypertension and organ ischemia in light of the most recent literature.

Nongenomic regulation by aldosterone of the epithelial NHE3 Na(+)/H(+) exchanger

The relevance of nongenomic pathways to regulation of epithelial function by aldosterone is poorly understood. Recently, we demonstrated that aldosterone inhibits transepithelial HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through a nongenomic pathway. Here, we examined the transport mechanism(s) responsible for this regulation, focusing on Na(+)/H(+) exchangers (NHE). In the MTAL, apical NHE3 mediates H(+) secretion necessary for HCO(3)(-) absorption; basolateral NHE1 influences HCO(3)(-) absorption by regulating apical NHE3 activity. In microperfused rat MTALs, the addition of 1 nM aldosterone rapidly decreased HCO(3)(-) absorption by 30%. This inhibition was unaffected by three maneuvers that inhibit basolateral Na(+)/H(+) exchange and was preserved in MTALs from NHE1 knockout mice, ruling out the involvement of NHE1. In contrast, exposure to aldosterone for 15 min caused a 30% decrease in apical Na(+)/H(+) exchange activity over the intracellular pH range from 6.5 to 7.7, due to a decrease in V(max). Inhibition of HCO(3)(-) absorption by aldosterone was not affected by 0.1 mM lumen Zn(2+) or 1 mM lumen DIDS, arguing against the involvement of an apical H(+) conductance or apical K(+)-HCO(3)(-) cotransport. These results demonstrate that aldosterone inhibits HCO(3)(-) absorption in the MTAL through inhibition of apical NHE3, and identify NHE3 as a target for nongenomic regulation by aldosterone. Aldosterone may influence a broad range of epithelial transport functions important for extracellular fluid volume and acid-base homeostasis through direct regulation of this exchanger.

Na+/H+ exchanger isoforms are differentially regulated in rat submandibular gland during acid/base disturbances in vivo

Acute metabolic acidosis and alkalosis cause a series of homeostatic adaptive responses in the kidney and other epithelia. We hypothesized that acid/base disturbances might affect the expression of Na(+)/H(+) exchanger (NHE) isoforms in salivary glands and determined the expression and cellular distribution of NHE3 and NHE4 in rat submandibular glands of controls and after imposed acute or chronic metabolic acidosis or alkalosis in vivo. Reverse transcription/polymerase chain reaction, in situ hybridization, and immunohistochemistry were applied by using specific primers, antisense probes, and antibodies, respectively. The results showed NHE3 and NHE4 transcript expression and protein abundance in rat submandibular gland. NHE3 was apically localized in duct cells, whereas NHE4 was found basolaterally distributed in acinar and duct cells. Acute acidosis and alkalosis and chronic acidosis had no effect on NHE3 and NHE4 expression and localization. In contrast, chronic metabolic alkalosis significantly decreased the number of apically stained NHE3 duct cells but had no effect on NHE3 mRNA expression. The results demonstrate, for the first time, the presence of NHE4 protein in salivary glands. The data also indicate the distinct regulation and adaptive changes of different isoforms of the same transporter in rat submandibular gland as a response to acid/base disturbances.

Evolutionary origins of eukaryotic sodium/proton exchangers

More than 200 genes annotated as Na+/H+ hydrogen exchangers (NHEs) currently reside in bioinformation databases such as GenBank and Pfam. We performed detailed phylogenetic analyses of these NHEs in an effort to better understand their specific functions and physiological roles. This analysis initially required examining the entire monovalent cation proton antiporter (CPA) superfamily that includes the CPA1, CPA2, and NaT-DC families of transporters, each of which has a unique set of bacterial ancestors. We have concluded that there are nine human NHE (or SLC9A) paralogs as well as two previously unknown human CPA2 genes, which we have named HsNHA1 and HsNHA2. The eukaryotic NHE family is composed of five phylogenetically distinct clades that differ in subcellular location, drug sensitivity, cation selectivity, and sequence length. The major subgroups are plasma membrane (recycling and resident) and intracellular (endosomal/TGN, NHE8-like, and plant vacuolar). HsNHE1, the first cloned eukaryotic NHE gene, belongs to the resident plasma membrane clade. The latter is the most recent to emerge, being found exclusively in vertebrates. In contrast, the intracellular clades are ubiquitously distributed and are likely precursors to the plasma membrane NHE. Yeast endosomal ScNHX1 was the first intracellular NHE to be described and is closely related to HsNHE6, HsNHE7, and HsNHE9 in humans. Our results link the appearance of NHE on the plasma membrane of animal cells to the use of the Na+/K(+)-ATPase to generate the membrane potential. These novel observations have allowed us to use comparative biology to predict physiological roles for the nine human NHE paralogs and to propose appropriate model organisms in which to study the unique properties of each NHE subclass.

Transepithelial HCO3- absorption is defective in renal thick ascending limbs from Na+/H+ exchanger NHE1 null mutant mice

In the medullary thick ascending limb (MTAL) of rat kidney, inhibiting basolateral Na(+)/H(+) exchange with either amiloride or nerve growth factor (NGF) results secondarily in inhibition of apical Na(+)/H(+) exchange, thereby decreasing transepithelial HCO(3)(-) absorption. To assess the possible role of the Na(+)/H(+) exchanger NHE1 in this regulatory process, MTALs from wild-type and NHE1 knockout (NHE1(-/-)) mice were studied using in vitro microperfusion. The rate of HCO(3)(-) absorption was decreased 60% in NHE1(-/-) MTALs (15.4 +/- 0.5 pmol.min(-1).mm(-1) wild-type vs. 6.0 +/- 0.5 pmol.min(-1).mm(-1) NHE1(-/-)). Transepithelial voltage, an index of the NaCl absorption rate, did not differ in wild-type and NHE1(-/-) MTALs. Basolateral addition of 10 microM amiloride or 0.7 nM NGF decreased HCO(3)(-) absorption by 45-49% in wild-type MTALs but had no effect on HCO(3)(-) absorption in NHE1(-/-) MTALs. Inhibition of HCO(3)(-) absorption by vasopressin and stimulation by hyposmolality, both of which regulate MTAL HCO(3)(-) absorption through primary effects on apical Na(+)/H(+) exchange, were similar in wild-type and NHE1(-/-) MTALs. Thus the regulatory defect in NHE1(-/-) MTALs is specific for factors (bath amiloride and NGF) shown previously to inhibit HCO(3)(-) absorption through primary effects on basolateral Na(+)/H(+) exchange. These findings demonstrate a novel role for NHE1 in transepithelial HCO(3)(-) absorption in the MTAL, in which basolateral NHE1 controls the activity of apical NHE3. Paradoxically, a reduction in NHE1-mediated H(+) extrusion across the basolateral membrane leads to a decrease in apical Na(+)/H(+) exchange activity that reduces HCO(3)(-) absorption.

Differentiated thick ascending limb (TAL) cultured cells derived from SV40 transgenic mice express functional apical NHE2 isoform: effect of nitric oxide

Studying the apical Na/H exchanger NHE2 is difficult in the intact thick ascending limb (TAL) because of its weak expression and transport activity compared with the co-expressed NHE3. From a mouse transgenic for a recombinant plasmid adeno-SV(40) (PK4), we developed an immortalized TAL cell line, referred to as MKTAL, which selectively expresses NHE2 protein and activity. The immortalized cells retain the main properties of TAL cells. They have a stable homogeneous epithelial-like phenotype, express SV(40) T antigen and exhibit polarity with an apical domain bearing few microvilli and separated from lateral domains by typical epithelial-type junctional complexes expressing ZO1 protein. Tamm-Horsfall protein is present on the apical membrane. MKTAL cells express NHE2 and NHE1 proteins but not NHE3 and NHE4, whereby NHE2 protein is expressed selectively in the apical domain of the plasma membrane. NHE2 contributed about half of the total Na/H exchange activity. mRNAs for the Na-K-2Cl cotransporter-2 (NKCC2) and the anion exchangers AE2 and AE3 were also present. While acute exposure to NO donors did not alter NHE2 activity, chronic exposure inhibited NHE2 activity selectively and down-regulated NHE2 mRNA abundance. In conclusion, MKTAL cells retain structural and functional properties of their in vivo TAL counterparts and express functional NHE2 protein in the apical membrane, which may be inhibited by NO. Thus, MKTAL cells may be an appropriate model for studying the cellular mechanisms of NHE2 regulation.

Macula densa cell signaling

Macula densa cells are renal sensor elements that detect changes in distal tubular fluid composition and transmit signals to the glomerular vascular elements. This tubuloglomerular feedback mechanism plays an important role in regulating glomerular filtration rate and blood flow. Macula densa cells detect changes in luminal sodium chloride concentration through a complex series of ion transport-related intracellular events. NaCl entry via a Na:K:2Cl cotransporter and Cl exit through a basolateral channel lead to cell depolarization and increases in cytosolic calcium. Na/H exchange (NHE2) results in cell alkalization, whereas intracellular [Na] is regulated by an apically located H(Na)-K ATPase and not by the traditional basolateral Na:K ATPase. Communication from macula densa cells to the glomerular vascular elements involves ATP release across the macula densa basolateral membrane through a maxi-anion channel. The adaptation of multi-photon microscopy is providing new insights into macula densa-glomerular signaling.