Collecting duct-specific Rh C glycoprotein deletion alters basal and acidosis-stimulated renal ammonia excretion

Characteristics of mammalian Rh glycoproteins (SLC42 transporters) and their role in acid-base transport

The mammalian Rh glycoproteins belong to the solute transporter family SLC42 and include RhAG, present in red blood cells, and two non-erythroid members RhBG and RhCG that are expressed in various tissues, including kidney, liver, skin and the GI tract. The Rh proteins in the red blood cell form an "Rh complex" made up of one D-subunit, one CE-subunit and two RhAG subunits. The Rh complex has a well-known antigenic effect but also contributes to the stability of the red cell membrane. RhBG and RhCG are related to the NH4(+) transporters of the yeast and bacteria but their exact function is yet to be determined. This review describes the expression and molecular properties of these membrane proteins and their potential role as NH3/NH4(+) and CO2 transporters. The likelihood that these proteins transport gases such as CO2 or NH3 is novel and significant. The review also describes the physiological importance of these proteins and their relevance to human disease.

Expression of the rhesus glycoproteins, ammonia transporter family members, RHCG and RHBG in male reproductive organs

The rhesus glycoproteins, Rh B glycoprotein (RHBG) and Rh C glycoprotein (RHCG), are recently identified ammonia transporters. Rhcg expression is necessary for normal male fertility, but its specific cellular expression is unknown, and Rhbg has not been reported to be expressed in the male reproductive tract. This study sought to determine the specific cellular expression of Rhcg, to determine whether Rhbg is expressed in the male reproductive tract, and, if so, to determine which cells express Rhbg using real-time RT-PCR, immunoblot analysis, and immunohistochemistry. Both Rhbg and Rhcg were expressed throughout the male reproductive tract. In the testis, high levels of Rhbg were expressed in Leydig cells, and Rhcg was expressed in spermatids during the later stages of their maturation (steps 13-16) in stages I-VIII of the seminiferous epithelium cycle. In the epididymis, basolateral Rhbg was present in narrow cells in the initial segment, in principal cells in the upper corpus, and in clear cells throughout the epididymis. Apical Rhcg immunolabel was present in principal cells in the caput and upper corpus epididymidis and in clear cells in the middle and lower corpus and cauda epididymidis. In the vas deferens, apical Rhcg immunolabel and basolateral Rhbg immunolabel were present in some principal cells and colocalized with H(+)-ATPase immunolabel. We conclude that both Rhbg and Rhcg are highly expressed in specific cells in the male reproductive tract where they can contribute to multiple components of male fertility.

Expression of glutamine synthetase in the mouse kidney: localization in multiple epithelial cell types and differential regulation by hypokalemia

Renal glutamine synthetase catalyzes the reaction of NH4+ with glutamate, forming glutamine and decreasing the ammonia available for net acid excretion. The purpose of the present study was to determine glutamine synthetase's specific cellular expression in the mouse kidney and its regulation by hypokalemia, a common cause of altered renal ammonia metabolism. Glutamine synthetase mRNA and protein were present in the renal cortex and in both the outer and inner stripes of the outer medulla. Immunohistochemistry showed glutamine synthetase expression throughout the entire proximal tubule and in nonproximal tubule cells. Double immunolabel with cell-specific markers demonstrated glutamine synthetase expression in type A intercalated cells, non-A, non-B intercalated cells, and distal convoluted tubule cells, but not in principal cells, type B intercalated cells, or connecting segment cells. Hypokalemia induced by feeding a nominally K+ -free diet for 12 days decreased glutamine synthetase expression throughout the entire proximal tubule and in the distal convoluted tubule and simultaneously increased glutamine synthetase expression in type A intercalated cells in both the cortical and outer medullary collecting duct. We conclude that glutamine synthetase is widely and specifically expressed in renal epithelial cells and that the regulation of expression differs in specific cell populations. Glutamine synthetase is likely to mediate an important role in renal ammonia metabolism.

Expression of the ammonia transporter family member, Rh B Glycoprotein, in the human kidney

The ammonia transporter family member, Rh B Glycoprotein (RhBG/Rhbg), is essential for ammonia transport by the rodent kidney, but in the human kidney mRNA but not protein expression has been reported. Because ammonia transport is fundamental for acid-base homeostasis, the current study addressed RhBG expression in the human kidney. Two distinct RhBG mRNA sequences have been reported, with different numbers of consecutive cytosines at nt1265 and thus encoding different carboxy-tails. Sequencing the region of difference in both human kidney and liver mRNA showed eight sequential cytosines, not seven as in some reports. Knowing the correct mRNA sequence for RhBG, we then assessed RhBG protein expression using antibodies against the correct amino acid sequence. Immunoblot analysis demonstrated RhBG protein expression in human kidney and immunohistochemistry identified basolateral RhBG in connecting segment (CNT) and the cortical and outer medullary collecting ducts. Colocalization of RhBG with multiple cell-specific markers demonstrated that that CNT cells and collecting duct type A intercalated cells express high levels of RhBG, and type B intercalated cells and principal cells do not express detectable RhBG. Thus, these studies identify the correct mRNA and thus protein sequence for human RhBG and show that the human kidney expresses basolateral RhBG protein in CNT, type A intercalated cells, and non-A, non-B cells. We conclude that RhBG can mediate an important role in human renal ammonia transport.

Intercalated cell-specific Rh B glycoprotein deletion diminishes renal ammonia excretion response to hypokalemia

The ammonia transporter family member, Rh B Glycoprotein (Rhbg), is an ammonia-specific transporter heavily expressed in the kidney and is necessary for the normal increase in ammonia excretion in response to metabolic acidosis. Hypokalemia is a common clinical condition in which there is increased renal ammonia excretion despite the absence of metabolic acidosis. The purpose of this study was to examine Rhbg's role in this response through the use of mice with intercalated cell-specific Rhbg deletion (IC-Rhbg-KO). Hypokalemia induced by feeding a K(+)-free diet increased urinary ammonia excretion significantly. In mice with intact Rhbg expression, hypokalemia increased Rhbg protein expression in intercalated cells in the cortical collecting duct (CCD) and in the outer medullary collecting duct (OMCD). Deletion of Rhbg from intercalated cells inhibited hypokalemia-induced changes in urinary total ammonia excretion significantly and completely prevented hypokalemia-induced increases in urinary ammonia concentration, but did not alter urinary pH. We conclude that hypokalemia increases Rhbg expression in intercalated cells in the cortex and outer medulla and that intercalated cell Rhbg expression is necessary for the normal increase in renal ammonia excretion in response to hypokalemia.

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.

Haploinsufficiency of the ammonia transporter Rhcg predisposes to chronic acidosis: Rhcg is critical for apical and basolateral ammonia transport in the mouse collecting duct

Ammonia secretion by the collecting duct (CD) is critical for acid-base homeostasis and, when defective, causes distal renal tubular acidosis (dRTA). The Rhesus protein RhCG mediates NH(3) transport as evident from cell-free and cellular models as well as from Rhcg-null mice. Here, we investigated in a Rhcg mouse model the metabolic effects of Rhcg haploinsufficiency, the role of Rhcg in basolateral NH(3) transport, and the mechanisms of adaptation to the lack of Rhcg. Both Rhcg(+/+) and Rhcg(+/-) mice were able to handle an acute acid load, whereas Rhcg(-/-) mice developed severe metabolic acidosis with reduced ammonuria and high mortality. However, chronic acid loading revealed that Rhcg(+/-) mice did not fully recover, showing lower blood HCO(3)(-) concentration and more alkaline urine. Microperfusion studies demonstrated that transepithelial NH(3) permeability was reduced by 80 and 40%, respectively, in CDs from Rhcg(-/-) and Rhcg(+/-) mice compared with controls. Basolateral membrane permeability to NH(3) was reduced in CDs from Rhcg(-/-) mice consistent with basolateral Rhcg localization. Rhcg(-/-) responded to acid loading with normal expression of enzymes and transporters involved in proximal tubular ammoniagenesis but reduced abundance of the NKCC2 transporter responsible for medullary accumulation of ammonium. Consequently, tissue ammonium content was decreased. These data demonstrate a role for apical and basolateral Rhcg in transepithelial NH(3) transport and uncover an incomplete dRTA phenotype in Rhcg(+/-) mice. Haploinsufficiency or reduced expression of RhCG may underlie human forms of (in)complete dRTA.

Renal ammonia excretion in response to hypokalemia: effect of collecting duct-specific Rh C glycoprotein deletion

The Rhesus factor protein, Rh C glycoprotein (Rhcg), is an ammonia transporter whose expression in the collecting duct is necessary for normal ammonia excretion both in basal conditions and in response to metabolic acidosis. Hypokalemia is a common clinical condition associated with increased renal ammonia excretion. In contrast to basal conditions and metabolic acidosis, increased ammonia excretion during hypokalemia can lead to an acid-base disorder, metabolic alkalosis, rather than maintenance of acid-base homeostasis. The purpose of the current studies was to determine Rhcg's role in hypokalemia-stimulated renal ammonia excretion through the use of mice with collecting duct-specific Rhcg deletion (CD-Rhcg-KO). In mice with intact Rhcg expression, a K(+)-free diet increased urinary ammonia excretion and urine alkalinization and concurrently increased Rhcg expression in the collecting duct in the outer medulla. Immunohistochemistry and immunogold electron microscopy showed hypokalemia increased both apical and basolateral Rhcg expression. In CD-Rhcg-KO, a K(+)-free diet increased urinary ammonia excretion and caused urine alkalinization, and the magnitude of these changes did not differ from mice with intact Rhcg expression. In mice on a K(+)-free diet, CD-Rhcg-KO increased phosphate-dependent glutaminase (PDG) expression in the outer medulla. We conclude that hypokalemia increases collecting duct Rhcg expression, that this likely contributes to the hypokalemia-stimulated increase in urinary ammonia excretion, and that adaptive increases in PDG expression can compensate for the absence of collecting duct Rhcg.

Mechanisms of the effects of acidosis and hypokalemia on renal ammonia metabolism

Renal ammonia metabolism is the predominant component of net acid excretion and new bicarbonate generation. Renal ammonia metabolism is regulated by acid-base balance. Both acute and chronic acid loads enhance ammonia production in the proximal tubule and secretion into the urine. In contrast, alkalosis reduces ammoniagenesis. Hypokalemia is a common electrolyte disorder that significantly increases renal ammonia production and excretion, despite causing metabolic alkalosis. Although the net effects of hypokalemia are similar to metabolic acidosis, molecular mechanisms of renal ammonia production and transport have not been well understood. This mini review summarizes recent findings regarding renal ammonia metabolism in response to chronic hypokalemia.

Aquaporins as gas channels

Gas molecules play important roles in human physiology. Volatile substances produced by one cell often regulate neighboring cells in a paracrine fashion. While gaseous molecules have traditionally been thought to travel from cell to cell by free diffusion through the bilayer portion of the membrane, this does not explain their rapid physiological actions. The recent observations that: (1) water channels can transport other molecules besides water, and (2) aquaporins are often expressed in tissues where gas (but not water) transport is essential suggest that these channels conduct physiologically important gases in addition to water. This review summarizes recent findings on the role of aquaporins as gas transporters as well as their physiological significance.

Effect of hypokalemia on renal expression of the ammonia transporter family members, Rh B Glycoprotein and Rh C Glycoprotein, in the rat kidney

Hypokalemia is a common electrolyte disorder that increases renal ammonia metabolism and can cause the development of an acid-base disorder, metabolic alkalosis. The ammonia transporter family members, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), are expressed in the distal nephron and collecting duct and mediate critical roles in acid-base homeostasis by facilitating ammonia secretion. In the current studies, the effect of hypokalemia on renal Rhbg and Rhcg expression was examined. Normal Sprague-Dawley rats received either K(+)-free or control diets for 2 wk. Rats receiving the K(+)-deficient diet developed hypokalemia and metabolic alkalosis associated with significant increases in both urinary ammonia excretion and urine pH. Rhcg expression increased in the outer medullary collecting duct (OMCD). In OMCD intercalated cells, hypokalemia resulted in more discrete apical Rhcg expression and a marked increase in apical plasma membrane immunolabel. In principal cells, in the OMCD, hypokalemia increased both apical and basolateral Rhcg immunolabel intensity. Cortical Rhcg expression was not detectably altered by immunohistochemistry, although there was a slight decrease in total expression by immunoblot analysis. Rhbg protein expression was decreased slightly in the cortex and not detectably altered in the outer medulla. We conclude that in rat OMCD, hypokalemia increases Rhcg expression, causes more polarized apical expression in intercalated cells, and increases both apical and basolateral expression in the principal cell. Increased plasma membrane Rhcg expression in response to hypokalemia in the rat, particularly in the OMCD, likely contributes to the increased ammonia excretion and thereby to the development of metabolic alkalosis.

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.

Molecular physiology of the Rh ammonia transport proteins

PURPOSE OF REVIEW

Recent studies have identified a new family of ammonia-specific transporters, Rh glycoproteins, which enable NH3-specific transport. The purpose of this review is to summarize recent evidence regarding the role of Rh glycoproteins in renal ammonia transport.

RECENT FINDINGS

The Rh glycoproteins, RhAG/Rhag, RhBG/Rhbg and RhCG/Rhcg, transport ammonia in the form of molecular NH3, although there is some evidence suggesting the possibility of NH4 transport. RhAG/Rhag is expressed only in erythrocytes, and not in the kidney. Rhbg and Rhcg are expressed in distal nephron sites, from the distal convoluted tubule through the inner medullary collecting duct, with basolateral Rhbg expression and both apical and basolateral Rhcg expression. Whether Rhbg contributes to renal ammonia transport remains controversial. Rhcg expression parallels ammonia excretion in multiple experimental models and genetic deletion studies, both global and collecting duct-specific, demonstrate a critical role for Rhcg in both basal and acidosis-stimulated renal ammonia excretion. X-ray crystallography has defined critical structural elements in Rh glycoprotein-mediated ammonia transport. Finally, Rh glycoproteins may also function as CO2 transporters.

SUMMARY

No longer can NH3 transport be considered to occur only through diffusive NH3 movement. Transporter-mediated NH3 movement is fundamental to ammonia metabolism.

Sharpey-Schafer lecture: gas channels

The traditional dogma has been that all gases diffuse through all membranes simply by dissolving in the lipid phase of the membrane. Although this mechanism may explain how most gases move through most membranes, it is now clear that some membranes have no demonstrable gas permeability, and that at least two families of membrane proteins, the aquaporins (AQPs) and the Rhesus (Rh) proteins, can each serve as pathways for the diffusion of both CO₂ and NH₃. The knockout of RhCG in the renal collecting duct leads to the predicted consequences in acid–base physiology, providing a clear-cut role for at least one gas channel in the normal physiology of mammals. In our laboratory, we have found that surface-pH (pH_S) transients provide a sensitive approach for detecting CO₂ and NH₃ movement across the cell membranes of Xenopus oocytes. Using this approach, we have found that each tested AQP and Rh protein has its own characteristic CO₂/NH₃ permeability ratio, which provides the first demonstration of gas selectivity by a channel. Our preliminary AQP1 data suggest that all the NH₃ and less than half of the CO₂ move along with H₂O through the four monomeric aquapores. The majority of CO₂ takes an alternative route through AQP1, possibly the central pore at the four-fold axis of symmetry. Preliminary data with two Rh proteins, bacterial AmtB and human erythroid RhAG, suggest a similar story, with all the NH₃ moving through the three monomeric NH₃ pores and the CO₂ taking a separate route, perhaps the central pore at the three-fold axis of symmetry. The movement of different gases via different pathways is likely to underlie the gas selectivity that these channels exhibit.

pH sensitivity of ammonium transport by Rhbg

Rhbg is a membrane glycoprotein that is involved in NH(3)/NH(4)(+) transport. Several models have been proposed to describe Rhbg, including an electroneutral NH(4)(+)/H(+) exchanger, a uniporter, an NH(4)(+) channel, or even a gas channel. In this study, we characterized the pH sensitivity of Rhbg expressed in Xenopus oocytes. We used two-electrode voltage clamp and ion-selective microelectrodes to measure NH(4)(+)-induced [and methyl ammonium (MA(+))] currents and changes in intracellular pH (pH(i)), respectively. In oocytes expressing Rhbg, 5 mM NH(4)Cl (NH(3)/NH(4)(+)) at extracellular pH (pH(o)) of 7.5 induced an inward current, decreased pH(i), and depolarized the cell. Raising pH(o) to 8.2 significantly enhanced the NH(4)(+)-induced current and pH(i) changes, whereas decreasing bath pH to 6.5 inhibited these changes. Lowering pH(i) (decreased by butyrate) also inhibited the NH(4)(+)-induced current and pH(i) decrease. In oocytes expressing Rhbg, 5 mM methyl amine hydrochloride (MA/MA(+)), often used as an NH(4)Cl substitute, induced an inward current, a pH(i) increase (not a decrease), and depolarization of the cell. Exposing the oocyte to MA/MA(+) at alkaline bath pH (8.2) enhanced the MA(+)-induced current, whereas lowering bath pH to 6.5 inhibited the MA(+) current completely. Exposing the oocyte to MA/MA(+) at low pH(i) abolished the MA(+)-induced current and depolarization; however, pH(i) still increased. These data indicate that 1) transport of NH(4)(+) and MA/MA(+) by Rhbg is pH sensitive; 2) electrogenic NH(4)(+) and MA(+) transport are stimulated by alkaline pH(o) but inhibited by acidic pH(i) or pH(o); and 3) electroneutral transport of MA by Rhbg is likely but is less sensitive to pH changes.

Deletion of the pH sensor GPR4 decreases renal acid excretion

Proton receptors are G protein-coupled receptors that accept protons as ligands and function as pH sensors. One of the proton receptors, GPR4, is relatively abundant in the kidney, but its potential role in acid-base homeostasis is unknown. In this study, we examined the distribution of GPR4 in the kidney, its function in kidney epithelial cells, and the effects of its deletion on acid-base homeostasis. We observed GPR4 expression in the kidney cortex, in the outer and inner medulla, in isolated kidney collecting ducts, and in cultured outer and inner medullary collecting duct cells (mOMCD1 and mIMCD3). Cultured mOMCD1 cells exhibited pH-dependent accumulation of intracellular cAMP, characteristic of GPR4 activation; GPR4 knockdown attenuated this accumulation. In vivo, deletion of GPR4 decreased net acid secretion by the kidney and resulted in a nongap metabolic acidosis, indicating that GPR4 is required to maintain acid-base homeostasis. Collectively, these findings suggest that GPR4 is a pH sensor with an important role in regulating acid secretion in the kidney collecting duct.

Role of the Rhesus glycoprotein, Rh B glycoprotein, in renal ammonia excretion

Rh B glycoprotein (Rhbg) is a member of the Rh glycoprotein family of ammonia transporters. In the current study, we examine Rhbg's role in basal and acidosis-stimulated acid-base homeostasis. Metabolic acidosis induced by HCl administration increased Rhbg expression in both the cortex and outer medulla. To test the functional significance of increased Rhbg expression, we used a Cre-loxP approach to generate mice with intercalated cell-specific Rhbg knockout (IC-Rhbg-KO). On normal diet, intercalated cell-specific Rhbg deletion did not alter urine ammonia excretion, pH, or titratable acid excretion significantly, but it did decrease glutamine synthetase expression in the outer medulla significantly. After metabolic acidosis was induced, urinary ammonia excretion was significantly less in IC-Rhbg-KO than in control (C) mice on days 2-4 of acid loading, but not on day 5. Urine pH and titratable acid excretion and dietary acid intake did not differ significantly between acid-loaded IC-Rhcg-KO and C mice. In IC-Rhbg-KO mice, acid loading increased connecting segment (CNT) cell and outer medullary collecting duct principal cell Rhbg expression. In both C and IC-Rhbg-KO mice, acid loading decreased glutamine synthetase in both the cortex and outer medulla; the decrease on day 3 was similar in IC-Rhbg-KO and C mice, but on day 5 it was significantly greater in IC-Rhbg-KO than in C mice. We conclude 1) intercalated cell Rhbg contributes to acidosis-stimulated renal ammonia excretion, 2) Rhbg in CNT and principal cells may contribute to renal ammonia excretion, and 3) decreased glutamine synthetase expression may enable normal rates of ammonia excretion under both basal conditions and on day 5 of acid loading in IC-Rhbg-KO mice.

Expression of ammonia transporter family members, Rh B glycoprotein and Rh C glycoprotein, in the developing rat kidney

Ammonia metabolism is a primary component of acid-base homeostasis but is incompletely developed at time of birth. Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg) are recently recognized ammonia transporter family members expressed in the mammalian kidney. This study's purpose was to establish the expression and localization of Rhbg and Rhcg during kidney development. We examined kidneys from fetal days 16 (E16), 18 (E18), and 20 (E20), and from the first 21 days of postnatal development. Rhbg was expressed initially at E18, with expression only in the connecting tubule (CNT); at E20, Rhbg was expressed in both the CNT and the medullary collecting duct (MCD). In contrast, Rhcg was first expressed at E16 with basal expression in the ureteric bud; at E18, it was expressed in a subset of CNT cells with an apical pattern, followed by apical and basolateral expression in the MCD at E20. In the cortex, Rhbg and Rhcg expression increased in the CNT before expression in the cortical collecting duct during fetal development. In the MCD, both Rhbg and Rhcg expression was initially in cells in the papillary tip, with gradual removal from the tip during the late fetal period and transition during the early neonatal period to an adult pattern with predominant expression in the outer MCD and only rare expression in cells in the initial inner MCD. Double-labeling with intercalated cell-specific markers identified that Rhbg and Rhcg were expressed initially in CNT cells, CNT A-type intercalated cells and non-A, non-B intercalated cells, and in MCD A-type intercalated cells. We conclude that expression of Rhbg and Rhcg parallels intercalated cell development and that immature Rhbg and Rhcg expression at birth contributes to incomplete ammonia excretion capacity.

Function of human Rh based on structure of RhCG at 2.1 A

In humans, NH(3) transport across cell membranes is facilitated by the Rh (rhesus) family of proteins. Human Rh C glycoprotein (RhCG) forms a trimeric complex that plays an essential role in ammonia excretion and renal pH regulation. The X-ray crystallographic structure of human RhCG, determined at 2.1 A resolution, reveals the mechanism of ammonia transport. Each monomer contains 12 transmembrane helices, one more than in the bacterial homologs. Reconstituted into proteoliposomes, RhCG conducts NH(3) to raise internal pH. Models of the erythrocyte Rh complex based on our RhCG structure suggest that the erythrocytic Rh complex is composed of stochastically assembled heterotrimers of RhAG, RhD, and RhCE.

Effect of intercalated cell-specific Rh C glycoprotein deletion on basal and metabolic acidosis-stimulated renal ammonia excretion

Rh C glycoprotein (Rhcg) is an NH(3)-specific transporter expressed in both intercalated cells (IC) and principal cells (PC) in the renal collecting duct. Recent studies show that deletion of Rhcg from both intercalated and principal cells inhibits both basal and acidosis-stimulated renal ammonia excretion. The purpose of the current studies was to better understand the specific role of Rhcg expression in intercalated cells in basal and metabolic acidosis-stimulated renal ammonia excretion. We generated mice with intercalated cell-specific Rhcg deletion (IC-Rhcg-KO) using Cre-loxP techniques; control (C) mice were floxed Rhcg but Cre negative. Under basal conditions, IC-Rhcg-KO and C mice excreted urine with similar ammonia content and pH. Mice were then acid loaded by adding HCl to their diet. Ammonia excretion after acid loading increased similarly in IC-Rhcg-KO and C mice during the first 2 days of acid loading but on day 3 was significantly less in IC-Rhcg-KO than in C mice. During the first 2 days of acid loading, urine was significantly more acidic in IC-Rhcg-KO mice than in C mice; there was no difference on day 3. In IC-Rhcg-KO mice, acid loading increased principal cell Rhcg expression in both the cortex and outer medulla as well as expression of another ammonia transporter, Rh glycoprotein B (Rhbg), in principal cells in the outer medulla. We conclude that 1) Rhcg expression in intercalated cells is necessary for the normal renal response to metabolic acidosis; 2) principal cell Rhcg contributes to both basal and acidosis-stimulated ammonia excretion; and 3) adaptations in Rhbg expression occur in response to acid-loading.

Expression of the gas-transporting proteins, Rh B glycoprotein and Rh C glycoprotein, in the murine lung

A family of gas-transporting proteins, the Mep/Amt/Rh glycoprotein family, has been identified recently. These are integral membrane proteins, are widely expressed in sites of gas transport, and are known to transport the gaseous molecule, NH(3), and recent evidence indicates they can transport CO(2). Because the mammalian lung is a critical site for gas transport, the current studies examine the expression of the nonerythroid members of this extended family, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), in the normal mouse lung. Real-time RT-PCR and immunoblot analysis demonstrated both Rhbg and Rhcg mRNA and protein expression, respectively. Immunohistochemistry demonstrated both Rhbg and Rhcg were expressed in bronchial and bronchiolar epithelial cells. Rhbg was expressed by Clara cells, specifically, whereas all bronchial/bronchiolar epithelial cells, with the exception of goblet cells, expressed Rhcg. Rhbg expression was basolateral, whereas Rhcg exhibited apical and intracellular immunolabel, polarized expression similar to that observed in Rhbg- and Rhcg-expressing epithelial cells in other organs. There was no detectable expression of either Rhbg or Rhcg in alveolar endothelial or epithelial cells, in pneumocytes or in vascular tissue. In vitro studies using cultured bronchial epithelial cells confirm Rhbg and Rhcg expression, demonstrate that saturable, not diffusive, transport is the primary mechanism of ammonia/methylammonia transport, and show that the saturable transport mechanism has kinetics similar to those demonstrated previously for Rhbg and Rhcg. These findings suggest Rhbg and Rhcg may contribute to bronchial epithelial cell ammonia metabolism and suggest that they do not contribute to pulmonary CO(2) transport.

Renal handling of ammonium and Acid base regulation

Renal ammonium metabolism is the primary component of net acid excretion and thereby is critical for acid-base homeostasis. Briefly, ammonium is produced from glutamine in the proximal tubule in a series of biochemical reactions that result in equimolar bicarbonate. Ammonium is predominantly secreted into the luminal fluid via the apical Na⁺/H⁺ exchanger, NHE3. The thick ascending limb of the loop of Henle reabsorbs luminal ammonium, predominantly by transport of NH₄⁺ by the apical Na⁺/K⁺/2Cl^- cotransporter, BSC1/NKCC2. This process results in renal interstitial ammonium accumulation. Finally, the collecting duct secretes ammonium from the renal interstitium into the luminal fluid. Although in past ammonium was believed to move across epithelia entirely by passive diffusion, an increasing number of studies demonstrated that specific proteins contribute to renal ammonium transport. Recent studies have yielded important new insights into the mechanisms of renal ammonium transport. In this review, we will discuss renal handling of ammonium, with particular emphasis on the transporters involved in this process.

Expression of rh glycoproteins in the Mammalian kidney

Ammonia metabolism is a fundamental process in the maintenance of life in all living organisms. Recent studies have identified ammonia transporter family proteins in yeast (Mep), plants (Amt), and mammals (Rh glycoproteins). In mammalian kidneys, where ammonia metabolism and transport are critically important for the regulation of systemic acid-base homeostasis, basolateral Rh B glycoprotein and apical/basolateral Rh C glycoprotein are expressed along the distal nephron segments. Data from experimental animal models and knockout mice suggest that the Rh glycoproteins appear to mediate important roles in urinary ammonia excretion.