A new face of the Rhesus antigen

Syp1 regulates the clathrin-mediated and clathrin-independent endocytosis of multiple cargo proteins through a novel sorting motif

Internalization of proteins from the plasma membrane (PM) allows for cell-surface composition regulation, signaling of network modulation, and nutrient uptake. Clathrin-mediated endocytosis (CME) is a major internalization route for PM proteins. During CME, endocytic adaptor proteins bind cargoes at the cell surface and link them to the PM and clathrin coat. Muniscins are a conserved family of endocytic adaptors, including Syp1 in budding yeast and its mammalian orthologue, FCHo1. These adaptors bind cargo via a C-terminal μ-homology domain (μHD); however, few cargoes exhibiting muniscin-dependent endocytosis have been identified, and the sorting sequence recognized by the µHD is unknown. To reveal Syp1 cargo-sorting motifs, we performed a phage display screen and used biochemical methods to demonstrate that the Syp1 µHD binds DxY motifs in the previously identified Syp1 cargo Mid2 and the v-SNARE Snc1. We also executed an unbiased visual screen, which identified the peptide transporter Ptr2 and the ammonium permease Mep3 as Syp1 cargoes containing DxY motifs. Finally, we determined that, in addition to regulating cargo entry through CME, Syp1 can promote internalization of Ptr2 through a recently identified clathrin-independent endocytic pathway that requires the Rho1 GTPase. These findings elucidate the mechanism of Syp1 cargo recognition and its role in trafficking.

Acid-Base Homeostasis

Acid-base homeostasis and pH regulation are critical for both normal physiology and cell metabolism and function. The importance of this regulation is evidenced by a variety of physiologic derangements that occur when plasma pH is either high or low. The kidneys have the predominant role in regulating the systemic bicarbonate concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO3(-) and production of new bicarbonate to replace that consumed by normal or pathologic acids. This production or generation of new HCO3(-) is done by net acid excretion. Under normal conditions, approximately one-third to one-half of net acid excretion by the kidneys is in the form of titratable acid. The other one-half to two-thirds is the excretion of ammonium. The capacity to excrete ammonium under conditions of acid loads is quantitatively much greater than the capacity to increase titratable acid. Multiple, often redundant pathways and processes exist to regulate these renal functions. Derangements in acid-base homeostasis, however, are common in clinical medicine and can often be related to the systems involved in acid-base transport in the kidneys.

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.

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.

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.

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

NH3 movement across plasma membranes has traditionally been ascribed to passive, lipid-phase diffusion. However, ammonia-specific transporters, Mep/Amt proteins, are present in primitive organisms and mammals express orthologs of Mep/Amt proteins, the Rh glycoproteins. These findings suggest that the mechanisms of NH3 movement in mammalian tissues should be reexamined. Rh C glycoprotein (Rhcg) is expressed in the collecting duct, where NH3 secretion is necessary for both basal and acidosis-stimulated ammonia transport. To determine whether the collecting duct secretes NH3 via Rhcg or via lipid-phase diffusion, we generated mice with collecting duct-specific Rhcg deletion (CD-KO). CD-KO mice had loxP sites flanking exons 5 and 9 of the Rhcg gene (Rhcg(fl/fl)) and expressed Cre-recombinase under control of the Ksp-cadherin promoter (Ksp-Cre). Control (C) mice were Rhcg(fl/fl) but Ksp-Cre negative. We confirmed kidney-specific genomic recombination using PCR analysis and collecting duct-specific Rhcg deletion using immunohistochemistry. Under basal conditions, urinary ammonia excretion was less in KO vs. C mice; urine pH was unchanged. After acid-loading for 7 days, CD-KO mice developed more severe metabolic acidosis than did C mice. Urinary ammonia excretion did not increase significantly on the first day of acidosis in CD-KO mice, despite an intact ability to increase urine acidification, whereas it increased significantly in C mice. On subsequent days, urinary ammonia excretion slowly increased in CD-KO mice, but was always significantly less than in C mice. We conclude that collecting duct Rhcg expression contributes to both basal and acidosis-stimulated renal ammonia excretion, indicating that collecting duct ammonia secretion is, at least in part, mediated by Rhcg and not solely by lipid diffusion.

Basolateral expression of the ammonia transporter family member Rh C glycoprotein in the mouse kidney

Ammonia metabolism and transport are critical for acid-base homeostasis. The ammonia transporter family member Rh C glycoprotein (Rhcg) is expressed in distal renal tubular segments, and its expression is regulated in parallel with renal ammonia metabolism. However, there are inconsistencies in its reported subcellular distribution, with both apical and basolateral Rhcg reported in rat and human kidney and only apical expression in mouse kidney. Because the membrane location of Rhcg is critical for understanding its physiological role, we reassessed mouse Rhcg localization using refined immunolocalization methods. Two antibodies directed against different Rhcg-specific epitopes identified both apical and basolateral Rhcg immunolabel in mouse kidney. Immunogold electron microscopy both confirmed basolateral plasma membrane Rhcg expression and showed that apical immunolabel represented expression in both the apical plasma membrane and in subapical cytoplasmic vesicles. Immunoblots and Northern blots identified similar bands in Balb/c and C57BL/6 kidneys, suggesting basolateral Rhcg may result from alternative trafficking. Basolateral Rhcg intensity was strain dependent, with less basolateral Rhcg expression in the Balb/c mouse compared with the C57BL/6 mouse. In mice with collecting duct-specific Rhcg gene deletion, generated using Cre-loxP techniques, neither apical nor basolateral Rhcg immunolabel was identified in the collecting duct, confirming that basolateral Rhcg was the product of the same gene product as apical Rhcg. Although basolateral Rhcg expression differed between C57BL/6 and Balb/c mice, Rh B glycoprotein, which is exclusively basolateral, was expressed at similar levels in the two strains. We conclude that Rhcg is present in both the apical and basolateral plasma membrane in the mouse kidney, where it is likely to contribute to renal ammonia metabolism.

Ammonium transport in the colonic crypt cell line, T84: role for Rhesus glycoproteins and NKCC1

Although colonic lumen NH(4)(+) levels are high, 15-44 mM normal range in humans, relatively few studies have addressed the transport mechanisms for NH(4)(+). More extensive studies have elucidated the transport of NH(4)(+) in the kidney collecting duct, which involves a number of transporter processes also present in the distal colon. Similar to NH(4)(+) secretion in the renal collecting duct, we show that the distal colon secretory model, T84 cell line, has the capacity to secrete NH(4)(+) and maintain an apical-to-basolateral NH(4)(+) gradient. NH(4)(+) transport in the secretory direction was supported by basolateral NH(4)(+) loading on NKCC1, Na(+)-K(+)-ATPase, and the NH(4)(+) transporter, RhBG. NH(4)(+) was transported on NKCC1 in T84 cells nearly as well as K(+) as determined by bumetanide-sensitive (86)Rb-uptake. (86)Rb-uptake and ouabain-sensitive current measurement indicated that NH(4)(+) is transported by Na(+)-K(+)-ATPase in these cells to an equal extent as K(+). T84 cells expressed mRNA for the basolateral NH(4)(+) transporter RhBG and the apical NH(4)(+) transporter RhCG. Net NH(4)(+) transport in the secretory direction determined by (14)C-methylammonium (MA) uptake and flux occurred in T84 cells suggesting functional RhG protein activity. The occurrence of NH(4)(+) transport in the secretory direction within a colonic crypt cell model likely serves to minimize net absorption of NH(4)(+) because of surface cell NH(4)(+) absorption. These findings suggest that we rethink the present limited understanding of NH(4)(+) handling by the distal colon as being due solely to passive absorption.

Regulation of ammonia homeostasis by the ammonium transporter AmtA in Dictyostelium discoideum

Ammonia has been shown to function as a morphogen at multiple steps during the development of the cellular slime mold Dictyostelium discoideum; however, it is largely unknown how intracellular ammonia levels are controlled. In the Dictyostelium genome, there are five genes that encode putative ammonium transporters: amtA, amtB, amtC, rhgA, and rhgB. Here, we show that AmtA regulates ammonia homeostasis during growth and development. We found that cells lacking amtA had increased levels of ammonia/ammonium, whereas their extracellular ammonia/ammonium levels were highly decreased. These results suggest that AmtA mediates the excretion of ammonium. In support of a role for AmtA in ammonia homeostasis, AmtA mRNA is expressed throughout the life cycle, and its expression level increases during development. Importantly, AmtA-mediated ammonia homeostasis is critical for many developmental processes. amtA(-) cells are more sensitive to NH(4)Cl than wild-type cells in inhibition of chemotaxis toward cyclic AMP and of formation of multicellular aggregates. Furthermore, even in the absence of exogenously added ammonia, we found that amtA(-) cells produced many small fruiting bodies and that the viability and germination of amtA(-) spores were dramatically compromised. Taken together, our data clearly demonstrate that AmtA regulates ammonia homeostasis and plays important roles in multiple developmental processes in Dictyostelium.

The Escherichia coli AmtB protein as a model system for understanding ammonium transport by Amt and Rh proteins

The Escherichia coli ammonium transport protein (AmtB) has become the model system of choice for analysis of the process of ammonium uptake by the ubiquitous Amt family of inner membrane proteins. Over the past 6 years we have developed a range of genetic and biochemical tools in this system. These have allowed structure/function analysis to develop rapidly, offering insight initially into the membrane topology of the protein and most recently leading to the solution of high-resolution 3D structures. Genetic analysis has revealed a novel regulatory mechanism that is apparently conserved in prokaryotic Amt proteins and genetic approaches are also now being used to dissect structure/function relationships in Amt proteins. The now well-recognised homology between the Amt proteins, found in archaea, eubacteria, fungi and plants, and the Rhesus proteins, found characteristically in animals, also means that studies on E. coli AmtB can potentially shed light on structure/function relationships in the clinically important Rh proteins.

Modelling the human rhesus proteins: implications for structure and function

The mammalian rhesus (Rh) proteins that carry the Rh blood group antigens of red blood cells are related to the ammonium channel (Amt) proteins found in both pro- and eukaryotes. However, despite their clinical importance the structure of the Rh antigens is presently unknown. We have constructed homology models of the human Rh proteins, RhD and RhAG using the structure of the Escherichia coli ammonia channel AmtB as a template, together with secondary structure predictions and the extensive available biochemical data for the Rh proteins. These models suggest that RhAG and the homologous non-erythrocyte Rhesus glycoproteins, RhBG and RhCG, have a very similar channel architecture to AmtB. By comparison, RhD and RhCE have a different arrangement of residues, indicating that if they function as ammonia channels at all, they must do so by a different mechanism. The E. coli AmtB protein is a homotrimer and our models provoke a reassessment of the widely accepted tetrameric model of the organisation of the erythrocyte Rh complex. A critical analysis of previously published data, together with sequencing yield data, lead us to suggest that the erythrocyte Rh complex could indeed also be trimeric.

Expression of the ammonia transporter proteins Rh B glycoprotein and Rh C glycoprotein in the intestinal tract

Ammonia metabolism is important in multiple aspects of gastrointestinal physiology, but the mechanisms of ammonia transport in the gastrointestinal tract remain incompletely defined. The present study examines expression of the ammonia transporter family members Rh B glycoprotein (RhBG) and Rh C glycoprotein (RhCG) in the mouse gastrointestinal tract. Real-time RT-PCR amplification and immunoblot analysis identified mRNA and protein for both RhBG and RhCG were expressed in stomach, duodenum, jejunum, ileum, and colon. Immunohistochemistry showed organ and cell-specific expression of both RhBG and RhCG. In the stomach, both RhBG and RhCG were expressed in the fundus and forestomach, but not in the antrum. In the forestomach, RhBG was expressed by all nucleated squamous epithelial cells, whereas RhCG was expressed only in the stratum germinativum. In the fundus, RhBG and RhCG immunoreactivity was present in zymogenic cells but not in parietal or mucous cells. Furthermore, zymogenic cell RhBG and RhCG expression was polarized, with apical RhCG and basolateral RhBG immunoreactivity. In the duodenum, jejunum, ileum, and colon, RhBG and RhCG immunoreactivity was present in villous, but not in mucous or crypt cells. Similar to the fundic zymogenic cell, RhBG and RhCG expression in villous epithelial cells was polarized when apical RhCG and basolateral RhBG immunoreactivity was present. Thus the ammonia transporting proteins RhBG and RhCG exhibit cell-specific, axially heterogeneous, and polarized expression in the intestinal tract suggesting they function cooperatively to mediate gastrointestinal tract ammonia transport.

Characteristics of renal Rhbg as an NH4+ transporter

Rhbg is one of two recently cloned nonerythroid glycoproteins belonging to the Rh antigen family. Rhbg is expressed in basolateral membranes of intercalated cells of the kidney cortical collecting duct and some other cell types of the distal nephron and may function as NH4+ transporters. The aim of this study was to characterize the role of Rhbg in transporting NH4+. To do so, we expressed Rhbg in Xenopus laevis oocytes. Two-electrode voltage-clamp and H+-selective microlectrodes were used to measure NH4+ currents, current-voltage plots, and intracellular pH (pHi). In oocytes expressing Rhbg, 5 mM NH4+ induced an inward current of 93 ± 7.7 nA ( n = 20) that was significantly larger than that in control oocytes of −29 ± 7.1 nA ( P < 0.005). Whole cell conductance, at all tested potentials (−60 to +60 mV), was significantly more in oocytes expressing Rhbg compared with H2O-injected oocytes. In Rhbg oocytes, 5 mM NH4+ depolarized the oocyte by 28 ± 3.6 mV and decreased pHi by 0.30 ± 0.04 at a rate of −20 ± 2.5 × 10−4 pH/s. In control oocytes, 5 mM NH4+ depolarized Vm by only 20 ± 5.8 mV and pHi decreased by 0.07 ± 0.01 at a rate of −2.7 ± 0.6 × 10−4 pH/s. Raising bath [NH4+] in increments from 1 to 20 mM elicited a proportionally larger decrease in pHi (ΔpHi), larger depolarization (Δ Vm), and a faster rate of pHi decrease. Bathing Rhbg oocytes in 20 mM NH4+ induced an inward current of 140 ± 7 nA that was not significantly different from 178 ± 23 nA induced in H2O-injected (control) oocytes. The rate of pHi decrease induced by increasing external [NH4+] was significantly faster in Rhbg than in H2O-injected oocytes at all external NH4+ concentrations. In oocytes expressing Rhbg, net NH4+ influx (estimated from NH4+-induced H+ influx) as a function of external [NH4+] saturated at higher [NH4+] with a Vmax of ∼30.8 and an apparent Km of 2.3 mM ( R2 = 0.99). These data strongly suggest that Rhbg is a specific electrogenic transporter of NH4+.

The mechanism of ammonia transport based on the crystal structure of AmtB of Escherichia coli

Ammonium is one of the most important nitrogen sources for bacteria, fungi, and plants, but it is toxic to animals. The ammonium transport proteins (methylamine permeases/ammonium transporters/rhesus) are present in all domains of life; however, functional studies with members of this family have yielded controversial results with respect to the chemical identity (NH(4)(+) or NH(3)) of the transported species. We have solved the structure of wild-type AmtB from Escherichia coli in two crystal forms at 1.8- and 2.1-A resolution, respectively. Substrate transport occurs through a narrow mainly hydrophobic pore located at the center of each monomer of the trimeric AmtB. At the periplasmic entry, a binding site for NH(4)(+) is observed. Two phenylalanine side chains (F107 and F215) block access into the pore from the periplasmic side. Further into the pore, the side chains of two highly conserved histidine residues (H168 and H318) bridged by a H-bond lie adjacent, with their edges pointing into the cavity. These histidine residues may facilitate the deprotonation of an ammonium ion entering the pore. Adiabatic free energy calculations support the hypothesis that an electrostatic barrier between H168 and H318 hinders the permeation of cations but not that of the uncharged NH(3.) The structural data and energetic considerations strongly indicate that the methylamine permeases/ammonium transporters/rhesus proteins are ammonia gas channels. Interestingly, at the cytoplasmic exit of the pore, two different conformational states are observed that might be related to the inactivation mechanism by its regulatory partner.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Basolateral ammonium transport by the mouse inner medullary collecting duct cell (mIMCD-3)

The renal collecting duct is the primary site for the ammonia secretion necessary for acid-base homeostasis. Recent studies have identified the presence of putative ammonia transporters in the collecting duct, but whether the collecting duct has transporter-mediated ammonia transport is unknown. The purpose of this study was to examine basolateral ammonia transport in the mouse collecting duct cell (mIMCD-3). To examine mIMCD-3 basolateral ammonia transport, we used cells grown to confluence on permeable support membranes and quantified basolateral uptake of the radiolabeled ammonia analog [14C]methylammonia ([14C]MA). mIMCD-3 cell basolateral MA transport exhibited both diffusive and transporter-mediated components. Transporter-mediated uptake exhibited a Kmfor MA of 4.6 ± 0.2 mM, exceeded diffusive uptake at MA concentrations below 7.0 ± 1.8 mM, and was competitively inhibited by ammonia with a Kiof 2.1 ± 0.6 mM. Transporter-mediated uptake was not altered by inhibitors of Na+-K+-ATPase, Na+-K+-2Cl−cotransporter, K+channels or KCC proteins, by excess potassium, by extracellular sodium or potassium removal or by varying membrane potential, suggesting the presence of a novel, electroneutral ammonia-MA transport mechanism. Increasing the outwardly directed transmembrane H+gradient increased transport activity by increasing Vmax. Finally, mIMCD-3 cells express mRNA and protein for the putative ammonia transporter Rh B-glycoprotein (RhBG), and they exhibit basolateral RhBG immunoreactivity. We conclude that mIMCD-3 cells express a basolateral electroneutral NH4+/H+exchange activity that may be mediated by RhBG.

Identification of candidate cancer-causing genes in mouse brain tumors by retroviral tagging

Murine retroviruses may cause malignant tumors in mice by insertional mutagenesis of host genes. The use of retroviral tagging as a means of identifying cancer-causing genes has, however, almost entirely been restricted to hematopoietic tumors. The aim of this study was to develop a system allowing for the retroviral tagging of candidate genes in malignant brain tumors. Mouse gliomas were induced by a recombinant Moloney murine leukemia virus encoding platelet-derived growth factor (PDGF) B-chain. The underlying idea was that tumors evolve through a combination of PDGF-mediated autocrine growth stimulation and insertional mutagenesis of genes that cooperate with PDGF in gliomagenesis. Common insertion sites (loci that were tagged in more than one tumor) were identified by cloning and sequencing retroviral flanking segments, followed by blast searches of mouse genome databases. A number of candidate brain tumor loci (Btls) were identified. Several of these Btls correspond to known tumor-causing genes; these findings strongly support the underlying idea of our experimental approach. Other Btls harbor genes with a hitherto unproven role in transformation or oncogenesis. Our findings indicate that retroviral tagging with a growth factor-encoding virus may be a powerful means of identifying candidate tumor-causing genes in nonhematopoietic tumors.

Blood-brain barrier permeability to ammonia in liver failure: a critical reappraisal

In patients with acute liver failure (ALF), hyperammonemia is related to development of cerebral edema and herniation. The present review discusses the mechanisms for the cerebral uptake of ammonia. A mathematical framework is provided to allow a quantitative examination of whether published studies can be explained by the conventional view that cerebral uptake of ammonia is restricted to diffusion of the unprotonated form (NH(3)) (the diffusion hypothesis). An increase in cerebral blood flow (CBF) enhanced ammonia uptake more than expected, possibly due to recruitment or heterogeneity of brain capillaries. Reported effects of pH on ammonia uptake were in the direction predicted by the diffusion hypothesis, but often less pronounced than expected. The published effects of mannitol, cooling, and indomethacin in experimental animals and patients were difficult to explain by the diffusion hypothesis alone, unless dramatic changes of capillary surface area or permeability for ammonia were induced. Therefore we considered the possible role of membrane protein mediated transport of NH(4)(+) across the blood-brain barrier (BBB). Early tracer studies in Rhesus monkeys suggested that NH(4)(+) is responsible for 20% or even more of the transport of ammonia from plasma to brain. In other locations, such as in the thick ascending limb of Hendle's loop and in isolated astrocytes, transport protein mediated translocation of NH(4)(+) is predominant. Many of the ion-transporters involved in renal NH(4)(+) reabsorbtion are also present in brain capillary membranes and could mediate uptake of NH(4)(+). Astrocytic uptake of NH(4)(+) is associated with increased extracellular K(+), which is a potent cerebral vasodilator. Such interference between transport of NH(4)(+) and other cations could be clinically important because increased cerebral blood flow often precedes cerebral herniation in acute liver failure. We suggest that protein mediated transport of NH(4)(+) through the brain capillary wall is a realistic possibility that should be more intensely studied.

Non-erythroid Rh glycoproteins: a putative new family of mammalian ammonium transporters

The Rhesus (Rh) glycoproteins, originally described in human blood cells, are mostly recognized for their immunogenic characteristics and importance in pregnancy. The Rh proteins in the red blood cell are expressed as an "Rh complex" made up of one D-subunit, one CE-subunit and two Rh-associated glycoprotein (RhAG) subunits. In addition to its antigenic property, the Rh complex is thought to contribute to membrane stability and structure of red blood cells. The exact function is yet to be determined. Recently, two non-erythroid Rh glycoproteins were cloned from mice (Rhcg and Rhbg) and humans (RhCG and RhBG). RhCG is expressed at the membrane surface alone with no apparent need for heteromeric interaction with other glycoproteins. It is more similar to RhAG than to Rh CE/D, occurs late in development and is expressed abundantly and broadly in kidney and testis. In the kidney RhCG is localized to the apical cell membrane of the collecting duct. Rhbg and its human analog (RhBG) are expressed mainly in liver, skin and the kidney tubules. In the kidney collecting duct, Rhbg is localized to the basolateral membrane. Based on structural similarities to the methylammonium and ammonium permease/ammonium (MEP/Amt) transporters in yeast and their sequence homology, these proteins probably function as NH(4)(+) transporters. An initial study has indicated that RhAG or RhCG promote efflux of NH(4)(+), whereas another study has suggested that RhAG functions as an NH(4)(+)-H(+) exchanger. Evidence for such a function is still circumstantial and data indicating that Rh proteins function as NH(4)(+) transporters are indirect.

cDNA cloning, mapping and polymorphism of the porcine Rhesus (RH) gene

The Rhesus (Rh) gene superfamily in humans and mice contains four independent genes, RH, RHAG, RHBG, and RHCG/GK. Heretofore, only the RHBG cDNA has been cloned in pig. We have isolated the porcine RH cDNA; its complete open reading frame of 1269 nucleotides encoded 423 amino acids. Porcine RH protein shared 67.6% amino acid identity with bovine RH, 61.0% with human RhCE and 60.8% with human RhD. The RT-PCR revealed RH transcripts in the spleen and bone marrow, but not in the heart, kidney, or lung. In RH intron 4, a deletion of 17 nucleotides distinguished the shorter allele (allele 1) from the longer. As determined in 115 unrelated pigs from five breeds - Landrace (L, n = 23), Large White (LW, n = 28), Duroc (D, n = 24), Hampshire (H, n = 20) and Piétrain (n = 20) - allele 1 frequencies were 1.0 (L, H), 0.77 (LW), 0.70 (P) and 0.25 (D). Somatic cell hybrid mapping localized the porcine RH and RHBG genes to pig chromosomes 6q22-q23 and 4q21-q22, respectively. Genetic mapping suggested RH-(FUT1, S, GPI, EAH, A1BG)-PGD as the most probable locus order. Sequence homology, mapping data, and haematopoietic tissue expression suggest that this cDNA may indeed encode the porcine RH homologue.

Localization of the ammonium transporters, Rh B glycoprotein and Rh C glycoprotein, in the mouse liver

BACKGROUND & AIMS

Hepatic ammonium metabolism is critical for maintenance of normal health. Three mammalian members of an ammonium transporter family have recently been identified: Rh A glycoprotein (RhAG), Rh B glycoprotein (RhBG), and Rh C glycoprotein (RhCG). This study examined which of these are expressed in the mouse liver and in which cells they are expressed.

METHODS

Normal Balb/c mice were used. Messenger RNA (mRNA) expression was detected using either conventional or real-time reverse-transcription polymerase chain reaction (RT-PCR). Protein expression was examined using immunoblot analysis and either immunohistochemical or immunofluorescent microscopy.

RESULTS

We confirmed hepatic RhBG mRNA expression using real-time RT-PCR. Immunoblot analysis identified expression of a approximately 45-kilodalton protein. Immunohistochemical and immunofluorescent microscopy identified basolateral RhBG immunoreactivity in 1-2 cell layers of hepatocytes surrounding central veins. No immunoreactivity was identified in periportal or midzonal hepatocytes. Perivenous hepatocyte-specific expression was confirmed by colocalization with glutamine synthetase. A second ammonium transporter, RhCG, was expressed but at substantially lower levels. Real-time RT-PCR quantified hepatic RhCG mRNA expression at approximately 0.4% of RhBG mRNA expression. Immunoblot analysis confirmed RhCG protein expression, and immunofluorescence microscopy identified RhCG expression in bile duct epithelia. In contrast to RhBG and RhCG, RhAG mRNA was not identified by RT-PCR.

CONCLUSIONS

RhBG and RhCG are expressed by the mouse liver. Basolateral RhBG is expressed by perivenous hepatocytes, where it may mediate ammonium uptake, and RhCG immunoreactivity is present in bile duct epithelial cells, where it may contribute to ammonium secretion into bile fluid.

Localization of the ammonium transporter proteins RhBG and RhCG in mouse kidney

Ammonia is both produced and transported by renal epithelial cells, and it regulates renal ion transport. Recent studies have identified a family of putative ammonium transporters; mRNA for two members of this family, Rh B-glycoprotein (RhBG) and Rh C-glycoprotein (RhCG), is expressed in the kidney. The purpose of this study was to determine the cellular location of RhBG and RhCG protein in the mouse kidney. We generated RhBG- and RhCG-specific anti-peptide antibodies. Immunoblot analysis confirmed that both proteins were expressed in the mouse kidney. RhBG localization with immunohistochemistry revealed discrete basolateral labeling in the connecting segment (CNT) and in the majority of initial collecting tubule (ICT) and cortical collecting duct (CCD) cells. In the outer medullary collecting duct (OMCD) and inner medullary collecting duct (IMCD) only a subpopulation of cells exhibited basolateral immunoreactivity. Colocalization of RhBG with carbonic anhydrase II, the thiazide-sensitive transporter, and the anion exchangers AE1 and pendrin demonstrated RhBG immunoreactivity in all CNT cells and all CCD and ICT principal cells. In the ICT and CCD, basolateral RhBG immunoreactivity is also present in A-type intercalated cells but not in pendrin-positive CCD intercalated cells. In the OMCD and IMCD, only intercalated cells exhibit RhBG immunoreactivity. Immunoreactivity for a second putative ammonium transporter, RhCG, was present in the apical region of cells with almost the same distribution as RhBG. However, RhCG immunoreactivity was present in all CCD cells, and it was present in outer stripe OMCD principal cells, in addition to OMCD and IMCD intercalated cells. Thus the majority of RhBG and RhCG protein expression is present in the same epithelial cell types in the CNT and collecting duct but with opposite polarity. These findings suggest that RhBG and RhCG may play important and cell-specific roles in ammonium transport and signaling in these regions of the kidney.

Aquaporin water channels--from atomic structure to clinical medicine

The water permeability of biological membranes has been a longstanding problem in physiology, but the proteins responsible for this remained unknown until discovery of the aquaporin 1 (AQP1) water channel protein. AQP1 is selectively permeated by water driven by osmotic gradients. The atomic structure of human AQP1 has recently been defined. Each subunit of the tetramer contains an individual aqueous pore that permits single-file passage of water molecules but interrupts the hydrogen bonding needed for passage of protons. At least 10 mammalian aquaporins have been identified, and these are selectively permeated by water (aquaporins) or water plus glycerol (aquaglyceroporins). The sites of expression coincide closely with the clinical phenotypes--ranging from congenital cataracts to nephrogenic diabetes insipidus. More than 200 members of the aquaporin family have been found in plants, microbials, invertebrates and vertebrates, and their importance to the physiology of these organisms is being uncovered.

New Insights into the Rh Superfamily of Genes and Proteins in Erythroid Cells and Nonerythroid Tissues

The past decade has seen extensive studies of the erythrocyte Rh30 polypeptides and Rh-associated glycoprotein, which specify the clinically important Rh blood group system. Here we consider recent advances on these and other Rh homologues in the context of gene organization, molecular evolution, tissue-specific expression, protein structure, and potential biological functions. The Rh family is now known to contain a large number of homologues that form a unique branch in the eucarya life domain. The ancient origin and broad distribution imply central roles for the various Rh proteins in maintaining normal cellular and organismal homeostatic conditions. Rh homologues occur in the form of multiple chromosomal loci in mice and humans, but as single-copy genes in unicellular organisms (e.g., green alga and slime mold). While primitive Rh genes vary largely in exon/intron design, the mammalian Rh homologues bear a similar genomic organization. Sequence comparisons have revealed the signatures and a consensus 12-transmembrane fold characteristic of the Rh family. Phylogenetic analysis has placed all Rh homologues as a related cluster that intercepts ammonium transporter (Amt) clusters, indicating an intimate evolutionary and structural relationship between the Rh and Amt families. The biochemical identification and epithelial expression of RhBG and RhCG orthologues in mammalian kidney, liver, skin, testis, and brain suggest that they serve as transporters likely participating in ammonia homeostasis. Further inquires into the structure, function, biosynthesis, and interaction of Rh proteins will shed new light on ammonia homeostasis in a wide range of human physiological and pathological states.