ORAI1 channel gating and selectivity is differentially altered by natural mutations in the first or third transmembrane domain

KEY POINTS

Gain-of-function mutations in the highly selective Ca²⁺ channel ORAI1 cause tubular aggregate myopathy (TAM) characterized by muscular pain, weakness and cramping. TAM-associated mutations in ORAI1 first and third transmembrane domain facilitate channel opening by STIM1, causing constitutive Ca²⁺ influx and increasing the currents evoked by Ca²⁺ store depletion. Mutation V107M additionally decreases the channel selectivity for Ca²⁺ ions and its inhibition by acidic pH, while mutation T184M does not alter the channel sensitivity to pH or to reactive oxygen species. The ORAI blocker GSK-7975A prevents the constitutive activity of TAM-associated channels and might be used in therapy for patients suffering from TAM.

ABSTRACT

Skeletal muscle differentiation relies on store-operated Ca²⁺ entry (SOCE) mediated by STIM proteins linking the depletion of endoplasmic/sarcoplasmic reticulum Ca²⁺ stores to the activation of membrane Ca²⁺ -permeable ORAI channels. Gain-of-function mutations in STIM1 or ORAI1 isoforms cause tubular aggregate myopathy (TAM), a skeletal muscle disorder with muscular pain, weakness and cramping. Here, we characterize two overactive ORAI1 mutants from patients with TAM: V107M and T184M, located in the first and third transmembrane domain of the channel. When ectopically expressed in HEK-293T cells or human primary myoblasts, the mutated channels increased basal and store-operated Ca²⁺ entry. The constitutive activity of V107M, L138F, T184M and P245L mutants was prevented by low concentrations of GSK-7975A while the G98S mutant was resistant to inhibition. Electrophysiological recordings confirmed ORAI1-V107M constitutive activity and revealed larger STIM1-gated V107M- and T184M-mediated currents with conserved fast and slow Ca²⁺ -dependent inactivation. Mutation V107M altered the channel selectivity for Ca²⁺ ions and conferred resistance to acidic inhibition. Ca²⁺ imaging and molecular dynamics simulations showed a preserved sensitivity of T184M to the negative regulation by reactive oxygen species. Both mutants were able to mediate SOCE in Stim1^-/- /Stim2^-/- mouse embryonic fibroblasts expressing the binding-deficient STIM1-F394H mutant, indicating a higher sensitivity for STIM1-mediated gating, with ORAI1-T184M gain-of-function being strictly dependent on STIM1. These findings provide new insights into the permeation and regulatory properties of ORAI1 mutants that might translate into therapies against diseases with gain-of-function mutations in ORAI1.

Zinc transporters in prostate cancer

Prostate cancer is a major health concern as it has the second highest incidence rate among cancers in men. Despite progress in tumor diagnostics and therapeutic approaches, prognosis for men with advanced disease remains poor. In this review we provide insight into the changes of the intermediary metabolism in normal prostate and prostate cancer. In contrast to normal cells, prostate cancer cells are reprogrammed for optimal energy-efficiency with a functional Krebs cycle and minimal apoptosis rates. A key element in this relationship is the uniquely high zinc level of normal prostate epithelial cells. Zinc is transported by the SLC30 and SLC39 families of zinc transporters. However, in prostate cancer the intracellular zinc content is remarkably reduced and expression levels of certain zinc transporters are altered. Here, we summarize the role of different zinc transporters in the development of prostate cancer.

Trpv6 mediates intestinal calcium absorption during calcium restriction and contributes to bone homeostasis

Energy-dependent intestinal calcium absorption is important for the maintenance of calcium and bone homeostasis, especially when dietary calcium supply is restricted. The active form of vitamin D, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], is a crucial regulator of this process and increases the expression of the transient receptor potential vanilloid 6 (Trpv6) calcium channel that mediates calcium transfer across the intestinal apical membrane. Genetic inactivation of Trpv6 in mice (Trpv6(-/-)) showed, however, that TRPV6 is redundant for intestinal calcium absorption when dietary calcium content is normal/high and passive diffusion likely contributes to maintain normal serum calcium levels. On the other hand, Trpv6 inactivation impaired the increase in intestinal calcium transport following calcium restriction, however without resulting in hypocalcemia. A possible explanation is that normocalcemia is maintained at the expense of bone homeostasis, a hypothesis investigated in this study. In this study, we thoroughly analyzed the bone phenotype of Trpv6(-/-) mice receiving a normal (approximately 1%) or low (approximately 0.02%) calcium diet from weaning onwards using micro-computed tomography, histomorphometry and serum parameters. When dietary supply of calcium is normal, Trpv6 inactivation did not affect growth plate morphology, bone mass and remodeling parameters in young adult or aging mice. Restricting dietary calcium had no effect on serum calcium levels and resulted in a comparable reduction in bone mass accrual in Trpv6(+/+) and Trpv6(-/-) mice (-35% and 45% respectively). This decrease in bone mass was associated with a similar increase in bone resorption, whereas serum osteocalcin levels and the amount of unmineralized bone matrix were only significantly increased in Trpv6(-/-) mice. Taken together, our findings indicate that TRPV6 contributes to intestinal calcium transport when dietary calcium supply is limited and in this condition indirectly regulates bone formation and/or mineralization.

Inherited epithelial transporter disorders--an overview

In the late 1990s, the identification of transporters and transporter-associated genes progressed substantially due to the development of new cloning approaches such as expression cloning and, subsequently, to the implementation of the human genome project. Since then, the role of many transporter genes in human diseases has been elucidated. In this overview, we focus on inherited disorders of epithelial transporters. In particular, we review genetic defects of the genes encoding glucose transporters (SLC2 and SLC5 families) and amino acid transporters (SLC1, SLC3, SLC6 and SLC7 families).

[Physiology and biochemistry of uric acid]

In humans, uric acid is the final breakdown product of unwanted purine nucleotides. Uric acid is the last stage in purine degradation, because humans lack the enzyme uricase which converts uric acid into allantoin. Uric acid has profound beneficial effects since it scavenges potential harmful radicals in our body. However, in conjunction with genetic or environmental factors, uric acid can cause significant health problems, leading to kidney stones when it builds up in the kidneys and to gout when crystals accumulate in the joints. The levels of uric acid in the blood must be tightly controlled to minimize these detrimental effects. Normally, the body eliminates enough uric acid in the kidney, and in part also through the intestines, to keep its concentration at a healthy level in the blood (approximately 300 microM). In patients with gout or kidney stone disease, however, the body either produces excessive amounts of uric acid or its ability to eliminate uric acid is disturbed in some way. In the kidney, uric acid is reabsorbed via the uric acid transporter URAT1. This transporter is the major mechanism for regulating blood uric acid levels and therefore may prove an interesting target for future drug development.

Modulation of DMT1 activity by redox compounds

Iron(II) exacerbates the effects of oxidative stress via the Fenton reaction. A number of human diseases are associated with iron accumulation including ischemia-reperfusion injury, inflammation and certain neurodegenerative diseases. The functional properties and localization in plasma membrane of cells and endosomes suggest an important role for the divalent metal transporter DMT1 (also known as DCT1 and Nramp2) in iron transport and cellular iron homeostasis. Although iron metabolism is strictly controlled and the activity of DMT1 is central in controlling iron homeostasis, no regulatory mechanisms for DMT1 have been so far identified. Our studies show that the activity of DMT1 is modulated by compounds that affect its redox status. We also show that both iron and zinc are transported by DMT1 when expressed in Xenopus laevis oocytes. Radiotracer uptake and electrophysiological measurements revealed that H(2)O(2) and Hg(2+) treatments result in substantial inhibition of DMT1. These findings may have a profound relevance from a physiological and pathophysiological standpoint.

Iron-dependent regulation of the divalent metal ion transporter

The first step in intestinal iron absorption is mediated by the H(+)-coupled Fe(2+) transporter called divalent cation transporter 1/divalent metal ion transporter 1 (DCT1/DMT1) (also known as natural resistance-associated macrophage protein 2). DCT1/DMT1 mRNA levels in the duodenum strongly increase in response to iron depletion. To study the mechanism of iron-dependent DCT1/DMT1 mRNA regulation, we investigated the endogenous expression of DCT1/DMT1 mRNA in various cell types. We found that only the iron responsive element (IRE)-containing form, which corresponds to one of two splice forms of DCT1/DMT1, is responsive to iron treatment and this responsiveness was cell type specific. We also examined the interaction of the putative 3'-UTR IRE with iron responsive binding proteins (IRP1 and IRP2), and found that IRP1 binds to the DCT1/DMT1-IRE with higher affinity compared to IRP2. This differential binding of IRP1 and IRP2 was also reported for the IREs of transferrin receptors, erythroid 5-aminolevulinate synthase and mitochondrial aconitase. We propose that regulation of DCT1/DMT1 mRNA by iron involves post-transcriptional regulation through the binding of IRP1 to the transporter's IRE, as well as other as yet unknown factors.

Single-channel activities of the human epithelial Ca2+ transport proteins CaT1 and CaT2

The human epithelial channels, CaT1 and CaT2, were expressed in oocytes, and their single-channel characteristics were compared. In the presence of Na+ and K+ as charge carriers in the pipette solutions, channel activities were observed only when the the extracellular sides of the patches were exposed to nominally Ca2+- and Mg2+-free solutions. In patches of both CaT1- and CaT2-expressing oocytes, multiple channel openings were observed, but the current levels were higher in CaT2-expressing oocytes, particularly at more negative voltages. With K+ as a charge carrier in patches of CaT1-expressing oocytes, the channel activity was low at -10 to -60 mV, but increased dramatically at more negative potentials. This voltage dependence was observed in the presence of both Na+ and K+. The channel activity with Na+, however, was higher at all potentials. Differences between the voltage dependencies for the two cations were also observed in CaT2-expressing oocytes, but the channel activities were higher than those in CaT1-expressing oocytes, particularly in the presence of Na+. We also found that low concentrations of extracellular Mg2+ (5-50 microm) elicited a strong inhibitory action on the CaT channels. Activation of the CaT1 and CaT2 channels by hyperpolarization and other factors may promote increased Ca2+ entry that participates in stimulation of intestinal absorption and renal reabsorption and/or other Ca2+ transport mechanisms in epithelial cells.

Inhibition of the glutamate transporter EAAC1 expressed in Xenopus oocytes by phorbol esters

Recent evidence indicates that second messengers and protein kinases regulate the activity and expression of glutamate transporters. The aim of the present study was to determine if direct activation of protein kinases C or A modulates the activity of the sodium-dependent glutamate transporter EAAC1. EAAC1 modulation was studied in cRNA-injected Xenopus oocytes by measuring [3H]L-glutamate uptake or glutamate-evoked uptake currents. We found that activation of PKA was ineffective, whereas treatment with the PKC agonist phorbol 12-myristate 13-acetate (PMA) caused a significant decrease in EAAC1 transport activity (IC(50)=44.7+/-12 nM). PMA-induced EAAC1 inhibition was PKC-mediated because the inhibition could be blocked by specific PKC inhibitors and incubation with the inactive 4alpha-phorbol-12,13-didecanoate (4alpha-PDD) did not affect EAAC1. Saturation studies of glutamate-evoked uptake currents showed that PMA-mediated inhibition was due to a decrease in I(max) with no change in K(m). PMA simultaneously decreased membrane capacitance (C(m)) and transport-associated current and increased cytosolic accumulation of EAAC1 protein, compared to control. These results suggest that PKC activation inhibits EAAC1 by promoting its retrieval from the plasma membrane. PMA also significantly decreased glutamate uptake in a Madin-Darby canine kidney (MDCK) cell line stably transfected with EAAC1 but enhanced EAAC1-mediated glutamate uptake in the rat C6 glioma cells, consistent with previous observations. Because activation of PKC by phorbol esters leads to opposite effects on EAAC1 activity in different culture models, we conclude that the PKC-mediated regulation of EAAC1 is cell-type specific.

Structural conservation of the genes encoding CaT1, CaT2, and related cation channels

We report here the genomic structures of the genes encoding human calcium transport proteins CaT1 and CaT2, which belong to a recently identified class of highly selective calcium entry channels. The mRNA for CaT1 was expressed more abundantly than that for CaT2 in three major tissues involved in transcellular calcium transport, namely intestine, kidney, and placenta, as determined by quantitative PCR. The genes encoding CaT1 and CaT2, ECAC2 and ECAC1, respectively, are completely conserved in terms of exon size in the coding regions. They also share similar intron-exon structures with the genes encoding the closely related, nonselective cation channels VR1, VRL-1, OTRPC4 (also known as VR-OAC, Trp12, and VRL-2), and a hypothetical protein, VRL-3. We conclude that ECAC2 and ECAC1, which encode calcium selective channels, share a common ancestral gene with the genes encoding the related nonselective cation channels.

Colonic epithelial hPepT1 expression occurs in inflammatory bowel disease: transport of bacterial peptides influences expression of MHC class 1 molecules

BACKGROUND & AIMS

hPepT1 is an intestinal epithelial apical membrane transporter responsible for uptake of di/tripeptides (including bacterial derived proinflammatory n-formyl peptides). hPepT1 expression normally has a strict axial gradient-highest in the proximal small intestine with no expression in the colon.

METHODS

Small intestinal-like cells (Caco2-BBE), and colonic-like cells (HT29-Cl.19A), and colonic mucosa from diseased and control patients were used in the present study.

RESULTS

hPepT1 expression occurs aberrantly in the colon with chronic ulcerative colitis (6 patients) and Crohn's disease (4 patients), but not in normal colon (4 patients) or colon with microscopic colitis (4 patients). To model expression of hPepT1 by colonic-like cells in inflamed states, we stably transfected HT29-Cl.19A cells with a modified hPepT1 tagged on the N-terminus with green fluorescence protein. Analysis of transfected cells revealed that: GFP-hPepT1 protein, like the natural protein, is targeted to the apical plasma membrane. In addition, the tagged protein retains the capability of di/tripeptide absorption, and the expression of the tagged protein by HT29-Cl.19A cells permits absorption of N-formyl-methionyl-leucyl-phenylalanine (fMLP), as occurs in hPepT1 expressing Caco2-BBE cells. fMLP uptake by colonic cells expressing GFP-hPepT1 specifically enhances major histocompatibility complex class I surface expression.

CONCLUSIONS

These data collectively indicate that, in some states of chronic inflammation, hPepT1 may be anomolously expressed in the colon. Further, transport of fMLP by hPepT1 potentially stimulates expression of key accessory immune molecule, MHC-1.

Differential distribution of the glutamate transporters GLT-1 and GLAST in tanycytes of the third ventricle

The ventral one-third of the ventricular lining in the hypothalamus is formed by specialized ependymal cells called the tanycytes. These cells may serve a neuroendocrine transport function because of their structural specializations, which include apical microvili on the ventricular surface and long basal processes that terminate on blood vessels or on the glia limitans. Here, we describe the expression of mRNA and protein for the glutamate transporters GLT-1 and GLAST in unique tanycyte populations of the third ventricle in rat brain. Using nonisotopic in situ hybridization, we demonstrate GLAST mRNA labeling in tanycytes of the ventral floor and lateral walls in the tuberal and mammillary recess portions of the third ventricle. This GLAST mRNA labeling had a higher intensity than the labeling intensity observed in regular ependymal cells throughout the ventricular system. Furthermore, we have identified strong GLT-1 mRNA labeling in a population of tanycytes situated in the dorsolateral walls of caudal tuberal and mammillary recess portions. Immunocytochemical staining indicates that both GLT-1 and GLAST protein are expressed in the tanycyte populations as well. These data corroborate previous findings that third ventricle tanycytes are functionally heterogeneous. Furthermore, the GLT-1-expressing tanycytes represent a population of tanycytes that, to date, has not been recognized as functionally distinct. The strong GLAST expression by the ventral tanycytes in the hypophysiotropic area suggests a role of tanycyte-mediated glutamate transport in neuroendocrine activity. The functional role of GLT-1 in dorsal wall tanycytes remains to be explored.

Transport function of the naturally occurring pathogenic polycystin-2 mutant, R742X

Most patients with autosomal dominant polycystic kidney disease (ADPKD) harbor mutations truncating polycystin-1 (PC1) or polycystin-2 (PC2), products of the PKD1 and PKD2 genes, respectively. A third member of the polycystin family, polycystin-L (PCL), was recently shown to function as a Ca(2+)-modulated nonselective cation channel. More recently, PC2 was also shown to be a nonselective cation channel with comparable properties to PCL, though the membrane targeting of PC2 likely varies with cell types. Here we show that PC2 expressed heterologously in Xenopus oocytes is targeted to intracellular compartments. By contrast, a truncated form of mouse PC2 corresponding to a naturally occurring human mutation R742X is targeted predominantly to the plasma membrane where it mediates K(+), Na(+), and Ca(2+) currents. Unlike PCL, the truncated form does not display Ca(2+)-activated transport activities, possibly due to loss of an EF-hand at the C-terminus. We propose that PC2 forms ion channels utilizing structural components which are preserved in the R742X form of the protein. Implications for epithelial cell signaling are discussed.

CaT1 expression correlates with tumor grade in prostate cancer

Ca(2+) signaling is important for growth and survival of prostatic carcinoma (PCa) cells. Here we report that the gene for CaT1, a channel protein highly selective for Ca(2+), is expressed at high levels in human PCa and in the LNCaP PCa cell line. CaT1 mRNA levels were elevated in PCa specimens in comparison to benign prostatic hyperplasia (BPH) specimens and positively correlated with Gleason grade in a PCa series. CaT1 mRNA was suppressed by androgen and was induced by a specific androgen receptor antagonist in LNCaP cells, suggesting that the gene is negatively regulated by androgen. These findings are the first to implicate a Ca(2+) channel in PCa progression and suggest that CaT1 may be a novel target for therapy.

CaT1 manifests the pore properties of the calcium-release-activated calcium channel

The calcium-release-activated Ca2+channel, ICRAC, is a highly Ca2+-selective ion channel that is activated on depletion of either intracellular Ca2+ levels or intracellular Ca2+ stores. The unique gating of ICRAC has made it a favourite target of investigation for new signal transduction mechanisms; however, without molecular identification of the channel protein, such studies have been inconclusive. Here we show that the protein CaT1 (ref. 4), which has six membrane-spanning domains, exhibits the unique biophysical properties of ICRAC when expressed in mammalian cells. Like ICRAC, expressed CaT1 protein is Ca2+ selective, activated by a reduction in intracellular Ca2+ concentration, and inactivated by higher intracellular concentrations of Ca2+. The channel is indistinguishable from ICRAC in the following features: sequence of selectivity to divalent cations; an anomalous mole fraction effect; whole-cell current kinetics; block by lanthanum; loss of selectivity in the absence of divalent cations; and single-channel conductance to Na+ in divalent-ion-free conditions. CaT1 is activated by both passive and active depletion of calcium stores. We propose that CaT1 comprises all or part of the ICRAC pore.

Polycystin-2 is a novel cation channel implicated in defective intracellular Ca(2+) homeostasis in polycystic kidney disease

Mutations in polycystins-1 and -2 (PC1 and PC2) cause autosomal dominant polycystic kidney disease (ADPKD), which is characterized by progressive development of epithelial renal cysts, ultimately leading to renal failure. The functions of these polycystins remain elusive. Here we show that PC2 is a Ca(2+)-permeable cation channel with properties distinct from any known intracellular channels. Its kinetic behavior is characterized by frequent transitions between closed and open states over a wide voltage range. The activity of the PC2 channel is transiently increased by elevating cytosolic Ca(2+). Given the predominant endoplasmic reticulum (ER) location of PC2 and its unresponsiveness to the known modulators of mediating Ca(2+) release from the ER, inositol-trisphosphate (IP(3)) and ryanodine, these results suggest that PC2 represents a novel type of channel with properties distinct from those of the other Ca(2+)-release channels. Our data also show that the PC2 channel can be translocated to the plasma membranes by defined chemical chaperones and proteasome modulators, suggesting that in vivo, it may also function in the plasma membrane under specific conditions. The sensitivity of the PC2 channel to changes of intracellular Ca(2+) concentration is deficient in a mutant found in ADPKD patients. The dysfunction of such mutants may result in defective coupling of PC2 to intracellular Ca(2+) homeostasis associated with the pathogenesis of ADPKD.

An iron-regulated ferric reductase associated with the absorption of dietary iron

The ability of intestinal mucosa to absorb dietary ferric iron is attributed to the presence of a brush-border membrane reductase activity that displays adaptive responses to iron status. We have isolated a complementary DNA, Dcytb (for duodenal cytochrome b), which encoded a putative plasma membrane di-heme protein in mouse duodenal mucosa. Dcytb shared between 45 and 50% similarity to the cytochrome b561 family of plasma membrane reductases, was highly expressed in the brush-border membrane of duodenal enterocytes, and induced ferric reductase activity when expressed in Xenopus oocytes and cultured cells. Duodenal expression levels of Dcytb messenger RNA and protein were regulated by changes in physiological modulators of iron absorption. Thus, Dcytb provides an important element in the iron absorption pathway.

Molecular characterization of a novel urea transporter from kidney inner medullary collecting ducts

In the terminal part of the kidney collecting duct, rapid urea reabsorption is essential to maintaining medullary hypertonicity, allowing maximal urinary concentration to occur. This process is mediated by facilitated urea transporters on both apical and basolateral membranes. Our previous studies have identified three rat urea transporters involved in the urinary concentrating mechanism, UT1, UT2 and UT3, herein renamed UrT1-A, UrT1-B, and UrT2, which exhibit distinct spatial distribution in the kidney. Here we report the molecular characterization of an additional urea transporter isoform, UrT1-C, from rat kidney that encodes a 460-amino acid residue protein. UrT1-C has 70 and 62% amino acid identity to rat UrT1-B and UrT2 (UT3), respectively, and 99% identity to a recently reported rat isoform (UT-A3; Karakashian A, Timmer RT, Klein JD, Gunn RB, Sands JM, and Bagnasco SM. J Am Soc Nephrol 10: 230-237, 1999). We report the anatomic distribution of UrT1-C in the rat kidney tubule system as well as a detailed functional characterization. UrT1-C m RNA is primarily expressed in the deep part of the inner medulla. When expressed in Xenopus laevis oocytes, UrT1-C induced a 15-fold stimulation of urea uptake, which was inhibited almost completely by phloretin (0.7 mM) and 60-95% by thiourea analogs (150 mM). The characteristics are consistent with those described in perfusion studies with inner medullary collecting duct (IMCD) segments, but, contrary to UrT1-A, UrT1-C-mediated urea uptake was not stimulated by activation of protein kinase A. Our data show that UrT1-C is a phloretin-inhibitable urea transporter expressed in the terminal collecting duct that likely serves as an exit mechanism for urea at the basolateral membrane of IMCD cells.

Diurnal rhythmicity in intestinal SGLT-1 function, V(max), and mRNA expression topography

Mechanisms underlying the circadian rhythmicity in intestinal sugar absorption remain unclear. To test whether this rhythmicity is caused by changes in Na(+)-glucose cotransporter 1 (SGLT-1) function, we measured phloridzin-inhibitable sugar fluxes as an index of SGLT-1 activity. Jejunum obtained from rats killed at 6-h intervals during a 12-h light-dark cycle (CT0 is circadian time 0 h, time of light onset) were mounted in Ussing chambers, and 3-O-methylglucose (3-OMG) fluxes were calculated before and after addition of phloridzin. 3-OMG-induced change in short-circuit current and absorptive flux were significantly greater at CT9 than at CT3. This increase was phloridzin inhibitable. Kinetic studies indicated a significant increase in SGLT-1 maximal velocity (V(max)) at CT9. Food intake between CT3 and CT9 was <10% of the daily total, indicating that the increased SGLT-1 activity was anticipatory. Diurnicity of SGLT-1 mRNA was confirmed by Northern blotting. Expression topography analyzed by in situ hybridization revealed more intense labeling along the entire villus axis at CT9 and CT15 compared with CT3 and CT21. We conclude that diurnicity in intestinal sugar absorption is caused by periodicity in SGLT-1 V(max).

Inhibition of CaT1 channel activity by a noncompetitive IP3 antagonist

A newly cloned, human epithelial Ca2+ transport protein (CaT1) was expressed in Xenopus laevis oocytes, and its single channel characteristics were examined. The CaT1 channel shows a strong dependence upon hyperpolarizing voltages, being activated by very negative voltages. The probability of channel opening and mean open times increase substantially at more negative voltages in the range of -90 to -160 mV. In addition, CaT1 channel activity was markedly inhibited by micromolar levels of a noncompetitive antagonist of the IP3 receptor originally isolated from a marine sponge, Xestospongin C. This inhibitory effect could be mediated indirectly via the binding of Xestospongin C to the inositol-trisphosphate (IP3) receptor or, alternatively, by a direct action on the CaT1 channel itself. Independent of its mechanism of action in inhibiting CaT1, Xestospongin C will provide a useful tool for elucidating the physiological role(s) of this novel epithelial Ca2+ channel.

Amyotrophic lateral sclerosis-linked glutamate transporter mutant has impaired glutamate clearance capacity

We have investigated the functional impact of a naturally occurring mutation of the human glutamate transporter GLT1 (EAAT2), which had been detected in a patient with sporadic amyotrophic lateral sclerosis. The mutation involves a substitution of the putative N-linked glycosylation site asparagine 206 by a serine residue (N206S) and results in reduced glycosylation of the transporter and decreased uptake activity. Electrophysiological analysis of N206S revealed a pronounced reduction in transport rate compared with wild-type, but there was no alteration in the apparent affinities for glutamate and sodium. In addition, no change in the sensitivity for the specific transport inhibitor dihydrokainate was observed. However, the decreased rate of transport was associated with a reduction of the N206S transporter in the plasma membrane. Under ionic conditions, which favor the reverse operation mode of the transporter, N206S exhibited an increased reverse transport capacity. Furthermore, if coexpressed in the same cell, N206S manifested a dominant negative effect on the wild-type GLT1 activity, whereas it did not affect wild-type EAAC1. These findings provide evidence for a role of the N-linked glycosylation in both cellular trafficking and transport function. The resulting alteration in glutamate clearance capacity likely contributes to excitotoxicity that participates in motor neuron degeneration in amyotrophic lateral sclerosis.

Human calcium transport protein CaT1

Transcellular calcium transport occurs in many epithelial tissues including intestine, kidney, and placenta. We identified the human ortholog (hCaT1) of a recently cloned rat calcium transport protein, CaT1, that mediates intestinal calcium uptake. hCaT1 messenger RNA is present in the gastrointestinal tract, including esophagus, stomach, duodenum, jejunum, ileum, and colon. High levels of hCaT1 transcripts are also present in pancreas, placenta, prostate, and salivary gland, while moderate levels are present in liver, kidney, and testis. hCaT1 mRNA is also expressed in the colorectal cancer cell line, SW480, and the chronic myelogenous leukemia cell line, K-562. The hCaT1 gene was assigned to the long arm of chromosome 7, bands q33-34, by fluorescence in situ hybridization. When expressed in Xenopus laevis oocytes, hCaT1 promotes saturable Ca(2+) uptake with a Michaelis constant of 0.25 mM. Our studies suggest a role for hCaT1 in cellular calcium uptake in a variety of tissues, including the transcellular calcium transport pathway in intestine.

Na/HCO3 cotransporters in rat brain: expression in glia, neurons, and choroid plexus

We studied the expression and distribution of Na/HCO(3) cotransporters in rat brain using polynucleotide probes and polyclonal antibodies derived from the electrogenic rat kidney Na/HCO(3) cotransporter (rkNBC). In whole brain, we observed a single mRNA ( approximately 7.5 kb) by Northern hybridization and a major approximately 130 kDa protein by immunoblotting with a polyclonal antiserum directed against the C terminus of rkNBC. NBC mRNA and protein were present in cortex, brainstem-diencephalon, and cerebellum. In situ hybridization revealed NBC mRNA expression throughout the CNS, with particularly high levels in olfactory bulb, hippocampal dentate gyrus, and cerebellum. NBC mRNA was present in glial cells (e.g., Bergmann glia of cerebellum and hippocampal astrocytes) and neurons (e.g., granule cells of dentate gyrus and neurons of cortex or striatum). Double hybridization of mRNA encoding NBC and glutamate transporter 1 (glial marker) confirmed that both glia and neurons express NBC. Indirect immunofluorescence microscopy demonstrated NBC protein throughout the CNS, particularly in hippocampus and cerebellum. Although NBC mRNA was restricted to cell bodies, NBC protein was distributed diffusely, compatible with a localization in cell processes and perhaps cell bodies. Double labeling with glial fibrillary acidic protein (astrocytic marker), microtubule-associated protein 2 (neuronal marker), or 2',3'-cyclic mononucleotide 3'-phosphodiesterase (oligodendrocytic marker) demonstrated expression of NBC protein in specific subpopulations of both glia and neurons. Moreover, NBC protein was present in both cultured hippocampal astrocytes and cortical neurons. NBC mRNA and protein were also present in epithelial cells of choroid plexus, ependyma, and meninges. Our results are thus consistent with multiple novel roles for Na/HCO(3) cotransport in CNS physiology.

A rat kidney-specific calcium transporter in the distal nephron

Active absorption of calcium from the intestine and reabsorption of calcium from the kidney are major determinants of whole body calcium homeostasis. Two recently cloned proteins, CaT1 and ECaC, have been postulated to mediate apical calcium uptake by rat intestine and rabbit kidney, respectively. By screening a rat kidney cortex library with a CaT1 probe, we isolated a cDNA encoding a protein (CaT2) with 84.2 and 73.4% amino acid identities to ECaC and CaT1, respectively. Unlike ECaC, CaT2 is kidney-specific in the rat and was not detected in intestine, brain, adrenal gland, heart, skeletal muscle, liver, lung, spleen, thymus, and testis by Northern analysis or reverse transcription polymerase chain reaction. The expression pattern of CaT2 in kidney was similar to that of calbindin D(28K) and the sodium calcium exchanger 1, NCX1, by in situ hybridization of adjacent sections. Furthermore, the mRNAs for CaT2 and calbindin D(28K) were colocalized in the same cells. CaT2 mediated saturable calcium uptake with a Michaelis constant (K(m)) of 0.66 mm when expressed in Xenopus laevis oocytes. Under voltage clamp condition, CaT2 promoted inward currents in X. laevis oocytes upon external application of Ca(2+). Sr(2+) and Ba(2+) but not Mg(2+) also evoked inward currents in CaT2-expressing oocytes. Similar to the alkaline earth metal ions, application of Cd(2+) elicited inward current in CaT2-expressing oocytes with a K(m) of 1.3 mm. Cd(2+), however, also potently inhibited CaT2-mediated Ca(2+) uptake with an IC(50) of 5.4 micrometer. Ca(2+) evoked currents were reduced at low pH and increased at high pH and were only slightly affected by the L-type voltage-dependent calcium channel antagonists, nifedipine, verapamil, diltiazem, and the agonist, Bay K 8644, even at relatively high concentrations. In conclusion, CaT2 may participate in calcium entry into the cells of the distal convoluted tubule and connecting segment of the nephron, where active reabsorption of calcium takes place via the transcellular route. The high sensitivity of CaT2 to Cd(2+) also provides a potential explanation for Cd(2+)-induced hypercalciuria and resultant renal stone formation.

A novel system A isoform mediating Na+/neutral amino acid cotransport

A cDNA clone encoding a plasma membrane alanine-preferring transporter (SAT2) has been isolated from glutamatergic neurons in culture and represents the second member of the system A family of neutral amino acid transporters. SAT2 displays a widespread distribution and is expressed in most tissues, including heart, adrenal gland, skeletal muscle, stomach, fat, brain, spinal cord, colon, and lung, with lower levels detected in spleen. No signal is detected in liver or testis. In the central nervous system, SAT2 is expressed in neurons. SAT2 is significantly up-regulated during differentiation of cerebellar granule cells and is absent from astrocytes in primary culture. The functional properties of SAT2, examined using transfected fibroblasts and in cRNA-injected voltage-clamped Xenopus oocytes, show that small aliphatic neutral amino acids are preferred substrates and that transport is voltage- and Na(+)-dependent (1:1 stoichiometry), pH-sensitive, and inhibited by alpha-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of system A. Kinetic analyses of alanine and MeAIB uptake by SAT2 are saturable, with Michaelis constants (K(m)) of 200-500 microm. In addition to its ubiquitous role as a substrate for oxidative metabolism and a major vehicle of nitrogen transport, SAT2 may provide alanine to function as the amino group donor to alpha-ketoglutarate to provide an alternative source for neurotransmitter synthesis in glutamatergic neurons.

Functional roles of histidine and tyrosine residues in the H(+)-peptide transporter PepT1

Histidyl residues in peptide transporters PepT1 and PepT2 are believed to participate in proton and substrate binding and to be crucial to the transporters' functional activities. In the present study, we performed mutagenesis of rabbit PepT1. We mutated three histidine residues (H57, H111, and H121) predicted to reside in transmembrane segments, as well as tyrosine residues adjacent to H57. Functional analysis of wild-type and mutant PepT1 expressed in Xenopus oocytes, using both the radiotracer methods and two-microelectrode voltage-clamping, revealed that not only the H57 but also the aromatic residues near H57 were essential for the normal function of PepT1, in agreement with the concept that aromatic residues stabilize the charge on H(+) when interacting with H57. While mutagenesis at H111 did not significantly affect the activity of PepT1, mutagenesis at H121 had profound implications. The substrate affinities for H121 mutants were decreased depending both on the charge of the substrate and the charge on the substituted residues at position 121. We propose that H57 and H121 are intimately involved in the binding of the coupling ion H(+) and the recognition of transportable peptide substrates, respectively.

Distribution of the glutamate transporters GLAST and GLT-1 in rat circumventricular organs, meninges, and dorsal root ganglia

The glial glutamate transporters GLAST and GLT-1 are primarily responsible for the removal of glutamate from brain extracellular fluid. This study compares the distribution of GLAST and GLT-1 expression in the circumventricular organs of the brain, in the meninges, and in the dorsal root ganglion. By using a highly sensitive nonisotopic in situ hybridization method and immunostaining, we demonstrate marked differences in the expression patterns for the two transporters. In the three sensory circumventricular organs that contain neuronal elements, i.e., the subfornical organ, the vascular organ of the lamina terminalis, and the area postrema, GLAST is strongly expressed, whereas GLT-1 is faintly expressed or absent. Both transporters are absent from the choroid plexus, and only GLAST mRNA is found in the subcommisural organ. In the pineal gland, GLAST is expressed by astrocytic cells near the pineal stalk, whereas GLT-1 is expressed by pinealocytes throughout the gland. In the pituitary gland, GLAST is likely expressed by folliculo-stellate cells in the anterior lobe, by a group of astrocyte-like cells and by marginal cells in the intermediate lobe, and by pituicytes in the posterior lobe, whereas GLT-1 is expressed only by the astrocyte-like cells in the intermediate lobe. Finally, GLAST, but not GLT-1, is expressed by specific layers of the meninges, and by satellite cells in the dorsal root ganglion. These results show that GLAST is the primary glutamate transporter in the circumventricular organs. The data provide further evidence that these two glutamate transporters fulfill markedly different functions in the nervous system.

The vitamin C transporter SVCT2 is expressed by astrocytes in culture but not in situ

Ascorbic acid (vitamin C) is known to be selectively accumulated by brain cells through sodium-dependent vitamin C transporters. It is unclear however, whether this uptake occurs in neurons, astrocytes or both. Using Northern analysis we demonstrate that the recently cloned ascorbate transporter isoform SVCT2 is expressed by cultured astrocytes. In contrast, in situ hybridization experiments reveal that SVCT2 mRNA is expressed only in neurons and not in normal astrocytes or astrocytes stimulated by an intrastriatal injection of the neurotoxin quinolinic acid. We conclude that SVCT2 is neuron specific and that the majority of ascorbate storage occurs in neurons. Furthermore, we propose that the observed sodium-dependent ascorbate transport in cultured astrocytes may be due to artificial upregulation of SVCT2 during cell culturing.

Differential recognition of ACE inhibitors in Xenopus laevis oocytes expressing rat PEPT1 and PEPT2

PURPOSE

To examine the mechanism of inhibition of glycylsarcosine (GlySar) transport by quinapril and enalapril, and whether or not angiotensin converting enzyme (ACE) inhibitors are transported by PEPT2 as well as by PEPT1.

METHODS

Xenopus laevis oocytes were cRNA-injected with rat PEPT1 or PEPT2 and the transport kinetics of radiolabeled GlySar were studied in the absence and presence of quinapril and enalapril. The two-microelectrode voltage-clamp technique was also performed to probe the electrogenic uptake of captopril, quinapril and enalapril.

RESULTS

Kinetic analyses demonstrated that quinapril inhibited the uptake of GlySar in a noncompetitive manner in Xenopus oocytes injected with PEPT1 or PEPT2 (Ki = 0.8 or 0.4 mM, respectively). In contrast, a competitive interaction was observed between GlySar and enalapril (Ki = 10.8 mM for PEPT1 or 4.3 mM for PEPT2). Most significantly, captopril and enalapril, but not quinapril, induced inwardly-directed currents in both PEPT1- and PEPT2-expressed oocytes.

CONCLUSIONS

These results are unique in providing direct evidence for the substrate recognition and transport of some ACE inhibitors by the high- and low-affinity oligopeptide transporters. Our findings point to differences between PEPT1 and PEPT2 in their affinity to, rather than in their specificity for, ACE inhibitors.

Long-term regulation of urea transporter expression by vasopressin in Brattleboro rats

Regulation of urea concentration in the renal medullary interstitium is important for maintenance of hypertonicity and therefore the osmotic driving force for water reabsorption. Studies in Sprague-Dawley rats showed that restriction of water intake for 3 days results in upregulation of urea transporter (UT) mRNA in the inner stripe of outer medulla of the kidney (2.9-kb UT2) but not in the inner medulla (4.0-kb UT1). The present study was performed to investigate the role of vasopressin in long-term regulation of UT1 and UT2 in neurogenic diabetes insipidus (Brattleboro) rats treated with a 7-day continuous infusion of [Arg(8)]-vasopressin (AVP), [deamino-Cys(1), D-Arg(8)]-vasopressin (dDAVP) or vehicle. Northern analysis showed that water restriction alone had no effect on the level of UT2 mRNA in vehicle-treated Brattleboro rats but UT2 mRNA markedly increased and UT1 mRNA modestly decreased after treatment with dDAVP. In situ hybridization further demonstrated that the UT2 signal is upregulated and spread along the descending thin limbs of loops of Henle and that UT1 signal is downregulated in the inner medullary collecting ducts in vasopressin-treated rats, with a greater response for dDAVP compared with the AVP-treated group. Immunocytochemistry studies revealed that the UT1 and UT2 proteins are also modified in the same pattern as the transcript changes. Our studies reveal the role of vasopressin in long-term regulation of UT1 and UT2 expression during water restriction.

A novel duodenal iron-regulated transporter, IREG1, implicated in the basolateral transfer of iron to the circulation

Iron absorption by the duodenal mucosa is initiated by uptake of ferrous Fe(II) iron across the brush border membrane and culminates in transfer of the metal across the basolateral membrane to the portal vein circulation by an unknown mechanism. We describe here the isolation and characterization of a novel cDNA (Ireg1) encoding a duodenal protein that is localized to the basolateral membrane of polarized epithelial cells. Ireg1 mRNA and protein expression are increased under conditions of increased iron absorption, and the 5' UTR of the Ireg1 mRNA contains a functional iron-responsive element (IRE). IREG1 stimulates iron efflux following expression in Xenopus oocytes. We conclude that IREG1 represents the long-sought duodenal iron export protein and is upregulated in the iron overload disease, hereditary hemochromatosis.

Human vitamin C (L-ascorbic acid) transporter SVCT1

In human, vitamin C (l-ascorbic acid) is an essential micronutrient required for an array of biological functions including enzymatic reactions and antioxidation. We describe here the molecular cloning of a novel human cDNA encoding a vitamin C transporter SVCT1. SVCT1 is largely confined to bulk-transporting epithelia (e.g., kidney and small intestine) with a putative alternative-splice product present in thymus. Applying radiotracer and voltage-clamp approaches in cRNA-injected Xenopus oocytes, we found that SVCT1 mediates saturable, concentrative, high-affinity l-ascorbic acid transport (K(0.5) = 50-100 microM) that is electrogenic and can be inhibited by phloretin. SVCT1 displays exquisite substrate selectivity, greatly favoring l-ascorbic acid over its isomers d-isoascorbic acid and dehydroascorbic acid and 2- or 6-substituted analogues, whereas glucose and nucleobases are excluded. We have mapped the SLC23A2 gene (coding for SVCT1) to human chromosome 5 in band 5q31.2-31.3, within a region commonly deleted in malignant myeloid (leukemia) diseases. In addition, we have demonstrated that the human SLC23A1 gene product is a related high-affinity l-ascorbic acid transporter (SVCT2) that is widely distributed in brain, retina, and a host of endocrine and neuroendocrine tissues. The molecular identification of the human l-ascorbic acid transporters now provides the tools with which to investigate their roles in vitamin C metabolism in health and disease.

Yeast SMF1 mediates H(+)-coupled iron uptake with concomitant uncoupled cation currents

Yeast membrane proteins SMF1, SMF2, and SMF3 are homologues of the DCT1 metal ion transporter family. Their functional characteristics and the implications of these characteristics in vivo have not yet been reported. Here we show that SMF1 expressed in Xenopus oocytes mediates H(+)-dependent Fe(2+) transport and uncoupled Na(+) flux. SMF1-mediated Fe(2+) transport exhibited saturation kinetics (K(m) = 2.2 microM), whereas the Na(+) flux did not, although both processes were electrogenic. SMF1 is also permeable to Li(+), Rb(+), K(+), and Ca(2+), which likely share the same uncoupled pathway. SMF2 (but not SMF3) mediated significant increases in both Fe(2+) and Na(+) transport compared with control oocytes. These data are consistent with the concept that uptake of divalent metal ions by SMF1 and SMF2 is essential to yeast cell growth. Na(+) inhibited metal ion uptake mediated by SMF1 and SMF2 expressed in oocytes. Consistent with this, we found that increased sensitivity of yeast to EGTA in the high Na(+) medium is due to inhibition of SMF1- and SMF2-mediated metal ion transport by uncoupled Na(+) pathway. Interestingly, DCT1 also mediates Fe(2+)-activated uncoupled currents. We propose that uncoupled ion permeabilities in metal ion transporters protect cells from metal ion overload.

Functional and molecular characterization of the human neutral solute channel aquaporin-9

In metabolically active cells, the coordinated transport of water and solutes is important for maintaining osmotic homeostasis. We recently identified a broad selective-neutral solute channel, AQP9, from rat liver that allows the passage of a wide variety of water and neutral solutes (H. Tsukaguchi, C. Shayakul, U. V. Berger, B. Mackenzie, S. Devidas, W. B. Guggino, A. N. van Hoek, and M. A. Hediger. J. Biol. Chem. 273: 24737-24743, 1998). A human homolog (hAQP9) with 76% amino acid sequence identity to rat AQP9 (rAQP9) was described, but its permeability was found to be restricted to water and urea (K. Ishibashi, M. Kuwahara, Y. Gu, Y. Tanaka, F. Marumo, and S. Sasaki. Biochem. Biophys. Res. Commun. 244: 268-274, 1998). Here we report a reevaluation of the functional characteristics of hAQP9, its tissue distribution, the structure of its gene, and its chromosomal localization. When expressed in Xenopus oocytes, hAQP9 allowed passage of a wide variety of noncharged solutes, including carbamides, polyols, purines, and pyrimidines in a phloretin- and mercurial-sensitive manner. These functional characteristics are similar to those of rAQP9. Based on Northern blot analysis, both rat and human AQP9 are abundantly expressed in liver, whereas, in contrast to rAQP9, hAQP9 is also expressed in peripheral leukocytes and in tissues that accumulate leukocytes, such as lung, spleen, and bone marrow. The human AQP9 gene is composed of 6 exons and 5 introns distributed over approximately approximately 25 kb. The gene organization is strikingly similar to that reported for human AQP3 and AQP7, suggesting their evolution from a common ancestral gene. The promoter region contains putative tonicity and glucocorticoid-responsive elements, suggesting that AQP9 may be regulated by osmolality and catabolism. Fluorescence in situ hybridization assigned its locus to chromosome 15 q22.1-22.2. Our data show that hAQP9 serves as a promiscuous solute channel expressed in both liver and peripheral leukocytes, where it is ideally suited to transport of metabolites and/or nutrients into and out of these cells

Glutamate transporters in kidney and brain

Glutamate transporters play important roles in the termination of excitatory neurotransmission and in providing cells with glutamate for metabolic purposes. In the kidney, glutamate transporters are involved in reabsorption of filtered acidic amino acids, regulation of ammonia and bicarbonate production, and protection of cells against osmotic stress.

Introduction: glutamate transport, metabolism, and physiological responses

The material covered in this set of articles was originally presented at Experimental Biology '98, in San Francisco, CA, on April 20, 1998. Here, the participants recount important elements of current research on the role of glutamate transporter activity in cellular signaling, metabolism, and organ function. W. A. Fairman and S. G. Amara discuss the five subtypes of human excitatory amino acid transporters, with emphasis on the EAAT4 subtype. M. A. Hediger discusses the expression and action of EAAC1 subtype of the human excitatory amino acid transporter. I. Nissim provides an overview of the significant role of pH in regulating Gln/Glu metabolism in the kidney, liver, and brain. J. D. McGivan and B. Nicholson describe some characteristics of glutamate transport regulation with regard to a specific experimental model of the bovine renal epithelial cell line NBL-1. Finally, T. C. Welbourne and J. C. Matthews introduce the "functional unit" concept of glutamate transport and how this relates to both glutamine metabolism and paracellular permeability.

Polycystin-L is a calcium-regulated cation channel permeable to calcium ions

Polycystic kidney diseases are genetic disorders in which the renal parenchyma is progressively replaced by fluid-filled cysts. Two members of the polycystin family (polycystin-1 and -2) are mutated in autosomal dominant polycystic kidney disease (ADPKD), and polycystin-L is deleted in mice with renal and retinal defects. Polycystins are membrane proteins that share significant sequence homology, especially polycystin-2 and -L (50% identity and 71% similarity). The functions of the polycystins remain unknown. Here we show that polycystin-L is a calcium-modulated nonselective cation channel that is permeable to sodium, potassium and calcium ions. Patch-clamp experiments revealed single-channel activity with a unitary conductance of 137 pS. Channel activity was substantially increased when either the extracellular or intracellular calcium-ion concentration was raised, indicating that polycystin-L may act as a transducer of calcium-mediated signalling in vivo. Its large single-channel conductance and regulation by calcium ions distinguish it from other structurally related cation channels.

Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption

Calcium is a major component of the mineral phase of bone and serves as a key intracellular second messenger. Postnatally, all bodily calcium must be absorbed from the diet through the intestine. Here we report the properties of a calcium transport protein (CaT1) cloned from rat duodenum using an expression cloning strategy in Xenopus laevis oocytes, which likely plays a key role in the intestinal uptake of calcium. CaT1 shows homology (75% amino acid sequence identity) to the apical calcium channel ECaC recently cloned from vitamin D-responsive cells of rabbit kidney and is structurally related to the capsaicin receptor and the TRP family of ion channels. Based on Northern analysis of rat tissues, a 3-kilobase CaT1 transcript is present in rat duodenum, proximal jejunum, cecum, and colon, and a 6.5-kilobase transcript is present in brain, thymus, and adrenal gland. In situ hybridization revealed strong CaT1 mRNA expression in enterocytes of duodenum, proximal jejunum, and cecum. No signals were detected in kidney, heart, liver, lung, spleen, and skeletal muscle. When expressed in Xenopus oocytes, CaT1 mediates saturable Ca(2+) uptake with a Michaelis constant of 0.44 mM. Transport of Ca(2+) by CaT1 is electrogenic, voltage-dependent, and exhibits a charge/Ca(2+) uptake ratio close to 2:1, indicating that CaT1-mediated Ca(2+) influx is not coupled to other ions. CaT1 activity is pH-sensitive, exhibiting significant inhibition by low pH. CaT1 is also permeant to Sr(2+) and Ba(2+) (but not Mg(2+)), although the currents evoked by Sr(2+) and Ba(2+) are much smaller than those evoked by Ca(2+). The trivalent cations Gd(3+) and La(3+) and the divalent cations Cu(2+), Pb(2+), Cd(2+), Co(2+), and Ni(2+) (each at 100 microM) do not evoke currents themselves, but inhibit CaT1-mediated Ca(2+) transport. Fe(3+), Fe(2+), Mn(2+), and Zn(2+) have no significant effects at 100 microM on CaT1-mediated Ca(2+) transport. CaT1 mRNA levels are not responsive to 1,25-dihydroxyvitamin D(3) administration or to calcium deficiency. Our studies strongly suggest that CaT1 provides the principal mechanism for Ca(2+) entry into enterocytes as part of the transcellular pathway of calcium absorption in the intestine.

Metal ion transporters in mammals: structure, function and pathological implications

Despite the importance of metal ions in several catalytic functions, there has been, until recently, little molecular information available on the mechanisms whereby metal ions are actively taken up by mammalian cells. The classical concept for iron uptake into mammalian cells has been the endocytosis of transferrin-bound Fe3+ by the transferrin receptor. Studies with hypotransferrinaemic mice revealed that in the intestine mucosal transferrin is derived from the plasma and that its presence is not required in the intestinal lumen for dietary iron absorption. This suggests that, at least in the intestine, other non-receptor-mediated uptake systems exist. The molecular identification of metal ion transporters is of great importance, in particular since an increasing number of human diseases are thought to be related to disturbances in metal ion homeostasis, including metal ion overload and deficiency disorders (i.e. anaemia, haemochromatosis, Menkes disease, Wilson's disease), and neurodegenerative diseases (i.e. Alzheimer's, Friedreich's ataxia and Parkinson's diseases). Furthermore, susceptibilities to mycobacterial infections are caused by metal ion transporter defects. The pathological implications of disturbed metal ion homeostasis confirm the vital roles these metal ions play in the catalytic function of many enzymes, in gene regulation (zinc-finger proteins), and in free radical homeostasis. Recent insights have significantly advanced our knowledge of how metal ions are taken up or released by mammalian cells. The purpose of this review is to summarize these advances and to give an overview on the growing number of mammalian metal ion transporters.

A family of mammalian Na+-dependent L-ascorbic acid transporters

Vitamin C (L-ascorbic acid) is essential for many enzymatic reactions, in which it serves to maintain prosthetic metal ions in their reduced forms (for example, Fe2+, Cu+), and for scavenging free radicals in order to protect tissues from oxidative damage. The facilitative sugar transporters of the GLUT type can transport the oxidized form of the vitamin, dehydroascorbic acid, but these transporters are unlikely to allow significant physiological amounts of vitamin C to be taken up in the presence of normal glucose concentrations, because the vitamin is present in plasma essentially only in its reduced form. Here we describe the isolation of two L-ascorbic acid transporters, SVCT1 and SVCT2, from rat complementary DNA libraries, as the first step in investigating the importance of L-ascorbic acid transport in regulating the supply and metabolism of vitamin C. We find that SVCT1 and SVCT2 each mediate concentrative, high-affinity L-ascorbic acid transport that is stereospecific and is driven by the Na+ electrochemical gradient. Despite their close sequence homology and similar functions, the two isoforms of the transporter are discretely distributed: SVCT1 is mainly confined to epithelial systems (intestine, kidney, liver), whereas SVCT2 serves a host of metabolically active cells and specialized tissues in the brain, eye and other organs.

Distribution of peptide transporter PEPT2 mRNA in the rat nervous system

The signaling action of neuropeptides in the brain is terminated by breakdown through extracellular peptidases and subsequent removal of the peptide fragments from the extracellular fluid via specific transporter proteins. Here we describe the anatomical distribution in the rat nervous system of the recently isolated high affinity peptide transporter PEPT2. Using nonisotopic in situ hybridization we demonstrate that PEPT2 mRNA is expressed in brain by astrocytes, subependymal cells, ependymal cells and epithelial cells of choroid plexus. Furthermore, PEPT2 is expressed in retina by Müller cells and in dorsal root ganglia by satellite cells. The mRNA levels of PEPT2 in astrocytes are moderate and relatively homogenous throughout the brain except for an area in ventral forebrain where PEPT2 levels are below average. PEPT2 mRNA expression is weakly upregulated in reactive astrocytes that were stimulated through an injection of the glutamatergic neurotoxin quinolinic acid. These data suggest that removal of neuropeptide fragments from brain extracellular fluid occurs via PEPT2 expressed in astrocytes, ependymal cells and choroid plexus epithelial cells.

SOD1 mutants linked to amyotrophic lateral sclerosis selectively inactivate a glial glutamate transporter

The mechanism by which Cu2+/Zn2+ superoxide dismutase (SOD1) mutants lead to motor neuron degeneration in familial amyotrophic lateral sclerosis (FALS) is unknown. We show that oxidative reactions triggered by hydrogen peroxide and catalyzed by A4V and I113T mutant but not wild-type SOD1 inactivated the glutamate transporter human GLT1. Chelation of the copper ion of the prosthetic group of A4V prevented GLT1 inhibition. GLT1 was a selective target of oxidation mediated by SOD1 mutants, and its reactivity was confined to the intracellular carboxyl-terminal domain. The antioxidant Mn(III)TBAP rescued GLT1 from inhibition. Because inactivation of GLT1 results in neuronal degeneration, we propose that toxic properties of SOD1 mutants lead to neuronal death via an excitotoxic mechanism in SOD1-linked FALS.

Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Kidney proximal tubule cells take up Krebs cycle intermediates for metabolic purposes and for secretion of organic anions through dicarboxylate/organic anion exchange. Alteration in reabsorption of citrate is closely related to renal stone formation. The presence of distinct types of sodium-coupled dicarboxylate transporters has been postulated on either side of the polarized epithelial membrane in the kidney proximal tubule. Using a PCR-based approach, we isolated a novel member of the sodium-dependent dicarboxylate/sulfate transporter called SDCT2. SDCT2 is a 600-amino acid residue protein that has 47-48% amino acid identity to SDCT1 and NaDC-1, previously identified in kidney and intestine. Northern analysis gave a single band of 3.3 kb for SDCT2 in kidney, liver, and brain. In situ hybridization revealed that SDCT2 is prominently expressed in kidney proximal tubule S3 segments and in perivenous hepatocytes, consistent with the sites of high-affinity dicarboxylate transport identified based on vesicle studies. A signal was also detected in the meningeal layers of the brain. SDCT2 expressed in Xenopus oocytes mediated sodium-dependent transport of di- and tricarboxylates with substrate preference for succinate rather than citrate, but excluding monocarboxylates. SDCT2, unlike SDCT1, displayed a unique pH dependence for succinate transport (optimal pH 7.5-8.5) and showed a high affinity for dimethylsuccinate, two features characteristic of basolateral transport. These data help to interpret the mechanisms of renal citrate transport, their alteration in pathophysiological conditions, and their role in the elimination of organic anions and therapeutic drugs.

Localization of sodium bicarbonate cotransporter (NBC) protein and messenger ribonucleic acid in rat epididymis

An acidic environment is important for sperm maturation in the epididymis and also helps to maintain mature sperm in an immotile state during storage in this organ. Both an Na+/H+ exchanger and an H+ATPase have been implicated in this process. The H+ATPase is concentrated in specialized apical (and/or narrow) and clear cells of the epididymis, while the Na+/H+ exchanger has not yet been localized in situ. As in other proton-secreting epithelia, bicarbonate transport occurs in the epididymis, where it is implicated in luminal acidification. In this study we used an antibody raised against a fusion protein (maltose-binding protein: MBP-NBC-5) from the C-terminus of the recently cloned rat kidney Na+/HCO3- cotransporter (NBC) to localize this protein in the epididymis and vas deferens of the rat. The distribution of the respective mRNA was mapped by in situ hybridization. NBC message was strongly expressed in the initial segment and the intermediate zone of the epididymis, and the NBC-5 antibody gave a strong basolateral staining in both principal cells and apical/narrow cells in this region. Western blotting revealed a single band at about 160 kDa in the epididymis. The intensity of staining as well as mRNA levels decreased in the cauda epididymidis and in the vas deferens, where only weak staining was seen. Basolateral NBC may function in parallel with apical proton secretion to regulate luminal acidification and/or bicarbonate reabsorption in the excurrent duct system.

Stoichiometry and kinetics of the high-affinity H+-coupled peptide transporter PepT2

Proton-coupled peptide transporters mediate the absorption of a large variety of di- and tripeptides as well as peptide-like pharmacologically active compounds. We report a kinetic analysis of the rat kidney high-affinity peptide transporter PepT2 expressed in Xenopus oocytes. By use of simultaneous radioactive uptake and current measurements under voltage-clamp condition, the charge to substrate uptake ratio was found to be close to 2 for both D-Phe-L-Ala and D-Phe-L-Glu, indicating that the H+:substrate stoichiometry is 2:1 and 3:1 for neutral and anionic dipeptides, respectively. The higher stoichiometry for anionic peptides suggests that they are transported in the protonated form. For D-Phe-L-Lys, the charge:uptake ratio averaged 2.4 from pooled experiments, suggesting that Phe-Lys crosses the membrane via PepT2 either in its deprotonated (neutral) or its positively charged form, averaging a H+:Phe-Lys stoichiometry of 1.4:1. These findings led to the overall conclusion that PepT2 couples transport of one peptide molecule to two H+. This is in contrast to the low-affinity transporter PepT1 that couples transport of one peptide to one H+. Quinapril inhibited PepT2-mediated currents in presence or in absence of external substrates. Oocytes expressing PepT2 exhibited quinapril-sensitive outward currents. In the absence of external substrate, a quinapril-sensitive proton inward current (proton leak) was also observed which, together with the observed pH-dependent PepT2-specific presteady-state currents (Ipss), indicates that at least one H+ binds to the transporter prior to substrate. PepT2 exhibited Ipss in response to hyperpolarization at pH 6.5-8.0. However, contrary to previous observations on various transporters, 1) no significant currents were observed corresponding to voltage jumps returning from hyperpolarization, and 2) at reduced extracellular pH, no significant Ipss were observed in either direction. Together with observed lower substrate affinities and decreased PepT2-mediated currents at hyperpolarized Vm, our data are consistent with the concept that hyperpolarization exerts inactivation effects on the transporter which are enhanced by low pH. Our studies revealed distinct properties of PepT2, compared with PepT1 and other ion-coupled transporters.

hPepT1-mediated epithelial transport of bacteria-derived chemotactic peptides enhances neutrophil-epithelial interactions

Intestinal epithelial cells express hPepT1, an apical transporter responsible for the uptake of a broad array of small peptides. As these could conceivably include n-formyl peptides, we examined whether hPepT1 could transport the model n-formylated peptide fMLP and, if so, whether such cellular uptake of fMLP influenced neutrophil-epithelial interactions. fMLP uptake into oocytes was enhanced by hPepT1 expression. In addition, fMLP competitively inhibited uptake of a known hPepT1 substrate (glycylsarcosine) in hPepT1 expressing oocytes. hPepT1 peptide uptake was further examined in a polarized human intestinal epithelial cell line (Caco2-BBE) known to express this transporter. Epithelial monolayers internalized apical fMLP in a fashion that was competitively inhibited by other hPepT1 recognized solutes, but not by related solutes that were not transported by hPepT1. Fluorescence analyses of intracellular pH revealed that fMLP uptake was accompanied by cytosolic acidification, consistent with the known function of hPepT1 as a peptide H+ cotransporter. Lumenal fMLP resulted in directed movement of neutrophils across epithelial monolayers. Solutes that inhibit hPepT1-mediated fMLP transport decreased neutrophil transmigration by approximately 50%. Conversely, conditions that enhanced the rate of hPepT1-mediated fMLP uptake (cytosolic acidification) enhanced neutrophil-transepithelial migration by approximately 70%. We conclude that hPepT1 transports fMLP and uptake of these peptide influences neutrophil-epithelial interactions. These data (a) emphasize the importance of hPepT1 in mediating intestinal inflammation, (b) raise the possibility that modulating hPepT1 activity could influence states of intestinal inflammation, and (c) provide the first evidence of a link between active transepithelial transport and neutrophil-epithelial interactions.

The amino acid transport system y+L/4F2hc is a heteromultimeric complex

4F2hc is an almost ubiquitous transmembrane protein in mammalian cells; upon expression in Xenopus laevis oocytes, it induces amino acid transport with characteristics of system y+L. Indirect evidence fostered speculation that function requires the association of 4F2hc with another protein endogenous to oocytes and native tissues. We show that expression of system y+L-like amino acid transport activity by 4F2hc in oocytes is limited by an endogenous factor and that direct covalent modification of external cysteine residue(s) of an oocyte membrane protein blocks system y+L/4F2hc transport activity, based on the following. 1) Induction of system y+L-like activity saturates at very low doses of human 4F2hc cRNA (0.1 ng/oocyte). This saturation occurs with very low expression of 4F2hc at the oocyte surface, and further increased expression of the protein at the cell surface does not result in higher induction of system y+L-like activity. 2) Human 4F2hc contains only two cysteine residues (C109 and C330). We mutated these residues, singly and in combination, to serine (C109S; CS1, C330S; CS2 and C109S-C330S, Cys-less). Mutation CS2 had no effect on the expressed system y+L-like transport activity, whereas C109S-containing mutants (CS1 and Cys-less) retained only partial y+L-like transport activity (30 to 50% of wild type). 3) Hg2+, the organic mercury compounds pCMB, and the membrane-impermeant pCMBS almost completely inactivated system y+L-like induced by human 4F2hc wild type and all the mutants studied. This was reversed by ss-mercaptoethanol, indicating that external cysteine residue(s) are the target of this inactivation. 4) Sensitivity to Hg2+ inactivation is increased by pretreatment of oocytes with ss-mercaptoethanol or in the C109S-containing mutants (CS1 and Cys-less). The increased Hg2+ reactivity of C109S-containing mutants supports the possibility that C109 may be linked by a disulfide bond to the Hg2+-targeted cysteine residue of the associated protein. These results indicate that 4F2hc is intimately associated with a membrane oocyte protein for the expression of system y+L amino acid transport activity. To our knowledge, this is the first direct evidence for a heteromultimeric protein structure of an organic solute carrier in mammals.

Molecular characterization of a broad selectivity neutral solute channel

In all living cells, coordination of solute and water movement across cell membranes is of critical importance for osmotic balance. The current concept is that these processes are of distinct biophysical nature. Here we report the expression cloning of a liver cDNA encoding a unique promiscuous solute channel (AQP9) that confers high permeability for both solutes and water. AQP9 mediates passage of a wide variety of non-charged solutes including carbamides, polyols, purines, and pyrimidines in a phloretin- and mercury-sensitive manner, whereas amino acids, cyclic sugars, Na+, K+, Cl-, and deprotonated monocarboxylates are excluded. The properties of AQP9 define a new evolutionary branch of the major intrinsic protein family of aquaporin proteins and describe a previously unknown mechanism by which a large variety of solutes and water can pass through a single pore, enabling rapid cellular uptake or exit of metabolites with minimal osmotic perturbation.