Renal osmotic stress-induced cotransporter: expression in the newborn, adult and post-ischemic rat kidney

The SLC6A18 Transporter Is Most Likely a Na-Dependent Glycine/Urea Antiporter Responsible for Urea Secretion in the Proximal Straight Tubule: Influence of This Urea Secretion on Glomerular Filtration Rate

BACKGROUND

Urea is the major end-product of protein metabolism in mammals. In carnivores and omnivores, a large load of urea is excreted daily in urine, with a concentration that is 30-100 times above that in plasma. This is important for the sake of water economy. Too little attention has been given to the existence of energy-dependent urea transport that plays an important role in this concentrating activity.

SUMMARY

This review first presents functional evidence for an energy-dependent urea secretion that occurs exclusively in the straight part of the proximal tubule (PST). Second, it proposes a candidate transmembrane transporter responsible for this urea secretion in the PST. SLC6A18 is expressed exclusively in the PST and has been identified as a glycine transporter, based on findings in SLC6A18 knockout mice. We propose that it is actually a glycine/urea antiport, secreting urea into the lumen in exchange for glycine and Na. Glycine is most likely recycled back into the cell via a transporter located in the brush border. Urea secretion in the PST modifies the composition of the tubular fluid in the thick ascending limb and, thus, contributes, indirectly, to influence the "signal" at the macula densa that plays a crucial role in the regulation of the glomerular filtration rate (GFR) by the tubulo-glomerular feedback.

KEY MESSAGES

Taking into account this secondary active secretion of urea in the mammalian kidney provides a new understanding of the influence of protein intake on GFR, of the regulation of urea excretion, and of the urine-concentrating mechanism.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

The SLC6 orphans are forming a family of amino acid transporters

Transporters in the human genome are grouped in solute carrier families (SLC). The SLC6 family is one of the biggest transporter families in the human genome comprising 20 members. It is usually referred to as the neurotransmitter transporter family because its founding members encode transporters for the neurotransmitters GABA, noradrenaline, serotonin and dopamine. The family also includes a number of 'orphan' transporters, the function of which has remained elusive until recently. Identification of the broadly specific neutral amino acid transporter SLC6A19 (also called B(0)AT1) suggested that all orphan transporters may in fact be amino acid transporters. This was subsequently confirmed by the identification of SLC6A20 as the long-sought IMINO system, a proline transporter found in kidney, intestine and brain. Very recently, SLC6A15 was identified as the neutral amino acid transporter B(0)AT2. All amino acid transporters appear to cotransport only 1Na(+) together with the amino acid substrate. Both, B(0)AT1 and B(0)AT2 are chloride independent, whereas IMINO is chloride dependent. The amino acid transporters of the SLC6 family are functionally and sequence related to the recently crystallized leucine transporter from Aquifex aeolicus. The structure elegantly explains many of the mechanistic features of the SLC6 amino acid transporters.

Characterization of mouse amino acid transporter B0AT1 (slc6a19)

The mechanism of the mouse (m)B0AT1 (slc6a19) transporter was studied in detail using two electrode voltage-clamp techniques and tracer studies in the Xenopus oocyte expression system. All neutral amino acids induced inward currents at physiological potentials, but large neutral non-aromatic amino acids were the preferred substrates of mB0AT1. Substrates were transported with K0.5 values ranging from approx. 1 mM to approx. 10 mM. The transporter mediates Na+-amino acid co-transport with a stoichiometry of 1:1. No other ions were involved in the transport mechanism. An increase in the extracellular Na+ concentration reduced the K0.5 for leucine, and vice versa. Moreover, the K0.5 values and Vmax values of both substrates varied with the membrane potential. As a result, K0.5 and Vmax values are a complex function of the concentration of substrate and co-substrate and the membrane potential. A model is presented assuming random binding order and a positive charge associated with the ternary [Na+-substrate-transporter] complex, which is consistent with the experimental data.

The molecular basis of neutral aminoacidurias

Recent success in the molecular cloning and identification of apical neutral amino acid transporters has shed a new light on inherited neutral amino acidurias, such as Hartnup disorder and Iminoglycinuria. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. The molecular defect underlying Iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. A model is presented, involving all three transporters that can explain the phenotypic variability of iminoglycinuria.

Proximo-distal specialization of epithelial transport processes within the Xenopus pronephric kidney tubules

The embryonic kidneys of larval aquatic vertebrates such as fish and frogs serve as excellent model systems for exploring the early development of nephric organs. These experimental systems can easily be manipulated by microsurgery, microinjection, genetics, or combinations of these approaches. However, little is known about how physiologically similar these simple kidneys are to the more complex mammalian adult kidneys. In addition, almost nothing is known about proximo-distal patterning of nephrons in any organism. In order begin to explore the physiological specialization of the pronephric tubules along the proximo-distal axis, a combination of uptake assays using fluorescently tagged proteins, LDL particles and dextrans, and an informatics-targeted in situ screen for transport proteins have been performed on embryos of the frog, Xenopus laevis. Genes identified to be expressed within unique subdomains of the pronephric tubules include an ABC transporter, two amino acid cotransporters, two sodium bicarbonate cotransporters, a novel sodium glucose cotransporter, a sodium potassium chloride cotransporter (NKCC2), a sodium chloride organic solute cotransporter (ROSIT), and a zinc transporter. A novel combination of colorimetric and fluorescent whole-mount in situ hybridization (FCIS) was used to precisely map the expression domain of each gene within the pronephros. These data indicate specialized physiological function and define multiple novel segments of the pronephric tubules, which contain at least six distinct transport domains. Uptake studies identified functional transport domains and also demonstrated that early glomeral leakage can allow visualization of protein movement into the pronephric tubules and thus establish a system for investigating experimentally induced proteinuria and glomerulonephritis.

Hypertension and impaired glycine handling in mice lacking the orphan transporter XT2

A family of orphan transporters has been discovered that are structurally related to the Na(+)-Cl(-)-dependent neurotransmitter transporters, including the dopamine transporter. One member of this family, the mouse XT2 gene, is predominantly expressed in the kidney and has 95% homology to rat ROSIT (renal osmotic stress-induced Na(+)-Cl(-) organic solute cotransporter). To study the physiological functions of this transporter, we generated XT2-knockout mice by gene targeting. XT2(-/-) mice develop and survive normally with no apparent abnormalities. To attempt to identify potential substrates for XT2, we screened urine from XT2-knockout mice by high-pressure liquid chromatography and mass spectrometry and found significantly elevated concentrations of glycine. To study glycine handling, XT2(+/+) and XT2(-/-) mice were injected with radiolabeled glycine, and urine samples were collected to monitor glycine excretion. After 2 h, XT2(-/-) mice were found to excrete almost twice as much glycine as the XT2(+/+) controls (P = 0.03). To determine whether the absence of the XT2 transporter affected sodium and fluid homeostasis, we measured systolic blood pressure by computerized tail-cuff manometry. Systolic blood pressure was significantly higher in XT2(-/-) mice (127 +/- 3 mmHg) than in wild-type controls (114 +/- 2 mmHg; P < 0.001). This difference in systolic blood pressure was maintained on high and low salt feeding. To examine whether the alteration in blood pressure and the defect in glycine handling were related, we measured systolic blood pressure in the XT2(-/-) mice during dietary glycine supplementation. Glycine loading caused systolic blood pressure to fall in the XT2(-/-) mice from 127 +/- 3 to 115 +/- 3 mmHg (P < 0.001), a level virtually identical to that of the wild-type controls. These data suggest that the XT2 orphan transporter is involved in glycine reabsorption and that the absence of this transporter is sufficient to cause hypertension.

Altered expression pattern of polycystin-2 in acute and chronic renal tubular diseases

Polycystin-2 represents one of so far two proteins found to be mutated in patients with autosomal-dominant polycystic kidney disease. Evidence obtained from experiments carried out in cell lines and with native kidney tissue strongly suggests that polycystin-2 is located in the endoplasmic reticulum. In the kidney, polycystin-2 is highly expressed in cells of the distal and connecting tubules, where it is located in the basal compartment. It is not known whether the expression of polycystin-2 in the kidney changes or whether it can be manipulated under certain instances. Therefore, the distribution of polycystin-2 under conditions leading to acute and chronic renal failure was analyzed. During ischemic acute renal failure, which affects primarily the S3 segment of the proximal tubule, a pronounced upregulation of polycystin-2 and a predominantly combined homogeneous and punctate cytoplasmic distribution in damaged cells was observed. After thallium-induced acute injury to thick ascending limb cells, polycystin-2 staining assumed a chicken wire-like pattern in damaged cells. In the (cy/+) rat, a model for autosomal-dominant polycystic kidney disease in which cysts originate predominantly from the proximal tubule, polycystin-2 immunoreactivity was lost in some distal tubules. In kidneys from (pcy/pcy) mice, a model for autosomal-recessive polycystic kidney disease in which cyst formation primarily affects distal tubules and collecting ducts, a minor portion of cyst-lining cells cease to express polycystin-2, whereas in the remaining cells, polycystin-2 is retained in their basal compartment. Data show that the expression and cellular distribution of polycystin-2 in different kinds of renal injuries depends on the type of damage and on the nephron-specific response to the injury. After ischemia, polycystin-2 may be upregulated by the injured cells to protect themselves. It is unlikely that polycystin-2 plays a role in cyst formation in the (cy/+) rat and in the (pcy/pcy) mouse.

The influence of culture media on embryonic renal collecting duct cell differentiation

During kidney development the embryonic ampullar collecting duct (CD) epithelium changes its function. The capability for nephron induction is lost and the epithelium develops into a heterogeneously composed epithelium consisting of principal and intercalated cells. Part of this development can be mimicked under in vitro conditions, when embryonic collecting duct epithelia are isolated from neonatal rabbit kidneys and kept under perfusion culture. The differentiation pattern is quite different when the embryonic collecting duct epithelia are cultured in standard Iscove's modified Dulbecco's medium as compared to medium supplemented with additional NaCl. Thus, the differentiation behavior of embryonic CD epithelia is unexpectedly sensitive. To obtain more information about how much influence the medium has on cell differentiation, we tested medium 199, basal medium Eagle, Williams' medium E, McCoys 5A medium, and Dulbecco's modified Eagle medium under serum-free conditions. The experiments show that in general, all of the tested media are suitable for culturing embryonic collecting duct epithelia. According to morphological criteria, there is no difference in morphological epithelial cell preservation. The immunohistochemical data reveal two groups of expressed antigens. Constitutively expressed antigens such as cytokeratin 19, P CD 9, Na/K ATPase, and laminin are present in all cells of the epithelia independent of the culture media used. In contrast, a group of antigens detected by mab 703, mab 503, and PNA is found only in individual series. Thus, each culture medium produces epithelia with a very specific cell differentiation pattern.

Transport of monoamine transmitters by the organic cation transporter type 2, OCT2

The recently cloned apical renal transport system for organic cations (OCT2) exists in dopamine-rich tissues such as kidney and some brain areas (Gründemann, D., Babin-Ebell, J., Martel, F., Ording, N., Schmidt, A., and Schömig, E. (1997) J. Biol. Chem. 272, 10408-10413). The study at hand was performed to answer the question of whether OCT2 accepts dopamine and other monoamine transmitters as substrate. 293 cells were stably transfected with the OCT2r cDNA resulting in the 293OCT2r cell line. Expression of OCT2r in 293 cells induces specific transport of tritiated dopamine, noradrenaline, adrenaline, and 5-hydroxytryptamine (5-HT). Initial rates of specific 3H-dopamine, 3H-noradrenaline, 3H-adrenaline, and 3H-5-HT transport were saturable, the Km values being 2.1, 4.4, 1.9, and 3.6 mmol/liter. The corresponding Vmax values were 3.9, 1.0, 0. 59, and 2.5 nmol min-1.mg of protein-1, respectively. 1, 1'-diisopropyl-2,4'-cyanine (disprocynium24), a known inhibitor of OCT2 with a potent eukaliuric diuretic activity, inhibited 3H-dopamine uptake into 293OCT2r cells with an Ki of 5.1 (2.6, 9.9) nmol/liter. In situ hybridization reveals that, within the kidney, the OCT2r mRNA is restricted to the outer medulla and deep portions of the medullary rays indicating selective expression in the S3 segment of the proximal tubule. These findings open the possibility that OCT2r plays a role in renal dopamine handling.

Specific regulation of StAR expression in the rat adrenal zona glomerulosa. An in situ hybridization study

Steroid acute regulatory protein (StAR) plays an essential role in steroidogenesis because it is responsible for the transfer of cholesterol from cellular stores to the inner mitochondrial membrane. We investigated the distribution and regulation of StAR expression in association with aldosterone production in the rat adrenal gland in vivo. Using nonradioactive in situ hybridization, we demonstrate that the outermost five to seven parenchymal cell layers express the StAR gene only weakly and inhomogeneously. The strongest expression is found in the zona fasciculata and zona reticularis. In addition, some cells in the adrenal medulla also stained positively. To differentiate between functionally active glomerulosa and inactive intermediate cells, we compared the expression pattern of StAR with that of aldosterone synthase. The expression of the latter is localized to two or three cell layers only, located immediately below the capsule. However, the cells of the intermedia are capable of expressing both genes prominently, as shown after stimulation with bilateral nephrectomy for 2 days. All zones of the adrenal cortex by then expressed StAR gene to the same extent. This was accompanied by a 50-fold elevated plasma aldosterone concentration. Our data demonstrate that the width of the aldosterone-producing zone can increase within a short period of time by recruiting hormonally inactive cells to steroidogenesis.

The swelling-activated chloride channel ClC-2, the chloride channel ClC-3, and ClC-5, a chloride channel mutated in kidney stone disease, are expressed in distinct subpopulations of renal epithelial cells

The mammalian genome encodes at least nine different members of the ClC family of chloride channels. So far only two of them could be localized on a cellular level in the kidney. We now report on the precise intrarenal localization of the mRNAs coding for the chloride channels ClC-2, ClC-3 and ClC-5. Expression of ClC-2 mRNA, encoding a swelling-activated chloride channel, could be demonstrated in the S3 segment of the proximal tubule. The chloride channel ClC-3 mRNA and ClC-5 mRNA, coding for a chloride channel mutated in kidney stone disease, were both expressed in intercalated cells of the connecting tubule and collecting duct. Whereas ClC-3 mRNA expression was most prominent in the cortex of rat kidneys, ClC-5 mRNA was expressed from the cortex through the upper portion of the inner medulla. A detailed analysis revealed that ClC-3 was expressed by type B intercalated cells, whereas ClC-5 was expressed by type A intercalated cells. These findings have important implications for the pathogenesis of hereditary kidney stone disease caused by mutations in the CLCN5 gene.