In vivo suppression of the renal Na+/Pi cotransporter by antisense oligonucleotides

A 20-mer phosphorothioate oligonucleotide (AS1) was designed to hybridize to the message for the rat kidney sodium phosphate cotransporter NaPi-2 close to the translation initiation site. Single intravenous doses of this oligonucleotide were given to rats maintained on a low phosphorus diet to increase NaPi-2 expression. At 3 days after oligonucleotide infusion, rats receiving 2.5 micromol of AS1 exhibited a reduction in renal NaPi-2 to cyclophilin mRNA ratio by 40% +/- 17%, and rats receiving 7.5 micromol of AS1 exhibited a reduction in NaPi-2 to cyclophilin mRNA ratio by 46% +/- 21%. Reversed-sequence AS1 was without effect. The higher dose of 7.5 micromol of AS1 also reduced the rate of phosphate uptake into renal brush border membrane vesicles and the expression of NaPi-2 protein detected by Western blotting in these vesicles. Reversed sequence AS1 was again without effect on these parameters. These results suggest that systemically infused oligonucleotides can exert antisense effects in the renal proximal tubule.     

Properties of electrogenic Pi transport by a human renal brush border Na+/Pi transporter

Inorganic phosphate (Pi) induced an inward current (IP) in Xenopus oocytes expressing the human renal Na+/Pi cotransporter NaPi-3. At 100mM Na+, Pi-transport was independent of the holding potential and resulted in an apparent Km of 0.08 mM; lowering the Na+ concentration to 50 mM resulted in an increase of the apparent Km to 0.22 mM at -50 mV and to 0.31 mM at -90 mV. In contrast, the apparent Km for Na+ was not significantly influenced by the holding potential. A decrease of the pH from 7.8 to 6.8 resulted in a decrease of IP at 50 mM Na+, but not at 150 mM Na+. Arsenate induced inward currents through NaPi-3 and decreased the apparent Km in measurements of IP. Phosphonoformic acid itself induced no currents, but inhibited Pi-induced currents with an apparent Ki of 3.6 mM. In summary, NaPi-3 displays characteristic Na+/Pi cotransporter properties with relevant interactions with arsenate (transport substrate) and phosphonoformic acid (inhibitor). Monovalent and divalent Pi both appear to be transported by NaPi-3.     

Localization and regulation of renal Na+/myo-inositol cotransporter in diabetic rats

We have examined the effect of diabetes on sodium/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol content in the kidney to test the hypothesis that diabetes-induced changes in renal myo-inositol levels are due to the regulation of SMIT mRNA levels. In streptozotocin-induced diabetic rats, after 3, 7 and 28 days of diabetes, SMIT mRNA levels in the whole kidney were increased three to fivefold, and remained increased by about twofold after six months of diabetes. Insulin treatment of diabetic rats normalized blood glucose levels and prevented the increase in SMIT mRNA levels. Treating diabetic rats with sorbinil, an aldose reductase inhibitor, corrected the abnormal accumulation of sorbitol but had no effect on the diabetes-induced increase in renal SMIT mRNA levels. The regional distribution of SMIT mRNA from normal rats showed a relative abundance in cortex, outer medulla, and inner medulla of 1.0:3.4:7.0. After seven days of diabetes, the levels of SMIT mRNA and myo-inositol content were significantly increased only in the outer medulla. In situ hybridization studies revealed that SMIT mRNA in the outer medulla was predominately localized to the medullary thick ascending limbs of Henle's loop and was not localized to any specific cell in the inner medulla. This distribution pattern was unchanged in diabetic rats. These studies show that diabetes causes an increase in renal SMIT mRNA, which is primarily localized to the outer medulla. Accumulation of myo-inositol by the thick ascending limb of Henle's loop may account for most of the increase caused by diabetes.     

Developmental regulation of Na+ / myo-inositol cotransporter gene expression

myo-Inositol plays a role in many important aspects of cellular regulation including membrane structure, signal transduction and osmoregulation. It is taken up into the cells by the Na+ / myo-inositol cotransporter (SMIT). We investigated developmental changes in the expression of SMIT mRNA and protein in the rat. In the fetal rat brain, SMIT mRNA was abundantly and diffusely expressed throughout the whole brain and the spinal cord. Positive signals were expressed in neuronal and non-neuronal cells in these regions. SMIT is gradually down-regulated nearer birth, but intense signals were still detected in the brain at postnatal day one. In the adult rat brain, very weak hybridization signals were detected throughout whole brain except for the choroid plexus where SMIT mRNA expression remained high. In contrast, the pattern of developmental regulation of SMIT gene expression in the kidney was opposite to that seen in the brain. Signals in the kidney were very weak during embryonic stages, whereas SMIT expression increased significantly after birth. These results suggest that myo-inositol and its transporter play an important role in the CNS developmental stage.     

Expression cloning and characterization of a renal electrogenic Na+/HCO3- cotransporter

Bicarbonate transporters are the principal regulators of pH in animal cells, and play a vital role in acid-base movement in the stomach, pancreas, intestine, kidney, reproductive system and central nervous system. The functional family of HCO3- transporters includes Cl- -HCO3- exchangers, three Na+/HCO3- cotransporters, a K+/HCO3- cotransporter, and a Na+-driven Cl- -HCO3- exchanger. Molecular information is sparse on HCO3- transporters, apart from Cl- -HCO3- exchangers ('anion exchangers'), whose complementary DNAs were cloned several years ago. Attempts to clone other HCO3- transporters, based on binding of inhibitors, protein purification or homology with anion exchangers, have so far been unsuccessful. Here we monitor the intracellular pH and membrane voltage in Xenopus oocytes to follow the expression of the most electrogenic transporter known: the renal 1:3 electrogenic Na+/HCO3- cotransporter from the salamander Ambystoma tigrinum. We now report the successful cloning and characterization of a cDNA encoding a cation-coupled HCO3- transporter. The encoded protein is 1,035 amino acids long with several potential membrane-spanning domains. We show that when it is expressed in Xenopus oocytes, this protein is electrogenic, Na+ and HCO3- dependent, and blocked by the anion-transport inhibitor DIDS, and conclude that it is the renal electrogenic sodium bicarbonate cotransporter (NBC).     

Parathyroid hormone and renal excretion of phosphate and calcium in normal starlings

The renal handling of calcium, phosphate, sodium, and potassium was studied in normal and parathyroid extract (PTE)-injected starlings, Sturnus vulgaris. The birds were anesthetized with Equi-Thesin and infused intravenously with 2.5% mannitol containing [14C]inulin. Normal starlings actively reabsorb all four of these substances. After intravenous administration of 50 IU PTE/100 g body wt, the relative phosphate clearance (CPO4/CIn) as well as tubular transfer of phosphate (TPO4) increased significantly. Phosphate secretion occurred and usually persisted longer than 2 h. The relative calcium clearance also rose after PTE, but the TCa did not shift. This probably indicates that the tubular transport maximum (Tm) for calcium had been exceeded. The relative clearances of sodium and potassium also increased after PTE; however, only the rise in CNa/CIn was significantly different from the controls. The glomerular filtration rate (CIn) also increased significantly after PTE, but this effect was transient and cannot explain the longer lasting effects of PTE on excretion of phosphate, calcium, or sodium.     

IGF-I and vanadate stimulate Na/Pi-cotransport in OK cells by increasing type II Na/Pi-cotransporter protein stability

The nitric oxide (NO) signaling system, consisting of NO synthases, soluble guanylyl cyclase, and cGMP, plays a prominent role in salt handling and regulation of blood pressure. Soluble guanylyl cyclases are heme-containing heterodimers (alpha/beta). The alpha1/beta1 isoform has greater NO sensitivity than the alpha1/beta2. It has recently been shown that expression of the beta subunits is altered in the kidney of the Dahl salt-sensitive rat, ie, the beta1 subunit is decreased and the beta2 subunit increased. However, whether soluble guanylyl cyclase is linked to salt sensitivity is not known. In the present study, we investigated linkage of guanylyl cyclase genes to blood pressure. Alpha1 and beta1 gene loci for soluble guanylyl cyclase were mapped to rat chromosome 2, and the beta2 gene locus was mapped to rat chromosome 5 using fluorescent in situ metaphase hybridization. By use of a rat radiation hybrid panel, the gene loci were then further mapped with respect to known quantitative trait locus markers of salt-sensitive hypertension in the Dahl rat on chromosomes 2 and 5. Genes for alpha1 and beta1 were closely linked by two-point analysis to Na+,K+-ATPase alpha1 isoform (LOD of 15.1 and 14.0, respectively) and calmodulin-dependent protein kinase II-delta loci (LOD of 14.3 and 12.9, respectively), which have been previously shown to flank a quantitative trait locus for blood pressure in the Dahl rat. The alpha1 and beta1 genes were closely linked (LOD of 11.3; theta, 0.4). The beta2 gene locus was closely linked to the endothelin-2 (ET-2) locus (LOD of 13.0), which has been shown to cosegregate with blood pressure. We conclude that soluble guanylyl cyclase subunit loci, ie, alpha1, beta1, and beta2, are good candidates for genes controlling salt-sensitive hypertension in the Dahl rat.     

Sequence and functional characterization of a renal sodium/dicarboxylate cotransporter

The cDNA coding for a rabbit renal Na+/dicarboxylate cotransporter, designated NaDC-1, was isolated by functional expression in Xenopus oocytes. NaDC-1 cDNA is approximately 2.3 kilobases in length and codes for a protein of 593 amino acids. NaDC-1 protein contains eight putative transmembrane domains, and the sequence and secondary structure are related to the renal Na+/sulfate transporter, NaSi-1. Northern analysis shows that the NaDC-1 message is abundant in kidney and small intestine, and related transporters may be found in liver, lung, and adrenal. The transport of succinate by NaDC-1 was sodium-dependent, sensitive to inhibition by lithium, and inhibited by a range of di- and tricarboxylic acids. This transporter also carries citrate, but it does not transport lactate. In kinetic experiments, the Km for succinate was around 0.4 mM and the Vmax was 15 nmol/oocyte/h, while the Hill coefficient of Na+ activation of succinate transport was 1.9. The transport of succinate by NaDC-1 was insensitive to changes in pH, whereas the transport of citrate increased with decreasing pH, in parallel with the concentration of divalent citrate in the medium. The results of the functional characterization indicate that NaDC-1 likely corresponds to the renal brush-border Na+/dicarboxylate cotransporter.     

Role of microtubules in the adaptive response to low phosphate of Na/Pi cotransport in opossum kidney cells

The role of microtubules and actin microfilaments in adaptive changes of the apical Na-dependent transport of phosphate (Pi) was investigated in opossum kidney (OK) cells. Up-regulation of Na/Pi cotransport was achieved by incubating OK cells in a medium containing 0.1 mM Pi; down-regulation of Na/Pi cotransport was provoked by refeeding adapted cells with 2 mM Pi. Up-regulation of Na/Pi cotransport was found to be inhibited by approximately 50% after a pretreatment of the cells with the microtubule disrupting agents nocodozole and colchicine; indirect immunofluorescence indicated complete depolymerization of the microtubular network. No inhibition of the adaptive response was observed after treatment of the cells with cytochalasin B to depolymerize actin microfilaments. In adapted cells, depolymerization of microtubules by nocodozole led to a reversibility of Na/Pi cotransport similar to that observed after refeeding adapted cells with 2 mM Pi. No effects of the microtubule disrupting drugs were observed on Na/L-glutamic acid transport. Depolymerization of microtubules did not prevent parathyroid-hormone-mediated inhibition of Na/Pi cotransport. It is concluded that microtubules are (at least in part) involved in the correct insertion of newly synthesized apical Na/Pi cotransport systems and that microtubules are not involved in the internalization of Na/Pi cotransport systems.     

Osmotic regulation of the Na+/myo-inositol cotransporter and postinduction normalization

BACKGROUND: The mechanisms underlying the vascular effects of propofol are not fully understood. Vasopressin, a potent vasoactive peptide, stimulates the arachidonate cascade and the synthesis of prostacyclin (PGI2; the main metabolite of the cascade in vascular smooth muscle cells). Arachidonic acid (AA) release by phospholipases is the rate-limiting step in the cascade. We investigated the mechanisms underlying vasopressin-induced AA release and the effect of propofol on PGI2 synthesis in a rat aortic smooth muscle cell line: A10 cells. METHODS: In cultured A10 cells pretreated with propofol, the stimulation by vasopressin of AA release and PGI2 synthesis was evaluated by measuring [3H]AA and 6-keto PGF1alpha, respectively, in the culture medium. The effects of propofol on vasopressin-induced activation of phosphoinositide-hydrolyzing phospholipase C and phosphatidylcholine-hydrolyzing phospholipase D were evaluated by measuring inositol phosphate formation and choline formation, respectively. RESULTS: A phospholipase C inhibitor and a phosphatidic acid phosphohydrolase inhibitor both attenuated vasopressin-induced AA release and PGI2 synthesis, as did a phospholipase A2 inhibitor. Propofol inhibited vasopressin-induced activation of phosphoinositide-hydrolyzing phospholipase C and phosphatidylcholine-hydrolyzing phospholipase D, but this effect of propofol was significant only at supraclinical concentration (0.1 mM). Propofol reduced vasopressin-induced PGI2 synthesis. The inhibitory effect was observed at concentrations (10 microM-0.1 mM) higher than those used clinically. CONCLUSIONS: Propofol suppresses the arachidonate cascade caused by vasopressin at least partly by inhibiting phosphoinositide-hydrolyzing phospholipase C and phosphatidylcholine-hydrolyzing phospholipase D, resulting in the inhibition of PGI2 synthesis. Propofol-mediated inhibition of vasopressin-stimulated synthesis of PGI2 may reduce the vasorelaxation by propofol.     

E2P phosphoforms of Na,K-ATPase. II. Interaction of substrate and cation-binding sites in Pi phosphorylation of Na,K-ATPase

In this investigation the effects of alkali cations on the transient kinetics of Na,K-ATPase phosphoenzyme formation from either ATP (E2P) or Pi (E'2P) were characterized by chemical quench methods as well as by stopped-flow RH421 fluorescence experiments. By combining the two methods it was possible to characterize the kinetics of Na, K-ATPase from two sources, shark rectal glands and pig kidney. The rate of the spontaneous dephosphorylation of E2P and E'2P was identical with a rate constant of about 1.1 s-1 at 20 degreesC. However, whereas dephosphorylation of E2P formed from ATP was strongly stimulated by K+, dephosphorylation of E'2P formed from Pi in the absence of alkali cations was K+-insensitive, although in pig renal enzyme K+ binding to E'2P could be demonstrated with RH421 fluorescence. It appears, therefore, that in pig kidney enzyme the rapid binding of K+ to E'2P was followed by a slow transition to a nonfluorescent form. For shark enzyme the K+-induced decrease of RH421 fluorescence of Pi phosphorylated enzyme was due to K+ binding to the dephosphoenzyme (E1), thus shifting the equilibrium away from E'2P. When Pi phosphorylation was performed with enzyme equilibrated with K+ or its congeners Tl+, Rb+, and Cs+ but not with Na+ or Li+, both the phosphorylation and the dephosphorylation rates were considerably increased. This indicates that binding of cations modifies the substrate site in a cation-specific way, suggesting an allosteric interaction between the conformation of the cation-binding sites and the phosphorylation site of the enzyme.     

Membrane traffic and control of proximal tubular sodium phosphate (Na/Pi)-cotransport

Phosphate (P(i)) is freely filtered at the glomerular capillaries and largely reabsorbed in the proximal tubule by a Na-dependent, secondary active transport mechanism. Two different brush border membrane Na/P(i)-cotransporters have recently been "cloned" (type I and type II). Only the type II transporter undergoes physiological regulation (e.g., diet, acid/base, parathyroid hormone); it is also involved in pathophysiological alterations of renal Pi-handling (e.g., X-linked hypophosphatemia). In recent experiments on rats and on tissue culture cells (Opossum kidney cells, OK cells) id was documented that manoeuvres leading to increased uptake involve membrane insertion (fast changes) and new synthesis of type II transporters (slow changes), whereas decreased Na/Pi-cotransport activity is associated with their specific membrane retrieval (fast changes) and lysosomal degradation (slow changes).     

Metabolic pathway of the degradation of macromolecules by lysosomal enzymes

The lysosomes of most cells function principally in intracellular digestion. They contain several dozens of enzymes, mostly acid hydrolyases. Vacuolar-ATPase on lysosomal membrane establishes low-pH environment required for efficient expression of hydrolyase activity. Of several dozen disorders of human and various animals known to be due to lysosomal dysfunctions, most of the lysosomal storage diseases are of genetic origin. Metabolic pathways of intracellular macromolecules within lysosomes have been established from studies on the molecular and biochemical defects on lysosomal enzymes. In the lysosomal storage diseases most of all, the metabolic pathways of breakdown of lipids, glycosaminoglycans or oligosaccharides are severely affected. The degradation pathways of glycoproteins, proteoglycans, and glycolipids by lysosomal enzymes are described in this brief review.     

Presteady-state currents of the rabbit Na+/glucose cotransporter (SGLT1)

The rabbit Na+/glucose cotransporter (SGLT1) exhibits a presteady-state current after step changes in membrane voltage in the absence of sugar. These currents reflect voltage-dependent processes involved in cotransport, and provide insight on the partial reactions of the transport cycle. SGLT1 presteady-state currents were studied as a function of external Na+, membrane voltage Vm, phlorizin and temperature. Step changes in membrane voltage-from the holding Vh to test values, elicited transient currents that rose rapidly to a peak (at 3-4 msec), before decaying to the steady state, with time constants tau approximately 4-20 msec, and were blocked by phlorizin (Ki approximately 30 microm). The total charge Q was equal for the application of the voltage pulse and the subsequent removal, and was a function of Vm. The Q-V curves obeyed the Boltzmann relation: the maximal charge Qmax was 4-120 nC; V0.5, the voltage for 50% Qmax was -5 to +30 mV; and z, the apparent valence of the moveable charge, was 1. Qmax and z were independent of Vh (between 0 and -100 mV) and temperature (20-30 degrees C), while increasing temperature shifted V0.5 towards more negative values. Decreasing [Na+]o decreased Qmax, and shifted V0.5 to more negative voltages 9by -100 mV per 10-fold decrease in [Na+]o). The time constant tau was voltage dependent: the tau-V relations were bell-shaped, with maximal taumax 8-20 msec. Decreasing [Na+]o decreased taumax, and shifted the tau-V curves towards more negative voltages. Increasing temperature also shifted the tau-V curves, but did not affect taumax. The maximum temperature coefficient Q10 for tau was 3-4, and corresponds to an activation energy of 25 kcal/mole. Simulations of a 6-state ordered kinetic model for rabbit Na+/glucose cotransport indicate that charge-movements are due to Na+-binding/dissociation and a conformational change of the empty transporter. The model predicts that (i) transient currents rise to a peak before decay to steady-state; (ii) the tau-V relations are bell-shaped, and shift towards more negative voltages as [Na+]o is reduced; (iii) taumax is decreased with decreasing [Na+]o; and (iv) the Q-V relations are shifted towards negative voltages as [Na+]o is reduced. In general, the kinetic properties of the presteady-state currents are qualitatively predicted by the model.     

Parathyroid hormone-related peptide and the parathyroid hormone/parathyroid hormone-related peptide receptor in skeletal development

Desquamative gingivitis is a fairly common complaint. Typically seen in females who are middle-aged or older, it is predominantly a manifestation of a range of vesiculobullous disorders. The main complaint is of persistent soreness of the gingiva. Most cases are related to lichen planus or pemphigoid, but it is also important to exclude pemphigus, dermatitis herpetiformis, linear IgA disease, chronic ulcerative stomatitis, and other conditions. Biopsy is invariably required to confirm the diagnosis after a full history, general, and oral examination. Apart from improving the oral hygiene, immunosuppressive therapy is typically required to control the condition.     

Accumulation of pyrraline-modified albumin in phagocytes due to reduced degradation by lysosomal enzymes

Previous studies suggested that the interaction between proteins modified by advanced glycation end products (AGEs) and cells, such as macrophages, may be involved in diabetic angiopathy. Pyrraline is one of the AGEs and known to be elevated in plasma of diabetic rats and humans, and is present in vascular lesions of diabetic and elderly subjects. We examined whether modification of albumin by pyrraline influences its degradation by macrophage-like cell line, P388D1 cells. Degradation of pyrraline-modified albumin by these cells was diminished, causing accumulation of the albumin in these cells. The susceptibility of pyrraline-modified albumin to lysosomal proteolytic enzymes was reduced by approximately 40% in vitro, while lysosomal activity in the cells per se was not affected. This phenomenon was also observed when human monocytes were used instead of P388D1 cells. Our results suggest that accumulation of pyrraline-modified albumin in P388D1 cells is due to the reduced susceptibility of the protein to lysosomal enzymatic degradation. Such alterations in the interaction between AGEs-modified protein and phagocytes may contribute to angiopathy in elderly subjects and patients with diabetes.     

Neutralization of conservative charged transmembrane residues in the Na+/glucose cotransporter SGLT1

Our goal was to identify pairs of charged residues in the membrane domains of the Na+/glucose cotransporter (SGLT1) that form salt bridges, to obtain information about packing of the transmembrane helices. The strategy was to neutralize Glu225, Asp273, Asp294, and Lys321 in helices 6-8, express the mutants in oocytes, measure [14C]-alphaMDG uptake, and then attempt to find second-site mutations of opposite charge that restored function. alphaMDG uptake by E225A was identical to that by SGLT1, whereas transport was reduced by over 90% for D273A, D294A, and K321A and was not restored in the double mutants D273A/K321A or D294A/K321A. This suggested that K321 did not form salt bridges with D273 or D294 and that E225 was not involved in salt-bridging. Neutralization of K321 dramatically changed the Na+ uniport and Na+/glucose cotransport kinetics. The maximum rate of uniport in K321A increased 3-5-fold with a decrease in the apparent affinity for Na+ (70 vs 3 mM) and no change in apparent H+ affinity (0.5 microM). The change in Na+ affinity caused a +50 mV shift in the charge/voltage (Q/V) and relaxation time constant (tau)/voltage curves in the presteady-state kinetics. The presteady-state kinetics in H+ remained unchanged. The lower Na+ affinity resulted also in a 200-fold increase in the apparent K0.5 for alphaMDG and phlorizin. Replacements of K321 with alanine, valine, glutamine, arginine, or glutamic acid residues changed the steady-state kinetics in a similar way. Therefore, we suggest that K321 determines, directly or indirectly, (i) the rate and selectivity of SGLT1 uniport activity and (ii) the apparent affinities of SGLT1 for Na+, and indirectly sugar in the cotransport mode.     

Interrelationship between the Na+/glucose cotransporter and CFTR in Caco-2 cells: relevance to cystic fibrosis

Both the Na+-dependent glucose cotransporter (SGLT1) and the cystic fibrosis transmembrane conductance regulator (CFTR) modulate Na+ and fluid movement, although in opposite directions. Yet few studies have investigated a possible interrelationship between these two transporters. By using the Caco-2 human colon carcinoma cell line, we confirmed that the activities of these transporters increased with spontaneous differentiation to the enterocytic phenotype. We showed that SGLT1 was positively regulated by Cl- and that optimal activity of CFTR was dependent on the presence of glucose. We also demonstrated that inhibition of CFTR by glibenclamide or diphenylamine-2-carboxylate did not modify the activity of SGLT1 and inhibition of SGLT1 by phlorizin did not modify the activity of CFTR, although it resulted in inhibition of glycoconjugate synthesis. These results point to positive substrate-cross regulation of SGLT1 and CFTR and suggest that NaCl and glucose are important for not only Na+ absorption and fluid movement, but also for cAMP-dependent Cl- efflux, and glycoconjugate synthesis, functions that are known to be anomalous in cystic fibrosis.     

Topology and function of the Na+/proline transporter of Escherichia coli, a member of the Na+/solute cotransporter family

Penidiamide, a new tripetide containing dehydrotryptamine, glycine and anthranilic acid linked together by two amide bonds, and oxindole were isolated from submerged cultures of Penicillium sp. 62-92. Both compounds preferentially inhibited human synovial phospholipase A2, penidiamide with an IC50 of 30 microM and oxindole of 380 microM. With the exception of U 937 cells (leukemia, human), no cytotoxic activities were detected against HL-60- (leukemia, human), HeLa S3- (epitheloid carcinoma, human), BHK 21- (kidney fibroblasts, hamster), and L1210-cells (leukemia, mouse). No antimicrobial activity was detected for oxindole, and only weak antibacterial activity for penidiamide. The structure of penidiamide was elucidated by spectroscopic methods.