The chromosome locations of the rabbit (Oryctolagus cuniculus) Na+-phosphate cotransporter genes ... more The chromosome locations of the rabbit (Oryctolagus cuniculus) Na+-phosphate cotransporter genes NPT1 and NPT2 were determined by fluorescence in situ hybridization. Our results localize NPT1 to rabbit chromosome 12p11 and NPT2 to rabbit chromosome 3p11. The corresponding genes in the human map to chromosome bands 6p22 and 5q35, respectively. These assignments agree with the previously reported homology between rabbit chromosome 12 and human chromosome 6 and provide the basis for the establishment of a conserved syntenic group between rabbit chromosome 3 and human chromosome 5.
Effect of salicylic acid and its metabolites on inorganic sulfate in rats. J Pharmnacol Exp Ther ... more Effect of salicylic acid and its metabolites on inorganic sulfate in rats. J Pharmnacol Exp Ther 244: 945-949.
Dietary restriction of phosphate is a well-known stimulator (acting indirectly via vitamin D(3)) ... more Dietary restriction of phosphate is a well-known stimulator (acting indirectly via vitamin D(3)) of small intestinal apical Na-P(i) cotransport. In the present study, we document by Western blots and immunohistochemistry that, in mice, a low-P(i) diet given for several days leads (in parallel to a stimulation of Na-P(i) cotransport) to an increase of the abundance of the type IIb Na-P(i) cotransporter in the brush-border membrane of mouse enterocytes. Similar results were also obtained by an injection of cholecalciferol. The abundance of the type IIb transcript was investigated by Northern blots. These results indicated that the amount of the type IIb transcript was not changed by either low-P(i) diet or cholecalciferol. It is concluded that stimulation of intestinal Na-P(i) cotransport by low-P(i) diet and vitamin D(3) can be explained by an increased amount of type IIb Na-P(i) cotransporters in the brush-border membrane and that augmentation of type IIb Na-P(i) cotransporters is not related to an increased rate of transcription of the type IIb gene.
Type II Na-P(i) cotransporters (type IIa and type IIb) represent apically located Na-P(i) cotrans... more Type II Na-P(i) cotransporters (type IIa and type IIb) represent apically located Na-P(i) cotransporters in epithelia of proximal tubules (type IIa) and small intestine (type IIb). Here we provide evidence that the type IIb (but not the type IIa) Na-P(i) cotransporter is also expressed in the lung. With the use of immunohistochemistry, location of the type IIb protein was found exclusively in the apical membrane of type II cells of the alveolar epithelium. Such a location of the type IIb cotransporter suggests an involvement in the reuptake of phosphate necessary for the synthesis of surfactant. A possible regulation of the abundance of the type IIb cotransporter in the lung was studied after adaptation of mice to a low-P(i) diet. After a chronic adaptation to a low-P(i) diet, no changes in the type IIb protein and the type IIb transcript were observed. These results exclude dietary intake of phosphate as a regulatory factor of the type IIb Na-P(i) cotransporter in alveolar type II cells.
The uptake of inorganic phosphate (Pi) was analyzed in monolayers and in apical membrane vesicles... more The uptake of inorganic phosphate (Pi) was analyzed in monolayers and in apical membrane vesicles (AMV) of the established intestinal cell line CaCo-2. AMV, prepared by a MgCl2 precipitation technique, were enriched approximately 10-fold in alkaline phosphatase activity. Pi uptake into intact cells as well as into AMV was specifically dependent on the presence of sodium. In the presence of high sodium concentrations, the apparent Km for Pi was 214 +/- 17 mumol/l in monolayers and 300 +/- 19.7 mumol/l in AMV. Increasing the sodium concentration increased the apparent affinity of the transport system for Pi but hardly affected the maximal velocity (Vmax). At 0.1 mmol/l Pi and pH 7.4, the apparent Km for sodium was approximately 70 mmol/l in intact cells as well as in AMV. The results obtained in both systems suggested the involvement of two sodium ions and one phosphate ion in the transport process. Advancing confluence--independently of the age of the monolayers--reduced sodium-dependent uptake of Pi significantly by a decrease in Vmax, whereas the apparent Km for Pi remained unchanged. It is concluded that the apical membrane of CaCo-2 cells contains a sodium-dependent transport system for Pi.
The initial linear rate of phosphate uptake was analyzed in rat renal brush border membrane vesic... more The initial linear rate of phosphate uptake was analyzed in rat renal brush border membrane vesicles. An increase in medium pH from 6.0 to 8.0 increased the sodium gradient-dependent phosphate uptake about 20-fold. Sodium-independent phosphate uptake was not altered in this pH range. At pH 7.4 an intravesicular acid pH stimulated the initial linear uptake rate (20-25%). The apparent Km for sodium increased from about 100 to 200 mM when pH was decreased from 7.4 to 6.4. The Hill coefficient for sodium interaction was close to 2 and was unaffected by pH. Increasing external sodium reduced the apparent Km of the transport system for phosphate independent of pH. Variations of phosphate concentration had no influence on the apparent Km for sodium. At high sodium concentrations, small effects (20-30%) of pH on the apparent Vmax of the transport system were found; measured at saturating sodium concentrations, the apparent Km values calculated on the basis of total phosphate were increased (50-60%) when pH was decreased from 7.4 to 6.4. The data indicate that the major effect of pH is to modify the interaction of the transport system with sodium. At nonsaturating sodium concentrations, this resulted indirectly in a reduction in the affinity for phosphate related to a different occupancy of the sodium binding site. The differences of transport rate at low phosphate and high sodium concentrations could be explained by preferential transport of divalent phosphate as well as by pH effects on other carrier properties.
Heavy metal intoxication with Hg2+, Pb2+ and Cd2+ commonly leads to phosphaturia. In this study, ... more Heavy metal intoxication with Hg2+, Pb2+ and Cd2+ commonly leads to phosphaturia. In this study, we examined the effects of these heavy metals on Pi-induced currents (Ip) through NaPi-3, the human renal cotransporter for Na+ and Pi. Hg2+ inhibited Ip in a dose- and time-dependent fashion. Hg2+ decreased the extrapolated maximal current but did not alter the apparent affinity for Pi. This inhibition was also observed with the membrane-permeable oxidizing agent 2,2'-dithio-bis(5-nitropyridine) (DTNP) but not with the membrane-impermeable 5,5'-dithiobis(2-nitrobenzoic acid). Hg(2+)- and DTNP-mediated inhibition of Ip was reversible only in the presence of the reducing agent 2,3-dihydroxybutane-1,4-dithiol. Cd2+ and Pb2+ also inhibited Ip. However, while CD2+ did not significantly alter the apparent affinity for Pi, the apparent concentration needed for half-maximal current (Km) for Pi was increased by Pb2+. In contrast to Hg2+, the inhibition of Ip by Cd2+ and Pb2+ was rapidly reversible upon washout. In the presence of the Na(+)-K(+)-adenosinetriphosphatase inhibitor ouabain, Ip was not reduced, and the effects of the heavy metals were maintained. In summary, the three heavy metals Hg2+, Cd2+, and Pb2+ inhibit Ip through the Na+/Pi cotransporter NaPi-3 by distinct mechanisms. Heavy metal-mediated inhibition of NaPi-3 may be responsible for the phosphaturia observed after intoxication with these compounds.
One way to obtain a deeper understanding of the complex function of the small intestinal and rena... more One way to obtain a deeper understanding of the complex function of the small intestinal and renal proximal tubular epithelium is to dissect it into single components and then, having defined the components under wellcontrolled conditions, try to describe the behaviour of the whole system on the basis of the properties of the single components.
Over the last 10-15 years the use of cellular and molecular biology permitted the analysis of kid... more Over the last 10-15 years the use of cellular and molecular biology permitted the analysis of kidney function at a cellular/molecular level. These advances allow a more detailed understanding of physiological/pathophysiological mechanisms. By taking examples from our studies on the renal handling of inorganic phosphate this gain in knowledge' is illustrated.
Renal phosphate (Pi) absorption is mediated via the type II sodium/Pi cotransporter (NaPi-2) in t... more Renal phosphate (Pi) absorption is mediated via the type II sodium/Pi cotransporter (NaPi-2) in the brush border membrane (BBM) of proximal tubules. Simultaneous detection of NaPi-2 mRNA by in situ hybridization and of NaPi-2 immunoreactivity by immunohistochemistry was performed to investigate the distribution of the cotransporter in healthy control rats and during progression of autosomal dominant polycystic kidney disease (ADPKD). The purpose of the study was to disclose a relation between proximal tubular cell differentiation and NaPi-2 expression. In controls, NaPi-2 expression was present in the entire proximal tubule. In the Han:SPRD (cy/ϩ) model for ADPKD, the proximal nephron is primarily affected by the cystic changes. Epithelial proliferation and impaired epithelial-matrix interaction result in a loss of cell differentiation that eventually leads to cystic enlargement of
Parathyroid hormone (PTH)-induced inhibition of renal proximal tubular Na/P i cotransport involve... more Parathyroid hormone (PTH)-induced inhibition of renal proximal tubular Na/P i cotransport involves two consecutive steps: endocytosis followed by lysosomal degradation of the type IIa Na/P i cotransporter. Tyrosine-, dileucine-, and diacidic-based motifs are suggested to be involved in endocytosis and/or lysosomal targeting of different plasma membrane proteins. The rat type IIa cotransporter (NaPi2) contains two cytoplasmic tyrosine residues (Y) within sequences highly homologous to tyrosinebased motifs (GY 402 FAM and Y 509 RWF), three cytoplasmic dileucine (LL 101 , LL 374, and LI 591 ) and two cytoplasmic diacidic motifs (EE 81 and EE 616 ). We studied the role of these motifs on the PTH-induced retrieval and lysosomal degradation of the NaPi2 cotransporter. To follow its trafficking in vivo, the NaPi2 protein was fused to the carboxyl-terminal end of the enhanced green fluorescence protein. This fusion did not impair the apical targeting or the PTH-induced endocytosis of the wild-type cotransporter when transfected in opossum kidney cells. Single and multiple Y and LL mutants retained the apical targeting and the PTH-induced degradation. Mutations of the diacidic motifs were also without effect. These data suggest that the above three motifs are not required for the PTH-induced internalization and/or degradation of the cotransporter.
Parathyroid hormone (PTH) leads to the inhibition of Na-Pi cotransport activity and to the downre... more Parathyroid hormone (PTH) leads to the inhibition of Na-Pi cotransport activity and to the downregulation of the number of type II Na-Pi cotransporters in proximal tubules, as well as in opossum kidney (OK) cells. PTH is known also to lead to an activation of adenylate cyclase and phospholipase C in proximal tubular preparations, as well as in OK cells. In the present study, we investigated the involvement of these two regulatory pathways in OK cells in the PTH-dependent downregulation of the number of type II Na-Pi cotransporters. We have addressed this issue by using pharmacological activators of protein kinase A (PKA) and protein kinase C (PKC), i.e., 8-bromo-cAMP (8-BrcAMP) and beta-12-O-tetradecanoylphorbol 13-acetate (beta-TPA), respectively, as well as by the use of synthetic peptide fragments of PTH that activate adenylate cyclase and/or phospholipase C, i.e., PTH-(1-34) and PTH-(3-34), respectively. Our results show that PTH signal transduction via cAMP-dependent, as well as cAMP-independent, pathways leads to a membrane retrieval and degradation of type II Na-Pi cotransporters and, thereby, to the inhibition of Na-Pi cotransport activity. Thereby, the cAMP-independent regulatory pathway leads only to partial effects (approximately 50%).
Renal proximal tubular sulfate reabsorption is mediated by brush border membrane Na+/sulfate-cotr... more Renal proximal tubular sulfate reabsorption is mediated by brush border membrane Na+/sulfate-cotransport and basolateral Na+-independent sulfate transport. Injection of rat kidney cortex mRNA into Xenopus laevis oocytes induced Na+-dependent as well as Na+-independent sulfate transport. The inhibition pattern of Na+dependent uptake coincided with that known for the brush border membrane; the inhibition pattern of Na+independent uptake suggested that this activity could be related to the basolateral cell surface. By Northern blot hybridization of size-fractionated mRNA, we provide evidence that the Na+-dependent uptake is induced by an mRNA species related to a recently cloned cDNA encoding rat renal cortex Na+/S04 cotransport (Nasi-1; Markovich, D., Forgo, J., Stange, G., Biber, J., and Murer, H. (1993) Proc. Nutl. ACME. ; the Na'hdependent sulfate transport activity seems to be related to an mRNA species encoding a rat liver Na+independent sulfate transporter (Bissig, M., Hagenbuch, B., Stieger, B., Koller, T., and Meier, P. J. (1994) J. Biol. Chem. 269,3017-3021). Hybrid depletion experiments using antisense oligonucleotides provided further evidence for the association of the expressed transport activities to Nasi-1 and sat-1, respectively.
A type II Na/Pi-cotransporter located in the brush border membrane is the rate limiting and physi... more A type II Na/Pi-cotransporter located in the brush border membrane is the rate limiting and physiologically regulated step in proximal tubular phosphate (Pi) reabsorption. In states of altered Pi-reabsorption [for example, in response to parathyroid hormone (PTH) and to altered dietary intake of Pi or as a consequence of genetic abnormalities], brush border expression of the type II Na/Pi-cotransporter is accordingly modified. PTH initiates a regulatory cascade leading to membrane retrieval, followed by lysosomal degradation of this transporter; recovery from inhibition requires its de novo synthesis. Pi-deprivation leads to an increased brush border expression of transporters that does not appear to require de novo synthesis in the short term. Pi-overload leads to membrane retrieval and degradation of transporters. Finally, in animals with genetically altered Pi-handling (Hyp; Gy) the brush border membrane expression of the type II Na/Pi-cotransporter is also reduced, suggesting that a genetically altered protein (such as PEX in Hyp) controls the expression of this transporter.
The chromosome locations of the rabbit (Oryctolagus cuniculus) Na+-phosphate cotransporter genes ... more The chromosome locations of the rabbit (Oryctolagus cuniculus) Na+-phosphate cotransporter genes NPT1 and NPT2 were determined by fluorescence in situ hybridization. Our results localize NPT1 to rabbit chromosome 12p11 and NPT2 to rabbit chromosome 3p11. The corresponding genes in the human map to chromosome bands 6p22 and 5q35, respectively. These assignments agree with the previously reported homology between rabbit chromosome 12 and human chromosome 6 and provide the basis for the establishment of a conserved syntenic group between rabbit chromosome 3 and human chromosome 5.
Effect of salicylic acid and its metabolites on inorganic sulfate in rats. J Pharmnacol Exp Ther ... more Effect of salicylic acid and its metabolites on inorganic sulfate in rats. J Pharmnacol Exp Ther 244: 945-949.
Dietary restriction of phosphate is a well-known stimulator (acting indirectly via vitamin D(3)) ... more Dietary restriction of phosphate is a well-known stimulator (acting indirectly via vitamin D(3)) of small intestinal apical Na-P(i) cotransport. In the present study, we document by Western blots and immunohistochemistry that, in mice, a low-P(i) diet given for several days leads (in parallel to a stimulation of Na-P(i) cotransport) to an increase of the abundance of the type IIb Na-P(i) cotransporter in the brush-border membrane of mouse enterocytes. Similar results were also obtained by an injection of cholecalciferol. The abundance of the type IIb transcript was investigated by Northern blots. These results indicated that the amount of the type IIb transcript was not changed by either low-P(i) diet or cholecalciferol. It is concluded that stimulation of intestinal Na-P(i) cotransport by low-P(i) diet and vitamin D(3) can be explained by an increased amount of type IIb Na-P(i) cotransporters in the brush-border membrane and that augmentation of type IIb Na-P(i) cotransporters is not related to an increased rate of transcription of the type IIb gene.
Type II Na-P(i) cotransporters (type IIa and type IIb) represent apically located Na-P(i) cotrans... more Type II Na-P(i) cotransporters (type IIa and type IIb) represent apically located Na-P(i) cotransporters in epithelia of proximal tubules (type IIa) and small intestine (type IIb). Here we provide evidence that the type IIb (but not the type IIa) Na-P(i) cotransporter is also expressed in the lung. With the use of immunohistochemistry, location of the type IIb protein was found exclusively in the apical membrane of type II cells of the alveolar epithelium. Such a location of the type IIb cotransporter suggests an involvement in the reuptake of phosphate necessary for the synthesis of surfactant. A possible regulation of the abundance of the type IIb cotransporter in the lung was studied after adaptation of mice to a low-P(i) diet. After a chronic adaptation to a low-P(i) diet, no changes in the type IIb protein and the type IIb transcript were observed. These results exclude dietary intake of phosphate as a regulatory factor of the type IIb Na-P(i) cotransporter in alveolar type II cells.
The uptake of inorganic phosphate (Pi) was analyzed in monolayers and in apical membrane vesicles... more The uptake of inorganic phosphate (Pi) was analyzed in monolayers and in apical membrane vesicles (AMV) of the established intestinal cell line CaCo-2. AMV, prepared by a MgCl2 precipitation technique, were enriched approximately 10-fold in alkaline phosphatase activity. Pi uptake into intact cells as well as into AMV was specifically dependent on the presence of sodium. In the presence of high sodium concentrations, the apparent Km for Pi was 214 +/- 17 mumol/l in monolayers and 300 +/- 19.7 mumol/l in AMV. Increasing the sodium concentration increased the apparent affinity of the transport system for Pi but hardly affected the maximal velocity (Vmax). At 0.1 mmol/l Pi and pH 7.4, the apparent Km for sodium was approximately 70 mmol/l in intact cells as well as in AMV. The results obtained in both systems suggested the involvement of two sodium ions and one phosphate ion in the transport process. Advancing confluence--independently of the age of the monolayers--reduced sodium-dependent uptake of Pi significantly by a decrease in Vmax, whereas the apparent Km for Pi remained unchanged. It is concluded that the apical membrane of CaCo-2 cells contains a sodium-dependent transport system for Pi.
The initial linear rate of phosphate uptake was analyzed in rat renal brush border membrane vesic... more The initial linear rate of phosphate uptake was analyzed in rat renal brush border membrane vesicles. An increase in medium pH from 6.0 to 8.0 increased the sodium gradient-dependent phosphate uptake about 20-fold. Sodium-independent phosphate uptake was not altered in this pH range. At pH 7.4 an intravesicular acid pH stimulated the initial linear uptake rate (20-25%). The apparent Km for sodium increased from about 100 to 200 mM when pH was decreased from 7.4 to 6.4. The Hill coefficient for sodium interaction was close to 2 and was unaffected by pH. Increasing external sodium reduced the apparent Km of the transport system for phosphate independent of pH. Variations of phosphate concentration had no influence on the apparent Km for sodium. At high sodium concentrations, small effects (20-30%) of pH on the apparent Vmax of the transport system were found; measured at saturating sodium concentrations, the apparent Km values calculated on the basis of total phosphate were increased (50-60%) when pH was decreased from 7.4 to 6.4. The data indicate that the major effect of pH is to modify the interaction of the transport system with sodium. At nonsaturating sodium concentrations, this resulted indirectly in a reduction in the affinity for phosphate related to a different occupancy of the sodium binding site. The differences of transport rate at low phosphate and high sodium concentrations could be explained by preferential transport of divalent phosphate as well as by pH effects on other carrier properties.
Heavy metal intoxication with Hg2+, Pb2+ and Cd2+ commonly leads to phosphaturia. In this study, ... more Heavy metal intoxication with Hg2+, Pb2+ and Cd2+ commonly leads to phosphaturia. In this study, we examined the effects of these heavy metals on Pi-induced currents (Ip) through NaPi-3, the human renal cotransporter for Na+ and Pi. Hg2+ inhibited Ip in a dose- and time-dependent fashion. Hg2+ decreased the extrapolated maximal current but did not alter the apparent affinity for Pi. This inhibition was also observed with the membrane-permeable oxidizing agent 2,2'-dithio-bis(5-nitropyridine) (DTNP) but not with the membrane-impermeable 5,5'-dithiobis(2-nitrobenzoic acid). Hg(2+)- and DTNP-mediated inhibition of Ip was reversible only in the presence of the reducing agent 2,3-dihydroxybutane-1,4-dithiol. Cd2+ and Pb2+ also inhibited Ip. However, while CD2+ did not significantly alter the apparent affinity for Pi, the apparent concentration needed for half-maximal current (Km) for Pi was increased by Pb2+. In contrast to Hg2+, the inhibition of Ip by Cd2+ and Pb2+ was rapidly reversible upon washout. In the presence of the Na(+)-K(+)-adenosinetriphosphatase inhibitor ouabain, Ip was not reduced, and the effects of the heavy metals were maintained. In summary, the three heavy metals Hg2+, Cd2+, and Pb2+ inhibit Ip through the Na+/Pi cotransporter NaPi-3 by distinct mechanisms. Heavy metal-mediated inhibition of NaPi-3 may be responsible for the phosphaturia observed after intoxication with these compounds.
One way to obtain a deeper understanding of the complex function of the small intestinal and rena... more One way to obtain a deeper understanding of the complex function of the small intestinal and renal proximal tubular epithelium is to dissect it into single components and then, having defined the components under wellcontrolled conditions, try to describe the behaviour of the whole system on the basis of the properties of the single components.
Over the last 10-15 years the use of cellular and molecular biology permitted the analysis of kid... more Over the last 10-15 years the use of cellular and molecular biology permitted the analysis of kidney function at a cellular/molecular level. These advances allow a more detailed understanding of physiological/pathophysiological mechanisms. By taking examples from our studies on the renal handling of inorganic phosphate this gain in knowledge' is illustrated.
Renal phosphate (Pi) absorption is mediated via the type II sodium/Pi cotransporter (NaPi-2) in t... more Renal phosphate (Pi) absorption is mediated via the type II sodium/Pi cotransporter (NaPi-2) in the brush border membrane (BBM) of proximal tubules. Simultaneous detection of NaPi-2 mRNA by in situ hybridization and of NaPi-2 immunoreactivity by immunohistochemistry was performed to investigate the distribution of the cotransporter in healthy control rats and during progression of autosomal dominant polycystic kidney disease (ADPKD). The purpose of the study was to disclose a relation between proximal tubular cell differentiation and NaPi-2 expression. In controls, NaPi-2 expression was present in the entire proximal tubule. In the Han:SPRD (cy/ϩ) model for ADPKD, the proximal nephron is primarily affected by the cystic changes. Epithelial proliferation and impaired epithelial-matrix interaction result in a loss of cell differentiation that eventually leads to cystic enlargement of
Parathyroid hormone (PTH)-induced inhibition of renal proximal tubular Na/P i cotransport involve... more Parathyroid hormone (PTH)-induced inhibition of renal proximal tubular Na/P i cotransport involves two consecutive steps: endocytosis followed by lysosomal degradation of the type IIa Na/P i cotransporter. Tyrosine-, dileucine-, and diacidic-based motifs are suggested to be involved in endocytosis and/or lysosomal targeting of different plasma membrane proteins. The rat type IIa cotransporter (NaPi2) contains two cytoplasmic tyrosine residues (Y) within sequences highly homologous to tyrosinebased motifs (GY 402 FAM and Y 509 RWF), three cytoplasmic dileucine (LL 101 , LL 374, and LI 591 ) and two cytoplasmic diacidic motifs (EE 81 and EE 616 ). We studied the role of these motifs on the PTH-induced retrieval and lysosomal degradation of the NaPi2 cotransporter. To follow its trafficking in vivo, the NaPi2 protein was fused to the carboxyl-terminal end of the enhanced green fluorescence protein. This fusion did not impair the apical targeting or the PTH-induced endocytosis of the wild-type cotransporter when transfected in opossum kidney cells. Single and multiple Y and LL mutants retained the apical targeting and the PTH-induced degradation. Mutations of the diacidic motifs were also without effect. These data suggest that the above three motifs are not required for the PTH-induced internalization and/or degradation of the cotransporter.
Parathyroid hormone (PTH) leads to the inhibition of Na-Pi cotransport activity and to the downre... more Parathyroid hormone (PTH) leads to the inhibition of Na-Pi cotransport activity and to the downregulation of the number of type II Na-Pi cotransporters in proximal tubules, as well as in opossum kidney (OK) cells. PTH is known also to lead to an activation of adenylate cyclase and phospholipase C in proximal tubular preparations, as well as in OK cells. In the present study, we investigated the involvement of these two regulatory pathways in OK cells in the PTH-dependent downregulation of the number of type II Na-Pi cotransporters. We have addressed this issue by using pharmacological activators of protein kinase A (PKA) and protein kinase C (PKC), i.e., 8-bromo-cAMP (8-BrcAMP) and beta-12-O-tetradecanoylphorbol 13-acetate (beta-TPA), respectively, as well as by the use of synthetic peptide fragments of PTH that activate adenylate cyclase and/or phospholipase C, i.e., PTH-(1-34) and PTH-(3-34), respectively. Our results show that PTH signal transduction via cAMP-dependent, as well as cAMP-independent, pathways leads to a membrane retrieval and degradation of type II Na-Pi cotransporters and, thereby, to the inhibition of Na-Pi cotransport activity. Thereby, the cAMP-independent regulatory pathway leads only to partial effects (approximately 50%).
Renal proximal tubular sulfate reabsorption is mediated by brush border membrane Na+/sulfate-cotr... more Renal proximal tubular sulfate reabsorption is mediated by brush border membrane Na+/sulfate-cotransport and basolateral Na+-independent sulfate transport. Injection of rat kidney cortex mRNA into Xenopus laevis oocytes induced Na+-dependent as well as Na+-independent sulfate transport. The inhibition pattern of Na+dependent uptake coincided with that known for the brush border membrane; the inhibition pattern of Na+independent uptake suggested that this activity could be related to the basolateral cell surface. By Northern blot hybridization of size-fractionated mRNA, we provide evidence that the Na+-dependent uptake is induced by an mRNA species related to a recently cloned cDNA encoding rat renal cortex Na+/S04 cotransport (Nasi-1; Markovich, D., Forgo, J., Stange, G., Biber, J., and Murer, H. (1993) Proc. Nutl. ACME. ; the Na'hdependent sulfate transport activity seems to be related to an mRNA species encoding a rat liver Na+independent sulfate transporter (Bissig, M., Hagenbuch, B., Stieger, B., Koller, T., and Meier, P. J. (1994) J. Biol. Chem. 269,3017-3021). Hybrid depletion experiments using antisense oligonucleotides provided further evidence for the association of the expressed transport activities to Nasi-1 and sat-1, respectively.
A type II Na/Pi-cotransporter located in the brush border membrane is the rate limiting and physi... more A type II Na/Pi-cotransporter located in the brush border membrane is the rate limiting and physiologically regulated step in proximal tubular phosphate (Pi) reabsorption. In states of altered Pi-reabsorption [for example, in response to parathyroid hormone (PTH) and to altered dietary intake of Pi or as a consequence of genetic abnormalities], brush border expression of the type II Na/Pi-cotransporter is accordingly modified. PTH initiates a regulatory cascade leading to membrane retrieval, followed by lysosomal degradation of this transporter; recovery from inhibition requires its de novo synthesis. Pi-deprivation leads to an increased brush border expression of transporters that does not appear to require de novo synthesis in the short term. Pi-overload leads to membrane retrieval and degradation of transporters. Finally, in animals with genetically altered Pi-handling (Hyp; Gy) the brush border membrane expression of the type II Na/Pi-cotransporter is also reduced, suggesting that a genetically altered protein (such as PEX in Hyp) controls the expression of this transporter.
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Papers by Heini Murer