RESUMEN
Fibroblast growth factor 23 (FGF23) regulates phosphate homeostasis and is linked to cardiovascular disease and all-cause mortality in chronic kidney disease. FGF23 rises in patients with CKD stages 2-3, but in patients with autosomal dominant polycystic kidney disease, the increase of FGF23 precedes the first measurable decline in renal function. The mechanisms governing FGF23 production and effects in kidney disease are largely unknown. Here we studied the relation between FGF23 and mineral homeostasis in two animal models of PKD. Plasma FGF23 levels were increased 10-fold in 4-week-old cy/+ Han:SPRD rats, whereas plasma urea and creatinine concentrations were similar to controls. Plasma calcium and phosphate levels as well as TmP/GFR were similar in PKD and control rats at all time points examined. Expression and activity of renal phosphate transporters, the vitamin D3-metabolizing enzymes, and the FGF23 co-ligand Klotho in the kidney were similar in PKD and control rats through 8 weeks of age, indicating resistance to FGF23, although phosphorylation of the FGF receptor substrate 2α protein was enhanced. In the kidneys of rats with PKD, FGF23 mRNA was highly expressed and FGF23 protein was detected in cells lining renal cysts. FGF23 expression in bone and spleen was similar in control rats and rats with PKD. Similarly, in an inducible Pkd1 knockout mouse model, plasma FGF23 levels were elevated, FGF23 was expressed in kidneys, but renal phosphate excretion was normal. Thus, the polycystic kidney produces FGF23 but is resistant to its action.
Asunto(s)
Factores de Crecimiento de Fibroblastos/metabolismo , Riñón/metabolismo , Enfermedades Renales Poliquísticas/metabolismo , Proteínas Adaptadoras Transductoras de Señales/metabolismo , Animales , Biomarcadores/sangre , Calcitriol/metabolismo , Calcio/sangre , Creatinina/sangre , Modelos Animales de Enfermedad , Femenino , Factor-23 de Crecimiento de Fibroblastos , Factores de Crecimiento de Fibroblastos/sangre , Factores de Crecimiento de Fibroblastos/genética , Glucuronidasa/metabolismo , Riñón/patología , Proteínas Klotho , Masculino , Ratones Noqueados , Hormona Paratiroidea/sangre , Fosfatos/sangre , Fosforilación , Enfermedades Renales Poliquísticas/sangre , Enfermedades Renales Poliquísticas/genética , Enfermedades Renales Poliquísticas/patología , ARN Mensajero/metabolismo , Ratas , Transducción de Señal , Canales Catiónicos TRPP/deficiencia , Canales Catiónicos TRPP/genética , Regulación hacia Arriba , Urea/sangreRESUMEN
BACKGROUND/AIMS: Uric acid nephrolithiasis is prevalent among patients with type 2 diabetes and metabolic syndrome; it is correlated with an acidic urine and lower urinary ammonium excretion and is likely associated with insulin resistance. Insulin stimulates ammoniagenesis in renal cell lines via increased phosphate-dependent glutaminase (PDG) activity and glutamine metabolism. Ammonium excretion into the proximal tubule is mediated at least in part by the Na(+)/H(+)-exchanger NHE3 and in the collecting duct involving the Rhesus protein RhCG. Here we tested, whether obesity and insulin resistance in a diet-induced mouse model could contribute to deranged ammonium excretion. METHODS: Obesity was induced by diet in mice and the impact on key molecules of proximal tubular ammoniagenesis and urinary acid excretion tested. RESULTS: Diet-induced obesity was confirmed by pathological intraperitoneal glucose tolerance tests (IPGTT). Three groups of mice were compared: control mice; obese, glucose-intolerant with abnormal IPGTT (O-GI); or moderate weight with normal IPGTT (Non-Responders, NR). Basal urinary ammonium excretion did not differ among groups. However, acid loading increased urinary ammonium excretion in all groups, but to a lesser extent in the O-GI group. SNAT3 mRNA expression was enhanced in both obese groups. PDG expression was elevated only in acid-loaded O-GI mice, whereas PEPCK was enhanced in both O-GI and NR groups given NH4CI. NHE activity in the brush border membrane of the proximal tubule was strongly reduced in the O-GI group whereas RhCG expression was similar. CONCLUSION: In sum, obesity and glucose intolerance impairs renal ammonium excretion in response to NH4CI feeding most likely through reduced NHE activity. The stimulation of SNAT3 and ammoniagenic enzyme expression may be compensatory but futile.
Asunto(s)
Sistemas de Transporte de Aminoácidos Neutros/metabolismo , Amoníaco/metabolismo , Obesidad/metabolismo , Animales , Prueba de Tolerancia a la Glucosa , Masculino , Ratones , Ratones Endogámicos C57BL , Reacción en Cadena en Tiempo Real de la PolimerasaRESUMEN
Renal reabsorption of inorganic phosphate (Pi) is mediated by the phosphate transporters NaPi-IIa, NaPi-IIc, and Pit-2 in the proximal tubule brush border membrane (BBM). Dietary Pi intake regulates these transporters; however, the contribution of the specific isoforms to the rapid and slow phase is not fully clarified. Moreover, the regulation of PTH and FGF23, two major phosphaturic hormones, during the adaptive phase has not been correlated. C57/BL6 and NaPi-IIa(-/-) mice received 5 days either 1.2 % (HPD) or 0.1 % (LPD) Pi-containing diets. Thereafter, some mice were acutely switched to LPD or HPD. Plasma Pi concentrations were similar under chronic diets, but lower when mice were acutely switched to LPD. Urinary Pi excretion was similar in C57/BL6 and NaPi-IIa(-/-) mice under HPD. During chronic LPD, NaPi-IIa(-/-) mice lost phosphate in urine compensated by higher intestinal Pi absorption. During the acute HPD-to-LPD switch, NaPi-IIa(-/-) mice exhibited a delayed decrease in urinary Pi excretion. PTH was acutely regulated by low dietary Pi intake. FGF23 did not respond to low Pi intake within 8 h whereas the phospho-adaptator protein FRS2α necessary for FGF-receptor cell signaling was downregulated. BBM Pi transport activity and NaPi-IIa but not NaPi-IIc and Pit-2 abundance acutely adapted to diets in C57/BL6 mice. In NaPi-IIa(-/-), Pi transport activity was low and did not adapt. Thus, NaPi-IIa mediates the fast adaptation to Pi intake and is upregulated during the adaptation to low Pi despite persistently high FGF23 levels. The sensitivity to FGF23 may be regulated by adapting FRS2α abundance and phosphorylation.
Asunto(s)
Adaptación Fisiológica , Factores de Crecimiento de Fibroblastos/metabolismo , Túbulos Renales Proximales/metabolismo , Proteínas de la Membrana/metabolismo , Fósforo Dietético/metabolismo , Proteínas Adaptadoras Transductoras de Señales/metabolismo , Animales , Factor-23 de Crecimiento de Fibroblastos , Absorción Intestinal , Túbulos Renales Proximales/fisiología , Proteínas de la Membrana/genética , Ratones , Ratones Endogámicos C57BL , Ratones Noqueados , Fósforo Dietético/sangre , Fósforo Dietético/orina , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/genética , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismoRESUMEN
Cystinuria is an autosomal recessive disease caused by mutations in SLC3A1 (rBAT) and SLC7A9 (b(0,+)AT). Gene targeting of the catalytic subunit (Slc7a9) in mice leads to excessive excretion of cystine, lysine, arginine, and ornithine. Here, we studied this non-type I cystinuria mouse model using gene expression analysis, Western blotting, clearance, and brush-border membrane vesicle (BBMV) uptake experiments to further characterize the renal and intestinal consequences of losing Slc7a9 function. The electrogenic and BBMV flux studies in the intestine suggested that arginine and ornithine are transported via other routes apart from system b(0,+). No remarkable gene expression changes were observed in other amino acid transporters and the peptide transporters in the intestine and kidney. Furthermore, the glomerular filtration rate (GFR) was reduced by 30% in knockout animals compared with wild-type animals. The fractional excretion of arginine was increased as expected (â¼100%), but fractional excretions of lysine (â¼35%), ornithine (â¼16%), and cystine (â¼11%) were less affected. Loss of function of b(0,+)AT reduced transport of cystine and arginine in renal BBMVs and completely abolished the exchanger activity of dibasic amino acids with neutral amino acids. In conclusion, loss of Slc7a9 function decreases the GFR and increases the excretion of several amino acids to a lesser extent than expected with no clear regulation at the mRNA and protein level of alternative transporters and no increased renal epithelial uptake. These observations indicate that transporters located in distal segments of the kidney and/or metabolic pathways may partially compensate for Slc7a9 loss of function.
Asunto(s)
Sistemas de Transporte de Aminoácidos Básicos/deficiencia , Aminoácidos Diaminos/metabolismo , Cistina/metabolismo , Cistinuria/metabolismo , Sistemas de Transporte de Aminoácidos Básicos/genética , Animales , Cistinuria/fisiopatología , Modelos Animales de Enfermedad , Tasa de Filtración Glomerular/fisiología , Riñón/metabolismo , Riñón/fisiopatología , Masculino , Ratones , Ratones Endogámicos C57BL , Ratones NoqueadosRESUMEN
BACKGROUND: The oxidative stress-responsive kinase 1 (OSR1) participates in the WNK-(with no K) kinase dependent regulation of renal salt excretion and blood pressure. Little is known, however, about the role of OSR1 in the regulation of further renal transport systems. The present study analyzed the effect of OSR1 on NaPiIIa, the major renal tubular phosphate transporter. METHODS: Immunohistochemistry and confocal microscopy were employed to determine renal localization of OSR1 and NaPiIIa. To elucidate the effect of OSR on NaPiIIa activity, cRNA encoding NaPiIIa was injected into Xenopus oocytes with or without additional injection of cRNA encoding OSR1, and phosphate transport was estimated from phosphateinduced currents determined with dual electrode voltage clamp. To elucidate the in vivo significance of OSR1 serum phosphate and hormone concentrations as well as urinary phosphate output of mice carrying one allele of WNK-resistant OSR1 (osr1tg/(+)) were compared to the respective values of wild type mice (osr1(+/+)). RESULTS: NaPiIIa and OSR1 were both expressed in proximal renal tubule cells. Coexpression of OSR1 significantly up-regulated phosphate-induced currents in NaPiIIa-expressing Xenopus oocytes. Despite decreased serum phosphate concentration urinary phosphate excretion was significantly increased and NaPiIIa protein abundance in the brush border membrane significantly reduced in osr1tg/(+) mice as compared to osr1(+/+) mice. Serum PTH and calcitriol levels were similar in osr1tg/(+) mice and in osr1(+/+) mice, serum FGF23 concentration was, however, significantly higher in osr1tg/(+) mice than in osr1(+/+) mice. CONCLUSIONS: OSR1 is expressed in proximal renal tubules and participates in the regulation of FGF23 release and renal tubular phosphate transport.
Asunto(s)
Túbulos Renales/metabolismo , Fosfatos/metabolismo , Proteínas Serina-Treonina Quinasas/genética , Proteínas Serina-Treonina Quinasas/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismo , Absorción/fisiología , Animales , Calcitriol/sangre , Femenino , Factor-23 de Crecimiento de Fibroblastos , Factores de Crecimiento de Fibroblastos/sangre , Masculino , Ratones , Ratones Mutantes , Oocitos/citología , Oocitos/metabolismo , Hormona Paratiroidea/sangre , Técnicas de Placa-Clamp , Xenopus laevisRESUMEN
Angiotensin -converting enzyme 2 (ACE2) is a regulator of the renin angiotensin system involved in acute lung failure, cardiovascular functions and severe acute respiratory syndrome (SARS) infections in mammals. A gene encoding a homologue to ACE2, termed collectrin (Tmem27), has been identified in immediate proximity to the ace2 locus. The in vivo function of collectrin was unclear. Here we report that targeted disruption of collectrin in mice results in a severe defect in renal amino acid uptake owing to downregulation of apical amino acid transporters in the kidney. Collectrin associates with multiple apical transporters and defines a novel group of renal amino acid transporters. Expression of collectrin in Xenopus oocytes and Madin-Darby canine kidney (MDCK) cells enhances amino acid transport by the transporter B(0)AT1. These data identify collectrin as a key regulator of renal amino acid uptake.
Asunto(s)
Sistemas de Transporte de Aminoácidos/metabolismo , Aminoácidos/metabolismo , Riñón/metabolismo , Glicoproteínas de Membrana/metabolismo , Animales , Transporte Biológico , Línea Celular , Polaridad Celular , Perros , Regulación hacia Abajo , Femenino , Masculino , Glicoproteínas de Membrana/deficiencia , Glicoproteínas de Membrana/genética , Ratones , Ratones Endogámicos C57BL , Oocitos/metabolismo , Tirosina/metabolismo , XenopusRESUMEN
Insulin and IGF1-dependent signaling activates protein kinase B and serum and glucocorticoid inducible kinase (PKB/SGK), which together phosphorylate and inactivate glycogen synthase kinase GSK3. Because insulin and IGF1 increase renal tubular calcium and phosphorus reabsorption, we examined GSK3 regulation of phosphate transporter activity and determined whether PKB/SGK inactivates GSK3 to enhance renal phosphate and calcium transport. Overexpression of GSK3 and the phosphate transporter NaPi-IIa in Xenopus oocytes decreased electrogenic phosphate transport compared with NaPi-IIa-expressing oocytes. PKB/SGK serine phosphorylation sites in GSK3 were mutated to alanine to create gsk3(KI) mice resistant to PKB/SGK inactivation. Compared with wildtype animals, gsk3(KI) animals exhibited greater urinary phosphate and calcium clearances with higher excretion rates and lower plasma concentrations. Isolated brush border membranes from gsk3(KI) mice showed less sodium-dependent phosphate transport and Na-phosphate co-transporter expression. Parathyroid hormone, 1,25-OH vitamin D levels, and bone mineral density were decreased in gsk3(KI) mice, suggesting a global dysregulation of bone mineral metabolism. Taken together, PKB/SGK phosphorylation of GSK3 increases phosphate transporter activity and reduces renal calcium and phosphate loss.
Asunto(s)
Calcio/orina , Glucógeno Sintasa Quinasa 3/fisiología , Hipofosfatemia Familiar/etiología , Proteínas Inmediatas-Precoces/fisiología , Proteínas Serina-Treonina Quinasas/fisiología , Proteínas Proto-Oncogénicas c-akt/fisiología , Animales , Densidad Ósea , Calcitriol/sangre , Túbulos Renales/metabolismo , Ratones , Hormona Paratiroidea/sangre , Fosfatos/metabolismo , FosforilaciónRESUMEN
Insulin and growth factors activate the phosphatidylinositide-3-kinase pathway, leading to stimulation of several kinases including serum- and glucocorticoid-inducible kinase isoform SGK3, a transport regulating kinase. Here, we explored the contribution of SGK3 to the regulation of renal tubular phosphate transport. Coexpression of SGK3 and sodium-phosphate cotransporter IIa significantly enhanced the phosphate-induced current in Xenopus oocytes. In sgk3 knockout and wild-type mice on a standard diet, fluid intake, glomerular filtration and urine flow rates, and urinary calcium ion excretion were similar. However, fractional urinary phosphate excretion was slightly but significantly larger in the knockout than in wild-type mice. Plasma calcium ion, phosphate concentration, and plasma parathyroid hormone levels were not significantly different between the two genotypes, but plasma calcitriol and fibroblast growth factor 23 concentrations were significantly lower in the knockout than in wild-type mice. Moreover, bone density was significantly lower in the knockouts than in wild-type mice. Histological analysis of the femur did not show any differences in cortical bone but there was slightly less prominent trabecular bone in sgk3 knockout mice. Thus, SGK3 has a subtle but significant role in the regulation of renal tubular phosphate transport and bone density.
Asunto(s)
Densidad Ósea/fisiología , Hipofosfatemia Familiar/etiología , Proteínas Serina-Treonina Quinasas/deficiencia , Animales , Transporte Biológico Activo , Densidad Ósea/genética , Calcio/metabolismo , Femenino , Humanos , Hipofosfatemia Familiar/enzimología , Hipofosfatemia Familiar/genética , Técnicas In Vitro , Túbulos Renales/metabolismo , Ratones , Ratones Noqueados , Oocitos/metabolismo , Fosfatos/metabolismo , Proteínas Serina-Treonina Quinasas/genética , Proteínas Serina-Treonina Quinasas/metabolismo , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Transducción de Señal , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/genética , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismo , XenopusRESUMEN
Renal phosphate reabsorption across the brush border membrane (BBM) in the proximal tubule is mediated by at least three transporters, NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3), and Pit-2 (SLC20A2). Parathyroid hormone (PTH) is a potent phosphaturic factor exerting an acute and chronic reduction in proximal tubule phosphate reabsorption. PTH acutely induces NaPi-IIa internalization from the BBM and lysosomal degradation, but its effects on NaPi-IIc and Pit-2 are unknown. In rats adapted to low phosphate diet, acute (30 and 60 min) application of PTH decreased BBM phosphate transport rates both in the absence and the presence of phosphonoformic acid, an inhibitor of SLC34 but not SLC20 transporters. Immunohistochemistry showed NaPi-IIa expression in the S1 to the S3 segment of superficial and juxtamedullary nephrons; NaPi-IIc was only detectable in S1 segments and Pit-2 in S1 and weakly in S2 segments of superficial and juxtamedullary nephrons. PTH reduced NaPi-IIa staining in the BBM with increased intracellular and lysosomal appearance. NaPi-IIa internalization was most prominent in S1 segments of superficial nephrons. We did not detect changes in NaPi-IIc and Pit-2 staining over this time period. Blockade of lysosomal protein degradation with leupeptin revealed NaPi-IIa accumulation in lysosomes, but no lysosomal staining for NaPi-IIc or Pit-2 could be detected. Immunoblotting of BBM confirmed the reduction in NaPi-IIa abundance and the absence of any effect on NaPi-IIc expression. Pit-2 protein abundance was also significantly reduced by PTH. Thus, function and expression of BBM phosphate cotransporters are differentially regulated allowing for fine-tuning of renal phosphate reabsorption.
Asunto(s)
Riñón/metabolismo , Hormona Paratiroidea/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato/metabolismo , Animales , Riñón/ultraestructura , Lisosomas/metabolismo , Masculino , Microvellosidades/metabolismo , Fosfatos/metabolismo , Ratas , Ratas Wistar , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo III/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIc/metabolismoRESUMEN
Two highly similar regions in the predicted first intracellular (ICL-1) and third extracellular loop (ECL-3) of the type IIa Na+/P(i) cotransporter (NaPi-IIa) have been shown previously to contain functionally important sites by applying the substituted cysteine accessibility method (SCAM). Incubation in methanethiosulfonate (MTS) reagents of mutants that contain novel cysteines in both loops led to full inhibition of cotransport activity. To elucidate further the role these regions play in defining the transport mechanism, a double mutant (A203C-S460C) was constructed with novel cysteines in each region. The effect of cysteine modification by different MTS reagents on two electrogenic transport modes (leak and cotransport) was investigated. MTSEA (2-aminoethyl MTS hydrobromide) and MTSES (MTS ethylsulfonate) led to full inhibition of cotransport and increased the leak, whereas incubation in MTSET (2-[trimethylammonium]ethyl MTS bromide) inhibited only cotransport. The behavior of other double mutants with a cysteine retained at one site and hydrophobic or hydrophilic residues substituted at the other site, indicated that most likely only Cys-460 was modifiable, but the residue at Ala-203 was critical for conferring the leak and cotransport mode behavior. Substrate interaction with the double mutant was unaffected by MTS exposure as the apparent P(i) and Na+ affinities for P(i)-induced currents and respective activation functions were unchanged after cysteine modification. This suggested that the modified site did not interfere with substrate recognition/binding, but prevents translocation of the fully loaded carrier. The time-dependency of cotransport loss and leak growth during modification of the double cysteine mutant was reciprocal, which suggested that the modified site is a kinetic codeterminant of both transport modes. The behavior is consistent with a kinetic model for NaPi-IIa that predicts mutual exclusiveness of both transport modes. Together, these findings suggest that parts of the opposing linker regions are associated with the NaPi-IIa transport pathway.
Asunto(s)
Fosfatos/farmacocinética , Simportadores/química , Simportadores/fisiología , Sustitución de Aminoácidos , Animales , Transporte Biológico Activo/fisiología , Cisteína/genética , Transporte Iónico/fisiología , Cinética , Potenciales de la Membrana/fisiología , Microvellosidades/metabolismo , Mutagénesis Sitio-Dirigida , Oocitos/química , Conformación Proteica , Proteínas Cotransportadoras de Sodio-Fosfato , Relación Estructura-Actividad , Xenopus laevisRESUMEN
The putative first intracellular and third extracellular linkers are known to play important roles in defining the transport properties of the type IIa Na+-coupled phosphate cotransporter (Kohler, K., I.C. Forster, G. Stange, J. Biber, and H. Murer. 2002b. J. Gen. Physiol. 120:693-705). To investigate whether other stretches that link predicted transmembrane domains are also involved, the substituted cysteine accessibility method (SCAM) was applied to sites in the predicted first and fourth extracellular linkers (ECL-1 and ECL-4). Mutants based on the wild-type (WT) backbone, with substituted novel cysteines, were expressed in Xenopus oocytes, and their function was assayed by isotope uptake and electrophysiology. Functionally important sites were identified in both linkers by exposing cells to membrane permeant and impermeant methanethiosulfonate (MTS) reagents. The cysteine modification reaction rates for sites in ECL-1 were faster than those in ECL-4, which suggested that the latter were less accessible from the extracellular medium. Generally, a finite cotransport activity remained at the end of the modification reaction. The change in activity was due to altered voltage-dependent kinetics of the Pi-dependent current. For example, cys substitution at Gly-134 in ECL-1 resulted in rate-limiting, voltage-independent cotransport activity for V < or = -80 mV, whereas the WT exhibited a linear voltage dependency. After cys modification, this mutant displayed a supralinear voltage dependency in the same voltage range. The opposite behavior was documented for cys substitution at Met-533 in ECL-4. Modification of cysteines at two other sites in ECL-1 (Ile-136 and Phe-137) also resulted in supralinear voltage dependencies for hyperpolarizing potentials. Taken together, these findings suggest that ECL-1 and ECL-4 may not directly form part of the transport pathway, but specific sites in these linkers can interact directly or indirectly with parts of NaPi-IIa that undergo voltage-dependent conformational changes and thereby influence the voltage dependency of cotransport.
Asunto(s)
Membrana Celular/fisiología , Cisteína/metabolismo , Potenciales de la Membrana/fisiología , Mesilatos/farmacología , Oocitos/fisiología , Fosfatos/metabolismo , Simportadores/metabolismo , Secuencia de Aminoácidos , Animales , Células Cultivadas , Cisteína/química , Cisteína/genética , Relación Dosis-Respuesta a Droga , Líquido Extracelular/metabolismo , Potenciales de la Membrana/efectos de los fármacos , Datos de Secuencia Molecular , Mutagénesis Sitio-Dirigida , Oocitos/efectos de los fármacos , Subunidades de Proteína , Proteínas Recombinantes/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato , Relación Estructura-Actividad , Simportadores/química , Simportadores/genética , Xenopus laevisRESUMEN
The principal mediators of renal phosphate (P(i)) reabsorption are the SLC34 family proteins NaPi-IIa and NaPi-IIc, localized to the proximal tubule (PT) apical membrane. Their abundance is regulated by circulatory factors and dietary P(i). Although their physiological importance has been confirmed in knockout animal studies, significant P(i) reabsorptive capacity remains, which suggests the involvement of other secondary-active P(i) transporters along the nephron. Here we show that a member of the SLC20 gene family (PiT-2) is localized to the brush-border membrane (BBM) of the PT epithelia and that its abundance, confirmed by Western blot and immunohistochemistry of rat kidney slices, is regulated by dietary P(i). In rats treated chronically on a high-P(i) (1.2%) diet, there was a marked decrease in the apparent abundance of PiT-2 protein in kidney slices compared with those from rats kept on a chronic low-P(i) (0.1%) diet. In Western blots of BBM from rats that were switched from a chronic low- to high-P(i) diet, NaPi-IIa showed rapid downregulation after 2 h; PiT-2 was also significantly downregulated at 24 h and NaPi-IIc after 48 h. For the converse dietary regime, NaPi-IIa showed adaptation within 8 h, whereas PiT-2 and NaPi-IIc showed a slower adaptive trend. Our findings suggest that PiT-2, until now considered as a ubiquitously expressed P(i) housekeeping transporter, is a novel mediator of P(i) reabsorption in the PT under conditions of acute P(i) deprivation, but with a different adaptive time course from NaPi-IIa and NaPi-IIc.
Asunto(s)
Membrana Celular/metabolismo , Dieta , Túbulos Renales Proximales/metabolismo , Fosfatos/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo III/metabolismo , Adaptación Fisiológica , Animales , Transporte Biológico , Western Blotting , Polaridad Celular , Inmunohistoquímica , Masculino , Microvellosidades/metabolismo , Fosfatos/administración & dosificación , Fosfatos/deficiencia , Ratas , Ratas Wistar , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo II/metabolismo , Factores de TiempoRESUMEN
Human glomerulonephritis (GN) is characterized by sustained proteinuria, sodium retention, hypertension, and edema formation. Increasing quantities of filtered protein enter the renal tubule, where they may alter epithelial transport functions. Exaggerated endocytosis and consequent protein overload may affect proximal tubules, but intrinsic malfunction of distal epithelia has also been reported. A straightforward assignment to a particular tubule segment causing salt retention in GN is still controversial. We hypothesized that 1) trafficking and surface expression of major transporters and channels involved in volume regulation were altered in GN, and 2) proximal tubular endocytosis may influence locally as well as downstream expressed tubular transporters and channels. Effects of anti-glomerular basement membrane GN were studied in controls and megalin-deficient mice with blunted proximal endocytosis. Mice displayed salt retention and elevated systolic blood pressure when proteinuria had reached 10-15 mg/24 h. Surface expression of proximal Na(+)-coupled transporters and water channels was in part [Na(+)-P(i) cotransporter IIa (NaPi-IIa) and aquaporin-1 (AQP1)] increased by megalin deficiency alone, but unchanged (Na(+)/H(+) exchanger 3) or reduced (NaPi-IIa and AQP1) in GN irrespective of the endocytosis defect. In distal epithelia, significant increases in proteolytic cleavage products of alpha-epithelial Na(+) channel (ENaC) and gamma-ENaC were observed, suggesting enhanced tubular sodium reabsorption. The effects of glomerular proteinuria dominated over those of blunted proximal endocytosis in contributing to ENaC cleavage. Our data indicate that ENaC-mediated sodium entry may be the rate-limiting step in proteinuric sodium retention. Enhanced proteolytic cleavage of ENaC points to a novel mechanism of channel activation which may involve the action of filtered plasma proteases.
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Endocitosis , Glomerulonefritis/metabolismo , Túbulos Renales/metabolismo , Proteínas de Transporte de Membrana/metabolismo , Proteinuria/metabolismo , Sodio/metabolismo , Aldosterona/sangre , Animales , Acuaporina 1/metabolismo , Acuaporina 2/metabolismo , Autoanticuerpos , Presión Sanguínea , Ciclooxigenasa 2/sangre , Modelos Animales de Enfermedad , Canales Epiteliales de Sodio/metabolismo , Femenino , Glomerulonefritis/inmunología , Glomerulonefritis/fisiopatología , Túbulos Renales/enzimología , Túbulos Renales/inmunología , Proteína 2 Relacionada con Receptor de Lipoproteína de Baja Densidad/deficiencia , Proteína 2 Relacionada con Receptor de Lipoproteína de Baja Densidad/genética , Ratones , Ratones Noqueados , Transporte de Proteínas , Proteinuria/inmunología , Proteinuria/fisiopatología , Renina/sangre , Sodio/sangre , Sodio/orina , Intercambiador 3 de Sodio-Hidrógeno , Intercambiadores de Sodio-Hidrógeno/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismo , Proteína de Unión a Vitamina D/orinaRESUMEN
During metabolic acidosis (MA), urinary phosphate excretion increases and contributes to acid removal. Two Na(+)-dependent phosphate transporters, NaPi-IIa (Slc34a1) and NaPi-IIc (Slc34a3), are located in the brush border membrane (BBM) of the proximal tubule and mediate renal phosphate reabsorption. Transcriptome analysis of kidneys from acid-loaded mice revealed a large decrease in NaPi-IIc messenger RNA (mRNA) and a smaller reduction in NaPi-IIa mRNA abundance. To investigate the contribution of transporter regulation to phosphaturia during MA, we examined renal phosphate transporters in normal and Slc34a1-gene ablated (NaPi-IIa KO) mice acid-loaded for 2 and 7 days. In normal mice, urinary phosphate excretion was transiently increased after 2 days of acid loading, whereas no change was found in Slc34a1-/- mice. BBM Na/Pi cotransport activity was progressively and significantly decreased in acid-loaded KO mice, whereas in WT animals, a small increase after 2 days of treatment was seen. Acidosis increased BBM NaPi-IIa abundance in WT mice and NaPi-IIc abundance in WT and KO animals. mRNA abundance of NaPi-IIa and NaPi-IIc decreased during MA. Immunohistochemistry did not indicate any change in the localization of NaPi-IIa and NaPi-IIc along the nephron. Interestingly, mRNA abundance of both Slc20 phosphate transporters, Pit1 and Pit2, was elevated after 7 days of MA in normal and KO mice. These data demonstrate that phosphaturia during acidosis is not caused by reduced protein expression of the major Na/Pi cotransporters NaPi-IIa and NaPi-IIc and suggest a direct inhibitory effect of low pH mainly on NaPi-IIa. Our data also suggest that Pit1 and Pit2 transporters may play a compensatory role.
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Acidosis/complicaciones , Hipofosfatemia Familiar/etiología , Túbulos Renales Proximales/metabolismo , Fosfatos/orina , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismo , Acidosis/orina , Animales , Modelos Animales de Enfermedad , Concentración de Iones de Hidrógeno , Hipofosfatemia Familiar/orina , Masculino , Ratones , Ratones Endogámicos C57BL , Ratones Noqueados , Microvellosidades/metabolismo , ARN Mensajero/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo III/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/deficiencia , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/genética , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIc/metabolismo , Factores de TiempoRESUMEN
This protocol describes a method for the isolation and purification of renal proximal tubular brush-border membranes in high yield and high purity. Based on a different reactivity of the brush-border membrane compared to other cellular membranes with divalent cations, such as Mg2+, purified membrane vesicles can be obtained after a few differential centrifugation steps (within approximately 3 h) that are suitable for in vitro studies, such as transport experiments or protein and lipid analysis.
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Fraccionamiento Químico/métodos , Túbulos Renales Proximales , Membranas , Microvellosidades , Animales , Túbulos Renales Proximales/ultraestructura , Membranas/ultraestructura , Ratones , Microvellosidades/ultraestructura , RatasRESUMEN
Phosphate reabsorption in the renal proximal tubule occurs mostly via the type IIa Na(+)-phosphate cotransporter (NaP(i)-IIa) in the brush border membrane (BBM). The activity and localization of NaP(i)-IIa are regulated, among other factors, by parathyroid hormone (PTH). NaP(i)-IIa interacts in vitro via its last three COOH-terminal amino acids with the PDZ protein Na(+)/H(+)-exchanger isoform 3 regulatory factor (NHERF)-1 (NHERF1). Renal phosphate reabsorption in Nherf1-deficient mice is altered, and NaP(i)-IIa expression in the BBM is reduced. In addition, it has been proposed that NHERF1 and NHERF2 are important for the coupling of PTH receptors (PTHRs) to phospholipase C (PLC) and the activation of the protein kinase C pathway. We tested the role of NHERF1 in the regulation of NaP(i)-IIa by PTH in Nherf1-deficient mice. Immunohistochemistry and Western blotting demonstrated that stimulation of apical and basolateral receptors with PTH-(1-34) led to internalization of NaP(i)-IIa in wild-type and Nherf1-deficient mice. Stimulation of only apical receptors with PTH-(3-34) failed to induce internalization in Nherf1-deficient mice. Expression and localization of apical PTHRs were similar in wild-type and Nherf1-deficient mice. Activation of the protein kinase C- and A-dependent pathways with 1,2-dioctanoyl-sn-glycerol or 8-bromo-cAMP induced normal internalization of NaP(i)-IIa in wild-type, as well as Nherf1-deficient, mice. Stimulation of PLC activity due to apical PTHRs was impaired in Nherf1-deficient mice. These data suggest that NHERF1 in the proximal tubule is important for PTH-induced internalization of NaP(i)-IIa and, specifically, couples the apical PTHR to PLC.
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Túbulos Renales Proximales/metabolismo , Fosfoproteínas/fisiología , Receptor de Hormona Paratiroídea Tipo 1/metabolismo , Intercambiadores de Sodio-Hidrógeno/fisiología , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/metabolismo , Fosfolipasas de Tipo C/metabolismo , 8-Bromo Monofosfato de Adenosina Cíclica/farmacología , Animales , Proteínas Quinasas Dependientes de AMP Cíclico/metabolismo , Diglicéridos/farmacología , Endocitosis , Femenino , Técnicas In Vitro , Transporte Iónico , Masculino , Ratones , Ratones Noqueados , Microvellosidades/metabolismo , Hormona Paratiroidea/farmacología , Fragmentos de Péptidos/farmacología , Fosfatos/metabolismo , Fosfoproteínas/genética , Unión Proteica , Proteína Quinasa C/metabolismo , Receptor de Hormona Paratiroídea Tipo 1/agonistas , Intercambiadores de Sodio-Hidrógeno/genética , Teriparatido/análogos & derivados , Teriparatido/farmacologíaRESUMEN
The Na(+)/H(+) exchanger 3 (NHE3) is expressed in the brush border membrane (BBM) of proximal tubules (PT). Its activity is down-regulated on increases in intracellular cAMP levels. The aim of this study was to investigate the contribution of the protein kinase A (PKA) and the exchange protein directly activated by cAMP (EPAC) dependent pathways in the regulation of NHE3 by adenosine 3',5'-cyclic monophosphate (cAMP). Opossum kidney cells and murine kidney slices were treated with cAMP analogs, which selectively activate either PKA or EPAC. Activation of either pathway resulted in an inhibition of NHE3 activity. The EPAC-induced effect was independent of PKA as indicated by the lack of activation of the kinase and the insensitivity to the PKA inhibitor H89. Both PKA and EPAC inhibited NHE3 activity without inducing changes in the expression of the transporter in BBM. Activation of PKA, but not of EPAC, led to an increase of NHE3 phosphorylation. In contrast, activation of PKA, but not of EPAC, inhibited renal type IIa Na(+)-coupled inorganic phosphate cotransporter (NaPi-IIa), another Na-dependent transporter expressed in proximal BBM. PKA, but not EPAC, induced the retrieval of NaPi-IIa from BBM. Our results suggest that EPAC activation may represent a previously unrecognized mechanism involved in the cAMP regulation of NHE3, whereas regulation of NaPi-IIa is mediated by PKA but not by EPAC.
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Proteínas Quinasas Dependientes de AMP Cíclico/fisiología , AMP Cíclico/fisiología , Factores de Intercambio de Guanina Nucleótido/fisiología , Intercambiadores de Sodio-Hidrógeno/metabolismo , Animales , Línea Celular , Riñón/enzimología , Riñón/ultraestructura , Cinética , Ratones , Microvellosidades/enzimología , Intercambiador 3 de Sodio-HidrógenoRESUMEN
During metabolic acidosis, P(i) serves as an important buffer to remove protons from the body. P(i) is released from bone together with carbonate buffering protons in blood. In addition, in the kidney, the fractional excretion of phosphate is increased allowing for the excretion of more acid equivalents in urine. The role of intestinal P(i) absorption in providing P(i) to buffer protons and compensating for loss from bone during metabolic acidosis has not been clarified yet. Inducing metabolic acidosis (NH(4)Cl in drinking water) for 2 or 7 days in mice increased urinary fractional P(i) excretion twofold, whereas serum P(i) levels were not altered. Na(+)-dependent P(i) transport in the small intestine, however, was stimulated from 1.89 +/- 3.22 to 40.72 +/- 11.98 pmol/mg protein (2 days of NH(4)Cl) in brush-border membrane vesicles prepared from total small intestine. Similarly, the protein abundance of the Na(+)-dependent phosphate cotransporter NaPi-IIb in the brush-border membrane was increased 5.3-fold, whereas mRNA levels remained stable. According to immunohistochemistry and real-time PCR NaPi-IIb expression was found to be mainly confined to the ileum in the small intestine, and this distribution was not altered during metabolic acidosis. These results suggest that the stimulation of intestinal P(i) absorption during metabolic acidosis may contribute to the buffering of acid equivalents by providing phosphate and may also help to prevent excessive liberation of phosphate from bone.
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Acidosis/metabolismo , Intestino Delgado/metabolismo , Fosfatos/metabolismo , Sodio/fisiología , Simportadores/biosíntesis , Cloruro de Amonio/farmacología , Animales , Transporte Biológico Activo , Técnica del Anticuerpo Fluorescente , Técnicas In Vitro , Absorción Intestinal , Masculino , Ratones , Microvellosidades/metabolismo , ARN Mensajero/biosíntesis , ARN Mensajero/genética , Ratas , Ratas Sprague-Dawley , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Proteínas Cotransportadoras de Sodio-Fosfato , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIbRESUMEN
The Na(+)/phosphate cotransporter NaPi-IIa (SLC34A1) is the major transporter mediating the reabsorption of P(i) in the proximal tubule. Expression and activity of NaPi-IIa is regulated by several factors, including parathyroid hormone, dopamine, metabolic acidosis, and dietary P(i) intake. Dopamine induces natriuresis and phosphaturia in vivo, and its actions on several Na(+)-transporting systems such as NHE3 and Na(+)-K(+)-ATPase have been investigated in detail. Using freshly isolated mouse kidney slices, perfused proximal tubules, and cultured renal epithelial cells, we examined the acute effects of dopamine on NaPi-IIa expression and localization. Incubation of isolated kidney slices with the selective D(1)-like receptor agonists fenoldopam (10 microM) and SKF-38393 (10 microM) for 1 h induced NaPi-IIa internalization and reduced expression of NaPi-IIa in the brush border membrane (BBM). The D(2)-like selective agonist quinpirole (1 microM) had no effect. The D(1) and D(2) agonists did not affect the renal Na(+)/sulfate cotransporter NaSi in the BBM of the proximal tubule. Studies with isolated perfused proximal tubules demonstrated that activation of luminal, but not basolateral, D(1)-like receptors caused NaPi-IIa internalization. In kidney slices, inhibition of PKC (1 microM chelerythrine) or ERK1/2 (20 microM PD-098089) pathways did not prevent the fenoldopam-induced internalization. Inhibition with the PKA blocker H-89 (10 microM) abolished the effect of fenoldopam. Immunoblot demonstrated a reduction of NaPi-IIa protein in BBMs from kidney slices treated with fenoldopam. Incubation of opossum kidney cells transfected with NaPi-IIa-green fluorescent protein chimera shifted fluorescence from the apical membrane to an intracellular pool. In summary, dopamine induces internalization of NaPi-IIa by activation of luminal D(1)-like receptors, an effect that is mediated by PKA.
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Túbulos Renales Proximales/metabolismo , Receptores de Dopamina D1/metabolismo , Simportadores/metabolismo , Animales , Polaridad Celular/fisiología , Células Cultivadas , Proteínas Quinasas Dependientes de AMP Cíclico/metabolismo , Agonistas de Dopamina/farmacología , Regulación hacia Abajo , Endocitosis/fisiología , Fenoldopam/farmacología , Túbulos Renales Proximales/citología , Masculino , Ratones , Ratones Endogámicos C57BL , Microvellosidades/metabolismo , Zarigüeyas , Técnicas de Cultivo de Órganos , Quinpirol/farmacología , Receptores de Dopamina D1/agonistas , Receptores de Dopamina D2/agonistas , Receptores de Dopamina D2/metabolismo , Proteínas Cotransportadoras de Sodio-Fosfato , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIaRESUMEN
Inorganic phosphate (P(i)) is reabsorbed in the renal proximal tubule mainly via the type-IIa sodium-phosphate cotransporter (NaPi-IIa). This protein is regulated tightly by different factors, among them dietary P(i) intake and parathyroid hormone (PTH). A number of PDZ-domain-containing proteins have been shown to interact with NaPi-IIa in vitro, such as Na(+)/H(+) exchanger-3 regulatory factor-1 (NHERF1) and PDZK1. PDZK1 is highly abundant in kidney and co-localizes with NaPi-IIa in the brush border membrane of proximal tubules. Recently, a knock-out mouse model for PDZK1 (Pdzk1(-/-)) has been generated, allowing the role of PDZK1 in the expression and regulation of the NaPi-IIa cotransporter to be examined in in vivo and in ex vivo preparations. The localization of NaPi-IIa and other proteins interacting with PDZK1 in vitro [Na(+)/H(+) exchanger (NHE3), chloride-formate exchanger (CFEX)/putative anion transporter-1 (PAT1), NHERF1] was not altered in Pdzk1(-/-) mice. The abundance of NaPi-IIa adapted to acute and chronic changes in dietary P(i) intake, but steady-state levels of NaPi-IIa were reduced in Pdzk1(-/-) under a P(i) rich diet. This was paralleled by a higher urinary fractional P(i) excretion. The abundance of the anion exchanger CFEX/PAT1 (SLC26A6) was also reduced. In contrast, NHERF1 abundance increased in the brush border membrane of Pdzk1(-/-) mice fed a high-P(i) diet. Acute regulation of NaPi-IIa by PTH in vivo and by PTH and activators of protein kinases A, C and G (PKA, PKC and PKG) in vitro (kidney slice preparation) was not altered in Pdzk1(-/-) mice. In conclusion, loss of PDZK1 did not result in major changes in proximal tubule function or NaPi-IIa regulation. However, under a P(i)-rich diet, loss of PDZK1 reduced NaPi-IIa abundance indicating that PDZK1 may play a role in the trafficking or stability of NaPi-IIa under these conditions.