Parathyroid hormone and dietary phosphate provoke a lysosomal routing of the proximal tubular Na/Pi-cotransporter type II

BACKGROUND

A decrease of proximal tubular reabsorption of phosphate (Pi), which can be provoked by parathyroid hormone (PTH) or by a high Pi-diet, has been shown to correlate with a decrease of the number of type II Na/Pi-cotransporters residing in the brush border membrane. While both PTH and a high Pi-diet lead to an internalization of type II cotransporters, the further cellular routing of internalized cotransporters has not been established unequivocally.

METHODS

To prevent lysosomal degradation, rats were treated with leupeptin prior to the injection of PTH or feeding acutely with a high Pi-diet. Kidney cortex were recovered and used for immunohistochemistry. In parallel, brush border membranes and lysosomes were isolated and analyzed by Western blotting.

RESULTS

Under both conditions (PTH and high Pi-diet), a strong overlap of internalized type II cotransporters with the late endosomes/lysosomes was observed by immunohistochemistry. In agreement, the content of type II Na/Pi-cotransporters was increased in lysosomes isolated from the corresponding tissues.

CONCLUSIONS

These results suggest that in proximal tubular cells type II Na/Pi-cotransporters internalized due to the action of PTH and acute high Pi-diet are routed to the lysosomes, and likely do not enter a recycling compartment.

Renal Na/H exchanger NHE-3 and Na-PO4 cotransporter NaPi-2 protein expression in glucocorticoid excess and deficient states

Administration of pharmacologic doses of glucocorticoid in vivo increases renal proximal tubule apical membrane Na/H exchange and decreases Na/PO4 cotransport activity (1). Current data suggest that the NHE-3 and NaPi-2 proteins mediate significant fractions of proximal tubule apical membrane Na/H exchange and Na/PO4 cotransport, respectively. This study examines whether glucocorticoid excess or deficiency affects NHE-3 and NaPi-2 protein abundance and the intrarenal distribution of these transporters. Protein abundance of NHE-3 and NaPi-2 in control rats was compared to rats rendered glucocorticoid-deficient by bilateral adrenalectomy, and to rats receiving pharmacologic doses of dexamethasone using immunoblots and immunohistochemistry. Adrenalectomy had modest effects on NHE-3 protein abundance, but dexamethasone administration to either adrenalectomized or sham-operated rats significantly increased NHE-3 protein abundance in both the proximal tubule and thick ascending limb, but not the thin descending limb. Adrenalectomy increased NaPi-2 protein abundance in the proximal tubule, whereas dexamethasone administration dramatically suppressed NaPi-2 protein on the apical membrane in both adrenalectomized and sham-operated animals. No significant reciprocal increase in subapical NaPi-2 staining was seen in the dexamethasone-treated rats. The present study shows that glucocorticoids regulate proximal tubule apical membrane Na/H exchange and NaPi cotransport by changes in protein abundance of NHE-3 and NaPi-2, respectively.

Dietary sulfate regulates the expression of the renal brush border Na/Si cotransporter NaSi-1

Dietary inorganic sulfate (Si) intake is an important factor in the regulation of renal proximal tubular sodium-dependent Si transport (Na/Si cotransport). The purpose of the present study was to determine whether modulation of Na/Si cotransport activity by dietary Si is mediated through regulation of the renal expression of the recently cloned NaSi-1 protein located in the apical brush border membrane (BBM) of the proximal tubule. It was found that rats fed a high Si diet had a marked increase in the renal excretion of Si and a concomitant decrease in BBM Na/Si cotransport activity when compared with rats on a control Si diet. The 43% decrease in BBM Na/Si cotransport activity was associated with a 33% decrease in BBM NaSi-1 protein abundance, as determined by Western blotting, and a 2.7-fold decrease in cortical NaSi-1 mRNA abundance, as determined by Northern blotting. Furthermore, cortical mRNA from rats fed a high Si diet when injected into Xenopus laevis oocytes led to a 2.2-fold decrease in Na/Si cotransport activity compared with mRNA isolated from control Si diet rats. This study indicates that adaptation to a high Si diet is accompanied by a decrease in renal cortical NaSi-1 mRNA abundance, which results in reduced expression of the NaSi-1 protein at the level of the proximal tubular BBM.

[Molecular physiology and pathophysiology of renal phosphate excretion]

The kidney is a key organ in phosphate homeostasis. Phosphate excretion is largely determined by free glomerular filtration and by regulated reabsorption in the proximal tubule. The cellular/molecular mechanisms involved in phosphate reabsorption have been elucidated in great detail over the past few years. A brush border membrane protein with most probably 8 membrane-spanning regions represents the rate-limiting and physiologically/pathophysiologically modified transport mechanism. Altered phosphate reabsorption correlates with an altered brush border membrane transporter protein content, altered either by new synthesis/membrane insertion or by membrane retrieval/degradation. Current knowledge on the molecular/cellular level is a prerequisite for an understanding of kidney based alterations in phosphate homoeostasis.

A PDZ domain-containing protein with homology to Diphor-1 maps to human chromosome 1q21

The novel gene, Diphor-1, was recently cloned from rat kidney and shown to increase phosphate uptake in cells when co-expressed with a Na(+)-Pi cotransporter, indicating that it may play a substantial role in cellular phosphate balance. Previously, the phosphate wasting disorder, autosomal dominant hypophosphatemic rickets (ADHR) was mapped to chromosome 12p13 by linkage analysis. In the present work, PDZK1, a PDZ domain-containing protein highly homologous to rat Diphor-1, was shown to be expressed in human kidney. Based upon its sequence similarity to rat Diphor-1, we considered PDZK1 a feasible candidate gene for ADHR. PDZK1 was found to localize to human chromosome 1q21, thereby ruling it out as a candidate for ADHR.

Immunolocalization of sat-1 sulfate/oxalate/bicarbonate anion exchanger in the rat kidney

The rat liver sulfate/bicarbonate/oxalate exchanger (sat-1) transports sulfate across the canalicular membrane in exchange for either bicarbonate or oxalate. Sulfate/oxalate exchange has been detected in the proximal tubule of the kidney, where it is probably involved in the reabsorption of filtered sulfate and the secretion of oxalate and may contribute to oxalate-dependent chloride reabsorption. Screening of a renal cortex cDNA library determined that sat-1 is expressed in the rat kidney. To evaluate this anion exchanger, the sat-1 protein was expressed in Sf9 cells. Sodium-independent sulfate and oxalate uptake was enhanced 7.3-fold and 13.1-fold, respectively, in Sf9 cells expressing the sat-1 protein compared with cells infected with wild-type virus. We determined that sat-1 is glycosylated in the kidney; however, anion exchange via sat-1 is observed despite incomplete glycosylation of sat-1 in Sf9 cells. The sat-1 protein, with an added COOH-terminal 6-histidine tag, was purified on a metal affinity column and used to generate anti-sat-1 monoclonal antibodies. The sat-1 protein was localized to the basolateral membrane, but not the apical membrane, of the proximal tubule by both Western blot analysis and immunohistochemistry. These studies demonstrate that sulfate/oxalate exchange on the apical and basolateral membranes of the proximal tubule represents transport on two different anion exchangers.

Chloride conductance and Pi transport are separate functions induced by the expression of NaPi-1 in Xenopus oocytes

Expression of the protein NaPi-1 in Xenopus oocytes has previously been shown to induce an outwardly rectifying Cl- conductance (GCl), organic anion transport and Na+-dependent Pi-uptake. In the present study we investigated the relation between the NaPi-1 induced GCl and Pi-induced currents and transport. NaPi-1 expression induced Pi-transport, which was not different at 1-20 ng/oocyte NaPi-1 cRNA injection and was already maximal at 1-2 days after cRNA injection. In contrast, GCl was augmented at increased amounts of cRNA injection (1-20 ng/oocyte) and over a five day expression period. Subsequently all experiments were performed on oocytes injected with 20 ng/oocytes cRNA. Pi-induced currents (Ip) could be observed in NaPi-1 expressing oocytes at high concentrations of Pi (>/= 1 mm Pi). The amplitudes of Ip correlated well with GCl. Ip was blocked by the Cl- channel blocker NPPB, partially Na+-dependent and completely abolished in Cl- free solution. In contrast, Pi-transport in NaPi-1 expressing oocytes was not NPPB sensitive, stronger depending on extracellular Na+ and weakly affected by Cl- substitution. Endogenous Pi-uptake in water-injected oocytes amounted in all experiments to 30-50% of the Na+-dependent Pi-transport observed in NaPi-1 expressing oocytes. The properties of the endogenous Pi-uptake system (Km for Pi > 1 mM; partial Na+- and Cl--dependence; lack of NPPB block) were similar to the NaPi-1 induced Pi-uptake, but no Ip could be recorded at Pi-concentrations </=3 mM. In summary, the present data suggest that Ip does not reflect charge transfer related to Pi-uptake, but a Pi-mediated modulation of GCl.

The voltage dependence of a cloned mammalian renal type II Na+/Pi cotransporter (NaPi-2)

The voltage dependence of the rat renal type II Na+/Pi cotransporter (NaPi-2) was investigated by expressing NaPi-2 in Xenopus laevis oocytes and applying the two-electrode voltage clamp. In the steady state, superfusion with inorganic phosphate (Pi) induced inward currents (Ip) in the presence of 96 mM Na+ over the potential range -140 </= V </= +40 mV. With Pi as the variable substrate, the apparent affinity constant (KmPi) was strongly dependent on Na+, increasing sixfold for a twofold reduction in external Na+. KmPi increased with depolarizing voltage and was more sensitive to voltage at reduced Na+. The Hill coefficient was close to unity and the predicted maximum Ip (Ipmax) was 40% smaller at 50 mM Na+. With Na+ as the variable substrate, KmNa was weakly dependent on both Pi and voltage, the Hill coefficient was close to 3 and Ipmax was independent of Pi at -50 mV. The competitive inhibitor phosphonoformic acid suppressed the steady state holding current in a Na+-dependent manner, indicating the existence of uncoupled Na+ slippage. Voltage steps induced pre-steady state relaxations typical for Na+-coupled cotransporters. NaPi-2-dependent relaxations were quantitated by a single, voltage-dependent exponential. At 96 mM Na+, a Boltzmann function was fit to the steady state charge distribution (Q-V) to give a midpoint voltage (V0.5) in the range -20 to -50 mV and an apparent valency of approximately 0.5 e-. V0.5 became more negative as Na+ was reduced. Pi suppressed relaxations in a dose-dependent manner, but had little effect on their voltage dependence. Reducing external pH shifted V0.5 to depolarizing potentials and suppressed relaxations in the absence of Na+, suggesting that protons interact with the unloaded carrier. These findings were incorporated into an ordered kinetic model whereby Na+ is the first and last substrate to bind, and the observed voltage dependence arises from the unloaded carrier and first Na+ binding step.

Cellular/molecular control of renal Na/Pi-cotransport

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 renal type II Na+/phosphate cotransporter

A sodium-dependent phosphate transporter (type II Na/Pi-cotransporter) was isolated which is expressed in apical membranes of proximal tubules and exhibits transport characteristics similar as described for renal reabsorption of phosphate. Type II associated Na/Pi-cotransport is electrogenic and results obtained by electrophysiological measurements support a transport model having a stoichiometry of 3 Na+/HPO4=. Changes of transport such as by parathyroid hormone and altered dietary intake of phosphate correlate with changes of the number of type II cotransporters in the apical membrane. These data suggest that the type II Na/Pi-cotransporter represents the main target for physiological and pathophysiological regulation.

Cellular mechanisms involved in the acute adaptation of OK cell Na/Pi-cotransport to high- or low-Pi medium

Variations in dietary phosphate (Pi) intake in rats lead to alterations of renal Pi reabsorption. These effects are associated with corresponding changes in the abundance of the type II Na/Pi-cotransporter protein in proximal tubular brush-border membranes. In the present study we investigated the regulation of the type II Na/Pi-cotransporter in response to high- and low-Pi medium in opossum kidney (OK) cells, an epithelial cell-line of proximal tubular origin. We show that "acute" (4 h) and "chronic" (24 h) exposures of OK cells to high- or low-Pi medium lead to decreases or increases, respectively, in Na/Pi-cotransport activity which are paralleled by alterations in the total cellular amount of the corresponding type II Na/Pi-cotransporter protein (NaPi-4), but not by changes in the amount of the NaPi-4 mRNA. Also in OK cells transfected with the corresponding rat renal type II Na/Pi-cotransporter (NaPi-2) alterations in the Pi concentration in the medium lead to changes in the amount of NaPi-2 protein but not in the amount of NaPi-2 mRNA. Furthermore we show that lysosomal inhibitors prevent the degradation of the transporter, but do not interfere with its inhibition, in response to "acute" exposure of OK cells to high-Pi medium. Inhibition of lysosomal degradation also leads, in control conditions, to an accumulation of the transporter detectable on Western blot. It is concluded that the lysosomal proteolytic pathway is not only involved in the Pi-induced downregulation of the type II Na/Pi-cotransporter but also in its basic turnover.

Parathyroid hormone leads to the lysosomal degradation of the renal type II Na/Pi cotransporter

We have studied the involvement of proteolytic pathways in the regulation of the Na/Pi cotransporter type II by parathyroid hormone (PTH) in opossum kidney cells. Inhibition of lysosomal degradation (by leupeptin, ammonium chloride, methylamine, chloroquine, L-methionine methyl ester) prevented the PTH-mediated degradation of the transporter, whereas inhibition of the proteasomal pathway (by lactacystin) did not. Moreover it was found (i) that whereas lysosomal inhibitors prevented the PTH-mediated degradation of the transporter they did not prevent the PTH-mediated inhibition of the Na/Pi cotransport and (ii) that treating opossum kidney cells with lysosomal inhibitors led to an increased expression of the transporter without any concomitant increase in the Na/Pi cotransport. Further analysis by subcellular fractionation and morphological techniques showed (i) that the Na/Pi cotransporter is constitutively transported to and degraded within late endosomes/lysosomes and (ii) that PTH leads to the increased degradation of the transporter in late endosomes/lysosomes.

Functional expression and purification of histidine-tagged rat renal Na/Phosphate (NaPi-2) and Na/Sulfate (NaSi-1) cotransporters

Two mammalian sodium-dependent anion-cotransporters (NaPi-2 for phosphate and NaSi-1 for sulfate) have been expressed in Sf9 insect cells using the baculovirus expression system. A histidine tag was introduced at the C-termini in order to facilitate purification by metal-affinity chromatography. Sf9 cells infected with the histidine-tagged Ni/Pi-cotransporter exhibited more than 60-fold higher sodium-dependent transport of phosphate compared to noninfected cells. Expressed Na/Pi-cotransport exhibited a Km of Pi of 0.21 mm and an apparent Km of sodium of 92 mm. Infected cells expressed a 65 kDa polypeptide as detected by Western blotting and immunoprecipitation. Sf9 cells infected with the histidine-tagged NaSi-1 or untagged NaSi-1 protein expressed sodium-dependent sulfate cotransport up to 60-fold higher compared to noninfected cells. Transport of sulfate was highly dependent on sodium exhibiting a Km of SO2-4 of about 0.3-0.4 mm and a Km of sodium of 55 mm. By Western blotting and immunoprecipitation expressed NaSi-1 proteins were detected at 55-60 kDa. These studies demonstrate that histidine tagged proximal tubular Na-dependent cotransporters for phosphate and sulfate can be expressed functionally in Sf9 cells and that the kinetic characteristics were not altered by the introduction of a histidine tag at the C-termini. Furthermore, it is demonstrated that after solubilization under denaturing conditions histidine-tagged cotransporter proteins can be purified by metal-chelate affinity chromatography.

Electrophysiological characterization of the flounder type II Na+/Pi cotransporter (NaPi-5) expressed in Xenopus laevis oocytes

The two electrode voltage clamp technique was used to investigate the steady-state and presteady-state kinetic properties of the type II Na+/Pi cotransporter NaPi-5, cloned from the kidney of winter flounder (Pseudopleuronectes americanus) and expressed in Xenopus laevis oocytes. Steady-state Pi-induced currents had a voltage-independent apparent K(m) for Pi of 0.03 mM and a Hill coefficient of 1.0 at neutral pH, when superfusing with 96 mM Na+. The apparent K(m) for Na+ at 1 mM Pi was strongly voltage dependent (increasing from 32 mM at -70 mV to 77 mM at -30 mV) and the Hill coefficient was between 1 and 2, indicating cooperative binding of more than one Na+ ion. The maximum steady-state current was pH dependent, diminishing by 50% or more for a change from pH 7.8 to pH 6.3. Voltage jumps elicited presteady-state relaxations in the presence of 96 mM Na+ which were suppressed at saturating Pi (1 mM). Relaxations were absent in non-injected oocytes. Charge was balanced for equal positive and negative steps, saturated at extremes of potential and reversed at the holding potential. Fitting the charge transfer to a Boltzmann relationship typically gave a midpoint voltage (V0.5) close to zero and an apparent valency of approximately 0.6. The maximum steady-state transport rate correlated linearly with the maximum Pi-suppressed charge movement, indicating that the relaxations were NaPi-5-specific. The apparent transporter turnover was estimated as 35 sec-1. The voltage dependence of the relaxations was Pi-independent, whereas changes in Na+ shifted V0.5 to -60 mV at 25 mM Na+. Protons suppressed relaxations but contributed to no detectable charge movement in zero external Na+. The voltage dependent presteady-state behavior of NaPi-5 could be described by a 3 state model in which the partial reactions involving reorientation of the unloaded carrier and binding of Na+ contribute to transmembrane charge movement.

Identification of a new gene product (diphor-1) regulated by dietary phosphate

Chronic restriction of dietary Pi elicits an increased reabsorption of Pi in the kidney proximal tubules, which involves a stimulation of apical Na-Pi cotransport. This adaptation is in part a direct cellular response of which the mechanism(s) are poorly understood. In this study, the impact of dietary Pi restriction on the differential expression of rat kidney cortex mRNAs was visualized to identify gene products regulated by the Pi status. When kidney cortex mRNAs of rats fed a low- or a high-Pi diet were compared by differential display-polymerase chain reaction (DD-PCR), thirty modulated cDNA bands were observed, of which four were confirmed as being regulated. We focused on one of the upregulated bands, dietary Pi-regulated RNA-1 (diphor-1). A cDNA containing an open reading frame encoding a 52-kDa protein was cloned by library screening. Diphor-1 exhibits a high degree of identity to the Na/H exchanger regulatory factor and to a tyrosine kinase activating protein. Highest expression of diphor-1 mRNA was detected in the kidney (proximal tubules) and in small intestine. Expression experiments showed that diphor-1 specifically increases Na-Pi cotransport in oocytes of Xenopus laevis coinjected with renal type II Na-Pi contransporter cRNA. Further characterizations of diphor-1 will show whether diphor-1 is primarily or secondarily involved in the response to dietary Pi.

Abnormal sulfate metabolism in vitamin D-deficient rats

To explore the possibility that vitamin D status regulates sulfate homeostasis, plasma sulfate levels, renal sulfate excretion, and the expression of the renal Na-SO4 cotransporter were evaluated in vitamin D-deficient (D-D-) rats and in D-D- rats rendered normocalcemic by either vitamin D or calcium/lactose supplementation. D-D- rats had significantly lower plasma sulfate levels than control animals (0.93+/-0.01 and 1.15+/-0.05 mM, respectively, P < 0.05), and fractional sulfate renal excretion was approximately threefold higher comparing D-D- and control rats. A decrease in renal cortical brush border membrane Na-SO4 cotransport activity, associated with a parallel decrease in both renal Na-SO4 cotransport protein and mRNA content (78+/-3 and 73+/-3% decreases, respectively, compared with control values), was also observed in D-D- rats. Vitamin D supplementation resulted in a return to normal of plasma sulfate, fractional sulfate excretion, and both renal Na-SO4 cotransport mRNA and protein. In contrast, renal sulfate excretion and renal Na-SO4 cotransport activity, protein abundance, and mRNA remained decreased in vitamin D-depleted rats fed a diet supplemented with lactose and calcium, despite that these rats were normocalcemic, and had significantly lower levels of parathyroid hormone and 25(OH)- and 1,25(OH)2-vitamin D levels than the vitamin D-supplemented groups. These results demonstrate that vitamin D modulates renal Na-SO4 sulfate cotransport and sulfate homeostasis. The ability of vitamin D status to regulate Na-SO4 cotransport appears to be a direct effect, and is not mediated by the effects of vitamin D on plasma calcium or parathyroid hormone levels. Because sulfate is required for synthesis of essential matrix components, abnormal sulfate metabolism in vitamin D-deficient animals may contribute to producing some of the abnormalities observed in rickets and osteomalacia.

Differentiation and cell polarity during renal cyst formation in the Han:SPRD (cy/+) rat, a model for ADPKD

Despite the recent positional cloning of genes responsible for autosomal dominant polycystic kidney disease (ADPKD), the exact pathogenetic mechanisms underlying this disorder are still unclear. To learn more about cyst formation, we investigated cell differentiation and cell polarity in the Han:SPRD (cy/+) rat between 21 days and 60 wk of age. At early stages of cyst development, alkaline phosphatase, aquaporin-1, NaSi-1 cotransporter, and Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) were expressed normally. Clusterin mRNA was only sparsely expressed at the onset of cystic degeneration and increased thereafter, being highest in noncystic nephron segments. In cyst wall cells, clusterin on the one hand and alkaline phosphatase, aquaporin-1, NaSi-1-cotransporter, and Na(+)-K(+)-ATPase on the other were expressed in a mutually exclusive fashion. No change in cell polarity could be observed at any stage. Our data therefore argue against a change in cell polarity and against an early arrest in normal tubular development during cyst formation in the Han:SPRD (cy/+) rat model of ADPKD but favor the hypothesis that tubular epithelia develop in an orderly fashion and degenerate thereafter.

Acid-induced stimulation of Na-Pi cotransport in OK cells: molecular characterization and effect of dexamethasone

Alterations in systemic acid/base balance affect renal Pi excretion. In the present study, the effects of an acidic pH on apical Na-dependent Pi (Na-Pi) cotransport were analyzed using OK cells (opossum kidney cell line). Cells were maintained at either pH 7.4 or 7.1 (altered HCO3- concentration at constant PCO2). Incubation in acidic medium led to an increase in Na-Pi cotransport activity, which was characterized by a transient, initial response (2-4 h, 25% increase) followed by a sustained response (24 h, 75% increase). Increased Na-Pi cotransport activity (24 h) was sensitive to inhibition by parathyroid hormone. Actinomycin D did not abolish the acid-induced increases (initial and sustained responses). Cycloheximide abolished the increase in Na-Pi cotransport observed after 24 h. The increase in Na-Pi cotransport (24 h) was prevented by dexamethasone (2 x 10(-6) M). Western blots showed a twofold (3 h) and two- to threefold (24 h) increase in NaPi-4 protein after acid exposure. Cycloheximide prevented the late increase in NaPi-4 protein abundance. Also dexamethasone reduced the increase in specific protein content. In conclusion, the exposure of OK cells to an acidic medium causes a stimulation of the NaPi-4 cotransporter that is prevented by dexamethasone.

Parathyroid hormone-dependent degradation of type II Na+/Pi cotransporters

Parathyroid hormone (PTH) inhibits proximal tubular brush border membrane Na+/Pi cotransport activity; this decrease in the transport activity was found to be associated with a decrease in type II Na+/Pi cotransporter protein content in rat brush border membranes. In the present study we investigated the PTH-dependent regulation of the type II Na+/Pi cotransporter in opossum kidney cells, a previously established model to study cellular mechanisms involved in the regulation of proximal tubular Na+/Pi cotransport. We transfected opossum kidney cells with a cDNA coding for NaPi-2 (rat renal type II Na+/Pi cotransporter). This allowed the study of PTH-dependent regulation of the transfected NaPi-2 and of the corresponding intrinsic cotransporter (NaPi-4). The results show (i) that the intrinsic and the transfected cotransporters are functionally (transport) and morphologically (immunofluorescence) localized at the apical membrane, (ii) that the intrinsic as well as the transfected Na+/Pi cotransport activities are inhibited by PTH, (iii) that PTH leads to a retrieval of both cotransporters from the apical membrane, (iv) that both cotransporters are rapidly degraded in response to PTH, and (v) that the reappearance/recovery of type II Na+/Pi cotransporter protein and function from PTH inhibition requires de novo protein synthesis. These results document that PTH leads to a removal of type II Na+/Pi cotransporters from the apical membrane and to their subsequent degradation.

Structure-permeation relations of met-enkephalin peptide analogues on absorption and secretion mechanisms in Caco-2 monolayers

Due to the low effective permeabilities of peptides at many absorption sites, their structure-permeation relations are of high interest. In this work structure-permeation relations of Met-enkephalin analogues are presented using confluent Caco-2 cells as an in vitro permeation model. Four model peptides (Met-enkephalin, [D-Ala2]Met-enkephalin, [D-Ala2]Met-enkephalinamide, and metkephamid) were tested in terms of permeability, lipophilicity, charge, and molecular size. Permeability coefficients (P(eff)) across Caco-2 cells were low, 3.3 x 10(-8) to 9.5 x 10(-8) cm s-1, and were similar to typical paracellular markers. No correlation of permeability and the log(apparent octanol/buffer partition coefficient) was observed. A 40-fold increase of the permeability of metkephamid in the presence of 10 mM EDTA suggested a significant contribution of paracellular transport. Independent support for this conclusion was obtained by visualizing the pathway of the fluorescein isocyanate isomer I 1-metkephamid by confocal laser scanning microscopy (CLSM). The fluorophore-labeled peptide was observed in the intercallular space only. Metkephamid permeabilities were found to be direction-specific. Permeabilities from basolateral to apical (b-to-a) were significantly higher (ca. 4-fold) than in the opposite (a-to-b) direction. The addition of verapamil equalized the permeabilities in the a-to-b and b-to-a directions, suggesting the involvement of a P-glycoprotein-mediated secretion mechanism. Similar observations were obtained with [D-Ala2]Met-enkephalinamide, but not with Met-enkephalin and [D-Ala2]Met-enkephalin. In contrast to the other analogues, metkephamid and [D-Ala2]Met-enkephalinamide are positively charged at neutral pH, as demonstrated by their isoelectric points (pl = 8.6 for [D-Ala2]Met-enkephalinamide and metkephamid and 5.3 for [D-Ala2]Met-enkephalin and Met-enkephalin). The data is in agreement with the literature showing that most compounds secreted by the P-glycoprotein transporter carry a positive charge.

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).

Role of microtubules in the rapid regulation of renal phosphate transport in response to acute alterations in dietary phosphate content

Renal proximal tubular response to acute administration of a low Pi diet is characterized by a rapid adaptive increase in apical brush border membrane (BBM) Na-Pi cotransport activity and Na-Pi cotransporter protein abundance, independent of a change in Na-Pi cotransporter mRNA levels (Levi, M., M. Lötscher, V. Sorribas, M. Custer, M. Arar, B. Kaissling, H. Murer, and J. Biber. 1994. Am. J. Physiol. 267: F900-F908). The purposes of the present study were to determine if the acute adaptive response occurs independent of de novo protein synthesis, and if microtubules play a role in the rapid upregulation of the Na-Pi cotransporters at the apical BBM. We found that inhibition of transcription by actinomycin D and translation by cycloheximide did not prevent the rapid adaptive response. In addition, in spite of a 3.3-fold increase in apical BBM Na-Pi cotransporter protein abundance, there was no change in cortical homogenate Na-Pi cotransporter protein abundance. Pretreatment with colchicine, which resulted in almost complete disruption of the microtubular network, abolished the adaptive increases in BBM Na-Pi cotransport activity and Na-Pi cotransporter protein abundance. In contrast, colchicine had no effect on the rapid downregulation of Na-Pi cotransport in response to acute administration of a high Pi diet. We conclude that the rapid adaptive increase in renal proximal tubular apical BBM Na-Pi cotransport activity and Na-Pi cotransporter abundance is independent of de novo protein synthesis, and is mediated by microtubule-dependent translocation of presynthesized Na-Pi cotransporter protein to the apical BBM.

Renal endosomal phosphate (Pi) transport in normal and diabetic rats and response to chronic Pi deprivation

Chronic renal adaptation to dietary deprivation of Pi is accompanied by increased Na+/Pi co-transport across the brush border membrane of the renal proximal tubule. The increased activity of this co-transport system depends on de novo protein synthesis and insulin. The present study used normal and diabetic rats to determine if the endosomal pool of Na+/Pi co-transporters was altered by Pi deprivation and the possible role of insulin. In response to 5 days of dietary Pi deprivation there was a significant increase in endosomal Na+/Pi co-transport in control rats but there was no change in diabetic rats. The increase in endosomal Pi uptake was restored in diabetic rats treated with exogenous insulin. Na(+)-independent Pi uptake and proline uptake remained unchanged in all groups. The changes in endosomal Na+/Pi co-transport correlated with the abundance of the specific Na+/Pi co-transporter protein, as determined by Western blots. The pattern of endosomal changes paralleled that observed in brush border membranes. One possibility consistent with these findings is that the endosomal fraction contains newly synthesized Na+/Pi co-transporters targeted for delivery to the apical brush border membrane. Increased synthesis and delivery is required to maintain the adaptation to chronic Pi deprivation.

Identification of a cDNA/protein leading to an increased Pi-uptake in Xenopus laevis oocytes

In a previous report we documented an increased Na(+)-dependent transport of inorganic phosphate (P(i)) in Xenopus laevis oocytes injected with mRNA isolated from rabbit duodenum (Yagci et al., Pfluegers Arch. 422:211-216, 1992; ref 24). In the present study we have used expression cloning in oocytes to search for the cDNA/mRNA involved in this effect. The identified cDNA (provisionally named PiUS; for P(i)-uptake stimulator) lead to a 3-4-fold stimulation of Na(+)-dependent P(i)-uptake (10ng cRNA injected, 3-5 days of expression). Na(+)-independent uptake of P(i) was also affected but transport of sulphate and L-arginine (in the presence or absence of sodium) remained unchanged. The apparent K(m)-values for the induced Na(+)-dependent uptake were 0.26 +/- 0.04 mM for P(i) and 14.8 +/- 3.0 mM for Na+. The 1796 bp cDNA codes for a protein of 425 amino acids. Hydropathy analysis suggests a lack of transmembrane segments. In vitro translation resulted in a protein of 60 kDa and provided no evidence of glycosylation. In Northern blots a mRNA of approximately 2 kb was recognized in various tissues including different intestinal segments, kidney cortex, kidney medulla, liver and heart. Homology searches showed no similarity to proteins involved in membrane transport and its control. In conclusion, we have cloned from a rabbit small intestinal cDNA library a novel cDNA encoding a protein stimulating P(i)-uptake into Xenopus laevis oocytes, but which is not a P(i)-transporter itself.