Parathyroid hormone-induced alterations of protein content and phosphorylation in enriched apical membranes of opossum kidney cells

Expression cloning human and rat renal cortex Na/Pi cotransporters: behind the scenes in the Murer laboratory

In the pre-genomic era, the cloning of a cDNA represented a significant achievement, particularly if the gene of interest encoded a membrane protein. At the time, molecular probes such as partial peptide sequences, suitable nucleic acid sequences, or antibodies were unavailable for most proteins and the “sodium-phosphate transporter” was no exception. In contrast, brush-border membrane vesicles and epithelial cell culture experiments had established a reliable set of functional hallmarks that described Na-dependent phosphate transport activity in some detail. Moreover, aspects of hormonal regulation of phosphate homeostasis could be recapitulated in these model systems. Expression cloning elegantly combined functional protein expression in Xenopus laevis oocytes with molecular biology to overcome the lack of molecular probes.

An apical expression signal of the renal type IIc Na+-dependent phosphate cotransporter in renal epithelial cells

The type IIc Na(+)-dependent phosphate cotransporter (NaPi-IIc) is specifically targeted to, and expressed on, the apical membrane of renal proximal tubular cells and mediates phosphate transport. In the present study, we investigated the signals that determine apical expression of NaPi-IIc with a focus on the role of the N- and the C-terminal tails of mouse NaPi-IIc in renal epithelial cells [opossum kidney (OK) and Madin-Darby canine kidney cells]. Wild-type NaPi-IIc, the cotransporter NaPi-IIa, as well as several IIa-IIc chimeras and deletion mutants, were fused to enhanced green fluorescent protein (EGFP), and their cellular localization was analyzed in polarized renal epithelial cells by confocal microscopy and by cell-surface biotinylation. Fluorescent EGFP-fused NaPi-IIc transporter proteins are correctly expressed in the apical membrane of OK cells. The apical expression of N-terminal deletion mutants (deletion of N-terminal 25, 50, or 69 amino acids) was not affected by truncation. In contrast, C-terminal deletion mutants (deletion of C-terminal 45, 50, or 62 amino acids) did not have correct apical expression. A more detailed mutational analysis indicated that a domain (amino acids WLHSL) in the cytoplasmic C terminus is required for apical expression of NaPi-IIc in renal epithelial cells. We conclude that targeting of NaPi-IIc to the apical cell surface is regulated by a unique amino acid motif in the cytoplasmic C-terminal domain.

PTH-induced internalization of apical membrane NaPi2a: role of actin and myosin VI

Parathyroid hormone (PTH) plays a critical role in the regulation of renal phosphorous homeostasis by altering the levels of the sodium-phosphate cotransporter NaPi2a in the brush border membrane (BBM) of renal proximal tubular cells. While details of the molecular events of PTH-induced internalization of NaPi2a are emerging, the precise events governing NaPi2a removal from brush border microvilli in response to PTH remain to be fully determined. Here we use a novel application of total internal reflection fluorescence microscopy to examine how PTH induces movement of NaPi2a out of brush border microvilli in living cells in real time. We show that a dynamic actin cytoskeleton is required for NaPi2a removal from the BBM in response to PTH. In addition, we demonstrate that a myosin motor that has previously been shown to be coregulated with NaPi2a, myosin VI, is necessary for PTH-induced removal of NaPi2a from BBM microvilli.

A dibasic motif involved in parathyroid hormone-induced down-regulation of the type IIa NaPi cotransporter

Type II NaPi cotransporters are expressed in the apical membrane of P(i)-(re)absorbing epithelia: the type IIa in renal proximal tubule and the type IIb in small intestine. Parathyroid hormone (PTH) leads to a retrieval from the apical membrane of the type IIa NaPi cotransporter. The type IIa cotransporter is also expressed in opossum kidney (OK) cells, and its expression is under the control of PTH. In the present study, we identified the molecular "domains" involved in the PTH-induced retrieval of the type IIa NaPi cotransporter. Wild-type mouse type IIa (mIIa) and type IIb (mIIb) as well as several mIIa-mIIb chimeras and site-directed mutants were fused to the enhanced green fluorescent protein and transfected into OK cells. We found that mIIa but not mIIb was internalized and degraded after incubation with 1-34 (or 3-34) PTH. Using chimeras, we found that the N and C termini were not required in this effect, whereas a "domain" located between residues 216 and 658 seemed to be necessary. This region contains two putative intracellular loops with highly conserved sequences between mIIa and mIIb; in the last intracellular loop, two charged amino acids of type IIa (K(503)R(504)) are replaced by uncharged residues in type IIb (N(520)I(521)). We generated two mutants in which these residues were interchanged: mIIaNI and mIIbKR. Similarly to mIIa, the mIIbKR mutant was endocytosed in response to 1-34 PTH; in contrast, mIIaNI behaved as mIIb and was not internalized. In conclusion, a dibasic amino acid motif (K(503)R(504)) located in the last intracellular loop of the type IIa NaPi cotransporter is essential for its PTH-induced retrieval.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Expression of parathyroid hormone receptors in MDCK and LLC-PK1 cells

Parathyroid hormone (PTH) inhibits renal proximal tubular phosphate (Pi) and bicarbonate reabsorption by regulating the activity of apical Na/Pi cotransport and Na/H exchange. Two renal epithelial cell lines ["proximal tubular", LLC-PK1; "distal tubular", Madin-Darby canine kidney, (MDCK) cells] were stably transfected with complementary deoxyribonucleic acids (cDNAs) encoding a cloned PTH receptor in order to examine the polarity of transfected receptor function and whether or not intrinsic Pi transport is regulated by the transfected PTH receptor. The receptors are functionally coupled to the stimulation of adenosine 3':5' cyclic monophosphate (cAMP) production at both cell surfaces in LLC-PK1 cells, whereas this response is primarily limited to the basolateral surface in MDCK cells. Immunocytochemistry suggests an apical and basolateral localization of the transfected PTH receptor in LLC-PK1 cells and only a basolateral localization in MDCK cells. PTH activation of the transfected receptors is not coupled to the regulation of intrinsic Pi transport in either LLC-PK1 or MDCK cells.

Parathyroid hormone inhibits plasma membrane Pi transport without changing endocytic activity in opossum kidney cells

Parathyroid hormone (PTH) inhibits Na(+)-dependent Pi uptake in renal epithelial cells from opossum kidney (OK). This requires an intact endocytic pathway, suggesting that one action of PTH may be to promote endocytic removal of Na+/Pi cotransporters from the cell membrane. The present study tested if PTH, at a dose that inhibited membrane Pi transport, also produced an increase in endocytic activity. Pi transport was measured in isolated plasma membrane vesicles. Endocytosis was measured by allowing cells to take up horseradish peroxidase (HRP) followed by assay of triton-sensitive (latent) HRP activity in subcellular fractions isolated by density gradient centrifugation. Incubation of OK cells with 10(-7) M PTH for 3 h decreased Na+/Pi cotransport by membrane vesicles to 328 +/- 54 pmol/mg/min compared to 448 +/- 67 pmol/mg/min (mean +/- S.E., P < 0.03) in controls. Latent HRP content of endosomal fractions was dependent on the time and temperature used to load cells with HRP and on the concentration of HRP. However, incubation of OK cells with 10(-7) M PTH for either 1 or 3 h produced no change in latent HRP activity. Thus the action of PTH on the Na+/Pi cotransporter in the plasma membrane of OK cells does not require a change in the rate of endocytosis.

Identification of estrogen receptor mRNA and the estrogen modulation of parathyroid hormone-stimulated cyclic AMP accumulation in opossum kidney cells

The opossum kidney (OK) cell was used as a model to test the hypothesis that estrogen directly affects proximal renal tubular epithelial cells. To demonstrate the expression of estrogen receptor in OK cells, we developed an approach using reverse transcription and the polymerase chain reaction. Analysis of the DNA amplified with nested primers revealed the predicted size fragment and restriction enzyme digestion products. To demonstrate the functional effects of estrogen, OK cells at confluence were preincubated in serum-free medium for 7-10 days with or without 17 beta-estradiol. Bovine PTH(1-34) (bPTH(1-34)) then stimulated a dose-dependent intracellular accumulation of cAMP that was maximal after 1 min and then gradually declined. Cyclic AMP in the medium slowly increased over 60 min. Preincubation with 17 beta-estradiol did not affect cell proliferation as measured by total protein content but caused an inhibition of bPTH(1-34)-stimulated intracellular cAMP accumulation that was maximal at 10(-11) M 17 beta-estradiol (71 +/- 3% control, p less than .001). bPTH(1-34) also increased cAMP release into the medium, an effect maximal using 10(-10) M 17 beta-estradiol (118 +/- 3% control, p less than .001). Preincubation with the inactive isomer 17 alpha-estradiol caused no changes in cAMP accumulation or release. Coincubation with the antiestrogen tamoxifen blocked the effects of 17 beta-estradiol. Sodium-dependent phosphate transport was: (1) inhibited by 2-h incubations with 10(-8) or 10(-10) M bPTH(1-34) and not affected by preincubation with 17 beta-estradiol, and (2) not inhibited by a 20-min incubation with 10(-8) M bPTH(1-34) unless cells were preincubated with 10(-8) M 17 beta-estradiol, suggesting that any possible effects of estrogen on phosphate transport are not directly mediated by changes in cAMP. These studies demonstrate the presence of estrogen receptor mRNA in OK cells as well as direct and specific effects of physiologic concentrations of estrogen on cAMP accumulation in these cells. This system may be a good model for further study of estrogen and PTH effects on the kidney.