HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) All Versions of this Article: 293/4/C1272    most recent 00051.2006v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Barac-Nieto, M. Articles by Spitzer, A. Search for Related Content PubMed PubMed Citation Articles by Barac-Nieto, M. Articles by Spitzer, A.

EXTRACELLULAR MATRIX, CELL INTERACTIONS

Cell-matrix interactions modulate transepithelial phosphate transport in P_i-deprived OK cells

¹Department of Physiology, Faculty of Medicine, Kuwait University, Kuwait; ²Department of Pediatrics, Albert Einstein College of Medicine, New York, New York; and ³Department of Medicine, University of Maryland, Baltimore, Maryland

Submitted 5 February 2006 ; accepted in final form 20 July 2007

	ABSTRACT

TOP ABSTRACT METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
In opossum kidney (OK) cells as well as in kidney proximal tubules, P_i depletion increases apical (A) and basolateral (B) Na⁺-dependent P_i cell influxes. In OK cells' monolayers in contrast to proximal tubules, there is no increase in transepithelial P_i transport. This limitation may be due to altered cell-matrix interactions. A and B cell ³²P_i uptakes and transepithelial ³²P_i and [¹⁴C]mannitol fluxes were measured in OK cells grown on uncoated or on Matrigel-coated filter inserts. Cells were exposed overnight to solution of either low (0.25 mM) or high (2.5 mM) P_i. When grown on Matrigel, immunofluorescence of apical NaPi4 (an isoform of the sodium-phosphate cotransporter) transporters increased and A and B ³²P_i uptakes into P_i depleted cells were five and threefold higher than in P_i replete cells (P < 0.001). P_i deprivation resulted in larger increase in A to B (4.6x, P < 0.001) than in B to A (3.5x, P < 0.001) P_i flux and net P_i transport from A to B increased 10-fold (P < 0.001). With P_i depletion increases in B to A (3.4x) and A to B (3.3x) paracellular [¹⁴C]mannitol fluxes were similar, and its net flux was opposite to that of P_i. In cells grown on uncoated filters, transepithelial and paracellular unidirectional and net P_i fluxes decreased or did not change with P_i depletion, despite twofold increases in apical and basolateral P_i cell influxes. In summary, Matrigel-OK cell interactions, particularly in P_i-depleted cells, led to enhanced expression of apical NaPi4 transporters resulting in higher P_i transport rates across cell boundaries; apical P_i readily entered the transcellular transport pool and paracellular fluxes were smaller fractions of transepithelial P_i fluxes. These Matrigel-induced changes led to an increase in net transepithelial apical to basolateral P_i transport.

renal; mineral; transport; paracellular; transcellular; cell compartments

In contrast to what occurs in the proximal tubule in vivo, we (2) and others (12) have shown that in monolayers of kidney proximal tubule-like cells in culture there is no adaptive increment in transepithelial P_i transport despite a rise in A and basolateral (B) Na⁺ gradient-dependent P_i influx into the cells (2, 10). The absence of transepithelial P_i transport in vitro may be the consequence of large paracellular fluxes that may overwhelm and mask any increase in transcellular P_i transport. Indeed, we found that large (60–80% of the P_i flux) fluxes of mannitol occur across monoloyers of OK cells grown on uncoated filters (2).

We have also shown that in OK cells grown on uncoated filters, transcellular P_i fluxes and the intracellular P_i pool were reduced in response to severe P_i deprivation, despite a decrease in paracellular fluxes and increased influx of P_i across A and B cell membranes (1, 2). Effective permeability for cell P_i efflux across B or A cell membranes and the distribution of intracellular P_i among inorganic and organophosphates fractions were unaltered (2). These findings suggest that in severely P_i-deprived OK cells grown on uncoated filters, P_i enters cell compartments where it is not available for transcellular transport. Cell compartments depend on the structure of the cytoskeleton (14), which may be influenced by cell-matrix interactions through kinases and proteins linking the cytoskeleton to the cell membrane (NHERF1-PDZK1-MAP17-Ezrin) (4, 5, 7). Our findings on P_i-depleted (P_i,d) cells stimulated our interest to study transepithelial P_i transport in OK cells grown on a matrix more akin to the natural basement membrane.

	METHODS

TOP ABSTRACT METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
Monolayers of OK cells were grown to confluence in a 5% CO₂-95% air atmosphere at 37°C, on uncoated or Matrigel-coated Cyclopore (polyethylene terephtalate, PET) filter (0.45-µm pore diameter) inserts. Inserts were placed in the wells of plates containing 2 ml DMEM-Ham F12 10% FBS in the compartment (B) to which the basolateral cell membranes were exposed. Two millilters of the same medium were placed in the insert compartment (A) to which the apical cell membranes were exposed (2).

The cells were exposed 16 h to FBS-free Ham F12 solutions containing either low (0.25 mM) or high (2.5 mM) P_i. The cells were then exposed to 2 ml FBS-free Ham-F12 containing 0.25 mM ³²P_i and [¹⁴C]mannitol in the A or the B compartments and to 2 ml FBS-free Ham F12 containing 0.25 mM unlabeled P_i and 0.25 mM mannitol in the opposite compartment. Aliquots (25 µl) were sampled from the latter compartment (B or A, respectively) after 5 or 15, 30, and 60 min to determine transepithelial ³²P_i and [¹⁴C]mannitol fluxes, in triplicate wells.

Uptakes of ³²P_i from either the A or the B compartments into the OK cell monolayers were measured after 5 min exposure to 0.25 mM ³²P_i. Uptake was stopped by aspiration and by three rinsings of the two compartments with ice-cold 100 mM Mg gluconate. The cells were lysed in 2 ml 0.5% Triton X100 in 0.2 N KOH in a plate shaker for 1 h. Aliquots of the lysate were used for Liquid Scintillation counting in 10 ml Aquasol (2). Proteins were measured using the Bio-Rad Coomassie Blue reagent (2) and -globulin in 0.5% Triton 0.2 N KOH as standard.

Fluorescence microscopy. OK cells were grown to confluency on filter inserts uncoated or coated with Matrigel (1:10 in DMEM-F12), exposed for 16 h to 0.25 or 2.5 mM P_i in DMEM-F12, fixed for 20 min with 3% paraformaldehyde in PBS supplemented with Ca²⁺-Mg²⁺, and quenched for 10 min with 20 mM glycine before staining. After being blocked with 10% goat serum in PBS, cells were incubated overnight with COOH-terminal anti-NaPi4 (an isoform of the sodium-phosphate cotransporter) chicken primary antibody and the next day rinsed and incubated with Alexa 488 Fluor-conjugated secondary rabbit antibody (Molecular Probes, Eugene, OR) against chicken IgG/IgY. Samples were mounted in 90% glycerol (Merck, Darmstadt, Germany) and 10% PBS containing 2.5% 1,4-diazabicyclo[2.2.2]octane (Sigma). Imaging was done with a confocal microscope (LSM 410, C. Zeiss, Thornwood, NY) with x63 and x40 oil immersion objectives.

The OK cell line was kindly provided by Drs. Biber and Murer (Zurich, Switzerland). Cell culture supplies, including the PET filter inserts, were obtained from BD, Falcon (Lincoln Park, NJ), or GIBCO-BRL (Grand Island, NY). Radioisotopes were purchased from DuPont New England Nuclear (Boston, MA). Chemicals were procured from Sigma (St. Louis, MO) or Baker (Phillisburg, NJ).The NaPi4 antibody was kindely provided by Dr. Moshe Levy (Denver, CO).

Data were analyzed using paired or unpaired t-tests. A P < 0.05 was considered statistically significant. Values are presented as means ± SE of at least three experiments using in each, triplicate determinations for any given experimental condition.

	RESULTS

TOP ABSTRACT METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
Effects of P_i depletion. In cells grown on Matrigel (Fig. 1A), exposure to 0.25 mM instead of 2.5 mM P_i in the culture medium (P_i depletion) increased transepithelial unidirectional P_i fluxes (A to B by 4.6-fold, P < 0.001 and B to A by 3.5-fold, P < 0.001). Net transepithelial P_i transport was from A to B and increased 10-fold (P < 0.05) (see Table 1). The unidirectional [¹⁴C]mannitol flux (Fig. 1B) from B to A increased 3.4-fold,(P < 0.001) and its A to B flux increased 3.3-fold (P < 0.001). The net mannitol flux increased 4-fold and was from B to A, opposite in direction to the net transepithelial P_i flux (Table 1). Transcellular unidirectional P_i fluxes were estimated by assuming that paracellular P_i fluxes equaled those of mannitol (trancellular = transepithelial – paracellular). On Matrigel, transcellular P_i fluxes increased markedly with P_i depletion: A to B by 10-fold, B to A nearly 4-fold (both P < 0.05). The net transcellular P_i flux was from A to B and was enhanced 8-fold (P < 0.05) (Table 1).

View larger version (17K): [in this window] [in a new window]   Fig. 1. Cumulative Pi (A) and mannitol (B) fluxes in Pi deprived (Pi,d) and Pi (Pi,r) replete opossum kidney (OK) cells grown on Matrigel-coated porous supports. Fluxes of 0.25 mM 32Pi (A) and [14C]mannitol (B) were measured at 5 or 15, 30, and or 60 min. Cumulative fluxes increased linearly with time. Transepithelial fluxes in the apical (A) to basolateral (B) or B to A directions were measured in contiguous wells of the same culture plate. Net fluxes were calculated from the difference between these two unidirectional fluxes. Letters r and d refer to fluxes measured in Pi replete or Pi-deprived OK cells, respectively, as described in METHODS. Measurements were done in triplicate wells. Data points presented are means ± SE of the averages measured in at least three different cell cultures plates. *P < 0.05 comparing flux in Pi,d cells with the same flux measured in Pi,r cells.

View larger version (18K): [in this window] [in a new window]   Fig. 2. Cumulative Pi (A) and mannitol (B) fluxes in Pi,d and Pi,r OK cells grown on uncoated porous supports. Fluxes of 0.25 mM 32Pi (A) and [14C]mannitol (B) were measured at 5 or 15, 30, and or 60 min. Fluxes increased linearly with time. Transepithelial fluxes in the apical (A) to basolateral (B) or B to A directions were measured in contiguous wells of the same culture plate (Fig. 1). Net fluxes were calculated from the difference between these two unidirectional fluxes. Letter r and d refer to fluxes measured in Pi replete or Pi-deprived OK cells, respectively, as described in METHODS. Measurements were done in triplicate wells. Data points presented are means ± SE of the averages measured in at least three different cell cultures plates.*P < 0.05 comparing flux in Pi,d cells with the same flux measured in Pi,r cells.

The largest effects of Matrigel were seen in P_i,d OK cells: P_i,d cells grown on Matrigel-coated compared with uncoated filters had higher fluxes of both P_i and mannitol (Table 1). This was true for the transepithelial unidirectional ³²P_i flux from A to B, which was 10.6-fold higher (P < 0.001); for the transepithelial P_i flux from B to A, which was 7.7-fold higher (P < 0.01) and particularly for the net transepithelial P_i flux, which was 33-fold higher (P < 0.01). The paracellular mannitol fluxes (Table 1) were 3.3-fold (A to B, P < 0.01) and 3.7-fold (B to A, P < 0.001) higher and the net mannitol flux from B to A was 6.7-fold larger (P < 0.05) in P_i,d cells grown on Matrigel than in those grown on uncoated filters (Table 1). The estimated unidirectional transcellular P_i fluxes (Table 1) were 35-fold (transcellular A to B, P < 0.01; transcellular B to A, P < 0.05), and the net transcellular P_i flux was 20-fold higher (P < 0.05) in P_i,d OK cells grown on Matrigel than in those grown on uncoated filters. The increase in A to B net transcellular P_i flux was consequent to a larger Matrigel-associated increment in the A to B (102 nmol·mg^–1·h^–1) than in the B to A (45 nmol·mg^–1·h^–1) unidirectional P_i fluxes (Table 1).

The paracellular unidirectional fluxes (Table 1), represented smaller percentages of the respective transepithelial P_i fluxes in cells grown on Matrigel than in those grown on uncoated filters. Consequently, the estimated transcellular P_i fluxes represented a larger percentage of the transepithelial P_i fluxes in cells grown on Matrigel than in those grown on uncoated filters (Table 1).

Apical and basolateral P_i influx rates. A and B ³²P_i (0.25 mM, 4,000 dpm/µl) influx rates into the cells (C) were higher in P_i,d compared with P_i,r cells (Table 1): In P_i,d cells grown on Matrigel, A to C and B to C P_i influxes were 5- and 3.6-fold higher, respectively, than in those in P_i,r cells (both P < 0.001, Table 1). In P_i,d cells grown on uncoated filters A to C and B to C, P_i influx rates were 1.9- (P < 0.01) and 2-fold (P < 0.05) as high as in P_i,r cells (P < 0.01, Table 1).

In P_i,r cells grown on Matrigel and on uncoated filters and in P_i,d cells grown on Matrigel, the rates of apical P_i uptake into cells were similar in magnitude to the respective estimated transcellular A to B P_i fluxes (Table 1). Consequently, in all these conditions, the apical uptake rates set the pace of the transcellular A to B P_i fluxes. By contrast, in P_i,d cells grown on uncoated filters, the A to C P_i uptake rate was 4-fold higher than the estimated transcellular A to B P_i flux (Table 1). In all conditions, basolateral (B to C) P_i uptake into cells was larger than the estimated respective transcellular B to A P_i fluxes (Table 1). Thus the rate of transcellular B to A P_i flux is not set by the basolateral P_i uptake.

Immunofluorescence. Confocal imaging at x63 exhibited (Fig. 3, XY plane at center of each image) abundance of cells labeled by indirect NaPi4 immunofluorescence with a patchy distribution on the cell surface, in P_i,d cells grown on Matrigel but not on those grown on uncoated filters (Fig. 3). Orthogonal sections in the XZ plane (Fig. 3, top insets) and in the YZ plane (Fig. 3, right inset) confirmed the apical localization of the immunofluorescence in the Matrigel grown P_i-deprived cells and its scarcity in cells grown on uncoated filters. Conventional fluorescence imaging at x40 revealed that indirect NaPi4 inmunofluorescence in P_i,d OK cells was 2.5-fold stronger (P < 0.01) in cells grown on Matrigel than in those grown on uncoated filters (see Fig. 3) In turn, P_i,d cells exhibited stronger fluorescence intensity than P_i,r cells grown on uncoated or on Matrigel-coated filters, which hardly exhibited any inmunofluorescence. The enhanced expression of NaPi4 in P_i,d cells grown on Matrigel than in those grown on uncoated filters is congruent with the higher A to C and A to B P_i fluxes observed in these cells.

View larger version (44K): [in this window] [in a new window]   Fig. 3. Confocal images at x63 of NaPi4 indirect immunofluorescence in OK cells grown on Matrigel or on uncoated filters and exposed for 16 h to 0.25 mM Pi in DMEM-F12 medium. Note the expression of NaPi4 immunofluorescence in patches on the apical cell surfaces of cells grown on Matrigel (middle) in contrast to absence of these surface patches in cells growing on uncoated filters. Confocal XZ (top insets) and YZ (right insets) sections confirm the mostly apical localization of the immunofluorescence in cells grown on Matrigel but less so in those growing on uncoated filters. Conventional fluorescence imaging at x40 allowed us to estimate a 2.5-fold higher (P < 0.01) NaPi4 immunofluorescence intensity in cells exposed to 0.25 mM Pi (n = 4) and grown on Matrigel than in those grown on uncoated filters (6,000 ± 700 pixels above threshold/field, n = 5). Immunofluorescence in cells exposed to 2.5 mM Pi was almost totally absent and did not differ in cells grown on Matrigel or on uncoated filters. This was used to set the fluorescence threshold intensity.

Cell-matrix (Matrigel) interactions increased unidirectional and net transepithelial and transcellular P_i fluxes, as well as paracellular mannitol fluxes, in P_i,d OK cells. In P_i,r OK cells, matrix interactions reduced unidirectional P_i transepithelial and paracellular mannitol fluxes but did not significantly alter net transepithelial, transcellular P_i, or paracellular fluxes.

	DISCUSSION

TOP ABSTRACT METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
The major new finding of this study was the large increase in net transepithelial P_i flux from A to B compartments observed in P_i,d OK cells monolayers grown on Matrigel. This net increase was mainly due to a fivefold increase in the influx of P_i across the apical cell membrane and to entry of all P_i crossing the apical membrane into the transcellular P_i transport pool, such that the apical influx rate did not differ from the estimated transcellular A to B P_i flux rate. The large increment in apical P_i influx was associated with enhanced expression of NaPi4 cotransporters at the apical cell surface and was sufficient to result in a net increment in transepithelial P_i transport despite increases in net paracellular flux and unidirectional P_i flux in the opposite direction.

These findings differ from those in cells grown on uncoated filters (2). Under these conditions, the apical P_i uptake also increased with P_i depletion (10), but the change was smaller (2 vs. 5-fold) compared with that in cells growing on Matrigel. Since interactions between cell integrins and matrix can alter the cytoskeleton (7) and interactions between the cytoskeleton and the cell membrane are involved in the upregulation of P_i transport in response to P_i depletion (4), it is not surprising that the magnitude of the upregulatory response was found to be dependent on cell-matrix interactions. The higher P_i fluxes observed in P_i,d OK cells grown on Matrigel correspond with the enhanced expression of NaPi4 cotransporters observed by indirect inmunofluorescence confocal microscopy (Fig. 3).

In P_i,d cells grown on uncoated filters only a fraction (0.25) of P_i entering across the apical cell membrane reached the P_i transport pool and appeared on the B side. The restriction was not at the basolateral cell membrane. Indeed, the unidirectional basolateral P_i efflux (C to B) rate, estimated from the net transcellular P_i transport rate plus the measured basolateral (B to C) P_i uptake rate, was found to exceed by sixfold the magnitude of the transcellular A to B P_i flux . Thus basolateral P_i efflux could not have limited the access of P_i transported across the apical cell membrane to the B compartment. It is more likely that in P_i,d cells grown on uncoated filters, most P_i transported across the apical membrane reaches a P_i pool that is unavailable for transcellular transport, perhaps a subapical pool or another intracellular pool, from which it can backflow to the A compartment or enter other P_i pools. Apparently, changes in cell matrix interactions led to changes in the compartmentalization of the intracellular P_i pools possibly through altered cytoskeletal and membrane-cytoskeletal interactions (7, 14).

Cell-matrix (Matrigel) interactions also led to changes in paracellular (mannitol) fluxes in P_i,r and P_i,d cells. This is consitent with their influence of these interactions on the permeability of tight junctions (13). The effect of cell-matrix interactions on paracellular fluxes was found to depend on the P_i status of the cells. Paracellular fluxes were lower in P_i,r cells (0.7x) but higher (3.5x) in P_i,d cells, when grown on Matrigel compared with uncoated filters. This suggests that signaling initiated by cell-matrix interactions is altered by P_i depletion or repletion. However, the fraction of the transpithelial P_i flux that could be accounted for by paracellular flux in the same direction was smaller (for example, A to B flux, 0.33 vs. 0.68) in cells grown on Matrigel than in cells grown on uncoated filters, independently of their P_i status. This was due to the concomitant increases in transcellular P_i fluxes observed in cells grown on Matrigel (12x), which were particularly large in P_i,d cells (35x) That means that changes in transcellular P_i transport have a larger influence on transepithelial P_i transport in cells grown on Matrigel than in those grown on uncoated filters.

The interaction of cells with Matrigel was associated with larger basolateral P_i influx (B to C) than observed in cells grown on uncoated filters. This suggests that Matrigel-cell interactions promote the expression or the activity of yet to be identified P_i transporters at the B membrane, particularly in P_i,d cells. In all cases, however, the basolateral P_i uptake rates exceeded the transcellular B to A P_i fluxes, indicating that this transcellular flux was not limited by basolateral P_i uptake. Indeed except for P_i,d cells on uncoated filters, the transcellular B to A P_i flux was found to nearly equal the magnitude of the apical P_i backflux [estimated as the difference between the measured apical P_i uptake (A to C) rate and the net transcellular P_i flux]. This suggests that in these cells the pace of transcellular B to A P_i flux was set by the rate of P_i efflux across the apical cell membrane. However, in P_i,d cells grown on uncoated filters, the transcellular B to A P_i flux was lower than the estimated apical P_i efflux suggesting that, in this condition, there is entry of basolateral P_i into pools that do not participate in transcellular P_i transport. Such trapping of P_i in pools that do not exchange with the P_i transport pool reflects intracellular P_i compartmentalization, dependent on the nature of cell-matrix interactions in P_i,d cells. That P_i entering via A or B membranes go into different OK cell compartments was already suggested by the finding that adaptive upregulation of P_i transport in response to P_i depletion only occurs when the depletion is at the apical compartment (10).

Because the B to C P_i uptake rate was always larger than the transcellular B to A P_i flux, it is likely that a large fraction of the basolateral P_i uptake recycles across the basolateral membrane. Indeed, we have shown exchange of basolateral for intracellular P_i and transstimulation of basolateral P_i efflux from OK cells by P_i in the basolateral compartment (3). Exchange of basolateral HPO₄= for cell H₂PO₄- results in net P_i transport across the basolateral membrane if the exchange ratio is not 1. In all conditions described here, there was net transfer of P_i to the basolateral compartment, and the estimated basolateral P_i efflux exceeded the basolateral P_i uptake into the cells. The estimated basolateral P_i exchange ratios in P_i,d and P_i,r cells were 1.7 and 1.3 on Matrigel and 1.2 and 1.7 on uncoated filters, respectively. A high exchange ratio may be necessary, but is not sufficient, for net apical to basolateral transepithelial P_i transport, because in P_i replete cells grown on uncoated filters, the ratio was high but net transepithelial P_i transport was the lowest.

A potentially significant caveat is that, in our discussion, paracellular P_i fluxes have been equated with those of mannitol. This implies that P_i and mannitol have the same permeance across the paracellular pathway. Since size and charge differ between these molecules, the assumption may be inaccurate. If mannitol fluxes overestimate paracellular P_i fluxes, then transcellular fluxes may have been underestimated. However, in most conditions studied (except for P_i,d cells on uncoated filters) the transcellular A to B fluxes were found to closely match the magnitude of the P_i influxes measured across the apical cell membranes. This widely accepted characteristic of renal transepithelial P_i transport (6) lends support to our postulate that paracellular permeability to P_i and mannitol are similar. Since there is no reason to believe that paracellular P_i and mannitol permeabilities only differ in P_i,d cells grown on uncoated filters, we surmize that in cells grown on Matrigel and in P_i,r cells grown on uncoated filters, but not in P_i,d cells grown on uncoated filters, transcellular P_i fluxes are limited by apical membrane fluxes rather than by intracellular compartmentalization of P_i. Furthermore, in no condition basolateral membrane fluxes were found to limit trancellular P_i fluxes.

	GRANTS

TOP ABSTRACT METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
These studies were initiated at the division of Pediatric Nephrology, Albert Einstein College of Medicine and supported by National Institutes of Health Grant RO1 DK-28477. They were completed at Kuwait University, Faculty of Medicine, Dept. of Physiology, with support from Kuwait University Research Administration MY030.

	ACKNOWLEDGMENTS

 
Confocal microscopy was done at Dr. Edward J. Weinman laboratory with the expert help of Deborah Steplock. The technical assistance of Ashley Singleton is greatly valued. We also thank Drs. H. Murer and J. Biber for providing the OK cells and Dr. Moshe Levy for the NaPi4 antibody.

	FOOTNOTES

 

Address for reprint requests and other correspondence: M. Barac-Nieto, Dept. of Physiology, Kuwait Univ., POB 24923, Safat 13110 Kuwait (e-mail: mario{at}hsc.edu.kw)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

	REFERENCES

TOP ABSTRACT METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
1. Barac-Nieto M, Spitzer A. NMR visible intracellular P_i and phosphoesters during adaptive regulation of Na⁺-P_i cotransport in opossum kidney cells. Am J Physiol Cell Physiol 267: C915–C919, 1994.[Abstract/Free Full Text]

2. Barac-Nieto M, Alfred M, Spitzer A. Phosphate depletion in opossum kidney cells: apical but not basolateral or transepithelial adaptions of P_i transport. Exp Nephrol 9: 258–264, 2001.[CrossRef][ISI][Medline]

3. Barac-Nieto M, Alfred M, Spitzer A. Basolateral P_i transport in P_i deprived OK cells. Exp Biol Med (Maywood) 227: 626–631, 2002.[Abstract/Free Full Text]

4. Hernando N, Deliot N, Gisler SM, Lederer E, Weinman EJ, Biber J, Murer H. PDZ-domain interactions and apical expression of type IIa Na/P(i) cotransporters. Proc Natl Acad Sci USA 99: 11957–11962, 2002.[Abstract/Free Full Text]

5. Humphries MJ. Monoclonal antibodies as probes of integrin priming and activation. Biochem Soc Trans 32: 407–411, 2004.[CrossRef][ISI][Medline]

6. Murer H, Hernando N, Forster I, Biber J. Molecular aspects in the regulation of renal inorganic phosphate reabsorption: the type IIa sodium/inorganic phosphate co-transporter as the key player. Curr Opin Nephrol Hypertens 10: 555–561, 2001.[CrossRef][ISI][Medline]

7. Ng T, Parsons M, Hughes WE, Monypenny J, Zicha D, Gautreau A, Arpin M, Gschmeissner S, Verveer PJ, Bastiaens PI, Parker PJ. Ezrin is a downstream effector of trafficking PKC-integrin complexes involved in the control of cell motility. EMBO J 20: 2723–2741, 2001.[CrossRef][ISI][Medline]

8. Quamme GA, Mizgala CL, Wong NLM, Whiting SJ. Effects of intraluminal pH and dietary phosphate on phosphate transport in the proximal convoluted tubule. Am J Physiol Renal Fluid Electrolyte Physiol 249: F759–F768, 1985.[Abstract/Free Full Text]

9. Reshkin SJ, Forgo J, Murer H. Functional asymmetry of phosphate transport and its regulation in opossum kidney cells: phosphate transport. Pflügers Arch 416: 554–560, 1990.[CrossRef][ISI][Medline]

10. Reshkin SJ, Forgo J, Biber J, Murer H. Functional asymmetry of phosphate transport and its regulation in opossum kidney cells: phosphate adaptation. Pflügers Arch 419: 256–262, 1991.[CrossRef][ISI][Medline]

11. Saoncella S, Calautti E, Neveu W, Goetinck PF. Syndecan-4 regulates ATF-2 transcriptional activity in a Rac1-dependent manner. J Biol Chem 279: 47172–47176, 2004.[Abstract/Free Full Text]

12. Scheinman SJ, Reid R, Coulson R, Jones DB, Ford SM. Transepithelial phosphate transport in rabbit proximal tubule cells adapted to phosphate deprivation. Am J Physiol Cell Physiol 266: C1609–C1618, 1994.[Abstract/Free Full Text]

13. Tafazoli F, Holmstrom A, Forsberg A, Magnusson KE. Apically exposed, tight junction-associated beta1-integrins allow binding and YopE-mediated perturbation of epithelial barriers by wild-type Yersinia bacteria. Infect Immun 68: 5335–5343, 2000.[Abstract/Free Full Text]

14. Wozniak MA, Modzelewska K, Kwong L, Keely PJ. Focal adhesion regulation of cell behavior. Biochim Biophys Acta 692: 103–19, 2004.

This Article Abstract Full Text (PDF) All Versions of this Article: 293/4/C1272    most recent 00051.2006v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Barac-Nieto, M. Articles by Spitzer, A. Search for Related Content PubMed PubMed Citation Articles by Barac-Nieto, M. Articles by Spitzer, A.