1 Department of Medicine, University of Cincinnati and 2 Veterans Affairs Medical Center at Cincinnati, Cincinnati Ohio 45267-0585; and 3 Department of Medicine, Northwestern University, Chicago, Illinois 60611
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ABSTRACT |
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The identities of the apical
Cl/base exchangers in kidney proximal tubule and cortical
collecting duct (CCD) cells remain unknown. Pendrin (PDS), which is
expressed at high levels in the thyroid and its mutation causes
Pendred's syndrome, is shown to be an anion exchanger. We investigated
the renal distribution of PDS and its function. Our results demonstrate
that pendrin mRNA expression in the rat kidney is abundant and limited
to the cortex. Proximal tubule suspensions isolated from kidney cortex were highly enriched in pendrin mRNA. Immunoblot analysis studies localized pendrin to cortical brush-border membranes. Nephron segment
RT-PCR localized pendrin mRNA to proximal tubule and CCD. Expression
studies in HEK-293 cells demonstrated that pendrin functions in the
Cl
/OH
,
Cl
/HCO3
, and Cl
/formate
exchange modes. The conclusion is that pendrin is an apical
Cl
/base exchanger in the kidney proximal tubule and CCD
and mediates Cl
/OH
,
Cl
/HCO3
, and Cl
/formate exchange.
proximal tubule; cortical collecting duct; anion exchanger; apical membrane
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INTRODUCTION |
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THE KIDNEY IS THE MAJOR
ORGAN responsible for maintaining electrolyte balance and
acid-base homeostasis in mammals. This is accomplished predominantly by
absorption of NaCl and secretion of acid or base equivalents in
different segments of the nephron (2). The proximal tubule
is responsible for the bulk of Na+, Cl, and
HCO3
reabsorption via an apical
Na+/H+ exchanger and a Cl
/base
exchanger acting in parallel (2, 4, 5, 37). Several functional studies have indicated that the apical Cl
/base
exchanger in kidney proximal tubule likely functions in Cl
/formate exchange mode, with formate converting to
formic acid in the lumen and diffusing back into the cell (2,
5). In
-intercalated cells of the cortical collecting duct
(CCD), an apical Cl
/base exchanger is predominantly
responsible for Cl
reabsorption and
HCO3
secretion and as a result is involved in
acid-base and electrolyte homeostasis (2, 27, 35). Whereas
functional studies have characterized these exchangers, their molecular
identities remain unknown. None of the known AE exchangers (AE-1, -2 or
-3) is found in the apical membrane of kidney nephron segments.
Recent studies have identified a new class of anion exchangers, including those downregulated in adenoma (DRA) and pendrin (PDS) (14, 17, 23, 30, 33). Neither PDS nor DRA are structurally related to the AE (AE-1, -2, and -3) family. Indeed, the homology at the amino acid level between DRA or PDS and AE family members is <15%. (The GenBank accession numbers NP000333, NP003031, and NP005061 were used for AE-1, AE-2, and AE-3, respectively.) PDS is expressed at extremely high levels in the thyroid, where it is thought to be involved in iodine transport across apical membranes of thyroid follicular epithelial cells (14, 26, 30). PDS is also expressed in the inner ear (15), but its function in this organ is unknown. Mutations in PDS cause Pendred's syndrome, an autosomal recessive hereditary disorder characterized by prelingual deafness, goiter, and impaired iodine organification as evidenced by a positive perchlorate test (12-15, 20, 21, 24).
Functional studies in Xenopus oocytes indicate that
PDS transports Cl (9) and can
function in Cl
/formate exchange mode (30).
Pendrin mRNA is found in the whole kidney RNA preparation
(14). However, its renal distribution, membrane
localization, and physiological function have not been characterized.
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EXPERIMENTAL PROCEDURES |
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Proximal Tubular Suspension and Brush-Border and Basolateral Membrane Preparation
Proximal tubular suspensions were prepared with Percoll gradient centrifugation as described (36). Brush-border membrane (BBM) vesicles were isolated from rat kidney cortex by a Ca2+ aggregation method (34, 36). Basolateral membrane (BLM) vesicles were isolated by differential and Percoll gradient centrifugation (34, 36). The purification of BBM vesicles relative to the initial homogenate, as determined by alkaline phosphatase assay, was 8.2 ± 1.1-fold. The purification of BLM vesicles relative to the initial homogenate, as determined by Na+-K+-ATPase assay, was 9.2 ± 1.3-fold.RNA Isolation and Northern Blot Hybridization
Total cellular RNA was extracted from kidney cortex, outer medulla, inner medulla, and proximal tubule suspensions using TriReagent (10), quantitated spectrophotometrically, and stored atRT-PCR of Rat Pendrin
Total RNA was prepared from rat renal cortex, poly(A)+-, selected using Oligotex latex beads (Qiagen) and then reverse transcribed at 47°C using SuperScript II RT (Life Technologies) and oligo(dT) primers. Oligonucleotide primers (5'-CAT TCT GGG GCT GGA CCT C, and 5'-CCT TCG GGA CAT TCA CTT TCA C) were designed based on rat pendrin cDNA (GenBank accession number AF167412). After PCR, the product was gel purified (revealing a single band of 488 bp) and used as a probe for Northern blot hybridizations. Sequence analysis of the PCR product verified the sequence as rat pendrin.Nephron Segment RT-PCR
Single nephron segments (proximal tubule or CCD) were dissected from freshly killed rat kidney at 4-6°C. The dissection medium comprised of (in mM) 140 NaCl, 2.5 K2HPO4, 2 CaCl2, 1.2 MgSO4, 5.5 D-glucose, 1 Na-citrate, 4 Na-lactate, and 6 L-alanine, pH 7.4, and bubbled with 100% O2. Tubule length for proximal straight tubule (S2 segment) was ~0.7 mm and for CCD it was ~0.5 mm. The nephron segments were pooled in a small volume (5-10 µl) of ice-cold phosphate-buffered saline (PBS), three or four segments per pool. The tubules were centrifuged at 12,000 g for 1 min at room temperature, and the PBS was removed and replaced with 10 µl of a tubule lysis solution consisting of 0.9% Triton X-100, 5 mM dithiothreitol (DTT), and 1 U/µl rRNasin (Promega). After 5 min on ice the tubules were agitated gently by tapping the tube, and 1 µl (0.5 µg) oligo(dT) primer, 1 µl H2O, 4 µl 5× reverse transcription buffer, 2 µl DTT (0.1 M), and 1 µl dNTP (10 mM each) were added. The reaction was equilibrated to 42°C for 2 min, and 1 µl SuperScript II RT (Life Technologies) was added, mixed, and incubated for 1 h at 42°C. After reverse transcription, 30 µl of TE (10 mM Tris/ClAmplification of the pendrin cDNA by the PCR was performed using parameters previously established using renal cortex RNA (see RT-PCR of Rat Pendrin). Briefly, each PCR contained 10 µl cDNA, 5 µl 10× PCR buffer (with 20 mM MgCl2), 1 µl 10 mM dNTP, 10 pmol each primer, and 2.5 units Taq DNA polymerase in a final volume of 50 µl. Cycling parameters were 95°C, 45 s; 47°C, 45 s; and 72°C, 2 min.
Antibody Generation and Immunoblot Analysis
Two peptides corresponding to amino acids 734 to 752 (KSREGQDSLLETVARIRDC) and amino acids 36 to 54 (RERRLPERRTRLDSLARSC) of mouse PDS (GenBank accession number AF136751) were synthesized and used for antibody generation. The preimmune and immune sera of the third bleed were purified by IgG purification kit (Sigma) and used for immunoblot analysis. BBM and BLM from rat kidney cortex and microsomal membranes from tranfected HEK-293 cells were resolved by SDS-PAGE (5 µg/lane) and transferred to nitrocellulose membrane. The membrane was blocked with 5% milk proteins and then incubated for 6 h with 40 µl of PDS-immune serum diluted at 1:1,000. The secondary antibody was a donkey anti-rabbit IgG conjugated to horseradish peroxidase (Pierce). The site of antigen-antibody complexation on the nitrocellulose membranes was visualized using chemiluminescence method (SuperSignal Substrate, Pierce) and captured on light-sensitive imaging film (Kodak).Cloning of Pendrin cDNA
The human wild-type PDS cDNA was generated by RT-PCR using total RNA from normal human thyroid tissue and Pfu polymerase (Stratagene, LaJolla, CA). Primer sequences were based on the sequence published by Everett et al. (14). The sense primer was 5'-CGCGAGCAGAGACAGGTCATGGCAGCG-3' and the antisense primer was 5'-TCTGGATCCCGGATGCAAGTGTACGCATAGCCTC-3'. The primers contained appropriate linkers for subcloning the PDS cDNA into the XhoI and BamH I sites of pSVL (Amersham Pharmacia, Piscataway, NJ).Stable Transfection With the Cloned Pendrin cDNA
For stable transfection in HEK-293 cells, the PDS cDNA was released from the pSVL vector by digestion with Xho1 and BamH I, blunt ended, and subcloned into a pCIneo vector. HEK-293 cells, grown in 60-mm dishes, were transfected with 8 µg of the full length PDS cDNA construct according to established methods (9). Transfected cells were selected by the resistance to G418. The cells were grown in DMEM supplemented with 10% fetal calf serum, 50 U/ml penicillin, and 200 µg/ml of G418. The cells were maintained at 37°C in a 5% CO2:95% O2 air incubator. Six clones of putative transfectants were analyzed by Northern blot using full length PDS cDNA as probe. For experiments, two clones were extensively analyzed.Cell pH Measurement
Changes in intracellular pH (pHi) were monitored using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) (3, 8). HEK 293 cells were grown to confluence on a glass coverslip and incubated in the presence of 5 µM BCECF in a ClTo determine pHi values, the fluorescence ratio at
excitation wavelengths of 500 and 450 nm was used. Calibration curves
were established by KCl/nigericin. HCO3-free or
HCO3
-containing solutions were used to determine the
HCO3
dependence of the transporter. To examine the
Cl
/OH
exchanger activity, cells were
switched to a Cl
-free medium. This maneuver causes cell
alkalinization via reversal of the Cl
/OH
exchanger. On pHi stabilization in Cl
-free
medium, cells were switched back to Cl
-containing
solution, resulting in recovery of pHi to baseline levels
due via Cl
/OH
exchange. To examine
Cl
/HCO3
exchange activity, the above
experiments were repeated in HCO3
-containing
solutions. The rate of cell acidification immediately on switching to
the Cl
-containing solution was used as the initial rate
of Cl
/OH
or
Cl
/HCO3
exchanger activity. To examine
the Cl
/formate exchange activity, experiments were
performed in the presence of 1 mM potassium formate.
Buffering Power
The intrinsic buffering power (Materials
[32P]dCTP was purchased from New England Nuclear (Boston, MA). Nitrocellulose filters and other chemicals were purchased from Sigma Chemical (St. Louis, MO). RadPrime DNA labeling kit was purchased from GIBCO-BRL. BCECF was from Molecular Probes (Eugene, OR).Statistical Analyses
Values are expressed as means ± SE. The significance of difference between mean values were examined using ANOVA. P < 0.05 was considered statistically significant. ![]() |
RESULTS |
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Pendrin mRNA Expression in the Kidney
To examine the distribution of pendrin mRNA in rat kidney, Northern blots were prepared using total RNA from whole kidney, cortex, outer medulla, and inner medulla, and then probed with rat pendrin cDNA probe. The results were as shown in Fig. 1A. Pendrin mRNA is expressed exclusively in the cortex under normal conditions. Within the cortex both proximal and distal tubules are present. Figure 1B shows a Northern blot prepared from RNA isolated from proximal tubule suspensions and probed with pendrin cDNA. As indicated, pendrin mRNA is abundantly expressed in proximal tubules.
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In the next series of experiments we investigated the expression of pendrin in two distinct cortical nephron segments (proximal tubule and CCD) using nephron segment RT-PCR (see EXPERIMENTAL PROCEDURES). Figure 1C shows two representative ethidium bromide gel pictures from nephron segment RT-PCR experiments and demonstrates that in addition to proximal tubule, PDS mRNA is also expressed in CCD.
Immunoblotting of Pendrin in the Kidney
In this series of experiments the membrane localization of pendrin was examined. We first determined the specificity of pendrin immune serum in HEK-293 cells stably transfected with PDS cDNA. As shown in Fig. 2A, the pendrin immune serum labeled an ~100-kDa band in transfected HEK-293 cells. This reaction was specific because it was prevented by immune preadsorption (Fig. 2A). The PDS mRNA and protein were not detected in nontransfected cells (data not shown). To determine the expression of pendrin in the kidney cortex and examine its membrane domain localization, proteins from cortical microsomes, BBM and BLM, were resolved by SDS-PAGE and probed with the immune serum. As indicated in Fig. 2B, a ~100-kDa protein was strongly labeled in the cortical microsomes, consistent with the expression of pendrin in the cortex. Pendrin localized to the BBM but not the BLM domain (Fig. 2B). This reaction was specific because it was prevented by immune preadsorption (Fig. 2B). These results are consistent with the localization of pendrin in cortical BBM. Repeated attempts in our laboratory in performing immunocytochemical studies with PDS-specific polyclonal antibodies have not been successful. As such, we could not examine the protein expression of PDS in CCDs.
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Functional Identity of Pendrin
In these series of experiments the functional identity of pendrin was examined in HEK-293 cells stably transfected with PDS cDNA.Cl/OH
exchange.
In the first series of functional studies we examined whether pendrin
can operate in Cl
/OH
exchange mode. Cells
were loaded with BCECF in the presence of a NaCl-containing,
HCO3
-free solution (EXPERIMENTAL
PROCEDURES) and monitored for pHi. The representative
pHi tracings in Fig. 3
demonstrate that switching to a Cl
-free solution resulted
in a rapid intracellular alkalinization in cells expressing pendrin.
Switching back to the Cl
-containing solution caused a
rapid pHi return to normal. Nontransfected HEK-293 cells
did not demonstrate any pHi alteration in response to
exposure to the Cl
-free medium (Fig. 3A).
DIDS, at 0.5 mM, almost abolished the pHi alteration in
transfected cells (Fig. 3A). These results are consistent
with pendrin functioning as a
Cl
/OH
(hydroxyl) exchanger. The averaged
results of multiple experiments (Fig. 3B) indicated that the
rate of Cl
/OH
exchange activity was
0.16 ± 0.02 in transfected cells (n = 11) and
0.01 ± 0.002 in nontransfected cells (n = 7, P < 0.001). The Cl
/OH
exchanger activity was inhibited by ~88% in the presence of 0.5 mM
DIDS (n = 5), with the exchanger activity decreasing to
0.02 ± 0.002 (P < 0.001 vs. no DIDS).
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Cl/HCO3
exchange.
To determine whether pendrin can also function in
Cl
/HCO3
exchange mode, the experiments
were repeated in the presence of 25 mM HCO3
. The
results of experiments demonstrated that in the presence of
HCO3
, the rate of Cl
/base exchange
activity was 0.22 ± 0.02 and 0.05 ± 0.01 pH/min in
transfected and nontransfected cells, respectively (n = 6 for each group, P < 0.01). When adjusted for the
minimal endogenous Cl
/HCO3
exchange
activity in nontransfected cells (which was around 0.05 pH/min), it
appears that the rate of Cl
/base exchange activity
(expressed as dpHi/dt, pH/min) in
PDS-expressing cells is not affected by the presence of
HCO3
. However, after adjusting for the total
buffering power in cultured cells (see EXPERIMENTAL
PROCEDURES and legend to Fig. 3C), it becomes evident
that in the presence of HCO3
, base equivalent
transport rate is increased in cells transfected with pendrin (Fig.
3C). Taken together, these results indicate that pendrin can
also function in Cl
/HCO3
exchange mode.
Cl/formate exchange.
We lastly examined whether pendrin can function in
Cl
/formate exchange mode, as has been demonstrated in
oocytes (29). The experiments were performed in the
presence of formate and the absence of a formate gradient (all
solutions had 1 mM formate). The purpose of having no formate gradient
was to prevent the diffusion of formate (in the form of formic acid)
across the plasma membrane, which in turn can alter the cell pH. Figure
4 demonstrates representative tracings in
the presence of formate in control or transfected cells. In the
presence of 1 mM formate, switching to a Cl
-free solution
resulted in a significant intracellular alkalinization only in
transfected cells. The pHi returned to baseline on
switching back to the Cl
-containing solution.
Nontransfected cells showed minimal pHi alterations in
response to switching to the Cl
-free solution (or back to
the normal solution) (Fig. 4A). The results further
indicated that the intracellular alkalinization was significantly
increased in the presence of formate (vs. Fig. 3). The averaged results
of multiple experiments (Fig. 4B) indicated that
Cl
/base exchange activity was 0.45 ± 0.04 in
transfected cells (n = 10) and 0.03 ± 0.003 pH/min in nontransfected cells (n = 7, P < 0.001). Comparison of the results indicates that
the rate of Cl
/base exchange activity is approximately
three times faster in the presence of formate than in its absence in
PDS transfected cells (dpHi/dt was
0.16 ± 0.02 in the absence of formate and 0.45 ± 0.04 in
the presence of formate, P < 0.01). The
formate-mediated Cl
/base exchanger activity in
transfected cells decreased to 0.13 ± 0.02 (n = 4) in the presence of 0.5 mM DIDS (P < 0.01 vs. no DIDS).
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DISCUSSION |
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The results of our experiments indicate that pendrin is expressed
in the kidney cortex and is absent in the medulla (Fig. 1A).
Proximal tubules isolated from kidney cortex demonstrated high levels
of pendrin mRNA expression (Fig. 1B). Nephron segment RT-PCR
localized this exchanger to the proximal tubule and CCD (Fig.
1C). Immunoblotting studies in membrane proteins isolated from kidney cortex localized pendrin to the BBM domain (Fig. 2). Expression studies in cultured HEK-293 cells demonstrated that pendrin
functions in Cl/OH
/HCO3
and Cl
/formate exchange modes (Figs. 3 and 4).
Mutations in pendrin have been shown to be the cause of Pendred's syndrome, an autosomal recessive hereditary disorder characterized by goiter, positive perchlorate test, and deafness (16). Using linkage analysis and positional cloning approaches, the gene mutated in Pendred's syndrome (PDS) was identified (12-14, 20, 21). Pendrin encodes an mRNA of ~5 kb in humans and is abundantly expressed in the thyroid (14). Lower levels of pendrin mRNA are expressed in the kidney (14). cDNA analysis indicates that pendrin is closely related to a family of anion transport proteins that include the rat sulfate-anion transporter (Sat-1), the human diastrophic dysplasia sulfate transporter (DTD), and the downregulated in adenoma gene (DRA) (7, 16, 17, 23, 28, 33). Sequence comparison revealed that pendrin has 29, 32, and 45% homology with Sat-1, DTD, and DRA, respectively, at the amino acid level (7, 14, 16, 28). Pendrin encodes a protein of ~95- to 100-kDa size, is localized on the apical domain of thyroid follicular cells (26), and is thought to be responsible for the transport of iodide (14, 30).
Our studies indicate that the mRNA expression of pendrin is abundantly
present in the kidney proximal tubule (and CCD) but is absent in the
medulla. This does not conflict with the studies in human kidney, where
mRNA levels of pendrin were found to be low (14). Those
studies utilized RNA isolated from the whole kidney, rather than the
cortex (14). Immunoblot analysis localized pendrin to the
apical membrane of kidney cortex, and functional studies demonstrated
that it can exchange Cl for OH
(HCO3
) or formate. Taken together, these results
indicate that pendrin is an apical Cl
/base exchanger in
the kidney cortex.
Studies in BBM vesicles isolated from rabbit kidney cortex indicate
that the apical Cl/base exchanger in the kidney proximal
tubule can function in Cl
/formate exchange mode (5,
18, 19). In perfused rabbit proximal tubule, the rate of fluid
reabsorption increased significantly in the presence of formate
(5). This process was inhibited in the presence of DIDS,
indicating that enhanced fluid reabsorption by formate is mediated via
Cl
/formate exchanger. Taken together, these results
indicate that Cl
/formate exchanger is responsible for the
bulk of Cl
reabsorption in proximal tubule. It has been
proposed that the formate, which is secreted into the lumen via
Cl
/formate exchange, is recycled back to the cell,
thereby providing substrate for the transporter (5).
In addition to the proximal tubule, -intercalated cells of CCD
express a Cl
/base exchange on their apical membrane
(2, 27, 38). The identity of this exchanger has remained
unknown. The expression of PDS mRNA in CCD raises an
intriguing possibility that this exchanger might be expressed in
-intercalated cells. We have not examined the subcellular
localization (apical vs. basolateral) of pendrin in CCD in detail.
However, based on the apical localization of pendrin in the proximal
tubule and our preliminary immunohistochemical studies in
CCD,1 we suggest that this
exchanger is located apically in
-intercalated cells of CCD.
An intriguing aspect of the current studies is the functioning of
pendrin in Cl/OH
exchange mode. Several
investigators have examined the presence of
Cl
/OH
exchange in BBM vesicles or intact
proximal tubule. Studies in BBM vesicles have been conflicting; whereas
one study found the presence of Cl
/OH
exchange, the other did not (31, 41). In perfused intact proximal tubule, however, two reports have identified the presence of
apical Cl
/OH
exchange (22,
32). Our studies are in full agreement with these latter two
studies and confirm that the apical Cl
/base exchanger in
kidney proximal tubule can function in
Cl
/OH
exchange mode.
The current studies indicate that pendrin functions as
Cl/OH
,
Cl
/HCO3
, and Cl
/formate
exchanger, suggesting that the apical Cl
/base exchanger
in kidney cortex can function in several distinct modes. This is in
agreement with studies in perfused rat kidney proximal tubule, where
the presence of Cl
/formate and
Cl
/OH
/HCO3
exchangers was
identified in apical membrane domain (1). Those experiments were performed in the presence of HCO3
only and therefore could not discriminate between
Cl
/OH
and
Cl
/HCO3
exchange. Interestingly and
similar to our current experiments, the rate of cell alkalinization in
response to luminal Cl
removal (an index of
Cl
/base exchange activity) was approximately threefold
higher in the presence of formate than in its absence (1),
suggesting that formate is a better substrate for the exchanger.
Studies have shown that formate increases Cl reabsorption
in distal tubule, presumably via an apical Cl
/base
exchanger (5, 39). However, whether formate can be transported via the CCD apical Cl
/base exchanger remains
speculative. Based on the presence of pendrin mRNA and protein in CCD
and based on the observation that pendrin can function in
Cl
/formate exchange mode (Fig. 4 and Ref. 30), we suggest
that the apical Cl
/base exchanger in
-intercalated
cells might also function in Cl
/formate exchange mode. It
is worth mentioning that the apical Cl
/base exchanger in
-intercalated cells functions predominantly in
Cl
/HCO3
exchange mode (and not
Cl
/OH
exchange mode) (2, 27,
38). This raises the possibility that the affinity of pendrin
for OH
vs. HCO3
is tissue specific.
Studies in our laboratories had demonstrated that liposomes
reconstituted with BBM proteins displayed Cl/formate and
Cl
/HCO3
/OH
exchange
activities (35). A 162-kDa protein was identified as a
likely candidate to mediate both processes (35). A partial amino acid sequence analysis shows that the 162-kDa protein is a novel
protein and shows no significant homology to any known protein (data
not shown). We have not obtained the full length cDNA of the 162-kDa
protein; however, the current results raise the possibility that
Cl
/formate exchange may be mediated by more than one
transporter. Additional studies are needed to address this issue.
It is not clear whether patients with Pendred's syndrome have enhanced
renal Cl excretion, as urinary electrolyte profiles have
not been studied in these patients. Such a conclusion would be
plausible based on the Cl
-absorbing ability and the
location of pendrin. However, downstream Cl
absorbing
transporters (the apical Na-K-2Cl cotransporter in the thick ascending
limb and the apical Na-Cl cotransporter in distal convoluted tubules)
may be upregulated in response to increased delivery of
Cl
from proximal tubule to more distal nephron segments.
Indeed, in animals with metabolic acidosis, which demonstrate
downregulation of proximal tubule apical Cl
/base
(formate) exchanger (40), the apical Na-K-2Cl
cotransporter is upregulated (6). Such a compensatory
defense mechanism would increase the reabsorption of Cl
and attenuate the loss of Cl
into the urine. With respect
to the role of pendrin as the apical Cl
/HCO3
exchanger in
-intercalated
cells, it is likely that patients with Pendred's syndrome might show
an impairment in urine alkalinization. To unmask such a defect,
however, patients need to be subjected to HCO3
loading or metabolic alkalosis.
In conclusion, pendrin is expressed in the kidney, with mRNA expression
limited to the proximal tubule and CCD. Immunoblotting localized
pendrin to the apical domain of cortical membranes. Functional studies
demonstrated that pendrin functions in
Cl/OH
,
Cl
/HCO3
, and Cl
/formate
exchange modes. We propose that pendrin is an apical Cl
/base exchanger in the kidney proximal tubule and
-intercalated cells of CCD.
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ACKNOWLEDGEMENTS |
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These studies were supported by National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-46789, DK-52281, and DK-54430, a Merit Review grant, a Cystic Fibrosis Foundation grant and grants from Dialysis Clinic to M. Soleimani and a grant from Northwestern Memorial Hospital to P. Kopp.
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FOOTNOTES |
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Address for reprint requests and other correspondence: M. Soleimani, Div. of Nephrology and Hypertension, Dept. of Medicine, Univ. of Cincinnati, 231 Bethesda Ave., MSB 5502, Cincinnati, OH 45267-0585 (E-mail: manoocher.soleimani{at}uc.edu).
1
Preliminary immunocytochemical studies in our
laboratory indicate specific apical staining in a subpopulation of CCD
cells, consistent with the expression of pendrin in -intercalated cells.
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.
Received 22 July 2000; accepted in final form 23 October 2000.
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