Basic Science Articles
Chloride/Bicarbonate Exchanger SLC26A7 Is Localized in
Endosomes in Medullary Collecting Duct Cells and Is
Targeted to the Basolateral Membrane in Hypertonicity and
Potassium Depletion
Jie Xu,* Roger T. Worrell,† Hong C. Li,* Sharon L. Barone,* Snezana Petrovic,*
Hassane Amlal,* and Manoocher Soleimani*‡
Departments of *Medicine and †Surgery, University of Cincinnati, and ‡Veterans Affairs Medical Center at Cincinnati,
Cincinnati, Ohio
SLC26A7 is a Clⴚ/HCO3ⴚ exchanger that is expressed on the basolateral membrane and in the cytoplasm of two distinct
acid-secreting epithelial cells: The A-intercalated cells in the kidney outer medullary collecting duct and the gastric parietal
cells. The intracellular localization of SLC26A7 suggests the possibility of trafficking between cell membrane and intracellular
compartments. For testing this hypothesis, full-length human SLC26A7 cDNA was fused with green fluorescence protein and
transiently expressed in MDCK epithelial cells. In monolayer cells in isotonic medium, SLC26A7 showed punctate distribution throughout the cytoplasm. However, in medium that was made hypertonic for 16 h, SLC26A7 was detected predominantly
in the plasma membrane. The presence of mitogen-activated protein kinase inhibitors blocked the trafficking of SLC26A7 to
the plasma membrane. Double-labeling studies demonstrated the localization of SLC26A7 to the transferrin receptor–positive
endosomes. A chimera that was composed of the amino terminal fragment of SLC26A7 and the carboxyl terminal fragment of
SLC26A1, and a C-terminal–truncated SLC26A7 were retained in the cytoplasm in hypertonicity. In separate studies, SLC26A7
showed predominant localization in plasma membrane in potassium-depleted isotonic medium (0.5 or 2 mEq/L KCl) versus
cytoplasmic distribution in normal potassium isotonic medium (4 mEq/L). It is concluded that SLC26A7 is present in
endosomes, and its targeting to the basolateral membrane is increased in hypertonicity and potassium depletion. The
trafficking to the cell surface suggests novel functional upregulation of SLC26A7 in states that are associated with hypokalemia or increased medullary tonicity. Additional studies are needed to ascertain the role of SLC26A7 in enhanced bicarbonate
absorption in outer medullary collecting duct in hypokalemia and in acid-base regulation in conditions that are associated
with increased medullary tonicity.
J Am Soc Nephrol 17: 956 –967, 2006. doi: 10.1681/ASN.2005111174
T
he solute carrier families 4 and 26 (SLC4 and SLC26,
respectively) encode two distinct groups of anion exchangers. Several members of the SLC4 family, designated as A1 (SLC4A1⬃AE1), A2 (SLC4A2⬃AE2), A3
(SLC4A3⬃AE3), and A4 (SLC4A9⬃AE4) are shown to mediate
Cl⫺/HCO3⫺ exchange (1,2). SLC26 is a new family of anion
exchangers that is composed of 10 distinct genes (3). Members
of the SLC26 family display very specific and limited tissue
distribution. Functional studies demonstrate that a number of
exchangers from this family, including SLC26A3 (DRA),
SLC26A4 (pendrin), SLC26A6 (PAT1 or CFEX), SLC26A7, and
SLC26A9 (4 –9) mediate Cl⫺/HCO3⫺ exchange. SLC26A4, A6,
and A7 are expressed in the kidney, whereas SLC26A3 and A9
Received November 9, 2005. Accepted January 26, 2006.
Published online ahead of print. Publication date available at www.jasn.org.
Address correspondence to: Dr. Manoocher Soleimani, Division of Nephrology
and Hypertension, Department of Medicine, University of Cincinnati, 231 Albert
Sabin Way, MSB 259G, Cincinnati, OH 45267-0585. Phone: 513-558-5471; Fax:
513-558-4309; E-mail: manoocher.soleimani@uc.edu
Copyright © 2006 by the American Society of Nephrology
are not. In the kidney, SLC26A4 (pendrin) is expressed on the
apical membranes of B-intercalated and non-A non-B–intercalated cells, whereas SLC26A6 (PAT1 or CFEX) is expressed on
the brush border membranes of the proximal tubule. In the
kidney, pendrin mediates bicarbonate secretion and chloride
reabsorption in the connecting segment and cortical collecting
duct (5,10 –12), whereas PAT1 is involved in transcellular chloride reabsorption in the proximal tubule (6,13–16).
SLC26A7 is a recently cloned member of the SLC26 family
(17,18). Functional and molecular studies from our laboratory
demonstrated that SLC26A7 is a chloride/bicarbonate exchanger (7). In the stomach, SLC26A7 is expressed on the
basolateral membrane of the acid-secreting gastric parietal cells
(7), whereas in the kidney, it localizes on the basolateral membrane of acid-secreting A-intercalated (A-IC) cells of the outer
medullary collecting duct (OMCD) (8,19). OMCD has the highest rate of acid secretion among the collecting duct segments
(20). Proton (acid) secretion across the apical membrane of A-IC
cells in OMCD via vacuolar H⫹-ATPase (and to some extent via
H⫹/K⫹ ATPase) results in the generation of intracellular bicarISSN: 1046-6673/1704-0956
J Am Soc Nephrol 17: 956 –967, 2006
bonate, which will be transported across the basolateral membrane into the peritubular space via basolateral Cl⫺/HCO3⫺
exchanger (20,21). SLC26A7 co-localizes with AE1 (SLC4A1) on
the basolateral membrane of A-IC cells of OMCD, indicating
possible distinct roles for these two Cl⫺/HCO3⫺ exchangers in
acid secretion and bicarbonate absorption (8,19). A recent study
indicated that SLC26A7 can function as a Cl⫺ channel that is
regulated by intracellular pH in the heterologous expression
system (22). The reason for the discrepancy between those
results and our observations that consistently demonstrate mediation of Cl⫺/HCO3⫺ exchange by SLC26A7 is currently unclear. The discrepancy may be due in part to the utilization of
different expression systems as well as differences in the interpretation of the results. In support of this latter possibility,
removal of perfusate chloride resulted in cell alkalinization in
SLC26A7-expressing HEK 293 cells (22), an observation that is
consistent with SLC26A7’s being a Cl⫺/HCO3⫺ exchanger.
Expression of human SLC26A7 in Xenopus oocytes increased
chloride-dependent cell alkalinization in the presence of CO2/
HCO3⫺, supporting the role of SLC26A7 as a Cl⫺/HCO3⫺
exchanger (S.L. Alper, Harvard Medical School, personal communication, February 2005).
Our immunofluorescent labeling studies in kidney and stomach demonstrated that in addition to membrane localization,
SLC26A7 shows abundant cytoplasmic, submembrane localization, raising the possibility that alteration in trafficking between
cell membrane and intracellular compartments may be a major
mechanism of functional regulation of this anion exchanger.
Toward this end, epitope-tagged SLC26A7 cDNA was expressed in MDCK epithelial cells that were exposed to hypertonicity or potassium depletion and visualized by confocal
microscopy. Our results demonstrate that SLC26A7 is present
in recycling endosomes in the cytoplasm in isotonic normal
medium but is moved to the membrane in hypertonic or potassium-depleted medium. The significance of the results is
discussed.
Materials and Methods
Construction of Epitope-Tagged SLC26A7, C-Terminal–
Truncated SLC26A7, Full-Length SLC26A1, and
A7/A1 Chimera
The full-length and C-terminal–truncated SLC26A7 were generated
by PCR, using the human full-length SLC26A7 DNA (5280 bp and 656
amino acid [aa] residues; Genebank NM_052832). The SLC26A1 (2773
bp and 704 aa residues; Genebank AF349043) and SLC26A7/A1 chimera were generated using mouse cDNA as templates. The resulting
wild-type SLC26A7, C-terminal–truncated SLC26A7, SLC26A1, and
A7/A1 mutants were amplified and fused translationally in-frame to
green fluorescence protein (GFP) by cloning into pcDNA3.1/NT-GFPTOPO vector (Invitrogen, Carlsbad, CA).
Full-length human SLC26A7 was amplified using the following
primers: Primer-237, 5⬘-GAA ATG ACA GGA GCA AAG AG (sense),
and primer-2328, 5⬘-GTT ATT GTA GCA GAG GTC ATC (antisense).
The PCR product was cloned into the GFP fusion TOPO vector
(pcDNA3.1/NT-GFP-TOPO vector). This resulted in the expression of
GFP-SLC26A7 fusion protein with the GFP fusing to the N-terminus of
SLC26A7. Using a similar approach, primer-237, GAA ATG ACA GGA
GCA AAG AG (sense), primer-2162, TCA GAT ATG ACT TAT TGC
SLC26A7, Endosomes, and Membrane Trafficking
957
AG (antisense), and primer-1919, TCA TTC TTC ATT GCA GTT G
(antisense), were used to generate the SLC26A7-CD16 cDNA, which
lacked the last 16 aa residues on the carboxyl terminal end. Full-length
mouse SLC26A1 was amplified using the following primers: Primer-22,
5⬘-GAC AGG ATG GAT GCT TCT C (sense), and primer-2204, 5⬘-ATT
CAC ACC ACT CCT ACA G (antisense) from mouse kidney. The PCR
product was cloned into the GFP fusion TOPO vector (pcDNA3.1/NTGFP-TOPO vector). This resulted in the expression of GFP-SLC26A1
fusion protein with the GFP fusing to the N-terminus of SLC26A1
(GFP-SLC26A1). For generation of A7/A1 chimeras, the amino-terminal end of mouse A7 was fused to the carboxy-terminal end of mouse
A1 to generate A7n/A1c chimera, with n and c designating the Nterminal and C-terminal ends, respectively. Toward this end, the DNA
fragment encoding aa residues 1 to 473 of mouse SLC26A7 was generated using primers A7–107, AAA ATG ACG GGA GCA AAG AG
(sense), and A7–1528, GAA TTC TGG GAA ACG TCC TAA CAC
(antisense), and fused in frame to the DNA fragment encoding the aa
residues 499 to 705 of mouse SLC26A1 using primers A1, GAATTCTTCTTCTCCCTGCTTAGCCTG (sense), and A1, ATTCACACCACTCCTACAG (antisense). This resulted in the expression of the GFPSLC26A7/A1 mutant fusion protein, which is composed of the aminoterminal fragment of A7 and the carboxyl terminal fragment of A1.
Transient Expression of Epitope-Tagged SLC26A7,
Truncated SLC26A7, SLC26A1, and A7/A1 Mutant in
MDCK Cells
MDCK cells were grown on glass coverslips and transiently transfected
with the epitope-tagged SLC26 isoforms or variants (above) and studied
48 h later according to established methods (23,24). Briefly, cells were
plated in 24-well plates and transfected with various cDNA fragments at
80% confluence using 0.8 g of DNA and 4 l of Lipofectamine 2000
(Invitrogen). All cells were co-labeled with Alexa Fluor 568 phalloidin
(Molecular Probes, Eugene, OR) as a marker of apical membrane labeling.
All cells were fixed 48 h after transfection, irrespective of the duration of
exposure to hypertonic or low-potassium medium, such that the total
length of exposure to isotonic plus hypertonic or normal-potassium plus
low-potassium medium was 48 h.
In separate studies, MDCK cells were grown on permeable polycarbonate membrane Transwell filters (cat. no. 3401; Corning Inc., Corning, NY)
at a density of approximately 105 cells/cm2. Cells achieved confluence
within 4 to 5 d and then were transiently transfected from the apical
surface with the GFP-SLC26A7 construct and studied 48 h later.
Confocal Microscopy and Immunofluorescence Labeling
MDCK cells were washed three times with PBS, fixed for 20 min with
3% formaldehyde in PBS, and washed three more times with PBS.
Afterward, cells were permeabilized with 0.1% TX-100 in PBS for 3 min,
washed three times with PBS, and co-stained with Alexa Fluor 568
phalloidin. Cells then were washed and mounted on glass slides in
Fluoromount-G (Southern Biotechnology Associates, Inc., Birmingham,
AL). Images were taken on a Zeiss LSM510 confocal microscope. Both
Z-line and Z-stack images were obtained using the LSM 5 Image
software to analyze the membrane targeting of GFP-fusion proteins
(23,24).
Water Loading in Rats
Sprague-Dawley rats that weighed 150 to 200 g were subjected to
water loading for 5 d according to established protocols. Briefly, the
control group (n ⫽ 4) was allowed tap water ad libitum, whereas the
water-loaded rats (n ⫽ 4) were induced to drink water abundantly by
adding glucose (50 g/1000 ml) to their drinking water.
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Journal of the American Society of Nephrology
Antibodies
A rabbit polyclonal antibody raised against a mouse SLC26A7 peptide with the aa residues CGAKRKKRSVLWGKMHTP (using the
mouse EST with Genbank accession no. BB666404) and an antibody
raised against human SLC26A7 were used for immunofluorescence
labeling (8,19).
Immunofluorescence Labeling Studies in Mouse Kidney
and Stomach
Immunofluorescence labeling was performed as described previously (7,8,19). Alexa Fluor 488 (green) or Alexa Fluor 568 (red) goat
anti-rabbit antibody was used as a secondary antibody. Sections were
examined on the epifluorescent microscope Eclipse 600 (Nikon Bioscience, Melville, NY) equipped with SPOT digital camera (Diagnostic
Instruments, Inc., Sterling Heights, MI). Digital images were acquired
using the Spot Advanced software provided with the camera.
Western Blot Analysis
Microsomal membrane and cytoplasmic fractions were isolated from
rat outer medulla according to established methods (25). Immunoblotting experiments were carried out as described previously (8,13).
Briefly, the solubilized proteins were size-fractionated on 8% SDS polyacrylamide minigels (Novex, San Diego, CA) under denaturing conditions, electrophoretically transferred to nitrocellulose membranes,
blocked with 5% milk proteins, and then probed with the affinitypurified anti-SLC26A7 immune serum at a dilution of 1:400. The secondary antibody was donkey anti-rabbit IgG conjugated to horseradish
peroxidase (Pierce, Rockford, IL). The sites of antigen-antibody complex formation on the nitrocellulose membranes were visualized using
chemiluminescence method (SuperSignal Substrate; Pierce) and captured on light-sensitive imaging film (Kodak, Rochester, NY).
Materials
All chemicals were purchased from Sigma Chemical Co. (St. Louis,
MO). RadPrime DNA labeling kit was purchased from Invitrogen
(Carlsbad, CA). mMACHINE kit was purchased from Ambion (Austin,
TX). Alexa Fluor– conjugated secondary antibodies and Hoechst 33342
were purchased from Molecular Probes Inc. The CT-GFP fusion expression kit, which contains the pcDNA3.1/CT-GFP TOPO vector, was
purchased from Invitrogen. Glyceraldehyde-3-phosphate dehydrogenase polyclonal antibodies were from Abcam (Cambridge, MA).
Statistical Analyses
The experiments were performed in duplicate (two slides per each
condition) and repeated at least three separate times for each maneuver. Quantification of SLC26A7 abundance in membrane or cytoplasm
was performed using MetaMorph imaging system software (Universal
Imaging Corp., West Chester, PA) by measuring the fluorescence intensity of SLC26A7-GFP in multiple square areas corresponding to
regions in membrane or cytoplasm. More than 16 separate fields were
analyzed per each transfected cell, with the total of 160 fields for 10
transfected cells per each slide. A total of 20 or 30 transfected cells from
three separate experiments were analyzed for each maneuver. Values
are expressed as arithmetic mean ⫾ SE. Comparisons were done by
using unpaired t test, and P ⬍ 0.05 was considered statistically significant. Microsoft Excel, ProStat (Philscience, South Korea), and PSI-Plot
(Philscience) were commercial software packages used for statistical
analysis.
J Am Soc Nephrol 17: 956 –967, 2006
Results
Our immunofluorescence labeling with purified antibodies
demonstrated the localization of SLC26A7 on the basolateral
membrane and in the cytoplasm of parietal cells in the stomach
and A-IC cells in kidney OMCD (7,8,19). These results have
been confirmed in our new immunofluorescence labeling studies in Figure 1, A and B. Figure 1A shows the subcellular
distribution of SLC26A7 in gastric parietal cells when images
with gastric H-K-ATPase are merged. Figure 1B shows the
subcellular distribution of SLC26A7 in kidney OMCD cells. The
low- and high-magnification images in Figure 1B (left and right,
respectively) along with the image in Figure 1A clearly demonstrate the localization of SLC26A7 on the basolateral membrane and in the cytoplasm of acid-secreting gastric parietal
cells and OMCD cells.
To verify the cytoplasmic expression of SLC26A7 quantitatively, we performed Western blotting on membrane and cytoplasmic proteins that were isolated from rat outer medulla, the
site of SLC26A7 expression. As indicated, Western blotting
detected SLC26A7 as an approximately 90-kD band, with abundant expression in both the membrane and the cytoplasmic
fractions from two separate rats (Figure 1C, left), confirming the
subcellular distribution of the exchanger in the kidney cells.
Preadsorption of the antibody with the antigen (synthetic peptide) completely prevented the labeling of SLC26A7 in the
kidney outer medulla fractions (Figure 1C, right), indicating the
specificity of the antibody.
Expression and Subcellular Distribution of SLC26A7 in
MDCK Cells
The cytoplasmic localization of the exchanger in Figure 1
raises the possibility of alteration in SLC26A7 abundance at the
cell surface by affecting the trafficking between cell membrane
and intracellular compartments. To analyze this possibility, we
examined the expression of full-length GFP with or without
SLC26A7 cDNA insert in MDCK cells. The cell membrane was
labeled with the actin-binding dye phalloidin. Figure 2A is a
Z-line merged image of phalloidin and GFP labeling and shows
that transfection with GFP vector alone (no SLC26A7 insert)
results in the accumulation of GFP in the cytoplasm with no
localization on the membrane. It is interesting that when transfection with GFP-SLC26A7 full-length cDNA was performed,
the acquired images demonstrated punctate distribution
through the cytoplasm, with no labeling on the plasma membrane (Figure 2B).
Effect of Hypertonicity on Distribution of SLC26A7 in
MDCK Cells
The expression of SLC26A7 in the kidney is limited predominantly to the medullary collecting duct, a segment that is
exposed to a hypertonic environment in vivo. The purpose of
the next series of experiments was to examine the effect of
hypertonicity on subcellular distribution of SLC26A7. Toward
this end, cells were transiently transfected with GFP-SLC26A7
cDNA; and 32 h later, the medium was made hypertonic by the
addition of 50 mM NaCl. The pH was maintained at 7.4. The
cells were fixed for 16 h after switching to the hypertonic
J Am Soc Nephrol 17: 956 –967, 2006
SLC26A7, Endosomes, and Membrane Trafficking
959
Figure 2. Expression and subcellular distribution of green fluorescence protein (GFP)-SLC26A7 in MDCK cells. (A) Transfection with GFP vector alone (no SLC26A7 insert) results in GFP
accumulation in the cytoplasm (Z-line images). (B) GFPSLC26A7 is expressed in punctate cytoplasmic structures, with
no labeling on the membrane. Red, phalloidin; green, GFP.
Figure 1. SLC26A7 expression in mouse kidney outer medullary
collecting duct (OMCD) and stomach parietal cells. (A) Immunofluorescence labeling of SLC26A7 and gastric H-K-ATPase in
gastric parietal cells. Merged image of SLC26A7 and gastric
H-K-ATPase labeling is shown. As indicated, SLC26A7 shows
significant intracellular as well as basolateral membrane localization in gastric parietal cells. (B) Immunofluorescence labeling of SLC26A7 in kidney outer medulla. SLC26A7 shows
labeling on the basolateral membrane as well as in the cytoplasm in a subpopulation of OMCD cells (left, low magnification; right, high magnification). (C) Immunoblotting of
SLC26A7 in membrane and cytoplasmic fractions in kidney
outer medulla. Microsomal membranes and cytoplasmic fractions from outer medulla of two rat kidneys were loaded at 100
g/lane onto lanes. Immunoblotting with the purified immune
serum shows a band with a molecular weight of approximately
90 kD in both the membrane and cytoplasmic fractions (left).
The labeling of the 90-kD band was abolished with preadsorbed immune serum in both fractions (right).
medium and analyzed microscopically. As shown in Figure 3A,
SLC26A7 was detected predominantly in the plasma membrane, with little residual labeling in the cytoplasm. Analysis of
30 transfected cells in hypertonic medium and 30 cells in isotonic medium by MetaMorph imaging system software (see
Materials and Methods) showed that 86 ⫾ 5% of SLC26A7
labeling was detected in the membrane in hypertonic medium,
whereas only 13 ⫾ 2% of the labeling was detected in the
membrane in isotonic medium (P ⬍ 0.0001). Subcellular distribution studies in hypertonic medium (Z-stack images) indicate
the targeting of SLC26A7 to the basolateral membrane (Figure
3B), consistent with published reports (7,8,19). Transfection
with the control construct (empty GFP with no SLC26A7 insert)
showed that the GFP was retained in the cytoplasm in hypertonic medium at 2 and 16 h (data not shown) in a manner that
was indistinguishable from the isotonic medium (Figure 2A).
The purpose of the next series of experiments was to determine the rapidity with which the shift in SLC26A7 distribution
from cytoplasm to the membrane occurs in hypertonic medium. Accordingly, cells were transfected with SLC26A7,
switched to the hypertonic medium 46 h later, and fixed for 2 h
after switching to the hypertonic medium. As shown in Figure
3C, exposure to the hypertonic medium for 2 h did not significantly increase the abundance of SLC26A7 in the membrane.
The hypertonicity in the above experiments was generated
by the addition of 50 mM NaCl. In the next series of experiments, hypertonicity was generated by the addition of sodiumfree solute. Toward this end, mannitol (100 Mm) was added to
the medium, and cells were fixed and studied 16 h later. As
shown in Figure 3D, SLC26A7 was detected predominantly in the
membrane in hypertonic medium. Analysis of 30 transfected cells
from three separate experiments showed that 79 ⫾ 5% of
SLC26A7 labeling was detected in the membrane after 16 h of
incubation with mannitol (P ⬍ 0.0001, versus isotonic medium in
Figure 3A).
The above experiments were performed on cells that were
grown on nonpermeable support. The next series of experiments were performed in cells that were grown on permeable
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J Am Soc Nephrol 17: 956 –967, 2006
Figure 3. Effect of hypertonicity on the expression and subcellular distribution of SLC26A7 in MDCK cells. (A) Expression of
SLC26A7 in hypertonic medium (top) versus isotonic medium (bottom). As shown, SLC26A7 was detected almost exclusively in
the plasma membrane in hypertonic medium (top) versus predominantly in intracellular compartments in isotonic medium
(bottom). Red, phalloidin; green, SLC26A7-GFP. (B) Subcellular distribution studies in hypertonic medium. Merged Z-stack image
of GFP-SLC26A7 and phalloidin indicates the targeting of SLC26A7 to the basolateral membrane. (C) Effect of 2 h of exposure to
hypertonic medium on SLC26A7 distribution in MDCK cells. As shown, SLC26A7 was detected predominantly in the cytoplasm
after 2 h of incubation in hypertonic medium. Red, phalloidin; green, SLC26A7-GFP. (D) Effect of mannitol on SLC26A7
distribution in MDCK cells. As shown, SLC26A7 was detected predominantly in the membrane after 16 h of incubation in
hypertonic medium. Red, phalloidin; green, SLC26A7-GFP. (E) SLC26A7 expression in cells that were grown on permeable
support. As shown, cells that were grown on permeable support were detected predominantly in the cytoplasm in isotonic
medium. However, in cells that were exposed to hypertonic medium from the basolateral and apical surfaces or from the
basolateral surface alone, SLC26A7 was detected predominantly on the plasma membrane. When hypertonicity was applied to the
apical surface alone, SLC26A7 showed mild abundance in the membrane with significant retention in the cytoplasm.
J Am Soc Nephrol 17: 956 –967, 2006
Figure 4. Effect of mitogen activated protein kinase (MAPK)
inhibition on subcellular distribution of SLC26A7 in hypertonicity. (Top) Hypertonic medium without MAPK inhibitor. As
indicated, SLC26A7 is targeted predominantly to the plasma
membrane in a hypertonic environment. (Bottom) Hypertonic
medium with MAPK inhibitor. MAPK inhibitor completely
blocked the trafficking of SLC26A7 to the membrane. Red,
phalloidin; green, SLC26A7-GFP.
support (see Materials and Methods). Toward this end, MDCK
cells were plated on Transwell filters and transfected with the
SLC26A7-GFP construct from the apical surface after becoming
SLC26A7, Endosomes, and Membrane Trafficking
961
confluent. Thirty-two hours after transfection, the medium that
faced the basolateral surface, the apical surface, or both surfaces
was made hypertonic by the addition of 50 mM NaCl. The cells
were fixed for 16 h after switching to the hypertonic medium
and analyzed microscopically. As shown in Figure 3E,
SLC26A7 was detected predominantly in the cytoplasm in isotonic medium (left) but in the plasma membrane in hypertonic
medium added to both the apical and basolateral sides (second
panel from left). In cells that were exposed to hypertonic medium from the basolateral side alone (right), the abundance of
SLC26A7 in the membrane was similar to that in cells that were
exposed to hypertonic medium from both sides (second panel
from left). In cells that were exposed to hypertonic medium from
the apical surface alone, SLC26A7 was detected predominantly in
the cytoplasm with some labeling in the membrane (second panel
from right).
In an attempt to identify the signal(s) that mediates the
targeting of SLC26A7 to the membrane in hypertonicity, we
investigated the role of mitogen-activates protein kinase
(MAPK). This assumption was based on published literature
demonstrating the activation of MAPK by high osmolarity
(26 –28). Toward this end, the experiments in hypertonic medium were repeated in the presence or absence of SB203580/
S-8307 (Sigma-Aldrich, St. Louis, MO), a specific inhibitor of
p38 MAPK (27,28). Accordingly, MDCK cells were transfected
with the GFP-SLC26A7 cDNA and then incubated with 10 M
SB203580 upon switching to the hypertonic medium. As shown
in Figure 4, the presence of the MAPK inhibitor SB203580
Figure 5. SLC26A7 resides in recycling endosomes in MDCK cells. (A) Isotonic medium. As shown, SLC26A7 (right) and
transferrin receptor (left) co-localize to the same intracellular compartments (middle: merged). (B) Hypertonic medium. SLC26A7
and transferrin receptor co-localize to the plasma membrane in hypertonic medium.
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J Am Soc Nephrol 17: 956 –967, 2006
completely blocked the trafficking of SLC26A7 to the membrane (bottom) when compared with its absence (top). Analysis
of the epitope-tagged insert in 10 transfected cells with the p38
MAPK inhibitor and 10 cells without the inhibitor in hypertonic
medium showed that only 9 ⫾ 2% of SLC26A7 labeling was in the
membrane in the presence of the inhibitor versus 83 ⫾ 7% in its
absence (P ⬍ 0.0001, three separate experiments). Similarly, addition of PD 98059 (Sigma-Aldrich), an inhibitor of the upstream
regulatory protein kinase MAP/extracellular signal–regulated kinase kinase (27,28), also blocked the trafficking of SLC26A7 to the
membrane in hypertonic medium in a manner similar to that
shown in Figure 4 (data not shown).
ysis of 20 transfected cells in hypertonic or isotonic medium
(see Materials and Methods) showed that 79 ⫾ 7% of SLC26A7
labeling in isotonic medium and 69 ⫾ 6% in hypertonic medium was detected in the cytoplasm (P ⬎ 0.05). These results
suggest that the signal that directs the targeting of SLC26A7 to
the membrane in hypertonic medium likely resides in the Cterminal end of SLC26A7. It is worth mentioning that mouse
SLC26A7, which was used for domain-swapping experiments
in Figure 6, shows a subcellular distribution pattern similar to
the human SLC26A7 (data not shown), indicating that species
difference does not play any role in membrane targeting of
SLC26A7.
SLC26A7 Resides in Endosomes in MDCK Cells
Expression of C-Terminal–Truncated SLC26A7 in
MDCK Cells
The disappearance of SLC26A7 from the cytoplasm and its
appearance in the plasma membrane in hypertonic medium
(Figures 2 and 3) raises the possibility that SLC26A7 may reside
in the endosomes. To investigate this possibility, we performed
double labeling with SLC26A7-GFP and the transferrin receptor marker (transferrin conjugated with Alexa Fluor 568; Molecular Probes) in both isotonic and hypertonic media. As indicated in Figure 5A, the intracellular localization of GFPSLC26A7 (right) and transferrin receptor (left) significantly
overlapped in isotonic medium. Furthermore, the majority of
transferrin receptor (left) and GFP-SLC26A7 (right) labeling
was detected in the plasma membrane in hypertonic medium
(Figure 5B, merged). Analysis of the epitope-tagged insert in 10
transfected cells in isotonic medium and 10 cells in hypertonic
medium from three separate experiments showed significant
co-localization of SLC26A7 with the transferrin receptor in
hypertonic medium.
Expression of SLC26A1 and SLC26A7/A1 Chimera in
MDCK Cells
In the next series of experiments, we examined whether the
localization of SLC26A7 in the endosomes is unique to this
member of the SLC26 family or is a property that is shared by
other members of the SLC26 family. Toward this end, SLC26A1
(also know as SAT1, for sulfate anion transporters 1) (3,16,29)
was fused in frame with GFP (see Materials and Methods) and
expressed in MDCK cells in isotonic media at pH 7.4. As
indicated in Figure 6A, SLC26A1 is expressed predominantly in
the plasma membrane, with little expression in the cytoplasm
(Z-line image). Subcellular distribution studies in isotonic media (Z-stack image) indicate that SLC26A1 is localized to the
basolateral membrane (Figure 6B). The membrane abundance
of SLC26A1 did not change in hypertonic medium (data not
shown). These studies support the conclusion that the pattern
of distribution of SLC26A7 is unique to this isoform and does
not extend to other members of the SLC26 family.
Next, we examined the expression of the SLC26A7/A1 mutant, which encoded the N-terminal portion of A7 and the
C-terminal fragment of A1 in isotonic and hypertonic media
(see Materials and Methods). The results demonstrate that
A7/A1 chimera is localized predominantly to the cytoplasm in
isotonic medium (Figure 6C, left) and shows little expression in
the membrane in hypertonic medium (Figure 6C, right). Anal-
In the next series of experiments, the expression of the truncated SLC26A7 (see Materials and Methods) was examined in
isotonic medium and hypertonic medium. As indicated in Figure 6D (left), deletion of the last 16 aa residues from the Cterminal end resulted in a more diffuse intracellular distribution with little expression in the membrane (Z-line image). The
truncated SLC26A7 was retained predominantly in the cytoplasm with faint labeling in the plasma membrane in hypertonic medium (Figure 6D, right). Analysis of 20 transfected cells
in hypertonic or isotonic medium showed that 81 ⫾ 6% of
SLC26A7 labeling in isotonic medium and 74 ⫾ 7% in hypertonic medium was detected in the cytoplasm (P ⬎ 0.05). Taken
together, these data indicate that a domain within the last 16 aa
of the C-terminal fragment is responsible for the targeting of
SLC26A7 to the membrane in hypertonic medium.
Effect of Potassium Depletion on Expression GFP-SLC26A7
in MDCK Cells
Rats that are fed a potassium-free diet for 3 to 7 d demonstrate increased bicarbonate reabsorption in their OMCD (30),
suggesting the upregulation of apical and basolateral bicarbonate-absorbing transporters in A-IC cells (31). On the basis of the
studies showing the localization of SLC26A7 to the basolateral
membrane of A-IC cells in OMCD (8,19), we entertained the
possibility that potassium depletion might regulate the trafficking of SLC26A7 to the membrane. Toward this end, cells were
transiently transfected with GFP-SLC26A7 cDNA and thereafter were exposed to either very low potassium (0.5 mM/L) or
low potassium (2 mM/L) and compared with normal potassium (4 mEq/L) for 16 or 2 h. The medium was kept isotonic
with all potassium concentrations. Figure 7 compares the effect
of varying potassium concentration on subcellular distribution
of GFP-SLC26A7. As indicated, SLC26A7 was detected predominantly in the plasma membrane in very-low-potassium
medium (0.5 mM) for 16 h (right) versus in punctate cytoplasmic structures in normal-potassium medium (left). At 2 mM
potassium for 16 h, SLC26A7 shows significant abundance in
the membrane versus the isotonic medium, with moderate cytoplasmic localization (second panel from right) when compared with 0.5 mEq/L potassium (right). The abundance of
SLC26A7 in the membrane was estimated at approximately
23 ⫾ 4% in normal potassium, 58 ⫾ 5% in 2 mEq/L potassium
J Am Soc Nephrol 17: 956 –967, 2006
SLC26A7, Endosomes, and Membrane Trafficking
963
Figure 6. Expression of SLC26A1, SLC26A7/A1 chimera, and truncated SLC26A7 in MDCK cells. (A) Expression of SLC26A1 in
MDCK cells (Z-line image). SLC26A1 is expressed predominantly in plasma membrane, with no expression in the cytoplasm. (B)
Subcellular distribution of SLC26A1 in isotonic medium (Z-stack images) indicates that SLC26A1 is localized to the basolateral
membrane. Red, phalloidin; green, SLC26A1-GFP. (C) Expression of SLC26A7/A1 chimera in MDCK cells in isotonic and
hypertonic media (Z-line image). SLC26A7/A1 chimera is expressed predominantly in the cytoplasm with some labeling in the
plasma membrane (left, isotonic medium), and its distribution pattern does not change with increased osmolarity (right). (D) Truncated
SLC26A7. As shown, the C-terminal–truncated SLC26A7 shows cytoplasmic distribution in isotonic medium (left). The truncated
SLC26A7 remained predominantly in the cytoplasm with faint expression in the membrane in hypertonic medium (right).
964
Journal of the American Society of Nephrology
J Am Soc Nephrol 17: 956 –967, 2006
Figure 7. Effect of potassium depletion on the subcellular distribution of GFP-SLC26A7 in MDCK cells. (Left) Normal potassium
(4 mEq/L). (Middle two) Low potassium (2 mEq/L) for 2 and 16 h. (Right) Very low potassium (0.5 mEq/L) for 16 h. As
demonstrated, SLC26A7 is expressed predominantly in punctate intracellular structures in normal potassium media (left) but is
detected predominantly in the plasma in low-potassium medium for 16 h (right two). Second from left shows significant
cytoplasmic localization with little membrane expression after short (2 h) incubation in low-potassium medium. Red, phalloidin;
green, SLC26A7-GFP.
(P ⬍ 0.001 versus normal-potassium medium), and 81 ⫾ 5% in
0.5 mEq/L potassium for 16 h (P ⬍ 0.0001 versus normalpotassium medium), with a total of 30 cells analyzed for each
potassium concentration. A shorter exposure of MDCK cells (2
h) to low-potassium medium (2 mEq/L) did not increase the
membrane abundance of SLC26A7 (Figure 7, second panel
from left). Transfection with the control construct (empty GFP
with no SLC26A7 insert) showed that the GFP was retained in
the cytoplasm in low-potassium medium at 2 and 16 h (data not
shown) in a manner that was indistinguishable from the normal
medium (Figure 2A).
tween cell membrane and intracellular compartments. Epitopetagged full-length human SLC26A7 was detected predominantly
as punctate distribution in the cytoplasm (Figure 2). However,
in hypertonic medium (50 mM NaCl or 100 mM mannitol
added to the isotonic medium [pH 7.4]), SLC26A7 appeared
predominantly on the plasma membrane (Figure 3). The presence of MAPK inhibitors completely blocked the trafficking of
SLC26A7 to the plasma membrane (Figure 4). Double labeling
with the transferrin receptor markers demonstrated that
SLC26A7 shows significant expression in recycling endosomes
SLC26A7 Expression in Water-Loaded Rats
The results of the experiments in Figures 2, 3, and 4 as well as
a recently published report (32) demonstrate that increased interstitial tonicity increases, whereas decreased interstitial tonicity
decreases the membrane abundance of SLC26A7 in OMCD cells.
To examine the effect of reduced interstitial osmolarity on
SLC26A7 in a more detailed manner, we subjected rats to water
loading for 5 d (see Materials and Methods) and then examined
them by immunofluorescence labeling. Water-loaded rats displayed significant polyuria (increased urine output) and reduced
urine osmolarity versus control rats, consistent with published
reports. As demonstrated in low- and high-magnification images
of the kidney outer medulla (Figure 8, top and bottom, respectively), the number of OMCD cells that displayed SLC26A7 expression on their basolateral membrane decreased significantly in
water-loaded rats (Figure 8, right) versus normal control rats (Figure 8, left). The reduction in membrane expression was associated
with a reciprocal increase in SLC26A7 abundance in the cytoplasm
in OMCD cells (Figure 8).
Discussion
Immunocytochemical staining (7,8,19) as well as immunoblot
analysis (Figure 1) demonstrated that in addition to basolateral
membrane, significant intracellular localization of SLC26A7 is
observed in kidney medullary collecting or gastric parietal
cells, raising the possibility that alteration in SLC26A7 abundance at the cell surface may occur by changes in traffic be-
Figure 8. Effect of water loading on SLC26A7 expression in rat
kidney OMCD. (Top) Low magnification. (Bottom) High magnification. Immunofluorescence labeling demonstrates significant reduction in SLC26A7 expression on the basolateral membrane of OMCD in water-loaded rats (right) when compared
with normal rats (left). The reduction in membrane expression
was associated with reciprocal increase in SLC26A7 abundance
in the cytoplasmic compartment.
J Am Soc Nephrol 17: 956 –967, 2006
(Figure 5). The signal directing the trafficking of SLC26A7 between endosomes and the plasma membrane resides in its Cterminal end (Figure 6). Last, we observe that SLC26A7 was
detected predominantly in plasma membrane in potassium-depleted medium versus cytoplasmic localization in normal-potassium media (Figure 7). The trafficking to the membrane in hypertonic or potassium-depleted medium was time dependent. Water
loading, which decreases the medullary interstitial osmolality,
decreased the membrane abundance of SLC26A7 (Figure 8).
The detection of SLC26A7 in the plasma membrane of cells that
were exposed to hypertonic medium in vitro suggests that alterations in kidney medullary interstitial osmolarity in vivo can affect
the abundance of SLC26A7 in the basolateral membrane of A-IC
cells in OMCD. In support of this possibility, studies in Brattleboro
rats, which lack endogenous vasopressin and, as a result, have
reduced medullary interstitial osmolarity (33,34), showed very
little SLC26A7 expression on the basolateral membrane of medullary collecting duct cells (32). It is interesting that treatment with
vasopressin, which normalizes the medullary interstitial osmolarity (33,34), resulted in significant upregulation of SLC26A7 on the
basolateral membrane of OMCD cells without affecting its mRNA
expression levels (32). These results are consistent with posttranscriptional regulation of SLC26A7 by vasopressin in Brattleboro
rats and are in agreement with enhanced trafficking of SLC26A7
to the membrane by hypertonicity in vitro (Figures 3, 4, and 5). In
contrast to SLC26A7, AE1 is abundantly expressed in the basolateral membrane of A-IC in OMCD in Brattleboro rats, and its
expression actually decreases, albeit very mildly, in response to
vasopressin (32). The robust SLC26A7 appearance in basolateral
membranes and its distinct response versus AE1 to vasopressin
suggest differential regulation of AE1 and SLC26A7 in pathophysiologic states.
A recent report showed that in rats that were subjected to water
deprivation for 3 d, the mRNA and protein expression of
SLC26A7 in OMCD were enhanced (19). The transcriptional regulation of SLC26A7 in water deprivation does not conflict with its
posttranscriptional regulation by hypertonicity in cultured medullary collecting duct cells in vitro (Figures 3, 4, and 5) or by
vasopressin in Brattleboro rats (32). It is plausible that increased
interstitial tonicity of medulla can increase the expression of
SLC26A7 at both transcriptional and posttranscriptional levels,
with transcriptional regulation becoming the dominant regulatory
mode at longer duration or more severe degree of hypertonicity
such as 3 d of water deprivation. Alternatively, it is possible that
the transcriptional regulation of SLC26A7 in water deprivation is
due to factors other than increased medullary osmolarity such as
volume depletion, activation of renin angiotensin and sympathetic
systems, or decreased kidney perfusion, (35,36).
The trafficking of ion transporters, in particular acid-base
transporters such as H⫹-ATPase, between intracellular structures and the plasma membrane has been well described (37–
39). However, unlike these ion transporters, the trafficking of
SLC26A7 to the membrane is slow and time dependent (Figures
3 and 4). As a Cl⫺/HCO3⫺ exchanger that is adapted to hypertonicity, SLC26A7 activation results in the entry of chloride,
which subsequently regulates cell volume. In this regard,
SLC26A7 function may be similar to betaine transporter, which
SLC26A7, Endosomes, and Membrane Trafficking
965
is involved in cell volume regulation and shows a time-dependent trafficking in hypertonicity that is very similar to SLC26A7
(40). The bicarbonate exit that is coupled to chloride entry
suggests that SLC26A7, which exchanges chloride for bicarbonate across the basolateral membrane of acid-secreting OMCD
cells, will also regulate cell pH and/or bicarbonate exit in
hypertonicity in a time-dependent manner. The lack of acute
regulation of SLC26A7 in hypertonicity suggests either that
AE1, which co-localizes with SLC26A7, is not acutely inhibited
by hypertonicity or that other acid-base transporters may be
activated immediately after the generation of hypertonicity.
The localization of SLC26A7 in the endosomes is unique
among SLC26 members. SLC26A1 (SAT1) was expressed predominantly on the basolateral membrane of MDCK cells in
isotonic medium (Figure 6, B and C) and remained in the
membrane in hypertonic medium (data not shown). Furthermore, our preliminary studies demonstrate that epitope-tagged
AE1, which co-localizes with SLC26A7 on the basolateral membrane of cells in OMCD (8,19), is expressed predominantly in
plasma membrane in MDCK cells in isotonic medium (data not
shown), confirming published reports (41). Taken together,
these results suggest that SLC26A7 trafficking is distinct from
SLC4A1 and other SLC26 anion exchangers.
The targeting of SLC26A7 to the membrane was completely
prevented in the presence of MAPK inhibitors (Figure 4), indicating that MAPK is activated and plays an important role in enhanced cell surface expression of SLC26A7 in a hypertonic environment. These findings are consistent with published reports
indicating that hypertonicity increases the membrane targeting
and/or activity of several ion transporters, including Na-K-ATPase, Glut 4, and AE2 (42– 44). Whether the activation of MAPK is
in response to the hypertonicity or the consequent cell shrinkage
remains speculative (27,28). Authors could not find any published
studies in mammalian cells demonstrating a critical role for
MAPK in the alteration in endosomal/surface membrane trafficking of acid-base transporters in a high osmotic environment. As
such, these results may be the first report on such finding. Furthermore, whereas the trafficking of the recycling endosomes to
the membrane has been demonstrated in acute hypertonicity, little
information is available regarding their targeting to the membrane
in long-term (16 h) hypertonicity. In addition to blocking membrane targeting, p38 MAPK inhibitor seemed to reduce the overall
signal intensity of SLC26A7, suggesting a possible effect on its
abundance (Figure 4). Whether MAPK inhibitors directly reduce
the synthesis of new SLC26A7 protein remains speculative. A
more plausible explanation is that the SLC26A7 protein that is
destined for the plasma membrane undergoes enhanced degradation in the lysosomes after being retained in the cytoplasm.
The truncated SLC26A7, lacking the last 16 aa, showed a
distribution pattern that was very distinct from the full-length
protein in isotonic and hypertonic media (Figure 6, A and B).
Similar to the truncated SLC26A7, the A7/A1 chimera was
expressed diffusely in the cytoplasm in isotonic medium and
showed little membrane expression in hypertonic medium (Figure 6D). These latter experiments confirm the results of the
studies with the C-terminal–truncated mutant and suggest that
the determinant site that directs the trafficking of SLC26A7
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Journal of the American Society of Nephrology
from the recycling endosomes to the membrane resides in its
C-terminal end. Future studies should focus on the identification of the aa residues that are responsible for the targeting of
SLC26A7 to the endosomes and its trafficking to the membrane
in pathophysiologic states.
Potassium depletion causes metabolic alkalosis in mammals
(45,46), in large part as a result of increased absorption of
bicarbonate in the kidney proximal tubule and the collecting
duct (30,31,47,48). Rats that were fed a potassium-free diet
developed significant hypokalemia or intracellular potassium
depletion as early as 24 h after ingestion of the diet (49). In vitro
microperfusion studies in the collecting duct demonstrated
increased absorption of bicarbonate in OMCD in rats that were
fed a potassium-free diet for 7 d (30). Molecular studies demonstrated increased abundance of apical H⫹-ATPase and colonic H⫹-K⫹-ATPase in OMCD of potassium-depleted rats
and mice (50,51). However, the identity of the basolateral Cl⫺/
HCO3⫺ exchanger in the OMCD cells that is upregulated in
potassium depletion, whether SLC26A7 or AE1, remained and
still remains speculative. These results demonstrate that
SLC26A7 abundance in the membrane was increased significantly in potassium-depleted medium (Figure 7). On the basis
of our in vitro experiments, we propose that potassium depletion increases the abundance of SLC26A7 in basolateral membrane of rat OMCD in vivo, thereby increasing net bicarbonate
absorption in hypokalemia.
J Am Soc Nephrol 17: 956 –967, 2006
5.
6.
7.
8.
9.
10.
11.
Conclusion
SLC26A7 displays unique subcellular distribution in kidney
cells, with predominant abundance in endosomes in normalpotassium isotonic medium and almost exclusive detection in
the membrane in either hypertonic or potassium-depleted medium. The trafficking to the cell surface suggests novel functional upregulation of SLC26A7 in states that are associated
with increased medullary tonicity or hypokalemia. Additional
studies in pathophysiologic conditions in rats and more specifically in genetically engineered mice that lack SLC26A7 should
clarify the role of SLC26A7 in enhanced bicarbonate absorption
in OMCD in hypokalemia and in acid-base regulation in conditions that are associated with increased medullary tonicity.
12.
14.
13.
15.
Acknowledgments
These studies were supported by National Institutes of Health grant
DK 62809, a Merit Review Grant, a Cystic Fibrosis Foundation grant,
and grants from Dialysis Clinic Incorporated (to M.S.).
References
1. Alper SL, Darman RB, Chernova MN, Dahl NK: The AE
gene family of Cl/HCO3⫺ exchangers. J Nephrol 15[Suppl
5]: S41–S53, 2002
2. Soleimani M, Burnham CE: Na⫹:HCO(3⫺) cotransporters
(NBC): Cloning and characterization. J Membr Biol 183:
71– 84, 2001
3. Mount DB, Romero MF: The SLC26 gene family of multifunctional anion exchangers. Pflugers Arch 447: 710 –721, 2004
4. Melvin JE, Park K, Richardson L, Schultheis PJ, Shull GE:
Mouse down-regulated in adenoma (DRA) is an intestinal
Cl(⫺)/HCO(3)(⫺) exchanger and is up-regulated in colon
16.
17.
18.
19.
of mice lacking the NHE3 Na(⫹)/H(⫹) exchanger. J Biol
Chem 274: 22855–22861, 1999
Soleimani M, Greeley T, Petrovic S, Wang Z, Amlal H,
Kopp P, Burnham CE: Pendrin: An apical Cl⫺/OH⫺/
HCO3⫺ exchanger in the kidney cortex. Am J Physiol Renal
Physiol 280: F356 –F364, 2001
Wang Z, Petrovic S, Mann E, Soleimani M: Identification of
an apical Cl(⫺)/HCO3(⫺) exchanger in the small intestine.
Am J Physiol Gastrointest Liver Physiol 282: G573–G579, 2002
Petrovic S, Ju X, Barone S, Seidler U, Alper SL, Lohi H,
Kere J, Soleimani M: Identification of a basolateral Cl⫺/
HCO3⫺ exchanger specific to gastric parietal cells. Am J
Physiol Gastrointest Liver Physiol 284: G1093–G1103, 2003
Petrovic S, Barone S, Xu J, Conforti L, Ma L, Kujala M, Kere J,
Soleimani M: SLC26A7: A basolateral Cl⫺/HCO3⫺ exchanger
specific to intercalated cells of the outer medullary collecting
duct. Am J Physiol Renal Physiol 286: F161–F169, 2004
Xu J, Henriksnas J, Barone S, Witte D, Shull GE, Forte JG,
Holm L, Soleimani M: SLC26A9 is expressed in gastric
surface epithelial cells, mediates Cl⫺/HCO3⫺ exchange,
and is inhibited by NH4⫹. Am J Physiol Cell Physiol 289:
C493–C505, 2005
Royaux IE, Wall SM, Karniski LP, Everett LA, Suzuki K,
Knepper MA, Green ED: Pendrin, encoded by the Pendred
syndrome gene, resides in the apical region of renal intercalated cells and mediates bicarbonate secretion. Proc Natl
Acad Sci U S A 98: 4221– 4226, 2001
Wall SM, Hassell KA, Royaux IE, Green ED, Chang JY, Shipley GL, Verlander JW: Localization of pendrin in mouse
kidney. Am J Physiol Renal Physiol 284: F229 –F241, 2003
Wagner CA, Finberg KE, Stehberger PA, Lifton RP,
Giebisch GH, Aronson PS, Geibel JP: Regulation of the
expression of the Cl⫺/anion exchanger pendrin in mouse
kidney by acid-base status. Kidney Int 62: 2109 –2117, 2002
Wang Z, Wang T, Petrovic S, Tuo B, Riederer B, Barone S,
Lorenz JN, Seidler U, Aronson PS, Soleimani M: Renal and
intestinal transport defects in Slc26a6-null mice. Am J
Physiol Cell Physiol 288: C957–C965, 2005
Knauf F, Yang CL, Thomson RB, Mentone SA, Giebisch G,
Aronson PS: Identification of a chloride-formate exchanger
expressed on the brush border membrane of renal proximal
tubule cells. Proc Natl Acad Sci U S A 98: 9425–9430, 2001
Petrovic S, Ma L, Wang Z, Soleimani M: Identification of an
apical Cl⫺/HCO3⫺ exchanger in rat kidney proximal tubule. Am J Physiol Cell Physiol 285: C608 –C617, 2003
Xie Q, Welch R, Mercado A, Romero MF, Mount DB:
Molecular characterization of the murine Slc26a6 anion
exchanger: Functional comparison with Slc26a1. Am J
Physiol Renal Physiol 283: F826 –F838, 2002
Lohi H, Kujala M, Makela S, Lehtonen E, Kestila M,
Saarialho-Kere U, Markovich D, Kere J: Functional characterization of three novel tissue-specific anion exchangers
SLC26A7, -A8, and -A9. J Biol Chem 277: 14246 –14254, 2002
Vincourt JB, Jullien D, Kossida S, Amalric F, Girard JP:
Molecular cloning of SLC26A7, a novel member of the
SLC26 sulfate/anion transporter family, from high endothelial venules and kidney. Genomics 79: 249 –256, 2002
Barone S, Amlal H, Xu J, Kujala M, Kere J, Petrovic S, Soleimani M: Differential regulation of basolateral Cl⫺/
HCO3⫺ exchangers SLC26A7 and AE1 in kidney outer medullary collecting duct. J Am Soc Nephrol 15: 2002–2011, 2004
J Am Soc Nephrol 17: 956 –967, 2006
20. Schuster VL: Function and regulation of collecting duct
intercalated cells. Annu Rev Physiol 55: 267–288, 1993
21. Weiner ID, Wingo CS, Hamm LL: Regulation of intracellular
pH in two cell populations of inner stripe of rabbit outer
medullary collecting duct. Am J Physiol 265: F406 –F415, 1993
22. Kim KH, Shcheynikov N, Wang Y, Muallem S: SLC26A7 is
a Cl⫺ channel regulated by intracellular pH. J Biol Chem
280: 6463– 6470, 2005
23. Li HC, Worrell RT, Matthews JB, Husseinzadeh H,
Neumeier L, Petrovic S, Conforti L, Soleimani M: Identification of a carboxyl-terminal motif essential for the targeting of Na⫹-HCO3⫺ cotransporter NBC1 to the basolateral
membrane. J Biol Chem 279: 43190 – 43197, 2004
24. Li HC, Szigligeti P, Worrell RT, Matthews JB, Conforti L,
Soleimani M: Missense mutations in Na⫹:HCO3⫺ cotransporter NBC1 show abnormal trafficking in polarized kidney cells: A basis of proximal renal tubular acidosis. Am J
Physiol Renal Physiol 289: F61–F71, 2005
25. Kim SW, Kim JW, Choi KC, Ma SK, Oh Y, Jung JY, Kim J,
Lee J: Indomethacin enhances shuttling of aquaporin-2
despite decreased abundance in rat kidney. J Am Soc Nephrol 15: 2998 –3005, 2004
26. Wojtaszek PA, Heasley LE, Berl T: In vivo regulation of
MAP kinases in Ratus norvegicus renal papilla by water
loading and restriction. J Clin Invest 102: 1874 –1881, 1998
27. Roger F, Martin PY, Rousselot M, Favre H, Feraille E: Cell
shrinkage triggers the activation of mitogen-activated protein kinases by hypertonicity in the rat kidney medullary
thick ascending limb of the Henle’s loop. Requirement of
p38 kinase for the regulatory volume increase response.
J Biol Chem 274: 34103–34110, 1999
28. Sheikh-Hamad D, Di Mari J, Suki WN, Safirstein R, Watts
BA 3rd, Rouse D: p38 kinase activity is essential for osmotic induction of mRNAs for HSP70 and transporter for
organic solute betaine in Madin-Darby canine kidney cells.
J Biol Chem 273: 1832–1837, 1998
29. Lee A, Beck L, Markovich D: The mouse sulfate anion transporter gene Sat1 (Slc26a1): Cloning, tissue distribution, gene
structure, functional characterization, and transcriptional regulation thyroid hormone. DNA Cell Biol 22: 19 –31, 2003
30. Nakamura S, Wang Z, Galla JH, Soleimani M: K⫹ depletion increases. Am J Physiol 274: F687–F692, 1998
31. Wingo CS: Active proton secretion and potassium absorption in the rabbit outer medullary collecting duct. Functional evidence for proton-potassium-activated adenosine
triphosphatase. J Clin Invest 84: 361–365, 1989
32. Petrovic S, Amlal H, Ma L, Sun X, Karet F, Barone S,
Soleiman M: Vasopressin induces the expression of the
Cl⫺/HCO3⫺ exchanger SLC26A7 in the kidney medullary collecting duct of Brattleboro rats. Am J Physiol Renal
Physiol 2006, in press
33. Blumenfeld JD, Hebert SC, Heilig CW, Balschi JA, Stromski
ME, Gullans SR: Organic osmolytes in inner medulla of
Brattleboro rat: Effects of ADH and dehydration. Am J
Physiol 256: F916 –F922, 1989
34. DiGiovanni SR, Nielsen S, Christensen EI, Knepper MA:
Regulation of collecting duct water channel expression by
vasopressin in Brattleboro rat. Proc Natl Acad Sci U S A 91:
8984 – 8988, 1994
35. Bird JE, Blantz RC: Acute renal failure: The glomerular and
tubular connection. Pediatr Nephrol 1: 348 –358, 1987
SLC26A7, Endosomes, and Membrane Trafficking
967
36. Ishikawa E: Experimental study of effect of water deprivation-induced dehydration on renal function in rats. Hinyokika Kiyo 33: 1342–1348, 1987
37. Sautin YY, Lu M, Gaugler A, Zhang L, Gluck SL: Phosphatidylinositol 3-kinase-mediated effects of glucose on vacuolar H⫹-ATPase assembly, translocation, and acidification
of intracellular compartments in renal epithelial cells. Mol
Cell Biol 25: 575–589, 2005
38. Brown D, Breton S: H(⫹)V-ATPase-dependent luminal
acidification in the kidney collecting duct and the epididymis/vas deferens: Vesicle recycling and transcytotic pathways. J Exp Biol 203: 137–145, 2000
39. Thurmond DC, Ceresa BP, Okada S, Elmendorf JS, Coker
K, Pessin JE: Regulation of insulin-stimulated GLUT4
translocation by Munc18c in 3T3L1 adipocytes. J Biol Chem
273: 33876 –33883, 1998
40. Kempson SA, Parikh V, Xi L, Chu S, Montrose MH: Subcellular redistribution of the renal betaine transporter during hypertonic stress. Am J Physiol Cell Physiol 285: C1091–C1100, 2003
40. Beckmann R, Toye AM, Smythe JS, Anstee DJ, Tanner MJ:
An N-terminal GFP tag does not alter the functional expression to the plasma membrane of red cell and kidney
anion exchanger (AE1) in mammalian cells. Mol Membr Biol
19: 187–200, 2002
42. Chernova MN, Stewart AK, Jiang L, Friedman DJ, Kunes YZ,
Alper SL: Structure-function relationships of AE2 regulation
by Ca(i)(2⫹)-sensitive stimulators NH(4⫹) and hypertonicity. Am J Physiol Cell Physiol 284: C1235–C1246, 2003
43. Randhawa VK, Thong FS, Lim DY, Li D, Garg RR, Rudge
R, Galli T, Rudich A, Klip A: Insulin and hypertonicity
recruit GLUT4 to the plasma membrane of muscle cells
by using N-ethylmaleimide-sensitive factor-dependent
SNARE mechanisms but different v-SNAREs: Role of TIVAMP. Mol Biol Cell 15: 5565–5573, 2004
44. Pihakaski-Maunsbach K, Tokonabe S, Vorum H, Rivard CJ,
Capasso JM, Berl T, Maunsbach AB: The gamma-subunit of
Na-K-ATPase is incorporated into plasma membranes of
mouse IMCD3 cells in response to hypertonicity. Am J
Physiol Renal Physiol 288: F650 –F657, 2005
45. Hulter HN, Sigala JF, Sebastian A: K⫹ deprivation potentiates the renal alkalosis-producing effect of mineralocorticoid. Am J Physiol 235: F298 –F309, 1978
46. Alpern RJ, Emmett M, Seldin DW: Metabolic alkalosis. In: The
Kidney: Physiology and Pathophysiology, 2nd Ed., edited by Seldin
DW, Giebish G, New York, Raven Press, 1992, pp 2733–2756
47. Kunau RT Jr, Frick A, Rector FC Jr, Seldin DW: Micropuncture study of the proximal tubular factors responsible for
the maintenance of alkalosis during potassium deficiency
in the rat. Clin Sci 34: 223–231, 1968
48. Soleimani M, Bergman JA, Hosford MA, McKinney TD:
Potassium depletion increases luminal Na⫹/H⫹ exchange
and basolateral Na⫹:CO3⫺:HCO3⫺ cotransport in rat renal cortex. J Clin Invest 86: 1076 –1083, 1990
49. Amlal H, Krane CM, Chen Q, Soleimani M: Early polyuria
and urinary concentrating defect in potassium deprivation.
Am J Physiol Renal Physiol 279: F655–F663, 2000
50. DuBose TD Jr, Codina J, Burges A, Pressley TA: Regulation
of H(⫹)-K(⫹)-ATPase expression in kidney. Am J Physiol
269: F500 –F507, 1995
51. Silver RB, Soleimani M: H⫹-K⫹-ATPases: Regulation and role
in pathophysiological states. Am J Physiol 276: F799–F811, 1999