From the Service de Biologie Cellulaire,
Département de Biologie Cellulaire et Moléculaire,
CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France, and the
§ Institut des Sciences Végétales, CNRS, 91198 Gif-sur-Yvette Cedex, France
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ABSTRACT |
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A facilitated diffusion for glycerol is present in human erythrocytes. Glycerol transporters identified to date belong to the Major Intrinsic Protein (MIP) family of integral membrane proteins, and one of them, aquaporin-3 (AQP3), has been characterized in mammals. Using an antibody raised against a peptide corresponding to the rat AQP3 carboxyl terminus, we examined the presence of AQP3 in normal and Colton-null (aquaporin-1 (AQP1)-deficient) human erythrocytes. Three immunoreactive bands were detected on immunoblots of both normal and Colton-null red cells, very similar to the bands revealed in rat kidney, a material in which AQP3 has been extensively studied. By immunofluorescence, anti-AQP3 antibodies stained the plasma membranes of both normal and Colton-null erythrocytes. Glycerol transport was measured on intact erythrocytes by stopped-flow light scattering and on one-step pink ghosts by a rapid filtration technique. Glycerol permeability values, similar in both cell types, suggest that AQP1 does not represent the major path for glycerol movement across red blood cell membranes. Furthermore, pharmacological studies showed that Colton-null red cells remain sensitive to water and glycerol flux inhibitors, supporting the idea that another proteinaceous path, probably AQP3, mediates most of the glycerol movements across red cell membranes and represents part of the residual water transport activity found in AQP1-deficient red cells.
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INTRODUCTION |
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Human erythrocytes are highly permeable to water, urea, and glycerol (1-3). The existence of membrane proteins that facilitate water and solute movements in these cells has been postulated, but most of these proteins remain to be characterized. In 1992, Agre and co-workers identified a very abundant protein of the human red blood cell as the first water-selective channel that was named aquaporin-1 (AQP1)1 (4). Colton-null erythrocytes, lacking AQP1, exhibit a reduced osmotic water permeability (5). However, these cells are found to be residually permeable to water, suggesting the presence of additional, non-AQP1, water channels.
The existence of a protein-mediated glycerol transport in erythrocytes has relied mostly on pharmacological evidence. In particular, inhibition of glycerol transport by sulfhydryl reagents, phloretin, and copper ions has been reported (1, 3, 6). Yet, the identity of the erythrocyte glycerol carrier remains unknown. The glycerol facilitators identified to date all belong to the Major Intrinsic Protein (MIP) family (7). They have been characterized in several organisms: GlpF, a bacterial glycerol permease facilitator (8); Fps1p, a yeast glycerol exporter (9); aquaporin-3 (AQP3), initially characterized in mammalian kidney (10-12); and AQP7, recently identified in rat testis (13). Compared with other mammalian aquaporins, which are selective mostly for water, AQP3 is moderately permeable to water, but highly permeable to glycerol and possibly to urea (11, 12, 14). AQP3 expression has been reported in several mammalian tissues, including kidney, intestine, stomach, spleen, and eye (11, 15, 16).
The aim of this study was to investigate the nature of the red cell glycerol transporter. Due to the high permeability of AQP3 to glycerol, we hypothesized that AQP3, if expressed in human erythrocytes, could account for their facilitated glycerol permeability. Since AQP1 itself was previously suggested to transport glycerol (17), in our study we used red cells from an individual who does not express AQP1, associated to the Colton group antigens (18). We found no difference in glycerol transport between the normal and Colton-null (Co(a-b-)) red cells, suggesting that AQP1 does not constitute a major pathway for glycerol. By contrast, our immunological data demonstrate that AQP3 is expressed in both normal and Colton-null red cells. Pharmacological evidence strongly supports the hypothesis that AQP3 could account for the high glycerol permeability of these cells and that, in Colton-null erythrocytes, AQP3 could constitute a residual, mercury-sensitive pathway for water.
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EXPERIMENTAL PROCEDURES |
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Materials-- Control and Co(a-b-) red cells (18) were obtained from the Institut National de Transfusion Sanguine (Paris, France). Rabbit polyclonal antibodies against purified human AQP1 were raised as described previously (19). Polyclonal antibodies against a 26-amino acid synthetic peptide corresponding to the carboxyl terminus of rat kidney AQP3 were raised in rabbits (Neosystem, France). The anti-AQP3 serum was affinity purified by chromatography (immunobilization Kit 2, Pierce). The secondary antibodies used in Western blot experiments (goat anti-rabbit-peroxidase) were from Promega (Madison, WI); those used in immunofluorescence (mouse anti-rabbit-CY3) were from Jackson Immunoresearch. 14C-Glycerol (120 mCi/mM) was synthesized by the Service des Molécules Marquées (CEA/Saclay, France). Phenylmethylsulfonyl fluoride, pepstatin, DIDS, and phloretin were from Sigma Immunochemicals. Reagents used in electrophoresis and immunoblotting were from Pharmacia LKB Biotechnology (Uppsala, Sweden) and from Amersham Corporation (Buckinghamshire, United Kingdom). All other reagents were at least analytical grade.
Red Cell Preparation-- Thawed normal and Co(a-b-) red cells were washed three times in a Carlsen buffer (3) (in mM): NaCl, 154; D-glucose, 5; KH2PO4, 0.25; Na2HPO4, 0.25; pH 7.4, 300 mosm/kg H2O, by centrifugations at 2,000 × g for 10 min at 8 °C. The cells were then resuspended in the Carlsen buffer at a hematocrit of 1.5%, corresponding to ~107 cells/ml and directly used for stopped-flow experiments.
Stopped-flow Experiments--
Kinetics of red cell volume
changes were followed at 26 °C, by 90° light scattering
(exc = 600 nm) using a stopped-flow spectrophotometer (SFM3, Biologic, Claix, France). An emission wavelength > 500 nm
was obtained by using a cut-on filter (Specivex, J526a). Cell osmotic
water permeability was measured by mixing 100 µl of cells with an
equal volume of a hyperosmotic solution of sucrose to produce a 100 mosm/kg H2O inwardly-directed osmotic gradient. Data from
at least 10 time-courses were averaged and fitted to single exponential
functions (k is the rate constant, in s
1) by
using the Simplex procedure of the BIOKINE software (Biologic, France).
The osmotic water permeability coefficient,
Pf, in cm/s, was determined using the
following equation (5),
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(Eq. 1) |
One-step Pink Ghost Preparation and Glycerol Efflux Measurements-- The hemoglobin-free erythrocyte ghosts were prepared by the method described by Ojcius (22). Briefly, washed normal and Colton-null red cells were lysed in a hypotonic medium, 5 mM Na2HPO4, pH 8.0, containing 20 µg/ml phenylmethylsulfonyl fluoride, and 2 µg/ml pepstatin, and centrifuged at 25,000 × g for 20 min at 4 °C. The membranes were resuspended in 5 mM Na2HPO4, 100 mM NaCl, and 100 mM glycerol, pH 8.0, at 108 ghosts/ml, and sealed by incubation at 37 °C for 1 h. Glycerol effluxes were measured at 20 °C by a rapid filtration method (23). Samples of 6 × 107 ghosts were loaded for 1 h with 14C-glycerol (5 µCi/ml) and 3H-mannitol (5 µCi/ml). Mannitol was used as an impermeant intracellular marker, to quantify the ghosts retained on glass-fiber filters (Whatman GF/A). The filters were rinsed over a preset period of 500 ms to 10 s at a flow rate of 1 ml/s. The cold washout solution contained 100 mM raffinose instead of glycerol. The retained radioactivity (14C/3H) was quantified by liquid scintillation. Glycerol permeability coefficient (Pgly, in cm/s) was calculated from the rate constant of the exponential time course of glycerol efflux and from ghost surface area to initial volume ratio.
Electrophoresis and Immunoblotting-- Human and rat hemoglobin-free ghosts were prepared as described above. Purified human AQP1 was obtained as described previously (17). Bloodless rat kidney outer medulla was prepared according to Ecelbarger et al. (15). Protein concentration of the membrane fractions was measured using the Pierce BCA Protein Assay reagent kit. Five µg of membrane proteins and 0.6 µg of pure AQP1 were denatured at 65 °C for 10 min in Laemmli sample buffer (24), and separated by 12.5% SDS-polyacrylamide gel electrophoresis. Proteins were transferred to PVDF membranes (NEN Life Science Products) and probed with the affinity-purified anti-AQP3 at approximately 0.25 µg/ml or the anti-AQP1 whole serum diluted 1:500. Controls were carried out by using the affinity-purified anti-AQP3 preabsorbed for 10 min with 0.2 mg/ml of immunizing peptide. Immunoreactive proteins were revealed by the ECL Western blotting technique (Enhanced ChemiLuminescence, Amersham Pharmacia Biotech).
Indirect Immunofluorescence--
Erythrocytes washed in PBS were
fixed for 1 h in 4% paraformaldehyde in PBS, followed by PBS
washes. After a 3-min centrifugation at 2000 × g, the
cell pellet was infiltrated in PBS containing 2.3 M
sucrose. The samples were frozen in liquid nitrogen, and 0.75 µm
sections were cut on an ultracryomicrotome at 70 °C (Reichert Ultracut, Leica, Wien, Austria) and collected on Superfrost Plus glass
slides. Adult Sprague-Dawley male rats were anesthetized (pentobarbital, 5 mg/100 g of body weight, intraperitoneally) and
perfused with a fixative containing 7.1 mM
Na2HPO4, 30.4 mM NaH2PO4, 75 mM lysine, 10 mM sodium periodate, and 2% paraformaldehyde (pH 7.4).
Kidneys were removed, sliced, and kept in fixative overnight at
4 °C, followed by extensive PBS washes. Tissue slices were infiltrated overnight in PBS containing 30% sucrose and frozen in
liquid N2. Cryosections of 5 µm were cut on a cryostat
(Leica) and collected on Superfrost Plus glass slides. The slides were incubated for 5 min in PBS containing 1% bovine serum albumin (PBS/BSA), followed by 1-h incubation in primary antibody (1:300 dilution of anti-AQP3 antiserum with or without preincubation of the
serum with the peptide used for immunization, or 1:100 dilution of
anti-AQP1 anti-serum) in PBS/BSA. The sections were then washed 3 × 10 min in PBS, followed by a 45-min incubation in CY3-conjugated
mouse anti-rabbit antibodies (6 µg/ml) in PBS/BSA. The sections were
washed 2 × 10 min in PBS and mounted for observation under a
fluorescence microscope (Olympus Vanox-T).
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RESULTS |
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Immunoblot Analysis of AQP3 Expression in Various Tissues-- Fig. 1A shows results from immunoblots probed with affinity-purified antibodies raised against a rat AQP3 carboxyl-terminus peptide. In rat renal outer medulla (Fig. 1A, lane 4), a characteristic profile of three stained bands was observed: one band at ~25 kDa and two broader bands at 33-40 and >67 kDa. None of these bands was detected when the anti-AQP3 antibody was preabsorbed with the immunizing peptide (Fig. 1A, lane 1). A similar profile with three stained bands was also observed in rat red cell membranes probed with the anti-AQP3 antibody (Fig. 1A, lane 5), suggesting that, in addition to kidney, AQP3 was also expressed in rat erythrocytes. Again, no signal was detected when the anti-AQP3 was preincubated with the immunizing peptide (Fig. 1A, lane 2). Although the 26 amino acids of human AQP3 carboxyl terminus differ from those of rat by three residues, the anti-AQP3 antibody also recognized three bands in human normal ghosts (Fig. 1A, lane 7): a faint 25-kDa band and two broad 37-48-kDa and >70-kDa bands. An identical signal was observed in Co(a-b-) ghosts (Fig. 1A, lane 6). Similar to controls with rat membrane samples, no signal was detected in human red cells when the anti-AQP3 was first blocked by the immunizing peptide (Fig. 1A, lane 3). The anti-AQP3 antibody failed to recognize human AQP1 purified from red blood cells (Fig. 1A, lane 8), thus confirming the specificity of anti-AQP3 antibodies. Most importantly, our results indicate that AQP3 is present in normal and Colton-null human red cells.
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Indirect Immunofluorescence-- In experiments on human red blood cells, the anti-AQP3 serum strongly stained the plasma membranes of both normal (Fig. 2a) and Colton-null cells (Figs. 2, c and d). No staining was observed when the antiserum was preincubated with the peptide used for immunization (Fig. 2b, normal cells). In contrast, the polyclonal antiserum against purified AQP1 stained the plasma membranes of normal human red blood cells (Fig. 2e) but not those of Colton-null cells (Fig. 2f). In rat kidney, the anti-AQP3 antibodies recognized the basolateral plasma membranes of collecting duct principal cells (Fig. 2g) but not those of adjacent intercalated cells (Fig. 2g, arrowheads). Although no staining was observed in other cell types of the rat kidney nephron, as previously reported (16), we also found staining of rat red blood cells (Fig. 2h). Altogether, these results confirm the presence of AQP3 in both human (normal and Colton-null) and rat red blood cell membranes.
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Water and Glycerol Permeabilities of Normal and Colton-null Red
Cells--
Fig. 3A shows a
typical time course of osmotic shrinkage induced by a 100 mosm/kg
H2O sucrose gradient, carried out with thawed normal and
Colton-null red cells at 26 °C. Averaged Pf was
(1.32 ± 0.09) × 102 cm/s and (2.16 ± 0.16) × 10
3 cm/s (n = 6) for normal and
Co(a-b-), respectively. As previously reported (5), the
Pf values in Co(a-b-) cells were significantly lower
than in normal cells.
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Inhibition Studies-- The effects of transport inhibitors were tested on normal and Colton-null red cells using the stopped-flow technique. Glycerol effluxes were followed in isoosmotic conditions, to avoid initial water movements. Fig. 5 shows the effects of 0.2 mM DIDS, 0.1 or 0.5 mM phloretin, 0.1 mM CuSO4, or 0.5 mM HgCl2 on glycerol (Fig. 5A, normal; Fig. 5B, Co(a-b-)) and water (Fig. 5C, normal; Fig. 5D, Co(a-b-)) efflux rates. DIDS inhibited the glycerol outflow from normal and Co(a-b-) cells by approximately 45%. It also reduced the rate of water efflux of both cell types, with a greater efficiency in Co(a-b-) erythrocytes. A large inhibition of glycerol transport in normal and Co(a-b-) red cells was observed in the presence of 0.1 mM (80 and 67% inhibition, respectively) and 0.5 mM phloretin (93 and 73% inhibition, respectively). In addition, phloretin markedly inhibited water transport in Colton-null erythrocytes, at the 2 concentrations tested (~53% inhibition). CuSO4 significantly reduced the glycerol permeability of normal (69% inhibition) and Co(a-b-) (33% inhibition) red cells, and also strongly reduced the water permeability of Colton-null erythrocytes (60% inhibition). HgCl2 dramatically inhibited both glycerol and water transport in normal and Co(a-b-) erythrocytes. Percentages of inhibition of glycerol efflux were 75 and 72% for normal and Colton-null cells, respectively, and the water efflux was inhibited by 90% in normal and by 50% in Colton-null erythrocytes. This indicates the presence, in the latter, of a residual mercury-sensitive path for water. In conclusion, all the reagents inhibited glycerol transport in human normal and Colton-null red blood cells in a similar manner. In contrast, these molecules had differential effects on water transport in both cell genotypes. These data support the idea that the major path for water transport was disrupted in Colton-null cells while that for glycerol was not.
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DISCUSSION |
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AQP1 is the major water channel of human red blood cells. However, as we showed here and as previously reported (5), a significant mercury-sensitive part of osmotic water permeability remains in AQP1-knocked out (Colton-null) red cells. The existence of additional, non-AQP1, protein pathways for water transport has been discussed by Toon and Solomon (26). Red blood cells also exhibit a high glycerol transport capacity, whose molecular bases have remained unknown. AQP3 is a mammalian aquaporin which, in addition to being moderately permeable to water, is highly permeable to glycerol and, to a lesser extent, to urea (11, 12, 14). In the present work, we provide evidence that AQP3 is expressed in normal and Colton-null red cells and likely mediates not only the residual water transport, but also most of glycerol transport in human erythrocytes.
Human AQP3 was detected by Western blotting in both normal and Colton-null red cells and exhibited a migration profile very similar to that of rat AQP3 in renal outer medulla and erythrocytes. The broad high molecular weight bands revealed in human tissues appeared slightly larger than those of rat tissues and may correspond to different glycosylation patterns. Indirect immunofluorescence confirmed the presence of AQP3 in the plasma membranes of normal and Colton-null cells. Interestingly, the presence in red blood cells of AQP1 and AQP3 constitutes a new example of aquaporin colocalization on the same plasma membrane. AQP3 and AQP4 were colocalized on the basolateral membrane of kidney collecting duct principal cells (16). AQP1 and AQP7, another aquaporin permeable to glycerol (13), are colocalized in the kidney proximal tubule brush-border membrane (27). AQP7 (13) and AQP8 (28) were both detected in rat testis.
To investigate the relevance of AQP3-mediated transport in
erythrocytes, we characterized glycerol transport in both normal and
Colton-null red cells. For this purpose, two distinct methods were
used. The rates of red cell volume changes were estimated by
light-scattering stopped-flow experiments on intact cells submitted to
an osmotic glycerol gradient. This technique, which requires small
amounts of material, was particularly useful for the study of
Colton-null cells, which are not easily available. However, stopped-flow light scattering measurements do not allow accurate calculations of solute permeability coefficients, because of
uncertainties in red cell surface area changes and refractive index
effects (2, 29). Consequently, we considered in these experiments only
the shrinking or swelling rate constants related to glycerol movements.
In a second type of experiment, the glycerol permeability was directly
measured by a rapid filtration method using one-step pink ghosts
equilibrated in 14C-glycerol. In this case, the
Pgly can be calculated from the rate of
radioactive glycerol efflux and was 1.4 and 1.6 × 106 cm/s in normal and Colton-null ghosts, respectively,
i.e. in the range of already published values for human red
blood cells (3, 25). These values are higher than the
Pgly of bovine red cells, lacking the
facilitated diffusion mechanism for glycerol transport (30). These
values remain also 2 orders of magnitude lower than the
Pf of Colton-null cells, indicating that, even in
these cells, the rate of water flow was not limiting (29). Thus, both
stopped-flow and tracer methods indicate a similar glycerol transport
capacity in normal and AQP1-deficient red blood cells. This means that,
in its native environment, AQP1 does not constitute a major pathway for
glycerol. We previously reported that AQP1 itself had a small glycerol
permeability (17). However, the glycerol transport capacity of AQP1,
estimated after expression in Xenopus oocytes, is much lower
than that of AQP3,2 and may
therefore not be detectable in the intact red cell membrane. The
unitary Pgly of the bacterial GlpF is 2 × 105 molecules/s (31). If we assume that the unitary
Pgly of AQP3 is in the same range, the number of
AQP3 copies that account for Pgly in
erythrocytes would be around 15,000, i.e. at least ten times
lower than the number of AQP1 in these cells (32). Conversely, AQP1
appears to have a higher unitary Pf than AQP3. This, with its higher abundance, explains why AQP1 dominates water transport in the red blood cell membrane.
A pharmacological characterization of glycerol and water transport was performed in normal and Colton-null red cells. All the reagents tested exerted similar inhibitory effects on glycerol transport in normal and Colton-null red cells, confirming the idea that a protein, distinct of AQP1, mediates most of glycerol transport in the red cell membrane. Some of the inhibitors tested can have unspecific membrane effects (33), and indeed, their use may be controversial. In particular, Ma et al. (12) did not observe any inhibitory effect of 0.2 mM DIDS on glycerol transport in human erythrocytes. In contrast, an inhibition of water transport by DIDS has been reported by Toon and Solomon (34) in human red blood cells. In our experiments, DIDS was inhibitory both on glycerol and water effluxes. Copper was found to affect the glycerol transport, as already described (3), and also the water transport in an unexplained fashion. Both mercury and phloretin have been shown to alter water transport in AQP3-injected oocytes (11, 14). HgCl2 had the strongest inhibitory effects, suggesting that this reagent blocked the protein-mediated pathways for both water and glycerol transport. Phloretin markedly inhibited the residual water transport of the Colton-null cells, further supporting the hypothesis that AQP3 could be the phloretin-sensitive water channel of these AQP1-deficient cells.
Taken together, our results demonstrate the presence of functional AQP3 in the human red cell membrane, which can account for the glycerol permeability of these cells and the residual water permeability of AQP1-deficient erythrocytes. Our findings may help explain the lack of clinical defects in Colton-null patients (18). While AQP1 transports mostly water, the significance of AQP3 expression in red blood cells may reside in its ability to transport solutes. However, the role devoted to a glycerol transporter in erythrocytes is not elucidated. In contrast to testis or liver, where an important metabolism of glycerol has been described (13, 35), glycerol appears not to be metabolized by erythrocytes. The presence of a glycerol facilitator in red cells could make them less susceptible to osmotic stress upon local exposure to high glycerol concentrations. A similar hypothesis has been advanced by Macey (36), who suggested that the high urea permeability of human red blood cells could protect these cells when they reach the deeper regions of the renal medulla. AQP3 can also transport urea to some extent and yet to be discovered solutes that can be important for the red cell osmoregulation. Also, the possibility that a glycerol facilitator serves physiologically for another purpose should not be dismissed.
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ACKNOWLEDGEMENTS |
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We are very grateful to Dr. Pascal Bailly (Institut National de Transfusion Sanguine, Paris) for generously providing us with the Colton-null phenotype human red blood cells and for helpful discussions. We also thank Dr. Jean Labarre (Service de Biochimie et Génétique Moléculaire, CEA/Saclay) for providing radiolabeled 14C-glycerol. The discussions concerning Pf calculations with Dr. Jean Thiéry (CEA/Cadarache) are greatly acknowledged. We also thank our colleague Dr. Germain Rousselet for helpful discussions and suggestions and Dr. Florent Guillain for advice in stopped-flow experiments.
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FOOTNOTES |
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* 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.
¶ To whom correspondence should be addressed. Tel.: 33-1-69-08-74-00; Fax: 33-1-69-08-80-46; E-mail: tacnet{at}dsvidf.cea.fr.
1 The abbreviations used are: AQP1, aquaporin-1; AQP3, aquaporin-3; MIP, Major Intrinsic Protein; Pf, osmotic water permeability coefficient; Pgly, glycerol permeability; DIDS, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid; PBS, phosphate-buffered saline; Co(a-b-), Colton-null; BSA, bovine serum albumin.
2 N. Roudier, unpublished data.
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REFERENCES |
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