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* This work was supported by the Sundhedsvidenskabelige Forskningsråd and the NOVO NORDISK Foundation.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.
Aquaporins (AQPs) were expressed in Xenopus laevis oocytes in order to study the effects of external pH and solute structure on permeabilities. For AQP3 the osmotic water permeability, Lp, was abolished at acid pH values with a pK of 6.4 and a Hill coefficient of 3. TheLp values of AQP0, AQP1, AQP2, AQP4, and AQP5 were independent of pH. For AQP3 the glycerol permeabilityPGl, obtained from [14C]glycerol uptake, was abolished at acid pH values with a pK of 6.1 and a Hill coefficient of 6. Consequently, AQP3 acts as a glycerol and water channel at physiological pH, but predominantly as a glycerol channel at pH values around 6.1. The pH effects were reversible. The interactions between fluxes of water and straight chain polyols were inferred from reflection coefficients (ς). For AQP3, water and glycerol interacted by competing for titratable site(s): ςGl was 0.15 at neutral pH but doubled at pH 6.4. The ς values were smaller for polyols in which the —OH groups were free to form hydrogen bonds. The activation energy for the transport processes was around 5 kcal mol−1. We suggest that water and polyols permeate AQP3 by forming successive hydrogen bonds with titratable sites.
). Here we combine this method with tracer measurements in order to study the effects of H+, temperature, and solute structure on transport of water, glycerol, and other straight chain polyols. The study is performed predominantly in AQP3, but also in AQP0, AQP1, AQP2, AQP4, and AQP5.
At present, transport models are restricted to the use of macrophysical concepts such as pore diameter and pore length. This is mainly due to the lack of knowledge about the structure of the putative pore and the nature of the chemical interactions with the permeating molecules (
). Our data for AQP3 suggest a model where the permeation of water and polyols are determined by the formation of hydrogen bonds between the pore and the permeating molecule. From a physiological point of view, it is interesting that both the glycerol and the water transport through the AQP3 exhibited a strong, immediate, and reversible pH dependence. Such short term and direct gating of transport is a novel feature of aquaporins.
MATERIALS AND METHODS
Details of the preparation of mRNA, the preparation and injection of Xenopus oocytes, and the set-up forLp and ς measurements have been described in detail previously (
). For measurements of the water permeability (Lp) and reflection coefficients (ς) oocytes were placed in a chamber (30 μl) in which solution changes could be accomplished within 5 s (90% complete). The oocytes were stabilized by the insertion of two microelectrodes, which also recorded the membrane potential. The presence of the microelectrodes did not affect the measurements (
). Oocyte volumes were monitored on-line with an accuracy of 0.03% equal to about 0.4 nl, via an inverted microscope connected to a charge coupled device camera.Lp and ς were obtained from the initial rate of volume decrease induced by the addition of 20 mosmol of test solute to the control bathing solution. The ς of a given polyol was obtained from the ratio between the volume changes induced by the polyol and mannitol. The control bathing solution contained in mm: 90 NaCl, 20 mosmol of mannitol, 2 KCl, 1 CaCl2, 1 MgCl2, 10 HEPES or MES, osmolarity 214 mosmol; pH was adjusted according to the type of experiment. Test solutions were obtained by adding 20 mm mannitol or one of the following straight chain polyols (mole weights and boiling points (°C), respectively, in parentheses): ethylene glycol, EG (62, 197); 1,2-propanediol, PD (76, 187); glycerol, Gl (92, 193); 1,2-butandiol, B12 (90, 192); 1,3-butandiol, B13 (90, 204); 1,4-butandiol, B14 (90, 230); 2,3-butandiol, B23 (90, 184); 1,2-pentandiol, P12 (104, 206); 1,4-pentandiol, P14 (104, not detected); 1,5-pentandiol, P15 (104, 242), and 2,4-pentandiol, P24 (104, 201); the structures are indicated in Fig. 1B. The isotonic test solution was obtained by replacing 20 mosmol of mannitol in the control solution by 20 mosmol of glycerol.
The glycerol permeability, PGl, was derived from [14C]glycerol uptakes. The oocytes were equilibrated for at least 2.5 min in the given test solution before being transferred for 0.5–10 min to 3 ml of the stirred test solution to which 4 μCi ml−1 [14C]glycerol (Amersham Pharmacia Biotech, code CFB 174, ethanol-free) had been added. To terminate uptake, oocytes were washed twice in ice-cold medium and transferred to scintillation vials, incubated for approximately 1 h at room temperature with 1 ml of 20% SDS before addition of 15 ml of scintillation fluid (Packard Opti-Fluor), and counting in a scintillation counter (Packard Tri-Carb).
The Lp and PGl were analyzed as functions of external pH by fitting to a Boltzmann function: exp[(pH − pK)/ΔH]/(1 + exp[(pH − pK)/ΔH]). ΔH is a measure of the steepness of the curve around pK and is related to the Hill coefficient n = d(logLp)/d(pH) via n = 1/(2.3 ΔH).
A phenomenological analysis of the volume and solute fluxes and the coupling between them was obtained from irreversible thermodynamics (
where JV is the volume flow, determined from the initial rate of change in oocyte volume. JGlis the flux of glycerol, determined from tracer uptake. A is the true oocyte surface area: with an average diameter of 1.35-mm, oocytes have an apparent surface area of 5.9 mm2. Folding of the membrane increases this area by a factor of nine (
) to give the true surface area A = 0.53 cm2.R is the gas constant and T the absolute temperature. ΔCi is the transmembrane concentration difference of impermeable solutes such as mannitol; ΔCGl is the difference in glycerol concentration. CGl′ is the average concentration of glycerol in the aqueous pore. The coupling between the solute and volume fluxes in the aquaporin can be characterized by ς (Equations 10–56 in Ref.
where Δx/ϕw is a constant that gives the ratio of the membrane thickness to the volume fraction of water,fsw is a formal frictional factor that gives the coupling between the solute (glycerol) and water in the pore. Changes in ς, which arise from partial molar volume effects, are ignored because they were too small to affect the measured values. It follows that a ς significantly smaller than 1 is evidence for interaction between solute and water and that 1 − ςGl is proportional to the glycerol permeability, PGl, times the friction, fsw. We will refer to this term as the interaction and relate Arrhenius activation energies (Ea) to this.
The parameters presented have been corrected for the fluxes taking place via the membrane of the native oocytes. All numbers are given as means ± S.E., unless otherwise stated the number in parentheses is the number of experiments in at least four oocytes.
Lp and ς at pH 7.4
For AQP3 theLp was 6.9 ± 0.3 (10−5 cm s−1 (osmol liter−1)−1) (29 oocytes). The ς values for the polyols were all smaller than 1, for data see Fig. 1. For AQP1 theLp was 6.8 ± 0.3 (10−5 cm s−1 (osmol liter−1)−1) (8 oocytes). ςGl was 0.94 ± 0.01 (35), ςPD was 0.93 ± 0.02 (32), and ςEG was 0.95 ± 0.01 (36), all significantly lower than 1.
The possibility that the test solute crosses the membrane via another route than the AQP and diminishes the driving force during the measurement can be excluded (
). Consider in the present investigation the most permeable test solute, EG. If a flux of EG (via the membrane or in between the membrane and the AQPs) should lead to any significant intracellular accumulation within the test period of 10–20 s, then ςEG would be small for both AQP3 and AQP1. In fact, ςEG for AQP3 was close to zero, for AQP1 it was about one.
The permeability arising from the native oocyte membrane was small. In the native oocytes addition of mannitol gave an Lpof 0.33 ± 0.02 (10−5 cm s−1 (osmol liter−1)−1) (nine oocytes), which was about 20 times smaller than that of oocytes expressing AQP0 to AQP5. The ς values were all smaller than one: for Gl, 0.90; PD, 0.83; EG, 0.63; 12B, 0.45; 13B, 0.59; 14B, 0.83; 23B, 0.69; 12P, 0.56; 14P, 0.70; 15P, 0.73; 24P, 0.53, all with S.E. values around 10% (four oocytes).
pH Dependence of Lp, ς, and Membrane Potential
The Lp of AQP3 was immediately affected by shifts in external pH. When the pH of the bath solution was changed to acidic values, the Lp began to decrease within 1–3 s (compare Fig. 2,A and B). The inhibition was complete in about 60 s, at which time Lp equaled that of native oocytes. To ensure steady states at a given pH, oocytes were adapted for at least 2 min before Lp and ς were recorded. The steady state Lp had a marked pH dependence with a pK of 6.4 ± 0.01, a Hill coefficient of 2.7 ± 0.3 (χ2 = 0.017, 119 recordings from nine oocytes), and complete inhibition at pH values below 5.5 (Fig. 2C). The inhibition was reversible; at the return to external pH of 7.4,Lp immediately began to recover (Fig. 2B) and was normalized in about 60 s. In about one-third of the oocytes, changes in bathing solution pH under isotonic conditions resulted in short lasting (typically 2 s) transient changes in oocyte volume of about 0.1%; we have no explanation of this phenomenon. The Lp values of the other AQPs were insensitive to external pH. The Lp of AQP1 was measured for pH values between 7.4 and 4.5 (Fig. 2C). For AQP0, AQP2, AQP4, and AQP5 there were no significant differences between the Lp values measured at pH 7.4 and at 4.5 (data not shown).
For AQP3 the reflection coefficients for glycerol (ςGlyc) and formamide (ςForm) were sensitive to external pH. In steady state ςGl was 0.15 ± 0.01 (
Acid pH reduced the membrane potential of the oocytes. At pH 7.4 the average potential was −43 ± 2 mV (35 oocytes). When the external pH was lowered the potential began to depolarize within 1 s and became stable after about 60 s. At pH 4.5 the steady state potential was −18 ± 2 mV (nine oocytes); between these values the steady state potential was a linear function of external pH. The membrane potential began to recover within 1 s when external pH was returned to 7.4 and stabilized after about 60 s. The pH effect was independent of which type of aquaporin that was expressed and must be ascribed to reversible modulations by pH of ion channel activity in the oocyte plasma membrane.
For AQP3, ςGl decreased with increasing temperature from 0.24 ± 0.01 at 15.6 °C (
). In the temperature range 15.6 to 31 °C, the Arrhenius activation energy (Ea in kcal mol−1) for (1 − ςGl) was 4.8 ± 0.23 (54).Ea was higher in the range 31 to 22 °C (7.2 ± 0.45 kcal mol−1) than in the range 22 to 15.6 °C (3.4 ± 0.6 kcal mol−1). Oocytes only survived about 10 min at 31 °C as judged from the membrane potential.Ea for (1 − ς12B) was 4.5 ± 1.6 kcal mol−1 (four oocytes) and for (1 − ς23B) it was 4.8 kcal mol−1 (two oocytes), measurements at 15 and 22 °C.
Glycerol Uptake by AQP3
The uptake rate for glycerol decreased with time, but was practically constant for the first min in the following experiments (Fig. 3A). In hypertonic test solutions, where 20 mm of glycerol was added to the control bathing solution at pH 7.4, the initial rate of uptake (JGl) was 2.4 ± 0.15 (10−9mol min−1 oocyte−1) (15 oocytes). Given the average surface area of 0.53 cm2 this gives aPGl of 2.8 ± 0.25 (10−6 cm s−1) (Fig. 3B). The uptake into native oocytes was measured to be less than 4% of JGl (Fig. 3A) and has been corrected for inPGl. PGl was independent of whether it was recorded during an influx or an efflux of water. Oocytes swelled in isotonic test solution where 20 mmglycerol replaced mannitol (
). Under this conditionsPGl was 0.99 ± 0.06 (15 oocytes) times thePGl recorded in the hyperosmolar test solution used above. This shows that solvent drag in the pore is insignificant,i.e. that the second term on the right-hand side of Equation 2 can be disregarded and that the derivation ofPGl by means of hypertonic solutions is valid.
PGl was reduced at lower temperatures. A comparison of PGl measured at 23 and 10 °C gave an Ea of 5.6 ± 0.5 kcal mol−1 (eight oocytes).
PGl was a function of the pH of the test solutions (Fig. 3B). It was independent of pH values down to about 6.25, decreased steeply for more acid pH values, and was abolished at a pH of about 5.6. The pK was 6.1 ± 0.04 and the Hill coefficient 6.2 ± 1.6 (χ2 = 0.04, 90 oocytes).
AQP3 has been found to act as a channel for both water and glycerol transport. The fluxes are linear functions of their respective chemical driving forces (
). Our data show that the transports are gated by H+. In principle, three simultaneously active groups of titratable sites could be responsible for this behavior: one group in which titration led to closure of the channel(s), another that controlled theLp, and finally one that controlled thePGl. We will discuss the simplest possibility: that these groups are, at least partially, identical.
Mechanisms and Coupling of Water and Glycerol Fluxes
The behavior of AQP3 cannot be interpreted in terms of a physical pore. AQP3 remained open to glycerol transport in the pH range 5.8–6.2 while being closed for the smaller water molecule. The data can be interpreted by means of an Eyring energy barrier model (
). On this model, the molecule permeates by a series of jumps, the energy barriers of which are determined by the chemical bonds between the molecule and specific sites in the pathway. For AQP3, the Ea forLp was low, around 5 kcal mol−1, which suggests that the water molecule at neutral pH crosses this aquaporin by forming a succession of single hydrogen bonds. TheLp exhibited an immediate and reversible dependence on external pH (Fig. 2, A and B), under steady state conditions the Lp depended in a sigmoidal manner on external pH with a pK of 6.4 and a Hill coefficient of about 3. This suggests that at least three cooperating titratable sites determine the Lp. In the simplest, but not the only, model these sites are located in the aqueous pathway and determine the energy barriers for water permeation. Titration of the sites would abolish their hydrogen bonding capacity and render them effectively hydrophobic. In analogy to the Lp, thePGl had low activation energy and a marked dependence on external pH. But the pK was lower, 6.1, and the Hill coefficient larger, about 6 (Fig. 3B). This would suggest that glycerol also permeates by forming successive hydrogen bonds, the Hill coefficient indicates at least six.
The difference between pK values and Hill coefficients raises the question whether it is the same titratable groups that determine the Lp and PGl. We suggest it is and that the difference arises from two effects. First, glycerol with its three —OH groups might have to make and break more hydrogen bonds than water in order to cross the aquaporin; this would lead to a higher Hill coefficient. Second, the pK for glycerol transport could be shifted due to a competitive interaction between H+ and glycerol at the sites. Such competition has been described in intact human red blood cells (
). In these cells pK forPGl was about 6.0 at external glycerol concentrations of 1 mm and 5.5 at external glycerol concentrations of 2 m. In addition, the Hill coefficients were estimated to be larger than 2 (
). These values are in agreement with those of the present study where glycerol concentrations of 20 mm were employed. It appears that glycerol, when close to the titratable site, to a certain extent displaces water molecules, an effect that would be enhanced by the confinement of the pore. The resulting lower molar fraction of water near the site would result in a lower local H+ concentration and consequently in a decrease of the effective pK. The hypothesis of a pathway shared by water and glycerol in AQP3 is supported by the finding that ςGl doubled when external pH was lowered to 6.4. This shows that the pathways for water and glycerol has at least one titratable site in common with a pK around 6.4. Titration reduces the availability of this site, and the interaction between the fluxes is reduced. At sufficiently acid pH values both water and glycerol would be unable to cross the channel. The fact that ς for formamide also doubled at pH 6.4 supports the notion that it is the —OH groups of the solute rather than its backbone that is responsible for interaction.
Our model suggest a mechanism of how another member of the major integral protein family, the glycerol facilitator GlpF, can act as a glycerol channel without letting through water (
). If the pore of GlpF had a titratable site that was protonated at normal pH, it would be hydrophobic and in effect prevent the passage of water. If the site allowed competitive interaction between glycerol and H+ as described above, the glycerol molecule would be able to remove the H+ and use the site for transport.
The model is not directly applicable to AQP0, AQP1, AQP2, AQP4, and AQP5, since they did not exhibit any pH sensitivity. One possibility is that the sites responsible for Lp in these aquaporins are not accessible to H+. AQP1 has a small but significant permeability to glycerol (
), and the ς values for the smaller polyols, EG and PD, were of the same size as that of glycerol. This shows that although these polyols interacts with water in AQP1, some structural incompatibility, not found in AQP3, prevents them from permeating at any larger rate.
The Role of Polyol Composition
In general the ς values for AQP3 increased with the number of —OH groups and number of carbons of the test solute (Fig. 1B). The importance of —OH groups available for hydrogen bonding was particularly clear when ς values of the butanols B12, B13, B14, and B23 were compared. For these polyols the location and intramolecular interactions of the two —OH groups had significant specific effects on ς values. The ς values were larger if the two —OH groups were located next to each other and engaged in intramolecular bonding (ςB23 > ςB12 > ςB13 ≅ ςB14). The extent of intramolecular bonding was mirrored by the boiling points, which were lower for B12 and B23 than for B13 and B14. The —OH groups in B12 and B23 were therefore not available for interaction with the sites in the aquaporin to the same degree as the —OH groups of B13 and B14. This would result in smaller fluxes (smaller P) and/or smaller frictions with the water (smaller fsw) and therefore in larger ς values for B23 and B12 (Equation 3). The effects of the locations of the —OH groups on ς were absent for the pentanols. Most likely the longer carbon chain mitigate the strength of intramolecular bonding between —OH groups as witnessed by the small variations between the boiling points among this group.
The picture that emerges is one where the test molecules, viewed as cylinders of different lengths and roughly similar diameters, cross the pore of AQP3 with their axis parallel to the pore. During permeation the —OH groups of the solute form a succession of single hydrogen bonds with the aquaporin as indicated by the low activation energies of around 5 kcal mol−1 observed forJGl, 1 − ςGl, 1 − ςB12, and 1 − ςB23.
Comparison with Other Studies
The numerical values for the transport parameters derived here and in an earlier paper (
) was higher than the one determined here, 0.15. The oocytes in the present study had more negative membrane potentials, on average −43 mV compared with the −25 mV in the previous study. As ςGlwas found to increase with acidity, we suggest that the oocytes used previously, being more stressed, might have had a more acid intracellular pH.
Relation between Primary Structure and Permeation in AQP3
The ability of AQP3 to transport both water and larger solutes is shared by AQP7 (
). Functionally, this places these three aquaporins in between those members of the major integral protein family that transport predominantly water (i.e. AQP0, AQP1, AQP2, AQP4, AQP5) and those that are impermeable to water,i.e. the glycerol facilitator GlpF (
). In view of the unique dependence of AQP3 on pH, titratable residues common for AQP3 and the water transporting aquaporins may not be relevant to explain the transport properties, while homology with the glycerol transporting aquaporins may be more important. If single residues are focused upon, an obvious guess for the titratable sites would be histidines, which qua their imidazole ring have pK values of 6.0–7.0 when incorporated into proteins. Other candidates with hydrophilic side groups are aspartate and glutamate residues, which may have pK values as high as 7 in proteins. All three amino acid residues are known to participate in hydrogen bonding (
). It should be investigated whether this phenomena and the one described by us have a common basis. The rapid and reversible effects of H+ observed by us, however, do not per seimplicate structural or major conformational changes.
AQP3 has been localized in several mammalian tissues: eye, kidney, stomach, spleen, intestine, and erythrocytes (
. The reduction of the Lp of AQP3 by H+suggests that these tissues under anaerobic conditions protect themselves against excessive cellular swelling by a reduction of the passive water permeability. The lower pK forPGl than for Lp shows that the cells strive to retain their capacity for glycerol uptake under acidosis.
The technical assistance of Inge Kjeldsen, Tove Soland, and Svend Christoffersen is gratefully acknowledged. Valuable suggestions have been given by Anne-Kristine Meinild, M. Grunnet, Nanna Boxenbaum, Dr. J. Brahm, Dr. S. Dissing, and Dr. W. D. Stein.