INTRODUCTION

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Vasopressin is, by virtue of its antidiuretic action, the key regulator of water homeostasis in vertebrates. In mammals, the peptide hormone acts by redistributing the water channel AQP2¹ from intracellular vesicles to the apical membrane of kidney epithelial (principal) cells of the renal collecting duct (reviewed in Refs. 1 and 2). This event causes a rapid increase in the water permeability of the epithelial monolayer, thereby permitting reabsorption of water from the lumen of the collecting duct. As a consequence, urine osmolality increases and urinary output decreases.

In principal cells, vasopressin activates basolaterally located V2 receptors coupled to adenylyl cyclase by the cholera toxin-sensitive G protein G_s. The translocation of AQP2 is initiated by the hormone-induced rise in intracellular cAMP and the subsequent activation of cAMP-dependent protein kinase (3). The molecular events following this step, which eventually lead to the fusion of AQP2-bearing vesicles with the apical membrane, are not known.

GTP-binding proteins of remarkable diversity are key components in the regulation of vesicle movement between compartments of the exocytic and endocytic pathways (reviewed in Ref. 4). In particular, monomeric GTP-binding proteins of the Rab/YPT family have been assigned to various intracellular transport pathways (reviewed in Ref. 5). Rab3, a constituent of vesicles undergoing regulated excoytosis, has also been recently found in a kidney preparation enriched for AQP2-bearing vesicles (6); in further experiments the subtype has been determined as Rab 3a.² Heterotrimeric G proteins, traditionally thought to be transducer molecules confined to the plasma membrane, are also present on intracellular vesicles, and an increasing number of studies suggest that they participate in various intracellular transport pathways (reviewed in Refs. 7 and 8). In secretory, e.g. insulin-secreting cells (reviewed in Ref. 9), PTX, by uncoupling G proteins of the G_i/G_o family from activated receptors by ADP-ribosylation of their  subunits, inhibits regulated exocytosis independent of signaling events across the plasma membrane.

To identify proteins involved in the AQP2 shuttle we exploited a recently developed cell line (CD8) (10), established by stably transfecting the RC.SV3 rabbit cortical collecting duct cells (11) with cDNA encoding rat AQP2. CD8 cells respond to vasopressin or forskolin with a 4-6-fold increase in the osmotic water permeability coefficient, P_f, and redistribution of AQP2 from an intracellular compartment to the apical membrane. Thus CD8 represent a unique model system, possessing the functional key properties of principal cells in situ. Here we show that a G protein of the G_i family is required for cAMP-triggered trafficking of AQP2.

	EXPERIMENTAL PROCEDURES

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Culture and PTX Treatment of CD8 Cells-- CD8 cells (10) were established by stably transfecting the RC.SV3 rabbit cortical collecting duct cells (11) with cDNA encoding rat AQP2. CD8 cells were grown at 37 °C as described in a hormonally defined medium (10). Confluent monolayers were used at days 3-5 after plating. Cells grown on coverslips were incubated in the absence or presence of PTX (2 µg/ml, List Biological Laboratories) for 3 h at 37 °C and either stimulated with vasopressin or forskolin (15-20 min at 37 °C) or left under basal conditions. Thereafter, cells were prepared for TIR microfluorimetry or immunofluorescence microscopy.

Osmotic Water Permeability Measurements by TIR Microfluorimetry-- The osmotically induced cell volume changes in adherent cells were measured by TIR microfluorimetry (13). Application of the procedure to CD8 cells and calculation of P_f values have been described (10).

Antibodies-- An affinity-purified antiserum (AS AQP2) raised against an AQP2 peptide has been characterized (10). A panel of rabbit antisera raised against synthetic peptides deduced from G protein subunits were employed: AS 8 raised against a peptide ((C)GAGESGKSTIVKQMK, G_c) common to various G protein  subunits including G_s, G_i, G_o, and G_t; AS 398 raised against a peptide ((C)TDDGMAVATGSWDDSFLKIWN, G_c) common to G protein  subunits (subtypes 1-4); AS 6 (affinity-purified) raised against a peptide ((C)NLKEDGISAAKDVK, G_o) specific for G_o  subunits; AS 190 (affinity-purified) raised against a peptide (LDRIAQPNYI, G_i1) specific for the G_i1  subunit; AS 373 raised against a peptide (DVIIKNNLKDCGLF, G_i1/2) specific for G_i1 and G_i2  subunits; AS 269 raised against a peptide ((C)TGANKYDEAAS, G_i2) specific for the G_i2  subunit; and AS 86 raised against a peptide (KNNLKECGLY, G_i3) specific for the G_i3  subunits. An N-terminal C (in parentheses) in the sequence indicates an additional C used for coupling to keyhole limpet hemocyanin. Excepting antisera 190 and 373 the production of antisera and their specificity (as tested in Western blot and/or in immunoprecipitation experiments with recombinant G protein subunits) have been described (14, 15). In Western blot experiments AS 190 recognized specifically the G_i1  subunit, and AS 373 recognized specifically G_i1 and G_i2  subunits. The subunits recognized by the other antisera are mentioned under "Results."

Conventional Immunofluorescence and Video Confocal Microscopy-- Conventional immunofluorescence and video confocal microscopy (16, 17) of CD8 cells grown on coverslips have been described (10). For video confocal microscopy, images were taken in the xy plane at steps of 320 nm, using an oil-immersion objective (40 × 1.40 normal aperture); xz sections were extracted from a set of planar images, the overall depth corresponding to about 6 µm.

Effect of Peptides on cAMP-induced AQP2 Translocation and Quantitative Immunofluorescence-- CD8 cells grown on coverslips were washed three times in "intracellular" (IC) buffer (140 mM potassium glutamate, 20 mM Hepes, 5 mM MgCl₂, 5 mM EGTA, 5 mM NaCl, pH 7.4) at 37 °C. Permeabilization of cells was achieved using staphylococcal  toxin (18). The permeabilization was monitored by the cellular uptake of carboxyfluorescein (1.24 mM). Conditions in which more than 95% of cells were rendered fluorescent were selected. As an additional control for permeabilization, cAMP, which is membrane-impermeable, was used to elicit redistribution of AQP2 (see below). Cells were incubated for 10 min at 37 °C in the presence of  toxin in IC buffer devoid of peptides (control) or in IC buffer supplemented with peptides corresponding to the C termini of G protein  subunits (19). Thereafter, cells were washed three times in IC buffer and either incubated with 10 mM cAMP for 10 min at 37 °C or left under basal condition, fixed and immunostained with AS AQP2 (10). In addition to the G_i1/2 and G_i3 peptides (see above), a peptide (YLGLEKLNNK) with a reversed sequence with respect to the G_i3 peptide was employed. To quantify the effect of different peptides on cAMP-induced AQP2 redistribution in CD8 cells, AQP2 immunofluorescence intensity was analyzed in different experimental conditions. Briefly, planar images obtained by conventional immunofluorescence were analyzed using the Image Tool software which assigns to the brightest fluorescence detectable in the image a score of 255 and to the least fluorescence detectable a value of 1. All images had exactly the same pixel number in order to determine differences in the distribution of fluorescence between the different experimental groups. At least four randomly chosen boxes from different fields of the coverslips (corresponding to approximately 20 cells each) were analyzed from at least three separate experiments. Background fluorescence was measured and normalized. Images were processed, and the distribution of pixel intensity as a function of their frequency was determined for each image and relative parameters as standard deviation, skewness, and kurtosis were determined. Skewness is a measure of the symmetry of a profile about the mean pixel intensity value. Kurtosis describes the randomness of the shape of the profile relative to that of a perfectly random pixel intensity distribution. Statistical analyses were performed using the unpaired t test.

Preparation of Cell Homogenates and Membranes-- Homogenates were prepared as described previously (10). For the preparation of crude membranes, cell monolayers were trypsinized, washed twice with phosphate-buffered saline, and homogenized with a glass/Teflon homogenizer in ice-cold buffer H (300 mM mannitol, 12 mM Hepes/Tris, pH 7.4). The suspension was centrifuged at 2,500 × g for 15 min, and the pellet containing nuclei and unbroken cells was discarded. The supernatant was spun at 10,000 × g for 10 min. The supernatant and the homogenized pellet were recombined, centrifuged at 100,000 g for 60 min, and the pellet was resuspended in phosphate-buffered saline, 0.1 mM phenylmethylsulfonyl fluoride. Crude membranes from rat kidney papilla or rabbit brain were prepared by cutting the tissues into small slices and homogenizing them in ice-cold buffer H. For the preparation of crude membranes, the resulting suspension was subjected to the protocol described above. For the preparation of high speed pellet (HSP) from rabbit kidney papilla, the papillae were excised and homogenized with a glass/Teflon homogenizer in ice-cold buffer H. The suspension was centrifuged at 700 × g for 10 min at 4 °C. The supernatant was centrifuged at 17,000 × g for 45 min at 4 °C. The supernatant was spun at 200,000 × g in a Beckman Rotor 60 Ti for 60 min at 4 °C. The final pellet (HSP) enriched in intracellular vesicles, was recovered in buffer H and used for immunoblot experiments.

Preparation of AQP2-containing Vesicles ("Endosomes")-- Endosomes were prepared from rat kidney papillae or CD8 cells according to Sabolic et al. (20) with slight modifications. Briefly, the kidney papillae from six rats were excised and cut into small pieces. These pieces or CD8 cells from confluent monolayers grown in six 175-cm² culture flasks were homogenized with a glass/Teflon homogenizer in ice-cold buffer H. The suspension was centrifuged at 2,500 × g at 4 °C, and the supernatant was recentrifuged at 20,000 × g for 20 min at 4 °C. The obtained supernatant and the upper part of the pellet were centrifuged at 45,000 × g for 60 min at 4 °C. The pellet was recovered, resuspended in 1.5 ml of buffer H, homogenized with a manual glass Potter, loaded on an 18% Percoll gradient and centrifuged at 45,000 × g for 50 min at 4 °C. The opalescent part of the gradient located in the bottom third of the gradient was recovered, resuspended in KCl buffer (300 mM mannitol, 100 mM KCl, 5 mM MgSO₄, 15 mM Hepes/Tris, pH 7.0), and centrifuged at 45,000 × g for 60 min at 4 °C. The pellet containing AQP2-enriched vesicles was recovered in buffer H.

[³²P]ADP-ribosylation of Proteins and Western Blotting-- PTX-catalyzed [³²P]ADP-ribosylation of proteins and analysis of radiolabeled proteins were carried out as described previously (15). Western blotting was performed as described elsewhere (10) except that proteins were separated by 10% SDS gels in the presence of 1 M urea. Filters were incubated with primary antibodies diluted as indicated and thereafter with alkaline phosphatase-conjugated goat anti-rabbit IgG (1:5000, Sigma). Immunoreactive proteins were visualized by a color reaction (10).

	RESULTS

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PTX Inhibits the Forskolin-induced Increase in Water Permeability and AQP2 Trafficking-- To test whether G proteins are involved in the AQP2 redistribution downstream of the cAMP/cAMP-dependent protein kinase signal, we treated CD8 cells with PTX which ADP-ribosylates and thereby uncouples G proteins of the G_i and G_o types from their cognate receptors. The time course of cell swelling in response to changes in perfusate osmolality was measured by TIR microfluorimetry (Fig. 1; see "Experimental Procedures"). In control cells, forskolin, a strong direct activator of adenylyl cyclase, accelerated cell swelling in response to a decrease in perfusate osmolality by 200 mM (Fig. 1A); it also increased the extent of swelling in the observed time interval (45 s). The change in fluorescence caused by forskolin corresponds to an about 5-fold increase in the P_f (Fig. 1C). When cells had been treated with PTX, the time course of cell swelling (Fig. 1B) and water permeability (Fig. 1C) were not modified by forskolin. The data suggest that PTX either inhibits AQP2 trafficking or impairs AQP2 function.

View larger version (20K): [in this window] [in a new window]   Fig. 1.   . Panels A and B, representative TIR fluorescence time courses of CD8 cells at 10 °C in response to a 200 mOsm inwardly directed NaCl gradient. In the absence of PTX (PTX, panel A), forskolin (FSK; 104 M for 15 min) caused a dramatic increase in the rate of cell swelling. When cells were pretreated with PTX (+PTX, panel B) FSK had no effect on the rate of cell swelling. Panel C, mean values ± S.E. (n = 3) of Pf [(× 104) cm/s] calculated from TIR fluorescence time courses.

View larger version (27K): [in this window] [in a new window]   Fig. 2.   Video confocal microscopy of CD8 cells. Cells grown on coverslips were incubated in the absence (PTX) or presence of PTX (+PTX) and either stimulated with vasopressin (AVP; 10 nM for 20 min at 37 °C) or left under basal conditions (Control). The cells were fixed and immunostained with AS AQP2. Shown is an xz section calculated from a set of xy images (bottom, plane of coverslip).

Identity of the G Protein Involved in AQP2 Trafficking-- Further experiments were performed to identify the PTX substrates involved in AQP2 trafficking. Fractions from CD8 cells (total cell homogenate, crude membranes and a fraction enriched for AQP2-bearing vesicles traditionally termed endosomes; see "Experimental Procedures") were incubated with [³²P]NAD in the absence or presence of PTX, and [³²P]ADP-ribosylated proteins were analyzed by high resolution SDS-polyacrylamide gel electrophoresis followed by autoradiography (Fig. 3). In brain membranes, a rich source for PTX-sensitive G proteins of the G_i and G_o families, a strong signal was detected at 39-45 kDa. Among the various preparations of CD8 cells the strongest signal was observed in the endosome fraction. Whereas a weak signal was seen in the homogenate, apparently two major substrates were found in the 43-kDa region in the crude membrane and endosome preparations. As shown below, the PTX substrates in CD8 cells correspond to  subunits of the G_i family. The relatively high apparent molecular masses of PTX substrates (G_i/G_o  subunits) are due to the presence of 6 M urea during SDS-polyacrylamide gel electrophoresis (15).

View larger version (74K): [in this window] [in a new window]   Fig. 3.   PTX substrates in CD8 cells. The indicated fractions from rat brain and CD8 cells were incubated with [32P]NAD in the absence () or (presence (+) of PTX. Proteins were separated by SDS-polyacrylamide gel electrophoresis in the presence of 6 M urea, and 32P-labeled proteins were visualized by autoradiography. The selected film shows substrates in all preparations (including a weak signal in CD8 cell homogenate).

View larger version (44K): [in this window] [in a new window]   Fig. 4.   Effect of peptides on cAMP-induced AQP2 translocation in permeabilized CD8 cells. Cells grown on coverslips were permeabilized with  toxin in the absence of peptides (no peptide) or in the presence of Gi1/2 peptide (100 µg/ml), Gi3 peptide (100 µg/ml), or a peptide (100 µg/ml) having a reversed sequence with respect to the Gi3 peptide (irrelevant peptide). Cells were then washed and incubated with 10 µM cAMP (+cAMP). Control experiments were performed with permeabilized cells left under basal conditions (cAMP; top panel). The cells where fixed, stained with AS AQP2, and analyzed by video confocal microscopy (xz scans).

Quantification of the Peptide Effect on cAMP-triggered AQP2 Translocation-- To quantify the effect of different peptides, we next applied image analysis methods based on AQP2 signal intensity in CD8 cells. Cells were stained with AS AQP2 under different experimental conditions, and planar images, obtained by conventional immunofluorescence microscopy, were analyzed using the Image Tool software that assigns to the brightest fluorescence detectable in the image a score of 255 and to the least fluorescence detectable a value of 1 (see "Experimental Procedures"). A representative experiment is shown in Fig. 5. Under basal conditions, a punctate intracellular AQP2 staining is visible. As previously shown (10), addition of vasopressin causes the disappearance of intracellular staining because of the redistribution of AQP2 in the apical membrane; forskolin has a similar effect (not shown). The relative pixel intensity plotted as a function of their frequency (lower panels) is substantially different under nonstimulated and stimulated conditions. Whereas a random pixel intensity distribution characterizes the basal condition, most of the pixels are in a narrow range of a low intensity gray scale under stimulated conditions.

View larger version (36K): [in this window] [in a new window]   Fig. 5.   Representative distribution of relative pixel intensity as a function of their frequency in CD8 cells stained with AS AQP2 under basal condition and after stimulation with vasopressin (10 nM). After hormonal stimulation, most of the pixels are concentrated in a narrow range of gray scale characterized by a low intensity, whereas a random pixel intensity distribution is typical for the basal condition.

View larger version (14K): [in this window] [in a new window]   Fig. 6.   Quantification of the effects of peptides on cAMP-triggered AQP2 translocation by quantitative image analysis. Cells were incubated with different concentrations of the Gi3 peptide (), the G i1/2 peptide (), or the irrelevant peptide () and stimulated with cAMP. Two parameters, kurtosis (A) and skewness (B), were calculated using Image Tool software (see "Experimental Procedures"). Each experimental point represents the mean ± S.D. of the parameters obtained from four images of a representative experiment. Similar results were obtained in at least three independent experiments.

G Protein Subunits in CD8 Cells-- As a next step, we analyzed the subcellular distribution of G protein subunits in CD8 cells by performing Western blot experiments with a panel of anti-peptide antibodies. In contrast to Fig. 3, SDS gels with 1 M urea were used to separate proteins prior to blotting (Fig. 7). In some panels, an immunoreactive band at 42 kDa is visible in the membrane preparation from rabbit kidney enriched in endosomes (HSP). Further analysis revealed that this signal was entirely due to an unspecific staining by a batch of secondary antibodies; it was not observed in the absence of primary antibodies.

View larger version (29K): [in this window] [in a new window]   Fig. 7.   Detection of AQP2, G protein  and  subunits in CD8 cells, rabbit and rat kidney papilla/medulla, and rabbit brain. Onto each lane of an SDS gel, 60 µg of protein were loaded. Proteins were separated by SDS-polyacrylamide gel electrophoresis and transferred onto membranes, which were incubated with the primary antipeptide antisera indicated on the bottom right of each panel. The AQP2 antiserum and AS 190 (Gi1) were subjected to affinity chromatography prior to use. The dilutions of the other antisera were 1:5000. Immunoreactive proteins were visualized by a color reaction. Molecular masses are indicated on the left.

G

G

G

G

G

View larger version (89K): [in this window] [in a new window]   Fig. 8.   . Localization of AQP2 and Gi  subunits in CD8 cells. Cells were analyzed by conventional immunofluorescence microscopy. The primary anti-peptide-antisera were: AS AQP2 (a), AS 86 (raised against the Gi3 peptide; c) and AS 373 (raised against the Gi1/2 peptide; e). Control experiments with peptide-adsorbed antibodies are shown on the corresponding right panels. Magnification, × 700.

	DISCUSSION

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Vasopressin exerts its antidiuretic effect by translocating AQP2 from intracellular compartments to the apical plasma membrane of renal collecting duct principal cells. The primary cellular targets of vasopressin are V2 receptors coupled to adenylyl cyclase via the cholera toxin-sensitive G protein G_s. Activation of this pathway leads to an increase in intracellular cAMP levels and activation of cAMP-dependent protein kinase. It has been suggested that phosphorylation of AQP2 by cAMP-dependent protein kinase plays a role in hormonally induced AQP2 trafficking (23, 24). Here we show that a second G protein is essential for the vasopressin-induced shuttle of AQP2. This G protein is sensitive to PTX and is involved in the pathway downstream of the cAMP/cAMP-dependent protein kinase signal (Fig. 9). Confocal microscopy and quantification of AQP2 signal intensity by image analysis revealed that a synthetic peptide corresponding to the C terminus of the G_i3  subunit (G_i3 peptide) inhibited the cAMP-induced AQP2 translocation in a concentration-dependent manner, reaching a maximal inhibitory effect at 50-100 µg/ml. At this concentration, the redistribution of AQP2 appeared to be completely suppressed. Employed at concentrations up to 100 µg/ml, a peptide corresponding to the C termini of the  subunits of G_i1 and G_i2 (G_i1/2 peptide) was inactive; a 20-25% inhibition was observed at a concentration of 200 µg/ml. A peptide with the amino acid composition of the G_i3 peptide but with a reversed sequence was inactive even at a high concentration (200 µg/ml). The latter finding indicates that the inhibitory effects of the G_i1/2 and G_i3 peptides are specific, i.e. are due to functional inhibition of G_i proteins.

View larger version (36K): [in this window] [in a new window]   Fig. 9.   A cellular model for vasopressin/cAMP-induced AQP2-trafficking in renal collecting duct principal cells. The mechanisms underlying docking and fusion of AQP2-bearing vesicles are not known. The detection of the small GTP-binding protein Rab3a (6) (U. Liebenhoff and W. Rosenthal, unpublished results), synaptobrevin 2, here referred to as VAMP2 (6, 30, 32), and syntaxin 4 (31) in principal cells suggests that these proteins are involved in AQP2 trafficking. We show here that a G protein of the Gi family is required for AQP2 trafficking downstream of the cAMP/cAMP-dependent protein kinase (PKA) signal. Its precise role remains to be determined (see "Discussion").

The C termini of the G_i  subunits show a very high degree of sequence identity. In fact the G_i1/2 and G_i3 peptides differ by only two amino acids (see "Experimental Procedures"). It is therefore plausible that the two peptides loose their subtype specificity if employed at a high concentration. Alternatively, several G_i subtypes may in principle be capable of regulating AQP2 trafficking, indicating redundancy with regard to the regulation of a key component controlling the excretion of water. The higher potency of the G_i3 peptide compared with that of the G_i1/2 peptide suggests that under physiological conditions, G_i3 is primarily involved in AQP2 trafficking. In ATP-permeabilized mast cells the G_i3 peptide inhibits exocytosis, whereas a peptide (KENLKDCGLF) with 90% identity to a C-terminal portion of the G_i2  subunit is less potent (19).

The finding that PTX, an inhibitor of G protein function, also inhibits AQP2 trafficking strongly suggests that PTX-sensitive G proteins stimulate or facilitate insertion of AQP2 into the apical membrane. They may either promote early events (i.e. targeting of AQP-bearing vesicles to the apical plasma membrane) or late events (docking/fusion of AQP-bearing vesicles to/with the apical plasma membrane) (Fig. 9). The abundance of G_i proteins on intracellular structures of CD8 cells suggests that G proteins at these locations rather than those associated with the plasma membrane are involved in AQP2 trafficking, although in mast cells the G_i3  subunit involved in regulated exocytosis has been located to the plasma membrane (19). During revision of the manuscript, the association of G_i3  subunit with secretory vesicles was reported (25). Interestingly, the authors suggest an involvement of G_i3 in vesicle swelling, a potentially important prerequisite for the fusion of vesicles with the cell plasma membrane. For a proximal kidney tubule cell line (LLC-PK1) an inhibitory role for the G_i3  subunit in the constitutive secretion of heparan sulfate proteoglycan has been reported (26). Thus in kidney epithelial cells G_i3 may have a dual role in vesicular transport, i.e. inhibition of constitutive exocytosis and promotion of regulated exocytosis. Alternatively, the G_i3  subunit may serve different functions in CD8 and LLC-PK1 cells.

Sands et al. (27), by applying the Western blot technique, reported the presence of the  subunits of G_q/11, G_i1, G_i2 and G_i3 in isolated rat kidney endosomes. Our data are in agreement with theirs regarding the presence of all known G_i  subunits in the rat kidney preparation. In rabbit kidney, however, G_i1 appears to be expressed at low levels. While Sands et al. postulate an inhibitory role of G_i  subunits in translocation of AQP2 to the apical membrane (counteracting the effect of vasopressin), we demonstrate here their involvement in the vasopressin-induced insertion of AQP2 in the apical membrane.

The mechanisms by which G proteins control vesicular transport is not understood (7, 8). Although we have not addressed this problem, the present data provide some hints. G proteins in fractions enriched in AQP2-bearing vesicles are excellent substrates for PTX (Fig. 3), suggesting that they are present as heterotrimers. Consistent with this view, both G protein  and  subunits were detected in these preparations. In general, the heterotrimeric form of a G protein is required for its interaction with activated plasma membrane receptors, and PTX prevents receptor-mediated but not receptor-independent activation of G proteins (reviewed in Ref. 28). The inhibitory effect of PTX on receptor/G protein coupling and a plethora of other data (reviewed in Ref. 29) show that the C termini of G protein  subunits are important contact sites for receptors. We show here that PTX and peptides corresponding to the C termini of G_i  subunits inhibit AQP2 trafficking. These findings suggest that similar to the well known receptor/G protein interactions, a heterotrimeric G protein is involved in AQP2 trafficking and that the C terminus of its  subunit is required for the interaction with an unknown, possibly upstream, signaling component.

The detection of synaptobrevin 2, Rab3a, and syntaxin 4 in principal cells suggests that these proteins, known to be involved in the Ca²⁺-triggered exocytosis in secretory cells, also participate in the cAMP-triggered fusion of AQP2-bearing vesicles with the apical plasma membrane (6, 30, 31). A role for synaptobrevin 2 in this process is supported by the finding that tetanus toxin, which cleaves synaptobrevin 2, inhibits the homotypic fusion of purified AQP2-bearing vesicles in vitro (32). We show here the functional involvement of a PTX-sensitive G protein in cAMP-induced insertion of AQP2 in the apical plasma membrane of principal cells in vivo, thereby defining a component required for cAMP-triggered exocytosis in renal epithelial cells. Future studies should lead to the identification of further proteins involved in this process, in particular of those forming the functional link between the cAMP/cAMP-dependent protein kinase signal and vesicle targeting, docking, or fusion.