Introduction

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-Aminobutyric acid (GABA)-activated chloride channels represent a pharmacologically diverse class of receptors that mediate inhibitory synaptic activity in the central nervous system (CNS). At least two classes of fast inhibitory GABA receptors exist in the CNS: GABA_A receptors composed of , , , , , , and  subunits (Schofield et al., 1987; Lolait et al., 1989; Pritchett et al., 1989; Shivers et al., 1989; Ymer et al., 1989; Hedblom and Kirkness, 1997; Bonnert et al., 1999) and GABA_C receptors presumably composed of  subunits (Cutting et al., 1991). Both classes have a Cl-selective pore, but in contrast to GABA_A, GABA_C receptors are insensitive to bicuculline, barbiturates and benzodiazepines (Sivilotti and Nistri, 1989; Polenzani et al., 1991; Johnston, 1996).

The distribution of GABA_A and GABA_C receptors in the CNS is well documented (Richards et al., 1987; Houser et al., 1988; Benke et al., 1991; Zimprich et al., 1991; Wisden et al., 1992; Enz et al., 1996; Koulen et al., 1998), although the mechanism responsible for delivery of receptors to and from the membrane surface is poorly understood. Although GABA_A and GABA_C receptors can be localized in the same neuron, they exist at separate synaptic sites (Koulen et al., 1998). In addition to surface receptors, a significant cytoplasmic pool exists in cells (Mammen et al., 1997; Wan et al., 1997). Whether these receptors can be dynamically shuttled back and forth between an intracellular pool and the cell surface is not well established.

GABA_A receptors can be potentiated or inhibited by direct phosphorylation via protein kinase C (PKC), PKA, or tyrosine protein kinases (Moss et al., 1992; Moss et al., 1995; Lin et al., 1996; McDonald et al., 1998). The results from these studies, however, have been controversial. Recombinant 112 receptors expressed in HEK293 cells and GABA_A (mRNA injection) or recombinant 112S receptors expressed in Xenopus oocytes demonstrated a reduction in the amplitude of the GABA-activated current in response to PKC activators (Sigel and Baur, 1988; Kellenberger et al., 1992; Krishek et al., 1994; Chapell et al., 1998). In contrast, introduction of catalytically active PKC into L929 fibroblasts expressing recombinant 112L receptors caused an enhancement of the GABA-mediated current (Lin et al., 1996). Previously, for GABA_C receptors, we provided evidence that internalization is a potential mechanism for phosphorylation-dependent inactivation of 1 receptors transfected in HEK cells (Filippova et al., 1999). Both recombinant GABA_A and GABA_C receptors are inactivated in a PKC-dependent manner by phorbol 12-myristate 13-acetate (PMA) in Xenopus oocytes (Sigel and Baur, 1988; Chapell et al., 1998; Kusama et al., 1998).

To gain insight into the mechanism of PKC-dependent modulation of recombinant GABA_A and GABA_C receptors, we used: 1) the two-electrode voltage clamp to measure GABA-activated currents, 2) fluorescence microscopy to visualize green fluorescence protein (GFP)-tagged GABA receptor subunits on the membrane surface, and 3) [³H]GABA binding in intact oocytes to estimate changes in the number of receptors on the cell surface. We found that activation of PKC induced the internalization of GABA receptors and these internalized receptors could, in time, return back to the cell surface. Thus, by regulating GABA receptor surface expression, PKC may play a key role in the regulation of GABA-mediated inhibition.

	Materials and Methods

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Clones, Constructs, and In Vitro Transcription. The human 1 and rat 1, 2, 2 subunits were obtained via the polymerase chain reaction as previously described (Amin et al., 1994; Amin and Weiss, 1994), and subcloned into the p-ALTER-1 vector (Promega, Madison, WI). Henceforth, 1, 2, and 2 will be referred to as , , and , respectively. Altered Sites (Promega) was used for the site directed mutagenesis that was verified by cDNA sequencing.

Xenopus Oocyte Expression. X. laevis (Xenopus I, Ann Arbor, MI) were anesthetized by MS-222, and oocytes were surgically removed and placed in a solution that consisted of 85.5 mM NaCl, 2.5 mM KCl, 5 mM HEPES, 1 mM CaCl₂, 1 mM MgCl₂, 1 mM Na₂HPO₄, 50 U/ml penicillin, and 50 µg/ml streptomycin, pH 7.5. Oocytes were dispersed in this solution without Ca²⁺, but in the presence of 0.3% Collagenase A (Boehringer-Mannheim, Indianapolis, IN). After isolation, stage 6 oocytes were thoroughly rinsed and maintained at 18°C in the above-mentioned solution plus 1 mM Ca²⁺. Micropipettes for injecting cRNA were pulled on a Sutter P87 horizontal puller. cRNA was diluted 2- to 10-fold with pyrocarbonate-treated water. A total of 0.5 to 5 ng of cRNA was injected into each oocyte.

Voltage Clamp of Oocytes. Two-electrode voltage-clamp procedures were used for current recording 3 or 4 days after cRNA injection. Oocytes were placed on a 300-µm nylon mesh suspended in a small volume chamber (<100 µl). The oocyte was perfused continuously with a solution containing 92.5 mM NaCl, 2.5 mM KCl, 5 mM HEPES, 1 mM CaCl₂, 1 mM MgCl₂, and 0.5 mM Na₂PO₄, pH 7.5. The solution was switched to the test solution, which is identical with the perfusion solution plus drug (e.g., GABA, carbachol). In experiments with nACh receptors, atropine (0.5 µM) was added to block muscarinic ACh receptors. All experiments were performed at room temperature.

(1)

(2)

Fluorescence Analysis. The intensity of the fluorescence from -GFP receptors was analyzed with a Diaphot 300 fluorescence microscope (Nikon, Tokyo, Japan). The GABA-activated current from oocytes injected with -GFP cRNA was tested using two-electrode voltage clamp before examining the fluorescence.

Western Blot Analysis. Noninjected oocytes and oocytes injected with the cRNA coding for the 1 IC loop (40 per each condition) were homogenized, centrifuged, and layered on a discontinuous 20% sucrose gradient in the presence of a protease inhibitor cocktail (Sigma Chemical Co.), 1 mM dithiothreotol, and 5 mM MgCl₂ in a Tris-buffered saline. After centrifugation, the top layer (cytosol) and pellet (membrane) fractions were collected. The proteins from the membrane fraction were solubilized by 60 mM urea. The membrane and cytosolic fractions were then boiled in SDS-polyacrylamide gel electrophoresis loading buffer and separated on a 10% polyacrylamide gel and transferred to a nitroceullulose membrane. The immunoblotting procedure included a 2-h incubation with a mouse polyclonal antibody (1:500) raised against the M3-M4 IC loop of the 1 receptor followed by a 1-h incubation in an alkaline phosphatase-conjugated anti-IgG antibody diluted 1:1000. A 14-kDa band from the cytosol fraction was detected as the IC loop protein.

[³H] Binding Assay. To estimate [³H]GABA binding, we used the single-oocyte binding method as previously described (Chang and Weiss, 1999). Briefly, single oocytes injected with 1 cRNA were held by suction at the end of a pipette tip. The oocyte was first incubated in [³H]GABA for 60 to 90 s (specific activity = 94 Ci/mmol; Amersham life Sciences, Arlington Heights, IL), then rinsed for 6 s in a 150 ml of ice-cold stirring bath to remove free [³H]GABA from the oocyte surface. Next, the oocyte was held in a 250-µl incubation solution for 90 s to allow the bound [³H]GABA to dissociate from the receptor. The released counts per minute were then determined in a liquid scintillation counter.

In Vitro Phosphorylation. The GST-IC loop fusion protein was purified from the bacterial cell line Epicurian Coli BL21-Gold(DE3)pLysS (Stratagene, La Jolla, CA) with glutathione-Sepharose (Amersham, Uppsala, Sweden) under nondenaturing conditions. The in vitro phosphorylation assay was carried out with the fusion protein still bound to the Sepharose. The catalytic domain of protein kinase C (Calbiochem, San Diego, CA) was added to the beads in the presence of [-³²P]ATP for 25 min. The Sepharose was then rinsed, boiled, and ran on a denaturing 15% polyacrylamide gel and subjected to autoradiography.

	Results

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PMA-Induced Inactivation of Homomeric 1 GABA_C and -GABA_A Receptors. The GABA-activated current from homomeric 1 receptors expressed in Xenopus oocytes was stable during long-term recording (up to 2 h). The activator of PKC, PMA (100 nM; 10-min application) induced a continuous decrease in the amplitude of the GABA-activated current (normalized value was 0.32 ± 0.02; n = 4 after 55 min of recording) (Fig. 1, A and B). The inactive analog of PMA, PMA (1 µM; 10-min application), had no significant effect on the GABA-evoked current (1.03 ± 0.06; n = 4) after 30 min of recording (Fig. 1, A and B).

View larger version (29K): [in this window] [in a new window]   Fig. 1.   PMA-induced inactivation of recombinant 1 receptors. A, PMA (100 nM; 10 min) decreased the GABA-evoked current to 0.36 of its original value after 35 min of recording. The inactive analog of PMA, PMA (1 µM; 10 min), was ineffective during 55 min of recording. B, average of the normalized GABA-activated current amplitude after PMA () and PMA () treatment during prolonged recording. Note the amplitude of the GABA-activated current after PMA treatment did not significantly change (1.03 ± 0.06; n = 4, P < .05). After PMA treatment, however, the amplitude declined to 0.32 ± 0.02 (n = 4) of the initial value.

View larger version (27K): [in this window] [in a new window]   Fig. 2.   PMA-induced inactivation of recombinant receptors. A, PMA (100 nM; 10 min) decreased the GABA-evoked current to 0.1 of the original value during 45 min of recording. B, inactive analog of PMA, PMA (1 µM; 10 min), was ineffective during 45 min of recording. C, average of the normalized GABA-activated current amplitude after PMA and PMA treatment. Note the amplitude of the GABA-activated current declined to 0.22 ± 0.06 (n = 5) of the initial value during 45 min of recording after PMA treatment. With PMA treatment, the amplitude was 0.95 ± 0 0.03 (n = 5) after 45 min of recording.

PKC Is Involved in PMA-Induced Inactivation of 1 and Receptors. To verify the role of PKC in the PMA-induced inactivation of GABA receptors, the degree of inactivation was determined in the presence of the PKC inhibitor calphostin C (1 µM), injected into the oocyte 1 h before PMA (100 nM) treatment (Fig. 3, A and B). Calphostin C, on its own, did not significantly modify the amplitude of the GABA-activated current (0.96 ± 0.06; n = 5; data not shown), although it decreased the degree of PMA-induced inactivation of 1 and receptors (Fig. 3, B and C, respectively). It is possible that calphostin C slowed the rate of the PMA effect. Nevertheless, these data confirm that PKC is involved in the time-dependent inactivation.

View larger version (33K): [in this window] [in a new window]   Fig. 3.   PKC is involved in the modulation of GABA receptors. A, current traces evoked by GABA (20 µM) from receptors after 1 nM (top) and 5 nM (middle) PMA treatment. The lower traces are in the presence of 5 nM PMA and 1 µM calphostin C. B, plot of the amplitude of the GABA-activated current from -GABA receptors as a function of time in the presence of 20 nM PMA either with () or without () 1 µM calphostin C. After 55 min of recording, the normalized amplitudes were 0.12 ± 0.05 (n = 4) and 0.37 ± 0.11 (n = 4) in the absence and presence of calphostin C, respectively. C, plot of the amplitude of the GABA-activated current from 1 GABA receptors as a function of time in the presence of 50 nM PMA either with () or without () 1 µM calphostin C. After 40 min of recording, the normalized amplitudes were 0.10 ± 0.05 (n = 7) and 0.44 ± 0.10 (n = 5) in the absence and presence of calphostin C, respectively.

PMA-Induced Inactivation Was Not Caused by Direct Phosphorylation of 1 Receptor. We next set out to determine whether the PMA-induced inactivation was the result of a direct phosphorylation of the GABA receptor. The 1 subunit has three consensus PKC sites in the IC loop between the third and fourth transmembrane domains (S410, S419, and S426) (Cutting et al., 1991) and it has been demonstrated that mutation of these three sites does not prevent the PMA-induced inactivation (Kusama et al., 1998). However, it is not known which, if any of these three sites can be phosphorylated. In addition, other nonstandard PKC consensus sites may exist (Kennelly and Krebs, 1991). To identify potential PKC sites, we carried out an in vitro phosphorylation assay with a GST 1 IC loop fusion protein. A summary autoradiogram is presented in Fig. 4A. Mutation of the serines at positions 419 and 426 to alanines eliminated a majority of the phosphorylation (compare lanes 1 and 2). Most of the phosphorylation occurred at residue 419 (data not shown). Mutation of the serine doublet at positions 422 and 423 to alanines eliminated the remainder of the phosphorylation (lane 3). At present, we do not know which of these two serines (422 or 423) is the actual site of phosphorylation (perhaps it is both), but nevertheless, mutation of these four serines produced an IC loop that was not phosphorylated in vitro.

View larger version (36K): [in this window] [in a new window]   Fig. 4.   Mutation of the PKC phosphorylation sites on the 1 receptor did not prevent the inactivation of IGABA. A, lower gel is an autoradiogram from an in vitro phosphorylation assay of the GST-1 IC loop fusion protein. Note that the S419A and S426A mutations eliminated a majority of the phosphorylation, whereas the S422SA and S423A mutations eliminated the remainder of the phosphorylation. The gel on top is the corresponding coomassie stain of the autoradiogram, indicating similar amounts of protein were loaded in each lane. Alignment of the autoradiogram with the stained gel, as well as a comparison of the predicted and observed molecular weight of the GST-IC loop fusion protein, indicated that the upper band (indicated by the arrow) corresponded to the phosphorylated band in the autoradiogram. Additional nonphosphorylated bands could be identified as smaller fragments of the fusion protein without the PKC phosphorylation sites. B, average of the normalized GABA-activated current amplitude from S419A/S422A/S423A/S426A receptors after PMA (100 nM; 10 min) treatment. Note the decline in the current amplitude was similar to that of the wild-type 1 receptor, indicating that these mutations did not alter the PMA-induced inactivation of IGABA.

PMA Does Not Alter Sensitivity for GABA. One possible explanation for the PMA-induced inactivation could be a change in the sensitivity for GABA. To investigate this possibility, we recorded the GABA sensitivity of 1 and receptors before and after inactivation (Fig. 5). In control conditions, 1 and receptors have a GABA EC₅₀ of 51 ± 5 and 0.87 ± 0.05 µM, respectively (Fig. 5A). Based on these mean EC₅₀ values, we selected two GABA concentrations (dashed lines in Fig. 5A) and compared the ratio of the amplitudes at these concentrations before and after inactivation. The ratio between the current amplitude activated at 1 and 5 µM GABA for 1 receptors was similar before and after PMA treatment (Fig. 5B and Fig. 5C, left), although the current amplitude decreased to 0.23 ± 0.06 (n = 5) of its pretreatment value (Fig. 5C, right). The normalized traces in Fig. 5B are the scaled and superimposed responses to 1 µM GABA before and after PMA treatment. Note the responses are identical, indicating that there was no change in the activation and deactivation kinetics. The ratio between current amplitude activated at 20 and 50 µM GABA for receptors was similar before and after PMA treatment (Fig. 5D, left), although the current amplitude decreased to 0.04 ± 0.02 (n = 4) of its pretreatment value (Fig. 5D, right). Thus, PMA does not alter the GABA sensitivity of 1 and receptors.

View larger version (31K): [in this window] [in a new window]   Fig. 5.   PMA did not alter agonist sensitivity. A, dose-response relationship for 1 (left) and (right) receptors. The continuous line is the best fit of the Hill equation (eq. 1) to the data. The vertical dashed lines are the responses at 1 and 5 µM GABA for 1 and 20 and 50 µM GABA for used to assess changes in sensitivity. B, current traces from receptors at 1 and 5 µM GABA before (left) and after (middle) PMA exposure (100 nM; 10 min). On the right are the 1 µM GABA traces before and after PMA trratment superimposed. C, graph on the left plots the ratio of the 1 and 5 µM response before and after PMA treatment; the right graph plots the normalized amplitude of the current in response to 5 µM GABA before and after PMA treatment. Note the ratio did not significantly change after PMA-induced inactivation with a normalized value of 0.9 ± 0.11 (n = 5) after PMA exposure. The current amplitude, however, decreased to 0.23 ± 0 0.06 (n = 5) of the initial value. D, graph on the left plots the ratio of the 20 and 50 µM response before and after PMA treatment. The right graph plots the normalized amplitude of the current in response to 50 µM GABA before and after PMA treatment. Note the ratio did not significantly change after PMA-induced inactivation with a normalized value of 0.93 ± 0.15 (n = 4) after PMA exposure. The current amplitude, however, decreased to 0.04 ± 0.02 (n = 4) of its initial value.

Internalization Is a Possible Mechanism of PMA-Induced Inactivation of 1 and Receptors. A second possible mechanism of the PMA-induced inactivation of the GABA-activated current is removal of GABA receptors from the membrane surface. To estimate changes in the number of receptors on the membrane surface, we examined [³H]GABA binding to surface 1 receptors and fluorescence of GFP-tagged -GABA_A receptors during PMA-induced inactivation.

View larger version (27K): [in this window] [in a new window]   Fig. 6.   PMA reduced surface binding of [3H]GABA to 1 receptors. Oocytes were exposed to PMA (100 nM; 10 min) and the current amplitude and surface binding of [3H]GABA (Chang and Weiss, 1999) were examined concomitantly at 10-min intervals. The average of the normalized amplitude () and [3H]GABA binding () are plotted. Note the similar time course of the decline.

View larger version (59K): [in this window] [in a new window]   Fig. 7.   PMA-induced internalization of -GFP-receptors. A, GFP was fused to the C terminus of the  subunit and the , -GFP, and  subunits were coexpressed in Xenopus oocytes. Representative currents in response to 20 and 500 µM GABA from the -GFP construct. Although the expression level was reduced somewhat in comparison to wild-type receptors, the activation features of the GFP-tagged receptors were similar to that of the wild type. B, surface fluorescence from an oocyte expressing GFP receptors. Note the continuous decline of fluorescence after PMA treatment.

Specificity of PMA-Induced Internalization of GABA Receptors. To examine the specificity of PMA-induced internalization, we coexpressed nACh receptors (₇ or ₄₂) along with 1 receptors. Carbachol (1 mM) or GABA (20 µM) were applied at 15-min intervals to oocytes expressing both nACh and GABA receptors. PMA (100 nM; 10-min treatment) produced inactivation of 1 receptors (Fig. 8A) but did not significantly change the current amplitude from ₇ or ₄₂ receptors (Fig. 8, B and C). After 30 min of recording, the amplitude normalized to the initial value was 0.36 ± 0.05 (n = 4) for 1 receptors, 1.24 ± 0.18 (n = 4) for ₇ receptors, and 1.01 ± 0.1 (n = 5) for ₄₂ receptors, respectively (Fig. 8D). Thus, PMA specifically internalizes GABA receptors but does not modulate the amplitude of neuronal nACh receptors. These data suggest that the internalization of GABA receptors was not simply due to a PKC-dependent nonspecific plasma membrane retrieval.

View larger version (30K): [in this window] [in a new window]   Fig. 8.   PMA induced inactivation of 1 receptors but did not decrease the current amplitude of neuronal nACh receptors. A-C, traces are currents activated by 20 µM GABA for 1 receptors and 1 mM carbamycholine for 7 and 42 nACh receptors, respectively. D, normalized averages of the current amplitudes from 1, 7, and 42 receptors after PMA treatment (50 nM; 10 min).

Coexpression or Coinjection of Large IC Loop. The IC loop of 1 and 2 subunits has been shown to interact with the cytoskeleton (Hanley et al., 1999; Wang et al., 1999) and this interaction may be important in the PKC-dependent receptor internalization. To test this possibility, we coexpressed 1 receptors with the M3-M4 IC loop in Xenopus oocytes. If the IC loop of the 1 receptor interacts with a component of the cytoskeleton and this interaction was necessary for the PMA-induced inactivation, then overexpression of the IC loop should compete with the PMA-induced inactivation. Figure 9A shows a Western blot from oocytes confirming the expression of the IC loop in the cytosol. With PMA treatment (0.1 µM; 10 min), the amplitude of the GABA-activated current normalized to the initial value was 0.22 ± 0.03 (n = 5) and 0.24 ± 0.08 (n = 4) after 50 min of recording with and without the coexpression of the IC loop, respectively (Fig. 9B, right). Similar results were obtained with direct injection of recombinant IC loop protein. These data suggest that the PMA-induced inactivation of 1 receptors may not require a specific interaction with the M3-M4 IC loop. We cannot rule out the possibility, however, that for reasons of aberrant IC loop conformation or improper subcellular location, the IC loop protein was unable to compete with the intact IC loop.

View larger version (26K): [in this window] [in a new window]   Fig. 9.   Coexpression of the IC loop with 1 receptors did not prevent the PMA-mediated inactivation. A, Western blot from oocytes expressing the 1 IC loop. An ~15-kDa band was detected in the cytosol that was not present in the membrane fraction or the cytosolic fraction of noncRNA-injected oocytes. B, current amplitude before and after PMA treatment in oocytes expressing the 1 receptor and oocytes expressing the 1 receptor along with the IC loop. The degree of PMA-induced inactivation was similar, indicating that the IC loop did not compete with the receptor internalization.

Receptors Could Return to Cell Surface after PMA-Dependent Internalization. We next examined if the receptors that were internalized by PKC activation could return to the cell surface. To answer this question, we assessed the recovery of functional GABA-activated currents after PMA-induced internalization. The amplitude of the GABA-evoked current from 1 receptors normalized to the initial value recovered from 0.08 ± 0.03 to 0.8 ± 0.1 (n = 9) after 24 h (PMA = 100 nM; 10 min) (Fig. 10, A and B). The GABA-activated current did not significantly change during 24 h in oocytes without PMA treatment [amplitude normalized to the initial value was 0.98 ± 0.1 (n = 5); Fig. 10B, right]. Twenty-four hours after the PMA-induced internalization, the GABA-activated current from receptors recovered from 0.06 ± 0.05 (n = 4) to 0.60 ± 0.12 (n = 4) of the initial value. These data suggest that receptors internalized by PMA activation can exist as a pool that can be returned to the cell surface.

View larger version (22K): [in this window] [in a new window]   Fig. 10.   Recovery from PMA-induced internalization of GABA receptors.A,GABA-activated current from 1 receptors before and after PMA treatment (100 nM; 10 min). Note that the current almost completely recovered within 20 h. B, plot of the normalized current amplitude before and after PMA treatment.  represent the GABA-activated current amplitude in a group of oocytes that were not exposed to PMA. Note that in the absence of PMA, the current amplitude remained stable over this time course. C, GABA-activated current from receptors before and after PMA treatment (100 nM; 10 min). As for the 1 receptors, the current almost completely recovered within 20 h. D, plot of the normalized current amplitude before and after PMA treatment in receptors.  represent the GABA-activated current amplitude in a group of oocytes that were not exposed to PMA.

View larger version (26K): [in this window] [in a new window]   Fig. 11.   Internalized receptors can return to the cell surface. A, traces show the block of a 3 µM GABA-activated current by different concentrations of Zn2+ for the wild-type (top) and 6HIS (bottom) constructs. The graph to the right plots the fractional block as a function of the Zn2+ concentration. The continuous lines are fits of eq. 2 to these data, which yielded IC50 values/Hill coefficients of 44 ± 3/1.8 ± 0.2 (n = 4) and 10.2 ± 0.1/1.0 ± 0.05 (n = 8) for the wild-type and 6HIS receptors, respectively. B, block of the 6HIS receptor by 2.5 µM GABA before (left) and after (middle) a 120-min incubation with the exoprotease carboxypeptidase A that cleaved the histidines and reverted the Zn2+ sensitivity to that of the wild-type receptor. The rightmost traces show the Zn2+ sensitivity after the same oocyte was exposed to 100 nM PMA causing internalization and was then allowed to recover for 24 h. Note that the Zn2+ sensitivity was similar to that of the wild type (cleaved 6HIS), indicating the internalized receptors were returned to the cell surface. C, averages from four oocytes showing the fractional activation of a 3 µM GABA-activated current by 2.5 µM Zn2+ before exposure to carboxypeptidase A (), after exposure (), and after exposure to 100 nM PMA with a subsequent 24-h recovery period (). Percentage of inactivation by Zn2+ is shown for the wild-type 1 receptor for comparison (). D, plot of the fraction of current remaining after 100 nM PMA () and after the 24-h recovery period (). The averages are for the same data set as in Fig. 11C.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

Previous investigations on the PKC-dependent regulation of GABA_A receptors have been controversial because both an inhibition (Sigel and Baur, 1988; Kellenberger et al., 1992; Chapell et al., 1998) and potentiation (Lin et al., 1996) have been observed. For GABA_C receptors, a negative modulation by PKC has been reported for both native GABA_C (Feigenspan and Bormann, 1994) and recombinant 1 (Kusama et al., 1998; Filippova et al., 1999) receptors.

Our results document a role for PKC in the modulation of recombinant GABA receptors expressed in Xenopus oocytes. PKC induces receptor internalization as revealed by a decrease in surface binding of [³H]GABA, and the disappearance of GFP-tagged GABA receptors from the cell surface. This confirms previous findings that activation of PKC can reduce the amplitude of GABA-activated currents in oocytes (Sigel and Baur, 1988) and is in agreement with the conclusion that receptor internalization is the mechanism of this inactivation for GABA_A receptors expressed in oocytes (Chapell et al., 1998) and 1 receptors expressed in HEK293 cells (Filippova et al., 1999). That a similar effect of PKC activation was observed for native GABA_C receptors (Feigenspan and Bormann, 1994) as well as recombinant 1 receptors transiently expressed in HEK293 cells (Filippova et al., 1999) indicates that this effect is not unique to the oocyte expression system.

Internalization of membrane receptors by PKC activation is not without precedent because PMA has been shown to internalize the transferrin receptor, epidermal growth factor receptor, sodium, K-ATPase, a retinal taurine transporter, and the Na⁺/glucose transporter (for review, see Backer and King, 1991). In many cases, and similar to our findings, this internalization does not depend on receptor phosphorylation (Backer and King, 1991). Internalization of receptors in Xenopus oocytes is not, however, a nonspecific phenomenon because PKC-internalized GABA receptors but not nACh receptors expressed in the same oocyte. Although this phenomenon may reflect a specificity for interaction with the internalization machinery or with PKC (receptor for activated C kinase proteins; for review, see Mochly-Rosen, 1995), we cannot rule out the possibility that the nACh and GABA receptors are clustered in different regions of the oocyte and that endocytosis is region- (e.g., pole-) specific. Although we have not mapped the location of the GABA and nACh receptors, the -GFP receptors are clearly clustered on the cell surface (Fig. 7).

The details of PKC-dependent modulation of GABA_C receptors are still unclear. Assuming the residues identified in the in vitro phosphorylation studies are the same as in the full-length 1 receptor, direct phosphorylation of the 1 receptor itself was not required for the PKC-dependent internalization. Our present working hypothesis is that other protein(s) along the pathway for internalization require phosphorylation by PKC. Phorbol esters have been shown to alter the organization of microtubules, microfilaments, and other components of the cytoskeletal network (Phaire-Washington et al., 1980; Backer and King, 1991). Presumably, an interaction between GABA receptors and the cytoskeleton could be crucial for receptor shuttling to and from the cell surface. Recently, two proteins have been identified, mitogen-activated protein-1B and GABARAP, that are candidates for linking GABA_C and GABA_A receptors to the cytoskeleton, respectively (Hanley et al., 1999; Wang et al., 1999). The cytoskeleton has already been implicated in membrane receptor modulation (Connolly, 1984; Rosenmund and Westbrook, 1993), including recombinant 1 GABA_C receptors expressed in HEK293 cells (Filippova et al., 1999).

Concerning the particular domains of the GABA receptor that could interact with the machinery responsible for internalization and retrieval, the obvious candidate was the putative large IC loop between M3 and M4. This region has already been implicated in the interaction with mitogen-activated protein-1B (Hanley et al., 1999). Coexpression or coinjection of the 1 IC loop did not interfere with the PMA-induced internalization although several technical problems could account for these findings. First, the amount of IC loop could have been insufficient to block the requisite protein-protein interactions. Second, the coexpressed IC loop may exist in a different subcellular compartment than the 1 receptors (e.g., cytosol versus membrane). Third, the conformation of the IC loop protein could be different from that of the IC loop in the 1 receptor. Alternatively, other putative intracellular regions such as the loop between the first and second membrane-spanning domains could serve as a tether for internalization. And last, the present picture of the membrane topology of the GABA receptor subunit is only a model based on a hydrophobicity analysis. It is possible that other regions of the subunit are available for intracellular protein-protein interactions.

The reported half-life of receptors on the membrane surface of native neurons varies from 12 to 30 h at different stages of development (Steinbach, 1981; Killisch et al., 1991; Mammen et al., 1997). During this time, receptors can change their surface distribution (Mammen et al., 1997; O'Brien et al., 1997). In addition to the surface receptors, a second intracellular pool exists (Mammen et al., 1997). Whether receptors can be dynamically shuttled between the membrane surface and an intracellular pool during their lifetime is still controversial. Concerning GABA receptors, activation of tyrosine kinases has been shown to recruit GABA_A receptors to the postsynaptic region (Wan et al., 1997). In this study, we demonstrated a PKC-dependent internalization of GABA receptors from the cell surface and, as evidenced by functional recovery, these internalized receptors can be reinserted in the plasma membrane. A similar recovery of internalized receptors has been observed for 1 receptors expressed in HEK293 cells (Filippova et al., 1999). Thus, by controlling surface expression, PKC may play a key role in regulating GABA-mediated inhibition in the CNS.