Simultaneous stimulation of GABA and beta adrenergic receptors stabilizes isotypes of activated adenylyl cyclase heterocomplex
© Robichon et al; licensee BioMed Central Ltd. 2004
Received: 29 July 2003
Accepted: 09 June 2004
Published: 09 June 2004
We investigated how the synthesis of cAMP, stimulated by isoproterenol acting through β-adrenoreceptors and Gs, is strongly amplified by simultaneous incubation with baclofen. Baclofen is an agonist of δ-aminobutyric acid type B receptors [GABAB], known to inhibit adenylyl cyclase via Gi. Because these agents have opposite effects on cAMP levels, the unexpected increase in cAMP synthesis when they are applied simultaneously has been intensively investigated. From previous reports, it appears that cyclase type II contributes most significantly to this phenomenon.
We found that simultaneous application of isoproterenol and baclofen specifically influences the association/dissociation of molecules involved in the induction and termination of cyclase activity. Beta/gamma from [GABA]B receptor-coupled Gi has a higher affinity for adenylyl cyclase isoform(s) when these isoforms are co-associated with Gs. Our data also suggest that, when beta/gamma and Gαs are associated with adenylyl cyclase isoform(s), beta/gamma from [GABA]B receptor-coupled Gi retards the GTPase activity of Gαs from adrenergic receptor. These reciprocal regulations of subunits of the adenylyl cyclase complex might be responsible for the drastic increase of cAMP synthesis in response to the simultaneous signals.
Simultaneous signals arriving at a particular synapse converge on molecular detectors of coincidence and trigger specific biochemical events. We hypothesize that this phenomenon comes from the complex molecular architectures involved, including scaffolding proteins that make reciprocal interactions between associated molecules possible. The biochemistry of simultaneous signaling is addressed as a key to synaptic function.
The defensive withdrawal reflex of Aplysia affords a model of classical conditioning. This involves a conditioned stimulus (touching the siphon) and an unconditioned stimulus (shocking the tail), and has been interpreted in terms of the integration of two separate simultaneous signals in the same motor neuron: Ca++ and Gαs-GTP [1, 2]. Genetic analysis in Drosophila showed that a mutant of learning and memory, rutabaga, has a defect in the gene coding for an isoform of adenylyl cyclase (type II), which is regulated by both Ca++ and Gα s [3, 4]. This mutant shows low and unregulated levels of cAMP. Paradoxically, elevation of cAMP levels in dnc mutants (defect in the cAMP phosphodiesterase gene) causes an identical learning and memory deficit. The strongest phenotype comes from an allele that increases the cAMP level to as much as eight times normal [4–6]. Moreover, the double mutant dnc/rut has a normal cAMP level but still shows a learning and memory deficit, which suggests that these behavioral defects depend on failure to regulate cAMP levels . Studies of these molecules in mutants and/or transgenic animals have led to an accumulation of interesting data in behavioral analysis of memory, learning and associative competence . Moreover and surprisingly, it has been demonstrated that a high level of cAMP also follows the simultaneous activation of Gi by GABA and G s by isoproterenol in rat neurons, whose combined (opposing) effects were expected to restore the basal cAMP level [2, 8–10]. Biochemical analysis has revealed the role of adenylyl cyclase type II in this phenomenon, which is highly active when complexed with both beta/gamma and Gαs. Interestingly, beta/gamma in such complexes comes from the Gi component [2, 11, 12]. Six beta and 13 gamma isoforms and a large family of cyclase isoforms have been cloned [13–18], which suggests that only specific combinations might be involved in coincidental signaling. Molecules that act as detectors of coincidental signals allow us to understand how the topology of the synaptic network influences its biochemistry. Our analysis of the molecular components involved in simultaneous GABA/isoproterenol signaling has been facilitated by previously reported data on the role of beta4/gamma2 and beta1/gamma2 in the activation of cyclase type II [19, 20]. In the present study we investigated whether beta/gamma might influence the GTPase activity of Gαs when beta/gamma, Gαs and cyclase are associated in a complex form and conversely, whether Gαs pre-associated with a cyclase, might increase the affinity of beta/gamma for this complex.
Simultaneous signaling determines the scale of cAMP synthesis
The affinity of beta/gamma subunits for cyclase isoform(s) depends on co-association with Gαs
After solubilizing the membranes with nonionic detergent, we performed a 'pull down' experiment with Forskolin-agarose to isolate the cyclase complex. Lubrol is known to inhibit G protein GTPase activity, which allowed us to 'freeze' the G protein activated state while we isolated the cyclase complex. ADP-ribosylation by cholera toxin inhibits Gαs GTPase. ADP-ribosylation by pertussis toxin uncouples receptors and Gi [36, 37]. We measured the amount of radioactive ADP-ribosylated Gαs in the cyclase complex isolated with Forskolin-agarose.
Isotopic ADP ribosylation was not performed quantitatively (in conditions that eliminate "back ribosylations"). We observed that these optimal conditions saturated the signal.Gαs from inactivated receptors represents a huge amount compared to Gαs activated by isoproterenol. This interference might overwhelm the specific signal induced by isoproterenol if a probable artefactual slow exchange GDP/GTP occurs at silent Gαs. We carried out therefore the radioactive ADP ribosylation for a short time and a limited concentration of reagent in order to label preferentially the isoproterenol activated Gαs. This experimental design allowed us to compare relative levels of labeling as an index when membranes were treated with different drugs. The quantification of radioactivity was normalized with the quantity of protein present in each individual 'pull down'.
We tried also to quantify the ratio of molar amounts Gαs/beta attached to affinity chromatography when membranes were incubated with drugs. Analysis by densitometry of immunostained bands was not carried out because the relation quantity/staining was not linear. We proceeded also as follows: proteins isolated with forskolin-agarose were dissociated with 1 M NaCl and derivatized with the reagent: N-hydroxysuccinimido biotin. The biotinyl proteins were then precipitated with avidin-agarose. These complex were incubated with an antibody against Gαs or Gbeta then a rabbit antibody-alkaline phosphatase in order to quantify these molecules. Precise quantity of Gαs and Gbeta (few μg of immunoprecipitated material) were submitted to the same protocol of biotinylation (data not shown). Unfortunately this method gave us variable data to some extent. We decide therefore to shift to the Bolton Hunter iodination method that has the advantage to be faster and more accurate. The labeled material attached to affinity chromatography was submitted to gel electrophoresis, then we measured the radioactivity in 35 and 45 Kda bands of dried electrophoresis gels although these iodinated bands might include unrelated proteins and don't discriminate between isoforms of Gα. We obtained the following ratio: isoproterenol + baclofen, Gα/Gbeta = 1.8+/-0.5, isoproterenol, Gα/Gbeta= 5+/-2.2, baclofen,Gα /Gbeta = 1.1+/-0.2, isoproterenol + pertussis toxin, Gα/Gbeta= 15+/-4). Again, this method don't discriminate Gαs from Gαi. The amount of cyclase(s) was >10 fold to the amount of Ga and/or Gbeta. The quantification of Bolton Hunter labeled bands were normalized with the quantity of protein present in each individual 'pull down'.
A representative colloidal blue stained gel of attached material to affinity chromatography is shown (fig. 2c). We see two bands at 45 and 35 kDa, corresponding to the Gαs and Gbeta subunits respectively. The 35 kDa band vanished when membranes were treated with pertussis toxin and a strong doublet appeared when membranes were incubated with iso+bac. Moreover, the specificity of the 'pull down' procedure was assessed with membranes incubated with GDP versus GTPyS+iso+bac (fig. 2c). We see clearly the absence of the doublet with the GDP treated membranes and the non specific proteins attached to agarose. We did also competition experiment with samples pre-incubated with free Forskolin at 10 nM and 10 μM. We observed that two bands (45 and 35 kDa) disappeared while the non specific binding was unchanged (data non shown).
The GTPase activity of Gαs bound to the cyclase(s) complex is decreased by co-association with beta/gamma subunits
Adenylyl cyclases contain two conserved homologous cytoplasmic domains (C1 and C2) that form the catalytic core of the enzyme . Forskolin appears to induce the assembly of these two domains by interacting with the catalytic cleft [21–23]. The affinity between C1 and C2 is also facilitated by Gαs binding. These data have been confirmed by the finding that Forskolin and Gαs stimulate synergistically the cyclase activity . In the presence of Gαs the affinity of Forskolin for the dimer C1/C2 is high (Kd = 0.1 μM), which suggests a stronger affinity for the complete cyclase molecule . The site of interaction of the cyclase (type I or II) for beta/gamma has been located in the C1 b region. This site is independent of the Gαs and Forskolin interaction domains. These findings argue in favor of isolating the cyclase(s) and its associated proteins using Forskolin-agarose affinity chromatography although this procedure enriches indistinctly the different isoforms of the cyclase family. The authors who described this method  reported that the complex could be dissociated with high salt concentrations in order to purify the cyclase(s). Our 'pull down' material is heterogeneous because this isolation procedure does not discriminate between the different isotypes of cyclase. Furthermore, Gαs recognizes all the isoforms of adenylate cyclase whereas the association Gbeta/cyclase is limited to two isotypes (type I and II) which share similar features. Consequently, this mismatch makes the comparison of Gαs/Gbeta precipitated in the cyclase 'pull down' difficult. Inconstant data were observed in experiments of kinetics where both molecules were quantified on the same gel. On the other hand, Gαs-GTP in the Gαs/cyclase(s) complex might have a Kcat for the hydrolysis of bound GTP different from one isoform of cyclase to another. The heterogeneity of cyclases in the 'pull down' makes the analysis of co-associated Gαs uncertain, specially if we aim to parallel the dynamics of association/dissociation of Gbeta and Gαs. Furthermore, our analysis might be hampered by variable elution/retention of components from the affinity column and the yield of their isolation from crude membranes. These limitations are difficult to assess.
On the other hand, the drastic increase of cAMP synthesis by simultaneous Gi/Gs activation finely analyzed in rat hippocampus  is known to be attributable to the type II isoform . Type I is also regulated by beta/gamma and Gαs but here beta/gamma is inhibitory. High concentrations of beta/gamma do not eliminate the cyclase type I activity induced by Gαs, but stabilize it at intermediate levels, which suggests that the conformation of cyclase type I is rearranged by association with beta/gamma . The two isoforms show similar affinities for beta/gamma (half-maximal effect about 5 nM) . We postulate therefore that the molecular events described in this report mostly highlight the relevance of cyclase type II, despite isotype I contamination. We might also speculate that the intra-complex regulatory mechanisms described here have the same characteristics for both isotypes, although they induce opposite effects on cyclase activity.
Furthermore, we carried out our experiments with intact membranes, in order to preserve the architecture of the multiple associated proteins that might be relevant to the biochemistry involved in coincidental phenomenon. For example, regulators of G proteins (RGS) interact mostly with the Gα subunit of Gi/o/q by accelerating their intrinsic GTPase activity, which alters the amplitude of the effect of a stimulated receptor . Each member of this large family of genes (16 isotypes have been described) displays differential selectivity for these G proteins . Interestingly, RGS 14 and 12 are unique in that they inhibit the guanine nucleotide exchange of Gi in addition to their GAP (GTPase activating protein) activity. A recent report shows that PKA phosphorylation of RGS 14 increases the inhibition of nucleotide exchange 3-fold, thereby increasing the binding Gi/GDP and consequently limiting the interactions of Gi with effectors . RGS 14 is expressed in brain , which might be of interest in relation to our work. By increasing PKA activity, high cAMP concentrations should inhibit more Gi molecules enhancing the effect of the simultaneous stimulation (Gαs and Gbeta) of cyclase type II. After a while beta/gamma would then be captured by Gi/GDP, ending the simultaneous stimulation of cyclase type II by a retro-control loop.
In this report, we have investigated whether beta/gamma bound to cyclase(s) might influence the rate of GTP hydrolysis of Gαs, or conversely whether Gαs bound to cyclase(s) might modify its affinity for beta/gamma. We found that the GTPase activity of Gαs coupled to the cyclase(s) complex decreases when beta adrenergic and GABA receptors are simultaneously stimulated. We also demonstrated that the affinity of beta/gamma (from Gi) for the cyclase(s) is increased when the latter is associated with GTP-Gαs. Our data strongly suggest that simultaneous application of stimuli, which individually exert opposing effects on the levels of second messengers, triggers specific kinetics of protein association and enzyme catalysis. This highlights the paradoxically high level of cAMP obtained by simultaneous GABA/beta adrenergic receptor activation. Adenylate cyclase was reported to function as a GTPase activating protein for Gαs . Our data support the idea that beta/gamma from [GABA]b receptor-coupled Gi retards the GTPase activity of adrenergic receptor-coupled Gαs when these molecules are complexed with adenylyl cyclase isoforms.
Cyclases are not the only molecules to be regulated 'à la carte' by simultaneous signaling. Various molecules in different tissues correspond to the definition of 'detectors of simultaneity': for example, muscle phosphorylase kinase, a tetramer constituted of a catalytic subunit, two cAMP dependent regulatory subunits and calmodulin. Two stimuli (Ca++ and cAMP) synergistically stimulate phosphorylase kinase activity . Furthermore, some molecules fine-tune their activities, depending on the local context, in 'reading' the temporal order of their regulatory signals. For example, phosphatase I is associated with a potent protein inhibitor: this inhibitor has a site that is phosphorylated by caseine kinase I. This phosphorylation protects an adjacent phosphorylated site from calcineurin and consequently these two phospho sites in the inhibitor controls the activity of the phosphatase . Another example is the endothelial nitric oxide synthase (eNOS). This enzyme has two major phosphorylation sites, which have opposite functions. One, phosphorylated by PKA, stimulates the enzyme activity, the other, phosphorylated by PKC, inhibits it. Interestingly, PKA and/or PKC phosphorylation of one site induces dephosphorylation of the other. Two phosphatases associated with eNOS have one or the other phosphorylated site as specific substrate. When one phosphatase is active the other is inactive . All these coordinated controls display different modes used by key molecules acting as sensors of multiple converging second messengers. Specific molecular events induced by simultaneous signals and/or sequentially ordered signals imply interactions of components in a large complex. This seems the essential paradigm of synaptic function.
Western blotting was carried out with a polyclonal anti-Beta antibody (Santa Cruz/Biotechnology, 2161 Delaware Avenue, Santa Cruz Ca 95060), which recognizes all the isoforms of beta. Cholera toxin (activated subunit), pertussis toxin and NAD+ were purchased from Sigma (St. Louis Missouri 63178-9916 P.O. Box 14508). Radioactive [γ-32P] GTP (NEG004Z, 6000 Ci/mmol) and [32P] NAD+ (NEG023X, 800 Ci/mmol) were purchased from NEN Life Sciences Products (PerkinElmer life Sciences, 549 Albany Street, Boston MA 02118–2512). Cyclic AMP was determined using a radioactive kit purchased from NEN (RIA kit using [125I] NEK033) with membranes in experimental conditions described in figure legends. Bolton Hunter reagent (diiodinated), [125 I] (NEX120H) was purchased from NEN. Saclofen, propanolol, isoproterenol and baclofen were purchased from Sigma (cell signaling and neuroscience). Forskolin-agarose was purchased from Sigma.
Preparation of plasma membrane of isolated nerve terminals (synaptosomes)
Synaptosomes were prepared by homogenization of rat brain (Sprague Dawley, 150 g) in 310 mM sucrose, 10 mM Hepes (pH 7.4), 1 mM EDTA. The homogenate was centrifuged for 5 min at 900 gmax at 4°C. The supernatants were combined and centrifuged at 11 000 gmax for 15 min. Pellets were re-suspended and this synaptosome-enriched fraction was layered on to each of two discontinuous gradients (12% (w/v), 9% and 6% Ficoll) and centrifuged for 30 min at 75,000 gmax according to the protocol described elsewhere . The intermediate band (within the 9% layer) was resuspended in HEPES 5 mM (pH 7.4), EDTA 1 mM for 1 h in ice and the plasma membrane fraction released by osmotic shock was re-centrifuged for 5 min at 10,000 gmax. The isolated plasma membranes were incubated at 30°C for 15 min with anti proteases (PMSF: 1 μg/ml, leupeptin: 1 μg/ml and aprotinin: 1 μg/ml) in order to wash out endogenous neuromediators, then aliquoted and frozen at -70°C.
Isolation of the cyclase complex
Membranes (50 μg protein) were assayed as described in the figure legends, then solubilized in Triton X-100 1%, 150 mM NaCl, 10 mM Tris (pH 7.4), 1 mM EDTA. Samples were briefly centrifuged and the supernatant was diluted five times (final concentration of Triton X-100 = 0.2%) in 150 mM NaCl, 10 mM Tris (pH 7.4), 1 mM EDTA plus 0.5 mM Mg. Lubrol at these concentrations potently inhibits GTPase activity by preventing the release of hydrolysis products, as reported elsewhere . We observed that Triton X-100 or Tween 20 (Sigma) gave similar effects. Forskolin-agarose (25 μl) was added to the extract and incubated for 10 min at 4°C. Samples were constantly re-suspended by tube rotation. Samples were then centrifuged briefly, the supernatants discarded and the pellets washed once with the same buffer. Molecules attached to affinity beads were separated by SDS-PAGE alongside pre-stained molecular weight markers (Sigma). The electrophoresis gels were stained with colloidal blue (Sigma).
ADP-ribosylation of Gs and Gi by Pertussis toxin and cholera toxin
ADP-ribosylation using pertussis toxin (Sigma) was carried out as indicated in the figure legends, in the presence of 1 mM EDTA, 2 mM DTT, 5 mM thymidine, 10 mM HEPES (pH 7.4), 25 μg/ml pertussis toxin and 10 μM NAD+, according to the protocol described elsewhere . ADP-ribosylation was also carried out with 10 μg/ml cholera toxin as described elsewhere  with a slight modification: the endogenous cofactor for cholera toxin (ARF) was activated with only GTP, as indicated in the figures. For radioactive labeling by cholera toxin (10 μg/ml toxin), 10 μM [32P] NAD+ (50 000 cpm/pmol) were incubated with membranes in a buffer with 10 mM thymidine and 200 mM Nacl as indicated in figure legends. Radioactivity attached to the isolated cyclase complex was counted and normalized with the amount of proteins precipitated by the affinity chromatography as follows: an aliquot of each sample was incubated with 5 μl of trypsin-agarose (Sigma) for 30 min in 10 mM phosphate buffer pH = 8 and the released peptides were measured at 220 nm in 150 μl of the same buffer.
Cold ADP-ribosylation by cholera toxin was carried out with membranes 20 min at 30°C with 30 μg/ml toxin, 50 μM NAD+ and the concentration of GTP indicated in figure legends. Cold ADP-ribosylation by pertussis toxin was carried out with membranes with 50 μg/ml toxin and 50 μM NAD+ for 20 min at 30°C.
The material bound to Forskolin-agarose was submitted to SDS acrylamide gel electrophoresis (12%) using pre-stained standards (Sigma). To monitor the accumulation of beta/gamma, the gels were blotted on to nitrocellulose for Western analysis using antibodies at the dilution indicated by Santa Cruz Biotech. The anti-beta used (T-20) is broadly reactive with beta 1, 2, 3 and 4. The blots were developed by alkaline phosphatase conjugated anti rabbit IgG (Sigma). For cross-linking experiment, the precipitate was incubated with glutaraldehyde 1% in 0.2 M Na carbonate pH = 8 for few minutes then the reaction was stopped with 10 μl of lysine solution (1 mM). Experimental conditions for the kinetic analysis are given in the figure legends. To quantify the beta subunit, the material bound to Forskolin-agarose was labeled with Bolton Hunter reagent (NEN) (20 000 cpm/samples) in 150 μl phosphate buffer 10 mM (pH 7.4) for few minutes, then molecules were separated by SDS polyacrylamide gel electrophoresis. Gels were dried and the bands around 35 kDa were excised and counted. Radiolabeled ADP-ribosylated Gαs was counted according to the same method. Counts were normalized with the amount of protein in each individual 'pull down'.
To quantify the molar amount of precipitated Gα and beta, the material attached to affinity chromatography was labeled with Bolton Hunter reagent as above. This material was first submitted to high salt treatment in 50 μl of 500 mM Nacl and 10 mM phosphate pH = 8 for 1 hour to dissociate proteins, then samples were diluted 3 times before the labeling. This material was precipitated by trichloracetic acid (5%), the pellet was neutralized with Tris buffer and loaded on electrophoresis gel (10% acrylamide). The respective bands were excised from the dried gel and counted. We assumed that the intensity of I125 labeling is proportional to the size of the protein and we divided the counts by an index of molecular weight: 1 forbeta, 1.28 for Gα and 3.15 for cyclase (beta/beta = 1, Gα/beta = 1.28 and Cyclase/beta = 3.15) in order to compare the numbers. Counts were normalized with the amount of protein as described above and the ratio of normalized counts are reported as the ratio of molar amount of molecules. Results represent the average of three experiments +/-SE.
Determination of cAMP levels
Adenylate cyclase activity was estimated in membranes in a medium consisting of 20 mM MOPS (pH 7.4), 10 mM creatine phosphate, 50 μg per ml of creatine kinase, 5 mM MgCl2 and 500 μM radioactive ATP (200 cpm/pmol) using a RIA kit (NEN).
GTPase activity was measured by the release of [32Pi] from [γ32P] GTP previously bound to Gαs. GTP hydrolytic activity was detected by pre-equilibrating the G proteins with [γ32P] GTP in the absence of Mg, then the GTPase reaction was started by adding MgCl2 at 0°C (5 mM final concentration). The decrease of the radiolabeled GTP/G protein complex was quantified by nitrocellulose filtration. This method, described elsewhere , was used with slight modifications in the experimental conditions described in the figure legends.
Single turnover GTPase assay
This assay, described elsewhere , was slightly modified. Membranes (50 μg protein) in 250 μl final volume were incubated with 1 μM [γ32P] GTP (10,000 cpm/pmol) in 50 mM Hepes (pH 8.0), 1 mM DTT, 1 mM EDTA, at 20°C for 20 min. Samples were placed in ice for 5 min. The GTPase reaction was initiated at 0°C (in ice) by the addition of MgCl2 and GTP to final concentrations of 5 mM and 1 mM respectively. At the indicated time points, samples were solubilized and the cyclase complex was isolated with 25 μl of Forskolin-agarose in presence of NaF (100 μM). The attached material was re-suspended in 150 μl of the initial buffer at 0°C (without Mg) and submitted to immediate filtration on nitrocellulose. Filters were counted in Beckman beta counter.
Determination of fractional occupancy by GTP
This assay was adapted from the procedure described elsewhere [40, 41]. Membranes (50 μg protein) were incubated at 20°C for 20 min with 1 μM [γ32P] GTP (10,000 cpm/pmol) in 50 mM Hepes (pH 8.0), 1 mM DTT, 1 mM EDTA and drugs. Control membranes were treated with cholera toxin (CTx) as indicated above. The membranes (incubated with drugs or CTx according to the figure legends) were placed in ice, then the GTPase reaction was initiated by 5 mM MgCl2. At the indicated times, the cyclase complex was isolated in presence of NaF (100 μM) and radioactivity was counted as above.
Beta, gamma: subunits of G protein, Gs: stimulatory G protein, Gi: inhibitory G protein, Gα: alpha subunit of G protein, CTx: cholera toxin, PTx: pertussis toxin, Gαs: stimulatory alpha subunit of G protein, iso: isoproterenol, bac: baclofen
This work was supported by CNRS and a grant from the local government of Burgundy. We are grateful to Pierre Hericourt and Amélie Garnier for technical assistance and discussions. We thank Sue Broughton, Ralph Greenspan and Paul Salin for the many discussions that initiated this work.
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