Modulation of 5-HT(1A) receptor activation by its interaction with wild-type and mutant g(alphai3) proteins

How much do we know about the coupling of G-proteins to serotonin receptors?

Serotonin receptors are G-protein-coupled receptors (GPCRs) involved in a variety of psychiatric disorders. G-proteins, heterotrimeric complexes that couple to multiple receptors, are activated when their receptor is bound by the appropriate ligand. Activation triggers a cascade of further signalling events that ultimately result in cell function changes. Each of the several known G-protein types can activate multiple pathways. Interestingly, since several G-proteins can couple to the same serotonin receptor type, receptor activation can result in induction of different pathways. To reach a better understanding of the role, interactions and expression of G-proteins a literature search was performed in order to list all the known heterotrimeric combinations and serotonin receptor complexes. Public databases were analysed to collect transcript and protein expression data relating to G-proteins in neural tissues. Only a very small number of heterotrimeric combinations and G-protein-receptor complexes out of the possible thousands suggested by expression data analysis have been examined experimentally. In addition this has mostly been obtained using insect, hamster, rat and, to a lesser extent, human cell lines. Besides highlighting which interactions have not been explored, our findings suggest additional possible interactions that should be examined based on our expression data analysis.

Protean agonism at the dopamine D2 receptor: (S)-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of Go1 but an antagonist/inverse agonist for Gi1,Gi2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D(2) receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins could be expressed in an inducible fashion. Varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [(35)S]GTPgammaS onto each of Galpha(i1),Galpha(i2),Galpha(i3), and Galpha(o1). By contrast, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when Galpha(o1) was the target G protein but an antagonist/inverse agonist at Galpha(i1),Galpha(i2), and Galpha(i3). In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D(2) receptor-G protein fusion proteins, and the high-affinity state was eliminated by guanine nucleotide. (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine bound to an apparent single state of the constructs in which the D(2) receptor was fused to Galpha(i1),Galpha(i2), or Galpha(i3). However, it bound to distinct high- and low-affinity states of the D(2) receptor-Galpha(o1) fusion, with the high-affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D(2) receptor when expression of pertussis toxin-resistant forms of each of Galpha(i1), Galpha(i2), Galpha(i3), and Galpha(o1) was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of Galpha(o1). p-Tyramine displayed a protean ligand profile similar to that of (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine but with lower potency. These results demonstrate (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine to be a protean agonist at the D(2) receptor and may explain in vivo actions of this ligand.

8-OH-DPAT as a 5-HT7 agonist: phase shifts of the circadian biological clock through increases in cAMP production

Neurons in the suprachiasmatic nucleus (SCN), the site of the endogenous biological clock in mammals, fire spontaneously, peaking in firing rate near ZT6 or at the midpoint of the light phase in a 12:12 light-dark cycle. In rat hypothalamic slices, tissue incubations with drugs can produce a shift in this daily rhythm, the magnitude of which is dependent upon dose and the time of treatment. Previous work with 8-OH-DPAT had noted its ability to produce a phase advance, an earlier occurrence of the peak in neuronal firing, when applied at ZT6. Activation of 5-HT7 receptors was thought to be responsible for the shift, despite the clear preference of 8-OH-DPAT for 5-HT1A sites in terms of receptor binding affinity. In the present work, the actions of 8-OH-DPAT in SCN slices were confirmed and expanded to include additional dose-response and antagonist treatments. By itself, 8-OH-DPAT produced a concentration-dependent phase advance that was sensitive to co-application with 5-HT7 antagonists (ritanserin, mesulergine, SB-269970), but not to 5-HT1A antagonists (WAY-100,635, UH-301). Assignment of the receptor mechanisms for the antagonists employed was accomplished in experiments measuring binding affinities and the generation of cAMP, the latter monitored in a HEK-293 cell line expressing the r5-HT7 receptor and in tissue derived from rat SCN. The results indicate that the increases observed in cAMP levels are small but appear to be sufficient to produce a pharmacological resetting of the clock pacemaker. By aiding in the identification of the 5-HT receptor subtype responsible for the observed phase shifts and cAMP changes, 8-OH-DPAT represents an important pharmacological tool for 5-HT7 receptor activation, essentially broadening its role as the prototypical 5-HT1A agonist to one combining these two receptor activities.

Mutation in a protein kinase C phosphorylation site of the 5-HT1A receptor preferentially attenuates Ca2+ responses to partial as opposed to higher-efficacy 5-HT1A agonists

The Thr(149)Ala mutation in a putative protein kinase C phosphorylation site of the 5-HT(1A) receptor's second intracellular loop has been shown to affect the closing of Ca(2+) channels and Ca(2+) mobilisation without interfering with the inhibitory cAMP pathway (Mol Pharmacol 52 (1997) 164). Here, the Ca(2+) responses for a series of 5-HT(1A) agonists were compared between the wild-type (wt) and mutant Thr(149)Ala 5-HT(1A) receptor as part of a fusion protein containing a G(alpha)(15) protein. Neither the mutation nor the fusion process modified the [(3)H]WAY 100635-based ligand binding profile of the fusion proteins as compared to the wt 5-HT(1A) receptor protein. Whereas at the wt 5-HT(1A) receptor, 5-HT induced a Ca(2+) response in CHO-K1 cells via endogenous G(i/o) proteins, the Ca(2+) response to 5-HT at the mutant Thr(149)Ala 5-HT(1A) receptor was fully dependent on either the co-expression or the fusion to a recombinant G(alpha)(15) protein. Buspirone, flesinoxan and 8-OH-DPAT produced a graded partial response (26 to 62%) at the wt 5-HT(1A):G(alpha)(15) fusion protein; F 13640, 5-CT and F 14679 behaved as higher-efficacy agonists with maximal Ca(2+) responses similar to 5-HT. The maximal Ca(2+) responses at the mutant Thr(149)Ala 5-HT(1A):G(alpha)(15) fusion protein were significantly attenuated for flesinoxan and 8-OH-DPAT (-45 and -36%, respectively); the response to the other 5-HT agonists was not significantly affected. A similar effect was observed upon treatment with phorbol 12-myristate 13-acetate at the Thr(149)Ala 5-HT(1A):G(alpha)(15) fusion protein. In conclusion, the amplitude of the Ca(2+) responses induced by partial, but not that to fuller 5-HT(1A) receptor agonists, is affected by the Thr(149)Ala mutation of the 5-HT(1A):G(alpha)(15) fusion protein.

Effective information transfer from the alpha 1b-adrenoceptor to Galpha 11 requires both beta/gamma interactions and an aromatic group four amino acids from the C terminus of the G protein

Co-expression of the alpha(1b)-adrenoreceptor and Galpha(11) in cells derived from a Galpha(q)/Galpha(11) knock-out mouse allows agonist-mediated elevation of intracellular Ca(2+) levels that is transduced by beta/gamma released from the G protein alpha subunit. Mutation of Tyr(356) of Galpha(11) to Phe, within a receptor contact domain, had little effect on function but this was reduced greatly by alteration to Ser and virtually eliminated by conversion to Asp. This pattern was replicated following incorporation of each form of Galpha(11) into fusion proteins with the alpha(1b)-adrenoreceptor. Following a [(35)S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding assay, immunoprecipitation of the wild type alpha(1b)-adrenoreceptor-Galpha(11) fusion protein indicated that the agonist phenylephrine stimulated guanine nucleotide exchange on Galpha(11) more than 30-fold. Information transfer by agonist was controlled in residue 356 Galpha(11) mutants with rank order Tyr > Phe > Trp > Ile > Ala = Gln = Arg > Ser > Asp, although these alterations did not alter the binding affinity of either phenylephrine or an antagonist ligand. Mutation of a beta/gamma contact interface in the alpha(1b)-adrenoreceptor-Tyr(356) Galpha(11) fusion protein did not alter ligand binding affinity but did reduce greatly beta/gamma binding and phenylephrine stimulation of [(35)S]GTPgammaS binding. It also prevented agonist elevation of intracellular Ca(2+) levels, as did a mutation in Galpha(11) that prevents G protein subunit dissociation. These results indicate that a bulky aromatic group is required four amino acids from the C terminus of Galpha(11) to maximize information transfer from an agonist-occupied receptor and disprove the hypothesis that tyrosine phosphorylation of this residue is required for G protein activation (Umemori, H., Inoue, T., Kume, S., Sekiyama, N., Nagao, M., Itoh, H., Nakanishi, S., Mikoshiba, K., and Yamamoto, T. (1997) Science 276, 1878-1881). This is distinct from Galpha(i1), where hydrophobicity of the amino acid is the key determinant at this location. They also further demonstrate a key role for the beta/gamma complex in enhancing receptor to G protein alpha subunit information transfer.

Differential signalling of both wild-type and Thr(343)Arg dopamine D(2short) receptor by partial agonists in a G-protein-dependent manner

G-protein activation and Ca(2+) responses by the wild-type D(2short) receptor and a mutation Thr(343)Arg, in the distal BBXXB motif of its third intracellular loop, were investigated in CHO-K1 cells in terms of ligand:receptor:G-protein interactions. No evidence was obtained for constitutive, agonist-independent receptor activation, but differences in the ligand-mediated activation profiles of both the wild-type and mutant Thr(343)Arg D(2short) receptor were observed. Most of the partial agonists, but not bromocriptine, displayed an enhanced response at the mutant D(2short) receptor, suggesting that the mutation brings the receptor in a more active state. This enhancement was apparent both at the level of G-protein activation ([35S]GTPgammaS binding) and at the effector (Ca(2+) response) and occurred with different G(alpha)-proteins. Partial agonists were also found to act differently via the wild-type D(2short) receptor depending on the involved G(alpha)-protein. Compared with higher efficacy agonists, partial agonists displayed Ca(2+) responses with slower and dissimilar kinetic properties. Lisuride and in particular bromocriptine produced a more potent response in the co-presence of a G(alphao) protein instead of a chimeric G(alphaq/o)- or a promiscuous G(alpha15)-protein. S(+)-propylnorapomorphine showed a similar partial response irrespective of the combined G(alpha)-protein. Bromerguride and (+)-UH 232 induced weak (16 to 21% versus dopamine) intrinsic activity in the co-presence of a G(alphaq/o)-protein in contrast to their silent properties with a G(alpha15)- or a G(alphao)Cys(351)Ile-protein. In conclusion, the present data strongly suggest that multiple activation binding sites are involved with these ligands at the D(2short) receptor, and that their activation may be unravelled by either the mutation or co-expressed G(alpha)-proteins being investigated.

Analytical pharmacology of G protein-coupled receptors by stoichiometric expression of the receptor and G(alpha) protein subunits

The description of a new family of recombinant proteins, which are constructed by the covalent fusion of the cDNA encoding a G protein-coupled receptor with that of a G(alpha) protein subunit, has recently been introduced as an original strategy to explore receptor pharmacology under defined experimental conditions. As such, a controlled 1:1 stoichiometry of receptor and G(alpha) protein expression can be achieved, as well as a forced spatial proximity to each other. Fusion proteins have been revealed as active at the receptor ligand binding level and functional at the G(alpha) protein and effector level. Insights on analytical pharmacological data are discussed for wild-type and mutant receptors interacting with a given G(alpha) protein subunit and different subtypes of either wild-type or mutant G(alpha) proteins activated by a single receptor subtype. A possible alteration of the receptor:G(alpha) protein selectivity may occur due either to the spatial proximity of both protein partners or to a constraint receptor state unable to accommodate to different G(alpha) protein states. Coactivation of endogenous G(alpha) proteins in host cells expressing a fusion protein has also been observed, but depends mainly on the coupling efficiency of the receptor and G(alpha) protein engaged in the fusion process. The ligand's apparent intrinsic activity has been shown to be either enhanced, attenuated, or unmodified when the functional responses of a fusion protein are compared to the coexpression of both fusion protein partners.

Diverse signalling by 5-hydroxytryptamine (5-HT) receptors

Fourteen different receptor subtypes might be regarded as a diversity that is sufficient to accommodate the wide-ranging physiological roles of 5-hydroxytryptamine (5-HT). However, it is becoming clear that, for 5-HT as for other neurotransmitters, the concept of a receptor as a gatekeeper for a specific cellular process or event is too restrictive. Multiple receptor-mediated biochemical cascades can be activated in cells in response to an agonist by a number of mechanisms. Whereas it is well established that different agonists do not necessarily elicit the same magnitude of response, they probably also select between various possible signal transduction pathways. Receptor signalling may be diverse via a single receptor subtype as a consequence of specific agonist-receptor-G protein interactions. 5-HT receptors are even more heterogeneous when one considers that the amino acid sequence of these receptor subtypes may vary from individual to individual, and that there is an increasing number of receptor isoforms due to alternative splicing and RNA editing of 5-HT receptor transcripts. Activation, in particular constitutive, agonist-independent activation, of some of these receptor isoforms has been reported to be altered. This implies that ligands with similar binding affinities may display different pharmacological properties (partial agonist, antagonist, or inverse agonist) versus these receptor isoforms, depending on their activation state. Therefore, intervention with receptor ligands to modify hampered neurotransmission pathways is a difficult task, and one needs to consider the growing evidence of diversity in G protein-coupled receptor signalling.