Regulation of the avidity of ternary complexes containing the human 5-HT(1A) receptor by mutation of a receptor contact site on the interacting G protein alpha subunit

The serotonin 5-HT7 receptors: two decades of research

Like most neurotransmitters, serotonin possesses a simple structure. However, the pharmacological consequences are more complex and diverse. Serotonin is involved in numerous functions in the human body including the control of appetite, sleep, memory and learning, temperature regulation, mood, behavior, cardiovascular function, muscle contraction, endocrine regulation, and depression. Low levels of serotonin may be associated with several disorders, namely increase in aggressive and angry behaviors, clinical depression, Parkinson's disease, obsessive-compulsive disorder, eating disorders, migraine, irritable bowel syndrome, tinnitus, and bipolar disease. These effects are mediated via different serotonin (5-HT) receptors. In this review, we will focus on the last discovered member of this serotonin receptor family, the 5-HT7 receptor. This receptor belongs to the G protein-coupled receptor superfamily and was cloned two decades ago. Later, different splice variants were described but no major functional differences have been described so far. All 5-HT7 receptor variants are coupled to Gαs proteins and stimulate cAMP formation. Recently, several interacting proteins have been reported, which can influence receptor signaling and trafficking.

Homologous down-regulation of histamine H3 receptors in rat striatal slices

Preincubation of striatal slices with the selective histamine H3-receptor agonist immepip (100 nM) decreased the specific binding of N-alpha-[methyl-3H]-histamine ([3H]-NMHA) to membranes obtained from the treated slices. The binding decrease was significant after 5 min, remained at similar reduced levels between 5- and 30-min incubations with agonist, and only a partial recovery was observed after 90-min washout (34, 41, and 44% at 90, 120, and 150 min, respectively). Saturation analysis showed a significant decrease in both receptor density (-44% +/- 9%) and affinity (dissociation constant, Kd 1.15 +/- 0.23 nM from 0.59 +/- 0.17 nM). The effect of immepip was mimicked by histamine and the H3 agonists imetit and R-alpha-methylhistamine, and was blocked by the H3 antagonist thioperamide. The reduction in [3H]-NMHA binding was fully and partially prevented by incubation at 4 degrees C and in hypertonic medium, respectively, but not by the endocytosis inhibitor phenylarsine oxide (10 microM). None of the following protein kinase inhibitors, Ro-318220 and Gö-6976 (PKC), H-89 (PKA) and staurosporine (general inhibitor) prevented the effect of immepip. In [3H]-adenine-labeled slices the preincubation with immepip (100 nM, 15 min) prevented the inhibitory effect of H3 receptor activation on forskolin-induced [3H]-cAMP accumulation (99% +/- 9% vs. 76% +/- 4% of control values). Taken together our results indicate that agonist binding promotes the down-regulation of striatal H3 receptors resulting in a significant loss of function.

A highly conserved glycine within linker I and the extreme C terminus of G protein alpha subunits interact cooperatively in switching G protein-coupled receptor-to-effector specificity

Numerous studies have attested to the importance of the extreme C terminus of G protein alpha subunits in determining their selectivity of receptor recognition. We have previously reported that a highly conserved glycine residue within linker I is important for constraining the fidelity of receptor recognition by Galpha(q) proteins. Herein, we explored whether both modules (linker I and extreme C terminus) interact cooperatively in switching G protein-coupled receptor (GPCR)-to-effector specificity and created as models mutant Galpha(q) proteins in which glycine was replaced with various amino acids and the C-terminal five Galpha(q) residues with the corresponding Galpha(i) or Galpha(s) sequence. Coupling properties of the mutated Galpha(q) proteins were determined after coexpression with a panel of 13 G(i)-and G(s) -selective receptors and compared with those of Galpha proteins modified in only one module. Galpha proteins modified in both modules are significantly more efficacious in channeling non-G(q) -selective receptors to G(q)-mediated signaling events compare with those containing each module alone. Additive effects of both modules were observed even if individual modules lacked an effect on GPCR-to-effector specificity. Dually modified Galpha proteins were also superior in conferring high-affinity agonist sites onto a coexpressed GPCR in the absence, but not in the presence, of guanine nucleotides. Together, our data suggest that receptor-G protein coupling selectivity involves cooperative interactions between the extreme C terminus and linker I of Galpha proteins and that distinct determinants of selectivity exist for individual receptors.