G protein coupling and signaling of Gs-coupled serotonin receptors

Abstract

G protein-coupled receptors (GPCRs) constitute a large group of proteins involved in conveying extracellular signals to the inside of the cell. There are different classes of GPCRs, which can be activated by different signals. GPCRs are important targets for drug discovery, and over 30% of all drugs currently on the market activate or inhibit one or several GPCRs. In addition to G protein activation, several GPCRs can also activate other signalling pathways. This thesis addresses how 5-HT7 and 5-HT4 serotonin receptors interact with G proteins and investigates the localized intracellular cAMP signalling elicited by 5-HT4 receptors in normal and failing hearts.

Most GPCRs interact with and activate G proteins in a dynamic manner. Data presented in this thesis demonstrate with the use of FRET (Fluorescence Resonance Energy Transfer) and FRAP (Fluorescence Recovery After Photobleaching) technology that in HEK293 cells, the 5-HT7 receptor is associated with G protein (Gs) in the absence of ligand, as opposed to the 5-HT4 and the β1 adrenergic receptor (β1AR). Further, upon agonist activation of the 5-HT7 receptor, the Gα subunit undergoes a fast conformational change followed by a relatively slow dissociation of the Gγ subunit from both receptor and Gα. Moreover, through the construction of a series of 5-HT4/5-HT7 chimeric receptors, the structural domains responsible for this preassociation were elucidated. The thesis reveals that the same domains responsible for preassociation, found to be located in the intracellular loop 3 and C-terminal tail, were involved in low potency activation of adenylyl cyclase, indicating a possible connection between the two phenomena, and thus unveiling a novel aspect of GPCR pharmacology. Stimulation of 5-HT4 receptors in failing hearts induces a positive inotropic effect, through the second messenger cAMP, similar to the more studied β-ARs. However, the 5-HT4 receptors appear more efficiently coupled to increased contractility than β-AR, as they induce a comparable contractile response, despite eliciting a lower total cAMP increase. Thus, another hypothesis tested in the thesis was that 5-HT4-dependent cAMP increase was compartmented close to the contractile apparatus of the cell. Employing FRET sensors targeted to key subcellular domains, localized cAMP dynamics were studied by live imaging of adult cardiomyocytes from failing and sham operated hearts. We found a serotonin-elicited cAMP response comparable to that elicited by β-AR-activation in failing heart in proximity of Troponin I. In addition, the relative contribution of phosphodiesterases 3 and 4 (PDE3 and PDE4, respectively) appears altered in failing cardiomyocytes, possibly due to downregulation of PDE4, which confers increased relevance to PDE3.