Novel cAMP signalling paradigms: therapeutic implications for airway disease

Abstract

Since its discovery over 50 years ago, cAMP has been the archetypal second messenger introducing students to the concept of cell signalling at the simplest level. As explored in this review, however, there are many more facets to cAMP signalling than the path from Gs-coupled receptor to adenylyl cyclase (AC) to cAMP to PKA to biological effect. After a brief description of this canonical cAMP signalling pathway, a snapshot is provided of the novel paradigms of cAMP signalling. As in the airway the cAMP pathway relays the major bronchorelaxant signal and as such is the target for frontline therapy for asthma and COPD, particular emphasis is given to airway disease and therapy. Areas discussed include biased agonism, continued signalling following internalization, modulation of cAMP by AC, control of cAMP degradation, cAMP and calcium crosstalk, Epac-mediated signalling and finally the implications of altered genotypes will be considered. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

Figure 1

The canonical cAMP signalling pathway. Following the binding of agonist, a Gs-protein coupled receptor (GsPCR) such as the β₂-adrenoceptor, or a Gi-protein coupled receptor (GiPCR); for example, muscarinic M2 Acetylcholine receptor undergoes a conformational change promoting the dissociation of the Gα subunit from the Gβγ dimer. Whereas the Gαi-protein has an inhibitory effect on AC, the Gαs-protein stimulates AC inducing it to catalyse the formation of cAMP from ATP. Binding of cAMP to PKA results in the release of the PKA catalytic subunits allowing them to phosphorylate a wide range of cellular targets, with the net result being bronchorelaxation (see text for further details).

Figure 2

Distribution of PKA-based and Epac-based probes in cultured human airway smooth muscle cells under basal conditions and following exposure to 10 µM Isoprenaline. Panel A shows the distribution of CFP-Epac(dDEP,CD)-VENUS in untreated human airway smooth muscle cells. Panels B through E utilize a pseudocolour scale to demonstrate the emission ratio from CFP/YFP with blue corresponding to probe activity observed under basal conditions (B) and red corresponding to maximal probe activity following stimulation with isoprenaline (E). Panels C and D show probe activation at timepoints intermediate to these. Panel F shows six regions of interest selected from the same cell, and the plots of emission ratio for each of these are shown in panel G. The time point at which 10 µM isoprenaline added is marked (+Iso). Panels H through J show a human airway smooth muscle cell expressing both the catalytic subunits of PKA fused to YFP and the regulatory regions of PKA fused to CFP. Panel H simply shows the location of the probe under basal conditions (the YFP emission is shown but no difference was observed between this and the distribution of CFP). Again utilizing a pseudocolour range, panel I shows the cell under basal conditions, whilst panel J is the cell following exposure to 10 µM isoprenaline.

Figure 3

Novel cAMP signalling paradigms. (A) Biased agonism; (B) continued signalling following internalization; (C) modulation of cAMP by AC; (D) control of cAMP degradation; (E) cAMP and calcium crosstalk; (F) Epac-mediated signalling; (G) spatiotemporal control of cAMP; (H) genetic considerations.