Alpha-2 Adrenergic Receptors and Signal Transduction Effector Output in Relation to G-Protein Coupling and Signalling Cross-Talk BY

Näsman, J. 2001. Alpha-2 Adrenergic Receptors and Signal Transduction. Effector Output in Relation to G-Protein Coupling and Signalling Cross-Talk. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 1105. 64 pp. Uppsala. ISBN 91-554-5195-0. The alpha-2 adrenergic receptor (α2-AR) subfamily includes three different subtypes, α2A-, α2Band α2C-AR, all believed to exert their function through heterotrimeric Gi/o-proteins. The present study was undertaken in order to investigate subtype differences in terms of cellular response and to explore other potential signalling pathways of α2-ARs. Evidence is provided for a strong Gs-protein coupling capability of the α2B-AR, leading to stimulation of adenylyl cyclase (AC). The difference between the α2Aand α2B-AR subtypes, in this respect, was shown to be due to differences in the second intracellular loops of the receptor proteins. Substitution of the second loop in α2A-AR with the corresponding domain of α2B-AR enrolled the chimeric α2A/α2B receptor with functional α2B-AR properties. Dual Gi and Gs coupling can have different consequences for AC output. Using coexpression of receptors and G-proteins, it was shown that the ultimate cellular response of α2B-AR activation is largely dependent on the ratio of Gito Gs-protein amounts in the cell. Also Giand Go-proteins appear to have different regulatory influences on AC. Heterologous expression of AC2 together with Gi or Go and the α2A-AR resulted in receptor-mediated inhibition of protein kinase C-stimulated AC2 activity through Go, whereas activation of Gi potentiated the activity. α2-ARs mobilize Ca in response to agonists in some cell types. This response was shown to depend on tonic purinergic receptor activity in transfected CHO cells. Elimination of the tonic receptor activity almost completely inhibited the Ca response of α2-ARs. In conclusion, α2-ARs can couple to multiple G-proteins, including Gi, Go and Gs. The cellular response to α2-AR activation depends on which receptor subtype is expressed, which cellular signalling constituents are engaged (G-proteins and effectors), and the signalling status of the effectors (dormant or primed).