Differential Intracellular Signaling of the GalR1 and GalR2 Galanin Receptor Subtypes

Abstract

The diverse physiological functions exerted by the neuropeptide galanin may be regulated by multiple G protein-coupled receptor subtypes and intracellular signaling pathways. Three galanin receptor subtypes (GalRs) have been recently cloned, but the G protein coupling profiles of these receptors are not completely understood. We have generated GalR1- and GalR2-expressing Chinese hamster ovary (CHO) cell lines and systematically examined the potential for these two receptors to couple to the Gs, Gi, Go, and Gq proteins. Galanin did not stimulate an increase in cAMP levels in GalR1/CHO or GalR2/CHO cells, suggesting an inability of either receptor to couple to Gs. Galanin inhibited forskolin-stimulated cAMP production in GalR1/CHO cells by 70% and in GalR2/CHO cells by 30%, suggesting a strong coupling of GalR1 to Gi and a more modest coupling between GalR2 and Gi. GalR1 and GalR2 both mediated pertussis toxin-sensitive MAPK activity (2-3-fold). The stimulation mediated by GalR1 was inhibited by expression of the C-terminus of beta-adrenergic receptor kinase (beta ARKct), which specifically inhibits G beta gamma signaling, but was not affected by the protein kinase C (PKC) inhibitor, bis[indolylmaleimide], or cellular depletion of PKC. In contrast, GalR2-mediated MAPK activation was not affected by beta ARKct expression but was abolished by inhibition of PKC activity. The data demonstrate that GalR1 is coupled to a Gibetagamma signaling pathway to mediate MAPK activation. In contrast, GalR2 utilizes a distinct signaling pathway to mediate MAPK activation, which is consistent with Go-mediated MAPK activation in CHO cells. Galanin was unable to stimulate inositol phosphate (IP) accumulation in CHO or COS-7 cells expressing GalR1. In contrast, galanin stimulated a 7-fold increase in IP production in CHO or COS-7 cells expressing GalR2. The GalR2-mediated IP production was not affected by pertussis toxin, suggesting a linkage of GalR2 with Gq/G11. Thus, the GalR1 receptor appears to activate only the Gi pathway. By contrast, GalR2 is capable of stimulating signaling which is consistent with activation of Go, Gq/G11, and Gi. The differential signaling profiles and the tissue distribution patterns of GalR1 and GalR2 may underlie the functional spectra of galanin action mediated by these galanin receptors and regulate the diverse physiological functions of galanin.

Expression of galanin receptor messenger RNAs in different regions of the rat gastrointestinal tract

“…This effect is likely to be mediated by GalR2 [10], the only GalR that signals through a stimulatory Gq/11 pathway, thus its activation by galanin could result in smooth muscle contraction [6]. Participation of GalR2 in the effect of galanin on gastric motility is supported by our findings of high levels of GalR2 mRNA in the stomach and is consistent with functional studies indicating the GalR2 as the main GalR implicated in the excitatory mechanism for smooth muscle contraction [27]. Indeed, galanin induces contraction of the jejunum and colon by direct effect on longitudinal smooth muscle [1,26].…”

“…Galanin receptor activation involves different signaling pathways. GalR1 and GalR3 activate intracellular effectors through pertussis-toxin sensitive Gi/o proteins [27] resulting in inhibition of adenylyl cyclase activity and activation of inward K + current [23]. By contrast, GalR2 predominantly couples positively to phospholipase C through a Gq/11 protein to mediate inositol phospholipid turnover and Ca 2+ mobilization with a pertussis-toxin independent mechanism, even though it also signals through Gi/Go proteins [22,27].…”

Differential Role of Galanin Receptors in the Regulation of Depression-Like Behavior and Monoamine/Stress-Related Genes at the Cell Body Level

“…Among these receptors, GalR1 and GalR3 mainly activate G i/o types of G proteins mediating inhibitory actions of galanin (Habert-Ortoli et al, 1994;Burgevin et al, 1995;Parker et al, 1995;see Branchek et al, 2000). In contrast, the GalR2 subtype can transmit either stimulatory effects of galanin, for example, on neurotransmitter release, acting via G q/11 types of G proteins (Smith et al, 1997;Wang et al, 1997;Fathi et al, 1998), or it can inhibit neurotransmission via G i/o types (Fathi et al, 1998;Wang et al, 1998;see Branchek et al, 2000). Thus, the physiological effects of galanin are varied and complex.…”

Galanin inhibits calcium channels via Gαi-protein mediated by GalR1 in rat nucleus tractus solitarius

“…Both GalR1 and GalR3 signal via G i-protein and decrease cyclic AMP levels by inhibiting adenylate cyclase (Smith et al, 1997). On the other hand, the main pathway downstream from GalR2 is through coupling to G q/11-protein and activates phospholipase C (PLC) to increase inositol triphosphate accumulation and the increase of intracellular Ca 2+ (Wang et al, 1998;Lundstrom et al, 2005); this would presumably stimulate neuronal activity and neurotransmitter release. In this study, AR-M1896 (GalR2 agonist) did not modulate IBa in NTS.…”