Pertussistoxin-sensitive Signalwege von alpha-MSH und AgRP an Melanocortin-4-Rezeptoren
Der Melanocortin-4-Rezeptor (MC4R) ist ein G-Protein-gekoppelter Rezeptor (GP- CR), dessen Signaltransduktion von zentraler Bedeutung für die Regulation der Körpergewichtshomöostase im Hypothalamus ist. Mutationen im MC4R sind die häufigste monogenetische Ursache für Adipositas. Bislang wurde die...
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The Melanocortin-4 receptor (MC4R) is a G-protein-coupled receptor (GPCR), which signalling is essential for the control of homeostasis of body weight in the hypothalamus. Mutations in the MC4R are the most frequent monogenetic cause of severe adiposity. So far, the signalling of the MC4R has been exclusively attributed to its coupling to Gs-proteins. The endogenous MC4R-agonist Melanocyt-stimulating hormone Ð (Ð-MSH) acts anorexigenic by an increase of cAMP-accumulation due to Gs-activation. Consistent with this finding the D90N-mutation of the MC4R, which selctively lacks in Gs-coupling is associated with severe obesity. A special cha- racteristic of the MC4R is the existence of endogenous antagonists: Agouti-related protein (AgRP) acts orexigenic by an decrease of cAMP-accumulation. This decre- ase in cAMP-accumulation is independent from the presence of agonists, i.e. AgRP has autonomous signalling apart from displacement of agonists. This autonomous signalling of AgRP has been described as inverse agonism, which means the stabi- lization of a conformation of the receptor with a lower degree of Gs-coupling than the basal one in absence of an agonist. Interestingly the physiologic relevance and the detailed mechanism of this signalling are still unknown. For the exploration of the detailed molecular mechanism of action of Ð-MSH and AgRP at the MC4R, overexpressing HEK293-cells and endogenous expressing GT1-7-cells were used. G- protein-activation was examined by GTPÒ35S-assay and the cAMP-accumulation 112 6. Abstract by a cAMP-assay. Surprisingly we found that not only the agonist Ð-MSH but also the antagonist AgRP activates G-proteins at the MC4R. The G-protein-activation due to AgRP was PTX-sensitiv, which characterizes the activated G-proteins as Gi/o. In addition the PTX-sensitiv activation of G-proteins of the Gs-deficient mu- tation documents the involvement of Gi/o-proteins in the signalling of the MC4R in general. Conclusively, we could establish the dual coupling of the MC4R to Gs- and Gi/o-proteins in an endogenous cell system. AgRP is able to activate selectively Gi/o-proteins and can regulate independently from Ð-MSH the homeostasis of bo- dy weight. In addition to this important findings for the melanocortin system, the definition of AgRP as an endogenous biased agonist at an established GPCR has relevant impact on the understanding of this receptor family in general.