Medizinische Universität Graz Austria/Österreich - Forschungsportal - Medical University of Graz
Gewählte Publikation:
Kargl, J.
Trafficking and Signaling of the G protein-coupled receptor 55
[ Dissertation ] Medical University of Graz; 2012. pp. 139
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- Autor*innen der Med Uni Graz:
- Kargl Julia
- Betreuer*innen:
- Groschner Klaus
- Waldhoer Maria
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- Abstract:
- The endocannabinoid system consists of endogenous cannabinoids, two seven transmembrane spanning / G protein-coupled receptors (7TM/GPCRs), the cannabinoid 1 (CB1) and cannabinoid 2 (CB2) receptors, as well as enzymes synthesizing and degrading endocannabinoids. Cannabinoids are compounds that can (i) be extracted form Cannabis sativa, (ii) are endogenously expressed mediators or (iii) are synthetic drugs, all of which display high affinity for cannabinoid receptors. Cannabinoids play an important role in medicine due to their psychoactive, analgesic and anti-inflammatory properties. Recently, the G protein-coupled receptor 55 (GPR55) was characterized as a novel receptor for synthetic cannabinoids and small lipid mediators. Besides being activated by the endogenous lipid ligand L-¿-lysophosphatidylinositol (LPI), several synthetic CB1 inverse agonists/antagonists, such as AM251, AM281 and rimonabant (SR141716A), have been shown to activate GPR55. Rimonabant has further attracted attention since it was marketed to induce weight loss and reduce smoking. However, due to severe side effects, such as the development of anxiety and depression, rimonabant was withdrawn from the market. In my thesis I elucidate the post-endocytic fate of GPR55 in vitro and in vivo. One regulatory mechanism to guarantee appropriate 7TM/GPCR expression levels in physiological conditions is that of downregulating 7TM/GPCRs via the GPCR-associated sorting protein 1 (GASP-1), thus leading to an attenuation of cellular signaling events. I provide evidence that GPR55 is targeted to the lysosomal/degradative pathway upon agonist stimulation and that this process involves a direct protein ¿ protein interaction with GASP-1. Disrupting the GPR55-GASP-1 interaction prevents receptor degradation, and thereby allows receptor recycling. Further, I investigated downregulation of GPR55 and rimonabant induced side-effects in wild type (WT) C57BL/6 mice and GASP-1 knock-out (GASP-1 KO) mice. Repetitive rimonabant administration led to a significant downregulation of GPR55 in WT but not in GASP-1 KO mice. Interestingly, GASP-1 KO - rather than WT - mice show a higher degree of anxiety- and depression like behavior after chronic rimonabant administration. These data implicate GASP-1 as an important regulator of ligand mediated downregulation of GPR55 and provide tangible evidence that GPR55 ¿ GASP-1 interactions play a key role in rimonabant induced side effects. In addition, the concept of receptor homo- and heteromerization is well-established. Receptor homo- and heteromerization has been shown to impact the trafficking and signaling properties of the involved 7TM/GPCRs. I demonstrate that GPR55 can form heteromers with the cannabinoid 1 receptor. I show that the co-expression of CB1 receptors and GPR55 specifically inhibits both, GPR55 mediated transcription factor and ERK1/2 MAP-Kinase activation. However, once the CB1 receptor is activated, GPR55 mediated signaling is restored. In addition, I observe that the presence of GPR55 enhances CB1R mediated ERK1/2 and transcription factor activation. These data provide first evidence that GPR55 can form heteromers with another 7TM/GPCR and that this interaction with the CB1 receptor has functional consequences in vitro. This work provides first insights into the cellular regulation of GPR55 by various processes, such as (i) the direct interaction with the sorting protein GASP-1, resulting in receptor degradation upon ligand stimulation and (ii) the heteromerization and cross-talk with the cannabinoid 1 receptor.