A mutation in the second transmembrane region of the CB1 receptor selectively disrupts G protein signaling and prevents receptor internalization

New mechanisms in heptahelical receptor signaling to mitogen activated protein kinase cascades

Activation of classical second messenger cascades cannot fully explain the recently appreciated roles of heptahelical, or G-protein coupled receptors (GPCRs), in stimulation of mitogen activated protein kinase (MAPK) cascades. Rather, several distinct signaling mechanisms appear to contribute to GPCR-mediated MAPK activation. These include transactivation of the Epidermal Growth Factor Receptor (EGFR) via the autocrine/paracrine release of EGF-like ligands at the cell surface and scaffolding of MAPK cascades. A significant advance in the understanding of how GPCRs activate MAPK cascades is the discovery that beta-arrestin, a protein well known for its roles in both receptor desensitization and internalization, serves as a scaffolding protein for at least two GPCR stimulated MAPK cascades, the extracellular signal regulated kinase (ERK) cascade and the c-jun N-terminal kinase 3 (JNK3) cascade. Together, these novel mechanisms of GPCR-mediated MAPK regulation may permit GPCRs in specific situations to control the temporal and spatial activity of MAPKs and thereby determine the consequences of GPCR stimulation with respect to transcriptional activation, cell proliferation and apoptosis.

Separation of cannabinoid receptor affinity and efficacy in delta-8-tetrahydrocannabinol side-chain analogues

1. The activities of a number of side-chain analogues of delta-8-tetrahydrocannabinol (Delta(8)-THC) in rat cerebellar membrane preparations were tested. 2. The affinities of each compound for the CB(1) receptor were compared by their respective abilities to displace [(3)H]-SR141716A and their efficacies compared by stimulation of [(35)S]-GTPgammaS binding. 3. It was found that the affinities varied from 0.19+/-0.03 nM for 3-norpentyl-3-[6'-cyano,1',1'-dimethyl]hexyl-Delta(8)-THC to 395+/-66.3 nM for 5'-[N-(4-chlorophenyl)]-1',1'-dimethyl-carboxamido-Delta(8)-THC. 4. The efficacies of these compounds varied greatly, ranging from the very low efficacy exhibited to acetylenic compounds such as 1'-heptyn-Delta(8)-THC and 4'-octyn-Delta(8)-THC to higher efficacy compounds such as 5'-(4-cyanophenoxy)-1',1'-dimethyl-Delta(8)-THC and 5'-[N-(4-aminosulphonylphenyl)]-1',1' dimethyl-carboxamido Delta(8)-THC. All agonist activities were antagonized by the CB(1)-selective antagonist SR141716A. 5. It was found that a ligand's CB(1) affinity and efficacy are differentially altered by modifications in the side-chain. Decreasing the flexibility of the side-chain reduced efficacy but largely did not alter affinity. Additionally, the positioning of electrostatic moieties, such as cyano groups, within the side-chain also has contrasting effects on these two properties. 6. In summary, this report details the characterization of a number of novel Delta(8)-THC analogues in rat cerebellar membranes. It provides the first detailed pharmacological analysis of how the inclusion of electrostatic moieties in the side-chain and also how alteration of the side-chain's flexibility may differentially affect a CB(1) cannabinoid receptor ligand's affinity and efficacy.

Cannabinoid receptors and the regulation of immune response

Cannabinoid research underwent a tremendous increase during the last 10 years. This progress was made possible by the discovery of cannabinoid receptors and the endogenous ligands for these receptors. Cannabinoid research is developing in two major directions: neurobehavioral properties of cannabinoids and the impact of cannabinoids on the immune system. Recent studies characterized the cannabinoid-induced response as a very complex process because of the involvement of multiple signalling pathways linked to cannabinoid receptors or effects elicited by cannabinoids without receptor participation. The objective of this review is to present this complexity as it applies to immune response. The functional properties of cannabinoid receptors, signalling pathways linked to cannabinoid receptors and the modulation of immune response by cannabinoid receptor ligands are discussed. Special attention is given to 'endocannabinoids' as immunomodulatory molecules.

Role of endocytosis in the activation of the extracellular signal-regulated kinase cascade by sequestering and nonsequestering G protein-coupled receptors

Acting through a number of distinct pathways, many G protein-coupled receptors (GPCRs) activate the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) cascade. Recently, it has been shown that in some cases, clathrin-mediated endocytosis is required for GPCR activation of the ERK/MAPK cascade, whereas in others it is not. Accordingly, we compared ERK activation mediated by a GPCR that does not undergo agonist-stimulated endocytosis, the alpha(2A) adrenergic receptor (alpha(2A) AR), with ERK activation mediated by the beta(2) adrenergic receptor (beta(2) AR), which is endocytosed. Surprisingly, we found that in COS-7 cells, ERK activation by the alpha(2A) AR, like that mediated by both the beta(2) AR and the epidermal growth factor receptor (EGFR), is sensitive to mechanistically distinct inhibitors of clathrin-mediated endocytosis, including monodansylcadaverine, a mutant dynamin I, and a mutant beta-arrestin 1. Moreover, we determined that, as has been shown for many other GPCRs, both alpha(2A) and beta(2) AR-mediated ERK activation involves transactivation of the EGFR. Using confocal immunofluorescence microscopy, we found that stimulation of the beta(2) AR, the alpha(2A) AR, or the EGFR each results in internalization of a green fluorescent protein-tagged EGFR. Although beta(2) AR stimulation leads to redistribution of both the beta(2) AR and EGFR, activation of the alpha(2A) AR leads to redistribution of the EGFR but the alpha(2A) AR remains on the plasma membrane. These findings separate GPCR endocytosis from the requirement for clathrin-mediated endocytosis in EGFR transactivation-mediated ERK activation and suggest that it is the receptor tyrosine kinase or another downstream effector that must engage the endocytic machinery.