The cannabinoid CB1 receptor (CB1R) is a G protein-coupled receptor, which couples to the G i/o family of heterotrimeric G proteins. The receptor displays both basal and agonist-induced signaling and internalization. Although basal activity of CB1Rs is attributed to constitutive (agonist-independent) receptor activity, studies in neurons suggested a role of postsynaptic endocannabinoid (eCB) release in the persistent activity of presynaptic CB1Rs. To elucidate the role of eCBs in basal CB1R activity, we have investigated the role of diacylglycerol lipase (DAGL) in this process in Chinese hamster ovary (CHO) cells, which are not targeted specifically with eCBs. Agonist-induced G protein activation was determined by detecting dissociation G protein subunits expressed in CHO cells with bioluminescence resonance energy transfer (BRET), after labeling the ␣ and ␤ subunits with Renilla luciferase and enhanced yellow fluorescent protein (EYFP), respectively. Preincubation of the cells with tetrahydrolipstatin (THL), a known inhibitor of DAGLs, caused inhibition of the basal activity of CB1R. Moreover, preincubation of CHO and cultured hippocampal neurons with THL increased the number of CB1Rs on the cell membrane, which reflects its inhibitory action on CB1R internalization in non-simulated cells. In CHO cells co-expressing CB1R and angiotensin AT 1 receptors, angiotensin II-induced G o protein activation that was blocked by both a CB1R antagonist and THL. These data indicate that cell-derived eCB mediators have a general role in the basal activity of CB1Rs in non-neural cells and neurons, and that this mechanism can be stimulated by AT 1 receptor activation.

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In the past years, the relationship between the endocannabinoid system (ECS) and other hormonal and neuromodulatory systems has been intensively studied. G protein-coupled receptors (GPCRs) can stimulate endocannabinoid (eCB) production via activation of Gq/11 proteins and, in some cases, Gs proteins. In this review, we summarize the pathways through which GPCR activation can trigger eCB release, as well as the best known examples of this process throughout the body tissues. Angiotensin II-induced activation of AT1 receptors, similar to other Gq/11-coupled receptors, can lead to the formation of 2-arachido-noylglycerol (2-AG), an important eCB. The importance of eCB formation in angiotensin II action is supported by the finding that the hypertensive effect of angiotensin II, injected directly into the hypothalamic paraventricular nucleus of anaesthetized rats, can be abolished by AM251, an inverse agonist of CB1 cannabinoid receptors (CB1Rs). We conclude that activation of the ECS should be considered as a general consequence of the stimulation of Gq/11-coupled receptors, and may mediate some of the physiological effects of GPCRs.

CB 1 cannabinoid receptor (CB 1 R) undergoes both constitutive and agonist-induced internalization, but the underlying mechanisms of these processes and the role of β-arrestins in the regulation of CB 1 R function are not completely understood. In this study, we followed CB 1 R internalization using confocal microscopy and bioluminescence resonance energy transfer measurements in HeLa and Neuro-2a cells. We found that upon activation CB 1 R binds β-arrestin2 (β-arr2), but not β-arrestin1. Furthermore, both the expression of dominant-negative β-arr2 (β-arr2-V54D) and siRNA-mediated knock-down of β-arr2 impaired the agonist-induced internalization of CB 1 R. In contrast, neither β-arr2-V54D nor β-arr2-specific siRNA had a significant effect on the constitutive internalization of CB 1 R. However, both constitutive and agonist-induced internalization of CB 1 R were impaired by siRNA-mediated depletion of clathrin heavy chain. We conclude that although clathrin is required for both constitutive and agonist-stimulated internalization of CB 1 R, β-arr2 binding is only required for agonist-induced internalization of the receptor suggesting that the molecular mechanisms underlying constitutive and agonist-induced internalization of CB 1 R are different.

β-arrestins are key regulators and signal transducers of G protein-coupled receptors (GPCRs). The interaction between receptorsand β-arrestins is generally believed to require both receptor activity and phosphorylation by GPCR kinases. In this study, we investigated whether β-arrestins are able to bind second messenger kinase-phosphorylated, but inactive receptors as well. Since heterologous phosphorylation is a common phenomenon among GPCRs, this mode of β-arrestin activation may represent a novel mechanism of signal transduction and receptor cross-talk.Here we demonstrate that activation of protein kinase C (PKC) by phorbol myristate acetate, G q/11-coupled GPCR or epidermal growth factor receptor stimulation promotes β-arrestin2 recruitment to unliganded AT1 angiotensin receptor (AT1R). We found that this interaction depends on the stability lock, a structure responsible for the sustained binding between GPCRs and β-arrestins, formed by phosphorylated serine-threonine clusters in the receptor's C-terminus and two conserved phosphate-binding lysines in the β-arrestin2 Ndomain. Using improved FlAsH-based β-arrestin2 conformational biosensors, we also show that the stability lock not only stabilizes the receptor-β-arrestin interaction, but also governs the structural rearrangements within β-arrestins. Furthermore, we found that β-arrestin2 binds to PKC-phosphorylated AT1R in a distinct active conformation, which triggers MAPK recruitment and receptor internalization. Our results provide new insights into the activation of β-arrestins and reveal their novel role in receptor cross-talk.The family of G protein-coupled receptors (GPCRs) consists of ~800 members in humans and about 30% of modern drugs target these molecules (1). GPCRs respond to a wide variety of endogenous ligands, including hormones, neurotransmitters, and lipids. Despite their huge diversity, the signal transduction mechanisms of GPCRs share several common features: agonist binding is followed by the activation of a relatively small number of heterotrimeric G proteins, which initiate complex intracellular signaling cascades. Receptor responsiveness to further stimulation is attenuated by a multistep process, called desensitization (2). In case of homologous desensitization, active GPCRs are phosphorylated by GPCR kinases (GRKs) followed by the recruitment of β-arrestin proteins (β-arrestin1 and JBC C o n f i d e n t i a lHeterologous regulation of inactive receptors via β-arrestin 2 β-arrestin2, also known as arrestin-2 and arrestin-3, respectively). β-arrestins uncouple the receptors from G proteins and initiate receptor internalization (3), thereby serving as the key regulators of GPCRs' function. In contrast, heterologous desensitization is mediated by second-messenger activated kinases, such as protein kinase C (PKC), which can phosphorylate active and inactive receptors. Heterologous desensitization was originally thought to be independent of β-arrestins, however, some data have challenged this concept (4-6). In addition to their rol...

The role of the highly conserved 'DRY' motif in the signaling of the CB 1 cannabinoid receptor (CB 1 R) was investigated by inducing single-, double-, and triple-alanine mutations into this site of the receptor. We found that the CB 1 R-R3.50A mutant displays a partial decrease in its ability to activate heterotrimeric G o proteins (w80% of WT CB 1 R (CB 1 R-WT)). Moreover, this mutant showed an enhanced basal b-arrestin2 (b-arr2) recruitment. More strikingly, the double-mutant CB 1 R-D3.49A/R3.50A was biased toward b-arrs, as it gained a robustly increased b-arr1 and b-arr2 recruitment ability compared with the WT receptor, while its G-protein activation was decreased. In contrast, the double-mutant CB 1 R-R3.50A/Y3.51A proved to be G-protein-biased, as it was practically unable to recruit b-arrs in response to agonist stimulus, while still activating G-proteins, although at a reduced level (w70% of CB 1 R-WT). Agonist-induced ERK1/2 activation of the CB 1 R mutants showed a good correlation with their b-arr recruitment ability but not with their G-protein activation or inhibition of cAMP accumulation. Our results suggest that G-protein activation and b-arr binding of the CB 1 R are mediated by distinct receptor conformations, and the conserved 'DRY' motif plays different roles in the stabilization of these conformations, thus mediating both G-protein-and b-arr-mediated functions of CB 1 R.

13Heterodimerization between angiotensin type 1A receptor (AT 1 R) and β 2 -adrenergic 14 receptor (β 2 AR) has been shown to modulate G protein-mediated effects of these GPCRs.Our aim was to study the effect of heterodimerization on β-arrestin 18 coupling. We found that β-arrestin binding of β 2 AR is affected by activation of AT 1 Rs. 19Costimulation with angiotensin II and isoproterenol markedly enhanced the 20 interaction between β 2 AR and β-arrestins, by prolonging the lifespan of β 2 AR-induced 21 β-arrestin2 clusters at the plasma membrane. While candesartan, a conventional 22AT 1 R antagonist, had no effect on the β-arrestin2 binding to β 2 AR, TRV120023, a β-23 arrestin biased agonist, enhanced the interaction. 24These findings reveal a new crosstalk mechanism between AT 1 R and β 2 AR, and 25 suggest that enhanced β-arrestin2 binding to β 2 AR can contribute to the 26 pharmacological effects of biased AT 1 R agonists. 28 29Highlights: 30Heterodimerization between AT 1 R and β 2 AR enhances β-arrestin coupling of β 2 AR. 31Heterodimerization increases the lifespan of β-arrestin2 clusters after β 2 AR 32 stimulation. 33Biased AT 1 R ligands alter the function of heterodimerized β 2 AR.

CB 1 cannabinoid receptor (CB 1 R) undergoes both constitutive and agonist-induced internalization, but the underlying mechanisms of these processes and the role of βarrestins in the regulation of CB 1 R function are not completely understood. In this study, we followed CB 1 R internalization using confocal microscopy and bioluminescence resonance energy transfer measurements in HeLa and Neuro-2a cells. We found that upon activation CB 1 R binds β-arrestin2 (β-arr2), but not β-arrestin1. Furthermore, both the expression of dominant-negative β-arr2 (β-arr2-V54D) and siRNA-mediated knock-down of β-arr2 impaired the agonist-induced internalization of CB 1 R. In contrast, neither β-arr2-V54D nor β-arr2-specific siRNA had a significant effect on the constitutive internalization of CB 1 R. However, both constitutive and agonist-induced internalization of CB 1 R were impaired by siRNA-mediated depletion of clathrin heavy chain. We conclude that although clathrin is required for both constitutive and agonist-stimulated internalization of CB 1 R, β-arr2 binding is only required for agonist-induced internalization of the receptor suggesting that the molecular mechanisms underlying constitutive and agonist-induced internalization of CB 1 R are different.