SUMMARY Virus infection is sensed in the cytoplasm by retinoic acid-inducible gene I (RIG-I, also known as DDX58), which requires RNA and polyubiquitin binding to induce type I interferon (IFN), and activate cellular innate immunity. We show that the human IFN-inducible oligoadenylate synthetases-like (OASL) protein had antiviral activity and mediated RIG-I activation by mimicking polyubiquitin. Loss of OASL expression reduced RIG-I signaling and enhanced virus replication in human cells. Conversely, OASL expression suppressed replication of a number of viruses in a RIG-I-dependent manner and enhanced RIG-I-mediated IFN induction. OASL interacted and colocalized with RIG-I, and through its C-terminal ubiquitin-like domain specifically enhanced RIG-I signaling. Bone marrow derived macrophages from mice deficient for Oasl2 showed that among the two mouse orthologs of human OASL; Oasl2 is functionally similar to human OASL. Our findings show a mechanism by which human OASL contributes to host antiviral responses by enhancing RIG-I activation.

An agonist that acts through a single receptor can activate numerous signaling pathways. Recent studies have suggested that different ligands can differentially activate these pathways by stabilizing a limited range of receptor conformations, which in turn preferentially drive different downstream signaling cascades. This concept, termed “biased signaling” represents an exciting therapeutic opportunity to target specific pathways that elicit only desired effects, while avoiding undesired effects mediated by different signaling cascades. The cannabinoid receptors CB1 and CB2 each activate multiple pathways, and evidence is emerging for bias within these pathways. This review will summarize the current evidence for biased signaling through cannabinoid receptor subtypes CB1 and CB2.

Recreational consumption of synthetic cannabinoid receptor agonists (SCRAs) is a growing crisis in public health in many parts of the world. AMB-FUBINACA is a member of this class of drugs and is responsible for a large proportion of SCRA-related toxicity both in New Zealand and internationally. Strikingly, little is currently known about the mechanisms by which SCRAs exert toxic effects or whether their activity through the CB 1 cannabinoid receptor (the mediator of cannabinoid-related psychoactivity) is sufficient to explain clinical observations. The current study therefore set out to perform a basic molecular pharmacology characterization of AMB-FUBINACA (in comparison to traditional research cannabinoids CP55,940, WIN55,212-2, and Δ 9 -THC) in fundamental pathways of receptor activity, including cAMP inhibition, pERK activation, ability to drive CB 1 internalization, and ability to induce translocation of β-arrestins-1 and -2. Activity pathways were then compared by operational analysis to indicate whether AMB-FUBINACA may be a biased ligand. Results revealed that AMB-FUBINACA is highly efficacious and potent in all pathways assayed. However, surprisingly, bias analysis suggested that Δ 9 -THC, not AMB-FUBINACA, may be a biased ligand, with it being less active in both arrestin pathways than predicted by the activity of the other ligands tested. These data may help predict molecular characteristics of SCRAs. However, more research is required to determine whether these molecular effects manifest in toxicity at tissue/system level.

Arrestin translocation and signaling have come to the fore of the G protein-coupled receptor molecular pharmacology field. Some receptor–arrestin interactions are relatively well understood and considered responsible for specific therapeutic or adverse outcomes. Coupling of arrestins with cannabinoid receptors 1 (CB 1 ) and 2 (CB 2 ) has been reported, though the majority of studies have not systematically characterized the differential ligand dependence of this activity. In addition, many prior studies have utilized bovine (rather than human) arrestins, and the most widely applied assays require reporter-tagged receptors, which prevent meaningful comparison between receptor types. We have employed a bioluminescence resonance energy transfer (BRET) method that does not require the use of tagged receptors and thereby allows comparisons of arrestin translocation between receptor types, as well as with cells lacking the receptor of interest – an important control. The ability of a selection of CB 1 and CB 2 agonists to stimulate cell surface translocation of human and bovine β-arrestin-1 and -2 was assessed. We find that some CB 1 ligands induce moderate β-arrestin-2 translocation in comparison with vasopressin V 2 receptor (a robust arrestin recruiter); however, CB 1 coupling with β-arrestin-1 and CB 2 with either arrestin elicited low relative efficacies. A range of efficacies between ligands was evident for both receptors and arrestins. Endocannabinoid 2-arachidonoylglycerol stood out as a high efficacy ligand for translocation of β-arrestin-2 via CB 1 . Δ 9 -tetrahydrocannabinol was generally unable to elicit translocation of either arrestin subtype via CB 1 or CB 2 ; however, control experiments revealed translocation in cells not expressing CB 1 /CB 2 , which may assist in explaining some discrepancy with the literature. Overexpression of GRK2 had modest influence on CB 1 /CB 2 -induced arrestin translocation. Results with bovine and human arrestins were largely analogous, but a few instances of inconsistent rank order potencies/efficacies between bovine and human arrestins raise the possibility that subtle differences in receptor conformation stabilized by these ligands manifest in disparate affinities for the two arrestin species, with important potential consequences for interpretation in ligand bias studies. As well as contributing important information regarding CB 1 /CB 2 ligand-dependent arrestin coupling, our study raises a number of points for consideration in the design and interpretation of arrestin recruitment assays.

The members of the oligoadenylate synthetase (OAS) family of proteins are antiviral restriction factors that target a wide range of RNA and DNA viruses. They function as intracellular double-stranded RNA (dsRNA) sensors that, upon binding to dsRNA, undergo a conformational change and are activated to synthesize 2=-5=-linked oligoadenylates (2-5As). 2-5As of sufficient length act as second messengers to activate RNase L and thereby restrict viral replication. We expressed human OAS3 using the baculovirus system and purified it to homogeneity. We show that recombinant OAS3 is activated at a substantially lower concentration of dsRNA than OAS1, making it a potent in vivo sensor of dsRNA. Moreover, we find that OAS3 synthesizes considerably longer 2-5As than previously reported, and that OAS3 can activate RNase L intracellularly. The combined high affinity for dsRNA and the capability to produce 2-5As of sufficient length to activate RNase L suggests that OAS3 is a potent activator of RNase L. In addition, we provide experimental evidence to support one active site of OAS3 located in the C-terminal OAS domain and generate a low-resolution structure of OAS3 using SAXS. IMPORTANCEWe are the first to purify the OAS3 enzyme to homogeneity, which allowed us to characterize the mechanism utilized by OAS3 and identify the active site. We provide compelling evidence that OAS3 can produce 2=-5=-oligoadenylates of sufficient length to activate RNase L. This is contrary to what is described in the current literature but agrees with recent in vivo data showing that OAS3 harbors an antiviral activity requiring RNase L. Thus, our work redefines our understanding of the biological role of OAS3. Furthermore, we used a combination of mutagenesis and small-angle X-ray scattering to describe the active site and lowresolution structure of OAS3.T he 2=-5=-oligoadenylate synthetases (OAS) are a family of interferon (IFN)-and virus-induced antiviral restriction factors (1-3) that offer protection against a wide spectrum of RNA and DNA viruses (for a review, see Silverman [4]). The OAS family belongs to a nucleotidyltransferase superfamily (5). Instead of the 3=-5=-phosphodiester linkage found in RNA and DNA, when activated by dsRNA the OAS enzymes synthesize 2=-5=-phosphodiester-linked oligoadenylates (2-5As) with the general formula p 3 A(2=p5=A) n , where n Ն 1 (6, 7). The OAS proteins share structural homology and the ability to make 2= specific phosphodiester bonds with the recently discovered pattern recognition receptor for cytosolic dsDNA, cGAS (8-11). In humans, the OAS family consists of four members: OAS1, OAS2, OAS3, and OASL (12, 13). OAS1, OAS2, and OAS3 have 2=-5=-oligoadenylate synthetase activity (14), whereas OASL is devoid of this activity despite sharing significant sequence similarity with the other OAS proteins (15, 16). The 42-kDa OAS1 consists of one OAS domain, while the 69-kDa OAS2 consists of two OAS domains and the 120-kDa OAS3 consists of three OAS domains (16-18). The active site of all polymerases (to...