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Receptor Structures for a Caldron of Cannabinoids

2019; Cell Press; Volume: 176; Issue: 3 Linguagem: Inglês

10.1016/j.cell.2019.01.012

1097-4172

Emily Lorenzen, Thomas P. Sakmar,

Neuroscience and Neuropharmacology Research

Structures of the cannabinoid receptor 1 (CB1) in complex with an "ultrapotent" synthetic cannabinoid and its G protein (Krishna Kumar et al., 2019Krishna Kumar K. Shalev-Benami M. Robertson M.J. Hu H. Banister S.D. Hollingsworth S.A. Latorraca N.R. Kato H.E. Hilger D. Maeda S. et al.Structure of a Signaling Cannabinoid Receptor 1-G protein complex.Cell. 2019; 176 (this issue): 448-458Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar) and CB2 in complex with a new rationally designed inverse agonist (Li et al., 2019Li X. Hua T. Vemuri K. Ho J.-H. Wu Y. Wu L. Popov P. Benchama O. Zvonok N. Locke K. et al.Crystal Structure of the Human Cannabinoid Receptor CB2.Cell. 2019; 176 (this issue): 459-467Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar) provide unique snapshots of the molecular pharmacology of cannabinoids. Structures of the cannabinoid receptor 1 (CB1) in complex with an "ultrapotent" synthetic cannabinoid and its G protein (Krishna Kumar et al., 2019Krishna Kumar K. Shalev-Benami M. Robertson M.J. Hu H. Banister S.D. Hollingsworth S.A. Latorraca N.R. Kato H.E. Hilger D. Maeda S. et al.Structure of a Signaling Cannabinoid Receptor 1-G protein complex.Cell. 2019; 176 (this issue): 448-458Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar) and CB2 in complex with a new rationally designed inverse agonist (Li et al., 2019Li X. Hua T. Vemuri K. Ho J.-H. Wu Y. Wu L. Popov P. Benchama O. Zvonok N. Locke K. et al.Crystal Structure of the Human Cannabinoid Receptor CB2.Cell. 2019; 176 (this issue): 459-467Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar) provide unique snapshots of the molecular pharmacology of cannabinoids. Even in the current era of the "opioid epidemic," an emerging class of synthetic cannabinoids poses yet another major public health concern. On a warm breezy summer day in July 2016 in New York City, an "ultrapotent" synthetic cannabinoid caused the mass intoxication of 33 people and helped usher in the new era of "zombie" drugs in the United States. The culprit ingredient in the street drug AK-47 24 Karat Gold was eventually identified as the synthetic cannabinoid AMB-FUBINACA (Adams et al., 2017Adams A.J. Banister S.D. Irizarry L. Trecki J. Schwartz M. Gerona R. "Zombie" Outbreak Caused by the Synthetic Cannabinoid AMB-FUBINACA in New York.N. Engl. J. Med. 2017; 376: 235-242Crossref PubMed Scopus (198) Google Scholar). In this issue of Cell, Krishna Kumar et al., 2019Krishna Kumar K. Shalev-Benami M. Robertson M.J. Hu H. Banister S.D. Hollingsworth S.A. Latorraca N.R. Kato H.E. Hilger D. Maeda S. et al.Structure of a Signaling Cannabinoid Receptor 1-G protein complex.Cell. 2019; 176 (this issue): 448-458Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar report the cryoelectron microscopy (cryo-EM) structure of a derivative of AMB-FUBINACA in complex with the activated type 1 cannabinoid (CB1) receptor and its heterotrimeric G protein partner. Another paper also reports the first high-resolution crystal structure of the other main CB receptor subtype, CB2, in complex with a novel synthetic inverse agonist ligand (Li et al., 2019Li X. Hua T. Vemuri K. Ho J.-H. Wu Y. Wu L. Popov P. Benchama O. Zvonok N. Locke K. et al.Crystal Structure of the Human Cannabinoid Receptor CB2.Cell. 2019; 176 (this issue): 459-467Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar). The two new structures provide a fascinating portrait of how this important class of G protein-coupled receptors (GPCRs) binds their high-potency ligands, become activated, and then couple to downstream signaling systems. Among the vast family of GPCRs, the cannabinoid receptors CB1 and CB2 are a particularly interesting story of sleuth, failed opportunity and promise. CB1 receptor was originally an "orphan" receptor—a receptor expressed at high levels in the brain without an identified endogenous ligand—that was shown to bind to Δ-9 tetrahydrocannabinol (THC), the primary active ingredient in cannabis extracts (Matsuda et al., 1990Matsuda L.A. Lolait S.J. Brownstein M.J. Young A.C. Bonner T.I. Structure of a cannabinoid receptor and functional expression of the cloned cDNA.Nature. 1990; 346: 561-564Crossref PubMed Scopus (4196) Google Scholar). The subsequent identification of another orphan receptor, CB2, in the periphery (Munro et al., 1993Munro S. Thomas K.L. Abu-Shaar M. Molecular characterization of a peripheral receptor for cannabinoids.Nature. 1993; 365: 61-65Crossref PubMed Scopus (4128) Google Scholar) helped to explain many of the non-psychoactive effects of cannabinoids, including analgesia, immunosuppression, a decrease in intraocular pressure in glaucoma, and anti-emetic properties. Eventually, the endogenous arachidonate-derived ligands for CB receptors were identified, primarily anandamide (N-arachidonoyl-ethanolamine) and 2-AG (2-arachidonoylglycerol) (Devane et al., 1992Devane W.A. Hanus L. Breuer A. Pertwee R.G. Stevenson L.A. Griffin G. Gibson D. Mandelbaum A. Etinger A. Mechoulam R. Isolation and structure of a brain constituent that binds to the cannabinoid receptor.Science. 1992; 258: 1946-1949Crossref PubMed Scopus (4664) Google Scholar), which are generally considered to be "on-demand" retrograde neurotransmitters. But the original cannabis nomenclature stuck, and the endogenous ligands were termed endocannabinoids. THC was chemically an easier target to modify than the endocannabinoids, so a caldron full of THC derivatives has been synthesized, most notably in the early phases by J.W. Huffman and colleagues (Figure 1A). The potential promise of CB receptors as a drug target suffered a major setback when the first CB1 receptor inverse agonist, Rimonabant, was pulled from the market in 2009 due to serious psychiatric adverse effects. The cryo-EM structure of the CB1 receptor in complex with the agonist ligand MDMB-FUBINACA (FUB) and its heterotrimeric G protein signaling partner (the inhibitory Gi) is one of only several active GPCR-G protein complex structures, and the information content of the complex is outstanding. One nice feature of the structure is that it is relatively "native," with only the single-chain variable fragment, scFv16, along with a positive allosteric modulator (PAM) needed to stabilize the complex (although the PAM, ZCZ-011, does not appear in the structure). The FUB binding pocket in the transmembrane (TM) region of CB1 is "capped" from the extracellular aqueous surface by extracellular loop 2, a feature that is also found in the visual pigment rhodopsin, suggesting that hydrophobic ligands enter the binding pocket from a pore between TM helices as might be expected. Another feature of the CB1 structure that might be of particular interest to natural products medicinal chemists is the elucidation of how the indazole ring of FUB serves a similar role to the gem-dimethyl of another synthetic cannabinoid, AM11542 (Hua et al., 2017Hua T. Vemuri K. Nikas S.P. Laprairie R.B. Wu Y. Qu L. Pu M. Korde A. Jiang S. Ho J.-H. et al.Crystal structures of agonist-bound human cannabinoid receptor CB1.Nature. 2017; 547: 468-471Crossref PubMed Scopus (288) Google Scholar). "Gems" are a common and important chemical feature of many natural product GPCR ligands. Perhaps the main highlight of the structure is that the binding mode of FUB shows how agonist ligand binding can explain the dramatic outward movement of TM helix 6 upon receptor activation, which is the sine qua non of GPCR activation. The indazole ring of the "C-shaped" FUB structure directly impacts a conserved molecular "toggle switch" that links conserved amino acid residues on TM helix 3 and 6. The phenyl ring of a phenylalanine residue on TM helix 3 flips out to form a strong π-π interaction with indazole of the FUB, allowing the conserved tryptophan residue of the switch—located on TM helix 6—to rotate, which causes a loss of interhelical constraints, along with a dramatic rearrangement of TM helix 4. These agonist-induced conformational changes couple the ligand-binding domain to the intracellular surface, where a relatively rigid TM helix 5 in CB1, which lacks a proline residue found in most other GPCRs, extends to interact with the G protein. This structure, along with the structure of CB2 described below, establishes once and for all the universal mechanism of GPCR activation: agonist-initiated movement of the toggle switch Trp—first implicated in spectroscopic studies on rhodopsin (Lin and Sakmar, 1996Lin S.W. Sakmar T.P. Specific tryptophan UV-absorbance changes are probes of the transition of rhodopsin to its active state.Biochemistry. 1996; 35: 11149-11159Crossref PubMed Scopus (224) Google Scholar) and advanced by mutagenesis studies in CB receptors (McAllister et al., 2004McAllister S.D. Hurst D.P. Barnett-Norris J. Lynch D. Reggio P.H. Abood M.E. Structural mimicry in class A G protein-coupled receptor rotamer toggle switches: the importance of the F3.36(201)/W6.48(357) interaction in cannabinoid CB1 receptor activation.J. Biol. Chem. 2004; 279: 48024-48037Crossref PubMed Scopus (127) Google Scholar)—followed by significant outward helical movement. The high-resolution X-ray crystal structure of the CB2 receptor in complex with a novel rationally designed antagonist, AM10257, also provides significant insights about CB receptor pharmacology and GPCR activation mechanism. The new structure means that a direct comparison between binding modes in CB1 and CB2 receptors is finally possible, which is vital to provide a structural basis to design subtype-specific CB receptor ligands (Figure 1B). And the strong similarities between the ligand-binding pockets of CB1 and CB2 show why it has been so difficult to design subtype-specific ligands. The relatively spacious binding pocket of the CB2 receptor allows the almost triskelion structure of the AM10257, which includes a bulky adamantyl group, to nestle into the binding pocket with a pose that is distinctly different from its location in CB1. Remarkably, the structure of the extracellular domain of the AM10257-bound CB2 receptor shares conformational similarity with an earlier agonist-bound CB1 structure (Hua et al., 2017Hua T. Vemuri K. Nikas S.P. Laprairie R.B. Wu Y. Qu L. Pu M. Korde A. Jiang S. Ho J.-H. et al.Crystal structures of agonist-bound human cannabinoid receptor CB1.Nature. 2017; 547: 468-471Crossref PubMed Scopus (288) Google Scholar). The authors show that AM10257 has an unexpected pharmacological profile of agonism at the CB1 receptor, but antagonism at the CB2 receptor, suggesting that it should be possible to design ligands that have dual agonist/antagonist properties at the two CB receptor subtypes—a complex but potentially useful proposition. It remains to be seen how CB receptors that are not engineered to facilitate crystallization will behave in cellular signaling assays that measure G protein-coupling efficiency. The use of ultrapotent synthetic cannabinoids to facilitate structural studies, as well as associated molecular dynamics simulations, has provided unusual insights into the molecular pharmacology and molecular mechanism of activation of CB receptors. Precisely how the receptors function in a membrane bilayer, how endocannabinoids engage the receptor, and how membrane phospholipids might act as allosteric modulators of receptor function remain to be determined. In addition, comparison of the CB1-FUB-Gi complex structure with other active GPCR-G protein complexes provides some new insights but does not yet fully explain the structural basis for G protein subtype specificity or why some receptors can couple to more than one different G protein (Lorenzen et al., 2018Lorenzen E. Ceraudo E. Berchiche Y.A. Rico C.A. Fürstenberg A. Sakmar T.P. Huber T. G protein subtype-specific signaling bias in a series of CCR5 chemokine analogs.Sci. Signal. 2018; 11Crossref PubMed Scopus (21) Google Scholar). It has been suggested that marijuana was the real winner in the recent midterm elections in the United States. Referendums in several states approved legalization, or regulated medicinal availability, of cannabis. Cannabinoids are clearly popular with the public. But although both natural and synthetic cannabinoids have medicinal potential, ultrapotent designer synthetic cannabinoids pose a significant risk for abuse. The important state-of-the art structural studies published in the current issue of Cell suggest that although much progress has been made, much additional work is needed to understand the complexities of the endocannabinoid receptor system in human biology and medicine. We wish to thank the Eleanor Schwartz Charitable Foundation and the Nicholson Fund for support. T.P.S. is the Richard M. & Isabel P. Furlaud Professor at The Rockefeller University. Crystal Structure of the Human Cannabinoid Receptor CB2Li et al.CellJanuary 10, 2019In BriefThe structure of the human cannabinoid receptor CB2 reveals how small molecules affect CB2 differently than CB1 and point to principles that could inform rational and selective drug design. Full-Text PDF Open ArchiveStructure of a Signaling Cannabinoid Receptor 1-G Protein ComplexKrishna Kumar et al.CellJanuary 10, 2019In BriefLooking at how a toxic, synthetic ligand locks cannabinoid receptor 1 into a signaling conformation points to ways to understand and modulate receptor activity. Full-Text PDF Open Archive