The cannabinoid receptor type 1
The cannabinoid receptor type 1 (CB1R) is a G-protein coupled receptor (GPCR), abundant in the central nervous system, that is activated by the key psychoactive component in Cannabis species, delta-9-tetrahydrocannabinol (THC).1 Beyond its well-described role in psychopharmacology, CB1R is also strongly implicated in regulating food intake, and inappropriate CB1R signalling contributes to the development of obesity and other metabolic conditions, especially type 2 diabetes.2,3 Indeed, an inverse agonist of CB1R (a compound that produces the opposite signal from a receptor, rather than simply blocking it), rimonabant, was briefly marketed as an anti-obesity and anti-diabetic drug in the late 2000s. However, rimonabant was withdrawn worldwide scant years later amid serious concerns over its safety prompted by unexpected psychiatric side effects.4
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Allosteric modulators
Allosteric modulators are compounds that bind to receptors, such as CB1R, at a different place than the compound or hormone our bodies produce to interact with the receptor (known as the orthosteric site).5,6 In this way, they can enhance (positive allosteric modulation; PAM) or diminish (negative allosteric modulation; NAM) the existing natural signalling of the receptor.7,8 The essential promise of allosteric modulators as drugs is that they can influence the activity of their targets in a more subtle way.
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For psychedelic drug research, there are obvious advantages to this approach. Many GPCRs, such as CB1R and the serotonin receptors, produce potentially undesirable psychoactive effects when activated, which complicates the use of agonists (activating drugs that act on the orthosteric site).9 Furthermore, direct inactivation, or inverse agonism, can be dangerous, as seen with rimonabant. Allosteric modulation may therefore offer ways to avoid undesirable “on-target” effects for receptors involved in human responses to psychedelics.
X-ray crystal structure of CB1R with a PAMClick Here
In an important paper recently published in Nature Chemical Biology, Yang and colleagues used a combination of cutting-edge techniques to convincingly reveal how allosteric modulators can bind to CB1R and influence its function.5
Firstly, they employed x-ray crystallography to obtain the structure of CB1R (grey) bound simultaneously to a synthetic cannabinoid, CP 55,940, and a positive allosteric modulator, ZCZ011. In this technique, large amounts of purified receptor protein are mixed with one or more drugs of interest, and grown carefully into crystals. When high intensity x-rays are fired through these crystals, they scatter into specific patterns. With the help of computers, scientists can use these patterns to reconstruct the precise 3D structures of the receptor and drugs.
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From this experiment, Yang and colleagues found that ZCZ011, the PAM (yellow), inserts into a cleft on one side of the protein, which sits in between three of CB1R’s eight helix domains (pale green) (Figure 1). ZCZ011 pushes parts of these helices inward, towards the receptor’s centre, where agonists bind (Figure 1, right panel). This physically moves amino acids inside the orthosteric site, particularly serine 173 (S173, magenta), closer to the agonist CP 55,940 (blue), improving its binding to CB1R. Indeed, S173 must be important for the activating effect of this synthetic cannabinoid, as CB1R activation by CP 55,940 drops 60-fold when S173 is mutated to a different amino acid.Details
FIGURE 1: ZCZ011, A PAM, BINDS IN A CLEFT BETWEEN HELICES 2, 3, AND 4 (PALE GREEN) OF CB1R. INSIDE THE ORTHOSTERIC BINDING SITE, AN AMINO ACID S173 (MAGENTA) IMPORTANT FOR ACTIVATION BY THE AGONIST CP 55,940 (PALE BLUE) IS PUSHED CLOSER TO THE AGONIST, STRENGTHENING ITS BINDING. THE MOLECULAR COORDINATES FOR THIS X-RAY CRYSTAL STRUCTURE WERE OBTAINED FROM THE PROTEIN DATABANK, PDB: 7FEE.
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Cryogenic electron microscopy structure of CB1R with a PAM
It has previously been shown that agonists, such as the synthetic cannabinoid MDMB-Fubinaca, insert deep into the centre of the receptor and disrupt an essential “toggle switch” that physically holds the receptor in an inactive state (Figure 2A).10 Disrupting this switch forces CB1R to undergo a series of shape changes, ultimately leading to a large outward movement of the receptor’s 6th helix. This creates a larger cavity on the side of the receptor inside the cell, allowing G proteins to interact with this side and begin signalling (Figure 2B).