Researchers in Europe and the United States collaborated to successfully develop a two-step photoaffinity probe to monitor endogenous G-protein coupled receptor (GPCR) cannabinoid receptor 2 (CB2R) expression and ligand engagement. The group made up of scientists from institutions in The Netherlands, Switzerland, and the US examined the potential for CB2R use in the treatment of tissue injury and inflammatory diseases. The CBR2 is found in immune system cells and is upregulated as a result of pathophysiological conditions. They published their results in the Journal of the American Chemical Society in February 2018.
There are hundreds of described G-protein coupled receptors (GPCR) in the literature; GCPRs are a large and diverse family of transmembrane domain receptors that transmit extracellular signals to the cell. Each contains seven segments that spans the width of the membrane. GPCRs communicate through G-proteins to activate Ras or other signaling proteins to trigger intracellular signaling pathways.
The authors write that the CB2R plays a critical role in cell migration and immunosuppression and shares 44% overall homology with the cannabinoid receptor 1 (CB1R) which is found primarily in the central nervous system. CB2R activation alone may provide therapeutic benefits without side effects of CB1R activation so there has been work done to identify a CB2R agonist. CB2R agonists like LEI101, HU308, and HU910, as reported by these authors, have demonstrated efficacy for chronic and inflammatory pain, neuro- and nephropathy, and other disorders in animal models.
The researchers faced challenges in determining the type of labeling that would work because of the variable surface expression level of GPCRs. In addition, normal biochemical methods using antibodies against GPCR cannot be used because CB2R does not currently have specific antibodies against it. Lastly, because GPCRs change conformation upon the ligand binding, labeling cannot inhibit those changes and signaling cascade process while also needing to avoid degradation.
The result is the development of a two-step photoaffinity-based protein profiling process which utilizes a small ligation handle, a fluorescent tag, and an element to cross-link the product (ligand with receptor) to assess endogenous expression using LEI121. This is important to help drive research forward through demonstration of CB2R occupancy at the site of inflammation in patients.