Abstract
The endocannabinoid system (ECS) is a retrograde messenger system, consisting of lipid signaling molecules that bind to at least two G-protein-coupled receptors, Cannabinoid receptor 1 and 2 (CB1 and 2). As CB2 is primarily expressed on immune cells such as B cells, T cells, macrophages, dendritic cells, and microglia, it is of great interest how CB2 contributes to immune cell development and function in health and disease. Here, understanding the mechanisms of CB2 involvement in immune-cell function as well as the trafficking and regulation of CB2 expressing cells are crucial issues. Up to now, CB2 antibodies produce unclear results, especially those targeting the murine protein. Therefore, we have generated BAC transgenic GFP reporter mice (CB2-GFPTg) to trace CB2 expression in vitro and in situ. Those mice express GFP under the CB2 promoter and display GFP expression paralleling CB2 expression on the transcript level in spleen, thymus and brain tissue. Furthermore, by using fluorescence techniques we show that the major sources for GFP-CB2 expression are B cells in spleen and blood and microglia in the brain. This novel CB2-GFP transgenic reporter mouse line represents a powerful resource to study CB2 expression in different cell types. Furthermore, it could be used for analyzing CB2-mediated mobilization and trafficking of immune cells as well as studying the fate of recruited immune cells in models of acute and chronic inflammation.
Highlights
The CB2 receptor was discovered in 1993 by Munro et al [1] in a study showing that CB2 mRNA was present in splenocytes, but not detectable in the brain
Through screening for GFP expression of peripheral blood mononuclear cells (PBMCs) isolated from tail vein blood by flow cytometry, we identified one line (CB2-GFPTg 4) that exhibited reliable expression in the FITC channel (S2C Fig)
Highest expression of GFP and CB2 mRNA was detected by quantitative real-time PCR in spleen tissue
Summary
The CB2 receptor was discovered in 1993 by Munro et al [1] in a study showing that CB2 mRNA was present in splenocytes, but not detectable in the brain. Baek et al [11] recently examined CB2 immunolabeling using various commercially available CB2 antibodies on uninjured brain tissue of WT and CB2 KO mice. They found that interpretation of immunohistochemistry results with CB2 antibodies in brain tissue should be done with great caution.
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