Species differences in cannabinoid receptor 2 (<i>CNR2</i>gene): identification of novel human and rodent CB2 isoforms, differential tissue expression and regulation by cannabinoid receptor ligands

Cannabinoids, endocannabinoids and marijuana activate two well-characterized cannabinoid receptors (CB-Rs), CB1-Rs and CB2-Rs. The expression of CB1-Rs in the brain and periphery has been well studied, but neuronal CB2-Rs have received much less attention than CB1-Rs. Many studies have now identified and characterized functional glial and neuronal CB2-Rs in the central nervous system. However, many features of CB2-R gene structure, regulation and variation remain poorly characterized in comparison with the CB1-R. In this study, we report on the discovery of a novel human CB2 gene promoter transcribing testis (CB2A) isoform with starting exon located ca 45 kb upstream from the previously identified promoter transcribing the spleen isoform (CB2B). The 5' exons of both CB2 isoforms are untranslated 5'UTRs and alternatively spliced to the major protein coding exon of the CB2 gene. CB2A is expressed higher in testis and brain than CB2B that is expressed higher in other peripheral tissues than CB2A. Species comparison found that the CB2 gene of human, rat and mouse genomes deviated in their gene structures and isoform expression patterns. mCB2A expression was increased significantly in the cerebellum of mice treated with the CB-R mixed agonist, WIN55212-2. These results provide much improved information about CB2 gene structure and its human and rodent variants that should be considered in developing CB2-R-based therapeutic agents.