AN AMINO-TERMINAL VARIANT OF THE CENTRAL CANNABINOID RECEPTOR RESULTING FROM ALTERNATIVE SPLICING

The cDNA sequences encoding the central cannabinoid receptor, CB1, are known for two species, rat and human. However, little information con cerning the flanking, noncoding regions is presently available. We hav e isolated two overlapping clones from a human lung cDNA Library with CB1 cDNA inserts. One of these, cann7, contains a short stretch of the CB1 coding region and 4 kilobase pairs Orb) of the 3'-untranslated re gion (UTR), including two polyadenylation signals. The other, cann6, i s identical to cann7 upstream from the first polyadenylation signal, a nd in addition, it contains the whole coding region and extends for 1. 8 kb into the 5'-UTR. Comparison of cann6 with the published sequence (Gerard, C. M., Mollereau, C., Vassart, G., and Parmentier, M. (1991) Biochem. J. 279, 129-134) shows the coding regions to be identical, bu t reveals important differences in the flanking regions. Notably, the cann6 sequence appears to be that of an immature transcript, containin g 1.8 kb of an intronic sequence in the 5'-UTR. In addition, polymeras e chain reaction amplification of the CB1 coding region in the IM-9 ce ll line cDNA resulted in two fragments, one containing the whole CB1 c oding region and the second lacking a 167-base pair intron within the sequence encoding the amino-terminal tail of the receptor. This altern atively spliced form would translate to an NH2-terminal modified isofo rm (CB1A) of the receptor, shorter than CB1 by 61 amino acids. In addi tion, the first 28 amino acids of the putative truncated receptor are completely different from those of CB1, containing more hydrophobic re sidues. Rat CB1 mRNA is similarly alternatively spliced. A study of th e distribution of the human CB1 and CB1A mRNAs by reverse transcriptio n-polymerase chain reaction analysis showed the presence of both CB1 a nd CB1A throughout the brain and in all the peripheral tissues examine d, with CB1A being present in amounts of up to 20% of CB1.