This year, our group collaborated with the GCAP transgenic lab to study the distribution and orientation of catechol-O-methyltransferase (COMT). COMT is a schizophrenia risk gene and its enzyme product is critical for the inactivation and metabolism of dopamine and other catechol compounds. COMT produces 2 transcripts with the long form is translated into the membrane-bound (MB) and the soluble (S) form. MB-COMT is primarily found in the brain and is implicated in dopamine signaling, whereas S-COMT is found in peripheral tissues and has a role in detoxification and catechol degradation in the liver and blood. Currently, there is no MB-COMT specific inhibitor. Tolcapone, a drug with L-Dopamine for treatment of Parkinsons disease, is used to study dopamine-dependent brain functions inhibits both MB-COMT and S-COMT. The inhibition of S-COMT may play a role in liver toxicity which limits its therapeutic use. Therefore, it will be advantageous to develop MB-COMT specific inhibitors. COMT gene has a well characterized functional polymorphism, Val158Met, which effects COMT activity in humans. The met allele is associated with having much lower enzyme activity and better cognitive function but increases sensitivity to pain. The knowledge that there exist other functional polymorphisms in COMT that effect enzyme activity, makes individualized doses of catechol-based drugs quite reasonable. The Clinical Brain Disorders Branch/Genes, Cognition and Psychosis Program as well as other labs have well characterized the role of COMT in cortically mediated cognition. Clinical assessments of tolcapone have suggested improvement of cortical function. However, given all the information on COMT, the cell biology of MB-COMT remains unclear. This study examined MB-COMT orientation in human lymphoblastoid cells and cellular distribution of MB-COMT on pyramidal neurons. To determine the orientation of MB-COMT, we used transfected human lymphoblastoid cells, which resulted in MB-COMT tagged on either terminus with green fluorescent protein (GFP). Our results showed only the C-terminal GFP tag upon treatment with anti-GFP antibody conjugated with R-Phycoerythrin. This finding indicated that the C-terminus of MB-COMT is outside of the cell and that the catalytic domain in the C-terminal region is in the extracellular space. This orientation renders MB-COMT capable of catalyzing the transfer of a methyl group from S-adenosylmethionine to catecholamines in synaptic and extracellular spaces, while not impacting cross-membrane transport. This suggests that newly developed COMT inhibitors impermeable to the cell membrane will be MB-COMT specific in the brain. Future studies will be conducted which will aid in the development of new therapeutic compounds for treatment of complex psychiatric disorders.

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U.S. National Institute of Mental Health
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