Identification of a human 5-HT sub(6) receptor variant produced by alternative splicing

The complexity of the 5-hydroxytryptamine (5-HT) (serotonin) receptor family has been increased by the findings that isoforms or splice variants exist for subtypes such as the 5-HT sub(2B), 5-HT sub(2C), 5-HT sub(4) and 5-HT sub(7) subtypes. Further molecular biological studies in our laboratory have demonstrated that a splice variant of the 5-HT sub(6) receptor exists in the human brain. Experiments performed using a degenerate PCR approach from human caudate cDNA revealed a 5-HT sub(6) receptor clone with a 289 bp deletion of the region coding for transmembrane IV through the third intracellular loop. This deletion produces a frameshift creating a downstream stop codon which results in a truncated protein containing 10 unique amino acids at its carboxyl end. The variant transcript occurs as a result of alternative splicing using an upstream donor site and the acceptor site from the first intron in the 5-HT sub(6) receptor gene. The splicing pattern seen for this transcript was not detected in rat or mouse whole brain cDNA by PCR due to the lack of a consensus 5' donor site. Coexpression of the variant 5-HT sub(6) transcript and the full length 5-HT sub(6) transcript was observed in caudate and substantia nigra but not in hippocampus, cortex, cerebellum and thalamus. Transient transfection of a 5-HT sub(6) variant construct into Cos-7 cells demonstrated that a truncated receptor was translocated to the membrane but appeared nonfunctional.