FUNCTIONAL-CHARACTERIZATION OF 2 CHOLECYSTOKININ-B GASTRIN RECEPTOR ISOFORMS - A PREFERENTIAL SPLICE DONOR SITE IN THE HUMAN RECEPTOR GENE

The cholecystokinin-B and gastrin receptor is encoded by a single gene composed of five exons and spanning over 10 kilobases on human chromo some lip 15.5-->15.4. Exon 4 has two possible alternative splicing don or sites that seem to be conserved in other species such as the canine , rat, Mastomys, and mouse. They could generate two receptor isoforms (short- and long-form), which differ in their putative third cytoplasm ic domain of the serpentine C-protein-coupled receptors. In the presen t study, we examined whether an alternative splicing is operated in a tissue-specific manner and whether two receptor isoforms have function al differences. RNase-protection assay and S1 nuclease mapping demonst rated the preferential expression of the short-form in the human brain as well as the digestive organs, stomach and pancreas. The two putati ve isoforms of the cholecystokinin-B/gastrin receptor expressed in mou se fibroblasts showed the same characteristics in their ligand-binding s, the major signal transduction such as phosphoinositides production, cytoplasmic Ca2+ increase, tyrosine phosphorylation of focal adhesion kinase, activation of mitogen-activated protein kinase, and the induc tion of early-responsive genes such as c-fos, c-myc, and c-jun. Moreov er, the ligand-dependent trophic effect was seen in both receptor isof orms, Taken together with the absence of tissue-specific expression of two receptor isoforms, these results suggest a species-specific domin ant splice donor site in exon 4 of the human receptor gene.