FUNCTIONALLY DIFFERENT ISOFORMS OF THE HUMAN CALCITONIN RECEPTOR RESULT FROM ALTERNATIVE SPLICING OF THE GENE TRANSCRIPT

Two subtypes of the human calcitonin receptor (hCTR) have been describ ed which differ from one another by the presence or absence of a 16-am ino acid insert in the first intracellular loop, Both isoforms were st ably expressed in baby hamster kidney cells to compare their ligand bi nding and second messenger coupling. The binding affinity and the on/o ff rate of binding for salmon CT were identical for the two receptor i soforms. However, the presence of the insert significantly reduced the ability of the receptor to couple to both adenylate cyclase and phosp holipase C, Stimulation of a transient calcium response was only obser ved with the insert-negative receptor, Similarly, the ED50 for the cAM P response is 100-fold higher for the insert-positive form compared wi th the insert-negative form of the receptor, However, the maximal cAMP response was equivalent for both receptor isoforms, The rate of inter nalization of the insert-positive form of the receptor is significantl y impaired relative to the insert-negative receptor, which suggests th at this process may be dependent on the stimulation of a second messen ger pathway. Cloning and characterization of the relevant portion of t he hCTR gene revealed that these isoforms are generated by alternative splicing. We also discovered a third isoform of the hCTR, which can b e generated by alternative splicing at the same position. The presence of a stop codon in this newly described alternative exon would lead t o premature termination of the receptor at the C-terminal end of the f irst transmembrane domain.