Investigation of the role of the third extracellular loop of the human calcitonin receptor in peptide interactions

Abstract

The calcitonin-family of peptides comprises calcitonin, amylin, calcitonin gene-related peptides (CGRPs), adrenomedullin (AM) and AM2 that show broad pharmacological potential as drug targets for treatments of diseases such as diabetes, migraine and osteoporosis. Their receptors are calcitonin receptor-like receptor (CLR) or calcitonin receptor (CTR) and receptor activitymodifying protein (RAMP) pairings. For example, amylin receptors are formed from CTR interactions with RAMPs. Multiple peptides can also activate these receptors with different potencies. For instance, CTR can bind other peptides in the calcitonin peptide family including amylin and CGRP with low affinity. On the otherhand, CLR and RAMP parings are mainly activated by CGRP and AM. Small molecule or modified peptide agonists of CTR would represent advances in the treatment of bone diseases. To develop these, an understanding of how the peptides in the calcitonin peptide family bind to and activate their receptors is needed. Studies have shown that two regions in class B GPCRs are particularly important for peptide binding: the N-terminus and the juxtamembrane region, including the extracellular loops (ECLs). However, there is very little known about the role of the ECLs in CTR. Thus, alanine scanning mutagenesis was first performed for five residues at positions 367 to 371 ECL3 of the human CTR. These residues in the 367-371 region of CTR are also different from those in CLR, which led to the hypothesis that the different pharmacology that CTR and CLR exhibit might arise from their different ECL3 sequences. Therefore, the same residues in CTR were individually mutated to those corresponding residues in CLR. The biological activities of these mutants were investigated using a cAMP assay when stimulated with human calcitonin, rat amylin and human αCGRP. Cell surface expression was measured by ELISA and found not to be altered for all mutants. In cAMP assays, the mutants were also equivalent to wildtype, except for I371A Amylin receptor (AMY1(a)) showed a five-fold decrease in potency when stimulated by rat amylin. However this effect was context dependent and further investigation is required to see whether this was a real effect. Overall, these data suggest that this region of ECL3, 367-371 of human CTR appears to have little role in peptide interactions, and other regions are more likely to be involved in peptide-receptor interactions in the CTR and AMY1(a) receptor.