Molecular cloning and expression of equine calcitonin, calcitonin gene-related peptide-I, and calcitonin gene-related peptide-II.

In this study, we describe the cloning and tissue expression of equine calcitonin (CT), calcitonin-gene related peptide (CGRP)-I, and CGRP-II cDNA. We also describe a novel divergent form of CGRP (CGRP-I). Equine CT has greatest homology (>85%) to human, rat and mouse subgroups of calcitonins. Equine CGRP-I has low homology (<59%) to CGRPs of other species. The signal and N-terminal peptides for equine CT and CGRP-I were identical, indicating that these peptides are encoded by a gene equivalent to the human CALC-I gene. Equine CGRP-II has >80% homology to chicken, human, rat, ovine, swine, and bovine CGRPs. The homology between equine CGRP-I and CGRP-II is low (56%). The high homology of equine CGRP-II and the low homology of equine CGRP-I to CGRP in other species were unexpected findings. Northern blot analysis revealed that CT mRNA expression was restricted to the thyroid gland; however, RT-PCR revealed that CT mRNA expression was also present in the pituitary gland and in the liver. CGRP-I and CGRP-II mRNA expression was present in several regions of the nervous system and other tissues of neuroectodermal origin. An unexpected finding was CGRP-I expression in the kidney by both Northern analysis and by RT-PCR. Based on these results, CT gene expression in the horse was not restricted to the thyroid gland, and CT may be important in regulating pituitary cell function. CGRPs are widely expressed in tissues of the central and peripheral nervous system. Information from this study will be valuable to study the role of CT, CGRP-I, and CGRP-II in equine health and disease.