Calcitonin of the puffer fish (Fugu Rubripes)
MS Clark, PM Ingleton, LA Bendell & DM Power
The evolution of aquatic vertebrates to dry land required adaptation of physiological systems concerned with water and mineral ion homeostasis. Two factors, calcitonin (CT) and parathyroid hormone (PTH), have ultimately become the principal hypo- and hyper-calcaemic factors respectively, in terrestrial vertebrates. Fish CT has been isolated and shown to be hypocalcaemic in mammals; its role in fish physiology is less well defined, although it may control calcium influx via the gills. For further studies of the roles of CT in teleost fishes we have cloned the gene from the puffer fish, an advanced teleost, determined its genomic organisation and gene expression in tissues by in situ hybridisation and RT-PCR.
The gene shows intron/exon boundaries also conserved in human, rat, chicken and salmon CT. It can undergo alternate splicing to produce CT and calcitonin gene-related peptide (CGRP). Only one CT has so far been identified in Fugu and the mature peptide is 32 amino acids long including 2 cysteine residues which form a disulphide bond. Comparative mapping shows conserved linkage of four genes from human chromosomal region 11p15. By in situ hybridisation gene expression was most abundant in the ultimobranchial gland, the dense neuronal layer of the mid-brain, supramedullary cells of the brain stem, ova and in pituitary secretory cells, apparently with greatest expression in somatotrophs, and with limited expression in gill chloride cells. RT-PCR demonstrated calcitonin mRNA in the brain, spinal cord, gills and eye, but other tissues had virtually undetectable levels of transcript. These observations suggest calcitonin is involved in neurotransmission, development and that it may act as an endocrine factor in Fugu.