Molecular identification of single hormone-encoding proglucagon cDNA isoforms from squamates and their abundant...

Among ectothermic reptiles, the order Squamata has adapted most successfully to the terrestrial environment. However, the physiological background of this success remains unknown. Since the regulation of energy metabolism provides an important insight into terrestrial adaption by ectothermic animals, we focused on proglucagon-derived peptides (PGDPs). In the process of cloning proglucagon mRNA in geckos, we identified several novel proglucagon (PG) cDNA isoforms. They were tissue-specifically and strongly expressed in the pancreas and small intestine of the geckos, suggesting their biological relevance. Therefore, in order to clarify whether these novel cDNA isoforms are phylogenetically conserved, we performed the additional molecular characterization of proglucagon cDNAs from several representative species of the Squamata and Testudine clade and examined the expression of proglucagon mRNAs in the small intestine and pancreas. In the present study, a total of 7 proglucagon cDNA isoforms were identified and divided into two groups (Classes A and B) based on the 3'-UTR sequence of each isoform. The longest isoform of each group (named PG-A₁ and PG-B₁, respectively) had the same molecular characteristics as those previously reported from chickens and reptiles, namely, PG-A and PG-B. Other 5 isoforms were novel-type cDNAs, and were the products of exon skipping (named PG-A₂, PG-A_2s, PG-B₂, PG-B_2s, and PG-B₃). Some of these isoforms coded for only one peptide hormone (GLP-1 or GLP-2). This is the first identification of single hormone-encoding proglucagon cDNAs in vertebrates. Moreover, an expression analysis of these isoforms revealed that single hormone-encoding proglucagon mRNAs were predominantly expressed with tissue and lineage specificities in the reptile clade. Collectively, the present results suggest an independent regulatory system for GLP-1 and GLP-2 secretion and indicate the plasticity of proglucagon genes in expressing different isoforms in different tissues in Squamata. These results also provide insights into the plastic energy metabolic system of Squamata in accordance with various habitats in the terrestrial environment, supporting their successful prosperity.