Characterization of the Cholecystokinin and Gastrin Genes from the Bullfrog, Rana catesbeiana: Evolutionary Conservation of Primary and Secondary Sites of Gene Expression

Abstract

The gastrin and cholecystokinin (CCK) genes, and the complementary DNAs they encode, have been isolated and sequenced from the bullfrog, Rana catesbeiana. The CCK gene promoter region possess the same four well characterized transcriptional control elements as the human CCK gene, namely an E-box, AP-1 binding site, Sp1 site, and TATA box. In contrast, no obvious regulatory motifs are conserved in the gastrin gene. Alignment of the bullfrog preprohormone sequences with other members of the CCK/gastrin peptide family showed that preproCCK has been conserved to a greater degree during evolution than preprogastrin. In mammalian species, gastrin gene expression is typically associated with the antrum, and CCK with the small intestine and brain. However numerous secondary sites of CCK/gastrin gene expression have also been found. RT-PCR showed a high degree of conservation of both primary and secondary sites of CCK/gastrin production between mammals and the bullfrog, with gastrin messenger RNA being detected in the antrum, duodenum, colon, pancreas, brain, and testes, whereas CCK mRNA was observed in the brain, lung, testes, and throughout the length of the small intestine. In situ hybridization using radiolabeled gene specific antisense oligonucleotides uncovered CCK and gastrin messenger RNA in distinct areas of the bullfrog central nervous system and pituitary gland. Notably, the gastrin gene was expressed in the pituitary gland and hypothalamus of the bullfrog, as previously seen in mammals. This highly preserved tissue expression pattern suggests that gastrin plays specific roles in the hypothalamus and pituitary gland that are distinct from those of CCK. Our findings show that in spite of the structural resemblance, bullfrog CCK and gastrin constitute independent neuroendocrine peptide systems.