Development of ghrelin transgenic mice for elucidation of clinical implication of ghrelin

To elucidate the clinical implication of ghrelin, we have been trying to generate variable models of transgenic (Tg) mice overexpressing ghrelin. We generated Tg mice overexpressing des-acyl ghrelin in a wide variety of tissues under the control of β-actin promoter. While plasma des-acyl ghrelin level in the Tg mice was 44-fold greater than that of control mice, there was no differences in the plasma ghrelin level between des-acyl ghrelin Tg and the control mice. The des-acyl ghrelin Tg mice exhibited the lower body weight and the shorter body length due to modulation of GH-IGF-1 axis. We tried to generate Tg mice expressing a ghrelin analog, which possessed ghrelin-like activity (Trp³-ghrelin Tg mice). The plasma Trp³-ghrelin concentration in Trp³-ghrelin Tg mice was approximately 85-fold higher than plasma ghrelin (acylated ghrelin) concentration seen in the control mice. Because Trp³-ghrelin is approximately 24-fold less potent than ghrelin, the plasma Trp³-ghrelin concentration in Trp³-ghrelin Tg mice was calculated to have approximately 3.5-fold biological activity greater than that of ghrelin (acylated ghrelin) in the control mice. Trp³-ghrelin Tg mice did not show any phenotypes except for reduced insulin sensitivity in 1-year old. After the identification of ghrelin O-acyltransferase (GOAT), we generated doubly Tg mice overexpressing both mouse des-acyl ghrelin and mouse GOAT in the liver by cross-mating the two kinds of Tg mice. The plasma ghrelin concentration of doubly Tg mice was approximately 2-fold higher than that of the control mice. No apparent phenotypic changes in body weight and food intake were observed in doubly Tg mice. Further studies are ongoing in our laboratory to generate Tg mice with the increased plasma ghrelin level to a greater extent. The better understanding of physiological and pathophysiological significance of ghrelin from experiments using an excellent animal model may provide a new therapeutic approach for human diseases.