Generation of Transgenic Mice Overexpressing a Ghrelin Analog

After the discovery of ghrelin, we attempted to generate ghrelin gene transgenic (Tg) mice. These animals, however, produced only des-acyl ghrelin, which lacked the n-octanoyl modification at Ser3 necessary to manifest ghrelin activity. Because the mechanism for acyl-modification of ghrelin had been unclear until the recent identification of GOAT (ghrelin O-acyltransferase), it had been difficult to generate Tg mice overexpressing ghrelin using standard procedures. Therefore, we planned to generate Tg mice overexpressing a ghrelin analog, which possessed ghrelin-like activity in the absence of acylation at Ser3 and could be synthesized in vivo. As the replacement of Ser3 of ghrelin with Trp3 (Trp3-ghrelin) preserves a low level of ghrelin activity and Trp3-ghrelin can be synthesized in vivo, we generated mice overexpressing Trp3-ghrelin by using the hSAP (human serum-amyloid-P) promoter. Plasma Trp3-ghrelin concentrations in the Tg mice were approximately 85-fold higher than plasma ghrelin concentrations in non-Tg littermates. Because Trp3-ghrelin is approximately 1/10–1/20 less potent than ghrelin in vivo, plasma Trp3-ghrelin concentrations in Tg mice were calculated to have an activity approximately 6-fold greater than that of acylated ghrelin seen in non-Tg mice (85-fold x 1/10–1/20). Tg mice exhibited a normal growth and glucose metabolism in their early life stage. However, 1-yr-old Tg mice demonstrated impaired glucose tolerance and reduced insulin sensitivity. This model will be useful to evaluate the long-term effects of ghrelin or ghrelin analogs. In addition, this technique may be a useful method to generate gain-of-activity models for hormones that require posttranscriptional modifications. Impaired glucose tolerance was observed in the mice overexpressing a ghrelin analog.