Transgene-driven protein expression specific to the intermediate pituitary melanotrope cells of Xenopus laevis

In the present study, we examined the amphibian Xenopus laevis as a model for stable transgenesis and in particular targeted transgene protein expression to the melanotrope cells in the intermediate pituitary. For this purpose, we have fused a Xenopus proopiomelanocortin (POMC) gene promoter fragment to the gene encoding the reporter green fluorescent protein (GFP). The transgene was integrated into the Xenopus genome as short concatemers at one to six different integration sites and at a total of one to approximately 20 copies. During early development the POMC gene promoter fragment gave rise to GFP expression in the total prosencephalon, whereas during further development expression became more restricted. In free-swimming stage 40 embryos, GFP was found to be primarily expressed in the melanotrope cells of the intermediate pituitary. Immunohistochemical analysis of cryosections of brains/pituitaries from juvenile transgenic frogs revealed the nearly exclusive expression of GFP in the intermediate pituitary. Metabolic labelling of intermediate and anterior pituitaries showed newly synthesized GFP protein to be indeed primarily expressed in the intermediate pituitary cells. Hence, stable Xenopus transgenesis with the POMC gene promoter is a powerful tool to study the physiological role of proteins in a well-defined neuroendocrine system and close to the in vivo situation.