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Placenta-Specific Expression of the Rat Growth Hormone-Releasing Hormone Gene Promoter in Transgenic Mice¹

Department of Biochemistry and Molecular Biology, Faculty of Chemistry, (N.N., M.P.-R., A.B.), and the Department of Animal, Plant, and Cellular Biology, Faculty of Biology (J.A.D.R., E.S.), University of Barcelona, 08028 Barcelona, Spain; and the Howard Hughes Medical Institute and Section of Immunobiology, Yale University School of Medicine (R.A.F.), New Haven, Connecticut 06510-8023

Address all correspondence and requests for reprints to: Dr. Núria Nogués, Department of Medicine (M/C 640), University of Illinois College of Medicine, Chicago, Illinois 60612-7323. E-mail: NNogues{at}uic.edu

GH-releasing hormone (GHRH) is a hypothalamic peptide that plays a critical role in controlling the synthesis and secretion of GH in the anterior pituitary. Along with many other hypothalamic hormones, GHRH is also expressed in the placenta, although its physiological role in this tissue has not yet been determined. The placental prepro-GHRH is identical to that found in the hypothalamus. However, the placental and hypothalamic GHRH messenger RNAs differ in the region corresponding to the untranslated exon 1. A combined mechanism involving the use of tissue-specific promoters and the differential splicing of exon 1 generates the mature GHRH messenger RNAs in placenta and hypothalamus. As a first step toward the localization of the regulatory elements involved in the placenta-specific expression of the GHRH gene, we have generated transgenic mice containing constructs in which potential regulatory sequences of the rat GHRH gene were fused to the chloramphenicol acetyltransferase (CAT) reporter gene. Construct GHRH-CAT1, which contains 7.5 kilobases of flanking sequences upstream to the placental transcription start site, did not promote CAT expression in the transgenic animals. In contrast, construct GHRH-CAT2, which differs from construct GHRH-CAT1 in having additional sequences located downstream to placental exon 1, exhibited high levels of CAT expression in brain and placenta. Our results show that the sequences included in construct GHRH-CAT2 contain the cis-acting regulatory elements necessary to direct developmentally regulated and cell type-specific expression of the CAT gene in the placenta. Unexpectedly, the expression of the transgene in the brain was detected in glial cells of different areas, but not in the hypothalamus.

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