PLACENTA-SPECIFIC EXPRESSION OF THE RAT GROWTH HORMONE-RELEASING HORMONE GENE PROMOTER IN TRANSGENIC MICE

GH-releasing hormone (GHRH) is a hypothalamic peptide that plays a cri tical role in controlling the synthesis and secretion of GH in the ant erior pituitary. Along with many other hypothalamic hormones, GHRH is also expressed in the placenta, although its physiological role in thi s tissue has not Set been determined. The placental prepro-GHRH is ide ntical to that found in the hypothalamus. However, the placental and h ypothalamic GHRH messenger RNAs differ in the region corresponding to the untranslated exon 1. A combined mechanism involving the use of tis sue-specific promoters and the differential splicing of exon 1 generat es the mature GHRH messenger RNAs in placenta and hypothalamus. As a f irst step toward the localization of the regulatory elements involved in the placenta-specific expression of the GHRH gene, we have generate d transgenic mice containing constructs in which potential regulatory sequences of the rat GHRH gene were fused to the chloramphenicol acety ltransferase (CAT) reporter gene. Construct GHRH-CAT1, which contains 7.5 kilobases of flanking sequences upstream to the placental transcri ption start site, did not promote CAT expression in the transgenic ani mals. In contrast, construct GHRH-CAT2, which differs from construct G HRH-CAT1 in having additional sequences located downstream to placenta l exon 1, exhibited high levels of CAT expression in brain and placent a. Our results show that the sequences included in construct GHRH-CAT2 contain the cis-acting regulatory elements necessary to direct develo pmentally 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 h ypothalamus.