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.

译文

GH释放激素 (GHRH) 是一种下丘脑肽,在控制垂体前叶GH的合成和分泌中起关键作用。GHRH与许多其他下丘脑激素一样,也在胎盘中表达,尽管尚未确定其在该组织中的生理作用。胎盘前GHRH与下丘脑中的相同。然而,胎盘和下丘脑GHRH信使rna在对应于未翻译外显子1的区域有所不同。涉及使用组织特异性启动子和外显子1的差异剪接的联合机制在胎盘和下丘脑中产生成熟的GHRH信使rna。作为定位GHRH基因的胎盘特异性表达所涉及的调控元件的第一步,我们已经产生了包含构建体的转基因小鼠,其中大鼠GHRH基因的潜在调控序列与氯霉素乙酰转移酶 (CAT) 报告基因融合。在胎盘转录起始位点上游包含7.5千碱基侧翼序列的构建GHRH-CAT1不促进转基因动物中的CAT表达。相反,与构建体GHRH-CAT1不同的构建体GHRH-CAT2具有位于胎盘外显子1下游的附加序列,在脑和胎盘中表现出高水平的CAT表达。我们的结果表明,包含在构建体GHRH-CAT2中的序列包含指导胎盘中CAT基因的发育调控和细胞类型特异性表达所必需的顺式作用调控元件。出乎意料的是,在不同区域的神经胶质细胞中检测到转基因在大脑中的表达,而在下丘脑中却没有。

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