TROPHOBLAST GENE EXPRESSION AND DIFFERENTIATION

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The placenta functions in several important ways in promoting fetal development. This organ serves to implant the developing embryo in the uterus; as a conduit for the exchange of nutritional supplies and excreted waste between the mother and the fetus; as a barrier preventing attack on the fetus by the maternal immune system, and preventing transport into the fetal circulation of hazardous compounds such as heavy metals; and as a source of secreted proteins that regulate both maternal and fetal physiology. In regard to this last role, the placenta in several species (including rodents, sheep, cows, and humans) is the site of synthesis of proteins that are structurally related to pituitary prolactin. These hormones regulate maternal ovarian steroidogenesis (inducing progesterone production to maintain pregnancy), liver metabolism (to release glucose from storage forms for use by the fetus), and mammary development (to prepare for lactation); these hormones also enter the fetal compartment, where they may have direct effects on fetal development, for example in lung development. In the mouse, the prolactin family of proteins synthesized specifically in the placenta includes placental lactogen I, placental lactogen II, proliferin, and proliferin-related protein. We propose to define the genetic elements that regulate the trophoblast- and temporal-specific expression of these prolactin family genes in the mouse. Initial efforts will focus on placental lactogen I and proliferin-related protein, since the activity of these gene promoters is trophoblast-specific in cell culture transfection assays. Placental lactogen I provides the further advantage of being expressed early during gestation, and therefore may represent a target for a transcription/differentiation factor acting at an early stage of placental development; the placental lactogen I gene promoter will therefore also be used to generate immortalized trophoblast cell lines. The proliferin-related protein, meanwhile, provides a unique handle on mouse trophoblast differentiation since it is expressed in two trophoblast cell types, unlike the other members of the prolactin family. Both transient transfections into rodent choriocarcinoma cells and in vivo expression in transgenic mice will be assayed to define these regulatory elements. Once defined, these elements will then be used to identify a trans-acting regulatory factor(s) responsible for the expression of these genes in trophoblast cells. The expression of this factor during placental and fetal development, as well as in the adult, will then be investigated. Identification and characterization of this factor(s) will provide a means to examine placental trophoblast cell differentiation on a molecular level. Since the trophoblast cells are intimately involved in embryonic development, identifying the genetic elements and factors involved in trophoblast cell differentiation and function will aid in detecting, preventing, and treating birth defects.