PLACENTAL AMINO ACID TRANSPORT: MOLECULAR ASPECTS

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The primary objective of the project is to obtain a clear understanding of the mechanisms involved in the transport of a unique amino acid, taurine, across the human placenta. Taurine, though a non-protein amino acid, is the most abundant free amino in several tissues (e.g. brain, heart, and retina) of the human fetus. In animals, induction of taurine deficiency in the mother during pregnancy produces deleterious effects on the developing fetus, causing functional impairment of those organs which accumulate high levels of taurine. The effects of taurine deficiency in utero include retinal degeneration, cardiac myopathy and impairment of cerebellar development. These studies indicate that taurine plays an important role in the growth and development of the fetus. Paradoxically however, neither the human fetus nor the human placenta can synthesize taurine endogenously. Maternal-to-fetal transfer across the placenta is the only mechanisms available for the fetus to obtain this important nutrient. A model is proposed for the transport of taurine across the placenta according to which the syncytiotrophoblast expresses two similar, yet distinct, taurine transporters, one in the maternal-facing brush border membrane and the other in the fetal-facing basal membrane and a functional coordination between these two transporters bring about the transfer of taurine from mother to fetus. The project seeks to obtain supporting evidence for this model, by establishing the molecular and biochemical identity of these two transporters. This goal will be achieved by isolating the cDNAs encoding these transporters from a human placental cDNA library and by functionally expressing the cloned transporters in mammalian cells and in Xenopus laevis oocytes. Polyclonal and monoclonal antibodies against the cloned transport proteins will be generated to be used in immunohistochemical studies to establish the polarized localization of the two taurine transporters in the syncytiotrophoblast. Characterization of taurine transport across the brush border and basal membranes will also be done with purified membrane vesicles from normal placentas. These studies will be complemented with an intact cell system in which placental choriocarcinoma cells will be grown on permeable filters allowing access to the brush border and basolateral membranes for differential assessment and characterization of taurine transport across these membranes. Another goal of the project is to investigate the hormonal regulation of the placental taurine transporter(s) and the cellular mechanisms involved therein. It is proposed that the transporter is regulated by posttranslational modification involving phosphorylation and that protein kinase C and calcineurin (a Ca2+- calmodulin-dependent phosphoprotein phosphatase) participate in this process. The taurine transporter cDNA(s) and the antitransporter antibodies will provide excellent experimental tools for these studies. This project will result in a thorough understanding of the molecular and cellular mechanisms responsible for the maternal-to-fetal transfer of taurine across the human placenta.