Dissecting IGF regulation of cell turnover in an integrated cellular system: the human placenta as a model

During pregnancy, growth of the fetus depends on the transfer of food and oxygen from the mother's blood, and transfer of fetal waste products in the opposite direction. These exchange processes are carried out by the placenta. Development of a placenta very early in pregnancy is therefore a prerequisite for normal fetal growth, and indeed the placenta is larger than the fetus until about 4 months of pregnancy. If the placenta doesn't develop properly then the nutrient and oxygen supply to the fetus is compromised, resulting in impaired growth. 3-10% of all babies will have suffered impaired fetal growth. Many of these babies die or, if they do survive, they are more likely to be ill or disabled during childhood. In addition, being small at birth has a life-long impact on health as the risk of developing heart disease or diabetes in adulthood is much greater in these individuals. Studies of mouse genetics have shown that a family of hormones known as insulin-like growth factors (IGFs) are required to help the placenta grow. We now need to know whether this is also true in women; if so, placental and fetal growth might be enhanced by administering IGF to the mother. There are obvious ethical barriers to asking this question directly, but we have developed new methods by which pieces of placenta can be kept alive for several days in the test tube, a time sufficiently long to be able to manipulate and measure growth. We have already obtained results that show IGF indeed makes human placental cells grow. In this project we will use placental tissue maintained in a controlled laboratory environment to investigate how IGF delivered from the maternal side stimulates growth. First we will find out how IGF gets into the placenta. The placental surface (called the syncytium) acts as a barrier that prevents bacteria and other harmful agents from reaching the fetus. IGF surmounts this barrier in ways that are not yet understood. We will establish how IGF can get into the placenta, by looking at how it can cross the syncytium. We will determine if proteins called phosphatases (PTPs), which are known to help other cells and organs grow, act in concert with IGFs. In order to do this, we will need to develop some new methods for eliminating ('knock down') phosphatases from living placental tissue, and then we will see if IGFs can still make cells in the placenta grow. In addition to finding out how IGFs work, which may eventually lead to treatments for pregnancies in which the baby doesn't grow properly, the technological developments proposed in this project are exciting because the placenta is a readily accessible, complex multicellular system that provides a better model for human cellular signalling than commonly used cell lines, animals or lower organisms. Our results will give insight into how growth signals are coordinated in tissues and organisms, and we intend they should lead on to the development of the placenta as a widely applicable target system for drug screening.

In the human placenta, syncytiotrophoblast acts as the primary maternal-fetal barrier, transporting solutes and excluding pathogens. Villous cytotrophoblast is a progenitor cell for terminally differentiated, non-proliferative syncytiotrophoblast. A balance between the proliferation, differentiation and fusion of cytotrophoblast is critical for normal development; however the events that regulate these processes are poorly understood. A major unanswered question is how placental growth is regulated by maternal signals. Cytotrophoblast proliferation has been difficult to study in human because in vitro these cells rapidly and irreversibly exit the cell cycle. We have developed a placental explant model in which proliferation and differentiation are maintained and, critically, kinetics can be sensitively regulated from the maternal-facing side of the placenta. IGF delivered at the maternal surface stimulates increased cytotrophoblast proliferation, while inhibitors of the tyrosine phosphatase SHP-2, which is linked to IGF signalling pathways, reduce proliferation. It is clear that signals from soluble mediators must be transmitted across the syncytium to the cytotrophoblast. We will use high resolution imaging, pharmacological inhibition and siRNAi knockdown of downstream gene products including SHP-1 and SHP-2, to ask how IGF delivered to the maternal surface alters cytotrophoblast kinetics. These studies will dissect the events involved in regulating cell turnover in a complex multicellular system and will lead to a better understanding of placental development and growth. The technological advances in prospect are also exciting because the human placenta is a readily accessible system that provides a better model for human cellular signalling than commonly used cell lines, animals or lower organisms; consequently, we intend our research to lead on to the development of the placenta as a widely applicable target system for drug screening.