The placenta is a temporary organ that forms in the uterus during pregnancy, serving as the interface between the mother and the developing fetus. This remarkable organ facilitates the exchange of oxygen, nutrients, and waste products, acting as the baby’s life support system. Beyond its role as a conduit, the placenta is an active endocrine organ, producing a wide array of unique proteins that are fundamental to a healthy pregnancy and fetal development.
The Placenta’s Protein Production
The placenta functions as an active endocrine gland, synthesizing numerous proteins essential for its growth and for communication between the mother and fetus. The syncytiotrophoblast, the outer layer of the placenta, is primarily responsible for this extensive protein synthesis and secretion. This layer is in direct contact with maternal blood, allowing these proteins to be released into the mother’s circulation.
Approximately 65% of all human protein-coding genes are expressed in the placenta, with about 288 genes showing elevated expression compared to other tissue types. This extensive genetic expression underscores the placenta’s dynamic role and its capacity to produce proteins unique to pregnancy. These proteins are involved in various processes, from influencing maternal physiology to directly supporting fetal growth and development.
Diverse Functions of Placenta Proteins
Placental proteins serve a wide range of functions, each contributing to a healthy pregnancy.
Transporters
Some proteins act as transporters, facilitating the movement of essential substances from the mother to the fetus. For instance, the small neutral amino-acid transporter 4 (SNAT4) transports amino acids across the placenta, which are necessary for fetal development. Deficiencies in SNAT4 can lead to developmental abnormalities and a higher risk of miscarriage.
Hormones
Other placental proteins function as hormones, orchestrating significant physiological changes in both the mother and fetus. Human chorionic gonadotropin (hCG), often called the “pregnancy hormone,” is one of the first proteins produced by the placenta after implantation. It maintains the corpus luteum, which produces progesterone to sustain the uterine lining and prevent early menstruation. Human placental lactogen (hPL) regulates maternal metabolism, decreasing insulin sensitivity and increasing maternal blood glucose levels, ensuring a sufficient energy supply for the fetus.
Immune Modulation and Development
Placental proteins also modulate the maternal immune system, preventing the mother’s body from rejecting the fetus while providing protection against infections. Proteins like Placenta-specific protein 1 (PLAC1) are involved in the development and maintenance of the placenta itself. PLAC1 is expressed most between 22 and 40 weeks of pregnancy, and its absence has been linked to issues like placentomegaly and intrauterine growth restriction. Proper placental development, supported by these proteins, is essential for a successful pregnancy.
Placenta Proteins and Pregnancy Health
Abnormal levels or functions of placenta proteins can indicate or contribute to various pregnancy complications.
Pre-eclampsia
Pre-eclampsia, a condition characterized by high blood pressure and potential organ damage, is associated with an imbalance of specific placenta-derived proteins. This includes higher concentrations of soluble fms-like tyrosine kinase-1 (sFlt-1) and lower concentrations of placental growth factor (PlGF) in the mother’s circulation. This imbalance can lead to issues with blood vessel development and function, affecting both maternal and fetal health.
Fetal Growth and Gestational Diabetes
Problems with nutrient transport proteins can lead to restricted fetal growth, where the fetus does not reach its full growth potential. Placental dysfunction, which can involve these proteins, impairs the transfer of nutrients and oxygen, affecting approximately 5-10% of pregnancies. Additionally, placental hormones and proteins can influence insulin resistance in the mother, potentially contributing to gestational diabetes. The placental proteome is often altered in gestational diabetes, particularly in cases with large-for-gestational-age newborns. These alterations can impact both maternal health and fetal development.
Monitoring Pregnancy
Some placenta proteins are measured in blood tests to monitor pregnancy health and assess the risk of certain complications. For instance, hCG is widely used in pregnancy tests. While the sFlt-1/PlGF ratio has been studied for its potential in diagnosing pre-eclampsia, its use is not universally standardized for diagnosis or exclusion of the condition. These protein markers offer insights into placental function and fetal well-being throughout pregnancy. Ongoing research continues to explore the full diagnostic and prognostic potential of these and other placental protein markers.