The role of side-population trophoblasts in normal and abnormal placentation

Abstract

The human placenta is a fetal organ that is vital for healthy growth of the baby. Adequate placentation requires the normal function of specialised epithelial cells called trophoblasts, of which there are three main populations: cytotrophoblasts, extravillous trophoblasts (EVTs) and the syncytiotrophoblast. Impaired trophoblast differentiation might result in fetal growth restriction (FGR) where babies are born dangerously small. All mature trophoblasts are derived from a putative human trophoblast stem cell (TSC). Side-population trophoblasts are a transcriptomically distinct candidate human TSC that have been isolated from first trimester placentae, but require further characterisation to demonstrate their stem cell phenotype (James et al. 2015). The aim of this thesis was to further characterise sidepopulation trophoblasts and determine their role in normal and abnormal placentation. Side-population trophoblasts have a distinct methylome compared to both cytotrophoblasts and EVTs, and this results from differences in the pattern rather than the degree of CpG methylation. First trimester side-population trophoblasts can be propagated in colonies only when grown in TSC medium, and can be differentiated into 1) ß4 integrin expressing cytotrophoblast-like cells, 2) multinucleated syncytiotrophoblast-like cells that express syncytiotrophoblast makers (syncytin-1 and human choriogonadotrophin (hCG)) and secrete hCG, and 3) HLA-G positive EVT-like cells that invade through Matrigel™. Collectively, these data suggest that side-population trophoblasts are likely to be human TSC. Side-population trophoblasts were isolated from third trimester placentae for the first time. Importantly, the proportion of side-population trophoblast isolated from late gestation and first trimester placentae was not significantly different, but the side-population in FGR placentae was significantly reduced (0.32%) compared to normal third trimester placentae (3.01%, p<0.0001). Changes in the expression of apoptotic (DOK1, KLF11, KCNQ1OT1), and cell proliferation (TMSB4X, SPAG9, KLF9 and EFNB2) genes in FGR side-population trophoblasts may explain their reduced numbers in FGR. The depletion of such a stem cell population in the placenta would likely have significant functional consequences for placental growth and trophoblast differentiation, and impair placental exchange function and thus fetal growth. Future understanding of the functional role of sidepopulation trophoblasts play in FGR may allow us to therapeutically target the root cause of placental dysfunction in FGR.