I-7: Maternal Signalling to the Placenta

Background: Though it is well established that maternal blood-borne signals influence highly the growth of the placenta, the mechanisms are not known. In vitro trophoblast culture models are limited by an inability to reconstruct the polarised bilayer of the human hemochorial placenta. We have used a first trimester villous tissue explant system to investigate how growth factors interact with placental cells. We have in particular been interested in how signals arriving at the microvillous membrane of the post-mitotic syncytium can stimulate proliferation in cytotrophoblast progenitor cells. Materials and Methods: Tissues were obtained at pregnancy termination (8-13 weeks). Explants were established and maintained in serum-free DMEM/F12 and siRNA knockdown achieved either in the syncytium alone or in cytotrophoblast as well. Avidin-conjugated quantum dots were loaded with biotinylated growth factor for tracking by high resolution microscopy. Explants were incubated with BrdU before fixation and immunostaining to detect cells that had passed through S phase during culture. IGFs or TGFβ were used to stimulate placental tissue. Phosphorylation pathways were detected immunochemically.Results: IGF-I and -II stimulate proliferation in cytotrophoblasts through a pathway that depends on IGFR1. Downstream activation of the activating tyrosine phosphatase SHP-2 occurs in cytotrophoblast but not in the syncytium, leading to MAP kinase-dependent proliferation and PI3 kinase-dependent protection from apoptosis. IGF-conjugated quantum dots were taken up by syncytiotrophoblast and delivered to the underlying cytotrophoblast. ERK phosphorylation occurs in syncytium in response to IGF, indicating the existence of an independent signalling pathway that might be targeted directly at syncytial function. TGFβ1 stimulates cytotrophoblast proliferation via TβRI/II and not TβRV. SMAD2 is activated in both cytotrophoblast and some syncytiotrophoblast nuclei, again suggesting that multiple downstream pathways are targeted as a result of ligand binding to the syncytial surface. SMAD2 is also activated in response to another ligand, IGFBP3, but no pERK is seen and proliferation is inhibited in both the presence and absence of IGF.Conclusion: Syncytiotrophoblast plays a selective as well as a protective role, decoding and transmitting specific signals to the underlying cytotrophoblast in order to maintain or expand the progenitor cell layer. When considering treatments for fetal growth restriction, targeting signalling components downstream of receptor may be needed to discriminate between growth and differentiation effects.