Thyroid hormones (TH) are vital for the developing human fetus; either too much or too little results in severe impairment of neurological development. As the fetal thyroid gland does not produce TH until mid-gestation, normal fetal development relies on transfer of adequate amounts of TH from the maternal circulation. There is increasing clinical evidence that even mild maternal hypothyroidism in early pregnancy adversely affects the child’s intellectual development. There is also strong clinical evidence that TH transfer continues late in gestation, at least in cases of fetal thyroid gland development failure.
The maternal and fetal circulations are separated in placenta by a layer of trophoblast cells, which abundantly express type III deiodinase (D3). This enzyme is responsible for deiodination of T4 to the inactive reverse tri-iodothyronine (rT3). In vitro perfusion studies of human placenta demonstrate that very little T4 in the maternal circuit escapes deiodination to rT3, suggesting this is an important regulatory step in the materno-fetal transport of TH. Importantly however, in the case of hypothyroid fetuses, TH transfer occurs without reduced D3 expression. This suggests that additional and as yet unidentified factor(s) exist in the placental TH economy that allows T4 to avoid D3 inactivation.
TH binding proteins are an integral part of the TH economy with only unbound TH considered active. Both serum TH binding proteins and cytosolic TH binding proteins transport TH through aqueous environments. These proteins provide a large, temporarily inactive pool of transportable TH. Their synthesis by placenta has not previously been examined. It is proposed that there are placental TH binding proteins, and that binding to these proteins may protect TH from deiodination and allow it to reach the fetal circulation. The aim of this thesis therefore is to examine the placenta for TH binding proteins and characterise their role in the trophoblast TH economy.
TH binding proteins in the placental cytosol were isolated, and identified using a I-T3 binding assay, a series of chromatography columns, Bis/Tris gels and MALDI-TOF MS. These experiments failed to identify a novel cytosolic TH binding protein, but did identify the extra-cellular distributing proteins, thyroxine binding globulin (TBG) transthyretin (TTR) and albumin. Subsequent analysis confirmed that trophoblasts cells synthesised both TTR and albumin. Although the identification of albumin synthesis by the placenta is a novel finding, the remainder of this thesis focussed on TTR and its role in the TH economy of the placenta. TTR is synthesised and secreted as a complex with retinol binding protein (RBP) by the liver, choroid plexus (CP) and retinal pigment epithelia, and is involved in the TH transport in these epithelial tissues. Apically secreted trophoblast TTR would increase the local concentration at the maternal interface, whereas basal secretion would increase the fetal concentration, both of which would have important implications for the TH economy of the mother and fetus. Using both placental explants and JEG-3 choriocarcinoma cells it was established that TTR is secreted from placenta and that this secretion, at least in transfected JEG-3 cells, is predominantly apical and independent of RBP. As only free T4 is believed to enter cells an increase in local concentration of TTR in the maternal circulation will reduce the free T4 concentration at the maternal interface. Recently the internalisation into target cells of hormone and vitamins bound to their carrier proteins has received increasing attention. Internalisation of TTR has been confirmed in several tissues. In the proximal tubules of the kidney it occurs through a megalin-mediated process, which is enhanced when T4 is bound to TTR. Megalin is expressed at the maternal membrane of trophoblast cells. Results from the final chapter of this thesis confirm internalisation of TTR in both placenta and JEG-3 cells, and demonstrates that in JEG-3 cells this occurs through a megalin-mediated process. In summary this thesis has contributed to knowledge of the TH economy of the placenta and possible modes of transplacental transfer of T4 by identifying the synthesis of TTR and albumin by trophoblast cells, the subsequent secretion of TTR into maternal and fetal spaces and reuptake at the apical membrane via a megalin-mediated process. This provides an alternative route of entry for TH into trophoblasts that may avoid deiodination via D3.