Preeclampsia is a hypertensive disease affecting 3-5% of otherwise healthy pregnancies. The pathogenesis of preeclampsia is unclear but it is known that placental toxin(s) trigger the disease. Transthyretin, a carrier protein for thyroxine/retinol, which is aggregated and cytotoxic in preeclamptic serum may be one such toxin. We undertook this study to investigate whether transthyretin production is altered in preeclamptic placentae and whether this potential “toxin” is carried into the maternal circulation by placental extracellular vesicles.
RNA and protein levels of transthyretin in preeclamptic and control placentae were investigated by qRT-PCR, immunohistochemistry and Western blotting. Macro-, micro- and nano- vesicles were harvested from cultured placentae by differential centrifugation (2000g, 20000g, 100000g). Total aggregated protein in each vesicle fraction was quantified by Proteostat® assay and the levels of monomeric/aggregated transthyretin were determined by Western blotting.
The level of transthyretin protein (n=7; p<0.011), but not transthyretin mRNA (n=8), was increased in preeclamptic placentae. Transthyretin was detected in micro- and nano- vesicles, but not macro-vesicles, from first trimester (n=5) and term placentae (n=5). The level of monomeric and aggregated transthyretin was significantly increased in nano-vesicles, but not micro-vesicles, from preeclamptic placentae compared to control placentae (n=8; p<0.0127). Nano-vesicles from preeclamptic placentae also contained higher total levels of protein aggregation (n=7; p=0.0136).
We have shown that the placenta actively “secretes” transthyretin into the maternal circulation throughout gestation via placental micro- and nano- vesicles. That the levels of transthyretin mRNA in preeclamptic placentae was unchanged, while the protein level in both the placenta and nano-vesicles was increased, suggests that post-transcriptional modifications to transthyretin are affected in preeclampsia, leading to increased transthyretin aggregation, and transport into the maternal blood via nano-vesicles. The presence of aggregated transthyretin in nano-vesicles may alter the interaction between these vesicles and maternal cells, contributing to the clinical symptoms of preeclampsia.