Saving placental thrombomodulin

In this issue of Blood, Kohli et al¹  report a noncoagulation role of thrombomodulin (TM) expressed on trophoblast cells in maintaining placental growth and healthy embryogenesis. They show that (1) the inflammatory cytokine interleukin-1β (IL-1β) suppressed TM synthesis from trophoblast stem cells in culture and induced the ectodomain shedding of TM from these cells; (2) the TM shedding was also induced in pregnant C57BL/6J mice infused with endothelial cell-derived extracellular vesicles (eEVs), which cause placental inflammation; (3) the IL-1β receptor antagonist anakinra prevented TM shedding in these eEV-infused pregnant mice and reduced placental abnormalities in these mice; (4) the soluble TM that resists proteolysis and oxidation (solulin) reduced TM shedding in the pregnant C57BL/6J mice infused with eEVs and prevented fetal death, intrauterine growth restriction, placental inflammation, and growth suppression; and (5) the protection offered by solulin was reproduced in transgenic mice with enhanced expression of TM in embryonic tissue, including trophoblasts. The key findings from the pregnant mice were reproduced in well-controlled in vitro experiments using trophoblastic stem cells in culture and further validated by studying placentas collected from patients with preeclampsia.

TM (CD141) is a type 1 transmembrane glycoprotein protein that is expressed on endothelial cells.²  It forms a complex with thrombin in a 1:1 molar ratio to drastically accelerate protein C activation. TM is also expressed on trophoblasts and their differentiated offspring syncytiotrophoblasts,³  which cover the placental villi exposed to maternal blood. The TM expression on trophoblasts and syncytiotrophoblasts increases with gestational age.⁴ 

This study is important for several reasons. First, TM has long been established as a key anticoagulant on the surface of endothelial cells, but its function on trophoblasts remained ambiguous. This study provides evidence that trophoblast-expressed TM is crucial for placental development and fetal survival. It extends previous findings, by the same group of investigators, that deletion of the TM gene THBD causes embryonic death at day 8.5 post coitum,⁵  and that death is prevented by reconstituting TM expression in extraembryonic tissue.⁶  The lethality is caused by severe consumptive coagulopathy induced by endothelial TM deficiency and by developmental defects of placenta due to the growth arrest of trophoblast cells.

Second, this study demonstrates that inflammasome activation does not directly cause embryonic death in TM-deficient mice, but does so indirectly by inducing TM shedding from the surface of trophoblast cells to impair the proliferation of these cells and placental development. This finding adds a new dimension to the pathogenesis of preeclampsia, which is characterized by poor placentation and endothelial dysfunction and manifests clinically as new onset hypertension and proteinuria during late pregnancy. Preeclampsia is caused or initiated by placental factors, as it resolves rapidly or improves significantly after delivery or pregnancy termination. Among these placental factors, placental inflammation is widely recognized as a key causal factor for preeclampsia. This study identifies TM shedding from trophoblasts as an intermediate process between placental inflammation and preeclampsia.

Third, the study reaffirms the critical interplay between extracellular vesicles and platelets in causing placental inflammation.⁷  This finding is consistent with multiple and paradoxical activities of extracellular vesicles from diverse parental cells during normal pregnancy and in the pathogenesis of preeclampsia. For example, placental extracellular vesicles released during normal pregnancy are necessary for inducing maternal adaptive changes such as tolerance, but excessive shedding of these placental vesicles often indicates placental pathologies and systemic changes associated with preeclampsia. Pregnant and nonpregnant mice develop hypertension and kidney injuries rapidly after they are infused with syncytiotrophoblast-derived extracellular vesicles.⁸ 

The study also raises several important questions.¹  How does TM shedding cause the growth arrest of placenta? TM has a short cytoplasmic tail that contains potential phosphorylation sites, suggesting that it may induce intracellular signaling, but the nature and function of TM-induced signaling remain largely unknown. Alternatively, trophoblasts also express endothelial protein C receptor that is as efficient as that on endothelial cells.⁹  It is therefore possible that TM shedding impairs the trophoblastic TM-protein C system, resulting in coagulation and fibrin deposition on the surface of syncytiotrophoblast at the fetal-maternal interface to inflict cellular injuries. This is important to investigate because women with normal pregnancies often develop a hypercoagulable state that worsens significantly in patients with preeclampsia.²  This study shows that solulin prevents inflammation-induced TM shedding from trophoblasts and preeclampsia-like pathology in pregnant mice infused with eEVs. However, the study does not provide information as to whether solulin prevents TM shedding or is anchored to trophoblasts to replace shed TM. If the former, does solulin block the enzyme that sheds TM? If the latter, the question becomes why soluble TM shed from trophoblasts or endothelial cells, which has been found in the plasma of patients with preeclampsia,¹⁰  lacks the same activity. Is solulin active because of its resistance to oxidative stress, which is common in patients with preeclampsia and could render intrinsic soluble TM inactive? Alternatively, trophoblast cells may shed TM through microvesiculation,³  and extracellular vesicle-bound TM may be inactive. The study shows that transgenic mice with enhanced expression of TM in embryonic tissue did not develop the extracellular vesicle–induced phenotype of preeclampsia. Does the TM expressed on the embryonic tissue of the transgenic mice resist shedding or slow the kinetics of shedding? The answers to these questions could help identify the role of TM in the placental development and pathogenesis of pregnancy-associated vascular disorders.