Hemostatic Activation and Glycocalyx Shedding during Pregnancy

Background: Pregnancy has been described as a hypercoagulable state and is characterized by an increased risk of thrombotic events; yet the absolute incidence is low. The glycocalyx is a network of proteoglycans and glycoproteins projecting from the endothelial cell surface that plays a key role in maintaining endothelial integrity. Syndecan-1 is a component of the endothelial glycocalyx and is substituted with heparan sulfate chains which promote its interactions with key anticoagulants such as tissue factor pathway inhibitor and antithrombin. Proteolytic release of components of the glycocalyx (i.e. shedding) has been shown to occur in response to a number of physiologic challenges. Thrombomodulin, another endothelial cell transmembrane protein, is a key component of the primary dynamic anticoagulant mechanism limiting thrombus growth and is also subject to proteolytic release. In this study, we longitudinally evaluate hemostatic activation (D-dimer), glycocalyx shedding (syndecan-1) and thrombomodulin release in healthy women who became pregnant.

Methods: Healthy nulliparous women (N=26) were prospectively enrolled in a longitudinal study at the University of Vermont Medical Center in a protocol approved by the Institutional Review Board. Study visits were conducted prior to pregnancy in the follicular phase (Visit 1), at the end of the first trimester (Visit 2), and the mid-third trimester (Visit 3). Blood was collected into citrate, and platelet poor plasma aliquots were prepared. Syndecan-1 and soluble thrombomodulin were measured on all samples by ELISA, and D-dimer was measured at Visits 1 and 3. For analyses, comparisons were made across Visits using a paired Student's t-test. Data are expressed as mean ± SD.

Results: D-dimer levels were increased (3-13 fold across all women) significantly at Visit 3 (7.6 ± 3.9 μg/mL) compared to Visit 1 (1.2 ± 0.5 μg/mL), p < 0.001. Syndecan-1 levels were elevated at Visit 2 relative to Visit 1 in 25 of the 26 women; Visit 2 levels (43.7 ± 12.7 ng/mL) were significantly higher than Visit 1 (25.6 ± 18.0 ng/mL), p < 0.001. At Visit 3, an average 20-fold increase (range of 2.7 - 52.9-fold) in syndecan-1 levels (Visit 3, 518.5 ± 248.5 ng/mL) compared to Visit 1, p < 0.001 was observed. Soluble thrombomodulin did not change between Visit 1 (2.93 ± 0.5 μg/mL) and Visit 2 (2.88 ± 0.6 μg/mL); however there was a significant increase by Visit 3 (3.59 ± 0.7 μg/mL), p<0.001. Correlation analyses relating syndecan-1 and D-dimer levels and syndecan-1 and soluble thrombomodulin levels at each visit or relating fold changes in these markers between Visit 1 and Visit 3 showed no significant associations.

Conclusions: By Visit 3 (the third trimester) women show significant increases in hemostatic activation, glycocalyx shedding and the release of another endothelial cell membrane protein. Increasing the levels of syndecan-1 in circulation may have a heparin-like effect on antithrombin activity while increasing soluble thrombomodulin levels may increase systemic protein C activation. However, the release of two potent anticoagulants from the endothelial surface can reduce the ability of the endothelial surface to help mediate and restrict thrombus growth at the site of injury. Future studies are needed to address whether there may be a prothrombotic contribution from the degraded endothelial surface that is offset by the release of anticoagulants into the circulatory system.