Soluble FMS-Like Tyrosine Kinase-1 (sFLT-1) and Soluble Endoglin (sEng) Induce Platelet Hyperfunction in a Rat Model of Severe Preeclampsia

Background: Placental ischemia is a key initiating event in the pathophysiology of preeclampsia (PE) that triggers the release of anti-angiogenic factors into the maternal circulation, including sFLT-1 and sEng. These placental factors cause systemic vascular dysfunction and ultimately hypertension and end-organ damage in PE. Furthermore, PE has been associated with placental thrombosis and increased risk of thrombotic events during and after pregnancy, being recognized as a hypercoagulable state. However, the mechanisms of hypercoagulability in PE remain to be fully elucidated. Previous studies have shown that, similar to patients with severe PE, chronic sFLT-1 and sEng infusion into pregnant rats promotes placental pathology, hypertension, and organ damage failure. Our study aimed to determine the contributions of platelets versus fibrin to the coagulation process in this model of severe PE. Methods: 12-14-week-old Sprague-Dawley rats were either implanted intraperitoneally with an osmotic mini-pump containing sFLT-1 and another one loaded with sEng (4.7 and 7.0 µg/kg/day, respectively) or underwent a sham surgery on gestational day (GD)12 (n=4-8/group, depending on the assay). On GD19, arterial blood was drawn into collection tubes with anticoagulant to run viscoelastic assays to measure all phases of hemostasis using the thromboelastography (TEG) 6s Hemostasis Analyzer. Citrated blood was used in global hemostasis with lysis assay to assess the relative contribution of platelets and fibrinogen to overall clot strength, whereas heparinized blood was utilized in platelet mapping assay to assess adenosine diphosphate (ADP)- as well as arachidonic acid (AA)-stimulated platelet clot strength. EDTA plasma was used in qRT-PCR to quantify cell-free mitochondrial (mt) DNA, a damage-associated molecular pattern released from hypoxic and injured tissue with platelet-activation effects. Results: sFLT-1/sEng-infused pregnant rats had decreased platelet cell count compared to sham pregnant rats (5.85±0.28 vs. 6.9 ±0.34 x 10^8 cells/mL; P=0.0274). TEG-based citrated-kaolin (CK) and heparinized-kaolin heparinase (HKH) assays, which identify the hemostatic characteristics of blood in response to thrombin and kaolin showed that maximal amplitude (MA) are similar in sFLT-1/sEng-infused and sham pregnant rats (74.30±0.66 vs. 74.50±1.55 and 75.61±0.76 vs. 76.42±1.02 mm, respectively; both p>0.05). In the platelet-mapping TEG assay, reptilase and Factor XIII (activator F; ActF) are utilized to initiate fibrin-only clot formation, allowing the relative contribution of fibrin polymerization and platelet aggregation to total MA to be distinguished. ActF MA was lower in sFLT-1/sEng-infused pregnant rats than in sham counterparts (25.42±1.53 vs. 34.27±3.85 mm; P= 0.0426). Consequently, platelet contribution to HKH MA was higher in sFLT-1/sEng-infused pregnant rats than in sham counterparts (66.17±1.98 vs. 55.49±4.50 %; P=0.0405). Heparinized-ADP and -AA assays showed similar MA values between sFLT-1/sEng-infused and sham pregnant rats (40.13±4.89 vs. 46.76±5.06 and 73.45±0.77 vs. 73.52±0.62 mm, respectively; both p>0.05). Finally, plasma mtDNA levels were increased in sFLT-1/sEng-infused pregnant rats compared to sham pregnant rats on GD19 (0.067±0.029 vs. 0.003±0.001 ng/mL; P= 0.0121). Conclusion: These findings indicate that platelets have a larger contribution to coagulation than fibrin in sFLT-1/sEng-infused pregnant rats, despite exhibiting reduced number of circulating platelets. In addition, sFLT-1/sEng induced increased circulating mtDNA levels, which could serve as one of the mechanisms leading to platelet hyperfunction in our model of severe PE. This research was supported by the National Institutes of Health (K01HL159032, K08GM138812, R01HL148191, and U54GM115428). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.