Thrombin-cleaved syndecan-3/-4 ectodomain fragments mediate endothelial barrier dysfunction

The endothelial glycocalyx is a critical barrier structure that protects endothelial cells from harmful interactions with circulating cells and mediators. When this layer degrades, endothelial integrity is compromised, potentially leading to increased permeability. Syndecan proteins are essential components of the glycocalyx and are shed during pathological conditions involving elevated protease activity, such as thrombin. However, the specific role of thrombin-cleaved syndecan fragments in endothelial dysfunction remains unclear. Prior research has shown that syndecans contribute to glycocalyx stability and barrier function, but the direct effects of their cleavage products on endothelial permeability have not been fully explored. This gap motivated the current investigation into whether thrombin-generated syndecan fragments can directly alter endothelial barrier function. The study aimed to determine if these fragments could act as permeability mediators, independent of other known signaling pathways. Understanding this mechanism could provide insights into vascular dysfunction in diseases like sepsis or thrombosis. The current work builds on existing knowledge of glycocalyx structure and protease activity in endothelial injury. This paper addresses a specific knowledge gap regarding the functional consequences of syndecan cleavage by thrombin.

This study aimed to investigate whether thrombin can cleave syndecan-3 and syndecan-4 ectodomains and whether the resulting fragments can directly affect endothelial barrier function. The specific problem addressed is the lack of understanding about how syndecan cleavage products contribute to vascular permeability. The motivation for this study stems from the observation that syndecan shedding occurs in disease states with elevated thrombin activity, such as sepsis and thrombotic disorders. The authors sought to determine if thrombin-generated syndecan fragments could act as direct mediators of endothelial barrier dysfunction. The study also aimed to assess whether these effects are independent of known signaling pathways like glycosylation or thrombin receptor PAR1. By focusing on syndecan-3 and -4, the research targeted specific glycocalyx components that are known to be shed during pathological conditions. The investigation sought to clarify the role of these fragments in promoting paracellular permeability. This work contributes to the broader understanding of vascular dysfunction mechanisms in thrombin-activated diseases.

The study used transmission electron microscopy to visualize the glycocalyx in human lung microvasculature. The researchers confirmed the presence of all syndecan subtypes on the endothelial surface of agarose-inflated human lungs. ELISA and western blot analysis were employed to detect thrombin-induced cleavage of syndecan-3 and -4 ectodomains. In vivo experiments involved administering syndecan-3 ectodomain fragments to mouse lungs and measuring albumin-bound Evans blue extravasation as an indicator of plasma protein leakage. Transendothelial electrical resistance was used to assess barrier integrity in cultured endothelial cells. The effects of syndecan fragments were tested in the presence of a Rho kinase inhibitor to determine pathway dependence. The study also examined VE-cadherin-based adherens junctions and F-actin stress fibers using fluorescent imaging techniques. These methods allowed the researchers to evaluate the direct impact of syndecan fragments on endothelial permeability and junctional integrity.

The study found that thrombin can cleave syndecan-3 and -4 ectodomains, generating fragments that directly affect endothelial barrier function. Administration of syndecan-3 ectodomain fragments in mouse lungs increased albumin-bound Evans blue extravasation, indicating plasma protein leakage. Transendothelial electrical resistance was significantly reduced in endothelial cells exposed to these fragments. This reduction was sensitive to a Rho kinase inhibitor, suggesting involvement of Rho kinase signaling. The effects of syndecan fragments were independent of glycosylation and thrombin receptor PAR1. Fluorescent imaging revealed rapid disorganization of VE-cadherin-based adherens junctions in endothelial cells. Increased F-actin stress fibers were also observed, supporting the role of syndecan fragments in promoting paracellular permeability. These findings suggest that thrombin-cleaved syndecan-3/-4 fragments directly contribute to endothelial barrier dysfunction.

The authors concluded that thrombin can cleave syndecan-3 and -4 ectodomains into fragments that interact with endothelial cells, leading to paracellular hyperpermeability. The study demonstrated that these fragments increase plasma protein leakage and reduce transendothelial electrical resistance. These effects were not mediated by glycosylation or thrombin receptor PAR1. The cleavage products caused disorganization of VE-cadherin junctions and increased F-actin stress fibers. The findings support the hypothesis that syndecan-3/-4 fragments act as direct mediators of endothelial barrier dysfunction. The study suggests that thrombin-generated syndecan fragments may play a role in vascular dysfunction during sepsis or thrombotic disease states. The results highlight the importance of syndecan cleavage in endothelial permeability regulation. These conclusions are based on experimental evidence from in vitro and in vivo models.