Platelets, inflammatory cells, von Willebrand factor, syndecan-1, fibrin, fibronectin, and bacteria co-localize in the liver thrombi of Bacillus anthracis-infected mice

Syndecan-1: a key player in health and disease

Syndecan-1 (SDC-1) is a transmembrane protein localized on the basolateral surface of epithelial cells, encompassing a core protein with heparin sulfate and chondroitin sulfate glycosaminoglycan side chains. SDC-1 is involved in a panoply of cellular mechanisms including cell-to-cell adhesion, extracellular matrix interactions, cell cycle modulation, and lipid clearance. Alterations in the expression and function of SDC-1 are implicated in numerous disease entities, making it an attractive diagnostic and therapeutic target. However, despite its broad involvement in several disease processes, the underlying mechanism contributing to its diverse functions, pathogenesis, and therapeutic uses remains underexplored. Therefore, this review examines the role of SDC-1 in health and disease, focusing on liver pathologies, inflammatory diseases, infectious diseases, and cancer, and sheds light on SDC-1-based therapeutic approaches. Moreover, it delves into the mechanisms through which SDC-1 contributes to these diseases, emphasizing cell-type specific mechanisms. By comprehensively summarizing the significance of SDC-1, its association with several diseases, and its underlying mechanisms of action, the findings of this review could inform future research directions toward the development of targeted therapies and early diagnosis for a multitude of disease entities.

Longitudinal Assessment of Plasma Syndecan-1 Predicts 60-Day Mortality in Patients with COVID-19

Background: Endotheliopathy is a common pathologic finding in patients with acute and long COVID-19. It may be associated with disease severity and predispose patients to long-term complications. Plasma levels of a proteoglycan, syndecan-1, are found to be significantly elevated in patients with COVID-19, but its roles in assessing disease severity and predicting long-term outcome are not fully understood. Methods: A total of 124 consecutive hospitalized patients with SARS-CoV-2 infection were prospectively enrolled and blood samples were collected on admission (T1), 3−4 days following treatment (T2), and 1−2 days prior to discharge or death (T3). Plasma levels of syndecan-1 were determined using an immunosorbent assay; various statistical analyses were performed to determine the association between plasma syndecan-1 levels and disease severity or the 60-day mortality rate. Results: Compared with those in the healthy controls, plasma levels of syndecan-1 in patients with critical COVID-19 were significantly higher (p < 0.0001). However, there was no statistically significant difference among patients with different disease severity (p > 0.05), resulting from large individual variability. Longitudinal analysis demonstrated that while the levels fluctuated during hospitalization in all patients, plasma syndecan-1 levels were persistently elevated from baseline in critical COVID-19 patients. Cox proportional hazard regression analyses revealed that elevated plasma levels of syndecan-1 (>260 ng/mL at T1, >1018 ng/mL at T2, and >461 ng/mL at T3) were significantly associated with the 60-day mortality rate. Conclusions: Endotheliopathy, marked by glycocalyx degradation and elevated plasma syndecan-1, occurs in nearly all hospitalized patients with SARS-CoV-2 infection; elevated plasma syndecan-1 is associated with increased mortality in COVID-19 patients.

The Association of Syndecan-1, Hypercoagulable State and Thrombosis and in Patients With Nephrotic Syndrome

The aim of this study is to investigate whether Syndecan-1 (SDC-1), an indicator of endothelial glycocalyx injury, would increase the risk of hypercoagulable state and thrombosis in patients with nephrotic syndrome (NS). The prospective study was conducted among patients undergoing renal biopsy in the Department of Nephrology in our hospital from May to September 2018. We enrolled in patients with NS as the experimental group and patients with normal serum creatinine and proteinuria less than 1 g as the control group. Patients’ characteristics including age, sex, laboratory test results and blood samples were collected for each patient. The blood samples were taken before the renal biopsy. The samples were immediately processed and frozen at −80°C for later measurement of Syndecan-1. One hundred and thirty-six patients were enrolled in the study. Patients with NS and hypercoagulability had a higher level of SDC-1 compared with control group. Patients with membranous nephropathy occupied the highest SDC-1 level (P = 0.012). Logistic regression showed that highly increased level of SDC-1 (>53.18 ng/ml) was an independent predicator for predicting hypercoagulable state. The elevated level of SDC-1 indicated that endothelial injury, combined with its role of accelerating hypercoagulable state, might be considered of vital importance in the pathophysiological progress of thrombosis formation in patients with NS.

Understanding Infection-Induced Thrombosis: Lessons Learned From Animal Models

Thrombosis is a common consequence of infection that is associated with poor patient outcome. Nevertheless, the mechanisms by which infection-associated thrombosis is induced, maintained and resolved are poorly understood, as is the contribution thrombosis makes to host control of infection and pathogen spread. The key difference between infection-associated thrombosis and thrombosis in other circumstances is a stronger inflammation-mediated component caused by the presence of the pathogen and its products. This inflammation triggers the activation of platelets, which may accompany damage to the endothelium, resulting in fibrin deposition and thrombus formation. This process is often referred to as thrombo-inflammation. Strikingly, despite its clinical importance and despite thrombi being induced to many different pathogens, it is still unclear whether the mechanisms underlying this process are conserved and how we can best understand this process. This review summarizes thrombosis in a variety of models, including single antigen models such as LPS, and infection models using viruses and bacteria. We provide a specific focus on Salmonella Typhimurium infection as a useful model to address all stages of thrombosis during infection. We highlight how this model has helped us identify how thrombosis can appear in different organs at different times and thrombi be detected for weeks after infection in one site, yet largely be resolved within 24 h in another. Furthermore, we discuss the observation that thrombi induced to Salmonella Typhimurium are largely devoid of bacteria. Finally, we discuss the value of different therapeutic approaches to target thrombosis, the potential importance of timing in their administration and the necessity to maintain normal hemostasis after treatment. Improvements in our understanding of these processes can be used to better target infection-mediated mechanisms of thrombosis.

The endothelial glycocalyx anchors von Willebrand factor fibers to the vascular endothelium

The dynamic change from a globular conformation to an elongated fiber determines the ability of von Willebrand factor (VWF) to trap platelets. Fiber formation is favored by the anchorage of VWF to the endothelial cell surface, and VWF-platelet aggregates on the endothelium contribute to inflammation, infection, and tumor progression. Although P-selectin and ανβ3-integrins may bind VWF, their precise role is unclear, and additional binding partners have been proposed. In the present study, we evaluated whether the endothelial glycocalyx anchors VWF fibers to the endothelium. Using microfluidic experiments, we showed that stabilization of the endothelial glycocalyx by chitosan oligosaccharides or overexpression of syndecan-1 (SDC-1) significantly supports the binding of VWF fibers to endothelial cells. Heparinase-mediated degradation or impaired synthesis of heparan sulfate (HS), a major component of the endothelial glycocalyx, reduces VWF fiber-dependent platelet recruitment. Molecular interaction studies using flow cytometry and live-cell fluorescence microscopy provided further evidence that VWF binds to HS linked to SDC-1. In a murine melanoma model, we found that protection of the endothelial glycocalyx through the silencing of heparanase increases the number of VWF fibers attached to the wall of tumor blood vessels. In conclusion, we identified HS chains as a relevant binding factor for VWF fibers at the endothelial cell surface in vitro and in vivo.

Sphingosine-1-phosphate in Endothelial Cell Recellularization Improves Patency and Endothelialization of Decellularized Vascular Grafts In Vivo

Background: S1P has been shown to improve the endothelialization of decellularized vascular grafts in vitro. Here, we evaluated the potential of tissue-engineered vascular grafts (TEVGs) constructed by ECs and S1P on decellularized vascular scaffolds in a rat model. Methods: Rat aorta was decellularized mainly by 0.1% SDS and characterized by histology. Rat ECs, were seeded onto decellularized scaffolds, and the viability of the ECs was evaluated by biochemical assays. Then, we investigated the in vivo patency rate and endothelialization for five groups of decellularized vascular grafts (each n = 6) in a rat abdominal aorta model for 14 days. The five groups included (1) rat allogenic aorta (RAA); (2) decellularized RAA (DRAA); (3) DRAA with S1P (DRAA/S1P); (4) DRAA with EC recellularization (DRAA/EC); and (5) DRAA with S1P and EC recellularization (DRAA/EC/S1P). Results: In vitro, ECs were identified by the uptake of Dil-Ac-LDL. S1P enhanced the expression of syndecan-1 on ECs and supported the proliferation of ECs on decellularized vascular grafts. In vivo, RAA and DRAA/EC/S1P both had 100% patency without thrombus formation within 14 days. Better endothelialization, more wall structure maintenance and less inflammation were noted in the DRAA/EC/S1P group. In contrast, there was thrombus formation in the DRAA, DRAA/S1P and DRAA/EC groups. Conclusion: S1P could inhibit thrombus formation to improve the patency rate of EC-covered decellularized vascular grafts in vivo and may play an important role in the construction of TEVGs.

Sphingosine-1-phosphate improves endothelialization with reduction of thrombosis in recellularized human umbilical vein graft by inhibiting syndecan-1 shedding in vitro

Sphingosine-1-phosphate (S1P) has been known to promote endothelial cell (EC) proliferation and protect Syndecan-1 (SDC1) from shedding, thereby maintaining this antithrombotic signal. In the present study, we investigated the effect of S1P in the construction of a functional tissue-engineered blood vessel by using human endothelial cells and decellularized human umbilical vein (DHUV) scaffolds. Both human umbilical vein endothelial cells (HUVEC) and human cord blood derived endothelial progenitor cells (EPC) were seeded onto the scaffold with or without the S1P treatment. The efficacy of re-cellularization was determined by using the fluorescent marker CellTracker CMFDA and anti-CD31 immunostaining. The antithrombotic effect of S1P was examined by the anti-aggregation tests measuring platelet adherence and clotting time. Finally, we altered the expression of SDC1, a major glycocalyx protein on the endothelial cell surface, using MMP-7 digestion to explore its role using platelet adhesion tests in vitro. The result showed that S1P enhanced the attachment of HUVEC and EPC. Based on the anti-aggregation tests, S1P-treated HUVEC recellularized vessels when grafted showed reduced thrombus formation compared to controls. Our results also identified reduced SDC1 shedding from HUVEC responsible for inhibition of platelet adherence. However, no significant antithrombogenic effect of S1P was observed on EPC. In conclusion, S1P is an effective agent capable of decreasing thrombotic risk in engineered blood vessel grafts.

STATEMENT OF SIGNIFICANCE

Sphingosine-1phosphate (S1P) is a low molecular-weight phospholipid mediator that regulates diverse biological activities of endothelial cell, including survival, proliferation, cell barrier integrity, and also influences the development of the vascular system. Based on these characters, we the first time to use it as an additive during the process of a small caliber blood vessel construction by decellularized human umbilical vein and endothelial cell/endothelial progenitor. We further explored the function and mechanism of S1P in promoting revascularization and protection against thrombosis in this tissue engineered vascular grafts. The results showed that S1P could not only accelerate the generation but also reduce thrombus formation of small caliber blood vessel.

Platelets and infection

The primary function of platelets is to patrol the vasculature and seal vessel breaches to limit blood loss. However, it is becoming increasingly clear that they also contribute to pathophysiological conditions like thrombosis, atherosclerosis, stroke and infection. Severe sepsis is a devastating disease that claims hundreds of thousands of lives every year in North America and is a major burden to the public health system. Platelet surface receptors like GPIb, αIIbβ3, TLR2 and TLR4 are involved in direct platelet-bacteria interactions. Plasma proteins like fibrinogen and vWF enable indirect interactions. Furthermore, platelet granules contain a plethora of proteins that modulate the immune response as well as microbicidal agents which can directly lyse bacteria. Bacterial toxins are potent platelet activators and can cause intravascular platelet aggregation. Platelets contribute to the antibacterial response of the host involving Kupffer cells, neutrophils and the complement system. In this review we summarize the current knowledge about platelet-bacteria interactions and highlight recent advances in the field.

Nucleation of platelets with blood-borne pathogens on Kupffer cells precedes other innate immunity and contributes to bacterial clearance

Using intravital imaging of the liver, we unveil a collaborative role for platelets with Kupffer cells (KCs) in eradicating bloodborne bacterial infections. Under basal conditions, platelets via glycoprotein Ib (GPIb) formed transient “touch-and-go” interactions with von Willebrand factor (vWF) constitutively expressed on KCs. Bacteria, such as Bacillus cereus and Methicillin-resistant Staphylococcus aureus (MRSA), were rapidly caught by KCs and triggered platelets to switch from “touch-and-go” to sustained GPIIb-mediated adhesion on the KC surface to encase the bacterium. Infected GpIbα^−/− mice demonstrated increased endothelial and KC damage, leading to increased fluid leakage, significant polycythemia and rapid mortality. This study identifies a novel surveillance mechanism of intravascular macrophage by platelets that rapidly converts to a critical host response against bloodborne bacteria.

Activated protein C ameliorates Bacillus anthracis lethal toxin-induced lethal pathogenesis in rats

BACKGROUND

Lethal toxin (LT) is a major virulence factor of Bacillus anthracis. Sprague Dawley rats manifest pronounced lung edema and shock after LT treatments, resulting in high mortality. The heart failure that is induced by LT has been suggested to be a principal mechanism of lung edema and mortality in rodents. Since LT-induced death occurs more rapidly in rats than in mice, suggesting that other mechanisms in addition to the heart dysfunction may be contributed to the fast progression of LT-induced pathogenesis in rats. Coagulopathy may contribute to circulatory failure and lung injury. However, the effect of LT on coagulation-induced lung dysfunction is unclear.

METHODS

To investigate the involvement of coagulopathy in LT-mediated pathogenesis, the mortality, lung histology and coagulant levels of LT-treated rats were examined. The effects of activated protein C (aPC) on LT-mediated pathogenesis were also evaluated.

RESULTS

Fibrin depositions were detected in the lungs of LT-treated rats, indicating that coagulation was activated. Increased levels of plasma D-dimer and thrombomodulin, and the ameliorative effect of aPC further suggested that the activation of coagulation-fibrinolysis pathways plays a role in LT-mediated pathogenesis in rats. Reduced mortality was associated with decreased plasma levels of D-dimer and thrombomodulin following aPC treatments in rats with LT-mediated pathogenesis.

CONCLUSIONS

These findings suggest that the activation of coagulation in lung tissue contributes to mortality in LT-mediated pathogenesis in rats. In addition, anticoagulant aPC may help to develop a feasible therapeutic strategy.