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

Abstract

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.

Graphical abstract

Figure 1.

Binding of platelets to endothelial-released VWF. (A) Snapshot taken during a representative microfluidic experiment. HUVECs were perfused at a constant shear stress of 6 dyne/cm². VWF release was induced by 50 µM of histamine, and adhering fluorescent platelets (false color-coded in white) were tracked by fluorescence microscopy. VWF fibers (green) were stained with a FITC-conjugated antibody during the experiment. Scale bar, 100 µm. (B) VWF-mediated binding of platelets (white) to the endothelium. The endothelial gylcocalyx was manipulated through treatment with heparinase-I (hep’nase), COS, or both compounds (COS + hep’nase). Only histamine-treated HUVECs were used as control (CTL). Scale bar, 100 µm. (C) Quantitative evaluation of the VWF-dependent platelet coverage (n = 3-9). *P ≤ .05, **P ≤ .01 (1-way analysis of variance and Bonferroni post hoc test). rel., relative.

Figure 2.

COS-induced modification of the endothelial glycocalyx promotes VWF binding. (A) Surfaces of HUVECs, treated with COSs (2µg/mL) or untreated (CTL), imaged by AFM. Height images provide 3-dimensional topography of the cellular surface. The height is false color coded. Black corresponds to a height of 0 µm and white to a height of 2 µm. Overview images are further magnified as indicated by square regions (i-ii). Corresponding lateral deflection images show the interaction between the scanning tip and the cellular surface in the depicted areas (i-ii). Further magnification of the lateral deflection images indicate an increased lateral deflection (white circle) on HUVECs treated with COSs in comparison with nontreated cells (CTL) (iii-iv). Scale bars, 1 µm. (B) Fluorescence microscopic analysis of the endothelial glycocalyx (red) and anchored VWF fibers (green). Cell nuclei are labeled in blue. The endothelial glycocalyx staining was more pronounced in COS-treated HUVECs. The lack of the endothelial glycocalyx staining beneath VWF fibers, which were in close proximity to the HUVEC surface, indicates an occupation of the HS chains by VWF. Scale bars, 10 µm. (C) Quantitative evaluations of VWF fluorescence staining of at least 10 fields of view of 3 independent experiments. **P ≤ .01 (Student t test).

Figure 3.

Coexpression of SDC-1 and VWF in melanoma tissue. (A) Representative overview image of a tumor tissue section comprising the primary melanoma and adjacent peritumoral skin. The image was assembled from 156 single immune fluorescence images. Scale bar, 2 mm. The white box marks a skin blood vessel adjacent to the melanoma tissue. The corresponding magnification is depicted in panel B. VWF is shown in green, SDC-1 in red, and cell nuclei in blue. Scale bar, 100 µm. (C) Colocalization of SDC-1 and VWF was quantified in 1649 blood vessels. The dashed line indicates the linear correlation between the expression of VWF and SDC-1. R² = 0.87; P < .0001 (Spearman’s rank correlation). (D) Transcription analysis confirms correlation of SDC-1 and VWF expression in primary tumors of 479 patients with malignant melanoma. *P ≤ .05, **P ≤ .01 (1-way analysis of variance and Bonferroni post hoc test). mRNA, messenger RNA; RFU, relative fluorescence unit.

Figure 4.

Interaction between endothelial SDC-1 and VWF fibers. Blood vessel of tumor tissue generated by control melanoma cells (shCTL) (A) or melanoma cells (B) with silenced HPSE (shHPSE). (A-B) White boxes mark the area of the depicted magnifications shown below. Scale bars, 20 µm. SDC-1 is shown in red, VWF in green, and nuclei in blue. (C) Quantifications of the endothelial SDC-1 (eSDC-1) and the total VWF content in shCTL and shHPSE tumors (n = 4). Signal levels were expressed as mean fluorescence intensity (MFI). (D) Change of VWF fiber amount retained at the blood vessel wall in shCTL and shHPSE tumors (n = 4). Differences in signal levels were expressed as change in MFI (ΔMFI). *P ≤ .05, **P ≤ .01 (Student t test).

Figure 5.

Interaction between endothelial SDC-1 and VWF fibers in vitro. (A) Representative images of empty vector (EV)–transfected HUVECs and cells that overexpress SDC-1 (SDC-1⁺). Magnified regions (white boxes) are shown below. Scale bars, 10 µm. SDC-1 is shown in red, VWF in green, and nuclei in blue. (B) Endothelial SDC-1 promotes the VWF-mediated binding of platelets. HUVECs transfected with an EV control or with an SDC-1 vector (SDC-1⁺) were perfused at a constant shear stress of 6 dyne/cm². VWF release was induced by 50 µM of histamine, and adhering fluorescent platelets were followed by fluorescence microscopy. Where indicated, the endothelial glycocalyx was trimmed by heparinase-I (hep’nase). Scale bar, 100 µm. (C) Quantitative evaluation of the VWF-dependent platelet coverage (n = 11). **P ≤ .01 (1-way analysis and Bonferroni post hoc test).

Figure 6.

Interaction between recombinant VWF and recombinant SDC-1 exposed by HEK293 cells. (A) Fluorescence microscopic images of cells expressing human rhSDC-1 fused to the red fluorescent protein DsRed (SDC-1⁺) were incubated with or without 20 µg/mL of enhanced green fluorescent protein (eGFP)–labeled human VWF. HEK293 cells that were transfected with an EV served as a control. Scale bar, 10 µm. Inset shows the colocalization between the VWF and membranous SDC-1. (B) Flow cytometric measurement of the dose-dependent binding of VWF to SDC-1⁺ and EV cells (n = 3). (C) Binding of VWF to HEK cells in the presence of unfractionated heparin (UFH) or after manipulation of the glycocalyx with heparinase (hep’nase) and/or COS (n = 3-9). **P ≤ .01 (1-way analysis of variance and Bonferroni post hoc test).

Figure 7.

Proposed model aligning the obtained data to recent literature. VWF fibers are tethered to SDC-1 as part of the endothelial glycocalyx. Platelets are trapped by the VWF fibers and further activated in the course of coagulation (eg, through thrombin). Platelet-derived MMPs and HPSE are able to shed the endothelial glycocalyx. Soluble SDC-1 prevents further blood clotting, whereas the endothelial glycocalyx–free endothelial layer promotes the recruitment of immune cells.