Heparanase Is a Putative Mediator of Endothelial Glycocalyx Damage in COVID-19 – A Proof-of-Concept Study

Coronavirus disease 2019 (COVID-19) is a systemic disease associated with injury (thinning) of the endothelial glycocalyx (eGC), a protective layer on the vascular endothelium. The aim of this translational study was to investigate the role of the eGC-degrading enzyme heparanase (HPSE), which is known to play a central role in the destruction of the eGC in bacterial sepsis. Excess activity of HPSE in plasma from COVID-19 patients correlated with several markers of eGC damage and perfused boundary region (PBR, an inverse estimate of glycocalyx dimensions of vessels with a diameter 4-25 µm). In a series of translational experiments, we demonstrate that the changes in eGC thickness of cultured cells exposed to COVID-19 serum correlated closely with HPSE activity in concordant plasma samples (R = 0.82, P = 0.003). Inhibition of HPSE by a nonanticoagulant heparin fragment prevented eGC injury in response to COVID-19 serum, as shown by atomic force microscopy and immunofluorescence imaging. Our results suggest that the protective effect of heparin in COVID-19 may be due to an eGC-protective off-target effect.

The endothelial glycocalyx (eGC) is a delicate gel-like layer coating the luminal surface of the vascular endothelium (5,6). It is up to 3 µm thick, largely consists of highly sulfated glycosaminoglycans and proteoglycans, and it plays a pivotal role in the maintenance of microcirculatory homeostasis (7,8). Specifically, the eGC acts as a negatively charged "firewall" to reduce leukocyte-endothelialinteractions (9). Its carbohydrate-rich matrix provides resistance to water permeability and contributes to the proportion of albumin molecules "reflected" back into plasma by the vessel wall (10,11). Beyond that, the glycocalyx contributes to the regulation of the redox state and is crucially involved in the mediation of shear-induced nitric oxide release as well as physiologic anticoagulation (8,12,13).
At least in bacterial sepsis, the final common pathway of eGC damage appears to be remarkably consistent: the activation and release of the heparan sulfate (HS)-degrading enzyme heparanase (HPSE) (14,15). Besides cleaving HS from the cell surface, HPSE also enhances shedding of transmembrane HS proteoglycan syndecan-1 (Syn-1) by upregulating the expression of matrix metalloproteinase 9 (MMP9), which is a syndecan sheddase. Therefore, HPSE (in)directly contributes to increased HS and Syn-1 plasma levels, which are both markers of endothelial glycocalyx degradation. The aim of this translational proof-ofconcept study was to investigate the role of HPSE in COVID-19induced eGC damage.

Study Design and Study Population
Sublingual video microscopy and blood sampling were performed in 16 adult PCR-confirmed COVID-19 patients, prospectively enrolled from May to June 2020 at the intensive care units (ICU) of the University Hospital Münster and three local teaching hospitals in a non-consecutive manner. Plasma samples were obtained, centrifuged, and stored at -80°C until analysis. Twelve apparently healthy, age-matched volunteers served as controls ( Table 1). Some of the participants were already included in a previous study (4). The study was performed in accordance with the Declaration of Helsinki and approved by the Ethics Committee of the General Medical Council Westfalen-Lippe and the WWU Münster, Germany (file number: amendment to 2016-073-f-S). Written informed consent to participate has been obtained from all individuals.

In Vivo Assessment of Sublingual Glycocalyx Dimensions
Real-time intravital microscopy was performed at the bedside with a sidestream dark field (SDF) camera (CapiScope HVCS, KK Technology, Honiton, UK) to visualize the sublingual microvasculature (microvessel diameter 4-25 µm) as reported previously in detail (16,17). In brief, the SDF camera uses green light emitting stroboscopic diodes (540nm) to detect the hemoglobin of passing red blood cells (RBCs). Image acquisition and analysis was performed by GlycoCheck ™ Software (Microvascular Health Solutions Inc., Salt Lake City, UT, USA). It detects the dynamic lateral RBC movement into the glycocalyx, which is expressed as the perfused boundary region (PBR, in µm). An altered or degraded glycocalyx allows more RBCs to penetrate deeply toward the endothelial surface, with a consequent increase in the PBR. In every patient, we performed two complete measurements which were averaged to account for spatial heterogeneity of the sublingual microvasculature.

HPSE Activity and HS Competition Assay
The activity of HPSE in EDTA plasma was determined by a heparanase activity assay (Amsbio, Abingdon, UK, Cat# Ra001-02-K) according to the manufacturer's instructions but using an in-house developed HPSE buffer (18). The dose dependent inhibition of HPSE with HS isolated from bovine kidney was assessed with an in-house developed HPSE activity assay [11]. HS in EDTA plasma samples was quantified by a previously described HS competition assay. Importantly, this assay is specific to HS, therefore the measurement is not affected by the presence of heparin (18).

Statistical Analysis
Values are presented as absolute values with median and interquartile range (IQR

COVID-19 patients at the ICU had a median (IQR) Sequential
Organ Failure Assessment (SOFA) score of 9 (5 -15), were predominantly intubated (14/16, 88%) and showed an in-hospital mortality of 38% (6/16). Compared to healthy subjects, COVID-19 patients showed a significantly higher HPSE activity in plasma ( Figure 1A). Accordingly, increased levels of HS (the main substrate of HPSE), HA and Syn-1 demonstrated eGC shedding in COVID-19 ( Figure 1B, Table 1). This finding was further confirmed by high PBR values (i.e., a thin eGC) measured in COVID-19 patients ( Figure 1C and To further validate our findings, we exposed endothelial cells (ECs) to randomly selected sterile-filtered sera (5%; diluted in  (Figure 2A), indicating that the eGC damage seen in COVID-19 can be quantitatively reproduced in cultured ECs. Delta changes of eGC thickness measured by AFM also correlated closely with the respective plasma HPSE activity in the samples (R = 0.82, P = 0.003) ( Figure 2B). Competitive inhibition of heparanase by the nonanticoagulant heparin fragment NAH completely prevented the decline in eGC thickness in response to COVID-19 serum ( Figure 2C). Similarly, immunofluorescence imaging of the eGC showed a marked decrease in HS positivity and surface coverage in ECs incubated with COVID-19 serum, whereas COVID-19 serum supplemented with NAH caused only a very slight loss in staining intensity and coverage compared to incubation with serum from healthy controls ( Figures 2D, E).
To understand this finding in more detail, we attempted to roughly compensate for the presence of heparin [which dose dependently blocks HPSE (18)] by using activated partial thromboplastin time (aPTT) as an adjustment. The aPTT was significantly prolonged in COVID-19 ( Table 1) and correlated positively with HPSE activity (R = 0.58, P < 0.001), suggesting that the true HPSE activity in severely ill COVID-19 patients could be even higher than the values actually measured. Adjusting HPSE activity for aPTT (normalized HPSE/ normalized aPTT quotient) indeed revealed a moderate association with the PBR (R = 0.44, P = 0.025).
Finally, we investigated the relationship between HPSE and HS in more detail. Interestingly, separate analysis of the two groups revealed contrasting regression slopes, suggesting that excessive generation of HS fragments in severe COVID-19 (which was not present in controls) may have partially blocked HPSE activity in COVID-19 patients ( Figure 3A). Additional in vitro inhibition experiments with HS isolated from bovine kidney confirmed a dose dependent inhibition of HPSE at concentrations between 0 and 6.25 µg/ml; nearly full inhibition of heparanase was reached at a concentration of 6.25 µg/ml HS ( Figure 3B).

DISCUSSION
Since the beginning of the COVID-19 pandemic, evidence has emerged that COVID-19 is a vascular rather than purely respiratory illness (1). Here we show that HPSE, the only enzyme in mammals that degrades HS chains from HS proteoglycans of the glycocalyx, plays a key role in mediating eGC damage in COVID-19. HPSE is predominantly released by vascular endothelium, macrophages, and platelets. Its expression is up-regulated in endothelial cells by several factors, such as reactive oxygen species, inflammatory cytokines, high glucose, and advanced glycosylation products (22)(23)(24)(25). Two previous COVID-19 studies measured HPSE antigen levels (26,27) and two others determined enzymatic HPSE activity (18,28). All but one study reported significantly elevated HPSE levels in COVID-19 patients. This finding therefore fits well with our previous data showing that sublingual eGC thickness (PBR) appears to be an appropriate risk marker for hospital mortality in COVID-19 (4). The in vitro experiments in our current study show that HPSE is a critical factor for eGC damage, as glycocalyx thinning correlated with HPSE activity and could be completely prevented by HPSE inhibition. The latter finding is even more surprising because the protective effect was achieved despite the presence of numerous cytokines and mediators in the patients' sera ( Table 1). This makes HPSE a promising target for intervention in COVID-19.
The possible involvement of HPSE in COVID-19 pathology was predicted already at the beginning of the pandemic long before initial studies on the subject (22,29). It was speculated at the time that HPSE might contribute to the removal of HS chains from cell surfaces, facilitating virus release from host cells -a mechanism that has been well described for herpes simplex virus-1 (30,31). Accordingly, enzymatic removal of HS from the eGC by HPSE is probably not the only underlying mechanism. For example, HPSE can induce matrix metalloproteases, which in turn cut transmembrane heparan sulfate proteoglycans such as syndecan-1 and thus further enhance eGC damage (31). Consistent with this assessment, our patients showed markedly elevated blood levels of hyaluronan and syndecan-1. The finding that HPSE activity in healthy individuals and COVID-19 patients was inversely correlated with HS concentration suggests that a minimum amount of HPSE is required before systemic eGC degradation begins. If so, HPSE would be more likely to be a late marker of disease severity. However, to disrupt the interaction of Sars-CoV-2 with the host cell, early blockade of HPSE could be beneficial.
In bacterial sepsis, the potential of HPSE and eGC as future pharmacological targets has already been highlighted (14,15,32). In detail, prevention of eGC damage by inhibition of HPSE significantly abolished vascular hyperpermeability and subsequent lung injury in murine endotoxemia (12). Using AFM we could show earlier, that unfractionated heparin (UFH), which also saturates HPSE, completely abolished the HPSE-induced decline of eGC thickness in freshly isolated rat aorta (20). Furthermore, the heparin fragment NAH prevented thinning of the eGC on human ECs induced by serum from sepsis patients in vitro (21). Based on these data, it would be very reasonable to assume that blocking HPSE may also be an effective approach to protecting eGC in COVID-19. Indeed, by western blotting of EC lysates, Potje et al. showed that low molecular weight heparin (LMWH) reduced the loss of glycocalyx components induced by plasma from COVID-19 patients (26). Our translational study comparing, for the first time, HPSE activity with the corresponding glycocalyx dimensions in vivo and in vitro in matched pairs strongly confirms and extends this exciting finding.
At the clinical level, the administration of heparin and/or LMWH has been shown to provide a significant survival benefit in both entities (33)(34)(35). Hence, apart from the anticoagulant effect of heparin the non-anticoagulant and, among these, especially anti-inflammatory properties should not be neglected and deserve closer attention (36). It is conceivable that this protective effect could be partly due to an off-target effect of heparini.e., the inhibition of HPSE. Further studies, ideally with accompanying HPSE measurement and sublingual video microscopy, are needed to confirm this assumption. However, the paradoxical decrease in HPSE activity in the COVID-19 patients with the highest HS concentrations certainly complicates any interpretation of HPSE activity in plasma samples. It is interesting to speculate that this could be a feedback loop whose aim is to keep HPSE activity within a certain range and to dampen its deleterious effects on the eGC. However, our data emphasize the importance of analyzing the HPSE in relation to the HS (and also to UFH/LMWH) in future studies. Of note, the investigation of heparins or heparin mimetics in COVID-19 appears particularly promising as these drugs hold the potential to counteract disease onset and progression in various ways. Not only the inhibition of endothelial HPSE, as presented in the current study, and HPSE of viral origin, as mentioned above, emerge as relevant target structures for heparins. Recent studies revealed HS/heparinrelated binding sites on the SARS-CoV-2 spike protein and cobinding of HS and angiotensin-converting enzyme 2 seems to be necessary for docking to cellular surfaces. Heparins, on the other hand, were capable to potently block spike protein binding, probably due to blocking these specific binding sites (37). Consequently, not only heparins but also heparan sulfate mimeticsso far especially investigated in context of cancer therapycome to interest as potential drugs in future COVID-19 treatment studies (38). As our study is primarily hypothesis-generating, some limitations should be noted. First, the sample size of this proof-of-concept study was rather small. However, our dataset includes a wide range of glycocalyx and inflammatory markers, as well as a novel microscopy method to calculate the PBR in vivo. Although the PBR is not measured directly but estimated based on the radial displacement of red blood cells, this validated method is highly reliable (14,16,17,39). Differences in PBR between patients and controls matched well with values obtained by Stahl et al. in another COVID-19 ICU cohort (27). Second, the use of aPTT as a measure of normalization of HPSE activity is certainly subject to its own variance. It would have been better to use either an inhibitor constant of heparin/LMWH or anti-Xa activity for correction to derive the active HPSE level. As we do not have citrate retain samples, a more accurate correction for heparin/LMWH effect was not possible in this data set. Unfortunately, the type, dose and application route of the heparins used were very heterogeneous, so that we were unable to convert these data into meaningful equivalent concentrations. Third, the number of serum samples used for the in vitro studies is limited because the AFM technique is sophisticated and time-consuming.
In conclusion, our translational study indicates that HPSE is a putative mediator of endothelial glycocalyx damage in COVID- 19. Further studies are needed to clarify whether the benefit of heparin administration in COVID-19 is due to an eGCprotective off-target effect.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by Ethics Committee of the General Medical Council Westfalen-Lippe and the WWU Münster, Germany. The patients/participants provided their written informed consent to participate in this study.

AUTHOR CONTRIBUTIONS
CD designed and conducted the in vitro experiments, analyzed the data, prepared the figures and contributed to the manuscript; AR and IO enrolled patients, performed sublingual video microscopy, performed ELISA assays and contributed to the manuscript; MR performed AFM experiments and revised the manuscript; BB, RS, and JV performed the HPSE activity and HS competition assays and contributed to the manuscript; AL, HP, and WL advised on the study design, discussed the findings and revised the manuscript; PK had the initial idea, supervised the study, recruited and coordinated participating centers, contributed to the analysis and figures, and drafted the manuscript. All authors contributed to the article and approved the submitted version.

FUNDING
This work was supported by the German Research Foundation (rotational position of KFO 342 -ZA428/18-1 to CD and AR; KU 2873/3-1 to PK) and the fund Innovative Medical Research of the University of Münster Medical School (I-RO221907 to AR). The funding sources had no role in study design, data collection and analysis, the decision to publish, or preparation of the manuscript.

ACKNOWLEDGMENTS
We thank Jan Sackarnd, Phil-Robin Tepasse, Stephan Braune, Ulrich Göbel, Gerold Thölking and Andreas Gröschel for their help in recruiting patients, Manfred Fobker for assistance in sample processing and Hans Vink for assistance with regard to the Glycocheck ® system. We would like to acknowledge the support of the Open Access Publication Fund of the University of Münster.