The endothelial glycocalyx proves to be a crucial structure in COVID-19 pathophysiology, the degradation of which plays a central role in the vascular and systemic complications of the disease. It has been clearly demonstrated that COVID-19 leads to a significant detachment of the glycocalyx, with damage markers correlating with disease severity and progression. This damage to the glycocalyx contributes to endothelial dysfunction, increased vascular permeability, coagulation disorders and impaired myocardial function.
The mechanisms of glycocalyx damage in COVID-19 are complex and include inflammatory mediators, enzymatic degradation by sheddases, neutrophil activation with release of myeloperoxidase and oxidative stress. Conversely, an intact glycocalyx appears to protect endothelial cells from direct viral infection by shielding ACE2 receptors from the SARS-CoV-2 spike protein.
There is worrying evidence that damage to the glycocalyx persists long after the acute phase of COVID-19 and may contribute to long-term vascular dysfunction and cardiovascular complications. This persistent damage could explain some of the symptoms associated with “Long COVID” and should be further investigated.
The endothelial glycocalyx: structure and function
The endothelial glycocalyx is a complex gel-like layer of glycosylated lipid-protein mixtures. It covers the luminal surfaces of the endothelial cells that line the blood vessels, where it serves as a buffer between the circulating blood and the vessel wall. It plays a crucial role in maintaining vascular homeostasis and fulfills several essential functions under normal physiological conditions:
On the one hand, it regulates vascular permeability by acting as a selective barrier, mediating cell adhesion processes and exerting antithrombotic and anti-inflammatory effects[1]. On the other hand, this protective layer represents the first line of cellular defense against infection, with some of its molecular components also acting as receptors in intercellular interactions, including interaction with viruses[3].
Composition and structural organization
The endothelial glycocalyx consists primarily of proteoglycans (PG) with the associated glycosaminoglycan side chains (GAG) and forms a complex three-dimensional network. Proteoglycans such as syndecans and glypicans anchor to the endothelial cell membrane and extend outward, while glycosaminoglycans such as heparan sulfate, chondroitin sulfate and hyaluronic acid are bound to these core proteins[1].
Together with adsorbed molecules from the blood plasma, the glycocalyx forms the so-called endothelial surface layer[8]. This fragile barrier is dynamically maintained by a balance of continuous synthesis and detachment, which ensures its structural integrity and functionality under normal conditions[8].
Evidence of damage to the glycocalyx in COVID-19
Several studies have shown that the endothelial glycocalyx is significantly damaged in patients with COVID-19, with the extent of damage correlating with the severity of the disease. The primary evidence of glycocalyx damage is the increased concentration of glycocalyx components in the blood, indicating the detachment of this protective layer. In particular, plasma levels of syndecan-1, heparan sulphate (HS) and hyaluronan (HA) are significantly elevated in COVID-19 patients compared to healthy controls[1][5]. The higher the levels, the more severe the symptoms and the higher the mortality rate.
Direct visualization of glycocalyx damage
In addition to biomarkers in the blood, direct visualization techniques have confirmed the destruction of the glycocalyx in COVID-19. Observational studies have shown that a thinner vascular endothelial glycocalyx correlates with a significant reduction in vascular density and reduced red blood cell velocity. This is also associated with increased disease severity[2].
Mechanisms of damage to the glycocalyx in COVID-19
Several interlinked mechanisms contribute to the degradation of the glycocalyx during SARS-CoV-2 infection, with both direct viral effects and the host inflammatory response playing a role.
Inflammatory mediators and cytokine storm
COVID-19-induced inflammation plays an important role in the degradation of the glycocalyx. The inflammatory response to SARS-CoV-2 infection leads to the release of various inflammatory mediators that target the endothelial glycocalyx[1]. Severe COVID-19 infections are characterized by a “cytokine storm” with elevated levels of proinflammatory cytokines such as IL-1β, IL-6 and TNF-α[1]. These inflammatory cytokines have been shown to cause significant damage to the glycocalyx structure.
Enzymatic degradation by sheddases
The enzymatic degradation of the endothelial glycocalyx in COVID-19 is mediated by specific enzymes known as sheddases. These include heparanase, matrix metalloproteases (MMPs) and hyaluronidases[1]. Heparanase is the only known mammalian enzyme capable of cleaving heparan sulphate. Heparan sulfate is the most abundant glycosaminoglycan in the glycocalyx and mainly contributes to the negatively charged barrier[1].
Studies have shown that both the level and activity of heparanase are increased in COVID-19 patients[1]. This enzyme is synthesized in the endoplasmic reticulum and cleaved by cathepsin L to form the mature heparanase that can degrade the glycocalyx[1]. The increased activity of these sheddases in COVID-19 explains the increased degradation of the glycocalyx observed in patients.
Neutrophil activation and myeloperoxidase release
Recent evidence suggests that neutrophil activation is another key mechanism for glycocalyx damage in COVID-19. Neutrophils, the most abundant white blood cells in the bloodstream, secrete antimicrobial substances that have been linked to the development of severe COVID-19[4]. A specific neutrophil enzyme, myeloperoxidase (MPO), appears to be particularly important in the degradation of glycocalyx.
Research has shown that “MPO levels, MPO activity and levels of soluble EG proteins are significantly increased compared to controls, and concentrations increase in proportion to disease severity”[4]. Furthermore, MPO levels and activity correlate significantly with soluble glycocalyx levels, suggesting a causative role[4]. This relationship between neutrophil activation and glycocalyx detachment represents an important pathway in the pathophysiology of COVID-19.
Oxidative stress
Oxidative stress, characterized by an overproduction of reactive oxygen species (ROS), is another factor contributing to glycocalyx damage in COVID-19. The inflammatory response to SARS-CoV-2 infection leads to increased production of ROS, which can directly damage the glycocalyx structure[1]. Markers of oxidative stress, such as malondialdehyde (MDA), are elevated in COVID-19 patients compared to controls, with levels as high as 10.55 ± 2.45 versus 1.01 ± 0.50 nmol/L[10]. These markers of oxidative stress are associated with residual cardiovascular symptoms even four months after infection[10].
Protective function of the intact glycocalyx
Interestingly, research has shown that an intact glycocalyx can serve as a protective barrier against SARS-CoV-2 infection of endothelial cells. This protective function is based on the ability of the glycocalyx to shield the angiotensin-converting enzyme 2 (ACE2) receptor, which is the primary binding site for the SARS-CoV-2 spike protein.
Regulation of the binding of the spike protein to ACE2
In studies using force spectroscopy methods, ACE2- and glycocalyx-dependent adhesion forces between the viral spike protein and endothelial cells were measured directly[3]. These studies revealed a dual role of the endothelial glycocalyx in the control of viral adhesion and interaction with ACE2 receptors.
In healthy endothelium with a well-preserved glycocalyx layer, the glycocalyx “strongly binds the S protein but shields its interaction with ACE2″[3]. The glycocalyx acts as an “anchor” for the spike protein, but prevents it from reaching the ACE2 receptors that would allow viral entry. In contrast, “reduction of the glycocalyx layer exposes the ACE2 receptors and promotes their interaction with the S protein”[3]. This result suggests that “the susceptibility of ECs to COVID-19 infection may depend on the state of the glycocalyx”[3].
Implications for the susceptibility of the endothelium to infections
The protective role of the glycocalyx has important implications for understanding why some people develop more severe vascular complications from COVID-19. If the glycocalyx is already damaged due to pre-existing conditions such as diabetes, hypertension or atherosclerosis, endothelial cells may be more susceptible to direct infection by SARS-CoV-2[3]. This could explain the higher risk of severe COVID-19 infection in patients with these diseases, which are known to be associated with glycocalyx dysfunction.
Evaluation of cell elasticity supported this hypothesis and showed that the glycocalyx can prevent the interaction of the spike protein with ACE2 receptors in endothelial cells through the strong binding of the spike protein to heparan sulfate[3]. However, if the glycocalyx is damaged or reduced, this protective mechanism fails, potentially allowing more direct viral effects on the endothelium.
Consequences of damage to the glycocalyx in COVID-19
Degradation of the endothelial glycocalyx in COVID-19 leads to widespread vascular dysfunction and contributes significantly to the pathophysiology and clinical manifestations of the disease.
Vascular endothelial dysfunction
Endothelial damage is considered one of the most important features of severe COVID-19 patients, manifesting as systemic inflammatory response syndrome, acute respiratory distress syndrome, microvascular thrombosis, Kawasaki disease and multiple organ failure[1]. Damage to the vascular endothelial glycocalyx leads to vascular endothelial dysfunction, microvascular hyperpermeability, thrombosis and leukocyte adhesion[1]. These effects further exacerbate the development of COVID-19 and delay recovery from vascular dysfunction[1].
Measurements of vascular function have confirmed this dysfunction. COVID-19 patients have lower coronary flow reserve (CFR) and flow-mediated dilation (FMD) values than control subjects (2.39 ± 0.39 vs. 3.31 ± 0.59, p = 0.0122 and 5.12 ± 2.95% vs. 8.12 ± 2.23%, p = 0.006, respectively)[10]. These parameters are indicators of impaired vascular function.
Increased arterial stiffness
Damage to the glycocalyx contributes to increased arterial stiffness in COVID-19 patients. Studies have found a higher pulse wave velocity (PWV) in COVID-19 patients compared to controls (PWVc-f 12.32 ± 2.44 vs. 10.11 ± 1.85 m/sec, p = 0.033)[10]. Pulse wave velocity is a direct measure of arterial stiffness, with higher values indicating stiffer arteries and a higher cardiovascular risk.
Impaired myocardial function
The vascular dysfunction resulting from damage to the glycocalyx also extends to cardiac function. COVID-19 patients have reduced left ventricular global longitudinal strain (GLS) compared to controls (-19.11 ± 2.14% vs -20.41 ± 1.61%, p = 0.001)[10]. They also have a higher myocardial work index and higher wasted work, while myocardial efficiency is lower (94.8 ± 2.5 % vs. 96.06 ± 2.3 %, p = 0.008)[10]. These results suggest that “SARS-CoV-2 may cause vascular dysfunction followed by wasted cardiac work to compensate for the increased arterial stiffness”[10].
Coagulation disorders and thrombosis
The degradation of the glycocalyx contributes significantly to the coagulation disorders observed with COVID-19. The intact glycocalyx normally confers anticoagulant properties to the endothelial surface[6]. When it is damaged, this anticoagulant function is impaired, leading to a pro-thrombotic state.
The cytokine storm in COVID-19 targets the glycocalyx, which leads to subsequent coagulation disorders[7]. The combination of endothelial dysfunction and glycocalyx degradation creates an environment that favors thrombus formation, which explains the high incidence of thromboembolic complications in severe COVID-19 cases.
Long-term effects and recovery
A particularly worrying aspect of glycocalyx damage in COVID-19 is its persistence beyond the acute phase of infection, potentially contributing to long-term consequences.
Persistent glycocalyx damage in convalescent patients
Despite clinical recovery, markers of glycocalyx damage remain elevated in convalescent COVID-19 patients[4]. Studies have shown that “despite clinical recovery, protein concentrations remain significantly elevated”[4]. Interestingly, there is a trend towards increasing MPO activity in convalescent plasma in both the severe and non-severe groups[4]. This persistent dysfunction of the glycocalyx may explain some of the long-term cardiovascular and microvascular complications reported in COVID-19 survivors.
Impaired vascular function months after infection
In studies examining patients four months after COVID-19 infection, persistent impairment of endothelial glycocalyx, vascular function and myocardial efficiency was observed[10]. Compared to control subjects, these patients continued to have lower coronary flow reserve and flow-mediated dilation values, higher pulse wave velocity and impaired global left ventricular longitudinal strain[10]. Endothelial biomarkers such as von Willebrand factor and thrombomodulin remained higher in COVID-19 patients than in controls[10].
These findings suggest that vascular dysfunction persists long after the acute phase of COVID-19, which may contribute to the constellation of symptoms described as “long COVID” in some patients.
Outlook
Future research should focus on the development of specific therapies to protect and restore the endothelial glycocalyx in COVID-19 to mitigate both acute vascular complications and long-term consequences of this disease. In addition, glycocalyx biomarkers may serve as valuable tools for monitoring disease progression, predicting treatment outcomes and making therapeutic decisions in the treatment of COVID-19.
Sources
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