Introduction
The endothelial glycocalyx is a thin layer consisting of proteoglycans, glycoproteins and glycosaminoglycans that lines the luminal side of vascular endothelial cells. It acts as a barrier and contributes to the maintenance of vascular homeostasis and micro perfusion. During solid organ transplantation, the endothelial glycocalyx of the graft is damaged as part of Ischemia Reperfusion Injury (IRI), which is associated with impaired organ function. Although several substances are known to mitigate glycocalyx damage, it has not been possible to use these substances during graft storage on ice.
Normothermic machine perfusion (NMP) emerges as an alternative technology for organ preservation and allows for organ evaluation, but also offers the possibility to treat and thus improve organ quality during storage. This review highlights the current knowledge on glycocalyx injury during organ transplantation, presents ways to protect the endothelial glycocalyx and discusses potential glycocalyx protection strategies during normothermic machine perfusion.
Structure of the Glycocalyx
The endothelial glycocalyx is a complex 0.1–1 µm thick layer that lines the luminal surface of endothelial cells. Proteoglycans (e.g., syndecans) and glycoproteins form the backbone of this structure and are anchored in the endothelial cells. Heparin and chondroitin sulfate glycosaminoglycans covalently bind to these transmembrane proteins, thereby forming a negatively charged matrix where plasma components such as albumin and orosomucoids accumulate (Fig 1).
Function of the Endothelial Glycocalyx and Pathophysiological Implications
Even prior to the identification of the biochemical structure of the glycocalyx, the presence of a peripheral layer of plasma surrounding the red blood cell column was observed, and indicated the anatomical location of the glycocalyx.
It had previously been noticed, that the haematocrit in capillaries (i.e., the fractional volume of red blood cells within a capillary) was significantly lower than the core haematocrit and the arteriolar haematocrit. This layer, later identified as the glycocalyx protruding into the vessel lumen, is the reason why red blood cells flow in the center of the vessel.
Further studies have shown that the thickness of this layer can be reduced by treatment with heparinase, which allows the red blood cell column to flow closer to the vascular wall. It has become increasingly clear that the endothelial glycocalyx mainly regulates vessel wall permeability for fluids and proteins. It has been shown that the degree of destruction of the glycocalyx, due to pathological processes such as sepsis, is a predictive marker for the development of disseminated intravascular coagulation.
The Effect of Solid Organ Transplantation on the Endothelial Glycocalyx
IRI is a key driving factor for graft damage in heart, lung, liver and kidney transplantation. Recent studies suggested that damage to the endothelial glycocalyx significantly contributes to the development of IRI (Fig 2).
Significantly higher plasma levels of syndecan-1 in liver graft recipients after transplantation than before transplantation, indicating destruction of the endothelial glycocalyx . Furthermore, it was determined that effluent syndecan-1 concentrations were greater in recipients who had developed an early allograft dysfunction (EAD) than in patients who had not.
Taken together, these data suggest a possible role for glycocalyx degradation markers as monitoring parameters for liver graft function early after transplantation. Destruction of the glycocalyx has also been demonstrated in kidney grafts. After kidney transplantation from donation after circulatory death (DCD) donors, the so-called red blood cell exclusion zone of peritubular capillaries, measured by side stream darkfield microscopy, is significantly thinner when compared to kidneys from living donors, which indicates destruction of the endothelial glycocalyx during ex situ kidney preservation.
Glycocalyx-Protective and Restoration Strategies
In intensive care and perioperative medicine, adequate fluid therapy is a prerequisite for maintaining organ perfusion and oxygenation and for preventing tissue hypoxia. Due to its barrier function, the intact endothelial glycocalyx plays an important role in physiological fluid homeostasis and impedes rapid large transmural fluid shifts from the intravascular to the extravascular compartment. However, the endothelial glycocalyx is particularly sensitive to fluid loading conditions, i.e., hypervolemia.
Hypervolemia, as induced in the first patient group, was associated with a release of atrial natriuretic peptide and an increase in glycocalyx-shedding parameters in the plasma. In view of their effect on the endothelial glycocalyx, colloids seem to better protect its physiological functioning than crystalloids.
Additionally, microvascular barrier function, as assessed by measurements of microvascular permeability, seems to be better preserved with infusions of colloids than with crystalloid solutions. Hence, the type and amount of volume replacement fluids seem to play an important role in maintaining physiological glycocalyx function.
Alternative Organ Preservation Strategies and Their Influence on the Glycocalyx
A recently developed technique of organ preservation is normothermic machine perfusion (NMP), where the graft is perfused with oxygenated blood and nutrients at normothermic body temperature to provide a physiological environment.
Albumin
Importantly, it has been shown that albumin also exerts glycocalyx-protective properties. Studies showed that the glycocalyx is better protected by albumin than by saline or hydroxyethyl starch, measured by the extent of edema formation in the myocardium as an indicator of the vascular barrier function of the glycocalyx.
Administration of albumin to the traditional preservation solution “histidine-tryptophane-ketoglutarate” (HTK) resulted in significantly decreased syndecan-1 and heparan sulfate levels in the perfusion solution of transplanted guinea pig hearts after four hours of cold ischemic storage as compared to transplanted hearts perfused with HTK preservation solution. In summary, albumin appears to exert a glycocalyx-protective effect and protect the graft from interstitial edema in animal studies.
Antithrombin
Several studies have shown that antithrombin, one of the most important endogenous inhibitors of coagulation, exhibits glycocalyx-protective properties. Besides these anti-inflammatory characteristics, the tight binding of antithrombin to glycosaminoglycans, prevents destructive enzymes from docking to the glycocalyx, which may preserve its function.
Several studies have shown that glycocalyx destruction triggered by TNF alpha or by IRI can be prevented by the administration of antithrombin. Taken together, antithrombin appears to be an important factor in glycocalyx protection. In addition to the interaction with the glycocalyx which prevents clotting and inflammation of the endothelium, antithrombin stabilizes the glycocalyx and protects it from shedding.
Glucocorticoids
Glucocorticoids are known to directly inhibit the production of intracellular reactive oxygen species (ROS). As glucocorticoids constitute an essential part of immunosuppressive therapy after solid organ transplantation, glycocalyx protection should occur as a positive side-effect. Administration of glucocorticoids during normothermic machine perfusion could result in further protection of the glycocalyx during organ preservation.
Sulodexide
Sulodexide is a glycosaminoglycan composed of approximately 80% heparan sulfate and 20% dermatan sulfate and is approved for the treatment of chronic venous insufficiency and the prevention of recurrent venous thrombosis in certain countries. It has been shown that in septic mice, where the volume of the glycocalyx is significantly reduced, administration of sulodexide accelerated the recovery of the endothelial glycocalyx, reduced vascular permeability and improved survival rates.
Lidocaine
The amid-type local anesthetic lidocaine has been shown to protect the endothelial glycocalyx. A possible explanation for this finding could be the anti-inflammatory properties of lidocaine, which seem to mitigate IRI. Lidocaine mediates its anti-inflammatory properties by blocking ion channels, thus inhibiting the release of cytokines and histamine from mast cells and basophils.
Glycocalyx Components
In a mouse model of ischemia/reperfusion injury of the cremaster muscle, administration of hyaluronan resulted in glycocalyx repair. Interestingly, this effect was seen regardless of whether hyaluronan was given before or after IRI. This suggests the potential of hyaluronan to prevent glycocalyx destruction, but also its ability to restore the glycocalyx.
Other components of the endothelial glycocalyx, e.g., heparan sulfate or sphingosine 1-phosphate, have been successfully used to repair the glycocalyx in cell culture models. Several pharmacological interventions, such as adaptation of the perfusate to optimize microvascular perfusion of the graft, have already been shown to protect and regenerate the endothelial glycocalyx (Fig 2).
Conclusions
Several investigations have found that the glycocalyx’s integrity and function are critical in transplantation therapy. Glycocalyx injury, on the other hand, happens throughout the organ’s retrieval, storage, and reperfusion. Despite the fact that glycocalyx-damaging processes have been identified, there has yet to be a therapeutic use. Interventions aimed at improving specific organ deficiencies are technically difficult due to the conditions of cold storage.
Novel organ storage strategies, such as normothermic perfusion, have, on the other hand, been acknowledged as effective instruments for analyzing organ function while in storage. In addition, normothermic perfusion allows for ex situ medication therapy of organs. Despite the fact that some compounds with a protective or even regenerative impact on the glycocalyx have been discovered, they have yet to be employed in clinical practice.
NMP decreases IRI and may thus be linked to the preservation of glycocalyx integrity. Treatment with glycocalyx-protective chemicals during NMP may increase graft quality and function during storage, leading to a better result. The relevance of glycocalyx protection in transplantation therapy necessitates research into the effects of various substances on glycocalyx protection and regeneration.
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