Specific components associated with the endothelial glycocalyx are lost from brain capillaries in cerebral malaria

BACKGROUND

Cerebral malaria (CM) is a rare, but severe and frequently fatal outcome of infections with Plasmodium falciparum. Pathogenetic mechanisms include endothelial activation and sequestration of parasitized erythrocytes in the cerebral microvessels. Increased concentrations of glycosaminoglycans in urine and plasma of malaria patients have been described, suggesting involvement of endothelial glycocalyx.

METHODS

We used lectin histochemistry on postmortem samples to compare the distribution of multiple sugar epitopes on cerebral capillaries in children who died from CM and from non-malarial comas.

RESULTS

N-acetyl glucosamine residues detected by tomato lectin are generally reduced in children with CM compared to controls. We used the vascular expression of intercellular adhesion molecule-1 and mannose residues on brain capillaries of CM as evidence of local vascular inflammation, and both were expressed more highly in CM patients than controls. Sialic acid residues were found to be significantly reduced in patients with CM. By contrast, the levels of other sugar epitopes regularly detected on the cerebral vasculature were unchanged, and this suggests specific remodeling of cerebral microvessels in CM patients.

CONCLUSIONS

Our findings support and expand upon earlier reports of disruptions of the endothelial glycocalyx in children with severe malaria.

Vascular Dysfunction in Malaria: Understanding the Role of the Endothelial Glycocalyx

Malaria caused by Plasmodium falciparum results in over 400,000 deaths annually, predominantly affecting African children. In addition, non-falciparum species including vivax and knowlesi cause significant morbidity and mortality. Vascular dysfunction is a key feature in malaria pathogenesis leading to impaired blood perfusion, vascular obstruction, and tissue hypoxia. Contributing factors include adhesion of infected RBC to endothelium, endothelial activation, and reduced nitric oxide formation. Endothelial glycocalyx (eGC) protects the vasculature by maintaining vessel integrity and regulating cellular adhesion and nitric oxide signaling pathways. Breakdown of eGC is known to occur in infectious diseases such as bacterial sepsis and dengue and is associated with adverse outcomes. Emerging studies using biochemical markers and in vivo imaging suggest that eGC breakdown occurs during Plasmodium infection and is associated with markers of malaria disease severity, endothelial activation, and vascular function. In this review, we describe characteristics of eGC breakdown in malaria and discuss how these relate to vascular dysfunction and adverse outcomes. Further understanding of this process may lead to adjunctive therapy to preserve or restore damaged eGC and reduce microvascular dysfunction and the morbidity/mortality of malaria.

Degradation of endothelial glycocalyx in Tanzanian children with falciparum malaria

A layer of glycocalyx covers the vascular endothelium serving important protective and homeostatic functions. The objective of this study was to determine if breakdown of the endothelial glycocalyx (eGC) occurs during malaria infection in children. Measures of eGC integrity, endothelial activation, and microvascular reactivity were prospectively evaluated in 146 children: 44 with moderately severe malaria (MSM), 42 with severe malaria (SM), and 60 healthy controls (HC). Biochemical measures of eGC integrity included plasma syndecan-1 and total urinary glycosaminoglycans (GAG). Side-stream dark field imaging was used to quantitatively assess integrity of eGC. Plasma angiopoietin-2 (Ang-2) was measured as a marker of endothelial activation and also as a possible mediator of eGC breakdown. Our results show that urinary GAG, syndecan-1, and Ang-2 were elevated in patients with MSM and SM compared with HC. Syndecan-1 and GAG levels correlated significantly with each other and with plasma Ang-2. The eGC breakdown products also inversely correlated significantly with hemoglobin and platelet count. In the MSM group, imaging results provided further evidence for eGC degradation. Although not correlated with markers of eGC degradation, vascular function (assessed by non-invasive near infrared spectroscopy [NIRS]) demonstrated reduced microvascular reactivity, particularly affecting the SM group. Our findings provide further evidence for breakdown of eGC in falciparum malaria that may contribute to endothelial activation and adhesion of parasitized red blood cells, with reduced nitric oxide formation, and vascular dysfunction.

An improved in vitro model for studying the structural and functional properties of the endothelial glycocalyx in arteries, capillaries and veins

The endothelial glycocalyx is a dynamic structure integral to blood vessel hemodynamics and capable of tightly regulating a range of biological processes (ie, innate immunity, inflammation, and coagulation) through dynamic changes in its composition of the brush structure. Evaluating the specific roles of the endothelial glycocalyx under a range of pathophysiologic conditions has been a challenge in vitro as it is difficult to generate functional glycocalyces using commonly employed 2D cell culture models. We present a new multi-height microfluidic platform that promotes the growth of functional glycocalyces by eliciting unique shear stress forces over a continuous human umbilical vein endothelial cell monolayer at magnitudes that recapitulate the physical environment in arterial, capillary and venous regions of the vasculature. Following 72 hours of shear stress, unique glycocalyx structures formed within each region that were distinct from that observed in short (3 days) and long-term (21 days) static cell culture. The model demonstrated glycocalyx-specific properties that match the characteristics of the endothelium in arteries, capillaries and veins, with respect to surface protein expression, platelet adhesion, lymphocyte binding and nanoparticle uptake. With artery-to-capillary-to-vein transition on a continuous endothelial monolayer, this in vitro platform is an improved system over static cell culture for more effectively studying the role of the glycocalyx in endothelial biology and disease.

Unveiling the Sugary Secrets of Plasmodium Parasites

Plasmodium parasites cause malaria disease, one of the leading global health burdens for humanity, infecting hundreds of millions of people each year. Different glycans on the parasite and the host cell surface play significant roles in both malaria pathogenesis and host defense mechanisms. So far, only small, truncated N- and O-glycans have been identified in Plasmodium species. In contrast, complex glycosylphosphatidylinositol (GPI) glycolipids are highly abundant on the parasite’s cell membrane and are essential for its survival. Moreover, the parasites express lectins that bind and exploit the host cell surface glycans for different aspects of the parasite life cycle, such as adherence, invasion, and evasion of the host immune system. In parallel, the host cell glycocalyx and lectin expression serve as the first line of defense against Plasmodium parasites and directly dictate susceptibility to Plasmodium infection. This review provides an overview of the glycobiology involved in Plasmodium-host interactions and its contribution to malaria pathogenesis. Recent findings are presented and evaluated in the context of potential therapeutic exploitation.

Surface charge, glycocalyx, and blood-brain barrier function

The negative surface charge of brain microvessel endothelial cells is derived from the special composition of their membrane lipids and the thick endothelial surface glycocalyx. They are important elements of the unique defense systems of the blood-brain barrier. The tissue-specific properties, components, function and charge of the brain endothelial glycocalyx have only been studied in detail in the past 15 years. This review highlights the importance of the negative surface charge in the permeability of macromolecules and nanoparticles as well as in drug interactions. We discuss surface charge and glycoxalyx changes in pathologies related to the brain microvasculature and protective measures against glycocalyx shedding and damage. We present biophysical techniques, including a microfluidic chip device, to measure surface charge of living brain endothelial cells and imaging methods for visualization of surface charge and glycocalyx.

The neurovascular extracellular matrix in health and disease

The blood-brain barrier (BBB) is a vital interface that supports normal brain functions. Endothelial cells (ECs) are the main component of the BBB and are highly specialized to govern the transfer of substances into brain. The EC lumen is enmeshed with an extracellular matrix (ECM), known as the endothelial glycocalyx layer (EGL). The lumen-facing EGL is primarily comprised of proteoglycans (PGs) and glycosaminoglycans (GAGs), which function as the first line of defense for blood-to-brain transfer of substances. Circulating factors must first penetrate the EGL before interacting with the EC. The abundance and composition of the PG and GAGs can dictate EGL function, and determine which circulating substances communicate with the ECs. The EGL can interact with circulating factors through physio-chemical interactions with the EC. Some disease states reveal a "thinning" of the EGL that may increase EC interactions with components of the systemic circulation and alter BBB function. EGL changes may also contribute to the cognitive complications of systemic diseases, such as sepsis and diabetes. For decades, researchers have measured how genetic and environmental factors influence the peripheral EGL constituents; however, much less is known about the neurovascular EGL. In this mini-review, we introduce components of the EGL and innovative ways to measure their abundance and composition that may contribute to BBB dysfunction.

Malaria link of hypertension: a hidden syndicate of angiotensin II, bradykinin and sphingosine 1-phosphate

In malaria-endemic countries, the burden of hypertension is on the rise. Although malaria and hypertension seem to have no direct link, several studies in recent years support their possible link. Three bioactive molecules such as angiotensin II (Ang II), bradykinin (BK) and sphingosine 1-phosphate (S1P) are crucial in regulating blood pressure. While the increased level of Ang II and S1P are responsible for inducing hypertension, BK is arthero-protective and anti-hypertensive. Therefore, in the present review, based on available literatures we highlight the present knowledge on the production and bioavailability of these molecules, the mechanism of their regulation of hypertension, and patho-physiological role in malaria. Further, a possible link between malaria and hypertension is hypothesized through various arguments based on experimental evidence. Understanding of their mechanisms of blood pressure regulation during malaria infection may open up avenues for drug therapeutics and management of malaria in co-morbidity with hypertension.

The Glycocalyx and Its Role in Vascular Physiology and Vascular Related Diseases

Purpose

In 2007 the two senior authors wrote a review on the structure and function of the endothelial glycocalyx layer (Weinbaum in Annu Rev Biomed Eng 9:121–167, 2007). Since then there has been an explosion of interest in this hydrated gel-like structure that coats the luminal surface of endothelial cells that line our vasculature due to its important functions in (A) basic vascular physiology and (B) vascular related diseases. This review will highlight the major advances that have occurred since our 2007 paper.

Methods

A literature search mainly focusing on the role of the glycocalyx in the two major areas described above was performed using electronic databases.

Results

In part (A) of this review, the new formulation of the century old Starling principle, now referred to as the Michel–Weinbaum glycoclayx model or revised Starling hypothesis, is described including new subtleties and physiological ramifications. New insights into mechanotransduction and release of nitric oxide due to fluid shear stress sensed by the glycocalyx are elaborated. Major advances in understanding the organization and function of glycocalyx components, and new techniques for measuring both its thickness and spatio-chemical organization based on super resolution, stochastic optical reconstruction microscopy (STORM) are presented. As discussed in part (B) of this review, it is now recognized that artery wall stiffness associated with hypertension and aging induces glycocalyx degradation, endothelial dysfunction and vascular disease. In addition to atherosclerosis and cardiovascular diseases, the glycocalyx plays an important role in lifestyle related diseases (e.g., diabetes) and cancer. Infectious diseases including sepsis, Dengue, Zika and Corona viruses, and malaria also involve the glycocalyx. Because of increasing recognition of the role of the glycocalyx in a wide range of diseases, there has been a vigorous search for methods to protect the glycocalyx from degradation or to enhance its synthesis in disease environments.

Conclusion

As we have seen in this review, many important developments in our basic understanding of GCX structure, function and role in diseases have been described since the 2007 paper. The future is wide open for continued GCX research.

Sphingosine 1-Phosphate in Malaria Pathogenesis and Its Implication in Therapeutic Opportunities

Sphingosine 1-Phosphate (S1P) is a bioactive lipid intermediate in the sphingolipid metabolism, which exist in two pools, intracellular and extracellular, and each pool has a different function. The circulating extracellular pool, specifically the plasma S1P is shown to be important in regulating various physiological processes related to malaria pathogenesis in recent years. Although blood cells (red blood cells and platelets), vascular endothelial cells and hepatocytes are considered as the important sources of plasma S1P, their extent of contribution is still debated. The red blood cells (RBCs) and platelets serve as a major repository of intracellular S1P due to lack, or low activity of S1P degrading enzymes, however, contribution of platelets toward maintaining plasma S1P is shown negligible under normal condition. Substantial evidences suggest platelets loss during falciparum infection as a contributing factor for severe malaria. However, platelets function as a source for plasma S1P in malaria needs to be examined experimentally. RBC being the preferential site for parasite seclusion, and having the ability of trans-cellular S1P transportation to EC upon tight cell-cell contact, might play critical role in differential S1P distribution and parasite growth. In the present review, we have summarized the significance of both the S1P pools in the context of malaria, and how the RBC content of S1P can be channelized in better ways for its possible implication in therapeutic opportunities to control malaria.

Pathophysiology and neurologic sequelae of cerebral malaria

Cerebral malaria (CM), results from Plasmodium falciparum infection, and has a high mortality rate. CM survivors can retain life-long post CM sequelae, including seizures and neurocognitive deficits profoundly affecting their quality of life. As the Plasmodium parasite does not enter the brain, but resides inside erythrocytes and are confined to the lumen of the brain's vasculature, the neuropathogenesis leading to these neurologic sequelae is unclear and under-investigated. Interestingly, postmortem CM pathology differs in brain regions, such as the appearance of haemorragic punctae in white versus gray matter. Various host and parasite factors contribute to the risk of CM, including exposure at a young age, parasite- and host-related genetics, parasite sequestration and the extent of host inflammatory responses. Thus far, several proposed adjunctive treatments have not been successful in the treatment of CM but are highly needed. The region-specific CM neuro-pathogenesis leading to neurologic sequelae is intriguing, but not sufficiently addressed in research. More attention to this may lead to the development of effective adjunctive treatments to address CM neurologic sequelae.

This was the year that was: brain barriers and brain fluid research in 2019

This editorial highlights advances in brain barrier and brain fluid research published in 2019, as well as addressing current controversies and pressing needs. Topics include recent advances related to: the cerebral endothelium and the neurovascular unit; the choroid plexus, arachnoid membrane; cerebrospinal fluid and the glymphatic hypothesis; the impact of disease states on brain barriers and brain fluids; drug delivery to the brain; and translation of preclinical data to the clinic. This editorial also mourns the loss of two important figures in the field, Malcolm B. Segal and Edward G. Stopa.

In Vivo Imaging of the Buccal Mucosa Shows Loss of the Endothelial Glycocalyx and Perivascular Hemorrhages in Pediatric Plasmodium falciparum Malaria

Severe malaria is mostly caused by Plasmodium falciparum, resulting in considerable, systemic inflammation and pronounced endothelial activation. The endothelium forms an interface between blood and tissue, and vasculopathy has previously been linked with malaria severity. We studied the extent to which the endothelial glycocalyx that normally maintains endothelial function is involved in falciparum malaria pathogenesis by using incident dark-field imaging in the buccal mucosa. This enabled calculation of the perfused boundary region, which indicates to what extent erythrocytes can permeate the endothelial glycocalyx. The perfused boundary region was significantly increased in severe malaria patients and mirrored by an increase of soluble glycocalyx components in plasma. This is suggestive of a substantial endothelial glycocalyx loss. Patients with severe malaria had significantly higher plasma levels of sulfated glycosaminoglycans than patients with uncomplicated malaria, whereas other measured glycocalyx markers were raised to a comparable extent in both groups. In severe malaria, the plasma level of the glycosaminoglycan hyaluronic acid was positively correlated with the perfused boundary region in the buccal cavity. Plasma hyaluronic acid and heparan sulfate were particularly high in severe malaria patients with a low Blantyre coma score, suggesting involvement in its pathogenesis. In vivo imaging also detected perivascular hemorrhages and sequestering late-stage parasites. In line with this, plasma angiopoietin-1 was decreased while angiopoietin-2 was increased, suggesting vascular instability. The density of hemorrhages correlated negatively with plasma levels of angiopoietin-1. Our findings indicate that as with experimental malaria, the loss of endothelial glycocalyx is associated with vascular dysfunction in human malaria and is related to severity.

Glycocalyx Breakdown Is Associated With Severe Disease and Fatal Outcome in Plasmodium falciparum Malaria

BACKGROUND

Interactions between the endothelium and infected erythrocytes play a major role in the pathogenesis of falciparum malaria, with microvascular dysfunction and parasite sequestration associated with worsening outcomes. The glycocalyx is a carbohydrate-rich layer that lines the endothelium, with multiple roles in vascular homeostasis. The role of the glycocalyx in falciparum malaria and the association with disease severity has not been investigated.

METHODS

We prospectively enrolled Indonesian inpatients (aged ≥18 years) with severe (SM) or moderately severe (MSM) falciparum malaria, as defined by World Health Organization criteria, and healthy controls (HCs). On enrollment, blood and urine samples were collected concurrently with measurements of vascular nitric oxide (NO) bioavailability. Urine was assayed for glycocalyx breakdown products (glycosaminoglycans) using a dimethylmethylene blue (GAG-DMMB) and liquid chromatography-tandem mass spectrometry (GAG-MS) assay.

RESULTS

A total of 129 patients (SM = 43, MSM = 57, HC=29) were recruited. GAG-DMMB and GAG-MS (g/mol creatinine) were increased in SM (mean, 95% confidence interval: 3.98, 2.44-5.53 and 6.82, 5.19-8.44) compared to MSM patients (1.78, 1.27-2.29 and 4.87, 4.27-5.46) and HCs (0.22, 0.06-0.37 and 1.24, 0.89-1.59; P < 0.001). In SM patients, GAG-DMMB and GAG-MS were increased in those with a fatal outcome (n = 3; median, interquartile range: 6.72, 3.80-27.87 and 12.15, 7.88-17.20) compared to survivors (n = 39; 3.10, 0.46-4.5 and 4.64, 2.02-15.20; P = 0.03). Glycocalyx degradation was significantly associated with parasite biomass in both MSM (r = 0.48, GAG-DMMB and r = 0.43, GAG-MS; P < 0.001) and SM patients (r = 0.47, P = 0.002 and r = 0.33, P = 0.04) and inversely associated with endothelial NO bioavailability.

CONCLUSIONS

Increased endothelial glycocalyx breakdown is associated with severe disease and a fatal outcome in adults with falciparum malaria.

Glycocalyx breakdown is increased in African children with cerebral and uncomplicated falciparum malaria

Cerebral malaria (CM) from Plasmodium falciparum infection is associated with endothelial dysfunction and parasite sequestration. The glycocalyx (GCX), a carbohydrate-rich layer lining the endothelium, is crucial in vascular homeostasis. To evaluate the role of its loss in the pathogenesis of pediatric CM, we measured GCX degradation in Tanzanian children with World Health Organization-defined CM (n = 55), uncomplicated malaria (UM; n = 20), and healthy controls (HCs; n = 25). Urine GCX breakdown products [glycosaminoglycans (GAGs)] were quantified using dimethylmethylene blue (DMMB) and liquid chromatography-tandem mass spectrometry assays. DMMB-GAG and mass spectrometry (MS)-GAG (g/mol creatinine) were increased in CM and UM compared with HCs (P < 0.001), with no differences in DMMB-GAG and MS-GAG between CM and UM children or between those with and without a fatal outcome. In CM survivors, urinary GCX DMMB-GAG normalized by d 3. After adjusting for disease severity, DMMB-GAG was significantly associated with parasitemia [partial correlation coefficient (P_corr) = 0.34; P = 0.01] and plasma TNF (P_corr = 0.26; P = 0.04) and inversely with plasma and urine NO oxidation products [P_corr = -0.31 (P = 0.01) and P_corr = -0.26 (P = 0.03), respectively]. GCX breakdown is increased in children with falciparum malaria, with similar elevations in CM and UM. Endothelial GCX degradation may impair endothelial NO production, exacerbate adhesion-molecule expression, exposure, and parasite sequestration, and contribute to malaria pathogenesis.-Yeo, T. W., Bush, P. A., Chen, Y., Young, S. P., Zhang, H., Millington, D. S., Granger, D. L., Mwaikambo, E. D., Anstey, N. M., Weinberg, J. B. Glycocalyx breakdown is increased in African children with cerebral and uncomplicated falciparum malaria.