Glycocalyx: Function & Structure

Jamila Siamwala, Amanda Robb
  • Author
    Jamila Siamwala

    Jamila H. Siamwala, PhD has a doctoral degree in Cell Biology, Masters in Biomedical Genetics and Bachelors in Zoology. Her post-doctoral training is from University of California, San Diego from 2013-2018 and she was recruited as an Assistant Professor (Research) in Brown University from 2018-2021. She has over 10 years of experience in academic writing, mentoring and research in wide range of subjects related to human physiology, basic cardiovascular sciences, space effects, mitochondrial biology, metabolism and cell biology.

  • Instructor
    Amanda Robb

    Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

Learn when Glycocalyx was first described and what role does it play in humans and bacteria. See the structure of Glycocalyx and know how it is disrupted. Updated: 11/18/2021

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Glycocalyx structure in Humans

What is Glycocalyx?

Glycocalyx (sweet husk) was first described by Bennett HS in 1963 based on their polysaccharide composition of glycoproteins and glycolipids and later by Luft (1965) from the electron micrographs of microvessels. These are gel-like negatively charged filamentous structures are found on the luminal surface of blood vessels and act as molecular sieves controlling vascular homeostasis, inflammation, coagulation, and permeability. The glycocalyx comprises the scaffolding glycoproteins (mucins, selectins, adhesion molecules), glycosaminoglycans (Hyaluronan), and proteoglycans (heparin sulfate) associated with blood flow and blood components. Glycocalyx regulates the movement of fluids between the endothelial cells and functions as barriers to macromolecules. In addition to the filter function, the glycocalyx is involved in cell-cell recognition, adhesion, membrane bending, tabulation, and molding of the plasma membrane. Mucins are involved in tubulation and release of copious amounts of extracellular vesicles involved in carcinogenesis, tumor angiogenesis, and metastasis. Glycocalyx structural and functional defects lead to an inflammatory response in the blood vessels associated with vascular diseases including type 2 diabetes, atherosclerosis and sepsis.

Structure of Glycocalyx

The most contentious problem to date is to determine the exact shape, size, dynamics, and molecular interactions of glycocalyx due to its hydrophilic nature and lack of techniques. Vink E et al used intravital microscopy to follow the movement of red blood cells and fluorescent dextran in the plasma is filtered by the glycocalyx. In doing so, they identified a zone that was neither plasma nor red blood cells but extended into the endothelium. The size of this zone obtained by subtracting red blood cells and plasma zone from the fluorescent walls of the vessels is approx. 0.5 micron. Wheat germ agglutinin is used recently to follow the activities and structural modification of glycocalyx since it specifically binds to sugar molecules and allows the visualization of the entire glycocalyx architecture. The structure of glycocalyx is not yet fully elucidated. Based on the biochemical analysis, the glycocalyx is primarily composed of proteoglycans, glycosaminoglycans, and glycoproteins. The proteoglycans are either free-floating or attached with a long core and unbranched carbohydrate side chains and provide the backbone in the extracellular space.

The membrane-bound proteoglycans are attached to the cell membrane in two ways. First, by the syndecans of the single-span transmembrane domain. Then, the proteoglycans attach by their COOH terminus to the cell membrane through glycosylphosphotidylinositol (GPI) anchor (like Glypican). GPI anchors play a role in localizing protein to the luminal surface of the glycocalyx. The free-floating proteoglycans lack the transmembrane domains and are not connected to the cell membrane. However, they have strong interactions with the other components of the extracellular matrix like Perlecans, mimecan, decorin, versican, and biglycan). The perlecan's can bind to growth factors as well and help in activating the cells. The GAG's are long, linear polysaccharide molecules composed of repeating disaccharide units (heparan sulfate, chondroitin sulfate, dermatan sulfate, and Karastan sulfate). The most abundant and highly investigated GAG is heparin sulfate, accounting for 90% of endothelial cell proteoglycans and consisting of glucuronic acid and N-acetylglucosamine. Keratan sulfate consists of galacturonic acid and N-acetylglucosamine; chondroitin sulfate consists of glucuronic acid and N-acetylgalactosamine, and dermatan sulfate consists of glucuronic acid and N-acetylgalactosamine. These moieties undergo deacetylation and sulfation after the formation of the polysaccharide chain.

The glycoproteins differ from proteoglycans in having small branched carbohydrate side chains as opposed to long unbranched side chains. The adhesion molecules such as selectin, integrins, and immunoglobulins belong to the glycoprotein group. E-selectin and P-selectin are specific to endothelial cells and are overexpressed when endothelial cells are activated by inflammatory mediators. The integrins connect the extracellular matrix to the cytoskeleton and are involved in signal transduction. The intracellular adhesion molecule (ICAM's), platelet /endothelial cell adhesion molecule (PECAM-1), and vascular cell adhesion molecule (VCAM-1) belong to the immunoglobulin superfamily, which serves as ligands for growth factors and activates the intracellular signaling pathways.

Structure of cellular membrane with adhesion protein and cytoskeleton anchor

Glycocalyx structure in Endothelial Cells

Cobblestone-shaped endothelial cells form the inner lining of the blood vessels that are in angiogenesis, vasculogenesis, barrier functions, and tissue remodeling. The glycocalyx is found lining the endothelial cells and act as gatekeepers allowing movement of molecules and interacting with the components of the blood. Therefore, glycocalyx resides in the capillaries of almost all the vessels as endothelial cells occupy a vast surface area of all the vessels in the body. The heparan sulfate proteoglycans in the glycocalyx provide sites for plasma protein binding and hyaluronan binds to water molecules, conferring weight to the glycocalyx. Systemic glycocalyx volume, a measure of glycocalyx thickness varies with disease conditions wherein healthy glycocalyx is ~0.9um compared to ~0.5um in diabetes mellitus. The glycosaminoglycans provide support to the glycocalyx in the endothelium. The concentration of enzymes and growth factors is important to control the vascular tone. The endothelial glycocalyx acts as a control center, interacting with various proteins and enzymes like:

  • antithrombin III
  • heparin cofactor II,
  • Tissue factor pathway inhibitor (TFP1),
  • Lipoprotein lipase (LPL),
  • Low-density lipoprotein (LDL),
  • Vascular Endothelial Growth Factor (VEGF),
  • Transforming growth factor-beta 1 or 2 (TGF beta1/2),
  • Fibroblast growth factor (FGF), and
  • Interleukins and Regulated on Activation, Normal T Expressed and Secreted (RANTES)

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  • 0:00 What is the Glycocalyx?
  • 1:11 Glycocalyx in Humans
  • 2:36 Glycocalyx in Bacteria
  • 4:05 Medical Applications
  • 5:04 Lesson Summary
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Frequently Asked Questions

How does glycocalyx contribute to disease?

Glycocalyx regulates the movement of fluids between the endothelial cells and functions as barriers and prevents leakage of the plasma components. The disintegration of glycocalyx leads to increased permeability and loss of vascular integrity, thereby contributing to the disease phenotypes.

What are the components of glycocalyx?

The main components of the glycocalyx are glycoproteins (mucins, selectins, adhesion molecules), glycosaminoglycans (Hyaluronan), and proteoglycans (heparin sulfate). They participate in various capacities to perform the glycocalyx functions.

What is the purpose of the glycocalyx?

The glycocalyx is the protective layer of the endothelial cells found on the lumen side of the vessels. The glycocalyx acts as a molecular sieve allowing only selective molecules to enter or leave the vessels. In the prokaryotes, the glycocalyx prevents recognition and destruction of the bacteria by the immune cells. Therefore, the glycocalyx is involved in maintaining vascular integrity and pathological conditions such as diabetes, sepsis, cancer are associated with reduction of the glycocalyx.

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