The Emerging Role of the Mammalian Glycocalyx in Functional Membrane Organization and Immune System Regulation

Abstract

All cells in the human body are covered by a dense layer of sugars and the proteins and lipids to which they are attached, collectively termed the "glycocalyx." For decades, the organization of the glycocalyx and its interplay with the cellular state have remained enigmatic. This changed in recent years. Latest research has shown that the glycocalyx is an organelle of vital significance, actively involved in and functionally relevant for various cellular processes, that can be directly targeted in therapeutic contexts. This review gives a brief introduction into glycocalyx biology and describes the specific challenges glycocalyx research faces. Then, the traditional view of the role of the glycocalyx is discussed before several recent breakthroughs in glycocalyx research are surveyed. These results exemplify a currently unfolding bigger picture about the role of the glycocalyx as a fundamental cellular agent.

Keywords: KRAS; cancer; cancer immune therapy; glycocalyx; immune system; immunosynapse; membrane organization; siglecs.

FIGURE 1

Schematic depiction of the glycocalyx. The glycocalyx is a central constituent of any cell, consisting of sugars and the proteins and lipids to which they are attached. For simplicity, the different sugars found within the glycocalyx are depicted with the same symbol (blue hexagons). Note that the depiction is roughly to scale, i.e., membrane proteins are buried under sugars.

FIGURE 2

Important sugars found in humans and examples of sugar conjugates found in the glycocalyx. (A) Common sugars found in humans and their pictorial representation according to the symbol nomenclature for glycans (SNFG). Note that this list is not exhaustive. Glc, glucose; Gal, galactose; Man, mannose; NeuAc, N-acetylneuraminic acid; GlcNAc, N-acetylglucosamine; GalNAc, N-acetylgalactosamine; GlaA, glucuronic acid; Xyl, xylose; Fuc, fucose. (B) Five examples for sugar conjugates found in the glycocalyx: GM4, a glycolipid; an extended core 4 O-glycosylation structure; the core structure of complex N-glycosylation; a side chain found in chondroitin sulfate; hyaluronan or hyaluronic acid. For simplicity, the linkage chemistry of the glycosidic bond between each monosaccharide is not specified.

FIGURE 3

The influence of mucins on cell morphology. (A) Schematic depiction of a mucin. Mucins have a bottlebrush structure: A heavily O-glycosylated protein backbone is attached to the membrane via a membrane anchor. (B) Increased mucin density on the cell membrane causes a fundamental change in cell morphology from a flat, adherent phenotype to strong membrane tubularization and cell lifting. (C) At low densities, mucins adopt a compact mushroom phenotype. When the mucin density increases above a threshold, the mucins extend to a polymer brush. The increased order imposes an entropic penalty on the system, which is reduced via bending of the membrane, giving the mucins more orientational degrees of freedom.

FIGURE 4

The picket-fence model and its influence onto membrane protein diffusion. (A) CD44, an abundant transmembrane protein, binds the glycocalyx component hyaluronic acid with its extracellular domain. Intracellularly, it engages with the cortical actin cytoskeleton via ezrin and other linker proteins. (B) Hyaluronan, CD44, and cortical actin demarcate membrane domains (red dotted line) that regulate diffusion of membrane proteins (red dot).

FIGURE 5

The cancer glycocalyx in integrin-mediated cell survival and its alteration upon oncogenic events. (A) Cells adhere to extracellular matrix (ECM) components via integrins. If the glycocalyx is thin, the integrins extend beyond the glycocalyx. (B) A thick glycocalyx as frequently expressed by cancer cells extends significantly further into the extracellular space than active integrins. As a result, interactions between integrins and the ECM are not possible in most areas of the cell surface, however, hotspots of integrin-ECM-binding are created via a kinetic funnel. (C) Upon proto-oncogenic events such as epithelial-to-mesenchymal transition (EMT) or oncogenic RAS activation (e.g., KRAS^G12D), the height of the glycocalyx increases, which fosters tumor progression and survival. The oncogenic effect is mediated by mediators such as GALNT7 and others.

FIGURE 6

Siglecs and sialic acid in immune system regulation and specific targeting of the cancer sialome for treatment strategies. (A) Sialylated proteins of lipids on the cancer cell bind to siglec receptors on immune cells, e.g., natural killer (NK) cells. Siglecs contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) and immunoreceptor tyrosine-based switch motifs (ITSMs), which recruit phosphatases such as Src homology 2 domain-containing protein tyrosine phosphatase 1 and 2 (SHP1 and -2), which causes activity reduction of the NK cell. (B) The antibody-sialidase conjugate T-Sia 2.0 to specifically desialylate cancer cells. (C) T-Sia 2.0 binds to membrane proteins characteristically overexpressed by the cancer. The sialidase causes desialylation of the cancer, which abolishes binding of NK cell siglecs and prevents NK cell downregulation.