Glycocalyx Curving the Membrane: Forces Emerging from the Cell Exterior

Abstract

Morphological transitions are typically attributed to the actions of proteins and lipids. Largely overlooked in membrane shape regulation is the glycocalyx, a pericellular membrane coat that resides on all cells in the human body. Comprised of complex sugar polymers known as glycans as well as glycosylated lipids and proteins, the glycocalyx is ideally positioned to impart forces on the plasma membrane. Large, unstructured polysaccharides and glycoproteins in the glycocalyx can generate crowding pressures strong enough to induce membrane curvature. Stress may also originate from glycan chains that convey curvature preference on asymmetrically distributed lipids, which are exploited by binding factors and infectious agents to induce morphological changes. Through such forces, the glycocalyx can have profound effects on the biogenesis of functional cell surface structures as well as the secretion of extracellular vesicles. In this review, we discuss recent evidence and examples of these mechanisms in normal health and disease.

Keywords: cancer; cell shape; glycocalyx; membrane morphology; microvesicle; mucin.

Figure 1

The molecular crowding of glycopolymers bends membranes outward into a plethora of shapes.(a) Unstructured glycopolymers such as mucin and hyaluronan (HA) are anchored to the cell surface. Increasing polymer density achieves the coverage necessary for molecular crowding. (b) Estimates of the energetic driving forces and two-dimensional tangential pressures in a theoretical mucin 1 (MUC1) brush on a flat segment of membrane. The free energy per area of the brush is normalized by the thermal energy, k_BT. Complete details of the theoretical model and parameter estimates are in Gandhi et al. (2019). (c, top) Steric interactions between crowded polymers exert increasing pressure to bend a flat surface into blebs and tubular membrane protrusions that pearl or vesiculate tips to release microvesicles. (Bottom) Micrographs showing examples of these cell morphologies. Figure adapted with permission from Shurer et al. (2019).

Figure 2

Microvesicle release from membrane protrusions. (a) Electron micrograph and diagrams depict the presence of an actin core in glycocalyx-coated tubular membrane protrusions. Upon cytoskeletal depolymerization, pressures from the glycocalyx can vesiculate protrusion tips or undulate protrusions. Protrusions with vesiculated tips may be stabilized by actin retained at the base. Myosin accumulation at protrusion tips can mediate membrane fissure for microvesicle release. Alternatively, the spontaneous fissure of undulating protrusions can lead to microvesicle release. (b) Fluorescence image shows myosin accumulation at the vesiculated tips of hyaluronan synthase 3 (HAS3)-induced tubular protrusions. (c) Protrusion undulation and pearled morphologies have been observed for mucin 1 (MUC1)-mediated processes, as depicted in this scanning electron micrograph. Figure adapted with permission from Koistinen et al. (2015) and Shurer et al. (2019).

Figure 3

Glycolipid-induced membrane structures. (a) Glycolipid clustering induces outward membrane protrusions for neurite outgrowth. Major bilayer-forming lipids such as phosphatidylcholine (PC) have a cylindrical shape (orange). The membrane bends away from large headgroups that shape glycolipids such as GM1 into a cone (pink). Factors such as matrix proteins bind glycolipids to induce curvature. PC is depicted with a choline (gray) headgroup, while GM1 is depicted with a headgroup of five sugar monomers (blue, yellow, and purple). (b) Glycolipids organized by carbohydrate-binding proteins (lectins) can bend membranes inward to form clathrin-independent endocytic buds and tubules. (Left) Glycolipid binding to the pentameric toxin subunit B (TxB) of Shiga and cholera toxins and polyomavirus SV40 VP1 causes membrane curvature. Glycolipid-TxB/VP1 complexes are clustered by membrane fluctuation and induce lipid compression and reorganization for tubulation. (Right) Endogenous galectin-3 first binds cargo glycoproteins before oligomerization occurs for binding glycolipids and inducing membrane bending for endocytosis.