2026
Volume 26, Number 1, pp. 9-22
SARS-CoV-2 spikes bind epithelial cell surface heparan sulfate not the sterically hidden sialic acid inaccessible in deeper clefts
J.N. Mehrishi
Former Department of Radiotherapeutics, University of Cambridge, UK
Endothelial-epithelial cells (ECs) are covered with heparan sulfate (HS), which bears negative charges (quantified as electrophoretic mobility, EPM). The EPM is related to the ζ-potential, in turn related to the surface charge density. Heparanase removes HS, hence decreasing the EPM. SARS-CoV-2 spike protein interacts with HS for attachment and with acetylcholine esterase (ACE2) in order to enter cells, and moves into ACE2-negative cells via the independent receptor TMEM106B (a protein encoded by the TMEM106B gene, found primarily within neurons and oligodendrocytes). Neuraminidase treatment of cells bearing sialic acid-related electrostatic charge from ionized N-acetylneuraminic acid (NANA or NA) releases NANA, decreasing cell EPM, but ECs treated with neuraminidase (EC 3.2.18) or proteases do not show decreased EPM, indicating that no sialic acid is present (although vascular endothelial surfaces bear high concentrations). This is comparable to cat red blood cell (RBC) membrane structure, these cells show no decrease of cell EPM when treated with neuraminidase, suggesting absence of sialic acid. Interestingly, cat RBCs are covered with gangliosides, glycolipid NANA, removable by pronase and which, when removed, reveal hidden sialic acid–glycoprotein–NANA removable by neuraminidase. Pronase-treated cat RBCs further treated with neuraminidase showed decreased EPM, confirming exposed sialic acid. It is suggested that the EC membrane structure incorporates sialic acid (in deeper clefts) sterically hidden behind HS and inaccessible as a receptor. This requires confirmation by investigating cells sequentially treated with heparanase (EC 3.2.1.166) and neuraminidase, using cell electrophoresis and flow cytometry of cells labeled with Sambucus nigra-FITC. The conclusion is that although sialic acid is indeed a receptor for influenza virus HCOV-OC43, it is by no means a universal receptor for viruses.
Keywords: ACE2, coronavirus, mutants, receptors, steric hindrance, THEM106B1