Polymer exclusion, cell adhesion and membrane fusion

Abstract

FROM physicochemical studies on the adhesion of fibroblasts to synthetic substrata^1,2 I suggested that, in general, surface-bound polymers (for example adsorbed serum proteins) probably inhibit cell attachment because of their steric exclusion volume. This effect is well known in colloid science (Faraday used gelatin to stabilise gold sols³) but does not seem to be widely appreciated in cell biology. Thus, a recent review on the role of proteins and divalent cations in cell adhesion, viewed as a problem in colloid stability, discusses only charge repulsion and calcium bridging⁴. Furthermore, standard texts on the cell surface⁵ present the classical DLVO (Derjaguin, Landau, Verwey and Overbeek) theory of charge repulsion against dispersive attraction, but omit the later development by Overbeek and coworkers of the principle that even uncharged surfaces, if covered by adsorbed polymer molecules, may repel each other⁶. I now show that steric exclusion by glycocalyx polymers on the plasma membrane would explain some recent observations on fusion and attachment, by a variety of cells. The term steric exclusion³ comprises not only geometric exclusion by rigid molecules, such as rods and spheres, but also unfavourable thermodynamic parameters such as free energy, summed up over many segments, and, for random coil polymers, entropy resulting from the ability of each molecule to claim extra lebensraum by flexing its segments.