On the role of metal cations in cellular adhesion: effect on cell surface charge.

Reprinted from THE JOURNAL or Exrnnrmzxru. Zoonoc! Vol. 163, No. 1, October 1966 On the Role of Metal Cations in Cellular Adhesion: Effect on Cell Surface Charge‘ PETER B. ARMSTRONG 2 Department of Biology, The Johns Hopkins University, Baltimore, Maryland ABSTRACT An examination has been made of the hypothesis that the role of divalent cations in promoting reaggregation of dissociated cells is principally to reduce the negative electrostatic charge at the cell surface. This hypothesis would suggest that cells in the presence of different divalent cations would reaggregate at identical rates if the concentrations of the different cations were so adjusted that in all cases the cells possess the same charge. This expectation has not been fulﬁlled. Magne- sium ion was most eﬂ-‘ective in promoting reaggregation, calcium somewhat less so, and strontium and barium had little or no eifect. It seems well established that the ability of individual animal cells to adhere to one another normally requires the presence of some divalent cation in the external milieu. In a now classic study, Herbst (’00) showed that the cells of the sea- urchin larva separated from one another when the larva was placed in sea water which was lacking in calcium and mag- nesium ions. Subsequent ﬁndings that a variety of other tissues could also be dis- sociated upon removal of calcium and magnesium reinforced the notion that these ions play an essential role in the process of cell adhesion. A variety of hypotheses have been pro- posed to account for this dependence of cell adhesion upon the presence of dival- ent cations in the medium (Gray, ’31; Schmitt, ’41; Steinberg, ’58). One of the most appealing of these arises as a con- sequence of a more general discussion re- cently presented by Curtis (’60, ’62, ’66) concerning the question of just what phys- ical forces are of signiﬁcance in the phe- nomenon of cellular adhesion. Curtis has proposed that the forces oper- ative in cell adhesion are the same as those which are of greatest importance in deter- mining the properties of lyophobic colloids. Considerable success has been achieved in interpreting the behavior of these latter sys- tems if it is assumed that the most impor- tant forces between the colloid particles are van der Waals forces of attraction and electrostatic forces of repulsion (Derjaguin J. EXP. ZooL., 163: 99-110. and Landau, ’41; Verwey and Overbeek, &#39;48). The latter forces result from an elec- trical charge carried at the surfaces of the colloid particles. The particles which make up a lyophobic colloid system can be caused to adhere to one another by addition of electrolyte to the medium. The added electrolyte exerts its role primarily by reducing the electrostatic forces of repulsion. Like colloid particles, cells hear an elec- trical charge at their surfaces. Under physiological conditions, the net surface charge of most investigated cells is nega- tive in sign (Abramson et al., ’42; Bang- ham and Pethica, ’60; Collins, ’66; Dan, ’34; Nevo et al., ’61). This charge appar- ently is due mainly to ionogenic groups which are a part of the surface structure. The hypothesis proposed by Curtis sug- gests that the primary role of the positively charged divalent cations in promoting cell adhesion is to lower the cells’ net nega- tive surface charge (Curtis, ’66). This would reduce the electrostatic repulsive forces between individual cells. Adhesion could occur upon sufﬁcient reduction of these repulsive forces. The present study is an attempt to test the hypothesis that Ca” and Mg” are required for cells to adhere mainly because these ions lower surface charge. If it could be shown that other cations were also able 1 This work was performed in partial fulﬁllment of the requirements for the Ph.D. degree at the Johns Hopkins University. 3Present address: Department of Zoology, Univer- sity of California, Davis. Davis, California. 99