Liposomas

1354

Biophysical Journal

Volume 80

March 2001

1354 –1358

Ultraweak Sugar-Sugar Interactions for Transient Cell Adhesion
Frederic Pincet,* Tugdual Le Bouar,* Yongmin Zhang,† Jacques Esnault,† Jean-Maurice Mallet,† Eric Perez,* ´ ´ and Pierre Sinay† ¨
*Laboratoire de Physique Statistique de l’Ecole Normale Superieure, UMR 8550 associee au CNRS et aux Universites Paris 6 et Paris 7,´ ´ ´ and †Departement de Chimie de l’Ecole Normale Superieure, UMR 8642, 75231 Paris cedex 05, France ´ ´

ABSTRACT Carbohydrate-carbohydrate interactions are rarely considered in biologically relevant situations such as cell recognition and adhesion. One Ca2 -mediated homotypic interaction between two Lewisx determinants (Lex) has been proposed to drive cell adhesion in murine embryogenesis.Here, we confirm the existence of this specific interaction by reporting the first direct quantitative measurements in an environment akin to that provided by membranes. The adhesion between giant vesicles functionalized with Lex was obtained by micropipette aspiration and contact angle measurements. This interaction is below the thermal energy, and cell-cell adhesion will require a large numberof molecules, as illustrated by the Lex concentration peak observed at the cell membranes during the morula stage of the embryo. This adhesion is ultralow and therefore difficult to measure. Such small interactions explain why the concept of specific interactions between carbohydrates is often neglected.

INTRODUCTION Molecular features governing the selectivity in cell-cell recognition andadhesion are key elements to understanding morphogenesis and organogenesis. All living organisms are characterized by the presence of glycoproteins and glycosphingolipids on the cytoplasmic membrane. Carbohydrate chains, as exposed structures at the cell surface, should play a key role in early events of cell-cell recognition. Indeed, they have firmly been recognized as interaction sites in celladhesion processes such as leukocyte recruitment or hostpathogen interaction. Such events are commonly attributed to lectin or lectin-like proteins and the corresponding specific carbohydrate ligands. It has been suggested that specific carbohydrate-carbohydrate interactions may also play this important role (Hakomori, 1991). Several direct measurements of adhesion between surfaces decorated withglycolipids have been reported in the past (Marra, 1985, 1988; Evans, 1987; Rand and Parsegian, 1989; Luckham et al., 1993; Yu et al., 1998; Ricoul et al., 1998). These strong adhesions, obtained through various techniques (surface force apparatus, osmotic stress method, and vesicle micropipette aspiration), were due to van der Waals or other nonspecific forces. However, some carbohydrate-carbohydrateinteractions found in biological processes have been proposed to be specific (Hakomori, 1991; Sharon and Lis, 1993; Bovin, 1996). One of them, the Lewisx determinant (Lex), has been identified as presumably playing a biological role by means of intermembrane interactions. According to a current hypothesis, it has a calcium-mediated specific interaction with itself. Lex has been shown to be involvedin murine and human embryogenesis. This carbohydrate is present on the cell membrane in the polar headgroup of glycolipids. It is not expressed at the cell surface until the eight-cell stage, shows maximal expression at the morula stage of mouse embryogenesis, and declines after compaction (Solter and Knowles, 1998; Fenderson et al., 1986). This compaction stage could be inhibited either byanti-Lex antibodies or Lex itself, or by inactivating the calcium with EDTA. As Lex is not charged, the role of the calcium is not electrostatic in nature. These results (Eggens et al., 1989; Kojima et al., 1994) led the authors to propose a calcium mediated Lex-Lex-specific homotypic interaction as a basis of cell adhesion in preimplantation embryos, and in various tumor cells. Other observations...