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The natural LewisX-bearing lipids promote membrane adhesion: influence of ceramide on carbohydrate-carbohydrate recognition.

Carbohydate–carbohydrate recognition has recently emerged as a potentially important interaction in cell adhesion processes. One carbohydrate, the Lewis determinant (Le), is involved in murine embryogenesis, although the precise mechanism underlying this role is as yet unclear. Camediated homotypic interaction between two Le determinants has been proposed to initiate cell adhesion during the compaction stage of the embryo. Several recent studies support the existence of such calcium-mediated homotypic recognition and have also provided a body of information on the geometry, structural requirements, and energetics of a Le–Le interaction. However, in these studies the local environment of the Le was always very different from that existing at a typical cell surface. In cells, the Lebearing molecules are usually composed of a ceramide connected to the Le trisaccharide through a lactose group. This geometry considerably restricts the possible orientations of the Le [13] compared to those of soluble forms, 8] or to the large freedom in orientation provided by long flexible spacers. The ceramide in the natural Le-bearing molecules may therefore have a very strong influence on the recognition of Le borne by opposite cells, by inhibiting or enhancing the recognition. To test more directly the hypothesis that Le could serve as a promoter for cell adhesion, the challenge is to determine if the natural Le-bearing molecules allow the Le–Le recognition between two membranes. Two natural glycosphingolipids have been synthesized for this purpose. The first one, called CerLLe, is composed of a Le trisaccharide (Galb1!4[Fuca1!3]GlcNAc) attached to a ceramide (Cer) unit (two hydrophobic tails: one sphingosine and one stearic acid) through a lactose (L) group (Figure 1a). The second one, CerLLe, is used as a control and is composed of the same ceramide and lactose moieties, but has a Lewis a (Le) trisaccharide as headgroup instead of a Le determinant. Le is an isomer of Le, and the only difference between the two determinants is the position of the