Asymmetric synthesis of water-soluble analogues of galactosylceramide, an HIV-1 receptor: new tools to study virus-glycolipid interactions.

Galactosylceramide (GalCer) is a glycosphingolipid (GSL) receptor that allows HIV-1 infection of CD4-negative cells from neural and intestinal tissues. A water-soluble analogue of GalCer that features its polar head and the characteristic galactose-ceramide linkage but lacks the carbohydrate chains was prepared as a single enantiomer from (S)-serine. This analogue was not recognized in binding tests with the HIV-1 surface envelope glycoprotein gp120 in solution, which revealed the crucial importance of the ceramide alkyl chains. Two series of water-soluble GalCer analogues that contained either a hexanoic or a decanoic acyl unit and a saturated nine-carbon sphingosine moiety were designed by using molecular modeling results from natural GSLs and analogues with truncated alkyl chains. The longer chain compounds exhibit the characteristic fundamental conformation of GalCer. Seven analogues were prepared from Garner's aldehyde according to a straightforward and efficient asymmetric synthesis. All of these compounds proved to be water soluble but did not bind to gp120 in a solid-phase binding assay. These analogues were thus tested by using surface pressure measurements on a monomolecular film of GalCer, which served as a model of the plasma membrane. The incorporation of analogues very similar to GalCer into a GalCer monolayer prevented the insertion of gp120, whereas a structurally different derivative was not active. Based on these data, the molecular bases for recognition of GSLs by gp120 were elucidated. The essential importance of the GSL conformation in the primary interaction event and the crucial role of the alkyl chains of the ceramide moiety in the secondary interactions and the insertion process were clearly established.