Glycosphingolipid functions

Abstract

The combination of carbohydrate and lipid generates unusual molecules in which the two distinctive halves of the glycoconjugate influence the function of each other. Membrane glycolipids can act as primary receptors for carbohydrate binding proteins to mediate transmembrane signaling despite restriction to the outer bilayer leaflet. The extensive heterogeneity of the lipid moiety plays a significant, but still largely unknown, role in glycosphingolipid function. Potential interplay between glycolipids and their fatty acid isoforms, together with their preferential interaction with cholesterol, generates a complex mechanism for the regulation of their function in cellular physiology.

Figure 1.

Synthetic pathways for the major GSL species. Glucosylceramide is the key precursor for most GSLs and lactosyl ceramide provides the branch point for the different GSL series.

Figure 2.

The glycolipid binding motif in the HIV adhesin gp120 is contained within the chemokine receptor binding sequence of the V3 loop. Red amino acids are required for chemokine receptor binding, green for GSL binding, and yellow are required for both. GSL sugar stacking against the ring of the aromatic amino acid is the key to binding.

Figure 3.

GSL fatty acid mixing can “switch on” ligand binding. Gp120 bound Gb₃/cholesterol vesicles separated to the top of a sucrose gradient (fraction 1) after equilibrium ultracentrifugation. Gp120 binding in this fraction varied according to the Gb₃ fatty acid content and the mixture of Gb₃ fatty acid isoforms present. Effective binding to the human renal Gb₃ mixture was seen. The C16 Gb₃ fatty acid isoform was also effectively bound but C18 and C20 alone were not recognized. Binding to C22 and C24 Gb₃ was effective but no binding to C24:1 Gb₃ was detected. A mixture of all these Gb₃ fatty acid isoforms (mix) was effectively bound but removal of the C24:1 Gb₃ (mix w/o 24:1) resulted in a loss of gp120 binding. Removing in addition, the C18 and C20 Gb₃ fatty acid isoforms (mix w/o24:1, 18, 20), each of which alone are unbound by gp120, induced gp120 binding. Thus, the presence of C18 and C20 together can “switch off” gp120-Gb₃ binding, whereas C24:1 Gb₃ can “switch on” gp120-Gb₃ binding. Remarkably, the combination of C24:1 and C18 Gb₃ fatty acid isoforms (which individually do not bind gp120), generated vesicles highly reactive with gp120. Similar results were obtained for VT1 binding to these Gb₃ fatty acid isoforms. (Adapted from Mahfoud et al. 2009; reprinted with permission from ASBMB Journals © 2009.)

Figure 4.

Composite GSL membrane receptor foci as dynamic positional “bar code” markers during vesicular transport. Differential GSL receptor function according to fatty acid isoform mix or GSL “coreceptor” presence provides a mechanism to generate precise positional foci for ligand binding during dynamic membrane processes. Complementary (which in combination, bind ligand) lipid microdomains in vesicular compartments A and B are indicated in red and green. The red domains in vesicle A contain GSL fatty acid isoform mixtures, which either do not bind ligand within a cholesterol matrix, or are missing a coreceptor GSL able to facilitate GSL presentation for ligand binding within a cholesterol matrix. In vesicle B, the green lipid microdomains contain the appropriate GSL fatty acid isoform to induce ligand binding within the red domains, or the appropriate coreceptor GSL to promote GSL receptor function in the red lipid microdomains of vesicle A. On vesicle fusion (C), the initially separate domains can now aggregate and can show a time-dependent initiation of GSL receptor competency via fusion of the red and green domains (yellow domains). At later times (D) these domains may separate as shown, or remain stable and additional domains of GSL receptor competency (yellow) can be formed. Although shown as domains, intersection of GSL diffusion gradients could achieve the same ends.