Leishmania lipophosphoglycan: how to establish structure-activity relationships for this highly complex and multifunctional glycoconjugate?

Abstract

A key feature of many pathogenic microorganisms is the presence of a dense glycocalyx at their surface, composed of lipid-anchored glycoproteins and non-protein-bound polysaccharides. These surface glycolipids are important virulence factors for bacterial, fungal and protozoan pathogens. The highly complex glycoconjugate lipophosphoglycan (LPG) is one of the dominant surface macromolecules of the promastigote stage of all Leishmania parasitic species. LPG plays critical pleiotropic roles in parasite survival and infectivity in both the sandfly vector and the mammalian host. Here, we review the composition of the Leishmania glycocalyx, the chemical structure of LPG and what is currently known about its effects in the mammalian host, specifically. We will then discuss the current approaches employed to elucidate LPG functions. Finally, we will provide a viewpoint on future directions that this area of investigation could take to unravel in detail the biological activity of the specific molecular elements composing the structurally complex LPG.

Keywords: LPG function; LPG structure; LPG structure-activity relationships; Leishmania glycoconjugates; chemical synthesis; lipophosphoglycan.

Figure 1

Structure of the Leishmania LPG. Top and bottom panels show two different representations of the LPG structure of Leishmania parasite. LPG is constituted of four key domains. The GPI anchor and the glycan core are shown in red. The linear conserved phosphoglycan chain is in blue. The terminating oligosaccharide cap is shown in pink. In each domains, the residues that are not conserved among the LPG of the different Leishmania species are represented in a different color (Man, X, Y and Glc). The Glucose phosphate branched on the mannose residue of the glycan core (in green) is present in the LPG of L. donovani, L. mexicana and some subspecies of L. major but absent in some other subspecies. The linear phosphoglycan chain of L. donovani can be substituted with different residues. X represents the substitutent group express in the LPG L. mexicana, L. major and L. tropica. Y is a substitutent present in the LPG of L. aethopica. Gal, galactose; Man, Mannose; Glc, glucose; Galf, galactofuranose; GlcN, glucosamine; Ino, inositol.

Figure 2

Localization of LPG at the membrane of the Leishmania-containing phagosome. Bone marrow macrophages were infected with L.donovani promastigote for 1 h at 37°C then macrophages were washed to remove extracellular parasites and incubated in new medium for 24 h. Infected cells were processed for immunofluorescence staining. The LPG molecules were stained using the anti-PG antibody CA7AE (Red), the lysosome and phagosome compartments were stained using the anti-LAMP-1 antibody (Green), the host cell and the parasite nuclei were stained using the Hoechst dye (Blue). Images are analyzed using the Imaris software. (A) Represents a single Z section of 0.7 μm. (B) Represents a zoomed image area of (A). Panel (C) is a 3D reconstruction of the entire Z stack. Arrow points to two Leishmania-containing vacuoles showing LPG at the phagosomal membranes. Scale bar, 10 μm.