The structure of a Cryptococcus neoformans polysaccharide motif recognized by protective antibodies: A combined NMR and MD study

• Published in: Proceedings of the National Academy of Sciences - PNAS 2024-02, Vol.121 (7), p.1
• Main Authors: Hargett, Audra A., Azurmendi, Hugo F., Crawford, Conor J., Wear, Maggie P., Oscarson, Stefan, Casadevall, Arturo, Freedberg, Darón I.
• Format: Article
• Language: English
• Subjects: Biological Sciences; Capsular polysaccharides; Cryptococcosis; Cryptococcus neoformans; Esters; Exopolysaccharides; Fungi; Glucuronoxylomannan; Mannan; Meningitis; Monoclonal antibodies; NMR; Nuclear magnetic resonance; Phagocytosis; Polymers; Polysaccharides; Saccharides; Vaccines; Virulence factors
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2315733121

Cryptococcus neoformans is a fungal pathogen responsible for cryptococcosis and cryptococcal meningitis. The C. neoformans ’ capsular polysaccharide and its shed exopolysaccharide function both as key virulence factors and to protect the fungal cell from phagocytosis. Currently, a glycoconjugate of these polysaccharides is being explored as a vaccine to protect against C. neoformans infection. In this study, NOE and J -coupling values from NMR experiments were consistent with a converged structure of the synthetic decasaccharide, GXM10-Ac 3 , calculated from MD simulations. GXM10-Ac 3 was designed as an extension of glucuronoxylomannan (GXM) polysaccharide motif (M2) which is common in the clinically predominant serotype A strains and is recognized by protective forms of GXM-specific monoclonal antibodies. The M2 motif is a hexasaccharide with a three-residue α-mannan backbone, modified by β-(1→2)-xyloses (Xyl) on the first two mannoses (Man) and a β-(1→2)-glucuronic acid (GlcA) on the third Man. Combined NMR and MD analyses reveal that GXM10-Ac 3 adopts an extended structure, with Xyl/GlcA branches alternating sides along the α-mannan backbone. O -acetyl esters also alternate sides and are grouped in pairs. MD analysis of a twelve M2-repeating unit polymer supports the notion that the GXM10-Ac 3 structure is uniformly represented throughout the polysaccharide. This derived GXM model displays high flexibility while maintaining a structural identity, yielding insights to further explore intermolecular interactions between polysaccharides, interactions with anti-GXM mAbs, and the cryptococcal polysaccharide architecture.