Cryptococcus neoformans capsule regrowth experiments reveal dynamics of enlargement and architecture

Abstract

The polysaccharide capsule of fungal pathogen Cryptococcus neoformans is a critical virulence factor that has historically evaded complete characterization. Cryptococcal polysaccharides are known to either remain attached to the cell as capsular polysaccharides (CPSs) or to be shed into the extracellular space as exopolysaccharides (EPSs). While many studies have examined the properties of EPS, far less is known about CPS. In this work, we detail the development of new physical and enzymatic methods for the isolation of CPS which can be used to explore the architecture of the capsule and isolated capsular material. We show that sonication or Glucanex enzyme cocktail digestion yields soluble CPS preparations, while use of a French pressure cell press or Glucanex digestion followed by cell disruption removed the capsule and produced cell wall–associated polysaccharide aggregates that we call “capsule ghosts”, implying an inherent organization that allows the CPS to exist independent of the cell wall surface. Since sonication and Glucanex digestion were noncytotoxic, it was also possible to observe the cryptococcal cells rebuilding their capsule, revealing the presence of reducing end glycans throughout the capsule. Finally, analysis of dimethyl sulfoxide-extracted and sonicated CPS preparations revealed the conservation of previously identified glucuronoxylomannan motifs only in the sonicated CPS. Together, these observations provide new insights into capsule architecture and synthesis, consistent with a model in which the capsule is assembled from the cell wall outward using smaller polymers, which are then compiled into larger ones.