Date Published: May 16, 2013
Publisher: Public Library of Science
Author(s): Freda E. C. Jen, Matthew J. Warren, Benjamin L. Schulz, Peter M. Power, W. Edward Swords, Jeffery N. Weiser, Michael A. Apicella, Jennifer L. Edwards, Michael P. Jennings, Thomas Rudel.
Pili of pathogenic Neisseria are major virulence factors associated with adhesion, twitching motility, auto-aggregation, and DNA transformation. Pili of N. meningitidis are subject to several different post-translational modifications. Among these pilin modifications, the presence of phosphorylcholine (ChoP) and a glycan on the pilin protein are phase-variable (subject to high frequency, reversible on/off switching of expression). In this study we report the location of two ChoP modifications on the C-terminus of N. meningitidis pilin. We show that the surface accessibility of ChoP on pili is affected by phase variable changes to the structure of the pilin-linked glycan. We identify for the first time that the platelet activating factor receptor (PAFr) is a key, early event receptor for meningococcal adherence to human bronchial epithelial cells and tissue, and that synergy between the pilin-linked glycan and ChoP post-translational modifications is required for pili to optimally engage PAFr to mediate adherence to human airway cells.
Type IV fimbriae, or pili, are long filamentous structures that extend from the bacterial surface and primarily consist of the monomer pilin protein . Pili are shown to play a major role in promoting colonization of the mucosal epithelium by several human pathogens and, thus, are generally considered to be major virulence determinants for bacterial pathogens. Roles attributed to the Type IV pili expressed by the pathogenic Neisseria (N. gonorrhoeae and N. meningitidis) include: adhesion, cytotoxicity, twitching motility, auto-aggregation, and DNA transformation ; . Pili of the pathogenic Neisseria are post-translationally modified. These post-translational modifications (PTMs) include: a glycan , phosphorylcholine (ChoP) , and/or a phosphoglycerol . Roles for these PTMs in the pathogenesis of N. gonorrhoeae and N. meningitidis are proposed. For example, during N. gonorrhoeae challenge of primary human cervical epithelial cells, the pilin glycan mediates the activation state of complement receptor 3 (CR3); this, in turn, modulates gonococcal adherence, invasion, and ultimately their intra-epithelial cell survival . Additionally, the pili-linked phosphoglycerol is proposed to trigger N. meningitidis dissemination . The role of the ChoP PTM has not been previously determined and is the subject of this study.
The presence of ChoP on pilin is subject to phase variation, and this results from alterations in the length of the homopolymeric guanine tract in pptA, the gene encoding the transferase responsible for linking ChoP to pilin . The presence of surface exposed, phase variable ChoP is not unique to Neisseria. For example, ChoP is a covalently linked, post-translational modification found on the surface of predominate respiratory pathogens, e. g. H. influenzae, S. pneumoniae, and Pseudomonas aeruginosa for which a role in mediating adhesion and invasion to host epithelia is described for ChoP. Additionally, C-reactive protein (CRP) present in blood has the potential to bind to ChoP that, in turn, can result in complement-mediated killing of H. influenzae bearing ChoP on their surface . In this scenario, expression of ChoP would serve as a selective disadvantage to the microorganism. This has led to the idea that phase variation of ChoP provides one mechanism by which invasive pathogens adapt to a transitional lifestyle that occurs with respect to colonization of the respiratory mucosa and, subsequent, survival during systemic infections . We could not demonstrate CRP/complement-mediated killing of strain C311#3 using the same methodology as described for H. influenzae (20); however, Casey et al. (2008) have shown ChoP-dependent opsonophagocytic killing of strain C311#3, suggesting that expression of ChoP by meningococci may be disadvantageous in some environments. As ChoP is clearly important in the pathobiology of other respiratory pathogens, we sought to analyse its role in pili-related functions in N. meningitidis.