Characterisation of the p38 signalling pathway in candidalysin-induced epithelial activation

Abstract

Candida albicans is an important human fungal pathogen, contributing significantly to morbidity and mortality, especially in immunocompromised patients. C. albicans secretes the toxin candidalysin, which damages epithelial cells and drives innate inflammatory responses. Previous studies showed that candidalysin induces epithelial cytokine release via the epidermal growth factor receptor (EGFR) and mitogen-activated protein kinase (MAPK) pathways including p38, ERK1/2 and the transcription factor c-Fos. How these MAPKs are interconnected with EGFR and orchestrated along with other EGFR-activated pathways, such as protein kinase C (PKC), to promote downstream effector responses is not understood. In addition, candidalysin induces EGFR internalization via a mechanism which is yet to be identified.  
  
This thesis work demonstrates that candidalysin activates parallel and independent MAPK signalling pathways in oral epithelial cells, with distinct signalling outputs. Candidalysin induces ERK1/2 activation via EGFR, resulting in c-Fos activation and the release of the neutrophil-activating chemokines GM-CSF and G-CSF. In parallel, candidalysin activates p38 contributing to interleukin (IL-6) release and heat-shock protein (Hsp27) phosphorylation. Notably, p38 is not triggered by EGFR but rather by a circuit requiring both MAPK-kinases (MKKs) and Src in a non- redundant fashion. Moreover, use of the known p38 stimulus, anisomycin, and live cell imaging provide novel insight into the role of the Src-p38 pathway in mediating candidalysin-induced EGFR phosphorylation and internalization independently of ligands. Finally, in vivo experiments reveal a role for p38 in early and efficient clearance of C. albicans in a c-Fos independent manner during murine oropharyngeal candidiasis.  
  
This study delineates how candidalysin activates the p38 and ERK1/2 MAPK pathways that contribute to immune activation during C. albicans infection. It also provides novel insight into a previously unknown mechanism of p38 activation leading to ligand-independent endocytosis of EGFR.

Date of Award	1 Mar 2022
Original language	English
Awarding Institution	King's College London
Supervisor	Peter Parker (Supervisor) & Julian Naglik (Supervisor)