The ‘black fungus’: elucidating cell wall changes in the Mucor circinelloides germinating spore and its influence on host interaction

Ghuman, Harlene (2023). The ‘black fungus’: elucidating cell wall changes in the Mucor circinelloides germinating spore and its influence on host interaction. University of Birmingham. Ph.D.

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Abstract

The filamentous fungal pathogen, Mucor circinelloides, is a causative agent of the fatal invasive fungal infection, mucormycosis, recently termed the ‘black fungus’. During the COVID-19 pandemic there was a dramatic increase in the incidence of mucormycosis, which has mortality rates approaching 100%. The objective of this research was to enhance our understanding of M. circinelloides spore germination, and determine how this influences the interaction between spores and the host immune cells, macrophages and platelets. Firstly, (i) RNA-sequencing was employed to determine differentially expressed genes and processes during M. circinelloides spore germination, with an emphasis on those implicated in cell wall biosynthesis, and (ii) transmission electron microscopy was utilised to investigate the ultrastructure of spores during germination, and (iii) fluorescence microscopy and flow cytometry were employed to determine cell wall compositional changes. Elucidation of significantly differentially expressed genes during germination, showed that from resting spore to fully swollen spore, there is an upregulation of genes associated with cell wall biosynthesis pathways including, mannoprotein glycosylation, β-glucan biosynthesis and chitin biosynthesis. Accordingly, investigating the ultrastructure of the M. circinelloides spore cell wall during germination, showed extensive cell wall remodelling. Germination is at the crux of mucormycetes pathogenicity, and the first point-of-contact for host immune cells is the fungal cell wall. To investigate whether the developmental stage – where significant compositional cell wall changes occur - of M. circinelloides spores influences host-spore interaction, the interplay between spores at different stages of germination with macrophages and platelets was investigated. This research showed that M. circinelloides spore developmental stage plays a significant role in its interaction with macrophages and platelets, with spores at the mid-point swelling displaying the greatest phagocytic uptake and inducing the most platelet aggregation. Furthermore, phagocytosis of M. circinelloides spores was shown to be dependent upon mannose receptors, and characterisation of the platelet activation pathway showed platelet IgG receptor FcyRIIa and surface integrin aIIbβ3 to mediate platelet activation in response to M. circinelloides spores. Collectively, this research provides findings that could be further investigated in the bid to identify novel targets for mucormycosis treatment.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):