Role of MNN1 gene family in Candida albicans in mannan structure and host immune recognition

Bhawna Yadav bhawna.yadav@abdn.ac.uk 1 Katja Schaefer 1 Douglas W. Lowman 2 Michael D. Kruppa 2 David L. Williams 2 Neil A.R. Gow 1
1Aberdeen Fungal Group, MRC Centre for Medical Mycology, Institute of Medical Sciences, University of Aberdeen, Aberdeen, UK
2Department of Surgery, Centre of Excellence in Inflammation, Infectious Disease and Immunity, Quillen College of Medicine, East Tennessee State University, Johnson City, Tennessee, USA

Cell surface molecules are significant in host-pathogen interactions as the host defence system is capable of recognizing a range of cell surface pathogen-associated molecular patterns (PAMPs) with the help of specific pathogen recognition receptors (PRRs).

The outer N-mannan-rich microfibrillar layer of the Candida albicans cell wall is complex, heterogeneous and differs in chemistry in different cell morphotypes. The N-mannan is composed of an α-1,6 mannose backbone with side chains predominantly of α-1,2 mannose capped with α-1,3 mannose. Because the α-1,3 mannose capping sugars are likely to be important for mannan recognition by C-type lectins and other mannan-recognising PRRs we aimed to create and characterise a complex mutant lacking all six members of the Mnn1 α-1,3 mannosyltransferases. .

A previous study reported the characterization of individual null mutants of the MNN1 family members, however, with the exception of the mnn14 null, none of the other mutants displayed any defects – most likely due to functional redundancies between the family members (1).

We created a mnn1Δ6 mutant, disrupting all twelve alleles of MNN1 family, using CRISPR-Cas9 mediated gene disruption. Analysis of mannan structure by NMR in ten independent mnn1Δ6 mutants showed an expected loss of peak corresponding to α-1,3 mannan residues and longer mannan side chains suggesting that α-1,3 mannose capping contributed to side-chain termination. However, no change in cell wall porosity was observed in the hextuple mutants. Immune recognition by human monocytes and mouse RAW macrophage cell line measured by cytokine production were altered in the mnn1Δ6 mutants.

Our results thus suggest for the first time that the terminal capping mannan residues regulate mannan chain length in C. albicans, and PAMP recognition by host PRRs leading to altered recognition and immune response.

References:

  1. Bates S et al., BMC Notes Res, 2013








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