Chitin synthesis and fungal pathogenesis

Abstract

Chitin is an essential part of the carbohydrate skeleton of the fungal cell wall and is a molecule that is not represented in humans and other vertebrates. Complex regulatory mechanisms enable chitin to be positioned at specific sites throughout the cell cycle to maintain the overall strength of the wall and enable rapid, life-saving modifications to be made under cell wall stress conditions. Chitin has also recently emerged as a significant player in the activation and attenuation of immune responses to fungi and other chitin-containing parasites. This review summarises latest advances in the analysis of chitin synthesis regulation in the context of fungal pathogenesis.

Figure 1

Chitin structure and diversity in fungi. Chitin is a β(1,4)-homopolymer of N-acetylglucosamine that folds in an anti-parallel manner forming intra-chain hydrogen bonds. Chitin chains are cross-linked covalently to β(1,3)-glucan (green) to form the inner skeleton of most fungi. Examples of shadow cast electron microscopy images of chitin from (a)Neurospora crassa; (b)Coprinus cinereus; (c) chitin–chitosan from Mucor mucedo; and (d)Candida albicans. In (e) and (f), the structure of chitin from C. albicans is shown in a chs3Δ and chs8Δ mutant, respectively, demonstrating that the architecture of chitin is genetically determined [24].

Figure 2

Signalling pathways that regulate Candida albicansCHS gene expression. The HOG, PKC and the Ca²⁺/calcineurin signalling pathways regulate chitin synthesis and CHS gene expression [20]. The Rlm1 transcription factor downstream of the PKC MAP kinase cascade controls the expression of a number of cell wall related genes in S. cerevisiae. Putative Rlm1 binding motifs (red boxes) in the promoters of CaCHS2 and CaCHS8 contribute to their cell wall stress-activated regulation [21]. Activation of the calcineurin pathway results in de-phosphorylation of the CaCrz1 transcription factor. CaCrz1 then moves to the nucleus and induces expression of genes with CDREs (calcium-dependent response elements) within their promoter sequences. C. albicans has significant re-wiring of signalling pathways compared to S. cerevisiae, for example, the role of the CaSko1 transcription factor in response to caspofungin is independent of the CaHog1 MAP kinase but involves the CaPsk1 PAS kinase [26]. Potential CDREs and Sko1 binding sites identified in silico upstream of CaCHS2 and CaCHS8 (grey boxes) were not required for regulation of gene expression [21]. Sequences in the first 347 bp and 125 bp of the CaCHS2 and CaCHS8 promoters governed expression through these three signalling pathways [21].

Figure 3

Phosphorylation of Candida albicans Chs3 on a specific serine residue is required to target the CHS to sites of polarised growth. CaChs3 was tagged with yellow-fluorescent protein (YFP) and the localisation of the CHS in growing hyphae was observed by time-lapse fluorescence microscopy. (a)CaChs3-YFP localises to the tip of the growing hypha and then flashes at the site of septum formation. (b) Mislocalisation of CaChs3 is observed when the serine at position 139 has been mutated to an alanine that cannot be phosphorylated (Chs3^S139A-YFP).