Loss of Cell Wall Mannosylphosphate in Candida albicans Does Not Influence Macrophage Recognition

Richard P Hobson,Carol A Munro,Steven Bates,Donna M Maccallum,Jim E Cutler,Sigrid E.M Heinsbroek,Gordon D Brown,Frank C Odds,Neil A.R Gow

doi:10.1074/jbc.m405003200

Abstract

The outer layer of the cell wall of the human pathogenic fungus Candida albicans is enriched with heavily mannosylated glycoproteins that are the immediate point of contact between the fungus and cells of the host, including phagocytes. Previous work had identified components of the acid-labile fraction of N-linked mannan, comprising beta-1,2-linked mannose residues attached via a phosphodiester bond, as potential ligands for macrophage receptors and modulators of macrophage function. We therefore isolated and disrupted the CaMNN4 gene, which is required for mannosyl phosphate transfer and hence the attachment of beta-1,2 mannose oligosaccharides to the acid-labile N-mannan side chains. With the mannosylphosphate eliminated, the mnn4Delta null mutant was unable to bind the charged cationic dye Alcian Blue and was devoid of acid-labile beta-1,2-linked oligomannosaccharides. The mnn4Delta mutant was unaffected in cell growth and morphogenesis in vitro and in virulence in a murine model of systemic C. albicans infection. The null mutant was also not affected in its interaction with macrophages. Mannosylphosphate is therefore not required for macrophage interactions or for virulence of C. albicans.

Highlights

The outer layer of the cell wall of the human pathogenic fungus Candida albicans is enriched with heavily mannosylated glycoproteins that are the immediate point of contact between the fungus and cells of the host, including phagocytes
Because most ␤-1,2-mannose is attached via mannosylphosphate in the C. albicans cell wall, we investigated the role of this fraction in macrophage recognition by constructing a null mutant in the C. albicans homolog of the Saccharomyces cerevisiae MNN4, which is required for mannosylphosphorylation
The C. albicans MNN4 gene was isolated as the closest homolog to the ScMNN4 gene, which is involved in the regulation of mannosylphosphate production

Summary

Introduction

The outer layer of the cell wall of the human pathogenic fungus Candida albicans is enriched with heavily mannosylated glycoproteins that are the immediate point of contact between the fungus and cells of the host, including phagocytes. Previous work had identified components of the acid-labile fraction of N-linked mannan, comprising ␤-1,2-linked mannose residues attached via a phosphodiester bond, as potential ligands for macrophage receptors and modulators of macrophage function. The outer cell wall of C. albicans is enriched with glycoproteins that are modified by highly branched N-linked glycosylation including a mannosylphosphate-containing fraction that has been implicated in macrophage interactions [18, 19]. The other known source of ␤-1,2-linked mannose residues in the C. albicans cell wall is phospholipomannan, which consists of a lipid (ceramide) attached by a unique mannosylphosphate/ manno-inositol phosphate spacer to an unbranched ␤-1,2-oligomannoside chain of up to 19 residues in length [15]. Purified ␤-1,2-manno-oligosaccharides bind to macrophages [24], inhibit binding of C. albicans to macrophage-like cells in competition assays [25, 26], inhibit nitric oxide production [27], This paper is available on line at http://www.jbc.org

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