N-Acetylglucosamine (GlcNAc) Induction of Hyphal Morphogenesis and Transcriptional Responses in Candida albicans Are Not Dependent on Its Metabolism

Abstract

N-Acetylglucosamine (GlcNAc) stimulates important signaling pathways in a wide range of organisms. In the human fungal pathogen Candida albicans, GlcNAc stimulates hyphal cell morphogenesis, virulence genes, and the genes needed to catabolize GlcNAc. Previous studies on the GlcNAc transporter (NGT1) indicated that GlcNAc has to be internalized to induce signaling. Therefore, the role of GlcNAc catabolism was examined by deleting the genes required to phosphorylate, deacetylate, and deaminate GlcNAc to convert it to fructose-6-PO 4 (HXK1, NAG1, and DAC1). As expected, the mutants failed to utilize GlcNAc. Surprisingly, GlcNAc inhibited the growth of the nag1⌬ and dac1⌬ mutants in the presence of other sugars, suggesting that excess GlcNAc-6-PO 4 is deleterious. Interestingly, both hxk1⌬ and an hxk1⌬ nag1⌬ dac1⌬ triple mutant could be efficiently stimulated by GlcNAc to form hyphae. These mutants could also be stimulated to express GlcNAcregulated genes. Because GlcNAc must be phosphorylated by Hxk1 to be catabolized, and also for it to enter the anabolic pathways that form chitin, N-linked glycosylation, and glycosylphosphatidylinositol anchors, the mutant phenotypes indicate that GlcNAc metabolism is not needed to induce signaling in C. albicans. Thus, these studies in C. albicans reveal a novel role for GlcNAc in cell signaling that may also regulate critical pathways in other organisms.

Fungal Morphogenesis

“…GlcNAc transporters, NGT1 and NGT2, are necessary for H. capsulatum cells to robustly filament in response to GlcNAc and in standard glucose medium, suggesting that Ngt1 and Ngt2 monitor endogenous levels of GlcNAc to control filamentous growth in response to temperature (Gilmore et al 2013). Interestingly, GlcNAc metabolism is not required for GlcNAc signaling in these fungi (Naseem et al 2011;Gilmore et al 2013). Current data suggest that either internal GlcNAc acts as a signal or GlcNAc transporters can also function as a signal transducer.…”

Regulation of Hyphal Growth and N-Acetylglucosamine Catabolism by Two Transcription Factors in Candida albicans

“…These cassettes carry the GFPg variant that is more photostable (Zhang and Konopka 2010). PCR primers were used that carry 70 bp of homology to the sequences adjacent to the termination codon of NGT1 or HXK1, as described previously (Zhang and Konopka 2010;Naseem et al 2011). The resulting PCR products were then used to transform the corresponding strains to create GFP fusion genes.…”

“…GlcNAc does not have to be catabolized to induce hyphal growth, because a strain lacking the genes needed for GlcNAc metabolism (hxk1D nag1D dac1D) can be induced to form hyphae (Naseem et al 2011. This led to the proposal that cells sensitively distinguish exogenous GlcNAc taken up from the environment vs. GlcNAc synthesized inside the cell, because exogenous GlcNAc is not phosphorylated whereas cells only synthesize phosphorylated forms of this sugar (e.g., GlcNAc-6-PO4) (Naseem et al 2012).…”

Streptococcus gordonii comCDE (competence) operon modulates biofilm formation with Candida albicans

“…The SspB-Als3 interaction is involved in biofilm development in which bacteria and fungi cooperate at the cellular and molecular level to generate a robust mixed microbial community (Whitmore & Lamont, 2011). Community composition may be regulated by extracellular signalling molecules such as autoinducer-2 (AI-2) (McNab et al, 2003;Bamford et al, 2009), farnesol (Bamford et al, 2009;Lu et al, 2014), Nacetyl-D-glucosamine (Naseem et al, 2011) and homoserine lactones (Hogan et al, 2004). In addition, the quorum-sensing (QS) molecule, competence stimulating peptide (CSP), produced by Streptococcus mutans inhibits hypha formation by C. albicans (Jarosz et al, 2009).…”