Date Published: April 13, 2016
Publisher: Public Library of Science
Author(s): Delma S. Childers, Ingrida Raziunaite, Gabriela Mol Avelar, Joanna Mackie, Susan Budge, David Stead, Neil A. R. Gow, Megan D. Lenardon, Elizabeth R. Ballou, Donna M. MacCallum, Alistair J. P. Brown, Robert A. Cramer.
Efficient carbon assimilation is critical for microbial growth and pathogenesis. The environmental yeast Saccharomyces cerevisiae is “Crabtree positive”, displaying a rapid metabolic switch from the assimilation of alternative carbon sources to sugars. Following exposure to sugars, this switch is mediated by the transcriptional repression of genes (carbon catabolite repression) and the turnover (catabolite inactivation) of enzymes involved in the assimilation of alternative carbon sources. The pathogenic yeast Candida albicans is Crabtree negative. It has retained carbon catabolite repression mechanisms, but has undergone posttranscriptional rewiring such that gluconeogenic and glyoxylate cycle enzymes are not subject to ubiquitin-mediated catabolite inactivation. Consequently, when glucose becomes available, C. albicans can continue to assimilate alternative carbon sources alongside the glucose. We show that this metabolic flexibility promotes host colonization and virulence. The glyoxylate cycle enzyme isocitrate lyase (CaIcl1) was rendered sensitive to ubiquitin-mediated catabolite inactivation in C. albicans by addition of a ubiquitination site. This mutation, which inhibits lactate assimilation in the presence of glucose, reduces the ability of C. albicans cells to withstand macrophage killing, colonize the gastrointestinal tract and cause systemic infections in mice. Interestingly, most S. cerevisiae clinical isolates we examined (67%) have acquired the ability to assimilate lactate in the presence of glucose (i.e. they have become Crabtree negative). These S. cerevisiae strains are more resistant to macrophage killing than Crabtree positive clinical isolates. Moreover, Crabtree negative S. cerevisiae mutants that lack Gid8, a key component of the Glucose-Induced Degradation complex, are more resistant to macrophage killing and display increased virulence in immunocompromised mice. Thus, while Crabtree positivity might impart a fitness advantage for yeasts in environmental niches, the more flexible carbon assimilation strategies offered by Crabtree negativity enhance the ability of yeasts to colonize and infect the mammalian host.
Microbes must acquire nutrients efficiently if they are to compete effectively in complex microenvironments. A common microbial strategy is to focus resources on the utilization of energetically favourable carbon sources when they are available, and then, once they become exhausted, switch to alternative, less favourable carbon sources. For example, organisms such as Escherichia coli and Saccharomyces cerevisiae assimilate sugars such as glucose before switching to less favourable carbon sources such as alcohols and organic acids [1, 2]. This behaviour reflects the niches these microbes occupy. In these environments, microbes often compete for survival during cycles of “feast and famine”, where periods of growth on less favourable carbon sources or starvation are punctuated by episodes of sugar availability . These organisms have evolved elegant regulatory mechanisms, such as the lac operon  and GAL regulon , that mediate the efficient, sequential assimilation of sugars and alternative carbon sources. In contrast, microbes that have evolved in niches that contain limiting sugar concentrations display alternative modes of carbon assimilation. For example, the fungal pathogen Candida albicans is able to assimilate sugars and alternative carbon sources simultaneously . We show here that this type of metabolic flexibility enhances the ability of yeasts to colonise and infect the mammalian host.
Yeast pathogenicity is a polygenic trait. A range of virulence factors contribute to pathogenicity, including yeast-hypha morphogenesis, adhesins and invasins, and secreted hydrolases . Pathogenicity is also enhanced by fitness attributes such as robust stress responses and efficient metabolic adaptation [43–45]. Clearly the ability to assimilate the variety of carbon sources available in host niches is advantageous to pathogenic yeasts. For example, the loss of glycolysis or glucose sensing attenuates the dissemination and pathogenicity of the basidiomycete yeast Cryptococcus neoformans . Also, mutations that block glycolysis, gluconeogenesis or the glyoxylate cycle attenuate the virulence of C. albicans, an ascomycete yeast [25, 28, 29]. We now show that the coordinated regulation of these pathways is important for virulence in C. albicans and S. cerevisiae. Our data suggest that evolutionary pressures in host niches have selected for yeasts with enhanced metabolic flexibility that permits the simultaneous exploitation of sugars and alternative carbon sources. This view is supported by several key observations.