The Rewiring of Ubiquitination Targets in a Pathogenic Yeast Promotes Metabolic Flexibility, Host Colonization and Virulence

Abstract

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.

The Snf1‐activating kinase Sak1 is a key regulator of metabolic adaptation and in vivo fitness of Candida albicans

“…Metabolic flexibility is considered as an important aspect in the ability of C. albicans to colonise the mammalian gastrointestinal tract and cause infections (Brock, 2009;Brown et al, 2014;Ene et al, 2014;Childers et al, 2016). Snf1 is a core regulator of nutritional adaptation, and our results A. YPD overnight cultures of the wild-type strain SC5314 and mutant derivatives containing a single wild-type SNF1 allele or a mutated SNF1 T208A allele were appropriately diluted and plated on different media.…”

“…Metabolic flexibility is considered as an important aspect in the ability of C. albicans to colonise the mammalian gastrointestinal tract and cause infections (Brock, ; Brown et al ., ; Ene et al ., ; Childers et al ., ). Snf1 is a core regulator of nutritional adaptation, and our results show that in C. albicans Sak1 is a crucial activator of this essential kinase.…”

“…The metabolic flexibility of C. albicans, which enables it to utilise many different carbon and nitrogen sources, is considered as a key aspect of both commensalism and infection (Barelle et al, 2006;Brown et al, 2014;Ene et al, 2014). The preferred carbon source glucose is present only at low levels in many host niches, and the ability to utilise alternative carbon sources is critical for the virulence of C. albicans; mutants lacking glyoxylate cycle or gluconeogenesis genes, which are required for growth on nonfermentable carbon sources, are attenuated in mouse models of gastrointestinal colonisation and systemic candidiasis (Lorenz and Fink, 2001;Barelle et al, 2006;Ramirez and Lorenz, 2007;Childers et al, 2016). Nutrient availability influences the physiological status of the cells, and growth of C. albicans on alternative carbon sources has been shown to impact the structure of the cell wall, stress resistance and recognition by the immune system (Ene et al, 2012(Ene et al, , 2013.…”

The Cell Biology of the Trichosporon-Host Interaction

“…Indeed, this fungus is specialized in assimilating a large variety of carbon and nitrogen sources, a finding highlighting its close rapport with the local microbiome via nutrient recycling. The rewiring of metabolic pathways conveying a more flexible carbon assimilation strategy has been associated with the shift between the commensal and pathogenic states of C. albicans (Sandai et al, 2012; Childers et al, 2016). Furthermore, C. albicans has evolved mechanisms to bypass the nutritional limitations imposed by the host through the expression of micronutrient transporters (Crawford and Wilson, 2015) or redundant enzymes that use alternative micronutrients (Li et al, 2015).…”

Recent advances on Candida albicans biology and virulence

“…While transcriptional regulation is important for this process, post-translational circuitry has also been found to be critical. Recent evidence on the ubiquitination-controlled degradation of enzymes involved in carbohydrate metabolism has established that differences in the enzymes targeted for turnover play a critical role in distinguishing how S. cerevisiae and C. albicans process sugar 14 . These data suggest that C. albicans is much more flexible in its use of carbon sources and that this flexibility allows the pathogen to exploit varied niches within the mammalian host.…”