The Regulatory Proteins Rtg1/3 Govern Sphingolipid Homeostasis in the Human-Associated Yeast Candida albicans.

Integrating nutrient sensing with the synthesis of complex molecules is a central feature of metabolism. Yet the regulatory mechanisms underlying such integration are often unknown. Here, we establish that the transcription regulators Rtg1/3 are key determinants of sphingolipid homeostasis in the human fungal pathogen Candida albicans. Quantitative analysis of the C. albicans lipidome reveals Rtg1/3-dependent alterations in all complex sphingolipids and their precursors, ceramides. Mutations in the regulators render the fungus susceptible to myriocin, a sphingolipid synthesis inhibitor. Rtg1/3 exert control on the expression of several enzymes involved in the synthesis of sphingolipids' building blocks, and the regulators are activated upon engulfment of C. albicans cells by human neutrophils. We demonstrate that Rtg1p and Rtg3p are regulated at two levels, one in response to sphingolipids and the other by the nutrient sensor TOR. Our findings, therefore, indicate that the Rtg1/3 system integrates nutrient sensing into the synthesis of complex lipids.

The yeast form of the fungus Candida albicans promotes persistence in the gut of gnotobiotic mice.

Many microorganisms that cause systemic, life-threatening infections in humans reside as harmless commensals in our digestive tract. Yet little is known about the biology of these microbes in the gut. Here, we visualize the interface between the human commensal and pathogenic fungus Candida albicans and the intestine of mice, a surrogate host. Because the indigenous mouse microbiota restricts C. albicans settlement, we compared the patterns of colonization in the gut of germ free and antibiotic-treated conventionally raised mice. In contrast to the heterogeneous morphologies found in the latter, we establish that in germ free animals the fungus almost uniformly adopts the yeast cell form, a proxy of its commensal state. By screening a collection of C. albicans transcription regulator deletion mutants in gnotobiotic mice, we identify several genes previously unknown to contribute to in vivo fitness. We investigate three of these regulators-ZCF8, ZFU2 and TRY4-and show that indeed they favor the yeast form over other morphologies. Consistent with this finding, we demonstrate that genetically inducing non-yeast cell morphologies is detrimental to the fitness of C. albicans in the gut. Furthermore, the identified regulators promote adherence of the fungus to a surface covered with mucin and to mucus-producing intestinal epithelial cells. In agreement with this result, histology sections indicate that C. albicans dwells in the murine gut in close proximity to the mucus layer. Thus, our findings reveal a set of regulators that endows C. albicans with the ability to endure in the intestine through multiple mechanisms.