Nature of β-1,3-Glucan-Exposing Features on Candida albicans Cell Wall and Their Modulation

Candida albicans exists as a commensal of mucosal surfaces and the gastrointestinal tract without causing pathology. However, this fungus is also a common cause of mucosal and systemic infections when antifungal immune defenses become compromised. The activation of antifungal host defenses depends on the recognition of fungal pathogen-associated molecular patterns (PAMPs), such as β-1,3-glucan. In C. albicans, most β-1,3-glucan is present in the inner cell wall, concealed by the outer mannan layer, but some β-1,3-glucan becomes exposed at the cell surface. In response to host signals, such as lactate, C. albicans induces the Xog1 exoglucanase, which shaves exposed β-1,3-glucan from the cell surface, thereby reducing phagocytic recognition. We show here that β-1,3-glucan is exposed at bud scars and punctate foci on the lateral wall of yeast cells, that this exposed β-1,3-glucan is targeted during phagocytic attack, and that lactate-induced masking reduces β-1,3-glucan exposure at bud scars and at punctate foci. β-1,3-Glucan masking depends upon protein kinase A (PKA) signaling. We reveal that inactivating PKA, or its conserved downstream effectors, Sin3 and Mig1/Mig2, affects the amounts of the Xog1 and Eng1 glucanases in the C. albicans secretome and modulates β-1,3-glucan exposure. Furthermore, perturbing PKA, Sin3, or Mig1/Mig2 attenuates the virulence of lactate-exposed C. albicans cells in Galleria. Taken together, the data are consistent with the idea that β-1,3-glucan masking contributes to Candida pathogenicity. IMPORTANCE Microbes that coexist with humans have evolved ways of avoiding or evading our immunological defenses. These include the masking by these microbes of their "pathogen-associated molecular patterns" (PAMPs), which are recognized as "foreign" and used to activate protective immunity. The commensal fungus Candida albicans masks the proinflammatory PAMP β-1,3-glucan, which is an essential component of its cell wall. Most of this β-1,3-glucan is hidden beneath an outer layer of the cell wall on these microbes, but some can become exposed at the fungal cell surface. Using high-resolution confocal microscopy, we examine the nature of the exposed β-1,3-glucan at C. albicans bud scars and at punctate foci on the lateral cell wall, and we show that these features are targeted by innate immune cells. We also reveal that downstream effectors of protein kinase A (Mig1/Mig2, Sin3) regulate the secretion of major glucanases, modulate the levels of β-1,3-glucan exposure, and influence the virulence of C. albicans in an invertebrate model of systemic infection. Our data support the view that β-1,3-glucan masking contributes to immune evasion and the virulence of a major fungal pathogen of humans.

A Suppressor Mutation in the β-Subunit Kis1 Restores Functionality of the SNF1 Complex in Candida albicans snf4Δ Mutants

The heterotrimeric protein kinase SNF1 is a key regulator of metabolic adaptation in the pathogenic yeast Candida albicans, and mutants with a defective SNF1 complex cannot grow on carbon sources other than glucose. We identified a novel type of suppressor mutation in the β-subunit Kis1 that rescued the growth defects of cells lacking the regulatory γ-subunit Snf4 of the SNF1 complex. Unlike wild-type Kis1, the mutated Kis1^A396T could bind to the catalytic α-subunit Snf1 in the absence of Snf4. Binding of Kis1^A396T did not enhance phosphorylation of Snf1 by the upstream activating kinase Sak1, which is impaired in snf4Δ mutants. Nevertheless, the mutated Kis1^A396T reestablished SNF1-dependent gene expression, confirming that SNF1 functionality was restored. The repressor proteins Mig1 and Mig2 were phosphorylated even in the absence of Snf1, but their phosphorylation patterns were altered, indicating that SNF1 regulates Mig1 and Mig2 activity indirectly. In contrast to wild-type cells, mutants lacking Snf4 were unable to reduce the amounts of Mig1 and Mig2 when grown on alternative carbon sources, and this deficiency was also remediated by the mutated Kis1^A396T. These results provide novel insights into the regulation of SNF1 and the repressors Mig1 and Mig2 in the metabolic adaptation of C. albicans.

IMPORTANCE The highly conserved protein kinase SNF1 plays a key role in the metabolic adaptation of the pathogenic yeast Candida albicans, but it is not clear how it regulates its downstream targets in this fungus. We show that the repressor proteins Mig1 and Mig2 are phosphorylated also in cells lacking the catalytic α-subunit Snf1 of the SNF1 complex, but the amounts of both proteins were reduced in wild-type cells when glucose was replaced by alternative carbon sources, pointing to an indirect mechanism of regulation. Mutants lacking the regulatory γ-subunit Snf4 of the SNF1 complex, which cannot grow on alternative carbon sources, were unable to downregulate Mig1 and Mig2 levels. We identified a novel type of suppressor mutation, an amino acid substitution in the β-subunit Kis1, which enabled Kis1 to bind to Snf1 in the absence of Snf4, thereby restoring Mig1 and Mig2 downregulation, SNF1-dependent gene expression, and growth on alternative carbon sources. These results provide new insights into the SNF1 signaling pathway in C. albicans.