Exposure of Candida albicans (1,3)-glucan is promoted by activation of the Cek1 pathway

Candida albicans is among the most common causes of human fungal infections and is an important source of mortality. C. albicans is able to diminish its detection by innate immune cells through masking of (1,3)-glucan in the inner cell wall with an outer layer of heavily glycosylated mannoproteins (mannan). However, mutations or drugs that disrupt the cell wall can lead to exposure of (1,3)-glucan (unmasking) and enhanced detection by innate immune cells through receptors like Dectin-1, the C-type signaling lectin. Previously, our lab showed that the pathway for synthesizing the phospholipid phosphatidylserine (PS) plays a role in (1,3)-glucan masking. The homozygous PS synthase knockout mutant, cho1/, exhibits increased exposure of (1,3)-glucan. Several Mitogen Activated Protein Kinase (MAPK) pathways and their upstream Rho-type small GTPases are important for regulating cell wall biogenesis and remodeling. In the cho1/ mutant, both the Cek1 and Mkc1 MAPKs are constitutively activated, and they act downstream of the small GTPases Cdc42 and Rho1, respectively. In addition, Cdc42 activity is up-regulated in cho1/. Thus, it was hypothesized that activation of Cdc42 or Rho1 and their downstream kinases cause unmasking. Disruption of MKC1 does not decrease unmasking in cho1/, and hyperactivation of Rho1 in wild-type cells increases unmasking and activation of both Cek1 and Mkc1. Moreover, independent hyperactivation of the MAP kinase kinase kinase Ste11 in wild-type cells leads to Cek1 activation and increased (1,3)-glucan exposure. Thus, upregulation of the Cek1 MAPK pathway causes unmasking, and may be responsible for unmasking in cho1/. Author summary Candida albicans causes fungal infections in the oral cavities and bloodstreams of patients with weakened immune function, such as AIDS or cancer patients. The immune system detects fungal infections, in part, by detecting the antigenic cell wall polysaccharide (1,3)-glucan. The ability to mask (1,3)-glucan from immune detection is a virulence factor of C. albicans and a range of fungal pathogens. If synthesis of the phospholipid phosphatidylserine is disrupted in C. albicans (cho1/ mutation), then cho1/ exhibits significantly increased exposure of (1,3)-glucan to immune detection compared to wild-type. Intracellular signaling cascades that regulate cell wall synthesis are upregulated in the cho1/ mutant. It was hypothesized that upregulation of these pathways might be responsible for unmasking in this mutant. Genetic approaches were used to activate these pathways independently of the cho1/ mutation. It was discovered that activation of one pathway, Cdc42-Cek1, leads to (1,3)-glucan exposure. Thus, this pathway can cause (1,3)-glucan exposure, and its upregulation may be the cause of unmasking in the cho1/ mutant.