Cell wall glucan remodeling is required for Candida albicans adhesion and invasion

The fungal cell wall plays a crucial role in host-pathogen interactions. Its formation is the result of the coordinated activity of several extracellular enzymes which assemble the constituents, remodel and hydrolyze them in the extracellular space. Candida albicans Phr1 and Phr2 proteins belong to Family GH72 of (1,3)-glucanosyltransferases and play a crucial role in cell wall assembly. PHR1 and PHR2 are differently regulated by extracellular pH. PHR1 is expressed when ambient pH is 5.5 or higher whereas PHR2 has the reverse expression pattern. Their deletion causes a pH-conditional defect in morphogenesis and virulence. In this work we explored whether PHR1 deletion affects C. albicans potential to invade human epithelia. We exploited a reconstituted human epithelium (RHE) as a model system. After 24 h from the exposure of RHE to the control cells (CAI-10) or to a PHR1 null mutant (CAS-10) the effects on invasion were scored. Control cells penetrated the entire epithelium layer very efficiently and invaded the underlying collagen matrix whereas the incubation with the mutant cells did not result in penetration of the epithelium and consequently no invasion of the matrix was detectable. The lack of cells in proximity of the epithelium layer suggested that adhesion might also be affected. Thus we studied the behavior of delta-phr1 cells in different adhesion assays. The mutant cells exhibited a marked reduction in the adhesion to abiotic surfaces. About 80% of the control cells were adhered within 30 min from transfer to adhesion conditions increasing to about 95% by 2h. The extent of adherence of PHR1 null mutant cells was greatly reduced since only 20% adhered by 30 min increasing to a maximum of about 30% by 2h. Next we tested the ability of PHR1 null mutant to adhere to monolayers of human epithelial cells. A similar defect in adhesion was detected. Transcription profiling of selected hyphal-specific and adhesin-encoding genes during adhesion indicates that in the PHR1 null mutant HWP1 and ECE1 transcript levels are markedly reduced. Our results suggest that expression of PHR1 strongly contributes to adhesion and invasion, two processes that promote the establishment of C. albicans infections and progression.