The Molecular Mechanism By Which Hyphal Development Is Induced From Log Phase Cells In Candida Albicans

Candida albicans is the most prevalent fungal pathogen of humans.It can switch between yeast and hyphae growth forms,which is critical for its pathogenesis.The usual method for inducing hyphal formation is by diluting cells from a saturated culture into fresh medium at 37℃.The activation of the cAMP–PKA pathway at 37℃ down-regulates the expression of NRG1,the major repressor of hyphal development.Meanwhile,the inoculation releases the inhibition of farnesol,which triggers the protein degradation of Nrg1.Both pathways are required for the rapid clearing of Nrg1 protein,and neither one is sufficient for hyphal induction.It is reported that N‐acetylglucosamine(GlcNAc),serum or neutral pH could stimulate filamentation in log phase cells,whereas how C.albicans develops hyphae without inoculation remains unknown.Here,we uncovered the molecular mechanism for how C.albicans develops hyphae from log phase cells.The expression level of NRG1,the major repressor of hyphal development,decreases dramatically during hyphal development from log phase cells upon N-acetylglucosamine(GlcNAc)induction,whereas the protein degradation of Nrg1 does not change in log phase cells upon GlcNAc treatment.These results indicate that GlcNAc stimulates filamentation in log phase cells through transcriptional down-regulation of NRG1.Instead of cAMP–PKA pathway,GlcNAc sensor Ngs1 is responsible for this process.Through a genetic screen,we find that deletion of BRG1 blocked hyphal induction and the expression of hyphal specific genes in response to GlcNAc.BRG1 expression is activated in the presence of GlcNAc.Importantly,constitutively expressed BRG1 induces hyphal development in log phase cells even in the absence of GlcNAc or Ngs1.Our data indicates that GlcNAc activates hyphal growth in log phase cells through upregulation of BRG1 expression.Chromatin immunoprecipitation(ChIP)assay showed that Ngs1 bound to the UAS region of BRG1 promoter.Brg1 functions upstream of Nrg1 in hyphal development of log phase cells,as deletion of NRG1 in brg1 mutant resembles the phenotype of nrg1 mutant.We further show that Ngs1 binds to GlcNAc to activate its N-acetyltransferase activity,leading to the induction of BRG1 expression.The increased level of BRG1 could repress NRG1 transcripts,resulting in hyphal growth.Intrestingly,Serum or neutral pH‐induced filamentation in log phase cells is also through Brg1‐mediated NRG1 down‐regulation.Our study provides a molecular mechanism for how C.albicans forms hyphae in different cell states.This flexibility may facilitate C.albicans to adapt varied host environment during infection.