Functional Study Of Swi/Snf Complex In Hyphal Development Of Candida Albicans

The ability of dimorphic transition is one of the most determinants of Candida albicans for its pathogenicity, which is regulated by several signal transduction pathways. By functional complementation, we have isolated genes from the genomic library of C. albicans. The product of one of the genes shared highest sequence similarity with Swi1 in S. cerevisiae, and could partially complement the defect of S. cerevisiae swi1 mutant in utilization of glycerol, indicating that this gene is a homologue of S. cerevisiae SWI1 (ScSWI1), so we designated it as C. albicans SWI1 (CaSWI1).Knockout of C. albicans SWI1 also caused defect in utilization of glycerol, and most importantly, prevented the hyphal development of C. albicans. swi1/swi1 mutant showed defect in hyphal formation, expression of hypha-specific and virulence-related genes and hypha-associated actin polarization. These results indicate that Swi1 functions as an activator of hyphal development by regulating the expression of hypha-specific genes and reorganizing cellular distribution of actin fibre.C. albicans SNF2 gene, which was identified from C. albicans genomic database, shares highest similarity with S. cerevisiae SNF2 gene. C. albicans Snf2 and Swi1 could interact with each other in vivo and were localized in nuclear independent of growth forms. Furthermore, snf2/snf2 mutant exhibited identical phenotypes with swi1/swi1 mutant including constitutive pseudohyphal growth, defect in hyphal formation and avirulence in a mouse systemic infection model. These results indicate that a conserved Swi/Snf complex is required for hyphal development and pathogenicity in C. albicans, as an activator in hyphal formation and as a repressor in pseudohyphal development. In S. cerevisiae, deletion of ScSWI1 led to defect of invasive growth, pseudohyphal formation and expression of FLO11. However, CaSWI1 could only partially complement the defect of scswi1 mutant in invasion but not in pseudohyphal growth. Over-expression of CaSWI1 could activate pseudohyphal and invasive growth in S. cerevisiae independent of either cAMP/PKA or MAPK pathway. But ScSWI1 could only bypass the requirement of cAMP/PKA pathway in activation of pseudohyphal growth, and required both pathways in activation of invasive growth. These results indicate that though both CaSwi1 and ScSwi1 function as activators in filamentous growth, they mediate signaling via different pathways in S. cerevisiae.Tup1 functions as a repressor in filamentous formation in C. albicans. In liquid media, the swi1/swi1 tup1/tup1 double mutant showed similar phenotypes with swi1/swi1 single mutant including constitutive pseudohyphal formation, defect in expression of hypha-specific genes and defect in hypha-associated actin polarization. However, swi1/swi1 tup1/tup1 double mutant resembled tup1/tup1 mutant in invasive growth on solid media and filamentous growth in embedded growth condition. These results suggest that antagonistic interplay between Swi/Snf complex and Tup1 on morphogenesis of C. albicans is dependent on the extracellular stimuli.Crk1 is a protein kinase involved in regulation of hyphal development of C. albicans. crk1/crk1, swi1/swi1 and snf2/snf2 mutants showed similar cell morphology. Interestingly, Crk1 protein was absent in snf2/snf2 mutant, while CRK1 gene is normally expressed, suggesting that Swi/Snf complex may be invoved in regulation of Crk1 protein level other than the traditional transcriptional modulation for the hyphal development of C. albicans. Furthermore, crk1/crk1, swi1/swi1 and snf2/snf2 mutants showed sensitivity to 6-azauracil, which could be suppressed by guanine, suggesting that Swi/Snf complex and Crk1 may contribute to RNA polymerase II-dependent efficient transcriptional elongation during hyphal development of C. albicans.