Cdc42p GTPase Regulates the Budded-to-Hyphal-Form Transition and Expression of Hypha-Specific Transcripts in Candida albicans

Abstract

The yeast Candida albicans is a major opportunistic pathogen of immunocompromised individuals. It can grow in several distinct morphological states, including budded and hyphal forms, and the ability to make the dynamic transition between these forms is strongly correlated with virulence. Recent studies implicating the Cdc42p GTPase in hypha formation relied on cdc42 mutations that affected the mitotic functions of the protein, thereby precluding any substantive conclusions about the specific role of Cdc42p in the budded-to-hypha-form transition and virulence. Therefore, we took advantage of several Saccharomyces cerevisiae cdc42 mutants that separated Cdc42p's mitotic functions away from its role in filamentous growth. The homologous cdc42-S26I, cdc42-E100G, and cdc42-S158T mutations in C. albicans Cdc42p caused a dramatic defect in the budded-tohypha-form transition in response to various hypha-inducing signals without affecting normal budded growth, strongly supporting the conclusion that Cdc42p has an integral function in orchestrating the morphological transition in C. albicans. In addition, the cdc42-S26I and cdc42-E100G mutants demonstrated a reduced ability to damage endothelial cells, a process that is strongly correlated to virulence. The three mutants also had reduced expression of several hypha-specific genes, including those under the regulation of the Efg1p transcription factor. These data indicate that Cdc42p-dependent signaling pathways regulate the budded-to-hyphaform transition and the expression of hypha-specific genes.

Environmental Sensing and Signal Transduction Pathways Regulating Morphopathogenic Determinants of Candida albicans

“…Certain point mutants of Cdc42 do not affect its mitotic functions but strongly affect morphogenesis upon serum stimulation. Consistent with this phenotype, the expression of hypha-specific genes is also reduced or transient in these mutants (23,306 Inputs into the Cek1 MAPK pathway may also occur by quorum sensing. Quorum-sensing molecules allow bacteria to monitor their growth and to control cell density-dependent phenomena.…”

“…Cdc42 binds with high affinity to the Cst20 kinase, as well as to a second PAK kinase, Cla4 (172,287). Cdc42 and its exchange factor, Cdc24, are both required for hyphal growth (22,305,306). Certain point mutants of Cdc42 do not affect its mitotic functions but strongly affect morphogenesis upon serum stimulation.…”

Regulation of the Cdc42/Cdc24 GTPase Module during Candida albicans Hyphal Growth

“…In the budding yeast Saccharomyces cerevisiae and the pathogenic yeast Candida albicans, Cdc42 and its exchange factor Cdc24 are necessary for polarized growth during budding (22,56). Furthermore, these two proteins have been shown to be specifically required for mating in S. cerevisiae (6,33,37,42) and invasive hyphal growth of C. albicans (7,48,49). In S. cerevisiae, these proteins are necessary both for establishing and maintaining cell polarity (8,21,36,51).…”

Site-Directed and Random Insertional Mutagenesis in Medically Important Fungi

“…The mutant alleles were introduced into exogenous loci under the expression of constitutive or regulatable promoters (Bassilana and Arkowitz, 2006;Feng et al, 1999;Sanchez-Martinez and Perez-Martin, 2002). However, since CDC42 is essential, the mutated alleles were introduced at the endogenous locus in a CDC42/cdc42 heterozygote (VandenBerg et al, 2004). These studies demonstrated the existence of different hyphal induction pathways, cross-talk between the MAPK and cAMP pathways, and distinction between growth and morphogenesis.…”

“…Much of the initial molecular dissection of the signaling pathways involved in morphogenesis was deciphered by constructing dominant-active and dominant-negative alleles of the G signaling proteins. Site-specific mutations were introduced in CDC42, RAC1, GPA2, RAS1 and RAS2 based on homology to Saccharomyces and mammalian G proteins (Bassilana and Arkowitz, 2006;Feng et al, 1999;Sanchez-Martinez and Perez-Martin, 2002;vandenBerg et al, 2004). The mutant alleles were introduced into exogenous loci under the expression of constitutive or regulatable promoters (Bassilana and Arkowitz, 2006;Feng et al, 1999;Sanchez-Martinez and Perez-Martin, 2002).…”