Role of Actin Cytoskeletal Dynamics in Activation of the Cyclic AMP Pathway and HWP1 Gene Expression in Candida albicans

Abstract

Changes in gene expression during reversible bud-hypha transitions of the opportunistic fungal pathogen Candida albicans permit adaptation to environmental conditions that are critical for proliferation in host tissues. Our previous work has shown that the hypha-specific adhesin gene HWP1 is up-regulated by the cyclic AMP (cAMP) signaling pathway. However, little is known about the potential influences of determinants of cell morphology on HWP1 gene expression. We found that blocking hypha formation with cytochalasin A, which destabilizes actin filaments, and with latrunculin A, which sequesters actin monomers, led to a loss of HWP1 gene expression. In contrast, high levels of HWP1 gene expression were observed when the F-actin stabilizer jasplakinolide was used to block hypha formation, suggesting that HWP1 expression could be regulated by actin structures. Mutants defective in formin-mediated nucleation of F-actin were reduced in HWP1 gene expression, providing genetic support for the importance of actin structures. Kinetic experiments with wildtype and actin-deficient cells revealed two distinct phases of HWP1 gene expression, with a slow, actinindependent phase preceding a fast, actin-dependent phase. Low levels of HWP1 gene expression that appeared to be independent of stabilized actin and cAMP signaling were detected using indirect immunofluorescence. A connection between actin structures and the cAMP signaling pathway was shown using hyper-and hypomorphic cAMP mutants, providing a possible mechanism for up-regulation of HWP1 gene expression by stabilized actin. The results reveal a new role for F-actin as a regulatory agent of hypha-specific gene expression at the bud-hypha transition.Candida albicans is an opportunistic fungal pathogen with a tropism for the gastrointestinal mucosa. The progression to candidiasis from the usual state of commensalism is prompted by changes in the immune status of the host that relay signals for proliferation and culminate in tissue invasion. Growth in host tissues is characterized by the presence of yeast, pseudohyphal, and hyphal morphologies in response to a number of environmental parameters within the host that may include temperature, pH, and nutrition (31,33,66) as well as other unknown factors. The different morphologies of C. albicans are accompanied by differences in gene expression patterns that are critical to the development of candidiasis. The mechanisms that mediate changes in gene expression in concert with changes in morphology are directly relevant to the regulation of virulence of C. albicans.The hypha-specific gene HWP1 (hyphal wall protein 1) is an attractive candidate for studying morphology-specific gene regulation since it is abundantly expressed in hyphal growth mode and is barely detectable in the yeast morphology (73, 74). Pseudohyphae vary in the level of HWP1 gene expression (71). HWP1 encodes a glycosylphosphatidylinositol-linked cell wall protein that serves as a transglutaminase substrate and enables C. albicans to form covalent attachments ...

Rsr1 Focuses Cdc42 Activity at Hyphal Tips and Promotes Maintenance of Hyphal Development in Candida albicans

“…One possibility is that Cdc42 localization sets up a physical platform for actin-dependent regulation of hyphal signaling pathways. Indeed, it has been shown that ECE1, HGC1, HWP1, and HYR1 are induced via the cyclic AMP (cAMP)-dependent protein kinase A signaling pathway and that their expression is reduced in the face of genetic and chemical disruptions of the actin cytoskeleton (2,42). The recent discovery that actin and adenylate cyclase physically interact with each other in a complex and that this complex is required for the efficient production of cAMP (43) provides strong support for the idea that the cellular status of polarized actin modulates gene transcription during hyphal morphogenesis.…”

Whi2p links nutritional sensing to actin-dependent Ras-cAMP-PKA regulation and apoptosis in yeast

“…The influence of actin dynamics on cAMP-PKA signalling activity has recently been noted in both S. cerevisiae and C. albicans Wolyniak and Sundstrom, 2007). In addition, the activity of Ras-cAMP-PKA signalling is known to be a critical component in the regulation of mitochondrial activity, stress response and cell cycle arrest in response to nutritional status (Hlavata et al, 2003;Hlavata et al, 2008).…”

Modulation of Morphogenesis in Candida albicans by Various Small Molecules

“…Various inhibitors of actin dynamics, including latrunculin, jasplakinolide, and cytochalasin A, also blocked the Y-H transition induced in serum-containing, Spider, or M199 medium at 37°C (1,50,135,147). 1,4-Diamino-2-butanone (DAB) blocked the Y-H transition induced in RPMI 1640 medium at 37°C by reducing polyamine levels, decreasing CYR1 mRNA levels, and reducing cAMP cellular levels (137).…”