Expression levels of a filament-specific transcriptional regulator are sufficient to determine Candida albicans morphology and virulence

Abstract

Candida albicans, the major human fungal pathogen, undergoes a reversible morphological transition from single yeast cells to pseudohyphal and hyphal filaments (elongated cells attached end-to-end). Because typical C. albicans infections contain a mixture of these morphologies it has, for many years, been difficult to assess the relative contribution of each form to virulence. In addition, the regulatory mechanisms that determine growth in pseudohyphal and hyphal morphologies are largely unknown. To address these questions we have generated a C. albicans strain that can be genetically manipulated to grow completely in the hyphal form under non-filament-inducing conditions in vitro. This was achieved by inducing high-level constitutive expression of UME6, a recently identified filament-specific transcriptional regulator of C. albicans hyphal extension. We show that high-level UME6 expression significantly increases hyphal formation and promotes virulence in a mouse model of systemic candidiasis. Our results strongly suggest that shifting the morphology of a C. albicans population toward the hyphal form, and/or increasing hyphalspecific gene expression, during the course of infection is sufficient to improve virulence potential. We also demonstrate that lower levels of UME6 expression specify growth largely in the pseudohyphal form and that increasing UME6 levels is sufficient to cause cells to gradually shift from pseudohyphal to hyphal morphology. In addition, we show that UME6 levels differentially induce the expression of several known filament-specific transcripts. These findings suggest that a common transcriptional regulatory mechanism functions to specify both pseudohyphal and hyphal morphologies in a dosage-dependent manner.filamentous growth ͉ transcriptional regulation C andida albicans is the most important human fungal pathogen because of its frequency of isolation and the amount of morbidity and mortality it causes. Although normally found as a commensal in the human gastrointestinal tract, C. albicans is responsible for a wide variety of mucosal infections such as oral and vaginal thrush. C. albicans also infects every organ and tissue in the human body (1-3). Approximately 70% of all women will experience at least one episode of vaginal candidiasis during their lifetime (4), and systemic candidiasis is now the fourthleading cause of hospital-acquired bloodstream infections in the United States, with a mortality rate approaching 35% (5). AIDS patients, organ transplant recipients, cancer patients on chemotherapy, recipients of artificial joints and prosthetic devices, and other immunocompromised individuals are particularly susceptible to infection (for reviews see refs. 2, 3, and 6).One property known to contribute to C. albicans virulence is the ability to undergo a morphological transition from yeast (single, oval budding cells) to filaments (elongated cells attached end-to-end) (for reviews see refs. 7 and 8). This transition is known to occur in response to a variety of inducing signals present...

Trimorphic stepping stones pave the way to fungal virulence

“…The findings of Carlisle et al (5) have broader evolutionary implications. The finding that pseudohyphae are a developmental way station between yeast and hyphae suggests stepwise evolution (Fig.…”

“…1). This question, and implications for virulence, were addressed by Carlisle et al with engineered strains (5).…”

“…S. cerevisiae pseudohyphal growth long eluded detection-it requires special conditions/strains and was lost during domestication (4). How pseudohyphae are related to yeast and hyphae (as a distinct fate or a continuum) was unknown until the report of Carlisle et al (5) in this issue of PNAS. They reveal that pseudohyphae are intermediate between yeast and hyphae, with implications for pathogen-host interactions and fungal evolution.…”

“…The studies of Carlisle et al (5) involve controlling expression of Ume6, a zinc-finger transcription factor downstream of Nrg1 in circuits for dimorphic transition and virulence (13,14). In their studies, Ume6 was expressed from the doxycycline-regulatable promoter (tet-UME6), leading to controlled consequences: yeast with the promoter off (doxϩ), and hyphae in response to high-level Ume6 (doxϪ), even under non-filamentinducing conditions.…”

Fungal Morphogenesis

“…The combination of hypoxia and high CO 2 , but neither condition alone, maintains C. albicans hyphal elongation, even in mutants lacking the nutrient responsive chromatin-remodeling pathway . One key downstream target of Hda1-mediated chromatin regulation is Ume6, a hypha-specific transcription factor that controls the level and duration of hypha-specific transcription (Banerjee et al 2008;Carlisle et al 2009;Zeidler et al 2009;Lu et al 2012). Ume6 is stabilized via regulation by Ofd1, a prolyl hydroxylase family member that is inhibited by hypoxia and an uncharacterized pathway that senses high CO 2 .…”