Identification and Characterization of anSKN7Homologue inCryptococcus neoformans

Abstract

Cryptococcus neoformans is an encapsulated fungal pathogen that primarily infects the central nervous system of immunocompromised individuals, causing life-threatening meningoencephalitis. The capacity of C. neoformans to subvert host defenses and disseminate by intracellular parasitism of alveolar macrophages in the immune-compromised host has led to studies to evaluate genes associated with C. neoformans resistance to oxidative stress. In the present study, we identify and characterize a C. neoformans homologue to SKN7, a transcription factor in Saccharomyces cerevisiae that regulates the oxidative stress response, cell cycle, and cell wall biosynthesis. To examine the contribution of SKN7 in the pathogenesis of fungal infections, we created skn7 mutants via targeted disruption. The skn7 mutants were observed to be more susceptible to reactive oxygen species in vitro and were significantly less virulent than the wild-type strain and a reconstituted strain as measured by cumulative survival in the mouse inhalational model. The Skn7 protein was observed to be important for expression of thioredoxin reductase in response to oxidative challenge. Interestingly, skn7 mutants were also observed to flocculate following in vitro culture, a novel phenotype not observed in skn7 mutants derived from other fungi. These findings demonstrate that SKN7 contributes to the virulence composite but is not required for pathogenicity in C. neoformans. In addition, flocculation of C. neoformans skn7 mutants suggests a potentially unique function of SKN7 not previously observed in other cryptococcal strains or skn7 mutants.Cryptococcus neoformans, the etiological agent of cryptococcosis, is an encapsulated fungal pathogen that infects the central nervous system of immunocompromised individuals, causing life-threatening meningoencephalitis (31). Cryptococcosis occurs in approximately 5 to 25% of AIDS patients worldwide (37), and studies have shown that 2.8% of organ transplant recipients can develop C. neoformans infections, resulting in an overall death rate of 42% (16). Although highly active antiretroviral therapy has contributed to a significant decrease in the incidence of cryptococcosis in AIDS patients in developed countries (2), increases in organ transplant recipients and patients undergoing extensive corticosteroid therapy forecast a rise of cryptococcosis in other high-risk populations. In medically advanced countries the acute mortality rate is between 10 and 25% (37), and conventional antifungal agents are often excessively toxic, lack potent fungicidal properties, or are being rendered less effective by the emergence of drug-resistant strains. Therefore, continued studies are needed to identify novel targets for the development of drugs or vaccines to combat cryptococcal infections.Obligate aerobic microorganisms, such as C. neoformans, are under persistent exposure to endogenous oxidative stress caused by the incomplete reduction of oxygen to water, which yields reactive oxygen species (ROS) such as H 2 O 2 , the hydrox...

Posttranslational, Translational, and Transcriptional Responses to Nitric Oxide Stress in Cryptococcus neoformans : Implications for Virulence

“…Interestingly, thioredoxin reductase, TRR1, which is induced by microarray analysis in response to nitric oxide, has also been shown to be induced during peroxide stress (34). This oxidative stress induction of TRR1 was shown to be dependent on the putative transcription factor Skn7 (52). We can also show by real-time PCR that TRR1, but not FHB1, induction during nitrosative stress is dependent on Yap4 (data not shown).…”

The SskA and SrrA Response Regulators Are Implicated in Oxidative Stress Responses of Hyphae and Asexual Spores in the Phosphorelay Signaling Network ofAspergillus nidulans

“…47,48) This suggests that a characteristic stress-responsive signaling pathway consisting of the prokaryotic and the eukaryotic signaling modes is highly conserved within a variety of species belonging to fungi. However, it should be emphasized that S. cerevisiae has only one HK and S. pombe three, while A. nidulans has 15.…”

Rewiring of Signaling Networks Modulating Thermotolerance in the Human Pathogen Cryptococcus neoformans

“…In this process, Skn7, which has a similar domain structure as that of Hsf1, directly interacts with Hsf1 to induce HSPs in response to oxidative stresses (Raitt et al 2000). Although interaction between Skn7 and Hsf1 is not known at this point in C. neoformans, the previous findings that Skn7 is involved in oxidative stress response in C. neoformans (Wormley et al 2005;Bahn et al 2006;Coenjaerts et al 2006) indicates a potential cooperative interaction between the two TFs. Therefore, it is possible that Hsf1 may mediate distinct transcriptional and post-transcriptional responses to varying environmental stresses in C. neoformans.…”