iCryptococcus neoformans/i Histone Acetyltransferase Gcn5 Regulates Fungal Adaptation to the Host

Cryptococcus neoformans is an environmental fungus and an opportunistic human pathogen. Previous studies have demonstrated major alterations in its transcriptional profile as this microorganism enters the hostile environment of the human host. To assess the role of chromatin remodeling in host-induced transcriptional responses, we identified the C. neoformans Gcn5 histone acetyltransferase and demonstrated its function by complementation studies of Saccharomyces cerevisiae. The C. neoformans gcn5Δ mutant strain has defects in high-temperature growth and capsule attachment to the cell surface, in addition to increased sensitivity to FK506 and oxidative stress. Treatment of wild-type cells with the histone acetyltransferase inhibitor garcinol mimics cellular effects of the gcn5Δ mutation. Gcn5 regulates the expression of many genes that are important in responding to the specific environmental conditions encountered by C. neoformans inside the host. Accordingly, the gcn5Δ mutant is avirulent in animal models of cryptococcosis. Our study demonstrates the importance of chromatin remodeling by the conserved histone acetyltransferase Gcn5 in regulating the expression of specific genes that allow C. neoformans to respond appropriately to the human host.

Microorganisms must rapidly induce and repress various transcriptional networks in order to adapt to the stressful conditions of the infected host. One aspect of this rapid transcriptional response is the ability to remodel chromatin, allowing transcription factors access to the promoters of important stress response genes. Acetylation of specific lysine residues of histone proteins is one mechanism for chromatin remodeling that leads to altered transcription (62). Acetylated histones are preferentially associated with regions of active transcription, and they are thought to recruit the assembly of transcriptional complexes (for reviews, see references 5 and 31). Mutation of histone proteins to prevent acetylation results in defects in transcription and cell growth (32, 39, 58, 68). In eukaryotes, histone proteins are acetylated by the Gcn5 protein as part of larger, multisubunit, chromatin remodeling complexes (11, 19, 31, 55).

These large histone acetyltransferase complexes are recruited to specific gene promoters to modify local chromatin structure and thus regulate transcription. In the budding yeast Saccharomyces cerevisiae , the Spt3-Ada2-Gcn5 (SAGA) complex is involved in global transcriptional regulation. However, further studies of SAGA function, through mapping the localization of the complex to specific loci, have shown that binding is enriched at particular stress-responsive genes. In S. cerevisiae , Gcn5 is involved in regulating transcriptional responses to common environmental stresses, such as high temperature, oxidative damage, high osmolarity, and nutrient deprivation. In the fission yeast Schizosaccharomyces pombe , Gcn5 is involved in a narrower group of stress responses and plays an important role in resistance to salt. Microarray studies of both of these species revealed that acetylation by the SAGA complex is able to remodel the chromatin to transcriptionally regulate genes important for responding to environmental stresses and that these genes tend to be coordinately regulated (33).

Cryptococcus neoformans is an opportunistic human fungal pathogen that causes meningoencephalitis in immunocompromised hosts. Inside the host, C. neoformans lives extracellularly as well as within phagocytic cells. One way in which it responds to these varied stresses is by inducing important virulence factors such as the antioxidant melanin and antiphagocytic polysaccharide capsule (1, 18, 69). It also adapts to grow under conditions of high temperature, oxidative stress, high concentrations of salt, human physiological pH, and high levels of CO2. Previous work with C. neoformans determined that the Gpa1/cyclic AMP (cAMP) and Hog1/mitogen-activated protein kinase (MAPK) pathways, among others, are important in regulating responses to these common stresses by leading to the transcriptional activation of stress-related genes (35). However, the role of chromatin remodeling in the transcriptional response of C. neoformans to stress is still unknown. Therefore, in this work we examine the function of C. neoformans Gcn5 (CnGcn5) in adaptation to the host environment, paying particular attention to its role in transcriptional regulation of host stress response genes. We demonstrate that histone acetyltransferase activity is necessary for the C. neoformans response to specific conditions associated with the host environment, and it is therefore essential for virulence.