Genetic bypass of Aspergillus nidulans crzA function in calcium homeostasis

Fungal Lysine Deacetylases in Virulence, Resistance, and Production of Small Bioactive Compounds

The growing number of immunocompromised patients begs for efficient therapy strategies against invasive fungal infections. As conventional antifungal treatment is increasingly hampered by resistance to commonly used antifungals, development of novel therapy regimens is required. On the other hand, numerous fungal species are industrially exploited as cell factories of enzymes and chemicals or as producers of medically relevant pharmaceuticals. Consequently, there is immense interest in tapping the almost inexhaustible fungal portfolio of natural products for potential medical and industrial applications. Both the pathogenicity and production of those small metabolites are significantly dependent on the acetylation status of distinct regulatory proteins. Thus, classical lysine deacetylases (KDACs) are crucial virulence determinants and important regulators of natural products of fungi. In this review, we present an overview of the members of classical KDACs and their complexes in filamentous fungi. Further, we discuss the impact of the genetic manipulation of KDACs on the pathogenicity and production of bioactive molecules. Special consideration is given to inhibitors of these enzymes and their role as potential new antifungals and emerging tools for the discovery of novel pharmaceutical drugs and antibiotics in fungal producer strains.

RcLS2F - A Novel Fungal Class 1 KDAC Co-repressor Complex in Aspergillus nidulans

The fungal class 1 lysine deacetylase (KDAC) RpdA is a promising target for prevention and treatment of invasive fungal infection. RpdA is essential for survival of the most common air-borne mold pathogen Aspergillus fumigatus and the model organism Aspergillus nidulans. In A. nidulans, RpdA depletion induced production of previously unknown small bioactive substances. As known from yeasts and mammals, class 1 KDACs act as components of multimeric protein complexes, which previously was indicated also for A. nidulans. Composition of these complexes, however, remained obscure. In this study, we used tandem affinity purification to characterize different RpdA complexes and their composition in A. nidulans. In addition to known class 1 KDAC interactors, we identified a novel RpdA complex, which was termed RcLS2F. It contains ScrC, previously described as suppressor of the transcription factor CrzA, as well as the uncharacterized protein FscA. We show that recruitment of FscA depends on ScrC and we provide clear evidence that ΔcrzA suppression by ScrC depletion is due to a lack of transcriptional repression caused by loss of the novel RcLS2F complex. Moreover, RcLS2F is essential for sexual development and engaged in an autoregulatory feed-back loop.

Function of crzA in Fungal Development and Aflatoxin Production in Aspergillus flavus

The calcineurin pathway is an important signaling cascade for growth, sexual development, stress response, and pathogenicity in fungi. In this study, we investigated the function of CrzA, a key transcription factor of the calcineurin pathway, in an aflatoxin-producing fungus Aspergillus flavus (A. flavus). To examine the role of the crzA gene, crzA deletion mutant strains in A. flavus were constructed and their phenotypes, including fungal growth, spore formation, and sclerotial formation, were examined. Absence of crzA results in decreased colony growth, the number of conidia, and sclerocia production. The crzA-deficient mutant strains were more susceptible to osmotic pressure and cell wall stress than control or complemented strains. Moreover, deletion of crzA results in a reduction in aflatoxin production. Taken together, these results demonstrate that CrzA is important for differentiation and mycotoxin production in A. flavus.

Calcium signaling pathway is involved in non-CYP51 azole resistance in Aspergillus fumigatus

The opportunistic fungal pathogen Aspergillus fumigatus, which is one of the primary airborne ascomycete pathogens and allergens worldwide, causes invasive fungal infections, which have high morbidity and mortality rates among immunosuppressed patients. The abuse of azole antifungals results in serious drug resistance in clinical therapy. Thus, a thorough understanding of the azole drug resistance mechanism and screening of antifungal agents with a novel mode of action and new drug targets are required to fight against drug resistance. Current studies suggest that there are three major azole resistance mechanisms in fungal pathogens, including changes of the drug target Cyp51, activation of drug efflux pumps and induction of cellular stress responses. Fungi must adapt to a variety of external environmental stressors to survive. These obstacles include stress to the plasma membrane after azole antifungal treatments, high temperature, pH variation, and oxidative stress. As a filamentous fungus, A. fumigatus has evolved numerous signal-transduction systems to sense and respond to azole stresses to survive and proliferate in harsh environmental conditions. Among these signal-transduction systems, the Ca2+ signaling pathway is one of the most important response systems, which has been verified to be involved in stress adaptation. In this review, we have summarized how the components of the calcium-signaling pathway and their interaction network are involved in azole stress response in A. fumigatus.

Modulation of calcineurin activity in Aspergillus nidulans: the roles of high magnesium concentrations and of transcriptional factor CrzA

A proper response to elevated extracellular calcium levels helps to most organisms to keep this secondary messenger under strict control, thereby preventing inadequate activation or inhibition of many regulatory activities into cells. In fungi, the calcineurin responsive zinc-finger Crz1/CrzA transcription factor transduces calcium signaling to gene expression. In Aspergillus nidulans, absence of CrzA activity leads to alkaline pH sensitivity and loss of tolerance to high levels of extracellular calcium. Disruption of calcium uptake mechanisms or the presence of high levels of Mg²⁺ partially suppresses this calcium-sensitive phenotype of null crzA strain. The effects of Mg²⁺ on CrzA phosphorylation and perturbations that reduce calcineurin phosphatase activity on CrzA demonstrate that the calcium sensitive phenotype of null crzA strain is a consequence of up-regulated calcineurin activity under calcium-induced conditions.

Calcineurin and Calcium Channel CchA Coordinate the Salt Stress Response by Regulating Cytoplasmic Ca2+ Homeostasis in Aspergillus nidulans

The eukaryotic calcium/calmodulin-dependent protein phosphatase calcineurin is crucial for the environmental adaption of fungi. However, the mechanism of coordinate regulation of the response to salt stress by calcineurin and the high-affinity calcium channel CchA in fungi is not well understood. Here we show that the deletion of cchA suppresses the hyphal growth defects caused by the loss of calcineurin under salt stress in Aspergillus nidulans Additionally, the hypersensitivity of the ΔcnaA strain to extracellular calcium and cell-wall-damaging agents can be suppressed by cchA deletion. Using the calcium-sensitive photoprotein aequorin to monitor the cytoplasmic Ca(2+) concentration ([Ca(2+)]c) in living cells, we found that calcineurin negatively regulates CchA on calcium uptake in response to external calcium in normally cultured cells. However, in salt-stress-pretreated cells, loss of either cnaA or cchA significantly decreased the [Ca(2+)]c, but a deficiency in both cnaA and cchA switches the [Ca(2+)]c to the reference strain level, indicating that calcineurin and CchA synergistically coordinate calcium influx under salt stress. Moreover, real-time PCR results showed that the dysfunction of cchA in the ΔcnaA strain dramatically restored the expression of enaA (a major determinant for sodium detoxification), which was abolished in the ΔcnaA strain under salt stress. These results suggest that double deficiencies of cnaA and cchA could bypass the requirement of calcineurin to induce enaA expression under salt stress. Finally, YvcA, a member of the transient receptor potential channel (TRPC) protein family of vacuolar Ca(2+) channels, was proven to compensate for calcineurin-CchA in fungal salt stress adaption.IMPORTANCE The feedback inhibition relationship between calcineurin and the calcium channel Cch1/Mid1 has been well recognized from yeast. Interestingly, our previous study (S. Wang et al., PLoS One 7:e46564, 2012, http://dx.doi.org/10.1371/journal.pone.0046564) showed that the deletion of cchA could suppress the hyphal growth defects caused by the loss of calcineurin under salt stress in Aspergillus nidulans In this study, our findings suggest that fungi are able to develop a unique mechanism for adapting to environmental salt stress. Compared to cells cultured normally, the NaCl-pretreated cells had a remarkable increase in transient [Ca(2+)]c Furthermore, we show that calcineurin and CchA are required to modulate cellular calcium levels and synergistically coordinate calcium influx under salt stress. Finally, YvcA, a member of of the TRPC family of vacuolar Ca(2+) channels, was proven to compensate for calcineurin-CchA in fungal salt stress adaption. The findings in this study provide insights into the complex regulatory links between calcineurin and CchA to maintain cytoplasmic Ca(2+) homeostasis in response to different environments.

Calcineurin as a Multifunctional Regulator: Unraveling Novel Functions in Fungal Stress Responses, Hyphal Growth, Drug Resistance, and Pathogenesis

Calcineurin signaling plays diverse roles in fungi in regulating stress responses, morphogenesis and pathogenesis. Although calcineurin signaling is conserved among fungi, recent studies indicate important divergences in calcineurin-dependent cellular functions among different human fungal pathogens. Fungal pathogens utilize the calcineurin pathway to effectively survive the host environment and cause life-threatening infections. The immunosuppressive calcineurin inhibitors (FK506 and cyclosporine A) are active against fungi, making targeting calcineurin a promising antifungal drug development strategy. Here we summarize current knowledge on calcineurin in yeasts and filamentous fungi, and review the importance of understanding fungal-specific attributes of calcineurin to decipher fungal pathogenesis and develop novel antifungal therapeutic approaches.