Calcineurin regulation in fungi and beyond

Constitutive Overexpression of CRZ1 in Trichoderma reesei Reveals Its Ability to Enhance Recombinant Lipase Production

The filamentous fungus Trichoderma reesei is extensively utilized in the realm of recombinant protein expression owing to its well-established protein production systems. However, the potential for efficient and convenient protein production in T. reesei has not been fully harnessed. To further increase the production of recombinant lipase Candida antarctica lipase B (CalB), we overexpressed seven transcription activators and found that overexpression of the calcineurin CRZ1 could significantly enhance CalB production by 2.08-fold, and then through employing a synthetic strong promoter Pcbh7IR and knocking out ace1, a competitor of crz1, CalB production was further increased 4.2-fold. Furthermore, we found that increasing the Ca2+ concentration enabled the enhancement of CalB production in the CRZ1-overexpressing strains. Additionally, transcription analysis revealed a significant upregulation of calcium-related pathway genes and ER chaperone genes in T. reesei, revealing that overexpression of CRZ1 can increase the production of the recombinant protein by activation of the calcium signaling pathway. In this study, by systematically manipulating the recombinant protein expression module and its regulatory transcription activators, we found a positive effect of crz1-mediated calcium channels in boosting the expression of the recombinant protein.

Calcineurin activity in Fonsecaea pedrosoi: tacrolimus and cyclosporine A inhibited conidia growth, filamentation and showed synergism with itraconazole

Fonsecaea pedrosoi is a melanized fungus that causes chromoblastomycosis (CBM), a tropical neglected disease responsible for chronic and disability-related subcutaneous mycosis. Given the challenging nature of CBM treatment, the study of new targets and novel bioactive drugs capable of improving patient life quality is urgent. In the present work, we detected a calcineurin activity in F. pedrosoi conidial form, employing primarily colorimetric, immunoblotting and flow cytometry assays. Our findings reveal that the calcineurin activity of F. pedrosoi was stimulated by Ca2+/calmodulin, inhibited by EGTA and specific inhibitors, such as tacrolimus (FK506) and cyclosporine A (CsA), and proved to be insensitive to okadaic acid. In addition, FK506 and CsA were able to affect the cellular viability and the fungal proliferation. This effect was corroborated by transmission electron microscopy that showed both calcineurin inhibitors promoted profound changes in the ultrastructure of conidia, causing mainly cytoplasm condensation and intense vacuolization that are clear indication of cell death. Our data indicated that FK506 exhibited the highest effectiveness, with a minimum inhibitory concentration (MIC) of 3.12 mg/L, whereas CsA required 15.6 mg/L to inhibit 100% of conidial growth. Interestingly, when both were combined with itraconazole, they demonstrated anti-F. pedrosoi activity, exhibiting a synergistic effect. Moreover, the fungal filamentation was affected after treatment with both calcineurin inhibitors. These data corroborate with other calcineurin studies in fungal cells and open up further discussions aiming to establish the role of this enzyme as a potential target for antifungal therapy against CBM infections.

Modes-of-action of antifungal compounds: Stressors and (target-site-specific) toxins, toxicants, or Toxin-stressors

Fungi and antifungal compounds are relevant to the United Nation's Sustainable Development Goals. However, the modes-of-action of antifungals-whether they are naturally occurring substances or anthropogenic fungicides-are often unknown or are misallocated in terms of their mechanistic category. Here, we consider the most effective approaches to identifying whether antifungal substances are cellular stressors, toxins/toxicants (that are target-site-specific), or have a hybrid mode-of-action as Toxin-stressors (that induce cellular stress yet are target-site-specific). This newly described 'toxin-stressor' category includes some photosensitisers that target the cell membrane and, once activated by light or ultraviolet radiation, cause oxidative damage. We provide a glossary of terms and a diagrammatic representation of diverse types of stressors, toxic substances, and Toxin-stressors, a classification that is pertinent to inhibitory substances not only for fungi but for all types of cellular life. A decision-tree approach can also be used to help differentiate toxic substances from cellular stressors (Curr Opin Biotechnol 2015 33: 228-259). For compounds that target specific sites in the cell, we evaluate the relative merits of using metabolite analyses, chemical genetics, chemoproteomics, transcriptomics, and the target-based drug-discovery approach (based on that used in pharmaceutical research), focusing on both ascomycete models and the less-studied basidiomycete fungi. Chemical genetic methods to elucidate modes-of-action currently have limited application for fungi where molecular tools are not yet available; we discuss ways to circumvent this bottleneck. We also discuss ecologically commonplace scenarios in which multiple substances act to limit the functionality of the fungal cell and a number of as-yet-unresolved questions about the modes-of-action of antifungal compounds pertaining to the Sustainable Development Goals.

Transcriptome analysis of Armillaria gallica 012 m in response to auxin

Gastrodia elata is an achlorophyllous and fully mycoheterotrophic orchid which obtains carbon and other nutrients from Armillaria species in its life cycle. Many researchers suggested that plant hormones, as signing molecules, play a central role in the plant-fungi interaction. In the process of Armillaria gallica 012 m cultivation, both exogenous indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) distinctly stimulated the growth of mycelia in solid media. The differential expression genes (DEGs) of A. gallica 012 m with IAA versus blank control (BK) and IBA versus BK were investigated. The results showed that more than 80% of DEGs of the IAA group were coincident with the DEGs of the IBA group, and more than half of upregulated DEGs and most of the downregulated DEGs of the IAA group coincided with those DEGs of the IBA group. Above research implied that A. gallica 012 m could perceive IAA and IBA, and possess similar responses and signaling pathways to IAA and IBA. The overlapping differential genes of the IAA group and IBA group were analyzed by GO term, and the results showed that several DEGs identified were related to biological processes including positive regulation of the biological process and biological process. The downregulated NmrA-like and FKBP_C genes might be benefit to the growth of mycelia. Those results can explain that exiguous IAA and IBA improved the growth of A. gallica to some extent. We speculate that IAA and IBA are signaling molecules, and regulate the expression of growth-related genes of A. gallica 012 m by the same signaling pathway.

Cellular Calcium Levels Influenced by NCA-2 Impact Circadian Period Determination in Neurospora

Intracellular calcium signaling has been implicated in the control of a variety of circadian processes in animals and plants, but its role in microbial clocks has remained largely cryptic. To examine the role of intracellular Ca²⁺ in the Neurospora clock, we screened mutants with knockouts of calcium transporter genes and identified a gene encoding a calcium exporter, nca-2, uniquely as having significant period effects. The loss of NCA-2 results in an increase in the cytosolic calcium level, and this leads to hyper-phosphorylation of core clock components, FRQ and WC-1, and a short period, as measured by both the core oscillator and the overt clock. Genetic analyses showed that mutations in certain frq phospho-sites and in Ca²⁺-calmodulin-dependent kinase 2 (camk-2) are epistatic to nca-2 in controlling the pace of the oscillator. These data are consistent with a model in which elevated intracellular Ca²⁺ leads to the increased activity of CAMK-2, leading to enhanced FRQ phosphorylation, accelerated closure of the circadian feedback loop, and a shortened circadian period length. At a mechanistic level, some CAMKs undergo more auto-phosphorylations in the Δnca-2 mutant, consistent with high calcium levels in the Δnca-2 mutant influencing the enzymatic activities of CAMKs. NCA-2 interacts with multiple proteins, including CSP-6, a protein known to be required for circadian output. Most importantly, the expression of nca-2 is circadian clock-controlled at both the transcriptional and translational levels, and this in combination with the period effects seen in strains lacking NCA-2 firmly places calcium signaling within the larger circadian system, where it acts as both an input to and an output from the core clock.

Calcineurin-dependent regulation of endocytosis by a plasma membrane ubiquitin ligase adaptor, Rcr1

Rsp5, the Nedd4 family member in yeast, is an E3 ubiquitin ligase involved in numerous cellular processes, many of which require Rsp5 to interact with PY-motif containing adaptor proteins. Here, we show that two paralogous transmembrane Rsp5 adaptors, Rcr1 and Rcr2, are sorted to distinct cellular locations: Rcr1 is a plasma membrane (PM) protein, whereas Rcr2 is sorted to the vacuole. Rcr2 is delivered to the vacuole using ubiquitin as a sorting signal. Rcr1 is delivered to the PM by the exomer complex using a newly uncovered PM sorting motif. Further, we show that Rcr1, but not Rcr2, is up-regulated via the calcineurin/Crz1 signaling pathway. Upon exogenous calcium treatment, Rcr1 ubiquitinates and down-regulates the chitin synthase Chs3. We propose that the PM-anchored Rsp5/Rcr1 ubiquitin ligase-adaptor complex can provide an acute response to degrade unwanted proteins under stress conditions, thereby maintaining cell integrity.

Structure-guided approaches to targeting stress responses in human fungal pathogens

Fungi inhabit extraordinarily diverse ecological niches, including the human body. Invasive fungal infections have a devastating impact on human health worldwide, killing ∼1.5 million individuals annually. The majority of these deaths are attributable to species of Candida, Cryptococcus, and Aspergillus Treating fungal infections is challenging, in part due to the emergence of resistance to our limited arsenal of antifungal agents, necessitating the development of novel therapeutic options. Whereas conventional antifungal strategies target proteins or cellular components essential for fungal growth, an attractive alternative strategy involves targeting proteins that regulate fungal virulence or antifungal drug resistance, such as regulators of fungal stress responses. Stress response networks enable fungi to adapt, grow, and cause disease in humans and include regulators that are highly conserved across eukaryotes as well as those that are fungal-specific. This review highlights recent developments in elucidating crystal structures of fungal stress response regulators and emphasizes how this knowledge can guide the design of fungal-selective inhibitors. We focus on the progress that has been made with highly conserved regulators, including the molecular chaperone Hsp90, the protein phosphatase calcineurin, and the small GTPase Ras1, as well as with divergent stress response regulators, including the cell wall kinase Yck2 and trehalose synthases. Exploring structures of these important fungal stress regulators will accelerate the design of selective antifungals that can be deployed to combat life-threatening fungal diseases.

Three proline rotamases involved in calcium homeostasis play differential roles in stress tolerance, virulence and calcineurin regulation of Beauveria bassiana

FK506-sensitive proline rotamases (FPRs), also known as FK506-binding proteins (FKBPs), can mediate immunosuppressive drug resistance in budding yeast but their physiological roles in filamentous fungi remain opaque. Here, we report that three FPRs (cytosolic/nuclear 12.15-kD Fpr1, membrane-associated 14.78-kD Fpr2 and nuclear 50.43-kD Fpr3) are all equally essential for cellular Ca²⁺ homeostasis and contribute significantly to calcineurin activity at different levels in the insect-pathogenic fungus Beauveria bassiana although the deletion of fpr1 alone conferred resistance to FK506. Radial growth, conidiation, conidial viability and virulence were less compromised in the absence of fpr1 or fpr2 than in the absence of fpr3, which abolished almost all growth on scant media and reduced growth moderately on rich media. The Δfpr3 mutant was more sensitive to Na⁺ , K⁺ , Mn²⁺ , Ca²⁺ , Cu²⁺ , metal chelate, heat shock and UVB irradiation than was Δfpr2 while both mutants were equally sensitive to Zn²⁺ , Mg²⁺ , Fe²⁺ , H₂ O₂ and cell wall-perturbing agents. In contrast, the Δfpr1 mutant was less sensitive to fewer stress cues. Most of 32 examined genes involved in DNA damage repair, Na⁺ /K⁺ detoxification or osmotolerance and Ca²⁺ homeostasis were downregulated sharply in Δfpr2 and Δfpr3 but rarely so affected in Δfpr1, coinciding well with their phenotypic changes. These findings uncover important, but differential, roles of three FPRs in the fungal adaptation to insect host and environment and provide novel insight into their essential roles in calcium signalling pathway.

Characterization of three FK506-binding proteins in the entomopathogenic fungus Beauveria bassiana

FK506 binding proteins (FKBPs) participate in regulation of diverse biological processes. However, the role of these proteins in insect-pathogenic fungi is far from well understood. To investigate the functions of FKBPs in Beauveria bassiana, a widely used entomopathogenic fungus for control of insect pests, we identify three putative FKBP genes, Bbfkbp12, Bbfkbp15, and Bbfkbp50, in the fungus. Gene-disruption experiments show that loss of Bbfkbp12 results in a significant increase of resistance of B. bassiana against the immunosuppressive compounds FK506 and rapamycin, while loss of Bbfkbp50 leads to the resistance to the ergosterol synthesis inhibitor lovastatin. Transcription assays of calcineurin (CaN)- and mTOR (mammalian target of rapamycin)-downstream target genes confirm that BbFKBP12 is the target of both FK506 and rapamycin, associated with CaN- and mTOR-signal pathways in B. bassiana. GFP-tagging of the proteins shows that BbFKBP12 and BbFKBP15 localize in cytoplasm while BbFKBP50 in nucleus. Our results provide useful information for the study of functions of CaN- and mTOR-mediated signaling, and ergosterol synthesis in the entomopathogenic fungi.

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.

A closer look at Aspergillus: online monitoring via scattered light enables reproducible phenotyping

BACKGROUND

Filamentously growing microorganisms offer unique advantages for biotechnological processes, such as extraordinary secretion capacities, going along with multiple obstacles due to their complex morphology. However, limited experimental throughput in bioprocess development still hampers taking advantage of their full potential. Miniaturization and automation are powerful tools to accelerate bioprocess development, but so far the application of such technologies has mainly been focused on non-filamentous systems. During cultivation, filamentous fungi can undergo remarkable morphological changes, creating challenging cultivation conditions. Depending on the process and product, only one specific state of morphology may be advantageous to achieve e.g. optimal productivity or yield. Different approaches to control morphology have been investigated, such as microparticle enhanced cultivation. However, the addition of solid microparticles impedes the optical measurements typically used by microbioreactor systems and thus alternatives are needed.

RESULTS

Aspergillus giganteus IfGB 0902 was used as a model system to develop a time-efficient and robust workflow allowing microscale cultivation with increased throughput. The effect of microtiter plate geometry, shaking frequency and medium additives (talc and calcium chloride) on homogeneity of culture morphology as well as reproducibility were analyzed via online biomass measurement, microscopic imaging and cell dry weight. While addition of talc severely affected online measurements, 2% (w v-1) calcium chloride was successfully applied to obtain a highly reproducible growth behavior with homogenous morphology. Furthermore, the influence of small amounts of complex components was investigated for the applied model strain. By correlation to cell dry weight, it could be shown that optical measurements are a suitable signal for biomass concentration. However, each correlation is only applicable for a specific set of cultivation parameters. These optimized conditions were used in micro as well as lab-scale bioreactor cultivation in order to verify the reproducibility and scalability of the setup.

CONCLUSION

A robust workflow for A. giganteus was developed, allowing for reproducible microscale cultivation with online monitoring, where calcium chloride is an useful alternative to microparticle enhanced cultivation in order to control the morphology. Independent of the cultivation volume, comparable phenotypes were observed in microtiter plates and in lab-scale bioreactor.

Disruption of calcineurin catalytic subunit (cnaA) in Epichloë festucae induces symbiotic defects and intrahyphal hyphae formation

Calcineurin is a conserved calcium/calmodulin-dependent protein phosphatase, consisting of a catalytic subunit A and a regulatory subunit B, which is involved in calcium-dependent signalling and regulation of various important cellular processes. In this study, we functionally characterized the catalytic subunit A (CnaA) of the endophytic fungus Epichloë festucae which forms a symbiotic association with the grass host Lolium perenne. We deleted the CnaA-encoding gene cnaA in E. festucae and examined its role in hyphal growth, cell wall integrity and symbiosis. This ΔcnaA strain had a severe growth defect with loss of radial growth and hyper-branched hyphae. Transmission electron microscopy and confocal microscopy analysis of the mutant revealed cell wall defects, aberrant septation and the formation of intrahyphal hyphae, both in culture and in planta. The mutant strain also showed a reduced infection rate in planta. The fluorescence of mutant hyphae stained with WGA-AF488 was reduced, indicating reduced chitin accessibility. Together, these results show that E. festucae CnaA is required for fungal growth, maintaining cell wall integrity and host colonization.

Tacrolimus Increases the Effectiveness of Itraconazole and Fluconazole against Sporothrix spp

Calcineurin inhibitors – such as the clinically used drug tacrolimus – are active against important fungal pathogens, particularly when combined with azoles. However, tacrolimus has not been tested against sporotrichosis, an endemic subcutaneous mycosis with worldwide distribution. Here, we evaluated the activity of tacrolimus and cyclosporine A in vitro – as monotherapy and in combination with itraconazole or fluconazole – against yeasts of Sporothrix brasiliensis and S. schenckii, the main sporotrichosis agents in Brazil. We also analyzed the effect of tacrolimus treatment on intracellular neutral lipid levels, which typically increase after azole treatment. Tacrolimus inhibited the growth of yeasts from S. brasiliensis and S. schenckii reference isolates, with minimum inhibitory concentration (MIC) values (required for ≥50% growth inhibition) of 1 and 2 mg/L, respectively. Importantly, the combination of tacrolimus and azoles exhibited high synergy toward reference Sporothrix isolates. Tacrolimus combined with itraconazole significantly increased neutral lipid accumulation in S. brasiliensis, but not in S. schenckii. Clinical isolates of S. brasiliensis and S. schenckii were more sensitive to tacrolimus as monotherapy than feline-borne isolates, however, synergy between tacrolimus and azoles was only observed for feline-borne isolates. Cyclosporine A was effective against S. brasiliensis and S. schenckii as monotherapy (MIC = 1 mg/L), but exhibited no synergy with itraconazole and fluconazole. We conclude that tacrolimus has promising antifungal activity against sporotrichosis agents, and also increases the activity of the current anti-sporotrichosis therapy (itraconazole and fluconazole) in combination assays against S. brasiliensis feline-borne isolates.

Transcriptomic Analysis Reveals Genes Mediating Salt Tolerance through Calcineurin/CchA-Independent Signaling in Aspergillus nidulans

Adaptation to changes in the environment is crucial for the viability of all organisms. Although the importance of calcineurin in the stress response has been highlighted in filamentous fungi, little is known about the involvement of ion-responsive genes and pathways in conferring salt tolerance without calcium signaling. In this study, high-throughput RNA-seq was used to investigate salt stress-induced genes in the parent, ΔcnaB, and ΔcnaBΔcchA strains of Aspergillus nidulans, which differ greatly in salt adaption. In total, 2,884 differentially expressed genes including 1,382 up- and 1,502 downregulated genes were identified. Secondary transporters, which were upregulated to a greater extent in ΔcnaBΔcchA than in the parent or ΔcnaB strains, are likely to play important roles in response to salt stress. Furthermore, 36 genes were exclusively upregulated in the ΔcnaBΔcchA under salt stress. Functional analysis of differentially expressed genes revealed that genes involved in transport, heat shock protein binding, and cell division processes were exclusively activated in ΔcnaBΔcchA. Overall, our findings reveal that secondary transporters and stress-responsive genes may play crucial roles in salt tolerance to bypass the requirement for the CchA-calcineurin pathway, contributing to a deeper understanding of the mechanisms that influence fungal salt stress adaption in Aspergillus.

Pheromone independent unisexual development in Cryptococcus neoformans

The fungus Cryptococcus neoformans can undergo a-α bisexual and unisexual reproduction. Completion of both sexual reproduction modes requires similar cellular differentiation processes and meiosis. Although bisexual reproduction generates equal number of a and α progeny and is far more efficient than unisexual reproduction under mating-inducing laboratory conditions, the α mating type dominates in nature. Population genetic studies suggest that unisexual reproduction by α isolates might have contributed to this sharply skewed distribution of the mating types. However, the predominance of the α mating type and the seemingly inefficient unisexual reproduction observed under laboratory conditions present a conundrum. Here, we discovered a previously unrecognized condition that promotes unisexual reproduction while suppressing bisexual reproduction. Pheromone is the principal stimulus for bisexual development in Cryptococcus. Interestingly, pheromone and other components of the pheromone pathway, including the key transcription factor Mat2, are not necessary but rather inhibitory for Cryptococcus to complete its unisexual cycle under this condition. The inactivation of the pheromone pathway promotes unisexual reproduction despite the essential role of this pathway in non-self-recognition during bisexual reproduction. Nonetheless, the requirement for the known filamentation regulator Znf2 and the expression of hyphal or basidium specific proteins remain the same for pheromone-dependent or independent sexual reproduction. Transcriptome analyses and an insertional mutagenesis screen in mat2Δ identified calcineurin being essential for this process. We further found that Znf2 and calcineurin work cooperatively in controlling unisexual development in this fungus. These findings indicate that Mat2 acts as a repressor of pheromone-independent unisexual development while serving as an activator for a-α bisexual development. The bi-functionality of Mat2 might have allowed it to act as a toggle switch for the mode of sexual development in this ubiquitous eukaryotic microbe.

Characterization of calcineurin from Cryptococcus humicola and the application of calcineurin in aluminum tolerance

BACKGROUND

Calcineurin (CaN) is a Ca2+- and calmodulin (CaM)-dependent serine/threonine phosphatase. Previous studies have found that CaN is involved in the regulation of the stress responses.

RESULTS

In this study, the growth of Cryptococcus humicola was inhibited by the CaN inhibitor tacrolimus (FK506) under aluminum (Al) stress. The expression of CNA encoding a catalytic subunit A (CNA) and its interaction with CaM were upregulated when the concentration of Al was increased. A CaM-binding domain and key amino acids responsible for interaction with CaM were identified. ∆CNAb with a deletion from S454 to A639 was detected to bind to CaM, while ∆CNAa with a deletion from R436 to A639 showed no binding to CaM. The binding affinities of CNA1 and CNA2, in which I439 or I443 were replaced by Ala, were decreased relative to wild-type CNA. The phosphatase activities of ∆CNAa, CNA1 and CNA2 were lower than the wild-type protein. These results suggest that the region between R436 and S454 is essential for the interaction with CaM and I439, I443 are key amino acids in this region. The ability of the CNA transgenic yeast to develop resistance to Al was significantly higher than that of control yeast. Residual Al in the CNA transgenic yeast culture media was significantly lower than the amount of Al originally added to the media or the residual Al remaining in the control yeast culture media. These findings suggest that CNA confers Al tolerance, and the mechanism of Al tolerance may involve absorption of active Al.

CONCLUSIONS

Al stress up-regulated the expression of CNA. CaM-binding domain and key amino acids responsible for interaction with CaM were identified and both are required for phosphatase activities. CNA conferred yeast Al resistance indicating that the gene has a potential to improve Al-tolerance through gene engineering.

The putative flavin carrier family FlcA-C is important for Aspergillus fumigatus virulence

Aspergillus fumigatus is an opportunistic fungal pathogen and the most important species causing pulmonary fungal infections. The signaling by calcium is very important for A. fumigatus pathogenicity and it is regulated by the transcription factor CrzA. We have previously used used ChIP-seq (Chromatin Immunoprecipitation DNA sequencing) aiming to identify gene targets regulated by CrzA. We have identified among several genes regulated by calcium stress, the putative flavin transporter, flcA. This transporter belongs to a small protein family composed of FlcA, B, and C. The ΔflcA null mutant showed several phenotypes, such as morphological defects, increased sensitivity to calcium chelating-agent ethylene glycol tetraacetic acid (EGTA), cell wall or oxidative damaging agents and metals, repre-sentative of deficiencies in calcium signaling and iron homeostasis. Increasing calcium concentrations improved significantly the ΔflcA growth and conidiation, indicating that ΔflcA mutant has calcium insufficiency. Finally, ΔflcA-C mutants showed reduced flavin adenine dinucleotide (FAD) and were avirulent in a low dose murine infection model.

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.

In-depth 2-DE reference map of Aspergillus fumigatus and its proteomic profiling on exposure to itraconazole

Aspergillus fumigatus (A. fumigatus) is a medically important opportunistic fungus that may lead to invasive aspergillosis in humans with weak immune system. Proteomic profiling of this fungus on exposure to itraconazole (ITC), an azole antifungal drug, may lead to identification of its molecular targets and better understanding on the development of drug resistance against ITC in A. fumigatus. Here, proteome analysis was performed using 2-DE followed by mass spectrometric analysis which resulted in identification of a total of 259 unique proteins. Further, proteome profiling of A. fumigatus was carried out on exposure to ITC, 0.154 μg/ml, the minimum inhibitory concentration (MIC50). Image analysis showed altered levels of 175 proteins (66 upregulated and 109 downregulated) of A. fumigatus treated with ITC as compared to the untreated control. Peptide mass fingerprinting led to the identification of 54 proteins (12 up-regulated and 42 down-regulated). The differentially expressed proteins include proteins related to cell stress, carbohydrate metabolism and amino acid metabolism. We also observed four proteins, including nucleotide phosphate kinase (NDK), that are reported to interact with calcineurin, a protein involved in regulation of cell morphology and fungal virulence. Comparison of differentially expressed proteins on exposure to ITC with artemisinin (ART), an antimalarial drug with antifungal activity(1), revealed a total of 26 proteins to be common among them suggesting that common proteins and pathways are targeted by these two antifungal agents. The proteins targeted by ITC may serve as important leads for development of new antifungal drugs.

Calcineurin Undergoes a Conformational Switch Evoked via Peptidyl-Prolyl Isomerization

A limited repertoire of PPP family of serine/threonine phosphatases with a highly conserved catalytic domain acts on thousands of protein targets to orchestrate myriad central biological roles. A major structural reorganization of human calcineurin, a ubiquitous Ser/Thr PPP regulated by calcium and calmodulin and targeted by immunosuppressant drugs cyclosporin A and FK506, is unveiled here. The new conformation involves trans- to cis-isomerization of proline in the SAPNY sequence, highly conserved across PPPs, and remodels the main regulatory site where NFATc transcription factors bind. Transitions between cis- and trans-conformations may involve peptidyl prolyl isomerases such as cyclophilin A and FKBP12, which are known to physically interact with and modulate calcineurin even in the absence of immunosuppressant drugs. Alternative conformations in PPPs provide a new perspective on interactions with substrates and other protein partners and may foster development of more specific inhibitors as drug candidates.

RNA-sequencing analysis of Trichophyton rubrum transcriptome in response to sublethal doses of acriflavine

BACKGROUND

The dermatophyte Trichophyton rubrum is an anthropophilic filamentous fungus that infects keratinized tissues and is the most common etiologic agent isolated in human dermatophytoses. The clinical treatment of these infections is challenging because only few antifungal drugs are commercially available. To understand the mode of action of cytotoxic drugs against fungi, we evaluated the time-dependent effects of acriflavine on T. rubrum transcriptome using high-throughput RNA-sequencing (RNA-seq) technology.

RESULTS

RNA-seq analysis generated approximately 200 million short reads that were mapped to the Broad Institute's Dermatophyte Comparative Database before differential gene expression analysis was performed. By employing a stringent cut-off threshold of -1.5 and 1.5 log₂-fold changes in gene expression, a subset of 490 unique genes were found to be modulated in T. rubrum in response to acriflavine exposure. Among the selected genes, 69 genes were modulated at all exposure time points. Functional categorization indicated the putative involvement of these genes in various cellular processes such as oxidation-reduction reaction, transmembrane transport, and metal ion binding. Interestingly, genes putatively involved in the pathogenicity of dermatophytoses were down-regulated suggesting that this drug interferes with the virulence of T. rubrum. Moreover, we identified 159 novel putative transcripts in intergenic regions and two transcripts in intron regions of T. rubrum genome.

CONCLUSION

The results provide insights into the molecular events underlying the stress responses of T. rubrum to acriflavine, revealing that this drug interfered with important molecular events involved in the establishment and maintenance of fungal infection in the host. In addition, the identification of novel transcripts will further enable the improvement of gene annotation and open reading frame prediction of T. rubrum and other dermatophyte genomes.

ChIP-seq reveals a role for CrzA in the Aspergillus fumigatus high-osmolarity glycerol response (HOG) signalling pathway

Aspergillus fumigatus is an opportunistic pathogen and allergen of mammals. Calcium signalling is essential for A. fumigatus pathogenicity and is regulated by the CrzA transcription factor. We used ChIP-seq (Chromatin Immunoprecipitation DNA sequencing) to explore CrzA gene targets in A. fumigatus. In total, 165 potential binding peaks including 102 directly regulated genes were identified, resulting in the prediction of the A[GT][CG]CA[AC][AG] CrzA-binding motif. The 102 CrzA putatively regulated genes exhibited a diverse array of functions. The phkB (Afu3g12530) histidine kinase and the sskB (Afu1g10940) MAP kinase kinase kinase of the HOG (high-osmolarity glycerol response) pathway were regulated by CrzA. Several members of the two-component system (TCS) and the HOG pathway were more sensitive to calcium. CrzA::GFP was translocated to the nucleus upon osmotic stress. CrzA is important for the phosphorylation of the SakA MAPK in response to osmotic shock. The ΔsskB was more sensitive to CaCl2 , NaCl, and paraquat stress, while being avirulent in a murine model of invasive pulmonary aspergillosis. The presence of CaCl2 and osmotic stresses resulted in synergistic inhibition of ΔcrzA and ΔsskB growth. These results suggest there is a genetic interaction between the A. fumigatus calcineurin-CrzA and HOG pathway that is essential for full virulence.

Five vacuolar Ca(2+) exchangers play different roles in calcineurin-dependent Ca(2+)/Mn(2+) tolerance, multistress responses and virulence of a filamentous entomopathogen

Multiple Vcx1 (vacuolar calcium exchanger) paralogues exist in many filamentous fungi but are functionally unexplored unlike a single Vcx1 ortholog well characterized in yeasts. Here we show that five Vcx1 paralogues (Vcx1A-E) in Beauveria bassiana are conditionally functional for intracellular Ca(2+) homeostasis and contribute differentially to multistress tolerance and virulence in the filamentous entomopathogen. Each vcx1 deletion drastically upregulated transcriptional expressions of four other partners and six P-type Ca(2+)-ATPases, resulting in elevated or lowered intracellular Ca(2+) concentration in some deletion mutants treated with Ca(2+) stress or untreated at 25 and 30 °C. When calcineurin was inactivated by cyclosporine A, Ca(2+) tolerance decreased by 11-17% in five Δvcx1 mutants, but Mn(2+) sensitivity increased only in Δvcx1A and Δvcx1D, at optimal 25 °C. These two mutants were also more sensitive to Ca(2+) stress at 30 °C when calcineurin was active, and showed minor growth defect at 25 and 30 °C when calcineurin was inactive. Moreover, all the Δvcx1 mutants were more sensitive to dithiothreitol (stress-response trigger to endoplasmic reticulum) and Congo red (cell wall stressor); three of them were consistently less tolerant to the oxidants menadione and H2O2. The fungal virulence to Galleria mellonella larvae decreased by 15-40% in four Δvcx1 mutants excluding Δvcx1E, which was uniquely defective in conidial thermotolerance. All the changes were restored by each vcx1 complementation. Our findings indicate that the five Vcx1 paralogues in B. bassiana contribute differentially to calcineurin-dependent Ca(2+)/Mn(2+) tolerance, multistress responses and virulence, and recall attention to multifunctional Vcx1 paralogues in filamentous fungi.

Protein kinase C overexpression suppresses calcineurin-associated defects in Aspergillus nidulans and is involved in mitochondrial function

In filamentous fungi, intracellular signaling pathways which are mediated by changing calcium levels and/or by activated protein kinase C (Pkc), control fungal adaptation to external stimuli. A rise in intracellular Ca2+ levels activates calcineurin subunit A (CnaA), which regulates cellular calcium homeostasis among other processes. Pkc is primarily involved in maintaining cell wall integrity (CWI) in response to different environmental stresses. Cross-talk between the Ca2+ and Pkc-mediated pathways has mainly been described in Saccharomyces cerevisiae and in a few other filamentous fungi. The presented study describes a genetic interaction between CnaA and PkcA in the filamentous fungus Aspergillus nidulans. Overexpression of pkcA partially rescues the phenotypes caused by a cnaA deletion. Furthermore, CnaA appears to affect the regulation of a mitogen-activated kinase, MpkA, involved in the CWI pathway. Reversely, PkcA is involved in controlling intracellular calcium homeostasis, as was confirmed by microarray analysis. Furthermore, overexpression of pkcA in a cnaA deletion background restores mitochondrial number and function. In conclusion, PkcA and CnaA-mediated signaling appear to share common targets, one of which appears to be MpkA of the CWI pathway. Both pathways also regulate components involved in mitochondrial biogenesis and function. This study describes targets for PkcA and CnaA-signaling pathways in an A. nidulans and identifies a novel interaction of both pathways in the regulation of cellular respiration.

Calcineurin phosphatase and phospholipase C are required for developmental and pathological functions in the citrus fungal pathogen Alternaria alternata

Excessive Ca(2+) or compounds interfering with phosphoinositide cycling have been found to inhibit the growth of the tangerine pathotype of Alternaria alternata, suggesting a crucial role of Ca(2+) homeostasis in this pathotype. The roles of PLC1, a phospholipase C-coding gene and CAL1, a calcineurin phosphatase-coding gene were investigated. Targeted gene disruption showed that both PLC1 and CAL1 were required for vegetative growth, conidial formation and pathogenesis in citrus. Fungal strains lacking PLC1 or CAL1 exhibited extremely slow growth and induced small lesions on calamondin leaves. Δplc1 mutants produced fewer conidia, which germinated at slower rates than wild-type. Δcal1 mutants produced abnormal hyphae and failed to produce any mature conidia, but instead produced highly melanized bulbous hyphae with distinct septae. Fluorescence microscopy using Fluo-3 dye as a Ca(2+) indicator revealed that the Δplc1 mutant hyphae emitted stronger cytosolic fluorescence, and the Δcal1 mutant hyphae emitted less cytosolic fluorescence, than those of wild-type. Infection assessed on detached calamondin leaves revealed that application of CaCl2 or neomycin 24 h prior to inoculation provided protection against Alt. alternata. These data indicate that a dynamic equilibrium of cellular Ca(2+) is critical for developmental and pathological processes of Alt. alternata.

Genetic bypass of Aspergillus nidulans crzA function in calcium homeostasis

After dephosphorylation by the phosphatase calcineurin, the fungal transcription factor CrzA enters the nucleus and activates the transcription of genes responsible for calcium homeostasis and many other calcium-regulated activities. A lack of CrzA confers calcium-sensitivity to the filamentous fungus Aspergillus nidulans. To further understand calcium signaling in filamentous fungi and to identify genes that interact genetically with CrzA, we selected for mutations that were able to suppress crzAΔ calcium intolerance and identified three genes. Through genetic mapping, gene sequencing, and mutant rescue, we were able to identify these as cnaB (encoding the calcineurin regulatory subunit), folA (encoding an enzyme involved in folic acid biosynthesis, dihydroneopterin aldolase), and scrC (suppression of crzA^-, encoding a hypothetical protein). By using a calcium indicator, Fluo-3, we were able to determine that the wild-type and the suppressor strains were either able to regulate intracellular calcium levels or were able to take up and or store calcium correctly. The increased expression of calcium transporters, pmcA and/or pmcB, in suppressor mutants possibly enabled tolerance to high levels of calcium. Our results suggest that a cnaB suppressor mutation confers calcium tolerance to crzAΔ strains through restoration of calcium homeostasis. These results stress that in A. nidulans there are calcineurin-dependent and CrzA-independent pathways. In addition, it is possible that CrzA is able to contribute to the modulation of folic acid biosynthesis.

Phospho-regulation and nucleocytoplasmic trafficking of CrzA in response to calcium and alkaline-pH stress in Aspergillus nidulans

Tolerance to abiotic stresses by microorganisms require of appropriate signalling and regulatory pathways. Calcineurin phosphatases mediate calcium-dependent signalling pathways which are widely distributed among phylogeny. In Saccharomyces cerevisiae, calcineurin mediates the post-translational modification of downstream effectors, most of them transcription factors, being the best-characterized calcineurin-regulated zinc-finger factor 1, Crz1p. Here we study the signalling process of CrzA, a filamentous fungal Crz orthologue, in response to calcium and ambient-pH alkalinization. In Aspergillus nidulans resting cells CrzA locates in the cytoplasm being excluded from nuclei. CrzA is a phospho-protein and upon calcium, manganese or alkaline-pH stresses, accumulates in nuclei in a calcineurin-dependent manner. Functional analysis of CrzA defined the presence of a nuclear-export and two nuclear-localization signals as well as a PSINVE sequence that constitutes the major calcineurin-docking domain. First 450 amino acids of CrzA contain these functional motifs and in this region is where phosphorylated residues locate. Different phosphorylation steps are identified in CrzA and activities of casein kinase 1 homologue, CkiA, and of glycogen synthase kinase-3β, identified for the first time here as GskA, are involved. The phospho-signalling process and nucleocytoplasmic trafficking of CrzA shows similarities to those described in yeast for Crz1p homologues and of NFATs in mammals.

Molecular characterization of calcineurin B from the non-virulent Trypanosoma rangeli kinetoplastid indicates high gene conservation

Calcineurin B, the regulatory subunit of calcineurin, a serine/threonine protein phosphatase, is highly conserved throughout the evolutionary scale including trypanosomatids such as Trypanosoma cruzi, and Leishmania major. Thus, in these flagellates the protein is required for mammalian host cell invasion and virulence and stress responses. With the aim of determining the presence of calcineurin B in Trypanosoma rangeli, a non-virulent trypanosome for mammals, the respective gene was amplified by PCR, cloned and sequenced. Two sequences of 531 bp in length showing a nucleotide polymorphism (314A>C) were obtained in spite of a single-copy gene was revealed by Southern blot. These sequences, probably the alleles from the gene, showed a 79% of identity with those from T. cruzi and clustered as the sister group of this trypanosome species in a Maximum Parsimony analysis. Deduced amino acid sequence comparison with trypanosomatids and other organisms through the phylogenetic scale as well as the obtained protein structural homology model suggested the presence of the four potential EF-hand regions and the corresponding calcium binding sites of the last three of these domains. Having assessed the expression of this protein in T. rangeli epimastigotes, and taking into account the following facts: (i) calcineurin inhibitors have inhibitory effect on the in vitro replication of T. cruzi, (ii) L. major promastigote growth is inhibited by chelating agents, and (iii) T. rangeli does not seem to productively infect mammalian cells, it is hypothesized herein that the function of this protein in T. rangeli is required for epimastigote growth.

Host-induced gene silencing of wheat leaf rust fungus Puccinia triticina pathogenicity genes mediated by the Barley stripe mosaic virus

Rust fungi are devastating plant pathogens and several Puccinia species have a large economic impact on wheat production worldwide. Disease protection, mostly offered by introgressed host-resistance genes, is often race-specific and rapidly overcome by newly-emerging virulent strains. Extensive new genomic resources have identified vital pathogenicity genes but their study is hampered because of the biotrophic life styles of rust fungi. In cereals, Barley stripe mosaic virus (BSMV)-induced RNAi has emerged as a useful tool to study loss-of-function phenotypes of candidate genes. Expression of pathogen-derived gene fragments in this system can be used to obtain in planta-generated silencing of corresponding genes inside biotrophic pathogens, a technique termed host-induced gene silencing (HIGS). Here we test the effectiveness of BSMV-mediated HIGS in the wheat leaf rust fungus Puccinia triticina (Pt) by targeting three predicted pathogenicity genes, a MAPK, a cyclophilin, and a calcineurin regulatory subunit. Inoculation of BSMV RNAi constructs generated fungal gene-specific siRNA molecules in systemic leaves of wheat plant. Subsequent Pt inoculation resulted in a suppressed disease phenotype and a reduction in endogenous transcript levels of the targeted fungal genes indicating translocation of siRNA molecules from host to fungal cells. Efficiency of this host-generated trans-specific RNAi was enhanced by using BSMV silencing vectors defective in coat protein coupled with introducing fungal gene sequences simultaneously in sense and antisense orientation. The disease suppression indicated the likely involvement of these fungal genes in pathogenicity. This study demonstrates that BSMV-mediated in planta-generated RNAi is an effective strategy for functional genomics in rust fungi.

Functional characterization of calcineurin homologs PsCNA1/PsCNB1 in Puccinia striiformis f. sp. tritici using a host-induced RNAi system

Calcineurin plays a key role in morphogenesis, pathogenesis and drug resistance in most fungi. However, the function of calcineurin genes in Puccinia striiformis f. sp. tritici (Pst) is unclear. We identified and characterized the calcineurin genes PsCNA1 and PsCNB1 in Pst. Phylogenetic analyses indicate that PsCNA1 and PsCNB1 form a calcium/calmodulin regulated protein phosphatase belonging to the calcineurin heterodimers composed of subunits A and B. Quantitative RT-PCR analyses revealed that both PsCNA1 and PsCNB1 expression reached their maximum in the stage of haustorium formation, which is one day after inoculation. Using barely stripe mosaic virus (BSMV) as a transient expression vector in wheat, the expression of PsCNA1 and PsCNB1 in Pst was suppressed, leading to slower extension of fungal hyphae and reduced production of urediospores. The immune-suppressive drugs cyclosporin A and FK506 markedly reduced the germination rates of urediospores, and when germination did occur, more than two germtubes were produced. These results suggest that the calcineurin signaling pathway participates in stripe rust morphogenetic differentiation, especially the formation of haustoria during the early stage of infection and during the production of urediospores. Therefore PsCNA1 and PsCNB1 can be considered important pathogenicity genes involved in the wheat-Pst interaction.

Cellular alterations and modulation of protein expression in bitumen-challenged human osteoblast cells

PURPOSE

There are many arguments on the carcinogenic potential of bitumen extract. The mechanism of bitumen-induced damage is not well understood at the molecular level. Therefore, in the present study, cell-transforming and tumor-inducing potential of bitumen extract was studied using in vitro [human osteosarcoma (HOS) cells] and in vivo [nude and severe combined immunodeficiency (SCID) mice] models.

METHODS

Gas chromatography/mass spectrometry (GC/MS) analysis was carried out to find out the existence of carcinogenic compounds in the bitumen extract. Cell transformation test, anchorage independence assay, karyotyping assay, tumorigenicity assay, and 2-DE analysis were used to find out the effect of bitumen using the in vitro and in vivo models.

RESULTS

GC/MS analysis showed the existence of carcinogenic compounds in the bitumen extract. HOS cells were treated with different concentrations (25, 50, and 100 μl/ml) of bitumen extract. Compared to the parental HOS cells, bitumen transformants (HOS T1 and HOS T2) showed the characteristics of anchorage independency, chromosomal anomaly, and cellular transformation. Interestingly, bitumen transformants were not able to form tumor in nude/SCID mice. Proteomic analysis revealed the existence of 19 differentially expressed proteins involved in progression of cancer, angiogenesis, cell adhesion, etc.

CONCLUSIONS

Exposure of bitumen extract to HOS cells results in the cellular transformation similar to cancer cells and can modulate proteins involved in the progression of cancer. We state that the non-tumorogenic potential of bitumen transformant in nude/SCID mice can be attributed to the downregulation of galectin-1, chromodomain helicase DNA-binding protein 1-like gene, and membrane-associated guanylate kinase 2 protein.

Functional characterization of an Aspergillus fumigatus calcium transporter (PmcA) that is essential for fungal infection

Aspergillus fumigatus is a primary and opportunistic pathogen, as well as a major allergen, of mammals. The Ca(+2)-calcineurin pathway affects virulence, morphogenesis and antifungal drug action in A. fumigatus. Here, we investigated three components of the A. fumigatus Ca(+2)-calcineurin pathway, pmcA,-B, and -C, which encode calcium transporters. We demonstrated that CrzA can directly control the mRNA accumulation of the pmcA-C genes by binding to their promoter regions. CrzA-binding experiments suggested that the 5'-CACAGCCAC-3' and 5'-CCCTGCCCC-3' sequences upstream of pmcA and pmcC genes, respectively, are possible calcineurin-dependent response elements (CDREs)-like consensus motifs. Null mutants were constructed for pmcA and -B and a conditional mutant for pmcC demonstrating pmcC is an essential gene. The ΔpmcA and ΔpmcB mutants were more sensitive to calcium and resistant to manganese and cyclosporin was able to modulate the sensitivity or resistance of these mutants to these salts, supporting the interaction between calcineurin and the function of these transporters. The pmcA-C genes have decreased mRNA abundance into the alveoli in the ΔcalA and ΔcrzA mutant strains. However, only the A. fumigatus ΔpmcA was avirulent in the murine model of invasive pulmonary aspergillosis.

Aspergillus fumigatus calcineurin interacts with a nucleoside diphosphate kinase

The Ca(2+)-calcineurin pathway affects virulence and morphogenesis in filamentous fungi. Here, we identified 37 CalA-interacting proteins that interact with the catalytic subunit of calcineurin (CalA) in Aspergillus fumigatus, including the nucleoside diphosphate kinase (SwoH). The in vivo interaction between CalA and SwoH was validated by bimolecular fluorescence complementation. A. fumigatus swoH is an essential gene. Therefore, a temperature-sensitive conditional mutant strain with a point mutation in the active site, SwoH(V83F), was constructed, which demonstrated reduced growth and increased sensitivity to elevated temperatures. The SwoH(V83F) mutation did not cause a loss in virulence in the Galleria mellonella infection model. Taken together these results imply that CalA interacts with SwoH.

Calcineurin and its regulator sra/DSCR1 are essential for sleep in Drosophila

Sleep is a fundamental biological process for all animals. However, the molecular mechanisms that regulate sleep are still poorly understood. Here we report that sleep-like behavior in Drosophila is severely impaired by mutations in sarah (sra), a member of the Regulator of Calcineurin (RCAN) family of genes. Sleep reduction in sra mutants is highly correlated with decreases in Sra protein levels. Pan-neural expression of sra rescues this behavioral phenotype, indicating that neuronal sra function is required for normal sleep. Since Sra regulates calcineurin (CN), we generated and examined the behavior of knock-out mutants for all Drosophila CN genes: CanA-14F, Pp2B-14D, and CanA1 (catalytic subunits), and CanB and CanB2 (regulatory subunits). While all mutants show at least minor changes in sleep, CanA-14F(KO) and CanB(KO) have striking reductions, suggesting that these are the major CN subunits regulating sleep. In addition, neuronal expression of constitutively active forms of CN catalytic subunits also significantly reduces sleep, demonstrating that both increases and decreases in CN activity inhibit sleep. sra sleep defects are suppressed by CN mutations, indicating that sra and CN affect sleep through a common mechanism. Our results demonstrate that CN and its regulation by Sra are required for normal sleep in Drosophila and identify a critical role of Ca(2+)/calmodulin-dependent signaling in sleep regulation.

Impacts of calcium signal transduction on the fermentation production of antitumor ganoderic acids by medicinal mushroom Ganoderma lucidum

Recently signal transduction engineering of secondary metabolism is receiving great interest as a powerful tool towards efficient production of valuable secondary metabolites. This work found that the calcineurin-signal transduction was significant to triterpene biosynthesis by higher fungus (mushroom). Addition of calcium ion (at 10mM) to static liquid cultures of Ganoderma lucidum, a famous traditional medicinal mushroom, was proved as a useful strategy to enhance the production of antitumor ganoderic acids (GAs), which resulted in 3.7-, 2.6-, 4.5-, 3.2- and 3.8-fold improvement of total GAs, individual GA-Mk, -T, -S, and -Me, respectively. Experiments using Ca2+ sensor inhibitors indicated the involvement of calcineurin signal in regulating GAs biosynthesis. Quantitative gene transcription analysis revealed that the expression levels of genes of GAs biosynthesis and Ca2+ sensor were up-regulated with calcium addition while down-regulated under the inhibitors addition, suggesting that higher GAs production may be resulted from higher expressions of those genes. Based on the results obtained, a possible model on the effect of external calcium ion on the GAs biosynthesis via calcineurin signal transduction pathway was proposed.

Association of calcineurin with the COPI protein Sec28 and the COPII protein Sec13 revealed by quantitative proteomics

Calcineurin is a calcium-calmodulin-dependent serine/threonine specific protein phosphatase operating in key cellular processes governing responses to extracellular cues. Calcineurin is essential for growth at high temperature and virulence of the human fungal pathogen Cryptococcus neoformans but the underlying mechanism is unknown. We performed a mass spectrometry analysis to identify proteins that associate with the calcineurin A catalytic subunit (Cna1) in C. neoformans cells grown under non-stress and high temperature stress conditions. A novel prioritization strategy for mass spectrometry data from immunoprecipitation experiments identified putative substrates and proteins potentially operating with calcineurin in common pathways. Cna1 co-purified with proteins involved in membrane trafficking including the COPI component Sec28 and the COPII component Sec13. The association of Cna1 with Sec28 and Sec13 was confirmed by co-immunoprecipitation. Cna1 exhibited a dramatic change in subcellular localization during high temperature stress from diffuse cytoplasmic to ER-associated puncta and the mother-bud neck and co-localized with Sec28 and Sec13.

Functional characterization of the regulators of calcineurin in Candida glabrata

The serine-threonine-specific protein phosphatase calcineurin is a key mediator of various stress responses in fungi. Herein, we characterized functions of the endogenous regulators of calcineurin (RCNs), Rcn1 and Rcn2, in the pathogenic fungus Candida glabrata. Rcn1 exerted both inhibitory and stimulatory effects on calcineurin signaling, but Rcn2 displayed only inhibitory activity. Phenotypic analyses of C. glabrata strains lacking either RCNs, calcineurin, or both revealed that calcineurin requires Rcn1, but not Rcn2, for antifungal tolerance in C. glabrata.

Calcineurin controls drug tolerance, hyphal growth, and virulence in Candida dubliniensis

Candida dubliniensis is an emerging pathogenic yeast species closely related to Candida albicans and frequently found colonizing or infecting the oral cavities of HIV/AIDS patients. Drug resistance during C. dubliniensis infection is common and constitutes a significant therapeutic challenge. The calcineurin inhibitor FK506 exhibits synergistic fungicidal activity with azoles or echinocandins in the fungal pathogens C. albicans, Cryptococcus neoformans, and Aspergillus fumigatus. In this study, we show that calcineurin is required for cell wall integrity and wild-type tolerance of C. dubliniensis to azoles and echinocandins; hence, these drugs are candidates for combination therapy with calcineurin inhibitors. In contrast to C. albicans, in which the roles of calcineurin and Crz1 in hyphal growth are unclear, here we show that calcineurin and Crz1 play a clearly demonstrable role in hyphal growth in response to nutrient limitation in C. dubliniensis. We further demonstrate that thigmotropism is controlled by Crz1, but not calcineurin, in C. dubliniensis. Similar to C. albicans, C. dubliniensis calcineurin enhances survival in serum. C. dubliniensis calcineurin and crz1/crz1 mutants exhibit attenuated virulence in a murine systemic infection model, likely attributable to defects in cell wall integrity, hyphal growth, and serum survival. Furthermore, we show that C. dubliniensis calcineurin mutants are unable to establish murine ocular infection or form biofilms in a rat denture model. That calcineurin is required for drug tolerance and virulence makes fungus-specific calcineurin inhibitors attractive candidates for combination therapy with azoles or echinocandins against emerging C. dubliniensis infections.

Isolation of transcripts overexpressed in the human pathogenTrichophyton rubrumgrown in lipid as carbon source

Trichophyton rubrum is the most common etiological agent of human dermatophytosis. Despite the incidence and medical importance of this dermatophyte, little is known about the mechanisms of host invasion and pathogenicity. Host invasion depends on the adaptive cellular responses of the pathogen that allow it to penetrate the skin layers, which are mainly composed of proteins and lipids. In this study, we used suppression subtractive hybridization to identify transcripts overexpressed in T. rubrum cultured in lipid as carbon source. Among the subtractive cDNA clones isolated, 85 clones were positively screened by cDNA array dot blotting and were sequenced. The putative proteins encoded by the isolated transcripts showed similarities to fungal proteins involved in metabolism, signaling, defense, and virulence, such as the MDR/ABC transporter, glucan 1,3-β-glucosidase, chitin synthase B, copper-sulfate-regulated protein, and serine/threonine phosphatase (calcineurin A). These results provide the first molecular insight into the genes differentially expressed during the adaptation of T. rubrum to a lipidic carbon source.

Calcineurin is required for Leishmania major stress response pathways and for virulence in the mammalian host

Leishmania parasites must adapt to elevated temperatures and other environmental stresses during infection of their mammalian hosts. How these environmental cues are sensed is poorly understood. In this study we show that calcium uptake is required for parasite thermotolerance at 34-37°C. To identify potential downstream targets of calcium influx, a Leishmania major mutant lacking the essential regulatory subunit (CnB) of the Ca(2+) /calmodulin-dependent serine/threonine-specific phosphatase, calcineurin, was generated. The Δcnb mutant grew as well as wild-type parasites at 27°C and differentiated normally to infective metacyclic promastigotes. However, Δcnb parasites lost viability when exposed to increased temperature (34°C) and were hypersensitive to endoplasmic reticulum and membrane stress, induced by tunicamycin and inhibitors of sterol and sphingolipid biosynthesis respectively. Δcnb promastigotes were internalized by macrophages, but their differentiation to the heat adapted amastigote stage was delayed and the resulting parasites failed to proliferate. Strikingly, the Δcnb parasites were completely cleared by susceptible BALB/c mice. Complementation of Δcnb parasites with CnB restored thermotolerance and infectivity in both macrophages and animal models. Our results suggest that Ca(2+) influx and calcineurin signalling are required for both early and long-term adaptive parasite responses to environmental stresses encountered in the mammalian host.

Response to environmental stresses, cell-wall integrity, and virulence are orchestrated through the calcineurin pathway in Ustilago hordei

In eukaryotes, several biological processes are regulated through calcium signaling. Calcineurin is a calcium-calmodulin-regulated serine/threonine phosphatase consisting of catalytic subunit A and regulatory subunit B. Phosphatase activity resides in the catalytic subunit, which activates by dephosphorylation downstream components such as transcription factor Crz1. The importance of this pathway to respond to environmental stress has been explored in several fungal pathogens. The basidiomycete Ustilago hordei causes covered smut of barley. We addressed the role of the Ca(2+)-calcineurin activated pathway by deleting UhCna1 and UhCnb1. These genes were not essential in U. hordei but the corresponding mutants displayed a variety of phenotypes when applying environmental stress such as sensitivity to pH, temperature, H₂O₂, mono- and divalent cations; and to genotoxic, acid, or oxidative stresses. Cell-wall integrity was compromised and mutants displayed altered cell morphologies. Mating was delayed but not abolished, and combined sensitivities likely explained a severely reduced virulence toward barley plants. Expression analyses revealed that response to salt stress involved the induction of membrane ATPase genes UhEna1 and UhEna2, which were regulated through the calcineurin pathway. Upregulation of UhFKS1, a 1,3-β-d-glucan synthase gene, correlated with the increased amount of 1,3-β-d-glucan in the calcineurin mutants grown under salt stress.