Activity of the calcium channel pore Cch1 is dependent on a modulatory region of the subunit Mid1 in Cryptococcus neoformans

Unraveling the transcriptional features and gene expression networks of pathogenic and saprotrophic Ophiostoma species during the infection of Ulmus americana

American elm (Ulmus americana), highly prized for its ornamental value, has suffered two successive outbreaks of Dutch elm disease (DED) caused by ascomycete fungi belonging to the genus Ophiostoma. To identify the genes linked to the pathogenicity of different species and lineages of Ophiostoma, we inoculated 2-year-old U. americana saplings with six strains representing three species of DED fungi, and one strain of the saprotroph Ophiostoma quercus. Differential expression analyses were performed following RNA sequencing of fungal transcripts recovered at 3- and 10-days post-infection. Based on a total of 8,640 Ophiostoma genes, we observed a difference in fungal gene expression depending on the strain inoculated and the time of incubation in host tissue. Some genes overexpressed in the more virulent strains of Ophiostoma encode hydrolases that possibly act synergistically. A mutant of Ophiostoma novo-ulmi in which the gene encoding the ogf1 transcription factor had been deleted did not produce transcripts for the gene encoding the hydrophobin cerato-ulmin and was less virulent. Weighted gene correlation network analyses identified several candidate pathogenicity genes distributed among 13 modules of interconnected genes.IMPORTANCEOphiostoma is a genus of cosmopolitan fungi that belongs to the family Ophiostomataceae and includes the pathogens responsible for two devastating pandemics of Dutch elm disease (DED). As the mechanisms of action of DED agents remain unclear, we carried out the first comparative transcriptomic study including representative strains of the three Ophiostoma species causing DED, along with the phylogenetically close saprotrophic species Ophiostoma quercus. Statistical analyses of the fungal transcriptomes recovered at 3 and 10 days following infection of Ulmus americana saplings highlighted several candidate genes associated with virulence and host-pathogen interactions wherein each strain showed a distinct transcriptome. The results of this research underscore the importance of investigating the transcriptional behavior of different fungal taxa to understand their pathogenicity and virulence in relation to the timeline of infection.

High-affinity iron and calcium transport pathways are involved in U(VI) uptake in the budding yeast Saccharomyces cerevisiae

Uranium (U) is a naturally-occurring radionuclide that is toxic for all living organisms. To date, the mechanisms of U uptake are far from being understood. Here we provide a direct characterization of the transport machineries capable of transporting U, using the yeast Saccharomyces cerevisiae as a unicellular eukaryote model. First, we evidenced a metabolism-dependent U transport in yeast. Then, competition experiments with essential metals allowed us to identify calcium, iron and copper entry pathways as potential routes for U uptake. The analysis of various metal transport mutants revealed that mutant affected in calcium (mid1Δ and cch1Δ) and Fe(III) (ftr1Δ) transport, exhibited highly reduced U uptake rates and accumulation, demonstrating the implication of the calcium channel Mid1/Cch1 and the iron permease Ftr1 in U uptake. Finally, expression of the Mid1 gene into the mid1Δ mutant restored U uptake levels of the wild type strain, underscoring the central role of the Mid1/Cch1 calcium channel in U absorption process in yeast. Our results also open up the opportunity for rapid screening of U-transporter candidates by functional expression in yeast, before their validation in more complex higher eukaryote model systems.

Calcium Binding Protein Ncs1 Is Calcineurin Regulated in Cryptococcus neoformans and Essential for Cell Division and Virulence

Intracellular calcium (Ca2+) is crucial for signal transduction in Cryptococcus neoformans, the major cause of fatal fungal meningitis. The calcineurin pathway is the only Ca2+-requiring signaling cascade implicated in cryptococcal stress adaptation and virulence, with Ca2+ binding mediated by the EF-hand domains of the Ca2+ sensor protein calmodulin. In this study, we identified the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) as a member of the EF-hand superfamily. We demonstrated that Ncs1 has a role in Ca2+ homeostasis under stress and nonstress conditions, as the ncs1Δ mutant is sensitive to a high Ca2+ concentration and has an elevated basal Ca2+ level. Furthermore, NCS1 expression is induced by Ca2+, with the Ncs1 protein adopting a punctate subcellular distribution. We also demonstrate that, in contrast to the case with Saccharomyces cerevisiae, NCS1 expression in C. neoformans is regulated by the calcineurin pathway via the transcription factor Crz1, as NCS1 expression is reduced by FK506 treatment and CRZ1 deletion. Moreover, the ncs1Δ mutant shares a high temperature and high Ca2+ sensitivity phenotype with the calcineurin and calmodulin mutants (cna1Δ and cam1Δ), and the NCS1 promoter contains two calcineurin/Crz1-dependent response elements (CDRE1). Ncs1 deficiency coincided with reduced growth, characterized by delayed bud emergence and aberrant cell division, and hypovirulence in a mouse infection model. In summary, our data show that Ncs1 has a significant role as a Ca2+ sensor in C. neoformans, working with calcineurin to regulate Ca2+ homeostasis and, consequently, promote fungal growth and virulence.IMPORTANCECryptococcus neoformans is the major cause of fungal meningitis in HIV-infected patients. Several studies have highlighted the important contributions of Ca2+ signaling and homeostasis to the virulence of C. neoformans Here, we identify the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) and demonstrate its role in Ca2+ homeostasis, bud emergence, cell cycle progression, and virulence. We also show that Ncs1 function is regulated by the calcineurin/Crz1 signaling cascade. Our work provides evidence of a link between Ca2+ homeostasis and cell cycle progression in C. neoformans.

Mid1 affects ion transport, cell wall integrity, and host penetration of the entomopathogenic fungus Metarhizium acridum

Calcium signaling plays important roles in stress tolerance and virulence in fungi. Mid1, an accessory protein of Cch1 calcium channel, has been discussed in baker's yeast and some filamentous fungi. However, functions of the Mid1 gene in entomopathogenic fungi are not clear. In this study, the Mid1 gene was functionally characterized by deleting it in the entomopathogenic fungus Metarhizium acridum. The growth of the ΔMaMid1 mutant was similar as the wild type on normal growth medium, but inhibited by exogenous Ca²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Li⁺, and calcium chelator ethylene glycol tetraacetic acid (EGTA). Cation transportation-related genes were upregulated and intracellular calcium concentration was decreased in ΔMaMid1. Deletion of the MaMid1 gene impaired the tolerance to cell wall-disrupting agents but had no impact on heat or ultraviolet irradiation tolerance compared with the wild type. Bioassays showed that ΔMaMid1 had decreased virulence, with defects in the ability to penetrate the host cuticle. Compared with the wild type, appressorium formation on locust wings and fungal growth in the insect hemocoel were significantly decreased in the ΔMaMid1 mutant in a bioassay through topical inoculation. The phenotypes of ΔMaMid1 were fully restored in a complementation strain. Taken together, our study demonstrates that the MaMid1 affects intracellular ion homeostasis and contributes to virulence by affecting the initial penetration process in M. acridum.

Repurposing drugs to fast-track therapeutic agents for the treatment of cryptococcosis

Many infectious diseases disproportionately affect people in the developing world. Cryptococcal meningitis is one of the most common mycoses in HIV-AIDS patients, with the highest burden of disease in sub-Saharan Africa. Current best treatment regimens still result in unacceptably high mortality rates, and more effective antifungal agents are needed urgently. Drug development is hampered by the difficulty of developing effective antifungal agents that are not also toxic to human cells, and by a reluctance among pharmaceutical companies to invest in drugs that cannot guarantee a high financial return. Drug repurposing, where existing drugs are screened for alternative activities, is becoming an attractive approach in antimicrobial discovery programs, and various compound libraries are now commercially available. As these drugs have already undergone extensive optimisation and passed regulatory hurdles this can fast-track their progress to market for new uses. This study screened the Screen-Well Enzo library of 640 compounds for candidates that phenotypically inhibited the growth of Cryptococcus deuterogattii. The anthelminthic agent flubendazole, and L-type calcium channel blockers nifedipine, nisoldipine and felodipine, appeared particularly promising and were tested in additional strains and species. Flubendazole was very active against all pathogenic Cryptococcus species, with minimum inhibitory concentrations of 0.039-0.156 μg/mL, and was equally effective against isolates that were resistant to fluconazole. While nifedipine, nisoldipine and felodipine all inhibited Cryptococcus, nisoldipine was also effective against Candida, Saccharomyces and Aspergillus. This study validates repurposing as a rapid approach for finding new agents to treat neglected infectious diseases.

Roles of three Fusarium graminearum membrane Ca2+ channels in the formation of Ca2+ signatures, growth, development, pathogenicity and mycotoxin production

Similar to animals and plants, external stimuli cause dynamic spatial and temporal changes of cytoplasmic Ca²⁺ in fungi. Such changes are referred as the Ca²⁺ signature and control cellular responses by modulating the activity or location of diverse Ca²⁺-binding proteins (CBPs) and also indirectly affecting proteins that interact with CBPs. To understand the mechanism underpinning Ca²⁺ signaling, therefore, characterization of how Ca²⁺ moves to and from the cytoplasm to create Ca²⁺ signatures under different conditions is fundamental. Three genes encoding plasma membrane Ca²⁺ channels in a Fusarium graminearum strain that expresses a fluorescent protein-based Ca²⁺ indicator in the cytoplasm were mutagenized to investigate their roles in the generation of Ca²⁺ signatures under different growth conditions and genetic backgrounds. The genes disrupted include CCH1 and MID1, which encode a high affinity Ca²⁺ uptake system, and FIG1, encoding a low affinity Ca²⁺ channel. Resulting mutants were also analyzed for growth, development, pathogenicity and mycotoxin production to determine how loss of each of the genes alters these traits. To investigate whether individual genes influence the function and expression of other genes, phenotypes and Ca²⁺ signatures of their double and triple mutants, as well as their expression patterns, were analyzed.

PpMID1 Plays a Role in the Asexual Development and Virulence of Phytophthora parasitica

Phytophthora parasitica is a notorious oomycete pathogen that causes severe disease in a wide variety of crop species. Infection of plants involves mainly its asexual life stage, including papillate sporangia and biflagellated zoospores, which are the primary dispersal and infection agents of this pathogen. Calcium signaling has been thought as the key regulator for sporangium formation and zoospore differentiation. However, not much is known about the molecular players involved in these processes. In Saccharomyces cerevisiae, mating pheromone-induced death 1 (MID1) encodes a component of a putative calcium channel. Here, we identified and characterized the function of PpMID1, an MID1 homolog from P. parasitica. The expression of PpMID1 was high in sporangia. Gene silencing of PpMID1 resulted in the formation of sporangia that lacked papilla and showed a tendency for direct germination. Notably, in response to cold shock to induce zoospore formation, these sporangia showed no sign of cytoplasmic cleavage and thereby failed to form zoospores. Nonetheless, the addition of CaCl₂ or MgCl₂ partially recovered the silenced sporangia phenotype, with the formation of papillate sporangia similar to those of the wild type and the release of zoospores upon cold shock. As well, virulence toward Nicotiana benthamiana was reduced in the PpMID1-silenced transformants. These results indicate a role of PpMID1 in the asexual development and virulence of P. parasitica.

The Cch1-Mid1 High-Affinity Calcium Channel Contributes to the Virulence of Cryptococcus neoformans by Mitigating Oxidative Stress

Pathogenic fungi have developed mechanisms to cope with stresses imposed by hosts. For Cryptococcus spp., this implies active defense mechanisms that attenuate and ultimately overcome the onslaught of oxidative stresses in macrophages. Among cellular pathways within Cryptococcus neoformans' arsenal is the plasma membrane high-affinity Cch1-Mid1 calcium (Ca(2+)) channel (CMC). Here we show that CMC has an unexpectedly complex and disparate role in mitigating oxidative stress. Upon inhibiting the Ccp1-mediated oxidative response pathway with antimycin, strains of C. neoformans expressing only Mid1 displayed enhanced growth, but this was significantly attenuated upon H2O2 exposure in the absence of Mid1, suggesting a regulatory role for Mid1 acting through the Ccp1-mediated oxidative stress response. This notion is further supported by the interaction detected between Mid1 and Ccp1 (cytochrome c peroxidase). In contrast, Cch1 appears to have a more general role in promoting cryptococci survival during oxidative stress. A strain lacking Cch1 displayed a growth defect in the presence of H2O2 without BAPTA [(1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, cesium salt] or additional stressors such as antimycin. Consistent with a greater contribution of Cch1 to oxidative stress tolerance, an intracellular growth defect was observed for the cch1Δ strain in the macrophage cell line J774A.1. Interestingly, while the absence of either Mid1 or Cch1 significantly compromises the ability of C. neoformans to tolerate oxidative stress, the absence of both Mid1 and Cch1 has a negligible effect on C. neoformans growth during H2O2 stress, suggesting the existence of a compensatory mechanism that becomes active in the absence of CMC.

Activation of a TRP-like channel and intracellular Ca2+ dynamics during phospholipase-C-mediated cell death

The model organism Neurospora crassa undergoes programmed cell death when exposed to staurosporine. Here, we show that staurosporine causes defined changes in cytosolic free Ca²⁺ ([Ca²⁺]_c) dynamics and a distinct Ca²⁺ signature that involves Ca²⁺ influx from the external medium and internal Ca²⁺ stores. We investigated the molecular basis of this Ca²⁺ response by using [Ca²⁺]_c measurements combined with pharmacological and genetic approaches. Phospholipase C was identified as a pivotal player during cell death, because modulation of the phospholipase C signaling pathway and deletion of PLC-2, which we show to be involved in hyphal development, results in an inability to trigger the characteristic staurosporine-induced Ca²⁺ signature. Using Δcch-1, Δfig-1 and Δyvc-1 mutants and a range of inhibitors, we show that extracellular Ca²⁺ entry does not occur through the hitherto described high- and low-affinity Ca²⁺ uptake systems, but through the opening of plasma membrane channels with properties resembling the transient receptor potential (TRP) family. Partial blockage of the response to staurosporine after inhibition of a putative inositol-1,4,5-trisphosphate (IP₃) receptor suggests that Ca²⁺ release from internal stores following IP₃ formation combines with the extracellular Ca²⁺ influx.

Ancient association between cation leak channels and Mid1 proteins is conserved in fungi and animals

Neuronal resting potential can tune the excitability of neural networks, affecting downstream behavior. Sodium leak channels (NALCN) play a key role in rhythmic behaviors by helping set, or subtly changing neuronal resting potential. The full complexity of these newly described channels is just beginning to be appreciated, however. NALCN channels can associate with numerous subunits in different tissues and can be activated by several different peptides and second messengers. We recently showed that NALCN channels are closely related to fungal calcium channels, which they functionally resemble. Here, we use this relationship to predict a family of NALCN-associated proteins in animals on the basis of homology with the yeast protein Mid1, the subunit of the yeast calcium channel. These proteins all share a cysteine-rich region that is necessary for Mid1 function in yeast. We validate this predicted association by showing that the Mid1 homolog in Drosophila, encoded by the CG33988 gene, is coordinately expressed with NALCN, and that knockdown of either protein creates identical phenotypes in several behaviors associated with NALCN function. The relationship between Mid1 and leak channels has therefore persisted over a billion years of evolution, despite drastic changes to both proteins and the organisms in which they exist.

FigA, a putative homolog of low-affinity calcium system member Fig1 in Saccharomyces cerevisiae, is involved in growth and asexual and sexual development in Aspergillus nidulans

Calcium-mediated signaling pathways are widely employed in eukaryotes and are implicated in the regulation of diverse biological processes. In Saccharomyces cerevisiae, at least two different calcium uptake systems have been identified: the high-affinity calcium influx system (HACS) and the low-affinity calcium influx system (LACS). Compared to the HACS, the LACS in fungi is not well known. In this study, FigA, a homolog of the LACS member Fig1 from S. cerevisiae, was functionally characterized in the filamentous fungus Aspergillus nidulans. Loss of figA resulted in retardant hyphal growth and a sharp reduction of conidial production. Most importantly, FigA is essential for the homothallic mating (self-fertilization) process; further, FigA is required for heterothallic mating (outcrossing) in the absence of HACS midA. Interestingly, in a figA deletion mutant, adding extracellular Ca(2+) rescued the hyphal growth defects but could not restore asexual and sexual reproduction. Furthermore, quantitative PCR results revealed that figA deletion sharply decreased the expression of brlA and nsdD, which are known as key regulators during asexual and sexual development, respectively. In addition, green fluorescent protein (GFP) tagging at the C terminus of FigA (FigA::GFP) showed that FigA localized to the center of the septum in mature hyphal cells, to the location between vesicles and metulae, and between the junctions of metulae and phialides in conidiophores. Thus, our findings suggest that FigA, apart from being a member of a calcium uptake system in A. nidulans, may play multiple unexplored roles during hyphal growth and asexual and sexual development.

The bacterial secondary metabolite 2,4-diacetylphloroglucinol impairs mitochondrial function and affects calcium homeostasis in Neurospora crassa

The bacterial secondary metabolite 2,4-diacetylphloroglucinol (DAPG) is of interest as an active ingredient of biological control strains of Pseudomonas fluorescens and as a potential lead pharmaceutical molecule because of its capacity to inhibit growth of diverse microbial and non-microbial cells. The mechanism by which this occurs is unknown and in this study the filamentous fungus Neurospora crassa was used as a model to investigate the effects of DAPG on a eukaryotic cell. Colony growth, conidial germination and cell fusion assays confirmed the inhibitory nature of DAPG towards N. crassa. A number of different fluorescent dyes and fluorescent protein reporters were used to assess the effects of DAPG treatment on mitochondrial and other cellular functions. DAPG treatment led to changes in mitochondrial morphology, and rapid loss of mitochondrial membrane potential. These effects are likely to be responsible for the toxicity of DAPG. It was also found that DAPG treatment caused extracellular calcium to be taken up by conidial germlings leading to a transient increase in cytosolic free Ca(2+) with a distinct concentration dependent Ca(2+) signature.