Calcium uptake and efflux during the yeast to mycelium transition in Sporothrix schenckii

Current Progress in Sporothrix brasiliensis Basic Aspects

Sporotrichosis is known as a subacute or chronic infection, which is caused by thermodimorphic fungi of the genus Sporothrix. It is a cosmopolitan infection, which is more prevalent in tropical and subtropical regions and can affect both humans and other mammals. The main etiological agents causing this disease are Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa, which have been recognized as members of the Sporothrix pathogenic clade. Within this clade, S. brasiliensis is considered the most virulent species and represents an important pathogen due to its distribution and prevalence in different regions of South America, such as Brazil, Argentina, Chile, and Paraguay, and Central American countries, such as Panama. In Brazil, S. brasiliensis has been of great concern due to the number of zoonotic cases that have been reported over the years. In this paper, a detailed review of the current literature on this pathogen and its different aspects will be carried out, including its genome, pathogen-host interaction, resistance mechanisms to antifungal drugs, and the caused zoonosis. Furthermore, we provide the prediction of some putative virulence factors encoded by the genome of this fungal species.

Biological and Clinical Attributes of Sporothrix globosa, a Causative Agent of Sporotrichosis

Sporotrichosis is an important subcutaneous mycosis with high prevalence and threat to human and animal health worldwide. Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa are the main etiological agents of this disease; and even though many efforts have been made recently to understand the Sporothrix-host interaction, little is known about S. globosa, an underestimated species. This organism shows the lowest virulence among the members of the Sporothrix pathogenic clade and represents an important pathogenic agent due to its global distribution. Here, we offer a review with all the known information about S. globosa, including its genome and proteomic information, and compare it with S. schenckii and S. brasiliensis, to explain the differences observed among these species, in terms of virulence, the host immune response, and the antifungal sensitivity. Also, we provide the gene prediction of some S. globosa putative virulence factors.

Virulence Factors of Sporothrix schenckii

Sporothrix schenckii is one of the etiological agents of sporotrichosis. In this review, we discuss the virulence factors that have been proven to participate in the S. schenckii-host interaction. Among these known factors, we can find cell wall glycoproteins, adhesins, melanin, extracellular vesicles, and dimorphism. Furthermore, the morphological transition of S. schenckii in response to environmental conditions such as pH and temperature represents a means by which the fungus is able to establish mycosis in mammals. One of the key features in the development of sporotrichosis is the adhesion of the fungus to the host extracellular matrix. This event represents the first step to developing the mycosis, which involves adhesins such as the glycoproteins Gp70, Hsp60, and Pap1, which play a key role during the infection. The production of melanin helps the fungus to survive longer in the tissues and to neutralize or diminish many of the host’s attacks, which is why it is also considered a key factor in pathogenesis. Today, the study of human fungal pathogens’ virulence factors is a thriving area of research. Although we know some of the virulence factors in S. schenckii, much remains to be understood about the complex process of sporotrichosis development and the factors involved during the infection.

Sporothrix schenckii complex biology: environment and fungal pathogenicity

Sporothrix schenckii is a complex of various species of fungus found in soils, plants, decaying vegetables and other outdoor environments. It is the aetiological agent of sporotrichosis in humans and several animals. Humans and animals can acquire the disease through traumatic inoculation of the fungus into subcutaneous tissue. Despite the importance of sporotrichosis, it being currently regarded as an emergent disease in several countries, the factors driving its increasing medical importance are still largely unknown. There have only been a few studies addressing the influence of the environment on the virulence of these pathogens. However, recent studies have demonstrated that adverse conditions in its natural habitats can trigger the expression of different virulence factors that confer survival advantages both in animal hosts and in the environment. In this review, we provide updates on the important advances in the understanding of the biology of Spor. schenckii and the modification of its virulence linked to demonstrated or putative environmental factors.

Calcium/calmodulin kinase1 and its relation to thermotolerance and HSP90 in Sporothrix schenckii: an RNAi and yeast two-hybrid study

Background

Sporothrix schenckii is a pathogenic dimorphic fungus of worldwide distribution. It grows in the saprophytic form with hyaline, regularly septated hyphae and pyriform conidia at 25°C and as the yeast or parasitic form at 35°C. Previously, we characterized a calcium/calmodulin kinase in this fungus. Inhibitors of this kinase were observed to inhibit the yeast cell cycle in S. schenckii.

Results

The presence of RNA interference (RNAi) mechanism in this fungus was confirmed by the identification of a Dicer-1 homologue in S. schenckii DNA. RNAi technology was used to corroborate the role of calcium/calmodulin kinase I in S. schenckii dimorphism. Yeast cells were transformed with the pSilent-Dual2G (pSD2G) plasmid w/wo inserts of the coding region of the calcium/calmodulin kinase I (sscmk1) gene. Transformants were selected at 35°C using resistance to geneticin. Following transfer to liquid medium at 35°C, RNAi transformants developed as abnormal mycelium clumps and not as yeast cells as would be expected. The level of sscmk1 gene expression in RNAi transformants at 35°C was less than that of cells transformed with the empty pSD2G at this same temperature. Yeast two-hybrid analysis of proteins that interact with SSCMK1 identified a homologue of heat shock protein 90 (HSP90) as interacting with this kinase. Growth of the fungus similar to that of the RNAi transformants was observed in medium with geldanamycin (GdA, 10 μM), an inhibitor of HSP90.

Conclusions

Using the RNAi technology we silenced the expression of sscmk1 gene in this fungus. RNAi transformants were unable to grow as yeast cells at 35°C showing decreased tolerance to this temperature. The interaction of SSCMK1 with HSP90, observed using the yeast two-hybrid assay suggests that this kinase is involved in thermotolerance through its interaction with HSP90. SSCMK1 interacted with the C terminal domain of HSP90 where effector proteins and co-chaperones interact. These results confirmed SSCMK1 as an important enzyme involved in the dimorphism of S. schenckii, necessary for the development of the yeast phase of this fungus. Also this study constitutes the first report of the transformation of S. schenckii and the use of RNAi to study gene function in this fungus.

Cellular roles of neuronal calcium sensor-1 and calcium/calmodulin-dependent kinases in fungi

The neuronal calcium sensor-1 (NCS-1) possesses a consensus signal for N-terminal myristoylation and four EF-hand Ca(2+)-binding sites, and mediates the effects of cytosolic Ca(2+). Minute changes in free intracellular Ca(2+) are quickly transformed into changes in the activity of several kinases including calcium/calmodulin-dependent protein kinases (Ca(2+)/CaMKs) that are involved in regulating many eukaryotic cell functions. However, our current knowledge of NCS-1 and Ca(2+)/CaMKs comes mostly from studies of the mammalian enzymes. Thus far very few fungal homologues of NCS-1 and Ca(2+)/CaMKs have been characterized and little is known about their cellular roles. In this minireview, we describe the known sequences, interactions with target proteins and cellular roles of NCS-1 and Ca(2+)/CaMKs in fungi.

Cytosolic phospholipase A2: a member of the signalling pathway of a new G protein alpha subunit in Sporothrix schenckii

Background

Sporothrix schenckii is a pathogenic dimorphic fungus, the etiological agent of sporotrichosis, a lymphocutaneous disease that can remain localized or can disseminate, involving joints, lungs, and the central nervous system. Pathogenic fungi use signal transduction pathways to rapidly adapt to changing environmental conditions and S. schenckii is no exception. S. schenckii yeast cells, either proliferate (yeast cell cycle) or engage in a developmental program that includes proliferation accompanied by morphogenesis (yeast to mycelium transition) depending on the environmental conditions. The principal intracellular receptors of environmental signals are the heterotrimeric G proteins, suggesting their involvement in fungal dimorphism and pathogenicity. Identifying these G proteins in fungi and their involvement in protein-protein interactions will help determine their role in signal transduction pathways.

Results

In this work we describe a new G protein α subunit gene in S. schenckii, ssg-2. The cDNA sequence of ssg-2 revealed a predicted open reading frame of 1,065 nucleotides encoding a 355 amino acids protein with a molecular weight of 40.9 kDa. When used as bait in a yeast two-hybrid assay, a cytoplasmic phospholipase A₂ catalytic subunit was identified as interacting with SSG-2. The sspla₂ gene, revealed an open reading frame of 2538 bp and encoded an 846 amino acid protein with a calculated molecular weight of 92.62 kDa. The principal features that characterize cPLA₂ were identified in this enzyme such as a phospholipase catalytic domain and the characteristic invariable arginine and serine residues. A role for SSPLA₂ in the control of dimorphism in S. schenckii is suggested by observing the effects of inhibitors of the enzyme on the yeast cell cycle and the yeast to mycelium transition in this fungus. Phospholipase A₂ inhibitors such as AACOCF3 (an analogue of archidonic acid) and isotetrandrine (an inhibitor of G protein PLA₂ interactions) were found to inhibit budding by yeasts induced to re-enter the yeast cell cycle and to stimulate the yeast to mycelium transition showing that this enzyme is necessary for the yeast cell cycle.

Conclusion

A new G protein α subunit gene was characterized in S. schenckii and protein-protein interactions studies revealed this G protein alpha subunit interacts with a cPLA₂ homologue. The PLA₂ homologue reported here is the first phospholipase identified in S. schenckii and the first time a PLA₂ homologue is identified as interacting with a G protein α subunit in a pathogenic dimorphic fungus, establishing a relationship between these G proteins and the pathogenic potential of fungi. This cPLA₂ homologue is known to play a role in signal transduction and fungal pathogenesis. Using cPLA₂ inhibitors, this enzyme was found to affect dimorphism in S. schenckii and was found to be necessary for the development of the yeast or pathogenic form of the fungus.

Functional, genetic and bioinformatic characterization of a calcium/calmodulin kinase gene in Sporothrix schenckii

Background

Sporothrix schenckii is a pathogenic, dimorphic fungus, the etiological agent of sporotrichosis, a subcutaneous lymphatic mycosis. Dimorphism in S. schenckii responds to second messengers such as cAMP and calcium, suggesting the possible involvement of a calcium/calmodulin kinase in its regulation. In this study we describe a novel calcium/calmodulin-dependent protein kinase gene in S. schenckii, sscmk1, and the effects of inhibitors of calmodulin and calcium/calmodulin kinases on the yeast to mycelium transition and the yeast cell cycle.

Results

Using the PCR homology approach a new member of the calcium/calmodulin kinase family, SSCMK1, was identified in this fungus. The cDNA sequence of sscmk1 revealed an open reading frame of 1,221 nucleotides encoding a 407 amino acid protein with a predicted molecular weight of 45.6 kDa. The genomic sequence of sscmk1 revealed the same ORF interrupted by five introns. Bioinformatic analyses of SSCMK1 showed that this protein had the distinctive features that characterize a calcium/calmodulin protein kinase: a serine/threonine protein kinase domain and a calmodulin-binding domain. When compared to homologues from seven species of filamentous fungi, SSCMK1 showed substantial similarities, except for a large and highly variable region that encompasses positions 330 – 380 of the multiple sequence alignment. Inhibition studies using calmodulin inhibitor W-7, and calcium/calmodulin kinase inhibitors, KN-62 and lavendustin C, were found to inhibit budding by cells induced to re-enter the yeast cell cycle and to favor the yeast to mycelium transition.

Conclusion

This study constitutes the first evidence of the presence of a calcium/calmodulin kinase-encoding gene in S. schenckii and its possible involvement as an effector of dimorphism in this fungus. These results suggest that a calcium/calmodulin dependent signaling pathway could be involved in the regulation of dimorphism in this fungus. The results suggest that the calcium/calmodulin kinases of yeasts are evolutionarily distinct from those in filamentous fungi.

Expression of a Pho85 cyclin-dependent kinase is repressed during the dimorphic transition in Sporothrix schenckii

Sporothrix schenckii is a pathogenic fungus that undergoes a dimorphic transition from yeast to mycelium in response to environmental conditions such as cell density, temperature, and calcium. We identified a homolog of the Pho85 cyclin-dependent kinase (Cdk) that mediates cellular responses to environmental conditions in other organisms. By Western blot, three proteins containing the PSTAIRE motif, which characterize the cyclin-dependent protein kinases, were identified in S. schenckii. The gene encoding a Pho85 homolog, PhoSs, was identified and sequenced. The phoSs gene consists of 990bp, contains one intron, and encodes a protein of 306 amino acids. The S. schenckii Pho85 homolog shares features with Cdks, including the PSTAIRE motif, an ATP binding domain, and a serine-threonine kinase domain. By quantitative competitive RT-PCR, expression of the phoSs gene was found to decrease 30-fold during the yeast to mycelium transition. The addition of extracellular calcium accelerated the dimorphic transition and restored phoSs expression. These findings suggest PhoSs may participate in the control of the yeast to mycelium transition in S. schenckii.

Characterization of a protein kinase C gene in Sporothrix schenckii and its expression during the yeast-to-mycelium transition

The yeast-to-mycelium transition in Sporothrix schenckii has been shown to respond to protein kinase C (PKC) effectors, indicating the involvement of PKC in this regulation. In this study, we identified the presence of two pkcl-like genes in S. schenckii. Using fungal genomic DNA as template and primers targeted to conserved sequences in the Saccharomyces cerevisiae pkc1 gene, two partially overlapping extra long polymerase chain reaction (XL-PCR) products were obtained. These XL-PCR products were sequenced and found to encode part of the C3/C4 domains of two different PKC-like proteins. The presence of two different genes was confirmed by Southern blot analysis. These two genes were named pkcSs-1 and pkcSs-2. The sequence of the pkcSs-2 gene was completed and revealed an open reading frame of 3942 nucleotides interrupted by five introns. A transcript of 8.7 kb was detected in northern blot analysis of poly A+ RNA. The pkcSs-2 gene encodes a protein of 1194 amino acids and 132.84 kDa that contains the characteristic structure and domains of other fungal PKCs reported to date. Using reverse transcription-PCR (RT-PCR), the pkcSs-2 gene was found to be expressed at all intervals tested during the yeast-to-mycelium transition.

Presence of a pertussis toxin-sensitive G protein alpha subunit in Sporothrix schenckii

As an initial step in the study of the role of G proteins in signal transduction in Sporothrix schenckii, we identified a Galphai subunit using different experimental approaches. Western blots of fungal membrane preparations using anti-Galphacommon and anti-Galphai1-Galphai2 antibodies identified a band of approximately 41 kDa. Pertussis toxin-catalyzed adenosine diphosphate (ADP)-ribosylation of these membrane fractions confirmed the presence of a protein substrate of 41 kDa. A 357 bp polymerase chain reaction (PCR) product obtained using fungal DNA as template and primers targeted to conserved Galphai sequences, was used as a probe to isolate a clone from an S. schenckii genomic library. A partial sequence for a Galphai subunit was obtained from this clone. The sequence was completed using the rapid amplification of cDNA ends (RACE) technique with mycelium and yeast cDNA. The cDNA sequence revealed a 1059 bp open reading frame encoding a 353 amino acid Galphai subunit of 41 kDa, more than 90% identical to the CPG-1 of Cryphonectria parasitica, and GNA-1 of Neurospora crassa. The genomic sequence was obtained by PCR using fungal DNA, and revealed a 1250 bp sequence and the presence of three introns. These results provide evidence for the first time of the presence and expression of a Galphai homolog in a pathogenic dimorphic fungus.

Different protein kinase C isoforms are present in the yeast and mycelium forms of Sporothrix schenckii

Protein kinase C (PKC) plays an important role in the control of proliferation and differentiation of a wide range of cell types, and fungi are no exception. Previous results reported by us on the effects of the phorbol ester, 12-myristate-13-acetate phorbol (PMA) and other PKC effector molecules, on dimorphism in Sporothrix schenckii suggested the presence of this enzyme in the fungus and its involvement in the control of morphogenetic transitions. The work summarized here confirms the presence of PKC in yeast and mycelium extracts of S. schenckii. Different isoforms of this enzyme were found to be present in the yeast and mycelium forms of the fungus and were identified by Western blot analysis using affinity purified anti-PKC isoforms specific antibodies: the gamma and zeta isoforms were detected in both the yeast and mycelium forms of the fungus, while the beta isoform was only detected in the yeast form. The presence of PKC was confirmed biochemically by measuring total enzyme activity in both forms of the fungus. No significant differences were observed for the PKC activity level recorded for both the mycelium and yeast forms of the fungus (p < or = 0.05). These data confirm the presence of PKC activity in Sporothrix schenckii and constitutes the first evidence concerning the differential expression of PKC isoforms in the mycelium and yeast forms of a dimorphic fungus, supporting the possible involvement of this important signal transduction enzyme in the control of morphogenesis in this fungus.

Symptomless endophytic colonization of maize byFusarium moniliforme

Fusarium moniliforme Sheldon is a nonobligate parasite of maize that produces several mycotoxins. This fungus causes systemic infections of maize kernels, which then serve as dissemination vehicles and inoculum sources. Infected plants may or may not show disease symptoms. Symptomless infection was studied in maize plants and was compared with plants that eventually showed symptoms of seedling disease. Three isolates of F. moniliforme, a hygromycin-resistant mutant of two of these isolates, and Fusarium fujikuroi were used to infect surface and internally sterilized maize kernels, and symptomatic and symptomless infections were observed for 8 weeks. The results indicated that in symptomless infected plants, hyphae were intercellular only and distributed throughout the plant, whereas in plants showing disease symptoms, the fungus was both intercellular and intracellular. Symptomless plants remained symptomless throughout the observation period, and at the ultrastructural level there was no evidence of an antagonistic relationship. This indicates that the symptomless state persists beyond the seedling stage and could contribute, without visual signs, to the total mycotoxin contaminants of maize both before and during kernel development. Keywords: Gibberella fujikuroi, endophyte, toxic fungus, maize, fusarium toxins, corn disease.

Calcium stimulates molecular and cellular events during the yeast-to-mycelium transition in Sporothrix schenckii

Calcium ions (Ca2+) have been identified as mediators of proliferative and morphogenetic processes in many eukaryotic cells. The effects of these ions on the cellular and macromolecular processes that accompany the dimorphic transition from the yeast-to-mycelial form of Sporothrix schenckii have been studied. Ca2+ were found to stimulate germ tube formation and growth in these cells at an optimal concentration of 1.0 mM. Studies concerning the effects of this cation on the molecular processes that precede germ tube formation revealed that the earliest molecular event which was stimulated by 1.0 mM Ca2+ was RNA synthesis. An increased incorporation of radioactivity into RNA in the presence of 1.0 mM Ca2+ was first observed at 0-3 h, and subsequently at all other times tested, following inoculation. A stimulation in rRNA and tRNA synthesis was detected in the presence of 1.0 mM Ca2+. The incorporation of radioactivity into proteins was stimulated 3-5 h following induction in the presence of Ca2+ suggesting a specific effect of Ca2+ on protein synthesis. This increased incorporation takes place prior to the start of DNA synthesis. Incorporation of radioactivity into DNA was also stimulated in the presence of Ca2+, 6 and 9 h after inoculation. This stimulation resulted in nuclear division taking place with a shorter lag period and proceeding with increased kinetics. The results reported here are evidence that Ca2+ plays a role in the control of the early molecular and cellular processes that accompany the yeast-to-mycelium transition in S. schenckii and offer an explanation of how Ca2+ can control the expression of the dimorphic potential of this fungus.