Putative PmrA and PmcA are important for normal growth, morphogenesis and cell wall integrity, but not for viability in Aspergillus nidulans

Regulatory Role of Vacuolar Calcium Transport Proteins in Growth, Calcium Signaling, and Cellulase Production in Trichoderma reesei

Recent research has revealed the calcium signaling significance in the production of cellulases in Trichoderma reesei. While vacuoles serve as the primary calcium storage within cells, the function of vacuolar calcium transporter proteins in this process remains unclear. In this study, we conducted a functional characterization of four vacuolar calcium transport proteins in T. reesei. This was accomplished by the construction of the four mutant strains ∆trpmc1, ∆tryvc1, ∆tryvc3, and ∆tryvc4. These mutants displayed enhanced growth when subjected to arabinose, xylitol, and xylose. Furthermore, the mutants ∆trpmc1, ∆tryvc1, and ∆tryvc4 showed a reduction in growth under conditions of 100 mM MnCl2, implying their role in manganese resistance. Our enzymatic activity assays revealed a lack of the expected augmentation in cellulolytic activity that is typically seen in the parental strain following the introduction of calcium. This was mirrored in the expression patterns of the cellulase genes. The vacuolar calcium transport genes were also found to play a role in the expression of genes involved with the biosynthesis of secondary metabolites. In summary, our research highlights the crucial role of the vacuolar calcium transporters and, therefore, of the calcium signaling in orchestrating cellulase and hemicellulase expression, sugar utilization, and stress resistance in T. reesei.

The Development of Aspergillus flavus and Biosynthesis of Aflatoxin B1 are Regulated by the Golgi-Localized Mn2+ Transporter Pmr1

Microorganisms rely on diverse ion transport and trace elements to sustain growth, development, and secondary metabolism. Manganese (Mn2+) is essential for various biological processes and plays a crucial role in the metabolism of human cells, plants, and yeast. In Aspergillus flavus, we confirmed that Pmr1 localized in cis- and medial-Golgi compartments was critical in facilitating Mn2+ transport, fungal growth, development, secondary metabolism, and glycosylation. In comparison to the wild type, the Δpmr1 mutant displayed heightened sensitivity to environmental stress, accompanied by inhibited synthesis of aflatoxin B1, kojic acid, and a substantial reduction in pathogenicity toward peanuts and maize. Interestingly, the addition of exogenous Mn2+ effectively rectified the developmental and secondary metabolic defects in the Δpmr1 mutant. However, Mn2+ supplement failed to restore the growth and development of the Δpmr1Δgdt1 double mutant, which indicated that the Gdt1 compensated for the functional deficiency of pmr1. In addition, our results showed that pmr1 knockout leads to an upregulation of O-glycosyl-N-acetylglucose (O-GlcNAc) and O-GlcNAc transferase (OGT), while Mn2+ supplementation can restore the glycosylation in A. flavus. Collectively, this study indicates that the pmr1 regulates Mn2+ via Golgi and maintains growth and metabolism functions of A. flavus through regulation of the glycosylation.

The plasma membrane H+-ATPase is critical for cell growth and pathogenicity in Penicillium digitatum

The plasma membrane H⁺-ATPase (PMA1) is a major cytosolic pH regulator and a potential candidate for antifungal drug discovery due to its fungal specificity and criticality. In this study, the function of Penicillum digitatum PMA1 was characterized through RNA interference (RNAi) and overexpression technology. The results showed that silencing the PMA1 gene reduces cell growth and pathogenicity, and increases susceptibility of P. digitatum to proton pump inhibitors (PPIs). Under scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examination, cell morphology was significantly altered in the PMA1- silenced mutant (si57). When compared with wild type (WT) and the overexpressed mutant (oe9), the cell walls of the si57 mutant were thicker and their cell membrane damage manifested particularly at sites of polarized growth. Consistent with the morphological change on the cell wall, chitin and glucan content of the cell wall of si57 were significantly lower and accompanied with increased activities of chitinase and glucanase. The lower ergosterol content in the si57 mutant then increased cell membrane permeability, ultimately leading to leakage of cytoplasmic contents such as ions, reduced sugars and soluble proteins. Furthermore, significantly decreased activity of cell wall degrading enzymes of si57 during citrus fruit infections indicates a reduced pathogenicity in this mutant. We conclude that PMA1 in P. digitatum plays an important role in maintaining pathogenesis and PMA1 could be a candidate novel fungicidal drug discovery for citrus green mold. KEY POINTS: Silencing PMA1 gene decreased the growth and pathogenicity of P. digitatum. Silencing PMA1 gene damaged cell wall and cell membrane integrity of P. digitatum. PMA1 appears to be a suitable fungicidal target against citrus green mold.

A Transient Receptor Potential-like Calcium Ion Channel in the Filamentous Fungus Aspergillus nidulans

Transient Receptor Potential (TRP) proteins constitute a superfamily that encodes transmembrane ion channels with highly diverse permeation and gating properties. Filamentous fungi possess putative TRP channel-encoded genes, but their functions remain elusive. Here, we report that a putative TRP-like calcium channel, trpR, in the filamentous fungus Aspergillus nidulans, performs important roles in conidiation and in adapting to cell wall disruption reagents in a high temperature-induced defect-dependent manner, especially under a calcium-limited culture condition. The genetic and functional relationship between TrpR and the previously identified high-affinity calcium channels CchA/MidA indicates that TrpR has an opposite response to CchA/MidA when reacting to cell wall disruption reagents and in regulating calcium transients. However, a considerable addition of calcium can rescue all the defects that occur in TrpR and CchA/MidA, meaning that calcium is able to bypass the necessary requirement. Nevertheless, the colocalization at the membrane of the Golgi for TrpR and the P-type Golgi Ca²⁺ ATPase PmrA suggests two channels that may work as ion transporters, transferring Ca²⁺ from the cytosol into the Golgi apparatus and maintaining cellular calcium homeostasis. Therefore, combined with data for the trpR deletion mutant revealing abnormal cell wall structures, TrpR works as a Golgi membrane calcium ion channel that involves cell wall integration.

Calcineurin signaling pathway influences Aspergillus niger biofilm formation by affecting hydrophobicity and cell wall integrity

BACKGROUND

Biofilms, as a kind of fixed-cell community, can greatly improve industrial fermentation efficiency in immobilized fermentation, but the regulation process is still unclear, which restricts their application. Ca2+ was reported to be a key factor affecting biofilm formation. However, the effect of Ca2+ on biofilm structure and microbiology was yet only studied in bacteria. How Ca2+-mediated calcineurin signaling pathway (CSP) alters biofilm formation in bacteria and fungi has rarely been reported. On this basis, we investigated the regulation of CSP on the formation of biofilm in Aspergillus niger.

RESULTS

Deletion of the key genes MidA, CchA, CrzA or CnaA in the CSP lowered the Ca2+ concentration in the mycelium to a different extent, inhibited the formation of A. niger biofilm, reduced the hydrophobicity and adhesion of spores, destroyed the cell wall integrity of hyphae, and reduced the flocculation ability of hyphae. qRT-PCR results showed that the expression of spore hydrophobic protein RodA, galactosaminogalactan (GAG) biosynthesis genes (uge3, uge5, agd3, gtb3), and α-1,3-glucan biosynthesis genes (ags1, ags3) in the ∆MidA, ∆CchA, ∆CrzA, ∆CnaA strains were significantly down-regulated compared with those of the wild type (WT). In addition, the transcription levels of the chitin synthesis gene (chsB, chsD) and β-1,3-glucan synthesis gene (FksA) were consistent with the change in chitin and β-1,3-glucan contents in mutant strains.

CONCLUSION

These results indicated that CSP affected the hydrophobicity and adhesion of spores, the integrity of mycelial cell walls and flocculation by affecting Ca2+ levels in mycelium, which in turn affected biofilm formation. This work provides a possible explanation for how CSP changes the formation of A. niger biofilm, and reveals a pathway for controlling biofilm formation in industrial immobilized fermentation.

Calcium Transport Proteins in Fungi: The Phylogenetic Diversity of Their Relevance for Growth, Virulence, and Stress Resistance

The key players of calcium (Ca²⁺) homeostasis and Ca²⁺ signal generation, which are Ca²⁺ channels, Ca²⁺/H⁺ antiporters, and Ca²⁺-ATPases, are present in all fungi. Their coordinated action maintains a low Ca²⁺ baseline, allows a fast increase in free Ca²⁺ concentration upon a stimulus, and terminates this Ca²⁺ elevation by an exponential decrease – hence forming a Ca²⁺ signal. In this respect, the Ca²⁺ signaling machinery is conserved in different fungi. However, does the similarity of the genetic inventory that shapes the Ca²⁺ peak imply that if “you’ve seen one, you’ve seen them all” in terms of physiological relevance? Individual studies have focused mostly on a single species, and mechanisms elucidated in few model organisms are usually extrapolated to other species. This mini-review focuses on the physiological relevance of the machinery that maintains Ca²⁺ homeostasis for growth, virulence, and stress responses. It reveals common and divergent functions of homologous proteins in different fungal species. In conclusion, for the physiological role of these Ca²⁺ transport proteins, “seen one,” in many cases, does not mean: “seen them all.”

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

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

Phosphoproteomic and transcriptomic analyses reveal multiple functions for Aspergillus nidulans MpkA independent of cell wall stress

The protein kinase MpkA plays a prominent role in the cell wall integrity signaling (CWIS) pathway, acting as the terminal MAPK activating expression of genes which encode cell wall biosynthetic enzymes and other repair functions. Numerous studies focus on MpkA function during cell wall perturbation. Here, we focus on the role MpkA plays outside of cell wall stress, during steady state growth. In an effort to seek other, as yet unknown, connections to this pathway, an mpkA deletion mutant (ΔmpkA) was subjected to phosphoproteomic and transcriptomic analysis. When compared to the control (isogenic parent of ΔmpkA), there is strong evidence suggesting MpkA is involved with maintaining cell wall strength, branching regulation, and the iron starvation pathway, among others. Particle-size analysis during shake flask growth revealed ΔmpkA mycelia were about 4 times smaller than the control strain and more than 90 cell wall related genes show significantly altered expression levels. The deletion mutant had a significantly higher branching rate than the control and phosphoproteomic results show putative branching-regulation proteins, such as CotA, LagA, and Cdc24, have a significantly different level of phosphorylation. When grown in iron limited conditions, ΔmpkA had no difference in growth rate or production of siderophores, whereas the control strain showed decreased growth rate and increased siderophore production. Transcriptomic data revealed over 25 iron related genes with altered transcript levels. Results suggest MpkA is involved with regulation of broad cellular functions in the absence of stress.

The Lectin Chaperone Calnexin Is Involved in the Endoplasmic Reticulum Stress Response by Regulating Ca2+ Homeostasis in Aspergillus nidulans

The Ca²⁺-mediated signaling pathway is crucial for environmental adaptation in fungi. Here we show that calnexin, a molecular chaperone located in the endoplasmic reticulum (ER), plays an important role in regulating the cytosolic free calcium concentration ([Ca²⁺]_c) in Aspergillus nidulans Inactivation of calnexin (ClxA) in A. nidulans caused severe defects in hyphal growth and conidiation under ER stress caused by the ER stress-inducing agent dithiothreitol (DTT) or high temperature. Importantly, defects in the ΔclxA mutant were restored by the addition of extracellular calcium. Furthermore, the CchA/MidA complex (the high-affinity Ca²⁺ channels), calcineurin (calcium/calmodulin-dependent protein phosphatase), and PmrA (secretory pathway Ca²⁺ ATPase) were required for extracellular calcium-based restoration of the DTT/thermal stress sensitivity in the ΔclxA mutant. Interestingly, the ΔclxA mutant exhibited markedly reduced conidium formation and hyphal growth defects under the low-calcium condition, which is similar to defects caused by mutations in MidA/CchA. Moreover, the phenotypic defects were further exacerbated in the ΔclxA ΔmidA ΔcchA mutant, which suggested that ClxA and MidA/CchA are both required under the calcium-limiting condition. Using the calcium-sensitive photoprotein aequorin to monitor [Ca²⁺]_c in living cells, we found that ClxA and MidA/CchA complex synergistically coordinate transient increase in [Ca²⁺]_c in response to extracellular calcium. Moreover, ClxA, in particular its luminal domain, plays a role in mediating the transient [Ca²⁺]_c in response to DTT-induced ER stress in the absence of extracellular calcium, indicating ClxA may mediate calcium release from internal calcium stores. Our findings provide new insights into the role of calnexin in the regulation of calcium-mediated response in fungal ER stress adaptation.IMPORTANCE Calnexin is a well-known molecular chaperone conserved from yeast to humans. Although it contains calcium binding domains, little is known about the role of calnexin in Ca²⁺ regulation. In this study, we demonstrate that calnexin (ClxA) in the filamentous fungus Aspergillus nidulans, similar to the high-affinity calcium uptake system (HACS), is required for normal growth and conidiation under the calcium-limiting condition. The ClxA dysfunction decreases the transient cytosolic free calcium concentration ([Ca²⁺]_c) induced by a high extracellular calcium or DTT-induced ER stress. Our findings provide the direct evidence that calnexin plays important roles in regulating Ca²⁺ homeostasis in addition to its role as a molecular chaperone in fungi. These results provide new insights into the roles of calnexin and expand knowledge of fungal stress adaptation.

Palmitoylation of the Cysteine Residue in the DHHC Motif of a Palmitoyl Transferase Mediates Ca2+ Homeostasis in Aspergillus

Finely tuned changes in cytosolic free calcium ([Ca²⁺]_c) mediate numerous intracellular functions resulting in the activation or inactivation of a series of target proteins. Palmitoylation is a reversible post-translational modification involved in membrane protein trafficking between membranes and in their functional modulation. However, studies on the relationship between palmitoylation and calcium signaling have been limited. Here, we demonstrate that the yeast palmitoyl transferase ScAkr1p homolog, AkrA in Aspergillus nidulans, regulates [Ca²⁺]_c homeostasis. Deletion of akrA showed marked defects in hyphal growth and conidiation under low calcium conditions which were similar to the effects of deleting components of the high-affinity calcium uptake system (HACS). The [Ca²⁺]_c dynamics in living cells expressing the calcium reporter aequorin in different akrA mutant backgrounds were defective in their [Ca²⁺]_c responses to high extracellular Ca²⁺ stress or drugs that cause ER or plasma membrane stress. All of these effects on the [Ca²⁺]_c responses mediated by AkrA were closely associated with the cysteine residue of the AkrA DHHC motif, which is required for palmitoylation by AkrA. Using the acyl-biotin exchange chemistry assay combined with proteomic mass spectrometry, we identified protein substrates palmitoylated by AkrA including two new putative P-type ATPases (Pmc1 and Spf1 homologs), a putative proton V-type proton ATPase (Vma5 homolog) and three putative proteins in A. nidulans, the transcripts of which have previously been shown to be induced by extracellular calcium stress in a CrzA-dependent manner. Thus, our findings provide strong evidence that the AkrA protein regulates [Ca²⁺]_c homeostasis by palmitoylating these protein candidates and give new insights the role of palmitoylation in the regulation of calcium-mediated responses to extracellular, ER or plasma membrane stress.

Palmitoylation is a reversible post-translational modification catalyzed by palmitoyl acyltransferases (PATs) and proteins that undergo this modification are involved in numerous intracellular functions. Yeast Akr1p was the first characterized PAT whilst HIP14, an Akr1p homolog in human, is one of the most highly conserved of 23 human PATs that catalyze the addition of palmitate to the Huntington protein which is of major importance in Huntington’s disease. Calcium serves numerous signaling and structural functions in all eukaryotes. However, studies on the relationship between calcium signaling and palmitoylation are lacking. In this study, we demonstrate that the palmitoyl transferase Akr1 homolog in the filamentous fungus Aspergillus nidulans, similar to the high-affinity calcium uptake system (HACS), is required for normal growth and sporulation in the presence of low extracellular calcium. We find that AkrA dysfunction decreases the transient increase in cytosolic free calcium induced by a high extracellular calcium stress, tunicamycin (which induces endoplasmic reticulum stress) or the antifungal agent itraconazole (which induces plasma membrane stress). The influence of AkrA on all of these processes involves its DHHC motif, which is required for palmitoylation of various proteins associated with many processes including calcium signaling and membrane trafficking. Our findings provide evidence for a crucial link between calcium signaling and palmitoylation, suggesting a possible role in the mechanistic basis of human PAT-related diseases. These results also indicate that regulators of posttranslational modification may provide promising antifungal targets for new therapies.