Identification and Characterization of Key Charged Residues in the Cofilin Protein Involved in Azole Susceptibility, Apoptosis, and Virulence of Aspergillus fumigatus

Achieving Resilience in Aging: How Mitochondrial Modulation Drives Age-associated Fluconazole Tolerance in Cryptococcus neoformans

Cryptococcus neoformans ( Cn ) is an opportunistic fungal microorganism that causes life-threatening meningoencephalitis. During the infection, the microbial population is heterogeneously composed of cells with varying generational ages, with older cells accumulating during chronic infections. This is attributed to their enhanced resistance to phagocytic killing and tolerance of antifungals like fluconazole (FLC). In this study, we investigated the role of ergosterol synthesis, ATP-binding cassette (ABC) transporters, and mitochondrial metabolism in the regulation of age-dependent FLC tolerance. We find that old Cn cells increase the production of ergosterol and exhibit upregulation of ABC transporters. Old cells also show transcriptional and phenotypic characteristics consistent with increased metabolic activity, leading to increased ATP production. This is accompanied by increased production of reactive oxygen species (ROS), which results in mitochondrial fragmentation. This study demonstrates that the metabolic changes occurring in the mitochondria of old cells drive the increase in ergosterol synthesis and the upregulation of ABC transporters, leading to FLC tolerance.

IMPORTANCE

Infections caused by Cryptococcus neoformans cause more than 180,000 deaths annually. Estimated one-year mortality for patients receiving care ranges from 20% in developed countries to 70% in developing countries, suggesting that current treatments are inadequate. Some fungal cells can persist and replicate despite the usage of current antifungal regimens, leading to death or treatment failure. In replicative aging, older cells display a resilient phenotype, characterized by their enhanced tolerance against antifungals and resistance to killing by host cells. This study shows that age-dependent increase in mitochondrial reactive oxygen species drive changes in ABC transporters and ergosterol synthesis, ultimately leading to the heightened tolerance against fluconazole in old C. neoformans cells. Understanding the underlying molecular mechanisms of this age-associated antifungal tolerance will enable more targeted antifungal therapies for cryptococcal infections.

Requirement of a putative mitochondrial GTPase, GemA, for azole susceptibility, virulence, and cell wall integrity in Aspergillus fumigatus

Drug resistance in fungal pathogens is a new challenge in clinical aspergillosis treatment. Mitochondria as dynamic organelles are involved in numerous biological processes in fungi, including drug resistance. However, little is known about the mechanism underlying mitochondrial regulation of the response of fungal pathogens to antifungal drugs. Here, we showed that a putative mitochondrial GTPase, GemA, a yeast Gem1 homolog, is crucial for the azole response and cell wall integrity in the opportunistic pathogen Aspergillus fumigatus. The fluorescence observation showed that GFP-labeled GemA is located in mitochondria, and loss of gemA results in aberrant giant mitochondrial morphology and abnormal mitochondrial membrane potential. Moreover, a ΔgemA mutant attenuates fungal virulence in the Galleria mellonella model of aspergillosis. Furthermore, gemA loss increases resistance to azoles and terbinafine but not to amphotericin B. Of note, RNA-seq combined with RT-qPCR showed that a series of drug efflux pumps were upregulated in the gemA deletion mutant. Deleting mdr1 or inhibiting the expression of drug efflux pumps can partially decrease the resistance to azoles resulting from the gemA mutant, implying that GemA influences azole response by affecting the expression of drug efflux pumps. Importantly, the ΔgemA mutant is susceptible to the cell wall-perturbing reagent CR, but not to CFW, and this defect can be partly rescued by hyperosmotic stress. TEM revealed that the cell wall of ΔgemA was thicker than that of the WT strain, demonstrating that GemA plays a role in cell wall composition and integrity. Collectively, we identified a putative mitochondrial GTPase, GemA, which is critical for hyphal growth, virulence, azole susceptibility, and cell wall integrity and acts by affecting mitochondrial function.

A Cecropin-4 Derived Peptide C18 Inhibits Candida albicans by Disturbing Mitochondrial Function

Global burden of fungal infections and related health risk has accelerated at an incredible pace, and multidrug resistance emergency aggravates the need for the development of new effective strategies. Candida albicans is clinically the most ubiquitous pathogenic fungus that leads to high incidence and mortality in immunocompromised patients. Antimicrobial peptides (AMPs), in this context, represent promising alternatives having potential to be exploited for improving human health. In our previous studies, a Cecropin-4-derived peptide named C18 was found to possess a broader antibacterial spectrum after modification and exhibit significant antifungal activity against C. albicans. In this study, C18 shows antifungal activity against C. albicans or non-albicans Candida species with a minimum inhibitory concentration (MIC) at 4∼32 μg/ml, and clinical isolates of fluconazole (FLZ)-resistance C. tropicalis were highly susceptible to C18 with MIC value of 8 or 16 μg/ml. Additionally, C18 is superior to FLZ for killing planktonic C. albicans from inhibitory and killing kinetic curves. Moreover, C18 could attenuate the virulence of C. albicans, which includes damaging the cell structure, retarding hyphae transition, and inhibiting biofilm formation. Intriguingly, in the Galleria mellonella model with C. albicans infection, C18 could improve the survival rate of G. mellonella larvae to 70% and reduce C. albicans load from 5.01 × 107 to 5.62 × 104 CFU. For mechanistic action of C18, the level of reactive oxygen species (ROS) generation and cytosolic Ca2 + increased in the presence of C18, which is closely associated with mitochondrial dysfunction. Meanwhile, mitochondrial membrane potential (△Ψm) loss and ATP depletion of C. albicans occurred with the treatment of C18. We hypothesized that C18 might inhibit C. albicans via triggering mitochondrial dysfunction driven by ROS generation and Ca2 + accumulation. Our observation provides a basis for future research to explore the antifungal strategies and presents C18 as an attractive therapeutic candidate to be developed to treat candidiasis.

Low Glucose Mediated Fluconazole Tolerance in Cryptococcus neoformans

Chronic meningoencephalitis is caused by Cryptococcus neoformans and is treated in many parts of the world with fluconazole (FLC) monotherapy, which is associated with treatment failure and poor outcome. In the host, C. neoformans propagates predominantly under low glucose growth conditions. We investigated whether low glucose, mimicked by growing in synthetic media (SM) with 0.05% glucose (SM^lowglu), affects FLC-resistance. A > 4-fold increase in FLC tolerance was observed in seven C. neoformans strains when minimum inhibitory concentration (MIC) was determined in SM^lowglu compared to MIC in SM with normal (2%) glucose (SM^nlglu). In SM^lowglu, C. neoformans cells exhibited upregulation of efflux pump genes AFR1 (8.7-fold) and AFR2 (2.5-fold), as well as decreased accumulation (2.6-fold) of Nile Red, an efflux pump substrate. Elevated intracellular ATP levels (3.2-fold and 3.4-fold), as well as decreased mitochondrial reactive oxygen species levels (12.8-fold and 17-fold), were found in the presence and absence of FLC, indicating that low glucose altered mitochondrial function. Fluorescence microscopy revealed that mitochondria of C. neoformans grown in SM^lowglu were fragmented, whereas normal glucose promoted a reticular network of mitochondria. Although mitochondrial membrane potential (MMP) was not markedly affected in SM^lowglu, it significantly decreased in the presence of FLC (12.5-fold) in SM^nlglu, but remained stable in SM^lowglu-growing C. neoformans cells. Our data demonstrate that increased FLC tolerance in low glucose-growing C. neoformans is the result of increased efflux pump activities and altered mitochondrial function, which is more preserved in SM^lowglu. This mechanism of resistance is different from FLC heteroresistance, which is associated with aneuploidy of chromosome 1 (Chr1).

Galleria mellonella as a screening tool to study virulence factors of Aspergillus fumigatus

The invertebrate Galleria mellonella has increasingly and widely been used in the last few years to study complex host–microbe interactions. Aspergillus fumigatus is one of the most pathogenic fungi causing life-threatening diseases in humans and animals. Galleria mellonella larvae has been proven as a reliable model for the analysis of pathogenesis and virulence factors, enable to screen a large number of A. fumigatus strains. This review describes the different uses of G. mellonella to study A. fumigatus and provides a comparison of the different protocols to trace fungal pathogenicity. The review also includes a summary of the diverse mutants tested in G. mellonella, and their respective contribution to A. fumigatus virulence. Previous investigations indicated that G. mellonella should be considered as an interesting tool even though a mammalian model may be required to complete and verify initial data.

Role of cofilin‑1 in arsenic trioxide‑induced apoptosis of NB4‑R1 cells

All-trans retinoic acid (ATRA) and arsenic trioxide (As₂O₃) are currently first-line treatments for acute promyelocytic leukemia (APL). However, a number of patients with APL are resistant to ATRA but still sensitive to As₂O₃, and the underlying mechanisms of this remain unclear. In the present study, two-dimensional gel electrophoresis, mass spectrometry and other proteomic methods were applied to screen and identify the differentially expressed proteins between the retinoic acid-sensitive cell lines and drug-resistant cell lines. The results demonstrated that in retinoic acid-resistant NB4-R1 cells, the protein expression of cofilin-1 was markedly increased compared with that in the drug-sensitive NB4 cells. Subsequently, the effects of cofilin-1 on As₂O₃-induced apoptosis in NB4-R1 cells were further investigated. The results revealed that cell viability was markedly suppressed and apoptosis was increased in the As₂O₃-treated NB4-R1 cells, with increased expression levels of cleaved-poly (ADP-ribose) polymerase and cleaved-caspase 12. Cofilin-1 expression was significantly decreased at both the mRNA and protein levels in the As₂O₃-treated group compared with the control. Western blotting further revealed that As₂O₃ treatment decreased the cytoplasmic cofilin-1 level but increased its expression in the mitochondrion. However, the opposite effects of As₂O₃ on the cytochrome C distribution were found in NB4-R1 cells. This suggested that As₂O₃ can induce the transfer of cofilin-1 from the cytoplasm to mitochondria and trigger the release of mitochondrial cytochrome C in NB4-R1 cells. Moreover, cofilin-1 knockdown by its specific short hairpin RNA significantly suppressed As₂O₃-induced NB4-R1 cell apoptosis and inhibited the release of mitochondrial cytochrome C. Whereas, overexpression of cofilin-1 using a plasmid vector carrying cofilin-1 increased the release of cytochrome C into the cytoplasm from the mitochondria in As₂O₃-treated NB4-R1 cells. In conclusion, cofilin-1 played a role in As₂O₃-induced NB4-R1 cell apoptosis and it might be a novel target for APL treatment.

Mitochondria-Mediated Azole Drug Resistance and Fungal Pathogenicity: Opportunities for Therapeutic Development

In recent years, the role of mitochondria in pathogenic fungi in terms of azole resistance and fungal pathogenicity has been a rapidly developing field. In this review, we describe the molecular mechanisms by which mitochondria are involved in regulating azole resistance and fungal pathogenicity. Mitochondrial function is involved in the regulation of drug efflux pumps at the transcriptional and posttranslational levels. On the one hand, defects in mitochondrial function can serve as the signal leading to activation of calcium signaling and the pleiotropic drug resistance pathway and, therefore, can globally upregulate the expression of drug efflux pump genes, leading to azole drug resistance. On the other hand, mitochondria also contribute to azole resistance through modulation of drug efflux pump localization and activity. Mitochondria further contribute to azole resistance through participating in iron homeostasis and lipid biosynthesis. Additionally, mitochondrial dynamics play an important role in azole resistance. Meanwhile, mitochondrial morphology is important for fungal virulence, playing roles in growth in stressful conditions in a host. Furthermore, there is a close link between mitochondrial respiration and fungal virulence, and mitochondrial respiration plays an important role in morphogenetic transition, hypoxia adaptation, and cell wall biosynthesis. Finally, we discuss the possibility for targeting mitochondrial factors for the development of antifungal therapies.

Fungal Lanosterol 14α-demethylase: A target for next-generation antifungal design

The cytochrome P450 enzyme lanosterol 14α-demethylase (LDM) is the target of the azole antifungals used widely in medicine and agriculture as prophylaxis or treatments of infections or diseases caused by fungal pathogens. These drugs and agrochemicals contain an imidazole, triazole or tetrazole substituent, with one of the nitrogens in the azole ring coordinating as the sixth axial ligand to the LDM heme iron. Structural studies show that this membrane bound enzyme contains a relatively rigid ligand binding pocket comprised of a deeply buried heme-containing active site together with a substrate entry channel and putative product exit channel that reach to the membrane. Within the ligand binding pocket the azole antifungals have additional affinity determining interactions with hydrophobic side-chains, the polypeptide backbone and via water-mediated hydrogen bond networks. This review will describe the tools that can be used to identify and characterise the next generation of antifungals targeting LDM, with the goal of obtaining highly potent broad-spectrum fungicides that will be able to avoid target and drug efflux mediated antifungal resistance.

Role of Downregulation and Phosphorylation of Cofilin in Polarized Growth, MpkA Activation and Stress Response of Aspergillus fumigatus

Aspergillus fumigatus causes most of aspergillosis in clinic and comprehensive function analysis of its key protein would promote anti-aspergillosis. In a previous study, we speculated actin depolymerizing factor cofilin might be essential for A. fumigatus viability and found its overexpression upregulated oxidative response and cell wall polysaccharide synthesis of this pathogen. Here, we constructed a conditional cofilin mutant to determine the essential role of cofilin. And the role of cofilin downregulation and phosphorylation in A. fumigatus was further analyzed. Cofilin was required for the polarized growth and heat sensitivity of A. fumigatus. Downregulation of cofilin caused hyphal cytoplasmic leakage, increased the sensitivity of A. fumigatus to sodium dodecyl sulfonate but not to calcofluor white and Congo Red and farnesol, and enhanced the basal phosphorylation level of MpkA, suggesting that cofilin affected the cell wall integrity (CWI) signaling. Downregulation of cofilin also increased the sensitivity of A. fumigatus to alkaline pH and H₂O₂. Repressing cofilin expression in A. fumigatus lead to attenuated virulence, which manifested as lower adherence and internalization rates, weaker host inflammatory response and shorter survival rate in a Galleria mellonella model. Expression of non-phosphorylated cofilin with a mutation of S5A had little impacts on A. fumigatus, whereas expression of a mimic-phosphorylated cofilin with a mutation of S5E resulted in inhibited growth, increased phospho-MpkA level, and decreased pathogenicity. In conclusion, cofilin is crucial to modulating the polarized growth, stress response, CWI and virulence of A. fumigatus.