A mutation in C. neoformans mitochondrial NADH dehydrogenase results in increased virulence in mice

The dynamics of fungal genome organization and its impact on host adaptation and antifungal resistance

Fungi are a diverse kingdom characterized by remarkable genomic plasticity that facilitates pathogenicity and adaptation to adverse environmental conditions. In this review, we delve into the dynamic organization of fungal genomes and its implications for host adaptation and antifungal resistance. We examine key features and the heterogeneity of genomes across different fungal species, including but not limited to their chromosome content, DNA composition, distribution and arrangement of their content across chromosomes, and other major traits. We further highlight how this variability in genomic traits influences their virulence and adaptation to adverse conditions. Fungal genomes exhibit large variations in size, gene content, and structural features, such as abundance of transposable elements (TEs), compartmentalization into gene-rich and TE-rich regions, and the presence or absence of dispensable chromosomes. Genomic structural variations are equally diverse in fungi, ranging from whole-chromosome duplications that may enhance tolerance to antifungal compounds, to targeted deletion of effector encoding genes that may promote virulence. Finally, the often-overlooked fungal mitochondrial genomes can also affect virulence and resistance to fungicides. Such and other features of fungal genome organization are reviewed and discussed in the context of host-microbe interactions and antifungal resistance.

The interplay between electron transport chain function and iron regulatory factors influences melanin formation in Cryptococcus neoformans

Mitochondrial functions are critical for the ability of the fungal pathogen Cryptococcus neoformans to cause disease. However, mechanistic connections between key functions such as the mitochondrial electron transport chain (ETC) and virulence factor elaboration have yet to be thoroughly characterized. Here, we observed that inhibition of ETC complex III suppressed melanin formation, a major virulence factor. This inhibition was partially overcome by defects in Cir1 or HapX, two transcription factors that regulate iron acquisition and use. In this regard, loss of Cir1 derepresses the expression of laccase genes as a potential mechanism to restore melanin, while HapX may condition melanin formation by controlling oxidative stress. We hypothesize that ETC dysfunction alters redox homeostasis to influence melanin formation. Consistent with this idea, inhibition of growth by hydrogen peroxide was exacerbated in the presence of the melanin substrate L-DOPA. In addition, loss of the mitochondrial chaperone Mrj1, which influences the activity of ETC complex III and reduces ROS accumulation, also partially overcame antimycin A inhibition of melanin. The phenotypic impact of mitochondrial dysfunction was consistent with RNA-Seq analyses of WT cells treated with antimycin A or L-DOPA, or cells lacking Cir1 that revealed influences on transcripts encoding mitochondrial functions (e.g., ETC components and proteins for Fe-S cluster assembly). Overall, these findings reveal mitochondria-nuclear communication via ROS and iron regulators to control virulence factor production in C. neoformans.IMPORTANCEThere is a growing appreciation of the importance of mitochondrial functions and iron homeostasis in the ability of fungal pathogens to sense the vertebrate host environment and cause disease. Many mitochondrial functions such as heme and iron-sulfur cluster biosynthesis, and the electron transport chain (ETC), are dependent on iron. Connections between factors that regulate iron homeostasis and mitochondrial activities are known in model yeasts and are emerging for fungal pathogens. In this study, we identified connections between iron regulatory transcription factors (e.g., Cir1 and HapX) and the activity of complex III of the ETC that influence the formation of melanin, a key virulence factor in the pathogenic fungus Cryptococcus neoformans. This fungus causes meningoencephalitis in immunocompromised people and is a major threat to the HIV/AIDS population. Thus, understanding how mitochondrial functions influence virulence may support new therapeutic approaches to combat diseases caused by C. neoformans and other fungi.

Alternative Oxidase - Aid or obstacle to combat the rise of fungal pathogens?

Fungal pathogens present a growing threat to both humans and global health security alike. Increasing evidence of antifungal resistance in fungal populations that infect both humans and plant species has increased reliance on combination therapies and shown the need for new antifungal therapeutic targets to be investigated. Here, we review the roles of mitochondria and fungal respiration in pathogenesis and discuss the role of the Alternative Oxidase enzyme (Aox) in both human fungal pathogens and phytopathogens. Increasing evidence exists for Aox within mechanisms that underpin fungal virulence. Aox also plays important roles in adaptability that may prove useful within dual targeted fungal-specific therapeutic approaches. As improved fungal specific mitochondrial and Aox inhibitors are under development we may see this as an emerging target for future approaches to tackling the growing challenge of fungal infection.

The interplay between electron transport chain function and iron regulatory factors influences melanin formation in Cryptococcus neoformans

Mitochondrial functions are critical for the ability of the fungal pathogen Cryptococcus neoformans to cause disease. However, mechanistic connections between key functions such as the mitochondrial electron transport chain (ETC) and virulence factor elaboration have yet to be thoroughly characterized. Here, we observed that inhibition of ETC complex III suppressed melanin formation, a major virulence factor. This inhibition was partially blocked upon loss of Cir1 or HapX, two transcription factors that regulate iron acquisition and use. In this regard, loss of Cir1 derepresses the expression of laccase genes as a potential mechanism to restore melanin, while HapX may condition melanin formation by controlling oxidative stress. We hypothesize that ETC dysfunction alters redox homeostasis to influence melanin formation. Consistent with this idea, inhibition of growth by hydrogen peroxide was exacerbated in the presence of the melanin substrate L-DOPA. Additionally, loss of the mitochondrial chaperone Mrj1, which influences the activity of ETC complex III and reduces ROS accumulation, also partially blocked antimycin A inhibition of melanin. The phenotypic impact of mitochondrial dysfunction was consistent with RNA-Seq analyses of WT cells treated with antimycin A or L-DOPA, or cells lacking Cir1 that revealed influences on transcripts encoding mitochondrial functions (e.g., ETC components and proteins for Fe-S cluster assembly). Overall, these findings reveal mitochondria-nuclear communication via ROS and iron regulators to control virulence factor production in C. neoformans.

Models for Inducing Experimental Cryptococcosis in Mice

Cryptococcus neoformans and Cryptococcus gattii are the predominant etiological agents of cryptococcosis, a particularly problematic disease in immunocompromised individuals. The increased clinical use of immunosuppressive drugs, the inherent ability of Cryptococcus species to suppress and evade host immune responses, and the emergence of drug-resistant yeast support the need for model systems that facilitate the design of novel immunotherapies and antifungals to combat disease progression. The mouse model of cryptococcosis is a widely used system to study Cryptococcus pathogenesis and the efficacy of antifungal drugs in vivo. In this chapter, we describe three commonly used strategies to establish cryptococcosis in mice: intranasal, intratracheal, and intravenous inoculations. Also, we discuss the methodology for delivering drugs to mice via intraperitoneal injection.

Alternative Non-Mammalian Animal and Cellular Methods for the Study of Host-Fungal Interactions

In the study of fungal pathogenesis, alternative methods have gained prominence due to recent global legislation restricting the use of mammalian animals in research. The principle of the 3 Rs (replacement, reduction, and refinement) is integrated into regulations and guidelines governing animal experimentation in nearly all countries. This principle advocates substituting vertebrate animals with other invertebrate organisms, embryos, microorganisms, or cell cultures. This review addresses host-fungus interactions by employing three-dimensional (3D) cultures, which offer more faithful replication of the in vivo environment, and by utilizing alternative animal models to replace traditional mammals. Among these alternative models, species like Caenorhabditis elegans and Danio rerio share approximately 75% of their genes with humans. Furthermore, models such as Galleria mellonella and Tenebrio molitor demonstrate similarities in their innate immune systems as well as anatomical and physiological barriers, resembling those found in mammalian organisms.

Juxtaposing Caenorhabditis elegans-Pathogenic Mould Model with Other Models; How Reliable Is This Nematode Model? A Mini Review

The application of Caenorhabditis elegans as a pathogenic model has spanned decades. Its use for pathogenic mould modeling has been attracting some attention lately, though not without some reservations. Several studies have shown C. elegans to be a reliable model for evaluating moulds' virulence factors and patterns as well as for screening the pathogenicity of mutant strains alongside their parental/wild type and revertant/complementary strains. There is a very high degree of reported similarities between the virulence patterns demonstrated in C. elegans and those of other invertebrate and vertebrate models. We have here presented several works in which this nematode model was adopted for virulence evaluation, and other comparative research in which virulence in C. elegans model were juxtaposed with other models. We have further presented possible reasons why there might have been variations of virulence in a few cases, thereby validating C. elegans to be an effective and reliable tool in the study of pathogenic moulds.

Succinate Dehydrogenase, Succinate, and Superoxides: A Genetic, Epigenetic, Metabolic, Environmental Explosive Crossroad

Research focused on succinate dehydrogenase (SDH) and its substrate, succinate, culminated in the 1950s accompanying the rapid development of research dedicated to bioenergetics and intermediary metabolism. This allowed researchers to uncover the implication of SDH in both the mitochondrial respiratory chain and the Krebs cycle. Nowadays, this theme is experiencing a real revival following the discovery of the role of SDH and succinate in a subset of tumors and cancers in humans. The aim of this review is to enlighten the many questions yet unanswered, ranging from fundamental to clinically oriented aspects, up to the danger of the current use of SDH as a target for a subclass of pesticides.