Emergence and evolution of virulence in human pathogenic fungi.

One billion people worldwide are affected by fungal pathogens, of which 1.6 million succumb to fungal infections per year. This review discusses the emergence and evolution of fungal pathogenesis in humans in the form of opportunistic commensal and environmental fungi. We explore the attributes that contribute to their success as pathogens and the scenarios which may have caused the evolutionary selection of virulence factors. This includes antivirulence and avirulence genes, notions that are new for fungal pathogens of humans but which are based on well established concepts in bacterial pathogens and phytopathogenic fungi. These ideas will ultimately help us to better understand the pathogenicity of fungi that infect humans: from the emergence to the finer adjustment of virulence to promote pathogen persistence.

Transient Mitochondria Dysfunction Confers Fungal Cross-Resistance against Phagocytic Killing and Fluconazole.

Loss or inactivation of antivirulence genes is an adaptive strategy in pathogen evolution. Candida glabrata is an important opportunistic pathogen related to baker's yeast, with the ability to both quickly increase its intrinsic high level of azole resistance and persist within phagocytes. During C. glabrata's evolution as a pathogen, the mitochondrial DNA polymerase CgMip1 has been under positive selection. We show that CgMIP1 deletion not only triggers loss of mitochondrial function and a petite phenotype, but increases C. glabrata's azole and endoplasmic reticulum (ER) stress resistance and, importantly, its survival in phagocytes. The same phenotype is induced by fluconazole and by exposure to macrophages, conferring a cross-resistance between antifungals and immune cells, and can be found in clinical isolates despite a slow growth of petite strains. This suggests that petite constitutes a bet-hedging strategy of C. glabrata and, potentially, a relevant cause of azole resistance. Mitochondrial function may therefore be considered a potential antivirulence factor. IMPORTANCE Candida glabrata is an opportunistic pathogen whose incidence has been increasing in the last 40 years. It has risen to become the most prominent non-Candida albicans Candida (NCAC) species to cause candidemia, constituting about one-third of isolates in the United States, and steadily increasing in European countries and in Australia. Despite its clinical importance, C. glabrata's pathogenicity strategies remain poorly understood. Our research shows that loss of mitochondrial function and the resulting petite phenotype is advantageous for C. glabrata to cope with infection-related stressors, such as antifungals and host immune defenses. The (cross-)resistance against both these factors may have major implications in the clinical outcome of infections with this major fungal pathogen.

The involvement of the Candida glabrata trehalase enzymes in stress resistance and gut colonization.

Candida glabrata is an opportunistic human fungal pathogen and is frequently present in the human microbiome. It has a high relative resistance to environmental stresses and several antifungal drugs. An important component involved in microbial stress tolerance is trehalose. In this work, we characterized the three C. glabrata trehalase enzymes Ath1, Nth1 and Nth2. Single, double and triple deletion strains were constructed and characterized both in vitro and in vivo to determine the role of these enzymes in virulence. Ath1 was found to be located in the periplasm and was essential for growth on trehalose as sole carbon source, while Nth1 on the other hand was important for oxidative stress resistance, an observation which was consistent by the lower survival rate of the NTH1 deletion strain in human macrophages. No significant phenotype was observed for Nth2. The triple deletion strain was unable to establish a stable colonization of the gastrointestinal (GI) tract in mice indicating the importance of having trehalase activity for colonization in the gut.

Candida pathogens induce protective mitochondria-associated type I interferon signalling and a damage-driven response in vaginal epithelial cells.

Vaginal candidiasis is an extremely common disease predominantly caused by four phylogenetically diverse species: Candida albicans; Candida glabrata; Candida parapsilosis; and Candida tropicalis. Using a time course infection model of vaginal epithelial cells and dual RNA sequencing, we show that these species exhibit distinct pathogenicity patterns, which are defined by highly species-specific transcriptional profiles during infection of vaginal epithelial cells. In contrast, host cells exhibit a homogeneous response to all species at the early stages of infection, which is characterized by sublethal mitochondrial signalling inducing a protective type I interferon response. At the later stages, the transcriptional response of the host diverges in a species-dependent manner. This divergence is primarily driven by the extent of epithelial damage elicited by species-specific mechanisms, such as secretion of the toxin candidalysin by C. albicans. Our results uncover a dynamic, biphasic response of vaginal epithelial cells to Candida species, which is characterized by protective mitochondria-associated type I interferon signalling and a species-specific damage-driven response.

Antivirulence and avirulence genes in human pathogenic fungi.

Opportunistic commensal and environmental fungi can cause superficial to systemic diseases in humans. But how did these pathogens adapt to infect us and how does host-pathogen co-evolution shape their virulence potential? During evolution toward pathogenicity, not only do microorganisms gain virulence genes, but they also tend to lose non-adaptive genes in the host niche. Additionally, virulence factors can become detrimental during infection when they trigger host recognition. The loss of non-adaptive genes as well as the loss of the virulence potential of genes by adaptations to the host has been investigated in pathogenic bacteria and phytopathogenic fungi, where they are known as antivirulence and avirulence genes, respectively. However, these concepts are nearly unknown in the field of pathogenic fungi of humans. We think that this unnecessarily limits our view of human-fungal interplay, and that much could be learned if we applied a similar framework to aspects of these interactions. In this review, we, therefore, define and adapt the concepts of antivirulence and avirulence genes for human pathogenic fungi. We provide examples for analogies to antivirulence genes of bacterial pathogens and to avirulence genes of phytopathogenic fungi. Introducing these terms to the field of pathogenic fungi of humans can help to better comprehend the emergence and evolution of fungal virulence and disease.

ZNF518B gene up-regulation promotes dissemination of tumour cells and is governed by epigenetic mechanisms in colorectal cancer.

Most of colorectal cancer CRC-related death is due to metastasis and the finding of markers for prognosis of invasiveness, constitutes an appealing challenge. Here, after analysing cDNA array containing 43 tumour and 5 normal mucosa samples, we report that the expression of the ZNF518B gene as a whole and that of its two major splicing isoforms are significantly increased in tumours. The canonical isoform was also up-regulated in a patients' cohort containing 70 tumour and 69 adjacent tissue samples. The effects of silencing ZNF518B on the phenotype of CRC cell lines were then studied. The gene does not affect cell proliferation, but plays a significant role in cell migration and invasiveness and induces changes in the epithelial-to-mesenchymal transition markers, suggesting that ZNF518B favours tumour cell dissemination. To study the regulation of the gene, transcription-related changes in nucleosomal organisation and epigenetic marks around the transcriptional start site were analysed. The positioning of a nucleosome over the transcription start site and the differential presence of the epigenetic marks H3K9ac, H3K27ac, H3K4me3 and H3K9me3 correlate with gene expression. Inhibition of histone deacetylases increases the transcription of ZNF518B, which may be a candidate for invasiveness prognosis in CRC and a target for epigenetic drugs.