The Fungus Among Us: Why the Treatment of Fungal Infections Is So Problematic

When we think of microbes that can make us sick, it is usually bacteria that cross our minds first. We tend to forget about another major microbial type that can also cause severe diseases: the fungi. Yeasts and molds make up the majority of microscopic fungi and both types can cause various infections in humans, from mild skin rashes to deadly blood infections. These fungi have found several ways to cause us harm, such as using the body’s nutrients, escaping the surveillance of the immune system, or hijacking and destroying our cells. On cellular level, we have a lot in common with fungi. These common features between human cells and fungal cells makes the development of antibiotics and vaccines to treat fungal infections very difficult. In this article, we will describe some fungal infections and explain current options for their treatment

Integrated analysis of SR-like protein kinases Sky1 and Sky2 links signaling networks with transcriptional regulation in Candida albicans

Fungal infections are a major global health burden where Candida albicans is among the most common fungal pathogen in humans and is a common cause of invasive candidiasis. Fungal phenotypes, such as those related to morphology, proliferation and virulence are mainly driven by gene expression, which is primarily regulated by kinase signaling cascades. Serine-arginine (SR) protein kinases are highly conserved among eukaryotes and are involved in major transcriptional processes in human and S. cerevisiae. Candida albicans harbors two SR protein kinases, while Sky2 is important for metabolic adaptation, Sky1 has similar functions as in S. cerevisiae. To investigate the role of these SR kinases for the regulation of transcriptional responses in C. albicans, we performed RNA sequencing of sky1Δ and sky2Δ and integrated a comprehensive phosphoproteome dataset of these mutants. Using a Systems Biology approach, we study transcriptional regulation in the context of kinase signaling networks. Transcriptomic enrichment analysis indicates that pathways involved in the regulation of gene expression are downregulated and mitochondrial processes are upregulated in sky1Δ. In sky2Δ, primarily metabolic processes are affected, especially for arginine, and we observed that arginine-induced hyphae formation is impaired in sky2Δ. In addition, our analysis identifies several transcription factors as potential drivers of the transcriptional response. Among these, a core set is shared between both kinase knockouts, but it appears to regulate different subsets of target genes. To elucidate these diverse regulatory patterns, we created network modules by integrating the data of site-specific protein phosphorylation and gene expression with kinase-substrate predictions and protein-protein interactions. These integrated signaling modules reveal shared parts but also highlight specific patterns characteristic for each kinase. Interestingly, the modules contain many proteins involved in fungal morphogenesis and stress response. Accordingly, experimental phenotyping shows a higher resistance to Hygromycin B for sky1Δ. Thus, our study demonstrates that a combination of computational approaches with integration of experimental data can offer a new systems biological perspective on the complex network of signaling and transcription. With that, the investigation of the interface between signaling and transcriptional regulation in C. albicans provides a deeper insight into how cellular mechanisms can shape the phenotype

The role of released ATP in killing Candida albicans and other extracellular microbial pathogens by cationic peptides

A unifying theme common to the action of many cationic peptides that display lethal activities against microbial pathogens is their specific action at microbial membranes that results in selective loss of ions and small nucleotides chiefly ATP. One model cationic peptide that induces non-lytic release of ATP from the fungal pathogen Candida albicans is salivary histatin 5 (Hst 5). The major characteristic of Hst 5-induced ATP release is that it occurs rapidly while cells are still metabolically active and have polarized membranes, thus precluding cell lysis as the means of release of ATP. Other cationic peptides that induce selective release of ATP from target microbes are lactoferricin, human neutrophil defensins, bactenecin, and cathelicidin peptides. The role of released extracellular ATP induced by cationic peptides is not known, but localized increases in extracellular ATP concentration may serve to potentiate cell killing, facilitate further peptide uptake, or function as an additional signal to activate the host innate immune system at the site of infection

Modulation of host pH by the phytopathogen <i>Colletotrichum gloeosporioides</i> increases fungal virulence.

<p><i>C</i>. <i>gloeosporioides</i> infects the tomato fruit in a process initiated upon attachment of the fungal conidia to the plant surface. During the quiescent stage of infection, fruit physiological factors such as nutrient availability, acidic pH, and surface waxes determine the rate of fungal growth and germination. As the fruit ripens, conidia germinate into a specialized structure, named appressorium, which eventually becomes melanized. Melanin alters the permeability of the plant cell wall, creating a hypertonic environment that allows the fungus to penetrate the host epidermis using turgor pressure. This process is accompanied by active metabolism of amino acids, such as glutamate and glutamine, and gradual environmental alkalization. The fungus transitions into the necrotrophic stage, characterized by a dramatic shift in fungal metabolism and activation of pathogenicity factors, such as proteases and lyases, resulting in anthracnose fruit rot.</p

Modulation of Phagosomal pH by <i>Candida albicans</i> Promotes Hyphal Morphogenesis and Requires Stp2p, a Regulator of Amino Acid Transport

<div><p><i>Candida albicans</i>, the most important fungal pathogen of humans, has a unique interaction with macrophages in which phagocytosis induces a switch from the yeast to hyphal form, allowing it to escape by rupturing the immune cell. While a variety of factors induce this switch in vitro, including neutral pH, it is not clear what triggers morphogenesis within the macrophage where the acidic environment should inhibit this transition. In vitro, <i>C. albicans</i> grown in similar conditions in which amino acids are the primary carbon source generate large quantities of ammonia to raise the extracellular pH and induce the hyphal switch. We show here that <i>C. albicans</i> cells neutralize the macrophage phagosome and that neutral pH is a key inducer of germination in phagocytosed cells by using a mutant lacking <i>STP2</i>, a transcription factor that regulates the expression of multiple amino acid permeases, that is completely deficient in alkalinization <i>in vitro</i>. Phagocytosed <i>stp2Δ</i> mutant cells showed significant reduction in hypha formation and escaped from macrophages less readily compared to wild type cells; as a result <i>stp2Δ</i> mutant cells were killed at a higher rate and caused less damage to RAW264.7 macrophages. Stp2p-regulated import leads to alkalinization of the phagosome, since the majority of the wild type cells fail to co-localize with acidophilic dyes, whereas the <i>stp2Δ</i> mutant cells were located in acidic phagosomes. Furthermore, <i>stp2Δ</i> mutant cells were able to form hyphae and escape from neutral phagosomes, indicating that the survival defect in these cells was pH dependent. Finally, these defects are reflected in an attenuation of virulence in a mouse model of disseminated candidiasis. Altogether our results suggest that <i>C. albicans</i> utilizes amino acids to promote neutralization of the phagosomal pH, hyphal morphogenesis, and escape from macrophages.</p></div