Control of morphogenesis, protease secretion and gene expression in Candida albicans by the preferred nitrogen source ammonium

Amino Acid Sensing and Assimilation by the Fungal Pathogen Candida albicans in the Human Host

Nutrient uptake is essential for cellular life and the capacity to perceive extracellular nutrients is critical for coordinating their uptake and metabolism. Commensal fungal pathogens, e.g., Candida albicans, have evolved in close association with human hosts and are well-adapted to using diverse nutrients found in discrete host niches. Human cells that cannot synthesize all amino acids require the uptake of the “essential amino acids” to remain viable. Consistently, high levels of amino acids circulate in the blood. Host proteins are rich sources of amino acids but their use depends on proteases to cleave them into smaller peptides and free amino acids. C. albicans responds to extracellular amino acids by pleiotropically enhancing their uptake and derive energy from their catabolism to power opportunistic virulent growth. Studies using Saccharomyces cerevisiae have established paradigms to understand metabolic processes in C. albicans; however, fundamental differences exist. The advent of CRISPR/Cas9-based methods facilitate genetic analysis in C. albicans, and state-of-the-art molecular biological techniques are being applied to directly examine growth requirements in vivo and in situ in infected hosts. The combination of divergent approaches can illuminate the biological roles of individual cellular components. Here we discuss recent findings regarding nutrient sensing with a focus on amino acid uptake and metabolism, processes that underlie the virulence of C. albicans.

The histone chaperone HIR maintains chromatin states to control nitrogen assimilation and fungal virulence

Adaptation to changing environments and immune evasion is pivotal for fitness of pathogens. Yet, the underlying mechanisms remain largely unknown. Adaptation is governed by dynamic transcriptional re-programming, which is tightly connected to chromatin architecture. Here, we report a pivotal role for the HIR histone chaperone complex in modulating virulence of the human fungal pathogen Candida albicans. Genetic ablation of HIR function alters chromatin accessibility linked to aberrant transcriptional responses to protein as nitrogen source. This accelerates metabolic adaptation and increases the release of extracellular proteases, which enables scavenging of alternative nitrogen sources. Furthermore, HIR controls fungal virulence, as HIR1 deletion leads to differential recognition by immune cells and hypervirulence in a mouse model of systemic infection. This work provides mechanistic insights into chromatin-coupled regulatory mechanisms that fine-tune pathogen gene expression and virulence. Furthermore, the data point toward the requirement of refined screening approaches to exploit chromatin modifications as antifungal strategies.

Cation Transporters of Candida albicans-New Targets to Fight Candidiasis?

Candidiasis is the wide-spread fungal infection caused by numerous strains of yeast, with the prevalence of Candida albicans. The current treatment of candidiasis is becoming rather ineffective and costly owing to the emergence of resistant strains; hence, the exploration of new possible drug targets is necessary. The most promising route is the development of novel antibiotics targeting this pathogen. In this review, we summarize such candidates found in C. albicans and those involved in the transport of (metal) cations, as the latter are essential for numerous processes within the cell; hence, disruption of their fluxes can be fatal for C. albicans.

The expression, secretion and activity of the aspartic protease MpAPr1 in Metschnikowia pulcherrima IWBT Y1123

Protease-secreting yeasts have broad biotechnological potential for application to various industrial processes, including winemaking. However, this activity is influenced by the yeast response to environmental factors such as nitrogen and protein sources, as are found in grape juice. In this study, the wine-relevant yeast Metschnikowia pulcherrima IWBT Y1123, with known protease-secreting ability, was subjected to different nitrogen-containing compounds to monitor their impact on protease secretion and activity. Protease activity increased above basal levels for haemoglobin-containing treatments, indicating an inductive influence of proteins. On the other hand, treatments containing both haemoglobin and assimilable nitrogen sources led to a delayed increase in protease activity and protein degradation, suggesting a nitrogen catabolite repression mechanism at work. Protease activity and expression were furthermore evaluated in grape juice, which revealed increased expression and activity levels over time as promising results for further investigations into the impact of this yeast on wine properties.

Understanding the regulation of extracellular protease gene expression in fungi: a key step towards their biotechnological applications

The secretion of proteases by certain species of yeast and filamentous fungi is of importance not only for their biological function and survival, but also for their biotechnological application to various processes in the food, beverage, and bioprocessing industries. A key step towards understanding the role that these organisms play in their environment, and how their protease-secreting ability may be optimally utilised through industrial applications, involves an evaluation of those factors which influence protease production. The objective of this review is to provide an overview of the findings from investigations directed at elucidating the regulatory mechanisms underlying extracellular protease secretion in yeast and filamentous fungi, and the environmental stimuli that elicit these responses. The influence of nitrogen-, carbon-, and sulphur-containing compounds, as well as proteins, temperature, and pH, on extracellular protease regulation, which is frequently exerted at the transcriptional level, is discussed in particular depth. Protease-secreting organisms of biotechnological interest are also presented in this context, in an effort to explore the areas of industrial significance that could possibly benefit from such knowledge. In this way, the establishment of a platform of existing knowledge regarding fungal protease regulation is attempted, with the particular goal of aiding in the practical application of these organisms to processes that require secretion of this enzyme.

The Candida albicans stress response gene Stomatin-Like Protein 3 is implicated in ROS-induced apoptotic-like death of yeast phase cells

The ubiquitous presence of SPFH (Stomatin, Prohibitin, Flotillin, HflK/HflC) proteins in all domains of life suggests that their function would be conserved. However, SPFH functions are diverse with organism-specific attributes. SPFH proteins play critical roles in physiological processes such as mechanosensation and respiration. Here, we characterize the stomatin ORF19.7296/SLP3 in the opportunistic human pathogen Candida albicans. Consistent with the localization of stomatin proteins, a Slp3p-Yfp fusion protein formed visible puncta along the plasma membrane. We also visualized Slp3p within the vacuolar lumen. Slp3p primary sequence analyses identified four putative S-palmitoylation sites, which may facilitate membrane localization and are conserved features of stomatins. Plasma membrane insertion sequences are present in mammalian and nematode SPFH proteins, but are absent in Slp3p. Strikingly, Slp3p was present in yeast cells, but was absent in hyphal cells, thus categorizing it as a yeast-phase specific protein. Slp3p membrane fluorescence significantly increased in response to cellular stress caused by plasma membrane, cell wall, oxidative, or osmotic perturbants, implicating SLP3 as a general stress-response gene. A slp3Δ/Δ homozygous null mutant had no detected phenotype when slp3Δ/Δ mutants were grown in the presence of a variety of stress agents. Also, we did not observe a defect in ion accumulation, filamentation, endocytosis, vacuolar structure and function, cell wall structure, or cytoskeletal structure. However, SLP3 over-expression triggered apoptotic-like death following prolonged exposure to oxidative stress or when cells were induced to form hyphae. Our findings reveal the cellular localization of Slp3p, and for the first time associate Slp3p function with the oxidative stress response.

Growth of Candida albicans in human saliva is supported by low-molecular-mass compounds

Saliva plays a key role in the maintenance of a stable oral microflora. It contains antimicrobial compounds but also functions as a substrate for growth of bacteria under conditions of low external nutrient supply. Besides bacteria, yeasts, in particular Candida albicans, commonly inhabit the oral cavity. Under immunocompromised conditions, instantaneous outgrowth of this yeast occurs in oral carriers of C. albicans, suggesting that this yeast is able to survive in the oral cavity with saliva as sole source of growth substrate. The aim of the present study was to identify the salivary constituents that are used by C. albicans for growth and survival in saliva. In addition, we have explored the effect of growth in saliva on the susceptibility of C. albicans to histatin 5, a salivary antifungal peptide. It was found that C. albicans was able to grow in human saliva without addition of glucose, and in the stationary phase could survive for more than 400 h. Candida albicans grown in saliva was more than 10 times less susceptible for salivary histatin 5 than C. albicans cultured in Sabouraud medium.

Candida survival strategies

Only few Candida species, e.g., Candida albicans, Candida glabrata, Candida dubliniensis, and Candida parapsilosis, are successful colonizers of a human host. Under certain circumstances these species can cause infections ranging from superficial to life-threatening disseminated candidiasis. The success of C. albicans, the most prevalent and best studied Candida species, as both commensal and human pathogen depends on its genetic, biochemical, and morphological flexibility which facilitates adaptation to a wide range of host niches. In addition, formation of biofilms provides additional protection from adverse environmental conditions. Furthermore, in many host niches Candida cells coexist with members of the human microbiome. The resulting fungal-bacterial interactions have a major influence on the success of C. albicans as commensal and also influence disease development and outcome. In this chapter, we review the current knowledge of important survival strategies of Candida spp., focusing on fundamental fitness and virulence traits of C. albicans.