Identification of small molecules that disrupt vacuolar function in the pathogen Candida albicans

Drug repurposing for fungal infections

The rise of multidrug-resistant bacteria is a well-recognized threat to world health, necessitating the implementation of effective treatments. This issue has been identified as a top priority on the global agenda by the World Health Organization. Certain strains, such as Candida glabrata, Candida krusei, Candida lusitaniae, Candida auris, select cryptococcal species, and opportunistic Aspergillus or Fusarium species, have significant intrinsic resistance to numerous antifungal medicines. This inherent resistance and subsequent suboptimal clinical outcomes underscore the critical imperative for enhanced therapeutic alternatives and management protocols. The challenge of effectively treating fungal infections, compounded by the protracted timelines involved in developing novel drugs, underscores the pressing need to explore alternative therapeutic avenues. Among these, drug repurposing emerges as a particularly promising and expeditious solution, providing cost-effective solutions and safety benefits. In the fight against life-threatening resistant fungal infections, the idea of repurposing existing medications has encouraged research into both established and new compounds as a last-resort therapy. This chapter seeks to provide a comprehensive overview of contemporary antifungal drugs, as well as their key resistance mechanisms. Additionally, it seeks to provide insight into the antimicrobial properties of non-traditional drugs, thereby offering a holistic perspective on the evolving landscape of antifungal therapeutics.

A mechanism of lysosomal calcium entry

Lysosomal calcium (Ca2+) release is critical to cell signaling and is mediated by well-known lysosomal Ca2+ channels. Yet, how lysosomes refill their Ca2+ remains hitherto undescribed. Here, from an RNA interference screen in Caenorhabditis elegans, we identify an evolutionarily conserved gene, lci-1, that facilitates lysosomal Ca2+ entry in C. elegans and mammalian cells. We found that its human homolog TMEM165, previously designated as a Ca2+/H+ exchanger, imports Ca2+ pH dependently into lysosomes. Using two-ion mapping and electrophysiology, we show that TMEM165, hereafter referred to as human LCI, acts as a proton-activated, lysosomal Ca2+ importer. Defects in lysosomal Ca2+ channels cause several neurodegenerative diseases, and knowledge of lysosomal Ca2+ importers may provide previously unidentified avenues to explore the physiology of Ca2+ channels.

Can nonsteroidal anti-inflammatory drugs (NSAIDs) be repurposed for fungal infection?

Nonsteroidal anti-inflammatory drugs (NSAIDs) are an important class of anti-inflammatory drugs widely used for the treatment of musculoskeletal disorders, mild-to-moderate pain, and fever. This review aimed to explain the functional role and possible mechanisms of the antifungal effects of NSAIDs alone or in combination with antifungal drugs in vitro and in vivo. Several studies reported that NSAIDs such as aspirin, ibuprofen, diclofenac, indomethacin, ketorolac, celecoxib, flurbiprofen, and nimesulide had antifungal activities in vitro, either fungistatic or fungicidal, against different strains of Candida, Aspergillus, Cryptococcus, Microsporum, and Trichophyton species. These drugs inhibited biofilm adhesion and development, and yeast-to-hypha conversion which may be related to a prostaglandin E2 (PGE2)/PGEx-dependent mechanism. Modulating PGE2 levels by NSAIDs during fungal infection can be introduced as a possible mechanism to overcome. In addition, some important mechanisms of the antifungal activities of NSAIDs and their new derivatives on fungi and host immune responses are summarized. Overall, we believe that using NSAIDs along with classical antifungal drugs has the potential to be investigated as a novel therapeutic strategy in clinical studies. Furthermore, combination therapy can help manage resistant strains, increase the efficacy of antifungal drugs, and reduce toxicity.

Weapons against Themselves: Identification and Use of Quorum Sensing Volatile Molecules to Control Plant Pathogenic Fungi Growth

Quorum sensing (QS) is often defined as a mechanism of microbial communication that can regulate microbial behaviors in accordance with population density. Much is known about QS mechanisms in bacteria, but fungal QS research is still in its infancy. In this study, the molecules constituting the volatolomes of the plant pathogenic fungi Fusarium culmorum and Cochliobolus sativus have been identified during culture conditions involving low and high spore concentrations, with the high concentration imitating overpopulation conditions (for QS stimulation). We determined that volatolomes emitted by these species in conditions of overpopulation have a negative impact on their mycelial growth, with some of the emitted molecules possibly acting as QSM. Candidate VOCs related to QS have then been identified by testing the effect of individual volatile organic compounds (VOCs) on mycelial growth of their emitting species. The antifungal effect observed for the volatolome of F. culmorum in the overpopulation condition could be attributed to ethyl acetate, 2-methylpropan-1-ol, 3-methylbutyl ethanoate, 3-methylbutan-1-ol, and pentan-1-ol, while it could be attributed to longifolene, 3-methylbutan-1-ol, 2-methylpropan-1-ol, and ethyl acetate for C. sativus in the overpopulation condition. This work could pave the way to a sustainable alternative to chemical fungicides.

Small molecule based anti-virulence approaches against Candida albicans infections

Fungi are considered "silent killers" due to the difficulty of, and delays in diagnosis of infections and lack of effective antifungals. This challenge is compounded by the fact that being eukaryotes, fungi share several similarities with human cellular targets, creating obstacles to drug discovery. Candida albicans, a ubiquitous microbe in the human body is well-known for its role as an opportunistic pathogen in immunosuppressed people. Significantly, C. albicans is resistant to all the three classes of antifungals that are currently clinically available. Over the past few years, a paradigm shift has been recommended in the management of C. albicans infections, wherein anti-virulence strategies are considered an alternative to the discovery of new antimycotics. Small molecules, with a molecular weight <900 Daltons, can easily permeate the cell membrane and modulate the signal transduction pathways to elicit desired virulence inhibitory actions against pathogens. This review dissects in-depth, the discoveries that have been made with small-molecule anti-virulence approaches to tackle C. albicans infections.

Cdc42 regulates reactive oxygen species production in the pathogenic yeast Candida albicans

Across eukaryotes, Rho GTPases such as Rac and Cdc42 play important roles in establishing cell polarity, which is a key feature of cell growth. In mammals and filamentous fungi, Rac targets large protein complexes containing NADPH oxidases (NOX) that produce reactive oxygen species (ROS). In comparison, Rho GTPases of unicellular eukaryotes were believed to signal cell polarity without ROS, and it was unclear whether Rho GTPases were required for ROS production in these organisms. We document here the first example of Rho GTPase-mediated post-transcriptional control of ROS in a unicellular microbe. Specifically, Cdc42 is required for ROS production by the NOX Fre8 of the opportunistic fungal pathogen Candida albicans. During morphogenesis to a hyphal form, a filamentous growth state, C. albicans FRE8 mRNA is induced, which leads to a burst in ROS. Fre8-ROS is also induced during morphogenesis when FRE8 is driven by an ectopic promoter; hence, Fre8 ROS production is in addition controlled at the post-transcriptional level. Using fluorescently tagged Fre8, we observe that the majority of the protein is associated with the vacuolar system. Interestingly, much of Fre8 in the vacuolar system appears inactive, and Fre8-induced ROS is only produced at sites near the hyphal tip, where Cdc42 is also localized during morphogenesis. We observe that Cdc42 is necessary to activate Fre8-mediated ROS production during morphogenesis. Cdc42 regulation of Fre8 occurs without the large NOX protein complexes typical of higher eukaryotes and therefore represents a novel form of ROS control by Rho GTPases.

Drug Repurposing in Medical Mycology: Identification of Compounds as Potential Antifungals to Overcome the Emergence of Multidrug-Resistant Fungi

Immunodepression, whether due to HIV infection or organ transplantation, has increased human vulnerability to fungal infections. These conditions have created an optimal environment for the emergence of opportunistic infections, which is concomitant to the increase in antifungal resistance. The use of conventional antifungal drugs as azoles and polyenes can lead to clinical failure, particularly in immunocompromised individuals. Difficulties related to treating fungal infections combined with the time required to develop new drugs, require urgent consideration of other therapeutic alternatives. Drug repurposing is one of the most promising and rapid solutions that the scientific and medical community can turn to, with low costs and safety advantages. To treat life-threatening resistant fungal infections, drug repurposing has led to the consideration of well-known and potential molecules as a last-line therapy. The aim of this review is to provide a summary of current antifungal compounds and their main resistance mechanisms, following by an overview of the antifungal activity of non-traditional antimicrobial drugs. We provide their eventual mechanisms of action and the synergistic combinations that improve the activity of current antifungal treatments. Finally, we discuss drug repurposing for the main emerging multidrug resistant (MDR) fungus, including the Candida auris, Aspergillus or Cryptococcus species.

Candida albicans, a reservoir of Listeria monocytogenes?

Listeria monocytogenes is a pathogen causing serious or mortal infections in human risk populations. Its infectivity is in part due to its ability to infect diverse eukaryotic cells. Since several bacteria can enter into yeast cells, including Candida albicans, the aims of this work were to evaluate if L. monocytogenes was able to harbor, retaining its viability, within C. albicans cells and to evaluate the effect of temperature and an antibiotic as stressing factors in its rate of entry into yeast cells. Both microorganisms were co-incubated in BHI broth during 48 h and the entry of bacteria into yeast cells was evaluated at different times. Then, yeasts free of extracellular bacteria were obtained seeding samples of the co-culture on YGC agar, which contains chloramphenicol, to obtain extracellular bacteria-free yeasts. These extracellular bacteria free yeasts were used to search for bacterial DNA in total yeast DNA and to evaluate the viability of intra-yeast bacteria. Finally, the effect of temperature and of chloramphenicol as inducers of stress on the rate of bacterial entry into yeast cells were investigated. After co-culturing both microorganisms, wet mount optical microscopy showed the presence of moving bacteria within yeasts and transmission electron microscopy confirmed the presence of intra-yeast bacteria. PCR allowed to amplify L. monocytogenes iap gene in C. albicans total DNA obtained from yeasts free of extracellular bacteria. Moreover, the SYTO 9 green fluorescence observed in bacterial cells within vacuoles of yeasts suggests that intra-yeast bacteria remain viable. Furthermore, the entry of L. monocytogenes into yeasts cells was favored by the presence of stressing factors (chloramphenicol and temperature). Therefore, yeasts may be reservoirs of viable L. monocytogenes and might spread them to the following generations of yeasts.

In Vitro Incorporation of Helicobacter pylori into Candida albicans Caused by Acidic pH Stress

Yeasts can adapt to a wide range of pH fluctuations (2 to 10), while Helicobacter pylori, a facultative intracellular bacterium, can adapt to a range from pH 6 to 8. This work analyzed if H. pylori J99 can protect itself from acidic pH by entering into Candida albicans ATCC 90028. Growth curves were determined for H. pylori and C. albicans at pH 3, 4, and 7. Both microorganisms were co-incubated at the same pH values, and the presence of intra-yeast bacteria was evaluated. Intra-yeast bacteria-like bodies were detected using wet mounting, and intra-yeast binding of anti-H. pylori antibodies was detected using immunofluorescence. The presence of the H. pylori rDNA 16S gene in total DNA from yeasts was demonstrated after PCR amplification. H. pylori showed larger death percentages at pH 3 and 4 than at pH 7. On the contrary, the viability of the yeast was not affected by any of the pHs evaluated. H. pylori entered into C. albicans at all the pH values assayed but to a greater extent at unfavorable pH values (pH 3 or 4, p = 0.014 and p = 0.001, respectively). In conclusion, it is possible to suggest that H. pylori can shelter itself within C. albicans under unfavorable pH conditions.

Antifungal effects of statins

Fungal infections are estimated to be responsible for 1.5 million deaths annually. Global anti-microbial resistance is also observed for fungal pathogens, and scientists are looking for new antifungal agents to address this challenge. One potential strategy is to evaluate currently available drugs for their possible antifungal activity. One of the suggested drug classes are statins, which are commonly used to decrease plasma cholesterol and reduce cardiovascular risk associated with low density lipoprotein cholesterol (LDL-c). Statins are postulated to possess pleiotropic effects beyond cholesterol lowering; improving endothelial function, modulating inflammation, and potentially exerting anti-microbial effects. In this study, we reviewed in-vitro and in-vivo studies, as well as clinical reports pertaining to the antifungal efficacy of statins. In addition, we have addressed various modulators of statin anti-fungal activity and the potential mechanisms responsible for their anti-fungal effects. In general, statins do possess anti-fungal activity, targeting a broad spectrum of fungal organisms including human opportunistic pathogens such as Candida spp. and Zygomycetes, Dermatophytes, alimentary toxigenic species such as Aspergillus spp., and fungi found in device implants such as Saccharomyces cerevisiae. Statins have been shown to augment a number of antifungal drug classes, for example, the azoles and polyenes. Synthetic statins are generally considered more potent than the first generation of fungal metabolites. Fluvastatin is considered the most effective statin with the broadest and most potent fungal inhibitory activity, including fungicidal and/or fungistatic properties. This has been demonstrated with plasma concentrations that can easily be achieved in a clinical setting. Additionally, statins can potentiate the efficacy of available antifungal drugs in a synergistic fashion. Although only a limited number of animal and human studies have been reported to date, observational cohort studies have confirmed that patients using statins have a reduced risk of candidemia-related complications. Further studies are warranted to confirm our findings and expand current knowledge of the anti-fungal effects of statins.

Repurposing of Ribavirin as an Adjunct Therapy against Invasive Candida Strains in an In Vitro Study

The use of antifungal agents in clinical settings is limited by the appearance of drug resistance and adverse side effects. There is, therefore, an urgent need to develop new drugs to strengthen the treatment of invasive fungal diseases. The aim of this study is to describe the potential repurposing of ribavirin as an adjunct therapy against Candida spp. Primary screening of a Prestwick Chemical library against Candida albicans ATCC 90028 and fluconazole-resistant Candida albicans strains was performed. Subsequently, we evaluated the responses of 100 Candida sp. strains to ribavirin, an antiviral agent, using the broth microdilution method as recommended by CLSI. We checked the involvement of efflux pump activity in the development of ribavirin resistance. We studied time-kill curves and performed a checkerboard assay for a ribavirin-antifungal combination study. Twenty-one nonstandard antifungal compounds were identified, including ribavirin. Ribavirin had antifungal activity in vitro against 63 Candida strains, including strains of C. albicans, C. parapsilosis, and C. tropicalis, with MICs ranging from 0.37 to 3.02 μg/ml, while MICs for C. krusei, C. glabrata, C. lusitaniae, and some C. albicans strains remained high (≥24.16 μg/ml). No relation was observed between efflux pump activity and ribavirin resistance. Ribavirin exhibited fungistatic activity against multidrug-resistant (MDR) C. albicans and fungicidal activity against a C. parapsilosis strain. In addition, ribavirin acted synergistically with azoles against Candida strains for which ribavirin MICs were <24.4 μg/ml. This study highlights the potential clinical application of ribavirin, alone or in association with other antifungal agents, as an adjunct anti-Candida drug.

In Vivo Indicators of Cytoplasmic, Vacuolar, and Extracellular pH Using pHluorin2 in Candida albicans

Candida albicans is an opportunistic fungal pathogen that colonizes the reproductive and gastrointestinal tracts of its human host. It can also invade the bloodstream and deeper organs of immunosuppressed individuals, and thus it encounters enormous variations in external pH in vivo. Accordingly, survival within such diverse niches necessitates robust adaptive responses to regulate intracellular pH. However, the impact of antifungal drugs upon these adaptive responses, and on intracellular pH in general, is not well characterized. Furthermore, the tools and methods currently available to directly monitor intracellular pH in C. albicans, as well as other fungal pathogens, have significant limitations. To address these issues, we developed a new and improved set of pH sensors based on the pH-responsive fluorescent protein pHluorin. This includes a cytoplasmic sensor, a probe that localizes inside the fungal vacuole (an acidified compartment that plays a central role in intracellular pH homeostasis), and a cell surface probe that can detect changes in extracellular pH. These tools can be used to monitor pH within single C. albicans cells or in cell populations in real time through convenient and high-throughput assays.

Environmental or chemically induced stresses often trigger physiological responses that regulate intracellular pH. As such, the capacity to detect pH changes in real time and within live cells is of fundamental importance to essentially all aspects of biology. In this respect, pHluorin, a pH-sensitive variant of green fluorescent protein, has provided an invaluable tool to detect such responses. Here, we report the adaptation of pHluorin2 (PHL2), a substantially brighter variant of pHluorin, for use with the human fungal pathogen Candida albicans. As well as a cytoplasmic PHL2 indicator, we describe a version that specifically localizes within the fungal vacuole, an acidified subcellular compartment with important functions in nutrient storage and pH homeostasis. In addition, by means of a glycophosphatidylinositol-anchored PHL2-fusion protein, we generated a cell surface pH sensor. We demonstrated the utility of these tools in several applications, including accurate intracellular and extracellular pH measurements in individual cells via flow cytometry and in cell populations via a convenient plate reader-based protocol. The PHL2 tools can also be used for endpoint as well as time course experiments and to conduct chemical screens to identify drugs that alter normal pH homeostasis. These tools enable observation of the highly dynamic intracellular pH shifts that occur throughout the fungal growth cycle, as well as in response to various chemical treatments.

IMPORTANCECandida albicans is an opportunistic fungal pathogen that colonizes the reproductive and gastrointestinal tracts of its human host. It can also invade the bloodstream and deeper organs of immunosuppressed individuals, and thus it encounters enormous variations in external pH in vivo. Accordingly, survival within such diverse niches necessitates robust adaptive responses to regulate intracellular pH. However, the impact of antifungal drugs upon these adaptive responses, and on intracellular pH in general, is not well characterized. Furthermore, the tools and methods currently available to directly monitor intracellular pH in C. albicans, as well as other fungal pathogens, have significant limitations. To address these issues, we developed a new and improved set of pH sensors based on the pH-responsive fluorescent protein pHluorin. This includes a cytoplasmic sensor, a probe that localizes inside the fungal vacuole (an acidified compartment that plays a central role in intracellular pH homeostasis), and a cell surface probe that can detect changes in extracellular pH. These tools can be used to monitor pH within single C. albicans cells or in cell populations in real time through convenient and high-throughput assays.