Cell-wall dyes interfere with Cryptococcus neoformans melanin deposition

Characterization and Biological Activities of Melanin from the Medicinal Fungi Ophiocordyceps sinensis

Melanin is a complex natural pigment that is widely present in fungi. The mushroom Ophiocordyceps sinensis has a variety of pharmacological effects. The active substances of O. sinensis have been extensively studied, but few studies have focused on the O. sinensis melanin. In this study, the production of melanin was increased by adding light or oxidative stress, namely, reactive oxygen species (ROS) or reactive nitrogen species (RNS), during liquid fermentation. Subsequently, the structure of the purified melanin was characterized using elemental analysis, ultraviolet-visible absorption spectrum, Fourier transform infrared (FTIR), electron paramagnetic resonance (EPR), and pyrolysis gas chromatography and mass spectrometry (Py-GCMS). Studies have shown that O. sinensis melanin is composed of C (50.59), H (6.18), O (33.90), N (8.19), and S (1.20), with maximum absorbance at 237 nm and typical melanin structures such as benzene, indole, and pyrrole. Additionally, the various biological activities of O. sinensis melanin have been discovered; it can chelate heavy metals and shows a strong ultraviolet-blocking ability. Moreover, O. sinensis melanin can reduce the levels of intracellular reactive oxygen species and counteract the oxidative damage of H₂O₂ to cells. These results can help us to develop applications of O. sinensis melanin in radiation resistance, heavy metal pollution remediation, and antioxidant use.

Melanization of Candida auris Is Associated with Alteration of Extracellular pH

Candida auris is a recently emerged global fungal pathogen, which causes life-threatening infections, often in healthcare settings. C. auris infections are worrisome because the fungus is often resistant to multiple antifungal drug classes. Furthermore, C. auris forms durable and difficult to remove biofilms. Due to the relatively recent, resilient, and resistant nature of C. auris, we investigated whether it produces the common fungal virulence factor melanin. Melanin is a black-brown pigment typically produced following enzymatic oxidation of aromatic precursors, which promotes fungal virulence through oxidative stress resistance, mammalian immune response evasion, and antifungal peptide and pharmaceutical inactivation. We found that certain strains of C. auris oxidized L-DOPA and catecholamines into melanin. Melanization occurred extracellularly in a process mediated by alkalinization of the extracellular environment, resulting in granule-like structures that adhere to the fungus' external surface. C. auris had relatively high cell surface hydrophobicity, but there was no correlation between hydrophobicity and melanization. Melanin protected the fungus from oxidative damage, but we did not observe a protective role during infection of macrophages or Galleria mellonella larvae. In summary, C. auris alkalinizes the extracellular medium, which promotes the non-enzymatic oxidation of L-DOPA to melanin that attaches to its surface, thus illustrating a novel mechanism for fungal melanization.

Ethylene Promotes Expression of the Appressorium- and Pathogenicity-Related Genes via GPCR- and MAPK-Dependent Manners in Colletotrichum gloeosporioides

Ethylene (ET) represents a signal that can be sensed by plant pathogenic fungi to accelerate their spore germination and subsequent infection. However, the molecular mechanisms of responses to ET in fungi remain largely unclear. In this study, Colletotrichum gloeosporioides was investigated via transcriptomic analysis to reveal the genes that account for the ET-regulated fungal development and virulence. The results showed that ET promoted genes encoding for fungal melanin biosynthesis enzymes, extracellular hydrolases, and appressorium-associated structure proteins at 4 h after treatment. When the germination lasted until 24 h, ET induced multiple appressoria from every single spore, but downregulated most of the genes. Loss of selected ET responsive genes encoding for scytalone dehydratase (CgSCD1) and cerato-platanin virulence protein (CgCP1) were unable to alter ET sensitivity of C. gloeosporioides in vitro but attenuated the influence of ET on pathogenicity. Knockout of the G-protein-coupled receptors CgGPCR3-1/2 and the MAPK signaling pathway components CgMK1 and CgSte11 resulted in reduced ET sensitivity. Taken together, this study in C. gloeosporioides reports that ET can cause transcription changes in a large set of genes, which are mainly responsible for appressorium development and virulence expression, and these processes are dependent on the GPCR and MAPK pathways.

The infection cushion of Botrytis cinerea: a fungal 'weapon' of plant-biomass destruction

The necrotrophic plant-pathogen fungus Botrytis cinerea produces multicellular appressoria dedicated to plant penetration, named infection cushions (IC). A microarray analysis was performed to identify genes upregulated in mature IC. The expression data were validated by RT-qPCR analysis performed in vitro and in planta, proteomic analysis of the IC secretome and biochemical assays. 1231 upregulated genes and 79 up-accumulated proteins were identified. The data support the secretion of effectors by IC: phytotoxins, ROS, proteases, cutinases, plant cell wall-degrading enzymes and plant cell death-inducing proteins. Parallel upregulation of sugar transport and sugar catabolism-encoding genes would indicate a role of IC in nutrition. The data also reveal a substantial remodelling of the IC cell wall and suggest a role for melanin and chitosan in IC function. Lastly, mutagenesis of two upregulated genes in IC identified secreted fasciclin-like proteins as actors in the pathogenesis of B. cinerea. These results support the role of IC in plant penetration and also introduce other unexpected functions for this fungal organ, in colonization, necrotrophy and nutrition of the pathogen.

Pathogenic Delivery: The Biological Roles of Cryptococcal Extracellular Vesicles

Extracellular vesicles (EVs) are produced by all domains of life. In fungi, these structures were first described in Cryptococcus neoformans and, since then, they were characterized in several pathogenic and non-pathogenic fungal species. Cryptococcal EVs participate in the export of virulence factors that directly impact the Cryptococcus-host interaction. Our knowledge of the biogenesis and pathogenic roles of Cryptococcus EVs is still limited, but recent methodological and scientific advances have improved our understanding of how cryptococcal EVs participate in both physiological and pathogenic events. In this review, we will discuss the importance of cryptococcal EVs, including early historical studies suggesting their existence in Cryptococcus, their putative mechanisms of biogenesis, methods of isolation, and possible roles in the interaction with host cells.

Helping Scholars Overcome Socioeconomic Barriers to Medical and Biomedical Careers: Creating a Pipeline Initiative

Problem: To achieve their potential in medical and biomedical careers, students (scholars) from under-resourced backgrounds must build sophisticated skills and develop confidence and professionalism. To flourish in an advanced educational system that may be unfamiliar, these scholars also need networks of mentors and role models. These challenges can affect scholars at multiple stages of their education. Intervention: To meet these challenges, we created a broad and innovative biomedical research-focused pipeline program: the Johns Hopkins Initiative for Careers in Science in Medicine (CSM Initiative). This initiative targets three levels: high school, undergraduate, and post-baccalaureate/pre-doctoral (graduate and medical). We provide training in essential academic, research, professional, and social skills to meet the unique challenges of our scholars from under-resourced backgrounds. Scholars also build relationships with mentors who provide career guidance and support. We present an overview of the training and assessment at each level of this initiative. Context: The initiative took place at an institution located in the greater Baltimore area and that is endowed with exceptional doctoral and postdoctoral trainees, staff, and faculty including clinicians, physician-scientists, and scientists who served as key role models and mentors. Our pipeline program draws from local high school students and a local and national pool of undergraduates and post-baccalaureates preparing for medical or graduate school. Impact: Our goals for the high school scholars are significant improvement in academic skills, increased confidence, and matriculation into higher education systems. Currently, at least 83% of high school scholars have matriculated into four-year college programs and 73% have chosen science, technology, engineering, math, and medicine (STEMM)-related majors. Among undergraduate participants, 42% have matriculated thus far into medical or biomedical graduate programs and this number is expected to rise as more scholars graduate from college and either enter graduate training or pursue STEMM careers. Another 25% have returned to our post-baccalaureate program. Among post-baccalaureate scholars, 71% have now matriculated into doctoral-level graduate biomedical programs (medical or graduate school) and the remaining 29% are pursuing careers in STEMM-related fields such as biomedical research with some still aiming at graduate-level education. Our long-term goal is to see a large majority of our scholars become successful professionals in medicine, biomedical research, allied healthcare, or other STEMM fields. Analysis of the early phases of the CSM initiative demonstrates such outcomes are attainable. Lessons Learned: This program provides experiences in which scholars develop and practice core competencies essential for developing their self-identity as scientists and professionals. The most important lesson learned is that mentorship teams must be highly dynamic, flexible, thoughtful, and personal in responding to the wide range of challenges and obstacles that scholars from under-resourced backgrounds must overcome to achieve career success.

Unveiling the Properties of Thai Stingless Bee Propolis via Diminishing Cell Wall-Associated Cryptococcal Melanin and Enhancing the Fungicidal Activity of Macrophages

Cryptococcus neoformans, a life-threatening human yeast pathogen, has the ability to produce melanin, which is one of the common virulence factors contributing to cryptococcal pathogenesis. This virulence factor is closely associated with the cryptococcal cell wall, specifically chitin and chitosan polysaccharides, a complex structure that is essential for maintaining cellular structure and integrity. In this study, we aim to investigate the effects of two stingless bee (SLB) propolis from Tetragonula laeviceps and Tetrigona melanoleuca against cell wall-associated melanin in C. neoformans, and its immune response in RAW 264.7 macrophage. The ethanolic extract of SLB propolis (EEP) has strongly exhibited anti-cryptococcal activity. Moreover, EEP from both sources reduced chitin/chitosan and melanin production against C. neoformans in a dose-dependent manner. Likewise, the mRNA expression level of CDA1, IPC1-PKC1 and LAC1 genes involved in the cryptococcal melanization pathway was significantly decreased at 2 mg/mL in EEP treatment. Additionally, pretreatment with EEP prior to yeast infection dramatically reduced intracellular replication of C. neoformans in RAW 264.7 macrophages in a dose-dependent manner. This study might be a new insight to use a natural powerful source, not only acting to target cell wall-associated molecules, but also being capable to explore a novel strategy by which dysregulation of these molecules leads to promote immunomodulatory activity.

Exploring Cryptococcus neoformans capsule structure and assembly with a hydroxylamine-armed fluorescent probe

Chemical biology is an emerging field that enables the study and manipulation of biological systems with probes whose reactivities provide structural insights. The opportunistic fungal pathogen Cryptococcus neoformans possesses a polysaccharide capsule that is a major virulence factor, but is challenging to study. We report here the synthesis of a hydroxylamine-armed fluorescent probe that reacts with reducing glycans and its application to study the architecture of the C. neoformans capsule under a variety of conditions. The probe signal localized intracellularly and at the cell wall-membrane interface, implying the presence of reducing-end glycans at this location where the capsule is attached to the cell body. In contrast, no fluorescence signal was detected in the capsule body. We observed vesicle-like structures containing the reducing-end probe, both intra- and extracellularly, consistent with the importance of vesicles in capsular assembly. Disrupting the capsule with DMSO, ultrasound, or mechanical shear stress resulted in capsule alterations that affected the binding of the probe, as reducing ends were exposed and cell membrane integrity was compromised. Unlike the polysaccharides in the assembled capsule, isolated exopolysaccharides contained reducing ends. The reactivity of the hydroxylamine-armed fluorescent probe suggests a model for capsule assembly whereby reducing ends localize to the cell wall surface, supporting previous findings suggesting that this is an initiation point for capsular assembly. We propose that chemical biology is a promising approach for studying the C. neoformans capsule and its associated polysaccharides to unravel their roles in fungal virulence.

Melanization in Cryptococcus neoformans Requires Complex Regulation

The fungal human pathogen Cryptococcus neoformans undergoes melanization in response to nutrient starvation and exposure to exogenous melanin precursors. Melanization protects the fungus against host defense mechanisms such as oxidative damage and other environmental stressors (e.g., heat/cold stress, antimicrobial compounds, ionizing radiation). Conversely, the melanization process generates cytotoxic intermediates, and melanized cells are potentially susceptible to overheating and to certain melanin-binding drugs. Despite the importance of melanin in C. neoformans biology, the signaling mechanisms regulating its synthesis are poorly understood. The recent report by D. Lee, E.-H. Jang, M. Lee, S.-W. Kim, et al. [mBio 10(5):e02267-19, 2019, https://doi.org/10.1128/mBio.02267-19] provides new insights into how C. neoformans regulates melanization. The authors identified a core melanin regulatory network consisting of transcription factors and kinases required for melanization under low-nutrient conditions. The redundant and epistatic connections of this melanin-regulating network demonstrate that C. neoformans melanization is complex and carefully regulated at multiple levels. Such complex regulation reflects the multiple functions of melanin in C. neoformans biology.

The Role of Melanin in Fungal Pathogenesis for Animal Hosts

Melanins are a class of pigments that are ubiquitous throughout biology. They play incredibly diverse and important roles ranging from radiation protection to immune defense, camouflage, and virulence. Fungi have evolved to use melanin to be able to persist in the environment and within organisms. Fungal melanins are often located within the cell well and are able to neutralize reactive oxygen species and other radicals, defend against UV radiation, bind and sequester non-specific peptides and compounds, and produce a physical barrier that defends the cell. For this reason, melanized fungi are often well-suited to be human pathogens-melanin allows fungi to neutralize the microbicidal oxidative bursts of our innate immune system, bind and inactivate to antimicrobial peptides and enzymes, sequester antifungal pharmaceuticals, and create a shield to block immune recognition of the fungus. Due to the importance and pervasiveness of melanin in fungal virulence, mammalian immune systems have evolved antifungal strategies that involve directly detecting and binding to fungal melanins. Such strategies include the use of melanin-specific antibody responses and C-type lectins like the newly discovered melanin-specific MelLec receptor.

Extracellular vesicles of human pathogenic fungi

Extracellular vesicles play a significant role in many aspects of cellular life including cell-to-cell communication, pathogenesis and cancer progression. However very little is known about their role in fungi and we are just at the beginning of understanding their influence on fungal pathophysiology and host-pathogen interactions. Recent findings have revealed a role for fungal vesicles in triggering anti-microbial activities as well as in modulating virulence strategies, suggesting potential new avenues for antifungal therapies. In this review, we summarize our current understanding of fungal extracellular vesicles, including their biogenesis, secretion and size variation, and discuss how they may influence the human immune response and some key questions that remain unanswered.