Invasion of Cryptococcus neoformans into human brain microvascular endothelial cells is mediated through the lipid rafts-endocytic pathway via the dual specificity tyrosine phosphorylation-regulated kinase 3 (DYRK3)

Mechanisms and Virulence Factors of Cryptococcus neoformans Dissemination to the Central Nervous System

Cryptococcosis is a prevalent fungal infection of the central nervous system (CNS) caused by Cryptococcus neoformans, a yeast with a polysaccharide capsule in the basidiomycete group. Normally, C. neoformans infects the respiratory tract and then breaches the blood-brain barrier (BBB), leading to meningitis or meningoencephalitis, which leads to hundreds of thousands of deaths each year. Although the mechanism by which C. neoformans infiltrates the BBB to invade the brain has yet to be fully understood, research has revealed that C. neoformans can cross the BBB using transcellular penetration, paracellular traversal, and infected phagocytes (the "Trojan horse" mechanism). The secretion of multiple virulence factors by C. neoformans is crucial in facilitating the spread of infection after breaching the BBB and causing brain infections. Extensive research has shown that various virulence factors play a significant role in the dissemination of infection beyond the lungs. This review explores the mechanisms of C. neoformans entering the CNS and explains how it bypasses the BBB. Additionally, it aims to understand the interplay between the regulatory mechanisms and virulence factors of C. neoformans.

The pathways and the mechanisms by which Cryptococcus enters the brain

Generally, Cryptococcus initially infects the respiratory tract, but can spread, eventually crossing the blood-brain barrier (BBB) and causing meningitis or meningoencephalitis. Specifically, Cryptococcus invades the vascular endothelial cells of the BBB, from which it enters the brain. The main mechanisms through which Cryptococcus crosses the BBB are transcellular traversal, the paracellular pathway, and via Trojan horse. In this paper, the mechanisms by which Cryptococcus crosses the BBB were explained in detail. In addition to pathways of entry to the brain, this paper presents a discussion on some rare cryptococcal infections and provides some insights for future research directions.

Macropinocytosis as a potential mechanism driving neurotropism of Cryptococcus neoformans

Cryptococcus neoformans can invade the central nervous system by crossing the blood-brain barrier via a transcellular mechanism that relies on multiple host factors. In this narrative, we review the evidence that a direct interplay between C. neoformans and brain endothelial cells forms the basis for invasion and transmigration across the brain endothelium. Adherence and internalization of C. neoformans is dependent on transmembrane proteins, including a hyaluronic acid receptor and an ephrin receptor tyrosine kinase. We consider the role of EphA2 in facilitating the invasion of the central nervous system by C. neoformans and highlight experimental evidence supporting macropinocytosis as a potential mechanism of internalization and transcytosis. How macropinocytosis might be conclusively demonstrated in the context of C. neoformans is also discussed.

A dissemination-prone morphotype enhances extrapulmonary organ entry by Cryptococcus neoformans

Environmental pathogens move from ecological niches to mammalian hosts, requiring adaptation to dramatically different environments. Microbes that disseminate farther, including the fungal meningitis pathogen Cryptococcus neoformans, require additional adaptation to diverse tissues. We demonstrate that the formation of a small C. neoformans morphotype-called "seed" cells due to their colonizing ability-is critical for extrapulmonary organ entry. Seed cells exhibit changes in fungal cell size and surface expression that result in an enhanced macrophage update. Seed cell formation is triggered by environmental factors, including C. neoformans' environmental niche, and pigeon guano with phosphate plays a central role. Seed cells show the enhanced expression of phosphate acquisition genes, and mutants unable to acquire phosphate fail to adopt the seed cell morphotype. Additionally, phosphate can be released by tissue damage, potentially establishing a feed-forward loop of seed cell formation and dissemination. Thus, C. neoformans' size variation represent inducible morphotypes that change host interactions to facilitate microbe spread.

Host cell membrane microdomains and fungal infection

Lipid microdomains or lipid rafts are dynamic and tightly ordered regions of the plasma membrane. In mammalian cells, they are enriched in cholesterol, glycosphingolipids, Glycosylphosphatidylinositol-anchored and signalling-related proteins. Several studies have suggested that mammalian pattern recognition receptors are concentrated or recruited to lipid domains during host-pathogen association to enhance the effectiveness of host effector processes. However, pathogens have also evolved strategies to exploit these domains to invade cells and survive. In fungal organisms, a complex cell wall network usually mediates the first contact with the host cells. This cell wall may contain virulence factors that interfere with the host membrane microdomains dynamics, potentially impacting the infection outcome. Indeed, the microdomain disruption can dampen fungus-host cell adhesion, phagocytosis and cellular immune responses. Here, we provide an overview of regulatory strategies employed by pathogenic fungi to engage with and potentially subvert the lipid microdomains of host cells. TAKE AWAY: Lipid microdomains are ordered regions of the plasma membrane enriched in cholesterol, glycosphingolipids (GSL), GPI-anchored and signalling-related proteins. Pathogen recognition by host immune cells can involve lipid microdomain participation. During this process, these domains can coalesce in larger complexes recruiting receptors and signalling proteins, significantly increasing their signalling abilities. The antifungal innate immune response is mediated by the engagement of pathogen-associated molecular patterns to pattern recognition receptors (PRRs) at the plasma membrane of innate immune cells. Lipid microdomains can concentrate or recruit PRRs during host cell-fungi association through a multi-interactive mechanism. This association can enhance the effectiveness of host effector processes. However, virulence factors at the fungal cell surface and extracellular vesicles can re-assembly these domains, compromising the downstream signalling and favouring the disease development. Lipid microdomains are therefore very attractive targets for novel drugs to combat fungal infections.

Transcriptomic analysis of human brain microvascular endothelial cells exposed to laminin binding protein (adhesion lipoprotein) and Streptococcus pneumoniae

Streptococcus pneumoniae invades the CNS and triggers a strong cellular response. To date, signaling events that occur in the human brain microvascular endothelial cells (hBMECs), in response to pneumococci or its surface adhesins are not mapped comprehensively. We evaluated the response of hBMECs to the adhesion lipoprotein (a laminin binding protein—Lbp) or live pneumococci. Lbp is a surface adhesin recently identified as a potential ligand, which binds to the hBMECs. Transcriptomic analysis was performed by RNA-seq of three independent biological replicates and validated with qRT-PCR using 11 genes. In total 350 differentially expressed genes (DEGs) were identified after infection with S. pneumoniae, whereas 443 DEGs when challenged with Lbp. Total 231 DEGs were common in both treatments. Integrative functional analysis revealed participation of DEGs in cytokine, chemokine, TNF signaling pathways and phagosome formation. Moreover, Lbp induced cell senescence and breakdown, and remodeling of ECM. This is the first report which maps complete picture of cell signaling events in the hBMECs triggered against S. pneumoniae and Lbp. The data obtained here could contribute in a better understanding of the invasion of pneumococci across BBB and underscores role of Lbp adhesin in evoking the gene expression in neurovascular unit.

Quantitative analysis reveals internalisation of Cryptococcus neoformans by brain endothelial cells in vivo

Migration of Cryptococcus neoformans from the blood to the brain parenchyma is crucial to cause fatal meningoencephalitis. Although mechanisms involved in brain migration of C. neoformans have been widely studied in vitro, less is known about how the fungus crosses the blood-brain barrier (BBB) in vivo. This is in part because of the lack of an approach to quantitatively analyse the dynamics of fungal transmigration into the brain across the BBB in vivo. In this study, we report a novel approach to quantitatively analyse the interactions between C. neoformans and brain endothelial cells in a mouse model using flow cytometry. Using this system, we show that C. neoformans was internalised by brain endothelial cells in vivo and that mice infected with acapsular or heat-killed C. neoformans yeast cells displayed a lower frequency of brain endothelial cells containing the yeast cell compared to mice infected with wild-type or viable yeast cells, respectively. We further demonstrate that brain endothelial cells were invaded by serotype A strain (H99 strain) at a higher rate compared to serotype D strain (52D strain). Our experiments established that internalisation of C. neoformans by brain endothelial cells occurred in vivo and offered a powerful approach to quantitatively analyse fungal migration into the brain.

Mechanisms of fungal dissemination

Fungal infections are an increasing threat to global public health. There are more than six million fungal species worldwide, but less than 1% are known to infect humans. Most of these fungal infections are superficial, affecting the hair, skin and nails, but some species are capable of causing life-threatening diseases. The most common of these include Cryptococcus neoformans, Aspergillus fumigatus and Candida albicans. These fungi are typically innocuous and even constitute a part of the human microbiome, but if these pathogens disseminate throughout the body, they can cause fatal infections which account for more than one million deaths worldwide each year. Thus, systemic dissemination of fungi is a critical step in the development of these deadly infections. In this review, we discuss our current understanding of how fungi disseminate from the initial infection sites to the bloodstream, how immune cells eliminate fungi from circulation and how fungi leave the blood and enter distant organs, highlighting some recent advances and offering some perspectives on future directions.

The Transcription Factor Pdr802 Regulates Titan Cell Formation and Pathogenicity of Cryptococcus neoformans

The pathogenic yeast Cryptococcus neoformans presents a worldwide threat to human health, especially in the context of immunocompromise, and current antifungal therapy is hindered by cost, limited availability, and inadequate efficacy. After the infectious particle is inhaled, C. neoformans initiates a complex transcriptional program that integrates cellular responses and enables adaptation to the host lung environment.

Cryptococcus neoformans is a ubiquitous, opportunistic fungal pathogen that kills almost 200,000 people worldwide each year. It is acquired when mammalian hosts inhale the infectious propagules; these are deposited in the lung and, in the context of immunocompromise, may disseminate to the brain and cause lethal meningoencephalitis. Once inside the host, C. neoformans undergoes a variety of adaptive processes, including secretion of virulence factors, expansion of a polysaccharide capsule that impedes phagocytosis, and the production of giant (Titan) cells. The transcription factor Pdr802 is one regulator of these responses to the host environment. Expression of the corresponding gene is highly induced under host-like conditions in vitro and is critical for C. neoformans dissemination and virulence in a mouse model of infection. Direct targets of Pdr802 include the quorum sensing proteins Pqp1, Opt1, and Liv3; the transcription factors Stb4, Zfc3, and Bzp4, which regulate cryptococcal brain infectivity and capsule thickness; the calcineurin targets Had1 and Crz1, important for cell wall remodeling and C. neoformans virulence; and additional genes related to resistance to host temperature and oxidative stress, and to urease activity. Notably, cryptococci engineered to lack Pdr802 showed a dramatic increase in Titan cells, which are not phagocytosed and have diminished ability to directly cross biological barriers. This explains the limited dissemination of pdr802 mutant cells to the central nervous system and the consequently reduced virulence of this strain. The role of Pdr802 as a negative regulator of Titan cell formation is thus critical for cryptococcal pathogenicity.

An Antivirulence Approach for Preventing Cryptococcus neoformans from Crossing the Blood-Brain Barrier via Novel Natural Product Inhibitors of a Fungal Metalloprotease

Fungal infections like cryptococcal meningitis are difficult to resolve because of the limited therapies available. The small arsenal of antifungal drugs reflect the difficulty in finding available targets in fungi because like mammalian cells, fungi are eukaryotes. The limited efficacy, toxicity, and rising resistance of antifungals contribute to the high morbidity and mortality of fungal infections and further underscore the dire but unmet need for new antifungal drugs. The traditional approach in antifungal drug development has been to target fungal growth, but an attractive alternative is to target mechanisms of pathogenesis. An important attribute of Cryptococcus neoformans (Cn) pathogenesis is its ability to enter the central nervous system. Here, we describe a large-scale screen that identified three natural products that prevented Cn from crossing the blood-brain barrier by inhibiting the virulence factor Mpr1 without affecting the growth of Cn. We propose that compounds identified here could be further developed as antivirulence therapy that would be administered preemptively or serve as a prophylactic in patients at high risk for developing cryptococcal meningitis.

Cryptococcus neoformans (Cn) is the leading cause of fungal meningitis, a deadly disease with limited therapeutic options. Dissemination to the central nervous system hinges on the ability of Cn to breach the blood-brain barrier (BBB) and is considered an attribute of Cn virulence. Targeting virulence instead of growth for antifungal drug development has not been fully exploited despite the benefits of this approach. Mpr1 is a secreted fungal metalloprotease not required for fungal growth, but rather, it functions as a virulence factor by facilitating Cn migration across the BBB. This central role for Mpr1, its extracellular location, and lack of expression in mammalian cells make Mpr1 a high-value target for an antivirulence approach aimed at developing therapeutics for cryptococcal meningitis. To test this notion, we devised a large-scale screen to identify compounds that prohibited Cn from crossing the BBB by selectively blocking Mpr1 proteolytic activity, without inhibiting the growth of Cn. A phytochemical natural product-derived library was screened to identify new molecular scaffolds of prototypes unique to a Cn microecosystem. Of the 240 pure natural products examined, 3 lead compounds, abietic acid, diosgenin, and lupinine inhibited Mpr1 proteolytic activity with 50% inhibitory concentration (IC₅₀) values of <10 μM, displayed little to no mammalian cell toxicity, and did not affect Cn growth. Notably, the lead compounds blocked Cn from crossing the BBB, without damaging the barrier integrity, suggesting the bioactive molecules had no off-target effects. We propose that these new drug scaffolds are promising candidates for the development of antivirulence therapy against cryptococcal meningitis.

VCAM1/VLA4 interaction mediates Ly6Clow monocyte recruitment to the brain in a TNFR signaling dependent manner during fungal infection

Monocytes exist in two major populations, termed Ly6C^hi and Ly6C^low monocytes. Compared to Ly6C^hi monocytes, less is known about Ly6C^low monocyte recruitment and mechanisms involved in the recruitment of this subset. Furthermore, the role of Ly6C^low monocytes during infections is largely unknown. Here, using intravital microscopy, we demonstrate that Ly6C^low monocytes are predominantly recruited to the brain vasculature following intravenous infection with Cryptococcus neoformans, a fungal pathogen causing meningoencephalitis. The recruitment depends primarily on the interaction of VCAM1 expressed on the brain endothelium with VLA4 expressed on Ly6C^low monocytes. Furthermore, TNFR signaling is essential for the recruitment through enhancing VLA4 expression on Ly6C^low monocytes. Interestingly, the recruited Ly6C^low monocytes internalized C. neoformans and carried the organism while crawling on and adhering to the luminal wall of brain vasculature and migrating to the brain parenchyma. Our study reveals a substantial recruitment of Ly6C^low monocytes to the brain and highlights important properties of this subset during infection.

Monocytes are white blood cells, circulating in the bloodstream and playing important roles during infections. There are two subsets of monocytes in mice: Ly6C^hi and Ly6C^low monocytes. In contrast to the recruitment of Ly6C^hi monocytes shown in other infection models, we observed the predominant recruitment of Ly6C^low monocytes to the brain post-capillary venules during intravenous infection with C. neoformans, a fungal pathogen causing brain infection. The recruitment is mainly mediated by the interaction of VCAM1 and VLA4, which are expressed on the brain endothelium and monocytes, respectively. We further demonstrate that TNFR signaling plays an essential role during Ly6C^low monocyte recruitment through enhancing VLA4 expression on monocytes. We also observed that Ly6C^low monocytes internalize C. neoformans and, together with the ingested organism, crawl along the luminal wall of brain vasculatures and migrate to the brain parenchyma. Thus, VCAM1/VLA4 interaction mediates Ly6C^low monocyte recruitment to the brain in a TNFR signaling dependent manner during fungal infection.

In vitro expression of vital virulent genes of clinical and environmental isolates of Cryptococcus neoformans/gattii in endothelial cells of human blood-brain barrier

BACKGROUND

Evaluation of the pathogenesis of clinical and environmental cryptococcal isolates to the central nervous system is necessary for understanding the risk. This study was designed to determine the in vitro expression of six important virulent genes of Cryptococcus neoformans/gattii in Human Brain Microvascular Endothelial cells (hBMEC).

METHODS

The hBMEC were infected with Cryptococcus to determine invasion and survival rate at 3, 12 and 24hours by subsequent colony count of internalized yeasts. The whole RNA of the intracellular Cryptococcus was extracted to quantify the expression of CAP10, PLB1, ENA1, URE1, LAC1, and MATα genes by real-time quantitative PCR for 3 and 12hours of infection.

RESULTS

Invasion and survival rates were higher in clinical and standard strains of C. neoformans. A significant difference was observed among the clinical and environmental isolates for the expression of CAP10, ENA1, LAC1, MATα and URE1 at 3hours, and ENA1, LAC1, MATα, PLB1 and URE1 at 12hours. Clinical isolates showed significant upregulation of all the genes except PLB1, which was higher in environmental isolates. Relative expressions at the two time-points showed statistically significant (P=0.043) changes for the clinical isolates and no significance (P=0.063) for environmental isolates.

CONCLUSION

The C. gattii (VGI) isolates showed significantly lower invasion and survival than C. neoformans (VNI, and VNII) irrespective of their sources. Clinical isolates exhibited higher expression for the majority of the virulent genes until 12hours of infection, probably due to their better adaptation in the host system and enhanced pathogenicity than the environmental counterparts.

Endocytic Markers Associated with the Internalization and Processing of Aspergillus fumigatus Conidia by BEAS-2B Cells

Conidia from the fungus Aspergillus fumigatus are notorious for their ability to stay airborne. This characteristic is believed to allow conidia to penetrate into the cleanest environments. Several hundred conidia are thought to be inhaled each day by a given individual and then expelled by mucociliary clearance. Given that airway epithelial cells make up a significant portion of the pulmonary-air interface, we set out to determine the percentage of conidia that are actually internalized after initial contact with airway epithelial cells. We determined this through an in vitro assay using an immortalized bronchial airway epithelial cell line known as BEAS-2B. Our results suggest a small fraction of conidia are internalized by BEAS-2B cells, while the majority stay adherent to the surface of cells or are washed away during sample processing. Internalization of conidia was observed at 6 h postchallenge and not prior. Our data also indicate conidia are rendered metabolically inactive within 3 h of challenge, suggesting BEAS-2B cells process a large number of conidia without internalization in this early time frame. We have also identified several host endocytosis markers that localize around internalized conidia as well as contribute to the processing of conidia. Understanding how these host endocytosis markers affect the processing of internal and/or external conidia may provide a novel avenue for therapeutic development.

Aspergillus fumigatus is a ubiquitous mold that produces small airborne conidia capable of traversing deep into the respiratory system. Recognition, processing, and clearance of A. fumigatus conidia by bronchial airway epithelial cells are thought to be relevant to host defense and immune signaling. Using z-stack confocal microscopy, we observed that only 10 to 20% of adherent conidia from the AF293 clinical isolate are internalized by BEAS-2B cells 6 h postchallenge and not prior. Similar percentages of internalization were observed for the CEA10 clinical isolate. A large subset of both AF293 and CEA10 conidia are rendered metabolically inactive without internalization at 3 h postchallenge by BEAS-2B cells. A significantly larger percentage of CEA10 conidia are metabolically active at 9 and 12 h postchallenge in comparison to the AF293 isolate, demonstrating heterogeneity among clinical isolates. We identified 7 host markers (caveolin, flotillin-2, RAB5C, RAB8B, RAB7A, 2xFYVE, and FAPP1) that consistently localized around internalized conidia 9 h postchallenge. Transient gene silencing of RAB5C, PIK3C3, and flotillin-2 resulted in a larger population of metabolically active conidia. Our findings emphasize the abundance of both host phosphatidylinositol 3-phosphate (PI3P) and PI4P around internalized conidia, as well as the importance of class III PI3P kinase for conidial processing. Therapeutic development focused on RAB5C-, PIK3C3-, and flotillin-2-mediated pathways may provide novel opportunities to modulate conidial processing and internalization. Determination of how contacted, external conidia are processed by airway epithelial cells may also provide a novel avenue to generate host-targeted therapeutics.

IMPORTANCE Conidia from the fungus Aspergillus fumigatus are notorious for their ability to stay airborne. This characteristic is believed to allow conidia to penetrate into the cleanest environments. Several hundred conidia are thought to be inhaled each day by a given individual and then expelled by mucociliary clearance. Given that airway epithelial cells make up a significant portion of the pulmonary-air interface, we set out to determine the percentage of conidia that are actually internalized after initial contact with airway epithelial cells. We determined this through an in vitro assay using an immortalized bronchial airway epithelial cell line known as BEAS-2B. Our results suggest a small fraction of conidia are internalized by BEAS-2B cells, while the majority stay adherent to the surface of cells or are washed away during sample processing. Internalization of conidia was observed at 6 h postchallenge and not prior. Our data also indicate conidia are rendered metabolically inactive within 3 h of challenge, suggesting BEAS-2B cells process a large number of conidia without internalization in this early time frame. We have also identified several host endocytosis markers that localize around internalized conidia as well as contribute to the processing of conidia. Understanding how these host endocytosis markers affect the processing of internal and/or external conidia may provide a novel avenue for therapeutic development.

Pathogen interactions with endothelial cells and the induction of innate and adaptive immunity

There are over 10 trillion endothelial cells (EC) that line the vasculature of the human body. These cells not only have specialized functions in the maintenance of homeostasis within the circulation and various tissues but they also have a major role in immune function. EC also represent an important replicative niche for a subset of viral, bacterial, and parasitic organisms that are present in the blood or lymph; however, there are major gaps in our knowledge regarding how pathogens interact with EC and how this influences disease outcome. In this article, we review the literature on EC-pathogen interactions and their role in innate and adaptive mechanisms of resistance to infection and highlight opportunities to address prominent knowledge gaps.

Cryptococcal Traits Mediating Adherence to Biotic and Abiotic Surfaces

Several species in the genus Cryptococcus are facultative intracellular pathogens capable of causing disease associated with high mortality and morbidity in humans. These fungi interact with other organisms in the soil, and these interactions may contribute to the development of adaptation mechanisms that function in virulence by promoting fungal survival in animal hosts. Fungal adhesion molecules, also known as adhesins, have been classically considered as cell-surface or secreted proteins that play critical roles in microbial pathogenesis or in biofilm formation as structural components. Pathogenic Cryptococcus spp. differ from other pathogenic yeasts in having a polysaccharide capsule that covers the cell wall surface and precludes interactions of those structures with host cell receptors. Hence, pathogenic Cryptococcus spp. use unconventional tools for surface attachment. In this essay, we review the unique traits and mechanisms favoring adhesion of Cryptococcus spp. to biotic and abiotic surfaces. Knowledge of the traits that mediate adherence could be exploited in the development of therapeutic, biomedical, and/or industrial products.

Cryptococcus neoformans urease affects the outcome of intracellular pathogenesis by modulating phagolysosomal pH

Cryptococcus neoformans is a facultative intracellular pathogen and its interaction with macrophages is a key event determining the outcome of infection. Urease is a major virulence factor in C. neoformans but its role during macrophage interaction has not been characterized. Consequently, we analyzed the effect of urease on fungal-macrophage interaction using wild-type, urease-deficient and urease-complemented strains of C. neoformans. The frequency of non-lytic exocytosis events was reduced in the absence of urease. Urease-positive C. neoformans manifested reduced and delayed intracellular replication with fewer macrophages displaying phagolysosomal membrane permeabilization. The production of urease was associated with increased phagolysosomal pH, which in turn reduced growth of urease-positive C. neoformans inside macrophages. Interestingly, the ure1 mutant strain grew slower in fungal growth medium which was buffered to neutral pH (pH 7.4). Mice inoculated with macrophages carrying urease-deficient C. neoformans had lower fungal burden in the brain than mice infected with macrophages carrying wild-type strain. In contrast, the absence of urease did not affect survival of yeast when interacting with amoebae. Because of the inability of the urease deletion mutant to grow on urea as a sole nitrogen source, we hypothesize urease plays a nutritional role involved in nitrogen acquisition in the environment. Taken together, our data demonstrate that urease affects fitness within the mammalian phagosome, promoting non-lytic exocytosis while delaying intracellular replication and thus reducing phagolysosomal membrane damage, events that could facilitate cryptococcal dissemination when transported inside macrophages. This system provides an example where an enzyme involved in nutrient acquisition modulates virulence during mammalian infection.

Neuro-Immune Mechanisms of Anti-Cryptococcal Protection

Cryptococcal meningitis (CM) is a life-threatening fungal disease affecting both immunosuppressed and immunocompetent people. The main causative agent of CM is Cryptococcus neoformans, a basidiomycete fungus prevalent in the environment. Our understanding of the immune mechanisms controlling C. neoformans growth within the central nervous system (CNS) is poor. However, there have been several recent advances in the field of neuroimmunology regarding how cells resident within the CNS, such as microglia and neurons, can participate in immune surveillance and control of infection. In this mini-review, the cells of the CNS are discussed with reference to what is currently known about how they control C. neoformans infection.

Innate immune evasion strategies against Cryptococcal meningitis caused by Cryptococcus neoformans

As an infectious fungus that affects the respiratory tract, Cryptococcus neoformans (C. neoformans) commonly causes asymptomatic pulmonary infection. C. neoformans may target the brain instead of the lungs and cross the blood-brain barrier (BBB) in the early phase of infection; however, this is dependent on successful evasion of the host innate immune system. During the initial stage of fungal infection, a complex network of innate immune factors are activated. C. neoformans utilizes a number of strategies to overcome the anti-fungal mechanisms of the host innate immune system and cross the BBB. In the present review, the defensive mechanisms of C. neoformans against the innate immune system and its ability to cross the BBB were discussed, with an emphasis on recent insights into the activities of anti-phagocytotic and anti-oxidative factors in C. neoformans.

The Metalloprotease, Mpr1, Engages AnnexinA2 to Promote the Transcytosis of Fungal Cells across the Blood-Brain Barrier

Eukaryotic pathogens display multiple mechanisms for breaching the blood-brain barrier (BBB) and invading the central nervous system (CNS). Of the fungal spp., that cause disease in mammals, only some cross brain microvascular endothelial cells which constitute the BBB, and invade the brain. Cryptococcus neoformans, the leading cause of fungal meningoencephalitis, crosses the BBB directly by transcytosis or by co-opting monocytes. We previously determined that Mpr1, a secreted fungal metalloprotease, facilitates association of fungal cells to brain microvascular endothelial cells and we confirmed that the sole expression of CnMPR1 endowed S. cerevisiae with an ability to cross the BBB. Here, the gain of function conferred onto S. cerevisiae by CnMPR1 (i.e., Sc<CnMPR1> strain) was used to identify targets of Mpr1 that might reside on the surface of the BBB. Following biotin-labeling of BBB surface proteins, Sc<CnMPR1>-associated proteins were identified by LC-MS/MS. Of the 62 proteins identified several were cytoskeleton-endocytosis-associated including AnnexinA2 (AnxA2). Using an in vitro model of the human BBB where AnxA2 activity was blocked, we found that the lack of AnxA2 activity prevented the movement of S. cerevisiae across the BBB (i.e., transcytosis of Sc<CnMPR1> strain) but unexpectedly, TEM analysis revealed that AnxA2 was not required for the association or the internalization of Sc<CnMPR1>. Additionally, the co-localization of AnxA2 and Sc<CnMPR1> suggest that successful crossing of the BBB is dependent on an AxnA2-Mpr1-mediated interaction. Collectively the data suggest that AnxA2 plays a central role in fungal transcytosis in human brain microvascular endothelial cells. The movement and exocytosis of Sc<CnMPR1> is dependent on membrane trafficking events that involve AnxA2 but these events appear to be independent from the actions of AnxA2 at the host cell surface. We propose that Mpr1 activity promotes cytoskeleton remodeling in brain microvascular endothelial cells and thereby engages AnxA2 in order to facilitate fungal transcytosis of the BBB.

Protective and Pathological Immunity during Central Nervous System Infections

The concept of immune privilege of the central nervous system (CNS) has dominated the study of inflammatory processes in the brain. However, clinically relevant models have highlighted that innate pathways limit pathogen invasion of the CNS and adaptive immunity mediates control of many neural infections. As protective responses can result in bystander damage, there are regulatory mechanisms that balance protective and pathological inflammation, but these mechanisms might also allow microbial persistence. The focus of this review is to consider the host-pathogen interactions that influence neurotropic infections and to highlight advances in our understanding of innate and adaptive mechanisms of resistance as key determinants of the outcome of CNS infection. Advances in these areas have broadened our comprehension of how the immune system functions in the brain and can readily overcome immune privilege.

Trojan Horse Transit Contributes to Blood-Brain Barrier Crossing of a Eukaryotic Pathogen

The blood-brain barrier (BBB) protects the central nervous system (CNS) by restricting the passage of molecules and microorganisms. Despite this barrier, however, the fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that is estimated to kill over 600,000 people annually. Cryptococcal infection begins in the lung, and experimental evidence suggests that host phagocytes play a role in subsequent dissemination, although this role remains ill defined. Additionally, the disparate experimental approaches that have been used to probe various potential routes of BBB transit make it impossible to assess their relative contributions, confounding any integrated understanding of cryptococcal brain entry. Here we used an in vitro model BBB to show that a “Trojan horse” mechanism contributes significantly to fungal barrier crossing and that host factors regulate this process independently of free fungal transit. We also, for the first time, directly imaged C. neoformans-containing phagocytes crossing the BBB, showing that they do so via transendothelial pores. Finally, we found that Trojan horse crossing enables CNS entry of fungal mutants that cannot otherwise traverse the BBB, and we demonstrate additional intercellular interactions that may contribute to brain entry. Our work elucidates the mechanism of cryptococcal brain invasion and offers approaches to study other neuropathogens.

The fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that kills hundreds of thousands of people each year. One route that has been proposed for this brain entry is a Trojan horse mechanism, whereby the fungus crosses the blood-brain barrier (BBB) as a passenger inside host phagocytes. Although indirect experimental evidence supports this intriguing mechanism, it has never been directly visualized. Here we directly image Trojan horse transit and show that it is regulated independently of free fungal entry, contributes to cryptococcal BBB crossing, and allows mutant fungi that cannot enter alone to invade the brain.

CD44-mediated monocyte transmigration across Cryptococcus neoformans-infected brain microvascular endothelial cells is enhanced by HIV-1 gp41-I90 ectodomain

Background

Cryptococcus neoformans (Cn) is an important opportunistic pathogen in the immunocompromised people, including AIDS patients, which leads to fatal cryptococcal meningitis with high mortality rate. Previous researches have shown that HIV-1 gp41-I90 ectodomain can enhance Cn adhesion to and invasion of brain microvascular endothelial cell (BMEC), which constitutes the blood brain barrier (BBB). However, little is known about the role of HIV-1 gp41-I90 in the monocyte transmigration across Cn-infected BBB. In the present study, we provide evidence that HIV-1 gp41-I90 and Cn synergistically enhance monocytes transmigration across the BBB in vitro and in vivo. The underlying mechanisms for this phenomenon require further study.

Methods

In this study, the enhancing role of HIV-1 gp41-I90 in monocyte transmigration across Cn-infected BBB was demonstrated by performed transmigration assays in vitro and in vivo.

Results

Our results showed that the transmigration rate of monocytes are positively associated with Cn and/or HIV-1 gp41-I90, the co-exposure (HIV-1 gp41-I90 + Cn) group showed a higher THP-1 transmigration rate (P < 0.01). Using CD44 knock-down HBMEC or CD44 inhibitor Bikunin in the assay, the facilitation of transmigration rates of monocyte enhanced by HIV-1 gp41-I90 was significantly suppressed. Western blotting analysis and biotin/avidin enzyme-linked immunosorbent assays (BA-ELISAs) showed that Cn and HIV-1 gp41-I90 could increase the expression of CD44 and ICAM-1 on the HBMEC. Moreover, Cn and/or HIV-1 gp41-I90 could also induce CD44 redistribution to the membrane lipid rafts. By establishing the mouse cryptococcal meningitis model, we found that HIV-1 gp41-I90 and Cn could synergistically enhance the monocytes transmigration, increase the BBB permeability and injury in vivo.

Conclusions

Collectively, our findings suggested that HIV-1 gp41-I90 ectodomain can enhance the transmigration of THP-1 through Cn-infected BBB, which may be mediated by CD44. This novel study enlightens the future prospects to elaborate the inflammatory responses induced by HIV-1 gp41-I90 ectodomain and to effectively eliminate the opportunistic infections in AIDS patients.

Plasma membrane lipids and their role in fungal virulence

There has been considerable evidence in recent years suggesting that plasma membrane lipids are important regulators of fungal pathogenicity. Various glycolipids have been shown to impart virulent properties in several fungal species, while others have been shown to play a role in host defense. In addition to their role as virulence factors, lipids also contribute to other virulence mechanisms such as drug resistance, biofilm formation, and release of extracellular vesicles. In addition, lipids also affect the mechanical properties of the plasma membrane through the formation of packed microdomains composed mainly of sphingolipids and sterols. Changes in the composition of lipid microdomains have been shown to disrupt the localization of virulence factors and affect fungal pathogenicity. This review gathers evidence on the various roles of plasma membrane lipids in fungal virulence and how lipids might contribute to the different processes that occur during infection and treatment. Insight into the role of lipids in fungal virulence can lead to an improved understanding of the process of fungal pathogenesis and the development of new lipid-mediated therapeutic strategies.

How Cryptococcus interacts with the blood-brain barrier

Cryptococcus demonstrates predilection for invasion of the brain, but the mechanism by which Cryptococcus crosses the blood-brain barrier (BBB) to cause brain invasion is largely unknown. In order for Cryptococcus to cross the BBB, there must be a way to either cross human brain microvascular endothelial cells, which are the main constitute of the BBB, or go in between tight junctions. Recent evidence of human brain microvascular endothelial cell responses to transcellular brain invasions includes membrane rearrangements, intracellular signaling pathways and cytoskeletal activations. Several Cryptococcal genes related to the traversal of BBB have been identified, including CPS1, ITR1a, ITR3c, PLB1, MPR1, FNX1 and RUB1. In addition, Cryptococcus neoformans-derived microvesicles may contribute to cryptococcal brain invasion. Paracellularly, Cryptococcus may traverse across BBB using either routes utilizing plasmin, ammonia or macrophages in a Trojan horse mechanism.

Alpha7 nicotinic acetylcholine receptor is required for blood-brain barrier injury-related CNS disorders caused by Cryptococcus neoformans and HIV-1 associated comorbidity factors

Background

Cryptococcal meningitis is the most common fungal infection of the central nervous system (CNS) in HIV/AIDS. HIV-1 virotoxins (e.g., gp41) are able to induce disorders of the blood-brain barrier (BBB), which mainly consists of BMEC. Our recent study suggests that α7 nAChR is an essential regulator of inflammation, which contributes to regulation of NF-κB signaling, neuroinflammation and BBB disorders caused by microbial (e.g., HIV-1 gp120) and non-microbial [e.g., methamphetamine (METH)] factors. However, the underlying mechanisms for multiple comorbidities are unclear.

Methods

In this report, an aggravating role of α7 nAChR in host defense against CNS disorders caused by these comorbidities was demonstrated by chemical [inhibitor: methyllycaconitine (MLA)] and genetic (α7^−/− mice) blockages of α7 nAChR.

Results

As shown in our in vivo studies, BBB injury was significantly reduced in α7^−/− mice infected with C. neoformans. Stimulation by the gp41 ectodomain peptide (gp41-I90) and METH was abolished in the α7^−/− animals. C. neoformans and gp41-I90 could activate NF-κB. Gp41-I90- and METH-induced monocyte transmigration and senescence were significantly inhibited by MLA and CAPE (caffeic acid phenethyl ester, an NF-κB inhibitor).

Conclusions

Collectively, our data suggest that α7 nAChR plays a detrimental role in the host defense against C. neoformans- and HIV-1 associated comorbidity factors-induced BBB injury and CNS disorders.

Dancing cheek to cheek: Cryptococcus neoformans and phagocytes

Meningoencephalitis caused by Cryptococcus neoformans (Cn) has become one of the leading causes of mortality in AIDS patients. Understanding the interactions between Cn and phagocytes is fundamental in exploring the pathogenicity of cryptococcal meningoencephalitis. Cn may be extracellular or contained in the monocytes, macrophages, neutrophils, dendritic cells and even endothelial cells. The internalized Cn may proliferate inside the host cells, or cause the lysis of host cells, or leave the host cells via non-lytic exocytosis, or even hijack the host cells (Trojan horse) for the brain dissemination, which are regulated by microbe factors and also immune molecules. Coexistence of protective and deleterious roles of phagocytes in the progression of cryptococcosis warrant further investigation.

From the blood to the brain: avenues of eukaryotic pathogen dissemination to the central nervous system

Infection of the central nervous system (CNS) is a significant cause of morbidity and mortality, and treatments available to combat the highly debilitating symptoms of CNS infection are limited. The mechanisms by which pathogens in the circulation overcome host immunity and breach the blood-brain barrier are active areas of investigation. In this review, we discuss recent work that has significantly advanced our understanding of the avenues of pathogen dissemination to the CNS for four eukaryotic pathogens of global health importance: Toxoplasma gondii, Plasmodium falciparum, Trypanosoma brucei, and Cryptococcus neoformans. These studies highlight the remarkable diversity of pathogen strategies for trafficking to the brain and will ultimately contribute to an improved ability to combat life-threatening CNS disease.

Host cell invasion by medically important fungi

To infect the host and cause disease, many medically important fungi invade normally nonphagocytic host cells, such as endothelial cells and epithelial cells. Host cell invasion is a two-step process consisting of adherence followed by invasion. There are two general mechanisms of host cell invasion, induced endocytosis and active penetration. Furthermore, fungi can traverse epithelial or endothelial cell barriers either by proteolytic degradation of intercellular tight junctions or via a Trojan horse mechanism in which they are transported by leukocytes. Although these mechanisms of host cell invasion have been best studied using Candida albicans and Cryptococcus neoformans, it is probable that other invasive fungi also use one or more of these mechanisms to invade host cells. Identification of these invasion mechanisms holds promise to facilitate the development of new approaches to inhibit fungal invasion and thereby prevent disease.

Cryptococcus inositol utilization modulates the host protective immune response during brain infection

BACKGROUND

Cryptococcus neoformans is the most common cause of fungal meningitis among individuals with HIV/AIDS, which is uniformly fatal without proper treatment. The underlying mechanism of disease development in the brain that leads to cryptococcal meningoencephalitis remains incompletely understood. We have previously demonstrated that inositol transporters (ITR) are required for Cryptococcus virulence. The itr1aΔ itr3cΔ double mutant of C. neoformans was attenuated for virulence in a murine model of intra-cerebral infection; demonstrating that Itr1a and Itr3c are required for full virulence during brain infection, despite a similar growth rate between the mutant and wild type strains in the infected brain.

RESULTS

To understand the immune pathology associated with infection by the itr1aΔ itr3cΔ double mutant, we investigated the molecular correlates of host immune response during mouse brain infection. We used genome-wide transcriptome shotgun sequencing (RNA-Seq) and quantitative real-time PCR (qRT-PCR) methods to examine the host gene expression profile in the infected brain. Our results show that compared to the wild type, infection of mouse brains by the mutant leads to significant activation of cellular networks/pathways associated with host protective immunity. Most of the significantly differentially expressed genes (SDEG) are part of immune cell networks such as tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ) regulon, indicating that infection by the mutant mounts a stronger host immune response compared to the wild type. Interestingly, a significant reduction in glucuronoxylomannan (GXM) secretion was observed in the itr1aΔ itr3cΔ mutant cells, indicating that inositol utilization pathways play a role in capsule production.

CONCLUSIONS

Since capsule has been shown to impact the host response during Cryptococcus-host interactions, our results suggest that the reduced GXM production may contribute to the increased immune activation in the mutant-infected animals.

Invasion of the central nervous system by Cryptococcus neoformans requires a secreted fungal metalloprotease

Cryptococcus spp. cause life-threatening fungal infection of the central nervous system (CNS), predominantly in patients with a compromised immune system. Why Cryptococcus neoformans has this remarkable tropism for the CNS is not clear. Recent research on cerebral pathogenesis of C. neoformans revealed a predominantly transcellular migration of cryptococci across the brain endothelium; however, the identities of key fungal virulence factors that function specifically to invade the CNS remain unresolved. Here we found that a novel, secreted metalloprotease (Mpr1) that we identified in the extracellular proteome of C. neoformans (CnMpr1) is required for establishing fungal disease in the CNS. Mpr1 belongs to a poorly characterized M36 class of fungalysins that are expressed in only some fungal species. A strain of C. neoformans lacking the gene encoding Mpr1 (mpr1Δ) failed to breach the endothelium in an in vitro model of the human blood-brain barrier (BBB). A mammalian host infected with the mpr1Δ null strain demonstrated significant improvement in survival due to a reduced brain fungal burden and lacked the brain pathology commonly associated with cryptococcal disease. The in vivo studies further indicate that Mpr1 is not required for fungal dissemination and Mpr1 likely targets the brain endothelium specifically. Remarkably, the sole expression of CnMPR1 in Saccharomyces cerevisiae resulted in a robust migration of yeast cells across the brain endothelium, demonstrating Mpr1's specific activity in breaching the BBB and suggesting that Mpr1 may function independently of the hyaluronic acid-CD44 pathway. This distinct role for Mpr1 may develop into innovative treatment options and facilitate a brain-specific drug delivery platform.

IMPORTANCE

Cryptococcus neoformans is a medically relevant fungal pathogen causing significant morbidity and mortality, particularly in immunocompromised individuals. An intriguing feature is its strong neurotropism, and consequently the hallmark of cryptococcal disease is a brain infection, cryptococcal meningoencephalitis. For C. neoformans to penetrate the central nervous system (CNS), it first breaches the blood-brain barrier via a transcellular pathway; however, the identities of fungal factors required for this transmigration remain largely unknown. In an effort to identify extracellular fungal proteins that could mediate interactions with the brain endothelium, we undertook a proteomic analysis of the extracellular proteome and identified a secreted metalloprotease (Mpr1) belonging to the M36 class of fungalysins. Here we found that Mpr1 promotes migration of C. neoformans across the brain endothelium and into the CNS by facilitating attachment of cryptococci to the endothelium surface, thus underscoring the critical role of M36 proteases in fungal pathogenesis.

Cryptococcus neoformans is internalized by receptor-mediated or 'triggered' phagocytosis, dependent on actin recruitment

Cryptococcosis by the encapsulated yeast Cryptococcus neoformans affects mostly immunocompromised individuals and is a frequent neurological complication in AIDS patients. Recent studies support the idea that intracellular survival of Cryptococcus yeast cells is important for the pathogenesis of cryptococcosis. However, the initial steps of Cryptococcus internalization by host cells remain poorly understood. Here, we investigate the mechanism of Cryptococcus neoformans phagocytosis by peritoneal macrophages using confocal and electron microscopy techniques, as well as flow cytometry quantification, evaluating the importance of fungal capsule production and of host cell cytoskeletal elements for fungal phagocytosis. Electron microscopy analyses revealed that capsular and acapsular strains of C. neoformans are internalized by macrophages via both 'zipper' (receptor-mediated) and 'trigger' (membrane ruffle-dependent) phagocytosis mechanisms. Actin filaments surrounded phagosomes of capsular and acapsular yeasts, and the actin depolymerizing drugs cytochalasin D and latrunculin B inhibited yeast internalization and actin recruitment to the phagosome area. In contrast, nocodazole and paclitaxel, inhibitors of microtubule dynamics decreased internalization but did not prevent actin recruitment to the site of phagocytosis. Our results show that different uptake mechanisms, dependent on both actin and tubulin dynamics occur during yeast internalization by macrophages, and that capsule production does not affect the mode of Cryptococcus uptake by host cells.

Glucose-coated gold nanoparticles transfer across human brain endothelium and enter astrocytes in vitro

The blood-brain barrier prevents the entry of many therapeutic agents into the brain. Various nanocarriers have been developed to help agents to cross this barrier, but they all have limitations, with regard to tissue-selectivity and their ability to cross the endothelium. This study investigated the potential for 4 nm coated gold nanoparticles to act as selective carriers across human brain endothelium and subsequently to enter astrocytes. The transfer rate of glucose-coated gold nanoparticles across primary human brain endothelium was at least three times faster than across non-brain endothelia. Movement of these nanoparticles occurred across the apical and basal plasma membranes via the cytosol with relatively little vesicular or paracellular migration; antibiotics that interfere with vesicular transport did not block migration. The transfer rate was also dependent on the surface coating of the nanoparticle and incubation temperature. Using a novel 3-dimensional co-culture system, which includes primary human astrocytes and a brain endothelial cell line hCMEC/D3, we demonstrated that the glucose-coated nanoparticles traverse the endothelium, move through the extracellular matrix and localize in astrocytes. The movement of the nanoparticles through the matrix was >10 µm/hour and they appeared in the nuclei of the astrocytes in considerable numbers. These nanoparticles have the correct properties for efficient and selective carriers of therapeutic agents across the blood-brain barrier.

Cryptococcus neoformans promotes its transmigration into the central nervous system by inducing molecular and cellular changes in brain endothelial cells

Cryptococcus spp. cause fungal meningitis, a life-threatening infection that occurs predominately in immunocompromised individuals. In order for Cryptococcus neoformans to invade the central nervous system (CNS), it must first penetrate the brain endothelium, also known as the blood-brain barrier (BBB). Despite the importance of the interrelation between C. neoformans and the brain endothelium in establishing CNS infection, very little is known about this microenvironment. Here we sought to resolve the cellular and molecular basis that defines the fungal-BBB interface during cryptococcal attachment to, and internalization by, the human brain endothelium. In order to accomplish this by a systems-wide approach, the proteomic profile of human brain endothelial cells challenged with C. neoformans was resolved using a label-free differential quantitative mass spectrometry method known as spectral counting (SC). Here, we demonstrate that as brain endothelial cells associate with, and internalize, cryptococci, they upregulate the expression of several proteins involved with cytoskeleton, metabolism, signaling, and inflammation, suggesting that they are actively signaling and undergoing cytoskeleton remodeling via annexin A2, S100A10, transgelin, and myosin. Transmission electronic microscopy (TEM) analysis demonstrates dramatic structural changes in nuclei, mitochondria, the endoplasmic reticulum (ER), and the plasma membrane that are indicative of cell stress and cell damage. The translocation of HMGB1, a marker of cell injury, the downregulation of proteins that function in transcription, energy production, protein processing, and the upregulation of cyclophilin A further support the notion that C. neoformans elicits changes in brain endothelial cells that facilitate the migration of cryptococci across the BBB and ultimately induce endothelial cell necrosis.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Can host receptors for fungi be targeted for treatment of fungal infections?

The invasion and stimulation of normally non-phagocytic host cells, such as epithelial and endothelial cells, is a key step in the pathogenesis of many fungal infections. In most cases, host cell invasion and/or stimulation of a proinflammatory response is induced when proteins or carbohydrates on the fungal cell surface bind to receptors on the host cell. Although many of these fungal-host cell interactions have only been investigated in vitro, the therapeutic efficacy of blocking the host cell receptors for Candida albicans and Rhizopus oryzae has been demonstrated in experimental animal models of infection. We summarize recent studies of the fungal receptors on normally non-phagocytic host cells and the therapeutic implications of blocking these receptors.

Mechanisms of infection by the human fungal pathogen Cryptococcus neoformans

Brain infection by the fungus Cryptococcus neoformans results in inflammation of the meninges and brain parenchyma, a condition known as meningoencephalitis. One million people are estimated to suffer cryptococcal meningitis globally and >60% of these cases die within 3 months of diagnosis. Humans are believed to contract infection by inhalation of spores or dried yeast cells, which subsequently colonize the lung tissue. In the lungs, cryptococci may be cleared by the lung phagocytes, stay latent, cause pulmonary infection and/or disseminate to other body parts, preferentially the brain, culminating in cryptococcal meningoencephalitis. In this review, we discuss the pathogenesis of C. neoformans from the environment to the brain, the current understanding of the mechanisms of cryptococcal transmission into the brain and cryptococcal meningitis. We also give an insight into future cryptococcosis research and the development of novel therapies.

Brain inositol is a novel stimulator for promoting Cryptococcus penetration of the blood-brain barrier

Cryptococcus neoformans is the most common cause of fungal meningitis, with high mortality and morbidity. The reason for the frequent occurrence of Cryptococcus infection in the central nervous system (CNS) is poorly understood. The facts that human and animal brains contain abundant inositol and that Cryptococcus has a sophisticated system for the acquisition of inositol from the environment suggests that host inositol utilization may contribute to the development of cryptococcal meningitis. In this study, we found that inositol plays an important role in Cryptococcus traversal across the blood-brain barrier (BBB) both in an in vitro human BBB model and in in vivo animal models. The capacity of inositol to stimulate BBB crossing was dependent upon fungal inositol transporters, indicated by a 70% reduction in transmigration efficiency in mutant strains lacking two major inositol transporters, Itr1a and Itr3c. Upregulation of genes involved in the inositol catabolic pathway was evident in a microarray analysis following inositol treatment. In addition, inositol increased the production of hyaluronic acid in Cryptococcus cells, which is a ligand known to binding host CD44 receptor for their invasion. These studies suggest an inositol-dependent Cryptococcus traversal of the BBB, and support our hypothesis that utilization of host-derived inositol by Cryptococcus contributes to CNS infection.

Cryptococcus neoformans is an AIDS-associated human fungal pathogen that annually causes over 1 million cases of meningitis world-wide, and more than 600,000 attributable deaths. Cryptococcus often causes lung and brain infection and is the leading cause of fungal meningitis in immunosuppressed patients. Why Cryptococcus frequently infects the central nervous system to cause fatal meningitis is an unanswered critical question. Our previous studies revealed a sophisticated inositol acquisition system in Cryptococcus that plays a central role in utilizing environmental inositol to complete its sexual cycle. Here we further demonstrate that inositol acquisition is also important for fungal infection in the brain, where abundant inositol is available. We found that inositol promotes the traversal of Cryptococcus across the blood-brain barrier (BBB), and such stimulation is fungal inositol transporter dependent. We also identified the effects of host inositol on fungal cellular functions that contribute to the stimulation of fungal penetration of the BBB. We propose that inositol utilization is a novel virulence factor for CNS cryptococcosis. Our work lays an important foundation for understanding how fungi respond to available host inositol and indicates the impact of host inositol acquisition on the development of cryptococcal meningitis.

Sphingolipids in neuroinflammation

Sphingolipids, the main component of cellular membranes, are cellular 'jack-of-all-trades', influencing a variety of functions including signal transduction, cell activation, membrane fluidity and cell-cell interactions.In the last few years, sphingolipids have begun to be investigated in the pathophysiology of major diseases of the brain, e.g. multiple sclerosis and dementia. Modulation of neuroinflammatory responses, such as lymphocyte behaviour, is a chance to intervene in the pathways that cause disease. There is much research still to be done in this field, but the prospect of treating previously untreatable medical conditions compels us onwards. Here, we review the current knowledge of the link between sphingolipids and neuroinflammation.

Induction of brain microvascular endothelial cell urokinase expression by Cryptococcus neoformans facilitates blood-brain barrier invasion

The invasive ability of the blood-borne fungal pathogen Cryptococcus neoformans can be enhanced through interactions with host plasma components, such as plasminogen. Previously we showed by in vitro studies that plasminogen coats the surface of C. neoformans and is converted to the active serine protease, plasmin, by host plasminogen activators. Viable, but not formaldehyde- or sodium azide-killed, cryptococcal strains undergo brain microvascular endothelial cell-dependent plasminogen-to-plasmin activation, which results in enhanced, plasmin-dependent cryptococcal invasion of primary bovine brain microvascular endothelial cells and fungal ability to degrade plasmin substrates. In the present work, brain microvascular endothelial cells cultured with viable, but not killed, cryptococcal strains led to significant increases in both urokinase mRNA transcription and cell-associated urokinase protein expression. Soluble urokinase was also detected in conditioned medium from brain microvascular endothelial cells cultured with viable, but not killed, C. neoformans. Exposure of plasminogen pre-coated viable C. neoformans to conditioned medium from strain-matched brain microvascular endothelial cell-fungal co-cultures resulted in plasminogen-to-plasmin activation and plasmin-dependent cryptococcal invasion. siRNA-mediated silencing of urokinase gene expression or the use of specific inhibitors of urokinase activity abrogated both plasminogen-to-plasmin activation on C. neoformans and cryptococcal-brain microvascular endothelial cell invasion. Our results suggest that pathogen exploitation of the host urokinase-plasmin(ogen) system may contribute to C. neoformans virulence during invasive cryptococcosis.

Cryptococcus neoformans-derived microvesicles enhance the pathogenesis of fungal brain infection

Cryptococcal meningoencephalitis is the most common fungal disease in the central nervous system. The mechanisms by which Cryptococcus neoformans invades the brain are largely unknown. In this study, we found that C. neoformans-derived microvesicles (CnMVs) can enhance the traversal of the blood-brain barrier (BBB) by C. neoformans invitro. The immunofluorescence imaging demonstrates that CnMVs can fuse with human brain microvascular endothelial cells (HBMECs), the constituents of the BBB. This activity is presumably due to the ability of the CnMVs to activate HBMEC membrane rafts and induce cell fusogenic activity. CnMVs also enhanced C. neoformans infection of the brain, found in both infected brains and cerebrospinal fluid. In infected mouse brains, CnMVs are distributed inside and around C. neoformans-induced cystic lesions. GFAP (glial fibrillary acidic protein)-positive astrocytes were found surrounding the cystic lesions, overlapping with the 14-3-3-GFP (14-3-3-green fluorescence protein fusion) signals. Substantial changes could be observed in areas that have a high density of CnMV staining. This is the first demonstration that C. neoformans-derived microvesicles can facilitate cryptococcal traversal across the BBB and accumulate at lesion sites of C. neoformans-infected brains. Results of this study suggested that CnMVs play an important role in the pathogenesis of cryptococcal meningoencephalitis.

Real-time in vivo imaging of fungal migration to the central nervous system

Recent technical advances have afforded valuable new insights into the pathogenesis of fungal infections in the central nervous system (CNS), which continue to cause devastating complications, particularly in immunocompromised individuals. To cause CNS mycosis, organisms such as Cryptococcus neoformans become blood borne and progress through a series of pathogenic checkpoints that culminate in fungal replication in the brain. Critical steps include fungal arrest in the vasculature of the brain, interaction and signalling of the fungal and endothelial cells leading to transmigration with subsequent parenchymal invasion and fungal replication in the CNS. Previous studies that made use of in vitro and ex vivo approaches contributed greatly to our understanding of brain invasion by fungi. However, the knowledge gained from previous studies relied on in vitro models that did not account for vascular haemodynamics. For this reason, more refined approaches that model blood flow and vascular anatomy are required, andultimately studying fungal invasion and dissemination in vivo. Indeed, in vivo imaging (also known as intravital imaging) has emerged as a valuable technique to probe host-pathogen interactions. In this review, with a focus on C. neoformans, we will provide an overview of the applications of the prior techniques and recent advances, their strengths and limitations in characterizing the migration of fungi into the brain, and unanswered questions that may provide new directions for research.

Identification of genes from the fungal pathogen Cryptococcus neoformans related to transmigration into the central nervous system

Background

A mouse brain transmigration assessment (MBTA) was created to investigate the central nervous system (CNS) pathogenesis of cryptococcal meningoencephalitis.

Methodology/Principal Findings

Two cryptococcal mutants were identified from a pool of 109 pre-selected mutants that were signature-tagged with the nourseothricin acetyltransferase (NAT) resistance cassette. These two mutants displayed abnormal transmigration into the central nervous system. One mutant displaying decreased transmigration contains a null mutation in the putative FNX1 gene, whereas the other mutant possessing a null mutation in the putative RUB1 gene exhibited increased transmigration into the brain. Two macrophage adhesion-defective mutants in the pool, 12F1 and 3C9, showed reduced phagocytosis by macrophages, but displayed no defects in CNS entry suggesting that transit within macrophages (the “Trojan horse” model of CNS entry) is not the primary mechanism for C. neoformans migration into the CNS in this MBTA.

Conclusions/Significance

This research design provides a new strategy for genetic impact studies on how Cryptococcus passes through the blood-brain barrier (BBB), and the specific isolated mutants in this assay support a transcellular mechanism of CNS entry.

Catch me if you can: phagocytosis and killing avoidance by Cryptococcus neoformans

After inhalation of infectious particles, Cryptococcus neoformans resides in the alveolar spaces, where it can survive and replicate in the extracellular environment. This yeast has developed different mechanisms to avoid internalization by phagocytic cells, the main one being a polysaccharide capsule around the cell body, which inhibits the uptake of the yeast by macrophages. In addition, capsule-independent mechanisms have also been described, such as the production of antiphagocytic proteins. Despite these mechanisms, phagocytosis can occur in the presence of opsonins, and once C. neoformans is internalized, multiple outcomes are possible, including pathogen killing or intracellular replication and escape from macrophages. For this reason, C. neoformans is considered a facultative intracellular pathogen. As alveolar macrophages are the first component of the host immune system to confront C. neoformans, the outcome of this interaction could determine the degree of infection, producing either a severe disseminated disease or a latency state. In this review, we will tackle the complexity of the interaction between C. neoformans and macrophages, including the phagocytic avoidance mechanisms and all the possible outcomes that have been described for this interaction. Finally, we will discuss the consequences of the different outcomes for the type of infection produced in the host.

Cryptococcus neoformans activates RhoGTPase proteins followed by protein kinase C, focal adhesion kinase, and ezrin to promote traversal across the blood-brain barrier

Cryptococcus neoformans is an opportunistic fungal pathogen that causes meningoencephalitis. Previous studies have demonstrated that Cryptococcus binding and invasion of human brain microvascular endothelial cells (HBMEC) is a prerequisite for transmigration across the blood-brain barrier. However, the molecular mechanism involved in the cryptococcal blood-brain barrier traversal is poorly understood. In this study we examined the signaling events in HBMEC during interaction with C. neoformans. Analysis with inhibitors revealed that cryptococcal association, invasion, and transmigration require host actin cytoskeleton rearrangement. Rho pulldown assays revealed that Cryptococcus induces activation of three members of RhoGTPases, e.g. RhoA, Rac1, and Cdc42, and their activations are required for cryptococcal transmigration across the HBMEC monolayer. Western blot analysis showed that Cryptococcus also induces phosphorylation of focal adhesion kinase (FAK), ezrin, and protein kinase C α (PKCα), all of which are involved in the rearrangement of host actin cytoskeleton. Down-regulation of FAK, ezrin, or PKCα by shRNA knockdown, dominant-negative transfection, or inhibitors significantly reduces cryptococcal ability to traverse the HBMEC monolayer, indicating their positive role in cryptococcal transmigration. In addition, activation of RhoGTPases is the upstream event for phosphorylation of FAK, ezrin, and PKCα during C. neoformans-HBMEC interaction. Taken together, our findings demonstrate that C. neoformans activates RhoGTPases and subsequently FAK, ezrin, and PKCα to promote their traversal across the HBMEC monolayer, which is the critical step for cryptococcal brain infection and development of meningitis.

Capsule independent uptake of the fungal pathogen Cryptococcus neoformans into brain microvascular endothelial cells

Cryptococcosis is a life-threatening fungal disease with a high rate of mortality among HIV/AIDS patients across the world. The ability to penetrate the blood-brain barrier (BBB) is central to the pathogenesis of cryptococcosis, but the way in which this occurs remains unclear. Here we use both mouse and human brain derived endothelial cells (bEnd3 and hCMEC/D3) to accurately quantify fungal uptake and survival within brain endothelial cells. Our data indicate that the adherence and internalisation of cryptococci by brain microvascular endothelial cells is an infrequent event involving small numbers of cryptococcal yeast cells. Interestingly, this process requires neither active signalling from the fungus nor the presence of the fungal capsule. Thus entry into brain microvascular endothelial cells is most likely a passive event that occurs following ‘trapping’ within capillary beds of the BBB.

Hyaluronic acid receptor CD44 deficiency is associated with decreased Cryptococcus neoformans brain infection

Cryptococcus neoformans is a pathogenic yeast that can invade the brain and cause meningoencephalitis. Our previous in vitro studies suggested that the interaction between C. neoformans hyaluronic acid and human brain endothelial CD44 could be the initial step of brain invasion. In this report, we used a CD44 knock-out (KO or CD44(-/-)) mouse model to explore the importance of CD44 in C. neoformans brain invasion. Our results showed that C. neoformans-infected CD44 KO mice survived longer than the infected wild-type mice. Consistent with our in vitro results, the brain and cerebrospinal fluid fungal burden was reduced in CD44-deficient mice. Histopathological studies showed smaller and fewer cystic lesions in the brains of CD44 KO mice. Interestingly, the cystic lesions contained C. neoformans cells embedded within their polysaccharide capsule and were surrounded by host glial cells. We also found that a secondary hyaluronic acid receptor, RHAMM (receptor of hyaluronan-mediated motility), was present in the CD44 KO mice. Importantly, our studies demonstrated an in vivo blocking effect of simvastatin. These results suggest that the CD44 and RHAMM receptors function on membrane lipid rafts during invasion and that simvastatin may have a potential therapeutic role in C. neoformans infections of the brain.

Molecular mechanisms of cryptococcal meningitis

Fungal meningitis is a serious disease caused by a fungal infection of the central nervous system (CNS) mostly in individuals with immune system deficiencies. Fungal meningitis is often fatal without proper treatment, and the mortality rate remains unacceptably high even with antifungal drug interventions. Currently, cryptococcal meningitis is the most common fungal meningitis in HIV-1/AIDS, and its disease mechanism has been extensively studied. The key steps for fungi to infect brain and cause meningitis after establishment of local infection are the dissemination of fungal cells to the bloodstream and invasion through the blood brain barrier to reach the CNS. In this review, we use cryptococcal CNS infection as an example to describe the current molecular understanding of fungal meningitis, including the establishment of the infection, dissemination, and brain invasion. Host and microbial factors that contribute to these infection steps are also discussed.