Increased permeability of blood-brain barrier is mediated by serine protease during Cryptococcus meningitis

Ascorbic acid as serine protease inhibitor in lung cancer cell line and human serum albumin

Serine proteases (SPs) are distributed among all living cells accounting for almost one-third of all proteases. Dysregulation of SPs during inflammation and/or infection can result in devastating consequences, such as skin and lung inflammation, neuroinflammation, arthritis, as well as metastasis of cancerous cells. Such activities are tightly regulated by various inhibitors known as serine protease inhibitors (SERPIN). The thermodynamic investigations previously revealed that L-ascorbic acid binds to trypsin more firmly than pepsin and the binding force of L-ascorbic acid is driven by hydrogen bonds and van der Waals forces. However, the physiochemical effects of such interaction on trypsin and/or pepsin have not yet been reported. Ascorbic acid, also known as vitamin C, is one of the essential nutrients and most common food supplements, fortificants, and preservatives. The aim of this study was to explore the inhibitory effects of ascorbic acid on serine proteases at various concentrations on the in-vitro digestion and/or hydrolysis of intercellular matrix of cell monolayer and human serum albumin (HSA). The inhibitory effects of ascorbic on trypsin are investigated by qualitative and quantitative analysis using SDS-PAGE imaging and NIH densitometric software. Upon the addition of ascorbic acid in both indicator systems, the detachment and/or dissociation of cell monolayer and the digestion of HSA were inhibited in the presence of EDTA-Trypsin. The inhibitory effect of ascorbic acid on the digestion of intercellular matrix and/or hydrolysis of HSA showed a dose-dependent trend until it reached the maximum extent of inhibition. At an equal concentration (2.5mg/mL) ascorbic acid and EDTA-Trypsin exhibited the most potent inhibitory effect on the in vitro digestion of protein either in the form of intercellular matrix in cell monolayer and/or HSA respectively. Overall, our results based on two indicator systems strongly indicate that ascorbic acid may function as a serine protease inhibitor (SERPIN) beyond other important functions.

Cryptococcal proteases exhibit the potential to activate the latent SARS-CoV-2 spike protein

BACKGROUND

The COVID-19 pandemic has affected more than 650 million people and resulted in over 6.8 million deaths. Notably, the disease could co-manifest with microbial infections, like cryptococcosis, which also presents as a primary lung infection.

OBJECTIVE

In this contribution, we sought to determine if cryptococcal supernatant (which contains secreted furin-like proteases) could activate the SARS-CoV-2 spike protein.

METHODS

Molecular docking of the crystal structures of the SARS-CoV-2 spike protein (target) and selected cryptococcal proteases (ligands) was executed using the high ambiguity driven protein-protein docking (HADDOCK) server, with the furin protease serving as a reference ligand. The furin protease is found in human cells and typically activates the SARS-CoV-2 spike protein. Importantly, in order to provide experimental evidence for enzymatic activity, we also assessed the biochemical efficiency of cryptococcal proteases to initiate viral entry into HEK-293 T cells by SARS-CoV-2 spike pseudotyped Lentivirus.

RESULTS

We show that the selected cryptococcal proteases could interact with the spike protein, and some had a better or comparable binding affinity for the spike protein than furin protease following an in silico comparative analysis of the molecular docking parameters. Furthermore, it was noted that the biochemical efficiency of the cryptococcal supernatant to transduce HEK-293 T cells with SARS-CoV-2 pseudovirions was comparable (p > 0.05) to that of recombinant furin.

CONCLUSIONS

Taken together, these data show that cryptococcal proteases could activate the SARS-CoV-2 spike protein. In practice, it may be critical to determine if patients have an underlying cryptococcal infection, as this microbe could secrete proteases that may further activate the SARS-CoV-2 viral particles, thus undermining COVID-19 intervention measures.

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 multi-arm, parallel, preclinical study investigating the potential benefits of acetazolamide, candesartan, and triciribine in combination with fluconazole for the treatment of cryptococcal meningoencephalitis

Cryptococcus neoformans, an opportunistic fungal pathogen, primarily infects immunodeficient patients frequently causing cryptococcal meningoencephalitis (CM). Increased intracranial pressure (ICP) is a serious complication responsible for increased morbidity and mortality in CM patients. Non-invasive pharmacological agents that mitigate ICP could be beneficial in treating CM patients. The objective of the study was to investigate the efficacy of acetazolamide (AZA), candesartan (CAN), and triciribine (TCBN), in combination with the antifungal fluconazole, on C. neoformans-induced endothelial, brain, and lung injury in an experimental mouse model of CM. Our study shows that C. neoformans increases the expression of brain endothelial cell (BEC) junction proteins Claudin-5 (Cldn5) and VE-Cadherin to induce pathological cell-barrier remodeling and gap formation associated with increased Akt and p38 MAPK activation. All three agents inhibited C. neoformans-induced endothelial gap formation, only CAN and TCBN significantly reduced C. neoformans-induced Cldn5 expression, and only TCBN was effective in inhibiting Akt and p38MAPK. Interestingly, although C. neoformans did not cause brain or lung edema in mice, it induced lung and brain injuries, which were significantly reversed by AZA, CAN, or TCBN. Our study provides novel insights into the direct effects of C. neoformans on BECs in vitro, and the potential benefits of using AZA, CAN, or TCBN in the management of CM patients.

What fungal CNS infections can teach us about neuroimmunology and CNS-specific immunity

Immunity to fungal infections of the central nervous system (CNS) is one of the most poorly understood subjects within the field of medical mycology. Yet, the majority of deaths from invasive fungal infections are caused by brain-tropic fungi. In recent years, there have been several significant discoveries in the regulation of neuroinflammation and the role of the immune system in tissue homeostasis within the CNS. In this review, I highlight five important advances in the neuroimmunology field over the last decade and discuss how we should capitalise on these discoveries to better understand the pathogenesis of fungal CNS infections. In addition, the latest insights into fungal invasion tactics, microglia-astrocyte crosstalk and regulation of antifungal adaptive immune responses are summarised in the context of our contemporary understanding of CNS-specific immunity.

More than Just Protein Degradation: The Regulatory Roles and Moonlighting Functions of Extracellular Proteases Produced by Fungi Pathogenic for Humans

Extracellular proteases belong to the main virulence factors of pathogenic fungi. Their proteolytic activities plays a crucial role in the acquisition of nutrients from the external environment, destroying host barriers and defenses, and disrupting homeostasis in the human body, e.g., by affecting the functions of plasma proteolytic cascades, and playing sophisticated regulatory roles in various processes. Interestingly, some proteases belong to the group of moonlighting proteins, i.e., they have additional functions that contribute to successful host colonization and infection development, but they are not directly related to proteolysis. In this review, we describe examples of such multitasking of extracellular proteases that have been reported for medically important pathogenic fungi of the Candida, Aspergillus, Penicillium, Cryptococcus, Rhizopus, and Pneumocystis genera, as well as dermatophytes and selected endemic species. Additional functions of proteinases include supporting binding to host proteins, and adhesion to host cells. They also mediate self-aggregation and biofilm formation. In addition, fungal proteases affect the host immune cells and allergenicity, understood as the ability to stimulate a non-standard immune response. Finally, they play a role in the proper maintenance of cellular homeostasis. Knowledge about the multifunctionality of proteases, in addition to their canonical roles, greatly contributes to an understanding of the mechanisms of fungal pathogenicity.

Cryptococcus neoformans Infection in the Central Nervous System: The Battle between Host and Pathogen

Cryptococcus neoformans (C. neoformans) is a pathogenic fungus with a global distribution. Humans become infected by inhaling the fungus from the environment, and the fungus initially colonizes the lungs. If the immune system fails to contain C. neoformans in the lungs, the fungus can disseminate to the blood and invade the central nervous system, resulting in fatal meningoencephalitis particularly in immunocompromised individuals including HIV/AIDS patients. Following brain invasion, C. neoformans will encounter host defenses involving resident as well as recruited immune cells in the brain. To overcome host defenses, C. neoformans possesses multiple virulence factors capable of modulating immune responses. The outcome of the interactions between the host and C. neoformans will determine the disease progression. In this review, we describe the current understanding of how C. neoformans migrates to the brain across the blood–brain barrier, and how the host immune system responds to the invading organism in the brain. We will also discuss the virulence factors that C. neoformans uses to modulate host immune responses.

Cryptococcus neoformans Genotypic Diversity and Disease Outcome among HIV Patients in Africa

Cryptococcal meningoencephalitis, a disease with poor patient outcomes, remains the most prevalent invasive fungal infection worldwide, accounting for approximately 180,000 deaths each year. In several areas of sub-Saharan Africa with the highest HIV prevalence, cryptococcal meningitis is the leading cause of community-acquired meningitis, with a high mortality among HIV-infected individuals. Recent studies show that patient disease outcomes are impacted by the genetics of the infecting isolate. Yet, there is still limited knowledge of how these genotypic variations contribute to clinical disease outcome. Further, it is unclear how the genetic heterogeneity of C. neoformans and the extensive phenotypic variation observed between and within isolates affects infection and disease. In this review, we discuss current knowledge of how various genotypes impact disease progression and patient outcome in HIV-positive populations in sub-Saharan African, a setting with a high burden of cryptococcosis.

Elevated Intracranial Pressure in Cryptococcal Meningoencephalitis: Examining Old, New, and Promising Drug Therapies

Despite the availability of effective antifungal therapy, cryptococcal meningoencephalitis (CM) remains associated with elevated mortality. The spectrum of symptoms associated with the central nervous system (CNS) cryptococcosis is directly caused by the high fungal burden in the subarachnoid space and the peri-endothelial space of the CNS vasculature, which results in intracranial hypertension (ICH). Management of intracranial pressure (ICP) through aggressive drainage of cerebrospinal fluid by lumbar puncture is associated with increased survival. Unfortunately, these procedures are invasive and require specialized skills and supplies that are not readily available in resource-limited settings that carry the highest burden of CM. The institution of pharmacologic therapies to reduce the production or increase the resorption of cerebrospinal fluid would likely improve clinical outcomes associated with ICH in patients with CM. Here, we discuss the potential role of multiple pharmacologic drug classes such as diuretics, corticosteroids, and antiepileptic agents used to decrease ICP in various neurological conditions as potential future therapies for CM.

Reversal of CSF HIV-1 Escape during Treatment of HIV-Associated Cryptococcal Meningitis in Botswana

Cerebrospinal fluid (CSF) viral escape has been poorly described among people with HIV-associated cryptococcal meningitis. We determined the prevalence of CSF viral escape and HIV-1 viral load (VL) trajectories in individuals treated for HIV-associated cryptococcal meningitis. A retrospective longitudinal study was performed using paired CSF and plasma collected prior to and during the antifungal treatment of 83 participants recruited at the Botswana site of the phase-3 AMBITION-cm trial (2018−2021). HIV-1 RNA levels were quantified then CSF viral escape (CSF HIV-1 RNA ≥ 0.5 log10 higher than plasma) and HIV-1 VL trajectories were assessed. CSF viral escape occurred in 20/62 (32.3%; 95% confidence interval [CI]: 21.9−44.6%), 13/52 (25.0%; 95% CI: 15.2−38.2%) and 1/33 (3.0%; 95% CI: 0.16−15.3%) participants at days 1, 7 and 14 respectively. CSF viral escape was significantly lower on day 14 compared to days 1 and 7, p = 0.003 and p = 0.02, respectively. HIV-1 VL decreased significantly from day 1 to day 14 post antifungal therapy in the CSF but not in the plasma (β = −0.47; 95% CI: −0.69 to −0.25; p < 0.001). CSF viral escape is high among individuals presenting with HIV-associated cryptococcal meningitis; however, antifungal therapy may reverse this, highlighting the importance of rapid initiation of antifungal therapy in these patients.

Cryptococcal Protease(s) and the Activation of SARS-CoV-2 Spike (S) Protein

In this contribution, we report on the possibility that cryptococcal protease(s) could activate the SARS-CoV-2 spike (S) protein. The S protein is documented to have a unique four-amino-acid sequence (underlined, SPRRAR↓S) at the interface between the S1 and S2 sites, that serves as a cleavage site for the human protease, furin. We compared the biochemical efficiency of cryptococcal protease(s) and furin to mediate the proteolytic cleavage of the S1/S2 site in a fluorogenic peptide. We show that cryptococcal protease(s) processes this site in a manner comparable to the efficiency of furin (p > 0.581). We conclude the paper by discussing the impact of these findings in the context of a SARS-CoV-2 disease manifesting while there is an underlying cryptococcal infection.

Peptidases: promising antifungal targets of the human fungal pathogen, Cryptococcus neoformans

Cryptococcus neoformans is a globally important fungal pathogen, primarily inflicting disease on immunocompromised individuals. The widespread use of antifungal agents in medicine and agriculture supports the development of antifungal resistance through evolution, and the emergence of new strains with intrinsic resistance drives the need for new therapeutics. For C. neoformans, the production of virulence factors, including extracellular peptidases (e.g., CnMpr-1 and May1) with mechanistic roles in tissue invasion and fungal survival, constitute approximately 2% of the fungal proteome and cover five classes of enzymes. Given their role in fungal virulence, peptidases represent promising targets for anti-virulence discovery in the development of new approaches against C. neoformans. Additionally, intracellular peptidases, which are involved in resistance mechanisms against current treatment options (e.g., azole drugs), as well as capsule biosynthesis and elaboration of virulence factors, present additional opportunities to combat the pathogen. In this review, we highlight key cryptococcal peptidases with defined or predicted roles in fungal virulence and assess sequence alignments against their human homologs. With this information, we define the feasibility of the select peptidases as “druggable” targets for inhibition, representing prospective therapeutic options against the deadly fungus.

The Possible Role of Microbial Proteases in Facilitating SARS-CoV-2 Brain Invasion

SARS-CoV-2 has been shown to display proclivity towards organs bearing angiotensin-converting enzyme (ACE2) expression cells. Of interest herein is the ability of the virus to exhibit neurotropism. However, there is limited information on how this virus invades the brain. With this contribution, we explore how, in the context of a microbial co-infection using a cryptococcal co-infection as a model, SARS-CoV-2 could reach the brain. We theorise that the secretion of proteases by disseminated fungal cells might also activate the S2 domain of the viral spike glycoprotein for membrane fusion with brain endothelial cells leading to endocytosis. Understanding this potential invasion mechanism could lead to better SARS-CoV-2 intervention measures, which may also be applicable in instances of co-infection, especially with protease-secreting pathogens.

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.

Diverse species in the genus Cryptococcus: Pathogens and their non-pathogenic ancestors

The genus Cryptococcus comprises of more than 30 species. It consists of clinically significant pathogenic Cryptococcus neoformans/Cryptococcus gattii species complex comprising of a minimum of seven species. These pathogens cost more than 200,000 lives annually by causing cryptococcal meningoencephalitis. The evolution of the pathogenic species from closely related non-pathogenic species of the Cryptococcus amylolentus complex is of particular importance and several advances have been made to understand their phylogenetic and genomic relationships. The current review briefly describes the sexual reproduction process followed by an individual description of the members focusing on their key attributes and virulence mechanisms of the pathogenic species. A special section on phylogenetic studies is aimed at understanding the evolutionary divergence of pathogens from non-pathogens. Recent findings from our group pertaining to parameters affecting codon usage bias in six pathogenic and three non-pathogenic ancestral species and their corroboration with existing phylogenetic reports are also included in the current review.

[A clinical analysis of bacterial meningitis in full-term and preterm infants]

OBJECTIVE

To study the clinical features and prognosis of bacterial meningitis in full-term and preterm infants.

METHODS

A retrospective analysis was performed for the clinical data of 102 neonates with bacterial meningitis. According to the gestational age, they were divided into a preterm group (n=46) and a full-term group (n=56). The two groups were compared in terms of clinical manifestations, laboratory markers, imaging findings, and clinical outcomes.

RESULTS

Poor response and apnea were the major clinical manifestations in the preterm group (P<0.05), while pyrexia and convulsions were more common in the full-term group (P<0.05). The full-term group had a significantly higher glucose level in cerebrospinal fluid (CSF) than the preterm group (P<0.05). Compared with the full-term group, the preterm group had significantly higher C-reactive protein level, positive rate of blood culture, and incidence rate of poor prognosis (P<0.05). There were no significant differences between the two groups in leukocyte count in peripheral blood, levels of leukocytes and protein in CSF, and positive rate of CSF culture (P>0.05).

CONCLUSIONS

There are certain differences in the clinical manifestations between full-term and preterm infants with bacterial meningitis. Preterm infants tend to have a higher incidence rate of poor prognosis.

Reduced phagocytosis and killing of Cryptococcus neoformans biofilm-derived cells by J774.16 macrophages is associated with fungal capsular production and surface modification

Cryptococcus neoformans is an opportunistic encapsulated pathogen that causes life-threatening meningoencephalitis in individuals with immunosuppression. We compared the interactions of C. neoformans planktonic and biofilm-derived cells with J774.16 macrophage-like cells. Planktonic cells are more phagocytized and killed by J774.16 cells than biofilm-derived fungal cells. Biofilm-derived cryptococci possess larger capsule size and release significantly more capsular polysaccharide than planktonic cells in culture. Biofilm-derived fungi exhibited upregulation of genes involved in capsular production. Capsular-specific monoclonal antibody 18B7 demonstrated differential binding to the surface of planktonic and biofilm-derived cryptococci providing a plausible strategy for fungal evasion of macrophages and persistence. Future studies are necessary to elucidate how C. neoformans biofilm-derived cells regulate their virulence factors when interacting with cells of the immune system.

Cryptococcal Meningitis and Anti-virulence Therapeutic Strategies

Fungal infections of the central nervous system are responsible for significant morbidity and mortality. Cryptococcus neoformans (Cn) is the primary cause of fungal meningitis. Infection begins in the lung after inhalation of fungal spores but often spreads to other organs, particularly the brain in immunosuppressed individuals. Cn’s ability to survive phagocytosis and endure the onslaught of oxidative attack imposed by the innate immune response facilitates dissemination to the central nervous system (CNS). Despite the success of Cn at bypassing innate immunity, entry into the heavily protected brain requires that Cn overwhelm the highly restricted blood-brain barrier (BBB). This is a formidable task but mounting evidence suggests that Cn expresses surface-bound and secreted virulence factors including urease, metalloprotease, and hyaluronic acid that can undermine the BBB. In addition, Cn can exploit multiple routes of entry to gain access to the CNS. In this review, we discuss the cellular and molecular interface of Cn and the BBB, and we propose that the virulence factors mediating BBB crossing could be targeted for the development of anti-virulence drugs aimed at preventing fungal colonization of the CNS.

Mechanisms of Cryptococcus neoformans-Mediated Host Damage

Cryptococcus neoformans is not usually considered a cytotoxic fungal pathogen but there is considerable evidence that this microbe can damage host cells and tissues. In this essay, we review the evidence that C. neoformans damages host cells and note that the mechanisms involved are diverse. We consider C. neoformans-mediated host damage at the molecular, cellular, tissue, and organism level. Direct mechanisms of cytotoxicity include lytic exocytosis, organelle dysfunction, phagolysosomal membrane damage, and cytoskeletal alterations. Cytotoxicity contributes to pathogenesis by interfering with immune effector cell function and disrupting endothelial barriers thus allowing dissemination. When C. neoformans-mediated and immune-mediated host damage is sufficient to affect homeostasis, cryptococcosis occurs at the organism level.

Population Pharmacokinetic Modeling to Describe the Total Plasma and Free Brain Levels of Fluconazole in Healthy and Cryptococcus neoformans Infected Rats: How Does the Infection Impact the Drug's Levels on Biophase?

PURPOSE

The present work aimed to evaluate the influence of experimental meningitis caused by C. neoformans on total plasma and free brain concentrations of fluconazole (FLC) in Wistar rats.

METHOD

The infection was induced by the administration of 100 μL of inoculum (1.10⁵ CFU) through the tail vein. Free drug in the brain was assessed by microdialisys (μD). Blood and μD samples were collected at pre-determined time points up to 12 h after intravenous administration of FLC (20 mg/kg) to healthy and infected rats. The concentration-time profiles were analyzed by non-compartmental and population pharmacokinetics approaches.

RESULTS

A two-compartmental popPK model was able to simultaneously describe plasma and free drug concentrations in the brain for both groups investigated. Analysis of plasma and μD samples showed a better FLC distribution on the brain of infected than healthy animals (1.04 ± 0.31 vs 0.69 ± 0.14, respectively). The probability of target attainment was calculated by Monte Carlo simulations based on the developed popPK model for 125 mg/kg dose for rats and 400-2000 mg for humans.

CONCLUSIONS

FLC showed a limited use in monotherapy to the treatment of criptoccocosis in rats and humans to value of MIC >8 μg/mL.

Overview of carbon and nitrogen catabolite metabolism in the virulence of human pathogenic fungi

It is estimated that fungal infections, caused most commonly by Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, result in more deaths annually than malaria or tuberculosis. It has long been hypothesized the fungal metabolism plays a critical role in virulence though specific nutrient sources utilized by human pathogenic fungi in vivo has remained enigmatic. However, the metabolic utilisation of preferred carbon and nitrogen sources, encountered in a host niche-dependent manner, is known as carbon catabolite and nitrogen catabolite repression (CCR, NCR), and has been shown to be important for virulence. Several sensory and uptake systems exist, including carbon and nitrogen source-specific sensors and transporters, that allow scavenging of preferred nutrient sources. Subsequent metabolic utilisation is governed by transcription factors, whose functions and essentiality differ between fungal species. Furthermore, additional factors exist that contribute to the implementation of CCR and NCR. The role of the CCR and NCR-related factors in virulence varies greatly between fungal species and a substantial gap in knowledge exists regarding specific pathways. Further elucidation of carbon and nitrogen metabolism mechanisms is therefore required in a fungal species- and animal model-specific manner in order to screen for targets that are potential candidates for anti-fungal drug development.

Diversity of Cell Wall Related Proteins in Human Pathogenic Fungi

The cell wall is one of the major keys to fungal identity. Fungi use their cell wall to sense the environment, and localize nutrients and competing microorganism. Pathogenic species additionally modify their cell walls to hide from a host's immune system. With the growing number of fungal infections and alarming shortage of available drugs, we are in need of new approaches to fight pathogens. The cell wall seems to be a natural target, since animal host cells are devoid of it. The current knowledge about fungal cell wall components is often limited, and there is huge diversity both in structure and composition between species. In order to compare the distribution of diverse proteins involved in cell wall biosynthesis and maintenance, we performed sequence homology searches against 24 fungal proteomes from distinct taxonomic groups, all reported as human pathogens. This approach led to identification of 4014 cell wall proteins (CWPs), and enabled us to speculate about cell wall composition in recently sequenced pathogenic fungi with limited experimental information. We found large expansions of several CWP families, in particular taxa, and a number of new CWPs possibly involved in evading host immune recognition. Here, we present a comprehensive evolutionary history of fungal CWP families in the context of the fungal tree of life.

Overview of selected virulence attributes in Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Trichophyton rubrum, and Exophiala dermatitidis

The incidence of fungal diseases has been increasing since 1980, and is associated with excessive morbidity and mortality, particularly among immunosuppressed patients. Of the known 625 pathogenic fungal species, infections caused by the genera Aspergillus, Candida, Cryptococcus, and Trichophyton are responsible for more than 300 million estimated episodes of acute or chronic infections worldwide. In addition, a rather neglected group of opportunistic fungi known as black yeasts and their filamentous relatives cause a wide variety of recalcitrant infections in both immunocompetent and immunosuppressed hosts. This article provides an overview of selected virulence factors that are known to suppress host immunity and enhance the infectivity of these fungi.

A critical review on serine protease: Key immune manipulator and pathology mediator

Proteolytic activity is fundamental to survival, so it is not surprising that all living organisms have proteases, especially seine protease. This enzyme in its numerous isoforms and homologues, constitutes the quintessential offence and defence factors, in the form of surface proteins, secreted molecules, gut digestive enzymes, venom in specialised glands or plant latex, among other manifestations. Occurring as trypsin, chymotrypsin, elastase, collagenase, thrombin, subtilisin etc., it mediates a diverse array of functions, including pathological roles as inflammatory, coagulatory to haemorrhagic. This review emphasizes that despite the superficial differences in mechanisms, most health issues, be they infectious, allergic, metabolic, or neural have a common conduit. This enzyme, in its various glycosylated forms leads to signal misinterpretations, wreaking havoc. However, organisms are endowed with serine protease inhibitors which might restrain this ubiquitous yet deleterious enzyme. Hence, serine proteases-driven pathogenesis and antagonising role of inhibitors is the focal point of this critical review.

Genetic, Transcriptome, Proteomic, and Epidemiological Evidence for Blood-Brain Barrier Disruption and Polymicrobial Brain Invasion as Determinant Factors in Alzheimer's Disease

Diverse pathogens are detected in Alzheimer's disease (AD) brains. A bioinformatics survey showed that AD genome-wide association study (GWAS) genes (localized in bone marrow, immune locations and microglia) relate to multiple host/pathogen interactomes (Candida albicans, Cryptococcus neoformans, Bornavirus, Borrelia burgdorferri, cytomegalovirus, Ebola virus, HSV-1, HERV-W, HIV-1, Epstein-Barr, hepatitis C, influenza, Chlamydia pneumoniae, Porphyrymonas gingivalis, Helicobacter pylori, Toxoplasma gondii, Trypanosoma cruzi). These interactomes also relate to the AD hippocampal transcriptome and to plaque or tangle proteins. Upregulated AD hippocampal genes match those upregulated by multiple bacteria, viruses, fungi, or protozoa in immunocompetent cells. AD genes are enriched in GWAS datasets reflecting pathogen diversity, suggesting selection for pathogen resistance, as supported by the old age of AD patients, implying resistance to earlier infections. APOE4 is concentrated in regions of high parasitic burden and protects against childhood tropical infections and hepatitis C. Immune/inflammatory gain of function applies to APOE4, CR1, and TREM2 variants. AD genes are also expressed in the blood-brain barrier (BBB), which is disrupted by AD risk factors (age, alcohol, aluminum, concussion, cerebral hypoperfusion, diabetes, homocysteine, hypercholesterolemia, hypertension, obesity, pesticides, pollution, physical inactivity, sleep disruption, smoking) and by pathogens, directly or via olfactory routes to basal-forebrain BBB control centers. The BBB benefits from statins, NSAIDs, estrogen, melatonin, memantine, and the Mediterranean diet. Polymicrobial involvement is supported by upregulation of bacterial, viral, and fungal sensors/defenders in the AD brain, blood, or cerebrospinal fluid. AD serum amyloid-β autoantibodies may attenuate its antimicrobial effects favoring microbial survival and cerebral invasion leading to activation of neurodestructive immune/inflammatory processes, which may also be augmented by age-related immunosenescence. AD may thus respond to antibiotic, antifungal, or antiviral therapy.

Improved Method for the Establishment of an In Vitro Blood-Brain Barrier Model Based on Porcine Brain Endothelial Cells

The aim of this protocol presents an optimized procedure for the purification and cultivation of pBECs and to establish in vitro blood-brain barrier (BBB) models based on pBECs in mono-culture (MC), MC with astrocyte-conditioned medium (ACM), and non-contact co-culture (NCC) with astrocytes of porcine or rat origin. pBECs were isolated and cultured from fragments of capillaries from the brain cortices of domestic pigs 5-6 months old. These fragments were purified by careful removal of meninges, isolation and homogenization of grey matter, filtration, enzymatic digestion, and centrifugation. To further eliminate contaminating cells, the capillary fragments were cultured with puromycin-containing medium. When 60-95% confluent, pBECs growing from the capillary fragments were passaged to permeable membrane filter inserts and established in the models. To increase barrier tightness and BBB characteristic phenotype of pBECs, the cells were treated with the following differentiation factors: membrane permeant 8-CPT-cAMP (here abbreviated cAMP), hydrocortisone, and a phosphodiesterase inhibitor, RO-20-1724 (RO). The procedure was carried out over a period of 9-11 days, and when establishing the NCC model, the astrocytes were cultured 2-8 weeks in advance. Adherence to the described procedures in the protocol has allowed the establishment of endothelial layers with highly restricted paracellular permeability, with the NCC model showing an average transendothelial electrical resistance (TEER) of 1249 ± 80 Ω cm², and paracellular permeability (Papp) for Lucifer Yellow of 0.90 10^-6 ± 0.13 10^-6 cm sec^-1 (mean ± SEM, n=55). Further evaluation of this pBEC phenotype showed good expression of the tight junctional proteins claudin 5, ZO-1, occludin and adherens junction protein p120 catenin. The model presented can be used for a range of studies of the BBB in health and disease and, with the highly restrictive paracellular permeability, this model is suitable for studies of transport and intracellular trafficking.

Fungal lifestyle reflected in serine protease repertoire

Fungi are able to switch between different lifestyles in order to adapt to environmental changes. Their ecological strategy is connected to their secretome as fungi obtain nutrients by secreting hydrolytic enzymes to their surrounding and acquiring the digested molecules. We focus on fungal serine proteases (SPs), the phylogenetic distribution of which is barely described so far. In order to collect a complete set of fungal proteases, we searched over 600 fungal proteomes. Obtained results suggest that serine proteases are more ubiquitous than expected. From 54 SP families described in MEROPS Peptidase Database, 21 are present in fungi. Interestingly, 14 of them are also present in Metazoa and Viridiplantae - this suggests that, except one (S64), all fungal SP families evolved before plants and fungi diverged. Most representatives of sequenced eukaryotic lineages encode a set of 13-16 SP families. The number of SPs from each family varies among the analysed taxa. The most abundant are S8 proteases. In order to verify hypotheses linking lifestyle and expansions of particular SP, we performed statistical analyses and revealed previously undescribed associations. Here, we present a comprehensive evolutionary history of fungal SP families in the context of fungal ecology and fungal tree of life.

Cryptococcus neoformans ADS lyase is an enzyme essential for virulence whose crystal structure reveals features exploitable in antifungal drug design

There is significant clinical need for new antifungal agents to manage infections with pathogenic species such as Cryptococcus neoformans Because the purine biosynthesis pathway is essential for many metabolic processes, such as synthesis of DNA and RNA and energy generation, it may represent a potential target for developing new antifungals. Within this pathway, the bifunctional enzyme adenylosuccinate (ADS) lyase plays a role in the formation of the key intermediates inosine monophosphate and AMP involved in the synthesis of ATP and GTP, prompting us to investigate ADS lyase in C. neoformans. Here, we report that ADE13 encodes ADS lyase in C. neoformans. We found that an ade13Δ mutant is an adenine auxotroph and is unable to successfully cause infections in a murine model of virulence. Plate assays revealed that production of a number of virulence factors essential for dissemination and survival of C. neoformans in a host environment was compromised even with the addition of exogenous adenine. Purified recombinant C. neoformans ADS lyase shows catalytic activity similar to its human counterpart, and its crystal structure, the first fungal ADS lyase structure determined, shows a high degree of structural similarity to that of human ADS lyase. Two potentially important amino acid differences are identified in the C. neoformans crystal structure, in particular a threonine residue that may serve as an additional point of binding for a fungal enzyme-specific inhibitor. Besides serving as an antimicrobial target, C. neoformans ADS lyase inhibitors may also serve as potential therapeutics for metabolic disease; rather than disrupt ADS lyase, compounds that improve the stability the enzyme may be used to treat ADS lyase deficiency disease.

Complex Roles of Annexin A2 in Host Blood-Brain Barrier Invasion by Cryptococcus neoformans

INTRODUCTION

Fungal transversal across the brain microvascular endothelial cells (BMECs) is the essential step for the development of cryptococcal meningoencephalitis. Annexin A2 (AnxA2) is an important signaling protein involved in several intracellular processes such as membrane trafficking, endocytosis, and exocytosis.

AIM

To investigate the roles and mechanism of AnxA2 during cryptococcal transversal of BMECs.

RESULTS

Cryptococcus neoformans infection initiated upregulation of AnxA2 in mouse BMECs. Blockade with anti-AnxA2 antibody led to a reduction in fungal transcytosis activity but no change in its adhesion efficiency. Intriguingly, AnxA2 depletion caused a significant increase in fungal association activity but had no effect on their transcytosis. AnxA2 suppression resulted in marked reduction in its partner protein S100A10, and S100A10 suppression in BMECs significantly reduced the cryptococcal transcytosis efficiency. Furthermore, AnxA2 dephosphorylation at Tyr23 and dephosphorylation of downstream cofilin were required for cryptococcal transversal of BMECs, both of which might be primarily involved in the association of C. neoformans with host cells.

CONCLUSIONS

Our work indicated that AnxA2 played complex roles in traversal of C. neoformans across host BMECs, which might be dependent on downstream cofilin to inhibit fungal adhesion but rely on its partner S100A10 to promote cryptococcal transcytosis.

Fungal colonization of the brain: anatomopathological aspects of neurological cryptococcosis

Brain infection by the fungus Cryptococcus neoformans results in an estimated 500,000 human deaths per annum. Colonization of the central nervous system (CNS) by C. neoformans causes different clinical syndromes that involve interaction of a number of fungal components with distinct brain cells. In this manuscript, our literature review confirmed the notion that the Cryptococcus field is expanding rapidly, but also suggested that studies on neuropathogenesis still represent a small fraction of basic research activity in the field. We therefore discussed anatomical and physiological aspects of the brain during infection by C. neoformans, in addition to mechanisms by which brain resident cells interact with the fungus. This review suggests that multiple efforts are necessary to improve the knowledge on how C. neoformans affects brain cells, in order to enable the generation of new therapeutic tools in a near future.

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.

Phenotypic characteristics associated with virulence of clinical isolates from the Sporothrix complex

The Sporothrix complex members cause sporotrichosis, a subcutaneous mycosis with a wide spectrum of clinical manifestations. Several specific phenotypic characteristics are associated with virulence in many fungi, but studies in this field involving the Sporothrix complex species are scarce. Melanization, thermotolerance, and production of proteases, catalase, and urease were investigated in 61 S. brasiliensis, one S. globosa, and 10 S. schenckii strains. The S. brasiliensis strains showed a higher expression of melanin and urease compared with S. schenckii. These two species, however, presented similar thermotolerances. Our S. globosa strain had low expression of all studied virulence factors. The relationship between these phenotypes and clinical aspects of sporotrichosis was also evaluated. Strains isolated from patients with spontaneous regression of infection were heavily melanized and produced high urease levels. Melanin was also related to dissemination of internal organs and protease production was associated with HIV-coinfection. A murine sporotrichosis model showed that a S. brasiliensis strain with high expression of virulence factors was able to disseminate and yield a high fungal burden in comparison with a control S. schenckii strain. Our results show that virulence-related phenotypes are variably expressed within the Sporothrix complex species and might be involved in clinical aspects of sporotrichosis.

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.