Secreted aspartyl proteases family: a perspective review on the regulation of fungal pathogenesis

Functional Characterization of FgAsp, a Gene Coding an Aspartic Acid Protease in Fusarium graminearum

Aspartic proteases (APs), hydrolases with aspartic acid residues as catalytic active sites, are closely associated with processes such as plant growth and development and fungal and bacterial pathogenesis. F. graminearum is the dominant pathogenic fungus that causes Fusarium head blight (FHB) in wheat. However, the relationship of APs to the growth, development, and pathogenesis of F. graminearum is not clear. Therefore, we selected the FGSG_09558 gene, whose function annotation is aspartate protease, for further study. In this study, FGSG_09558 was found to contain a conserved structural domain and signal peptide sequence of aspartic acid protease and was therefore named FgAsp. The function of FgAsp in F. graminearum was investigated by constructing the knockout and complementation mutants of this gene. The results showed that with respect to the wild type (PH-1), the knockout mutant showed a significant reduction in mycelial growth, asexual spore production, and sexual spore formation, highlighting the key role of FgAsp in the growth and development of F. graminearum. In addition, the mutants showed a significant reduction in the virulence and accumulation level of deoxynivalenol (DON) content on maize whiskers, wheat germ sheaths, and wheat ears. DON, as a key factor of virulence, plays an important role in the F. graminearum infection of wheat ears, suggesting that FgAsp is involved in the regulation of F. graminearum pathogenicity by affecting the accumulation of the DON toxin. FgAsp had a significant effect on the ability of F. graminearum to utilize various sugars, especially arabinose. In response to the stress, hydrogen peroxide inhibited the growth of the mutant most significantly, indicating the important function of FgAsp in the strain's response to environmental stress. Finally, FgAsp plays a key role in the regulation of F. graminearum growth and development, pathogenicity, and environmental stress response.

Exploring the effects of culture conditions on Yapsin (YPS) gene expression in Nakaseomyces glabratus

Nakaseomyces glabratus, previously known as Candida glabrata, has the great potential to cause systemic fungal infections despite its similarity to baker's yeast. Its pathogenicity is attributed to the production of numerous virulence factors, among which the YPS genes (YPS1-YPS11) encoding aspartyl proteases have yet to be sufficiently characterized, and limited studies suggest their involvement in cellular homeostasis. The study's novelty is an investigation of the role of YPS in N. glabratus's ability to adapt to different host environments. For this purpose, we isolated RNA from N. glabratus cells grown in both host niche-mimicking culture media, such as artificial saliva (AS) and vagina-simulating media (VS), as well as standard yeast media (RPMI 1640 and YPDA). We then performed quantitative real-time PCR to evaluate YPS gene expression at different growth phases. At the early logarithmic phase, we observed a general increase in the expression levels of YPS genes; however, at the stationary phase, high expression levels were maintained for YPS7 in RPMI 1640 and YPDA media and YPS6 in RPMI 1640 and AS media. In addition, although the VS medium does not promote the proliferation of N. glabratus, the yeast can survive in an acidic environment, and the significantly overexpressed gene is YPS7. These findings underscore the significant modulation of N. glabratus YPS gene expression in response to external environmental conditions. This research provides insights into the molecular basis of N. glabratus pathogenicity and highlights new potential targets for antifungal therapy.

Nanoparticles in the battle against Candida auris biofilms: current advances and future prospects

Candida auris has emerged as a significant global health threat due to its multidrug resistance and ability to form robust biofilms, particularly on medical devices and hospital surfaces. Biofilms protect C. auris from antifungal treatments and the host immune response, making infections persistent and difficult to control. This review explores the potential of nanoparticles to overcome the limitations of traditional antifungal therapies in combating C. auris biofilms. Nanoparticles, with their unique physicochemical properties, offer promising strategies to penetrate biofilm matrices, deliver antifungal agents, and disrupt biofilm structure. Various types of nanoparticles, including metallic, polymeric, lipid-based, and cyclodextrin-based, demonstrate enhanced biofilm penetration and antifungal activity. Their ability to generate reactive oxygen species, disrupt cell adhesion, and release antifungals in a controlled manner makes them ideal candidates for biofilm-targeted therapies. This review presents the current advancements in nanoparticle-based solutions, emphasizing the need for further research into their mechanisms of action, safety, and clinical application. By addressing the challenge of C. auris biofilms specifically, this review provides a critical synthesis of existing knowledge and identifies future directions for developing effective antifungal therapies using nanotechnology.

Cryptococcus neoformans infections: aspartyl protease potential to improve outcome in susceptible hosts

Though a confined or a broad population is exposed respectively to endemic or pandemic infections, in the same environment, some individuals resist the development of infections. The attributed reason is the inheritance of a set of immune system genes that can efficiently deal with the pathogens. In this study, we show how outbred mice differentially respond to Cryptococcus neoformans, a fungal pathogen, and the mechanism through which the surviving mice mount a protective immune defense. We identified that those mice developing antibodies specifically against Pep1p, an aspartic protease secreted by C. neoformans, had significantly improved survival. Vaccination (either prophylactic or therapeutic) with a recombinant Pep1p significantly increased the survival of the mice by decreasing the fungal load and stimulating a protective immune response. Passive immunization of C. neoformans-infected mice with monoclonal antibodies developed against Pep1p also improves the survival of the mice by increasing phagocytosis of C. neoformans and decreasing the multiplication of this fungus. Together, these data demonstrate the prophylactic and therapeutic potentials of the C. neoformans antigenic protein Pep1p or Pep1p-specific antibodies against this fungal infection. Also, this study suggests that the immunological interaction and thereby the responses developed against a pathogen guide the hosts to behave differentially against microbial pathogenicity.

IMPORTANCE

Vaccination and immunotherapies against fungal pathogens still remain a challenge. Here, we show using an in vivo model based on outbred mice that development of antibodies against Pep1p, an antigenic protein of the fungal pathogen Cryptococcus neoformans, confers resistance to this fungal infection. In support of this observation, prophylactic or therapeutic immunization of the mice with recombinant Pep1p could improve their survival when infected with a lethal dose of C. neoformans. Moreover, passive therapy with monoclonal anti-Pep1p antibodies also enhanced survival of the mice from C. neoformans infection. The associated antifungal mechanisms were mounting of a protective immune response and the development of fungal specific antibodies that decrease the fungal burden due to an increase in their phagocytosis and/or inhibit the fungal multiplication. Together, our study demonstrates (a) the mode of host-fungal interaction and the immune response developed thereby play a crucial role in developing resistance against C. neoformans; (b) Pep1p, an aspartic protease as well as an antigenic protein secreted by C. neoformans, can be exploited for vaccination (both prophylactic and therapeutic) or immunotherapy to improve the host defense during this fungal infection.

Monoterpene antifungal activities: evaluating geraniol, citronellal, and linalool on Candida biofilm, host inflammatory responses, and structure-activity relationships

Introduction: Despite the rising concern with fungal resistance, a myriad of molecules has yet to be explored. Geraniol, linalool, and citronellal are monoterpenes with the same molecular formula (C10H18O), however, neither the effect of these compounds on inflammatory axis induced by Candida spp. nor the antibiofilm Structure-Activity Relationship (SAR) have been well-investigated. Herein we analyzed geraniol, linalool and citronellal antifungal activity, cytotoxicity, and distinctive antibiofilm SAR, also the influence of geraniol on Candida spp induced dysregulated inflammatory axis, and in vivo toxicity. Methods: Minimal inhibitory (MIC) and fungicidal (MFC) concentrations against Candida spp were defined, followed by antibiofilm activity (CFU-colony forming unit/mL/g of dry weight). Cytotoxic activity was assessed using human monocytes (THP-1) and oral squamous cell (TR146). Geraniol was selected for further analysis based on antifungal, antibiofilm and cytotoxic results. Geraniol was tested using a dual-chamber co-culture model with TR146 cells infected with C. albicans, and THP-1 cells, used to mimic oral epithelium upon fungal infection. Expression of Candida enzymes (phospholipase-PLB and aspartyl proteases-SAP) and host inflammatory cytokines (interleukins: IL-1β, IL-6, IL-17, IL-18, IL-10, and Tumor necrosis factor-TNF) were analyzed. Lastly, geraniol in vivo toxicity was assessed using Galleria mellonella. Results: MIC values obtained were 1.25-5 mM/mL for geraniol, 25-100 mM/mL for linalool, and 100-200 mM/mL for citronellal. Geraniol 5 and 50 mM/mL reduced yeast viability during biofilm analysis, only 500 mM/mL of linalool was effective against a 72 h biofilm and no biofilm activity was seen for citronellal. LD50 for TR146 and THP-1 were, respectively: geraniol 5.883 and 8.027 mM/mL; linalool 1.432 and 1.709 mM/mL; and citronellal 0.3006 and 0.1825 mM/mL. Geraniol was able to downregulate expression of fungal enzymes and host pro-inflammatory cytokines IL-1β, IL-6, and IL-18. Finally, safety in vivo parameters were observed up to 20 mM/Kg. Discussion: Despite chemical similarities, geraniol presented better antifungal, antibiofilm activity, and lower cytotoxicity when compared to the other monoterpenes. It also showed low in vivo toxicity and capacity to downregulate the expression of fungal enzymes and host pro-inflammatory cytokines. Thus, it can be highlighted as a viable option for oral candidiasis treatment.

Candida parapsilosis: Heterogeneous and strain-specific expression of secreted aspartic proteases (Sapp1 and Sapp2)

The increasing prevalence of Candida parapsilosis as a causative agent of fungal infections underscores the need to comprehensively understand its virulence factors. Secreted aspartic proteases (Saps) play a significant role in adhesion events, promoting biofilm formation, causing tissue damage and evading the host's immune response. In C. parapsilosis, three Saps have been identified: Sapp1, Sapp2 and Sapp3. The present study investigates the production dynamics of Sapp1 and Sapp2 across 10 clinical isolates of C. parapsilosis using various approaches. Each fungal isolate demonstrated the capability to utilize bovine serum albumin (BSA) as the sole nitrogen source, as evidenced by its degradation in a cell-free culture medium, forming low molecular mass polypeptides. Interestingly, the degradation of different proteinaceous substrates, such as BSA, human serum albumin (HSA), gelatin and hemoglobin, was typically isolate-dependent. Notably, higher proteolysis of HSA compared to BSA, gelatin and hemoglobin was observed. A quantitative assay revealed that the cleavage of a peptide fluorogenic substrate (cathepsin D) was isolate-specific, ranging from 44.15 to 270.61 fluorescence arbitrary units (FAU), with a mean proteolysis of 150.7 FAU. The presence of both Sapp1 and Sapp2 antigens on the cell surface of these fungal isolates was confirmed through immunological detection employing specific anti-Sapp1 and anti-Sapp2 antibodies. The surface levels of Sapp1 were consistently higher, up to fourfold, compared to Sapp2. Similarly, higher levels of Sapp1 than Sapp2 were detected in fungal secretions. This study provides insights into the dynamic expression and regulation of Sapps in C. parapsilosis, highlighting a known virulence factor that is considered a potential target for drug development against this increasingly prominent pathogen.

Saps1-3 Antigens in Candida albicans: Differential Modulation Following Exposure to Soluble Proteins, Mammalian Cells, and Infection in Mice

The secreted aspartic peptidases (Saps) of Candida albicans play crucial roles in various steps of fungal-host interactions. Using a flow cytometry approach, this study investigated the expression of Saps1-3 antigens after (i) incubation with soluble proteins, (ii) interaction with mammalian cells, and (iii) infection in immunosuppressed BALB/c mice. Supplementation strategies involving increasing concentrations of bovine serum albumin (BSA) added to yeast carbon base (YCB) medium as the sole nitrogenous source revealed a positive and significant correlation between BSA concentration and both the growth rate and the percentage of fluorescent cells (%FC) labeled with anti-Saps1-3 antibodies. Supplementing the YCB medium with various soluble proteins significantly modulated the expression of Saps1-3 antigens in C. albicans. Specifically, immunoglobulin G, gelatin, and total bovine/human sera significantly reduced the %FC, while laminin, human serum albumin, fibrinogen, hemoglobin, and mucin considerably increased the %FC compared to BSA. Furthermore, co-cultivating C. albicans yeasts with either live epithelial or macrophage cells induced the expression of Saps1-3 antigens in 78% (mean fluorescence intensity [MFI] = 152.1) and 82.7% (MFI = 178.2) of the yeast cells, respectively, compared to BSA, which resulted in 29.3% fluorescent cells (MFI = 50.9). Lastly, the yeasts recovered from the kidneys of infected immunosuppressed mice demonstrated a 4.8-fold increase in the production of Saps1-3 antigens (MFI = 246.6) compared to BSA, with 95.5% of yeasts labeled with anti-Saps1-3 antibodies. Altogether, these results demonstrated the positive modulation of Saps' expression in C. albicans by various key host proteinaceous components, as well as by in vitro and in vivo host challenges.

Multiplexed DNA and Protease Detection with Orthogonal Energy Transfer on a Single Quantum Dot Scaffolded Biosensor

Almost all pathogens, whether viral or bacterial, utilize key proteolytic steps in their pathogenesis. The ability to detect a pathogen's genomic material along with its proteolytic activity represents one approach to identifying the pathogen and providing initial evidence of its viability. Here, we report on a prototype biosensor design assembled around a single semiconductor quantum dot (QD) scaffold that is capable of detecting both nucleic acid sequences and proteolytic activity by using orthogonal energy transfer (ET) processes. The sensor consists of a central QD assembled via peptidyl-PNA linkers with multiple DNA sequences that encode complements to genomic sequences originating from the Ebola, Influenza, and COVID-19 viruses, which we use as surrogate targets. These are hybridized to complement strands labeled with a terbium (Tb) chelate, AlexaFluor647 (AF647), and Cy5.5 dyes, giving rise to two potential FRET cascades: the first includes Tb → QD → AF647 → Cy5.5 (→ = ET step), which is detected in a time-gated modality, and QD → AF647 → Cy5.5, which is detected from direct excitation. The labeled DNA-displaying QD construct is then further assembled with a RuII-modified peptide, which quenches QD photoluminescence by charge transfer and is recognized by a protease to yield the full biosensor. Each of the labeled DNAs and peptides can be ratiometrically assembled to the QD in a controllable manner to tune each of the ET pathways. Addition of a given target DNA displaces its labeled complement on the QD, disrupting that FRET channel, while protease addition disrupts charge transfer quenching of the central QD scaffold and boosts its photoluminescence and FRET relay capabilities. Along with characterizing the ET pathways and verifying biosensing in both individual and multiplexed formats, we also demonstrate the ability of this construct to function in molecular logic and perform Boolean operations; this highlights the construct's ability to discriminate and transduce signals between different inputs or pathogens. The potential application space for such a sensor device is discussed.

Candida die-off: Adverse effect and neutralization with phytotherapy approaches

Candida albicans is the main species that causes 3rd most common bloodstream infection candidiasis in hospitalization. Once it has been diagnosed and treated with antifungal medications accurately, large amounts of Candida cells are killed off rapidly known as Candida die-off or Jarisch-Herxheimer reactions. When Candida cells are killed off quickly, a large no. of toxic substances are released simultaneously. This flood of endotoxins is noxious (harmful) and causes the kidneys and liver to work overtime to try and remove them which causes worsening of symptoms in patients. As a complementary and holistic approach to addressing Candida die-off and its associated symptoms, plant-based remedies i.e., phytotherapy have been gaining increased attention. In this review paper, we have discussed major factors involved in provoking Candida die-off, their management by phytotherapy, challenges associated with the toxic effects due to die-off, and neutralization of Candida die-off through phytotherapy to manage this problem and challenges. In conclusion, this article serves as a meticulous compilation of knowledge on the intriguing subject of Candida die-off, presenting a distinct and informative perspective that has the potential to pave the way for new insights in the realm of plant-based antifungal therapeutics.

Biofilm Formation and Aspartyl Proteinase Activity and Their Association with Azole Resistance Among Candida albicans Causing Vulvovaginal Candidiasis, Egypt

Background

Candida albicans (C. albicans) is a major cause of vulvovaginal candidiasis (VVC), a condition that is commonly treated with azole agents. Biofilm formation and aspartyl proteinase production are important virulence factors that could be linked to azole resistance in C. albicans impeding therapy.

Aim

To find out the association of both factors with azole resistance among C. albicans isolated from VVC cases in Egyptian nonpregnant women of childbearing age.

Patients and Methods

In a cross-sectional study, C. albicans was isolated from nonpregnant females diagnosed clinically as having VVC during a 1-year study period. Susceptibility to azole agents was tested using the disc diffusion method. Biofilm formation and aspartyl proteinase production were assessed phenotypically. Additionally, two biofilm-related genes (ALS1 and HWP1) and three proteinase genes (SAP2, SAP4, and SAP6) were screened for using polymerase chain reaction (PCR).

Results

Among 204 C. albicans isolates, azole resistance ratios were as follows: voriconazole (30.4%), itraconazole (17.6%), fluconazole (11.3%) and econazole (6.4%). Biofilm-producing capacity was detected in 63.2% of isolates, and 63.2% were proteinase producers. The frequencies of ALS1 and HWP1 were 69.6% and 74.5%, respectively, while SAP2, SAP4, and SAP6 were 69.2%, 88.7%, and 64.7%, respectively. Biofilm formation was significantly associated with azole resistance (P < 0.001 for each tested azole agent) as was proteinase production (P < 0.001 for fluconazole, voriconazole, and econazole resistance and P = 0.047 for itraconazole).

Conclusion

Among nonpregnant Egyptian women of childbearing age, azole resistance in C. albicans causing VVC is significantly associated with biofilm formation and proteinase production. The development of new therapeutic agents that can target these factors is warranted.