Candida parapsilosis expresses and secretes two aspartic proteinases

Secreted Aspartic Proteinases: Key Factors in Candida Infections and Host-Pathogen Interactions

Extracellular proteases are key factors contributing to the virulence of pathogenic fungi from the genus Candida. Their proteolytic activities are crucial for extracting nutrients from the external environment, degrading host defenses, and destabilizing the internal balance of the human organism. Currently, the enzymes most frequently described in this context are secreted aspartic proteases (Saps). This review comprehensively explores the multifaceted roles of Saps, highlighting their importance in biofilm formation, tissue invasion through the degradation of extracellular matrix proteins and components of the coagulation cascade, modulation of host immune responses via impairment of neutrophil and monocyte/macrophage functions, and their contribution to antifungal resistance. Additionally, the diagnostic challenges associated with Candida infections and the potential of Saps as biomarkers were discussed. Furthermore, we examined the prospects of developing vaccines based on Saps and the use of protease inhibitors as adjunctive therapies for candidiasis. Given the complex biology of Saps and their central role in Candida pathogenicity, a multidisciplinary approach may pave the way for innovative diagnostic strategies and open new opportunities for innovative clinical interventions against candidiasis.

Candida-Epithelial Interactions

A plethora of intricate and dynamic molecular interactions occur between microbes and the epithelial cells that form the mucosal surfaces of the human body. Fungi, particularly species of Candida, are commensal members of our microbiota, continuously interacting with epithelial cells. Transient and localised perturbations to the mucosal environment can facilitate the overgrowth of fungi, causing infection. This minireview will examine the direct and indirect mechanisms by which Candida species and epithelial cells interact with each other, and explore the factors involved in the central processes of adhesion, invasion, and destruction of host mucosal surfaces.

Atomic resolution crystal structure of Sapp2p, a secreted aspartic protease from Candida parapsilosis

The virulence of the Candida pathogens is enhanced by the production of secreted aspartic proteases, which therefore represent possible targets for drug design. Here, the crystal structure of the secreted aspartic protease Sapp2p from Candida parapsilosis was determined. Sapp2p was isolated from its natural source and crystallized in complex with pepstatin A, a classical aspartic protease inhibitor. The atomic resolution of 0.83 Å allowed the protonation states of the active-site residues to be inferred. A detailed comparison of the structure of Sapp2p with the structure of Sapp1p, the most abundant C. parapsilosis secreted aspartic protease, was performed. The analysis, which included advanced quantum-chemical interaction-energy calculations, uncovered molecular details that allowed the experimentally observed equipotent inhibition of both isoenzymes by pepstatin A to be rationalized.

Release of biologically active kinin peptides, Met-Lys-bradykinin and Leu-Met-Lys-bradykinin from human kininogens by two major secreted aspartic proteases of Candida parapsilosis

In terms of infection incidence, the yeast Candida parapsilosis is the second after Candida albicans as causative agent of candidiases in humans. The major virulence factors of C. parapsilosis are secreted aspartic proteases (SAPPs) which help the pathogen to disseminate, acquire nutrients and dysregulate the mechanisms of innate immunity of the host. In the current work we characterized the action of two major extracellular proteases of C. parapsilosis, SAPP1 and SAPP2, on human kininogens, proteinaceous precursors of vasoactive and proinflammatory bradykinin-related peptides, collectively called the kinins. The kininogens, preferably the form with lower molecular mass, were effectively cleaved by SAPPs, with the release of two uncommon kinins, Met-Lys-bradykinin and Leu-Met-Lys-bradykinin. While optimal at acidic pH (4-5), the kinin release yield was only 2-3-fold lower at neutral pH. These peptides were able to interact with cellular kinin receptors of B2 subtype and to stimulate the human endothelial cells HMEC-1 to increased secretion of proinflammatory interleukins (ILs), IL-1β and IL-6. The analysis of the stability of SAPP-generated kinins in plasma suggested that they are biologically equivalent to bradykinin, the best agonist of B2 receptor subtype and can be quickly converted to des-Arg(9)-bradykinin, the agonist of inflammation-inducible B1 receptors.

Evidence for the presence of proteolytically active secreted aspartic proteinase 1 of Candida parapsilosis in the cell wall

Pathogenic yeasts of the genus Candida produce secreted aspartic proteinases, which are known to enhance virulence. We focused on Sapp1p proteinase secreted by Candida parapsilosis and studied the final stage of its passage through the cell wall and release into the extracellular environment. We found that Sapp1p displays enzyme activity prior to secretion, and therefore, it is probably fully folded within the upper layer of the cell wall. The positioning of cell surface-associated Sapp1p was detected by cell wall protein labeling using biotinylation agents, extraction of cell wall proteins by β-mercaptoethanol, immunochemical detection, and mass spectrometry analysis. All lysine residues present in the structure of soluble, purified Sapp1p were labeled with biotin. In contrast, the accessibility of individual lysines in cell wall-associated Sapp1p varied with the exception of four lysine residues that were biotinylated in all experiments performed, suggesting that Sapp1p has a preferred orientation in the cell wall. As the molecular weight of this partially labeled Sapp1p did not differ among the experiments, we can assume that the retaining of Sapp1p in the cell wall is not a totally random process and that pathogenic yeasts might use this cell-associated proteinase activity to enhance degradation of appropriate substrates.

Increased expression of keratinase and other peptidases by Candida parapsilosis mutants

Keratinases are enzymes of great importance involved in pathogenic processes of some fungi. They also have a widespread ecological role since they are responsible for the degradation and recycling of keratin. On the one hand, studying them furthers our knowledge of pathogenicity mechanisms, which has important implications for human health, and on the other hand, understanding their ecological role in keratin recycling has biotechnological potential. Here, a wild-type keratinolytic Candida parapsilosis strain isolated from a poultry farm was treated with ethyl methanesulfonate in order to generate mutants with increased keratinase activity. Mutants were then cultured on media with keratin extracted from chicken feathers as the sole source of nitrogen and carbon. Approximately 500 mutants were screened and compared with the described keratinolytic wild type. Three strains, H36, I7 and J5, showed enhanced keratinase activity. The wild-type strain produced 80 U/mL of keratinolytic activity, strain H36 produced 110 U/mL, strain I7, 130 U/mL, and strain J5, 140 U/mL. A 70% increase in enzyme activity was recorded for strain J5. Enzymatic activity was evaluated by zymograms with proteic substrates. A peptidase migrating at 100 kDa was detected with keratin, bovine serum albumin and casein. In addition, a peptidase with a molecular mass of 50 kDa was observed with casein in the wild-type strain and in mutants H36 and J5. Gelatinase activity was detected at 60 kDa. A single band of 35 kDa was found in wild-type C. parapsilosis and in mutants with hemoglobin substrate.

Candida parapsilosis: a review of its epidemiology, pathogenesis, clinical aspects, typing and antimicrobial susceptibility

The Candida parapsilosis family has emerged as a major opportunistic and nosocomial pathogen. It causes multifaceted pathology in immuno-compromised and normal hosts, notably low birth weight neonates. Its emergence may relate to an ability to colonize the skin, proliferate in glucose-containing solutions, and adhere to plastic. When clusters appear, determination of genetic relatedness among strains and identification of a common source are important. Its virulence appears associated with a capacity to produce biofilm and production of phospholipase and aspartyl protease. Further investigations of the host-pathogen interactions are needed. This review summarizes basic science, clinical and experimental information about C. parapsilosis.

Candida parapsilosis, an emerging fungal pathogen

SUMMARY

Candida parapsilosis is an emerging major human pathogen that has dramatically increased in significance and prevalence over the past 2 decades, such that C. parapsilosis is now one of the leading causes of invasive candidal disease. Individuals at the highest risk for severe infection include neonates and patients in intensive care units. C. parapsilosis infections are especially associated with hyperalimentation solutions, prosthetic devices, and indwelling catheters, as well as the nosocomial spread of disease through the hands of health care workers. Factors involved in disease pathogenesis include the secretion of hydrolytic enzymes, adhesion to prosthetics, and biofilm formation. New molecular genetic tools are providing additional and much-needed information regarding C. parapsilosis virulence. The emerging information will provide a deeper understanding of C. parapsilosis pathogenesis and facilitate the development of new therapeutic approaches for treating C. parapsilosis infections.

Development of a set of plasmid vectors for genetic manipulations of the pathogenic yeast Candida parapsilosis

A system for genetic transformation of the yeast Candida parapsilosis, recently developed in our laboratory, opened a venue for investigation of this pathogenic species at the molecular level. In this study we extend the range of available experimental tools by construction of a genomic DNA library suitable for screening and isolation of genes by functional complementation of yeast mutants and a set of replicative plasmid vectors for genetic manipulation of C. parapsilosis cells. The plasmids are based on auxotrophic (CpGAL1, CpURA3, CpMET2, CpLYS4) and dominant (CaIMH3) selection markers. In addition, we constructed plasmid derivatives containing reporter genes yEGFP3 and KlLAC4 coding for enhanced version of the green fluorescent protein and Kluyveromyces lactis beta-galactosidase, respectively. The vectors facilitate propagation and expression of cloned genes in C. parapsilosis cells and allow intracellular localization of gene products and/or monitoring the activity of promoter sequences.

Cloning and characterization of Sapp2p, the second aspartic proteinase isoenzyme from Candida parapsilosis

The human fungal pathogen Candida parapsilosis possesses at least three genes encoding secreted aspartic proteinases. Whereas the Sapp1p isoenzyme has already been biochemically characterized, the SAPP2 and SAPP3 gene products have not. The Sapp2p precursor, pro-Sapp2p, was therefore expressed in Escherichia coli and purified. Autoactivation of pro-Sapp2p in acidic conditions was inefficient and resulted in a protein extended by eight amino acids at the N-terminus (Sapp2p(+8)). The correct promature junction KR/SSPSS was cleaved by trypsin or by a membrane-bound Kex2-like proteinase from Candida parapsilosis. The mature Sapp2p obtained by the assisted activation was proteolytically active. Its activity was more than twofold higher than that of the self-processed protein species Sapp2p(+8), as measured by the hemoglobin cleavage test. The substrate specificity of Sapp2p differs from that of Sapp1p. Peptides containing aromatic residues in the P1 and P1' positions are cleaved poorly by Sapp2p. A fluorogenic substrate was synthesized to facilitate further studies.

Hydrolytic enzymes as virulence factors of Candida albicans

Candida albicans is a facultative pathogenic micro-organism that has developed several virulence traits enabling invasion of host tissues and avoidance of host defence mechanisms. Virulence factors that contribute to this process are the hydrolytic enzymes. Most of them are extracellularly secreted by the fungus. The most discussed hydrolytic enzymes produced by C. albicans are secreted aspartic proteinases (Saps). The role of these Saps for C. albicans infections was carefully evaluated in numerous studies, whereas only little is known about the physiological role of the secreted phospholipases (PL) and almost nothing about the involvement of lipases (Lip) in virulence. They may play an important role in the pathogenicity of candidosis and their hydrolytic activity probably has a number of possible functions in addition to the simple role of digesting molecules for nutrition. Saps as the best-studied member of this group of hydrolytic enzymes contribute to host tissue invasion by digesting or destroying cell membranes and by degrading host surface molecules. There is also some evidence that hydrolytic enzymes are able to attack cells and molecules of the host immune system to avoid or resist antimicrobial activity. High hydrolytic activity with broad substrate specificity has been found in several Candida species, most notably in C. albicans. This activity is attributed to multigene families with at least 10 members for Saps and Lips and several members for PL B. Distinct members of these gene families are differentially regulated in various Candida infections. In future, prevention and control of Candida infections might be achieved by pharmacological or immunological tools specifically modulated to inhibit virulence factors, e.g. the family of Saps.

Genetic manipulation of the pathogenic yeast Candida parapsilosis

Candida parapsilosis is an important human pathogen, responsible for severe cases of systemic candidiasis and one of the leading causes of mortality in neonates. In this report, we describe the first system for genetic manipulation of C. parapsilosis. We isolated and subsequently determined DNA sequences of genes encoding galactokinase ( CpGAL1) and orotidine-5'-phosphate decarboxylase ( CpURA3) from a genomic DNA library of C. parapsilosis by functional complementation of corresponding mutations in Saccharomyces cerevisiae. The predicted protein products, Gal1p and Ura3p, displayed a high degree of homology with corresponding sequences of C. albicans and S. cerevisiae, respectively. A collection of galactokinase-deficient ( gal1) strains of C. parapsilosis was prepared using direct selection of mutagenized cells on media containing 2-deoxy-galactose. Additionally, we constructed a plasmid vector carrying CpGAL1 as a selection marker and a genomic DNA fragment with an autonomously replicating sequence activity that transforms the C. parapsilosis gal1 mutant strain with high efficiency. This system for genetic transformation of C. parapsilosis may significantly advance the study of this human pathogen, greatly improving our understanding of its biology and virulence, with implications for drug development.

Secreted aspartic proteases of Candida albicans, Candida tropicalis, Candida parapsilosis and Candida lusitaniae. Inhibition with peptidomimetic inhibitors

The frequency of Candida infections has increased in recent years and it has been accompanied by a significant rise in morbidity and mortality. The secretion of aspartic proteases by Candida spp. was demonstrated to be one of the virulence determinants. Candida albicans is classified as the major human pathogen in the genus Candida. However, other species of this genus have been found to cause an increasing number of candidiases. We isolated secreted aspartic proteases (Saps) of C. albicans (Sap2p), C. tropicalis (Sapt1p), C. parapsilosis (Sapp1p), and C. lusitaniae (Saplp) from culture media. All the isolated proteases were N-terminally sequenced. Their specific proteolytic activities and sensitivity to series of peptidomimetic inhibitors modified in the type of scissile bond replacement as well as in the N- and C-termini were analyzed. The most divergent substrate specificity was observed for the Sap of C. tropicalis. The specificity of Sap of C. lusitaniae is most closely related to that of Sap of C. parapsilosis. We designed and prepared an inhibitor containing phenylstatine isoster that was equipotent towards all four proteases within the range of 10-10-10-9 M. The HIV-1 protease inhibitors ritonavir, saquinavir, indinavir, and nelfinavir were also tested for the inhibition of four Saps. Only ritonavir and saquinavir inhibited Sap2p, Sapt1p, Sapp1p, and Saplp in micromolar concentrations.

Enzymological Characterization of Secreted Proteinases from Candida parapsilosis and Candida lusitaniae

Opportunistic pathogens of the genusCandidaproduce secreted aspartic proteinases (Saps) that are considered as one of the virulence factors. While Saps ofC. albicansandC. tropicalishave been studied extensively, information concerning proteinases secreted by other pathogenicCandidaspecies is scarce. To our knowledge, enzymologic characterizations of Sap ofC. parapsilosis(Sapp1p) andC. lusitaniae(Saplp) are limited. We have purified Saps ofC. parapsilosisandC. lusitaniaefrom yeast culture supernatants and detected only one gene product of both Sapp1p and Saplp using isoelectric focusing and N-terminal sequencing. Molecular weight of Saplp determined by SDS-PAGE is approximately 42 000. For the highest enzyme activity, both Sapp1p and Saplp require pH ranging from 3 to 4 and temperature between 27-40 °C for Saplp and 45 °C for Sapp1p, respectively. At physiological temperature, both the proteinases are stable. The characterization of Saps of clinical isolates ofC. parapsilosisandC. lusitaniaeshould contribute to design of novel drugs specifically targeted on proteinases.

Biotyping and virulence properties of skin isolates of Candida parapsilosis

The biotype and virulence of skin isolates of Candida parapsilosis were compared with blood isolates of the same fungus. Morphotype, resistotype, and electrophoretic karyotype determinations did not reveal any special cluster with a unique or dominant pathogenic feature among all of the isolates, regardless of their source. However, all cutaneous isolates had uniformly elevated secretory aspartyl-protease (Sap) activity, more than four times higher than the enzyme activity of the blood isolates. They were also highly vaginopathic in a rat vaginitis model, being significantly more virulent than blood isolates in this infection model. In contrast, skin isolates were nonpathogenic in systemic infection of cyclophosphamide-immunodepressed mice, while some blood isolates were, in this model, highly pathogenic (median survival time, 2 days, with internal organ invasion at autopsy). Finally, skin isolates did not differ, as a whole, from blood isolates in their adherence to plastic. This property was associated with a morphotype, as defined by a colony with continuous fringe, which was present among both skin and blood isolates. While confirming the genetic heterogenicity of C. parapsilosis, our data strongly suggest that the potential of this fungus to cause mucosal disease is associated with Sap production and is substantially distinct from that of systemic invasion.

Multiplicity of genes encoding secreted aspartic proteinases in Candida species

The secreted aspartic proteinases (SAP) of Candida sp. are presumed to be potential virulence factors. In the opportunistic pathogen Candida albicans the proteinase genes identified to date, SAP1, SAP2, SAP3 and SAP4, constitute a multigene family. Before addressing the possible role of each proteinase in virulence, we sought to isolate all the members of this multigene family by screening a genomic library with a SAP1 probe for additional C. albicans SAP genes using low-stringency hybridization conditions. Three putative new members, SAP5, SAP6 and SAP7 were isolated and sequenced. The N-terminal segments of the deduced amino acid sequences of SAP5 and SAP6 contained secretion signal sequences similar to those of other Candida SAPs. Upon comparison and alignment with the other reported SAP amino acid sequences, SAP7 is not only the most divergent protein but also exhibits a much longer putative pro-sequence with a single Lys-Lys putative processing site. Using SAP1 to SAP7 as probes, the overall number of SAP genes in C. albicans was tentatively estimated by low-stringency hybridization to EcoRI-digested genomic DNA. While each isolated SAP gene could be assigned to distinct EcoRI bands, the existence of two additional genes not isolated after screening of the C. albicans gene library was inferred. Furthermore, evidence was obtained for the existence of SAP multigene families in other Candida species such as C. tropicalis, C. parapsilosis and C. guillermondii.