Isolation and expression of a gene which encodes a wall-associated proteinase of Coccidioides immitis

Coccidioides Species: A Review of Basic Research: 2022

Coccidioides immitis and posadasii are closely related fungal species that cause coccidioidomycosis. These dimorphic organisms cause disease in immunocompetent as well as immunocompromised individuals and as much as 40% of the population is infected in the endemic area. Although most infections resolve spontaneously, the infection can be prolonged and, in some instances, fatal. Coccidioides has been studied for more than 100 years and many aspects of the organism and the disease it causes have been investigated. There are over 500 manuscripts concerning Coccidioides (excluding clinical articles) referenced in PubMed over the past 50 years, so there is a large body of evidence to review. We reviewed the most accurate and informative basic research studies of these fungi including some seminal older studies as well as an extensive review of current research. This is an attempt to gather the most important basic research studies about this fungus into one publication. To focus this review, we will discuss the mycology of the organism exclusively rather than the studies of the host response or clinical studies. We hope that this review will be a useful resource to those interested in Coccidioides and coccidioidomycosis.

Production of extracellular proteolytic activity by Histoplasma capsulatum grown in Histoplasma-macrophage medium is limited to restriction fragment length polymorphism class 1 isolates

Extracellular proteolytic activity was studied for 28 strains of Histoplasma capsulatum var. capsulatum and 2 strains of H. capsulatum var. duboisii. Secreted protease activity assessed by skim milk agarose clearance was limited solely to H. capsulatum var. capsulatum restriction fragment length polymorphism (RFLP) class 1 strains. There was a difference in proteolytic activity levels among class 1 strains. Extracellular proteolytic activity was further determined during growth of those strains in liquid medium using azodye-impregnated protein substrates. In general, the highest activities were measured when azocollagen was used, whereas azocasein and azoalbumin were cleaved less efficiently. The activity was inhibited by phenylmethylsulfonyl fluoride, 4-(2-aminoethyl) benzenesulfonyl fluoride, antipain, and chymostatin, indicating, thereby, the presence of chymotrypsin-like serine proteases. Chromatographic analyses as well as variable substrate use at different culture times revealed production of at least 2 different enzyme pools of the same serine-like protease family. Our results demonstrate a distinctive ability of RFLP class 1 isolates to produce and secrete serine proteinase-type activity. This peculiarity may be relevant to the biology and pathogenesis of this particular clade of H. capsulatum isolates. Overall, the feature of extracellular proteolytic activity production enables a convenient and unequivocal identification of RFLP class 1 isolates and, thereby, can be used in H. capsulatum strain differentiation and typing.

Characterisation of an extracellular serine protease gene (nasp gene) from Dermatophilus congolensis

A partial amino acid sequence of a serine protease from Dermatophilus congolensis allowed the design of oligonucleotide primers that were complemented with additional ones from previously published partial sequences of the gene encoding the enzyme. The polymerase chain reaction (PCR), using combinations of specific and degenerate oligonucleotide primers, allowed the amplification of a 1738-bp internal fragment of the gene, which was finally characterised by inverse PCR as the first full-length sequenced serine protease gene (nasp) from Dermatophilus congolensis. The deduced amino acid sequence of this enzyme, probably involved in the pathogenesis of dermatophilosis, links it to the subtilisin family of proteases.

Molecular approaches to the study of Coccidioides immitis

The study of the molecular biology of Coccidioides sp. is only just beginning. As the importance of coccidioidomycosis grows as a public health problem, our need for understanding of pathogenesis, immune responses, and improved antifungal therapy also increases in proportion. Tools have now become available to study gene manipulation in this pathogen and this will allow molecular approaches to be used. Genetic experiments will also be accelerated by the availability of the whole coccidioidal genome, expected to be made public in the spring of 2003 (see http://www.tigr.org/tdb/tgi/cigi/GenInfo.html). Thus, there seems to be several reasons to expect considerable progress in the coming years.

Secreted proteases from pathogenic fungi

Many species of human pathogenic fungi secrete proteases in vitro or during the infection process. Secreted endoproteases belong to the aspartic proteases of the pepsin family, serine proteases of the subtilisin family, and metalloproteases of two different families. To these proteases has to be added the non-pepsin-type aspartic protease from Aspergillus niger and a unique chymotrypsin-like protease from Coccidioides immitis. Pathogenic fungi also secrete aminopeptidases, carboxypeptidases and dipeptidyl-peptidases. The function of fungal secreted proteases and their importance in infections vary. It is evident that secreted proteases are important for the virulence of dermatophytes since these fungi grow exclusively in the stratum corneum, nails or hair, which constitutes their sole nitrogen and carbon sources. The aspartic proteases secreted by Candida albicans are involved in the adherence process and penetration of tissues, and in interactions with the immune system of the infected host. For Aspergillus fumigatus, the role of proteolytic activity has not yet been proved. Although the secreted proteases have been intensively investigated as potential virulence factors, knowledge on protease substrate specificities is rather poor and few studies have focused on the research of inhibitors. Knowledge of substrate specificities will increase our understanding about the action of each protease secreted by pathogenic fungi and will help to determine their contribution to virulence.

Aspects of fungal pathogenesis in humans

Fungal diseases have become increasingly important in the past few years. Because few fungi are professional pathogens, fungal pathogenic mechanisms tend to be highly complex, arising in large part from adaptations of preexisting characteristics of the organisms' nonparasitic lifestyles. In the past few years, genetic approaches have elucidated many fungal virulence factors, and increasing knowledge of host reactions has also clarified much about fungal diseases. The literature on fungal pathogenesis has grown correspondingly; this review, therefore, will not attempt to provide comprehensive coverage of fungal disease but focuses on properties of the infecting fungus and interactions with the host. These topics have been chosen to make the review most useful to two kinds of readers: fungal geneticists and molecular biologists who are interested in learning about the biological problems posed by infectious diseases, and physicians who want to know the kinds of basic approaches available to study fungal virulence.

[The significance of secretory and structure-associated proteases of Aspergillus fumigatus for the pathogenesis of invasive aspergillosis]

In the course of invasive aspergillosis, Aspergillus fumigatus is capable of penetrating any tissue of the host. Secretory proteinases of the fungus might facilitate the hyphae to grow through fibrillar proteins like elastin and collagen. However, using systemic infection models, no significantly reduced virulence could be shown with fungal mutants deficient for all known secretory proteinases. Thus, secretory proteinases might be of minor relevance for the pathogenesis of invasive aspergillosis. In addition, microscopic examination of aspergilli penetrating vessel walls did not reveal obvious lysis of wall proteins, thus emphasizing a mechanical disruption of fibrillar proteins by the growing hyphae. However, a strictly localized proteolysis at the tips of growing hyphae caused by wall associated proteinases might be involved. Candidates for such a mechanism are the activities of aspartic and serine proteinases which we have discovered in the cell wall fraction of A. fumigatus.

Concordance of gene genealogies reveals reproductive isolation in the pathogenic fungus Coccidioides immitis

Simple cladogenetic theory suggests that gene genealogies can be used to detect mixis in a population and delineate reproductively isolated groups within sexual taxa. We have taken this approach in a study of Coccidioides immitis, an ascomycete fungus responsible for a recent epidemic of coccidioidomycosis (Valley fever) in California. To test whether this fungus represents a single sexual species throughout its entire geographic range, we have compared genealogies from fragments of five nuclear genes. The five genealogies show multiple incompatibilities indicative of sex, but also share a branch that partitions the isolates into two reproductively isolated taxa, one centered in California and the other outside California. We conclude that coccidioidomycosis can be caused by two distinct noninterbreeding taxa. This result should aid the future study of the disease and illustrates the utility of the genealogical approach in population genetics.

Virulence factors of medically important fungi

Human fungal pathogens have become an increasingly important medical problem with the explosion in the number of immunocompromised patients as a result of cancer, steroid therapy, chemotherapy, and AIDS. Additionally, the globalization of travel and expansion of humankind into previously undisturbed habitats have led to the reemergence of old fungi and new exposure to previously undescribed fungi. Until recently, relatively little was known about virulence factors for the medically important fungi. With the advent of molecular genetics, rapid progress has now been made in understanding the basis of pathogenicity for organisms such as Aspergillus species and Cryptococcus neoformans. The twin technologies of genetic transformation and "knockout" deletion construction allowed for genetic tests of virulence factors in these organisms. Such knowledge will prove invaluable for the rational design of antifungal therapies. Putative virulence factors and attributes are reviewed for Aspergillus species, C. neoformans, the dimorphic fungal pathogens, and others, with a focus upon a molecular genetic approach. Candida species are excluded from coverage, having been the subject of numerous recent reviews. This growing body of knowledge about fungal pathogens and their virulence factors will significantly aid efforts to treat the serious diseases they cause.

Expression of PZ-peptidases by cultures of several pathogenic fungi. Purification and characterization of a collagenase from Trichophyton schoenleinii

Peptidolytic activity was studied in the broken-cell extracts of 17 isolates of pathogenic fungi tested with phenylazobenzyloxycarbonyl-Pro-Leu-Gly-Pro-Arg (PZ-PLGPA) as a substrate. All the fungi studied except Candida spp., Cryptococcus neoformans and two actinomycetes hydrolyzed the substrate and therefore contained a so-called PZ-peptidase activity. Of all the positive strains, Trichophyton schoenleinii, a pathogenic fungus showed the highest activity and was therefore chosen as a source for PZ-peptidase purification. The four chromatographic steps, a 'negative' dye column, a 'positive' dye column, hydroxyapatite Ultrogel, and modified TSK (HW 55), gave a highly purified peptidase with a 12% overall yield. Inhibitor studies suggested that the 82 000 M(r) PZ-peptidase is a metalloproteinase. Moreover it cleaved native rat type I collagen. Partial peptide sequencing showed a strong sequence homology to regions of two metalloproteinases previously identified in the yeast Saccharomyces cerevisiae and in rat.

Molecular and biochemical characterization of a Coccidioides immitis-specific antigen

Results of earlier investigations have indicated that the saprobic phase of Coccidioides immitis produces a heat-stable, 19-kDa antigen with serine proteinase activity which has been suggested to be specific for this pathogenic fungus. In the present study we have determined the N-terminal and partial internal amino acid sequences of the purified, 19-kDa antigen, cloned the gene which encodes this polypeptide, and confirmed that the secreted proteinase is a Coccidioides-specific antigen (CS-Ag). Both the genomic and cDNA sequences are reported and reveal that the csa gene which encodes this antigen has no introns. A 543-bp open reading frame encodes a 181-amino-acid-containing protein with a predicted molecular mass of 19.8 kDa and an isoelectric point of 8.3. The csa gene was localized on chromosome I of three representative C. immitis clinical isolates on the basis of Southern hybridizations. Expression of the csa gene in Escherichia coli using the pET21a plasmid vector yielded a recombinant protein that was recognized in immunoblot assays by antibody raised to the purified 19-kDa CS-Ag. Secretion of the native antigen is suggested to occur by cleavage of a putative 23-residue signal peptide. The native CS-Ag showed a low degree of glycosylation. Analysis of the carbohydrate composition of the CS-Ag revealed xylose, mannose, galactose, and glucose. However, the purified antigen showed no affinity for concanavalin A. A PCR method with specificity and high sensitivity for detection of C. immitis genomic DNA, using a pair of synthetic oligonucleotide primers whose sequences were based on that of the csa gene, was developed. A 520-bp product was amplified only when C. immitis genomic DNA was used as the template. The lower limits of DNA detection using this PCR method were 1 pg of C. immitis genomic DNA by ethidium bromide staining and 100 fg after Southern hybridization. The csa gene-based PCR method for detection of C. immitis DNA is useful for culture identification and may have clinical applications for the diagnosis of coccidioidal infections.

Families of serine peptidases

This chapter examines families of serine peptidases. Serine peptidases are found in viruses, bacteria, and eukaryotes. They include exopeptidases, endopeptidases, oligopeptidases, and omega peptidases. On the basis of three-dimensional structures, most of the serine peptidase families can be grouped together into about six clans that may have common ancestors. The structures are known for members of four of the clans, chymotrypsin, subtilisin, carboxypeptidase C, and Escherichia D-Ala-D-Ala peptidase A. The peptidases of chymotrypsin, subtilisin, and carboxypeptidase C clans have a common “catalytic triad” of three amino acids—namely, serine (nucleophile), aspartate (electrophile), and histidine (base). The geometric orientations of these are closely similar between families; however the protein folds are quite different. The arrangements of the catalytic residues in the linear sequences of members of the various families commonly reflect their relationships at the clan level. The members of the chymotrypsin family are almost entirely confined to animals. 10 families are included in chymotrypsin clan (SA), and all the active members of these families are endopeptidases. The order of catalytic residues in the polypeptide chain in clan SA is His/Asp/Ser.

A seroreactive 120-kilodalton beta-1,3-glucanase of Coccidioides immitis which may participate in spherule morphogenesis

A beta-glucosidase of Coccidioides immitis was identified in electrophoresis gel separations of the concanavalin A-bound mycelial culture-filtrate-plus-lysate preparation. p-Nitrophenol-beta-D-glucopyranoside was used as the substrate to visualize the enzymatically active fraction in nonreducing gels. The gel-isolated, chromatographically purified enzyme has an optimal pH of 8.0 and cleaves beta-1,3-glycosyl linkages. The alkaline beta-glucosidase was further characterized by a pI of 3.8 to 4.0, optimal activity at 37 to 40 degrees C, and molecular size of 120 kDa as identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified beta-glucosidase is identical to a previously reported 120-kDa antigen (Ag) which reacts with immunoglobulin M (IgM) tube precipitin (TP) antibody in sera from patients with coccidioidomycosis. The TP-Ag was described as a valuable serodiagnostic reagent for detection of specific IgM in patients with early coccidioidal infections. The beta-glucosidase, like the TP-Ag, was localized in the cell wall and cytoplasmic vesicles of parasitic cells (spherules) by immunofluorescence and immunoelectron microscopy with specific antiserum raised against the purified enzyme. The boiled cell wall fraction isolated from these same young (presegmented) spherules was partially digested by the beta-glucosidase. Addition of a potent beta-glucosidase inhibitor, 1-deoxynojirimycin, to the parasitic-phase culture medium at a concentration of 200 microM blocked or retarded conversion of arthroconidia to spherules. Antibody was raised in guinea pigs against chromatographically purified 1-deoxynojirimycin which was conjugated with bovine serum albumin. The inhibitor was localized by immunofluorescence in the wall of the 1-deoxynojirimycin-treated cells. We suggest that the spherule wall-associated, alkaline hydrolase functions as a beta-1,3-glucanase to provide for wall plasticity as well as intussusception of newly synthesized wall polymers during the period of rapid diametric growth of parasitic cells of C. immitis.