Purification and characterization of a 315 kDa keratinolytic subtilisin-like serine protease fromMicrosporum canisand evidence of its secretion in naturally infected cats

Aminopeptidases and dipeptidyl-peptidases secreted by the dermatophyte Trichophyton rubrum

The nature of secreted aminopeptidases in Trichophyton rubrum was investigated by using a reverse genetic approach. T. rubrum genomic and cDNA libraries were screened with Aspergillus spp. and Saccharomyces cerevisiae aminopeptidase genes as the probes. Two leucine aminopeptidases, ruLap1 and ruLap2, and two dipeptidyl-peptidases, ruDppIV and ruDppV, were characterized and compared to orthologues secreted by Aspergillus fumigatus using a recombinant protein from Pichia pastoris. RuLap1 is a 33 kDa nonglycosylated protein, while ruLap2 is a 58-65 kDa glycoprotein. The hydrolytic activity of ruLap1, ruLap2 and A. fumigatus orthologues showed various preferences for different aminoacyl-7-amido-4-methylcoumarin substrates, and various sensitivities to inhibitors and cations. ruDppIV and ruDppV showed similar activities to A. fumigatus orthologues. In addition to endopeptidases, the four aminopeptidases ruLap1, ruLap2, ruDppIV and ruDppV were produced by T. rubrum in a medium containing keratin as the sole nitrogen source. Synergism between endo- and exopeptidases is likely to be essential for dermatophyte virulence, since these fungi grow only in keratinized tissues.

Allergy and dermatophytes

Tinea pedis (athlete's foot) and onychomycosis (infection of the toenails) caused by the dermatophyte fungus Trichophyton are highly prevalent in adults. Several Trichophyton allergens have been identified based on elicitation of immunoglobulin E antibody-mediated immediate-hypersensitivity (IH) responses. Evidence of an etiologic role for Trichophyton in asthma in some subjects with IH and chronic dermatophytosis is provided by bronchial reactivity to Trichophyton. Improvement of asthma after systemic antifungal treatment corroborates this link. A unique feature of Trichophyton allergens is the ability of the same antigen to elicit delayed-type hypersensitivity (DTH) in individuals who lack IH reactivity. Delayed responses appear to confer protection, while IH responses do not, based on the association with acute versus chronic skin infection. The amino acid sequence identity of Trichophyton allergens with diverse enzyme families supports a dual role for these proteins in fungal pathogenesis and allergic disease. Characterizing the immunologic properties of Trichophyton allergens and defining immune mechanisms which drive dichotomous responses are pivotal to understanding the dermatophyte-allergy relationship. Recent studies have identified DTH-associated major T-cell epitopes which could facilitate the development of peptide vaccines. Characterization of additional molecular targets by using new techniques may aid not only in the eradication of infection but also in the resolution of allergic symptoms.

Proteolytic peptide patterns as indicators for fungal infections and nonfungal affections of human nails measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The discrimination of onychomycoses from endogenous diseases showing macroscopically similar symptoms is difficult. Long-lasting but ineffective antifungal therapies using systemic medicaments with often severe adverse reactions may be the consequence. We introduce a novel mass spectrometric method for the discrimination of fungal infections and nonfungal affections. Horn samples from patients infected by Trichophyton rubrum, from patients with psoriasis affecting nails, and from healthy persons were investigated. Onychomycoses are basically associated with proteolytic attacks of the virulent fungi-secreting proteases partly hydrolyzing the horn material. Endogenous diseases lack these proteolytic activities, conserving intact structural proteins. Tryptical digestion of horn material produced cleavage peptides detectable by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Mass spectra of horn material infected by T. rubrum were clearly different from those originating from healthy test persons and from patients with psoriasis. Two methods were successfully applied to quantify the differences between groups of samples. One is based on the Euclidean match factor, and the other is based on the identification of specific peptide peaks occurring exclusively within one group of persons. The Euclidean match factor distributions and the occurrence of specific peptide peaks allowed a clear differentiation of T. rubrum infections from psoriasis patients and healthy test persons. No differences were found between healthy test persons and psoriasis patients. The method is rapid and does not require any cultivation.

Secreted subtilisin gene family in Trichophyton rubrum

Secreted proteases constitute potential virulence factors of dermatophytes. A total of seven genes encoding putative serine proteases of the subtilisin family (SUB) were isolated in Trichophyton rubrum. Based on sequence data and intron-exon structure, a phylogenetic analysis of subtilisins from T. rubrum and other fungi revealed a presumed ancestral lineage comprising T. rubrum SUB2 and Aspergillus SUBs. All other SUBs (SUB1, SUB3-7) are dermatophyte-specific and have apparently emerged more recently, through successive gene duplication events. We showed that two subtilisins, Sub3 and Sub4, were detected in culture supernatants of T. rubrum grown in a medium containing soy protein as a sole nitrogen source. Both recombinant enzymes produced in Pichia pastoris are highly active on keratin azure suggesting that these proteases play an important role in invasion of keratinised tissues by the fungus. The set of deduced amino acid sequences of T. rubrum SUB ORFs allowed the identification of orthologous Subs secreted by other dermatophyte species using proteolysis and mass spectrometry.

Multiplication of an ancestral gene encoding secreted fungalysin preceded species differentiation in the dermatophytes Trichophyton and Microsporum

Dermatophytes are human and animal pathogenic fungi which cause cutaneous infections and grow exclusively in the stratum corneum, nails and hair. In a culture medium containing soy proteins as sole nitrogen source a substantial proteolytic activity was secreted by Trichophyton rubrum, Trichophyton mentagrophytes and Microsporum canis. This proteolytic activity was 55-75 % inhibited by o-phenanthroline, attesting that metalloproteases were secreted by all three species. Using a consensus probe constructed on previously characterized genes encoding metalloproteases (MEP) of the M36 fungalysin family in Aspergillus fumigatus, Aspergillus oryzae and M. canis, a five-member MEP family was isolated from genomic libraries of T. rubrum, T. mentagrophytes and M. canis. A phylogenetic analysis of genomic and protein sequences revealed a robust tree consisting of five main clades, each of them including a MEP sequence type from each dermatophyte species. Each MEP type was remarkably conserved across species (72-97 % amino acid sequence identity). The tree topology clearly indicated that the multiplication of MEP genes in dermatophytes occurred prior to species divergence. In culture medium containing soy proteins as a sole nitrogen source secreted Meps accounted for 19-36 % of total secreted protein extracts; characterization of protein bands by proteolysis and mass spectrometry revealed that the three dermatophyte species secreted two Meps (Mep3 and Mep4) encoded by orthologous genes.

Crystal structure of fervidolysin from Fervidobacterium pennivorans, a keratinolytic enzyme related to subtilisin

Structure-forming fibrous proteins like keratins, gelatins and collagens are degraded only by a few proteases as their tight packing limits access to the potential cleavage sites. To understand the keratin degradation in detail, we describe the first crystal structure of a keratin-degrading enzyme (keratinase), fervidolysin, from Fervidobacterium pennivorans as an immature form with propeptide (PD)-bound. The 1.7A resolution crystal structure shows that the protease is composed of four domains: a catalytic domain (CD), two beta-sandwich domains (SDs), and the PD domain. A structural alignment shows a distant relationship between the PD-CD substructure of fervidolysin and pro-subtilisin E. Tight binding of PD to the remaining part of the protease is mediated by hydrogen bonds along the domain surfaces and around the active cleft, and by the clamps to SD1 and SD2. The crystal structure of this multi-domain protein fervidolysin provides insights into proenzyme activation and the role of non-catalytic domains, suggesting a functional relationship to the fibronectin (FN)-like domains of the human promatrix metalloprotease-2 (proMMP-2) that degrades the fibrous polymeric substrate gelatin.

Recombinant expression and antigenic properties of a 31.5-kDa keratinolytic subtilisin-like serine protease from Microsporum canis

A secreted 31.5-kDa keratinolytic subtilase (SUB3; AJ431180) is thought to be a Microsporum canis virulence factor and represents a candidate for vaccination trials. In this study, the recombinant keratinase (r-SUB3) was produced by the Pichia pastoris expression system and purified to homogeneity. Recombinant SUB3 displayed identical biochemical properties with the native protease. Experimentally cutaneously infected guinea pigs showed specific lymphoproliferative response towards r-SUB3, while no specific humoral immune response was induced except for one animal. The heterologous expression of SUB3 provides a valuable tool for addressing further investigations on the role of this keratinase in the specific cellular immune response and on its use in vaccination trials in the cat.

Serum-specific antibodies in rabbits naturally infected with Trichophyton mentagrophytes

In the present study, we investigated the humoral immune response of rabbits to Trichophyton mentagrophytes (sensu lato) proteins obtained from keratin-rich media in vitro. The test rabbits were naturally infected with T. mentagrophytes. The production of keratinolytic enzymes in T. mentagrophytes was stimulated by growing cultures with keratin as a sole nitrogen source. The proteins were isolated from a protein extract prepared from the fungal mat. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) showed three bands. Bands with Mr 20 and 30 kDa were glycosylated, whereas a band of 18 kDa was not. The rabbits' humoral immune responses to the total protein extract of T. mentagrophytes and to the proteins with keratinolytic activity was studied by immunoblotting. IgG from infected rabbits' sera revealed eight dominant bands with apparent molecular weights between 20 and 75 kDa. Bands of 20, 30 and 33 kDa appeared with a frequency rate of 76% only on immunoblots of infected rabbit serum. Using an indirect enzyme-linked immunosorbent assay (ELISA), we observed a significant increase in specific antibodies in a group of infected rabbits compared to a control group (P < 0.001). The ELISA exhibited 95% sensitivity and 83% specificity at the optimal cut-off value, with 90% predictive values of a positive and a negative result. Under these conditions, it could be used in the accurate detection of specific antibodies in sera of infected rabbits.

Humoral and cellular immune response to aMicrosporumcanisrecombinant keratinolytic metalloprotease (r-MEP3) in experimentally infected guinea pigs

In order to better understand the host-fungus relationship in Microsporum canis dermatophytosis and to identify major fungal antigens, the immune response to a crude exoantigen preparation and to a purified recombinant keratinolytic metalloprotease (r-MEP3) was evaluated in guinea pigs experimentally infected with M. canis. Humoral and cellular immune responses were assessed from day 0 to day 57 post-infection (PI), the former by enzyme-linked immunosorbent assay (ELISA) and the latter via a lymphocyte proliferation assay. Infected guinea pigs developed humoral and cellular responses to both M. canis exoantigen and r-MEP3, while no specific immune response to these antigens was observed in control animals. This is the first report on the development of both humoral and cell-mediated immune responses to a purified keratinase in M. canis dermatophytosis.

Secreted metalloprotease gene family of Microsporum canis

Keratinolytic proteases secreted by dermatophytes are likely to be virulence-related factors. Microsporum canis, the main agent of dermatophytosis in dogs and cats, causes a zoonosis that is frequently reported. Using Aspergillus fumigatus metalloprotease genomic sequence (MEP) as a probe, three genes (MEP1, MEP2, and MEP3) were isolated from an M. canis genomic library. They presented a quite-high percentage of identity with both A. fumigatus MEP and Aspergillus oryzae neutral protease I genes. At the amino acid level, they all contained an HEXXH consensus sequence, confirming that these M. canis genes (MEP genes) encode a zinc-containing metalloprotease gene family. Furthermore, MEP3 was found to be the gene encoding a previously isolated M. canis 43.5-kDa keratinolytic metalloprotease, and was successfully expressed as an active recombinant enzyme in Pichia pastoris. Reverse transcriptase nested PCR performed on total RNA extracted from the hair of M. canis-infected guinea pigs showed that at least MEP2 and MEP3 are produced during the infection process. This is the first report describing the isolation of a gene family encoding potential virulence-related factors in dermatophytes.

Isolation of a Microsporum canis gene family encoding three subtilisin-like proteases expressed in vivo

Microsporum canis is the main agent of dermatophytosis in dogs and cats and is responsible for frequent zoonosis. The pathogenesis of the disease remains largely unknown, however. Among potential fungal virulence factors are secreted keratinolytic proteases, whose molecular characterization would be an important step towards the understanding of dermatophytic infection pathogenesis. M. canis secretes a 31.5 kDa keratinolytic subtilisin-like protease as the major component in a culture medium containing cat keratin as the sole nitrogen source. Using a probe corresponding to a gene's internal fragment, which was obtained by polymerase chain reaction, the entire gene encoding this protease named SUB3 was cloned from a M. canislambdaEMBL3 genomic library. Two closely related genes, termed SUB1 and SUB2, were also cloned from the library using as a probe the gene coding for Aspergillus fumigatus 33 kDa alkaline protease (ALP). Deduced amino acid sequence analysis revealed that SUB1, SUB2, and SUB3 are secreted proteases and show large regions of identity between themselves and with subtilisin-like proteases of other filamentous fungi. Interest ingly, mRNA of SUB1, SUB2, and SUB3 were detected by reverse transcriptase nested-polymerase chain reaction from hair of experimentally infected guinea pigs. These results show that SUB1, SUB2, and SUB3 encode a family of subtilisin-like proteases and strongly suggest that these proteases are produced by M. canis during the invasion of keratinized structures. This is the first report describing the isolation of a gene family encoding potential virulence-related factors in dermatophytes.

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.

Keratin degradation: a cooperative action of two enzymes from Stenotrophomonas sp

A novel keratin-degrading bacterium Stenotrophomonas sp. strain D-1, isolated from deer fur, produced two types of extracellular proteins: proteolytic and disulfide bond-reducing. The results on the biochemical properties suggest that this protease belongs to the serine protease, and the disulfide bond-reducing protein could be the disulfide reductase type. None of these enzymes showed keratinolytic activity independently. However, after mixing of the two enzymes, the keratinolytic activity was increased tremendously (more than 50-fold) over that of the protease only. This keratinolytic activity was more than 2-fold higher than that of the combination with proteinase K (also known for its high keratinolytic activity). Since the two enzymes discovered in this study acted cooperatively and resulted in higher keratinolytic activity, a new mechanism of keratin degradation has been revealed. To our knowledge, this is the first report on the cooperative action of two enzymes resulting in the effective degradation of keratin.

Production of extracellular enzymes by Microsporum canis and their role in its virulence

Microsporum canis is the most prevalent dermatophyte of domestic animals. Several enzymes produced by dermatophytes, particularly keratinases, are considered to play a role in the virulence of this fungus. To investigate the possible relationship between the clinical status of M. canis infection and enzymatic activity of isolates, we studied the relationship between keratinase, elastase, lipase and DNase levels produced in vitro by different isolates and virulence as expressed in a guinea pig model. Samples isolated from symptomatic dogs and cats showed a statistically significantly (P < 0.05) higher keratinase activity than samples isolated from asymptomatic animals. Experimental infection of guinea pigs showed that a strain with high in vitro keratinase activity induced acute infection, which resolved clinically and mycologically faster than the infection induced by a strain with low keratinase activity. This suggested a strong correlation between high keratinase activity and the development of symptoms. The same correlation was not observed for other enzymes tested.

Purification and characterization of a 43.5 kDa keratinolytic metalloprotease from Microsporum canis

A keratinolytic protease secreted by a feline clinical isolate of Microsporum canis cultivated in a broth containing feline keratin as the sole nitrogen source was purified from the culture filtrate by affinity chromatography on bacitracin-agarose and by hydrophobic chromatography on octyl-agarose. The enzyme had an apparent molecular mass of 43.5 kDa and the pI was 7.7. It had a significant activity against keratin azure, elastin-Congo red and denatured type I collagen (azocoll). Using the latter substrate, the optimum pH was around 8 and the apparent optimum temperature around 50 degrees C. The protease was strongly inhibited by 1,10-phenanthroline, phosphoramidon and EDTA. The first 13 N-terminal amino acid sequence showed a 61% homology with that of the extracellular metalloprotease of Aspergillus fumigatus and with the neutral protease I of A. oryzae, confirming that this 43.5 kDa keratinase is a metalloprotease. This keratinolytic metalloprotease could be a virulence-related factor involved in pathophysiological mechanisms of M. canis dermatophytosis.