Identification of a novel family of non-lysosomal aspartic proteases in nematodes

Characterization of a novel aspartyl protease inhibitor from Haemonchus contortus

Background

Aspartyl protease inhibitor (API) was thought to protect intestinal parasitic nematodes from their hostile proteolytic environment. Studies on Ostertagia ostertagi, Ascaris suum and Brugia malayi indicated that aspins might play roles in nematode infection. In a recent study, proteins differentially expressed between free-living third-stage larvae (L3) and activated L3 (xL3) of Haemonchus contortus were identified by 2D-DIGE. API was found downregulated in xL3 when compared with L3. However, there was no report about the functions of H. contortus API in the parasite-host interaction. In this study, the gene encoding API from H. contortus was cloned, expressed, and part of its biological characteristics were studied.

Results

A DNA fragment of 681 bp was amplified by RT-PCR. Ninety one percent of the amino acid sequence was similar with that for aspin from O. ostertagi. The recombinant API protein was fusion-expressed with a molecular weight of 48 × 10³. Results of Western blot showed that the recombinant API could be recognized by serum from goat infected with H. contortus. It was found that API was localized exclusively in the subcutaneous tissue and epithelial cells of the gastrointestinal tract in adult H. contortus. qRT-PCR suggested that the API gene was differentially transcribed in different life-cycle stages, with the lowest level in female adults and the highest in free-living L3 larvae. Enzyme inhibition assay indicated that the recombinant API can inhibit the activity of pepsin significantly, and the optimal reaction pH and temperature were 4.0 and 37–50 °C respectively. In vitro study showed that the recombinant API could induce goat PBMCs to express IFN-γ, IL-4 and IL-10.

Conclusions

A new aspartyl protease inhibitor was cloned from H. contortus and its characteristics were studied for the first time. The results indicate that API may regulate the immune response of the host and play roles in the infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2137-1) contains supplementary material, which is available to authorized users.

Cloning and molecular analysis of the aspartic protease Sc-ASP110 gene transcript in Steinernema carpocapsae

Many protease genes have previously been shown to be involved in parasitism and in the development of Steinernema carpocapsae, including a gene predicted to encode an aspartic protease, Sc-ASP110, which was cloned and was analysed in this study. A cDNA encoding Sc-ASP110 was cloned based on an expressed sequence tag (EST) fragment from our EST library. The full-length cDNA of Sc-ASP110 consists of 1112 nucleotides with a catalytic aspartic domain (aa18-337). The putative 341 amino acid residues have a calculated molecular mass of 37·1 kDa and a theoretical pI of 4·7. BLASTp analysis of the Sc-ASP110 amino acid sequence showed 45-77% amino acid sequence identity to parasitic and non-parasitic nematode aspartic proteases. An expression analysis showed that the sc-asp110 gene was upregulated during the late parasitic stage, L4, and 24 h after induction of in vitro nematodes. A sequence comparison revealed that Sc-ASP110 was a member of an aspartic protease family; additionally, a phylogenetic analysis indicated that Sc-ASP110 was clustered with the closely related nematode Steinernema feltiae. In situ hybridization showed that sc-asp110 was expressed in the body walls of dorsal cells. The upregulated Sc-ASP110 expression revealed that this protease could play a role in the late parasitic process. In this study, we have cloned and analysed the gene transcript of Sc-ASP110 in S. carpocapsae.

Molecular characterization of 45 kDa aspartic protease of Trichinella spiralis

In a previous study, we identified an aspartic protease gene (Ts-Asp) from the Trichinella spiralis muscle stage larva cDNA library. The gene sequence of Ts-Asp was 1281 bp long and was found to encode a protein consisting of 405 amino acids, with a molecular mass of 45.248 kD and a pI of 5.95. The deduced Ts-Asp has a conserved catalytic motif with catalytic aspartic acid residues in the active site, a common characteristic of aspartic proteases. In addition, the deduced amino acid sequence of Ts-Asp was found to possess significant homology (above 50%) with aspartic proteases from nematode parasites. Results of phylogenetic analysis indicated a close relationship of Ts-Asp with cathepsin D aspartic proteases. For production of recombinant Ts-Asp (rTs-Asp), the pGEX4T expression system was used. Like other proteases, the purified rTs-Asp was able to digest collagen matrix in vitro. Abundant expression of Ts-Asp was observed in muscle stage larva. Ts-Asp was detected in ES proteins, and was able to elicit the production of specific antibodies. It is the first report of molecular characterization of aspartic protease isolated from T. spiralis.

Identification of novel aspartic proteases from Strongyloides ratti and characterisation of their evolutionary relationships, stage-specific expression and molecular structure

BACKGROUND

Aspartic proteases are known to play an important role in the biology of nematode parasitism. This role is best characterised in blood-feeding nematodes, where they digest haemoglobin, but they are also likely to play important roles in the biology of nematode parasites that do not feed on blood. In the present work, we investigate the evolution and expression of aspartic proteases in Strongyloides ratti, which permits a unique comparison between parasitic and free-living adult forms within its life-cycle.

RESULTS

We identified eight transcribed aspartic protease sequences and a further two genomic sequences and compared these to homologues in Caenorhabditis elegans and other nematode species. Phylogenetic analysis demonstrated a complex pattern of gene evolution, such that some S. ratti sequences had a one-to-one correspondence with orthologues of C. elegans but that lineage-specific expansions have occurred for other aspartic proteases in these two nematodes. These gene duplication events may have contributed to the adaptation of the two species to their different lifestyles. Among the set of S. ratti aspartic proteases were two closely-related isoforms that showed differential expression during different life stages: ASP-2A is highly expressed in parasitic females while ASP-2B is predominantly found in free-living adults. Molecular modelling of the ASP-2 isoforms reveals that their substrate specificities are likely to be very similar, but that ASP-2B is more electrostatically negative over its entire molecular surface than ASP-2A. This characteristic may be related to different pH values of the environments in which these two isoforms operate.

CONCLUSIONS

We have demonstrated that S. ratti provides a powerful model to explore the genetic adaptations associated with parasitic versus free-living life-styles. We have discovered gene duplication of aspartic protease genes in Strongyloides and identified a pair of paralogues differentially expressed in either the parasitic or the free-living phase of the nematode life-cycle, consistent with an adaptive role for aspartic proteases in the evolution of nematode parasitism.

Screening of different classes of proteases in microfilarial and adult stages of Setaria cervi

Many of the filarial proteases involved in critical physiological functions are expressed in stage-specific manner and belong to various mechanistic classes. Setaria cervi, a bovine filarial parasite express different classes of proteases. This parasite shows strong antigenic cross-reactivity with human filarial parasites Wuchereria bancrofti and Brugia malayi. Somatic extracts of S. cervi microfilariae (mf) and adult stages as well as their excretory-secretory (ES) products were screened for the presence of different classes of proteases using general (casein, bovine hemoglobin) and class specific substrates. Detergent-soluble extracts of male and female worms were also screened. Significant enzyme activity was detected in ES products both at pH 5.0 and 7.0 with casein. Cathepsin B-like activity was found to be much higher in membrane-bound extract than in the crude-soluble extract. However, it was also found to be actively secreted by both mf and adult worms. Cathepsin D-like activity assayed at pH 3.0 was very low both in somatic extract as well as in ES products. Collagenase activity at neutral pH showed higher levels, both in somatic extract and ES products. Cathepsin L-like activity was detected only in crude-soluble extract but was below detectable limit in ES products. Leucine aminopeptidase activity was significant both in crude-soluble extract and ES products. This study, thus, might be helpful for a better understanding of host-parasite interaction and identification of appropriate virulence factors that may be targeted as vaccine and/or drug targets against lymphatic filariasis.

Meloidogyne incognita: molecular cloning and characterization of a cDNA encoding a cathepsin D-like aspartic proteinase

Herein we describe the cloning and characterization of a cDNA encoding an aspartic proteinase from the root-knot nematode Meloidogyne incognita. Using PCR techniques, a 1471-bp cDNA fragment encoding a cathepsin D-like (Mi-asp1) transcript was isolated from second-stage larvae mRNA. Its predicted amino acid sequence comprises a pro-region of 71 amino acid residues and a mature protease of 378 amino acid residues with a predicted molecular mass of 41.502kDa. Protein sequence comparisons of Mi-asp1 with GenBank (DQ360827) sequences showed 59-71% identity with nematode-specific cathepsin D-like aspartic proteinases. Southern blot analysis, RT-PCR amplification and EST mining suggest the existence of a developmentally expressed gene family encoding aspartic proteinases in M. incognita. Mi-asp1 may represent a potential target for molecular intervention for the purposes of plant-parasitic nematode control.

Genome-wide investigation reveals pathogen-specific and shared signatures in the response of Caenorhabditis elegans to infection

Microarray analysis of the transcriptional response of C. elegans to four bacterial pathogens revealed that different infections trigger responses, some of which are common to all four pathogens, such as necrotic cell death, which has been associated with infection in humans.

Background

There are striking similarities between the innate immune systems of invertebrates and vertebrates. Caenorhabditis elegans is increasingly used as a model for the study of innate immunity. Evidence is accumulating that C. elegans mounts distinct responses to different pathogens, but the true extent of this specificity is unclear. Here, we employ direct comparative genomic analyses to explore the nature of the host immune response.

Results

Using whole-genome microarrays representing 20,334 genes, we analyzed the transcriptional response of C. elegans to four bacterial pathogens. Different bacteria provoke pathogen-specific signatures within the host, involving differential regulation of 3.5-5% of all genes. These include genes that encode potential pathogen-recognition and antimicrobial proteins. Additionally, variance analysis revealed a robust signature shared by the pathogens, involving 22 genes associated with proteolysis, cell death and stress responses. The expression of these genes, including those that mediate necrosis, is similarly altered following infection with three bacterial pathogens. We show that necrosis aggravates pathogenesis and accelerates the death of the host.

Conclusion

Our results suggest that in C. elegans, different infections trigger both specific responses and responses shared by several pathogens, involving immune defense genes. The response shared by pathogens involves necrotic cell death, which has been associated with infection in humans. Our results are the first indication that necrosis is important for disease susceptibility in C. elegans. This opens the way for detailed study of the means by which certain bacteria exploit conserved elements of host cell-death machinery to increase their effective virulence.

First identification of a phosphorylcholine-substituted protein from Caenorhabditis elegans: isolation and characterization of the aspartyl protease ASP-6

Caenorhabditis elegans is a widely accepted model system for parasitic nematodes, drug screening and developmental studies. Similar to parasitic worms, C. elegans expresses glycosphingolipids and glycoproteins carrying, in part, phosphorylcholine (PCho) substitutions, which might play important roles in nematode development, fertility and, at least in the case of parasites, survival within the host. With the exception of a major secretory/excretory product from Acanthocheilonema viteae (ES-62), no protein carrying this epitope has been studied in detail yet. Here we report on the identification, characterization and localization of the aspartyl protease ASP-6 of C. elegans, which is excreted by the nematode in a PCho-substituted form. Within the worm, most prominent expression of the protein is observed in the intestine, while muscle and epithelial cells express asp-6 to a lesser extent. In animals harboring an ASP-6::GFP fusion protein, diffuse fluorescence throughout the body cavity of adult worms indicates that the chimeric protein is secreted.

An aspartyl protease inhibitor of Ostertagia ostertagi: Molecular cloning, analysis of stage and tissue specific expression and vaccine trial

Protease inhibitors are thought to protect intestinal parasitic nematodes from their hostile proteolytic environment. In a previous study, screening of Ostertagia ostertagi cDNA libraries with local antibody probes of the abomasal lymph nodes and mucus revealed a (28 kDa) aspartyl protease inhibitor (API), which was exclusively recognised by antibodies from immune calves. Here we report the molecular characterization of Oo-API (sequence analysis, developmental expression and localization) and a vaccine trial in cattle with the native and recombinant baculo-expressed antigen. The full-length open reading frame of api encodes a protein of 28 kDa. The sequence showed 82% significant homology to an Aspin homologue from Trichostrongylus colubriformis (AA034715). The cDNA encoding the full-length sequence was cloned in a bacterial pET expression vector and the pVec 35 baculovirus vector. Polyclonal rabbit serum against the Escherichia coli-expressed protein was used to develop Western Blots of extracts and ES and to localize the antigen on L3, L4 and adult worm sections. The protein was expressed in all life stages, which was confirmed by real-time polymerase chain reaction (RT-PCR), and was mainly localized in the cuticle of L3, the intestinal cells of L4, and the gut and sphincter of adult worms. Polyclonal serum was also used to affinity purify the native protein. Vaccination of calves with native Oo-API and baculovirus-expressed Oo-rbAPI in combination with QuilA resulted in no protection against Ostertagia challenge infections.

Cloning and characterisation of an aspartyl protease inhibitor (API-1) from Ancylostoma hookworms

Hookworm infection persists as a public health problem in developing nations. Vaccine-based strategies offer the best chance of long-term control. Aspartyl protease inhibitors from parasitic nematodes are highly immunogenic, and have been suggested as potential vaccine antigens. An aspartyl protease inhibitor, API-1, was cloned and characterised from the hookworms Ancylostoma caninum and Ancylostoma ceylanicum. Using sequence from the hookworm expressed sequence tag project, specific primers were designed and used to amplify Ac-api-1 from A. caninum infective L3 cDNA by PCR. Amplicons from the 5' and 3' ends were cloned, sequenced, and combined to create an 874-bp full-length composite sequence of the Ac-api-1 gene. The A. ceylanicum api-1 cDNA of 878 bp was cloned from L3 cDNA using the A. caninum primers. The amino acid sequences of hookworm orthologues were nearly identical, and database searching indicated they belonged to the aspin family, a group of nematode specific aspartyl protease inhibitors that includes the Ascaris pepsin inhibitor PI-3. Ac-api-1 mRNA was detected by reverse transcriptase PCR in eggs, L1, L3 and adult life cycle stages. A polyclonal antiserum against Escherichia coli expressed recombinant Ac-API-1 detected the protein in adult A. caninum excretory/secretory products, but not in those from activated infective larvae. Immunolocalisation experiments using the antiserum indicated that Ac-API-1 is present primarily in the pseudocoelomic fluid in adult hookworms. Soluble, yeast-expressed Ac-API-1 failed to inhibit pepsin or a hookworm gut aspartyl protease in vitro, but inhibited approximately 30% of the proteolytic activity of adult excretory/secretory products. The pseudocoleomic location, presence in all life cycle stages, lack of inhibitory activity against pepsin, and inhibitory activity against excretory/secretory products suggest that Ac-API-1 inhibits an unidentified, putative aspartyl protease secreted by adult hookworms, and may be released as an enzyme-inhibitor complex. The highly immunogenic properties of nematode aspins suggest that Ac-API-1 represents a promising target for a recombinant hookworm vaccine.

Onchocerca volvulus: expression and immunolocalization of a nematode cathepsin D-like lysosomal aspartic protease

The N-terminal region of the cathepsin D-like aspartic protease from the human filarial parasite Onchocerca volvulus was expressed as His-tag fusion protein. Light and electron microscopic immunohistology using antibodies against the recombinant protein showed labeling of lysosomes in the hypodermis and epithelia of the intestine and the reproductive organs of Onchocerca. While developing oocytes were negative, mature oocytes and early morulae showed strong labeling. In older embryos and mature microfilariae, stained lysosomes were only found in a few cells. Cell death in degenerating microfilariae of patients untreated and treated with microfilaricidal drugs was associated with strong expression of aspartic protease. IgG1, IgG4, and IgE antibodies reactive with the recombinant protein were demonstrated in sera from onchocerciasis patients indicating exposure and recognition of the enzyme by the host's defence system. The aspartic protease of O. volvulus appears to function in intestinal digestion and tissue degradation of the filaria.

Identification and characterisation of an aspartyl protease inhibitor homologue as a major allergen of Trichostrongylus colubriformis

Allergens were identified from the gastrointestinal nematode of sheep, Trichostrongylus colubriformis, by probing Western blots of infective larvae (third stage) somatic antigen with IgE purified from the serum of sheep grazed on worm contaminated pasture. A 31 kDa allergen was frequently recognised by sera from immune sheep, particularly those deriving from a line that has been genetically selected over 23 years for parasite resistance. Using a proteomic approach, the 31 kDa allergen was identified as an aspartyl protease inhibitor homologue. The entire coding sequence of T. colubriformis aspartyl protease inhibitor (Tco-api-1) was obtained and the mature protein expressed in Escherichia coli. Anti-Tco-API-1 antibodies revealed that a commonly observed 21 kDa T. colubriformis allergen species is a truncated form of Tco-API-1. Specific IgE responses to T. colubriformis aspartyl protease inhibitor were significantly correlated with the degree of resistance to nematode infection as measured by faecal egg count in sheep. Surprisingly, IgE responses to Tco-API-1 were not correlated with breech soiling (dag score), which is thought to be caused, in part, by allergic hypersensitivity to worms. Therefore, a specific IgE response to this allergen may be a suitable marker for identifying lambs at an early age that will develop strong immunity to gastrointestinal nematodes.

Cleavage of hemoglobin by hookworm cathepsin D aspartic proteases and its potential contribution to host specificity

Hookworms routinely reach the gut of nonpermissive hosts but fail to successfully feed, develop, and reproduce. To investigate the effects of host-parasite coevolution on the ability of hookworms to feed in nonpermissive hosts, we cloned and expressed aspartic proteases from canine and human hookworms. We show here that a cathepsin D-like protease from the canine hookworm Ancylosotoma caninum (Ac-APR-1) and the orthologous protease from the human hookworm Necator americanus (Na-APR-1) are expressed in the gut and probably exert their proteolytic activity extracellularly. Both proteases were detected immunologically and enzymatically in somatic extracts of adult worms. The two proteases were expressed in baculovirus, and both cleaved human and dog hemoglobin (Hb) in vitro. Each protease digested Hb from its permissive host between twofold (whole molecule) and sixfold (synthetic peptides) more efficiently than Hb from the nonpermissive host, despite the two proteases' having identical residues lining their active site clefts. Furthermore, both proteases cleaved Hb at numerous distinct sites and showed different substrate preferences. The findings suggest that the paradigm of matching the molecular structure of the food source within a host to the molecular structure of the catabolic proteases of the parasite is an important contributing factor for host-parasite compatibility and host species range.

Aspartic proteases from the nematode Caenorhabditis elegans. Structural organization and developmental and cell-specific expression of asp-1

A Caenorhabditis elegans gene (asp-1) and cDNA that encode a homologue of cathepsin D aspartic protease were cloned and characterized. The asp-1 mRNA is transcribed from a single exon, and it begins with the SL1 trans-splice leader sequence. The protein (ASP-1) is expressed as a 396-amino acid, 42.7-kDa pre-pro-peptide that is post-translationally processed into a approximately 40-kDa lysosomal protein. ASP-1 shares approximately 60% sequence identity with the aspartic protease precursor from the nematode Strongyloides stercoralis. The amino acid sequences adjacent to the two active site aspartic acid residues in ASP-1 are 100% identical to those in other eukaryotic aspartic proteases. In addition, ASP-1 contains conserved, potential disulfide bond-forming cysteine residues and N-glycosylation sites. The asp-1 gene is exclusively transcribed in the intestinal cells, with the highest levels of expression observed at late embryonic and early larval stages of development. asp-1 transcription is not observed in adult nematodes or mature larvae. Furthermore, transcription predominantly occurs in eight anterior cells of the intestine (int6-int8). Analyses of ASP-1 nucleotide and amino acid sequences revealed the presence of five additional C. elegans aspartic proteases.

Genomic and genetic research on bursate nematodes: significance, implications and prospects

Molecular genetic research on parasitic nematodes (order Strongylida) is of major significance for many fundamental and applied areas of medical and veterinary parasitology. The advent of gene technology has led to some progress for this group of nematodes, particularly in studying parasite systematics, drug resistance and population genetics, and in the development of diagnostic assays and the characterisation of potential vaccine and drug targets. This paper gives an account of the molecular biology and genetics of strongylid nematodes, mainly of veterinary socio-economic importance, indicates the implications of such research and gives a perspective on genome research for this important parasite group, in light of recent technological advances and knowledge of the genomes of other metazoan organisms.