Preliminary molecular characterization of the human pathogen Angiostrongylus cantonensis

De novo transcriptome reveals blood coagulation/antithrombin factors and infection mechanisms in Angiostrongylus cantonensis adult worms

Angiostrongylus cantonensis is the main aetiological agent of eosinophilic meningoencephalitis in humans. Several outbreaks have been documented around the world, cementing its status as an emerging global public health concern. As a result, new strategies for the diagnosis, prophylaxis and treatment of cerebral angiostrongyliasis are urgently needed. In this study, we report on the de novo assembly of the A. cantonensis transcriptome, its full functional annotation and a reconstruction of complete metabolic pathways. All results are available at AngiostrongylusDB (http://angiostrongylus.lad.pucrs.br/admin/welcome). The aim of this study was to identify the active genes and metabolic pathways involved in the mechanisms of infection and survival inside Rattus norvegicus. Among 389 metabolic mapped pathways, the blood coagulation/antithrombin pathways of heparan sulphate/heparin are highlighted. Moreover, we identified genes codified to GP63 (leishmanolysin), CALR (calreticulin), ACE (peptidyl-dipeptidase A), myoglobin and vWD (von Willebrand factor type D domain protein) involved in the infection invasion and survival of the parasite. The large dataset of functional annotations provided and the full-length transcripts identified in this research may facilitate future functional genomics studies and provides a basis for the development of new techniques for the diagnosis, prevention and treatment of cerebral angiostrongyliasis.

The opposite roles of PAS-5 and Galectin-1 in immune response during the early infection of Angiostrongylus cantonensis

BACKGROUND

Angiostrongylus cantonensis is a human zoonotic nematode parasite. Our previous studies found that PAS-5 and Galectin-1 (Gal-1) proteins of A. cantonensis could be strongly recognized by sera from mice infected with A. cantonensis. In this study, we further evaluated the potential roles of these two proteins in the induction of immune response in mice.

METHODS

Mice were immunized with recombinant PAS-5 or Gal-1 and then challenged with 30 infective A. cantonensis larvae following the last immunization. We then examined the infected mice for changes in serum antibodies and cytokines by ELISA, CD4+ T cells and CD4+CD25+FoxP3+ regulatory T cells (Tregs) by flow cytometry, and tissue damage severity by hematoxylin-eosin (H&E) staining.

RESULTS

Compared with control mice, the PAS-5-immunized mice exhibited increased levels of serum antibodies and cytokines (except for IL-10) at different time points post-infection. PAS-5 immunization promoted significant proliferation of CD4+ T cells, and caused more damage in the brain tissue. Vaccination with Gal-1 inhibited the production of antibodies (except for IgG1) and IFN-γ, but promoted the expression of IL-4 and IL-10. Gal-1 immunization results in significant increases in the levels of CD4+CD25+FoxP3+ Tregs, and mild inflammatory changes.

CONCLUSIONS

Taken together, our findings show that PAS-5 enhances, but Gal-1 inhibits the immune response in the early stage of A. cantonensis infections.

Angiostrongylus cantonensis: a review of its distribution, molecular biology and clinical significance as a human pathogen

Angiostrongylus cantonensis is a metastrongyloid nematode found widely in the Asia-Pacific region, and the aetiological agent of angiostrongyliasis; a disease characterized by eosinophilic meningitis. Rattus rats are definitive hosts of A. cantonensis, while intermediate hosts include terrestrial and aquatic molluscs. Humans are dead-end hosts that usually become infected upon ingestion of infected molluscs. A presumptive diagnosis is often made based on clinical features, a history of mollusc consumption, eosinophilic pleocytosis in cerebral spinal fluid, and advanced imaging such as computed tomography. Serological tests are available for angiostrongyliasis, though many tests are still under development. While there is no treatment consensus, therapy often includes a combination of anthelmintics and corticosteroids. Angiostrongyliasis is relatively rare, but is often associated with morbidity and sometimes mortality. Recent reports suggest the parasites' range is increasing, leading to fatalities in regions previously considered Angiostrongylus-free, and sometimes, delayed diagnosis in newly invaded regions. Increased awareness of angiostrongyliasis would facilitate rapid diagnosis and improved clinical outcomes. This paper summarizes knowledge on the parasites' life cycle, clinical aspects and epidemiology. The molecular biology of Angiostrongylus spp. is also discussed. Attention is paid to the significance of angiostrongyliasis in Australia, given the recent severe cases reported from the Sydney region.

Emergence of neural angiostrongyliasis in eastern Australia

Despite an apparent increase in cases of angiostrongyliasis in humans and animals in Australia, the epidemiology of infection with the two species of Angiostrongylus that co-exist in this country, namely A. cantonensis and A. mackerrasae, is poorly understood. This knowledge gap is particularly important with respect to A. mackerrasae, a species evidently native to Australia, as its ability to cause disease in humans is unknown. Likewise, there is little information on the roles of native and introduced species of rodents and molluscs as hosts of Angiostrongylus species in Australia. This review focuses on the gaps in the knowledge about the two species, highlighting the need for epidemiological and pathogenesis studies on the native lungworm A. mackerrasae.

The genome and transcriptome of the zoonotic hookworm Ancylostoma ceylanicum identify infection-specific gene families

Hookworms infect over 400 million people, stunting and impoverishing them. Sequencing hookworm genomes and finding which genes they express during infection should help in devising new drugs or vaccines against hookworms. Unlike other hookworms, Ancylostoma ceylanicum infects both humans and other mammals, providing a laboratory model for hookworm disease. We determined an A. ceylanicum genome sequence of 313 Mb, with transcriptomic data throughout infection showing expression of 30,738 genes. Approximately 900 genes were upregulated during early infection in vivo, including ASPRs, a cryptic subfamily of activation-associated secreted proteins (ASPs). Genes downregulated during early infection included ion channels and G protein-coupled receptors; this downregulation was observed in both parasitic and free-living nematodes. Later, at the onset of heavy blood feeding, C-lectin genes were upregulated along with genes for secreted clade V proteins (SCVPs), encoding a previously undescribed protein family. These findings provide new drug and vaccine targets and should help elucidate hookworm pathogenesis.

Identification and characterisation of microRNAs in young adults of Angiostrongylus cantonensis via a deep-sequencing approach

Angiostrongylus cantonensis is an important causative agent of eosinophilic meningitis and eosinophilic meningoencephalitis in humans. MicroRNAs (miRNAs) are small non-coding RNAs that participate in a wide range of biological processes. This study employed a deep-sequencing approach to study miRNAs from young adults of A. cantonensis. Based on 16,880,456 high-quality reads, 252 conserved mature miRNAs including 10 antisense miRNAs that belonging to 90 families, together with 10 antisense miRNAs were identified and characterised. Among these sequences, 53 miRNAs from 25 families displayed 50 or more reads. The conserved miRNA families were divided into four groups according to their phylogenetic distribution and a total of nine families without any members showing homology to other nematodes or adult worms were identified. Stem-loop real-time polymerase chain reaction analysis of aca-miR-1-1 and aca-miR-71-1 demonstrated that their level of expression increased dramatically from infective larvae to young adults and then decreased in adult worms, with the male worms exhibiting significantly higher levels of expression than female worms. These findings provide information related to the regulation of gene expression during the growth, development and pathogenesis of young adults of A. cantonensis.

MicroRNA expression profile in the third- and fourth-stage larvae of Angiostrongylus cantonensis

The pathogenesis of angiostrongyliasis, resulting from the third-stage and the fourth-stage Angiostrongylus cantonensis larvae invasion of the human central nervous system, remains elusive. MicroRNAs are important regulators of gene expression and involved in many biological processes. The aim of this study was to determine and characterize miRNAs of third (L3) and fourth (L4) larvae of A. cantonensis by Solex deep sequencing. A total of 629 conserved miRNAs (526 and 376 miRNAs in L3 and L4 larvae of A. cantonensis, respectively) and three novel candidate miRNA from L3 and L4 larva of A. cantonensis were identified with bioinformatic analysis. There were 163 miRNAs upregulated and 54 miRNAs downregulated (fold changes ≥5.0) in the L4 of A. cantonensis compared with that of L3 of A. cantonensis. Interestingly, Gene Ontology "biological process" classifications revealed that 26 miRNAs of significantly differential expression are associated with the immune system, which implies that these miRNAs might participate in the pathogenesis of angiostrongyliasis by regulating genes involved in immune response pathways. Furthermore, the differential expression patterns of 26 conserved miRNAs between L3 and L4 of A. cantonensis were verified. The results of real-time PCR and Northern blot showed that the aca-miR-124 and aca-miR-146a-5p have a low level expression in L3 larvae but high level expression in L4 larvae. Transfection of aca-miR-124 mimics alone significantly downregulated the mRNA expression of IL-6 and IL-1β and TNF-a in the N9 cells, compared to the combination transfection of aca-miR-124 mimics and inhibitor (P < 0.05), suggesting that miR-124 of A. cantonensis have an important role in the suppression of microglia activation. In conclusion, the study presents a general picture of the expression of small RNAs in L3 and L4 of A. cantonensis and highlights conserved miRNAs differentially expressed between L3 and L4 larvae. Our data revealed that miRNAs of parasite may mediate important roles in A. cantonensis immune evasion and aca-miR-146a-5p can serve as a potential biomarker to diagnose angiostrongyliasis.

A RAB family antigen of Angiostrongylus cantonensis induces a Th1-type immune response in vitro and in vivo

A cDNA library was constructed from an Angiostrongylus cantonensis young adult and the encoded proteins were expressed in Escherichia coli. One reactive antigen, a RAB-2 protein, was selected using an immunoscreening technique. The expression of the Th1-type cytokine IFN-γ was elicited in mouse splenic cells that were co-cultured with the recombinant RAB-2 protein and in the sera of mice that were immunised with this protein and adjuvant (50 μg at 2-week intervals). In the A. cantonensis-infected groups, the mice were orally infected with 35 infective larvae, and a subset of the infected mice were immunised with the recombinant RAB-2 protein in adjuvant. Serum samples were collected every week for ELISA, and the pathological examinations were performed at 14 days post infection (dpi). An increase in IFN-γ expression was noted in the blood, and the brain sections revealed moderate eosinophilic meningitis in the immunised mice. The RAB-2 antigen of A. cantonensis induced a Th1-type immune response both in vitro and in vivo.

Molecular cloning, expression, and characterization of a putative activation-associated secreted protein from Angiostrongylus cantonensis

Activation-associated secreted protein (ASP) had been found in many helminthes, which was associated with pathogenesis and stage transition. A complementary DNA (cDNA) sequence encoding a putative two-domain ASP was obtained from an Angiostrongylus cantonensis fourth-stage larvae cDNA library, which we designated as AgASP. The cDNA of AgASP contains an open reading frame encoding 424 amino acids, the first 19 residues being a putative secretion signal. The expression pattern of this protein was investigated by real-time polymerase chain reaction and Western blot. We found that this protein expressed most highly in the brain-stage larvae (Lbr) of this parasite and existed in the excretory/secretory products of this stage. Immunofluorescence showed it existed in the lumen of the Lbr. The recombinant protein can be recognized by the infection sera from mice (nonpermissive host), while it cannot be recognized by infection sera from rats (permissive host). The infiltration of neutrophils in infected nonpermissive host can be lessened by immunizing this host with this protein (immunized vs control group, 13.7 ± 10.2 vs 65.5 ± 19.2). These findings suggest that this protein plays a role in the pathogenesis of human angiostrongyliasis and is worthy of further study.

Genomics and transcriptomics across the diversity of the Nematoda

The diversity of biology in nematodes is reflected in the diversity of their genomes. Parasitic species in particular have evolved mechanisms to invade and outwit their hosts, and these offer opportunities for the development of control measures. Genomic analyses can reveal the molecular underpinnings of phenotypes such as parasitism and thus, initiate and support research programmes that explore the manipulation of host and parasite physiologies to achieve favourable outcomes. Wide sampling across nematode diversity allows phylogenetically informed formulation of research hypotheses, identification of core features shared by all species or important evolutionary novelties present in isolated clades. Many nematode species have been investigated through the use of the expressed sequence tag approach, which samples from the transcribed genome. Gene catalogues generated in this way can be explored to reveal the patterns of expression associated with parasitism and candidates for testing as drug targets or vaccine components. Analysis environments, such as NEMBASE facilitate exploitation of these data. The development of new high-throughput DNA-sequencing technologies has facilitated transcriptomic and genomic approaches to parasite biology. Whole genome sequencing offers more complete catalogues of genes and assists a systems approach to phenotype dissection. These efforts are being coordinated through the 959 Nematode Genomes initiative.

Molecular characterization and immunolocalization of a protein disulfide isomerase from Angiostrongylus cantonensis

Protein disulfide isomerases (PDIs), belonging to the thioredoxin superfamily, are oxidoreductases that catalyze the formation, reduction, and isomerization of disulfide bonds among cysteine residues of proteins. In this study, we report the cloning and characterization of a cDNA encoding a protein disulfide isomerase (AcPDI) from a cDNA library of fourth-stage larvae of Angiostrongylus cantonensis. The deduced amino acid sequence contains two thioredoxin domains and exhibits high identity to the homologues from other species. Quantitative real-time PCR (qRT-PCR) was performed at the third-stage larvae, fourth-stage larvae, and adult stage of A. cantonensis, and the results revealed that the AcPDI mRNA, while expressed at all three stages, is expressed at a significantly higher level in female adult worms. Results of immunohistochemical studies indicated that the AcPDI expression was specifically localized in the tegument and uterus wall of female adult worms. Biochemical analysis showed that recombinant AcPDI was biologically active in vitro and exhibited the typical biochemical functions of PDIs: oxidase/isomerase and reductase activities. Collectively, these results implied that AcPDI may be a female-enriched protein and associated with the reproductive development of A. cantonensis. In addition, considering its biochemical properties, AcPDI may be involved in the formation of the cuticle of A. cantonensis.

Cloning and characterization of a novel cathepsin B-like cysteine proteinase from Angiostrongylus cantonensis

Cysteine protease plays a key role in host-parasite interactions. In this study, we identified a novel gene encoding a cathepsin B-like cysteine protease (AcCBL1) from the cDNA library of Angiostrongysus cantonensis fourth-stage larvae (L4) and characterized its biological role in the parasite. Sequence and phylogeny analysis showed that AcCBL1 is related to other cathepsin B family members with the conserved catalytic triad (Cys, His, Asn) and diagnostic occluding loop. In addition, the sequence contains a specific "hemoglobinase motif" and might have a hemoglobinase (Hb)-degrading function. The recombinant AcCBL1 (rAcCBL1) exhibited the protease activity by gelation SDS/PAGE assay; rAcCBL1 can cleave the fluorogenic substrate Z-Arg-Arg-AMC, and the optimum pH was 5.5. The enzyme can hydrolyse several host proteins including Hb and human IgG in acidic pH, but low levels of hydrolysis were observed in neutral pH. Reverse transcription polymerase chain reaction revealed that AcCBL1 expression was detected throughout various developmental stages, L3, L4, adult male and female worms. Western blotting analysis indicated that AcCBL1 was an excretory/secretory product of L4 in mature form of protease. Immunolocalization demonstrated that AcCBL1 was mainly localized in the intestine of L4. These results suggest that rAcCBL1 may play an important role in the parasite nutrition uptake.

Human Angiostrongylus cantonensis: an update

Angiostrongylus cantonensis was first discovered in 1935 and has become an important emerging pathogen causing human angiostrongyliasis. Major outbreaks of human angiostrongyliasis have been reported in endemic regions. Thousands of cases of human angiostrongyliasis have been documented worldwide. A. cantonensis has spread from its traditional endemic regions of the Pacific islands and Southeast Asia to the American continent including the USA, Caribbean islands and Brazil. Humans acquire A. cantonensis by consumption of raw or undercooked intermediate snail hosts or paratenic hosts. The main clinical manifestations of human angiostrongyliasis are eosinophilic meningitis and ocular angiostrongyliasis. The treatment of this disease includes supportive treatment, corticosteroid therapy, and combined therapy with corticosteroids and anthelminthics. The most effective method for prevention is to persuade people not to eat raw or undercooked intermediate and paratenic hosts.

A transcriptomic study on the pepsin-activated infective larvae of Angiostrongylus cantonensis

To investigate the pepsin-activated infective (third-stage) larvae of Angiostrongylus cantonensis at the transcriptomic level, 1496 ESTs were generated from a cDNA library and clustered into 161 contigs and 757 singletons. Among these unigenes, 54.5% had significant sequence homology with known proteins. The most abundantly expressed transcripts were cathepsin B-like cysteine protease 1 and 2, metalloprotease I, metalloprotease 1 precursor, and extracellular superoxide dismutase. Protein complex was the most common Gene Ontology classification within the 'cellular component' category, embryonic development ending in birth or egg hatching within 'biological process', and protein binding within 'molecular function'. Moreover, 280 clusters were mapped to 158 KEGG pathways and 134 had unique EC numbers. These findings suggest that treatment with pepsin-HCl not only digests the tissues of the snail host but also activates the infective larvae.

Molecular cloning and characterization of a cathepsin B from Angiostrongylus cantonensis

Cysteine proteases, a superfamily of hydrolytic enzymes, have numerous functions in parasites. Here, we reported the cloning and characterization of a cDNA encoding a cathepsin B (AcCPB) from Angiostrongylus cantonensis fourth-stage larvae cDNA library. The deduced amino acid sequence analysis indicated AcCPB is related to other cathepsin B family members with an overall conserved architecture. AcCPB is evolutionarily more close to other parasitic nematode cathepsin B than the ones from hosts, sharing 43-53% similarities to the homologues from other organisms. Real-time quantitative PCR analysis revealed that AcCPB was expressed significantly higher in the fourth-stage larvae (L4) and the fifth-stage larvae (L5) than that in the third-stage larvae (L3) and adult worms (Aw). Unexpectedly, AcCPB was expressed at a higher level in L4 and L5 derived from mice than the larvae at the same stages derived from rats. The protease activity of recombinant AcCPB (rAcCPB) expressed in Escherichia coli showed high thermostability and acidic pH optima. The role in ovalbumin digestion and enzyme activity of rAcCPB could be evidently inhibited by cystatin from A.cantonensis. Furthermore, we found rAcCPB increased the expression levels of CD40, MHC II, and CD80 on LPS-stimulated dendritic cells (DCs). In this study, we provided the first experimental evidence for the expression of cathepsin B in A.cantonensis. Besides its highly specific expression in the stages of L4 and L5 when the worms cause dysfunction of the blood-brain barrier of hosts, AcCPB displayed different expression profiles in non-permissive host- and permissive host-derived larval stages and was involved in the maturation of DCs, suggesting a potential role in the central nervous system invasion and the immunoregulation during parasite-host interactions.

Molecular cloning and characterization of cystatin, a cysteine protease inhibitor, from Angiostrongylus cantonensis

Cystatins are thiol proteinase inhibitors ubiquitously present in mammalian body and serve various important physiological functions. In the present study, a novel cystatin molecule (AcCystatin) was cloned from a cDNA library of Angiostrongylus cantonensis fourth-stage larvae. The putative 14-kDa protein contained 120 residues with cystatin-conserved motifs known to interact with the active site of cysteine peptidases and showed high identities with cystatins from other nematodes. RT-PCR analysis revealed that the expression pattern of AcCystatin was equal at the time points of third-stage larvae, fourth-stage larvae, and adults of the parasite life cycle. The recombinant AcCystatin (rAcCystatin) expressed and purified from Escherichia coli has been demonstrated to possess an obvious inhibitory activity against cathepsin B and could significantly upregulate nitric oxide production from IFN-gamma activated RAW 264.7 macrophages. Sera from mice (non-permissive host) infected with A. cantonensis detected rAcCystatin by Western blot, while the sera from infected rats (permissive host) could not. The results implied that AcCystatin might be an immunoregulator in A. cantonensis infection.