Organic and inorganic acids as the stimulus for exsheathment of infective juveniles of nematodes

Untargeted Multimodal Metabolomics Investigation of the Haemonchus contortus Exsheathment Secretome

In nematodes that invade the gastro-intestinal tract of the ruminant, the process of larval exsheathment marks the transition from the free-living to the parasitic stages of these parasites. To investigate the secretome associated with larval exsheathment, a closed in vitro system that effectively reproduces the two basic components of an anaerobic rumen environment (CO2 and 39 °C) was developed to trigger exsheathment in one of the most pathogenic and model gastrointestinal parasitic nematodes, Haemonchus contortus (barber‘s pole worm). This study reports the use of multimodal untargeted metabolomics and lipidomics methodologies to identify the metabolic signatures and compounds secreted during in vitro larval exsheathment in the H. contortus infective third-stage larva (iL3). A combination of statistical and chemoinformatic analyses using three analytical platforms revealed a panel of metabolites detected post exsheathment and associated with amino acids, purines, as well as select organic compounds. The major lipid classes identified by the non-targeted lipidomics method applied were lysophosphatidylglycerols, diglycerides, fatty acyls, glycerophospholipids, and a triglyceride. The identified metabolites may serve as metabolic signatures to improve tractability of parasitic nematodes for characterizing small molecule host–parasite interactions related to pathogenesis, vaccine and drug design, as well as the discovery of metabolic biomarkers.

Elucidating the molecular and developmental biology of parasitic nematodes: Moving to a multiomics paradigm

In the past two decades, significant progress has been made in the sequencing, assembly, annotation and analyses of genomes and transcriptomes of parasitic worms of socioeconomic importance. This progress has somewhat improved our knowledge and understanding of these pathogens at the molecular level. However, compared with the free-living nematode Caenorhabditis elegans, the areas of functional genomics, transcriptomics, proteomics and metabolomics of parasitic nematodes are still in their infancy, and there are major gaps in our knowledge and understanding of the molecular biology of parasitic nematodes. The information on signalling molecules, molecular pathways and microRNAs (miRNAs) that are known to be involved in developmental processes in C. elegans and the availability of some molecular resources (draft genomes, transcriptomes and some proteomes) for selected parasitic nematodes provide a basis to start exploring the developmental biology of parasitic nematodes. Indeed, some studies have identified molecules and pathways that might associate with developmental processes in related, parasitic nematodes, such as Haemonchus contortus (barber's pole worm). However, detailed information is often scant and 'omics resources are limited, preventing a proper integration of 'omic data sets and comprehensive analyses. Moreover, little is known about the functional roles of pheromones, hormones, signalling pathways and post-transcriptional/post-translational regulations in the development of key parasitic nematodes throughout their entire life cycles. Although C. elegans is an excellent model to assist molecular studies of parasitic nematodes, its use is limited when it comes to explorations of processes that are specific to parasitism within host animals. A deep understanding of parasitic nematodes, such as H. contortus, requires substantially enhanced resources and the use of integrative 'omics approaches for analyses. The improved genome and well-established in vitro larval culture system for H. contortus provide unprecedented opportunities for comprehensive studies of the transcriptomes (mRNA and miRNA), proteomes (somatic, excretory/secretory and phosphorylated proteins) and lipidomes (e.g., polar and neutral lipids) of this nematode. Such resources should enable in-depth explorations of its developmental biology at a level, not previously possible. The main aims of this review are (i) to provide a background on the development of nematodes, with a particular emphasis on the molecular aspects involved in the dauer formation and exit in C. elegans; (ii) to critically appraise the current state of knowledge of the developmental biology of parasitic nematodes and identify key knowledge gaps; (iii) to cover salient aspects of H. contortus, with a focus on the recent advances in genomics, transcriptomics, proteomics and lipidomics as well as in vitro culturing systems; (iv) to review recent advances in our knowledge and understanding of the molecular and developmental biology of H. contortus using an integrative multiomics approach, and discuss the implications of this approach for detailed explorations of signalling molecules, molecular processes and pathways likely associated with nematode development, adaptation and parasitism, and for the identification of novel intervention targets against these pathogens. Clearly, the multiomics approach established recently is readily applicable to exploring a wide range of interesting and socioeconomically significant parasitic worms (including also trematodes and cestodes) at the molecular level, and to elucidate host-parasite interactions and disease processes.

Abomasal nematode species differ in their in vitro response to exsheathment triggers

A crucial step in the infection process of grazing ruminants by gastro-intestinal nematodes is the exsheathment of the infective third-stage larva following ingestion. Recently, heat shock was shown to play an important role in the carbon dioxide (CO₂)-dependent exsheathment response in Haemonchus contortus. The current in vitro study set out to evaluate the role of heat shock in other abomasal species. In rumen fluid, all species tested exsheathed rapidly and efficiently in response to heat shock and CO_2. This response was significantly higher compared to slow temperature changes, supporting the hypothesis that heat shock plays an important role in vivo. However, in artificial buffer, the effect of heat shock was species-dependent. For H. contortus and Ostertagia leptospicularis, the response in artificial buffer was similar to rumen fluid. In contrast, Ostertagia ostertagi and Teladorsagia circumcincta exsheathment was significantly lower and/or slower in artificial buffer, and there was no benefit of heat shock. For these two species, it appears that there are co-factors in the rumen fluid, in addition to heat shock and CO₂, contributing to exsheathment. Overall, the data indicate that there are significant differences between abomasal species in their response to exsheathment triggers.

Heat shock, but not temperature, is a biological trigger for the exsheathment of third-stage larvae of Haemonchus contortus

Gastrointestinal parasites are an important health issue in grazing ruminants. Understanding the processes involved in the transition from the free living to the parasitic life stage of these nematodes is one avenue to identifying new targets amenable to future intervention. The transition to parasitism is initiated by exsheathment and is triggered by the sudden change in environment after ingestion of the infective larva by the host. Two major changes in environment are the increases in temperature and carbon dioxide (CO₂) levels. For CO₂ a role in exsheathment has been described previously, but the exact role of temperature was unclear. The current study is the first to investigate the importance of temperature in triggering exsheathment of Haemonchus contortus. Carbon dioxide induced exsheathment in H. contortus proved to be temperature dependent, as no exsheathment was observed at room temperatures. However, the temperature requirement to trigger exsheathment was quite specific. A rapid change in temperature (heat shock) very efficiently induced high levels of exsheathment. In contrast, when the larvae were exposed to a slow increase in temperature, the exsheathment response was smaller and delayed. Further investigation revealed that timing of the heat shock in relation to the CO₂ administration was crucial, as well as the final temperature and magnitude of the heat shock. In conclusion, these data indicate that heat shock rather than temperature itself is a crucial aspect in triggering the biological exsheathment cascade, and thus infection process, of H. contortus.

Strongyloides stercoralis daf-2 encodes a divergent ortholog of Caenorhabditis elegans DAF-2

We hypothesise that developmental arrest in infectious larvae of parasitic nematodes is regulated by signalling pathways homologous to Caenorhabditis elegans DAF (dauer formation) pathways. Alignment of Strongyloides stercoralis (Ss) DAF-2 with DAF-2 of C. elegans and homologs of other species shows that most structural motifs in these insulin-like receptors are conserved. However, the catalytic domain of Ss-DAF-2 contains two substitutions (Q1242 and Q1256), that would result in constitutive dauer formation in C. elegans or diabetes in vertebrate animals. Ss-daf-2 also shows two alternately spliced isoforms, the constitutively expressed Ss-daf-2a, and Ss-daf-2b, which is only expressed in stages leading to parasitism.

The kinetics of exsheathment of infective nematode larvae is disturbed in the presence of a tannin-rich plant extract (sainfoin) both in vitro and in vivo

The mode of action of bioactive plants on gastrointestinal nematodes remains obscure. Previous in vitro studies showed that exsheathment was significantly disturbed after contact with tannin-rich extracts. However, the role of important factors (extract concentration, parasite species) has not been assessed and no information is available on the occurrence in vivo. These questions represent the objectives of this study. The model incorporated the parasites Haemonchus contortus and Trichostrongylus colubriformis with sainfoin as the bioactive plant. A set of in vitro assays was performed, measuring the changes observed, after 3 h of contact with increasing concentrations of sainfoin, on the rate of artificial exsheathment. The results indicated that sainfoin extracts interfered with exsheathment in a dose-dependent manner and the process overall was similar for both nematodes. The restoration of control values observed after adding PEG to extracts confirms a major role for tannins. A second study was performed in vivo on rumen-cannulated sheep fed with different proportions of sainfoin in the diet to verify these in vitro results. The consumption of a higher proportion of sainfoin was indeed associated with significant delays in Haemonchus exsheathment. Overall, the results confirmed that interference with the early step of nematode infection might be one of the modes of action that contributes to the anthelmintic properties of tanniniferous plants.

Characterization of excretory-secretory products from larval stages of Haemonchus contortus cultured in vitro

To determine biochemical changes associated with early parasite development, Haemonchus contortus larvae were cultured in vitro to the fourth stage (L4). Infective larvae developed from third to fourth stage in 48-96 h. Metabolic activity increased following stimulus of infective stages by CO2 secretion/excretion of significant amounts of protein into cultures and larval feeding did not occur until larvae had molted to the fourth stage. Larval feeding, as monitored by the ability of larvae to ingest fluorescein-labeled albumin, correlated with molting to the fourth stage and only fourth stage larvae were observed to feed. Fourth stage larvae secreted/excreted several enzymes into culture media including a metalloprotease, an acid phosphohydrolase, a cathepsin C-like enzyme, a phospholipase C-like enzyme and an N-acetyl-beta-D-glucosaminidase. Excretory-secretory (ES) products produced by L4 had antigenic homologies with parasite products produced during the second molt and with proteins and glycoproteins extracted from third and fourth stage larvae. ES products were recognized by sera from sheep infected with H. contortus. The enzymes identified here serve as markers for maturation to the fourth larval stage as well as the initiation of feeding and are likely to be involved in extracorporeal digestion. Further, they might serve as potential targets for immune or chemical control of trichostrongyle infections.

The role of adult worms in suppressing functional protective immunity to Heligmosomoides polygyrus bakeri challenge infections

Adult Heligmosomoides polygyrus bakeri, radiolabelled with [35S]-methionine were successfully transferred to naive NIH mice by oral gavage. Adult worms and radiolabel could be detected up to 45 days post-infection. Adult worms gavaged into immune NIH mice, immunized with a drug abbreviated larval infection, were rejected within 45 days. These adults worms were unable to ablate the development of a functional protective response to a larval challenge infection in the NIH strain. In fact 50 adult worms were sufficient to significantly immunize NIH mice against a larval challenge infection. However, adult worms were able to suppress the development of a functional protective response in an outbred CFLP strain. Although a protective immune response could not be elicited to a challenge infection in CBA mice, the presence of gavaged adult worms was shown to increase the susceptibility of mice to a challenge infection. For all mouse strains, no significant difference in levels of L4 antigen-specific serum IgG, IgG1, IgG2a, and IgA existed between immune mice and groups of mice immunosuppressed by adult worms. Levels of L4 antigen-specific serum IgG1 were significantly lower in the poorly immunizable CBA strain compared to CFLP and NIH strains. No correlation was found across mouse strains between the intensity of the antibody response and the mean worm burdens per animal group. In addition, no correlation was found between levels of L4 antigen-specific antibody within each mouse and the loss of worms by individual mice.

External stimuli and intracellular signalling in the modification of the nematode surface during transition to the mammalian host environment

Previous work has shown that the surface of infective larvae of parasitic nematodes will not bind the fluorescent lipid analogue 5-N-(octadecanoyl)aminofluorescein (AF18) until after exposure of the parasite to mammalian tissue-culture conditions. In this study, culture media which are permissive or non-permissive for the acquisition of lipophilicity for AF18 were altered in order to examine possible stimuli involved. This showed that external alkaline pH and high sodium ion concentration were highly stimulatory. The internal signalling pathways which may be involved in the surface alteration were then examined using agents which are known to affect intracellular signalling in mammalian cells. The results indicated that elevation of cGMP levels was stimulatory whereas inhibition of a putative Na+/H+ antiporter or calcium mobilization was inhibitory, and it is argued that high intracellular levels of cAMP may be inhibitory. Whilst the precise effects of the agents used on nematode cells remain to be established, these results provide a framework for the examination of the processes involved in the modification of the nematode surface which takes place immediately after the infection event.

The isolation of nucleic acid from nematodes requires an understanding of the parasite and its cuticular structure

As parasitology increasingly becomes the domain of molecular biologists, it is important to keep in mind that a fundamental understanding of the whole parasite, its structure and behaviour can help to solve complex problems of molecular biology. Hugh Dawkins and Terence Spencer discuss the preparation of DNA from filaform larvae of Trichostrongylus colubriformis and Ostertagia circumcincta and how a detailed knowledge of the morphology and life cycle of each parasite helps in this process.

The physiology of infection with nematodes: the role of intracellular pH in the development of the early parasitic stage

1. H2CO3, the host's signal which induced (less than 25 min) the Ca2+-dependent development of the first parasitic stage of Haemonchus contortus, produced a diphasic change in the pHi (less than 30 min), measured with 5,5-dimethyl-2,4-oxazolidinedione, in the infective stage. The intensity of the diphasic change was related to the [H2CO3] and so to the efficiency of the stimulus for development. 2. MES and HEPES buffers induced a rapid rise in pHi of infective stages in step with pHo. Ortho-and pyrophosphate induced greater changes in pHi. Such buffers did not initiate development. 3. These and other results suggest that the stimulus for development, H2CO3, induced an energy-dependent Ca2+/H+ exchange mediated by mitochondria of the infective stage, which initiated development of the parasitic stage.

The nature and action of host signals

It is clear that excystations in vitro of the coccidia so far examined involves two steps, in the first of which CO2 is important, and the second, in which an external source of chymotrypsin and surface-active agents are required. However, the details of the mechanism of excystment are not clear. We do not know how the presence of CO2 changes the permeability of the oocyst wall. We do not know whether CO2 does anything to the sporozoite or sporocyst; the circumstance that mechanically-released sporocysts readily excyst under appropriate conditions without the necessity for high concentrations of, or perhaps any, CO2 suggests it does not. Circumstantial evidence suggests that the substrate in which chymotrypsin acts is the Stieda body, but whether the enzyme has other roles we do not know. Similarly, the role of bile is ill-defined, although it does seem that the induction of activity is important--but how is this brought about? The techniques available to excyst oocysts are, for many species, very efficient. If CO2 is, as it seems to be, a fundamental stimulus, then efficiency might be enhanced if more attention was given, not so much to increasing the time of exposure and amount of CO2 in the gas phase, but rather to the pH of the medium, which is rarely stated or apparently, controlled. The pH determines the proportion of the different carbonate species in solution, which may be of greater significance than the partial pressure of CO2 in the gas phase (see also Section V A). Although high numbers of excysted sporocysts can be obtained with a particular technique, this does not necessarily mean that all the signals supplied by the host are reproduced in vitro. Jackson (1962) found it necessary to wash oocysts in water or dilute buffers between the primary phase and the secondary phase, a step which implies a deficiency in the methods he used. Commonly, oocysts are exposed to a strong solution of L-cysteine. Does this reflect a general deficiency in the technique, or a counterpart of strongly reducing conditions in ruminant and non-ruminant alike? It seems that we have only a very general outline of excystment, and that we do not understand the details. Yet the problem seems to have been put aside; the most recent relevant reference we have found is dated 1983.