The effect of invertebrate hormones and potential hormone inhibitors on the third larval moult of the filarial nematode, Dirofilaria immitis, in vitro

Neuromodulation and the toolkit for behavioural evolution: can ecdysis shed light on an old problem?

The geneticist Thomas Dobzhansky famously declared: 'Nothing in biology makes sense except in the light of evolution'. A key evolutionary adaptation of Metazoa is directed movement, which has been elaborated into a spectacularly varied number of behaviours in animal clades. The mechanisms by which animal behaviours have evolved, however, remain unresolved. This is due, in part, to the indirect control of behaviour by the genome, which provides the components for both building and operating the brain circuits that generate behaviour. These brain circuits are adapted to respond flexibly to environmental contingencies and physiological needs and can change as a function of experience. The resulting plasticity of behavioural expression makes it difficult to characterize homologous elements of behaviour and to track their evolution. Here, we evaluate progress in identifying the genetic substrates of behavioural evolution and suggest that examining adaptive changes in neuromodulatory signalling may be a particularly productive focus for future studies. We propose that the behavioural sequences used by ecdysozoans to moult are an attractive model for studying the role of neuromodulation in behavioural evolution.

Developmental regulation of Dirofilaria immitis microfilariae and evaluation of ecdysone signaling pathway transcript level using droplet digital PCR

BACKGROUND

Current measures for the prevention of dirofilariasis, caused by the dog heartworm, Dirofilaria immitis, rely on macrocyclic lactones, but evidence of drug-resistant isolates has called for alternative approaches to disease intervention. As microfilariae are known to be in a state of developmental arrest in their mammalian host and then undergo two molts once inside the arthropod, the aim of this study was to look at the developmental regulation of D. immitis microfilariae that occurs in their arthropod host using in vitro approaches and to investigate the role of the ecdysone signaling system in this development regulation.

METHODS

Dirofilaria immitis microfilariae extracted from dog blood were incubated under various culture conditions to identify those most suitable for in vitro culture and development of the microfilariae, and to determine the effects of fetal bovine serum (FBS), mosquito cells, and ecdysteroid on the development of the microfilariae. Transcript levels of the ecdysone signaling pathway components were measured with droplet digital PCR (ddPCR).

RESULTS

In vitro conditions that best promote early development of D. immitis microfilariae to the "late sausage stage" have been identified, although shedding of the cuticle was not observed. FBS had inhibitory effects on the development and motility of the microfilariae, but media conditioned with Anopheles gambiae cells were favorable to microfilarial growth. The transcript level study using ddPCR also showed that ecdysone signaling system components were upregulated in developing microfilariae and that 20-hydroxyecdysone increased the proportion of larvae developing to the sausage and late sausage stages in vitro.

CONCLUSIONS

The arthropod host environment provides cues required for the rapid development of D. immitis microfilariae, and the ecdysone signaling system may play an important role in filarial nematode developmental transitions. This study contributes to a better understanding of the developmental process of D. immitis microfilariae.

Prediction pipeline for discovery of regulatory motifs associated with Brugia malayi molting

Filarial nematodes can cause debilitating diseases in humans. They have complicated life cycles involving an insect vector and mammalian hosts, and they go through a number of developmental molts. While whole genome sequences of parasitic worms are now available, very little is known about transcription factor (TF) binding sites and their cognate transcription factors that play a role in regulating development. To address this gap, we developed a novel motif prediction pipeline, Emotif Alpha, that integrates ten different motif discovery algorithms, multiple statistical tests, and a comparative analysis of conserved elements between the filarial worms Brugia malayi and Onchocerca volvulus, and the free-living nematode Caenorhabditis elegans. We identified stage-specific TF binding motifs in B. malayi, with a particular focus on those potentially involved in the L3-L4 molt, a stage important for the establishment of infection in the mammalian host. Using an in vitro molting system, we tested and validated three of these motifs demonstrating the accuracy of the motif prediction pipeline.

Diseases caused by parasitic worms such as the filariae are among the leading causes of morbidity in the developing world. Very little is known about how development is regulated in these vector-transmitted parasites. We have developed a computational method to identify motifs that correspond to transcription factor binding sites in the genome of the parasitic worm, Brugia malayi, one of the causative agents of lymphatic filariasis. Using this approach, we were able to predict stage-specific transcription factor binding sites involved in a stage of the molting process important for the establishment of the infection. We validated the role of these motifs using an in vitro molting system.

Identification of Ecdysone Hormone Receptor Agonists as a Therapeutic Approach for Treating Filarial Infections

Background

A homologue of the ecdysone receptor has previously been identified in human filarial parasites. As the ecdysone receptor is not found in vertebrates, it and the regulatory pathways it controls represent attractive potential chemotherapeutic targets.

Methodology/ Principal Findings

Administration of 20-hydroxyecdysone to gerbils infected with B. malayi infective larvae disrupted their development to adult stage parasites. A stable mammalian cell line was created incorporating the B. malayi ecdysone receptor ligand-binding domain, its heterodimer partner and a secreted luciferase reporter in HEK293 cells. This was employed to screen a series of ecdysone agonist, identifying seven agonists active at sub-micromolar concentrations. A B. malayi ecdysone receptor ligand-binding domain was developed and used to study the ligand-receptor interactions of these agonists. An excellent correlation between the virtual screening results and the screening assay was observed. Based on both of these approaches, steroidal ecdysone agonists and the diacylhydrazine family of compounds were identified as a fruitful source of potential receptor agonists. In further confirmation of the modeling and screening results, Ponasterone A and Muristerone A, two compounds predicted to be strong ecdysone agonists stimulated expulsion of microfilaria and immature stages from adult parasites.

Conclusions

The studies validate the potential of the B. malayi ecdysone receptor as a drug target and provide a means to rapidly evaluate compounds for development of a new class of drugs against the human filarial parasites.

The human filarial parasites are the causative agents of two neglected tropical diseases targeted for elimination by the international community. The current elimination programs rely upon the mass distribution of a limited number of drugs, leaving the programs open to failure in the event that resistance develops. Thus, there is a critical need for novel chemotherapeutic agents to supplement the current arsenal. The filarial parasites are ecdysozoans, whose developmental processes are controlled by a master regulator, the ecdysone receptor. Here we validate the potential of the filarial ecdysone receptor as a chemotherapeutic target and report the development of high throughput and virtual screening assays that may be used to compounds that target it.

Draft genome of Brugia pahangi: high similarity between B. pahangi and B. malayi

BACKGROUND

Efforts to completely eradicate lymphatic filariasis from human population may be challenged by the emergence of Brugia pahangi as another zoonotic lymphatic filarial nematode. In this report, a genomic study was conducted to understand this species at molecular level.

METHODS

After blood meal on a B. pahangi-harbouring cat, the Aedes togoi mosquitoes were maintained to harvest infective third stage larvae, which were then injected into male Mongolian gerbils. Subsequently, adult B. pahangi were obtained from the infected gerbil for genomic DNA extraction. Sequencing and subsequently, construction of genomic libraries were performed. This was followed by genomic analyses and gene annotation analysis. By using archived protein sequences of B. malayi and a few other nematodes, clustering of gene orthologs and phylogenetics were conducted.

RESULTS

A total of 9687 coding genes were predicted. The genome of B. pahangi shared high similarity to that B. malayi genome, particularly genes annotated to fundamental processes. Nevertheless, 166 genes were considered to be unique to B. pahangi, which may be responsible for the distinct properties of B. pahangi as compared to other filarial nematodes. In addition, 803 genes were deduced to be derived from Wolbachia, an endosymbiont bacterium, with 44 of these genes intercalate into the nematode genome.

CONCLUSIONS

The reporting of B. pahangi draft genome contributes to genomic archive. Albeit with high similarity to B. malayi genome, the B. pahangi-unique genes found in this study may serve as new focus to study differences in virulence, vector selection and host adaptability among different Brugia spp.

The genome of the nematode Pristionchus pacificus encodes putative homologs of RXR/Usp and EcR

Ecdysteroid signaling is an important regulator of arthropod development and reproduction. However, the role of ecdysteroid signaling in another Ecdysozoan animal, the nematode, remains unclear. We report here the identification, cloning, and temporal expression of genes encoding putative homologs of the two nuclear receptor components of the ecdysone receptor, RXR/Usp (NR2B) and EcR (NR1H), in the nematode Pristionchus pacificus. The P. pacificus genes Ppa-pnhr-1 and Ppa-pnhr-2 encode nuclear receptors with strong sequence similarity to RXR/Usp and EcR, respectively. Maximum likelihood analysis incorporating both DNA-binding and ligand-binding domains places the two proteins in the NR2B and NR1H groups with strong bootstrap support. RT-PCR analysis reveals that both Ppa-pnhr-1 and Ppa-pnhr-2 are expressed during larval development and that Ppa-pnhr-1 expression oscillates with the molting cycle. The identification of a putative ecdysone receptor in a nematode amenable to genetic analysis provides a powerful system to investigate the function and evolution of ecdysone receptor signaling in the Nematoda.

An ortholog of the ecdysone receptor protein (EcR) from the parasitic nematode Haemonchus contortus

High concentrations (> or =4.2mM) of 20E inhibit the development of Haemonchus contortus eggs to the L3 larval stage. We report the cloning of cDNA encoding an EcR ortholog (HcEcR) from H. contortus mRNA expressed during L3. Phylogenetically, this and the putative EcR from Brugia malayi form a separate branch between arthropod EcRs and liver X receptors. Two isoforms of HcEcR differ in the inclusion/omission of a 3-residue segment in the A/B domain. Single nucleotide polymorphisms at 49 positions can be grouped into two major patterns in the A/BC segment and two in the DE/F segment. Some 35% of the highly conserved ecdysteroid-contacting residues in insect EcRs are also conserved in the HcEcR ligand binding domain, but it contains unusual residue choices at other ligand-contacting positions. Recombinant co-expression of HcEcR DE/F segments with a phthirapteran USP DE/F segment in insect cells resulted in stable proteins which did not heterodimerize or bind [(3)H]ponasterone A.

Molecular evidence for a functional ecdysone signaling system in Brugia malayi

Background

Filarial nematodes, including Brugia malayi, the causative agent of lymphatic filariasis, undergo molting in both arthropod and mammalian hosts to complete their life cycles. An understanding of how these parasites cross developmental checkpoints may reveal potential targets for intervention. Pharmacological evidence suggests that ecdysteroids play a role in parasitic nematode molting and fertility although their specific function remains unknown. In insects, ecdysone triggers molting through the activation of the ecdysone receptor: a heterodimer of EcR (ecdysone receptor) and USP (Ultraspiracle).

Methods and Findings

We report the cloning and characterization of a B. malayi EcR homologue (Bma-EcR). Bma-EcR dimerizes with insect and nematode USP/RXRs and binds to DNA encoding a canonical ecdysone response element (EcRE). In support of the existence of an active ecdysone receptor in Brugia we also cloned a Brugia rxr (retinoid X receptor) homolog (Bma-RXR) and demonstrate that Bma-EcR and Bma-RXR interact to form an active heterodimer using a mammalian two-hybrid activation assay. The Bma-EcR ligand-binding domain (LBD) exhibits ligand-dependent transactivation via a GAL4 fusion protein combined with a chimeric RXR in mammalian cells treated with Ponasterone-A or a synthetic ecdysone agonist. Furthermore, we demonstrate specific up-regulation of reporter gene activity in transgenic B. malayi embryos transfected with a luciferase construct controlled by an EcRE engineered in a B. malayi promoter, in the presence of 20-hydroxy-ecdysone.

Conclusions

Our study identifies and characterizes the two components (Bma-EcR and Bma-RXR) necessary for constituting a functional ecdysteroid receptor in B. malayi. Importantly, the ligand binding domain of BmaEcR is shown to be capable of responding to ecdysteroid ligands, and conversely, ecdysteroids can activate transcription of genes downstream of an EcRE in live B. malayi embryos. These results together confirm that an ecdysone signaling system operates in B. malayi and strongly suggest that Bma-EcR plays a central role in it. Furthermore, our study proposes that existing compounds targeting the insect ecdysone signaling pathway should be considered as potential pharmacological agents against filarial parasites.

Filarial parasites such as Brugia malayi and Onchocerca volvulus are the causative agents of the tropical diseases lymphatic filariasis and onchocerciasis, which infect 150 million people, mainly in Africa and Southeast Asia. Filarial nematodes have a complex life cycle that involves transmission and development within both mammalian and insect hosts. The successful completion of the life cycle includes four molts, two of which are triggered upon transmission from one host to the other, human and mosquito, respectively. Elucidation of the molecular mechanisms involved in the molting processes in filarial nematodes may yield a new set of targets for drug intervention. In insects and other arthropods molting transitions are regulated by the steroid hormone ecdysone that interacts with a specialized hormone receptor composed of two different proteins belonging to the family of nuclear receptors. We have cloned from B. malayi two members of the nuclear receptor family that show many sequence and biochemical properties consistent with the ecdysone receptor of insects. This finding represents the first report of a functional ecdysone receptor homolog in nematodes. We have also established a transgenic hormone induction assay in B. malayi that can be used to discover ecdysone responsive genes and potentially lead to screening assays for active compounds for pharmaceutical development.

Tetracycline treatment targeting Wolbachia affects expression of an array of proteins in Brugia malayi parasite

Wolbachia is an intracellular endosymbiont of Brugia malayi parasite whose presence is essential for the survival of the parasite. Treatment of B. malayi-infected jirds with tetracycline eliminates Wolbachia, which affects parasite survival and fitness. In the present study we have tried to identify parasite proteins that are affected when Wolbachia is targeted by tetracycline. For this Wolbachia depleted parasites (B. malayi) were obtained by tetracycline treatment of infected Mongolian jirds (Meriones unguiculatus) and their protein profile after 2-DE separation was compared with that of untreated parasites harboring Wolbachia. Approximately 100 protein spots could be visualized followed by CBB staining of 2-D gel and included for comparative analysis. Of these, 54 showed differential expressions, while two new protein spots emerged (of 90.3 and 64.4 kDa). These proteins were subjected to further analysis by MALDI-TOF for their identification using Brugia coding sequence database composed of both genomic and EST sequences. Our study unravels two crucial findings: (i) the parasite or Wolbachia proteins, which disappeared/down-regulated appear be essential for parasite survival and may be used as drug targets and (ii) tetracycline treatment interferes with the regulatory machinery vital for parasites cellular integrity and defense and thus could possibly be a molecular mechanism for the killing of filarial parasite. This is the first proteomic study substantiating the wolbachial genome integrity with its nematode host and providing functional genomic data of human lymphatic filarial parasite B. malayi.

Helminth genomics: The implications for human health

More than two billion people (one-third of humanity) are infected with parasitic roundworms or flatworms, collectively known as helminth parasites. These infections cause diseases that are responsible for enormous levels of morbidity and mortality, delays in the physical development of children, loss of productivity among the workforce, and maintenance of poverty. Genomes of the major helminth species that affect humans, and many others of agricultural and veterinary significance, are now the subject of intensive genome sequencing and annotation. Draft genome sequences of the filarial worm Brugia malayi and two of the human schistosomes, Schistosoma japonicum and S. mansoni, are now available, among others. These genome data will provide the basis for a comprehensive understanding of the molecular mechanisms involved in helminth nutrition and metabolism, host-dependent development and maturation, immune evasion, and evolution. They are likely also to predict new potential vaccine candidates and drug targets. In this review, we present an overview of these efforts and emphasize the potential impact and importance of these new findings.

Brugia malayi gene expression in response to the targeting of the Wolbachia endosymbiont by tetracycline treatment

Background

Brugia malayi, like most human filarial parasite species, harbors an endosymbiotic bacterium of the genus Wolbachia. Elimination of the endosymbiont leads to sterilization of the adult female. Previous biochemical and genetic studies have established that communication with its endobacterium is essential for survival of the worm.

Methodology/Principal findings

We used electron microscopy to examine the effects of antibiotic treatment on Wolbachia cell structure. We have also used microarray and quantitative RT-PCR analyses to examine the regulation of the B. malayi transcripts altered in response to the anti-Wolbachia treatment. Microscopy of worms taken from animals treated with tetracycline for 14 and 21 days (14 d and 21 d) demonstrated substantial morphologic effects on the Wolbachia endobacterium by 14 d and complete degeneration of the endobacterial structures by 21 d. We observed upregulation of transcripts primarily encoding proteins involved in amino acid synthesis and protein translation, and downregulation of transcripts involved in cuticle biosynthesis after both 7 d and 14 d of treatment. In worms exposed to tetracycline in culture, substantial effects on endobacteria morphology were evident by day 3, and extensive death of the endobacteria was observed by day 5. In a detailed examination of the expression kinetics of selected signaling genes carried out on such cultured worms, a bimodal pattern of regulation was observed. The selected genes were upregulated during the early phase of antibiotic treatment and quickly downregulated in the following days. These same genes were upregulated once more at 6 days post-treatment.

Conclusions/Significance

Upregulation of protein translation and amino acid synthesis may indicate a generalized stress response induced in B. malayi due to a shortage of essential nutrients/factors that are otherwise supplied by Wolbachia. Downregulation of transcripts involved in cuticle biosynthesis perhaps reflects a disruption in the normal embryogenic program. This is confirmed by the expression pattern of transcripts that may be representative of the worms' response to Wolbachia in different tissues; the early peak potentially reflects the effect of bacteria death on the embryogenic program while the second peak may be a manifestation of the adult worm response to the affected bacteria within the hypodermis.

Filarial parasites afflict hundreds of millions of individuals worldwide, and cause significant public health problems in many of the poorest countries in the world. Most of the human filarial parasite species, including Brugia malayi, harbor endosymbiotic bacteria of the genus Wolbachia. Elimination of the endosymbiont leads to sterilization of the adult female worm. The need exists for the development of new chemotherapeutic approaches that can practically exploit the vulnerability of the filaria to the loss of the Wolbachia. In this study we performed ultrastructural and microarray analyses of female worms collected from infected jirds treated with tetracycline. Results suggest that the endosymbiotic bacteria were specifically affected by the antibiotic. Furthermore, in response to the targeting of the endosymbiont, the parasites modulated expression of their genes. When exposed to tetracycline, the parasites over-expressed genes involved in protein synthesis. Expression of genes involved in cuticle biosynthesis and energy metabolism was, on the other hand, limited.

Endocrine disruption in nematodes: effects and mechanisms

This paper reviews the current knowledge on endocrine disruption in nematodes. These organisms have received little attention in the field of ecotoxicology, in spite of their important role in aquatic ecosystems. Research on endocrine regulation and disruption in nematodes, especially the more recent studies, concentrate mainly on one species, Caenorhabditis elegans. Although an endocrine system is not known in nematodes, there is evidence that many processes are regulated via hormonal pathways. As vertebrate hormones, such as steroids, may have endocrine functions in nematodes as well, endocrine disrupting chemicals (EDCs) defined for vertebrates may also be able to influence nematodes. The studies that are reviewed here, and own data showed that potential EDCs can affect nematodes on all organizational levels, from molecules to communities. It is concluded that nematodes, notably its prominent species C. elegans, are a promising organism group for the development of biomonitoring tools, provided that more mechanistic evidence is gathered on hormonal processes within these animals.

Setaria digitata secreted filarial lipids modulate IL-12 signaling through JAK-STAT pathway leading to the development of Th1 response

Filariasis is a debilitating parasitic disease in many tropical countries. Despite the highly evolved immune system, the filarial parasites successfully evade host immunity to persist for a sustained period of time. Earlier studies have shown that the filarial parasites achieve this long-term survival through release of immunosuppressive materials in the host. In this study, we show that the secreted filarial lipids (SFL) isolated from Setaria digitata suppress Th1 immune response. While immunization with myelin antigen induces Th1 response in mice, in vitro treatment with SFL resulted in a dose-dependent decrease in myelin antigen-induced proliferation and secretion of IL-12 and IFNgamma. The SFL also inhibited IL-12-induced T cell proliferation and Th1 differentiation in vitro. The inhibition of T cell responses by SFL associates with the blockade of IL-12-induced activation of JAK-STAT signaling pathway in T cells. These findings suggest that the SFL modulates Th1 immune response by blocking IL-12 signaling in T cells and thus play a role in host immune evasion of filarial parasites.

The annotated genome of Wolbachia from the filarial nematode Brugia malayi: what it means for progress in antifilarial medicine

Filarial nematodes contain endosymbiotic bacteria of the genus Wolbachia. As described in the April 2005 issue of PLoS Biology, Foster et al. have sequenced the genome of the Wolbachia that lives in the nematode Brugia malayi. What are the clinical implications?

The Wolbachia genome of Brugia malayi: endosymbiont evolution within a human pathogenic nematode

Complete genome DNA sequence and analysis is presented for Wolbachia, the obligate alpha-proteobacterial endosymbiont required for fertility and survival of the human filarial parasitic nematode Brugia malayi. Although, quantitatively, the genome is even more degraded than those of closely related Rickettsia species, Wolbachia has retained more intact metabolic pathways. The ability to provide riboflavin, flavin adenine dinucleotide, heme, and nucleotides is likely to be Wolbachia's principal contribution to the mutualistic relationship, whereas the host nematode likely supplies amino acids required for Wolbachia growth. Genome comparison of the Wolbachia endosymbiont of B. malayi (wBm) with the Wolbachia endosymbiont of Drosophila melanogaster (wMel) shows that they share similar metabolic trends, although their genomes show a high degree of genome shuffling. In contrast to wMel, wBm contains no prophage and has a reduced level of repeated DNA. Both Wolbachia have lost a considerable number of membrane biogenesis genes that apparently make them unable to synthesize lipid A, the usual component of proteobacterial membranes. However, differences in their peptidoglycan structures may reflect the mutualistic lifestyle of wBm in contrast to the parasitic lifestyle of wMel. The smaller genome size of wBm, relative to wMel, may reflect the loss of genes required for infecting host cells and avoiding host defense systems. Analysis of this first sequenced endosymbiont genome from a filarial nematode provides insight into endosymbiont evolution and additionally provides new potential targets for elimination of cutaneous and lymphatic human filarial disease.

Analysis of this Wolbachia genome, which resides within filarial parasites, offers insight into endosymbiont evolution and the promise of new strategies for the elimination of human filarial disease

Wolbachia.Bacterial Endosymbionts of Filarial Nematodes

Filarial nematodes are important helminth parasites of the tropics and a leading cause of global disability. They include species responsible for onchocerciasis, lymphatic filariasis and dirofilariasis. A unique feature of these nematodes is their dependency upon a symbiotic intracellular bacterium, Wolbachia, which is essential for normal development and fertility. Advances in our understanding of the symbiosis of Wolbachia bacteria with filarial nematodes have made rapid progress in recent years. Here we summarise our current understanding of the evolution of the symbiotic association together with insights into the functional basis of the interaction derived from genomic analysis. Also we discuss the contribution of Wolbachia to inflammatory-mediated pathogenesis and adverse reactions to anti-filarial drugs and describe the outcome of recent field trials using antibiotics as a promising new tool for the treatment of filarial infection and disease.

Genotoxic effects of cadmium chloride and azadirachtin treated singly and in combination in fish

The genotoxic effects of cadmium chloride (CdCl(2)) and azadirachtin (Aza) were assessed singly and conjointly in a fish, Oreochromis mossambicus, with endpoints such as chromosome aberrations, abnormal red cell nuclei, abnormal sperm morphology, and protein content (both qualitative and quantitative) of selected tissues, namely, muscle, heart, eye, brain, gill, liver, spleen, and kidney. The primary objectives were, first, to examine if CdCl(2), a common pollutant, and Aza, a natural product of the neem plant used extensively as an 'ecofriendly' agent for many purposes, had any genotoxic effect of their own on nontarget aquatic organisms of economic importance; and second, if Aza could have any ameliorating effect on CdCl(2)-induced genotoxicity in O. mossambicus tissues. As compared with distilled water-treated controls, both CdCl(2) and Aza induced genotoxicity in O. mossambicus, the former in greater quantity than that produced by Aza. However, Cd-induced toxicity in O. mossambicus appeared to be ameliorated to some extent by Aza.

Expression and function of conserved nuclear receptor genes in Caenorhabditis elegans

The Caenorhabditis elegans genome encodes 284 nuclear receptor (NR) genes. Among these 284 NR genes are 15 genes conserved among the Metazoa. Here, we analyze the expression and function of eight heretofore uncharacterized conserved C. elegans NR genes. Reporter gene analysis demonstrates that these genes have distinct expression patterns and that a majority of the C. elegans cell types express a conserved NR gene. RNA interference with NR gene function resulted in visible phenotypes for three of the genes, revealing functions in various processes during postembryonic development. Five of the conserved NR genes are orthologs of NR genes that function during molting and metamorphosis in insects. Functional studies confirm a role for most of these 'ecdysone cascade' NR orthologs during the continuous growth and dauer molts. Transcript levels for these genes fluctuate in a reiterated pattern during the molting cycles, reminiscent of the expression hierarchy observed in the insect ecdysone response. Together, these analyses provide a foundation for further dissecting the role of NRs in nematode development as well as for evaluating conservation of NR functions among the Metazoa.

An rxr/usp homolog from the parasitic nematode, Dirofilaria immitis

Filarial parasites are responsible for several serious human diseases with symptoms such as lymphoedema, elephantiasis, and blindness. An understanding of how these parasites pass through developmental checkpoints may suggest potential targets for intervention. A useful model system for the study of the human parasites is the closely related nematode Dirofilaria immitis, the causative agent of dog heartworm disease. In D. immitis, molting from the third to the fourth larval stage can be induced in vitro by the insect molting hormone, 20-hydroxyecdysone, suggesting that this, or some related steroid, may play a similar role in the development of D. immitis. The holoreceptor of 20-hydroxyecdysone consists of two nuclear receptors (NRs) ecdysone receptor (EcR) and ultraspiracle (USP), USP being the insect orthologue of the vertebrate RXR. We have identified a D. immitis rxr/usp, Di-rxr-1 (NR2B4). Di-RXR-1 can bind in vitro to EcR and DHR38, both known insect USP partners. Like, USP, it activates transcription in Drosophila Schneider S2 cells in a 20-hydroxyecdysone-dependent manner, via its interaction with the endogenous EcR protein. By Northern blot analysis, Di-rxr-1 mRNA is detected in adult females, but not in males. This is the first characterization of a nematode rxr/usp.

Ecdysteroids and oocyte development in the black fly Simulium vittatum

BACKGROUND

Oocyte development was studied in the autogenous black fly, Simulium vittatum (Diptera, Nematocera), a vector of Onchocerca volvulus, the causative agent of onchocerciasis.

RESULTS

Oocyte growth was nearly linear between adult eclosion and was complete by 72 hours at 21 degrees C. The oocyte became opaque at 14 hours after eclosion indicating the initiation of protein yolk deposition. The accumulation of vitellogenin was measured using SDS-PAGE. The density of the yolk protein bands at about 200 and 65 kDa increased during the first and second days after eclosion. The amount of protein in the 200 kDa band of vitellogenin, determined using densitometry, rapidly increased between 12 and 25 hours after eclosion. Ecdysteroid levels were measured using a competitive ELISA. Ecdysteroid levels increased rapidly and subsequently declined during the first day after eclosion.

CONCLUSION

These data show a correlation between the appearance of vitellogenin in the oocyte, and the rise in ecdysteroids. A possible relationship to molting of the nematode, Onchocerca volvulus, is discussed.

Molting, ecdysis, and reproduction of Trichinella spiralis are supported in vitro by intestinal epithelial cells

Trichinella spiralis is an obligate parasite of animals that has an unusual intracellular life cycle. Investigation of parasitism at the cellular and molecular levels has been challenging because of a shortage of tools for in vitro cultivation of T. spiralis. We have found that T. spiralis larvae molt, ecdyse, develop to adulthood, and reproduce when they are inoculated onto cultured intestinal epithelial cells. Initially, larvae invade and migrate through cells in a monolayer (T. ManWarren, L. Gagliardo, J. Geyer, C. McVay, S. Pearce-Kelling, and J. Appleton, Infect. Immun. 65:4806-4812, 1997). During prolonged culture in Caco-2 epithelial cells, L1 larvae molted and ecdysed with efficiencies as high as 50%. Molting and ecdysis in vitro required entry of the parasite into cells; conditions that prevented entry into cells also prevented ecdysis. When larvae were inoculated at a low density and cultured for 5 to 9 days, as many as 50% of the larvae developed to adult stages. Low numbers of mature male worms with copulatory appendages were observed in these cultures. The majority of worms that survived for five or more days were unfertilized females. Low-density cultures supported development of female worms with embryos at rates of 4 to 5%. These results show that the intestinal life cycle of T. spiralis can be supported entirely by host epithelial cells. Our model should allow more detailed investigation of intracellular parasitism by T. spiralis.

Dirofilaria immitis encodes Di-nhr-7, a putative orthologue of the Drosophila ecdysone-regulated E78 gene

Filarial parasites are responsible for several serious human diseases with symptoms such as lymphoedema, elephantiasis, and blindness. An understanding of how these parasites pass through developmental checkpoints may elucidate the general mechanisms of these illnesses and suggest potential targets for intervention. A useful model system for the study of human filariasis is the related nematode Dirofilaria immitis, the causative agent of dog heartworm disease. In D. immitis, molting from the third to the fourth larval stage can be induced in vitro by the insect hormone 20-OH ecdysone, suggesting that ecdysone, or some related hormone, may play a similar role in the development of D. immitis. Ecdysone has a well-characterized developmental role in insects, where it is involved in the control of molting and metamorphosis. We have identified a D. immitis orthologue of the Drosophila ecdysone response early gene E78, a member of the nuclear receptor (NR) superfamily. The D. immitis gene, Di-nhr-7 (NR1E1) encodes at least three isoforms, including two potential negative regulatory isoforms, and is expressed in a sex-specific manner. An MBP/Di-NHR-7 fusion protein is able to bind to DNA response elements that are recognized by the closely related mammalian NR Rev-erb(alpha).

Identification and characterization of an aromatic amino acid decarboxylase from the filarial nematode, Dirofilaria immitis

A novel secreted aromatic amino acid decarboxylase-like molecule was identified in the excretory/secretory products of L3/L4 larvae as well as in an extract of adult Dirofilaria immitis. The secretion of the enzyme was developmentally regulated. Peak enzyme activities were detected in the culture medium before and after the molting of L3 larvae in vitro. The enzyme was purified from D. immitis adult extracts and the excretory/secretory products of L3/L4 larvae using different chromatographic methods followed by isoelectric focusing and SDS-PAGE. The enzyme has a molecular mass of 48 kDa and a pI of 5.6, and shows a specific enzymatic activity towards the aromatic amino acid substrates phenylalanine, tyrosine and tryptophan. The enzyme's activity did not show an absolute requirement for exogenous pyridoxal-5-phosphate. However, addition of pyridoxal-5-phosphate at 5 microM in the reaction increased the enzyme activity greatly. The enzyme had the ability to catalyze the formation of dopamine from L-dopa. Studies on the effects of inhibitors on the enzyme activity showed that the enzyme was sensitive to Pefabloc and p-chloromercuribenzoic acid, but not to diisopropyl flurophosphate. The Km values of the enzyme for H-Phe-AMC, H-Tyr-AMC and H-Trp-AMC were calculated to be 32.1 microM, 35.1 microM and 29.1 microM, respectively.

Identification of ligands and coligands for the ecdysone-regulated gene switch

The ecdysone-inducible gene switch is a useful tool for modulating gene expression in mammalian cells and transgenic animals. We have identified inducers derived from plants as well as certain classes of insecticides that increase the versatility of this gene regulation system. Phytoecdysteroids share the favorable kinetics of steroids, but are inert in mammals. The gene regulation properties of one of these ecdysteroids have been examined in cell culture and in newly developed strains of ecdysone-system transgenic mice. Ponasterone A is a potent regulator of gene expression in cells and transgenic animals, enabling reporter genes to be turned on and off rapidly. A number of nonsteroidal insecticides have been identified that also activate the ecdysone system. Because the gene-controlling properties of the ecdysone switch are based on a heterodimer composed of a modified ecdysone receptor (VgEcR) and the retinoid X receptor (RXR), we have tested the effect of RXR ligands on the VgEcR/RXR complex. Used alone, RXR ligands display no activity on the ecdysone switch. However, when used in combination with a VgEcR ligand, RXR ligands dramatically enhance the absolute levels of induction. This property of the heterodimer has allowed the development of superinducer combinations that increase the dynamic range of the system.

The position of the Arthropoda in the phylogenetic system

Traditionally, Panarthropoda (Euarthropoda, Onychophora, Tardigrada) are regarded as being closely related to Annelida in a taxon Articulata, but this is not supported by molecular analyses. Comparisons of gene sequences suggest that all molting taxa (Panarthropoda, Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera) are related in a monophyletic taxon Ecdysozoa. An examination of the characters supporting Articulata reveals that only segmentation with a teloblastic segment formation and the existence of segmental coelomic cavities with nephridia support the Articulata, whereas all other characters are modified or reduced in the panarthropod lineage. Another set of characters is presented that supports the monophyly of Ecdysozoa: molting under influence of ecdysteroid hormones, loss of locomotory cilia, trilayered cuticle and the formation of the epicuticle from the tips of epidermal microvilli. Comparative morphology suggests Gastrotricha as the sister group of Ecdysozoa with the synapomorphies: triradiate muscular sucking pharynx and terminal mouth opening. Thus there are morphological characters that support Articulata, but molecular as well as morphological data advocate Ecdysozoa. Comparison of both hypotheses should prompt further thorough and targeted investigations. J. Morphol. 238:263-285, 1998. © 1998 Wiley-Liss, Inc.

Effects of azadirachtin on Ctenocephalides felis in the dog and the cat

Azadirachtin-containing neem seed extract is a powerful insect growth regulator, a feeding deterrent and repellent with low toxicity. Unfortunately, azadirachtin degrades rapidly in light, excessive heat or alkalinity. Evaluations of azadirachtin on ectoparasites on animals have been scarce. The purpose of this work was to describe the effects of normal and potentiated azadirachtin on Ctenocephalides felis in the dog or cat. Groups of kennelled greyhounds and domestic cats infested with C. felis were sprayed once with azadirachtin containing neem seed extract with or without diethyltoluamide (Deet) and/or citronella. Methanolic extracts with 200, 1000 or 2400 ppm azadirachtin reduced fleas in a dose-dependent manner. Compared with fleas counted on treated dogs just before treatment and untreated infested dogs, 1000-2400 ppm azadirachtin reduced fleas 93-53% for 19 days. However, combined with 500 ppm Deet and 33% w/v citronella, only 500 ppm azadirachtin reduced fleas 95-62% for 20 days. On cats inoculated with 50 fleas 2 days before treatment, the combination reduced fleas and eggs 100% to day 6 and 83-51% from day 7 to 9. On petri dishes, the combination achieved 100% egg mortality up to day 7 and 80% to day 14 and 48-52% to days 21-28. Deet, with or without neem seed extract or citronella, and citronella, with or without neem, did not reduce fleas significantly. The results show that azadirachtin reduced fleas in a dose-dependent manner in flea-contaminated environments. In cats, the combination killed most fleas within 24 h, providing effective flea control for 7 days. The results suggest that Deet with citronella potentiated the effect of azadirachtin on C. felis.