A highly conserved nematode protein folding operon in Caenorhabditis elegans and Caenorhabditis briggsae

Operon conservation and the evolution of trans-splicing in the phylum Nematoda

The nematode Caenorhabditis elegans is unique among model animals in that many of its genes are cotranscribed as polycistronic pre-mRNAs from operons. The mechanism by which these operonic transcripts are resolved into mature mRNAs includes trans-splicing to a family of SL2-like spliced leader exons. SL2-like spliced leaders are distinct from SL1, the major spliced leader in C. elegans and other nematode species. We surveyed five additional nematode species, representing three of the five major clades of the phylum Nematoda, for the presence of operons and the use of trans-spliced leaders in resolution of polycistronic pre-mRNAs. Conserved operons were found in Pristionchus pacificus, Nippostrongylus brasiliensis, Strongyloides ratti, Brugia malayi, and Ascaris suum. In nematodes closely related to the rhabditine C. elegans, a related family of SL2-like spliced leaders is used for operonic transcript resolution. However, in the tylenchine S. ratti operonic transcripts are resolved using a family of spliced leaders related to SL1. Non-operonic genes in S. ratti may also receive these SL1 variants. In the spirurine nematodes B. malayi and A. suum operonic transcripts are resolved using SL1. Mapping these phenotypes onto the robust molecular phylogeny for the Nematoda suggests that operons evolved before SL2-like spliced leaders, which are an evolutionary invention of the rhabditine lineage.

The genome of the nematode worm Caenorhabditis elegans was the first of any animal to be completely sequenced. One surprising finding in this worm's genome was that about one-fifth of its genes were organised as sets of from two to eight genes expressed from the same promoter, similar to bacterial “operons.” The pre-mRNAs made from these operons are processed by an intermolecular ligation process called SL trans-splicing. Other animal genomes, such as the human genome or that of the fruit fly contain neither operons nor SL trans-splicing. In this article, Guiliano and Blaxter have investigated whether this curious facet of genome organisation is peculiar to C. elegans and close relatives by examining the genomes of a wide range of parasitic and free-living nematodes. The authors find that both operons and trans-splicing are present across the nematodes, and that operons evolve as other genome features do. All of the species surveyed use trans-splicing to resolve their multigene pre-mRNAs into single-gene mRNAs, but the details differ significantly from the process in C. elegans. In particular, the short piece of RNA that is attached to the beginning of operon-derived mRNAs has changed independently in many nematode groups.

Peptidyl-prolyl cis-trans isomerases (immunophilins) and their roles in parasite biochemistry, host-parasite interaction and antiparasitic drug action

Immunophilin is the collective name given to the cyclophilin and FK506-binding protein families. As the name suggests, these include the major binding proteins of certain immunosuppressive drugs: cyclophilins for the cyclic peptide cyclosporin A and FK506-binding proteins for the macrolactones FK506 and rapamycin. Both families, although dissimilar in sequence, possess peptidyl-prolyl cis-trans isomerase activity in vitro and can play roles in protein folding and transport, RNA splicing and the regulation of multi-protein complexes in cells. In addition to enzymic activity, many immunophilins act as molecular chaperones. This property may be conferred by the isomerase domain and/or by additional domains. Recent years have seen a great increase in the number of known immunophilin genes in parasitic protozoa and helminths and in many cases their products have been characterised biochemically and their temporal and spatial expression patterns have been examined. Some of these genes represent novel types: one example is a Toxoplasma gondii gene encoding a protein with both cyclophilin and FK506-binding protein domains. Likely roles in protein folding and oligomerisation, RNA splicing and sexual differentiation have been suggested for parasite immunophilins. In addition, unexpected roles in parasite virulence (Mip FK506-binding protein of Trypanosoma cruzi) and host immuno-modulation (e.g. 18-kDa cyclophilin of T. gondii) have been established. Furthermore, in view of the potent antiparasitic activities of cyclosporins, macrolactones and non-immunosuppressive derivatives of these compounds, immunophilins may mediate drug action and/or may themselves represent potential drug targets. Investigation of the mechanisms of action of these agents may lead to the design of potent and selective antimalarial and other antiparasitic drugs. This review discusses the properties of immunophilins in parasites and the 'animal model'Caenorhabditis elegans and relates these to our understanding of the roles of these proteins in cellular biochemistry, host-parasite interaction and the antiparasitic mechanisms of the drugs that bind to them.

Enzymes involved in the biogenesis of the nematode cuticle

Nematodes include species that are significant parasites of man, his domestic animals and crops, and cause chronic debilitating diseases in the developing world; such as lymphatic filariasis and river blindness caused by filarial species. Around one third of the World's population harbour parasitic nematodes; no vaccines exist for prevention of infection, limited effective drugs are available and drug resistance is an ever-increasing problem. A critical structure of the nematode is the protective cuticle, a collagen-rich extracellular matrix (ECM) that forms the exoskeleton, and is critical for viability. This resilient structure is synthesized sequentially five times during nematode development and offers protection from the environment, including the hosts' immune response. The detailed characterization of this complex structure; it's components, and the means by which they are synthesized, modified, processed and assembled will identify targets that may be exploited in the future control of parasitic nematodes. This review will focus on the nematode cuticle. This structure is predominantly composed of collagens, a class of proteins that are modified by a range of co- and post-translational modifications prior to assembly into higher order complexes or ECMs. The collagens and their associated enzymes have been comprehensively characterized in vertebrate systems and some of these studies will be addressed in this review. Conversely, the biosynthesis of this class of essential structural proteins has not been studied in such detail in the nematodes. As with all morphogenetic, functional and developmental studies in the Nematoda phylum, the free-living species Caenorhabditis elegans has proven to be invaluable in the characterization of the cuticle and the cuticle collagen gene family, and is now proving to be an excellent model in the study of cuticle collagen biosynthetic enzymes. This model system will be the main focus of this review.

The Caenorhabditis elegans ERp60 homolog protein disulfide isomerase-3 has disulfide isomerase and transglutaminase-like cross-linking activity and is involved in the maintenance of body morphology

A novel protein disulfide isomerase gene, pdi-3, was isolated from the nematode Caenorhabditis elegans. This gene encodes an enzyme related to the ERp60 class of thioredoxin proteins and was found to exhibit unusual enzymatic properties. Recombinant protein displayed both disulfide bond isomerase activity and calcium-dependent transglutaminase-like cross-linking activity. The pdi-3 transcript was developmentally constitutively expressed, and the encoded protein is present in many tissues including the gut and the hypodermis. The nematode hypodermis synthesizes the essential collagenous extracellular matrix (ECM) called the cuticle. Transcript disruption via double-stranded RNA interference resulted in dramatic and specific synthetic phenotypes in several C. elegans mutant alleles with weakened cuticles: sqt-3(e2117), dpy-18(e364, ok162, and bx26). These nematodes displayed severe dumpy phenotypes and disrupted lateral alae, a destabilized cuticle and abnormal male and hermaphrodite tail morphologies. These defects were confirmed to be consistent with hypodermal seam cell abnormalities and corresponded with the severe disruption of a cuticle collagen. Wild type nematodes did not exhibit observable morphological defects; however, cuticle collagen localization was mildly disrupted following pdi-3 RNA interference. The unusual thioredoxin enzyme, protein disulfide isomerase-3, may therefore play a role in ECM assembly. This enzyme is required for the proper maintenance of post-embryonic body shape in strains with a weakened cuticle, perhaps through ECM stabilization via cross-linking activity, disulfide isomerase protein folding activity, protein disulfide isomerase chaperone activity, or via multifunctional events.

Microsporidian invasion apparatus: identification of a novel polar tube protein and evidence for clustering of ptp1 and ptp2 genes in three Encephalitozoon species

Microsporidia are unicellular eukaryotes occurring as obligate intracellular parasites which produce resistant spores. A unique motile process is represented by the sudden extrusion of the sporal polar tube for initiating entry of the parasite into a new host cell. The complete sequence of an acidic proline-rich polar tube protein (renamed PTP1) has been previously reported for Encephalitozoon cuniculi and E. hellem. Our immunological investigations provided evidence for an additional PTP in E. cuniculi, termed PTP2. The corresponding gene was sequenced and then expressed in Escherichia coli. As expected, mouse antibodies raised against the recombinant protein reacted specifically with the polar tube. The single copy ptp1 and ptp2 genes of E. cuniculi were tandemly arranged on chromosome VI. Polyadenylation of the mRNAs was demonstrated. Identification and sequencing of homologous genes in the two other human-infecting Encephalitozoon species (ptp2 in E. hellem and ptp1 and ptp2 in E. intestinalis) were facilitated by conserved gene clustering. PTP2 appears as a novel structural protein (30 kDa) with a basic lysine-rich core and an acidic tail. Unlike PTP1, this protein is devoid of large tandem repeats. The interspecies conservation of cysteine residues supports a major role of disulfide bridges in polar tube assembly. The two PTPs should serve as both molecular markers of spore differentiation and diagnostic tools.

Prolyl 4-hydroxylase is an essential procollagen-modifying enzyme required for exoskeleton formation and the maintenance of body shape in the nematode Caenorhabditis elegans

The multienzyme complex prolyl 4-hydroxylase catalyzes the hydroxylation of proline residues and acts as a chaperone during collagen synthesis in multicellular organisms. The beta subunit of this complex is identical to protein disulfide isomerase (PDI). The free-living nematode Caenorhabditis elegans is encased in a collagenous exoskeleton and represents an excellent model for the study of collagen biosynthesis and extracellular matrix formation. In this study, we examined prolyl 4-hydroxylase alpha-subunit (PHY; EC 1.14.11.2)- and beta-subunit (PDI; EC 5.3.4.1)-encoding genes with respect to their role in collagen modification and formation of the C. elegans exoskeleton. We identified genes encoding two PHYs and a single associated PDI and showed that all three are expressed in collagen-synthesizing ectodermal cells at times of maximal collagen synthesis. Disruption of the pdi gene via RNA interference resulted in embryonic lethality. Similarly, the combined phy genes are required for embryonic development. Interference with phy-1 resulted in a morphologically dumpy phenotype, which we determined to be identical to the uncharacterized dpy-18 locus. Two dpy-18 mutant strains were shown to have null alleles for phy-1 and to have a reduced hydroxyproline content in their exoskeleton collagens. This study demonstrates in vivo that this enzyme complex plays a central role in extracellular matrix formation and is essential for normal metazoan development.

Identification of potential vaccine and drug target candidates by expressed sequence tag analysis and immunoscreening of Onchocerca volvulus larval cDNA libraries

The search for appropriate vaccine candidates and drug targets against onchocerciasis has so far been confronted with several limitations due to the unavailability of biological material, appropriate molecular resources, and knowledge of the parasite biology. To identify targets for vaccine or chemotherapy development we have undertaken two approaches. First, cDNA expression libraries were constructed from life cycle stages that are critical for establishment of Onchocerca volvulus infection, the third-stage larvae (L3) and the molting L3. A gene discovery effort was then initiated by random expressed sequence tag analysis of 5,506 cDNA clones. Cluster analyses showed that many of the transcripts were up-regulated and/or stage specific in either one or both of the cDNA libraries when compared to the microfilariae, L2, and both adult stages of the parasite. Homology searches against the GenBank database facilitated the identification of several genes of interest, such as proteinases, proteinase inhibitors, antioxidant or detoxification enzymes, and neurotransmitter receptors, as well as structural and housekeeping genes. Other O. volvulus genes showed homology only to predicted genes from the free-living nematode Caenorhabditis elegans or were entirely novel. Some of the novel proteins contain potential secretory leaders. Secondly, by immunoscreening the molting L3 cDNA library with a pool of human sera from putatively immune individuals, we identified six novel immunogenic proteins that otherwise would not have been identified as potential vaccinogens using the gene discovery effort. This study lays a solid foundation for a better understanding of the biology of O. volvulus as well as for the identification of novel targets for filaricidal agents and/or vaccines against onchocerciasis based on immunological and rational hypothesis-driven research.