Brugia spp. and Litomosoides carinii: identification of a covalently cross-linked microfilarial sheath matrix protein (shp2)

Use of RNA interference to investigate gene function in the human filarial nematode parasite Brugia malayi

We describe the successful use of the reverse genetic technique RNA interference (RNAi) to investigate gene function in the human filarial nematode parasite Brugia malayi. We used fluorescently labelled double stranded RNA (dsRNA) to demonstrate that 300 bp molecules are able to enter adult females in culture while they remain excluded from microfilariae (mf). We have developed an optimised microvolume culture system to allow the exposure of parasites to high concentrations of dsRNA for extended periods. Culturing of adult female parasites in this system for 24h does not significantly reduce parasite lifespan or mf release in culture. Three B. malayi genes, beta-tubulin (Bm-tub-1), RNA polymerase II large subunit (Bm-ama-1) and B. malayi mf sheath protein 1/mf22 (Bm-shp-1) were targeted by soaking adult female B. malayi in dsRNA complementary to these transcripts in the optimised culture system. Targeting of the two housekeeping genes Bm-tub-1 and Bm-ama-1 led to a reduction in the levels of their transcripts, as assessed by reverse transcriptase coupled PCR (RT-PCR), and resulted in parasite death in culture. In contrast, targeting of the Bm-shp-1 gene was not lethal to adult females in culture. A marked reduction in mf release was observed for shp-1 RNAi parasites compared to controls and in addition 50% of mf released did not have fully elongated sheaths. This "short" phenotype correlated with the loss of the stockpiled shp-1 transcript from developing mf in treated adult female gonads. From these data we conclude that RNAi may be a useful method for assessment of drug target potential of genes identified in filarial gene discovery projects.

Cloning and expression analysis of two mucin-like genes encoding microfilarial sheath surface proteins of the parasitic nematodes Brugia and Litomosoides

In several filarial genera the first stage larvae (microfilariae) are enclosed by an eggshell-derived sheath that provides a major interface between the parasite and the host immune system. Analysis of the polypeptide constituents of the microfilarial sheath from the cotton rat filaria Litomosoides sigmodontis identified two abundant surface glycoproteins: Shp3a and Shp3. The corresponding genes and the orthologues of the human parasite Brugia malayi and the rodent filaria Brugia pahangi were cloned and sequenced. They encode secreted, mucin-like proteins with N-terminal Ser/Thr-rich repeats and a C-terminal anchor domain rich in aromatic amino acids. About 75% of the protein molecular masses result from post-translational modifications. The Ser/Thr-rich motifs are supposed to serve as targets for dimethylaminoethanol-phosphate substitutions. These modifications were detected only on the sheaths of the late developmental stage of stretched microfilariae, corresponding with the expression of the proteins in the epithelium of the distal part of the uterus and the specific transcription of shp3 and shp3a in the anterior female worm segment. Genomic analysis of all three species demonstrated a conserved linkage of the two genes. Their transcripts undergo cis- and trans-splicing. The transcription start sites of the primary transcripts were determined for the L. sigmodontis genes. The core promoter regions are remarkably conserved between the paralogue genes Ls-shp3a and Ls-shp3 and their orthologues in Brugia, implicating conserved regulatory elements.

Molecular characterization of a Litomosoides sigmodontis protein involved in the development of the microfilarial sheath during embryogenesis

A cDNA clone Ls110 was isolated from a female Litomosoides sigmodontis expression library using an antiserum raised against the microfilarial sheath. The complete cDNA encodes a protein (Ls110) of 382 amino acids. Southern and PCR analyses revealed the presence of Ls110 in L. sigmodontis as a single copy gene. The transcription of the Ls110 gene was limited to female worms. In these worms the transcription was confined to the epithelial cells of the uterus. The protein Ls110 was detected not only in the epithelial layer of the uterus but also secreted in the lumen of the uterus. All the intra-uterine embryonic stages showed the protein bound to their egg shell/sheath, except the early multicellular embryonic stages and fully developed microfilariae. The transient occurrence of Ls110 on these structures of intra-uterine stages besides the presence of a cysteine-rich N-terminal region (SXC-like domain) suggest that the protein may play a role in the formation of the microfilarial sheath during embryogenesis.

Transglutaminase-catalyzed reactions in the growth, maturation and development of parasitic nematodes

Parasitic nematodes cause several debilitating diseases in humans and animals. New drugs that are parasite specific and minimally toxic to the host are needed to counter these infections effectively. The identification and inhibition of enzymes that are vital for the growth and survival of parasites offer new approaches for developing effective chemotherapeutic agents. Several enzymes in nematodes fall into this category. Here, Ramaswamy Chandrashekar and Kapil Mehta examine in detail the role of transglutaminase, a protein-crosslinking enzyme, in the normal growth and development of nematodes, with an emphasis on filarial parasites.

cut-1-like genes are present in the filarial nematodes, Brugia pahangi and Brugia malayi, and, as in other nematodes, code for components of the cuticle

A fragment of a cut-1 like gene from the filarial nematode Brugia pahangi (designated Bp-cut-1) was isolated by PCR from genomic DNA. The sequence was used to design primers for use in RT-PCR and resulted in the isolation of a cDNA fragment from larvae in the process of the L3-L4 moult. Screening of a B. malayi genomic library identified a single clone, Bm-cut-1. Using primers designed from the Brugia sequences, semi-quantitative RT-PCR was carried out on 11 different life cycle stages chosen to cover periods around the moult and inter-moult periods. This analysis demonstrated that the cut-1 mRNA was most abundant preceding the moult, consistent with its function as a cuticular protein. Immuno-gold electron microscopy using an affinity purified antiserum raised to the highly conserved region of Ascaris CUT-1 confirmed that the protein was restricted to a tight band in the median layer of the cuticle. Despite the fact that no transcripts could be detected in mature adult worms by RT-PCR, immuno-gold microscopy revealed staining of the microfilarial cuticle within the uterus of the adult female worm, suggesting that other cut-1-like genes are present in Brugia.

Identification of a molecular marker for the Y chromosome of Brugia malayi

Random amplification of polymorphic DNA (RAPD) was used to analyse genomic DNA from virgin females and males of Brugia malayi, with a view to identifying sex-specific differences predicted by an XX/XY system of chromosomal sex determination. A product of 2338 bp, amplified with the arbitrary primer 5' GTTGCGATCC 3', was obtained exclusively from males. Primers based on the sequence of this product amplified a DNA fragment of the expected size from each of two independent isolates of B. malayi (from Malaysia and Indonesia) by PCR. No reaction product was obtained from the closely related species Brugia pahangi. In a genetic cross between B. malayi males and B. pahangi females, F1 hybrid microfilariae were PCR-positive, indicating that the locus is paternally-inherited. Southern blotting demonstrated that the target sequence resides in the high molecular weight fraction of genomic DNA, confirming that it is of chromosomal, rather than mitochondrial, origin. Sequencing of the locus revealed significant similarity with members of a family of reverse transcriptase-like genes in Caenorhabditis elegans. In-frame stops indicate that the gene is non-functional, but multiple bands of hybridisation in Southern blots suggest that the RT sequence may be the relic of a transposable element. Multiple repeats of the dinucleotide AT occurred in another region of the sequence. These varied in number between the two isolates of B. malayi in the manner of a microsatellite, surprisingly the first to be described from the B. malayi genome. Because of its association with the Y chromosome, we have given the locus the acronym TOY (Tag On Y). Identification of this chromosome-specific marker confirms the XX/XY heterogametic karyotype in B. malayi and opens the way to elucidation of the role of Y in sex determination.

Expression of the microfilarial sheath protein 2 (shp2) of the filarial parasites Litomosoides sigmodontis and Brugia malayi

The microfilarial sheaths of the filarial parasites Brugia malayi, Brugia pahangi, and Litomosoides sigmodontis consist of several parasite proteins, probably ranging between 7 and 10. The gene encoding sheath protein 2 (shp2), which is the object of this study, is transcribed in embryos and in the uterine epithelium; at least in B. malayi, it is translated in both tissues. Apparently, shp2 is synthesized as a monomer, exported by the respective cells, and integrated into the microfilarial sheath. In the sheath, it exists as a highly polymerized molecule cross-linked by cysteine formation and other covalent bonds, presumably epsilon-(gamma-glutamyl)-lysine links.

Stage-specific gene expression in lymphatic filarial nematodes

Lymphatic filarial nematodes remain a significant cause of morbidity throughout much of the tropics. One approach to the development of rational control methods is an improved understanding of the basic biology of these organisms in relation to the mechanisms used to complete their life cycles. In this article, Eileen Devaney, Sam Martin and Fiona Thompson review new approaches to defining stage-specific molecules in filarial nematodes, and discuss their recent work on the isolation and characterization of stage-regulated cDNAs from Brugia pahangi.

Identification of a region of genetic variability among Bacillus anthracis strains and related species

The identification of a region of sequence variability among individual isolates of Bacillus anthracis as well as the two closely related species, Bacillus cereus and Bacillus mycoides, has made a sequence-based approach for the rapid differentiation among members of this group possible. We have identified this region of sequence divergence by comparison of arbitrarily primed (AP)-PCR "fingerprints" generated by an M13 bacteriophage-derived primer and sequencing the respective forms of the only polymorphic fragment observed. The 1,480-bp fragment derived from genomic DNA of the Sterne strain of B. anthracis contained four consecutive repeats of CAATATCAACAA. The same fragment from the Vollum strain was identical except that two of these repeats were deleted. The Ames strain of B. anthracis differed from the Sterne strain by a single-nucleotide deletion. More than 150 nucleotide differences separated B. cereus and B. mycoides from B. anthracis in pairwise comparisons. The nucleotide sequence of the variable fragment from each species contained one complete open reading frame (ORF) (designated vrrA, for variable region with repetitive sequence), encoding a potential 30-kDa protein located between the carboxy terminus of an upstream ORF (designated orf1) and the amino terminus of a downstream ORF (designated lytB). The sequence variation was primarily in vrrA, which was glutamine- and proline-rich (30% of total) and contained repetitive regions. A large proportion of the nucleotide substitutions between species were synonymous. vrrA has 35% identity with the microfilarial sheath protein shp2 of the parasitic worm Litomosoides carinii.