Characterization of the manganese superoxide dismutase cDNA and gene from the human parasite Onchocerca volvulus

Manganese homeostasis at the host-pathogen interface and in the host immune system

Manganese serves as an indispensable catalytic center and the structural core of various enzymes that participate in a plethora of biological processes, including oxidative phosphorylation, glycosylation, and signal transduction. In pathogenic microorganisms, manganese is required for survival by maintaining basic biochemical activity and virulence; in contrast, the host utilizes a process known as nutritional immunity to sequester manganese from invading pathogens. Recent epidemiological and animal studies have shown that manganese increases the immune response in a wide range of vertebrates, including humans, rodents, birds, and fish. On the other hand, excess manganese can cause neurotoxicity and other detrimental effects. Here, we review recent data illustrating the essential role of manganese homeostasis at the host-pathogen interface and in the host immune system. We also discuss the accumulating body of evidence that manganese modulates various signaling pathways in immune processes. Finally, we discuss the key molecular players involved in manganese's immune regulatory function, as well as the clinical implications with respect to cancer immunotherapy.

Genomic organization of the cytosolic manganese superoxide dismutase gene from the Pacific white shrimp, Litopenaeus vannamei, and its response to thermal stress

Cytosolic manganese superoxide dismutase (cMnSOD) is an important antioxidant enzyme which catalyzes the conversion of superoxides to oxygen and hydrogen peroxide in several organisms. In the Pacific white shrimp, Litopenaeus vannamei, three cMnSOD genes (LvcMnSOD1-3) have previously been characterized. Here, the genomic structure of LvcMnSOD2 and its mRNA expression in response to thermal stress was examined. Analysis of the nucleotide sequence demonstrated that LvcMnSOD2 is comprised of 2392 bp spanning from the ATG translation start site to the stop codon and contains six exons interrupted by five introns. The 5' region upstream of the LvcMnSOD2 gene contains several putative regulatory elements but lacks the accepted TATA sequence. The putative transcription factor binding elements that may be involved in LvcMnSOD2 mRNA expression level include activator protein-1 (AP-1), cAMP response element binding protein (CREB), upstream stimulatory factor (USF), CAAT-enhancer binding protein (C/EBP), nuclear factor-κB (NF-κB) and heat shock regulatory element (HSE). In addition, we compared the 5' upstream sequences of the LvcMnSOD2 gene between two shrimp strains that are resistant or susceptible to Taura syndrome virus (TSV), respectively, which revealed the absence of the USF and C/EBP elements at positions -2125 and -1986, respectively, in the TSV-susceptible shrimp line. Moreover, genomic variations between the two shrimp strains were detected in some of the putative C/EBP, USF, HSE and NF-κB transcription factor binding elements. That these genomic variations might be involved in the TSV resistance as well as in stress responses remains to be evaluated. The presence of 15 putative HSEs suggests that the expression of LvcMnSOD2 is regulated under thermal stress. Here, we found that in response to a 1 or 3 h thermal stress (35 °C), the mRNA expression levels of LvcMnSOD2 were significantly increased and then gradually decreased in the recovering phase at room temperature (25 °C) to control levels by 3 h after the heat shock. Thus, the antioxidant system may be induced to protect cells from the oxidative damage caused by thermal stress. The genomic organization of LvcMnSOD2 likely provides a clue to the mechanisms that might regulate the antioxidant defense pathway in shrimps and so potentially in marine invertebrates.

Cytosolic manganese superoxide dismutase genes from the white shrimp Litopenaeus vannamei are differentially expressed in response to lipopolysaccharides, white spot virus and during ontogeny

Manganese superoxide dismutase (MnSOD) is an antioxidant enzyme usually located in mitochondria. There are only a few examples of cytosolic MnSOD (cMnSOD). In the shrimp Litopenaeus vannamei, we have previously characterized three cMnSOD cDNAs and their differential tissue-specific expression. To obtain insights about their genomic organization, we characterized the three corresponding cMnSOD genes, named them cMnsod1, cMnsod2, and cMnsod3 and studied their specific expression during ontogeny, response to lipopolysaccharides (LPS) and white spot virus infection (WSSV) in hemocytes from shrimp. The first two genes contain five introns flanked by canonical 5'-GT-AG-3' intron splice-site junctions, while the third one is intron-less. We analyzed 995 nucleotides upstream cMnsod2, but no classical promoter sequences were found. The deduced products of the three cMnSOD genes differ in two amino acids and there are four silent changes. cMnsod3 expression is modulated by WSSV and cMnsod2 by LPS. cMnsod2 is expressed from eggs to post larval stage during ontogeny. This is the first report of crustacean cMnSOD multigenes that are differently induced during the defense response and ontogeny.

Shrimp invertebrate lysozyme i-lyz: gene structure, molecular model and response of c and i lysozymes to lipopolysaccharide (LPS)

The invertebrate lysozyme (i-lyz or destabilase) is present in shrimp. This protein may have a function as a peptidoglycan-breaking enzyme and as a peptidase. Shrimp is commonly infected with Vibrio sp., a Gram-negative bacteria, and it is known that the c-lyz (similar to chicken lysozyme) is active against these bacteria. To further understand the regulation of lysozymes, we determined the gene sequence and modeled the protein structure of i-lyz. In addition, the expression of i-lyz and c-lyz in response to lipopolysaccharide (LPS) was studied. The shrimp i-lyz gene is interrupted by two introns with canonical splice junctions. The expression of the shrimp i-lyz was transiently down-regulated after LPS injection followed by induction after 6 h in hepatopancreas. In contrast, c-lyz was up-regulated in hepatopancreas 4 h post-injection and slightly down-regulated in gills. The L. vannamei i-lyz does not contain the catalytic residues for muramidase (glycohydrolase) neither isopeptidase activities; however, it is known that the antibacterial activity does not solely rely on the enzymatic activity of the protein. The study of invertebrate lysozyme will increase our understanding of the regulatory process of the defense mechanisms.

Cu/Zn- and Mn-superoxide dismutase (SOD) from the copepod Tigriopus japonicus: molecular cloning and expression in response to environmental pollutants

Superoxide dismutase (SOD) is an important antioxidant enzyme which catalyzes conversion of superoxide to oxygen and hydrogen peroxide in aerobic organisms. Here, we cloned and sequenced the full-length cDNA and genomic DNA of two SODs from the copepod, Tigriopus japonicus: copper/zinc SOD (TJ-Cu/Zn-SOD) and manganese SOD (TJ-Mn-SOD). To define whether TJ-Mn-SOD is a cytosolic or a mitochondrial protein, a phylogenetic analysis was performed. The genomic structure of both TJ-SOD genes was determined with the promoter region sequences. In order to investigate their potential role in response to environmental pollutants, T. japonicus were treated with heavy metal (copper, zinc, and silver; 0, 10, 25, 50, and 100 μg L(-1)) and industrial chemicals (benzo[α]pyrene, 4-nonylphenol, and tributyltin; 0, 1, 5, 10, and 20 μg L(-1)) for 96 h. Subsequently, the TJ-Cu/Zn-SOD and TJ-Mn-SOD mRNA level was measured with quantitative real-time RT-PCR along with total SOD activity. The deduced amino acid residues of TJ-Cu/Zn-SOD and TJ-Mn-SOD possessed evolutionary conserved domains that are required for metal binding and Cu/ZnSOD-conserved signature sequences. The phylogenetic analysis revealed that TJ-Mn-SOD was closely clustered to mitochondrial Mn-SOD of another copepod, Lepeophtheirus salmonis. TJ-Cu/Zn-SOD gene had four exons and three introns, while the TJ-Mn-SOD gene consisted of two exons interrupted by one intron. In the 5'-flanking region of TJ-Cu/Zn-SOD and TJ-Mn-SOD, we observed several transcription regulatory elements such as p53, XRE, MRE, and ERE-half sites. In the response to heavy metals, Cu, Zn, and Ag, both TJ-Cu/Zn-SOD and TJ-Mn-SOD transcript levels along with enzyme levels were significantly increased at high concentrations (50 μg L(-1) and 100 μg L(-1)). Particularly, in the Cu- and Ag-exposed group, the expression of TJ-Mn-SOD mRNA was regulated more sensitively than the TJ-Cu/Zn-SOD mRNA level, indicating that the chemical susceptibility would be not correlated with the form of chemicals. B[a]P treatment showed a significant increase in the expression of both TJ-SODs mRNA level and enzyme level from 5 μg L(-1) concentration, while TBT decreased its expression at high concentrations (10 μg L(-1) and 20 μg L(-1)). 4-NP increased both TJ-SODs mRNA level at 1 μg L(-1) concentration, and then inhibited its expression from 5 μg L(-1) concentration to a lower level than the control. This finding suggests that TJ-Cu/Zn-SOD and TJ-Mn-SOD would be an inducible gene upon exposure to heavy metals and B[α]P, and could be used as a potential biomarker for the risk assessment of these environmental pollutants. This is the first report to elucidate response of SOD to environmental pollutants in copepods. Therefore, this study would give a clue to better understand the mode of action of antioxidant genes and enzymes under oxidative stress in marine invertebrates.

Identification and characterization of a mitochondrial manganese superoxide dismutase of Spirometra erinacei

A gene encoding the manganese superoxide dismutase (Mn-SOD) of Spirometra erinacei was identified, and the biochemical properties of the recombinant enzyme were partially characterized. The S. erinacei Mn-SOD gene consisted of 669 bp, which encoded 222 amino acids. A sequence analysis of the gene showed that it had typical molecular structures, including characteristic metal-binding residues and motifs that were conserved in Mn-SODs. An analysis of the N-terminal presequence of S. erinacei Mn-SOD revealed that it had physiochemical characteristics commonly found in mitochondria-targeting sequences and predicted that the enzyme is located in the mitochondria. A biochemical analysis also revealed that the enzyme is a typical Mn-SOD. The enzyme was consistently expressed in both S. erinacei plerocercoid larvae and adult worms. Our results collectively suggested that S. erinacei Mn-SOD is a typical mitochondrial Mn-SOD and may play an important role in parasite physiology, detoxifying excess superoxide radicals generated in the mitochondria.

A manganese superoxide dismutase in blood clam Tegillarca granosa: molecular cloning, tissue distribution and expression analysis

Superoxide dismutase (SOD, EC 1.15.1.1) is one of the central enzymes involved in scavenging the high level of reactive oxygen species (ROS) by transforming O₂⁻ into hydrogen peroxide and oxygen. The full-length mitochondrial Mn-SOD cDNA of blood clam Tegillarca granosa (denoted as TgmMnSOD) was identified from haemocytes by homology cloning and rapid amplification of cDNA ends (RACE) approaches. The nucleotide sequence of TgmMnSOD consisted of 1106bp with a 5' UTR of 195bp, a 3' UTR of 227bp with a candidate polyadenylation signal sequence ATTAAA and a short polyA tail, and an open reading frame (ORF) of 648bp encoding a secreted polypeptide of 227 amino acids residues. The deduced amino acid sequence of TgmMnSOD shared significant homology to mMnSODs from other species, indicating that TgmMnSOD should be a novel member of the mMnSOD family. Several highly conserved motifs including three mMnSOD signatures, amino acid residues responsible for coordinating the manganese and the putative active center were almost completely conserved in the deduced amino acid of TgmMnSOD. The mRNA expression of TgmMnSOD in the tissues of mantle, foot, gill, haemocytes and hepatopancreas was examined by quantitative real-time PCR (qT-PCR) and mRNA transcripts of TgmMnSOD were mainly detected in hepatopancreas, haemocytes, and gill and weakly detected in the mantle and foot. The temporal expression of TgmMnSOD in haemocytes after heavy metal exposure revealed that TgmMnSOD could be induced by the three pollutants with different response profiles. The polyclonal antibodies generated from the recombinant product of TgmMnSOD could specifically identify not only the recombinant product, but also the native protein from haemocytes. The present results strongly suggested that TgmMnSOD was a cute response protein involved in marine heavy metal contaminants challenge in T. granosa.

Functional expression and characterization of a cytosolic copper/zinc-superoxide dismutase of Spirometra erinacei

Spirometra erinacei is a pseudophyllidean tapeworm which inhabits the intestines of cats and dogs. The infections are usually asymptomatic in these animals, but the infection of the plerocercoid larvae of the parasite, spargana, cause sparganosis in other vertebrates, including human. In this study, we identified a gene encoding the copper/zinc-superoxide dismutase of S. erinacei (SeCuZn-SOD) and partially characterized the biochemical and functional properties of the enzyme. The open reading frame of SeCuZnSOD was 465 bp that encodes 154 amino acids. The characteristic amino acid residues and motifs required for coordinating copper and zinc enzymatic function were well conserved. The genomic length of the SeCuZnSOD was 1,985 bp consisting of three exons that are separated by two introns. SeCuZnSOD is a typical cytosolic form which shares similar biochemical properties, including broad pH optima and inhibition profile by KCN and H(2)O(2), with cytosolic Cu/Zn-SODs of other organisms. SeCuZnSOD was functionally expressed in both S. erinacei plerocercoid larvae and adult worms, and its expression level was significantly increased when the plerocercoid larvae were treated with paraquat. The enzyme may play essential roles for survival of the parasite not only by protecting itself from endogenous oxidative stress, but also by detoxifying oxidative killing of the parasite by host immune effector cells.

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.

Enzymatic antioxidant systems in helminth parasites

Parasitic helminths have a coexistence with mammalian hosts whereby they survive for several years in known hostile conditions of their hosts. Many explanations exist describing how these parasitic helminths are able to survive. In the last years, a lot of studies have focused on both enzymatic and non-enzymatic antioxidant systems now shown to exist in these parasites and which may serve as defence tactics against the host-generated oxygen radicals. The relevance of antioxidant enzymes is confirmed by the fact that some of these molecules represent putative protective anti-parasite vaccines (i.e. in schistosomiasis). This review tries to compile what is known to date of the enzymatic antioxidant systems in selected parasitic helminths.

Cloning, Sequence, and Expression Analysis of a New MnSOD-Encoding Gene from the Root-Knot Nematode Meloidogyne incognita

A gene encoding a manganese superoxide dismutase (MnSOD) enzyme (Mi-mnsod) was identified and characterized in second-stage juveniles of the root-knot nematode Meloidogyne incognita. The Mi-mnsod gene was found to possess five exons and four introns with (GT/AG) consensus splice-site junctions. The deduced amino acid sequence of Mi-mnsod encodes a putative 25 KDa protein, with conserved amino acid residues of the MnSOD family, including the Parker-Blake signature and four metal-binding sites. The derived amino acid sequence showed high similarity to other eukaryotic MnSODs, including a 23 amino acid N-terminal putative mitochondrial transit peptide. Gene expression was observed throughout the posterior nematode body region with elevated signal intensities at the anterior portion of the intestine. DNA blot analysis and sequencing data showed the occurrence of three putative copies of the MnSOD gene with nucleotide polymorphisms found at the fourth exon and the 3' un-translated region.

Laser-assisted microdissection for the study of the ecology of parasites in their hosts

The population biology of internal parasites is difficult to study because the adult parasites are often inaccessible, deep within the host's body. Developing stages, such as eggs in the faeces or larvae in the skin are more easily obtained, but are difficult to handle because they are often very small and with a tough cuticle. This has limited their use in molecular ecology for estimating population biology parameters of the adults (their parents). We have used Onchocerca ochengi (a filarial nematode parasite of cattle) to describe a novel and generally applicable method of easily and conveniently isolating individual larvae (microfilariae) from the host using laser-assisted microdissection. Furthermore, we have been able to improve the isolation of DNA by using the laser to bisect the larva to release DNA from the tissues enclosed within the parasite cuticle, and in this way we have achieved amplification of fragments over 1400 bp, and routinely PCR-amplified single-copy sequences from 5% of the DNA from a single larva (the equivalent of approximately 15 nuclei), and regularly from 0.5%.

Cloning and differential expression of manganese superoxide dismutase (Mn-SOD) of Trichinella pseudospiralis

The complete coding sequence of manganese superoxide dismutase (Mn-SOD) of Trichinella pseudospiralis (Tp) was obtained and characterized for the first time by degenerative reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of complementary DNA ends (RACE) reactions. The open reading frame of Tp Mn-SOD contained 663 nucleotides, encoding 220 amino acid residues. This included the conserved histidine and aspartate residues for metal binding, cysteine residues for disulfide bond formation, and arginine residue for directing the superoxide ion to the protein. The presence of mitochondrial transit peptides and maturation cleavage site suggest that the cloned Tp Mn-SOD gene is a mitochondrial enzyme. It is a single copy gene containing three introns. Northern blotting suggested that the expression level of Mn-SOD is lower than Cu/Zn SOD in infective stage larvae. Semi-quantitative RT-PCR demonstrated that a single dominant transcript of Tp Mn-SOD was highly expressed in the infective-stage larvae but not in adult worms. The information provides a better understanding of the highly compartmentalized superoxide dismutases of adenophorean nematodes.

Molecular characterization of two superoxide dismutases from Hydra vulgaris

Apparent full-length cDNA sequences coding for manganese superoxide dismutase (HvMnSOD) and extracellular superoxide dismutase (HvEC-SOD) were isolated from Hydra vulgaris in order to understand their expression and 3D structures; and explore their possibility of being used as for biomarkers for environmental stress and toxicity. The deduced HvMnSOD protein consists of 219 amino acids of which first 21 amino acids constitute a presumed mitochondria-targeting signal peptide whereas HvEC-SOD protein consists of 189 amino acids of which first 19 amino acids constitute a presumed signal peptide. Molecular model generated for HvMnSOD displayed the N-terminal long alpha antiparallel hairpin and the C-terminal mixed alpha/beta fold characteristic of MnSODs and that for HvEC-SOD displayed the characteristic CuZnSOD â-barrel fold. Hydrae subjected to thermal, starvation, metal and oxidative stress responded by regulating MnSOD and EC-SOD mRNA transcription. These results indicated that these genes are involved in the cellular stress response and (anti)oxidative processes triggered by stressor and contaminant exposure. Hence the expression of these SODs in hydra may have potential as molecular biomarkers for assessing stress, toxicity and pro-oxidant quality of chemicals and aquatic environmental quality.

Cloning and characterization of the Cu/Zn superoxide dismutase of Trichinella pseudospiralis

Copper/zinc (Cu/Zn) superoxide dismutase (SOD) activity was identified for the first time in both crude somatic extracts (CE) and excretory/secretory (E/S) products of Trichinella pseudospiralis. It was the dominant SOD in infective-stage larvae. Native polyacrylamide gel electrophoresis of CE and E/S products yielded a prominent band, which was cyanide-sensitive and was partly inhibited by hydrogen peroxide in SOD assay. Cytosolic Cu/Zn SOD was cloned. The 471-bp full-length cDNA sequence contained an open reading frame of 157 amino acids. The gene contained three introns. Quantitative reverse transcription-polymerase chain reaction indicated that the expression of cytosolic Cu/Zn SOD was substantially higher in infective-stage larvae than in adult worms. Cluster analysis showed that the sequence of the Cu/Zn SOD of T. pseudospiralis, an adenophorean nematode, is related to those of Brugia pahangi, Acanthocheilonema viteae, Onchocerca volvulus, and Haemonchus contortus (all belonging to the sercenentean group).

Isolation and analysis of partial cDNA sequence coding for superoxide dismutase in Wuchereria bancrofti

Molecular characterization of Wuchereria bancrofti is essential to develop suitable anti-filarial drugs and vaccines. We describe here isolation, sequence analysis and cloning of a partial cDNA of an enzyme superoxide dismutase from this parasite. The immunoscreening of a lambda zap W. bancrofti microfilarial (Mf) cDNA library with microfilaremic sera had resulted in the isolation of several seroreactive clones including, WbSOD. This clone contained a 309 bp insert and showed significant nucleotide and deduced amino acid sequence homologies to the superoxide dismutases of other nematode parasites. The antioxidant property of this enzyme may have important contribution in the defense mechanism of the parasite against host immune response.

Molecular cloning and characterization of the copper/zinc and manganese superoxide dismutase genes from the human parasiteClonorchis sinensis

A copper/zinc superoxide dismutase (Cu/ZnSOD) gene and a manganese superoxide dismutase (MnSOD) gene of the human parasiteClonorchis sinensishave been cloned and their gene products functionally characterized. GenesCu/ZnSODandMnSODencode proteins of 16 kDa and 25·4 kDa, respectively. The deduced amino acid sequences of the two genes contained highly conserved residues required for activity and secondary structure formation of Cu/ZnSOD and MnSOD, respectively, and show up to 73·7% and 75·4% identities with their counterparts in other animals. The genomic DNA sequence analysis of Cu/ZnSOD gene revealed this as an intronless gene. Inhibitor studies with purified recombinant Cu/ZnSOD and MnSOD, both of which were functionally expressed inEscherichia coli, confirmed that they are copper/zinc and manganese-containing SOD, respectively. Immunoblots showed that bothC. sinensisCu/ZnSOD and MnSOD should be antigenic for humans, and both, especially theC. sinensisMnSOD, exhibit extensive cross-reactions with sera of patients infected by other trematodes or cestodes. RT-PCR and SOD activity staining of parasite lysates indicate that there are no significant differences in mRNA level or SOD activity for both species of SOD, indicating cytosolic Cu/ZnSOD and MnSOD might play a comparatively important role in theC. sinensisantioxidant system.

Molecular cloning, characterization and regulation by cadmium of a superoxide dismutase from the ectomycorrhizal fungus Paxillus involutus

The gene encoding a superoxide dismutase (PiSOD) was cloned by suppressive subtractive hybridization from cDNA library of the ectomycorrhizal fungus, Paxillus involutus, grown under cadmium-stress conditions. The encoded protein was presumed to be localized in the peroxisomes because it contained a C-terminal peroxisomal localization peptide (SKL) and lacked an N-terminal mitochondrial transit peptide. Complementation of an Escherichia coli SOD null strain that is unable to grow in the presence of paraquat or cadmium indicated that cloned Pisod encoded a functional superoxide dismutase. Sensitivity of PiSOD activity to H2O2 but not KCN, and sequence homologies to other SODs strongly suggest that it is a manganese-containing superoxide dismutase. Monitoring PiSOD transcript, immunoreactive polypeptide and superoxide dismutase activity following cadmium stress suggests that the principal level of control is post-translational. This is, to our knowledge, the first insight in the characterization of molecular events that take place in an ectomycorrhizal fungus during exposure to heavy metals.

mRNA trans-splicing in the human parasitic cestode Echinococcus multilocularis

An identical 36-nucleotide exon was identified at the 5' termini of different mRNAs from the cestode Echinococcus multilocularis. We provide evidence that this exon constitutes a new spliced leader (SL) that is obviously trans-spliced to echinococcal pre-mRNAs, donated by a non-polyadenylated, trimethylguanosine-capped SL-RNA of 104 nucleotides. Sequence comparisons indicated that cestode and trematode SLs are likely to be derived from a common ancestor gene. No conservation was, however, observed concerning the spectrum of mRNAs that is trans-spliced in cestodes and trematodes, indicating that trans-splicing of a particular flatworm mRNA is not correlated with the function of the encoded protein. We also show that the echinococcal gene elp, encoding a member of the ezrin/radixin/moesin protein family, is expressed via two alternative transcripts, spliced either cis or trans at an identical splice acceptor site. This was accompanied by the formation of different elp primary transcripts, harboring a complete or a truncated upstream intron, which supports the hypothesis that alternative cis/trans-splicing depends on the presence or absence of an upstream splice donor site. A putative SL gene was also identified on chromosomal DNA of Echinococcus granulosus, indicating widespread utilization of trans-splicing in the genus.

Molecular cloning and expression of Cu/Zn-containing superoxide dismutase from Fasciola hepatica

The cytosolic superoxide dismutase (SOD) of Fasciola hepatica, a causative agent of fascioliasis, was purified and characterized. The enzyme consists of two identical subunits, each with an apparent molecular mass of 17.5 kDa. An analysis of the enzyme's primary structure and inhibition studies revealed that the enzyme is a copper/zinc-containing SOD (Cu/Zn-SOD). The enzyme activity was relatively stable in a broad pH range, from pH 7.0 to 10.0, and the enzyme showed maximum activity at pH 7.5. This enzyme also displayed strong antigenicity against sera of bovine and human subjects with fascioliasis. The SOD gene fragment was amplified by PCR with degenerate oligonucleotide primers derived from amino acid sequences conserved in the Cu/Zn-SODs of other organisms. An F. hepatica cDNA library was screened with the SOD gene fragment as a probe. As a result, a complete gene encoding the Cu/Zn-SOD was identified, and its nucleotide sequence was determined. The gene had an open reading frame of 438 bp and 146 deduced amino acids. Comparison of the deduced amino acid sequence of the enzyme with previously reported Cu/Zn-SOD amino acid sequences revealed considerably high homologies. The coding region of the F. hepatica Cu/Zn-SOD was cloned and expressed in Escherichia coli. Staining of native polyacrylamide gel for SOD activity of the expressed protein revealed SOD activity that was inactivated by potassium cyanide and hydrogen peroxide but not by sodium azide. This means that the presence of the recombinant fusion protein is indicative of Cu/Zn-SOD. The expressed protein also reacted with sera of bovine and human subjects with fascioliasis, but it did not react with sera of uninfected bovine and human subjects.

The genomes of Onchocerca volvulus

Onchocerca volvulus, the filarial parasite that causes onchocerciasis or river blindness, contains three distinct genomes. These include the nuclear genome, the mitochondrial genome and the genome of an intracellular endosymbiont of the genus Wolbachia. The nuclear genome is roughly 1.5x10(8) bp in size, and is arranged on four chromosome pairs. Analysis of expressed sequence tags from different life-cycle stages has resulted in the identification of transcripts from roughly 4000 O. volvulus genes. Several of these transcripts are highly abundant, including those encoding collagen and cuticular proteins. Analysis of several gene sequences from O. volvulus suggests that the nuclear genes of O. volvulus are relatively compact and are interrupted relatively frequently by small introns. The intron-exon boundaries of these genes generally follow the GU-AG rule characteristic of the splice donor and acceptors of other vertebrate organisms. The nuclear genome also contains at least one repeated sequence family of a 150 bp repeat which is arranged in tandem arrays and appears subject to concerted evolution. The mitochondrial genome of O. volvulus is remarkably compact, only 13747 bp in size. Consistent with the small size of the genome, four gene pairs overlap, eight contain no intergenic regions and the remaining gene pairs are separated by small intergenic domains ranging from 1 to 46 bp. The protein-coding genes of the O. volvulus mitochondrial genome exhibit a striking codon bias, with 15/20 amino acids having a single codon preference greater than 70%. Intraspecific variation in both the nuclear and mitochondrial genomes appears to be quite limited, consistent with the hypothesis that O. volvulus has suffered a genetic bottleneck in the recent past.

Onchocerca volvulus superoxide dismutase genes: identification of functional promoters for pre-mRNA transcripts which undergo trans-splicing

The genes encoding three forms of superoxide dismutase, the cytosolic and extracellular CuZn superoxide dismutases and the mitochondrial Mn superoxide dismutase, were isolated from an Onchocerca volvulus lambda fix II genomic library. Genomic Southern blot analyses indicate single-copy genes in the O. volvulus genome. The O. volvulus cytosolic and extracellular CuZnSOD genes (Ov-sod-1 and Ov-sod-2) are separated by 0.8 kb of sequence and are convergently transcribed. Since the transcripts from all three sod genes are trans-spliced, the transcription start point of each gene was determined in a heterologous system that lacks trans-splicing machinery by in vitro transcription using Drosophila embryo nuclear extracts, followed by primer extension experiments. The ability of the 5' flanking region of the genes encoding the three Ov-SODs to promote transcription was further examined in transient transfections of Chinese hamster ovary cells. In firefly luciferase reporter assays, the Ov-sod-1 and -2 and the MnSOD (Ov-sod-3) gene promoters showed minimal, strong, and moderate levels of activity in these cells, respectively. Both Ov-sod-2 and -3 gene promoter regions showed an initial increase in activity in response to 5' deletions. The results from the in vitro transcription experiments and the luciferase reporter assays were consistent and suggest the presence of Inr-like elements in the promoter regions of the Ov-sod genes.

Onchocerca volvulus: immunolocalization of the extracellular CuZn superoxide dismutase using antibodies raised against a 15-mer epitope of this enzyme

The study describes the immunohistological localization of the extracellular CuZn superoxide dismutase (SOD2) in the parasitic nematode Onchocerca volvulus. Using specific antiserum raised against a 15-amino-acid peptide from the N-terminal region of the mature protein, this enzyme is detected primarily in the intestinal epithelium of the adult worms and to a lesser extent in the muscle cells of the uterine wall. A blocking experiment with the SOD2 peptide reduced the staining significantly, confirming specificity. The localization profile of SOD2 correlates extremely well with the localization of iron deposits in the gut and uterine muscle cells of adult O. volvulus. The detection of SOD2 in the functional intestine of O. volvulus, together with the evidence that it is a secreted protein, indicates that this enzyme in parasitic nematodes is in a position to interact with host molecules. It also demonstrates the accessibility of the parasite enzyme to an inhibitor or blocking antibody.

Resistance of filarial nematode parasites to oxidative stress

All filariae examined to date express a comprehensive repertoire of both cytoplasmic and secreted anti-oxidant enzymes, although significant differences exist between species and life-cycle stages. Adult Brugia malayi, Dirofilaria immitis and Onchocerca volvulus secrete CuZn superoxide dismutases, and the former two species also secrete a selenocysteine-independent glutathione peroxidase. This enzyme has been localised to the cuticular matrix of B. malayi, and the preferential reduction of fatty acid- and phospholipid hydroperoxides suggests that it may protect cuticular membranes from oxidative damage rather than directly metabolise hydrogen peroxide. Adult O. volvulus may compensate for an apparent deficiency in expression of this enzyme via a secreted variant of glutathione S-transferase. Recent studies have identified a highly expressed family of enzymes collectively termed peroxiredoxins, which most probably play an essential role in reduction of hydroperoxides. Data from cDNA cloning exercises indicate that all filarial species examined thus far express at least two peroxiredoxin variants which have been localised to diverse tissues. In-vitro studies have shown that B. malayi are particularly resistant to oxidative stress, and that the parasites do not rely solely on enzymatic mechanisms of defence. Cuticular lipids are relatively resistant to lipid peroxidation due to the low unsaturation indices of the constituent fatty acyl residues, but complete protection is afforded by the presence of alpha-tocopherol, presumably assimilated from host extracellular fluids. Brugia malayi are also relatively resistant to nitric oxide-mediated toxicity, and this may be due in part to incomplete dependence on aerobic metabolism. Little is known of potential mechanisms for detoxification of nitric oxide derivatives and adaptive responses to oxidative stress in general, and these represent goals for future research.

Thioredoxin peroxidase from Onchocerca volvulus: a major hydrogen peroxide detoxifying enzyme in filarial parasites

Random screening of an Onchocerca volvulus third-stage (L3) cDNA library identified a highly abundant cDNA encoding a newly discovered antioxidant enzyme, thioredoxin peroxidase (TPx), a member of the peroxidoxin superfamily. This TPx cDNA (Ov-tpx-2) encodes a polypeptide of 199 amino acid residues with a calculated molecular weight of 21,890 Da. The Ov-tpx-2 cDNA represents roughly 2.5% of the total cDNAs from the L3 cDNA library. The gene was expressed in Escherichia coli and the protein product was shown to have antioxidant activity. Antiserum raised against Ov-TPX-2 recognized a native protein from extracts of both the L3 and adult-stages with a molecular weight of 22 kD. The localization and stage-specificity of Ov-TPX-2 protein was analyzed by immunocytochemistry and immunoelectron microscopy using monospecific antibodies. Expression was detected in late first-stage larvae during development in the vector and increased in intensity during differentiation to the infective L3-stage. The antigen was also detected in post-infective larvae and adult worms. In larvae, Ov-TPX-2 protein was predominantly localized to the hypodermis and cuticle, with additional sites in the hypodermal chords and multivesicular bodies. In adult worms, the primary sites of expression were the uterine epithelium and intestine, with additional labeling of the body wall and cuticle. Developing embryos and microfilariae in utero were bathed in Ov-TPX-2 protein discharged from epithelial cells. These results suggest that Ov-TPX-2 may protect the parasites from being damaged by host-generated oxidative stress and that Ov-TPX-2 protein provides the H2O2-detoxifying activity predicted but not previously identified in filarial parasites. Its highly upregulated expression in infective larvae may aid in parasite establishment following transmission to the definitive host.

Localization and functional analysis of the cytosolic and extracellular CuZn superoxide dismutases in the human parasitic nematode Onchocerca volvulus

This study describes the histological localization of two CuZn superoxide dismutases (SOD1 and SOD2) in the parasitic nematode Onchocerca volvulus, and a functional characterization of the 'extracellular' form of this enzyme (SOD2) which provides evidence that it is involved in the defense against environmental superoxide anion radicals. These essential enzymes are detected in larval and adult stages of the parasite, determined at the mRNA and protein levels by in situ hybridization and immunolocalization studies. These proteins are distributed throughout the worm, at various concentrations with particularly high levels produced in the hypodermis. In vitro maintenance of parasites indicated that SOD2 was secreted outside the parasite into the medium. Baculovirus constructs designed to test the ability of the SOD2 hydrophobic N-terminal region to function in processing and secretion confirmed the ability of this polypeptide sequence to direct the secretion of a marker protein, as well as of the mature SOD2 enzyme. Analyses of the native, mature SOD2 enzyme molecular mass, and the primary and quaternary structure, indicate that unlike other extracellular SODs, the SOD2 is active as a non-glycosylated dimer, rather than as a tetrameric glycoprotein. The detection of SOD2 outside of the parasite maintained in vitro, and the confirmation that the SOD2 is a secreted enzyme, indicate that this enzyme plays a role in the interactive biology of parasitic nematodes with their hosts.

Molecular characterization and expression of Onchocerca volvulus glutathione reductase

Glutathione metabolism represents a potential target for anti-parasite drug design. The central role of glutathione reductase (GR) in maintenance of the thiol redox state and in anti-oxidative defence has to be evaluated in more detail in order to establish the essential function of this enzyme for the survival of the filarial parasite Onchocerca volvulus. The O. volvulus GR (OvGR) gene was cloned and sequenced. The gene is composed of 13 exons and 12 introns and spans 4065 bp. The first intron is located within the 5'-untranslated region of the gene, 16 nucleotides upstream of the translation initiation codon. Southern-blot analysis and structural characterization of the genomic sequence indicate that OvGR is encoded by a single-copy gene. Isolation of various cDNA clones revealed a polymorphism of polyadenylation initiation with no consensus polyadenylation sites in any of the cDNAs analysed. The entire cDNA is 1977 bp long and carries the nematode-specific spliced leader sequence SL1 at its 5' end, 236 nucleotides upstream of the first in-frame methionine. The cDNA codes for a polypeptide of 462 amino acids with 53.5% sequence identity with human GR (HsGR). A total of 18 out of 19 residues contributing to glutathione binding are identical in OvGR and HsGR. However, one of the arginine residues (Arg-224 in HsGR) involved in discrimination between NADPH and NADH in all known GRs is substituted by tryptophan (Trp-207 in OvGR). The coding region of OvGR was expressed in Escherichia coli as a histidine-fusion protein, and it was established that the parasite protein still favours the binding of NADPH (Km 10.9 microM) over NADH (Km 108 microM). The histidine-fusion protein has a subunit size of 54 kDa and is active as a homodimer of 110 kDa.