[Complete mitochondrial genomes of Paragonimus westermani in China and phylogenetic analysis of various geographical isolates]

OBJECTIVE

To compare the complete mitochondrial genome sequences of two phenotypes of Paragonimus westermani isolated from Fujian Province with different sizes of metacercariae, and perform a phylogenetic analysis of various geographical isolates of P. westermani from Asia, so as to identify the possible genetic characteristics associated with the P. westermani phenotypes.

METHODS

P. westermani metacercariae with different sizes (large metacercariae, 380-420 μm in diameter; small metacercariae, 320-340 μm) isolated from freshwater crabs were used to infect dogs, and the eggs and adult worms of P. westermani were collected from the dog stool samples and lung tissues. Then, the egg size and morphology were compared. In addition, genomic DNA was extracted from the adult worms of the two phenotypes of P. westermani and used for the PCR amplification to yield the complete mitochondrial genome sequence. Sequence structure and phylogenetic analyses were performed based on the complete mitochondrial genome of P. westermani.

RESULTS

Following infection with large and small P. westermani metacercariae, the adult worms recovered from the dog lung had a thick body, and had oral and ventral suckers. The ventral sucker was located slightly in front of the midline of the body, and testes, ovary and vitelline gland were seen in the adult worms. Following fixation, the adults appeared oval, with an approximately 1.7∶1 of the length-width ratio. The length and width of the eggs isolated from the fecal samples of dogs infected with large and small P. westermani metacercariae varied significantly, and the large metacercariae produced bigger eggs than the smaller metacercariae. Based on the morphological features of adults and eggs and the ITS2 sequences, both phenotypes were identified as P. westermani. The complete mitochondrial genome sequence analysis of adults showed almost consistent sequences in the protein-coding region of the mitochondrial genome of adult worms derived from large and small metacercariae, with a major variation seen in the former non-coding region. Sliding window analysis revealed the most polymorphic region within the ND4 gene across the mitochondrial genome from various geographical isolates of P. westermani, and phylogenetic analysis showed that both phenotypes were clustered into the Chinese branch of P. westermani, which was close to the Japanese branch and distinct from the South/Southeast Asian branch.

CONCLUSIONS

The genetic distance between the phenotypes of P. westermani isolated from Fujian Province is near at a mitochondrial genome level, with no remarkable genetic differentiation seen; however, the mutation and structural changes in the non-coding region may result in the phenotypic variations. In addition, there is a distinct variation of the evolutionary rate in the mitochondrial coding genes, suggesting the selection of appropriate molecular markers during the phylogenic researches.

Whole-genome sequence of the oriental lung fluke Paragonimus westermani

Background

Foodborne infections caused by lung flukes of the genus Paragonimus are a significant and widespread public health problem in tropical areas. Approximately 50 Paragonimus species have been reported to infect animals and humans, but Paragonimus westermani is responsible for the bulk of human disease. Despite their medical and economic importance, no genome sequence for any Paragonimus species is available.

Results

We sequenced and assembled the genome of P. westermani, which is among the largest of the known pathogen genomes with an estimated size of 1.1 Gb. A 922.8 Mb genome assembly was generated from Illumina and Pacific Biosciences (PacBio) sequence data, covering 84% of the estimated genome size. The genome has a high proportion (45%) of repeat-derived DNA, particularly of the long interspersed element and long terminal repeat subtypes, and the expansion of these elements may explain some of the large size. We predicted 12,852 protein coding genes, showing a high level of conservation with related trematode species. The majority of proteins (80%) had homologs in the human liver fluke Opisthorchis viverrini, with an average sequence identity of 64.1%. Assembly of the P. westermani mitochondrial genome from long PacBio reads resulted in a single high-quality circularized 20.6 kb contig. The contig harbored a 6.9 kb region of non-coding repetitive DNA comprised of three distinct repeat units. Our results suggest that the region is highly polymorphic in P. westermani, possibly even within single worm isolates.

Conclusions

The generated assembly represents the first Paragonimus genome sequence and will facilitate future molecular studies of this important, but neglected, parasite group.

[Geographic distribution and gene sequencing of Paragonimus westermani in some areas of Guangdong Province]

OBJECTIVE

To investigate the current distribution of Paragonimus westermani in Guangdong Province.

METHOD

Snails and crabs collected from mountain streams in regional survey sites were dissected to detect cercarial and metacercarial infections of P. westermani. Domestic cats and dogs artificially infected with the collected metacercariae were also dissected to detect adult worms of P. westermani. The COI and ITS2 gene sequences of those adult worms were compared with those of known Paragonimus specimen deposited in the GenBank.

RESULTS

All of the first intermediate hosts in five survey sites of Liangkou, Nankun, Mountain, Dadong, Muxi, Guowu, were identified as Semisulcospira libertina, whose cercariae infection rates were 0.33%, 0.15%, 0.058%, 0.10%, and 0.05%, respectively; the second intermediate hosts in above five sites were all identified as Sinopotamon denticulatum, whose metacercariae infection rates were 100%, 100%, 38.09%, 55.36%, and 65.26%, respectively. The numbers of metacercariae in the five sites were 79.4, 105.66, 9.16, 16.18, and 15.6 per positive crab, respectively, and 11.12, 7.87, 0.58, 0.69, and 0.85 per gram of crab, respectively. All the metacercariae were identical to those of P. westermani. Adult worms and eggs of P. westermani were found in both reservoir hosts of domestic cats and dogs infected artificially. By comparing the COI genes of five representative samples from each survey site with that of Paragonimus #AF219379.21, AF540958.1 from GenBank, we found out the homology to be 99%, 99%, 99%, 98%, and 99%, respectively. In addition, a comparison of the ITS2 gene sequences between the above five samples and Paragonimus #DQ836243.1, DQ351845.1, AB354217.1 from GenBank revealed 98%, 99%, 98%, 98%, and 98% gene homology, respectively.

CONCLUSION

Two ultra-high and three high endemic areas of P. westermani are discovered in Guangdong Province. No obvious differences were found among the types of P. westermani in the above five endemic areas.

Molecular cloning and enzymatic characterization of a class mu glutathione S-transferase of Paragonimus westermani

Glutathione S-transferase (GST) is a component of a second line of defense against bioreactive radicals derived from host immune attack. Paragonimus westermani causes acute or chronic lung diseases in mammals. A cDNA clone, PwGST#11, of adult P. westermani produced in the present study was 748 bp long and encoded an open reading frame of 217 amino acids with a starting methionine. The molecular mass of this putative polypeptide, Pw26GST, was estimated to be 25.1 kDa with an isoelectric point of 5.7. Pw26GST was homologous with the 26-kDa GSTs of trematodes and vertebrates. Nine of the ten amino acid residues lining the glutathione-binding pocket were conserved. Putative Pw26GST polypeptide was clustered with 26-kDa GSTs of trematodes belonging to the class mu. Recombinant Pw26GST protein generated bacterially, revealed GST enzyme activity toward an universal and class mu-specific substrates. Mouse antisera to recombinant Pw26GST protein recognized native 26-kDa GST of P. westermani but not the GSTs of any other trematodes. Collectively, Pw26GST was found to be a member of class mu GSTs of P. westermani.

Molecular characterization of two myoglobins of Paragonimus westermani

Myoglobins (Mbs), globin proteins, are present in high concentrations in trematodes. In Paragonimus westermani, 2 cDNAs were found to encode Mbs. The first clone, Pwmyo1, codes a total of 149 amino acids with a calculated mass of 16.6 kDa. The second, Pwmyo2, encodes a 146-amino acid protein with a calculated mass of 16.2 kDa. The predicted secondary structures showed the presence of 8 helices, which is the basic characteristic of Mbs. Sequence alignment revealed a high homology with the other trematode Mbs. The 2 clones contained the characteristic tyrosyl residues at helical positions B10 and distal E7, which are substitutions that have been previously shown to contribute to the high oxygen affinity of Mbs. Polyclonal antibodies against the recombinant Mbs were raised with no cross-reactivity observed. Immunolocalization revealed the proteins to be distributed generally throughout the parenchymal tissues, but absent from the tegument and reproductive organs. The cell mass of the eggs of the worm stained positive to Pwmyo2 but not Pwmyo1, suggesting the stage-specific expression of these Mbs.

Paragonimus westermani: Biochemical and immunological characterizations of paramyosin

Paramyosin of the helminth parasite is a muscle protein that plays multifunctional roles in host-parasite relationships. In this study, we have cloned a gene encoding Paragonimus westermani paramyosin (PwPmy) and characterized biochemical and immunological properties of the recombinant protein. The recombinant PwPmy (rPwPmy) was shown to bind both human immunoglobulin G (IgG) and collagen. The protein was constitutively expressed in various developmental stages of the parasite and its expression level increased progressively as the parasite matured. Immunohistological analysis revealed that PwPmy was mainly localized in subtegumental muscle, tegument and cells surrounding the oral sucker, intestine, and ovary of the parasite. Sera from patients with paragonimiasis showed antibody reactivity against rPwPmy, and IgG1 and IgG4 were predominant. Immunization of mice with rPwPmy also induced high IgG responses. Biochemical and immunological characterization of PwPmy may provide valuable information for the further study to develop a vaccine or a chemotherapeutic agent for paragonimiasis.

Differential expression of Paragonimus westermani eggshell proteins during the developmental stages

Eggs of trematode parasites are comprised of numerous vitelline cells and one fertilized ovum, and are encapsulated within a protein shell provided by the vitellocytes. In this study, we isolated two full-length cDNA clones that showed substantial levels of sequence identity with trematode-specific eggshell precursor proteins from the human lung fluke, Paragonimus westermani. These cDNAs, designated Pw-Vit20 (868-bp-long) and Pw-Vit36 (883-bp-long), shared a 76% identity with one another at the nucleotide level, and each encoded a 261-amino acid (aa) polypeptide. The deduced aa sequences contained a N-terminal hydrophobic segment, as well as a sequence motif of Gly-Gly-Gly-Tyr-Asp-Asn/Thr-Tyr-Gly-Lys/Gln, which is highly homologous with the eggshell proteins of Fasciola hepatica. With the high frequencies of tyrosine, glycine and lysine, the positions occupied by tyrosine, which has been proved to be converted into dihydroxyphenylalanine, were well preserved. Pw-Vit20 and Pw-Vit36 were found to be monoexonic genes with variably diverged variants scattered into multiple genomic loci. Their protein products were localized in the vitelline follicles and eggshells. Expression of Pw-Vit20 was restricted to the egg and adult stages, thus suggesting a critical involvement of Pw-Vit20 in the parasite's fecundity activity. Conversely, Pw-Vit36 was constitutively expressed in the metacercariae and juvenile stages in the vitelline follicles and ducts, which suggested that the prepositioning of stem or primordial vitelline cells within the juveniles prior to sexual maturation. Pw-Vit36 might acquire a unique or additional function relevant to the maturation and/or development of the vitelline cells/follicles during the evolutionary period of P. westermani. Differential biological implications of multiple eggshell precursor proteins may provide insight into the molecular mechanism of eggshell formation and the developmental process of the vitelline follicles in the parasitic trematode.

Molecular and biochemical characterization of hemoglobinase, a cysteine proteinase, in Paragonimus westermani

The mammalian trematode Paragonimus westermani is a typical digenetic parasite, which can cause paragonimiasis in humans. Host tissues and blood cells are important sources of nutrients for development, growth and reproduction of P. westermani. In this study, a cDNA clone encoding a 47 kDa hemoglobinase of P. westermani was characterized by sequencing analysis, and its localization was investigated immunohistochemically. The phylogenetic tree prepared based on the hemoglobinase gene showed high homology with hemoglobinases of Fasciola hepatica and Schistosoma spp. Moreover, recombinant P. westermani hemoglobinase degradaded human hemoglobin at acidic pH (from 3.0 to 5.5) and its activity was almost completely inhibited by E-64, a cysteine proteinase inhibitor. Immunohistochemical studies showed that P. westermani hemoglobinase was localized in the epithelium of the adult worm intestine implying that the protein has a specific function. These observations suggest that hemoglobinase may act as a digestive enzyme for acquisition of nutrients from host hemoglobin. Further investigations may provide insights into hemoglobin catabolism in P. westermani.

COMPARISON OF GENE REPRESENTATION BETWEEN DIPLOID AND TRIPLOID PARAGONIMUS WESTERMANI BY EXPRESSED SEQUENCE TAG ANALYSES

Expressed sequence tag (EST) analysis of the diploid and triploid Paragonimus westermani genes was done to have a rapid and informative outlook of the gene-expression profiles of the parasites. Totals of 506 and 505 ESTs were generated from the diploid and triploid P. westermani cDNA libraries. Based on the BLASTx search results of the diploid P. westermani ESTs, 308 (60.9%) matched significantly with formerly identified genes and 198 (39.1%) showed no significant homology in the GenBank database. A similar homology pattern was shown from the triploid EST BLASTx search results with 346 (68.5%) sharing homology with previously identified genes and 159 (31.5%) showing no significant homology. The EST data from both libraries were analyzed and grouped into 9 categories. Comparison of the 2 EST pools revealed high similarities among the categories of the significantly matched genes. Single genes matched repeatedly were also observed in the 2 EST data. Some genes were found that are not yet characterized in P. westermani; these genes were matched by both the diploid and triploid ESTs. Further study of these genes may provide us with more understanding on the parasite's biology and their specific functions in the 2 strains.

Critical roles for excretory-secretory cysteine proteases during tissue invasion of Paragonimus westermani newly excysted metacercariae

Paragonimus westermani is a trematode parasite, which causes pulmonary and/or extrapulmonary granulomatous disease in humans. Successful invasion of the host tissue is critical for the survival of this tissue-invasive parasite. The enzymatic hydrolysis of host proteins is clearly a prerequisite of this process. In this study, we have investigated the functional roles of the excretory-secretory cysteine proteases of P. westermani newly excysted metacercariae (PwNEM) in tissue invasion. The 27 and 28 kDa enzymes (PwMc27 and PwMc28) purified from PwNEM excretory-secretory products (ESP), preferentially degraded fibrillar proteins, but not globular proteins. PwMc28 significantly facilitated the invasion of PwNEM into mouse peritoneum, whereas a diffusible cysteine protease inhibitor, trans-epoxysuccinyl-L-leuciloamido-(4-guanidino) butane (E-64) inhibited this process dose-dependently. Two distinct isoforms of PwMc28 (PwMc28a and PwMc28b), which exhibited two amino acid differences in their mature domains, were identified by tandem mass spectrometry and sequence analysis. Both enzymes were localized at the tegument on the anterior border and on the oral sucker, which suggests excretion-secretion via exocytosis or via the excretory canal network. The mRNA transcripts of PwMc28a and b were expressed abundantly during the active invasion/migration through the host's tissues, suggesting their relevant function to tissue invasion/migration in the definitive host.

MOLECULAR CLONING AND CHARACTERIZATION OF COPPER/ZINC-SUPEROXIDE DISMUTASE OF PARAGONIMUS WESTERMANI

Superoxide dismutases (SODs; EC 1.15.1.1) play important roles in the protection of the parasites against cellular oxygen-mediated killing of the hosts. A copper/zinc-containing SOD (Cu/Zn-SOD) was identified previously from lung fluke, Paragonimus westermani. To expand our understanding of P. westermani SOD, we isolated a complementary DNA encoding a Cu/Zn-SOD, expressed the active enzyme in Escherichia coli, and characterized its biochemical properties. The deduced amino acid (aa) sequence of the gene shared up to 73.7% identities with Cu/Zn-SODs of other helminths and shared well-conserved characteristic motifs and essential aa residues involved in coordinating copper and zinc enzymatic functions. Recombinant Cu/ Zn-SOD exhibited comparable biochemical properties with that of the native enzyme, including pH optima and potassium cyanide-and hydrogen peroxide-sensitive inhibition profiles. The active enzyme consisted of 2 identical subunits covalently linked by disulfide bonds. The enzyme was constitutively expressed throughout various developmental stages of the parasite. The levels increased as P. westermani matured and plateaued in adult stage. Our result suggests the enzyme might play an important role for parasites to survive in the hosts through its superoxide anion-detoxifying function.

Molecular discrimination between individual metacercariae of Paragonimus heterotremus and P. westermani occurring in Thailand

To accurately discriminate between individual metacercariae of Paragonimus heterotremus and P. westermani occurring in Thailand, polymerase chain reaction (PCR)-based molecular methods were established and subjected to an evaluation. We first amplified and sequenced the second internal transcribed spacer (ITS2) region of the nuclear ribosomal DNA of the two species. Based on their nucleotide differences, P. heterotremus and P. westermani were unequivocally discriminated from each other. These nucleotide differences were further utilized to select the ApaL1 endonuclease site for PCR-restriction fragment length polymorphism (PCR-RFLP) analyses and to design species-specific primers for multiplex PCR reactions. Both PCR-RFLP and multiplex PCR methods allowed a more rapid and labor-effective species discrimination. Furthermore, the multiplex PCR method enabled the most efficient discrimination because species identification involved a single round of PCR in a single tube. In Thailand, P. heterotremus is the only species affecting humans. Thus, the methods established in the present study can be used as reliable tools to identify the lung fluke metacercariae that cause human disease.

Genomic characterization of lung flukes, Paragonimus heterotremus, P. siamensis, P. harinasutai, P. westermani and P. bangkokensis by RAPD markers

Random amplified polymorphic DNA (RAPD) markers were assayed in an attempt to discriminate among five species of Paragonimus. Genomic DNAs of two strains of Paragonimus heterotremus from two provinces in Thailand, Saraburi and Phitsanulok, as well as of P. siamensis, P. harinasutai, P. westermani and P. bangkokensis were extracted and amplified by an arbitrary primer, namely P2 (5-GTTTCGCTCC-3). RAPD patterns showed that those five species were genetically distinct, although they shared genomic DNA to some extent. This primer could also distinguish between two strains of P. heterotremus. The polymorphism observed allowed to construct a relationship dendrogram. The phylogenetic dendrogram showed that the P. heterotremus strains were closest to P. harinasutai, followed by P. siamensis, P. bangkokensis and P. westermani.