Host-related DNA sequences are localized in the body of schistosome adults

Tissue Transglutaminase-Regulated Transformed Growth Factor-β1 in the Parasite Links Schistosoma japonicum Infection with Liver Fibrosis

Transforming growth factor (TGF-β1) is among the strongest factors of liver fibrogenesis, but its association with Schistosoma-caused liver fibrosis is controversial. Tissue transglutaminase (tTG) is the principal enzyme controlling TGF-β1 maturation and contributes to Sj-infected liver fibrosis. Here we aim to explore the consistency between tTG and TGF-β1 and TGF-β1 source and its correlation with liver fibrosis after Sj-infection. TGF-β1 was upregulated at weeks 6 and 8 upon liver fibrosis induction. During tTG inhibition, TGF-β1 level decreased in sera and liver of infected mice. TGF-β1 showed positive staining in liver containing Sj adult worms and eggs. TGF-β1 was also detected in Sj adult worm sections, soluble egg antigen and Sj adult worm antigen, and adult worms' culture medium. The TGF-β1 mature peptide cDNA sequence and its extended sequence were amplified through RT-PCR and RACE-PCR using adult worms as template, and sequence is analyzed and loaded to NCBI GenBank (number GQ338152.1). TGF-β1 transcript in Sj eggs was higher than in adult worms. In Sj-infected liver, transcriptional level of TGF-β1 from Sj, but not mouse liver, correlated with liver fibrosis extent. This study provides evidence that tTG regulates TGF-β1 and illustrates the importance of targeting tTG in treating Sj infection-induced fibrosis.

The role of helminth infections in carcinogenesis

This review examines the significant literature on the role of helminth infections in carcinogenesis. Both parasitic infections and cancer have complex natural histories and long latent periods during which numerous exogenous and endogenous factors interact to obfuscate causality. Although only two helminths, Schistosoma haematobium and Opisthorchis viverrini, have been proven to be definitely carcinogenic to humans, others have been implicated in facilitating malignant transformation. The known mechanisms of helminth-induced cancer include chronic inflammation, modulation of the host immune system, inhibition of intracellular communication, disruption of proliferation-antiproliferation pathways, induction of genomic instability and stimulation of malignant stem cell progeny. Approximately 16% of all cancer cases worldwide are attributable to pathogenic agents, including schistosomes and liver flukes. This equates to 1,375,000 preventable cancer deaths per year. Means to reduce the incidence of helminth-associated malignancies are discussed.

Schistosome albumin is of host, not parasite, origin

Recent work has implicated schistosome albumin as part of a mechanism for neutralizing the oxidative assault by host immune defenses and suggested that the gene had been acquired by horizontal transfer from the mammalian host. In the course of proteomic analyses of Schistosoma mansoni adult worm vomitus and eggs recovered from mice, we identified numerous peptides, largely derived from murine rather than parasite albumin. We therefore conjectured that the supposed S. mansoni albumin sequence deposited on GenBank might be the result of contamination rather than horizontal gene transfer. Based on phylogenetic analysis the most likely source was the Syrian (golden) hamster Mesocricetus auratus. Proteomic analysis of Syrian hamster albumin generated peptide identities to S. mansoni as the top hit, with a high ion score >1,500 and 63% coverage of the translated cDNA sequence. RT-PCR using specific primers permitted amplification of the M. auratus albumin transcript, which is identical to the deposited S. mansoni albumin sequence. PCR amplification of a fragment of the M. auratus albumin gene from genomic DNA suggests a homologous structure to the Mus musculus albumin gene. We were unable to find the S. mansoni albumin gene sequence by in silico searching on either version 3 of the S. mansoni genome assembly or the >3 million shotgun DNA reads. Finally, Southern blotting detected the albumin gene in M. auratus but not in S. mansoni genomic DNA, even when the latter was present in a 10-fold excess. Collectively, our data make the strongest case that the schistosome albumin protein described in previous reports is of host origin and all nucleotide-derived data are the result of contamination with host material. By analogy, we suggest that other reported examples of horizontal gene transfer to schistosomes might similarly be explained by complementary/genomic DNA contamination.

Heterogeneity of class I and class II MHC sequences in Schistosoma mansoni

We investigated the genetic variations in class I and class II major histocompatibility complex (MHC) genes of Schistosoma mansoni and the effects of host MHC genotypes. S. mansoni was maintained in combinations of two mouse strains with different MHC genotypes, and the MHC gene sequences of the cercariae were investigated. The detected class I MHC gene sequences were variable, with high similarity between the H-2D(b) murine host and the parasite. For other combinations, however, the parasite sequence was homologous to those of anthropoids. All class II MHC sequences detected in S. mansoni were homologous to those of anthropoids. Our results suggest that the genetic variation in the MHC sequences of S. mansoni is derived in part from the current host, indicating horizontal transfer of the sequences from mammal to parasite.

Complement C2 receptor inhibitor trispanning: from man to schistosome

Horizontal gene transfer (HGT), in relation to genetic transfer between hosts and parasites, is a little described mechanism. Since the complement inhibitor CRIT was first discovered in the human Schistosoma parasite (the causative agent of Bilharzia) and in Trypanosoma cruzi (a parasite causing Chagas' disease), it has been found to be distributed amongst various species, ranging from the early teleost cod to rats and humans. In terms of evolutionary distance, as measured in a phylogenetic analysis of these CRIT genes at nucleotide level, the parasitic species are as removed from their human host as is the rat sequence, suggesting HGT. The hypotheses that CRIT in humans and schistosomes is orthologous and that the presence of CRIT in schistosomes occurs as a result of host-to-parasite HGT are presented in the light of empirical data and the growing body of data on mobile genetic elements in human and schistosome genomes. In summary, these data indicate phylogenetic proximity between Schistosoma and human CRIT, identity of function, high nucleotide/amino acid identity and secondary protein structure, as well as identical genomic organization.

Existence of host DNA sequences in schistosomes—horizontal and vertical transmission

The localization of repetitive DNA sequences in the mouse genome such as mouse type 2 Alu sequence (B2) and mouse retrovirus-related sequences was shown in the body of adult Schistosoma japonicum and Schistosoma mansoni by applying an in situ PCR and hybridization technique. Using the same method, mouse major histocompatibility complex (MHC) class I sequence was also found in schistosomes. Furthermore, mouse MHC class I sequence and type A retroviral sequence were detected in S. japonicum and S. mansoni cercarial DNA by blot hybridization. These findings indicated that horizontal and vertical transmission of host DNA sequences occurred in schistosomes. The incorporation and propagation of host sequences in schistosomes and the roles played by such host sequences form the focus of this brief review.

Genomics and the biology of parasites

Despite the advances of modern medicine, the threat of chronic illness, disfigurement, or death that can result from parasitic infection still affects the majority of the world population, retarding economic development. For most parasitic diseases, current therapeutics often leave much to be desired in terms of administration regime, toxicity, or effectiveness and potential vaccines are a long way from market. Our best prospects for identifying new targets for drug, vaccine, and diagnostics development and for dissecting the biological basis of drug resistance, antigenic diversity, infectivity and pathology lie in parasite genome analysis, and international mapping and gene discovery initiatives are under way for a variety of protozoan and helminth parasites. These are far from ideal experimental organisms, and the influence of biological and genomic characteristics on experimental approaches is discussed, progress is reviewed and future prospects are examined.

Some molecular insights into schistosome evolution

Robust phylogenies based on molecular data for species within the genus Schistosoma have been generated in recent years. The considerable progress made in understanding the relationships between many of the 19 recognised species of Schistosoma is reviewed with particular attention being given to the detection and analysis of parasite variation as shown by studies on ribosomal RNA genes, mitochondrial DNA and RAPDs. For the most part, molecular phylogenies agree with observations based on morphological or life-history characteristics. It is clear that the parasites of man do not form a monophyletic group and that close relationships exist between parasites within species groups, especially in the S. haematobium group of species. The S. japonicum group appears to be the most divergent of the species groups and yet little DNA sequence variation has been observed between various isolates of S: japonicum. Some of the less studied schistosomes have yet to be examined at the molecular level and may prove to be interesting links between the species groups as has recently been shown with S. hippopotami. The power of molecular approaches for the analysis of schistosomes at the population and individual level is now apparent, especially for S. mansoni. Important questions remain concerning the maintenance of parasite diversity and how schistosomes respond to selection pressures imposed either during natural progression through the life-cycle or through drug treatment or vaccination. Gene discovery and gene mapping projects are leading to a better understanding of the schistosome genome and can be expected to contribute significantly to future comparative evolutionary studies.

Host-like sequences in the schistosome genome

A series of recent papers has indicated that widespread genomic rearrangements take place in the genome of schistosomes during the life cycle of the parasite. These results have been controversial since genomic rearrangements are not common in eukaryotes, probably because excessive genome plasticity would carry a heavy evolutionary price. Here, Karen Clough, Alec Drew and Paul Brindley present data that ostensibly support the concept of widespread genomic rearrangements, but for which they suggest a different interpretation. They conclude that artefactual contamination of schistosome genome preparations with host DNA can probably explain the Southern hybridization results which led to the original hypothesis of developmental, genomic rearrangements.

Detection of host DNA sequences including the H-2 locus of the major histocompatibility complex in schistosomes

The mouse type 2 Alu (B2) sequence was detected in both DNAs of Schistosoma mansoni and S.japonicum except for the cercarial stage by the polymerase chain reaction (PCR). Using several kinds of mouse STMS (sequence tagged microsatellite site) primer sets, PCR products related to the host were found in the DNAs of S. mansoni as well as of S.japonicum. Products could be detected only in the DNA of S. japonicum using certain STMS primer sets. The fact that no products could be amplified from the DNAs of both parasites when other kinds of STMS primer sets were used suggests unequal incorporation of the host DNA into the schistosomes. Furthermore, the sequence of the N-terminal domain of H-2, the mouse major histocompatibility complex (MHC), was detected in the DNAs from S. mansoni miracidium, male adult and S. japonicum adults, whereas the sequence of the C2 domain of H-2 was found only in the DNAs of S. japonicum adults. This evidence that host DNA sequences, including the class I MHC, exist heterogeneously in the DNAs of schistosomes might provide an important insight for further understanding of host-parasite immune interactions.

Parasite infection and cancer: with special emphasis on Schistosoma japonicum infections (Trematoda). A review

This article contains a review of current knowledge on the association of parasite infections and cancer formation, especially that of Schistosoma japonicum (Trematoda) in man and experimental animals. The association of S. haematobium infection and bladder cancer is well known and documented. However, S. japonicum infection has also been reported to be associated with cancer, in this case hepatocellular carcinoma and/or colorectal cancer. Pathological records and analyses have shown a correlation between this infection and cancer, and pathohistological descriptions have been numerous, together with clinical case reports. Epidemiological analyses have been conducted in China and Japan and support a role of S. japonicum infection as one of the risk factors in cancer formation, along with others, such as hepatitis virus infection and alcoholic intake. Experimental results have also shown that cancer appears early and in larger numbers in experimentally infected animals given a known carcinogen. In spite of these positive end-point associations, the mechanism of schistosome-mediated enhancement of carcinogenesis is obscure. A suggestive observation is that in S. japonicum-infected mice carcinogen-metabolizing hepatic activity including P-450 was decreased so that an administered carcinogen persisted for a longer period than in uninfected mice. Further studies, both epidemiological and experimental, are needed to firmly establish the relationship between schistosome infection and cancer.

Physiological bases for parasite-induced alterations of host behaviour

Parasitism is defined in various ways as an intimate relationship in which one partner, the parasite, lives on or in another, the host, generally at the expense of the latter. Parasitism commonly results in a unique array of host physiological responses and adaptations. Most studies of the physiological effects of parasitism have focused on the pathological consequence of infection and disease. While many physiological changes contribute to pathogenesis, it is now recognized that parasitic infections at sub-clinical levels also produce physiological effects that either ameliorate or may not contribute to the disease process. Moreover, these physiological changes are often manifested by altered host behaviour. Behavioural studies have enabled an ecological- and evolutionary-oriented evaluation of host responses. In this fashion, physiological effects may be assessed as to whether they affect fitness and confer benefit or harm to one or both of the symbionts involved. We briefly examine how these physiological responses, specifically neural, endocrine, neuromodulatory, and immunomodulatory components, may interact to modify host behaviors. We consider the adaptiveness of these responses and how the behavioural patterns elicited may simultaneously appear adaptive for the parasite as well as the host. In addition, we address how parasite-host physiological and behavioural interactions may be altered during the course of parasitism.