RNA Seq analysis of the Eimeria tenella gametocyte transcriptome reveals clues about the molecular basis for sexual reproduction and oocyst biogenesis

Background

The protozoan Eimeria tenella is a common parasite of chickens, causing avian coccidiosis, a disease of on-going concern to agricultural industries. The high prevalence of E. tenella can be attributed to the resilient oocyst stage, which is transmitted between hosts in the environment. As in related Coccidia, development of the eimerian oocyst appears to be dependent on completion of the parasite’s sexual cycle. RNA Seq transcriptome profiling offers insights into the mechanisms governing the biology of E. tenella sexual stages (gametocytes) and the potential to identify targets for blocking parasite transmission.

Results

Comparisons between the sequenced transcriptomes of E. tenella gametocytes and two asexual developmental stages, merozoites and sporozoites, revealed upregulated gametocyte transcription of 863 genes. Many of these genes code for proteins involved in coccidian sexual biology, such as oocyst wall biosynthesis and fertilisation, and some of these were characterised in more depth. Thus, macrogametocyte-specific expression and localisation was confirmed for two proteins destined for incorporation into the oocyst wall, as well as for a subtilisin protease and an oxidoreductase. Homologues of an oocyst wall protein and oxidoreductase were found in the related coccidian, Toxoplasma gondii, and shown to be macrogametocyte-specific. In addition, a microgametocyte gamete fusion protein, EtHAP2, was discovered.

Conclusions

The need for novel vaccine candidates capable of controlling coccidiosis is rising and this panel of gametocyte targets represents an invaluable resource for development of future strategies to interrupt parasite transmission, not just in Eimeria but in other Coccidia, including Toxoplasma, where transmission blocking is a relatively unexplored strategy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1298-6) contains supplementary material, which is available to authorized users.

Stage-specific expression of protease genes in the apicomplexan parasite, Eimeria tenella

Background

Proteases regulate pathogenesis in apicomplexan parasites but investigations of proteases have been largely confined to the asexual stages of Plasmodium falciparum and Toxoplasma gondii. Thus, little is known about proteases in other Apicomplexa, particularly in the sexual stages. We screened the Eimeria tenella genome database for proteases, classified these into families and determined their stage specific expression.

Results

Over forty protease genes were identified in the E. tenella genome. These were distributed across aspartic (three genes), cysteine (sixteen), metallo (fourteen) and serine (twelve) proteases. Expression of at least fifteen protease genes was upregulated in merozoites including homologs of genes known to be important in host cell invasion, remodelling and egress in P. falciparum and/or T. gondii. Thirteen protease genes were specifically expressed or upregulated in gametocytes; five of these were in two families of serine proteases (S1 and S8) that are over-represented in the coccidian parasites, E. tenella and T. gondii, distinctive within the Apicomplexa because of their hard-walled oocysts. Serine protease inhibitors prevented processing of EtGAM56, a protein from E. tenella gametocytes that gives rise to tyrosine-rich peptides that are incorporated into the oocyst wall.

Conclusion

Eimeria tenella possesses a large number of protease genes. Expression of many of these genes is upregulated in asexual stages. However, expression of almost one-third of protease genes is upregulated in, or confined to gametocytes; some of these appear to be unique to the Coccidia and may play key roles in the formation of the oocyst wall, a defining feature of this group of parasites.

Chasing the golden egg: vaccination against poultry coccidiosis

Eimeria species, of the Phylum Apicomplexa, cause the disease coccidiosis in poultry, resulting in severe economic losses every year. Transmission of the disease is via the faecal-oral route, and is facilitated by intensive rearing conditions in the poultry industry. Additionally, Eimeria has developed drug resistance against most anticoccidials used today, which, along with the public demand for chemical free meat, has lead to the requirement for an effective vaccine strategy. This review focuses on the history and current status of anticoccidial vaccines, and our work in developing the transmission-blocking vaccine, CoxAbic (Netanya, Israel). The vaccine is composed of affinity-purified antigens from the wall-forming bodies of macrogametocytes of Eimeria maxima, which are proteolytically processed and cross-linked via tyrosine residues to form the environmentally resistant oocyst wall. The vaccine is delivered via maternal immunization, where vaccination of laying hens leads to protection of broiler offspring. It has been extensively tested for efficacy and safety in field trials conducted in five countries and involving over 60 million offspring chickens from immunized hens and is currently the only subunit vaccine against any protozoan parasite to reach the marketplace.

Oocyst wall formation and composition in coccidian parasites

The oocyst wall of coccidian parasites is a robust structure that is resistant to a variety of environmental and chemical insults. This resilience allows oocysts to survive for long periods, facilitating transmission from host to host. The wall is bilayered and is formed by the sequential release of the contents of two specialized organelles - wall forming body 1 and wall forming body 2 - found in the macrogametocyte stage of Coccidia. The oocyst wall is over 90% protein but few of these proteins have been studied. One group is cysteine-rich and may be presumed to crosslink via disulphide bridges, though this is yet to be investigated. Another group of wall proteins is rich in tyrosine. These proteins, which range in size from 8-31 kDa, are derived from larger precursors of 56 and 82 kDa found in the wall forming bodies. Proteases may catalyze processing of the precursors into tyrosine-rich peptides, which are then oxidatively crosslinked in a reaction catalyzed by peroxidases. In support of this hypothesis, the oocyst wall has high levels of dityrosine bonds. These dityrosine crosslinked proteins may provide a structural matrix for assembly of the oocyst wall and contribute to its resilience.