Abstract
BackgroundThe apicomplexan parasite Toxoplasma gondii is cosmopolitan in nature, largely as a result of its highly flexible life cycle. Felids are its only definitive hosts and a wide range of mammals and birds serve as intermediate hosts. The latent bradyzoite stage is orally infectious in all warm-blooded vertebrates and establishes chronic, transmissible infections. When bradyzoites are ingested by felids, they transform into merozoites in enterocytes and expand asexually as part of their coccidian life cycle. In all other intermediate hosts, however, bradyzoites differentiate exclusively to tachyzoites, and disseminate extraintestinally to many cell types. Both merozoites and tachyzoites undergo rapid asexual population expansion, yet possess different effector fates with respect to the cells and tissues they develop in and the subsequent stages they differentiate into.ResultsTo determine whether merozoites utilize distinct suites of genes to attach, invade, and replicate within feline enterocytes, we performed comparative transcriptional profiling on purified tachyzoites and merozoites. We used high-throughput RNA-Seq to compare the merozoite and tachyzoite transcriptomes. 8323 genes were annotated with sequence reads across the two asexually replicating stages of the parasite life cycle. Metabolism was similar between the two replicating stages. However, significant stage-specific expression differences were measured, with 312 transcripts exclusive to merozoites versus 453 exclusive to tachyzoites. Genes coding for 177 predicted secreted proteins and 64 membrane- associated proteins were annotated as merozoite-specific. The vast majority of known dense-granule (GRA), microneme (MIC), and rhoptry (ROP) genes were not expressed in merozoites. In contrast, a large set of surface proteins (SRS) was expressed exclusively in merozoites.ConclusionsThe distinct expression profiles of merozoites and tachyzoites reveal significant additional complexity within the T. gondii life cycle, demonstrating that merozoites are distinct asexual dividing stages which are uniquely adapted to their niche and biological purpose.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1225-x) contains supplementary material, which is available to authorized users.
Highlights
The apicomplexan parasite Toxoplasma gondii is cosmopolitan in nature, largely as a result of its highly flexible life cycle
Using genome-wide comparative transcriptomics, we show that merozoites express distinct gene families in a stage-specific fashion, and fail to express the majority of annotated rhoptry proteins (ROPs), GRA and MIC proteins which are upregulated during tachyzoite replication
Purification of merozoites from feline enterocytes for RNA-Seq analysis T. gondii parasite preparations were generated from enterocytes from an infected cat at onset of patency (5d post infection)
Summary
The apicomplexan parasite Toxoplasma gondii is cosmopolitan in nature, largely as a result of its highly flexible life cycle. In all other intermediate hosts, bradyzoites differentiate exclusively to tachyzoites, and disseminate extraintestinally to many cell types Both merozoites and tachyzoites undergo rapid asexual population expansion, yet possess different effector fates with respect to the cells and tissues they develop in and the subsequent stages they differentiate into. Toxoplasma gondii is an intracellular zoonotic parasite that chronically infects 30% of the world’s human population [1] It has a complex life cycle, infecting a wide range of mammals and birds as intermediate hosts but with felids as the only definitive hosts. The parasite population that develops in cat enterocytes undergoes a classical coccidian cycle involving several rounds of asexual division and amplification followed by differentiation into macro- and microgamonts, the dimorphic stages of sexual development. The sexual phase continues within the oocyst as meiosis ensues, followed by mitosis to produce infectious sporozoites, encased within sporocysts inside the oocysts
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