title: Asexual expansion of Toxoplasma gondii merozoites is distinct from tachyzoites and entails expression of non-overlapping gene families to attach, invade, and replicate within feline enterocytes.
abstract: BACKGROUND
  The 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.
  
  
  RESULTS
  To 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Institute of Parasitology-University of Zurich, Winterthurerstrasse 266a, Zürich, 8057, Switzerland. adrian.hehl@uzh.ch.
authors.forename: Adrian B
authors.initials: AB
authors.lastname: Hehl
authors.affiliation: Institute of Parasitology-University of Zurich, Winterthurerstrasse 266a, Zürich, 8057, Switzerland. walter.basso@access.uzh.ch.
authors.forename: Walter U
authors.initials: WU
authors.lastname: Basso
authors.affiliation: Institute of Parasitology-University of Zurich, Winterthurerstrasse 266a, Zürich, 8057, Switzerland. christoph.lippuner@dkf.unibe.ch.|Current address: Department of Anaesthesiology and Pain Medicine, Inselspital, University of Bern, Freiburgstrasse, Bern, 3010, Switzerland. christoph.lippuner@dkf.unibe.ch.
authors.forename: Christoph
authors.initials: C
authors.lastname: Lippuner
authors.affiliation: Institute of Parasitology-University of Zurich, Winterthurerstrasse 266a, Zürich, 8057, Switzerland. chandra.ramakrishnan@uzh.ch.
authors.forename: Chandra
authors.initials: C
authors.lastname: Ramakrishnan
authors.affiliation: Functional Genomics Center Zurich, Winterthurerstrasse 190, Zürich, 8057, Switzerland. michal.okoniewski@id.ethz.ch.
authors.forename: Michal
authors.initials: M
authors.lastname: Okoniewski
authors.affiliation: Institute of Parasitology-University of Zurich, Winterthurerstrasse 266a, Zürich, 8057, Switzerland. robert.walker1@jcu.edu.au.|Queensland Tropical Health Alliance Research Laboratory, Faculty of Medicine, Health and Molecular Sciences, James Cook University, Cairns Campus, McGregor Road, Smithfield, QLD, 4878, Australia. robert.walker1@jcu.edu.au.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Walker
authors.affiliation: Molecular Parasitology Section, Laboratory of Parasitic Diseases, NIAID, NIH, Bethesda, Maryland, USA. griggm@niaid.nih.gov.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Grigg
authors.affiliation: Queensland Tropical Health Alliance Research Laboratory, Faculty of Medicine, Health and Molecular Sciences, James Cook University, Cairns Campus, McGregor Road, Smithfield, QLD, 4878, Australia. nicholas.smith@jcu.edu.au.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Smith
authors.affiliation: Institute of Parasitology-University of Zurich, Winterthurerstrasse 266a, Zürich, 8057, Switzerland. deplazesp@access.uzh.ch.
authors.forename: Peter
authors.initials: P
authors.lastname: Deplazes
mesh_terms: D000818:Animals; D002415:Cats; D055028:Comparative Genomic Hybridization; D020895:Enterocytes / Q000469:parasitology*; D018507:Gene Expression Regulation, Developmental; D018503:Genome, Protozoan; D008018:Life Cycle Stages / Q000235:genetics; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism; D050436:Regulatory Elements, Transcriptional / Q000235:genetics; D017423:Sequence Analysis, RNA; D014122:Toxoplasma / Q000235:genetics* / Q000254:growth & development / Q000472:pathogenicity; D014124:Toxoplasmosis, Animal / Q000469:parasitology / Q000473:pathology
chemical_list: D015800:Protozoan Proteins