Dissection of the in vitro developmental program of Hammondia hammondi reveals a link between stress sensitivity and life cycle flexibility in Toxoplasma gondii

Abstract

Most eukaryotic parasites are obligately heteroxenous, requiring sequential infection of different host species in order to survive. Toxoplasma gondii is a rare exception to this rule, having a uniquely facultative heteroxenous life cycle. To understand the origins of this phenomenon, we compared development and stress responses in T. gondii to those of its its obligately heteroxenous relative, Hammondia hammondi and have identified multiple H. hammondi growth states that are distinct from those in T. gondii. Of these, the most dramatic difference was that H. hammondi was refractory to stressors that robustly induce cyst formation in T. gondii, and this was reflected most dramatically in its unchanging transcriptome after stress exposure. We also found that H. hammondi could be propagated in vitro for up to 8 days post-excystation, and we exploited this to generate the first ever transgenic H. hammondi line. Overall our data show that H. hammondi zoites grow as stringently regulated, unique life stages that are distinct from T. gondii tachyzoites, and implicate stress sensitivity as a potential developmental innovation that increased the flexibility of the T. gondii life cycle.

Head-to-head comparisons of Toxoplasma gondii and its near relative Hammondia hammondi reveal dramatic differences in the host response and effectors with species-specific functions

“…H. hammondi was first discovered in the feces of a cat in Iowa, USA in 1975 [19], and while morphologically indistinguishable from T. gondii, H. hammondi has a comparatively restricted natural intermediate host range (being incapable of infecting birds) [20,21]. Prior work shows that H. hammondi shares >95% of its~8000 genes in near perfect synteny with T. gondii and expresses functional orthologs of key T. gondii effectors such as ROP18 and ROP5 [22,23]. This is important from an epidemiological and evolutionary perspective, as H. hammondi is not known to cause clinical disease in any naturally infected intermediate (human and other animals) or definitive host [20].…”

“…This is important from an epidemiological and evolutionary perspective, as H. hammondi is not known to cause clinical disease in any naturally infected intermediate (human and other animals) or definitive host [20]. In addition, in tissue culture H. hammondi replicates for a short period of time and then enters into a unique terminally differentiated bradyzoite state that is a) unable to be subcultured in vitro, b) incapable of infecting an intermediate mouse host and c) only capable of infecting a feline host [20,23]. Paradoxically, despite this natural bradyzoite developmental program and expression of canonical bradyzoite genes during tachyzoite-like replication [23,24], H. hammondi cannot be induced to form bradyzoites using stresses like high pH medium that robustly induce cyst formation in T. gondii [23].…”

“…In addition, in tissue culture H. hammondi replicates for a short period of time and then enters into a unique terminally differentiated bradyzoite state that is a) unable to be subcultured in vitro, b) incapable of infecting an intermediate mouse host and c) only capable of infecting a feline host [20,23]. Paradoxically, despite this natural bradyzoite developmental program and expression of canonical bradyzoite genes during tachyzoite-like replication [23,24], H. hammondi cannot be induced to form bradyzoites using stresses like high pH medium that robustly induce cyst formation in T. gondii [23]. While it is not yet known if H. hammondi is capable of infecting humans, and there are no tests as yet capable of distinguishing these closely-related species serologically [25], H. hammondi and T. gondii oocysts are known to co-circulate, suggesting that humans are likely to encounter this parasite [26].…”

Transcriptional profiling reveals T cells cluster around neurons injected withToxoplasma gondiiproteins

“…To further explore these data, we compared the cluster of T. gondii genes with higher abundance in type II TINs versus type III TINs transcriptomes with published transcript abundance from a different type III strain (VEG) after bradyzoite conversion via high pH ( Fig. 5C; data taken from Sokol et al 52 ). In this comparison, we found that many of the genes in the higherin-type II TINs cluster were also induced in VEG bradyzoites compared to tachyzoites.…”

“…In this comparison, we found that many of the genes in the higherin-type II TINs cluster were also induced in VEG bradyzoites compared to tachyzoites. When we performed Gene Set Enrichment Analysis (GSEA) 53 using the previously published data 52 to create "bradyzoite" and "tachyzoite" gene sets, we found significant enrichment of the bradyzoite gene set in type II TINs transcriptome and the tachyzoite gene set in type III TINs transcriptome ( Fig. S3B).…”