A Comparison of Stage Conversion in the Coccidian Apicomplexans Toxoplasma gondii, Hammondia hammondi, and Neospora caninum

Abstract

Stage conversion is a critical life cycle feature for several Apicomplexan parasites as the ability to switch between life forms is critical for replication, dissemination, pathogenesis and ultimately, transmission to a new host. In order for these developmental transitions to occur, the parasite must first sense changes in their environment, such as the presence of stressors or other environmental signals, and then respond to these signals by initiating global alterations in gene expression. As our understanding of the genetic components required for stage conversion continues to broaden, we can better understand the conserved mechanisms for this process and unique components and their contribution to pathogenesis by comparing stage conversion in multiple closely related species. In this review, we will discuss what is currently known about the mechanisms driving stage conversion in Toxoplasma gondii and its closest relatives Hammondia hammondi and Neospora caninum. Work by us and others has shown that these species have some important differences in the way that they (1) progress through their life cycle and (2) respond to stage conversion initiating stressors. To provide a specific example of species-specific complexities associated with stage conversion, we will discuss our recent published and unpublished work comparing stress responses in T. gondii and H. hammondi.

The transcriptome from asexual to sexual in vitro development of Cystoisospora suis (Apicomplexa: Coccidia)

“…In Plasmodium , they play a role in stage conversion 76 and are related to sexual commitment of blood-stages 77 – 79 . Currently, 67 ApiAP2 domain-containing proteins are annotated in the Toxoplasma genome, with 24 being expressed cyclically during the tachyzoite division cycle 18 , 80 and six in bradyzoite development 81 . The genome of C. suis encodes 64 AP2 factors of which we found 23 AP2 factors downregulated and 7 AP2 factors upregulated in the sexual stages, possibly linked to the observed stage conversion from type II merozoites to gamonts and onwards to gametes.…”

Advances towards the complete in vitro life cycle of Toxoplasma gondii

“…A variety of differentiation-inducing stresses for in vitro cultures have been sought for and identified. They have been described in several recent reviews 37 – 39 . Arguably, the most frequently used trigger is cell culture medium with an alkaline pH (8.3).…”

Mining the Proteome of Toxoplasma Parasites Seeking Vaccine and Diagnostic Candidates

“…The different T. gondii life stages employ specific mechanisms for triggering stage conversion, and these could be related to pathobiology within the host [21]. Changes in the expression levels of some proteins also occur as parasites progress through their life cycles, and it is likely that particular proteins have important functions in restricted life stages [22].…”