The Unfolded Protein Response in the Protozoan Parasite Toxoplasma gondii Features Translational and Transcriptional Control

Abstract

The unfolded protein response (UPR) is an important regulatory network that responds to perturbations in protein homeostasis in the endoplasmic reticulum (ER). In mammalian cells, the UPR features translational and transcriptional mechanisms of gene expression aimed at restoring proteostatic control. A central feature of the UPR is phosphorylation of the ␣ subunit of eukaryotic initiation factor-2 (eIF2) by PERK (EIF2AK3/PEK), which reduces the influx of nascent proteins into the ER by lowering global protein synthesis, coincident with preferential translation of key transcription activators of genes that function to expand the processing capacity of this secretory organelle. Upon ER stress, the apicomplexan parasite Toxoplasma gondii is known to induce phosphorylation of Toxoplasma eIF2␣ and lower translation initiation. To characterize the nature of the ensuing UPR in this parasite, we carried out microarray analyses to measure the changes in the transcriptome and in translational control during ER stress. We determined that a collection of transcripts linked with the secretory process are induced in response to ER stress, supporting the idea that a transcriptional induction phase of the UPR occurs in Toxoplasma. Furthermore, we determined that about 500 gene transcripts showed enhanced association with translating ribosomes during ER stress. Many of these target genes are suggested to be involved in gene expression, including JmjC5, which continues to be actively translated during ER stress. This study indicates that Toxoplasma triggers a UPR during ER stress that features both translational and transcriptional regulatory mechanisms, which is likely to be important for parasite invasion and development.

Endoplasmic Reticulum Stress Triggers Gametocytogenesis in the Malaria Parasite

“…Based on the previously known target genes, AP2 factors alone accounted for about 60% of the 155 genes induced upon DTT exposure. Involvement of the AP2 family of transcription factors may emerge as a common mechanism among protozoans as Toxoplasma is also known to involve AP2 factors in response to tunicamycin-induced ER stress (32).…”

Translational Control in the Latency of Apicomplexan Parasites

“…, eIF2 is phosphorylated in response to stressful insults and has been shown to accompany all latent stages thus far examined, which include extracellular tachyzoites, bradyzoites, rings, schizonts, gametocytes, and salivary gland sporozoites [9,31,36–41,47,48,50]. Furthermore, while replacement of endogenous eIF2 with a non-phosphorylatable version is lethal in Plasmodium berghei , this substitution results in reduced viability for extracellular Toxoplasma tachyzoites, implicating the ISR plays in the maintenance of latency [9,38].…”

“…A decrease in protein synthesis indeed occurs in these parasites upon eIF2 phosphorylation, indicating that the translational repression that is observed upon canonical ISR activation is conserved in apicomplexans [9,37,38,48,50]. Preferential translation of stress-responsive transcripts is another feature of the ISR that promotes adaptation or cellular differentiation [53].…”

“…In Toxoplasma extracellular tachyzoites, a number of transcripts that are lowly translated in non-stressed conditions become heavily translated following ER stress, indicating increased efficiency of their translation [50]. These preferentially translated mRNAs encode heat shock proteins, aminoacyl tRNA synthases and other translational machinery, as well as those that mediate transcriptional responses like chromatin remodeling and transcription factors.…”

The unfolded protein response (UPR) pathway inCryptococcus

“…38 Moreover, some protozoans do not possess Ire1 or Hac1/ Xbp1 homologs. 39,40 Therefore, it seems that the Ire1-dependent UPR pathway has undergone extensive divergence during evolution, particularly in terms of the regulatory mechanism of its downstream bZIP transcription factors.…”