A single master regulator controls asexual cell cycle division patterns inToxoplasma gondii

Abstract

All apicomplexan parasites have complex life cycles exhibiting division characterized by a tightly regulated cell cycle control, resulting in the emergence of daughter parasites in possession of a single nucleus and a complete set of organelles. Apicomplexa have evolved efficient and distinctive strategies for intracellular replication where the timing of emergence of the daughter cells, a process termed "budding", is a decisive element. However, the molecular mechanisms that provide the proper timing of parasite budding remain unknown.Using Toxoplasma gondii as a model Apicomplexa, we identified a master regulator that controls the timing of the budding process. We show that an ApiAP2 transcription factor, TgAP2IX-5, controls cell cycle events downstream of centrosome duplication including organelle division and segregation. TgAP2IX-5 binds to the promoter of hundreds of genes and controls the activation of the budding-specific cell cycle expression program. We show that TgAP2IX-5 regulates the expression of specific transcription factors that are necessary for the completion of the budding cycle. TgAP2IX-5 acts as a licensing factor that ensures that asexual proliferation continues by promoting the inhibition of the differentiation pathway at each round of the cell cycle. Therefore, TgAP2IX-5 is a master regulator that controls both cell cycle and developmental pathways.

The extracellular milieu ofToxoplasma’s lytic cycle drives lab-adaptation and promotes changes in lipid metabolism primarily driven by transcriptional reprogramming

“…Lab adaptation therefore seems to be driven beyond the inhibition of ‘state change’ controlled by APIX-9. To be comprehensive, we included data for all TFs with known targets [17, 18, 35, 37-41]. Genes controlled by BFD1 are upregulated during lab adaptation, again consistent with genes shared with bradyzoite differentiation.…”

“…The apicomplexan Apetala 2 (ApiAP2) family of transcription factors (TFs) is with 68 annotated members on ToxoDB the most expanded. Numerous biological traits have been associated with specific AP2s [17, 18, 35–40]. In addition, Myb TFs are another significant family (14 annotated genes on ToxoDB) associated with parasites specific functions such as BFD1 with bradyzoite differentiation [41].…”

“…Trends of the AP2s indicate their key role in extracellular transcriptional mechanism. b Previous studies have identified direct (by AP2 ChIP-Seq [17], ChIP-qPCR [35, 37, 39], ChIP-chip [40] or cut and run [41]) and indirect (by AP2 ikD or KO followed by RNA-seq [17, 18] or microarray analysis [38]) targets of AP2 and Myb TFs. GSEA comparing these direct/indirect transcription factor targets to lab-adaptive DEGs identified significant enrichments, indicating AP2 TFs as potential regulators of lab-adaptation.…”

“…Genes controlled by BFD1 are upregulated during lab adaptation, again consistent with genes shared with bradyzoite differentiation. Expression of most genes controlled by AP2XI-5 [40], AP2X-5 [18], and AP2IX-5 [17, 42] trends down. This is not surprising since these clusters associated with cell division, and extracellular parasites do not divide.…”

“…Many of these genes are hypothetical but experimental validation of select genes confirmed their roles in GT1’s evolution. Finally, the transcriptional data were used to assemble a transcriptional network, which consolidated the defined roles of several characterized transcription factors [17, 18]. Taken together, we successfully applied lab-adaptation and E&R as a tool to identify a rich set of 300 host-independent virulence factors.…”