Cell cycle-regulated ApiAP2s and parasite development: the Toxoplasma paradigm

Abstract

The cell division cycle of T. gondii is driven by cyclically expressed ApiAP2 transcription factors (AP2s) that promote gene sets (regulons) associated with specific biological functions. AP2s drive other AP2s, thereby propelling the progressive gene expression waves defining the lytic cycle. AP2s can act as dimers by themselves, in combination with other AP2s (constitutive or cyclical) or in complexes with epigenetic factors. Exit from the cell cycle into either the extracellular state or differentiation into bradyzoites results in major changes in gene expression. Surprisingly, both transitions lead to expression of a shared set of unique AP2s that suggest a shared stress response that, governed by the specific conditions, leads to different outcomes.

Fig 1.. AP2 genes.

A. Plot marks the chromosomal location of AP2s. Blue dot indicate peak expression timing during asexual cell division (G1, S, M, C color coded as indicated). Constitutively expressed AP2 are marked in red. AP2s not detected (AP2III-3, AP2III-4, AP2IV-1, AP2VIIa-8, AP2IX-10, AP2X-3, AP2X-2) are marked in gray. B. Fourier analysis identified 32 cyclic AP2s clustered into 5 groups (C1-C5) with similar expression profile during the tachyzoite asexual cell replication cycle. A total of 28 AP2s are constitutively expressed at moderate level (cluster C6).

Fig 2.. UMAP projection and analyses of scRNA-seq data.

Integrated scRNA-seq data sets representing different cell cycle and environmental/development stages. P6: a cell cluster in 0 hr stressed, tachyzoite state Prugniaud parasites; P1: Prugniaud parasites under alkaline stress (bradyzoite differentiation); intracellular: asynchronously replicating parasites from 0 hr stressed Prugniaud and asynchronously replicating RH parasites; extracellular: tachyzoites incubated for 6 hrs under extracellular stress in normal growth medium at 37°C. All Prugniaud data from Xue et al [45], RH intracellular from Lou et al [14] and RH extracellular was generated here following protocols as in [14,27], A. Overlay off all data sets annotated by inferred cell division cycle phases (transferred from [45]). Arrow represents the direction of cell division cycle progression. B. Overlay off all data sets annotated by development/environmental state. C. Each data set split by development/environmental state (top) and cell cycle stage (bottom).

Figure 3.. The AP2 interface of lytic replication cycle entry, exit, checkpoints and the tie in of stress.

A. Cell division cycle of tachyzoites with checkpoints as follows: 1. Restriction; 2. DNA licensing; 3. Centrosome duplication; 4. Daughter assembly; 5. Spindle assembly [4]. The restriction checkpoint permits exit and entry either to the extracellular G0 stage tachyzoites, or stage differentiation. AP2 factors that have experimentally been validated are marked at the stages they act. Further details in Table 1. B. The shared AP2-mediated stress response between extracellular tachyzoites and differentiating bradyzoites. Different stresses indicated by lightning bolts. BFD1 (bradyzoite formation deficient 1) and ENO1 (enolase 1) are other unique TFs founds in bradyzoites.