Saturday, December 17, 2016

Opposing transcriptional mechanisms regulate Toxoplasma development

Opposing transcriptional mechanisms regulate Toxoplasma development.

Dong-Pyo HongJoshua B RadkeMichael W White



The Toxoplasma biology that underlies human chronic infection is developmental conversion of the acute tachyzoite stage into the latent bradyzoite stage. We investigated the role of two alkaline-stress induced ApiAP2 transcription factors, AP2IV-3 and AP2IX-9, in bradyzoite development. These factors were expressed in two overlapping waves during bradyzoite development with AP2IX-9 increasing expression earlier than AP2IV-3, which peaked as AP2IX-9 expression was declining. Disruption of the AP2IX-9 gene enhanced, while deletion of AP2IV-3 gene decreased tissue cyst formation demonstrating these factors have opposite functions in bradyzoite development. Conversely, conditional overexpression of FKBP-modified of AP2IX-9 or AP2IV-3 with the small molecule Shield 1 had a reciprocal effect on tissue cyst formation confirming the conclusions of the knockout experiments. The AP2IX-9 repressor and AP2IV-3 activator tissue cyst phenotypes were borne out in gene expression studies that determined many of the same bradyzoite genes were regulated in an opposite manner by these transcription factors. A common gene target was the canonical bradyzoite marker, BAG1, and mechanistic experiments determined that like AP2IX-9, AP2IV-3 regulates a BAG1 promoter-luciferase reporter and specific binds the BAG1 promoter in parasite chromatin. Altogether, these results suggest the AP2IX-9 transcriptional repressor and AP2IV-3 transcriptional activator likely compete to control bradyzoite gene expression, which may permit Toxoplasma to better adapt to different tissue environments and select a suitable host cell for long term survival of the dormant tissue cyst.

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