|Ph.D Student||Hod-Dvorai Reut|
|Subject||Dual Function of Polycomb Group Proteins in T Helper|
|Department||Department of Medicine||Supervisor||MS. Orly Avni|
|Full Thesis text|
Following antigen recognition, naive T helper (Th; CD4) cells can differentiate toward one of several effector lineages such as Th1, Th2 and Th17; each expressing distinctive transcriptional profiles of cytokine genes. These cytokines eventually instruct the strategy of the immune response. In our search for factors that propagate the transcriptional programs of differentiated Th cells, we found that Polycomb group (PcG) proteins, which are known as epigenetic regulators that maintain repressive chromatin states, bind differentially the signature cytokine genes. Unexpectedly, their binding was correlated with transcriptional activation. The recruitment of the PcG proteins Mel-18 and Ezh2 to the cytokine promoters was inhibited in the presence of cyclosporine A, suggesting the involvement of NFAT. Using the RNAi approach, we found that downregulation of Ezh2 was consistent with its function as positive regulator of the signature cytokine genes in primary Th2 and Th17, and established Th1 and Th2 cells. Nevertheless, our results suggest that PcG proteins can function also as conventional transcriptional repressors in Th cells, for example of Tbx21 in Th17 cells. This dual function of the PcG proteins may participate in the mechanisms underlying the commitment and plasticity of the Th phenotypes.
Our results suggest that in Th17 cells the TCR and polarizing cytokines synergize to modulate the binding activity of Mel-18 and the Th17 lineage-specifying transcription factor, RORgt, at the Il17a promoter, and consequently to facilitate Il17a expression. All together, these data support a model whereby the non-differentially expressed PcG proteins are recruited in a Th-lineage specific manner to their target genes to enforce the maintenance of specific transcriptional programs as transcriptional repressors or activators.
In addition, we found that the active cytokine promoters of Ifng and Il4 are differentially acetylated at H3K27 in correlation with gene expression, suggesting that this modification might prevent Ezh2 from catalyzing its typical repressive modification.
We also studied the coordinate regulation of PcG proteins by microRNAs. We found that the expression of the PcG proteins Mel-18 and Eed is regulated by the microRNA mir-181a in HEK-293T cells.
Our data suggest that the function of the PcG proteins is more versatile and complex than it was previously appreciated, and additional experiments are necessary to explore the mechanisms underlying their differential targeting and dual functions.