The plasticity of cells in a multicellular organism is progressively lost during differentiation. This is reflected in studies involving the ectopic misexpression of transcription factors (TFs) — certain master regulator or terminal selector TFs that can efficiently activate target genes early in development, lose this ability as cells differentiate. While this phenomenon of cell fate restriction is widely observed, the mechanisms orchestrating it are poorly understood. In C. elegans, we have used the ubiquitous over expression of Zn-finger TF CHE-1, the terminal selector for ASE chemosensory neurons, as an assay to understand these mechanisms. When CHE-1 is ubiquitously expressed in embryonic stages, it activates ASE gene expression in many cell types, while in adults it can only act in a few neurons and the pharynx. To uncover factors that inhibit plasticity of all other cells, we used a candidate approach to look for mutants in which CHE-1 can continue to act in adults. We hypothesized that terminal selector TFs themselves, in addition to specifying cellular identity by controlling large gene sets, may also act to inhibit plasticity. To this end, we first tested the activity of CHE-1 in mutants of COE TF unc-3, the terminal selector for a subset of cholinergic motor neurons (MNs). We find that contrary to the case in wildtype worms, affected unc-3 mutant MNs remain plastic, and CHE-1 is indeed able to induce expression of ASE genes even in adults. A similar effect is not observed in mutants of mab-9 and unc-55, two TFs that do not act as terminal selectors but control small subsets of identity genes in cholinergic MNs. Overall, in 5 of 7 terminal selector mutants tested, CHE-1 can induce expression of ASE genes in the affected cells of adult mutant worms significantly more than in wildtype, raising the exciting possibility that this a a common function among selector TFs. To further understand this phenomenon we asked if terminal selector TFs affect histone modifications at specific loci. Using the LacI/LacO system to tag the promoter of one CHE-1 target, gcy-5, we performed immunostaining for repressive histone marks H3K27me3 and H3K9me3. Our data indicate no significant difference in the presence of these marks at the gcy-5 promoter in wt and unc-3, suggesting that at least these marks are not affected by TFs. Taken together, our results provide evidence that the restriction of a cell fate requires the activity of terminal selector-like TFs. These factors may orchestrate the restriction of cell fates by directly interacting with chromatin complexes or by maintaining transcription of genes that either organize the chromatin or otherwise create an inhibitory environment at non-transcribed genes.
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