Title: Progressive restriction of CNS cell-fate potential by the transiently expressed transcription factor Nkx2.2
Description: The progressive loss of developmental potential is a hallmark of all differentiating cells in multicellular organisms. At the chromatin level, this restriction in cell-fate plasticity is established through the silencing of active and poised lineage-specific genes that are incompatible with the terminal fate of the maturing cell type. The effective and stable inhibition of gene expression relies on the coordinated action of transcriptional repressors. These repressors are often transiently expressed only at the time of cell-fate specification and direct lineage decisions by suppressing alternative developmental programs. However, compared to the numerous studies examining the mechanisms by which cell-type specific transcriptional activators program cellular identity, little is currently known regarding how transient repressors execute permanent silencing of gene regulatory networks. To address this question, I have examined the mechanisms through which the transiently expressed transcription factor (TF) Nkx2.2 represses the acquisition of motor neuron (MN) identity in V3 neuronal progenitors.

While it is well-established that Nkx2.2 functions as a transcriptional repressor through its interactions with the Groucho (Grg) family of co-repressors, how these interactions manifest in gene silencing has remained unknown. Moreover, the effects of Nkx2.2 occupancy on chromatin modifications have not been determined. In this dissertation, I demonstrate that surprisingly, Nkx2.2 decommissions enhancers of the MN developmental program not through the recruitment of additional co-repressor proteins but rather through the eviction of co-activator complexes. While this displacement is dependent upon an intact Grg-interacting domain, Nkx2.2 binding does not increase Grg enrichment. In addition, extensive profiling of Nkx2.2 genome-wide binding events in neural precursors unexpectedly revealed that Nkx2.2 occupies not only enhancers of MN progenitor genes acutely repressed by Nkx2.2 but also enhancers of genes expressed exclusively in postmitotic MNs, long after Nkx2.2 expression has been down- regulated.

In vivo lineage tracing experiments and in vitro genomic analyses demonstrated that Nkx2.2 also functions in a repressive capacity at these poised regulatory regions. Here, Nkx2.2 binding prevents the activation of postmitotic genetic networks through a preferential enlistment of histone deacetylase complex 2 (HDAC2) proteins. However, this binding is not accompanied by the deposition of repressive chromatin modifications, and removal of Nkx2.2 in differentiating V3 neurons leads to the ectopic expression of the postmitotic MN TFs Isl1 and Hb9. Collectively, these studies indicate that transiently expressed repressors may establish gene suppression by counteracting the activities of transcriptional activators, rather than by directly establishing repressive chromatin signatures. As transcriptional reprogramming of differentiated cell linages often fails to adequately silence the expression programs of the starting population, these results may help to inform new methodologies for instructing cell conversions.