The use of embryonic stem cells for therapeutic purposes holds great promise in the field of regenerative medicine. Transcriptional programming is of utmost importance for this objective but remains inefficient due to our lack of understanding of the obstacles which must be overcome in order to redirect cell-fate. During development, the differentiation of multipotent progenitor cells into postmitotic cells is accompanied by the progressive restriction of cell-fate potential. At the biochemical level, this is achieved through he silencing of enhancer and promoter elements regulating genes whose expression is incompatible with the final fate of the developing cell type. While many studies have examined the mechanism by which transcriptional activators program cell identity, little is known regarding the mechanisms by which transcriptional repressors silence their target genes. Gaining a deeper understanding of these mechanisms is pivotal to identifying and creating novel strategies for surmounting the current barriers of reprogramming protocols. To address these questions, I have performed a detailed examination of the transcription factor (TF) Nkx2.2. In the developing spinal cord, Nkx2.2 is transiently expressed in motor neuron (MN) progenitors, where it acts to suppress the MN developmental program in order to establish V3 interneuron identity. While it is well known that Nkx2.2 is a repressor of MN cell fate, the direct targets of Nkx2.2 are unknown, and its effects on the chromatin architecture remain to be elucidated. More generally, it is unclear whether TFs transiently expressed in progenitor cells can stably silence the genetic programs of postmitotic, mature neurons. Interestingly, genome binding analysis of Nkx2.2 expressed in MN progenitors has revealed that Nkx2.2 binds not only to enhancer elements that control MN progenitor identity but also to enhancers of postmitotic MN genes, which become activated much later in development. These findings raise the intriguing possibility that Nkx2.2 represses MN identity in part through the stable and prospective silencing of genes that regulate postmitotic MN identity. To define the effects of Nkx2.2 on MN differentiation, I will: i) determine the impact of Nkx2.2 binding on the chromatin landscape of MN progenitor and postmitotic genes and regulatory elements and ii) examine whether Nkx2.2 directly and stably represses the postmitotic MN program. These experiments will elucidate the still poorly understood mechanisms by which transiently expressed TFs control genetic regulatory networks. The findings from this study may inform more efficient strategies for transcriptional reprogramming of cellular identity, a process that will undoubtedly be useful for the generation of clinically relevant cell types in neurodegenerative treatments.
The potential use of embryonic stem cells for therapeutic purposes depends on our ability to correctly direct the differentiation of clinically relevant cell types in vitro. While several studies have examined how transcriptional activators program identity, little is currently known about how transcriptional repressors silence their target genes and the lack of this information hinders our ability to produce specific cell populations. This project aims to identify the mechanisms underlying the repression of motor neuron identity in order to provide insight into more effective methods for cellular differentiation strategies.
