AbstractThe RE1-silencing transcription factor (REST) regulates the expression of neural specific genes by acting as a transcriptional repressor in non-neuronal cells. REST binds to repressor elements (RE1s) in the DNA and recruits histone deacetylases (HDACs) and other histone modifying and chromatin remodeling proteins through its corepressors mSin3 and CoREST. RE1s are located on a large number of genes that are required for the development and maintenance of the neuronal phenotype and are normally repressed in non-neural cells. By regulating gene expression, REST participates in the orderly developmental transition from a neuroepithelial precursor or stem cell to a functional neuron. Glial cells also develop from neuroepithelial stem cells but little is known about the role of REST in the development of glia. Here, I examine the expression and function of REST in glial cells, with a focus on oligodendrocytes and their precursors. Oligodendrocytes are the myelin forming cells of the central nervous system. They develop from an identified precursor known as oligodendrocyte progenitor cells (OPCs). The differentiation of an OPC into a mature oligodendrocyte occurs in a step or stage-wise progression and is regulated by both genetic and epigenetic mechanism. OPCs are highly plastic cells that share a common lineage with some classes of neurons, have some properties usually associated with neurons, and are capable of being reprogrammed to act as neural stem-like cells that can develop into functional neurons. In this thesis, I demonstrate that REST is an important regulator in oligodendrocyte differentiation and OPC cell fate determination.REST is expressed in glial cells and functions as a transcriptional repressor in OPCs. REST transcript and protein expression increase 4-fold during the first 48hrs of oligodendrocyte differentiation. During this maturation period, expression of REST regulated genes decreases as oligodendrocyte specific genes are activated. Perturbing REST function by overexpressing dominant negative REST (DnREST) or REST-VP16 (a chimeric protein containing the DNA binding domain of REST fused to the activation domain of VP16) results in a decrease in oligodendrocyte formation as demonstrated by immunofluorescence, qRT-PCR, immunoblot, and clonal analysis. Additionally, REST loss of function inhibits the ability of OPCs to develop into process bearing GFAP-positive type II astrocytes. Perturbing REST function in differentiating OPCs results in a significant increase in neuronal-like cells at the expense of glia. Through this loss-of-function approach, I demonstrated that REST has 2 functions in the oligodendrocyte lineage: it is required for the timely and complete differentiation of OPCs into oligodendrocytes and it prevents the expression of neuronal properties, suggesting that REST is an important regulator of OPC lineage plasticity.