Year 3

Transcription factors are central for establishing the cell type-specific gene expression patterns. In pluripotent cells, SOX2 is a transcription factor that is central to the pluripotency expression program. Transcription factors like SOX2 are often regulated by post-translational modifications (PTMs) that occur in response to developmental and environmental cues. These PTMs alter transcription factor activity to facilitate changes in gene expression associated with differentiation or to promote growth under altered nutrient or stress conditions. We have found that SOX2 is post-translationally modified by the enzyme OGT, which is central in the glucose nutrient-sensing pathway, in embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) (mouse and human). Our preliminary data in the mouse system showed that mutation of the modified residue affects SOX2 function during reprogramming to pluripotency and during self-renewal of pluripotent cells. Our preliminary data also implicate this PTM in regulating human SOX2, as the equivalent residue is also modified in human pluripotent cells. We have begun experiments to gain a molecular understanding of the OGT-dependent modification, termed O-GlcNAcylation, of SOX2 in reprogramming to pluripotency in human cells. Our aims are 1) to examine whether mutation of the modified SOX2 residue enhances human reprogramming, 2) to examine whether increasing OGT amounts or activity enhance human reprogramming (since OGT is expressed at much lower levels in somatic cells than pluripotent cells), and 3) to determine the molecular consequences of expressing the mutant form of SOX2 during reprogramming and in pluripotent cells (in this third aim we use mouse as a more genetically tractable model system to first investigate molecular mechanism and then examine the mechanisms implicated in the human system). Our results to date provide a molecular basis for the increased reprogramming efficiency observed with mutant SOX2: we find that SOX2 that cannot be modified causes i) increased expression of epigenetic modifiers that are necessary for successful reprogramming, ii) increases SOX2 association with proteins that promote pluripotency and decreases SOX2 interaction with proteins that promote differentiation, and iii) alters the genome-wide distribution of SOX2 . In addition we found that two other reprogramming factors, KLF4 and MYC, are also O-GlcNAcylated. We continue to build on these findings to understand how this nutrient sensitive modification impacts the activity of reprogramming factors to promote pluripotency.