Shared mechanisms regulate transcription-factor activity to control cell fate in neural stem cells and the embryo

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ABSTRACT Development requires that cells robustly exit one cell fate and enter another. In the early embryo the specified germ cells must be rapidly reprogrammed to the pluripotent cells that can subsequently generate an entirely new organism. Conversely, following asymmetric stem-cell division the two daughter cells must adopt different fates with one regenerating the stem cell and the other exiting the multipotent state and initiating differentiation. These essential developmental transitions require that the activity of those factors that drive pluripotency be precisely controlled as both too many and too few stem cells are detrimental to the organism. While the transcription factors that drive stem-cell fate have been well characterized in culture, less is known about how the activity of these factors is tightly controlled within the context of a developing organism. Our preliminary data demonstrate a shared role for the transcription factor Zelda (ZLD) as a master regulator of the mulitpotent state in both the early embryo and larval neural stem cells of Drosophila melanogaster. We have recently demonstrated that in both the embryo and the larva ZLD can reprogram cells to a multipotent fate and increased ZLD activity is deleterious. Thus, ZLD activity must be tightly controlled to allow for development to proceed. Our preliminary data suggest that chromatin structure may limit the ability of ZLD to engage the genome and reprogram cell fate. ZLD activity is additionally regulated by post-transcriptional mechanisms that control ZLD levels. Based on these preliminary data we are well positioned to elucidate general mechanisms by which the activities of master regulators of stem-cell fate are precisely controlled to maintain a balance between self-renewal and differentiation. We will use genetic, genomic, and biochemical strategies to (1) identify mechanisms by which chromatin structure influences ZLD activity and (2) determine how post- transcriptional regulation of zld RNA controls ZLD protein levels in both neural stem cells and the early embryo. Together these results will have important implications for understanding how the balance between the multipotent and differentiated states are precisely controlled during development.