Among the earliest and most fundamental of differentiation events in many species is the distinction between somatic cells and the germ cells. In addition to their unique ability to undergo meiosis, the germ cells are different from most somatic cells in two respects; they are capable of producing numerous generations of their own progeny (self-renewal), and are capable of producing multiple distinct differentiation fates (pluripotency). Pluripotency, in particular, has been recognized as a key mechanism for the maintenance of stem cells in general, and can be seen, in large part, as the absence of epigenetic chromatin modifications that progressively occur during differentiation of somatic tissues. However, the mechanism by which stem cells prevent the installation of epigenetic markers is not known. In C. elegans, early germline cells appear to completely lack mRNA transcription, and a germline- specific nuclear protein, PIE-l, is required for this global repression. It has been proposed that this function of PIE-1 accounts for the absence of differentiation in the gerrnline cells and therefore the maintenance of germ cell potential. This proposal presents new evidence suggesting that PIE-1 also downregulates a putative histone deacetylaselchromatin remodeling activity, called the MEP-1/LET- 418/HDA-1 complex and related to the mammalian NuRD (nucleosome remodeling and histone deacetylase) complex. A model is presented in which PIE-1 prevents the activation of this complex during early phases of embryogenesis, and thus protects germ cell-specific chromatin organization. The current study will characterize biochemical activities of the MEP-1/LET-418/HDA-1 complex and investigate the physiological significance of the MEP-1/LET-418/HDA-1 inhibition by PIE-1 in the early germ cells. In addition, the mechanism by which PIE-1 inhibits the MEP-1/LET-418/HDA-1 complex will be investigated through genetic screens. The overall goals of this study are to identify the mechanism of germ cell development in C. elegans, with which embryonic development can be studied at the molecular and cellular levels, and to provide insights into how transcriptional competence is regulated in stem cell in general.
