The long-term goal of my laboratory is understanding the molecular mechanism(s) by which maternal contribution shapes the early development of an embryo, and how the transition to zygotic control is carried out. This proposal focuses on maternal factors controlling the development of germline precursors in early C. elegans embryos. Germ cell precursors, like stem cells, are maintained transcriptionally inactive in a wide variety of animals. In C. elegans, germ cell precursors are generated through four rounds of asymmetric stem cell-like divisions, beginning with the 1-cell zygote, P0. Each division gives rise to a smaller transcriptionally repressed germline blastomere (P1-P4), and a larger somatic sister cell which soon becomes transcriptionally active. This transcriptional repression has been shown to require a maternally- supplied protein, PIE-1. Although PIE-1 protein is in all germline blastomeres, it does not account for repression in P0 or P1. We show that two closely related maternal proteins, OMA-1 and -2, are required for transcriptional repression in all germline blastomeres, through a mechanism distinct from PIE-1 repression. OMA-1 and OMA-2 are expressed only in oocytes and 1-cell embryos, and their degradation after the 1-cell stage requires developmentally-regulated phosphorylation events. OMA-1 and OMA-2 repress transcription directly in 1-cell and early 2-cell embryos by sequestering an essential component of the basal transcription complex, TAF-4, in the cytoplasm. OMA proteins do not sequester TAF-4 to the cytoplasm in oocytes. In addition, we propose that OMA-1/2 prevent PIE-1 from being degraded in the gonad, thereby indirectly maintaining transcriptional repression in later germline blastomeres. Our data are consistent with OMA-1/2 preventing PIE-1 degradation through translational repression of a factor regulating PIE-1 degradation.
Aim 1 investigates the relationship between regulation of OMA-1 degradation and TAF-4 binding. Surprisingly, the same phosphorylation event that earmarks OMA-1 for degradation is required for TAF-4 binding.
Aim 2 investigates the mechanism by which OMA proteins maintain PIE-1 stability in oocytes. OMA-1 and -2 belong to a protein family with RNA- binding activity, and preliminary results support OMA binding to the mRNA for a protein critical for PIE-1 degradation and inhibiting its translation specifically in oocytes.
Aim 3 investigates how the phosphorylation events that occur following fertilization convert OMA function from stabilizing PIE- 1 protein in oocytes via translational inhibition to regulating germ cell precursor development in the early embryo by transcriptional repression. Insights gained should be relevant to our understanding of the oocyte-to-embryo transition, germ cell development and stem cell biology.
One of the most exciting areas of molecular medicine, and one with potentially immediate and broad impact upon public health, is that of stem cells, and in particular adult stem cells. As research on these various stem cell populations, including germ cells, continues, it appears that many, if not all of these cells, may retain pluripotency, even possibly totipotency. Our studies address the question of how germ cell precursors are maintained totipotent in the early embryo, and any insights gained from our experimental system are expected to shed light on the regulation of stem cell populations in general.
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