Abstract

The mechanisms mediating the establishment of totipotency during the egg-to-embryo transition in mammals remain poorly understood. It is clear, however, that unique factors stored in the oocyte cytoplasm are critical for orchestrating this complex cellular event. The cytoplasmic lattices (CPLs), an abundant structure found in mammalian oocytes and preimplantation embryos, have long been predicted to function as a storage form for maternal factors in the oocyte. Here, I present evidence further supporting this model and demonstrating that oocyte- and early embryo-restricted peptidylarginine deiminase 6 (PADI6) is required for CPL formation. PADI6 is the most evolutionarily divergent member of the PADI family. To date, no enzymatic activity has been demonstrated for PADI6. Based on in silico studies, I present a model explaining how PADI6 may differ from other PADI homologues. I also report efforts aimed at establishing a somatic cell system for testing this model. Next, given my observation that CPLs do not form in PADI6-/- mouse oocytes, I test a number of hypotheses regarding CPL composition and function using this in vivo model. By better characterizing the PADI6-/- phenotype, I demonstrate that PADI6 functions as a novel maternal effect gene that is required for development past the two-cell stage. I also show that the abundance, solubility, and/or localization of stored maternal factors including ribosomal protein S6 and α-tubulin are dramatically affected in PADI6-/- oocytes and embryos. Finally, I demonstrate that PADI6-/- two-cell embryos do not properly activate their genome. Taken together, I propose that the CPLs constitute a translationally repressed, paracrystalline mRNP network: an epigenomic counterpart to heterochromatin. Furthermore, the observation that PADI6 localizes asymmetrically to the actin-rich cytocortex at the embryonic cleavage stages supports the model that polarity is established very early in mammalian development, akin to early development in insects and other vertebrates. The molecular basis of this polarity is still largely undiscovered. Without PADI6, actin localization is not disrupted, however a number of maternal factors fail to achieve proper cytocortical localization. Thus, I present a model implicating the cytocortex in mammalian translational control and PADI6 and/or the CPLs as critical regulators of its proper nucleation and function.