Defining the mechanism of actin-mediated spindle position sensing in mouse oocytes

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Recent work has revealed that cytoplasmic actin networks are required for the first meiotic division of oocytes at the beginning of animal life. These actin networks are responsible for transporting chromosomes over long distances and for asymmetric positioning of the meiotic spindle. Although several lines of evidence support the notion that the actin nucleators formin and Spir might be required to form these networks, we do not at all understand their molecular mechanism of action under physiological conditions. The principal aim of the present proposal is therefore to understand how Spir and formin nucleation complexes interact to nucleate actin filaments in animal oocytes, how these nucleation complexes are regulated and how the actin filaments are organized into a network to mediate chromosome/spindle positioning. To address these questions, we will take advantage of two biological model systems to study meiosis, namely starfish and mouse, because they allow us to combine genetics, biochemistry and advanced live cell imaging methods in animal oocytes. Our approach has three major complementary and integrated parts. First, we will use quantitative live cell imaging assays to characterize the function of formin and Spir in available knock out mouse models. Second, we will identify new meiotic formin/Spir accessory factors and regulators by biochemical purification and mass spectrometry in starfish. And third, we will characterize and validate these newly identified candidates under physiological conditions using both starfish and mouse oocytes. Studying the two systems will allow us to efficiently dissect the molecular mechanism underlying the formation and function of oocyte specific actin networks and reveal their evolutionarily conserved principles. Answering these questions will be of key importance for understanding the basic mechanisms of oocyte meiosis and at the same time provide one of the first physiological models to investigating the conserved Spir-formin interaction in actin filament nucleation. Both of these problems are of great interest to the field of cell biology and have important implications for human health, in particular for understanding infertility.