The role of actin in ensuring mitotic fidelity during early mammalian development - Summary The goal of this proposal is to test a novel role for actin during mitosis in the preimplantation mouse embryo. Following fertilization, the embryo starts to undergo mitotic divisions and must accurately propagate its genetic material in order to avoid aneuploidy. However, early mammalian embryos lack efficient spindle assembly mechanisms and it remains unclear how proper chromosome segregation is achieved during early development. Using advanced imaging, we discovered actin cables inside the cell nucleus in the early embryo. Upon mitosis, these cables capture chromosomes and undergo contraction. This initiates a chromosome congression-like process before the mitotic spindle completes its assembly, which is disrupted by manipulation of F-actin, but not of microtubules. Moreover, when the spindle is assembled, we found another actin network that encloses the spindle and regulates its growth and position, which likely helps to ensure proper cell division patterns. Our data support a new paradigm for the regulation of mitosis in early development whereby nuclear actin cables and peri-spindle actin regulate key steps of mitosis, typically carried out by the mitotic spindle in centrosomal cells. Thus, we propose to dissect the mechanisms by which these actin networks form, how they are regulated, their importance for ensuring mitotic fidelity, and their developmental regulation. Aim 1 will determine how nuclear actin cables form, capture chromosomes, and initiate their congression. For this, we will combine high-resolution live-imaging of actin, chromosome, and microtubule dynamics with biophysical and molecular manipulations in the intact-developing mouse embryo. Aim 2 will use similar approaches to mechanistically dissect how the peri-spindle actin network forms, and how it regulates spindle growth and position to ensure proper chromosome segregation and cell division patterns. In addition to testing a new actin-dependent model for the regulation of mitotic fidelity during early development, the project may eventually help to reduce mitotic errors in IVF embryos, a significant barrier in reproductive medicine.
