Plk1 is required for nuclear envelope remodeling in the C. elegans early embryo

Animal cells undergo open mitosis, where nuclear envelope breakdown (NEBD) allows microtubules emanating from centrosomes to connect to the duplicated chromosomes. At the end of mitosis, the nuclear envelope reforms around the segregated chromosomes, generating a single nucleus in each daughter cell. Polo-like kinase 1 (PLK1) is a conserved kinase involved in multiple steps of mitosis and meiosis. However, a critical role in NEBD has yet to be shown. In C. elegans,complete inactivation of PLK1 leads to a failure in meiotic progression. However, we observed that a partial inactivation of PLK1, using a temperature sensitive (ts) mutant allele grown at a semi-permissive temperature, led to the appearance of embryos with a pair of nuclei in each cell. In these embryos, both nuclear pore complexes (NPC) and the nuclear lamina persisted around the chromatin throughout mitosis, suggesting a defect in NEBD. Normally, following fertilization, the maternal and paternal pronuclei meet, their nuclear envelopes disassemble and their chromosomes align on a single metaphase plate. In plk-1 ts embryos, however, after fertilization the nuclear envelopes of the maternal and paternal nuclei failed to disassemble, forming two distinct nuclei attached to each other. Interestingly, failure in NEBD did not prevent progression through mitosis in early embryonic cells. Rather, the maternal and paternal nuclei underwent separate mitoses, creating cells with paired nuclei. Electron micrographs revealed that the paired nuclei are juxtaposed but are not fused. We hypothesized that the defect in NEBD in plk1 ts cells is due to defect in disassembly of one of more NE components. If that were the case, then a reduction in NE components by RNAi would facilitate NEBD and re-establish the formation of a single nucleus in plk1 cells. Indeed, we found that a reduction in certain NE components, such as, Nup98, Nup107, Nup35, suppressed the paired-nuclei phenotype of plk1 ts cells. Whether these are direct PLK1 phosphorylation targets is under investigation. This study is the first demonstration of an involvement of PLK1 in NEBD in an intact organism.