Observations from several cell types suggest that this sudden rupture requires mechanical force generated by the cytoskeleton. As this dramatic change is easily visible, even by transmitted light microscopy, this second step is commonly identified as ‘NEBD,’ marking the transition between the prophase and the prometaphase of cell division. In all of the species and cell types investigated to date, the slow, phosphorylation-driven weakening of the NE is followed by a sudden rupture of the NE leading to rapid and complete mixing of cyto- and nucleoplasm. ribosomes and microtubules) is maintained ( Terasaki et al., 2001 Lénárt et al., 2003).
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Importantly, however, during this first phase of NEBD, the overall structure of the NE (as observed by electron microscopy (EM)) is still intact and the compartmentalization of large protein complexes (e.g. Furthermore, it is likely that the mechanical properties of the NE are affected, that is the NE is weakened and destabilized as a result of the phosphorylation of lamins and lamina-associated proteins ( Ungricht and Kutay, 2017). This allows proteins, and smaller dextrans up to ~70 kDa, to leak in or out of the nucleus ( Lénárt et al., 2003). In all species in which nuclear envelope breakdown (NEBD) has been investigated in detail, including somatic cells and oocytes from various species, NEBD begins with a partial permeabilization of the NE resulting from phosphorylation-driven disassembly of the NPCs and other NE components ( Dultz et al., 2008 Mühlhäusser and Kutay, 2007 Terasaki et al., 2001 Lénárt et al., 2003 Martino et al., 2017 Linder et al., 2017).
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In somatic mammalian cells, NE disassembly involves the complete dismantling of the NPCs, depolymerization of the lamina, and re-absorption of the nuclear membranes into the ER ( Hetzer, 2010 Ungricht and Kutay, 2017). In Drosophila and Caenorhabditis elegans embryos, the NE and the lamina remains partially intact during cell division, whereas in vertebrates and deuterostomes (including the echinoderm starfish), the complex NE structure is fully disassembled during division. Depending on the species and nuclear architecture, there is a broad diversity in disassembly mechanisms. The NE must be dismantled at the onset of every cell division to give microtubules access to chromosomes, and then reassembled at the end of division once the chromosomes are segregated. The oocyte NE is densely packed with NPCs that serve as a stockpile of these complexes (rendering oocytes a popular model in which to study NPCs), and the lamina is thick so that it is able to provide mechanical support for this very large structure ( Goldberg and Allen, 1995). Oocytes have a very specialized nuclear architecture with an exceptionally large nucleus, also known as the germinal vesicle, which stores nuclear components that are necessary to support early embryonic development. For example, the composition of the lamina is adapted to provide the high mechanical stability that is necessary in muscle cells, or sufficient flexibility in immune cells, which need to squeeze through confined spaces ( Thiam et al., 2016). This complex NE membrane structure is mechanically supported by a network of intermediate filaments, the lamina, which lines the nucleoplasmic side ( Burke and Ellenberg, 2002).Īcross species and cell types a considerable diversity of nuclear structure allows adaptation to physiological function. The inner and outer NE is fused at nuclear pore complexes (NPCs) to mediate nucleo-cytoplasmic transport. The nuclear envelope (NE), composed of inner and outer nuclear membranes, is a specialized sub-compartment of the endoplasmic reticulum (ER) that separates the nucleus and the cytoplasm in eukaryotic cells. We reveal a new function for actin-mediated membrane shaping in nuclear rupture that is likely to have implications in other contexts, such as nuclear rupture observed in cancer cells.
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Packed NPCs sort into a distinct membrane network, while breaks appear in ER-like, pore-free regions. These F-actin spikes protrude pore-free nuclear membranes, whereas the adjoining stretches of membrane accumulate NPCs that are associated with the still-intact lamina. We show that actin is nucleated within the lamina, sprouting filopodia-like spikes towards the nuclear membranes. Here, we address the mechanism of F-actin-driven NE rupture by correlated live-cell, super-resolution and electron microscopy. We showed that breakdown of this specialized NE is mediated by an Arp2/3-nucleated F-actin ‘shell’ in starfish oocytes, in contrast to microtubule-driven tearing in mammalian fibroblasts. The nucleus of oocytes (germinal vesicle) is unusually large and its nuclear envelope (NE) is densely packed with nuclear pore complexes (NPCs) that are stockpiled for embryonic development.