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The Nuclear Envelope During Mitosis

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The Nuclear Envelope During Mitosis Available online at www.sciencedirect.com ScienceDirect § Breaking down the wall: the nuclear envelope during mitosis 1 1,2 Christine J Smoyer and Sue L Jaspersen A defining feature of eukaryotic cells is the nucleus, which assemble an intranuclear spindle. As we discuss below, houses the genome inside the nuclear envelope (NE): a double eukaryotic cells utilize both strategies, as well as vari- lipid bilayer that separates the nuclear and cytoplasmic ations in between (Figure 1), offering glimpses into how materials. Although the NE is commonly viewed as a barrier that the nucleus evolved as an organelle to organize and is overcome only by embedded nuclear pore complexes protect the genome and why mutations in NE proteins (NPCs) that facilitate nuclear-cytoplasmic trafficking, recent result in a broad spectrum of human diseases, including work in a wide range of eukaryotes reveals that the NE is a cancer [1,2]. dynamic organelle that is modified each time the cell divides to ultimately establish two functional daughter nuclei. Here, we The nuclear envelope review how studies of divergent mitotic strategies have helped Although multiple theories exist on how the eukaryotic elucidate common properties of NE biology that allow it to nucleus arose during evolution, most share the idea that function throughout the cell cycle. the NE and its associated proteins have ancient roots in Addresses division of the genome [1,3]. The nucleus likely co- 1 Stowers Institute for Medical Research, Kansas City, MO 64110, United evolved with other parts of the endomembrane system. States 2 However, unlike other cellular membranes, the NE is a Department of Molecular and Integrative Physiology, University of double lipid bilayer composed of an outer (ONM) and Kansas Medical Center, Kansas City, KS 66160, United States inner membrane (INM) separated by a perinuclear space. Corresponding author: Jaspersen, Sue L ([email protected]) The ONM is contiguous with the ER and is thought to contain many of the same proteins and lipids. In contrast, the INM is distinct—proteomic analyses suggest that it Current Opinion in Cell Biology 2014, 26:1–9 contains at least 100 unique components and may contain This review comes from a themed issue on Cell architecture over 1000 proteins, many of which interact with chromo- Edited by Sue Biggins and Matthew D Welch somes and/or the nuclear lamina [4,5,6]. For a complete overview see the Issue and the Editorial The NE is typically regarded as a hydrophobic barrier: Available online 4th September 2013 transport of macromolecules into and out of the nucleus 0955-0674/$ – see front matter, # 2013 The Authors. Published by occurs through nuclear pore complexes (NPCs) that are Elsevier Ltd. This is an open access article under the CC BY-NC-SA embedded in the NE at sites where the ONM and INM license (http://creativecommons.org/licenses/by-nc-sa/3.0/. are contiguous (reviewed in [7,8]). The NPC has often http://dx.doi.org/10.1016/j.ceb.2013.08.002 been considered to be the sole route for crossing the NE; however, a recent study found that large ribonucleopro- tein particles involved in Wnt signaling at the neuromus- Introduction cular junction of Drosophila larvae cross the NE by a During cell division, genetic material and cytoplasmic vesicle budding and fusion pathway that is NPC-inde- components are partitioned into two daughter cells. In pendent [9 ]. This pathway, in which a vesicle buds from eukaryotic cells, the DNA is segregated away from the the INM and then docks and fuses with the ONM, is cytoplasm by the nuclear envelope (NE). After DNA is reminiscent of the nuclear egress mechanism used by duplicated, it must be physically separated, which is certain virus particles to escape the NE (reviewed in facilitated by a microtubule-based structure known as [10,11]). It is unclear if vesicle-mediated transport can the mitotic spindle. Key regulators of microtubule nuclea- be used for nuclear entry as well as for nuclear export or if tion, including tubulin, microtubule-associated proteins it is a widely used mechanism of nuclear-cytoplasmic and the microtubule-organizing center (MTOC), are pre- trafficking, but it is tantalizing to speculate that there dominately localized in the cytoplasm of interphase cells. are multiple routes across the NE border. In order to allow microtubules access to chromosomes, the NE must either become permeable, or tubulin and The budding and fission yeast closed mitosis microtubule-associated proteins need to be imported into Although the NE barrier is advantageous in terms of the nucleus and the MTOC inserted into the NE to regulation of transcription, translation and DNA repli- cation, it poses a unique challenge during mitosis: how § to distribute chromosomes inside the nucleus into the This is an open-access article distributed under the terms of the two daughter cells using the cytoplasmic microtubule Creative Commons Attribution-NonCommercial-ShareAlike License, which permits non-commercial use, distribution, and reproduction in cytoskeleton. Some organisms, such as the budding any medium, provided the original author and source are credited. yeast Saccharomyces cerevisiae and the fission yeast www.sciencedirect.com Current Opinion in Cell Biology 2014, 26:1–9 2 Cell architecture Figure 1 interphase mitosis open semi-open closed closed-rupture Current Opinion in Cell Biology Mitotic strategies used in eukaryotes. Shown are interphase (left) and mitotic (right) cells undergoing open, semi-open and closed mitosis as well as the closed mitosis of S. japonicus in which the NE ruptures during anaphase. MTOCs are depicted in purple, chromosomes in blue, microtubules in black and membranes in white. Schizzosaccharomyces pombe, undergo a closed mitosis Cytological and molecular analysis of the SPB in where the NE does not break down (Figure 1). In budding yeast suggests that its assembly is similar to order to facilitate the organization of chromosomes de novo assembly of NPCs in all eukaryotes. Both on the mitotic spindle, the MTOC, known as the SPBs and NPCs are assembled as subcomplexes in spindle pole body (SPB), is anchored in the NE during the cytoplasm and nucleoplasm; NE remodeling and all or part of the cell cycle [12–14]. As we discuss below, stabilization are required to allow these complexes to despite structural differences between the fungal SPB associate in the NE to create an intact and and the metazoan centrosome, analysis of SPB dupli- functional SPB or NPC (Figure 2) (reviewed in cation and tethering in the NE in both budding and [7,15]). Because SPB insertion into the NE occurs at fission yeast has led to important insights into NE a specific time (late G1 phase) and place (adjacent to biology. the existing SPB), it is a useful model for analysis of Current Opinion in Cell Biology 2014, 26:1–9 www.sciencedirect.com Nuclear envelope during mitosis Smoyer and Jaspersen 3 Figure 2 Figure 3 ONM reticulons/Yop1 Poms INM ONM approximation INM hemifusion fusion pore membrane ALPS proteins insertion Current Opinion in Cell Biology Molecular mechanism of pore membrane formation. It is thought that the INM and ONM approximation is achieved by lumenal interactions or pore membrane proteins (Poms), which may also recruit other NPC SPB membrane remodeling enzymes such as the reticulons/Yop1 that insert Current Opinion in Cell Biology into the outer leaflet to induce membrane curvature. SUN proteins on the INM have been proposed to result in localized changes in membrane composition to facilitate membrane curvature, which is then stabilized at The pore membrane. Depicted are the double lipid bilayers of the ONM least in part by ALPS proteins, such as Nbp1. and INM. Formation of the pore membrane is thought to begin by approximation of the inner membrane leaflets, which fuse to form a hemifused membrane structure. The outer leaflets then fuse, and the pore membrane is created. Further expansion of the pore allows for assembly of a NPC or SPB into the NE. Both complexes are anchored in the NPC assembly defects seen may be due to titration of the pore membrane by integral membrane proteins, which appear as proteins away from the NPC to the SPB [23]. hooks in the electron microscope. Converting a lipid bilayer to a curved structure requires specialized proteins that specifically act on the membrane events at the NE required for complex insertion to induce curvature. Reticulons and the related Yop1/ (reviewed in [15,16]). DP1 are ER membrane bending proteins that are thought to induce membrane curvature by insertion into the outer Examination of NE structure by electron microscopy and leaflet of the membrane (Figure 3) [7,24]. Previous work tomography shows that SPBs, like NPCs, are located in in yeast and Xenopus illustrated a role for the reticulons in pores: regions of the NE where the ONM and INM are NPC assembly, most likely through the creation of a NE contiguous [12,17,18]. Localized changes in the NE are pore, although other models have also been proposed thought to allow transition from a lipid bilayer to a curved [25,26]. Further analysis of S. cerevisiae cells lacking both pore membrane for insertion of SPBs and NPCs. The RTN1 (reticulon) and YOP1 revealed a defect in SPB connection between membrane remodeling events integrity, including an inability to tether the SPB in the required for SPB insertion and NPC assembly was origin- NE [27 ]. Rtn1 and Yop1 physically interact with Ndc1, ally proposed by Chial and colleagues following charac- pointing to a model in which Ndc1 recruits membrane- terization of Ndc1, an evolutionarily conserved integral remodeling factors to the site of SPB and/or NPC assem- membrane protein that is a component of both complexes bly [27 ]. [19]. Mutations in NDC1 result primarily in SPB dupli- cation defects, although some alleles, as well as NDC1 Ndc1 may recruit additional factors to the SPB to form the depletion, display defects in NPC assembly [19–22].
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