Nuclear envelope disassembly in mitotic extract requires functional nuclear pores and a nuclear lamina

Gaussian curvature dilutes the nuclear lamina, favoring nuclear rupture, especially at high strain rate

Nuclear rupture has long been associated with deficits or defects in lamins, with recent results also indicating a role for actomyosin stress, but key physical determinants of rupture remain unclear. Here, lamin-B filaments stably interact with the nuclear membrane at sites of low Gaussian curvature yet dilute at high curvature to favor rupture, whereas lamin-A depletion requires high strain-rates. Live-cell imaging of lamin-B1 gene-edited cancer cells is complemented by fixed-cell imaging of rupture in: iPS-derived progeria patients cells, cells within beating chick embryo hearts, and cancer cells with multi-site rupture after migration through small pores. Data fit a model of stiff filaments that detach from a curved surface.Rupture is modestly suppressed by inhibiting myosin-II and by hypotonic stress, which slow the strain-rates. Lamin-A dilution and rupture probability indeed increase above a threshold rate of nuclear pulling. Curvature-sensing mechanisms of proteins at plasma membranes, including Piezo1, might thus apply at nuclear membranes.Summary statement: High nuclear curvature drives lamina dilution and nuclear envelope rupture even when myosin stress is inhibited. Stiff filaments generally dilute from sites of high Gaussian curvature, providing mathematical fits of experiments.

Evaluation of mammalian cell-free systems of nuclear disassembly and assembly

Mammalian cell-free systems are very useful for the biochemical and structural study of nuclear disassembly and assembly. Through experimental manipulations, the role of specific proteins in these processes can be studied. Recently, we intended to examine the involvement of integral and peripheral inner nuclear membrane proteins in nuclear disassembly and assembly. However, we could not achieve proper disassembly when isolated interphase HeLa nuclei were exposed to mitotic soluble extracts obtained from the same cell line and containing cyclin B1. Homogenates of synchronized mitotic HeLa cells left to reassemble their nuclei generated incomplete nuclear envelopes on chromatin masses. Digitonin-permeabilized mitotic cells also assembled incomplete nuclei, generating a lot of cytoplasmic inclusions of inner nuclear membrane proteins as an intermediate. These results were therefore used as a basis for a critical evaluation of mammalian cell-free systems. We present here evidence that cell synchronization itself can interfere with the progress of nuclear assembly, possibly by causing aberrant nuclear disassembly and/or by inducing the formation of an abnormal number of mitotic spindles.

The nuclear envelope: form and reformation

The membrane system that encloses genomic DNA is referred to as the nuclear envelope. However, with emerging roles in signaling and gene expression, these membranes clearly serve as more than just a physical barrier separating the nucleus and cytoplasm. Recent progress in our understanding of nuclear envelope architecture and composition has also revealed an intriguing connection between constituents of the nuclear envelope and human disease, providing further impetus to decipher this cellular structure and the dramatic remodeling process it undergoes with each cell division.

A Light-Activated Probe of Intracellular Protein Kinase Activity

The first example of a photoactivated probe of intracellular enzymatic activity is described. The caged derivative of a fluorescent protein kinase C peptide-based sensor was prepared by modifying the free hydroxyl group of a phosphorylatable serine moiety with a photolabile appendage that blocks phosphoryl transfer. We have demonstrated that the caged sensor allows one to (1) sample PKC activity with exquisite temporal precision, (2) control the relative amount of active sensor available for phosphorylation, and (3) examine protein kinase activity at multiple time points.

A carboxyl-terminal interaction of lamin B1 is dependent on the CAAX endoprotease Rce1 and carboxymethylation

The mammalian nuclear lamina protein lamin B1 is posttranslationally modified by farnesylation, endoproteolysis, and carboxymethylation at a carboxyl-terminal CAAX motif. In this work, we demonstrate that the CAAX endoprotease Rce1 is required for lamin B1 endoproteolysis, demonstrate an independent pool of proteolyzed but nonmethylated lamin B1, as well as fully processed lamin B1, in interphase nuclei, and show a role for methylation in the organization of lamin B1 into domains of the nuclear lamina. Deficiency in the endoproteolysis or methylation of lamin B1 results in loss of integrity and deformity of the nuclear lamina. These data show that the organization of the nuclear envelope and lamina is dependent on a mechanism involving the methylation of lamin B1, and they identify a potential mechanism of laminopathy involving a B-type lamin.

Presence of a nucleus or nucleus-deriving factors is indispensable for the formation of the spindle, the diastema and the cleavage furrow in the blastomere of the Xenopus embryo

The present study examines the indispensability of a nucleus or nucleus-deriving factors in the induction of cleavage in Xenopus eggs by testing cleavage in Xenopus eggs fertilized with ultraviolet (UV)-damaged sperm and deprived of the female nucleus. These eggs, which contain only one UV-damaged nucleus with one set of centrioles, undergo unique cleavages. Cleavage takes place in only one of the two blastomeres formed by the immediately preceding cleavage. Histologically, only one nucleus, which does not appear to be organized into typical chromosomes, is found in one of the two blastomeres formed by the immediately preceding cleavage. The typical bipolar spindle and the diastema, or a slit of astral rays, are formed in the blastomere that contains the nucleus. By contrast, only asters lacking the spindle and the diastema are formed in the remaining blastomeres, which do not contain a nucleus. The same results are obtained in eggs that contain two UV-damaged nuclei with one set of centrioles. In these eggs, cleavage appears to occur in one or two blastomeres that contain either or both of the nuclei and one bipolar spindle. In eggs that contain one intact and one UV-damaged nuclei, cleavage takes place quite normally with each blastomere containing one nucleus or one set of chromosomes as well as one bipolar spindle. Thus, there is a very close correlation between the presence of a nucleus and the formation of the mitotic spindle, the diastema and the cleavage furrow in the blastomeres of Xenopus embryos. We conclude that the presence of a nucleus or nucleus-deriving factors is indispensable for the formation of the bipolar spindle, the diastema and the cleavage furrow in the blastomeres of the Xenopus embryos.

Nuclear pores and nuclear assembly

Communication between the nucleus and cytoplasm occurs through large macromolecular structures, the nuclear pores. Quantitative scanning transmission electron microscopy has estimated the mass of a nuclear pore to be 60 million Daltons in yeast and 120 million Daltons in vertebrates. The past two years were noteworthy in that they saw: 1) the purification of both the yeast and vertebrate nuclear pores, 2) the initial description of routes through the pore for specific transport receptors, 3) glimpses of intranuclear organization imposed by the nuclear pores and envelope and 4) the revelation of new and pivotal roles for the small GTPase Ran not only in nuclear import but in spindle assembly and nuclear membrane fusion.

Identification of tyrosine-phosphorylated proteins associated with the nuclear envelope

The nuclear envelope separates the nucleoplasm from the rest of the cell. Throughout the cell cycle, its structural integrity is controlled by reversible protein phosphorylation. Whereas its phosphorylation-dependent disassembly during mitosis is well characterized, little is known about phosphorylation events at this structure during interphase. The few characterized examples cover protein phosphorylation at serine and threonine residues, but not tyrosine phosphorylation at the nuclear envelope. Here, we demonstrate that tyrosine phosphorylation and dephosphorylation occur at the nuclear envelope of intact Neuro2a mouse neuroblastoma cells. Tyrosine kinase and phosphatase activities remain associated with purified nuclear envelopes. A similar pattern of tyrosine-phosphorylated nuclear envelope proteins suggests that the same tyrosine kinases act at the nuclear envelope of intact cells and at the purified nuclear envelope. We have also identified eight tyrosine-phosphorylated nuclear envelope proteins by 2D BAC/SDS/PAGE, immunoblotting with phosphotyrosine-specific antibodies, tryptic in-gel digestion, and MS analysis of tryptic peptides. These proteins are the lamina proteins lamin A, lamin B1, and lamin B2, the inner nuclear membrane protein LAP2beta, the heat shock protein hsc70, and the DNA/RNA-binding proteins PSF, hypothetical 16-kDa protein, and NonO, which copurify with the nuclear envelope.

The Ketel(D) dominant-negative mutations identify maternal function of the Drosophila importin-beta gene required for cleavage nuclei formation

The Ketel(D) dominant female-sterile mutations and their ketel(r) revertant alleles identify the Ketel gene, which encodes the importin-beta (karyopherin-beta) homologue of Drosophila melanogaster. Embryogenesis does not commence in the Ketel(D) eggs deposited by the Ketel(D)/+ females due to failure of cleavage nuclei formation. When injected into wild-type cleavage embryos, cytoplasm of the Ketel(D) eggs does not inhibit nuclear protein import but prevents cleavage nuclei formation following mitosis. The Ketel(+) transgenes slightly reduce effects of the Ketel(D) mutations. The paternally derived Ketel(D) alleles act as recessive zygotic lethal mutations: the Ketel(D)/- hemizygotes, like the ketel(r)/ketel(r) and the ketel(r)/- zygotes, perish during second larval instar. The Ketel maternal dowry supports their short life. The Ketel(D)-related defects originate most likely following association of the Ketel(D)-encoded mutant molecules with a maternally provided partner. As in the Ketel(D) eggs, embryogenesis does not commence in eggs of germline chimeras with ketel(r)/- germline cells and normal soma, underlining the dominant-negative nature of the Ketel(D) mutations. The ketel(r) homozygous clones are fully viable in the follicle epithelium in wings and tergites. The Ketel gene is not expressed in most larval tissues, as revealed by the expression pattern of a Ketel promoter-lacZ reporter gene.

HA95 is a protein of the chromatin and nuclear matrix regulating nuclear envelope dynamics

We report a role for HA95, a nuclear protein with high homology to the nuclear A-kinase anchoring protein AKAP95, in the regulation of nuclear envelope-chromatin interactions. Biochemical and photobleaching data indicate that HA95 is tightly associated with chromatin and the nuclear matrix/lamina network in interphase, and bound to chromatin at mitosis. HA95 resides in a complex together with lamin B receptor (LBR), lamina-associated polypeptide (LAP)2 and emerin, integral proteins of the inner nuclear membrane. Cross-linking experiments, however, illustrate a tight association of HA95 with LBR and LAP2 only. Intra-nuclear blocking of HA95 with anti-HA95 antibodies abolishes nuclear breakdown in a mitotic HeLa cell extract. The antibodies inhibit nuclear membrane breakdown and chromatin condensation - the latter independently of nuclear membranes. However, lamina disassembly is not affected, as judged by immunological analyses of A/C- and B-type lamins. In contrast, immunoblocking of HA95 bound to condensed chromosomes does not impair chromatin decondensation, nuclear membrane reassembly or lamina reformation. Our results argue for a role for HA95 in anchoring nuclear membranes and lamins to chromatin in interphase, and in releasing membranes from chromatin at mitosis. The data also suggest that HA95 is not involved in initial binding of membranes to chromatin upon nuclear reassembly. We propose that HA95 is a central platform at the chromatin/nuclear matrix interface implicated in regulating nuclear envelope-chromatin interactions during the cell cycle.

C. elegans nuclear envelope proteins emerin, MAN1, lamin, and nucleoporins reveal unique timing of nuclear envelope breakdown during mitosis

Emerin, MAN1, and LAP2 are integral membrane proteins of the vertebrate nuclear envelope. They share a 43-residue N-terminal motif termed the LEM domain. We found three putative LEM domain genes in Caenorhabditis elegans, designated emr-1, lem-2, and lem-3. We analyzed emr-l, which encodes Ce-emerin, and lem-2, which encodes Ce-MAN1. Ce-emerin and Ce-MAN1 migrate on SDS-PAGE as 17- and 52-kDa proteins, respectively. Based on their biochemical extraction properties and immunolocalization, both Ce-emerin and Ce-MAN1 are integral membrane proteins localized at the nuclear envelope. We used antibodies against Ce-MAN1, Ce-emerin, nucleoporins, and Ce-lamin to determine the timing of nuclear envelope breakdown during mitosis in C. elegans. The C. elegans nuclear envelope disassembles very late compared with vertebrates and Drosophila. The nuclear membranes remained intact everywhere except near spindle poles during metaphase and early anaphase, fully disassembling only during mid-late anaphase. Disassembly of pore complexes, and to a lesser extent the lamina, depended on embryo age: pore complexes were absent during metaphase in >30-cell embryos but existed until anaphase in 2- to 24-cell embryos. Intranuclear mRNA splicing factors disassembled after prophase. The timing of nuclear disassembly in C. elegans is novel and may reflect its evolutionary position between unicellular and more complex eukaryotes.

Rearrangements of sea urchin egg cytoplasmic membrane domains at fertilization

Fertilization in the sea urchin is accompanied by rapid reorganization of the egg endoplasmic reticulum (ER). ER-derived vesicles contribute to one of three classes of membranes used in assembling the male pronuclear envelope in vitro. We provide here biochemical evidence for the rearrangement of sea urchin egg cytoplasmic membrane domains at fertilization up to the first mitosis, with respect to two nuclear envelope markers, lamin B and lamin B receptor (LBR), using purified vesicles prepared from homogenates fractionated by floatation on sucrose gradients. In unfertilized eggs, immunoprecipitation data indicate that most of lamin B and LBR are localized in the same vesicles but do not interact. By 3 min post-fertilization, both proteins are more widely distributed across the gradients and by 12 min most of lamin B and LBR are localized in vesicles of different densities. This partitioning is maintained throughout S phase. At mitosis, most lamin B and LBR remain in distinct vesicles, while a small proportion of lamin B and LBR, likely derived from the disassembled nuclear envelope, associate in a minor subset of vesicles. The results illustrate a dynamic reorganization of egg cytoplasmic membranes at fertilization, and the establishment of distinct membrane domains enriched in specific nuclear envelope markers during the first cell cycle of sea urchin development. Additionally, we demonstrate that male pro-nuclear membrane assembly occurs only when both cytosol and membranes originate from fertilized but not unfertilized eggs, suggesting that fertilization-induced membrane rearrangements contribute to the ability of the egg to assemble the male pronuclear envelope.

The A-kinase-anchoring protein AKAP95 is a multivalent protein with a key role in chromatin condensation at mitosis

Protein kinase A (PKA) and the nuclear A-kinase-anchoring protein AKAP95 have previously been shown to localize in separate compartments in interphase but associate at mitosis. We demonstrate here a role for the mitotic AKAP95-PKA complex. In HeLa cells, AKAP95 is associated with the nuclear matrix in interphase and redistributes mostly into a chromatin fraction at mitosis. In a cytosolic extract derived from mitotic cells, AKAP95 recruits the RIIalpha regulatory subunit of PKA onto chromatin. Intranuclear immunoblocking of AKAP95 inhibits chromosome condensation at mitosis and in mitotic extract in a PKA-independent manner. Immunodepletion of AKAP95 from the extract or immunoblocking of AKAP95 at metaphase induces premature chromatin decondensation. Condensation is restored in vitro by a recombinant AKAP95 fragment comprising the 306-carboxy-terminal amino acids of the protein. Maintenance of condensed chromatin requires PKA binding to chromatin-associated AKAP95 and cAMP signaling through PKA. Chromatin-associated AKAP95 interacts with Eg7, the human homologue of Xenopus pEg7, a component of the 13S condensin complex. Moreover, immunoblocking nuclear AKAP95 inhibits the recruitment of Eg7 to chromatin in vitro. We propose that AKAP95 is a multivalent molecule that in addition to anchoring a cAMP/PKA-signaling complex onto chromosomes, plays a role in regulating chromosome structure at mitosis.

In vitro nuclear assembly with affinity-purified nuclear envelope precursor vesicle fractions, PV1 and PV2

Nuclear envelope precursor vesicles were affinity purified from a Xenopus egg extract by a chromatin binding method. Vesicles bound to chromatin at 4 degrees C were dissociated with a high salt buffer and further fractionated into nuclear envelope precursor vesicle fractions 1 (PV1) and 2 (PV2) by differential centrifugation. PV1 contained larger vesicles. When chromatin was incubated in a Xenopus egg cytosol fraction supplemented with PV1, vesicles bound to chromatin, fused with each other, formed a bilayered nuclear envelope, and assembled into spherical small nuclei. However, the thus assembled nuclei did not grow to the normal size. Nuclear pore complexes were not found on the thus assembled nuclei. On the other hand, PV2 contained smaller vesicles. PV2 vesicles bound to chromatin, fused little with each other in the Xenopus egg cytosol fraction, and no nuclei were assembled. When PV1 supplemented with PV2 was used for the nuclear assembly reaction, the assembled nuclei grew to the normal size. Nuclear pore complexes existed in the thus assembled nuclear envelopes. These results suggested that 1) two vesicle populations, PV1 and PV2, are necessary for the assembly of normal sized nuclei, 2) PV1 contains a chromatin targeting molecule(s) and membrane fusion machinery, 3) PV2 contains a chromatin targeting molecule(s) and a molecule(s) necessary for nuclear pore complex assembly, and 4) PV1 has the ability to assemble a nuclear membrane, and PV2 is necessary for the assembly of nuclear pore complexes and for nuclei to grow to the normal size. An in vitro nuclear assembly system constituted with affinity-purified vesicle fractions, PV1 and PV2, was established.

Subcellular distribution of the Xenopus p58/lamin B receptor in oocytes and eggs

The p58/lamin B receptor of vertebrates is localized in the inner nuclear membrane. Antibodies raised against the bacterially expressed amino-terminal half of Xenopus p58 (Xp58) revealed that in Xenopus oocytes the vast majority of this membrane protein is localized in cytoplasmic membranes. Only very small amounts of p58 not detectable by immunofluorescence microscopy were contained in the oocyte nuclear envelope. In contrast, nuclear membranes of 2-cell stage embryos were successfully stained with p58 antibodies, nuclei reconstituted in vitro in Xenopus egg extracts contained p58, and the nucleoplasmic domain of Xp58 could be specifically bound to sperm chromatin in vitro. One major difference between oocytes and early embryonic cells is that no chromatin is associated with the oocyte inner nuclear membrane whereas the complement of lamins is identical in both cell types. To gain insight into the properties of oocyte p58 we microinjected isolated nuclei of cultured rat cells into the cytoplasm of Xenopus oocytes. The oocyte p58 was detectable by immunofluorescence microscopy within 16-20 hours in the nuclear membrane of rat nuclei. Our data indicate that the peripheral chromatin but not lamins are required for the retention of p58 in the inner nuclear membrane. Sucrose step gradient centrifugation of total oocyte membranes revealed that the oocyte p58 was predominantly recovered in membrane fractions that did not contain lamins whereas membrane associated lamins and p58 of unfertilized eggs were found in the same fractions. By electron microscopical immunolocalizations one major population of meiotic p58 vesicles was identified that contained exclusively p58 and a second minor population (ca. 11% of p58 vesicles) contained in addition to p58 membrane bound B-type lamins. Egg vesicles containing pore membrane proteins were predominantly recovered in gradient fractions that did not contain p58 and B-type lamins. Our data indicate that the targeting of p58 to chromatin at the end of mitosis in the early Xenopus embryo is a process independent from that of lamin targeting. Comparable to the situation in oocytes and eggs, a significant proportion of p58 of interphase cells could be recovered in fractions that did not contain lamins. This population of p58 molecules could be extracted from A6-cells with buffers containing 1% Triton X-100/0.15 M NaCl and could be pelleted by a 50,000 g centrifugation. A- and B-type lamins were not detectable in the p58 containing pellet.

Nucleo-cytoplasmic interactions that control nuclear envelope breakdown and entry into mitosis in the sea urchin zygote

In sea urchin zygotes and mammalian cells nuclear envelope breakdown (NEB) is not driven simply by a rise in cytoplasmic cyclin dependent kinase 1-cyclin B (Cdk1-B) activity; the checkpoint monitoring DNA synthesis can prevent NEB in the face of mitotic levels of Cdk1-B. Using sea urchin zygotes we investigated whether this checkpoint prevents NEB by restricting import of regulatory proteins into the nucleus. We find that cyclin B1-GFP accumulates in nuclei that cannot complete DNA synthesis and do not break down. Thus, this checkpoint limits NEB downstream of both the cytoplasmic activation and nuclear accumulation of Cdk1-B1. In separate experiments we fertilize sea urchin eggs with sperm whose DNA has been covalently cross-linked to inhibit replication. When the pronuclei fuse, the resulting zygote nucleus does not break down for >180 minutes (equivalent to three cell cycles), even though Cdk1-B activity rises to greater than mitotic levels. If pronuclear fusion is prevented, then the female pronucleus breaks down at the normal time (average 68 minutes) and the male pronucleus with cross-linked DNA breaks down 16 minutes later. This male pronucleus has a functional checkpoint because it does not break down for >120 minutes if the female pronucleus is removed just prior to NEB. These results reveal the existence of an activity released by the female pronucleus upon its breakdown, that overrides the checkpoint in the male pronucleus and induces NEB. Microinjecting wheat germ agglutinin into binucleate zygotes reveals that this activity involves molecules that must be actively translocated into the male pronucleus.

Active transgenes in zebrafish are enriched in acetylated histone H4 and dynamically associate with RNA Pol II and splicing complexes

We have investigated the functional organization of active and silent integrated luciferase transgenes in zebrafish, with the aim of accounting for the variegation of transgene expression in this species. We demonstrate the enrichment of transcriptionally active transgenes in acetylated histone H4 and the dynamic association of the transgenes with splicing factor SC35 and RNA Pol II. Analysis of interphase nuclei and extended chromatin fibers by immunofluorescence and in situ hybridization reveals a co-localization of transgenes with acetylated H4 in luciferase-expressing animals only. Enrichment of expressed transgenes in acetylated H4 is further demonstrated by their co-precipitation from chromatin using anti-acetylated H4 antibodies. Little correlation exists, however, between the level of histone acetylation and the degree of transgene expression. In transgene-expressing zebrafish, most transgenes co-localize with Pol II and SC35, whereas no such association occurs in non-expressing individuals. Inhibition of Pol II abolishes transgene expression and disrupts association of transgenes with SC35, although inactivated transgenes remains enriched in acetylated histones. Exposure of embryos to the histone deacetylation inhibitor TSA induces expression of most silent transgenes. Chromatin containing activated transgenes becomes enriched in acetylated histones and the transgenes recruit SC35 and Pol II. The results demonstrate a correlation between H4 acetylation and transgene activity, and argue that active transgenes dynamically recruit splicing factors and Pol II. The data also suggest that dissociation of splicing factors from transgenes upon Pol II inhibition is not a consequence of changes in H4 acetylation.

Sequential PKC- and Cdc2-mediated phosphorylation events elicit zebrafish nuclear envelope disassembly

Molecular markers of the zebrafish inner nuclear membrane (NEP55) and nuclear lamina (L68) were identified, partially characterized and used to demonstrate that disassembly of the zebrafish nuclear envelope requires sequential phosphorylation events by first PKC, then Cdc2 kinase. NEP55 and L68 are immunologically and functionally related to human LAP2beta and lamin B, respectively. Exposure of zebrafish nuclei to meiotic cytosol elicits rapid phosphorylation of NEP55 and L68, and disassembly of both proteins. L68 phosphorylation is completely inhibited by simultaneous inhibition of Cdc2 and PKC and only partially blocked by inhibition of either kinase. NEP55 phosphorylation is completely prevented by inhibition or immunodepletion of cytosolic Cdc2. Inhibition of cAMP-dependent kinase, MEK or CaM kinase II does not affect NEP55 or L68 phosphorylation. In vitro, nuclear envelope disassembly requires phosphorylation of NEP55 and L68 by both mammalian PKC and Cdc2. Inhibition of either kinase is sufficient to abolish NE disassembly. Furthermore, novel two-step phosphorylation assays in cytosol and in vitro indicate that PKC-mediated phosphorylation of L68 prior to Cdc2-mediated phosphorylation of L68 and NEP55 is essential to elicit nuclear envelope breakdown. Phosphorylation elicited by Cdc2 prior to PKC prevents nuclear envelope disassembly even though NEP55 is phosphorylated. The results indicate that sequential phosphorylation events elicited by PKC, followed by Cdc2, are required for zebrafish nuclear disassembly. They also argue that phosphorylation of inner nuclear membrane integral proteins is not sufficient to promote nuclear envelope breakdown, and suggest a multiple-level regulation of disassembly of nuclear envelope components during meiosis and at mitosis.