Nuclei that lack a lamina accumulate karyophilic proteins and assemble a nuclear matrix

Insertional mutation of the Drosophila nuclear lamin Dm0 gene results in defective nuclear envelopes, clustering of nuclear pore complexes, and accumulation of annulate lamellae

Nuclear lamins are thought to play an important role in disassembly and reassembly of the nucleus during mitosis. Here, we describe a Drosophila lamin Dm0 mutant resulting from a P element insertion into the first intron of the Dm0 gene. Homozygous mutant animals showed a severe phenotype including retardation in development, reduced viability, sterility, and impaired locomotion. Immunocytochemical and ultrastructural analysis revealed that reduced lamin Dm0 expression caused an enrichment of nuclear pore complexes in cytoplasmic annulate lamellae and in nuclear envelope clusters. In several cells, particularly the densely packed somata of the central nervous system, defective nuclear envelopes were observed in addition. All aspects of the mutant phenotype were rescued upon P element-mediated germline transformation with a lamin Dm0 transgene. These data constitute the first genetic proof that lamins are essential for the structural organization of the cell nucleus.

Formation of the male pronuclear lamina in Drosophila melanogaster

Upon fertilization, a sperm nucleus reorganizes to become a male pronucleus. This reorganization includes breakdown and reformation of the nuclear envelope of the male pronucleus. In this study, we used a maternally encoded nuclear lamina protein, YA, in parallel with another lamina protein, lamin Dm, as probes to study the formation of the male pronuclear lamina in Drosophila melanogaster. Ectopically expressed YA is present in the nuclear envelopes of spermatocytes, but not in mature sperm, similar to endogenous lamin Dm. This suggests that the nuclear envelope of Drosophila sperm differs from that of somatic cells. Upon fertilization, YA and lamin Dm are recruited to the periphery of the male-derived nucleus before or during the early stages of migration by the male pronucleus. Using a paternal effect mutation, snky, we found that recruitment of lamina proteins to the male pronucleus requires, and probably accompanies, reorganization of the sperm nucleus. In order to identify factors that affect the recruitment of nuclear lamina proteins to the male pronucleus, we examined the subcellular localization of YA and lamin Dm in mutant embryos defective for the function of either the male pronucleus (mh, K81, and pal or both pronuclei (gnu, png, and plu). None of these mutations affect the recruitment of YA or lamin Dm to the male pronuclear envelope, suggesting that the mutations affect processes independent of, or after, reorganization of the nuclear envelope. Double mutant analyses between Ya and gnu suggest that YA plays a role in the nuclear envelope permissive for rounds of DNA replication.

Disruption of nuclear lamin organization alters the distribution of replication factors and inhibits DNA synthesis

The nuclear lamina is a fibrous structure that lies at the interface between the nuclear envelope and the nucleoplasm. The major proteins comprising the lamina, the nuclear lamins, are also found in foci in the nucleoplasm, distinct from the peripheral lamina. The nuclear lamins have been associated with a number of processes in the nucleus, including DNA replication. To further characterize the specific role of lamins in DNA replication, we have used a truncated human lamin as a dominant negative mutant to perturb lamin organization. This protein disrupts the lamin organization of nuclei when microinjected into mammalian cells and also disrupts the lamin organization of in vitro assembled nuclei when added to Xenopus laevis interphase egg extracts. In both cases, the lamina appears to be completely absent, and instead the endogenous lamins and the mutant lamin protein are found in nucleoplasmic aggregates. Coincident with the disruption of lamin organization, there is a dramatic reduction in DNA replication. As a consequence of this disruption, the distributions of PCNA and the large subunit of the RFC complex, proteins required for the elongation phase of DNA replication, are altered such that they are found within the intranucleoplasmic lamin aggregates. In contrast, the distribution of XMCM3, XORC2, and DNA polymerase alpha, proteins required for the initiation stage of DNA replication, remains unaltered. The data presented demonstrate that the nuclear lamins may be required for the elongation phase of DNA replication.

Enzymatic activities involved in the DNA resynthesis step of nucleotide excision repair are firmly attached to chromatin

In this study the role of nuclear architecture in nucleotide excision repair (NER) was investigated by gentle dismantling of the cell and probing the capability of chromatin to carry out repair in vitro. The rationale behind this approach is that compartmentalization of NER at nuclear structures would make the enzymatic activities refractory to extraction by buffers that solubilize cellular membranes. In order to obtain intact chromatin primary human fibroblasts were encapsulated in agarose microbeads and lysed in isotonic buffers containing the non-ionic detergent Triton X-100. Under these conditions the majority of cellular proteins diffuse out of the beads, but the remaining chromatin is able to replicate and to transcribe DNA in the presence of triphosphates and Mg2+. UV irradiation of confluent repair-proficient human fibroblasts prior to lysis stimulated the incorporation of deoxynucleotide triphosphates in Triton X-100-isolated chromatin, even under stringent lysis conditions. In addition, experiments with UV-sensitive xeroderma pigmentosum (complementation groups A and C) and Cockayne's syndrome fibroblasts (complementation group A) revealed that this repair synthesis was due to global genome repair activity. Transcription-coupled repair was only detectable in cells permeabilized by streptolysin O (SLO). Repair synthesis in Triton X-100-isolated chromatin amounted to 15% of the total repair synthesis as measured in SLO-permeabilized cells. To allow the detection of these activities in vitro, presynthesis complexes have to be formed in intact cells, indicating that chromatin from Triton X-100-lysed cells is unable to initiate NER in vitro. Our data indicate that the components involved in the resynthesis step of NER are tightly associated with chromatin. A substantial fraction of total proliferating cell nuclear antigen (PCNA), which is required for the resynthesis step in NER, has been reported to become Triton X-100 non-extractable and tightly associated with nuclear structures after UV irradiation of cells. We propose that Triton X-100-resistant repair synthesis might be mediated by this chromatin-bound fraction of total PCNA.

Nuclear lamina and nuclear matrix organization in sperm pronuclei assembled in Xenopus egg extract

Nuclear lamina and matrices were prepared from sperm pronuclei assembled in Xenopus egg extracts using a fractionation and extraction procedure. Indirect immunofluorescence revealed that while chromatin was efficiently removed from nuclei during the extraction procedure, the distribution of lamins was unaffected. Consistent with this data, the amount of lamin B3, determined by immunoblotting, was not affected through the extraction procedure. Nuclear matrices were visualised in DGD sections by TEM. Within these sections filaments were observed both at the boundary of the nucleus (the lamina) and within the body of the nucleus (internal nuclear matrix filaments). To improve resolution, nuclear matrices were also prepared as whole mounts and viewed using field emission in lens scanning electron microscopy (FEISEM). This technique revealed two distinct networks of filaments. Filaments lying at the surface of nuclear matrices interconnected nuclear pores. These filaments were readily labelled with monoclonal anti-lamin B3 antibodies. Filaments lying within the body of the nuclear matrix were highly branched but were not readily labelled with antilamin B3 antibodies. Nuclear matrices were also prepared from sperm pronuclei assembled in lamin B3 depleted extracts. Using FEISEM, filaments were also detected in these preparations. However, these filaments were poorly organised and often appeared to aggregate. To confirm these results nuclear matrices were also observed as whole mounts using TEM. Nuclear matrices prepared from control nuclei contained a dense array of interconnected filaments. Many (but not all) of these filaments were labelled with anti-lamin B3 antibodies. In contrast, nuclear matrices prepared from "lamin depleted nuclei' contained poorly organised or aggregated filaments which were not specifically labelled with anti-lamin B3 antibodies.

The dynamic properties and possible functions of nuclear lamins

The nuclear lamins are thought to form a thin fibrous layer called the nuclear lamina, underlying the inner nuclear envelope membrane. In this review, we summarize data on the dynamic properties of nuclear lamins during the cell cycle and during development. We discuss the implications of dynamics for lamin functions. The lamins may be involved in DNA replication, chromatin organization, differentiation, nuclear structural support, and nuclear envelope reassembly. Emphasis is placed on recent data that indicate that the lamina, contrary to previous views, is not a static structure. For example, the lamins form nucleoplasmic foci, distinct from the peripheral lamina, which vary in their patterns of distribution as well as their composition in a cell cycle-dependent manner. During the S phase, these foci colocalize with chromatin and sites of DNA replication. At other points during the cell cycle, they may represent sites of lamin post-translation processing that take place prior to incorporation into the lamina. Secondary modifications of the lamins such as isoprenylation and phosphorylation are involved in the regulation of the dynamic properties and the assembly of lamins. In addition, a number of lamin-associated proteins have been recently identified and these are described along with their potential functions.

The structural basis of nuclear function

Most models for transcription and replication involve polymerases that track along the template. We review here experiments that suggest an alternative in which polymerization occurs as the template slides past a polymerase fixed to a large structure in the eukaryotic nucleus--a "factory" attached to a nucleoskeleton. This means that higher-order structure dictates how and when DNA is replicated or transcribed.

Origins of replication and the nuclear matrix: the DHFR domain as a paradigm

The eukaryotic genome appears to be organized in a loopwise fashion by periodic attachment to the nuclear matrix. The proposal that a chromatin loop corresponds to a functional domain has stirred interest in the properties of the DNA sequences at the bases of these loops, the matrix-attached regions (MARs). Evidence has been presented suggesting that certain MARs act as boundary elements isolating domains from their chromosomal context. MARs have also been found in the vicinity of promoters and enhancers and they could act by displacing these cis-regulatory elements into the proper nuclear subcompartment. Attachment to the matrix might also play a role in DNA replication. A large body of evidence indicates that replication occurs on the nuclear matrix. This implies that any DNA sequence will be attached to the matrix at a certain time during the cell cycle. This transient mode of attachment contrasts with the proposed permanent attachment of origins of DNA replication with the nuclear matrix. While some data exist that support this suggestion, the current lack of understanding of the mammalian replication origin precludes definitive conclusions regarding the role of MARs in the initiation process.

S-phase phosphorylation of lamin B2

Lamin B2 modification in synchronously dividing populations of human diploid fibroblasts was determined by 2-dimensional gel electrophoresis and [32P]orthophosphate labelling. In quiescent (G0) and G1 cultures of HDF, lamin B2 migrated as 2 spots on 2-dimensional gels. In contrast, in S-phase populations of HDF lamin B2 migrated as a single basic species. The level of lamin B2 phosphorylation was determined after immunoisolation from [32P]orthophosphate labelled cells. The results of these experiments indicated a 2-3-fold increase in the steady state level of lamin B2 phosphorylation in S-phase HDF compared with G0 HDF. Consistent with this evidence, tryptic peptide maps revealed the presence of a phosphopeptide in S-phase lamin B2 which was absent from G0 lamin B2. Since all of the phosphate incorporated into S-phase and G0 lamin B2 was recovered in serine residues we conclude that the S-phase specific phosphopeptide did not represent either of the cdc2 sites associated with entry nuclear lamina breakdown.

Xenopus lamin B3 has a direct role in the assembly of a replication competent nucleus: evidence from cell-free egg extracts

Xenopus egg extracts which assemble replication competent nuclei in vitro were depleted of lamin B3 using monoclonal antibody L6 5D5 linked to paramagnetic beads. After depletion, the extracts were still capable of assembling nuclei around demembranated sperm heads. Using field emission in lens scanning electron microscopy (FEISEM) we show that most nuclei assembled in lamin B3-depleted extracts have continuous nuclear envelopes and well formed nuclear pores. However, several consistent differences were observed. Most nuclei were small and only attained diameters which were half the size of controls. In a small number of nuclei, nuclear pore baskets, normally present on the inner aspect of the nuclear envelope, appeared on its outer surface. Finally, the assembly of nuclear pores was slower in lamin B3-depleted extracts, indicating a slower overall rate of nuclear envelope assembly. The results of FEISEM were confirmed using conventional TEM thin sections, where again the majority of nuclei assembled in lamin B3-depleted extracts had well formed double unit membranes containing a high density of nuclear pores. Since nuclear envelope assembly was mostly normal but slow in these nuclei, the lamin content of 'depleted' extracts was investigated. While lamin B3 was recovered efficiently from cytosolic and membrane fractions by our procedure, a second minor lamin isoform, which has characteristics similar to those of the somatic lamin B2, remained in the extract. Thus it is likely that this lamin is necessary for nuclear envelope assembly. However, while lamin B2 did not co-precipitate with lamin B3 during immunodepletion experiments, several protein species did specifically associate with lamin B3 on paramagnetic immunobeads. The major protein species associated with lamin B3 migrated with molecular masses of 102 kDa and 57 kDa, respectively, on one-dimensional polyacrylamide gels. On two-dimensional O'Farrell gels the mobility of the 102 kDa protein was identical to the mobility of a major nuclear matrix protein, indicating a specific association between lamin B3 and other nuclear matrix proteins. Nuclei assembled in lamin B3-depleted extracts did not assemble a lamina, judged by indirect immunofluorescence, and failed to initiate semi-conservative DNA replication. However, by reinoculating depleted extracts with purified lamin B3, nuclear lamina assembly and DNA replication could both be rescued. Thus it seems likely that the inability of lamin-depleted extracts to assemble a replication competent nucleus is a direct consequence of a failure to assemble a lamina.

Both cdc2 and cdk2 promote S phase initiation in Xenopus egg extracts

Xenopus egg extracts induce S phase DNA replication in added sperm pronuclei in a highly regulated manner, similar to events in vivo. Removal of cyclin-dependant kinases (cdks) or cdk2 from these extracts using affinity matrices severely inhibits initiation of S phase. We have used p13suc1 beads to remove both cdk2 and cdc2 proteins from egg extracts and developed a method to replace either protein alone to assess their capacity to initiate DNA replication. Re-addition of either cdk2 or cdc2 proteins to depleted extracts, through translation of their respective mRNAs, restimulated replication, judged by both total synthesis and labelling index. An ATP-binding-site mutant cdk2 mRNA (cdk2.R33) failed to stimulate replication and inhibited S phase initiation in mock-depleted extracts. Both human and Xenopus cdc2 mRNAs rescued replication in this system. Human mutant mRNAs have been used to show that the stimulation induced requires cdc2 catalytic activity, though not its mitotically active form. Rescue of replication by p34cdc2 is also observed in extracts depleted of cdks with a cdk2 antibody, which still retain much of their endogenous cdc2 protein. We conclude that newly synthesised p34cdc2, but not the inherited ‘old’ form, can induce S phase and in this form may overlap in function with p33cdk2.

Lamin proteins form an internal nucleoskeleton as well as a peripheral lamina in human cells

The nuclear lamina forms a protein mesh that underlies the nuclear membrane. In most mammalian cells it contains the intermediate filament proteins, lamins A, B and C. As their name indicates, lamins are generally thought to be confined to the nuclear periphery. We now show that they also form part of a diffuse skeleton that ramifies throughout the interior of the nucleus. Unlike their peripheral counterparts, these internal lamins are buried in dense chromatin and so are inaccessible to antibodies, but accessibility can be increased by removing chromatin. Knobs and nodes on an internal skeleton can then be immunolabelled using fluorescein- or gold-conjugated anti-lamin A antibodies. These results suggest that the lamins are misnamed as they are also found internally.

The role of protein phosphorylation in the assembly of a replication competent nucleus: investigations in Xenopus egg extracts using the cyanobacterial toxin microcystin-LR

Cell-free extracts of Xenopus eggs support nuclear assembly and DNA replication in vitro. Extracts supplemented with the protein phosphatase inhibitor microcystin-LR displayed various inhibitory effects at different concentrations of the toxin. In the presence of cycloheximide, additions of microcystin did not induce histone H1-kinase activity. Nevertheless, increasing concentrations of microcystin did sequentially prevent DNA replication, nuclear lamina assembly and nuclear envelope assembly. DNA replication was prevented when microcystin was added at 250 nM. Furthermore, this effect could be reversed after the addition of the catalytic sub-unit of protein phosphatase 2A to inhibited extracts. At a concentration of 250 nM microcystin, nuclear membrane assembly, nuclear lamina assembly and nuclear transport all occurred in egg extracts. In addition single-stranded M13 DNA replication was also permitted. However, it appeared that replicase assembly was not completed, since nuclei assembled in microcystin-treated extracts displayed an unusual distribution of proliferating cell nuclear antigen (PCNA). Although PCNA was located at sites that resembled pre-replication foci, this nuclear protein was readily solubilised when nuclei were isolated and extracted sequentially with Triton, nucleases and salts. Despite this, nuclei containing pre-assembled replication forks could synthesise DNA when transferred into microcystin-treated extracts.

Local and global changes in the morphology and distribution of replication centres in rapidly expanding nuclei

Sperm pronuclei assembled in Xenopus egg extracts undergo a 10-fold increase in volume over a typical 150-min incubation. Double immunofluorescence with anti-proliferating cell nuclear antigen (PCNA) antibodies and biotinylated anologues of thymidine can be used to distinguish different phases of S-phase. The mean diameter of pronuclei at initiation was 12.42 microns. During the elongation phase of S-phase the mean diameter increased to 17.44 microns, equal to a sevenfold increase in nuclear volume. Confocal microscopy was used to investigate the morphology and distribution of replication centres through S-phase. In early S-phase nuclei, the mean centre-to-centre distance between any replication centre and its 10 nearest neighbours was 1.24 microns; this decreased significantly to 0.90 micron in mid S-phase nuclei and 0.99 micron in late S-phase nuclei. In addition, the morphology of replication foci also changed. In early S-phase nuclei, each replication focus was a discrete entity. However, as S-phase progressed neighbouring replication foci appeared to fuse giving rise to ring-like structures or string-like structures. Three-dimensional reconstructed images of z-series collected through early and late S-phase nuclei indicated that as nuclei enlarge replication centres appear to accumulate at the nuclear periphery. A model is proposed to accommodate the results described below.

Weaving a pattern from disparate threads: lamin function in nuclear assembly and DNA replication

The major residual structure that remains associated with the nuclear envelope following extraction of isolated nuclei or oocyte germinal vesicles with non-ionic detergents, nucleases and high salt is the lamina (Fawcett, 1966; Aaronson and Blobel, 1975; Dwyer and Blobel, 1976). The nuclear lamina is composed of intermediate filament proteins, termed lamins (Gerace and Blobel, 1980; Shelton et al., 1980), which polymerise to form a basket-weave lattice of fibrils, which covers the entire inner surface of the nuclear envelope and interlinks nuclear pores (Aebi et al., 1986; Stewart and Whytock, 1988; Goldberg and Allen, 1992). At mitosis, the nuclear envelope and the lamina both break down to allow chromosome segregation. As a consequence, each structure has to be rebuilt during anaphase and telophase, allowing cells an opportunity to reposition chromosomes (Heslop-Harrison and Bennett, 1990) and to reorganise looped chromatin domains (Franke, 1974; Franke et al., 1981; Hochstrasser et al., 1986), which may in turn control the use of subsets of genes. Because of the position that it occupies, its dynamics during mitosis and the fact that it is an essential component of proliferating cells, the lamina has been assigned a number of putative roles both in nuclear metabolism and in nuclear envelope assembly (Burke and Gerace, 1986; Nigg, 1989). However, to date there is little clear cut evidence that satisfactorily explains the function of the lamina in relation to its structure. In this Commentary we will describe some of the recent work that addresses this problem and attempt to provide a unified model for the role of lamins in nuclear envelope assembly and for the lamina in the initiation of DNA replication.