Experimental disintegration of the nuclear envelope. Evidence for pore-connecting fibrils

Real-Time Cellular Thermal Shift Assay to Monitor Target Engagement

Determining a molecule's mechanism of action is paramount during chemical probe development and drug discovery. The cellular thermal shift assay (CETSA) is a valuable tool to confirm target engagement in cells for a small molecule that demonstrates a pharmacological effect. CETSA directly detects biophysical interactions between ligands and protein targets, which can alter a protein's unfolding and aggregation properties in response to thermal challenge. In traditional CETSA experiments, each temperature requires an individual sample, which restricts throughput and requires substantial optimization. To capture the full aggregation profile of a protein from a single sample, we developed a prototype real-time CETSA (RT-CETSA) platform by coupling a real-time PCR instrument with a CCD camera to detect luminescence. A thermally stable Nanoluciferase variant (ThermLuc) was bioengineered to withstand unfolding at temperatures greater than 90 °C and was compatible with monitoring target engagement events when fused to diverse targets. Utilizing well-characterized inhibitors of lactate dehydrogenase alpha, RT-CETSA showed significant correlation with enzymatic, biophysical, and other cell-based assays. A data analysis pipeline was developed to enhance the sensitivity of RT-CETSA to detect on-target binding. RT-CETSA technology advances capabilities of the CETSA method and facilitates the identification of ligand-target engagement in cells, a critical step in assessing the mechanism of action of a small molecule.

Computational analyses reveal spatial relationships between nuclear pore complexes and specific lamins

Nuclear lamin isoforms form fibrous meshworks associated with nuclear pore complexes (NPCs). Using datasets prepared from subpixel and segmentation analyses of 3D-structured illumination microscopy images of WT and lamin isoform knockout mouse embryo fibroblasts, we determined with high precision the spatial association of NPCs with specific lamin isoform fibers. These relationships are retained in the enlarged lamin meshworks of Lmna-/- and Lmnb1-/- fibroblast nuclei. Cryo-ET observations reveal that the lamin filaments composing the fibers contact the nucleoplasmic ring of NPCs. Knockdown of the ring-associated nucleoporin ELYS induces NPC clusters that exclude lamin A/C fibers but include LB1 and LB2 fibers. Knockdown of the nucleoporin TPR or NUP153 alters the arrangement of lamin fibers and NPCs. Evidence that the number of NPCs is regulated by specific lamin isoforms is presented. Overall the results demonstrate that lamin isoforms and nucleoporins act together to maintain the normal organization of lamin meshworks and NPCs within the nuclear envelope.

The Quest for the Blueprint of the Nuclear Pore Complex

During my postdoc interview in June of 1998, I asked Günter why he was moving more towards the nucleus in his latest studies. He said, "Well Joe, that's where everything starts." By the end of the interview, I accepted the postdoc. He had a way of making everything sound so cool. Günter's progression was natural, since the endoplasmic reticulum and the nucleus are the only organelles that share the same membrane. The nuclear envelope extends into a double membrane system with nuclear pore complexes embedded in the pore membrane openings. Even while writing this review, I remember Günter stressing; it is the nuclear pore complex. Just saying nuclear pore doesn't encompass the full magnitude of its significance. The nuclear pore complex is one of the largest collection of proteins that fit together for an overall function: transport. This review will cover the Blobel lab contributions in the quest for the blueprint of the nuclear pore complex from isolation of the nuclear envelope and nuclear lamin to the ring structures.

The intriguing plant nuclear lamina

The nuclear lamina is a complex protein mesh attached to the inner nuclear membrane (INM), which is also associated with nuclear pore complexes. It provides mechanical support to the nucleus and nuclear envelope, and as well as facilitating the connection of the nucleoskeleton to the cytoskeleton, it is also involved in chromatin organization, gene regulation, and signaling. In metazoans, the nuclear lamina consists of a polymeric layer of lamins and other interacting proteins responsible for its association with the INM and chromatin. In plants, field emission scanning electron microscopy of nuclei, and thin section transmission electron microscopy of isolated nucleoskeletons, reveals the lamina to have a similar structure to that of metazoans. Moreover, although plants lack lamin genes and the genes encoding most lamin-binding proteins, the main functions of the lamina are fulfilled in plants. Hence, it would appear that the plant lamina is not based on lamins and that other proteins substitute for lamins in plant cells. The nuclear matrix constituent proteins are the best characterized structural proteins in the plant lamina. Although these proteins do not display strong sequence similarity to lamins, their predicted secondary structure and sub-nuclear distribution, as well as their influence on nuclear size and shape, and on heterochromatin organization, suggest they could be functional lamin analogs. In this review we shall summarize what is currently known about the organization and composition of the plant nuclear lamina and its interacting complexes, and we will discuss the activity of this structure in the plant cell and its nucleus.

Nuclear lamina at the crossroads of the cytoplasm and nucleus

The nuclear lamina is a protein meshwork that lines the nuclear envelope in metazoan cells. It is composed largely of a polymeric assembly of lamins, which comprise a distinct sequence homology class of the intermediate filament protein family. On the basis of its structural properties, the lamina originally was proposed to provide scaffolding for the nuclear envelope and to promote anchoring of chromatin and nuclear pore complexes at the nuclear surface. This viewpoint has expanded greatly during the past 25 years, with a host of surprising new insights on lamina structure, molecular composition and functional attributes. It has been established that the self-assembly properties of lamins are very similar to those of cytoplasmic intermediate filament proteins, and that the lamin polymer is physically associated with components of the cytoplasmic cytoskeleton and with a multitude of chromatin and inner nuclear membrane proteins. Cumulative evidence points to an important role for the lamina in regulating signaling and gene activity, and in mechanically coupling the cytoplasmic cytoskeleton to the nucleus. The significance of the lamina has been vaulted to the forefront by the discovery that mutations in lamins and lamina-associated polypeptides lead to an array of human diseases. A key future challenge is to understand how the lamina integrates pathways for mechanics and signaling at the molecular level. Understanding the structure of the lamina from the atomic to supramolecular levels will be essential for achieving this goal.

Invertebrate lamins

Lamins are the main component of the nuclear lamina and considered to be the ancestors of all intermediate filament proteins. They are localized mainly at the nuclear periphery where they form protein complexes with integral proteins of the nuclear inner membrane, transcriptional regulators, histones and chromatin modifiers. Studying lamins in invertebrate species has unique advantages including the smaller number of lamin genes in the invertebrate genomes and powerful genetic analyses in Caenorhabditis elegans and Drosophila melanogaster. These simpler nuclear lamina systems allow direct analyses of their structure and functions. Here we give an overview of recent advances in the field of invertebrate nuclear lamins with special emphasis on their evolution, assembly and functions.

Nuclear envelope, nuclear lamina, and inherited disease

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. In recent years, mutations in nuclear-envelope proteins have been shown to cause a surprisingly wide array of inherited diseases. While the mutant proteins are generally expressed in most or all differentiated somatic cells, many mutations cause fairly tissue-specific disorders. Perhaps the most dramatic case is that of mutations in A-type lamins, intermediate filament proteins associated with the inner nuclear membrane. Different mutations in the same lamin proteins have been shown to cause striated muscle diseases, partial lipodystrophy syndromes, a peripheral neuropathy, and disorders with features of severe premature aging. In this review, we summarize fundamental aspects of nuclear envelope structure and function, the inherited diseases caused by mutations in lamins and other nuclear envelope proteins, and possible pathogenic mechanisms.

Viscoelastic properties of the cell nucleus

Mechanical factors play an important role in the regulation of cell physiology. One pathway by which mechanical stress may influence gene expression is through a direct physical connection from the extracellular matrix across the plasma membrane and to the nucleus. However, little is known of the mechanical properties or deformation behavior of the nucleus. The goal of this study was to quantify the viscoelastic properties of mechanically and chemically isolated nuclei of articular chondrocytes using micropipet aspiration in conjunction theoretical viscoelastic model. Isolated nuclei behaved as viscoelastic solid materials similar to the cytoplasm, but were 3-4 times stiffer and nearly twice as viscous as the cytoplasm. Quantitative information of the biophysical properties and deformation behavior of the nucleus may provide further insight on the relationships between the stress-strain state of the nucleus and that of the extracellular matrix, as well as potential mechanisms of mechanical signal transduction.

Assembly of Drosophila lamin Dm0 and C mutant proteins studied with the baculovirus system

Despite extensive knowledge of the in vitro polymerization properties of nuclear lamins, it is still not well understood how the nuclear lamina assembles in vivo. To learn more about the relationship between in vitro and in vivo polymerization of nuclear lamins, we expressed Drosophila lamin Dm0, mutant proteins, having well defined alterations of their in vitro polymerization properties, in Sf9 cells using the baculovirus system. All lamin Dm0 mutants assembled into fibrillar aggregates indistinguishable in morphology from those assembled by the wild-type protein. However, in contrast to wild-type lamin Dm0, mutant proteins were extracted with buffers of physiological ionic strength and pH containing Triton X-100. These results indicate that various types of lamin dimer-dimer interactions can be disrupted without affecting the morphology of the lamin Dm0 polymer. However, all types of dimer-dimer interactions tested appear to be important for full polymer stability. In addition, we analyzed the polymer formation of two Drosophila lamin C mutants and found that a segment in the carboxy-terminal tail domain is required for assembly of lamin C paracrystals at the nuclear lamina.

Dynamics of nuclear pore distribution in nucleoporin mutant yeast cells

To follow the dynamics of nuclear pore distribution in living yeast cells, we have generated fusion proteins between the green fluorescent protein (GFP) and the yeast nucleoporins Nup49p and Nup133p. In nup133- dividing cells that display a constitutive nuclear pore clustering, in vivo analysis of GFP-Nup49p localization revealed changes in the distribution of nuclear pore complex (NPC) clusters. Furthermore, upon induction of Nup133p expression in a GAL-nup133 strain, a progressive fragmentation of the NPC aggregates was observed that in turn led to a wild-type nuclear pore distribution. To try to uncouple Nup133p-induced NPC redistribution from successive nuclear divisions and nuclear pore biogenesis, we devised an assay based on the formation of heterokaryons between nup133- mutants and cells either expressing or overexpressing Nup133p. Under these conditions, the use of GFP-Nup133p and GFP-Nup49p fusion proteins revealed that Nup133p can be rapidly targeted to the clustered nuclear pores, where its amino-terminal domain is required to promote the redistribution of preexisting NPCs.

Immunolocalization of lamins and nuclear pore complex proteins by atomic force microscopy

The nuclear envelope functions as a selective barrier separating the nuclear from the cytosolic compartment. Nuclear pore complexes (NPCs) mediate nuclear import and export of macromolecules and, therefore, are potential regulators of gene expression. In this study we applied atomic force microscopy (AFM) to visualize the three dimensional (3D) structure of individual NPCs in the absence and presence of two different antibodies, one directed against a pore protein (gp62) and another directed against Xenopus lamin LIII, a component of the nuclear lamina, a filament meshwork localized on the nucleoplasmic side of the nuclear envelope (NE) adjacent to and interacting with NPCs. Using 12-nm gold-labelled secondary antibodies and transmission electron microscopy we could clearly localize the primary single anti-gp62 antibody on NPCs and the primary single anti-LIII antibody between NPCs. Using AFM, the secondary antibodies against anti-gp62 could be detected as particles 7 nm in height on the nucleoplasmic face of NPCs. The secondary antibodies against anti-LIII could be clearly identified between NPCs. The secondary antibodies, attached to a 12-nm colloidal gold particle and visualized on glass, revealed similar shapes and heights as found on NEs. According to the 3D images, the volume of a single gold particle conjugated with secondary antibodies was 10203 nm3. This volume is equivalent to the volume of 38 IgG molecules associated with one individual gold particle. A similar volume of 11987 nm3 was calculated from a model assuming that the 150-kDa IgG molecules perfectly cover the spherical gold particle. We conclude that AFM can be used for identifying antibodies or other macromolecules associated with biomembranes.

Intranuclear filaments containing a nuclear pore complex protein

Nuclear pore complexes (NPCs) are anchoring sites of intranuclear filaments of 3-6 nm diameter that are coaxially arranged on the perimeter of a cylinder and project into the nuclear interior for lengths varying in different kinds of cells. Using a specific monoclonal antibody we have found that a polypeptide of approximately 190 kD on SDS-PAGE, which appears to be identical to the recently described NPC protein "nup 153," is a general constituent of these intranuclear NPC-attached filaments in different types of cells from diverse species, including amphibian oocytes where these filaments are abundant and can be relatively long. We have further observed that during mitosis this filament protein transiently disassembles, resulting in a distinct soluble molecular entity of approximately 12.5 S, and then disperses over most of the cytoplasm. Similarly, the amphibian oocyte protein appears in a soluble form of approximately 16 S during meiotic metaphase and can be immunoprecipitated from egg cytoplasmic supernatants. We conclude that this NPC protein can assemble into a filamentous form at considerable distance from the nuclear envelope and discuss possible functions of these NPC-attached filaments, from a role as guidance structure involved in nucleocytoplasmic transport to a form of excess storage of NPC proteins in oocytes.

Characterization of nuclear pore distribution in freeze-fracture replicas of seminiferous tubules isolated by transillumination

Transilluminated seminiferous tubules were staged and utilized to determine the distribution of nuclear pore complexes in seminiferous tubules of the rat. Segments of seminiferous tubules of adult albino rats were separated and identified (in stages VII-VIII, IX-XI, XII-XIV, and V-VI), and then processed by freeze-fracture. Type A spermatogonia, the only spermatogonia located in seminiferous segments possessing stages IX-XI and XII-XIV, are oval cells in contact with the basal lamina. They either exhibit a random distribution of nuclear pores or a slight degree of clumping. Type B spermatogonia, found in segments possessing stages V-VI, exhibit, instead, a noticeable pore clustering. The identification of intermediate spermatogonia was not undertaken in this study. Preleptotene spermatocytes are easily identified in freeze-fracture by their location in segments with stages VII-VIII, by their arrangement in numerous groups between the basal lamina and the pachytene spermatocytes, and by their comparatively small size. They exhibit noticeable pore clustering. Leptotene (segments containing stages IX-XI) and zygotene (XII-XIV) spermatocytes show a more homogeneous distribution of nuclear pores. Pachytene spermatocytes are identified by their large size, by consistent detachment from the basal lamina and by being rather numerous and found in all the stages explored. Diplotene spermatocytes have the largest nuclei of all germ cells. They are always detached from the basal lamina and found only in seminiferous segments containing stage XIII. Pachytenes display a regular geometric array of pore aggregation with striking clustering, whereas diplotene nuclear pores takes on a random distribution. Secondary spermatocytes, only present in stage XIV intermingled with metaphase-anaphase profiles, are characterized in replicas by a paucity of evenly distributed nuclear pores.

Genesis of hadacidin-induced cleft palate in hamster: morphogenesis, electron microscopy, and determination of DNA synthesis, cAMP, and enzyme acid phosphatase

A morphological, electron microscopic, and biochemical study was undertaken to analyze the genesis of hadacidin-induced cleft palate in hamster fetuses. Gross and light microscopic observations indicated that hadacidin affected the growth of vertical palatal shelves to induce cleft palate. Electron microscopic observations showed that initial hadacidin-induced changes were seen in the mesenchymal cells. Within 12 hr of drug administration, the perinuclear space was swollen and a lysosomal response injury was evident in the mesenchymal cells. Subsequently, 24 hr after hadacidin treatment, lysosomes appeared in the epithelial cells; changes were also seen in the basal lamina which included separation of the lamina densa from the basal cells, duplication of lamina densa, and complete loss of basal lamina. Between 36 and 42 hr post-treatment, the cellular and basal lamina changes subsided, and the epithelium of vertical shelves underwent stratification. Biochemical determination of enzyme acid phosphatase indicated that the levels of enzyme activity in both the control and treated palatal tissues corresponded to the appearance of lysosomes. Measurement of cAMP levels suggested that the peak activity of cAMP corresponded to that of enzyme acid phosphatase and cell injury. The cAMP activity in hadacidin-injured cells, however, was significantly lower in comparison to that of the dying cells of control palates. Hadacidin treatment also affected DNA synthesis in the developing primordia of the palate. It was suggested that hadacidin injures the precursor cells of the palate prior to the appearance of the primordia, and subsequently affects their proliferative behavior, stunting the vertical growth of the palatal shelves and inducing a cleft palate.

Pathogenesis of bromodeoxyuridine-induced cleft palate in hamster

In the present study, the morphological, histochemical, biochemical, and cellular aspects of the pathogenesis of bromodeoxyuridine (BrdU)-induced cleft palate in hamster fetuses were analyzed. Morphological observations indicated that BrdU interferes with the growth of the vertical shelves and thus induces cleft palate. At an ultrastructural level, BrdU-induced changes were first seen in the mesenchymal cells. Eighteen hours after drug administration, the initial alterations were characterized by swelling of the nuclear membrane and the appearance of lysosomes in the mesenchymal cells of the roof of the oronasal cavity. During the next 6 hr, as the palatal primordia developed, lysosomes were also seen in the overlying epithelial cells. The appearance of lysosomal activity, which was verified by acid phosphatase histochemistry, was temporally abnormal and was interpreted as a sublethal response to BrdU treatment. Later the cellular alterations subsided; 48 hr after BrdU treatment, they were absent in both the epithelial and mesenchymal cells of the vertically developing palatal shelves. Subsequently, unlike controls (in which the palatal shelves undergo reorientation and fusion), the BrdU-treated shelves remained vertical until term. Biochemical determination of DNA synthesis indicated that although there was an inhibition of DNA synthesis at the time of appearance of palatal primordia, a catch-up growth during the ensuing 12 hr may have restored the number of cells available for the formation of a vertical palatal shelf. It was suggested that BrdU affected cytodifferentiation in the palatal tissues during the critical phase of early vertical development to induce a cleft palate.

On the cell-free association of lamins A and C with metaphase chromosomes

Nuclear envelopes have previously been shown to assemble spontaneously around endogenous chromosomes in cell-free homogenates of mitotic Chinese hamster ovary cells. In order to further analyze the mechanisms underlying nuclear envelope reformation and the functions of the individual nuclear lamin polypeptides, a fractionated cell-free nuclear envelope reassembly system involving purified chromosomes and either a postchromosomal supernatant or a cytosol fraction from mitotic cells has been devised. Results obtained with this fractionated system show that lamins A and C will associate with the surfaces of chromosomes in the absence of lamin B and membranes, this association being inhibitable by ATP-gamma-S. However, in the absence of membranes chromatin decondensation never occurs. Using the reversible swelling of chromosomes in low ionic strength buffers lacking divalent cations as the basis of a simple assay, it is demonstrated that the association of lamins A and C with the surfaces of chromosomes has a pronounced and easily observable effect on chromatin organization.

Specific disruption of intermediate filaments and the nuclear lamina by the 19-kDa product of the adenovirus E1B oncogene

The 19-kDa protein encoded within the adenovirus E1B gene is essential for transformation by adenovirus and for proper regulation of viral early gene transcription. In order to investigate the biological function of the 19-kDa E1B protein, vectors were constructed to produce the 19-kDa protein in mammalian cells under the direction of heterologous promoters. Surprisingly, during transient expression, the E1B 19-kDa protein specifically associated with and disrupted the organization of intermediate filaments and the nuclear lamina, without disturbing the organization of other cytoskeletal networks. These results directly demonstrate an effect of a viral transforming protein on the cytoskeleton and suggest a role for intermediate filaments and the nuclear lamina in modulation of viral gene expression and the process of oncogenic transformation.

The conserved carboxy-terminal cysteine of nuclear lamins is essential for lamin association with the nuclear envelope

We have analyzed the interaction of soluble nuclear lamins with the nuclear envelope by microinjection of normal and mutated lamins into the cytoplasm of Xenopus laevis oocytes. Our results demonstrate that the conserved cysteine of the carboxy-terminal tetrapeptide Cys Ala/Ser Ile Met of lamins is essential for their association with the nuclear envelope. Removal of this sequence or replacement of the cysteine by serine resulted in Xenopus lamin L1 remaining in a soluble, non-envelope-associated state within the nucleus. Similar mutations of Xenopus lamin A resulted in only partial reduction of nuclear envelope association, indicating that lamin A contains additional signals that can partially compensate for the lack of the cysteine. Mammalian lamin C lacks this tetrapeptide and is not associated with the nuclear envelope in our experimental system. Cloning of the tetrapeptide Cys Ala Ile Met to the carboxy terminus of human lamin C resulted in lamin being found in a nuclear envelope-associated form in oocytes. Mutations at the amino terminus and in the alpha-helical region of lamin L1 revealed that the carboxy terminus mediates the association of lamins with the nuclear envelope; however, this alone is insufficient for maintenance of a stable association with the nuclear envelope.

Organization of the nuclear pore complex in Saccharomyces cerevisiae

Fractions enriched for nuclear pore complexes (NPCs) have been isolated from Saccharomyces cerevisiae. The sequential extraction of nuclei with detergent, nucleases, and salt reveals an organization of the yeast NPC similar to other eukaryotes. Yeast NPCs contain a 30-nm "ring" structure not previously described in other organisms. This structure appears to organize 10-nm filaments into an assembly which exhibits an eight-fold rotational symmetry. Some proteins in the NPC fraction are capable of forming intermediate-sized filaments. These studies suggest that some component of the nuclear pore complex organizes an interaction between nuclear and cytoplasmic networks of intermediate filaments.

Cellular carbohydrate components in human, rabbit and rat lacrimal gland. Studies using fluorescein and peroxidase labelled lectins

Orbital lacrimal glands from adult male and female rabbits, rats and humans were examined for the presence of intracellular receptors of four lectins: concanavalin-A agglutinin, lutus tetragonolobus agglutinin, ricinus comunis-60 agglutinin and wheat-germ agglutinin using fluorescein-conjugated lectin and peroxidase labelling methods for fluorescence and electron microscopy, respectively. Lectins were used as specific probes to detect carbohydrate moiety of the lacrimal gland. The pattern of labelling with the lectins suggests that N-acetyl-glucosamine, N-acetyl-D-galactosamine, D-galactose, D-mannose, sialic acid and L-fucose are contained in the lacrimal gland of the three species. The significance of these findings is discussed.

Incorporation of a product of mevalonic acid metabolism into proteins of Chinese hamster ovary cell nuclei

We have examined the nuclear localization of isoprenylated proteins in CHO-K1 cells labeled with [14C]mevalonate. Nuclear proteins of 68, 70, and 74 kD, posttranslationally modified by an isoprenoid, are also components of a nuclear matrix-intermediate filament preparation from CHO cells. Furthermore, the 68-, 70-, and 74-kD isoprenylated polypeptides are immunoprecipitated from cell extracts with two different anti-lamin antisera. Based on exact two-dimensional comigration with lamin B, both from rat liver lamin and CHO nuclear matrix-intermediate filament preparations, and its immunoprecipitation with anti-lamin antisera, we conclude that the 68-kD isoprenylated protein found in nuclei from [14C]mevalonate-labeled CHO cells is lamin B. The more basic 74-kD isoprenylated nuclear protein is similar in molecular mass and isoelectric pH variants to the lamin A precursor polypeptide reported by others. Starving cells for mevalonate results in a dramatic accumulation of a polypeptide that comigrates on two-dimensional, non-equilibrium pH gradient electrophoresis (NEPHGE) gels with the 74-kD isoprenylated protein. The 70-kD isoprenylated protein, which is resolved on NEPHGE gels as being higher in molecular mass and slightly more basic than lamin B, has not yet been identified.

Preparation of shadowed nuclear envelopes from Xenopus oocyte germinal vesicles for electron microscopy

Methods for examining the structure of the nuclear envelope of oocytes of Xenopus laevis by electron microscopy using metal shadowing have been developed and evaluated. Minor modifications were made to existing methods for preparing specimens by freeze drying, mainly to eliminate unnecessary steps and a rapid method for examining the structure and arrangement of nuclear envelope components, based on dehydration in an ethanol series followed by amyl acetate and then air drying, was also developed. The preservation of the lamina and connections between the nuclear pore complexes using the rapid air drying method was satisfactory for observing the fibrous components of the envelope and their attachment to the pores. Furthermore, air drying required only simple laboratory apparatus and, moreover, offered several advantages compared to freeze drying when assessing the effect of various disruptive treatments on the nuclear envelope or examining the connections between its components. In specimens prepared by either the more rapid air drying method or by freeze drying, the lamina meshwork beneath the nuclear face of the envelope was clear, but the fine structure of the nuclear pore complexes was superior in freeze dried preparations. In views of the nucleoplasmic face of the envelope, the lamina meshwork was suspended above the support film in freeze dried preparations, but collapsed in most air dried specimens. This collapse was not without its advantages, however, as it facilitated observation of the connections between nuclear pore complexes and lamina fibres, which were often masked in freeze dried preparations.

Involvement of nuclear lamins in postmitotic reorganization of chromatin as demonstrated by microinjection of lamin antibodies

The nuclear lamins are major components of a proteinaceous polymer that is located at the interface of the nuclear membrane and chromatin; these lamins are solubilized and dispersed throughout the cytoplasm during mitosis. It has been postulated that these proteins, assembled into the lamina, provide an architectural framework for the organization of the cell nucleus. To test this hypothesis we microinjected lamin antibodies into cultured PtK2 cells during mitosis, thereby decreasing the soluble pool of lamins. The antibody injected was identified, together with the lamins, in cytoplasmic aggregates by immunoelectron microscopy. We show that microinjected cells are not able to form normal daughter nuclei, in contrast to cells injected with other immunoglobulins. Although cells injected with lamin antibodies are able to complete cytokinesis, the chromatin of their daughter nuclei remains arrested in a telophase-like configuration, and the telophase-like chromatin remains inactive as judged from its condensed state and by the absence of nucleoli. These results indicate that lamins and the nuclear lamina structure are involved in the functional organization of the interphase chromatin.

The nuclear lamina is a meshwork of intermediate-type filaments

The nuclear lamina, a protein meshwork lining the nucleoplasmic surface of the inner nuclear membrane, is thought to provide a framework for organizing nuclear envelope structure and an anchoring site at the nuclear periphery for interphase chromatin. In several higher eukaryotic cells, the lamina appears to be a polymer comprised mainly of one to three immunologically related polypeptides of relative molecular mass (Mr) 60,000-75,000 (60-70K) termed lamins. Three lamins (A, B, and C) are typically present in mammalian somatic cells. Previous studies on nuclear envelopes of rat liver and Xenopus oocytes suggested that the lamina has a fibrillar or filamentous substructure. Interestingly, protein sequences recently deduced for human lamins A and C from complementary DNA clones indicate that both of these polypeptides contain a region of approximately 350 amino acids very similar in sequence to the coiled-coil alpha-helical rod domain that characterizes all intermediate-type filament (IF) proteins. Here we analyse the supramolecular organization of the native nuclear lamina and the structure and assembly properties of purified lamins, and show that the lamins constitute a previously unrecognized class of IF polypeptides.

The nuclear lamins. A multigene family of proteins in evolution and differentiation

The nuclear lamina consists of a proteinaceous layer or meshwork situated subjacent to the inner nuclear membrane. It is a karyoskeletal structure formed by a polymer containing one to three major polypeptides collectively termed the lamins. In all cells examined of vertebrates and invertebrates, the lamins exhibit very similar Mr ranging from 60 000 to 80 000. In vertebrates, two groups of lamins can be distinguished by their isoelectric value, one being near-neutral and the other acidic (isoelectric pH values of 5.6 and lower). The lamins represent a family of polypeptides with regions highly conserved during evolution. In certain species, e.g., the amphibian, Xenopus laevis, they exhibit cell type-specific expression during embryonic development, terminal differentiation of certain somatic cells, and gametogenesis. The nuclear lamina of diverse cell types can be composed of one, two or three different lamin polypeptides, without obvious differences in its morphology.

Change of karyoskeleton during spermatogenesis of Xenopus: expression of lamin LIV, a nuclear lamina protein specific for the male germ line

Lamins are the major constituent proteins of the nuclear lamina. In the frog, Xenopus laevis, they are the products of a multigene family whose expression can be correlated to certain routes of cell differentiation. For example, lamins LI (Mr, 72,000) and LII (Mr, 68,000) is expressed, together with LI/LII, in certain highly differentiated cell types such as neurons and muscle cells and is the only lamin present in diplotene oocytes. Here we report the identification by means of two monoclonal antibodies of a fourth lamin (LIV) of Mr 75,000, which is expressed specifically during the later stages of spermatogenesis. In the seminiferous tubules, Sertoli cells contain LI/LII and LIII whereas, among the spermatogenic cells, spermatogonia contain only LI and LII. In contrast, in spermatids and sperm cells these lamins are completely replaced by lamin LIV. Primary spermatocytes are negative with both antibodies, indicating that a switch in the expression of lamins occurs early in spermatogenesis. Lamin LIV is distributed in patches along the nuclear envelopes of elongated spermatids and sperm cells rather than in the characteristic continuous lamina pattern found in most other cells. We hypothesize that the specific expression of lamin LIV is related to the conspicuous changes of nuclear architecture and chronmatin composition that are known to take place during the late stages of sperm development.

Characterization of lamin proteins in BHK cells

Lamins are structural proteins found in rat liver nuclear envelope and are major constituents of the nuclear matrix. 2-D gel electrophoresis indicates that BHK cell nuclear matrix is composed of four major proteins (62 kD, 68 kD, 70 kD and 72 kD). Three of these proteins are very similar to lamins A, B and C of rat liver nuclear envelope according to their molecular mass and isoelectric points. An anti-serum specific to BHK matrix proteins has been raised. On 2-D immunoblot, this serum detects all the 62, 68 and 72 kD polypeptide isovariants but only one of the two isovariants of the 70 kD polypeptide. Rat lamins A, B and C react with the anti-BHK matrix serum. However, when a monoclonal antibody to rat liver lamins A, B and C is used (Burke, B, Tooze, J & Warren, G, EMBO j 2 (1983) 361 [23]), only the 72 kD (lamin A-like) and the 62 kD (lamin C-like) BHK polypeptides are detected. Our results suggest that although a strong similarity exists between BHK and rat lamins, there is no identical cross-reactivity between the two species.

Electron microscopic immunolocalization of a karyoskeletal protein of molecular weight 145 000 in nucleoli and perinucleolar bodies of Xenopus laevis

Amplified nucleoli of Xenopus laevis oocytes contain a major karyoskeletal protein of Mr 145 000 insoluble in low- and high-salt buffer as well as in non-denaturing detergents. Electron microscopic localization on native and high-salt extracted nucleoli using specific murine antibodies against this polypeptide and gold-coupled antibodies for visualization reveals that the Mr 145 000 protein is located in coils of filaments of ca 4 nm diameter. In addition, this protein occurs in the medusoid filament bodies (MFBs) present in the nucleolar cortex and free in the nucleoplasm. In somatic cells of tissues and in A6 kidney epithelial cells grown in vitro the Mr 145 000 polypeptide or an immunologically related protein is also organized in coiled aggregates of filaments 4-12 nm in diameter present both in the periphery of nucleoli and free in the nucleoplasm. We discuss a possible role of this protein as a karyoskeletal support involved in the storage and transport of preribosomal particles.

Patterns of nuclear lamins in diverse animal and plant cells and in germ cells as revealed by immunofluorescence microscopy with polyclonal and monoclonal antibodies

Polyclonal and monoclonal antibodies against rat liver nuclear lamins have been used to evaluate the immunological cross-reactivity of lamins with a given antibody in a variety of animal and plant cells. The results indicated that lamins of all vertebrate cells but not invertebrate cells share at least one antigenic determinant, resulting in immunological cross-reaction with polyclonal antisera to lamina from rat liver. The range of cross-reaction with monoclonal antibody to rat lamins includes all mammalian cells tested but we observed no reaction with other vertebrate and invertebrate cells. Thus, by means of immunological cross-reaction a less stringently conserved pattern is observed for lamins than, for example, cytoskeletal proteins. We have also investigated the fate of the nuclear lamins during meiosis in testes and ovaries of the mouse. Lamins are absent from male meiotic cells and during oogenesis in meiotic prophases.

A monoclonal antibody against nuclear lamina proteins reveals cell type-specificity in Xenopus laevis

Immunofluorescence microscopy shows that the monoclonal murine antibody PKB8 stains the nuclear lamina of various somatic cells from vertebrates as diverse as mammals, birds and amphibia. It also decorates the nuclear periphery of oocytes from rat and chicken but does not react with spermatocytes, spermatids and spermatozoa. Immunoblotting experiments demonstrate reaction with lamina polypeptides A, B and C of rat, with lamina polypeptide A of chicken, and with lamina polypeptides LI and LII of erythrocytes of the frog, Xenopus laevis. Antibody PKB8 does, however, not bind, on blotted polypeptides and on sections through ovaries, to the pore complex-lamina polypeptide of Mr 68000 present in Xenopus oocytes. These results reveal the existence of a common antigenic determinant in all three lamina polypeptides of mammals, in one lamina polypeptide of chicken and in two amphibian lamina polypeptides. The immunological data also indicate that, in Xenopus laevis, pore complex-lamina polypeptides of somatic cells and oocytes are distinct. The Mr 68000 protein of Xenopus oocytes is also different from polypeptides LI and LII of somatic Xenopus cells by tryptic peptide mapping. The results suggest that nuclear pore complex-lamina polypeptides represent a family of related polypeptides containing regions highly conserved during evolution and that these polypeptides can be differentially expressed in cells of at least one species, Xenopus laevis.

Localization of a major nuclear envelope protein by differential solubilization

The nuclear envelope (NE) is the interface of the two major compartments of the cell. We used differential solubilization in conjunction with ultrastructural visualization to localize components of the NE in the surf clam Spisula solidissima. The high salt-resistant NE fraction can be separated into a pore complex-containing supernatant (4 M urea extract) and a membrane pellet devoid of pore complexes or pore remnants. Urea extraction of the membrane pellet reveals two major proteins with an apparent molecular weight (MWapp) of 67 000 (clam lamin) and 200 000 that are also found in the high-salt and detergent-extracted NE containing pore complexes. Urea extraction of the clam NE under reducing conditions removes the clam lamin. The 200 000 D protein remaining in the NE after removal of the pore complex is not solubilized by detergent extraction and thus can be localized on the inner nuclear part of the NE.

Identification of a major polypeptide of the nuclear pore complex

The nuclear pore complex is a prominent structural component of the nuclear envelope that appears to regulate nucleoplasmic molecular movement. Up to now, none of its polypeptides have been defined. To identify possible pore complex proteins, we fractionated rat liver nuclear envelopes and microsomal membranes with strong protein perturbants into peripheral and intrinsic membrane proteins, and compared these fractions on SDS gels. From this analysis, we identified a prominent 190-kilodalton intrinsic membrane polypeptide that occurs specifically in nuclear envelopes. Lectin binding studies indicate that this polypeptide (gp 190) is the major nuclear envelope glycoprotein. Upon treatment of nuclear envelopes with Triton X-100, gp 190 remains associated with a protein substructure of the nuclear envelope consisting of pore complexes and nuclear lamina. We prepared monospecific antibodies to gp 190 for immunocytochemical localization. Immunofluorescence staining of tissue culture cells suggests that gp 190 occurs exclusively in the nucleus during interphase. This polypeptide becomes dispersed throughout the cell in mitotic prophase when the nuclear envelope is disassembled, and subsequently returns to the nuclear surfaces during telophase when the nuclear envelope is reconstructed. Immunoferritin labeling of Triton-treated rat liver nuclei demonstrates that gp 190 occurs exclusively in the nuclear pore complex, in the regions of the cytoplasmic (and possibly nucleoplasmic) pore complex annuli. A polypeptide that cross-reacts with gp 190 is present in diverse vertebrate species, as shown by antibody labeling of nitrocellulose SDS gel transfers. On the basis of its biochemical characteristics, we suggest that gp 190 may be involved in anchoring the pore complex to nuclear envelope membranes.

Immunological localization of a major karyoskeletal protein in nucleoli of oocytes and somatic cells of Xenopus laevis

Oocyte nuclei of Xenopus laevis contain two major karyoskeletal proteins characterized by their resistance to extractions in high salt buffers and the detergent Triton X-100, i.e. a polypeptide of 68,000 mol wt which is located in the core complex-lamina structure and a polypeptide of 145,000 mol wt enriched in nucleolar fractions. Both proteins are also different by tryptic peptide maps and immunological determinants. Mouse antibodies were raised against insoluble karyoskeletal proteins from Xenopus oocytes and analyzed by immunoblotting procedures. Affinity purified antibodies were prepared using antigens bound to nitrocellulose paper. In immunofluorescence microscopy of Xenopus oocytes purified antibodies against the polypeptide of 145,000 mol wt showed strong staining of nucleoli, with higher concentration in the nucleolar cortex, and of smaller nucleoplasmic bodies. In various other cells including hepatocytes, Sertoli cells, spermatogonia, and cultured kidney epithelial cells antibody staining was localized in small subnucleolar granules. The results support the conclusion that this "insoluble" protein is a major nucleus-specific protein which is specifically associated with--and characteristic of--nucleoli and certain nucleolus-related nuclear bodies. It represents the first case of a positive localization of a karyoskeletal protein in the nuclear interior, i.e. away from the pore complex-lamina structure of the nuclear cortex.

A nucleolar skeleton of protein filaments demonstrated in amplified nucleoli of Xenopus laevis

The amplified, extrachromosomal nucleoli of Xenopus oocytes contain a meshwork of approximately 4-nm-thick filaments, which are densely coiled into higher-order fibrils of diameter 30-40 nm and are resistant to treatment with high- and low-salt concentrations, nucleases (DNase I, pancreatic RNase, micrococcal nuclease), sulfhydryl agents, and various nonionic detergents. This filamentous "skeleton" has been prepared from manually isolated nuclear contents and nucleoli as well as from nucleoli isolated by fluorescence-activated particle sorting. The nucleolar skeletons are observed in light and electron microscopy and are characterized by ravels of filaments that are especially densely packed in the nucleolar cortex. DNA as well as RNA are not constituents of this structure, and precursors to ribosomal RNAs are completely removed from the extraction-resistant filaments by treatment with high-salt buffer or RNase. Fractions of isolated nucleolar skeletons show specific enrichment of an acidic major protein of 145,000 mol wt and an apparent pI value of approximately 6.15, accompanied in some preparations by various amounts of minor proteins. The demonstration of this skeletal structure in "free" extrachromosomal nucleoli excludes the problem of contaminations by nonnucleolar material such as perinucleolar heterochromatin normally encountered in studies of nucleoli from somatic cells. It is suggested that this insoluble protein filament complex forms a skeleton specific to the nucleolus proper that is different from other extraction-resistant components of the nucleus such as matrix and lamina and is involved in the spatial organization of the nucleolar chromatin and its transcriptional products.