Colocalization of intranuclear lamin foci with RNA splicing factors

Proteome analysis of a rat liver nuclear insoluble protein fraction and localization of a novel protein, ISP36, to compartments in the interchromatin space

A rat liver nuclear insoluble protein fraction was analyzed to investigate candidate proteins participating in nuclear architecture formation. Proteins were subjected to two-dimensional separation by reversed-phase HPLC in 60% formic acid and SDS/PAGE. The method produced good resolution of insoluble proteins. One hundred and thirty-eight proteins were separated, and 28 of these were identified. The identified proteins included one novel protein, seven known nuclear proteins and 12 known nuclear matrix proteins. The novel 36 kDa protein was further investigated for its subnuclear localization. The human ortholog of the protein was expressed in Escherichia coli and antibodies were raised against the recombinant protein. Exclusive localization of the protein to the nuclear insoluble protein fraction was confirmed by cell fractionation followed by immunoblotting. Immunostaining of mouse C3H cells suggested that the 36 kDa protein was a constituent of an insoluble macromolecular complex spread throughout the interchromatin space of the nucleus. The protein was designated 'interchromatin space protein of 36 kDa', ISP36.

Remodelling of the nuclear lamina and nucleoskeleton is required for skeletal muscle differentiation in vitro

Changes in the expression and distribution of nuclear lamins were investigated during C2C12 myoblast differentiation. The expression of most lamins was unchanged during myogenesis. By contrast, lamin-B2 expression increased and LAP2alpha expression decreased twofold. These changes were correlated with reduced solubility and redistribution of A-type lamins. When C2C12 myoblasts were transfected with a lamin-A mutant that causes autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD), the mutant protein accumulated in the nucleoplasm and exerted dominant influences over endogenous lamins. Myoblasts transfected with wild-type lamins differentiated, albeit more slowly, whereas myoblasts transfected with mutant lamins failed to differentiate. Myoblast differentiation requires dephosphorylation of the retinoblastoma protein Rb. During myogenesis, Rb was rapidly and progressively dephosphorylated. Underphosphorylated Rb formed complexes with LAP2alpha in proliferating myoblasts and postmitotic myoblasts. In myoblasts transfected with the mutant lamins, this complex was disrupted. These data suggest that remodelling of the nucleoskeleton is necessary for skeletal-muscle differentiation and for correct regulation of Rb pathways.

Differential nuclear remodeling of mammalian somatic cells by Xenopus laevis oocyte and egg cytoplasm

The mechanisms governing nuclear reprogramming have not been fully elucidated yet; however, recent studies show a universally conserved ability of both oocyte and egg components to reprogram gene expression in somatic cells. The activation of genes associated with pluripotency by oocyte/egg components may require the remodeling of nuclear structures, such that they can acquire the features of early embryos and pluripotent cells. Here, we report on the remodeling of the nuclear lamina of mammalian cells by Xenopus oocyte and egg extracts. Lamin A/C is removed from somatic cells incubated in oocyte and egg extracts in an active process that requires permeable nuclear pores. Removal of lamin A/C is specific, since B-type lamins are not changed, and it is not dependent on the incorporation Xenopus egg specific lamin III. Moreover, transcriptional activity is differentially regulated in somatic cells incubated in the extracts. Pol I and II transcriptions are maintained in cells in oocyte extracts; however, both activities are abolished in egg extracts. Our study shows that components of oocyte and egg extracts can modify the nuclear lamina of somatic cells and that this nuclear remodeling induces a structural change in the nucleus which may have implications for transcriptional activity. These experiments suggest that modifications in the nuclear lamina structure by the removal of somatic proteins and the incorporation of oocyte/egg components may contribute to the reprogramming of somatic cell nuclei and may define a characteristic configuration of pluripotent cells.

Nuclear envelopathies--raising the nuclear veil

The nuclear envelope separates the chromosomes from cytoplasm in eukaryotic cells and consists of three main domains: inner and outer nuclear membranes and nuclear pore complexes. The inner nuclear membrane maintains close associations with the underlying chromatin and nuclear lamina. For many years, the nuclear envelope was thought to function mainly as an architectural stabilizer of the nucleus, participating in assembly and disassembly processes during mitosis. However, recent findings demonstrate that nuclear envelope proteins are involved in fundamental nuclear functions, such as gene transcription and DNA replication, and that inherited or de novo mutated proteins cause human diseases, termed "nuclear envelopathies." These findings emphasize the importance of understanding the functions of this cellular domain, in both physiologic and pathologic states. To date, mutations in the genes encoding the nuclear envelope proteins emerin, MAN1, lamin A/C, and lamin B receptor were found to cause nuclear envelopathies. The diseases that are caused by mutations in LMNA gene are collectively called "laminopathies." Nuclear envelopathies have diverse clinical phenotypes, ranging from cardiac and skeletal myopathies to partial lipodystrophy, peripheral neuropathy, and premature aging. This raises the question of how do such ubiquitously expressed proteins give rise to tissue-specific disease phenotypes. One possible explanation is the involvement of nuclear envelope proteins in the regulation of gene transcription, a novel mechanism that has been the focus of research in our lab in recent years. In this review, we describe recent discoveries in the field of nuclear envelopathies and discuss current proposed pathophysiological mechanisms underlying these diseases.

Both lamin A and lamin C mutations cause lamina instability as well as loss of internal nuclear lamin organization

We have applied the fluorescence loss of intensity after photobleaching (FLIP) technique to study the molecular dynamics and organization of nuclear lamin proteins in cell lines stably transfected with green fluorescent protein (GFP)-tagged A-type lamin cDNA. Normal lamin A and C proteins show abundant decoration of the inner layer of the nuclear membrane, the nuclear lamina, and a generally diffuse localization in the nuclear interior. Bleaching studies revealed that, while the GFP-tagged lamins in the lamina were virtually immobile, the intranuclear fraction of these molecules was partially mobile. Intranuclear lamin C was significantly more mobile than intranuclear lamina A. In search of a structural cause for the variety of inherited diseases caused by A-type lamin mutations, we have studied the molecular organization of GFP-tagged lamin A and lamin C mutants R453W and R386K, found in Emery-Dreifuss muscular dystrophy (EDMD), and lamin A and lamin C mutant R482W, found in patients with Dunnigan-type familial partial lipodystrophy (FPLD). In all mutants, a prominent increase in lamin mobility was observed, indicating loss of structural stability of lamin polymers, both at the perinuclear lamina and in the intranuclear lamin organization. While the lamin rod domain mutant showed overall increased mobility, the tail domain mutants showed mainly intranuclear destabilization, possibly as a result of loss of interaction with chromatin. Decreased stability of lamin mutant polymers was confirmed by flow cytometric analyses and immunoblotting of nuclear extracts. Our findings suggest a loss of function of A-type lamin mutant proteins in the organization of intranuclear chromatin and predict the loss of gene regulatory function in laminopathies.

Sequestration of pRb by cyclin D3 causes intranuclear reorganization of lamin A/C during muscle cell differentiation

The A-type lamins that localize in nuclear domains termed lamin speckles are reorganized and antigenically masked specifically during myoblast differentiation. This rearrangement was observed to be linked to the myogenic program as lamin speckles, stained with monoclonal antibody (mAb) LA-2H10, were reorganized in MyoD-transfected fibroblasts induced to transdifferentiate to muscle cells. In C2C12 myoblasts, speckles were reorganized early during differentiation in cyclin D3-expressing cells. Ectopic cyclin D3 induced lamin reorganization in C2C12 myoblasts but not in other cell types. Experiments with adenovirus E1A protein that can bind to and segregate the retinoblastoma protein (pRb) indicated that pRb was essential for the cyclin D3-mediated reorganization of lamin speckles. Cyclin D3-expressing myoblasts displayed site-specific reduction of pRb phosphorylation. Furthermore, disruption of lamin structures by overexpression of lamins inhibited expression of the muscle regulatory factor myogenin. Our results suggest that the reorganization of internal lamins in muscle cells is mediated by key regulators of the muscle differentiation program.

Satellite DNA binding and cellular localisation of RNA helicase P68

We purified a 68-kDa protein from the mouse nuclear matrix using ion exchange and affinity chromatography. Column fractions were tested for specific binding to mouse minor satellite DNA using a gel mobility shift assay. The protein was identified by mass spectrometry as RNA helicase P68. In fixed cells, P68 was found to shuttle in and out of SC35 domains, forming fibres and granules in a cell-cycle dependent manner. Analysis of the P68 sequence revealed a short potential coiled-coil domain that might be involved in the formation of P68 fibres. Contacts between centromeres and P68 granules were observed during all phases of the cycle but they were most prominent in mitosis. At this stage, P68 was found in both the centromeric regions and the connections between chromosomes. Direct interaction of P68/DEAD box RNA helicase with satellite DNAs in vitro has not been demonstrated for any other members of the RNA helicase family.

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.

Laminopathies

Nuclear lamins form a fibrous nucleoskeletal network of intermediate-sized filaments that underlies the inner nuclear membrane. It associates with this membrane through interactions with specific integral nuclear membrane proteins, while within this flattened lamin lattice the nuclear pore complexes are embedded. Next to this peripheral network, the lamins can form intranuclear structures. The lamins are the evolutionary progenitors of the cytoplasmic intermediate filament proteins and have profound influences on nuclear structure and function. These influences require that lamins have dynamic properties and dual identities as structural building blocks on the one hand, and transcription regulators on the other. Which of these identities underlies the laminopathies, a myriad of genetic diseases caused by mutations in lamins or lamin-associated proteins, is a topic of intense debate.

Immunolocalization of detergent-susceptible nucleoplasmic lamin A/C foci by a novel monoclonal antibody

The A-type lamins are localized in the interior of the nucleus as well as on the nuclear periphery. In this study, we have characterized a monoclonal antibody LA-2F9 produced against recombinant rat lamin A which stains a subpopulation of various cell types in a pattern of small nucleoplasmic foci that are unusually susceptible to mild detergent/salt extraction. The specific reactivity of mAb LA-2F9 towards lamins was confirmed by immunoblotting of HeLa and C3H10T(1/2) whole cell lysates and nuclear lysates. The epitope for LA-2F9 was narrowed down to amino acid residues 268-278 (SAKLDNARQSA). To check whether the appearance of lamin foci was cell-cycle-dependent, C3H10T(1/2) cells were serum-starved and then refed to trigger cells to enter the G(1) phase of the cell-cycle. The intensity of staining increased 3.5-fold within 6 h of refeeding, when the maximum number of cells were labeled with LA-2F9. We also checked whether the LA-2F9 foci colocalized with nuclear proteins known to be distributed in small foci such as hnRNPs, snRNPs, SC-35, and p80 coilin, but did not find evidence of colocalization. Our studies suggest that LA-2F9 has a novel and specific reactivity towards detergent-susceptible lower order lamin structures that are likely to be assembly intermediates.

LAP2α and BAF transiently localize to telomeres and specific regions on chromatin during nuclear assembly

Lamina-associated polypeptide (LAP) 2α is a LEM (lamina-associated polypeptide emerin MAN1) family protein associated with nucleoplasmic A-type lamins and chromatin. Using live cell imaging and fluorescence microscopy we demonstrate that LAP2α was mostly cytoplasmic in metaphase and associated with telomeres in anaphase. Telomeric LAP2α clusters grew in size, formed `core' structures on chromatin adjacent to the spindle in telophase, and translocated to the nucleoplasm in G1 phase. A subfraction of lamin C and emerin followed LAP2α to the core region early on, whereas LAP2β, lamin B receptor and lamin B initially bound to more peripheral regions of chromatin, before they spread to core structures with different kinetics. Furthermore, the DNA-crosslinking protein barrier-to-autointegration factor (BAF) bound to LAP2α in vitro and in mitotic extracts, and subfractions of BAF relocalized to core structures with LAP2α. We propose that LAP2α and a subfraction of BAF form defined complexes in chromatin core regions and may be involved in chromatin reorganization during early stages of nuclear assembly.

A-type lamins: Guardians of the soma?

The gene LMNA encodes the proteins lamins A and C and is implicated in nine different laminopathies - inherited diseases that are linked to premature ageing. Recent evidence has demonstrated that lamins A and C have essential functions in protecting cells from physical damage, as well as in maintaining the function of transcription factors required for the differentiation of adult stem cells. Thus, the degenerative nature of laminopathies is explained because these lamins are essential for maintenance of somatic tissues in adulthood.

Heat stress-induced localization of small heat shock proteins in mouse myoblasts: intranuclear lamin A/C speckles as target for αB-crystallin and Hsp25

We examined the effect of heat stress on localization of two sHsps, alphaB-crystallin and Hsp25, and of Hsc70, a member of a different class of heat shock proteins (Hsps), in both undifferentiated and differentiated mouse C2C12 cells. Under normal conditions, alphaB-crystallin and Hsp25 are found in the cytoplasm; only alphaB-crystallin is also found in the nucleus, distributed in a speckled pattern. Hsc70 is found to be homogeneously distributed throughout the cell. On heat stress, all these proteins translocate almost entirely into the nucleus and upon recovery relocate to the cytoplasm. Dual staining experiments using C2C12 myoblasts show that alphaB-crystallin and Hsp25, but not Hsc70, colocalize with the intranuclear lamin A/C and the splicing factor SC-35, suggesting interactions of sHsps and intranuclear lamin A/C. Interestingly, none of these proteins are found in the myotube nuclei. Upon heat stress, only Hsc70 translocates into the myotube nuclei. This differential entry of alphaB-crystallin and Hsp25 into the nuclei of myoblasts and myotubes upon heat stress may have functional role in the development and/or in the maintenance of muscle cells. Our study therefore suggests that these sHsps may be a part of the intranuclear lamin A/C network or stabilizing this specific network.

Formation of nuclear splicing factor compartments is independent of lamins A/C

Nuclear lamins are major architectural elements of the mammalian cell nucleus, and they have been implicated in the functional organization of the nuclear interior, possibly by providing structural support for nuclear compartments. Colocalization studies have suggested a structural role for lamins in the formation and maintenance of pre-mRNA splicing factor compartments. Here, we have directly tested this hypothesis by analysis of embryonic fibroblasts from knock-out mice lacking A- and C-type lamins. We show that the morphology and cellular properties of splicing factor compartments are independent of A- and C-type lamins. Genetic loss of lamins A/C has no effect on the cellular distribution of several pre-mRNA splicing factors and does not affect the compartment morphology as examined by light and electron microscopy. The association of splicing factors with the nuclear matrix fraction persists in the absence of lamins A/C. Live cell microscopy demonstrates that the intranuclear positional stability of splicing factor compartments is maintained and that the exchange dynamics of SF2/ASF between the compartments and the nucleoplasm is not affected by loss of lamin A/C. Our results demonstrate that formation and maintenance of intranuclear splicing factor compartments is independent of lamins A/C, and they argue against an essential structural role of lamins A/C in splicing factor compartment morphology.

Lco1 is a novel widely expressed lamin-binding protein in the nuclear interior

A-type lamins are localized at the nuclear envelope and in the nucleoplasm, and are implicated in human diseases called laminopathies. In a yeast two-hybrid screen with lamin C, we identified a novel widely expressed 171-kDa protein that we named Lamin companion 1 (Lco1). Three independent biochemical assays showed direct binding of Lco1 to the C-terminal tail of A-type lamins with an affinity of 700 nM. Lco1 also bound the lamin B1 tail with lower affinity (2 microM). Ectopic Lco1 was found primarily in the nucleoplasm and colocalized with endogenous intranuclear A-type lamins in HeLa cells. Overexpression of prelamin A caused redistribution of ectopic Lco1 to the nuclear rim together with ectopic lamin A, confirming association of Lco1 with lamin A in vivo. Whereas the major C-terminal lamin-binding fragment of Lco1 was cytoplasmic, the N-terminal Lco1 fragment localized in the nucleoplasm upon expression in cells. Furthermore, full-length Lco1 was nuclear in cells lacking A-type lamins, showing that A-type lamins are not required for nuclear targeting of Lco1. We conclude that Lco1 is a novel intranuclear lamin-binding protein. We hypothesize that Lco1 is involved in organizing the internal lamin network and potentially relevant as a laminopathy disease gene or modifier.

Histone deacetylase inhibitors – a new tool to treat cancer

Transcriptional regulation in eukaryotes is a multilevel hierarchical process. It is becoming clear that higher-order chromatin structure, occurring via modifications of histones in their nucleosome structure, plays a crucial role in regulating gene expression, both in normal and pathological states. Deacetylation of histones by histone deacetylases (HDACs) modifies the chromatin from an open gene active euchromatin structure to a closed gene silenced heterochromatin structure. Several cancer promoting mutations and chromosomal translocations result in repression of transcription through abnormal recruitment and activation of HDACs, leading to neoplastic transformation. This is the rationale for the evolvement of HDAC inhibitors as a new class in cancer therapy. Trials have shown anti-proliferation effect of histone deacetylase inhibitors in cell culture, animal models and in human with both hematological and solid tumors. The exact mechanism by which histone deacetylase inhibitors exert their effect is still obscure. Reversal of the alteration in gene expression by fusion transcription factors or overexpressed repressors is just one of several possible explanations. The territory of heterochromatin in the vicinity of the nuclear periphery raised the possibility of involvement of nuclear envelope proteins in the regulation of transcription. Our laboratory is interested in the transcription repression mechanism induced by the nuclear envelope lamina associated polypeptide 2 (LAP2) family of proteins through chromatin modification. Here, we will describe the structure of the nucleosome, review regulation of gene expression by acetylation of histones and give an update on the current phase I and phase II clinical trials with histone deacetylase inhibitors.

Genome-scale expression profiling of Hutchinson-Gilford progeria syndrome reveals widespread transcriptional misregulation leading to mesodermal/mesenchymal defects and accelerated atherosclerosis

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disease with widespread phenotypic features resembling premature aging. HGPS was recently shown to be caused by dominant mutations in the LMNA gene, resulting in the in-frame deletion of 50 amino acids near the carboxyl terminus of the encoded lamin A protein. Children with this disease typically succumb to myocardial infarction or stroke caused by severe atherosclerosis at an average age of 13 years. To elucidate further the molecular pathogenesis of this disease, we compared the gene expression patterns of three HGPS fibroblast cell strains heterozygous for the LMNA mutation with three normal, age-matched cell strains. We defined a set of 361 genes (1.1% of the approximately 33,000 genes analysed) that showed at least a 2-fold, statistically significant change. The most prominent categories encode transcription factors and extracellular matrix proteins, many of which are known to function in the tissues severely affected in HGPS. The most affected gene, MEOX2/GAX, is a homeobox transcription factor implicated as a negative regulator of mesodermal tissue proliferation. Thus, at the gene expression level, HGPS shows the hallmarks of a developmental disorder affecting mesodermal and mesenchymal cell lineages. The identification of a large number of genes implicated in atherosclerosis is especially valuable, because it provides clues to pathological processes that can now be investigated in HGPS patients or animal models.

A Phosphorylation Cluster in the Chromatin-binding Region Regulates Chromosome Association of LAP2α

LAP2alpha is a LEM family protein associated with nucleoplasmic A-type lamins and chromatin in interphase. Like lamins and other lamina proteins LAP2alpha is cytoplasmic in metaphase, but it associates with chromosomes prior to nuclear envelope formation in late anaphase to telophase. In vitro phosphorylation analysis and mass spectrometry identified a cluster of at least three mitotic cyclin-dependent kinase 1 phosphorylation sites in the C-terminal chromatin-binding region of LAP2alpha as well as four additional potential sites in the cluster, some of which were targeted alternatively in LAP2alpha mutated at the major sites. LAP2alpha mutants containing serine --> alanine mutations at all seven sites revealed a clear phenotype. Mutated LAP2alpha remained associated with chromosomes throughout mitosis, but the dissociation of lamins into the cytoplasm and nuclear envelope disassembly were not affected. These data demonstrate the in vivo significance of mitotic phosphorylation for the dynamic behavior of LAP2alpha in the cell cycle and show that, unlike the interaction with lamins, the chromatin association of LAP2alpha is regulated by multiple mitosis-specific phosphorylation at sites clustered within a defined region in the C terminus of the protein.

Lamin B1 is required for mouse development and nuclear integrity

Lamins are key structural components of the nuclear lamina, an intermediate filament meshwork that lies beneath the inner nuclear membrane. Lamins play a role in nuclear architecture, DNA replication, and gene expression. Mutations affecting A-type lamins have been associated with a variety of human diseases, including muscular dystrophy, cardiomyopathy, lipodystrophy, and progeria, but mutations in B-type lamins have never been identified in humans or in experimental animals. To investigate the in vivo function of lamin B1, the major B-type lamin, we generated mice with an insertional mutation in Lmnb1. The mutation resulted in the synthesis of a mutant lamin B1 protein lacking several key functional domains, including a portion of the rod domain, the nuclear localization signal, and the CAAX motif (the carboxyl-terminal signal for farnesylation). Homozygous Lmnb1 mutant mice survived embryonic development but died at birth with defects in lung and bone. Fibroblasts from mutant embryos grew under standard cell-culture conditions but displayed grossly misshapen nuclei, impaired differentiation, increased polyploidy, and premature senescence. Thus, the lamin B1 mutant mice provide evidence for a broad and nonredundant function of lamin B1 in mammalian development. These mutant mice and cell lines derived from them will be useful models for studying the role of the nuclear lamina in various cellular processes.

Proteomic analysis of interchromatin granule clusters

A variety of proteins involved in gene expression have been localized within mammalian cell nuclei in a speckled distribution that predominantly corresponds to interchromatin granule clusters (IGCs). We have applied a mass spectrometry strategy to identify the protein composition of this nuclear organelle purified from mouse liver nuclei. Using this approach, we have identified 146 proteins, many of which had already been shown to be localized to IGCs, or their functions are common to other already identified IGC proteins. In addition, we identified 32 proteins for which only sequence information is available and thus these represent novel IGC protein candidates. We find that 54% of the identified IGC proteins have known functions in pre-mRNA splicing. In combination with proteins involved in other steps of pre-mRNA processing, 81% of the identified IGC proteins are associated with RNA metabolism. In addition, proteins involved in transcription, as well as several other cellular functions, have been identified in the IGC fraction. However, the predominance of pre-mRNA processing factors supports the proposed role of IGCs as assembly, modification, and/or storage sites for proteins involved in pre-mRNA processing.

Localized effects of cAMP mediated by distinct routes of protein kinase A

More than 20% of the human genome encodes proteins involved in transmembrane and intracellular signaling pathways. The cAMP-protein kinase A (PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells and is involved in regulation of cellular functions in almost all tissues in mammals. Various extracellular signals converge on this signal pathway through ligand binding to G protein-coupled receptors, and the cAMP-PKA pathway is therefore tightly regulated at several levels to maintain specificity in the multitude of signal inputs. Ligand-induced changes in cAMP concentration vary in duration, amplitude, and extension into the cell, and cAMP microdomains are shaped by adenylyl cyclases that form cAMP as well as phosphodiesterases that degrade cAMP. Different PKA isozymes with distinct biochemical properties and cell-specific expression contribute to cell and organ specificity. A kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP-PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphatases and cAMP-specific phosphodiesterases as well as components of other signaling pathways into multiprotein signaling complexes that serve as crossroads for different paths of cell signaling. Targeting of PKA and integration of a wide repertoire of proteins involved in signal transduction into complex signal networks further increase the specificity required for the precise regulation of numerous cellular and physiological processes.

Failure of lamin A/C to functionally assemble in R482L mutated familial partial lipodystrophy fibroblasts: altered intermolecular interaction with emerin and implications for gene transcription

Familial partial lipodystrophy is an autosomal dominant disease caused by mutations of the LMNA gene encoding alternatively spliced lamins A and C. Abnormal distribution of body fat and insulin resistance characterize the clinical phenotype. In this study, we analyzed primary fibroblast cultures from a patient carrying an R482L lamin A/C mutation by a morphological and biochemical approach. Abnormalities were observed consisting of nuclear lamin A/C aggregates mostly localized close to the nuclear lamina. These aggregates were not bound to either DNA-containing structures or RNA splicing intranuclear compartments. In addition, emerin did not colocalize with nuclear lamin A/C aggregates. Interestingly, emerin failed to interact with lamin A in R482L mutated fibroblasts in vivo, while the interaction with lamin C was preserved in vitro, as determined by coimmunoprecipitation experiments. The presence of lamin A/C nuclear aggregates was restricted to actively transcribing cells, and it was increased in insulin-treated fibroblasts. In fibroblasts carrying lamin A/C nuclear aggregates, a reduced incorporation of bromouridine was observed, demonstrating that mutated lamin A/C in FPLD cells interferes with RNA transcription.

Structure-specific DNA-binding proteins as the foundation for three-dimensional chromatin organization

Any functions of tandem repetitive sequences need proteins that specifically bind to them. Telomere-binding TRF2/MTBP attaches telomeres to the nuclear envelope in interphase due to its rod-domain-like motif. Interphase nuclei organized as a number of sponge-like ruffly round chromosome territories that could be rotated from outside. SAF-A/hnRNP-U and p68-helicase are proteins suitable to do that. Their location in the interchromosome territory space, ATPase domains, and the ability to be bound by satellite DNAs (satDNA) make them part of the wires used to help chromosome territory rotates. In case of active transcription p68-helicase can be involved in the formation of local "gene expression matrices" and due to its satDNA-binding specificity cause the rearrangement of the local chromosome territory. The marks of chromatin rearrangement, which have to be heritable, could be provided by SAF-A/hnRNP-U. During telophase unfolding the proper chromatin arrangement is restored according to these marks. The structural specificity of both proteins to the satDNAs provides a regulative but relatively stable mode of binding. The structural specificity of protein binding could help to find the "magic" centromeric sequence. With future investigations of proteins with the structural specificity of binding during early embryogenesis, when heterochromatin formation goes on, the molecular mechanisms of the "gene gating" hypothesis (Blobel, 1985) will be confirmed.

Architectural defects in pronuclei of mouse nuclear transplant embryos

Reprogramming somatic nuclear function by transplantation of nuclei into recipient oocytes is associated with a morphological remodeling of the somatic nucleus. Successful cloning of animals by nuclear transplantation (NT) demonstrates that reprogramming somatic cell function is possible. However, low pregnancy rates and high frequencies of lethal abnormalities in animals born suggest that reprogramming is rarely complete. To address this issue, we tested the hypothesis that nuclear transplantation leads to nuclear remodeling deficiencies. We report the identification of several markers of morphological remodeling, or lack thereof, of mouse cumulus cell nuclei after transplantation into oocytes. Notably, nuclear transplant mouse embryos exhibit nuclear assembly of the differentiated cell-specific A-type lamins at the one-cell stage, as a result of misregulation of lamin A gene expression. The transplanted nuclei also display enhanced concentration of the nuclear matrix-associated protein NuMA as a result of translation from maternal mRNA and de novo transcription. The A-kinase anchoring protein 95 (AKAP95), a marker of the nuclear envelope-chromatin interface, is of somatic origin. Furthermore, greater resistance of AKAP95 and DNA to in situ extractions of one-cell stage NT embryos with non-ionic detergent, DNase, RNase and NaCl reflects an enhanced proportion of heterochromatin in these embryos. Passage through first embryonic mitosis does not rescue the defects detected in one-cell stage embryos. We propose that somatic nuclear reprogramming deficiencies by NT might emanate from, at least in part, failure to remodel the somatic nucleus morphologically into a functional embryonic nucleus.

Nuclear speckles: a model for nuclear organelles

Speckles are subnuclear structures that are enriched in pre-messenger RNA splicing factors and are located in the interchromatin regions of the nucleoplasm of mammalian cells. At the fluorescence-microscope level they appear as irregular, punctate structures, which vary in size and shape, and when examined by electron microscopy they are seen as clusters of interchromatin granules. Speckles are dynamic structures, and both their protein and RNA-protein components can cycle continuously between speckles and other nuclear locations, including active transcription sites. Studies on the composition, structure and behaviour of speckles have provided a model for understanding the functional compartmentalization of the nucleus and the organization of the gene-expression machinery.

Trl-GAGA directly interacts with lola like and both are part of the repressive complex of Polycomb group of genes

Epigenetic inheritance to maintain the expression state of the genome is essential during development. In Drosophila, the cis regulatory elements, called the Polycomb Response Elements (PREs) function to mark the epigenetic cellular memory of the corresponding genomic region with the help of PcG and trxG proteins. While the PcG genes code for the repressor proteins, the trxG genes encode activator proteins. The observations that some proteins may function both as PcG and trxG member and that both these group of proteins act upon common cis elements indicate at least a partial functional overlap among these proteins. Trl-GAGA was initially identified as a trxG member but later was shown to be essential for PcG function on several PREs. In order to understand how Trl-GAGA functions in PcG context, we have looked for the interactors of this protein. We identified lola like, aka batman, as a strong interactor of GAGA factor in a yeast two-hybrid screen. lolal also interacts with polyhomeotic and, like Trl, both lolal and ph are needed for iab-7PRE mediated pairing dependent silencing of mini-white transgene. These observations suggest a possible mechanism of how Trl-GAGA plays a role in maintaining the repressed state of target genes involving lolal, which may function as a mediator to recruit PcG complexes.

AKAP149 is a novel PP1 specifier required to maintain nuclear envelope integrity in G1 phase

Reassembly of the nuclear envelope (NE) at the end of mitosis requires targeting of the B-type lamin protein phosphatase, PP1, to the envelope by A-kinase anchoring protein AKAP149. We show here that NE-associated AKAP149 is a novel PP1-specifying subunit involved in maintaining nuclear architecture through G1 phase. PP1 remains associated with NE-bound AKAP149 during G1 but is released from AKAP149 upon S phase entry, as AKAP149 becomes serine-phosphorylated. NE-associated AKAP149 inhibits PP1 activity towards glycogen phosphorylase but enhances PP1 phosphatase activity towards B-type lamins, indicating that AKAP149 is a B-type lamin specifying subunit of PP1. In vivo dissociation of PP1 from NE-bound AKAP149 in G1-phase nuclei triggers phosphorylation and depolymerization of A- and B-type lamins. The lamins solubilize intranuclearly without affecting the inner nuclear membrane or pore complex distribution. This correlates with the induction of a G1 arrest and, ultimately, apoptosis. We propose that AKAP149-regulated PP1 activity at the NE during G1 is required to maintain nuclear integrity and cell survival.

Reversible changes in the nuclear lamina induced by hyperthermia

The nuclear matrix (NM) has been identified as a potential target for heat-induced cell killing. Previous studies have shown that heat-shock may significantly modulate lamin B content. Since changes in NM structure have often been accompanied by changes in protein composition, we investigated whether hyperthermia induced changes in nuclear lamina (NL) structure in non-tolerant and thermotolerant cells, and the implications of these changes on cell survival. Using indirect immunofluorescence techniques and confocal microscopy, we found that heating cells at 42 or 45.5 degrees C caused invaginations and other distortions of the peripheral NL. While hyperthermia did not alter the number or structure of internal lamin B foci, heat-induced alterations to the peripheral NL were dose-dependent. Interestingly, NL structure recovered with time after heating in cells that were destined to live or die. Thermotolerant cells heated at 45.5 degrees C showed similar initial changes in the NL compared to non-tolerant cells, but recovery occurred much faster. Taken together, these results suggest that the amount of initial damage to the peripheral NL is not correlated with heat-induced cell killing. However, the possibility that an increased rate of recovery might confer a survival advantage cannot be discounted.

Lamin expression in normal human skin, actinic keratosis, squamous cell carcinoma and basal cell carcinoma

BACKGROUND

Aberrant expression patterns of nuclear lamins have been described in various types of cancer depending on the subtype of cancer, its aggressiveness, proliferative capacity and degree of differentiation. In general, the expression of A-type lamins (lamins A and C) has been correlated with a non-proliferating, differentiated state of cells and tissues.

OBJECTIVES

To establish and compare the expression patterns of lamins in normal human skin, actinic keratosis (AK), squamous cell carcinoma (SCC) and basal cell carcinoma (BCC).

METHODS

Expression patterns of the individual lamin subtypes were studied immunohistochemically. The proliferation capacity of the tumour cells was detected using a specific antibody to Ki-67, and was related to the A-type lamin expression patterns.

RESULTS

In normal skin, lamin A was expressed in the suprabasal cell compartment of the epidermis, whereas the basal cells were mostly unstained. BCCs and SCCs stained positive in most cells, while the epidermis overlying BCC and SCC and the epidermis in AK stained homogeneously and strongly in the basal cells in addition to the suprabasal cells. Lamin C was expressed in some basal cells of normal epidermis while the suprabasal cells stained strongly positive. Both BCCs and SCCs stained strongly positive for lamin C, with the difference that in BCC the staining was predominantly present in nucleolar structures with occasional staining of the nuclear envelope. The epidermis overlying SCC showed strong positivity in the lamina of virtually all cells. The expression of lamin C in the basal cells of AK resembled the expression pattern seen in the epidermis overlying BCC, i.e. a nucleolar staining next to nuclear envelope staining. Lamin B1 and B2 were found in virtually all cells in normal epidermis, AK, BCC, SCC and the epidermis overlying cancer. The percentage of Ki-67-expressing cells was highest in BCC (45%), and gradually decreased via epidermis overlying BCC, AK, SCC, and epidermis overlying SCC, to normal skin (11%). Simultaneous expression of A-type lamins and Ki-67 occurred in approximately 50% of the proliferating (Ki-67 positive) cells in BCC and SCC.

CONCLUSIONS

Significant changes occur in the expression patterns of A-type lamins in both premalignant and malignant lesions of the skin. The profound overlap of lamin A and Ki-67 staining patterns indicates that the proliferating tumour cells may obtain a certain degree of differentiation. Finally, lamin A expression in the basal cell layer of the apparently normal epidermis overlying BCC may suggest its involvement in the primary process.

Lamin A/C speckles mediate spatial organization of splicing factor compartments and RNA polymerase II transcription

The A-type lamins have been observed to colocalize with RNA splicing factors in speckles within the nucleus, in addition to their typical distribution at the nuclear periphery. To understand the functions of lamin speckles, the effects of transcriptional inhibitors known to modify RNA splicing factor compartments (SFCs) were examined. Treatment of HeLa cells with alpha-amanitin or 5,6-dichlorobenzimidazole riboside (DRB) inhibited RNA polymerase II (pol II) transcription and led to the enlargement of lamin speckles as well as SFCs. Removal of the reversible inhibitor DRB resulted in the reactivation of transcription and a rapid, synchronous redistribution of lamins and splicing factors to normal-sized speckles, indicating a close association between lamin speckles and SFCs. Conversely, the expression of NH2-terminally modified lamin A or C in HeLa cells brought about a loss of lamin speckles, depletion of SFCs, and down-regulation of pol II transcription without affecting the peripheral lamina. Our results suggest a unique role for lamin speckles in the spatial organization of RNA splicing factors and pol II transcription in the nucleus.

Lamins: building blocks or regulators of gene expression?

Intermediate filament (IF) proteins are the building blocks of cytoskeletal filaments, the main function of which is to maintain cell shape and integrity. The lamins are thought to be the evolutionary progenitors of IF proteins and they have profound influences on both nuclear structure and function. These influences require the lamins to have dynamic properties and dual identities--as building blocks and transcriptional regulators. Which one of these identities underlies a myriad of genetic diseases is a topic of intense debate.

Association of the nuclear matrix component NuMA with the Cajal body and nuclear speckle compartments during transitions in transcriptional activity in lens cell differentiation

The transcriptional status of cells can be deduced from the staining pattern of various nuclear markers such as the Cajal body, nucleolus and nuclear speckles. In this study we have used these markers to correlate transcriptional status with cell differentiation in the lens. As a closed system with no cell loss and with each stage being spatially preserved, it is particularly well suited to such studies. To confirm that the nuclear markers in lens cells follow the same trends as in other cells, primary bovine lens epithelial cells were cultured and then treated with actinomycin D to inhibit transcription. This reduced the Cajal body markers to one or two foci per nucleus and the nucleoli became compacted as revealed by fibrillarin staining. The nuclear speckles, containing snRNPs (e.g. Sm) and the splicing factor, SC35, also became larger and more numerous while the signal for trimethylguanine (TMG) decreased suggesting a role hierarchy for the various speckle factors during transcriptional shutdown. The signal for survival of motor neurones gene product (SMN) also decreased at this point. In the lens epithelium, postmitotic cells near the equatorial region had one or two Cajal bodies per nucleus, indicating these cells had only basal levels of transcription. Sm was also present as large foci in these cells. Interestingly, both the speckles and Cajal bodies were NuMA-positive in these post-mitotic cells. At the epithelial-fibre cell transition, Cajal body number increased, while their size decreased indicative of increased transcriptional activity. Fibrillarin adopted the open floret pattern indicating increased transcriptional activity. The nuclear speckles adopted a more diffuse nucleoplasmic pattern, although some spots were still observed. All NuMA colocalisation with the Cajal bodies and nuclear speckles was lost at this stage of lens cell differentiation. Transcriptional shutdown occurs at a later stage in fibre cell differentiation, prior to programmed nuclear destruction. In the lens, both the Cajal bodies and nuclear speckles again became NuMA-positive, although separate NuMA spots were also formed during transcriptional shutdown. These data suggest the nuclear matrix is important in the concentration of Cajal body and speckle components into large, distinct spots in transcriptionally inactive nuclei and also suggest a new role for NuMA in post-mitotic cells to assist in these sub-nuclear reorganisations.

Distinct functions of the unique C terminus of LAP2alpha in cell proliferation and nuclear assembly

The non-membrane-bound lamina-associated polypeptide 2 isoform, LAP2alpha, forms nucleoskeletal structures with A-type lamins and interacts with chromosomes in a cell cycle-dependent manner. LAP2alpha contains a LEM (LAP2, emerin, and MAN1) domain in the constant N terminus that binds to chromosomal barrier-to-autointegration factor, and a C-terminal unique region that is essential for chromosome binding. Here we show that C-terminal LAP2alpha fragment efficiently bound to mitotic chromosomes and inhibited assembly of endogenous LAP2alpha, nuclear membranes, and lamins A/C in in vitro nuclear assembly assays. Full-length recombinant LAP2alpha, which bound to chromosomes, and N-terminal fragment, which did not bind, had no effect on assembly. This suggested an essential role for the LAP2alpha C terminus in chromosome association and for the N-terminal LEM domain in subsequent assembly stages. In vivo analysis upon transient expression of GFP-tagged LAP2alpha fragments confirmed that, unlike the N-terminal fragment, the C-terminal fragment was able to bind to chromosomes during mitosis, if expressed weakly. At higher expression levels, C-terminal LAP2alpha fragment and full-length protein led to cell cycle arrest in interphase and apoptosis, as shown by fluorescence-activated cell sorter analysis, time lapse microscopy, and BrdUrd incorporation assays. These data indicated distinct functions of LAP2alpha in cell cycle progression during interphase and in nuclear reassembly during mitosis.

Disassembly of interchromatin granule clusters alters the coordination of transcription and pre-mRNA splicing

To examine the involvement of interchromatin granule clusters (IGCs) in transcription and pre-mRNA splicing in mammalian cell nuclei, the serine-arginine (SR) protein kinase cdc2-like kinase (Clk)/STY was used as a tool to manipulate IGC integrity in vivo. Both immunofluorescence and transmission electron microscopy analyses of cells overexpressing Clk/STY indicate that IGC components are completely redistributed to a diffuse nuclear localization, leaving no residual structure. Conversely, overexpression of a catalytically inactive mutant, Clk/STY(K190R), causes retention of hypophosphorylated SR proteins in nuclear speckles. Our data suggest that the protein-protein interactions responsible for the clustering of interchromatin granules are disrupted when SR proteins are hyperphosphorylated and stabilized when SR proteins are hypophosphorylated. Interestingly, cells without intact IGCs continue to synthesize nascent transcripts. However, both the accumulation of splicing factors at sites of pre-mRNA synthesis as well as pre-mRNA splicing are dramatically reduced, demonstrating that IGC disassembly perturbs coordination between transcription and pre-mRNA splicing in mammalian cell nuclei.

Distinct changes in intranuclear lamin A/C organization during myoblast differentiation

Intranuclear lamin foci or speckles have been observed in various cell types. In order to explore the possibility of changes in internal lamin organization during muscle differentiation, we have examined the appearance of A-type lamin speckles that associate with RNA splicing factor speckles in C2C12 myoblasts and myotubes. Lamin speckles were observed in dividing myoblasts but disappeared early during the course of differentiation in postmitotic myocytes, and were absent in myotubes and muscle fibers. However, no changes were seen in the typical peripheral organization of lamins A/C or B1 or in RNA splicing factor speckles. Lamin speckles were also absent in quiescent myoblasts but reappeared as cells were reactivated to enter the cell cycle. These changes were not observed in other quiescent cell types. Immunoblot analysis indicated that the abundance and migration of lamins A and C was not altered in differentiated myoblasts. When myotube or quiescent myoblast nuclei were extracted with nucleases and detergent, a uniformly stained internal lamina was revealed, indicating that lamins A/C were antigenically masked in these cells, probably owing to structural reorganization of the lamina during differentiation or quiescence. Our results suggest that muscle cell differentiation is accompanied by regulated rearrangements in the organization of the A-type lamins.

Early localization of NPA58, a rat nuclear pore-associated protein, to the reforming nuclear envelope during mitosis

We have studied the mitotic reassembly of the nuclear envelope, using antibodies to nuclear marker proteins and NPA58 in F-111 rat fibroblast cells. In earlier studies we have proposed that NPA58, a 58 kDa rat nuclear protein, is involved in nuclear protein import. In this report, NPA58 is shown to be localized on the cytoplasmic face of the envelope in interphase cells, in close association with nuclear pores. In mitotic cells NPA58 is dispersed in the cytoplasm till anaphase. The targeting of NPA58 to the reforming nuclear envelope in early telophase coincides with the recruitment of a well-characterized class of nuclear pore proteins recognized by the antibody mAb 414, and occurs prior to the incorporation of lamin B1 into the envelope. Significant protein import activity is detectable only after localization of NPA58 in the newly-formed envelope. The early targeting of NPA58 is consistent with its proposed role in nuclear transport.

Lamins in disease: why do ubiquitously expressed nuclear envelope proteins give rise to tissue-specific disease phenotypes?

The nuclear lamina is a filamentous structure composed of lamins that supports the inner nuclear membrane. Several integral membrane proteins including emerin, LBR, LAP1 and LAP2 bind to nuclear lamins in vitro and can influence lamin function and dynamics in vivo. Results from various studies suggest that lamins function in DNA replication and nuclear envelope assembly and determine the size and shape of the nuclear envelope. In addition, lamins also bind chromatin and certain DNA sequences, and might influence chromosome position. Recent evidence has revealed that mutations in A-type lamins give rise to a range of rare, but dominant, genetic disorders, including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy with conduction-system disease and Dunnigan-type familial partial lipodystrophy. An examination of how lamins A/C, emerin and other integral membrane proteins interact at the INM provides the basis for a novel model for how mutations that promote disease phenotypes are likely to influence these interactions and therefore cause cellular pathology through a combination of weakness of the lamina or altered gene expression.

Phosphorylation of NPA58, a rat nuclear pore-associated protein, correlates with its mitotic distribution

At the onset of mitosis in higher eukaryotic cells, the nuclear envelope and its components including subunits of the nuclear pore complexes are disassembled, and these are reassembled toward the end of mitosis. We have studied the role of protein phosphorylation in this process, by investigating the phosphorylation status of a specific pore-associated protein during mitosis. Using a monoclonal antibody, mAb E2, earlier shown to inhibit nuclear protein import in rat fibroblast cells, we have identified a 58-kDa protein termed NPA58 that is partially associated with nuclear pores based on a high degree of coincident immunofluorescence in dual labeling experiments with mAb 414, a well-studied pore-complex-reactive antibody. NPA58 is specifically phosphorylated during mitosis and dephosphorylated upon release from metaphase arrest. Confocal microscopy analysis shows that NPA58 is dispersed in the cytoplasm early in mitosis when it is phosphorylated, while its relocalization in the reforming nuclear envelope during telophase temporally correlates with its dephosphorylation upon release from metaphase arrest. Our data provide in vivo evidence that the modifications mediated by phosphorylation and dephosphorylation are required for regulating the mitotic localization of a nuclear-pore-associated protein.

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.

Lamina-associated polypeptide 2alpha binds intranuclear A-type lamins

The nucleoskeletal protein lamina-associated polypeptide 2(α) (LAP2*) contains a large, unique C terminus and differs significantly from its alternatively spliced, mostly membrane-integrated isoforms, such as LAP2beta. Unlike lamin B-binding LAP2beta, LAP2alpha was found by confocal immunofluorescence microscopy to colocalize preferentially with A-type lamins in the newly formed nuclei assembled after mitosis. While only a subfraction of lamins A and C (lamin A/C) was associated with the predominantly nuclear LAP2alpha in telophase, the majority of lamin A/C colocalized with LAP2alpha in G(1)-phase nuclei. Furthermore, selective disruption of A-type lamin structures by overexpression of lamin mutants in HeLa cells caused a redistribution of LAP2alpha. Coimmunoprecipitation experiments revealed that a fraction of lamin A/C formed a stable, SDS-resistant complex with LAP2alpha in interphase cells and in postmetaphase cell extracts. Blot overlay binding studies revealed a direct binding of LAP2alpha to exclusively A-type lamins and located the interaction domains to the C-terminal 78 amino acids of LAP2alpha and to residues 319–566 in lamin A/C, which include the C terminus of the rod and the entire tail common to lamin A/C. These findings suggest that LAP2alpha and A-type lamins cooperate in the organization of internal nuclear structures.

Omega speckles - a novel class of nuclear speckles containing hnRNPs associated with noncoding hsr-omega RNA in Drosophila

Fluorescence RNA:RNA in situ hybridization studies in various larval and adult cell types of Drosophila melanogaster showed that the noncoding hsr-omega nuclear (hsromega-n) transcripts were present in the form of many small speckles. These speckles, which we name ‘omega speckles’, were distributed in the interchromatin space in close proximity to the chromatin. The only chromosomal site where hsromega-n transcripts localized was the 93D locus or the hsromega gene itself. The number of nucleoplasmic speckles varied in different cell types. Heat shock, which inhibits general chromosomal transcription, caused the individual speckles to coalesce into larger but fewer clusters. In extreme cases, only a single large cluster of hsromega-n transcripts localizing to the hsromega locus was seen in each nucleus. In situ immunocytochemical staining using antibodies against heterogenous nuclear RNA binding proteins (hnRNPs) like HRB87F, Hrp40, Hrb57A and S5 revealed that, in all cell types, all the hnRNPs gave a diffuse staining of chromatin areas and in addition, were present as large numbers of speckles. Colocalization studies revealed an absolute colocalization of the hnRNPs and the omegaspeckles. Heat shock caused all the hnRNPs to cluster together exactly, following the hsromega-n transcripts. Immunoprecipitation studies using the hnRNP antibodies further demonstrated a physical association of hnRNPs and hsromega transcripts. The omegaspeckles are distinct from interchromatin granules since nuclear speckles containing serine/arginine-rich SR-proteins like SC35 and SRp55 did not colocalize with the ω speckles. The speckled distribution of hnRNPs was completely disrupted in hsromega nullosomics. We conclude that the hsromega-n transcripts play essential structural and functional roles in organizing and establishing the hnRNP-containing omega speckles and thus regulate the trafficking and availability of hnRNPs and other related RNA binding proteins in the cell nucleus.

Recruitment of protein phosphatase 1 to the nuclear envelope by A-kinase anchoring protein AKAP149 is a prerequisite for nuclear lamina assembly

Subcellular targeting of cAMP-dependent protein kinase (protein kinase A [PKA]) and of type 1 protein phosphatase (PP1) is believed to enhance the specificity of these enzymes. We report that in addition to anchoring PKA, A-kinase anchoring protein AKAP149 recruits PP1 at the nuclear envelope (NE) upon somatic nuclear reformation in vitro, and that PP1 targeting to the NE is a prerequisite for assembly of B-type lamins. AKAP149 is an integral membrane protein of the endoplasmic reticulum/NE network. The PP1-binding domain of AKAP149 was identified as K(153)GVLF(157). PP1 binds immobilized AKAP149 in vitro and coprecipitates with AKAP149 from purified NE extracts. Affinity isolation of PP1 from solubilized NEs copurifies AKAP149. Upon reassembly of somatic nuclei in interphase extract, PP1 is targeted to the NE. Targeting is inhibited by a peptide containing the PP1-binding domain of AKAP149, abolished in nuclei assembled with membranes immunodepleted of AKAP149, and restored after reincorporation of AKAP149 into nuclear membranes. B-type lamins do not assemble into a lamina when NE targeting of PP1 is abolished, and is rescued upon recruitment of PP1 to the NE. We propose that kinase and phosphatase anchoring at the NE by AKAP149 plays in a role in modulating nuclear reassembly at the end of mitosis.

Review: the dynamics of the nuclear lamins during the cell cycle-- relationship between structure and function

The nuclear lamins are members of the intermediate filament (IF) family of proteins. The lamins have an essential role in maintaining nuclear integrity, as do the other IF family members in the cytoplasm. Also like cytoplasmic IFs, the organization of lamins is dynamic. The lamins are found not only at the nuclear periphery but also in the interior of the nucleus, as distinct nucleoplasmic foci and possibly as a network throughout the nucleus. Nuclear processes such as DNA replication may be organized around these structures. In this review, we discuss changes in the structure and organization of the nuclear lamins during the cell cycle and during cell differentiation. These changes are correlated with changes in nuclear structure and function. For example, the interactions of lamins with chromatin and nuclear envelope components occur very early during nuclear assembly following mitosis. During S-phase, the lamins colocalize with markers of DNA replication, and proper lamin organization must be maintained for replication to proceed. When cells differentiate, the expression pattern of lamin isotypes changes. In addition, changes in lamin organization and expression patterns accompany the nuclear alterations observed in transformed cells. These lamin structures may modulate nuclear function in each of these processes.