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PubMed1997

[Chromosomal localization of M/SAR from DNA of the clone lambda20p1.4 in Drosophila melanogaster salivary glands].

M/SAR of Drosophila melanogaster DNA clone gamma 20p 1.4 was localized on chromosomes by in situ hybridization of the gamma 20p 1.4 DNA fragment and its deletion mutants with salivary gland chromosomes of third instar larvae of the lines Canton S and Oregon R. The M/SAR sequence, capable of specifically binding to lamin in vitro, was detected in the pericentric regions 20CD, 40EF, 41AB, the proximal part of region 81 of chromosome 3, and region 101 of chromosome 4, which attach to the nuclear envelope in cells of D. melanogaster salivary glands, and in region 49D, which does not come into contact with the nuclear periphery. This is the first M/SAR localized in pericentric chromatin of D. melanogaster.

Animals↗
PubMed2006

Calcium microdomains in mitochondria and nucleus.

Endomembranes modify the progression of the cytosolic Ca(2+) wave and contribute to generate Ca(2+) microdomains, both in the cytosol and inside the own organella. The concentration of Ca(2+) in the cytosol ([Ca(2+)](C)), the mitochondria ([Ca(2+)](M)) and the nucleus ([Ca(2+)](N)) are similar at rest, but may become very different during cell activation. Mitochondria avidly take up Ca(2+) from the high [Ca(2+)](C) microdomains generated during cell activation near Ca(2+) channels of the plasma membrane and/or the endomembranes and prevent propagation of the high Ca(2+) signal to the bulk cytosol. This shaping of [Ca(2+)](C) signaling is essential for independent regulation of compartmentalized cell functions. On the other hand, a high [Ca(2+)](M) signal is generated selectively in the mitochondria close to the active areas, which tunes up respiration to the increased local needs. The progression of the [Ca(2+)](C) signal to the nucleus may be dampened by mitochondria, the nuclear envelope or higher buffering power inside the nucleoplasm. On the other hand, selective [Ca(2+)](N) signals could be generated by direct release of stored Ca(2+) into the nucleoplasm. Ca(2+) release could even be restricted to subnuclear domains. Putative Ca(2+) stores include the nuclear envelope, their invaginations inside the nucleoplasm (nucleoplasmic reticulum) and nuclear microvesicles. Inositol trisphosphate, cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate have all been reported to produce release of Ca(2+) into the nucleoplasm, but contribution of these mechanisms under physiological conditions is still uncertain.

Animals↗
PubMed2003

Nuclear accumulation of the small GTPase Gsp1p depends on nucleoporins Nup133p, Rat2p/Nup120p, Nup85p, Nic96p, and the acetyl-CoA carboxylase Acc1p.

The small GTPase Ran/Gsp1p plays an essential role in nuclear trafficking of macromolecules, as Ran/Gsp1p regulates many transport processes across the nuclear pore complex (NPC). To determine the role of nucleoporins in the generation of the nucleocytoplasmic Gsp1p concentration gradient, mutations in various nucleoporin genes were analyzed in the yeast Saccharomyces cerevisiae. We show that the nucleoporins Nup133p, Rat2p/Nup120p, Nup85p, Nic96p, and the enzyme acetyl-CoA carboxylase (MTR7) control the distribution and cellular concentration of Gsp1p. At the restrictive temperature the reporter protein GFP-Gsp1p, which is too large to diffuse across the nuclear envelope, fails to concentrate in nuclei of nup133delta, rat2-1, nup85delta, nic96deltaC, and mtr7-1 cells, demonstrating that GFP-Gsp1p nuclear import is deficient. In addition, the concentration of Gsp1p is severely reduced in mutants nup133Delta and mtr7-1 under these conditions. We have now identified the molecular mechanisms that contribute to the dissipation of the Gsp1p concentration gradient in these mutants. Loss of the Gsp1p gradient in nup133delta and rat2-1 can be explained by reduced binding of the Gsp1p nuclear carrier Ntf2p to NPCs. Likewise, nup85delta cells that mislocalize GFP-Gsp1p at the permissive as well as non-permissive temperature have a diminished association of Ntf2p-GFP with nuclear envelopes under both conditions. Moreover, under restrictive conditions Prp20p, the guanine nucleotide exchange factor for Gsp1p, mislocalizes to the cytoplasm in nup85delta, nic96deltaC, and mtr7-1 cells, thereby contributing to a collapse of the Gsp1p gradient. Taken together, components of the NPC subcomplex containing Rat2p/Nup120p, Nup133p, and Nup85p, in addition to proteins Nic96p and Mtr7p, are shown to be crucial for the formation of a nucleocytoplasmic Gsp1p gradient.

Acetyltransferases↗
PubMed1995

Localization of non-conventional protein kinase C isoforms in bovine brain cell nuclei.

Using Western blotting and immunofluorescence microscopy we detected the protein kinase C isoforms delta, epsilon and zeta in isolated cell nuclei from bovine cerebral cortex. Both protein kinase C (PKC) delta and PKC epsilon are present in higher concentrations in neuronal than in glial nuclei and are located inside the nucleus and at the nuclear envelope. There they give a punctate staining in immunofluorescence microscopy. PKC zeta is also present both in the nucleoplasm and at the nuclear envelope. PKC eta could not be detected in the cell nuclei and, even in the homogenate of cerebral cortex, this isoform is present only in very low concentrations. The antibody against PKC eta bound strongly to a nucleoplasmic protein with an apparent molecular mass of 99 kDa. The localization of non-conventional PKC isoforms at the cell nucleus strongly indicates that these isoforms are directly involved in the regulation of nuclear processes.

Animals↗
PubMed1999

Characterization of cyclic nucleotide phosphodiesterase isoforms associated to isolated cardiac nuclei.

The identity and location of nuclear cyclic nucleotide phosphodiesterases (PDE) has yet to be ascertained. Intact cardiac nuclei and subnuclear fractions from ovine hearts were isolated to determine cAMP-specific PDE activity which was 3-fold greater than that of cGMP PDE, the latter being insensitive to Ca-calmodulin and zaprinast. Specific hydrolytic activities of the cardiac nuclear envelopes (NE) were similar to those measured in the corresponding intact nuclei, thus suggesting that most PDE activity is associated with the nuclear membrane. Moreover, the main hydrolytic activities in cardiac nuclei were attributed to PDE4 (56%) and PDE3 (44%). The pharmacological sensitivity of each isoform in terms of IC(50), K(m) and K(i) values was typical of previously characterized cardiac PDE 3 and 4 isoforms. PDE2 (cGMP-stimulated PDE) represented a minor component (8-9%) of total hydrolytic activity. Solubilization of nuclear envelopes and HPLC separation also yielded rolipram-sensitive PDE activities. Upon 1% Triton X-100 extractions, the presence of PDE3 and PDE4 was revealed in a low speed, nucleopore complex-enriched, P1 pellet. In addition, Western blot analysis demonstrated the presence of PDE4B and PDE4D subtypes in the nuclei as well as enrichment in NE. However, in the same preparations, the presence of PDE4A could not be ascertained. Altogether, these results suggest an intrinsic and predominant association of these nuclear PDEs with the NE and much likely with nucleopore complexes.

1-Methyl-3-isobutylxanthine↗
PubMed2005

Syne proteins anchor muscle nuclei at the neuromuscular junction.

Vertebrate skeletal muscle fibers contain hundreds of nuclei, of which three to six are functionally specialized and stably anchored beneath the postsynaptic membrane at the neuromuscular junction (NMJ). The mechanisms that localize synaptic nuclei and the roles they play in neuromuscular development are unknown. Syne-1 is concentrated at the nuclear envelope of synaptic nuclei; its Caenorhabditis elegans orthologue ANC-1 functions to tether nuclei to the cytoskeleton. To test the involvement of Syne proteins in nuclear anchoring, we generated transgenic mice overexpressing the conserved C-terminal Klarsicht/ANC-1/Syne homology domain of Syne-1. The transgene acted in a dominant interfering fashion, displacing endogenous Syne-1 from the nuclear envelope. Muscle nuclei failed to aggregate at the NMJ in transgenic mice, demonstrating that localization and positioning of synaptic nuclei require Syne proteins. We then exploited this phenotype to show that synaptic nuclear aggregates are dispensable for maturation of the NMJ.

Animals↗
PubMed1989

Functional role of newly formed pore complexes in postmitotic nuclear reorganization.

Many nuclear proteins are released into the cytoplasm at prometaphase and are transported back into the daughter nuclei at the end of mitosis. To determine the role of this reentry in nuclear remodelling during early interphase, we experimentally manipulated nuclear protein uptake in dividing cells. Recently we and others have shown that signal-dependent, pore complex-mediated uptake of nuclear protein is blocked in living cells on microinjection of the lectin wheat germ agglutinin (WGA), or of antibodies such as PI1 that are directed against WGA-binding pore complex glycoproteins. In the present study, we microinjected mitotic PtK2 cells with WGA or antibody PI1 and followed nuclear reorganization of the daughter cells by immunofluorescence and electron microscopy. The inhibitory effect on nuclear protein uptake was monitored by co-injection of the karyophilic protein nucleoplasmin. When injected by itself early in mitosis, nucleoplasmin became sequestered into the daughter nuclei as they entered telophase. In contrast, nucleoplasmin was excluded from the daughter nuclei in the presence of WGA or antibody PI1. Although PtK2 cells with blocked nuclear protein uptake completed cytokinesis, their nuclei showed a telophase-like completed cytokinesis, their nuclei showed a telophase-like organization characterized by highly condensed chromatin surrounded by a nuclear envelope containing a few pore complexes. These findings suggest that pore complexes become functional as early as telophase, in close coincidence with nuclear envelope reformation. They further indicate that the extensive structural rearrangement of the nucleus during the telophase-G1 transition is dependent on the influx of karyophilic proteins from the cytoplasm through the pore complexes, and is not due solely to chromosome-associated components.

Animals↗
PubMed1990

Immunocytochemical identification of cytoskeletal linkages to smooth muscle cell nuclei and mitochondria.

In avian smooth muscle cells, desmin-containing intermediate filaments (IFs) are a prominent component of the cytoskeleton and are readily seen in several domains, including the axial intermediate filament bundle (IFB). Both the nucleus and some of the mitochondria are partly surrounded by elements of the IFB. By using anti-desmin and protein-A-colloidal gold labeling, we have identified intermediate filaments that form linkages with the nuclear envelope and with mitochondria. These linkage regions seem to occupy a proportionately greater part of the mitochondrial surface than of the nuclear envelope. The existence of these linkages in smooth muscle cells is consistent with results that support similar linkages to mitochondria and other cellular structures in various cells that contain either vimentin or keratin IFs. These linkages could functionally restrain or assist in homeostatically restoring organelles to their normal position after the rearrangement that accompanies the substantial shortening of smooth muscle cells.

Animals↗
PubMed2003

Expression of lamin A mutated in the carboxyl-terminal tail generates an aberrant nuclear phenotype similar to that observed in cells from patients with Dunnigan-type partial lipodystrophy and Emery-Dreifuss muscular dystrophy.

Autosomal dominantly inherited missense mutations in lamins A and C cause familial partial lipodystrophy of the Dunnigan-type (FPLD), and myopathies including Emery-Dreifuss muscular dystrophy (EDMD). While mutations responsible for FPLD are restricted to the carboxyl-terminal tails, those responsible for EDMD are spread throughout the molecules. We observed here the same structural abnormalities in the nuclear envelope and chromatin of fibroblasts from patients with FPLD and EDMD, harboring missense mutations at codons 482 and 453, respectively. Similar nuclear alterations were generated in fibroblasts, myoblasts, and preadipocytes mouse cell lines overexpressing lamin A harboring either of these two mutations. A large variation in sensitivity to lamin A overexpression was observed among the three cell lines, which was correlated with their variable endogenous content in A-type lamins and emerin. The occurrence of nuclear abnormalities was reduced when lamin B1 was coexpressed with mutant lamin A, emphasizing the functional interaction of the two types of lamins. Transfected cells therefore develop similar phenotypes when expressing lamins mutated in the carboxyl-terminal tail at sites responsible for FPLD or EDMD.

Adipocytes↗
PubMed1998

Nuclear lamins: their structure, assembly, and interactions.

Nuclear lamins are intermediate filament-type proteins that are the major building blocks of the nuclear lamina, a fibrous proteinaceous meshwork underlying the inner nuclear membrane. Lamins can also be localized in the nuclear interior, in a diffuse or spotted pattern. Nuclei assembled in vitro in the absence of lamins are fragile, indicating that lamins mechanically stabilize the cell nucleus. Available evidence also indicates a role for lamins in DNA replication, chromatin organization, spatial arrangement of nuclear pore complexes, nuclear growth, and anchorage of nuclear envelope proteins. In this review we summarize the current state of knowledge on the structure, assembly, and possible functional roles of nuclear lamins, emphasizing the information concerning the ability of nuclear lamins to self-assemble into distinct oligomers and polymers.

Animals↗
PubMed1984

A morphometric study of developing pancreatic acinar cells of rats during prenatal life.

The pancreatic acinar cells of rat embryos obtained at 15, 17, 19 and 21 days of gestation have been examined using fine-structural and morphometric techniques. Morphometric analysis demonstrates significant variations in the average volume of the cell, nucleus and cytoplasm, and the volume, surface and numerical densities of various cytoplasmic organelles during fetal life. In particular, the volume and surface densities of rER exhibit maximal values at 19 days of gestation, suggesting that secretory proteins are produced most actively at this time. Furthermore, membrane continuity between the nuclear envelope and rER is frequently discernible in acinar cells, indicating that at this stage the rER is mainly derived from the nuclear envelope. Zymogen granules first appear at 17 days of gestation. By 21 days, they occupy the greater part of the cytoplasm of the acinar cells, no polarity being seen in their distribution pattern. No direct evidence for the secretion of zymogen granules has been observed during fetal life. It therefore appears that membrane transport involved with intracellular movement of newly synthesized proteins from rER via the Golgi complex to zymogen granules occurs in one direction and lacks regulation.

Animals↗
PubMed2000

Nuclear-specific degradation of Far1 is controlled by the localization of the F-box protein Cdc4.

Far1 is a bifunctional protein that is required to arrest the cell cycle and establish cell polarity during yeast mating. Here we show that SCF(Cdc4) ubiquitylates Far1 in the nucleus, which in turn targets the multi-ubiquitylated protein to 26S proteasomes most likely located at the nuclear envelope. In response to mating pheromones, a fraction of Far1 was stabilized after its export into the cytoplasm by Ste21/Msn5. Preventing nuclear export destabilized Far1, while conversely cytoplasmic Far1 was stabilized, although the protein was efficiently phosphorylated in a Cdc28-Cln-dependent manner. The core SCF subunits Cdc53, Hrt1 and Skp1 were distributed in the nucleus and the cytoplasm, whereas the F-box protein Cdc4 was exclusively nuclear. A cytoplasmic form of Cdc4 was unable to complement the growth defect of cdc4-1 cells, but it was sufficient to degrade Far1 in the cytoplasm. Our results illustrate the importance of subcellular localization of F-box proteins, and provide an example of how an extracellular signal regulates protein stability at the level of substrate localization.

Amino Acid Sequence↗
PubMed1986

Spectrin subtypes in mammalian brain: an immunoelectron microscopic study.

Spectrin is a major cytoskeletal component of the brain. At least 2 distinct spectrin subtypes are found in mammalian brain: brain spectrin(240/235) and brain spectrin(240/235E). In the present study spectrin subtypes were localized in the adult mouse brain by immunoelectron microscopy using antibodies that recognize each subtype. Brain spectrin(240/235E) was concentrated in neuronal cell bodies, dendrites, and postsynaptic terminals. It was also prominently associated with the plasma membrane, microtubules, filaments, mitochondria, endoplasmic reticulum, and nuclear envelope, and it appeared to interconnect structural elements within the cell. Brain spectrin(240/235E) also was localized to the plasma membrane, nuclear envelope, and cytoplasmic organelles of glial cell bodies. Brain spectrin(240/235) was detected in axons and presynaptic elements, where it was associated with the plasma membrane, microtubules, filaments, synaptic vesicles, and mitochondria. These results show that spectrin is distributed throughout the cytoplasm of neural cells, the location of spectrin is dependent on subtype, and the cytoplasmic surface of plasma membrane and organelles contains an extensive and intricate spectrin meshwork.

Animals↗
PubMed1983

The effects of taxol on the organization of the cytoskeleton in cultured ovarian granulosa cells.

Exposure of ovarian granulosa cells to taxol, a potent microtubule assembly promoting and stabilization agent, results in a time and dose-dependent alteration in the organization of cytoplasmic filaments (microtubules, microfilaments, intermediate filaments) and organelles. Transmission electron microscopy and fluorescence microscopy are used in conjunction with various antimitotic drugs to evaluate the action of taxol on suspension and monolayer cultures. Taxol treatment (1.0 microM) of freshly isolated suspended cells for 4 h leads to the formation of multiple bundles of microtubules emanating from a single centrosomal organizing center. Within 4 h after addition to monolayer cultures, 1.0 microM taxol induces a lateral aggregation of microtubules which, by 12 to 24 h of treatment, results in the appearance of dense bundles of tightly-packed microtubules located near the centrosome, close to the outer nuclear envelope, and at peripheral cytoplasmic sites. By electron microscopy, both lateral and end-on associations between bundled microtubules and the nuclear envelope are apparent. During the course of taxol-induced bundling, intermediate filaments are altered from a normally dispersed fibrous network into perinuclear aggregates. Microtubule bundling is also associated with a rearrangement of actin filaments from stress fibers into a marginal distribution, and actin appears to be excluded from sites of bundle formation. When granulosa cells are treated with equivalent concentrations of the microtubule-disrupting drugs, colchicine or nocodazole, either before or during taxol treatment, bundle formation is prevented suggesting that an intact microtubule network is required for taxol-induced bundling. Colchicine, but not nocodazole, is able to reverse the effects of taxol on bundle formation. The data suggest that the cellular distribution of cytoplasmic organelles, intermediate filaments, and microfilaments are regulated by the cytoplasmic microtubule complex.

Alkaloids↗
PubMed1984

Lamin B from rat liver nuclei exists both as a lamina protein and as an intrinsic membrane protein.

Rat liver nuclear matrix structures were isolated while preserving the integrity of the nuclear envelope, i.e. in the absence of any detergent extraction. In order to determine the relationships between the nuclear membranes and peripheral lamina, nuclear matrix-envelope preparations were submitted to sodium carbonate extraction (0.1 M, pH 11.5), a solvent which solubilizes both peripheral membrane proteins and membrane-enclosed contents. One-dimensional and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of material insoluble in sodium carbonate confirmed that intrinsic membrane proteins were indeed retained in the membrane structures. Approximately 50 to 60% of the lamin B present in matrix-envelope preparations was found in these insoluble membranes while a smaller amount of lamin A and even less of lamin C resisted complete extraction. The identity of the lamins was confirmed by their migration on two-dimensional gels and by comparison of one-dimensional peptide maps. The same results were obtained using nuclear membranes prepared by a milder heparin procedure. The location of lamin B as an intrinsic membrane protein was also established by photoaffinity labeling with the membrane-penetrating reagent azidopyrene. A small but reproducible amount of labeling occurred as well on lamin A polypeptides. These results support the hypothesis that the peripheral lamina is attached to the nuclear envelope and anchored there via the presence of lamin B molecules within the bilayer of the inner nuclear membrane.

Animals↗
PubMed1998

LBR, a chromatin and lamin binding protein from the inner nuclear membrane, is proteolyzed at late stages of apoptosis.

Chromatin condensation and apposition to the nuclear envelope is an important feature of the execution phase of apoptosis. During this process, lamin proteins that are located between the inner nuclear membrane and heterochromatin are proteolyzed by the apoptosis-specific protease caspase 6. We have investigated the fate of nuclear membranes during apoptosis by studying the lamin B receptor (LBR), a transmembrane protein of the inner nuclear membrane. LBR interacts through its nucleoplasmic amino-terminal domain with both heterochromatin and B-type lamins, and is phosphorylated throughout the cell cycle, but on different sites in interphase and mitosis. We report here that: (i) the amino-terminal domain of LBR is specifically cleaved during apoptosis to generate an approximately 20 kDa soluble fragment; (ii) the cleavage of LBR is a late event of apoptosis and occurs subsequent to lamin B cleavage; (iii) the phosphorylation of LBR during apoptosis is similar to that occurring in interphase. As the association of condensed chromatin with the inner nuclear membrane persists until the late stages of apoptosis, we suggest that the chromatin binding protein LBR plays a major role in maintaining this association.

Animals↗
PubMed1994

Lamins and lamin-associated proteins.

A variety of morphological and biochemical studies have established that the nuclear lamins play an important role in nuclear structure and dynamics. Recent work reveals the existence of specialized lamin isotypes and novel pathways of modulation of lamin import into the nucleus via phosphorylation by protein kinase C. Other studies also unveil a wide spectrum of molecular interactions between the lamin proteins and integral membrane components of the nuclear envelope.

Animals↗
PubMed1977

Interrelationship between annulate lamellae and the cytoplasmic microtubule complex in tumor cells in vivo and in vitro.

Annulate lamellae were observed in untreated in vivo ascites tumor cells with a diminished cytoplasmic microtubule complex. The ascites tumor cells in vitro responded to prolonged colchicine treatment with the formation of annulate lamellae. Simultaneous treatment with dibutyryl cycle adenosine monophosphate and colchicine seemed to enhance the formation of annulate lamellae. Single pore complexes were found in the granular endoplasmic reticulum in untreated tumor cell in vitro, and a close association of microtubules with the nuclear envelope was observed. Our results suggest the existence of an interrelationship between the cytoplasmic microtubule complex and certain other cell structures, i.e. the nuclear envelope, annulate lamellae, and single pore complexes.

Animals↗