Transfection of human lamins A and C into mouse embryonal carcinoma cells possessing only lamin B

Parietal endoderm cell line from a rat blastocyst

We have derived a cell line, RE1, from a pre-implantation rat blastocyst, resembling morphologically the L2 cell line from a parietal yolk sac carcinoma of the rat, as well as parietal endoderm cell lines of the mouse. The sub-cellular organization and epithelial characteristics of RE1 cells are described. The cells express cytokeratins of simple epithelia, and vimentin; and demonstrate synthesis of proteins of the extracellular matrix, such as laminin and collagen IV. Extensive Reichert's-like basement membrane is formed by RE1 cells when grown in suspension as aggregates. Cells have a microvillous surface morphology and abundant, rough endoplasmic reticulum which is swollen with apparent secretory material. These morphological and cytochemical features are characteristic of parietal endoderm cells in vivo, and the RE1 cell line is deduced to be rat parietal endoderm. In addition, RE1 cells were examined for expression of stage-specific embryonic antigens: cells reacted with antibody against SSEA-1/TEC-1 and EMA-1, constituting the first observation of parietal endoderm cells expressing the respective epitopes. RE1-cell monolayers did not generate transepithelial resistances or potential differences in vitro, consistent with their formation of leaky epithelia. Our observations on RE1-cell morphology and ultrastructure are consistent with the occurrence of epithelial-mesenchyme transitions in culture.

Ectopic overexpression of Drosophila lamin C is stage-specific lethal

To gain insight into the function of the developmentally regulated A-type lamins we transformed Drosophila melanogaster with a construct containing the hsp70 promoter followed by the Drosophila lamin C (an analog of vertebrate A-type lamins) cDNA. Lamin C was expressed ectopically after heat shock of embryos and localized to the nucleus. No phenotypic change was observed after lamin C expression in embryos that normally do not contain lamin C. However, ectopic expression of lamin C during most larval (but not pupal) stages stalled growth, inhibited ecdysteroid signaling (in particular during the larval-prepupal transition), resulted in development of melanotic tumors, and finally caused death. During pupation in control animals, when massive apoptosis of larval tissues takes place, lamin C is proteolyzed into a fragment with a size similar to that predicted by caspase cleavage. The ectopically expressed lamin C is identically cleaved, resulting in a large increase of the steady-state level of the lamin C fragment. A null mutation of the dcp-1 gene, one of the two known Drosophila caspase genes, also results in development of melanotic tumors and larval death, suggesting that the ectopically expressed lamin C inhibits apoptosis through competitive inhibition of caspase activity.

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.

Regulation of prelamin A endoprotease activity by prelamin A

The maturation of lamin A is completed by the endoproteolytic cleavage of its farnesylated precursor protein, prelamin A. In the absence of this cleavage, prelamin A can neither give rise to lamin A nor assemble into the nuclear lamina. We call the enzyme which catalyzes this endoproteolytic step the 'prelamin A endoprotease'. In this study, we begin characterization of the regulation of prelamin A endoprotease. In particular, we address the question as to whether prelamin A endoprotease activity is constitutive in cells or responds to expression of prelamin A. To do this, we compared the activity of this novel endoprotease in cells which express prelamin A with those that do not. Our data shows that the enzymatic activity of prelamin A endoprotease is enhanced by the expression of prelamin A.

Isolation and characterization of a feeder-dependent, porcine trophectoderm cell line obtained from a 9-day blastocyst

We have established in culture a feeder-dependent cell line, termed TE1, from a 9 day, pre-implantation, porcine embryo. TE1 cells were observed by light and electron microscopy, and characterized by immunocytochemistry: the morphology, cytology and ultrastructure of this cell line are described. The cells display epithelial characteristics, as revealed using immunofluorescence microscopy with antibody against cytokeratins of simple epithelia, but not with antibody against vimentin. The cells demonstrate many morphological and cytochemical features in common with trophectoderm of the intact porcine blastocyst. For example, TE1 cells are polarized and possess tight junctions at their borders, similar to those found in trophectoderm of the pre-implantation embryo. Moreover, TE1 cells label positively for the porcine trophectoderm-specific monoclonal antibody, SN1/38. Thus, by several important criteria TE1 is deduced to be a porcine trophectoderm cell line.

Expression of prelamin A but not mature lamin A confers sensitivity of DNA biosynthesis to lovastatin on F9 teratocarcinoma cells

The role of inhibition of prelamin A processing in the inhibition of DNA synthesis by lovastatin was examined by expressing prelamin A in F9 teratocarcinoma cells. These cells, normally lacking expression of the A/C lamins, were transfected with constructs expressing either prelamin A or mature lamin A and the effect of lovastatin on DNA biosynthesis was assessed. It was found that expression of prelamin A specifically conferred sensitivity to inhibition of DNA biosynthesis by lovastatin on F9 cells.

Regulation of the expression of lamins A and C is post-transcriptional in P19 embryonal carcinoma cells

The polypeptide composition of the nuclear lamina can display important variations: undifferentiated cells express only lamin B and they acquire lamins A and C only after differentiation. We have analyzed the expression of lamins A and C in P19 pluripotent mouse embryonal carcinoma cells. Undifferentiated P19 cells are completely devoid of lamins A and C. We show that undifferentiated P19 cells contain low, but detectable steady-state levels of RNAs for lamins A and C that begin to increase by 24 h of retinoic acid-induced differentiation. However, the rate of transcription of the lamin A and C gene(s), analyzed by run-on transcription assays, remains unchanged during the differentiation process. These results demonstrate that, at least in P19 embryonal carcinoma cells, regulation of the expression of lamins A and C is a post-transcriptional event.

Phorbol esters induce transient changes in the accessibility of the carboxy-terminal domain of nuclear lamin A

Treatment of human epithelial cells in culture with phorbol esters (TPA) gives rise to a transient and reversible loss of accessibility to antibodies of the nonhelical carboxy-terminal domain of nuclear lamin A that distinguishes it from lamin C. No change in the accessibility of epitopes present in the common domain of lamins A and C was observed. Loss of accessibility of lamin A was not due to proteolytic degradation nor to modification of the isoelectric point of lamin A and did not depend upon protein kinase C activation nor protein synthesis. Perturbation of desmosome organization by growth in low calcium blocked the effect of TPA on lamin A. Prolonged exposure to nocodazole, one of the effects of which is a perinuclear collapse of intermediate filaments, also blocked the effect of TPA on lamin A. These results suggest that the initial target of TPA may be at the level of cell-cell contacts and that the perturbation induced by TPA may be propagated via the structural link formed by intermediate filaments between the cell surface and the nucleus, giving rise to a change in conformation of the carboxy-terminal domain of lamin A or to an interaction of this domain with another nuclear component. These results form the basis for the hypothesis that the interphase nuclear lamina may play an active role in the process of mechanochemical signal transduction.

Autoantibodies to major and minor nuclear lamins are not restricted to autoimmune diseases

Autoantibodies to lamins, the major polypeptide components of the nuclear lamina, have been reported in selected sera from patients with autoimmune diseases, including anti-lamin B in systemic lupus erythematosus (SLE) and anti-lamins AC in autoimmune chronic active hepatitis (CAH). We have studied the frequency, specificity, and isotypy of autoantibodies to major and minor lamins by immunoblotting on purified rat liver lamins in 190 sera from normal controls (n = 62), rheumatic disease controls (n = 42), and autoimmune disease patients (n = 86). The frequency of anti-lamin in normal controls was 85.5%, and ranged from 77 to 100% in the other groups. Anti-lamin frequency was not related to age, sex, or disease duration. Reactivity with lamin A or with minor lamins only was observed with 7 various sera and 2 normal sera, respectively. Between groups, the proportions of reactive sera were not different for lamins AC (18-47%) and for lamin B (22-36%). In particular, anti-lamin B and anti-lamins AC were not more common in SLE or CAH than in normal sera. The most frequent lamin specificity of SLE sera was anti-lamins ABC. Anti-lamin isotypes were IgG and/or IgM. Titers of IgM antibodies were not higher in any group. However, IgG anti-lamin titers were higher in CAH than in normal, ankylosing spondylitis, or SLE sera. The highest end point titers (greater than or equal to 1:3200) were observed with CAH, SLE, and rheumatoid arthritis (RA) sera with IgG anti-lamins AC, B, or ABC, or with IgM anti-lamins ABC. None of these SLE and RA patients had evidence of liver disease. Reactivity with minor lamins was more frequent in CAH. We conclude that anti-lamin autoantibodies are present in sera from most individuals and that the highest titers are found in sera from patients with autoimmune diseases.

Assembly-disassembly of the nuclear lamina

Recently, progress in the study of lamins has been made in three areas: signals required for targetting newly synthesized lamins to the correct subnuclear compartment have been identified; information on lamina assembly has been obtained from in vitro studies using bacterially expressed proteins; and a mechanistic explanation for how the nuclear lamina is diassembled at the onset of mitosis is emerging.

Internuclear exchange of an inner nuclear membrane protein (p55) in heterokaryons: in vivo evidence for the interaction of p55 with the nuclear lamina

The movement between nuclei of an integral protein of the inner nuclear membrane has been studied in rat/mouse and rat/hamster heterokaryons. This protein, p55, was found to equilibrate between nuclei over a period of approximately 6 h in the absence of new protein synthesis. When rat/mouse heterokaryons were constructed using an undifferentiated murine embryonal carcinoma (P19), which lacks lamins A and C, no accumulation of p55 in the mouse cell nucleus was observed. However, P19 nuclei could be rendered competent to accumulate p55 by transfecting the parent cells with human lamin A before cell fusion, supporting the notion that p55 may interact with the nuclear lamina. Since p55 does not appear to be able to dissociate from the nuclear membrane, it is concluded that this exchange between nuclei does not occur in the aqueous phase and instead is probably membrane mediated. It is proposed that this protein may be free to move between the inner and outer nuclear membranes via the continuities at the nuclear pore complexes and that transfer between nuclei occurs via lateral diffusion through the peripheral ER, which appears to form a single continuous membrane system in these heterokaryons. One implication of these observations is that accumulation of at least some integral proteins in the inner nuclear membrane may be mediated by interactions with other nuclear components and may not require a single defined targeting sequence.

Cellular intermediate filament networks and their derangement in alcoholic hepatitis

Intermediate filaments are major components of most eukaryotic cells that form from the polymerization of protein subunits that are expressed in tissue and development specific fashions. The interactions of intermediate filaments with a myriad of other cellular proteins and structures give rise to a complex overall cellular architecture that is likely responsible for cellular well-being. The mature 10-nm filaments are relatively stable cellular structures, but the intermediate filaments undergo major morphological and biochemical changes, especially during mitosis, differentiation, and in response to certain drugs. Evidence exists that hepatocyte intermediate filaments (keratin filaments) are deranged in alcoholic hepatitis, an inflammatory liver disease of alcoholics and heavy spree drinkers. The classical and characteristic pathological hepatocyte inclusion bodies of alcoholic hepatitis, Mallory bodies, are composed in part of normal keratins that likely derive from the pre-existing hepatocyte intermediate filament network. It is unclear if intermediate filament network derangement in alcoholic hepatitis is directly caused by the actions of ethanol or its metabolites on intermediate filaments or their associated structures, or whether alcohol causes a cellular insult or injury elsewhere and a subsequent response (e.g., immune) causes intermediate filament network derangement. The precise mechanisms responsible for intermediate filament derangement remain to be elucidated; however, experimental data exist that support and refute several hypotheses. Hopefully, further studies will help determine a better overall understanding of the abnormalities of intermediate filaments and their relationship to the pathophysiology of alcoholic hepatitis and other diseases.

Differential accessibility of the tail domain of nuclear lamin A in interphase and mitotic cells

Human autoantibodies reactive against the tail domain exclusive to lamin A and absent from lamin C have been used for immunofluorescence studies on human fibroblast and epithelial cells. These autoantibodies were seen to react on mitotic cells where lamin A is present in a soluble depolymerized form and to react against lamin A in assembled interphase nuclear lamina after in situ extraction of chromatin. Taken together, these results support the suggestion that the tail domain of lamin A may be involved in the putative interaction of lamin A with chromatin.