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

DNA-lipid interactions in vitro and in vivo

The data on lipid-nucleic interactions and their role in vitro and in vivo are presented. The results of study of DNA-lipid complexes in absence and in presence of divalent metal cations (triple complexes) are discussed. The triple complexes represent the generation of cellular structures such as pore complexes of eucaryotes and "Bayer's junctions" of procaryotes. The participation of triple complexes in the formation of structure of bacterial and eucaryotic nucleoid and nuclear matrix is analysed. A model of formation of triple complexes and cellular structures and their role in DNA-lipid interactions are discussed.

Cell-free assays for gamma-secretase activity

The amyloid b-protein (Ab) deposited in Alzheimer's disease (AD) is a normally secreted proteolytic product of the amyloid b-protein precursor (APP). Generation of Ab from the APP requires two sequential proteolytic events: an initial b-secretase cleavage at the amino terminus of the Ab sequence followed by g-secretase cleavage at the carboxyl terminus of Ab. We describe the development of a robust in vitro assay for g-secretase cleavage by showing de novo Ab production in vitro and establish that this assay monitors authentic gamma-secretase activity by documenting the production of a cognate g-CTF, confirming the size of the Ab produced by mass spectrometry, and inhibiting cleavage in this system with multiple inhibitors that alter g-secretase activity in living cells. Using this assay, we demonstrate that the g-secretase activity 1) is tightly associated with the membrane, 2) can be solubilized, 3) has a pH optimum of 6.8 but is active from pH 6.0 to pH >8.4, and 4) ascertain that activities of the g-40 and g-42 are indeed pharmacologically distinct. These studies should facilitate the purification of the protease or proteases that are responsible for this unusual activity, which is a major therapeutic target for the treatment of AD.

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.

Host nuclear abnormalities and depletion of nuclear antigens induced in Trichinella spiralis-infected muscle cells by the anthelmintic mebendazole

Infection by the parasitic nematode Trichinella spiralis induces cell cycle repositioning (chronic suspension in apparent G2/M) and genetic reprogramming in differentiated mammalian skeletal muscle cells. These changes occur in association with dramatic enlargement of infected host cell nuclei (as large as 17 microm in diameter) and nucleoli. Nuclear antigens (NA) that colocalize with host chromatin have been detected by antibodies to T. spiralis antigens, but the functions of these NA are unresolved. Mebendazole (MBZ) preferentially binds parasite versus host beta-tubulins, is implicated in inhibiting secretion in nematodes and induces cytoplasmic changes in muscle cells infected with T. spiralis. These infected cell changes might be indirect via MBZ inhibition of parasite secretions. This effect would have implications for host/parasite interactions and was evaluated here. MBZ treatment of chronically infected mice caused: (1) a significant deformation of host nuclei and diminution of nucleoli by 4 and 6 days of treatment (dot), respectively; (2) a reduction of nuclear lamins A/C in infected cell nuclei that was concomitant with nuclear deformation; and (3) significant reductions in total RNA, general protein and acid phosphatase activity levels. These changes were associated with the depletion of NA from host nuclei detected by 4 dot. However, DNA content of infected cell nuclei was not detectably reduced and muscle gene expression was not reactivated. The cellular changes documented are likely to account for previously described cytoplasmic alterations induced by MBZ. Concomitant depletion of NA from infected cell nuclei suggests a role of these products in regulating nuclear functions of host cells.

Does the nuclear envelope contain two types of ligand-gated Ca2+ release channels?

The nuclear envelope is composed of two membranes deliminating a perinuclear space which displays functional properties similar to those of a Ca2+-storing compartment. ATP-driven Ca2+ uptake and InsP3-induced Ca2+ release processes have been described in isolated nuclei. Recently, it was reported that cADP-ribose and InsP3 can trigger a nucleoplasmic Ca2+ increase. It was hypothesized that the inner nuclear membrane possesses Ca2+ channels that are regulated by ryanodine or InsP3. Radio-ligand binding assays and Western blot experiments were performed in order to investigate their presence in sheep cardiac and rat liver nuclear envelopes. Ryanodine receptors (RyR) were not detected in liver nuclear envelopes by either binding assay or Western blot analysis. However, cardiac nuclear envelopes were found to retain a very low level of specific ryanodine binding, which was not detected on immuno-blots obtained with three types of isoform-specific RyR antibodies. In contrast, nuclear InsP3-binding sites were consistently detected in both cardiac and liver nuclear envelopes. Altogether, these results provide evidence for the major contributor InsP3-gated Ca2+ channels in control of Ca2+ release from the perinuclear space in liver and cardiac cells.

The nuclear matrix and the regulation of chromatin organization and function

Nuclear DNA is organized into loop domains, with the base of the loop being bound to the nuclear matrix. Loops with transcriptionally active and/or potentially active genes have a DNase I-sensitive chromatin structure, while repressed chromatin loops have a condensed configuration that is essentially invisible to the transcription machinery. Core histone acetylation and torsional stress appear to be responsible for the generation and/or maintenance of the open potentially active chromatin loops. The transcriptionally active region of the loop makes several dynamic attachments with the nuclear matrix and is associated with core histones that are dynamically acetylated. Histone acetyltransferase and deacetylase, which catalyze this rapid acetylation and deacetylation, are bound to the nuclear matrix. Several transcription factors are components of the nuclear matrix. Histone acetyltransferase, deacetylase, and transcription factors may contribute to the dynamic attachment of the active chromatin domains with the nuclear matrix at sites of ongoing transcription.

The differential expression of lamin epitopes during mouse spermatogenesis

The presence of lamin proteins in mouse spermatogenic cells has been examined by using an anti-lamin AC and an anti-lamin B antisera which recognize somatic lamins A and C, and somatic lamin B, respectively. Anti-lamin B binds to the nuclear periphery of all cell types examined, including Sertoli cells, primitive type A spermatogonia, preleptotene, leptotene, zygotene and pachytene spermatocytes, and round spermatids. In sperm nuclei, the antigenic determinants are localized to a narrow domain of the nucleus. However, after removing the perinuclear theca, anti-lamin B localizes to the entire nuclear periphery in a punctate pattern, suggesting that it is binding to determinants previously covered by the theca constituents. On immunoblots anti-lamin B reacts with a approximately 68 kD polypeptide in all germ cells and, to a lesser extent, with four additional polypeptides present only in meiotic and post-meiotic nuclear matrices. Anti-lamin AC also reacts with the perinuclear region of the somatic cells in the testes, in particular, those of the interstitium and also the Sertoli cells of the seminiferous epithelium. In contrast to anti-lamin B, anti-lamin AC does not bind to the germ cells at any stage of spermatogenesis. In addition, nuclear matrix proteins from isolated spermatogenic cells do not bind anti-lamin AC on immunoblots, suggesting the lack of reactivity is not due to the masking of any antigenic sites. These data demonstrate that germ cells contain lamin B throughout spermatogenesis, even during meiosis and spermiogenesis when the nuclear periphery lacks a distinct fibrous lamina.

Internal lamin structures within G1 nuclei of human dermal fibroblasts

The nuclear lamina is a mesh-like network of fibres subjacent to the inner nuclear membrane that is believed to be involved in the specific spatial reorganisation of chromatin after mitosis. To determine how the lamina might be involved in chromatin reorganisation, we have performed indirect immunofluorescence studies on quiescent and proliferating human dermal fibroblasts (HDF). Two monoclonal antibodies recognising human lamins A and C and three different fixation methods were employed. In indirect immunofluorescence studies, cultures of quiescent cells displayed a uniform perinuclear distribution of the antibodies. In proliferating cultures two distinct populations of cells were observed: one population displayed a typical perinuclear antibody distribution, while the second population displayed an unusual pattern consisting of a series of spots and fibres within the nucleus. By inducing cell-cycle synchrony in cultures we were able to determine that the unusual internal distribution of the lamin antibodies was restricted to cells in G1. Optical sectioning and 3-D reconstruction of the lamina structures in G1 nuclei was performed with a confocal laser scanning microscope (CLSM). This revealed that the internal lamin structures consisted of small foci and fibres proliferating throughout the nucleus. These structures were shown to be closely associated with areas of condensed chromatin but not nuclear membrane. As cells progress towards S phase the internal lamin foci disappear.

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.

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

The peripheral lamina of eukaryotic nuclei is composed of polypeptides called lamins that vary in number from one to four according to organism, cell type, and differentiated state of the cells. Early embryonic cells and stem cells of mammals generally possess only lamin B while lamins A and C appear later during differentiation. To study the role of the late appearance of lamins A and C in the differentiated phenotype, we have performed transfection of cDNAs coding for human lamins A or C into mouse embryonal carcinoma (EC) cell lines F9 and P19 lacking these two lamins. Transient transfections have shown that lamins A or C could be expressed, translocated to the peripheral lamina, and distributed into daughter cell nuclei after mitosis. These results demonstrated that EC cells devoid of lamins A and C nevertheless possessed the appropriate mechanisms for the localization and mitotic redistribution of exogenous lamins A and C.

Cloning and sequencing of cDNA clones encoding chicken lamins A and B1 and comparison of the primary structures of vertebrate A- and B-type lamins

Nuclear lamins are intermediate-filament-type proteins forming a fibrillar meshwork underlying the inner nuclear membrane. The existence of multiple isoforms of lamin proteins in vertebrates is believed to reflect functional specializations during cell division and differentiation. Although biochemical criteria may be used to classify many lamin isoforms into A- and B-type subfamilies, the structural features distinguishing the members of these subfamilies remain to be characterized fully. Here, we report the complete primary structures of chicken lamins A and B1, as they are deduced from cloned cDNAs; in the accompanying paper we present the complete sequence of lamin B2, a second avian B-type lamin. Comparisons of the chicken lamin sequences with each other and with those of other lamins allow us to establish structural features that are common to members of both subfamilies. Conversely, multiple sequence alignments make it possible to identify a number of structural motifs that clearly differentiate B-type lamins from A-type lamins. With this information at hand, we attempt to correlate different biochemical properties of A- and B-type lamins with the presence or absence of specific sequence motifs.

Lamin A is not synthesized as a larger precursor polypeptide

Isolation of rat liver nuclei in the presence of N-ethylmaleimide (NEM) led to the recovery in the final nuclear matrix of a higher molecular weight form of lamin A. The 2 kDa larger form was identified as lamin A by isoelectric point determination, recognition by an anti-lamin A and C monoclonal antibody and peptide mapping using V8 protease and N-chlorosuccinimide. The 2 kDa extension was tentatively localized to the carboxy-terminus of lamin A. Pulse-chase labeling and immunoprecipitation studies using baby hamster kidney cells showed that lysis of the cells in the presence of NEM allowed the recovery of a stable higher molecular weight form of lamin A. We conclude from these results that NEM prevented the degradation of the native form of lamin A previously thought to represent a higher molecular weight transient precursor form.

The occurrence and distribution of lamin proteins during mammalian spermatogenesis and early embryonic development

Based on current evidence it is apparent that the lamins undergo a dynamic reorganization during both spermatogenesis and early embryonic development, processes that presumably underscore unusual requirements in germ-cell differentiation and embryonic development.

Lamins A, B and C share an epitope with the common domain of intermediate filament proteins

Peripheral lamina were isolated from rat liver nuclei and probed with a monoclonal antibody reactive against a common domain of all intermediate filament (IF) proteins. The antibody reacted against lamins A and C as expected from their known sequence homology with IF proteins. Lamin B was also recognized by the antibody. These results indicate that lamin B, despite important biochemical differences vs lamins A and C, also shares sequence homology with IF proteins.

Synthesis and stability of nuclear matrix proteins in resting and serum-stimulated Swiss 3T3 cells

The major [35S]methionine-radiolabeled nuclear matrix proteins of mouse 3T3 cells were isolated, and most of these were found to be similar in molecular weight, charge, and protease fingerprint to the nuclear matrix proteins of African green monkey kidney cells, which are found tightly bound to simian virus 40 chromosomes. These nuclear matrix proteins were found to be synthesized in quiescent and serum-stimulated cells and to be turned over slowly during pulse-chase experiments. In contrast, a 70-Kd (kilodalton) neutral protein identified as lamin a was found to be turned over rapidly, producing a 68-Kd protein with a similar isoelectric point. In addition, we observed a decrease in the amounts of two chromatin-bound matrix proteins and a relative increase in lamin a following labeling in the presence of 2 micrograms/ml actinomycin D. However, these effects do not appear to be a result of inhibition of transcription, since they were not observed with other inhibitors (alpha-amanitin and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole).

Cloning of hybridomas by a single-cell transfer technique

A method for cloning hybridomas is described which involves transfer of single cells with a capillary tubing connected to a suction apparatus. This method enables the efficient recovery of antibody-producing clones of differing abundances from the parent hybridoma cultures. The 4 cell lines submitted to this cloning procedure produced a 100% yield of positive wells upon recloning by the same method.