Functional interaction of nuclear transport-defective simian virus 40 large T antigen with chromatin and nuclear matrix

Telomerase-independent telomere length maintenance in the absence of alternative lengthening of telomeres-associated promyelocytic leukemia bodies

Immortal tumor cells and cell lines employ a telomere maintenance mechanism that allows them to escape the normal limits on proliferative potential. In the absence of telomerase, telomere length may be maintained by an alternative lengthening of telomeres (ALT) mechanism. All human ALT cell lines described thus far have nuclear domains of unknown function, termed ALT-associated promyelocytic leukemia bodies (APB), containing promyelocytic leukemia protein, telomeric DNA and telomere binding proteins. Here we describe telomerase-negative human cells with telomeres that contain a substantial proportion of nontelomeric DNA sequences (like telomerase-null Saccharomyces cerevisiae survivor type I cells) and that are maintained in the absence of APBs. In other respects, they resemble typical ALT cell lines: the telomeres are highly heterogeneous in length (ranging from very short to very long) and undergo rapid changes in length. In addition, these cells are capable of copying a targeted DNA tag from one telomere into other telomeres. These data show that APBs are not always essential for ALT-mediated telomere maintenance.

Growth factor regulation of cell growth and proliferation in the nervous system. A new intracrine nuclear mechanism

This article discusses a novel intracrine mechanism of growth-factor action in the nervous system whereby fibroblast growth factor-2 (FGF-2) and its receptor accumulate in the cell nucleus and act as mediators in the control of cell growth and proliferation. In human and rat brain the levels and subcellular localization of FGF-2 differ between quiescent and reactive astrocytes. Quiescent cells express a low level of FGF-2, which is located predominantly within the cytoplasm. In reactive astrocytes, the expression of FGF-2 increases and the proteins are found in both the cytoplasm and nucleus. In glioma tumors, FGF-2 is overexpressed in the nuclei of neoplastic cells. Similar changes in FGF-2 expression and localization are found in vitro. The nuclear accumulation of FGF-2 reflects a transient activation of the FGF-2 gene by potentially novel transactivating factors interacting with an upstream regulatory promoter region. In parallel with FGF-2, the nuclei of astrocytes contain the high-affinity FGF-2 receptor, FGFR1. Nuclear FGFR1 is full length, retains kinase activity, and is localized within the nuclear interior in association with the nuclear matrix. Transfection of either FGF-2 or FGFR1 into cells that do not normally express these proteins results in their nuclear accumulation and concomitant increases in cell proliferation. A similar regulation of nuclear FGF-2 and FGFR1 is observed in neural crestderived adrenal medullary cells and of FGF-2 in the nuclei of cerebellar neurons. Thus, the regulation of the nuclear content of FGF-2 and FGFR1 could serve as a novel mechanism controlling growth and proliferation of glial and neuronal cells.

Dynamic continuity of nuclear and mitotic matrix proteins in the cell cycle

The eukaryotic cell nucleus is a membrane-enclosed compartment containing the genome and associated molecules supported by a highly insoluble filamentous network known as the nucleoskeleton or nuclear matrix. The nuclear matrix is believed to play roles in maintaining nuclear architecture and organizing nuclear metabolism. Recently, advances in microscopic techniques and the availability of new molecular probes have made it possible to localize functional domains within the nuclear matrix and demonstrate dynamic interactions between both soluble and insoluble components involved in the control of multiple nuclear transactions. Like the cytoplasm and its skeleton, the nucleoplasm is highly structured and very crowded with an equally complex skeletal framework. In fact, there is growing evidence that the two skeletal systems are functionally contiguous, providing a dynamic cellular matrix connecting the cell surface with the genome. If we impose cell cycle dynamics upon this skeletal organization, it is obvious that the genome and associated nuclear matrix must undergo a major structural transition during mitosis, being disassembled and/or reorganized in late G2 and reassembled again in daughter nuclei. However, recent evidence from our laboratory and elsewhere suggests that much of the nuclear matrix is used to form the mitotic apparatus (MA). Indeed, both facultative and constitutive matrix-associated proteins such as NuMA, CENP-B, CENP-F, and the retinoblastoma protein (Rb) associate within and around the MA. During mitosis, the nuclear matrix proteins may either become inert "passengers" or assume critical functions in partitioning the genome into newly formed G1 nuclei. Therefore, we support the view that the nuclear matrix exists as a dynamic architectural continuum, embracing the genome and maintaining cellular regulation throughout the cell cycle.

Nuclear accumulation of fibroblast growth factor receptors is regulated by multiple signals in adrenal medullary cells

In an effort to determine the localization of fibroblast growth factor (FGF) receptors (FGFR) that could mediate the intracellular action of FGF-2, we discovered the presence of high-affinity. FGF-2 binding sites in the nuclei of bovine adrenal medullary cells (BAMC). Western blot analysis demonstrated the presence of 103-, 118-, and 145-kDa forms of FGFR1 in nuclei isolated from BAMC. 125I-FGF-2 cross-linking to nuclear extracts followed by FGFR1 immunoprecipitation showed that FGFR1 can account for the nuclear FGF-2 binding sites. Nuclear FGFR1 has kinase activity and undergoes autophosphorylation. Immunocytochemistry with the use of confocal and electron microscopes demonstrated the presence of FGFR1 within the nuclear interior. Nuclear subfractionation followed by Western blot or immunoelectron microscopic analysis showed that the nuclear FGFR1 is contained in the nuclear matrix and the nucleoplasm. Agents that induce translocation of endogenous FGF-2 to the nucleus (forskolin, carbachol, or angiotensin II) increased the intranuclear accumulation of FGFR1. This accumulation was accompanied by an overall increase in FGF-2-inducible tyrosine kinase activity. Our findings suggest a novel mode for growth factor action whereby growth factor receptors translocate to the nucleus in parallel with their ligand and act as direct mediators of nuclear responses to cell stimulation.

Chromatin domains and prediction of MAR sequences

Polynuceosomes are constrained into loops or domains and are insulated from the effects of chromatin structure and torsional strain from flanking domains by the cross-complexation of matrix-attached regions (MARs) and matrix proteins. MARs or SARs have an average size of 500 bp, are spaced about every 30 kb, and are control elements maintaining independent realms of gene activity. A fraction of MARs may cohabit with core origin replication (ORIs) and another fraction might cohabit with transcriptional enhancers. DNA replication, transcription, repair, splicing, and recombination seem to take place on the nuclear matrix. Classical AT-rich MARs have been proposed to anchor the core enhancers and core origins complexed with low abundancy transcription factors to the nuclear matrix via the cooperative binding to MARs of abundant classical matrix proteins (topoisomerase II, histone H1, lamins, SP120, ARBP, SATB1); this creates a unique nuclear microenvironment rich in regulatory proteins able to sustain transcription, replication, repair, and recombination. Theoretical searches and experimental data strongly support a model of activation of MARs and ORIs by transcription factors. A set of 21 characteristics are deduced or proposed for MAR/ORI sequences including their enrichment in inverted repeats, AT tracts, DNA unwinding elements, replication initiator protein sites, homooligonucleotide repeats (i.e., AAA, TTT, CCC), curved DNA, DNase I-hypersensitive sites, nucleosome-free stretches, polypurine stretches, and motifs with a potential for left-handed and triplex structures. We are establishing Banks of ORI and MAR sequences and have undertaken a large project of sequencing a large number of MARs in an effort to determine classes of DNA sequences in these regulatory elements and to understand their role at the origins of replication and transcriptional enhancers.

The retinoblastoma gene product is a cell cycle-dependent, nuclear matrix-associated protein

The retinoblastoma gene product (Rb) has been established as a tumor suppressor and cell cycle regulator, although its mechanism of action remains obscure. The observations that several Rb-binding viral oncoproteins all associate with the nuclear matrix suggest that these interactions may occur on this structure. To determine whether Rb itself is a component of the matrix, we extracted synchronized cultured cells to isolate matrix proteins while preserving nuclear architecture. Immunoblot and immunolabeling data show that a significant portion of hypophosphorylated Rb associates with the matrix only during early G1. Mutant Rb in tumor cells did not associate with the matrix, whereas Rb-reconstituted cells contained abundant matrix-bound Rb. Rb is distributed widely throughout the matrix, particularly concentrated at the nuclear periphery and in nucleolar remnants. Core filaments of the matrix contained no detectable Rb. Our screening of expression libraries for potential Rb-associated proteins has identified several that are part of the matrix. Specifically, the peripheral matrix proteins lamin A and C bound Rb in vitro. We therefore suggest that Rb interactions with the nuclear matrix may be important for its ability to regulate cell cycle progression.

Adenovirus precursor to terminal protein interacts with the nuclear matrix in vivo and in vitro

The adenovirus precursor to the terminal protein (pTP), expressed in a vaccinia virus expression system or in native adenovirus, was assayed for its ability to interact with the nuclear matrix. Biochemical function was measured by determining the relative amount of pTP protein or of adenovirus DNA that remained associated with the nuclear matrix after extensive washing. pTP was retained on the matrix whereas beta-galactosidase was not, as assayed by quantitative immunoblot analysis. Nuclear matrix isolated from adenovirus-infected HeLa cells retained bound adenovirus DNA even when washed with 1 M guanidine hydrochloride; this interaction could be inhibited by added purified pTP protein. Analogous experiments with matrix isolated from HeLa cells infected with a recombinant vaccinia virus that expressed pTP showed a similar retention of pTP protein; this association could also be inhibited by added pTP protein. Binding of pTP to nuclear matrix isolated from uninfected cells was saturable, with an apparent Kd of 250 nM and an estimated 2.8 x 10(6) sites for pTP binding per cell nucleus. The association of pTP with matrix is postulated to help direct adenovirus replication complexes to the appropriate locale within the nucleus.