Resinless section electron microscopy of HeLa cell mitotic architecture

Mitotic partitioning and selective reorganization of tissue-specific transcription factors in progeny cells

Postmitotic gene expression requires restoration of nuclear organization and assembly of regulatory complexes. The hematopoietic and osteogenic Runx (Cbfa/AML) transcription factors are punctately organized in the interphase nucleus and provide a model for understanding the subnuclear organization of tissue-specific regulatory proteins after mitosis. Here we have used quantitative in situ immunofluorescence microscopy and quantitative image analysis to show that Runx factors undergo progressive changes in cellular localization during mitosis while retaining a punctate distribution. In comparison, the acetyl transferase p300 and acetylated histone H4 remain localized with DNA throughout mitosis while the RNA processing factor SC35 is excluded from mitotic chromatin. Subnuclear organization of Runx foci is completely restored in telophase, and Runx proteins are equally partitioned into progeny nuclei. In contrast, subnuclear organization of SC35 is restored subsequent to telophase. Our results show a sequential reorganization of Runx and its coregulatory proteins that precedes restoration of RNA processing speckles. Thus, mitotic partitioning and spatiotemporal reorganization of regulatory proteins together render progeny cells equivalently competent to support phenotypic gene expression.

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.

A normally masked nuclear matrix antigen that appears at mitosis on cytoskeleton filaments adjoining chromosomes, centrioles, and midbodies

mAbs were generated against HeLa nuclear matrix proteins and one, HIB2, which selectively stained mitotic cells, was selected for further study. Western blot analysis showed H1B2 antibody detected a protein of 240 kD in the nuclear matrix fractions. The H1B2 antigen was completely masked in immunofluorescently stained interphase cells. However, removing chromatin with DNase I digestion and 0.25 M ammonium sulfate extraction exposed the protein epitope. The resulting fluorescence pattern was bright, highly punctate, and entirely nuclear. Further extraction of the nuclear matrix with 2 M NaCl uncovers an underlying, anastomosing network of 9-13 nm core filaments. Most of the H1B2 antigen was retained in the fibrogranular masses enmeshed in the core filament network and not in the filaments themselves. The H1B2 antigen showed remarkable behavior at mitosis. As cells approached prophase the antigen became unmasked to immunofluorescent staining without the removal of chromatin. First appearing as a bright spot, the antibody staining spread through the nucleus finally concentrating in the region around the condensed chromosomes. The antibody also brightly stained the spindle poles and, more weakly, in a punctate pattern in the cytoskeleton around the spindle. As the chromosomes separated at anaphase, H1B2 remained with the separating daughter sets of chromosomes. The H1B2 antigen returned to the reforming nucleus at telophase, but left a bright staining region in the midbody. Immunoelectron microscopy of resinless sections showed that, in the mitotic cell, the H1B2 antibody did not stain chromosomes and centrioles themselves, but decorated a fibrogranular network surrounding and connected to the chromosomes and a fibrogranular structure surrounding the centriole.

Formation of lentivirus particles by mammalian cells infected with recombinant fowlpox virus

Recombinant fowlpox viruses (FPV) containing the env or gag-pol genes of simian immunodeficiency virus from macaques (SIVmac) were constructed. The env, gag, and pol-encoded polypeptides were efficiently expressed and processed in avian cells productively infected with FPV as well as in mammalian cells, in which FPV infection is abortive. In addition, the recombinant FPV expressing the gag-pol genes directed the formation of defective, lentivirus-like particles which were released into the culture medium of infected cells. Coinfection of cells with the env and gag-pol recombinant viruses resulted in the generation of particles containing SIVmac envelope glycoprotein. The applications of this system to vaccine development are discussed.

Localization of heterogeneous nuclear ribonucleoprotein in the interphase nuclear matrix core filaments and on perichromosomal filaments at mitosis

Although heterogeneous nuclear RNA (hnRNA) has been localized to the core filament substructure of the nuclear matrix, its precise location in the filament network has been unknown. The fA12 monoclonal antibody can localize, at high resolution, hn ribonucleoproteins (hnRNPs) and, presumably, hnRNA. Gold bead immunolabeling of resinless electron microscopy sections showed the fA12 antigens were in the fibrogranular material enmeshed in the filament network and not in the filaments themselves. At mitosis, hnRNP antigens became dispersed into a halo surrounding the chromosomes and spindle poles. Immunogold staining showed fA12 stained fibrogranular material associated with perichromosomal and pericentriolar filaments distinct from the mitotic spindle fibers. fA12 also labeled the midbody remaining after cytokinesis.

Core filaments of the nuclear matrix

The nuclear matrix is concealed by a much larger mass of chromatin, which can be removed selectively by digesting nuclei with DNase I followed by elution of chromatin with 0.25 M ammonium sulfate. This mild procedure removes chromatin almost completely and preserves nuclear matrix morphology. The complete nuclear matrix consists of a nuclear lamina with an interior matrix composed of thick, polymorphic fibers and large masses that resemble remnant nucleoli. Further extraction of the nuclear matrices of HeLa or MCF-7 cells with 2 M sodium chloride uncovered a network of core filaments. A few dark masses remained enmeshed in the filament network and may be remnants of the nuclear matrix thick fibers and nucleoli. The highly branched core filaments had diameters of 9 and 13 nm measured relative to the intermediate filaments. They may serve as the core structure around which the matrix is constructed. The core filaments retained 70% of nuclear RNA. This RNA consisted both of ribosomal RNA precursors and of very high molecular weight hnRNA with a modal size of 20 kb. Treatment with RNase A removed the core filaments. When 2 M sodium chloride was used directly to remove chromatin after DNase I digestion without a preceding 0.25 M ammonium sulfate extraction, the core filaments were not revealed. Instead, the nuclear interior was filled with amorphous masses that may cover the filaments. This reflected a requirement for a stepwise increase in ionic strength because gradual addition of sodium chloride to a final concentration of 2 M without an 0.25 M ammonium sulfate extraction uncovered core filaments.