Nuclear morphology during the S phase

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

The formation of the olfactory nerve and olfactory bulb (OB) glomeruli begins embryonically in mice. However, the development of the olfactory system continues throughout life with the addition of new olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). Much attention has been given to the perinatal innervation of the OB by OSN axons, but in the young adult the process of OSN maturation and axon targeting to the OB remains controversial. To address this gap in understanding, we used BrdU to label late-born OSNs in young adult mice at postnatal day 25 (P25-born OSNs) and timed their molecular maturation following basal cell division. We show that OSNs in young adults undergo a sequential molecular development with the expression of GAP 43 (growth-associated protein 43) > AC3 (adenylyl cyclase 3) > OMP (olfactory marker protein), consecutively, in a time frame of ∼8 d. To assess OSN axon development, we implemented an in vivo fate-mapping strategy to label P25-born OSNs with ZsGreen. Using sampling intervals of 24 h, we demonstrate the progressive extension of OSN axons in the OE, through the foramen of the cribriform plate, and onto the surface of the OB. OSN axons reached the OB and began to target and robustly innervate specific glomeruli ∼10 d following basal cell division, a time point at which OMP expression becomes evident. Our data demonstrate a sequential process of correlated axon extension and molecular maturation that is similar to that seen in the neonate, but on a slightly longer timescale and with regional differences in the OE.

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

The formation of the olfactory nerve and olfactory bulb (OB) glomeruli begins embryonically in mice. However, the development of the olfactory system continues throughout life with the addition of new olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). Much attention has been given to the perinatal innervation of the OB by OSN axons, but in the young adult the process of OSN maturation and axon targeting to the OB remains controversial. To address this gap in understanding, we used BrdU to label late-born OSNs in young adult mice at postnatal day 25 (P25-born OSNs) and timed their molecular maturation following basal cell division. We show that OSNs in young adults undergo a sequential molecular development with the expression of GAP 43 (growth-associated protein 43) > AC3 (adenylyl cyclase 3) > OMP (olfactory marker protein), consecutively, in a time frame of ∼8 d. To assess OSN axon development, we implemented an in vivo fate-mapping strategy to label P25-born OSNs with ZsGreen. Using sampling intervals of 24 h, we demonstrate the progressive extension of OSN axons in the OE, through the foramen of the cribriform plate, and onto the surface of the OB. OSN axons reached the OB and began to target and robustly innervate specific glomeruli ∼10 d following basal cell division, a time point at which OMP expression becomes evident. Our data demonstrate a sequential process of correlated axon extension and molecular maturation that is similar to that seen in the neonate, but on a slightly longer timescale and with regional differences in the OE.

Nuclear α1-antichymotrypsin promotes chromatin condensation and inhibits proliferation of human hepatocellular carcinoma cells

BACKGROUND & AIMS

α1-Antichymotrypsin (α1-ACT), a member of the serpin family (SERPINA3), is an acute-phase protein secreted by hepatocytes in response to cytokines such as oncostatin M. α1-ACT is a protease inhibitor thought to limit tissue damage produced by excessive inflammation-associated proteolysis. However, α1-ACT also is detected in the nuclei of cells, where its activities are unknown. Expression of α1-ACT is down-regulated in human hepatocellular carcinoma (HCC) tissues and cells; we examined its roles in liver regeneration and HCC proliferation.

METHODS

We measured levels of α1-ACT messenger RNA in human HCC samples and healthy liver tissue. We reduced levels of α1-ACT using targeted RNA interference in human HCC (HepG2) and mouse hepatocyte (AML12) cell lines, and overexpressed α1-ACT from lentiviral vectors in Huh7 (HCC) cells and adeno-associated viral vectors in livers of mice. We assessed proliferation, differentiation, and chromatin compaction in cultured cells, and liver regeneration and tumor formation in mice.

RESULTS

Reducing levels of α1-ACT promoted proliferation of HCC cells in vitro. Oncostatin M up-regulated α1-ACT expression and nuclear translocation, which inhibited HCC cell proliferation and activated differentiation of mouse hepatocytes. We identified amino acids required for α1-ACT nuclear localization, and found that α1-ACT inhibits cell-cycle progression and anchorage-independent proliferation of HCC cells. HCC cells that overexpressed α1-ACT formed smaller tumors in mice than HCC cells that did not express the protein. α1-ACT was observed to self-associate and polymerize in the nuclei of cells; nuclear α1-ACT strongly bound chromatin to promote a condensed state that could prevent cell proliferation.

CONCLUSIONS

α1-ACT localizes to the nuclei of hepatic cells to control chromatin condensation and proliferation. Overexpression of α1-ACT slows the growth of HCC xenograft tumors in nude mice.

Cytosolic calcium regulates liver regeneration in the rat

Liver regeneration is regulated by growth factors, cytokines, and other endocrine and metabolic factors. Calcium is important for cell division, but its role in liver regeneration is not known. The purpose of this study was to understand the effects of cytosolic calcium signals in liver growth after partial hepatectomy (PH). The gene encoding the calcium-binding protein parvalbumin (PV) targeted to the cytosol using a nuclear export sequence (NES), and using a discosoma red fluorescent protein (DsR) marker, was transfected into rat livers by injecting it, in recombinant adenovirus (Ad), into the portal vein. We performed two-thirds PH 4 days after Ad-PV-NES-DsR or Ad-DsR injection, and liver regeneration was analyzed. Calcium signals were analyzed with fura-2-acetoxymethyl ester in hepatocytes isolated from Ad-infected rats and in Ad-infected Hela cells. Also, isolated hepatocytes were infected with Ad-DsR or Ad-PV-NES-DsR and assayed for bromodeoxyuridine incorporation. Ad-PV-NES-DsR injection resulted in PV expression in the hepatocyte cytosol. Agonist-induced cytosolic calcium oscillations were attenuated in both PV-NES-expressing Hela cells and hepatocytes, as compared to DsR-expressing cells. Bromodeoxyuridine incorporation (S phase), phosphorylated histone 3 immunostaining (mitosis), and liver mass restoration after PH were all significantly delayed in PV-NES rats. Reduced cyclin expression and retinoblastoma protein phosphorylation confirmed this observation. PV-NES rats exhibited reduced c-fos induction and delayed extracellular signal-regulated kinase 1/2 phosphorylation after PH. Finally, primary PV-NES-expressing hepatocytes exhibited less proliferation and agonist-induced cyclic adenosine monophosphate responsive element binding and extracellular signal-regulated kinase 1/2 phosphorylation, as compared with control cells.

CONCLUSION

Cytosolic calcium signals promote liver regeneration by enhancing progression of hepatocytes through the cell cycle.

A high-temporal resolution technology for dynamic proteomic analysis based on 35S labeling

As more and more research efforts have been attracted to dynamic or differential proteomics, a method with high temporal resolution and high throughput is required. In present study, a (35)S in vivo Labeling Analysis for Dynamic Proteomics (SiLAD) was designed and tested by analyzing the dynamic proteome changes in the highly synchronized A549 cells, as well as in the rat liver 2/3 partial hepatectomy surgery. The results validated that SiLAD technique, in combination with 2-Dimensional Electrophoresis, provided a highly sensitivity method to illustrate the non-disturbed endogenous proteins dynamic changes with a good temporal resolution and high signal/noise ratio. A significant number of differential proteins can be discovered or re-categorized by this technique. Another unique feature of SiLAD is its capability of quantifying the rate of protein expression, which reflects the cellular physiological turn points more effectively. Finally, the prescribed SiLAD proteome snapshot pattern could be potentially used as an exclusive symbol for characterizing each stage in well regulated biological processes.

A role for CK2 upon interkinetic nuclear migration in the cell cycle of retinal progenitor cells

In developing retina, the nucleus of the elongated neuroepithelial cells undergoes interkinetic nuclear migration (INM), that is it migrates back and forth across the proliferative layer during the cell cycle. S-phase occurs at the basal side, while M-phase occurs at the apical margin of the retinal progenitors. G1 and G2-phases occur along the nuclear migration pathway. We tested whether this feature of the retinal cell cycle is controlled by CK2, which, among its many substrates, phosphorylates both molecular motors and cytoskeletal components. Double immunolabeling showed that CK2 is contained in BrdU-labeled retinal progenitors. INM was examined after pulse labeling the retina of newborn rats with BrdU, by plotting nuclear movement from basal to apical sides of the retinal progenitors during G2. The CK2 specific inhibitor 4,5,6,7-tetrabromobenzotriazole inhibited the activity of rat retinal CK2, and blocked nuclear movement proper in a dose-dependent way. No apoptosis was detected, and total numbers of BrdU-labeled nuclei remained constant following treatment. Immunohistochemistry showed that, following inhibition of CK2, the tubulin cytoskeleton is disorganized, with reduced acetylated and increased tyrosinated tubulin. This indicates a reduction in stable microtubules, with accumulation of free tubulin dimers. The results show that CK2 activity is required for INM in retinal progenitor cells.

Immunogold-labeled S-phase neoblasts, total neoblast number, their distribution, and evidence for arrested neoblasts in Macrostomum lignano (Platyhelminthes, Rhabditophora)

Neoblasts in Platyhelminthes are the only cells to proliferate and differentiate into all cell types. In Macrostomum lignano, the incorporation of 5'-bromo-2'-deoxyuridine (BrdU) in neoblasts confirmed the distribution of S-phase cells in two lateral bands. BrdU labeling for light and for transmission electron microscopy (TEM) identified three populations of proliferating cells: somatic neoblasts located between the epidermis and gastrodermis (mesodermal neoblasts), neoblasts located within the gastrodermis (gastrodermal neoblasts), and gonadal S-phase cells. In adults, three stages of mesodermal neoblasts (2, 2-3, and 3) defined by their ultrastructure were found. Stage 1 neoblasts where only seen in hatchlings. These stages either were phases within the S-phase of one neoblast pool or were subsequent stages of differentiating neoblasts, each with its own cell cycle. Regular TEM and immunogold labeling provided the basis for calculating the total number of neoblasts and the ratio of labeled to non-labeled neoblasts. Somatic neoblasts represented 6.5% of the total number of cells. Of these, 27% were labeled in S-phase. Of this fraction, 33% were in stage 2, 46% in stage 2-3, and 21% in stage 3. Immunogold labeling substantiated results concerning the differentiation of neoblasts into somatic cells. Non-labeled stage 2 neoblasts were present, even after a 2-week BrdU exposure. Double labeling of mitoses and FMRF-amide revealed a close spatial relationship of mesodermal neoblasts with the nervous system. Immunogold-labeled sections showed that nearly 70% of S-phase cells were in direct contact or within 5 microm from nerve cords.

Proteasome dynamics during cell cycle in rat Schwann cells

The proteasome is responsible for most of the protein degradation that takes place in the cytoplasm and nucleus. Immunofluorescence and electron microscopy are used to study proteasome dynamics during the cell cycle in rat Schwann cells. During interphase, the proteasome is present in the nucleus and cytoplasm and shows no colocalization with cytoskeletal components. Some cytoplasmic proteasomes always localize in the centrosome both in interphase and in mitotic cells and only associate with microtubules during mitosis. The proteasome exits the nucleus during prophase. In anaphase, the proteasome becomes prominent in the region between the two sets of migrating chromosomes and in association with interzonal microtubules and stem bodies. In telophase, the proteasome begins to reenter the nucleus and is prominent in the midbody region until the end of cytokinesis. The proteasome does not colocalize with actin or vimentin during mitosis, except for colocalization with actin in the sheet-like lamellipodia, which serve as substrate attachments for the cell during mitosis. During S phase, nuclear proteasomes colocalize with foci of BrdU incorporation, but this association changes with time: maximal at early S phase and declining as S phase progresses to the end. These results are discussed in relation to the biochemical pathways involved in cell cycle progression.

Fine structural in situ analysis of nascent DNA movement following DNA replication

Nascent DNA (newly replicated DNA) was visualized in situ with regard to the position of the previously replicated DNA and to chromatin structure. Localization of nascent DNA at the replication sites can be achieved through pulse labeling of cells with labeled DNA precursors during very short periods of time. We were able to label V79 Chinese Hamster cells for as shortly as 2 min with BrdU; Br-DNA, detected by immunoelectron microscopy, occurs at the periphery of dense chromatin, at individual dispersed chromatin fibers, and within dispersed chromatin areas. In these regions DNA polymerase alpha was also visualized. After a 5-min BrdU pulse, condensed chromatin also became labeled. When the pulse was followed by a chase, a larger number of gold particles occurred on condensed chromatin. Double-labeling experiments, consisting in first incubating cells with IdU for 20 min, chased for 10 min and then labeled for 5 min with CldU, reveal CldU-labeled nascent DNA on the periphery of condensed chromatin, while previously replicated IdU-labeled DNA has been internalized into condensed chromatin. Altogether, these results show that the sites of DNA replication correspond essentially to perichromatin regions and that the newly replicated DNA moves rapidly from replication sites toward the interior of condensed chromatin areas.

Nuclear accumulation of S-adenosylhomocysteine hydrolase in transcriptionally active cells during development of Xenopus laevis

The oocyte nuclear antigen of the monoclonal antibody 32-5B6 of Xenopus laevis is subject to regulated nuclear translocation during embryogenesis. It is distributed in the cytoplasm during oocyte maturation, where it remains during cleavage and blastula stages, before it gradually reaccumulates in the nuclei during gastrulation. We have now identified this antigen to be the enzyme S-adenosylhomocysteine hydrolase (SAHH). SAHH is the only enzyme that cleaves S-adenosylhomocysteine, a reaction product and an inhibitor of all S-adenosylmethionine-dependent methylation reactions. We have compared the spatial and temporal patterns of nuclear localization of SAHH and of nuclear methyltransferase activities during embryogenesis and in tissue culture cells. Nuclear localization of Xenopus SAHH did not temporally correlate with DNA methylation. However, we found that SAHH nuclear localization coincides with high rates of mRNA synthesis, a subpopulation colocalizes with RNA polymerase II, and inhibitors of SAHH reduce both methylation and synthesis of poly(A)(+) RNA. We therefore propose that accumulation of SAHH in the nucleus may be required for efficient cap methylation in transcriptionally active cells. Mutation analysis revealed that the C terminus and the N terminus are both required for efficient nuclear translocation in tissue culture cells, indicating that more than one interacting domain contributes to nuclear accumulation of Xenopus SAHH.

Scanning electron microscopic detection of nuclear structures involved in DNA replication

In order to evaluate at the ultrastructural level the three dimensional chromatin arrangement during interphase and particularly during the S phase, the immunogold detection of Bromodeoxyuridine (BrdU), as a marker of DNA synthesis, was performed in human HeLa, HL60, and in murine Friend leukemia cells (FLC). Field emission in lens scanning electron microscopy analysis of ultrathin cryosections revealed the presence of a regular three-dimensional network of fibers in dispersed chromatin. This spatial architecture was apparently constituted mainly of 10 nm filaments organized in loops of about 80-100 nm. Nodal points and the overlapping of such coils appeared as thicker structures of about 30 nm in diameter. Thin filaments of about 5 nm did not show a regular distribution. This three-dimensional fiber organization seemed quite constant in the dispersed chromatin of all the cell lines analyzed. The DNase treatment of the samples selectively removed the 10 nm class fibers, whereas the BrdU labeling confirmed the presence of newly synthesized DNA organized into chromatin units with a regular arrangement. These data suggest that the 10 nm chromatin fiber likely represents the DNA condensation order at which DNA duplication starts and the main weft of a three dimensional network within the interphase nucleus.

Ultrastructural aspects of the DNA polymerase alpha distribution during the cell cycle

We studied the nuclear topography of the replicating enzyme DNA polymerase alpha in HeLa cells by transmission electron microscopy and field emission in lens scanning electron microscopy. Cells were synchronized at the G1/S-phase boundary and samples of the different phases of the cell cycle were labeled with an anti-DNA polymerase alpha antibody detected by an immunogold reaction. DNA synthesis was detected by immunogold labeling after bromodeoxyuridine administration. The typical labeling pattern of DNA polymerase alpha observed in G1- and S-phase cells was represented by circular structures 80-100 nm in diameter surrounding an electron-dense area. In double labeled samples these circular structures were associated with bromodeoxyuridine-containing DNA replication sites, forming rosette-like structures. Field emission scanning electron microscopy performed on ultrathin cryosections revealed the chromatin fibers underlying DNA polymerase alpha complexes and showed that the size of the rosette-like structures corresponded to the diameter of chromatin foldings. G2- and M-phase cells showed a spread distribution of DNA polymerase alpha. The evidence of DNA polymerase alpha circular arrangement exclusively in G1- and S-phase cells, obtained by such different approaches, allowed us to consider the three-dimensional structures as DNA replication areas.

The eleven stages of the cell cycle, with emphasis on the changes in chromosomes and nucleoli during interphase and mitosis

Since we had subdivided the cell cycle into 11 stages--four for mitosis and seven for the interphase--and since we had experience in detecting DNA in the electron microscope (EN) by the osmium-amine procedure of Cogliati and Gauthier (Compt. Rend. Acad. Sci., 1973;276:3041-3044), we combined the two approaches for the analysis of DNA-containing structures at all stages of the cell cycle. Thin Epon sections of formaldehyde-fixed mouse duodenum were stained by osmium-amine for electron microscopic examination of the stages in the 12.3-hr long cell cycle of mouse duodenal crypt columnar cells. In addition, semi-thin Lowicryl sections of mouse duodenal crypts and cultured rat kidney cells were stained with the DNA-specific Hoechst 33258 dye and examined in the fluorescence microscope. The DNA detected by osmium-amine is in the form of nucleofilaments, seen at high magnification as long rows of 11 nm-wide rings (consisting of stained DNA encircling unstained histones). At all stages of the cycle as well as in nondividing cells, nucleofilaments are of three types: 'free,' 'attached' to chromatin accumulations, and 'compacted' in all chromatin accumulations, the form of dense spirals within. At stage I of the cycle, besides free and attached nucleofilaments, compacted ones are observed in the three heterochromatin forms (peripheral, nucleolus-associated, clumped). Soon after the S phase begins, chromatin 'aggregates' appear, which are small at stage II, mid-sized at stage III, and large at stage IV. Chromatin 'bulges' also appear at stage III and enlarge at stage IV, while heterochromatins disappear. At stage V, aggregates and bulges accrete into 'chromomeres,' a process responsible for the apparent chromosome condensation observed at prophase. The chromomeres gradually line up in rows and, at stage VIa (prometaphase), approach one another within each row and coalesce to build up the metaphase chromosomes which are fully formed at stage VIb (metaphase). Daughter chromosomes arising at stage VII (anaphase) are eventually packed into a chromosomal mass at each pole of the cell. During stage VIII (telophase), the chromosomal mass is split into large chunks. In the course of the G1 phase, the chunks thin out to give rise to irregular 'bands' at stage IX, the bands are then cleaved into central and peripheral fragments at stage X, and finally the central fragments are replaced by free nucleofilaments and clumps at stage XI, while the peripheral fragments are replaced by peripheral heterochromatin. The "nucleoli" at stages I-III are associated with stained heterochromatin but otherwise appear as unstained lucent areas, except for weakly stained patches composed of histone-free DNA filaments. During stage IV, nucleoli lose patches and associated heterochromatin, while weakly lucent, pale vesicles appear within nucleoli and in the nucleoplasm. By the end of substage VIa, nucleoli generally disappear, while pale vesicles persist around the chromosomes appearing at substage VIb. At stages VIII and IX, the vesicles seem to become strongly lucent and, at stages IX and X, they associate and fuse to yield homogeneous lucent areas, the 'prenucleolar bodies,' which include histone-free DNA patches. During stage XI, groups of these bodies associate to give rise to nucleoli. In conclusion, the cell cycle DNA changes can be classified into 4 broad periods (Fig. 6): 1) Stage I is a 2-hr long interphase "pause," during which the stained DNA shows no signs of either chromosome condensation or decondensation, while the overall nuclear pattern is similar to that in nondividing cell nuclei. Nucleoli are fully developed. 2) From stage II to VIa, the "chromosome condensation" period extends over about 7 hr, during which the events are interpreted as follows. Throughout the S phase (stages II-IV), newly-synthesized segments of nucleofilaments approach one another, adhere and thus build aggregates and later bulges on nuclear matrix sites. (ABSTRACT TRUNCATED)