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)     

Mutations in nucleolar proteins lead to nucleolar accumulation of polyA+ RNA in Saccharomyces cerevisiae

Synthesis of mRNA and rRNA occur in the chromatin-rich nucleoplasm and the nucleolus, respectively. Nevertheless, we here report that a Saccharomyces cerevisiae gene, MTR3, previously implicated in mRNA transport, codes for a novel essential 28-kDa nucleolar protein. Moreover, in mtr3-1 the accumulated polyA+ RNA actually colocalizes with nucleolar antigens, the nucleolus becomes somewhat disorganized, and rRNA synthesis and processing are inhibited. A strain with a ts conditional mutation in RNA polymerase I also shows nucleolar accumulation of polyA+ RNA, whereas strains with mutations in the nucleolar protein Nop1p do not. Thus, in several mutant backgrounds, when mRNA cannot be exported i concentrates in the nucleolus. mRNA may normally encounter nucleolar components before export and proteins such as Mtr3p may be critical for export of both mRNA and ribosomal subunits.     

Differential subcellular localization of protein phosphatase-1 alpha, gamma1, and delta isoforms during both interphase and mitosis in mammalian cells

Protein phosphatase-1 (PP-1) is involved in the regulation of numerous metabolic processes in mammalian cells. The major isoforms of PP-1, alpha, gamma1, and delta, have nearly identical catalytic domains, but they vary in sequence at their extreme NH2 and COOH termini. With specific antibodies raised against the unique COOH-terminal sequence of each isoform, we find that the three PP-1 isoforms are each expressed in all mammalian cells tested, but that they localize within these cells in a strikingly distinct and characteristic manner. Each isoform is present both within the cytoplasm and in the nucleus during interphase. Within the nucleus, PP-1 alpha associates with the nuclear matrix, PP-1 gamma1 concentrates in nucleoli in association with RNA, and PP-1 delta localizes to nonnucleolar whole chromatin. During mitosis, PP-1 alpha is localized to the centrosome, PP-1 gamma1 is associated with microtubules of the mitotic spindle, and PP-1 delta strongly associates with chromosomes. We conclude that PP-1 isoforms are targeted to strikingly distinct and independent sites in the cell, permitting unique and independent roles for each of the isoforms in regulating discrete cellular processes.     

Messenger ribonucleoprotein complexes of cryptobiotic embryos of Artemia salina

Poly(A)-containing ribonucleoprotein (poly(A)+-RNP) particles in the post-mitochondrial supernatant of cryptobiotic embryos of Artemia salina were characterized by hybridization to [3H]-poly(U). By sucrose isopycnic centrifugation, approximately 2/3 of poly(A)+-RNPs was found to band at 1.27-1.30 (g/cm3) and the rest 1+/3 at 1.20-1.23 (g/cm3) and below 1.20 (g/cm3). The 1.27-1.30 RNPs could be separated into two density classes, 1.27-1.28 and 1.30 (g/cm3) respectively. The latter RNP class was apparently complexed with ribosomal components because they were completely converted to the former RNP class (free RNPs) by 25 mM EDTA treatment. Further, the 1.30 (g/cm3) RNPs were resolved into several RNP species having sedimentation coefficients above 50 S. which were transformed mostly to 20-30 S rnps in the presence of 25 mM EDTA. The free 20-30 S RNPs contained 8-14 S poly(A)+-RNAs, having the highest template activity in a wheat embryo cell-free system, whereas the 1.20-1.23 poly(A)+-RNPs consisted of 10 S and 16 S RNPs, both of which contained 4 S poly(A)-containing sequences without any template activity.     

Repositioning of human interphase chromosomes by nucleolar dynamics in the reverse transformation of HT1080 fibrosarcoma cells

OBJECTIVES: The effects of norepinephrine on expression of cardiac genes during pathological cardiac growth and heart failure are not fully understood. Tissue insulin-like growth factor 1 (IGF-1) and its receptor (IGF-1R) play an important role in the regulation of the hyperplastic capacity of cardiac myocytes. Sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2), on the other hand, is important in regulating cardiac contractile function. The present study examined the effects of elevated levels of NE on expression of IGF-1/IGF-1R and SERCA2 mRNAs. METHODS: Rats were infused with NE using osmotic minipumps for 3 and 6 days at a rate of 50 micrograms/kg/h and also at a higher dose (130 micrograms/kg/h) for 6 and 14 days. Levels of expression of IGF-1/IGF-1R and SERCA2 mRNAs were determined by ribonuclease protection assay and by Northern blotting, respectively. RESULTS: NE treatment significantly increased IGF-1 mRNA levels in both left- and right-ventricle; however, levels of IGF-1R increased in the left- but not the right-ventricle. By contrast, NE infusion at both the lower dose and the higher dose failed to alter expression of SERCA2 mRNA. CONCLUSION: Our results suggest that NE treatment differentially regulates expression of IGF-1 and IGF-1R in the ventricles of rat heart and that NE appears not to affect expression of SERCA2 mRNA.     

Nop5p is a small nucleolar ribonucleoprotein component required for pre-18 S rRNA processing in yeast

We have identified a novel nucleolar protein, Nop5p, that is essential for growth in Saccharomyces cerevisiae. Monoclonal antibodies B47 and 37C12 recognize Nop5p, which has a predicted size of 57 kDa and possesses a KKX repeat motif at its carboxyl terminus. Truncations that removed the KKX motif were functional and localized to the nucleolus, but conferred slow growth at 37 degreesC. Nop5p shows significant sequence homology with yeast Sik1p/Nop56p, and putative homologues in archaebacteria, plants, and human. Depletion of Nop5p in a GAL-NOP5 strain lengthened the doubling time about 5-fold, and selectively reduced steady-state levels of 40 S ribosomal subunits and 18 S rRNA relative to levels of free 60 S subunits and 25 S rRNA. Northern blotting and primer extension analyses showed that Nop5p depletion impairs processing of 35 S pre-rRNA at the A0 and A2 cleavage sites. Nop5p is associated with the small nucleolar RNAs U3, snR13, U14, and U18. Depletion of Nop5p caused the nucleolar protein Nop1p (yeast fibrillarin) to be localized to the nucleus and cytosol. Also, 37C12 co-immunoprecipitated Nop1p. These results suggest that Nop5p functions with Nop1p in the execution of early pre-rRNA processing steps that lead to formation of 18 S rRNA.     

Evolution of the nucleolar apparatus during oogenesis in Acipenseridae

Evolution of the nucleoli has been followed during oogenesis in the Acipenserid fishes, Acipenser ruthenus (the sterlet) and A. guidenstadti (the sturgeon) using light and electron microscopes. In the ovaries of adults, the oogonial nuclei usually have a single nucleolus with an adjacent mass of paranucleolar fibrillar material. The cytoplasm of the oogonia contains two dense bodies peculiar only to gonocytes, one being electron dense and containing RNA and the otherbeing electron-lucent and lacking RNA...     

Increased in vitro phosphorylation of rat liver nucleolar proteins following triiodothyronine administration

It has been shown that triiodothyronine (Ta) administration to thyroidectomized rats induces an increase in the in vitro net 32P uptake into liver nucleolar proteins. Such an increase depends on a stimulation of the nucleolus-associated protein kinase activity and not on a lower dephosphorylation rate.     

Four distinct regions in the auxiliary domain of heterogeneous nuclear ribonucleoprotein C-related proteins

The purpose of this presentation of pathological data is to demonstrate how our ideas have developed from a chronological point of view to our present interest in elementary cognitive processes. The present relative rarity of significant data in brain imagery, compared with what is observed in other fields of neuro-psychology, is probably due to the complexity of this cognitive processes.     

Argyrophilic proteins in the nucleolar organizer in the differential diagnosis of cervical dysplasias and cancer

Prospects of using the nucleolar organizer region (NOR) identification by silver nitrate for the diagnosis of dysplasias and cervical cancer were studied. Four groups of patients (22 cases) have been examined: group 1 (7 cases)--unchanged endocervical epithelium, group 2 (6 cases)--dysplasia of cervical epithelium of degree II-III, group 3 (3 cases)--carcinoma in situ, group 4 (6 cases)--invasive squamous cell nonkeratinizing carcinoma. The investigation showed that there are reliable differences in the quantity of silver granules per nucleus and also of NOR index between normal ectocervical epithelium and dysplasia, normal ectocervical epithelium and cancer, dysplasia and cervical cancer. The data obtained suggest that NOR argyrophilia may be a diagnostic marker of the tumor malignancy degree.     

The vertebrate cell kinetochore and its roles during mitosis

A replicated chromosome possesses two discrete, complex, dynamic, macromolecular assemblies, known as kinetochores, that are positioned on opposite sides of the primary constriction of the chromosome. Here, the authors review how kinetochores control chromosome segregation during mitosis in vertebrates. They attach the chromosome to the opposing spindle poles by trapping the dynamic plus-ends of microtubules growing from the poles. They then produce much of the force for chromosome poleward motion, regulate when this force is applied, and act as a site for microtubule assembly and disassembly. Finally, they control the metaphase-anaphase transition by inhibiting chromatid separation until the chromatids are properly attached.     

The Golgi and endoplasmic reticulum remain independent during mitosis in HeLa cells

Partitioning of the mammalian Golgi apparatus during cell division involves disassembly at M-phase. Despite the importance of the disassembly/reassembly pathway in Golgi biogenesis, it remains unclear whether mitotic Golgi breakdown in vivo proceeds by direct vesiculation or involves fusion with the endoplasmic reticulum (ER). To test whether mitotic Golgi is fused with the ER, we compared the distribution of ER and Golgi proteins in interphase and mitotic HeLa cells by immunofluorescence microscopy, velocity gradient fractionation, and density gradient fractionation. While mitotic ER appeared to be a fine reticulum excluded from the region containing the spindle-pole body, mitotic Golgi appeared to be dispersed small vesicles that penetrated the area containing spindle microtubules. After cell disruption, M-phase Golgi was recovered in two size classes. The major breakdown product, accounting for at least 75% of the Golgi, was a population of 60-nm vesicles that were completely separated from the ER using velocity gradient separation. The minor breakdown product was a larger, more heterogenously sized, membrane population. Double-label fluorescence analysis of these membranes indicated that this portion of mitotic Golgi also lacked detectable ER marker proteins. Therefore we conclude that the ER and Golgi remain distinct at M-phase in HeLa cells. To test whether the 60-nm vesicles might form from the ER at M-phase as the result of a two-step vesiculation pathway involving ER-Golgi fusion followed by Golgi vesicle budding, mitotic cells were generated with fused ER and Golgi by brefeldin A treatment. Upon brefeldin A removal, Golgi vesicles did not emerge from the ER. In contrast, the Golgi readily reformed from similarly treated interphase cells. We conclude that Golgi-derived vesicles remain distinct from the ER in mitotic HeLa cells, and that mitotic cells lack the capacity of interphase cells for Golgi reemergence from the ER. These experiments suggest that mitotic Golgi breakdown proceeds by direct vesiculation independent of the ER.     

Partitioning of the Golgi apparatus during mitosis in living HeLa cells

The Golgi apparatus of HeLa cells was fluorescently tagged with a green fluorescent protein (GFP), localized by attachment to the NH2-terminal retention signal of N-acetylglucosaminyltransferase I (NAGT I). The location was confirmed by immunogold and immunofluorescence microscopy using a variety of Golgi markers. The behavior of the fluorescent Golgi marker was observed in fixed and living mitotic cells using confocal microscopy. By metaphase, cells contained a constant number of Golgi fragments dispersed throughout the cytoplasm. Conventional and cryoimmunoelectron microscopy showed that the NAGT I-GFP chimera (NAGFP)-positive fragments were tubulo-vesicular mitotic Golgi clusters. Mitotic conversion of Golgi stacks into mitotic clusters had surprisingly little effect on the polarity of Golgi membrane markers at the level of fluorescence microscopy. In living cells, there was little self-directed movement of the clusters in the period from metaphase to early telophase. In late telophase, the Golgi ribbon began to be reformed by a dynamic process of congregation and tubulation of the newly inherited Golgi fragments. The accuracy of partitioning the NAGFP-tagged Golgi was found to exceed that expected for a stochastic partitioning process. The results provide direct evidence for mitotic clusters as the unit of partitioning and suggest that precise regulation of the number, position, and compartmentation of mitotic membranes is a critical feature for the ordered inheritance of the Golgi apparatus.     

Localization of the 26S proteasome during mitosis and meiosis in fission yeast

The 26S proteasome is a large multisubunit complex involved in degrading both cytoplasmic and nuclear proteins. We have investigated the localization of this complex in the fission yeast, Schizosaccharomyces pombe. Immunofluorescence microscopy shows a striking localization pattern whereby the proteasome is found predominantly at the nuclear periphery, both in interphase and throughout mitosis. Electron microscopic analysis revealed a concentration of label near the inner side of the nuclear envelope. The localization of green fluorescent protein (GFP)-tagged 26S proteasomes was analyzed in live cells during mitosis and meiosis. Throughout mitosis the proteasome remained predominantly at the nuclear periphery. During meiosis the proteasome was found to undergo dramatic changes in its localization. Throughout the first meiotic division, the signal is more dispersed over the nucleus. During meiosis II, there was a dramatic re-localization, and the signal became restricted to the area between the separating DNA until the end of meiosis when the signal dispersed before returning to the nuclear periphery during spore formation. These findings strongly imply that the nuclear periphery is a major site of protein degradation in fission yeast both in interphase and throughout mitosis. Furthermore they raise interesting questions as to the spatial organization of protein degradation during meiosis.     

EDTA-and puromycin-derived duck- and rabbit globin-messenger ribonucleoprotein complexes isolated by oligo (dT)-cellulose chromatography

Duck- and rabbit globin messenger ribonucleoprotein complexes isolated by oligo(dT) cellulose chromatography reveal an identical protein pattern-two main proteins of molecular weights of 73,000 and 49,000 daltons and minor components-whether the complexes have been liberated from polyribosomes with the EDTA- or the puromycin-high-salt method. In the globin messenger ribonucleoprotein particles of both species predominantly the protein with a molecular weight of 73,000 daltons is attached to poly(A)-containing regions of the messenger RNAs.     

The mechanism of Golgi segregation during mitosis is cell type-specific

Golgi membranes in Drosophila embryos and tissue culture cells are found as discrete units dispersed in the cytoplasm. We provide evidence that Golgi membranes do not undergo any dramatic change in their organization during the rapid mitotic divisions of the nuclei in the syncitial embryo or during cell division postcellularization. By contrast, in Drosophila tissue culture cells, the Golgi membranes undergo complete fragmentation during mitosis. Our studies show that the mechanism of Golgi partitioning during cell division is cell type-specific.     

Genetic control of mitosis, meiosis and cellular differentiation during mammalian spermatogenesis

Gametogenesis in both the male and female mammal represents a specialized and highly regulated series of cell cycle events, involving both mitosis and meiosis as well as subsequent differentiation. Recent advances in our understanding of the genetic control of the eukaryotic cell cycle have underscored the evolutionarily-conserved nature of these regulatory processes. However, most of the data have been obtained from yeast model systems and mammalian cell lines. Furthermore, most of the observations focus on regulation of mitotic cell cycles. In the present paper: (i) aspects of gametogenesis in mammals that represent unique cell-cycle control points are highlighted; (ii) current knowledge on the regulation of the germ cell cycle, in the context of what is known in yeast and other model eukaryotic systems, is summarized; and (iii) strategies that can be used to identify additional cell cycle regulating genes are outlined.     

The release and absorption of small ribonucleoprotein complexes (alpha RNPs) in cultures of transformed rat fibroblasts and human epidermoid carcinoma

The release of alpha RNPs and their absorbtion by the cells from culture medium were studied. The rat fibroblasts of parental serum-dependent cell line LRec-1, and of selected serum-free cell line LRec-1sf rapidly released and absorbed alpha RNPs. In the latter case, both auto- and heterologous alpha RNPs were seen to penetrate, whereas only autologous alpha RNPs entered LRec-1 cells. Besides, the ability to rapid export and absorbtion of autocrine alpha RNPs was demonstrated for human epidermoid carcinoma cell line A431. Both LRec-1 and LRec-1sf cells expressed mRNAs with homology to ID-like nucleotide sequences, the level of mRNA expression decreasing markedly when serum-dependent LRec-1 cells were grown in serum-free medium.