Pascal Roussel

Nucleolus: the fascinating nuclear body

Nucleoli are the prominent contrasted structures of the cell nucleus. In the nucleolus, ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins. RNA polymerase I synthesizes the ribosomal RNAs and this activity is cell cycle regulated. The nucleolus reveals the functional organization of the nucleus in which the compartmentation of the different steps of ribosome biogenesis is observed whereas the nucleolar machineries are in permanent exchange with the nucleoplasm and other nuclear bodies. After mitosis, nucleolar assembly is a time and space regulated process controlled by the cell cycle. In addition, by generating a large volume in the nucleus with apparently no RNA polymerase II activity, the nucleolus creates a domain of retention/sequestration of molecules normally active outside the nucleolus. Viruses interact with the nucleolus and recruit nucleolar proteins to facilitate virus replication. The nucleolus is also a sensor of stress due to the redistribution of the ribosomal proteins in the nucleoplasm by nucleolus disruption. The nucleolus plays several crucial functions in the nucleus: in addition to its function as ribosome factory of the cells it is a multifunctional nuclear domain, and nucleolar activity is linked with several pathologies. Perspectives on the evolution of this research area are proposed.

Cyclin-dependent kinases govern formation and maintenance of the nucleolus

In higher eukaryotic cells, the nucleolus is a nuclear compartment assembled at the beginning of interphase, maintained during interphase, and disorganized during mitosis. Even if its structural organization appears to be undissociable from its function in ribosome biogenesis, the mechanisms that govern the formation and maintenance of the nucleolus are not elucidated. To determine if cell cycle regulators are implicated, we investigated the putative role of the cyclin-dependent kinases (CDKs) on ribosome biogenesis and nucleolar organization. Inhibition of CDK1–cyclin B during mitosis leads to resumption of rDNA transcription, but is not sufficient to induce proper processing of the pre-rRNA and total relocalization of the processing machinery into rDNA transcription sites. Similarly, at the exit from mitosis, both translocation of the late processing machinery and pre-rRNA processing are impaired in a reversible manner by CDK inhibitors. Therefore, CDK activity seems indispensable for the building of functional nucleoli. Furthermore, inhibition of CDKs in interphasic cells also hampered proper pre-rRNA processing and induced a dramatic disorganization of the nucleolus. Thus, we propose that the mechanisms governing both formation and maintenance of functional nucleoli involve CDK activities and couple the cell cycle to ribosome biogenesis.

Involvement of SIRT7 in resumption of rDNA transcription at the exit from mitosis

Sirtuins, also designated class III histone deacetylases, are implicated in the regulation of cell division, apoptosis, DNA damage repair, genomic silencing and longevity. The nucleolar Sirtuin7 (SIRT7) was reported to be involved in the regulation of ribosomal gene (rDNA) transcription, but there are no data concerning the regulation of SIRT7 during the cell cycle. Here we have analyzed the behavior of endogenous SIRT7 during mitosis, while rDNA transcription is repressed. SIRT7 remains associated with nucleolar organizer regions, as does the RNA polymerase I machinery. SIRT7 directly interacts with the rDNA transcription factor UBF. Moreover, SIRT7 is phosphorylated via the CDK1-cyclin B pathway during mitosis and dephosphorylated by a phosphatase sensitive to okadaic acid at the exit from mitosis before onset of rDNA transcription. Interestingly, dephosphorylation events induce a conformational modification of the carboxy-terminal region of SIRT7 before the release of mitotic repression of rDNA transcription. As SIRT7 activity is required to resume rDNA transcription in telophase, we propose that this conformational modification regulates onset of rDNA transcription.

Amount of the two major Ag‐NOR proteins, nucleolin, and protein B23 is cell‐cycle dependent

To know the biological basis allowing the use of Ag-NOR protein expression as proliferation marker in human malignancies, the relationship between cell cycle and amount of Ag-NOR protein was analyzed. The quantification of the two major Ag-NOR proteins, nucleolin and protein B23, was performed in exponentially growing, serum-deprived, and cell-cycle stimulated cells. Expression of nucleolin was low in serum-deprived cells and increased mostly in S phase during cell-cycle stimulation. Conversely, expression of protein B23 was slightly repressed in serum-deprived cells, and increased progressively until G2 phase during cell-cycle stimulation. The accumulation of nucleolin and protein B23 in G2 compared to G1 was demonstrated using sorted phase-specific cells. In G0, cells sorted according to their very low RNA content, and the amount of Ag-NOR proteins was half of that found in G1 cells, nucleolin being only weakly detectable. Therefore, the expression of nucleolin increased between G0-G1 and G1-S phases. These data support the hypothesis that quantification of Ag-NOR proteins is an estimation of the percentage of cells in each cell cycle phase because their amount is high in S-G2 and low in G1 phases.

Crystal structure determination of α, β and γ-Bi4V2O11 polymorphs. Part I: γ and β-Bi4V2O11

Abstract Using combined X-ray single crystal and neutron powder thermodiffraction data, the crystal structure of the high temperature γ -form of Bi 4 V 2 O 11 was confirmed and accurately refined in the I4/mmm space group and that of the β -form was entirely determined in the centrosymmetric Amam space group. The two-fold superlattice characterising the β structure is the result of an ordering process involving corner-sharing V–O tetrahedra and disordered trigonal bipyramids. A possible scheme for the γ ↔ β phase transition is proposed.

Common and reversible regulation of wild-type p53 function and of ribosomal biogenesis by protein kinases in human cells

Two specific inhibitors of cyclin-dependent kinase 2 (Cdk2), roscovitine and olomoucine, have been shown recently to induce nuclear accumulation of wt p53 and nucleolar unravelling in interphase human untransformed IMR-90 and breast tumor-derived MCF-7 cells. Here, we show that the early response of MCF-7 cells to roscovitine is fully reversible since a rapid restoration of nucleolar organization followed by an induction of p21WAF1/CIP1, a downregulation of nuclear wt p53 and normal cell cycle resumption occurs if the compound is removed after 4 h. Interestingly, similar reversible effects are also induced by the casein kinase II (CKII) inhibitor, 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole. Upon short-term treatment also, both compounds significantly, but reversibly, reduce the level of 45S precursor ribosomal RNA. Cells exposed to the two types of protein kinase inhibitors for longer times keep exhibiting altered nucleolar and wt p53 features, yet they strikingly differentiate in that most roscovitine-treated cells fail to ever accumulate high levels of p21WAF1/CIP1 in contrast with DRB-treated ones. In both cases, however, the cells eventually fall into an irreversible state and die. Moreover, we found that constitutive overexpression of p21WAF1/CIP1 alters the nucleolar unravelling process in the presence of DRB, but not of roscovitine, suggesting a role for this physiological Cdk inhibitor in the regulation of nucleolar function. Our data also support the notion that both roscovitine- and DRB-sensitive protein kinases, probably including Cdk2 and CKII, via their dual implication in the p53-Rb pathway and in ribosomal biogenesis, would participate in coupling cell growth with cell division.

Unprecedented dual behaviour of a half-sandwich scandium-based initiator for both highly selective isoprene and styrene polymerisation

Highly cis-1,4 polymerisation of isoprene and syndiospecific polymerisation of styrene were conducted using the same borohydrido half-sandwich scandium pre-initiator.

An uranyl citrate coordination polymer with a 3D open-framework involving uranyl cation-cation interactions

A novel uranyl-organic compound incorporating citrate linker exhibits a 3D open-framework. It is built up from the connection of infinite chains of uranyl-centered polyhedra with citrate ligands, generating layers, which are further linked perpendicularly by isolated uranium tetranuclear motifs, involving uranyl cation-cation interactions.

Amount variability of total and individual Ag-NOR proteins in cells stimulated to proliferate.

Ribosomal genes are associated with a subset of acidic proteins called Ag-NOR proteins. The amount of nucleolar Ag-NOR proteins varies, depending on nucleolar activity and/or cell proliferation. To understand the linkage between the amount of Ag-NOR proteins, ribosome biogenesis, and cell proliferation, we investigated the variability of Ag-NOR proteins in rRNA-stimulated cells maintained in G1 and in rRNA-stimulated cells entering the mitotic cycle. Rat hepatocytes were stimulated with cortisol for rRNA synthesis (1, 4, and 8 hr) and the cell cycle was induced by hepatectomy in regenerating hepatocytes (3-21 hr). In non-stimulated hepatocytes, nucleolin and protein B23 were the two major Ag-NOR proteins, corresponding to 70% of total Ag-NOR staining. In hepatocytes stimulated for rRNA synthesis in G1, the amount of Ag-NOR proteins was only slightly increased, whereas in cycle-stimulated cells it was increased 3.04-fold. This is the consequence of a differential increase of the major Ag-NOR proteins that appears earlier and is proportionally more important for nucleolin (3.5-fold) than for protein B23 (twofold) and also for the increase of several minor Ag-NOR proteins. We conclude that, in dividing cells, the mean value of the Ag-NOR proteins measured reflects the percentage of cells in the different phases. This could explain why the amount of Ag-NOR proteins can be used as a marker of cell proliferation.

Synthesis and photocatalytic activity of iodine-doped ZnO nanoflowers

The paper reports on the preparation and photocatalytic activity of thin films of iodine-doped ZnO nanoflowers deposited on glass substrate using a simple growth process based on hydrothermal synthesis. Addition of iodic acid (5–20 vol%) in the reaction mixture allows the introduction of iodine ions in the form of I− or IO3− in the ZnO lattice, as suggested by X-ray photoelectron spectroscopy. Doping ZnO nanostructured films with iodine did not impact their morphology, while it has a significant influence on their optical properties. Indeed, the nanostructured ZnO films, prepared in the presence of iodic acid, display a large increase of the visible luminescence, which reaches a maximum at a concentration of 10 vol%. Finally, the photocatalytic activity of the ZnO nanostructured films for the photodegradation of a model pollutant, rhodamine B, was evaluated under UV and visible light irradiation. While under UV light irradiation, both undoped and iodine-doped ZnO films show a similar behavior, the photocatalytic performance of iodine-doped under visible light irradiation is significantly enhanced in comparison to that of undoped ZnO.