Cell cycle regulation of Greatwall kinase nuclear localization facilitates mitotic progression

Cell division requires the coordination of critical protein kinases and phosphatases. Greatwall (Gwl) kinase activity inactivates PP2A-B55 at mitotic entry to promote the phosphorylation of cyclin B–Cdk1 substrates, but how Gwl is regulated is poorly understood. We found that the subcellular localization of Gwl changed dramatically during the cell cycle in Drosophila. Gwl translocated from the nucleus to the cytoplasm in prophase. We identified two critical nuclear localization signals in the central, poorly characterized region of Gwl, which are required for its function. The Polo kinase associated with and phosphorylated Gwl in this region, promoting its binding to 14-3-3ε and its localization to the cytoplasm in prophase. Our results suggest that cyclin B–Cdk1 phosphorylation of Gwl is also required for its nuclear exclusion by a distinct mechanism. We show that the nucleo-cytoplasmic regulation of Gwl is essential for its functions in vivo and propose that the spatial regulation of Gwl at mitotic entry contributes to the mitotic switch.     

Cell cycle regulation

Drosophila researchers met in sunny San Diego for the 49(th) Annual Meeting of The Genetics Society of America. It was cold outside and even colder inside. Like last year, 'Mitosis, Meiosis and Cell Division' was no longer a session. Instead, we searched out and covered talks and posters in 'Cell Division and Growth Control', 'Gametogenesis', 'Cytoskeleton and Cell Biology' and 'Genome and Chromosome Structure'. We split up for maximal coverage and re-grouped later for the Workshop on Cell Cycle and Checkpoints. We apologize in advance for the brevity or omission of some reports.     

TRIM16 inhibits neuroblastoma cell proliferation through cell cycle regulation and dynamic nuclear localization

Neuroblastoma is the most common solid tumor in childhood and represents 15% of all children’s cancer deaths. We have previously demonstrated that tripartite motif 16 (TRIM16), a member of the RING B-box coiled-coil (RBCC)/tripartite totif (TRIM) protein family, has significant effects on neuroblastoma proliferation and migration in vitro and tumorigenicity in vivo. However, the mechanism by which this putative tumor suppressor influences cell proliferation and tumorigenicity was undetermined. Here we show, for the first time, TRIM16’s striking pattern of expression and dynamic localization during cell cycle progression and neuroblastoma tumor development. In a tyrosine hydroxylase MYCN (TH-MYCN) neuroblastoma mouse model, immunohistochemical staining revealed strong nuclear TRIM16 expression in differentiating ganglia cells but not in the tumor-initiating cells. Furthermore in vitro studies clearly demonstrated that during G₁ cell cycle phase, TRIM16 protein expression is upregulated and shifts to the nucleus of cells. TRIM16 also plays a role in cell cycle progression through changes in Cyclin D1 and p27 expression. Importantly, using TRIM16 deletion mutants, an uncharacterized protein domain of TRIM16 was found to be required for both TRIM16’s growth inhibitory effects and its nuclear localization. Taken together, our data suggest that TRIM16 acts as a novel regulator of both neuroblastoma G₁/S progression and cell differentiation.     

Cell cycle dependent regulation of protein kinase CK2 signaling to the nuclear matrix

Protein kinase CK2 is a ubiquitous protein serine/threonine kinase that is involved in cell growth and proliferation as well as suppression of apoptosis. Several studies have suggested that the kinase plays a role in cell cycle progression; however, changes in enzyme activity during phases of cell cycle have not been detected. Nuclear matrix is a key locus for CK2 signaling in the nucleus. We therefore examined CK2 signaling to the nuclear matrix in distinct phases of cell cycle by employing synchronized ALVA-41 prostate cancer cells. Removal of serum from the culture medium resulted in G0/G1 arrest, and a reduction in the nuclear matrix-associated CK2 activity which was rapidly reversed on addition of serum. Arresting the cells in G(0)/G(1) phase with hydroxyurea and subsequent release to S phase by serum gave similar results. Cells arrested in the G(2)/M phase by treatment with nocodazole demonstrated an extensive reduction in the nuclear matrix-associated CK2 which was reversed rapidly on addition of serum. Changes in the immunoreactive CK2 protein were concordant with the activity data reflecting a dynamic trafficking of the kinase in distinct phases of cell cycle. Under the same conditions, CK2 activity in total cellular lysate remained essentially unaltered. These results provide the first direct evidence of discrete modulations of CK2 in the nuclear matrix during the cell cycle progression. Inducible overexpression of CK2 in CHO cells yielded only a modest increase in CK2 activity even though a significant increase in expression was apparent at the level of CK2 alpha-specific message. Stably transfected ALVA-41 cells, however, did not show a significant change in CK2 levels despite increased expression at the message level. Not surprisingly, both types of the stably transfected cells failed to show any alteration in cell cycle progression. Distribution of the CK2 activity in the cytosolic versus nuclear matrix fractions in normal cells appears to be different from that in the cancer cells such that the ratio of nuclear matrix to cytosolic activity is much higher in the latter. Considering that nuclear matrix is central to several nuclear functions, this pattern of intracellular distribution of CK2 may have implications for its role in the oncogenic process. Published 2003 Wiley-Liss, Inc.     

Cell cycle regulation in Schizosaccharomyces pombe

Cdc2, a cyclin-dependent kinase, controls cell cycle progression in fission yeast. New details of Cdc2 regulation and function have been uncovered in recent studies. These studies involve cyclins that associate with Cdc2 in G1-phase and the proteins that regulate inhibitory phosphorylation of Cdc2 during S-phase and G2-phase. Recent investigations have also provided a better understanding of proteins that regulate DNA replication and that are directly or indirectly controlled by Cdc2.     

Cell cycle regulation by environmental pH

Purified populations of quiescent human tumour cells were isolated from plateau phase cultures of PMC-22 cells by centrifugal elutriation. Dilution into fresh medium resulted in these quiescent cells entering S phase exponentially with a t1/2 of 12 hr, after a 18-20-hr lag period during which cellular RNA content increased. Subsequent studies showed that recruitment of quiescent cells into the cell cycle could be regulated by extracellular pH. When exponentially growing PMC-22 cells were exposed to acidic extracellular pH levels, three growth patterns were observed: (1) Normal growth between pH 7.2 to pH 6.8; (2) A reduction in growth rate associated with accumulation of cells with a G₁ DNA content between pH 6.7 and 6.4 (this was also shown to occur in a number of other tumour cell lines); (3) Non-cell-cycle-phase-specific arrest of growth at pH levels less than 6.3. Further studies with purified quiescent cell populations showed the possible existence of a pH-dependent restriction point in the G₁ phase of these tumour cells. The implications of these observations to tumour biology are discussed.     

Cell cycle regulation and interneuron production

The regulation of progenitor proliferation in developing brain in has been extensively studied in the cerebral cortex, but relatively little is known about progenitor divisions in ventral germinal zones. Recent observations pertinent to interneuron genesis in the ventral forebrain, especially in the medial ganglionic eminence, indicate similarities to cerebral cortical neurogenesis and hint at some interesting differences between ventral and dorsal telencephalon progenitors. Proliferation within the ganglionic eminences is discussed from the vantage point of neural precursor cell cycles, especially G1-phase, and current models of neurogenic divisions in cortex that may apply to ventral forebrain as well.     

Cell cycle regulation in bacteria.

Significant progress has been made in the study of ftsZ expression and the topology of FtsZ protein localization in Escherichia coli cells. Exciting results on the identification of new genes required for chromosome resolution and partitioning after the completion of DNA synthesis have also been reported. A recent area of study is asymmetric cell division and its role in differentiation in Bacillus subtilis and Caulobacter crescentus. Biochemical activities of bacterial cell division gene products are also beginning to be addressed.     

Cell cycle regulation in higher plants

The isolation of plant genes homologous to cdk and cyclin components from yeast and animals proves the existence of a basic cell cycle machinery in all eukaryotes. cdk and cyclin expression has been shown to be involved in the spatial and temporal control of cell division in a variety of developmental processes. In plants, cell division and development are closely interlinked processes that are regulated by phytohormones. cdks and cyclins were found to be under control of phytohormones underscoring their integral role in mediating different developmental pathways. Furthermore, studies on cdk and cyclin expression not only correlate with actual cell cycle activity but also with cell division competence providing a working model to understand regeneration capacity at the molecular level.     

PTEN的表达调控及入核机制

抑癌基因磷酸酶与张力蛋白同源物（phosphatase and tensin homolog,PTEN）基因的功能缺失往往会形成多种类型的癌症,它的表达在转录和转录后水平都受到严格的调控。本文对PTEN表达的调控机制以及PTEN的相关核质转运机制进行了综述。     

Cell cycle regulation of flagellar genes.

The expression of the flagellar master operon, flhDC, peaked in the middle of three consecutive cell cycles. The level of expression was lowest at the time of cell division. The expression of the second-level operon, flhB, peaked at cell division. The swimming speed of individual cells was also highest at the time of cell division.     

Cell cycle regulation of the microtubular cytoskeleton

The microtubular element of the plant cytoskeleton undergoes dramatic architectural changes in the course of the cell cycle, specifically at the entry into and exit from mitosis. These changes underlie the acquisition of specialized properties and functions involved, for example, in the equal segregation of chromosomes and the correct positioning and formation of the new cell wall. Here we review some of the molecular mechanisms by which the dynamics and the organization of microtubules are regulated and suggest how these mechanisms may be under the control of cell cycle events.