Toward an understanding of cell growth and the cell division cycle of unicellular photoautotrophic cyanobacteria

The cell division cycle of Synechococcus sp. strain PCC 6301 in light is characterized by the sequential and orderly appearance of macromolecular synthesis periods. In the dark, macromolecular synthesis and cell division are severely curtailed. When dark-incubated cultures are reexposed to light, a new cell cycle is initiated. The pattern of the cell events displayed by Synechococcus in light and the absence of sustained growth in dark incubation conditions suggests that light-activated regulatory molecules control macromolecular synthesis and the cell division cycle. For example, ribosomal RNA synthesis is stimulated by a light-activated DNA binding factor in light but not in the dark. Light/dark conditions induce cell synchrony in Prochlorococcus. Distinct G1, S and G2 phases characterize cell cycles of marine Synechococcus and Prochlorococcus. Cell division in Synechococcus elongatus PCC 7942 and marine Synechococcus is controlled by circadian oscillators.     

An increase in Sendai virus-induced cell fusion of erythrocytes infected with Plasmodium chabaudi

The extent of cell fusion induced by Sendai virus was examined in erythrocytes infected with Plasmodium chabaudi. An increase in cell fusion of erythrocytes with Ehrlich tumor cells and of erythrocytes with erythrocytes was observed with the infected erythrocytes. However, agglutination by the virus was not changed between erythrocytes of normal and malarial mice. These results indicate that the increase in cell fusion occurred in the process of membrane fusion, suggesting that some membrane property of Plasmodium-parasitized erythrocytes is changed in terms of Sendai virus-induced cell fusion.     

Satellite cells in denervated muscles.

It is known that, in a denervated striated muscle, the satellite cells multiply by mitotic division. A liaison between these satellite cells and the Schwann cell in front of the post-synaptic membrane in denervated frog muscle has been observed. It is probable that such cell connections help in the subsistence of the Schwann cell in a denervated muscle.     

p27 small interfering RNA induces cell death through elevating cell cycle activity in cultured cortical neurons: a proof-of-concept study

Recent research has demonstrated that cell cycle-associated molecules are activated in multiple forms of cell death in mature neurons, and raised a hypothesis that unscheduled cell cycle activity leads to neuronal cell death. But there is little evidence that changes in endogenous level of these molecules are causally associated with neuronal cell death. Here we transfected small interfering RNA (siRNA) targeting cyclin-dependent kinase (CDK) inhibitor p27, which plays an important role in cell cycle arrest at G1-S phase, into cultured cortical neurons. Transfection of p27 siRNA reduced neuronal viability in a time-dependent manner. p27 siRNA induced phosphorylation of retinoblastoma protein (Rb), a marker of cell cycle progression at late G1 phase. Moreover, phosphorylation of Rb and neuronal cell death provoked by p27 siRNA were abrogated by pharmacological CDK inhibitors, olomoucine and purvalanol A. Our data demonstrate that a decrease in endogenous p27 induces neuronal cell death through elevating cell cycle activity.     

Satellite cells in denervated muscles

Summary It is known that, in a denervated striated muscle, the satellite cells multiply by mitotic division. A liaison between these satellite cells and the Schwann cell in front of the post-synaptic membrane in denervated frog muscle has been observed. It is probable that such cell connections help in the subsistence of the Schwann cell in a denervated muscle.     

Cell–cell junctional mechanotransduction in endothelial remodeling

The vasculature is one of the most dynamic tissues that encounter numerous mechanical cues derived from pulsatile blood flow, blood pressure, activity of smooth muscle cells in the vessel wall, and transmigration of immune cells. The inner layer of blood and lymphatic vessels is covered by the endothelium, a monolayer of cells which separates blood from tissue, an important function that it fulfills even under the dynamic circumstances of the vascular microenvironment. In addition, remodeling of the endothelial barrier during angiogenesis and trafficking of immune cells is achieved by specific modulation of cell–cell adhesion structures between the endothelial cells. In recent years, there have been many new discoveries in the field of cellular mechanotransduction which controls the formation and destabilization of the vascular barrier. Force-induced adaptation at endothelial cell–cell adhesion structures is a crucial node in these processes that challenge the vascular barrier. One of the key examples of a force-induced molecular event is the recruitment of vinculin to the VE-cadherin complex upon pulling forces at cell–cell junctions. Here, we highlight recent advances in the current understanding of mechanotransduction responses at, and derived from, endothelial cell–cell junctions. We further discuss their importance for vascular barrier function and remodeling in development, inflammation, and vascular disease.     

MSAP enhances migration of C6 glioma cells through phosphorylation of the myosin regulatory light chain

A key regulatory mechanism in cell motility is the control of myosin activity, which in non-muscle cells is determined by phosphorylation of the myosin regulatory light chain (MRLC). Here we show that MRLC-interacting protein (MIR)-interacting saposin-like protein (MSAP) enhances cell spreading in fibroblasts and migration of rat C6 glioma cells through increases in MRLC phosphorylation. Overexpression of MSAP enhanced the motility of glioma cells measured in matrigel invasion chambers and using a scratch assay. Downregulation of MSAP by RNA interference significantly decreased glioma cell migration and phosphorylation of MRLC. Inhibition of the corresponding MRLC kinase by ML-7 did not affect migration of MSAP-overexpressing cells. The present results show that MSAP controls glioma cell migration via enhancement of MRLC phosphorylation. This effect is independent of the activity of MRLC kinase. Thus, MSAP is a novel modulator of cell motility that influences migration of glioma cells and possibly other tumors.Received 9 February 2005; received after revision 2 March 2005; accepted 21 March 2005     

Resetting epigenetic signatures to induce somatic cell reprogramming

Somatic cell reprogramming consists of the induction of a complex sequence of events that results in the modification of the developmental state of the cell. It is now routinely possible to reprogram fully differentiated cells back to pluripotent cells, and to transdifferentiate cells of a given type in cells of a totally different lineage origin. However, whether there are key initiating factors that are distinct from those that control stem-cell renewal and that can initiate the reprogramming process remains unknown. In contrast, what is clear is that, by modifying the epigenetic status of a cell, its reprogramming can be initiated. Here, we review the current literature that shows how the plasticity of a cell can be modulated by modifying its epigenetic status, and we discuss how epigenetic barriers can be removed, to induce an efficient reprogramming process.     

Die Wirkung energiereicher Strahlen auf die Atmungsfermentkette

Summary The irradiation sensitivity of the cytochrome system was investigated, using as examples baker's yeast andE. coli. A method is described with which the proportion of reduced cytochrome in cell suspensions could be measured spectrometrically. In contrast to isolated cytochrome, which is easily oxydized by Roentgen rays, the cytochrome of living cells is very stable to Roentgen rays. The electrone transport is not influenced by high energy rays. It is probable that the hydrogen transport is also undisturbed. Only if the cell is so damaged by high doses of rays that every exchange of substances is impossible, i.e. if the cell is dead, is the ferrocytochrome oxydized and perhaps also destroyed. The oxydation is thus a secondary process resulting from death of the cell by rays. It is supposed that the changes in energy metabolism described in the literature, especially the disturbances of oxydative phosphorylation, are also secondary processes.The concepts developed in radiochemistry on the action of high energy rays on organic compounds must therefore be especially tested in each case for their application in the living cell.     

Measurement of phosphatase activity in the cell wall obtained with the help of a nonionic detergent (Triton X 100)

The phosphatase activity of cell walls isolated by treating the intact roots with Triton X 100 is very steady. It is similar to that of living roots and that of cell walls isolated by centrifugation after the grinding of the tissues.     

Nitric oxide can inhibit apoptosis or switch it into necrosis

Nitric oxide (NO) and its related molecules are important messengers that play central roles in pathophysiology. Redox modulation of thiol groups on protein cysteine residues by S-nitrosylation can modulate protein function. NO has emerged as a potent regulator of apoptosis in many cell types, either preventing cell death or driving an apoptotic response into a necrotic one. NO protects neuroblastoma cells from retinoid- and cisplatin-induced apoptosis, without significantly increasing necrotic cell damage. Nitrosylation of thiol groups of several critical factors may be important for cell survival. Indeed, S-nitrosylation of the active-site cysteine residue of apoptotic molecules, such as caspases and tissue transglutaminase, results in the inhibition of their catalytic activities and has important implications for the regulation of apoptosis by NO. On the other hand, NO is able to shift the anti-CD95- and ceramide-triggered apoptotic response of Jurkat T cells into necrotic cell death. In these apoptotic models, NO is therefore unable to solely inhibit cell death, indicating that it may act below the point of no return elicited by CD95-ligation and ceramide stimulation.     

The spread of Tobacco mosaic virus infection: insights into the cellular mechanism of RNA transport

Interactions of plant cells with pathogens or other biotic or abiotic environmental factors can give rise to systemic defense responses that rely upon the cell-to-cell and systemic transport of specific signals. A novel type of systemic signaling was revealed by recent evidence indicating the existence of RNA species that travel cell to cell and through the vasculature. The most compelling evidence for intercellular and systemic transport of RNA in plants is provided by viroids and viruses that apparently use the endogenous transport machinery to spread infection. The cell to cell movement of plant viruses occurs through small pores in the cell wall known as plasmodesmata and depends on virus-encoded 'movement proteins'. This review summarizes current knowledge of Tobacco mosaic virus infection with emphasis on the mechanism by which this virus targets its RNA genome from sites of replication to plasmodesmata to achieve intercellular spread.     

DNA-dependent-DNA-polymerase: Possible limiting influence on cell reproduction during viral leukemogenesis

Summary Evidence is presented that during viral leukemogenesis spleen cell nuclei show an increase in labelling index and mean grain count, that is not accompanied by any changes in the nuclear level of DNA-polymerase-. It is suggested that polymerase production remains under the control of the normal cell mechanisms and the virus may affect cell proliferation by altering the primer-template levels.Acknowledgment. This work was supported in part by NIH-NCI grant No. 1-PO₂-CA-10438 and by USERDA contract No. E (11-1) 3097, P.G.B. was also supported by contract NCI-CB-43899.     

Lysozyme activates Enterococcus faecium to induce necrotic cell death in macrophages

Enterococci are commensal organisms in the alimentary tract. However, they can cause a variety of life-threatening infections, especially in nosocomial settings. We hypothesized that induction of cell death might enable these facultative pathogenic bacteria to evade the innate immune response and to cause infections of their host. We demonstrate that E. faecium when exposed to lysozyme induces cell death in macrophages in vitro and in vivo. Flow cytometric analyses of J774A.1 macrophages infected with E. faecium revealed loss of cell membrane integrity indicated by uptake of propidium iodide and decrease of the inner mitochondrial transmembrane potential ΔΨ_m. Inhibition of caspases, treatment of macrophages with cytochalasin D, or rifampicin did not prevent cells from dying, suggesting cell death mechanisms that are independent of caspase activation, bacterial uptake, and intracellular bacterial replication. Characteristics of necrotic cell death were demonstrated by both lack of procaspase 3 activation and cell shrinkage, electron microscopy, and release of lactate dehydrogenase. Pretreatment of E. faecium with lysozyme and subsequently with broad spectrum protease considerably reduced cell death, suggesting that a bacterial surface protein is causative for cell death induction. Moreover, in a mouse peritonitis model we demonstrated that E. faecium induces cell death of peritoneal macrophages in vivo. Altogether, our results show that enterococci, under specific conditions such as exposure to lysozyme, induce necrotic cell death in macrophages, which might contribute to disseminated infections by these facultative pathogenic bacteria.     

Regulation of G1 phase progression by growth factors and the extracellular matrix

Cell cycle progression is regulated by both intracellular and extracellular control mechanisms. Intracellular controls ensure that cell cycle progression is stopped in response to irregularities such as DNA damage or faulty spindle assembly, whereas extracellular factors may determine cell fate such as differentiation, proliferation or programmed cell death (apoptosis). When extracellular factors bind to receptors at the outside of the cell, signal transduction cascades are activated inside the cell that eventually lead to cellular responses. We have shown previously that MAP kinase (MAPK), one of the proteins involved in several signal transduction processes, is phosphorylated early after mitosis and translocates to the nucleus around the restriction point. The activation of MAPK is independent of cell attachment, but does require the presence of growth factors. Moreover, it appears that in Chinese hamster ovary cells, a transformed cell line, growth factors must be present early in the G1 phase for a nuclear translocation of MAPK and subsequent DNA replication to occur. When growth factors are withdrawn from the medium immediately after mitosis, MAPK is not phosphorylated, cell cycle progression is stopped and cells appear to enter a quiescent state, which may lead to apoptosis. Furthermore, in addition to this growth-factor-regulated decision point in early G1 phase, another growth-factor-sensitive period can be distinguished at the end of the G1 phase. This period is suggested to correlate with the classical restriction point (R) and may be related to cell differentiation.     

Mitochondrial control of caspase-dependent and -independent cell death

Mitochondria control whether a cell lives or dies. The role mitochondria play in deciding the fate of a cell was first identified in the mid-1990s, because mitochondria-enriched fractions were found to be necessary for activation of death proteases, the caspases, in a cell-free model of apoptotic cell death. Mitochondrial involvement in apoptosis was subsequently shown to be regulated by Bcl-2, a protein that was known to contribute to cancer in specific circumstances. The important role of mitochondria in promoting caspase activation has therefore been a major focus of apoptosis research; however, it is also clear that mitochondria contribute to cell death by caspase-independent mechanisms. In this review, we will highlight recent findings and discuss the mechanism underlying the mitochondrial control of apoptosis and caspase-independent cell death.     

Non-caspase proteases: triggers or amplifiers of apoptosis?

Caspases are the most important effectors of apoptosis, the major form of programmed cell death (PCD) in multicellular organisms. This is best reflected by the appearance of serious development defects in mice deficient for caspase-8, -9, and -3. Meanwhile, caspase-independent PCD, mediated by other proteases or signaling components has been described in numerous publications. Although we do not doubt that such cell death exists, we propose that it has evolved later during evolution and is most likely not designed to execute, but to amplify and speed-up caspase-dependent cell death. This review shall provide evidence for such a concept.     

Phosphorylation and activation of the atypical kinase p53-related protein kinase (PRPK) by Akt/PKB

p53-related protein kinase (PRPK), the human homologue of yeast Bud32, belonging to a small subfamily of atypical protein kinases, is inactive unless it is previously incubated with cell lysates. Here we show that such an activation of PRPK is mediated by another kinase, Akt/PKB, which phosphorylates PRPK at Ser250. We show that recombinant PRPK is phosphorylated in vitro by Akt and its phospho-form is recognized by a Ser250-phospho-specific antibody; that cell co-transfection with Akt along with wild-type PRPK, but not with its Ser250Ala mutant, results in increased PRPK phosphorylation; and that the phosphorylation of p53 at Ser15, the only known substrate of PRPK, is markedly increased by co-transfection of Akt with wild-type PRPK, but not PRPK dead mutant, and is abrogated by cell treatment with the Akt pathway inhibitor LY294002. Our data disclose an unanticipated mechanism by which PRPK can be activated and provide a functional link between this enigmatic kinase and the Akt signaling pathway.