Calcium transients during mitosis: observations in flux

A secreted chitinase-like protein (OsCLP) supports root growth through calcium signaling in Oryza sativa

Chitinases belong to a conserved protein family and play multiple roles in defense, development and growth regulation in plants. Here, we identified a secreted chitinase-like protein, OsCLP, which functions in rice growth. A T-DNA insertion mutant of OsCLP (osclp) showed significant retardation of root and shoot growth. A comparative proteomic analysis was carried out using root tissue of wild-type and the osclp mutant to understand the OsCLP-mediated rice growth retardation. Results obtained revealed that proteins related to glycolysis (phosphoglycerate kinase), stress adaption (chaperonin) and calcium signaling (calreticulin and CDPK1) were differentially regulated in osclp roots. Fura-2 molecular probe staining, which is an intracellular calcium indicator, and inductively coupled plasma-mass spectrometry (ICP-MS) analysis suggested that the intracellular calcium content was significantly lower in roots of osclp as compared with the wild-type. Exogenous application of Ca²⁺ resulted in successful recovery of both primary and lateral root growth in osclp. Moreover, overexpression of OsCLP resulted in improved growth with modified seed shape and starch structure; however, the overall yield remained unaffected. Taken together, our results highlight the involvement of OsCLP in rice growth by regulating the intracellular calcium concentrations.

Spontaneous [Ca2+]i oscillations in G1/S phase-synchronized cells

Ca(2+) signaling controls a wide range of cellular functions such as division, fertilization, apoptosis and necrosis. Specifically, calcium signaling is thought to play a crucial role in driving cells through the different stages of the cell-division cycle. In most cells, however, this fact is far from being established. Few studies have examined this question from a different perspective: whether cells exhibit some characteristic cell cycle-dependent intracellular calcium-signaling patterns. This approach is effective in discerning the causal relationship between Ca(2+) signaling and the cell cycle. Through synchronization of the cell cycle, flow cytometry and confocal scanning microscopic intracellular calcium ion concentration ([Ca(2+)](i)) imaging, the present study shows that the G1/S phase transition is uniquely characterized by spontaneous [Ca(2+)](i) oscillations that last for up to 40 min. Most likely, these oscillations emanate from the [Ca(2+)](i) signaling that accompanies DNA replication as the cell prepares for the next division cycle. These temporal signals further affirm the significance of Ca(2+) in the cell cycle.

An isoform of myosin XI is responsible for the translocation of endoplasmic reticulum in tobacco cultured BY-2 cells

The involvement of myosin XI in generating the motive force for cytoplasmic streaming in plant cells is becoming evident. For a comprehensive understanding of the physiological roles of myosin XI isoforms, it is necessary to elucidate the properties and functions of each isoform individually. In tobacco cultured BY-2 cells, two types of myosins, one composed of 175 kDa heavy chain (175 kDa myosin) and the other of 170 kDa heavy chain (170 kDa myosin), have been identified biochemically and immunocytochemically. From sequence analyses of cDNA clones encoding heavy chains of 175 kDa and 170 kDa myosin, both myosins have been classified as myosin XI. Immunocytochemical studies using a polyclonal antibody against purified 175 kDa myosin heavy chain showed that the 175 kDa myosin is distributed throughout the cytoplasm as fine dots in interphase BY-2 cells. During mitosis, some parts of 175 kDa myosin were found to accumulate in the pre-prophase band (PPB), spindle, the equatorial plane of a phragmoplast and on the circumference of daughter nuclei. In transgenic BY-2 cells, in which an endoplasmic reticulum (ER)-specific retention signal, HDEL, tagged with green fluorescent protein (GFP) was stably expressed, ER showed a similar behaviour to that of 175 kDa myosin. Furthermore, this myosin was co-fractionated with GFP-ER by sucrose density gradient centrifugation. From these findings, it was suggested that the 175 kDa myosin is a molecular motor responsible for translocating ER in BY-2 cells.

Quantitative imaging of subcellular calcium stores in mammalian LLC-PK1 epithelial cells undergoing mitosis by SIMS ion microscopy

Quantitative 3-D total calcium gradients, representing subcellular stored calcium, were imaged with a CAMECA IMS-3f SIMS ion microscope in cryogenically prepared frozen freeze-dried LLC-PK1 cells captured in interphase and various stages of mitosis. 39K and 23Na concentrations were also measured in the same cells. Correlative optical (or SEM) and SIMS analysis of cells revealed a redistribution of the interphase Golgi calcium store in prophase and prometaphase cells. In metaphase cells, simultaneous SIMS imaging of total calcium in both the spindle and the non-spindle cytoplasm of individual cells revealed a gradual and dynamic alignment of calcium stores in both half-spindles prior to the onset of anaphase. The anaphase cells revealed the highest local total calcium concentrations in the spindle regions behind the daughter chromosomes and the lowest in the central spindle region. The pericentriolar material in telophase cells contained calcium stores. Quantitatively, a typical metaphase cell with well-aligned calcium stores in the spindle region contained 1.1 mM total calcium in each half-spindle, 0.8 mM total calcium in the non-spindle cytoplasm, and 0.5mM total calcium in the chromosomes. At the submicron scale, the distribution of total calcium was heterogeneous in the chromosomes, metaphase spindle, and non-spindle cytoplasm. An increased binding of calcium to chromosomes is not a physiological requirement for chromosomal condensation in mitosis, since interphase nuclei and mitotic chromosomes contained comparable total calcium concentrations measured per unit volume. A significant reduction of total calcium in the non-spindle cytoplasm was observed in the metaphase, anaphase, and telophase cells, which is indicative of the limited storage of the releasable calcium pool in these specific stages of mitosis. Direct total calcium measurements in subcellular regions confirmed that both the spindle and the non-spindle cytoplasm of metaphase cells contained inositol 1,4,5-trisphosphate (IP3)-sensitive calcium stores sensitive to arginine vasopressin, thapsigargin, and calcium ionophore A23187. The dynamic alignment of calcium stores in both half-spindles may be an integral part of the time-dependent process of a cell's overall preparation for exiting the metaphase stage in mammalian LLC-PK1 cells.

Parthenogenetic and nuclear transfer rabbit embryo development and apoptosis after activation treatments

Previous studies mainly evaluated the effect of culture conditions on preimplantation embryo apoptosis. In order to inhibit apoptosis of nuclear transfer (NT) embryos, putative apoptosis inhibitors were used to treat donor cells. However, little is known about the effect of activation treatments on embryo apoptosis. We firstly investigated the effect of various parthenogenetic activation (PA) treatments on embryo development, blastocyst cell number, and apoptosis, and then one of these activation treatments proved to be most efficient was selected for activation rabbit NT embryos. The activation by electrical pulses and 30 min later, electroporation with 25 muM D-myoinositol 1,4,5-trisphosphate (IP3) in Ca(2+)- and Mg(2+)-free PBS, then exposure to 2.0 mM 6-dimethylaminopurine (6-DMAP) for 3 hr effectively activated rabbit oocytes, and resulted in significantly a higher blastocyst development rate (72.7%) and total cell number (175 +/- 14.1), and markedly lower apoptosis level of blastocyst (4.3 +/- 0.5) than all the other groups. When the same activation protocol was applied in NT embryo activation, we found that exposure of the embryos to 6-DMAP for 3 hr could decrease the apoptosis level of blastocyst and increase blastocyst rate and cell number. The results demonstrate that oocyte activation affects not only embryo development and quality but also embryo apoptosis.

Studies of cell division (mitosis and cytokinesis) by dynamic secondary ion mass spectrometry ion microscopy: LLC-PK1 epithelial cells as a model for subcellular isotopic imaging

The feasibility of the renal epithelial LLC-PK1 cell line as a model for cell division studies with secondary ion mass spectrometry (SIMS) was tested. In this cell line, cells undergoing all stages of mitosis and cytokinesis remained firmly attached to the substrate and could be cryogenically prepared. Fractured freeze-dried mitotic cells showed well-preserved organelles as revealed by fluorescence imaging of rhodamine-123 and C6-NBD-ceramide by confocal laser scanning microscopy. Secondary electron microscopy analysis of fractured freeze-dried dividing cells revealed minimal surface topography that does not interfere in isotopic imaging of both positive (39K, 23Na, 24Mg, 40Ca, etc.) and negative (31P, 35Cl, etc.) secondaries with a CAMECA IMS-3f ion microscope. Mitotic cells revealed well-preserved intracellular ionic composition of even the most diffusible ions (total concentrations of 39K+ and 23Na+) as revealed by K : Na ratios of approximately 10. Structurally damaged mitotic cells could be identified by their reduced K : Na ratios and an excessive loading of calcium. Quantitative three-dimensional SIMS analysis was required for studying subcellular calcium distribution in dividing cells. The LLC-PK1 model also allowed SIMS studies of M-phase arrested cells with mitosis-arresting drugs (taxol, monastrol and nocodazole). This study opens new avenues of cell division research related to ion fluxes and chemical composition with SIMS.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Activation of rabbit oocytes: the impact of the Ca2+ signal regime on development

Postfertilization manipulation of mammalian embryos results in various developmental alterations. To determine whether the manipulation of the Ca2+ regime causing oocyte activation is a valuable experimental means in helping understand the biological process by which embryos integrate signals from outside and later regulate gene expression, we linked Ca2+ signal parameters i.e. amplitude, number and frequency, with the efficiency and quality of postimplantation development. Freshly ovulated rabbit oocytes were subjected to repetitive and modulated Ca2+ influx. The results provide three major pieces of information. Firstly, the Ca2+ stimulus is the most efficient signal activating mammalian eggs when it is applied in a repetitive manner, the amplitude being the crucial factor. Secondly, the dynamics of early cleavage does not appear to be determined by either the frequency or the amplitude of modulation of the Ca2+ signal that activates the oocyte. Thirdly, amplitude and temporal modulation of the Ca2+ signal in the early minutes influences the developmental performance and the morphology of the rabbit parthenogenetic conceptus at day 11.5 of pregnancy. The results demonstrate the importance of epigenetic events during postfertilization as well as the possible uses of Ca2+ modulation in studying long term developmental effects.

Arrangement of actin filaments and cytoplasmic granules in the sea urchin egg after TPA treatment

Elongation of microvilli and formation of actin filaments after treatment with a phorbol ester, TPA, were investigated in unfertilized eggs of Hemicentrotus pulcherrimus. Microvilli on the egg surface were examined by scanning electron microscopy. Actin filaments in the cortical layer of the eggs were observed by fluorescence microscopy using rhodamine-labeled phalloidin. The actin molecules were polymerized and bundled to form long filaments inside the cortical layer of eggs after TPA treatment. Arrangement of the actin filaments was followed by spiral elongation of microvilli. Transmission electron microscopic studies showed that the cortical granules under the cell membrane of sea urchin eggs were transferred after TPA treatment from the surface to the interior of the cell [Ciapa et al., 1988: Dev. Biol. 128:142-149]. This movement of the cortical granules was inhibited by cytochalasin B, but not by nocodazole. Furthermore, the distribution of clear granules was changed following TPA treatment. From these results we conclude that intracellular actin filaments may cause the transport of cortical granules and clear granules into the central area of the egg by the activation of protein kinase C. The possible involvement of actin in the inward displacement of granules might be the result of the rearrangement of actin filaments in the cortical layer.

Calcium signalling during zebrafish embryonic development

Calcium signals appear throughout the first 24 hours of zebrafish development. These begin at egg activation, then continue to be generated throughout the subsequent zygote, cleavage, blastula, gastrula, and segmentation periods. They are thus associated with the major phases of pattern formation: cell proliferation, cell differentiation, axis determination, the generation of primary germ layers, the emergence of rudimentary organ systems, and therefore the establishment of the basic vertebrate body plan. When signals need to be transmitted across significant distances they take the form of waves, either intracellular waves when the cell size is large, or later in development when the cell size is reduced, intercellular waves. We will consider both types of calcium signals and their integration into signalling networks, and discuss their possible functions and developmental significance with regard to pattern formation. BioEssays 22:113-123, 2000.

Irregular nuclear localization and anucleate cell production in Escherichia coli induced by a Ca2+ chelator, EGTA

A screening system for specific inhibitors of chromosome partitioning in Escherichia coli was constructed using the blue assay method developed for detection of anucleate cell production. Effects of known chemical compounds and antibiotics were examined in the system. It was found that a calcium-chelating reagent, EGTA, induced blue zones around the paper disks containing EGTA at concentrations which did not induce growth inhibition zones. Induction of anucleate cell production by EGTA was confirmed by fluorescence microscopy after DAPI staining. Even in the nucleated cells, irregular intracellular localization of nucleoids was frequently observed. The effect of EGTA was reversed by addition of Ca(2+). These results suggest the possible role of calcium ion in the process of chromosome partitioning in E. coli.

Effect of intracellular Ca2+ chelation with the acetoxymethyl ester-derived form of bis(o-aminophenoxy)ethane-N,N,N,N',N'-tetraacetic acid on meiotic division and chromosomal segregation in mouse oocytes

PURPOSE

Our purpose was to ascertain the effect of intracellular Ca2+ chelation on the chromosomal distribution and segregation of mouse oocytes during maturation in vitro.

METHODS

Germinal vesicle oocytes were loaded with the acetoxymethyl ester-derived form of bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). Chromosomal distribution and segregation of control and BAPTA-AM-treated metaphase II (MII) oocytes were evaluated at 16 hr, and intracellular ATP content at 0, 1, and 16 hr after BAPTA-AM loading.

RESULTS

BAPTA-AM treatment decreased (P < or = 0.05) the potential for in vitro maturation, increased (P < or = 0.0001) the percentage of oocytes displaying an abnormal distribution of metaphase II chromosomes in the meiosis II spindle and aneuploidy, and decreased (P < or = 0.005) the ATP content at 0, 1, and 16 hr of culture compared to the control groups.

CONCLUSIONS

These findings raise some concern about any other condition/drug that may directly or indirectly decrease the intracellular Ca2+ concentration in human oocytes.

Effect of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, on the first mitotic divisions of the fertilized sea urchin egg

We have reported earlier that the polyphosphoinositide messenger system may control mitosis in sea urchin eggs. Besides phospholipase C activation and its second messengers, phosphatidylinositol (PI) 3-kinase has been proposed to affect a wide variety of cellular processes in other cellular systems. Therefore, we have investigated whether PI 3-kinase could play a role in regulating the sea urchin early embryonic development. Our data presented here suggest that PI 3-kinase is present in sea urchin eggs. We found that wortmannin, an inhibitor of PI 3-kinase, led to arrest of the cell cycle. Chromosome condensation, nuclear envelope breakdown, microtubular aster polymerization, protein and DNA synthesis were not affected when fertilization was performed in the presence of the drug. However, maturation-promoting factor (MPF) activation was inhibited and centrosome duplication was perturbed preventing the formation of a bipolar mitotic spindle in wortmannin treated eggs. We discuss how PI 3-kinase might be involved in the cascade of events leading to the first mitotic divisions of the fertilized sea urchin egg.

Localized calcium transients accompany furrow positioning, propagation, and deepening during the early cleavage period of zebrafish embryos

Through the injection of f-aequorin (a calcium-specific luminescent reporter) and the use of an imaging photon detector, we see a distinct localized elevation of intracellular calcium that accompanies the appearance of the first furrow arc at the blastodisc surface: the furrow positioning signal. As the leading edges of the arc progress outward toward the margins of the blastodisc, they are accompanied by two subsurface slow calcium waves moving at about 0.2 micron/s: the furrow propagation signal. As these wave fronts approach the edge of the blastodisc, another calcium signal arises in the central region where the positioning signal originally appeared. Like the propagation signal, it extends outward to the margins of the blastodisc, but in this case it also moves downward, accompanying the deepening process that separates the daughter cells: the furrow deepening signal. Both of these furrow deepening progressions move at around 0.1 to 0.2 micron/s. The deepening signal begins to diminish from the center outward, returning to precleavage resting levels on completion of cytokinesis. The signaling sequence is repeated during the second cell division cycle. These localized transients do not require external calcium and they can be dissipated after they have begun by introducing calcium shuttle buffers, resulting in furrow delocalization and regression. They also occur in parthenogenetically activated eggs in which, in an attenuated form, they accompany abortive cleavages.

Virus-like particles, bacteria and microsporidia affect spindle-associated membranes in spermatocytes of Lepidoptera species

Larval testes of four Lepidoptera species were examined using electron microscopy. The testes of one species, the Mediterranean mealmoth Ephestia kuehniella (Pyralidae), were devoid of intracellular pathogens and serve as a control. In this species, metaphase spindles of primary spermatocytes showed a thick layer of perispindle membranes. The membranes were structurally very similar to the agranular endoplasmic reticulum. Membranes of this type occurred also at high frequency throughout the spindle matrix. The analysis of larval testes of Pieris brassicae (Pieridae) revealed virus-like particles within spermatocytes. In another species, Philudoria potatoria (Lasiocampidae), the spermatocytes possessed intracellular bacteria. Whereas the pathogens were found within the germ cells in these species, a fourth species, Plutella xylostella (Plutellide), showed microsporidia within somatic cells of the testis sheath. In all the infected animals, the mass of perispindle membranes was reduced in comparison with spermatocytes of E. kuehniella. However, spindle structure appeared regular in the infected animals. This indicates that a thick layer of perispindle membranes is not decisive for spindle assembly and function in male meiosis of Lepidopera.

Transient release of calcium from inositol 1,4,5-trisphosphate-specific stores regulates mouse preimplantation development

Inositol 1,4,5-trisphosphate can regulate growth and differentiation by modulating the release of intracellular Ca2+ in a variety of cellular systems, and it is involved in oocyte activation. Recent studies suggest that mammalian preimplantation development may also be regulated by the release of Ca2+ from intracellular stores. The rate of cavitation and cell division was accelerated after a transient elevation of intracellular Ca2+ levels was induced in morulae by exposure to ethanol or ionomycin. Embryos exposed to BAPTA-AM, a chelator of intracellular Ca2+, exhibited a brief dose-dependent reduction in basal Ca2+ levels, a temporal inhibition of ionophore-induced Ca2+ signalling and a subsequent delay in blastocoel formation. BAPTA-AM at 0.5 microM did not significantly alter the basal intracellular calcium level, but chelated Ca2+ that was released after ethanol exposure and thereby attenuated the ethanol-induced acceleration of cavitation. BAPTA-AM also inhibited cell division to the 16-cell stage in a dose-dependent manner, which correlated with the inhibition of cavitation. Thimerosal and inositol 1,4,5-trisphosphate significantly elevated the intracellular Ca2+ concentration in mouse morula-stage embryos, providing evidence for the existence of inositol 1,4,5-trisphosphate-sensitive Ca2+ stores. Although caffeine failed to release intracellular Ca2+, ryanodine induced a small biphasic release of Ca2+, suggesting that ryanodine-sensitive Ca2+ stores may also exist in mouse embryos. Morulae exposed to the calmodulin inhibitor W-7 exhibited a dose-dependent delay in blastocoel formation. A 4 hour exposure to 10 microM W-7 did not significantly alter cavitation, but attenuated the ionophore-induced stimulation of blastocoel formation. This finding suggests that the developmental effects produced through Ca2+ signalling are mediated by calmodulin. Our results demonstrate that Ca2+ release in mouse morulae occurs predominantly through the inositol 1,4,5-trisphosphate receptor, and that alteration of intracellular Ca2+ levels can accelerate or delay embryonic growth and differentiation, providing a mechanistic link between the regulation of oocyte and embryonic development.

Calcium, BOBs, QEDs, microdomains and a cellular decision: control of mitotic cell division in sand dollar blastomeres

The role of Ca2+ in controlling cell processes (e.g. mitosis) presents an enigma in its ubiquity and selectivity. Intracellular free Ca2+ (Ca2+i) is an essential regulator of specific biochemical and physiological aspects of mitosis (e.g. nuclear envelope breakdown (NEB)). Changes in Ca2+i concentrations during mitosis in second cell-cycle sand dollar (Echinaracnius parma) blastomeres were imaged as Ca(2+)-dependent luminescence of the photoprotein aequorin with multi-spectral analytical video microscopy. Photons of this luminescence were seen as bright observable blobs (BOBs). Spatiotemporal patterns of BOBs were followed through one or more cell cycles to detect directly changes in Ca2+i, and were seen to change in a characteristic fashion prior to NEB, the onset of anaphase chromosome movement, and during cytokinesis. These patterns were observed from one cell cycle to the next in a single cell, from cell to cell, and from egg batch to egg batch. In both mitosis and synaptic transmission increases in Ca2+i concentration occurs in discrete, short-lived, highly localized pulses we name quantum emission domains (QEDs) within regions we named microdomains. Signal and statistical optical analyses of spatiotemporal BOB patterns show that many BOBs are linked by constant displacements in space-time (velocity). Linked BOBs are thus nonrandom and are classified as QEDS. Analyses of QED patterns demonstrated that the calcium signals required for NEB are nonrandom, and are evoked by an agent(s) generated proximal to a Ca2+i-QED; models of waves, diffusible agonists and Ca(2+)-activated Ca2+ release do not fit pre-NEB cell data. Spatial and temporal resolution of this multispectral approach significantly exceeds that reported for other methods, and avoids the perturbations associated with many fluorescent Ca2+ reporters that interfere with cells being studied (Ca(2+)-buffering, UV toxicity, etc.). Spatiotemporal patterns of Ca2+i-QED can control so many different processes, i.e. specific frequencies used to control particular processes. Predictive and structured patterns of calcium signals (e.g. a language expressed in Ca2+) may selectively regulate specific Ca(2+)-dependent cellular processes.

A cell cycle-associated change in Ca2+ releasing activity leads to the generation of Ca2+ transients in mouse embryos during the first mitotic division

We have used Ca2+-sensitive fluorescent dyes to monitor intracellular Ca2+ during mitosis in one-cell mouse embryos. We find that fertilized embryos generate Ca2+ transients at nuclear envelope breakdown (NEBD) and during mitosis. In addition, fertilized embryos arrested in metaphase using colcemid continue to generate Ca2+ transients. In contrast, parthenogenetic embryos produced by a 2-h exposure to strontium containing medium do not generate detectable Ca2+ transients at NEBD or in mitosis. However, when parthenogenetic embryos are cultured continuously in strontium containing medium Ca2+ transients are detected in mitosis but not in interphase. This suggests that mitotic Ca2+ transients are detected in the presence of an appropriate stimulus such as fertilization or strontium. The Ca2+ transient detected in fertilized embryos is not necessary for inducing NEBD since parthenogenetic embryos undergo nuclear envelope breakdown (NEBD). Also the first sign that NEBD is imminent occurs several minutes before the Ca2+ transient. The Ca2+ transient at NEBD appears to be associated with the nucleus since nuclear transfer experiments show that the presence of a karyoplast from a fertilized embryo is essential. Finally, we show that the intracellular Ca2+ chelator Bapta inhibits NEBD in fertilized and parthenogenetic embryos in a dose-dependent manner. These studies show that during mitosis there is an endogenous increase in Ca2+ releasing activity that leads to the generation of Ca2+ transients specifically during mitosis. The ability of Ca2+ buffers to inhibit NEBD regardless of the presence of global Ca2+ transients suggests that the underlying cell cycle-associated Ca2+ releasing activity may take the form of localized Ca2+ transients.

Calcium and cell cycle control in early embryos

Over the past few years, we have witnessed a burgeoning series of papers addressing the role of calcium signalling in cell cycle control. In this review I will attempt to bring together all the diverse threads and discuss new concepts that have arisen from the most recent data. Because the major part of the data concerns mitosis/meiosis entry and exit, I have focused on these areas. I will jointly refer to meiotic and mitotic phases of the cell cycle as M-phase because these phases are highly comparable. Studies of the cell cycle involve a huge range of species, from plants to humans. I will, however, restrict this review to the work performed in early embryos. I apologise in advance to contributors to this field whose names I do not mention because they do not work on embryos.

Maitotoxin, a calcium channel activator, inhibits cell cycle progression through the G1/S and G2/M transitions and prevents CDC2 kinase activation in GH4C1 cells

Calcium regulates progression through several checkpoints in the cell cycle, including the G1/S-phase transition, G2/M-phase transition, and exit from mitosis. In the GH4C1 rat pituitary cell line, calcium mobilizing polypeptides and calcium channel activation inhibit cell proliferation. This report examines the effects of maitotoxin (MTX), an activator of type L voltage-dependent calcium channels (L-VDCC), on calcium influx and cell cycle progression in GH4C1 cells. MTX causes both a block from G1 to S-phase and a concentration-dependent accumulation of cells in G2+M. MTX does not increase the mitotic index; thus, sustained calcium channel activation by MTX results in an accumulation of cells in G2. In order to temporally localize the MTX-induced G2 block relative to cell cycle regulatory events at the G2/M transition, we assessed the relative activity of two cell cycle regulatory protein kinases, CDC2 and CDK2, in MTX-treated cells. CDC2-specific histone kinase activity in MTX-treated cells is lower than either in cells blocked in mitosis with the microtubule destabilizing agent demecolcine or in randomly cycling cells. In contrast, the activity of CDK2 is highest in MTX-treated cells, consistent with a G2 block prior to CDC2 activation. Together, these results implicate with a G2 block prior to CDC2 activation. Together, these results implicate calcium as an intracellular signal required for progression through G2 phase of the cell cycle prior to CDC2 kinase activation.

The timing of protein kinase activation events in the cascade that regulates mitotic progression in Tradescantia stamen hair cells

Stamen hair cells of the spiderwort plant Tradescantia virginiana exhibit unusually predictable rates of progression through mitosis, particularly from the time of nuclear envelope breakdown (NEBD) through the initiation of cytokinesis. The predictable rate of progression through prometaphase and metaphase has made these cells a useful model system for the determination of the timing of regulatory events that trigger entry into anaphase. A number of studies suggest that the elevation of one or more protein kinase activities is a necessary prerequisite for entry into anaphase. The current experiments employ two strategies to test when these elevations in protein kinase activity actually occur during metaphase. In perfusions, we added the protein kinase inhibitors K-252a, staurosporine, or calphostin C to living stamen hair cells for 10-min intervals at known times during prometaphase or metaphase and monitored the subsequent rate of progression into anaphase. Metaphase transit times were altered as a function of the time of addition of K-252a or staurosporine to the cells; metaphase transit times were extended significantly by treatments initiated in prometaphase through early metaphase and again late in metaphase. Transit times were normal after treatments initiated in mid-metaphase, approximately 15 to 21 min after NEBD. Calphostin C had no significant effect on the metaphase transit times. In parallel, cells were microinjected with known quantities of a general-purpose protein kinase substrate peptide, VRKRTLRRL, at predefined time points during prometaphase and metaphase. At a cytosolic concentration of 100 nM to 1 microM, the peptide doubled or tripled the metaphase transit times when injected into the cytosol of mitotic cells within the first 4 min after NEBD, at any point from 7.5 to 9 min after NEBD, at any point from 14 to 16 min after NEBD, at 21 min after NEBD, or at 24 min after NEBD. At the concentration used and during these brief intervals, the peptide appeared to act as a competitive inhibitor to reveal inflection points when protein kinase activation was occurring or when endogenous substrate levels approached levels of the peptide. The timing of these inflection points coincides with the changes in protein kinase activities during prometaphase and metaphase, as indicated by our perfusions of cells with the broad spectrum kinase inhibitors. Collectively, our results suggest that the cascade that culminates in anaphase is complex and involves several successive protein kinase activation steps punctuated by the activation of one or more protein phosphatases in mid-metaphase.

Calcium, calmodulin and cell cycle progression

Proliferation of mammalian cells both in vivo and in vitro is dependent upon physiological concentrations of extracellular Ca2+. Growth factor stimulation of quiescent cells at the G0/G1 border usually results in a rapid mobilization of Ca2+ from both intra- and extracellular pools. However, Ca2+ influx is also required for later phases of cell cycle transition, especially in the late G1 phase for initiation of DNA synthesis. Available evidence indicates that calmodulin plays the major and essential roles in the Ca(2+)-dependent regulation of cell proliferation. Ca2+ and calmodulin act at multiple points in the cell cycle, including the initiation of the S phase and both initiation and completion of the M phase. Ca2+ and calmodulin stimulate the expression of genes involved in the cell cycle progression, leading to activation of cyclin-dependent kinases p33cdk2 and p34cdc2. Ca2+ and calmodulin are also involved in activation of enzymes participating in nucleotide metabolism and DNA replication, as well as nuclear envelope breakdown and cytokinesis. Ca2+/calmodulin-dependent protein kinase II and protein phosphatase calcineurin are both involved in the Ca2+ and calmodulin-mediated signalling of growth regulation. As compared to normal cells, growth of transformed cells is independent of extracellular Ca2+ and much less sensitive to calmodulin antagonists, suggesting the existence of derangements in the Ca2+ and calmodulin-mediated growth regulation mechanisms.

New horizons for cytokinesis

The mechanism of cytokines is an old problem in cell biology that has received fresh attention recently with a large variety of powerful approaches and experimental systems. Significant advances have been made on the structure of the cortical cytoskeleton, the identification of proteins and genes involved, and the regulatory mechanism. Many surprises have surfaced within the past two years, leading us toward a major revision in our understanding of this important process.

Modulation of fibroblast response to maitotoxin along the cell division cycle

Maitotoxin (MTX) induces an increase of [Ca2+]i and of phosphoinositide breakdown in various cell types. The [Ca2+]i increase followed with fluorescent probes on cell suspensions has been described as slow and lasting, in contrast to the "signal" induced by calcium ionophores such as ionomycin. MTX effects have been studied on two fibroblastic cell lines, BHK21 C13 and FR 3T3, synchronized by serum deprivation treatment performed in an isoleucine-free medium for BHK21 C13 cells. In BHK21 C13 cells, flow cytometry analysis showed that two stages, G1/S and G2/M, were particularly susceptible to MTX treatment. Scanning laser cytometry demonstrated that calcium response of FR 3T3 fibroblasts followed with Indo-1 varied during the cell division cycle. The [Ca2+]i increase was almost always vertical, but its delay after MTX addition lasted from zero (S and G2/M transition) to 10-20 min (G1) or more (G2). No [Ca2+]i change could be detected during mitosis. The [Ca2+]i response at the S phase was biphasic. These observations suggest that (1) the lasting response described in the literature represents a global cell population effect, and (2) cells are more sensitive to MTX at specific stages of the cell division cycle, which could correspond to periods when calcium signals have been detected in different cell types.

The role of calcium in cell division

Calcium ions (Ca2+) appear to participate in the regulation of several aspects of cell division. Evidence is accumulating that transients or local gradients in the [Ca2+] contribute to different events including nuclear envelope breakdown and reformation, cleavage furrow formation and growth, and cell plate formation. At present there is little direct evidence that Ca2+ transients trigger the onset of anaphase. However, studies with exogenously applied Ca2+ indicate that spindle fibers and the movement of chromosomes at anaphase are exquisitely sensitive to the ion at physiological levels. Although Ca2+ is involved with many processes there are many gaps in our understanding, particularly pertaining to exactly when and where the ion concentration changes are expressed, which events and macromolecules are targeted, and what the processes are that control Ca2+.

Isolation and characterization of a sea urchin zygote cortex that supports in vitro contraction and reactivation of furrowing

The isolation of the cortex of the sea urchin blastomere by detergent lysis was explored with the aim of analyzing components important in the structure and function of the cortical cytoskeleton, and their relationship to such phenomena as contraction. Buffered EGTA medium supplemented with isotonic glycerol and with magnesium, at a level close to the reported internal cellular concentration, yields stable cytoskeletal cortices that retain their spherical shape. Cortices prepared this way contain actin, myosin, fascin and spectrin, components normally associated with the cortical cytoskeleton in a similar distribution to that in intact zygotes. They retain the organized cortical filamentous structure, including the actin-fascin bundles that form cores of microvilli. ATP and NaCl caused changes in cortical shape, described as either contraction or expansion, respectively. Spectrin, but not myosin, was partially extracted by NaCl, resulting in expansion of the cortex that suggests a role for spectrin in maintenance of cortical structure. ATP (but not ADP nor ATP gamma S), which caused the partial removal of myosin and spectrin, led to the contraction of the cortex, consistent with a role for myosin in cortical tension. In cortices isolated from dividing eggs, the zygotes retained their cleavage furrows and ATP induced continuation of furrow progression. This preparation appears to be a useful in vitro model for cytokinesis.

Intracellular free calcium oscillations in normal and cleavage-blocked embryos and artificially activated eggs of Xenopus laevis

We have measured levels of intracellular free calcium ([Ca2+]i) in albino Xenopus laevis embryos using recombinant aequorin and a photon-counting system. We observed sinusoidal oscillations in [Ca2+]i that had the same frequency as cleavage, with cleavage occurring when [Ca2+]i was lowest. An increase in calcium was seen to precede first cleavage. The cyclic changes in calcium were superimposed on a secondary pattern that increased, peaked between third and fifth cleavages and then slowly declined to a level similar to that measured before first cleavage. The amplitude of the oscillations was small during the first few cleavages but became larger with each cycle, with the largest oscillations occurring when the secondary pattern peaked (between third and fifth cleavage). As the secondary pattern declined, the amplitude of the oscillations also became smaller. The oscillations are due to release of calcium from intracellular stores, since the signal was the same in calcium-free solution as in normal medium. When cleavage was blocked with the microtubule-disrupting drugs colchicine or nocodazole, the [Ca2+]i oscillations persisted. Calcium oscillations of a similar magnitude and frequency were also present in artificially activated eggs. The secondary pattern was different in cleavage-blocked embryos and artificially activated eggs, the baseline increasing until about the third cycle and then remaining elevated for the rest of the recording (&gt; 8 hours). By fixing embryos at various points in the calcium cycle, we determined that mitosis began shortly after calcium levels reached their peak and was complete before the calcium level dropped to its lowest point.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ triggers premature inactivation of the cdc2 protein kinase in permeabilized sea urchin embryos

Exit from mitosis requires inactivation of the cyclin B-p34cdc2 protein kinase complex. Since increased cytosolic Ca2+ has been implicated as a potential trigger of mitotic progression, we directly tested the possibility that Ca2+ triggers the pathway responsible for inactivating the cdc2 kinase, using sea urchin embryos permeabilized at various stages of the cell cycle. In cells permeabilized during late interphase and prophase, micromolar Ca2+ induced premature inactivation of the cdc2 kinase without affecting the absolute amount of p34cdc2 protein. Inactivation was selective for the cdc2 kinase, as elevated Ca2+ had no effect on cAMP-dependent protein kinase activity. Premature cdc2 kinase inactivation did not require cyclin B destruction, but did coincide with the dissociation of cyclin B-p34cdc2 complexes. In cells permeabilized during prometaphase and metaphase, cdc2 kinase inactivation was Ca(2+)-independent, presumably because at these later times the inactivating pathway had been enabled prior to permeabilization. This work provides evidence that Ca2+ is the physiological trigger enabling cdc2 kinase inactivation during mitosis.

Differences in the effect of Ca2+ on isolated microtubules from cod and cow brain

Isolated microtubules from cod and cow brains were compared with respect to their response to calcium ions. The effect of Ca2+ on cod microtubules was found to be temperature dependent. In contrast to cow microtubules, cod microtubules assembled at 18 degrees C. At this temperature the assembly was inhibited by Ca2+ concentrations of 2 mM and higher. This was also found for cow microtubules at 37 degrees C. However, at 30 degrees C there was no effect of 2 mM Ca2+ of the amount of assembly or disassembly of cod microtubules consisting of only tubulin or of tubulin and microtubule-associated proteins (MAPs). The morphology was affected though, since some coiled ribbons formed from tubulin and MAPs. The calcium-binding calmodulin did not alter the effect of calcium on cod microtubules markedly. At higher Ca2+ concentrations (> 4 mM), coiled ribbons were formed from cod tubulin and MAPs, but mainly amorphous aggregates and very few coiled ribbons were formed from cod tubulin alone, indicating that the Ca2+ effect is modulated by cod MAPs. The modulatory effect of cod MAPs was however not species specific, since both cod and cow MAPs had the same effect on cod microtubules, in spite of a different protein composition. A MAP-dependent effect of Ca2+ was also found for cow microtubule proteins. The assembly of pure cow tubulin, as well as that of cow tubulin and MAPs, was inhibited by 2 mM Ca2+. In the presence of 10 and 20 mM Ca2+, pure cow tubulin formed amorphous aggregates, rings, and even paracrystals, while the assembly of cow tubulin and MAPs was inhibited. Our results suggest therefore that the effect of Ca2+ can be moderated by MAPs, but depends on intrinsic properties of the different tubulins.

Calcium buffer injections inhibit cytokinesis in Xenopus eggs

A slow cortical wave of high calcium accompanies the elongation of cleavage furrows in medaka fish eggs as well as in Xenopus eggs. We explored the role of such waves by injecting calcium buffers into Xenopus eggs at various times before and during first and second cleavage. Injection earlier than about 15 minutes before first cleavage normally starts delays it for hours. Injection between about 15 minutes and a few minutes before cleavage normally starts allows a (short) furrow to form on time but usually yields an eccentric one. This forms away from the injection side, often as far off-center as the egg's equator, and then regresses. Injection soon after it starts quickly arrests elongation of the furrow and eventually induces its regression; while injection a bit later likewise soon arrests elongation but allows delocalized furrow deepening to continue. The dependence of these inhibitory actions upon the dissociation constants and final cytosolic concentrations of the injected buffers indicates that they act as shuttle buffers to suppress needed zones of high calcium in the micromolar range. We conclude that the high calcium that is found within these furrows is needed to induce them, to extend them and even to maintain them. Moreover, while short, eccentric furrows often form as far off center as the equator, they somehow always form along a meridian through the animal pole. This seems difficult to explain by the orthodox, diastral model. Rather, it suggests that the cleavage furrows in Xenopus--and perhaps in animal cells quite generally--are directly induced by a diastema or telophase disc rather than by the asters.

Heterogeneity of intracellular calcium responses to parathyroid hormone and thrombin in primary osteoblast-like cells and UMR106-01 cells: correlations with culture conditions, intracellular calcium concentration and differentiation state

The present study evaluates the effect of parathyroid hormone (PTH) on intracellular calcium. Intracellular calcium ion concentrations ([Ca2+]i) in fetal rat osteoblasts in primary culture (ROB) and in UMR106-01 osteogenic sarcoma cells were monitored as changes in the ratio (R) of Fura-2 fluorescence intensities in single cells as well as populations of cells. In both single ROB and UMR106-01 cells, addition of 10(-7) M rat PTH1-34 and 3 NIH units/ml human thrombin resulted in heterogeneous responses in R values and therefore [Ca2+]i. PTH-induced calcium responsiveness of ROB was dependent on culture conditions, such that response frequencies were positively correlated with the percentage of fetal calf serum in the culture medium. PTH responsive ROB and UMR106-01 cells had significantly higher resting [Ca2+]i than unresponsive cells. PTH- or thrombin-mediated calcium signalling appeared not to be correlated to alkaline phosphatase activity in single ROB. Low percentages of cells responded to PTH in comparison to thrombin suggesting that an increase in [Ca2+]i is not a common PTH signalling pathway in osteoblasts in primary culture. Our data suggest that activation of this signalling pathway by PTH is culture condition dependent, possibly via a cell-cycle related increase in sensitivity of the pathway.

A mutation in PLC1, a candidate phosphoinositide-specific phospholipase C gene from Saccharomyces cerevisiae, causes aberrant mitotic chromosome segregation

We identified a putative Saccharomyces cerevisiae homolog of a phosphoinositide-specific phospholipase C (PI-PLC) gene, PLC1, which encodes a protein most similar to the delta class of PI-PLC enzymes. The PLC1 gene was isolated during a study of yeast strains that exhibit defects in chromosome segregation. plc1-1 cells showed a 10-fold increase in aberrant chromosome segregation compared with the wild type. Molecular analysis revealed that PLC1 encodes a predicted protein of 101 kDa with approximately 50 and 26% identity to the highly conserved X and Y domains of PI-PLC isozymes from humans, bovines, rats, and Drosophila melanogaster. The putative yeast protein also contains a consensus EF-hand domain that is predicted to bind calcium. Interestingly, the temperature-sensitive and chromosome missegregation phenotypes exhibited by plc1-1 cells were partially suppressed by exogenous calcium.

Calcium buffer injections delay cleavage in Xenopus laevis blastomeres

Microinjection of calcium buffers into the two-cell Xenopus laevis embryo delays cell division in a dose-dependent manner. Four calcium buffers in the BAPTA series with different affinities for calcium were used to distinguish between a localized calcium gradient regulating cleavage and the global calcium concentration regulating this event. DibromoBAPTA (Kd = 1.5 microM) was found to delay cleavage at the lowest intracellular concentration (1.3 mM) of the four buffers tested. The effectiveness of the calcium buffers was dependent upon the buffer dissociation constant but not in a linear fashion. The concentration of buffer required to delay cleavage increased as the buffer's dissociation constant shifted above or below that of the optimum buffer, dibromoBAPTA. This relationship between a calcium buffer's effectiveness at delaying cleavage and its calcium affinity provides support for the hypothesis that a calcium concentration gradient is required for normal cell cycle progression (Speksnijder, J. E., A. L. Miller, M. H. Weisenseel, T.-H. Chen, and L. F. Jaffe. 1989. Proc. Natl. Acad. Sci. USA. 86:6607-6611). DibromoBAPTA was also injected with two different amounts of coinjected calcium to test the possibility that the free calcium concentration of the buffer solution is the important parameter for delaying cleavage. However, we found that changes in buffer concentration have a much stronger effect than changes in the free calcium concentration. This observation supports the hypothesis that BAPTA-type buffers exert their effect by shuttling calcium from regions of high concentration to those of lower concentration, reducing any calcium concentration gradients present in the Xenopus embryo.

Rapid and Reversible High-Affinity Binding of the Dinitroaniline Herbicide Oryzalin to Tubulin from Zea mays L

Oryzalin, a dinitroaniline herbicide, was previously reported to bind to plant tubulin with a moderate strengthe interaction (dissociation constant [Kd] = 8.4 [mu]M) that appeared inconsistent with the nanomolar concentrations of drug that cause the loss of microtubules, inhibit mitosis, and produce herbicidal effects in plants (L.C. Morejohn, T.E. Bureau, J. Mole-Bajer, A.S. Bajer, D.E. Fosket [1987] Planta 172: 252-264). To characterize further the mechanism of action of oryzalin, both kinetic and quasi-equilibrium ligand-binding methods were used to examine the interaction of [14C]-oryzalin with tubulin from cultured cells of maize (Zea mays L. cv Black Mexican Sweet). Oryzalin binds to maize tubulin dimer via a rapid and pH-dependent interaction to form a tubulin-oryzalin complex. Both the tubulin-oryzalin binding strength and stoichiometry are underestimated substantially when measured by kinetic binding methods, because the tubulin-oryzalin complex dissociates rapidly into unliganded tubulin and free oryzalin. Also, an uncharacterized factor(s) that is co-isolated with maize tubulin was found to noncompetitively inhibit oryzalin binding to the dimer. Quasi-equilibrium binding measurements of the tubulin-oryzalin complex using purified maize dimer afforded a Kd of 95 nM (pH 6.9; 23[deg]C) and an estimated maximum molar binding stoichiometry of 0.5. No binding of oryzalin to pure bovine brain tubulin was detected by equilibrium dialysis, and oryzalin has no discernible effect on microtubules in mouse 3T3 fibroblasts, indicating an absence of the oryzalin-binding site on mammalian tubulin. Oryzalin binds to pure taxol-stabilized maize microtubules in a polymer mass- and number-dependent manner, although polymerized tubulin has a much lower oryzalin-binding capacity than unpolymerized tubulin. Much more oryzalin is incorporated into polyment during taxol-induced assembly of pure maize tubulin, and half-maximal inhibition of the rapid phase of taxol-induced polymerization of 5 [mu]M tubulin is obtained with 700 [mu]M oryzalin. The data are consistent with a molecular mechanism whereby oryzalin binds rapidly, reversibly, and with high affinity to the plant tubulin dimer to form a tubulin-oryzalin complex that, at concentrations substoichiometric to tubulin, copolymerizes with unliganded tubulin and slows further assembly. Because half-maximal inhibition of maize callus growth is produced by 37 nM oryzalin, the herbicidal effects of oryzalin appear to result from a substoichiometric poisoning of microtubules.

A mutation in PLC1, a candidate phosphoinositide-specific phospholipase C gene from Saccharomyces cerevisiae, causes aberrant mitotic chromosome segregation

We identified a putative Saccharomyces cerevisiae homolog of a phosphoinositide-specific phospholipase C (PI-PLC) gene, PLC1, which encodes a protein most similar to the delta class of PI-PLC enzymes. The PLC1 gene was isolated during a study of yeast strains that exhibit defects in chromosome segregation. plc1-1 cells showed a 10-fold increase in aberrant chromosome segregation compared with the wild type. Molecular analysis revealed that PLC1 encodes a predicted protein of 101 kDa with approximately 50 and 26% identity to the highly conserved X and Y domains of PI-PLC isozymes from humans, bovines, rats, and Drosophila melanogaster. The putative yeast protein also contains a consensus EF-hand domain that is predicted to bind calcium. Interestingly, the temperature-sensitive and chromosome missegregation phenotypes exhibited by plc1-1 cells were partially suppressed by exogenous calcium.

Topoisomerase II inhibition prevents anaphase chromatid segregation in mammalian cells independently of the generation of DNA strand breaks

Yeast temperature-sensitive mutants of DNA topoisomerase II are incapable of chromosome condensation and anaphase chromatid segregation. In mammalian cells, topoisomerase II inhibitors such as etoposide (VP-16-123) have similar effects. Unfortunately, conclusions drawn from work with mammalian cells have been limited by the fact that the standard inhibitors of topoisomerase II also generate DNA strand breaks, which when produced by other agents (e.g. ionizing radiation) are known to affect progression into and through mitosis. Here we show that the anti-tumour agent ICRF-193, recently identified as a topoisomerase II inhibitor operating by a non-standard mechanism, generates neither covalent complexes between topoisomerase II and DNA, nor adjacent DNA strand breaks, in mitotic HeLa. However, the drug does prevent anaphase segregation in HeLa and PtK2 cells, with effects similar to those of etoposide. We therefore conclude that topoisomerase II function is required for anaphase chromosome segregation in mammalian cells, as it is in yeast.

The S-100 protein causes an increase of intracellular calcium and death of PC12 cells

The S-100 protein-PC12 cell interaction has been studied as a model system of the possible physiological role played by S-100 protein in the nervous system. The data reported demonstrate that S-100 exerts a cytotoxic action which eventually leads to PC12 cell death, regardless of the cell cycle phase. The effect is specific for the S-100 isoforms, which are made up of two identical subunits and is abolished by a monoclonal antibody directed against the same isoforms. Other isoforms and/or calcium-binding proteins, such as troponin or calmodulin, do not induce the same effects. The action of S-100 on cell viability is not detectable in other cell lines of different embryological origin, such as 3T3, L1210, GH3. S-100 causes a rapid and considerable increase (two- to three-fold) of intracellular Ca2+ concentration in PC12 cells accompanied by cytostatic and cytotoxic action. It is postulated that this action also occurs in vivo, as part of the physiological action of this protein.

Intracellular free Ca2+ in the cell cycle in human fibroblasts: transitions between G1 and G0 and progression into S phase

Intracellular free calcium ([Ca2+]i) has been proposed to play an important part in the regulation of the cell cycle. Although a number of studies have shown that stimulation of quiescent cells with growth factors causes an immediate rise in [Ca2+]i (Rabinovitch et al., 1986; Vincentini and Villereal, 1986; Hesketh et al., 1988; Tucker et al., 1989, Wahl et al., 1990), a causal relationship between the [Ca2+]i transient and the ability of the cells to reenter the cell cycle has not been firmly established. We have found that blocking the mitogen-induced elevation of [Ca2+]i with the cytoplasmic [Ca2+]i buffer dimethyl BAPTA (dmBAPTA) also blocks subsequent entry of cells into S phase. The dose response curves for inhibition of serum stimulation of [Ca2+]i and DNA synthesis by dmBAPTA are virtually identical including an anomalous stimulation observed at low levels of dmBAPTA. Reversal of the [Ca2+]i buffering effect of dmBAPTA by transient exposure of the cells to the Ca2+ ionophore ionomycin also reverses the inhibition of DNA synthesis 20-24 h later. Ionomycin by itself does not stimulate DNA synthesis. These data are consistent with the conclusion that a transient increase in [Ca2+]i occurring shortly after serum stimulation of quiescent fibroblasts is necessary but not sufficient for subsequent entry of the cells into S phase. This study is the first to show a direct relationship between early serum stimulated Cai2+ increase and subsequent DNA synthesis in human cells. It also goes beyond recent studies on BALB/3T3 cells by providing dose response data and demonstrating reversibility, which are strong indications of a cause and effect relationship.

Thigmomorphogenesis: the effect of mechanical perturbation on plants

Thigmomorphogenetic responses occur in many environmental settings. The most pronounced effects are found under conditions of extremely high rates of turbulent wind or water flow. However, it is an ubiquitous phenomenon, since mechanical perturbations are to be encountered under all but the most stringent laboratory conditions. Our present understanding of these phenomena is the result of studies at the ecological, anatomical, physiological, biochemical, biophysical and molecular biological levels.

Real-time imaging of neurons retrogradely and anterogradely labelled with calcium-sensitive dyes

Membrane-impermeant calcium indicator dyes were used to retrogradely label dorsal root ganglia, spinal motoneurons and interneurons in the spinal cord of the chick embryo. The dyes were also used to label anterogradely primary afferent axons in the spinal cord and synaptic endings in the ciliary ganglion. Labelled neurons were imaged using digital videomicroscopy. Motoneurons and dorsal root ganglion cells exhibited a frequency-dependent change in fluorescence during antidromic stimulation. Single antidromic stimuli resulted in fluorescence transients that could be resolved in individual cells in real time. In addition, fluorescence changes could be recorded in motoneurons during episodes of bursting generated by rhythmic synaptic inputs from premotor networks. Stimulus-induced fluorescence signals were also detected in axons and synaptic endings labelled anterogradely. Optical signals were largely abolished in the absence of extracellular calcium. The results show that calcium changes can now be measured in identified populations of neurons and presynaptic terminals. The strong dependence of these signals on impulse activity suggests that the technique will be useful for monitoring the activity of identified neuronal populations. The calcium-dependent fluorescence signal probably results from cytosolic dye derived from diffusion which may limit the technique to situations in which the dye can be applied close (< 1 cm) to cell bodies.

Dynamics of organelles in the mitotic spindles of living cells: membrane and microtubule interactions

The distribution and dynamics of the membranous organelles in two cell types were investigated during cell division. Live cells (either PtK2 or LLC-PK1) labeled with the vital dye 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] were observed via serial optical sectioning with the laser-scanning confocal microscope. Z-series of labeled, dividing cells were collected every 1-2 minutes throughout mitosis, beginning at prophase and extending to the spreading of the daughter cells. Membrane distribution began to change from the onset of prophase in both cell types. When the mitotic spindle formed in prometaphase, fine tubular membranes, similar to those extending out to the edges of interphase cells aligned along the kinetochore spindle fibers. The lacy polygonal network typical of interphase cells persisted beneath the spindle, and a membrane network was also associated with the dorsal layer of the cell. As PtK2 cells reached metaphase, their spindles were nearly devoid of membrane staining, whereas the spindles of LLC-PK1 cells contained many tubular and small vesicular membranous structures. X-Z series of the LLC-PK1 metaphase spindle revealed a small cone of membranes that was separated from the rest of the cytoplasm by kinetochore MTs. In both cell types, as chromosome separation proceeded, the interzone remained nearly devoid of membranes until the onset of anaphase B. At this time the elongating interzonal microtubules were closely associated with the polygonal network of endoplasmic reticulum. Cytokinesis caused a compression, and then an exclusion of organelles from the midbody. Immunofluorescence staining with anti-tubulin antibodies suggested that spindle membranes were associated with microtubules throughout mitosis. In addition, taxol induced a dense and extensive collection of small vesicles to collect at the spindle poles of both cell types. Nocodazole treatment induced a distinct loss of organization of the membranous components of the spindles. Together these results suggest that microtubules organize the membrane distribution in mitotic cells, and that this organization may vary in different cell types depending on the quantity of microtubules within the spindle.