Intracellular free calcium and mitosis in mammalian cells: anaphase onset is calcium modulated, but is not triggered by a brief transient

Plasma albumin induces calcium waves in rat cortical astrocytes

Changes in intracellular calcium were monitored in cultured cortical astrocytes stimulated with albumin. Albumin elicited intracellular calcium mobilisation from intracellular stores, inducing repetitive intracellular calcium oscillations. The oscillations were not blocked by ryanodine, a blocker of the Ca-induced Ca release mechanism, and the release occurred from the same store as is accessed by glutamate and bradykinin, both of which release calcium by an IP3-dependent mechanism. Calcium signals induced by albumin appear therefore to occur via a pure IP3-dependent mechanism. When albumin was applied to confluent monolayers of astrocytes, the oscillations in individual cells were initially unsynchronised, but after several minutes of application, the Ca2 oscillations were observed to synchronise and spread through the astrocyte network as a wave. These intercellular calcium waves were inhibited by the gap junction blocker halothane. Using the fluorescence recovery after photobleaching (FRAP) technique, we demonstrate that the development of propagated waves with prolonged exposure to albumin does not result from an increase in cell coupling. The development of calcium waves on exposure to albumin may be important in the formation of glial scars in the CNS after breakdown of the blood-brain barrier.

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.

Ethanol inhibits muscarinic receptor-stimulated phosphoinositide metabolism and calcium mobilization in rat primary cortical cultures

In recent years, it has been hypothesized that muscarinic receptor-stimulated phosphoinositide (PI) metabolism may represent a relevant target for the developmental neurotoxicity of ethanol. Age-, brain region-, and receptor-specific inhibitory effects of ethanol on this system have been found, both in vitro and after in vivo administration. As a direct consequence of this action, alterations of calcium homeostasis would be expected, through alterations of inositol trisphosphate formation, which mediates intracellular calcium mobilization. In the present study, the effects of ethanol (50-500 mM) on carbachol-stimulated PI metabolism and free intracellular calcium levels were investigated in rat primary cortical cultures, by measuring release of inositol phosphates and utilizing the two calcium probes fluo-3 and indo-1 on an ACAS (Adherent Cell Analysis and Sorting) Laser Cytometer. Ethanol exerted a concentration-dependent inhibition of carbachol-stimulated PI metabolism. In addition, ethanol's inhibitory effect paralleled the temporal development of the muscarinic receptor signal transduction system, with the strongest inhibition (25-50%) occurring when maximal stimulation by carbachol occurs (days 5-7). Ethanol also exerted a concentration-dependent decrease in free intracellular calcium levels following carbachol stimulation. Both initial calcium spike amplitude, seen in all responsive cells, as well as the total number of cells responding to carbachol, were decreased by ethanol. The inhibitory effects of ethanol seemed dependent upon preincubation time, in that a longer preincubation (30 min) with the lowest dose (50 mM), showed almost the same decrease in responding cell number and reduction in spike amplitude in responding cells, as a shorter incubation (10 min) with the highest ethanol dose (500 mM). The specificity of the response to carbachol was demonstrated by blocking the response with 10 microM atropine. Moreover, experiments with carbachol in calcium-free buffer with 1 mM EGTA indicated that the initial calcium spike was due to intracellular calcium mobilization from intracellular stores. Since calcium is believed to play important roles in cell proliferation and differentiation, these results support the hypothesis that this intracellular signal-transduction pathway may be a target for ethanol, contributing to its developmental neurotoxicity.

Chelation of intracellular Ca2+ inhibits murine keratinocyte differentiation in vitro

The role of intracellular Ca2+ in the regulation of Ca(2+)-induced terminal differentiation of mouse keratinocytes was investigated using the intracellular Ca2+ chelator 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). A cell permeable acetoxymethyl (AM) ester derivative BAPTA (BAPTA/AM) was loaded into primary mouse keratinocytes in 0.05 mM Ca2+ medium, and then the cells were induced to differentiate by medium containing 0.12 or 0.5 mM Ca(2+) Intracellular BAPTA loaded by BAPTA/AM (15-30 microM) inhibited the expression of epidermal differentiation-specific proteins keratin 1 (K1), keratin 10 (K10), filaggrin and loricrin as detected by immunoblotting. The differentiation-associated redistribution of E-cadherin on the cell membrane was delayed but not inhibited as determined by immunofluorescence. BAPTA also inhibited the expression of K1, K10 and loricrin mRNA. Furthermore, BAPTA prevented the decrease in DNA synthesis induced by 0.12 and 0.5 mM Ca2+, indicating the drug was inhibiting differentiation but was not toxic to keratinocytes. To evaluate the influence of BAPTA on intracellular Ca2+, the concentration of intracellular free Ca2+ (Cai) in BAPTA-loaded keratinocytes was examined by digital image analysis using the Ca(2+)-sensitive fluorescent probe fura-2, and Ca2+ influx was measured by 45Ca2+ uptake studies. Increase in extracellular Ca2+ (Cao) in the culture medium of keratinocytes caused a sustained increase in both Cai and Ca2+ localized to ionomycin-sensitive intracellular stores in keratinocytes. BAPTA lowered basal Cai concentration and prevented the Cai increase. After 12 hours of BAPTA treatment, the basal level of Cai returned to the control value, but the Ca2+ localized in intracellular stores was substantially decreased. 45Ca2+ uptake was initially (within 30 min) increased in BAPTA-loaded cells. However, the total 45Ca2+ accumulation over 24 hours in BAPTA-loaded cells remained unchanged from control values. These results indicate that keratinocytes can maintain Cai and total cellular Ca2+ content in the presence of increased amount of intracellular Ca2+ buffer (e.g., BAPTA) by depleting intracellular Ca2+ stores over a long period. The inhibition by BAPTA of keratinocyte differentiation marker expression may result from depletion of the Ca(2+)-stores since this is the major change in intracellular Ca2+ detected at the time keratinocytes express the differentiation markers. In contrast, the redistribution of E-cadherin on the cell membrane may be more directly associated with Cai change.

Tissue transglutaminase and apoptosis: sense and antisense transfection studies with human neuroblastoma cells

In this report, we show that the overexpression of tissue transglutaminase (tTG) in the human neuroblastoma cell line SK-N-BE(2) renders these neural crest-derived cells highly susceptible to death by apoptosis. Cells transfected with a full-length tTG cDNA, under the control of a constitutive promoter, show a drastic reduction in proliferative capacity paralleled by a large increase in cell death rate. The dying tTG-transfected cells exhibit both cytoplasmic and nuclear changes characteristic of cells undergoing apoptosis. The tTG-transfected cells express high Bcl-2 protein levels as well as phenotypic neural cell adhesion molecule markers (NCAM and neurofilaments) of cells differentiating along the neuronal pathway. In keeping with these findings, transfection of neuroblastoma cells with an expression vector containing segments of the human tTG cDNA in antisense orientation resulted in a pronounced decrease of both spontaneous and retinoic acid (RA)-induced apoptosis. We also present evidence that (i) the apoptotic program of these neuroectodermal cells is strictly regulated by RA and (ii) cell death by apoptosis in the human neuroblastoma SK-N-BE(2) cells preferentially occurs in the substrate-adherent phenotype. For the first time, we report here a direct effect of tTG in the phenotypic maturation toward apoptosis. These results indicate that the tTG-dependent irreversible cross-linking of intracellular protein represents an important biochemical event in the induction of the structural changes featuring cells dying by apoptosis.

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+.

Tissue transglutaminase and apoptosis: sense and antisense transfection studies with human neuroblastoma cells

In this report, we show that the overexpression of tissue transglutaminase (tTG) in the human neuroblastoma cell line SK-N-BE(2) renders these neural crest-derived cells highly susceptible to death by apoptosis. Cells transfected with a full-length tTG cDNA, under the control of a constitutive promoter, show a drastic reduction in proliferative capacity paralleled by a large increase in cell death rate. The dying tTG-transfected cells exhibit both cytoplasmic and nuclear changes characteristic of cells undergoing apoptosis. The tTG-transfected cells express high Bcl-2 protein levels as well as phenotypic neural cell adhesion molecule markers (NCAM and neurofilaments) of cells differentiating along the neuronal pathway. In keeping with these findings, transfection of neuroblastoma cells with an expression vector containing segments of the human tTG cDNA in antisense orientation resulted in a pronounced decrease of both spontaneous and retinoic acid (RA)-induced apoptosis. We also present evidence that (i) the apoptotic program of these neuroectodermal cells is strictly regulated by RA and (ii) cell death by apoptosis in the human neuroblastoma SK-N-BE(2) cells preferentially occurs in the substrate-adherent phenotype. For the first time, we report here a direct effect of tTG in the phenotypic maturation toward apoptosis. These results indicate that the tTG-dependent irreversible cross-linking of intracellular protein represents an important biochemical event in the induction of the structural changes featuring cells dying by apoptosis.

Microtubule tracks can be detected in mouse oocytes with an antibody directed against a calcium transporter

In metaphase II-arrested mouse oocytes, most microtubules are found in the meiotic spindle, a structure that remains stable for hours despite microtubule instability. Microtubule organizing centres (MTOCs) are present at the poles of the spindle and in the cytoplasm, but the latter nucleate very few microtubules. This particular organization of the microtubule network enabled us to observe the unexpected behaviour of a protein that can associate with microtubules. We compared the distribution of a mitosis-activated calcium transport system with that of the microtubule network, by immunofluorescence, using two monoclonal antibodies, one directed against a component of the calcium transport system (7/13), and the other against the common tyrosinated form of alpha-tubulin (YL1/2). The 7/13 staining was associated with the spindle microtubules and with the kinetochore area. In addition, we observed many asters in the cytoplasm, around the cytoplasmic MTOCs. The majority of these asters were not stained with the antitubulin antibody. Moreover, these 7/13 asters either disappeared after nocodazole treatment or were enlarged after taxol treatment. Using a confocal microscope, we observed single fibres that were stained with both antibodies: the extremity furthest from the MTOC (corresponding to the + end of the microtubule) being detected by the 7/13 antibody only. All these observations suggest that the 7/13 antigen is associated with microtubule tracks that persist a few minutes after microtubule depolymerization. The possible role of these tracks in microtubule regrowth is discussed.

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.

Change in K+ current of HeLa cells with progression of the cell cycle studied by patch-clamp technique

The K+ channel of HeLa S3 cells in metaphase was analyzed by inside-out and whole cell patch-clamp techniques. The channel had the characteristics of strong inward rectification, small conductance (22 pS at -100 mV), and dependence on intracellular Ca2+. We investigated the cell cycle dependency of the channel, using cells synchronized by harvesting them at the mitotic stage. The cell capacitance increased gradually with increases in the cell volume toward the S phase. The inward K+ currents through the channel at fixed membrane potentials were highest in early G1 and then decreased with time to a minimum in the S phase, increasing again in the M phase. The permeabilities at fixed membrane potentials were also highest in early G1, decreased to minima in the S phase, and increased again toward the next mitosis. In contrast, mean amplitude and the open probability of the single channel at a fixed membrane potential (-60 mV) did not change significantly during the cell cycle. Therefore the capacitance increases with progression of the cell cycle, whereas the permeability decreases from early G1 to an apparent minimum in the S phase. These changes may be caused by cell cycle-dependent changes in the number of channels.

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.

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.

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.

Adenosine triphosphate stimulates phosphoinositide metabolism, mobilizes intracellular calcium, and inhibits terminal differentiation of human epidermal keratinocytes

During wound healing, release of ATP from platelets potentially exposes the epidermis to concentrations of ATP known to alter cellular functions mediated via changes in inositol trisphosphate (IP3) and intracellular calcium (Cai) levels. Therefore, we determined whether keratinocytes respond to ATP with a rise in Cai and IP3 and whether such increases are accompanied by a change in their proliferation and differentiation. Changes in Cai were measured in Indo-1-loaded neonatal human foreskin keratinocytes after stimulation with extracellular ATP. Extracellular ATP evoked a transient and acute increase in Cai of keratinocytes both in the presence and in the absence of extracellular calcium. ATP also induced the phosphoinositide turnover of keratinocytes, consistent with its effect in releasing calcium from intracellular sources. ATP did not permeabilize keratinocytes, nor did it promote Ca influx into the cells. The half-maximal effect of ATP was at 10 microM, and saturation was observed at 30-100 microM. UTP, ITP, and ATP gamma S were as effective as ATP in releasing Cai from intracellular stores and competed with ATP for their response, whereas AMP and adenosine were ineffective, suggesting the specificity of P2 purinergic receptors in mediating the ATP response in keratinocytes. Single cell measurements revealed heterogeneity in the calcium response to ATP. This heterogeneity did not appear to be due to differences in the initial Cai response but to subsequent removal of increased Cai by these cells. ATP inhibited terminal differentiation of keratinocytes as measured by [35S]methionine incorporation into cornified envelopes and modestly stimulated incorporation of [3H]thymidine into DNA. Chelation of Cai by bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid reduced basal Cai, blocked the Cai response to ATP, inhibited the basal rate of DNA synthesis, and blocked the ATP-induced increase in DNA synthesis. We conclude that extracellular ATP may be an important physiological regulator of epidermal growth and differentiation acting via IP3 and Cai.

Sister chromatid separation in frog egg extracts requires DNA topoisomerase II activity during anaphase

We have produced metaphase spindles and induced them to enter anaphase in vitro. Sperm nuclei were added to frog egg extracts, allowed to replicate their DNA, and driven into metaphase by the addition of cytoplasm containing active maturation promoting factor (MPF) and cytostatic factor (CSF), an activity that stabilizes MPF. Addition of calcium induces the inactivation of MPF, sister chromatid separation and anaphase chromosome movement. DNA topoisomerase II inhibitors prevent chromosome segregation at anaphase, demonstrating that the chromatids are catenated at metaphase and that decatenation occurs at the start of anaphase. Topoisomerase II activity towards exogenous substrates does not increase at the metaphase to anaphase transition, showing that chromosome separation at anaphase is not triggered by a bulk activation of topoisomerase II.

Intracellular cyclic AMP not calcium, determines the direction of vesicle movement in melanophores: direct measurement by fluorescence ratio imaging

Intracellular movement of vesiculated pigment granules in angelfish melanophores is regulated by a signalling pathway that triggers kinesin and dyneinlike microtubule motor proteins. We have tested the relative importance of intracellular Ca2+ ([Ca2+]i) vs cAMP ([cAMP]i) in the control of such motility by adrenergic agonists, using fluorescence ratio imaging and many ways to artificially stimulate or suppress signals in these pathways. Fura-2 imaging reported a [Ca2+]i elevation accompanying pigment aggregation, but this increase was not essential since movement was not induced with the calcium ionophore, ionomycin, nor was movement blocked when the increases were suppressed by withdrawal of extracellular Ca2+ or loading of intracellular BAPTA. The phosphatase inhibitor, okadaic acid, blocked aggregation and induced dispersion at concentrations that suggested that the protein phosphatase PP-1 or PP-2A was continuously turning phosphate over during intracellular motility. cAMP was monitored dynamically in single living cells by microinjecting cAMP-dependent kinase in which the catalytic and regulatory subunits were labeled with fluorescein and rhodamine respectively (Adams et al., 1991. Nature (Lond.). 349:694- 697). Ratio imaging of F1CRhR showed that the alpha 2-adrenergic receptor-mediated aggregation was accompanied by a dose-dependent decrease in [cAMP]i. The decrease in [cAMP]i was both necessary and sufficient for aggregation, since cAMP analogs or microinjected free catalytic subunit of A kinase-blocked aggregation or caused dispersal, whereas the cAMP antagonist RpcAMPs or the microinjection of the specific kinase inhibitor PKI5-24 amide induced aggregation. Our conclusion that cAMP, not calcium, controls bidirectional microtubule dependent motility in melanophores might be relevant to other instances of non-muscle cell motility.

Meiosis, egg activation, and nuclear envelope breakdown are differentially reliant on Ca2+, whereas germinal vesicle breakdown is Ca2+ independent in the mouse oocyte

During early development, intracellular Ca2+ mobilization is not only essential for fertilization, but has also been implicated during other meiotic and mitotic events, such as germinal vesicle breakdown (GVBD) and nuclear envelope breakdown (NEBD). In this study, the roles of intracellular and extracellular Ca2+ were examined during meiotic maturation and reinitiation at parthenogenetic activation and during first mitosis in a single species using the same methodologies. Cumulus-free metaphase II mouse oocytes immediately resumed anaphase upon the induction of a large, transient Ca2+ elevation. This resumption of meiosis and associated events, such as cortical granule discharge, were not sensitive to extracellular Ca2+ removal, but were blocked by intracellular Ca2+ chelators. In contrast, meiosis I was dependent on external Ca2+; in its absence, the formation and function of the first meiotic spindle was delayed, the first polar body did not form and an interphase-like state was induced. GVBD was not dependent on external Ca2+ and showed no associated Ca2+ changes. NEBD at first mitosis in fertilized eggs, on the other hand, was frequently, but not always associated with a brief Ca2+ transient and was dependent on Ca2+ mobilization. We conclude that GVBD is Ca2+ independent, but that the dependence of NEBD on Ca2+ suggests regulation by more than one pathway. As cells develop from Ca(2+)-independent germinal vesicle oocytes to internal Ca(2+)-dependent pronuclear eggs, internal Ca2+ pools increase by approximately fourfold.

Intracellular Ca2+ response of rabbit oocytes to electrical stimulation

Electrical stimulation is known to cause activation in mammalian oocytes, possibly by eliciting an elevation in intracellular calcium (Ca2+). This study reports intracellular Ca2+ concentrations in mature rabbit oocytes using the Ca2+ indicator fura-2. Calcium levels were determined prior to, during, and after the administration of an electrical pulse (3.6 kV/cm for 60 microseconds). Baseline Ca2+ levels ranged from 30 to 90 nM. The intracellular Ca2+ transient evoked by a pulse, peaked at 11 sec, was highly variable in amplitude (40-300 nM) and returned to prepulse levels within 300 sec. Electrically stimulated oocytes did not exhibit repetitive Ca2+ transients. The size of the cytoplasmic Ca2+ rise was influenced by the duration of the pulse, the field strength and the concentrations of external Ca2+ rise was influenced by the duration of the pulse, the field strength and the concentrations of external Ca2+ (P less than 0.05). Oocytes electrically stimulated in the presence of 100 microM CaCl2, which evoked Ca2+ transients with a mean magnitude of 120 nM, activated at a higher rate (P less than 0.05) than oocytes stimulated in the presence of either higher or lower levels of external Ca2+. Although oocytes electrically shocked at 16-18 hr after administration of human chorionic gonadotropin (hphCG) activated at a lower rate than oocytes stimulated at 22-24 hphCG (P less than 0.05), their intracellular Ca2+ response to the pulse was similar (P less than 0.05). These results indicate that electrical pulse parameters and extracellular Ca2+ concentrations can be used to modulate intracellular Ca2+ levels and optimize oocyte activation rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Association of cytoplasmic free Ca2+ gradients with subcellular organelles

Previous investigations have identified gradients of intracellular free (Ca2+)i (Ca2+i) in the cytoplasm of human fibroblasts. In this study we have compared the spatial distribution of these gradients with the subcellular distribution of cytoplasmic organelles. Using the Ca(2+)-sensitive dye fura-2 and organelle-specific fluorescent dyes, we have found that the highest Ca2+ concentrations are found in the perinuclear cytoplasm and that these regions co-localize with the Golgi apparatus. The area occupied by the endoplasmic reticulum, which includes the Golgi region plus an adjacent area, is also significantly elevated above the average cellular (Ca2+)i. Most mitochondria are located in regions different from those with the highest (Ca2+)i. A variety of phenomena which could have given rise to artifactual (Ca2+)i gradients have been ruled out, including compartmentalization of fura-2 in subcellular organelles, incomplete hydrolysis of fura-2AM esters, and the presence of pH gradients which might change the Ca2+ binding characteristics of fura-2. The existence of gradients in (Ca2+)i between ER and Golgi containing regions of the cytoplasm supports the hypothesis (Sambrook: Cell 61:197-199, 1990) that the traffic of membrane bound vesicles from ER to Golgi is directed by local variations in (Ca2+)i.

Spontaneous changes in intracellular calcium concentration in type I astrocytes from rat cerebral cortex in primary culture

Measurement of fura-2 fluorescence in type I astrocytes from rat cerebral cortex showed that the intracellular calcium ion concentration undergoes very large spontaneous changes. These spike-like changes ranged from resting levels of calcium of 50-250 nM to as high as 1-2 microM. The spikes were found to be irregular in frequency and amplitude and were frequently synchronous in confluent cultures. The synchronous events appeared as propagating waves that spread over many cells. The spontaneous spikes persisted when the extracellular calcium concentration was reduced to below micromolar levels suggesting that the source for the increases in [Ca2+]i was intracellular. Treatment of the astrocytes with tetrodotoxin did not abolish the spontaneous changes, nor did blockade of voltage-dependent calcium channels with nimodipine and D-600. Ryanodine, a blocker of the sarcoplasmic reticulum calcium-induced calcium release channel, was also without effect. These changes in [Ca2+]i were different in character from both agonist-induced oscillations and depolarization-induced increases in intracellular calcium concentration. Depolarization using 25-100 mM [K+]o resulted in a prompt rise in intracellular calcium concentration, which returned to near resting levels, and this response was sensitive to removal of extracellular calcium and voltage-gated calcium channel antagonists. L-glutamate (0.5-100 microM) caused large increases in [Ca2+]i that were associated with discrete periodic oscillations in some cells. The cellular trigger for the spontaneous spikes is currently not understood. We conclude that spontaneous changes in [Ca2+]i in astrocytes are distinct from agonist-induced and membrane potential depolarization-induced changes.

Feedback control of mitosis in budding yeast

We have investigated the feedback control that prevents cells with incompletely assembled spindles from leaving mitosis. We isolated budding yeast mutants sensitive to the anti-microtubule drug benomyl. Mitotic arrest-deficient (mad) mutants are the subclass of benomyl-sensitive mutants in which the completion of mitosis is not delayed in the presence of benomyl and that die as a consequence of their premature exit from mitosis. A number of properties of the mad mutants indicate that they are defective in the feedback control over the exit from mitosis: their killing by benomyl requires passage through mitosis; their benomyl sensitivity can be suppressed by an independent method for delaying the exit from mitosis; they have normal microtubules; and they have increased frequencies of chromosome loss. We cloned MAD2, which encodes a putative calcium-binding protein whose disruption is lethal. We discuss the role of feedback controls in coordinating events in the cell cycle.

Cell cycle-related fluctuations in transcellular ionic currents and plasma membrane Ca2+/Mg2+ ATPase activity during early cleavages of Lymnaea stagnalis embryos

During the first four mitotic division cycles of Lymnaea stagnalis embryos, we have detected cell cycle-dependent changes in the pattern of transcellular ionic currents and membrane-bound Ca²⁺-stimulated ATPase activity. Ionic currents ranging from 0.05 to 2.50 μA/cm² have been measured using the vibrating probe technique. Enzyme activity was detected using Ando's cytochemical method (Ando et al. 1981) which reveals Ca²⁺/Mg²⁺ ATPase localization at the ultrastructural level, and under high-stringency conditions with respect to calcium availability, it reveals Ca²⁺-stimulated ATPase. The ionic currents and Ca²⁺-stimulated ATPase localization have in common that important changes occur during the M-phase of the cell cycles. Minimal outward current at the vegetal pole coincides with metaphase/anaphase. Maximal inward current at the animal pole coincides with the onset of cytokinesis at that pole. Ca²⁺-stimulated ATPase is absent from one half of the embryo at metaphase/anaphase of the two- and four-cell stage, whereas it is present in all cells during the remaining part of the cell cycle. Since fluctuations of cytosolic free calcium concentrations appear to correlate with both karyokinesis and cytokinesis, we speculate that part of the cyclic pattern of Ca²⁺-stimulated ATPase localization and of the transcellular ionic currents reflects the elevation of cytosolic free calcium concentration during the M-phase.

Spectrofluorimetric and image recordings of spontaneous and lectin-induced cytosolic calcium oscillations in Jurkat T cells

Intracellular variations in Ca2+ concentrations have been measured in single Jurkat T lymphocyte variants (77 6.8 and E6.1) using Fura-2 as a probe. Under basal conditions, the cytosolic Ca2+ level is stable but some cells show spontaneous Ca2+ oscillations (frequency, 0.30 +/- 0.06 Hz). These oscillations are sensitive to the external concentration of Ca2+ since they can no longer be observed when the bathing solution is replaced (superfusion) with a Ca(2+)-free medium or when a Ca2+ chelator (EGTA) is added. Various changes in the cytosolic concentration of Ca2+ ([Ca2+]i) can be observed when the cells are exposed to the mitogenic lectin phytohemagglutinin (PHA, 80 nM). For instance, in the case of non-oscillating cells, the lectin induces either a rapid increase in [Ca2+]i that is followed by a sustained response (plateau) or it triggers Ca2+ spikes. In the case of experiments done in Ca(2+)-free medium, only the initial spike was observed. In the case of spontaneously oscillating cells, PHA induces a rapid increase in [Ca2+]i that is followed by a plateau where oscillations are absent. In every case, the PHA-dependent Ca2+ response is abrogated in a Ca(2+)-free medium. Computer simulations based on the model of Goldbeter et al. [27] show that the various Ca2+ responses of Jurkat cells are related to the cytosolic level of free Ca2+. Video imaging analyses show that the cellular Ca2+ responses are not homogeneous whether the observations are made in spontaneously oscillating Jurkat cells or when they are exposed to PHA.

Intracellular free calcium oscillates during cell division of Xenopus embryos

In Xenopus embryos, previous results failed to detect changes in the activity of free calcium ions (Ca2+i) during cell division using Ca2(+)-selective microelectrodes, while experiments with aequorin yielded uncertain results complicated by the variation during cell division of the aequorin concentration to cell volume ratio. We now report, using Ca2(+)-selective microelectrodes, that cell division in Xenopus embryos is accompanied by periodic oscillations of the Ca2+i level, which occur with a periodicity of 30 min, equal to that of the cell cycle. These Ca2+i oscillations were detected in 24 out of 35 experiments, and had a mean amplitude of 70 nM, around a basal Ca2+i level of 0.40 microM. Ca2+i oscillations did not take place in the absence of cell division, either in artificially activated eggs or in cleavage-blocked embryos. Therefore, Ca2+i oscillations do not represent, unlike intracellular pH oscillations (Grandin, N., and M. Charbonneau. J. Cell Biol. 111:523-532. 1990), a component of the basic cell cycle ("cytoplasmic clock" or "master oscillator"), but appear to be more likely related to some events of mitosis.

A model for intracellular calcium signaling and the coordinate regulation of hormone biosynthesis, receptors and secretion

A two-state model for the stimulus-induced nongraded response of a single cell is formulated. Individual metestrus gonadotropes stimulated with LHRH operate as a simple switch: either on or off. At a given concentration of stimulus some gonadotropes switch on, while others do not switch on, secretion. The probability of a gonadotrope being in the secretory state is enhanced with each increment of LHRH concentration. Individual gonadotropes in a secretory state are envisioned to decrease their number of LHRH receptors and to switch off LH biosynthesis. On the other hand, individual gonadotropes that are not in a secretory state are thought to increase their number of LHRH receptors and to switch on LH biosynthesis. The group of individuals in the population that have thresholds falling in the range of a given stimulus initiate secretion. And, the group of individuals in the population that have thresholds that fall above the range of a given stimulus do not initiate secretion. More remarkable is evidence that the cells that are protected from hormone secretion nevertheless respond with a set of intracellular signals and this provides a new perspective of how they switch on hormone biosynthesis and up-regulate the LHRH receptors. These changes are envisioned to reduce the threshold of an individual cell and accordingly to enhance the probability that the cell responds in the secretory state with the next stimulus. This scheme would appear to lead to automatic cycles of secretion and biosynthesis since an individual cell can occupy only one of two states at any time and occupancy of either state promotes change to the other. This may provide a solution to the problem of how an endocrine gland might reconcile differences in the time-course of hormone secretion which occurs rapidly and hormone biosynthesis that requires a longer period of time. Parenthetically, the model may also be adapted to the case where the vast majority of individuals in the population are generally subthreshold in relation to the physiological stimulus: such an adaption leads to interesting ways of viewing the mammalian reproductive cycle and the regulation of the preovulatory LH surge. A two-state model of the internal Ca2+ store is outlined here to stimulate thought on how the intracellular signals of each binary state may switch a variety of cellular responses either on or off. The model provides a new perspective on the coordinate regulation of hormone biosynthesis, receptors, and secretion that may be useful in the final reconciliation of population studies with insights about individual cells.

Voltage-dependent calcium channels regulate GH4 pituitary cell proliferation at two stages of the cell cycle

Calcium is an intracellular signal implicated in the regulation of cell proliferation. We have examined the growth regulatory role of voltage-dependent calcium channels (VDCC) in a rat pituitary cell line (GH4C1) that expresses two well-characterized VDCC subtypes (L and T) and is growth-inhibited by several agents known to enhance calcium entry. Thyrotropin-releasing hormone (TRH), tetradecanoylphorbol acetate (TPA), and epidermal growth factor (EGF), each known to enhance calcium entry in GH4 cells, decrease GH4 cell number and incorporation of [3H]-thymidine. The growth inhibitory action of these agents is cytostatic with a predominant effect to block G1 cells from entering S-phase. We next examined the growth regulatory action of pharmacologic agents that interact directly and specifically with type L VDCC. Activation of type L VDCC with the dihydropyridine BAY K8644 inhibits GH4 proliferation as measured by cell number and [3H]-thymidine incorporation. This action of BAY K8644 is enhanced by a submaximal K(+)-maintained depolarization, and the growth inhibitory action of these agents is also cytostatic as evident by the block of G1 cells from entering S-phase. Nimodipine, an antagonist specific for type L VDCC blocks (IC50 = 30 nM) BAY K8644-inhibited cell proliferation by substantially reducing the S-phase block. Taken together these findings indicate that calcium entry through type L VDCC inhibits GH4 cell proliferation by blocking entry into S-phase. By contrast, nimodipine caused only a small reversal of the TRH-induced S-phase block, suggesting that TRH inhibits proliferation by a mechanism that differs at least in part from L-channel activation. Unexpectedly, nimodipine, given alone, caused a substantial inhibition of GH4 cell proliferation. This action of nimodipine was cytostatic, yet differed from calcium channel activators in that the percentage of S-phase cells was unchanged whereas G2-M-phase cells increased with a parallel decrease in G1-phase cells. Similar effects were also observed with other classes of calcium channel blockers. Taken together these results indicate that calcium entry through VDCC regulates GH4 cell proliferation differently depending on the stage of the cell cycle. In G1-phase cells, sustained entry of calcium through type L VDCC blocks entry into S-phase. In G2-M-phase cells entry of calcium promotes progression through mitosis.

Effect of calcium and calcium antagonists on 45Ca influx and cellular growth of human prostatic tumor cells

Calcium and calmodulin play significant roles in DNA synthesis and cell proliferation. In this work the effects of verapamil, trifluoperazine, and tamoxifen on 45Ca uptake and cell growth in human prostatic tumor cells (DU 145) and human fibroblast cells (1 BR) were studied. Although the maximum proliferation was achieved at a concentration of around 2 mM CaCl2 in both DU 145 and 1 BR, growth of DU 145 cells was considerably greater than 1 BR at all calcium concentrations (0.1-4 mM). Calcium uptake experiments, using 45Ca, revealed that the unstimulated 45Ca uptake in 1 BR fibroblasts was 4-5 times higher than in DU 145 cancer cells. Depolarization with high extracellular K caused a 2-3-fold increase in 45Ca influx in 1 BR but only 25-55% increase in DU 145 cells. Verapamil caused a significant inhibition of cell growth with an IC50 value of 55 microM. Verapamil paradoxically increased 45Ca uptake in both unstimulated and K-stimulated DU 145 cells. Whereas unstimulated 45Ca uptake could be blocked by very low concentrations of lathanum (10 microM), much higher concentrations (1-10 mM) were required to completely block uptake in K-depolarized cells. Both trifluoperazine and tamoxifen also inhibited cell proliferation with an IC50 concentration of approximately 5 microM. These drugs, had, however, no effect on 45Ca uptake either in unstimulated or depolarized cells. The results suggest that voltage-gated calcium channels exist in both DU 145 cancer cells and fibroblasts. However, verapamil, in contrast to 1 BR, failed to block these channels in DU 145 cells. The mechanism of antiproliferative action of verapamil may be related to the observed, although paradoxical, increase in cellular calcium. The effect of trifluoperazine and tamoxifen does not involve changes in transmembrane calcium movements but could be mediated by their inhibition of calmodulin-mediated reactions within the cell.

Localization of the centrin-related 165,000-Mr protein of PtK2 cells during the cell cycle

In this study, we follow changes in localization of the centrin-related 165,000-Mr protein of PtK2 cells during the cell cycle. This protein is a component of a pericentriolar lattice that consists of pericentriolar satellites, pericentriolar matrix, and basal feet (Baron A.T., and J.L. Salisbury, J. Cell Biol. 107:2669-2678, 1988). By immunofluorescence microscopy, the 165,000-Mr protein is seen as a constellation of pericentrosomal spots. We observe that cells in late G1 and S are characterized by a dense centrosomal focus of spots with additional spots dispersed throughout the cytoplasm. In G2, one bright centrosomal focus of clustered spots is observed. As the cells proceed through prophase this single focus divides, forming two foci that move toward opposite sides of the nucleus. During prometaphase, each polar focus of spots disperses. At metaphase, the spots are distributed throughout each half-cytoplast from the poles to the chromosomes. During anaphase chromosome movement, some spots are seen beside and behind the trailing chromosome arms while others are clustered at the poles. At telophase, pericentrosomal spots radiate from the poles to surround each mass of chromatin. In early G1, pericentrosomal spots surround each newly formed nucleus. We conclude that the 165,000-Mr protein is a dynamic component of both the centrosome (pericentriolar matrix) and the mitotic apparatus (spindle matrix).

Properties of a potassium-selective ion channel in human melanoma cells

Currents through ion channels were measured from cells of a human melanin-producing melanoma cell line (IRG 1) with the patch clamp technique. In these cells the most frequently observed channel is a potassium channel. The channel activates slowly at depolarizing voltage steps but does not inactivate. Single channel potassium currents can be measured in cell-attached patches at the resting potential of melanoma cells. The channel has a conductance of approximately 10 pS. As measured from the reversal potentials of single channel currents, the permeability ratio for sodium and potassium, PNa/PK, is between 0.03 and 0.04. Open probability is increased at positive potentials. Mean open times are prolonged at voltage steps to more positive potentials. Closed time histograms are fitted by two exponentials. The slow shut time is decreased at positive potentials. In whole cell measurements, cell conductance measured between -20 and + 70 mV was reduced by 10 mM tetraethylammonium chloride from 6.4 +/- 1.2 nS (n = 4) to 0.8 +/- 0.3 nS (n = 3). Application of isoproterenol decreases the probability of the channel being open without any change in the single channel conductance. A possible role of the 10 pS potassium channel in the growth of melanoma cells is discussed.

Active involvement of Ca2+ in mitotic progression of Swiss 3T3 fibroblasts

Global Ca2+ transients have been observed to precede nuclear envelope breakdown and the onset of anaphase in Swiss 3T3 fibroblasts in 8% (vol/vol) FBS. The occurrence of these Ca2+ transients was dependent on intracellular stores. These Ca2+ transients could be (a) abolished by serum removal without halting mitosis, and (b) eliminated by increasing intracellular Ca2+ buffering capacity through loading the cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) buffer, via the tetra(acetoxymethyl) ester, without hindering the transition into anaphase. Microinjection of sufficient concentrations of BAPTA buffer could block nuclear envelope breakdown. Pulses of Ca2+ generated by flash photolysis of intracellularly trapped nitr-5, a "caged" Ca2+, could precipitate precocious nuclear envelope breakdown in prophase cells. In metaphase cells, photochemically generated Ca2+ pulses could cause changes in the appearance of the chromosomes, but the length of time required for cells to make the transition from metaphase to anaphase remained essentially unchanged regardless of whether a Ca2+ pulse was photoreleased during metaphase. The results from these photorelease experiments were not dependent on the presence of serum in the medium. Discharging intracellular Ca2+ stores with ionomycin in the presence of 1.8 mM extracellular Ca2+ doubled the time for cells to pass from late metaphase into anaphase, whereas severe Ca2+ deprivation by treatment with ionomycin in EGTA-containing medium halted mitosis. Our results collectively indicate that Ca2+ is actively involved in nuclear envelope breakdown, but Ca2+ signals are likely unnecessary for the metaphase-anaphase transition in Swiss 3T3 fibroblasts. Additional studies of intracellular Ca2+ concentrations in mitotic REF52 and PtK1 cells revealed that Ca2+ transients are not observed at all mitotic stages in all cells. The absence of observable global Ca2+ transients, where calcium buffers can block and pulses of Ca2+ can advance mitotic stages, may imply that the relevant Ca2+ movements are too local to be detected.

Histamine receptors in human fibroblasts: inositol phosphates, Ca2+, and cell growth

Histamine stimulated inositol phosphate formation by human skin fibroblasts. The effect of histamine was reduced but still readily apparent in the absence of extracellular Ca2+. Histamine caused a transient increase in intracellular free Ca2+ as detected by indo-1 and fura-2 fluorescence studies on cell populations and on individual cells. Similar increases were observed in the absence of extracellular Ca2+, indicating that the effect was primarily due to mobilization of Ca2+ from intracellular stores, presumably by inositol trisphosphate (IP3). The effects of histamine on phosphoinositide metabolism and intracellular Ca2+ were inhibited by pretreatment of the cells with phorbol esters, suggesting that the histamine receptor in fibroblasts is subject to feedback regulation by protein kinase C. Histamine inhibited the incorporation of [3H]-thymidine into DNA. The effects of histamine on inositol phosphate formation, intracellular Ca2+, and thymidine incorporation were blocked by the H1 receptor antagonist mepyramine. Our results indicate that human skin fibroblasts have H1 receptors coupled to the formation of inositol phosphates and mobilization of intracellular Ca2+. We suggest that this H1 receptor also mediates a block of the cell cycle and that histamine may play a physiological role in the regulation of fibroblast proliferation.

Protein kinase C acts downstream of calcium at entry into the first mitotic interphase of Xenopus laevis

Transit into interphase of the first mitotic cell cycle in amphibian eggs is a process referred to as activation and is accompanied by an increase in intracellular free calcium [( Ca2+]i), which may be transduced into cytoplasmic events characteristic of interphase by protein kinase C (PKC). To investigate the respective roles of [Ca2+]i and PKC in Xenopus laevis egg activation, the calcium signal was blocked by microinjection of the calcium chelator BAPTA, or the activity of PKC was blocked by PKC inhibitors sphingosine or H7. Eggs were then challenged for activation by treatment with either calcium ionophore A23187 or the PKC activator PMA. BAPTA prevented cortical contraction, cortical granule exocytosis, and cleavage furrow formation in eggs challenged with A23187 but not with PMA. In contrast, sphingosine and H7 inhibited cortical granule exocytosis, cortical contraction, and cleavage furrow formation in eggs challenged with either A23187 or PMA. Measurement of egg [Ca2+]i with calcium-sensitive electrodes demonstrated that PMA treatment does not increase egg [Ca2+]i in BAPTA-injected eggs. Further, PMA does not increase [Ca2+]i in eggs that have not been injected with BAPTA. These results show that PKC acts downstream of the [Ca2+]i increase to induce cytoplasmic events of the first Xenopus mitotic cell cycle.

Inositol phosphates and cell signalling

Inositol 1,4,5-trisphosphate is a second messenger which regulates intracellular calcium both by mobilizing calcium from internal stores and, perhaps indirectly, by stimulating calcium entry. In these actions it may function with its phosphorylated metabolite, inositol 1,3,4,5-tetrakisphosphate. The subtlety of calcium regulation by inositol phosphates is emphasized by recent studies that have revealed oscillations in calcium concentration which are perhaps part of a frequency-encoded second-messenger system.