The role of the phosphatidylinositol cycle in mitosis in sea urchin zygotes. Lithium inhibition is overcome by myo-inositol but not by other cyclitols or sugars

Phosphoinositides: Lipids with informative heads and mastermind functions in cell division

Phosphoinositides are low abundant but essential phospholipids in eukaryotic cells and refer to phosphatidylinositol and its seven polyphospho-derivatives. In this review, we summarize our current knowledge on phosphoinositides in multiple aspects of cell division in animal cells, including mitotic cell rounding, longitudinal cell elongation, cytokinesis furrow ingression, intercellular bridge abscission and post-cytokinesis events. PtdIns(4,5)P₂production plays critical roles in spindle orientation, mitotic cell shape and bridge stability after furrow ingression by recruiting force generator complexes and numerous cytoskeleton binding proteins. Later, PtdIns(4,5)P₂hydrolysis and PtdIns3P production are essential for normal cytokinesis abscission. Finally, emerging functions of PtdIns3P and likely PtdIns(4,5)P₂have recently been reported for midbody remnant clearance after abscission. We describe how the multiple functions of phosphoinositides in cell division reflect their distinct roles in local recruitment of protein complexes, membrane traffic and cytoskeleton remodeling. This article is part of a Special Issue entitled Phosphoinositides.

Phosphoinositides and cytokinesis: the "PIP" of the iceberg

Phosphoinositides [Phosphatidylinositol (PtdIns), phosphatidylinositol 3-monophosphate (PtdIns3P), phosphatidylinositol 4-monophosphate (PtdIns4P), phosphatidylinositol 5-monophosphate (PtdIns5P), phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2) ), phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2) ), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2) ), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3) )] are lowly abundant acidic lipids found at the cytosolic leaflet of the plasma membrane and intracellular membranes. Initially discovered as precursors of second messengers in signal transduction, phosphoinositides are now known to directly or indirectly control key cellular functions, such as cell polarity, cell migration, cell survival, cytoskeletal dynamics, and vesicular traffic. Phosphoinositides actually play a central role at the interface between membranes and cytoskeletons and contribute to the identity of the cellular compartments by recruiting specific proteins. Increasing evidence indicates that several phosphoinositides, particularly PtdIns(4,5)P(2) , are essential for cytokinesis, notably after furrow ingression. The present knowledge about the specific phosphoinositides and phosphoinositide modifying-enzymes involved in cytokinesis will be first presented. The review of the current data will then show that furrow stability and cytokinesis abscission require that both phosphoinositide production and hydrolysis are regulated in space and time. Finally, I will further discuss recent mechanistic insights on how phosphoinositides regulate membrane trafficking and cytoskeletal remodeling for successful furrow ingression and intercellular bridge abscission. This will highlight unanticipated connections between cytokinesis and enzymes implicated in human diseases, such as the Lowe syndrome.

Phosphoinositide function in cytokinesis

In systems as diverse as yeast, slime mold and animal cells, the levels and distribution of phosphatidylinositol phosphates (PIPs) must be strictly regulated for successful cell cleavage. The precise mechanism by which PIPs function in this process remains unknown. Recent experiments are beginning to shed light on the cellular pathways in which PIPs make key contributions during cytokinesis. In particular, PIPs promote proper actin cytoskeletal organization and direct membrane trafficking in dividing cells. Future research will uncover temporal and spatial regulation of the different PIPs, thus elucidating their role in cytoskeletal and membrane events that drive cell cleavage.

Phosphoinositide signaling plays a key role in cytokinesis

To perform the vital functions of motility and division, cells must undergo dramatic shifts in cell polarity. Recent evidence suggests that polarized distributions of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate, which are clearly important for regulating cell morphology during migration, also play an important role during the final event in cell division, which is cytokinesis. Thus, there is a critical interplay between the membrane phosphoinositides and the cytoskeletal cortex that regulates the complex series of cell shape changes that accompany these two processes.

Continuous phosphatidylinositol metabolism is required for cleavage of crane fly spermatocytes

Successful cleavage of animal cells requires co-ordinated regulation of the actomyosin contractile ring and cleavage furrow ingression. Data from a variety of systems implicate phosphoinositol lipids and calcium release as potential regulators of this fundamental process. Here we examine the requirement for various steps of the phosphatidylinositol (PtdIns) cycle in dividing crane fly (Nephrotoma suturalis) spermatocytes. PtdIns cycle inhibitors were added to living cells after cleavage furrows formed and began to ingress. Inhibitors known to block PtdIns recycling (lithium), PtdIns phosphorylation (wortmannin, LY294002) or phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] hydrolysis [U73122 (U7)] all stopped or slowed furrowing. The effect of these drugs on cytokinesis was quite rapid (within 0-4 minutes), so continuous metabolism of PtdIns appears to be required for continued cleavage furrow ingression. U7 caused cleavage furrow regression concomitant with depletion of F-actin from the contractile ring, whereas the other inhibitors caused neither regression nor depletion of F-actin. That U7 depletes furrow-associated actin seems counterintuitive, as inhibition of phospholipase C would be expected to increase cellular levels of PtdIns(4,5)P2 and hence increase actin polymerization. Our confocal images suggest, however, that F-actin might accumulate at the poles of U7-treated cells, consistent with the idea that PtdIns(4,5)P2 hydrolysis may be required for actin filaments formed at the poles to participate in contractile ring assembly at the furrow.

Patterning mechanisms in the evolution of derived developmental life histories: the role of Wnt signaling in axis formation of the direct-developing sea urchin Heliocidaris erythrogramma

A number of echinoderm species have replaced indirect development with highly modified direct-developmental modes, and provide models for the study of the evolution of early embryonic development. These divergent early ontogenies may differ significantly in life history, oogenesis, cleavage pattern, cell lineage, and timing of cell fate specification compared with those of indirect-developing species. No direct-developing echinoderm species has been studied at the level of molecular specification of embryonic axes. Here we report the first functional analysis of Wnt pathway components in Heliocidaris erythrogramma, a direct-developing sea urchin. We show by misexpression and dominant negative knockout construct expression that Wnt8 and TCF are functionally conserved in the generation of the primary (animal/vegetal) axis in two independently evolved direct-developing sea urchins. Thus, Wnt pathway signaling is an overall deeply conserved mechanism for axis formation that transcends radical changes to early developmental ontogenies. However, the timing of expression and linkages between Wnt8, TCF, and components of the PMC-specification pathway have changed. These changes correlate with the transition from an indirect- to a direct-developing larval life history.

Lithium blocks cell cycle transitions in the first cell cycles of sea urchin embryos, an effect rescued by myo-inositol

Lithium is a classical inhibitor of the phosphoinositide pathway and is teratogenic. We report the effects of lithium on the first cell cycles of sea urchin (Lytechinus pictus) embryos. Embryos cultured in 400 mM lithium chloride sea water showed marked delay to the cell cycle and a tendency to arrest prior to nuclear envelope breakdown, at metaphase and at cytokinesis. After removal of lithium, the block was reversed and embryos developed to form normal late blastulae. The lithium-induced block was also reversed by myo- but not epi-inositol, indicating that lithium was acting via the phosphoinositide pathway. Lithium microinjection before fertilization caused arrest prior to nuclear envelope breakdown at much lower concentrations (3-5 mM). Co-injection of myo-inositol prevented the block. Microinjection of 1–2 mM lithium led to block at the cleavage stage. This was also reversed by coinjection of myo-inositol. Embryos blocked by lithium microinjection proceeded rapidly into mitosis after photolysis of caged inositol 1,4,5-trisphosphate. These data demonstrate that a patent phosphoinositide signalling pathway is essential for the proper timing of cell cycle transitions and offer a possible explanation for lithium's teratogenic effects.

Local perinuclear calcium signals associated with mitosis-entry in early sea urchin embryos

Using calcium-sensitive dyes together with their dextran conjugates and confocal microscopy, we have looked for evidence of localized calcium signaling in the region of the nucleus before entry into mitosis, using the sea urchin egg first mitotic cell cycle as a model. Global calcium transients that appear to originate from the nuclear area are often observed just before nuclear envelope breakdown (NEB). In the absence of global increases in calcium, confocal microscopy using Calcium Green-1 dextran indicator dye revealed localized calcium transients in the perinuclear region. We have also used a photoinactivatable calcium chelator, nitrophenyl EGTA (NP-EGTA), to test whether the chelator-induced block of mitosis entry can be reversed after inactivation of the chelator. Cells arrested before NEB by injection of NP-EGTA resume the cell cycle after flash photolysis of the chelator. Photolysis of chelator triggers calcium release. TreatmenT with caFfeine to enhance calcium-induced calcium release increases the amplitude of NEB-associated calcium transients. These results indicate that calcium increases local to the nucleus are required to trigger entry into mitosis. Local calcium transients arise in the perinuclear region and can spread from this region into the cytoplasm. Thus, cell cycle calcium signals are generated by the perinuclear mitotic machinery in early sea urchin embryos.

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.

Inositol 1,4,5-trisphosphate mass changes from fertilization through first cleavage in Xenopus laevis

After fertilization in Xenopus laevis, inositol 1,4,5-trisphosphate (IP3) mass increased from 53 to 261 fmol/cell and returned to near basal by 10 min after insemination. IP3 was also elevated over control egg levels during first mitosis and first cleavage. Because IP3 levels and the fertilization calcium wave decline at about the same time and because calcium ionophore or pricking the egg increased IP3, the fertilization calcium wave may be due to calcium-induced IP3 production. In addition, the onset of sperm motility was associated with an increase, whereas the acrosomal reaction was accompanied by a decrease in IP3 mass. Combining our published data with this report, the first chronology of the levels of IP3 from the induction of meiosis (maturation) through fertilization and cleavage in one cellular system is summarized. These data suggest an in vivo dose response for IP3 and calcium release. A small (17 fmol/cell) IP3 change during the induction of meiosis may not be associated with a calcium change. Larger IP3 changes at cleavage (40 fmol/cell) and mitosis (125 fmol/cell) are associated with localized small calcium increases, whereas the largest IP3 change (208 fmol/cell) is associated with the large calcium increase at fertilization.

Bombesin impairs spindle function in mitotic V79 Chinese hamster cells by a receptor-dependent mechanism

Bombesin belongs to a family of peptides acting as local hormones with roles in growth regulation, neural function and secretion. Upon binding to its receptor bombesin primarily elicits an increase of inositolphosphates and diacylglycerol, events leading to increased [Ca2+]i and activation of protein kinase C. When asynchronously growing V79 Chinese hamster cells were treated with bombesin in the 10(-9)-10(-7) M concentration range their content of inositolphosphates increased and so did the frequency of mitotic cells with abnormal chromosomal arrangements (c-mitoses). Both effects were abolished by simultaneous addition of the synthetic peptide antagonist D-Arg1,D-Phe5,D-Trpu7,9-Leu11-substance P that binds to certain bombesin receptors. These results demonstrate that the V79 cells most probably have receptors for bombesin and that the weak but significant c-mitotic effect is mediated by such receptors.

Inhibitory effect of a Microcystis sp (cyanobacteria) toxin on development of preimplantation mouse embryos

1. A soluble toxin, purified from the algae bloom of an eutrophic lake dominated by Microcystis, is a very effective inhibitor of early embryo development in a dose-response relationship. 2. Two- and 8-cell mouse embryos under the influence of Microcystis toxin do not reach the developmental stages of morula and blastocyst, respectively. 3. Actin cortex is disorganized without change in the microtubules structure. 4. Results are discussed in terms of the possible mechanisms by which the toxin arrests development considering, specifically, effects on the cytoskeleton and/or on voltage-insensitive transmembrane Ca2+ channels.

Reducing inositol lipid hydrolysis, Ins(1,4,5)P3 receptor availability, or Ca2+ gradients lengthens the duration of the cell cycle in Xenopus laevis blastomeres

We have microinjected a mAb specifically directed to phosphatidylinositol 4,5-bisphosphate (PIP2) into one blastomere of two-cell stage Xenopus laevis embryos. This antibody binds to endogenous PIP2 and reduces its rate of hydrolysis by phospholipase C. Antibody-injected blastomeres undergo partial or complete arrest of the cell cycle whereas the uninjected sister blastomeres divided normally. Since PIP2 hydrolysis normally produces diacylglycerol (DG) and inositol 1,4,5-triphosphate (Ins[1,4,5]P3), we attempted to measure changes in the levels of DG following stimulation of PIP2 hydrolysis in antibody-injected oocytes. The total amount of DG in antibody-injected oocytes was significantly reduced compared to that of water-injected ones following stimulation by either acetylcholine or progesterone indicating that the antibody does indeed suppress PIP2 hydrolysis. We also found that the PIP2 antibodies greatly reduced the amount of intracellular Ca2+ released in the egg cortex during egg activation. As an indirect test for Ins(1,4,5)P3 involvement in the cell cycle we injected heparin which competes with Ins(1,4,5)P3 for binding to its receptor, and thus inhibits Ins(1,4,5)P3-induced Ca2+ release. Microinjection of heparin into one blastomere of the two-cell stage embryo caused partial or complete arrest of the cell cycle depending upon the concentration of heparin injected. We further investigated the effect of reducing any [Ca2+]i gradients by microinjecting dibromo-BAPTA into the blastomere. Dibromo-BAPTA injection completely blocked mitotic cell division when a final concentration of 1.5 mM was used. These results suggest that PIP2 turnover as well as second messenger activity influence cell cycle duration during embryonic cell division in frogs.

Lithium chloride inhibits the phosphorylation of newly synthesized neurofilament protein, NF-M, in cultured chick sensory neurons

The middle and high molecular weight members of the neurofilament triplet, NF-M and NF-H, undergo extensive posttranslational polyphosphorylation, a process requiring 24 h or more for completion. We have investigated ways of perturbing this process in intact cells and have found that phosphorylation of newly synthesized NF-M in cultured chick sensory neurons is inhibited by Li+. [35S]Methionine pulse-chase experiments were carried out with pure neuronal cultures, and the phosphorylation of newly synthesized NF-M was monitored by following the accompanying change, with chase time, in apparent size and charge of the polypeptide. Addition of LiCl to the medium inhibited this mobility shift in a dose-dependent manner over concentrations between 2 and 25 mM. Incorporation of 32P into NF-M, as well as NF-H, was also inhibited, whereas incorporation into the low molecular weight neurofilament protein, beta-tubulin, and total protein was unaffected. Protein synthesis was not altered. Exposure to 25 mM LiCl for up to 72 h was not toxic, and the inhibition of NF-M phosphorylation was completely reversible. When 25 mM Li+ was added after NF-M had become partially phosphorylated, further progression was blocked, but there was no net dephosphorylation or degradation of NF-M. Additional experiments suggest that this action of Li+ is probably not due to effects on second messenger levels or to effects on tubulin metabolism and assembly state presented in our accompanying article, but rather to interference by Li+ itself, with the phosphorylation of NF-M and NF-H by specific neurofilament kinase(s).

Rapid degradation of newly synthesized tubulin in lithium-treated sensory neurons

When cultured chick sensory neurons were labeled with [35S]methionine for 1 h or longer in the presence of 5-25 mM LiCl, we found a dose-dependent reduction in the level of radiolabeled tubulin, to one third of control levels, with no noticeable effect on other proteins. The magnitude of this response was identical after a 1-h or 72-h preincubation in 25 mM LiCl and returned to control values within 1 h after removal of LiCl. Short (5-min) pulse-chase experiments revealed that tubulin synthesis was not affected by Li+, but that newly synthesized tubulin was rapidly degraded, such that 50% of the labeled beta-tubulin was lost within 5 min. There was no enhanced degradation of tubulin present before exposure to Li+. Addition of LiCl at various times before and after a 10-min pulse suggested that tubulin becomes completely refractory to Li(+)-induced degradation within 10 min after translation. Although Li+ treatment resulted in a decrease in the fraction of extant tubulin present in the unassembled form, the Li(+)-induced degradation of nascent tubulin is not a consequence of shifts in assembly state, because colcemid or taxol treatment did not lead to rapid degradation of newly synthesized tubulin, and neither drug altered the response to Li+. We suggest that Li+ interferes with the correct folding of tubulin polypeptides, exposing sites, normally hidden, to the action of a protease(s).

Identification of cultured cells expressing ligand-gated cationic channels

We have identified cultured cells that express ligand-gated cation channels using a simple method which may also be applied to the screening of chemical agents for their use as agonists or antagonists. This assay is based upon the observation that many ligand-gated cation channels are permeable to lithium and agonists induce the flux of lithium into the cells which contain them. Since the accumulation of intracellular lithium can alter the cell cycle, the measurement of [3H]thymidine ([3H]thy) incorporation should reflect this occurrence. This expectation was realized using the PC12 cell line which expresses neuronal-like nicotinic acetylcholine receptor (nAChR). When cholinergic agonists are applied to PC12 cells in the presence of lithium-containing buffer and cells are subsequently pulsed with [3H]thy, the radiolabel incorporation into these cells relative to controls is reduced. If cholinergic antagonists are included or if the concentration of agonist either rapidly desensitizes receptors or is insufficient to induce channel opening, the reduction in [3H]thy incorporation is not observed. This method also provides a rapid way to screen cultured cell lines for those that express ligand-gated cation channels. This assay offers the potential to be automated for the low cost screening of drugs which act upon ligand-gated ion channels.

Analysis of inducible contractile rings suggests a role for protein kinase C in embryonic cytokinesis and wound healing

A semi-in vitro system derived from Xenopus oocytes which allows induction of contractile ring (CR) formation and closure is described and exploited to elucidate regulatory and structural features of cytokinesis. The inducible CRs (ICRs) are composed of actin filaments and closure is actin filament-dependent as is cytokinesis in vivo. ICR closure in this system is calcium-dependent and pH-sensitive, as is cytokinesis in permeabilized cells (Cande: Journal of Cell Biology 87:326, 1980). Closure of ICRs proceeds at a rate and with a kinetic pattern similar to embryonic cytokinesis. Collectively, these data demonstrate that this system is a faithful mimic of cytokinesis in vivo. ICR formation and closure is protein kinase C (PKC)-dependent and neomycin-sensitive, indicating that the PKC branch of the polyphosphoinositide pathway regulates formation of the actomyosin ring which is the effector of cytokinesis. Kinetic measurements show that the rate of ICR closure reaches a peak of 4-8 microns/sec. Since the maximum measured velocity of actin filament translocation by vertebrate, non-muscle myosins is 0.04 micron/sec, the later observations support a model in which the CR is segmented, containing multiple sites where filaments overlap in a "sliding filament" fashion. Because the rate decreases after reaching a peak, the results also suggest that the number of overlap sites decrease with time.

Reduced inositol polyphosphate accumulation and inositol supply induced by lithium in stimulated cerebral cortex slices

The ability of lithium to interfere with phosphoinositide metabolism in rat cerebral cortex slices has been examined by monitoring the accumulation of CMP-phosphatidate (CMP-PtdOH) and the reduction in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 levels. A small accumulation of [14C]CMP-PtdOH was seen in slices prelabelled with [14C]cytidine and stimulated with carbachol (1 mM) or Li+ (1 mM). However, simultaneous addition of both agents for 30 min produced a 22-fold accumulation, with Li+ producing a half-maximal effect at a concentration of 0.61 +/- 0.19 mM. Kinetic studies revealed that the effects of carbachol and Li+ on CMP-PtdOH accumulation occurred with no initial lag apparent under these conditions and that preincubation with myo-inositol (10 or 30 mM) dramatically attenuated CMP-PtdOH accumulation. myo-Inositol could also attenuate the rate of accumulation of CMP-PtdOH when added 20 min after carbachol and Li+; these effects were not observed when equimolar concentrations of scyllo-inositol were added. Use of specific radioreceptor assays allowed the mass accumulations of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 to be monitored. Following a lag of 5-10 min, Li+ resulted in a marked reduction in the accumulation of both inositol polyphosphates resulting from muscarinic-cholinergic stimulation. Preincubation of cerebral cortex slices with myo- (but not scyllo-) inositol delayed, but did not prevent, the reduction in the accumulation of Ins(1,4,5)P3 or Ins(1,3,4,5)P4. The results suggest that cerebral cortex, at least in vitro, is very sensitive to myo-inositol depletion under conditions of muscarinic receptor stimulation. The relationship of such depletion to the generation of inositol polyphosphate second messengers is discussed.

Activation of protein kinase C triggers premature compaction in the four-cell stage mouse embryo

During mouse preimplantation development, the cells of the mouse embryo undergo a progressive subcellular reorganization at compaction, which eventually results in the formation of two distinct cell types. We have investigated the effect that activators of the Ca2(+)-phospholipid-dependent protein kinase (PKC) have on mouse compaction. Phorbol ester activation of PKC caused premature compaction of four-cell embryos within a few minutes of addition followed by a prolonged decompaction phase after 1 hr. This response was dose-dependent to concentrations as low as 250 pg/ml. Diacylglycerides also caused compaction; however, it was more sustained than with phorbol esters and was not followed by a phase of decompaction. Inhibition of PKC with sphingosine blocks induced compaction in a dose-dependent manner and also blocks normal compaction of eight-cell embryos. A monoclonal antibody to the cell adhesion molecule, E-cadherin, which mediates mouse embryo compaction, completely blocks compaction induced by these activators of PKC. Indirect immunofluorescence with a monoclonal antibody to E-cadherin indicates that PKC activation causes a rapid shift in the localization of this cell adhesion molecule, which coincides with the observed compaction. These results suggest that PKC plays a role in the initiation of compaction through its effect either directly or indirectly on E-cadherin.

1,2-Dioctanoylglycerol accelerates or retards mitotic progression in Tradescantia stamen hair cells as a function of the time of its addition

We have treated living, intact stamen hair cells from the spiderwort plant, Tradescantia virginiana, with 0.5 microgram/ml or 60 micrograms/ml 1,2-dioctanoylglycerol, a potent and permeant activator of protein kinase C, and have observed the rates of progression of mitosis from prophase through anaphase. We have found that in addition to the concentration used, the time of initial treatment with 1,2-dioctanoylglycerol defines the response by the cells. The cells rapidly undergo nuclear envelope breakdown when this diglyceride is added in very late prophase, 0 to approximately 8 min prior to the time of normal nuclear envelope breakdown. Anaphase onset occurs 28 min after nuclear envelope breakdown, rather than after the 33 min interval observed in untreated cells. Rapid progression through metaphase is also observed if cells are treated with 0.5 microgram/ml 1,2-dioctanoylglycerol during prometaphase, up to 15 min after nuclear envelope breakdown. The addition of 0.5 microgram/ml 1,2-dioctanoylglycerol in late metaphase, approximately 26 min after nuclear envelope breakdown, results in sister chromatid separation slightly ahead of its normal time, 33 min after nuclear envelope breakdown, and in precocious cell plate vesicle aggregation, 3-5 min earlier than that observed in untreated cells. Treatment of cells with 60 micrograms/ml of 1,2-dioctanoylglycerol at any point during the interval from 0 to approximately 5 min prior to nuclear envelope breakdown results in precocious entry into anaphase. If cells are treated with either 0.5 microgram/ml or 60 micrograms/ml 1,2-dioctanoylglycerol earlier than 20 min before nuclear envelope breakdown, they do not enter mitosis, but instead revert to interphase without dividing. When 1,2-dioctanoylglycerol is added at other times during mitosis, the rate of subsequent mitotic progression is dramatically slowed; the cells require greater than 55 min to progress from nuclear envelope breakdown to anaphase onset, though once in anaphase, the cells progress onward to cytokinesis at normal rates. Treatments o of cells with 1,3-dioctanoylglycerol at any point during prophase, prometaphase, or metaphase are without effect on the rate of subsequent mitotic progression. The shifts in response by cells treated at specific times with 1,2-dioctanoylglycerol during mid- and late metaphase may be indicative of the existence of one or more regulatory switch points (i.e., checkpoints) just prior to anaphase onset.

Propranolol, a beta-adrenergic receptor blocker, affects microfilament organization, but not microtubules, during the first division in sea urchin eggs

Propranolol, a beta-adrenergic receptor blocker, blocks the formation of the cleavage furrow, while karyokinesis is unaffected during first division in the sea urchins Paracentrotus lividus or Lytechinus pictus. This effect is reversed by adrenalin, indicating that it is mediated by an adrenergic mechanism. The staining of F-actin microfilaments by rhodamine phalloidin in eggs in which the cleavage is blocked by the drug has revealed that propranolol affects both the distribution and the organization of actin microfilaments. A low-voltage scanning electron microscopy (LVSEM) study of microvilli in these eggs shows an extensive rearrangement of the egg surface. Anti-tubulin immunofluorescence microscopy of eggs treated with propranolol shows that they form normal mitotic asters. This indicates that while cleavage is affected, mitotic spindle formation is not. These results suggest that neurotransmitter monoamines known to be present in the sea urchin egg might be involved in the reorganization of the actin cytoskeleton underlying the formation of the cleavage furrow.

Lithium-induced teratogenesis in frog embryos prevented by a polyphosphoinositide cycle intermediate or a diacylglycerol analog

Microinjection of LiCl into prospective ventral blastomeres of the 32-cell Xenopus embryo gives rise to duplication of dorsoanterior structures such as the notochord, neural tube, eyes, and cement gland. We report here that this teratogenic effect of Li+ is prevented by coinjection of equimolar myo-inositol, an intermediate of the polyphosphoinositide cycle. In contrast, epi-inositol, a nonbiological positional isomer of inositol not employed in this cycle, is ineffective at rescuing Li+-injected embryos. Treatment of embryos at stage 7 with the tumor promoter, phorbol myristate acetate (an analog of the polyphosphoinositide cycle-derived second messenger, diacylglycerol), also prevents dorsoanterior duplication of Li+ embryos, while the nontransforming analog, phorbol myristate acetate-4-O-methyl ether, is without effect. Both of these rescuing agents are without obvious effects on development when administered alone (i.e., without Li+). Li+-selective microelectrode measurements demonstrate that intracellular Li+ levels are identical when Li+ is injected with or without myo-inositol. Clonal analysis shows that blastomeres injected with Li+ plus myo-inositol make a normal contribution of progeny to the later embryo. Because Li+ is a well-established inhibitor of the polyphosphoinositide cycle and can thereby have profound effects on cellular myo-inositol and diacylglycerol levels, these observations concerning inositol-mediated rescue suggest a role for altered polyphosphoinositide cycle activity in lithium-induced teratogenesis.

Mechanism of action of antimitotic drugs: a new hypothesis based on the role of cellular calcium

The antimitotic drugs such as colchicine, podophyllotoxin, etc. are currently believed to exert their cytotoxic and antimitotic effects due to binding of the drug-tubulin complex to the growing ends of microtubules (MTs), leading to an "end-capping or poisoning" effect. However, to account for a number of apparently puzzling observations regarding antimitotic drugs (which cannot be readily explained by the current model) and the mitotic process, a new hypothesis regarding the mechanism of action of antimitotic drugs is proposed. The key observations in this context are as follows: (i) The antimitotic drugs bind specifically to free tubulin. (ii) Cell growth by these drugs is specifically blocked in metaphase, and interphase microtubules do not seem to play any role in the drugs' cytotoxic or antimitotic effects. (iii) Tubulin is specifically associated with a number of membranous organelles (viz. mitochondria, plasma membranes, endoplasmic reticulum) which are responsible for intracellular Ca+2 homeostasis. (iv) Fluorescent derivatives of antimitotic drugs also bind to the above membranous organelles and not to MTs. (v) Ca+2 plays a central role in the control of MT assembly/disassembly in vivo and a Ca+2 pulse is necessary for the metaphase to anaphase transition. (vi) Cellular mutants which exhibit specific resistance to various antimitotic drugs are altered in either tubulin(s) or mitochondrial matrix proteins. To account for these observations, it is suggested that free tubulin present in the above membranous organelles serves as the cellular receptor for these drugs and this binding interferes with the Ca+2 regulatory/signalling mechanism essential for anaphase chromosome movement. The effect of these drugs on interphase MTs appears to be a secondary consequence of this alteration in Ca+2 regulation. The observed changes in mitochondrial matrix proteins in many of the mutants resistant to antimitotic drugs further indicate that mitochondria should play an important role in Ca+2 homeostasis, as it relates to mitosis. The possible mechanisms by which these drugs may interfere with the Ca+2 regulation and some implications of this hypothesis are discussed.

Influence of lithium on the positive inotropic effect of phenylephrine and isoprenaline in guinea-pig heart

The influence of lithium on the positive inotropic effects mediated by alpha- and beta-adrenoceptor stimulation was studied in isolated guinea-pig ventricular preparations stimulated at 1 Hz. Lithium chloride (10(-3)-2 X 10(-2) M) shifted the concentration-response curve for the inotropic effect of phenylephrine to the right in a dose-dependent manner. [3H]Prazosin binding was not inhibited by 10(-2) M lithium. The antagonistic effect of lithium was almost completely prevented by the presence of 10(-2) M myoinositol, and was potentiated by 10(-4) M 2-2'-anhydro-2-C-(hydroxymethyl)-myo-inositol, an antagonist of myo-inositol. The positive inotropic effect of isoprenaline was completely unaffected by either 10(-2) M lithium, 10(-2) M myo-inositol, or 10(-4) M 2-2'-anhydro-2-C-(hydroxymethyl)-myo-inositol. Since it is known that lithium interferes with inositol phosphate metabolism, our results produce further indirect evidence of an involvement of inositol phosphates in the myocardial response to alpha-adrenoceptor, but not to beta-adrenoceptor, stimulation. Our findings also suggest that chronic lithium treatment could interfere with the adrenergic modulation of myocardial contractility in those physio-pathological conditions in which the role of myocardial alpha-adrenoceptors becomes predominant.