Calcium buffer injections inhibit cytokinesis in Xenopus eggs

A localized calcium transient and polar body abscission

Polar body emission is a special form of cytokinesis in oocyte meiosis that ensures the correct number of chromosomes in reproduction-competent eggs. The molecular mechanism of the last step, polar body abscission, is poorly understood. While it has been proposed that Ca²⁺ signaling plays important roles in embryonic cytokinesis, to date transient increases in intracellular free Ca²⁺ have been difficult to document in oocyte meiosis except for the global Ca²⁺ wave induced by sperm at fertilization. Here, we find that microinjection of the calcium chelator dibromo-BAPTA inhibits polar body abscission in Xenopus laevis oocytes. Using a novel, microtubule-targeted ratio-metric calcium sensor, we detected a calcium transient that is focused at the contractile ring-associated plasma membrane and which occurred after anaphase and constriction of the contractile ring but prior to abscission. This calcium transient was confirmed by mobile calcium probes. Further, the Ca²⁺-sensitive protein kinase Cβ C2 domain transiently translocated to the contractile ring-associated membrane simultaneously with the calcium transient. Collectively, these results demonstrate that a calcium transient, apparently originating at the contractile ring-associated plasma membrane, promotes polar body abscission.

Mitosis, Focus on Calcium

The transformation of a single fertilised egg into an adult human consisting of tens of trillions of highly diverse cell types is a marvel of biology. The expansion is largely achieved by cell duplication through the process of mitosis. Mitosis is essential for normal growth, development, and tissue repair and is one of the most tightly regulated biological processes studied. This regulation is designed to ensure accurate segregation of chromosomes into each new daughter cell since errors in this process can lead to genetic imbalances, aneuploidy, that can lead to diseases including cancer. Understanding how mitosis operates and the molecular mechanisms that ensure its fidelity are therefore not only of significant intellectual value but provide unique insights into disease pathology. The purpose of this review is to revisit historical evidence that mitosis can be influenced by the ubiquitous second messenger calcium and to discuss this in the context of new findings revealing exciting new information about its role in cell division.

Microtubule-dependent subcellular organisation of pluripotent cells

With the advancement of cutting-edge live imaging technologies, microtubule remodelling has evolved as an integral regulator for the establishment of distinct differentiated cells. However, despite their fundamental role in cell structure and function, microtubules have received less attention when unravelling the regulatory circuitry of pluripotency. Here, we summarise the role of microtubule organisation and microtubule-dependent events required for the formation of pluripotent cells in vivo by deciphering the process of early embryogenesis: from fertilisation to blastocyst. Furthermore, we highlight current advances in elucidating the significance of specific microtubule arrays in in vitro culture systems of pluripotent stem cells and how the microtubule cytoskeleton serves as a highway for the precise intracellular movement of organelles. This Review provides an informed understanding of the intrinsic role of subcellular architecture of pluripotent cells and accentuates their regenerative potential in combination with innovative light-inducible microtubule techniques.

Imaging early embryonic calcium activity with GCaMP6s transgenic zebrafish

Intracellular Ca²⁺ signaling regulates cellular activities during embryogenesis and in adult organisms. We generated stable Tg[βactin2:GCaMP6s]^stl351 and Tg[ubi:GCaMP6s]^stl352 transgenic lines that combine the ubiquitously-expressed Ca²⁺ indicator GCaMP6s with the transparent characteristics of zebrafish embryos to achieve superior in vivo Ca²⁺ imaging. Using the Tg[βactin2:GCaMP6s]^stl351 line featuring strong GCaMP6s expression from cleavage through gastrula stages, we detected higher frequency of Ca²⁺ transients in the superficial blastomeres during the blastula stages preceding the midblastula transition. Additionally, GCaMP6s also revealed that dorsal-biased Ca²⁺ signaling that follows the midblastula transition persisted longer during gastrulation, compared with earlier studies. We observed that dorsal-biased Ca²⁺ signaling is diminished in ventralized ichabod/β-catenin2 mutant embryos and ectopically induced in embryos dorsalized by excess β-catenin. During gastrulation, we directly visualized Ca²⁺ signaling in the dorsal forerunner cells, which form in a Nodal signaling dependent manner and later give rise to the laterality organ. We found that excess Nodal increases the number and the duration of Ca²⁺ transients specifically in the dorsal forerunner cells. The GCaMP6s transgenic lines described here enable unprecedented visualization of dynamic Ca²⁺ events from embryogenesis through adulthood, augmenting the zebrafish toolbox.

Ca2+ Signalling and Membrane Dynamics During Cytokinesis in Animal Cells

Interest in the role of Ca²⁺ signalling as a possible regulator of the combinatorial processes that result in the separation of the daughter cells during cytokinesis, extend back almost a 100 years. One of the key processes required for the successful completion of cytokinesis in animal cells (especially in the large holoblastically and meroblastically dividing embryonic cells of a number of amphibian and fish species), is the dynamic remodelling of the plasma membrane. Ca²⁺ signalling was subsequently demonstrated to regulate various different aspects of cytokinesis in animal cells, and so here we focus specifically on the role of Ca²⁺ signalling in the remodelling of the plasma membrane. We begin by providing a brief history of the animal models used and the research accomplished by the early twentieth century investigators, with regards to this aspect of animal cell cytokinesis. We then review the most recent progress made (i.e., in the last 10 years), which has significantly advanced our current understanding on the role of cytokinetic Ca²⁺ signalling in membrane remodelling. To this end, we initially summarize what is currently known about the Ca²⁺ transients generated during animal cell cytokinesis, and then we describe the latest findings regarding the source of Ca²⁺ generating these transients. Finally, we review the current evidence about the possible targets of the different cytokinetic Ca²⁺ transients with a particular emphasis on those that either directly or indirectly affect plasma membrane dynamics. With regards to the latter, we discuss the possible role of the early Ca²⁺ signalling events in the deformation of the plasma membrane at the start of cytokinesis (i.e., during furrow positioning), as well as the role of the subsequent Ca²⁺ signals in the trafficking and fusion of vesicles, which help to remodel the plasma membrane during the final stages of cell division. As it is becoming clear that each of the cytokinetic Ca²⁺ transients might have multiple, integrated targets, deciphering the precise role of each transient represents a significant (and ongoing) challenge.

Spindle function in Xenopus oocytes involves possible nanodomain calcium signaling

Injection of dibromo-BAPTA caused immediate collapse of meiotic spindles in frog oocytes. In contrast, EGTA had no effect on the spindle or polar body emission. The disruption of spindle integrity by the fast but not slow calcium chelators suggests that meiotic spindle function in the oocytes involves nanodomain calcium signaling.

Intracellular calcium transients are a universal phenomenon at fertilization and are required for egg activation, but the exact role of Ca²⁺ in second-polar-body emission remains unknown. On the other hand, similar calcium transients have not been demonstrated during oocyte maturation, and yet, manipulating intracellular calcium levels interferes with first-polar-body emission in mice and frogs. To determine the precise role of calcium signaling in polar body formation, we used live-cell imaging coupled with temporally precise intracellular calcium buffering. We found that BAPTA-based calcium chelators cause immediate depolymerization of spindle microtubules in meiosis I and meiosis II. Surprisingly, EGTA at similar or higher intracellular concentrations had no effect on spindle function or polar body emission. Using two calcium probes containing permutated GFP and the calcium sensor calmodulin (Lck-GCaMP3 and GCaMP3), we demonstrated enrichment of the probes at the spindle but failed to detect calcium increase during oocyte maturation at the spindle or elsewhere. Finally, endogenous calmodulin was found to colocalize with spindle microtubules throughout all stages of meiosis. Our results—most important, the different sensitivities of the spindle to BAPTA and EGTA—suggest that meiotic spindle function in frog oocytes requires highly localized, or nanodomain, calcium signaling.

Selective Vulnerability of Cancer Cells by Inhibition of Ca(2+) Transfer from Endoplasmic Reticulum to Mitochondria

In the absence of low-level ER-to-mitochondrial Ca(2+) transfer, ATP levels fall, and AMPK-dependent, mTOR-independent autophagy is induced as an essential survival mechanism in many cell types. Here, we demonstrate that tumorigenic cancer cell lines, transformed primary human fibroblasts, and tumors in vivo respond similarly but that autophagy is insufficient for survival, and cancer cells die while their normal counterparts are spared. Cancer cell death is due to compromised bioenergetics that can be rescued with metabolic substrates or nucleotides and caused by necrosis associated with mitotic catastrophe during their proliferation. Our findings reveal an unexpected dependency on constitutive Ca(2+) transfer to mitochondria for viability of tumorigenic cells and suggest that mitochondrial Ca(2+) addiction is a feature of cancer cells.

Modulation of phosphatidylinositol 4-phosphate levels by CaBP7 controls cytokinesis in mammalian cells

For more than 25 years, lysosomes have been known to cluster at the intercellular bridge during cytokinesis, but why has remained a mystery. This study provides evidence that phosphoinositide metabolism is important for this clustering and that lysosome activity is required for cytokinesis.

Calcium and phosphoinositide signaling regulate cell division in model systems, but their significance in mammalian cells is unclear. Calcium-binding protein-7 (CaBP7) is a phosphatidylinositol 4-kinaseIIIβ (PI4KIIIβ) inhibitor required during cytokinesis in mammalian cells, hinting at a link between these pathways. Here we characterize a novel association of CaBP7 with lysosomes that cluster at the intercellular bridge during cytokinesis in HeLa cells. We show that CaBP7 regulates lysosome clustering and that PI4KIIIβ is essential for normal cytokinesis. CaBP7 depletion induces lysosome mislocalization, extension of intercellular bridge lifetime, and cytokinesis failure. These data connect phosphoinositide and calcium pathways to lysosome localization and normal cytokinesis in mammalian cells.

Mitochondria localize to the cleavage furrow in mammalian cytokinesis

Mitochondria are dynamic organelles with multiple cellular functions, including ATP production, calcium buffering, and lipid biosynthesis. Several studies have shown that mitochondrial positioning is regulated by the cytoskeleton during cell division in several eukaryotic systems. However, the distribution of mitochondria during mammalian cytokinesis and whether the distribution is regulated by the cytoskeleton has not been examined. Using live spinning disk confocal microscopy and quantitative analysis of mitochondrial fluorescence intensity, we demonstrate that mitochondria are recruited to the cleavage furrow during cytokinesis in HeLa cells. After anaphase onset, the mitochondria are recruited towards the site of cleavage furrow formation, where they remain enriched as the furrow ingresses and until cytokinesis completion. Furthermore, we show that recruitment of mitochondria to the furrow occurs in multiple mammalian cells lines as well as in monopolar, bipolar, and multipolar divisions, suggesting that the mechanism of recruitment is conserved and robust. Using inhibitors of cytoskeleton dynamics, we show that the microtubule cytoskeleton, but not actin, is required to transport mitochondria to the cleavage furrow. Thus, mitochondria are specifically recruited to the cleavage furrow in a microtubule-dependent manner during mammalian cytokinesis. Two possible reasons for this could be to localize mitochondrial function to the furrow to facilitate cytokinesis and / or ensure accurate mitochondrial inheritance.

Characterization of Ca(2+) signaling in the external yolk syncytial layer during the late blastula and early gastrula periods of zebrafish development

Preferential loading of the complementary bioluminescent (f-aequorin) and fluorescent (Calcium Green-1 dextran) Ca(2+) reporters into the yolk syncytial layer (YSL) of zebrafish embryos, revealed the generation of stochastic patterns of fast, short-range, and slow, long-range Ca(2+) waves that propagate exclusively through the external YSL (E-YSL). Starting abruptly just after doming (~4.5h post-fertilization: hpf), and ending at the shield stage (~6.0hpf) these distinct classes of waves propagated at mean velocities of ~50 and ~4μm/s, respectively. Although the number and pattern of these waves varied between embryos, their initiation site and arcs of propagation displayed a distinct dorsal bias, suggesting an association with the formation and maintenance of the nascent dorsal-ventral axis. Wave initiation coincided with a characteristic clustering of YSL nuclei (YSN), and their associated perinuclear ER, in the E-YSL. Furthermore, the inter-YSN distance (IND) appeared to be critical such that Ca(2+) wave propagation occurred only when this was <~8μm; an IND >~8μm was coincidental with wave termination at shield stage. Treatment with the IP3R antagonist, 2-APB, the Ca(2+) buffer, 5,5'-dibromo BAPTA, and the SERCA-pump inhibitor, thapsigargin, resulted in a significant disruption of the E-YSL Ca(2+) waves, whereas exposure to the RyR antagonists, ryanodine and dantrolene, had no significant effect. These findings led us to propose that the E-YSL Ca(2+) waves are generated mainly via Ca(2+) release from IP3Rs located in the perinuclear ER, and that the clustering of the YSN is an essential step in providing a CICR pathway required for wave propagation. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Calcium sensitivity of α-actinin is required for equatorial actin assembly during cytokinesis

The actin cross-linking protein, α-actinin, plays a crucial role in mediating furrow ingression during cytokinesis. However, the mechanism by which its dynamics are regulated during this process is poorly understood. Here we have investigated the role of calcium sensitivity of α-actinin in the regulation of its dynamics by generating a functional calcium-insensitive mutant (EFM). GFP-tagged EFM (EFM-GFP) localized to the equatorial regions during cell division. However, the maximal equatorial accumulation of EFM-GFP was significantly smaller in comparison to α-actinin-GFP when it was expressed in normal cells and cells depleted of endogenous α-actinin. No apparent defects in cytokinesis were observed in these cells. However, F-actin levels at the equator were significantly reduced in cells expressing EFM-GFP as compared with α-actinin-GFP at furrow initiation but were recovered during furrow ingression. These results suggest that calcium sensitivity of α-actinin is required for its equatorial accumulation that is crucial for the initial equatorial actin assembly but is dispensable for cytokinesis. Equatorial RhoA localization was not affected by EFM-GFP overexpression, suggesting that equatorial actin assembly is predominantly driven by the RhoA-dependent mechanism. Our observations shed new light on the role and regulation of the accumulation of pre-existing actin filaments in equatorial actin assembly during cytokinesis.

Disruption of transient receptor potential canonical channel 1 causes incomplete cytokinesis and slows the growth of human malignant gliomas

Despite decades of research, primary brain tumors, gliomas, lack effective treatment options and present a huge clinical challenge. Particularly, the most malignant subtype, Glioblastoma multiforme, proliferates extensively and cells often undergo incomplete cell divisions, resulting in multinucleated cells. We now present evidence that multinucleated glioma cells result from the functional loss of transient receptor potential canonical 1 (TRPC1) channels, plasma membrane proteins involved in agonist-induced calcium entry and reloading of intracellular Ca(2+) stores. Pharmacological inhibition or shRNA mediated suppression of TRPC1 causes loss of functional channels and store-operated calcium entry in D54MG glioma cells. This is associated with reduced cell proliferation and, frequently, with incomplete cell division. The resulting multinucleated cells are reminiscent of those found in patient biopsies. In a flank tumor model, tumor size was significantly decreased when TRPC1 expression was disrupted using a doxycycline inducible shRNA knockdown approach. These results suggest that TRPC1 channels play an important role in glioma cell division most likely by regulating calcium signaling during cytokinesis.

Calcium waves and development

Those calcium oscillations which go deep into cells take the form of 'fast' calcium waves. In fully active cells at room temperature, these move at 15-30 microns/s and are propagated by a reaction-diffusion mechanism governed by the Luther equation in which calcium ions are the only propagators and calcium-induced calcium release is the only reaction. However, they may be initiated by a second mode of Ca(2+)-induced Ca2+ release within the lumen of the endoplasmic reticulum (ER). In sea urchin fertilization, this second mode of Ca(2+)-induced Ca2+ release is in turn begun by calcium entering the sperm and thence the ER. Subsurface calcium waves include an important class of surface contraction waves which move at 0.3-3 microns/s and are called 'slow' waves. Their prototype is the 0.5 micron/s wave which accompanies and controls cytokinesis in large eggs. Slow waves may be propagated by mechanical tension rather than by diffusion. Recent work with Dictyostelium transfected with apoaequorin has provided the first views of free calcium patterns within a developing, multicellular organism. During most or all of development, those regions which will differentiate into stalk or stalk-like cells (as opposed to spores) exhibit frequent calcium pulses. These pulses are believed to be fast calcium waves and to feed back on these regions so as to favour non-spore differentiation.

Regulation of the actin cytoskeleton by PIP2 in cytokinesis

Cytokinesis is a sequential process that occurs in three phases: assembly of the cytokinetic apparatus, furrow progression and fission (abscission) of the newly formed daughter cells. The ingression of the cleavage furrow is dependent on the constriction of an equatorial actomyosin ring in many cell types. Recent studies have demonstrated that this structure is highly dynamic and undergoes active polymerization and depolymerization throughout the furrowing process. Despite much progress in the identification of contractile ring components, little is known regarding the mechanism of its assembly and structural rearrangements. PIP2 (phosphatidylinositol 4,5-bisphosphate) is a critical regulator of actin dynamics and plays an essential role in cell motility and adhesion. Recent studies have indicated that an elevation of PIP2 at the cleavage furrow is a critical event for furrow stability. In this review we discuss the role of PIP2-mediated signalling in the structural maintenance of the contractile ring and furrow progression. In addition, we address the role of other phosphoinositides, PI(4)P (phosphatidylinositol 4-phosphate) and PIP3 (phosphatidylinositol 3,4,5-triphosphate) in these processes.

A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs

Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase-anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase-anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.

Endoplasmic reticulum generates calcium signalling microdomains around the nucleus and spindle in syncytial Drosophila embryos

Cell cycle calcium signals are generated by inositol trisphosphate-mediated release of calcium from internal stores [Ciapa, Pesando, Wilding and Whitaker (1994) Nature (London) 368, 875–878; Groigno and Whitaker (1998) Cell 92, 193–204]. The major internal calcium store is the ER (endoplasmic reticulum): the spatial organization of the ER during mitosis is important in defining a microdomain around the nucleus and mitotic spindle in early Drosophila embryos [Parry, McDougall and Whitaker (2005) J. Cell Biol. 171, 47–59]. Nuclear divisions in syncytial Drosophila embryos are accompanied by both cortical and nuclear localized calcium transients. Mitosis is prevented by the InsP3 antagonists Xestospongin C and heparin. Nuclear-localized transients and cortical transients rely on extraembryonic calcium, suggesting that ER calcium levels are maintained by calcium influx.

Microdomains bounded by endoplasmic reticulum segregate cell cycle calcium transients in syncytial Drosophila embryos

Cell cycle calcium signals are generated by the inositol trisphosphate (InsP₃)–mediated release of calcium from internal stores (Ciapa, B., D. Pesando, M. Wilding, and M. Whitaker. 1994. Nature. 368:875–878; Groigno, L., and M. Whitaker. 1998. Cell. 92:193–204). The major internal calcium store is the endoplasmic reticulum (ER); thus, the spatial organization of the ER during mitosis may be important in shaping and defining calcium signals. In early Drosophila melanogaster embryos, ER surrounds the nucleus and mitotic spindle during mitosis, offering an opportunity to determine whether perinuclear localization of ER conditions calcium signaling during mitosis. We establish that the nuclear divisions in syncytial Drosophila embryos are accompanied by both cortical and nuclear localized calcium transients. Constructs that chelate InsP₃ also prevent nuclear division. An analysis of nuclear calcium concentrations demonstrates that they are differentially regulated. These observations demonstrate that mitotic calcium signals in Drosophila embryos are confined to mitotic microdomains and offer an explanation for the apparent absence of detectable global calcium signals during mitosis in some cell types.

Ca2+-dependent interaction of BAPTA with phospholipids

Starting from a comparative study of different Ca2+ chelators on the G-protein-induced inhibition of the CaV2.1 Ca channels, we demonstrate that BAPTA and DM-nitrophen are able to interact, in a Ca2+- and lipid-dependent manner, with phospholipid monolayers. Critical insertion pressure and sensitivity to charged lipids indicated that insertion in the lipid film may occur in biological membranes as those found on Xenopus oocytes. This novel property is not found for EGTA and EDTA and may participate to the unusual ability of BAPTA-related molecules to chelate Ca2+ ions in the very close vicinity of the plasma membrane, where most of the Ca2+-dependent signalling triggered by voltage-gated Ca2+ currents occurs.

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.

The NO pathway acts late during the fertilization response in sea urchin eggs

Both the inositol 1,4,5-trisphosphate (InsP(3)) and ryanodine receptor pathways contribute to the Ca(2+) transient at fertilization in sea urchin eggs. To date, the precise contribution of each pathway has been difficult to ascertain. Evidence has accumulated to suggest that the InsP(3) receptor pathway has a primary role in causing Ca(2+) release and egg activation. However, this was recently called into question by a report implicating NO as the primary egg activator. In the present study we pursue the hypothesis that NO is a primary egg activator in sea urchin eggs and build on previous findings that an NO/cGMP/cyclic ADP-ribose (cADPR) pathway is active at fertilization in sea urchin eggs to define its role. Using a fluorescence indicator of NO levels, we have measured both NO and Ca(2+) at fertilization and establish that NO levels rise after, not before, the Ca(2+) wave is initiated and that this rise is Ca(2+)-dependent. By inhibiting the increase in NO at fertilization, we find not that the Ca(2+) transient is abolished but that the duration of the transient is significantly reduced. The latency and rise time of the transient are unaffected. This effect is mirrored by the inhibition of cGMP and cADPR signaling in sea urchin eggs at fertilization. We establish that cADPR is generated at fertilization, at a time comparable to the time of the rise in NO levels. We conclude that NO is unlikely to be a primary egg activator but, rather, acts after the initiation of the Ca(2+) wave to regulate the duration of the fertilization Ca(2+) transient.

Localized calcium signals along the cleavage furrow of the Xenopus egg are not involved in cytokinesis

It has been proposed that a localized calcium (Ca) signal at the growing end of the cleavage furrow triggers cleavage furrow formation in large eggs. We have examined the possible role of a Ca signal in cleavage furrow formation in the Xenopus laevis egg during the first cleavage. We were able to detect two kinds of Ca waves along the cleavage furrow. However, the Ca waves appeared after cleavage furrow formation in late stages of the first cleavage. In addition, cleavage was not affected by injection of dibromoBAPTA or EGTA into the eggs at a concentration sufficient to suppress the Ca waves. Furthermore, even smaller classes of Ca release such as Ca puffs and Ca blips do not occur at the growing end of the cleavage furrow. These observations demonstrate that localized Ca signals in the cleavage furrow are not involved in cytokinesis. The two Ca waves have unique characteristics. The first wave propagates only in the region of newly inserted membrane along the cleavage furrow. On the other hand, the second wave propagates along the border of new and old membranes, suggesting that this wave might be involved in adhesion between two blastomeres.

The Dictyostelium LvsA protein is localized on the contractile vacuole and is required for osmoregulation

LvsA is a Dictyostelium protein that is essential for cytokinesis and that is related to the mammalian beige/LYST family of proteins. To better understand the function of this novel protein family we tagged LvsA with GFP using recombination techniques. GFP-LvsA is primarily associated with the membranes of the contractile vacuole system and it also has a punctate distribution in the cytoplasm. Two markers of the Dictyostelium contractile vacuole, the vacuolar proton pump and calmodulin, show extensive colocalization with GFP-LvsA on contractile vacuole membranes. Interestingly, the association of LvsA with contractile vacuole membranes occurs only during the discharge phase of the vacuole. In LvsA mutants the contractile vacuole becomes disorganized and calmodulin dissociates from the contractile vacuole membranes. Consequently, the contractile vacuole is unable to function normally, it can swell but seems unable to discharge and the LvsA mutants become osmosensitive. These results demonstrate that LvsA can associate transiently with the contractile vacuole membrane compartment and that this association is necessary for the function of the contractile vacuole during osmoregulation. This transient association with specific membrane compartments may be a general property of other BEACH-domain containing proteins.

The molecular mechanism of targeted vesicle transport in cytokinesis

Recent studies have demonstrated that vesicle transport to cleavage furrow is indispensable for cytokinesis. Some animal and plant cells form distinct structures during cell division known as central spindle and phragmoplast, respectively. Several essential factors involved in the vesicle transport have been isolated so far. SNARE proteins and molecular motors play a central role in this process. For future research of cytokinesis, it is important to investigate these factors as well as cytoskeletal components of the contractile ring in detail. This review focuses on the molecular mechanism of targeted vesicle transport in cytokinesis.

Contractile ring formation in Xenopus egg and fission yeast

How actin filaments (F-actin) and myosin II (myosin) assemble to form the contractile ring was investigated with fission yeast and Xenopus egg. In fission yeast cells, an aster-like structure composed of F-actin cables is formed at the medial cortex of the cell during prophase to metaphase, and a single F-actin cable(s) extends from this structure, which seems to be a structural basis of the contractile ring. In early mitosis, myosin localizes as dots in the medial cortex independently of F-actin. Then they fuse with each other and are packed into a thin contractile ring. At the growing ends of the cleavage furrow of Xenopus eggs, F-actin at first assembles to form patches. Next they fuse with each other to form short F-actin bundles. The short bundles then form long bundles. Myosin seems to be transported by the cortical movement to the growing end and assembles there as spots earlier than F-actin. Actin polymerization into the patches is likely to occur after accumulation of myosin. The myosin spots and the F-actin patches are simultaneously reorganized to form the contractile ring bundles. The idea that a Ca signal triggers cleavage furrow formation was tested with Xenopus eggs during the first cleavage. We could not detect any Ca signals such as a Ca wave, Ca puffs or even Ca blips at the growing end of the cleavage furrow. Furthermore, cleavages are not affected by Ca-chelators injected into the eggs at concentrations sufficient to suppress the Ca waves. Thus we conclude that formation of the contractile ring is not induced by a Ca signal at the growing end of the cleavage furrow.

Imaging patterns of calcium transients during neural induction in Xenopus laevis embryos

Through the injection of f-aequorin (a calcium-sensitive bioluminescent reporter) into the dorsal micromeres of 8-cell stage Xenopus laevis embryos, and the use of a Photon Imaging Microscope, distinct patterns of calcium signalling were visualised during the gastrulation period. We present results to show that localised domains of elevated calcium were observed exclusively in the anterior dorsal part of the ectoderm, and that these transients increased in number and amplitude between stages 9 to 11, just prior to the onset of neural induction. During this time, however, no increase in cytosolic free calcium was observed in the ventral ectoderm, mesoderm or endoderm. The origin and role of these dorsal calcium-signalling patterns were also investigated. Calcium transients require the presence of functional L-type voltage-sensitive calcium channels. Inhibition of channel activation from stages 8 to 14 with the specific antagonist R(+)BayK 8644 led to a complete inhibition of the calcium transients during gastrulation and resulted in severe defects in the subsequent formation of the anterior nervous system. BayK treatment also led to a reduction in the expression of Zic3 and geminin in whole embryos, and of NCAM in noggin-treated animal caps. The possible role of calcium transients in regulating developmental gene expression is discussed.

Relationships between the central spindle and the contractile ring during cytokinesis in animal cells

During late anaphase and telophase, animal cells develop a bundle of antiparallel, interdigitating microtubules between the two daughter nuclei. Recent data indicate that this structure, called the central spindle, plays an essential role during cytokinesis. Studies in Drosophila and on vertebrate cells strongly suggest that the molecular signals for cytokinesis specifically emanate from the central spindle midzone. Moreover, the analysis of Drosophila mutants defective in cytokinesis has revealed a cooperative interaction between the central spindle microtubules and the contractile ring: when either of these structures is perturbed, the proper assembly of the other is disrupted. Based on these results we propose a model for the role of the central spindle during cytokinesis. We suggest that the interaction between central spindle microtubules and cortical actin filaments leads to two early events crucial for cytokinesis: the positioning of the contractile ring, and the stabilization of the plus ends of the interdigitating microtubules that comprise the central spindle. The latter event would provide the cell with a specialized microtubule scaffold that could mediate the translocation of plus-end-directed molecular motors to the cell's equator. Among the cargoes transported by these motors could be proteins involved in the regulation and execution of cytokinesis.

Multiple types of calcium signals are associated with cell division in zebrafish embryo

Recent studies suggested that a Ca(2+) signal is involved in the regulation of cell division. For example, using a confocal imaging technique, we have shown that a localized Ca(2+) elevation was clearly associated with the onset of cytokinesis in zebrafish embryo [Chang and Meng (1995) J. Cell Biol. 131:1539-1545]. This finding was later confirmed in studies using aequorin as a Ca(2+) probe. Here, we used a 4-D confocal measurement technique to further characterize the properties of the Ca(2+) signal associated with cell division. We found evidence that there were three types of Ca(2+) signals associated with different stages of cell cleavage in embryonic cell. The first type was repetitive Ca(2+) spikes that emerged several minutes before the first cell cleavage began. These Ca(2+) spikes were first distributed broadly over the central region of the blastodisc and then gradually localized in the equatorial region; they appeared to play the role of determining the position of the first cleavage plane. The second type was a calcium wave that propagated along the cleavage furrow and appeared to guide the furrow extension during the progression of cytokinesis. The third type was a group of post-cleavage calcium spikes that appeared to be responsible for furrow deepening and maintenance of the contractile band. When this type of Ca(2+) transient was blocked by injecting BAPTA or heparin, cell cleavage regressed and the structure of the contractile band could no longer be maintained.

Timing of calcium and protein synthesis requirements for the first mitotic cell cycle in fertilised Xenopus eggs

Mitosis is governed by the activity of the M-phase promoting factor (MPF). In some systems, particularly early embryos, transient increases in calcium concentration have been shown to be necessary for mitosis and regulate its timing. By microinjection of the calcium buffer, dibromoBAPTA, into fertilised Xenopus eggs, we have assessed whether calcium events are required to initiate MPF activation and inactivation. Since initial experiments showed that this buffer inhibited protein synthesis, we measured when mitosis and cleavage became independent of translation. We found that, after a period of protein synthesis essential for cleavage, there was a phase during which continued translation affected the timing of cleavage, but was not essential for its occurrence. Measurement of MPF activity in single embryos injected with calcium buffer at different times in the first cell cycle, showed that there were two sensitive periods. The first period of sensitivity blocked MPF activation and coincided with the time at which cleavage became completely independent of protein synthesis. The second sensitive period occurred just before histone kinase activity peaked, and was necessary for kinase inactivation. Preventing inactivation in this way arrested egg extracts in mitosis. These results support the view that transient increases in free calcium concentration contribute to mitotic progression by first triggering MPF activation and subsequently, with elevated MPF activity, inducing its inactivation.

A small, physiological electric field orients cell division

We report on an observation that the orientation of cell division is directed by small, applied electric fields (EFs). Cultured human corneal epithelial cells were exposed to a direct-current EF of physiological magnitude. Cells divided while attached to the culture dish, and most did so with a cleavage plane perpendicular to the EF vector. There are many instances in which cell divisions in vivo occur in the presence of direct-current physiological EF, for example, during embryonic morphogenesis, neuronal and epithelial differentiation, wound healing, or tumor formation. Endogenous physiological EFs may play important roles in some or all of these processes by regulating the axis of cell division and, hence, the positioning of daughter cells.

Usefulness and limitations of linear approximations to the understanding of Ca++ signals

Simple approximations to some limiting cases of Ca++ signalling provide insight into the complex problems of buffered diffusion and of Ca++ homeostasis in the presence of buffers. Three cases are presented, where the influence of Ca++ buffers can readily be understood in the limit of small signals: the return of global cellular [Ca++] following a short stimulus in a 'Single Compartment', buffered diffusion along a cylindrical axon in the 'Rapid Buffer Approximation', and nonequilibrium microdomains of elevated [Ca++] in the immediate vicinity of open Ca++ channels.

Patterns of free calcium in zebrafish embryos

Direct knowledge of Ca2+ patterns in vertebrate development is largely restricted to early stages, in which they control fertilization, ooplasmic segregation and cleavage. To explore new roles of Ca2+ in vertebrate development, we injected the Ca2+ indicator aequorin into zebrafish eggs and imaged Ca2+ throughout the first day of development. During early cleavages, a high Ca2+ zone is seen in the cleavage furrows. The high Ca2+ zone during first cleavage spreads as a slow wave (0.5 microm/second) and is preceded by three Ca2+ pulses within the animal pole region of the egg. When Ca2+ concentrations are clamped at the resting level by BAPTA buffer injection into the zygote, all signs of development are blocked. In later development, Ca2+ patterns are associated with cell movements during gastrulation, with neural induction, with brain regionalization, with formation of the somites and neural keel, with otic placode formation, with muscle movements and with formation of the heart. Particularly remarkable is a sharp boundary between high Ca2+ in the presumptive forebrain and midbrain versus low Ca2+ in the presumptive hindbrain starting at 10 hours of development. When Ca2+ changes are damped by injection of low concentrations of BAPTA, fish form with greatly reduced eyes and hearts. The present study provides a first overview of Ca2+ patterns during prolonged periods of vertebrate development and points to new roles of Ca2+ in cellular differentiation and pattern formation.

Calcium transients accompany ooplasmic segregation in zebrafish embryos

Through the injection of f-aequorin (a calcium-specific luminescent reporter), and the use of an imaging photon detector, transient localized elevations of free cytosolic calcium in the forming blastodisc (BD) and animal hemisphere cortex were visualized that correlated with ooplasmic segregation. The introduction of an appropriate concentration of the weak (KD = 1.5 micromol/L) calcium buffer 5,5'-dibromo-BAPTA results in the dissipation of these calcium domains, and inhibits cytoplasmic streaming and the subsequent formation of a BD at the animal pole. These inhibitory actions are dependent on the final cytosolic concentration of buffer within the egg: > or = 1.3 mmol/L blocks ooplasmic streaming; < 1.3 mmol/L eggs segregate normally. Injection of 5,5'-dimethyl-BAPTA (KD = 0.15 micromol/L) to a final concentration of 1.5 mmol/L as a control has no effect on ooplasmic streaming. These results suggest that localized domains of elevated free cytosolic calcium are essential for ooplasmic segregation in zebrafish. Furthermore, a hypothetical model is presented linking these calcium transients to the contraction of a cortically located actin microfilament network as a possible mechanism providing the driving force for segregation.

Calcium and mitosis

Calcium signals often accompany mitosis. The most obvious example of calcium as a mitotic signal is at fertilization in vertebrate eggs, where the calcium transient induces anaphase onset. New imaging methods have demonstrated smaller calcium signals that control mitosis entry and mitosis exit in sea urchin embryos. Other experiments in mouse and frog embryos indicate that similar signals with similar function may play a part in these embryos, too. The links between these calcium control signals and mitotic kinase activation are adumbrated. It appears that calcium oscillations are a property of the mitotic state. A case is made that calcium may be a universal mitotic signal, with the possible exception of early meiotic events in oocytes.

Requirement for microtubules in new membrane formation during cytokinesis of Xenopus embryos

In cleaving Xenopus eggs, exposure to nocodazole or cold shock prevents the addition of new plasma membrane to the cleavage plane and causes furrows to recede, suggesting a specific role for microtubules in cytokinesis. Whole-mount confocal immunocytochemistry reveals a ring of radially arranged, acetylated microtubule bundles at the base of all advancing cleavage furrows, from the first cleavage through the midblastula stage. We hypothesize that this novel microtubular structure is involved in transporting maternal stores of membrane in the subcortex to a site of membrane addition near the leading edge of the furrow.

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.

Microtubule-microfilament synergy in the cytoskeleton

This review describes examples of structural and functional synergy of the microtubule and actin filament cytoskeleton. An analysis of basal body (centriole)-associated fibrillar networks includes studies of ciliated epithelium, neurosensory epithelium, centrosomes, and ciliated protozoa. Microtubule and actin filament interactions in cell division and development are illustrated by centrosome motility, cleavage furrow positioning, centriole migration, nuclear migration, dynamics in the phragmoplast, growth cone motility, syncytial organization, and ring canals. Model systems currently used for studies on organelle transport are described in relation to mitochondrial transport in axons and vesicular transport in polarized epithelium. Evidence that both anterograde and retrograde motors are associated with one organelle is also discussed. The final section reviews proteins that bind both microtubules and actin filaments and are possible regulators of microtubule-microfilament interactions. Regulatory roles for posttranslational modifications, microtubule and microfilament dynamics, and multisubunit complexes are considered.

The dmf1/mid1 gene is essential for correct positioning of the division septum in fission yeast

Little is known about the mechanisms that establish the position of the division plane in eukaryotic cells. Wild-type fission yeast cells divide by forming a septum in the middle of the cell at the end of mitosis. Dmf1 mutants complete mitosis and initiate septum formation, but the septa that form are positioned at random locations and angles in the cell, rather than in the middle. We have cloned the dmf1 gene as a suppressor of the cdc7-24 mutant. The dmf1 mutant is allelic with mid1. The gene encodes a novel protein containing a putative nuclear localization signal, and a carboxy-terminal PH domain. In wild-type cells, Dmf1p is nuclear during interphase, and relocates to form a medial ring at the cell cortex coincident with the onset of mitosis. This relocalization occurs before formation of the actin ring and is associated with increased phosphorylation of Dmf1p. The Dmf1p ring can be formed in the absence of an actin ring, but depends on some of the genes required for actin ring formation. When the septum is completed and the cells separate, Dmf1p staining is once again nuclear. These data implicate Dmf1p as an important element in assuring correct placement of the division septum in Schizosaccharomyces pombe cells.

Calcium requirements during mitotic cdc2 kinase activation and cyclin degradation in Xenopus egg extracts

Activation of p34cdc2 kinase is essential for entry into mitosis while subsequent deactivation and cyclin degradation are associated with exit. In Xenopus embryos, both of these phases are regulated by post-translation modifications and occur spontaneously on incubation of extracts prepared late in the first cell cycle. Even though high levels of calcium buffer were initially used to prepare these extracts, we found that free calcium levels in them remained in the observed physiological range (200-500 nM). Further addition of calcium buffers only slightly reduced free calcium levels, but inhibited histone H1 (cdc2A) kinase deactivation and cyclin degradation. Higher buffer concentrations slowed the kinase activation phase. Reducing the free buffer concentration by premixing with calcium reversed the effects of the buffer, indicating that the inhibitory effects arose from the calcium-chelating properties of the buffer rather than non-specific side effects. Furthermore, additions of calcium buffer at the end of the H1 kinase activation phase did not prevent deactivation. From these results, and the order of effectiveness of different calcium buffers in disrupting the H1 kinase cycle, we suggest that local transient increases in free calcium influence the rate of cdc2 kinase activation and are required to initiate the pathway leading to cyclin degradation and kinase inactivation in mitotic cell cycles.

Introduction of calcium buffers into the cytosol of Dictyostelium discoideum amoebae alters cell morphology and inhibits chemotaxis

Differentiating Dictyostelium discoideum amoebae respond chemotactically towards the attractant cAMP. To test whether chemotaxis requires the establishment of a spatial gradient of the cytosolic calcium concentration ([Ca2+]i) we scrape-loaded calcium chelating agents with different affinities for Ca2+ into the cytosol of the cells. The buffers were 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and its derivatives. Parameters analyzed were general cell morphology and the capability to protrude pseudopods and to migrate towards a cAMP-filled capillary. The chelators dose- and time-dependently inhibited spreading of the amoebae on the substrate. Both oriented pseudopod formation and locomotion of the cells were reduced. This effect was overcome by extracellular Ca2+, but not Mg2+. The effects of BAPTA derivatives were compared to the inhibition by BAPTA. A dose-response curve was obtained; 5,5'-difluoro-BAPTA was the most potent analogue. We conclude that a [Ca2+]i-gradient is necessary for orientation and locomotion. Chemotaxis experiments performed in the presence of extracellular EGTA revealed that liberation of Ca2+ from intracellular stores is sufficient for pseudopod formation; yet under physiological conditions influx of extracellular Ca2+ is also used to establish the gradient.

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.

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+.

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)