A localized elevation of cytosolic free calcium is associated with cytokinesis in the zebrafish embryo

Localized calcium transients in phragmoplast regulate cytokinesis of tobacco BY-2 cells

KEY MESSAGE

Plants exhibit a unique pattern of cytosolic Ca2+ dynamics to correlate with microtubules to regulate cytokinesis, which significantly differs from those observed in animal and yeast cells. Calcium (Ca2+) transients mediated signaling is known to be essential in cytokinesis across eukaryotic cells. However, the detailed spatiotemporal dynamics of Ca2+ during plant cytokinesis remain largely unexplored. In this study, we employed GCaMP5, a genetically encoded Ca2+ sensor, to investigate cytokinetic Ca2+ transients during cytokinesis in Nicotiana tabacum Bright Yellow-2 (BY-2) cells. We validated the effectiveness of GCaMP5 to capture fluctuations in intracellular free Ca2+ in transgenic BY-2 cells. Our results reveal that Ca2+ dynamics during BY-2 cell cytokinesis are distinctly different from those observed in embryonic and yeast cells. It is characterized by an initial significant Ca2+ spike within the phragmoplast region. This spike is followed by a decrease in Ca2+ concentration at the onset of cytokinesis in phragmoplast, which then remains elevated in comparison to the cytosolic Ca2+ until the completion of cell plate formation. At the end of cytokinesis, Ca2+ becomes uniformly distributed in the cytosol. This pattern contrasts with the typical dual waves of Ca2+ spikes observed during cytokinesis in animal embryonic cells and fission yeasts. Furthermore, applications of pharmaceutical inhibitors for either Ca2+ or microtubules revealed a close correlation between Ca2+ transients and microtubule organization in the regulation of cytokinesis. Collectively, our findings highlight the unique dynamics and crucial role of Ca2+ transients during plant cell cytokinesis, and provides new insights into plant cell division mechanisms.

Membrane stretching activates calcium permeability of a putative channel Pkd2 during fission yeast cytokinesis

Pkd2 is the fission yeast homologue of polycystins. This putative ion channel localizes to the plasma membrane. It is required for the expansion of cell volume during interphase growth and cytokinesis, the last step of cell division. However, the channel activity of Pkd2 remains untested. Here, we examined the calcium permeability and mechanosensitivity of Pkd2 through in vitro reconstitution and calcium imaging of pkd2 mutant cells. Pkd2 was translated and inserted into the lipid bilayers of giant unilamellar vesicles using a cell-free expression system. The reconstituted Pkd2 permeated calcium when the membrane was stretched via hypoosmotic shock. In vivo, inactivation of Pkd2 through a temperature-sensitive mutation pkd2-B42 reduced the average intracellular calcium level by 34%. Compared with the wild type, the hypomorphic mutation pkd2-81KD reduced the amplitude of hypoosmotic shock-triggered calcium spikes by 59%. During cytokinesis, mutations of pkd2 reduced the calcium spikes, accompanying cell separation and the ensuing membrane stretching, by 60%. We concluded that fission yeast polycystin Pkd2 allows calcium influx when activated by membrane stretching, representing a likely mechanosensitive channel that contributes to the cytokinetic calcium spikes.

The mechanosensitive Piezo1 channel controls endosome trafficking for an efficient cytokinetic abscission

Mechanical forces are exerted throughout cytokinesis, the final step of cell division. Yet, how forces are transduced and affect the signaling dynamics of cytokinetic proteins remains poorly characterized. We now show that the mechanosensitive Piezo1 channel is activated at the intercellular bridge (ICB) connecting daughter cells to regulate abscission. Inhibition of Piezo1 caused multinucleation both in vitro and in vivo. Piezo1 positioning at the ICB during cytokinesis depends on Pacsin3. Pharmacological and genetic inhibition of Piezo1 or Pacsin3 resulted in mislocation of Rab11-family-interacting protein 3 (Rab11-FIP3) endosomes, apoptosis-linked gene 2-interacting protein X (ALIX), and endosomal sorting complex required for transport III (ESCRT-III). Furthermore, we identified FIP3 as the link between Piezo1-generated Ca²⁺ signals and ALIX delivery to the ICB, where ALIX recruits the ESCRT-III component charged multivesicular body protein 4B, which promotes abscission. These results provide a different view of how mechanical forces participate in cytokinesis and identify Piezo1 as a key modulator of endosome trafficking.

Calcium spikes accompany cleavage furrow ingression and cell separation during fission yeast cytokinesis

The role of calcium signaling in cytokinesis has long remained ambiguous. Past studies of embryonic cell division discovered that calcium concentration increases transiently at the division plane just before cleavage furrow ingression, suggesting that these calcium transients could trigger contractile ring constriction. However, such calcium transients have only been found in animal embryos and their function remains controversial. We explored cytokinetic calcium transients in the fission yeast Schizosaccharomyces pombe by adopting GCaMP, a genetically encoded calcium indicator, to determine the intracellular calcium level of this model organism. We validated GCaMP as a highly sensitive calcium reporter in fission yeast, allowing us to capture calcium transients triggered by osmotic shocks. We identified a correlation between the intracellular calcium level and cell division, consistent with the existence of calcium transients during cytokinesis. Using time-lapse microscopy and quantitative image analysis, we discovered calcium spikes both at the start of cleavage furrow ingression and the end of cell separation. Inhibition of these calcium spikes slowed the furrow ingression and led to frequent lysis of daughter cells. We conclude that like the larger animal embryos, fission yeast triggers calcium transients that may play an important role in cytokinesis (197).

Role of ion channels during cell division

Ion channels are transmembrane proteins whose canonical function is the transport of ions across the plasma membrane to regulate cell membrane potential and play an essential role in neural communication, nerve conduction, and muscle contraction. However, over the last few years, non-canonical functions have been identified for many channels, having active roles in phagocytosis, invasiveness, proliferation, among others. The participation of some channels in cell proliferation has raised the question of whether they may play an active role in mitosis. There are several reports showing the participation of channels during interphase, however, the direct participation of ion channels in mitosis has received less attention. In this article, we summarize the current evidence on the participation of ion channels in mitosis. We also summarize some tools that would allow the study of ion channels and cell cycle regulatory molecules in individual cells during mitosis.

The role of the cytoskeleton in germ plasm aggregation and compaction in the zebrafish embryo

The transmission of genetic information from one generation to another is crucial for survival of animal species. This is accomplished by the induction of primordial germ cells (PGCs) that will eventually establish the germline. In some animals the germline is induced by signals in gastrula, whereas in others it is specified by inheritance of maternal determinants, known as germ plasm. In zebrafish, aggregation and compaction of maternally derived germ plasm during the first several embryonic cell cycles is essential for generation of PGCs. These processes are controlled by cellular functions associated with the cellular division apparatus. Ribonucleoparticles containing germ plasm components are bound to both the ends of astral microtubules and a dynamic F-actin network through a mechanism integrated with that which drives the cell division program. In this chapter we discuss the role that modifications of the cell division apparatus, including the cytoskeleton and cytoskeleton-associated proteins, play in the regulation of zebrafish germ plasm assembly.

Calcium Imaging in the Zebrafish

The zebrafish (Danio rerio) has emerged as a widely used model system during the last four decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging, including calcium imaging of intracellular transients in many different tissues. While being a vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function in health and disease. Since the mid 90's, the development and neuronal function of the embryonic, larval, and later, adult zebrafish have been studied using calcium imaging methods. This updated chapter is reviewing the advances in methods and research findings of zebrafish calcium imaging during the last decade. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Synthetic calcium indicators, conjugated to dextrans and acetoxymethyl (AM) esters, are still used to label specific neuronal cell types in the hindbrain and the olfactory system. However, genetically encoded calcium indicators, such as aequorin and the GCaMP family of indicators, expressed in various tissues by the use of cell-specific promoters, are now the choice for most applications, including brain-wide imaging. Calcium imaging in the zebrafish has contributed greatly to our understanding of basic biological principles during development and adulthood, and the function of disease-related genes in a vertebrate system.

The Use of Complementary Luminescent and Fluorescent Techniques for Imaging Ca2+ Signaling Events During the Early Development of Zebrafish (Danio rerio)

We have visualized many of the Ca²⁺ signaling events that occur during the early stages of zebrafish development using complementary luminescent and fluorescent imaging techniques. We initially microinject embryos with the luminescent Ca²⁺ reporter, f-holo-aequorin, and using a custom-designed luminescent imaging system, we can obtain pan-embryonic visual information continually for up to the first ~24 h postfertilization (hpf). Once we know approximately when and where to look for these Ca²⁺ signaling events within a complex developing embryo, we then repeat the experiment using a fluorescent Ca²⁺ reporter such as calcium green-1 dextran and use confocal laser scanning microscopy to provide time-lapse series of higher-resolution images. These protocols allow us to identify the specific cell types and even the particular subcellular domain (e.g., nucleus or cytoplasm) generating the Ca²⁺ signal. Here, we outline the techniques we use to precisely microinject f-holo-aequorin or calcium green-1 dextran into embryos without affecting their viability or development. We also describe how to inject specific regions of early embryos in order to load localized embryonic domains with a particular Ca²⁺ reporter. These same techniques can also be used to introduce other membrane-impermeable reagents into embryos, including Ca²⁺ channel antagonists, Ca²⁺ chelators, fluorescent dyes, RNA, and DNA.

Slow calcium waves mediate furrow microtubule reorganization and germ plasm compaction in the early zebrafish embryo

Zebrafish germ plasm ribonucleoparticles (RNPs) become recruited to furrows of early zebrafish embryos through their association with astral microtubules ends. During the initiation of cytokinesis, microtubules are remodeled into a furrow microtubule array (FMA), which is thought to be analogous to the mammalian midbody involved in membrane abscission. During furrow maturation, RNPs and FMA tubules transition from their original distribution along the furrow to enrichments at the furrow distal ends, which facilitates germ plasm mass compaction. We show that nebel mutants exhibit reduced furrow-associated slow calcium waves (SCWs), caused at least in part by defective enrichment of calcium stores. RNP and FMA distal enrichment mirrors the medial-to-distal polarity of SCWs, and inhibition of calcium release or downstream mediators such as Calmodulin affects RNP and FMA distal enrichment. Blastomeres with reduced or lacking SCWs, such as early blastomeres in nebel mutants and wild-type blastomeres at later stages, exhibit medially bundling microtubules similar to midbodies in other cell types. Our data indicate that SCWs provide medial-to-distal directionality along the furrow to facilitate germ plasm RNP enrichment at the furrow ends.

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.

SOCE proteins, STIM1 and Orai1, are localized to the cleavage furrow during cytokinesis of the first and second cell division cycles in zebrafish embryos

In zebrafish embryos, distinct Ca2+ transients are localized to the early cleavage furrows during the first few cell division cycles. These transients are generated mainly by release via IP3Rs in the endoplasmic reticulum, and they are necessary for furrow positioning, propagation, deepening and apposition. We previously showed, via the use of inhibitors, that store-operated Ca2+ entry (SOCE) also appears to be essential for maintaining the IP3R-mediated elevated levels of [Ca2+]i for the extended periods required for the completion of successful furrow deepening and daughter cell apposition in these large embryonic cells. Here, newly fertilized, dechorionated embryos were fixed at various times during the first and second cell division cycles and immunolabelled with antibodies to STIM1 and/or Orai1 (key components of SOCE). We show that both of these proteins have a dynamic pattern of localization during cytokinesis of the first two cell division cycles. These new data help to support our previous inhibitor results, and provide additional evidence that SOCE contributes to the maintenance of the sustained elevated Ca2+ that is required for the successful completion of cytokinesis in the large cells of embryonic zebrafish.

Insecticide cytotoxicology in China: Current status and challenges

The insecticide cytotoxicology, as a new branch of toxicology, has rapidly developed in China. During the past twenty years, thousands of investigations have sprung up to evaluate the damages and clarify the mechanisms of insecticidal chemical substances to insect cells in vivo or in vitro. The mechanisms of necrosis, apoptosis or autophagy induced by synthetic or biogenic pesticides and virus infections have been systematically illuminated in many important models, including S2, BmN, SL-1, Sf21 and Sf9 cell lines. In addition, a variety of methods have also been applied to examine the effects of insecticides and elaborate the modes of action. As a result, many vital factors and pathways, such as cytochrome c, the Bcl-2 family and caspases, in mitochondrial signaling pathways, intracellular free calcium and lysosome signal pathways have been illuminated and drawn much attention. Benefiting from the application of insecticide cytotoxicology, natural products purifications, biological activities assessments of synthetic compounds and high throughput screening models have been accelerated in China. However, many questions remained, and there exist great challenges, especially in theory system, evaluation criterion, evaluation model, relationship between activity in vitro and effectiveness in vivo, and the toxicological mechanism. Fortunately, the generation of "omics" could bring opportunities for the development of insecticide cytotoxicology.

Expression and reconstitution of the bioluminescent Ca(2+) reporter aequorin in human embryonic stem cells, and exploration of the presence of functional IP3 and ryanodine receptors during the early stages of their differentiation into cardiomyocytes

In order to develop a novel method of visualizing possible Ca(2+) signaling during the early differentiation of hESCs into cardiomyocytes and avoid some of the inherent problems associated with using fluorescent reporters, we expressed the bioluminescent Ca(2+) reporter, apo-aequorin, in HES2 cells and then reconstituted active holo-aequorin by incubation with f-coelenterazine. The temporal nature of the Ca(2+) signals generated by the holo-f-aequorin-expressing HES2 cells during the earliest stages of differentiation into cardiomyocytes was then investigated. Our data show that no endogenous Ca(2+) transients (generated by release from intracellular stores) were detected in 1-12-day-old cardiospheres but transients were generated in cardiospheres following stimulation with KCl or CaCl2, indicating that holo-f-aequorin was functional in these cells. Furthermore, following the addition of exogenous ATP, an inositol trisphosphate receptor (IP3R) agonist, small Ca(2+) transients were generated from day 1 onward. That ATP was inducing Ca(2+) release from functional IP3Rs was demonstrated by treatment with 2-APB, a known IP3R antagonist. In contrast, following treatment with caffeine, a ryanodine receptor (RyR) agonist, a minimal Ca(2+) response was observed at day 8 of differentiation only. Thus, our data indicate that unlike RyRs, IP3Rs are present and continually functional at these early stages of cardiomyocyte differentiation.

Ca2+-Dependent Endoplasmic Reticulum Stress Regulates Mechanical Stress-Mediated Cartilage Thinning

Our previous study identified that endoplasmic reticulum stress (ERS) plays a critical role in chondrocyte apoptosis and mandibular cartilage thinning in response to compressive mechanical force, although the underlying mechanisms remain elusive. Because the endoplasmic reticulum (ER) is a primary site of intracellular Ca(2+) storage, we hypothesized that Ca(2+)-dependent ERS might be involved in mechanical stress-mediated mandibular cartilage thinning. In this study, we used in vitro and in vivo models to determine Ca(2+) concentrations, histological changes, subcellular changes, apoptosis, and the expression of ERS markers in mandibular cartilage and chondrocytes. The results showed that in chondrocytes, cytosolic Ca(2+) ([Ca(2+)]i) was dramatically increased by compressive mechanical force. Interestingly, the inhibition of Ca(2+) channels by ryanodine and 2-aminoethoxydiphenyl borate, inhibitors of ryanodine receptors and inositol trisphosphate receptors, respectively, partially rescued mechanical force-mediated mandibular cartilage thinning. Furthermore, chondrocyte apoptosis was also compromised by inhibiting the increase in [Ca(2+)]i that occurred in response to compressive mechanical force. Mechanistically, the ERS induced by compressive mechanical force was also repressed by [Ca(2+)]i inhibition, as demonstrated by a decrease in the expression of the ER stress markers 78 kDa glucose-regulated protein (GRP78) and 94 kDa glucose-regulated protein (GRP94) at both the mRNA and protein levels. Collectively, these data identified [Ca(2+)]i as a critical mediator of the pathological changes that occur in mandibular cartilage under compressive mechanical force and shed light on the treatment of mechanical stress-mediated cartilage degradation.

Inhibitory gene expression of the Cav3.1 T-type calcium channel to improve neuronal injury induced by lidocaine hydrochloride

Cav3.1 is a low-voltage-activated (LVA) calcium channel that plays a key role in regulating intracellular calcium ion levels. In this study, we observed the effects of lidocaine hydrochloride on the pshRNA-CACNA1G-SH-SY5Y cells that silenced Cav3.1 mRNA by RNA interference, and investigated the roles of p38 MAPK in these effects. We constructed the pNC-puro-CACNA1G-SH-SY5Y cells and pshRNA-CACNA1G -SH-SY5Y cells by the RNA interference. All the cells were cultured with or without 10mM lidocaine hydrochloride for 24 h. The cell morphology, cell viability, Cav3.1 and p38 protein expression, cell apoptosis rate and intracellular calcium ion concentration were detected. We found that all cells treated with 10mM lidocaine hydrochloride for 24 h showed cellular rounding, axonal regression, and cellular floating. Compared with the cells in SH-SY5Y+Lido group and NC+Lido group, those in the RNAi+Lido group showed similar changes, but of smaller magnitude. Additionally, following lidocaine hydrochloride all cells displayed increased Cav3.1 and p38 MAPK protein, apoptosis rate, and intracellular calcium ion levels; however,these changes in the RNAi+Lido group were less pronounced than in the SH-SY5Y+Lido and NC+Lido groups. The cell viability decreased following lidocaine hydrochloride treatment, but viability of the cells in the RNAi+Lido group was higher than in the SH-SY5Y+Lido and NC+Lido groups. The results showed that Cav3.1 may be involved in neuronal injury induced by lidocaine hydrochloride and that p38 MAPK phosphorylation was reduced upon Cav3.1 gene silencing.

[Excessive fluoride inducing calcium overload and apoptosis of ameloblasts]

OBJECTIVE

To study the effect of excessive fluoride on calcium overload and apoptosis in cultured rat ameloblasts in vitro.

METHODS

Logarithmic-phase ameloblasts (HAT-7) were treated with 0, 0.4, 0.8, 1.6, 3.2, and 6.4 mmol · L(-1) sodium fluoride (NaF) solution. Cell activities were detected by using a Cell Counting Kit 8 (CCK-8) assay after 48 h of treatment. The effect of fluoride on cell apoptosis was analyzed by using flow cytometry. Excessive fluoride-induced calcium concentration and calreticulin expression changes in ameloblasts were detected by using laser scanning confocal microscopy, Western blot analysis, and real-time quantitative polymerase chain reaction.

RESULTS

NaF inhibited ameloblast activity at 1.6, 3.2, and 6.4 mmol · L(-1) (dose-dependent) after 48 h of induction. The Ca2+ fluorescence intensity of HAT-7 cells incubated with 1.6 and 3.2 mmol · L(-1) NaF was higher than that in the control group. The fluoride-induced early-stage apoptosis of ameloblasts after 48 h of induction and the early-stage apoptosis rate was positively correlated with fluoride concentration. Calreticulin mRNA expression in HAT-7 cells was higher than that in the control group after 48 h of incubation with 0.8, 1.2, and 1.6 mmol · L(-1) NaF.

CONCLUSION

Excessive fluoride-induced calcium overload in ameloblasts and further caused endoplasmic reticulum stress-mediated apoptosis.

Calcium signaling in extraembryonic domains during early teleost development

It is becoming recognized that the extraembryonic domains of developing vertebrates, that is, those that make no cellular contribution to the embryo proper, act as important signaling centers that induce and pattern the germ layers and help establish the key embryonic axes. In the embryos of teleost fish, in particular, significant progress has been made in understanding how signaling activity in extraembryonic domains, such as the enveloping layer, the yolk syncytial layer, and the yolk cell, might help regulate development via a combination of inductive interactions, cellular dynamics, and localized gene expression. Ca(2+) signaling in a variety of forms that include propagating waves and standing gradients is a feature found in all three teleostean extraembryonic domains. This leads us to propose that in addition to their other well-characterized signaling activities, extraembryonic domains are well suited (due to their relative stability and continuity) to act as Ca(2+) signaling centers and conduits.

Effects of soybean agglutinin on mechanical barrier function and tight junction protein expression in intestinal epithelial cells from piglets

In this study, we sought to investigate the role of soybean agglutinin (SBA) in mediating membrane permeability and the mechanical barrier function of intestinal epithelial cells. The IPEC-J2 cells were cultured and treated with 0, 0.5, 1.0, 1.5, 2.0, 2.5, or 3.0 mg/mL SBA. Transepithelial electrical resistance (TEER) and alkaline phosphatase (AP) activity were measured to evaluate membrane permeability. The results showed a significant decrease in TEER values (p < 0.05) in a time- and dose-dependent manner, and a pronounced increase in AP activity (p < 0.05). Cell growth and cell morphology were used to evaluate the cell viability. A significant cell growth inhibition (p < 0.05) and alteration of morphology were observed when the concentration of SBA was increased. The results of western blotting showed that the expression levels of occludin and claudin-3 were decreased by 31% and 64% compared to those of the control, respectively (p < 0.05). In addition, immunofluorescence labeling indicated an obvious decrease in staining of these targets and changes in their localizations. In conclusion, SBA increased the membrane permeability, inhibited the cell viability and reduced the levels of tight junction proteins (occludin and claudin-3), leading to a decrease in mechanical barrier function in intestinal epithelial cells.

Biphasic assembly of the contractile apparatus during the first two cell division cycles in zebrafish embryos

The large and optically clear embryos of the zebrafish provide an excellent model system in which to study the dynamic assembly of the essential contractile band components, actin and myosin, via double fluorescent labelling in combination with confocal microscopy. We report the rapid appearance (i.e. within <2 min) of a restricted arc of F-actin patches along the prospective furrow plane in a central, apical region of the blastodisc cortex. These patches then fused with each other end-to-end forming multiple actin cables, which were subsequently bundled together forming an F-actin band. During this initial assembly phase, the F-actin-based structure did not elongate laterally, but was still restricted to an arc extending ~15° either side of the blastodisc apex. This initial assembly phase was then followed by an extension phase, where additional F-actin patches were added to each end of the original arc, thus extending it out to the edges of the blastodisc. The dynamics of phosphorylated myosin light chain 2 (MLC2) recruitment to this F-actin scaffold also reflect the two-phase nature of the contractile apparatus assembly. MLC2 was not associated with the initial F-actin arc, but MLC2 clusters were recruited and assembled into the extending ends of the band. We propose that the MLC2-free central region of the contractile apparatus acts to position and then extend the cleavage furrow in the correct plane, while the actomyosin ends alone generate the force required for furrow ingression. This biphasic assembly strategy may be required to successfully divide the early cells of large embryos.

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.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

Calcium imaging in the zebrafish

The zebrafish (Danio rerio) has emerged as a new model system during the last three decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging. While being the vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function. Since the mid 1990s, the embryonic development and neuronal function of the larval, and later, adult zebrafish have been studied using calcium imaging methods. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Dextran indicators have been used to label cells in the developing embryo from dye injection into the one-cell stage. Dextrans have also been useful for retrograde labeling of spinal cord neurons and cells in the olfactory system. Acetoxymethyl (AM) esters permit labeling of larger areas of tissue such as the tectum, a region responsible for visual processing. Genetically encoded calcium indicators have been expressed in various tissues by the use of cell-specific promoters. These studies have contributed greatly to our understanding of basic biological principles during development and adulthood, and of the function of disease-related genes in a vertebrate system.

Aequorin-based genetic approaches to visualize Ca2+ signaling in developing animal systems

BACKGROUND

In recent years, as our understanding of the various roles played by Ca2+ signaling in development and differentiation has expanded, the challenge of imaging Ca2+ dynamics within living cells, tissues, and whole animal systems has been extended to include specific signaling activity in organelles and non-membrane bound sub-cellular domains.

SCOPE OF REVIEW

In this review we outline how recent advances in genetics and molecular biology have contributed to improving and developing current bioluminescence-based Ca2+ imaging techniques. Reporters can now be targeted to specific cell types, or indeed organelles or domains within a particular cell.

MAJOR CONCLUSIONS

These advances have contributed to our current understanding of the specificity and heterogeneity of developmental Ca2+ signaling. The improvement in the spatial resolution that results from specifically targeting a Ca2+ reporter has helped to reveal how a ubiquitous signaling messenger like Ca2+ can regulate coincidental but different signaling events within an individual cell; a Ca2+ signaling paradox that until now has been hard to explain.

GENERAL SIGNIFICANCE

Techniques used to target specific reporters via genetic means will have applications beyond those of the Ca2+ signaling field, and these will, therefore, make a significant contribution in extending our understanding of the signaling networks that regulate animal development. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Calcium-release channels in paramecium. Genomic expansion, differential positioning and partial transcriptional elimination

The release of Ca²⁺ from internal stores is a major source of signal Ca²⁺ in almost all cell types. The internal Ca²⁺ pools are activated via two main families of intracellular Ca²⁺-release channels, the ryanodine and the inositol 1,4,5-trisphosphate (InsP₃) receptors. Among multicellular organisms these channel types are ubiquitous, whereas in most unicellular eukaryotes the identification of orthologs is impaired probably due to evolutionary sequence divergence. However, the ciliated protozoan Paramecium allowed us to prognosticate six groups, with a total of 34 genes, encoding proteins with characteristics typical of InsP₃ and ryanodine receptors by BLAST search of the Paramecium database. We here report that these Ca²⁺-release channels may display all or only some of the characteristics of canonical InsP₃ and ryanodine receptors. In all cases, prediction methods indicate the presence of six trans-membrane regions in the C-terminal domains, thus corresponding to canonical InsP₃ receptors, while a sequence homologous to the InsP₃-binding domain is present only in some types. Only two types have been analyzed in detail previously. We now show, by using antibodies and eventually by green fluorescent protein labeling, that the members of all six groups localize to distinct organelles known to participate in vesicle trafficking and, thus, may provide Ca²⁺ for local membrane-membrane interactions. Whole genome duplication can explain radiation within the six groups. Comparative and evolutionary evaluation suggests derivation from a common ancestor of canonical InsP₃ and ryanodine receptors. With one group we could ascertain, to our knowledge for the first time, aberrant splicing in one thoroughly analyzed Paramecium gene. This yields truncated forms and, thus, may indicate a way to pseudogene formation. No comparable analysis is available for any other, free-living or parasitic/pathogenic protozoan.

Necessary role for intracellular Ca2+ transients in initiating the apical-basolateral thinning of enveloping layer cells during the early blastula period of zebrafish development

During the early blastula period of zebrafish embryos, the outermost blastomeres begin to undergo a significant thinning in the apical/basolateral dimension to form the first distinct cellular domain of the embryo, the enveloping layer (EVL). During this shape transformation, only the EVL-precursor cells generate a coincidental series of highly restricted Ca(2+) transients. To investigate the role of these localized Ca(2+) transients in this shape-change process, embryos were treated with a Ca(2+) chelator (5,5'-difluoro BAPTA AM; DFB), or the Ca(2+) ionophore (A23187), to downregulate and upregulate the transients, respectively, while the shape-change of the forming EVL cells was measured. DFB was shown to significantly slow, and A23187 to significantly facilitate the shape change of the forming EVL cells. In addition, to investigate the possible involvement of the phosphoinositide and Wnt/Ca(2+) signaling pathways in the Ca(2+) transient generation and/or shape-change processes, embryos were treated with antagonists (thapsigargin, 2-APB and U73122) or an agonist (Wnt-5A) of these pathways. Wnt-5A upregulated the EVL-restricted Ca(2+) transients and facilitated the change in shape of the EVL cells, while 2-APB downregulated the Ca(2+) transients and significantly slowed the cell shape-change process. Furthermore, thapsigargin and U73122 also both inhibited the EVL cell shape-change. We hypothesize, therefore, that the highly localized and coincidental Ca(2+) transients play a necessary role in initiating the shape-change of the EVL cells.

Regulator of g protein signaling 3 modulates wnt5b calcium dynamics and somite patterning

Vertebrate development requires communication among cells of the embryo in order to define the body axis, and the Wnt-signaling network plays a key role in axis formation as well as in a vast array of other cellular processes. One arm of the Wnt-signaling network, the non-canonical Wnt pathway, mediates intracellular calcium release via activation of heterotrimeric G proteins. Regulator of G protein Signaling (RGS) proteins can accelerate inactivation of G proteins by acting as G protein GTPase-activating proteins (GAPs), however, the possible role of RGS proteins in non-canonical Wnt signaling and development is not known. Here, we identify rgs3 as having an overlapping expression pattern with wnt5b in zebrafish and reveal that individual knockdown of either rgs3 or wnt5b gene function produces similar somite patterning defects. Additionally, we describe endogenous calcium release dynamics in developing zebrafish somites and determine that both rgs3 and wnt5b function are required for appropriate frequency and amplitude of calcium release activity. Using rescue of gene knockdown and in vivo calcium imaging assays, we demonstrate that the activity of Rgs3 requires its ability to interact with Gα subunits and function as a G protein GAP. Thus, Rgs3 function is necessary for appropriate frequency and amplitude of calcium release during somitogenesis and is downstream of Wnt5 activity. These results provide the first evidence for an essential developmental role of RGS proteins in modulating the duration of non-canonical Wnt signaling.

Vertebrate development requires communication among cells in order to define the body axis (front/back, head/tail, or left/right). Secreted factors such as Wnts play key roles in a vast array of cellular processes, including patterning of the body axis. One arm of the Wnt-signaling network, the non-canonical pathway, mediates intracellular calcium release via activation of heterotrimeric G proteins. Regulator of G protein Signaling (RGS) proteins can accelerate inactivation of G proteins by acting as G protein GAPs and are uniquely situated to control the amplitude of a Wnt signal. Here, we combine cellular, molecular, and genetic analyses with high resolution calcium imaging to identify a role for RGS modulation of Wnt-mediated calcium release dynamics and developmental patterning events. We find that loss of rgs3 gene function produced body patterning defects like those observed with loss of wnt5b gene function. Analysis of endogenous calcium release dynamics in developing zebrafish revealed that both rgs3 and wnt5b are required for appropriate frequency and amplitude of calcium release. Our results provide new evidence that a member of the RGS protein family is essential for modulating the non-canonical Wnt network to assure normal tissue patterning during development.

Vertebrate maternal-effect genes: Insights into fertilization, early cleavage divisions, and germ cell determinant localization from studies in the zebrafish

In the earliest stages of animal development prior to the commencement of zygotic transcription, all critical cellular processes are carried out by maternally-provided molecular products accumulated in the egg during oogenesis. Disruption of these maternal products can lead to defective embryogenesis. In this review, we focus on maternal genes with roles in the fundamental processes of fertilization, cell division, centrosome regulation, and germ cell development with emphasis on findings from the zebrafish, as this is a unique and valuable model system for vertebrate reproduction.

Cancer as a metabolic disease

Emerging evidence indicates that impaired cellular energy metabolism is the defining characteristic of nearly all cancers regardless of cellular or tissue origin. In contrast to normal cells, which derive most of their usable energy from oxidative phosphorylation, most cancer cells become heavily dependent on substrate level phosphorylation to meet energy demands. Evidence is reviewed supporting a general hypothesis that genomic instability and essentially all hallmarks of cancer, including aerobic glycolysis (Warburg effect), can be linked to impaired mitochondrial function and energy metabolism. A view of cancer as primarily a metabolic disease will impact approaches to cancer management and prevention.

Cell Cycle-Dependent Localization of Voltage-Dependent Calcium Channels and the Mitotic Apparatus in a Neuroendocrine Cell Line(AtT-20)

Changes in intracellular calcium are necessary for the successful progression of mitosis in many cells. Both elevation and reduction in intracellular calcium can disrupt mitosis by mechanisms that remain ill defined. In this study we explore the role of transmembrane voltage-gated calcium channels (CaV channels) as regulators of mitosis in the mouse corticotroph cell line (AtT-20). We report that the nifedipine-sensitive isoform CaV1.2 is localized to the "poleward side" of kinetechores during metaphase and at the midbody during cytokinesis. A second nifedipine-sensitive isoform, CaV1.3, is present at the mid-spindle zone in telophase, but is also seen at the midbody. Nifedipine reduces the rate of cell proliferation, and, utilizing time-lapse microscopy, we show that this is due to a block at the prometaphase stage of the cell cycle. Using Fluo-4 we detect calcium fluxes at sites corresponding to the mid-spindle zone and the midbody region. Another calcium dye, Fura PE3/AM, causes an inhibition of mitosis prior to anaphase that we attribute to a chelation of intracellular calcium. Our results demonstrate a novel, isoform-specific localization of CaV1 channels during cell division and suggest a possible role for these channels in the calcium-dependent events underlying mitotic progression in pituitary corticotrophs.

Analysis of signaling pathways in zebrafish development by microinjection

The zebrafish oocyte differs substantially from the zygote and cleavage-stage embryo with regard to the ease with which it can be microinjected with proteins or reagents that modify subsequent development. The objective of this chapter is to describe methods developed in this and other laboratories for microinjection and calcium imaging in the unfertilized zebrafish egg. Methods of immobilizing the oocyte include a holding chamber and a holding pipette. The holding chamber allows imaging of three or four oocytes simultaneously, while the holding pipette facilitates imaging of localized regions in the oocyte. Injection of calcium green dextran via holding chambers allowed detection of global changes in Ca2+ release following fertilization and development through early blastula stages. Injection and imaging with the holding pipette method allowed discrimination of calcium changes in the egg cortex from that in the central regions of the cell. The results demonstrate the highly localized nature of calcium signaling in the zebrafish zygote and the implications of this signaling for embryonic development.

Calcium signalling during the cleavage period of zebrafish development

Imaging studies, using both luminescent and fluorescent Ca(2+)-sensitive reporters, have revealed that during the first few meroblastic cleavages of the large embryos of teleosts, localized elevations of intracellular Ca(2+) accompany positioning, propagation, deepening and apposition of the cleavage furrows. Here, we will review the Ca(2+) transients reported during the cleavage period in these embryos, with reference mainly to that of the zebrafish (Danio rerio). We will also present the latest findings that support the proposal that Ca(2+) transients are an essential feature of embryonic cytokinesis. In addition, the potential upstream triggers and downstream targets of the different cytokinetic Ca(2+) transients will be discussed. Finally, we will present a hypothetical model that summarizes what has been suggested to be the various roles of Ca(2+) signalling during cytokinesis in teleost embryos.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Multiple roles of the furrow deepening Ca2+ transient during cytokinesis in zebrafish embryos

The generation of a required series of localized Ca(2+) transients during cytokinesis in zebrafish embryos suggests that Ca(2+) plays a necessary role in regulating this process. Here, we report that cortical actin remodeling, characterized by the reorganization of the contractile band and the formation during furrow deepening of pericleavage F-actin enrichments (PAEs), requires a localized increase in intracellular Ca(2+), which is released from IP(3)-sensitive stores. We demonstrate that VAMP-2 vesicle fusion at the deepening furrow also requires Ca(2+) released via IP(3) receptors, as well as the presence of PAEs and the action of calpains. Finally, by expressing a dominant-negative form of the kinesin-like protein, kif23, we demonstrate that its recruitment to the furrow region is required for VAMP-2 vesicle transport; and via FRAP analysis, that kif23 localization is also Ca(2+)-dependent. Collectively, our data demonstrate that a localized increase in intracellular Ca(2+) is involved in regulating several key events during furrow deepening and subsequent apposition.

Cytokinesis: placing and making the final cut

Cytokinesis is the process by which cells physically separate after the duplication and spatial segregation of the genetic material. A number of general principles apply to this process. First the microtubule cytoskeleton plays an important role in the choice and positioning of the division site. Once the site is chosen, the local assembly of the actomyosin contractile ring remodels the plasma membrane. Finally, membrane trafficking to and membrane fusion at the division site cause the physical separation of the daughter cells, a process termed abscission. Here we will discuss recent advances in our understanding of the mechanisms of cytokinesis in animals, yeast, and plants.

Ca2+SIGNALLING AND EARLY EMBRYONIC PATTERNING DURING ZEBRAFISH DEVELOPMENT

1. It has been proposed that Ca2+ signalling, in the form of pulses, waves and steady gradients, may play a crucial role in key pattern-forming events during early vertebrate development. 2. With reference to the embryo of the zebrafish (Danio rerio), herein we review the Ca2+ transients reported from the cleavage to segmentation periods. This time-window includes most of the major pattern-forming events of early development, which transform a single-cell zygote into a complex multicellular embryo with established primary germ layers and body axes. 3. Data are presented to support our proposal that intracellular Ca2+ waves are an essential feature of embryonic cytokinesis and that propagating intercellular Ca2+ waves (both long and short range) may play a crucial role in: (i) the establishment of the embryonic periderm and the coordination of cell movements during epiboly, convergence and extension; (ii) the establishment of the basic embryonic axes and germ layers; and (iii) definition of the morphological boundaries of specific tissue domains and embryonic structures, including future organ anlagen. 4. The potential downstream targets of these Ca2+ transients are also discussed, as well as how they may integrate with other pattern-forming signalling pathways known to modulate early developmental events.

Molecular and functional characterization of inositol trisphosphate receptors during early zebrafish development

Fluctuations in cytosolic Ca(2+) are crucial for a variety of cellular processes including many aspects of development. Mobilization of intracellular Ca(2+) stores via the production of inositol trisphosphate (IP(3)) and the consequent activation of IP(3)-sensitive Ca(2+) channels is a ubiquitous means by which diverse stimuli mediate their cellular effects. Although IP(3) receptors have been well studied at fertilization, information regarding their possible involvement during subsequent development is scant. In the present study we examined the role of IP(3) receptors in early development of the zebrafish. We report the first molecular analysis of zebrafish IP(3) receptors which indicates that, like mammals, the zebrafish genome contains three distinct IP(3) receptor genes. mRNA for all isoforms was detectable at differing levels by the 64 cell stage, and IP(3)-induced Ca(2+) transients could be readily generated (by flash photolysis) in a controlled fashion throughout the cleavage period in vivo. Furthermore, we show that early blastula formation was disrupted by pharmacological blockade of IP(3) receptors or phospholipase C, by molecular inhibition of the former by injection of IRBIT (IP(3) receptor-binding protein released with IP(3)) and by depletion of thapsigargin-sensitive Ca(2+) stores after completion of the second cell cycle. Inhibition of Ca(2+) entry or ryanodine receptors, however, had little effect. Our work defines the importance of IP(3) receptors during early development of a genetically and optically tractable model vertebrate organism.

Ca2+ signaling and early embryonic patterning during the Blastula and Gastrula Periods of Zebrafish and Xenopus development

It has been proposed that Ca(2+) signaling, in the form of pulses, waves and steady gradients, may play a crucial role in key pattern forming events during early vertebrate development [L.F. Jaffe, Organization of early development by calcium patterns, BioEssays 21 (1999) 657-667; M.J. Berridge, P. Lipp, M.D. Bootman, The versatility and universality of calcium signaling, Nat. Rev. Mol. Cell Biol. 1 (2000) 11-21; S.E. Webb, A.L. Miller, Calcium signalling during embryonic development, Nat. Rev. Mol. Cell Biol. 4 (2003) 539-551]. With reference to the embryos of zebrafish (Danio rerio) and the frog, Xenopus laevis, we review the Ca(2+) signals reported during the Blastula and Gastrula Periods. This developmental window encompasses the major pattern forming events of epiboly, involution, and convergent extension, which result in the establishment of the basic germ layers and body axes [C.B. Kimmel, W.W. Ballard, S.R. Kimmel, B. Ullmann, T.F. Schilling, Stages of embryonic development of the zebrafish, Dev. Dyn. 203 (1995) 253-310]. Data will be presented to support the suggestion that propagating waves (both long and short range) of Ca(2+) release, followed by sequestration, may play a crucial role in: (1) Coordinating cell movements during these pattern forming events and (2) Contributing to the establishment of the basic embryonic axes, as well as (3) Helping to define the morphological boundaries of specific tissue domains and embryonic structures, including future organ anlagen [E. Gilland, A.L. Miller, E. Karplus, R. Baker, S.E. Webb, Imaging of multicellular large-scale rhythmic calcium waves during zebrafish gastrulation, Proc. Natl. Acad. Sci. USA 96 (1999) 157-161; J.B. Wallingford, A.J. Ewald, R.M. Harland, S.E. Fraser, Calcium signaling during convergent extension in Xenopus, Curr. Biol. 11 (2001) 652-661]. The various potential targets of these Ca(2+) transients will also be discussed, as well as how they might integrate with other known pattern forming pathways known to modulate early developmental events (such as the Wnt/Ca(2+)pathway; [T.A. Westfall, B. Hjertos, D.C. Slusarski, Requirement for intracellular calcium modulation in zebrafish dorsal-ventral patterning, Dev. Biol. 259 (2003) 380-391]).

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.

Recruitment and SNARE-mediated fusion of vesicles in furrow membrane remodeling during cytokinesis in zebrafish embryos

Cytokinesis is the final stage in cell division that serves to partition cytoplasm and daughter nuclei into separate cells. Membrane remodeling at the cleavage plane is a required feature of cytokinesis in many species. In animal cells, however, the precise mechanisms and molecular interactions that mediate this process are not yet fully understood. Using real-time imaging in live, early stage zebrafish embryos, we demonstrate that vesicles labeled with the v-SNARE, VAMP-2, are recruited to the cleavage furrow during deepening in a microtubule-dependent manner. These vesicles then fuse with, and transfer their VAMP-2 fluorescent label to, the plasma membrane during both furrow deepening and subsequent apposition. This observation indicates that new membrane is being inserted during these stages of cytokinesis. Inhibition of SNAP-25 (a cognate t-SNARE of VAMP-2), using a monoclonal antibody, blocked VAMP-2 vesicle fusion and furrow apposition. Transient expression of mutant forms of SNAP-25 also produced defects in furrow apposition. SNAP-25 inhibition by either method, however, did not have any significant effect on furrow deepening. Thus, our data clearly indicate that VAMP-2 and SNAP-25 play an essential role in daughter blastomere apposition, possibly via the delivery of components that promote the cell-to-cell adhesion required for the successful completion of cytokinesis. Our results also support the idea that new membrane addition, which occurs during late stage cytokinesis, is not required for furrow deepening that results from contractile band constriction.

Requirement for a localized, IP3R-generated Ca2+transient during the furrow positioning process in zebrafish zygotes

We report that the first localized Ca2+transient visualized in the blastodisc cortex of post-mitotic zebrafish zygotes has unique features. We confirm that this initial ‘furrow positioning’ Ca2+transient precedes the physical appearance of the first cleavage furrow at the blastodisc surface and that it has unique dynamics, which distinguish it from the subsequent furrow propagation transients that develop from it. This initial transient displays a distinct rising phase that peaks prior to the initiation of the two linear, subsurface, self-propagating Ca2+waves that constitute the subsequent furrow propagation transient. Through the carefully timed introduction of the Ca2+buffer, dibromo-BAPTA, we also demonstrate the absolute requirement of this initial rising phase Ca2+transient in positioning the furrow at the blastodisc surface: no rising phase transient, no cleavage furrow. Likewise, the introduction of the inositol 1,4,5-trisphosphate receptor (IP3R) antagonist, 2-aminoethoxydiphenyl borate, eliminates both the rising phase transient and the appearance of the furrow at the cell surface. On the other hand, antagonists of the ryanodine receptor and NAADP-sensitive channels, or simply bathing the zygote in Ca2+-free medium, have no effect on the generation of the rising phase positioning transient or the appearance of the furrow at the surface. This suggests that like the subsequent propagation and deepening/zipping Ca2+transients, the rising phase furrow positioning transient is also generated specifically by Ca2+released via IP3Rs. We propose, however, that despite being generated by a similar Ca2+release mechanism, the unique features of this initial transient suggest that it might be a distinct signal with a specific function associated with positioning the cleavage furrow at the blastodisc surface.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

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.