Imaging of multicellular large-scale rhythmic calcium waves during zebrafish gastrulation

II. Model building: an electrical theory of control of growth and development in animals, prompted by studies of exogenous magnetic field effects (paper I), and evidence of DNA current conduction, in vitro

A theory of control of cellular proliferation and differentiation in the early development of metazoan systems, postulating a system of electrical controls "parallel" to the processes of molecular biochemistry, is presented. It is argued that the processes of molecular biochemistry alone cannot explain how a developing organism defies a stochastic universe. The demonstration of current flow (charge transfer) along the long axis of DNA through the base-pairs (the "pi-way) in vitro raises the question of whether nature may employ such current flows for biological purposes. Such currents might be too small to be accessible to direct measurement in vivo but conduction has been measured in vitro, and the methods might well be extended to living systems. This has not been done because there is no reasonable model which could stimulate experimentation. We suggest several related, but detachable or independent, models for the biological utility of charge transfer, whose scope admittedly outruns current concepts of thinking about organization, growth, and development in eukaryotic, metazoan systems. The ideas are related to explanations proposed to explain the effects demonstrated on tumors and normal tissues described in Article I (this issue). Microscopic and mesoscopic potential fields and currents are well known at sub-cellular, cellular, and organ systems levels. Not only are such phenomena associated with internal cellular membranes in bioenergetics and information flow, but remarkable long-range fields over tissue interfaces and organs appear to play a role in embryonic development (Nuccitelli, 1992 ). The origin of the fields remains unclear and is the subject of active investigation. We are proposing that similar processes could play a vital role at a "sub-microscopic level," at the level of the chromosomes themselves, and could play a role in organizing and directing fundamental processes of growth and development, in parallel with the more discernible fields and currents described.

Wnt/Fz signaling and the cytoskeleton: potential roles in tumorigenesis

Wnt/beta-catenin regulates cellular functions related to tumor initiation and progression, cell proliferation, differentiation, survival, and adhesion. Beta-catenin-independent Wnt pathways have been proposed to regulate cell polarity and migration, including metastasis. In this review, we discuss the possible roles of both beta-catenin-dependent and -independent signaling pathways in tumor progression, with an emphasis on their regulation of Rho-family GTPases, cytoskeletal remodeling, and relationships with cell-cell adhesion and cilia/ciliogenesis.

Inhibition of stored Ca2+ release disrupts convergence-related cell movements in the lateral intermediate mesoderm resulting in abnormal positioning and morphology of the pronephric anlagen in intact zebrafish embryos

Ca(2+) is a highly versatile intra- and intercellular signal that has been reported to regulate a variety of different pattern-forming processes during early development. To investigate the potential role of Ca(2+) signaling in regulating convergence-related cell movements, and the positioning and morphology of the pronephric anlagen, we treated zebrafish embryos from 11.5 h postfertilization (hpf; i.e. just before the pronephric anlagen are morphologically distinguishable in the lateral intermediate mesoderm; LIM) to 16 hpf, with a variety of membrane permeable pharmacological reagents known to modulate [Ca(2+)](i). The effect of these treatments on pronephric anlagen positioning and morphology was determined in both fixed and live embryos via in situ hybridization using the pronephic-specific probes, cdh17, pax2.1 and sim1, and confocal imaging of BODIPY FL C(5)-ceramide-labeled embryos, respectively. We report that Ca(2+) released from intracellular stores via inositol 1,4,5-trisphosphate receptors plays a significant role in the positioning and morphology of the pronephric anlagen, but does not affect the fate determination of the LIM cells that form these primordia. Our data suggest that when Ca(2+) release is inhibited, the resulting effects on the pronephric anlagen are a consequence of the disruption of normal convergence-related movements of LIM cells toward the embryonic midline.

A novel role for MAPKAPK2 in morphogenesis during zebrafish development

One of the earliest morphogenetic processes in the development of many animals is epiboly. In the zebrafish, epiboly ensues when the animally localized blastoderm cells spread, thin over, and enclose the vegetally localized yolk. Only a few factors are known to function in this fundamental process. We identified a maternal-effect mutant, betty boop (bbp), which displays a novel defect in epiboly, wherein the blastoderm margin constricts dramatically, precisely when half of the yolk cell is covered by the blastoderm, causing the yolk cell to burst. Whole-blastoderm transplants and mRNA microinjection rescue demonstrate that Bbp functions in the yolk cell to regulate epiboly. We positionally cloned the maternal-effect bbp mutant gene and identified it as the zebrafish homolog of the serine-threonine kinase Mitogen Activated Protein Kinase Activated Protein Kinase 2, or MAPKAPK2, which was not previously known to function in embryonic development. We show that the regulation of MAPKAPK2 is conserved and p38 MAP kinase functions upstream of MAPKAPK2 in regulating epiboly in the zebrafish embryo. Dramatic alterations in calcium dynamics, together with the massive marginal constrictive force observed in bbp mutants, indicate precocious constriction of an F-actin network within the yolk cell, which first forms at 50% epiboly and regulates epiboly progression. We show that MAPKAPK2 activity and its regulator p38 MAPK function in the yolk cell to regulate the process of epiboly, identifying a new pathway regulating this cell movement process. We postulate that a p38 MAPKAPK2 kinase cascade modulates the activity of F-actin at the yolk cell margin circumference allowing the gradual closure of the blastopore as epiboly progresses.

One of the earliest cell movement processes in the development of many animals is epiboly. In the zebrafish, epiboly ensues when the blastoderm cells spread over and enclose the yolk cell. Only a few factors are known to function in this fundamental process. We identified a maternal-effect mutant, betty boop (bbp), which displays a novel defect in epiboly, wherein the blastoderm margin constricts dramatically, precisely when half of the yolk cell is covered by the blastoderm, causing the yolk cell to burst. We demonstrate that Bbp functions in the yolk cell to regulate epiboly. We positionally cloned the bbp mutant gene and identified it as the serine-threonine kinase Mitogen Activated Protein Kinase Activated Protein Kinase 2, or MAPKAPK2, which was not previously known to function in embryonic development. We show that the regulation of MAPKAPK2 is conserved within a p38 MAP kinase pathway, thus identifying a new pathway in the regulation of this fundamental cell movement process. We postulate that a p38 MAPKAPK2 kinase cascade modulates F-actin contraction at the yolk cell margin circumference, allowing the gradual closure of the cells over the yolk cell as epiboly progresses.

Timing in cellular Ca2+ signaling

Calcium (Ca2+) signals are generated across a broad time range. Kinetic considerations impact how information is processed to encode and decode Ca2+ signals, the choreography of responses that ensure specific and efficient signaling and the overall temporal amplification such that ephemeral Ca2+ signals have lasting physiological value. The reciprocal importance of timing for Ca2+ signaling, and Ca2+ signaling for timing is exemplified by the altered kinetic profiles of Ca2+ signals in certain diseases and the likely role of basal Ca2+ fluctuations in the perception of time itself.

Chemical technologies for probing embryonic development

Embryogenesis is a remarkable program of cell proliferation, migration, and differentiation that transforms a single fertilized egg into a complex multicellular organism. Understanding this process at the molecular and systems levels will require an interdisciplinary approach, including the concepts and technologies of chemical biology. This tutorial review provides an overview of chemical tools that have been used in developmental biology research, focusing on methods that enable spatiotemporal control of gene function and the visualization of embryonic patterning. Limitations of current approaches and future challenges are also discussed.

Calcium dynamics integrated into signalling pathways that influence vertebrate axial patterning

Many aspects of animal development including fertilization as well as organ formation and function are dependent upon the dynamic release of calcium (Ca(2+)) ions. Although the controlled release and/or accumulation of Ca(2+) ions has been extensively studied, how the release dynamics produce a specific biological output in embryonic development is less clear. We will briefly summarize Ca(2+) sources, highlight data on endogenous Ca(2+) release in vertebrate embryos relevant to body plan formation and cell movement, and integrate pharmacological and molecular-genetic studies to lend insight into the signalling pathways involved. Finally, based on in vivo imaging in zebrafish genetic mutants, we will put forward the model that distinct Ca(2+) release dynamics lead to antagonism of the developmentally important Wnt/beta-catenin signalling pathway, while sustained Ca(2+) release modulates cell polarization or directed migration.

Automated detection of intercellular signaling in astrocyte networks using the converging squares algorithm

Intercellular calcium waves in central nervous system astrocyte networks underline the principle mechanism of cell signaling in astrocyte syncsytiums, which putatively contribute to the modulation of neuronal signaling and metabolic regulation. In support of carrying out systems level analyses of astrocyte networks, we have optimized and validated the converging squares image segmentation algorithm to automatically detect the relative spatial locations of all cells in a visible network as a preliminary step towards analyzing the dynamics of astrocyte intracellular calcium transients, which are the signals that mediate intercellular calcium waves. We used the temporal derivatives of pixel intensities as the data source for the algorithm. The method works by converging progressively smaller squares until the signal peak is reached. It is robust to noise and performs comparably to manual cell signal identification, but is much faster and efficient. This is the first reported application of this algorithm to glial networks that we are aware of.

Beta-catenin-independent Wnt pathways: signals, core proteins, and effectors

Wnt signaling activates several distinct intracellular pathways, which are important for cell proliferation, differentiation, and polarity. Wnt proteins are secreted molecules that typically signal across the membrane via interaction with the transmembrane receptor Frizzled. Following interaction with Frizzled, the downstream effect of the most widely studied Wnt pathway is stabilization and nuclear translocation of the cytosolic protein, beta-catenin. In this chapter, we discuss two beta-catenin-independent branches of Wnt signaling: 1) Wnt/planar cell polarity (PCP), a Wnt pathway that signals through the small GTPases, Rho and Rac, to promote changes in the actin cytoskeleton, and 2) Wnt/Ca2+, a Wnt pathway that promotes intracellular calcium transients and negatively regulates the Wnt/beta-catenin pathway. Finally, during the course of our discussion, we highlight areas that require future research.

Calcium fluxes in dorsal forerunner cells antagonize beta-catenin and alter left-right patterning

Establishment of the left-right axis is essential for normal organ morphogenesis and function. Ca(2+) signaling and cilia function in the zebrafish Kuppfer's Vesicle (KV) have been implicated in laterality. Here we describe an endogenous Ca(2+) release event in the region of the KV precursors (dorsal forerunner cells, DFCs), prior to KV and cilia formation. Manipulation of Ca(2+) release to disrupt this early flux does not impact early DFC specification, but results in altered DFC migration or cohesion in the tailbud at somite stages. This leads to disruption of KV formation followed by bilateral expression of asymmetrical genes, and randomized organ laterality. We identify beta-catenin inhibition as a Ca(2+)-signaling target and demonstrate that localized loss of Ca(2+) within the DFC region or DFC-specific activation of beta-catenin is sufficient to alter laterality in zebrafish. We identify a previously unknown DFC-like cell population in Xenopus and demonstrate a similar Ca(2+)-sensitive stage. As in zebrafish, manipulation of Ca(2+) release results in ectopic nuclear beta-catenin and altered laterality. Overall, our data support a conserved early Ca(2+) requirement in DFC-like cell function in zebrafish and Xenopus.

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.

Bioluminescent imaging of Ca2+ activity reveals spatiotemporal dynamics in glial networks of dark-adapted mouse retina

Glial Ca(2+) excitability plays a key role in reciprocal neuron-glia communication. In the retina, neuron-glia signalling is expected to be maximal in the dark, but the glial Ca(2+) signal characteristics under such conditions have not been evaluated. To address this question, we used bioluminescence imaging to monitor spontaneous Ca(2+) changes under dark conditions selectively in Müller cells, the principal retinal glial cells. By combining this imaging approach with network analysis, we demonstrate that activity in Müller cells is organized in networks of coactive cells, involving 2-16 cells located distantly and/or in clusters. We also report that spontaneous activity of small networks (2-6 Müller cells) repeat over time, sometimes in the same sequential order, revealing specific temporal dynamics. In addition, we show that networks of coactive glial cells are inhibited by TTX, indicating that ganglion and/or amacrine neuronal cells probably regulate Müller cell network properties. These results represent the first demonstration that spontaneous activity in adult Müller cells is patterned into correlated networks that display repeated sequences of coactivations over time. Furthermore, our bioluminescence technique provides a novel tool to study the dynamic characteristics of glial Ca(2+) events in the retina under dark conditions, which should greatly facilitate future investigations of retinal dark-adaptive processes.

Differential expression of CaMK-II genes during early zebrafish embryogenesis

CaMK-II is a highly conserved Ca(2+)/calmodulin-dependent protein kinase expressed throughout the lifespan of all vertebrates. During early development, CaMK-II regulates cell cycle progression and "non-canonical" Wnt-dependent convergent extension. In the zebrafish, Danio rerio, CaMK-II activity rises within 2 hr after fertilization. At the time of somite formation, zygotic expression from six genes (camk2a1, camk2b1, camk2g1, camk2g2, camk2d1, camk2d2) results in a second phase of increased activity. Zebrafish CaMK-II genes are 92-95% identical to their human counterparts in the non-variable regions. During the first three days of development, alternative splicing yields at least 20 splice variants, many of which are unique. Whole-mount in situ hybridization reveals that camk2g1 comprises the majority of maternal expression. All six genes are expressed strongly in ventral regions at the 18-somite stage. Later, camk2a1 is expressed in anterior somites, heart, and then forebrain. Camk2b1 is expressed in somites, mid- and forebrain, gut, retina, and pectoral fins. Camk2g1 appears strongly along the midline and then in brain, gut, and pectoral fins. Camk2g2 is expressed early in the midbrain and trunk and exhibits the earliest retinal expression. Camk2d1 is elevated early at somite boundaries, then epidermal tissue, while camk2d2 is expressed in discrete anterior locations, steadily increasing along either side of the dorsal midline and then throughout the brain, including the retina. These findings reveal a complex pattern of CaMK-II gene expression consistent with pleiotropic roles during development.

Calcium signaling in vertebrate embryonic patterning and morphogenesis

Signaling pathways that rely on the controlled release and/or accumulation of calcium ions are important in a variety of developmental events in the vertebrate embryo, affecting cell fate specification and morphogenesis. One such major developmentally important pathway is the Wnt/calcium signaling pathway, which, through its antagonism of Wnt/beta-catenin signaling, appears to regulate the formation of the early embryonic organizer. In addition, the Wnt/calcium pathway shares components with another non-canonical Wnt pathway involved in planar cell polarity, suggesting that these two pathways form a loose network involved in polarized cell migratory movements that fashion the vertebrate body plan. Furthermore, left-right axis determination, neural induction and somite formation also display dynamic calcium release, which may be critical in these patterning events. Finally, we summarize recent evidence that propose a role for calcium signaling in stem cell biology and human developmental disorders.

The spatial distribution of inositol 1,4,5-trisphosphate receptor isoforms shapes Ca2+ waves

Cytosolic Ca(2+) is a versatile second messenger that can regulate multiple cellular processes simultaneously. This is accomplished in part through Ca(2+) waves and other spatial patterns of Ca(2+) signals. To investigate the mechanism responsible for the formation of Ca(2+) waves, we examined the role of inositol 1,4,5-trisphosphate receptor (InsP3R) isoforms in Ca(2+) wave formation. Ca(2+) signals were examined in hepatocytes, which express the type I and II InsP3R in a polarized fashion, and in AR4-2J cells, a nonpolarized cell line that expresses type I and II InsP3R in a ratio similar to what is found in hepatocytes but homogeneously throughout the cell. Expression of type I or II InsP3R was selectively suppressed by isoform-specific DNA antisense in an adenoviral delivery system, which was delivered to AR4-2J cells in culture and to hepatocytes in vivo. Loss of either isoform inhibited Ca(2+) signals to a similar extent in AR4-2J cells. In contrast, loss of the basolateral type I InsP3R decreased the sensitivity of hepatocytes to vasopressin but had little effect on the initiation or spread of Ca(2+) waves across hepatocytes. Loss of the apical type II isoform caused an even greater decrease in the sensitivity of hepatocytes to vasopressin and resulted in Ca(2+) waves that were much slower and delayed in onset. These findings provide evidence that the apical concentration of type II InsP3Rs is essential for the formation of Ca(2+) waves in hepatocytes. The subcellular distribution of InsP3R isoforms may critically determine the repertoire of spatial patterns of Ca(2+) signals.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

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]).

Voltage-dependent calcium influx mediates maturation of myofibril arrangement in ascidian larval muscle

Calcium signaling is important for multiple events during embryonic development. However, roles of calcium influx during embryogenesis have not been fully understood since routes of calcium influx are often redundant. To define roles of voltage-gated calcium channel (Cav) during embryogenesis, we have isolated an ascidian Cav beta subunit gene (TuCavbeta) and performed gene knockdown using the morpholino antisense oligonucleotide (MO). The suppression of Cav activity by TuCavbetaMO remarkably perturbed gastrulation and tail elongation. Further, larvae with normal morphology also failed to exhibit motility. Phalloidin-staining showed that arrangement of myofibrils was uncoordinated in muscle cells of TuCavbetaMO-injected larvae with normal tail. To further understand the roles of Cav activity in myofibrillogenesis, we tested pharmacological inhibitions with ryanodine, curare, and N-benzyl-p-toluensulphonamide (BTS). The treatment with ryanodine, an intracellular calcium release blocker, did not significantly affect the motility and establishment of the myofibril orientation. However, treatment with curare, an acetylcholine receptor blocker, and BTS, an actomyosin ATPase specific inhibitor, led to abnormal motility and irregular orientation of myofibrils that was similar to those of TuCavbetaMO-injected larvae. Our results suggest that contractile activation regulated by voltage-dependent calcium influx but not by intracellular calcium release is required for proper arrangement of myofibrils.

Ca2+ signaling during vertebrate somitogenesis

A variety of Ca2+ signals, in the form of intercellular pulses and waves, have been reported to be associated with the various sequential stages of somitogenesis: from convergent extension and the formation of the paraxial mesoderm; during the patterning of the paraxial mesoderm to establish segmental units; throughout the formation of the morphological boundaries that delineate the segmental units, and finally from within the maturing somites as they undergo subsequent development and differentiation. Due to both the technical challenges presented in imaging intact, developing embryos, and the subtle nature of the Ca2+ transients generated, they have proved to be difficult to visualize. However, a combination of cultured cell preparations and improvements in explant and whole embryo imaging techniques has begun to reveal a new and exciting class of developmental Ca2+ signals. In this chapter, we review the small, but expanding, number of reports in the literature and attempt to identify common characteristics of the somitogenic Ca2+ transients, such as their mode of generation, as well as their spatial and temporal features. This may help to elucidate the significance and function of these intriguing Ca2+ transients and thus integrate them into the complex signaling networks that orchestrate early developmental events.

Role of Fyn kinase in signaling associated with epiboly during zebrafish development

The function of Fyn kinase during zebrafish development through the blastula stage was investigated through the use of dominant-negative constructs designed to suppress the function of zebrafish c-Fyn. Microinjection of SH2 domain-containing fusion protein or mRNA encoding the mutated, catalytically inactive Fyn at 45 min post-insemination had no significant effect during cleavage and did not inhibit formation of the yolk syncitial layer. Smoothing of the enveloping cell layer at the midblastula transition occurred normally and expression of bon/mixer and mezzo, zygotic transcription factors indicated that activation of the zygotic genome did occur. Signaling pathways involved with axis determination such as beta-catenin, activin, and nodal appeared to function normally as evidenced by expression of boz, goosecoid, and mezzo. However, while formation of the yolk syncitial layer was normal, the marginal blastomeres failed to migrate toward the vegetal pole and epiboly did not occur, a phenotype similar but distinct from that resulting from suppression of c-Yes kinase. The block to development was prevented by co-injection of c-Fyn mRNA with the dominant-negative construct indicating that it was a specific effect. Injection of the dominant-negative mRNA into individual blastomeres indicated that the effect was exerted on the intrinsic ability of the individual blastomeres to respond to signals directing epiboly and not on the signals themselves. Analysis of the pattern of calcium signaling in experimental and control embryos demonstrated that the elevated [Ca2+]i characteristic of the marginal blastomeres was suppressed. Together, these observations indicate that Fyn kinase plays an important role in epiboly, possibly through its effects in calcium signaling.

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.

Calcium transients and neural induction in vertebrates

Evidence indicates that a variety of different types of Ca2+ transients (i.e., standing gradients, pulses and propagating waves) may be occurring both simultaneously and sequentially during neural induction in vertebrate embryos. Transients have been observed in the dorsal marginal zone and in the presumptive neural ectoderm, suggesting that they may be generated within two distinct germ layers at separate embryological locations. It has been proposed that the Ca2+ transients might have multiple roles during the period of neural induction, ranging from: activating the expression of early neural genes; contributing to the inhibition of BMP-4 signalling; generating secretion gradients of morphogens; regulating and co-ordinating convergent extension; and establishing and reinforcing dorsoventral axis specification. Both intra- and extracellular stores (either acting separately or in combination) have been shown to generate the neuralizing Ca2+ transients via well-established release mechanisms, and transients have been shown to propagate between connected cells, suggesting an intercellular signalling dimension. Thus, good evidence is accumulating to suggest that Ca2+ might be a key central regulator in the process of neural induction.

Left-right asymmetry in embryonic development: a comprehensive review

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well characterized, the left-right (LR) axis has only recently begun to be understood at the molecular level. The mechanisms which ensure invariant LR asymmetry of the heart, viscera, and brain represent a thread connecting biomolecular chirality to human cognition, along the way involving fundamental aspects of cell biology, biophysics, and evolutionary biology. An understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes. This review summarizes the current knowledge regarding LR patterning in a number of vertebrate and invertebrate species, discusses several poorly understood but important phenomena, and highlights some important open questions about the evolutionary origin and conservation of mechanisms underlying embryonic asymmetry.

Xenopus p21-activated kinase 5 regulates blastomeres' adhesive properties during convergent extension movements

The p21-activated kinase (PAK) proteins regulate many cellular events including cell cycle progression, cell death and survival, and cytoskeleton rearrangements. We previously identified X-PAK5 that binds the actin and microtubule networks, and could potentially regulate their coordinated dynamics during cell motility. In this study, we investigated the functional importance of this kinase during gastrulation in Xenopus. X-PAK5 is mainly expressed in regions of the embryo that undergo extensive cell movements during gastrula such as the animal hemisphere and the marginal zone. Expression of a kinase-dead mutant inhibits convergent extension movements in whole embryos and in activin-treated animal cap by modifying behavior of cells. This phenotype is rescued in embryo by adding back X-PAK5 catalytic activity. The active kinase decreases cell adhesiveness when expressed in animal hemisphere and inhibits the calcium-dependent reassociation of cells, while dead X-PAK5 kinase localizes to cell-cell junctions and increases cell adhesion. In addition, endogenous X-PAK5 colocalizes with adherens junction proteins and its activity is regulated by extracellular calcium. Taken together, our results suggest that X-PAK5 regulates convergent extension movements in vivo by modulating the calcium-mediated cell-cell adhesion.

Approaches to measuring calcium in zebrafish: focus on neuronal development

Calcium ions are known to act as important cellular signals during nervous system development. In vitro studies have provided significant information on the role of calcium signals during neuronal development; however, the function of this messenger in nervous system maturation in vivo remains to be established. The zebrafish has emerged as a valuable model for the study of vertebrate embryogenesis. Fertilisation is external and the rapid growth of the transparent embryo, including development of internal organs, can be observed easily making it well suited for imaging studies. The developing nervous system is relatively simple and has been well characterised, allowing individual neurons to be identified. Using the zebrafish model, both intracellular and intercellular calcium signals throughout embryonic development have been characterised. This review summarises technical approaches to measure calcium signals in developing embryonic and larval zebrafish, and includes recent developments that will facilitate the study of calcium signalling in vivo. The application of calcium imaging techniques to investigate the action of this messenger during embryogenesis in intact zebrafish is illustrated by discussion of their contribution to our understanding of neuronal development in vivo.

The calcium pump of the endoplasmic reticulum plays a role in midline signaling during early zebrafish development

During early vertebrate development, a signaling network is activated along the midline of the embryo. This signaling network induces the neural tube floor plate and ventral brain regions. In turn, induction of the ventral brain region is important for bilateral division of the forebrain and bilateral separation of the eyes. The present study provides direct evidence for a role of the endoplasmic reticulum Ca(2+) pump in zebrafish midline signaling. The endoplasmic reticulum Ca(2+) pump was inhibited in zebrafish embryos using thapsigargin or cyclopiazonic acid. Inhibition of the endoplasmic reticulum Ca(2+) pump during early gastrulation induces cyclopia, mimicking defects observed in cyclops, squint, one-eyed pinhead, and silberblick mutant embryos. In contrast, inhibition of the endoplasmic reticulum Ca(2+) pump during mid-gastrulation does not induce cyclopia, but does induce tail defects, mimicking defects observed in no-tail mutant embryos. This study is the first to relate thapsigargin and cyclopiazonic acid with induction of cyclopia. In addition, obtained results provide new information on the roles of Ca(2+) in embryonic development and may lead to new insights on the mechanisms underlying holoprosencephaly, a relatively common brain defect in human development.

Organization and function of microfilaments during late epiboly in zebrafish embryos

We report that, during epiboly in zebrafish, three F-actin--based structures appear only after the blastoderm migrates past the embryonic equator. They are composed of two ring-like F-actin structures that form at the deep cell and enveloping layer margins of the blastoderm and a punctate actin band that develops in the external yolk syncytial layer. Treatment with cytochalasin B or the calcium chelator dibromo-BAPTA results in the disruption of all three of these actin-based structures, leading to the slowing or immediate arrest of epiboly, respectively, followed by a failure of yolk cell occlusion and the eventual lysis of the embryo through the vegetal pole region. We suggest, therefore, that these structures function in the occlusion of the vegetal portion of the yolk cell during the latter stages of epiboly. Possible roles for these new structures, their modulation by Ca2+, as well as the functions of other previously described F-actin--based structures observed throughout epiboly, are discussed.

Systematic screening for signaling molecules expressed during somitogenesis by the signal sequence trap method

We describe a systematic screening to search for molecules that act as an extracellular signal during somitogenesis in vertebrates. Somitogenesis, which gives rise to segmented structures of axial bones and muscles, is a consequence of cooperative morphogenetic movements caused by precisely regulated cell and tissue interactions. We employed a strategy that combined subtractive hybridization to enrich paraxial mesoderm/somite-specific cDNAs and the signal sequence trap method, which selects signal sequence-containing molecules. Ninety-two independent cDNAs found to possess a putative signal sequence or a transmembrane domain are presented with a data base accession number for each. These clones include cDNAs which were previously identified with a function characterized, cDNAs previously identified with an undetermined function, and also cDNAs with no similarity to known sequences. Among them, 16 clones exhibited peculiar patterns of expression in the presomitic mesoderm/somites revealed by whole-mount and section in situ hybridization techniques, with some clones also being expressed in the forming neural tube. This is the first report in which an elaborate strategy combining three different screening steps was employed to identify signaling molecules relevant to a particular morphogenetic process.

Calcium signalling during embryonic development

Consider a hypothetical design specification for an integrated communication-control system within an embryo. It would require short-range (subcellular) and long-range (pan-embryonic) abilities, it would have to be flexible and, at the same time, robust enough to operate in a dynamically changing environment without information being lost or misinterpreted. Although many signalling elements appear, disappear and sometimes reappear during development, it is becoming clear that embryos also depend on a ubiquitous, persistent and highly versatile signalling system that is based around a single messenger, Ca2+.

Localization of the nitric oxide/cGMP signaling pathway-related genes and influences of morpholino knock-down of soluble guanylyl cyclase on medaka fish embryogenesis

To better understand the nitric oxide (NO) / cyclic GMP (cGMP) signaling pathway during embryogenesis, we examined the spatial and temporal expression pattern of the genes for neuronal nitric oxide synthase (nNOS), soluble guanylyl cyclase (soluble GC) subunit (OlGCS-alpha(1), OlGCS-alpha(2), and OlGCS-beta(1)), and cGMP-dependent protein kinase (cGK) I and II (cGK I and cGK II) in the medaka fish embryos. OlGCS-beta(1) and nNOS were expressed maternally and OlGCS-alpha(1), OlGCS-alpha(2),and cGK II were expressed zygotically. The zygotic expression of OlGCS-alpha(1) and cGK I was detected at stage 19, while that of OlGCS-alpha(2) was detected at stage 16. Whole-mount in situ hybridization showed that the expression of nNOS or cGK I was localized in tail bud, otic vesicles, thyroid, and brain ventricle, or in thymus, gill arch, and olfactory pits, respectively, and that of OlGCS-alpha(1), OlGCS-alpha(2), or OlGCS-beta(1) was dim and dispersed throughout the embryos. To clarify the "role of the NO/cGMP signaling pathway in embryogenesis, we examined the influences of morpholino antisense oligonucleotide of the soluble GC subunit gene (alpha(1)-MO, alpha(2)-MO or beta(1)-MO) on development of medaka fish embryos. Embryos injected with alpha(1)-MO or alpha(2)-MO mainly exhibited abnormalities in the central nervous system, including defects in the formation of forebrain, eye, and otic vesicles. alpha(2)-MO injection caused cell death at the tail bud of the embryos at stage 22, and beta(1)-MO injection inhibited the development of the embryos at late blastula. These results suggest that the NO/cGMP signaling pathway plays critical roles in early embryogenesis.

Spontaneous activity-independent intracellular calcium signals in the developing spinal cord of the zebrafish embryo

Calcium signals play an important role in a variety of processes necessary for neuronal development. Whilst the characteristics and function of calcium signals have been comprehensively examined in vitro, the significance of these signals during development in an intact embryo remains unclear. In this study, we have examined the spatial and temporal patterns of intracellular calcium signals in precursor cells (cells without processes) within the spinal cord of the intact zebrafish embryo aged between 17 and 27 h. In total, approximately one-third of cells displayed spontaneous intracellular calcium transients. The calcium transients had an average peak amplitude of 33.3 (+/-2.8%) above baseline, a duration of 52.2 (+/-6.3 s) and occurred with an average frequency of 4.6 (+/-0.4 per hour). Calcium transients were observed in precursor cells located throughout the spinal cord, with the highest percentage of active cells (35.1+/-8%) occurring at a developmental time of 21-22 h. Furthermore these intracellular calcium signals were observed in the presence of tricaine, indicating that they are not generated via sodium-dependent action potentials. In precursor cells loaded with the calcium buffer BAPTA both the frequency and the amplitude of the calcium transients was significantly reduced. The intracellular calcium transients may represent a common activity-independent calcium-mediated mechanism that contributes to the regulation of neuronal development in the spinal cord of the zebrafish embryo during the segmentation and early pharyngula period.

Control of intercalation is cell-autonomous in the notochord of Ciona intestinalis

Dishevelled signaling plays a critical role in the control of cell intercalation during convergent extension in vertebrates. This study presents evidence that Dishevelled serves a similar function in the Ciona notochord. Embryos transgenic for mutant Dishevelled fail to elongate their tails, and notochord cells fail to intercalate, though notochord cell fates are unaffected. Analysis of mosaic transgenics revealed that the effects of mutant Dishevelled on notochord intercalation are cell-autonomous in Ciona, though such defects have nonautonomous effects in Xenopus. Furthermore, our data indicate that notochord cell intercalation in Ciona does not require the progressive signals which coordinate cell intercalation in the Xenopus notochord, highlighting an important difference in how mediolateral cell intercalation is controlled in the two animals. Finally, this study establishes the Ciona embryo as an effective in vivo system for the study of the molecular control of morphogenetic cell movements in chordates.

Non-canonical Wnt signaling in Xenopus: regulation of axis formation and gastrulation

Wnt proteins form a family of secreted glycoproteins that are involved in different developmental processes such as differentiation, proliferation, cell migration and cell polarity. To exert its function, Wnt proteins activate different intracellular signaling cascades. Whereas the canonical, Wnt/beta-catenin pathway is well characterized, less is known about the function of non-canonical Wnt pathways in vertebrates. I here summarize recent findings implicating important roles for Wnt/Ca(2+) and Wnt/JNK signaling during different aspects of early Xenopus laevis development, namely axis formation and gastrulation movements.

Convergent extension: the molecular control of polarized cell movement during embryonic development

During development, vertebrate embryos undergo dramatic changes in shape. The lengthening and narrowing of a field of cells, termed convergent extension, contributes to a variety of morphogenetic processes. Focusing on frogs and fish, we review the different cellular mechanisms and the well-conserved signaling pathways that underlie this process.

The type II inositol 1,4,5-trisphosphate receptor can trigger Ca2+ waves in rat hepatocytes

BACKGROUND & AIMS

Ca2+ regulates cell functions through signaling patterns such as Ca2+ oscillations and Ca2+ waves. The type I inositol 1,4,5-trisphosphate receptor is thought to support Ca2+ oscillations, whereas the type III inositol 1,4,5-trisphosphate receptor is thought to initiate Ca2+ waves. The role of the type II inositol 1,4,5-trisphosphate receptor is less clear, because it behaves like the type III inositol 1,4,5-trisphosphate receptor at the single-channel level but can support Ca2+ oscillations in intact cells. Because the type II inositol 1,4,5-trisphosphate receptor is the predominant isoform in liver, we examined whether this isoform can trigger Ca2+ waves in hepatocytes.

METHODS

The expression and distribution of inositol 1,4,5-trisphosphate receptor isoforms was examined in rat liver by immunoblot and confocal immunofluorescence. The effects of inositol 1,4,5-trisphosphate on Ca2+ signaling were examined in isolated rat hepatocyte couplets by using flash photolysis and time-lapse confocal microscopy.

RESULTS

The type II inositol 1,4,5-trisphosphate receptor was concentrated near the canalicular pole in hepatocytes, whereas the type I inositol 1,4,5-trisphosphate receptor was found elsewhere. Stimulation of hepatocytes with vasopressin or directly with inositol 1,4,5-trisphosphate induced Ca2+ waves that began in the canalicular region and then spread to the rest of the cell. Inositol 1,4,5-Trisphosphate-induced Ca2+ signals also increased more rapidly in the canalicular region. Hepatocytes did not express the ryanodine receptor, and cyclic adenosine diphosphate-ribose had no effect on Ca2+ signaling in these cells.

CONCLUSIONS

The type II inositol 1,4,5-trisphosphate receptor establishes a pericanalicular trigger zone from which Ca2+ waves originate in hepatocytes.

Identification and characterization of a calcium channel gamma subunit expressed in differentiating neurons and myoblasts

Transient elevations of intracellular calcium (calcium transients) play critical roles in many developmental processes, including differentiation. Although the factors that regulate calcium transients are not clearly defined, calcium influx may be controlled by molecules interacting with calcium channels, including channel regulatory subunits. Here, we describe the chick gamma4 regulatory subunit (CACNG4), the first such subunit to be characterized in early development. CACNG4 is expressed early in the cranial neural plate, and later in the cranial and dorsal root ganglia; importantly, the timing of this later expression correlates precisely with the onset of neuronal differentiation. CACNG4 expression is also observed in nonneuronal tissues undergoing differentiation, specifically the myotome and a subpopulation of differentiating myoblasts in the limb bud. Finally, within the distal cranial ganglia, we show that CACNG4 is expressed in placode-derived cells (prospective neurons), but also, surprisingly, in neural crest-derived cells, previously shown to form only glia in this location; contrary to these previous results, we find that neural crest cells can form neurons in the distal ganglia. Given the proposed role of CACNG4 in modulating calcium channels and its expression in differentiating cells, we suggest that CACNG4 may promote differentiation via regulation of intracellular calcium levels.

Buffering intracellular calcium disrupts motoneuron development in intact zebrafish embryos

Numerous studies, performed mainly on dissociated cells, have shown that calcium signals have a role during different stages of neuronal development. However, the actions of calcium during neuronal development in vivo remain to be established. The present study has investigated the role of intracellular calcium signals during development of motoneurons in the spinal cord of intact zebrafish embryos. Loading blastomeres of early embryos with either the calcium buffer BAPTA or the calcium reporter dye Calcium Green, was shown to disrupt motoneuron development in the spinal cord of embryos at 24 h postfertilisation. Loading the calcium buffer BAPTA, at an intracellular concentration of 1 mM, into the blastomeres of early embryos did not alter the resting levels of intracellular calcium, but significantly dampened transient rises in intracellular calcium in the cells of later stage embryos. Loading cells with 1 mM BAPTA significantly decreased the number of motoneurons present in the spinal cord at 24 h, indicating that calcium signals are important for normal motoneuron differentiation. Furthermore, in those BAPTA-filled cells that did adopt a motoneuron cell fate, axogenesis was found to be inhibited, suggestive of a role for calcium signalling in neurite initiation. This work provides evidence that calcium signals are necessary at several stages of motoneuron development in vivo.

XenopusDishevelled signaling regulates both neural and mesodermal convergent extension: parallel forces elongating the body axis

During amphibian development, non-canonical Wnt signals regulate the polarity of intercalating dorsal mesoderm cells during convergent extension. Cells of the overlying posterior neural ectoderm engage in similar morphogenetic cell movements. Important differences have been discerned in the cell behaviors associated with neural and mesodermal cell intercalation, raising the possibility that different mechanisms may control intercalations in these two tissues. In this report, targeted expression of mutants of Xenopus Dishevelled (Xdsh) to neural or mesodermal tissues elicited different defects that were consistent with inhibition of either neural or mesodermal convergent extension. Expression of mutant Xdsh also inhibited elongation of neural tissues in vitro in Keller sandwich explants and in vivo in neural plate grafts. Targeted expression of other Wnt signaling antagonists also inhibited neural convergent extension in whole embryos. In situ hybridization indicated that these defects were not due to changes in cell fate. Examination of embryonic phenotypes after inhibition of convergent extension in different tissues reveals a primary role for mesodermal convergent extension in axial elongation, and a role for neural convergent extension as an equalizing force to produce a straight axis. This study demonstrates that non-canonical Wnt signaling is a common mechanism controlling convergent extension in two very different tissues in the Xenopus embryo and may reflect a general conservation of control mechanisms in vertebrate convergent extension.

Vertebrate gastrulation: calcium waves orchestrate cell movements

A recent study reveals that the propagation of intercellular calcium signals is closely associated with the generation of convergent extension movements during Xenopus gastrulation. Such signals provide a mechanism whereby large populations of cells can communicate to generate orchestrated cell movements.

Calcium signaling during convergent extension in Xenopus

BACKGROUND

During Xenopus gastrulation, cell intercalation drives convergent extension of dorsal tissues. This process requires the coordination of motility throughout a large population of cells. The signaling mechanisms that regulate these movements in space and time remain poorly understood.

RESULTS

To investigate the potential contribution of calcium signaling to the control of morphogenetic movements, we visualized calcium dynamics during convergent extension using a calcium-sensitive fluorescent dye and a novel confocal microscopy system. We found that dramatic intercellular waves of calcium mobilization occurred in cells undergoing convergent extension in explants of gastrulating Xenopus embryos. These waves arose stochastically with respect to timing and position within the dorsal tissues. Waves propagated quickly and were often accompanied by a wave of contraction within the tissue. Calcium waves were not observed in explants of the ventral marginal zone or prospective epidermis. Pharmacological depletion of intracellular calcium stores abolished the calcium dynamics and also inhibited convergent extension without affecting cell fate. These data indicate that calcium signaling plays a direct role in the coordination of convergent extension cell movements.

CONCLUSIONS

The data presented here indicate that intercellular calcium signaling plays an important role in vertebrate convergent extension. We suggest that calcium waves may represent a widely used mechanism by which large groups of cells can coordinate complex cell movements.

Plant calcium signaling and monitoring: pros and cons and recent experimental approaches

This review focusses on Ca(2+)-mediated plant cell signaling and optical methods for in vivo [Ca2+] monitoring and imaging in plants. The cytosolic free calcium concentration has long been considered the central cellular key in plants. However, more and more data are turning up that critically question this view. Conflicting arguments show that there are still many open questions. One conclusion is that the cytosolic free Ca2+ concentration is just one of many cellular network parameters orchestrating complex cellular signaling. Novel experimental strategies which unveil interference of cellular parameters and communication of transduction pathways are required to understand this network. To date only optical methods are able to provide both kinetic and spatial information about cellular key parameters simultaneously. Focussing on calcium there are currently three classes of calcium indicators employed (i.e., chemical fluorescent dyes, luminescent indicators, and green-fluorescent-protein-based indicators). Properties and capabilities as well as advantages and disadvantages of these indicators when used in plant systems are discussed. Finally, general experimental strategies are mentioned which are able to answer open questions raised here.

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.

The versatility and universality of calcium signalling

The universality of calcium as an intracellular messenger depends on its enormous versatility. Cells have a calcium signalling toolkit with many components that can be mixed and matched to create a wide range of spatial and temporal signals. This versatility is exploited to control processes as diverse as fertilization, proliferation, development, learning and memory, contraction and secretion, and must be accomplished within the context of calcium being highly toxic. Exceeding its normal spatial and temporal boundaries can result in cell death through both necrosis and apoptosis.

Cell-type-specific calcium responses to drought, salt and cold in the Arabidopsis root

Little is known about the signalling processes involved in the response of roots to abiotic stresses. The Arabidopsis root is a model system of root anatomy with a simple architecture and is amenable to genetic manipulation. Although it is known that the root responds to cold, drought and salt stress with increases in cytoplasmic free calcium, there is currently no information about the role(s) of the functionally diverse cell types that comprise the root. Transgenic Arabidopsis with enhancer-trapped GAL4 expression in specific cell types was used to target the calcium reporting protein, aequorin, fused to a modified yellow fluorescent protein (YFP). The luminescence output of targeted aequorin enabled in vivo measurement of changes in cytosolic free calcium concentrations ([Ca2+]cyt) in specific cell types during acute cold, osmotic and salt stresses. In response to an acute cold stress, all cell types tested as well as plants constitutively expressing aequorin displayed rapid [Ca2+]cyt peaks. However, there were significant quantitative differences between different cell types in terms of their response to cold stress, osmotic stress (440 mM mannitol) and salt stress (220 mM NaCl), implying specific roles for certain cell types in the detection and/or response to these stimuli. In response to osmotic and salt stress, the endodermis and pericycle displayed prolonged oscillations in cytosolic calcium that were distinct from the responses of the other cell types tested. Targeted expression of aequorin circumvented the technical difficulties involved in fluorescent dye injection as well as the lack of cell specificity of constitutively expressed aequorin, and revealed a new level of complexity in root calcium signalling.

Calcium signalling during zebrafish embryonic development

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