Calcium transients accompany ooplasmic segregation in zebrafish embryos

Maternal regulation of the vertebrate oocyte-to-embryo transition

Maternally-loaded factors in the egg accumulate during oogenesis and are essential for the acquisition of oocyte and egg developmental competence to ensure the production of viable embryos. However, their molecular nature and functional importance remain poorly understood. Here, we present a collection of 9 recessive maternal-effect mutants identified in a zebrafish forward genetic screen that reveal unique molecular insights into the mechanisms controlling the vertebrate oocyte-to-embryo transition. Four genes, over easy, p33bjta, poached and black caviar, were found to control initial steps in yolk globule sizing and protein cleavage during oocyte maturation that act independently of nuclear maturation. The krang, kazukuram, p28tabj, and spotty genes play distinct roles in egg activation, including cortical granule biology, cytoplasmic segregation, the regulation of microtubule organizing center assembly and microtubule nucleation, and establishing the basic body plan. Furthermore, we cloned two of the mutant genes, identifying the over easy gene as a subunit of the Adaptor Protein complex 5, Ap5m1, which implicates it in regulating intracellular trafficking and yolk vesicle formation. The novel maternal protein Krang/Kiaa0513, highly conserved in metazoans, was discovered and linked to the function of cortical granules during egg activation. These mutant genes represent novel genetic entry points to decipher the molecular mechanisms functioning in the oocyte-to-embryo transition, fertility, and human disease. Additionally, our genetic adult screen not only contributes to the existing knowledge in the field but also sets the basis for future investigations. Thus, the identified maternal genes represent key players in the coordination and execution of events prior to fertilization.

Development Features on the Selection of Animal Models for Teratogenic Testing

Today, the use of animal models from different species continues to represent a fundamental step in teratogenic testing, despite the increase in alternative solutions that provide an important screening to the enormous quantity of new substances that aim to enter the market every year. The maintenance of these models is due to the sharing of similar development processes with humans, and in this way they represent an important contribution to the safety in the use of the compounds tested. Furthermore, the application of advances in embryology to teratology, although hampered by the complexity of reproductive processes, continues to prove the importance of sensitivity during embryonic and fetal development to detect potential toxicity, inducing mortality/abortion and malformations.In this chapter, essential periods of development in different models are outlined, highlighting the similarities and differences between species, the advantages and disadvantages of each group, and specific sensitivities for teratogenic testing. Models can be divided into invertebrate species such as earthworms of the species Eisenia fetida/Eisenia andrei, Caenorhabditis elegans, and Drosophila melanogaster, allowing for rapid results and minor ethical concerns. Vertebrate nonmammalian species Xenopus laevis and Danio rerio are important models to assess teratogenic potential later in development with fewer ethical requirements. Finally, the mammalian species Mus musculus, Rattus norvegicus, and Oryctolagus cuniculus, phylogenetically closer to humans, are essential for the assessment of complex specialized processes, occurring later in development.Regulations for the development of toxicology tests require the use of mammalian species. Although ethical concerns and costs limit their use in large-scale screening. On the other hand, invertebrate and vertebrate nonmammalian species are increasing as alternative animal models, as these organisms combine low cost, less ethical requirements, and culture conditions compatible with large-scale screening. Their main advantage is to allow high-throughput screening in a whole-animal context, in contrast to the in vitro techniques, not dependent on the prior identification of a target. Better knowledge of the development pathways of animal models will allow to maximize human translation and reduce the number of animals used, leading to a selection of compounds with an improved safety profile and reduced time to market for new drugs.

Actin-mediated endocytosis in the E-YSL helps drive epiboly in zebrafish

In zebrafish, a punctate band of F-actin is reported to develop in the external yolk syncytial layer (E-YSL) during the latter part of epiboly in zebrafish embryos. Here, electron microscopy (EM) and fluorescence confocal microscopy were conducted to investigate dynamic changes in the E-YSL membrane during epiboly. Using scanning EM, we report that the surface of the E-YSL is highly convoluted, consisting of a complex interwoven network of branching membrane surface microvilli-like protrusions. The region of membrane surface protrusions was relatively wide at 30% epiboly but narrowed as epiboly progressed. This narrowing was coincident with the formation of the punctate actin band. We also demonstrated using immunogold transmission EM that actin clusters were localized at the membrane surface mainly within the protrusions as well as in deeper locations of the E-YSL. Furthermore, during the latter part of epiboly, the punctate actin band was coincident with a region of highly dynamic endocytosis. Treatment with cytochalasin B led to the disruption of the punctate actin band and the membrane surface protrusions, as well as the attenuation of endocytosis. Together, our data suggest that, in the E-YSL, the region encompassing the membrane surface protrusions and its associated punctate actin band are likely to be an integral part of the localized endocytosis, which is important for the progression of epiboly in zebrafish embryos.

Temporal and spatial expression of zebrafish mctp genes and evaluation of frameshift alleles of mctp2b

MCTPs (multiple C2 domain proteins with two transmembrane regions) have been proposed as novel endoplasmic reticulum calcium sensors; however, their function remains largely unknown. Here we report the structure of the four mctp genes from zebrafish (mctp1a, mctp1b, mctp2a and mctp2b), their diversity, expression pattern during embryonic development and in adult tissue and the effect of knocking down the expression of Mctp2b by CRISPR/Cas9. The four mctp genes are expressed from early development and exhibit differential expression patterns but are found mainly in the nervous and muscular systems. Mctp2b tagged with fluorescent proteins and expressed in HEK-293 cells and neurons of the fish spinal cord localized mostly in the endoplasmic reticulum but also in lysosomes and late and recycling endosomes. Knocking down mctp2b expression impaired embryonic development, suggesting that the functional participation of this gene is relevant, at least during the early stages of development.

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.

Somatic cell nuclear transfer in non-enucleated goldfish oocytes: understanding DNA fate during oocyte activation and first cellular division

Nuclear transfer consists in injecting a somatic nucleus carrying valuable genetic information into a recipient oocyte to sire a diploid offspring which bears the genome of interest. It requires that the oocyte (maternal) DNA is removed. In fish, because enucleation is difficult to achieve, non-enucleated oocytes are often used and disappearance of the maternal DNA was reported in some clones. The present work explores which cellular events explain spontaneous erasure of maternal DNA, as mastering this phenomenon would circumvent the painstaking procedure of fish oocyte enucleation. The fate of the somatic and maternal DNA during oocyte activation and first cell cycle was studied using DNA labeling and immunofluorescence in goldfish clones. Maternal DNA was always found as an intact metaphase within the oocyte, and polar body extrusion was minimally affected after oocyte activation. During the first cell cycle, only 40% of the clones displayed symmetric cleavage, and these symmetric clones contributed to 80% of those surviving at hatching. Maternal DNA was often fragmented and located under the cleavage furrow. The somatic DNA was organized either into a normal mitotic spindle or abnormal multinuclear spindle. Scenarios matching the DNA behavior and the embryo fate are proposed.

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.

Bulk Actin Dynamics Drive Phase Segregation in Zebrafish Oocytes

Segregation of maternal determinants within the oocyte constitutes the first step in embryo patterning. In zebrafish oocytes, extensive ooplasmic streaming leads to the segregation of ooplasm from yolk granules along the animal-vegetal axis of the oocyte. Here, we show that this process does not rely on cortical actin reorganization, as previously thought, but instead on a cell-cycle-dependent bulk actin polymerization wave traveling from the animal to the vegetal pole of the oocyte. This wave functions in segregation by both pulling ooplasm animally and pushing yolk granules vegetally. Using biophysical experimentation and theory, we show that ooplasm pulling is mediated by bulk actin network flows exerting friction forces on the ooplasm, while yolk granule pushing is achieved by a mechanism closely resembling actin comet formation on yolk granules. Our study defines a novel role of cell-cycle-controlled bulk actin polymerization waves in oocyte polarization via ooplasmic segregation.

Formation and dynamics of cytoplasmic domains and their genetic regulation during the zebrafish oocyte-to-embryo transition

Establishment and movement of cytoplasmic domains is of great importance for the emergence of cell polarity, germline segregation, embryonic axis specification and correct sorting of organelles and macromolecules into different embryonic cells. The zebrafish oocyte, egg and zygote are valuable material for the study of cytoplasmic domains formation and dynamics during development. In this review we examined how cytoplasmic domains form and are relocated during zebrafish early embryogenesis. Distinct cortical cytoplasmic domains (also referred to as ectoplasm domains) form first during early oogenesis by the localization of mRNAs to the vegetal or animal poles of the oocyte or radially throughout the cortex. Cytoplasmic segregation in the late oocyte relocates non-cortical cytoplasm (endoplasm) into the preblastodisc and yolk cell. The preblastodisc is a precursor to the blastodisc, which gives rise to the blastoderm and most the future embryo. After egg activation, the blastodisc enlarges by transport of cytoplasm from the yolk cell to the animal pole, along defined pathways or streamers that include a complex cytoskeletal meshwork and cytoplasmic movement at different speeds. A powerful actin ring, assembled at the margin of the blastodisc, appears to drive the massive streaming of cytoplasm. The fact that the mechanism(s) leading to the formation and relocation of cytoplasmic domains are affected in maternal-effect mutants indicates that these processes are under maternal control. Here, we also discuss why these mutants represent outstanding genetic entry points to investigate the genetic basis of cytoplasmic segregation. Functional studies, combined with the analysis of zebrafish mutants, generated by forward and reverse genetic strategies, are expected to decipher the molecular mechanism(s) by which the maternal factors regulate cytoplasmic movements during early vertebrate development.

Biological Concerns on the Selection of Animal Models for Teratogenic Testing

During pregnancy fetus can be exposed to a variety of chemicals which may induce abortion and malformations. Due to the amounts of new substances coming into the market every year, a high demand for a rapid, reliable, and cost-effective method to detect potential toxicity is necessary. Different species have been used as animal models for teratogen screening, most of them sharing similar development processes with humans. However, the application of embryology knowledge to teratology is hampered by the complexity of the reproduction processes.The present chapter outlines the essential development periods in different models, and highlights the similarities and differences between species, advantages and disadvantages of each group, and specific sensitivities for teratogenic tests. These models can be organized into the following categories: (1) invertebrate species such Caenorhabditis elegans and Drosophila melanogaster, which have become ideal for screening simple mechanisms in the early periods of reproductive cycle, allowing for rapid results and minor ethical concerns; (2) vertebrate nonmammalian species such Xenopus laevis and Danio rerio, important models to assess teratogenic potential in later development with fewer ethical requirements; and (3) the mammalian species Mus musculus, Rattus norvegicus, and Oryctolagus cuniculus, phylogenetically more close to humans, essential to assess complex specialized processes, that occur later in development.Rules for development toxicology tests require the use of mammalian species. However, ethical concerns and costs limit their use in large-scale screening. By contrast, invertebrate and vertebrate nonmammalian species are increasing as alternative animal models, as these organisms combine less ethical requirements, low costs and culture conditions compatible with large-scale screening. In contrast to the in vitro techniques, their main advantage is to allow for high-throughput screening in a whole-animal context, not dependent on the prior identification of a target. In this chapter, the biological development of the animals most used in teratogenic tests is adressed with the aims of maximizing human translation, reducing the number of animals used, and the time to market for new drugs.

Ca2+ release via two-pore channel type 2 (TPC2) is required for slow muscle cell myofibrillogenesis and myotomal patterning in intact zebrafish embryos

We recently demonstrated a critical role for two-pore channel type 2 (TPC2)-mediated Ca²⁺ release during the differentiation of slow (skeletal) muscle cells (SMC) in intact zebrafish embryos, via the introduction of a translational-blocking morpholino antisense oligonucleotide (MO). Here, we extend our study and demonstrate that knockdown of TPC2 with a non-overlapping splice-blocking MO, knockout of TPC2 (via the generation of a tpcn2^dhkz1a mutant line of zebrafish using CRISPR/Cas9 gene-editing), or the pharmacological inhibition of TPC2 action with bafilomycin A1 or trans-ned-19, also lead to a significant attenuation of SMC differentiation, characterized by a disruption of SMC myofibrillogenesis and gross morphological changes in the trunk musculature. When the morphants were injected with tpcn2-mRNA or were treated with IP₃/BM or caffeine (agonists of the inositol 1,4,5-trisphosphate receptor (IP₃R) and ryanodine receptor (RyR), respectively), many aspects of myofibrillogenesis and myotomal patterning (and in the case of the pharmacological treatments, the Ca²⁺ signals generated in the SMCs), were rescued. STED super-resolution microscopy revealed a close physical relationship between clusters of RyR in the terminal cisternae of the sarcoplasmic reticulum (SR), and TPC2 in lysosomes, with a mean estimated separation of ~52-87nm. Our data therefore add to the increasing body of evidence, which indicate that localized Ca²⁺ release via TPC2 might trigger the generation of more global Ca²⁺ release from the SR via Ca²⁺-induced Ca²⁺ release.

Sterol O-Acyltransferase 2 Contributes to the Yolk Cholesterol Trafficking during Zebrafish Embryogenesis

To elucidate whether Sterol O-acyltransferase (Soat) mediates the absorption and transportation of yolk lipids to the developing embryo, zebrafish soat1 and soat2 were cloned and studied. In the adult zebrafish, soat1 was detected ubiquitously while soat2 mRNA was detected specifically in the liver, intestine, brain and testis. Whole mount in situ hybridization demonstrated that both soat1 and soat2 expressed in the yolk syncytial layer, hatching gland and developing cardiovascular as well as digestive systems, suggesting that Soats may play important roles in the lipid trafficking and utilization during embryonic development. The enzymatic activity of zebrafish Soat2 was confirmed by Oil Red O staining in the HEK293 cells overexpressing this gene, and could be quenched by Soat2 inhibitor Pyripyropene A (PPPA). The zebrafish embryos injected with PPPA or morpholino oligo against soat2 in the yolk showed significantly larger yolk when compared with wild-type embryos, especially at 72 hpf, indicating a slower rate of yolk consumption. Our result indicated that zebrafish Soat2 is catalytically active in synthesizing cholesteryl esters and contributes to the yolk cholesterol trafficking during zebrafish embryogenesis.

Cortical depth and differential transport of vegetally localized dorsal and germ line determinants in the zebrafish embryo

In zebrafish embryos, factors involved in both axis induction and primordial germ cell (PGC) development are localized to the vegetal pole of the egg. However, upon egg activation axis induction factors experience an asymmetric off-center shift whereas PGC factors undergo symmetric animally-directed movement. We examined the spatial relationship between the proposed dorsal genes wnt8a and grip2a and the PGC factor dazl at the vegetal cortex. We find that RNAs for these genes localize to different cortical depths, with the RNA for the PGC factor dazl at a deeper cortical level than those for axis-inducing factors. In addition, and in contrast to the role of microtubules in the long-range transport of dorsal determinants, we find that germ line determinant transport depends on the actin cytoskeleton. Our results support a model in which vegetal cortex differential RNA transport behavior is facilitated by RNA localization along cortical depth and differential coupling to cortical transport.

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.

Electro-microinjection of fish eggs with an immobile capillary electrode

Microinjection with ultra-fine glass capillaries is widely used to introduce cryoprotective agents and other foreign molecules into animal cells, oocytes, and embryos. The fragility of glass capillaries makes difficult the microinjection of fish eggs and embryos, which are usually protected by a hard outer shell, called the chorion. In this study, we introduce a new electromechanical approach, based on the electropiercing of fish eggs with a stationary needle electrode. The electropiercing setup consists of two asymmetric electrodes, including a μm-scaled nickel needle placed opposite to a mm-scaled planar counter-electrode. A fish egg is immersed in low-conductivity solution and positioned between the electrodes. Upon application of a short electric pulse of sufficient field strength, the chorion is electroporated and the egg is attracted to the needle electrode by positive dielectrophoresis. As a result, the hard chorion and the subjacent yolk membrane are impaled by the sharp electrode tip, thus providing direct access to the egg yolk plasma. Our experiments on early-stage medaka fish embryos showed the applicability of electro-microinjection to fish eggs measuring about 1 mm in diameter. We optimized the electropiercing of medaka eggs with respect to the field strength, pulse duration, and conductivity of bathing medium. We microscopically examined the injection of dye solution into egg yolk and the impact of electropiercing on embryos' viability and development. We also analyzed the mechanisms of electropiercing in comparison with the conventional mechanical microinjection. The new electropiercing method has a high potential for automation, e.g., via integration into microfluidic devices, which would allow a large-scale microinjection of fish eggs for a variety of applications in basic research and aquaculture.

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.

Introduction of aequorin into zebrafish embryos for recording Ca(2+) signaling during the first 48 h of development

Ca(2+) signals, whether transient pulses, propagating waves, or long-duration, steady gradients, are generally considered to play an important role in the pattern-forming events that occur during vertebrate development. One vertebrate that has long been a favorite of embryologists because of its ex utero development and the optical clarity of its embryos is the zebrafish, Danio rerio. Using the bioluminescent Ca(2+) reporter aequorin, distinct Ca(2+) signals have been reported for at least the first 48 h of zebrafish development, with signals becoming progressively more complex as the embryo develops. Here we provide a general introduction to aequorin and its use in monitoring Ca(2+) signals and discuss methods for introducing aequorin into zebrafish embryos.

Microinjecting holo-aequorin into dechorionated and intact zebrafish embryos

The injection of holo-aequorin into embryos at the one-cell stage, along with the use of a simple photomultiplier tube or luminescence imaging system, allows transient localized elevations of free cytosolic Ca(2+) to be recorded and observed during the first 24 h of zebrafish development. The technique for loading dechorionated or intact one-cell stage zebrafish embryos with holo-aequorin is described here.

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.

Dynamic microtubules at the vegetal cortex predict the embryonic axis in zebrafish

In zebrafish, as in many animals, maternal dorsal determinants are vegetally localized in the egg and are transported after fertilization in a microtubule-dependent manner. However, the organization of early microtubules, their dynamics and their contribution to axis formation are not fully understood. Using live imaging, we identified two populations of microtubules, perpendicular bundles and parallel arrays, which are directionally oriented and detected exclusively at the vegetal cortex before the first cell division. Perpendicular bundles emanate from the vegetal cortex, extend towards the blastoderm, and orient along the animal-vegetal axis. Parallel arrays become asymmetric on the vegetal cortex, and orient towards dorsal. We show that the orientation of microtubules at 20 minutes post-fertilization can predict where the embryonic dorsal structures in zebrafish will form. Furthermore, we find that parallel microtubule arrays colocalize with wnt8a RNA, the candidate maternal dorsal factor. Vegetal cytoplasmic granules are displaced with parallel arrays by ~20°, providing in vivo evidence of a cortical rotation-like process in zebrafish. Cortical displacement requires parallel microtubule arrays, and probably contributes to asymmetric transport of maternal determinants. Formation of parallel arrays depends on Ca(2+) signaling. Thus, microtubule polarity and organization predicts the zebrafish embryonic axis. In addition, our results suggest that cortical rotation-like processes might be more common in early development than previously thought.

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

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

Calcium 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.

Optimization of somatic cell injection in the perspective of nuclear transfer in goldfish

Background

Nuclear transfer has the potential to become one strategy for fish genetic resources management, by allowing fish reconstruction from cryopreserved somatic cells. Survival rates after nuclear transfer are still low however. The part played by unsuitable handling conditions is often questioned, but the different steps in the procedure are difficult to address separately. In this work led on goldfish (Carassius auratus), the step of somatic cells injection was explored. Non-enucleated metaphase II oocytes were used as a template to explore the toxicity of the injection medium, to estimate the best location where the cell should be injected, and to assess the delay necessary between cell injection and oocyte activation.

Results

Trout coelomic fluid was the most suitable medium to maintain freshly spawned oocytes at the metaphase II stage during oocyte manipulation. Oocytes were then injected with several media to test their toxicity on embryo development after fertilization. Trout coelomic fluid was the least toxic medium after injection, and the smallest injected volume (10 pL) allowed the same hatching rates as the non injected controls (84.8% ± 23). In somatic cell transfer experiments using non enucleated metaphase II oocytes as recipient, cell plasma membrane was ruptured within one minute after injection. Cell injection at the top of the animal pole in the oocyte allowed higher development rates than cell injection deeper within the oocyte (respectively 59% and 23% at mid-blastula stage). Embryo development rates were also higher when oocyte activation was delayed for 30 min after cell injection than when activation was induced without delay (respectively 72% and 48% at mid-blastula stage).

Conclusions

The best ability of goldfish oocytes to sustain embryo development was obtained when the carrier medium was trout coelomic fluid, when the cell was injected close to the animal pole, and when oocyte activation was induced 30 min after somatic cell injection. Although the experiments were not designed to produce characterized clones, application of these parameters to somatic cell nuclear transfer experiments in enucleated metaphase II oocytes is expected to improve the quality of the reconstructed embryos.

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.

Reorganization of cytoplasm in the zebrafish oocyte and egg during early steps of ooplasmic segregation

The aim of this work is to determine when and how ooplasmic segregation is initiated in the zebrafish egg. To this end, the organization of the ooplasm and vitelloplasm were examined in oocytes and eggs shortly after activation. Ooplasmic segregation, initiated in the stage V oocyte, led to the formation of ooplasmic domains rich in organelles, and ribonucleoproteins. A linear array of closely arranged peripheral yolk globules separated an outer domain of ectoplasm from an inner domain of interconnected endoplasmic lacunae. The structure of this yolk array and the distribution of microinjected labeled tracers suggests that it may provide a barrier limiting ooplasm transit. Loosely arranged yolk globules at the animal hemisphere allow wide connections between the endoplasm and a preblastodisc domain. Activation caused further segregation of ooplasm, reorganization of endoplasmic lacunae, and blastodisc growth. The presence of an endoplasmic cytoskeleton suggests that these changes may be driven by microtubules and microfilaments.

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.

Fertilization triggers localized activation of Src-family protein kinases in the zebrafish egg

Fertilization triggers activation of Src-family kinases in eggs of various species including marine invertebrates and lower vertebrates. While immunofluorescence studies have localized Src-family kinases to the plasma membrane or cortical cytoplasm, no information is available regarding the extent to which these kinases are activated in different regions of the zygote. The objective of the present study was to detect the subcellular distribution of activated Src-family kinases in the fertilized zebrafish egg. An antibody specific for the active, non-phosphorylated form of Src-family PTKs was used to detect these activated kinases by immunofluorescence. The results demonstrate that Fyn, and possibly other Src family members are activated by dephosphorylation of the C-terminal tyrosine at fertilization. The activated Src-family kinases are asymmetrically distributed around the egg cortex with an area of higher kinase activity localized adjacent to the micropyle near the presumptive animal pole. Fertilization initially caused elevation of kinase activity in the cytoplasm underlying the micropyle, but this quickly spread to involve the entire zygote cortex. Later, during egg activation, formation of the blastodisc involved concentration of active Src-family kinase in the blastodisc cortex. As cytokinesis began, activated Src-family kinases were no longer limited to the cortex, but became more evenly distributed in the clear apical cytoplasm of the blastomeres. The results demonstrate that the cortex of the zebrafish egg is functionally differentiated and that fertilization triggers localized activation of Src-family kinases at the point of sperm entry, which subsequently progresses through the entire egg cortex.

Separate pathways of RNA recruitment lead to the compartmentalization of the zebrafish germ plasm

The maternal RNAs vasa, dead end, nanos1, and daz-like all become localized to the peripheral ends of the first and second cleavage furrows, where they form part of the zebrafish germ plasm. We show that aggregates of a first class of germ plasm components, which include dead end, nanos1, and vasa RNAs, are initially present in a wide cortical band at the animal pole. Aggregates containing these three RNAs appear to be associated with f-actin, which during the first cell cycle undergoes a microtubule-dependent movement towards the periphery as well as circumferential alignment. These cytoskeletal rearrangements lead to the further aggregation of particles containing these RNAs and their concomitant recruitment to the forming furrow. Aggregates containing a second class of germ plasm RNA components, which include the transcript for daz-like, translocate along the plane of the cortex towards the animal pole, where they are recruited to the germ plasm. After recruitment to the furrow, these two classes of RNAs occupy overlapping yet distinct regions of the germ plasm, and this arrangement is maintained during the early cleavage stages. Our observations suggest that separate pathways of RNA recruitment facilitate the compartmentalization of the zebrafish germ plasm.

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.

Identification of recessive maternal-effect mutations in the zebrafish using a gynogenesis-based method

In animal species, early developmental processes are driven by maternally derived factors. Here, we describe a forward genetics approach to identify recessive mutations in genes encoding such maternal factors in the zebrafish. We used a gynogenesis-based approach to identify 14 recessive maternal-effect mutations. Homozygosity for these mutations in adult females leads to the inviability of their offspring. Confocal microscopy of embryos labeled with a DNA dye and a membrane marker allowed us to further analyze mutant embryos for defects in nuclear and cellular divisions. The mutations result in a range of defects in early developmental processes, including egg activation, early nuclear events, mitosis, cytokinesis, axial patterning, and gastrulation. Our effort constitutes a systematic attempt to identify maternal-effect genes in a vertebrate species. The sample of mutations that we have identified reflects the diversity of maternally driven functions in early development and underscores the importance of maternal factors in this process.

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.

Maternal factors in zebrafish development

All processes that occur before the activation of the zygotic genome at the midblastula transition are driven by maternal products, which are produced during oogenesis and stored in the mature oocyte. Upon egg activation and fertilization, these maternal factors initiate developmental cascades that carry out the embryonic developmental program. Even after the initiation of zygotic gene expression, perduring maternal products continue performing essential functions, either together with other maternal factors or through interactions with newly expressed zygotic products. Advances in zebrafish research have placed this organism in a unique position to contribute to a detailed understanding of the role of maternal factors in early vertebrate development. This review summarizes our knowledge on the processes involved in the production and redistribution of maternal factors during zebrafish oogenesis and early development, as well as our understanding of the function of these factors in axis formation, germ layer and germ cell specification, and other early embryonic processes.

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.

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

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

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.

On the mechanism of ooplasmic segregation in single-cell zebrafish embryos

It has been previously shown that localized elevations of free cytosolic calcium are associated with a morphological contraction in the forming blastodisc and animal hemisphere cortex during ooplasmic segregation in zebrafish zygotes. It was subsequently proposed, in a hypothetical model, that these calcium transients might be linked to the contraction of a cortically located actin microfilament network as a potential driving force for segregation. Here, by labeling single-cell embryos during the major phase of segregation with rhodamine-phalloidin, direct evidence is presented to indicate that the surface contraction was generated by an actin-based cortical network. Furthermore, while zygotes incubated with colchicine underwent normal ooplasmic segregation, those incubated with cytochalasin B did not generate a constriction band or segregate to form a blastodisc. During segregation at the single-cell stage, ooplasm simultaneously moved in two directions: toward the blastodisc within the so-called axial streamers, and toward the vegetal pole in the peripheral ooplasm. The velocities of both axial and peripheral streaming movements are reported. By injection of a fluorescein isothiocyanate (FITC)-labeled 2000 kDa dextran into the peripheral ooplasm it was demonstrated that a portion of it feeds into the bases of the extending streamers, which helps to explain the lack of accumulation of ooplasm at the vegetal pole. These new data were incorporated into the original model to link the bipolar ooplasmic movements with the calcium-modulated, actin-mediated contraction of the animal hemisphere cortex as a means of establishing and driving ooplasmic segregation in zebrafish.

Ways of looking at calcium

What we understand about signalling pathways depends very much on the ways we can measure them. I review ways of measuring calcium and explore how changes in methods have led to new ways of thinking about calcium signals. I also suggest how the ways we have of looking at calcium will influence the analysis of other signalling pathways that, until now, have not been studied with the spatiotemporal precision available to those studying calcium signalling.

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

Oscillations of cytosolic free calcium levels have been shown to influence gene regulation and cell differentiation in a variety of model systems. Intercellular calcium waves thus present a plausible mechanism for coordinating cellular processes during embryogenesis. Herein we report use of aequorin and a photon imaging microscope to directly observe a rhythmic series of intercellular calcium waves that circumnavigate zebrafish embryos over a 10-h period during gastrulation and axial segmentation. These waves first appeared at about 65% epiboly and continued to arise every 5-10 min up to at least the 16-somite stage. The waves originated from loci of high calcium activity bordering the blastoderm margin. Several initiating loci were active early in the wave series, whereas later a dorsal marginal midline locus predominated. On completion of epiboly, the dorsal locus was incorporated into the developing tail bud and continued to generate calcium waves. The locations and timing at which calcium dynamics are most active appear to correspond closely to embryonic cellular and syncytial sites of known morphogenetic importance. The observations suggest that a panembryonic calcium signaling system operating in a clock-like fashion might play a role during vertebrate axial patterning.