Buffering intracellular calcium disrupts motoneuron development in intact zebrafish embryos

Rapid Zebrafish Behavioral Profiling Assay Accelerates the Identification of Environmental Neurodevelopmental Toxicants

Rapid and cost-effective in vivo assays to screen potential environmental neurodevelopmental toxicants are necessary to address the limitations of in vitro platforms, such as the inability to fully recapitulate the developmental and physiological processes of whole organisms. In the present study, a rapid zebrafish behavioral profiling assay was developed to characterize the neurodevelopmental effects of environmental substances by quantitatively evaluating multiple spontaneous movement features of zebrafish embryos. This video analysis-based assay automatically segmented every embryo and thus was able to accurately quantify spontaneous movement features, including frequency, duration, intensity, interval, and the number of continuous movements. When tested with eight environmental substances known to be neurodevelopmental toxicants, such as chlorpyrifos and bisphenol A, the assay successfully captured frequency alterations that were well-documented in previous studies while also providing additional information. Using an optimized procedure, we further assessed 132 potential neurotoxins that spanned a wide range of molecular targets, many of which were previously detected in environmental waterbodies. The distinct altered behavioral barcodes indicated that the spontaneous movement was impacted by diverse neuroactive substances, and the effects could be effectively evaluated with the developed assay. The web-based tool, named EMAnalysis, is further provided at http://www.envh.sjtu.edu.cn/zebrafish_contraction.jsp. Thus, this assay provides an efficient platform to accelerate the pace of neurotoxic environmental contaminant discoveries.

Bright and High-Performance Genetically Encoded Ca2+ Indicator Based on mNeonGreen Fluorescent Protein

Genetically encodable calcium ion (Ca²⁺) indicators (GECIs) based on green fluorescent proteins (GFP) are powerful tools for imaging of cell signaling and neural activity in model organisms. Following almost 2 decades of steady improvements in the Aequorea victoria GFP-based GCaMP series of GECIs, the performance of the most recent generation (i.e., jGCaMP7) may have reached its practical limit due to the inherent properties of GFP. In an effort to sustain the steady progression toward ever-improved GECIs, we undertook the development of a new GECI based on the bright monomeric GFP, mNeonGreen (mNG). The resulting indicator, mNG-GECO1, is 60% brighter than GCaMP6s in vitro and provides comparable performance as demonstrated by imaging Ca²⁺ dynamics in cultured cells, primary neurons, and in vivo in larval zebrafish. These results suggest that mNG-GECO1 is a promising next-generation GECI that could inherit the mantle of GCaMP and allow the steady improvement of GECIs to continue for generations to come.

Trpc1 as the Missing Link Between the Bmp and Ca2+ Signalling Pathways During Neural Specification in Amphibians

In amphibians, the inhibition of bone morphogenetic protein (BMP) in the dorsal ectoderm has been proposed to be responsible for the first step of neural specification, called neural induction. We previously demonstrated that in Xenopus laevis embryos, the BMP signalling antagonist, noggin, triggers an influx of Ca²⁺ through voltage-dependent L-type Ca²⁺ channels (LTCCs), mainly via Ca_V1.2, and we showed that this influx constitutes a necessary and sufficient signal for triggering the expression of neural genes. However, the mechanism linking the inhibition of BMP signalling with the activation of LTCCs remained unknown. Here, we demonstrate that the transient receptor potential canonical subfamily member 1, (Trpc1), is an intermediate between BMP receptor type II (BMPRII) and the Ca_V1.2 channel. We show that noggin induces a physical interaction between BMPRII and Trpc1 channels. This interaction leads to the activation of Trpc1 channels and to an influx of cations, which depolarizes the plasma membrane up to a threshold sufficient to activate Cav1.2. Together, our results demonstrate for the first time that during neural induction, Ca²⁺ entry through the Ca_V1.2 channel results from the noggin-induced interaction between Trpc1 and BMPRII.

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

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

Role of Active Contraction and Tropomodulins in Regulating Actin Filament Length and Sarcomere Structure in Developing Zebrafish Skeletal Muscle

Whilst it is recognized that contraction plays an important part in maintaining the structure and function of mature skeletal muscle, its role during development remains undefined. In this study the role of movement in skeletal muscle maturation was investigated in intact zebrafish embryos using a combination of genetic and pharmacological approaches. An immotile mutant line (cacnb1 (ts25) ) which lacks functional voltage-gated calcium channels (dihydropyridine receptors) in the muscle and pharmacological immobilization of embryos with a reversible anesthetic (Tricaine), allowed the study of paralysis (in mutants and anesthetized fish) and recovery of movement (reversal of anesthetic treatment). The effect of paralysis in early embryos (aged between 17 and 24 hours post-fertilization, hpf) on skeletal muscle structure at both myofibrillar and myofilament level was determined using both immunostaining with confocal microscopy and small angle X-ray diffraction. The consequences of paralysis and subsequent recovery on the localization of the actin capping proteins Tropomodulin 1 & 4 (Tmod) in fish aged from 17 hpf until 42 hpf was also assessed. The functional consequences of early paralysis were investigated by examining the mechanical properties of the larval muscle. The length-force relationship, active and passive tension, was measured in immotile, recovered and control skeletal muscle at 5 and 7 day post-fertilization (dpf). Recovery of muscle function was also assessed by examining swimming patterns in recovered and control fish. Inhibition of the initial embryonic movements (up to 24 hpf) resulted in an increase in myofibril length and a decrease in width followed by almost complete recovery in both moving and paralyzed fish by 42 hpf. In conclusion, myofibril organization is regulated by a dual mechanism involving movement-dependent and movement-independent processes. The initial contractile event itself drives the localization of Tmod1 to its sarcomeric position, capping the actin pointed ends and ultimately regulating actin length. This study demonstrates that both contraction and contractile-independent mechanisms are important for the regulation of myofibril organization, which in turn is necessary for establishing proper skeletal muscle structure and function during development in vivo in zebrafish.

Ca(2+) coding and decoding strategies for the specification of neural and renal precursor cells during development

During embryogenesis, a rise in intracellular Ca(2+) is known to be a widespread trigger for directing stem cells towards a specific tissue fate, but the precise Ca(2+) signalling mechanisms involved in achieving these pleiotropic effects are still poorly understood. In this review, we compare the Ca(2+) signalling events that appear to be one of the first steps in initiating and regulating both neural determination (neural induction) and kidney development (nephrogenesis). We have highlighted the necessary and sufficient role played by Ca(2+) influx and by Ca(2+) transients in the determination and differentiation of pools of neural or renal precursors. We have identified new Ca(2+) target genes involved in neural induction and we showed that the same Ca(2+) early target genes studied are not restricted to neural tissue but are also present in other tissues, principally in the pronephros. In this review, we also described a mechanism whereby the transcriptional control of gene expression during neurogenesis and nephrogenesis might be directly controlled by Ca(2+) signalling. This mechanism involves members of the Kcnip family such that a change in their binding properties to specific DNA sites is a result of Ca(2+) binding to EF-hand motifs. The different functions of Ca(2+) signalling during these two events illustrate the versatility of Ca(2+) as a second messenger.

Expression and functionality of transient receptor potential melastatin 4 (TRPM4)-like channels during development of the zebrafish

Calcium signaling, from localized spikes to coordinated waves, are linked to cleavage, patterning, differentiation, and growth during embryonic development. The basis for control of these Ca(2+) signals is poorly defined. In this study, the expression and functionality of the transient receptor potential melastatin 4 protein (TRPM4), an ion channel that controls Ca(2+) entry into cells, was examined in the zebrafish embryo and adult. Originating with the human TRPM4 gene, Ensembl ortholog, NCBI BLAST, and Homologene searches identified a zebrafish TRPM4 "like" gene encoding a predicted protein of 1199 amino acids and sharing a 42-43% sequence identity with the mouse, rat, and human. Custom-designed zebrafish primers identified TRPM4 transcripts throughout the 0-123h period of embryonic development with greatest and lowest relative expression at 12 and 123h post-fertilization, respectively. Perforated patch clamp recordings in 27h embryonic cells revealed Ca(2+)-activated currents with the characteristics of those described for mammalian TRPM4. Similarly, TRPM4-like expression and functionality was observed in brain and liver cells from adult fish. These findings suggest that a TRPM4-like channel is available for Ca(2+) regulation during early development of the zebrafish.

Kcnip1 a Ca²⁺-dependent transcriptional repressor regulates the size of the neural plate in Xenopus

In amphibian embryos, our previous work has demonstrated that calcium transients occurring in the dorsal ectoderm at the onset of gastrulation are necessary and sufficient to engage the ectodermal cells into a neural fate by inducing neural specific genes. Some of these genes are direct targets of calcium. Here we search for a direct transcriptional mechanism by which calcium signals are acting. The only known mechanism responsible for a direct action of calcium on gene transcription involves an EF-hand Ca²⁺ binding protein which belongs to a group of four proteins (Kcnip1 to 4). Kcnip protein can act in a Ca²⁺-dependent manner as a transcriptional repressor by binding to a specific DNA sequence, the Downstream Regulatory Element (DRE) site. In Xenopus, among the four kcnips, we show that only kcnip1 is timely and spatially present in the presumptive neural territories and is able to bind DRE sites in a Ca²⁺-dependent manner. The loss of function of kcnip1 results in the expansion of the neural plate through an increased proliferation of neural progenitors. Later on, this leads to an impairment in the development of anterior neural structures. We propose that, in the embryo, at the onset of neurogenesis Kcnip1 is the Ca²⁺-dependent transcriptional repressor that controls the size of the neural plate. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Zebrafish models of human motor neuron diseases: advantages and limitations

Motor neuron diseases (MNDs) are an etiologically heterogeneous group of disorders of neurodegenerative origin, which result in degeneration of lower (LMNs) and/or upper motor neurons (UMNs). Neurodegenerative MNDs include pure hereditary spastic paraplegia (HSP), which involves specific degeneration of UMNs, leading to progressive spasticity of the lower limbs. In contrast, spinal muscular atrophy (SMA) involves the specific degeneration of LMNs, with symmetrical muscle weakness and atrophy. Amyotrophic lateral sclerosis (ALS), the most common adult-onset MND, is characterized by the degeneration of both UMNs and LMNs, leading to progressive muscle weakness, atrophy, and spasticity. A review of the comparative neuroanatomy of the human and zebrafish motor systems showed that, while the zebrafish was a homologous model for LMN disorders, such as SMA, it was only partially relevant in the case of UMN disorders, due to the absence of corticospinal and rubrospinal tracts in its central nervous system. Even considering the limitation of this model to fully reproduce the human UMN disorders, zebrafish offer an excellent alternative vertebrate model for the molecular and genetic dissection of MND mechanisms. Its advantages include the conservation of genome and physiological processes and applicable in vivo tools, including easy imaging, loss or gain of function methods, behavioral tests to examine changes in motor activity, and the ease of simultaneous chemical/drug testing on large numbers of animals. This facilitates the assessment of the environmental origin of MNDs, alone or in combination with genetic traits and putative modifier genes. Positive hits obtained by phenotype-based small-molecule screening using zebrafish may potentially be effective drugs for treatment of human MNDs.

Evaluating the developmental toxicity of trypanocidal nitroaromatic compounds on zebrafish

Current therapies against African and American trypanosomiasis are problematic and with no immediate prospect of a vaccine there is an urgent need for cheap, more effective treatments. To aid the drug discovery pipeline, we report a novel in vivo screening approach using zebrafish (Danio rerio) embryos as a means of rapidly assessing a compounds developmental toxicity. This technique, amenable to high-throughput screening, was validated using several trypanocidal nitroaromatic prodrugs including nifurtimox and benznidazole.

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.

Ryanodine receptors, a family of intracellular calcium ion channels, are expressed throughout early vertebrate development

BACKGROUND

Calcium signals ([Ca2+]i) direct many aspects of embryo development but their regulation is not well characterised. Ryanodine receptors (RyRs) are a family of intracellular Ca2+ release channels that control the flux of Ca2+ from internal stores into the cytosol. RyRs are primarily known for their role in excitation-contraction coupling in adult striated muscle and ryr gene mutations are implicated in several human diseases. Current evidence suggests that RyRs do not have a major role to play prior to organogenesis but regulate tissue differentiation.

FINDINGS

The sequences of the five zebrafish ryr genes were confirmed, their evolutionary relationship established and the primary sequences compared to other vertebrates, including humans. RyRs are differentially expressed in slow (ryr1a), fast (ryr3) and both types (ryr1b) of developing skeletal muscle. There are two ryr2 genes (ryr2a and ryr2b) which are expressed exclusively in developing CNS and cardiac tissue, respectively. In addition, ryr3 and ryr2a mRNA is detectable in the initial stages of development, prior to embryonic axis formation.

CONCLUSIONS

Our work reveals that zebrafish ryr genes are differentially expressed throughout the developing embryo from cleavage onwards. The data suggests that RyR-regulated Ca2+ signals are associated with several aspects of embryonic development, from organogenesis through to the differentiation of the musculoskeletal, cardiovascular and nervous system. These studies will facilitate further work to explore the developmental function of RyRs in each of these tissue types.

Connexin 39.9 protein is necessary for coordinated activation of slow-twitch muscle and normal behavior in zebrafish

In many tissues and organs, connexin proteins assemble between neighboring cells to form gap junctions. These gap junctions facilitate direct intercellular communication between adjoining cells, allowing for the transmission of both chemical and electrical signals. In rodents, gap junctions are found in differentiating myoblasts and are important for myogenesis. Although gap junctions were once believed to be absent from differentiated skeletal muscle in mammals, recent studies in teleosts revealed that differentiated muscle does express connexins and is electrically coupled, at least at the larval stage. These findings raised questions regarding the functional significance of gap junctions in differentiated muscle. Our analysis of gap junctions in muscle began with the isolation of a zebrafish motor mutant that displayed weak coiling at day 1 of development, a behavior known to be driven by slow-twitch muscle (slow muscle). We identified a missense mutation in the gene encoding Connexin 39.9. In situ hybridization found connexin 39.9 to be expressed by slow muscle. Paired muscle recordings uncovered that wild-type slow muscles are electrically coupled, whereas mutant slow muscles are not. The further examination of cellular activity revealed aberrant, arrhythmic touch-evoked Ca(2+) transients in mutant slow muscle and a reduction in the number of muscle fibers contracting in response to touch in mutants. These results indicate that Connexin 39.9 facilitates the spreading of neuronal inputs, which is irregular during motor development, beyond the muscle cells and that gap junctions play an essential role in the efficient recruitment of slow muscle fibers.

Early neural development in vertebrates is also a matter of calcium

The calcium (Ca(2+)) signaling pathways have crucial roles in development from fertilization through differentiation to organogenesis. In the nervous system, Ca(2+) signals are important regulators for various neuronal functions, including formation and maturation of neuronal circuits and long-term memory. However, Ca(2+) signals are mainly involved in the earliest steps of nervous system development including neural induction, differentiation of neural progenitors into neurons, and the neuro-glial switch. This review examines when and how Ca(2+) signals are generated during each of these steps with examples taken from in vivo studies in vertebrate embryos and from in vitro assays using embryonic and neural stem cells. Also discussed is the highly specific nature of the Ca(2+) signaling pathway and its interaction with the other signaling pathways involved in early neural development.

Visualization of Ca²+ signaling during embryonic skeletal muscle formation in vertebrates

Dynamic changes in cytosolic and nuclear Ca(2+) concentration are reported to play a critical regulatory role in different aspects of skeletal muscle development and differentiation. Here we review our current knowledge of the spatial dynamics of Ca(2+) signals generated during muscle development in mouse, rat, and Xenopus myocytes in culture, in the exposed myotome of dissected Xenopus embryos, and in intact normally developing zebrafish. It is becoming clear that subcellular domains, either membrane-bound or otherwise, may have their own Ca(2+) signaling signatures. Thus, to understand the roles played by myogenic Ca(2+) signaling, we must consider: (1) the triggers and targets within these signaling domains; (2) interdomain signaling, and (3) how these Ca(2+) signals integrate with other signaling networks involved in myogenesis. Imaging techniques that are currently available to provide direct visualization of these Ca(2+) signals are also described.

Calcium signalling in growth cone migration

Growth cones, the motile structures at the tips of advancing axons and dendrites, respond to a wide range of cues by either turning towards or away from the cue. Cytosolic calcium signals appear to mediate a large fraction of both types of response. Calcium signals can be generated by influx through plasma membrane channels or by release from intracellular stores. While neurotransmitters can elicit calcium influx through ionotropic receptors, other chemical cues open plasma membrane voltage gated calcium channels by a mechanism other than a change of membrane voltage. In general attractive cues generate spatially and temporally restricted calcium increases that are difficult to detect using conventional indicators. One target for these calcium signals is calmodulin dependent protein kinase II. Repulsive cues generate spatially and temporally more diffuse calcium increases that can be more readily detected using fluorescent indicators. One target for these is the phosphatase calcineurin, which may act by dephosphorylating GAP43 and allowing the latter to cap actin filaments.

Acetylcholine and calcium signalling regulates muscle fibre formation in the zebrafish embryo

Nerve activity is known to be an important regulator of muscle phenotype in the adult, but its contribution to muscle development during embryogenesis remains unresolved. We used the zebrafish embryo and in vivo imaging approaches to address the role of activity-generated signals, acetylcholine and intracellular calcium, in vertebrate slow muscle development. We show that acetylcholine drives initial muscle contraction and embryonic movement via release of intracellular calcium from ryanodine receptors. Inhibition of this activity-dependent pathway at the level of the acetylcholine receptor or ryanodine receptor did not disrupt slow fibre number, elongation or migration but affected myofibril organisation. In mutants lacking functional acetylcholine receptors myofibre length increased and sarcomere length decreased significantly. We propose that calcium is acting via the cytoskeleton to regulate myofibril organisation. Within a myofibre, sarcomere length and number are the key parameters regulating force generation; hence our findings imply a critical role for nerve-mediated calcium signals in the formation of physiologically functional muscle units during development.

Teratogenic effects of azaspiracid-1 identified by microinjection of Japanese medaka (Oryzias latipes) embryos

Azaspiracid-1 (AZA-1) is a newly identified phycotoxin that accumulates in commercially important bivalve molluscs harvested in several European countries and causes severe human intoxications. Molluscan shellfish are known vectors for accumulation and subsequent transfer of phycotoxins such as brevetoxin and domoic acid through various trophic levels within food webs. Finfish can also accumulate phycotoxins, both directly from toxic algae or from consumption of contaminated shellfish and smaller intoxicated fish. To evaluate the teratogenic potential of AZA-1 and its relevancy to toxin accumulation in finfish, we have utilized a microinjection technique to mimic the maternal-egg toxin transfer of an AZA-1 reference standard and a shellfish extract containing azaspiracids in an embryonic Japanese medaka (Oryzias latipes) fish model. Microinjection of purified AZA-1 caused dose-dependent effects on heart rate, developmental rate, hatching success, and viability in medaka embryos. Within 4 days of exposure to doses > or = 40 pg AZA-1/egg, substantial retardation in development was observed as reduced somatic growth and yolk absorption, and delayed onset of blood circulation and pigmentation. Embryos treated to > or =40 pg AZA-1/egg had slower heart rates (bradycardia) for the 9 days in ovo period, followed by reduced hatching success. Microinjection of a contaminated mussel (Mytilus edulis) extract containing AZAs (AZA-1, -2, and -3), okadaic acid, and dinophysistoxin-2 resulted in similar responses from the fish embryos at equivalent doses. These studies demonstrate that AZA-1 is a potent teratogen to finfish. This work will complement future investigations on AZA-1 accumulation in marine food webs and provide a basis for understanding its toxicity at different trophic levels.

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.

"In vivo" monitoring of neuronal network activity in zebrafish by two-photon Ca(2+) imaging

The zebrafish larva is a powerful model for the analysis of behaviour and the underlying neuronal network activity during early stages of development. Here we employ a new approach of "in vivo" Ca(2+) imaging in this preparation. We demonstrate that bolus injection of membrane-permeable Ca(2+) indicator dyes into the spinal cord of zebrafish larvae results in rapid staining of essentially the entire spinal cord. Using two-photon imaging, we could monitor Ca(2+) signals simultaneously from a large population of spinal neurons with single-cell resolution. To test the method, Ca(2+) transients were produced by iontophoretic application of glutamate and, as observed for the first time in a living preparation, of GABA or glycine. Glycine-evoked Ca(2+) transients were blocked by the application of strychnine. Sensory stimuli that trigger escape reflexes in mobile zebrafish evoked Ca(2+) transients in distinct neurons of the spinal network. Moreover, long-term recordings revealed spontaneous Ca(2+) transients in individual spinal neurons. Frequently, this activity occurred synchronously among many neurons in the network. In conclusion, the new approach permits a reliable analysis with single-cell resolution of the functional organisation of developing neuronal networks.

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+.

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.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.