Lithium-sensitive production of inositol phosphates during amphibian embryonic mesoderm induction

Towards a Unified Understanding of Lithium Action in Basic Biology and its Significance for Applied Biology

Lithium has literally been everywhere forever, since it is one of the three elements created in the Big Bang. Lithium concentration in rocks, soil, and fresh water is highly variable from place to place, and has varied widely in specific regions over evolutionary and geologic time. The biological effects of lithium are many and varied. Based on experiments in which animals are deprived of lithium, lithium is an essential nutrient. At the other extreme, at lithium ingestion sufficient to raise blood concentration significantly over 1 mM/, lithium is acutely toxic. There is no consensus regarding optimum levels of lithium intake for populations or individuals-with the single exception that lithium is a generally accepted first-line therapy for bipolar disorder, and specific dosage guidelines for sufferers of that condition are generally agreed on. Epidemiological evidence correlating various markers of social dysfunction and disease vs. lithium level in drinking water suggest benefits of moderately elevated lithium compared to average levels of lithium intake. In contrast to other biologically significant ions, lithium is unusual in not having its concentration in fluids of multicellular animals closely regulated. For hydrogen ions, sodium ions, potassium ions, calcium ions, chloride ions, and magnesium ions, blood and extracellular fluid concentrations are closely and necessarily regulated by systems of highly selective channels, and primary and secondary active transporters. Lithium, while having strong biological activity, is tolerated over body fluid concentrations ranging over many orders of magnitude. The lack of biological regulation of lithium appears due to lack of lithium-specific binding sites and selectivity filters. Rather lithium exerts its myriad physiological and biochemical effects by competing for macromolecular sites that are relatively specific for other cations, most especially for sodium and magnesium. This review will consider what is known about the nature of this competition and suggest using and extending this knowledge towards the goal of a unified understanding of lithium in biology and the application of that understanding in medicine and nutrition.

New insights into negative effects of lithium on sea urchin Paracentrotus lividus embryos

The diffuse use of lithium in a number of industrial processes has produced a significant contamination of groundwater and surface water with it. The increased use of lithium has generated only scarce studies on its concentrations in ambient waters and on its effects on aquatic organisms. Only few contributions have focused on the toxicity of lithium in marine organisms (such as marine animals, algae and vegetables), showing that the toxic effect depends on the animal species. In the present study we describe the morphological and the molecular effects of lithium chloride (LiCl), using the sea urchin Paracentrotus lividus as a model organism. We show that LiCl, if added to the eggs before fertilization, induces malformations in the embryos in a dose-dependent manner. We have also followed by RT qPCR the expression levels of thirty seven genes (belonging to different classes of functional processes, such as stress, development, differentiation, skeletogenesis and detoxifications) to identify the molecular targets of LiCl. This study opens new perspectives for the understanding of the mechanism of action of lithium on marine organisms. The findings may also have relevance outside the world of marine organisms since lithium is widely prescribed for the treatment of human bipolar disorders.

Ca²⁺ influx-linked protein kinase C activity regulates the β-catenin localization, micromere induction signalling and the oral-aboral axis formation in early sea urchin embryos

Sea urchin embryos initiate cell specifications at the 16-cell stage by forming the mesomeres, macromeres and micromeres according to the relative position of the cells in the animal-vegetal axis. The most vegetal cells, micromeres, autonomously differentiate into skeletons and induce the neighbouring macromere cells to become mesoendoderm in the β-catenin-dependent Wnt8 signalling pathway. Although the underlying molecular mechanism for this progression is largely unknown, we have previously reported that the initial events might be triggered by the Ca2+ influxes through the egg-originated L-type Ca2+ channels distributed asymmetrically along the animal-vegetal axis and through the stretch-dependent Ca2+channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca2+ targets, protein kinase C (PKC), plays a role in cell specification upstream of β-catenin. To this end, we surveyed the expression pattern of β-catenin in early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotus pulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of β-catenin does not take place in the micromeres, but in the macromeres at the 16-cell stage. Using the HpPKC-I, we have demonstrated further that PKC not only determines cell-specific nucleation of β-catenin, but also regulates a variety of cell specification events in the early sea urchin embryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca2+ signalling, and the expression of key transcription factors.

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.

Synaptic polarity depends on phosphatidylinositol signaling regulated by myo-inositol monophosphatase in Caenorhabditis elegans

Although neurons are highly polarized, how neuronal polarity is generated remains poorly understood. An evolutionarily conserved inositol-producing enzyme myo-inositol monophosphatase (IMPase) is essential for polarized localization of synaptic molecules in Caenorhabditis elegans and can be inhibited by lithium, a drug for bipolar disorder. The synaptic defect of IMPase mutants causes defects in sensory behaviors including thermotaxis. Here we show that the abnormalities of IMPase mutants can be suppressed by mutations in two enzymes, phospholipase Cβ or synaptojanin, which presumably reduce the level of membrane phosphatidylinositol 4,5-bisphosphate (PIP₂). We also found that mutations in phospholipase Cβ conferred resistance to lithium treatment. Our results suggest that reduction of PIP₂ on plasma membrane is a major cause of abnormal synaptic polarity in IMPase mutants and provide the first in vivo evidence that lithium impairs neuronal PIP₂ synthesis through inhibition of IMPase. We propose that the PIP₂ signaling regulated by IMPase plays a novel and fundamental role in the synaptic polarity.

Calcium dynamics integrated into signalling pathways that influence vertebrate axial patterning

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

Cellular plasticity cascades in the pathophysiology and treatment of bipolar disorder

Bipolar disorder (BPD) is characterized by recurrent episodes of disturbed affect including mania and depression as well as changes in psychovegetative function, cognitive performance, and general health. A growing body of data suggests that BPD arises from abnormalities in synaptic and neuronal plasticity cascades, leading to aberrant information processing in critical synapses and circuits. Thus, these illnesses can best be conceptualized as genetically influenced disorders of synapses and circuits rather than simply as deficits or excesses in individual neurotransmitters. In addition, commonly used mood-stabilizing drugs that are effective in treating BPD have been shown to target intracellular signaling pathways that control synaptic plasticity and cellular resilience. In this article we draw on clinical, preclinical, neuroimaging, and post-mortem data to discuss the neurobiology of BPD within a conceptual framework while highlighting the role of neuroplasticity in the pathophysiology and treatment of this disorder.

Calcium signaling in vertebrate embryonic patterning and morphogenesis

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

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

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

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

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

Heteromeric, but not homomeric, connexin channels are selectively permeable to inositol phosphates

Previous work has shown that channels formed by both connexin (Cx)26 and Cx32 (heteromeric Cx26/Cx32 hemichannels) are selectively permeable to cAMP and cGMP. To further investigate differential connexin channel permeability among second messengers, and the influence of connexin channel composition on the selectivity, the permeability of inositol phosphates with one to four phosphate groups through homomeric Cx26, homomeric Cx32, and heteromeric Cx26/Cx32 channels was examined. Connexin channels were purified from transfected HeLa cells and from rat, mouse, and guinea pig livers, resulting in channels with a broad range of Cx26/Cx32 aggregate ratios. Permeability to inositol phosphates was assessed by flux through reconstituted channels. Surprisingly, myoinositol and all inositol phosphates tested were permeable through homomeric Cx32 and homomeric Cx26 channels. Even more surprising, heteromeric Cx26/Cx32 channels showed striking differences in permeability among inositol phosphates with three or four phosphate groups and among isomers of inositol triphosphate. Thus, heteromeric channels are selectively permeable among inositol phosphates, whereas the corresponding homomeric channels are not. There was no discernible difference in the permeability of channels with similar Cx26/Cx32 ratios purified from native and heterologous sources. The molecular selectivity of heteromeric channels among three inositol triphosphates could not be accounted for by simple connexin isoform stoichiometry distributions and therefore may depend on specific isoform radial arrangements within the hexameric channels. Dynamic regulation of channel composition in vivo may effectively and efficiently modulate intercellular signaling by inositol phosphates.

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.

Genomic organization and regulation of the LeIMP-1 and LeIMP-2 genes encoding myo-inositol monophosphatase in tomato

Myo-inositol (inositol) monophosphatase (IMP), an enzyme which catalyzes the synthesis of free inositol from various inositol monophosphates, is encoded by a small multigene family in many organisms. The tomato IMP gene family encodes three IMP isoforms with identical in vitro biochemical properties. To determine the role of each tomato LeIMP gene in plant growth, we isolated the genomic DNA copies of the LeIMP-1 and LeIMP-2 genes. The LeIMP-1 gene spans approximately 5.8 kb and consists of 12 exons, whereas the LeIMP-2 gene consists of an uninterrupted, single open reading frame (ORF). We have previously shown that steady-state levels of LeIMP-2 mRNA were very low in comparison to LeIMP-1 and LeIMP-3 mRNA levels. To determine whether LeIMP-2 gene expression was spatially restricted to a discreet domain within the plant we constructed transgenic plants containing an LeIMP-2 promoter::uidA gene fusion. Analysis of transgenic seedlings revealed that the LeIMP-2 promoter directed gene expression within epidermal and cortex cells of specific stem/leaf junctions in an abaxial-specific pattern and in the shoot apical meristem. Further, inositol, the product of IMP catalysis, and Li+, an inhibitor of IMP catalysis, decreased expression of the LeIMP-2 promoter as measured by a decrease in beta-glucuronidase activity after treatment.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Requirement for intracellular calcium modulation in zebrafish dorsal-ventral patterning

The phosphoinositide (PI) cycle is an important signal transduction pathway that, upon activation, generates intracellular second messengers and leads to calcium release. To determine whether PI cycle-mediated intracellular calcium release is required for body plan formation, we systematically dissect PI cycle function in the zebrafish (Danio rerio). We inhibit PI cycle function at three different steps and deplete internal calcium stores, demonstrating an impact on endogenous calcium release and Wnt/beta-catenin signaling. Inhibition of endogenous calcium modulation induces hyperdorsalized phenotypes in a dose-dependent manner. Ectopic dorsal-signaling centers are generated in PI cycle-inhibited embryos as demonstrated by altered beta-catenin subcellular localization and ectopic expression of beta-catenin target genes. These results provide evidence that modulation of calcium release is critical for early embryonic patterning and acts by influencing the stabilization of beta-catenin protein.

Statistical evaluation of differential expression on cDNA nylon arrays with replicated experiments

In this paper we focus on the detection of differentially expressed genes according to changes in hybridization signals using statistical tests. These tests were applied to 14 208 zebrafish cDNA clones that were immobilized on a nylon support and hybridized with radioactively labeled target mRNA from wild-type and lithium-treated zebrafish embryos. The methods were evaluated with respect to 16 control clones that correspond to eight different genes which are known to be involved in dorso-ventral axis specification. Moreover, 4608 Arabidopsis thaliana clones on the same array were used to judge statistical significance of expression changes and to control the false positive rates of the test decisions. Utilizing this special array design we show that differential expression of a high proportion of cDNA clones (15/16) and the respective genes (7/8) were identified, with a false positive error of <5% using the constant control data. Furthermore, we investigated the influence of the number of repetitions of experiments on the accuracy of the procedures with experimental and simulated data. Our results suggest that the detection of differential expression with repeated hybridization experiments is an accurate and sensitive way of identifying even small expression changes (1:1.5) of a large number of genes in parallel.

Molecular targets of lithium action

Lithium is highly effective in the treatment of bipolar disorder and also has multiple effects on embryonic development, glycogen synthesis, hematopoiesis, and other processes. However, the mechanism of lithium action is still unclear. A number of enzymes have been proposed as potential targets of lithium action, including inositol monophosphatase, a family of structurally related phosphomonoesterases, and the protein kinase glycogen synthase kinase-3. These potential targets are widely expressed, require metal ions for catalysis, and are generally inhibited by lithium in an uncompetitive manner, most likely by displacing a divalent cation. Thus, the challenge is to determine which target, if any, is responsible for a given response to lithium in cells. Comparison of lithium effects with genetic disruption of putative target molecules has helped to validate these targets, and the use of alternative inhibitors of a given target can also lend strong support for or against a proposed mechanism of lithium action. In this review, lithium sensitive enzymes are discussed, and a number of criteria are proposed to evaluate which of these enzymes are involved in the response to lithium in a given setting.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

Calmodulin differentially modulates Smad1 and Smad2 signaling

The members of the Smad protein family are intracellular mediators of transforming growth factor beta (TGF-beta) signaling. Smad1 transduces bone morphogenetic protein signals, inducing formation of ventral mesoderm in Xenopus embryos, whereas Smad2 transduces activin/TGF-beta signals, generating dorsal mesoderm. Calmodulin directly binds to many Smads and was shown to down-regulate Smad2 activity in a cell culture system (Zimmerman, C. M., Kariapper, M. S. T., and Mathews, L. S. (1997) J. Biol. Chem. 273, 677-680). Here, we extend those data and demonstrate that calmodulin alters Smad signaling in living embryos, increasing Smad1 activity while inhibiting Smad2 function. To characterize this regulation, we undertook a structure-function analysis and found that calmodulin binds to two distinct and conserved regions in both Smad1 and Smad2. Receptor tyrosine kinase signaling also modifies Smad activity (Kretzschmar, M., Doody, J., and Massagué, J. (1997) Nature 389, 618-622; Kretzschmar, M., Doody, J., Timokhina, I., and Massagué, J. (1999) Genes Dev. 13, 804-816; de Caestecker, M. P., Parks, W. T., Frank, C. J., Castagnino, P., Bottaro, D. P., Roberts, A. B., and Lechleider, R. J. (1998) Genes Dev. 12, 1587-1592). We show that calmodulin binding to Smads inhibits subsequent Erk2-dependent phosphorylation of Smads and vice versa. These observations suggest the presence of a cross-talk between three major signaling cascades as follows: Ca(2+)/calmodulin, receptor tyrosine kinase, and TGF-beta pathways.

Galphas family G proteins activate IP(3)-Ca(2+) signaling via gbetagamma and transduce ventralizing signals in Xenopus

During early embryonic development, IP(3)-Ca(2+) signaling transduces ventral signaling at the time of dorsoventral axis formation. To identify molecules functioning upstream in this signal pathway, we examined effects of a panel of inhibitory antibodies against Galphaq/11, Galphas/olf, or Galphai/o/t/z. While all these antibodies showed direct inhibition of their targets, their effects on redirection of the ventral mesoderm to a dorsal fate varied. Anti-Galphas/olf antibody showed strong induction of dorsal fate, anti-Galphai/o/t/z antibody did so weakly, and anti-Galphaq/11 antibody was without effect. Injection of betaARK, a Gbetagamma inhibitor, mimicked the dorsalizing effect of anti-Galphas/olf antibody, whereas injection of adenylyl cyclase inhibitors at a concentration which inhibited Galphas-coupled cAMP increase did not do so. The activation of Galphas-coupled receptor gave rise to Ca(2+) transients. All these results suggest that activation of the Galphas-coupled receptor relays dorsoventral signal to Gbetagamma, which then stimulates PLCbeta and then the IP(3)-Ca(2+) system. This signaling pathway may play a crucial role in transducing ventral signals.

The versatility and universality of calcium signalling

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

Desensitization of IP3-induced Ca2+ release by overexpression of a constitutively active Gqalpha protein converts ventral to dorsal fate in Xenopus early embryos

The constitutively active Gqalpha mutant construct (GqalphaQ-L) in Xenopus early embryos was overexpressed and the effects on dorsoventral patterning examined. It was found that prolonged stimulation of inositol 1,4,5-trisphosphate (IP3)-Ca2+ signaling by overexpression of GqalphaQ-L led to desensitization of IP3-induced Ca2+ release (IICR). Desensitization of IICR on the ventral side specifically induced an ectopic dorsal axis due to the conversion of ventral marginal mesoderm to adopt a dorsal fate. This effect of desensitization resembles that of inhibitory antibodies against the IP3 receptor, as reported previously. These results strengthen the earlier finding that active IP3-Ca2+ signaling functions in ventral signaling during the early embryonic development of Xenopus. Furthermore, the nature of downregulation of the Xenopus IP3 receptor through continuous stimulation of IP3-Ca2+ signaling might play a role in regulating endogenous IP3-Ca2+ signaling in Xenopus early development.

RGS proteins inhibit Xwnt-8 signaling in Xenopus embryonic development

RGS family members are GTPase activating proteins (GAPs) that antagonize signaling by heterotrimeric G proteins. Injection of Xenopus embryos with RNA encoding rat RGS4 (rRGS4), a GAP for G(i) and G(q), resulted in shortened trunks and decreased skeletal muscle. This phenotype is nearly identical to the effect of injection of either frzb or dominant negative Xwnt-8. Injection of human RGS2, which selectively deactivates G(q), had similar effects. rRGS4 inhibited the ability of early Xwnt-8 but not Xdsh misexpression to cause axis duplication. This effect is distinct from axin family members that contain RGS-like domains but act downstream of Xdsh. We identified two Xenopus RGS4 homologs, one of which, Xrgs4a, was expressed as a Spemann organizer component. Injection of Xenopus embryos with Xrgs4a also resulted in shortened trunks and decreased skeletal muscle. These results suggest that RGS proteins modulate Xwnt-8 signaling by attenuating the function of a G protein.

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.

The involvement of cAMP signaling pathway in axis specification in Xenopus embryos

The cAMP signaling system has been postulated to be involved in embryogenesis of many animal species, however, little is known about its role in embryonic axis formation in vertebrates. In this study, the role of the cAMP signaling pathway in patterning the body plan of the Xenopus embryo was investigated by expressing and activating the exogenous human 5-hydroxytryptamine type 1a receptor (5-HT(1a)R) which inhibits adenylyl cyclase through inhibitory G-protein in embryos in a spatially- and temporally-controlled manner. In embryos, ventral, but not dorsal expression and stimulation of this receptor during blastula and gastrula stages induced secondary axes but were lacking anterior structures. At the molecular level, 5-HT(1a)R stimulation induced expression of the dorsal mesoderm marker genes, and downregulated expression of the ventral markers but had no effect on expression of the pan mesodermal marker gene in ventral marginal zone explants. In addition, ventral expression and stimulation of the receptor partially restored dorsal axis of UV-irradiated axis deficient embryo. Finally, the total mass of cAMP differs between dorsal and ventral regions of blastula and gastrula embryos and this is regulated in a temporally-specific manner. These results suggest that the cAMP signaling system may be involved in the transduction of ventral signals in patterning early embryos.

Signalling pathways in the brain: cellular transduction of mood stabilisation in the treatment of manic-depressive illness

The long-term treatment of manic-depressive illness (MDI) likely involves the strategic regulation of signalling pathways and gene expression in critical neuronal circuits. Accumulated evidence has identified signalling pathways, in particular the family of protein kinase C (PKC) isozymes, as targets for the long-term action of lithium. Chronic lithium administration produces a reduction in the expression of PKC alpha and epsilon, as well as a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. More recently, studies have demonstrated robust effects of lithium on another kinase system, GSK-3beta, and on neuroprotective/neurotrophic proteins in the brain. Given the key roles of these signalling cascades in the amplification and integration of signals in the central nervous system, these findings have clear implications not only for research into the neurobiology of MDI, but also for the future development of novel and innovative treatment strategies.

Effects of lithium on pigmentation in the embryonic zebrafish (Brachydanio rerio)

Pigment cell precursors of the embryonic zebrafish give rise to melanophores, xanthophores and/or iridophores. Cell signaling mechanisms related to the development of pigmentation remain obscure. In order to examine the mechanisms involved in pigment cell signaling, we treated zebrafish embryos with various activators and inhibitors of signaling pathways. Among those chemicals tested, LiCl and LiCl/forskolin had a stimulatory effect on pigmentation, most notable in the melanophore population. We propose that the inositol phosphate (IP) pathway, is involved in pigment pattern formation in zebrafish through its involvement in the: (1) differentiation/proliferation of melanophores; (2) dispersion of melanosomes; and/or (3) synthesis/deposition of melanin. To discern at what level pigmentation was being effected we: (1) counted the number of melanophores in control and experimental animals 5 days after treatment; (2) measured tyrosinase activity and melanin content; and (3) employed immunoblotting techniques with anti-tyrosine-related protein-2 and anti-melanocyte-specific gene-1 as melanophore-specific markers. Although gross pigmentation increased dramatically in LiCl- and LiCl/forskolin treated embryos, the effect on pigmentation was not due to an increase in the proliferation of melanophores, but was possibly through an increase in melanin synthesis and/or deposition. Collectively, results from these studies suggest the involvement of an IP-signaling pathway in the stimulation of pigmentation in embryonic zebrafish through the synthesis/deposition of melanin within the neural crest-derived melanophores.

Crosstalk between the phosphatidylinositol cycle and MAP kinase signaling pathways in Xenopus mesoderm induction

Recent studies have established a role for the phosphoinositide (PI) cycle in the early patterning of Xenopus mesoderm. In explants, stimulation of this pathway in the absence of growth factors does not induce mesoderm, but when accompanied by growth factor treatment, simultaneous PI cycle stimulation results in profound morphological and molecular changes in the mesoderm induced by the growth factor. This suggests the possibility that the PI cycle exerts its influence via crosstalk, by modulating some primary mesoderm-inducing pathway. Given recent identification of mitogen-activated protein kinase (MAPK) as an intracellular mediator of some mesoderm-inducing signals, the present study explores MAPK as a potential site of PI cycle-mediated crosstalk. We report that MAPK activity, like PI cycle activity, increases in intact embryos during mesoderm induction. Phosphoinositide cycle stimulation during treatment of explants with basic fibroblast growth factor (bFGF) synergistically increases late-phase MAPK activity and potentiates bFGF-induced expression of Xbra, a MAPK-dependent mesodermal marker.

Role of inositol 1,4,5-trisphosphate receptor in ventral signaling in Xenopus embryos

The inositol 1,4,5-trisphosphate (IP3) receptor is a calcium ion channel involved in the release of free Ca2+ from intracellular stores. For analysis of the role of IP3-induced Ca2+ release (IICR) on patterning of the embryonic body, monoclonal antibodies that inhibit IICR were produced. Injection of these blocking antibodies into the ventral part of early Xenopus embryos induced modest dorsal differentiation. A close correlation between IICR blocking potencies and ectopic dorsal axis induction frequency suggests that an active IP3-Ca2+ signal may participate in the modulation of ventral differentiation.

Developmental expression of the inositol 1,4,5-trisphosphate receptor and localization of inositol 1,4,5-trisphosphate during early embryogenesis in Xenopus laevis

To study the role of inositol 1,4,5-trisphosphate (IP3) receptors during early embryogenesis in Xenopus, we examined the temporal-spatial localization of Xenopus IP3 receptor (XIP3R). XIP3R protein is enriched in the animal hemisphere of early cleavage stage embryos and becomes localized in the ectoderm and involuted mesoderm in gastrula stage embryos. Up to tailbud stages, expression of XIP3R is observed in the mesodermal tissues and in most subregions of the central nervous system. A quantitative analysis of endogenous IP3 mass during normal early embryogenesis revealed an increase in IP3 mass first observed at early gastrula stage of 10.5 with an enrichment in the ectoderm throughout the gastrula stages, implying a potential role during gastrulation.

Mutations causing achondroplasia and thanatophoric dysplasia alter bFGF-induced calcium signals in human diploid fibroblasts

Mutations in the fibroblast growth factor receptor (FGFR) gene family recently have been shown to underlie several hereditary disorders of bone development, with specific FGFR3 mutations causing achondroplasia (Ach) and thanatophoric dysplasia (TD). However, for none of these mutations has the defect in receptor function been demonstrated directly and, therefore, for none has the pathophysiological mechanism of the disease been defined. Using our established techniques for single-cell ratiometric real-time calcium image analysis, we defined the nature of the basic fibroblast growth factor (bFGF)-induced calcium signal in human diploid fibroblasts, and, in blinded studies, have analyzed the bFGF-induced signals from 18 independent fibroblast cell lines, including multiple lines from patients with known mutant alleles of FGFR3 and syndromes of Ach or TD. Control cells responded with transient increases in intracellular calcium, with many cells showing oscillatory calcium waves. Homozygous Ach cell lines failed to signal, whereas heterozygous Ach lines responded nearly normally. We observed heterogeneous signals in TD heterozygotes: the unresponsive lines all turned out to carry TD1 alleles, whereas all responsive lines had TD2 alleles. Since FGFR1, 2 and 3 receptors are known to be expressed in fibroblasts, our results suggest that specific mutant FGFR3 alleles can function in a dosage-dependent dominant-negative fashion to inactivate FGFR signaling.

Activation of the Wnt signaling pathway: a molecular mechanism for lithium action

Glycogen synthase kinase-3 beta (GSK-3 beta/zeste-white-3/shaggy) is a negative regulator of the wnt signaling pathway which plays a central role in the development of invertebrates and vertebrates; loss of function and dominant negative mutations in GSK-3 beta lead to activation of the wnt pathway in Drosophila and Xenopus. We now provide evidence that lithium activates downstream components of the wnt signaling pathway in vivo, leading to accumulation of beta-catenin protein. Our data indicate that this activation of the wnt pathway is a consequence of inhibition of GSK-3 beta by lithium. Using a novel assay for GSK-3 beta in oocytes, we show that lithium inhibits GSK-3 beta from species as diverse as Dictyostelium discoideum and Xenopus laevis, providing a biochemical mechanism for the action of lithium on the development of these organisms. Lithium treatment also leads to activation of an AP-1-luciferase reporter in Xenopus embryos, consistent with previous observations that GSK-3 beta inhibits c-jun activity. Activation of the wnt pathway with a dominant negative form of GSK-3 beta is inhibited by myo-inositol, similar to the previously described effect of coinjecting myo-inositol with lithium. The mechanism by which myo-inositol inhibits both dominant negative GSK-3 beta and lithium remains uncertain.

Gene cloning and characterization of CDP-diacylglycerol synthase from rat brain

A cDNA encoded a 462-amino acid protein, which showed CDP-diacylglycerol synthase (CDS) activity was cloned for the first time as the vertebrate enzyme molecule from rat brain cDNA library. The deduced molecular mass of this rat CDS was 53 kDa, and putative primary structure included several possible membrane- spanning regions. At the amino acid sequence level, rat CDS shared 55.5%, 31. 7%, and 20.9% identity with already known Drosophila, Saccharomyces cerevisiae, and Escherichia coli CDS, respectively. This rat CDS preferred 1-stearoyl-2-arachidonoyl phosphatidic acid as a substrate, and its activity was strongly inhibited by phosphatidylglycerol 4, 5-bisphosphate. By immunoblotting analysis of COS cells overexpressed with the epitope-tagged for rat CDS, a 60-kDa band was detected. By epitope-tag immunocytochemistry, the CDS protein was mainly localized in close association with the membrane of the endoplasmic reticulum of the transfected cells. The intense mRNA expression of CDS was localized in the cerebellar Purkinje cells, the pineal body, and the inner segment of photoreceptor cells. Additionally, very intense expression was detected in postmitotic spermatocytes and spermatids.

Modulation of embryonic intracellular Ca2+ signaling by Wnt-5A

Wnt genes encode secreted proteins which are implicated in receptor-mediated cell-cell signaling events important in embryogenesis, but the second messenger systems modulated by Wnts have not been identified. We report that ectopic expression of Xwnt-5A in zebrafish embryos enhances the frequency of intracellular Ca2+ transients in the enveloping layer of the blastodisc, whereas Xwnt-8 does not. These transients are independent of extracellular Ca2+. Consistent with the observed Ca2+ transients playing a role in responses of embryos to Xwnt-5A, we report that the ligand-activated serotonin type 1C receptor, which stimulates PI cycle activity and Ca2+ signaling independent of Wnts, phenocopies embryonic responses to Xwnt-5A. These results suggest that intercellular signaling by a subset of vertebrate Wnts involves modulation of a intracellular Ca2+ signaling pathway, which may arise from phosphatidylinositol cycle activity.

Modulation of Xenopus embryo mesoderm-specific gene expression and dorsoanterior patterning by receptors that activate the phosphatidylinositol cycle signal transduction pathway

A role for the phosphatidylinositol (PI) cycle signal transduction pathway in Xenopus mesoderm induction has been revealed by observations of PI cycle activation coincident with this process, combined with the demonstration that Li+ (a PI cycle inhibitor) blocks this response and hyperdorsalizes mesoderm induction in intact embryos or augments growth factor-mediated induction in animal caps. It has been suggested that spatially restricted PI cycle activity in the marginal zone might modulate (but not, itself, activate) mesoderm induction. To better characterize the ability of PI cycle activity to modulate the pattern of mesoderm-specific gene expression elicited by mesoderm-inducing growth factors we have expressed in the embryo exogenous 5-hydroxytryptamine receptors that activate the PI cycle. In embryos, ventral expression and activation of these receptors during mesoderm induction are without obvious effect, whereas dorsal expression and activation yield dorsoanterior-deficient tadpoles. In animal caps induced with activin, simultaneous activation of exogenous 5-hydroxytryptamine receptors inhibits both convergent extension movements associated with dorsal mesoderm induction and the expression of goosecoid, a dorsal-specific gene, but is without effect on expression of a 149 generic mesodermal marker, Xbra. All of these effects of a 149 PI cycle-stimulating receptor are the opposites of those previously reported for the PI cycle inhibitor, Li+. PI cycle activity thus proves able to modulate the dorsal/ventral character of early mesodermal gene expression elicited by growth factor, suggesting a model for mesodermal patterning.

Effect of activin and lithium on isolated Xenopus animal blastomeres and response alteration at the midblastula transition

Dorsoventral mesoderm patterning in the amphibian embryo involves a series of interactions mediated by several peptide growth factors. Animal blastomeres isolated at the 8-cell stage are useful for studying mesoderm patterning, since they contain the prospective (uninduced) mesoderm region and allow examination of the default state of animal cells. When activin is applied to these dorsal and ventral animal half explants, a competence prepattern for responding to activin is observed. In order to investigate the characteristics of prepatterning, we treated animal blastomeres with the embryo dorsalizing agent LiCl. Treatment with lithium alone did not induce normal trunk mesoderm in either blastomere. Lithium did, however, alter the competence of animal blastomeres to activin. Dorsal mesoderm was formed in the ventral blastomeres, as well as in the dorsal blastomeres. This result reveals that the early dorsoventral polarity in the animal hemisphere is not fixed. Using goosecoid(gsc) and Xwnt-8 genes as dorsal and ventral mesoderm markers, it was verified that lithium modifies the competence to activin. Unexpectedly, lithium treatment on its own resulted in gsc expression in the animal half explants. This suggests that embryo goosecoid expression may be induced by the effect of dorsal determination activity, but not by mesoderm induction. However, lithium induced also the expression of brachyury (Xbra) gene at very low levels. This would indicate the formation of dorsal-anterior mesoderm, which was not identified by the tissue observations. Expression of Xwnt-8, a ventral mesoderm marker usually induced in blastula animal caps by activin, was hardly induced in the blastomere explants. We isolated whole animal half explants at the 8-cell stage and exposed to activin at different stages. It was found that the same concentration of activin induces gsc before the midblastula stage, and induces Xwnt-8 at later stages. This suggests that the response of animal blastomeres alters depending on the stage of activin signaling.

Regulation of Spemann organizer formation by the intracellular kinase Xgsk-3

Dorsal axis formation in the Xenopus embryo can be induced by the ectopic expression of several Wnt family members. In Drosophila, the protein encoded by the Wnt family gene, wingless, signals through a pathway that antagonizes the effects of the serine/threonine kinase zeste-white 3/shaggy. We describe the isolation and characterization of a Xenopus homolog of zeste-white 3/shaggy, Xgsk-3. A kinase-dead mutant of Xgsk-3, Xgsk-3K--&gt;R, has a dominant negative effect and mimics the ability of Wnt to induce a secondary axis by induction of an ectopic Spemann organizer. Xgsk-3K--&gt;R, like Wnt, induces dorsal axis formation when expressed in the deep vegetal cells, which do not contribute to the axis. These results indicate that the dorsal fate is actively repressed by Xgsk-3, which must be inactivated for dorsal axis formation to occur. Furthermore, our work suggests that the effects of Xgsk-3K--&gt;R are mediated by an additional intercellular signal.

Regulation of PLC-mediated signalling in vivo by CDP-diacylglycerol synthase

CDP-diacylglycerol synthase (CDS) is an enzyme required for the regeneration of the signalling molecule phosphatidylinositol-4,5-bisphosphate (PtdlnsP2) from phosphatidic acid. A photo-receptor cell-specific isoform of CDS from Drosophila is a key regulator of phototransduction, a G-protein-coupled signalling cascade mediated by phospholipase C. cds mutants cannot sustain a light-activated current as a result of depletion of PtdlnsP2. Overexpression of CDS increases the amplitude of the light response, demonstrating that availability of PtdlnsP2 is a determinant in the gain of this pathway. cds mutants undergo light-dependent retinal degeneration which can be suppressed by a mutation in phospholipase C. Thus, enzymes involved in PtdlnsP2 metabolism regulate phosphoinositide-mediated signalling cascades in vivo.

Studies with a Xenopus BMP receptor suggest that ventral mesoderm-inducing signals override dorsal signals in vivo

We report the isolation of a Xenopus BMP receptor that is expressed maternally in the appropriate location to play a role in mesoderm induction. This receptor binds both BMP-2 and BMP-4 with high affinity. A truncated form of this BMP receptor specifically blocks BMP-4 signaling. Expression of this truncated BMP receptor during embryogenesis converts ventral mesoderm to dorsal mesoderm. Contrary to the popularly held view that ventral is the ground state for all mesoderm, our results suggest that formation of ventral mesoderm requires an active signal and that, in the absence of this ventral signal, dorsal mesoderm is formed.

Interleukin 4 receptor signaling in human monocytes and U937 cells involves the activation of a phosphatidylcholine-specific phospholipase C: a comparison with chemotactic peptide, FMLP, phospholipase D, and sphingomyelinase

Interleukin 4 (IL-4) diminishes cytokine activation of human macrophage. IL-4 binding to monocyte IL-4R is associated with protein kinase C (PKC) translocation to a nuclear fraction. The cleavage of diacyglycerol (DAG), an activator of PKC, from membrane phospholipids was investigated to define the proximal events of IL-4R signaling. IL-4 induced a statistically significant time-and dose-dependent generation of DAG. The IL-4-triggered production of DAG was not derived from phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis, since neither cytosolic calcium flux nor liberation of inositol phosphates was detected in response to IL-4. Experiments were performed using [14C-methyl]choline-labeled U937 cells and monocytes to determine whether IL-4R activated phospholipase C (PLC), PLD, or PLA2 to use membrane phosphatidylcholine (PC) to form DAG. IL-4 induced a time- and dose-dependent increase of phosphocholine (pchol) with concomitant degradation of membrane PC (p < 0.05 compared with control). The finding that the peak reduction of PC was equivalent to peak production of pchol suggested that IL-4R signaling involved the activation of a PC-specific PLC. Changes in choline (chol) or lyso-PC and glycerolphosphocholine, the respective products of PC cleavage by PLD or PLA2, were not detected in IL-4-treated cells. In contrast, exogenous PLD induced an increase in chol and concomitant loss of membrane PC. Additional investigation suggested that IL-4R signaling does not involve PLD. In cells labeled with L-lyso-3-PC 1-[1-14C]palmitoyl, PLD but not IL-4, increased the production of phosphatidic acid (PA) and phosphatidyl-ethanol when pretreated with ethanol. Propranolol, an inhibitor of phosphatidate phosphohydrolase, and calyculin A, a phosphatase 1 and 2A inhibitor, blocked DAG production in response to FMLP but not to IL-4. In propranolol pretreated cells, PMA but not IL-4 triggered the production of PA and lowered the amount of DAG. Evidence that PLA2 is not coupled to IL-4R is the detection of arachidonate production in response to FMLP but not to IL-4. Furthermore, IL-4R is not coupled to sphingomyelinase (SMase) since IL-4, unlike exogenous SMase, did not generate ceramide but induced the hydrolysis of PC to pchol that was comparable to exogenous PLC. In summary, IL-4R signaling in monocytes and U937 cells involves PLC and not PLD, PLA2, or SMase, and it uses PC and not PIP2 to form DAG.

Intracellular free calcium oscillations in normal and cleavage-blocked embryos and artificially activated eggs of Xenopus laevis

We have measured levels of intracellular free calcium ([Ca2+]i) in albino Xenopus laevis embryos using recombinant aequorin and a photon-counting system. We observed sinusoidal oscillations in [Ca2+]i that had the same frequency as cleavage, with cleavage occurring when [Ca2+]i was lowest. An increase in calcium was seen to precede first cleavage. The cyclic changes in calcium were superimposed on a secondary pattern that increased, peaked between third and fifth cleavages and then slowly declined to a level similar to that measured before first cleavage. The amplitude of the oscillations was small during the first few cleavages but became larger with each cycle, with the largest oscillations occurring when the secondary pattern peaked (between third and fifth cleavage). As the secondary pattern declined, the amplitude of the oscillations also became smaller. The oscillations are due to release of calcium from intracellular stores, since the signal was the same in calcium-free solution as in normal medium. When cleavage was blocked with the microtubule-disrupting drugs colchicine or nocodazole, the [Ca2+]i oscillations persisted. Calcium oscillations of a similar magnitude and frequency were also present in artificially activated eggs. The secondary pattern was different in cleavage-blocked embryos and artificially activated eggs, the baseline increasing until about the third cycle and then remaining elevated for the rest of the recording (&gt; 8 hours). By fixing embryos at various points in the calcium cycle, we determined that mitosis began shortly after calcium levels reached their peak and was complete before the calcium level dropped to its lowest point.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct activation of phospholipase C-gamma by fibroblast growth factor receptor is not required for mesoderm induction in Xenopus animal caps

Members of the fibroblast growth factor (FGF) family induce mesoderm formation in explants of Xenopus embryonic ectoderm (animal caps). Recent studies have been directed at determining signaling pathways downstream of the FGF receptor that are important in mesoderm induction. We have recently shown that a point mutation in the FGF receptor changing tyrosine 766 to phenylalanine (Y/F mutation) abolishes phospholipase C-gamma (PLC-gamma) activation in mammalian cells. To explore the role of PLC-gamma activation in FGF-stimulated mesoderm induction, we constructed two chimeric receptors, each consisting of the extracellular portion of the platelet-derived growth factor beta receptor, with one having the transmembrane and intracellular portions of the wild-type FGF receptor 1 (PR-FR wt) and the other having the corresponding region of the Y/F766 mutant FGF receptor 1 (PR-FR Y/F766). When expressed in Xenopus oocytes, only PR-FR wt was able to mediate PLC gamma phosphorylation, inositol-1,4,5-trisphosphate accumulation, and calcium efflux in response to platelet-derived growth factor stimulation. However, both receptors mediated mesoderm induction in Xenopus animal caps as measured by cap elongation, muscle-specific actin mRNA induction, and skeletal muscle formation. These results demonstrate that PLC gamma activation by the FGF receptor is not required for FGF-stimulated mesoderm induction.