Gdf16, a novel member of the growth/differentiation factor subgroup of the TGF-beta superfamily, is expressed in the hindbrain and epibranchial placodes

We have isolated and characterized the developmental expression of Xenopus gdf16, a novel member of the growth/differentiation factor (gdf) gene family. The gdf16 gene encodes a pre-proprotein of 413 amino acids and a mature peptide of 122 amino acids. Gdf16 is most closely related to the zebrafish genes dynamo and radar, but exhibits a completely different expression pattern. Gene expression is detected at early tailbud (stage 25) in the first two epibranchial placodes and in a hindbrain-specific domain. As development proceeds, the gene is expressed in all the epibranchial placodes, the hindbrain, and the diencephalon.

Conservation of sequence and expression of Xenopus and zebrafish dHAND during cardiac, branchial arch and lateral mesoderm development

dHAND and eHAND are related basic helix-loop-helix transcription factors that are expressed in the cardiac mesoderm and in numerous neural crest-derived cell types in chick and mouse. To better understand the evolutionary development of overlapping expression and function of the HAND genes during embryogenesis, we cloned the zebrafish and Xenopus orthologues. Comparison of dHAND sequences in zebrafish, Xenopus, chick, mouse and human demonstrated conservation throughout the protein. Expression of dHAND in zebrafish was seen in the earliest precursors of all lateral mesoderm at early gastrulation stages. At neurula and later stages, dHAND expression was observed in lateral precardiac mesoderm, branchial arch neural crest derivatives and posterior lateral mesoderm. At looping heart stages, cardiac dHAND expression remained generalized with no apparent regionalization. Interestingly, no eHAND orthologue was found in zebrafish. In Xenopus, dHAND and eHAND were co-expressed in the cardiac mesoderm without the segmental restriction seen in mice. Xenopus dHAND and eHAND were also expressed bilaterally in the lateral mesoderm without any left-right asymmetry. Within the branchial arches, XdHAND was expressed in a broader domain than XeHAND, similar to their mouse counterparts. Together, these data demonstrate conservation of HAND structure and expression across species.

Visualization of DNA and RNA molecules, and protein-DNA complexes for electron microscopy

This article provides step-by step instructions for the preparation of double- and single-stranded DNA and RNA molecules and protein-DNA complexes for electron microscopy (EM). Absorption, spreading, staining, dark-field imaging, and metal shadowing techniques are described in detail. A number of examples are illustrated on analysis of DNA replication, DNA repair and DNA recombination to demonstrate the usefulness of the technique for EM visualisation. Application of immunogold labeling of specific protein in DNA-protein complexes is also covered.

Dishevelled phosphorylation, subcellular localization and multimerization regulate its role in early embryogenesis

Dishevelled (Dsh) induces a secondary axis and can translocate to the membrane when activated by Frizzleds; however, dominant-negative approaches have not supported a role for Dsh in primary axis formation. We demonstrate that the Dsh protein is post-translationally modified at the dorsal side of the embryo: timing and position of this regulation suggests a role of Dsh in dorsal-ventral patterning in Xenopus. To create functional links between these properties of Dsh we analyzed the influence of endogenous Frizzleds and the Dsh domain dependency for these characteristics. Xenopus Frizzleds phosphorylate and translocate Xdsh to the membrane irrespective of their differential ectopic axes inducing abilities, showing that translocation is insufficient for axis induction. Dsh deletion analysis revealed that axis inducing abilities did not segregate with Xdsh membrane association. The DIX region and a short stretch at the N-terminus of the DEP domain are necessary for axis induction while the DEP region is required for Dsh membrane association and its phosphorylation. In addition, Dsh forms homomeric complexes in embryos suggesting that multimerization is important for its proper function.

Active dendritic membrane properties of Xenopus larval spinal neurons analyzed with a whole cell soma voltage clamp

Voltage- and current-clamp measurements of inwardly directed currents were made from the somatic regions of Xenopus laevis spinal neurons. Current-voltage (I-V) curves determined under voltage clamp, but not current clamp, were able to indicate a negative slope conductance in neurons that showed strong accommodating action potential responses to a constant current stimulation. Voltage-clamp I-V curves from repetitive firing neurons did not have a net negative slope conductance and had identical I-V plots under current clamp. Frequency domain responses indicate negative slope conductances with different properties with or without tetrodotoxin, suggesting that both sodium and calcium currents are present in these spinal neurons. The currents obtained from a voltage clamp of the somatic region were analyzed in terms of spatially controlled soma membrane currents and additional currents from dendritic potential responses. Linearized frequency domain analysis in combination with both voltage- and current-clamp responses over a range of membrane potentials was essential for an accurate determination of consistent neuronal model behavior. In essence, the data obtained at resting or hyperpolarized membrane potentials in the frequency domain were used to determine the electrotonic structure, while both the frequency and time domain data at depolarized potentials were required to characterize the voltage-dependent channels. Finally, the dendritic and somatic membrane properties were used to reconstruct the action potential behavior and quantitatively predict the dependence of neuronal firing properties on electrotonic structure. The reconstructed action potentials reproduced the behavior of two broad distributions of interneurons characterized by their degree of accommodation. These studies suggest that in addition to the ionic conductances, electrotonic structure is correlated with the action potential behavior of larval neurons.

Mesenchyme with fgf-10 expression is responsible for regenerative capacity in Xenopus limb buds

A young tadpole of an anuran amphibian can completely regenerate an amputated limb, and it exhibits an ontogenetic decline in the ability to regenerate its limbs. However, whether mesenchymal or epidermal tissue is responsible for this decrease of the capacity remains unclear. Moreover, little is known about the molecular interactions between these two tissues during regeneration. The results of this study showed that fgf-10 expression in the limb mesenchymal cells clearly corresponds to the regenerative capacity and that fgf-10 and fgf-8 are synergistically reexpressed in regenerating blastemas. However, neither fgf-10 nor fgf-8 is reexpressed after amputation of a nonregenerative limb. Nevertheless, nonregenerative epidermal tissue can reexpress fgf-8 under the influence of regenerative mesenchyme, as was demonstrated by experiments using a recombinant limb composed of regenerative limb mesenchyme and nonregenerative limb epidermis. Taken together, our data demonstrate that the regenerative capacity depends on mesenchymal tissue and suggest that fgf-10 is likely to be involved in this capacity.

Firing properties and electrotonic structure of Xenopus larval spinal neurons

Whole cell voltage- and current-clamp measurements were done on intact Xenopus laevis larval spinal neurons at developmental stages 42-47. Firing patterns and electrotonic properties of putative interneurons from the dorsal and ventral medial regions of the spinal cord at myotome levels 4-6 were measured in isolated spinal cord preparations. Passive electrotonic parameters were determined with internal cesium sulfate solutions as well as in the presence of active potassium conductances. Step-clamp stimuli were combined with white-noise frequency domain measurements to determine both linear and nonlinear responses at different membrane potential levels. Comparison of analytic and compartmental dendritic models provided a way to determine the number of compartments needed to describe the dendritic structure. The electrotonic structure of putative interneurons was correlated with their firing behavior such that highly accommodating neurons (Type B) had relatively larger dendritic areas and lower electrotonic lengths compared with neurons that showed sustained action potential firing in response to a constant current (Type A). Type A neurons had a wide range of dendritic areas and potassium conductances that were activated at membrane potentials more negative than observed in Type B neurons. The differences in the potassium conductances were in part responsible for a much greater rectification in the steady-state current voltage (I-V curve) of the strongly accommodating neurons compared with repetitively firing cells. The average values of the passive electrotonic parameters found for Rall Type A and B neurons were c(soma) = 3.3 and 2.6 pF, g(soma) = 187 and 38 pS, L = 0.36 and 0.21, and A = 3.3 and 6.5 for soma capacitance, soma conductance, electrotonic length, and the ratio of the dendritic to somatic areas, respectively. Thus these experiments suggest that there is a correlation between the electrotonic structure and the excitability properties elicited from the somatic region.

Ephrin-B regulates the Ipsilateral routing of retinal axons at the optic chiasm

In Xenopus tadpoles, all retinal ganglion cells (RGCs) send axons contralaterally across the optic chiasm. At metamorphosis, a subpopulation of EphB-expressing RGCs in the ventrotemporal retina begin to project ipsilaterally. However, when these metamorphic RGCs are grafted into embryos, they project contralaterally, suggesting that the embryonic chiasm lacks signals that guide axons ipsilaterally. Ephrin-B is expressed discretely at the chiasm of metamorphic but not premetamorphic Xenopus. When expressed prematurely in the embryonic chiasm, ephrin-B causes precocious ipsilateral projections from the EphB-expressing RGCs. Ephrin-B is also found in the chiasm of mammals, which have ipsilateral projections, but not in the chiasm of fish and birds, which do not. These results suggest that ephrin-B/EphB interactions play a key role in the sorting of axons at the vertebrate chiasm.

Voltage dependence of slow inactivation in Shaker potassium channels results from changes in relative K(+) and Na(+) permeabilities

Time constants of slow inactivation were investigated in NH(2)-terminal deleted Shaker potassium channels using macro-patch recordings from Xenopus oocytes. Slow inactivation is voltage insensitive in physiological solutions or in simple experimental solutions such as K(+)(o)//K(+)(i) or Na(+)(o)//K(+)(i). However, when [Na(+)](i) is increased while [K(+)](i) is reduced, voltage sensitivity appears in the slow inactivation rates at positive potentials. In such solutions, the I-V curves show a region of negative slope conductance between approximately 0 and +60 mV, with strongly increased outward current at more positive voltages, yielding an N-shaped curvature. These changes in peak outward currents are associated with marked changes in the dominant slow inactivation time constant from approximately 1.5 s at potentials less than approximately +60 mV to approximately 30 ms at more than +150 mV. Since slow inactivation in Shaker channels is extremely sensitive to the concentrations and species of permeant ions, more rapid entry into slow inactivated state(s) might indicate decreased K(+) permeation and increased Na(+) permeation at positive potentials. However, the N-shaped I-V curve becomes fully developed before the onset of significant slow inactivation, indicating that this N-shaped I-V does not arise from permeability changes associated with entry into slow inactivated states. Thus, changes in the relative contributions of K(+) and Na(+) ions to outward currents could arise either: (a) from depletions of [K(+)](i) sufficient to permit increased Na(+) permeation, or (b) from voltage-dependent changes in K(+) and Na(+) permeabilities. Our results rule out the first of these mechanisms. Furthermore, effects of changing [K(+)](i) and [K(+)](o) on ramp I-V waveforms suggest that applied potential directly affects relative permeation by K(+) and Na(+) ions. Therefore, we conclude that the voltage sensitivity of slow inactivation rates arises indirectly as a result of voltage-dependent changes in the ion occupancy of these channels, and demonstrate that simple barrier models can predict such voltage-dependent changes in relative permeabilities.

In vitro and in vivo characterization of conantokin-R, a selective NMDA receptor antagonist isolated from the venom of the fish-hunting snail Conus radiatus

The purification, characterization, and synthesis of conantokin-R (Con-R), an N-methyl-D-aspartate (NMDA) receptor peptide antagonist from the venom of Conus radiatus, are described. With the use of well defined animal seizure models, Con-R was found to possess an anticonvulsant profile superior to that of ifenprodil and dizocilpine (MK-801). With voltage-clamp recording of Xenopus oocytes expressing heteromeric NMDA receptors from cloned NR1 and NR2 subunit RNAs, Con-R exhibited the following order of preference for NR2 subunits: NR2B approximately NR2A > NR2C >> NR2D. Con-R was without effect on oocytes expressing the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 or the kainate receptor subunit GluR6. In mouse cortical neurons voltage-clamped at -60 mV, Con-R application produced a slowly developing block of inward currents evoked by 10 microM NMDA and 1 microM glycine (IC(50) = 350 nM). At 3 microM, Con-R did not affect gamma-aminobutyric acid- or kainate-evoked currents. Con-R prevented sound-induced tonic extension seizures in the Frings audiogenic seizure-susceptible mice at i.c.v. doses below toxic levels. It was also effective at nontoxic doses in CF#1 mice against tonic extension seizures induced by threshold (15 mA) and maximal (50 mA) stimulation, and it partially blocked clonic seizures induced by s.c. pentylenetetrazol. In contrast, MK-801 and ifenprodil were effective only at doses approaching (audiogenic seizures) or exceeding (electrical and pentylenetetrazol seizures) those required to produce significant behavioral impairment. These results indicate that the subtype selectivity and other properties of Con-R afford a distinct advantage over the noncompetitive NMDA antagonists MK-801 and ifenprodil. Con-R is a useful new pharmacological agent for differentiation between the anticonvulsant and toxic effects of NMDA antagonists.

Timing of calcium and protein synthesis requirements for the first mitotic cell cycle in fertilised Xenopus eggs

Mitosis is governed by the activity of the M-phase promoting factor (MPF). In some systems, particularly early embryos, transient increases in calcium concentration have been shown to be necessary for mitosis and regulate its timing. By microinjection of the calcium buffer, dibromoBAPTA, into fertilised Xenopus eggs, we have assessed whether calcium events are required to initiate MPF activation and inactivation. Since initial experiments showed that this buffer inhibited protein synthesis, we measured when mitosis and cleavage became independent of translation. We found that, after a period of protein synthesis essential for cleavage, there was a phase during which continued translation affected the timing of cleavage, but was not essential for its occurrence. Measurement of MPF activity in single embryos injected with calcium buffer at different times in the first cell cycle, showed that there were two sensitive periods. The first period of sensitivity blocked MPF activation and coincided with the time at which cleavage became completely independent of protein synthesis. The second sensitive period occurred just before histone kinase activity peaked, and was necessary for kinase inactivation. Preventing inactivation in this way arrested egg extracts in mitosis. These results support the view that transient increases in free calcium concentration contribute to mitotic progression by first triggering MPF activation and subsequently, with elevated MPF activity, inducing its inactivation.

Phosphatase 2A and polo kinase, two antagonistic regulators of cdc25 activation and MPF auto-amplification

The auto-catalytic activation of the cyclin-dependent kinase Cdc2 or MPF (M-phase promoting factor) is an irreversible process responsible for the entry into M phase. In Xenopus oocyte, a positive feed-back loop between Cdc2 kinase and its activating phosphatase Cdc25 allows the abrupt activation of MPF and the entry into the first meiotic division. We have studied the Cdc2/Cdc25 feed-back loop using cell-free systems derived from Xenopus prophase-arrested oocyte. Our findings support the following two-step model for MPF amplification: during the first step, Cdc25 acquires a basal catalytic activity resulting in a linear activation of Cdc2 kinase. In turn Cdc2 partially phosphorylates Cdc25 but no amplification takes place; under this condition Plx1 kinase and its activating kinase, Plkk1 are activated. However, their activity is not required for the partial phosphorylation of Cdc25. This first step occurs independently of PP2A or Suc1/Cks-dependent Cdc25/Cdc2 association. On the contrary, the second step involves the full phosphorylation and activation of Cdc25 and the initiation of the amplification loop. It depends both on PP2A inhibition and Plx1 kinase activity. Suc1-dependent Cdc25/Cdc2 interaction is required for this process.

Calcium signaling in the developing Xenopus myotome

Embryonic Xenopus myocytes generate spontaneous calcium (Ca(2+)) transients during differentiation in culture. Suppression of these transients disrupts myofibril organization and the formation of sarcomeres through an identified signal transduction cascade. Since transients often occur during myocyte polarization and migration in culture, we hypothesized they might play additional roles in vivo during tissue formation. We have tested this hypothesis by examining Ca(2+) dynamics in the intact Xenopus paraxial mesoderm as it differentiates into the mature myotome. We find that Ca(2+) transients occur in cells of the developing myotome with characteristics remarkably similar to those in cultured myocytes. Transients produced within the myotome are correlated with somitogenesis as well as myocyte maturation. Since transients arise from intracellular stores in cultured myocytes, we examined the functional distribution of both IP(3) and ryanodine receptors in the intact myotome by eliciting Ca(2+) elevations in response to photorelease of caged IP(3) and superfusion of caffeine, respectively. As in culture, transients in vivo depend on Ca(2+) release from ryanodine receptor (RyR) stores, and blocking RyR during development interferes with somite maturation.

BMP signalling in early Xenopus development

Bone morphogenetic proteins (BMPs) are typically members of the transforming growth factor beta (TGF-beta) family with diverse roles in embryonic development. At least five genes with homology to BMPs are expressed during Xenopus development, along with their receptors and intracellular signalling pathways. The evidence suggests that BMPs have roles to play in both mesoderm induction and dorsoventral patterning. Studies in Xenopus have also identified a number of inhibitory binding proteins for the classical BMPs, encoded by genes such as chordin and noggin. These proteins appear to be responsible for establishing a morphogen gradient of BMP4 activity, which specifies different dorsoventral fates in early gastrulae. An emerging theme is that inhibition of BMP signalling is an important mechanism regulating cell fate decisions in early development.

Xenopus GDF6, a new antagonist of noggin and a partner of BMPs

In Xenopus, ectodermal cell fates are determined by antagonistic interaction between the BMP subfamily of TGF-(beta) ligands and the organizer-specific secreted factors (e.g. noggin, chordin and follistatin). Inhibition of BMP function by these factors can convert cells from an epidermal to a neural cell fate. In this study, we report that GDF6, a new member of the Xenopus TGF-(beta) family, can function in antagonistic interaction with neural inducers. GDF6 induces epidermis and inhibits neural tissue in dissociated cells, and this activity is blocked by the presence of noggin. We demonstrate that GDF6 binds directly to the neural inducer noggin. Furthermore, we find that GDF6 and BMP2 can form heterodimers and the process seems to require cotranslation of the proteins in the same cells. In normal embryos, GDF6 and BMP2 are coexpressed in several places, including the edge of the neural plate at early neurula stages, suggesting that GDF6 may synergize with BMPs to regulate patterning of the ectoderm. Our data show for the first time that noggin can bind directly to and inhibit another TGF-(beta) family member: GDF6. In addition, BMP and GDF6 heterodimers may play an important role in vivo to regulate cell fate determination and patterning.

Regulation of dorsal gene expression in Xenopus by the ventralizing homeodomain gene Vox

Patterning in the vertebrate embryo is controlled by an interplay between signals from the dorsal organizer and the ventrally expressed BMPs. Here we examine the function of Vox, a homeodomain-containing gene that is activated by the ventralizing signal BMP-4. Inhibition of BMP signaling using a dominant negative BMP receptor (DeltaBMPR) leads to the ectopic activation of dorsal genes in the ventral marginal zone, and this activation is prevented by co-injection of Vox. chordin is the most strongly activated of those genes that are up-regulated by DeltaBMPR and is the gene most strongly inhibited by Vox expression. We demonstrate that Vox acts as a transcriptional repressor, showing that the activity of native Vox is mimicked by a Vox-repressor fusion (VoxEnR) and that a Vox-activator fusion (VoxG4A) acts as an antimorph, causing the formation of a partial secondary axis when expressed on the ventral side of the embryo. Although Vox can ectopically activate BMP-4 expression in whole embryos, we see no activation of BMP-4 by VoxG4A, demonstrating that this activation is indirect. Using a hormone-inducible version of VoxG4A, we find that a critical time window for Vox function is during the late blastula period. Using this construct, we demonstrate that only a subset of dorsal genes is directly repressed by Vox, revealing that there are different modes of regulation for organizer genes. Since the major direct target for Vox repression is chordin, we propose that Vox acts in establishing a BMP-4 morphogen gradient by restricting the expression domain of chordin.

A quantitative analysis of signal transduction from activin receptor to nucleus and its relevance to morphogen gradient interpretation

Previous work has shown that Xenopus blastula cells sense activin concentration by assessing the absolute number of occupied receptors per cell (100 and 300 molecules of bound activin activate Xbra and Xgsc transcription, respectively; a difference of only 3-fold). We now ask how quantitative differences in the absolute number of occupied receptors lead to the qualitatively distinct gene responses in the nucleus through SMAD2, a transducer of concentration-dependent gene responses to activin. We show that the injection of 0.2 or 0.6 ng of Smad2 mRNA activates Xbra or Xgsc transcription, respectively, involving, again, only a 3-fold difference. Furthermore, Xbra transcription is down-regulated by overexpression of SMAD2 as it is after activin signaling. We have developed a method to isolate nuclei from animal cap cells and subsequently have quantified the amount of nuclear SMAD2 protein. We find that the injection of 0.2 or 0.6 ng of Smad2 mRNA into an egg leads to only a 3-fold difference in the amount of SMAD2 protein in the nuclei of the blastula cells that express Xbra or Xgsc. We conclude that a 3-fold difference in the absolute number of occupied activin receptors can be maintained only as a 3-fold difference in the level of nuclear SMAD2 protein. Therefore, in this example of morphogen action, there appears to be no amplification of a key cytoplasmic transduction response, and a small but developmentally important change in extracellular signal concentration is relayed directly to the nucleus.

Chronic ethanol exposure enhances signaling through muscarinic receptors expressed by cRNA injection in Xenopus oocytes: implications for mechanism of action

The molecular mechanisms underlying the cerebral symptoms of ethanol withdrawal syndrome are poorly understood. In addition to ethanol's effect on GABA and NMDA receptors, ethanol affects muscarinic acetylcholine signaling. This interaction has attracted attention because of the importance of muscarinic signaling in consciousness. Chronic ethanol exposure increases muscarinic receptor binding. Increased transcription of receptor message has been suggested as the underlying mechanism, but this hypothesis has not been tested directly. Therefore, we studied the effects of ethanol on muscarinic signaling in a model that bypasses transcription of muscarinic receptor genes. We expressed rat m1 muscarinic receptors by cRNA microinjection in Xenopus oocytes. Cells were voltage-clamped at -70 mV and effects of prolonged (24, 48, and 72 hr) exposure to ethanol (25, 50, and 100 mM) on methylcholine-induced calcium-activated Cl- currents were determined. Effects of prolonged ethanol exposure on currents induced by stimulation of lysophosphatidate receptors, direct G protein activation, or inositol trisphosphate receptor activation were studied as well. Prolonged ethanol exposure enhanced methylcholine (or lysophosphatidate-)-induced currents in a time- and concentration-dependent manner. Thus, enhanced muscarinic gene transcription is not required for ethanol enhancement of muscarinic signaling. Lack of ethanol effect on inositol trisphosphate-induced signaling suggests that intracellular signaling systems downstream of phospholipase C are not involved. In contrast, currents induced by direct G protein stimulation were enhanced significantly. Therefore, one potential site of ethanol's action on muscarinic signaling is upregulation of the associated G protein or enhancement of its functioning.

Neighboring glycine residues are essential for P2X2 receptor/channel function

The roles of a glycine-rich region in the cloned P2X2 receptor/channel were evaluated by site-directed mutagenesis. Responsiveness to ATP was lost when Gly247 was replaced by alanine. The sensitivity to ATP was reduced when Gly248 was replaced by alanine, and the responsiveness to ATP was lost when Gly248 was replaced by valine. The results suggest that the neighboring glycine residues are essential for P2X2 receptor/channel function.

Cyclin-dependent kinase control of centrosome duplication

Centrosomes nucleate microtubules and duplicate once per cell cycle. This duplication and subsequent segregation in mitosis results in maintenance of the one centrosome/cell ratio. Centrosome duplication occurs during the G1/S transition in somatic cells and must be coupled to the events of the nuclear cell cycle; failure to coordinate duplication and mitosis results in abnormal numbers of centrosomes and aberrant mitoses. Using both in vivo and in vitro assays, we show that centrosome duplication in Xenopus laevis embryos requires cyclin/cdk2 kinase activity. Injection of the cdk (cyclin-dependent kinase) inhibitor p21 into one blastomere of a dividing embryo blocks centrosome duplication in that blastomere; the related cdk inhibitor p27 has a similar effect. An in vitro system using Xenopus extracts carries out separation of the paired centrioles within the centrosome. This centriole separation activity is dependent on cyclin/cdk2 activity; depletion of either cdk2 or of the two activating cyclins, cyclin A and cyclin E, eliminates centriole separation activity. In addition, centriole separation is inhibited by the mitotic state, suggesting a mechanism of linking the cell cycle to periodic duplication of the centrosome.

Bone morphogenetic protein antagonism of Spemann's organizer is independent of Wnt signaling

The Xenopus homeobox gene twin is involved in the Wnt-mediated induction of Spemann's organizer. Additionally, several lines of evidence indicate that bone morphogenetic proteins (BMPs) play a role in repressing the formation of the organizer by antagonizing the expression of genes involved in organizer establishment. In order to determine at what level BMPs exert their effect, we measured the activity of different genes expressed within the organizer region. We report that BMP signaling can antagonize the induction of the dorsal-specific gene goosecoid but is unable to affect Wnt signaling at the level of twin. These results suggest that the antagonistic activities of BMPs in organizer formation occur postzygotically, independent of twin regulation, and that Wnt-like dorsal determinant signaling pathways do not crosstalk with BMPs.

Modulation of a calcium-activated chloride current by Maitotoxin

The effect of Maitotoxin (MTX) on the calcium-activated chloride current (ICl-Ca) from Xenopus oocytes was studied, applying the two-electrode voltage clamp technique. MTX increased the current amplitude at all the voltages explored and reduced the time to reach the maximum current level (time to peak). At low toxin concentrations (15 pM), both effects were fully reversible. Activation of ICl-Ca by MTX was secondary to the increment in the intracellular Ca2+ concentration induced by this toxin, since incubation of the oocytes with the cell-permeant Ca2+ chelator BAPTA-AM, greatly reduced the effect of MTX on ICl-Ca. Furthermore, external chloride ions removal also diminished the MTX effect on the current, strongly suggesting that the main current activated by MTX is ICl-Ca. Subsequent applications of a fixed toxin concentration after toxin washout resulted in enhanced ICl-Ca, suggesting that the toxin effect potentiates.