[Neural determination in Xenopus laevis embryos: control of early neural gene expression by calcium]. / La détermination neurale dans l'embryon de Xenopus laevis: contrôle de l'expression des gènes neuraux précoces par le calcium.

In amphibian embryos the central nervous system derives from the dorsal region of the ectoderm. Molecular studies led to the formulation of the "neural default model" in which neural development is under the inhibitory control of members of the BMP family. These growth factors also act as epidermis inducers. The neural fate is revealed by factors secreted by the Spemann Organizer such as noggin, chordin, follistatin, Xnr3 and cerberus which act by blocking BMP signalling. We propose a new model for neural cell determination in which a signalling pathway controlled by an increase in intracellular calcium suppresses the epidermis fate and activates the neural fate instead. This increase in calcium is due to an influx through calcium channels of the L-type, expressed in ectodermal cells during gastrulation. The possible involvement of a calcium-dependent phosphatase (calcineurin) to inhibit the epidermis fate and a calcium-calmodulin kinase (CaMkinase II) which activates the neural fate is discussed.

Imaging patterns of calcium transients during neural induction in Xenopus laevis embryos.

Through the injection of f-aequorin (a calcium-sensitive bioluminescent reporter) into the dorsal micromeres of 8-cell stage Xenopus laevis embryos, and the use of a Photon Imaging Microscope, distinct patterns of calcium signalling were visualised during the gastrulation period. We present results to show that localised domains of elevated calcium were observed exclusively in the anterior dorsal part of the ectoderm, and that these transients increased in number and amplitude between stages 9 to 11, just prior to the onset of neural induction. During this time, however, no increase in cytosolic free calcium was observed in the ventral ectoderm, mesoderm or endoderm. The origin and role of these dorsal calcium-signalling patterns were also investigated. Calcium transients require the presence of functional L-type voltage-sensitive calcium channels. Inhibition of channel activation from stages 8 to 14 with the specific antagonist R(+)BayK 8644 led to a complete inhibition of the calcium transients during gastrulation and resulted in severe defects in the subsequent formation of the anterior nervous system. BayK treatment also led to a reduction in the expression of Zic3 and geminin in whole embryos, and of NCAM in noggin-treated animal caps. The possible role of calcium transients in regulating developmental gene expression is discussed.

L-type calcium channel activation controls the in vivo transduction of the neuralizing signal in the amphibian embryos.

We have analyzed the transduction pathways involved in the triggering of neural induction, in amphibian embryos, in vivo. Using a plasmid construction, we have targetted the bioluminescent calcium probe aequorin to the plasma membrane of ectoderm cells of the amphibian Pleurodeles waltl before gastrulation. We have demonstrated that the in vivo triggering of neural induction depends on the activation of calcium-dependent pathways and involves L-type calcium channels. Furthermore, on excised ectoderm taken at the gastrula stage, we show that noggin, a protein currently considered as one of the natural inducers, also activates L-type calcium channels. This activation represents the first necessary event to determine cells of the dorsal ectoderm toward the neural pathway.

Expression of membrane targeted aequorin in Xenopus laevis oocytes.

We described here a system for high level of expression of the calcium activated photoprotein aequorin. This protein has been targeted to the plasma membrane of Xenopus oocyte by nuclear microinjection of a plasmid containing a construction of a chimeric cDNA encoding a fusion protein composed of the photoprotein aequorin and the 5-HT1A receptor. The expression of this fusion protein is placed under the control of RSV promoter. Functional photoprotein was reconstituted in the oocyte by incubation with coelenterazine. The amount of photoprotein 24 h after nuclear microinjection of the plasmid was sufficient to trigger a detectable light emission following calcium entry. The efficiency of the expression is correlated with the dose of plasmid injected. Intracytoplasmic injection of the plasmid always failed in photoprotein expression. Targeting of the apoprotein was demonstrated by immunolocalization under confocal microscopy. In our experimental conditions, the apoprotein was always localized at the animal pole above the nucleus. We never observed expression and targeting to the plasma membrane of the vegetal pole. WE suggest that such expression might be of great interest for the study of numerous problems of developmental biology, in which calcium-dependent pathways are involved.

[Fertilization and development of axolotl oocytes with already grey crescent experimentally formed during their maturation]. / Fécondation et développement d'ovocytes d'axolotl déjà pourvus d'un croissant gris expérimentalement formé pendant leur maturation.

Pigmented axolotl coelomic oocytes were induced to form a gray crescent by simultaneous action of a gravity vector and of a heat-shock (36 +/- 0.5 degree C during 10-15 min), according to a previously described method. Those oocytes were subsequently reintroduced into the coelomic cavity of an albino recipient female, which had been previously inseminated. Among fertilized treated oocytes, more than 75% developed into embryos whose dorsal side corresponded to the gray crescent-forming area of the oocyte. It is known that in normally fertilized control eggs, a gray crescent forms just prior to the first cleavage and corresponds in most cases to the dorsal side of the future embryo. Therefore, our results strongly suggest that fertilization is not a primary compulsory step towards effective determination of the future dorsal area in an Amphibian egg, since the required cytoplasmic rearrangements can be elicited much earlier in a maturing oocyte and remain effective during subsequent development.