Calcium signalling during neural induction in Xenopus laevis embryos

In Xenopus, experiments performed with isolated ectoderm suggest that neural determination is a 'by default' mechanism, which occurs when bone morphogenetic proteins (BMPs) are antagonized by extracellular antagonists, BMP being responsible for the determination of epidermis. However, Ca2+ imaging of intact Xenopus embryos reveals patterns of Ca2+ transients which are generated via the activation of dihydropyridine-sensitive Ca2+ channels in the dorsal ectoderm but not in the ventral ectoderm. These increases in the concentration of intracellular Ca2+([Ca2+]i) appear to be necessary and sufficient to orient the ectodermal cells towards a neural fate as increasing the [Ca2+]i artificially results in neuralization of the ectoderm. We constructed a subtractive cDNA library between untreated and caffeine-treated ectoderms (to increase [Ca2+]i) and then identified early Ca2+-sensitive target genes expressed in the neural territories. One of these genes, an arginine methyltransferase, controls the expression of the early proneural gene, Zic3. Here, we discuss the evidence for the existence of an alternative model to the 'by default' mechanism, where Ca2+ plays a central regulatory role in the expression of Zic3, an early proneural gene, and in epidermal determination which only occurs when the Ca2+-dependent signalling pathways are inactive.