Calcium signalling during neural induction inXenopus laevisembryos

Abstract

InXenopus, 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 intactXenopusembryos 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⁺]_iartificially 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 ofZic3, an early proneural gene, and in epidermal determination which only occurs when the Ca²⁺-dependent signalling pathways are inactive.