Cell signalling during Xenopus development
University College London
Our current research has focussed on the roles of two gene families in regulating early Xenopus development: the Tolloid family of metalloproteases and the large family of G-protein coupled receptors (GPCRs) . Tolloid Metalloproteases : The Tolloid family of secreted metalloproteases are homologues of the Drosophila tolloid gene, which is required for dorsal-ventral patterning during the blastoderm stage of embryogenesis. In Xenopus, the three homologues that have so far been described are called BMP1 (a longer splice variant is usually called Tolloid), Xolloid, and Xolloid-related. All three genes are expressed during gastrulation where they appear to regulate the activity of BMP4, an extracellular signalling molecule that plays a central role in dorsal-ventral patterning. High concentrations of BMP4 specify ventral fates and low concentrations specify lateral fates, while dorsal fates require an absence of BMP. Tolloid metalloproteases regulate BMP4 signalling by cleaving Chordin, an inhibitory binding protein for BMP4 that is synthesised in the dorsal mesoderm of Xenopus gastrulae. Chordin that has been cleaved by Tolloids has a greatly reduced affinity for BMP4. While BMP1and Xolloid are widely distributed in Xenopus gastrulae, Xolloid-related is localised to ventral and lateral sectors of mesoderm suggesting that it could be the more important of the three in regulating Chordin. In addition to regulating Chordin, these proteases are also required to cleave components of the extracellular matrix, including the main fibrillar Collagens. The number of substrates known to be cleaved by these proteases continues to increase and a general theme is that they generate the mature protein from longer precursors.
G-Protein Coupled Receptors: The GPCR family is the largest family of cell-surface receptors, with thousands already described. In mice there are about 1000 GPCRs concerned with the sense of smell alone. They act as receptors for an enormous range of signals, including proteins, small peptides, amino acids, fatty acids, and nucleotides. All have a similar structure, a single polypeptide chain that threads back and forth across the lipid bilayer seven times, with an extracellular N-terminus and an intracellular C-terminus. They all activate trimeric GTP-binding proteins (G-proteins), relay molecules that activate a number of intracellular effector molecules (e.g. Ca2+ and cAMP).Despite their importance in adult physiology, relatively little is known about their roles in development. We have previously cloned a GPCR for extracellular nucleotides (e.g. ATP) that we have called P2Y8 and shown that it is localised to the developing neural plate. Transcripts are lost once the neural tube has formed, indicating that it has a role in the earliest phase of neural development. We have searched the Xenopus EST databases for GPCRs that are expressed during early development, concentrating on receptors belonging to the P2Y subclass. At least four P2Y receptors are expressed in Xenopus gastrulae and we are currently characterising their roles in development.