Final Report Summary - BODYBUILT (Building The Vertebrate Body)
Skeletal muscles of the body and the axial skeleton derive from an embryonic tissue called paraxial mesoderm. This tissue plays a key role in establishing the vertebrate body axis as it controls key patterning processes such as segmentation and posterior elongation. Major signaling pathways such as Notch, FGF, retinoic acid (RA) and Wnt signaling are involved in the regulation of these processes. The project Bodybuilt that was funded by ERC from 2009 to 2015 was focused on the study of cell signaling during patterning of the vertebrate paraxial mesoderm. By performing metabolomic coupled to transcriptomic analyses of the developing paraxial mesoderm, we showed that immature cells in the posterior paraxial mesoderm exhibit aerobic glycolysis with high lactate production, similar to the Warburg type of metabolism observed in cancer cells. As cells mature, glycolytic activity decreases and cells essentially rely on a respiration-based type of metabolism. Interfering with glycolysis selectively blocked body axis elongation but not segmentation whereas blocking respiration arrests segmentation without interfering with elongation. The switch between these metabolic states in the embryo correlates with major signaling transitions involved in the control of segmentation and body axis elongation, suggesting a cross-talk between energy metabolism and signaling. We showed that FGF signaling acts upstream of glycolysis while in turn, active glycolysis is required for Wnt signaling. Furthermore, we showed that in the tail bud, the high glycolytic activity results in low extracellular pH and high intracellular pH like in cancer cells. Our data suggest that the pH regulation downstream of glycolysis is required for Wnt signaling. Thus our data point to a strikingly similar regulation of cell physiology between the vertebrate tail bud and cancer cells and support a direct implication of energy metabolism in key patterning processes of the vertebrate embryo. Segmentation of the body axis in periodic units is a cardinal feature of vertebrates and of many invertebrates. Vertebrate segmentation was proposed to rely on positional information provided by an oscillator -the segmentation clock- and by a set of posterior gradients of FGF and Wnt signaling pathways-the wavefront- along the presomitic mesoderm (PSM). We have established an in vitro culture system in which sustained oscillations of a cyclic reporter producing target wave patterns can occur indefinitely in circular explants. We showed that a critical cell-density is required for the system to oscillate as in dynamic quorum-sensing systems. We also show that FGF signaling can modify the excitability threshold of the system, indicating that clock and wavefront are not independent identities as commonly presented. Together, our work indicates that the segmentation clock exhibits characteristics of an excitable system. We have also developed protocols to differentiate human and mouse pluripotent cells to a paraxial mesoderm fate allowing to study the differentiation of this tissue in vitro. Bilateral symmetry is a striking feature of the vertebrate body plan organization. The vertebral precursors, called somites, provide one of the best illustrations of embryonic symmetry. Maintenance of somitogenesis symmetry requires Retinoic acid (RA) and its coactivator Rere/Atrophin2. Using a proteomic approach we have identified a protein complex, containing Wdr5, Hdac1, Hdac2 and Rere (named WHHERE), which regulates RA signalling and controls embryonic symmetry. While the brain is overly symmetrical, several behaviors such as handedness show characteristic asymmetries. In mouse, 50% of the population exhibit either dextral or sinistral handedness. We showed that Rere mutation results in the majority of heterozygote mutants becoming dextral. MRI analysis of wild type dextral and Rere mutant animals identifies similar asymmetries in the right motor cortex suggesting that Rere is important in the development of these asymmetries. Thus our work identified a new molecular pathway involved in the establishment of brain asymmetry in mammals.