Olivier Pourquié

Avian hairy Gene Expression Identifies a Molecular Clock Linked to Vertebrate Segmentation and Somitogenesis

We have identified and characterized c-hairy1, an avian homolog of the Drosophila segmentation gene, hairy. c-hairy1 is strongly expressed in the presomitic mesoderm, where its mRNA exhibits cyclic waves of expression whose temporal periodicity corresponds to the formation time of one somite (90 min). The apparent movement of these waves is due to coordinated pulses of c-hairy1 expression, not to cell displacement along the anteroposterior axis, nor to propagation of an activating signal. Rather, the rhythmic c-hairy mRNA expression is an autonomous property of the paraxial mesoderm. These results provide molecular evidence for a developmental clock linked to segmentation and somitogenesis of the paraxial mesoderm, and support the possibility that segmentation mechanisms used by invertebrates and vertebrates have been conserved.

Lateral and Axial Signals Involved in Avian Somite Patterning: A Role for BMP4

In vertebrates, muscles of the limbs and body wall derive from the lateral compartment of the embryonic somites, and axial muscles derive from the medial compartment. Whereas the mechanisms that direct patterning of somites along the dorsoventral axis are beginning to be understood, little is known about the tissue interactions and signaling molecules that direct somite patterning along the mediolateral axis. We report the identification of a specific marker for the lateral somitic compartment and its early derivatives, cSim1, an avian homolog of the Drosophila single minded gene. Using this marker, we provide evidence that specification of the lateral somitic lineage results from the antagonistic actions of a diffusible medializing signal from the neural tube and a diffusible lateralizing signal from the lateral plate mesoderm, and we implicate bone morphogenetic protein 4(BMP4) in directing this lateralization.

Delta-1 Activation of Notch-1 Signaling Results in HES-1 Transactivation

ABSTRACT The Notch receptor is involved in many cell fate determination events in vertebrates and invertebrates. It has been shown inDrosophila melanogaster that Delta-dependent Notch signaling activates the transcription factor Suppressor of Hairless, leading to an increased expression of the Enhancer of Splitgenes. Genetic evidence has also implicated the kuzbaniangene, which encodes a disintegrin metalloprotease, in the Notch signaling pathway. By using a two-cell coculture assay, we show here that vertebrate Dl-1 activates the Notch-1 cascade. Consistent with previous data obtained with active forms of Notch-1 aHES-1-derived promoter construct is transactivated in cells expressing Notch-1 in response to Dl-1 stimulation. Impairing the proteolytic maturation of the full-length receptor leads to a decrease in HES-1 transactivation, further supporting the hypothesis that only mature processed Notch is expressed at the cell surface and activated by its ligand. Furthermore, we observed that Dl-1-inducedHES-1 transactivation was dependent both on Kuzbanian and RBP-J activities, consistent with the involvement of these two proteins in Notch signaling in Drosophila. We also observed that exposure of Notch-1-expressing cells to Dl-1 results in an increased level of endogenous HES-1 mRNA. Finally, coculture of Dl-1-expressing cells with myogenic C2 cells suppresses differentiation of C2 cells into myotubes, as previously demonstrated for Jagged-1 and Jagged-2, and also leads to an increased level of endogenousHES-1 mRNA. Thus, Dl-1 behaves as a functional ligand for Notch-1 and has the same ability to suppress cell differentiation as the Jagged proteins do.

Expression of the cell adhesion proteins BEN/SC1/DM‐GRASP and TAG‐1 defines early steps of axonogenesis in the human spinal cord

We have studied the expression pattern of two cell adhesion proteins of the immunoglobin (Ig) superfamily, BEN/SC1/DM‐GRASP (BEN) and the transient axonal glycoprotein TAG‐1, during the development of the human nervous system. This study was performed by immunocytochemistry on sections of human embryos ranging from 4 to 13 weeks postconception. The overall distribution of the two proteins during development is very similar to that reported in other vertebrate species, but several important differences have been observed. Both proteins exhibit a transient expression on selected neuronal populations, which include the motor and the sensory neurons. In addition, BEN was also detected on virtually all neurons derived from the neural crest as well as in nonneuronal tissues. A major difference of expression with the chick embryo is that, in the motor neurons, BEN expression was not observed at early stages of development, thus arguing against a role of this molecule in pathfinding and fasciculation. BEN was observed to be restricted to subsets of motor neurons, such as the medial column at the upper limb level. Expression was also detected in a laterodorsal population of the ventral horn cells, which are likely to correspond to migrating preganglionic neurons that originate from the motor pool at the thoracic level. TAG‐1 was found on commissural neurons and weakly on the sympathetic neurons; it was also detected on restricted nonneuronal populations. In addition, we observed TAG‐1 expression in fibers that could correspond either to subsets of dorsal root ganglia (DRGs) central afferences (including the Ia fibers) or to the axons of association interneurons and in scattered motoneurons likely to correspond either to preganglionic neurons, to γ‐motoneurons, or to late‐born motoneurons. Therefore, our results indicate that the molecular strategies used to establish the axonal scaffolding of the nervous system in humans are extremely conserved among the different vertebrates. J. Comp. Neurol. 379:415–427, 1997.

New Molecular Insights on the Development of the Peripheral Nervous System

In the study of cell differentiation, a necessary step is to determine the onset of specific gene activities, in the various cell types emerging during embryogenesis. Production of monoclonal antibodies (Mab) able to distinguish specific antigenic determinants carried by differentiated cells, constitutes a mean to fulfill such a goal.