Recently, mutations in genes for the Notch ligands Jagged1 and Delta-like3 were shown to be responsible for two human malsegmentation disorders, Alagille syndrome and Spondylocostal dysostosis. A new perspective towards an understanding of these diseases is offered by the discovery of genes within the Notch pathway which exhibit dynamic, oscillating expression patterns in the tissues that form the skeleton and muscles of the adult body. The formation of reiterated segments is a common theme in many organisms including humans. Further knowledge of how segmentation is achieved and how individual segments are given specific identities is crucial to an understanding of both normal and defective formation of bone, muscle and nervous system. Recently, molecules belonging to the Notch signaling pathway have been implicated in playing a major role during the segmentation of vertebrate mesoderm and its morphological consequence, the formation of somites. Interestingly, the mRNA levels of several components of this pathway oscillate with a periodicity equal to that of somite formation. The oscillations are themselves coordinated into a wavefront that progresses anteriorly through the unsegmented, pre-somitic mesoderm and culminates in a fixed pattern of expression concomitant with the formation of each finished somite. The molecular role of individual Notch signaling pathway components during vertebrate segmentation and the function of their unusual cyclic pattern of expression is presently unknown. This study proposes to use the embryologically and genetically tractable zebrafish to gain an understanding of the function of these gene oscillations in the segmentation process. Currently, the migratory trajectories and fate commitment of the cells that undergo oscillations in the pre-somitic mesoderm are not known and this fundamental information is vital to any proposal for oscillatory function. Additionally, the availability of the zebrafish mutant line fused somites, which makes no mesodermal segments, allows for the study of interactions of the mRNA oscillations with the processes that confer segment polarity and boundary formation. The experiments in this proposal will contribute to a better understanding of the function of mRNA oscillations with respect to cell movement, fate commitment, the formation of boundaries and the formation of segment polarity in the pre-somitic mesoderm of vertebrates. The proposed findings will allow the construction of a more accurate model of the segmentation process, and are applicable to the understanding of any human syndrome with a segmentation defect.
