The Roles Of Notch Signaling During Xenopus Somitogenesis PDF Download

Abstract: The Notch pathway is a highly conserved signaling mechanism used for cell-to-cell communication during development. Errors in this communication system can lead to birth defects and contribute to the development and progression of numerous diseases. The significance of Notch in cardiac development is evident from the identification of human mutations in various Notch pathway members that result in congenital cardiovascular malformations. Despite the importance of Notch signaling in cardiac development, a lack of functional studies examining the molecular mechanisms underlying these Notch implicated malformations represents a significant gap in the field of cardiovascular biology. We identified novel NOTCH1 mutations in patients with cardiac malformations involving the left ventricular outflow tract (LVOT) that reduce ligand-induced signaling through defective intracellular processing of the mutant Notch1 receptor protein. The Notch pathway also functions in a biological clock that is critical for skeletal development during somitogenesis. During somitogenesis, somites bud from the anterior end of the presomitic mesoderm (PSM) to give rise to the axial skeleton, the dermis of the back, and skeletal muscle. This developmental process proves useful for studying the Notch pathway, because in the PSM, Notch oscillations are coordinated, resulting in visible oscillatory expression of genes linked to the clock. Mutations that perturb these oscillations can be easily observed at the level of gene expression, and result in overt skeletal phenotypes that do not affect viability. Proper clock function requires that gene expression of Notch pathway members be tightly controlled during the time period of the clock. One of these oscillatory genes is Lunatic fringe (Lfng), which encodes a glycosyltransferase that modifies the Notch receptor to modulate Notch signaling (Hicks C, 2000). Understanding the mechanisms of Notch regulation in the segmentation clock is an active topic of research with many unanswered questions. We hypothesized that microRNAs (miRNAs), small molecules that regulate gene expression, have a conserved function in the segmentation clock through regulation of oscillatory genes in the Notch pathway. miRNA microarray analysis and RT-PCR identified miRNAs enriched in the mouse PSM. Preventing interactions between one of these miRNAs, miR-125a-5p, and Lfng in vivo leads to abnormal segmentation and perturbs cyclic gene expression in the PSM, indicating that miRNA regulation of Lfng is important for the clock mechanism. This work provides the first evidence supporting a role for miRNAs in the segmentation clock and reveals a novel mechanism of post-transcriptional regulation of oscillatory genes. In addition, since the miRNA microarray revealed a large number of miRNAs either enriched in the PSM or mature somites, future work in the lab will examine the function of these miRNAs in segmentation clock function and somite maturation.