The anterior/posterior polarity of somites is disrupted in paraxis-deficient mice

Establishing the anterior/posterior (A/P) boundary of individual somites is important for setting up the segmental body plan of all vertebrates. Resegmentation of adjacent sclerotomes to form the vertebrae and selective migration of neural crest cells during the formation of the dorsal root ganglia and peripheral nerves occur in response to differential expression of genes in the anterior and posterior halves of the somite. Recent evidence indicates that the A/P axis is established at the anterior end of the presomitic mesoderm prior to overt somitogenesis in response to both Mesp2 and Notch signaling. Here, we report that mice deficient for paraxis, a gene required for somite epithelialization, also display defects in the axial skeleton and peripheral nerves that are consistent with a failure in A/P patterning. Expression of Mesp2 and genes in the Notch pathway were not altered in the presomitic mesoderm of paraxis(-/-) embryos. Furthermore, downstream targets of Notch activation in the presomitic mesoderm, including EphA4, were transcribed normally, indicating that paraxis was not required for Notch signaling. However, genes that were normally restricted to the posterior half of somites were present in a diffuse pattern in the paraxis(-/-) embryos, suggesting a loss of A/P polarity. Collectively, these data indicate a role for paraxis in maintaining somite polarity that is independent of Notch signaling.     

The role of presenilin 1 during somite segmentation

The Notch signalling pathway plays essential roles during the specification of the rostral and caudal somite halves and subsequent segmentation of the paraxial mesoderm. We have re-investigated the role of presenilin 1 (Ps1; encoded by Psen1) during segmentation using newly generated alleles of the Psen1 mutation. In Psen1-deficient mice, proteolytic activation of Notch1 was significantly affected and the expression of several genes involved in the Notch signalling pathway was altered, including Delta-like3, Hes5, lunatic fringe (Lfng) and Mesp2. Thus, Ps1-dependent activation of the Notch pathway is essential for caudal half somite development. We observed defects in Notch signalling in both the caudal and rostral region of the presomitic mesoderm. In the caudal presomitic mesoderm, Ps1 was involved in maintaining the amplitude of cyclic activation of the Notch pathway, as represented by significant reduction of Lfng expression in Psen1-deficient mice. In the rostral presomitic mesoderm, rapid downregulation of the Mesp2 expression in the presumptive caudal half somite depends on Ps1 and is a prerequisite for caudal somite half specification. Chimaera analysis between Psen1-deficient and wild-type cells revealed that condensation of the wild-type cells in the caudal half somite was concordant with the formation of segment boundaries, while mutant and wild-type cells intermingled in the presomitic mesoderm. This implies that periodic activation of the Notch pathway in the presomitic mesoderm is still latent to segregate the presumptive rostral and caudal somite. A transient episode of Mesp2 expression might be needed for Notch activation by Ps1 to confer rostral or caudal properties. In summary, we propose that Ps1 is involved in the functional manifestation of the segmentation clock in the presomitic mesoderm.     

Interaction between Notch signalling and Lunatic fringe during somite boundary formation in the mouse.

BACKGROUND: The process of somitogenesis can be divided into three major events: the prepatterning of the mesoderm; the formation of boundaries between the prospective somites; and the cellular differentiation of the somites. Expression and functional studies have demonstrated the involvement of the murine Notch pathway in somitogenesis, although its precise role in this process is not yet well understood. We examined the effect of mutations in the Notch pathway elements Delta like 1 (Dll1), Notch1 and RBPJkappa on genes expressed in the presomitic mesoderm (PSM) and have defined the spatial relationships of Notch pathway gene expression in this region. RESULTS: We have shown that expression of Notch pathway genes in the PSM overlaps in the region where the boundary between the posterior and anterior halves of two consecutive somites will form. The Dll1, Notch1 and RBPJkappa mutations disrupt the expression of Lunatic fringe (L-fng), Jagged1, Mesp1, Mesp2 and Hes5 in the PSM. Furthermore, expression of EphA4, mCer 1 and uncx4.1, markers for the anterior-posterior subdivisions of the somites, is down-regulated to different extents in Notch pathway mutants, indicating a global alteration of pattern in the PSM. CONCLUSIONS: We propose a model for the mechanism of somite border formation in which the activity of Notch in the PSM is restricted by L-fng to a boundary-forming territory in the posterior half of the prospective somite. In this region, Notch function activates a set of genes that are involved in boundary formation and anterior-posterior somite identity.     

Discrete Notch signaling requirements in the specification of hematopoietic stem cells

Hematopoietic stem cells (HSCs) require multiple molecular inputs for proper specification, including activity of the Notch signaling pathway. A requirement for the Notch1 and dispensability of the Notch2 receptor has been demonstrated in mice, but the role of the remaining Notch receptors has not been investigated. Here, we demonstrate that three of the four Notch receptors are independently required for the specification of HSCs in the zebrafish. The orthologues of the murine Notch1 receptor, Notch1a and Notch1b, are each required intrinsically to fate HSCs, just prior to their emergence from aortic hemogenic endothelium. By contrast, the Notch3 receptor is required earlier within the developing somite to regulate HSC emergence in a non-cell-autonomous manner. Epistatic analyses demonstrate that Notch3 function lies downstream of Wnt16, which is required for HSC specification through its regulation of two Notch ligands, dlc and dld. Collectively, these findings demonstrate for the first time that multiple Notch signaling inputs are required to specify HSCs and that Notch3 performs a novel role within the somite to regulate the neighboring precursors of hemogenic endothelium.     

Oscillating signaling pathways during embryonic development

Oscillatory signaling pathway activity during embryonic development was first identified in the process of vertebrate somite formation. In mouse, this process is thought to be largely controlled by a cyclic signaling network involving the Notch, FGF, and Wnt pathways. Surprisingly, several recent genetic studies reveal that the core oscillation pacemaker is unlikely to involve periodic activation by these pathways. The mechanism(s) responsible for the production of oscillatory gene activity during somite formation remains, therefore, to be discovered. Oscillatory signaling activity has recently been identified in developmental processes distinct from somite formation. Both the processes of limb development in chick embryos and the maintenance of neural progenitors in mouse embryos involve oscillatory gene activity related to the Notch pathway. These discoveries indicate that oscillatory signaling activities during embryonic development might serve a more general function than previously thought.     

Compartmentalised expression of <Emphasis Type="Italic">Delta-like 1</Emphasis> in epithelial somites is required for the formation of intervertebral joints

Background  

Expression of the mouse Delta-like 1 (Dll1) gene in the presomitic mesoderm and in the caudal halves of somites of the developing embryo is required for the formation of epithelial somites and for the maintenance of caudal somite identity, respectively. The rostro-caudal polarity of somites is initiated early on within the presomitic mesoderm in nascent somites. Here we have investigated the requirement of restricted Dll1 expression in caudal somite compartments for the maintenance of rostro-caudal somite polarity and the morphogenesis of the axial skeleton. We did this by overexpressing a functional copy of the Dll1 gene throughout the paraxial mesoderm, in particular in anterior somite compartments, during somitogenesis in transgenic mice.     

Mutation of the fucose-specific β1,3 N-acetylglucosaminyltransferase LFNG results in abnormal formation of the spine

Notch signaling is an evolutionarily conserved mechanism that determines cell fate in a variety of contexts during development. This is achieved through different modes of action that are context dependent. One mode involves boundary formation between two groups of cells. With this mode of action, Notch signaling is central to vertebrate evolution as it drives the segmentation of paraxial mesoderm in the formation of somites, which are the precursors of the vertebra. In this case, boundary formation facilitates a mesenchymal to epithelial transition, leading to the creation of a somite. In addition, the boundary establishes a signaling center that patterns the somite, a feature that directly impacts on vertebral column formation. Studies in Xenopus, zebrafish, chicken and mouse have established the importance of Notch signaling in somitogenesis, and indeed in mouse how perturbations in somitogenesis affect vertebral column formation. Spondylocostal dysostosis is a congenital disorder characterized by formation of abnormal vertebrae. Here, mutation in Notch pathway genes demonstrates that Notch signaling is also required for normal somite formation and vertebral column development in humans; of particular interest here is mutation of the LUNATIC FRINGE (LFNG) gene, which causes SCD type 3. LUNATIC FRINGE encodes for a fucose-specific β1,3-N-acetylglucosaminyltransferase, which modifies Notch receptors and alters Notch signaling activity. This review will focus on Notch glycolsylation, and the role of LUNATIC FRINGE in somite formation and vertebral column development in mice and humans.