Dynamic expression of lunatic fringe suggests a link between notch signaling and an autonomous cellular oscillator driving somite segmentation

The bHLH regulator pMesogenin1 is required for maturation and segmentation of paraxial mesoderm

Paraxial mesoderm in vertebrates gives rise to all trunk and limb skeletal muscles, the trunk skeleton, and portions of the trunk dermis and vasculature. We show here that germline deletion of mouse pMesogenin1, a bHLH class gene specifically expressed in developmentally immature unsegmented paraxial mesoderm, causes complete failure of somite formation and segmentation of the body trunk and tail. At the molecular level, the phenotype features dramatic loss of expression within the presomitic mesoderm of Notch/Delta pathway components and oscillating somitic clock genes that are thought to control segmentation and somitogenesis. Subsequent patterning and specification steps for paraxial mesoderm also fail, leading to a complete absence of all trunk paraxial mesoderm derivatives, which include skeletal muscle, vertebrae, and ribs. We infer that pMesogenin1 is an essential upstream regulator of trunk paraxial mesoderm development and segmentation.

Notch signalling and the synchronization of the somite segmentation clock

In vertebrates with mutations in the Notch cell-cell communication pathway, segmentation fails: the boundaries demarcating somites, the segments of the embryonic body axis, are absent or irregular. This phenotype has prompted many investigations, but the role of Notch signalling in somitogenesis remains mysterious. Somite patterning is thought to be governed by a "clock-and-wavefront" mechanism: a biochemical oscillator (the segmentation clock) operates in the cells of the presomitic mesoderm, the immature tissue from which the somites are sequentially produced, and a wavefront of maturation sweeps back through this tissue, arresting oscillation and initiating somite differentiation. Cells arrested in different phases of their cycle express different genes, defining the spatially periodic pattern of somites and controlling the physical process of segmentation. Notch signalling, one might think, must be necessary for oscillation, or to organize subsequent events that create the somite boundaries. Here we analyse a set of zebrafish mutants and arrive at a different interpretation: the essential function of Notch signalling in somite segmentation is to keep the oscillations of neighbouring presomitic mesoderm cells synchronized.

Somite development in zebrafish

A full understanding of somite development requires knowledge of the molecular genetic pathways for cell determination as well as the cellular behaviors that underlie segmentation, somite epithelialization, and somite patterning. The zebrafish has long been recognized as an ideal organism for cellular and histological studies of somite patterning. In recent years, genetics has proven to be a very powerful complementary approach to these embryological studies, as genetic screens for zebrafish mutants defective in somitogenesis have identified over 50 genes that are necessary for normal somite development. Zebrafish is thus an ideal system in which to analyze the role of specific gene products in regulating the cell behaviors that underlie somite development. We review what is currently known about zebrafish somite development and compare it where appropriate to somite development in chick and mouse. We discuss the processes of segmentation and somite epithelialization, and then review the patterning of cell types within the somite. We show directly, for the first time, that muscle cell and sclerotome migrations occur at the same time. We end with a look at the many questions about somitogenesis that are still unanswered.

Oscillating expression of c-Hey2 in the presomitic mesoderm suggests that the segmentation clock may use combinatorial signaling through multiple interacting bHLH factors

Vertebrate somitogenesis comprises the generation of a temporal periodicity, the establishment of anteroposterior compartment identity, and the translation of the temporal periodicity into the metameric pattern of somites. Molecular players at each of these steps are beginning to be identified. Especially, members of the Notch signaling cascade appear to be involved in setting up the somitogenesis clock and subsequent events. We had previously demonstrated specific expression of the mHey1 and mHey2 basic helix-loop-helix (bHLH) factors during somitogenesis. Here we show that perturbed Notch signaling in Dll1 and Notch1 knockout mutants affects this expression in the presomitic mesoderm (PSM) and the somites. In the caudal PSM, however, mHey2 expression is maintained and thus is likely to be independent of Notch signaling. Furthermore, we analysed the dynamic expression of the respective chicken c-Hey1 and c-Hey2 genes during somitogenesis. Not only is c-Hey2 rhythmically expressed across the chicken presomitic mesoderm like c-hairy1, but its transcription is similarly independent of de novo protein synthesis. In contrast, the dynamic expression of c-Hey1 is restricted to the anterior segmental plate. Both c-Hey genes are coexpressed with c-hairy1 in the posterior somite half. Further in vitro and in vivo interaction assays demonstrated direct homo- and heterodimerisation between these hairy-related bHLH proteins, suggesting a combinatorial action in both the generation of a temporal periodicity and the anterior-posterior somite compartmentalisation.

Molecular and cellular biology of avian somite development

Much of our understanding of early vertebrate embryogenesis derives from experimental work done with the chick embryo. Studies of the avian somite have played a key role in elucidating the developmental history of this important structure, the source of most muscle and bone in the organism. Here we review the development of the avian somite including morphological and molecular data on the origin of paraxial mesoderm, maturation of the segmental plate, specification and formation of somite compartments, and somite cell differentiation into cartilage and skeletal muscle.

A chemical flow system mimics waves of gene expression during segmentation

The early vertebrate developmental process of somitogenesis involves bands of gene expression that form periodically at the posterior end of the presomitic mesoderm (PSM) and traverse it with decreasing width and velocity. We have constructed a chemical flow system that, based on the novel flow-distributed oscillator (FDO) mechanism of wave pattern formation, reproduces key physical features of the PSM and observe concentration waves having similar spatio-temporal behavior. This suggests that the gene expression waves can be understood qualitatively in terms of phase dynamics in an open flow of a self-oscillating medium and that chemical flow systems can be used to mimic and model biological pattern formation during axial growth. In fact, expressions for wavelength and wave velocity derived from phase dynamics are found to be in quantitative agreement with measurements from both the biological and the chemical systems. This indicates that they, despite their significant differences, have common dynamics.

Control of her1 expression during zebrafish somitogenesis by a Delta-dependent oscillator and an independent wave-front activity

Somitogenesis has been linked both to a molecular clock that controls the oscillation of gene expression in the presomitic mesoderm (PSM) and to Notch pathway signaling. The oscillator, or clock, is thought to create a prepattern of stripes of gene expression that regulates the activity of the Notch pathway that subsequently directs somite border formation. Here, we report that the zebrafish gene after eight (aei) that is required for both somitogenesis and neurogenesis encodes the Notch ligand DeltaD. Additional analysis revealed that stripes of her1 expression oscillate within the PSM and that aei/DeltaDsignaling is required for this oscillation.aei/DeltaD expression does not oscillate, indicating that the activity of the Notch pathway upstream ofher1 may function within the oscillator itself. Moreover, we found that her1 stripes are expressed in the anlage of consecutive somites, indicating that its expression pattern is not pair-rule. Analysis of her1 expression inaei/DeltaD, fused somites (fss), and aei;fss embryos uncovered a wave-front activity that is capable of continually inducing her1 expression de novo in the anterior PSM in the absence of the oscillation of her1. The wave-front activity, in reference to the clock and wave-front model, is defined as such because it interacts with the oscillator-derived pattern in the anterior PSM and is required for somite morphogenesis. This wave-front activity is blocked in embryos mutant for fssbut not aei/DeltaD. Thus, our analysis indicates that the smooth sequence of formation, refinement, and fading ofher1 stripes in the PSM is governed by two separate activities.

Zebrafish Mesp family genes, mesp-a and mesp-b are segmentally expressed in the presomitic mesoderm, and Mesp-b confers the anterior identity to the developing somites

Segmentation of a vertebrate embryo begins with the subdivision of the paraxial mesoderm into somites through a not-well-understood process. Recent studies provided evidence that the Notch-Delta and the FGFR (fibroblast growth factor receptor) signalling pathways are required for segmentation. In addition, the Mesp family of bHLH transcription factors have been implicated in establishing a segmental prepattern in the presomitic mesoderm. In this study, we have characterized zebrafish mesp-a and mesp-b genes that are closely related to Mesp family genes in other vertebrates. During gastrulation, only mesp-a is expressed in the paraxial mesoderm at the blastoderm margin. During the segmentation period, both genes are segmentally expressed in one to three stripes in the anterior parts of somite primordia. In fused somites (fss) embryos, in which all early somite boundary formation is blocked, initial mesp-a expression at the gastrula stage remains intact, but the expression of mesp-a and mesp-b is not detected during the segmentation period. This suggests that these genes are downstream targets of fss at the segmentation stage. Comparison with her1 expression (Muller, M., von Weizsacker, E. and Campos-Ortega, J. A. (1996) Development 122, 2071–2078) suggests that, like her1, mesp genes are not expressed in primordia of the first several somites. Furthermore, we found that zebrafish her1 expression oscillates in the presomitic mesoderm. The her1 stripe, which first appears in the tailbud region, moves in a caudal to rostral direction, and it finally overlaps the most rostral mesp stripe. Thus, in the trunk region, both her1 and mesp transcripts are detected in every somite primordium posterior to the forming somites. Ectopic expression of Mesp-b in embryos causes a loss of the posterior identity within the somite primordium, leading to a segmentation defect. These embryos show a reduction in expression of the posterior genes, myoD and notch5, with uniform expression of the anterior genes, FGFR1, papc and notch6. These observations suggest that zebrafish mesp genes are involved in anteroposterior specification within the presumptive somites, by regulating the essential signalling pathways mediated by Notch-Delta and FGFR.

Notch signalling is required for cyclic expression of the hairy-like gene HES1 in the presomitic mesoderm

Somitic segmentation provides the framework on which the segmental pattern of the vertebrae, some muscles and the peripheral nervous system is established. Recent evidence indicates that a molecular oscillator, the ‘segmentation clock’, operates in the presomitic mesoderm (PSM) to direct periodic expression of c-hairy1 and lunatic fringe (l-fng). Here, we report the identification and characterisation of a second avian hairy-related gene, c-hairy2, which also cycles in the PSM and whose sequence is closely related to the mammalian HES1 gene, a downstream target of Notch signalling in vertebrates. We show that HES1 mRNA is also expressed in a cyclic fashion in the mouse PSM, similar to that observed for c-hairy1 and c-hairy2 in the chick. In HES1 mutant mouse embryos, the periodic expression of l-fng is maintained, suggesting that HES1 is not a critical component of the oscillator mechanism. In contrast, dynamic HES1 expression is lost in mice mutant for Delta1, which are defective for Notch signalling. These results suggest that Notch signalling is required for hairy-like genes cyclic expression in the PSM.

A clock-work somite

Somites are transient structures which represent the most overt segmental feature of the vertebrate embryo. The strict temporal regulation of somitogenesis is of critical developmental importance since many segmental structures adopt a periodicity based on that of the somites. Until recently, the mechanisms underlying the periodicity of somitogenesis were largely unknown. Based on the oscillations of c-hairy1 and lunatic fringe RNA, we now have evidence for an intrinsic segmentation clock in presomitic cells. Translation of this temporal periodicity into a spatial periodicity, through somite formation, requires Notch signaling. While the Hox genes are certainly involved, it remains unknown how the metameric vertebrate axis becomes regionalized along the antero-posterior (AP) dimension into the occipital, cervical, thoracic, lumbar, and sacral domains. We discuss the implications of cell division as a clock mechanism underlying the regionalization of somites and their derivatives along the AP axis. Possible links between the segmentation clock and axial regionalization are also discussed. BioEssays 22:72-83, 2000.

Arbiter of differentiation and death: Notch signaling meets apoptosis

Notch-ligand interactions are a highly conserved mechanism that regulates cell fate decisions. Over the past few years, numerous observations have shown that this mechanism operates to regulate cell differentiation in an enormous variety of developmental and cell maturation processes. Recent studies indicate that in addition to cell differentiation, Notch signaling has direct effects on proliferation and programmed cell death. The picture emerging from these findings suggests that, depending on cellular and developmental context, Notch signaling may function as a general "arbiter" of cell fate, regulating differentiation potential, rate of proliferation, and apoptotic cell death. In this review, we briefly summarize the current knowledge of the structure and function of Notch receptors and discuss the recent evidence that Notch signaling regulates apoptotic cell death. The possible mechanisms of this effect and its potential implications for developmental biology, immunobiology, neuropathology, and tumor biology are discussed.

Fringe: defining borders by regulating the notch pathway

The Notch pathway mediates cell-cell interaction in many developmental processes. Multiple proteins regulate the Notch pathway, among these are the products of the fringe genes. The first fringe gene was identified in Drosophila, where it is involved in the formation of the dorsal/ventral border of the wing disc. It has now been found to be crucial for determining the dorsal/ventral border of the Drosophila eye. In vertebrates, fringe genes play roles in the formation of the apical ectodermal ridge, the dorsal/ventral border in the limb bud, and in the development of somitic borders. The roles of fringe in the neural tube or the eyes of vertebrate embryos are not clear, although it is unlikely that these roles are evolutionarily related to those in the same tissues in Drosophila. Genetic evidences suggest that Fringe protein functions by modulating the Notch signaling pathway, perhaps through differential regulation of Notch activation by different ligands; however, the mechanism underlying Fringe function remains to be investigated.

Notch around the clock

The establishment of a segmental pattern within the vertebrate body plan is achieved during embryogenesis by the somitogenesis process. Two molecular systems have been implicated in this phenomenon: a molecular clock linked to vertebrate segmentation and the Notch signalling pathway. Rhythmic expression of the Lunatic Fringe gene in the presomitic mesoderm has now provided a link between these two systems.

Fringe, Notch, and making developmental boundaries

Multiple mechanisms are involved in positioning and restricting specialized dorsal-ventral border cells in the Drosophila wing, including modulation of Notch signaling by Fringe, autonomous inhibition by Notch ligands, and inhibition of Notch target genes by Nubbin. Recent studies have revealed that Fringe also modulates a Notch-mediated signaling process between dorsal and ventral cells in the Drosophila eye, establishing an organizer of eye growth and patterning along the dorsal-ventral midline. Fringe-dependent modulation of Notch signaling also plays a key role in Drosophila leg segmentation and growth. Lunatic Fringe has been shown to be required for vertebrate somitogenesis, where it appears to act as a crucial link between a molecular clock and the regulation of Notch signaling.