Two linkedhairy/Enhancer of split-related zebrafish genes,her1andher7, function together to refine alternating somite boundaries

tortuga refines Notch pathway gene expression in the zebrafish presomitic mesoderm at the post-transcriptional level

We have identified the zebrafish tortuga (tor) gene by an ENU-induced mutation that disrupts the presomitic mesoderm (PSM) expression of Notch pathway genes. In tor mutants, Notch pathway gene expression persists in regions of the PSM where expression is normally off in wild type embryos. The expression of hairy/Enhancer of split-related 1 (her1) is affected first, followed by the delta genes deltaC and deltaD, and finally, by another hairy/Enhancer of split-related gene, her7. In situ hybridization with intron-specific probes for her1 and deltaC indicates that transcriptional bursts of expression are normal in tor mutants, suggesting that tor normally functions to refine her1 and deltaC message levels downstream of transcription. Despite the striking defects in Notch pathway gene expression, somite boundaries form normally in tor mutant embryos, although somitic mesoderm defects are apparent later, when cells mature to form muscle fibers. Thus, while the function of Notch pathway genes is required for proper somite formation, the tor mutant phenotype suggests that precise oscillations of Notch pathway transcripts are not essential for establishing segmental pattern in the presomitic mesoderm.

Interactions between muscle fibers and segment boundaries in zebrafish

The most obvious segmental structures in the vertebrate embryo are somites: transient structures that give rise to vertebrae and much of the musculature. In zebrafish, most somitic cells give rise to long muscle fibers that are anchored to intersegmental boundaries. Therefore, this boundary is analogous to the mammalian tendon in that it transduces muscle-generated force to the skeletal system. We have investigated interactions between somite boundaries and muscle fibers. We define three stages of segment boundary formation. The first stage is the formation of the initial epithelial somite boundary. The second "transition" stage involves both the elongation of initially round muscle precursor cells and somite boundary maturation. The third stage is myotome boundary formation, where the boundary becomes rich in extracellular matrix and all muscle precursor cells have elongated to form long muscle fibers. It is known that formation of the initial epithelial somite boundary requires Notch signaling; vertebrate Notch pathway mutants show severe defects in somitogenesis. However, many zebrafish Notch pathway mutants are homozygous viable suggesting that segmentation of their larval and adult body plans at least partially recovers. We show that epithelial somite boundary formation and slow-twitch muscle morphogenesis are initially disrupted in after eight (aei) mutant embryos (which lack function of the Notch ligand, DeltaD); however, myotome boundaries form later ("recover") in a Hedgehog-dependent fashion. Inhibition of Hedgehog-induced slow muscle induction in aei/deltaD and deadly seven (des)/notch1a mutant embryos suggests that slow muscle is necessary for myotome boundary recovery in the absence of initial epithelial somite boundary formation. Because we have previously demonstrated that slow muscle migration triggers fast muscle cell elongation in zebrafish, we hypothesize that migrating slow muscle facilitates myotome boundary formation in aei/deltaD mutant embryos by patterning coordinated fast muscle cell elongation. In addition, we utilized genetic mosaic analysis to show that somite boundaries also function to limit the extent to which fast muscle cells can elongate. Combined, our results indicate that multiple interactions between somite boundaries and muscle fibers mediate zebrafish segmentation.

Generation of segment polarity in the paraxial mesoderm of the zebrafish through a T-box-dependent inductive event

The first morphological sign of vertebrate postcranial body segmentation is the sequential production from posterior paraxial mesoderm of blocks of cells termed somites. Each of these embryonic structures is polarized along the anterior/posterior axis, a subdivision first distinguished by marker gene expression restricted to rostral or caudal territories of forming somites. To better understand the generation of segment polarity in vertebrates, we have studied the zebrafish mutant fused somites (fss), because its paraxial mesoderm lacks segment polarity. Previously examined markers of caudal half-segment identity are widely expressed, whereas markers of rostral identity are either missing or dramatically down-regulated, suggesting that the paraxial mesoderm of the fss mutant embryo is profoundly caudalized. These findings gave rise to a model for the formation of segment polarity in the zebrafish in which caudal is the default identity for paraxial mesoderm, upon which is patterned rostral identity in an fss-dependent manner. In contrast to this scheme, the caudal marker gene ephrinA1 was recently shown to be down-regulated in fss embryos. We now show that notch5, another caudal identity marker and a component of the Delta/Notch signaling system, is not expressed in the paraxial mesoderm of early segmentation stage fss embryos. We use cell transplantation to create genetic mosaics between fss and wild-type embryos in order to assay the requirement for fss function in notch5 expression. In contrast to the expression of rostral markers, which have a cell-autonomous requirement for fss, expression of notch5 is induced in fss cells at short range by nearby wild-type cells, indicating a cell-non-autonomous requirement for fss function in this process. These new data suggest that segment polarity is created in a three-step process in which cells that have assumed a rostral identity must subsequently communicate with their partially caudalized neighbors in order to induce the fully caudalized state.

beamter/deltaC and the role of Notch ligands in the zebrafish somite segmentation, hindbrain neurogenesis and hypochord differentiation

The Tübingen large-scale zebrafish genetic screen completed in 1996 identified a set of five genes required for orderly somite segmentation. Four of them have been molecularly identified and three were found to code for components of the Notch pathway, which are required for the coordinated oscillation of gene expression, known as the segmentation clock, in the presomitic mesoderm (PSM). Here, we show that the final member of the group, beamter (bea), codes for the Notch ligand DeltaC, and we present and characterize two new alleles, including one allele encoding for a protein truncated in the 7th EGF repeat and an allele deleting only the DSL domain which was previously shown to be necessary for ligand function. Interestingly however, when we over-express any of the mutant deltaC mRNAs, we observe antimorphic effects on both hindbrain neurogenesis and hypochord formation. Expression of bea/deltaC oscillates in the PSM, and a triple fluorescent in situ analysis of its oscillation in relation to that of other oscillating genes in the PSM reveals differences in subcellular localization of the oscillating mRNAs in individual cells in different oscillation phases. Mutations in aei/deltaD and bea/deltaC differ in the way they disrupt the oscillating expression of her1 and deltaC. Furthermore, we find that the double mutants have significantly stronger defects in hypochord formation but not in somitogenesis or hindbrain neurogenesis, indicating genetically that the two delta's may function either semi-redundantly or distinctly, depending upon context.

Integrinalpha5-dependent fibronectin accumulation for maintenance of somite boundaries in zebrafish embryos

Boundary formation and epithelialization are crucial processes in the morphological segmentation of vertebrate somites. By a genetic screening procedure with zebrafish, we identified two genes, integrinalpha5 (itga5) and fibronectin (fn), required for these processes. Fibronectin proteins accumulate at somite boundaries in accordance with epithelialization of the somites. Both Fibronectin accumulation and the epithelialization are dependent on itga5, which is expressed in the most medial part of somites. Although somite boundaries are initially formed, but not maintained, in the anterior trunk of the mutant embryos deficient in either gene, their maintenance is defective at all axial levels of embryos deficient for both of these genes. Therefore, Integrinalpha5-directed assembly of Fibronectin appears critical for epithelialization and boundary maintenance of somites. Furthermore, with an additional deficiency in ephrin-B2a, the segmental defect in itga5 or fn mutant embryos is expanded posteriorly, indicating that both Integrin-Fibronectin and Eph-Ephrin systems function cooperatively in maintaining somite boundaries.

Zebrafish hairy/enhancer of split protein links FGF signaling to cyclic gene expression in the periodic segmentation of somites

Notch and fibroblast growth factor (FGF) signaling pathways have been implicated in the establishment of proper periodicity of vertebrate somites. Here, we show evidence that a Hes6-related hairy/Enhancer of split-related gene, her13.2, links FGF signaling to the Notch-regulated oscillation machinery in zebrafish. Expression of her13.2 is induced by FGF-soaked beads and decreased by an FGF signaling inhibitor. her13.2 is required for periodic repression of the Notch-regulated genes her1 and her7, and for proper somite segmentation. Furthermore, Her13.2 augments autorepression of her1 in association with Her1 protein. Therefore, FGF signaling appears to maintain the oscillation machinery by supplying a binding partner, Her13.2, for Her1.

Cooperative function of deltaC and her7 in anterior segment formation

Segmentation of paraxial mesoderm in vertebrates is regulated by a genetic oscillator that manifests as a series of wavelike or cyclic gene expression domains in the embryo. In zebrafish, this oscillator involves members of the Delta/Notch intercellular signaling pathway, and its down-stream targets, the Her family of transcriptional repressors. Loss of function of any one of the genes of this system, such as her7, gives rise to segmentation defects in the posterior trunk and tail, concomitant with a disruption of cyclic expression domains, indicating that the oscillator is required for posterior segmentation. Control of segmentation in the anterior trunk, and its relationship to that of the posterior is, however, not yet well understood. A combined loss of the cyclic Her genes her1 and her7 disrupts segmentation of both anterior and posterior paraxial mesoderm, indicating that her genes function redundantly in anterior segmentation. To test whether this anterior redundancy is specific to the her gene family, or alternatively is a more global feature of the segmentation oscillator, we looked at anterior segmentation after morpholino knock down of the cyclic cell-surface Notch ligand deltaC (dlc), either alone or in combination with her7, or other Delta/Notch pathway genes. We find that dlc is required for coherence of wavelike expression domains of cyclic genes her1 and her7 and maintenance of their expression levels, as well as for cyclic transcription of dlc itself, confirming that dlc is a component of the segmentation oscillator. Dose dependent, posteriorly-restricted segmentation defects were seen in the dlc knock down, and in combination with the deltaD or notch1a mutants. However, combined reduction of function of dlc and her7 results in defective segmentation of both anterior and posterior paraxial mesoderm, and a failure of cyclic expression domains to initiate, similar to loss of both her genes. Thus, anterior segmentation requires the functions of both her and delta family members in a parallel manner, suggesting that the segmentation oscillator operates in paraxial mesoderm along the entire vertebrate axis.

Retinoic acid signalling links left–right asymmetric patterning and bilaterally symmetric somitogenesis in the zebrafish embryo

During embryogenesis, cells are spatially patterned as a result of highly coordinated and stereotyped morphogenetic events. In the vertebrate embryo, information on laterality is conveyed to the node, and subsequently to the lateral plate mesoderm, by a complex cascade of epigenetic and genetic events, eventually leading to a left-right asymmetric body plan. At the same time, the paraxial mesoderm is patterned along the anterior-posterior axis in metameric units, or somites, in a bilaterally symmetric fashion. Here we characterize a cascade of laterality information in the zebrafish embryo and show that blocking the early steps of this cascade (before it reaches the lateral plate mesoderm) results in random left-right asymmetric somitogenesis. We also uncover a mechanism mediated by retinoic acid signalling that is crucial in buffering the influence of the flow of laterality information on the left-right progression of somite formation, and thus in ensuring bilaterally symmetric somitogenesis.

Integrinα5 and Delta/Notch Signaling Have Complementary Spatiotemporal Requirements during Zebrafish Somitogenesis

Somitogenesis is the process by which the segmented precursors of the skeletal muscle and vertebral column are generated during vertebrate embryogenesis. While somitogenesis appears to be a serially homologous, reiterative process, we find that there are differences between the genetic control of early/anterior and late/posterior somitogenesis. We demonstrate that point mutations can cause segmentation defects in either the anterior, middle, or posterior somites in the zebrafish. We find that mutations in zebrafish integrinalpha5 disrupt anterior somite formation, giving a phenotype complementary to the posterior defects seen in the notch pathway mutants after eight/deltaD and deadly seven/notch1a. Double mutants between the notch pathway and integrinalpha5 display somite defects along the entire body axis, with a complete loss of the mesenchymal-to-epithelial transition and Fibronectin matrix assembly in the posterior. Our data suggest that notch- and integrinalpha5-dependent cell polarization and Fibronectin matrix assembly occur concomitantly and interdependently during border morphogenesis.

her7 and hey1, but not lunatic fringe show dynamic expression during somitogenesis in medaka (Oryzias latipes)

Epithelialized somites form repeatedly from the unsegmented presomitic mesoderm (PSM) in the tailbud of vertebrate embryos. Mutant analysis has shown that the Delta-Notch pathway is essential for the temporal and spatial control of somite formation. Several components of this pathway show cyclic transcription, which is driven by a molecular oscillator. This oscillator is thought to act similarly in different vertebrates. In this study, we used the Japanese Medaka (Oryzias latipes) to examine the expression of three factors of the Delta-Notch cascade that are known to show cyclic expression in the PSM of higher vertebrates. We report that in contrast to the situation in mice, lunatic fringe (lfng) in medaka is expressed in a non-dynamic fashion in the rostral halves of the formed somites and the anteriormost PSM. On the other hand, her7, a member of the hairy/Enhancer-of-split related (Her) gene family, shows cyclic expression in the medaka PSM. Although this is similar in zebrafish, there are important differences in the distribution of transcripts in the PSM indicating different modes of regulation in both fish species. Finally, we show that hey1, another Delta-Notch regulated bHLH gene, is dynamically expressed in the PSM of medaka, similar to hey1 in zebrafish and the hey2 orthologs in mice and chicken. Interestingly, medaka hey1 is also expressed in the dorsal aorta and the heart, two tissues where hey2, but not hey1, is expressed in zebrafish. This shows that several components of the Delta-Notch pathway are differently regulated during somitogenesis in different species.

Vertebrate myotome development

The embryonic myotome generates both the axial musculature and the appendicular muscle of the fins and limbs. Early in embryo development the mesoderm is segmented into somites, and within these the primary myotome forms by a complex series of cellular movements and migrations. A new model of primary myotome formation in amniotes has emerged recently. The myotome also includes the muscle progenitor cells that are known to contribute to the secondary formation of the myotome. The adult myotome contains satellite cells that play an important role in adult muscle regeneration. Recent studies have shed light on how the growth and patterning of the myotome occurs.

Segmentation

The three major taxa with metameric segmentation (annelids, arthropods, and chordates) appear to use three very different molecular strategies to generate segments. However, unexpected similarities are starting to emerge from characterization of pair-rule patterning and segmental border formation. Moreover, the existence of an ancestral segmentation clock based on Notch signaling has become likely. An old concept of comparative anatomy, the enterocoele theory, is compatible with a single origin of segmentation mechanisms and could therefore provide a conceptual framework for assessing these molecular similarities.

Somite polarity and segmental patterning of the peripheral nervous system

The analysis of the outgrowth pattern of spinal axons in the chick embryo has shown that somites are polarized into anterior and posterior halves. This polarity dictates the segmental development of the peripheral nervous system: migrating neural crest cells and outgrowing spinal axons traverse exclusively the anterior halves of the somite-derived sclerotomes, ensuring a proper register between spinal axons, their ganglia and the segmented vertebral column. Much progress has been made recently in understanding the molecular basis for somite polarization, and its linkage with Notch/Delta, Wnt and Fgf signalling. Contact-repulsive molecules expressed by posterior half-sclerotome cells provide critical guidance cues for axons and neural crest cells along the anterior-posterior axis. Diffusible repellents from surrounding tissues, particularly the dermomyotome and notochord, orient outgrowing spinal axons in the dorso-ventral axis ('surround repulsion'). Repulsive forces therefore guide axons in three dimensions. Although several molecular systems have been identified that may guide neural crest cells and axons in the sclerotome, it remains unclear whether these operate together with considerable overall redundancy, or whether any one system predominates in vivo.

The role of Suppressor of Hairless in Notch mediated signalling during zebrafish somitogenesis

Suppressor of Hairless (Su(H)) codes for a protein that interacts with the intracellular domain of Notch to activate the target genes of the Delta-Notch signalling pathway. We have cloned the zebrafish homologue of Su(H) and have analysed its function by morpholino mediated knockdown. While there are at least four notch and four delta homologues in zebrafish, there appears to be only one complete Su(H) homologue. We have analysed the function of Su(H) in the somitogenesis process and its influence on the expression of notch pathway genes, in particular her1, her7, deltaC and deltaD. The cyclic expression of her1, her7 and deltaC in the presomitic mesoderm is disrupted by the Su(H) knockdown mimicking the expression of these genes in the notch1a mutant deadly seven. deltaD expression is similarly affected by Su(H) knockdown like deltaC but shows in addition an ectopic expression in the developing neural tube. The inactivation of Su(H) in a fss/tbx24 mutant background leads furthermore to a clear breakdown of cyclic her1 and her7 expression, indicating that the Delta-Notch pathway is required for the creation of oscillation and not only for the synchronisation between neighbouring cells. The strongest phenotypes in the Su(H) knockdown embryos show a loss of all somites posterior to the first five to seven ones. This phenotype is stronger than the known amorphic phenotypes for notch1 (des) or deltaD (aei) in zebrafish, but mimicks the knockout phenotype of RBP-Jkappa gene in the mouse, which is the homologue of Su(H). This suggests that there is some functional redundancy among the Notch and Delta genes. This fact that the first five to seven somites are only weakly affected by Su(H) knockdown indicates that additional genetic pathways may be active in the specification of the most anterior somites.

The vertebrate segmentation clock

In vertebrate embryos, somite segmentation is controlled by a molecular clock, in the form of a transcriptional oscillator that operates in the presomitic mesoderm. Most of the genes implicated in the oscillator belong to the Notch pathway; a recently discovered exception is the Wnt pathway gene Axin2. Experiments have revealed several negative feedback loops that might generate oscillations, leading to at least four different theories. The simplest of these is based on direct autoinhibition of certain members of the hairy/E(spl) family of Notch target genes--Hes7 in the mouse, and her1 and her7 in the zebrafish. A mathematical account of this mechanism explains some surprising observations and suggests that the period of oscillation is chiefly determined by the transcriptional and translational delays--the times required to make a molecule of the mRNA and a molecule of the protein.

Modeling human hematopoietic and cardiovascular diseases in zebrafish

Zebrafish have emerged as a useful vertebrate model system in which unbiased large-scale screens have revealed hundreds of mutations affecting vertebrate development. Many zebrafish mutants closely resemble known human disorders, thus providing intriguing prospects for uncovering the genetic basis of human diseases and for the development of pharmacologic agents that inhibit or correct the progression of developmental disorders. The rapid pace of advances in genomic sequencing and map construction, in addition to morpholino targeting and transgenic techniques, have facilitated the identification and analysis of genes associated with zebrafish mutants, thus promoting the development of zebrafish as a model for human disorders. This review aims to illustrate how the zebrafish has been used to identify unknown genes, to assign function to known genes, and to delineate genetic pathways, all contributing valuable leads toward understanding human pathophysiology.

Mutations affecting somite formation in the Medaka (Oryzias latipes)

The metameric structure of the vertebrate trunk is generated by repeated formation of somites from the unsegmented presomitic mesoderm (PSM). We report the initial characterization of nine different mutants affecting segmentation that were isolated in a large-scale mutagenesis screen in Medaka (Oryzias latipes). Four mutants were identified that show a complete or partial absence of somites or somite boundaries. In addition, five mutations were found that cause fused somites or somites with irregular sizes and shapes. In situ hybridization analysis using specific markers involved in the segmentation clock and antero-posterior (A-P) polarity of somites revealed that the nine mutants can be compiled into two groups. In group 1, mutants exhibit defects in tailbud formation and PSM prepatterning, whereas A-P identity in the somites is defective in group 2 mutants. Three mutants (planlos, pll; schnelles ende, sne; samidare, sam) have characteristic phenotypes that are similar to those in zebrafish mutants affected in the Delta/Notch signaling pathway. The majority of mutants, however, exhibit somitic phenotypes distinct from those found in zebrafish, such as individually fused somites and irregular somite sizes. Thus, these Medaka mutants can be expected to provide clues to uncovering novel components essential for somitogenesis.

Instability of Hes7 protein is crucial for the somite segmentation clock

During somitogenesis, a pair of somites buds off from the presomitic mesoderm every 2 hours in mouse embryos, suggesting that somite segmentation is controlled by a biological clock with a 2-hour cycle. Expression of the basic helix-loop-helix factor Hes7, an effector of Notch signaling, follows a 2-hour oscillatory cycle controlled by negative feedback; this is proposed to be the molecular basis for the somite segmentation clock. If the proposal is correct, this clock should depend crucially on the short lifetime of Hes7. To address the biological importance of Hes7 instability, we generated mice expressing mutant Hes7 with a longer half-life (approximately 30 min compared with approximately 22 min for wild-type Hes7) but normal repressor activity. In these mice, somite segmentation and oscillatory expression became severely disorganized after a few normal cycles of segmentation. We simulated this effect mathematically using a direct autorepression model. Thus, instability of Hes7 is essential for sustained oscillation and for its function as a segmentation clock.

A Notch feeling of somite segmentation and beyond

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Modulation of Notch Signaling During Somitogenesis

The Notch signaling pathway is known to govern various aspects of tissue differentiation during embryonic development by mediating local cell-cell interactions that often control cell fate. The conserved components that underlie Notch signaling have been isolated in vertebrates, leading to a biochemical delineation of a core Notch signaling pathway and functional studies of this pathway during embryogenesis. Herein we highlight recent progress in determining how Notch signaling contributes to the development of the vertebrate embryo. We first discuss the role of Notch in the process of segmentation where rapid changes have been shown to occur in both the spatial and temporal aspects of Notch signaling, which are critical for segmental patterning. Indeed, the role of Notch in segmentation re-emphasizes a recurring question in Notch biology: how are the components involved in Notch signaling regulated to ensure their dynamic properties? Second, we address this question by discussing recent work on the biochemical mechanisms that potentially regulate Notch signaling during segmentation, including those that act on the receptors, ligands, and signal transduction apparatus.

Somitogenesis: breaking new boundaries

Segmentation is a fundamental process in vertebrate embryogenesis, and one of the earliest manifestations of segmental patterning is the generation of transient, serially repeated blocks of mesodermal cells known as somites. Disruption of the normal segmentation process in humans leads to vertebral abnormalities such as spondylocostal dysostosis. In this minireview, we discuss recent advances in the dynamic molecular and cellular mechanisms governing segmentation.

Autoinhibition with transcriptional delay: a simple mechanism for the zebrafish somitogenesis oscillator

BACKGROUND

The pattern of somites is traced out by a mechanism involving oscillating gene expression at the tail end of the embryo. In zebrafish, two linked oscillating genes, her1 and her7, coding for inhibitory gene regulatory proteins, are especially implicated in genesis of the oscillations, while Notch signaling appears necessary for synchronization of adjacent cells.

RESULTS

I show by mathematical simulation that direct autorepression of her1 and her7 by their own protein products provides a mechanism for the intracellular oscillator. This mechanism operates robustly even when one allows for the fact that gene regulation is an essentially noisy (stochastic) process. The predicted period is close to the observed period (30 min) and is dictated primarily by the transcriptional delay, the time taken to make an mRNA molecule. Through its coupling to her1/her7 expression, Notch signaling can keep the rapid oscillations in adjacent cells synchronized. When the coupling parameters are varied, however, the model system can switch to oscillations of a much longer period, resembling that of the mouse or chick somitogenesis oscillator and governed by the delays in the Notch pathway. Such Notch-mediated synchronous oscillations are predicted even in the absence of direct her1/her7 autoregulation, through operation of the standard Notch signaling pathway that is usually assumed simply to give lateral inhibition.

CONCLUSIONS

Direct autorepression of a gene by its own product can generate oscillations, with a period determined by the transcriptional and translational delays. Simple as they are, such systems show surprising behaviors. To understand them, unaided intuition is not enough: we need mathematics.

Activity and distribution of paxillin, focal adhesion kinase, and cadherin indicate cooperative roles during zebrafish morphogenesis

We investigated the focal adhesion proteins paxillin and Fak, and the cell-cell adhesion protein cadherin in developing zebrafish (Danio rerio) embryos. Cadherins are expressed in presomitic mesoderm where they delineate cells. The initiation of somite formation coincides with an increase in the phosphorylation of Fak, and the accumulation of Fak, phosphorylated Fak, paxillin, and fibronectin at nascent somite boundaries. In the notochord, cadherins are expressed on cells during intercalation, and phosphorylated Fak accumulates in circumferential rings where the notochord cells contact laminin in the perichordal sheath. Subsequently, changes in the orientations of collagen fibers in the sheath suggest that Fak-mediated adhesion allows longitudinal expansion of the notochord, but not lateral expansion, resulting in notochord elongation. Novel observations showed that focal adhesion kinase and paxillin concentrate at sites of cell-cell adhesion in the epithelial enveloping layer and may associate with actin cytoskeleton at epithelial junctions containing cadherins. Fak is phosphorylated at these epithelial junctions but is not phosphorylated on Tyr397, implicating a noncanonical mechanism of regulation. These data suggest that Fak and paxillin may function in the integration of cadherin-based and integrin-based cell adhesion during the morphogenesis of the early zebrafish embryo.

Oscillations, clocks and segmentation

Notch signalling molecules, such as the basic helix-loop-helix factors Hes1 and Hes7, periodically change their expression in the presomitic mesoderm, and each cycle of gene expression is associated with somite formation (every two hours in mouse). This cyclic expression is the manifestation of an intrinsic mechanism, called the segmentation clock, which is essential for coordinated somite segmentation. Interestingly, the oscillatory expression of Hes1 is observed in many cell types after serum stimulation, suggesting that this ultradian clock is not unique to presomitic mesoderm cells but widely distributed. This oscillation depends on the negative feedback loop, and once its promoter is constitutively activated, Hes1 seems to start oscillatory gene expression autonomously. Thus, Hes1 acts as a device that transduces a direct current of input into an alternating current, which ticks the hours in many biological systems.

Periodic repression by the bHLH factor Hes7 is an essential mechanism for the somite segmentation clock

Hes7, a bHLH gene essential for somitogenesis, displays cyclic expression of mRNA in the presomitic mesoderm (PSM). Here, we show that Hes7 protein is also expressed in a dynamic manner, which depends on proteasome-mediated degradation. Spatial comparison revealed that Hes7 and Lunatic fringe (Lfng) transcription occurs in the Hes7 protein-negative domains. Furthermore, Hes7 and Lfng transcription is constitutively up-regulated in the absence of Hes7 protein and down-regulated by stabilization of Hes7 protein. Thus, periodic repression by Hes7 protein is critical for the cyclic transcription of Hes7 and Lfng, and this negative feedback represents a molecular basis for the segmentation clock.

Playing by pair-rules?

Although in Drosophila pair-rule genes play crucial roles in the genetic hierarchy that subdivides the embryo into segments, the extent to which pair-rule patterning is utilized by different arthropods and other segmented phyla is unknown. Recent data of Dearden et al.1 and Henry et al.,2 however, hint that a pair-rule mechanism might play a role in the segmentation process of basal arthropods and vertebrates.

A Hes1-based oscillator in cultured cells and its potential implications for the segmentation clock

During somitogenesis an oscillatory mechanism termed the "segmentation" clock generates periodic waves of gene expression, which translate into the periodic spatial pattern manifest as somites. The dynamic expression of the clock genes shares the same periodicity as somitogenesis. Notch signaling is believed to play a role in the segmentation clock mechanism. The paper by Hirata et al.(1) identifies a biological clock in cultured cells that is dependent upon the Notch target gene Hes1, and which shows a periodicity similar to that of the segmentation clock. This finding opens the possibility that the same oscillator mechanism might also operate in other tissues or cell types.

Vertebrate segmentation: lunatic transcriptional regulation

Vertebrate segmentation relies on a molecular oscillator, the segmentation clock, which controls the periodic expression of genes such as lunatic fringe in the presomitic mesoderm. Oscillations of lunatic fringe transcripts have now been shown to be controlled at the transcriptional level by clock elements in the lunatic fringe promoter.