Tbx protein level critical for clock-mediated somite positioning is regulated through interaction between Tbx and Ripply

Cell-autonomous timing drives the vertebrate segmentation clock's wave pattern

Rhythmic and sequential segmentation of the growing vertebrate body relies on the segmentation clock, a multi-cellular oscillating genetic network. The clock is visible as tissue-level kinematic waves of gene expression that travel through the presomitic mesoderm (PSM) and arrest at the position of each forming segment. Here, we test how this hallmark wave pattern is driven by culturing single maturing PSM cells. We compare their cell-autonomous oscillatory and arrest dynamics to those we observe in the embryo at cellular resolution, finding similarity in the relative slowing of oscillations and arrest in concert with differentiation. This shows that cell-extrinsic signals are not required by the cells to instruct the developmental program underlying the wave pattern. We show that a cell-autonomous timing activity initiates during cell exit from the tailbud, then runs down in the anterior-ward cell flow in the PSM, thereby using elapsed time to provide positional information to the clock. Exogenous FGF lengthens the duration of the cell-intrinsic timer, indicating extrinsic factors in the embryo may regulate the segmentation clock via the timer. In sum, our work suggests that a noisy cell-autonomous, intrinsic timer drives the slowing and arrest of oscillations underlying the wave pattern, while extrinsic factors in the embryo tune this timer's duration and precision. This is a new insight into the balance of cell-intrinsic and -extrinsic mechanisms driving tissue patterning in development.

Cellular and molecular control of vertebrate somitogenesis

Segmentation is a fundamental feature of the vertebrate body plan. This metameric organization is first implemented by somitogenesis in the early embryo, when paired epithelial blocks called somites are rhythmically formed to flank the neural tube. Recent advances in in vitro models have offered new opportunities to elucidate the mechanisms that underlie somitogenesis. Notably, models derived from human pluripotent stem cells introduced an efficient proxy for studying this process during human development. In this Review, we summarize the current understanding of somitogenesis gained from both in vivo studies and in vitro studies. We deconstruct the spatiotemporal dynamics of somitogenesis into four distinct modules: dynamic events in the presomitic mesoderm, segmental determination, somite anteroposterior polarity patterning, and epithelial morphogenesis. We first focus on the segmentation clock, as well as signalling and metabolic gradients along the tissue, before discussing the clock and wavefront and other models that account for segmental determination. We then detail the molecular and cellular mechanisms of anteroposterior polarity patterning and somite epithelialization.

Vertebral pattern and morphology is determined during embryonic segmentation

BACKGROUND

The segmented nature of the adult vertebral column is based on segmentation of the paraxial mesoderm during early embryogenesis. Disruptions to embryonic segmentation, whether caused by genetic lesions or environmental stress, result in adult vertebral pathologies. However, the mechanisms linking embryonic segmentation and the details of adult vertebral morphology are poorly understood.

RESULTS

We induced border defects using two approaches in zebrafish: heat stress and misregulation of embryonic segmentation genes tbx6, mesp-ba, and ripply1. We assayed vertebral length, regularity, and polarity using microscopic and radiological imaging. In population studies, we find a correlation between specific embryonic border defects and specific vertebral defects, and within individual fish, we trace specific adult vertebral defects to specific embryonic border defects.

CONCLUSIONS

Our data reveal that transient disruptions of embryonic segment border formation led to significant vertebral anomalies that persist through adulthood. The spacing of embryonic borders controls the length of the vertebra. The positions of embryonic borders control the positions of ribs and arches. Embryonic borders underlie fusions and divisions between adjacent spines and ribs. These data suggest that segment borders have a dominant role in vertebral development.

Spatiotemporal control of pattern formation during somitogenesis

Spatiotemporal patterns widely occur in biological, chemical, and physical systems. Particularly, embryonic development displays a diverse gamut of repetitive patterns established in many tissues and organs. Branching treelike structures in lungs, kidneys, livers, pancreases, and mammary glands as well as digits and bones in appendages, teeth, and palates are just a few examples. A fascinating instance of repetitive patterning is the sequential segmentation of the primary body axis, which is conserved in all vertebrates and many arthropods and annelids. In these species, the body axis elongates at the posterior end of the embryo containing an unsegmented tissue. Meanwhile, segments sequentially bud off from the anterior end of the unsegmented tissue, laying down an exquisite repetitive pattern and creating a segmented body plan. In vertebrates, the paraxial mesoderm is sequentially divided into somites. In this review, we will discuss the most prominent models, the most puzzling experimental data, and outstanding questions in vertebrate somite segmentation.

Generation of patterns in the paraxial mesoderm

The Segmentation Clock is a tissue-level patterning system that enables the segmentation of the vertebral column precursors into transient multicellular blocks called somites. This patterning system comprises a set of elements that are essential for correct segmentation. Under the so-called "Clock and Wavefront" model, the system consists of two elements, a genetic oscillator that manifests itself as traveling waves of gene expression, and a regressing wavefront that transforms the temporally periodic signal encoded in the oscillations into a permanent spatially periodic pattern of somite boundaries. Over the last twenty years, every new discovery about the Segmentation Clock has been tightly linked to the nomenclature of the "Clock and Wavefront" model. This constrained allocation of discoveries into these two elements has generated long-standing debates in the field as what defines molecularly the wavefront and how and where the interaction between the two elements establishes the future somite boundaries. In this review, we propose an expansion of the "Clock and Wavefront" model into three elements, "Clock", "Wavefront" and signaling gradients. We first provide a detailed description of the components and regulatory mechanisms of each element, and we then examine how the spatiotemporal integration of the three elements leads to the establishment of the presumptive somite boundaries. To be as exhaustive as possible, we focus on the Segmentation Clock in zebrafish. Furthermore, we show how this three-element expansion of the model provides a better understanding of the somite formation process and we emphasize where our current understanding of this patterning system remains obscure.

Ripply suppresses Tbx6 to induce dynamic-to-static conversion in somite segmentation

The metameric pattern of somites is created based on oscillatory expression of clock genes in presomitic mesoderm. However, the mechanism for converting the dynamic oscillation to a static pattern of somites is still unclear. Here, we provide evidence that Ripply/Tbx6 machinery is a key regulator of this conversion. Ripply1/Ripply2-mediated removal of Tbx6 protein defines somite boundary and also leads to cessation of clock gene expression in zebrafish embryos. On the other hand, activation of ripply1/ripply2 mRNA and protein expression is periodically regulated by clock oscillation in conjunction with an Erk signaling gradient. Whereas Ripply protein decreases rapidly in embryos, Ripply-triggered Tbx6 suppression persists long enough to complete somite boundary formation. Mathematical modeling shows that a molecular network based on results of this study can reproduce dynamic-to-static conversion in somitogenesis. Furthermore, simulations with this model suggest that sustained suppression of Tbx6 caused by Ripply is crucial in this conversion.

The vertebrate Embryo Clock: Common players dancing to a different beat

Vertebrate embryo somitogenesis is the earliest morphological manifestation of the characteristic patterned structure of the adult axial skeleton. Pairs of somites flanking the neural tube are formed periodically during early development, and the molecular mechanisms in temporal control of this early patterning event have been thoroughly studied. The discovery of a molecular Embryo Clock (EC) underlying the periodicity of somite formation shed light on the importance of gene expression dynamics for pattern formation. The EC is now known to be present in all vertebrate organisms studied and this mechanism was also described in limb development and stem cell differentiation. An outstanding question, however, remains unanswered: what sets the different EC paces observed in different organisms and tissues? This review aims to summarize the available knowledge regarding the pace of the EC, its regulation and experimental manipulation and to expose new questions that might help shed light on what is still to unveil.

Congenital cervical spine malformation due to bi-allelic RIPPLY2 variants in spondylocostal dysostosis type 6

RIPPLY2 is an essential part of the formation of somite patterning during embryogenesis and in establishment of rostro-caudal polarity. Here, we describe three individuals from two families with compound-heterozygous variants in RIPPLY2 (NM_001009994.2): c.238A > T, p.(Arg80*) and c.240-4 T > G, p.(?), in two 15 and 20-year-old sisters, and a homozygous nonsense variant, c.238A > T, p.(Arg80*), in an 8 year old boy. All patients had multiple vertebral body malformations in the cervical and thoracic region, small or absent rib involvement, myelopathies, and common clinical features of SCDO6 including scoliosis, mild facial asymmetry, spinal spasticity and hemivertebrae. The nonsense variant can be classified as likely pathogenic based on the ACMG criteria while the splice variants must be classified as a variant of unknown significance. With this report on two further families, we confirm RIPPLY2 as the gene for SCDO6 and broaden the phenotype by adding myelopathy with or without spinal canal stenosis and spinal spasticity to the symptom spectrum.

Congenital posterior cervical spine malformation due to biallelic c.240-4T>G RIPPLY2 variant: A discrete entity

The clinical and radiological spectrum of spondylocostal dysostosis syndromes encompasses distinctive costo-vertebral anomalies. RIPPLY2 biallelic pathogenic variants were described in two distinct cervical spine malformation syndromes: Klippel-Feil syndrome and posterior cervical spine malformation. RIPPLY2 is involved in the determination of rostro-caudal polarity and somite patterning during development. To date, only four cases have been reported. The current report aims at further delineating the posterior malformation in three new patients. Three patients from two unrelated families underwent clinical and radiological examination through X-ray, 3D computed tomography and brain magnetic resonance imaging. After informed consent was obtained, family-based whole exome sequencing (WES) was performed. Complex vertebral segmentation defects in the cervico-thoracic spine were observed in all patients. WES led to the identification of the homozygous splicing variant c.240-4T>G in all subjects. This variant is predicted to result in aberrant splicing of Exon 4. The current report highlights a subtype of cervical spine malformation with major atlo-axoidal malformation compromising spinal cord integrity. This distinctive mutation-specific pattern of malformation differs from Klippel-Feil syndrome and broadens the current classification, defining a sub-type of RIPPLY2-related skeletal disorder. Of note, the phenotype of one patient overlaps with oculo-auriculo-vertebral spectrum disorder.

Transcriptional autoregulation of zebrafish tbx6 is required for somite segmentation

The presumptive somite boundary in the presomitic mesoderm (PSM) is defined by the anterior border of the expression domain of Tbx6 protein. During somite segmentation, the expression domain of Tbx6 is regressed by Ripply-meditated degradation of Tbx6 protein. Although the expression of zebrafish tbx6 remains restricted to the PSM, the transcriptional regulation of tbx6 remains poorly understood. Here, we show that the expression of zebrafish tbx6 is maintained by transcriptional autoregulation. We find that a proximal-located cis-regulatory module, TR1, which contains two putative T-box sites, is required for somite segmentation in the intermediate body and for proper expression of segmentation genes. Embryos with deletion of TR1 exhibit significant reduction of tbx6 expression at the 12-somite stage, although its expression is initially observed. Additionally, Tbx6 is associated with TR1 and activates its own expression in the anterior PSM. Furthermore, the anterior expansion of tbx6 expression in ripply gene mutants is suppressed in a TR1-dependent manner. The results suggest that the autoregulatory loop of zebrafish tbx6 facilitates immediate removal of Tbx6 protein through termination of its own transcription at the anterior PSM.

Phenotypic heterogeneity of kyphoscoliosis with vertebral and rib defects: a case series

Disorders associated with multiple vertebral segmentation defects may have additional rib anomalies in form of absence or hypoplastic ribs, fanning of ribs, etc. Spondylocostal dysostosis is genetic disorder with abnormal vertebral segmentation and rib anomalies. Diagnosis is often delayed because of non-familiarity with the characteristic features. There are six genes identified for spondylocostal dysostosis, of which SCDO5 is responsible for autosomal dominant form of the disorder. Retrospective study was conducted in Genetic and Metabolic unit of a tertiary hospital in north India over a period of 9 years. Twenty patients with a clinical diagnosis of congenital scoliosis were identified, and reviewed. Three patients were discussed in an earlier report and 11 subsequent patients, are described in this case series here. The median age at presentation was 34 months. The patients showed hemivertebrae, vertebral fusion, fusion of ribs, fanning of ribs. Hydrocephalus/ventriculomegaly was found in three cases and diastematomyelia was identified in one case. Other associated anomalies included corpus callosal agenesis, club foot and capillary malformation. One parent showed rib/spinal defects in two cases. Further studies are needed to characterise the phenotype and genetic basis of scoliosis in Indian patients.

Functional roles of the Ripply-mediated suppression of segmentation gene expression at the anterior presomitic mesoderm in zebrafish

Somites sequentially form with a regular interval by the segmentation from the anterior region of the presomitic mesoderm (PSM). The expression of several genes involved in the somite segmentation is switched off at the transition from the anterior PSM to somites. Zebrafish Ripply1, which down-regulates a T-box transcription factor Tbx6, is required for the suppression of segmentation gene expression. However, the functional roles of the Ripply-mediated suppression of segmentation gene expression at the anterior PSM remain elusive. In this study, we generated ripply1 mutants and examined genetic interaction between ripply1/2 and tbx6. Zebrafish ripply1^-/- embryos failed to form the somite boundaries as was observed in knockdown embryos. We found that somite segmentation defects in ripply1 mutants were suppressed by heterozygous mutation of tbx6 or partial translational inhibition of tbx6 by antisense morpholino. We further showed that somite boundaries that were recovered in tbx6^+/-; ripply1^-/- embryos were dependent on the function of ripply2, indicating that relative gene dosage between ripply1/2 and tbx6 plays a critical role in the somite formation. Interestingly, the expression of segmentation genes such mesp as was still not fully suppressed at the anterior PSM of tbx6^+/-; ripply1^-/- embryos although the somite formation and rostral-caudal polarity of somites were properly established. Furthermore, impaired myogenesis was observed in the segmented somites in tbx6^+/-; ripply1^-/- embryos. These results revealed that partial suppression of the segmentation gene expression by Ripply is sufficient to establish the rostral-caudal polarity of somites, and that stronger suppression of the segmentation gene expression by Ripply is required for proper myogenesis in zebrafish embryos.

Inverse agonism of retinoic acid receptors directs epiblast cells into the paraxial mesoderm lineage

We have investigated the differentiation of paraxial mesoderm from mouse embryonic stem cells utilizing a Tbx6-EYFP/Brachyury (T)-Cherry dual reporter system. Differentiation from the mouse ESC state directly into mesoderm via Wnt pathway activation was low, but augmented by treatment with AGN193109, a pan-retinoic acid receptor inverse agonist. After five days of differentiation, T⁺ cells increased from 12.2% to 18.8%, Tbx6⁺ cells increased from 5.8% to 12.7%, and T⁺/Tbx6⁺ cells increased from 2.4% to 14.1%. The synergism of AGN193109 with Wnt3a/CHIR99021 was further substantiated by the increased expression of paraxial mesoderm gene markers Tbx6, Msgn1, Meox1, and Hoxb1. Separate to inverse agonist treatment, when mouse ESCs were indirectly differentiated into mesoderm via a transient epiblast step the efficiency of paraxial mesoderm formation markedly increased. Tbx6⁺ cells represented 65-75% of the total cell population after just 3 days of differentiation and the expression of paraxial mesoderm marker genes Tbx6 and Msgn increased over 100-fold and 300-fold, respectively. Further evaluation of AGN193109 treatment on the indirect differentiation protocol suggested that RARs have two distinct roles. First, AGN193109 treatment at the epiblast step and mesoderm step promoted paraxial mesoderm formation over other mesoderm and endoderm lineage types. Second, continued treatment during mesoderm formation revealed its ability to repress the maturation of presomitic mesoderm into somitic paraxial mesoderm. Thus, the continuous treatment of AGN193109 during epiblast and mesoderm differentiation steps yielded a culture where ~90% of the cells were Tbx6⁺. The surprisingly early effect of inverse agonist treatment at the epiblast step of differentiation led us to further examine the effect of AGN193109 treatment during an extended epiblast differentiation protocol. Interestingly, while inverse agonist treatment had no impact on the conversion of ESCs into epiblast cells based on the expression of Rex1, Fgf5, and pluripotency marker genes Oct4, Nanog, and Sox2, after three days of differentiation in the presence of AGN193109 caudal epiblast and early paraxial mesoderm marker genes, T, Cyp26a1, Fgf8, Tbx6 and Msgn were all highly up-regulated. Collectively, our studies reveal an earlier than appreciated role for RARs in epiblast cells and the modulation of their function via inverse agonist treatment can promote their differentiation into the paraxial mesoderm lineage.

Ripply2 recruits proteasome complex for Tbx6 degradation to define segment border during murine somitogenesis

The metameric structure in vertebrates is based on the periodic formation of somites from the anterior end of the presomitic mesoderm (PSM). The segmentation boundary is defined by the Tbx6 expression domain, whose anterior limit is determined by Tbx6 protein destabilization via Ripply2. However, the molecular mechanism of this process is poorly understood. Here, we show that Ripply2 directly binds to Tbx6 in cultured cells without changing the stability of Tbx6, indicating an unknown mechanism for Tbx6 degradation in vivo. We succeeded in reproducing in vivo events using a mouse ES induction system, in which Tbx6 degradation occurred via Ripply2. Mass spectrometry analysis of the PSM-fated ES cells revealed that proteasomes are major components of the Ripply2-binding complex, suggesting that recruitment of a protein-degradation-complex is a pivotal function of Ripply2. Finally, we identified a motif in the T-box, which is required for Tbx6 degradation independent of binding with Ripply2 in vivo.

Travelling waves in somitogenesis: Collective cellular properties emerge from time-delayed juxtacrine oscillation coupling

The sculpturing of the vertebrate body plan into segments begins with the sequential formation of somites in the presomitic mesoderm (PSM). The rhythmicity of this process is controlled by travelling waves of gene expression. These kinetic waves emerge from coupled cellular oscillators and sweep across the PSM. In zebrafish, the oscillations are driven by autorepression of her genes and are synchronized via Notch signalling. Mathematical modelling has played an important role in explaining how collective properties emerge from the molecular interactions. Increasingly more quantitative experimental data permits the validation of those mathematical models, yet leads to increasingly more complex model formulations that hamper an intuitive understanding of the underlying mechanisms. Here, we review previous efforts, and design a mechanistic model of the her1 oscillator, which represents the experimentally viable her7;hes6 double mutant. This genetically simplified system is ideally suited to conceptually recapitulate oscillatory entrainment and travelling wave formation, and to highlight open questions. It shows that three key parameters, the autorepression delay, the juxtacrine coupling delay, and the coupling strength, are sufficient to understand the emergence of the collective period, the collective amplitude, and the synchronization of neighbouring Her1 oscillators. Moreover, two spatiotemporal time delay gradients, in the autorepression and in the juxtacrine signalling, are required to explain the collective oscillatory dynamics and synchrony of PSM cells. The highlighted developmental principles likely apply more generally to other developmental processes, including neurogenesis and angiogenesis.

microRNA-206 modulates an Rtn4a/Cxcr4a/Thbs3a axis in newly forming somites to maintain and stabilize the somite boundary formation of zebrafish embryos

Although microRNA-206 (miR-206) is known to regulate proliferation and differentiation of muscle fibroblasts, the role of miR-206 in early-stage somite development is still unknown. During somitogenesis of zebrafish embryos, reticulon4a (rtn4a) is specifically repressed by miR-206. The somite boundary was defective, and actin filaments were crossing over the boundary in either miR-206-knockdown or rtn4a-overexpressed embryos. In these treated embryos, C-X-C motif chemokine receptor 4a (cxcr4a) was reduced, while thrombospondin 3a (thbs3a) was increased. The defective boundary was phenocopied in either cxcr4a-knockdown or thbs3a-overexpressed embryos. Repression of thbs3a expression by cxcr4a reduced the occurrence of the boundary defect. We demonstrated that cxcr4a is an upstream regulator of thbs3a and that defective boundary cells could not process epithelialization in the absence of intracellular accumulation of the phosphorylated focal adhesion kinase (p-FAK) in boundary cells. Therefore, in the newly forming somites, miR-206-mediated downregulation of rtn4a increases cxcr4a. This activity largely decreases thbs3a expression in the epithelial cells of the somite boundary, which causes epithelialization of boundary cells through mesenchymal-epithelial transition (MET) and eventually leads to somite boundary formation. Collectively, we suggest that miR-206 mediates a novel pathway, the Rtn4a/Cxcr4a/Thbs3a axis, that allows boundary cells to undergo MET and form somite boundaries in the newly forming somites of zebrafish embryos.

Segmentation of the zebrafish axial skeleton relies on notochord sheath cells and not on the segmentation clock

Segmentation of the axial skeleton in amniotes depends on the segmentation clock, which patterns the paraxial mesoderm and the sclerotome. While the segmentation clock clearly operates in teleosts, the role of the sclerotome in establishing the axial skeleton is unclear. We severely disrupt zebrafish paraxial segmentation, yet observe a largely normal segmentation process of the chordacentra. We demonstrate that axial entpd5+ notochord sheath cells are responsible for chordacentrum mineralization, and serve as a marker for axial segmentation. While autonomous within the notochord sheath, entpd5 expression and centrum formation show some plasticity and can respond to myotome pattern. These observations reveal for the first time the dynamics of notochord segmentation in a teleost, and are consistent with an autonomous patterning mechanism that is influenced, but not determined by adjacent paraxial mesoderm. This behavior is not consistent with a clock-type mechanism in the notochord.

A Reduced Transcriptome Approach to Assess Environmental Toxicants Using Zebrafish Embryo Test

Omics approaches can monitor responses and alterations of biological pathways at genome-scale, which are useful to predict potential adverse effects by environmental toxicants. However, high throughput application of transcriptomics in chemical assessment is limited due to the high cost and lack of "standardized" toxicogenomic methods. Here, a reduced zebrafish transcriptome (RZT) approach was developed to represent the whole transcriptome and to profile bioactivity of chemical and environmental mixtures in zebrafish embryo. RZT gene set of 1637 zebrafish Entrez genes was designed to cover a wide range of biological processes, and to faithfully capture gene-level and pathway-level changes by toxicants compared with the whole transcriptome. Concentration-response modeling was used to calculate the effect concentrations (ECs) of DEGs and corresponding molecular pathways. To validate the RZT approach, quantitative analysis of gene expression by RNA-ampliseq technology was used to identify differentially expressed genes (DEGs) at 32 hpf following exposure to seven serial dilutions of reference chemical BPA (10-10E^-5μM) or each of four water samples ranging from wastewater to drinking water (relative enrichment factors 10-6.4 × 10^-4). The RZT-ampliseq-embryo approach was both sensitive and able to identify a wide spectrum of biological activities associated with BPA exposure. Water quality was benchmarked based on the sensitivity distribution curve of biological pathways detected using RZT-ampliseq-embryo. Finally, the most sensitive biological pathways were identified, including those linked with adverse reproductive outcomes, genotoxicity and development outcomes. RZT-ampliseq-embryo approach provides an efficient and cost-effective tool to prioritize toxicants based on responsiveness of biological pathways.

Modelling asymmetric somitogenesis: Deciphering the mechanisms behind species differences

Somitogenesis is one of the major hallmarks of bilateral symmetry in vertebrates. This symmetry is lost when retinoic acid (RA) signalling is inhibited, allowing the left-right determination pathway to influence somitogenesis. In all three studied vertebrate model species, zebrafish, chicken and mouse, the frequency of somite formation becomes asymmetric, with slower gene expression oscillations driving somitogenesis on the right side. Still, intriguingly, the resulting left-right asymmetric phenotypes differ significantly between these model species. While somitogenesis is generally considered as functionally equivalent among different vertebrates, substantial differences exist in the subset of oscillating genes between different vertebrate species. Variation also appears to exist in the way oscillations cease and somite boundaries become patterned. In addition, in absence of RA, the FGF8 gradient thought to constitute the determination wavefront becomes asymmetric in zebrafish and mouse, extending more anteriorly to the right, while remaining symmetric in chicken. Here we use a computational modelling approach to decipher the causes underlying species differences in asymmetric somitogenesis. Specifically, we investigate to what extent differences can be explained from observed differences in FGF asymmetry and whether differences in somite determination dynamics may also be involved. We demonstrate that a simple clock-and-wavefront model incorporating the observed left-right differences in somitogenesis frequency readily reproduces asymmetric somitogenesis in chicken. However, incorporating asymmetry in FGF signalling was insufficient to robustly reproduce mouse or zebrafish asymmetry phenotypes. In order to explain these phenoptypes we needed to extend the basic model, incorporating species-specific details of the somitogenesis determination mechanism. Our results thus demonstrate that a combination of differences in FGF dynamics and somite determination cause species differences in asymmetric somitogenesis. In addition,they highlight the power of using computational models as well as studying left-right asymmetry to obtain more insight in somitogenesis.

Quadruple zebrafish mutant reveals different roles of Mesp genes in somite segmentation between mouse and zebrafish

The segmental pattern of somites is generated by sequential conversion of the temporal periodicity provided by the molecular clock. Whereas the basic structure of this clock is conserved among different species, diversity also exists, especially in terms of the molecular network. The temporal periodicity is subsequently converted into the spatial pattern of somites, and Mesp2 plays crucial roles in this conversion in the mouse. However, it remains unclear whether Mesp genes play similar roles in other vertebrates. In this study, we generated zebrafish mutants lacking all four zebrafish Mesp genes by using TALEN-mediated genome editing. Contrary to the situation in the mouse Mesp2 mutant, in the zebrafish Mesp quadruple mutant embryos the positions of somite boundaries were clearly determined and morphological boundaries were formed, although their formation was not completely normal. However, each somite was caudalized in a similar manner to the mouse Mesp2 mutant, and the superficial horizontal myoseptum and lateral line primordia were not properly formed in the quadruple mutants. These results clarify the conserved and species-specific roles of Mesp in the link between the molecular clock and somite morphogenesis.

Molecular mechanism for cyclic generation of somites: Lessons from mice and zebrafish

The somite is the most prominent metameric structure observed during vertebrate embryogenesis, and its metamerism preserves the characteristic structures of the vertebrae and muscles in the adult body. During vertebrate somitogenesis, sequential formation of epithelialized cell boundaries generates the somites. According to the "clock and wavefront model," the periodical and sequential generation of somites is achieved by the integration of spatiotemporal information provided by the segmentation clock and wavefront. In the anterior region of the presomitic mesoderm, which is the somite precursor, the orchestration between the segmentation clock and the wavefront achieves morphogenesis of somites through multiple processes such as determination of somite boundary position, generation of morophological boundary, and establishment of the rostrocaudal polarity within a somite. Recently, numerous studies using various model animals including mouse, zebrafish, and chick have gradually revealed the molecular aspect of the "clock and wavefront" model and the molecular mechanism connecting the segmentation clock and the wavefront to the multiple processes of somite morphogenesis. In this review, we first summarize the current knowledge about the molecular mechanisms underlying the clock and the wavefront and then describe those of the three processes of somite morphogenesis. Especially, we will discuss the conservation and diversification in the molecular network of the somitigenesis among vertebrates, focusing on two typical model animals used for genetic analyses, i.e., the mouse and zebrafish. In this review, we described molecular mechanism for the generation of somites based on the spatiotemporal information provided by "segmentation clock" and "wavefront" focusing on the evidences obtained from mouse and zebrafish.

Segmental border is defined by Ripply2-mediated Tbx6 repression independent of Mesp2

The precise border of somites formed during mouse somitogenesis is defined by a Tbx6 expression domain, which is established by Mesp2-mediated Tbx6 suppression in the anterior part of the presomitic mesoderm (PSM). Ripply2, a target of Mesp2, is proposed to be involved in this down-regulation because Ripply2 deficiency causes an anterior expansion of the Tbx6 domain, resembling the Mesp2-null phenotype. However, it is unclear whether Ripply2 acts on Tbx6 independently or in association with Mesp2. To address this question, we generated three sets of transgenic mice with the following Ripply2 expression patterns: (1) overexpression in the endogenous expression domain, (2) expression instead of Mesp2 (Ripply2-knockin), and (3) ectopic expression in the entire PSM. We found accelerated Tbx6 degradation in the embryos showing Ripply2 overexpression. In the Ripply2-knockin embryos, the anterior limit of Tbx6 domain was generated by Ripply2 even in the absence of Mesp2. Ectopic Ripply2 expression along the entire PSM suppressed Tbx6 and induced Sox2-positive neural tube formation at the bilateral domain, resembling the Tbx6-null phenotype. This phenotype resulted from Tbx6 protein and not mRNA elimination, suggesting the post-translational down-regulation of Tbx6 by Ripply2. Taken together, our results demonstrate that Ripply2 represses Tbx6 in a Mesp2-independent manner, which contributes to the accurate segmental border formation.