Mutations affecting somite formation in the Medaka (Oryzias latipes)

Species-specific roles of the Notch ligands, receptors, and targets orchestrating the signaling landscape of the segmentation clock

Somitogenesis is a hallmark feature of all vertebrates and some invertebrate species that involves the periodic formation of block-like structures called somites. Somites are transient embryonic segments that eventually establish the entire vertebral column. A highly conserved molecular oscillator called the segmentation clock underlies this periodic event and the pace of this clock regulates the pace of somite formation. Although conserved signaling pathways govern the clock in most vertebrates, the mechanisms underlying the species-specific divergence in various clock characteristics remain elusive. For example, the segmentation clock in classical model species such as zebrafish, chick, and mouse embryos tick with a periodicity of ∼30, ∼90, and ∼120 min respectively. This enables them to form the species-specific number of vertebrae during their overall timespan of somitogenesis. Here, we perform a systematic review of the species-specific features of the segmentation clock with a keen focus on mouse embryos. We perform this review using three different perspectives: Notch-responsive clock genes, ligand-receptor dynamics, and synchronization between neighboring oscillators. We further review reports that use non-classical model organisms and in vitro model systems that complement our current understanding of the segmentation clock. Our review highlights the importance of comparative developmental biology to further our understanding of this essential developmental process.

Deformity or variation? Phenotypic diversity in the zebrafish vertebral column

Vertebral bodies are composed of two types of metameric elements, centra and arches, each of which is considered as a developmental module. Most parts of the teleost vertebral column have a one-to-one relationship between centra and arches, although, in all teleosts, this one-to-one relationship is lost in the caudal fin endoskeleton. Deviation from the one-to-one relationship occurs in most vertebrates, related to changes in the number of vertebral centra or to a change in the number of arches. In zebrafish, deviations also occur predominantly in the caudal region of the vertebral column. In-depth phenotypic analysis of wild-type zebrafish was performed using whole-mount stained samples, histological analyses and synchrotron radiation X-ray tomographic microscopy 3D reconstructions. Three deviant centra phenotypes were observed: (i) fusion of two vertebral centra, (ii) wedge-shaped hemivertebrae and (iii) centra with reduced length. Neural and haemal arches and their spines displayed bilateral and unilateral variations that resemble vertebral column phenotypes of stem-ward actinopterygians or other gnathostomes as well as pathological conditions in extant species. Whether it is possible to distinguish variations from pathological alterations and whether alterations resemble ancestral conditions is discussed in the context of centra and arch variations in other vertebrate groups and basal actinopterygian species.

Metabolic regulation of the ultradian oscillator Hes1 by reactive oxygen species

Ultradian oscillators are cyclically expressed genes with a period of less than 24h, found in the major signalling pathways. The Notch effector hairy and enhancer of split Hes genes are ultradian oscillators. The physiological signals that synchronise and entrain Hes oscillators remain poorly understood. We investigated whether cellular metabolism modulates Hes1 cyclic expression. We demonstrated that, in mouse myoblasts (C2C12), Hes1 oscillation depends on reactive oxygen species (ROS), which are generated by the mitochondria electron transport chain and by NADPH oxidases NOXs. In vitro, the regulation of Hes1 by ROS occurs via the calcium-mediated signalling. The modulation of Hes1 by ROS was relevant in vivo, since perturbing ROS homeostasis was sufficient to alter Medaka (Oryzias latipes) somitogenesis, a process that is dependent on Hes1 ultradian oscillation during embryo development. Moreover, in a Medaka model for human microphthalmia with linear skin lesions syndrome, in which mitochondrial ROS homeostasis was impaired, we documented important somitogenesis defects and the deregulation of Hes homologues genes involved in somitogenesis. Notably, both molecular and developmental defects were rescued by antioxidant treatments. Our studies provide the first evidence of a coupling between cellular redox metabolism and an ultradian biological oscillator with important pathophysiological implication for somitogenesis.

Patterning embryos with oscillations: structure, function and dynamics of the vertebrate segmentation clock

The segmentation clock is an oscillating genetic network thought to govern the rhythmic and sequential subdivision of the elongating body axis of the vertebrate embryo into somites: the precursors of the segmented vertebral column. Understanding how the rhythmic signal arises, how it achieves precision and how it patterns the embryo remain challenging issues. Recent work has provided evidence of how the period of the segmentation clock is regulated and how this affects the anatomy of the embryo. The ongoing development of real-time clock reporters and mathematical models promise novel insight into the dynamic behavior of the clock.

Tgfbeta3 regulation of chondrogenesis and osteogenesis in zebrafish is mediated through formation and survival of a subpopulation of the cranial neural crest

Zebrafish tgfbeta3 is strongly expressed in a subpopulation of the migrating neural crest cells, developing pharyngeal arches and neurocranial cartilages. To study the regulatory role of tgfbeta3 in head skeletal formation, we knocked down tgfbeta3 in zebrafish and found impaired craniofacial chondrogenesis, evident by malformations in selected neurocranial and pharyngeal arch cartilages. Over-expressing tgfbeta3 in embryos resulted in smaller craniofacial cartilages without any gross malformations. These defects suggest that tgfbeta3 is required for normal chondrogenesis. To address the cellular mechanisms that lead to the observed malformations, we analyzed cranial neural crest development in morphant and tgfbeta3 over-expressing fish. We observed reduced pre-migratory and migratory cranial neural crest, the precursors of the neurocranial cartilage and pharyngeal arches, in tgfbeta3 knockdown embryos. In contrast, only the migratory neural crest was reduced in embryos over-expressing tgfbeta3. This raised the possibility that the reduced number of cranial neural crest cells is a result of increased apoptosis. Consistent with this, markedly elevated TUNEL staining in the midbrain and hindbrain, and developing pharyngeal arch region was observed in morphants, while tgfbeta3 over-expressing embryos showed marginally increased apoptosis in the developing pharyngeal arch region. We propose that both Tgfbeta3 suppression and over-expression result in reduced chondrocyte and osteocyte formation, but to different degrees and through different mechanisms. In Tgfbeta3 suppressed embryos, this is due to impaired formation and survival of a subpopulation of cranial neural crest cells through markedly increased apoptosis in regions containing the cranial neural crest cells, while in Tgfbeta3 over-expressing embryos, the milder phenotype is also due to a slightly elevated apoptosis in these regions. Therefore, proper cranial neural crest formation and survival, and ultimately craniofacial chondrogenesis and osteogenesis, are dependent on tight regulation of Tgfbeta3 protein levels in zebrafish.

A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Resorption and remodelling of skeletal tissues is required for development and growth, mechanical adaptation, repair, and mineral homeostasis of the vertebrate skeleton. Here we review for the first time the current knowledge about resorption and remodelling of the skeleton in teleost fish, the largest and most diverse group of extant vertebrates. Teleost species are increasingly used in aquaculture and as models in biomedical skeletal research. Thus, detailed knowledge is required to establish the differences and similarities between mammalian and teleost skeletal remodelling, and between distantly related species such as zebrafish (Danio rerio) and medaka (Oryzias latipes). The cellular mechanisms of differentiation and activation of osteoclasts and the functions of teleost skeletal remodelling are described. Several characteristics, related to skeletal remodelling, distinguish teleosts from mammals. These characteristics include (a) the absence of osteocytes in most species; (b) the absence of haematopoietic bone marrow tissue; (c) the abundance of small mononucleated osteoclasts performing non-lacunar (smooth) bone resorption, in addition to or instead of multinucleated osteoclasts; and (d) a phosphorus- rather than calcium-driven mineral homeostasis (mainly affecting the postcranial dermal skeleton). Furthermore, (e) skeletal resorption is often absent from particular sites, due to sparse or lacking endochondral ossification. Based on the mode of skeletal remodelling in early ontogeny of all teleosts and in later stages of development of teleosts with acellular bone we suggest a link between acellular bone and the predominance of mononucleated osteoclasts, on the one hand, and cellular bone and multinucleated osteoclasts on the other. The evolutionary origin of skeletal remodelling is discussed and whether mononucleated osteoclasts represent an ancestral type of resorbing cells. Revealing the differentiation and activation of teleost skeletal resorbing cells, in the absence of several factors that trigger mammalian osteoclast differentiation, is a current challenge. Understanding which characters of teleost bone remodelling are derived and which characters are conserved should enhance our understanding of the process in fish and may provide insights into alternative pathways of bone remodelling in mammals.

Genetic analysis of somite formation in laboratory fish models

The repeated appearance of somites is one of the most fascinating aspects of vertebrate embryogenesis. Recent studies identified complex regulatory circuits that provide the molecular basis for the "clock and wave front" model, postulated almost 30 years ago by Cooke and Zeeman. The highly coordinated process of somite formation involves several networks of molecular cascades including the Delta/Notch, Wnt, FGF and retinoid signalling pathways. Studies in mouse, Xenopus and especially chicken over the last decade have helped to understand the role and interactions of these pathways in somitogenesis. More recently, this has been supplemented by experiments in zebrafish. This animal model offers the possibility of performing large scale mutagenesis screens to identify novel factors and pathways involved in somitogenesis. Molecular cloning of zebrafish somite mutants mainly resulted in genes that belong to the Delta/Notch pathway and therefore underlined the importance of this pathway during somitogenesis. The fact that other pathways have not yet been identified by genetic screening in this species was assumed to be caused by functional redundancy of duplicated genes in zebrafish. In 2000, a large-scale mutagenesis screen has been initiated in Kyoto, Japan using the related teleost medaka (Oryzias latipes). In this screen, mutants with unique phenotypes have been identified, which have not been described in zebrafish or mouse. In this chapter, we will review the progress that has been made in understanding the molecular control of somite formation in zebrafish and will discuss recent efforts to screen for novel phenotypes using medaka somitogenesis mutants.

The genetics and embryology of zebrafish metamerism

Somites are the most obvious metameric structures in the vertebrate embryo. They are mesodermal segments that form in bilateral pairs flanking the notochord and are created sequentially in an anterior to posterior sequence concomitant with the posterior growth of the trunk and tail. Zebrafish somitogenesis is regulated by a clock that causes cells in the presomitic mesoderm (PSM) to undergo cyclical activation and repression of several notch pathway genes. Coordinated oscillation among neighboring cells manifests as stripes of gene expression that pass through the cells of the PSM in a posterior to anterior direction. As axial growth continually adds new cells to the posterior tail bud, cells of the PSM become relatively less posterior. This gradual assumption of a more anterior position occurs over developmental time and constitutes part of a maturation process that governs morphological segmentation in conjunction with the clock. Segment morphogenesis involves a mesenchymal to epithelial transition as prospective border cells at the anterior end of the mesenchymal PSM adopt a polarized, columnar morphology and surround a mesenchymal core of cells. The segmental pattern influences the development of the somite derivatives such as the myotome, and the myotome reciprocates to affect the formation of segment boundaries. While somites appear to be serially homologous, there may be variation in the segmentation mechanism along the body axis. Moreover, whereas the genetic architecture of the zebrafish, mouse, and chick segmentation clocks shares many common elements, there is evidence that the gene networks have undergone independent modification during evolution.

Zebrafish cypher is important for somite formation and heart development

Mammalian CYPHER (Oracle, KIA0613), a member of the PDZ-LIM family of proteins (Enigma/LMP-1, ENH, ZASP/Cypher, RIL, ALP, and CLP-36), has been associated with cardiac and muscular myopathies. Targeted deletion of Cypher in mice is neonatal lethal possibly caused by myopathies. To further investigate the role of cypher in development, we have cloned the zebrafish orthologue. We present here the gene, domain structure, and expression pattern of zebrafish cypher during development. Cypher was not present as a maternal mRNA and was absent during early development. Cypher mRNA was first detected at the 3-somite stage in adaxial somites, and as somites matured, cypher expression gradually enveloped the whole somite. Later, cypher expression was also found in the heart, in head and jaw musculature, and in the brain. We further identified 13 alternative spliced forms of cypher from zebrafish heart and skeletal muscle tissue, among them a very short form containing the PDZ domain but lacking the ZM (ZASP-like) motif and the LIM domains. Targeted gene knock-down experiments using cypher antisense morpholinos led to severe defects, including truncation of the embryo, deformation of somites, dilatation of the pericardium, and thinning of the ventricular wall. The phenotype could be rescued by a cypher form, which contains the PDZ domain and the ZM motif, but lacks all three LIM domains. These findings indicate that a PDZ domain protein is important for normal somite formation and in normal heart development. Treatment of zebrafish embryos with cyclopamine, which disrupts hedgehog signaling, abolished cypher expression in 9 somite and 15-somite stage embryos. Taken together, our data suggest that cypher may play a role downstream of sonic hedgehog, in a late stage of somite development, when slow muscle fibers differentiate and migrate from the adaxial cells.

Transgenic analysis of the medaka mesp-b enhancer in somitogenesis

Somitogenesis is a critical step during the formation of metameric structures in vertebrates. Recent studies in mouse, chick, zebrafish and Xenopus have revealed that several factors, such as T-box genes, Notch/Delta, Wnt, retinoic acid and FGF signaling, are involved in the specification of nascent somites. By interacting with these pathways, the Mesp2-like bHLH transcription factors are transiently expressed in the anterior presomitic mesoderm and play a crucial role in somite formation. The regulatory mechanisms of Mesp2 and its related genes during somitogenesis have been studied in mouse and Xenopus. However, the precise mechanism that regulates the transcriptional activity of Mesp2 has yet to be determined. In our current report, we identify the essential enhancer element of medaka mesp-b, an orthologue of mouse Mesp2, using transgenic techniques and embryo manipulation. Our results demonstrate that a region of approximately 2.8 kb, upstream of the mesp-b gene, is responsible for both the initiation and anterior localization of mesp-b transcription within a somite primordium. Furthermore, putative motifs for both T-box transcription factors and Notch/Delta signaling are present in this enhancer region and are essential for activity.

Vascular anatomy of the developing medaka, Oryzias latipes: a complementary fish model for cardiovascular research on vertebrates

The zebrafish has become a very useful vertebrate model for cardiovascular research, but detailed morphogenetic studies have revealed that it differs from mammals in certain aspects of the primary circulatory system, in particular, the early vitelline circulation. We searched for another teleost species that might serve as a complementary model for the formation of these early primary vessels. Here (and online at http://www.shigen.nig.ac.jp/medaka/atlas/), we present a detailed characterization of the vascular anatomy of the developing medaka embryo from the stage 24 (1 day 20 hr) through stage 30 (3 days 10 hr). Three-dimensional images using confocal microangiography show that the medaka, Oryzias latipes, follows the common embryonic circulatory pattern consisting of ventral aorta, aortic arches, dorsal aorta, transverse vessels, vitelline capillary plexus, and marginal veins. The medaka, thus, may serve as a valuable model system for genetic analysis of the primary vasculature of vertebrates.

Comparative analysis of her genes during fish somitogenesis suggests a mouse/chick-like mode of oscillation in medaka

Somitogenesis is the key developmental step, which divides the vertebrate body axis into segmentally repeated structures. It requires an intricate process of pre-patterning, which is driven by an oscillator mechanism consisting of the Delta-Notch pathway and various hairy- and Enhancer of split-related (her) genes. The subset of her genes, which are necessary to set up the segmentation clock, reveal a complex scenario of interactions. To understand which her genes are essential core players in this process, we compared the expression patterns of somitogenesis-relevant her genes in zebrafish and medaka (Oryzias latipes). Most of the respective medaka genes (Ol-her) are duplicated like what has been shown for zebrafish (Dr-her) and pufferfish genes (Fr-her). However, zebrafish genes show some additional copies and significant differences in expression patterns. For the paralogues Dr-her1 and Dr-her11, only one copy exists in the medaka (Ol-her1/11), which combines the expression patterns found for both zebrafish genes. In contrast to Dr-her5, the medaka orthologue appears to play a role in somitogenesis because it is expressed in the presomitic mesoderm (PSM). PSM expression also suggests a role for both Ol-her13 genes, homologues of mouse Hes6 (mHes6), in this process, which would be consistent with a conserved mHes6 homologue gear in the segmentation clock exclusively in lower vertebrates. Members of the mHes5 homologue group seem to be involved in somite formation in all vertebrates (e.g. Dr- and Ol-her12), although different paralogues are additionally recruited in zebrafish (e.g. Dr-her15) and medaka (e.g. Ol-her4). We found that the linkage between duplicates is strongly conserved between pufferfish and medaka and less well conserved in zebrafish. Nevertheless, linkage and orientation of several her duplicates are identical in all three species. Therefore, small-scale duplications must have happened before whole genome duplication occurred in a fish ancestor. Expression of multiple stripes in the intermediate PSM, characteristic for the zebrafish orthologues, is absent in all somitogenesis-related her genes of the medaka. In fact, the expression mode of Ol-her1/11 and Ol-her5 indicates dynamism similar to the hairy clock genes in chicken and mouse. This suggests that Danio rerio shows a rather derived clock mode when compared to other fish species and amniotes or that, alternatively, the clock mode evolved independently in zebrafish, medaka and mouse or chicken.

Cell rearrangements during development of the somite and its derivatives

The generation of somites, and the subsequent formation of their major derivatives, muscle-, cartilage-, dermis- and tendon-cell lineages, is tightly orchestrated and, to different extents, these are also mutually supporting processes. They involve complex and timely reorganizations of the paraxial mesoderm, such as multiple phases of epithelial-mesenchymal rearrangements and vice-versa, cellular movements and migrations, and modifications of both cell shape and cell cycle properties. These morphogenetic changes are triggered by local environmental signals and are tightly associated to a genetic program imparting cell-specific fates. Elucidating these signals and their downstream effectors, in addition to determining the state of specification of responsive cell subsets and that of single progenitors in the various domains, is only beginning.

A systematic genome-wide screen for mutations affecting organogenesis in Medaka, Oryzias latipes

A large-scale mutagenesis screen was performed in Medaka to identify genes acting in diverse developmental processes. Mutations were identified in homozygous F3 progeny derived from ENU-treated founder males. In addition to the morphological inspection of live embryos, other approaches were used to detect abnormalities in organogenesis and in specific cellular processes, including germ cell migration, nerve tract formation, sensory organ differentiation and DNA repair. Among 2031 embryonic lethal mutations identified, 312 causing defects in organogenesis were selected for further analyses. From these, 126 mutations were characterized genetically and assigned to 105 genes. The similarity of the development of Medaka and zebrafish facilitated the comparison of mutant phenotypes, which indicated that many mutations in Medaka cause unique phenotypes so far unrecorded in zebrafish. Even when mutations of the two fish species cause a similar phenotype such as one-eyed-pinhead or parachute, more genes were found in Medaka than in zebrafish that produced the same phenotype when mutated. These observations suggest that many Medaka mutants represent new genes and, therefore, are important complements to the collection of zebrafish mutants that have proven so valuable for exploring genomic function in development.