Molecular clocks underlying vertebrate embryo segmentation: A 10-year-old hairy-go-round

Axial and limb muscle development: dialogue with the neighbourhood

Skeletal muscles are part of the musculoskeletal system which also includes nerves, tendons, connective tissue, bones and blood vessels. Here we review the development of axial and limb muscles in amniotes within the context of their surrounding tissues in vivo. We highlight the reciprocal dialogue mediated by signalling factors between cells of these adjacent tissues and developing muscles and also demonstrate its importance from the onset of muscle cell differentiation well into foetal development. Early embryonic tissues secrete factors which are important regulators of myogenesis. However, later muscle development relies on other tissue collaborators, such as developing nerves and connective tissue, which are in turn influenced by the developing muscles themselves. We conclude that skeletal muscle development in vivo is a compelling example of the importance of reciprocal interactions between developing tissues for the complete and coordinated development of a functional system.

Is the Epididymis a Series of Organs Placed Side By Side?

The mammalian epididymis is more than a highly convoluted tube divided into four regions: initial segment, caput, corpus and cauda. It is a highly segmented structure with each segment expressing its own and overlapping genes, proteins, and signal transduction pathways. Therefore, the epididymis may be viewed as a series of organs placed side by side. In this review we discuss the contributions of septa that divide the epididymis into segments and present hypotheses as to the mechanism by which septa form. The mechanisms of Wolffian duct segmentation are likened to the mechanisms of segmentation of the renal nephron and somites. The renal nephron may provide valuable clues as to how the Wolffian duct is patterned during development, whereas somitogenesis may provide clues as to the timing of the development of each segment. Emphasis is also placed upon how segments are differentially regulated, in support of the idea that the epididymis can be considered a series of multiple organs placed side by side. One region in particular, the initial segment, which consists of 2 or 4 segments in mice and rats, respectively, is unique with respect to its regulation and vascularity compared to other segments; loss of development of these segments leads to male infertility. Different ways of thinking about how the epididymis functions may provide new directions and ideas as to how sperm maturation takes place.

Patterning in time and space: HoxB cluster gene expression in the developing chick embryo

The developing embryo is a paradigmatic model to study molecular mechanisms of time control in Biology. Hox genes are key players in the specification of tissue identity during embryo development and their expression is under strict temporal regulation. However, the molecular mechanisms underlying timely Hox activation in the early embryo remain unknown. This is hindered by the lack of a rigorous temporal framework of sequential Hox expression within a single cluster. Herein, a thorough characterization of HoxB cluster gene expression was performed over time and space in the early chick embryo. Clear temporal collinearity of HoxB cluster gene expression activation was observed. Spatial collinearity of HoxB expression was evidenced in different stages of development and in multiple tissues. Using embryo explant cultures we showed that HoxB2 is cyclically expressed in the rostral presomitic mesoderm with the same periodicity as somite formation, suggesting a link between timely tissue specification and somite formation. We foresee that the molecular framework herein provided will facilitate experimental approaches aimed at identifying the regulatory mechanisms underlying Hox expression in Time and Space.

Timing embryo segmentation: dynamics and regulatory mechanisms of the vertebrate segmentation clock

All vertebrate species present a segmented body, easily observed in the vertebrate column and its associated components, which provides a high degree of motility to the adult body and efficient protection of the internal organs. The sequential formation of the segmented precursors of the vertebral column during embryonic development, the somites, is governed by an oscillating genetic network, the somitogenesis molecular clock. Herein, we provide an overview of the molecular clock operating during somite formation and its underlying molecular regulatory mechanisms. Human congenital vertebral malformations have been associated with perturbations in these oscillatory mechanisms. Thus, a better comprehension of the molecular mechanisms regulating somite formation is required in order to fully understand the origin of human skeletal malformations.

Joint interpretation of AER/FGF and ZPA/SHH over time and space underlies hairy2 expression in the chick limb

Embryo development requires precise orchestration of cell proliferation and differentiation in both time and space. A molecular clock operating through gene expression oscillations was first described in the presomitic mesoderm (PSM) underlying periodic somite formation. Cycles of HES gene expression have been further identified in other progenitor cells, including the chick distal limb mesenchyme, embryonic neural progenitors and both mesenchymal and embryonic stem cells. In the limb, hairy2 is expressed in the distal mesenchyme, adjacent to the FGF source (AER) and along the ZPA-derived SHH gradient, the two major regulators of limb development. Here we report that hairy2 expression depends on joint AER/FGF and ZPA/SHH signaling. FGF plays an instructive role on hairy2, mediated by Erk and Akt pathway activation, while SHH acts by creating a permissive state defined by Gli3-A/Gli3-R>1. Moreover, we show that AER/FGF and ZPA/SHH present distinct temporal and spatial signaling properties in the distal limb mesenchyme: SHH acts at a long-term, long-range on hairy2, while FGF has a short-term, short-range action. Our work establishes limb hairy2 expression as an output of integrated FGF and SHH signaling in time and space, providing novel clues for understanding the regulatory mechanisms underlying HES oscillations in multiple systems, including embryonic stem cell pluripotency.

Tissue regulation of somitic colloid-like1 gene expression

Body skeletal muscles formation starts with somite differentiation, due to signals from surrounding tissues. Somite ventral portion forms the sclerotome while its dorsal fraction constitutes the dermamyotome, and later the dermatome and myotome. Relative levels of BMP activity have been proposed to control several aspects of somite development, namely the time and location of myogenesis within the somite. The fine-tuning of BMP activity is primarily achieved via negative regulation by diffusible BMP inhibitors, such as Noggin and Chordin, and on a secondary level by proteins cleaving these inhibitors, such as BMP1/Tolloid metalloprotease family members. Herein, we carefully described the somitic expression of colloid-like1, one of the chick BMP1/Tolloid homologues, and found that this gene is specifically expressed in the 10 most anterior somites, suggesting that it may be involved in neck muscle formation. By using in ovo microsurgery and tridimensional embryo tissue culture techniques we assessed the function of surrounding structures, neural tube, notochord, surface ectoderm and lateral plate mesoderm, on the maintenance of somitic colloid-like1 gene expression. We unveil that a signal coming from the neural tube is responsible for this expression and rule out the main candidate pathway, Wnt. By comparing the somitic colloid-like1 gene expression with that of related signaling partners, such as BMP4, Noggin and Chordin, we propose that colloid-like1 plays a role in the reinforcement of BMP4 activity in the medial portion of the 10 most anterior dermomyotomes, thus belonging to the molecular machinery controlling neck muscle development in the chick.

Somite unit chronometry to analyze teratogen phase specificity in the paraxial mesoderm

Phase specificity, the temporal and tissue restriction of teratogen-induced defects during embryonic -development, is a poorly understood but common property of teratogens, an important source of human birth defects. Somite counting and somite units are novel chronometric tools used here to identify stages of paraxial mesoderm development that are sensitive to pulse-chase exposure (2 to >16 h) to 5-bromodeoxyuridine (BrdU). In all cases, it was the presomitic mesoderm (PSM) that was sensitive to BrdU induced segmentation anomalies. At high concentration (1.0 × 10(-2) M BrdU), PSM presegment stages ss-IV and earlier were irreversibly inhibited from completing segmentation. At low concentration (2.6 × 10(-6) M), BrdU induced periodic focal defects that predominantly trace back to PSM presegments between ss-V and ss-IX. Phase specificity is characteristic of both types of segmentation anomalies. Focal segmentation defects are phase-specific because they result from disruption of 2-3 presegments in the PSM while adjacent -rostral and caudal presegments are (apparently) unaffected. Irreversible inhibition of segmentation is also phase-specific because only PSM presegments ss-IV or earlier were affected while older segments (ss-III to ss-I) were able to complete segmentation. The presegments predominantly affected have not yet passed the determination front, the point at which the segmentation clock establishes somite rostro-caudal -polarity. Somite unit chronometry provides a means to identify specific PSM presegment stages that are susceptible to induced segmentation defects and the biological processes that underlie that vulnerability.

Left-right function of dmrt2 genes is not conserved between zebrafish and mouse

BACKGROUND

Members of the Dmrt family, generally associated with sex determination, were shown to be involved in several other functions during embryonic development. Dmrt2 has been studied in the context of zebrafish development where, due to a duplication event, two paralog genes dmrt2a and dmrt2b are present. Both zebrafish dmrt2a/terra and dmrt2b are important to regulate left-right patterning in the lateral plate mesoderm. In addition, dmrt2a/terra is necessary for symmetric somite formation while dmrt2b regulates somite differentiation impacting on slow muscle development. One dmrt2 gene is also expressed in the mouse embryo, where it is necessary for somite differentiation but with an impact on axial skeleton development. However, nothing was known about its role during left-right patterning in the lateral plate mesoderm or in the symmetric synchronization of somite formation.

METHODOLOGY/PRINCIPAL FINDINGS

Using a dmrt2 mutant mouse line, we show that this gene is not involved in symmetric somite formation and does not regulate the laterality pathway that controls left-right asymmetric organ positioning. We reveal that dmrt2a/terra is present in the zebrafish laterality organ, the Kupffer's vesicle, while its homologue is excluded from the mouse equivalent structure, the node. On the basis of evolutionary sub-functionalization and neo-functionalization theories we discuss this absence of functional conservation.

CONCLUSIONS/SIGNIFICANCE

Our results show that the role of dmrt2 gene is not conserved during zebrafish and mouse embryonic development.

Sonic hedgehog in temporal control of somite formation

Vertebrate embryo somite formation is temporally controlled by the cyclic expression of somitogenesis clock genes in the presomitic mesoderm (PSM). The somitogenesis clock is believed to be an intrinsic property of this tissue, operating independently of embryonic midline structures and the signaling molecules produced therein, namely Sonic hedgehog (Shh). This work revisits the notochord signaling contribution to temporal control of PSM segmentation by assessing the rate and number of somites formed and somitogenesis molecular clock gene expression oscillations upon notochord ablation. The absence of the notochord causes a delay in somite formation, accompanied by an increase in the period of molecular clock oscillations. Shh is the notochord-derived signal responsible for this effect, as these alterations are recapitulated by Shh signaling inhibitors and rescued by an external Shh supply. We have characterized chick smoothened expression pattern and have found that the PSM expresses both patched1 and smoothened Shh signal transducers. Upon notochord ablation, patched1, gli1, and fgf8 are down-regulated, whereas gli2 and gli3 are overexpressed. Strikingly, notochord-deprived PSM segmentation rate recovers over time, concomitant with raldh2 overexpression. Accordingly, exogenous RA supplement rescues notochord ablation effects on somite formation. A model is presented in which Shh and RA pathways converge to inhibit PSM Gli activity, ensuring timely somite formation. Altogether, our data provide evidence that a balance between different pathways ensures the robustness of timely somite formation and that notochord-derived Shh is a component of the molecular network regulating the pace of the somitogenesis clock.

Dynamic 3D cell rearrangements guided by a fibronectin matrix underlie somitogenesis

Somites are transient segments formed in a rostro-caudal progression during vertebrate development. In chick embryos, segmentation of a new pair of somites occurs every 90 minutes and involves a mesenchyme-to-epithelium transition of cells from the presomitic mesoderm. Little is known about the cellular rearrangements involved, and, although it is known that the fibronectin extracellular matrix is required, its actual role remains elusive. Using 3D and 4D imaging of somite formation we discovered that somitogenesis consists of a complex choreography of individual cell movements. Epithelialization starts medially with the formation of a transient epithelium of cuboidal cells, followed by cell elongation and reorganization into a pseudostratified epithelium of spindle-shaped epitheloid cells. Mesenchymal cells are then recruited to this medial epithelium through accretion, a phenomenon that spreads to all sides, except the lateral side of the forming somite, which epithelializes by cell elongation and intercalation. Surprisingly, an important contribution to the somite epithelium also comes from the continuous egression of mesenchymal cells from the core into the epithelium via its apical side. Inhibition of fibronectin matrix assembly first slows down the rate, and then halts somite formation, without affecting pseudopodial activity or cell body movements. Rather, cell elongation, centripetal alignment, N-cadherin polarization and egression are impaired, showing that the fibronectin matrix plays a role in polarizing and guiding the exploratory behavior of somitic cells. To our knowledge, this is the first 4D in vivo recording of a full mesenchyme-to-epithelium transition. This approach brought new insights into this event and highlighted the importance of the extracellular matrix as a guiding cue during morphogenesis.

Role of unusual O-glycans in intercellular signaling

In the last two decades, our knowledge of the role of glycans in development and signal transduction has expanded enormously. While most work has focused on the importance of N-linked or mucin-type O-linked glycosylation, recent work has highlighted the importance of several more unusual forms of glycosylation that are the focus of this review. In particular, the ability of O-fucose glycans on the epidermal growth factor-like (EGF) repeats of Notch to modulate signaling places glycosylation alongside phosphorylation as a means to modulate protein-protein interactions and their resultant downstream signals. The recent discovery that O-glucose modification of Notch EGF repeats is also required for Notch function has further expanded the range of glycosylation events capable of modulating Notch signaling. The prominent role of Notch during development and in later cell-fate decisions underscores the importance of these modifications in human biology. The role of glycans in intercellular signaling events is only beginning to be understood and appears ready to expand into new areas with the discovery that thrombospondin type 1 repeats are also modified with O-fucose glycans. Finally, a rare form of glycosylation called C-mannosylation modifies tryptophans in some signaling competent molecules and may be a further layer of complexity in the field. We will review each of these areas focusing on the glycan structures produced, the consequence of their presence, and the enzymes responsible.