The bHLH class protein pMesogenin1 can specify paraxial mesoderm phenotypes

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.

Characterization and expression of a presomitic mesoderm-specific mespo gene in zebrafish

A complete zebrafish mespo cDNA encoding a protein of 131 amino acids with a bHLH domain in the C-terminal has been isolated. The bHLH domain of zebrafish Mespo is highly similar to those in the mouse, chick and Xenopus, sharing 82.4%, 80.4% and 74.5% amino acid identity, respectively. At 50% epiboly, the zebrafish mespo is first detected in the marginal zone of the blastoderm but excluding the prospective shield. Subsequently, mespo expression is intensified in the involuting mesoderm at 60% epiboly, and then restricted to the presomitic mesoderm (PSM) at 95% epiboly. At the 1-somite stage, mespo expression becomes reduced in the most rostral PSM. During segmentation, mespo expression is gradually downregulated at the most rostral segmental plate where cells are being coalesced to form somites. In spadetail mutant embryos, most of mespo-expressing cells were missing.

Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish

The function of basic helix-loop-helix (bHLH) proteins in cell differentiation was shown to be conserved from Drosophila to vertebrates, exemplified by the function of MyoD in striated muscle differentiation. In phylogeny striated muscle tissue appears first in jellyfish and the question of its evolutionary position is controversially discussed. For this reason we have studied the developmental role of myogenic bHLH genes in medusa development. Based on their dimerization ability, four genes of the bHLH family of transcription factors were isolated from the hydrozoan jellyfish Podocoryne carnea. While the proteins Id and Ash group with cognate family members from bilaterians, Net-like and JellyD1 could not be unequivocally classified. Id is expressed during the medusa budding process and in the adult medusa, Ash and Net-like are expressed in all life cycle stages from egg to adult medusa and JellyD1 is expressed in the blastula and gastrula stages, the planula larva, and in late medusa bud stages. The dimerization specificity, the expression pattern, and the conservation of two residues specific for a MyoD bHLH domain suggest that JellyD1 is related to an ancestral MyoD gene. Id, Net-like, and JellyD1 are either expressed in the entocodon or its derived tissues, the striated and smooth muscle of the bell. These findings strengthen the hypothesis that the entocodon is a mesoderm-like structure and that the common ancestor of Cnidaria and Bilateria was more complex in cell-type architecture and body organization than commonly thought.

Distinct patterns of downstream target activation are specified by the helix-loop-helix domain of proneural basic helix-loop-helix transcription factors

Both gain- and loss-of-function analyses indicate that proneural basic/helix-loop-helix (bHLH) proteins direct not only general aspects of neuronal differentiation but also specific aspects of neuronal identity within neural progenitors. In order to better understand the function of this family of transcription factors, we have used hormone-inducible fusion constructs to assay temporal patterns of downstream target regulation in response to proneural bHLH overexpression. In these studies, we have compared two distantly related Xenopus proneural bHLH genes, Xash1 and XNgnr1. Our findings indicate that both Xash1 and XNgnr1 induce expression of the general neuronal differentiation marker, N-tubulin, with a similar time course in animal cap progenitor populations. In contrast, these genes each induce distinct patterns of early downstream target expression. Both genes induce expression of the HLH-containing gene, Xcoe2, at early time points, but only XNgnr1 induces early expression of the bHLH genes, Xath3 and XNeuroD. Structure:function analyses indicate that the distinct pattern of XNgnr1-induced downstream target activation is linked to the XNgnr1 HLH domain, demonstrating a novel role for this domain in mediating the differential function of individual members of the proneural bHLH gene family.

Vertebrate somitogenesis

In vertebrates, the paraxial mesoderm corresponds to the bilateral strips of mesodermal tissue flanking the notochord and neural tube and which are delimited laterally by the intermediate mesoderm and the lateral plate. The paraxial mesoderm comprises the head or cephalic mesoderm anteriorly and the somitic region throughout the trunk and the tail of the vertebrates. Soon after gastrulation, the somitic region of vertebrates starts to become segmented into paired blocks of mesoderm, termed somites. This process lasts until the number of somites characteristic of the species is reached. The somites later give rise to all skeletal muscles of the body, the axial skeleton, and part of the dermis. In this review I discuss the processes involved in the formation of the paraxial mesoderm and its segmentation into somites in vertebrates.

Dynamic expression of chicken cMeso2 in segmental plate and somites

Abstract Somitogenesis in vertebrates involves prepatterning of paraxial mesoderm into somitomeres, establishing of anteroposterior polarity within somite primordia, and boundary formation between individual somites. cMeso2 is a newly identified chicken gene encoding a bHLH transcription factor, which is expressed in a transient stripe pattern in anterior presomitic mesoderm before segmentation of somites. The expression pattern overlaps with that of cMeso1 and correlates in time with the formation cycle of somites, suggesting that it may have a role in this process. Unlike its homologues in other organisms cMeso2 transcripts in chicken locate to the posterior aspects of somitomeres and constitute a marker for the caudal half of somites. Initiation of cMeso2 expression in presomitic mesoderm as well as its maintenance appears to be independent from influences by surrounding tissues, suggesting that it is part of the intrinsic program underlying segmentation. Although cMeso1 contains a C-terminal activator domain that can be transferred onto an independent DNA-binding domain, no evidence for such a transactivator domain can be found in cMeso2. In contrast, cMeso2 exerts transcriptional inhibition when coexpressed with the cMeso1 transactivator and seems to contain a repressor domain. Thus, cMeso1 and cMeso2 may function in an antagonistic manner during somitogenesis.

The making of the somite: molecular events in vertebrate segmentation

The reiterated structures of the vertebrate axial skeleton, spinal nervous system and body muscle are based on the metameric structure of somites, which are formed in a dynamic morphogenetic process. Somite segmentation requires the activity of a biochemical oscillator known as the somite-segmentation clock. Although the molecular identity of the clock remains unknown, genetic and experimental evidence has accumulated that indicates how the periodicity of somite formation is generated, how the positions of segment borders are determined, and how the rostrocaudal polarity within somite primordia is generated.

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

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

Characterization of a maternal T-Box gene in Ciona intestinalis

A new T-box gene, CiVegTR, was isolated in the ascidian Ciona intestinalis. CiVegTR maternal RNAs become localized to the vegetal cytoplasm of fertilized eggs and are incorporated into muscle lineages derived from the B4.1 blastomere. The CiVegTR protein binds to specific sequences within a minimal, 262-bp enhancer that mediates Ci-snail expression in the tail muscles. Mutations in these binding sites abolish expression from an otherwise normal lacZ reporter gene in electroporated embryos. In addition to the previously identified AC-core E-box sequences, T-box recognition sequences are conserved in the promoter regions of many genes expressed in B4.1 lineages in both Ciona and the distantly related ascidian Halocynthia. These results suggest that CiVegTR encodes a component of the classical muscle determinant that was first identified in ascidians nearly 100 years ago.