Expression of avian Groucho-related genes (Grgs) during embryonic development

C-terminal binding proteins: central players in development and disease

C-terminal binding proteins (CtBPs) were initially identified as binding partners for the E1A-transforming proteins. Although the invertebrate genome encodes one CtBP protein, two CtBPs (CtBP1 and CtBP2) are encoded by the vertebrate genome and perform both unique and duplicative functions. CtBP1 and CtBP2 are closely related and act as transcriptional corepressors when activated by nicotinamide adenine dinucleotide binding to their dehydrogenase domains. CtBPs exert transcriptional repression primarily via recruitment of a corepressor complex to DNA that consists of histone deacetylases (HDACs) and histone methyltransferases, although CtBPs can also repress transcription through HDAC-independent mechanisms. More recent studies have demonstrated a critical function for CtBPs in the transcriptional repression of pro-apoptotic genes such as Bax, Puma, Bik, and Noxa. Nonetheless, although recent efforts have characterized the essential involvement of CtBPs in promoting cellular survival, the dysregulation of CtBPs in both neurodegenerative disease and cancers remains to be fully elucidated.

Prediction of gene network models in limb muscle precursors

The ventrolateral dermomyotome gives rise to all muscles of the limbs through the delamination and migration of cells into the limb buds. These cells proliferate and form myoblasts, withdraw from the cell cycle and become terminally differentiated. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Gene expression arrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1(EGFP), which resulted from the loss of Pitx2. Target genes of Pitx2 were clustered using the "David Bioinformatics Functional Annotation Tool" to bin genes according to enrichment of gene ontology keywords. This provided a way to both narrow the target genes and identify potential gene families regulated by Pitx2. Representative target genes in the most enriched bins were analyzed for the presence and evolutionary conservation of Pitx2 consensus binding sequence, TAATCY, on the -20kb, intronic, and coding regions of the genes. Fifteen Pitx2 target genes were selected based on the above analysis and were identified as having functions involving cytoskeleton organization, tissue specification, and transcription factors. Data from these studies suggest that Pitx2 acts to regulate cell motility and expression of muscle specific genes in the muscle precursors during forelimb muscle development. This work provides a framework to develop the gene network leading to skeletal muscle development, growth and regeneration.

Expression, purification, crystallization, and preliminary X-ray diffraction analysis of the human TLE1 Q domain

Human transducin-like enhancer of split 1 (TLE1) plays crucial roles in a number of developmental processes and is involved in pathogenesis of malignancy tumors. The N-terminal glutamine-rich domain (Q domain) of TLE1 mediates its tetramerization and interactions with different DNA-binding transcription factors to regulate Notch and Wnt signaling pathways. To better understand the molecular mechanism of TLE1's functions in these pathways, we cloned, purified, and crystallized the TLE1 Q domain (TLE1-Q). The crystals belong to space group C222(1), with the complete diffraction data of the native and Se-Met TLE1-Q collected to 3.5 and 4.1 Å resolutions, respectively. The phasing-solving and model building are in progress.

Identification of genes downstream of the Shh signalling in the developing chick wing and syn-expressed with Hoxd13 using microarray and 3D computational analysis

Sonic hedgehog (Shh) signalling by the polarizing region at the posterior margin of the chick wing bud is pivotal in patterning the digits but apart from a few key downstream genes, such as Hoxd13, which is expressed in the posterior region of the wing that gives rise to the digits, the genes that mediate the response to Shh signalling are not known. To find genes that are co-expressed with Hoxd13 in the posterior of chick wing buds and regulated in the same way, we used microarrays to compare gene expression between anterior and posterior thirds of wing buds from normal chick embryos and from polydactylous talpid³ mutant chick embryos, which have defective Shh signalling due to lack of primary cilia. We identified 1070 differentially expressed gene transcripts, which were then clustered. Two clusters contained genes predominantly expressed in posterior thirds of normal wing buds; in one cluster, genes including Hoxd13, were expressed at high levels in anterior and posterior thirds in talpid³ wing buds, in the other cluster, genes including Ptc1, were expressed at low levels in anterior and posterior thirds in talpid³ wing buds. Expression patterns of genes in these two clusters were validated in normal and talpid³ mutant wing buds by in situ hybridisation and demonstrated to be responsive to application of Shh. Expression of several genes in the Hoxd13 cluster was also shown to be responsive to manipulation of protein kinase A (PKA) activity, thus demonstrating regulation by Gli repression. Genes in the Hoxd13 cluster were then sub-clustered by computational comparison of 3D expression patterns in normal wing buds to produce syn-expression groups. Hoxd13 and Sall1 are syn-expressed in the posterior region of early chick wing buds together with 6 novel genes which are likely to be functionally related and represent secondary targets of Shh signalling. Other groups of syn-expressed genes were also identified, including a group of genes involved in vascularisation.

Meis2 competes with the Groucho co-repressor Tle4 for binding to Otx2 and specifies tectal fate without induction of a secondary midbrain-hindbrain boundary organizer

The transcription factor Otx2 is expressed throughout the anterior neuroectoderm and is required for the formation of all forebrain- and midbrain-derived structures. The molecular determinants that cooperate with Otx2 to subdivide its expression domain into distinct functional units are, however, poorly understood at present. We show here that the TALE-homeodomain protein Meis2 is expressed in the chick tectal anlage and is both necessary and sufficient for tectal development. Unlike known tectum-inducing genes, the ability of Meis2 to initiate tectal development does not involve the formation of a secondary midbrain-hindbrain boundary organizer, but instead requires direct interaction with Otx2. Using an Otx2-dependent reporter assay we demonstrate that Meis2 competes with the Groucho co-repressor Tle4 (Grg4) for binding to Otx2 and thereby restores Otx2 transcriptional activator function. Together, our data suggest a model in which the balance between a co-repressor and a co-activator, which compete for binding to Otx2 in the mesencephalic vesicle, provides spatial and temporal control over tectal development. Controlled formation of Meis2-containing higher order protein complexes might thus serve as a general mechanism to achieve subdivision of the anterior neuroectoderm into distinct functional units during embryogenesis.

The Groucho/TLE/Grg family of transcriptional co-repressors

The Groucho family of co-repressor proteins are essential for development and may also have a role in some human cancers.

Extrinsic versus intrinsic cues in avian paraxial mesoderm patterning and differentiation

Somitic and head mesoderm contribute to cartilage and bone and deliver the entire skeletal musculature. Studies on avian somite patterning and cell differentiation led to the view that these processes depend solely on cues from surrounding tissues. However, evidence is accumulating that some developmental decisions depend on information within the somitic tissue itself. Moreover, recent studies established that head and somitic mesoderm, though delivering the same tissue types, are set up to follow their own, distinct developmental programmes. With a particular focus on the chicken embryo, we review the current understanding of how extrinsic signalling, operating in a framework of intrinsically regulated constraints, controls paraxial mesoderm patterning and cell differentiation.

Ledgerline, a Novel Xenopus laevis Gene, Regulates Differentiation of Presomitic Mesoderm During Somitogenesis

Segmentation of the vertebrate body via the sequential formation of somites is an important process in embryogenesis. This sequential process is governed by the activation and regulation of Notch-related molecular oscillators by fibroblast growth factor and retinoic acid (RA) signaling. In this study, we identified ledgerline, a novel gene of Xenopus laevis expressed specifically in the presomitic mesoderm. Knockdown of ledgerline using antisense morpholino oligonucleotides shifted the developing somite front and altered the expression of genes that regulate molecular oscillation, including Delta2, ESR5, Hairy2a, and Thylacine1. Knockdown of ledgerline also downregulated RALDH-2 expression. Injection of RARalpha-CA, a constitutively active mutant of the RA receptor RARalpha, subsequently reduced the altered Thylacine1 expression. These results strongly suggest that ledgerline is essential for mesodermal RA activity and differentiation of the presomitic mesoderm during Xenopus somitogenesis.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.