Distinct regulatory elements direct delta1 expression in the nervous system and paraxial mesoderm of transgenic mice

Activity of the mouse Notch ligand DLL1 is sensitive to C-terminal tagging in vivo

OBJECTIVE

The mammalian Notch ligand DLL1 has essential functions during development. To visualise DLL1 in tissues, for sorting and enrichment of DLL1-expressing cells, and to efficiently purify DLL1 protein complexes we tagged DLL1 in mice with AcGFPHA or Strep/FLAG.

RESULTS

We generated constructs to express DLL1 that carried C-terminal in-frame an AcGFPHA tag flanked by loxP sites followed by a Strep/FLAG (SF) tag out of frame. Cre-mediated recombination replaced AcGFP-HA by SF. The AcGFPHAstopSF cassette was added to DLL1 for tests in cultured cells and introduced into endogenous DLL1 in mice by homologous recombination. Tagged DLL1 protein was detected by antibodies against GFP and HA or Flag, respectively, both in CHO cells and embryo lysates. In CHO cells the AcGFP fluorophore fused to DLL1 was functional. In vivo AcGFP expression was below the level of detection by direct fluorescence. However, the SF tag allowed us to specifically purify DLL1 complexes from embryo lysates. Homozygous mice expressing AcGFPHA or SF-tagged DLL1 revealed a vertebral column phenotype reminiscent of disturbances in AP polarity during somitogenesis, a process most sensitive to reduced DLL1 function. Thus, even small C-terminal tags can impinge on sensitive developmental processes requiring DLL1 activity.

Morphogenesis is transcriptionally coupled to neurogenesis during peripheral olfactory organ development

Sense organs acquire their distinctive shapes concomitantly with the differentiation of sensory cells and neurons necessary for their function. Although our understanding of the mechanisms controlling morphogenesis and neurogenesis in these structures has grown, how these processes are coordinated remains largely unexplored. Neurogenesis in the zebrafish olfactory epithelium requires the bHLH proneural transcription factor Neurogenin 1 (Neurog1). To address whether Neurog1 also controls morphogenesis, we analysed the migratory behaviour of early olfactory neural progenitors in neurog1 mutant embryos. Our results indicate that the oriented movements of these progenitors are disrupted in this context. Morphogenesis is similarly affected by mutations in the chemokine receptor gene, cxcr4b, suggesting it is a potential Neurog1 target gene. We find that Neurog1 directly regulates cxcr4b through an E-box cluster located just upstream of the cxcr4b transcription start site. Our results suggest that proneural transcription factors, such as Neurog1, directly couple distinct aspects of nervous system development.

Functionally distinct roles for T and Tbx6 during mouse development

The mouse T-box transcription factors T and Tbx6 are co-expressed in the primitive streak and have unique domains of expression; T is expressed in the notochord, while Tbx6 is expressed in the presomitic mesoderm. T-box factors are related through a shared DNA binding domain, the T-domain, and can therefore bind to similar DNA sequences at least in vitro. We investigated the functional similarities and differences of T and Tbx6 DNA binding and transcriptional activity in vitro and their interaction genetically in vivo. We show that at one target, Dll1, the T-domains of T and Tbx6 have different affinities for the binding sites present in the mesoderm enhancer. We further show using in vitro assays that T and Tbx6 differentially affect transcription with Tbx6 activating expression tenfold higher than T, that T and Tbx6 can compete at target gene enhancers, and that this competition requires a functional DNA binding domain. Next, we addressed whether T and Tbx6 can compete in vivo. First, we generated embryos that express Tbx6 at greater than wild-type levels embryos and show that these embryos have short tails, resembling the T heterozygous phenotype. Next, using the dominant-negative TWis allele, we show that Tbx6+/− TWis/+ embryos share similarities with embryos homozygous for the Tbx6 hypomorphic allele rib-vertebrae, specifically fusions of several ribs and malformation of some vertebrae. Finally, we tested whether Tbx6 can functionally replace T using a knockin approach, which resulted in severe T null-like phenotypes in chimeric embryos generated with ES cells heterozygous for a Tbx6 knockin at the T locus. Altogether, our results of differences in affinity for DNA binding sites and transcriptional activity for T and Tbx6 provide a potential mechanism for the failure of Tbx6 to functionally replace T and possible competition phenotypes in vivo.

Summary: Mouse Tbx6 fails to compensate for heterozygous loss of T; instead ectopic Tbx6 in the T expression-domain in knockin embryos generates T null-like phenotypes suggestive of competition.

E proteins sharpen neurogenesis by modulating proneural bHLH transcription factors' activity in an E-box-dependent manner

Class II HLH proteins heterodimerize with class I HLH/E proteins to regulate transcription. Here, we show that E proteins sharpen neurogenesis by adjusting the neurogenic strength of the distinct proneural proteins. We find that inhibiting BMP signaling or its target ID2 in the chick embryo spinal cord, impairs the neuronal production from progenitors expressing ATOH1/ASCL1, but less severely that from progenitors expressing NEUROG1/2/PTF1a. We show this context-dependent response to result from the differential modulation of proneural proteins’ activity by E proteins. E proteins synergize with proneural proteins when acting on CAGSTG motifs, thereby facilitating the activity of ASCL1/ATOH1 which preferentially bind to such motifs. Conversely, E proteins restrict the neurogenic strength of NEUROG1/2 by directly inhibiting their preferential binding to CADATG motifs. Since we find this mechanism to be conserved in corticogenesis, we propose this differential co-operation of E proteins with proneural proteins as a novel though general feature of their mechanism of action.

The brain and spinal cord are made up of a range of cell types that carry out different roles within the central nervous system. Each type of neuron is uniquely specialized to do its job. Neurons are produced early during development, when they differentiate from a group of cells called neural progenitor cells. Within these groups, molecules called proneural proteins control which types of neurons will develop from the neural progenitor cells, and how many of them.

Proneural proteins work by binding to specific patterns in the DNA, called E-boxes, with the help of E proteins. E proteins are typically understood to be passive partners, working with each different proneural protein indiscriminately. However, Le Dréau, Escalona et al. discovered that E proteins in fact have a much more active role to play.

Using chick embryos, it was found that E proteins influence the way different proneural proteins bind to DNA. The E proteins have preferences for certain E-boxes in the DNA, just like proneural proteins do. The E proteins enhanced the activity of the proneural proteins that share their E-box preference, and reined in the activity of proneural proteins that prefer other E-boxes. As a result, the E proteins controlled the ability of these proteins to instruct neural progenitor cells to produce specific, specialized neurons, and thus ensured that the distinct types of neurons were produced in appropriate amounts.

These findings will help shed light on the roles E proteins play in the development of the central nervous system, and the processes that control growth and lead to cell diversity. The results may also have applications in the field of regenerative medicine, as proneural proteins play an important role in cell reprogramming.

Deletion of the sclerotome-enriched lncRNA PEAT augments ribosomal protein expression

To define a complete catalog of the genes that are activated during mouse sclerotome formation, we sequenced RNA from embryonic mouse tissue directed to form sclerotome in culture. In addition to well-known early markers of sclerotome, such as Pax1, Pax9, and the Bapx2/Nkx3-2 homolog Nkx3-1, the long-noncoding RNA PEAT (Pax1 enhancer antisense transcript) was induced in sclerotome-directed samples. Strikingly, PEAT is located just upstream of the Pax1 gene. Using CRISPR/Cas9, we generated a mouse line bearing a complete deletion of the PEAT-transcribed unit. RNA-seq on PEAT mutant embryos showed that loss of PEAT modestly increases bone morphogenetic protein target gene expression and also elevates the expression of a large subset of ribosomal protein mRNAs.

Generation of an 870 kb deletion encompassing the Skt/Etl4 locus by combination of inter- and intra-chromosomal recombination

Background

Etl4^lacZ (Enhancer trap locus 4) and Skt^Gt (Sickle tail) are lacZ reporter gene integrations into the same locus on mouse chromosome 2 targeting a gene that is expressed in the notochord of early embryos and in multiple epithelia during later development. Both insertions caused recessive mutations that resulted exclusively in mild defects in the caudal vertebral column. Since notochord-derived signals are essential for formation of the vertebral column the phenotypes suggested that the lacZ insertions interfered with some notochord-dependent aspect of vertebral development. As both insertions occurred in introns it was unclear whether they represent hypomorphic alleles or abolish gene function. Here, we have generated a definitive null allele of the Skt/Etl4 gene and analysed homozygous mutants.

Results

We have introduced loxP sites into three positions of the gene based on additional upstream exons that we identified, and deleted approximately 870 kb of the locus by a combination of inter- and intra-chromosomal Cre-mediated recombinations in the female germ line of mice. This deletion removes about 90 % of the coding region and results in the loss of the SKT/ETL4 protein. Similar to the Etl4^lacZ and Skt^Gt alleles our deletion mutants are viable and fertile and show only mild defects in caudal vertebrae due to abnormal intervertebral disc development, although with higher penetrance. No other tissue with Skt/Etl4 expression that we analysed showed obvious defects.

Conclusion

The complete loss of Skt/Etl4 function affects only development of caudal notochord derivatives and is compensated for in its other expression domains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0302-0) contains supplementary material, which is available to authorized users.

Hedgehog/Patched-associated rhabdomyosarcoma formation from delta1-expressing mesodermal cells

Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma. In children, the 2 major RMS subtypes are alveolar and embryonal RMS. Aberrant Hedgehog/Patched1 (Hh/Ptch) signaling is a hallmark of embryonal RMS. We demonstrate that mice carrying a Ptch mutation in mesodermal Delta1-expressing cells develop embryonal-like RMS at a similar rate as mice harboring a Ptch mutation in the germline or the brachury-expressing mesoderm. The tumor incidence decreases dramatically when Ptch is mutated in Myf5- or Pax3-expressing cells. No RMS develop from Myogenin/Mef2c-expressing cells. This suggests that Hh/Ptch-associated RMS are derived from Delta1-positive, Myf5-negative, Myogenin-negative and Pax3-negative mesodermal progenitors that can undergo myogenic differentiation but lack stable lineage commitment. Additional preliminary genetic data and data on mesodermal progenitors further imply an interplay of Hh/Ptch and Delta/Notch signaling activity during RMS initiation. In contrast, Wnt signals supposedly suppress RMS formation because RMS multiplicity decreases after inactivation of the Wnt-inhibitor Wif1. Finally, our results strongly suggest that the tumor-initiating event determines the lineage of RMS origin.

O-fucosylation of DLL3 is required for its function during somitogenesis

Delta-like 3 (DLL3) is a member of the DSL family of Notch ligands in amniotes. In contrast to DLL1 and DLL4, the other Delta-like proteins in the mouse, DLL3 does not bind in trans to Notch and does not activate the receptor, but shows cis-interaction and cis-inhibitory properties on Notch signaling in vitro. Loss of the DSL protein DLL3 in the mouse results in severe somite patterning defects, which are virtually indistinguishable from the defects in mice that lack lunatic fringe (LFNG), a glycosyltransferase involved in modifying Notch signaling. Like LFNG, DLL3 is located within the trans-Golgi, however, its biochemical function is still unclear. Here, we show that i) both proteins interact, ii) epidermal growth factor like repeats 2 and 5 of DLL3 are O-fucosylated at consensus sites for POFUT1, and iii) further modified by FNG proteins in vitro. Embryos double homozygous for null mutations in Dll3 and Lfng are phenotypically indistinguishable from the single mutants supporting a potential common function. Mutation of the O-fucosylation sites in DLL3 does not disrupt the interaction of DLL3 with LFNG or full length Notch1or DLL1, and O-fucosylation-deficient DLL3 can still inhibit Notch in cis in vitro. However, in contrast to wild type DLL3, O-fucosylation-deficient DLL3 cannot compensate for the loss of endogenous DLL3 during somitogenesis in the embryo. Together our results suggest that the cis-inhibitory activity of DLL3 observed in cultured cells might not fully reflect its assumed essential physiological property, suggest that DLL3 and LFNG act together, and strongly supports that modification of DLL3 by O-linked fucose is essential for its function during somitogenesis.

Hoxb6 can interfere with somitogenesis in the posterior embryo through a mechanism independent of its rib-promoting activity

Formation of the vertebrate axial skeleton requires coordinated Hox gene activity. Hox group 6 genes are involved in the formation of the thoracic area due to their unique rib-promoting properties. We show here that the linker region (LR) connecting the homeodomain and the hexapeptide is essential for Hoxb6 rib-promoting activity. The LR-defective Hoxb6 protein was still able to bind a target enhancer together with Pax3 producing a dominant negative effect, indicating that the LR brings additional regulatory factors to target DNA elements. We also found an unexpected association between Hoxb6 and segmentation in the paraxial mesoderm. In particular, Hoxb6 can disturb somitogenesis and anterior-posterior somite patterning by deregulating Lfng expression. Interestingly, this interaction occurred differently in thoracic and more caudal embryonic areas, indicating functional differences in somitogenesis before and after the trunk to tail transition. Our results suggest the requirement of precisely regulated Hoxb6 expression for proper segmentation at tailbud stages.

Redox regulation by Pitx2 and Pitx3 is critical for fetal myogenesis

During development, major metabolic changes occur as cells become more specialized within a lineage. In the case of skeletal muscle, differentiation is accompanied by a switch from a glycolytic proliferative progenitor state to an oxidative postmitotic differentiated state. Such changes require extensive mitochondrial biogenesis leading to increased reactive oxygen species (ROS) production that needs to be balanced by an antioxidant system. Our analysis of double conditional Pitx2/3 mouse mutants, both in vivo during fetal myogenesis and ex vivo in primary muscle cell cultures, reveals excessive upregulation of ROS levels leading to DNA damage and apoptosis of differentiating cells. This is a consequence of downregulation of Nrf1 and genes for antioxidant enzymes, direct targets of Pitx2/3, leading to decreased expression of antioxidant enzymes, as well as impairment of mitochondrial function. Our analysis identifies Pitx2 and Pitx3 as key regulators of the intracellular redox state preventing DNA damage as cells undergo differentiation.

Switching axial progenitors from producing trunk to tail tissues in vertebrate embryos

The vertebrate body is made by progressive addition of new tissue from progenitors at the posterior embryonic end. Axial extension involves different mechanisms that produce internal organs in the trunk but not in the tail. We show that Gdf11 signaling is a major coordinator of the trunk-to-tail transition. Without Gdf11 signaling, the switch from trunk to tail is significantly delayed, and its premature activation brings the hindlimbs and cloaca next to the forelimbs, leaving extremely short trunks. Gdf11 activity includes activation of Isl1 to promote formation of the hindlimbs and cloaca-associated mesoderm as the most posterior derivatives of lateral mesoderm progenitors. Gdf11 also coordinates reallocation of bipotent neuromesodermal progenitors from the anterior primitive streak to the tail bud, in part by reducing the retinoic acid available to the progenitors. Our findings provide a perspective to understand the evolution of the vertebrate body plan.

Dynamic CREB family activity drives segmentation and posterior polarity specification in mammalian somitogenesis

The segmented body plan of vertebrates is prefigured by reiterated embryonic mesodermal structures called somites. In the mouse embryo, timely somite formation from the presomitic mesoderm (PSM) is controlled by the "segmentation clock," a molecular oscillator that triggers progressive waves of Notch activity throughout the PSM. Notch clock activity is suppressed in the posterior PSM by FGF signaling until it crosses a determination front at which its net activity is sufficiently high to effect segmentation. Here, Notch and Wnt signaling directs somite anterior/posterior (A/P) polarity specification and boundary formation via regulation of the segmentation effector gene Mesoderm posterior 2. How Notch and Wnt signaling becomes coordinated at this front is incompletely defined. Here we show that the activity of the cAMP responsive element binding protein (CREB) family of transcription factors exhibits Wnt3a-dependent oscillatory behavior near the determination front and is in unison with Notch activity. Inhibition of CREB family in the mesoderm causes defects in somite segmentation and a loss in somite posterior polarity leading to fusions of vertebrae and ribs. Among the CREB family downstream genes, several are known to be regulated by Wnt3a. Of those, we show that the CREB family occupies a conserved binding site in the promoter region of Delta-like 1, encoding a Notch ligand, in the anterior PSM as a mechanism to specify posterior identity of somites. Together, these data support that the CREB family acts at the determination front to modulate Wnt signaling and strengthen Notch signaling as a means to orchestrate cells for somite segmentation and anterior/posterior patterning.

Dynamic interactions between intermediate neurogenic progenitors and radial glia in embryonic mouse neocortex: potential role in Dll1-Notch signaling

The mammalian neocortical progenitor cell niche is composed of a diverse repertoire of neuroepithelial cells, radial glia (RG), and intermediate neurogenic progenitors (INPs). Previously, live-cell imaging experiments have proved crucial in identifying these distinct progenitor populations, especially INPs, which amplify neural output by undergoing additional rounds of proliferation before differentiating into new neurons. INPs also provide feedback to the RG pool by serving as a source of Delta-like 1 (Dll1), a key ligand for activating Notch signaling in neighboring cells, a well-known mechanism for maintaining RG identity. While much is known about Dll1-Notch signaling at the molecular level, little is known about how this cell-cell contact dependent feedback is transmitted at the cellular level. To investigate how RG and INPs might interact to convey Notch signals, we used high-resolution live-cell multiphoton microscopy (MPM) to directly observe cellular interactions and dynamics, in conjunction with Notch-pathway specific reporters in the neocortical neural stem cell niche in organotypic brain slices from embryonic mice. We found that INPs and RG interact via dynamic and transient elongate processes, some apparently long-range (extending from the subventricular zone to the ventricular zone), and some short-range (filopodia-like). Gene expression profiling of RG and INPs revealed further progenitor cell diversification, including different subpopulations of Hes1+ and/or Hes5+ RG, and Dll1+ and/or Dll3+ INPs. Thus, the embryonic progenitor niche includes a network of dynamic cell-cell interactions, using different combinations of Notch signaling molecules to maintain and likely diversify progenitor pools.

Role of a polymorphism in a Hox/Pax-responsive enhancer in the evolution of the vertebrate spine

Patterning of the vertebrate skeleton requires the coordinated activity of Hox genes. In particular, Hox10 proteins are essential to set the transition from thoracic to lumbar vertebrae because of their rib-repressing activity. In snakes, however, the thoracic region extends well into Hox10-expressing areas of the embryo, suggesting that these proteins are unable to block rib formation. Here, we show that this is not a result of the loss of rib-repressing properties by the snake proteins, but rather to a single base pair change in a Hox/Paired box (Pax)-responsive enhancer, which prevents the binding of Hox proteins. This polymorphism is also found in Paenungulata, such as elephants and manatees, which have extended rib cages. In vivo, this modified enhancer failed to respond to Hox10 activity, supporting its role in the extension of rib cages. In contrast, the enhancer could still interact with Hoxb6 and Pax3 to promote rib formation. These results suggest that a polymorphism in the Hox/Pax-responsive enhancer may have played a role in the evolution of the vertebrate spine by differently modulating its response to rib-suppressing and rib-promoting Hox proteins.

Regulatory role for a conserved motif adjacent to the homeodomain of Hox10 proteins

Development of the vertebrate axial skeleton requires the concerted activity of several Hox genes. Among them, Hox genes belonging to the paralog group 10 are essential for the formation of the lumbar region of the vertebral column, owing to their capacity to block rib formation. In this work, we explored the basis for the rib-repressing activity of Hox10 proteins. Because genetic experiments in mice demonstrated that Hox10 proteins are strongly redundant in this function, we first searched for common motifs among the group members. We identified the presence of two small sequences flanking the homeodomain that are phylogenetically conserved among Hox10 proteins and that seem to be specific for this group. We show here that one of these motifs is required but not sufficient for the rib-repressing activity of Hox10 proteins. This motif includes two potential phosphorylation sites, which are essential for protein activity as their mutation to alanines resulted in a total loss of rib-repressing properties. Our data indicates that this motif has a significant regulatory function, modulating interactions with more N-terminal parts of the Hox protein, eventually triggering the rib-repressing program. In addition, this motif might also regulate protein activity by alteration of the protein's DNA-binding affinity through changes in the phosphorylation state of two conserved tyrosine residues within the homeodomain.

Genetic and functional evidence implicating DLL1 as the gene that influences susceptibility to visceral leishmaniasis at chromosome 6q27

BACKGROUND

Visceral leishmaniasis (VL) is caused by Leishmania donovani and Leishmania infantum chagasi. Genome-wide linkage studies from Sudan and Brazil identified a putative susceptibility locus on chromosome 6q27.

METHODS

Twenty-two single-nucleotide polymorphisms (SNPs) at genes PHF10, C6orf70, DLL1, FAM120B, PSMB1, and TBP were genotyped in 193 VL cases from 85 Sudanese families, and 8 SNPs at genes PHF10, C6orf70, DLL1, PSMB1, and TBP were genotyped in 194 VL cases from 80 Brazilian families. Family-based association, haplotype, and linkage disequilibrium analyses were performed. Multispecies comparative sequence analysis was used to identify conserved noncoding sequences carrying putative regulatory elements. Quantitative reverse-transcription polymerase chain reaction measured expression of candidate genes in splenic aspirates from Indian patients with VL compared with that in the control spleen sample.

RESULTS

Positive associations were observed at PHF10, C6orf70, DLL1, PSMB1, and TBP in Sudan, but only at DLL1 in Brazil (combined P = 3 × 10(-4) at DLL1 across Sudan and Brazil). No functional coding region variants were observed in resequencing of 22 Sudanese VL cases. DLL1 expression was significantly (P = 2 × 10(-7)) reduced (mean fold change, 3.5 [SEM, 0.7]) in splenic aspirates from patients with VL, whereas other 6q27 genes showed higher levels (1.27 × 10(-6) < P < .01) than did the control spleen sample. A cluster of conserved noncoding sequences with putative regulatory variants was identified in the distal promoter of DLL1.

CONCLUSIONS

DLL1, which encodes Delta-like 1, the ligand for Notch3, is strongly implicated as the chromosome 6q27 VL susceptibility gene.

Resuscitating cancer immunosurveillance: selective stimulation of DLL1-Notch signaling in T cells rescues T-cell function and inhibits tumor growth

Deficiencies in immune function that accumulate during cancer immunoediting lead to a progressive escape from host immunosurveillance. Therapies that correct or overcome these defects could have a powerful impact on cancer management, but current knowledge of the types and mechanisms of immune escape is still incomplete. Here, we report a novel mechanism of escape from T-cell immunity that is caused by reduction in levels of the Delta family Notch ligands DLL1 and DLL4 in hematopoietic microenvironments. An important mediator of this effect was an elevation in the levels of circulating VEGF. Selective activation of the DLL1-Notch signaling pathway in bone marrow precursors enhanced T-cell activation and inhibited tumor growth. Conversely, tumor growth led to inhibition of Delta family ligand signaling through Notch in the hematopoietic environment, resulting in suppressed T-cell function. Overall, our findings uncover a novel mechanism of tumoral immune escape and suggest that a soluble multivalent form of DLL1 may offer a generalized therapeutic intervention to stimulate T-cell immunity and suppress tumor growth.

The Wnt3a/β-catenin target gene Mesogenin1 controls the segmentation clock by activating a Notch signalling program

Segmentation is an organizing principle of body plans. The segmentation clock, a molecular oscillator best illustrated by the cyclic expression of Notch signalling genes, controls the periodic cleavage of somites from unsegmented presomitic mesoderm during vertebrate segmentation. Wnt3a controls the spatiotemporal expression of cyclic Notch genes; however, the underlying mechanisms remain obscure. Here we show by transcriptional profiling of Wnt3a (-/-) embryos that the bHLH transcription factor, Mesogenin1 (Msgn1), is a direct target gene of Wnt3a. To identify Msgn1 targets, we conducted genome-wide studies of Msgn1 activity in embryonic stem cells. We show that Msgn1 is a major transcriptional activator of a Notch signalling program and synergizes with Notch to trigger clock gene expression. Msgn1 also indirectly regulates cyclic genes in the Fgf and Wnt pathways. Thus, Msgn1 is a central component of a transcriptional cascade that translates a spatial Wnt3a gradient into a temporal pattern of clock gene expression.

Tbx6-dependent Sox2 regulation determines neural or mesodermal fate in axial stem cells

The classical view of neural plate development held that it arises from the ectoderm, after its separation from the mesodermal and endodermal lineages. However, recent cell lineage tracing experiments indicate that the caudal neural plate and paraxial mesoderm are generated from common bipotential axial stem cells originating from the caudal lateral epiblast (CLE)1,2. Tbx6 null mutant mouse embryos which produce ectopic neural tubes at the expense of paraxial mesoderm3 must provide a clue to the regulatory mechanism underlying this neural versus mesodermal fate choice. Here we demonstrate that Tbx6-dependent regulation of Sox2 determines the fate of axial stem cells. In wild-type embryos, enhancer N1 of the neural primordial gene Sox2 is activated in the CLE, and the cells staying in the superficial layer sustain N1 activity and activate Sox2 expression in the neural plate4-6. In contrast, the cells destined to become mesoderm activate Tbx6 and turn off enhancer N1 before migrating into the paraxial mesoderm compartment. In Tbx6 mutant embryos, however, enhancer N1 activity persists in the paraxial mesoderm compartment, eliciting ectopic Sox2 activation and transforming the paraxial mesoderm into neural tubes. An enhancer N1-specific deletion mutation introduced into Tbx6 mutant embryos prevented this Sox2 activation in the mesodermal compartment and subsequent development of ectopic neural tubes, indicating that Tbx6 regulates Sox2 via enhancer N1. Tbx6-dependent repression of Wnt3a in the paraxial mesodermal compartment is implicated in this regulatory process. Paraxial mesoderm-specific misexpression of a Sox2 transgene in wild type embryos resulted in ectopic neural tube development. Thus, Tbx6 represses Sox2 by inactivating enhancer N1 to inhibit neural development, and this is an essential step for the specification of paraxial mesoderm from the axial stem cells.

A cluster of non-redundant Ngn1 binding sites is required for regulation of deltaA expression in zebrafish

Proneural genes encode bHLH transcription factors that are key regulator of neurogenesis in both vertebrates and invertebrates. How these transcription factors regulate targets required for neural determination and/or specification is beginning to be understood. In this study, we show that zebrafish deltaA is a transcriptional target of proneural factors. Using a combination of transient and stable transgenic reporters, we show that regulation of deltaA by one such proneural factor, Ngn1, requires three clustered E-box binding sites that act in a non-redundant manner. Furthermore, we show that as for other proneural targets, members of the different proneural families regulate deltaA expression via distinct cis-regulatory modules (CRMs). Interestingly, however, while the deltaA CRM regulated by a second proneural factor, Ascl1, has been conserved between delta genes of different species, we show that the Ngn1 CRM has not. These results suggest that evolutionary constraints on the mechanism by which Ngn1 regulates gene expression appear less strict than for Ascl1.

Evidence for a myotomal Hox/Myf cascade governing nonautonomous control of rib specification within global vertebral domains

Hox genes are essential for the patterning of the axial skeleton. Hox group 10 has been shown to specify the lumbar domain by setting a rib-inhibiting program in the presomitic mesoderm (PSM). We have now produced mice with ribs in every vertebra by ectopically expressing Hox group 6 in the PSM, indicating that Hox genes are also able to specify the thoracic domain. We show that the information provided by Hox genes to specify rib-containing and rib-less areas is first interpreted in the myotome through the regional-specific control of Myf5 and Myf6 expression. This information is then transmitted to the sclerotome by a system that includes FGF and PDGF signaling to produce vertebrae with or without ribs at different axial levels. Our findings offer a new perspective of how Hox genes produce global patterns in the axial skeleton and support a redundant nonmyogenic role of Myf5 and Myf6 in rib formation.

Additive and global functions of HoxA cluster genes in mesoderm derivatives

Hox genes encode transcription factors that play a central role in the specification of regional identities along the anterior to posterior body axis. In the developing mouse embryo, Hox genes from all four genomic clusters are involved in range of developmental processes, including the patterning of skeletal structures and the formation of several organs. However, the functional redundancy observed either between paralogous genes, or among neighboring genes from the same cluster, has hampered functional analyses, in particular when synergistic, cluster-specific functions are considered. Here, we report that mutant mice lacking the entire HoxA cluster in mesodermal lineages display the expected spectrum of postnatal respiratory, cardiac and urogenital defects, previously reported for single gene mutations. Likewise, mild phenotypes are observed in both appendicular and axial skeleton. However, a striking effect was uncovered in the hematopoietic system, much stronger than that seen for Hoxa9 inactivation alone, which involves stem cells (HSCs) as well as the erythroid lineage, indicating that several Hoxa genes are necessary for normal hematopoiesis to occur. Finally, the combined deletions of Hoxa and Hoxd genes reveal abnormalities in axial elongation as well as skin morphogenesis that are likely the results of defects in epithelial-mesenchymal interactions.

Acheate-scute like 1 (Ascl1) is required for normal delta-like (Dll) gene expression and notch signaling during retinal development

Delta gene expression in Drosophila is regulated by proneural basic helix-loop-helix (bHLH) transcription factors, such as acheate-scute. In vertebrates, multiple Delta-like and proneural bHLH genes are expressed during neurogenesis, especially in the retina. We recently uncovered a relationship between Acheate-scute like 1 (Ascl1), Delta-like genes, and Notch in chick retinal progenitors. Here, we report that mammalian retinal progenitors are also the primary source of Delta-like genes, likely signaling through Notch among themselves, while differentiating neurons expressed Jagged2. Ascl1 is coexpressed in Delta-like and Notch active progenitors, and required for normal Delta-like gene expression and Notch signaling. We also reveal a role for Ascl1 in the regulation of Hes6, a proneurogenic factor that inhibits Notch signaling to promote neural rather than glial differentiation. Thus, these results suggest a molecular mechanism whereby attenuated Notch levels coupled with reduced proneurogenic activity in progenitors leads to increased gliogenesis and decreased neurogenesis in the Ascl1-deficient retina.

Cdx and Hox genes differentially regulate posterior axial growth in mammalian embryos

Hox and Cdx transcription factors regulate embryonic positional identities. Cdx mutant mice display posterior body truncations of the axial skeleton, neuraxis, and caudal urorectal structures. We show that trunk Hox genes stimulate axial extension, as they can largely rescue these Cdx mutant phenotypes. Conversely, posterior (paralog group 13) Hox genes can prematurely arrest posterior axial growth when precociously expressed. Our data suggest that the transition from trunk to tail Hox gene expression successively regulates the construction and termination of axial structures in the mouse embryo. Thus, Hox genes seem to differentially orchestrate posterior expansion of embryonic tissues during axial morphogenesis as an integral part of their function in specifying head-to-tail identity. In addition, we present evidence that Cdx and Hox transcription factors exert these effects by controlling Wnt signaling. Concomitant regulation of Cyp26a1 expression, restraining retinoic acid signaling away from the posterior growth zone, may likewise play a role in timing the trunk-tail transition.

Notch: from neural development to neurological disorders

Notch is an integral membrane protein that functions as receptor for ligands such as jagged and delta that are associated with the surface of neighboring cells. Upon ligand binding, notch is proteolytically cleaved within its transmembrane domain by presenilin-1 (the enzymatic component of the gamma-secretase complex) resulting in the release of a notch intracellular domain which translocates to the nucleus where it regulates gene expression. Notch signaling plays multiple roles in the development of the CNS including regulating neural stem cell (NSC) proliferation, survival, self-renewal and differentiation. Notch is also present in post-mitotic neurons in the adult CNS wherein its activation influences structural and functional plasticity including processes involved in learning and memory. Recent findings suggest that notch signaling in neurons, glia, and NSCs may be involved in pathological changes that occur in disorders such as stroke, Alzheimer's disease and CNS tumors. Studies of animal models suggest the potential of agents that target notch signaling as therapeutic interventions for several different CNS disorders.

Identification of direct T-box target genes in the developing zebrafish mesoderm

The zebrafish genes spadetail (spt) and no tail (ntl) encode T-box transcription factors that are important for early mesoderm development. Although much has been done to characterize these genes, the identity and location of target regulatory elements remain largely unknown. Here, we survey the genome for downstream target genes of the Spt and Ntl T-box transcription factors. We find evidence for extensive additive interactions towards gene activation and limited evidence for combinatorial and antagonistic interactions between the two factors. Using in vitro binding selection assays to define Spt- and Ntl-binding motifs, we searched for target regulatory sequence via a combination of binding motif searches and comparative genomics. We identified regulatory elements for tbx6 and deltaD, and, using chromatin immunoprecipitation, in vitro DNA binding assays and transgenic methods, we provide evidence that both are directly regulated by T-box transcription factors. We also find that deltaD is directly activated by T-box factors in the tail bud, where it has been implicated in starting the segmentation clock, suggesting that spt and ntl act upstream of this process.

Tbx6 regulates left/right patterning in mouse embryos through effects on nodal cilia and perinodal signaling

Background

The determination of left/right body axis during early embryogenesis sets up a developmental cascade that coordinates the development of the viscera and is essential to the correct placement and alignment of organ systems and vasculature. Defective left-right patterning can lead to congenital cardiac malformations, vascular anomalies and other serious health problems. Here we describe a novel role for the T-box transcription factor gene Tbx6 in left/right body axis determination in the mouse.

Results

Embryos lacking Tbx6 show randomized embryo turning and heart looping. Our results point to multiple mechanisms for this effect. First, Dll1, a direct target of Tbx6, is down regulated around the node in Tbx6 mutants and there is a subsequent decrease in nodal signaling, which is required for laterality determination. Secondly, in spite of a lack of expression of Tbx6 in the node, we document a profound effect of the Tbx6 mutation on the morphology and motility of nodal cilia. This results in the loss of asymmetric calcium signaling at the periphery of the node, suggesting that unidirectional nodal flow is disrupted. To carry out these studies, we devised a novel method for direct labeling and live imaging cilia in vivo using a genetically-encoded fluorescent protein fusion that labels tubulin, combined with laser point scanning confocal microscopy for direct visualization of cilia movement.

Conclusions

We conclude that the transcription factor gene Tbx6 is essential for correct left/right axis determination in the mouse and acts through effects on notch signaling around the node as well as through an effect on the morphology and motility of the nodal cilia.

Characterization of the proneural gene regulatory network during mouse telencephalon development

Background

The proneural proteins Mash1 and Ngn2 are key cell autonomous regulators of neurogenesis in the mammalian central nervous system, yet little is known about the molecular pathways regulated by these transcription factors.

Results

Here we identify the downstream effectors of proneural genes in the telencephalon using a genomic approach to analyze the transcriptome of mice that are either lacking or overexpressing proneural genes. Novel targets of Ngn2 and/or Mash1 were identified, such as members of the Notch and Wnt pathways, and proteins involved in adhesion and signal transduction. Next, we searched the non-coding sequence surrounding the predicted proneural downstream effector genes for evolutionarily conserved transcription factor binding sites associated with newly defined consensus binding sites for Ngn2 and Mash1. This allowed us to identify potential novel co-factors and co-regulators for proneural proteins, including Creb, Tcf/Lef, Pou-domain containing transcription factors, Sox9, and Mef2a. Finally, a gene regulatory network was delineated using a novel Bayesian-based algorithm that can incorporate information from diverse datasets.

Conclusion

Together, these data shed light on the molecular pathways regulated by proneural genes and demonstrate that the integration of experimentation with bioinformatics can guide both hypothesis testing and hypothesis generation.

cis-Decoder discovers constellations of conserved DNA sequences shared among tissue-specific enhancers

: The use of cis-Decoder, a new tool for discovery of conserved sequence elements that are shared between similarly regulating enhancers, suggests that enhancers use overlapping repertoires of highly conserved core elements.

A systematic approach is described for analysis of evolutionarily conserved cis-regulatory DNA using cis-Decoder, a tool for discovery of conserved sequence elements that are shared between similarly regulated enhancers. Analysis of 2,086 conserved sequence blocks (CSBs), identified from 135 characterized enhancers, reveals most CSBs consist of shorter overlapping/adjacent elements that are either enhancer type-specific or common to enhancers with divergent regulatory behaviors. Our findings suggest that enhancers employ overlapping repertoires of highly conserved core elements.

Transcriptional Repression by the T-box Proteins Tbx18 and Tbx15 Depends on Groucho Corepressors

Tbox18 (Tbx18) and Tbox15 (Tbx15) encode a closely related pair of vertebrate-specific T-box (Tbx) transcription factors. Functional analyses in the mouse have proven the requirement of Tbx15 in skin and skeletal development and of Tbx18 in the formation of the vertebral column, the ureter, and the posterior pole of the heart. Despite the accumulation of genetic data concerning the embryological roles of these genes, it is currently unclear how Tbx18 and Tbx15 exert their function on the molecular level. Here, we have initiated a molecular analysis of Tbx18 and Tbx15 proteins and have characterized functional domains for nuclear localization, DNA binding, and transcriptional modulation. We show that both proteins homo- and heterodimerize, bind to various combinations of T half-sites, and repress transcription in a Groucho-dependent manner. Competition with activating T-box proteins may constitute one mode of action as we show that Tbx18 interacts with Gata4 and Nkx2-5 and competes Tbx5-mediated activation of the cardiac Natriuretic peptide precursor type a-promoter and that ectopic expression of Tbx18 down-regulates Tbx6-activated Delta-like 1 expression in the somitic mesoderm in vivo.

Expression of Msgn1 in the presomitic mesoderm is controlled by synergism of WNT signalling and Tbx6

The vertebral column and skeletal muscles of vertebrates are derived from the paraxial mesoderm, which is laid down initially as two stripes of mesenchymal cells alongside the neural tube and subsequently segmented. Previous work has shown that the wingless-type MMTV integration site family (WNT), fibroblast growth factor- and Delta-Notch signalling pathways control presomitic mesoderm (psm) formation and segmentation. Here, we show that the expression of mesogenin 1, a basic helix-loop-helix transcription factor, which is essential for psm maturation, is regulated by synergism between WNT signalling and the T-box 6 transcription factor, involving a feed-forward control mechanism. These findings emphasize the crucial role of WNT signalling in the control of psm formation, maturation and segmentation.

Compartmentalised expression of Delta-like 1 in epithelial somites is required for the formation of intervertebral joints

Background

Expression of the mouse Delta-like 1 (Dll1) gene in the presomitic mesoderm and in the caudal halves of somites of the developing embryo is required for the formation of epithelial somites and for the maintenance of caudal somite identity, respectively. The rostro-caudal polarity of somites is initiated early on within the presomitic mesoderm in nascent somites. Here we have investigated the requirement of restricted Dll1 expression in caudal somite compartments for the maintenance of rostro-caudal somite polarity and the morphogenesis of the axial skeleton. We did this by overexpressing a functional copy of the Dll1 gene throughout the paraxial mesoderm, in particular in anterior somite compartments, during somitogenesis in transgenic mice.

Results

Epithelial somites were generated normally and appeared histologically normal in embryos of two independent Dll1 over-expressing transgenic lines. Gene expression analyses of rostro-caudal marker genes suggested that over-expression of Dll1 without restriction to caudal compartments was not sufficient to confer caudal identity to rostral somite halves in transgenic embryos. Nevertheless, Dll1 over-expression caused dysmorphologies of the axial skeleton, in particular, in morphological structures that derive from the articular joint forming compartment of vertebrae. Accordingly, transgenic animals exhibited missing or reduced intervertebral discs, rostral and caudal articular processes as well as costal heads of ribs. In addition, the midline of the vertebral column did not develop normally. Transgenic mice had open neural arches and split vertebral bodies with ectopic pseudo-growth plates. Endochondral bone formation and ossification in the developing vertebrae were delayed.

Conclusion

The mice overexpressing Dll1 exhibit skeletal dysmorphologies that are also evident in several mutant mice with defects in somite compartmentalisation. The Dll1 transgenic mice demonstrate that vertebral dysmorphologies such as bony fusions of vertebrae and midline vertebral defects can occur without apparent changes in somitic rostro-caudal marker gene expression. Also, we demonstrate that the over-expression of the Dll1 gene in rostral epithelial somites is not sufficient to confer caudal identity to rostral compartments. Our data suggest that the restricted Dll1 expression in caudal epithelial somites may be particularly required for the proper development of the intervertebral joint forming compartment.

Proneural bHLH and Brn Proteins Coregulate a Neurogenic Program through Cooperative Binding to a Conserved DNA Motif

Proneural proteins play a central role in vertebrate neurogenesis, but little is known of the genes that they regulate and of the factors that interact with proneural proteins to activate a neurogenic program. Here, we demonstrate that the proneural protein Mash1 and the POU proteins Brn1 and Brn2 interact on the promoter of the Notch ligand Delta1 and synergistically activate Delta1 transcription, a key step in neurogenesis. Overexpression experiments in vivo indicate that Brn2, like Mash1, regulates additional aspects of neurogenesis, including the division of progenitors and the differentiation and migration of neurons. We identify by in silico screening a number of additional candidate target genes, which are recognized by Mash1 and Brn proteins through a DNA-binding motif similar to that found in the Delta1 gene and present a broad range of activities. We thus propose that Mash1 synergizes with Brn factors to regulate multiple steps of neurogenesis.

Non-cell-autonomous action of STAT3 in maintenance of neural precursor cells in the mouse neocortex

The transcription factor STAT3 promotes astrocytic differentiation of neural precursor cells (NPCs) during postnatal development of the mouse neocortex, but little has been known of the possible role of STAT3 in the embryonic neocortex. We now show that STAT3 is expressed in NPCs of the mouse embryonic neocortex and that the JAK-STAT3 signaling pathway plays an essential role in the maintenance of NPCs by fibroblast growth factor 2. Conditional deletion of the STAT3 gene in NPCs reduced their capacity to form neurospheres in vitro, as well as promoted neuronal differentiation both in vitro and in vivo. Furthermore, STAT3 was found to maintain NPCs in the undifferentiated state in a non-cell-autonomous manner. STAT3-dependent expression of the Notch ligand Delta-like1 (DLL1) appears to account for the non-cell-autonomous effect of STAT3 on NPC maintenance, as knockdown of DLL1 by RNA interference or inhibition of Notch activation with a gamma-secretase inhibitor abrogated the enhancement of neurosphere formation by STAT3. Our results reveal a previously unrecognized mechanism of interaction between the JAK-STAT3 and DLL1-Notch signaling pathways, as well as a pivotal role for this interaction in maintenance of NPCs during early neocortical development.

Mesodermal and neuronal retinoids regulate the induction and maintenance of limb innervating spinal motor neurons

During embryonic development, the generation, diversification and maintenance of spinal motor neurons depend upon extrinsic signals that are tightly regulated. Retinoic acid (RA) is necessary for specifying the fates of forelimb-innervating motor neurons of the Lateral Motor Column (LMC), and the specification of LMC neurons into medial and lateral subtypes. Previous studies implicate motor neurons as the relevant source of RA for specifying lateral LMC fates at forelimb levels. However, at the time of LMC diversification, a significant amount of retinoids in the spinal cord originates from the adjacent paraxial mesoderm. Here we employ mouse genetics to show that RA derived from the paraxial mesoderm is required for lateral LMC induction at forelimb and hindlimb levels, demonstrating that mesodermally synthesized RA functions as a second source of signals to specify lateral LMC identity. Furthermore, reduced RA levels in postmitotic motor neurons result in a decrease of medial and lateral LMC neurons, and abnormal axonal projections in the limb; invoking additional roles for neuronally synthesized RA in motor neuron maintenance and survival. These findings suggest that during embryogenesis, mesodermal and neuronal retinoids act coordinately to establish and maintain appropriate cohorts of spinal motor neurons that innervate target muscles in the limb.

Hox genes specify vertebral types in the presomitic mesoderm

We show here that expression of Hoxa10 in the presomitic mesoderm is sufficient to confer a Hox group 10 patterning program to the somite, producing vertebrae without ribs, an effect not achieved when Hoxa10 is expressed in the somites. In addition, Hox group 11-dependent vertebral sacralization requires Hoxa11 expression in the presomitic mesoderm, while their caudal differentiation requires that Hoxa11 is expressed in the somites. Therefore, Hox gene patterning activity is different in the somites and presomitic mesoderm, the latter being very prominent for Hox gene-mediated patterning of the axial skeleton. This is further supported by our finding that inactivation of Gbx2, a homeobox-containing gene expressed in the presomitic mesoderm but not in the somites, produced Hox-like phenotypes in the axial skeleton without affecting Hox gene expression.

Multiple POU-binding motifs, recognized by tissue-specific nuclear factors, are important for Dll1 gene expression in neural stem cells

We cloned the 5'-flanking region of the mouse homolog of the Delta gene (Dll1) and demonstrated that the sequence between nucleotide position -514 and -484 in the 5'-flanking region of Dll1 played a critical role in the regulation of its tissue-specific expression in neural stem cells (NSCs). Further, we showed that multiple POU-binding motifs, located within this short sequence of 30bp, were essential for transcriptional activation of Dll1 and also that multiple tissue-specific nuclear factors recognized these POU-binding motifs in various combinations through differentiation of NSCs. Thus, POU-binding factors may play an important role in Dll1 expression in developing NSCs.

Dll1 is a downstream target of Tbx6 in the paraxial mesoderm

Tbx6 is a member of the T-box family of transcription factors. In the mouse, Tbx6 is expressed in the primitive streak, tail bud, and presomitic mesoderm and is essential for the specification of posterior paraxial mesoderm; in its absence, posterior somites are replaced by ectopic neural tubes. Analysis of embryos expressing reduced levels of Tbx6 also revealed that it is required for the correct patterning of the somites as well as their initial specification. As a first step toward identifying downstream targets of Tbx6, we examined the DNA binding properties of Tbx6 and identified a Tbx6 consensus binding site. Previously, we have shown that expression of Dll1, which encodes a Notch ligand, is lost in the Tbx6 mutant and that Tbx6 and Dll1 genetically interact, indicating that Dll1 may be a direct target of Tbx6 in the paraxial mesoderm. We uncovered four putative Tbx6 binding sites within a Dll1 paraxial mesoderm enhancer and show that Tbx6 can bind two of these sites in vitro. Altogether, these results lend further support for Dll1 being a direct target of Tbx6 in the presomitic mesoderm.

LEF1-mediated regulation of Delta-like1 links Wnt and Notch signaling in somitogenesis

Wnt signaling, which is mediated by LEF1/TCF transcription factors, has been placed upstream of the Notch pathway in vertebrate somitogenesis. Here, we examine the molecular basis for this presumed hierarchy and show that a targeted mutation of Lef1, which abrogates LEF1 function and impairs the activity of coexpressed TCF factors, affects the patterning of somites and the expression of components of the Notch pathway. LEF1 was found to bind multiple sites in the Dll1 promoter in vitro and in vivo. Moreover, mutations of LEF1-binding sites in the Dll1 promoter impair expression of a Dll1-LacZ transgene in the presomitic mesoderm. Finally, the induced expression of LEF1-beta-catenin activates the expression of endogenous Dll1 in fibroblastic cells. Thus, Wnt signaling can affect the Notch pathway by a LEF1-mediated regulation of Dll1.

WNT signaling, in synergy with T/TBX6, controls Notch signaling by regulating Dll1 expression in the presomitic mesoderm of mouse embryos

Notch signaling in the presomitic mesoderm (psm) is critical for somite formation and patterning. Here, we show that WNT signals regulate transcription of the Notch ligand Dll1 in the tailbud and psm. LEF/TCF factors cooperate with TBX6 to activate transcription from the Dll1 promoter in vitro. Mutating either T or LEF/TCF sites in the Dll1 promoter abolishes reporter gene expression in vitro as well as in the tail bud and psm of transgenic embryos. Our results indicate that WNT activity, in synergy with TBX6, regulates Dll1 transcription and thereby controls Notch activity, somite formation, and patterning.

Distribution of presenilin 1 and 2 and their relation to Notch receptors and ligands in human embryonic/foetal central nervous system

Notch signaling in vertebrates is mediated by four Notch receptors (Notch-1, -2, -3, and -4) that are activated by interacting with at least five different Notch ligands, Jagged-1, Jagged-2, Delta-1, -2, and -3. Recent studies have shown that the gamma-secretase-like intramembranous cleavage of Notch receptors to release their cytoplasmic signaling domains requires the presenilin (PS) proteins 1 and 2 (PS1 and PS2). Here, we used immunohistochemistry to compare the distribution of all four Notch receptor proteins and three ligands in the context of co-localization with PS1 and PS2 in first trimester human central nervous system (CNS). In addition, we investigated Notch receptors and ligands expression by Western blotting. The study was performed on the forebrain and spinal cord of human embryonic/foetal CNS (5-11 gestational weeks). Results showed a divergent distribution of the different Notch receptor proteins with only Notch-1 being co-localized with PS1 and PS2. Notch-2 was only seen occasionally within the developing cortex and spinal cord. Notch-3 expression was restricted to neuroepithelial cells of the spinal cord and endothelial cells in blood vessels of both developing cerebral cortex and spinal cord. The weak, punctate staining of Notch-4 in the neuroepithelium of the spinal cord could not be confirmed with Western blotting. Neither Notch-2, nor -3 showed overlap with either PS1 or PS2 immunoreactivity. The ligand Jagged-1 was found sporadically in the neuroepithelial cell layer in cerebral cortex of the earlier stages of development and of the spinal cord during the first trimester while Jagged-2 was not detected. Jagged-1 and Jagged-2 immunoreactivities were not found in the 9-11-week cortex. No co-distribution of Jagged-1 and PS1 or PS2 was found. Delta-1 ligand expression was detected in neuroepithelial cells of the ventricular zone of the cerebral cortex, and also in maturating neurons in the cortical plate and ventral horns of the developing spinal cord. The presence of Notch-1, Delta-1 and Jagged-1 in the neuroepithelium of developing CNS indicates that Notch signaling in proliferating human progenitor cells only involves these two receptor ligands and that cleavage of Notch-1 is mediated both by PS1 and PS2. The strong immunoreactivity of Notch-1, Delta-1 and PS1 in the cortical plate and in maturating neurons of the spinal cord also suggests that these proteins may regulate the maturation processes of post-mitotic neurons. The pronounced PS1 immunoreactivity in neurites in the hindbrain and spinal cord without detectable expression of any Notch receptor or ligand suggests that a possible role for PS1 in neurite growth involves either gamma-secretase-mediated cleavage of other substrates or gamma-secretase-independent mechanisms.

The T-box transcription factor Tbx18 maintains the separation of anterior and posterior somite compartments

The compartmentalization of somites along their anterior-posterior (AP) axis is pivotal to the segmental organization of the vertebrate axial skeleton and the peripheral nervous system. Anterior and posterior somite halves contribute to different vertebral elements. They are also characterized by different proliferation rates and properties with respect to neural crest cell migration and spinal nerve passage. AP-somite polarity is generated in the anterior presomitic mesoderm by Mesp2 and Delta/Notch signaling. Here, we demonstrate that maintenance of AP-somite polarity is mediated by the T-box transcription factor Tbx18. Mice deficient for Tbx18 show expansion of pedicles with transverse processes and proximal ribs, elements derived from the posterior lateral sclerotome. AP-somite polarity is established in Tbx18 mutant embryos but is not maintained. During somite maturation, posterior somite compartments expand most likely because of posterior cells invading the anterior somite half. In the anterior lateral sclerotome, Tbx18 acts as an antiapoptotic factor. Ectopic expression experiments suggest that Tbx18 can promote anterior at the expense of posterior somite compartments. In summary, Tbx18 appears to act downstream of Mesp2 and Delta/Notch signaling to maintain the separation of anterior and posterior somite compartments.

Specification of vertebral identity is coupled to Notch signalling and the segmentation clock

To further analyse requirements for Notch signalling in patterning the paraxial mesoderm, we generated transgenic mice that express in the paraxial mesoderm a dominant-negative version of Delta1. Transgenic mice with reduced Notch activity in the presomitic mesoderm as indicated by loss of Hes5 expression were viable and displayed defects in somites and vertebrae consistent with known roles of Notch signalling in somite compartmentalisation. In addition, these mice showed with variable expressivity and penetrance alterations of vertebral identities resembling homeotic transformations, and subtle changes of Hox gene expression in day 12.5 embryos. Mice that carried only one functional copy of the endogenous Delta1 gene also showed changes of vertebral identities in the lower cervical region, suggesting a previously unnoticed haploinsufficiency for Delta1. Likewise, in mice carrying a null allele of the oscillating Lfng gene, or in transgenic mice expressing Lfngconstitutively in the presomitic mesoderm, vertebral identities were changed and numbers of segments in the cervical and thoracic regions were reduced,suggesting anterior shifts of axial identity. Together, these results provide genetic evidence that precisely regulated levels of Notch activity as well as cyclic Lfng activity are critical for positional specification of the anteroposterior body axis in the paraxial mesoderm.

A Notch feeling of somite segmentation and beyond

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Transcriptional oscillation of lunatic fringe is essential for somitogenesis

A molecular oscillator that controls the expression of cyclic genes such as lunatic fringe (Lfng) in the presomitic mesoderm has been shown to be coupled with somite formation in vertebrate embryos. To address the functional significance of oscillating Lfng expression, we have generated transgenic mice expressing Lfng constitutively in the presomitic mesoderm in addition to the intrinsic cyclic Lfng activity. These transgenic lines displayed defects of somite patterning and vertebral organization that were very similar to those of Lfng null mutants. Furthermore, constitutive expression of exogenous Lfng did not compensate for the complete loss of cyclic endogenous Lfng activity. Noncyclic exogenous Lfng expression did not abolish cyclic expression of endogenous Lfng in the posterior presomitic mesoderm (psm) but affected its expression pattern in the anterior psm. Similarly, dynamic expression of Hes7 was not abolished but abnormal expression patterns were obtained. Our data are consistent with a model in which alternations of Lfng activity between ON and OFF states in the presomitic mesoderm prior to somite segmentation are critical for proper somite patterning, and suggest that Notch signaling might not be the only determinant of cyclic gene expression in the presomitic mesoderm of mouse embryos.

Defective somite patterning in mouse embryos with reduced levels of Tbx6

During vertebrate embryogenesis, paraxial mesoderm gives rise to somites, which subsequently develop into the dermis, skeletal muscle, ribs and vertebrae of the adult. Mutations that disrupt the patterning of individual somites have dramatic effects on these tissues, including fusions of the ribs and vertebrae. The T-box transcription factor, Tbx6, is expressed in the paraxial mesoderm but is downregulated as somites develop. It is essential for the formation of posterior somites, which are replaced with ectopic neural tubes in Tbx6-null mutant embryos. We show that partial restoration of Tbx6 expression in null mutants rescues somite development, but that rostrocaudal patterning within them is defective, ultimately resulting in rib and vertebral fusions, demonstrating that Tbx6 activity in the paraxial mesoderm is required not simply for somite specification but also for their normal patterning. Somite patterning is dependent upon Notch signaling and we show that Tbx6 genetically interacts with the Notch ligand, delta-like 1 (Dll1). Dll1 expression, which is absent in the Tbx6-null mutant, is restored at reduced levels in the partially rescued mutants, suggesting that Dll1 is a target of Tbx6. We also identify the spontaneous mutation rib-vertebrae as a hypomorphic mutation in Tbx6. The similarity in the phenotypes we describe here and that of some human birth defects, such as spondylocostal dysostosis, raises the possibility that mutations in Tbx6 or components of this pathway may be responsible for these defects.

Wnt3a plays a major role in the segmentation clock controlling somitogenesis

The vertebral column derives from somites generated by segmentation of presomitic mesoderm (PSM). Somitogenesis involves a molecular oscillator, the segmentation clock, controlling periodic Notch signaling in the PSM. Here, we establish a novel link between Wnt/beta-catenin signaling and the segmentation clock. Axin2, a negative regulator of the Wnt pathway, is directly controlled by Wnt/beta-catenin and shows oscillating expression in the PSM, even when Notch signaling is impaired, alternating with Lfng expression. Moreover, Wnt3a is required for oscillating Notch signaling activity in the PSM. We propose that the segmentation clock is established by Wnt/beta-catenin signaling via a negative-feedback mechanism and that Wnt3a controls the segmentation process in vertebrates.