Development of chick axial mesoderm: specification of prechordal mesoderm by anterior endoderm-derived TGFbeta family signalling.

Two populations of axial mesoderm cells can be recognised in the chick embryo, posterior notochord and anterior prechordal mesoderm. We have examined the cellular and molecular events that govern the specification of prechordal mesoderm. We report that notochord and prechordal mesoderm cells are intermingled and share expression of many markers as they initially extend out of Hensen's node. In vitro culture studies, together with in vivo grafting experiments, reveal that early extending axial mesoderm cells are labile and that their character may be defined subsequently through signals that derive from anterior endodermal tissues. Anterior endoderm elicits aspects of prechordal mesoderm identity in extending axial mesoderm by repressing notochord characteristics, briefly maintaining gsc expression and inducing BMP7 expression. Together these experiments suggest that, in vivo, signalling by anterior endoderm may determine the extent of prechordal mesoderm. The transforming growth factor (beta) (TGFbeta) superfamily members BMP2, BMP4, BMP7 and activin, all of which are transiently expressed in anterior endoderm mimic distinct aspects of its patterning actions. Together our results suggest that anterior endoderm-derived TGFbetas may specify prechordal mesoderm character in chick axial mesoderm.

Family of Ebf/Olf-1-related genes potentially involved in neuronal differentiation and regional specification in the central nervous system.

Two novel mouse genes, Ebf2 and Ebf3, have been identified which show high similarity to the rodent Ebf/Olf-1 and the Drosophila collier genes. The strong conservation of the protein regions corresponding to the DNA binding and dimerisation domains previously defined in Ebf strongly suggests that Ebf2 and Ebf3 also constitute DNA sequence-specific transcription factors. Determination of the chromosomal locations of the two genes indicated that the different members of this novel mouse multigene family are not clustered. A detailed analysis of the expression of each of the three Ebf genes in the developing central nervous system revealed partially overlapping patterns with two salient features: 1) In the region extending from the midbrain to the spinal cord, the expression of the three genes correlated with neuronal maturation, with a general activation in early post-mitotic cells, followed by specific patterns of extinction also consistent with the neurogenic gradient. 2) In the forebrain area, although the patterns of expression of the Ebf genes also reflected neuronal maturation, they appeared in addition to be region specific. These data suggest that Ebf genes may be involved in the control of neuronal differentiation in the CNS and in enforcing regional diversity in populations of post-mitotic forebrain neurons.

Cooperation of BMP7 and SHH in the induction of forebrain ventral midline cells by prechordal mesoderm.

Ventral midline cells at different rostrocaudal levels of the central nervous system exhibit distinct properties but share the ability to pattern the dorsoventral axis of the neural tube. We show here that ventral midline cells acquire distinct identities in response to the different signaling activities of underlying mesoderm. Signals from prechordal mesoderm control the differentiation of rostral diencephalic ventral midline cells, whereas notochord induces floor plate cells caudally. Sonic hedgehog (SHH) is expressed throughout axial mesoderm and is required for the induction of both rostral diencephalic ventral midline cells and floor plate. However, prechordal mesoderm also expresses BMP7 whose function is required coordinately with SHH to induce rostral diencephalic ventral midline cells. BMP7 acts directly on neural cells, modifying their response to SHH so that they differentiate into rostral diencephalic ventral midline cells rather than floor plate cells. Our results suggest a model whereby axial mesoderm both induces the differentiation of overlying neural cells and controls the rostrocaudal character of the ventral midline of the neural tube.

[Role of the Krox-20 gene in the development of rhombencephalon]. / Rôle du gène Krox-20 dans le développement du rhombencéphale.

In the hindbrain region of the developing CNS, anteroposterior patterning involves a transient segmentation process which leads to the formation of morphological bulges called rhombomeres. The rhombomeres constitute cell lineage restriction units and participate in the establishment of a metameric pattern which is responsible for the segmental organisation of motor and reticular neurons. Like Drosophila compartments, rhombomeres also constitute domains of specific gene expression. Genes expressed in a rhombomere-specific manner so far identified encode various types of putative regulatory molecules, including transcription factors, like Hox proteins, the zinc finger protein Krox-20 and the basic domain leucine-zipper protein kreisler, and receptor type molecules, like Sek-1, a member of the EPH family of tyrosine kinase receptors. Such genes are thought to play a role either in the definition of segmental territories or in the specification of the identity of the rhombomeres. Initial analysis of the function of some of these genes have indeed supported this hypothesis. This is the case for the Krox-20 gene. It is expressed within the developing hindbrain in two transverse domains which prefigure and then coincide with r3 and r5. We have inactivated Krox-20 by homologous recombination in ES cells and demonstrated that the mutation leads to the deletion of r3 and r5. The mutation introduced into the Krox-20 gene involved the in-frame insertion of the lacZ coding sequence. This allowed us to follow the late expression pattern of the gene and to identify two additional phenotypes, affecting myelination of the peripheral nervous system and endochondral ossification. The lacZ reporter also permitted a detailed analysis of the expression of Krox-20 in peripheral glial cells, revealing important steps in the control of their development. Recently we have performed a detailed analysis of specific neuronal populations affected by the mutation which shed new light on the role of Krox-20 in the segmentation and on the physiological consequences of its inactivation. We have also identified several new members of the Sek-1 family of receptor tyrosine kinases, which are also expressed in a rhombomere-specific manner. Finally, we have provided evidence that Krox-20 is as a key regulator of r3/r5-specific transcription, controlling the expression of at least five other regulator genes in these rhombomeres. In three cases, Hoxb-2, Hoxa-2 and Sek-1, we could demonstrate that Krox-20 was directly involved in the transcriptional activation of these genes.

Hoxb-2 transcriptional activation in rhombomeres 3 and 5 requires an evolutionarily conserved cis-acting element in addition to the Krox-20 binding site.

Segmentation is a key feature of the development of the vertebrate hindbrain where it involves the generation of repetitive morphological units termed rhombomeres (r). Hox genes are likely to play an essential role in the specification of segmental identity and we have been investigating their regulation. We show here that the mouse and chicken Hoxb-2 genes are dependent for their expression in r3 and r5 on homologous enhancer elements and on binding to this enhancer of the r3/r5-specific transcriptional activator Krox-20. Among the three Krox-20 binding sites of the mouse Hoxb-2 enhancer, only the high-affinity site is absolutely necessary for activity. In contrast, we have identified an additional cis-acting element, Box1, essential for r3/r5 enhancer activity. It is conserved both in sequence and in position respective to the high-affinity Krox-20 binding site within the mouse and chicken enhancers. Furthermore, a short 44 bp sequence spanning the Box1 and Krox-20 sites can act as an r3/r5 enhancer when oligomerized. Box1 may therefore constitute a recognition sequence for another factor cooperating with Krox-20. Taken together, these data demonstrate the conservation of Hox gene regulation and of Krox-20 function during vertebrate evolution.

The conserved role of Krox-20 in directing Hox gene expression during vertebrate hindbrain segmentation.

Transient segmentation in the hindbrain is a fundamental morphogenetic phenomenon in the vertebrate embryo, and the restricted expression of subsets of Hox genes in the developing rhombomeric units and their derivatives is linked with regional specification. Here we show that patterning of the vertebrate hindbrain involves the direct upregulation of the chicken and pufferfish group 2 paralogous genes, Hoxb-2 and Hoxa-2, in rhombomeres 3 and 5 (r3 and r5) by the zinc finger gene Krox-20. We identified evolutionarily conserved r3/r5 enhancers that contain high affinity Krox-20. binding sites capable of mediating transactivation by Krox-20. In addition to conservation of binding sites critical for Krox-20 activity in the chicken Hoxa-2 and pufferfish Hoxb-2 genes, the r3/r5 enhancers are also characterized by the presence of a number of identical motifs likely to be involved in cooperative interactions with Krox-20 during the process of hindbrain patterning in vertebrates.

Segmental expression of Hoxa-2 in the hindbrain is directly regulated by Krox-20.

The hindbrain is a segmented structure divided into repeating metameric units termed rhombomeres (r). The Hox family, vertebrate homologs of the Drosophila HOM-C homeotic selector genes, are expressed in rhombomere-restricted patterns and are believed to participate in regulating segmental identities. Krox-20, a zinc finger gene, has a highly conserved pattern of expression in r3 and r5 and is functionally required for their maintenance in mouse embryos. Krox-20 has been shown to directly regulate the Hoxb-2 gene and we wanted to determine if it was involved in regulating multiple Hox genes as a part of its functional role. Hoxa-2 is the only known paralog of Hoxb-2, and we examined the patterns of expression of the mouse Hoxa-2 gene with particular focus on r3 and r5 in wild type and Krox-20-/- mutant embryos. There was a clear loss of expression in r3, which indicated that Hoxa-2 was downstream of Krox-20. Using transgenic analysis with E. coli lacZ reporter genes we have identified and mapped an r3/r5 enhancer in the 5' flanking region of the Hoxa-2 gene. Deletion analysis narrowed this region to an 809 bp Bg/II fragment, and in vitro binding and competition assays with bacterially expressed Krox-20 protein identified two sites within the enhancer. Mutation of these Krox-20 sites in the regulatory region specifically abolished r3/r5 activity, but did not affect neural crest and mesodermal components. This indicated that the two Krox-20 sites are required in vivo for enhancer function. Furthermore, ectopic expression of Krox-20 in r4 was able to transactivate the Hoxa 2/lacZ reporter in this rhombomere. Together our findings suggest that Krox-20 directly participates in the transcriptional regulation of Hoxa-2 during hindbrain segmentation, and is responsible for the upregulation of the r3 and r5 domains of expression of both vertebrate group 2 Hox paralogs. Therefore, the segmental phenotypes in the Krox-20 mutants are likely to reflect the role of Krox-20 in directly regulating multiple Hox genes.

The zinc finger gene Krox20 regulates HoxB2 (Hox2.8) during hindbrain segmentation.

The zinc finger gene Krox20 and many Hox homeobox genes are expressed in segment-restricted domains in the hindbrain. The restricted expression patterns appear before morphological segmentation, suggesting that these transcription factors may play an early role in the establishment and identity of rhombomeric segments. In this paper, we show that the HoxB2 (Hox2.8) gene is normally upregulated in rhombomeres (r) 3, 4, and 5, and we identify an enhancer region upstream of the gene that imposes r3/r5 expression in transgenic mice. This enhancer contains three Krox20-binding sites required in vitro for complex formation with Krox20 protein and in vivo for rhombomere-restricted expression. In transgenic mice, Krox20 expressed in ectopic domains can transactivate a reporter construct containing the HoxB2 r3/r5 enhancer. These data demonstrate that Krox20 is a part of the upstream transcriptional cascade that directly regulates HoxB2 expression during hindbrain segmentation.

[Molecular analysis of the development of the rhombencephalon]. / Analyse moléculaire du développement du rhombencéphale.

Hindbrain development involves a segmentation process which in particular governs cranial-motor nerves organization. We have initiated a molecular analysis of this morphogenetic process by searching for genes expressed in specific hindbrain segments (rhombomeres). We have identified a gene encoding a transcription factor, Krox-20, which is specific of rhombomeres 3 and 5 and several genes encoding receptor tyrosine kinases which are also segment-specific. The study of Krox-20 has allowed the identification of its nucleotide target, the localization of the functional domains of the protein and the demonstration of its involvement in the transcriptional activation of the Hoxb-2 gene and in the control of normal hindbrain development.

Mapping functional regions of the segment-specific transcription factor Krox-20.

Krox-20, a zinc finger transcription factor with similarity to Sp1, is likely to play an important role in the development of the vertebrate central nervous system. A knowledge of its molecular properties will help to understand its physiological functions. We have therefore performed a structure-function analysis of the protein to identify the regions involved in DNA-binding and transcriptional activation. Our data suggest that only the zinc fingers are required for high affinity, specific DNA-binding. Transcriptional activation was not affected by deletion of the C-terminal tail of the protein. In contrast, deletion of the N-terminal half, upstream of the zinc fingers, completely abolished transactivation without affecting DNA-binding or nuclear localization. Two transcriptional activation domains were identified in this region. They cooperate to establish full activity. They are rich in negatively-charged amino acids and are therefore may constitute acidic activation domains. Comparative analysis of the amino acid sequences of several zinc finger proteins belonging to the Krox-20 subfamily indicates that they contain acidic regions at similar locations within their N-terminal region, suggesting that the functional organization of these proteins has been conserved during evolution.

Base sequence discrimination by zinc-finger DNA-binding domains.

Zinc fingers constitute important eukaryotic DNA-binding domains, being present in many transcription factors. The Cys2/His2 zinc-finger class has conserved motifs of 28-30 amino acids which are usually present as tandem repeats. The structure of a Cys2/His2 zinc finger has been determined by nuclear magnetic resonance, but details of its interaction with DNA were not established. Here we identify amino acids governing DNA-binding specificity using in vitro directed mutagenesis guided by similarities between the zinc fingers of transcription factors Sp1 and Krox-20. Krox-20 is a serum-inducible transcription activator which is possibly involved in the regulation of hindbrain development; it contains three zinc fingers similar to those of Sp1 and binds to a 9-base-pair target sequence which is related to that of Sp1. Our results show that each finger spans three nucleotides and indicate two positions in Krox-20 zinc fingers that are important for base-pair selectivity. Modelling with molecular graphics suggests that these residues could bind directly with the bases and that other amino acid-base contacts are also possible.

The serum-inducible mouse gene Krox-24 encodes a sequence-specific transcriptional activator.

The mouse gene Krox-24 is transiently activated during cell cycle reentry. It encodes a protein with three zinc fingers similar to those of the transcription factor Sp1. Here we present a biochemical characterization of the gene products. Krox-24 mRNA is translated into two proteins of 82 and 88 kilodaltons, designated p82Krox-24 and p88Krox-24, respectively. p82Krox-24 is initiated at the first AUG codon of the open reading frame, whereas synthesis of p88Krox-24 starts at a non-AUG codon located upstream. Both proteins were synthesized in HeLa cells infected with recombinant vaccinia viruses expressing Krox-24 cDNAs. Under these conditions, they were found phosphorylated on serine residues and glycosylated. The availability of the proteins made possible the determination of the DNA recognition sequence. In vitro, Krox-24 bound specifically to the sequence 5'-GCG(C/G)GGGCG-3'. This sequence is similar but not identical to the Sp1 target sequence. Insertion of an oligomer for the binding site in cis, close to the herpes simplex virus thymidine kinase promoter, rendered this promoter responsive to Krox-24. Krox-24 is therefore a sequence-specific transcriptional activator. Krox-24-binding sites were found upstream of several serum-inducible genes, raising the possibility that Krox-24 is involved in the regulation of these genes.

The segment-specific gene Krox-20 encodes a transcription factor with binding sites in the promoter region of the Hox-1.4 gene.

Krox-20 is a mouse zinc finger gene expressed in a segment-specific manner in the early central nervous system, which makes it a potential developmental control gene. In this report, we show that the Krox-20 protein binds in vitro to two specific DNA sites located upstream from the homeobox containing gene Hox-1.4. The nucleotide sequence recognized by Krox-20 is closely related to the Sp1 target sequence, which is consistent with the similarity existing between the zinc fingers of the two proteins. In co-transfection experiments in cultured cells, Krox-20 dramatically activates transcription from the herpes simplex virus thymidine kinase promoter when an oligomer of its binding site is present in cis close to the promoter. Analysis of mutated binding sites demonstrates that the level of activation by Krox-20 correlates with the affinity of the protein for the mutant sequence. These data indicate that Krox-20 constitutes a sequence-specific DNA-binding transcription factor. Parallel analysis of the expression of Krox-20 and Hox-1.4 in the neural tube by in situ hybridization revealed no overlap, arguing against direct interactions between these two genes. The possible involvement of Krox-20 in the regulation of the transcription of other homeobox genes is discussed in view of their respective patterns of expression.

Presence of globin gene transcripts in chicken oocytes and of a partially processed globin RNA in early embryos.

RNA isolated from chicken oocytes and early embryos of various stages of development were probed with cloned cDNA of the alpha type (pi, alpha D and alpha A) and beta type (beta A, globin genes. Transcripts of all genes were present, although at a very low level, in the RNA of oocytes, and of embryos of the blastula and gastrula stages, prior to the onset of globin synthesis at about 30 h incubation. Interestingly, Northern blotting of electrophoretically fractionated embryonic RNA made it possible to observe, at all stages of development and for all genes tested, RNA molecules several hundred nucleotides longer than mature mRNA. PCR amplification of the pi globin transcripts indicates that these additional sequences are localized upstream of the CAP site. These higher-MW forms were found to be replaced by normal-size globin mRNA several hours after the onset of globin synthesis. The relevance of these data to comprehension of how globin gene expression is controlled during development is discussed.