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At least 1,063 records · Page 59Linked to original sources
PubMed1998

Patterning the floral meristem.

Flowers are reproductive structures unique to the angiosperms. Flowers, which develop from small mounds of cells called floral meristems, show a number of universal patterns such as the arrangement of organs of different type along the floral axis. However, other conserved patterns, such as floral asymmetry, are merely conserved within different subgroups. This review discusses the emerging picture of early operating developmental mechanisms, which pattern floral meristems along the radial and dorsiventral axes, and of later-acting ones, which pattern tissue differentiation within floral organs.

Body Patterning↗
PubMedDate not supplied

Genetic analysis of axon pattern formation in the embryonic CNS of Drosophila.

The major axon tracts in the embryonic CNS of Drosophila are organised in a simple, ladder-like pattern. Each neuromere contains two commissures which connect the contra-lateral sides and two longitudinal connectives which connect the different neuromeres along the anterior-posterior axis. The commissures form in close association with only few cells located at the CNS midline. The formation of longitudinal connectives depends in part on the presence of specific lateral glial cells. To unravel the genes underlying the formation of the embryonic CNS axon pattern, we conducted a saturating F2 EMS mutagenesis, screening for mutations, which disrupt this process. The analyses of the identified mutations lead to a simple sequential model on axon pattern formation in embryonic CNS.

Animals↗
PubMed2005

Choice of either beta-catenin or Groucho/TLE as a co-factor for Xtcf-3 determines dorsal-ventral cell fate of diencephalon during Xenopus development.

Co-repressor Groucho/Transducin-Like Enhancer of split (TLE) interacts with transcription factors that are expressed in the central nervous system (CNS), and regulates transcriptional activities. In this study, we examined the contribution of Groucho/TLE to CNS development in Xenopus. The functional inhibition of Groucho/TLE using the WRPW motif as a competitor resulted in the conversion of the ventral cell into the dorsal fate in the prospective diencephalon. We also found that the neural plate was expanded laterally without inhibiting neural crest development. In tailbud, the disturbance of trigeminal ganglion development was observed. These observations allow us to conclude that Groucho/TLE plays important roles in the induction and patterning of distinct CNS territories. We found that Xtcf-3 is involved in some of the patterning in these territories. We generated the variant of Xtcf-3, Xtcf-3BDN-, which is suspected to interfere with the interaction between endogenous Groucho/TLE and Xtcf-3. The transcriptional activation of the Xtcf-3-target genes in response to endogenous Wnt/beta-catenin signaling by the overexpression of Xtcf-3BDN- led to a reduction of the ventral diencephalon. This result indicates that transcriptional repression by the Groucho/TLE-Xtcf-3 complex is important for ventral diencephalon patterning. This idea is supported by the finding that the overexpression of the dominant-negative form of Xtcf-3 or axil causes the expansion of the ventral diencephalon. Based on these data, we propose that the localized activation of Wnt/beta-catenin signaling, which converts Tcf from a repressor to an activator, is required for the establishment of dorsal-ventral patterning in the prospective diencephalon.

Amino Acid Motifs↗
PubMed2005

Vessels and nerves: marching to the same tune.

The patterning of the nervous system is achieved through coordinated action of a variety of repulsive or attractive neuronal guidance factors that direct the growth of growing axons to specific pathways. Recent studies suggest that the same or similar sets of factors also guide migration of endothelial cells, helping to direct the stereotypical embryonic patterning of the developing vertebrate vascular system.

Animals↗
PubMed2005

Preferential adhesion mediated by Hibris and Roughest regulates morphogenesis and patterning in the Drosophila eye.

Cell adhesion is essential for morphogenesis; however, the mechanisms by which cell adhesion coordinates precisely regulated morphogenesis are poorly understood. Here we analyze the morphogenetic processes that organize the interommatidial precursor cells (IPCs) of the Drosophila pupal eye. We demonstrate that the Drosophila immunoglobulin superfamily members Hibris and Roughest are essential for IPC morphogenesis in the eye. The two loci are expressed in complementary cell types, and Hibris and Roughest proteins bind directly in vivo. Primary pigment cells employ Hibris to function as organizers in this process; IPCs minimize contacts with neighboring IPCs and utilize Roughest to maximize contacts with primaries. In addition, we provide evidence that interactions between Hibris and Roughest promote junction formation and that levels of Roughest in individual cells determine their capacity for competition. Our results demonstrate that preferential adhesion mediated by heterophilic interacting cell-adhesion molecules can create a precise pattern by minimizing surface free energy.

Animals↗
PubMed2005

Genomic regulatory networks and animal development.

The synthesis of gene expression data and cis-regulatory analysis permits the elucidation of genomic regulatory networks. These networks provide a direct visualization of the functional interconnections among the regulatory genes and signaling components leading to cell-specific patterns of gene activity. Complex developmental processes are thereby illuminated in ways not revealed by the conventional analysis of individual genes. In this review, we describe emerging networks in several different model systems, and compare them with the gene regulatory network that controls dorsoventral patterning of the Drosophila embryo.

Animals↗
PubMed2005

The Drosophila LIM-homeo domain protein Islet antagonizes pro-neural cell specification in the peripheral nervous system.

The pattern of the external sensory organs (SO) in Drosophila depends on the activity of the basic helix-loop-helix (bHLH) transcriptional activators Achaete/Scute (Ac/Sc) that are expressed in clusters of cells (pro-neural clusters) and provide the cells with the potential to develop a neural fate. In the mesothorax, the GATA1 transcription factor Pannier (Pnr), together with its cofactor Chip, activates ac/sc genes directly through binding to the dorso-central enhancer (DC) of ac/sc. We identify the LIM-homeo domain (LIM-HD) transcription factor Islet (Isl) by genetic screening and investigate its role in the thoracic pre-patterning. We show that isl loss-of-function mutations result in expanded Ac expression in DC and scutellar (SC) pro-neural clusters and formation of ectopic sensory organs. Overexpression of Isl decreases pro-neural expression and suppresses bristle development. Moreover, Isl is coexpressed with Pnr in the posterior region of the mesothorax. In the DC pro-neural cluster, Isl antagonizes Pnr activity both by dimerization with the DNA-binding domain of Pnr and via competitive inhibition of the Chip-bHLH interaction. We propose that sensory organ pre-patterning relies on the antagonistic activity of individual Chip-binding factors. The differential affinities of these binding-factors and their precise stoichiometry are crucial in specifying pre-patterns within the different pro-neural clusters.

Animals↗
PubMed2003

Wnt signaling during development of the gastrointestinal tract.

Wnt signaling pathways have been demonstrated to play important roles in controlling tissue patterning and cell proliferation. In the gastrointestinal tract, mutations that lead to activation of the canonical Wnt pathway through beta-catenin result in familial and sporadic colon cancers. The downstream transcription factor Tcf4 is required to maintain the proliferative stem cell compartment in the crypts of the small intestine. Activation of TCF-dependent transcription is a good correlate to neoplastic transformation. Despite its association with cancer in the colon, little is known of the role for Wnt signaling during development and patterning of the gut tube. We conducted a comprehensive expression screen for Wnt signaling components during different stages of gut development in the chick. Conserved expression patterns of these genes indicate that they likely play essential roles in gut morphogenesis. Based on the expression profiles of putative components of each pathway, we are able to postulate specific roles for the various pathways during gut development. Predictions of roles for canonical signaling in the developing gizzard, duodenum, and large intestine in chick were tested by viral misexpression of dominant-negative (DN) forms of the downstream cofactors Tcf4 and Lef1. In the chick, Tcf4 is expressed in the posterior gizzard mesoderm. Misexpression of DN-Tcf4 in the splanchnic mesoderm resulted in the failure of the gizzard epithelium to form microvilli. Lef1 is expressed in the chick duodenum and large intestine mesoderm. Viral misexpression of DN-Lef1 resulted in diminished mesoderm and overproliferation of the large intestine endoderm, leading to stenosis of the lumen. The results from these misexpression studies in the chick, together with evidence from colorectal lesions, indicate that the canonical Wnt pathway plays critical roles in balancing cell proliferation versus cell differentiation during gut development. The expression profiles of the Wnt signaling components presented in this paper should prove valuable in deciphering additional roles of the Wnt pathways during patterning of the vertebrate gut tube.

Animals↗
PubMed2001

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus.

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

Animals↗
PubMed2001

Correlated patterns of neuron differentiation and Hox gene expression in the hindbrain: a comparative analysis.

Hindbrain neurons are organized into coherent subpopulations with characteristic projection patterns and functions. Many of these serve vital functions that have been conserved throughout the vertebrate radiation, but diversification to modified or highly specialized functions has also occurred. The differentiation of identifiable neuron groups in specific spatial domains must involve the regional expression of determinants within the hindbrain neuroepithelium. The Hox genes are involved in longitudinal regionalization of the neural tube, and their expression patterns in the hindbrain are closely related to the rhombomeres which partition the hindbrain into morphogenetic units. Hox gene expression also exhibits conserved patterning as well as phylogenetic variation. One plausible mechanism that may have contributed to evolutionary diversification in hindbrain neuron populations is therefore the emergence of species-specific differences in Hox gene expression. This article presents a comparative overview of the regional patterning of selected Hox genes and hindbrain neuron populations in several embryologically important species. Although tantalizing correlations exist, the relationship between Hox genes and neuronal patterning is complex, and complicated by dynamic features in each. Much more comparative and developmental data must be obtained before the link between Hox gene expression and hindbrain neuron patterning can be elucidated satisfactorily in an evolutionary context.

Animals↗
PubMed2003

Polycomb group regulation of Hox gene expression in C. elegans.

Polycomb group (PcG) chromatin proteins regulate homeotic genes in both animals and plants. In Drosophila and vertebrates, PcG proteins form complexes and maintain early patterns of Hox gene repression, ensuring fidelity of developmental patterning. PcG proteins in C. elegans form a complex and mediate transcriptional silencing in the germline, but no role for the C. elegans PcG homologs in somatic Hox gene regulation has been demonstrated. Surprisingly, we find that the PcG homologs MES-2 [E(Z)] and MES-6 (ESC), along with MES-3, a protein without known homologs, do repress Hox expression in C. elegans. mes mutations cause anteroposterior transformations and disrupt Hox-dependent neuroblast migration. Thus, as in Drosophila, vertebrates, and plants, C. elegans PcG proteins regulate key developmental patterning genes to establish positional identity.

Alleles↗
PubMed2003

A Tcf4-positive mesodermal population provides a prepattern for vertebrate limb muscle patterning.

Nai;ve myogenic cells migrate from the somites into the developing vertebrate limb, where they simultaneously differentiate into myotubes and form distinct anatomical muscles. Limb signals have been hypothesized to direct the pattern of muscles formed, but the molecular nature of these signals and the identity of the cells that produce them have remained unclear. We have identified a population of lateral plate-derived limb mesodermal cells in both chick and mouse that expresses the transcription factor Tcf4 in a muscle-specific pattern independently of the muscle cells themselves. Functional experiments in the chick demonstrate that TCF4 and the Wnt-beta-catenin pathway in these limb mesodermal cells are critical for muscle patterning. We propose that Tcf4-expressing cells establish a prepattern in the limb mesoderm that determines the sites of myogenic differentiation and thus establishes the basic pattern of limb muscles.

Animals↗
PubMed1998

A dorsal/ventral boundary established by Notch controls growth and polarity in the Drosophila eye.

In the Drosophila compound eye the dorsal and ventral fields of eye units (ommatidia) meet along the dorsoventral midline, forming a line of mirror image symmetry called the equator. The molecular mechanism establishing the equator is not fully understood, but it involves the transcription factors encoded by the Iroquois gene complex. The Iroquois genes are expressed in the dorsal half of the eye and here we show that they regulate the expression of the secreted molecule Fringe. A boundary between fringe-expressing and fringe-non-expressing cells is essential, from the time of the second larval instar, for eye growth and formation of the equator. Boundaries of fringe expression determine where the transmembrane receptor Notch is activated. We find that Notch is activated at the dorsoventral midline, where it is required to promote growth and set up the axis of mirror symmetry. As boundaries of fringe expression and Notch activation are also important during Drosophila wing formation and vertebrate somitogenesis, we suggest that these boundaries constitute a general mechanism that directs growth and patterning of large fields of cells.

Animals↗
PubMed1998

Gli/Zic factors pattern the neural plate by defining domains of cell differentiation.

Three cell types differentiate in the early frog neural plate: neural crest at the lateral edges, floorplate at the midline and primary neurons in three bilateral stripes. Floorplate cells and ventral neurons are induced by Sonic hedgehog (Shh) and neural crest and dorsal neurons are induced by epidermal factors such as bone morphogenetic proteins (BMPs). Neurogenesis in a subset of cells within the stripes involves lateral inhibition. However, the process by which pools of precursors are defined in stereotypic domains in response to inductive signals is unknown. Here we show that frog Zic2 encodes a zinc-finger transcription factor of the Gli superfamily which is expressed in stripes that alternate with those in which primary neurons differentiate and overlap the domains of floorplate and neural crest progenitors. Zic2 inhibits neurogenesis and induces neural crest differentiation. Conversely, Gli proteins are widely expressed, induce neurogenesis and inhibit neural crest differentiation. Zic2 is therefore a vertebrate pre-pattern gene, encoding anti-neurogenic and crest-inducing functions that counteract the neurogenic but not the floorplate-inducing activity of Gli proteins. We propose that the combined function of Gli/Zic genes responds to inductive signals and induces patterned neural cell differentiation.

Animals↗
PubMed2006

Making digit patterns in the vertebrate limb.

The vertebrate limb has been a premier model for studying pattern formation - a striking digit pattern is formed in human hands, with a thumb forming at one edge and a little finger at the other. Classic embryological studies in different model organisms combined with new sophisticated techniques that integrate gene-expression patterns and cell behaviour have begun to shed light on the mechanisms that control digit patterning, and stimulate re-evaluation of the current models.

Animals↗
PubMedDate not supplied

Anteroposterior patterning by mutual repression of orthodenticle and caudal-type transcription factors.

Members of the Otx (orthodenticle) and Cdx (caudal) families of homeodomain transcription factors are expressed in similar embryonic regions in all animal groups and have been shown to be directly involved in anteroposterior patterning in a number of species. In the amphibian Xenopus laevis, the Otx family gene Xotx2 and the Cdx family gene Xcad3 are both expressed within the early dorsal organizer. We show that they have mutually repressive activities, suggesting that they play a crucial role in the early regionalization of the organizer into anterior and posterior territories. Xotx2 can act both as an activator and repressor of gene expression depending on context. A form of Xotx2 that acts exclusively as a repressor (OtxEn-R) was made by fusing the Xotx2 homeodomain to the Drosophila melanogaster engrailed transcriptional repressor domain. Overexpression of this protein in vivo indicates that OtxEn-R antagonizes the activating function of endogenous Xotx2 for anterior marker genes such as XCG and goosecoid but retains the ability to repress the expression of posterior markers such as Xcad3 and Xbra. OtxEn-R overexpression causes a severe derangement of anterior development, resulting in the loss of cement gland, eyes, stomodeal opening, and pharynx. The specification and development of anterior neural structures is dramatically abnormal up to and including the isthmic signaling center at the midbrain/hindbrain junction. This study provides good evidence that Xenopus Otx2 is required for normal head patterning and the process of anterior neural specification. We propose that a mutually antagonistic relationship between Otx and Cdx factors is a basic aspect of anteroposterior patterning in all vertebrates.

Animals↗
PubMed1997

Mouse Eya genes are expressed during limb tendon development and encode a transcriptional activation function.

Vertebrate limb tendons are derived from connective cells of the lateral plate mesoderm. Some of the developmental steps leading to the formation of vertebrate limb tendons have been previously identified; however, the molecular mechanisms responsible for tendinous patterning and maintenance during embryogenesis are largely unknown. The eyes absent (eya) gene of Drosophila encodes a novel nuclear protein of unknown molecular function. Here we show that Eya1 and Eya2, two mouse homologues of Drosophila eya, are expressed initially during limb development in connective tissue precursor cells. Later in limb development, Eya1 and Eya2 expression is associated with cell condensations that form different sets of limb tendons. Eya1 expression is largely restricted to flexor tendons, while Eya2 is expressed in the extensor tendons and ligaments of the phalangeal elements of the limb. These data suggest that Eya genes participate in the patterning of the distal tendons of the limb. To investigate the molecular functions of the Eya gene products, we have analyzed whether the highly divergent PST (proline-serine-threonine)-rich N-terminal regions of Eya1-3 function as transactivation domains. Our results demonstrate that Eya gene products can act as transcriptional activators, and they support a role for this molecular function in connective tissue patterning.

Animals↗
PubMed2005

Hoxd13 and Hoxa13 directly control the expression of the EphA7 Ephrin tyrosine kinase receptor in developing limbs.

Hoxa and Hoxd genes, related to the Drosophila Abd-B gene, display regionally restricted expression patterns and are necessary for the formation of the limb skeletal elements. Hox genes encode transcription factors, which are supposed to control the expression of a series of downstream target genes, whose nature has remained largely elusive. Several genes were identified that are differentially expressed in relation to Hox gene activity; few studies, however, explored their direct regulation by Hox proteins. Ephrin tyrosine kinase receptors and ephrins have been proposed as Hox targets, and recently, evidence was gained for their role in limb development. The expression of the EphA7 gene in developing limbs was shown to correlate with the expression of Hoxa13 and Hoxd13; however, its direct regulation by these genes has never been assessed. We have characterized the EphA7 promoter region and show that it contains multiple binding sites for paralog group 13 Hox proteins. We found that one of these sites is bound in vivo by HOXA13 and HOXD13 and by endogenous Hoxd13 in developing mouse limbs. Moreover, we show that HOXD13 and HOXA13 activate transcription from the EphA7 promoter and that a mutation of the HOXA13/HOXD13 binding site was sufficient to abolish activation. Conversely, the HOXD13(147L) mutation, identified in patients displaying a novel brachydactyly-polydactyly syndrome, does not bind to in vivo, and fails to transactivate the EphA7 promoter. These results establish that EphA7 is a direct downstream target of Hoxd13 and Hoxa13 during limb development, thus providing further insight into the regulatory networks that control limb patterning.

Amino Acid Sequence↗