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PubMed2007

Homology and the evolution of novelty during Danio adult pigment pattern development.

Recent studies using zebrafish and its relatives have provided insights into the development and evolution of adult pigment patterns. In this review, I describe how an iterative approach using a biomedical model organism and its close relatives can be used to elucidate both mechanistic and organismal aspects of pigment pattern formation. Such analyses have revealed critical roles for post-embryonic latent precursors as well as interactions among different pigment cell classes during adult pigment pattern formation and diversification. These studies also have started to reveal homologous and novel features of the underlying developmental processes.

Animals↗
PubMed2001

An acylatable residue of Hedgehog is differentially required in Drosophila and mouse limb development.

The Drosophila Hedgehog protein and its vertebrate counterpart Sonic hedgehog are required for a wide variety of patterning events throughout development. Hedgehog proteins are secreted from cells and undergo autocatalytic cleavage and cholesterol modification to produce a mature signaling domain. This domain of Sonic hedgehog has recently been shown to acquire an N-terminal acyl group in cell culture. We have investigated the in vivo role that such acylation might play in appendage patterning in mouse and Drosophila; in both species Hedgehog proteins define a posterior domain of the limb or wing. A mutant form of Sonic hedgehog that cannot undergo acylation retains significant ability to repattern the mouse limb. However, the corresponding mutation in Drosophila Hedgehog renders it inactive in vivo, although it is normally processed. Furthermore, overexpression of the mutant form has dominant negative effects on Hedgehog signaling. These data suggest that the importance of the N-terminal cysteine of mature Hedgehog in patterning appendages differs between species.

Acylation↗
PubMedDate not supplied

Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system.

Cnidarians represent the first animal phylum with an organized nervous system and a complex active behavior. The hydra nervous system is formed of sensory-motoneurons, ganglia neurons and mechanoreceptor cells named nematocytes, which all differentiate from a common stem cell. The neurons are organized as a nerve net and a subset of neurons participate in a more complex structure, the nerve ring that was identified in most cnidarian species at the base of the tentacles. In order to better understand the genetic control of this neuronal network, we analysed the expression of evolutionarily conserved regulatory genes in the hydra nervous system. The Prd-class homeogene prdl-b and the nuclear orphan receptor hyCOUP-TF are expressed at strong levels in proliferating nematoblasts, a lineage where they were found repressed during patterning and morphogenesis, and at low levels in distinct subsets of neurons. Interestingly, Prd-class homeobox and COUP-TF genes are also expressed during neurogenesis in bilaterians, suggesting that mechanoreceptor and neuronal cells derive from a common ancestral cell. Moreover, the Prd-class homeobox gene prdl-a, the Antp-class homeobox gene msh, and the thrombospondin-related gene TSP1, which are expressed in distinct subset of neurons in the adult polyp, are also expressed during early budding and/or head regeneration. These data strengthen the fact that two distinct regulations, one for neurogenesis and another for patterning, already apply to these regulatory genes, a feature also identified in bilaterian related genes.

Animals↗
PubMed2003

Shroom induces apical constriction and is required for hingepoint formation during neural tube closure.

BACKGROUND: The morphogenetic events of early vertebrate development generally involve the combined actions of several populations of cells, each engaged in a distinct behavior. Neural tube closure, for instance, involves apicobasal cell heightening, apical constriction at hingepoints, convergent extension of the midline, and pushing by the epidermis. Although a large number of genes are known to be required for neural tube closure, in only a very few cases has the affected cell behavior been identified. For example, neural tube closure requires the actin binding protein Shroom, but the cellular basis of Shroom function and how it influences neural tube closure remain to be elucidated. RESULTS: We show here that expression of Shroom is sufficient to organize apical constriction in transcriptionally quiescent, naive epithelial cells but not in non-polarized cells. Shroom-induced apical constriction was associated with enrichment of apically localized actin filaments and required the small GTPase Rap1 but not Rho. Endogenous Xenopus shroom was found to be expressed in cells engaged in apical constriction. Consistent with a role for Shroom in organizing apical constriction, disrupting Shroom function resulted in a specific failure of hingepoint formation, defective neuroepithelial sheet-bending, and failure of neural tube closure. CONCLUSIONS: These data demonstrate that Shroom is an essential regulator of apical constriction during neurulation. The finding that a single protein can initiate this process in epithelial cells establishes that bending of epithelial sheets may be patterned during development by the regulation of expression of single genes.

Actin Cytoskeleton↗
PubMed2005

Gastrulation movements: the logic and the nuts and bolts.

Gastrulation, the period during the early development of animals when major cell and tissue movements remodel an initially unstructured group of cells, requires coordinated control of different types of cellular activities in different cell populations. A hierarchy of genetic control mechanisms, involving cell signaling and transcriptional regulation, sets up the embryonic axes and specify the territories of the future germ layers. Cells in these territories modulate their cytoskeleton and their adhesive behavior, resulting in shape changes and movement. Similarities among different species in patterning and cell biological mechanisms are beginning to allow us to recognize general, conserved principles and speculate on possible ancestral mechanisms of gastrulation.

Animals↗
PubMed2006

Evolution of the ventral midline in insect embryos.

The ventral midline is a source of signals that pattern the nerve cord of insect embryos. In dipterans such as the fruitfly Drosophila melanogaster (D. mel.) and the mosquito Anopheles gambiae (A. gam.), the midline is narrow and spans just 1-2 cells. However, in the honeybee, Apis mellifera (A. mel.), the ventral midline is broad and encompasses 5-6 cells. slit and other midline-patterning genes display a corresponding expansion in expression. Evidence is presented that this difference is due to divergent cis regulation of the single-minded (sim) gene, which encodes a bHLH-PAS transcription factor essential for midline differentiation. sim is regulated by a combination of Notch signaling and a Twist (Twi) activator gradient in D. mel., but it is activated solely by Twi in A. mel. We suggest that the Twi-only mode of regulation--and the broad ventral midline--represents the ancestral form of CNS patterning in Holometabolous insects.

Animals↗
PubMed2003

Patterning of forelimb bud myogenic precursor cells requires retinoic acid signaling initiated by Raldh2.

Limb skeletal muscle is derived from cells of the dermomyotome that detach and migrate into the limb buds to form separate dorsal and ventral myogenic precursor domains. Myogenic precursor cell migration is dependent on limb bud mesenchymal expression of hepatocyte growth factor/scatter factor (Hgf), which encodes a secreted ligand that signals to dermomyotome through the membrane receptor tyrosine kinase Met. Here, we find that correct patterning of Hgf expression in forelimb buds is dependent on retinoic acid (RA) synthesized by retinaldehyde dehydrogenase 2 (Raldh2) expressed proximally. Raldh2(-/-) forelimb buds lack RA and display an anteroproximal shift in expression of Hgf such that its normally separate dorsal and ventral expression domains are joined into a single anterior-proximal domain. Met and MyoD are expressed in this abnormal domain, indicating that myogenic cell migration and differentiation are occurring in the absence of RA, but in an abnormal location. An RA-reporter transgene revealed that RA signaling in the forelimb bud normally exists in a gradient across the proximodistal axis, but uniformly across the anteroposterior axis, with all proximal limb bud cells exhibiting activity. Expression of Bmp4, an inhibitor of Hgf expression, is increased and shifted anteroproximally in Raldh2(-/-) limb buds, thus encroaching into the normal expression domain of Hgf. Our studies suggest that RA signaling provides proximodistal information for limb buds that counterbalances Bmp signaling, which in turn helps mediate proximodistal and anteroposterior patterning of Hgf expression to correctly direct migration of Met-expressing myogenic precursor cells.

Aldehyde Oxidoreductases↗
PubMed2000

Molecular motors: the driving force behind mammalian left-right development.

The molecular motors dynein and kinesin are large protein complexes that convert the energy generated by ATP hydrolysis into directional movement along the microtubule cytoskeleton. They are required for a myriad of cellular processes, including mitotic spindle movement, axonal and vesicular transport, and ciliary beating. Recently, it has been shown that, in addition, they have a unique role during embryonic patterning: they are required to orient and establish the left-right axis in early vertebrate development.

Animals↗
PubMed1997

Drosophila CBP is a co-activator of cubitus interruptus in hedgehog signalling.

The transcription factor CBP, originally identified as a coactivator for CREB, enhances transcription mediated by many other transcription factors. Mutations in the human CBP gene are associated with Rubinstein-Taybi syndrome, a haploinsufficiency disorder characterized by abnormal pattern formation, but the mechanism by which decreased CBP levels affect pattern formation is unclear. The hedgehog (hh) signalling pathway is an important determinant of pattern formation. cubitus interruptus (ci), a component in hh signalling, encodes a transcription factor homologous to the Gli family of proteins and is required for induction of the hh-dependent expression of patched (ptc), decapentaplegic (dpp) and wingless (wg). Haploinsufficiency for the ci-related transcription factor Gli3 causes phenotypic changes in mice (known as 'extra-toes) and humans (Greig's cephalopolysyndactyly syndrome) that have similarities to Rubinstein-Taybi syndrome. Here we show that Drosophila CBP (dCBP) functions as a coactivator of Ci, suggesting that the dCBP-Ci interaction may shed light on the contribution of CBP to pattern formation in mammals.

Animals↗
PubMed2003

Traveling stripes on the skin of a mutant mouse.

In the course of animal development, complex structures form autonomously from the apparently shapeless egg. How cells can produce spatial patterns that are much larger than each cell is one of the key issues in developmental biology. It has been suggested that spatial patterns in animals form through the same principles by which dispatched structures are formed in the nonbiological system. However, because of the complexity of biological systems, molecular details of such phenomena have been rarely clarified. In this article, we introduce an example of a pattern-forming phenomenon that occurs in the skin of mutant mice. The mutant mouse has a defect in splicing of the Foxn1 (Whn or nude) gene, which terminates hair follicle development just after pigment begins to accumulate in the follicle. The immature follicles are rapidly discharged, and a new hair cycle resumes. Eventually, the skin color of the mouse appears to oscillate. The color oscillation is synchronous in juvenile mice, but the phase gradually shifts among skin regions to eventually form traveling, evenly spaced stripes. Although the time scale is quite different, the pattern change in the mutant mouse shares characteristics with the nonlinear waves generated on excitable media, such as the Belousov-Zhabotinskii reaction, suggesting that a common principle underlies the wave pattern formation. Molecular details that underlie the phenomenon can be conjectured from recent molecular studies.

Age Factors↗
PubMed2005

Positional information and mobile transcriptional regulators determine cell pattern in the Arabidopsis root epidermis.

The root epidermis is a model system for deciphering the mechanism underpinning the formation of cellular pattern in planar groups of cells. The epidermis comprises rows of hair cells (H) and non-hair-bearing epidermal cells (N). Laser ablation and clonal analysis have shown that the fates of epidermal cells are flexible through development and that positional information which may be located in the cell wall or extracellular matrix determines cell fate. A leucine-rich repeat protein called SCRAMBLED is required for the development of cell pattern which may be involved in the perception of positional information. It is proposed that positional signals then initiate the cell-specific expression of a number of transcription factors that complete the patterning process, resulting in the expression of hair-promoting genes in hair cells (H) and their repression in the hairless cells (N).

Arabidopsis↗
PubMed1999

Ritonavir combination therapy restores intestinal function in children with advanced HIV disease.

OBJECTIVES: To investigate the intestinal absorptive processes in children with HIV infection before and after treatment with combination therapy that includes ritonavir. To test the hypothesis that combination therapy improves intestinal function. DESIGN: Intestinal function tests were performed in 10 children with advanced HIV disease at the enrollment and after 3 and 6 months of therapy with ritonavir combined with two HIV reverse transcriptase inhibitors. HIV viral load and CD4 cell counts were also determined; body weight was monitored. METHODS: The D-xylose absorption test, the steatocrit and the determination of fecal alpha1-antitrypsin concentration were used to evaluate carbohydrate and fat absorption, as well as fecal protein loss. Serum iron levels were measured to indirectly evaluate iron absorption. HIV-1 RNA-polymerase chain reaction (PCR) and immunofluorescence imaging were used to evaluate virologic and immunologic responses. RESULTS: In all, 9 children had carbohydrate malabsorption, 3 steatorrhea, 2 protein loss, and 7 iron deficiency. Most tests produced normal results after 3 months of therapy, and all abnormalities were abolished 6 months after institution of combination therapy. Mean results of each of four absorption tests were significantly changed on combination therapy. Viral load was progressively reduced and CD4 count was increased, with an inverse relationship. An evident shift of body weight pattern toward catch-up growth was observed in all children. CONCLUSIONS: Ritonavir combination therapy results in prompt and sustained restoration of intestinal function, which is associated with reduction in viral load, increase in CD4 counts, and gain in body weight.

Adolescent↗
PubMed2001

FoxH1 (Fast) functions to specify the anterior primitive streak in the mouse.

The node and the anterior visceral endoderm (AVE) are important organizing centers that pattern the mouse embryo by establishing the anterior-posterior (A-P), dorsal-ventral (D-V), and left-right (L-R) axes. Activin/nodal signaling through the Smad2 pathway has been implicated in AVE formation and in morphogenesis of the primitive streak, the anterior end of which gives rise to the node. The forkhead DNA-binding protein, FoxH1 (or Fast), functions as a Smad DNA-binding partner to regulate transcription in response to activin signaling. Here, we show that deletion of FoxH1 in mice results in failure to pattern the anterior primitive streak (APS) and form node, prechordal mesoderm, notochord, and definitive endoderm. In contrast, formation of the AVE can occur in the absence of FoxH1. The FoxH1 mutant phenotype is remarkably similar to that of mice deficient in the forkhead protein Foxa2 (HNF3beta), and we show that Foxa2 expression is dependent on FoxH1 function. These results show that FoxH1 functions in an activin/nodal-Smad signaling pathway that acts upstream of Foxa2 and is required specifically for patterning the APS and node in the mouse.

Activins↗
PubMed2000

Molecular analysis of the SCARECROW gene in maize reveals a common basis for radial patterning in diverse meristems.

Maize and Arabidopsis root apical meristems differ in several aspects of their radial organization and ontogeny. Despite the large evolutionary distance and differences in root radial patterning, analysis of the putative maize ortholog of the Arabidopsis patterning gene SCARECROW (SCR) revealed expression localized to the endodermis, which is similar to its expression in Arabidopsis. Expression in maize extends through the quiescent center, a population of mitotically inactive cells formerly thought to be undifferentiated and to lack radial pattern information. Zea mays SCARECROW (ZmSCR), the putative maize SCR ortholog, was used as a molecular marker to investigate radial patterning during regeneration of the root tip after either whole or partial excision. Analysis of the dynamic expression pattern of ZmSCR as well as other markers indicates the involvement of positional information as a primary determinant in regeneration of the root radial pattern.

Amino Acid Sequence↗
PubMed2005

Connexin 40, a target of transcription factor Tbx5, patterns wrist, digits, and sternum.

Haploinsufficiency of T-box transcription factor 5 (TBX5) causes human Holt-Oram syndrome (HOS), a developmental disorder characterized by skeletal and heart malformations. Mice carrying a Tbx5 null allele (Tbx5(+/Delta)) have malformations in digits, wrists, and sternum joints, regions where Tbx5 is expressed. We demonstrate that mice deficient in connexin 40 (Cx40), a Tbx5-regulated gap junction component, shared axial and appendicular skeletal malformations with Tbx5(+/Delta) mice. Although no role in skeleton patterning has been described for gap junctions, we demonstrate here that Cx40 is involved in formation of specific joints, as well as bone shape. Even a 50% reduction in either Tbx5 or Cx40 produces bone abnormalities, demonstrating their crucial control over skeletal development. Further, we demonstrate that Tbx5 exerts in part its key regulatory role in bone growth and maturation by controlling via Cx40 the expression of Sox9 (a transcription factor essential for chondrogenesis and skeleton growth). Our study strongly suggests that Cx40 deficiency accounts for many skeletal malformations in HOS and that Tbx5 regulation of Cx40 plays a critical role in the exquisite developmental patterning of the forelimbs and sternum.

Animals↗
PubMed2003

The T-box-encoding Dorsocross genes function in amnioserosa development and the patterning of the dorsolateral germ band downstream of Dpp.

Dpp signals are responsible for establishing a variety of cell identities in dorsal and lateral areas of the early Drosophila embryo, including the extra-embryonic amnioserosa as well as different ectodermal and mesodermal cell types. Although we have a reasonably clear picture of how Dpp signaling activity is modulated spatially and temporally during these processes, a better understanding of how these signals are executed requires the identification and characterization of a collection of downstream genes that uniquely respond to these signals. In the present study, we describe three novel genes, Dorsocross1, Dorsocross2 and Dorsocross3, which are expressed downstream of Dpp in the presumptive and definitive amnioserosa, dorsal ectoderm and dorsal mesoderm. We show that these genes are good candidates for being direct targets of the Dpp signaling cascade. Dorsocross expression in the dorsal ectoderm and mesoderm is metameric and requires a combination of Dpp and Wingless signals. In addition, a transverse stripe of expression in dorsoanterior areas of early embryos is independent of Dpp. The Dorsocross genes encode closely related proteins of the T-box domain family of transcription factors. All three genes are arranged in a gene cluster, are expressed in identical patterns in embryos, and appear to be genetically redundant. By generating mutants with a loss of all three Dorsocross genes, we demonstrate that Dorsocross gene activity is crucial for the completion of differentiation, cell proliferation arrest, and survival of amnioserosa cells. In addition, we show that the Dorsocross genes are required for normal patterning of the dorsolateral ectoderm and, in particular, the repression of wingless and the ladybird homeobox genes within this area of the germ band. These findings extend our knowledge of the regulatory pathways during amnioserosa development and the patterning of the dorsolateral embryonic germ band in response to Dpp signals.

Amino Acid Sequence↗
PubMed2004

Temporal requirement for hedgehog signaling in ventral telencephalic patterning.

Hedgehog signaling is required for multiple aspects of brain development, including growth, the establishment of both dorsal and ventral midline patterning and the generation of specific cell types such as oligodendrocytes and interneurons. To identify more precisely when during development hedgehog signaling mediates these events, we directed the removal of hedgehog signaling within the brain by embryonic day 9 of development, using a FoxG1(Cre) driver line to mediate the removal of a conditional smoothened null allele. We observed a loss of ventral telencephalic patterning that appears to result from an initial lack of specification of these structures rather than by changes in proliferation or cell death. A further consequence of the removal of smoothened in these mice is the near absence of both oligodendrocytes and interneurons. Surprisingly, the dorsal midline appears to be patterned normally in these mutants. Together with previous analyses, the present results demonstrate that hedgehog signaling in the period between E9.0 and E12 is essential for the patterning of ventral regions and the generation of cell types that are thought to largely arise from them.

Animals↗
PubMed2005

Patterning across the ascidian neural plate by lateral Nodal signalling sources.

Ascidians are invertebrate chordates with a simple larval tadpole form containing a notochord and an overlying dorsal neural tube. As in vertebrates, the neural tube of ascidian larvae displays positional differences along the rostral-caudal and dorsal-ventral axes in terms of neuronal cell types generated, morphology and gene expression. However, how these differences are established in this simple chordate remains largely unknown. In this study, we show that a single blastomere named b6.5, which is situated in a lateral position in the 32-cell-stage embryo, is a source of signal(s) required for patterning across the medial-lateral axis (future ventral-dorsal axis) of the neural plate. We identify this signal as a Ciona homologue of Nodal, Ci-Nodal. Transcriptional activation of Ci-Nodal in b6.5 depends upon vegetally derived Ci-FGF9/16/20. Using three distinct reagents to inhibit Nodal signals, we show that Nodal signalling is required for neural plate patterning across the medial-lateral axis and that, in the absence of this signal, the caudal-lateral part of the neural plate adopts a medial-like fate. Secondary muscle fate is similarly affected. We conclude that specification of the lateral neural plate is initiated by signalling sources laterally flanking the neural plate and involves a cell-fate choice between lateral and medial neural fates, with Nodal signalling promoting lateral fate. This role for Nodal signalling during ascidian neural plate patterning contrasts with that in vertebrates, where it is implicated in promoting a medial neural fate, the floor plate.

Animals↗