bluuebunny/crossref_metadata_2025 huggingface dataset:
Node and midline defects are associated with left-right development inDelta1mutant embryos
Authors: Przemeck Gerhard K. H. , Heinzmann Ulrich , Beckers Johannes , Hrabé de Angelis Martin
Published: December 2002

Axes formation is a fundamental process of early embryonic development. In addition to the anteroposterior and dorsoventral axes, the determination of the left-right axis is crucial for the proper morphogenesis of internal organs and is evolutionarily conserved in vertebrates. Genes known to be required for the normal establishment and/or maintenance of left-right asymmetry in vertebrates include, for example, components of the TGF-β family of intercellular signalling molecules and genes required for node and midline function. We report that Notch signalling, which previously had not been implicated in this morphogenetic process, is required for normal left-right determination in mice. We show, that the loss-of-function of the delta 1(Dll1) gene causes a situs ambiguous phenotype, including randomisation of the direction of heart looping and embryonic turning. The most probable cause for this left-right defect in Dll1 mutant embryos is a failure in the development of proper midline structures. These originate from the node, which is disrupted and deformed in Dll1 mutant embryos. Based on expression analysis in wild-type and mutant embryos, we suggest a model, in which Notch signalling is required for the proper differentiation of node cells and node morphology.
Mouse dispatched mutants fail to distribute hedgehog proteins and are defective in hedgehog signaling
Authors: Kawakami Takatoshi , Kawcak T'Nay , Li Ya-Jun , Zhang Wanhui , Hu Yongmei , Chuang Pao-Tien
Published: December 2002

Hedgehog (Hh) signaling plays a major role in multiple aspects of embryonic development, which involves both short- and long-range signaling from localized Hh sources. One unusual aspect of Hh signaling is the autoproteolytic processing of Hh followed by lipid modification. As a consequence, the N-terminal fragment of Hh becomes membrane anchored on the cell surface of Hh-producing cells. A key issue in Hh signaling is to understand the molecular mechanisms by which lipid-modified Hh protein is transported from its sites of synthesis and subsequently moves through the morphogenetic field. The dispatched gene, which encodes a putative multipass membrane protein, was initially identified in Drosophila and is required in Hh-producing cells, where it facilitates the transport of cholesterol-modified Hh. We report the identification of the mouse dispatched(Disp) gene and a phenotypic analysis of Disp mutant mice.Disp-null mice phenocopy mice deficient in the smoothened gene, an essential component for Hh reception, suggesting that Disp is essential for Hh signaling. This conclusion was further supported by a detailed molecular analysis of Disp knockout mice, which exhibit defects characteristic of loss of Hh signaling. We also provide evidence thatDisp is not required for Hh protein synthesis or processing, but rather for the movement of Hh protein from its sites of synthesis in mice. Taken together, our results reveal a conserved mechanism of Hh protein movement in Hh-producing cells that is essential for proper Hh signaling.
A role for midbrain arcs in nucleogenesis
Authors: Agarwala Seema , Ragsdale Clifton W.
Published: December 2002

Nuclei are fundamental units of vertebrate brain organization, but the mechanisms by which they are generated in development remain unclear. One possibility is that the early patterning of brain tissue into reiterated territories such as neuromeres and columns serves to allocate neurons to distinct nuclear fates. We tested this possibility in chick embryonic ventral midbrain, where a periodic pattern of molecularly distinct stripes (midbrain arcs) precedes the appearance of midbrain nuclei. We found that midbrain arc patterning has a direct relationship to the formation of nuclei. Both differential homeobox gene expression and diagnostic axon tracing studies established that the most medial arc contains primordia for two major midbrain nuclei: the oculomotor complex and the red nucleus. We tested the relationship of the medial arc to oculomotor complex and red nucleus development by perturbing arc pattern formation in Sonic Hedgehog and FGF8 misexpression experiments. We found that Sonic Hedgehog manipulations that induce ectopic arcs or expand the normal arc pattern elicit precisely parallel inductions or expansions of the red nucleus and oculomotor complex primordia. We further found that FGF8 manipulations that push the medial arc rostrally coordinately move both the red nucleus and oculomotor complex anlagen. Taken together,these findings suggest that arcs represent a patterning mechanism by which midbrain progenitor cells are allocated to specific nuclear fates.
A single cdk inhibitor, p27Xic1, functions beyond cell cycle regulation to promote muscle differentiation inXenopus
Authors: Vernon Ann E. , Philpott Anna
Published: December 2002

The molecular basis of the antagonism between cellular proliferation and differentiation is poorly understood. We have investigated the role of the cyclin-dependent kinase inhibitor p27Xic1 in the co-ordination of cell cycle exit and differentiation during early myogenesis in vivo usingXenopus embryos. In this report, we demonstrate that p27Xic1 is highly expressed in the developing myotome, that ablation of p27Xic1 protein prevents muscle differentiation and that p27Xic1 synergizes with the transcription factor MyoD to promote muscle differentiation. Furthermore, the ability of p27Xic1to promote myogenesis resides in an N-terminal domain and is separable from its cell cycle regulation function. This data demonstrates that a single cyclin-dependent kinase inhibitor, p27Xic1, controls in vivo muscle differentiation in Xenopus and that regulation of this process by p27Xic1 requires activities beyond cell cycle inhibition.
BMP signaling is required for septation of the outflow tract of the mammalian heart
Authors: Délot Emmanuèle C. , Bahamonde Matthew E. , Zhao Manxu , Lyons Karen M.
Published: December 2002

Bone morphogenetic proteins (BMPs) constitute a family of ∼20 growth factors involved in a tremendous variety of embryonic inductive processes. BMPs elicit dose-dependent effects on patterning during gastrulation and gradients of BMP activity are thought to be established through regulation of the relative concentrations of BMP receptors, ligands and antagonists. We tested whether later developmental events also are sensitive to reduced levels of BMP signaling. We engineered a knockout mouse that expresses a BMP type II receptor that lacks half of the ligand-binding domain. This altered receptor is expressed at levels comparable with the wild-type allele, but has reduced signaling capability. Unlike Bmpr2-null mice, mice homozygous for this hypomorphic receptor undergo normal gastrulation, providing genetic evidence of the dose-dependent effects of BMPs during mammalian development. Mutants,however, die at midgestation with cardiovascular and skeletal defects,demonstrating that the development of these tissues requires wild-type levels of BMP signaling. The most striking defects occur in the outflow tract of the heart, with absence of septation of the conotruncus below the valve level and interrupted aortic arch, a phenotype known in humans as persistent truncus arteriosus (type A4). In addition, semilunar valves do not form in mutants,while the atrioventricular valves appear unaffected. Abnormal septation of the heart and valve anomalies are the most frequent forms of congenital cardiac defects in humans; however, most mouse models display broad defects throughout cardiac tissues. The more restricted spectrum of cardiac anomalies inBmpr2ΔE2 mutants makes this strain a key murine model to understand the embryonic defects of persistent truncus arteriosus and impaired semilunar valve formation in humans.
Nmycupregulation by sonic hedgehog signaling promotes proliferation in developing cerebellar granule neuron precursors
Authors: Kenney Anna Marie , Cole Michael D. , Rowitch David H.
Published: December 2002

Hedgehog pathway activation is required for expansion of specific neuronal precursor populations during development and is etiologic in the human cerebellar tumor, medulloblastoma. We report that sonic hedgehog (Shh)signaling upregulates expression of the proto-oncogene Nmyc in cultured cerebellar granule neuron precursors (CGNPs) in the absence of new protein synthesis. The temporal-spatial expression pattern of Nmyc,but not other Myc family members, precisely coincides with regions of hedgehog proliferative activity in the developing cerebellum and is observed in medulloblastomas of Patched (Ptch) heterozygous mice. Overexpression of Nmyc promotes cell-autonomous G1 cyclin upregulation and CGNP proliferation independent of Shh signaling. Furthermore,Myc antagonism in vitro significantly decreases proliferative effects of Shh in cultured CGNPs. Together, these findings identify Nmyc as a direct target of the Shh pathway that functions to regulate cell cycle progression in cerebellar granule neuron precursors.
Transgenic mice expressing F3/contactin from the TAG-1 promoter exhibit developmentally regulated changes in the differentiation of cerebellar neurons
Authors: Bizzoca Antonella , Virgintino Daniela , Lorusso Loredana , Buttiglione Maura , Yoshida Lynn , Polizzi Angela , Tattoli Maria , Cagiano Raffaele , Rossi Ferdinando , Kozlov Serguei , Furley Andrew , Gennarini Gianfranco
Published: December 2002

F3/contactin (CNTN1) and TAG-1 (CNTN2) are closely related axonal glycoproteins that are differentially regulated during development. In the cerebellar cortex TAG-1 is expressed first as granule cell progenitors differentiate in the premigratory zone of the external germinal layer. However, as these cells begin radial migration, TAG-1 is replaced by F3/contactin. To address the significance of this differential regulation, we have generated transgenic mice in which F3/contactin expression is driven byTAG-1 gene regulatory sequences, which results in premature expression of F3/contactin in granule cells. These animals (TAG/F3mice) display a developmentally regulated cerebellar phenotype in which the size of the cerebellum is markedly reduced during the first two postnatal weeks but subsequently recovers. This is due in part to a reduction in the number of granule cells, most evident in the external germinal layer at postnatal day 3 and in the inner granular layer between postnatal days 8 and 11. The reduction in granule cell number is accompanied by a decrease in precursor granule cell proliferation at postnatal day 3, followed by an increase in the number of cycling cells at postnatal day 8. In the same developmental window the size of the molecular layer is markedly reduced and Purkinje cell dendrites fail to elaborate normally. These data are consistent with a model in which deployment of F3/contactin on granule cells affects proliferation and differentiation of these neurons as well as the differentiation of their synaptic partners, the Purkinje cells. Together,these findings indicate that precise spatio-temporal regulation of TAG-1 and F3/contactin expression is critical for normal cerebellar morphogenesis.
Fgfr3expression by astrocytes and their precursors: evidence that astrocytes and oligodendrocytes originate in distinct neuroepithelial domains
Authors: Pringle Nigel P. , Yu Wei-Ping , Howell Marisa , Colvin Jennifer S. , Ornitz David M. , Richardson William D.
Published: December 2002

The postnatal central nervous system (CNS) contains many scattered cells that express fibroblast growth factor receptor 3 transcripts (Fgfr3). They first appear in the ventricular zone (VZ) of the embryonic spinal cord in mid-gestation and then distribute into both grey and white matter —suggesting that they are glial cells, not neurones. TheFgfr3+ cells are interspersed with but distinct from platelet-derived growth factor receptor α (Pdgfra)-positive oligodendrocyte progenitors. This fits with the observation thatFgfr3 expression is preferentially excluded from the pMN domain of the ventral VZ where Pdgfra+ oligodendrocyte progenitors— and motoneurones — originate. Many glial fibrillary acidic protein (Gfap)- positive astrocytes co-express Fgfr3 in vitro and in vivo. Fgfr3+ cells within and outside the VZ also express the astroglial marker glutamine synthetase (Glns). We conclude that(1) Fgfr3 marks astrocytes and their neuroepithelial precursors in the developing CNS and (2) astrocytes and oligodendrocytes originate in complementary domains of the VZ. Production of astrocytes from cultured neuroepithelial cells is hedgehog independent, whereas oligodendrocyte development requires hedgehog signalling, adding further support to the idea that astrocytes and oligodendrocytes can develop independently. In addition,we found that mice with a targeted deletion in the Fgfr3 locus strongly upregulate Gfap in grey matter (protoplasmic) astrocytes, implying that signalling through Fgfr3 normally represses Gfap expression in vivo.
Six3 and Six6 activity is modulated by members of the groucho family
Authors: López-Ríos Javier , Tessmar Kristin , Loosli Felix , Wittbrodt Joachim , Bovolenta Paola
Published: December 2002

Six3 and Six6 are two genes required for the specification and proliferation of the eye field in vertebrate embryos,suggesting that they might be the functional counterparts of theDrosophila gene sine oculis (so). Phylogenetic and functional analysis have however challenged this idea, raising the possibility that the molecular network in which Six3 and Six6 act may be different from that described for SO. To address this, we have performed yeast two-hybrid screens, using either Six3 or Six6 as a bait. In this paper,we report the results of the latter screen that led to the identification of TLE1 (a transcriptional repressor of the groucho family) and AES (a potential dominant negative form of TLE proteins) as cofactors for both SIX6 and SIX3. Biochemical and mutational analysis shows that the Six domain of both SIX3 and SIX6 strongly interact with the QD domain of TLE1 and AES, but that SIX3 also interacts with TLE proteins via the WDR domain. Tle1and Aes are expressed in the developing eye of medaka fish(Oryzias latipes) embryos, overlapping with the distribution of bothSix3 and Six6. Gain-of-function studies in medaka show a clear synergistic activity between SIX3/SIX6 and TLE1,which, on its own, can expand the eye field. Conversely, AES alone decreases the eye size and abrogates the phenotypic consequences ofSIX3/6 over-expression. These data indicate that both Tle1and Aes participate in the molecular network that control eye development and are consistent with the view that both Six3 and Six6 act in combination with either Tle1 and/or Aes.
Impaired intervertebral disc formation in the absence ofJun
Authors: Behrens Axel , Haigh Jody , Mechta-Grigoriou Fatima , Nagy Andras , Yaniv Moshe , Wagner Erwin F.
Published: December 2002

Jun is a major component of the heterodimeric transcription factor AP-1 and is essential for embryonic development, as foetuses that lack Jun die at mid-gestation. Ubiquitous mosaic inactivation of a conditional Junallele by cre/LoxP-mediated recombination was used to screen for novel functions of Jun and revealed that its absence results in severe malformations of the axial skeleton. More-specific Jun deletion by collagen2a1-cre demonstrated the essential function of Jun in the notochord and sclerotome. Mutant notochordal cells showed increased apoptosis, resulting in hypocellularity of the intervertebral discs. Subsequently, fusion of vertebral bodies caused a scoliosis of the axial skeleton. Thus, Junis required for axial skeletogenesis by regulating notochord survival and intervertebral disc formation.
Genetic requirements ofvestigialin the regulation ofDrosophilawing development
Authors: Baena-López L. Alberto , García-Bellido Antonio
Published: December 2002

The gene vestigial has been proposed to act as a master gene because of its supposed capacity to initiate and drive wing development. We show that the ectopic expression of vestigial only induces ectopic outgrowths with wing cuticular differentiation and wing blade gene expression patterns in specific developmental and genetic contexts. In the process of transformation, wingless seems to be an essential but insufficient co-factor of vestigial. vestigial ectopic expression alone orvestigial plus wingless co-expression in clones differentiate `mixed' cuticular patterns (they contain wing blade trichomes and chaetae characteristic of the endogenous surrounding tissue) and express wing blade genes only in patches of cells within the clones. In addition, we have found that these clones, in the wing imaginal disc, may cause autonomous as well as non-autonomous cuticular transformations and wing blade gene expression patterns. These non-autonomous effects in surrounding cells result from recruitment or `inductive assimilation' of vestigial orwingless-vestigial overexpressing cells.
A component of the transcriptional mediator complex inhibits RAS-dependent vulval fate specification inC. elegans
Authors: Moghal Nadeem , Sternberg Paul W.
Published: December 2002

Negative regulation of receptor tyrosine kinase (RTK)/RAS signaling pathways is important for normal development and the prevention of disease in humans. We have used a genetic screen in C. elegans to identify genes that antagonize the activity of activated LET-23, a member of the EGFR family of RTKs. We identified two loss-of-function mutations in dpy-22,previously cloned as sop-1, that promote the ability of activated LET-23 to induce ectopic vulval fates. DPY-22 is a glutamine-rich protein that is most similar to human TRAP230, a component of a transcriptional mediator complex. DPY-22 has previously been shown to regulate WNT responses through inhibition of the β-catenin-like protein BAR-1. We provide evidence that DPY-22 also inhibits RAS-dependent vulval fate specification independently of BAR-1, and probably regulates the activities of multiple transcription factors during development. Furthermore, we demonstrate that although inhibition of BAR-1-dependent gene expression has been shown to require the C-terminal glutamine-rich region, this region is dispensable for inhibition of RAS-dependent cell differentiation. Thus, the glutamine-rich region contributes to specificity of this class of mediator protein.
The zinc finger transcription factorGfi1, implicated in lymphomagenesis, is required for inner ear hair cell differentiation and survival
Authors: Wallis Deeann , Hamblen Melanie , Zhou Yi , Venken Koen J. T. , Schumacher Armin , Grimes H. Leighton , Zoghbi Huda Y. , Orkin Stuart H. , Bellen Hugo J.
Published: December 2002

Gfi1 was first identified as causing interleukin 2-independent growth in T cells and lymphomagenesis in mice. Much work has shown thatGfi1 and Gfi1b, a second mouse homolog, play pivotal roles in blood cell lineage differentiation. However, neither Gfi1 norGfi1b has been implicated in nervous system development, even though their invertebrate homologues, senseless in Drosophila andpag-3 in C. elegans are expressed and required in the nervous system. We show that Gfi1 mRNA is expressed in many areas that give rise to neuronal cells during embryonic development in mouse, and that Gfi1 protein has a more restricted expression pattern. By E12.5Gfi1 mRNA is expressed in both the CNS and PNS as well as in many sensory epithelia including the developing inner ear epithelia. At later developmental stages, Gfi1 expression in the ear is refined to the hair cells and neurons throughout the inner ear. Gfi1 protein is expressed in a more restricted pattern in specialized sensory cells of the PNS, including the eye, presumptive Merkel cells, the lung and hair cells of the inner ear.Gfi1 mutant mice display behavioral defects that are consistent with inner ear anomalies, as they are ataxic, circle, display head tilting behavior and do not respond to noise. They have a unique inner ear phenotype in that the vestibular and ccchlear hair cells are differentially affected. AlthoughGfi1-deficient mice initially specify inner ear hair cells, these hair cells are disorganized in both the vestibule and cochlea. The outer hair cells of the cochlea are improperly innervated and express neuronal markers that are not normally expressed in these cells. Furthermore, Gfi1mutant mice lose all cochlear hair cells just prior to and soon after birth through apoptosis. Finally, by five months of age there is also a dramatic reduction in the number of cochlear neurons. Hence, Gfi1 is expressed in the developing nervous system, is required for inner ear hair cell differentiation, and its loss causes programmed cell death.
Pitx1andPitx2are required for development of hindlimb buds
Authors: Marcil Alexandre , Dumontier Émilie , Chamberland Michel , Camper Sally A. , Drouin Jacques
Published: December 2002

Two closely related homeobox transcription factors, Pitx1 and Pitx2, have been implicated in patterning of lateral plate mesoderm derivatives: Pitx1 for specification of hindlimb identity and Pitx2 for determination of laterality. We show that, together, Pitx1 and Pitx2 are required for formation of hindlimb buds and, when present in limited doses, for development of proximal (femur)and anterior (tibia and digit 1) hindlimb structures. Although Pitx1 is expressed throughout developing hindlimb buds, Pitx2 is not expressed in limb bud mesenchyme itself, but is co-expressed with Pitx1 in the presumptive hindlimb field before bud growth. Thus, Pitx1 and Pitx2genes are required for sustained hindlimb bud growth and formation of hindlimbs.
The cdk inhibitor p27Xic1 is required for differentiation of primary neurones inXenopus
Authors: Vernon Ann E. , Devine Christine , Philpott Anna
Published: December 2002

We have investigated the role of the cyclin-dependent kinase inhibitor,p27Xic1, in the coordination of cell cycle exit and differentiation during early neurogenesis. We demonstrate that p27Xic1 is highly expressed in cells destined to become primary neurones and is essential for an early stage of neurogenesis. Ablation of p27Xic1 protein prevents differentiation of primary neurones, while overexpressing p27Xic1promotes their formation. p27Xic1 may enhance neurogenesis by stabilising the bHLH protein, neurogenin. Moreover, the ability of p27Xic1 to stabilise neurogenin and enhance neurogenesis localises to an N-terminal domain of the molecule and is separable from its ability to inhibit the cell cycle.
Direct autoregulation and gene dosage compensation by POU-domain transcription factor Brn3a
Authors: Trieu May , Ma Ann , Eng S. Raisa , Fedtsova Natalia , Turner Eric E.
Published: December 2002

Brn3a is a POU-domain transcription factor expressed in peripheral sensory neurons and in specific interneurons of the caudal CNS. Sensory expression of Brn3a is regulated by a specific upstream enhancer, the activity of which is greatly increased in Brn3a knockout mice, implying that Brn3a negatively regulates its own expression. Brn3a binds to highly conserved sites within this enhancer, and alteration of these sites abolishes Brn3a regulation of reporter transgenes. Furthermore, endogenous Brn3a expression levels in the sensory ganglia of Brn3a+/+ andBrn3a+/- mice are similar, demonstrating that autoregulation can compensate for the loss of one allele by increasing transcription of the remaining gene copy. Conversely, transgenic overexpression of Brn3a in the trigeminal ganglion suppresses the expression of the endogenous gene. These findings demonstrate that the Brn3a locus functions as a self-regulating unit to maintain a constant expression level of this key regulator of neural development.
TheBLADE-ON-PETIOLE 1gene controls leaf pattern formation through the modulation of meristematic activity inArabidopsis
Authors: Ha Chan Man , Kim Gyung-Tae , Kim Byung Chul , Jun Ji Hyung , Soh Moon Soo , Ueno Yoshihisa , Machida Yasunori , Tsukaya Hirokazu , Nam Hong Gil
Published: December 2002

The plant leaf provides an ideal system to study the mechanisms of organ formation and morphogenesis. The key factors that control leaf morphogenesis include the timing, location and extent of meristematic activity during cell division and differentiation. We identified an Arabidopsis mutant in which the regulation of meristematic activities in leaves was aberrant. The recessive mutant allele blade-on-petiole1-1 (bop1-1)produced ectopic, lobed blades along the adaxial side of petioles of the cotyledon and rosette leaves. The ectopic organ, which has some of the characteristics of rosette leaf blades with formation of trichomes in a dorsoventrally dependent manner, was generated by prolonged and clustered cell division in the mutant petioles. Ectopic, lobed blades were also formed on the proximal part of cauline leaves that lacked a petiole. Thus, BOP1regulates the meristematic activity of leaf cells in a proximodistally dependent manner. Manifestation of the phenotypes in the mutant leaves was dependent on the leaf position. Thus, BOP1 controls leaf morphogenesis through control of the ectopic meristematic activity but within the context of the leaf proximodistality, dorsoventrality and heteroblasty.BOP1 appears to regulate meristematic activity in organs other than leaves, since the mutation also causes some ectopic outgrowths on stem surfaces and at the base of floral organs. Three class I knox genes,i.e., KNAT1, KNAT2 and KNAT6, were expressed aberrantly in the leaves of the bop1-1 mutant. Furthermore, the bop1-1 mutation showed some synergistic effect in double mutants with as1-1 oras2-2 mutation that is known to be defective in the regulation of meristematic activity and class I knox gene expression in leaves. Thebop1-1 mutation also showed a synergistic effect with thestm-1 mutation, a strong mutant allele of a class I knoxgene, STM. We, thus, suggest that BOP1 promotes or maintains a developmentally determinate state in leaf cells through the regulation of class I knox genes.
The glial cell undergoes apoptosis in the microchaete lineage ofDrosophila
Authors: Fichelson Pierre , Gho Michel
Published: December 2002

Apoptosis plays a major role in vertebrate and invertebrate development. The adult Drosophila thoracic microchaete is a mechanosensory organ whose development has been extensively studied as a model of how cell division and cell determination intermingle. This sensory organ arises from a cell lineage that produces a glial cell and four other cells that form the organ. In this study, using an in vivo approach as well as fixed material, we show that the glial cell undergoes nucleus fragmentation shortly after birth. Fragmentation was blocked after overexpression of the caspase inhibitor p35 or removal of the pro-apoptotic genes reaper, hid and grim,showing that the glial cell undergoes apoptosis. Moreover, it seems that fragments are eliminated from the epithelium by mobile macrophages. Forcing survival of the glial cells induces precocious axonal outgrowth but does not affect final axonal patterning and connectivity. However, under these conditions, glial cells do not fragment but leave the epithelium by a mechanism that is reminiscent of cell competition. Finally, we present evidences showing that glial cells are committed to apoptosis independently ofgcm and prospero expression. We suggest that apoptosis is triggered by a cell autonomous mechanism.
Dystroglycan is required for polarizing the epithelial cells and the oocyte inDrosophila
Authors: Deng Wu-Min , Schneider Martina , Frock Richard , Castillejo-Lopez Casimiro , Gaman Emily Anne , Baumgartner Stefan , Ruohola-Baker Hannele
Published: December 2002

The transmembrane protein Dystroglycan is a central element of the dystrophin-associated glycoprotein complex, which is involved in the pathogenesis of many forms of muscular dystrophy. Dystroglycan is a receptor for multiple extracellular matrix (ECM) molecules such as Laminin, agrin and perlecan, and plays a role in linking the ECM to the actin cytoskeleton;however, how these interactions are regulated and their basic cellular functions are poorly understood. Using mosaic analysis and RNAi in the model organism Drosophila melanogaster, we show that Dystroglycan is required cell-autonomously for cellular polarity in two different cell types,the epithelial cells (apicobasal polarity) and the oocyte (anteroposterior polarity). Loss of Dystroglycan function in follicle and disc epithelia results in expansion of apical markers to the basal side of cells and overexpression results in a reduced apical localization of these same markers. In Dystroglycan germline clones early oocyte polarity markers fail to be localized to the posterior, and oocyte cortical F-actin organization is abnormal. Dystroglycan is also required non-cell-autonomously to organize the planar polarity of basal actin in follicle cells, possibly by organizing the Laminin ECM. These data suggest that the primary function of Dystroglycan in oogenesis is to organize cellular polarity; and this study sets the stage for analyzing the Dystroglycan complex by using the power ofDrosophila molecular genetics.
A conserved role for the MEK signalling pathway in neural tissue specification and posteriorisation in the invertebrate chordate, the ascidianCiona intestinalis
Authors: Hudson Clare , Darras Sébastien , Caillol Danielle , Yasuo Hitoyoshi , Lemaire Patrick
Published: December 2002

Ascidians are invertebrate chordates with a larval body plan similar to that of vertebrates. The ascidian larval CNS is divided along the anteroposterior axis into sensory vesicle, neck, visceral ganglion and tail nerve cord. The anterior part of the sensory vesicle comes from the a-line animal blastomeres, whereas the remaining CNS is largely derived from the A-line vegetal blastomeres. We have analysed the role of the Ras/MEK/ERK signalling pathway in the formation of the larval CNS in the ascidian,Ciona intestinalis. We show evidence that this pathway is required,during the cleavage stages, for the acquisition of: (1) neural fates in otherwise epidermal cells (in a-line cells); and (2) the posterior identity of tail nerve cord precursors that otherwise adopt a more anterior neural character (in A-line cells). Altogether, the MEK signalling pathway appears to play evolutionary conserved roles in these processes in ascidians and vertebrates, suggesting that this may represent an ancestral chordate strategy.
`Cyclic alopecia' inMsx2mutants: defects in hair cycling and hair shaft differentiation
Authors: Ma Liang , Liu Jian , Wu Tobey , Plikus Maksim , Jiang Ting-Xin , Bi Qun , Liu Yi-Hsin , Müller-Röver Sven , Peters Heiko , Sundberg John P. , Maxson Rob , Maas Richard L. , Chuong Cheng-Ming
Published: December 2002

Msx2-deficient mice exhibit progressive hair loss, starting at P14 and followed by successive cycles of wavelike regrowth and loss. During the hair cycle, Msx2 deficiency shortens anagen phase, but prolongs catagen and telogen. Msx2-deficient hair shafts are structurally abnormal. Molecular analyses suggest a Bmp4/Bmp2/Msx2/Foxn1 acidic hair keratin pathway is involved. These structurally abnormal hairs are easily dislodged in catagen implying a precocious exogen. Deficiency in Msx2 helps to reveal the distinctive skin domains on the same mouse. Each domain cycles asynchronously — although hairs within each skin domain cycle in synchronized waves. Thus, the combinatorial defects in hair cycling and differentiation, together with concealed skin domains, account for the cyclic alopecia phenotype.
Localized JAK/STAT signaling is required for oriented cell rearrangement in a tubular epithelium
Authors: Johansen Katherine A. , Iwaki D. David , Lengyel Judith A.
Published: December 2002

Rearrangement of cells constrained within an epithelium is a key process that contributes to tubular morphogenesis. We show that activation in a gradient of the highly conserved JAK/STAT pathway is essential for orienting the cell rearrangement that drives elongation of a genetically tractable model. Using loss-of-function and gain-of-function experiments, we show that the components of the pathway from ligand to the activated transcriptional regulator STAT are required for cell rearrangement in the Drosophilaembryonic hindgut. The difference in effect between localized expression of ligand (Unpaired) and dominant active JAK (Hopscotch) demonstrates that the ligand plays a cell non-autonomous role in hindgut cell rearrangement. Taken together with the appearance of STAT92E in a gradient in the hindgut epithelium, these results support a model in which an anteroposterior gradient of ligand results in a gradient of activated STAT. These results provide the first example in which JAK/STAT signaling plays a required role in orienting cell rearrangement that elongates an epithelium.
Early determination and long-term persistence of adult-generated new neurons in the hippocampus of mice
Authors: Kempermann Gerd , Gast Daniela , Kronenberg Golo , Yamaguchi Masahiro , Gage Fred H.
Published: December 2002

New neurons are continually generated in the adult hippocampus, but the important question, whether adult neurogenesis is transient or leads to the lasting presence of new neurons, has not yet been answered. Dividing cells were labeled with bromodeoxyuridine (BrdU) and were investigated by means of immunofluorescence and confocal microscopy at several time-points 1 day to 11 months thereafter. BrdU-labeled neurons remained stable in number and in their relative position in the granule cell layer over at least 11 months. This finding implies that the addition of new neurons is not transient and that their final number and localization are determined early. By contrast,expression of immature markers β-III-tubulin and doublecortin in BrdU-labeled cells, peaked early after division and was not detectable after 4 weeks. In transgenic mice expressing enhanced green fluorescent protein under the nestin promoter none of the BrdU/nestin-positive cells early after division expressed the mature marker NeuN, confirming that no dividing neurons were detected. These new data suggest that new neurons are recruited early from the pool of proliferating progenitor cells and lead to a lasting effect of adult neurogenesis.
TheDrosophila pho-likegene encodes a YY1-related DNA binding protein that is redundant withpleiohomeoticin homeotic gene silencing
Authors: Brown J. Lesley , Fritsch Cornelia , Mueller Jürg , Kassis Judith A.
Published: December 2002

Polycomb group proteins (PcG) repress homeotic genes in cells where these genes must remain inactive during Drosophila and vertebrate development. This repression depends on cis-acting silencer sequences, called Polycomb group response elements (PREs). Pleiohomeotic (Pho), the only known sequence-specific DNA-binding PcG protein, binds to PREs but phomutants show only mild phenotypes compared with other PcG mutants. We characterize pho-like, a gene encoding a protein with high similarity to Pho. Pho-like binds to Pho-binding sites in vitro and pho-like,pho double mutants show more severe misexpression of homeotic genes than do the single mutants. These results suggest that Pho and Pho-like act redundantly to repress homeotic genes. We examined the distribution of five PcG proteins on polytene chromosomes from pho-like, pho double mutants. Pc, Psc, Scm, E(z) and Ph remain bound to polytene chromosomes at most sites in the absence of Pho and Pho-like. At a few chromosomal locations,however, some of the PcG proteins are no longer present in the absence of Pho and Pho-like, suggesting that Pho-like and Pho may anchor PcG protein complexes to only a subset of PREs. Alternatively, Pho-like and Pho may not participate in the anchoring of PcG complexes, but may be necessary for transcriptional repression mediated through PREs. In contrast to Pho and Pho-like, removal of Trithorax-like/GAGA factor or Zeste, two other DNA-binding proteins implicated in PRE function, does not cause misexpression of homeotic genes or reporter genes in imaginal disks.
EcR isoforms inDrosophila: testing tissue-specific requirements by targeted blockade and rescue
Authors: Cherbas Lucy , Hu Xiao , Zhimulev Igor , Belyaeva Elena , Cherbas Peter
Published: December 2002

The three Drosophila EcR isoforms differ only at their N termini;thus, they share the conserved ligand-binding domain transcriptional activation function (AF2) and only differ in the unconserved A/B region, which contains a second, isoform-specific, activation function (AF1). We have developed a dominant-negative mutant EcR (EcR-DN), expressed it in flies with the GAL4/UAS system, and used it to block ecdysone signaling in eight tissues or groups of tissues. Localized EcR-DN arrests ecdysone-dependent development in the target cells and often — because of a molting checkpoint —arrests development globally. Simultaneously expressing individual wild-type EcR isoforms in the same target tissues suppresses the EcR-DN phenotype and identifies the rescuing isoform as sufficient to support the development of the target. Every isoform, and even an N-terminal truncated EcR that lacks any AF1, supports development in the fat body, eye discs, salivary glands,EH-secreting neurosecretory cells and in the dpp expression domain,implying that AF1 is dispensable in these tissues. By contrast, only EcR-A is able to support development in the margins of the wing discs, and only EcR-B2 can do so in the larval epidermis and the border cells of the developing egg chamber. In light of our results, the simplest explanations for the widespread spatial and temporal variations in EcR isoform titers appear untenable.
Two modes of recruitment of E(spl) repressors onto target genes
Authors: Giagtzoglou Nikolaos , Alifragis Pavlos , Koumbanakis Konstantinos A. , Delidakis Christos
Published: December 2002

The decision of ectodermal cells to adopt the sensory organ precursor fate in Drosophila is controlled by two classes of basic-helix-loop-helix transcription factors: the proneural Ac and Sc activators promote neural fate,whereas the E(spl) repressors suppress it. We show here that E(spl) proteins m7 and mγ are potent inhibitors of neural fate, even in the presence of excess Sc activity and even when their DNA-binding basic domain has been inactivated. Furthermore, these E(spl) proteins can efficiently repress target genes that lack cognate DNA binding sites, as long as these genes are bound by Ac/Sc activators. This activity of E(spl)m7 and mγ correlates with their ability to interact with proneural activators, through which they are probably tethered on target enhancers. Analysis of reporter genes and sensory organ(bristle) patterns reveals that, in addition to this indirect recruitment of E(spl) onto enhancers via protein-protein interaction with bound Ac/Sc factors, direct DNA binding of target genes by E(spl) also takes place. Irrespective of whether E(spl) are recruited via direct DNA binding or interaction with proneural proteins, the co-repressor Groucho is always needed for target gene repression.
Activation of theknirpslocus links patterning to morphogenesis of the second wing vein inDrosophila
Authors: Lunde Karen , Trimble Jennifer L. , Guichard Annabel , Guss Kirsten A. , Nauber Ulrich , Bier Ethan
Published: December 2002

The adjacent knirps (kni) and knirps-related(knrl) genes encode functionally related zinc finger transcription factors that collaborate to initiate development of the second longitudinal wing vein (L2). kni and knrl are expressed in the third instar larval wing disc in a narrow stripe of cells just anterior to the broad central zone of cells expressing high levels of the related spaltgenes. Here, we identify a 1.4 kb cis-acting enhancer element from the kni locus that faithfully directs gene expression in the L2 primordium. We find that three independent ri alleles have alterations mapping within the L2-enhancer element and show that two of these observed lesions eliminate the ability of the enhancer element to direct gene expression in the L2 primordium. The L2 enhancer can be subdivided into distinct activation and repression domains. The activation domain mediates the combined action of the general wing activator Scalloped and a putative locally provided factor, the activity of which is abrogated by a single nucleotide alteration in the ri53j mutant. We also find that misexpression of genes in L2 that are normally expressed in veins other than L2 results in abnormal L2 development. These experiments provide a mechanistic basis for understanding how kni and knrl link AP patterning to morphogenesis of the L2 vein by orchestrating the expression of a selective subset of vein-promoting genes in the L2 primordium.
Mechanisms of cell positioning during C. elegans gastrulation
Authors: Lee Jen-Yi , Goldstein Bob
Published: December 2002

Cell rearrangements are crucial during development. In this study, we useC. elegans gastrulation as a simple model to investigate the mechanisms of cell positioning. During C. elegans gastrulation, two endodermal precursor cells move from the ventral surface to the center of the embryo, leaving a gap between these ingressing cells and the eggshell. Six neighboring cells converge under the endodermal precursors, filling this gap. Using an in vitro system, we observed that these movements occurred consistently in the absence of the eggshell and the vitelline envelope. We found that movement of the neighbors towards each other is not dependent on chemotactic signaling between these cells. We further found that C. elegans gastrulation requires intact microfilaments, but not microtubules. The primary mechanism of microfilament-based motility does not appear to be through protrusive structures, such as lamellipodia or filopodia. Instead, our results suggest an alternative mechanism. We found that myosin activity is required for gastrulation, that the apical sides of the ingressing cells contract, and that the ingressing cells determine the direction of movement of their neighboring cells. Based on these results, we propose that ingression is driven by an actomyosin-based contraction of the apical side of the ingressing cells, which pulls neighboring cells underneath. We conclude that apical constriction can function to position blastomeres in early embryos, even before anchoring junctions form between cells.
Dlx proteins position the neural plate border and determine adjacent cell fates
Authors: Woda Juliana M. , Pastagia Julie , Mercola Mark , Artinger Kristin Bruk
Published: December 2002

The lateral border of the neural plate is a major source of signals that induce primary neurons, neural crest cells and cranial placodes as well as provide patterning cues to mesodermal structures such as somites and heart. Whereas secreted BMP, FGF and Wnt proteins influence the differentiation of neural and non-neural ectoderm, we show here that members of the Dlx family of transcription factors position the border between neural and non-neural ectoderm and are required for the specification of adjacent cell fates. Inhibition of endogenous Dlx activity in Xenopus embryos with an EnR-Dlx homeodomain fusion protein expands the neural plate into non-neural ectoderm tissue whereas ectopic activation of Dlx target genes inhibits neural plate differentiation. Importantly, the stereotypic pattern of border cell fates in the adjacent ectoderm is re-established only under conditions where the expanded neural plate abuts Dlx-positive non-neural ectoderm. Experiments in which presumptive neural plate was grafted to ventral ectoderm reiterate induction of neural crest and placodal lineages and also demonstrate that Dlx activity is required in non-neural ectoderm for the production of signals needed for induction of these cells. We propose that Dlx proteins regulate intercellular signaling across the interface between neural and non-neural ectoderm that is critical for inducing and patterning adjacent cell fates.
Molecular identification of distinct neurogenic and melanogenic neural crest sublineages
Authors: Luo Rushu , Gao Juan , Wehrle-Haller Bernhard , Henion Paul D.
Published: December 2002

Clonal and lineage analyses have demonstrated that although some neural crest cells have the ability to generate multiple cell types and display self-renewal ability, other crest cells generate a single or limited repertoire of cell types. However, it is not yet clear when, and in what order, crest cells become specified to adopt a particular fate. We report that the receptor tyrosine kinases TrkC and C-Kit are expressed by distinct neural crest subpopulations in vitro. We then analyzed the lineages of individual receptor-expressing crest cells and found that TrkC-expressing cells that have just emerged from the neural tube give rise to clones containing neurons or glial cells, or both, but never produce melanocytes. A short time later,TrkC-expressing cells only generate pure neuronal clones. By contrast, from their earliest appearance in neural tube outgrowths, C-Kit-expressing cells invariably give rise to clones containing only melanocytes. Our results directly demonstrate that distinct neurogenic and melanogenic sublineages diverge before or soon after crest cells emerge from the neural tube, that fate-restricted precursors are present in nascent neural crest populations and that these sublineages can be distinguished by their cell type-specific expression of receptor tyrosine kinases.
Development of theDrosophilagenital disc requires interactions between its segmental primordia
Authors: Gorfinkiel Nicole , Sánchez Lucas , Guerrero Isabel
Published: December 2002

In both sexes, the Drosophila genital disc comprises three segmental primordia: the female genital primordium derived from segment A8,the male genital primordium derived from segment A9 and the anal primordium derived from segments A10-11. Each segmental primordium has an anterior (A)and a posterior (P) compartment, the P cells of the three segments being contiguous at the lateral edges of the disc. We show that Hedgehog (Hh)expressed in the P compartment differentially signals A cells at the AP compartment border and A cells at the segmental border. As in the wing imaginal disc, cell lineage restriction of the AP compartment border is defined by Hh signalling. There is also a lineage restriction barrier at the segmental borders, even though the P compartment cells of the three segments converge in the lateral areas of the disc. Lineage restriction between segments A9 and A10-11 depends on factors other than the Hh, En and Hox genes. The segmental borders, however, can be permeable to some morphogenetic signals. Furthermore, cell ablation experiments show that the presence of all primordia (either the anal or the genital primordium) during development are required for normal development of genital disc. Collectively, these findings suggest that interaction between segmental primordia is required for the normal development of the genital disc.
Hypogonadotropic hypogonadism and peripheral neuropathy inEbf2-null mice
Authors: Corradi Anna , Croci Laura , Broccoli Vania , Zecchini Silvia , Previtali Stefano , Wurst Wolfgang , Amadio Stefano , Maggi Roberto , Quattrini Angelo , Consalez G. Giacomo
Published: December 2002

Olf/Ebf transcription factors have been implicated in numerous developmental processes, ranging from B-cell development to neuronal differentiation. We describe mice that carry a targeted deletion within theEbf2 (O/E3) gene. In Ebf2-null mutants, because of defective migration of gonadotropin releasing hormone-synthesizing neurons, formation of the neuroendocrine axis (which is essential for pubertal development) is impaired, leading to secondary hypogonadism. In addition,Ebf2-/- peripheral nerves feature defective axon sorting,hypomyelination, segmental dysmyelination and axonal damage, accompanied by a sharp decrease in motor nerve conduction velocity. Ebf2-null mice reveal a novel genetic cause of hypogonadotropic hypogonadism and peripheral neuropathy in the mouse, disclosing an important role for Ebf2 in neuronal migration and nerve development.