bluuebunny/crossref_metadata_2025 huggingface dataset:
Neural crest emigration from the neural tube depends on regulated cadherin expression
Authors: Nakagawa Shinichi , Takeichi Masatoshi
Published: April 2021

During the emergence of neural crest cells from the neural tube, the expression of cadherins dynamically changes. In the chicken embryo, the early neural tube expresses two cadherins, N-cadherin and cadherin-6B (cad6B), in the dorsal-most region where neural crest cells are generated. The expression of these two cadherins is, however, downregulated in the neural crest cells migrating from the neural tube; they instead begin expressing cadherin-7 (cad7). As an attempt to investigate the role of these changes in cadherin expression, we overexpressed various cadherin constructs, including N-cadherin, cad7, and a dominant negative N-cadherin (cN390Δ), in neural crest-generating cells. This was achieved by injecting adenoviral expression vectors encoding these molecules into the lumen of the closing neural tube of chicken embryos at stage 14. In neural tubes injected with the viruses, efficient infection was observed at the neural crest-forming area, resulting in the ectopic cadherin expression also in migrating neural crest cells. Notably, the distribution of neural crest cells with the ectopic cadherins changed depending on which constructs were expressed. Many crest cells failed to escape from the neural tube when N-cadherin or cad7 was overexpressed. Moreover, none of the cells with these ectopic cadherins migrated along the dorsolateral (melanocyte) pathway. When these samples were stained for Mitf, an early melanocyte marker, positive cells were found accumulated within the neural tube, suggesting that the failure of their migration was not due to differentiation defects. In contrast to these phenomena, cells expressing non-functional cadherins exhibited a normal migration pattern. Thus, the overexpression of a neuroepithelial cadherin (N-cadherin) and a crest cadherin (cad7) resulted in the same blocking effect on neural crest segregation from neuroepithelial cells, especially for melanocyte precursors. These findings suggest that the regulation of cadherin expression or its activity at the neural crest-forming area plays a critical role in neural crest emigration from the neural tube.
Overexpression of the forebrain-specific homeobox gene six3 induces rostral forebrain enlargement in zebrafish
Authors: Kobayashi Makoto , Toyama Reiko , Takeda Hiroyuki , Dawid Igor B. , Kawakami Kiyoshi
Published: April 2021

The Drosophila homeobox gene sine oculis is expressed in the rostral region of the embryo in early development and is essential for eye and brain formation. Its murine homolog, Six3, is expressed in the anterior neural plate and eye anlage, and may have crucial functions in eye and brain development. In this study, we describe the cloning and expression of zebrafish six3, the apparent ortholog of the mouse Six3 gene. Zebrafish six3 transcripts are first seen in hypoblast cells in early gastrula embryos and are found in the anterior axial mesendoderm through gastrulation. six3 expression in the head ectoderm begins at late gastrula. Throughout the segmentation period, six3 is expressed in the rostral region of the prospective forebrain. Overexpression of six3 in zebrafish embryos induced enlargement of the rostral forebrain, enhanced expression of pax2 in the optic stalk and led to a general disorganization of the brain. Disruption of either the Six domain or the homeodomain abolish these effects, implying that these domains are essential for six3 gene function. Our results suggest that the vertebrate Six3 genes are involved in the formation of the rostral forebrain.
Sonic hedgehog is not required for the induction of medial floor plate cells in the zebrafish
Authors: Schauerte Heike E. , Eeden Fredericus J. M. van , Fricke Cornelia , Odenthal Jörg , Strähle Uwe , Haffter Pascal
Published: April 2021

Sonic hedgehog (Shh) is a secreted protein that is involved in the organization and patterning of several tissues in vertebrates. We show that the zebrafish sonic-you (syu) gene, a member of a group of five genes required for somite patterning, is encoding Shh. Embryos mutant for a deletion of syu display defects in patterning of the somites, the lateral floor plate cells, the pectoral fins, the axons of motorneurons and the retinal ganglion cells. In contrast to mouse embryos lacking Shh activity, syu mutant embryos do form medial floor plate cells and motorneurons. Since ectopic overexpression of shh in zebrafish embryos does not induce ectopic medial floor plate cells, we conclude that shh is neither required nor sufficient to induce this cell type in the zebrafish.
A signaling cascade involving endothelin-1, dHAND and Msx1 regulates development of neural-crest-derived branchial arch mesenchyme
Authors: Thomas Tiffani , Kurihara Hiroki , Yamagishi Hiroyuki , Kurihara Yukiko , Yazaki Yoshio , Olson Eric N. , Srivastava Deepak
Published: April 2021

Numerous human syndromes are the result of abnormal cranial neural crest development. One group of such defects, referred to as CATCH-22 (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate, hypocalcemia, associated with chromosome 22 microdeletion) syndrome, exhibit craniofacial and cardiac defects resulting from abnormal development of the third and fourth neural crest-derived branchial arches and branchial arch arteries. Mice harboring a null mutation of the endothelin-1 gene (Edn1), which is expressed in the epithelial layer of the branchial arches and encodes for the endothelin-1 (ET-1) signaling peptide, have a phenotype similar to CATCH-22 syndrome with aortic arch defects and craniofacial abnormalities. Here we show that the basic helix-loop-helix transcription factor, dHAND, is expressed in the mesenchyme underlying the branchial arch epithelium. Further, dHAND and the related gene, eHAND, are downregulated in the branchial and aortic arches of Edn1-null embryos. In mice homozygous null for the dHAND gene, the first and second arches are hypoplastic secondary to programmed cell death and the third and fourth arches fail to form. Molecular analysis revealed that most markers of the neural-crest-derived components of the branchial arch are expressed in dHAND-null embryos, suggesting normal migration of neural crest cells. However, expression of the homeobox gene, Msx1, was undetectable in the mesenchyme of dHAND-null branchial arches but unaffected in the limb bud, consistent with the separable regulatory elements of Msx1 previously described. Together, these data suggest a model in which epithelial secretion of ET-1 stimulates mesenchymal expression of dHAND, which regulates Msx1 expression in the growing, distal branchial arch. Complete disruption of this molecular pathway results in growth failure of the branchial arches from apoptosis, while partial disruption leads to defects of branchial arch derivatives, similar to those seen in CATCH-22 syndrome.
The transcription factor HNF3β is required in visceral endoderm for normal primitive streak morphogenesis
Authors: Dufort Daniel , Schwartz Lois , Harpal Kendraprasad , Rossant Janet
Published: April 2021

During early embryogenesis, the transcription factor HNF3β is expressed in visceral and definitive endoderm, node, notochord and floorplate. A targeted mutation in the HNF3β gene results in the lack of a definitive node and notochord. Furthermore, lack of HNF3β results in failure of proper primitive streak elongation. To address whether HNF3β is required in visceral endoderm, we have used tetraploid embryo-ES cell aggregations to generate chimeric mouse embryos with wild-type visceral endoderm and homozygous mutant HNF3β embryonic ectoderm or vice versa. Replacing the visceral endoderm of mutant HNF3β embryos rescued proper primitive streak elongation and, conversely, mutant visceral endoderm imposed a severe embryonic-extraembryonic constriction on wild-type embryonic ectoderm. Restoration of normal streak morphogenesis was not sufficient to allow formation of the node and notochord in HNF3β mutant embryos. Thus, our results demonstrate that HNF3β has two separate roles in primitive streak formation. One is to act within the visceral endoderm to promote proper streak morphogenesis. The second is autonomous to the node and its precursors and involves specification of node and notochord cell fates. HNF3β mutant embryos rescued for the embryonic-extraembryonic constriction developed further than mutant embryos, allowing examination of later roles for HNF3β. We show that such mutant embryos lack foregut and midgut endoderm. In addition, left-right asymmetry is affected in the mutant embryos.
Divergent roles for NK-2 class homeobox genes in cardiogenesis in flies and mice
Authors: Ranganayakulu Gogineni , Elliott David A. , Harvey Richard P. , Olson Eric N.
Published: April 2021

Recent evidence suggests that cardiogenesis in organisms as diverse as insects and vertebrates is controlled by an ancient and evolutionarily conserved transcriptional pathway. In Drosophila, the NK-2 class homeobox gene tinman (tin) is expressed in cardiac and visceral mesodermal progenitors and is essential for their specification. In vertebrates, the tin homologue Nkx2-5/Csx and related genes are expressed in early cardiac and visceral mesodermal progenitors. To test for an early cardiogenic function for Nkx2-5 and to examine whether cardiogenic mechanisms are conserved, we introduced the mouse Nkx2-5 gene and various mutant and chimeric derivatives into the Drosophila germline, and tested for their ability to rescue the tin mutant phenotype. While tin itself strongly rescued both heart and visceral mesoderm, Nkx2-5 rescued only visceral mesoderm. Other vertebrate ‘non-cardiac’ NK-2 genes rescued neither. We mapped the cardiogenic domain of tin to a unique region at its N terminus and, when transferred to Nkx2-5, this region conferred a strong ability to rescue heart. Thus, the cardiac and visceral mesodermal functions of NK-2 homeogenes are separable in the Drosophila assay. The results suggest that, while tin and Nkx2-5 show close functional kinship, their mode of deployment in cardiogenesis has diverged possibly because of differences in their interactions with accessory factors. The distinct cardiogenic programs in vertebrates and flies may be built upon a common and perhaps more ancient program for specification of visceral muscle.
Interactions between Wingless and DFz2 during Drosophila wing development
Authors: Zhang Jianjun , Carthew Richard W.
Published: April 2021

Drosophila Wingless (Wg) is a secreted signaling protein of the Wnt family. Mutations in the wg gene disrupt the patterning of embryonic segments and their adult derivatives. Wg protein has been shown in cell culture to functionally interact with DFz2, a receptor that is structurally related to the tissue polarity protein Frizzled (Fz). However, it has not been determined if DFz2 functions in the Wg signaling pathway during fly development. Here we demonstrate that overexpression of DFz2 increases Wg-dependent signaling to induce ectopic margin bristle formation in developing Drosophila wings. Overexpression of a truncated form of DFz2 acts in a dominant-negative manner to block Wg signaling at the wing margin, and this block is rescued by co-expression of full-length DFz2 but not full-length Fz. Our results suggest that DFz2 and not Fz acts in the Wg signaling pathway for wing margin development. However, a truncated form of Fz also blocks Wg signaling in embryo and wing margin development, and the truncated form of DFz2 affects ommatidial polarity during eye development. These observations suggest that a single dominant-negative form of Fz or DFz2 can block more than one type of Wnt signaling pathway and imply that truncated proteins of the Fz family lose some aspect of signaling specificity.
LIN-12 protein expression and localization during vulval development in C. elegans
Authors: Levitan Diane , Greenwald Iva
Published: April 2021

We have used a LIN-12::GFP fusion protein to examine LIN-12 accumulation during cell fate decisions important for vulval development. During the naturally variable anchor cell (AC)/ventral uterine precursor cell (VU) decision of the somatic gonad, a transcription-based feedback mechanism biases two equivalent cells so that one becomes the AC while the other becomes a VU. LIN-12::GFP accumulation reflects lin-12 transcription: LIN-12::GFP is initially present in both cells, but disappears from the presumptive AC and becomes restricted to the presumptive VU. During vulval precursor cell (VPC) fate determination, six equipotential cells uniformly transcribe lin-12 and have invariant fates that are specified by multiple cell-cell interactions. The pattern of LIN-12::GFP accumulation in VPCs and in the VPC lineages is dynamic and does not always reflect lin-12 transcription. In particular, LIN-12::GFP is expressed initially in all six VPCs, but appears to be reduced specifically in P6.p as a consequence of the activation of the Ras pathway by an EGF-like inductive signal from the AC. We propose that downregulation of LIN-12 stability or translation in response to inductive signalling helps impose a bias on lateral signalling and contributes to the invariant pattern of VPC fates.
Conserved and divergent roles for members of the Snail family of transcription factors in the chick and mouse embryo
Authors: Sefton Mark , Sánchez Silvia , Nieto M. Angela
Published: April 2021

The members of the Snail family of zinc-finger transcription factors have been implicated in the formation of distinct tissues within the developing vertebrate and invertebrate embryo. Two members of this family have been described in higher vertebrates, Snail (Sna) and Slug (Slu), where they have been implicated in the formation of tissues such as the mesoderm and the neural crest. We have isolated the mouse homologue of the Slu gene enabling us to analyse and compare the amino acid sequences and the patterns of expression of both Sna and Slu in the chick and mouse. We have detected features in the sequences that allow the unequivocal ascription of any family member to the Sna or Slu subfamilies and we have observed that, during early stages of development, many of the sites of Slu and Sna expression in the mouse and chick embryo are swapped. Later in development, the sites of expression of Slu and Sna are conserved between these two species. These data, together with the data available in other species, lead us to propose that Slu and Sna arose as a duplication of an ancestor gene and that an extra duplication in the fish lineage has given rise to two Sna genes. Furthermore, several early sites of Slu and Sna expression have been swapped in the avian lineage. Our analysis of the Snail family may also shed new light on the origin of the neural crest.
Neural precursor cell chain migration and division are regulated through different β1 integrins
Authors: Jacques Thomas S. , B. Relvas João , Nishimura Stephen , Pytela Robert , M. Edwards Gwynneth , Streuli Charles H. , ffrench-Constant* Charles
Published: April 2021

Proliferation and tangential migration of neural precursor cells are essential determinants of CNS development. We have established cell culture models of both these processes using neural precursor cells grown as neurospheres. The pattern of migration that we observe in these cells is homotypic and occurs in the absence of a glial or neuronal scaffold, and is therefore equivalent to that previously described as chain migration. To determine the role of integrins in proliferation and migration, we have analysed the expression pattern of integrins on neurosphere cells and then performed blocking peptide and antibody experiments. Neurosphere cells express five major integrins, α5β1, α6Aβ1, αvβ1, αvβ5 and αvβ8 and, in addition, express low levels of α6Bβ1. Chain migration is inhibited by blocking the α6β1 integrin. Proliferation, by contrast, is inhibited by blocking the other β1 integrins, αvβ1 and α5β1. These results show that integrins are important regulators of neural precursor cell behaviour, with distinct β1 integrins regulating proliferation and migration. They also demonstrate a novel role for the α6β1 integrin in the cell-cell interactions underlying homotypic chain migration.
Wnt-7a maintains appropriate uterine patterning during the development of the mouse female reproductive tract
Authors: Miller Cary , Sassoon David A.
Published: April 2021

The murine female reproductive tract differentiates along the anteroposterior axis during postnatal development. This process is marked by the emergence of distinct cell types in the oviduct, uterus, cervix and vagina and is dependent upon specific mesenchymal-epithelial interactions as demonstrated by earlier heterografting experiments. Members of the Wnt family of signaling molecules have been recently identified in this system and an early functional role in reproductive tract development has been demonstrated. Mice were generated using ES-mediated homologous recombination for the Wnt-7a gene (Parr, B. A. and McMahon, A. P. (1995) Nature 374, 350-353). Since Wnt-7a is expressed in the female reproductive tract, we examined the developmental consequences of lack of Wnt-7a in the female reproductive tract. We observe that the oviduct lacks a clear demarcation from the anterior uterus, and acquires several cellular and molecular characteristics of the uterine horn. The uterus acquires cellular and molecular characteristics that represent an intermediate state between normal uterus and vagina. Normal vaginas have stratified epithelium and normal uteri have simple columnar epithelium, however, mutant uteri have stratified epithelium. Additionally, Wnt-7a mutant uteri do not form glands. The changes observed in the oviduct and uterus are accompanied by a postnatal loss of hoxa-10 and hoxa-11 expression, revealing that Wnt-7a is not required for early hoxa gene expression, but is required for maintenance of expression. These clustered hox genes have been shown to play a role in anteroposterior patterning in the female reproductive tract. In addition to this global posterior shift in the female reproductive tract, we note that the uterine smooth muscle is disorganized, indicating development along the radial axis is affected. Changes in the boundaries and levels of other Wnt genes are detectable at birth, prior to changes in morphologies. These results suggest that a mechanism whereby Wnt-7a signaling from the epithelium maintains the molecular and morphological boundaries of distinct cellular populations along the anteroposterior and radial axes of the female reproductive tract.
Overlapping effects of different members of the FGF family on lens fiber differentiation in transgenic mice
Authors: Lovicu Frank J. , Overbeek Paul A.
Published: April 2021

Fibroblast growth factors (FGFs), such as FGF-1, have been shown to induce differentiation of lens epithelial cells both in tissue culture and in transgenic mice. In the present study, using the αA-crystallin promoter, we generated transgenic mice that express different FGFs (FGF-4, FGF-7, FGF-8, FGF-9) specifically in the lens. All four FGFs induced changes in ocular development. Microphthalmic eyes were evident in transgenic mice expressing FGF-8, FGF-9 and some lines expressing FGF-4. A developmental study of the microphthalmic eyes revealed that, by embryonic day 15, expression of these FGFs induced lens epithelial cells to undergo premature fiber differentiation. In less severely affected lines expressing FGF-4 or FGF-7, the lens epithelial cells exhibited a premature exit from the cell cycle and underwent a fiber differentiation response later in development, leading to cataract formation. The responsiveness of lens cells to different FGFs indicates that these proteins stimulate the same or overlapping downstream signalling pathway(s). These overlapping effects of different FGFs on a common cell type indicate that the normal developmental roles for these genes are determined by the temporal and spatial regulation of their expression patterns. The fact that any of these FGFs can induce ocular defects and loss of lens transparency implies that it is essential for the normal eye to maintain very specific spatial control over FGF expression in order to prevent cataract induction.
Molecular identification of spadetail: regulation of zebrafish trunk and tail mesoderm formation by T-box genes
Authors: Griffin Kevin J. P. , Amacher Sharon L. , Kimmel Charles B. , Kimelman David
Published: April 2021

Inhibition of fibroblast growth factor (FGF) signaling prevents trunk and tail formation in Xenopus and zebrafish embryos. While the T-box transcription factor Brachyury (called No Tail in zebrafish) is a key mediator of FGF signaling in the notochord and tail, the pathways activated by FGF in non-notochordal trunk mesoderm have been uncertain. Previous studies have shown that the spadetail gene is required for non-notochordal trunk mesoderm formation; spadetail mutant embryos have major trunk mesoderm deficiencies, but relatively normal tail and notochord development. We demonstrate here that spadetail encodes a T-box transcription factor with homologues in Xenopus and chick. Spadetail is likely to be a key mediator of FGF signaling in trunk non-notochordal mesoderm, since spadetail expression is regulated by FGF signaling. Trunk and tail development are therefore dependent upon the complementary actions of two T-box genes, spadetail and no tail. We show that the regulatory hierarchy among spadetail, no tail and a third T-box gene, tbx6, are substantially different during trunk and tail mesoderm formation, and propose a genetic model that accounts for the regional phenotypes of spadetail and no tail mutants.
Determination of the identity of the derivatives of the cephalic neural crest: incompatibility between Hox gene expression and lower jaw development
Authors: Couly Gérard , Grapin-Botton Anne , Coltey Pierre , Ruhin Blandine , Douarin Nicole M. Le
Published: April 2021

In addition to pigment cells, and neural and endocrine derivatives, the neural crest is characterized by its ability to yield mesenchymal cells. In amniotes, this property is restricted to the cephalic region from the mid-diencephalon to the end of rhombomere 8 (level of somites 4/5). The cephalic neural crest is divided into two domains: an anterior region corresponding to the diencephalon, mesencephalon and metencephalon (r1, r2) in which expression of Hox genes is never observed, and a posterior domain in which neural crest cells exhibit (with a few exceptions) the same Hox code as the rhombomeres from which they originate. By altering the normal distribution of neural crest cells in the branchial arches through appropriate embryonic manipulations, we have investigated the relationships between Hox gene expression and the level of plasticity that neural crest cells display when they are led to migrate to an ectopic environment. We made the following observations. (i) Hox gene expression is not altered in neural crest cells by their transposition to ectopic sites. (ii) Expression of Hox genes by the BA ectoderm does not depend upon an induction by the neural crest. This second finding further supports the concept of segmentation of the cephalic ectoderm into ectomeres (Couly and Le Douarin, 1990). According to this concept, metameres can be defined in large bands of ectoderm including not only the CNS and the neural crest but also the corresponding superficial ectoderm fated to cover craniofacial primordia. (iii) The construction of a lower jaw requires the environment provided by the ectomesodermal components of BA1 or BA2 associated with the Hox gene non-expressing neural crest cells. Hox gene-expressing neural crest cells are unable to yield the lower jaw apparatus including the entoglossum and basihyal even in the BA1 environment. In contrast, the posterior part of the hyoid bone can be constructed by any region of the neural crest cells whether or not they are under the regulatory control of Hox genes. Such is also the case for the neural and connective tissues (including those comprising the cardiovascular system) of neural crest origin, upon which no segmental restriction is imposed. The latter finding confirms the plasticity observed 24 years ago (Le Douarin and Teillet, 1974) for the precursors of the PNS.
A distinct developmental programme for the cranial paraxial mesoderm in the chick embryo
Authors: Hacker Adam , Guthrie Sarah
Published: April 2021

Cells of the cranial paraxial mesoderm give rise to parts of the skull and muscles of the head. Some mesoderm cells migrate from locations close to the hindbrain into the branchial arches where they undergo muscle differentiation. We have characterised these migratory pathways in chick embryos either by DiI-labelling cells before migration or by grafting quail cranial paraxial mesoderm orthotopically. These experiments demonstrate that depending on their initial rostrocaudal position, cranial paraxial mesoderm cells migrate to fill the core of specific branchial arches. A survey of the expression of myogenic genes showed that the myogenic markers Myf5, MyoD and myogenin were expressed in branchial arch muscle, but at comparatively late stages compared with their expression in the somites. Pax3 was not expressed by myogenic cells that migrate into the branchial arches despite its expression in migrating precursors of limb muscles. In order to test whether segmental plate or somitic mesoderm has the ability to migrate in a cranial location, we grafted quail trunk mesoderm into the cranial paraxial mesoderm region. While segmental plate mesoderm cells did not migrate into the branchial arches, somitic cells were capable of migrating and were incorporated into the branchial arch muscle mass. Grafted somitic cells in the vicinity of the neural tube maintained expression of the somitic markers Pax3, MyoD and Pax1. By contrast, ectopic somitic cells located distal to the neural tube and in the branchial arches did not express Pax3. These data imply that signals in the vicinity of the hindbrain and branchial arches act on migrating myogenic cells to influence their gene expression and developmental pathways.
The teratogenic Veratrum alkaloid cyclopamine inhibits Sonic hedgehog signal transduction
Authors: Incardona John P. , Gaffield William , Kapur Raj P. , Roelink Henk
Published: April 2021

The steroidal alkaloid cyclopamine produces cyclopia and holoprosencephaly when administered to gastrulation-stage amniote embryos. Cyclopamine-induced malformations in chick embryos are associated with interruption of Sonic hedgehog (Shh)-mediated dorsoventral patterning of the neural tube and somites. Cell types normally induced in the ventral neural tube by Shh are either absent or appear aberrantly at the ventral midline after cyclopamine treatment, while dorsal cell types normally repressed by Shh appear ventrally. Somites in cyclopamine-treated embryos show Pax7 expression throughout, indicating failure of sclerotome induction. Cyclopamine at concentrations of 20-100 nM blocks the response of neural plate explants to recombinant Shh-N in a dose-dependent manner. Similar concentrations have no effect on the post-translational modification of Shh by cholesterol in transfected COS-1 cells. Comparison of the effects of cyclopamine to those of the holoprosencephaly-inducing cholesterol synthesis inhibitor AY-9944 shows that cyclopamine does not induce malformations by interfering with cholesterol metabolism. Although AY-9944 does not interrupt Shh signaling in ovo, it blocks the response to Shh-N in explants cultured without an exogenous cholesterol source. As predicted by current models of the regulation of cholesterol metabolism, the response to Shh-N in AY-9944-treated explants is restored by providing exogenous cholesterol. However, exogenous cholesterol does not restore Shh signaling in cyclopamine-treated explants. These findings suggest that cyclopamine-induced teratogenesis is due to a more direct antagonism of Shh signal transduction.
Developmental regulation of a cyclin-dependent kinase inhibitor controls postembryonic cell cycle progression in Caenorhabditis elegans
Authors: Hong Yang , Roy Richard , Ambros Victor
Published: April 2021

C. elegans cki-1 encodes a member of the CIP/KIP family of cyclin-dependent kinase inhibitors, and functions to link postembryonic developmental programs to cell cycle progression. The expression pattern of cki-1::GFP suggests that cki-1 is developmentally regulated in blast cells coincident with G1, and in differentiating cells. Ectopic expression of CKI-1 can prematurely arrest cells in G1, while reducing cki-1 activity by RNA-mediated interference (RNAi) causes extra larval cell divisions, suggesting a role for cki-1 in the developmental control of G1/S. cki-1 activity is required for the suspension of cell cycling that occurs in dauer larvae and starved L1 larvae in response to environmental signals. In vulva precursor cells (VPCs), a pathway of heterochronic genes acts via cki-1 to maintain VPCs in G1 during the L2 stage.
Effects of SEL-12 presenilin on LIN-12 localization and function in Caenorhabditis elegans
Authors: Levitan Diane , Greenwald Iva
Published: April 2021

Presenilins have been implicated in the development of Alzheimer’s disease and in facilitating LIN-12/Notch activity. Here, we use genetic methods to explore the relationship between C. elegans LIN-12 and SEL-12 presenilin. Reducing sel-12 activity can suppress the effects of elevated lin-12 activity when LIN-12 is activated by missense mutations but not when LIN-12 is activated by removal of the extracellular and transmembrane domains. These results suggest that SEL-12 does not function downstream of activated LIN-12. An active SEL-12::GFP hybrid protein accumulates in the perinuclear region of the vulval precursor cells (VPCs) of living hermaphrodites, consistent with a localization in endoplasmic reticulum/Golgi membranes; when sel-12 activity is reduced, less LIN-12 protein accumulates in the plasma membranes of the VPCs. Together with the genetic interactions between lin-12 and sel-12, these observations suggest a role for SEL-12 in LIN-12 processing or trafficking. However, SEL-12 does not appear to be a general factor that influences membrane protein activity, since reducing sel-12 activity does not suppress or enhance hypomorphic mutations in other genes encoding membrane proteins. We discuss potential parallels for the role of SEL-12/presenilin in facilitating LIN-12/Notch activity and in amyloid precursor protein (APP) processing.
FGFR-3 and FGFR-4 function cooperatively to direct alveogenesis in the murine lung
Authors: Weinstein Michael , Xu Xiaoling , Ohyama Kyoji , Deng Chu-Xia
Published: April 2021

Mammalian lungs begin as an outpocket of the foregut, and depend on multiple stages of branching morphogenesis and alveogenesis to reach their final form. An examination of fgf receptor gene expression indicated that all four receptors (fgfr-1 to fgfr-4) are expressed in postnatal lungs at varying levels. We show that mice homozygous for a targeted mutation of fgfr-4 exhibited no overt abnormalities in the lungs or any other organ. However, mice doubly homozygous for disruptions of the fgfr-3 and fgfr-4 genes display novel phenotypes not present in either single mutant, which include pronounced dwarfism and lung abnormalities. Lungs of fgfr-3−/−fgfr-4−/− animals, which are normal at birth, are completely blocked in alveogenesis and do not form secondary septae to delimit alveoli. Consequently, air spaces in the lung are expanded and no alveoli can be seen. The mutant lungs failed to downregulate postnatal elastin deposition despite their normal levels of surfactant expression and cell proliferation. These data revealed a cooperative function of FGFR-3 and FGFR-4 to promote the formation of alveoli during postnatal lung development.
Regulation of the Tribolium homologues of caudal and hunchback in Drosophila: evidence for maternal gradient systems in a short germ embryo
Authors: Wolff Christian , Schröder Reinhard , Schulz Cordula , Tautz Diethard , Klingler Martin
Published: April 2021

In short germ embryos, the germ rudiment forms at the posterior ventral side of the egg, while the anterior-dorsal region becomes the extra-embryonic serosa. It is difficult to see how an anterior gradient like that of bicoid in Drosophila could in these embryos be directly involved in patterning of the germ rudiment. Moreover, since it has not yet been possible to recover a bicoid homologue from any species outside the diptera, it has been speculated that the anterior bicoid gradient could be a late addition during insect evolution. We addressed this question by analysing the regulation of potential target genes of bicoid in the short germ embryo of Tribolium castaneum. We demonstrate that homologues of caudal and hunchback from Tribolium are regulated by Drosophila bicoid. In Drosophila, maternal caudal RNA is translationally repressed by bicoid. We find that Tribolium caudal RNA is also translationally repressed by bicoid, when it is transferred into Drosophila embryos under a maternal promoter. This strongly suggests that a functional bicoid homologue must exist in Tribolium. The second target gene, hunchback, is transcriptionally activated by bicoid in Drosophila. Transfer of the regulatory region of Tribolium hunchback into Drosophila also results in regulation by early maternal factors, including bicoid, but in a pattern that is more reminiscent of Tribolium hunchback expression, namely in two early blastoderm domains. Using enhancer mapping constructs and footprinting, we show that caudal activates the posterior of these domains via a specific promoter. Our experiments suggest that a major event in the evolutionary transition from short to long germ embryogenesis was the switch from activation of the hunchback gap domain by caudal to direct activation by bicoid. This regulatory switch can explain how this domain shifted from a posterior location in short germ embryos to its anterior position in long germ insects, and it also suggest how an anterior gradient can pattern the germ rudiment in short germ embryos, i.e. by regulating the expression of caudal.
Sequence-specific protein interaction with a transcriptional enhancer involved in the autoregulated expression of cAMP receptor 1 in Dictyostelium
Authors: Mu Xiuqian , Lee Brian , Louis John M. , Kimmel Alan R.
Published: April 2021

Major stages of Dictyostelium development are regulated by secreted, extracellular cAMP through activation of a serpentine receptor family. During early development, oscillations of extracellular cAMP mobilize cells for aggregation; later, continuous exposure to higher extracellular cAMP concentrations downregulates early gene expression and promotes cytodifferentiation and cell-specific gene expression. The cAMP receptor 1 gene CAR1 has two promoters that are differentially responsive to these extracellular cAMP stimuli. The early CAR1 promoter is induced by nM pulses of cAMP, which in turn are generated by CAR1-dependent activation of adenylyl cyclase (AC). Higher, non-fluctuating concentrations of cAMP will adapt this AC stimulus-response, repress the activated early promoter and induce the dormant late promoter. We now identify a critical element of the pulse-induced CAR1 promoter and a nuclear factor with sequence-specific interaction. Mutation of four nucleotides within the element prevents both in vitro protein binding and in vivo expression of an otherwise fully active early CAR1 promoter and multimerization of the wild-type, but not mutant, sequence will confer cAMP regulation to a quiescent heterologous promoter. These cis and trans elements, thus, constitute a part of the molecular response to the cAMP transmembrane signal cascade that regulates early development of Dictyostelium.
Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function
Authors: Knipper Marlies , Bandtlow Christine , Gestwa Lika , Köpschall Iris , Rohbock Karin , Wiechers Barbara , Zenner Hans-Peter , Zimmermann Ulrike
Published: April 2021

All cranial nerves, as well as the VIIIth nerve which invades the cochlea, have a proximal end in which myelin is formed by Schwann cells and a distal end which is surrounded by oligodendrocytes. The question which arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. In the present paper, we examined the relative time course of the myelinogenesis of the distal part of the VIIIth nerve by analyzing the expression of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin markers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, long before the onset of cochlea function. The expression rapidly achieved saturation levels on the approach to postnatal day 12, the day on which the cochlea function commenced. Because of its importance for the neuronal and morphological maturation of the cochlea during this time, an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea. Furthermore, an analysis of the thyroid hormone receptors, TRα and TRβ, indicates that TRβ is necessary for myelinogenesis of the VIIIth nerve. Rapid thyroid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee nerve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conduction is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.
BMPs mediate lateral inhibition at successive stages in feather tract development
Authors: Noramly Selina , Morgan Bruce A.
Published: April 2021

The spacing of feather buds in a tract is thought to arise from the interaction between an inducing signal from the dermis and an inhibitory signal generated in the nascent buds. Local BMP-2 expression in the ectoderm precedes the formation of the ectodermal placodes, which are the first morphological sign of bud differentiation. We have altered the activity of BMP-2 or BMP-4 in the ectoderm of the feather field by expressing them or their inhibitor noggin using retroviral vectors. These experiments demonstrate that BMP-2 is necessary and sufficient to mediate the lateral inhibition that positions buds in a tract. After buds are initiated, BMP-2 and BMP-4 continue to inhibit the adoption of bud fates and help to specify the size and shape of the bud. They may do so in part by their regulation of Fgf receptor expression in both the ectoderm and dermis. Additional insights into pattern formation in the feather bud can be inferred from the effects of altered BMP activity on bud morphogenesis.
CLAVATA2, a regulator of meristem and organ development in Arabidopsis
Authors: Kayes Jeffrey M. , Clark Steven E.
Published: April 2021

Mutations at the CLAVATA2 (CLV2) locus of Arabidopsis result in enlarged shoot and flower meristems, as well as alterations in the development of the gynoecia, flower pedicels, and stamens. The shoot and flower meristem phenotypes of clv2 mutants are similar to weak clv1 and clv3 mutants. We present genetic analysis that CLV2 may function in the same pathway as CLV1 and CLV3 in the regulation of meristem development, but function separately in the regulation of organ development. We also present evidence that clv2 phenotypes are altered when the mutants are grown under short-day light conditions. These alterations include flower-to-shoot transformations, as well as a nearly complete suppression of the flower phenotypes, indicating that the requirement for CLV2 changes in response to different physiological conditions. The stm-1 mutation dominantly suppresses clv2, and clv2 mutations suppress the strong stm-1 allele, but not the weak stm-2 allele.
VEGF mediates angioblast migration during development of the dorsal aorta in Xenopus
Authors: Cleaver Ondine , Krieg Paul A.
Published: April 2021

Angioblasts are precursor cells of the vascular endothelium which organize into the primitive blood vessels during embryogenesis. The molecular mechanisms underlying patterning of the embryonic vasculature remain unclear. Mutational analyses of the receptor tyrosine kinase flk-1 and its ligand vascular endothelial growth factor, VEGF, indicate that these molecules are critical for vascular development. Targeted ablation of the flk-1 gene results in complete failure of blood and vascular development (F. Shalaby et al. (1995)Nature 376, 62-66), while targeted ablation of the VEGF gene results in gross abnormalities in vascular patterning (P. Carmeliet et al. (1996)Nature 380, 435-439; N. Ferrara et al. (1996)Nature 380, 439-442). Here we report a role for VEGF in patterning the dorsal aorta of the Xenopus embryo. We show that the diffusible form of VEGF is expressed by the hypochord, which lies at the embryonic midline immediately dorsal to the location of the future dorsal aorta. We find that, initially, no flk-1- expressing angioblasts are present at this location, but that during subsequent development, angioblasts migrate from the lateral plate mesoderm to the midline where they form a single dorsal aorta. We have demonstrated that VEGF can act as a chemoattractant for angioblasts by ectopic expression of VEGF in the embryo. These results strongly suggest that localized sources of VEGF play a role in patterning the embryonic vasculature.
The life history of an embryonic signaling center: BMP-4 induces p21 and is associated with apoptosis in the mouse tooth enamel knot
Authors: Jernvall Jukka , Åberg Thomas , Kettunen Päivi , Keränen Soile , Thesleff Irma
Published: April 2021

The enamel knot, a transient epithelial structure, appears at the onset of mammalian tooth shape development. Until now, the morphological, cellular and molecular events leading to the formation and disappearance of the enamel knot have not been described. Here we report that the cessation of cell proliferation in the enamel knot in mouse molar teeth is linked with the expression of the cyclin-dependent kinase inhibitor p21. We show that p21 expression is induced by bone morphogenetic protein 4 (BMP-4) in isolated dental epithelia. As Bmp-4 is expressed only in the underlying dental mesenchyme at the onset of the enamel knot formation, these results support the role of the cyclin-dependent kinase inhibitors as inducible cell differentiation factors in epithelial-mesenchymal interactions. Furthermore, we show that the expression of p21 in the enamel knot is followed by Bmp-4 expression, and subsequently by apoptosis of the differentiated enamel knot cells. Three-dimensional reconstructions of serial sections after in situ hybridization and Tunel-staining indicated an exact codistribution of Bmp-4 transcripts and apoptotic cells. Apoptosis was stimulated by BMP-4 in isolated dental epithelia, but only in one third of the explants. We conclude that Bmp-4 may be involved both in the induction of the epithelial enamel knot, as a mesenchymal inducer of epithelial cyclin-dependent kinase inhibitors, and later in the termination of the enamel knot signaling functions by participating in the regulation of programmed cell death. These results show that the life history of the enamel knot is intimately linked to the initiation of tooth shape development and support the role of the enamel knot as an embryonic signaling center.
Muscle and tendon morphogenesis in the avian hind limb
Authors: Kardon Gabrielle
Published: April 2021

The proper development of the musculoskeletal system in the tetrapod limb requires the coordinated development of muscle, tendon and cartilage. This paper examines the morphogenesis of muscle and tendon in the developing avian hind limb. Based on a developmental series of embryos labeled with myosin and tenascin antibodies in whole mount, an integrative description of the temporal sequence and spatial pattern of muscle and tendon morphogenesis and their relationship to cartilage throughout the chick hind limb is presented for the first time. Anatomically distinct muscles arise by the progressive segregation of muscle: differentiated myotubes first appear as a pair of dorsal and ventral muscle masses; these masses subdivide into dorsal and ventral thigh, shank and foot muscle masses; and finally these six masses segregate into individual muscles. From their initial appearance, most myotubes are precisely oriented and their pattern presages the pattern of future, individual muscles. Anatomically distinct tendons emerge from three tendon primordia associated with the major joints of the limb. Contrary to previous reports, comparison of muscle and tendon reveals that much of their morphogenesis is temporally and spatially closely associated. To test whether reciprocal muscle-tendon interactions are necessary for correct muscle-tendon patterning or whether morphogenesis of each of these tissues is autonomous, two sets of experiments were conducted: (1) tendon development was examined in muscleless limbs produced by coelomic grafting of early limb buds and (2) muscle development was analyzed in limbs where tendon had been surgically altered. These experiments demonstrate that in the avian hind limb the initial morphogenetic events, formation of tendon primordia and initial differentiation of myogenic precursors, occur autonomously with respect to one another. However, later morphogenetic events, such as subdivision of muscle masses and segregation of tendon primordia into individual tendons, do require to various degrees reciprocal interactions between muscle and tendon. The dependence of these later morphogenetic events on tissue interactions differs between different proximodistal regions of the limb.
Production and activity of spore differentiation factors (SDFs) in Dictyostelium
Authors: Anjard Christophe , Chang Wen Tsan , Gross Julian , Nellen Wolfgang
Published: April 2021

SDF-1 and SDF-2 are peptides that promote terminal spore differentiation under submerged conditions. The present study shows that they accumulate differentially and are released during the development of wild-type cells and can promote spore formation in cells disaggregated from wild-type culminants. SDF-1 accumulates during the slug stage and is released in a single burst at the onset of culmination while SDF-2 accumulates during early culmination and is released in a single burst from mid-culminants. The effects of SDF-1 and SDF-2 on stalk cell formation in cell monolayers were investigated. SDF-1 by itself induces stalk cell formation in some strains and also synergizes with the stalk-cell-inducing factor, DIF-1. cAMP has an inhibitory effect on stalk cell formation when either DIF-1 or SDF-1 are present on their own but is almost not inhibitory when both are present. SDF-2 alone does not induce stalk cell formation and appears to inhibit the response to DIF-1. At the same time, it increases the extent of vacuolization of the stalk cells that are produced. We propose that the release of SDF-1 and then of SDF-2 may mark irreversible steps in the developmental programme associated, respectively, with culmination and spore maturation.