bluuebunny/crossref_metadata_2025_split huggingface dataset:
Rac1 mutations produce aberrant epithelial differentiation in the developing and adult mouse small intestine
Authors: Stappenbeck Thaddeus S. , Gordon Jeffrey I. 
Published: April 2021

ABSTRACT
The mouse small intestinal epithelium undergoes continuous renewal throughout life. Previous studies suggest that differentiation of this epithelium is regulated by instructions that are received as cells migrate along crypt-villus units. The nature of the instructions and their intracellular processing remain largely undefined. In this report, we have used genetic mosaic analysis to examine the role of Rac1 GTPase-mediated signaling in controlling differentiation. A constitutively active mutation (Rac1Leu61) or a dominant negative mutation (Rac1Asn17) was expressed in the 129/Sv embryonic stem cell-derived component of the small intestine of C57Bl/6-ROSA26↔129/Sv mice. Rac1Leu61 induces precocious differentiation of members of the Paneth cell and enterocytic lineages in the proliferative compartment of the fetal gut, without suppressing cell division. Forced expression of the dominant negative mutation inhibits epithelial differentiation, without affecting cell division, and slows enterocytic migration along crypt-villus units. The effects produced by Rac1Leu61 or Rac1Asn17 in the 129/Sv epithelium do not spread to adjacent normal C57Bl/6 epithelial cells. These results provide in vivo evidence that Rac1 is involved in the import and intracellular processing of signals that control differentiation of a mammalian epithelium.
Function and regulation of homothorax in the wing imaginai disc of Drosophila
Authors: Azpiazu Natalia , Morata Ginés 
Published: April 2021

ABSTRACT
The gene homothorax (hth) is originally expressed uniformly in the wing imaginai disc but, during development, its activity is restricted to the cells that form the thorax and the hinge, where the wing blade attaches to the thorax, and eliminated in the wing pouch, which forms the wing blade. We show that hth repression in the wing pouch is a prerequisite for wing development; forcing hth expression prevents growth of the wing blade. Both the Dpp and the Wg pathways are involved in hth repression. Cells unable to process the Dpp (lacking thick veins or Mothers against Dpp activity) or the Wg (lacking dishevelled function) signal express hth in the wing pouch. We have identified vestigial (vg) as a Wg and Dpp response factor that is involved in hth control. In contrast to its repressing role in the wing pouch, wg upregulates hth expression in the hinge. We have also identified the gene teashirt (tsh) as a positive regulator of hth in the hinge. tsh plays a role specifying hinge structures, possibly in co-operation with hth.
Three-dimensional in vivo analysis of Dictyostelium mounds reveals directional sorting of prestalk cells and defines a role for the myosin II regulatory light chain in prestalk cell sorting and tip protrusion
Authors: Clow Patricia A. , Chen Tung-Ling L. , Chisholm Rex L. , McNally James G. 
Published: April 2021

ABSTRACT
During cell sorting in Dictyostelium, we observed that GFP-tagged prestalk cells (ecmAO-expressing cells) moved independently and directionally to form a cluster. This is consistent with a chemotaxis model for cell sorting (and not differential adhesion) in which a long-range signal attracts many of the prestalk cells to the site of cluster formation. Surprisingly, the ecmAO prestalk cluster that we observed was initially found at a random location within the mound of this Ax3 strain, defining an intermediate sorting stage not widely reported in Dictyostelium. The cluster then moved en masse to the top of the mound to produce the classic, apical pattern of ecmAO prestalk cells. Migration of the cluster was also directional, suggesting the presence of another long-range guidance cue. Once at the mound apex, the cluster continued moving upward leading to protrusion of the mound’s tip. To investigate the role of the cluster in tip protrusion, we examined ecmAO prestalk-cell sorting in a myosin II regulatory light chain (RLC) null in which tips fail to form. In RLC-null mounds, ecmAO prestalk cells formed an initial cluster that began to move to the mound apex, but then arrested as a vertical column that extended from the mound’s apex to its base. Mixing experiments with wild-type cells demonstrated that the RLC-null ecmAO prestalk-cell defect is cell autonomous. These observations define a specific mechanism for myosin’s function in tip formation, namely a mechanical role in the upward movement of the ecmAO prestalk cluster. The wild-type data demonstrate that cell sorting can occur in two steps, suggesting that, in this Ax3 strain, spatially and temporally distinct cues may guide prestalk cells first to an initial cluster and then later to the tip.
Region-specific activation of theXenopus Brachyurypromoter involves active repression in ectoderm and endoderm: a study using transgenic frog embryos
Authors: Lerchner Walter , Latinkic Branko V. , Remacle Jacques E. , Huylebroeck Danny , Smith James C. 
Published: April 2021

ABSTRACTTissue specification in the early embryo requires the integration of spatial information at the promoters of developmentally important genes. Although several response elements for signalling pathways have been identified in Xenopus promoters, it is not yet understood what defines the sharp borders that restrict expression to a specific tissue. Here we use transgenic frog embryos to study the spatial and temporal regulation of the Xbra promoter. Deletion analysis and point mutations in putative transcription factor-binding sites identified two repressor modules, which exert their main effects at different stages during gastrulation. One module is defined by a bipartite binding site for a Smad-interacting protein (SIP1) of the δEF1 repressor family and acts to confine expression to the marginal zone early in gastrulation. The other module is defined by two homeodomain-binding sites and is responsible for repression in dorsal mesoderm and ectoderm at mid-gastrula stages. In addition, an upstream region of the promoter is necessary to repress expression in neural tissues later in development. Together, our results show that repression plays an important role in the restriction of Xbra expression to the mesoderm, and we suggest that similar mechanisms may be involved in the spatial regulation of other genes in early embryonic development.
Analysis of cranial neural crest migratory pathways in axolotl using cell markers and transplantation
Authors: Epperlein Hans-Henning , Meulemans Daniel , Bronner-Fraser Marianne , Steinbeisser Herbert , Selleck Mark A. J. 
Published: April 2021

ABSTRACT
We have examined the ability of normal and heterotopically transplanted neural crest cells to migrate along cranial neural crest pathways in the axolotl using focal DiI injections and in situ hybridization with the neural crest marker, AP-2. DiI labeling demonstrates that cranial neural crest cells migrate as distinct streams along prescribed pathways to populate the maxillary and mandibular processes of the first branchial arch, the hyoid arch and gill arches 1-4, following migratory pathways similar to those observed in other vertebrates. Another neural crest marker, the transcription factor AP-2, is expressed by premigratory neural crest cells within the neural folds and migrating neural crest cells en route to and within the branchial arches. Rotations of the cranial neural folds suggest that premigratory neural crest cells are not committed to a specific branchial arch fate, but can compensate when displaced short distances from their targets by migrating to a new target arch. In contrast, when cells are displaced far from their original location, they appear unable to respond appropriately to their new milieu such that they fail to migrate or appear to migrate randomly. When trunk neural folds are grafted heterotopically into the head, trunk neural crest cells migrate in a highly disorganized fashion and fail to follow normal cranial neural crest pathways. Importantly, we find incorporation of some trunk cells into branchial arch cartilage despite the random nature of their migration. This is the first demonstration that trunk neural crest cells can form cartilage when transplanted to the head. Our results indicate that, although cranial and trunk neural crest cells have inherent differences in ability to recognize migratory pathways, trunk neural crest can differentiate into cranial cartilage when given proper instructive cues.
Endogenous patterns of TGFβ superfamily signaling during early Xenopus development
Authors: Faure Sandrine , Lee Michelle A. , Keller Tracy , Dijke Peter ten , Whitman Malcolm 
Published: April 2021

ABSTRACT
Transforming growth factor β (TGFβ) superfamily signaling has been implicated in patterning of the early Xenopus embryo. Upon ligand stimulation, TGFβ receptors phosphorylate Smad proteins at carboxy-terminal SS(V/M)S consensus motifs. Smads 1/5/8, activated by bone morphogenetic protein (BMP) signaling, induce ventral mesoderm whereas Smad2, activated by activin-like ligands, induces dorsal mesoderm. Although ectopic expression studies are consistent with roles for TGFβ signals in early Xenopus embryogenesis, when and where BMP and activin-like signaling pathways are active endogenously has not been directly examined.
In this study, we investigate the temporal and spatial activation of TGFβ superfamily signaling in early Xenopus development by using antibodies specific for the type I receptor-phosphorylated forms of Smad1/5/8 and Smad2. We find that Smad1/5/8 and two distinct isoforms of Smad2, full-length Smad2 and Smad2Δexon3, are phosphorylated in early embryos. Both Smad1/5/8 and Smad2/Smad2Δexon3 are activated after, but not before, the mid-blastula transition (MBT). Endogenous activation of Smad2/Smad2Δexon3 requires zygotic transcription, while Smad1/5/8 activation at MBT appears to involve transcription-independent regulation. We also find that the competence of embryonic cells to respond to TGFβ superfamily ligands is temporally regulated and may be a determinant of early patterning.
Levels of phospho-Smad1/5/8 and of phospho- Smad2/Smad2Δexon3 are asymmetrically distributed across both the animal-vegetal and dorsoventral axes. The timing of the development of these asymmetries differs for phospho- Smad1/5/8 and for phospho-Smad2/Smad2Δexon3, and the spatial distribution of phosphorylation of each Smad changes dramatically as gastrulation begins. We discuss the implications of our results for endogenous functions of BMP and activin-like signals as candidate morphogens regulating primary germ layer formation and dorsoventral patterning of the early Xenopus embryo.
The bHLH gene Hes6, an inhibitor of Hes1, promotes neuronal differentiation
Authors: Bae Soo-Kyung , Bessho Yasumasa , Hojo Masato , Kageyama Ryoichiro 
Published: April 2021

ABSTRACT
We have isolated the basic helix-loop-helix (bHLH) gene Hes6, a novel member of the family of mammalian homologues of Drosophila hairy and Enhancer of split. Hes6 is expressed by both undifferentiated and differentiated cells, unlike Hes1, which is expressed only by the former cells. Hes6 alone does not bind to the DNA but suppresses Hes1 from repressing transcription. In addition, Hes6 suppresses Hes1 from inhibiting Mash1-E47 heterodimer and thereby enables Mash1 and E47 to upregulate transcription in the presence of Hes1. Furthermore, misexpression of Hes6 with retrovirus in the developing retina promotes rod photoreceptor differentiation, like Mash1, in sharp contrast to Hes1, which inhibits cell differentiation. These results suggest that Hes6 is an inhibitor of Hes1, supports Mash1 activity and promotes cell differentiation. Mutation analysis revealed that Hes1-and Hes6-specific functions are, at least in part, interchangeable by alteration of the loop region, suggesting that the loop is not simply a nonfunctional spacer but plays an important role in the specific functions.
Mechanisms regulating target gene selection by the homeodomain-containing protein Fushi tarazu
Authors: Nasiadka Andrzej , Grill Allan , Krause Henry M. 
Published: April 2021

ABSTRACT
Homeodomain proteins are DNA-binding transcription factors that control major developmental patterning events. Although DNA binding is mediated by the homeodomain, interactions with other transcription factors play an unusually important role in the selection and regulation of target genes. A major question in the field is whether these cofactor interactions select target genes by modulating DNA binding site specificity (selective binding model), transcriptional activity (activity regulation model) or both. A related issue is whether the number of target genes bound and regulated is a small or large percentage of genes in the genome. In this study, we have addressed these issues using a chimeric protein that contains the strong activation domain of the viral VP16 protein fused to the Drosophila homeodomain-containing protein Fushi tarazu (Ftz). We find that genes previously thought not to be direct targets of Ftz remain unaffected by FtzVP16. Addition of the VP16 activation domain to Ftz does, however, allow it to regulate previously identified target genes at times and in regions that Ftz alone cannot. It also changes Ftz into an activator of two genes that it normally represses. Taken together, the results suggest that Ftz binds and regulates a relatively limited number of target genes, and that cofactors affect target gene specificity primarily by controlling binding site selection. Activity regulation then fine-tunes the temporal and spatial domains of promoter responses, the magnitude of these responses, and whether they are positive or negative.
Measuring dimensions: the regulation of size and shape
Authors: Day Stephen J. , Lawrence Peter A. 
Published: April 2021

ABSTRACT
Over many years evidence has accumulated that plants and animals can regulate growth with reference to overall size rather than cell number. Thus, organs and organisms grow until they reach their characteristic size and shape and then they stop – they can even compensate for experimental manipulations that change, over several fold, cell number or average cell size. If the cell size is altered, the organism responds with a change in cell number and vice versa. We look at the Drosophila wing in more detail: here, both extracellular and intracellular regulators have been identified that link cell growth, division and cell survival to final organ size. We discuss a hypothesis that the local steepness of a morphogen gradient is a measure of length in one axis, a measure that is used to determine whether there will be net growth or not.
Role of frizzled 7 in the regulation of convergent extension movements during gastrulation in Xenopus laevis
Authors: Djiane Alexandre , Riou Jean-François , Umbhauer Muriel , Boucaut Jean-Claude , Shi De-Li 
Published: April 2021

ABSTRACT
Wnt signalling plays a crucial role in the control of morphogenetic movements. We describe the expression and functional analyses of frizzled 7 (Xfz7) during gastrulation in Xenopus. Low levels of Xfz7 transcripts are expressed maternally during cleavage stages; its zygotic expression strongly increases at the beginning of gastrulation and is predominantly localized to the presumptive neuroectoderm and deep cells of the involuting mesoderm. Overexpression of Xfz7 in the dorsal equatorial region affects the movements of convergent extension and delays mesodermal involution. It alters the correct localization, but not the expression, of mesodermal and neural markers. These effects can be rescued by extra-Xfz7, which is a secreted form of the receptor that also weakly inhibits convergent extension when overexpressed. This suggests that the wild-type and truncated receptors have opposing effects when coexpressed and that overexpression of Xfz7 causes an increased signalling activity. Consistent with this, Xfz7 biochemically and functionally interacts with Xwnt11. In addition, Dishevelled, but not β-catenin, synergizes with Xfz7 to affect convergent extension. Furthermore, overexpression of Xfz7 and Xwnt11 also affects convergent extension in activin-treated animal caps, and this can be efficiently reversed by coexpression of Cdc42T17N, a dominant negative mutant of the small GTPase Cdc42 known as a key regulator of actin cytoskeleton. Conversely, Cdc42G12V, a constitutively active mutant, rescues the effects of extra-Xfz7 on convergent extension in a dose-dependent manner. That both gain-of-function and loss-of-function of both frizzled and dishevelled produce the same phenotype has been well described in Drosophila tissue polarity. Therefore, our results suggest an endogenous role of Xfz7 in the regulation of convergent extension during gastrulation.
Sonic hedgehog signaling at gastrula stages specifies ventral telencephalic cells in the chick embryo
Authors: Gunhaga Lena , Jessell Thomas M. , Edlund Thomas 
Published: April 2021

ABSTRACT
A secreted signaling factor, Sonic hedgehog (Shh), has a crucial role in the generation of ventral cell types along the entire rostrocaudal axis of the neural tube. At caudal levels of the neuraxis, Shh is secreted by the notochord and floor plate during the period that ventral cell fates are specified. At anterior prosencephalic levels that give rise to the telencephalon, however, neither the prechordal mesoderm nor the ventral neural tube expresses Shh at the time that the overt ventral character of the telencephalon becomes evident. Thus, the precise role and timing of Shh signaling relevant to the specification of ventral telencephalic identity remains unclear. By analysing neural cell differentiation in chick neural plate explants we provide evidence that neural cells acquire molecular properties characteristic of the ventral telencephalon in response to Shh signals derived from the anterior primitive streak/Hensen’s node region at gastrula stages. Exposure of prospective anterior prosencephalic cells to Shh at this early stage is sufficient to initiate a temporal program of differentiation that parallels that of neurons generated normally in the medial ganglionic eminence subdivision of the ventral telencephalon.
Targeting of the EphA4 tyrosine kinase receptor affects dorsal/ventral pathfinding of limb motor axons
Authors: Helmbacher Françoise , Schneider-Maunoury Sylvie , Topilko Piotr , Tiret Laurent , Charnay Patrick 
Published: April 2021

ABSTRACT
The Eph family of tyrosine kinase receptors has recently been implicated in various processes involving the detection of environmental cues such as axonal guidance, targeted cell migration and boundary formation. We have inactivated the mouse EphA4 gene to investigate its functions during development. Homozygous EphA4 mutant animals show peroneal muscular atrophy correlating with the absence of the peroneal nerve, the main dorsal nerve of the hindlimb. This phenotype is also observed, although with a lower penetrance, in heterozygotes. During normal hindlimb innervation, motor axons converge towards the sciatic plexus region at the base of the limb bud, where they must choose between dorsal and ventral trajectories within the limb. Among the axons emerging from the sciatic plexus, dorsal projections show higher levels of EphA4 protein than ventral axons. In EphA4 mutant mice, presumptive dorsal motor axons fail to enter the dorsal compartment of the limb and join the ventral nerve. Our data therefore suggest that the level of EphA4 protein in growing limb motor axons is involved in the selection of dorsal versus ventral trajectories, thus contributing to the topographic organisation of motor projections.
Jumeaux, a novel Drosophila winged-helix family protein, is required for generating asymmetric sibling neuronal cell fates
Authors: Cheah Peh Yean , Chia William , Yang Xiaohang 
Published: April 2021

ABSTRACT
The great majority of neurons in the Drosophila embryonic CNS are generated through two successive asymmetric cell divisions; neuroblasts (NBs) divide to produce another NB and a smaller ganglion mother cell (GMC); GMCs divide to generate two sibling neurons which can adopt distinct identities. During the division of the first born GMC from the NB 4-2 lineage, GMC4-2a, Inscuteable (Insc) is localised to the apical cortex, Pon/Numb is localised to the basal cortex and two daughters with distinct identities, the RP2 motoneuron and its sibling RP2sib, are born. Resolution of distinct sibling neuronal fates requires correct apical localisation of Insc to facilitate the asymmetric localisation and preferential segregation of Pon/Numb to the basal daughter destined to become RP2. Here we report that jumeaux (jumu), which encodes a new member of the winged-helix family of transcription factors, is required to mediate the asymmetric localisation and segregation of Pon/Numb but is dispensable for Insc apical localisation during the GMC4-2a cell division. In jumu mutants GMC4-2a Pon/Numb asymmetric localisation is defective and both daughter neurons can adopt the RP2 identity. Jumu protein shows nuclear localisation and within the NB4-2 lineage is first detected only after the first neuroblast cell division, in GMC4-2a. Our results suggest that in addition to the correct formation of an apical complex, transcription mediated by Jumu is also necessary to facilitate the correct asymmetric localisation and segregation of Pon/Numb.
A Caenorhabditis elegans type I TGFβ receptor can function in the absence of type II kinase to promote larval development
Authors: Gunther Cathy V. , Georgi Laura L. , Riddle Donald L. 
Published: April 2021

ABSTRACT
The daf-4 gene encodes a type II bone morphogenetic protein receptor in Caenorhabditis elegans that regulates dauer larva formation, body size and male tail patterning. The putative type I receptor partner for DAF-4 in regulating dauer larva formation is DAF-1. Genetic tests of the mechanism of activation of these receptors show that DAF-1 can signal in the absence of DAF-4 kinase activity. A daf-1 mutation enhances dauer formation in a daf-4 null background, whereas overexpression of daf-1 partially rescues a daf-4 mutant. DAF-1 alone cannot fully compensate for the loss of DAF-4 activity, indicating that nondauer development normally results from the activities of both receptors. DAF-1 signaling in the absence of a type II kinase is unique in the type I receptor family. The activity may be an evolutionary remnant, owing to daf-1’s origin near the type I/type II divergence, or it may be an innovation that evolved in nematodes. daf-1 and daf-4 promoters both mediated expression of green fluorescent protein in the nervous system, indicating that a DAF-1/DAF-4 receptor complex may activate a neuronal signaling pathway. Signaling from a strong DAF-1/DAF-4 receptor complex or a weaker DAF-1 receptor alone may provide larvae with more precise control of the dauer/nondauer decision in a range of environmental conditions.