pinecone/core-2020-05-10-deduplication huggingface dataset:
Keeping the Receptor Tyrosine Kinase Signaling Pathway in Check: Lessons from Drosophila

AbstractThe receptor tyrosine kinase (RTK) signaling network plays a central role in regulating cellular differentiation, proliferation, and survival in all metazoan animals. Excessive or continuous activation of the RTK pathway has been linked to carcinogenesis in mammals, underscoring the importance of preventing uncontrolled signaling. This review will focus on the inhibitory mechanisms that keep RTK-mediated signals in check, with emphasis on conserved principles discerned from studies using Drosophila as a model system. Two general strategies of inhibition will be discussed. The first, threshold regulation, postulates that an effective way of antagonizing RTK signaling is to erect and maintain high threshold barriers that prevent inappropriate responses to moderate signaling levels. Activation of the pathway above this level overcomes the inhibitory blocks and shifts the balance to allow a positive flow of inductive information. A second layer of negative regulation involving induction of negative feedback loops that limit the extent, strength, or duration of the signal prevents runaway signaling in response to the high levels of activation required to surmount the threshold barriers. Such autoinhibitory mechanisms attenuate signaling at critical points throughout the network, from the receptor to the downstream effectors
Affinity Regulates Spatial Range of EGF Receptor Autocrine Ligand Binding

AbstractProper spatial localization of EGFR signaling activated by autocrine ligands represents a critical factor in embryonic development as well as tissue organization and function, and ligand/receptor binding affinity is among the molecular and cellular properties suggested to play a role in governing this localization. We employ a computational model to predict how receptor-binding affinity affects local capture of autocrine ligand vis-a-vis escape to distal regions, and provide experimental test by constructing cell lines expressing EGFR along with either wild-type EGF or a low-affinity mutant, EGFL47M. The model predicts local capture of a lower affinity autocrine ligand to be less efficient when the ligand production rate is small relative to receptor appearance rate. Our experimental data confirm this prediction, demonstrating that cells can use ligand/receptor binding affinity to regulate ligand spatial distribution when autocrine ligand production is limiting for receptor signaling
Disabled-2 Is Essential for Endodermal Cell Positioning and Structure Formation during Mouse Embryogenesis

AbstractThe signal transduction adapter protein Disabled-2 (Dab2) is one of the two mammalian orthologs of the Drosophila Disabled. The brain-specific Disabled-1 (Dab1) functions in positional organization of brain cells during development. Dab2 is widely distributed and is highly expressed in many epithelial cell types. The dab2 gene was interrupted by in-frame insertion of β-galactosidase (LacZ) in embryonic stem cells and transgenic mice were produced. Dab2 expression was first observed in the primitive endoderm at E4.5, immediately following implantation. The homozygous Dab2-deficient mutant is embryonic lethal (earlier than E6.5) due to defective cell positioning and structure formation of the visceral endoderm. In E5.5 dab2 (−/−) conceptus, visceral endoderm-like cells are present in the deformed primitive egg cylinder; however, the visceral endoderm cells are not organized, the cells of the epiblast have not expanded, and the proamniotic cavity fails to form. Disorganization of the visceral endodermal layer is evident, as cells with positive visceral endoderm markers are scattered throughout the dab2 (−/−) conceptus. Only degenerated remains were observed at E6.5 for dab2 (−/−) embryos, and by E7.5, the defective embryos were completely reabsorbed. In blastocyst in vitro culture, initially cells with characteristics of endoderm, trophectoderm, and inner cell mass were observed in the outgrowth of the hatched dab2 (−/−) blastocysts. However, the dab2 (−/−) endodermal cells are much more dispersed and disorganized than those from wild-type blastocysts, the inner cell mass fails to expand, and the outgrowth degenerates by day 7. Thus, Dab2 is required for visceral endodermal cell organization during early mouse development. The absence of an organized visceral endoderm in Dab2-deficient conceptus leads to the growth failure of the inner cell mass. We suggest that Dab2 functions in a signal pathway to regulate endodermal cell organization using endocytosis of ligands from the blastocoel cavity as a positioning cue
Mouse GLI3 Regulates Fgf8 Expression and Apoptosis in the Developing Neural Tube, Face, and Limb Bud

AbstractThe zinc finger transcription factor GLI3 is considered a repressor of vertebrate Hedgehog (Hh) signaling. In humans, the absence of GLI3 function causes Greig cephalopolysyndactyly syndrome, affecting the development of the brain, eye, face, and limb. Because the etiology of these malformations is not well understood, we examined the phenotype of mouse Gli3−/− mutants as a model to investigate this. We observed an up-regulation of Fgf8 in the anterior neural ridge, isthmus, eye, facial primordia, and limb buds of mutant embryos, sites coinciding with the human disease. Intriguingly, endogenous apoptosis was reduced in Fgf8-positive areas in Gli3−/− mutants. Since SHH is thought to be involved in Fgf8 regulation, we compared Fgf8 expression in Shh−/− and Gli3−/−;Shh−/− mutant embryos. Whereas Fgf8 expression was almost absent in Shh−/− mutants, it was up-regulated in Gli3−/−;Shh−/− double mutants, suggesting that SHH is not required for Fgf8 induction, and that GLI3 normally represses Fgf8 independently of SHH. In the limb bud, we provide evidence that ectopic expression of Gremlin in Gli3−/− mutants might contribute to a decrease in apoptosis. Together, our data reveal that GLI3 limits Fgf8-expression domains in multiple tissues, through a mechanism that may include the induction or maintenance of apoptosis
The Regulation and Action of Myostatin as a Negative Regulator of Muscle Development during Avian Embryogenesis

AbstractMyostatin is a potent inhibitor of muscle growth. Genetic deletion of Myostatin leads to massive hyperplasia and hypertrophy of skeletal muscle. However, the overall muscle pattern is preserved. We show that, during chick embryonic development, Myostatin is expressed at stages and positions unlikely to influence qualitative muscle development. In the somites, Myostatin is predominantly expressed in a central domain of the dermomyotome but not at the dorsomedial and ventrolateral lips, where most cells for myotomal elongation are recruited. During limb bud development, Myostatin is transiently expressed at early stages in both myogenic and nonmyogenic regions. Myostatin is reexpressed during limb bud development at a time when splitting of muscle is underway. Heterotopically developed wing buds that fail to form muscle still express Myostatin. This demonstrates that, in the limb, not all Myostatin-expressing cells are of myogenic origin. Ectoderm and Sonic hedgehog have different effects on the expression of Myostatin dependent on stages at which the operation was performed and the length of the postoperative period. Finally, we show that application of Myostatin protein into the developing limb bud results in a down-regulation of Pax-3 and Myf-5, both genes associated with proliferation of myogenic cells; and, furthermore, Myostatin also prevents the expression of MyoD, a gene associated with muscle differentiation. The long-term effect of Myostatin treatment leads to a deficiency of limb muscle. Therefore, Myostatin negatively affects gene expression of transcription factors, which are necessary for establishing myogenic cell identity
Exogenous Growth Factors Stimulate the Regeneration of Ganglion Cells in the Chicken Retina

AbstractRecent reports have found that the posthatch chicken retina has the capacity for neuronal regeneration. The purpose of this study was to test whether the types of cells destroyed by neurotoxic lesions influence the types of cells that are regenerated, and whether exogenous growth factors stimulate neural regeneration in the chicken retina. N-methyl-d-aspartate (NMDA) was used to destroy amacrine and bipolar cells; kainate was used to destroy bipolar, amacrine, and ganglion cells; colchicine was used to selectively destroy ganglion cells. Following toxin-induced damage, bromo-deoxyuridine was used to label proliferating cells. In some animals, growth factors were injected into the vitreous chamber of the eye. We found that the proliferation of cells within the retina was stimulated by toxin-induced cell loss, and by insulin and FGF2. After either kainate- or colchicine-induced retinal damage, some of the newly generated cells expressed markers and had the morphology of ganglion cells. The combination of insulin and FGF2 stimulated the regeneration of ganglion cells in kainate- and colchicine-treated retinas. We conclude that exogenous growth factors can be used to stimulate neural regeneration in the retina. We propose that the type of neuron destroyed in the retina may allow or promote the regeneration of that neuronal type
The E3 Ubiquitin Ligase GREUL1 Anteriorizes Ectoderm during Xenopus Development

AbstractWe have identified a family of RING finger proteins that are orthologous to Drosophila Goliath (G1, Gol). One of the members, GREUL1 (Goliath Related E3 Ubiquitin Ligase 1), can convert Xenopus ectoderm into XAG-1- and Otx2-expressing cells in the absence of both neural tissue and muscle. This activity, combined with the finding that XGREUL1 is expressed within the cement gland, suggests a role for GREUL1 in the generation of anterior ectoderm. Although GREUL1 is not a direct inducer of neural tissue, it can activate the formation of ectopic neural cells within the epidermis of intact embryos. This suggests that GREUL1 can sensitize ectoderm to neuralizing signals. In this paper, we provide evidence that GREUL1 is an E3 ubiquitin ligase. Using a biochemical assay, we show that GREUL1 catalyzes the addition of polyubiquitin chains. These events are mediated by the RING domain since a mutation in two of the cysteines abolishes ligase activity. Mutation of these cysteines also compromises GREUL1's ability to induce cement gland. Thus, GREUL1's RING domain is necessary for both the ubiquitination of substrates and for the conversion of ectoderm to an anterior fate
A Role for Retinoic Acid in Regulating the Regeneration of Deer Antlers

AbstractDeer antlers are the only mammalian organs that can be repeatedly regenerated; each year, these complex structures are shed and then regrow to be used for display and fighting. To date, the molecular mechanisms controlling antler regeneration are not well understood. Vitamin A and its derivatives, retinoic acids, play important roles in embryonic skeletal development. Here, we provide several lines of evidence consistent with retinoids playing a functional role in controlling cellular differentiation during bone formation in the regenerating antler. Three receptors (α, β, γ) for both the retinoic acid receptor (RAR) and retinoid X receptor (RXR) families show distinct patterns of expression in the growing antler tip, the site of endochondral ossification. RARα and RXRβ are expressed in skin (“velvet”) and the underlying perichondrium. In cartilage, which is vascularised, RXRβ is specifically expressed in chondrocytes, which express type II collagen, and RARα in perivascular cells, which also express type I collagen, a marker of the osteoblast phenotype. High-performance liquid chromatography analysis shows significant amounts of Vitamin A (retinol) in antler tissues at all stages of differentiation. The metabolites all-trans-RA and 4-oxo-RA are found in skin, perichondrium, cartilage, bone, and periosteum. The RXR ligand, 9-cis-RA, is found in perichondrium, mineralised cartilage, and bone. To further define sites of RA synthesis in antler, we immunolocalised retinaldehyde dehydrogenase type 2 (RALDH-2), a major retinoic acid-generating enzyme. RALDH-2 is expressed in the skin and perichondrium and in perivascular cells in cartilage, although chondroprogenitors and chondrocytes express very low levels. At sites of bone formation, differentiated osteoblasts which express the bone-specific protein osteocalcin express high levels of RALDH2. The effect of RA on antler cell differentiation was studied in vitro; all-trans-RA inhibits expression of the chondrocyte phenotype, an effect that is blocked by addition of the RAR antagonist Ro41-5253. In monolayer cultures of mesenchymal progenitor cells, all-trans-RA increases the expression of alkaline phosphatase, a marker of the osteoblastic phenotype. In summary, this study has shown that antler tissues contain endogenous retinoids, including 9-cis RA, and the enzyme RALDH2 that generates RA. Sites of RA synthesis in antler correspond closely with the localisation of cells which express receptors for these ligands and which respond to the effects of RA
The Fused toes (Ft) Mouse Mutation Causes Anteroposterior and Dorsoventral Polydactyly

AbstractMouse mutants have been proven to be a valuable system to analyze the molecular network governing vertebrate limb development. In the present study, we report on the molecular and morphological consequences of the Fused toes (Ft) mutation on limb morphogenesis in homozygous embryos. We show that Ft affects all three axes as the mutant limbs display severe distal truncations of skeletal elements as well as an anteroposterior and an unusual form of dorsoventral polydactyly. Ectopic activation of the Shh signalling cascade in the distal-most mesoderm together with malformations of the AER likely account for these alterations. Moreover, we provide evidence that a deregulated control of programmed cell death triggered by Bmp-4 and Dkk-1 significantly contributes to the complex limb phenotype. In addition, our analysis reveals a specific requirement of the genes deleted by the Ft mutation in hindlimb morphogenesis
Constitutive Expression of Preproendothelin in the Cardiac Neural Crest Selectively Promotes Expansion of the Adventitia of the Great Vessels in Vivo

AbstractCardiac neural crest cells are essential for normal development of the great vessels and the heart, giving rise to a range of cell types, including both neuronal and non-neuronal adventitial cells and smooth muscle. Endothelin (ET) signaling plays an important role in the development of cardiac neural crest cell lineages, yet the underlying mechanisms that act to control their migration, differentiation, and proliferation remain largely unclear. We examined the expression patterns of the receptor, ETA, and the ET-specific converting enzyme, ECE-1, in the pharyngeal arches and great vessels of the developing chick embryo. In situ hybridization analysis revealed that, while ETA is expressed in the pharyngeal arch mesenchyme, populated by cardiac neural crest cells, ECE-1 expression is localized to the outermost ectodermal cells of the arches and then to the innermost endothelial cells of the great vessels. This dynamic pattern of expression suggests that only a subpopulation of neural crest cells in these regions is responsive to ET signaling at particular developmental time points. To test this, retroviral gene delivery was used to constitutively express preproET-1, a precursor of mature ET-1 ligand, in the cardiac neural crest. This resulted in a selective expansion of the outermost, adventitial cell population in the great vessels. In contrast, neither differentiation nor proliferation of neural crest-derived smooth muscle cells was significantly affected. These results suggest that constitutive expression of exogenous preproET-1 in the cardiac neural crest results in expansion restricted to an adventitial cell population of the developing great vessels
The Role of Neural Crest during Cardiac Development in a Mouse Model of DiGeorge Syndrome

AbstractThe velo-cardio-facial syndrome (VCFS)/DiGeorge syndrome (DGS) is a genetic disorder characterized by phenotypic abnormalities of the derivatives of the pharyngeal arches, including cardiac outflow tract defects. Neural crest cells play a major role in the development of the pharyngeal arches, and defects in these cells are likely responsible for the syndrome. Most patients are hemizygous for a 1.5- to 3.0-Mb region of 22q11, that is suspected to be critical for normal pharyngeal arch development. Mice hemizygous for a 1.5-Mb homologous region of chromosome 16 (Lgdel/+) exhibit conotruncal cardiac defects similar to those seen in affected VCFS/DGS patients. To investigate the role of Lgdel genes in neural crest development, we fate mapped neural crest cells in Lgdel/+ mice and we performed hemizygous neural crest-specific inactivation of Lgdel. Hemizygosity of the Lgdel region does not eliminate cardiac neural crest migration to the forming aortic arches. However, neural crest cells do not differentiate appropriately into smooth muscle in both fourth and sixth aortic arches and the affected aortic arch segments develop abnormally. Tissue-specific hemizygous inactivation of Lgdel genes in neural crest results in normal cardiovascular development. Based on our studies, we propose that Lgdel genes are required for the expression of soluble signals that regulate neural crest cell differentiation
Neurogenin1 Defines Zebrafish Cranial Sensory Ganglia Precursors

AbstractCells delaminate from epithelial placodes to form sensory ganglia in the vertebrate head. We describe the formation of cranial neurogenic placodes in the zebrafish, Danio rerio, using bHLH transcription factors as molecular markers. A single neurogenin gene, neurogenin1 (ngn1), is required for the development of all zebrafish cranial ganglia, which contrasts with other described vertebrates. Expression of ngn1 delineates zebrafish ganglionic placodes, including trigeminal, lateral line, and epibranchial placodes. In addition, ngn1 is expressed in a subset of cells within the otic vesicle that will delaminate to form the octaval (statoacoustic) ganglion. The trigeminal placode is the first to differentiate, and forms just lateral and adjacent to the neural crest. Expression of ngn1 is transient and prefigures expression of a related bHLH transcription factor, neuroD. Interfering with ngn1 function using a specific antisense morpholino oligonucleotide blocks differentiation of all cranial ganglia but not associated glial cells. Lateral line sensory neuromasts develop independently of ngn1 function, suggesting that two derivatives of lateral line placodes, ganglia and migrating primordia, are under separate genetic control
Morphogenetic Apoptosis: A Mechanism for Correcting Discontinuities in Morphogen Gradients

AbstractSmooth gradients of the morphogens Hh, Dpp, and Wg are required for proper development of Drosophila imaginal discs. Here, it is reported that, when a discontinuity is generated between two adjacent cells in the reception of either the Dpp or Wg signal, then cells on either side of the discontinuity boundary undergo apoptosis by activating the c-Jun N-terminal Kinase (JNK) pathway. Furthermore, in the medial region of the wing imaginal disc, the JNK pathway is also activated if cells do not receive the proper levels of Dpp and Hh signals. These observations suggest that cells within a developing field have the ability to access their spatial positions by comparing the level of morphogen signal they receive with that of their neighbors. This phenomenon is likely related to the process of cell competition, and we suggest that it is an evolutionarily important mechanism that helps prevent abnormal tissue specification and growth during development
BMP Pathways Are Involved in Otic Capsule Formation and Epithelial–Mesenchymal Signaling in the Developing Chicken Inner Ear

AbstractThe vertebrate inner ear consists of a complex labyrinth of epithelial cells that is surrounded by a bony capsule. The molecular mechanisms coordinating the development of the membranous and bony labyrinths are largely unknown. Previously, using avian retrovirus encoding Noggin (RCAS-Noggin) or beads soaked with Noggin protein, we have shown that bone morphogenetic proteins (BMPs) are important for the development of the otic epithelium in the chicken inner ear. Here, using two additional recombinant avian retroviruses, dominant negative and constitutively active forms of BMP receptors IB (BMPRIB), we show that BMPs, possibly acting through BMPRIB, are important for otic capsule formation. We also show that Bmp2 is strongly expressed in the prospective semicircular canals starting from the canal outpouch stage, suggesting that BMP2 plays an important role in canal formation. In addition, by correlating expression patterns of Bmps, their receptors, and localization of phosphorylated R-Smad (phospho R-Smad) immunoreactivity, an indicator of BMP activation, we show that BMPs emanating from the otic epithelium influence chondrogenesis of the otic capsule including the cartilage surrounding the semicircular canals
A Signaling Role for the Uncleaved Form of α6 Integrin in Differentiating Lens Fiber Cells

AbstractMany α integrin subunits are cleaved during their processing to yield heavy and light chains, which remain associated by disulfide bonds. While uncleaved α integrin subunits can form functional receptors that sometimes have distinct signaling roles from their better-characterized endoproteolytically cleaved counterparts, their expression at the cell surface and their association with signaling complexes have yet to be determined in vivo. In this study, we demonstrate that, in differentiating lens fiber cells, the uncleaved form of α6 integrin was expressed at the cell surface. This form of α6 integrin coimmunoprecipitated with both the signaling adaptor molecule Shc and its downstream effector Grb2, suggesting that, in lens fiber cells, uncleaved α6 integrin was associated with a Shc-mediated signaling complex. We show that expression of the cleaved form of α6 integrin progressively decreased relative to uncleaved α6 integrin as the state of lens cell differentiation increased, resulting in the predominance of uncleaved α6 integrin in the lens fiber cell zones. Interestingly, we previously have shown that α6 integrin is localized principally along the extensive cell–cell interfaces of these lens fiber cells, in the absence of its extracellular matrix ligand laminin. While we found that the cleaved form of α6 integrin contained both high mannose and complex sugars, the uncleaved form of α6 integrin contained only high mannose sugars. These properties suggest that the uncleaved form of α6 integrin may have a unique role in the embryonic lens. Its high association with Shc and Grb2 in the differentiating cortical fiber cell zone indicates that α6 integrin may provide a cell survival signal in the presence of the apoptotic-like processes that are initiated in this region of the embryonic lens to clear the lens cells of their organelles