198,542 research outputs found
Inhibitory effects of PG-H/aggrecan and PG-M/versican on avian neural crest cell migration
No full textAggrecans and PG-M/versicans represent two newly defined families of hyaluronan-binding proteoglycans for which the function is still poorly understood. Using the avian neural crest as a model system, we have examined the molecular mechanisms entailed in the cell-proteoglycan interaction during embryonic cell motility. Both the primary cartilage aggrecan of the avian embryo (PG-H/aggrecan) and the largest variant of the avian mesenchymal versican (PG-M/versican VO) failed to support neural crest cell adhesion and migration when topographically immobilized onto the substrate. Conversely, solely the PG-H/aggrecan, and similar aggrecans from other species, counteracted the migration-promoting effect of a number of matrix molecules lacking proteoglycan affinity. This inhibitory effect was not reproduced by the isolated glycosaminoglycan chains, the isolated core protein, the reduced and alkylated macromolecule, or the aggrecan in which the G1 hyaluronan-binding domain had been inactivated with hyaluronan fragments or antibodies. Limited depolymerization of the side chains and preincubation of the PG-H/aggrecan with anti-glycosaminoglycan antibodies differentially reduced the inhibitory activity of the proteoglycan on cell motility. The results demonstrate a diverse inhibitory effect of aggrecans and PG-M/versicans on embryonic cell movement and show that the inhibitory action of aggrecans is independent of substrate binding, is dependent on a G1 domain-mediated association of the intact proteoglycan with cell surface-bound hyaluronan, and is differentially mediated by its glycosaminoglycan side chains
Recent advances in defining the role of the extracellular Matrix in neural crest development
No full textDifferential neural crest cell migration on isolated collagens: role of RGD as a potential recognition site
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Molecular Mechanisms of Avian Neural Crest Cell Migration on Fibronectin and Laminin
No full textWe have examined the molecular interactions of avian neural crest cells with fibronectin and laminin in vitro during their initial migration from the neural tube. A 105-kDa proteolytic fragment of fibronectin encompassing the defined cell-binding domain (65 kDa) promoted migration of neural crest cells to the same extent as the intact molecule. Neural crest cell migration on both intact fibronectin and the 105-kDa fragment was reversibly inhibited by RGD-containing peptides. The 11.5-kDa fragment containing the RGDS cell attachment site was also able to support migration, whereas a 50-kDa fragment corresponding to the adjacent N-terminal portion of the defined cell-binding domain was unfavorable for neural crest cell movement. In addition to the putative “cell-binding domain,” neural crest cells were able to migrate on a 31-kDa fragment corresponding to the C-terminal heparin-binding (II) region of fibronectin, and were inhibited in their migration by exogenous heparin, but not by RGDS peptides. Heparin potentiated the inhibitory effect of RGDS peptides on intact fibronectin, but not on the 105-kDa fragment. On substrates of purified laminin, the extent of avian neural crest cell migration was maximal at relatively low substrate concentrations and was reduced at higher concentrations. The efficiency of laminin as a migratory substrate was enhanced when the glycoprotein occurred complexed with nidogen. Moreover, coupling of the laminin-nidogen complex to collagen type IV or the low density heparan sulfate proteoglycan further increased cell dispersion, whereas isolated nidogen or the proteoglycan alone were unable to stimulate migration and collagen type IV was a significantly less efficient migratory substrate than laminin-nidogen. Neural crest cell migration on laminin-nidogen was not affected by RGDS nor by YIGSR-containing peptides, but was reduced by 35% after addition of heparin. The predominant motility-promoting activity of laminin was localized to the E8 domain, possessing heparin-binding activity distinct from that of the N-terminal E3 domain. Migration on the E8 fragment was reduced by >70% after addition of heparin. The E1′ fragment supported a minimal degree of migration that was RGD-sensitive and heparin-insensitive, whereas the primary heparin-binding E3 fragment and the cell-adhesive P1 fragment were entirely nonpermissive for cell movement. Preincubation of laminin-nidogen substrates with antisera against the E8 fragment, but not against the E1′ or the E4 fragment, potently reduced migration on the complex, further suggesting that the E8 domain is the predominant motility-promoting region of laminin. We conclude that initial neural crest cell migration on fibronectin occurs primarily through an interaction with the RGDS site within the cell-binding domain, whereas other potential attachment/motility-promoting sites may act to stabilize cell-fibronectin linkages. Neural crest cell migration on laminin is primarily mediated by the E8 domain. The efficiency of this domain as well as the ability of other potential motility-promoting domains to stimulate cell movement may be influenced by the association of laminin with other extracellular matrix molecules
Collagens in avian neural crest development: distribution in vivo and migration-promoting ability in vitro
No full textThis study examines the spatiotemporal distribution of collagen (Col) types I-V and IX during neural crest development in vivo and their ability to support neural crest cell movement in vitro. Col I, III and IV were widespread throughout the embryo, including the neural crest migratory pathways, whereas Col II, V and IX preferentially localized to regions from which migrating neural crest cells were absent. Col I-IV and IX occurred both in association with basement membranes and within interstitial matrices, whereas Col V only was detected in juxtaposition to basement membranes. Although initially distributed throughout the rostrocaudal extent of the somitic sclerotome, Col I and III rearranged to the caudal portion with progressive neural crest cell migration through the rostral portion of the sclerotome. This rearrangement does not occur in neural crest-ablated embryos, suggesting that it is a direct consequence of neural crest cell migration. The perinotochordal matrix, avoided by neural crest cells, contained a metameric Col II/IX immunoreactivity along the rostrocaudal axis which alternated with that of Col I and III. In contrast, Col IV and V were not observed in this matrix, but lined the basement membranes of the notochord and ventrolateral neural tube. To determine their functional significance for neural crest cell migration in vivo, purified collagens were tested for their ability to promote neural crest cell motility in vitro. Neural crest cell migration on isolated collagens was most pronounced on Col I and IV, whereas Col II, V and the triple-helical fragment of Col VII were unable to support cell motility. Substrata created by copolymerization of Col I and fibronectin, or Col I and laminin-nidogen, supported cell motility better than Col I alone, whereas both Col V and a cartilage-type chondroitin sulfate proteoglycan reduced cell movement on Col I. Fibronectin bound to pre-immobilized monomeric Col I, II or V had a reduced ability to support neural crest cell movement when compared to fibronectin alone. A similar reduction was seen for Col IV bound to the low density heparan sulfate proteoglycan from the EHS mouse tumor. The results demonstrate that Col I-IX are differentially distributed in the early avian embryo. During neural crest development several of these collagens undergo dynamic reorganizations that correlate with the migration of neural crest cells. Furthermore, various collagens possess distinct abilities to support neural crest cell migration in vitro, and their migration-promoting activity can be modulated by their conformation and/or association with other matrix components
Molecular mechanisms of neural crest cell attachment and migration on types I and IV collagen
No full textWe have examined the mechanisms involved in the interaction of avian neural crest cells with collagen types I and IV (Col I and IV) during their adhesion and migration in vitro. For this purpose native Col IV was purified from chicken tissues, characterized biochemically and ultrastructurally. Purified chicken Col I and Col IV, and various proteolytic fragments of the collagens, were used in quantitative cell attachment and migration assays in conjunction with domain-specific collagen antibodies and antibodies to avian integrin subunits. Neural crest cells do not distinguish between different macromolecular arrangements of Col I during their initial attachment, but do so during their migration, showing a clear preference for polymeric Col I. Interaction with Col I is mediated by the alpha 1 beta 1 integrin, through binding to a segment of the alpha 1(I) chain composed of fragment CNBr3. Neural crest cell attachment and migration on Col IV involves recognition of conformation-dependent sites within the triple-helical region and the noncollagenous, carboxyl-terminal NC1 domain. This recognition requires integrity of inter- and intrachain disulfide linkages and correct folding of the molecule. Moreover, there also is evidence that interaction sites within the NC1 domain may be cryptic, being exposed during migration of the cells in the intact collagen as a result of the prolonged cell-substratum contact. In contrast to Col I, neural crest cell interaction with Col IV is mediated by beta 1-class integrins other than alpha 1 beta 1
Dr. Duane M. Jackson, Morehouse College, July 2011
No full textThis video is a conversation with Dr. Duane M. Jackson. Dr. Jackson talks about his paper, "Recall and the Serial Position Effect: The Role of Primacy and Recency on Accounting Students' Performance." Jackie Daniel, AUC Woodruff Library, is the interviewer
"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States" By M. Carey.
No full text"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States: containing bried sketches of the moral and political character of those states.
By M. Carey, member of the American philosophical, and of the American Antiquarian Society, and author of The Olive Branch, Cindiciae Hibernicae, essays on banking, on political economy, and on internal improvement.
To which are now added the English editor's comments on the subject; together with Important Advice to Emigrants, and Cautions Against Impositions Practiced in the Outports
Photoinduzierte und thermische Reaktionen funktionaler Moleküle auf Oberflächen
No full textIn order to introduce a band gap into graphene structures, charge carriers can
be laterally confined in quasi-one-dimensional graphene nanoribbons (GNR) with
nanometer scale widths. These GNR can be produced in an atomically precise
fashion and with well-defined electronic structure by thermally induced on-
surface synthesis from halogen-substituted molecular precursors. In this
thesis, photoemission techniques are used to characterize the electronic
structure of the different phases of such a bottom-up synthesis reaction: the
Br-substituted monomers after adsorption on a Au(111) surface, an intermediate
polymer and finally the fully conjugated GNR. Fundamental differences,
especially concerning the band gap, are observed which are attributed to the
different nature of the respective electronic systems. Using temperature-
programmed desorption, the reaction pathway of the Br atoms after
dehalogenation is followed, namely associative desorption of HBr during the
cyclodehydrogenation reaction in the last step of the on-surface synthesis. As
an alternative to the established, thermally induced fabrication method,
photo-induced covalent coupling of the molecular precursors on the surface is
investigated and dimerization is observed. The underlying process is found to
be an electronic excitation within the substrate followed by charge transfer
into the adsorbate, creating an anionic resonance. Additionally we find that
N-doping of molecular precursors allows to align the band gap with respect to
the Fermi level of the substrate without altering the size of the band gap.
Following a different approach for the development of functional
nanostructures, a photochromic molecule, di-meta-cyano-azobenzene, is studied
with photoelectron spectroscopies on the semi-metallic Bi(111) surface.
Azobenzenes in solution undergo reversible, photo-induced trans-cis
isomerization reactions and in this work, photoisomerization of the azobenzene
derivative is observed also for molecules directly adsorbed on the surface.
However, the process is not initiated by an intramolecular electronic
excitation as in the free molecule but rather by photon absorption in the
substrate followed by charge transfer into the unoccupied electronic states of
the adsorbate and formation of an anionic resonance which leads to the
conformational change. Photoisomerization can furthermore be initiated in the
multilayer where the efficiency of the process depends on the molecular
environment. In both coverage regimes, the cis isomers perform a thermally
induced reaction which is interpreted as a phenyl ring rotation.Um eine Bandlücke in Graphenstrukturen einzubringen, können Ladungsträger in
quasi-eindimensionalen Graphen-Nanobändern (engl. graphene nanoribbons, GNR)
mit Breiten im Nanometer-Bereich lateral eingeschränkt werden. Durch thermisch
induzierte, oberflächenassistierte Synthese ausgehend von
halogensubstituierten molekularen Vorläufermolekülen können diese GNR auf
atomar präzise Weise und mit wohldefinierter elektronischer Struktur
hergestellt werden. In dieser Arbeit werden Photoemissionsmethoden verwendet,
um die elektronische Struktur der verschiedenen Phasen einer solchen,
aufbauenden Synthesereaktion zu charakterisieren: die Br-substituierten
Monomere nach der Adsorption auf der Au(111)-Oberfläche, das intermediäre
Polymer und letztlich das voll konjugierte GNR. Fundamentale Unterschiede, vor
allem in Bezug auf die Bandlücke, werden beobachtet und dem unterschiedlichen
Wesen der jeweiligen elektronischen Systeme zugeschrieben. Mittels
Thermodesorptionsspektroskopie wird der Reaktionspfad der Br-Atome nach der
Dehalogenierung verfolgt, nämlich eine assoziative Desorption von HBr während
der Zyklodehydrogenierungsreaktion im letzten Schritt der
oberflächengebundenen Synthese. Als Alternative zur etablierten, thermischen
induzierten Herstellungsmethode wird eine lichtinduzierte kovalente Kopplung
der Vorläufermoleküle untersucht und eine Dimerisierung beobachtet. Der
zugrunde liegende Prozess wird als elektronische Anregung im Substrat erkannt,
gefolgt von Ladungstransfer ins Adsorbat, der eine anionische Resonanz
erzeugt. Desweiteren lernen wir, dass N-Dotierung der molekularen Vorläufer
eine Ausrichtung der Bandlücke relativ zum Ferminiveau des Substrats erlaubt,
ohne die Größe der Bandlücke zu ändern. Einem anderen Ansatz für die
Entwicklung funktionaler Nanostrukturen nachgehend wird ein photochromes
Molekül, di-meta-cyano-Azobenzol, mittels Photoelektronenspektroskopien auf
der halbmetallischen Bi(111)-Oberfläche untersucht. Azobenzol unterzieht sich
in Lösung einer reversiblen, lichtinduzierten trans-cis-
Isomerisierungsreaktion und in dieser Arbeit wird die Photoisomerisierung des
Azobenzolderivats auch für direkt an der Oberfläche adsorbierte Moleküle
beobachtet. Allerdings wird dieser Prozess nicht, wie im freien Molekül, durch
intramolekulare elektronische Anregung ausgelöst, sondern vielmehr durch
Photonenabsorption im Substrat gefolgt von Ladungstransfer in die unbesetzten
elektronischen Zustände des Adsorbats und die Bildung einer anionischen
Resonanz, die zur Konformationsänderung führt. Photoisomerisierung kann
außerdem in der Multilage ausgelöst werden, wo die Effizienz des Prozesses von
der molekularen Umgebung abhängt. In beiden Bedeckungsordnungen vollführt das
cis-Isomer eine thermisch ausgelöste Reaktion, die als Phenylringrotation
interpretiert wird
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