19 research outputs found

    Spatial Regulation of Gene Expression in Neurons During Synapse Formation and Synaptic Plasticity

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    mRNA localization and regulated translation allow individual neurons to locally regulate the proteome of each of their many subcellular compartments. To investigate the spatial regulation of gene expression during synaptic plasticity, we used a translational reporter system to demonstrate synapse- and stimulus-specific translation during long-term facilitation of Aplysia sensory-motor synapse. These studies revealed a role for a retrograde signal from the postsynaptic motor neuron in regulating translation in the presynaptic sensory neuron. Additional studies with the translational reporter demonstrated that distinct cis-acting localization elements were involved in targeting mRNA to distal neurites and to synapses. Our studies identified a 66 nucleotide long stem loop structure that directs mRNAs to synapses.In the final part of my thesis research, I addressed the question of whether and how synaptogenic signals direct mRNA targeting and spatially regulate gene expression during synapse formation. I cultured a bifurcated Aplysia sensory neuron contacting a nontarget motor neuron, with which it did not form chemical synapses, and a target motor neuron, with which it formed glutamatergic synapses, and imaged RNA and protein localization. I find that RNAs and translational machinery are delivered throughout the neuron, but that translation is enriched at sites of synaptic contact. Investigation of the molecular mechanisms that promote local translation revealed a role for netrin1-DCC signaling. Together, my research indicates that the spatial regulation of gene expression during synapse formation and during synaptic plasticity is mediated at the level of translation. This mechanism maximizes neuronal plasticity by rendering each compartment capable of locally changing its proteome in response to local cues

    A New Approach to Global Stability of Discrete Lotka-Volterra Predator-Prey Models

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    An Euler difference scheme for a three-dimensional predator-prey model is considered and we introduce a new approach to show the global stability of the scheme. For this purpose, we partition the three-dimensional space and calculate the sign of the rate change of population of species in each partitioned region. Our method is independent of dimension and then can be applicable to other dimensional discrete models. Numerical examples are presented to verify the results in this paper

    Seismic Fragility Analysis of Steel Liquid Storage Tanks Using Earthquake Ground Motions Recorded in Korea

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    Liquid-containing storage tanks are important structures in industrial complexes. Because earthquake damages to liquid storage tanks can cause structural collapse, fires, and hazardous material leaks, there have been continuous efforts to mitigate earthquake damages using seismic fragility analysis. In this regard, this study focuses on the seismic responses and fragility of liquid storage tanks. First, the characteristics of earthquake ground motions are a critical factor influencing the seismic fragility of structures; thus, this study employs real earthquake records observed in the target area, southeastern Korea, with the earthquake characteristics estimated based on the ratio of peak ground acceleration to peak ground velocity. When a liquid storage tank oscillates during an earthquake, additional forces can impact the tank wall owing to hydrodynamic pressures. Therefore, this study presents a sophisticated finite element (FE) model that reflects the hydrodynamic effect of an oscillating liquid. Another advantage of such an FE model is that detailed structural responses of the entire wall shells can be estimated; this is not possible in simplified lumped mass or surrogate models. Lastly, probabilistic seismic demand models are derived for three critical limit states: elastic buckling, elephant's foot buckling, and steel yielding. Using the real earthquake ground motion records, constructed FE model, and limit states, a seismic fragility analysis is performed for a typical anchored steel liquid storage tank in Korea. In addition, for comparison purposes, a ring-stiffened model is investigated to derive a seismic fragility curve. The results of the seismic fragility assessment show that elastic buckling is the most vulnerable damage state. In contrast, elephant's foot buckling and steel yielding indicate relatively severe damage levels. Furthermore, it is observed that ring stiffeners decrease the elastic buckling damage, although there is no practical effect on elephant's foot buckling and steel yielding in all ground motion intensities

    Hybrid fragility curve derivation of buildings based on post-earthquake reconnaissance data

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    This study proposes a new hybrid method that uses both of post-earthquake reconnaissance data and numerical analysis results based on a finite element (FE) model. As the uncertainty of a capacity threshold for a structural damage state needs to be estimated carefully, in the proposed method, the probabilistic distribution parameters of capacity thresholds are evaluated based on post-earthquake reconnaissance data. Subsequently, the hybrid fragility curves were derived for several damage states using the updated distribution parameters of capacity thresholds. To illustrate the detailed process of the proposed hybrid method, it was applied to piloti-type reinforce concrete (RC) buildings which were affected by the 2017 Pohang earthquake, Korea. In the example, analytical fragility curves were derived first, and then hybrid fragility curves were obtained using the distribution parameters of capacity thresholds which were updated based on actual post-earthquake reconnaissance data about the Pohang city. The results showed that the seismic fragility estimates approached to the empirical failure probability at 0.27 g PGA, corresponding to the ground motion intensity of the Pohang earthquake. To verify the proposed method, hybrid fragility curves were derived with the hypothetical reconnaissance data sets created based on assumed distribution parameters with errors of 10% and 1%. As a result, it was identified that the distribution parameters accurately converged to the assumed parameters and the case of 1% error had better convergence than that of 10% error

    Effects of Graphene Transfer and Thermal Annealing on Anticorrosive Properties of Stainless Steel

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    Stainless steel (STS) films were annealed in a thermal quartz tube and covered with graphene to improve their anticorrosive properties. Graphene was synthesized via the chemical vapor deposition method and transferred onto the surface of the STS film by the layer-by-layer approach. The structure of the STS film changed from alpha-Fe to gamma-Fe after annealing at 700 C for 1 h, resulting in an increase of 82.72% in the inhibition efficiency. However, one-layer graphene acted as a conductive pathway and therefore deteriorated the anticorrosive properties of the STS film. To overcome this problem, graphene was transferred layer by layer onto the STS film. It was found that transfer of three layers of graphene onto the STS film resulted in a 91.57% increase in the inhibition efficiency. Therefore, thermal annealing and transfer of multilayer graphene are considered to be effective in enhancing the anticorrosive properties of STS films

    A novel approach to assess the seismic performance of deteriorated bridge structures by employing UAV-based damage detection

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    All civil infrastructure, including bridges, deteriorates over time. Unmanned aerial vehicle (UAV) based visual inspection of bridges has been proposed to assess the condition of bridges. However, existing methods cannot determine the seismic performance of bridges based on the results of UAV-based visual inspection. In this study, a novel approach is proposed to assess the seismic performance of deteriorated bridges with the results of UAV-based damage detection. The proposed approach consists of two phases: (i) the damage detection phase using a UAV and (ii) the seismic performance assessment phase. The images obtained from UAV survey are used to conduct condition assessment for the bridge, based on a previously developed region-based convolutional neural network (R-CNN), and the damage grade is assigned. Note that here damage includes both seismic damage and deterioration. Subsequently, the finite element (FE) model of the intact bridge is updated to correspond to the assigned damage index. To demonstrate the proposed approach, an in-service prestressed concrete box-girder bridge is investigated. In particular, the seismic response of the deteriorated bridge is assessed based on a comparison with the intact bridge responses; focus is placed on the maximum moment and maximum displacement at the pier and the girder. Predictions indicate that the seismic responses of the deteriorated in-service bridge are 10% poorer than those of the intact bridge. These results demonstrate the potential for the UAV-based approach for evaluating the seismic performance of deteriorated bridges

    Development of a Master Sintering Curve for Al-Mg Alloy

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    A new master sintering curve (MSC) is proposed for Al-Mg alloy in order to effectively design the pressure-assisted sintering process. In this work, hot pressing experiments of Al-Mg alloy powders are performed. The changes of relative density during hot pressing are measured for the various heating rates of 5°C/min, 10°C/min, and 20°C/min at the fixed pressure of 50 MPa. A work of sintering, designated as Θ, is introduced and defined as Θ(t,T)=∫0t1/Texp-Q/RTdt. A work of sintering, Θ, could be interpreted as a measure for the amount of sintering work. The MSC in this work defines the relation between the apparent density and a work of sintering, Θ. Since the measurement of an apparent activation energy, Q, is very difficult, the correct value of Q is obtained numerically using a mean residual square method. Then, the master sintering curves for sintering of Al-Mg alloy powders are proposed for the sintering temperatures of 400°C and 500°C through scaling procedures. It is expected that the master sintering curves proposed in this work could help an engineer to design pressure-assisted sintering process for Al-Mg alloy
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