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The diffusional properties of dendrites depend on the density of dendritic spines
Santamaria, Fidel,Wils, Stefan,De Schutter, Erik,Augustine, George J. Blackwell Publishing Ltd 2011 The European journal of neuroscience Vol.34 No.4
<P><B>Abstract</B></P><P>We combined computational modeling and experimental measurements to determine the influence of dendritic structure on the diffusion of intracellular chemical signals in mouse cerebellar Purkinje cells and hippocamal CA1 pyramidal cells. Modeling predicts that molecular trapping by dendritic spines causes diffusion along spiny dendrites to be anomalous and that the value of the anomalous exponent (<I>d</I><SUB><I>w</I></SUB>) is proportional to spine density in both cell types. To test these predictions we combined the local photorelease of an inert dye, rhodamine dextran, with two‐photon fluorescence imaging to track diffusion along dendrites. Our results show that anomalous diffusion is present in spiny dendrites of both cell types. Further, the anomalous exponent is linearly related to the density of spines in pyramidal cells and <I>d</I><SUB><I>w</I></SUB> in Purkinje cells is consistent with such a relationship. We conclude that anomalous diffusion occurs in the dendrites of multiple types of neurons. Because spine density is dynamic and depends on neuronal activity, the degree of anomalous diffusion induced by spines can dynamically regulate the movement of molecules along dendrites.</P>
Santamaria, Fidel,Gonzalez, Jossina,Augustine, George J.,Raghavachari, Sridhar Public Library of Science 2010 PLoS computational biology Vol.6 No.5
<▼1><P>One mechanism of information storage in neurons is believed to be determined by the strength of synaptic contacts. The strength of an excitatory synapse is partially due to the concentration of a particular type of ionotropic glutamate receptor (AMPAR) in the post-synaptic density (PSD). AMPAR concentration in the PSD has to be plastic, to allow the storage of new memories; but it also has to be stable to preserve important information. Although much is known about the molecular identity of synapses, the biophysical mechanisms by which AMPAR can enter, leave and remain in the synapse are unclear. We used Monte Carlo simulations to determine the influence of PSD structure and activity in maintaining homeostatic concentrations of AMPARs in the synapse. We found that, the high concentration and excluded volume caused by PSD molecules result in molecular crowding. Diffusion of AMPAR in the PSD under such conditions is anomalous. Anomalous diffusion of AMPAR results in retention of these receptors inside the PSD for periods ranging from minutes to several hours in the absence of strong binding of receptors to PSD molecules. Trapping of receptors in the PSD by crowding effects was very sensitive to the concentration of PSD molecules, showing a switch-like behavior for retention of receptors. Non-covalent binding of AMPAR to anchored PSD molecules allowed the synapse to become well-mixed, resulting in normal diffusion of AMPAR. Binding also allowed the exchange of receptors in and out of the PSD. We propose that molecular crowding is an important biophysical mechanism to maintain homeostatic synaptic concentrations of AMPARs in the PSD without the need of energetically expensive biochemical reactions. In this context, binding of AMPAR with PSD molecules could collaborate with crowding to maintain synaptic homeostasis but could also allow synaptic plasticity by increasing the exchange of these receptors with the surrounding extra-synaptic membrane.</P></▼1><▼2><P><B>Author Summary</B></P><P>One of the most accepted theories of information storage in neurons is that it is partially localized in the strength of synaptic contacts. Evidence suggests that at the cellular level, in combination with other cellular mechanisms, this is implemented by increasing or decreasing the concentration of a particular type of membrane molecules. Two opposing mechanisms have to coexist in synapses to allow them to store information. On one hand, synapses have to be flexible, to allow the storage of new memories. On the other hand, synapses have to be stable to preserve previously learned information. Although much is known about the molecular identity of synapses, the biophysical mechanisms by which molecules can enter, leave and remain in the synapse are unclear. Our modeling work uses fundamental biophysical principles to quantify the effects of molecular collisions and biochemical reactions. Our results show that molecular collisions alone, between the diffusing proteins with anchored molecules in the synapse, can replicate known experimental results. Molecular collision in combination with biochemical binding can be fundamental biophysical principles used by synapses for the formation and preservation of memories.</P></▼2>
3D neutron tomography of a polymer electrolyte membrane fuel cell under sub-zero conditions
Santamaria, A.,Tang, H.Y.,Park, J.W.,Park, G.G.,Sohn, Y.J. Pergamon Press ; Elsevier Science Ltd 2012 International journal of hydrogen energy Vol.37 No.14
In this article, we implement both 2D and 3D based neutron imaging techniques on a polymer electrolyte membrane (PEFC) fuel cell under sub-zero conditions. A cell was run at steady state power, purged for 60 s, and then brought down to -5 <SUP>o</SUP>C inside an environmental chamber situated in front of a neutron beam. A series of 2D radiographs were taken as the cell dropped in temperature capturing the condensation and redistribution of flow field and gas diffusion layer (GDL) water. Immediately after this, 3D tomography was conducted while the cell remained at -5 <SUP>o</SUP>C. The image data was reconstructed into a 3D model in order to highlight regions where water/ice formations occur. The tomography results show where ice forms within the flow field and which regions are subject to blockages. Ice is observed predominately under channel areas due to water rejection by the GDL. The cathode side channel exit region displays higher ice content which correlates with elevated saturation levels from reaction water production during operation. Larger ice formations reside in the lower region of the flow field due to gravity. These blockages may pose significant issues to cold start of the cell as well as highlight potential drawbacks to shorter purge durations.
Catalytic steam reforming of biomass fast pyrolysis volatiles over Ni–Co bimetallic catalysts
Laura Santamaria,Gartzen Lopez,Aitor Arregi,Maite Artetxe,Maider Amutio,Javier Bilbao,Martin Olazar 한국공업화학회 2020 Journal of Industrial and Engineering Chemistry Vol.91 No.-
The influence of the metal selected as catalytic active phase in the two-step biomass pyrolysis-catalyticreforming strategy has been analyzed. The pyrolysis step was carried out in a conical spouted bed reactorat 500 C, whereas steam reforming was performed in afluidized bed reactor at 600 C. Ni/Al2O3,Co/Al2O3 and two bimetallic Ni-Co/Al2O3 catalysts with different metal loadings were synthesized by wetimpregnation method, and fresh and deactivated catalysts were characterized by N2 adsorption/desorption, X-ray Fluorescence (XRF), Temperature Programmed Reduction (TPR), X-Ray powderDiffraction (XRD), Temperature Programmed Oxidation (TPO), Scanning Electron Microscopy (SEM) andTransmission Electron Microscopy (TEM). Although Ni/Al2O3 and both bimetallic catalysts had similarinitial activity in terms of oxygenate conversion, (higher than 98%), the poorer metal dispersion observedin both bimetallic catalysts led to a fast decrease in conversion due to the promotion of coke formation onlarge particles. This occurred even though Ni–Co alloy formation has a positive influence by hindering theoxidation of Co0 species. The main cause for the deactivation of these catalysts is the formation of a cokewith amorphous structure. The poor initial performance of Co/Al2O3 catalyst is related to changes in theCo0 oxidation state induced by the presence of steam, which led to a fast deactivation of this catalyst.
M. Cortazar,L. Santamaria,G. Lopez,J. Alvarez,M. Amutio,J. Bilbao,M. Olazar 한국공업화학회 2021 Journal of Industrial and Engineering Chemistry Vol.99 No.-
The performance of Fe/olivine catalysts was tested in the continuous steam gasification of sawdust in abench scale plant provided with a fountain confined conical spouted bed reactor at 850 C. Olivine wasused as catalyst support and loaded with 5 wt%Fe. The activity and stability of the catalyst was monitoredby nitrogen adsorption-desorption, X-rayfluorescence spectroscopy, temperature programmedreduction, X-ray diffraction and X-ray photoelectron spectroscopy techniques, which were conductedbefore and after the runs. The fountain confined conical spouted bed performs well in the biomass steamgasification with primary catalysts. In fact, this reactor allows enhancing the gas-solid contact, andtherefore the catalytic activity by avoiding the elutriation offine catalyst particles. The uncatalysedefficiency of the gasification process, assessed based on the gas production and composition, H2production, tar concentration and composition, and carbon conversion efficiency, was considerabyimproved on the Fe/olivine catalyst, with tar reduction being especially remarkable (to 10.4 g Nm 3). After 140 min on stream, catalyst deactivation was particularly evident, as tar concentration increased to19.9 g Nm 3 (90% of that without catalyst). However, Fe/olivine catalyst was still active for WGS and CH4steam reforming reactions, with gas and H2 productions being 1.35 Nm3 kg 1 and 5.44 wt%, respectively. Metal iron oxidation to Fe3O4 caused catalyst deactivation, as the reaction environment shifted fromoxidizing to reducing conditions due to operational limitations.