Une solution généralisée de l'équation de Poisson

Grâce à une représentation analytique combinée avec des procédures numériques appropriées, nous avons pu développer une solution approximative généralisée de l'équation de Poisson. Ayant une allure relativement très simple ainsi qu'une exactitude très poussée, cette solution met en évidence la composition macroscopique de la charge d'espace constituée de porteurs libres

Limite de la région quasi neutre dans un semiconducteur

Nous proposons une définition généralisée de la limite de la région quasi neutre du semiconducteur contenant une zone d'accumulation. Cette limite est fixée à l'endroit où deux facteurs macroscopiques de perturbation électrique sont égaux. Ainsi la région quasi neutre garde toujours les mêmes propriétés électriques, i. e. du poentiel macroscopique, du champ électrique interne, de la densité de charge, indépendamment du matériau et de l'intensité de la perturbation de la neutralité électrique du cristal. Les facteurs de perturbation sont déterminés dans le cadre d'une analyse approfondie de la loi électrostatique des interfaces abruptes dans les semiconducteurs non dégénérés à l'état d'équilibre thermodynamique. Ils n'ont pas d'interprétation physique simple. L'un d'eux caractérise la perturbation électrique en fonction des propriétés du semiconducteur considéré, l'autre facteur est tout à fait indépendant de ces propriétés. Notre définition est du type macroscopique. Elle est déterminée dans le cadre de la statistique de Maxwell-Boltzmann, et a pour but de remplacer toutes les définitions plus ou moins arbitraires de l'épaisseur de la zone d'accumulation, jusqu'à présent couramment utilisées dans la modélisation ou simulation des dispositifs microélectronique. Elle peut être facilement généralisée à n'importe quelle perturbation électrique (statique ou dynamique) provoquant l'accumulation des porteurs libres

Microscopic analysis of Orai-mediated store-operated calcium entry in cells with experimentally altered levels of amyloid precursor protein

AbstractFamilial Alzheimer's disease (FAD)-causing mutations in presenilins were shown to alter intracellular calcium dynamics, including store-operated calcium entry (SOCE). However, the involvement of FAD-linked amyloid precursor protein (APP) in SOCE remains controversial. Here, we used gain-of-function and loss-of-function approaches to shed light on this issue. We found that Jurkat cells, which exhibit prominent SOCE mediated by Orai channels, maintain low APP levels. The ectopic expression of APP, either with wildtype sequence or FAD-causing Swedish mutation, had no effect on SOCE induced by calcium store depletion with cyclopiazonic acid (CPA). The overproduction of C99 fragments, mimicking amyloidogenic processing of APP, also had no effect. Moreover, there was no alteration in the CPA-evoked SOCE upon APP knockdown in HeLa cells, which natively express 100-fold more APP than Jurkat cells. Consistently, we found no evidence for APP-dependent changes in the mRNA or protein levels of main SOCE components. Altogether, these results suggest that APP does not modulate Orai-dependent SOCE following quantitative calcium store depletion

Analysis of calcium homeostasis in fresh lymphocytes from patients with sporadic Alzheimer's disease or mild cognitive impairment

AbstractAlzheimer's disease (AD) is the most widespread, age-related neurodegenerative disorder. Its two subtypes are sporadic AD (SAD) of unknown etiology and genetically encoded familial AD (FAD). The onset of AD is often preceded by mild cognitive impairment (MCI). Calcium dynamics were found to be dysregulated in FAD models, but little is known about the features of calcium dynamics in SAD. To explore calcium homeostasis during the early stages of SAD, we investigated store-operated calcium entry (SOCE) and inositol triphosphate receptor (IP3R)-mediated calcium release into the cytoplasm in unmodified B lymphocytes from MCI and SAD patients and compared them with non-demented subjects (NDS). Calcium levels in the endoplasmic reticulum and both the rising and falling SOCE slopes were very similar in all three groups. However, we found that SAD and MCI cells were more prone to IP3R activation than NDS cells, and increases in calcium levels in the cytoplasm were almost twice as frequent in SAD cells than in NDS cells. MCI cells and SAD cells exhibited an enhanced magnitude of calcium influx during SOCE. MCI cells but not SAD cells were characterized by higher basal cellular calcium levels than NDS cells. In summary, perturbed calcium homeostasis was observed in peripheral cells from MCI and SAD patients. Thus, lymphocytes obtained from MCI subjects may be promising in the early diagnosis of individuals who will eventually develop SAD. However, no conclusions are made regarding SAD due to the limited number patients. This article is part of a Special Issue entitled: 12th European Symposium on Calcium

Geometrical factor in multi-interface homostructures

In an experimental investigation carried out on multilayer silicon homostructures (of the n+-n-n+ type) an electric interaction between the two homointerfaces through majority carriers as a function of the geometrical factor has been shown. The two model homointerfaces were formed in the same monocrystal (CVD epitaxy) by only an abrupt change in the impurity doping level. Samples differing in the spacing of their parallel interfaces were characterized in the stationary and steady states (static and quasi-static regimes) to reveal modifications in their macroscopic transport. It has been demonstrated that the thermionic emission current, normally taken into account in the barrier transport (with large potential barriers) dominates in only two of five characteristic bias intervals. The two other phenomena, also related to the geometrical factor, i.e. the diffusion-drift and tunneling currents, dominate the conduction over practically the whole dc-bias range. These results allow the study of short and long range electrical interactions as well as the free carrier micromovement of simple and complex semiconductor interfaces of multilayer devices

Differential Roles for STIM1 and STIM2 in Store-Operated Calcium Entry in Rat Neurons

The interaction between Ca2+ sensors STIM1 and STIM2 and Ca2+ channel-forming protein ORAI1 is a crucial element of store-operated calcium entry (SOCE) in non-excitable cells. However, the molecular mechanism of SOCE in neurons remains unclear. We addressed this issue by establishing the presence and function of STIM proteins. Real-time polymerase chain reaction from cortical neurons showed that these cells contain significant amounts of Stim1 and Stim2 mRNA. Thapsigargin (TG) treatment increased the amount of both endogenous STIM proteins in neuronal membrane fractions. The number of YFP-STIM1/ORAI1 and YFP-STIM2/ORAI1 complexes was also enhanced by such treatment. The differences observed in the number of STIM1 and STIM2 complexes under SOCE conditions and the differential sensitivity to SOCE inhibitors suggest their distinct roles. Endoplasmic reticulum (ER) store depletion by TG enhanced intracellular Ca2+ levels in loaded with Fura-2 neurons transfected with YFP-STIM1 and ORAI1, but not with YFP-STIM2 and ORAI1, which correlated well with the number of complexes formed. Moreover, the SOCE inhibitors ML-9 and 2-APB reduced Ca2+ influx in neurons expressing YFP-STIM1/ORAI1 but produced no effect in cells transfected with YFP-STIM2/ORAI1. Moreover, in neurons transfected with YFP-STIM2/ORAI1, the increase in constitutive calcium entry was greater than with YFP-STIM1/ORAI1. Our data indicate that both STIM proteins are involved in calcium homeostasis in neurons. STIM1 mainly activates SOCE, whereas STIM2 regulates resting Ca2+ levels in the ER and Ca2+ leakage with the additional involvement of STIM1

Dynamic Interactions between a Silica Sphere and Deformable Interfaces in Organic Solvents Studied by Atomic Force Microscopy.

Recent studies have successfully measured surface forces using atomic force microscope (AFM) and modelled surface deformations using the Stokes-Reynolds-Young-Laplace (SRYL) equations for particle-droplet, particle-bubble, droplet-droplet and bubble-bubble systems in various solutions. The current work focuses on interactions between spherical silica particles and a viscoelastic interface of water droplets in crude oil. The self-assembly of surface active natural polyaromatic molecules (NPAMs) at the oil-water interface has previously been shown to change a viscous dominant oil-water interface to an elastic dominant interface, with interfacial aging due to gradual formation of rigid interfacial networks. AFM was used to measure the interactions between a small silica sphere (D ≈ 8 µm) and a deformable water droplet (D ≈ 70 µm), which exhibits time-dependent interfacial viscoelasticity in NPAM solutions. Unlike the systems studied previously, the measured deformation shown as a repulsive force over the constant compliance could not be modelled adequately by the conventional SRYL equations which are applicable only to purely Laplacian interfaces. As the water droplet ages in NPAM solutions, a rigid "skin" forms at the oil-water interface, with the interface exhibiting increased elasticity. Over a short aging period (up to 15 min), interfacial deformation is well predicted by the SRYL model. However upon further exposure to the NPAM solution, droplet deformation is under predicted by the model. Physical properties of this mechanical barrier as a function of time were further investigated by measuring interfacial tension, dilatational rheology and interfacial "crumpling" (non-smooth interface) upon droplet volume reduction. By introducing a viscoelasticity parameter to account for interfacial stiffening, we are able to correct this discrepancy and predict droplet deformation under AFM cantilever compression using experimentally-determined elasticity. This parameter appears to be important for modelling non-Laplacian systems with significant viscoelastic contributions, such as biological cell membranes or polymer blends