A Mechanism for the Polarity Formation of Chemoreceptors at the Growth Cone Membrane for Gradient Amplification during Directional Sensing

Accurate response to external directional signals is essential for many physiological functions such as chemotaxis or axonal guidance. It relies on the detection and amplification of gradients of chemical cues, which, in eukaryotic cells, involves the asymmetric relocalization of signaling molecules. How molecular events coordinate to induce a polarity at the cell level remains however poorly understood, particularly for nerve chemotaxis. Here, we propose a model, inspired by single-molecule experiments, for the membrane dynamics of GABA chemoreceptors in nerve growth cones (GCs) during directional sensing. In our model, transient interactions between the receptors and the microtubules, coupled to GABA-induced signaling, provide a positive-feedback loop that leads to redistribution of the receptors towards the gradient source. Using numerical simulations with parameters derived from experiments, we find that the kinetics of polarization and the steady-state polarized distribution of GABA receptors are in remarkable agreement with experimental observations. Furthermore, we make predictions on the properties of the GC seen as a sensing, amplification and filtering module. In particular, the growth cone acts as a low-pass filter with a time constant ∼10 minutes determined by the Brownian diffusion of chemoreceptors in the membrane. This filtering makes the gradient amplification resistent to rapid fluctuations of the external signals, a beneficial feature to enhance the accuracy of neuronal wiring. Since the model is based on minimal assumptions on the receptor/cytoskeleton interactions, its validity extends to polarity formation beyond the case of GABA gradient sensing. Altogether, it constitutes an original positive-feedback mechanism by which cells can dynamically adapt their internal organization to external signals

Dynamique de récepteurs uniques du GABAA dans le cône de croissance : rôle dans la détection de signaux de guidage

During the development of the nervous system, extending axons choose accurately theirdirections of growth. This step relies on the sensitive detection of guidance cues by receptorsin the membrane of the growth cone. We studied the dynamics of single GABAreceptors tagged by fluorescent nanocrystals. The receptors are initially homogeneouslydistributed in the growth cone membrane and are specifically relocalized toward the sourcewhen submitted to a GABA gradient. This reorganization is due to interactions with themicrotubules that have been identified by single receptor trajectory analysis and that displacethe receptors toward regions of high GABA concentration. The functional role ofthis phenomenon has been revealed by the measurement of a receptor redistribution inducedan amplification in the asymmetry of calcium concentration. These results supporta model of receptor self-organization providing an amplification of the external signal. Aweak asymmetric activation of receptors causes a weak asymmetry of calcium concentrationand of the cytoskeleton organization. This creates a positive feedback loop by favoringreceptor redistribution that amplifies the gradient induced signal by enhancing the asymmetryin the calcium concentration. The self-organization of the receptors can be correctlydescribed by a system of stochastic equations that has been analytically and numericallystudied. This study provides a general mechanism of amplification of external signals ingrowing axons and also in chemotactic or polarized systems.Lors du développement du système nerveux, les axones en croissance choisissent unedirection d'extension avec précision. Ce processus est permis par la détection sensible designaux de guidage par les récepteurs membranaires de la membrane du cône de croissance.Nous avons étudié la dynamique de récepteurs individuels du GABA marqués pardes nanocristaux fluorescents. Les récepteurs répartis également dans la membrane d'uncône de croissance soumis à un gradient de GABA sont spécifiquement redistribués vers sasource. Cette réorganisation est due à des interactions avec les microtubules déplaçant lesrécepteurs vers les régions à forte concentration de GABA, mises en évidence sur les trajectoiresdes récepteurs. Son rôle fonctionnel a été révélé par la mesure d'une amplificationde l'asymétrie de la concentration intracellulaire de calcium, qui régule l'organisation ducytosquelette. Ces observations permettent l'élaboration d'un modèle d'auto-organisationdes récepteurs, permettant une amplification du signal extérieur. Une faible activationasymétrique des récepteurs induit une asymétrie de concentration de calcium et de l'organisationdu cytosquelette. Ceci crée une boucle de rétroaction positive en favorisantla redistribution, qui renforce le signal induit par le gradient en amplifiant l'asymétriede la concentration de récepteurs et de calcium. L'auto-organisation des récepteurs estdécrite par un système d'équations stochastiques étudiées numériquement et analytiquement.Les travaux présentés permettent de proposer un mécanisme général d'amplificationde signaux extérieurs dans l'axone en croissance et dans les systèmes chimiotactiques oupolarisés

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Recording of GdVO4:Eu nanoparticle luminescence at 617 nm after injection in an anesthetized mouse ear (Nikon AxioZoom AZ100, Objective magnification x2, zoom 1.3, 6 mW.cm-2 excitation at 466 nm). Total duration 12 min and application of methylsalicylate after 2 mi

Particules d'oxyde à base de terres rares et utilisation notamment en imagerie

La présente demande concerne des produits composites multimodaux pour l'imagerie, en particulier pour l'imagerie diagnostique, et optionnellement pour thérapie, en particulier des produits composites capables d'être utilisés comme agents de contraste en particulier en imagerie par résonance magnétique (IRM), et/ou dans des techniques d'imagerie, comme par exemple en imagerie optique, en détection optique d'oxydants, en tomographie par émission de positrons (TEP), en tomodensitométrie (TDM) et/ou en imagerie par ultrasons et optionnellement simultanément utilisables en thérapie. Ces produits sont basés sur une particule comprenant ou consistant en une partie pourvue d'une activité d'agent de contraste et/ou d'une activité paramagnétique, et une partie pourvue d'une activité luminescente et optionnellement d'une activité de détection d'oxydant

Receptor displacement in the cell membrane by hydrodynamic force amplification through nanoparticles.

International audienceWe introduce an intrinsically multiplexed and easy to implement method to apply an external force to a biomolecule and thus probe its interaction with a second biomolecule or, more generally, its environment (for example, the cell membrane). We take advantage of the hydrodynamic interaction with a controlled fluid flow within a microfluidic channel to apply a force. By labeling the biomolecule with a nanoparticle that acts as a kite and increases the hydrodynamic interaction with the fluid, the drag induced by convection becomes important. We use this approach to track the motion of single membrane receptors, the Clostridium perfringens ε-toxin (CPεT) receptors that are confined in lipid raft platforms, and probe their interaction with the environment. Under external force, we observe displacements over distances up to 10 times the confining domain diameter due to elastic deformation of a barrier and return to the initial position after the flow is stopped. Receptors can also jump over such barriers. Analysis of the receptor motion characteristics before, during, and after a force is applied via the flow indicates that the receptors are displaced together with their confining raft platform. Experiments before and after incubation with latrunculin B reveal that the barriers are part of the actin cytoskeleton and have an average spring constant of 2.5 ± 0.6 pN/μm before vs. 0.6 ± 0.2 pN/μm after partial actin depolymerization. Our data, in combination with our previous work demonstrating that the ε-toxin receptor confinement is not influenced by the cytoskeleton, imply that it is the raft platform and its constituents rather than the receptor itself that encounters and deforms the barriers formed by the actin cytoskeleton