Le courant K+ rectifiant entrant du muscle (son rôle dans la plasticité à long terme de la synapse neuromusculaire)

LE KREMLIN-B.- PARIS 11-BU Méd (940432101) / SudocPARIS-BIUP (751062107) / SudocSudocFranceF

A novel synaptic plasticity rule explains homeostasis of neuromuscular transmission.

International audienceExcitability differs among muscle fibers and undergoes continuous changes during development and growth, yet the neuromuscular synapse maintains a remarkable fidelity of execution. Here we show in two evolutionarily distant vertebrates (Xenopus laevis cell culture and mouse nerve-muscle ex-vivo) that the skeletal muscle cell constantly senses, through two identified calcium signals, synaptic events and their efficacy in eliciting spikes. These sensors trigger retrograde signal(s) that control presynaptic neurotransmitter release, resulting in synaptic potentiation or depression. In the absence of spikes, synaptic events trigger potentiation. Once the synapse is sufficiently strong to initiate spiking, the occurrence of these spikes activates a negative retrograde feedback. These opposing signals dynamically balance the synapse in order to continuously adjust neurotransmitter release to a level matching current muscle cell excitability

Le NAADP, un messager libérant du calcium (étude de l'implication des réserves acides et rôle de CD38 dans la voie de synthèse)

LE KREMLIN-B.- PARIS 11-BU Méd (940432101) / SudocPARIS-BIUP (751062107) / SudocSudocFranceF

Confocal imaging and tracking of the exocytotic routes for D-serine-mediated gliotransmission.

D-Serine is an astrocyte-derived regulator for N-methyl-D-aspartate receptors, but the intracellular routes of its trafficking are still largely unknown. Here, we combined confocal microscopy with colocalization quantification to track the astrocytic organelles that store D-serine. We report that D-serine colocalizes with the transfected eGFP-synaptobrevin/VAMP2 and eGFP-cellubrevin/VAMP3, two v-SNAREs of the regulated secretory pathway. No significant colocalization was found with markers of the endosomal sorting and recycling system: EEA1, eGFP-endobrevin/VAMP8, eGFP-TI-VAMP/VAMP7, LAMP1, and CD63. Blockade of vesicular budding with colchicine shows that secretory vesicles import D-serine downstream to the Golgi apparatus. Finally, treatment of astrocytes with the Ca2+-ionophore A23187, glutamate agonists, or bradykinin trigger translocation of synaptobrevin/VAMP2 to the plasma membrane with a concomitant disappearance of D-serine from the regulated secretory pathway. Our results provide morphological evidence for a vesicular storage of D-serine in the regulated secretory pathway and the possible recruitment of these stores by Ca2+ mobilization to release D-serine

Glutamate receptor activation triggers a calcium-dependent and SNARE protein-dependent release of the gliotransmitter D-serine

The gliotransmitter d-serine is released upon (S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate and metabotropic glutamate receptor stimulation, but the mechanisms involved are unknown. Here, by using a highly sensitive bioassay to continuously monitor extracellular d-serine levels, we have investigated the pathways used in its release. We reveal that d-serine release is inhibited by removal of extracellular calcium and augmented by increasing extracellular calcium or after treatment with the Ca(2+) ionophore A23187. Furthermore, release of the amino acid is considerably reduced after depletion of thapsigargin-sensitive intracellular Ca(2+) stores or chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetate–acetoxymethyl ester. Interestingly, d-serine release also was markedly reduced by concanamycin A, a vacuolar-type H(+)-ATPase inhibitor, indicating a role for the vesicular proton gradient in the transmitter storage/release. In addition, agonist-evoked d-serine release was sensitive to tetanus neurotoxin. Finally, immunocytochemical and sucrose density gradient analysis revealed that a large fraction of d-serine colocalized with synaptobrevin/VAMP2, suggesting that it is stored in VAMP2-bearing vesicles. In summary, our study reveals the cellular mechanisms subserving d-serine release and highlights the importance of the glial cell exocytotic pathway in influencing CNS levels of extracellular d-serine

A conotoxin from Conus textile with unusual posttranslational modifications reduces presynaptic Ca(2+) influx

Cone snails are gastropod mollusks of the genus Conus that live in tropical marine habitats. They are predators that paralyze their prey by injection of venom containing a plethora of small, conformationally constrained peptides (conotoxins). We report the identification, characterization, and structure of a γ-carboxyglutamic acid-containing peptide, conotoxin ɛ-TxIX, isolated from the venom of the molluscivorous cone snail, Conus textile. The disulfide bonding pattern of the four cysteine residues, an unparalleled degree of posttranslational processing including bromination, hydroxylation, and glycosylation define a family of conotoxins that may target presynaptic Ca(2+) channels or act on G protein-coupled presynaptic receptors via another mechanism. This conotoxin selectively reduces neurotransmitter release at an Aplysia cholinergic synapse by reducing the presynaptic influx of Ca(2+) in a slow and reversible fashion. The three-dimensional structure, determined by two-dimensional (1)H NMR spectroscopy, identifies an electronegative patch created by the side chains of two γ-carboxyglutamic acid residues that extend outward from a cavernous cleft. The glycosylated threonine and hydroxylated proline enclose a localized hydrophobic region centered on the brominated tryptophan residue within the constrained intercysteine region