Antoine Triller

Docosahexaenoic Acid Protects from Dendritic Pathology in an Alzheimer's Disease Mouse Model

Learning and memory depend on dendritic spine actin assembly and docosahexaenoic acid (DHA), an essential n-3 (omega-3) polyunsaturated fatty acid (PFA). High DHA consumption is associated with reduced Alzheimers disease (AD) risk, yet mechanisms and therapeutic potential remain elusive. Here, we report that reduction of dietary n-3 PFA in an AD mouse model resulted in 80%-90% losses of the p85alpha subunit of phosphatidylinositol 3-kinase and the postsynaptic actin-regulating protein drebrin, as in AD brain. The loss of postsynaptic proteins was associated with increased oxidation, without concomitant neuron or presynaptic protein loss. n-3 PFA depletion increased caspase-cleaved actin, which was localized in dendrites ultrastructurally. Treatment of n-3 PFA-restricted mice with DHA protected against these effects and behavioral deficits and increased antiapoptotic BAD phosphorylation. Since n-3 PFAs are essential for p85-mediated CNS insulin signaling and selective protection of postsynaptic proteins, these findings have implications for neurodegenerative diseases where synaptic loss is critical, especially AD.

Otoferlin, Defective in a Human Deafness Form, Is Essential for Exocytosis at the Auditory Ribbon Synapse

The auditory inner hair cell (IHC) ribbon synapse operates with an exceptional temporal precision and maintains a high level of neurotransmitter release. However, the molecular mechanisms underlying IHC synaptic exocytosis are largely unknown. We studied otoferlin, a predicted C2-domain transmembrane protein, which is defective in a recessive form of human deafness. We show that otoferlin expression in the hair cells correlates with afferent synaptogenesis and find that otoferlin localizes to ribbon-associated synaptic vesicles. Otoferlin binds Ca(2+) and displays Ca(2+)-dependent interactions with the SNARE proteins syntaxin1 and SNAP25. Otoferlin deficient mice (Otof(-/-)) are profoundly deaf. Exocytosis in Otof(-/-) IHCs is almost completely abolished, despite normal ribbon synapse morphogenesis and Ca(2+) current. Thus, otoferlin is essential for a late step of synaptic vesicle exocytosis and may act as the major Ca(2+) sensor triggering membrane fusion at the IHC ribbon synapse.

A long nuclear-retained non-coding RNA regulates synaptogenesis by modulating gene expression

A growing number of long nuclear‐retained non‐coding RNAs (ncRNAs) have recently been described. However, few functions have been elucidated for these ncRNAs. Here, we have characterized the function of one such ncRNA, identified as metastasis‐associated lung adenocarcinoma transcript 1 (Malat1). Malat1 RNA is expressed in numerous tissues and is highly abundant in neurons. It is enriched in nuclear speckles only when RNA polymerase II‐dependent transcription is active. Knock‐down studies revealed that Malat1 modulates the recruitment of SR family pre‐mRNA‐splicing factors to the transcription site of a transgene array. DNA microarray analysis in Malat1‐depleted neuroblastoma cells indicates that Malat1 controls the expression of genes involved not only in nuclear processes, but also in synapse function. In cultured hippocampal neurons, knock‐down of Malat1 decreases synaptic density, whereas its over‐expression results in a cell‐autonomous increase in synaptic density. Our results suggest that Malat1 regulates synapse formation by modulating the expression of genes involved in synapse formation and/or maintenance.

The role of receptor diffusion in the organization of the postsynaptic membrane

Neurotransmitter receptor movement into and out of synapses is one of the core mechanisms for rapidly changing the number of functional receptors during synaptic plasticity. Recent data have shown that rapid gain and loss of receptors from synaptic sites are accounted for by endocytosis and exocytosis, as well as by lateral diffusion of receptors in the plane of the membrane. These events are interdependent and are regulated by neuronal activity and interactions with scaffolding proteins. Here we focus on the physical laws that govern receptor diffusion and stabilization, and how this might reshape how we think about the specific regulation of receptor accumulation at synapses.

Microglia activation triggers astrocyte-mediated modulation of excitatory neurotransmission

Fine control of neuronal activity is crucial to rapidly adjust to subtle changes of the environment. This fine tuning was thought to be purely neuronal until the discovery that astrocytes are active players of synaptic transmission. In the adult hippocampus, microglia are the other major glial cell type. Microglia are highly dynamic and closely associated with neurons and astrocytes. They react rapidly to modifications of their environment and are able to release molecules known to control neuronal function and synaptic transmission. Therefore, microglia display functional features of synaptic partners, but their involvement in the regulation of synaptic transmission has not yet been addressed. We have used a combination of pharmacological approaches with electrophysiological analysis on acute hippocampal slices and ATP assays in purified cell cultures to show that activation of microglia induces a rapid increase of spontaneous excitatory postsynaptic currents. We found that this modulation is mediated by binding of ATP to P2Y1R located on astrocytes and is independent of TNFα or NOS2. Our data indicate that, on activation, microglia cells rapidly release small amounts of ATP, and astrocytes, in turn, amplified this release. Finally, P2Y1 stimulation of astrocytes increased excitatory postsynaptic current frequency through a metabotropic glutamate receptor 5-dependent mechanism. These results indicate that microglia are genuine regulators of neurotransmission and place microglia as upstream partners of astrocytes. Because pathological activation of microglia and alteration of neurotransmission are two early symptoms of most brain diseases, our work also provides a basis for understanding synaptic dysfunction in neuronal diseases.

Dopamine D3 Receptors Expressed by All Mesencephalic Dopamine Neurons

A polyclonal antibody was generated using synthetic peptides designed in a specific sequence of the rat D3 receptor (D3R). Using transfected cells expressing recombinant D3R, but not D2 receptor, this antibody labeled 45–80 kDa species in Western blot analysis, immunoprecipitated a soluble fraction of [125I]iodosulpride binding, and generated immunofluorescence, mainly in the cytoplasmic perinuclear region of the cells. In rat brain, the distribution of immunoreactivity matched that of D3R binding, revealed using [125I]R(+)trans-7-hydroxy-2-[N-propyl-N-(3′-iodo-2′-propenyl)amino] tetralin ([125I]7-trans-OH-PIPAT), with dense signals in the islands of Calleja and mammillary bodies, and moderate to low signals in the shell of nucleus accumbens (AccSh), frontoparietal cortex, substantia nigra (SN), ventral tegmental area (VTA) and lobules 9 and 10 of the cerebellum. Very low or no signals could be detected in other rat brain regions, including dorsal striatum, or in D3R-deficient mouse brain. Labeling of perikarya of AccSh and SN/VTA appeared with a characteristic punctuate distribution, mostly at the plasma membrane where it was not associated with synaptic boutons, as revealed by synaptophysin immunoreactivity. In SN/VTA, D3R immunoreactivity was found on afferent terminals, arising from AccSh, in which destruction of intrinsic neurons by kainate infusions produced a loss of D3R binding in both AccSh and SN/VTA. D3R-immunoreactivity was also found in all tyrosine hydroxylase (TH)-positive neurons observed in SN, VTA and A8 retrorubral fields, where it could represent D3autoreceptors controlling dopamine neuron activities, in agreement with the elevated dopamine extracellular levels in projection areas of these neurons found in D3R-deficient mice.

Deleterious Effects of Amyloid β Oligomers Acting as an Extracellular Scaffold for mGluR5

Soluble oligomers of amyloid beta (Abeta) play a role in the memory impairment characteristic of Alzheimers disease. Acting as pathogenic ligands, Abeta oligomers bind to particular synapses and perturb their function, morphology, and maintenance. Events that occur shortly after oligomer binding have been investigated here in live hippocampal neurons by single particle tracking of quantum dot-labeled oligomers and synaptic proteins. Membrane-attached oligomers initially move freely, but their diffusion is hindered markedly upon accumulation at synapses. Concomitantly, individual metabotropic glutamate receptors (mGluR5) manifest strikingly reduced lateral diffusion as they become aberrantly clustered. This clustering of mGluR5 elevates intracellular calcium and causes synapse deterioration, responses prevented by an mGluR5 antagonist. As expected, clustering by artificial crosslinking also promotes synaptotoxicity. These results reveal a mechanism whereby Abeta oligomers induce the abnormal accumulation and overstabilization of a glutamate receptor, thus providing a mechanistic and molecular basis for Abeta oligomer-induced early synaptic failure.

Three-Dimensional Architecture of Presynaptic Terminal Cytomatrix

Presynaptic terminals are specialized for mediating rapid fusion of synaptic vesicles (SVs) after calcium influx. The regulated trafficking of SVs likely results from a highly organized cytomatrix. How this cytomatrix links SVs, maintains them near the active zones (AZs) of release, and organizes docked SVs at the release sites is not fully understood. To analyze the three-dimensional (3D) architecture of the presynaptic cytomatrix, electron tomography of presynaptic terminals contacting spines was performed in the stratum radiatum of the rat hippocampal CA1 area. To preserve the cytomatrix, hippocampal slices were immobilized using high-pressure freezing, followed by cryosubstitution and embedding. SVs are surrounded by a dense network of filaments. A given vesicle is connected to ∼1.5 neighboring ones. SVs at the periphery of this network are also linked to the plasma membrane, by longer filaments. More of these filaments are found at the AZ. At the AZ, docked SVs are grouped around presynaptic densities. Filaments with adjacent SVs emerge from these densities. Immunogold localizations revealed that synapsin is located in the presynaptic bouton, whereas Bassoon and CAST (ERC2) are at focal points next to the AZ. In synapsin triple knock-out mice, the number of SVs is reduced by 63%, but the size of the boutons is reduced by only 18%, and the mean distance of SVs to the AZ is unchanged. This 3D analysis reveals the morphological constraints exerted by the presynaptic molecular scaffold. SVs are tightly interconnected in the axonal bouton, and this network is preferentially connected to the AZ.

Surface trafficking of receptors between synaptic and extrasynaptic membranes: and yet they do move!

Concentration of neurotransmitter receptors at synapses is thought to result from stable binding to subsynaptic scaffold proteins. Recent data on synaptic plasticity have shown that changes in synaptic strength derive partly from modification of postsynaptic receptor numbers. This has led to the notion of receptor trafficking into and out of synapses. The proposed underlying mechanisms have under-evaluated the role of extrasynaptic receptors. Recent technological advances have allowed imaging of receptor movements at the single-molecule level, and these experiments demonstrate that receptors switch at unexpected rates between extrasynaptic and synaptic localizations by lateral diffusion. Variation in receptor numbers at postsynaptic sites is therefore likely to depend on regulation of diffusion by modification of the structure of the membrane and/or by transient interactions with scaffolding proteins. This review is part of the TINS Synaptic Connectivity series.

Fast and reversible trapping of surface glycine receptors by gephyrin

Variations in receptor number at a given synapse are known to contribute to synaptic plasticity, but methods used to establish this idea usually do not allow for the determination of the dynamics of these phenomena. We used single-particle tracking to follow in real time, on the cell surface, movements of the glycine receptor (GlyR) with or without the GlyR stabilizing protein gephyrin. GlyR alternated within seconds between diffusive and confined states. In the absence of gephyrin, GlyR were mostly freely diffusing. Gephyrin induced long confinement periods spatially associated with submembranous clusters of gephyrin. However, even when most receptors were stabilized, they still frequently made transitions through the diffusive state. These data show that receptor number in a cluster results from a dynamic equilibrium between the pools of stabilized and freely mobile receptors. Modification of this equilibrium could be involved in regulation of the number of receptors at synapses.