An improved method for the preparation of rat cerebellar glomeruli

Shisa7 is a GABAA receptor auxiliary subunit controlling benzodiazepine actions

The function and pharmacology of γ-aminobutyric acid type A receptors (GABA_ARs) are of great physiological and clinical importance and have long been thought to be determined by the channel pore-forming subunits. We discovered that Shisa7, a single-passing transmembrane protein, localizes at GABAergic inhibitory synapses and interacts with GABA_ARs. Shisa7 controls receptor abundance at synapses and speeds up the channel deactivation kinetics. Shisa7 also potently enhances the action of diazepam, a classic benzodiazepine, on GABA_ARs. Genetic deletion of Shisa7 selectively impairs GABAergic transmission and diminishes the effects of diazepam in mice. Our data indicate that Shisa7 regulates GABA_AR trafficking, function, and pharmacology and reveal a previously unknown molecular interaction that modulates benzodiazepine action in the brain.

GARLH Family Proteins Stabilize GABAA Receptors at Synapses

Ionotropic neurotransmitter receptors mediate fast synaptic transmission by functioning as ligand-gated ion channels. Fast inhibitory transmission in the brain is mediated mostly by ionotropic GABA_A receptors (GABA_ARs), but their essential components for synaptic localization remain unknown. Here, we identify putative auxiliary subunits of GABA_ARs, which we term GARLHs, consisting of LH4 and LH3 proteins. LH4 forms a stable tripartite complex with GABA_ARs and neuroligin-2 in the brain. Moreover, LH4 is required for the synaptic localization of GABA_ARs and inhibitory synaptic transmission in the hippocampus. Our findings propose GARLHs as the first identified auxiliary subunits for anion channels. These findings provide new insights into the regulation of inhibitory transmission and the molecular constituents of native anion channels in vivo.

Transcriptomic Analysis of Ribosome-Bound mRNA in Cortical Neurites In Vivo

Localized translation in neurites helps regulate synaptic strength and development. Dysregulation of local translation is associated with many neurological disorders. However, due to technical limitations, study of this phenomenon has largely been limited to brain regions with laminar organization of dendrites such as the hippocampus or cerebellum. It has not been examined in the cortex, a region of importance for most neurological disorders, where dendrites of each neuronal population are densely intermingled with cell bodies of others. Therefore, we have developed a novel method, SynapTRAP, which combines synaptoneurosomal fractionation with translating ribosome affinity purification to identify ribosome-bound mRNA in processes of genetically defined cell types. We demonstrate SynapTRAP's efficacy and report local translation in the cortex of mice, where we identify a subset of mRNAs that are translated in dendrites by neuronal ribosomes. These mRNAs have disproportionately longer lengths, enrichment for FMRP binding and G-quartets, and their genes are under greater evolutionary constraint in humans. In addition, we show that alternative splicing likely regulates this phenomenon. Overall, SynapTRAP allows for rapid isolation of cell-type-specific localized translation and is applicable to classes of previously inaccessible neuronal and non-neuronal cells in vivoSIGNIFICANCE STATEMENT Instructions for making proteins are found in the genome, housed within the nucleus of each cell. These are then copied as RNA and exported to manufacture new proteins. However, in the brain, memory is thought to be encoded by strengthening individual connections (synapses) between neurons far from the nucleus. Thus, to efficiently make new proteins specifically where they are needed, neurons can transport RNAs to sites near synapses to locally produce proteins. Importantly, several mutations that cause autism disrupt this process. It has been assumed this process occurs in all brain regions, but has never been measured in the cortex. We applied a newly developed method measure to study, for the first time, local translation in cortical neurons.

Behavioral and cerebellar transmission deficits in mice lacking the autism-linked gene islet brain-2

Deletion of the human SHANK3 gene near the terminus of chromosome 22q is associated with Phelan-McDermid syndrome and autism spectrum disorders. Nearly all such deletions also span the tightly linked IB2 gene. We show here that IB2 protein is broadly expressed in the brain and is highly enriched within postsynaptic densities. Experimental disruption of the IB2 gene in mice reduces AMPA and enhances NMDA receptor-mediated glutamatergic transmission in cerebellum, changes the morphology of Purkinje cell dendritic arbors, and induces motor and cognitive deficits suggesting an autism phenotype. These findings support a role for human IB2 mutation as a contributing genetic factor in Chr22qter-associated cognitive disorders.

Cerebellum as a target for toxic substances

The Purkinje cells and the granule cells are the most important targets in cerebellum for toxic substances. The Purkinje cells are among the largest neuron in the brain and are very sensitive to ischaemia, bilirubin, ethanol and diphenylhydantoin. The granule cells are small and seem to be sensitive to loss of intracellular glutathione. Granule cells are sensitive to methyl halides, thiophene, methyl mercury, 2-chloropropionic acid and trichlorfon. The Purkinje cells appear in the rat brain on pre-natal day 14-16, whereas the granule cells appear post-natally. Both cells are sensitive to excitotoxic chemicals and also to an effect on DNA or its repair mechanisms.

Muscimol and flunitrazepam binding sites in a subcellular fraction enriched in rat cerebellar glomeruli

In the internal granular layer of the cerebellar cortex the polysynaptic complexes called glomeruli consist mainly of homogeneous populations of glutamatergic and GABAergic synapses, both located on granule cell dendrites. A subcellular fraction enriched in glomeruli was prepared from rat cerebellum, and the distribution of GABAA and of benzodiazepine binding sites between membranes derived from this fraction (fraction G) and from a total cerebellar homogenate (fraction T) was studied. The benzodiazepine and GABA binding sites were measured by the binding of agonists [3H]flunitrazepam and [3H]muscimol, respectively. The results indicate that both binding sites are present, but only slightly enriched, in the glomerular synapses. We found a muscimol/flunitrazepam binding site ratio of two, which is consistent with the enrichment of muscimol binding sites in the granular layer shown by both autoradiographic with radioactive glutamatergic ligands and in situ hybridization experiments respectively.

Non-NMDA excitatory amino acid receptors in a subcellular fraction enriched in cerebellar glomeruli

In the internal granular layer of the cerebellar cortex the polysynaptic complexes called glomeruli consist mainly of homogeneous populations of glutamatergic and GABAergic synapses, both located on granule cell dendrites. A subcellular fraction enriched in glomeruli was prepared from rat cerebellum, and the distribution of the different types of NMDA and non-NMDA glutamate binding sites was studied in the membranes derived from this fraction (fraction G) as compared to that in the membranes prepared from a total cerebellar homogenate (fraction T). Cl-/Ca2+ independent [3H]glutamate binding sites were not abundant and could be reliably measured only in fraction G. Cl- dependent/Ca2+ activated [3H]glutamate binding sites were more abundant and exhibited a single KD in both fractions G and T. Quisqualate, NMDA, kainate, L-AP4 and trans-ACPD inhibited [3H]glutamate binding to different extents in the two membrane fractions. Quisqualate sensitive sites were predominant in all cases but more abundant in fraction T than in fraction G. An opposite distribution was observed for the NMDA sensitive binding sites while kainate sensitive binding sites were scarce everywhere. Trans-ACPD, a ligand presumed selective for metabotropic glutamate binding sites, displaced [3H]glutamate from fraction T but nor from fraction G, suggesting the absence of these sites from glomeruli. Similarly, no L-AP4 sensitive sites were present in fraction G while they were abundant in fraction T. Binding sites associated with ionotropic receptors of the quisqualate type were determined by measuring [3H]AMPA binding. The density of the high affinity [3H]AMPA binding sites in fraction T was twice as high as in fraction G, indicating that these sites are abundant in structures other than glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)