Microglia are highly motile phagocytic cells that infiltrate and take up residence in the developing brain, where they are thought to provide a surveillance and scavenging function. However, although microglia have been shown to engulf and clear damaged cellular debris after brain insult, it remains less clear what role microglia play in the uninjured brain. Here, we show that microglia actively engulf synaptic material and play a major role in synaptic pruning during postnatal development in mice. These findings link microglia surveillance to synaptic maturation and suggest that deficits in microglia function may contribute to synaptic abnormalities seen in some neurodevelopmental disorders.

The neurotransmitter GABA activates heteropentameric GABA A receptors, which are composed mostly of ␣, ␤, and ␥2 subunits. Regulated membrane trafficking and subcellular targeting of GABA A receptors is important for determining the efficacy of GABAergic inhibitory function. Of special interest is the ␥2 subunit, which is mostly dispensable for assembly and membrane insertion of functional receptors but essential for accumulation of GABA A receptors at synapses. In a search for novel receptor trafficking proteins, we have used the SOS-recruitment system and isolated a Golgi-specific DHHC zinc finger protein (GODZ) as a novel ␥2 subunit-interacting protein.GODZ is a member of the superfamily of DHHC cysteine-rich domain (DHHC-CRD) polytopic membrane proteins shown recently in yeast to represent palmitoyltransferases. GODZ mRNA is found in many tissues; however, in brain the protein is detected in neurons only and highly concentrated and asymmetrically distributed in the Golgi complex. GODZ interacts with a cysteine-rich 14-amino acid domain conserved specifically in the large cytoplasmic loop of ␥1-3 subunits but not in other GABA A receptor subunits. Coexpression of GODZ and GABA A receptors in heterologous cells results in palmitoylation of the ␥2 subunit in a cytoplasmic loop domain-dependent manner. Neuronal GABA A receptors are similarly palmitoylated. Thus, GODZ-mediated palmitoylation represents a novel posttranslational modification that is selective for ␥ subunit-containing GABA A receptor subtypes, a mechanism that is likely to be important for regulated trafficking of these receptors in the secretory pathway.

GABAA receptors (GABAA Rs) are ligand-gated Cl(-) channels that mediate most of the fast inhibitory neurotransmission in the central nervous system (CNS). Multiple GABAA R subtypes are assembled from a family of 19 subunit genes, raising the question of the significance of this heterogeneity. In this review, we discuss the evidence that GABAA R subtypes represent distinct receptor populations with a specific spatio-temporal expression pattern in the developing and adult CNS, being endowed with unique functional and pharmacological properties, as well as being differentially regulated at the transcriptional, post-transcriptional and translational levels. GABAA R subtypes are targeted to specific subcellular domains to mediate either synaptic or extrasynaptic transmission, and their action is dynamically regulated by a vast array of molecular mechanisms to adjust the strength of inhibition to the changing needs of neuronal networks. These adaptations involve not only changing the gating or kinetic properties of GABAA Rs, but also modifying the postsynaptic scaffold organised by gephyrin to anchor specific receptor subtypes at postsynaptic sites. The significance of GABAA R heterogeneity is particularly evident during CNS development and adult neurogenesis, with different receptor subtypes fulfilling distinct steps of neuronal differentiation and maturation. Finally, analysis of the specific roles of GABAA R subtypes reveals their involvement in the pathophysiology of major CNS disorders, and opens novel perspectives for therapeutic intervention. In conclusion, GABAA R subtypes represent the substrate of a multifaceted inhibitory neurotransmission system that is dynamically regulated and performs multiple operations, contributing globally to the proper development, function and plasticity of the CNS. genes, raising the question of the significance of this heterogeneity. In this review, we discuss the evidence that GABA A R subtypes represent distinct receptor populations with a specific spatiotemporal expression pattern in developing and adult CNS, being endowed with unique functional and pharmacological properties, as well as being differentially regulated at the (post-) transcriptional and translational levels. GABA A R subtypes are targeted to specific subcellular domains to mediate either synaptic or extrasynaptic transmission, and their action is dynamically regulated by a vast array of molecular mechanisms to adjust the strength of inhibition to the changing needs of neuronal networks.These adaptations take place not only by changing the gating or kinetic properties of GABA A Rs, but also by modifying the postsynaptic scaffold organized by gephyrin to anchor specific receptor subtypes at postsynaptic sites. The significance of GABA A R heterogeneity is particularly evident during CNS development and adult neurogenesis, with different receptor subtypes fulfilling distinct steps of neuronal differentiation and maturation. Finally, the analysis of the specific role of GABA A R subtypes reveals their im...

Non-technical summary To be effective, synaptic transmission requires precise alignment of the presynaptic terminal, releasing the neurotransmitter, with the postsynaptic density, where receptors are present at high density. Complex molecular mechanisms ensure this interplay between neurons and, in addition, stabilize receptors in the postsynaptic membrane. To explore these mechanisms at GABAergic synapses, which mediate inhibitory neurotransmission in the brain, we investigated here the consequences of 'removing' the receptors, using targeted gene deletion. Our results show that the receptors are dispensable for synapse formation, but are required for the postsynaptic aggregation of several proteins involved in receptor trafficking, anchoring and regulation. Defects in the molecular regulation of GABAergic synapses have been associated with neurodevelopmental disorders, mental retardation, anxiety and mood disorders, underscoring the relevance of fine tuning of GABAergic inhibition for proper brain function.Abstract Pyramidal cells express various GABA A receptor (GABA A R) subtypes, possibly to match inputs from functionally distinct interneurons targeting specific subcellular domains. Postsynaptic anchoring of GABA A Rs is ensured by a complex interplay between the scaffolding protein gephyrin, neuroligin-2 and collybistin. Direct interactions between these proteins and GABA A R subunits might contribute to synapse-specific distribution of GABA A R subtypes. In addition, the dystrophin-glycoprotein complex, mainly localized at perisomatic synapses, regulates GABA A R postsynaptic clustering at these sites. Here, we investigated how the functional and molecular organization of GABAergic synapses in CA1 pyramidal neurons is altered in mice lacking the GABA A R α2 subunit (α2-KO). We report a marked, layer-specific loss of postsynaptic gephyrin and neuroligin-2 clusters, without changes in GABAergic presynaptic terminals. Whole-cell voltage-clamp recordings in slices from α2-KO mice show a 40% decrease in GABAergic mIPSC frequency, with unchanged amplitude and kinetics. Applying low/high concentrations of zolpidem to discriminate between α1-and α2/α3-GABA A Rs demonstrates that residual mIPSCs in α2-KO mice are mediated by α1-GABA A Rs. Immunofluorescence analysis reveals maintenance of α1-GABA A R and neuroligin-2 clusters, but not gephyrin clusters, in perisomatic synapses of mutant mice, along with a complete loss of these three markers on the axon initial segment. This striking subcellular difference correlates with the preservation of dystrophin clusters, colocalized with neuroligin-2 and α1-GABA A Rs on pyramidal cell bodies of mutant mice. Dystrophin was not detected on the axon initial segment in either genotype. Collectively, these findings reveal synapse-specific anchoring of GABA A Rs at postsynaptic sites and suggest that the dystrophin-glycoprotein complex contributes to stabilize α1-GABA A R and neuroligin-2, but not gephyrin, in perisomatic postsynaptic densities.

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