SUMMARY Microglia are the resident CNS immune cells and active surveyors of the extracellular environment. While past work has focused on the role of these cells during disease, recent imaging studies reveal dynamic interactions between microglia and synaptic elements in the healthy brain. Despite these intriguing observations, the precise function of microglia at remodeling synapses and the mechanisms that underlie microglia-synapse interactions remain elusive. In the current study, we demonstrate a role for microglia in activity-dependent synaptic pruning in the postnatal retinogeniculate system. We show that microglia engulf presynaptic inputs during peak retinogeniculate pruning and engulfment is dependent upon neural activity and the microglia-specific phagocytic signaling pathway, complement receptor 3(CR3)/C3. Furthermore, disrupting microglia-specific CR3/C3 signaling resulted in sustained deficits in synaptic connectivity. These results define a role for microglia during postnatal development and identify underlying mechanisms by which microglia engulf and remodel developing synapses.

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Neuroligin (NLG), a postsynaptic adhesion molecule, is involved in the formation of synapses by binding to a cognate presynaptic ligand, neurexin. Here we report that neuroligin-1 (NLG1) undergoes ectodomain shedding at the juxtamembrane stalk region to generate a secreted form of NLG1 and a membrane-tethered C-terminal fragment (CTF) in adult rat brains in vivo as well as in neuronal cultures. Pharmacological and genetic studies identified ADAM10 as the major protease responsible for NLG1 shedding, the latter being augmented by synaptic NMDA receptor activation or interaction with soluble neurexin ligands. NLG1-CTF was subsequently cleaved by presenilin/γ-secretase. Secretion of soluble NLG1 was significantly upregulated under a prolonged epileptic seizure condition, and inhibition of NLG1 shedding led to an increase in numbers of dendritic spines in neuronal cultures. Collectively, neuronal activity-dependent proteolytic processing of NLG1 may negatively regulate the remodeling of spines at excitatory synapses.

Temporal lobe epilepsy (TLE) is accompanied by an abnormal location of granule cells in the dentate gyrus. Using a rat model of complex febrile seizures, which are thought to be a precipitating insult of TLE later in life, we report that aberrant migration of neonatal-generated granule cells results in granule cell ectopia that persists into adulthood. Febrile seizures induced an upregulation of GABA(A) receptors (GABA(A)-Rs) in neonatally generated granule cells, and hyperactivation of excitatory GABA(A)-Rs caused a reversal in the direction of granule cell migration. This abnormal migration was prevented by RNAi-mediated knockdown of the Na(+)K(+)2Cl(-) co-transporter (NKCC1), which regulates the excitatory action of GABA. NKCC1 inhibition with bumetanide after febrile seizures rescued the granule cell ectopia, susceptibility to limbic seizures and development of epilepsy. Thus, this work identifies a previously unknown pathogenic role of excitatory GABA(A)-R signaling and highlights NKCC1 as a potential therapeutic target for preventing granule cell ectopia and the development of epilepsy after febrile seizures.