Astrocytic uptake of GABA through GABA transporters (GATs) is an important mechanism regulating excitatory/inhibitory balance in the nervous system, however mechanisms by which astrocytes regulate GAT levels are undefined. Here we show at mid-pupal stages the Drosophila CNS neuropil is devoid of astrocyte membranes and synapses. Astrocyte membranes subsequently infiltrate the neuropil coordinate with synaptogenesis and a strocyte ablation reduces synapse numbers by half, indicating that Drosophila astrocytes are pro-synaptogenic. Shortly after synapses form in earnest, the GABA transporter, GAT, is up-regulated in astrocytes. Ablation or silencing of GABAergic neurons or disruption of metabotropic GABA receptor (GABABR1/2) signaling in astrocytes leads to decreased astrocytic GAT levels. Interestingly, developmental depletion of astrocytic GABABR1/2 signaling suppresses mechanosensory-induced seizure activity in mutants with hyperexcitable neurons. These data reveal astrocytes actively modulate GAT expression via metabotropic GABA receptor signaling, and highlight the importance of precise regulation of astrocytic GAT in modulation of seizure activity.

Developmental synaptic remodeling is important for the formation of precise neural circuitry, and its disruption has been linked to neurodevelopmental disorders such as autism and schizophrenia. Microglia prune synapses, but integration of this synapse pruning with overlapping and concurrent neurodevelopmental processes, remains elusive. Adhesion G protein‐coupled receptor ADGRG1/GPR56 controls multiple aspects of brain development in a cell type‐specific manner: In neural progenitor cells, GPR56 regulates cortical lamination, whereas in oligodendrocyte progenitor cells, GPR56 controls developmental myelination and myelin repair. Here, we show that microglial GPR56 maintains appropriate synaptic numbers in several brain regions in a time‐ and circuit‐dependent fashion. Phosphatidylserine (PS) on presynaptic elements binds GPR56 in a domain‐specific manner, and microglia‐specific deletion of Gpr56 leads to increased synapses as a result of reduced microglial engulfment of PS+ presynaptic inputs. Remarkably, a particular alternatively spliced isoform of GPR56 is selectively required for microglia‐mediated synaptic pruning. Our present data provide a ligand‐ and isoform‐specific mechanism underlying microglial GPR56‐mediated synapse pruning in the context of complex neurodevelopmental processes.

Astrocytes are important regulators of neural circuit function and behavior in the healthy and diseased nervous system. We screened for molecules in Drosophila astrocytes that modulate neuronal hyperexcitability and identified multiple components of focal adhesion complexes (FAs). Depletion of astrocytic Tensin, β-integrin, Talin, focal adhesion kinase (FAK), or matrix metalloproteinase 1 (Mmp1), resulted in enhanced behavioral recovery from genetic or pharmacologically induced seizure. Overexpression of Mmp1, predicted to activate FA signaling, led to a reciprocal enhancement of seizure severity. Blockade of FA-signaling molecules in astrocytes at basal levels of CNS excitability resulted in reduced astrocytic coverage of the synaptic neuropil and expression of the excitatory amino acid transporter EAAT1. However, induction of hyperexcitability after depletion of FA-signaling components resulted in enhanced astrocyte coverage and an approximately twofold increase in EAAT1 levels. Our work identifies FA-signaling molecules as important regulators of astrocyte outgrowth and EAAT1 expression under normal physiological conditions. Paradoxically, in the context of hyperexcitability, this pathway negatively regulates astrocytic process outgrowth and EAAT1 expression, and their blockade leading to enhanced recovery from seizure.

Schizophrenia risk is associated with increased gene copy number and brain expression of complement component 4 (C4). Because the complement system facilitates synaptic pruning, the C4 association has renewed interest in a hypothesis that excessive pruning contributes to schizophrenia pathogenesis. However, little is known about complement regulation in neural tissues or whether such regulation could be relevant to psychiatric illness. Intriguingly, common variation within CSMD1, which encodes a putative complement inhibitor, has consistently associated with schizophrenia at genome-wide significance. We found that Csmd1 is predominantly expressed in the brain by neurons, and is enriched at synapses; that human stem cell-derived neurons lacking CSMD1 are more vulnerable to complement deposition; and that mice lacking Csmd1 have increased brain complement activity, fewer synapses, aberrant complement-dependent development of a neural circuit, and synaptic elements that are preferentially engulfed by cultured microglia. These data suggest that CSMD1 opposes the complement cascade in neural tissues.