Astrocytes regulate synaptic connectivity in the CNS through secreted signals. Here we identified two astrocyte-secreted proteins, hevin and SPARC, as regulators of excitatory synaptogenesis in vitro and in vivo. Hevin induces the formation of synapses between cultured rat retinal ganglion cells. SPARC is not synaptogenic, but specifically antagonizes synaptogenic function of hevin. Hevin and SPARC are expressed by astrocytes in the superior colliculus, the synaptic target of retinal ganglion cells, concurrent with the excitatory synaptogenesis. Hevin-null mice had fewer excitatory synapses; conversely, SPARC-null mice had increased synaptic connections in the superior colliculus. Furthermore, we found that hevin is required for the structural maturation of the retinocollicular synapses. These results identify hevin as a positive and SPARC as a negative regulator of synapse formation and signify that, through regulation of relative levels of hevin and SPARC, astrocytes might control the formation, maturation, and plasticity of synapses in vivo.F ormation of the correct type and number of synaptic connections is crucial for the proper development and function of our nervous systems. In the past decade, astrocytes have emerged as important regulators of synaptic connectivity (1, 2).By using a purified retinal ganglion cell (RGC) culture system (3), we previously showed that astrocyte-secreted factors, including a family of ECM proteins, thrombospondins (TSPs), significantly increase the number of synapses formed between RGCs (4-6). These in vitro findings paved the way for recognition of astrocytes, and the TSPs they secrete, as important regulators of synapse formation and injury-mediated synaptic remodeling in vivo (4,5,7).TSPs belong to a subclass of secreted proteins called matricellular proteins. Matricellular proteins function by modulation of cell-cell and cell-matrix interactions, and thereby regulate the adhesion state of cells (8). Astrocytes express a number of matricellular proteins in addition to TSPs, and their expression is developmentally regulated and overlaps with early postnatal periods of synaptic development in the CNS (9, 10).In the present study we investigated whether other astrocytesecreted matricellular proteins could modulate synapse formation. Gene expression profiling of astrocytes suggested the matricellular proteins hevin [also known as secreted protein acidic and rich in cysteine (SPARC)-like 1] and SPARC as possible candidates. Hevin and SPARC are members of the SPARC family (11). Hevin was first identified as a synaptic glycoprotein and was initially termed synaptic cleft-1, or SC1 (12). It is localized to excitatory CNS synapses (13). Astrocytes in the developing brain express high levels of hevin and SPARC mRNA, with hevin mRNA being one of the highest-level mRNAs expressed by astrocytes (10). Unlike TSP1 and TSP2, the expression of which is decreased during maturation, hevin and SPARC mRNA levels remain high in the adult (9, 14-16).Here we investigated whether hevin and SPARC p...

SUMMARY Abnormalities in GABAergic interneurons, particularly fast-spiking interneurons (FSINs) that generate gamma (γ; ~30-120 Hz) oscillations, are hypothesized to disrupt prefrontal cortex (PFC)-dependent cognition in schizophrenia. Although γ rhythms are abnormal in schizophrenia, it remains unclear whether they directly influence cognition. Mechanisms underlying schizophrenia's typical post-adolescent onset also remain elusive. We addressed these issues using mice heterozygous for Dlx5/6, which regulate GABAergic interneuron development. In Dlx5/6+/− mice, FSINs become abnormal following adolescence, coinciding with the onset of cognitive inflexibility and deficient task-evoked γ oscillations. Inhibiting PFC interneurons in control mice reproduced these deficits, whereas stimulating them at γ-frequencies restored cognitive flexibility in adult Dlx5/6+/− mice. These pro-cognitive effects were frequency-specific and persistent. These findings elucidate a mechanism whereby abnormal FSIN development may contribute to the post-adolescent onset of schizophrenia endophenotypes. Furthermore, they demonstrate a causal, potentially therapeutic, role for PFC interneuron-driven gamma oscillations in cognitive domains at the core of schizophrenia.

SUMMARY Layer 5 pyramidal neurons comprise at least two subtypes: thick-tufted, subcortically-projecting Type A neurons, with prominent h-current, and thin-tufted, callosally-projecting Type B neurons, which lack prominent h-current. Using optogenetic stimulation, we find that these subtypes receive distinct forms of input that could subserve divergent functions. Repeatedly stimulating callosal inputs evokes progressively smaller excitatory responses in Type B but not Type A neurons. Callosal inputs also elicit more spikes in Type A neurons. Surprisingly, these effects arise via distinct mechanisms. Differences in the dynamics of excitatory responses reflect differences in presynaptic input, whereas differences in spiking depend on postsynaptic mechanisms. We also find that fast-spiking parvalbumin interneurons, but not somatostatin interneurons, preferentially inhibit Type A neurons, which leads to greater feedforward inhibition in this subtype. These differences may enable Type A neurons to detect salient inputs that are focused in space and time, while Type B neurons integrate across these dimensions.