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Astrocyte-neuron crosstalk through Hedgehog signaling mediates cortical circuit assembly

Neuron-glia relationships play a critical role in the regulation of synapse formation and neuronal specification. The cellular and molecular mechanisms by which neurons and astrocytes communicate and coordinate are not well understood. Here we demonstrate that the canonical Sonic hedgehog (Shh) pathway is active in cortical astrocytes, where it acts to coordinate layer-specific synaptic connectivity and functional circuit development. We show that Ptch1 is a Shh receptor that is expressed by cortical astrocytes during development and that Shh signaling is necessary and sufficient to promote the expression of layer-specific astrocyte genes involved in regulating synapse formation and function. Loss of Shh in layer V neurons reduces astrocyte complexity and coverage by astrocytic processes in tripartite synapses, moreover, cell-autonomous activation of Shh signaling in astrocytes promotes cortical excitatory synapse formation. Together, these results suggest that Shh secreted from deep layer cortical neurons acts to specialize the molecular and functional features of astrocytes during development to shape circuit assembly and function.