Astrocytic Neurexin-1 Orchestrates Functional Synapse Assembly

At tripartite synapses, astrocytes enmesh synaptic contacts, but how astrocytes contribute to the formation, maturation and plasticity of synapses remains elusive. Here we show that both astrocytes and neurons abundantly express neurexin-1, a presynaptic adhesion molecule that controls synaptic properties. Using super-resolution imaging, we demonstrate that presynaptic neuronal and astrocytic neurexin-1 form discrete nanoclusters at excitatory synapses. We find that distinct patterns of heparan sulfate modification and alternative splicing confer onto astrocytic and neuronal neurexin-1 different ligand specificities, thereby enabling compartment-specific signaling by neurexin-1. At hippocampal Schaffer-collateral synapses, deletion of neurexin-1 from either astrocytes or neurons did not alter synapse numbers, but differentially impaired synapse function. Neuronal neurexin-1 was essential for NMDA-receptor-mediated synaptic responses, whereas astrocytic neurexin-1 was required for maturation of silent synapses, AMPA-receptor recruitment, and long-term potentiation. Thus, astrocytes and neurons surprisingly use the same synaptic adhesion molecule to control distinct synapse properties.