Astrocytic Neurexin-1 Orchestrates Functional Synapse Assembly

At tripartite synapses, astrocytes surround synaptic contacts, but how astrocytes contribute to the assembly and function of synapses remains unclear. Here we show that neurexin-1α, a presynaptic adhesion molecule that controls synapse properties, is also abundantly expressed by astrocytes. Strikingly, astrocytic neurexin-1α, but not neuronal neurexin-1α, is highly modified by heparan sulfate. Moreover, astrocytic neurexin-1α is uniquely alternatively spliced and invariably contains an insert in splice-site #4, thereby restricting the range of ligands to which it binds. Deletion of neurexin-1 from astrocytes or neurons does not alter synapse numbers or synapse ultrastructure, but differentially impairs synapse function. At hippocampal Schaffer-collateral synapses, neuronal neurexin-1 is essential for normal NMDA-receptor-mediated synaptic responses, whereas astrocytic neurexin-1 is required for normal AMPA-receptor-mediated synaptic responses and for long-term potentiation of these responses. Thus, astrocytes directly shape synapse properties via a neurexin-1-dependent mechanism that involves a specific molecular program distinct from that of neuronal neurexin-1.