Molecular Logic Sculpting Cell-Specific Contributions of Neurexin-1 at the Tripartite Synapse

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ABSTRACT Substantial evidence implicates copy number variations (CNVs) of the gene encoding Neurexin-1 (Nrxn1), a synaptic cell adhesion molecule, in the pathoetiology of Autism spectrum disorder, schizophrenia, and Tourette syndrome. However, very little is known regarding the function and regulation of Nrxn1 at developing and adult synapses, an understanding of which may improve our knowledge of brain disorders and reveal novel therapeutic targets. My preliminary data show that Nrxn1 is not only expressed by neurons, but also by astrocytes, challenging the classical view of Nrxn1 as being exclusively presynaptic. Here, I propose that astrocytic Nrxn1 mediates an important form of communication between astrocytes and synapses that is required for normal excitatory synapse development and function. My preliminary findings show that astrocytic and neuronal Nrxn1 fundamentally differ in major isoform expression, alternative splicing, and heparan sulfate (HS) modification. Selective loss of astrocytic Nrxn1 leads to a significant and selective reduction in AMPA receptor (AMPAR)- mediated synaptic transmission without affecting synapse number in the hippocampus, a brain region important for consolidation of information from short-term memory to long-term memory and spatial memory. How does the postsynaptic membrane, which contains numerous neurexin ligands (e.g. LRRTMs, neuroligins, etc.), distinguish between presynaptic and astrocytic Nrxn1 to allow compartment-specific signaling within the tripartite synapse? In order to better understand the molecular mechanisms utilized by astrocytic Nrxn1 to instruct synapse development I will perform rescue experiments in Nrxn1 astrocyte conditional knockout (acKO) mice using AAVs to deliver Nrxn1 variants differing in major isoform identity, alternative splicing, HS modification, and intracellular signaling. Recordings of both spontaneous and evoked excitatory synaptic responses will be performed on CA1 pyramidal neurons to measure rescue efficacy. Next, I will extend our understanding of the functional consequences of Nrxn1 deletion through studying whether astrocytic Nrxn1 is required for basal and/or activity-induced changes in dendritic spine morphology and density, as well as the three-dimensional density of functional AMPARs. Finally, the synaptic deficits observed following deletion of astrocytic Nrxn1 may be a downstream consequence of impaired astrocyte function. Thus, I will measure several aspects of astrocyte function following Nrxn1 deletion in astrocytes, including physiological (i.e. membrane properties and channel currents), morphological (i.e. territory, gap-junction coupling, and association with synapses), and circuit-related (i.e. calcium dynamics) properties. It is anticipated that the proposed research will provide critical insights into the molecular and cellular basis of how Nrxn1 CNVs give rise to neurodevelopmental disorders and will shed light on a novel molecular program underpinning communication between neuronal and astrocytic compartments of the tripartite synapse.