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Abstract

Synaptic organizing molecules govern synapse assembly and control synaptic properties. This dissertation focuses on the synapse organizer neurexin 1, which is central to a genetic risk pathway for neuropsychiatric disorders. Using biochemistry, electrophysiology, immunofluorescence imaging, serial block-face electron microscopy, and behavior analyses, we study modes of regulating neurexin 1 function at hippocampal synapses. Neurexin’s role in synapse development was previously thought to be mediated purely by its protein domains. In Chapter 2, we show that it requires a post-translational glycan modification called heparan sulfate (HS). HS on neurexin 1 is essential for mouse survival and excitatory synaptic transmission. It serves as a distinct binding interface for postsynaptic partners neuroligins and LRRTMS, as well as novel ligands beyond canonically known partners. Because of heterogeneity in HS structure, our work provides a unique molecular basis for fine- tuning synapse function, and position glycan-binding motifs as potential therapeutic targets for diseases of the brain. Neurexins exemplify molecular diversity by way of alternative splicing and heparan sulfation. In Chapter 3, we reveal that these post-transcriptional and post-translational modes of regulation converge on neurexin 1 splice site 5 (S5). The S5 insert increases HS valency, an effect that is associated with reduced neurexin 1 protein level and lower glutamatergic transmission. Exclusion of neurexin 1 S5 in mice bolsters synaptic transmission while maintaining the AMPA/NMDA ratio and shifts behavior away from those associated with autism spectrum disorders. Our findings uncover NRXN1 S5 as a therapeutic site that can potentially be harnessed to restore function in neuropsychiatric disorders. In Chapter 4, we interrogate the consequences of a disease-associated monogenic loss of neurexin 1. In heterozygous Nrxn1 KO mice, we show a ~2-fold reduction in excitatory synaptic transmission while leaving synapse numbers unaffected. This deficit is rescued back to WT levels by the exclusion of S5 in the remaining WT Nrxn1 allele, consistent with a restoration of Nrxn1 protein level. Heterozygous deletion of Nrxn1 reduces presynaptic neurotransmitter release and AMPA/NMDA ratio, both rescued by the S5 exclusion strategy. Thus, we present a genetic platform whereby neurexin 1 S5 can be leveraged to potentially develop therapeutics to alleviate brain-based defects.