Dystroglycan Binding to α-Neurexin Competes with Neurexophilin-1 and Neuroligin in the Brain

Background: Extracellular matrix dystroglycan has essential functions at the neuromuscular junction and at inhibitory synapses in the brain. Results: Brain dystroglycan competes with neurexophilin-1 and neuroligins for binding to presynaptic -neurexins. Conclusion: Competition between -neurexin ligands in combination with alternative splicing determines formation of important trans-synaptic complexes. Significance: This is the first analysis of binding interference in -neurexin multiplexes. -Neurexins (-Nrxn) are mostly presynaptic cell surface molecules essential for neurotransmission that are linked to neuro-developmental disorders as autism or schizophrenia. Several interaction partners of -Nrxn are identified that depend on alternative splicing, including neuroligins (Nlgn) and dystroglycan (DAG). The trans-synaptic complex with Nlgn1 was extensively characterized and shown to partially mediate -Nrxn function. However, the interactions of -Nrxn with DAG, neurexophilins (Nxph1) and Nlgn2, ligands that occur specifically at inhibitory synapses, are incompletely understood. Using site-directed mutagenesis, we demonstrate the exact binding epitopes of DAG and Nxph1 on Nrxn1 and show that their binding is mutually exclusive. Identification of an unusual cysteine bridge pattern and complex type glycans in Nxph1 ensure binding to the second laminin/neurexin/sex hormone binding (LNS2) domain of Nrxn1, but this association does not interfere with Nlgn binding at LNS6. DAG, in contrast, interacts with both LNS2 and LNS6 domains without inserts in splice sites SS#2 or SS#4 mostly via LARGE (like-acetylglucosaminyltransferase)-dependent glycans attached to the mucin region. Unexpectedly, binding of DAG at LNS2 prevents interaction of Nlgn at LNS6 with or without splice insert in SS#4, presumably by sterically hindering each other in the u-form conformation of -Nrxn. Thus, expression of DAG and Nxph1 together with alternative splicing in Nrxn1 may prevent or facilitate formation of distinct trans-synaptic NrxnNlgn complexes, revealing an unanticipated way to contribute to the identity of synaptic subpopulations.