Cartography of Neurexins mapped by single-molecule long-read mRNA Sequencing

Neurexins are evolutionarily conserved presynaptic cell-adhesion molecules that are essential for normal synapse formation and synaptic transmission. Extensive alternative splicing of neurexin mRNAs was postulated to produce hundreds if not thousands of neurexin isoforms, but no direct evidence for such diversity has been available. We employed unbiased long-read single-molecule sequencing of full-length Nrxn1a, 1ß, 2ß, 3a, and 3ß mRNAs to systematically assess how many sites of alternative splicing are used in neurexins with a significant frequency, and whether alternative splicing at these sites is independent of each other. We found that especially a-neurexins may exhibit an even larger isoform diversity than previously envisioned, consistent with the notion that a-neurexins represents extracellular protein-interaction scaffolds, in which different LNS- and EGF-domains mediate distinct interactions that effect diverse functions and are independently regulated by the various events of alternative splicing. This could help generate the number of transsynaptic interactions needed to account for the enormous diversity of synaptic connections.