GluR?2 assembles four neurexins into trans-synaptic triad to trigger synapse formation.

ABSTRACT: Elucidation of molecular mechanisms of synapse formation is a prerequisite for the understanding of neural wiring, higher brain functions, and mental disorders. The trans-synaptic interaction of postsynaptic glutamate receptor ?2 (GluR?2) and presynaptic neurexins (NRXNs) through cerebellin precursor protein 1 (Cbln1) mediates synapse formation in vivo in the cerebellum. Here, we asked how the trans-synaptic triad induces synapse formation. Native GluR?2 existed as a tetramer in the membrane, whereas the N-terminal domain (NTD) of GluR?2 formed a stable homodimer. When incubated with cultured mouse cerebellar granule cells (GCs), dimeric GluR?2-NTD and Cbln1 exerted little effect on the accumulation of punctate immunostaining signals for Bassoon and vesicular glutamate transporter 1 in GC axons. However, tetramerized GluR?2-NTD stimulated the accumulation of these presynaptic proteins in the axons. Analysis of Cbln1 mutants suggested that the binding sites of GluR?2 and NRXN1? on Cbln1 are differential. Furthermore, there was no competition in the binding to Cbln1 between GluR?2-NTD and the extracellular domain (ECD) of NRXN1?. Thus, GluR?2 and Cbln1 interacted with each other rather independently of Cbln1-NRXN1? interaction and vice versa. Gel filtration and isothermal titration calorimetry analyses consistently showed that dimeric GluR?2-NTD and hexameric Cbln1 assembled in the 1:1 ratio, whereas hexameric Cbln1 and the laminin-neurexin-sex hormone-binding globulin domain of NRXN1?-ECD assembled in the 1:2 ratio. Thus, the synaptogenic triad is assembled from tetrameric GluR?2, hexameric Cbln1, and monomeric NRXN in the ratio of 1:2:4. These results suggest that GluR?2 triggers synapse formation by clustering four NRXNs through triad formation.

SUBMITTER: Lee SJ 

PROVIDER: S-EPMC6622077 | biostudies-literature | 2012 Mar

REPOSITORIES: biostudies-literature