Abstract

Synaptic vesicle fusion, as evoked by action potentials, is confined to presynaptic protein nanoclusters, which are closely aligned with concentrated postsynaptic receptors and their scaffolding proteins—an organization termed a ‘nanocolumn’. Efficient neurotransmission has long been suspected to require precise alignment between pre-synaptic vesicle release sites and post-synaptic receptors, but direct observations have been hampered by the physics of light-microscopy. Thomas Blanpied and colleagues use hyper-resolution microscopy — which overcomes the diffraction barrier — to reveal that vesicular fusion at single synapses, as evoked by action potentials, is confined to pre-synaptic protein nanoclusters. The nanoclusters are closely aligned with concentrated post-synaptic receptors and their scaffolding proteins. The resulting molecular 'nanocolumns' are reorganized during NMDA receptor-dependent plasticity, and the authors suggest that they may contribute to the maintenance and regulation of synaptic efficiency. Synaptic transmission is maintained by a delicate, sub-synaptic molecular architecture, and even mild alterations in synapse structure drive functional changes during experience-dependent plasticity and pathological disorders1,2. Key to this architecture is how the distribution of presynaptic vesicle fusion sites corresponds to the position of receptors in the postsynaptic density. However, while it has long been recognized that this spatial relationship modulates synaptic strength3, it has not been precisely described, owing in part to the limited resolution of light microscopy. Using localization microscopy, here we show that key proteins mediating vesicle priming and fusion are mutually co-enriched within nanometre-scale subregions of the presynaptic active zone. Through development of a new method to map vesicle fusion positions within single synapses in cultured rat hippocampal neurons, we find that action-potential-evoked fusion is guided by this protein gradient and occurs preferentially in confined areas with higher local density of Rab3-interacting molecule (RIM) within the active zones. These presynaptic RIM nanoclusters closely align with concentrated postsynaptic receptors and scaffolding proteins4,5,6, suggesting the existence of a trans-synaptic molecular ‘nanocolumn’. Thus, we propose that the nanoarchitecture of the active zone directs action-potential-evoked vesicle fusion to occur preferentially at sites directly opposing postsynaptic receptor–scaffold ensembles. Remarkably, NMDA receptor activation triggered distinct phases of plasticity in which postsynaptic reorganization was followed by trans-synaptic nanoscale realignment. This architecture suggests a simple organizational principle of central nervous system synapses to maintain and modulate synaptic efficiency.