Establishing synthetic ribbon-type active zones in a heterologous expression system.

Encoding of several sensory modalities into neural signals is mediated by ribbon synapses. The synaptic ribbon tethers synaptic vesicles at the presynaptic active zone (AZ) and may act as a super-scaffold organizing AZ topography. Here, we employed a synthetic biology approach to reconstitute structures mimicking ribbon-type AZs in human embryonic kidney 293 (HEK293) cells for probing minimal molecular requirements and studying presynaptic Ca2+ channel clustering. Co-expressing a membrane-targeted version of the AZ protein Bassoon and the ribbon core protein RIBEYE, we observed structures recapitulating basic aspects of ribbon-type AZs, which we call synthetic ribbons or SyRibbons. Super-resolution stimulated emission depletion (STED) microscopy and cryo-correlative electron tomography revealed SyRibbons were similar to native ribbons at AZs of cochlear inner hair cells in shape and size. SyRibbons with Ca2+ channel clusters formed upon additional expression of CaV1.3 Ca2+ channels and RIM-binding protein 2 (RBP2). CaV1.3 Ca2+ channel clusters associated with SyRibbons were larger than ribbonless CaV1.3 Ca2+ channel clusters, and functional analysis by Ca2+ imaging in combination with patch clamp showed partial confinement of the Ca2+ signal at SyRibbons. In summary, we identify Ca2+ channels, RBP, membrane-anchored Bassoon, and RIBEYE as minimal components for reconstituting a basic ribbon-type AZ. SyRibbons might complement animal studies on molecular interactions of AZ proteins.