Figure 4.

Molecular maturation of ribbon-type AZs. Molecular composition of a murine ribbon synapse between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) and its differential constitution during early postnatal development (prior to hearing onset) and in matured stages (after hearing onset). Along with the growth, membrane attachment and fusion of precursors, the establishment of a mature ribbon synapse involves the spatial confinement of its molecular components: Ca_Vs localize in tight clusters underneath the ribbon, multiple presynaptic densities merge to one single ribbon anchor and postsynaptically one continuous elongated postsynaptic density is formed that stabilizes a ring-like organization of AMPA receptors. A developmental switch in the regulation of presynaptic exocytosis is present in the downregulation of Synaptotagmin (Syt) 1 and 2, which are likely absent from mature ribbons. Here, the IHC-specific multi-C2-domain protein Otoferlin becomes responsible for the regulation of synaptic vesicle release in the early post-natal stages, despite expression from embryonic ages. (#) Bassoon is part of ribbon precursor spheres in the retina. The here illustrated divergence in the illustrated protein expression between immature and mature preparations does not necessarily reflect absence of expression in the young tissue per se but rather results from the current lack of experimental evidence from pre-hearing preparations. For more detailed information please refer to the respective parts in the text. Font color indicates association with the correspondingly colored pre-/postsynaptic compartment.