BAI1 localises AMPA receptors at the cochlear afferent postsynaptic density and is

Type I spiral ganglion neurons (SGNs) convey sound information to the central auditory pathway by forming axo-somatic synapses with inner hair cells (IHCs), the primary sensory receptors of the mammalian cochlea. Although IHC ribbon synapses have been extensively studied, the molecular mechanisms regulating the structure and function of the post-synaptic density (PSD) at the SGN afferent terminals are largely unknown. Using functional, morphological, and transcriptomic approaches, we demonstrate that brain-specific angiogenesis inhibitor 1 (BAI1), encoded by the Adgrb1 gene, is required for the clustering of the glutamate AMPA receptors GluR2-4 to the SGN post-synaptic density. Adult mice expressing BAI1 lacking the N-terminal thrombospondin type 1 repeats (TRS) have functional IHC synapses but fail to transmit acoustic information to the SGNs, leading to highly raised auditory thresholds. We also found that in adult Bai1-deficient mice, despite the almost complete absence of AMPA receptors, the SGN fibres innervating the IHCs did not degenerate. RNA sequencing and western blotting also indicate that AMPA receptors are expressed in the cochlea of Bai1-deficient mice, highlights that BAI1 is involved is trafficking or anchoring GluR2-4 to the PSDs. Moreover, these results have identified novel molecular and functional mechanisms required for sound encoding at cochlear ribbon synapses.