The ribbon is the structural hallmark of cochlear inner hair cell (IHC) afferent synapses, yet its role in information transfer to spiral ganglion neurons (SGNs) remains unclear. We investigated the ribbon’s contribution to IHC synapse formation and function using KO mice lacking RIBEYE. Despite loss of the entire ribbon structure, synapses retained their spatiotemporal development and KO mice had a mild hearing deficit. IHCs of KO had fewer synaptic vesicles and reduced exocytosis in response to brief depolarization; a high stimulus level rescued exocytosis in KO. SGNs exhibited a lack of sustained excitatory postsynaptic currents (EPSCs). We observed larger postsynaptic glutamate receptor plaques, potentially compensating for the reduced EPSC rate in KO. Surprisingly, large-amplitude EPSCs were maintained in KO, while a small population of low-amplitude slower EPSCs was increased in number. The ribbon facilitates signal transduction at physiological stimulus levels by retaining a larger residency pool of synaptic vesicles.

Background: Ryk acts as a Wnt receptor in several processes, including mouse neuronal development. Results: Ryk interacts with Wnt11 in zebrafish convergent extension and with Vangl2 in mouse cochlear development and neural tube closure. Conclusion: Ryk is required for Wnt/planar cell polarity signaling during mammalian development and signals via Vangl2 and RhoA. Significance: This study extends our knowledge of signaling downstream of Ryk.

Background In the cochlea patterning of the organ of Corti is tightly regulated to produce a single row of sound detecting inner hair cells and three rows of outer hair cells, which amplify and refine the signal. The recently identified R-Spondin family of signaling molecules usually act as co-activators of Wnt signaling; it is thought that they regulate turnover of Wnt receptors at the membrane. We sought to test whether R-Spondins function in the developing cochlea. Results Expression analysis of all four members of the R-Spondin family showed that only R-Spondin2 (Rspo2) is expressed in the cochlea during development of the sensory epithelium. Examination of an Rspo2 −/− mouse showed that loss of Rspo2 results in an additional single row of outer hair cells and disruption of peripheral innervation pattern. Addition of Rspo2 recombinant protein to organotypic cochlear cultures resulted in a small but significant decrease in the number of outer hair cells. Conclusions Rspo2 is required to limit the number of outer hair cells to three rows and for optimal arrangement of peripheral nerve fibers. The Rspo2 gain and loss-of-function studies show that in the ear, Rspo2 function is not consistent with its assigned role as a Wnt potentiator.