title: Adenomatous Polyposis Coli Protein Deletion in Efferent Olivocochlear Neurons Perturbs Afferent Synaptic Maturation and Reduces the Dynamic Range of Hearing.
abstract: Normal hearing requires proper differentiation of afferent ribbon synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) that carry acoustic information to the brain. Within individual IHCs, presynaptic ribbons show a size gradient with larger ribbons on the modiolar face and smaller ribbons on the pillar face. This structural gradient is associated with a gradient of spontaneous rates and threshold sensitivity, which is essential for a wide dynamic range of hearing. Despite their importance for hearing, mechanisms that direct ribbon differentiation are poorly defined. We recently identified adenomatous polyposis coli protein (APC) as a key regulator of interneuronal synapse maturation. Here, we show that APC is required for ribbon size heterogeneity and normal cochlear function. Compared with wild-type littermates, APC conditional knock-out (cKO) mice exhibit decreased auditory brainstem responses. The IHC ribbon size gradient is also perturbed. Whereas the normal-developing IHCs display ribbon size gradients before hearing onset, ribbon sizes are aberrant in APC cKOs from neonatal ages on. Reporter expression studies show that the CaMKII-Cre used to delete the floxed APC gene is present in efferent olivocochlear (OC) neurons, not IHCs or SGNs. APC loss led to increased volumes and numbers of OC inhibitory dopaminergic boutons on neonatal SGN fibers. Our findings identify APC in efferent OC neurons as essential for regulating ribbon heterogeneity, dopaminergic terminal differentiation, and cochlear sensitivity. This APC effect on auditory epithelial cell synapses resembles interneuronal and nerve-muscle synapses, thereby defining a global role for APC in synaptic maturation in diverse cell types.
  
  
  CONCLUSIONS
  This study identifies novel molecules and cellular interactions that are essential for the proper maturation of afferent ribbon synapses in sensory cells of the inner ear, and for normal hearing.
authors.affiliation: Department of Neuroscience, Sackler Biomedical Graduate School, Tufts University, School of Medicine, Boston, Massachusetts 02111.
authors.forename: Tyler T
authors.initials: TT
authors.lastname: Hickman
authors.affiliation: Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts 02111, and Eaton-Peabody Laboratory, Massachusetts Eye & Ear Infirmary, Boston, Massachusetts 02111.
authors.forename: M Charles
authors.initials: MC
authors.lastname: Liberman
authors.affiliation: Department of Neuroscience, Sackler Biomedical Graduate School, Tufts University, School of Medicine, Boston, Massachusetts 02111, Michele.Jacob@Tufts.edu.
authors.forename: Michele H
authors.initials: MH
authors.lastname: Jacob
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods; D025601:Adenomatous Polyposis Coli Protein / Q000172:deficiency* / Q000235:genetics; D000818:Animals; D003051:Cochlea / Q000378:metabolism* / Q000648:ultrastructure; D005260:Female; D017353:Gene Deletion; D006309:Hearing / Q000502:physiology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009475:Neurons, Afferent / Q000378:metabolism* / Q000648:ultrastructure; D009847:Olivary Nucleus / Q000378:metabolism* / Q000648:ultrastructure; D013569:Synapses / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D025601:Adenomatous Polyposis Coli Protein
keywords: adenomatous polyposis coli protein (APC); hair cell; hearing; olivocochlear efferent; ribbon; synapse