UC San Diego

Mechanosensory hair cells (HCs), key components for hearing and balancing in the auditory and vestibular systems, function by rapid and continuous synaptic vesicle release in their ribbon synapse, a specialized synapse found in some sensory cells characterized by the presence of an electron-dense structure called the synaptic ribbon. However, the molecular mechanisms underlying this characteristic function are largely unknown. In this thesis, using zebrafish as a model organism, I have investigated a protein that may play an important role in regulating hair cell synaptic function. Zebrafish Pinball Wizard (pwi) mutants are deficit of pinball or WRB (in mice and humans), a receptor that is presumably important in the processing and membrane insertion of tail-anchored (TA) proteins, such as the vesicle SNARE proteins and a deafness gene, Otoferlin. In the zebrafish insertional pwi mutant, the Ca2+ response in the post-synaptic afferent neurons originating from HC ribbon synapse activation and the acoustic-induced startle reflex is smaller and fatigued quickly. By using immunofluorescence, transmission electron microscopy (TEM), and a transgenic fluorescent ribbon-binding protein, I have found that the number of synaptic vesicles in the reserved pool surrounding the ribbon and the number of synaptic ribbons in mutant HCs are significantly reduced by half. These results, together with physiological and behavioral phenotypes, strongly suggest that pinball is required for the formation of a large reserve pool of vesicles to allow for a large secretion capacity and normal synaptic development in the HC