Molecular Physiology of Synaptic Sound Encoding

by Rohan Kapoor
Doctoral thesis
Date of Examination:2024-07-31
Date of issue:2025-04-10
Advisor:Prof. Dr. Tobias Moser
Referee:Prof. Dr. Erwin Neher
Referee:Prof. Dr. Silvio O. Rizzoli
Referee:Prof. Dr. Thomas Dresbach
Referee:Prof. Dr. Jeong Seop Rhee
Referee:Dr. Brett Carter
Persistent Address: http://dx.doi.org/10.53846/goediss-11197

 

 

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Abstract

English

Cochlear inner hair cells (IHCs) of the auditory system form specialised chemical synapses called ribbon synapses with type I spiral ganglion neurons (SGNs). These synapses encode sound information into neural signals for further transmission to the brain. The hallmark of these synapses is the synaptic ribbons - an electron-dense structure that tethers synaptic vesicles (SVs) close to the presynaptic active zones (AZ). This is believed to mediate fast, temporally precise, and tireless transmission of acoustic information essential for hearing. Despite major efforts, much remains unknown about these ribbon synapses, including the precise role of the synaptic ribbon in the auditory context and the molecular anatomy of these specialised synapses. Furthermore, absence of appropriate techniques to culture IHCs and a scarcity of IHCs inside the inner ear has hampered a detailed molecular dissection of these synapses, many components of which are likely unknown or not well understood. In this PhD thesis, we have attempted to shed light on the molecular structure and function of the IHC ribbon synapses by adopting both, top-down and bottom-up approaches. Firstly, we have investigated the role of Piccolino - a unique, shorter isoform of the AZ protein Piccolo, which is found exclusively at ribbon synapses and is a part of the synaptic ribbon scaffold. Our results show that Piccolino is necessary for maintaining ribbon architecture and for normal hearing. Secondly, we established reconstitution of synthetic ribbon-type AZs in a synapse-naïve heterologous expression system to investigate the minimal molecular requirements for such an assembly and assess Ca2+ channel clustering by ribbon components. Lastly, we established the use of MINFLUX nanoscopy in cochlear tissue samples to investigate the architecture of IHC ribbon synapses at a molecular level spatial resolution and chart the topography of various protein components, with special emphasis on presynaptic voltage-gated Ca2+ channels. Overall, this thesis attempts to bring new insights into our current understanding of IHC ribbon synapses, bridging gaps in existing knowledge and providing new perspectives into the molecular anatomy of these intriguing synapses.
Keywords: Ribbon Synapse; Inner Hair Cell; Auditory Neuroscience; Synapse Physiology
 
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