Charting the nanotopography of inner hair cell synapses using MINFLUX nanoscopy

For us to hear, the cochlea encodes sounds into neural signals at synapses
of inner hair cells (IHCs) and the auditory nerve with remarkable
fidelity. To achieve the high rates of temporally precise synaptic
transmission over long periods of time, IHCs employ sophisticated
ribbon-type active zones (AZ). In order for us to understand synaptic
sound encoding, we need to decipher the underpinning molecular topography
of these synapse which had remained challenging due to technological
limitations. In our study, we applied 3-dimensional minimal flux optical
nanoscopy (3D MINFLUX) to mouse IHC synapses to chart the position of key
pre- and postsynaptic proteins with single digit nanometre resolution of
imaging. We demonstrate that nanoclusters of channels and interacting
proteins govern the topography of AZs and postsynaptic densities (PSDs).
We count synaptic proteins, their nanoclusters and determine their spatial
organization feeding into computational modelling of AZ function. In
conclusion, our study reveals a nanocluster-based molecular AZ and PSD
topography, likely serving as functional modules in synaptic sound
encoding.