Maturational refinement of cochlear ribbon synapses

The perception of sound is performed by a specialized pathway that spans from the ear to the auditory cortex of the brain. The initial part of the auditory pathway involves the outer ear to capture the sound waves, the middle ear to conduct and enhance the signal, and the inner ear, where sensory cells translate the vibrations induced by sound to a message that can be transmitted to the brain. These auditory sensory cells, named inner hair cells (IHCs), contain microscopic hair-like processes on the apical cell pole, which bend upon deflection and thereby activate the IHC. The activated IHC can then communicate this signal to a partner auditory neuron via a highly specialized synaptic contact. The release of neurotransmitter from the IHC basal pole activates receptors on the connected neuron and thereby initiates the transmission of the sound stimulus. These synaptic connections between IHCs and auditory neurons are highly specialized and extremely efficient, thereby ensuring a precise and continuous transmission of auditory information into neural code. During initial development and subsequent maturation of the auditory pathway, various structural changes take place to maximize the efficiency of this specialized synapse, such as a reorganization and structural alignment of key scaffolding proteins as well as various types of neurotransmitter receptors. However, to date, these changes have mainly been studied on the side of the IHC, the presynaptic site, whereas the developmental refinement of the neuronal, postsynaptic site, has remained largely elusive thus far. In this project, we will hence investigate the development of the synaptic connections between IHCs and auditory neurons in mice with a special focus on the maturation of the postsynaptic compartment. Here, we aim to establish and functionally characterize the expression patterns of scaffolding proteins and neurotransmitter receptors residing in the postsynaptic compartment during the first two weeks of postnatal development. Also, we plan to assess the interactions between the different synaptic proteins and receptors in detail, and examine the effect of different states of neuronal activation on their spatial organization. To study this synaptic development, we will make use of a model system where the inner ear of young mice is extracted and preserved in a culture dish under physiological conditions. This method of organotypic culturing maintains the tissue structure and thus the connections between IHCs and connected auditory neurons. We will then combine tissue from different reporter mouse lines with chemical labelling and various live-cell and fixed tissue fluorescence microscopy techniques to visualize the different structures and proteins of interest in high resolution, thereby shedding light on the unexplored developmental processes taking place at the primary auditory synapse.