F1000Research 2016, 5(F1000 Faculty Rev):2081 Last updated: 26 AUG 2016 REVIEW New insights into cochlear sound encoding [version 1; referees: 2 approved] Tobias Moser1-4, Christian Vogl1,2 1Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany 2Auditory Neuroscience Group, Max-Planck-Institute for Experimental Medicine, Göttingen, Germany 3Synaptic Nanophysiology Group, Max-Planck-Institute for Biophysical Chemistry, Göttingen, Germany 4Auditory Neuroscience and Optogenetics Group, German Primate Center, Göttingen, Germany First published: 26 Aug 2016, 5(F1000 Faculty Rev):2081 (doi: Open Peer Review v1 10.12688/f1000research.8924.1) Latest published: 26 Aug 2016, 5(F1000 Faculty Rev):2081 (doi: 10.12688/f1000research.8924.1) Referee Status: Abstract Invited Referees The inner ear uses specialized synapses to indefatigably transmit sound 1 2 information from hair cells to spiral ganglion neurons at high rates with submillisecond precision. The emerging view is that hair cell synapses achieve version 1 their demanding function by employing an unconventional presynaptic published molecular composition. Hair cell active zones hold the synaptic ribbon, an 26 Aug 2016 electron-dense projection made primarily of RIBEYE, which tethers a halo of synaptic vesicles and is thought to enable a large readily releasable pool of vesicles and to contribute to its rapid replenishment. Another important F1000 Faculty Reviews are commissioned presynaptic player is otoferlin, coded by a deafness gene, which assumes a from members of the prestigious F1000 multi-faceted role in vesicular exocytosis and, when disrupted, causes auditory Faculty. In order to make these reviews as synaptopathy. A functional peculiarity of hair cell synapses is the massive heterogeneity in the sizes and shapes of excitatory postsynaptic currents. comprehensive and accessible as possible, Currently, there is controversy as to whether this reflects multiquantal release peer review takes place before publication; the with a variable extent of synchronization or uniquantal release through a referees are listed below, but their reports are dynamic fusion pore. Another important question in the field has been the not formally published. precise mechanisms of coupling presynaptic Ca2+ channels and vesicular Ca2+ sensors. This commentary provides an update on the current understanding of 1 Sonja Pyott, University Medical Center sound encoding in the cochlea with a focus on presynaptic mechanisms. Groningen Netherlands 2 Ian Russell, University of Brighton UK Discuss this article Comments (0) F1000Research Page 1 of 9 F1000Research 2016, 5(F1000 Faculty Rev):2081 Last updated: 26 AUG 2016 Corresponding author: Tobias Moser ([email protected]) How to cite this article: Moser T and Vogl C. New insights into cochlear sound encoding [version 1; referees: 2 approved] F1000Research 2016, 5(F1000 Faculty Rev):2081 (doi: 10.12688/f1000research.8924.1) Copyright: © 2016 Moser T and Vogl C. This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Grant information: This work was supported by grants from the German Research Foundation through the Collaborative Research Center 889 and the Leibniz Program (to TM). Competing interests: The authors declare that they have no competing interests. First published: 26 Aug 2016, 5(F1000 Faculty Rev):2081 (doi: 10.12688/f1000research.8924.1) F1000Research Page 2 of 9 F1000Research 2016, 5(F1000 Faculty Rev):2081 Last updated: 26 AUG 2016 Introduction In the mammalian cochlea, inner hair cells (IHCs)—the genuine sensory cells of the cochlea transform sound-induced mechanical signals into a neural code at their ribbon synapses. Upon hair bun- dle deflection, mechanotransducer channels, located in the stere- ociliar tips, provide hair cell depolarization. This process leads to presynaptic glutamate release from IHCs onto spiral ganglion neu- rons (SGNs), and ultimately activates the auditory pathway. Coding of sound at IHC ribbon synapses achieves impressive performance: each glutamatergic presynaptic active zone (AZ) of an IHC pro- vides the sole excitatory input to a postsynaptic SGN. Yet, each sin- gle AZ drives SGN spike rates at sound onset in the kilohertz range and supports firing at hundreds of hertz during ongoing stimulation. Moreover, these synapses are capable of transmitting information on the timing of the stimulus with submillisecond precision. The underlying mechanisms that mediate this performance have remained enigmatic but likely relate to the molecular and struc- tural specializations of the IHC ribbon-type AZ. Here, we briefly review the latest progress on the molecular anatomy and physiol- ogy of the IHC ribbon synapse, with a focus on the presynaptic AZ (for recent reviews of the postsynaptic SGN, see 1,2.) Dysfunc- tion or loss of IHC synapses causes a specific form of sensorineural hearing impairment: auditory synaptopathy (recently reviewed Figure 1. Inner hair cells drive sound encoding in several spiral in 3). We will then summarize recent experimental and theoreti- ganglion neurons. Schematic drawing of an inner hair cell (gray) cal work that has corroborated the Ca2+ nanodomain hypothesis of and its synapses with spiral ganglion neurons (black). Inset shows super-resolution (4Pi) images of an immunolabeled inner hair cell Ca2+ influx-exocytosis coupling at IHC ribbon synapses. Finally, synapse with the synaptic ribbon (red) placed opposite to the we will provide a brief overview of the current state of the debate center of the postsynaptic AMPA receptor cluster (green). Each on the mode of exocytosis at the hair cell AZ, which remains a spiral ganglion neuron is thought to receive input from one ribbon- hot topic of current research. type inner hair cell active zone at the postsynaptic swelling of its peripheral neurite. Unconventional presynaptic molecular composition The synaptic ribbon represents the most prominent structural devia- tion from “conventional” glutamatergic synapses of the vertebrate central nervous system (Figure 1). Depending on the cell type, ribbon21 and seems to assume multiple roles in hair cell exocy- developmental stage, and animal species under investigation, the tosis. For example, otoferlin has been suggested (i) to act as the ribbon can assume various shapes and sizes4–6. The main structural putative Ca2+ sensor in IHCs22,26, (ii) to facilitate vesicular prim- component of synaptic ribbons is RIBEYE7, a protein assembled ing and replenishment21,27, and (iii) to participate in exocytosis- from an aggregation-prone A domain and an enzymatically active B endocytosis coupling through direct interaction with the adaptor domain, both of which are transcribed from the CtBP2 gene8. The protein 2 (AP-2) complex28,29. It is tempting to speculate that synaptic ribbons help cluster large complements of Ca2+ channels IHCs evolved this unconventional molecular machinery in order and readily releasable vesicles at the IHC AZ, thereby enabling syn- to achieve the utmost performance. One possible requirement chronous auditory signaling and also promoting continuous vesicle could be a rapid and low-affinity engagement of synaptic vesicles replenishment9–11. Ribbons are also critical for sensory processing with release sites with molecular links to a nearby Ca2+ channel, in the retina, where they serve similar functions12–14, and, in addition, followed by rapid clearance of vesicular lipid and proteins from seem to play a role in coupling Ca2+ channels to release sites15. that site for it to be quickly reloaded. Clearly, more work is required to elucidate the molecular fusion machinery of IHCs. In addition to unexpectedly finding that IHC ribbon synapses appear to operate without neuronal SNAREs16 and the classic neuronal Besides the presence of otoferlin, IHC AZs are characterized by Ca2+ sensors synaptotagmin 1 and 217,18, we have recently come large Ca2+ channel clusters, which localize underneath the presyn- to realize that SNARE regulators such as complexins19,20, as well aptic density30 and consist predominantly of pore-forming L-type 21 31,32 33 as priming factors of the Munc13 and CAPS families which are CaV1.3 subunits , auxiliary CaVβ2 , and likely yet-to-be- 34 2+ critical for transmission at many synapses, also seem to be missing identified CaVα2δ subunits . Ca channel clustering depends on from IHCs. Instead, hair cells employ the multi-C2-domain protein multiple molecular scaffolds, such as Bassoon or the ribbon (or otoferlin22, a member of the ferlin family of membrane fusion- both)10,35 as well as RIM2α and β36. The seamless interplay and related proteins (reviewed in 23,24), which is a tail-anchored correct localization of these proteins is not only required for protein and requires the TRC40 pathway for efficient targeting to establishing a normal Ca2+ channel complement10,36 but also criti- the endoplasmic reticulum25. Otoferlin clusters below the synaptic cal to stabilize a large readily releasable pool of vesicles at the Page 3 of 9 F1000Research 2016, 5(F1000 Faculty Rev):2081 Last updated: 26 AUG 2016 AZ10,11,36. Moreover, IHC AZs contain additional scaffolds such governing AZ maturation, in particular, in regard to the progressive as Piccolino, a short splice variant of Piccolo37, and the Usher tightening of Ca2+ channel-synaptic vesicle coupling during early