Few CaV1.3 channels regulate the exocytosis of a synaptic vesicle at the hair cell ribbon synapse

Kif1a and intact microtubules maintain synaptic-vesicle populations at ribbon synapses in zebrafish hair cells

Sensory hair cells of the inner ear utilize specialized ribbon synapses to transmit sensory stimuli to the central nervous system. This transmission necessitates rapid and sustained neurotransmitter release, which depends on a large pool of synaptic vesicles at the hair-cell presynapse. While previous work in neurons has shown that kinesin motor proteins traffic synaptic material along microtubules to the presynapse, the mechanisms of this process in hair cells remain unclear. Our study demonstrates that the kinesin motor protein Kif1a, along with an intact microtubule network, is essential for enriching synaptic vesicles at the presynapse in hair cells. Through genetic and pharmacological approaches, we disrupt Kif1a function and impair microtubule networks in hair cells of the zebrafish lateral-line system. These manipulations led to a significant reduction in synaptic-vesicle populations at the presynapse in hair cells. Using electron microscopy, in vivo calcium imaging, and electrophysiology, we show that a diminished supply of synaptic vesicles adversely affects ribbon-synapse function. Kif1aa mutants exhibit dramatic reductions in spontaneous vesicle release and evoked postsynaptic calcium responses. Furthermore, kif1aa mutants exhibit impaired rheotaxis, a behaviour reliant on the ability of hair cells in the lateral line to respond to sustained flow stimuli. Overall, our results demonstrate that Kif1a-mediated microtubule transport is critical to enrich synaptic vesicles at the active zone, a process that is vital for proper ribbon-synapse function in hair cells. KEY POINTS: Kif1a mRNAs are present in zebrafish hair cells. Loss of Kif1a disrupts the enrichment of synaptic vesicles at ribbon synapses. Disruption of microtubules depletes synaptic vesicles at ribbon synapses. Kif1aa  mutants have impaired ribbon-synapse and sensory-system function.

A free intravesicular C-terminal of otoferlin is essential for synaptic vesicle docking and fusion at auditory inner hair cell ribbon synapses

Our understanding of how otoferlin, the major calcium sensor in inner hair cells (IHCs) synaptic transmission, contributes to the overall dynamics of synaptic vesicle (SV) trafficking remains limited. To address this question, we generated a knock-in mouse model expressing an otoferlin-GFP protein, where GFP was fused to its C-terminal transmembrane domain. Similar to the wild type protein, the GFP-tagged otoferlin showed normal expression and was associated with IHC SV. Surprisingly, while the heterozygote Otof+/GFP mice exhibited a normal hearing function, homozygote OtofGFP/GFP mice were profoundly deaf attributed to severe reduction in SV exocytosis. Fluorescence recovery after photobleaching revealed a markedly increased mobile fraction of the otof-GFP-associated SV in Otof GFP/GFP IHCs. Correspondingly, 3D-electron tomographic of the ribbon synapses indicated a reduced density of SV attached to the ribbon active zone. Collectively, these results indicate that otoferlin requires a free intravesicular C-terminal end for normal SV docking and fusion.

Nano-organization of synaptic calcium signaling

Recent studies suggest an exquisite structural nano-organization within single synapses, where sites of evoked fusion - marked by clustering of synaptic vesicles, active zone proteins and voltage-gated calcium channels - are directly juxtaposed to postsynaptic receptor clusters within nanocolumns. This direct nanometer scale alignment between presynaptic fusion apparatus and postsynaptic receptors is thought to ensure the fidelity of synaptic signaling and possibly allow multiple distinct signals to occur without interference from each other within a single active zone. The functional specificity of this organization is made possible by the inherent nano-organization of calcium signals, where all the different calcium sources such as voltage-gated calcium channels, intracellular stores and store-operated calcium entry have dedicated local targets within their nanodomain to ensure precision of action. Here, we discuss synaptic nano-organization from the perspective of calcium signals, where some of the principal findings from early work in the 1980s continue to inspire current studies that exploit new genetic tools and super-resolution imaging technologies.

Kif1a and intact microtubules maintain synaptic-vesicle populations at ribbon synapses in zebrafish hair cells

Sensory hair cells of the inner ear utilize specialized ribbon synapses to transmit sensory stimuli to the central nervous system. This sensory transmission necessitates rapid and sustained neurotransmitter release, which relies on a large pool of synaptic vesicles at the hair-cell presynapse. Work in neurons has shown that kinesin motor proteins traffic synaptic material along microtubules to the presynapse, but how new synaptic material reaches the presynapse in hair cells is not known. We show that the kinesin motor protein Kif1a and an intact microtubule network are necessary to enrich synaptic vesicles at the presynapse in hair cells. We use genetics and pharmacology to disrupt Kif1a function and impair microtubule networks in hair cells of the zebrafish lateral-line system. We find that these manipulations decrease synaptic-vesicle populations at the presynapse in hair cells. Using electron microscopy, along with in vivo calcium imaging and electrophysiology, we show that a diminished supply of synaptic vesicles adversely affects ribbon-synapse function. Kif1a mutants exhibit dramatic reductions in spontaneous vesicle release and evoked postsynaptic calcium responses. Additionally, we find that kif1a mutants exhibit impaired rheotaxis, a behavior reliant on the ability of hair cells in the lateral line to respond to sustained flow stimuli. Overall, our results demonstrate that Kif1a-based microtubule transport is critical to enrich synaptic vesicles at the active zone in hair cells, a process that is vital for proper ribbon-synapse function.

BAI1 localizes AMPA receptors at the cochlear afferent post-synaptic density and is essential for hearing

Type I spiral ganglion neurons (SGNs) convey sound information to the central auditory pathway by forming synapses with inner hair cells (IHCs) in the mammalian cochlea. The molecular mechanisms regulating the formation of the post-synaptic density (PSD) in the SGN afferent terminals are still unclear. Here, we demonstrate that brain-specific angiogenesis inhibitor 1 (BAI1) is required for the clustering of AMPA receptors GluR2-4 (glutamate receptors 2-4) at the PSD. Adult Bai1-deficient mice have functional IHCs but fail to transmit information to the SGNs, leading to highly raised hearing thresholds. Despite the almost complete absence of AMPA receptor subunits, the SGN fibers innervating the IHCs do not degenerate. Furthermore, we show that AMPA receptors are still expressed in the cochlea of Bai1-deficient mice, highlighting a role for BAI1 in trafficking or anchoring GluR2-4 to the PSDs. These findings identify molecular and functional mechanisms required for sound encoding at cochlear ribbon synapses.

Anion efflux mediates transduction in the hair cells of the zebrafish lateral line

Hair cells are the principal sensory receptors of the vertebrate auditory system, where they transduce sounds through mechanically gated ion channels that permit cations to flow from the surrounding endolymph into the cells. The lateral line of zebrafish has served as a key model system for understanding hair cell physiology and development, often with the belief that these hair cells employ a similar transduction mechanism. In this study, we demonstrate that these hair cells are exposed to an unregulated external environment with cation concentrations that are too low to support transduction. Our results indicate that hair cell excitation is instead mediated by a substantially different mechanism involving the outward flow of anions. Further investigation of hair cell transduction in a diversity of sensory systems and species will likely yield deep insights into the physiology of these unique cells.

Ca2+ regulation of glutamate release from inner hair cells of hearing mice

In our hearing organ, sound is encoded at ribbon synapses formed by inner hair cells (IHCs) and spiral ganglion neurons (SGNs). How the underlying synaptic vesicle (SV) release is controlled by Ca2+ in IHCs of hearing animals remained to be investigated. Here, we performed patch-clamp SGN recordings of the initial rate of release evoked by brief IHC Ca2+-influx in an ex vivo cochlear preparation from hearing mice. We aimed to closely mimic physiological conditions by perforated-patch recordings from IHCs kept at the physiological resting potential and at body temperature. We found release to relate supralinearly to Ca2+-influx (power, m: 4.3) when manipulating the [Ca2+] available for SV release by Zn2+-flicker-blocking of the single Ca2+-channel current. In contrast, a near linear Ca2+ dependence (m: 1.2 to 1.5) was observed when varying the number of open Ca2+-channels during deactivating Ca2+-currents and by dihydropyridine channel-inhibition. Concurrent changes of number and current of open Ca2+-channels over the range of physiological depolarizations revealed m: 1.8. These findings indicate that SV release requires ~4 Ca2+-ions to bind to their Ca2+-sensor of fusion. We interpret the near linear Ca2+-dependence of release during manipulations that change the number of open Ca2+-channels to reflect control of SV release by the high [Ca2+] in the Ca2+-nanodomain of one or few nearby Ca2+-channels. We propose that a combination of Ca2+ nanodomain control and supralinear intrinsic Ca2+-dependence of fusion optimally links SV release to the timing and amplitude of the IHC receptor potential and separates it from other IHC Ca2+-signals unrelated to afferent synaptic transmission.

From the outer ear to the nerve: A complete computer model of the peripheral auditory system

Computer models of the individual components of the peripheral auditory system - the outer, middle, and inner ears and the auditory nerve - have been developed in the past, with varying level of detail, breadth, and faithfulness of the underlying parameters. Building on previous work, we advance the modeling of the ear by presenting a complete, physiologically justified, bottom-up computer model based on up-to-date experimental data that integrates all of these parts together seamlessly. The detailed bottom-up design of the present model allows for the investigation of partial hearing mechanisms and their defects, including genetic, molecular, and microscopic factors. Also, thanks to the completeness of the model, one can study microscopic effects in the context of their implications on hearing as a whole, enabling the correlation with neural recordings and non-invasive psychoacoustic methods. Such a model is instrumental for advancing quantitative understanding of the mechanism of hearing, for investigating various forms of hearing impairment, as well as for devising next generation hearing aids and cochlear implants.

Diversity matters - extending sound intensity coding by inner hair cells via heterogeneous synapses

Our sense of hearing enables the processing of stimuli that differ in sound pressure by more than six orders of magnitude. How to process a wide range of stimulus intensities with temporal precision is an enigmatic phenomenon of the auditory system. Downstream of dynamic range compression by active cochlear micromechanics, the inner hair cells (IHCs) cover the full intensity range of sound input. Yet, the firing rate in each of their postsynaptic spiral ganglion neurons (SGNs) encodes only a fraction of it. As a population, spiral ganglion neurons with their respective individual coding fractions cover the entire audible range. How such "dynamic range fractionation" arises is a topic of current research and the focus of this review. Here, we discuss mechanisms for generating the diverse functional properties of SGNs and formulate testable hypotheses. We postulate that an interplay of synaptic heterogeneity, molecularly distinct subtypes of SGNs, and efferent modulation serves the neural decomposition of sound information and thus contributes to a population code for sound intensity.

mTORC2 regulates auditory hair cell structure and function

mTOR broadly controls cell growth, but little is known about the role of mTOR complex 2 (mTORC2) in the inner ear. To investigate the role of mTORC2 in sensory hair cells (HCs), we generated HC-specific Rictor knockout (HC-RicKO) mice. HC-RicKO mice exhibited early-onset, progressive, and profound hearing loss. Increased DPOAE thresholds indicated outer HC dysfunction. HCs are lost, but this occurs after hearing loss. Ultrastructural analysis revealed stunted and absent stereocilia in outer HCs. In inner HCs, the number of synapses was significantly decreased and the remaining synapses displayed a disrupted actin cytoskeleton and disorganized Ca2+ channels. Thus, the mTORC2 signaling pathway plays an important role in regulating auditory HC structure and function via regulation of the actin cytoskeleton. These results provide molecular insights on a central regulator of cochlear HCs and thus hearing.

Piccolino is required for ribbon architecture at cochlear inner hair cell synapses and for hearing

Cochlear inner hair cells (IHCs) form specialized ribbon synapses with spiral ganglion neurons that tirelessly transmit sound information at high rates over long time periods with extreme temporal precision. This functional specialization is essential for sound encoding and is attributed to a distinct molecular machinery with unique players or splice variants compared to conventional neuronal synapses. Among these is the active zone (AZ) scaffold protein piccolo/aczonin, which is represented by its short splice variant piccolino at cochlear and retinal ribbon synapses. While the function of piccolo at synapses of the central nervous system has been intensively investigated, the role of piccolino at IHC synapses remains unclear. In this study, we characterize the structure and function of IHC synapses in piccolo gene-trap mutant rats (Pclogt/gt ). We find a mild hearing deficit with elevated thresholds and reduced amplitudes of auditory brainstem responses. Ca2+ channel distribution and ribbon morphology are altered in apical IHCs, while their presynaptic function seems to be unchanged. We conclude that piccolino contributes to the AZ organization in IHCs and is essential for normal hearing.

Synaptic vesicle release during ribbon synapse formation of cone photoreceptors

Mammalian cone photoreceptors enable through their sophisticated synapse the high-fidelity transfer of visual information to second-order neurons in the retina. The synapse contains a proteinaceous organelle, called the synaptic ribbon, which tethers synaptic vesicles (SVs) at the active zone (AZ) close to voltage-gated Ca2+ channels. However, the exact contribution of the synaptic ribbon to neurotransmission is not fully understood, yet. In mice, precursors to synaptic ribbons appear within photoreceptor terminals shortly after birth as free-floating spherical structures, which progressively elongate and then attach to the AZ during the following days. Here, we took advantage of the process of synaptic ribbon maturation to study their contribution to SV release. We performed whole-cell patch-clamp recordings from cone photoreceptors at three postnatal (P) development stages (P8-9, P12-13, >P30) and measured evoked SV release, SV replenishment rate, recovery from synaptic depression, domain organization of voltage-sensitive Ca2+ channels, and Ca2+-sensitivity of exocytosis. Additionally, we performed electron microscopy to determine the density of SVs at ribbon-free and ribbon-occupied AZs. Our results suggest that ribbon attachment does not organize the voltage-sensitive Ca2+ channels into nanodomains or control SV release probability. However, ribbon attachment increases SV density at the AZ, increases the pool size of readily releasable SVs available for evoked SV release, facilitates SV replenishment without changing the SV pool refilling time, and increases the Ca2+- sensitivity of glutamate release.

Signal transmission in mature mammalian vestibular hair cells

The maintenance of balance and gaze relies on the faithful and rapid signaling of head movements to the brain. In mammals, vestibular organs contain two types of sensory hair cells, type-I and type-II, which convert the head motion-induced movement of their hair bundles into a graded receptor potential that drives action potential activity in their afferent fibers. While signal transmission in both hair cell types involves Ca2+-dependent quantal release of glutamate at ribbon synapses, type-I cells appear to also exhibit a non-quantal mechanism that is believed to increase transmission speed. However, the reliance of mature type-I hair cells on non-quantal transmission remains unknown. Here we investigated synaptic transmission in mammalian utricular hair cells using patch-clamp recording of Ca2+ currents and changes in membrane capacitance (ΔCm). We found that mature type-II hair cells showed robust exocytosis with a high-order dependence on Ca2+ entry. By contrast, exocytosis was approximately 10 times smaller in type-I hair cells. Synaptic vesicle exocytosis was largely absent in mature vestibular hair cells of CaV1.3 (CaV1.3−/−) and otoferlin (Otof−/−) knockout mice. Even though Ca2+-dependent exocytosis was small in type-I hair cells of wild-type mice, or absent in CaV1.3−/− and Otof−/−mice, these cells were able to drive action potential activity in the postsynaptic calyces. This supports a functional role for non-quantal synaptic transmission in type-I cells. The large vesicle pools in type-II cells would facilitate sustained transmission of tonic or low-frequency signals. In type-I cells, the restricted vesicle pool size, together with a rapid non-quantal mechanism, could allow them to sustain high-frequency phasic signal transmission at their specialized large calyceal synapses.

Resolving the molecular architecture of the photoreceptor active zone with 3D-MINFLUX

Cells assemble macromolecular complexes into scaffoldings that serve as substrates for catalytic processes. Years of molecular neurobiology research indicate that neurotransmission depends on such optimization strategies. However, the molecular topography of the presynaptic active zone (AZ), where transmitter is released upon synaptic vesicle (SV) fusion, remains to be visualized. Therefore, we implemented MINFLUX optical nanoscopy to resolve the AZ of rod photoreceptors. This was facilitated by a novel sample immobilization technique that we name heat-assisted rapid dehydration (HARD), wherein a thin layer of rod synaptic terminals (spherules) was transferred onto glass coverslips from fresh retinal slices. Rod ribbon AZs were readily immunolabeled and imaged in 3D with a precision of a few nanometers. Our 3D-MINFLUX results indicate that the SV release site in rods is a molecular complex of bassoon-RIM2-ubMunc13-2-Ca_v1.4, which repeats longitudinally on both sides of the ribbon.

Small Molecules as Modulators of Voltage-Gated Calcium Channels in Neurological Disorders: State of the Art and Perspectives

Voltage-gated calcium channels (VGCCs) are widely expressed in the brain, heart and vessels, smooth and skeletal muscle, as well as in endocrine cells. VGCCs mediate gene transcription, synaptic and neuronal structural plasticity, muscle contraction, the release of hormones and neurotransmitters, and membrane excitability. Therefore, it is not surprising that VGCC dysfunction results in severe pathologies, such as cardiovascular conditions, neurological and psychiatric disorders, altered glycemic levels, and abnormal smooth muscle tone. The latest research findings and clinical evidence increasingly show the critical role played by VGCCs in autism spectrum disorders, Parkinson's disease, drug addiction, pain, and epilepsy. These findings outline the importance of developing selective calcium channel inhibitors and modulators to treat such prevailing conditions of the central nervous system. Several small molecules inhibiting calcium channels are currently used in clinical practice to successfully treat pain and cardiovascular conditions. However, the limited palette of molecules available and the emerging extent of VGCC pathophysiology require the development of additional drugs targeting these channels. Here, we provide an overview of the role of calcium channels in neurological disorders and discuss possible strategies to generate novel therapeutics.

Optogenetics and electron tomography for structure-function analysis of cochlear ribbon synapses

Ribbon synapses of cochlear inner hair cells (IHCs) are specialized to indefatigably transmit sound information at high rates. To understand the underlying mechanisms, structure-function analysis of the active zone (AZ) of these synapses is essential. Previous electron microscopy studies of synaptic vesicle (SV) dynamics at the IHC AZ used potassium stimulation, which limited the temporal resolution to minutes. Here, we established optogenetic IHC stimulation followed by quick freezing within milliseconds and electron tomography to study the ultrastructure of functional synapse states with good temporal resolution in mice. We characterized optogenetic IHC stimulation by patch-clamp recordings from IHCs and postsynaptic boutons revealing robust IHC depolarization and neurotransmitter release. Ultrastructurally, the number of docked SVs increased upon short (17-25 ms) and long (48-76 ms) light stimulation paradigms. We did not observe enlarged SVs or other morphological correlates of homotypic fusion events. Our results indicate a rapid recruitment of SVs to the docked state upon stimulation and suggest that univesicular release prevails as the quantal mechanism of exocytosis at IHC ribbon synapses.

Macrophages Are Dispensable for Postnatal Pruning of the Cochlear Ribbon Synapses

Ribbon synapses of cochlear hair cells undergo pruning and maturation before the hearing onset. In the central nervous system (CNS), synaptic pruning was mediated by microglia, the brain-resident macrophages, via activation of the complement system. Whether a similar mechanism regulates ribbon synapse pruning is currently unknown. In this study, we report that the densities of cochlear macrophages surrounding hair cells were highest at around P8, corresponding well to the completion of ribbon synaptic pruning by P8–P9. Surprisingly, using multiple genetic mouse models, we found that postnatal pruning of the ribbon synapses and auditory functions were unaffected by the knockout of the complement receptor 3 (CR3) or by ablations of macrophages expressing either LysM or Cx3cr1. Our results suggest that unlike microglia in the CNS, macrophages in the cochlea do not mediate pruning of the cochlear ribbon synapses.

Sensory transduction is required for normal development and maturation of cochlear inner hair cell synapses

Acoustic overexposure and aging can damage auditory synapses in the inner ear by a process known as synaptopathy. These insults may also damage hair bundles and the sensory transduction apparatus in auditory hair cells. However, a connection between sensory transduction and synaptopathy has not been established. To evaluate potential contributions of sensory transduction to synapse formation and development, we assessed inner hair cell synapses in several genetic models of dysfunctional sensory transduction, including mice lacking transmembrane channel-like (Tmc) 1, Tmc2, or both, in Beethoven mice which carry a dominant Tmc1 mutation and in Spinner mice which carry a recessive mutation in transmembrane inner ear (Tmie). Our analyses reveal loss of synapses in the absence of sensory transduction and preservation of synapses in Tmc1-null mice following restoration of sensory transduction via Tmc1 gene therapy. These results provide insight into the requirement of sensory transduction for hair cell synapse development and maturation.

The Cholinergic Lateral Line Efferent Synapse: Structural, Functional and Molecular Similarities With Those of the Cochlea

Vertebrate hair cell (HC) systems are innervated by efferent fibers that modulate their response to external stimuli. In mammals, the best studied efferent-HC synapse, the cholinergic medial olivocochlear (MOC) efferent system, makes direct synaptic contacts with HCs. The net effect of MOC activity is to hyperpolarize HCs through the activation of α9α10 nicotinic cholinergic receptors (nAChRs) and the subsequent activation of Ca²⁺-dependent SK2 potassium channels. A serious obstacle in research on many mammalian sensory systems in their native context is that their constituent neurons are difficult to access even in newborn animals, hampering circuit observation, mapping, or controlled manipulation. By contrast, fishes and amphibians have a superficial and accessible mechanosensory system, the lateral line (LL), which circumvents many of these problems. LL responsiveness is modulated by efferent neurons which aid to distinguish between external and self-generated stimuli. One component of the LL efferent system is cholinergic and its activation inhibits LL afferent activity, similar to what has been described for MOC efferents. The zebrafish (Danio rerio) has emerged as a powerful model system for studying human hearing and balance disorders, since LL HC are structurally and functionally analogous to cochlear HCs, but are optically and pharmacologically accessible within an intact specimen. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of HC biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired HC dysfunction. With the rise of the zebrafish LL as a model in which to study auditory system function and disease, there has been an increased interest in studying its efferent system and evaluate the similarity between mammalian and piscine efferent synapses. Advances derived from studies in zebrafish include understanding the effect of the LL efferent system on HC and afferent activity, and revealing that an α9-containing nAChR, functionally coupled to SK channels, operates at the LL efferent synapse. In this review, we discuss the tools and findings of these recent investigations into zebrafish efferent-HC synapse, their commonalities with the mammalian counterpart and discuss several emerging areas for future studies.

Noise Exposure Potentiates Exocytosis From Cochlear Inner Hair Cells

Noise-induced hearing loss has gained relevance as one of the most common forms of hearing impairment. The anatomical correlates of hearing loss, principally cell damage and/or death, are relatively well-understood histologically. However, much less is known about the physiological aspects of damaged, surviving cells. Here we addressed the functional consequences of noise exposure on the capacity of inner hair cells (IHCs) to release synaptic vesicles at synapses with spiral ganglion neurons (SGNs). Mice of either sex at postnatal day (P) 15–16 were exposed to 1–12 kHz noise at 120 dB sound pressure level (SPL), for 1 h. Exocytosis was measured by tracking changes in membrane capacitance (ΔCm) from IHCs of the apical cochlea. Upon IHC depolarization to different membrane potentials, ΔC_m showed the typical bell-shaped curve that mirrors the voltage dependence of Ca²⁺ influx, in both exposed and unexposed cells. Surprisingly, from IHCs at 1-day after exposure (d.a.e.), we found potentiation of exocytosis at the peak of the bell-shaped curve. The increase in exocytosis was not accompanied by changes in whole-cell Ca²⁺ influx, suggesting a modification in coupling between Ca²⁺ channels and synaptic vesicles. Consistent with this notion, noise exposure also changed the Ca²⁺-dependence of exocytosis from linear to supralinear. Noise exposure did not cause loss of IHCs, but did result in a small reduction in the number of IHC-SGN synapses at 1-d.a.e. which recovered by 14-d.a.e. In contrast, a strong reduction in auditory brainstem response wave-I amplitude (representing synchronous firing of SGNs) and distortion product otoacoustic emissions (reflecting outer hair cell function) indicated a profound hearing loss at 1- and 14-d.a.e. To determine the role of glutamate release in the noise-induced potentiation of exocytosis, we evaluated vesicular glutamate transporter-3 (Vglut3) knock-out (KO) mice. Unlike WT, IHCs from Vglut3^KO mice showed a noise-induced reduction in ΔC_m and Ca²⁺ influx with no change in the Ca²⁺-dependence of exocytosis. Together, these results indicate that traumatic noise exposure triggers changes of IHC synaptic function including a Vglut3-dependent potentiation of exocytosis.

Resting and stimulated mouse rod photoreceptors show distinct patterns of vesicle release at ribbon synapses

The vertebrate visual system can detect and transmit signals from single photons. To understand how single-photon responses are transmitted, we characterized voltage-dependent properties of glutamate release in mouse rods. We measured presynaptic glutamate transporter anion current and found that rates of synaptic vesicle release increased with voltage-dependent Ca2+ current. Ca2+ influx and release rate also rose with temperature, attaining a rate of ∼11 vesicles/s/ribbon at -40 mV (35°C). By contrast, spontaneous release events at hyperpolarized potentials (-60 to -70 mV) were univesicular and occurred at random intervals. However, when rods were voltage clamped at -40 mV for many seconds to simulate maintained darkness, release occurred in coordinated bursts of 17 ± 7 quanta (mean ± SD; n = 22). Like fast release evoked by brief depolarizing stimuli, these bursts involved vesicles in the readily releasable pool of vesicles and were triggered by the opening of nearby ribbon-associated Ca2+ channels. Spontaneous release rates were elevated and bursts were absent after genetic elimination of the Ca2+ sensor synaptotagmin 1 (Syt1). This study shows that at the resting potential in darkness, rods release glutamate-filled vesicles from a pool at the base of synaptic ribbons at low rates but in Syt1-dependent bursts. The absence of bursting in cones suggests that this behavior may have a role in transmitting scotopic responses.

Effect of Sodium Salicylate on Calcium Currents and Exocytosis in Cochlear Inner Hair Cells: Implications for Tinnitus Generation

Sodium salicylate is an anti-inflammatory medication with a side-effect of tinnitus. Here, we used mouse cochlear cultures to explore the effects of salicylate treatment on cochlear inner hair cells (IHCs). We found that IHCs showed significant damage after exposure to a high concentration of salicylate. Whole-cell patch clamp recordings showed that 1-5 mmol/L salicylate did not affect the exocytosis of IHCs, indicating that IHCs are not involved in tinnitus generation by enhancing their neuronal input. Instead, salicylate induced a larger peak amplitude, a more negative half-activation voltage, and a steeper slope factor of Ca²⁺ current. Using noise analysis of Ca²⁺ tail currents and qRT-PCR, we further found that salicylate increased the number of Ca²⁺ channels along with Ca_V1.3 expression. All these changes could act synergistically to enhance the Ca²⁺ influx into IHCs. Inhibition of intracellular Ca²⁺ overload significantly attenuated IHC death after 10 mmol/L salicylate treatment. These results implicate a cellular mechanism for tinnitus generation in the peripheral auditory system.

Encoding sound in the cochlea: from receptor potential to afferent discharge

Ribbon-class synapses in the ear achieve analog to digital transformation of a continuously graded membrane potential to all-or-none spikes. In mammals, several auditory nerve fibres (ANFs) carry information from each inner hair cell (IHC) to the brain in parallel. Heterogeneity of transmission among synapses contributes to the diversity of ANF sound-response properties. In addition to the place code for sound frequency and the rate code for sound level, there is also a temporal code. In series with cochlear amplification and frequency tuning, neural representation of temporal cues over a broad range of sound levels enables auditory comprehension in noisy multi-speaker settings. The IHC membrane time constant introduces a low-pass filter that attenuates fluctuations of the receptor potential above 1-2 kHz. The ANF spike generator adds a high-pass filter via its depolarization-rate threshold that rejects slow changes in the postsynaptic potential and its phasic response property that ensures one spike per depolarization. Synaptic transmission involves several stochastic subcellular processes between IHC depolarization and ANF spike generation, introducing delay and jitter that limits the speed and precision of spike timing. ANFs spike at a preferred phase of periodic sounds in a process called phase-locking that is limited to frequencies below a few kilohertz by both the IHC receptor potential and the jitter in synaptic transmission. During phase-locking to periodic sounds of increasing intensity, faster and facilitated activation of synaptic transmission and spike generation may be offset by presynaptic depletion of synaptic vesicles, resulting in relatively small changes in response phase. Here we review encoding of spike-timing at cochlear ribbon synapses.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

RIM-Binding Proteins Are Required for Normal Sound-Encoding at Afferent Inner Hair Cell Synapses

The afferent synapses between inner hair cells (IHC) and spiral ganglion neurons are specialized to faithfully encode sound with sub-millisecond precision over prolonged periods of time. Here, we studied the role of Rab3 interacting molecule-binding proteins (RIM-BP) 1 and 2 – multidomain proteins of the active zone known to directly interact with RIMs, Bassoon and Ca_V1.3 – in IHC presynaptic function and hearing. Recordings of auditory brainstem responses and otoacoustic emissions revealed that genetic disruption of RIM-BPs 1 and 2 in mice (RIM-BP1/2^–/–) causes a synaptopathic hearing impairment exceeding that found in mice lacking RIM-BP2 (RIM-BP2^–/–). Patch-clamp recordings from RIM-BP1/2^–/– IHCs indicated a subtle impairment of exocytosis from the readily releasable pool of synaptic vesicles that had not been observed in RIM-BP2^–/– IHCs. In contrast, the reduction of Ca²⁺-influx and sustained exocytosis was similar to that in RIMBP2^–/– IHCs. We conclude that both RIM-BPs are required for normal sound encoding at the IHC synapse, whereby RIM-BP2 seems to take the leading role.

Recent advances in cochlear hair cell nanophysiology: subcellular compartmentalization of electrical signaling in compact sensory cells

In recent years, genetics, physiology, and structural biology have advanced into the molecular details of the sensory physiology of auditory hair cells. Inner hair cells (IHCs) and outer hair cells (OHCs) mediate two key functions: active amplification and non-linear compression of cochlear vibrations by OHCs and sound encoding by IHCs at their afferent synapses with the spiral ganglion neurons. OHCs and IHCs share some molecular physiology, e.g. mechanotransduction at the apical hair bundles, ribbon-type presynaptic active zones, and ionic conductances in the basolateral membrane. Unique features enabling their specific function include prestin-based electromotility of OHCs and indefatigable transmitter release at the highest known rates by ribbon-type IHC active zones. Despite their compact morphology, the molecular machineries that either generate electrical signals or are driven by these signals are essentially all segregated into local subcellular structures. This review provides a brief account on recent insights into the molecular physiology of cochlear hair cells with a specific focus on organization into membrane domains.

Phase-Locking Requires Efficient Ca2+ Extrusion at the Auditory Hair Cell Ribbon Synapse

Proper perception of sounds in the environment requires auditory signals to be encoded with extraordinary temporal precision up to tens of microseconds, but how it originates from the hearing organs in the periphery is poorly understood. In particular, sound-evoked spikes in auditory afferent fibers in vivo are phase-locked to sound frequencies up to 5 kHz, but it is not clear how hair cells can handle intracellular Ca²⁺ changes with such high speed and efficiency. In this study, we combined patch-clamp recording and two-photon Ca²⁺ imaging to examine Ca²⁺ dynamics in hair cell ribbon synapses in the bullfrog amphibian papilla of both sexes. We found that Ca²⁺ clearance from single synaptic ribbons followed a double exponential function, and the weight of the fast component, but not the two time constants, was significantly reduced for prolonged stimulation, and during inhibition of the plasma membrane Ca²⁺ ATPase (PMCA), the mitochondrial Ca²⁺ uptake (MCU), or the sarcolemma/endoplasmic reticulum Ca²⁺ ATPase (SERCA), but not the Na⁺/Ca²⁺ exchanger (NCX). Furthermore, we found that both the basal Ca²⁺ level and the Ca²⁺ rise during sinusoidal stimulation were significantly increased by inhibition of PMCA, MCU, or SERCA. Consistently, phase-locking of synaptic vesicle releases from hair cells was also significantly reduced by blocking PMCA, MCU, or SERCA, but not NCX. We conclude that, in addition to fast diffusion mediated by mobile Ca²⁺ buffer, multiple Ca²⁺ extrusion pumps are required for phase-locking at the auditory hair cell ribbon synapse.SIGNIFICANCE STATEMENT Hair cell synapses can transmit sound-driven signals precisely in the kHz range. However, previous studies of Ca²⁺ handling in auditory hair cells have often been conducted in immature hair cells, with elevated extracellular Ca²⁺ concentration, or through steady-state stimulation that may not be physiologically relevant. Here we examine Ca²⁺ clearance from hair cell synaptic ribbons in a fully mature preparation at physiological concentration of external Ca²⁺ and at physiological temperature. By stimulating hair cells with sinusoidal voltage commands that mimic pure sound tones, we recapitulated the phase-locking of hair cell exocytosis with an in vitro approach. This allowed us to reveal the Ca²⁺ extrusion mechanisms that are required for phase-locking at auditory hair cell ribbon synapses.

A sensory cell diversifies its output by varying Ca2+ influx-release coupling among active zones

The cochlea encodes sound pressures varying over six orders of magnitude by collective operation of functionally diverse spiral ganglion neurons (SGNs). The mechanisms enabling this functional diversity remain elusive. Here, we asked whether the sound intensity information, contained in the receptor potential of the presynaptic inner hair cell (IHC), is fractionated via heterogeneous synapses. We studied the transfer function of individual IHC synapses by combining patch‐clamp recordings with dual‐color Rhod‐FF and iGluSnFR imaging of presynaptic Ca²⁺ signals and glutamate release. Synapses differed in the voltage dependence of release: Those residing at the IHC' pillar side activated at more hyperpolarized potentials and typically showed tight control of release by few Ca²⁺ channels. We conclude that heterogeneity of voltage dependence and release site coupling of Ca²⁺ channels among the synapses varies synaptic transfer within individual IHCs and, thereby, likely contributes to the functional diversity of SGNs. The mechanism reported here might serve sensory cells and neurons more generally to diversify signaling even in close‐by synapses.

L-type voltage-gated calcium channel agonists mitigate hearing loss and modify ribbon synapse morphology in the zebrafish model of Usher syndrome type 1

The mariner (myo7aa^−/−) mutant is a zebrafish model for Usher syndrome type 1 (USH1). To further characterize hair cell synaptic elements in myo7aa^−/− mutants, we focused on the ribbon synapse and evaluated ultrastructure, number and distribution of immunolabeled ribbons, and postsynaptic densities. By transmission electron microscopy, we determined that myo7aa^−/− zebrafish have fewer glutamatergic vesicles tethered to ribbon synapses, yet maintain a comparable ribbon area. In myo7aa^−/− hair cells, immunolocalization of Ctbp2 showed fewer ribbon-containing cells in total and an altered distribution of Ctbp2 puncta compared to wild-type hair cells. myo7aa^−/− mutants have fewer postsynaptic densities – as assessed by MAGUK immunolabeling – compared to wild-type zebrafish. We quantified the circular swimming behavior of myo7aa^−/− mutant fish and measured a greater turning angle (absolute smooth orientation). It has previously been shown that L-type voltage-gated calcium channels are necessary for ribbon localization and occurrence of postsynaptic density; thus, we hypothesized and observed that L-type voltage-gated calcium channel agonists change behavioral and synaptic phenotypes in myo7aa^−/− mutants in a drug-specific manner. Our results indicate that treatment with L-type voltage-gated calcium channel agonists alter hair cell synaptic elements and improve behavioral phenotypes of myo7aa^−/− mutants. Our data support that L-type voltage-gated calcium channel agonists induce morphological changes at the ribbon synapse – in both the number of tethered vesicles and regarding the distribution of Ctbp2 puncta – shift swimming behavior and improve acoustic startle response.

Summary: We quantified behavioral and synaptic morphology differences between wild-type zebrafish larvae and the mariner (myo7aa^−/−) mutant, finding that these differences can be modified by L-type voltage-gated calcium channel agonists.

Protection of Cochlear Ribbon Synapses and Prevention of Hidden Hearing Loss

In the auditory system, ribbon synapses are vesicle-associated structures located between inner hair cells (IHCs) and spiral ganglion neurons that are implicated in the modulation of trafficking and fusion of synaptic vesicles at the presynaptic terminals. Synapse loss may result in hearing loss and difficulties with understanding speech in a noisy environment. This phenomenon happens without permanent hearing loss; that is, the cochlear synaptopathy is "hidden." Recent studies have reported that synapse loss might be critical in the pathogenesis of hidden hearing loss. A better understanding of the molecular mechanisms of the formation, structure, regeneration, and protection of ribbon synapses will assist in the design of potential therapeutic strategies. In this review, we describe and summarize the following aspects of ribbon synapses: (1) functional and structural features, (2) potential mechanisms of damage, (3) therapeutic research on protecting the synapses, and (4) the role of synaptic regeneration in auditory neuropathy and the current options for synapse rehabilitation.

Primary and secondary motoneurons use different calcium channel types to control escape and swimming behaviors in zebrafish

The escape response and rhythmic swimming in zebrafish are distinct behaviors mediated by two functionally distinct motoneuron (Mn) types. The primary (1°Mn) type depresses and has a large quantal content (Qc) and a high release probability (Pr). Conversely, the secondary (2°Mn) type facilitates and has low and variable Qc and Pr. This functional duality matches well the distinct associated behaviors, with the 1°Mn providing the strong, singular C bend initiating escape and the 2°Mn conferring weaker, rhythmic contractions. Contributing to these functional distinctions is our identification of P/Q-type calcium channels mediating transmitter release in 1°Mns and N-type channels in 2°Mns. Remarkably, despite these functional and behavioral distinctions, all ∼15 individual synapses on each muscle cell are shared by a 1°Mn bouton and at least one 2°Mn bouton. This blueprint of synaptic sharing provides an efficient way of controlling two different behaviors at the level of a single postsynaptic cell.

Exocytosis in mouse vestibular Type II hair cells shows a high-order Ca2+ dependence that is independent of synaptotagmin-4

Mature hair cells transduce information over a wide range of stimulus intensities and frequencies for prolonged periods of time. The efficiency of such a demanding task is reflected in the characteristics of exocytosis at their specialized presynaptic ribbons. Ribbons are electron-dense structures able to tether a large number of releasable vesicles allowing them to maintain high rates of vesicle release. Calcium entry through rapidly activating, non-inactivating CaV 1.3 (L-type) Ca2+ channels in response to cell depolarization causes a local increase in Ca2+ at the ribbon synapses, which is detected by the exocytotic Ca2+ sensors. The Ca2+ dependence of vesicle exocytosis at mammalian vestibular hair cell (VHC) ribbon synapses is believed to be linear, similar to that observed in mature cochlear inner hair cells (IHCs). The linear relation has been shown to correlate with the presence of the Ca2+ sensor synaptotagmin-4 (Syt-4). Therefore, we studied the exocytotic Ca2+ dependence, and the release kinetics of different vesicle pool populations, in Type II VHCs of control and Syt-4 knockout mice using patch-clamp capacitance measurements, under physiological recording conditions. We found that exocytosis in mature control and knockout Type II VHCs displayed a high-order dependence on Ca2+ entry, rather than the linear relation previously observed. Consistent with this finding, the Ca2+ dependence and release kinetics of the ready releasable pool (RRP) of vesicles were not affected by an absence of Syt-4. However, we did find that Syt-4 could play a role in regulating the release of the secondary releasable pool (SRP) in these cells. Our findings show that the coupling between Ca2+ influx and neurotransmitter release at mature Type II VHC ribbon synapses is faithfully described by a nonlinear relation that is likely to be more appropriate for the accurate encoding of low-frequency vestibular information, consistent with that observed at low-frequency mammalian auditory receptors.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

Simulation of the efficiency of inner hair cell secretion in the auditory pathway

Sound coding involves several stages of processing along the auditory path. Specifically, the Inner Hair Cells (IHC) act as sensory receptors and transduce acoustic information -frequency, intensity and duration of the stimulus- into neuronal signals. In this work, a stochastic model was implemented to achieve a better understanding of the IHC-auditory nerve synapse, specifically, the process of Ready Releasable Pool (RRP) vesicle exocytosis, a complicated process to study experimentally because current protocols do not provide adequate temporal resolution, in the order of milliseconds. The presented model allows predicting the efficiency of glutamate release towards explaining maturation changes or disease impacts in the auditory pathway.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Clustered Ca2+ Channels Are Blocked by Synaptic Vesicle Proton Release at Mammalian Auditory Ribbon Synapses

A Ca²⁺ current transient block (I_CaTB) by protons occurs at some ribbon-type synapses after exocytosis, but this has not been observed at mammalian hair cells. Here we show that a robust I_CaTB occurs at post-hearing mouse and gerbil inner hair cell (IHC) synapses, but not in immature IHC synapses, which contain non-compact active zones, where Ca²⁺ channels are loosely coupled to the release sites. Unlike I_CaTB at other ribbon synapses, I_CaTB in mammalian IHCs displays a surprising multi-peak structure that mirrors the EPSCs seen in paired recordings. Desynchronizing vesicular release with intracellular BAPTA or by deleting otoferlin, the Ca²⁺ sensor for exocytosis, greatly reduces I_CaTB, whereas enhancing release synchronization by raising Ca²⁺ influx or temperature increases I_CaTB. This suggests that I_CaTB is produced by fast multivesicular proton-release events. We propose that I_CaTB may function as a submillisecond feedback mechanism contributing to the auditory nerve's fast spike adaptation during sound stimulation.

Hair Cell Afferent Synapses: Function and Dysfunction

To provide a meaningful representation of the auditory landscape, mammalian cochlear hair cells are optimized to detect sounds over an incredibly broad range of frequencies and intensities with unparalleled accuracy. This ability is largely conferred by specialized ribbon synapses that continuously transmit acoustic information with high fidelity and sub-millisecond precision to the afferent dendrites of the spiral ganglion neurons. To achieve this extraordinary task, ribbon synapses employ a unique combination of molecules and mechanisms that are tailored to sounds of different frequencies. Here we review the current understanding of how the hair cell's presynaptic machinery and its postsynaptic afferent connections are formed, how they mature, and how their function is adapted for an accurate perception of sound.

Simultaneous Release of Multiple Vesicles from Rods Involves Synaptic Ribbons and Syntaxin 3B

First proposed as a specialized mode of release at sensory neurons possessing ribbon synapses, multivesicular release has since been described throughout the central nervous system. Many aspects of multivesicular release remain poorly understood. We explored mechanisms underlying simultaneous multivesicular release at ribbon synapses in salamander retinal rod photoreceptors. We assessed spontaneous release presynaptically by recording glutamate transporter anion currents (I_A(glu)) in rods. Spontaneous I_A(glu) events were correlated in amplitude and kinetics with simultaneously measured miniature excitatory postsynaptic currents in horizontal cells. Both measures indicated that a significant fraction of events is multiquantal, with an analysis of I_A(glu) revealing that multivesicular release constitutes ∼30% of spontaneous release events. I_A(glu) charge transfer increased linearly with event amplitude showing that larger events involve greater glutamate release. The kinetics of large and small I_A(glu) events were identical as were rise times of large and small miniature excitatory postsynaptic currents, indicating that the release of multiple vesicles during large events is highly synchronized. Effects of exogenous Ca²⁺ buffers suggested that multiquantal, but not uniquantal, release occurs preferentially near Ca²⁺ channels clustered beneath synaptic ribbons. Photoinactivation of ribbons reduced the frequency of spontaneous multiquantal events without affecting uniquantal release frequency, showing that spontaneous multiquantal release requires functional ribbons. Although both occur at ribbon-style active zones, the absence of cross-depletion indicates that evoked and spontaneous multiquantal release from ribbons involve different vesicle pools. Introducing an inhibitory peptide into rods to interfere with the SNARE protein, syntaxin 3B, selectively reduced multiquantal event frequency. These results support the hypothesis that simultaneous multiquantal release from rods arises from homotypic fusion among neighboring vesicles on ribbons and involves syntaxin 3B.

Localization of group II and III metabotropic glutamate receptors at pre- and postsynaptic sites of inner hair cell ribbon synapses

Glutamate is the major excitatory neurotransmitter in the CNS binding to a variety of glutamate receptors. Metabotropic glutamate receptors (mGluR1 to mGluR8) can act excitatory or inhibitory, depending on associated signal cascades. Expression and localization of inhibitory acting mGluRs at inner hair cells (IHCs) in the cochlea are largely unknown. Here, we analyzed expression of mGluR2, mGluR3, mGluR4, mGluR6, mGluR7, and mGluR8 and investigated their localization with respect to the presynaptic ribbon of IHC synapses. We detected transcripts for mGluR2, mGluR3, and mGluR4 as well as for mGluR7a, mGluR7b, mGluR8a, and mGluR8b splice variants. Using receptor-specific antibodies in cochlear wholemounts, we found expression of mGluR2, mGluR4, and mGluR8b close to presynaptic ribbons. Super resolution and confocal microscopy in combination with 3-dimensional reconstructions indicated a postsynaptic localization of mGluR2 that overlaps with postsynaptic density protein 95 on dendrites of afferent type I spiral ganglion neurons. In contrast, mGluR4 and mGluR8b were expressed at the presynapse close to IHC ribbons. In summary, we localized in detail 3 mGluR types at IHC ribbon synapses, providing a fundament for new therapeutical strategies that could protect the cochlea against noxious stimuli and excitotoxicity.-Klotz, L., Wendler, O., Frischknecht, R., Shigemoto, R., Schulze, H., Enz, R. Localization of group II and III metabotropic glutamate receptors at pre- and postsynaptic sites of inner hair cell ribbon synapses.

Clarin-2 is essential for hearing by maintaining stereocilia integrity and function

Hearing relies on mechanically gated ion channels present in the actin-rich stereocilia bundles at the apical surface of cochlear hair cells. Our knowledge of the mechanisms underlying the formation and maintenance of the sound-receptive structure is limited. Utilizing a large-scale forward genetic screen in mice, genome mapping and gene complementation tests, we identified Clrn2 as a new deafness gene. The Clrn2clarinet/clarinet mice (p.Trp4* mutation) exhibit a progressive, early-onset hearing loss, with no overt retinal deficits. Utilizing data from the UK Biobank study, we could show that CLRN2 is involved in human non-syndromic progressive hearing loss. Our in-depth morphological, molecular and functional investigations establish that while it is not required for initial formation of cochlear sensory hair cell stereocilia bundles, clarin-2 is critical for maintaining normal bundle integrity and functioning. In the differentiating hair bundles, lack of clarin-2 leads to loss of mechano-electrical transduction, followed by selective progressive loss of the transducing stereocilia. Together, our findings demonstrate a key role for clarin-2 in mammalian hearing, providing insights into the interplay between mechano-electrical transduction and stereocilia maintenance.

How to Build a Fast and Highly Sensitive Sound Detector That Remains Robust to Temperature Shifts

Frogs must have sharp hearing abilities during the warm summer months to successfully find mating partners. This study aims to understand how frog hair cell ribbon-type synapses preserve both sensitivity and temporal precision during temperature changes. Under room (∼24°C) and high (∼32°C) temperature, we performed in vitro patch-clamp recordings of hair cells and their afferent fibers in amphibian papillae of either male or female bullfrogs. Afferent fibers exhibited a wide heterogeneity in membrane input resistance (R_in) from 100 mΩ to 1000 mΩ, which may contribute to variations in spike threshold and firing frequency. At higher temperatures, most fibers increased their frequency of spike firing due to an increase in spontaneous EPSC frequencies. Hair cell resting membrane potential (V_rest) remained surprisingly stable during temperature increases, because Ca²⁺ influx and K⁺ outflux increased simultaneously. This increase in Ca²⁺ current likely enhanced spontaneous EPSC frequencies. These larger "leak currents" at V_rest also lowered R_in and produced higher electrical resonant frequencies. Lowering R_in will reduce the hair cells receptor potential and presumably moderate the systems sensitivity. Using membrane capacitance measurements, we suggest that hair cells can partially compensate for this reduced sensitivity by increasing exocytosis efficiency and the size of the readily releasable pool of synaptic vesicles. Furthermore, paired recordings of hair cells and their afferent fibers showed that synaptic delays shortened and multivesicular release becomes more synchronous at higher temperatures, which should improve temporal precision. Together, our results explain many previous in vivo observations on the temperature dependence of spikes in auditory nerves.SIGNIFICANCE STATEMENT The vertebrate inner ear detects and transmits auditory information over a broad dynamic range of sound frequency and intensity. It achieves remarkable sensitivity to soft sounds and precise frequency selectivity. How does the ear of cold-blooded vertebrates maintain its performance level as temperature changes? More specifically, how does the hair cell to afferent fiber synapse in bullfrog amphibian papilla adjust to a wide range of physiological temperatures without losing its sensitivity and temporal fidelity to sound signals? This study uses in vitro experiments to reveal the biophysical mechanisms that explain many observations made from in vivo auditory nerve fiber recordings. We find that higher temperature facilitates vesicle exocytosis and electrical tuning to higher sound frequencies, which benefits sensitivity and selectivity.

Genes Involved in the Development and Physiology of Both the Peripheral and Central Auditory Systems

The genetic approach, based on the study of inherited forms of deafness, has proven to be particularly effective for deciphering the molecular mechanisms underlying the development of the peripheral auditory system, the cochlea and its afferent auditory neurons, and how this system extracts the physical parameters of sound. Although this genetic dissection has provided little information about the central auditory system, scattered data suggest that some genes may have a critical role in both the peripheral and central auditory systems. Here, we review the genes controlling the development and function of the peripheral and central auditory systems, focusing on those with demonstrated intrinsic roles in both systems and highlighting the current underappreciation of these genes. Their encoded products are diverse, from transcription factors to ion channels, as are their roles in the central auditory system, mostly evaluated in brainstem nuclei. We examine the ontogenetic and evolutionary mechanisms that may underlie their expression at different sites.

Nanomachinery Organizing Release at Neuronal and Ribbon Synapses

A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.

Phase Locking of Auditory-Nerve Fibers Reveals Stereotyped Distortions and an Exponential Transfer Function with a Level-Dependent Slope

Phase locking of auditory-nerve-fiber (ANF) responses to the fine structure of acoustic stimuli is a hallmark of the auditory system's temporal precision and is important for many aspects of hearing. Period histograms from phase-locked ANF responses to low-frequency tones exhibit spike-rate and temporal asymmetries, but otherwise retain an approximately sinusoidal shape as stimulus level increases, even beyond the level at which the mean spike rate saturates. This is intriguing because apical cochlear mechanical vibrations show little compression, and mechanoelectrical transduction in the receptor cells is thought to obey a static sigmoidal nonlinearity, which might be expected to produce peak clipping at moderate and high stimulus levels. Here we analyze phase-locked responses of ANFs from cats of both sexes. We show that the lack of peak clipping is due neither to ANF refractoriness nor to spike-rate adaptation on time scales longer than the stimulus period. We demonstrate that the relationship between instantaneous pressure and instantaneous rate is well described by an exponential function whose slope decreases with increasing stimulus level. Relatively stereotyped harmonic distortions in the input to the exponential can account for the temporal asymmetry of the period histograms, including peak splitting. We show that the model accounts for published membrane-potential waveforms when assuming a power-of-three, but not a power-of-one, relationship to exocytosis. Finally, we demonstrate the relationship between the exponential transfer functions and the sigmoidal pseudotransducer functions obtained in the literature by plotting the maxima and minima of the voltage responses against the maxima and minima of the stimuli.SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing, but the mechanisms underlying phase locking are not fully understood. Intriguingly, period histograms retain an approximately sinusoidal shape across sound levels, even when the mean rate has saturated. We find that neither refractoriness nor spike-rate adaptation is responsible for this behavior. Instead, the peripheral auditory system operates as though it contains an exponential transfer function whose slope changes with stimulus level. The underlying mechanism is distinct from the comparatively weak cochlear mechanical compression in the cochlear apex, and likely resides in the receptor cells.

Endophilin-A regulates presynaptic Ca2+ influx and synaptic vesicle recycling in auditory hair cells

Ribbon synapses of cochlear inner hair cells (IHCs) operate with high rates of neurotransmission; yet, the molecular regulation of synaptic vesicle (SV) recycling at these synapses remains poorly understood. Here, we studied the role of endophilins-A1-3, endocytic adaptors with curvature-sensing and curvature-generating properties, in mouse IHCs. Single-cell RT-PCR indicated the expression of endophilins-A1-3 in IHCs, and immunoblotting confirmed the presence of endophilin-A1 and endophilin-A2 in the cochlea. Patch-clamp recordings from endophilin-A-deficient IHCs revealed a reduction of Ca2+ influx and exocytosis, which we attribute to a decreased abundance of presynaptic Ca2+ channels and impaired SV replenishment. Slow endocytic membrane retrieval, thought to reflect clathrin-mediated endocytosis, was impaired. Otoferlin, essential for IHC exocytosis, co-immunoprecipitated with purified endophilin-A1 protein, suggestive of a molecular interaction that might aid exocytosis-endocytosis coupling. Electron microscopy revealed lower SV numbers, but an increased occurrence of coated structures and endosome-like vacuoles at IHC active zones. In summary, endophilins regulate Ca2+ influx and promote SV recycling in IHCs, likely via coupling exocytosis to endocytosis, and contributing to membrane retrieval and SV reformation.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

In Vivo Calcium Imaging of Lateral-line Hair Cells in Larval Zebrafish

Sensory hair cells are mechanoreceptors found in the inner ear that are required for hearing and balance. Hair cells are activated in response to sensory stimuli that mechanically deflect apical protrusions called hair bundles. Deflection opens mechanotransduction (MET) channels in hair bundles, leading to an influx of cations, including calcium. This cation influx depolarizes the cell and opens voltage-gated calcium channels located basally at the hair-cell presynapse. In mammals, hair cells are encased in bone, and it is challenging to functionally assess these activities in vivo. In contrast, larval zebrafish are transparent and possess an externally located lateral-line organ that contains hair cells. These hair cells are functionally and structurally similar to mammalian hair cells and can be functionally assessed in vivo. This article outlines a technique that utilizes a genetically encoded calcium indicator (GECI), GCaMP6s, to measure stimulus-evoked calcium signals in zebrafish lateral-line hair cells. GCaMP6s can be used, along with confocal imaging, to measure in vivo calcium signals at the apex and base of lateral-line hair cells. These signals provide a real-time, quantifiable readout of both mechanosensation- and presynapse-dependent calcium activities within these hair cells. These calcium signals also provide important functional information regarding how hair cells detect and transmit sensory stimuli. Overall, this technique generates useful data about relative changes in calcium activity in vivo. It is less well-suited for quantification of the absolute magnitude of calcium changes. This in vivo technique is sensitive to motion artifacts. A reasonable amount of practice and skill are required for proper positioning, immobilization, and stimulation of larvae. Ultimately, when properly executed, the protocol outlined in this article provides a powerful way to collect valuable information about the activity of hair-cells in their natural, fully integrated states within a live animal.

The Transfer Characteristics of Hair Cells Encoding Mechanical Stimuli in the Lateral Line of Zebrafish

Hair cells transmit mechanical information by converting deflection of the hair bundle into synaptic release of glutamate. We have investigated this process in the lateral line of larval zebrafish (male and female) to understand how stimuli are encoded within a neuromast. Using multiphoton microscopy in vivo, we imaged synaptic release of glutamate using the reporter iGluSnFR as well as deflections of the cupula. We found that the neuromast is composed of a functionally diverse population of hair cells. Half the hair cells signaled cupula motion in both directions from rest, either by increasing glutamate release in response to a deflection in the positive direction or by reducing release in the negative direction. The relationship between cupula deflection and glutamate release demonstrated maximum sensitivity at displacements of just ∼40 nm in the positive direction. The remaining hair cells only signaled motion in one direction and were less sensitive, extending the operating range of the neuromast beyond 1 μm. Adaptation of the synaptic output was also heterogeneous, with some hair cells generating sustained glutamate release in response to a steady deflection of the cupula and others generating transient outputs. Finally, a distinct signal encoded a return of the cupula to rest: a large and transient burst of glutamate release from hair cells unresponsive to the initial stimulus. A population of hair cells with these different sensitivities, operating ranges, and adaptive properties will allow the neuromast to encode weak stimuli while maintaining the dynamic range to signal the amplitude and duration of stronger deflections.

SIGNIFICANCE STATEMENT Hair cells transmit information about mechanical stimuli by converting very small deflections of their hair bundle into changes in the release of the neurotransmitter glutamate. We have measured this input/output relation in the live fish using a fluorescent protein and find that different hair cells vary in their mechanical sensitivity and the time course of their response. These variations will allow the fish to sense the timing and duration of both very weak stimuli (∼40 nm deflections) and strong stimuli (∼1 μm), underlying the ability of the fish to avoid predators and maintain its body position in flowing water.

Balancing presynaptic release and endocytic membrane retrieval at hair cell ribbon synapses

The timely and reliable processing of auditory and vestibular information within the inner ear requires highly sophisticated sensory transduction pathways. On a cellular level, these demands are met by hair cells, which respond to sound waves - or alterations in body positioning - by releasing glutamate-filled synaptic vesicles (SVs) from their presynaptic active zones with unprecedented speed and exquisite temporal fidelity, thereby initiating the auditory and vestibular pathways. In order to achieve this, hair cells have developed anatomical and molecular specializations, such as the characteristic and name-giving 'synaptic ribbons' - presynaptically anchored dense bodies that tether SVs prior to release - as well as other unique or unconventional synaptic proteins. The tightly orchestrated interplay between these molecular components enables not only ultrafast exocytosis, but similarly rapid and efficient compensatory endocytosis. So far, the knowledge of how endocytosis operates at hair cell ribbon synapses is limited. In this Review, we summarize recent advances in our understanding of the SV cycle and molecular anatomy of hair cell ribbon synapses, with a focus on cochlear inner hair cells.

Mathematical Modeling and Analyses of Interspike-Intervals of Spontaneous Activity in Afferent Neurons of the Zebrafish Lateral Line

Without stimuli, hair cells spontaneously release neurotransmitter leading to spontaneous generation of action potentials (spikes) in innervating afferent neurons. We analyzed spontaneous spike patterns recorded from the lateral line of zebrafish and found that distributions of interspike intervals (ISIs) either have an exponential shape or an "L" shape that is characterized by a sharp decay but wide tail. ISI data were fitted to renewal-process models that accounted for the neuron refractory periods and hair-cell synaptic release. Modeling the timing of synaptic release using a mixture of two exponential distributions yielded the best fit for our ISI data. Additionally, lateral line ISIs displayed positive serial correlation and appeared to exhibit switching between faster and slower modes of spike generation. This pattern contrasts with previous findings from the auditory system where ISIs tended to have negative serial correlation due to synaptic depletion. We propose that afferent neuron innervation with multiple and heterogenous hair-cells synapses, each influenced by changes in calcium domains, can serve as a mechanism for the random switching behavior. Overall, our analyses provide evidence of how physiological similarities and differences between synapses and innervation patterns in the auditory, vestibular, and lateral line systems can lead to variations in spontaneous activity.