Hearing requires otoferlin-dependent efficient replenishment of synaptic vesicles in hair cells

Sharp Ca²⁺ nanodomains beneath the ribbon promote highly synchronous multivesicular release at hair cell synapses

Hair cell ribbon synapses exhibit several distinguishing features. Structurally, a dense body, or ribbon, is anchored to the presynaptic membrane and tethers synaptic vesicles; functionally, neurotransmitter release is dominated by large EPSC events produced by seemingly synchronous multivesicular release. However, the specific role of the synaptic ribbon in promoting this form of release remains elusive. Using complete ultrastructural reconstructions and capacitance measurements of bullfrog amphibian papilla hair cells dialyzed with high concentrations of a slow Ca²⁺ buffer (10 mM EGTA), we found that the number of synaptic vesicles at the base of the ribbon correlated closely to those vesicles that released most rapidly and efficiently, while the rest of the ribbon-tethered vesicles correlated to a second, slower pool of vesicles. Combined with the persistence of multivesicular release in extreme Ca²⁺ buffering conditions (10 mM BAPTA), our data argue against the Ca²⁺-dependent compound fusion of ribbon-tethered vesicles at hair cell synapses. Moreover, during hair cell depolarization, our results suggest that elevated Ca²⁺ levels enhance vesicle pool replenishment rates. Finally, using Ca²⁺ diffusion simulations, we propose that the ribbon and its vesicles define a small cytoplasmic volume where Ca²⁺ buffer is saturated, despite 10 mM BAPTA conditions. This local buffer saturation permits fast and large Ca²⁺ rises near release sites beneath the synaptic ribbon that can trigger multiquantal EPSCs. We conclude that, by restricting the available presynaptic volume, the ribbon may be creating conditions for the synchronous release of a small cohort of docked vesicles.

Functional assembly of mammalian cochlear hair cells

Hair cells in the mammalian inner ear convert sound into electrical signals that are relayed to the nervous system by the chemical neurotransmitter glutamate. Electrical information encoding sound is then passed through the central nervous system to the higher auditory centres in the brain, where it is used to construct a temporally and spatially accurate representation of the auditory landscape. To achieve this, hair cells must encode fundamental properties of sound stimuli at extremely high rates, not only during mechano-electrical transduction, which occurs in the hair bundles at the cell apex, but also during electrochemical transduction at the specialized ribbon synapses at the cell base. How is the development of such a sophisticated cell regulated? More specifically, to what extent does physiological activity contribute to the progression of the intrinsic genetic programmes that drive cell differentiation? Hair cell differentiation takes about 3 weeks in most rodents, from terminal mitosis during embryonic development to the onset of hearing around 2 weeks after birth. Until recent years, most of the molecules involved in hair cell development and function were unknown, which was mainly due to difficulties in working with the mammalian cochlea and the very small number of hair cells, about 16,000 in humans, present in the auditory organ. Recent advances in the ability to record from the acutely isolated cochlea maintained in near-physiological conditions, combined with the use of genetically modified mouse models, has allowed the identification of several proteins and molecular mechanisms that are crucial for the maturation and function of hair cells. In this article, I highlight recent findings from my laboratory that have furthered our understanding of how developing hair cells acquire the remarkable sensitivity of adult auditory sensory receptors.

Developmental acquisition of a rapid calcium-regulated vesicle supply allows sustained high rates of exocytosis in auditory hair cells

Auditory hair cells (HCs) have the remarkable property to indefinitely sustain high rates of synaptic vesicle release during ongoing sound stimulation. The mechanisms of vesicle supply that allow such indefatigable exocytosis at the ribbon active zone remain largely unknown. To address this issue, we characterized the kinetics of vesicle recruitment and release in developing chick auditory HCs. Experiments were done using the intact chick basilar papilla from E10 (embryonic day 10) to P2 (two days post-hatch) by monitoring changes in membrane capacitance and Ca(2+) currents during various voltage stimulations. Compared to immature pre-hearing HCs (E10-E12), mature post-hearing HCs (E18-P2) can steadily mobilize a larger readily releasable pool (RRP) of vesicles with faster kinetics and higher Ca(2+) efficiency. As assessed by varying the inter-pulse interval of a 100 ms paired-pulse depolarization protocol, the kinetics of RRP replenishment were found much faster in mature HCs. Unlike mature HCs, exocytosis in immature HCs showed large depression during repetitive stimulations. Remarkably, when the intracellular concentration of EGTA was raised from 0.5 to 2 mM, the paired-pulse depression level remained unchanged in immature HCs but was drastically increased in mature HCs, indicating that the Ca(2+) sensitivity of the vesicle replenishment process increases during maturation. Concomitantly, the immunoreactivity of the calcium sensor otoferlin and the number of ribbons at the HC plasma membrane largely increased, reaching a maximum level at E18-P2. Our results suggest that the efficient Ca(2+)-dependent vesicle release and supply in mature HCs essentially rely on the concomitant engagement of synaptic ribbons and otoferlin at the plasma membrane.

Ferlins: regulators of vesicle fusion for auditory neurotransmission, receptor trafficking and membrane repair

Ferlins are a family of multiple C2 domain proteins with emerging roles in vesicle fusion and membrane trafficking. Ferlin mutations are associated with muscular dystrophy (dysferlin) and deafness (otoferlin) in humans, and infertility in Caenorhabditis elegans (Fer-1) and Drosophila (misfire), demonstrating their importance for normal cellular functioning. Ferlins show ancient origins in eukaryotic evolution and are detected in all eukaryotic kingdoms, including unicellular eukaryotes and apicomplexian protists, suggesting origins in a common ancestor predating eukaryotic evolutionary branching. The characteristic feature of the ferlin family is their multiple tandem cytosolic C2 domains (five to seven C2 domains), the most of any protein family, and an extremely rare feature amongst eukaryotic proteins. Ferlins also bear a unique nested DysF domain and small conserved 60-70 residue ferlin-specific sequences (Fer domains). Ferlins segregate into two subtypes based on the presence (type I ferlin) or absence (type II ferlin) of the DysF and FerA domains. Ferlins have diverse tissue-specific and developmental expression patterns, with ferlin animal models united by pathologies arising from defects in vesicle fusion. Consistent with their proposed role in vesicle trafficking, ferlin interaction partners include cytoskeletal motors, other vesicle-associated trafficking proteins and transmembrane receptors or channels. Herein we summarize the research history of the ferlins, an intriguing family of structurally conserved proteins with a preserved ancestral function as regulators of vesicle fusion and receptor trafficking.

Short-term facilitation modulates size and timing of the synaptic response at the inner hair cell ribbon synapse

Inner hair cells (IHCs) in the mammalian cochlea are able to continuously release neurotransmitter in the presence of constant stimuli. Nonetheless, strong synaptic depression is observed over the first few milliseconds of stimulation. This process most likely underlies adaptation in the auditory nerve. In the present study we demonstrate that under certain conditions of stimulation, facilitation can occur at the IHC ribbon synapse. Using simultaneous whole-cell, voltage-clamp recordings from IHCs and afferent fiber endings in excised postnatal rat cochleae, we stimulated IHCs with 2 ms long test depolarizations from a holding potential of -89 mV. Synaptic currents in afferent fibers occurred with high failure rates of ∼ 50%. However, when a pre-depolarization to values of -55 to -49 mV was implemented before the test pulse, success rates of the synaptic response increased to 100%, the strength of the synaptic response increased ∼ 2.8-fold, and synaptic latency was reduced by ∼ 50%. When calcium influx was minimized during pre-depolarization, none of these effects were found, suggesting that calcium influx during pre-depolarizations is required for synaptic conditioning. Similarly, in response to paired-pulse protocols, short term facilitation occurred. The response to the second stimulus increased up to ∼ 5-fold, and its latency was reduced by up to 35% compared to the response to the first stimulus. We propose that at the IHC resting membrane potential, the ribbon synapse operates in a constantly facilitated mode caused by Ca(2+) influx, optimizing the size and timing of the postsynaptic response in auditory nerve fibers.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

Recovery from short-term depression and facilitation is ultrafast and Ca2+ dependent at auditory hair cell synapses

Short-term facilitation and depression coexist at many CNS synapses. Facilitation, however, has not been fully characterized at hair cell synapses. Using paired recordings and membrane capacitance measurements we find that paired-pulse plasticity at an adult frog auditory hair cell synapse depends on pulse duration and interpulse intervals. For short 20 ms depolarizing pulses, and interpulse intervals between 15 and 50 ms, facilitation occurred when hair cells were held at -90 mV. However, hair cells held at -60 mV displayed only paired-pulse depression. Facilitation was dependent on residual free Ca2+ levels because it was greatly reduced by the Ca2+ buffers EGTA and BAPTA. Furthermore, low external Ca2+ augmented facilitation, whereas depression was augmented by high external Ca2+, consistent with depletion of a small pool of fast releasing synaptic vesicles. Recovery from depression had a double-exponential time course with a fast component that may reflect the rapid replenishment of a depleted vesicle pool. We suggest that hair cells held at more depolarized in vivo-like resting membrane potentials have a tonic influx of Ca2+; they are thus in a dynamic state of continuous vesicle release, pool depletion and replenishment. Further Ca2+ influx during paired-pulse stimuli then leads to depression. However, at membrane potentials of -90 mV, ongoing release and pool depletion are minimized, so facilitation is revealed at time intervals when rapid vesicle pool replenishment occurs. Finally, we propose that vesicle pool replenishment kinetics is not rate limited by vesicle endocytosis, which is too slow to influence the rapid pool replenishment process.

Probing the functional equivalence of otoferlin and synaptotagmin 1 in exocytosis

Cochlear inner hair cells (IHCs) use Ca(2+)-dependent exocytosis of glutamate to signal sound information. Otoferlin (Otof), a C(2) domain protein essential for IHC exocytosis and hearing, may serve as a Ca(2+) sensor in vesicle fusion in IHCs that seem to lack the classical neuronal Ca(2+) sensors synaptotagmin 1 (Syt1) and Syt2. Support for the Ca(2+) sensor of fusion hypothesis for otoferlin function comes from biochemical experiments, but additional roles in late exocytosis upstream of fusion have been indicated by physiological studies. Here, we tested the functional equivalence of otoferlin and Syt1 in three neurosecretory model systems: auditory IHCs, adrenal chromaffin cells, and hippocampal neurons. Long-term and short-term ectopic expression of Syt1 in IHCs of Otof (-/-) mice by viral gene transfer in the embryonic inner ear and organotypic culture failed to rescue their Ca(2+) influx-triggered exocytosis. Conversely, virally mediated overexpression of otoferlin did not restore phasic exocytosis in Syt1-deficient chromaffin cells or neurons but enhanced asynchronous release in the latter. We further tested exocytosis in Otof (-/-) hippocampal neurons and in Syt1(-/-) IHCs but found no deficits in vesicle fusion. Expression analysis of different synaptotagmin isoforms indicated that Syt1 and Syt2 are absent from mature IHCs. Our data argue against a simple functional equivalence of the two C(2) domain proteins in exocytosis of IHC ribbon synapses, chromaffin cells, and hippocampal synapses.

Bassoon and the synaptic ribbon organize Ca²+ channels and vesicles to add release sites and promote refilling

At the presynaptic active zone, Ca²+ influx triggers fusion of synaptic vesicles. It is not well understood how Ca²+ channel clustering and synaptic vesicle docking are organized. Here, we studied structure and function of hair cell ribbon synapses following genetic disruption of the presynaptic scaffold protein Bassoon. Mutant synapses--mostly lacking the ribbon--showed a reduction in membrane-proximal vesicles, with ribbonless synapses affected more than ribbon-occupied synapses. Ca²+ channels were also fewer at mutant synapses and appeared in abnormally shaped clusters. Ribbon absence reduced Ca²+ channel numbers at mutant and wild-type synapses. Fast and sustained exocytosis was reduced, notwithstanding normal coupling of the remaining Ca²+ channels to exocytosis. In vitro recordings revealed a slight impairment of vesicle replenishment. Mechanistic modeling of the in vivo data independently supported morphological and functional in vitro findings. We conclude that Bassoon and the ribbon (1) create a large number of release sites by organizing Ca²+ channels and vesicles, and (2) promote vesicle replenishment.

Bassoon speeds vesicle reloading at a central excitatory synapse

Sustained rate-coded signals encode many types of sensory modalities. Some sensory synapses possess specialized ribbon structures, which tether vesicles, to enable high-frequency signaling. However, central synapses lack these structures, yet some can maintain signaling over a wide bandwidth. To analyze the underlying molecular mechanisms, we investigated the function of the active zone core component Bassoon in cerebellar mossy fiber to granule cell synapses. We show that short-term synaptic depression is enhanced in Bassoon knockout mice during sustained high-frequency trains but basal synaptic transmission is unaffected. Fluctuation and quantal analysis as well as quantification with constrained short-term plasticity models revealed that the vesicle reloading rate was halved in the absence of Bassoon. Thus, our data show that the cytomatrix protein Bassoon speeds the reloading of vesicles to release sites at a central excitatory synapse.

Information from cochlear potentials and genetic mutations helps localize the lesion site in auditory neuropathy

Auditory neuropathy (AN) is a disorder characterized by disruption of auditory nerve activity resulting from lesions involving the auditory nerve (postsynaptic AN), inner hair cells and/or the synapses with auditory nerve terminals (presynaptic AN). Affected subjects show impairment of speech perception beyond that expected for the hearing loss, abnormality of auditory brainstem potentials and preserved outer hair-cell activities. Furthermore, AN can be identified either as an isolated disorder or as an associated disorder with multisystem involvement including peripheral and optic neuropathies (non-isolated AN). Mutations in several nuclear and mitochondrial genes have been identified as underlying these forms of AN. Recently, new genes have been identified as involved in both isolated (DIAPH3, OTOF) and non-isolated AN (OPA1). Moreover, abnormal cochlear potentials have been recorded from patients with specific gene mutations by using acoustic stimuli or electrical stimulation through cochlear implant. In this review, different types of genetically based auditory neuropathies are discussed and the proposed molecular mechanisms underlying AN are reviewed.

The diverse roles of ribbon synapses in sensory neurotransmission

Sensory synapses of the visual and auditory systems must faithfully encode a wide dynamic range of graded signals, and must be capable of sustained transmitter release over long periods of time. Functionally and morphologically, these sensory synapses are unique: their active zones are specialized in several ways for sustained, rapid vesicle exocytosis, but their most striking feature is an organelle called the synaptic ribbon, which is a proteinaceous structure that extends into the cytoplasm at the active zone and tethers a large pool of releasable vesicles. But precisely how does the ribbon function to support tonic release at these synapses? Recent genetic and biophysical advances have begun to open the 'black box' of the synaptic ribbon with some surprising findings and promise to resolve its function in vision and hearing.

Control of exocytosis by synaptotagmins and otoferlin in auditory hair cells

In pre-hearing mice, vesicle exocytosis at cochlear inner hair cell (IHC) ribbon synapses is triggered by spontaneous Ca(2+) spikes. At the onset of hearing, IHC exocytosis is then exclusively driven by graded potentials, and is characterized by higher Ca(2+) efficiency and improved synchronization of vesicular release. The molecular players involved in this transition are still unknown. Here we addressed the involvement of synaptotagmins and otoferlin as putative Ca(2+) sensors in IHC exocytosis during postnatal maturation of the cochlea. Using cell capacitance measurements, we showed that Ca(2+)-evoked exocytosis in mouse IHCs switches from an otoferlin-independent to an otoferlin-dependent mechanism at postnatal day 4. During this early exocytotic period, several synaptotagmins (Syts), including Syt1, Syt2 and Syt7, were detected in IHCs. The exocytotic response as well as the release of the readily releasable vesicle pool (RRP) was, however, unchanged in newborn mutant mice lacking Syt1, Syt2 or Syt7 (Syt1(-/-), Syt2(-/-) and Syt7(-/-) mice). We only found a defect in RRP recovery in Syt1(-/-) mice which was apparent as a strongly reduced response to repetitive stimulations. In post-hearing Syt2(-/-) and Syt7(-/-) mutant mice, IHC synaptic exocytosis was unaffected. The transient expression of Syt1 and Syt2, which were no longer detected in IHCs after the onset of hearing, indicates that these two most common Ca(2+)-sensors in CNS synapses are not involved in mature IHCs. We suggest that otoferlin underlies highly efficient Ca(2+)-dependent membrane-membrane fusion, a process likely essential to increase the probability and synchrony of vesicle fusion events at the mature IHC ribbon synapse.