Calcium- and otoferlin-dependent exocytosis by immature outer hair cells

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.

Identification of the hair cell soma-1 antigen, HCS-1, as otoferlin

Hair cells, the mechanosensitive receptor cells of the inner ear, are critical for our senses of hearing and balance. The small number of these receptor cells in the inner ear has impeded the identification and characterization of proteins important for hair cell function. The binding specificity of monoclonal antibodies provides a means for identifying hair cell-specific proteins and isolating them for further study. We have generated a monoclonal antibody, termed hair cell soma-1 (HCS-1), which specifically immunolabels hair cells in at least five vertebrate classes, including sharks and rays, bony fish, amphibians, birds, and mammals. We used HCS-1 to immunoprecipitate the cognate antigen and identified it as otoferlin, a member of the ferlin protein family. Mutations in otoferlin underlie DFNB9, a recessive, nonsyndromic form of prelingual deafness characterized as an auditory neuropathy. Using immunocytochemistry, we find that otoferlin is associated with the entire basolateral membrane of the hair cells and with vesicular structures distributed throughout most of the hair cell cytoplasm. Biochemical assays indicate that otoferlin is tightly associated with membranes, as it is not solubilized by alterations in calcium or salt concentrations. HCS-1 immunolabeling does not co-localize with ribeye, a constituent of synaptic ribbons, suggesting that otoferlin may, in addition to its proposed function in synaptic vesicle release, play additional roles in hair cells.

Calcium balance and mechanotransduction in rat cochlear hair cells

Auditory transduction occurs by opening of Ca(2+)-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca(2+) clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca(2+) that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca(2+) uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca(2+) ATPase (PMCA) pump. Such manipulations raised intracellular Ca(2+) and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca(2+) loads, indicated a maximum Ca(2+) extrusion rate of 3.7 fmol x s(-1). The amplitude of the Ca(2+) transient decreased in proportion to the Ca(2+) concentration bathing the bundle and in artificial endolymph (160 mM K(+), 20 microM Ca(2+)), Ca(2+) carried 0.2% of the MT current. Nevertheless, MT currents in endolymph displayed fast adaptation with a submillisecond time constant. In endolymph, roughly 40% of the MT current was activated at rest when using 1 mM intracellular BAPTA compared with 12% with 1 mM EGTA, which enabled estimation of the in vivo Ca(2+) load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca(2+) diffusion, showing that the measured PMCA pump density could handle Ca(2+) loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.

Dysferlin interacts with tubulin and microtubules in mouse skeletal muscle

Dysferlin is a type II transmembrane protein implicated in surface membrane repair in muscle. Mutations in dysferlin lead to limb girdle muscular dystrophy 2B, Miyoshi Myopathy and distal anterior compartment myopathy. Dysferlin's mode of action is not well understood and only a few protein binding partners have thus far been identified. Using affinity purification followed by liquid chromatography/mass spectrometry, we identified alpha-tubulin as a novel binding partner for dysferlin. The association between dysferlin and alpha-tubulin, as well as between dysferlin and microtubules, was confirmed in vitro by glutathione S-transferase pulldown and microtubule binding assays. These interactions were confirmed in vivo by co-immunoprecipitation. Confocal microscopy revealed that dysferlin and alpha-tubulin co-localized in the perinuclear region and in vesicular structures in myoblasts, and along thin longitudinal structures reminiscent of microtubules in myotubes. We mapped dysferlin's alpha-tubulin-binding region to its C2A and C2B domains. Modulation of calcium levels did not affect dysferlin binding to alpha-tubulin, suggesting that this interaction is calcium-independent. Our studies identified a new binding partner for dysferlin and suggest a role for microtubules in dysferlin trafficking to the sarcolemma.

The passive cable properties of hair cell stereocilia and their contribution to somatic capacitance measurements

Somatic measurements of whole-cell capacitance are routinely used to understand physiologic events occurring in remote portions of cells. These studies often assume the intracellular space is voltage-clamped. We questioned this assumption in auditory and vestibular hair cells with respect to their stereocilia based on earlier studies showing that neurons, with radial dimensions similar to stereocilia, are not always isopotential under voltage-clamp. To explore this, we modeled the stereocilia as passive cables with transduction channels located at their tips. We found that the input capacitance measured at the soma changes when the transduction channels at the tips of the stereocilia are open compared to when the channels are closed. The maximum capacitance is felt with the transducer closed but will decrease as the transducer opens due to a length-dependent voltage drop along the stereocilium length. This potential drop is proportional to the intracellular resistance and stereocilium tip conductance and can produce a maximum capacitance error on the order of fF for single stereocilia and pF for the bundle.

Glutamate co-release at GABA/glycinergic synapses is crucial for the refinement of an inhibitory map

Many non-glutamatergic synaptic terminals in the mammalian brain contain the vesicular glutamate transporter 3 (VGLUT3), indicating that they co-release the excitatory neurotransmitter glutamate. However, the functional role of glutamate co-transmission at these synapses is poorly understood. In the auditory system, VGLUT3 expression and glutamate co-transmission are prominent in a developing GABA/glycinergic sound localization pathway. Here we show that mice with a genetic deletion of VGLUT3 exhibit disrupted glutamate-co-transmission and severe impairment in the refinement of this inhibitory pathway. Specifically, loss of glutamate co-transmission disrupts synaptic silencing and the strengthening of GABA/glycinergic connections that normally occur with maturation. Functional mapping studies further revealed that these deficits markedly degrade the precision of tonotopy in this inhibitory auditory pathway. These results demonstrate the crucial role of glutamate co-transmission in the synaptic reorganization and topographic specification of a developing inhibitory circuit.

Synaptotagmin IV determines the linear Ca2+ dependence of vesicle fusion at auditory ribbon synapses

Mammalian cochlear inner hair cells (IHCs) are specialized for the dynamic coding of continuous and finely graded sound signals. This ability is largely conferred by the linear Ca(2+) dependence of neurotransmitter release at their synapses, which is also a feature of visual and olfactory systems. The prevailing hypothesis is that linearity in IHCs occurs through a developmental change in the Ca(2+) sensitivity of synaptic vesicle fusion from the nonlinear (high order) Ca(2+) dependence of immature spiking cells. However, the nature of the Ca(2+) sensor(s) of vesicle fusion at hair cell synapses is unknown. We found that synaptotagmin IV was essential for establishing the linear exocytotic Ca(2+) dependence in adult rodent IHCs and immature outer hair cells. Moreover, the expression of the hitherto undetected synaptotagmins I and II correlated with a high-order Ca(2+) dependence in IHCs. We propose that the differential expression of synaptotagmins determines the characteristic Ca(2+) sensitivity of vesicle fusion at hair cell synapses.

The postsynaptic function of type II cochlear afferents

The mammalian cochlea is innervated by two classes of sensory neurons. Type I neurons make up 90-95% of the cochlear nerve and contact single inner hair cells (IHCs) to provide acoustic analysis as we know it. In contrast, the far less numerous Type II neurons arborize extensively among outer hair cells (OHCs) 1,2 and supporting cells3,4. Their scarcity, and smaller caliber axons, have made them the subject of much speculation, but little experimental progress for the past 50 years. Here we record from Type II fibers near their terminal arbors under OHCs to show that these receive excitatory glutamatergic synaptic input. The Type II peripheral arbor conducts action potentials, but the small and infrequent glutamatergic excitation implies a requirement for strong acoustic stimulation. Further, we show that Type II neurons are excited by adenosine tri-phosphate (ATP). Exogenous ATP depolarized Type II neurons both directly, and by evoking glutamatergic synaptic input 5. The present results prove that Type II neurons function as cochlear afferents, and can be modulated by ATP. The lesser magnitude of synaptic drive dictates a fundamentally different role in auditory signaling from that of Type I afferents.

Otoferlin is critical for a highly sensitive and linear calcium-dependent exocytosis at vestibular hair cell ribbon synapses

Otoferlin, a C2-domain-containing Ca(2+) binding protein, is required for synaptic exocytosis in auditory hair cells. However, its exact role remains essentially unknown. Intriguingly enough, no balance defect has been observed in otoferlin-deficient (Otof(-/-)) mice. Here, we show that the vestibular nerve compound action potentials evoked during transient linear acceleration ramps in Otof(-/-) mice display higher threshold, lower amplitude, and increased latency compared with wild-type mice. Using patch-clamp capacitance measurement in intact utricles, we show that type I and type II hair cells display a remarkable linear transfer function between Ca(2+) entry, flowing through voltage-activated Ca(2+) channels, and exocytosis. This linear Ca(2+) dependence was observed when changing the Ca(2+) channel open probability or the Ca(2+) flux per channel during various test potentials. In Otof(-/-) hair cells, exocytosis displays slower kinetics, reduced Ca(2+) sensitivity, and nonlinear Ca(2+) dependence, despite morphologically normal synapses and normal Ca(2+) currents. We conclude that otoferlin is essential for a high-affinity Ca(2+) sensor function that allows efficient and linear encoding of low-intensity stimuli at the vestibular hair cell synapse.

Localization and expression of clarin-1, the Clrn1 gene product, in auditory hair cells and photoreceptors

The Usher syndrome 3A (CLRN1) gene encodes clarin-1, which is a member of the tetraspanin family of transmembrane proteins. Although identified more than 6 years ago, little is known about its localization or function in the eye and ear. We developed a polyclonal antibody that react with all clarin-1 isoforms and used it to characterize protein expression in cochlea and retina. In the cochlea, we observe clarin-1expression in the stereocilia of P0 mice, and in synaptic terminals present at the base of the auditory hair cells from E18 to P6. In the retina, clarin-1 localizes to the connecting cilia, inner segment of photoreceptors and to the ribbon synapses. RT-PCR from P0 cochlea and P28 retina show mRNAs encoding only isoforms 2 and 3. Western blots show that only isoform 2 is present in protein extracts from these same tissues. We examined clarin-1 expression in the immortomouse-derived hair cell line UB/OC-1. Only isoform 2 is expressed in UB/OC-1 at both mRNA and protein levels, suggesting this isoform is biologically relevant to hair cell function. The protein co-localizes with microtubules and post-transgolgi vesicles. The subcellular localization of clarin-1 in hair cells and photoreceptors suggests it functions at both the basal and apical poles of neurosensoriepithelia.

EHD4 and CDH23 are interacting partners in cochlear hair cells

Cadherin 23 (CDH23), a transmembrane protein localized near the tips of hair cell stereocilia in the mammalian inner ear, is important for delivering mechanical signals to the mechano-electric transducer channels. To identify CDH23-interacting proteins, a membrane-based yeast two-hybrid screen of an outer hair cell (OHC) cDNA library was performed. EHD4, a member of the C-terminal EH domain containing a protein family involved in endocytic recycling, was identified as a potential interactor. To confirm the interaction, we first demonstrated the EHD4 mRNA expression in hair cells using in situ hybridization. Next, we showed that EHD4 co-localizes and co-immunoprecipitates with CDH23 in mammalian cells. Interestingly, the co-immunoprecipitation was found to be calcium-sensitive. To investigate the role of EHD4 in hearing, compound action potentials were measured in EHD4 knock-out (KO) mice. Although EHD4 KO mice have normal hearing sensitivity, analysis of mouse cochlear lysates revealed a 2-fold increase in EHD1, but no increase in EHD2 or EHD3, in EHD4 KO cochleae compared with wild type, suggesting that a compensatory increase in EHD1 levels may account for the absence of a hearing defect in EHD4 KO mice. Taken together, these data indicate that EHD4 is a novel CDH23-interacting protein that could regulate CDH23 trafficking/localization in a calcium-sensitive manner.

Identities and frequencies of mutations of the otoferlin gene (OTOF) causing DFNB9 deafness in Pakistan

Mutations in OTOF, encoding otoferlin, cause non-syndromic recessive hearing loss. The goal of our study was to define the identities and frequencies of OTOF mutations in a model population. We screened a cohort of 557 large consanguineous Pakistani families segregating recessive, severe-to-profound, prelingual-onset deafness for linkage to DFNB9. There were 13 families segregating deafness consistent with linkage to markers for DFNB9. We analyzed the genomic nucleotide sequence of OTOF and detected probable pathogenic sequence variants among all 13 families. These include the previously reported nonsense mutation p.R708X and 10 novel variants: 3 nonsense mutations (p.R425X, p.W536X, and p.Y1603X), 1 frameshift (c.1103_1104delinsC), 1 single amino acid deletion (p.E766del) and 5 missense substitutions of conserved residues (p.L573R, p.A1090E, p.E1733K, p.R1856Q and p.R1939W). OTOF mutations thus account for deafness in 13 (2.3%) of 557 Pakistani families. This overall prevalence is similar, but the mutation spectrum is different from those for Western populations. In addition, we demonstrate the existence of an alternative splice isoform of OTOF expressed in the human cochlea. This isoform must be required for human hearing because it encodes a unique alternative C-terminus affected by some DFNB9 mutations.

Does the brain connect before the periphery can direct? A comparison of three sensory systems in mice

The development of peripheral to central neural connections within the auditory, visual, and olfactory systems of mice is reviewed to address whether peripheral signaling may play an instructive role during initial synapse formation. For each sensory system, developmental times of histogenesis and the earliest ages of innervation and function are considered for peripheral and selected central relays. For the auditory and visual system, anatomical and functional reports indicate that central connections may form prior to synapse formation in the periphery. However, evidence from the olfactory system suggests that the peripheral olfactory sensory neurons form synaptic connections before more central olfactory connections are established. We find that significant gaps in knowledge exist for embryonic development of these systems in mice and that genetic tools have not yet been systematically directed to address these issues.

Functional maturation of the exocytotic machinery at gerbil hair cell ribbon synapses

Auditory afferent fibre activity in mammals relies on neurotransmission at hair cell ribbon synapses. Developmental changes in the Ca(2+) sensitivity of the synaptic machinery allow inner hair cells (IHCs), the primary auditory receptors, to encode Ca(2+) action potentials (APs) during pre-hearing stages and graded receptor potentials in adult animals. However, little is known about the time course of these changes or whether the kinetic properties of exocytosis differ as a function of IHC position along the immature cochlea. Furthermore, the role of afferent transmission in outer hair cells (OHCs) is not understood. Calcium currents and exocytosis (measured as membrane capacitance changes: DeltaC(m)) were measured with whole-cell recordings from immature gerbil hair cells using near-physiological conditions. The kinetics, vesicle pool depletion and Ca(2+) coupling of exocytosis were similar in apical and basal immature IHCs. This could indicate that possible differences in AP activity along the immature cochlea do not require synaptic specialization. Neurotransmission in IHCs became mature from postnatal day 20 (P20), although changes in its Ca(2+) dependence occurred at P9-P12 in basal and P12-P15 in apical cells. OHCs showed a smaller DeltaC(m) than IHCs that was reflected by fewer active zones in OHCs. Otoferlin, the proposed Ca(2+) sensor in cochlear hair cells, was similarly distributed in both cell types despite the high-order exocytotic Ca(2+) dependence in IHCs and the near-linear relation in OHCs. The results presented here provide a comprehensive study of the function and development of hair cell ribbon synapses.

Potassium currents induced by hydrostatic pressure modulate membrane potential and transmitter release in vestibular type II hair cells

Vestibular type II hair cells respond to increases in the hydrostatic pressure with pressure-dependent K(+) currents. We examined whether such currents may modulate transmitter release (assessed as membrane capacitance increments) by altering membrane potentials and voltage-gated Ca(2+) currents. Capacitance increments were dependent on voltage-gated Ca(2+) influx. Stimulating currents (0.7 nA) in current clamp induced depolarisations that were more negative by 8.7 +/- 2.1 mV when the bath height was elevated from 0.2 to 0.5 cm. In voltage clamp, protocols were used that simulated the time course of the membrane potential in current clamp at either low (control) or high hydrostatic pressure (high bath). The low bath protocol induced significantly larger Ca(2+) currents and increases in capacitance than the high bath protocol. We conclude that pressure-dependent K(+) currents may alter the voltage response of vestibular hair cells to an extent critical for Ca(2+) currents and transmitter release. This mechanism may contribute to vestibular dysfunction in Meniere's disease.

Hair cell afferent synapses

This review will cover advances in the study of hair cell afferent synaptic function occurring between 2005 and 2008. During this time, capacitance measurements of vesicular fusion have continued to be refined, optical methods have added insights regarding vesicle trafficking, and paired intracellular recordings have established the transfer function of the afferent synapse at high resolution. Further, genes have been identified with forms of deafness known as auditory neuropathy, and their role in afferent signaling explored in mouse models. With these advances, our view of the hair cell afferent synapse has continued to be refined, and surprising properties have been revealed that emphasize the unique role of this structure in neural function.

Tonotopic variation in the calcium dependence of neurotransmitter release and vesicle pool replenishment at mammalian auditory ribbon synapses

The mammalian cochlea is specialized to recognize and process complex auditory signals with remarkable acuity and temporal precision over a wide frequency range. The quality of the information relayed to the auditory afferent fibers mainly depends on the transfer characteristics of inner hair cell (IHC) ribbon synapses. To investigate the biophysical properties of the synaptic machinery, we measured changes in membrane capacitance (DeltaC(m)) in low-frequency (apical region, approximately 300 Hz) and high-frequency (basal, approximately 30 kHz) gerbil IHCs maintained in near physiological conditions (1.3 mm extracellular Ca(2+) and body temperature). With maturation, the Ca(2+) efficiency of exocytosis improved in both apical and basal IHCs and was more pronounced in the latter. Prehearing IHCs showed a similar Ca(2+) cooperativity of exocytosis despite the smaller DeltaC(m) in apical cells. After maturation, DeltaC(m) in high-frequency IHCs increased linearly with the Ca(2+) current, whereas, somewhat surprisingly, the relationship was significantly more nonlinear in low-frequency cells. This tonotopic difference seemed to be correlated with ribbon synapse morphology (spherical in apical and ellipsoid in basal IHCs) but not with the expression level of the proposed Ca(2+) sensor otoferlin or the spatial coupling between Ca(2+) channels and active zones. Repetitive stimulation of adult IHCs showed that vesicle pool refilling could become rate limiting for vesicle release, with high-frequency IHCs able to sustain greater release rates. Together, our findings provide the first evidence for a tonotopic difference in the properties of the synaptic machinery in mammalian IHCs, which could be essential for fine-tuning their receptor characteristics during sound stimulation.

Reciprocal synapses between outer hair cells and their afferent terminals: evidence for a local neural network in the mammalian cochlea

Cochlear outer hair cells (OHCs) serve both as sensory receptors and biological motors. Their sensory function is poorly understood because their afferent innervation, the type-II spiral ganglion cell, has small unmyelinated axons and constitutes only 5% of the cochlear nerve. Reciprocal synapses between OHCs and their type-II terminals, consisting of paired afferent and efferent specialization, have been described in the primate cochlea. Here, we use serial and semi-serial-section transmission electron microscopy to quantify the nature and number of synaptic interactions in the OHC area of adult cats. Reciprocal synapses were found in all OHC rows and all cochlear frequency regions. They were more common among third-row OHCs and in the apical half of the cochlea, where 86% of synapses were reciprocal. The relative frequency of reciprocal synapses was unchanged following surgical transection of the olivocochlear bundle in one cat, confirming that reciprocal synapses were not formed by efferent fibers. In the normal ear, axo-dendritic synapses between olivocochlear terminals and type-II terminals and/or dendrites were as common as synapses between olivocochlear terminals and OHCs, especially in the first row, where, on average, almost 30 such synapses were seen in the region under a single OHC. The results suggest that a complex local neuronal circuitry in the OHC area, formed by the dendrites of type-II neurons and modulated by the olivocochlear system, may be a fundamental property of the mammalian cochlea, rather than a curiosity of the primate ear. This network may mediate local feedback control of, and bidirectional communication among, OHCs throughout the cochlear spiral.