Modes and regulation of endocytic membrane retrieval in mouse auditory hair cells

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.

A cell type-specific approach to elucidate the role of miR-96 in inner ear hair cells

Introduction

Mutations in microRNA-96 (miR-96), a microRNA expressed within the hair cells (HCs) of the inner ear, result in progressive hearing loss in both mouse models and humans. In this study, we present the first HC-specific RNA-sequencing (RNA-seq) dataset from newborn Mir96Dmdo heterozygous, homozygous mutant, and wildtype mice.

Methods

Bulk RNA-seq was performed on HCs of newborn Mir96Dmdo heterozygous, homozygous mutant, and wildtype mice. Differentially expressed gene analysis was conducted on Mir96Dmdo homozygous mutant HCs compared to wildtype littermate controls, followed by GO term and protein-protein interaction analysis on these differentially expressed genes.

Results

We identify 215 upregulated and 428 downregulated genes in the HCs of the Mir96Dmdo homozygous mutant mice compared to their wildtype littermate controls. Many of the significantly downregulated genes in Mir96Dmdo homozygous mutant HCs have established roles in HC development and/or known roles in deafness including Myo15a, Myo7a, Ush1c, Gfi1, and Ptprq and have enrichment in gene ontology (GO) terms with biological functions such as sensory perception of sound. Interestingly, upregulated genes in Mir96Dmdo homozygous mutants, including possible miR-96 direct targets, show higher wildtype expression in supporting cells compared to HCs.

Conclusion

Our data further support a role for miR-96 in HC development, possibly as a repressor of supporting cell transcriptional programs in HCs. The HC-specific Mir96Dmdo RNA-seq data set generated from this manuscript are now publicly available in a dedicated profile in the gene expression analysis resource (gEAR-https://umgear.org/p?l=miR96).

Spatiotemporal expression of AP-2/myosin Ⅵ in mouse cochlear IHCs and correlation with auditory function

BACKGROUND

Recycling of synaptic vesicles plays an important role in vesicle pool replenishment, neurotransmitter release and synaptic plasticity. Clathrin-mediated endocytosis (CME) is considered to be the main mechanism for synaptic vesicle replenishment. AP-2 (adaptor-related protein complex 2) and myosin Ⅵ are known as key proteins that regulate the structure and dynamics of CME.

OBJECTIVE

This study aims to reveal the spatiotemporal expression of AP-2/myosin Ⅵ in inner hair cells (IHCs) of the mouse cochlea and its correlation with auditory function.

MATERIAL AND METHODS

Immunofluorescence was used to detect the localization and expression of AP-2 and myosin Ⅵ in cochlear hair cells (HCs) of CBA/CaJ mice of various ages. qRT-PCR was used to verify the differential expression of AP-2 and myosin Ⅵ mRNA in the mouse cochlea, and ABR tests were administered to mice of various ages. A preliminary analysis of the correlation between AP-2/myosin Ⅵ levels and auditory function was conducted.

RESULTS

AP-2 was located in the cytoplasmic region of IHCs and was mainly expressed in the basal region of IHCs and the area near ribbon synapses, while myosin Ⅵ was expressed in the cytoplasmic region of IHCs and OHCs. Furthermore, AP-2 and myosin Ⅵ were not significant detected in the cochleae of P7 mice; the expression level reached a peak at P35 and then decreased significantly with age. The expression patterns and expression levels of AP-2 and myosin Ⅵ in the cochleae of the mice were consistent with the development of the auditory system.

CONCLUSIONS AND SIGNIFICANCE

AP-2 and myosin Ⅵ protein expression may differ in mice of different ages, and this variation probably leads to a difference in the efficiency in CME; it may also cause a defect in IHC function.

Proteomic Analysis Reveals the Composition of Glutamatergic Organelles of Auditory Inner Hair Cells

In the ear, inner hair cells (IHCs) employ sophisticated glutamatergic ribbon synapses with afferent neurons to transmit auditory information to the brain. The presynaptic machinery responsible for neurotransmitter release in IHC synapses includes proteins such as the multi-C2-domain protein otoferlin and the vesicular glutamate transporter 3 (VGluT3). Yet, much of this likely unique molecular machinery remains to be deciphered. The scarcity of material has so far hampered biochemical studies which require large amounts of purified samples. We developed a subcellular fractionation workflow combined with immunoisolation of VGluT3-containing membrane vesicles, allowing for the enrichment of glutamatergic organelles that are likely dominated by synaptic vesicles (SVs) of IHCs. We have characterized their protein composition in mice before and after hearing onset using mass spectrometry and confocal imaging and provide a fully annotated proteome with hitherto unidentified proteins. Despite the prevalence of IHC marker proteins across IHC maturation, the profiles of trafficking proteins differed markedly before and after hearing onset. Among the proteins enriched after hearing onset were VAMP-7, syntaxin-7, syntaxin-8, syntaxin-12/13, SCAMP1, V-ATPase, SV2, and PKCα. Our study provides an inventory of the machinery associated with synaptic vesicle-mediated trafficking and presynaptic activity at IHC ribbon synapses and serves as a foundation for future functional studies.

Gentamicin administration leads to synaptic dysfunction in inner hair cells

Ototoxicity is a major side effect of aminoglycosides, which can cause irreversible hearing loss. Previous studies on aminoglycoside-induced ototoxicity have primarily focused on the loss of sensory hair cells. Recent investigations have revealed that aminoglycosides can also lead to the loss of ribbon synapses in inner hair cells (IHCs). However, the functional implications of ribbon synapse loss and the underlying mechanisms remain unclear. In this study, we intraperitoneally injected C57BL/6 J mice with 300 mg/kg gentamicin once daily for 3, 10, and 20 days. Then, we performed immunofluorescence staining, patch-clamp recording, proteomics analysis and western blotting to characterize the changes in ribbon synapses in IHCs and the associated mechanisms. After gentamicin treatment, the auditory brainstem response (ABR) threshold was elevated, and the ABR wave I amplitude was decreased. We also observed loss of ribbon synapses in IHCs. Interestingly, ribbon synapse loss occurred on both the modiolar and pillar sides of IHCs. Whole-cell patch-clamp recordings in IHCs revealed a reduction in the calcium current amplitude, along with a shifted half-activation voltage and altered calcium voltage dependency. Moreover, exocytosis of IHCs was reduced, consistent with the reduction in the ABR wave I amplitude. Through proteomic analysis, western blotting, and immunofluorescence staining, we found that gentamicin treatment resulted in downregulation of myosin VI, a protein crucial for synaptic vesicle recycling and replenishment in IHCs. Furthermore, we evaluated the kinetics of endocytosis and found a significant reduction in IHC exocytosis, possibly reflecting the impact of myosin VI downregulation on synaptic vesicle recycling. In summary, our findings demonstrate that gentamicin treatment leads to synaptic dysfunction in IHCs, highlighting the important role of myosin VI downregulation in gentamicin-induced synaptic damage.

Cochlear transcript diversity and its role in auditory functions implied by an otoferlin short isoform

Isoforms of a gene may contribute to diverse biological functions. In the cochlea, the repertoire of alternative isoforms remains unexplored. We integrated single-cell short-read and long-read RNA sequencing techniques and identified 236,012 transcripts, 126,612 of which were unannotated in the GENCODE database. Then we analyzed and verified the unannotated transcripts using RNA-seq, RT-PCR, Sanger sequencing, and MS-based proteomics approaches. To illustrate the importance of identifying spliced isoforms, we investigated otoferlin, a key protein involved in synaptic transmission in inner hair cells (IHCs). Upon deletion of the canonical otoferlin isoform, the identified short isoform is able to support normal hearing thresholds but with reduced sustained exocytosis of IHCs, and further revealed otoferlin functions in endocytic membrane retrieval that was not well-addressed previously. Furthermore, we found that otoferlin isoforms are associated with IHC functions and auditory phenotypes. This work expands our mechanistic understanding of auditory functions at the level of isoform resolution.

Nanoscale and functional heterogeneity of the hippocampal extracellular space

The extracellular space (ECS) and its constituents play a crucial role in brain development, plasticity, circadian rhythm, and behavior, as well as brain diseases. Yet, since this compartment has an intricate geometry and nanoscale dimensions, its detailed exploration in live tissue has remained an unmet challenge. Here, we used a combination of single-nanoparticle tracking and super-resolution microscopy approaches to map the nanoscale dimensions of the ECS across the rodent hippocampus. We report that these dimensions are heterogeneous between hippocampal areas. Notably, stratum radiatum CA1 and CA3 ECS differ in several characteristics, a difference that gets abolished after digestion of the extracellular matrix. The dynamics of extracellular immunoglobulins vary within these areas, consistent with their distinct ECS characteristics. Altogether, we demonstrate that ECS nanoscale anatomy and diffusion properties are widely heterogeneous across hippocampal areas, impacting the dynamics and distribution of extracellular molecules.

Effects of the clathrin inhibitor Pitstop-2 on synaptic vesicle recycling at a central synapse in vivo

Four modes of endocytosis and subsequent synaptic vesicle (SV) recycling have been described at the presynapse to ensure the availability of SVs for synaptic release. However, it is unclear to what extend these modes operate under physiological activity patterns in vivo. The coat protein clathrin can regenerate SVs either directly from the plasma membrane (PM) via clathrin-mediated endocytosis (CME), or indirectly from synaptic endosomes by SV budding. Here, we examined the role of clathrin in SV recycling under physiological conditions by applying the clathrin inhibitor Pitstop-2 to the calyx of Held, a synapse optimized for high frequency synaptic transmission in the auditory brainstem, in vivo. The effects of clathrin-inhibition on SV recycling were investigated by serial sectioning scanning electron microscopy (S3EM) and 3D reconstructions of endocytic structures labeled by the endocytosis marker horseradish peroxidase (HRP). We observed large endosomal compartments as well as HRP-filled, black SVs (bSVs) that have been recently recycled. The application of Pitstop-2 led to reduced bSV but not large endosome density, increased volumes of large endosomes and shifts in the localization of both types of endocytic compartments within the synapse. These changes after perturbation of clathrin function suggest that clathrin plays a role in SV recycling from both, the PM and large endosomes, under physiological activity patterns, in vivo.

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.

Characterization of cephalic and non-cephalic sensory cell types provides insight into joint photo- and mechanoreceptor evolution

Rhabdomeric opsins (r-opsins) are light sensors in cephalic eye photoreceptors, but also function in additional sensory organs. This has prompted questions on the evolutionary relationship of these cell types, and if ancient r-opsins were non-photosensory. A molecular profiling approach in the marine bristleworm Platynereis dumerilii revealed shared and distinct features of cephalic and non-cephalic r-opsin1-expressing cells. Non-cephalic cells possess a full set of phototransduction components, but also a mechanosensory signature. Prompted by the latter, we investigated Platynereis putative mechanotransducer and found that nompc and pkd2.1 co-expressed with r-opsin1 in TRE cells by HCR RNA-FISH. To further assess the role of r-Opsin1 in these cells, we studied its signaling properties and unraveled that r-Opsin1 is a Gαq-coupled blue light receptor. Profiling of cells from r-opsin1 mutants versus wild-types, and a comparison under different light conditions reveals that in the non-cephalic cells light - mediated by r-Opsin1 - adjusts the expression level of a calcium transporter relevant for auditory mechanosensation in vertebrates. We establish a deep-learning-based quantitative behavioral analysis for animal trunk movements and identify a light- and r-Opsin-1-dependent fine-tuning of the worm's undulatory movements in headless trunks, which are known to require mechanosensory feedback. Our results provide new data on peripheral cell types of likely light sensory/mechanosensory nature. These results point towards a concept in which such a multisensory cell type evolved to allow for fine-tuning of mechanosensation by light. This implies that light-independent mechanosensory roles of r-opsins may have evolved secondarily.

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.

Quantitative Fluorescent in situ Hybridization Reveals Differential Transcription Profile Sharpening of Endocytic Proteins in Cochlear Hair Cells Upon Maturation

The organ of Corti (OC) comprises two types of sensory cells: outer hair cells (OHCs) and inner hair cells (IHCs). While both are mechanotransducers, OHCs serve as cochlear amplifiers, whereas IHCs transform sound into transmitter release. Reliable sound encoding is ensured by indefatigable exocytosis of synaptic vesicles associated with efficient replenishment of the vesicle pool. Vesicle reformation requires retrieval of vesicle membrane from the hair cell’s membrane via endocytosis. So far, the protein machinery for endocytosis in pre-mature and terminally differentiated hair cells has only partially been deciphered. Here, we studied three endocytic proteins, dynamin-1, dynamin-3, and endophilin-A1, by assessing their transcription profiles in pre-mature and mature mouse OCs. State-of-the-art RNAscope^® fluorescent in situ hybridization (FISH) of whole-mount OCs was used for quantification of target mRNAs on single-cell level. We found that pre-mature IHCs contained more mRNA transcripts of dnm1 (encoding dynamin-1) and sh3gl2 (endophilin-A1), but less of dnm3 (dynamin-3) than OHCs. These differential transcription profiles between OHCs and IHCs were sharpened upon maturation. It is noteworthy that low but heterogeneous signal numbers were found between individual negative controls, which highlights the importance of corresponding analyses in RNAscope^® assays. Complementary immunolabeling revealed strong expression of dynamin-1 in the soma of mature IHCs, which was much weaker in pre-mature IHCs. By contrast, dynamin-3 was predominantly found in the soma and at the border of the cuticular plates of pre-mature and mature OHCs. In summary, using quantitative RNAscope^® FISH and immunohistochemistry on whole-mount tissue of both pre-mature and mature OCs, we disclosed the cellular upregulation of endocytic proteins at the level of transcription/translation during terminal differentiation of the OC. Dynamin-1 and endophilin-A1 likely contribute to the strengthening of the endocytic machinery in IHCs after the onset of hearing, whereas expression of dynamin-3 at the cuticular plate of pre-mature and mature OHCs suggests its possible involvement in activity-independent apical endocytosis.

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.

Synaptojanin2 Mutation Causes Progressive High-frequency Hearing Loss in Mice

Progressive hearing loss is very common in the human population but we know little about the underlying molecular mechanisms. Synaptojanin2 (Synj2) has been reported to be involved, as a mouse mutation led to a progressive increase in auditory thresholds with age. Synaptojanin2 is a phosphatidylinositol (PI) phosphatase that removes the five-position phosphates from phosphoinositides, such as PIP₂ and PIP₃, and is a key enzyme in clathrin-mediated endocytosis. To investigate the mechanisms underlying progressive hearing loss, we have studied a different mutation of mouse Synj2 to look for any evidence of involvement of vesicle trafficking particularly affecting the synapses of sensory hair cells. Auditory brainstem responses (ABR) developed normally at first but started to decline between 3 and 4 weeks of age in Synj2^tm1b mutants. At 6 weeks old, some evidence of outer hair cell (OHC) stereocilia fusion and degeneration was observed, but this was only seen in the extreme basal turn so cannot explain the raised ABR thresholds that correspond to more apical regions of the cochlear duct. We found no evidence of any defect in inner hair cell (IHC) exocytosis or endocytosis using single hair cell recordings, nor any sign of hair cell synaptic abnormalities. Endocochlear potentials (EP) were normal. The mechanism underlying progressive hearing loss in these mutants remains elusive, but our findings of raised distortion product otoacoustic emission (DPOAE) thresholds and signs of OHC degeneration both suggest an OHC origin for the hearing loss. Synaptojanin2 is not required for normal development of hearing but it is important for its maintenance.

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.

Different patterns of endocytosis in cochlear inner and outer hair cells of mice

Precise and efficient endocytosis is critical for sustained neurotransmission during continuous neuronal activity. Endocytosis is a prerequisite for maintaining the auditory function. However, the differences between the patterns of endocytosis in cochlear inner hair cells (IHCs) and outer hair cells (OHCs) remain unclear. Both IHCs and OHCs were obtained from adult C57 mice. Patterns of endocytosis in cells were estimated by analyzing the uptake of FM1-43, a fluorescent. The observations were made using live confocal imaging, fluorescence intensities were calculated statistically. Results revealed the details about following phenomenon, i) sites of entry: the FM1-43 dye was found to enter IHC at the apical area initially, the additional sites of entry were then found at basolateral membrane of the cells, The entry of the dye into OHCs initially appeared to be occurring around whole apical membranes area, which then diffused towards the other membrane surface of the cells, ii) capacity of endocytosis: fluorescence intensity in IHCs showed significantly higher than that of OHCs (P<0.01). We have found different patterns of endocytosis between IHCs and OHCs, this indicated functional distinctions between them. Moreover, FM1-43 dye can be potentially used as an indicator of the functional loss or repair of cochlear hair cells.

Viral Transfer of Mini-Otoferlins Partially Restores the Fast Component of Exocytosis and Uncovers Ultrafast Endocytosis in Auditory Hair Cells of Otoferlin Knock-Out Mice

Transmitter release at auditory inner hair cell (IHC) ribbon synapses involves exocytosis of glutamatergic vesicles during voltage activation of L-type Ca_v1.3 calcium channels. At these synapses, the fast and indefatigable release of synaptic vesicles by IHCs is controlled by otoferlin, a six-C2-domain (C2-ABCDEF) protein that functions as a high-affinity Ca²⁺ sensor. The molecular events by which each otoferlin C2 domain contributes to the regulation of the synaptic vesicle cycle in IHCs are still incompletely understood. Here, we investigate their role using a cochlear viral cDNA transfer approach in vivo, where IHCs of mouse lacking otoferlin (Otof ^-/- mice of both sexes) were virally transduced with cDNAs of various mini-otoferlins. Using patch-clamp recordings and membrane capacitance measurements, we show that the viral transfer of mini-otoferlin containing C2-ACEF, C2-EF, or C2-DEF partially restores the fast exocytotic component in Otof ^-/- mouse IHCs. The restoration was much less efficient with C2-ACDF, underlining the importance of the C2-EF domain. None of the mini-otoferlins tested restored the sustained component of vesicle release, explaining the absence of hearing recovery. The restoration of the fast exocytotic component in the transduced Otof ^-/- IHCs was also associated with a recovery of Ca²⁺ currents with normal amplitude and fast time inactivation, confirming that the C-terminal C2 domains of otoferlin are essential for normal gating of Ca_v1.3 channels. Finally, the reintroduction of the mini-otoferlins C2-EF, C2-DEF, or C2-ACEF allowed us to uncover and characterize for the first time a dynamin-dependent ultrafast endocytosis in IHCs.SIGNIFICANCE STATEMENT Otoferlin, a large six-C2-domain protein, is essential for synaptic vesicle exocytosis at auditory hair cell ribbon synapses. Here, we show that the viral expression of truncated forms of otoferlin (C2-EF, C2-DEF, and C2-ACEF) can partially rescue the fast and transient release component of exocytosis in mouse hair cells lacking otoferlin, yet cannot sustain exocytosis after long repeated stimulation. Remarkably, these hair cells also display a dynamin-dependent ultrafast endocytosis. Overall, our study uncovers the pleiotropic role of otoferlin in the hair cell synaptic vesicle cycle, notably in triggering both ultrafast exocytosis and endocytosis and recruiting synaptic vesicles to the active zone.

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.

Localized disorganization of the cochlear inner hair cell synaptic region after noise exposure

The prevalence and importance of hearing damage caused by noise levels not previously thought to cause permanent hearing impairment has become apparent in recent years. The damage to, and loss of, afferent terminals of auditory nerve fibres at the cochlear inner hair cell has been well established, but the effects of noise exposure and terminal loss on the inner hair cell are less known. Using three-dimensional structural studies in mice we have examined the consequences of afferent terminal damage on inner hair cell morphology and intracellular structure. We identified a structural phenotype in the pre-synaptic regions of these damaged hair cells that persists for four weeks after noise exposure, and demonstrates a specific dysregulation of the synaptic vesicle recycling pathway. We show evidence of a failure in regeneration of vesicles from small membrane cisterns in damaged terminals, resulting from a failure of separation of small vesicle buds from the larger cisternal membranes.

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.

AP180 promotes release site clearance and clathrin-dependent vesicle reformation in mouse cochlear inner hair cells

High-throughput neurotransmission at ribbon synapses of cochlear inner hair cells (IHCs) requires tight coupling of neurotransmitter release and balanced recycling of synaptic vesicles (SVs) as well as rapid restoration of release sites. Here, we examined the role of the adaptor protein AP180 for IHC synaptic transmission in AP180-KO mice using high-pressure freezing and electron tomography, confocal microscopy, patch-clamp membrane-capacitance measurements and systems physiology. AP180 was found predominantly at the synaptic pole of IHCs. AP180-deficient IHCs had severely reduced SV numbers, slowed endocytic membrane retrieval, and accumulated endocytic intermediates near ribbon synapses, indicating that AP180 is required for clathrin-dependent endocytosis and SV reformation in IHCs. Moreover, AP180 deletion led to a high prevalence of SVs in a multi-tethered or docked state after stimulation, a reduced rate of SV replenishment, and a hearing impairment. We conclude that, in addition to its role in clathrin recruitment, AP180 contributes to release site clearance in IHCs.

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.

Vesicle sub-pool organization at inner hair cell ribbon synapses

The afferent inner hair cell synapse harbors the synaptic ribbon, which ensures a constant vesicle supply. Synaptic vesicles (SVs) are arranged in morphologically discernable pools, linked via filaments to the ribbon or the presynaptic membrane. We propose that filaments play a major role in SV resupply and exocytosis at the ribbon. Using advanced electron microscopy, we demonstrate that SVs are organized in sub-pools defined by the filament number per vesicle and its connections. Upon stimulation, SVs increasingly linked to other vesicles and to the ribbon, whereas single-tethered SVs dominated at the membrane. Mutant mice for the hair cell protein otoferlin (pachanga, Otof ^Pga/Pga ) are profoundly deaf with reduced sustained release, serving as a model to investigate the SV replenishment at IHCs. Upon stimulation, multiple-tethered and docked vesicles (rarely observed in wild-type) accumulated at Otof ^Pga/Pga active zones due to an impairment downstream of docking. Conclusively, vesicles are organized in sub-pools at ribbon-type active zones by filaments to support vesicle supply, transport, and finally release.

Otoferlin acts as a Ca2+ sensor for vesicle fusion and vesicle pool replenishment at auditory hair cell ribbon synapses

Hearing relies on rapid, temporally precise, and sustained neurotransmitter release at the ribbon synapses of sensory cells, the inner hair cells (IHCs). This process requires otoferlin, a six C2-domain, Ca2+-binding transmembrane protein of synaptic vesicles. To decipher the role of otoferlin in the synaptic vesicle cycle, we produced knock-in mice (OtofAla515,Ala517/Ala515,Ala517) with lower Ca2+-binding affinity of the C2C domain. The IHC ribbon synapse structure, synaptic Ca2+ currents, and otoferlin distribution were unaffected in these mutant mice, but auditory brainstem response wave-I amplitude was reduced. Lower Ca2+ sensitivity and delay of the fast and sustained components of synaptic exocytosis were revealed by membrane capacitance measurement upon modulations of intracellular Ca2+ concentration, by varying Ca2+ influx through voltage-gated Ca2+-channels or Ca2+ uncaging. Otoferlin thus functions as a Ca2+ sensor, setting the rates of primed vesicle fusion with the presynaptic plasma membrane and synaptic vesicle pool replenishment in the IHC active zone.

Activity-Dependent Phosphorylation by CaMKIIδ Alters the Ca2+ Affinity of the Multi-C2-Domain Protein Otoferlin

Otoferlin is essential for fast Ca²⁺-triggered transmitter release from auditory inner hair cells (IHCs), playing key roles in synaptic vesicle release, replenishment and retrieval. Dysfunction of otoferlin results in profound prelingual deafness. Despite its crucial role in cochlear synaptic processes, mechanisms regulating otoferlin activity have not been studied to date. Here, we identified Ca²⁺/calmodulin-dependent serine/threonine kinase II delta (CaMKIIδ) as an otoferlin binding partner by pull-downs from chicken utricles and reassured interaction by a co-immunoprecipitation with heterologously expressed proteins in HEK cells. We confirmed the expression of CaMKIIδ in rodent IHCs by immunohistochemistry and real-time PCR. A proximity ligation assay indicates close proximity of the two proteins in rat IHCs, suggesting that otoferlin and CaMKIIδ also interact in mammalian IHCs. In vitro phosphorylation of otoferlin by CaMKIIδ revealed ten phosphorylation sites, five of which are located within C₂-domains. Exchange of serines/threonines at phosphorylated sites into phosphomimetic aspartates reduces the Ca²⁺ affinity of the recombinant C₂F domain 10-fold, and increases the Ca²⁺ affinity of the C₂C domain. Concordantly, we show that phosphorylation of otoferlin and/or its interaction partners are enhanced upon hair cell depolarization and blocked by pharmacological CaMKII inhibition. We therefore propose that otoferlin activity is regulated by CaMKIIδ in IHCs.

Promotion of endocytosis efficiency through an ATP-independent mechanism at rat calyx of Held terminals

KEY POINTS

At rat calyx of Held terminals, ATP was required not only for slow endocytosis, but also for rapid phase of compensatory endocytosis. An ATP-independent form of endocytosis was recruited to accelerate membrane retrieval at increased activity and temperature. ATP-independent endocytosis primarily involved retrieval of pre-existing membrane, which depended on Ca²⁺ and the activity of neutral sphingomyelinase but not clathrin-coated pit maturation. ATP-independent endocytosis represents a non-canonical mechanism that can efficiently retrieve membrane at physiological conditions without competing for the limited ATP at elevated neuronal activity.

ABSTRACT

Neurotransmission relies on membrane endocytosis to maintain vesicle supply and membrane stability. Endocytosis has been generally recognized as a major ATP-dependent function, which efficiently retrieves more membrane at elevated neuronal activity when ATP consumption within nerve terminals increases drastically. This paradox raises the interesting question of whether increased activity recruits ATP-independent mechanism(s) to accelerate endocytosis at the same time as preserving ATP availability for other tasks. To address this issue, we studied ATP requirement in three typical forms of endocytosis at rat calyx of Held terminals by whole-cell membrane capacitance measurements. At room temperature, blocking ATP hydrolysis effectively abolished slow endocytosis and rapid endocytosis but only partially inhibited excess endocytosis following intense stimulation. The ATP-independent endocytosis occurred at calyces from postnatal days 8-15, suggesting its existence before and after hearing onset. This endocytosis was not affected by a reduction of exocytosis using the light chain of botulinum toxin C, nor by block of clathrin-coat maturation. It was abolished by EGTA, which preferentially blocked endocytosis of retrievable membrane pre-existing at the surface, and was impaired by oxidation of cholesterol and inhibition of neutral sphingomyelinase. ATP-independent endocytosis became more significant at 34-35°C, and recovered membrane by an amount that, on average, was close to exocytosis. The results of the present study suggest that activity and temperature recruit ATP-independent endocytosis of pre-existing membrane (in addition to ATP-dependent endocytosis) to efficiently retrieve membrane at nerve terminals. This less understood endocytosis represents a non-canonical mechanism regulated by lipids such as cholesterol and sphingomyelinase.

5-Aryl-2-(naphtha-1-yl)sulfonamido-thiazol-4(5H)-ones as clathrin inhibitors

The development of a (Z)-5-((6,8-dichloro-4-oxo-4H-chromen-3-yl)methylene)-2-thioxothiazolidin-4-one (2), rhodanine-based lead that led to the Pitstop® 2 family of clathrin inhibitors is described herein. Head group substitution and bioisosteric replacement of the rhodanine core with a 2-aminothiazol-4(5H)-one scaffold eliminated off target dynamin activity. A series of N-substituents gave first phenylglycine (20, IC₅₀ ∼ 20 μM) then phenyl (25, IC₅₀ ∼ 7.1 μM) and 1-napthyl sulfonamide (26, Pitstop® 2 compound, IC₅₀ ∼ 1.9 μM) analogues with good activity, validating this approach. A final library exploring the head group resulted in three analogues displaying either slight improvements or comparable activity (33, 38, and 29 with IC₅₀ ∼ 1.4, 1.6 and 1.8 μM respectively) and nine others with IC₅₀ < 10 μM. These results were rationalized using in silico docking studies. Docking studies predicted enhanced Pitstop® 2 family binding, not a loss of binding, within the Pistop® groove of the reported clathrin mutant invalidating recent assumptions of poor selectivity for this family of clathrin inhibitors.

New insights into cochlear sound encoding

The inner ear uses specialized synapses to indefatigably transmit sound information from hair cells to spiral ganglion neurons at high rates with submillisecond precision. The emerging view is that hair cell synapses achieve their demanding function by employing an unconventional presynaptic molecular composition. Hair cell active zones hold the synaptic ribbon, an electron-dense projection made primarily of RIBEYE, which tethers a halo of synaptic vesicles and is thought to enable a large readily releasable pool of vesicles and to contribute to its rapid replenishment. Another important presynaptic player is otoferlin, coded by a deafness gene, which assumes a multi-faceted role in vesicular exocytosis and, when disrupted, causes auditory synaptopathy. A functional peculiarity of hair cell synapses is the massive heterogeneity in the sizes and shapes of excitatory postsynaptic currents. Currently, there is controversy as to whether this reflects multiquantal release with a variable extent of synchronization or uniquantal release through a dynamic fusion pore. Another important question in the field has been the precise mechanisms of coupling presynaptic Ca ²⁺ channels and vesicular Ca ²⁺ sensors. This commentary provides an update on the current understanding of sound encoding in the cochlea with a focus on presynaptic mechanisms.

Disruption of adaptor protein 2μ (AP-2μ) in cochlear hair cells impairs vesicle reloading of synaptic release sites and hearing

Active zones (AZs) of inner hair cells (IHCs) indefatigably release hundreds of vesicles per second, requiring each release site to reload vesicles at tens per second. Here, we report that the endocytic adaptor protein 2μ (AP-2μ) is required for release site replenishment and hearing. We show that hair cell-specific disruption of AP-2μ slows IHC exocytosis immediately after fusion of the readily releasable pool of vesicles, despite normal abundance of membrane-proximal vesicles and intact endocytic membrane retrieval. Sound-driven postsynaptic spiking was reduced in a use-dependent manner, and the altered interspike interval statistics suggested a slowed reloading of release sites. Sustained strong stimulation led to accumulation of endosome-like vacuoles, fewer clathrin-coated endocytic intermediates, and vesicle depletion of the membrane-distal synaptic ribbon in AP-2μ-deficient IHCs, indicating a further role of AP-2μ in clathrin-dependent vesicle reformation on a timescale of many seconds. Finally, we show that AP-2 sorts its IHC-cargo otoferlin. We propose that binding of AP-2 to otoferlin facilitates replenishment of release sites, for example, via speeding AZ clearance of exocytosed material, in addition to a role of AP-2 in synaptic vesicle reformation.

Molecularly and structurally distinct synapses mediate reliable encoding and processing of auditory information

Hearing impairment is the most common human sensory deficit. Considering the sophisticated anatomy and physiology of the auditory system, disease-related failures frequently occur. To meet the demands of the neuronal circuits responsible for processing auditory information, the synapses of the lower auditory pathway are anatomically and functionally specialized to process acoustic information indefatigably with utmost temporal precision. Despite sharing some functional properties, the afferent synapses of the cochlea and of auditory brainstem differ greatly in their morphology and employ distinct molecular mechanisms for regulating synaptic vesicle release. Calyceal synapses of the endbulb of Held and the calyx of Held profit from a large number of release sites that project onto one principal cell. Cochlear inner hair cell ribbon synapses exhibit a unique one-to-one relation of the presynaptic active zone to the postsynaptic cell and use hair-cell-specific proteins such as otoferlin for vesicle release. The understanding of the molecular physiology of the hair cell ribbon synapse has been advanced by human genetics studies of sensorineural hearing impairment, revealing human auditory synaptopathy as a new nosological entity.

Relating structure and function of inner hair cell ribbon synapses

In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus-secretion coupling, exocytosis mechanisms, exo-endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

Association of intracellular and synaptic organization in cochlear inner hair cells revealed by 3D electron microscopy

The ways in which cell architecture is modelled to meet cell function is a poorly understood facet of cell biology. To address this question, we have studied the cytoarchitecture of a cell with highly specialised organisation, the cochlear inner hair cell (IHC), using multiple hierarchies of three-dimensional (3D) electron microscopy analyses. We show that synaptic terminal distribution on the IHC surface correlates with cell shape, and the distribution of a highly organised network of membranes and mitochondria encompassing the infranuclear region of the cell. This network is juxtaposed to a population of small vesicles, which represents a potential new source of neurotransmitter vesicles for replenishment of the synapses. Structural linkages between organelles that underlie this organisation were identified by high-resolution imaging. Taken together, these results describe a cell-encompassing network of membranes and mitochondria present in IHCs that support efficient coding and transmission of auditory signals. Such techniques also have the potential for clarifying functionally specialised cytoarchitecture of other cell types.

Summary: 3D electron microscopy reconstructs the highly organised structure of the infranuclear region of the cochlear inner hair cell, which supports synaptic functions.

Proton-mediated block of Ca2+ channels during multivesicular release regulates short-term plasticity at an auditory hair cell synapse

Synaptic vesicles release both neurotransmitter and protons during exocytosis, which may result in a transient acidification of the synaptic cleft that can block Ca(2+) channels located close to the sites of exocytosis. Evidence for this effect has been reported for retinal ribbon-type synapses, but not for hair cell ribbon synapses. Here, we report evidence for proton release from bullfrog auditory hair cells when they are held at more physiological, in vivo-like holding potentials (Vh = -60 mV) that facilitate multivesicular release. During paired recordings of hair cells and afferent fibers, L-type voltage-gated Ca(2+) currents showed a transient block, which was highly correlated with the EPSC amplitude (or the amount of glutamate release). This effect was masked at Vh = -90 mV due to the presence of a T-type Ca(2+) current and blocked by strong pH buffering with HEPES or TABS. Increasing vesicular pH with internal methylamine in hair cells also abolished the transient block. High concentrations of intracellular Ca(2+) buffer (10 mm BAPTA) greatly reduced exocytosis and abolished the transient block of the Ca(2+) current. We estimate that this transient block is due to the rapid multivesicular release of ∼600-1300 H(+) ions per synaptic ribbon. Finally, during a train of depolarizing pulses, paired pulse plasticity was significantly changed by using 40 mm HEPES in addition to bicarbonate buffer. We propose that this transient block of Ca(2+) current leads to more efficient exocytosis per Ca(2+) ion influx and it may contribute to spike adaptation at the auditory nerve.

Exocytotic machineries of vestibular type I and cochlear ribbon synapses display similar intrinsic otoferlin-dependent Ca2+ sensitivity but a different coupling to Ca2+ channels

The hair cell ribbon synapses of the mammalian auditory and vestibular systems differ greatly in their anatomical organization and firing properties. Notably, vestibular Type I hair cells (VHC-I) are surrounded by a single calyx-type afferent terminal that receives input from several ribbons, whereas cochlear inner hair cells (IHCs) are contacted by several individual afferent boutons, each facing a single ribbon. The specificity of the presynaptic molecular mechanisms regulating transmitter release at these different sensory ribbon synapses is not well understood. Here, we found that exocytosis during voltage activation of Ca(2+) channels displayed higher Ca(2+) sensitivity, 10 mV more negative half-maximum activation, and a smaller dynamic range in VHC-I than in IHCs. VHC-I had a larger number of Ca(2+) channels per ribbon (158 vs 110 in IHCs), but their Ca(2+) current density was twofold smaller because of a smaller open probability and unitary conductance. Using confocal and stimulated emission depletion immunofluorescence microscopy, we showed that VHC-I had fewer synaptic ribbons (7 vs 17 in IHCs) to which Cav1.3 channels are more tightly organized than in IHCs. Gradual intracellular Ca(2+) uncaging experiments revealed that exocytosis had a similar intrinsic Ca(2+) sensitivity in both VHC-I and IHCs (KD of 3.3 ± 0.6 μM and 4.0 ± 0.7 μM, respectively). In otoferlin-deficient mice, exocytosis was largely reduced in VHC-I and IHCs. We conclude that VHC-I and IHCs use a similar micromolar-sensitive otoferlin Ca(2+) sensor and that their sensory encoding specificity is essentially determined by a different functional organization of Ca(2+) channels at their synaptic ribbons.

A new probe for super-resolution imaging of membranes elucidates trafficking pathways

mCLING is a novel membrane probe for the study of membrane trafficking with demonstrated value in both live and fixed cells across a wide range of biological systems.

The molecular composition of the organelles involved in membrane recycling is difficult to establish as a result of the absence of suitable labeling tools. We introduce in this paper a novel probe, named membrane-binding fluorophore-cysteine-lysine-palmitoyl group (mCLING), which labels the plasma membrane and is taken up during endocytosis. It remains attached to membranes after fixation and permeabilization and can therefore be used in combination with immunostaining and super-resolution microscopy. We applied mCLING to mammalian-cultured cells, yeast, bacteria, primary cultured neurons, Drosophila melanogaster larval neuromuscular junctions, and mammalian tissue. mCLING enabled us to study the molecular composition of different trafficking organelles. We used it to address several questions related to synaptic vesicle recycling in the auditory inner hair cells from the organ of Corti and to investigate molecular differences between synaptic vesicles that recycle actively or spontaneously in cultured neurons. We conclude that mCLING enables the investigation of trafficking membranes in a broad range of preparations.

Rapid kinetics of endocytosis at rod photoreceptor synapses depends upon endocytic load and calcium

Release from rods is triggered by the opening of L-type Ca2+ channels that lie beneath synaptic ribbons. After exocytosis, vesicles are retrieved by compensatory endocytosis. Previous work showed that endocytosis is dynamin-dependent in rods but dynamin-independent in cones. We hypothesized that fast endocytosis in rods may also differ from cones in its dependence upon the amount of Ca2+ influx and/or endocytic load. We measured exocytosis and endocytosis from membrane capacitance (C m) changes evoked by depolarizing steps in voltage clamped rods from tiger salamander retinal slices. Similar to cones, the time constant for endocytosis in rods was quite fast, averaging <200 ms. We manipulated Ca2+ influx and the amount of vesicle release by altering the duration and voltage of depolarizing steps. Unlike cones, endocytosis kinetics in rods slowed after increasing Ca2+ channel activation with longer step durations or more strongly depolarized voltage steps. Endocytosis kinetics also slowed as Ca2+ buffering was decreased by replacing BAPTA (10 or 1 mM) with the slower Ca2+ buffer EGTA (5 or 0.5 mM) in the pipette solution. These data provide further evidence that endocytosis mechanisms differ in rods and cones and suggest that endocytosis in rods is regulated by both endocytic load and local Ca2+ levels.