Rab3A immunolocalization in the mammalian vestibular end-organs during development and comparison with synaptophysin expression

Synaptic transmission at the vestibular hair cells of amniotes

A harmonized interplay between the central nervous system and the five peripheral end organs is how the vestibular system helps organisms feel a sense of balance and motion in three-dimensional space. The receptor cells of this system, much like their cochlear equivalents, are the specialized hair cells. However, research over the years has shown that the vestibular endorgans and hair cells evolved very differently from their cochlear counterparts. The structurally unique calyceal synapse, which appeared much later in the evolutionary time scale, and continues to intrigue researchers, is now known to support several forms of synaptic neurotransmission. The conventional quantal transmission is believed to employ the ribbon structures, which carry several tethered vesicles filled with neurotransmitters. However, the field of vestibular hair cell synaptic molecular anatomy is still at a nascent stage and needs further work. In this review, we will touch upon the basic structure and function of the peripheral vestibular system, with the focus on the various modes of neurotransmission at the type I vestibular hair cells. We will also shed light on the current knowledge about the molecular anatomy of the vestibular hair cell synapses and vestibular synaptopathy.

Neurosensory Differentiation and Innervation Patterning in the Human Fetal Vestibular End Organs between the Gestational Weeks 8-12

Balance orientation depends on the precise operation of the vestibular end organs and the vestibular ganglion neurons. Previous research on the assemblage of the neuronal network in the developing fetal vestibular organ has been limited to data from animal models. Insights into the molecular expression profiles and signaling moieties involved in embryological development of the human fetal inner ear have been limited. We present an investigation of the cells of the vestibular end organs with specific focus on the hair cell differentiation and innervation pattern using an uninterrupted series of unique specimens from gestational weeks 8-12. Nerve fibers positive for peripherin innervate the entire fetal crista and utricle. While in rodents only the peripheral regions of the cristae and the extra-striolar region of the statolithic organs are stained. At week 9, transcription factors PAX2 and PAX8 were observed in the hair cells whereas PAX6 was observed for the first time among the supporting cells of the cristae and the satellite glial cells of the vestibular ganglia. Glutamine synthetase, a regulator of the neurotransmitter glutamate, is strongly expressed among satellite glia cells, transitional zones of the utricle and supporting cells in the sensory epithelium. At gestational week 11, electron microscopic examination reveals bouton contacts at hair cells and first signs of the formation of a protocalyx at type I hair cells. Our study provides first-hand insight into the fetal development of the vestibular end organs as well as their pattern of innervation by means of immunohistochemical and EM techniques, with the aim of contributing toward our understanding of balance development.

Correlation between afferent rearrangements and behavioral deficits after local excitotoxic insult in the mammalian vestibule: a rat model of vertigo symptoms

Damage to inner ear afferent terminals is believed to result in many auditory and vestibular dysfunctions. The sequence of afferent injuries and repair, as well as their correlation with vertigo symptoms, remains poorly documented. In particular, information on the changes that take place at the primary vestibular endings during the first hours following a selective insult is lacking. In the present study, we combined histological analysis with behavioral assessments of vestibular function in a rat model of unilateral vestibular excitotoxic insult. Excitotoxicity resulted in an immediate but transient alteration of the balance function that was resolved within a week. Concomitantly, vestibular primary afferents underwent a sequence of structural changes followed by spontaneous repair. Within the first two hours after the insult, a first phase of pronounced vestibular dysfunction coincided with extensive swelling of afferent terminals. In the next 24 h, a second phase of significant but incomplete reduction of the vestibular dysfunction was accompanied by a resorption of swollen terminals and fiber retraction. Eventually, within 1 week, a third phase of complete balance restoration occurred. The slow and progressive withdrawal of the balance dysfunction correlated with full reconstitution of nerve terminals. Competitive re-innervation by afferent and efferent terminals that mimicked developmental synaptogenesis resulted in full re-afferentation of the sensory epithelia. By deciphering the sequence of structural alterations that occur in the vestibule during selective excitotoxic impairment, this study offers new understanding of how a vestibular insult develops in the vestibule and how it governs the heterogeneity of vertigo symptoms.

Summary: Early sequence of afferent injury and repair in vestibular sensory epithelium that correlates with balance disorders and functional restoration is detailed in a rodent model of excitotoxicity.

Plasticity of Scarpa's Ganglion Neurons as a Possible Basis for Functional Restoration within Vestibular Endorgans

In a previous study, we observed spontaneous restoration of vestibular function in young adult rodents following excitotoxic injury of the neuronal connections within vestibular endorgans. The functional restoration was supported by a repair of synaptic contacts between hair cells and primary vestibular neurons. This process was observed in 2/3 of the animals studied and occurred within 5 days following the synaptic damage. To assess whether repair capacity is a fundamental trait of vestibular endorgans and to decipher the cellular mechanisms supporting such a repair process, we studied the neuronal regeneration and synaptogenesis in co-cultures of vestibular epithelia and Scarpa's ganglion from young and adult rodents. We demonstrate that, under specific culture conditions, primary vestibular neurons from young mice or rats exhibit robust ability to regenerate nervous processes. When co-cultured with vestibular epithelia, primary vestibular neurons were able to establish de novo contacts with hair cells. Under the present paradigm, these contacts displayed morphological features of immature synaptic contacts. Preliminary observations using co-cultures of adult rodents suggest that this reparative capacity remained in older mice although to a lesser extent. Identifying the basic mechanisms underlying the repair process may provide a basis for novel therapeutic strategies to restore mature and functional vestibular synaptic contacts following damage or loss.

Molecular microdomains in a sensory terminal, the vestibular calyx ending

Many primary vestibular afferents form large cup-shaped postsynaptic terminals (calyces) that envelope the basolateral surfaces of type I hair cells. The calyceal terminals both respond to glutamate released from ribbon synapses in the type I cells and initiate spikes that propagate to the afferent's central terminals in the brainstem. The combination of synaptic and spike initiation functions in these unique sensory endings distinguishes them from the axonal nodes of central neurons and peripheral nerves, such as the sciatic nerve, which have provided most of our information about nodal specializations. We show that rat vestibular calyces express an unusual mix of voltage-gated Na and K channels and scaffolding, cell adhesion, and extracellular matrix proteins, which may hold the ion channels in place. Protein expression patterns form several microdomains within the calyx membrane: a synaptic domain facing the hair cell, the heminode abutting the first myelinated internode, and one or two intermediate domains. Differences in the expression and localization of proteins between afferent types and zones may contribute to known variations in afferent physiology.

The molecular architecture of ribbon presynaptic terminals

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

Ocsyn and mitochondrial-canalicular complexes in vestibular hair cells

Ocsyn, a syntaxin-interacting protein characterized by Safieddine et al. [Safieddine, S., Ly, C.D., Wang, Y.-X., Kachar, B., Petralia, R.S., Wenthold, R.J., 2002. Ocsyn, a novel syntaxin-interacting protein enriched in the subapical region of inner hair cells. Mol. Cell. Neurosci., 20, 343-353] in the guinea pig organ of Corti was primarily identified in organelles located at the subapical region of inner hair cells. They proposed that in cochlear inner hair cells, ocsyn was involved in protein trafficking associated to recycling endosomes. Ocsyn happens to be highly homologous to syntabulin with an almost identical syntaxin-binding domain. Syntabulin is believed to attach syntaxin-containing vesicles to kinesin for their axonal transport along microtubules. The present study shows the distribution of ocsyn in guinea pig and rat vestibular hair cells using immunocytochemistry and confocal microscopy. Ocsyn was characterized by intense immunolabeled spots distributed exclusively in type I and II vestibular hair cells. The subcuticular region under the cuticular plate exhibited particularly densely packed spots. In the neck region of the sensory cells, where microtubules are abundant, there was no colocalization of ocsyn and alpha-tubulin. Ocsyn labeled spots were also present in the medial and basal hair cell regions, particularly in the supranuclear and infranuclear regions. Mitochondria are particularly numerous in these three regions (subcuticular, supranuclear and infranuclear). Double labeling of ocsyn and cytochrome c showed that ocsyn was present in the same zones that mitochondria. This, together with the great similarity of ocsyn and syntabulin, suggest that, akin to syntabulin, ocsyn is involved in addressing organelles. We propose that ocsyn is involved in the formation of the canalicular-mitochondrial complexes depicted by Spicer et al. [Spicer, S.S., Thomopoulos, G.N., Schulte, B.A., 1999. Novel membranous structures in apical and basal compartments of inner hear cells. J. Comp. Neurol., 409, 424-437].

Neural connections between embryonic stem cell-derived neurons and vestibular hair cells in vitro

This study aimed to examine the potential of embryonic stem cell (ESC)-derived neural progenitors for restoration of the neural network in the peripheral vestibular system. Mouse ESC-derived neural progenitors were co-cultured with explants of vestibular sensory epithelia from neonatal mice. Histological analyses demonstrated that ESC-derived neurons substantially elongated their neurites towards vestibular hair cells, and attached to hair cells at the regions corresponding to the location of nerve endings in normal vestibular epithelia. Immunoreactivity for synaptophysin, a marker for synaptic vesicles, was present only in the cytoplasm of hair cells in sensory epithelia cultured alone, while the nerve endings of ESC-derived neurons attached to hair cells exhibited intense immunoreactivity for synaptophysin and some hair cells were moderately reactive in co-cultured specimens. The pattern of synaptophysin expression in co-cultured specimens was very similar to that observed in developing sensory epithelia, in which synaptic connections between hair cells and nerve endings are actively formed. These findings indicate that ESC-derived neurons have the potential to restore neural connections in the peripheral vestibular system.

Cod106, a novel synaptic protein expressed in sensory hair cells of the inner ear and in CNS neurons

The exquisite performance of the highly specialized mammalian inner ear requires a multitude of specific proteins. Yet, only a subset of these proteins has been identified and studied in detail. Here, we describe a novel gene expressed in the organ of Corti that encodes a membrane-associated protein of 106 kDa. The new protein, termed Cod106, lacks sequence homology to characterized gene products. As shown by in situ hybridization, it is expressed in auditory and vestibular hair cells, as well as in distinct sets of CNS neurons with particularly high abundance in hippocampus and cerebellum. Immunohistochemistry detected Cod106 at the basal, synaptic pole of cochlear outer hair cells and vestibular hair cells. In cultured hippocampal neurons, Cod106 immunofluorescence co-localized with the postsynaptic density protein 95 (PSD95), indicating a postsynaptic localization. Cell-type specificity and subcellular localization may be consistent with an involvement of Cod106 in synaptic processes.

Synapsin-like immunoreactivity is present in hair cells and efferent terminals of the toadfish crista ampullaris

The synapsins are presynaptic membrane-associated proteins involved in neurotransmitter release. They are differentially expressed in tissues and cells of the central and peripheral nervous system. In vestibular end organs of mammals, synapsin I-like immunoreactivity has been reported in efferent and afferent terminals and in afferent nerve calyces surrounding type I hair cells. In addition, synapsin I has recently been described in several non-neural cell lines. The present study was conducted to locate synapsin-like immunoreactivity in the neuronal and non-neuronal cells of the fish crista ampullaris, to examine the possibility that the non-neuronal sensory receptor cells express synapsins in vivo. Synapsin-like immunostaining was visualized by immunofluorescence detection in wholemounts of the toadfish crista ampullaris using multiphoton laser scanning microscopy and by electron microscopic visualization of post-embedding immunogold labeling. The results demonstrate that synapsin-like immunoreactivity is present in vestibular hair cells and efferent boutons of the toadfish crista ampullaris. Afferent endings are not labeled. Staining in hair cells is not associated with the synaptic ribbons, suggesting that there is an additional, non-synaptic role for the synapsins in some non-neuronal cells of vertebrates. Moreover, while the cristae of amniote and anamniote species share many functional attributes, differences in their synaptic vesicle-associated protein profiles appear to reflect their disparate hair cell populations.

Differential impact of hypergravity on maturating innervation in vestibular epithelia during rat development

Over the past decades, the new opportunity of space flights has revealed the importance of gravity as a mechanical constraint for terrestrial organisms as well as its influence on the somatosensory system. The lack of gravitational reference in orbital flight induces changes in equilibrium, with major modifications involving neuromorphological and physiological adaptations. However, few data have illustrated the putative effect of gravity on sensory vestibular epithelial development. We asked if gravity, the primary stimulus of utricles could act as an epigenetic factor. As sensorial deprivation linked to weightlessness is technically difficult, we used a ground-based centrifuge to increase the gravitational vector, in order to hyperstimulate the vestibule. In this study, 3 days after mating, pregnant females were submitted to hypergravity, 2 g (HG). Their embryos were raised, born and postnatally developed under HG. The establishment of connections between primary vestibular afferent neurons and hair cells in the utricle of these young rats was followed from birth to postnatal day 6 (PN6) and compared to embryos developed in normogravity (NG): Immunocytochemistry for neurofilaments and microvesicles revealed the differential effects of gravity on the late neuritogenic and synaptogenic processes in utricles. Taking type I hair cell innervation as a criterion of maturation, we found that primary afferent fibres reached the vestibular epithelium and enveloped hair cells in the same way, both under NG and HG. Thus, this phenomenon of leading growth cones to their epithelial target appears to be dependent on intrinsic genetic properties and not on an external stimulus. In contrast, the maturation of connection processes between type 1 hair cells and the afferent calyx, concerning specifically the microvesicles at their apex, was delayed under HG. Therefore, gravity appears to be an epigenetic factor influencing the late maturation of utricles. These differential effects of altered gravity on the development of the vestibular epithelium are discussed.

Confinement but not microgravity alters NMDA NR1 receptor expression in rat inner ear ganglia

Space flight produces changes in neuronal activity in the vestibular system. We studied the protein expression of the NMDA receptor subunit NR1 in the vestibular ganglia of rats exposed to microgravity for 17 days, beginning on postnatal day 8, as part of the NASA Neurolab mission. As a control, we studied the cochlear ganglia in the same way. NR1 expression in rats that had experienced microgravity (flight-FLT rats) was compared with that in two types of ground control. One control consisted of rats housed in regular cage conditions (VIV, vivarium); the other, asynchronous ground control (AGC), consisted of rats kept in cages similar to those used in flight (animal enclosure module, AEM), requiring no human care. After 8 days of flight, NR1 levels in the vestibular and cochlear neurons were similar in FLT, VIV and AGC rats. In contrast, 8 h after landing, the FLT and VIV animals showed similar, normal levels of NR1 staining, whereas the ganglia of the AGC animals displayed only very faint staining. Thus, microgravity did not modify NR1 expression in vestibular neurons. The lower levels of NR1 expression in the vestibular and cochlear neurons of AGC rats suggest an effect of confinement for 17 days in AEMs on the ground.

The expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina

In the present study, we generated a systematic overview of the expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina. Using indirect immunofluorescence microscopy, we analyzed the spatial and temporal distribution of 11 important membrane and membrane-associated synaptic proteins (syntaxin 1/3, SNAP-25, synaptobrevin 2, synaptogyrin, synaptotagmin I, SV2A, SV2B, Rab3A, clathrin light chains, CSP and neuroligin I) during synaptogenesis. The temporospatial distribution of these synaptic proteins was "normalized" by the simultaneous visualization of the synaptic vesicle protein synaptophysin, which served as an internal reference protein. We found that expression of various synaptic membrane proteins started at different time points and changed progressively during development. At early stages of development synaptic vesicle membrane proteins at extrasynaptic locations did not always colocalize with synaptophysin, indicating that these proteins probably do not reside in the same transport vesicles. Despite a non-synchronized onset of protein expression, clustering and colocalization of all synaptic membrane proteins at ribbon synapses roughly occurred in the same time window (between day 4 after birth, P4, and P5). Thus, the basic synaptic membrane machinery is already present in ribbon synapses before the well-known complete morphological maturation of ribbon synapses between P7 and P12. We conclude that ribbon synapse formation is a multistep process in which the concerted recruitment of synaptic membrane proteins is a relatively early event and clearly not the final step.

Structure suggests function: the case for synaptic ribbons as exocytotic nanomachines

Synaptic ribbons, the organelles identified in electron micrographs of the sensory synapses involved in vision, hearing, and balance, have long been hypothesized to play an important role in regulating presynaptic function because they associate with synaptic vesicles at the active zone. Their physiology and molecular composition have, however, remained largely unknown. Recently, a series of elegant studies spurred by technical innovation have finally begun to shed light on the ultrastructure and function of ribbon synapses. Electrical capacitance measurements have provided sub-millisecond resolution of exocytosis, evanescent-wave microscopy has filmed the fusion of single 30 nm synaptic vesicles, electron tomography has revealed the 3D architecture of the synapse, and molecular cloning has begun to identify the proteins that make up ribbons. These results are consistent with the ribbon serving as a vesicle "conveyor belt" to resupply the active zone, and with the suggestion that ribbon and conventional chemical synapses have much in common.

Distribution of frequenin in the mouse inner ear during development, comparison with other calcium-binding proteins and synaptophysin

Frequenin is a calcium-binding protein previously implicated in the regulation of neurotransmission. We report its immunocytochemical detection in the mouse inner ear, in the adult, and during embryonic (E) and postnatal (P) development. The distribution of frequenin was compared with those of other calcium-binding proteins (calbindin, calretinin, parvalbumin) and synaptophysin. In the adult mouse inner ear, frequenin immunostaining was observed in the afferent neuronal systems (vestibular and cochlear neurons, their processes and endings) and in the vestibular and cochlear efferent nerve terminals. Frequenin colocalized with synaptophysin in well characterized presynaptic compartments, such as the vestibular and cochlear efferent endings, and in putative presynaptic compartments, such as the apical part of the vestibular calyces. Frequenin was not found in vestibular hair cells and in cochlear inner and outer hair cells. During development, frequenin immunoreactivity was first detected on E11 in the neurons of the statoacoustic ganglion. On E14, frequenin was detected in the afferent neurites innervating the vestibular sensory epithelium, along with synaptophysin. On E16, frequenin was detected in the afferent neurites below the inner hair cells in the organ of Corti. The timing of frequenin detection in vestibular and cochlear afferent neurites was consistent with their sequences of maturation, and was earlier than synaptogenesis. Thus in the inner ear, frequenin is a very early marker of differentiated and growing neurons and is present in presynaptic and postsynaptic compartments.

Efferent function of vestibular afferent endings? Similar localization of N-type calcium channels, synaptic vesicle and synaptic membrane-associated proteins

We investigated the distribution of N-type voltage-dependent calcium channels that mediate Ca(2+) entry initiating transmitter release in the rat vestibular sensory epithelium. We used confocal microscopy to assess the in vitro labeling by fluorescent specific ligand binding, omega-conotoxin-GVIA and also the immunolabeling of presynaptic soluble N-ethylmaleimide-sensitive fusion factor attachment protein receptor (SNARE) proteins, syntaxin, 25,000 mol. wt synaptosome-associated protein and synaptotagmin: components of the neurotransmitter exocytosis machinery. We found that there was a close anatomical association between the voltage-gated calcium channels, the synaptic vesicle and synaptic membrane-associated proteins on the afferent nerve calyces and probably afferent boutons, which are postsynaptic compartments. Our data suggest that these peripheral afferent endings possess the presynaptic Ca(2+) channels and the components of the presynaptic SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. They provide additional evidence for a secretory function and efferent role of these endings in hair cell neurotransmission.

Glycine induced calcium concentration changes in vestibular type I sensory cells

Glutamate is the neurotransmitter of the synapse between vestibular type I hair cells and the afferent nerve calyx. This calyx may also be involved in local feedback, which may modify sensory cell activity via N-methyl-D-aspartate (NMDA) receptors. Glycine is the co-agonist of glutamate in NMDA receptor activation. Both agents have been detected by immunocytochemistry in the nerve calyx. Glutamate and NMDA stimulations cause changes in the intracellular calcium concentration ([Ca(2+)](i)) of isolated type I sensory cells. We investigated the effect of glycine stimulation on [Ca(2+)](i) in guinea pig type I sensory cells by spectrofluorimetry with fura-2. Glycine application to isolated type I sensory cells induced a rapid and transient increase in [Ca(2+)](i). The fluorescence ratio increased by 55% above the resting level. The peak was reached in 9 s and the return to basal level took about 20 s. A specific antagonist of the glycine site on NMDA receptors, 7-chlorokynurenate (10 microM), decreased the calcium response to glycine by 60%. Glycine may activate NMDA receptors. Glycine may also activate the strychnine-sensitive glycine receptor-gated channel. Strychnine (50 microM) decreased the calcium response to glycine by 60%. Thus, glycine probably induces calcium concentration changes in type I vestibular sensory cells via NMDA receptors and/or glycine receptors.

Patterns of synaptophysin expression during development of the inner ear in the chick

The onset of active neural connections between the periphery and the central nervous system is integral to the development of sensory systems. This study presents patterns of synaptogenesis in the chick basilar papilla (i.e., cochlea) by examining the immunohistochemical expression of synaptophysin with a specific monoclonal antibody, SBI 20.10. The initial onset of synaptophysin expression occurs in nerve fibers and ganglion cell bodies at a time when neurites reach the basement membrane of the chick cochlea on embryonic day 6-7 (ED 6-7). By ED 8, synaptophysin positive fibers invade the neural side of the entire length of the cochlea, so that by ED 9-10, fibers are forming multiple terminals on the basolateral ends of retracting receptor or hair cells. In contrast, on the abneural side, immunoreactive terminals are seen first as small, punctate contacts and then as large, synaptophysin positive calyceal endings beneath short hair cells. These terminals are sparse during early development, more numerous by ED 17-19, but still incomplete after 2 weeks posthatching. In comparison, hair cells show synaptophysin immunoreactivity in both supra- and infranuclear regions by ED 11-12, a time when efferent innervation is incomplete. Thus, during development, synaptophysin is expressed at both synaptic and nonsynaptic sites, is relatively selective in its regional distribution, and is expressed in hair cells at a time when auditory function begins. Our results present a framework with which to understand the potential role of synaptophysin in early synaptogenesis of the cochlea.

Ontogenesis of substance P in the rat peripheral vestibular system

Ontogenesis of substance P (SP) in rat vestibular receptors between gestational day 17 (GD 17) and postnatal day 17 (PD 17) was investigated by immunocytochemistry. SP immunoreactivity was first detected in the afferent nerve fibers of the utricular and saccular maculae on GD 19 and thereafter at birth (PD 0) in the cristae. SP immunoreactivity presented a characteristic pattern. It was strictly confined to the slopes of cristae and peripheral regions of the maculae. The pattern of SP immunoreactivity changed during the maturation of the afferent innervation pattern between PD 4 and PD 9, resulting in fewer fibers being stained. Then, the number of immunostained calyces and small bouton endings increased. On PD 17 the distribution of SP was similar to that of the adult stage. We suggest that SP, present in the epithelium at critical stages of development, may be involved in the maturation of vestibular receptors.