Mouse photoreceptor synaptic ribbons lose and regain material in response to illumination changes

A non-conducting role of the Ca v 1.4 Ca 2+ channel drives homeostatic plasticity at the cone photoreceptor synapse

In congenital stationary night blindness type 2 (CSNB2), a disorder involving the Cav1.4 (L-type) Ca2+ channel, visual impairment is mild considering that Cav1.4 mediates synaptic release from rod and cone photoreceptors. Here, we addressed this conundrum using a Cav1.4 knockout (KO) mouse and a knock-in (G369i KI) mouse expressing a non-conducting Cav1.4. Surprisingly, Cav3 (T-type) Ca2+ currents were detected in cones of G369i KI mice and Cav1.4 KO mice but not in cones of wild-type mouse, ground squirrel, and macaque retina. Whereas Cav1.4 KO mice are blind, G369i KI mice exhibit normal photopic (i.e., cone-mediated) visual behavior. Cone synapses, which fail to form in Cav1.4 KO mice, are present, albeit enlarged, and with some errors in postsynaptic wiring in G369i KI mice. While Cav1.4 KO mice lack evidence of cone synaptic responses, electrophysiological recordings in G369i KI mice revealed nominal transmission from cones to horizontal cells and bipolar cells. In CSNB2, we propose that Cav3 channels maintain cone synaptic output provided that the nonconducting role of Cav1.4 in cone synaptogenesis remains intact. Our findings reveal an unexpected form of homeostatic plasticity that relies on a non-canonical role of an ion channel.

Slow kinesin-dependent microtubular transport facilitates ribbon synapse assembly in developing cochlear inner hair cells

Sensory synapses are characterized by electron-dense presynaptic specializations, so-called synaptic ribbons. In cochlear inner hair cells (IHCs), ribbons play an essential role as core active zone (AZ) organizers, where they tether synaptic vesicles, cluster calcium channels and facilitate the temporally-precise release of primed vesicles. While a multitude of studies aimed to elucidate the molecular composition and function of IHC ribbon synapses, the developmental formation of these signalling complexes remains largely elusive to date. To address this shortcoming, we performed long-term live-cell imaging of fluorescently-labelled ribbon precursors in young postnatal IHCs to track ribbon precursor motion. We show that ribbon precursors utilize the apico-basal microtubular (MT) cytoskeleton for targeted trafficking to the presynapse, in a process reminiscent of slow axonal transport in neurons. During translocation, precursor volume regulation is achieved by highly dynamic structural plasticity - characterized by regularly-occurring fusion and fission events. Pharmacological MT destabilization negatively impacted on precursor translocation and attenuated structural plasticity, whereas genetic disruption of the anterograde molecular motor Kif1a impaired ribbon volume accumulation during developmental maturation. Combined, our data thus indicate an essential role of the MT cytoskeleton and Kif1a in adequate ribbon synapse formation and structural maintenance.

Age-dependent structural reorganization of utricular ribbon synapses

In mammals, spatial orientation is synaptically-encoded by sensory hair cells of the vestibular labyrinth. Vestibular hair cells (VHCs) harbor synaptic ribbons at their presynaptic active zones (AZs), which play a critical role in molecular scaffolding and facilitate synaptic release and vesicular replenishment. With advancing age, the prevalence of vestibular deficits increases; yet, the underlying mechanisms are not well understood and the possible accompanying morphological changes in the VHC synapses have not yet been systematically examined. We investigated the effects of maturation and aging on the ultrastructure of the ribbon-type AZs in murine utricles using various electron microscopic techniques and combined them with confocal and super-resolution light microscopy as well as metabolic imaging up to 1 year of age. In older animals, we detected predominantly in type I VHCs the formation of floating ribbon clusters, mostly consisting of newly synthesized ribbon material. Our findings suggest that VHC ribbon-type AZs undergo dramatic structural alterations upon aging.

Injury to Cone Synapses by Retinal Detachment: Differences from Rod Synapses and Protection by ROCK Inhibition

Attachment of a detached retina does not always restore vision to pre-injury levels, even if the attachment is anatomically successful. The problem is due in part to long-term damage to photoreceptor synapses. Previously, we reported on damage to rod synapses and synaptic protection using a Rho kinase (ROCK) inhibitor (AR13503) after retinal detachment (RD). This report documents the effects of detachment, reattachment, and protection by ROCK inhibition on cone synapses. Conventional confocal and stimulated emission depletion (STED) microscopy were used for morphological assessment and electroretinograms for functional analysis of an adult pig model of RD. RDs were examined 2 and 4 h after injury or two days later when spontaneous reattachment had occurred. Cone pedicles respond differently than rod spherules. They lose their synaptic ribbons, reduce invaginations, and change their shape. ROCK inhibition protects against these structural abnormalities whether the inhibitor is applied immediately or 2 h after the RD. Functional restoration of the photopic b-wave, indicating cone-bipolar neurotransmission, is also improved with ROCK inhibition. Successful protection of both rod and cone synapses with AR13503 suggests this drug will (1) be a useful adjunct to subretinal administration of gene or stem cell therapies and (2) improve recovery of the injured retina when treatment is delayed.

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Calcium Channels in Retinal Function and Disease

Voltage-gated Ca2+ (Cav) channels play pivotal roles in regulating gene transcription, neuronal excitability, and neurotransmitter release. To meet the spatial and temporal demands of visual signaling, Cav channels exhibit unusual properties in the retina compared to their counterparts in other areas of the nervous system. In this article, we review current concepts regarding the specific subtypes of Cav channels expressed in the retina, their intrinsic properties and forms of modulation, and how their dysregulation could lead to retinal disease.

Early Changes in Exo- and Endocytosis in the EAE Mouse Model of Multiple Sclerosis Correlate with Decreased Synaptic Ribbon Size and Reduced Ribbon-Associated Vesicle Pools in Rod Photoreceptor Synapses

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system that finally leads to demyelination. Demyelinating optic neuritis is a frequent symptom in MS. Recent studies also revealed synapse dysfunctions in MS patients and MS mouse models. We previously reported alterations of photoreceptor ribbon synapses in the experimental auto-immune encephalomyelitis (EAE) mouse model of MS. In the present study, we found that the previously observed decreased imunosignals of photoreceptor ribbons in early EAE resulted from a decrease in synaptic ribbon size, whereas the number/density of ribbons in photoreceptor synapses remained unchanged. Smaller photoreceptor ribbons are associated with fewer docked and ribbon-associated vesicles. At a functional level, depolarization-evoked exocytosis as monitored by optical recording was diminished even as early as on day 7 after EAE induction. Moreover compensatory, post-depolarization endocytosis was decreased. Decreased post-depolarization endocytosis in early EAE correlated with diminished synaptic enrichment of dynamin3. In contrast, basal endocytosis in photoreceptor synapses of resting non-depolarized retinal slices was increased in early EAE. Increased basal endocytosis correlated with increased de-phosphorylation of dynamin1. Thus, multiple endocytic pathways in photoreceptor synapse are differentially affected in early EAE and likely contribute to the observed synapse pathology in early EAE.

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.

Impaired Ca2+ Sensitivity of a Novel GCAP1 Variant Causes Cone Dystrophy and Leads to Abnormal Synaptic Transmission Between Photoreceptors and Bipolar Cells

Guanylate cyclase-activating protein 1 (GCAP1) is involved in the shutdown of the phototransduction cascade by regulating the enzymatic activity of retinal guanylate cyclase via a Ca²⁺/cGMP negative feedback. While the phototransduction-associated role of GCAP1 in the photoreceptor outer segment is widely established, its implication in synaptic transmission to downstream neurons remains to be clarified. Here, we present clinical and biochemical data on a novel isolate GCAP1 variant leading to a double amino acid substitution (p.N104K and p.G105R) and associated with cone dystrophy (COD) with an unusual phenotype. Severe alterations of the electroretinogram were observed under both scotopic and photopic conditions, with a negative pattern and abnormally attenuated b-wave component. The biochemical and biophysical analysis of the heterologously expressed N104K-G105R variant corroborated by molecular dynamics simulations highlighted a severely compromised Ca²⁺-sensitivity, accompanied by minor structural and stability alterations. Such differences reflected on the dysregulation of both guanylate cyclase isoforms (RetGC1 and RetGC2), resulting in the constitutive activation of both enzymes at physiological levels of Ca²⁺. As observed with other GCAP1-associated COD, perturbation of the homeostasis of Ca²⁺ and cGMP may lead to the toxic accumulation of second messengers, ultimately triggering cell death. However, the abnormal electroretinogram recorded in this patient also suggested that the dysregulation of the GCAP1–cyclase complex further propagates to the synaptic terminal, thereby altering the ON-pathway related to the b-wave generation. In conclusion, the pathological phenotype may rise from a combination of second messengers’ accumulation and dysfunctional synaptic communication with bipolar cells, whose molecular mechanisms remain to be clarified.

Transmission at rod and cone ribbon synapses in the retina

Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca²⁺ channel activity. While the opening of only a few Ca²⁺ channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.

Molecular Assembly and Structural Plasticity of Sensory Ribbon Synapses-A Presynaptic Perspective

In the mammalian cochlea, specialized ribbon-type synapses between sensory inner hair cells (IHCs) and postsynaptic spiral ganglion neurons ensure the temporal precision and indefatigability of synaptic sound encoding. These high-through-put synapses are presynaptically characterized by an electron-dense projection-the synaptic ribbon-which provides structural scaffolding and tethers a large pool of synaptic vesicles. While advances have been made in recent years in deciphering the molecular anatomy and function of these specialized active zones, the developmental assembly of this presynaptic interaction hub remains largely elusive. In this review, we discuss the dynamic nature of IHC (pre-) synaptogenesis and highlight molecular key players as well as the transport pathways underlying this process. Since developmental assembly appears to be a highly dynamic process, we further ask if this structural plasticity might be maintained into adulthood, how this may influence the functional properties of a given IHC synapse and how such plasticity could be regulated on the molecular level. To do so, we take a closer look at other ribbon-bearing systems, such as retinal photoreceptors and pinealocytes and aim to infer conserved mechanisms that may mediate these phenomena.

A dual role for Cav1.4 Ca2+ channels in the molecular and structural organization of the rod photoreceptor synapse

Synapses are fundamental information processing units that rely on voltage-gated Ca2+ (Cav) channels to trigger Ca2+-dependent neurotransmitter release. Cav channels also play Ca2+-independent roles in other biological contexts, but whether they do so in axon terminals is unknown. Here, we addressed this unknown with respect to the requirement for Cav1.4 L-type channels for the formation of rod photoreceptor synapses in the retina. Using a mouse strain expressing a non-conducting mutant form of Cav1.4, we report that the Cav1.4 protein, but not its Ca2+ conductance, is required for the molecular assembly of rod synapses; however, Cav1.4 Ca2+ signals are needed for the appropriate recruitment of postsynaptic partners. Our results support a model in which presynaptic Cav channels serve both as organizers of synaptic building blocks and as sources of Ca2+ ions in building the first synapse of the visual pathway and perhaps more broadly in the nervous system.

A pathoconnectome of early neurodegeneration: Network changes in retinal degeneration

Connectomics has demonstrated that synaptic networks and their topologies are precise and directly correlate with physiology and behavior. The next extension of connectomics is pathoconnectomics: to map neural network synaptology and circuit topologies corrupted by neurological disease in order to identify robust targets for therapeutics. In this report, we characterize a pathoconnectome of early retinal degeneration. This pathoconnectome was generated using serial section transmission electron microscopy to achieve an ultrastructural connectome with 2.18nm/px resolution for accurate identification of all chemical and gap junctional synapses. We observe aberrant connectivity in the rod-network pathway and novel synaptic connections deriving from neurite sprouting. These observations reveal principles of neuron responses to the loss of network components and can be extended to other neurodegenerative diseases.

Synaptic ribbons foster active zone stability and illumination-dependent active zone enrichment of RIM2 and Cav1.4 in photoreceptor synapses

Rod photoreceptor synapses use large, ribbon-type active zones for continuous synaptic transmission during light and dark. Since ribbons are physically connected to the active zones, we asked whether illumination-dependent changes of ribbons influence Cav1.4/RIM2 protein clusters at the active zone and whether these illumination-dependent effects at the active zone require the presence of the synaptic ribbon. We found that synaptic ribbon length and the length of presynaptic Cav1.4/RIM2 clusters are tightly correlated. Dark-adaptation did not change the number of ribbons and active zone puncta. However, mean ribbon length and length of presynaptic Cav1.4/RIM2 clusters increased significantly during dark-adaptation when tonic exocytosis is highest. In the present study, we identified by the analyses of synaptic ribbon-deficient RIBEYE knockout mice that synaptic ribbons are (1) needed to stabilize Cav1.4/RIM2 at rod photoreceptor active zones and (2) are required for the darkness-induced active zone enrichment of Cav1.4/RIM2. These data propose a role of the ribbon in active zone stabilization and suggest a homeostatic function of the ribbon in illumination-dependent active zone remodeling.

Structural Remodeling of Active Zones Is Associated with Synaptic Homeostasis

Perturbations to postsynaptic glutamate receptors (GluRs) trigger retrograde signaling to precisely increase presynaptic neurotransmitter release, maintaining stable levels of synaptic strength, a process referred to as homeostatic regulation. However, the structural change of homeostatic regulation remains poorly defined. At wild-type Drosophila neuromuscular junction synapse, there is one Bruchpilot (Brp) ring detected by superresolution microscopy at active zones (AZs). In the present study, we report multiple Brp rings (i.e., multiple T-bars seen by electron microscopy) at AZs of both male and female larvae when GluRs are reduced. At GluRIIC-deficient neuromuscular junctions, quantal size was reduced but quantal content was increased, indicative of homeostatic presynaptic potentiation. Consistently, multiple Brp rings at AZs were observed in the two classic synaptic homeostasis models (i.e., GluRIIA mutant and pharmacological blockade of GluRIIA activity). Furthermore, postsynaptic overexpression of the cell adhesion protein Neuroligin 1 partially rescued multiple Brp rings phenotype. Our study thus supports that the formation of multiple Brp rings at AZs might be a structural basis for synaptic homeostasis.SIGNIFICANCE STATEMENT Synaptic homeostasis is a conserved fundamental mechanism to maintain efficient neurotransmission of neural networks. Active zones (AZs) are characterized by an electron-dense cytomatrix, which is largely composed of Bruchpilot (Brp) at the Drosophila neuromuscular junction synapses. It is not clear how the structure of AZs changes during homeostatic regulation. To address this question, we examined the structure of AZs by superresolution microscopy and electron microscopy during homeostatic regulation. Our results reveal multiple Brp rings at AZs of glutamate receptor-deficient neuromuscular junction synapses compared with single Brp ring at AZs in wild type (WT). We further show that Neuroligin 1-mediated retrograde signaling regulates multiple Brp ring formation at glutamate receptor-deficient synapses. This study thus reveals a regulatory mechanism for synaptic homeostasis.

Circadian regulation in the retina: From molecules to network

The mammalian retina is the most unique tissue among those that display robust circadian/diurnal oscillations. The retina is not only a light sensing tissue that relays light information to the brain, it has its own circadian "system" independent from any influence from other circadian oscillators. While all retinal cells and retinal pigment epithelium (RPE) possess circadian oscillators, these oscillators integrate by means of neural synapses, electrical coupling (gap junctions), and released neurochemicals (such as dopamine, melatonin, adenosine, and ATP), so the whole retina functions as an integrated circadian system. Dysregulation of retinal clocks not only causes retinal or ocular diseases, it also impacts the circadian rhythm of the whole body, as the light information transmitted from the retina entrains the brain clock that governs the body circadian rhythms. In this review, how circadian oscillations in various retinal cells are integrated, and how retinal diseases affect daily rhythms.

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.

Angiotensin-Receptor-Associated Protein Modulates Ca2+ Signals in Photoreceptor and Mossy Fiber cells

Fast, precise and sustained neurotransmission requires graded Ca²⁺ signals at the presynaptic terminal. Neurotransmitter release depends on a complex interplay of Ca²⁺ fluxes and Ca²⁺ buffering in the presynaptic terminal that is not fully understood. Here, we show that the angiotensin-receptor-associated protein (ATRAP) localizes to synaptic terminals throughout the central nervous system. In the retinal photoreceptor synapse and the cerebellar mossy fiber-granule cell synapse, we find that ATRAP is involved in the generation of depolarization-evoked synaptic Ca²⁺ transients. Compared to wild type, Ca²⁺ imaging in acutely isolated preparations of the retina and the cerebellum from ATRAP knockout mice reveals a significant reduction of the sarcoendoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA) activity. Thus, in addition to its conventional role in angiotensin signaling, ATRAP also modulates presynaptic Ca²⁺ signaling within the central nervous system.

Circadian clock regulation of cone to horizontal cell synaptic transfer in the goldfish retina

Although it is well established that the vertebrate retina contains endogenous circadian clocks that regulate retinal physiology and function during day and night, the processes that the clocks affect and the means by which the clocks control these processes remain unresolved. We previously demonstrated that a circadian clock in the goldfish retina regulates rod-cone electrical coupling so that coupling is weak during the day and robust at night. The increase in rod-cone coupling at night introduces rod signals into cones so that the light responses of both cones and cone horizontal cells, which are post-synaptic to cones, become dominated by rod input. By comparing the light responses of cones, cone horizontal cells and rod horizontal cells, which are post-synaptic to rods, under dark-adapted conditions during day and night, we determined whether the daily changes in the strength of rod-cone coupling could account entirely for rhythmic changes in the light response properties of cones and cone horizontal cells. We report that although some aspects of the day/night changes in cone and cone horizontal cell light responses, such as response threshold and spectral tuning, are consistent with modulation of rod-cone coupling, other properties cannot be solely explained by this phenomenon. Specifically, we found that at night compared to the day the time course of spectrally-isolated cone photoresponses was slower, cone-to-cone horizontal cell synaptic transfer was highly non-linear and of lower gain, and the delay in cone-to-cone horizontal cell synaptic transmission was longer. However, under bright light-adapted conditions in both day and night, cone-to-cone horizontal cell synaptic transfer was linear and of high gain, and no additional delay was observed at the cone-to-cone horizontal cell synapse. These findings suggest that in addition to controlling rod-cone coupling, retinal clocks shape the light responses of cone horizontal cells by modulating cone-to-cone horizontal cell synaptic transmission.

Mapping developmental maturation of inner hair cell ribbon synapses in the apical mouse cochlea

Ribbon synapses of cochlear inner hair cells (IHCs) undergo molecular assembly and extensive functional and structural maturation before hearing onset. Here, we characterized the nanostructure of IHC synapses from late prenatal mouse embryo stages (embryonic days 14-18) into adulthood [postnatal day (P)48] using electron microscopy and tomography as well as optical nanoscopy of apical turn organs of Corti. We find that synaptic ribbon precursors arrive at presynaptic active zones (AZs) after afferent contacts have been established. These ribbon precursors contain the proteins RIBEYE and piccolino, tether synaptic vesicles and their delivery likely involves active, microtubule-based transport pathways. Synaptic contacts undergo a maturational transformation from multiple small to one single, large AZ. This maturation is characterized by the fusion of ribbon precursors with membrane-anchored ribbons that also appear to fuse with each other. Such fusion events are most frequently encountered around P12 and hence, coincide with hearing onset in mice. Thus, these events likely underlie the morphological and functional maturation of the AZ. Moreover, the postsynaptic densities appear to undergo a similar refinement alongside presynaptic maturation. Blockwise addition of ribbon material by fusion as found during AZ maturation might represent a general mechanism for modulating ribbon size.

Quantitative and Qualitative Evaluation of Photoreceptor Synapses in Developing, Degenerating and Regenerating Retinas

Quantitative and qualitative evaluation of synapses is crucial to understand neural connectivity. This is particularly relevant now, in view of the recent advances in regenerative biology and medicine. There is an urgent need to evaluate synapses to access the extent and functionality of reconstructed neural network. Most of the currently used synapse evaluation methods provide only all-or-none assessments. However, very often synapses appear in a wide spectrum of transient states such as during synaptogenesis or neural degeneration. Robust evaluation of synapse quantity and quality is therefore highly sought after. In this paper we introduce QUANTOS, a new method that can evaluate the number, likelihood, and maturity of photoreceptor ribbon synapses based on graphical properties of immunohistochemistry images. QUANTOS is composed of ImageJ Fiji macros, and R scripts which are both open-source and free software. We used QUANTOS to evaluate synaptogenesis in developing and degenerating retinas, as well as de novo synaptogenesis of mouse iPSC-retinas after transplantation to a retinal degeneration mouse model. Our analysis shows that while mouse iPSC-retinas are largely incapable of forming synapses in vitro, they can form extensive synapses following transplantation. The de novo synapses detected after transplantation seem to be in an intermediate state between mature and immature compared to wildtype retina. Furthermore, using QUANTOS we tested whether environmental light can affect photoreceptor synaptogenesis. We found that the onset of synaptogenesis was earlier under cyclic light (LD) condition when compared to constant dark (DD), resulting in more synapses at earlier developmental stages. The effect of light was also supported by micro electroretinography showing larger responses under LD condition. The number of synapses was also increased after transplantation of mouse iPSC-retinas to rd1 mice under LD condition. Our new probabilistic assessment of synapses may prove to be a valuable tool to gain critical insights into neural-network reconstruction and help develop treatments for neurodegenerative disorders.

Coding-in-Noise Deficits are Not Seen in Responses to Amplitude Modulation in Subjects with cochlear Synaptopathy Induced by a Single Noise Exposure

Since the first report of noise-induced synaptic damage in animals without permanent threshold shifts (PTSs), the concept of noise-induced hidden hearing loss (NIHHL) has been proposed to cover the functional deficits in hearing associated with noise-induced synaptopathy. Moreover, the potential functional deficit associated with the noise-induced synaptopathy has been largely attributed to the loss of auditory nerve fibers (ANFs) with a low spontaneous spike rate (SSR). As this group of ANFs is critical for coding at suprathreshold levels and in noisy background, coding-in-noise deficit (CIND) has been considered to be main consequence of the synaptopathy. However, such deficits have not been verified after a single, brief exposure to noise without PTS. In the present study, synaptopathy was generated by such noise exposure in both mice and guinea pigs. Responses to amplitude modulation (AM) were recorded at a high sound level in combination with masking to evaluate the existence of CINDs that might be associated with loss of low-SSR ANFs. An overall reduction in response amplitude was seen in AM-evoked compound action potential (CAP). However, no such reduction was seen in the scalp-recorded envelope following response (EFR), suggesting a compensation due to increased central gain. Moreover, there was no significant difference in masking effect between the control and noise groups. The results suggest that either there is no significant CIND after the synaptopathy we created, or the AM response tested with our protocol was not sufficiently sensitive to detect such a deficit; far-field EFR is not sensitive to cochlear pathology.

Noise-Induced Cochlear Synaptopathy and Ribbon Synapse Regeneration: Repair Process and Therapeutic Target

The synapse between the inner hair cells (IHCs) and the spiral ganglion neurons (SGNs) in mammalian cochleae is characterized as having presynaptic ribbons and therefore is called ribbon synapse. The special molecular organization is reviewed in this chapter in association with the functional feature of this synapse in signal processing. This is followed by the review on noise-induced damage to this synapse with a focus on recent reports in animal models in which the effect of brief noise exposures is observed without causing significant permanent threshold shift (PTS). In this regard, the potential mechanism of the synaptic damage by noise and the impact of this damage on hearing are summarized to clarify the concept of noise-induced hidden hearing loss, which is defined as the functional deficits in hearing without threshold elevation. A controversial issue is addressed in this review as whether the disrupted synapses can be regenerated. Moreover, the review summarizes the work of therapeutic research to protect the synapses or to promote the regeneration of the synapse after initial disruption. Lastly, several unresolved issues are raised for investigation in the future.

Noise-induced Cochlear Synaptopathy and Signal Processing Disorders

Noise-induced hidden hearing loss (NIHHL) has attracted great attention in hearing research and clinical audiology since the discovery of significant noise-induced synaptic damage in the absence of permanent threshold shifts (PTS) in animal models. Although the extant evidence for this damage is based on animal models, NIHHL likely occurs in humans as well. This review focuses on three issues concerning NIHHL that are somewhat controversial: (1) whether disrupted synapses can be re-established; (2) whether synaptic damage and repair are responsible for the initial temporal threshold shifts (TTS) and subsequent recovery; and (3) the relationship between the synaptic damage and repair processes and neural coding deficits. We conclude that, after a single, brief noise exposure, (1) the damaged and the totally destroyed synapses can be partially repaired, but the repaired synapses are functionally abnormal; (2) While deficits are observed in some aspects of neural responses related to temporal and intensity coding in the auditory nerve, we did not find strong evidence for hypothesized coding-in-noise deficits; (3) the sensitivity and the usefulness of the envelope following responses to amplitude modulation signals in detecting cochlear synaptopathy is questionable.

α2δ-4 Is Required for the Molecular and Structural Organization of Rod and Cone Photoreceptor Synapses

α2δ-4 is an auxiliary subunit of voltage-gated Cav1.4 L-type channels that regulate the development and mature exocytotic function of the photoreceptor ribbon synapse. In humans, mutations in the CACNA2D4 gene encoding α2δ-4 cause heterogeneous forms of vision impairment in humans, the underlying pathogenic mechanisms of which remain unclear. To investigate the retinal function of α2δ-4, we used genome editing to generate an α2δ-4 knock-out (α2δ-4 KO) mouse. In male and female α2δ-4 KO mice, rod spherules lack ribbons and other synaptic hallmarks early in development. Although the molecular organization of cone synapses is less affected than rod synapses, horizontal and cone bipolar processes extend abnormally in the outer nuclear layer in α2δ-4 KO retina. In reconstructions of α2δ-4 KO cone pedicles by serial block face scanning electron microscopy, ribbons appear normal, except that less than one-third show the expected triadic organization of processes at ribbon sites. The severity of the synaptic defects in α2δ-4 KO mice correlates with a progressive loss of Cav1.4 channels, first in terminals of rods and later cones. Despite the absence of b-waves in electroretinograms, visually guided behavior is evident in α2δ-4 KO mice and better under photopic than scotopic conditions. We conclude that α2δ-4 plays an essential role in maintaining the structural and functional integrity of rod and cone synapses, the disruption of which may contribute to visual impairment in humans with CACNA2D4 mutations.SIGNIFICANCE STATEMENT In the retina, visual information is first communicated by the synapse formed between photoreceptors and second-order neurons. The mechanisms that regulate the structural integrity of this synapse are poorly understood. Here we demonstrate a role for α2δ-4, a subunit of voltage-gated Ca2+ channels, in organizing the structure and function of photoreceptor synapses. We find that presynaptic Ca2+ channels are progressively lost and that rod and cone synapses are disrupted in mice that lack α2δ-4. Our results suggest that alterations in presynaptic Ca2+ signaling and photoreceptor synapse structure may contribute to vision impairment in humans with mutations in the CACNA2D4 gene encoding α2δ-4.

The synaptic ribbon is critical for sound encoding at high rates and with temporal precision

We studied the role of the synaptic ribbon for sound encoding at the synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) in mice lacking RIBEYE (RBE^KO/KO). Electron and immunofluorescence microscopy revealed a lack of synaptic ribbons and an assembly of several small active zones (AZs) at each synaptic contact. Spontaneous and sound-evoked firing rates of SGNs and their compound action potential were reduced, indicating impaired transmission at ribbonless IHC-SGN synapses. The temporal precision of sound encoding was impaired and the recovery of SGN-firing from adaptation indicated slowed synaptic vesicle (SV) replenishment. Activation of Ca²⁺-channels was shifted to more depolarized potentials and exocytosis was reduced for weak depolarizations. Presynaptic Ca²⁺-signals showed a broader spread, compatible with the altered Ca²⁺-channel clustering observed by super-resolution immunofluorescence microscopy. We postulate that RIBEYE disruption is partially compensated by multi-AZ organization. The remaining synaptic deficit indicates ribbon function in SV-replenishment and Ca²⁺-channel regulation.

Our sense of hearing relies on our ears quickly and tirelessly processing information in a precise manner. Sounds cause vibrations in a part of the inner ear called the cochlea. Inside the cochlea, the vibrations move hair-like structures on sensory cells that translate these movements into electrical signals. These hair cells are connected to specialized nerve cells that relay the signals to the brain, which then interprets them as sounds.

Hair cells communicate with the specialized nerve cells via connections known as chemical synapses. This means that the electrical signals in the hair cell activate channel proteins that allow calcium ions to flow in. This in turn triggers membrane-bound packages called vesicles inside the hair cell to fuse with its surface membrane and release their contents to the outside. The contents, namely chemicals called neurotransmitters, then travels across the space between the cells, relaying the signal to the nerve cell.

The junctions between the hair cells and the nerve cells are more specifically known as ribbon synapses. This is because they have a ribbon-like structure that appears to tether a halo of vesicles close to the active zone where neurotransmitters are released. However, the exact role of this synaptic ribbon has remained mysterious despite decades of study.

The ribbon is mainly composed of a protein called Ribeye, and now Jean, Lopez de la Morena, Michanski, Jaime Tobón et al. show that mutant mice that lack this protein do not have any ribbons at their “ribbon synapses”. Hair cells without synaptic ribbons are less able to timely and reliably send signals to the nerve cells, most likely because they cannot replenish the vesicles at the synapse quickly enough. Further analysis showed that the synaptic ribbon also helps to regulate the calcium channels at the synapse, which is important for linking the electrical signals in the hair cell to the release of the neurotransmitters.

Jean et al. also saw that hair cells without ribbons reorganize their synapses to form multiple active zones that could transfer neurotransmitter to the nerve cells. This could partially compensate for the loss of the ribbons, meaning the impact of their loss may have been underestimated. Future studies could explore this by eliminating the Ribeye protein only after the ribbon synapses are fully formed.

These findings may help scientists to better understand deafness and other hearing disorders in humans. They will also be of interest to neuroscientists who research synapses, hearing and other sensory processes.

Functional Roles of Complexin 3 and Complexin 4 at Mouse Photoreceptor Ribbon Synapses

Complexins (Cplxs) are SNARE complex regulators controlling the speed and Ca(2+) sensitivity of SNARE-mediated synaptic vesicle fusion. We have shown previously that photoreceptor ribbon synapses in mouse retina are equipped with Cplx3 and Cplx4 and that lack of both Cplxs perturbs photoreceptor ribbon synaptic function; however, Cplx3/4 function in photoreceptor synaptic transmission remained elusive. To investigate Cplx3/4 function in photoreceptor ribbon synapses, voltage-clamp recordings from postsynaptic horizontal cells were performed in horizontal slice preparations of Cplx3/4 wild-type (WT) and Cplx3/4 double knock-out (DKO) mice. We measured tonic activity in light and dark, current responses to changes in luminous intensity, and electrically evoked postsynaptic responses. Cplx3/4 decreased the frequency of tonic events and shifted their amplitude distribution to smaller values. Light responses were sustained in the presence of Cplx3/4, but transient in their absence. Finally, Cplx3/4 increased synaptic vesicle release evoked by electrical stimulation. Using electron microscopy, we quantified the number of synaptic vesicles at presynaptic ribbons after light or dark adaptation. In Cplx3/4 WT photoreceptors, the number of synaptic vesicles associated with the ribbon base close to the release site was significantly lower in light than in dark. This is in contrast to Cplx3/4 DKO photoreceptors, in which the number of ribbon-associated synaptic vesicles remained unchanged regardless of the adaptational state. Our results indicate a suppressing and a facilitating action of Cplx3/4 on Ca(2+)-dependent tonic and evoked neurotransmitter release, respectively, and a regulatory role in the adaptation-dependent availability of synaptic vesicles for release at photoreceptor ribbon synapses.

SIGNIFICANCE STATEMENT

Synaptic vesicle fusion at active zones of chemical synapses is executed by SNARE complexes. Complexins (Cplxs) are SNARE complex regulators and photoreceptor ribbon synapses are equipped with Cplx3 and Cplx4. The absence of both Cplxs perturbs ribbon synaptic function. Because we lack information on Cplx function in photoreceptor synaptic transmission, we investigated Cplx function using voltage-clamp recordings from postsynaptic horizontal cells of Cplx3/4 wild-type and Cplx3/4 double knock-out mice and quantified synaptic vesicle number at the ribbon after light and dark adaptation using electron microscopy. The findings reveal a suppressing action of Cplx3/4 on tonic neurotransmitter release, a facilitating action on evoked release, and a regulatory role of Cplx3/4 in the adaptation-dependent availability of synaptic vesicles at mouse photoreceptor ribbon synapses.

Signal transduction and signal transmission

Vision begins in highly specialized light-sensing neurons, the rod and cone photoreceptors. Their task is to absorb photons, transduce the physical stimulus into neuronal sig­nals, transmit the signals to the parallel signal processing pathways of the subsequent reti­nal network with the highest possible fidelity and continuously adapt to changes in stim­ulus intensities. If you imagine a pitch-black night with only a few photons hitting the ret­ina and being absorbed by the photoreceptors and a bright sunny day with the photore­ceptors being bombarded by billions of photons, you realize that a photoreceptor faces two fundamental challenges: it has to detect the light signal with the greatest sensitivity, e.g. a single photon leads to a change in the membrane potential of a rod photoreceptor and, at the same time, encode light intensities covering a broad dynamic range of sev­eral orders of magnitude. To fulfill these demands, photoreceptors have developed separate, structurally and functionally specialized compartments, which are the topic of this article: the outer segment for signal transduc­tion and the terminal with its highly complex ribbon synapse for signal transmission.

Presynaptic localization of GluK5 in rod photoreceptors suggests a novel function of high affinity glutamate receptors in the mammalian retina

Kainate receptors mediate glutamatergic signaling through both pre- and presynaptic receptors. Here, we studied the expression of the high affinity kainate receptor GluK5 in the mouse retina. Double-immunofluoresence labeling and electron microscopic analysis revealed a presynaptic localization of GluK5 in the outer plexiform layer. Unexpectedly, we found GluK5 almost exclusively localized to the presynaptic ribbon of photoreceptor terminals. Moreover, in GluK5-deficient mutant mice the structural integrity of synaptic ribbons was severely altered pointing to a novel function of GluK5 in organizing synaptic ribbons in the presynaptic terminals of rod photoreceptors.

Cochlear Synaptopathy and Noise-Induced Hidden Hearing Loss

Recent studies on animal models have shown that noise exposure that does not lead to permanent threshold shift (PTS) can cause considerable damage around the synapses between inner hair cells (IHCs) and type-I afferent auditory nerve fibers (ANFs). Disruption of these synapses not only disables the innervated ANFs but also results in the slow degeneration of spiral ganglion neurons if the synapses are not reestablished. Such a loss of ANFs should result in signal coding deficits, which are exacerbated by the bias of the damage toward synapses connecting low-spontaneous-rate (SR) ANFs, which are known to be vital for signal coding in noisy background. As there is no PTS, these functional deficits cannot be detected using routine audiological evaluations and may be unknown to subjects who have them. Such functional deficits in hearing without changes in sensitivity are generally called “noise-induced hidden hearing loss (NIHHL).” Here, we provide a brief review to address several critical issues related to NIHHL: (1) the mechanism of noise induced synaptic damage, (2) reversibility of the synaptic damage, (3) the functional deficits as the nature of NIHHL in animal studies, (4) evidence of NIHHL in human subjects, and (5) peripheral and central contribution of NIHHL.

Synaptic Vesicle Proteins and Active Zone Plasticity

Neurotransmitter is released from synaptic vesicles at the highly specialized presynaptic active zone (AZ). The complex molecular architecture of AZs mediates the speed, precision and plasticity of synaptic transmission. Importantly, structural and functional properties of AZs vary significantly, even for a given connection. Thus, there appear to be distinct AZ states, which fundamentally influence neuronal communication by controlling the positioning and release of synaptic vesicles. Vice versa, recent evidence has revealed that synaptic vesicle components also modulate organizational states of the AZ. The protein-rich cytomatrix at the active zone (CAZ) provides a structural platform for molecular interactions guiding vesicle exocytosis. Studies in Drosophila have now demonstrated that the vesicle proteins Synaptotagmin-1 (Syt1) and Rab3 also regulate glutamate release by shaping differentiation of the CAZ ultrastructure. We review these unexpected findings and discuss mechanistic interpretations of the reciprocal relationship between synaptic vesicles and AZ states, which has heretofore received little attention.

BRP-170 and BRP190 isoforms of Bruchpilot protein differentially contribute to the frequency of synapses and synaptic circadian plasticity in the visual system of Drosophila

In the first optic neuropil (lamina) of the optic lobe of Drosophila melanogaster, two classes of synapses, tetrad and feedback, show daily rhythms in the number and size of presynaptic profiles examined at the level of transmission electron microscopy (TEM). Number of tetrad presynaptic profiles increases twice a day, once in the morning and again in the evening, and their presynaptic ribbons are largest in the evening. In contrast, feedback synapses peak at night. The frequency of synapses is correlated with size of the presynaptic element measured as the platform size of so-called T-bars, with T-bar platforms being largest with increasing synapse frequency. The large scaffold protein Bruchpilot (BRP) is a major essential constituent of T-bars, with two major isoforms of 190 and 170 kD forming T-bars of the peripheral neuromuscular junctions (NMJ) synapses and in the brain. In addition to the analysis of cyclic plasticity of tetrad and feedback synapses in wild-type flies, we used TEM to examine daily changes in the size and distribution of synapses within isoform-specific BRP mutants, expressing BRP-190 (BRPΔ170) or BRP-170 (BRPΔ190) only. We found that the number and circadian plasticity of synapses depends on both isoforms. In the BRPΔ190 lacking BRP-190 there was almost 50% less tetrad synapses demonstrable than when both isoforms were present. The lack of BRP-170 and BRP-190 increased and decreased, respectively the number of feedback synapses, indicating that BRP-190 forms most of the feedback synapses. In both mutants, the daily plasticity of tetrad and feedback presynaptic profiles was abolished, except for feedback synapses in BRPΔ190. The oscillations in the number and size of presynaptic elements seem to depend on a different contribution of BRP isoforms in a presynaptic element at different time during the day and night and at various synapse types. The participation of both BRP isoforms may vary in different classes of synapses.

Molecular Remodeling of the Presynaptic Active Zone of Drosophila Photoreceptors via Activity-Dependent Feedback

Neural activity contributes to the regulation of the properties of synapses in sensory systems, allowing for adjustment to a changing environment. Little is known about how synaptic molecular components are regulated to achieve activity-dependent plasticity at central synapses. Here, we found that after prolonged exposure to natural ambient light the presynaptic active zone in Drosophila photoreceptors undergoes reversible remodeling, including loss of Bruchpilot, DLiprin-α, and DRBP, but not of DSyd-1 or Cacophony. The level of depolarization of the postsynaptic neurons is critical for the light-induced changes in active zone composition in the photoreceptors, indicating the existence of a feedback signal. In search of this signal, we have identified a crucial role of microtubule meshwork organization downstream of the divergent canonical Wnt pathway, potentially via Kinesin-3 Imac. These data reveal that active zone composition can be regulated in vivo and identify the underlying molecular machinery.

Characterization of Ribeye subunits in zebrafish hair cells reveals that exogenous Ribeye B-domain and CtBP1 localize to the basal ends of synaptic ribbons

Synaptic ribbons are presynaptic structures formed by the self-association of RIBEYE–the main structural component of ribbon synapses. RIBEYE consists of two domains: a unique N-terminal A-domain and a C-terminal B-domain that is identical to the transcription co-repressor C-terminal binding protein 2 (CtBP2). Previous studies in cell lines have shown that RIBEYE A-domain alone is sufficient to form ribbon-like aggregates and that both A- and B- domains form homo-and heterotypic interactions. As these interactions are likely the basis for synaptic-ribbon assembly and structural plasticity, we wanted to examine how zebrafish Ribeye A- and B- domains interact with synaptic ribbons in vivo. To that end, we characterized the localization of exogenously expressed Ribeye A- and B- domains and the closely related protein, CtBP1, in the hair cells of transgenic zebrafish larvae. Unexpectedly, exogenously expressed Ribeye A-domain showed variable patterns of localization in hair cells; one zebrafish paralog of A-domain failed to self-associate or localize to synaptic ribbons, while the other self-assembled but sometimes failed to localize to synaptic ribbons. By contrast, Ribeye B-domain/CtBP2 was robustly localized to synaptic ribbons. Moreover, both exogenously expressed B-domain/CtBP2 and CtBP1 were preferentially localized to the basal end of ribbons adjacent to the postsynaptic density. Overexpression of B-domain/CtBP2 also appeared to affect synaptic-ribbon composition; endogenous levels of ribbon-localized Ribeye were significantly reduced as hair cells matured in B-domain/CtBP2 transgenic larvae compared to wild-type. These results reveal how exogenously expressed Ribeye domains interact with synaptic ribbons, and suggest a potential organization of elements within the ribbon body.

In vivo knockdown of Piccolino disrupts presynaptic ribbon morphology in mouse photoreceptor synapses

Piccolo is the largest known cytomatrix protein at active zones of chemical synapses. A growing number of studies on conventional chemical synapses assign Piccolo a role in the recruitment and integration of molecules relevant for both endo- and exocytosis of synaptic vesicles, the dynamic assembly of presynaptic F-actin, as well as the proteostasis of presynaptic proteins, yet a direct function in the structural organization of the active zone has not been uncovered in part due to the expression of multiple alternatively spliced isoforms. We recently identified Piccolino, a Piccolo splice variant specifically expressed in sensory ribbon synapses of the eye and ear. Here we down regulated Piccolino in vivo via an adeno-associated virus-based RNA interference approach and explored the impact on the presynaptic structure of mouse photoreceptor ribbon synapses. Detailed immunocytochemical light and electron microscopical analysis of Piccolino knockdown in photoreceptors revealed a hitherto undescribed photoreceptor ribbon synaptic phenotype with striking morphological changes of synaptic ribbon ultrastructure.

Presynaptic [Ca(2+)] and GCAPs: aspects on the structure and function of photoreceptor ribbon synapses

Changes in intracellular calcium ions [Ca²⁺] play important roles in photoreceptor signaling. Consequently, intracellular [Ca²⁺] levels need to be tightly controlled. In the light-sensitive outer segments (OS) of photoreceptors, Ca²⁺ regulates the activity of retinal guanylate cyclases thus playing a central role in phototransduction and light-adaptation by restoring light-induced decreases in cGMP. In the synaptic terminals, changes of intracellular Ca²⁺ trigger various aspects of neurotransmission. Photoreceptors employ tonically active ribbon synapses that encode light-induced, graded changes of membrane potential into modulation of continuous synaptic vesicle exocytosis. The active zones of ribbon synapses contain large electron-dense structures, synaptic ribbons, that are associated with large numbers of synaptic vesicles. Synaptic coding at ribbon synapses differs from synaptic coding at conventional (phasic) synapses. Recent studies revealed new insights how synaptic ribbons are involved in this process. This review focuses on the regulation of [Ca²⁺] in presynaptic photoreceptor terminals and on the function of a particular Ca²⁺-regulated protein, the neuronal calcium sensor protein GCAP2 (guanylate cyclase-activating protein-2) in the photoreceptor ribbon synapse. GCAP2, an EF-hand-containing protein plays multiple roles in the OS and in the photoreceptor synapse. In the OS, GCAP2 works as a Ca²⁺-sensor within a Ca²⁺-regulated feedback loop that adjusts cGMP levels. In the photoreceptor synapse, GCAP2 binds to RIBEYE, a component of synaptic ribbons, and mediates Ca²⁺-dependent plasticity at that site. Possible mechanisms are discussed.

Developmental refinement of hair cell synapses tightens the coupling of Ca2+ influx to exocytosis

Cochlear inner hair cells (IHCs) develop from pre-sensory pacemaker to sound transducer. Here, we report that this involves changes in structure and function of the ribbon synapses between IHCs and spiral ganglion neurons (SGNs) around hearing onset in mice. As synapses matured they changed from holding several small presynaptic active zones (AZs) and apposed postsynaptic densities (PSDs) to one large AZ/PSD complex per SGN bouton. After the onset of hearing (i) IHCs had fewer and larger ribbons; (ii) CaV1.3 channels formed stripe-like clusters rather than the smaller and round clusters at immature AZs; (iii) extrasynaptic CaV1.3-channels were selectively reduced, (iv) the intrinsic Ca(2)(+) dependence of fast exocytosis probed by Ca(2)(+) uncaging remained unchanged but (v) the apparent Ca(2)(+) dependence of exocytosis linearized, when assessed by progressive dihydropyridine block of Ca(2)(+) influx. Biophysical modeling of exocytosis at mature and immature AZ topographies suggests that Ca(2)(+) influx through an individual channel dominates the [Ca(2)(+)] driving exocytosis at each mature release site. We conclude that IHC synapses undergo major developmental refinements, resulting in tighter spatial coupling between Ca(2)(+) influx and exocytosis.

Zebrafish Cacna1fa is required for cone photoreceptor function and synaptic ribbon formation

Mutations in the human CACNA1F gene cause incomplete congenital stationary night blindness type 2 (CSNB2), a non-progressive, clinically heterogeneous retinal disorder. However, the molecular mechanisms underlying CSNB2 have not been fully explored. Here, we describe the positional cloning of a blind zebrafish mutant, wait until dark (wud), which encodes a zebrafish homolog of human CACNA1F. We identified two zebrafish cacna1f paralogs and showed that the cacna1fa transcript (the gene mutated in wud) is expressed exclusively in the photoreceptor layer. We demonstrated that Cacna1fa localizes at the photoreceptor synapse and is absent from wud mutants. Electroretinograms revealed abnormal cone photoreceptor responses from wud mutants, indicating a defect in synaptic transmission. Although there are no obvious morphological differences, we found that wud mutants lacked synaptic ribbons and that wud is essential for the development of synaptic ribbons. We found that Ribeye, the most prominent synaptic ribbon protein, was less abundant and mislocalized in adult wud mutants. In addition to cloning wud, we identified synaptojanin 1 (synj1) as the defective gene in slacker (slak), a blind mutant with floating synaptic ribbons. We determined that Cacna1fa was expressed in slak photoreceptors and that Synj1 was initially expressed wud photoreceptors, but was absent by 5 days postfertilization. Collectively, our data demonstrate that Cacna1fa is essential for cone photoreceptor function and synaptic ribbon formation and reveal a previously unknown yet critical role of L-type voltage-dependent calcium channels in the expression and/or distribution of synaptic ribbon proteins, providing a new model to study the clinical variability in human CSNB2 patients.

P-Rex2, a Rac-guanine nucleotide exchange factor, is expressed selectively in ribbon synaptic terminals of the mouse retina

Background

Phosphatidylinositol (3,4,5)-trisphosphate-dependent Rac Exchanger 2 (P-Rex2) is a guanine nucleotide exchange factor (GEF) that specifically activates Rac GTPases, important regulators of actin cytoskeleton remodeling. P-Rex2 is known to modulate cerebellar Purkinje cell architecture and function, but P-Rex2 expression and function elsewhere in the central nervous system is unclear. To better understand potential roles for P-Rex2 in neuronal cytoskeletal remodeling and function, we performed widefield and confocal microscopy of specimens double immunolabeled for P-Rex2 and cell- and synapse-specific markers in the mouse retina.

Results

P-Rex2 was restricted to the plexiform layers of the retina and colocalized extensively with Vesicular Glutamate Transporter 1 (VGluT1), a specific marker for photoreceptor and bipolar cell terminals. Double labeling for P-Rex2 and peanut agglutinin, a cone terminal marker, confirmed that P-Rex2 was present in both rod and cone terminals. Double labeling with markers for specific bipolar cell types showed that P-Rex2 was present in the terminals of rod bipolar cells and multiple ON- and OFF-cone bipolar cell types. In contrast, P-Rex2 was not expressed in the processes or conventional synapses of amacrine or horizontal cells.

Conclusions

P-Rex2 is associated specifically with the glutamatergic ribbon synaptic terminals of photoreceptors and bipolar cells that transmit visual signals vertically through the retina. The Rac-GEF function of P-Rex2 implies a specific role for P-Rex2 and Rac-GTPases in regulating the actin cytoskeleton in glutamatergic ribbon synaptic terminals of retinal photoreceptors and bipolar cells and appears to be ideally positioned to modulate the adaptive plasticity of these terminals.

Disruption of the presynaptic cytomatrix protein bassoon degrades ribbon anchorage, multiquantal release, and sound encoding at the hair cell afferent synapse

Inner hair cells (IHCs) of the cochlea use ribbon synapses to transmit auditory information faithfully to spiral ganglion neurons (SGNs). In the present study, we used genetic disruption of the presynaptic scaffold protein bassoon in mice to manipulate the morphology and function of the IHC synapse. Although partial-deletion mutants lacking functional bassoon (Bsn(ΔEx4/5)) had a near-complete loss of ribbons from the synapses (up to 88% ribbonless synapses), gene-trap mutants (Bsn(gt)) showed weak residual expression of bassoon and 56% ribbonless synapses, whereas the remaining 44% had a loosely anchored ribbon. Patch-clamp recordings and synaptic CaV1.3 immunolabeling indicated a larger number of Ca(2+) channels for Bsn(gt) IHCs compared with Bsn(ΔEx4/5) IHCs and for Bsn(gt) ribbon-occupied versus Bsn(gt) ribbonless synapses. An intermediate phenotype of Bsn(gt) IHCs was also found by membrane capacitance measurements for sustained exocytosis, but not for the size of the readily releasable vesicle pool. The frequency and amplitude of EPSCs were reduced in Bsn(ΔEx4/5) mouse SGNs, whereas their postsynaptic AMPA receptor clusters were largely unaltered. Sound coding in SGN, assessed by recordings of single auditory nerve fibers and their population responses in vivo, was similarly affected in Bsn(gt) and Bsn(ΔEx4/5) mice. Both genotypes showed impaired sound onset coding and reduced evoked and spontaneous spike rates. In summary, reduced bassoon expression or complete lack of full-length bassoon impaired sound encoding to a similar extent, which is consistent with the comparable reduction of the readily releasable vesicle pool. This suggests that the remaining loosely anchored ribbons in Bsn(gt) IHCs were functionally inadequate or that ribbon independent mechanisms dominated the coding deficit.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Properties of ribbon and non-ribbon release from rod photoreceptors revealed by visualizing individual synaptic vesicles

Vesicle release from rod photoreceptors is regulated by Ca(2+) entry through L-type channels located near synaptic ribbons. We characterized sites and kinetics of vesicle release in salamander rods by using total internal reflection fluorescence microscopy to visualize fusion of individual synaptic vesicles. A small number of vesicles were loaded by brief incubation with FM1-43 or a dextran-conjugated, pH-sensitive form of rhodamine, pHrodo. Labeled organelles matched the diffraction-limited size of fluorescent microspheres and disappeared rapidly during stimulation. Consistent with fusion, depolarization-evoked vesicle disappearance paralleled electrophysiological release kinetics and was blocked by inhibiting Ca(2+) influx. Rods maintained tonic release at resting membrane potentials near those in darkness, causing depletion of membrane-associated vesicles unless Ca(2+) entry was inhibited. This depletion of release sites implies that sustained release may be rate limited by vesicle delivery. During depolarizing stimulation, newly appearing vesicles approached the membrane at ∼800 nm/s, where they paused for ∼60 ms before fusion. With fusion, vesicles advanced ∼18 nm closer to the membrane. Release events were concentrated near ribbons, but lengthy depolarization also triggered release from more distant non-ribbon sites. Consistent with greater contributions from non-ribbon sites during lengthier depolarization, damaging the ribbon by fluorophore-assisted laser inactivation (FALI) of Ribeye caused only weak inhibition of exocytotic capacitance increases evoked by 200-ms depolarizing test steps, whereas FALI more strongly inhibited capacitance increases evoked by 25 ms steps. Amplifying release by use of non-ribbon sites when rods are depolarized in darkness may improve detection of decrements in release when they hyperpolarize to light.

Strain differences in illumination-dependent structural changes at mouse photoreceptor ribbon synapses

Photoreceptor cells encode light signals over a wide range of intensities with graded changes in their membrane potential. At their highly specialized ribbon synapses they transmit the signals to the postsynaptic neurons by the tonic release of glutamate, which is continuously adjusted to changes in light intensity. Such a level of performance requires adaptive mechanisms, and it is suggested that illumination-dependent changes in ribbon shape and size are one of these adaptive processes. In this study we compared structural properties of synaptic ribbons under various illumination conditions between three mouse strains: the pigmented C57BL/6 and the two albino strains Balb/c and B6(Cg)-Tyr(c-2J) /J (coisogenic to C57BL/6). In addition, electroretinograms (ERGs) recorded in the same groups were compared. In the C57BL/6 mouse a change in illumination did not result in structural alterations of the synaptic ribbon. Similarly, in the B6(Cg)-Tyr(c-2J) /J mouse only minor structural changes were detected. In contrast, the state of adaptation had a large influence on the ribbon structure of the Balb/c mouse. The ERG recordings showed only small functional differences between C57BL/6 and B6(Cg)-Tyr(c-2J) /J mice, but the retinal function of Balb/c mice was strongly compromised. We conclude that illumination-dependent changes of photoreceptor ribbon structure differ between strains and thus cannot be regarded as a general mechanism for light adaptation.

Oral proton pump inhibitors disrupt horizontal cell-cone feedback and enhance visual hallucinations in macular degeneration patients

PURPOSE

Visual hallucinations (VHs) occur in macular degeneration patients with poor vision but normal cognitive function. The underlying mechanisms are poorly understood. We report the identification of pharmaceutical agents that enhance VH and use these agents to examine the contribution of retinal neurons to this syndrome.

METHODS

We detail clinical observations on VH in five macular degeneration patients treated with proton pump inhibitors having the core structure, 2-pyridyl-methylsulfinyl-benzimidazole. We tested possible retinal mechanisms using paired whole cell recordings to examine effects of these compounds on feedback interactions between horizontal cells and cones in amphibian retina.

RESULTS

Five patients with advanced wet macular degeneration described patterned VHs that were induced or enhanced by oral proton pump inhibitors. The abnormal images increased with light, disappeared in the dark, and originated in the retina, based on ophthalmodynamometry. Simultaneous paired whole cell recordings from amphibian cones and horizontal cells showed that 2-pyridyl-methylsulfinyl-benzimidazoles blocked the negative shift in voltage dependence and increase in amplitude of the calcium current (ICa) in cones that is induced by changes in horizontal cell membrane potential. These effects disrupt the negative feedback from horizontal cells to cones that is important for the formation of center-surround receptive fields in bipolar and ganglion cells, and thus for normal spatial and chromatic perception.

CONCLUSIONS

Our study suggests that changes in the output of retinal neurons caused by disturbances in outer retinal feedback mechanisms can enhance patterned visual hallucinations.

Presynaptic CaV1.3 channels regulate synaptic ribbon size and are required for synaptic maintenance in sensory hair cells

L-type calcium channels (Ca(V)1) are involved in diverse processes, such as neurotransmission, hormone secretion, muscle contraction, and gene expression. In this study, we uncover a role for Ca(V)1.3a in regulating the architecture of a cellular structure, the ribbon synapse, in developing zebrafish sensory hair cells. By combining in vivo calcium imaging with confocal and super-resolution structured illumination microscopy, we found that genetic disruption or acute block of Ca(V)1.3a channels led to enlargement of synaptic ribbons in hair cells. Conversely, activating channels reduced both synaptic-ribbon size and the number of intact synapses. Along with enlarged presynaptic ribbons in ca(V)1.3a mutants, we observed a profound loss of juxtaposition between presynaptic and postsynaptic components. These synaptic defects are not attributable to loss of neurotransmission, because vglut3 mutants lacking neurotransmitter release develop relatively normal hair-cell synapses. Moreover, regulation of synaptic-ribbon size by Ca(2+) influx may be used by other cell types, because we observed similar pharmacological effects on pinealocyte synaptic ribbons. Our results indicate that Ca(2+) influx through Ca(V)1.3 fine tunes synaptic ribbon size during hair-cell maturation and that Ca(V)1.3 is required for synaptic maintenance.

Evidence for synergistic and complementary roles of Bassoon and darkness in organizing the ribbon synapse

Ribbon synapses are tonically active high-throughput synapses. The performance of the ribbon synapse is accomplished by a specialization of the cytomatrix at the active zone (CAZ) referred to as the synaptic ribbon (SR). Progress in our understanding of the structure-function relationship at the ribbon synapse has come from observations that, in photoreceptors lacking a full-size scaffolding protein Bassoon (Bsn(ΔEx4/5)), dissociation of SRs coincides with perturbed signal transfer. The aim of the present study has been to elaborate the role of Bassoon as a structural organizer of the ribbon synapse and to differentiate it with regard to the ambient lighting conditions. The ultrastructure of retinal ribbon synapses has been compared between wild-type (Wt) and Bsn(ΔEx4/5) mice adapted to light (low activity) and darkness (high activity). The results obtained suggest that Bassoon and environmental illumination synergistically and complementarily act as organizers of the ribbon synapse. Thus, light-dependent and Bassoon-independent regulation involves initial SR tethering to the membrane and a basic shape transition of ribbon material from spherical to rod-like, since darkness induces these features in Bsn(ΔEx4/5) rod spherules. However, the tight anchorage of the SR via an arciform density and the proper assembly of SRs to the full-sized horseshoe-shaped complex depend on Bassoon, as these steps fail in Bsn(ΔEx4/5) rod spherules.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Deletion of the presynaptic scaffold CAST reduces active zone size in rod photoreceptors and impairs visual processing

How size and shape of presynaptic active zones are regulated at the molecular level has remained elusive. Here we provide insight from studying rod photoreceptor ribbon-type active zones after disruption of CAST/ERC2, one of the cytomatrix of the active zone (CAZ) proteins. Rod photoreceptors were present in normal numbers, and the a-wave of the electroretinogram (ERG)--reflecting their physiological population response--was unchanged in CAST knock-out (CAST(-/-)) mice. Using immunofluorescence and electron microscopy, we found that the size of the rod presynaptic active zones, their Ca(2+) channel complement, and the extension of the outer plexiform layer were diminished. Moreover, we observed sprouting of horizontal and bipolar cells toward the outer nuclear layer indicating impaired rod transmitter release. However, rod synapses of CAST(-/-) mice, unlike in mouse mutants for the CAZ protein Bassoon, displayed anchored ribbons, normal vesicle densities, clustered Ca(2+) channels, and essentially normal molecular organization. The reduction of the rod active zone size went along with diminished amplitudes of the b-wave in scotopic ERGs. Assuming, based on the otherwise intact synaptic structure, an unaltered function of the remaining release apparatus, we take our finding to suggest a scaling of release rate with the size of the active zone. Multielectrode-array recordings of retinal ganglion cells showed decreased contrast sensitivity. This was also observed by optometry, which, moreover, revealed reduced visual acuity. We conclude that CAST supports large active zone size and high rates of transmission at rod ribbon synapses, which are required for normal vision.

Overexpression of guanylate cyclase activating protein 2 in rod photoreceptors in vivo leads to morphological changes at the synaptic ribbon

Guanylate cyclase activating proteins are EF-hand containing proteins that confer calcium sensitivity to retinal guanylate cyclase at the outer segment discs of photoreceptor cells. By making the rate of cGMP synthesis dependent on the free intracellular calcium levels set by illumination, GCAPs play a fundamental role in the recovery of the light response and light adaptation. The main isoforms GCAP1 and GCAP2 also localize to the synaptic terminal, where their function is not known. Based on the reported interaction of GCAP2 with Ribeye, the major component of synaptic ribbons, it was proposed that GCAP2 could mediate the synaptic ribbon dynamic changes that happen in response to light. We here present a thorough ultrastructural analysis of rod synaptic terminals in loss-of-function (GCAP1/GCAP2 double knockout) and gain-of-function (transgenic overexpression) mouse models of GCAP2. Rod synaptic ribbons in GCAPs-/- mice did not differ from wildtype ribbons when mice were raised in constant darkness, indicating that GCAPs are not required for ribbon early assembly or maturation. Transgenic overexpression of GCAP2 in rods led to a shortening of synaptic ribbons, and to a higher than normal percentage of club-shaped and spherical ribbon morphologies. Restoration of GCAP2 expression in the GCAPs-/- background (GCAP2 expression in the absence of endogenous GCAP1) had the striking result of shortening ribbon length to a much higher degree than overexpression of GCAP2 in the wildtype background, as well as reducing the thickness of the outer plexiform layer without affecting the number of rod photoreceptor cells. These results indicate that preservation of the GCAP1 to GCAP2 relative levels is relevant for maintaining the integrity of the synaptic terminal. Our demonstration of GCAP2 immunolocalization at synaptic ribbons at the ultrastructural level would support a role of GCAPs at mediating the effect of light on morphological remodeling changes of synaptic ribbons.