The first synapse in vision in the aging mouse retina.

Vision is our primary sense, and maintaining it throughout our lifespan is crucial for our well-being. However, the retina, which initiates vision, suffers from an age-related, irreversible functional decline. What causes this functional decline, and how it might be treated, is still unclear. Synapses are the functional hub for signal transmission between neurons, and studies have shown that aging is widely associated with synaptic dysfunction. In this study, we examined the first synapse of the visual system - the rod and cone photoreceptor ribbon synapse - in the mouse retina using light and electron microscopy at 2-3 months, ~1 year, and >2 years of age. We asked, whether age-related changes in key synaptic components might be a driver of synaptic dysfunction and ultimately age-related functional decline during normal aging. We found sprouting of horizontal and bipolar cells, formation of ectopic photoreceptor ribbon synapses, and a decrease in the number of rod photoreceptors and photoreceptor ribbon synapses in the aged retina. However, the majority of the photoreceptors did not show obvious changes in the structural components and protein composition of their ribbon synapses. Noteworthy is the increase in mitochondrial size in rod photoreceptor terminals in the aged retina.

Heterogeneous Presynaptic Distribution of Munc13 Isoforms at Retinal Synapses and Identification of an Unconventional Bipolar Cell Type with Dual Expression of Munc13 Isoforms: A Study Using Munc13-EXFP Knock-in Mice.

Munc13 isoforms are constituents of the presynaptic compartment of chemical synapses, where they govern important steps in preparing synaptic vesicles for exocytosis. The role of Munc13-1, -2 and -3 is well documented in brain neurons, but less is known about their function and distribution among the neurons of the retina and their conventional and ribbon-type chemical synapses. Here, we examined the retinae of Munc13-1-, -2-, and -3-EXFP knock-in (KI) mice with a combination of immunocytochemistry, physiology, and electron microscopy. We show that knock-in of Munc13-EXFP fusion proteins did not affect overall retinal anatomy or synapse structure, but slightly affected synaptic transmission. By labeling Munc13-EXFP KI retinae with specific antibodies against Munc13-1, -2 and -3, we found that unlike in the brain, most retinal synapses seem to operate with a single Munc13 isoform. A surprising exception to this rule was type 6 ON bipolar cells, which expressed two Munc13 isoforms in their synaptic terminals, ubMunc13-2 and Munc13-3. The results of this study provide an important basis for future studies on the contribution of Munc13 isoforms in visual signal processing in the mammalian retina.

A Multiple Piccolino-RIBEYE Interaction Supports Plate-Shaped Synaptic Ribbons in Retinal Neurons.

Active zones at chemical synapses are highly specialized sites for the regulated release of neurotransmitters. Despite a high degree of active zone protein conservation in vertebrates, every type of chemical synapse expresses a given set of protein isoforms and splice variants adapted to the demands on neurotransmitter release. So far, we know little about how specific active zone proteins contribute to the structural and functional diversity of active zones. In this study, we explored the nanodomain organization of ribbon-type active zones by addressing the significance of Piccolino, the ribbon synapse-specific splice variant of Piccolo, for shaping the ribbon structure. We followed up on previous results, which indicated that rod photoreceptor synaptic ribbons lose their structural integrity in a knockdown of Piccolino. Here, we demonstrate an interaction between Piccolino and the major ribbon component RIBEYE that supports plate-shaped synaptic ribbons in retinal neurons. In a detailed ultrastructural analysis of three different types of retinal ribbon synapses in Piccolo/Piccolino-deficient male and female rats, we show that the absence of Piccolino destabilizes the superstructure of plate-shaped synaptic ribbons, although with variable manifestation in the cell types examined. Our analysis illustrates how the expression of a specific active zone protein splice variant (e.g., Piccolino) contributes to structural diversity of vertebrate active zones.SIGNIFICANCE STATEMENT Retinal ribbon synapses are a specialized type of chemical synapse adapted for the regulated fast and tonic release of neurotransmitter. The hallmark of retinal ribbon synapses is the plate-shaped synaptic ribbon, which extends from the release site into the terminals' cytoplasm and tethers hundreds of synaptic vesicles. Here, we show that Piccolino, the synaptic ribbon specific splice variant of Piccolo, interacts with RIBEYE, the main component of synaptic ribbons. This interaction occurs via several PxDLS-like motifs located at the C terminus of Piccolino, which can connect multiple RIBEYE molecules. Loss of Piccolino disrupts the characteristic plate-shaped structure of synaptic ribbons, indicating a role of Piccolino in synaptic ribbon assembly.

Signal transmission at invaginating cone photoreceptor synaptic contacts following deletion of the presynaptic cytomatrix protein Bassoon in mouse retina.

AIM: A key feature of the mammalian retina is the segregation of visual information in parallel pathways, starting at the photoreceptor terminals. Cone photoreceptors establish synaptic contacts with On bipolar and horizontal cells at invaginating, ribbon-containing synaptic sites, whereas Off bipolar cells form flat, non-ribbon-containing contacts. The cytomatrix protein Bassoon anchors ribbons at the active zone, and its absence induces detachment of ribbons from the active zone. In this study we investigate the impact of a missing Bassoon on synaptic transmission at the first synapse of the visual system. METHODS: Release properties of cone photoreceptors were studied in wild-type and mutant mouse retinae with a genetic disruption of the presynaptic cytomatrix protein Bassoon using whole-cell voltage-clamp recordings. Light and electron microscopy revealed the distribution of Ca2+ channels and synaptic vesicles, respectively, in both mouse lines. RESULTS: Whole-cell recordings from postsynaptic horizontal cells of the two mouse lines showed that the presence of Bassoon (and a ribbon) enhanced the rate of exocytosis during tonic and evoked release by increasing synaptic vesicle pool size and replenishment rate, while at the same time slowing synaptic vesicle release. Furthermore, the number of Cav 1.4 channels and synaptic vesicles was significantly higher at wild-type than at Bassoon mutant synaptic sites. CONCLUSION: The results of our study demonstrate that glutamate release from cone photoreceptor terminals can occur independent of a synaptic ribbon, but seems restricted to active zones, and they show the importance of a the synaptic ribbon in sustained and spatially and temporally synchronized neurotransmitter release.

Functional analyses of Pericentrin and Syne-2 interaction in ciliogenesis.

Pericentrin (Pcnt) is a multifunctional scaffold protein and mutations in the human PCNT gene are associated with several diseases, including ciliopathies. Pcnt plays a crucial role in ciliary development in olfactory receptor neurons, but its function in the photoreceptor-connecting cilium is unknown. We downregulated Pcnt in the retina ex vivo and in vivo via a virus-based RNA interference approach to study Pcnt function in photoreceptors. ShRNA-mediated knockdown of Pcnt impaired the development of the connecting cilium and the outer segment of photoreceptors, and caused a nuclear migration defect. In protein interaction screens, we found that the outer nuclear membrane protein Syne-2 (also known as Nesprin-2) is an interaction partner of Pcnt in photoreceptors. Syne-2 is important for positioning murine photoreceptor cell nuclei and for centrosomal migration during early ciliogenesis. CRISPR/Cas9-mediated knockout of Syne-2 in cell culture led to an overexpression and mislocalization of Pcnt and to ciliogenesis defects. Our findings suggest that the Pcnt-Syne-2 complex is important for ciliogenesis and outer segment formation during retinal development and plays a role in nuclear migration.

Analysis of RIM Expression and Function at Mouse Photoreceptor Ribbon Synapses.

RAB3A-interacting molecule (RIM) proteins are important regulators of transmitter release from active zones. At conventional chemical synapses, RIMs contribute substantially to vesicle priming and docking and their loss reduces the readily releasable pool of synaptic vesicles by up to 75%. The priming function of RIMs is mediated via the formation of a tripartite complex with Munc13 and RAB3A, which brings synaptic vesicles in close proximity to Ca2+ channels and the fusion site and activates Munc13. We reported previously that, at mouse photoreceptor ribbon synapses, vesicle priming is Munc13 independent. In this study, we examined RIM expression, distribution, and function at male and female mouse photoreceptor ribbon synapses. We provide evidence that RIM1α and RIM1ß are highly likely absent from mouse photoreceptors and that RIM2α is the major large RIM isoform present at photoreceptor ribbon synapses. We show that mouse photoreceptors predominantly express RIM2 variants that lack the interaction domain for Munc13. Loss of full-length RIM2α in a RIM2α mutant mouse only marginally perturbs photoreceptor synaptic transmission. Our findings therefore strongly argue for a priming mechanism at the photoreceptor ribbon synapse that is independent of the formation of a RIM-Munc13-RAB3A complex and thus provide further evidence for a fundamental difference between photoreceptor ribbon synapses and conventional chemical synapses in synaptic vesicle exocytosis.SIGNIFICANCE STATEMENT RAB3A-interacting molecules 1 and 2 (RIM1/2) are essential regulators of exocytosis. At conventional chemical synapses, their function involves Ca2+ channel clustering and synaptic vesicle priming and docking through interactions with Munc13 and RAB3A, respectively. Examining wild-type and RIM2 mutant mice, we show here that the sensory photoreceptor ribbon synapses most likely lack RIM1 and predominantly express RIM2 variants that lack the interaction domain for Munc13. Our findings demonstrate that the photoreceptor-specific RIM variants are not essential for synaptic vesicle priming at photoreceptor ribbon synapses, which represents a fundamental difference between photoreceptor ribbon synapses and conventional chemical synapses with respect to synaptic vesicle priming mechanisms.

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

UNLABELLED: 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.

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.

Evidence for a Clathrin-independent mode of endocytosis at a continuously active sensory synapse.

Synaptic vesicle exocytosis at chemical synapses is followed by compensatory endocytosis. Multiple pathways including Clathrin-mediated retrieval of single vesicles, bulk retrieval of large cisternae, and kiss-and-run retrieval have been reported to contribute to vesicle recycling. Particularly at the continuously active ribbon synapses of retinal photoreceptor and bipolar cells, compensatory endocytosis plays an essential role to provide ongoing vesicle supply. Yet, little is known about the mechanisms that contribute to endocytosis at these highly complex synapses. To identify possible specializations in ribbon synaptic endocytosis during different states of activity, we exposed mice to controlled lighting conditions and compared the distribution of endocytotic proteins at rod and cone photoreceptor, and ON bipolar cell ribbon synapses with light and electron microscopy. In mouse ON bipolar cell terminals, Clathrin-mediated endocytosis seemed to be the dominant mode of endocytosis at all adaptation states analyzed. In contrast, in mouse photoreceptor terminals in addition to Clathrin-coated pits, clusters of membranously connected electron-dense vesicles appeared during prolonged darkness. These clusters labeled for Dynamin3, Endophilin1, and Synaptojanin1, but not for AP180, Clathrin LC, and hsc70. We hypothesize that rod and cone photoreceptors possess an additional Clathrin-independent mode of vesicle retrieval supporting the continuous synaptic vesicle supply during prolonged high activity.

Photoreceptor degeneration in two mouse models for congenital stationary night blindness type 2.

Light-dependent conductance changes of voltage-gated Cav1.4 channels regulate neurotransmitter release at photoreceptor ribbon synapses. Mutations in the human CACNA1F gene encoding the α1F subunit of Cav1.4 channels cause an incomplete form of X-linked congenital stationary night blindness (CSNB2). Many CACNA1F mutations are loss-of-function mutations resulting in non-functional Cav1.4 channels, but some mutations alter the channels' gating properties and, presumably, disturb Ca(2+) influx at photoreceptor ribbon synapses. Notably, a CACNA1F mutation (I745T) was identified in a family with an uncommonly severe CSNB2-like phenotype, and, when expressed in a heterologous system, the mutation was shown to shift the voltage-dependence of channel activation, representing a gain-of-function. To gain insight into the pathomechanism that could explain the severity of this disorder, we generated a mouse model with the corresponding mutation in the murine Cacna1f gene (I756T) and compared it with a mouse model carrying a loss-of-function mutation (ΔEx14-17) in a longitudinal study up to eight months of age. In ΔEx14-17 mutants, the b-wave in the electroretinogram was absent, photoreceptor ribbon synapses were abnormal, and Ca(2+) responses to depolarization of photoreceptor terminals were undetectable. In contrast, I756T mutants had a reduced scotopic b-wave, some intact rod ribbon synapses, and a strong, though abnormal, Ca(2+) response to depolarization. Both mutants showed a progressive photoreceptor loss, but degeneration was more severe and significantly enhanced in the I756T mutants compared to the ΔEx14-17 mutants.

Identification and immunocytochemical characterization of Piccolino, a novel Piccolo splice variant selectively expressed at sensory ribbon synapses of the eye and ear.

Piccolo is one of the largest cytomatrix proteins present at active zones of chemical synapses, where it is suggested to play a role in recruiting and integrating molecules relevant for both synaptic vesicle exo- and endocytosis. Here we examined the retina of a Piccolo-mutant mouse with a targeted deletion of exon 14 in the Pclo gene. Piccolo deficiency resulted in its profound loss at conventional chemical amacrine cell synapses but retinal ribbon synapses were structurally and functionally unaffected. This led to the identification of a shorter, ribbon-specific Piccolo variant, Piccolino, present in retinal photoreceptor cells, bipolar cells, as well as in inner hair cells of the inner ear. By RT-PCR analysis and the generation of a Piccolino-specific antibody we show that non-splicing of intron 5/6 leads to premature translation termination and generation of the C-terminally truncated protein specifically expressed at active zones of ribbon synapse containing cell types. With in situ proximity ligation assays we provide evidence that this truncation leads to the absence of interaction sites for Bassoon, Munc13, and presumably also ELKS/CAST, RIM2, and the L-type Ca(2) (+) channel which exist in the full-length Piccolo at active zones of conventional chemical synapses. The putative lack of interactions with proteins of the active zone suggests a function of Piccolino at ribbon synapses of sensory neurons different from Piccolo's function at conventional chemical synapses.

Stability of active zone components at the photoreceptor ribbon complex.

PURPOSE: Photoreceptor ribbon synapses translate light-dependent changes of membrane potential into graded transmitter release over several orders of magnitude in intensity. A specialized organelle at the active zone--the synaptic ribbon--is a key player in this process, and it is well known that the ribbon undergoes illumination and thus activity-dependent structural changes. However, the molecular basis for these changes is unknown. The aim of this study was to correlate the known ultrastructural ribbon changes to the distribution of proteins of the presynaptic ribbon complex. METHODS: In an in vitro assay, two distinct structural ribbon states--club-shaped and spherical-shaped--were enriched and the distribution of presynaptic proteins at the rod photoreceptor ribbon complex was analyzed with immunocytochemistry and light and electron microscopy. RESULTS: We show that structural changes of the ribbon correlate with the redistribution of selected presynaptic proteins. The disassembly of the ribbon complex seems to be a multistep process, which starts with the removal of spherical ribbon material while arciform density and active zone plasma membrane proteins remain largely unchanged at their synaptic location. Only later, in a second phase following the removal of ribbon material, the arciform density and plasma membrane proteins are redistributed from their synaptic localization and active zones disappear. CONCLUSIONS: The results of our study show that photoreceptor ribbon and arciform density/plasma membrane components might be influenced differentially by activity-driven processes, thus providing a molecular basis for further investigation of regulatory and adaptive processes in photoreceptor ribbon synaptic transmission.

Strumpellin is a novel valosin-containing protein binding partner linking hereditary spastic paraplegia to protein aggregation diseases.

Mutations of the human valosin-containing protein gene cause autosomal-dominant inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia. We identified strumpellin as a novel valosin-containing protein binding partner. Strumpellin mutations have been shown to cause hereditary spastic paraplegia. We demonstrate that strumpellin is a ubiquitously expressed protein present in cytosolic and endoplasmic reticulum cell fractions. Overexpression or ablation of wild-type strumpellin caused significantly reduced wound closure velocities in wound healing assays, whereas overexpression of the disease-causing strumpellin N471D mutant showed no functional effect. Strumpellin knockdown experiments in human neuroblastoma cells resulted in a dramatic reduction of axonal outgrowth. Knockdown studies in zebrafish revealed severe cardiac contractile dysfunction, tail curvature and impaired motility. The latter phenotype is due to a loss of central and peripheral motoneuron formation. These data imply a strumpellin loss-of-function pathogenesis in hereditary spastic paraplegia. In the human central nervous system strumpellin shows a presynaptic localization. We further identified strumpellin in pathological protein aggregates in inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia, various myofibrillar myopathies and in cortical neurons of a Huntington's disease mouse model. Beyond hereditary spastic paraplegia, our findings imply that mutant forms of strumpellin and valosin-containing protein may have a concerted pathogenic role in various protein aggregate diseases.

Absence of functional active zone protein Bassoon affects assembly and transport of ribbon precursors during early steps of photoreceptor synaptogenesis.

The retinal photoreceptor ribbon synapse is a structurally and functionally unique type of chemical synapse, specialized for tonic release of neurotransmitter in the dark. It is characterized by the presynaptic ribbon, an electron-dense organelle at the active zone, which is covered by hundreds of synaptic vesicles. Recently we showed that photoreceptor ribbon complexes are assembled from non-membranous, spherical densities--the precursor spheres--during the first two postnatal weeks of photoreceptor synaptogenesis. A core component of the precursor spheres and a key player in attaching the ribbon to the active zone is the presynaptic cytomatrix protein Bassoon. In this study, we examined in a comprehensive light and electron microscopic analysis whether Bassoon plays a role in the formation of the precursor spheres using Bassoon mutant mice lacking functional Bassoon. We report that developing Bassoon mutant photoreceptors contain fewer and smaller precursor spheres and that transport of precursor spheres to nascent synapses is delayed compared to wild-type controls. Moreover, western blot analyses of homogenates from postnatal day 0 (P0) to P14 Bassoon mutant retinae exhibit lower RIBEYE and Piccolo protein levels compared to the wild type, indicating elevated protein degradation in the absence of Bassoon. Our findings reveal a novel function of Bassoon in the early formation and delivery of precursor spheres to nascent ribbon synaptic sites in addition to its known role in ribbon anchoring during later stages of photoreceptor ribbon synaptogenesis.

Aberrant function and structure of retinal ribbon synapses in the absence of complexin 3 and complexin 4.

Complexins regulate the speed and Ca(2+) sensitivity of SNARE-mediated synaptic vesicle fusion at conventional synapses. Two of the vertebrate complexins, Cplx3 and Cplx4, are specifically localized to retinal ribbon synapses. To test whether Cplx3 and Cplx4 contribute to the highly efficient transmitter release at ribbon synapses, we studied retina function and structure in Cplx3 and Cplx4 single- and double-knockout mice. Electroretinographic recordings from single and double mutants revealed a cooperative perturbing effect of Cplx3 and Cplx4 deletion on the b-wave amplitude, whereas most other detected effects in both plexiform synaptic layers were additive. Light and electron microscopic analyses uncovered a disorganized outer plexiform layer in the retinae of mice lacking Cplx3 and Cplx4, with a significant proportion of photoreceptor terminals containing spherical free-floating ribbons. These structural and functional aberrations were accompanied by behavioural deficits indicative of a vision deficit. Our results show that Cplx3 and Cplx4 are essential regulators of transmitter release at retinal ribbon synapses. Their loss leads to aberrant adjustment and fine-tuning of transmitter release at the photoreceptor ribbon synapse, alterations in transmission at bipolar cell terminals, changes in the temporal structure of synaptic processing in the inner plexiform layer of the retina and perturbed vision.

Early steps in the assembly of photoreceptor ribbon synapses in the mouse retina: the involvement of precursor spheres.

The retinal photoreceptor ribbon synapse is a chemical synapse structurally and functionally specialized for the tonic release of neurotransmitter. It is characterized by the presynaptic ribbon, an electron-dense organelle at the active zone covered by hundreds of synaptic vesicles. In conventional synapses, dense-core transport vesicles carrying a set of active zone proteins are implicated in early steps of synapse formation. In photoreceptor ribbon synapses, synaptic spheres are suggested to be involved in ribbon synapse assembly, but nothing is known about the molecular composition of these organelles. With light, electron, and stimulated emission depletion microscopy and immunocytochemistry, we investigated a series of presynaptic proteins during photoreceptor synaptogenesis. The cytomatrix proteins Bassoon, Piccolo, RIBEYE, and RIM1 appear early in synaptogenesis. They are transported in nonmembranous, electron-dense, spherical transport units, which we called precursor spheres, to the future presynaptic site. Other presynaptic proteins, i.e., Munc13, CAST1, RIM2, and an L-type Ca(2+) channel alpha1 subunit are not associated with the precursor spheres. They cluster directly at the active zone some time after the first set of cytomatrix proteins has arrived. By quantitative electron microscopy, we found an inverse correlation between the numbers of spheres and synaptic ribbons in the postnatally developing photoreceptor synaptic terminals. From these results, we suggest that the precursor spheres are the transport units for proteins of the photoreceptor ribbon compartment and are involved in the assembly of mature synaptic ribbons.

Structural and functional remodeling in the retina of a mouse with a photoreceptor synaptopathy: plasticity in the rod and degeneration in the cone system.

Knowledge about the plastic and regenerative capacity of the retina is of key importance for therapeutic approaches to restore vision in patients who suffer from degenerative retinal diseases. In the retinae of mice, mutant for the presynaptic scaffolding protein Bassoon, signal transfer at photoreceptor ribbon synapses is disturbed due to impaired ribbon attachment to the active zone. In a long-term study we observed, with light and electron microscopic immunocytochemistry and electroretinographic recordings, two overlapping events in the Bassoon mutant retina, i.e. loss of photoreceptor synapses in the outer plexiform layer, and structural remodeling and formation of ectopic photoreceptor synapses in the outer nuclear layer, a region usually devoid of synapses. Formation of ectopic synaptic sites starts around the time when photoreceptor synaptogenesis is completed in wild-type mice and progresses throughout life. The result is a dense plexus of ectopic photoreceptor synapses with significantly altered but considerable synaptic transmission. Ectopic synapse formation is led by the sprouting of horizontal cells followed by the extension of rod bipolar cell neurites that fasciculate with and grow along the horizontal cell processes. Although only the rod photoreceptors and their postsynaptic partners show structural and functional remodeling, our study demonstrates the potential of the retina for long-lasting plastic changes.

Expression of the vesicular glutamate transporter vGluT2 in a subset of cones of the mouse retina.

Cone photoreceptors have a continuous release of glutamate that is modulated by light. Vesicular glutamate transporters (vGluT) play an essential role for sustaining this release by loading synaptic vesicles in the cone synapse, the so-called cone pedicle. In the present study mouse retinas were immunostained for vGluT1 and vGluT2. vGluT1 was localized to all cone pedicles and rod spherules, whereas vGluT2 was found in only 10% of the cone pedicles. The vGluT2-expressing cones were characterized in more detail. They are distributed in a regular array, suggesting they are a distinct type. Their proportion does not differ between dorsal (L-cone-dominated) and ventral (S-cone-dominated) retina, and they are not the genuine blue cones of the mouse retina. During development, vGluT1 and vGluT2 expression in cones starts at around P0 and right from the beginning vGluT2 is only expressed in a subset of cones. Bipolar cells contact the vGluT2-expressing cones and other cones nonselectively. The possible functional role of vGluT2 expression in a small fraction of cones is discussed.