Differential localization of SNARE complex proteins SNAP-25, syntaxin, and VAMP during development of the mammalian retina

Botulinum neurotoxin serotype A inhibited ocular angiogenesis through modulating glial activation via SOCS3

BACKGROUND

Pathological angiogenesis causes significant vision loss in neovascular age-related macular degeneration and other retinopathies with neovascularization (NV). Neuronal/glial-vascular interactions influence the release of angiogenic and neurotrophic factors. We hypothesized that botulinum neurotoxin serotype A (BoNT/A) modulates pathological endothelial cell proliferation through glial cell activation and growth factor release.

METHODS

A laser-induced choroidal NV (CNV) was employed to investigate the anti-angiogenic effects of BoNT/A. Fundus fluorescence angiography, immunohistochemistry, and real-time PCR were used to assess BoNT/A efficacy in inhibiting CNV and the molecular mechanisms underlying this inhibition. Neuronal and glial suppressor of cytokine signaling 3 (SOCS3) deficient mice were used to investigate the molecular mechanisms of BoNT/A in inhibiting CNV via SOCS3.

FINDINGS

In laser-induced CNV mice with intravitreal BoNT/A treatment, CNV lesions decreased > 30%; vascular leakage and retinal glial activation were suppressed; and Socs3 mRNA expression was induced while vascular endothelial growth factor A (Vegfa) mRNA expression was suppressed. The protective effects of BoNT/A on CNV development were diminished in mice lacking neuronal/glial SOCS3.

CONCLUSION

BoNT/A suppressed laser-induced CNV and glial cell activation, in part through SOCS3 induction in neuronal/glial cells. BoNT/A treatment led to a decrease of pro-angiogenic factors, including VEGFA, highlighting the potential of BoNT/A as a therapeutic intervention for pathological angiogenesis in retinopathies.

SNAP-25, but not SNAP-23, is essential for photoreceptor development, survival, and function in mice

SNARE-mediated vesicular transport is thought to play roles in photoreceptor glutamate exocytosis and photopigment delivery. However, the functions of Synaptosomal-associated protein (SNAP) isoforms in photoreceptors are unknown. Here, we revisit the expression of SNAP-23 and SNAP-25 and generate photoreceptor-specific knockout mice to investigate their roles. Although we find that SNAP-23 shows weak mRNA expression in photoreceptors, SNAP-23 removal does not affect retinal morphology or vision. SNAP-25 mRNA is developmentally regulated and undergoes mRNA trafficking to photoreceptor inner segments at postnatal day 9 (P9). SNAP-25 knockout photoreceptors develop normally until P9 but degenerate by P14 resulting in severe retinal thinning. Photoreceptor loss in SNAP-25 knockout mice is associated with abolished electroretinograms and vision loss. We find mistrafficked photopigments, enlarged synaptic vesicles, and abnormal synaptic ribbons which potentially underlie photoreceptor degeneration. Our results conclude that SNAP-25, but not SNAP-23, mediates photopigment delivery and synaptic functioning required for photoreceptor development, survival, and function.

Age-associated changes in lineage composition of the enteric nervous system regulate gut health and disease

The enteric nervous system (ENS), a collection of neural cells contained in the wall of the gut, is of fundamental importance to gastrointestinal and systemic health. According to the prevailing paradigm, the ENS arises from progenitor cells migrating from the neural crest and remains largely unchanged thereafter. Here, we show that the lineage composition of maturing ENS changes with time, with a decline in the canonical lineage of neural-crest derived neurons and their replacement by a newly identified lineage of mesoderm-derived neurons. Single cell transcriptomics and immunochemical approaches establish a distinct expression profile of mesoderm-derived neurons. The dynamic balance between the proportions of neurons from these two different lineages in the post-natal gut is dependent on the availability of their respective trophic signals, GDNF-RET and HGF-MET. With increasing age, the mesoderm-derived neurons become the dominant form of neurons in the ENS, a change associated with significant functional effects on intestinal motility which can be reversed by GDNF supplementation. Transcriptomic analyses of human gut tissues show reduced GDNF-RET signaling in patients with intestinal dysmotility which is associated with reduction in neural crest-derived neuronal markers and concomitant increase in transcriptional patterns specific to mesoderm-derived neurons. Normal intestinal function in the adult gastrointestinal tract therefore appears to require an optimal balance between these two distinct lineages within the ENS.

Primary and secondary olfactory centres in human ontogeny

The olfactory centres are the evolutionary oldest and most conservative area of the telencephalon. Olfactory deficiencies are involved in a large spectrum of neurologic disorders and neurodegenerative diseases. The growing interest in human olfaction has been also been driven by COVID-19-induced transitional anosmia. Nevertheless, recent data on the human olfactory centres concerning normal histology and morphogenesis are rare. Published data in the field are mainly restricted to classic studies with non-uniform nomenclature and varied definitions of certain olfactory areas. While the olfactory system in model animals (rats, mice, and more rarely non-human primates) has been extensively investigated, the developmental timetable of olfactory centres in both human prenatal and postnatal ontogeny are poorly understood and unsystemised, which complicates the process of analysing human material, including medical researches. The main purpose of this review is to provide and discuss relevant morphological data on the normal ontogeny of the human olfactory centres, with a focus on the timetable of maturation and developmental cytoarchitecture, and with special reference to the definitions and terminology of certain olfactory areas.

Proteomic Analysis of Retinal Conditioned Medium: The Effect on Early Differentiation of Embyonic Stem Cells into Retina

Stem cell replacement therapy has emerged as one of the most promising treatment options for retinal degenerative diseases, which are the main causes of irreversible vision loss. Three-dimensional (3D) retinal organoid culture is a cutting-edge technology for differentiating embryonic stem cells into retinal cells by forming a laminated retinal structure. However, 3D culture systems have strict requirements with respect to the experimental environment and culture technologies. Our study aimed to investigate the effect of retinal conditioned medium (RCM) at different developmental stages on the early differentiation of embryonic stem cells into retina in a 3D culture system. Here, we added RCM to the 3D culture system and found it could promote the differentiation of mouse embryonic stem cells (mESCs) into neuroretina. We further explored the possible mechanisms of RCM that regulate differentiation through proteomic analysis. RCM at different time points disclosed different protein profiles. Proteins which improved energy metabolism of mESCs might help improve the viability of embryonic bodies (EBs). We then screened out Snap25, Cntn1, Negr1, Dpysl2, Dpysl3, and Crmp1 as candidate proteins that might play roles in the differentiation and neurogenesis processes of mESCs, hoping to provide a basis for optimizing a retinal differentiation protocol from embryonic stem cells.

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Horizontal Cell Feedback to Cone Photoreceptors in Mammalian Retina: Novel Insights From the GABA-pH Hybrid Model

How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is the transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts as a thermostat to keep the synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse are reviewed. This novel inter-neuronal messaging system carries feedback signals using two separate, but interwoven regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, has obscured it’s being defined. The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca²⁺ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl⁻ and HCO₃⁻ permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO₃⁻ efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H⁺ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.

Presynaptic SNAP-25 regulates retinal waves and retinogeniculate projection via phosphorylation

Patterned spontaneous activity periodically displays in developing retinas termed retinal waves, essential for visual circuit refinement. In neonatal rodents, retinal waves initiate in starburst amacrine cells (SACs), propagating across retinal ganglion cells (RGCs), further through visual centers. Although these waves are shown temporally synchronized with transiently high PKA activity, the downstream PKA target important for regulating the transmission from SACs remains unidentified. A t-SNARE, synaptosome-associated protein of 25 kDa (SNAP-25/SN25), serves as a PKA substrate, implying a potential role of SN25 in regulating retinal development. Here, we examined whether SN25 in SACs could regulate wave properties and retinogeniculate projection during development. In developing SACs, overexpression of wild-type SN25b, but not the PKA-phosphodeficient mutant (SN25b-T138A), decreased the frequency and spatial correlation of wave-associated calcium transients. Overexpressing SN25b, but not SN25b-T138A, in SACs dampened spontaneous, wave-associated, postsynaptic currents in RGCs and decreased the SAC release upon augmenting the cAMP-PKA signaling. These results suggest that SN25b overexpression may inhibit the strength of transmission from SACs via PKA-mediated phosphorylation at T138. Moreover, knockdown of endogenous SN25b increased the frequency of wave-associated calcium transients, supporting the role of SN25 in restraining wave periodicity. Finally, the eye-specific segregation of retinogeniculate projection was impaired by in vivo overexpression of SN25b, but not SN25b-T138A, in SACs. These results suggest that SN25 in developing SACs dampens the spatiotemporal properties of retinal waves and limits visual circuit refinement by phosphorylation at T138. Therefore, SN25 in SACs plays a profound role in regulating visual circuit refinement.

Loss-of-function of the ciliopathy protein Cc2d2a disorganizes the vesicle fusion machinery at the periciliary membrane and indirectly affects Rab8-trafficking in zebrafish photoreceptors

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous organelles involved in transduction of environmental signals such as light sensation in photoreceptors. Concentration of signal detection proteins such as opsins in the ciliary membrane is achieved by RabGTPase-regulated polarized vesicle trafficking and by a selective barrier at the ciliary base, the transition zone (TZ). Dysfunction of the TZ protein CC2D2A causes Joubert/Meckel syndromes in humans and loss of ciliary protein localization in animal models, including opsins in retinal photoreceptors. The link between the TZ and upstream vesicle trafficking has been little explored to date. Moreover, the role of the small GTPase Rab8 in opsin-carrier vesicle (OCV) trafficking has been recently questioned in a mouse model. Using correlative light and electron microscopy and live imaging in zebrafish photoreceptors, we provide the first live characterization of Rab8-mediated trafficking in photoreceptors in vivo. Our results support a possibly redundant role for both Rab8a/b paralogs in OCV trafficking, based on co-localization of Rab8 and opsins in vesicular structures, and joint movement of Rab8-tagged particles with opsin. We further investigate the role of the TZ protein Cc2d2a in Rab8-mediated trafficking using cc2d2a zebrafish mutants and identify a requirement for Cc2d2a in the latest step of OCV trafficking, namely vesicle fusion. Progressive accumulation of opsin-containing vesicles in the apical portion of photoreceptors lacking Cc2d2a is caused by disorganization of the vesicle fusion machinery at the periciliary membrane with mislocalization and loss of the t-SNAREs SNAP25 and Syntaxin3 and of the exocyst component Exoc4. We further observe secondary defects on upstream Rab8-trafficking with cytoplasmic accumulation of Rab8. Taken together, our results support participation of Rab8 in OCV trafficking and identify a novel role for the TZ protein Cc2d2a in fusion of incoming ciliary-directed vesicles, through organization of the vesicle fusion machinery at the periciliary membrane.

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous organelles involved in transduction of environmental signals to the cells. Concentration and regulation of signal detection proteins in the ciliary membrane is therefore tightly regulated through polarized vesicle trafficking and through a selective barrier at the ciliary base called the transition zone (TZ). Dysfunction of TZ proteins leads to human ciliopathies and to aberrant localization of ciliary proteins in animal models. In this work, we use zebrafish retinal photoreceptors as a model to explore the relationship between the TZ and upstream vesicle trafficking. Relying on modern technologies such as correlative light and electron microscopy and live imaging of fluorescently-tagged proteins, we identify a role for the TZ protein CC2D2A in organizing the components required for vesicle fusion at the periciliary membrane. We also characterize the movement dynamics of vesicles carrying light-detection proteins (opsins) towards the ciliary compartment of photoreceptors in vivo and provide novel data in support of the recently questioned involvement of the small GTPase Rab8 in opsin-carrier vesicle trafficking in photoreceptors.

A Presynaptic Group III mGluR Recruits Gβγ/SNARE Interactions to Inhibit Synaptic Transmission by Cone Photoreceptors in the Vertebrate Retina

G-protein βγ subunits (Gβγ) interact with presynaptic proteins and regulate neurotransmitter release downstream of Ca²⁺ influx. To accomplish their roles in sensory signaling, photoreceptor synapses use specialized presynaptic proteins that support neurotransmission at active zone structures known as ribbons. While several G-protein coupled receptors (GPCRs) influence synaptic transmission at ribbon synapses of cones and other retinal neurons, it is unknown whether Gβγ contributes to these effects. We tested whether activation of one particular GPCR, a metabotropic glutamate receptor (mGluR), can reduce cone synaptic transmission via Gβγ in tiger salamander retinas. In recordings from horizontal cells, we found that an mGluR agonist (L-AP4) reduced cone-driven light responses and mEPSC frequency. In paired recordings of cones and horizontal cells, L-AP4 slightly reduced cone I_Ca (∼10%) and caused a larger reduction in cone-driven EPSCs (∼30%). Proximity ligation assay revealed direct interactions between SNAP-25 and Gβγ subunits in retinal synaptic layers. Pretreatment with the SNAP-25 cleaving protease BoNT/A inhibited L-AP4 effects on synaptic transmission, as did introduction of a peptide derived from the SNAP-25 C terminus. Introducing Gβγ subunits directly into cones reduced EPSC amplitude. This effect was inhibited by BoNT/A, supporting a role for Gβγ/SNAP-25 interactions. However, the mGluR-dependent reduction in I_Ca was not mimicked by Gβγ, indicating that this effect was independent of Gβγ. The finding that synaptic transmission at cone ribbon synapses is regulated by Gβγ/SNAP-25 interactions indicates that these mechanisms are shared by conventional and ribbon-type synapses. Gβγ liberated from other photoreceptor GPCRs is also likely to regulate synaptic transmission.SIGNIFICANCE STATEMENT Dynamic regulation of synaptic transmission by presynaptic G-protein coupled receptors shapes information flow through neural circuits. At the first synapse in the visual system, presynaptic metabotropic glutamate receptors (mGluRs) regulate cone photoreceptor synaptic transmission, although the mechanisms and functional impact of this are unclear. We show that mGluRs regulate light response encoding across the cone synapse, accomplished in part by triggering G-protein βγ subunits (Gβγ) interactions with SNAP-25, a core component of the synaptic vesicle fusion machinery. In addition to revealing a role in visual processing, this provides the first demonstration that Gβγ/SNAP-25 interactions regulate synaptic function at a ribbon-type synapse, contributing to an emerging picture of the ubiquity of Gβγ/SNARE interactions in regulating synaptic transmission throughout the nervous system.

Transplantation of Neuro2a Cells into the Developing Postnatal Mouse Eye

The present study aimed to investigate the influence of the host retinal microenvironment on cell migration and differentiation using Neuro2a (N2a) cells transduced with green fluorescent protein. N2a cells were transplanted into the vitreous cavities of developing mouse eyes (C57BL/6) on postnatal days 1, 5, and 10 (P1, 5, and 10). To analyze the effects of the host microenvironment on neural differentiation of N2a cells in vitro, cells were treated with a conditioned medium (CM) collected from retinal cells cultured at each developmental stage. We observed that numerous cells transplanted into P5 mice eyes migrated into all layers of the host retina, and the presence of processes indicated morphological differentiation. Some transplanted N2a cells expressed several neural markers. However, cells transplanted into the P1 and 10 mice eyes only proliferated within the vitreous cavity. Neurite length increased in N2a cells treated with CM collected from the cultured retinal cells from P5 and 10 mice, while western blotting revealed that the levels of proteins related to neural differentiation were not significantly altered in N2a cells treated with CM. We show that the migration and differentiation capacities of transplanted cells were differentially influenced by the microenvironment of the retinal postnatal ontogeny.

Synaptotagmin I regulates patterned spontaneous activity in the developing rat retina via calcium binding to the C2AB domains

Background

In neonatal binocular animals, the developing retina displays patterned spontaneous activity termed retinal waves, which are initiated by a single class of interneurons (starburst amacrine cells, SACs) that release neurotransmitters. Although SACs are shown to regulate wave dynamics, little is known regarding how altering the proteins involved in neurotransmitter release may affect wave dynamics. Synaptotagmin (Syt) family harbors two Ca²⁺-binding domains (C2A and C2B) which serve as Ca²⁺ sensors in neurotransmitter release. However, it remains unclear whether SACs express any specific Syt isoform mediating retinal waves. Moreover, it is unknown how Ca²⁺ binding to C2A and C2B of Syt affects wave dynamics. Here, we investigated the expression of Syt I in the neonatal rat retina and examined the roles of C2A and C2B in regulating wave dynamics.

Methodology/Principal Findings

Immunostaining and confocal microscopy showed that Syt I was expressed in neonatal rat SACs and cholinergic synapses, consistent with its potential role as a Ca²⁺ sensor mediating retinal waves. By combining a horizontal electroporation strategy with the SAC-specific promoter, we specifically expressed Syt I mutants with weakened Ca²⁺-binding ability in C2A or C2B in SACs. Subsequent live Ca²⁺ imaging was used to monitor the effects of these molecular perturbations on wave-associated spontaneous Ca²⁺ transients. We found that targeted expression of Syt I C2A or C2B mutants in SACs significantly reduced the frequency, duration, and amplitude of wave-associated Ca²⁺ transients, suggesting that both C2 domains regulate wave temporal properties. In contrast, these C2 mutants had relatively minor effects on pairwise correlations over distance for wave-associated Ca²⁺ transients.

Conclusions/Significance

Through Ca²⁺ binding to C2A or C2B, the Ca²⁺ sensor Syt I in SACs may regulate patterned spontaneous activity to shape network activity during development. Hence, modulating the releasing machinery in presynaptic neurons (SACs) alters wave dynamics.

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

Transplantation of bone marrow-derived mesenchymal stem cells into the developing mouse eye

Mesenchymal stem cells (MSCs) have been studied widely for their potential to differentiate into various lineage cells including neural cells in vitro and in vivo. To investigate the influence of the developing host environment on the integration and morphological and molecular differentiation of MSCs, human bone marrow-derived mesenchymal stem cells (BM-MSCs) were transplanted into the developing mouse retina. Enhanced green fluorescent protein (GFP)-expressing BM-MSCs were transplanted by intraocular injections into mice, ranging in ages from 1 day postnatal (PN) to 10 days PN. The survival dates ranged from 7 days post-transplantation (DPT) to 28DPT, at which time an immunohistochemical analysis was performed on the eyes. The transplanted BM-MSCs survived and showed morphological differentiation into neural cells and some processes within the host retina. Some transplanted cells expressed microtubule associated protein 2 (MAP2ab, marker for mature neural cells) or glial fibrillary acid protein (GFAP, marker for glial cells) at 5PN 7DPT. In addition, some transplanted cells integrated into the developing retina. The morphological and molecular differentiation and integration within the 5PN 7DPT eye was greater than those of other-aged host eye. The present findings suggest that the age of the host environment can strongly influence the differentiation and integration of BM-MSCs.

The retina of the PCD/PCD mouse as a model of photoreceptor degeneration. A structural and functional study

In this work, we used the pcd (Purkinje cell degeneration) mutant mouse with a slow temporal progression of photoreceptor degeneration in order to analyze the structural and functional modifications in the neuronal populations of the outer and inner retina. Retinal immunocytochemistry and functional electroretinography were performed on the pcd/pcd mutant mice and control wild type animals of the C57/DBA strain at 45, 90, 180 and 270 post-natal days. Immunohistochemical studies were performed for a series of protein markers: calbindin, calretinin, PKCα, bassoon, synapsin, syntaxin and islet1. Full field electroretinography recordings were performed on control and dystrophic mice. Rod and mixed responses, and oscillatory potentials, were recorded in dark adapted conditions; cone and flicker responses were recorded under light adaptation. Our results show significant structural modifications in the photoreceptor populations and neurons of the inner retina. Changes in cell morphology affect mainly to the bipolar cells, which gradually lose their dendritic tufts. The electroretinography records reveal that in the pcd retinas the rod and cone systems show a reduction in the amplitude of the electrical signals. This decrease progresses slowly with the passage of time, although for the most advanced stage of photoreceptor degeneration considered, 270 post-natal days, it is still possible to record light induced responses. We conclude that pcd mice experience a loss of retinal function in correlation with the loss of photoreceptors with age, and significant changes in retinal synaptic processes.

SNAP25 expression in mammalian retinal horizontal cells

Horizontal cells mediate inhibitory feedforward and feedback lateral interactions in the outer retina at photoreceptor terminals and bipolar cell dendrites; however, the mechanisms that underlie synaptic transmission from mammalian horizontal cells are poorly understood. The localization of a vesicular γ-aminobutyric acid (GABA) transporter (VGAT) to horizontal cell processes in primate and rodent retinae suggested that mammalian horizontal cells release transmitter in a vesicular manner. Toward determining whether the molecular machinery for vesicular transmitter release is present in horizontal cells, we investigated the expression of SNAP25 (synaptosomal-associated protein of 25 kDa), a key SNARE protein, by immunocytochemistry with cell type-specific markers in the retinae of mouse, rat, rabbit, and monkey. Different commercial antibodies to SNAP25 were tested on vertical sections of retina. We report the robust expression of SNAP25 in both plexiform layers. Double labeling with SNAP25 and calbindin antibodies demonstrated that horizontal cell processes and their endings in photoreceptor triad synapses were strongly labeled for both proteins in mouse, rat, rabbit, and monkey retinae. Double labeling with parvalbumin antibodies in monkey retina verified SNAP25 immunoreactivity in all horizontal cells. Pre-embedding immunoelectron microscopy in rabbit retina confirmed expression of SNAP25 in lateral elements within photoreceptor triad synapses. The SNAP25 immunoreactivity in the plexiform layers and outer nuclear layer fell into at least three patterns depending on the antibody, suggesting a differential distribution of SNAP25 isoforms. The presence of SNAP25a and SNAP25b isoforms in mouse retina was established by reverse transcriptase-polymerase chain reaction. SNAP25 expression in mammalian horizontal cells along with other SNARE proteins is consistent with vesicular exocytosis.

Enrichment and differential targeting of complexins 3 and 4 in ribbon-containing sensory neurons during zebrafish development

Background

In sensory systems with broad bandwidths, polarized receptor cells utilize highly specialized organelles in their apical and basolateral compartments to transduce and ultimately transmit signals to the rest of the nervous system. While progress has been made in elucidating the assembly of the transduction apparatus, the development of synaptic ribbon-containing terminals remains poorly understood. To begin to delineate the targeting of the exocytotic machinery specifically in ribbon-containing neurons, we have examined the expression of complexins 3 and 4 in the zebrafish visual and acousticolateral systems during the first week of development.

Results

We have identified five members of the complexin 3/4 subfamily in zebrafish that show 50 to 75% amino acid identity with mammalian complexins 3 and 4. Utilizing a polyclonal antibody that recognizes all five orthologs, we demonstrate that these proteins are enriched in ribbon-containing sensory neurons. Complexin 3/4 is rapidly targeted to presynaptic terminals in the pineal organ and retina concomitantly with RIBEYE b, a component of ribbons. In hair cells of the inner ear and lateral line, however, complexin 3/4 immunoreactivity clusters on the apical surfaces of hair cells, among their stereocilia, rather than along the basolateral plasma membrane with RIBEYE b. A complexin 4a-specific antibody selectively labels the presynaptic terminals of visual system ribbon-containing neurons.

Conclusions

These results provide evidence for the concurrent transport and/or assembly of multiple components of the active zone in developing ribbon terminals. Members of the complexin 3/4 subfamily are enriched in these terminals in the visual system and in hair bundles of the acousticolateral system, suggesting that these proteins are differentially targeted and may have multiple roles in ribbon-containing sensory neurons.

Progression of neuronal and synaptic remodeling in the rd10 mouse model of retinitis pigmentosa

The Pde6b(rd10) (rd10) mouse has a moderate rate of photoreceptor degeneration and serves as a valuable model for human autosomal recessive retinitis pigmentosa (RP). We evaluated the progression of neuronal remodeling of second- and third-order retinal cells and their synaptic terminals in retinas from Pde6b(rd10) (rd10) mice at varying stages of degeneration ranging from postnatal day 30 (P30) to postnatal month 9.5 (PNM9.5) using immunolabeling for well-known cell- and synapse-specific markers. Following photoreceptor loss, changes occurred progressively from outer to inner retina. Horizontal cells and rod and cone bipolar cells underwent morphological remodeling that included loss of dendrites, cell body migration, and the sprouting of ectopic processes. Gliosis, characterized by translocation of Müller cell bodies to the outer retina and thickening of their processes, was evident by P30 and became more pronounced as degeneration progressed. Following rod degeneration, continued expression of VGluT1 in the outer retina was associated with survival and expression of synaptic proteins by nearby second-order neurons. Rod bipolar cell terminals showed a progressive reduction in size and ectopic bipolar cell processes extended into the inner nuclear layer and ganglion cell layer by PNM3.5. Putative ectopic conventional synapses, likely arising from amacrine cells, were present in the inner nuclear layer by PNM9.5. Despite these changes, the laminar organization of bipolar and amacrine cells and the ON-OFF organization in the inner plexiform layer was largely preserved. Surviving cone and bipolar cell terminals continued to express the appropriate cell-specific presynaptic proteins needed for synaptic function up to PNM9.5.

Immunocytochemical evidence for SNARE protein-dependent transmitter release from guinea pig horizontal cells

Horizontal cells are lateral interneurons that participate in visual processing in the outer retina but the cellular mechanisms underlying transmitter release from these cells are not fully understood. In non-mammalian horizontal cells, GABA release has been shown to occur by a non-vesicular mechanism. However, recent evidence in mammalian horizontal cells favors a vesicular mechanism as they lack plasmalemmal GABA transporters and some soluble NSF attachment protein receptor (SNARE) core proteins have been identified in rodent horizontal cells. Moreover, immunoreactivity for GABA and the molecular machinery to synthesize GABA have been found in guinea pig horizontal cells, suggesting that if components of the SNARE complex are expressed they could contribute to the vesicular release of GABA. In this study we investigated whether these vesicular and synaptic proteins are expressed by guinea pig horizontal cells using immunohistochemistry with well-characterized antibodies to evaluate their cellular distribution. Components of synaptic vesicles including vesicular GABA transporter, synapsin I and synaptic vesicle protein 2A were localized to horizontal cell processes and endings, along with the SNARE core complex proteins, syntaxin-1a, syntaxin-4 and synaptosomal-associated protein 25 (SNAP-25). Complexin I/II, a cytosolic protein that stabilizes the activated SNARE fusion core, strongly immunostained horizontal cell soma and processes. In addition, the vesicular Ca(2+)-sensor, synaptotagmin-2, which is essential for Ca(2+)-mediated vesicular release, was also localized to horizontal cell processes and somata. These morphological findings from guinea pig horizontal cells suggest that mammalian horizontal cells have the capacity to utilize a regulated Ca(2+)-dependent vesicular pathway to release neurotransmitter, and that this mechanism may be shared among many mammalian species.

A battery of cell- and structure-specific markers for the adult porcine retina

The pig is becoming an increasingly used non-primate model in experimental studies of human retinal diseases and disorders. The anatomy, size, and vasculature of the porcine eye and retina closely resemble their human counterparts, which allows for application of standard instrumentation and diagnostics used in the clinic. Despite many reports that demonstrate immunohistochemistry as a useful method for exploring neuropathological changes in the mammalian central nervous system, including the pig, the porcine retina has been sparsely described. Hence, to facilitate further immunohistochemical analysis of the porcine retina, we report on the successful use of a battery of antibodies for staining of paraformaldehyde-fixed cryosectioned retina. The following antibodies were evaluated for neuronal cells and structures: recoverin (cones and rods), Rho4D2 (rods), transducin-gamma (cones), ROM-1 (photoreceptor outer segments), calbindin (horizontal cells), PKC-alpha (bipolar cells), parvalbumin (amacrine and displaced amacrine cells), and NeuN (ganglion cells and displaced amacrines). For detecting synaptic connections in fiber layers, we used an antibody against synaptobrevin. For detecting retinal pigment epithelium, we studied antibodies against cytokeratin and RPE65, respectively. The glial cell markers used were bFGF (Müller cells and displaced amacrine cells), GFAP (Müller cells and astrocytes), and vimentin (Müller cells). Each staining effect was evaluated with regard to its specificity, sensitivity, and reproducibility in the identification of individual cells, specific cell structures, and fiber layers, respectively. The markers parvalbumin and ROM-1 were tested here for the first time for the porcine retina. All antibodies tested resulted in specific staining of high quality. In conclusion, all immunohistochemical protocols presented here will be applicable in fixed, cryosectioned pig retina.

Changes in retinoblastoma cell adhesion associated with optic nerve invasion

In the 1970s, several human retinoblastoma cell lines were developed from cultures of primary tumors. As the human retinoblastoma cell lines were established in culture, growth properties and changes in cell adhesion were described. Those changes correlated with the ability of the human retinoblastoma cell lines to invade the optic nerve and metastasize in orthotopic xenograft studies. However, the mechanisms that underlie these changes were not determined. We used the recently developed knockout mouse models of retinoblastoma to begin to characterize the molecular, cellular, and genetic changes associated with retinoblastoma tumor progression and optic nerve invasion. Here we report the isolation and characterization of the first mouse retinoblastoma cell lines with targeted deletions of the Rb family. Our detailed analysis of these cells as they were propagated in culture from the primary tumor shows that changes in cadherin-mediated cell adhesion are associated with retinoblastoma invasion of the optic nerve prior to metastasis. In addition, the same changes in cadherin-mediated cell adhesion correlate with the invasive properties of the human retinoblastoma cell lines isolated decades ago, providing a molecular mechanism for these earlier observations. Most importantly, our studies are in agreement with genetic studies on human retinoblastomas, suggesting that changes in this pathway are involved in tumor progression.

Syntaxin 3 and SNAP-25 pairing, regulated by omega-3 docosahexaenoic acid, controls the delivery of rhodopsin for the biogenesis of cilia-derived sensory organelles, the rod outer segments

The biogenesis of cilia-derived sensory organelles, the photoreceptor rod outer segments (ROS), is mediated by rhodopsin transport carriers (RTCs). The small GTPase Rab8 regulates ciliary targeting of RTCs, but their specific fusion sites have not been characterized. Here, we report that the Sec6/8 complex, or exocyst, is a candidate effector for Rab8. We also show that the Qa-SNARE syntaxin 3 is present in the rod inner segment (RIS) plasma membrane at the base of the cilium and displays a microtubule-dependent concentration gradient, whereas the Qbc-SNARE SNAP-25 is uniformly distributed in the RIS plasma membrane and the synapse. Treatment with omega-3 docosahexaenoic acid [DHA, 22:6(n-3)] causes increased co-immunoprecipitation and colocalization of SNAP-25 and syntaxin 3 at the base of the cilium, which results in the increased delivery of membrane to the ROS. This is particularly evident in propranolol-treated retinas, in which the DHA-mediated increase in SNARE pairing overcomes the tethering block, including dissociation of Sec8 into the cytosol. Together, our data indicate that the Sec6/8 complex, syntaxin 3 and SNAP-25 regulate rhodopsin delivery, probably by mediating docking and fusion of RTCs. We show further that DHA, an essential polyunsaturated fatty acid of the ROS, increases pairing of syntaxin 3 and SNAP-25 to regulate expansion of the ciliary membrane and ROS biogenesis.

The molecular architecture of ribbon presynaptic terminals

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

Distribution of plasma membrane-associated syntaxins 1 through 4 indicates distinct trafficking functions in the synaptic layers of the mouse retina

Background

Syntaxins 1 through 4 are SNAP receptor (SNARE) proteins that mediate vesicular trafficking to the plasma membrane. In retina, syntaxins 1 and 3 are expressed at conventional and ribbon synapses, respectively, suggesting that synaptic trafficking functions differ among syntaxin isoforms. To better understand syntaxins in synaptic signaling and trafficking, we further examined the cell- and synapse-specific expression of syntaxins 1 through 4 in the mouse retina by immunolabeling and confocal microscopy.

Results

Each isoform was expressed in the retina and showed a unique distribution in the synaptic layers of the retina, with little or no colocalization of isoforms. Syntaxin 1 was present in amacrine cell bodies and processes and conventional presynaptic terminals in the inner plexiform layer (IPL). Syntaxin 2 was present in amacrine cells and their processes in the IPL, but showed little colocalization with syntaxin 1 or other presynaptic markers. Syntaxin 3 was found in glutamatergic photoreceptor and bipolar cell ribbon synapses, but was absent from putative conventional glutamatergic amacrine cell synapses. Syntaxin 4 was localized to horizontal cell processes in the ribbon synaptic complexes of photoreceptor terminals and in puncta in the IPL that contacted dopaminergic and CD15-positive amacrine cells. Syntaxins 2 and 4 often were apposed to synaptic active zones labeled for bassoon.

Conclusion

These results indicate that each syntaxin isoform has unique, non-redundant functions in synaptic signaling and trafficking. Syntaxins 1 and 3 mediate presynaptic transmitter release from conventional and ribbon synapses, respectively. Syntaxins 2 and 4 are not presynaptic and likely mediate post-synaptic trafficking.

Status and applications of genomic resources for the gray, short-tailed opossum, Monodelphis domestica, an American marsupial model for comparative biology

Owing to its small size, favourable reproductive characteristics, and simple husbandry, the gray, short-tailed opossum, Monodelphis domestica, has become the most widely distributed and intensively utilised laboratory-bred research marsupial in the world today. This article provides an overview of the current state and future projections of genomic resources for this species and discusses the potential impact of this growing resource base on active research areas that use M. domestica as a model system. The resources discussed include: fully arrayed, bacterial artificial chromosome (BAC) libraries; an expanding linkage map; developing full-genome BAC-contig and chromosomal fluorescence in situ hybridisation maps; public websites providing access to the M. domestica whole-genome-shotgun sequence trace database and the whole-genome sequence assembly; and a new project underway to create an expressed-sequence database and microchip expression arrays for functional genomics applications. Major research areas discussed span a variety of genetic, evolutionary, physiologic, reproductive, developmental, and behavioural topics, including: comparative immunogenetics; genomic imprinting; reproductive biology; neurobiology; photobiology and carcinogenesis; genetics of lipoprotein metabolism; developmental and behavioural endocrinology; sexual differentiation and development; embryonic and fetal development; meiotic recombination; genome evolution; molecular evolution and phylogenetics; and more.

Cellular distribution and subcellular localization of molecular components of vesicular transmitter release in horizontal cells of rabbit retina

The mechanism underlying transmitter release from retinal horizontal cells is poorly understood. We investigated the possibility of vesicular transmitter release from mammalian horizontal cells by examining the expression of synaptic proteins that participate in vesicular transmitter release at chemical synapses. Using immunocytochemistry, we evaluated the cellular and subcellular distribution of complexin I/II, syntaxin-1, and synapsin I in rabbit retina. Strong labeling for complexin I/II, proteins that regulate a late step in vesicular transmitter release, was found in both synaptic layers of the retina, and in somata of A- and B-type horizontal cells, of gamma-aminobutyric acid (GABA)- and glycinergic amacrine cells, and of ganglion cells. Immunoelectron microscopy demonstrated the presence of complexin I/II in horizontal cell processes postsynaptic to rod and cone ribbon synapses. Syntaxin-1, a core protein of the soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) complex known to bind to complexin, and synapsin I, a synaptic vesicle-associated protein involved in the Ca(2+)-dependent recruitment of synaptic vesicles for transmitter release, were also present in the horizontal cells and their processes at photoreceptor synapses. Photoreceptors and bipolar cells did not express any of these proteins at their axon terminals. The presence of complexin I/II, syntaxin-1, and synapsin I in rabbit horizontal cell processes and tips suggests that a vesicular mechanism may underlie transmitter release from mammalian horizontal cells.

Developmental plasticity of photoreceptors

During development, retinal ganglion cells undergo conspicuous structural remodeling as they gradually attain their mature morphology and connectivity. Alterations in their dendritic organization and in their axonal projections can also be achieved following early insult to their targets or their afferents. Other retinal cell types are thought not to display this same degree of developmental plasticity. The present review will consider the evidence, drawn largely from recent experimental studies in the carnivore retina, that photoreceptors also undergo structural remodeling, extending their terminals transiently into inner plexiform layer before retracting to the outer plexiform layer. The determinants of this transient targeting to the inner plexiform layer are considered, and the role of cholinergic amacrine cells is discussed. The factors triggering this retraction are also considered, including the concurrent maturational changes in outer segment formation and in the differentiation of the outer plexiform layer. These results provide new insight into the life history of the photoreceptor cell and its connectivity, and suggest a transient role for the photoreceptors in the circuitry of the inner retina during early development, prior to the onset of phototransduction.

Expression of vesicular glutamate transporter 1 in the mouse retina reveals temporal ordering in development of rod vs. cone and ON vs. OFF circuits

Glutamatergic transmission is crucial to the segregation of ON and OFF pathways in the developing retina. The temporal sequence of maturation of vesicular glutamatergic transmission in rod and cone photoreceptor and ON and OFF bipolar cell terminals is currently unknown. Vesicular glutamate transporters (VGLUTs) that load glutamate into synaptic vesicles are necessary for vesicular glutamatergic transmission. To understand better the formation and maturation of glutamatergic transmission in the rod vs. cone and ON vs. OFF pathways of the retina, we examined the developmental expression of VGLUT1 and VGLUT2 immunocytochemically in the mouse retina. Photoreceptor and bipolar cell terminals showed only VGLUT1-immunoreactivity (-IR); no VGLUT2-IR was present in any synapses of the developing or adult retina. VGLUT1-IR was first detected in cone photoreceptor terminals at postnatal day 2 (P2), several days before initiation of ribbon synapse formation at P4-P5. Rod terminals showed VGLUT1-IR by P8, when they invade the outer plexiform layer (OPL) and initiate synaptogenesis. Developing OFF bipolar cell terminals showed VGLUT1-IR around P8, 2-3 days after bipolar terminals were first identified in the inner plexiform layer (IPL) by labeling for the photoreceptor and bipolar cell terminal marker, synaptic vesicle protein 2B. Although terminals of ON bipolar cells were present in the IPL by P6-P8, most did not show VGLUT1-IR until P8-P10 and increased dramatically from P12. These data suggest a hierarchical development of glutamatergic transmission in which cone circuits form prior to rod circuits in both the OPL and IPL, and OFF circuits form prior to ON circuits in the IPL.

Synaptic plasticity and functionality at the cone terminal of the developing zebrafish retina

Previous studies have analyzed photoreceptor development, some inner retina cell types, and specific neurotransmitters in the zebrafish retina. However, only minor attention has been paid to the morphology of the synaptic connection between photoreceptors and second order neurons even though it represents the transition from the light sensitive receptor to the neuronal network of the visual system. Here, we describe the appearance and differentiation of pre- and postsynaptic elements at cone synapses in the developing zebrafish retina together with the maturation of the directly connecting second order neurons and a dopaminergic third order feedback-neuron from the inner retina. Zebrafish larvae were examined at developmental stages from 2 to 7dpf (days postfertilization) and in the adult. Synaptic maturation at the photoreceptor terminals was examined with antibodies against synapse associated proteins. The appearance of synaptic plasticity at the so-called spinule-type synapses between cones and horizontal cells was assessed by electron microscopy, and the maturation of photoreceptor downstream connection was identified by immunocytochemistry for GluR4 (AMPA-type glutamate receptor subunit), protein kinase beta(1) (mixed rod-cone bipolar cells), and tyrosine hydroxylase (dopaminergic interplexiform cells). We found that developing zebrafish retinas possess first synaptic structures at the cone terminal as early as 3.5dpf. Morphological maturation of these synapses at 3.5-4dpf, together with the presence of synapse associated proteins at 2.5dpf and the maturation of second order neurons by 5dpf, indicate functional synaptic connectivity and plasticity between the cones and their second order neurons already at 5dpf. However, the mere number of spinules and ribbons at 7dpf still remains below the adult values, indicating that synaptic functionality of the zebrafish retina is not entirely completed at this stage of development.

Heterogeneous expression of SNAP-25 and synaptic vesicle proteins by central and peripheral inputs to sympathetic neurons

Neurons in prevertebral sympathetic ganglia receive convergent synaptic inputs from peripheral enteric neurons in addition to inputs from spinal preganglionic neurons. Although all inputs are functionally cholinergic, inputs from these two sources have distinctive neurochemical and functional profiles. We used multiple-labeling immunofluorescence, quantitative confocal microscopy, ultrastructural immunocytochemistry, and intracellular electrophysiologic recordings to examine whether populations of inputs to the guinea pig coeliac ganglion express different levels of synaptic proteins that could influence synaptic strength. Boutons of enteric intestinofugal inputs, identified by immunoreactivity to vasoactive intestinal peptide, showed considerable heterogeneity in their immunoreactivity to synaptosome-associated protein of 25 kDa (SNAP-25), synapsin, synaptophysin, choline acetyltransferase, and vesicular acetylcholine transporter. Mean levels of immunoreactivity to these proteins were significantly lower in terminals of intestinofugal inputs compared with terminals of spinal preganglionic inputs. Nevertheless, many boutons with undetectable levels of SNAP-25 immunoreactivity formed morphologically normal synapses with target neurons. Treatment with botulinum neurotoxin type A (20-50 nM for 2 hours in vitro) generated significant cleavage of SNAP-25 and produced similar dose- and time-dependent inhibitions of synaptic transmission from all classes of inputs, regardless of their mean level of SNAP-25 expression. The simplest interpretation of these results is that only synaptic boutons with detectable levels of SNAP-25 immunoreactivity contribute significantly to fast cholinergic transmission. Consequently, the low synaptic strength of intestinofugal inputs to final motor neurons in sympathetic pathways may be due in part to the low proportion of their boutons that express SNAP-25 and other synaptic proteins.

Redistribution of syntaxin mRNA in neuronal cell bodies regulates protein expression and transport during synapse formation and long-term synaptic plasticity

Syntaxin has an important role in regulating vesicle docking and fusion essential for neurotransmitter release. Here, we demonstrate that the distribution of syntaxin mRNA in cell bodies of sensory neurons (SNs) of Aplysia maintained in cell culture is affected by synapse formation, synapse stabilization, and long-term facilitation (LTF) produced by 5-HT. The distribution of the mRNA in turn regulates expression and axonal transport of the protein. Syntaxin mRNA and protein accumulated at the axon hillock of SNs during the initial phase of synapse formation. Significant numbers of granules containing syntaxin were detected in the SN axon. When synaptic strength was stable, both mRNA and protein were targeted away from the axon hillock, and the number of syntaxin granules in the SN axon was reduced. Dramatic increases in mRNA and protein accumulation at the axon hillock and number of syntaxin granules in the SN axon were produced when cultures with stable connections were treated with 5-HT that evoked LTF. Anisomycin (protein synthesis inhibitor) or KT5720 (protein kinase A inhibitor) blocked LTF, accumulation of syntaxin mRNA and protein at the axon hillock, and the increase in syntaxin granules in SN axons. The results indicate that without significant effects on overall mRNA expression, both target interaction and 5-HT via activation of protein kinase A pathway regulate expression of syntaxin and its packaging for transport into axons by influencing the distribution of its mRNA in the SN cell body.

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

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

Exocytosis, endocytosis, and development

During development, metazoans are faced with the daunting task of generating many different cell types in a temporally and spatially precise manner. This orderly process of cell generation relies on creating localized signals that activate or inhibit specific cellular pathways. Recent work has shown that some of these localized signals require the targeted secretion of proteins, or their uptake by endocytosis. The importance of these protein trafficking pathways in localized signal generation is further substantiated by endo- and exocytosis mutants which can phenocopy many developmental mutants. Genetic and molecular techniques that increase our ability to inhibit exocytosis and endocytosis in a temporal and cell-type specific manner are likely to further elucidate the complexities of development.

Expression of SNAP-25 during mammalian retinal development: thinking outside the synapse

The SNARE complex is the core machinery required for vesicle fusion events. Numerous structural, functional, and genetic studies have led to a better understanding of mechanisms that regulate vesicle fusion events during neural development. Studies using the mammalian retina as a model system have increased our understanding of the dynamic patterns of expression of SNARE proteins. In particular, the SNARE complex protein SNAP-25 is expressed in a dynamic fashion during the development of cholinergic amacrine cells in a number of mammalian species. SNAP-25 is also likely to play a crucial role during the development of vertebrate photoreceptors. The integration of comparative studies examining SNARE proteins, such as SNAP-25, provides a powerful approach for the study of CNS development.

Synaptic protein expression by regenerating adult photoreceptors

Regeneration of functionally normal synapses is required for functional recovery after degenerative central nervous system insults and requires proper expression and targeting of presynaptic proteins by regenerating neurons. The reconstitution of presynaptic terminals by regenerating adult neurons is poorly understood, however. We examined the intrinsic ability of regenerating adult retinal photoreceptors to reconstitute properly differentiated presynaptic terminals in the absence of target contact. The expression and localization of vesicle-associated membrane protein (VAMP), synaptic vesicle protein 2 (SV2), synaptophysin, synapsin I, and synaptosomal-associated protein of 25 kDa (SNAP-25) was assessed immunocytochemically. Photoreceptor terminals in the intact retina contain VAMP, SV2, synaptophysin, and SNAP-25, but not synapsin I. Isolated, regenerating adult photoreceptors intrinsically expressed the proper complement of synaptic vesicle proteins in the absence of target contact: VAMP, SV2, and synaptophysin were present at all stages of regenerative growth; synapsin I was never expressed. At early stages of regenerative growth, VAMP, SV2, and synaptophysin were diffusely localized in the cell, with prominent VAMP labeling distributed along the plasma membrane. SV2 and synaptophysin rapidly localized to regenerated terminals, but VAMP accumulated much more slowly, indicating that these proteins are trafficked independently. In contrast, labeling for SNAP-25, which is associated with the presynaptic plasma membrane, was undetectable in regenerating photoreceptors, suggesting that SNAP-25 expression is target-regulated. Thus, regenerating photoreceptors can intrinsically regulate the expression of the proper set of synaptic vesicle proteins. Proper expression of other presynaptic proteins, such as SNAP-25, and proper subcellular localization of synaptic proteins such as VAMP, however, may require extrinsic cues such as target contact.