RIBEYE recruits Munc119, a mammalian ortholog of the Caenorhabditis elegans protein unc119, to synaptic ribbons of photoreceptor synapses

Rabconnectin-3α/DMXL2 Is Locally Enriched at the Synaptic Ribbon of Rod Photoreceptor Synapses

Ribbon synapses reliably transmit synaptic signals over a broad signalling range. Rod photoreceptor ribbon synapses are capable of transmitting signals generated by the absorption of single photons. The high precision of ribbon synapses emphasizes the need for particularly efficient signalling mechanisms. Synaptic ribbons are presynaptic specializations of ribbon synapses and are anchored to the active zone. Synaptic ribbons bind many synaptic vesicles that are delivered to the active zone for continuous and faithful signalling. In the present study we demonstrate with independent antibodies at the light- and electron microscopic level that rabconnectin-3α (RC3α)-alternative name Dmx-like 2 (DMXL2)-is localized to the synaptic ribbons of rod photoreceptor synapses in the mouse retina. In the brain, RC3α-containing complexes are known to interact with important components of synaptic vesicles, including Rab3-activating/inactivating enzymes, priming proteins and the vesicular H⁺-ATPase that acidifies the synaptic vesicle lumen to promote full neurotransmitter loading. The association of RC3α/DMXL2 with rod synaptic ribbons of the mouse retina could enable these structures to deliver only fully signalling-competent synaptic vesicles to the active zone thus contributing to reliable synaptic communication.

A Novel Role for UNC119 as an Enhancer of Synaptic Transmission

Mammalian UNC119 is a ciliary trafficking chaperone highly expressed in the inner segment of retinal photoreceptors. Previous research has shown that UNC119 can bind to transducin, the synaptic ribbon protein RIBEYE, and the calcium-binding protein CaBP4, suggesting that UNC119 may have a role in synaptic transmission. We made patch-clamp recordings from retinal slices in mice with the UNC119 gene deleted and showed that removal of even one gene of UNC119 has no effect on the rod outer segment photocurrent, but acted on bipolar cells much like background light: it depolarized membrane potential, decreased sensitivity, accelerated response decay, and decreased the Hill coefficient of the response-intensity relationship. Similar effects were seen on rod bipolar-cell current and voltage responses, and after exposure to bright light to translocate transducin into the rod inner segment. These findings indicate that UNC119 deletion reduces the steady-state glutamate release rate at rod synapses, though no change in the voltage dependence of the synaptic Ca current was detected. We conclude that UNC119, either by itself or together with transducin, can facilitate the release of glutamate at rod synapses, probably by some interaction with RIBEYE or other synaptic proteins rather than by binding to CaBP4 or calcium channels.

Ribbon Synapses and Retinal Disease: Review

Synaptic ribbons are presynaptic protein complexes that are believed to be important for the transmission of sensory information in the visual system. Ribbons are selectively associated with those synapses where graded changes in membrane potential drive continuous neurotransmitter release. Defective synaptic transmission can arise as a result of the mutagenesis of a single ribbon component. Visual diseases that stem from malfunctions in the presynaptic molecular machinery of ribbon synapses in the retina are rare. In this review, we provide an overview of synaptopathies that give rise to retinal malfunction and our present understanding of the mechanisms that underlie their pathogenesis and discuss muscular dystrophies that exhibit ribbon synapse involvement in the pathology.

Ciliary Proteins Repurposed by the Synaptic Ribbon: Trafficking Myristoylated Proteins at Rod Photoreceptor Synapses

The Unc119 protein mediates transport of myristoylated proteins to the photoreceptor outer segment, a specialized primary cilium. This transport activity is regulated by the GTPase Arl3 as well as by Arl13b and Rp2 that control Arl3 activation/inactivation. Interestingly, Unc119 is also enriched in photoreceptor synapses and can bind to RIBEYE, the main component of synaptic ribbons. In the present study, we analyzed whether the known regulatory proteins, that control the Unc119-dependent myristoylated protein transport at the primary cilium, are also present at the photoreceptor synaptic ribbon complex by using high-resolution immunofluorescence and immunogold electron microscopy. We found Arl3 and Arl13b to be enriched at the synaptic ribbon whereas Rp2 was predominantly found on vesicles distributed within the entire terminal. These findings indicate that the synaptic ribbon could be involved in the discharge of Unc119-bound lipid-modified proteins. In agreement with this hypothesis, we found Nphp3 (Nephrocystin-3), a myristoylated, Unc119-dependent cargo protein enriched at the basal portion of the ribbon in close vicinity to the active zone. Mutations in Nphp3 are known to be associated with Senior–Løken Syndrome 3 (SLS3). Visual impairment and blindness in SLS3 might thus not only result from ciliary dysfunctions but also from malfunctions of the photoreceptor synapse.

Conditional Deletion of Cytoplasmic Dynein Heavy Chain in Postnatal Photoreceptors

Purpose

Cytoplasmic dynein-1 (henceforth dynein) moves cargo in conjunction with dynactin toward the minus end of microtubules. The dynein heavy chain, DYNC1H1, comprises the backbone of dynein, a retrograde motor. Deletion of Dync1h1 abrogates dynein function. The purpose of this communication is to demonstrate effects of photoreceptor dynein inactivation during late postnatal development and in adult retina.

Methods

We mated Dync1h1F/F mice with iCre75 and Prom1-CreERT2 mice to generate conditional rod and tamoxifen-induced knockout in rods and cones, respectively. We documented retina degeneration with confocal microscopy at postnatal day (P) 10 to P30 for the iCre75 line and 1 to 4 weeks post tamoxifen induction (wPTI) for the Prom1-CreERT2 line. We performed scotopic and photopic electroretinography (ERG) at P16 to P30 in the iCre75 line and at 1-week increments in the Prom1-CreERT2 line. Results were evaluated statistically using Student's t-test, two-factor ANOVA, and Welch's ANOVA.

Results

Cre-induced homologous recombination of Dync1h1F/F mice truncated DYNC1H1 after exon 23. rodDync1h1-/- photoreceptors degenerated after P14, reducing outer nuclear layer (ONL) thickness and combined inner segment/outer segment (IS/OS) length significantly by P18. Scotopic ERG a-wave amplitudes decreased by P16 and were extinguished at P30. Cones were stable under rod-knockout conditions until P21 but inactive at P30. In tamDync1h1-/- photoreceptors, the IS/OS began shortening by 3wPTI and were nearly eliminated by 4wPTI. The ONL shrank significantly over this interval, indicating rapid photoreceptor degeneration following the loss of dynein.

Conclusions

Our results demonstrate dynein is essential for the secretory pathway, formation of outer segments, and photoreceptor maintenance.

Functional compartmentalization of photoreceptor neurons

Retinal photoreceptors are neurons that convert dynamically changing patterns of light into electrical signals that are processed by retinal interneurons and ultimately transmitted to vision centers in the brain. They represent the essential first step in seeing without which the remainder of the visual system is rendered moot. To support this role, the major functions of photoreceptors are segregated into three main specialized compartments-the outer segment, the inner segment, and the pre-synaptic terminal. This compartmentalization is crucial for photoreceptor function-disruption leads to devastating blinding diseases for which therapies remain elusive. In this review, we examine the current understanding of the molecular and physical mechanisms underlying photoreceptor functional compartmentalization and highlight areas where significant knowledge gaps remain.

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.

Compartmentalization of Photoreceptor Sensory Cilia

Functional compartmentalization of cells is a universal strategy for segregating processes that require specific components, undergo regulation by modulating concentrations of those components, or that would be detrimental to other processes. Primary cilia are hair-like organelles that project from the apical plasma membranes of epithelial cells where they serve as exclusive compartments for sensing physical and chemical signals in the environment. As such, molecules involved in signal transduction are enriched within cilia and regulating their ciliary concentrations allows adaptation to the environmental stimuli. The highly efficient organization of primary cilia has been co-opted by major sensory neurons, olfactory cells and the photoreceptor neurons that underlie vision. The mechanisms underlying compartmentalization of cilia are an area of intense current research. Recent findings have revealed similarities and differences in molecular mechanisms of ciliary protein enrichment and its regulation among primary cilia and sensory cilia. Here we discuss the physiological demands on photoreceptors that have driven their evolution into neurons that rely on a highly specialized cilium for signaling changes in light intensity. We explore what is known and what is not known about how that specialization appears to have driven unique mechanisms for photoreceptor protein and membrane compartmentalization.

Transducin Partners Outside the Phototransduction Pathway

Transducin mediates signal transduction in a classical G protein-coupled receptor (GPCR) phototransduction cascade. Interactions of transducin with the receptor and the effector molecules had been extensively investigated and are currently defined at the atomic level. However, partners and functions of rod transducin α (Gα_{t 1}) and βγ (Gβ₁γ₁) outside the visual pathway are not well-understood. In particular, light-induced redistribution of rod transducin from the outer segment to the inner segment and synaptic terminal (IS/ST) allows Gα_t1 and/or Gβ₁γ₁ to modulate synaptic transmission from rods to rod bipolar cells (RBCs). Protein-protein interactions underlying this modulation are largely unknown. We discuss known interactors of transducin in the rod IS/ST compartment and potential pathways leading to the synaptic effects of light-dispersed Gα_t1 and Gβ₁γ₁. Furthermore, we show that a prominent non-GPCR guanine nucleotide exchange factor (GEF) and a chaperone of Gα subunits, resistance to inhibitors of cholinesterase 8A (Ric-8A) protein, is expressed throughout the retina including photoreceptor cells. Recent structures of Ric-8A alone and in complexes with Gα subunits have illuminated the structural underpinnings of the Ric-8A activities. We generated a mouse model with conditional knockout of Ric-8A in rods in order to begin defining the functional roles of the protein in rod photoreceptors and the retina. Our analysis suggests that Ric-8A is not an obligate chaperone of Gα_t1. Further research is needed to investigate probable roles of Ric-8A as a GEF, trafficking chaperone, or a mediator of the synaptic effects of Gα_t1.

The RAS-interacting chaperone UNC119 drives the RASSF6-MDM2-p53 axis and antagonizes RAS-mediated malignant transformation

The gene encoding the proto-oncogene GTPase RAS is frequently mutated in human cancers. Mutated RAS proteins trigger antiapoptotic and cell-proliferative signals and lead to oncogenesis. However, RAS also induces apoptosis and senescence, which may contribute to the eradication of cells with RAS mutations. We previously reported that Ras association domain family member 6 (RASSF6) binds MDM2 and stabilizes the tumor suppressor p53 and that the active form of KRAS promotes the interaction between RASSF6 and MDM2. We also reported that Unc-119 lipid-binding chaperone (UNC119A), a chaperone of myristoylated proteins, interacts with RASSF6 and regulates RASSF6-mediated apoptosis. In this study, using several human cancer cell lines, quantitative RT-PCR, RNAi-based gene silencing, and immunoprecipitation/-fluorescence and cell biology assays, we report that UNC119A interacts with the active form of KRAS and that the C-terminal modification of KRAS is required for this interaction. We also noted that the hydrophobic pocket of UNC119A, which binds the myristoylated peptides, is not involved in the interaction. We observed that UNC119A promotes the binding of KRAS to RASSF6, enhances the interaction between RASSF6 and MDM2, and induces apoptosis. Conversely, UNC119A silencing promoted soft-agar colony formation, migration, and invasiveness in KRAS-mutated cancer cells. We conclude that UNC119A promotes KRAS-mediated p53-dependent apoptosis via RASSF6 and may play a tumor-suppressive role in cells with KRAS mutations.

The absence of BBSome function decreases synaptogenesis and causes ectopic synapse formation in the retina

Photoreceptors possess ribbon synapses distinct from the conventional synapses in the brain. Little is known about the function of the BBSome, a complex integral in ciliary and intracellular trafficking, in ribbon synaptic formation. We performed immunohistochemistry using retinas from Bardet-Biedl Syndrome (BBS) mouse models and found that BBS mutant animals have significantly fewer ribbon synapses in the outer plexiform layer and increased ectopic synapses in the outer nuclear layer compared to controls. Many ectopic synapses in BBS mutant retinas are associated with horizontal cell axonal processes that aberrantly intrude into the outer nuclear layer. To determine whether this horizontal cell phenotype is a consequence of retinal degeneration, we examined this phenotype in mice with photoreceptor-specific inactivation of the BBSome induced by Cre recombinase driven by the rhodopsin promoter. At three months of age, despite retinal degeneration, Bbs8^{floxed/floxed}; Rho-Cre⁺ mice lack the aberrant intrusion of horizontal cell processes. At 6 months, some horizontal cell processes intrude into the outer nuclear layer in Bbs8^{floxed/floxed}; Rho-Cre⁺ mice, but the phenotype does not recapitulate the phenotypic severity observed in young congenital BBS mutant mice. Therefore, the lack of BBSome function negatively impacts retinal synaptogenesis, and causes horizontal cell defects in a potentially cell-autonomous fashion.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Early auto-immune targeting of photoreceptor ribbon synapses in mouse models of multiple sclerosis

Optic neuritis is one of the first manifestations of multiple sclerosis. Its pathogenesis is incompletely understood, but considered to be initiated by an auto‐immune response directed against myelin sheaths of the optic nerve. Here, we demonstrate in two frequently used and well‐validated mouse models of optic neuritis that ribbon synapses in the myelin‐free retina are targeted by an auto‐reactive immune system even before alterations in the optic nerve have developed. The auto‐immune response is directed against two adhesion proteins (CASPR1/CNTN1) that are present both in the paranodal region of myelinated nerves as well as at retinal ribbon synapses. This occurs in parallel with altered synaptic vesicle cycling in retinal ribbon synapses and altered visual behavior before the onset of optic nerve demyelination. These findings indicate that early synaptic dysfunctions in the retina contribute to the pathology of optic neuritis in multiple sclerosis.

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

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

Noise-induced Cochlear Synaptopathy and Signal Processing Disorders

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

UNC119 is a binding partner of tumor suppressor Ras-association domain family 6 and induces apoptosis and cell cycle arrest by MDM2 and p53

Ras-association domain family 6 (RASSF6) is a tumor suppressor that interacts with MDM2 and stabilizes p53. Caenorhabditis elegans unc-119 encodes a protein that is required for normal development of the nervous system. Humans have 2 unc-119 homologues, UNC119 and UNC119B. We have identified UNC119 as a RASSF6-interacting protein. UNC119 promotes the interaction between RASSF6 and MDM2 and stabilizes p53. Thus, UNC119 induces apoptosis by RASSF6 and p53. UNC119 depletion impairs DNA repair after DNA damage and results in polyploid cell generation. These findings support that UNC119 is a regulator of the RASSF6-MDM2-p53 axis and functions as a tumor suppressor.

Coordinating the uncoordinated: UNC119 trafficking in cilia

Constructing the distinct subcellular environment of the cilium relies in a large part upon intraflagellar transport (IFT) proteins, which traffic cargo both to and within the cilium. However, evidence from the last 10 years suggests that IFT alone is not sufficient to generate the ciliary environment. One essential factor is UNC119, which interacts with known IFT molecular switches to transport ciliary cargos. Despite its apparent importance in ciliary trafficking though, human UNC119 mutations have only rarely been associated with diseases commonly linked with ciliopathies. This review will outline the trafficking pathways required for constructing the cilium by highlighting UNC119's role and the complexities involved in ciliary trafficking. Finally, despite important roles for UNC119 in cilia, UNC119 proteins also interact with non-ciliary proteins to affect other cellular processes.

RIBEYE(B)-domain binds to lipid components of synaptic vesicles in an NAD(H)-dependent, redox-sensitive manner

Synaptic ribbons are needed for fast and continuous exocytosis in ribbon synapses. RIBEYE is a main protein component of synaptic ribbons and is necessary to build the synaptic ribbon. RIBEYE consists of a unique A-domain and a carboxyterminal B-domain, which binds NAD(H). Within the presynaptic terminal, the synaptic ribbons are in physical contact with large numbers of synaptic vesicle (SV)s. How this physical contact between ribbons and synaptic vesicles is established at a molecular level is not well understood. In the present study, we demonstrate that the RIBEYE(B)-domain can directly interact with lipid components of SVs using two different sedimentation assays with liposomes of defined chemical composition. Similar binding results were obtained with a SV-containing membrane fraction. The binding of liposomes to RIBEYE(B) depends upon the presence of a small amount of lysophospholipids present in the liposomes. Interestingly, binding of liposomes to RIBEYE(B) depends on NAD(H) in a redox-sensitive manner. The binding is enhanced by NADH, the reduced form, and is inhibited by NAD⁺, the oxidized form. Lipid-mediated attachment of vesicles is probably part of a multi-step process that also involves additional, protein-dependent processes.

The Disease Protein Tulp1 Is Essential for Periactive Zone Endocytosis in Photoreceptor Ribbon Synapses

Mutations in the Tulp1 gene cause severe, early-onset retinitis pigmentosa (RP14) in humans. In the retina, Tulp1 is mainly expressed in photoreceptors that use ribbon synapses to communicate with the inner retina. In the present study, we demonstrate that Tulp1 is highly enriched in the periactive zone of photoreceptor presynaptic terminals where Tulp1 colocalizes with major endocytic proteins close to the synaptic ribbon. Analyses of Tulp1 knock-out mice demonstrate that Tulp1 is essential to keep endocytic proteins enriched at the periactive zone and to maintain high levels of endocytic activity close to the synaptic ribbon. Moreover, we have discovered a novel interaction between Tulp1 and the synaptic ribbon protein RIBEYE, which is important to maintain synaptic ribbon integrity. The current findings suggest a new model for Tulp1-mediated localization of the endocytic machinery at the periactive zone of ribbon synapses and offer a new rationale and mechanism for vision loss associated with genetic defects in Tulp1.

How to make a synaptic ribbon: RIBEYE deletion abolishes ribbons in retinal synapses and disrupts neurotransmitter release

Synaptic ribbons are large proteinaceous scaffolds at the active zone of ribbon synapses that are specialized for rapid sustained synaptic vesicles exocytosis. A single ribbon-specific protein is known, RIBEYE, suggesting that ribbons may be constructed from RIBEYE protein. RIBEYE knockdown in zebrafish, however, only reduced but did not eliminate ribbons, indicating a more ancillary role. Here, we show in mice that full deletion of RIBEYE abolishes all presynaptic ribbons in retina synapses. Using paired recordings in acute retina slices, we demonstrate that deletion of RIBEYE severely impaired fast and sustained neurotransmitter release at bipolar neuron/AII amacrine cell synapses and rendered spontaneous miniature release sensitive to the slow Ca(2+)-buffer EGTA, suggesting that synaptic ribbons mediate nano-domain coupling of Ca(2+) channels to synaptic vesicle exocytosis. Our results show that RIBEYE is essential for synaptic ribbons as such, and may organize presynaptic nano-domains that position release-ready synaptic vesicles adjacent to Ca(2+) channels.

Ribbon Synapses and Visual Processing in the Retina

The first synapses transmitting visual information contain an unusual organelle, the ribbon, which is involved in the transport and priming of vesicles to be released at the active zone. The ribbon is one of many design features that allow efficient refilling of the active zone, which in turn enables graded changes in membrane potential to be transmitted using a continuous mode of neurotransmitter release. The ribbon also plays a key role in supplying vesicles for rapid and transient bursts of release that signal fast changes, such as the onset of light. We increasingly understand how the physiological properties of ribbon synapses determine basic transformations of the visual signal and, in particular, how the process of refilling the active zone regulates the gain and adaptive properties of the retinal circuit. The molecular basis of ribbon function is, however, far from clear.

ArfGAP3 is a component of the photoreceptor synaptic ribbon complex and forms an NAD(H)-regulated, redox-sensitive complex with RIBEYE that is important for endocytosis

Ribbon synapses are tonically active synapses in the retina and inner ear with intense vesicle traffic. How this traffic is organized and regulated is still unknown. Synaptic ribbons, large presynaptic structures associated with numerous synaptic vesicles, appear to be essential for this process. The base of the synaptic ribbon is anchored at the active zone and is a hotspot of exocytosis. The synaptic ribbon complex is also important for vesicle replenishment. RIBEYE is a unique and major component of synaptic ribbons. It consists of a unique A-domain and an NAD(H)-binding, C-terminal B-domain. In the present study, we show that the Arf-GTPase activating protein-3 (ArfGAP3), a well characterized regulator of vesicle formation at the Golgi apparatus, is also a component of the synaptic ribbon complex in photoreceptor synapses of the mouse retina and interacts with RIBEYE as shown by multiple, independent approaches. ArfGAP3 binds to RIBEYE(B)-domain in an NAD(H)-dependent manner. The interaction is redox sensitive because NADH is more efficient than the oxidized NAD(+) in promoting ArfGAP3-RIBEYE interaction. RIBEYE competes with the GTP-binding protein Arf1 for binding to ArfGAP3. Thus, binding of RIBEYE(B) to ArfGAP3 could prevent inactivation of Arf1 by ArfGAP3 and provides the synaptic ribbon with the possibility to control Arf1 function. The interaction is relevant for endocytic vesicle trafficking because overexpression of ArfGAP3 in photoreceptors strongly inhibited endocytotic uptake of FM1-43.

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

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

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

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

A local, periactive zone endocytic machinery at photoreceptor synapses in close vicinity to synaptic ribbons

Photoreceptor ribbon synapses are continuously active synapses with large active zones that contain synaptic ribbons. Synaptic ribbons are anchored to the active zones and are associated with large numbers of synaptic vesicles. The base of the ribbon that is located close to L-type voltage-gated Ca(2+) channels is a hotspot of exocytosis. The continuous exocytosis at the ribbon synapse needs to be balanced by compensatory endocytosis. Recent analyses indicated that vesicle recycling at the synaptic ribbon is also an important determinant of synaptic signaling at the photoreceptor synapse. To get insights into mechanisms of vesicle recycling at the photoreceptor ribbon synapse, we performed super-resolution structured illumination microscopy and immunogold electron microscopy to localize major components of the endocytotic membrane retrieval machinery in the photoreceptor synapse of the mouse retina. We found dynamin, syndapin, amphiphysin, and calcineurin, a regulator of activity-dependent endocytosis, to be highly enriched around the active zone and the synaptic ribbon. We present evidence for two clathrin heavy chain variants in the photoreceptor terminal; one is enriched around the synaptic ribbon, whereas the other is localized in the entry region of the terminal. The focal enrichment of endocytic proteins around the synaptic ribbon is consistent with a focal uptake of endocytic markers at that site. This endocytic activity functionally depends on dynamin. These data propose that the presynaptic periactive zone surrounding the synaptic ribbon complex is a hotspot of endocytosis in photoreceptor ribbon synapses.

Transducin translocation contributes to rod survival and enhances synaptic transmission from rods to rod bipolar cells

In rod photoreceptors, several phototransduction components display light-dependent translocation between cellular compartments. Notably, the G protein transducin translocates from rod outer segments to inner segments/spherules in bright light, but the functional consequences of translocation remain unclear. We generated transgenic mice where light-induced transducin translocation is impaired. These mice exhibited slow photoreceptor degeneration, which was prevented if they were dark-reared. Physiological recordings showed that control and transgenic rods and rod bipolar cells displayed similar sensitivity in darkness. After bright light exposure, control rods were more strongly desensitized than transgenic rods. However, in rod bipolar cells, this effect was reversed; transgenic rod bipolar cells were more strongly desensitized than control. This sensitivity reversal indicates that transducin translocation in rods enhances signaling to rod bipolar cells. The enhancement could not be explained by modulation of inner segment conductances or the voltage sensitivity of the synaptic Ca(2+) current, suggesting interactions of transducin with the synaptic machinery.

Habenular commissure formation in zebrafish is regulated by the pineal gland-specific gene unc119c

BACKGROUND

The zebrafish pineal gland (epiphysis) is a site of melatonin production, contains photoreceptor cells, and functions as a circadian clock pacemaker. Since it is located on the surface of the forebrain, it is accessible for manipulation and, therefore, is a useful model system to analyze pineal gland function and development. We previously analyzed the pineal transcriptome during development and showed that many genes exhibit a highly dynamic expression pattern in the pineal gland.

RESULTS

Among genes preferentially expressed in the zebrafish pineal gland, we identified a tissue-specific form of the unc119 gene family, unc119c, which is highly preferentially expressed in the pineal gland during day and night at all stages examined from embryo to adult. When expression of unc119c was inhibited, the formation of the habenular commissure (HC) was specifically compromised. The Unc119c interacting factors Arl3l1 and Arl3l2 as well as Wnt4a also proved indispensible for HC formation.

CONCLUSIONS

We suggest that Unc119c, together with Arl3l1/2, plays an important role in modulating Wnt4a production and secretion during HC formation in the forebrain of the zebrafish embryo.

The ribbon-associated protein C-terminal-binding protein 1 is not essential for the structure and function of retinal ribbon synapses

PURPOSE

Synaptic ribbons are organelles found at presynaptic active zones of sensory neurons that generate sustained graded electrical signals in response to stimuli, including retinal photoreceptor cells and bipolar neurons. RIBEYE is the major and specific protein constituent of ribbons; however, over the past decade an increasing number of other proteins have been identified at ribbon active zones, including C-terminal-binding protein 1 (CtBP1; a regulator of transcription and membrane trafficking that might bind to the B domain of RIBEYE). The appearance of CtBP1 together with RIBEYE suggests that it may contribute to ribbon function, but the possible role of CtBP1 at ribbon synapses has not yet been examined. Using CtBP1-knockout mice, we tested for functional effects of absence of CtBP1 protein.

METHODS

Confocal microscopy, electrophysiology, and electron microscopy were used to examine the structure and function of ribbon synapses in the retina and in isolated bipolar neurons from CtBP1 null mice compared with their wild-type littermates.

RESULTS

Expression of ribbons appeared to be normal in CtBP1 null mouse retina as revealed by immunofluorescence with an antibody to the B domain of RIBEYE and by binding studies using a fluorescent peptide that binds to RIBEYE in ribbons of living bipolar cells. Electron microscopy also showed grossly normal pre- and postsynaptic organization of ribbon synapses in both photoreceptors and bipolar cells. Synaptic vesicles were normal in size, but the overall density of reserve vesicles was reduced by ~20% in the cytoplasm of CtBP1 null ribbon synaptic terminals. However, the reduced vesicle density did not detectably alter synaptic function of bipolar neurons as revealed by activity-dependent loading of synaptic vesicles with FM4-64, presynaptic calcium current, capacitance measurements of synaptic exocytosis, and destaining of FM dye upon stimulation.

CONCLUSIONS

Overall the results suggest that CtBP1 protein is not essential for the formation of functional ribbon synapses in the retina.

UNC119a bridges the transmission of Fyn signals to Rab11, leading to the completion of cytokinesis

Src family kinases (SFKs) regulate the completion of cytokinesis through signal transduction pathways that lead to the Rab11-dependent phosphorylation of ERK and its localization to the midbody of cytokinetic cells. We find that UNC119a, a known activator of SFKs, plays essential roles in this signaling pathway. UNC119a localizes to the centrosome in interphase cells and begins to translocate from the spindle pole to the spindle midzone after the onset of mitosis; it then localizes to the intercellular bridge in telophase cells and to the midbody in cytokinetic cells. We show that the midbody localization of UNC119a is dependent on Rab11, and that knocking down UNC119a inhibits the Rab11-dependent phosphorylation and midbody localization of ERK and cytokinesis. Moreover, we demonstrate that UNC119a interacts with a Src family kinase, Fyn and is required for the activation of this kinase. These results suggest that UNC119a plays a key role in the Fyn signal transduction pathway, which regulates the completion of cytokinesis via Rab11.

Expression and subcellular distribution of UNC119a, a protein partner of transducin α subunit in rod photoreceptors

A recently discovered interaction of rod transducin α subunit (Gα(t1)) with UNC119a is thought to be important for transducin trafficking in photoreceptors. In this study, we analyzed the subcellular distribution of UNC119a under different conditions of illumination in vivo. Analyses by immunofluorescence and Western blotting of retina serial tangential sections demonstrated that UNC119a resides predominantly in the rod inner segment, with a small fraction of UNC119a also appearing to infiltrate the rod outer segment. Such a distribution is consistent with the proposed role of UNC119a in facilitating transducin transport from the rod inner segment to the outer segment in the dark. In addition, UNC119a was present in smaller amounts in the cell body and synaptic region of rods. The profile of UNC119a subcellular distribution remained largely unchanged under all tested conditions of illumination, and correlated with the profile of Gα(t1) following its light-dependent translocation. Quantification by Western blotting suggested that mouse retina contains ~17 pmol of UNC119a, giving a ~1 to 4 molar ratio of UNC119a to Gα(t1). Hence, light-translocated Gα(t1) can serve as a major partner of UNC119a. Supporting this role, the levels of UNC119a were downregulated by about 2-fold in mouse retina lacking Gα(t1). As a dominant partner, Gα(t1) may potentially modulate the function of other known UNC119a-interacting proteins involved in photoreceptor ciliary trafficking and synaptic regulation, in a light-dependent manner.

Uncoordinated (UNC)119: coordinating the trafficking of myristoylated proteins

The mechanism by which myristoylated proteins are targeted to specific subcellular membrane compartments is poorly understood. Two novel acyl-binding proteins, UNC119A and UNC119B, have been shown recently to function as chaperones/co-factors in the transport of myristoylated G protein α-subunits and src-type tyrosine kinases. UNC119 polypeptides feature an immunoglobulin-like β-sandwich fold that forms a hydrophobic pocket capable of binding lauroyl (C12) and myristoyl (C14) side chains. UNC119A in rod photoreceptors facilitates the transfer of transducin α subunits (Tα) from inner segment to outer segment membranes by forming an intermediate diffusible UNC119-Tα complex. Similar complexes are formed in other sensory neurons, as the G proteins ODR-3 and GPA-13 in Caenorhabditis elegans unc-119 mutants traffic inappropriately. UNC119B knockdown in IMCD3 cells prevents trafficking ofmyristoylated nephrocystin-3 (NPHP3), a protein associated with nephronophthisis, to cilia. Further, UNC119A was shown to transport myristoylated src-type tyrosine kinases to cell membranes and to affect T-cell receptor (TCR) and interleukin-5 receptor (IL-5R) activities. These interactions establish UNC119 polypeptides as novel lipid-binding chaperones with specificity for a diverse subset of myristoylated proteins.

High-resolution optical imaging of zebrafish larval ribbon synapse protein RIBEYE, RIM2, and CaV 1.4 by stimulation emission depletion microscopy

The synaptic ribbon is a unique presynaptic structure with an intricate morphology in photoreceptors. Because of the resolution limit in conventional fluorescence microscopy, investigating ribbon protein locations has been challenging, especially in the early development stages of model animals. Here, we used stimulated emission depletion microscopy, a super-resolution imaging technique, to look at retina sections in 4 days post-fertilization (dpf) zebrafish. We observed that in photoreceptor cells, RIBEYE and RIM2 are expressed along the synaptic ribbon, with RIM2 consistently located inside of the horseshoe-shaped synaptic ribbon structure with RIBEYE located on the outside. The L-type calcium channel subunit, CACNA1F, exhibited small spot-like staining beneath the RIM2 and RIBEYE structures. Using morpholino antisense oligonucleotides to knock down RIBEYE expression, we observed fewer and shorter ribbons in the photoreceptor outer plexiform layers of 4 dpf fish retina as well as a reduction in RIM2 expression. The clustering of CACNA1F in these blind fish was no longer observed, but instead showed a diffuse expression in the photoreceptor terminal.

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.

Basigin interacts with both MCT1 and MCT2 in murine spermatozoa

Lactate is provided to spermatogenic cells by Sertoli cells as an energy substrate and its transport is regulated by H(+)-monocarboxylate co-transporters (MCTs). In the case of several cell types it is known that MCT1 is associated with basigin and MCT2 with embigin. Here we demonstrate co-localization and co-immunoprecipitation of basigin with both MCT1 and MCT2 in sperm, whereas no interaction with embigin was detectable. An investigation of the functional activity of MCT proteins revealed that it was mainly the application of L-lactate which resulted in a decrease in pH(i) . The pH(i) changes were blocked with α-cyano-4-OH cinnamate and the preference for L-lactate-as opposed to D-Lactate-was demonstrated by the determination of ATP after exposure to both lactate isomers. We propose that basigin interacts with MCT1 and MCT2 to locate them properly in the membrane of spermatogenic cells and that this may enable sperm to utilize lactate as an energy substrate contributing to cell survival.

EF hand-mediated Ca- and cGMP-signaling in photoreceptor synaptic terminals

Photoreceptors, the light-sensitive receptor neurons of the retina, receive and transmit a plethora of visual informations from the surrounding world. Photoreceptors capture light and convert this energy into electrical signals that are conveyed to the inner retina. For synaptic communication with the inner retina, photoreceptors make large active zones that are marked by synaptic ribbons. These unique synapses support continuous vesicle exocytosis that is modulated by light-induced, graded changes of membrane potential. Synaptic transmission can be adjusted in an activity-dependent manner, and at the synaptic ribbons, Ca²⁺- and cGMP-dependent processes appear to play a central role. EF-hand-containing proteins mediate many of these Ca²⁺- and cGMP-dependent functions. Since continuous signaling of photoreceptors appears to be prone to malfunction, disturbances of Ca²⁺- and cGMP-mediated signaling in photoreceptors can lead to visual defects, retinal degeneration (rd), and even blindness. This review summarizes aspects of signal transmission at the photoreceptor presynaptic terminals that involve EF-hand-containing Ca²⁺-binding proteins.

The synaptic ribbon is a site of phosphatidic acid generation in ribbon synapses

Ribbon synapses continuously transmit graded membrane potential changes into changes of synaptic vesicle exocytosis and rely on intense synaptic membrane trafficking. The synaptic ribbon is considered central to this process. In the present study we asked whether tonically active ribbon synapses are associated with the generation of certain lipids, specifically the highly active signaling phospholipid phosphatidic acid (PA). Using PA-sensor proteins, we demonstrate that PA is enriched at mouse retinal ribbon synapses in close vicinity to the synaptic ribbon in situ. As shown by heterologous expression, RIBEYE, a main component of synaptic ribbons, is responsible for PA binding at synaptic ribbons. Furthermore, RIBEYE is directly involved in the synthesis of PA. Using various independent substrate binding and enzyme assays, we demonstrate that the B domain of RIBEYE possesses lysophosphatidic acid (LPA) acyltransferase (LPAAT) activity, which leads to the generation of PA from LPA. Since an LPAAT-deficient RIBEYE mutant does not recruit PA-binding proteins to artificial synaptic ribbons, whereas wild-type RIBEYE supports PA binding, we conclude that the LPAAT activity of the RIBEYE(B) domain is a physiologically relevant source of PA generation at the synaptic ribbon. We propose that PA generated at synaptic ribbons likely facilitates synaptic vesicle trafficking.

UNC119 is required for G protein trafficking in sensory neurons

UNC119 is widely expressed among vertebrates and other phyla. We found that UNC119 recognized the acylated N terminus of the rod photoreceptor transducin α (Tα) subunit and Caenorhabditis elegans G proteins ODR-3 and GPA-13. The crystal structure of human UNC119 at 1.95-Å resolution revealed an immunoglobulin-like β-sandwich fold. Pulldowns and isothermal titration calorimetry revealed a tight interaction between UNC119 and acylated Gα peptides. The structure of co-crystals of UNC119 with an acylated Tα N-terminal peptide at 2.0 Å revealed that the lipid chain is buried deeply into UNC119's hydrophobic cavity. UNC119 bound Tα-GTP, inhibiting its GTPase activity, thereby providing a stable UNC119-Tα-GTP complex capable of diffusing from the inner segment back to the outer segment after light-induced translocation. UNC119 deletion in both mouse and C. elegans led to G protein mislocalization. Thus, UNC119 is a Gα subunit cofactor essential for G protein trafficking in sensory cilia.

HIF signaling and overall gene expression changes during hypoxia and prolonged exercise differ considerably

Exercise as well as hypoxia cause an increase in angiogenesis, changes in mitochondrial density and alterations in metabolism, but it is still under debate whether the hypoxia inducible factor (HIF) is active during both situations. In this study gene expression analysis of zebrafish larvae that were raised under normoxic, hypoxic, or training conditions were compared, using microarray analysis, quantitative real-time PCR and protein data. Although HIF expression is posttranslationally regulated, mRNA expression levels of all three isoforms ( HIF-1α, HIF-2α, and HIF-3α) differed in each of the experimental groups, but the changes observed in hypoxic animals were much smaller than in trained larvae. Prominent changes were seen for Hif-2α expression, which significantly increased after the first day of exercise and then decreased down to values significantly below control values. HIF-3α mRNA expression in turn increased significantly, and at the end of the training period (9–15 days postfertilization) it was elevated three times. At the protein level a transient increase in HIF-1α was observed in hypoxic larvae, whereas in the exercise group the amount of HIF-1α protein even decreased below the level of control animals. The analyzed transcriptome was more affected in hypoxic zebrafish larvae, and hardly any genes were similarly altered by both treatments. These results clearly showed that HIF proteins played different roles in trained and hypoxic zebrafish larvae and that the exercise-induced transition to a more aerobic phenotype was not achieved by persistent activation of the hypoxic signaling pathway.

TbUNC119 and its binding protein complex are essential for propagation, motility, and morphogenesis of Trypanosoma brucei procyclic form cells

Flagellum-mediated motility of Trypanosoma brucei is considered to be essential for the parasite to complete stage development in the tsetse fly vector, while the mechanism by which flagellum-mediated motility is controlled are not fully understood. We thus compared T. brucei whole gene products (amino acid sequence) with Caenorhabditis elegans UNC (uncoordinated) proteins, in order to find uncharacterized motility-related T. brucei genes. Through in silico analysis, we found 88 gene products which were highly similar to C. elegans UNC proteins and categorized them as TbCEUN (T. brucei gene products which have high similarity to C. elegansUNC proteins). Approximately two thirds of the 88 TbCEUN gene products were kinesin-related molecules. A gene product highly similar to C. elegans UNC119 protein was designated as TbUNC119. RNAi-mediated depletion of TbUNC119 showed no apparent phenotype. However, knock-down analysis of both TbUNC119 and its binding protein (TbUNC119BP) which was found by yeast two-hybrid analysis showed characteristic phenotypes, including reduced motility, morphological change (extended cell shape), and cellular apoptosis. Based on the observed phenotypes, possible function of the TbUNC119 and TbUNC119BP is discussed.

Molecular anatomy of the hair cell's ribbon synapse

Hearing depends on reliable and temporally precise neurotransmission by cochlear hair cells. The wide dynamic range and high sensitivity with which these cells encode acoustic stimuli are associated with a presynaptic specialization termed the presynaptic dense body or synaptic ribbon. Apposed to the presynaptic density, this spherical or flattened structure tethers a layer of synaptic vesicles and is thought to facilitate their exocytotic fusion. Although defining the molecular constituents of the hair cell's synaptic ribbon should contribute to our understanding of neurotransmitter release at this synapse, accomplishing this task has been slowed by the difficulty of obtaining sufficient amounts of starting material for protein analysis from hair cells. We isolated synaptic material from chicken cochleas, purified synaptic ribbons with specific immunological reagents, and identified the associated proteins by tandem mass spectrometry. Purification of the ribbons revealed a predominant composition of C-terminal-binding proteins, especially ribeye, in association with the small GTPase Rab3, which is possibly involved in attaching vesicles to the ribbon. In comparison with the components of conventional synapses and of retinal ribbon synapses, we observed that certain regulatory proteins are excluded from the hair cell's synapse. Using antisera against several of the novel proteins and membrane-trafficking components that we had identified, we documented their localization in isolated hair cells. Our results indicate that the ribbon synapses of hair cells display modifications to the presynaptic machinery that are associated with the high-fidelity transmission of acoustic signals to the brain.

Nicotinamide adenine dinucleotide-dependent binding of the neuronal Ca2+ sensor protein GCAP2 to photoreceptor synaptic ribbons

Guanylate cyclase activating protein 2 (GCAP2) is a recoverin-like Ca2+-sensor protein known to modulate guanylate cyclase activity in photoreceptor outer segments. GCAP2 is also present in photoreceptor ribbon synapses where its function is unknown. Synaptic ribbons are active zone-associated presynaptic structures in the tonically active photoreceptor ribbon synapses and contain RIBEYE as a unique and major protein component. In the present study, we demonstrate by various independent approaches that GCAP2 specifically interacts with RIBEYE in photoreceptor synapses. We show that the flexible hinge 2 linker region of RIBEYE(B) domain that connects the nicotinamide adenine dinucleotide (NADH)-binding subdomain with the substrate-binding subdomain (SBD) binds to the C terminus of GCAP2. We demonstrate that the RIBEYE-GCAP2 interaction is induced by the binding of NADH to RIBEYE. RIBEYE-GCAP2 interaction is modulated by the SBD. GCAP2 is strongly expressed in synaptic terminals of light-adapted photoreceptors where GCAP2 is found close to synaptic ribbons as judged by confocal microscopy and proximity ligation assays. Virus-mediated overexpression of GCAP2 in photoreceptor synaptic terminals leads to a reduction in the number of synaptic ribbons. Therefore, GCAP2 is a prime candidate for mediating Ca2+-dependent dynamic changes of synaptic ribbons in photoreceptor synapses.

The making of synaptic ribbons: how they are built and what they do

Ribbon synapses in the retina and inner ear maintain tonic neurotransmitter release at high rates to transduce a broad bandwidth of stimulus intensities. In ribbon synapses, synaptic vesicles can be released by a slow, sustained mode and by fast, synchronous mechanisms. The high release rates require structural and functional specializations. The synaptic ribbon is the key structural specialization of ribbon synapses. Synaptic ribbons are large, electron-dense structures that immobilize numerous synaptic vesicles next to presynaptic release sites. A main component of synaptic ribbons is the protein RIBEYE that has the capability to build the scaffold of the synaptic ribbon via multiple RIBEYE-RIBEYE interactions. A modular assembly model of synaptic ribbons has been proposed in which synaptic ribbons are formed from individual RIBEYE subunits. The scaffold of the synaptic ribbon provides a docking site for RIBEYE-associated proteins that could execute specific synaptic ribbon functions. Multiple functions have been assigned to synaptic ribbons including roles in exocytosis, endocytosis, and synaptic membrane trafficking. Recent studies demonstrated the importance of synaptic ribbons for fast, synchronous release and emphasized the need of a tight and efficient coupling between presynaptic Ca(2+) signaling and exocytosis. The present review summarizes recent advances on structure and function of synaptic ribbons.

GCAP1 mutations associated with autosomal dominant cone dystrophy

We discuss the heterogeneity of autosomal dominant cone and cone-rod dystrophies (adCD, and adCORD, respectively). As one of the best characterized adCD genes, we focus on the GUCA1A gene encoding guanylate cyclase activating protein 1 (GCAP1), a protein carrying three high affinity Ca(2+) binding motifs (EF hands). GCAP1 senses changes in cytoplasmic free [Ca(2+)] and communicates these changes to GC1, by either inhibiting it (at high free [Ca(2+)]), or stimulating it (at low free [Ca(2+)]). A number of missense mutations altering the structure and Ca(2+) affinity of EF hands have been discovered. These mutations are associated with a gain of function, producing dominant cone and cone rod dystrophy phenotypes. In this article we review these mutations and describe the consequences of specific mutations on GCAP1 structure and GC stimulation.We discuss the heterogeneity of autosomal dominant cone and cone-rod dystrophies (adCD, and adCORD, respectively). As one of the best characterized adCD genes, we focus on the GUCA1A gene encoding guanylate cyclase activating protein 1 (GCAP1), a protein carrying three high affinity Ca(2+) binding motifs (EF hands). GCAP1 senses changes in cytoplasmic free [Ca(2+)] and communicates these changes to GC1, by either inhibiting it (at high free [Ca(2+)]), or stimulating it (at low free [Ca(2+)]). A number of missense mutations altering the structure and Ca(2+) affinity of EF hands have been discovered. These mutations are associated with a gain of function, producing dominant cone and cone rod dystrophy phenotypes. In this article we review these mutations and describe the consequences of specific mutations on GCAP1 structure and GC stimulation.

What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

IQ-ArfGEF/BRAG1 is associated with synaptic ribbons in the mouse retina

IQ-ArfGEF/BRAG1 is a guanine nucleotide exchange factor for ADP ribosylation factors (Arfs), which are implicated in membrane trafficking and actin cytoskeleton dynamics. In this study, we examined the immunohistochemical localization of IQ-ArfGEF/BRAG1 in the adult mouse retina using light and electron microscopy. IQ-ArfGEF/BRAG1 was distributed in a punctate manner and colocalized well with RIBEYE in both the outer and inner plexiform layers. Immunoelectron microscopic analysis showed that IQ-ArfGEF/BRAG1 was localized at the synaptic ribbons of photoreceptors. When heterologously expressed in HeLa cells, IQ-ArfGEF/BRAG1 was recruited to RIBEYE-containing clusters and formed an immunoprecipitable complex with RIBEYE. Furthermore, immunoprecipitation analysis showed that anti-IQ-ArfGEF/BRAG1 antibody efficiently pulled down RIBEYE from retinal lysates. These findings indicate that IQ-ArfGEF/BRAG1 is a novel component of retinal synaptic ribbons and forms a protein complex with RIBEYE.

Transcriptome analysis of the zebrafish pineal gland

The zebrafish pineal gland (epiphysis) is a site of melatonin production, contains photoreceptor cells, and functions as a circadian clock pace maker. Here, we have used microarray technology to study the zebrafish pineal transcriptome. Analysis of gene expression at three larval and two adult stages revealed a highly dynamic transcriptional profile, revealing many genes that are highly expressed in the zebrafish pineal gland. Statistical analysis of the data based on Gene Ontology annotation indicates that many transcription factors are highly expressed during larval stages, whereas genes dedicated to phototransduction are preferentially expressed in the adult. Furthermore, several genes were identified that exhibit day/night differences in expression. Among the multiple candidate genes suggested by these data, we note the identification of a tissue-specific form of the unc119 gene with a possible role in pineal development.