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

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

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

Loss of Function of RIMS2 Causes a Syndromic Congenital Cone-Rod Synaptic Disease with Neurodevelopmental and Pancreatic Involvement

Congenital cone-rod synaptic disorder (CRSD), also known as incomplete congenital stationary night blindness (iCSNB), is a non-progressive inherited retinal disease (IRD) characterized by night blindness, photophobia, and nystagmus, and distinctive electroretinographic features. Here, we report bi-allelic RIMS2 variants in seven CRSD-affected individuals from four unrelated families. Apart from CRSD, neurodevelopmental disease was observed in all affected individuals, and abnormal glucose homeostasis was observed in the eldest affected individual. RIMS2 regulates synaptic membrane exocytosis. Data mining of human adult bulk and single-cell retinal transcriptional datasets revealed predominant expression in rod photoreceptors, and immunostaining demonstrated RIMS2 localization in the human retinal outer plexiform layer, Purkinje cells, and pancreatic islets. Additionally, nonsense variants were shown to result in truncated RIMS2 and decreased insulin secretion in mammalian cells. The identification of a syndromic stationary congenital IRD has a major impact on the differential diagnosis of syndromic congenital IRD, which has previously been exclusively linked with degenerative IRD.

Characterization of GUCA1A-associated dominant cone/cone-rod dystrophy: low prevalence among Japanese patients with inherited retinal dystrophies

GUCA1A gene variants are associated with autosomal dominant (AD) cone dystrophy (COD) and cone-rod dystrophy (CORD). GUCA1A-associated AD-COD/CORD has never been reported in the Japanese population. The purpose of this study was to investigate clinical and genetic features of GUCA1A-associated AD-COD/CORD from a large Japanese cohort. We identified 8 variants [c.C50_80del (p.E17VfsX22), c.T124A (p.F42I), c.C204G (p.D68E), c.C238A (p.L80I), c.T295A (p.Y99N), c.A296C (p.Y99S), c.C451T (p.L151F), and c.A551G (p.Q184R)] in 14 families from our whole exome sequencing database composed of 1385 patients with inherited retinal diseases (IRDs) from 1192 families. Three variants (p.Y99N, p.Y99S, and p.L151F), which are located on/around EF-hand domains 3 and 4, were confirmed as "pathogenic", whereas the other five variants, which did not co-segregate with IRDs, were considered "non-pathogenic". Ophthalmic findings of 9 patients from 3 families with the pathogenic variants showed central visual impairment from early to middle-age onset and progressive macular atrophy. Electroretinography revealed severely decreased or non-recordable cone responses, whereas rod responses were highly variable, ranging from nearly normal to non-recordable. Our results indicate that the three pathogenic variants, two of which were novel, underlie AD-COD/CORD with progressive retinal atrophy, and the prevalence (0.25%, 3/1192 families) of GUCA1A-associated IRDs may be low among Japanese patients.

RIM1/2-Mediated Facilitation of Cav1.4 Channel Opening Is Required for Ca2+-Stimulated Release in Mouse Rod Photoreceptors

Night blindness can result from impaired photoreceptor function and a subset of cases have been linked to dysfunction of Cav1.4 calcium channels and in turn compromised synaptic transmission. Here, we show that active zone proteins RIM1/2 are important regulators of Cav1.4 channel function in mouse rod photoreceptors and thus synaptic activity. The conditional double knock-out (cdko) of RIM1 and RIM2 from rods starting a few weeks after birth did not change Cav1.4 protein expression at rod ribbon synapses nor was the morphology of the ribbon altered. Heterologous overexpression of RIM2 with Cav1.4 had no significant influence on current density when examined with BaCl2 as the charge carrier. Nonetheless, whole-cell voltage-clamp recordings from cdko rods revealed a profound reduction in Ca(2+) currents. Concomitantly, we observed a 4-fold reduction in spontaneous miniature release events from the cdko rod terminals and an almost complete absence of evoked responses when monitoring changes in membrane incorporation after strong step depolarizations. Under control conditions, 49 and 83 vesicles were released with 0.2 and 1 s depolarizations, respectively, which is close to the maximal number of vesicles estimated to be docked at the base of the ribbon active zone, but without RIM1/2, only a few vesicles were stimulated for release after a 1 s stimulation. In conclusion, our study shows that RIM1/2 potently enhance the influx of Ca(2+) into rod terminals through Cav1.4 channels, which is vitally important for the release of vesicles from the rod ribbon. Significance statement: Active zone scaffolding proteins are thought to bring multiple components involved in Ca(2+)-dependent exocytosis into functional interactions. We show that removal of scaffolding proteins RIM1/2 from rod photoreceptor ribbon synapses causes a dramatic loss of Ca(2+) influx through Cav1.4 channels and a correlated reduction in evoked release, yet the channels remain localized to synaptic ribbons in a normal fashion. Our findings strongly argue that RIM1/2 facilitate Ca(2+) entry and in turn Ca(2+) evoked release by modulating Cav1.4 channel openings; however, RIM1/2 are not needed for the retention of Cav1.4 at the synapse. In summary, a key function of RIM1/2 at rod ribbons is to enhance Cav1.4 channel activity, possibly through direct or indirect modulation of the channel.

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.

Structural insights into activation of the retinal L-type Ca²⁺ channel (Cav1.4) by Ca²⁺-binding protein 4 (CaBP4)

CaBP4 modulates Ca(2+)-dependent activity of L-type voltage-gated Ca(2+) channels (Cav1.4) in retinal photoreceptor cells. Mg(2+) binds to the first and third EF-hands (EF1 and EF3), and Ca(2+) binds to EF1, EF3, and EF4 of CaBP4. Here we present NMR structures of CaBP4 in both Mg(2+)-bound and Ca(2+)-bound states and model the CaBP4 structural interaction with Cav1.4. CaBP4 contains an unstructured N-terminal region (residues 1-99) and four EF-hands in two separate lobes. The N-lobe consists of EF1 and EF2 in a closed conformation with either Mg(2+) or Ca(2+) bound at EF1. The C-lobe binds Ca(2+) at EF3 and EF4 and exhibits a Ca(2+)-induced closed-to-open transition like that of calmodulin. Exposed residues in Ca(2+)-bound CaBP4 (Phe(137), Glu(168), Leu(207), Phe(214), Met(251), Phe(264), and Leu(268)) make contacts with the IQ motif in Cav1.4, and the Cav1.4 mutant Y1595E strongly impairs binding to CaBP4. We conclude that CaBP4 forms a collapsed structure around the IQ motif in Cav1.4 that we suggest may promote channel activation by disrupting an interaction between IQ and the inhibitor of Ca(2+)-dependent inactivation domain.

Endogenous calcium buffering at photoreceptor synaptic terminals in salamander retina

Calcium operates by several mechanisms to regulate glutamate release at rod and cone synaptic terminals. In addition to serving as the exocytotic trigger, Ca2+ accelerates replenishment of vesicles in cones and triggers Ca2+-induced Ca2+ release (CICR) in rods. Ca2+ thereby amplifies sustained exocytosis, enabling photoreceptor synapses to encode constant and changing light. A complete picture of the role of Ca2+ in regulating synaptic transmission requires an understanding of the endogenous Ca2+ handling mechanisms at the synapse. We therefore used the "added buffer" approach to measure the endogenous Ca2+ binding ratio (κendo ) and extrusion rate constant (γ) in synaptic terminals of photoreceptors in retinal slices from tiger salamander. We found that κendo was similar in both cell types-∼25 and 50 in rods and cones, respectively. Using measurements of the decay time constants of Ca2+ transients, we found that γ was also similar, with values of ∼100 s(-1) and 160 s(-1) in rods and cones, respectively. The measurements of κendo differ considerably from measurements in retinal bipolar cells, another ribbon-bearing class of retinal neurons, but are comparable to similar measurements at other conventional synapses. The values of γ are slower than at other synapses, suggesting that Ca2+ ions linger longer in photoreceptor terminals, supporting sustained exocytosis, CICR, and Ca2+ -dependent ribbon replenishment. The mechanisms of endogenous Ca2+ handling in photoreceptors are thus well-suited for supporting tonic neurotransmission. Similarities between rod and cone Ca2+ handling suggest that neither buffering nor extrusion underlie differences in synaptic transmission kinetics.

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.

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

Light-dependent conductance changes of voltage-gated Ca_v1.4 channels regulate neurotransmitter release at photoreceptor ribbon synapses. Mutations in the human CACNA1F gene encoding the α1F subunit of Ca_v1.4 channels cause an incomplete form of X-linked congenital stationary night blindness (CSNB2). Many CACNA1F mutations are loss-of-function mutations resulting in non-functional Ca_v1.4 channels, but some mutations alter the channels’ gating properties and, presumably, disturb Ca²⁺ influx at photoreceptor ribbon synapses. Notably, a CACNA1F mutation (I745T) was identified in a family with an uncommonly severe CSNB2-like phenotype, and, when expressed in a heterologous system, the mutation was shown to shift the voltage-dependence of channel activation, representing a gain-of-function. To gain insight into the pathomechanism that could explain the severity of this disorder, we generated a mouse model with the corresponding mutation in the murine Cacna1f gene (I756T) and compared it with a mouse model carrying a loss-of-function mutation (ΔEx14–17) in a longitudinal study up to eight months of age. In ΔEx14–17 mutants, the b-wave in the electroretinogram was absent, photoreceptor ribbon synapses were abnormal, and Ca²⁺ responses to depolarization of photoreceptor terminals were undetectable. In contrast, I756T mutants had a reduced scotopic b-wave, some intact rod ribbon synapses, and a strong, though abnormal, Ca²⁺ response to depolarization. Both mutants showed a progressive photoreceptor loss, but degeneration was more severe and significantly enhanced in the I756T mutants compared to the ΔEx14–17 mutants.

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.

A calcium-relay mechanism in vertebrate phototransduction

Calcium-signaling in cells requires a fine-tuned system of calcium-transport proteins involving ion channels, exchangers, and ion-pumps but also calcium-sensor proteins and their targets. Thus, control of physiological responses very often depends on incremental changes of the cytoplasmic calcium concentration, which are sensed by calcium-binding proteins and are further transmitted to specific target proteins. This Review will focus on calcium-signaling in vertebrate photoreceptor cells, where recent physiological and biochemical data indicate that a subset of neuronal calcium sensor proteins named guanylate cyclase-activating proteins (GCAPs) operate in a calcium-relay system, namely, to make gradual responses to small changes in calcium. We will further integrate this mechanism in an existing computational model of phototransduction showing that it is consistent and compatible with the dynamics that are characteristic for the precise operation of the phototransduction pathways.

Bcl-xL-mediated remodeling of rod and cone synaptic mitochondria after postnatal lead exposure: electron microscopy, tomography and oxygen consumption

PURPOSE

Postnatal lead exposure produces rod-selective and Bax-mediated apoptosis, decreased scotopic electroretinograms (ERGs), and scotopic and mesopic vision deficits in humans and/or experimental animals. Rod, but not cone, inner segment mitochondria were considered the primary site of action. However, photoreceptor synaptic mitochondria were not examined. Thus, our experiments investigated the structural and functional effects of environmentally relevant postnatal lead exposure on rod spherule and cone pedicle mitochondria and whether Bcl-xL overexpression provided neuroprotection.

METHODS

C57BL/6N mice pups were exposed to lead only during lactation via dams drinking water containing lead acetate. The blood [Pb] at weaning was 20.6±4.7 µg/dl, which decreased to the control value by 2 months. To assess synaptic mitochondrial structural differences and vulnerability to lead exposure, wild-type and transgenic mice overexpressing Bcl-xL in photoreceptors were used. Electron microscopy, three-dimensional electron tomography, and retinal and photoreceptor synaptic terminal oxygen consumption (QO(2)) studies were conducted in adult control, Bcl-xL, lead, and Bcl-xL/lead mice.

RESULTS

The spherule and pedicle mitochondria in lead-treated mice were swollen, and the cristae structure was markedly changed. In the lead-treated mice, the mitochondrial cristae surface area and volume (abundance: measure correlated with ATP (ATP) synthesis) were decreased in the spherules and increased in the pedicles. Pedicles also had an increased number of crista segments per volume. In the lead-treated mice, the number of segments/crista and fraction of cristae with multiple segments (branching) similarly increased in spherule and pedicle mitochondria. Lead-induced remodeling of spherule mitochondria produced smaller cristae with more branching, whereas pedicle mitochondria had larger cristae with more branching and increased crista junction (CJ) diameter. Lead decreased dark- and light-adapted photoreceptor and dark-adapted photoreceptor synaptic terminal QO(2). Bcl-xL partially blocked many of the lead-induced alterations relative to controls. However, spherules still had partially decreased abundance, whereas pedicles still had increased branching, increased crista segments per volume, and increased crista junction diameter. Moreover, photoreceptor and synaptic QO(2) were only partially recovered.

CONCLUSIONS

These findings reveal cellular and compartmental specific differences in the structure and vulnerability of rod and cone inner segment and synaptic mitochondria to postnatal lead exposure. Spherule and pedicle mitochondria in lead-exposed mice displayed complex and distinguishing patterns of cristae and matrix damage and remodeling consistent with studies showing that synaptic mitochondria are more sensitive to Ca(2+) overload, oxidative stress, and ATP loss than non-synaptic mitochondria. The lead-induced decreases in QO(2) likely resulted from the decreased spherule cristae abundance and smaller cristae, perhaps due to Bax-mediated effects as they occurred in apoptotic rod inner segments. The increase in pedicle cristae abundance and CJ diameter could have resulted from increased Drp1-mediated fission, as small mitochondrial fragments were observed. The mechanisms of Bcl-xL-mediated remodeling might occur via interaction with formation of CJ protein 1 (Fcj1), whereas the partial protection of synaptic QO(2) might result from the enhanced efficiency of energy metabolism via Bcl-xL's direct interaction with the F1F0 ATP synthase and/or regulation of cellular redox status. These lead-induced alterations in photoreceptor synaptic terminal mitochondria likely underlie the persistent scotopic and mesopic deficits in lead-exposed children, workers, and experimental animals. Our findings stress the clinical and scientific importance of examining synaptic dysfunction following injury or disease during development, and developing therapeutic treatments that prevent synaptic degeneration in retinal and neurodegenerative disorders even when apoptosis is blocked.