Localization of voltage-sensitive L-type calcium channels in the chicken retina

Circadian regulation in the retina: From molecules to network

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

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

D-cis-Diltiazem Can Produce Oxidative Stress in Healthy Depolarized Rods In Vivo

Purpose

New perspectives are needed to understand decades of contradictory reports on the neuroprotective effects of the Cav1.2 L-type calcium channel blocker d-cis-diltiazem in retinitis pigmentosa (RP) models. Here, we address, in vivo, the following two knowledge gaps regarding d-cis-diltiazem's actions in the murine outer retina: (1) do normal mouse rods contain d-cis-diltiazem-insensitive Cav1.2 L-type calcium channels? (2) Can d-cis-diltiazem modify the normal rod redox environment?

Methods

First, transretinal Cav1.2 L-type calcium channels were noninvasively mapped with manganese-enhanced magnetic resonance imaging (MRI) following agonist Bay K 8644 in C57BL/6 (B6) and in Cav1.2 L-type calcium channel BAY K 8644-insensitive mutant B6 mice. Second, d-cis-diltiazem-treated oxidative stress-vulnerable (B6) or -resistant [129S6 (S6)] mice were examined in vivo (QUEnch-assiSTed [QUEST] MRI) and in whole retina ex vivo (lucigenin). Retinal thickness was measured using MRI.

Results

The following results were observed: (1) manganese uptake patterns in BAY K 8644-treated controls and mutant mice identified in vivo Cav1.2 L-type calcium channels in inner and outer retina; and (2) d-cis-diltiazem induced rod oxidative stress in dark-adapted B6 mice but not in light-adapted B6 mice or dark-adapted S6 mice (QUEST MRI). Oxidative stress in vivo was limited to inferior outer retina in dark-adapted B6 mice approximately 1-hour post d-cis-diltiazem. By approximately 4 hours post, only superior outer retina oxidative stress was observed and whole retinal superoxide production was supernormal. All groups had unremarkable retinal thicknesses.

Conclusions

D-cis-diltiazem's unexpectedly complex spatiotemporal outer retinal oxidative stress pattern in vivo was dependent on genetic background and rod membrane depolarization, but not apparently dependent on Cav1.2 L-type calcium channels, providing a potential rationale for contradictory results in different RP models.

The Contribution of L-Type Cav1.3 Channels to Retinal Light Responses

L-type voltage-gated calcium channels (LTCCs) regulate tonic neurotransmitter release from sensory neurons including retinal photoreceptors. There are three types of LTCCs (Ca_v1.2, Ca_v1.3, and Ca_v1.4) expressed in the retina. While Ca_v1.2 is expressed in all retinal cells including the Müller glia and neurons, Ca_v1.3 and Ca_v1.4 are expressed in the retinal neurons with Ca_v1.4 exclusively expressed in the photoreceptor synaptic terminals. Mutations in the gene encoding Ca_v1.4 cause incomplete X-linked congenital stationary night blindness in humans. Even though Ca_v1.3 is present in the photoreceptor inner segments and the synaptic terminals in various vertebrate species, its role in vision is unclear, since genetic alterations in Ca_v1.3 are not associated with severe vision impairment in humans or in Ca_v1.3-null (Ca_v1.3^-/-) mice. However, a failure to regulate Ca_v1.3 was found in a mouse model of Usher syndrome, the most common cause of combined deafness and blindness in humans, indicating that Ca_v1.3 may contribute to retinal function. In this report, we combined physiological and morphological data to demonstrate the role of Ca_v1.3 in retinal physiology and function that has been undervalued thus far. Through ex vivo and in vivo electroretinogram (ERG) recordings and immunohistochemical staining, we found that Ca_v1.3 plays a role in retinal light responses and synaptic plasticity. Pharmacological inhibition of Ca_v1.3 decreased ex vivo ERG a- and b-wave amplitudes. In Ca_v1.3^-/- mice, their dark-adapted ERG a-, b-wave, and oscillatory potential amplitudes were significantly dampened, and implicit times were delayed compared to the wild type (WT). Furthermore, the density of ribbon synapses was reduced in the outer plexiform layer of Ca_v1.3^-/- mice retinas. Hence, Ca_v1.3 plays a more prominent role in retinal physiology and function than previously reported.

Retinoschisin Facilitates the Function of L-Type Voltage-Gated Calcium Channels

Modulation of ion channels by extracellular proteins plays critical roles in shaping synaptic plasticity. Retinoschisin (RS1) is an extracellular adhesive protein secreted from photoreceptors and bipolar cells, and it plays an important role during retinal development, as well as in maintaining the stability of retinal layers. RS1 is known to form homologous octamers and interact with molecules on the plasma membrane including phosphatidylserine, sodium-potassium exchanger complex, and L-type voltage-gated calcium channels (LTCCs). However, how this physical interaction between RS1 and ion channels might affect the channel gating properties is unclear. In retinal photoreceptors, two major LTCCs are Cav1.3 (α1D) and Cav1.4 (α1F) with distinct biophysical properties, functions and distributions. Cav1.3 is distributed from the inner segment (IS) to the synaptic terminal and is responsible for calcium influx to the photoreceptors and overall calcium homeostasis. Cav1.4 is only expressed at the synaptic terminal and is responsible for neurotransmitter release. Mutations of the gene encoding Cav1.4 cause X-linked incomplete congenital stationary night blindness type 2 (CSNB2), while null mutations of Cav1.3 cause a mild decrease of retinal light responses in mice. Even though RS1 is known to maintain retinal architecture, in this study, we present that RS1 interacts with both Cav1.3 and Cav1.4 and regulates their activations. RS1 was able to co-immunoprecipitate with Cav1.3 and Cav1.4 from porcine retinas, and it increased the LTCC currents and facilitated voltage-dependent activation in HEK cells co-transfected with RS1 and Cav1.3 or Cav1.4, thus providing evidence of a functional interaction between RS1 and LTCCs. The interaction between RS1 and Cav1.3 did not change the calcium-dependent inactivation of Cav1.3. In mice lacking RS1, the expression of Cav1.3 and Cav1.4 in the retina decreased, while in mice with Cav1.4 deletion, the retinal level of RS1 decreased. These results provide important evidence that RS1 is not only an adhesive protein promoting cell-cell adhesion, it is essential for anchoring other membrane proteins including ion channels and enhancing their function in the retina.

Relevance of tissue specific subunit expression in channelopathies

Channelopathies are a diverse group of human disorders that are caused by mutations in genes coding for ion channels or channel-regulating proteins. Several dozen channelopathies have been identified that involve both non-excitable cells as well as electrically active tissues like brain, skeletal and smooth muscle or the heart. In this review, we start out from the general question which ion channel genes are expressed tissue-selectively. We mined the human gene expression database Human Protein Atlas (HPA) for tissue-enriched ion channel genes and found 85 genes belonging to the ion channel families. Most of these genes were enriched in brain, testis and muscle and a complete list of the enriched ion channel genes is provided. We further focused on the tissue distribution of voltage-gated calcium channel (VGCC) genes including different brain areas and the retina based on the human gene expression from the FANTOM5 dataset. The expression data is complemented by an overview of the tissue-dependent aspects of L-type calcium channel (LTCC) function, dysfunction and pharmacology, as well as of their splice variants. Finally, we focus on the pathology of tissue-restricted LTCC channelopathies and their treatment options. This article is part of the Special Issue entitled 'Channelopathies.'

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Localization of Rod Bipolar Cells in the Mammalian Retina Using an Antibody Against the α1c L-type Ca(2+) Channel

Bipolar cells transmit stimuli via graded changes in membrane potential and neurotransmitter release is modulated by Ca²⁺ influx through L-type Ca²⁺ channels. The purpose of this study was to determine whether the α₁c subunit of L-type voltage-gated Ca²⁺ channel (α₁c L-type Ca²⁺ channel) colocalizes with protein kinase C alpha (PKC-α), which labels rod bipolar cells. Retinal whole mounts and vertical sections from mouse, hamster, rabbit, and dog were immunolabeled with antibodies against PKC-α and α₁c L-type Ca²⁺ channel, using fluorescein isothiocyanate (FITC) and Cy5 as visualizing agents. PKC-α-immunoreactive cells were morphologically identical to rod bipolar cells as previously reported. Their cell bodies were located within the inner nuclear layer, dendritic processes branched into the outer plexiform layer, and axons extended into the inner plexiform layer. Immunostaining showed that α₁c L-type Ca²⁺ channel colocalized with PKC-α in rod bipolar cells. The identical expression of PKC-α and α₁c L-type Ca²⁺ channel indicates that the α₁c L-type Ca²⁺ channel has a specific role in rod bipolar cells, and the antibody against the α₁c L-type Ca²⁺ channel may be a useful marker for studying the distribution of rod bipolar cells in mouse, hamster, rabbit, and dog retinas.

Differential calcium signaling mediated by voltage-gated calcium channels in rat retinal ganglion cells and their unmyelinated axons

Aberrant calcium regulation has been implicated as a causative factor in the degeneration of retinal ganglion cells (RGCs) in numerous injury models of optic neuropathy. Since calcium has dual roles in maintaining homeostasis and triggering apoptotic pathways in healthy and injured cells, respectively, investigation of voltage-gated Ca channel (VGCC) regulation as a potential strategy to reduce the loss of RGCs is warranted. The accessibility and structure of the retina provide advantages for the investigation of the mechanisms of calcium signalling in both the somata of ganglion cells as well as their unmyelinated axons. The goal of the present study was to determine the distribution of VGCC subtypes in the cell bodies and axons of ganglion cells in the normal retina and to define their contribution to calcium signals in these cellular compartments. We report L-type Ca channel α1C and α1D subunit immunoreactivity in rat RGC somata and axons. The N-type Ca channel α1B subunit was in RGC somata and axons, while the P/Q-type Ca channel α1A subunit was only in the RGC somata. We patch clamped isolated ganglion cells and biophysically identified T-type Ca channels. Calcium imaging studies of RGCs in wholemounted retinas showed that selective Ca channel antagonists reduced depolarization-evoked calcium signals mediated by L-, N-, P/Q- and T-type Ca channels in the cell bodies but only by L-type Ca channels in the axons. This differential contribution of VGCC subtypes to calcium signals in RGC somata and their axons may provide insight into the development of target-specific strategies to spare the loss of RGCs and their axons following injury.

Chicken embryos as a potential new model for early onset type I diabetes

Diabetic retinopathy (DR) is the leading cause of blindness among the American working population. The purpose of this study is to establish a new diabetic animal model using a cone-dominant avian species to address the distorted color vision and altered cone pathway responses in prediabetic and early diabetic patients. Chicken embryos were injected with either streptozotocin (STZ), high concentration of glucose (high-glucose), or vehicle at embryonic day 11. Cataracts occurred in varying degrees in both STZ- and high glucose-induced diabetic chick embryos at E18. Streptozotocin-diabetic chicken embryos had decreased levels of blood insulin, glucose transporter 4 (Glut4), and phosphorylated protein kinase B (pAKT). In STZ-injected E20 embryos, the ERG amplitudes of both a- and b-waves were significantly decreased, the implicit time of the a-wave was delayed, while that of the b-wave was significantly increased. Photoreceptors cultured from STZ-injected E18 embryos had a significant decrease in L-type voltage-gated calcium channel (L-VGCC) currents, which was reflected in the decreased level of L-VGCCα1D subunit in the STZ-diabetic retinas. Through these independent lines of evidence, STZ-injection was able to induce pathological conditions in the chicken embryonic retina, and it is promising to use chickens as a potential new animal model for type I diabetes.

Cav1.4 IT mouse as model for vision impairment in human congenital stationary night blindness type 2

Mutations in the CACNA1F gene encoding the Cav1.4 Ca (2+) channel are associated with X-linked congenital stationary night blindness type 2 (CSNB2). Despite the increasing knowledge about the functional behavior of mutated channels in heterologous systems, the pathophysiological mechanisms that result in vision impairment remain to be elucidated. This work provides a thorough functional characterization of the novel IT mouse line that harbors the gain-of-function mutation I745T reported in a New Zealand CSNB2 family. (1) Electroretinographic recordings in IT mice permitted a direct comparison with human data. Our data supported the hypothesis that a hyperpolarizing shift in the voltage-dependence of channel activation-as seen in the IT gain-of-function mutant (2)-may reduce the dynamic range of photoreceptor activity. Morphologically, the retinal outer nuclear layer in adult IT mutants was reduced in size and cone outer segments appeared shorter. The organization of the outer plexiform layer was disrupted, and synaptic structures of photoreceptors had a variable, partly immature, appearance. The associated visual deficiency was substantiated in behavioral paradigms. The IT mouse line serves as a specific model for the functional phenotype of human CSNB2 patients with gain-of-function mutations and may help to further understand the dysfunction in CSNB.

A new functional role for mechanistic/mammalian target of rapamycin complex 1 (mTORC1) in the circadian regulation of L-type voltage-gated calcium channels in avian cone photoreceptors

In the retina, the L-type voltage-gated calcium channels (L-VGCCs) are responsible for neurotransmitter release from photoreceptors and are under circadian regulation. Both the current densities and protein expression of L-VGCCs are significantly higher at night than during the day. However, the underlying mechanisms of circadian regulation of L-VGCCs in the retina are not completely understood. In this study, we demonstrated that the mechanistic/mammalian target of rapamycin complex (mTORC) signaling pathway participated in the circadian phase-dependent modulation of L-VGCCs. The activities of the mTOR cascade, from mTORC1 to its downstream targets, displayed circadian oscillations throughout the course of a day. Disruption of mTORC1 signaling dampened the L-VGCC current densities, as well as the protein expression of L-VGCCs at night. The decrease of L-VGCCs at night by mTORC1 inhibition was in part due to a reduction of L-VGCCα1 subunit translocation from the cytosol to the plasma membrane. Finally, we showed that mTORC1 was downstream of the phosphatidylionositol 3 kinase-protein kinase B (PI3K-AKT) signaling pathway. Taken together, mTORC1 signaling played a role in the circadian regulation of L-VGCCs, in part through regulation of ion channel trafficking and translocation, which brings to light a new functional role for mTORC1: the modulation of ion channel activities.

Calcineurin serves in the circadian output pathway to regulate the daily rhythm of L-type voltage-gated calcium channels in the retina

The L-type voltage-gated calcium channels (L-VGCCs) in avian retinal cone photoreceptors are under circadian control, in which the protein expression of the α1 subunits and the current density are greater at night than during the day. Both Ras-mitogen-activated protein kinase (MAPK) and Ras-phosphatidylionositol 3 kinase-protein kinase B (PI3K-AKT) signaling pathways are part of the circadian output that regulate the L-VGCC rhythm, while cAMP-dependent signaling is further upstream of Ras to regulate the circadian outputs in photoreceptors. However, there are missing links between cAMP-dependent signaling and Ras in the circadian output regulation of L-VGCCs. In this study, we report that calcineurin, a Ca2+/calmodulin-dependent serine (ser)/threonine (thr) phosphatase, participates in the circadian output pathway to regulate L-VGCCs through modulating both Ras-MAPK and Ras-PI3K-AKT signaling. The activity of calcineurin, but not its protein expression, was under circadian regulation. Application of a calcineurin inhibitor, FK-506 or cyclosporine A, reduced the L-VGCC current density at night with a corresponding decrease in L-VGCCα1D protein expression, but the circadian rhythm of L-VGCCα1D mRNA levels were not affected. Inhibition of calcineurin further reduced the phosphorylation of ERK and AKT (at thr 308) and inhibited the activation of Ras, but inhibitors of MAPK or PI3K signaling did not affect the circadian rhythm of calcineurin activity. However, inhibition of adenylate cyclase significantly dampened the circadian rhythm of calcineurin activity. These results suggest that calcineurin is upstream of MAPK and PI3K-AKT but downstream of cAMP in the circadian regulation of L-VGCCs.

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.

Location of release sites and calcium-activated chloride channels relative to calcium channels at the photoreceptor ribbon synapse

Vesicle release from photoreceptor ribbon synapses is regulated by L-type Ca(2+) channels, which are in turn regulated by Cl(-) moving through calcium-activated chloride [Cl(Ca)] channels. We assessed the proximity of Ca(2+) channels to release sites and Cl(Ca) channels in synaptic terminals of salamander photoreceptors by comparing fast (BAPTA) and slow (EGTA) intracellular Ca(2+) buffers. BAPTA did not fully block synaptic release, indicating some release sites are <100 nm from Ca(2+) channels. Comparing Cl(Ca) currents with predicted Ca(2+) diffusion profiles suggested that Cl(Ca) and Ca(2+) channels average a few hundred nanometers apart, but the inability of BAPTA to block Cl(Ca) currents completely suggested some channels are much closer together. Diffuse immunolabeling of terminals with an antibody to the putative Cl(Ca) channel TMEM16A supports the idea that Cl(Ca) channels are dispersed throughout the presynaptic terminal, in contrast with clustering of Ca(2+) channels near ribbons. Cl(Ca) currents evoked by intracellular calcium ion concentration ([Ca(2+)](i)) elevation through flash photolysis of DM-nitrophen exhibited EC(50) values of 556 and 377 nM with Hill slopes of 1.8 and 2.4 in rods and cones, respectively. These relationships were used to estimate average submembrane [Ca(2+)](i) in photoreceptor terminals. Consistent with control of exocytosis by [Ca(2+)] nanodomains near Ca(2+) channels, average submembrane [Ca(2+)](i) remained below the vesicle release threshold (∼ 400 nM) over much of the physiological voltage range for cones. Positioning Ca(2+) channels near release sites may improve fidelity in converting voltage changes to synaptic release. A diffuse distribution of Cl(Ca) channels may allow Ca(2+) influx at one site to influence relatively distant Ca(2+) channels.

Multiple Ca2+-dependent mechanisms regulate L-type Ca2+ current in retinal amacrine cells

Understanding the regulation of L-type voltage-gated Ca(2+) current is an important component of elucidating the signaling capabilities of retinal amacrine cells. Here we ask how the cytosolic Ca(2+) environment and the balance of Ca(2+)-dependent effectors shape native L-type Ca(2+) channel function in these cells. To achieve this, whole cell voltage clamp recordings were made from cultured amacrine cells under conditions that address the contribution of mitochondrial Ca(2+) uptake (MCU), Ca(2+)/calmodulin (CaM)-dependent channel inactivation (CDI), protein kinase A (PKA), and Ca(2+)-induced Ca(2+) release (CICR). Under control conditions, repeated activation of the L-type channels produces a progressive enhancement of the current. Inhibition of MCU causes a reduction in the Ca(2+) current amplitude that is dependent on Ca(2+) influx as well as cytosolic Ca(2+) buffering, consistent with CDI. Including the Ca(2+) buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) internally can shift the balance between enhancement and inhibition such that inhibition of MCU can enhance the current. Inhibition of PKA can remove the enhancing effect of BAPTA suggesting that cyclic AMP-dependent phosphorylation is involved. Inhibition of CaM suppresses CDI but spares the enhancement, consistent with the substantially higher sensitivity of the Ca(2+)-sensitive adenylate cyclase 1 (AC1) to Ca(2+)/CaM. Inhibition of the ryanodine receptor reduces the current amplitude, suggesting that CICR might normally amplify the activation of AC1 and stimulation of PKA activity. These experiments reveal that the amplitude of L-type Ca(2+) currents in retinal amacrine cells are both positively and negatively regulated by Ca(2+)-dependent mechanisms.

Differential role for synaptojanin 1 in rod and cone photoreceptors

Synaptojanin 1 (SynJ1) is a polyphosphoinositide phosphatase involved in clathrin-mediated endocytosis in conventional synapses. Studies with the zebrafish mutant nrc have revealed that loss of SynJ1 also affects cone photoreceptor ribbon synapses, causing pronounced morphological and functional abnormalities. In this study we continue to examine the role of SynJ1 in photoreceptors. Using a newly generated antibody specific for zebrafish SynJ1, we localized this protein predominantly to cone photoreceptors. We then used blastula stage transplantation experiments to demonstrate that rods from nrc mutants lacking SynJ1 develop normally and do not have the pronounced morphological defects detected in cones. Given the known involvement of SynJ1 in synaptic vesicle endocytosis, we hypothesize that rods and cones use distinct mechanisms for vesicle recycling.

Glutamate-induced internalization of Ca(v)1.3 L-type Ca(2+) channels protects retinal neurons against excitotoxicity

Glutamate-induced rise in the intracellular Ca(2+) level is thought to be a major cause of excitotoxic cell death, but the mechanisms that control the Ca(2+) overload are poorly understood. Using immunocytochemistry, electrophysiology and Ca(2+) imaging, we show that activation of ionotropic glutamate receptors induces a selective internalization of Ca(v)1.3 L-type Ca(2+) channels in salamander retinal neurons. The effect of glutamate on Ca(v)1.3 internalization was blocked in Ca(2+)-free external solution, or by strong buffering of internal Ca(2+) with BAPTA. Downregulation of L-type Ca(2+) channel activity in retinal ganglion cells by glutamate was suppressed by inhibitors of dynamin-dependent endocytosis. Stabilization of F-actin by jasplakinolide significantly reduced the ability of glutamate to induce internalization suggesting it is mediated by Ca(2+)-dependent reorganization of actin cytoskeleton. We showed that the Ca(v)1.3 is the primary L-type Ca(2+) channel contributing to kainate-induced excitotoxic death of amacrine and ganglion cells. Block of Ca(v)1.3 internalization by either dynamin inhibition or F-actin stabilization increased vulnerability of retinal amacrine and ganglion cells to kainate-induced excitotoxicity. Our data show for the first time that Ca(v)1.3 L-type Ca(2+) channels are subject to rapid glutamate-induced internalization, which may serve as a negative feedback mechanism protecting retinal neurons against glutamate-induced excitotoxicity.

Kinetics of synaptic transmission at ribbon synapses of rods and cones

The ribbon synapse is a specialized structure that allows photoreceptors to sustain the continuous release of vesicles for hours upon hours and years upon years but also respond rapidly to momentary changes in illumination. Light responses of cones are faster than those of rods and, mirroring this difference, synaptic transmission from cones is also faster than transmission from rods. This review evaluates the various factors that regulate synaptic kinetics and contribute to kinetic differences between rod and cone synapses. Presynaptically, the release of glutamate-laden synaptic vesicles is regulated by properties of the synaptic proteins involved in exocytosis, influx of calcium through calcium channels, calcium release from intracellular stores, diffusion of calcium to the release site, calcium buffering, and extrusion of calcium from the cytoplasm. The rate of vesicle replenishment also limits the ability of the synapse to follow changes in release. Post-synaptic factors include properties of glutamate receptors, dynamics of glutamate diffusion through the cleft, and glutamate uptake by glutamate transporters. Thus, multiple synaptic mechanisms help to shape the responses of second-order horizontal and bipolar cells.

The expression of L-type voltage-gated calcium channels in retinal photoreceptors is under circadian control

Photoreceptors are non-spiking neurons, and their synapses mediate the continuous release of neurotransmitters under the control of L-type voltage-gated calcium channels (VGCCs). Photoreceptors express endogenous circadian oscillators that play important roles in regulating photoreceptor physiology and function. Here, we report that the L-type VGCCs in chick cone photoreceptors are under circadian control. The L-type VGCC currents are greater when measured during the subjective night than during the subjective day. Using antibodies against the VGCCalpha1C and VGCCalpha1D subunits, we found that the immunofluorescence intensities of both VGCCalpha1C and VGCCalpha1D in photoreceptors are higher during the subjective night. However, the mRNA levels of VGCCalpha1D, but not VGCCalpha1C, are rhythmic. Nocturnal increases in L-type VGCCs are blocked by manumycin A, PD98059, and KN93, which suggest that the circadian output pathway includes Ras, Erk, and calcium-calmodulin dependent kinase II. In summary, four independent lines of evidence show that the L-VGCCs in cone photoreceptors are under circadian control.

C-terminal modulator controls Ca2+-dependent gating of Cav1.4 L-type Ca2+ channels

Tonic neurotransmitter release at sensory cell ribbon synapses is mediated by calcium (Ca2+) influx through L-type voltage-gated Ca2+ channels. This tonic release requires the channels to inactivate slower than in other tissues. Ca(v)1.4 L-type voltage-gated Ca2+ channels (LTCCs) are found at high densities in photoreceptor terminals, and alpha1 subunit mutations cause human congenital stationary night blindness type-2 (CSNB2). Ca(v)1.4 voltage-dependent inactivation is slow and Ca2+-dependent inactivation (CDI) is absent. We show that removal of the last 55 or 122 (C122) C-terminal amino acid residues of the human alpha1 subunit restores calmodulin-dependent CDI and shifts voltage of half-maximal activation to more negative potentials. The C terminus must therefore form part of a mechanism that prevents calmodulin-dependent CDI of Ca(v)1.4 and controls voltage-dependent activation. Fluorescence resonance energy transfer experiments in living cells revealed binding of C122 to C-terminal motifs mediating CDI in other Ca2+ channels. The absence of this modulatory mechanism in the CSNB2 truncation mutant K1591X underlines its importance for normal retinal function in humans.

Group III metabotropic glutamate receptors and exocytosed protons inhibit L-type calcium currents in cones but not in rods

Light responses of photoreceptors (rods and cones) are transmitted to the second-order neurons (bipolar cells and horizontal cells) via glutamatergic synapses located in the outer plexiform layer of the retina. Although it has been well established that postsynaptic group III metabotropic glutamate receptors (mGluRs) of ON bipolar cells contribute to generating the ON signal, presynaptic roles of group III mGluRs remain to be elucidated at this synaptic connection. We addressed this issue by applying the slice patch-clamp technique to the newt retina. OFF bipolar cells and horizontal cells generate a steady inward current in the dark and a transient inward current at light offset, both of which are mediated via postsynaptic non-NMDA receptors. A group III mGluR-specific agonist, L-2-amino-4-phosphonobutyric acid (L-AP-4), inhibited both the steady and off-transient inward currents but did not affect the glutamate-induced current in these postsynaptic neurons. L-AP-4 inhibited the presynaptic L-type calcium current (ICa) in cones by shifting the voltage dependence of activation to more positive membrane potentials. The inhibition of ICa was most prominent around the physiological range of cone membrane potentials. In contrast, L-AP-4 did not affect L-type ICa in rods. Paired recordings from photoreceptors and the synaptically connected second-order neurons confirmed that L-AP-4 inhibited both ICa and glutamate release in cones but not in rods. Furthermore, we found that exocytosed protons also inhibited ICa in cones but not in rods. Selective modulation of ICa in cones may help broaden the dynamic range of synaptic transfer by controlling the amount of transmitter release from cones.

Effects of congenital stationary night blindness type 2 mutations R508Q and L1364H on Cav1.4 L-type Ca2+ channel function and expression

At least 48 mutations in the CACNA1F gene encoding retinal Ca(v)1.4 L-type Ca(2+) channels have been linked to X-linked recessive congenital stationary night blindness type 2 (CSNB2). A large number of these are missense mutations encoding full-length alpha1-subunits that can potentially form functional channels. We have previously shown that such missense mutations can confer their phenotype by different pathological mechanisms, such as complete lack of alpha1 subunit protein expression or dramatic changes in channel gating. Here we investigated the functional consequences of CSNB2 missense mutations R508Q and L1364H. We found no (R508Q) or only minor (L1364H) changes in the gating properties of both mutants after heterologous expression in Xenopus laevis oocytes (at 20 degrees C). However, both mutants resulted in altered expression density of Ca(v)1.4 currents. When expressed in the mammalian cell line tsA-201, the current amplitude of L1364H channels was reduced when cells were grown at 30 degrees C and both mutations affected total alpha1 protein expression. This effect was temperature dependent. Our data provide evidence that, in contrast to previously characterized CSNB2 missense mutations, the clinical phenotype of R508Q and L1364H is unlikely to be explained by changes in channel gating. Instead, these mutations affect the protein expression of Ca(v)1.4 Ca(2+) channels.

Activation of mGluR5 modulates Ca2+ currents in retinal amacrine cells from the chick

In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2(+)-dependent adenylate cyclase.

Cav1.4alpha1 subunits can form slowly inactivating dihydropyridine-sensitive L-type Ca2+ channels lacking Ca2+-dependent inactivation

The neuronal L-type calcium channels (LTCCs) Cav1.2alpha1 and Cav1.3alpha1 are functionally distinct. Cav1.3alpha1 activates at lower voltages and inactivates more slowly than Cav1.2alpha1, making it suitable to support sustained L-type Ca2+ inward currents (ICa,L) and serve in pacemaker functions. We compared the biophysical and pharmacological properties of human retinal Cav1.4alpha1 using the whole-cell patch-clamp technique after heterologous expression in tsA-201 cells with other L-type alpha1 subunits. Cav1.4alpha1-mediated inward Ba2+ currents (IBa) required the coexpression of alpha2delta1 and beta3 or beta2a subunits and were detected in a lower proportion of transfected cells than Cav1.3alpha1. IBa activated at more negative voltages (5% activation threshold; -39mV; 15 mm Ba2+) than Cav1.2alpha1 and slightly more positive than Cav1.3alpha1. Voltage-dependent inactivation of IBa was slower than for Cav1.2alpha1 and Cav1.3alpha1( approximately 50% inactivation after 5 sec; alpha2delta1 + beta3 coexpression). Inactivation was not increased with Ca2+ as the charge carrier, indicating the absence of Ca2+-dependent inactivation. Cav1.4alpha1 exhibited voltage-dependent, G-protein-independent facilitation by strong depolarizing pulses. The dihydropyridine (DHP)-antagonist isradipine blocked Cav1.4alpha1 with approximately 15-fold lower sensitivity than Cav1.2alpha1 and in a voltage-dependent manner. Strong stimulation by the DHP BayK 8644 was found despite the substitution of an otherwise L-type channel-specific tyrosine residue in position 1414 (repeat IVS6) by a phenylalanine. Cav1.4alpha1 + alpha2delta1 + beta channel complexes can form LTCCs with intermediate DHP antagonist sensitivity lacking Ca2+-dependent inactivation. Their biophysical properties should enable them to contribute to sustained ICa,L at negative potentials, such as required for tonic neurotransmitter release in sensory cells and plateau potentials in spiking neurons.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.