In vitro phosphorylation in isolated horizontal cells of the fish retina: effects of the state of light adaptation

Application of proteomic technology in eye research: a mini review

Proteomics is a rapidly growing research area for the study of the protein cognate of genomic data. This review gives a brief overview of the modern proteomic technology. In addition to general applications of proteomics, we highlight its contribution to studying the physiology of different ocular tissues. We also summarise the published proteomic literature in the broad context of ophthalmic diseases, such as cataract, age-related maculopathy, diabetic retinopathy, glaucoma and myopia. The proteomic technology is a useful research tool and it will continue to advance our understanding of a variety of molecular processes in ocular tissues and diseases.

Calcium-binding protein Caldendrin and CaMKII are localized in spinules of the carp retina

Calcium-binding proteins translate the influx of Ca(2+) at excitatory synapses into spatiotemporal signals that regulate a variety of processes underlying synaptic plasticity. In the fish retina, the synaptic connectivity between photoreceptors and horizontal cells undergoes a remarkable plasticity, triggered by the ambient light conditions. With increasing light, the synaptic dendrites of horizontal cells form numerous spinules that are dissolved during dark adaptation. The dynamic regulation of this process is calcium-dependent and involves the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), but astonishingly its principal regulator Calmodulin (CaM) could not be localized to spinules. Here, we show that antibodies directed against Caldendrin (CaBP1), a member of the EF-hand calcium-binding protein family, strongly label the terminal dendrites of horizontal cells invaginating cone pedicles. Double-labeling experiments revealed that this label is closely associated with label for CaMKII. This association was confirmed at the ultrastructural level. Caldendrin immunoreactivity and CaMKII immunoreactivity are both present in horizontal cell dendrites flanking the synaptic ribbon within the cone pedicle and in particular in spinules formed by these terminals. Comparison of light- and dark-adapted retinas revealed a shift of the membrane-associated label for Caldendrin from the terminal dendrites into the spinules during light adaptation. These results suggest that Caldendrin is involved in the dynamic regulation of spinules and confirms the assumed potential of Caldendrin as a neural calcium sensor for synaptic plasticity.

Characterization of retinoic acid neuromodulation in the carp retina

Visual sensation in vertebrates starts with the isomerization of 11-cis retinaldehyde into all-trans retinaldehyde. Aldehyde dehydrogenases, present in the pigment epithelium and some retinal cells, convert all-trans retinaldehyde into all-trans retinoic acid (at-RA). Evidence in the retina and the hippocampus has accumulated, showing that at-RA, besides being a morphogenetic factor, also acts as a neuromodulator. In mature retina, at-RA affects visual processing by acting on gap junctional conductances and the synaptic transfer between photoreceptors and horizontal cells. We present evidence supporting a neuromodulatory role of at-RA in the carp retina. High performance liquid chromatography (HPLC) measurements and an RA bioassay indicate a light dependency of at-RA formation, which can explain the observed effects of at-RA on spinule formation at horizontal cell dendrites in this retina. Furthermore, inhibiting endogenous metabolism and catabolism of at-RA affects formation and persistence of spinules in a way, supporting a direct involvement of at-RA in this light-dependent mechanism of synaptic plasticity. The action of at-RA, however, seems independent of the dopaminergic system, known for its light-signaling role in the retina, because at-RA effects on spinule formation persisted in retina depleted of dopaminergic neurons or in the presence of haloperidol. Together, these data indicate that at-RA acts effectively as a direct neuromodulator in carp retina, transmitting information about ambient light conditions to the neuronal retina.

Molecular Cloning and Functional Expression of zfCx52.6

Gap junction-mediated electrical coupling contributes to synchronous oscillatory activities of neurons, and considerable progress has been made in defining the molecular composition of gap junction channels. In particular, cloning and functional expression of gap junction proteins (connexins (Cx)) from zebrafish retina have shown that this part of the brain possesses a high degree of connexin diversity that may account for differential functional properties of electrical synapses. Here, we report the cloning and functional characterization of a new connexin, designated zebrafish Cx52.6 (zfCx52.6). This connexin shows little similarity to known connexins from fish and higher vertebrates. By combining in situ hybridization with Laser Capture Microdissection and RT-PCR, we found that this novel fish connexin is expressed in horizontal cells in the inner nuclear layer of the retina. Functional expression of zfCx52.6 in neuroblastoma cells and Xenopus oocytes led to functional gap junctional channels and, in single oocytes, induced large non-junctional membrane currents indicative of the formation of hemichannels, which were inhibited in reversible fashion by raising extracellular Ca(2+) concentrations.

Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina

The subunit composition of ionotropic glutamate receptors (GluRs) is extremely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, it is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immuno light and electron microscopy to localize AMPA and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. These receptors are also inserted into the membrane of spinules, light-dependent protrusions of the horizontal cell dendrites, flanking the synaptic ribbon of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Colocalization data suggest that each subunit is expressed by a distinct bipolar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 could also be detected on dendrites of bipolar cells that, based on their localization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement of the different subunits within the cavity of the cone pedicle appeared not to be random: GluR2/3 was found predominantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two.

Molecular and functional diversity of neural connexins in the retina

Electrical synapses (gap junctions) in neuronal circuits have become a major focus in the study of network properties such as synchronization and oscillation (Galarreta and Hestrin, 1999; Gibson et al., 1999). Despite the recent progress made in unraveling the contribution of gap junctions to network behavior, little is known about the molecular composition of the junctional constituents. By cloning gap junction proteins [connexins (Cxs)] from zebrafish retina and through functional expression, we demonstrate that the retina possesses a high degree of connexin diversity, which may account for differential functional properties of electrical synapses. Three new Cxs, designated as zebrafish Cx27.5 (zfCx27.5), zfCx44.1, and zfCx55.5, and the carp ortholog of mammalian Cx43 were cloned. By in situ hybridization and in situ RT-PCR, we demonstrate that the four fish connexin mRNAs show differential localization in the retina. Transient functional expression in paired Xenopus oocytes and in the neuroblastoma N2A cell line indicate an extreme range of electrophysiological properties of these connexins in terms of voltage dependence and unitary conductance. For instance, the new zfCx44.1 exhibited high sensitivity to voltage-induced closure with currents decaying rapidly for transjunctional potentials >10 mV, whereas zfCx55.5 channels showed an opposite voltage dependence in response to voltage steps of either polarity. Moreover, although zfCx44.1 channels showed unitary conductance as high as any previously reported for junctional channels (nearly 300 pS), zfCx55. 5 and zfCx27.5 exhibited much lower unitary conductances (<60 pS).

Ca2+ regulation by the Na(+)-Ca2+ exchanger in retinal horizontal cells depolarized by L-glutamate

This study is concerned with regulation of the intracellular Ca2+ concentration ([Ca2+]i) of horizontal cells isolated from cyprinid fish retinae, with the main emphasis on the role of the (Na+)-Ca2+ exchanger. An inward current was blocked by Ca2+ (4 mM) during prolonged (> 1 h) depolarization by L-glutamate (100 microM) in the whole-cell voltage-clamp configuration, suggesting the persistent activation of voltage-gated Ca2+ channels. This (Co2+)-sensitive current was absent when extracellular Na+ was replaced by Li+ to suppress (Na+)-Ca2+ exchange. Measurement of [Ca2+]i using the Fura-2 ratiometric method gave the following results. (1) L-Glutamate (100 microM) caused [Ca2+]i to increase from the resting level of 75.4+/-36.8 nM (mean +/-S.D., n = 11) to the maximum level (2.2+/-1.4 microM, n = 11) within 15 s and then to decrease to a steady level of 0.59+/-0.23 microM (n = 11). (2) Nifedipine (100 microM) lowered the L-glutamate-induced steady [Ca2+]i level, which was still higher than the resting level. (3) L-Glutamate caused [Ca2+]i to increase even after blockading the voltage-gated Ca2+ channels by nifedipine or by clamping the membrane voltage at -55 mV. (4) (Na+)-free superfusate elevated the L-glutamate-induced steady [Ca2+]i level. (5) The time course of the [Ca2+]i decrease from the L-glutamate-induced steady level to the resting level was prolonged in the (Na+)-free superfusate. These results suggest that the (Na+)-Ca2+ exchanger extrudes intracellular Ca2+ to maintain a low [Ca2+]i level by counteracting the continuous Ca2+ influx through the voltage-gated Ca2+ channels and glutamate-gated channels when horizontal cells in situ are tonically depolarized by L-glutamate released from the photoreceptors. The (Na+)-Ca2+ exchange current isolated by a voltage-clamp experiment depends exponentially on the membrane potential.

Retinoic acid has light-adaptive effects on horizontal cells in the retina

Ambient light conditions affect the morphology of synaptic elements within the cone pedicle and modulate the spatial properties of the horizontal cell receptive field. We describe here that the effects of retinoic acid on these properties are similar to those of light adaptation. Intraorbital injection of retinoic acid into eyes of dark-adapted carp that subsequently were kept in complete darkness results in the formation of numerous spinules at the terminal dendrites of horizontal cells, a typical feature of light-adapted retinae. The formation of these spinules during light adaptation is impaired in the presence of citral, a competitive inhibitor of the dehydrogenase responsible for the generation of retinoic acid in vivo. Intracellularly recorded responses of horizontal cells from dark-adapted eyecup preparations superfused with retinoic acid reveal typical light-adapted spatial properties. Retinoic acid thus appears to act as a light-signaling modulator. Its activity appears not to be at the transcriptional level because its action was not blocked by actinomycin.

Ca(2+)-dependency of spinule plasticity at dendrites of retinal horizontal cells and its possible implication for the functional role of spinules

Calcium is involved in many aspects of synaptic plasticity and we have analyzed its involvement in spinule dynamics at retinal horizontal cell dendrites. We show here that in particular the retraction of spinules is a Ca(2+)-dependent process. Inhibiting calmodulin or CaMKII, blocked the retraction that was also impaired in low calcium Ringer. Changes of the cytosolic Ca(2+)-concentration through depletion of internal Ca(2+)-stores were without effect. This suggested that Ca(2+)-influx during dark adaption and subsequent activation of CaMKII is an important step for spinule retraction. Voltage dependent Ca(2+)-channels were not responsible for the Ca(2+)-influx, rather Ca2+ leaking through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-gated channels. This suggested a close local link between AMPA/kainate receptors and CaMKII indicating a possible postsynaptic function of spinules. The distribution of bound, omega-shaped vesicles within the cone pedicles and its dependence on artificial depolarization further supported the idea of a postsynaptic function of spinules.

Retraction of spinule-type neurites from carp retinal horizontal cell dendrites during dark adaptation involves the activation of Ca2+/calmodulin-dependent protein kinase II

The formation of spinules at the terminal dendrites of retinal horizontal cells with the onset of light and their subsequent retraction during darkness is a remarkable example of synaptic plasticity where sensory experience modifies reversibly, and on a time scale of minutes the ultrastructure of synaptic connectivity. The signals and the subsequent intracellular cascades underlying the prominent morphological alterations are only partially understood. We show here that lowering the external calcium concentration did prevent dark- and AMPA-induced retraction of spinules in a eyecup preparation. Furthermore, spinule retraction was prevented in vivo by the injection of calmidazolium, an inhibitor of calmodulin, into the eyeball, and also by the injection of KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase (CaMkII). We conclude that local Ca2+ influx through AMPA-gated channels followed by activation of CaMkII is an important step for spinule retraction during dark adaptation. The phosphorylation patterns of phosphoproteins derived from purified horizontal cells was affected by the inhibitors of calmodulin and CaMkII respectively. Some of the affected phosphoproteins appeared to be cytoskeleton-associated proteins, including GAP-43. Based on these observations, a putative scenario for the retraction of spinules is proposed.

Evidence for calcium/calmodulin dependence of spinule retraction in retinal horizontal cells

Horizontal cells of the carp retina alter their synaptic connections with cones during dark and light adaptation. At light onset, dendrites of horizontal cells, which are positioned laterally at the ribbon synapse, form "spinules," little processes with membrane densities. Spinules are retracted again during dark adaptation. Spinule retraction is also elicited upon glutamate application to the retina. In the present study, we address the question whether calcium/calmodulin-dependent pathways are involved in dark- and glutamate-evoked spinule retraction. Light-adapted retinas were isolated and subsequently dark adapted during incubation in media of different calcium concentrations. Spinule retraction was clearly blocked in low-calcium solutions (5 microM and 50 nM CaCl2). Incubation in medium containing cobalt chloride (2 mM) had the same effect. Both treatments blocked the glutamate-induced spinule retraction as well. These results indicate that spinule retraction is induced by a calcium influx into horizontal cells. To investigate whether calmodulin, the primary calcium receptor in eukaryotic cells, is present at the site of spinule formation, light- and dark-adapted retinas, embedded in LR White resin, were labelled with an antibody against calmodulin and gold-conjugated secondary antibodies. Horizontal cell dendrites at the ribbon synapse revealed strong calmodulin immunoreactivity, which was more than twice as high in light- as in dark-adapted retinas. The incubation of isolated retinas with the calmodulin antagonists W5 and W13 inhibited spinule retraction. In summary, these results suggest that spinule retraction may be regulated by calcium influx into horizontal cells and subsequent calcium/calmodulin-dependent pathways.

Two types of mitochondria are evidenced by protein kinase C immunoreactivity in the Müller cells of the carp retina

The localization of protein kinase C (PKC) was studied immunocytochemically in the Müller cells of the carp retina. Electron microscope immunocytochemistry (using a monoclonal antibody to the alpha and beta isoenzymes of PKC) showed PKC-immunoreactivity mainly inside some mitochondria, especially along the mitochondrial cristae whereas other mitochondria in the same Müller cells showed no staining. Despite a detailed analysis we did not find any significant morphological difference between labeled and unlabeled mitochondria. These results demonstrate, for the first time, the presence of PKC immunoreactivity inside mitochondria and suggest that individual mitochondria may differ in signal transduction pathway.

Protein content and cAMP-dependent phosphorylation of fractionated white perch retina

In the retinas of teleost fish dopamine, released from interplexiform cells, modulates synaptic transmission at both the chemical and electrical synapses of retinal horizontal cells. This modulation is due to activation of adenylate cyclase and phosphorylation by protein kinase A, perhaps of the synaptic ion channel proteins themselves. In this study we have fractionated the white perch retina by Percoll density gradient centrifugation in order to identify proteins which coenrich with horizontal cells. In addition we have tested retinal fractions for phosphorylation by native cAMP-dependent kinase. Our findings indicate that there are at least 3 proteins of molecular weights 28, 43/44 and 50 kDa which coenrich with horizontal cells and 3 proteins of 30/31 kDa, 35 kDa (putative rhodopsin) and 48 kDa (putative arrestin) which coenrich with photoreceptor fractions. The 43/44 kDa phosphoprotein is a target for cAMP-dependent protein phosphorylation and thus is apparently an element of the dopaminergic modulatory pathway in perch horizontal cells.