ON and OFF Signaling Pathways in the Retina and the Visual System

Visual processing starts at the retina of the eye, and signals are then transferred primarily to the visual cortex and the tectum. In the retina, multiple neural networks encode different aspects of visual input, such as color and motion. Subsequently, multiple neural streams in parallel convey unique aspects of visual information to cortical and subcortical regions. Bipolar cells, which are the second order neurons of the retina, separate visual signals evoked by light and dark contrasts and encode them to ON and OFF pathways, respectively. The interplay between ON and OFF neural signals is the foundation for visual processing for object contrast which underlies higher order stimulus processing. ON and OFF pathways have been classically thought to signal in a mirror-symmetric manner. However, while these two pathways contribute synergistically to visual perception in some instances, they have pronounced asymmetries suggesting independent operation in other cases. In this review, we summarize the role of the ON-OFF dichotomy in visual signaling, aiming to contribute to the understanding of visual recognition.

Characterization and staging of outer plexiform layer development in human retina and retinal organoids

The development of the first synapse of the visual system between photoreceptors and bipolar cells in the outer plexiform layer (OPL) of the human retina is critical for visual processing but poorly understood. By studying the maturation state and spatial organization of photoreceptors, depolarizing bipolar cells, and horizontal cells in the human fetal retina, we establish a pseudo-temporal staging system for OPL development that we term OPL-Stages 0 to 4. This was validated through quantification of increasingly precise subcellular localization of Bassoon to the OPL with each stage (p<0.0001). By applying these OPL staging criteria to human retinal organoids (HROs) derived from human embryonic and induced pluripotent stem cells, we observed comparable maturation from OPL-Stage 0 at day 100 in culture up to OPL-Stage 3 by day 160. Quantification of presynaptic protein localization confirmed progression from OPL-Stage 0 to 3 (p<0.0001). Overall, this study defines stages of human OPL development through mid-gestation and establishes HROs as a model system that recapitulates key aspects of human photoreceptor-bipolar cell synaptogenesis in vitro.

Broad locations of antigenic regions for anti-TRPM1 autoantibodies in paraneoplastic retinopathy with retinal ON bipolar cell dysfunction

PURPOSE

Cancer-associated retinal ON bipolar cell dysfunction (CARBD), which includes melanoma-associated retinopathy (MAR), has been reported to be caused by autoantibodies against the molecules expressed in ON bipolar cells, including TRPM1. The purpose of this study was to determine the antigenic regions of the autoantibodies against TRPM1 in the sera of CARBD patients, in whom we previously detected anti-TRPM1 autoantibodies.

METHODS

The antigenic regions against TRPM1 in the sera of eight CARBD patients were examined by Western blots using HEK293T cells transfected with the plasmids expressing FLAG-tagged TRPM1 fragments. The clinical course of these patients was also documented.

RESULTS

The clinical course differed among the patients. The electroretinograms (ERGs) and symptoms were improved in three patients, deteriorated in one patient, remained unchanged for a long time in one patient, and were not followable in three patients. Seven of the eight sera possessed multiple antigenic regions: two sera contained at least four antigen recognition regions, and three sera had at least three regions. The antigen regions were spread over the entire TRPM1 protein: five sera in the N-terminal intracellular domain, six sera in the transmembrane-containing region, and six sera in the C-terminal intracellular domain. No significant relationship was observed between the location of the antigen epitope and the patients' clinical course.

CONCLUSIONS

The antigenic regions of anti-TRPM1 autoantibodies in CARBD patients were present not only in the N-terminal intracellular domain, which was reported in an earlier report, but also in the transmembrane-containing region and in the C-terminal intracellular domain. In addition, the antigenic regions for TRPM1 were found to vary among the CARBD patients examined, and most of the sera had multiple antigenic regions.

Development and diversification of bipolar interneurons in the mammalian retina

The bipolar interneurons of the mammalian retina have evolved as a diverse set of cells with distinct subtype characteristics, which reflect specialized contributions to visual circuitry. Fifteen subtypes of bipolar interneurons have been identified in the mouse retina, each with characteristic gene expression, morphology, and light responses. This review provides an overview of the developmental events that underlie the generation of the diverse bipolar cell class, summarizing the current knowledge of genetic programs that establish and maintain bipolar subtype fates, as well as the events that shape the final distribution of bipolar subtypes. With much left to be discovered, bipolar interneurons present an ideal model system for studying the interplay between cell-autonomous and non-cell-autonomous mechanisms that influence neuronal subtype development within the central nervous system.

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.

ON-bipolar cell gene expression during retinal degeneration: Implications for optogenetic visual restoration

Purpose

Retinal bipolar cells survive even in the later stages of inherited retinal degenerations (IRDs) and so are attractive targets for optogenetic approaches to vision restoration. However, it is not known to what extent the remodelling that these cells undergo during degeneration affects their function. Specifically, it is unclear if they are free from metabolic stress, receptive to adeno-associated viral vectors, suitable for opsin-based optogenetic tools and able to propagate signals by releasing neurotransmitter.

Methods

Fluorescence activated cell sorting (FACS) was performed to isolate labelled bipolar cells from dissociated retinae of litter-mates with or without the IRD mutation Pde6b^rd1/rd1 selectively expressing an enhanced yellow fluorescent protein (EYFP) as a marker in ON-bipolar cells. Subsequent mRNA extraction allowed Illumina® microarray comparison of gene expression in bipolar cells from degenerate to those of wild type retinae. Changes in four candidate genes were further investigated at the protein level using retinal immunohistochemistry over the course of degeneration.

Results

A total of sixty differentially expressed transcripts reached statistical significance: these did not include any genes directly associated with native primary bipolar cell signalling, nor changes consistent with metabolic stress. Four significantly altered genes (Srm2, Slf2, Anxa7 & Cntn1), implicated in synaptic remodelling, neurotransmitter release and viral vector entry had immunohistochemical staining colocalising with ON-bipolar cell markers and varying over the course of degeneration.

Conclusion

Our findings suggest relatively few gene expression changes in the context of degeneration: that despite remodelling, bipolar cells are likely to remain viable targets for optogenetic vision restoration. In addition, several genes where changes were seen could provide a basis for investigations to enhance the efficacy of optogenetic therapies.

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Bipolar cells are attractive targets for therapeutic optogenetics in IRDs.

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This is the first cell specific transcriptomic analysis of bipolar cells in an IRD model.

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Bipolar cells maintain expression of genes essential to act as targets for optogenetics.

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Protein staining relating to four candidate genes (Anxa7, Cntn1, Srm2, Sulf2) is confirmed using immunohistochemistry.

Functional and Evolutionary Diversification of Otx2 and Crx in Vertebrate Retinal Photoreceptor and Bipolar Cell Development

Otx family homeoproteins Otx2 and Crx are expressed in photoreceptor precursor cells and bind to the common DNA-binding consensus sequence, but these two proteins have distinct functions in retinal development. To examine the functional substitutability of Otx2 and Crx, we generate knockin mouse lines in which Crx is replaced by Otx2 and vice versa. We find that Otx2 and Crx cannot be substituted in photoreceptor development. Subsequently, we investigate the function of Otx2 in photoreceptor and bipolar cell development. High Otx2 levels induce photoreceptor cell fate but not bipolar cell fate, whereas reduced Otx2 expression impairs bipolar cell maturation and survival. Furthermore, we identify Otx2 and Crx in the lamprey genome by using synteny analysis, suggesting that the last common ancestor of vertebrates possesses both Otx2 and Crx. We find that the retinal Otx2 expression pattern is different between lampreys and mice, suggesting that neofunctionalization of Otx2 occurred in the jawed vertebrate lineage.

Sox2 knockdown in the neonatal retina causes cell fate to switch from amacrine to bipolar

Transcription factor Sox2 is widely recognized for its critical roles in the nervous system, including the neural retina. Here, we aimed to reveal the function of Sox2 in the process of mouse postnatal development. After the suppression of Sox2 at P0, there was an increase number in bipolar cells but a decrease in amacrine cells. Inhibited Sox2 expression also led to decreased visual function. Furthermore, we found a distinctive type of retinal cells expressing the characteristic proteins of both bipolar cells and amacrine cells at P6, which may be an intermediate state in which amacrine cells were transforming into bipolar cells. Transcription factors associated with the development of bipolar cells and amacrine cells also support those changes. Our work indicated that inhibition of Sox2 could change cell fate by affecting transcription factors in the development of bipolar cells and amacrine cells, may provide new directions for the study and treatment of retinal genetic diseases and retinal dysplasia.

A subset of cone bipolar cells expresses the Na+ channel SCN2A in the human retina

Some bipolar cells in the human retina are known to express voltage-gated Na⁺ channels. However, it is unclear which types of channels are expressed, and whether Na⁺ channel expression is limited to specific types of bipolar cells. In the present study, we examined the types of voltage-gated Na⁺ channels expressed in human bipolar cells and the morphology of bipolar cells with voltage-gated Na⁺ currents. To investigate the expression of voltage-gated Na⁺ channels in human bipolar cells, we examined whether Na⁺ channel transcripts could be detected in single bipolar cells using the reverse transcription polymerase chain reaction (RT-PCR) technique. The voltage-gated Na⁺ current was recorded from isolated bipolar cells using the patch-clamp recording technique. Types of bipolar cells that have the Na⁺ currents were investigated by analyzing their morphology after staining with Lucifer yellow. Using RT-PCR, the SCN2A Na⁺ channel was detected in 5 of 6 isolated bipolar cells. This suggests that a subset of human bipolar cells expresses the SCN2A Na⁺ channel. Under voltage-clamp conditions, depolarizing voltage steps induced a fast transient inward current in cone bipolar cells with axon terminal boutons that stratified at the ON layer, which includes the stratum 3, 4, and 5 of the inner plexiform layer (IPL, n = 2/11 cells). The fast transient inward current of isolated bipolar cells was blocked by 1 μM of tetrodotoxin (TTX), a voltage-gated Na⁺ channel blocker. No fast transient inward current was recorded with axon terminals that stratify at the OFF layer, which includes stratum 1 and 2 of the IPL (n = 4). Thus, a subset of ON cone bipolar cells at least expresses the putative voltage-gated Na⁺ channel SCN2A in the human retina. The Na⁺ channels in the bipolar cells may serve to amplify the release of neurotransmitter, glutamate, when membrane potential is rapidly depolarized and thereby selectively accelerating light responses.

Early diabetes impairs ON sustained ganglion cell light responses and adaptation without cell death or dopamine insensitivity

Retinal signaling under dark-adapted conditions is perturbed during early diabetes. Additionally, dopamine, the main neuromodulator of retinal light adaptation, is diminished in diabetic retinas. However, it is not known if this dopamine deficiency changes how the retina responds to increased light or dopamine. Here we determine whether light adaptation is impaired in the diabetic retina, and investigate potential mechanism(s) of impairment. Diabetes was induced in C57BL/6J male mice via 3 intraperitoneal injections of streptozotocin (75 mg/kg) and confirmed by blood glucose levels more than 200 mg/dL. After 6 weeks, whole-cell recordings of light-evoked and spontaneous inhibitory postsynaptic currents (IPSCs) or excitatory postsynaptic currents (EPSCs) were made from rod bipolar cells and ON sustained ganglion cells, respectively. Light responses were recorded before and after D1 receptor (D1R) activation (SKF-38393, 20 μM) or light adaptation (background of 950 photons·μm^-2 ·s^-1). Retinal whole mounts were stained for either tyrosine hydroxylase and activated caspase-3 or GAD65/67, GlyT1 and RBPMS and imaged. D1R activation and light adaptation both decreased inhibition, but the disinhibition was not different between control and diabetic rod bipolar cells. However, diabetic ganglion cell light-evoked EPSCs were increased in the dark and showed reduced light adaptation. No differences were found in light adaptation of spontaneous EPSC parameters, suggesting upstream changes. No changes in cell density were found for dopaminergic, glycinergic or GABAergic amacrine cells, or ganglion cells. Thus, in early diabetes, ON sustained ganglion cells receive excessive excitation under dark- and light-adapted conditions. Our results show that this is not attributable to loss in number or dopamine sensitivity of inhibitory amacrine cells or loss of dopaminergic amacrine cells.

Inhibitory components of retinal bipolar cell receptive fields are differentially modulated by dopamine D1 receptors

During adaptation to an increase in environmental luminance, retinal signaling adjustments are mediated by the neuromodulator dopamine. Retinal dopamine is released with light and can affect center-surround receptive fields, the coupling state between neurons, and inhibitory pathways through inhibitory receptors and neurotransmitter release. While the inhibitory receptive field surround of bipolar cells becomes narrower and weaker during light adaptation, it is unknown how dopamine affects bipolar cell surrounds. If dopamine and light have similar effects, it would suggest that dopamine could be a mechanism for light-adapted changes. We tested the hypothesis that dopamine D1 receptor activation is sufficient to elicit the magnitude of light-adapted reductions in inhibitory bipolar cell surrounds. Surrounds were measured from OFF bipolar cells in dark-adapted mouse retinas while stimulating D1 receptors, which are located on bipolar, horizontal, and inhibitory amacrine cells. The D1 agonist SKF-38393 narrowed and weakened OFF bipolar cell inhibitory receptive fields but not to the same extent as with light adaptation. However, the receptive field surround reductions differed between the glycinergic and GABAergic components of the receptive field. GABAergic inhibitory strength was reduced only at the edges of the surround, while glycinergic inhibitory strength was reduced across the whole receptive field. These results expand the role of retinal dopamine to include modulation of bipolar cell receptive field surrounds. Additionally, our results suggest that D1 receptor pathways may be a mechanism for the light-adapted weakening of glycinergic surround inputs and the furthest wide-field GABAergic inputs to bipolar cells. However, remaining differences between light-adapted and D1 receptor-activated inhibition demonstrate that non-D1 receptor mechanisms are necessary to elicit the full effect of light adaptation on inhibitory surrounds.

Differential expression of PKCα and -β in the zebrafish retina

The retina is a complex neural circuit, which processes and transmits visual information from light perceiving photoreceptors to projecting retinal ganglion cells. Much of the computational power of the retina rests on signal integrating interneurons, such as bipolar cells. Commercially available antibodies against bovine and human conventional protein kinase C (PKC) α and -β are frequently used as markers for retinal ON-bipolar cells in different species, despite the fact that it is not known which bipolar cell subtype(s) they actually label. In zebrafish (Danio rerio) five prkc genes (coding for PKC proteins) have been identified. Their expression has not been systematically determined. While prkcg is not expressed in retinal tissue, the other four prkc (prkcaa, prkcab, prkcba, prkcbb) transcripts were found in different parts of the inner nuclear layer and some as well in the retinal ganglion cell layer. Immunohistochemical analysis in adult zebrafish retina using fluorescent in situ hybridization and PKC antibodies showed an overlapping immunolabeling of ON-bipolar cells that are most likely of the BON s6 and BON s6L or RRod type. However, comparison of transcript expression with immunolabeling, implies that these antibodies are not specific for one single zebrafish conventional PKC, but rather detect a combination of PKC -α and -β variants.

DSCAM differentially modulates pre- and postsynaptic structural and functional central connectivity during visual system wiring

BACKGROUND

Proper patterning of dendritic and axonal arbors is a critical step in the formation of functional neuronal circuits. Developing circuits rely on an array of molecular cues to shape arbor morphology, but the underlying mechanisms guiding the structural formation and interconnectivity of pre- and postsynaptic arbors in real time remain unclear. Here we explore how Down syndrome cell adhesion molecule (DSCAM) differentially shapes the dendritic morphology of central neurons and their presynaptic retinal ganglion cell (RGC) axons in the developing vertebrate visual system.

METHODS

The cell-autonomous role of DSCAM, in tectal neurons and in RGCs, was examined using targeted single-cell knockdown and overexpression approaches in developing Xenopus laevis tadpoles. Axonal arbors of RGCs and dendritic arbors of tectal neurons were visualized using real-time in vivo confocal microscopy imaging over the course of 3 days.

RESULTS

In the Xenopus visual system, DSCAM immunoreactivity is present in RGCs, cells in the optic tectum and the tectal neuropil at the time retinotectal synaptic connections are made. Downregulating DSCAM in tectal neurons significantly increased dendritic growth and branching rates while inducing dendrites to take on tortuous paths. Overexpression of DSCAM, in contrast, reduced dendritic branching and growth rate. Functional deficits mediated by tectal DSCAM knockdown were examined using visually guided behavioral assays in swimming tadpoles, revealing irregular behavioral responses to visual stimulus. Functional deficits in visual behavior also corresponded with changes in VGLUT/VGAT expression, markers of excitatory and inhibitory transmission, in the tectum. Conversely, single-cell DSCAM knockdown in the retina revealed that RGC axon arborization at the target is influenced by DSCAM, where axons grew at a slower rate and remained relatively simple. In the retina, dendritic arbors of RGCs were not affected by the reduction of DSCAM expression.

CONCLUSIONS

Together, our observations implicate DSCAM in the control of both pre- and postsynaptic structural and functional connectivity in the developing retinotectal circuit, where it primarily acts as a neuronal brake to limit and guide postsynaptic dendrite growth of tectal neurons while it also facilitates arborization of presynaptic RGC axons cell autonomously.

Requirements for Neurogenin2 during mouse postnatal retinal neurogenesis

During embryonic retinal development, the bHLH factor Neurog2 regulates the temporal progression of neurogenesis, but no role has been assigned for this gene in the postnatal retina. Using Neurog2 conditional mutants, we found that Neurog2 is necessary for the development of an early, embryonic cohort of rod photoreceptors, but also required by both a subset of cone bipolar subtypes, and rod bipolars. Using transcriptomics, we identified a subset of downregulated genes in P2 Neurog2 mutants, which act during rod differentiation, outer segment morphogenesis or visual processing. We also uncovered defects in neuronal cell culling, which suggests that the rod and bipolar cell phenotypes may arise via more complex mechanisms rather than a simple cell fate shift. However, given an overall phenotypic resemblance between Neurog2 and Blimp1 mutants, we explored the relationship between these two factors. We found that Blimp1 is downregulated between E12-birth in Neurog2 mutants, which probably reflects a dependence on Neurog2 in embryonic progenitor cells. Overall, we conclude that the Neurog2 gene is expressed and active prior to birth, but also exerts an influence on postnatal retinal neuron differentiation.

Bipolar cell gap junctions serve major signaling pathways in the human retina

Connexin36 (Cx36) constituent gap junctions (GJ) throughout the brain connect neurons into functional syncytia. In the retina they underlie the transmission, averaging and correlation of signals prior conveying visual information to the brain. This is the first study that describes retinal bipolar cell (BC) GJs in the human inner retina, whose function is enigmatic even in the examined animal models. Furthermore, a number of unique features (e.g. fovea, trichromacy, midget system) necessitate a reexamination of the animal model results in the human retina. Well-preserved postmortem human samples of this study are allowed to identify Cx36 expressing BCs neurochemically. Results reveal that both rod and cone pathway interneurons display strong Cx36 expression. Rod BC inputs to AII amacrine cells (AC) appear in juxtaposition to AII GJs, thus suggesting a strategic AII cell targeting by rod BCs. Cone BCs serving midget, parasol or koniocellular signaling pathways display a wealth of Cx36 expression to form homologously coupled arrays. In addition, they also establish heterologous GJ contacts to serve an exchange of information between parallel signaling streams. Interestingly, a prominent Cx36 expression was exhibited by midget system BCs that appear to maintain intimate contacts with bistratified BCs serving other pathways. These findings suggest that BC GJs in parallel signaling streams serve both an intra- and inter-pathway exchange of signals in the human retina.

NO signaling in retinal bipolar cells

Nitric oxide (NO) is a neuromodulator involved in physiological and pathological processes in the retina. In the inner retina, a subgroup of amacrine cells have been shown to synthesize NO, but bipolar cells remain controversial as NO sources. This study correlates NO synthesis in dark-adapted retinas, through labeling with the NO marker DAF-FM, with neuronal nitric oxide synthase (nNOS) and inducible NOS expression, and presence of the NO receptor soluble guanylate cyclase in bipolar cells. NO containing bipolar cells were morphologically identified by dialysis of DAF fluorescent cells with intracellular dyes, or by DAF labeling followed by immunohistochemistry for nNOS and other cellular markers. DAF fluorescence was observed in all types of bipolar cells that could be identified, but the most intense DAF fluorescence was observed in bipolar cells with severed processes, supporting pathological NO signaling. Among nNOS expressing bipolar cells, type 9 was confirmed unequivocally, while types 2, 3a, 3b, 4, 5, 7, 8 and the rod bipolar cell were devoid of this enzyme. These results establish specific bipolar cell types as NO sources in the inner retina, and support the involvement of NO signaling in physiological and pathological processes in the inner retina.

Characterization of connexin36 gap junctions in the human outer retina

Retinal connexins (Cx) form gap junctions (GJ) in key circuits that transmit average or synchronize signals. Expression of Cx36, -45, -50 and -57 have been described in many species but there is still a disconcerting paucity of information regarding the Cx makeup of human retinal GJs. We used well-preserved human postmortem samples to characterize Cx36 GJ constituent circuits of the outer plexiform layer (OPL). Based on their location, morphometric characteristics and co-localizations with outer retinal neuronal markers, we distinguished four populations of Cx36 plaques in the human OPL. Three of these were comprised of loosely scattered Cx36 plaques; the distalmost population 1 formed cone-to-rod GJs, population 2 in the mid-OPL formed cone-to-cone GJs, whereas the proximalmost population 4 likely connected bipolar cell dendrites. The fourth population (population 3) of Cx36 plaques conglomerated beneath cone pedicles and connected dendritic tips of bipolar cells that shared a common presynaptic cone. Overall, we show that the human outer retina displays a diverse cohort of Cx36 GJ that follows the general mammalian scheme and display a great functional diversity.

Endothelins Inhibit Osmotic Swelling of Rat Retinal Glial and Bipolar Cells by Activation of Growth Factor Signaling

Water accumulation in retinal glial (Müller) and neuronal cells resulting in cellular swelling contributes to the development of retinal edema and neurodegeneration. Here, we show that endothelin-1 (ET-1) dose-dependently inhibits the hypoosmotic swelling of Müller cells in freshly isolated retinal slices of control and diabetic rats, with a maximal inhibition at 100 nM. Osmotic Müller cell swelling was also inhibited by ET-2. The effect of ET-1 was mediated by activation of ET_A and ET_B receptors resulting in transactivation of metabotropic glutamate receptors, purinergic P2Y₁, and adenosine A₁ receptors. ET-1 (but not ET-2) also inhibited the osmotic swelling of bipolar cells in retinal slices, but failed to inhibit the swelling of freshly isolated bipolar cells. The inhibitory effect of ET-1 on the bipolar cell swelling in retinal slices was abrogated by inhibitors of the FGF receptor kinase (PD173074) and of TGF-β1 superfamily activin receptor-like kinase receptors (SB431542), respectively. Both Müller and bipolar cells displayed immunoreactivities of ET_A and ET_B receptor proteins. The data may suggest that neuroprotective effects of ETs in the retina are in part mediated by prevention of the cytotoxic swelling of retinal glial and bipolar cells. ET-1 acts directly on Müller cells, while the inhibitory effect of ET-1 on bipolar cell swelling is indirectly mediated, via stimulation of the release of growth factors like bFGF and TGF-β1 from Müller cells.

Retinal remodeling in human retinitis pigmentosa

Retinitis Pigmentosa (RP) in the human is a progressive, currently irreversible neural degenerative disease usually caused by gene defects that disrupt the function or architecture of the photoreceptors. While RP can initially be a disease of photoreceptors, there is increasing evidence that the inner retina becomes progressively disorganized as the outer retina degenerates. These alterations have been extensively described in animal models, but remodeling in humans has not been as well characterized. This study, using computational molecular phenotyping (CMP) seeks to advance our understanding of the retinal remodeling process in humans. We describe cone mediated preservation of overall topology, retinal reprogramming in the earliest stages of the disease in retinal bipolar cells, and alterations in both small molecule and protein signatures of neurons and glia. Furthermore, while Müller glia appear to be some of the last cells left in the degenerate retina, they are also one of the first cell classes in the neural retina to respond to stress which may reveal mechanisms related to remodeling and cell death in other retinal cell classes. Also fundamentally important is the finding that retinal network topologies are altered. Our results suggest interventions that presume substantial preservation of the neural retina will likely fail in late stages of the disease. Even early intervention offers no guarantee that the interventions will be immune to progressive remodeling. Fundamental work in the biology and mechanisms of disease progression are needed to support vision rescue strategies.

Ischemic regulation of brain-derived neurotrophic factor-mediated cell volume and TrkB expression in glial (Müller) and bipolar cells of the rat retina

BACKGROUND

Osmotic swelling of neurons and glial cells contributes to retinal edema and neurodegeneration. BDNF, a major neuroprotectant in the retina, was shown to inhibit osmotic swelling of glial (Müller) and bipolar cells in the rat retina; the effect of BDNF on the bipolar cell swelling is mediated by inducing a release of neuroprotective cytokines from Müller cells (Berk et al., Neuroscience 295:175-186, 2015). We determined whether BDNF-mediated cell volume regulation was altered after transient retinal ischemia.

METHODS

Retinal slices from the eyes of rats that underwent a 1-h pressure-induced retinal ischemia and from control eyes were superfused with a hypoosmotic solution.

RESULTS

Exogenous BDNF prevented osmotic swelling of Müller cells in both control and post-ischemic retinal slices. BDNF also prevented osmotic swelling of bipolar cells in the control retina, but not in the ischemic retina. On the other hand, exogenous bFGF prevented the swelling of both Müller and bipolar cells in the ischemic retina. Freshly isolated Müller cells of control retinas displayed immunoreactivity of truncated but not full-length TrkB. In contrast, Müller cells of post-ischemic retinas displayed immunoreactivity of both TrkB isoforms. Bipolar cells isolated from control and post-ischemic retinas were immunolabeled for both TrkB isoforms.

CONCLUSIONS

The data may suggest that the ischemic abrogation of the BDNF effect in bipolar cells is related to altered BDNF receptor expression in Müller cells. Glial upregulation of full-length TrkB may support the survival of Müller cells in the ischemic retina, but may impair the BDNF-induced release of neuroprotective cytokines such as bFGF from Müller cells.

Sigma-1 receptor activation inhibits osmotic swelling of rat retinal glial (Müller) cells by transactivation of glutamatergic and purinergic receptors

Water accumulation in retinal glial (Müller) and neuronal cells resulting in cellular swelling contributes to the development of retinal edema and neurodegeneration. Sigma (σ) receptor activation is known to have neuroprotective effects in the retina. Here, we show that the nonselective σ receptor agonist ditolylguanidine, and the selective σ1 receptor agonist PRE-084, inhibit the osmotic swelling of Müller cell somata induced by superfusion of rat retinal slices with a hypoosmotic solution containing barium ions. In contrast, PRE-084 did not inhibit the osmotic swelling of bipolar cell somata. The effects of σ receptor agonists on the Müller cell swelling were abrogated in the presence of blockers of metabotropic glutamate and purinergic P2Y1 receptors, respectively, suggesting that σ receptor activation triggers activation of a glutamatergic-purinergic signaling cascade which is known to prevent the osmotic Müller cell swelling. The swelling-inhibitory effect of 17β-estradiol was prevented by the σ1 receptor antagonist BD1047, suggesting that the effect is mediated by σ1 receptor activation. The data may suggest that the neuroprotective effect of σ receptor activation in the retina is in part mediated by prevention of the cytotoxic swelling of retinal glial cells.

TIMP-1 affects the spatial distribution of dendritic processes of second-order neurons in a rat model of Retinitis Pigmentosa

Retinitis Pigmentosa (RP) is an inherited disorder that may lead to blindness. In the rhodopsin S334ter-line-3 rat model of RP, the death of rods induces spatial rearrangement of cones into regular ring mosaics. Using this model, we discovered that the ring mosaics are restored to a homogeneous distribution upon application of tissue inhibitor of metalloproteinase-1 (TIMP-1). In this study, we further investigated the cone migration and spatial distribution of second-order neurons and their connections to cones in the presence or absence of TIMP-1 using immunohistochemistry to identify retinal neurons and their connections with cones. M-opsin cell bodies and their outer segments were evaluated to determine whether TIMP-1 delays the degeneration of outer segments of cones. We observed that during cone rearrangement into ring mosaics in RP retina, dendritic processes of second-order neurons undergo remodeling to maintain their synaptic connections with the cones in the rings. TIMP-1 treatment induced the cones to rearrange and dendritic processes of second-order neurons to return to a more homogeneous spatial distribution. In addition, TIMP-1 treatment protected the outer segments of cones at later stages of retinal degeneration. Our findings clearly demonstrate that despite their dramatic spatial rearrangement, cones and second-order neuron processes maintain their synaptic connections before and after TIMP-1 treatment.

Synthetic 3D diamond-based electrodes for flexible retinal neuroprostheses: Model, production and in vivo biocompatibility

Two retinal implants have recently received the CE mark and one has obtained FDA approval for the restoration of useful vision in blind patients. Since the spatial resolution of current vision prostheses is not sufficient for most patients to detect faces or perform activities of daily living, more electrodes with less crosstalk are needed to transfer complex images to the retina. In this study, we modelled planar and three-dimensional (3D) implants with a distant ground or a ground grid, to demonstrate greater spatial resolution with 3D structures. Using such flexible 3D implant prototypes, we showed that the degenerated retina could mould itself to the inside of the wells, thereby isolating bipolar neurons for specific, independent stimulation. To investigate the in vivo biocompatibility of diamond as an electrode or an isolating material, we developed a procedure for depositing diamond onto flexible 3D retinal implants. Taking polyimide 3D implants as a reference, we compared the number of neurones integrating the 3D diamond structures and their ratio to the numbers of all cells, including glial cells. Bipolar neurones were increased whereas there was no increase even a decrease in the total cell number. SEM examinations of implants confirmed the stability of the diamond after its implantation in vivo. This study further demonstrates the potential of 3D designs for increasing the resolution of retinal implants and validates the safety of diamond materials for retinal implants and neuroprostheses in general.

Deletion of myosin VI causes slow retinal optic neuropathy and age-related macular degeneration (AMD)-relevant retinal phenotype

The unconventional myosin VI, a member of the actin-based motor protein family of myosins, is expressed in the retina. Its deletion was previously shown to reduce amplitudes of the a- and b-waves of the electroretinogram. Analyzing wild-type and myosin VI-deficient Snell’s Waltzer mice in more detail, the expression pattern of myosin VI in retinal pigment epithelium, outer limiting membrane, and outer plexiform layer could be linked with differential progressing ocular deficits. These encompassed reduced a-waves and b-waves and disturbed oscillatory potentials in the electroretinogram, photoreceptor cell death, retinal microglia infiltration, and formation of basal laminar deposits. A phenotype comprising features of glaucoma (neurodegeneration) and age-related macular degeneration could thus be uncovered that suggests dysfunction of myosin VI and its variable cargo adaptor proteins for membrane sorting and autophagy, as possible candidate mediators for both disease forms.

AII amacrine cells discriminate between heterocellular and homocellular locations when assembling connexin36-containing gap junctions

Electrical synapses (gap junctions) rapidly transmit signals between neurons and are composed of connexins. In neurons, connexin36 (Cx36) is the most abundant isoform; however, the mechanisms underlying formation of Cx36-containing electrical synapses are unknown. We focus on homocellular and heterocellular gap junctions formed by an AII amacrine cell, a key interneuron found in all mammalian retinas. In mice lacking native Cx36 but expressing a variant tagged with enhanced green fluorescent protein at the C-terminus (KO-Cx36-EGFP), heterocellular gap junctions formed between AII cells and ON cone bipolar cells are fully functional, whereas homocellular gap junctions between two AII cells are not formed. A tracer injected into an AII amacrine cell spreads into ON cone bipolar cells but is excluded from other AII cells. Reconstruction of Cx36-EGFP clusters on an AII cell in the KO-Cx36-EGFP genotype confirmed that the number, but not average size, of the clusters is reduced - as expected for AII cells lacking a subset of electrical synapses. Our studies indicate that some neurons exhibit at least two discriminatory mechanisms for assembling Cx36. We suggest that employing different gap-junction-forming mechanisms could provide the means for a cell to regulate its gap junctions in a target-cell-specific manner, even if these junctions contain the same connexin.