Expression of vesicular glutamate transporter 1 in the mouse retina reveals temporal ordering in development of rod vs. cone and ON vs. OFF circuits

Glutamatergic neuronal activity regulates angiogenesis and blood-retinal barrier maturation via Norrin/β-catenin signaling

Interactions among neuronal, glial, and vascular components are crucial for retinal angiogenesis and blood-retinal barrier (BRB) maturation. Although synaptic dysfunction precedes vascular abnormalities in many retinal pathologies, how neuronal activity, specifically glutamatergic activity, regulates retinal angiogenesis and BRB maturation remains unclear. Using in vivo genetic studies in mice, single-cell RNA sequencing (scRNA-seq), and functional validation, we show that deep plexus angiogenesis and paracellular BRB maturation are delayed in Vglut1-/- retinas where neurons fail to release glutamate. By contrast, deep plexus angiogenesis and paracellular BRB maturation are accelerated in Gnat1-/- retinas, where constitutively depolarized rods release excessive glutamate. Norrin expression and endothelial Norrin/β-catenin signaling are downregulated in Vglut1-/- retinas and upregulated in Gnat1-/- retinas. Pharmacological activation of endothelial Norrin/β-catenin signaling in Vglut1-/- retinas rescues defects in deep plexus angiogenesis and paracellular BRB maturation. Our findings demonstrate that glutamatergic neuronal activity regulates retinal angiogenesis and BRB maturation by modulating endothelial Norrin/β-catenin signaling.

Neural extracellular matrix regulates visual sensory motor integration

Visual processing depends on sensitive and balanced synaptic neurotransmission. Extracellular matrix proteins in the environment of cells are key modulators in synaptogenesis and synaptic plasticity. In the present study, we provide evidence that the combined loss of the four extracellular matrix components, brevican, neurocan, tenascin-C, and tenascin-R, in quadruple knockout mice leads to severe retinal dysfunction and diminished visual motion processing in vivo. Remarkably, impaired visual motion processing was accompanied by a developmental loss of cholinergic direction-selective starburst amacrine cells. Additionally, we noted imbalance of inhibitory and excitatory synaptic signaling in the quadruple knockout retina. Collectively, the study offers insights into the functional importance of four key extracellular matrix proteins for retinal function, visual motion processing, and synaptic signaling.

Glutamatergic neuronal activity regulates angiogenesis and blood-retinal barrier maturation via Norrin/β-catenin signaling

Interactions among neuronal, glial and vascular components are crucial for retinal angiogenesis and blood-retinal barrier (BRB) maturation. Although synaptic dysfunction precedes vascular abnormalities in many retinal pathologies, how neuronal activity, specifically glutamatergic activity, regulates retinal angiogenesis and BRB maturation remains unclear. Using in vivo genetic studies in mice, single-cell RNA-sequencing and functional validation, we show that deep plexus angiogenesis and paracellular BRB maturation are delayed in Vglut1 -/- retinas where neurons fail to release glutamate. In contrast, deep plexus angiogenesis and paracellular BRB maturation are accelerated in Gnat1 -/- retinas where constitutively depolarized rods release excessive glutamate. Norrin expression and endothelial Norrin/β-catenin signaling are downregulated in Vglut1 -/- retinas, and upregulated in Gnat1 -/- retinas. Pharmacological activation of endothelial Norrin/β-catenin signaling in Vglut1 -/- retinas rescued defects in deep plexus angiogenesis and paracellular BRB maturation. Our findings demonstrate that glutamatergic neuronal activity regulates retinal angiogenesis and BRB maturation by modulating endothelial Norrin/β-catenin signaling.

miRNA Profiling of Developing Rat Retina in the First Three Postnatal Weeks

The morphogenesis of the mammalian retina depends on the precise control of gene expression during development. Small non-coding RNAs, including microRNAs play profound roles in various physiological and pathological processes via gene expression regulation. A systematic analysis of the expression profile of small non-coding RNAs in developing Wistar rat retinas (postnatally day 5 (P5), P7, P10, P15 and P21) was executed using IonTorrent PGM next-generation sequencing technique to reveal the crucial players in the early postnatal retinogenesis. Our analysis reveals extensive regulatory potential of microRNAs during retinal development. We found a group of microRNAs that show constant high abundance (miR-19, miR-101; miR-181, miR-183, miR-124 and let-7) during the development process. Others are present only in the early stages (miR-20a, miR-206, miR-133, miR-466, miR-1247, miR-3582), or at later stages (miR-29, miR-96, miR-125, miR-344 or miR-664). Further miRNAs were detected which are differentially expressed in time. Finally, pathway enrichment analysis has revealed 850 predicted target genes that mainly participate in lipid-, amino acid- and glycan metabolisms in the examined time-period (P5-P21). P5-P7 transition revealed the importance of miRNAs in glutamatergic synapse and gap junction pathways. Significantly downregulated miRNAs rno-miR-30c1 and 2, rno-miR-205 and rno-miR-503 were detected to target Prkx (ENSRNOG00000003696), Adcy6 (ENSRNOG00000011587), Gnai3 (ENSRNOG00000019465) and Gja1 (ENSRNOG00000000805) genes. The dataset described here will be a valuable resource for clarifying new regulatory mechanisms for retinal development and will greatly contribute to our understanding of the divergence and function of microRNAs.

Role of glutamate in the development of visual pathways

Glutamate is an important amino acid, metabolite and excitatory neurotransmitter, which is found in its free form in the extracellular spaces of the central nervous system (CNS). More than half of all synapses in CNS release glutamate. It is the main neurotransmitter driving the light responses in the retina. All types of photoreceptors, bipolar, ganglion and one type of glycinergic amacrine cells express specific subtypes of vesicular glutamate transporters and are the main source of endogenous glutamate in retina, besides Müller glia that are responsible for glutamate homeostasis, release and reuptake. Reduced or excessive extracellular glutamate was detected in the synaptic clefts of several naturally occurring or transgenic eye disease models, in which network rewiring and altered functions were observed. These led to the hypothesis that glutamate is one of the extrinsic signals for visual pathway development. This minireview examines experimental evidences supporting, or refuting, the influence of glutamate on prenatal and postnatal retinal development.

Developmental light deprivation transiently reduces the expression of vGluT2 and GluN2B in the rat ventral suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the most important circadian clock in mammals. The SCN synchronizes to environmental light via the retinohypothalamic tract (RHT), which is an axon cluster derived from melanopsin-expressing intrinsic photosensitive retinal ganglion cells. Investigations on the development of the nonimage-forming pathway and the RHT are scarce. Previous studies imply that light stimulation during postnatal development is not needed to make the RHT functional at adult stages. Here, we examined the effects of light deprivation (i.e., constant darkness (DD) rearing) during postnatal development on the expression in the ventral SCN of two crucial proteins for the synchronization of circadian rhythms to light: the presynaptic vesicular glutamate transporter type 2 (vGluT2) and the GluN2B subunit of the postsynaptic NMDA receptor. We found that animals submitted to DD conditions exhibited a transitory reduction in the expression of vGluT2 (at P12-19) and of GluN2B (at P7-9) that was compensated at older stages. These findings support the hypothesis that visual stimulation during early ages is not decisive for normal development of the RHT-SCN pathway.

Dynamic functional and structural remodeling during retinal regeneration in zebrafish

Introduction

Zebrafish regenerate their retinas following damage, resulting in restoration of visual function. Here we evaluate recovery of retinal function through qualitative and quantitative analysis of the electroretinogram (ERG) over time following retinal damage, in correlation to histological features of regenerated retinal tissue.

Methods

Retinas of adult zebrafish were lesioned by intravitreal injection of 10 μM (extensive lesion; destroys all neurons) or 2 μM (selective lesion; spares photoreceptors) ouabain. Unlesioned contralateral retinas served as controls. Function of retinal circuitry was analyzed at selected timepoints using ERG recordings from live zebrafish, and whole eyes were processed for histological analyses immediately thereafter.

Results

Qualitative and quantitative assessment of waveforms during retinal regeneration revealed dynamic changes that were heterogeneous on an individual level within each sampling time, but still followed common waveform recovery patterns on a per-fish and population-level basis. Early in the regeneration period (13-30 days post injury; DPI), for both lesion types, b-waves were essentially not detected, and unmasked increased apparent amplitudes, implicit times, and half-widths of a-waves (vs. controls). In control recordings, d-waves were not obviously detected, but apparent d-waves (OFF-bipolar responses) from regenerating retinas of several fish became prominent by 30DPI and dominated the post-photoreceptor response (PPR). Beyond 45DPI, b-waves became detectable, and the ratio of apparent d- to b-wave contributions progressively shifted with most, but not all, fish displaying a b-wave dominated PPR. At the latest timepoints (extensive, 90DPI; selective, 80DPI), recordings with measurable b-waves approached a normal waveform (implicit times and half-widths), but amplitudes were not restored to control levels. Histological analyses of the retinas from which ERGs were recorded showed that as regeneration progressed, PKCa + ON-bipolar terminals and parvalbumin + amacrine cell processes became more stereotypically positioned within the deep sublaminae of the INL over recovery time after each lesion type, consistent with the shift in PPR seen in the ERG recordings.

Discussion

Taken together, these data suggest that photoreceptor-OFF-bipolar component/connectivity may functionally recover and mature earlier during regeneration compared to the photoreceptor-ON-bipolar component, though the timeframe in which such recovery happens is heterogeneous on a per-fish basis. Collectively our studies suggest gradual restoration of ON-bipolar functional circuitry during retinal regeneration.

Long-term progression of retinal degeneration in a preclinical model of CLN7 Batten disease as a baseline for testing clinical therapeutics

BACKGROUND

Batten disease is characterized by cognitive and motor impairment, retinal degeneration, and seizures leading to premature death. Recent studies have shown efficacy for a gene therapy approach for CLN7 Batten disease. This gene therapy approach is promising to treat cognitive and motor impairment, but is not likely to delay vision loss. Additionally, the natural progression of retinal degeneration in CLN7 Batten disease patients is not well-known.

METHODS

We performed visual examinations on five patients with CLN7 Batten disease and found that patients were far progressed in degeneration within their first five years of life. To better understand the disease progression, we characterized the retina of a preclinical mouse model of CLN7 Batten disease, through the age at which mice present with paralysis and premature death.

FINDINGS

We found that this preclinical model shows signs of photoreceptor to bipolar synaptic defects early, and displays rod-cone dystrophy with late loss of bipolar cells. This vision loss could be followed not only via histology, but using clinical live imaging similar to that used in human patients.

INTERPRETATION

Natural history studies of rare paediatric neurodegenerative conditions are complicated by the rapid degeneration and limited availability of patients. Characterization of degeneration in the preclinical model allows for future experiments to better understand the mechanisms underlying the retinal disease progression in order to find therapeutics to treat patients, as well as to evaluate these therapeutic options for future human clinical trials.

FUNDING

Van Sickle Family Foundation Inc., NIHP30EY030413, Morton Fichtenbaum Charitable Trust and 5T32GM131945-03.

Improved Fluorescent Signal in Expansion Microscopy Using Fluorescent Fab Fragment Secondary Antibodies

Expansion microscopy (ExM) is a microscopic imaging approach that can achieve super-resolution visualization of fluorescently labeled biological samples using conventional fluorescence microscopy. The method is based on embedding of a fluorescently labeled biological sample in a hydrogel matrix followed by the physical expansion of the specimen, which is then viewed using a conventional fluorescent microscope. Variations of the method can be used to visualize endogenously expressed fluorescent proteins, such as GFP, fluorescently tagged antibodies, nucleic acids, or other fluorescently tagged molecules. A significant challenge of the method is that the physical expansion of the specimen produces a concommitant reduction in fluorescence intensity, which can make imaging difficult. We describe an approach for amplifying fluorescence signal following expansion of immunolabeled tissue sections by applying fluorescently labeled Fab fragment secondary antibodies to intensify fluorescent signal and enhance detection of labeling using conventional fluorescent microscopy.

A method to increase immunofluorescence signal intensity of Expansion Microscopy specimens is described.

Method utilizes commercially available reagents.

Enhances ability to acquire useful images in expanded tissue samples.

Impact of Primary RPE Cells in a Porcine Organotypic Co-Cultivation Model

The pathological events of age-related macular degeneration are characterized by degenerative processes involving the photoreceptor cells, retinal pigment epithelium (RPE), and the Bruch's membrane as well as choroidal alterations. To mimic in vivo interactions between photoreceptor cells and RPE cells ex vivo, complex models are required. Hence, the aim of this study was to establish a porcine organotypic co-cultivation model and enlighten the interactions of photoreceptor and RPE cells, with a special emphasis on potential neuroprotective effects. Porcine neuroretina explants were cultured with primary porcine RPE cells (ppRPE) or medium derived from these cells (=conditioned medium). Neuroretina explants cultured alone served as controls. After eight days, RT-qPCR and immunohistology were performed to analyze photoreceptors, synapses, macroglia, microglia, complement factors, and pro-inflammatory cytokines (e.g., IL1B, IL6, TNF) in the neuroretina samples. The presence of ppRPE cells preserved photoreceptors, whereas synaptical density was unaltered. Interestingly, on an immunohistological as well as on an mRNA level, microglia and complement factors were comparable in all groups. Increased IL6 levels were noted in ppRPE and conditioned medium samples, while TNF was only upregulated in the ppRPE group. IL1B was elevated in conditioned medium samples. In conclusion, a co-cultivation of ppRPE cells and neuroretina seem to have beneficial effects on the neuroretina, preserving photoreceptors and maintaining synaptic vesicles in vitro. This organotypic co-cultivation model can be used to investigate the complex interactions between the retina and RPE cells, gain further insight into neurodegenerative pathomechanisms occurring in retinal diseases, and evaluate potential therapeutics.

Visual Stimulation Induces Distinct Forms of Sensitization of On-Off Direction-Selective Ganglion Cell Responses in the Dorsal and Ventral Retina

Experience-dependent modulation of neuronal responses is a key attribute in sensory processing. In the mammalian retina, the On-Off direction-selective ganglion cell (DSGC) is well known for its robust direction selectivity. However, how the On-Off DSGC light responsiveness dynamically adjusts to the changing visual environment is underexplored. Here, we report that On-Off DSGCs tuned to posterior motion direction [i.e. posterior DSGCs (pDSGCs)] in mice of both sexes can be transiently sensitized by prior stimuli. Notably, distinct sensitization patterns are found in dorsal and ventral pDSGCs. Although responses of both dorsal and ventral pDSGCs to dark stimuli (Off responses) are sensitized, only dorsal cells show the sensitization of responses to bright stimuli (On responses). Visual stimulation to the dorsal retina potentiates a sustained excitatory input from Off bipolar cells, leading to tonic depolarization of pDSGCs. Such tonic depolarization propagates from the Off to the On dendritic arbor of the pDSGC to sensitize its On response. We also identified a previously overlooked feature of DSGC dendritic architecture that can support dendritic integration between On and Off dendritic layers bypassing the soma. By contrast, ventral pDSGCs lack a sensitized tonic depolarization and thus do not exhibit sensitization of their On responses. Our results highlight a topographic difference in Off bipolar cell inputs underlying divergent sensitization patterns of dorsal and ventral pDSGCs. Moreover, substantial crossovers between dendritic layers of On-Off DSGCs suggest an interactive dendritic algorithm for processing On and Off signals before they reach the soma.SIGNIFICANCE STATEMENT Visual neuronal responses are dynamically influenced by the prior visual experience. This form of plasticity reflects the efficient coding of the naturalistic environment by the visual system. We found that a class of retinal output neurons, On-Off direction-selective ganglion cells, transiently increase their responsiveness after visual stimulation. Cells located in dorsal and ventral retinas exhibit distinct sensitization patterns because of different adaptive properties of Off bipolar cell signaling. A previously overlooked dendritic morphologic feature of the On-Off direction-selective ganglion cell is implicated in the cross talk between On and Off pathways during sensitization. Together, these findings uncover a topographic difference in the adaptive encoding of upper and lower visual fields and the underlying neural mechanism in the dorsal and ventral retinas.

Coculture of ARPE-19 Cells and Porcine Neural Retina as an Ex Vivo Retinal Model

Neural retinal organ cultures are used to investigate ocular pathomechanisms. However, these cultures lack the essential retinal pigment epithelium (RPE) cells, which are part of the actual in vivo retina. To simulate a more realistic ex vivo model, porcine neural retina explants were cocultured with ARPE-19 cells (ARPE-19 group), which are derived from human RPE. To identify whether the entire cells or just the cell factors are necessary, in a second experimental group, porcine neural retina explants were cultured with medium derived from ARPE-19 cells (medium group). Individually cultured neural retina explants served as controls (control group). After 8 days, all neural retinas were analysed to evaluate retinal thickness, photoreceptors, microglia, complement factors and synapses (n = 6-8 per group). The neural retina thickness in the ARPE-19 group was significantly better preserved than in the control group (p = 0.031). Also, the number of L-cones was higher in the ARPE-19 group, as compared to the control group (p < 0.001). Furthermore, the ARPE-19 group displayed an increased presynaptic glutamate uptake (determined via vGluT1 labelling) and enhanced post-synaptic density (determined via PSD-95 labelling). Combined Iba1 and iNOS detection revealed only minor effects of ARPE-19 cells on microglial activity, with a slight downregulation of total microglia activity apparent in the medium group. Likewise, only minor beneficial effects on photoreceptors and synaptic structure were found in the medium group. This novel system offers the opportunity to investigate interactions between the neural retina and RPE cells, and suggests that the inclusion of a RPE feeder layer has beneficial effects on the ex vivo maintenance of neural retina. By modifying the culture conditions, this coculture model allows a better understanding of photoreceptor death and photoreceptor-RPE cell interactions in retinal diseases.

An uncommon neuronal class conveys visual signals from rods and cones to retinal ganglion cells

Neurons in the central nervous system (CNS) are distinguished by the neurotransmitter types they release, their synaptic connections, morphology, and genetic profiles. To fully understand how the CNS works, it is critical to identify all neuronal classes and reveal their synaptic connections. The retina has been extensively used to study neuronal development and circuit formation. Here, we describe a previously unidentified interneuron in mammalian retina. This interneuron shares some morphological, physiological, and molecular features with retinal bipolar cells, such as receiving input from photoreceptors and relaying visual signals to retinal ganglion cells. It also shares some features with amacrine cells (ACs), particularly Aii-ACs, such as their neurite morphology in the inner plexiform layer, the expression of some AC-specific markers, and possibly the release of the inhibitory neurotransmitter glycine. Thus, we unveil an uncommon interneuron, which may play an atypical role in vision.

Substantial restoration of night vision in adult mice with congenital stationary night blindness

Complete congenital stationary night blindness (cCSNB) due to mutations in TRPM1, GRM6, GPR179, NYX, or leucine-rich repeat immunoglobulin-like transmembrane domain 3 (LRIT3) is an incurable inherited retinal disorder characterized by an ON-bipolar cell (ON-BC) defect. Since the disease is non-degenerative and stable, treatment could theoretically be administrated at any time in life, making it a promising target for gene therapy. Until now, adeno-associated virus (AAV)-mediated therapies lead to significant functional improvements only in newborn cCSNB mice. Here we aimed to restore protein localization and function in adult Lrit3 ^{-/ -} mice. LRIT3 localizes in the outer plexiform layer and is crucial for TRPM1 localization at the dendritic tips of ON-BCs and the electroretinogram (ERG)-b-wave. AAV2-7m8-Lrit3 intravitreal injections were performed targeting either ON-BCs, photoreceptors (PRs), or both. Protein localization of LRIT3 and TRPM1 at the rod-to-rod BC synapse, functional rescue of scotopic responses, and ON-responses detection at the ganglion cell level were achieved in a few mice when ON-BCs alone or both PRs and ON-BCs, were targeted. More importantly, a significant number of treated adult Lrit3 ^-/- mice revealed an ERG b-wave recovery under scotopic conditions, improved optomotor responses, and on-time ON-responses at the ganglion cell level when PRs were targeted. Functional rescue was maintained for at least 4 months after treatment.

Neuroprotection of the Inner Retina Also Prevents Secondary Outer Retinal Pathology in a Mouse Model of Glaucoma

Purpose

We examined structural and functional changes in the outer retina of a mouse model of glaucoma. We examined whether these changes are a secondary consequence of damage in the inner retina and whether neuroprotection of the inner retina also prevents outer retinal changes.

Methods

We used an established microbead occlusion model of glaucoma whereby intraocular pressure (IOP) was elevated. Specific antibodies were used to label rod and cone bipolar cells (BCs), horizontal cells (HCs), and retinal ganglion cells (RGCs), as well as synaptic components in control and glaucomatous eyes, to assess structural damage and cell loss. ERG recordings were made to assess outer retina function.

Results

We found structural and functional damage of BCs, including significant cell loss and dendritic/axonal remodeling of HCs, following IOP elevation. The first significant loss of both BCs occurred at 4 to 5 weeks after microbead injection. However, early changes in the dendritic structure of RGCs were observed at 3 weeks, but significant changes in the rod BC axon terminal structure were not seen until 4 weeks. We found that protection of inner retinal neurons in glaucomatous eyes by pharmacological blockade of gap junctions or genetic ablation of connexin 36 largely prevented outer retinal damage.

Conclusions

Together, our results indicate that outer retinal impairments in glaucoma are a secondary sequalae of primary damage in the inner retina. The finding that neuroprotection of the inner retina can also prevent outer retinal damage has important implications with regard to the targets for effective neuroprotective therapy.

Helical Fasciculation of Bipolar and Horizontal Cell Neurites for Wiring With Photoreceptors in Macaque and Mouse Retinas

Purpose

The three-dimensional configurations of rod and cone bipolar cell (BC) dendrites and horizontal cell (HC) processes outside rod and cone synaptic terminals have not been fully elucidated. We reveal how these neurites are mutually arranged to coordinate formation and maintenance of the postsynaptic complex of ribbon synapses in mouse and monkey retinas.

Methods

Serial section transmission electron microscopy was utilized to reconstruct BC and HC neurites in macaque monkey and mouse, including metabotropic glutamate receptor 6 (mGluR6)-knockout mice.

Results

Starting from sporadically distributed branching points, rod BC and HC neurites (B and H, respectively) took specific paths to rod spherules by gradually adjusting their mutual positions, which resulted in a closed alternating pattern of H‒B‒H‒B neurites at the rod spherule aperture. This order corresponded to the array of elements constituting the postsynaptic complex of ribbon synapses. We identified novel helical coils of HC processes surrounding the rod BC dendrite in both mouse and macaque retinas, and these structures occurred more frequently in mGluR6-knockout than wild-type mouse retinas. Horizontal cell processes also formed hook-like protrusions that encircled cone BC and HC neurites below the cone pedicles in the macaque retina.

Conclusions

Bipolar and horizontal cell neurites take specific paths to adjust their mutual positions at the rod spherule aperture. Some HC processes are helically coiled around rod BC dendrites or form hook-like protrusions around cone BC dendrites and HC processes. Loss of mGluR6 signaling may be one factor promoting unbalanced neurite growth and compensatory neurite coiling.

Synaptic Remodeling in the Cone Pathway After Early Postnatal Horizontal Cell Ablation

The first synapse of the visual pathway is formed by photoreceptors, horizontal cells and bipolar cells. While ON bipolar cells invaginate into the photoreceptor terminal and form synaptic triads together with invaginating horizontal cell processes, OFF bipolar cells make flat contacts at the base of the terminal. When horizontal cells are ablated during retina development, no invaginating synapses are formed in rod photoreceptors. However, how cone photoreceptors and their synaptic connections with bipolar cells react to this insult, is unclear so far. To answer this question, we specifically ablated horizontal cells from the developing mouse retina. Following ablation around postnatal day 4 (P4)/P5, cones initially exhibited a normal morphology and formed flat contacts with OFF bipolar cells, but only few invaginating contacts with ON bipolar cells. From P15 on, synaptic remodeling became obvious with clustering of cone terminals and mislocalized cone somata in the OPL. Adult cones (P56) finally displayed highly branched axons with numerous terminals which contained ribbons and vesicular glutamate transporters. Furthermore, type 3a, 3b, and 4 OFF bipolar cell dendrites sprouted into the outer nuclear layer and even expressed glutamate receptors at the base of newly formed cone terminals. These results indicate that cones may be able to form new synapses with OFF bipolar cells in adult mice. In contrast, cone terminals lost their invaginating contacts with ON bipolar cells, highlighting the importance of horizontal cells for synapse maintenance. Taken together, our data demonstrate that early postnatal horizontal cell ablation leads to differential remodeling in the cone pathway: whereas synapses between cones and ON bipolar cells were lost, new putative synapses were established between cones and OFF bipolar cells. These results suggest that synapse formation and maintenance are regulated very differently between flat and invaginating contacts at cone terminals.

Knock-Out of Tenascin-C Ameliorates Ischemia-Induced Rod-Photoreceptor Degeneration and Retinal Dysfunction

Retinal ischemia is a common pathomechanism in various eye diseases. Recently, evidence accumulated suggesting that the extracellular matrix (ECM) glycoprotein tenascin-C (Tnc) plays a key role in ischemic degeneration. However, the possible functional role of Tnc in retinal ischemia is not yet known. The aim of our study was to explore retinal function and rod-bipolar/photoreceptor cell degeneration in wild type (WT) and Tnc knock-out (KO) mice after ischemia/reperfusion (I/R) injury. Therefore, I/R was induced by increasing intraocular pressure in the right eye of wild type (WT I/R) and Tnc KO (KO I/R) mice. The left eye served as untreated control (WT CO and KO CO). Scotopic electroretinogram (ERG) recordings were performed to examine rod-bipolar and rod-photoreceptor cell function. Changes of Tnc, rod-bipolar cells, photoreceptors, retinal structure and apoptotic and synaptic alterations were analyzed by immunohistochemistry, Hematoxylin and Eosin staining, Western blot, and quantitative real time PCR. We found increased Tnc protein levels 3 days after ischemia, while Tnc immunoreactivity decreased after 7 days. Tnc mRNA expression was comparable in the ischemic retina. ERG measurements after 7 days showed lower a-/b-wave amplitudes in both ischemic groups. Nevertheless, the amplitudes in the KO I/R group were higher than in the WT I/R group. We observed retinal thinning in WT I/R mice after 3 and 7 days. Although compared to the KO CO group, retinal thinning was not observed in the KO I/R group until 7 days. The number of PKCα⁺ rod-bipolar cells, recoverin⁺ photoreceptor staining and Prkca and Rcvrn expression were comparable in all groups. However, reduced rhodopsin protein as well as Rho and Gnat1 mRNA expression levels of rod-photoreceptors were found in the WT I/R, but not in the KO I/R retina. Additionally, a lower number of activated caspase 3⁺ cells was observed in the KO I/R group. Finally, both ischemic groups displayed enhanced vesicular glutamate transporter 1 (vGlut1) levels. Collectively, KO mice showed diminished rod-photoreceptor degeneration and retinal dysfunction after I/R. Elevated vGlut1 levels after ischemia could be related to an impaired glutamatergic photoreceptor-bipolar cell signaling and excitotoxicity. Our study provides novel evidence that Tnc reinforces ischemic retinal degeneration, possibly by synaptic remodeling.

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.

Neurofascin Is a Novel Component of Rod Photoreceptor Synapses in the Outer Retina

Neural circuit formation is an intricate and complex process where multiple neuron types must come together to form synaptic connections at a precise location and time. How this process is orchestrated during development remains poorly understood. Cell adhesion molecules are known to play a pivotal role in assembling neural circuits. They serve as recognition molecules between corresponding synaptic partners. In this study, we identified a new player in assembling neural circuits in the outer retina, the L1-family cell adhesion molecule Neurofascin (Nfasc). Our data reveals Nfasc is expressed in the synaptic layer where photoreceptors make synaptic connections to their respective partners. A closer examination of Nfasc expression shows high levels of expression in rod bipolars but not in cone bipolars. Disruption of Nfasc using a conditional knockout allele results in selective loss of pre- and post-synaptic proteins in the rod synaptic layer but not in the cone synaptic layer. Electron microscopic analysis confirms that indeed there are abnormal synaptic structures with less dendrites of rod bipolars innervating rod terminals in loss of Nfasc animals. Consistent with these findings, we also observe a decrease in rod-driven retinal responses with disruption of Nfasc function but not in cone-driven responses. Taken together, our data suggest a new role of Nfasc in rod synapses within the mouse outer retina.

Dystrophin Is Required for the Proper Timing in Retinal Histogenesis: A Thorough Investigation on the mdx Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked muscular disease caused by defective expression of the cytoskeletal protein dystrophin (Dp427). Selected autonomic and central neurons, including retinal neurons, express Dp427 and/or dystrophin shorter isoforms. Because of this, DMD patients may also experience different forms of cognitive impairment, neurological and autonomic disorders, and specific visual defects. DMD-related damages to the nervous system are established during development, suggesting a role for all dystrophin isoforms in neural circuit development and differentiation; however, to date, their function in retinogenesis has never been investigated. In this large-scale study, we analyzed whether the lack of Dp427 affects late retinogenesis in the mdx mouse, the most well studied animal model of DMD. Retinal gene expression and layer maturation, as well as neural cell proliferation, apoptosis, and differentiation, were evaluated in E18 and/or P0, P5, P10, and adult mice. In mdx mice, expression of Capn3, Id3 (E18-P5), and Dtnb (P5) genes, encoding proteins involved in different aspects of retina development and synaptogenesis (e.g., Calpain 3, DNA-binding protein inhibitor-3, and β-dystrobrevin, respectively), was transiently reduced compared to age-matched wild type mice. Concomitantly, a difference in the time required for the retinal ganglion cell layer to reach appropriate thickness was observed (P0–P5). Immunolabeling for specific cell markers also evidenced a significant dysregulation in the number of GABAergic amacrine cells (P5–P10), a transient decrease in the area immunopositive for the Vesicular Glutamate Transporter 1 (VGluT1) during ribbon synapse maturation (P10) and a reduction in the number of calretinin⁺ retinal ganglion cells (RGCs) (adults). Finally, the number of proliferating retinal progenitor cells (P5–P10) and apoptotic cells (P10) was reduced. These results support the hypothesis of a role for Dp427 during late retinogenesis different from those proposed in consolidated neural circuits. In particular, Dp427 may be involved in shaping specific steps of retina differentiation. Notably, although most of the above described quantitative alterations recover over time, the number of calretinin⁺ RGCs is reduced only in the mature retina. This suggests that alterations subtler than the timing of retinal maturation may occur, a hypothesis that demands further in-depth functional studies.

LKB1 coordinates neurite remodeling to drive synapse layer emergence in the outer retina

Structural changes in pre and postsynaptic neurons that accompany synapse formation often temporally and spatially overlap. Thus, it has been difficult to resolve which processes drive patterned connectivity. To overcome this, we use the laminated outer murine retina. We identify the serine/threonine kinase LKB1 as a key driver of synapse layer emergence. The absence of LKB1 in the retina caused a marked mislocalization and delay in synapse layer formation. In parallel, LKB1 modulated postsynaptic horizontal cell refinement and presynaptic photoreceptor axon growth. Mislocalized horizontal cell processes contacted aberrant cone axons in LKB1 mutants. These defects coincided with altered synapse protein organization, and horizontal cell neurites were misdirected to ectopic synapse protein regions. Together, these data suggest that LKB1 instructs the timing and location of connectivity in the outer retina via coordinate regulation of pre and postsynaptic neuron structure and the localization of synapse-associated proteins.

Molecular mechanisms underlying selective synapse formation of vertebrate retinal photoreceptor cells

In vertebrate central nervous systems (CNSs), highly diverse neurons are selectively connected via synapses, which are essential for building an intricate neural network. The vertebrate retina is part of the CNS and is comprised of a distinct laminar organization, which serves as a good model system to study developmental synapse formation mechanisms. In the retina outer plexiform layer, rods and cones, two types of photoreceptor cells, elaborate selective synaptic contacts with ON- and/or OFF-bipolar cell terminals as well as with horizontal cell terminals. In the mouse retina, three photoreceptor subtypes and at least 15 bipolar subtypes exist. Previous and recent studies have significantly progressed our understanding of how selective synapse formation, between specific subtypes of photoreceptor and bipolar cells, is designed at the molecular level. In the ON pathway, photoreceptor-derived secreted and transmembrane proteins directly interact in trans with the GRM6 (mGluR6) complex, which is localized to ON-bipolar cell dendritic terminals, leading to selective synapse formation. Here, we review our current understanding of the key factors and mechanisms underlying selective synapse formation of photoreceptor cells with bipolar and horizontal cells in the retina. In addition, we describe how defects/mutations of the molecules involved in photoreceptor synapse formation are associated with human retinal diseases and visual disorders.

Rod Bipolar Cells Require Horizontal Cells for Invagination Into the Terminals of Rod Photoreceptors

In the central nervous system, neuronal processing relies on the precisely orchestrated formation of synapses during development. The first synapse of the visual system is a triad synapse, comprising photoreceptors, horizontal cells and bipolar cells. During the second postnatal week, the axon terminal processes of horizontal cells invaginate rod spherules, followed by rod bipolar cell dendrites. Both elements finally oppose the synaptic ribbon (the release site of glutamate). However, it has not been fully elucidated whether horizontal cells are essential for rod bipolar cell dendrites to find their way into the rod terminal. In the present study, we investigated this question by specifically ablating horizontal cells from the early postnatal mouse retina. We monitored the formation of the rod-to-rod bipolar cell synapse during retinal maturation until postnatal day 21. Based on quantitative electron microscopy, we found that without horizontal cells, the dendrites of rod bipolar cells never entered rod terminals. Furthermore, rods displayed significantly fewer and shorter presynaptic ribbons, suggesting that glutamate release is decreased, which coincided with significantly reduced expression of postsynaptic proteins (mGluR6, GPR179) in rod bipolar cells. Collectively, our findings uncover that horizontal cells are indeed necessary guideposts for rod bipolar cells. Whether horizontal cells release diffusible guidance cues or provide structural guidance by expressing specific cell adhesion molecules remains to be seen.

Progressive myoclonic epilepsy-associated gene Kctd7 regulates retinal neurovascular patterning and function

Neuron function relies on and instructs the development and precise organization of neurovascular units that in turn support circuit activity. However, our understanding of the molecular cues that regulate this relationship remains sparse. Using a high-throughput screening pipeline, we recently identified several new regulators of vascular patterning. Among these was the potassium channel tetramerization domain-containing protein 7 (KCTD7). Mutations in KCTD7 are associated with progressive myoclonic epilepsy, but how KCTD7 regulates neural development and function remains poorly understood. To begin to identify such mechanisms, we focus on mouse retina, a tractable part of the central nervous system that contains precisely ordered neuron subtypes supported by a trilaminar vascular network. We find that deletion of Kctd7 induces defective patterning of the adult retina vascular network, resulting in increased branching, vessel length, and lacunarity. These alterations reflect early and specific defects in vessel development, as emergence of the superficial and deep vascular layers were delayed. These defects are likely due to a role for Kctd7 in inner retina neurons. Kctd7 is absent from vessels but present in neurons in the inner retina, and its deletion resulted in a corresponding increase in the number of bipolar cells in development and increased vessel branching in adults. These alterations were accompanied by retinal function deficits. Together, these data suggest that neuronal Kctd7 drives growth and patterning of the vasculature and that neurovascular interactions may participate in the pathogenesis of KCTD7-related human diseases.

Dynamic assembly of ribbon synapses and circuit maintenance in a vertebrate sensory system

Ribbon synapses transmit information in sensory systems, but their development is not well understood. To test the hypothesis that ribbon assembly stabilizes nascent synapses, we performed simultaneous time-lapse imaging of fluorescently-tagged ribbons in retinal cone bipolar cells (BCs) and postsynaptic densities (PSD95-FP) of retinal ganglion cells (RGCs). Ribbons and PSD95-FP clusters were more stable when these components colocalized at synapses. However, synapse density on ON-alpha RGCs was unchanged in mice lacking ribbons (ribeye knockout). Wildtype BCs make both ribbon-containing and ribbon-free synapses with these GCs even at maturity. Ribbon assembly and cone BC-RGC synapse maintenance are thus regulated independently. Despite the absence of synaptic ribbons, RGCs continued to respond robustly to light stimuli, although quantitative examination of the responses revealed reduced frequency and contrast sensitivity.

Regional differences in mitral cell development in mouse olfactory bulb

Olfactory sensory neurons (OSNs) located in the dorsomedial and ventromedial regions of the olfactory epithelium (OE) are distinguished from one another based on their molecular expression patterns. This difference is reflected in the separation of the glomerular layer of the olfactory bulb (OB) into dorsomedial and ventrolateral regions. However, it is unclear whether a complementary separation is also evident in the projection neurons that innervate the OB glomeruli. In this study, we compared the development of the OB between different regions by focusing on the transcription factor, Tbx21, which is expressed by mitral and tufted cells in the mature OB. Examining the OB at different developmental ages, we found that Tbx21 expression commenced in the anteromedial region called the tongue-shaped area, followed by the dorsomedial and then ventrolateral areas. We also showed that the tongue-shaped area was innervated by the OSNs located in the most dorsomedial part of the ventrolateral OE, the V-zone:DM. Interestingly, the generation of OSNs occurred first in the dorsomedial zone including the V-zone:DM, suggesting a correlation between the time course of OSN generation in the OE and Tbx21 expression in their target region of the OB. In contrast, expression of vGluT1, which is also found in all mitral cells in the mature OB, was first detected in the ventrolateral region during development. Our findings demonstrate that the development of projection neurons occurs in a compartmentalized manner in the OB; tongue-shaped, dorsomedial, and ventrolateral areas, and that not all projection neurons follow the same developmental pathway.

Human-Induced Pluripotent Stem Cells Generate Light Responsive Retinal Organoids with Variable and Nutrient-Dependent Efficiency

The availability of in vitro models of the human retina in which to perform pharmacological and toxicological studies is an urgent and unmet need. An essential step for developing in vitro models of human retina is the ability to generate laminated, physiologically functional, and light-responsive retinal organoids from renewable and patient specific sources. We investigated five different human-induced pluripotent stem cell (iPSC) lines and showed a significant variability in their efficiency to generate retinal organoids. Despite this variability, by month 5 of differentiation, all iPSC-derived retinal organoids were able to generate light responses, albeit immature, comparable to the earliest light responses recorded from the neonatal mouse retina, close to the period of eye opening. All iPSC-derived retinal organoids exhibited at this time a well-formed outer nuclear like layer containing photoreceptors with inner segments, connecting cilium, and outer like segments. The differentiation process was highly dependent on seeding cell density and nutrient availability determined by factorial experimental design. We adopted the differentiation protocol to a multiwell plate format, which enhanced generation of retinal organoids with retinal-pigmented epithelium (RPE) and improved ganglion cell development and the response to physiological stimuli. We tested the response of iPSC-derived retinal organoids to Moxifloxacin and showed that similarly to in vivo adult mouse retina, the primary affected cell types were photoreceptors. Together our data indicate that light responsive retinal organoids derived from carefully selected and differentiation efficient iPSC lines can be generated at the scale needed for pharmacology and drug screening purposes. Stem Cells 2018;36:1535-1551.

Retinal dysfunction parallels morphologic alterations and precede clinically detectable vascular alterations in Meriones shawi, a model of type 2 diabetes

Diabetic retinopathy is a major cause of reduced visual acuity and acquired blindness. The aim of this work was to analyze functional and vascular changes in diabetic Meriones shawi (M.sh) an animal model of metabolic syndrome and type 2 diabetes. The animals were divided into four groups. Two groups were fed a high fat diet (HFD) for 3 and 7 months, two other groups served as age-matched controls. Retinal function was assessed using full field electroretinogram (Ff-ERG). Retinal thickness and vasculature were examined by optical coherence tomography, eye fundus and fluorescein angiography. Immunohistochemistry was used to examine key proteins of glutamate metabolism and synaptic transmission. Diabetic animals exhibited significantly delayed scotopic and photopic ERG responses and decreases in scotopic and photopic a- and b-wave amplitudes at both time points. Furthermore, a decrease of the amplitude of the flicker response and variable changes in the scotopic and photopic oscillatory potentials was reported. A significant decrease in retinal thickness was observed. No evident change in the visual streak area and no sign of vascular abnormality was present; however, some exudates in the periphery were visible in 7 months diabetic animals. Imunohistochemistry detected a decrease in the expression of glutamate synthetase, vesicular glutamate transporter 1 and synaptophysin proteins. Results indicate that a significant retinal dysfunction was present in the HFD induced diabetes involving both rod and cone pathways and this dysfunction correlate well with the morphological abnormalities reported previously. Furthermore, neurodegeneration and abnormalities in retinal function occur before vascular alterations would be detectable in diabetic M.sh.

Role for Wnt Signaling in Retinal Neuropil Development: Analysis via RNA-Seq and In Vivo Somatic CRISPR Mutagenesis

Screens for genes that orchestrate neural circuit formation in mammals have been hindered by practical constraints of germline mutagenesis. To overcome these limitations, we combined RNA-seq with somatic CRISPR mutagenesis to study synapse development in the mouse retina. Here synapses occur between cellular layers, forming two multilayered neuropils. The outer neuropil, the outer plexiform layer (OPL), contains synapses made by rod and cone photoreceptor axons on rod and cone bipolar dendrites, respectively. We used RNA-seq to identify selectively expressed genes encoding cell surface and secreted proteins and CRISPR-Cas9 electroporation with cell-specific promoters to assess their roles in OPL development. Among the genes identified in this way are Wnt5a and Wnt5b. They are produced by rod bipolars and activate a non-canonical signaling pathway in rods to regulate early OPL patterning. The approach we use here can be applied to other parts of the brain.

Melanopsin ganglion cell outer retinal dendrites: Morphologically distinct and asymmetrically distributed in the mouse retina

A small population of retinal ganglion cells expresses the photopigment melanopsin and function as autonomous photoreceptors. They encode global luminance levels critical for light-mediated non-image forming visual processes including circadian rhythms and the pupillary light reflex. There are five melanopsin ganglion cell subtypes (M1-M5). M1 and displaced M1 (M1d) cells have dendrites that ramify within the outermost layer of the inner plexiform layer. It was recently discovered that some melanopsin ganglion cells extend dendrites into the outer retina. Outer Retinal Dendrites (ORDs) either ramify within the outer plexiform layer (OPL) or the inner nuclear layer, and while present in the mature retina, are most abundant postnatally. Anatomical evidence for synaptic transmission between cone photoreceptor terminals and ORDs suggests a novel photoreceptor to ganglion cell connection in the mammalian retina. While it is known that the number of ORDs in the retina is developmentally regulated, little is known about the morphology, the cells from which they originate, or their spatial distribution throughout the retina. We analyzed the morphology of melanopsin-immunopositive ORDs in the OPL at different developmental time points in the mouse retina and identified five types of ORDs originating from either M1 or M1d cells. However, a pattern emerges within these: ORDs from M1d cells are generally longer and more highly branched than ORDs from conventional M1 cells. Additionally, we found ORDs asymmetrically distributed to the dorsal retina. This morphological analysis provides the first step in identifying a potential role for biplexiform melanopsin ganglion cell ORDs.

The TRPM1 Channel Is Required for Development of the Rod ON Bipolar Cell-AII Amacrine Cell Pathway in the Retinal Circuit

Neurotransmission plays an essential role in neural circuit formation in the central nervous system (CNS). Although neurotransmission has been recently clarified as a key modulator of retinal circuit development, the roles of individual synaptic transmissions are not yet fully understood. In the current study, we investigated the role of neurotransmission from photoreceptor cells to ON bipolar cells in development using mutant mouse lines of both sexes in which this transmission is abrogated. We found that deletion of the ON bipolar cation channel TRPM1 results in the abnormal contraction of rod bipolar terminals and a decreased number of their synaptic connections with amacrine cells. In contrast, these histological alterations were not caused by a disruption of total glutamate transmission due to loss of the ON bipolar glutamate receptor mGluR6 or the photoreceptor glutamate transporter VGluT1. In addition, TRPM1 deficiency led to the reduction of total dendritic length, branch numbers, and cell body size in AII amacrine cells. Activated Goα, known to close the TRPM1 channel, interacted with TRPM1 and induced the contraction of rod bipolar terminals. Furthermore, overexpression of Channelrhodopsin-2 partially rescued rod bipolar cell development in the TRPM1^-/- retina, whereas the rescue effect by a constitutively closed form of TRPM1 was lower than that by the native form. Our results suggest that TRPM1 channel opening is essential for rod bipolar pathway establishment in development.SIGNIFICANCE STATEMENT Neurotransmission has been recognized recently as a key modulator of retinal circuit development in the CNS. However, the roles of individual synaptic transmissions are not yet fully understood. In the current study, we focused on neurotransmission between rod photoreceptor cells and rod bipolar cells in the retina. We used genetically modified mouse models which abrogate each step of neurotransmission: presynaptic glutamate release, postsynaptic glutamate reception, or transduction channel function. We found that the TRPM1 transduction channel is required for the development of rod bipolar cells and their synaptic formation with subsequent neurons, independently of glutamate transmission. This study advances our understanding of neurotransmission-mediated retinal circuit refinement.

Development of cone photoreceptors and their synapses in the human and monkey fovea

During retinal development, ribbon synapse assembly in the photoreceptors is a crucial step involving numerous molecules. While the developmental sequence of plexiform layers in human retina has been characterized, the molecular steps of synaptogenesis remain largely unknown. In the present study, we focused on the central rod-free region of primate retina, the fovea, to specifically investigate the development of cone photoreceptor ribbon synapses. Immunocytochemistry and electron microscopy were utilized to track the expression of photoreceptor transduction proteins and ribbon and synaptic markers in fetal human and Macaca retina. Although the inner plexiform layer appears earlier than the outer plexiform layer, synaptic proteins, and ribbons are first reliably recognized in cone pedicles. Markers first appear at fetal week 9. Both short (S) and medium/long (M/L) wavelength-selective cones express synaptic markers in the same temporal sequence; this is independent of opsin expression which takes place in S cones a month before M/L cones. The majority of ribbon markers, presynaptic vesicular release and postsynaptic neurotransduction-related machinery is present in both plexiform layers by fetal week 13. By contrast, two crucial components for cone to bipolar cell glutamatergic transmission, the metabotropic glutamate receptor 6 and voltage-dependent calcium channel α1.4, are not detected until fetal week 22 when bipolar cell invagination is present in the cone pedicle. These results suggest an intrinsically programmed but nonsynchronous expression of molecules in cone synaptic development. Moreover, functional ribbon synapses and active neurotransmission at foveal cone pedicles are possibly present as early as mid-gestation in human retina.

Top2b is involved in the formation of outer segment and synapse during late-stage photoreceptor differentiation by controlling key genes of photoreceptor transcriptional regulatory network

Topoisomerase II beta (Top2b) is an enzyme that alters the topologic states of DNA during transcription. Top2b deletion in early retinal progenitor cells causes severe defects in neural differentiation and affects cell survival in all retinal cell types. However, it is unclear whether the observed severe phenotypes are the result of cell-autonomous/primary defects or non-cell-autonomous/secondary defects caused by alterations of other retinal cells. Using photoreceptor cells as a model, we first characterized the phenotypes in Top2b conditional knockout. Top2b deletion leads to malformation of photoreceptor outer segments (OSs) and synapses accompanied by dramatic cell loss at late-stage photoreceptor differentiation. Then, we performed mosaic analysis with shRNA-mediated Top2b knockdown in neonatal retina using in vivo electroportation to target rod photoreceptors in neonatal retina. Top2b knockdown causes defective OS without causing a dramatic cell loss, suggesting a Top2b cell-autonomous function. Furthermore, RNA-seq analysis reveals that Top2b controls the expression of key genes in the photoreceptor gene-regulatory network (e.g., Crx, Nr2e3, Opn1sw, Vsx2) and retinopathy-related genes (e.g., Abca4, Bbs7, Pde6b). Together, our data establish a combinatorial cell-autonomous and non-cell-autonomous role for Top2b in the late stage of photoreceptor differentiation and maturation. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Accelerated retinal aging in PACAP knock-out mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide. PACAP and its receptors are widely distributed in the retina. A number of reports provided evidence that PACAP is neuroprotective in retinal degenerations. The current study compared retina cell type-specific differences in young (3-4months) and aged adults (14-16months), of wild-type (WT) mice and knock-out (KO) mice lacking endogenous PACAP production during the course of aging. Histological, immunocytochemical and Western blot examinations were performed. The staining for standard neurochemical markers (tyrosine hydroxylase for dopaminergic cells, calbindin 28 kDa for horizontal cells, protein kinase Cα for rod bipolar cells) of young adult PACAP KO retinas showed no substantial alterations compared to young adult WT retinas, except for the specific PACAP receptor (PAC1-R) staining. We could not detect PAC1-R immunoreactivity in bipolar and horizontal cells in young adult PACAP KO animals. Some other age-related changes were observed only in the PACAP KO mice only. These alterations included horizontal and rod bipolar cell dendritic sprouting into the photoreceptor layer and decreased ganglion cell number. Also, Müller glial cells showed elevated GFAP expression compared to the aging WT retinas. Furthermore, Western blot analyses revealed significant differences between the phosphorylation state of ERK1/2 and JNK in KO mice, indicating alterations in the MAPK signaling pathway. These results support the conclusion that endogenous PACAP contributes to protection against aging of the nervous system.

Somatic and neuritic spines on tyrosine hydroxylase-immunopositive cells of rat retina

Dopamine- and tyrosine hydroxylase-immunopositive cells (TH cells) modulate visually driven signals as they flow through retinal photoreceptor, bipolar, and ganglion cells. Previous studies suggested that TH cells release dopamine from varicose axons arborizing in the inner and outer plexiform layers after glutamatergic synapses depolarize TH cell dendrites in the inner plexiform layer and these depolarizations propagate to the varicosities. Although it has been proposed that these excitatory synapses are formed onto appendages resembling dendritic spines, spines have not been found on TH cells of most species examined to date or on TH cell somata that release dopamine when exposed to glutamate receptor agonists. By use of protocols that preserve proximal retinal neuron morphology, we have examined the shape, distribution, and synapse-related immunoreactivity of adult rat TH cells. We report here that TH cell somata, tapering and varicose inner plexiform layer neurites, and varicose outer plexiform layer neurites all bear spines, that some of these spines are immunopositive for glutamate receptor and postsynaptic density proteins (viz., GluR1, GluR4, NR1, PSD-95, and PSD-93), that TH cell somata and tapering neurites are also immunopositive for a γ-aminobutyric acid (GABA) receptor subunit (GABA_A R_α1 ), and that a synaptic ribbon-specific protein (RIBEYE) is found adjacent to some colocalizations of GluR1 and TH in the inner plexiform layer. These results identify previously undescribed sites at which glutamatergic and GABAergic inputs may stimulate and inhibit dopamine release, especially at somata and along varicose neurites that emerge from these somata and arborize in various levels of the retina. J. Comp. Neurol. 525:1707-1730, 2017. © 2016 Wiley Periodicals, Inc.

Retinal interneuron survival requires non-cell-autonomous Atrx activity

ATRX is a chromatin remodeling protein that is mutated in several intellectual disability disorders including alpha-thalassemia/mental retardation, X-linked (ATR-X) syndrome. We previously reported the prevalence of ophthalmological defects in ATR-X syndrome patients, and accordingly we find morphological and functional visual abnormalities in a mouse model harboring a mutation occurring in ATR-X patients. The visual system abnormalities observed in these mice parallels the Atrx-null retinal phenotype characterized by interneuron defects and selective loss of amacrine and horizontal cells. The mechanisms that underlie selective neuronal vulnerability and neurodegeneration in the central nervous system upon Atrx mutation or deletion are unknown. To interrogate the cellular specificity of Atrx for its retinal neuroprotective functions, we employed a combination of temporal and lineage-restricted conditional ablation strategies to generate five different conditional knockout mouse models, and subsequently identified a non-cell-autonomous requirement for Atrx in bipolar cells for inhibitory interneuron survival in the retina. Atrx-deficient retinal bipolar cells exhibit functional, structural and molecular alterations consistent with impairments in neuronal activity and connectivity. Gene expression changes in the Atrx-null retina indicate defective synaptic structure and neuronal circuitry, suggest excitotoxic mechanisms of neurodegeneration, and demonstrate that common targets of ATRX in the forebrain and retina may contribute to similar neuropathological processes underlying cognitive impairment and visual dysfunction in ATR-X syndrome.

Sox2-Deficient Müller Glia Disrupt the Structural and Functional Maturation of the Mammalian Retina

PURPOSE

Müller glia (MG), the principal glial cells of the vertebrate retina, display quiescent progenitor cell characteristics. They express key progenitor markers, including the high mobility group box transcription factor SOX2 and maintain a progenitor-like morphology. In the embryonic and mature central nervous system, SOX2 maintains neural stem cell identity. However, its function in committed Müller glia has yet to be determined.

METHODS

We use inducible, MG-specific genetic ablation of Sox2 in vivo at the peak of MG genesis to analyze its function in the maturation of murine MG and effects on other cells in the retina. Histologic and functional analysis of the Sox2-deficient retinas is conducted at key points in postnatal development.

RESULTS

Ablation of Sox2 in the postnatal retina results in disorganization of MG processes in the inner plexiform layer and mislocalized cell bodies in the nuclear layers. This disorganization is concurrent with a thinning of the neural retina and disruption of neuronal processes in the inner and outer plexiform layers. Functional analysis by electroretinography reveals a decrease in the b-wave amplitude. Disruption of MG maturation due to Sox2 ablation therefore negatively affected the function of the retina.

CONCLUSIONS

These results demonstrate a novel role for SOX2 in glial process outgrowth and adhesion, and provide new insights into the essential role Müller glia play in the development of retinal cytoarchitecture. Prior to this work, SOX2 was known to have a primary role in determining cell fate. Our experiments bypass cell fate conversion to establish a new role for SOX2 in a committed cell lineage.

Amyloid Precursor-Like Protein 2 deletion-induced retinal synaptopathy related to congenital stationary night blindness: structural, functional and molecular characteristics

Background

Amyloid precursor protein knockout mice (APP-KO) have impaired differentiation of amacrine and horizontal cells. APP is part of a gene family and its paralogue amyloid precursor-like protein 2 (APLP2) has both shared as well as distinct expression patterns to APP, including in the retina. Given the impact of APP in the retina we investigated how APLP2 expression affected the retina using APLP2 knockout mice (APLP2-KO).

Results

Using histology, morphometric analysis with noninvasive imaging technique and electron microscopy, we showed that APLP2-KO retina displayed abnormal formation of the outer synaptic layer, accompanied with greatly impaired photoreceptor ribbon synapses in adults. Moreover, APLP2-KO displayed a significant decease in ON-bipolar, rod bipolar and type 2 OFF-cone bipolar cells (36, 21 and 63 %, respectively). Reduction of the number of bipolar cells was accompanied with disrupted dendrites, reduced expression of metabotropic glutamate receptor 6 at the dendritic tips and alteration of axon terminals in the OFF laminae of the inner plexiform layer. In contrast, the APP-KO photoreceptor ribbon synapses and bipolar cells were intact. The APLP2-KO retina displayed numerous phenotypic similarities with the congenital stationary night blindness, a non-progressive retinal degeneration disease characterized by the loss of night vision. The pathological phenotypes in the APLP2-KO mouse correlated to altered transcription of genes involved in pre- and postsynatic structure/function, including CACNA1F, GRM6, TRMP1 and Gα0, and a normal scotopic a-wave electroretinogram amplitude, markedly reduced scotopic electroretinogram b-wave and modestly reduced photopic cone response. This confirmed the impaired function of the photoreceptor ribbon synapses and retinal bipolar cells, as is also observed in congenital stationary night blindness. Since congenital stationary night blindness present at birth, we extended our analysis to retinal differentiation and showed impaired differentiation of different bipolar cell subtypes and an altered temporal sequence of development from OFF to ON laminae in the inner plexiform layer. This was associated with the altered expression patterns of bipolar cell generation and differentiation factors, including MATH3, CHX10, VSX1 and OTX2.

Conclusions

These findings demonstrate that APLP2 couples retina development and synaptic genes and present the first evidence that APLP2 expression may be linked to synaptic disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0245-z) contains supplementary material, which is available to authorized users.

ON and OFF retinal ganglion cells differentially regulate serotonergic and GABAergic activity in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN), the major source of serotonergic input to the forebrain, receives excitatory input from the retina that can modulate serotonin levels and depressive-like behavior. In the Mongolian gerbil, retinal ganglion cells (RGCs) with alpha-like morphological and Y-like physiological properties innervate the DRN with ON DRN-projecting RGCs out numbering OFF DRN-projecting RGCs. The DRN neurons targeted by ON and OFF RGCs are unknown. To explore retino-raphe anatomical organization, retinal afferents labeled with Cholera toxin B were examined for association with the postsynaptic protein PSD-95. Synaptic associations between retinal afferents and DRN serotonergic and GABAergic neurons were observed. To explore retino-raphe functional organization, light-evoked c-fos expression was examined. Light significantly increased the number of DRN serotonergic and GABAergic cells expressing c-Fos. When ON RGCs were rendered silent while enhancing the firing rate of OFF RGCs, c-Fos expression was greatly increased in DRN serotonergic neurons suggesting that OFF DRN-projecting RGCs predominately activate serotonergic neurons whereas ON DRN-projecting RGCs mainly target GABAergic neurons. Direct glutamatergic retinal input to DRN 5-HT neurons contributes to the complex excitatory drive regulating these cells. Light, via the retinoraphe pathway can modify DRN 5-HT neuron activity which may play a role in modulating affective behavior.

Shared and distinct retinal input to the mouse superior colliculus and dorsal lateral geniculate nucleus

Our results suggest that the mouse superior colliculus (SC) has access to input from most of the retinal ganglion cells (RGCs) that innervate the dorsal lateral geniculate nucleus (dLGN). By comparison, a number of RGC types appear to innervate the SC but not the dLGN; these RGCs generally exhibit more transient responses and respond best to small stimuli.

The mammalian retina conveys the vast majority of information about visual stimuli to two brain regions: the dorsal lateral geniculate nucleus (dLGN) and the superior colliculus (SC). The degree to which retinal ganglion cells (RGCs) send similar or distinct information to the two areas remains unclear despite the important constraints that different patterns of RGC input place on downstream visual processing. To resolve this ambiguity, we injected a glycoprotein-deficient rabies virus coding for the expression of a fluorescent protein into the dLGN or SC; rabies virus labeled a smaller fraction of RGCs than lipophilic dyes such as DiI but, crucially, did not label RGC axons of passage. Approximately 80% of the RGCs infected by rabies virus injected into the dLGN were colabeled with DiI injected into the SC, suggesting that many dLGN-projecting RGCs also project to the SC. However, functional characterization of RGCs revealed that the SC receives input from several classes of RGCs that largely avoid the dLGN, in particular RGCs in which 1) sustained changes in light intensity elicit transient changes in firing rate and/or 2) a small range of stimulus sizes or temporal fluctuations in light intensity elicit robust activity. Taken together, our results illustrate several unexpected asymmetries in the information that the mouse retina conveys to two major downstream targets and suggest that differences in the output of dLGN and SC neurons reflect, at least in part, differences in the functional properties of RGCs that innervate the SC but not the dLGN.

Diabetic Retinopathy: Animal Models, Therapies, and Perspectives

Diabetic retinopathy (DR) is one of the major complications of diabetes. Although great efforts have been made to uncover the mechanisms underlying the pathology of DR, the exact causes of DR remain largely unknown. Because of multifactor involvement in DR etiology, currently no effective therapeutic treatments for DR are available. In this paper, we review the pathology of DR, commonly used animal models, and novel therapeutic approaches. Perspectives and future directions for DR treatment are discussed.

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.

Inner retinal change in a novel rd1-FTL mouse model of retinal degeneration

While photoreceptor loss is the most devastating result of inherited retinal degenerations such as retinitis pigmentosa, inner retinal neurons also undergo significant alteration. Detailing these changes has become important as many vision restorative therapies target the remaining neurons. In this study, the rd1-Fos-Tau-LacZ (rd1-FTL) mouse model was used to explore inner retinal change at a late stage of retinal degeneration, after the loss of photoreceptor nuclei. The rd1-FTL model carries a mutation in the phosphodiesterase gene, Pde6b, and an axonally targeted transgenic beta galactosidase reporter system under the control of the c-fos promoter. Retinae of transgenic rd1-FTL mice and control FTL animals aged 2-12 months were processed for indirect fluorescence immunocytochemistry. At 2 months of age, a time when the majority of photoreceptor nuclei are lost, there was negligible c-fos reporter (FTL) expression, however, from 4 months, reporter expression was observed to increase within subpopulations of amacrine and ganglion cells within the central retina. These areas of inner retinal FTL expression coincided with regions that contained aberrant Müller cells. Specifically, these cells exhibited reduced glutamine synthetase and Kir4.1 immunolabelling, whilst showing evidence of proliferative gliosis (increased cyclinD1 and glial fibrillary acidic protein expression). These changes were limited to distinct regions where cone photoreceptor terminals were absent. Overall, these results highlight that distinct areas of the rd1-FTL central retina undergo significant glial alterations after cone photoreceptor loss. These areas coincide with up-regulation of the c-fos reporter in the inner retina, which may represent a change in neuronal function/plasticity. The rd1-FTL mouse is a useful model system to probe changes that occur in the inner retina at later stages of retinal degeneration.

Synaptic pathology and therapeutic repair in adult retinoschisis mouse by AAV-RS1 transfer

Strategies aimed at invoking synaptic plasticity have therapeutic potential for several neurological conditions. The human retinal synaptic disease X-linked retinoschisis (XLRS) is characterized by impaired visual signal transmission through the retina and progressive visual acuity loss, and mice lacking retinoschisin (RS1) recapitulate human disease. Here, we demonstrate that restoration of RS1 via retina-specific delivery of adeno-associated virus type 8-RS1 (AAV8-RS1) vector rescues molecular pathology at the photoreceptor-depolarizing bipolar cell (photoreceptor-DBC) synapse and restores function in adult Rs1-KO animals. Initial development of the photoreceptor-DBC synapse was normal in the Rs1-KO retina; however, the metabotropic glutamate receptor 6/transient receptor potential melastatin subfamily M member 1-signaling (mGluR6/TRPM1-signaling) cascade was not properly maintained. Specifically, the TRPM1 channel and G proteins Gαo, Gβ5, and RGS11 were progressively lost from postsynaptic DBC dendritic tips, whereas the mGluR6 receptor and RGS7 maintained proper synaptic position. This postsynaptic disruption differed from other murine night-blindness models with an electronegative electroretinogram response, which is also characteristic of murine and human XLRS disease. Upon AAV8-RS1 gene transfer to the retina of adult XLRS mice, TRPM1 and the signaling molecules returned to their proper dendritic tip location, and the DBC resting membrane potential was restored. These findings provide insight into the molecular plasticity of a critical synapse in the visual system and demonstrate potential therapeutic avenues for some diseases involving synaptic pathology.

Rax Homeoprotein Regulates Photoreceptor Cell Maturation and Survival in Association with Crx in the Postnatal Mouse Retina

The Rax homeobox gene plays essential roles in multiple processes of vertebrate retina development. Many vertebrate species possess Rax and Rax2 genes, and different functions have been suggested. In contrast, mice contain a single Rax gene, and its functional roles in late retinal development are still unclear. To clarify mouse Rax function in postnatal photoreceptor development and maintenance, we generated conditional knockout mice in which Rax in maturing or mature photoreceptor cells was inactivated by tamoxifen treatment (Rax iCKO mice). When Rax was inactivated in postnatal Rax iCKO mice, developing photoreceptor cells showed a significant decrease in the level of the expression of rod and cone photoreceptor genes and mature adult photoreceptors exhibited a specific decrease in cone cell numbers. In luciferase assays, we found that Rax and Crx cooperatively transactivate Rhodopsin and cone opsin promoters and that an optimum Rax expression level to transactivate photoreceptor gene expression exists. Furthermore, Rax and Crx colocalized in maturing photoreceptor cells, and their coimmunoprecipitation was observed in cultured cells. Taken together, these results suggest that Rax plays essential roles in the maturation of both cones and rods and in the survival of cones by regulating photoreceptor gene expression with Crx in the postnatal mouse retina.

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.