Number and distribution of mouse retinal cone photoreceptors: differences between an albino (Swiss) and a pigmented (C57/BL6) strain

An ethologically relevant paradigm to assess defensive response to looming visual contrast stimuli

In the animal kingdom, threat information is perceived mainly through vision. The subcortical visual pathway plays a critical role in the rapid processing of visual information-induced fear, and triggers a response. Looming-evoked behavior in rodents, mimicking response to aerial predators, allowed identify the neural circuitry underlying instinctive defensive behaviors; however, the influence of disk/background contrast on the looming-induced behavioral response has not been examined, either in rats or mice. We studied the influence of the dark disk/gray background contrast in the type of rat and mouse defensive behavior in the looming arena, and we showed that rat and mouse response as a function of disk/background contrast adjusted to a sigmoid-like relationship. Both sex and age biased the contrast-dependent response, which was dampened in rats submitted to retinal unilateral or bilateral ischemia. Moreover, using genetically manipulated mice, we showed that the three type of photoresponsive retinal cells (i.e., cones, rods, and intrinsically photoresponsive retinal ganglion cells (ipRGCs)), participate in the contrast-dependent response, following this hierarchy: cones > > rods > >  > ipRGCs. The cone and rod involvement was confirmed using a mouse model of unilateral non-exudative age-related macular degeneration, which only damages canonical photoreceptors and significantly decreased the contrast sensitivity in the looming arena.

Retinal metabolism displays evidence for uncoupling of glycolysis and oxidative phosphorylation via Cori-, Cahill-, and mini-Krebs-cycle

The retina consumes massive amounts of energy, yet its metabolism and substrate exploitation remain poorly understood. Here, we used a murine explant model to manipulate retinal energy metabolism under entirely controlled conditions and utilised 1H-NMR spectroscopy-based metabolomics, in situ enzyme detection, and cell viability readouts to uncover the pathways of retinal energy production. Our experimental manipulations resulted in varying degrees of photoreceptor degeneration, while the inner retina and retinal pigment epithelium were essentially unaffected. This selective vulnerability of photoreceptors suggested very specific adaptations in their energy metabolism. Rod photoreceptors were found to rely strongly on oxidative phosphorylation, but only mildly on glycolysis. Conversely, cone photoreceptors were dependent on glycolysis but insensitive to electron transport chain decoupling. Importantly, photoreceptors appeared to uncouple glycolytic and Krebs-cycle metabolism via three different pathways: (1) the mini-Krebs-cycle, fuelled by glutamine and branched chain amino acids, generating N-acetylaspartate; (2) the alanine-generating Cahill-cycle; (3) the lactate-releasing Cori-cycle. Moreover, the metabolomics data indicated a shuttling of taurine and hypotaurine between the retinal pigment epithelium and photoreceptors, likely resulting in an additional net transfer of reducing power to photoreceptors. These findings expand our understanding of retinal physiology and pathology and shed new light on neuronal energy homeostasis and the pathogenesis of neurodegenerative diseases.

ERG Responses in Albinism, Idiopathic Infantile Nystagmus, and Controls

Purpose

Our primary aim was to compare adult full-field ERG (ffERG) responses in albinism, idiopathic infantile nystagmus (IIN), and controls. A secondary aim was to investigate the effect of within-subject changes in nystagmus eye movements on ffERG responses.

Methods

Dilated Ganzfeld flash ffERG responses were recorded using DTL electrodes under conditions of dark (standard and dim flash) and light adaptation in 68 participants with albinism, 43 with IIN, and 24 controls. For the primary aim, the effect of group and age on ffERG responses was investigated. For the secondary aim, null region characteristics were determined using eye movements recorded prior to ffERG recordings. ffERG responses were recorded near and away from the null regions of 18 participants also measuring the success rate of recordings.

Results

For the primary aim, age-adjusted photopic a- and b-wave amplitudes were consistently smaller in IIN compared with controls (P < 0.0001), with responses in both groups decreasing with age. In contrast, photopic a-wave amplitudes increased with age in albinism (P = 0.0035). For the secondary aim, more intense nystagmus significantly reduced the success rate of measurable responses. Within-subject changes in nystagmus intensity generated small, borderline significant differences in photopic b-wave peak times and a-and b-wave amplitudes under scotopic conditions with standard flash.

Conclusions

Age-adjusted photopic ffERG responses are significantly reduced in IIN adding to the growing body of evidence of retinal abnormalities in IIN. Differences between photopic responses in albinism and controls depend on age. Success at obtaining ffERG responses could be improved by recording responses at the null region.

An ethologically relevant paradigm to assess visual contrast sensitivity in rodents

In the animal kingdom, threat information is perceived mainly through vision. The subcortical visual pathway plays a critical role in the rapid processing of visual information-induced fear, and triggers a response. Looming-evoked behavior in rodents, mimicking response to aerial predators, allowed identify the neural circuitry underlying instinctive defensive behaviors; however, the influence of disk/background contrast on the looming-induced behavioral response has not been examined, either in rats or mice. We studied the influence of the dark disk/gray background contrast in the type of rat and mouse defensive behavior in the looming arena, and we showed that rat and mouse response as a function of disk/background contrast adjusted to a sigmoid-like relationship. Both sex and age biased the contrast-dependent response, which was dampened in rats submitted to retinal unilateral or bilateral ischemia. Moreover, using genetically manipulated mice, we showed that the three type of photoresponsive retinal cells (i.e., cones, rods, and intrinsically photoresponsive retinal ganglion cells (ipRGCs)), participate in the contrast-dependent response, following this hierarchy: cones ˃> rods ˃>>ipRGCs. The cone and rod involvement was confirmed using a mouse model of unilateral non-exudative age-related macular degeneration, which only damages canonical photoreceptors and significantly decreased the contrast sensitivity in the looming arena.

Gpr75 knockout mice display age-dependent cone photoreceptor cell loss

GPR75 is an orphan G protein-coupled receptor for which there is currently limited information and its function in physiology and disease is only recently beginning to emerge. This orphan receptor is expressed in the retina but its function in the eye is unknown. The earliest studies on GPR75 were conducted in the retina, where the receptor was first identified and cloned and mutations in the receptor were identified as a possible contributor to retinal degenerative disease. Despite these sporadic reports, the function of GPR75 in the retina and in retinal disease has yet to be explored. To assess whether GPR75 has a functional role in the retina, the retina of Gpr75 knockout mice was characterized. Knockout mice displayed a mild progressive retinal degeneration, which was accompanied by oxidative stress. The degeneration was because of the loss of both M-cone and S-cone photoreceptor cells. Housing mice under constant dark conditions reduced oxidative stress but did not prevent cone photoreceptor cell loss, indicating that oxidative stress is not a primary cause of the observed retinal degeneration. Studies here demonstrate an important role for GPR75 in maintaining the health of cone photoreceptor cells and that Gpr75 knockout mice can be used as a model to study cone photoreceptor cell loss.

A Protocol to Evaluate and Quantify Retinal Pigmented Epithelium Pathologies in Mouse Models of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a debilitating retinal disorder in aging populations. It is widely believed that dysfunction of the retinal pigmented epithelium (RPE) is a key pathobiological event in AMD. To understand the mechanisms that lead to RPE dysfunction, mouse models can be utilized by researchers. It has been established by previous studies that mice can develop RPE pathologies, some of which are observed in the eyes of individuals diagnosed with AMD. Here, we describe a phenotyping protocol to assess RPE pathologies in mice. This protocol includes the preparation and evaluation of retinal cross-sections using light microscopy and transmission electron microscopy, as well as that of RPE flat mounts by confocal microscopy. We detail the common types of murine RPE pathologies observed by these techniques and ways to quantify them through unbiased methods for statistical testing. As proof of concept, we use this RPE phenotyping protocol to quantify the RPE pathologies observed in mice overexpressing transmembrane protein 135 (Tmem135) and aged wild-type C57BL/6J mice. The main goal of this protocol is to present standard RPE phenotyping methods with unbiased quantitative assessments for scientists using mouse models of AMD.

Assessment of Murine Retinal Acuity Ex Vivo Using Multielectrode Array Recordings

Purpose

Visual acuity, measured by resolution of optotypes on a standard eye chart, is a critical clinical test for function of the visual system in humans. Behavioral tests in animals can be used to estimate visual acuity. However, such tests may be limited in the study of mutants or after synthetic vision restoration techniques. Because the total response of the retina to a visual scene is encoded in spiking patterns of retinal ganglion cells, it should be possible to estimate visual acuity in vitro from the retina by analyzing retinal ganglion cell output in response to test stimuli.

Methods

We created a method, EyeCandy, that combines a visual stimulus-generating engine with analysis of multielectrode array retinal recordings via a machine learning approach to measure murine retinal acuity in vitro. Visual stimuli included static checkerboards, drifting gratings, and letter optotypes.

Results

In retinas from wild-type C57Bl/6 mice, retinal acuity measurement for a drifting grating was 0.4 cycles per degree. In contrast, retinas from adult rd1 mice with outer retinal degeneration showed no detectable acuity. A comparison of acuities among different regions of the retina revealed substantial variation, with the inferior-nasal quadrant having highest RA. Letter classification accuracy of a projected Early Treatment Diabetic Retinopathy eye chart reached 99% accuracy for logMAR 3.0 letters. EyeCandy measured a restored RA of 0.05 and 0.08 cycles per degree for static and dynamic stimuli respectively from the retina of the rd1 mouse treated with the azobenzene photoswitch BENAQ.

Conclusions

Machine learning may be used to estimate retinal acuity.

Translational Relevance

The use of ex vivo retinal acuity measurement may allow determination of effects of mutations, drugs, injury, or other manipulations on retinal visual function.

Alpha retinal ganglion cells in pigmented mice retina: number and distribution

Purpose: To identify and characterize numerically and topographically the population of alpha retinal ganglion cells (αRGCs) and their subtypes, the sustained-response ON-center αRGCs (ONs-αRGCs), which correspond to the type 4 intrinsically photosensitive RGCs (M4-ipRGCs), the transient-response ON-center αRGCs (ONt-αRGCs), the sustained-response OFF-center αRGCs (OFFs-αRGCs), and the transient-response OFF-center αRGCs (OFFt-αRGCs) in the adult pigmented mouse retina.

Methods: The αRGC population and its subtypes were studied in flat-mounted retinas and radial sections immunodetected against non-phosphorylated high molecular weight neurofilament subunit (SMI-32) or osteopontin (OPN), two αRGCs pan-markers; Calbindin, expressed in ONs-αRGCs, and amacrines; T-box transcription factor T-brain 2 (Tbr2), a key transcriptional regulator for ipRGC development and maintenance, expressed in ipRGCs and GABA-displaced amacrine cells; OPN4, an anti-melanopsin antibody; or Brn3a and Brn3c, markers of RGCs. The total population of RGCs was counted automatically and αRGCs and its subtypes were counted manually, and color-coded neighborhood maps were used for their topographical representation.

Results: The total mean number of αRGCs per retina is 2,252 ± 306 SMI32+αRGCs and 2,315 ± 175 OPN+αRGCs (n = 10), representing 5.08% and 5.22% of the total number of RGCs traced from the optic nerve, respectively. αRGCs are distributed throughout the retina, showing a higher density in the temporal hemiretina. ONs-αRGCs represent ≈36% [841 ± 110 cells (n = 10)] of all αRGCs and are located throughout the retina, with the highest density in the temporal region. ONt-αRGCs represent ≈34% [797 ± 146 cells (n = 10)] of all αRGCs and are mainly located in the central retinal region. OFF-αRGCs represent the remaining 32% of total αRGCs and are divided equally between OFFs-αRGCs and OFFt-αRGCs [363 ± 50 cells (n = 10) and 376 ± 36 cells (n = 10), respectively]. OFFs-αRGCs are mainly located in the supero-temporal peripheral region of the retina and OFFt-αRGCs in the mid-peripheral region of the retina, especially in the infero-temporal region.

Conclusions: The combination of specific antibodies is a useful tool to identify and study αRGCs and their subtypes. αRGCs are distributed throughout the retina presenting higher density in the temporal area. The sustained ON and OFF response subtypes are mainly located in the periphery while the transient ON and OFF response subtypes are found in the central regions of the retina.

Transducin-Deficient Rod Photoreceptors Evaluated With Optical Coherence Tomography and Oxygen Consumption Rate Energy Biomarkers

Purpose

To test the hypothesis that rod energy biomarkers in light and dark are similar in mice without functional rod transducin (Gnat1rd17).

Methods

Gnat1rd17 and wildtype (WT) mice were studied in canonically low energy demand (light) and high energy demand (dark) conditions. We measured rod inner segment ellipsoid zone (ISez) profile shape, external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness, and magnitude of a hyporeflective band (HB) intensity dip located between photoreceptor tips and apical RPE; antioxidants were given in a subset of mice. Oxygen consumption rate (OCR) and visual performance indexes were also measured.

Results

The lower energy demand expected in light-adapted wildtype retinas was associated with an elongated ISez, thicker ELM-RPE, and higher HB magnitude, and lower OCR compared to high energy demand conditions in the dark. Gnat1rd17 mice showed a wildtype-like ISez profile shape at 20 minutes of light that became rounder at 60 minutes; at both times, ELM-RPE was smaller than wildtype values, and the HB magnitude was unmeasurable. OCR was higher than in the dark. Light-adapted Gnat1rd17 mice biomarkers were unaffected by anti-oxidants. Gnat1rd17 mice showed modest outer nuclear layer thinning and no reduction in visual performance indexes.

Conclusions

Light-stimulated changes in all biomarkers in WT mice are consistent with the established light-induced decrease in net energy demand. In contrast, biomarker changes in Gnat1rd17 mice raise the possibility that light increases net energy demand in the absence of rod phototransduction.

Immune recognition of syngeneic, allogeneic and xenogeneic stromal cell transplants in healthy retinas

Background

Advanced therapies using adult mesenchymal stromal cells (MSCs) for neurodegenerative diseases are not effectively translated into the clinic. The cross talk between the transplanted cells and the host tissue is something that, despite its importance, is not being systematically investigated.

Methods

We have compared the response of the mouse healthy retina to the intravitreal transplantation of MSCs derived from the bone marrow in four modalities: syngeneic, allogeneic, xenogeneic and allogeneic with immunosuppression using functional analysis in vivo and histology, cytometry and protein measurement post-mortem. Data were considered significant (p < 0.05) after nonparametric suitable statistical tests.

Results

Transplanted cells remain in the vitreous and are cleared by microglial cells a process that is quicker in allotransplants regardless of immunosuppression. All transplants cause anatomical remodelling which is more severe after xenotransplants. Xeno- and allotransplants with or without immunosuppression cause macro- and microglial activation and retinal functional impairment, being xenotransplants the most detrimental and the only ones that recruit CD45⁺Iba1⁻cells. The profile of proinflammatory cytokines changes in all transplantation settings. However, none of these changes affect the retinal ganglion cell population.

Conclusions

We show here a specific functional and anatomical retinal response depending on the MSC transplantation modality, an aspect that should be taken into consideration when conducting preclinical studies if we intend a more realistic translation into clinical practice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03129-y.

Inverse Problem Reveals Conditions for Characteristic Retinal Degeneration Patterns in Retinitis Pigmentosa Under the Trophic Factor Hypothesis

Retinitis pigmentosa (RP) is the most common inherited retinal dystrophy with a prevalence of about 1 in 4,000, affecting approximately 1.5 million people worldwide. Patients with RP experience progressive visual field loss as the retina degenerates, destroying light-sensitive photoreceptor cells (rods and cones), with rods affected earlier and more severely than cones. Spatio-temporal patterns of retinal degeneration in human RP have been well characterised; however, the mechanism(s) giving rise to these patterns have not been conclusively determined. One such mechanism, which has received a wealth of experimental support, is described by the trophic factor hypothesis. This hypothesis suggests that rods produce a trophic factor necessary for cone survival; the loss of rods depletes this factor, leading to cone degeneration. In this article, we formulate a partial differential equation mathematical model of RP in one spatial dimension, spanning the region between the retinal centre (fovea) and the retinal edge (ora serrata). Using this model we derive and solve an inverse problem, revealing for the first time experimentally testable conditions under which the trophic factor mechanism will qualitatively recapitulate the spatio-temporal patterns of retinal regeneration observed in human RP.

ERG and Behavioral CFF in Light-Damaged Albino Rats

The full-field ERG is useful for index rod- or cone-mediated retinal function in rodent models of retinal degeneration. However, the relationship between the ERG response amplitudes and visually guided behavior, such as flicker detection, is not well understood. A comparison of ERG to behavioral responses in a light-damage model of retinal degeneration allows us to better understand the functional implications of electrophysiological changes. Flicker-ERG and behavioral responses to flicker were used to determine critical flicker frequency (CFF) under scotopic and photopic conditions before and up to 90 d after a 10-day period of low-intensity light damage. Dark- and light-adapted ERG flash responses were significantly reduced after light damage. The a-wave was permanently reduced, while the b-wave amplitude recovered over three weeks after light damage. There was a small, but significant dip in scotopic ERG CFF. Photopic behavioral CFF was slightly lower following light damage. The recovery of the b-wave amplitude and flicker sensitivity demonstrates the plasticity of retinal circuits following photopic injury.

Cone Photoreceptor Loss in Light-Damaged Albino Rats

We investigated the etiology of decreased cone-driven vision in a light damage (LD) model of retinal degeneration. To induce slow, moderate degeneration, albino rats underwent low-intensity light exposure for 10 days. Electroretinography was utilized to assess physiologic function of the rod- and cone-driven retinal function in LD and control rats. Immunohistochemistry targeting cone arrestin allowed for quantification of cone density and for comparison of the decline in function. Photoreceptor loss was quantified by outer nuclear layer thickness decreases, as observed by optical coherence tomography and histology. The LD rats showed decreased rod- and cone-driven function with partial recovery 30 days after cessation of light exposure. In addition, LD rats showed decreased cone photoreceptor densities in the central retinal region compared to control rats. Our results demonstrate that the loss of cone-driven visual function induced by light damage is at least partially due to the death of cone photoreceptors.

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.

Asymmetric Distributions of Achromatic Bipolar Cells in the Mouse Retina

In the retina, evolutionary changes can be traced in the topography of photoreceptors. The shape of the visual streak depends on the height of the animal and its habitat, namely, woods, prairies, or mountains. Also, the distribution of distinct wavelength-sensitive cones is unique to each animal. For example, UV and green cones reside in the ventral and dorsal regions in the mouse retina, respectively, whereas in the rat retina these cones are homogeneously distributed. In contrast with the abundant investigation on the distribution of photoreceptors and the third-order neurons, the distribution of bipolar cells has not been well understood. We utilized two enhanced green fluorescent protein (EGFP) mouse lines, Lhx4-EGFP (Lhx4) and 6030405A18Rik-EGFP (Rik), to examine the topographic distributions of bipolar cells in the retina. First, we characterized their GFP-expressing cells using type-specific markers. We found that GFP was expressed by type 2, type 3a, and type 6 bipolar cells in the Rik mice and by type 3b, type 4, and type 5 bipolar cells in the Lhx4 mice. All these types are achromatic. Then, we examined the distributions of bipolar cells in the four cardinal directions and three different eccentricities of the retinal tissue. In the Rik mice, GFP-expressing bipolar cells were more highly observed in the nasal region than those in the temporal retina. The number of GFP cells was not different along with the ventral-dorsal axis. In contrast, in the Lhx4 mice, GFP-expressing cells occurred at a higher density in the ventral region than in the dorsal retina. However, no difference was observed along the nasal-temporal axis. Furthermore, we examined which type of bipolar cells contributed to the asymmetric distributions in the Rik mice. We found that type 3a bipolar cells occurred at a higher density in the temporal region, whereas type 6 bipolar cells were denser in the nasal region. The asymmetricity of these bipolar cells shaped the uneven distribution of the GFP cells in the Rik mice. In conclusion, we found that a subset of achromatic bipolar cells is asymmetrically distributed in the mouse retina, suggesting their unique roles in achromatic visual processing.

Glial Cell Activation and Oxidative Stress in Retinal Degeneration Induced by β-Alanine Caused Taurine Depletion and Light Exposure

We investigate glial cell activation and oxidative stress induced by taurine deficiency secondary to β-alanine administration and light exposure. Two months old Sprague-Dawley rats were divided into a control group and three experimental groups that were treated with 3% β-alanine in drinking water (taurine depleted) for two months, light exposed or both. Retinal and external thickness were measured in vivo at baseline and pre-processing with Spectral-Domain Optical Coherence Tomography (SD-OCT). Retinal cryostat cross sections were immunodetected with antibodies against various antigens to investigate microglial and macroglial cell reaction, photoreceptor outer segments, synaptic connections and oxidative stress. Taurine depletion caused a decrease in retinal thickness, shortening of photoreceptor outer segments, microglial cell activation, oxidative stress in the outer and inner nuclear layers and the ganglion cell layer and synaptic loss. These events were also observed in light exposed animals, which in addition showed photoreceptor death and macroglial cell reactivity. Light exposure under taurine depletion further increased glial cell reaction and oxidative stress. Finally, the retinal pigment epithelial cells were Fluorogold labeled and whole mounted, and we document that taurine depletion impairs their phagocytic capacity. We conclude that taurine depletion causes cell damage to various retinal layers including retinal pigment epithelial cells, photoreceptors and retinal ganglion cells, and increases the susceptibility of the photoreceptor outer segments to light damage. Thus, beta-alanine supplements should be used with caution.

Beyond Sector Retinitis Pigmentosa: Expanding the Phenotype and Natural History of the Rhodopsin Gene Codon 106 Mutation (Gly-to-Arg) in Autosomal Dominant Retinitis Pigmentosa

Sector and pericentral are two rare, regional forms of retinitis pigmentosa (RP). While usually defined as stable or only very slowly progressing, the available literature to support this claim is limited. Additionally, few studies have analyzed the spectrum of disease within a particular genotype. We identified all cases (9 patients) with an autosomal dominant Rhodopsin variant previously associated with sector RP (RHO c.316G > A, p.Gly106Arg) at our institution. Clinical histories were reviewed, and testing included visual fields, multimodal imaging, and electroretinography. Patients demonstrated a broad phenotypic spectrum that spanned regional phenotypes from sector-like to pericentral RP, as well as generalized disease. We also present evidence of significant intrafamilial variability in regional phenotypes. Finally, we present the longest-reported follow-up for a patient with RHO-associated sector-like RP, showing progression from sectoral to pericentral disease over three decades. In the absence of comorbid macular disease, the long-term prognosis for central visual acuity is good. However, we found that significant progression of RHO p.Gly106Arg disease can occur over protracted periods, with impact on peripheral vision. Longitudinal widefield imaging and periodic ERG reassessment are likely to aid in monitoring disease progression.

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.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

7,8-Dihydroxiflavone Maintains Retinal Functionality and Protects Various Types of RGCs in Adult Rats with Optic Nerve Transection

To analyze the neuroprotective effects of 7,8-Dihydroxyflavone (DHF) in vivo and ex vivo, adult albino Sprague-Dawley rats were given a left intraorbital optic nerve transection (IONT) and were divided in two groups: One was treated daily with intraperitoneal (ip) DHF (5 mg/kg) (n = 24) and the other (n = 18) received ip vehicle (1% DMSO in 0.9% NaCl) from one day before IONT until processing. At 5, 7, 10, 12, 14, and 21 days (d) after IONT, full field electroretinograms (ERG) were recorded from both experimental and one additional naïve-control group (n = 6). Treated rats were analyzed 7 (n = 14), 14 (n = 14) or 21 d (n = 14) after IONT, and the retinas immune stained against Brn3a, Osteopontin (OPN) and the T-box transcription factor T-brain 2 (Tbr2) to identify surviving retinal ganglion cells (RGCs) (Brn3a⁺), α-like (OPN⁺), α-OFF like (OPN⁺Brn3a⁺) or M4-like/α-ON sustained RGCs (OPN⁺Tbr⁺). Naïve and right treated retinas showed normal ERG recordings. Left vehicle-treated retinas showed decreased amplitudes of the scotopic threshold response (pSTR) (as early as 5 d), the rod b-wave, the mixed response and the cone response (as early as 10 d), which did not recover with time. In these retinas, by day 7 the total numbers of Brn3a⁺RGCs, OPN⁺RGCs and OPN⁺Tbr2⁺RGCs decreased to less than one half and OPN⁺Brn3a⁺RGCs decreased to approximately 0.5%, and Brn3a⁺RGCs showed a progressive loss with time, while OPN⁺RGCs and OPN⁺Tbr2⁺RGCs did not diminish after seven days. Compared to vehicle-treated, the left DHF-treated retinas showed significantly greater amplitudes of the pSTR, normal b-wave values and significantly greater numbers of OPN⁺RGCs and OPN⁺Tbr2⁺RGCs for up to 14 d and of Brn3a⁺RGCs for up to 21 days. DHF affords significant rescue of Brn3a⁺RGCs, OPN⁺RGCs and OPN⁺Tbr2⁺RGCs, but not OPN⁺Brn3a⁺RGCs, and preserves functional ERG responses after IONT.

Animal Models of LED-Induced Phototoxicity. Short- and Long-Term In Vivo and Ex Vivo Retinal Alterations

Phototoxicity animal models have been largely studied due to their degenerative communalities with human pathologies, e.g., age-related macular degeneration (AMD). Studies have documented not only the effects of white light exposure, but also other wavelengths using LEDs, such as blue or green light. Recently, a blue LED-induced phototoxicity (LIP) model has been developed that causes focal damage in the outer layers of the superior-temporal region of the retina in rodents. In vivo studies described a progressive reduction in retinal thickness that affected the most extensively the photoreceptor layer. Functionally, a transient reduction in a- and b-wave amplitude of the ERG response was observed. Ex vivo studies showed a progressive reduction of cones and an involvement of retinal pigment epithelium cells in the area of the lesion and, in parallel, an activation of microglial cells that perfectly circumscribe the damage in the outer retinal layer. The use of neuroprotective strategies such as intravitreal administration of trophic factors, e.g., basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) or pigment epithelium-derived factor (PEDF) and topical administration of the selective alpha-2 agonist (Brimonidine) have demonstrated to increase the survival of the cone population after LIP.

Mathematical Models of Retinitis Pigmentosa: The Trophic Factor Hypothesis

Retinitis pigmentosa (RP) is the term used to denote a group of inherited retinal-degenerative conditions that cause progressive sight loss. Individuals with this condition lose their light-sensitive photoreceptor cells, known as rods and cones, over a period of years to decades; degeneration starting in the retinal periphery, and spreading peripherally and centrally over time. RP is a rod-cone dystrophy, meaning that rod health and function are affected earlier and more severely than that of cones. Rods degenerate due to an underlying mutation, whereas the reasons for cone degeneration are unknown. A number of mechanisms have been proposed to explain secondary cone loss and the spatio-temporal patterns of retinal degeneration in RP. One of the most promising is the trophic factor hypothesis, which suggests that rods produce a factor necessary for cone survival, such that, when rods degenerate, cone degeneration follows. In this paper we formulate and analyse mathematical models of human RP under the trophic factor hypothesis. These models are constructed as systems of reaction-diffusion partial differential equations in one spatial dimension, and are solved and analysed using a combination of numerical and analytical methods. We predict the conditions under which cones will degenerate following the loss of a patch of rods from the retina, the critical trophic factor treatment rate required to prevent cone degeneration following rod loss and the spatio-temporal patterns of cone loss that would result if the trophic factor mechanism alone were responsible for retinal degeneration.

Short- and Long-Term Study of the Impact of Focal Blue Light-Emitting Diode-Induced Phototoxicity in Adult Albino Rats

Background: In adult rats we study the short- and long-term effects of focal blue light-emitting diode (LED)-induced phototoxicity (LIP) on retinal thickness and Iba-1⁺ activation. Methods: The left eyes of previously dark-adapted Sprague Dawley (SD) rats were photoexposed to a blue LED (20 s, 200 lux). In vivo longitudinal monitoring of retinal thickness, fundus images, and optical retinal sections was performed from 1 to 30 days (d) after LIP with SD-OCT. Ex vivo, we analysed the population of S-cone and Iba-1⁺ cells within a predetermined fixed-size circular area (PCA) centred on the lesion. Results: LIP resulted in a circular focal lesion readily identifiable in vivo by fundus examination, which showed within the PCAs a progressive thinning of the outer retinal layer, and a diminution of the S-cone population to 19% by 30 d. In parallel to S-cone loss, activated Iba-1⁺ cells delineated the lesioned area and acquired an ameboid morphology with peak expression at 3 d after LIP. Iba-1⁺ cells adopted a more relaxed-branched morphology at 7 d and by 14–30 d their morphology was fully branched. Conclusion: LIP caused a progressive reduction of the outer retina with loss of S cones and a parallel dynamic activation of microglial cells in the lesioned area.

Oxidative Stress Resistance 1 Gene Therapy Retards Neurodegeneration in the rd1 Mutant Mouse Model of Retinopathy

Purpose

Oxidative stress is a major factor underlying many neurodegenerative diseases. However, antioxidant therapy has had mixed results, possibly because of its indiscriminate activity. The purpose of our study was to determine if the human OXR1 (hOXR1) antioxidant regulatory gene could protect neurons from oxidative stress and delay photoreceptor cell death.

Methods

The cone-like 661W cell line was transfected to stably express the hOXR1 gene. Oxidative stress was induced by the addition of hydrogen peroxide (H2O2). Intracellular levels of reactive oxygen species (ROS), caspase cleavage, and cellular resistance to oxidative stress were determined and compared between the control and hOXR1 cells. For in vivo analysis, AAV8-hOXR1 was injected subretinally into the rd1 mouse model of retinal degeneration. Functional and structural integrity of the photoreceptors were assessed using electroretinography (ERG), histology, and immunofluorescence analysis.

Results

Expression of hOXR1 increased cellular resistance and reduced ROS levels and caspase cleavage in the 661W cell line after H2O2-induced oxidative stress. Subretinal injection of AAV8-hOXR1 in the rd1 mice improved their photoreceptor light response, expression and localization of photoreceptor-specific proteins, and delayed retinal degeneration.

Conclusions

Our results suggest that OXR1 is a potential therapy candidate for retinal degeneration. Because OXR1 targets oxidative stress, a common feature of many retinal degenerative diseases, it should be of therapeutic value to multiple retinal degenerative diseases.

Establishing the ground squirrel as a superb model for retinal ganglion cell disorders and optic neuropathies

Retinal ganglion cell (RGC) death occurs after optic nerve injury due to acute trauma or chronic degenerative conditions such as optic neuropathies (e.g., glaucoma). Currently, there are no effective therapies to prevent permanent vision loss resulting from RGC death, underlining the need for research on the pathogenesis of RGC disorders. Modeling human RGC/optic nerve diseases in non-human primates is ideal because of their similarity to humans, but has practical limitations including high cost and ethical considerations. In addition, many retinal degenerative disorders are age-related making the study in primate models prohibitively slow. For these reasons, mice and rats are commonly used to model RGC injuries. However, as nocturnal mammals, these rodents have retinal structures that differ from primates - possessing less than one-tenth of the RGCs found in the primate retina. Here we report the diurnal thirteen-lined ground squirrel (TLGS) as an alternative model. Compared to other rodent models, the number and distribution of RGCs in the TLGS retina are closer to primates. The TLGS retina possesses ~600,000 RGCs with the highest density along the equatorial retina matching the location of the highest cone density (visual streak). TLGS and primate retinas also share a similar interlocking pattern between RGC axons and astrocyte processes in the retina nerve fiber layer (RNFL). In addition, using TLGS we establish a new partial optic nerve injury model that precisely controls the extent of injury while sparing a portion of the retina as an ideal internal control for investigating the pathophysiology of axon degeneration and RGC death. Moreover, in vivo optical coherence tomography (OCT) imaging and ex vivo microscopic examinations of the retina in optic nerve injured TLGS confirm RGC loss precedes proximal axon degeneration, recapitulating human pathology. Thus, the TLGS retina is an excellent model, for translational research in neurodegeneration and therapeutic neuroprotection.

An in vivo model of focal light emitting diode-induced cone photoreceptor phototoxicity in adult pigmented mice: Protection with bFGF

PURPOSE

To develop a model of focal injury by blue light-emitting diode (LED)-induced phototoxicity (LIP) in pigmented mouse retinas and to study the effects on cone, Iba-1⁺ cells and retinal pigment epithelium (RPE) cell populations after administration of basic fibroblast growth factor (bFGF) and minocycline, alone or combined.

METHODS

In anesthetized dark-adapted adult female pigmented C57BL/6 mice, left pupils were dilated and the eye exposed to LIP (500 lux, 45 s). The retina was monitored longitudinally in vivo with SD-OCT for 7 days (d). Ex vivo, the effects of LIP and its protection with bFGF (0.5 μg) administered alone or combined with minocycline (45 mg/kg) were studied in immunolabeled arrestin-cone outer segments (a⁺OS) and quantified within a predetermined fixed-size circular area (PCA) centered on the lesion in flattened retinas at 1, 3, 5 or 7d. Moreover, Iba-1⁺ cells and RPE cell morphology were analysed with Iba-1 and ZO-1 antibodies, respectively.

RESULTS

LIP caused a focal lesion within the superior-temporal retina with retinal thinning, particularly the outer retinal layers (116.5 ± 2.9 μm to 36.8 ± 6.3 μm at 7d), and with progressive diminution of a⁺OS within the PCA reaching minimum values at 7d (6218 ± 342 to 3966 ± 311). Administration of bFGF alone (4519 ± 320) or in combination with minocycline (4882 ± 446) had a significant effect on a⁺OS survival at 7d and Iba-1⁺ cell activation was attenuated in the groups treated with minocycline. In parallel, the RPE cell integrity was progressively altered after LIP and administration of neuroprotective components had no restorative effect at 7d.

CONCLUSIONS

LIP resulted in progressive outer retinal damage affecting the OS cone population and RPE. Administration of bFGF increased a⁺OS survival but did not prevent RPE deterioration.

In vivo potency testing of subretinal rAAV5.hCNGB1 gene therapy in the Cngb1 knockout mouse model of retinitis pigmentosa

Retinitis pigmentosa type 45 (RP45) is an autosomal-recessively inherited blinding disease caused by mutations in the cyclic nucleotide gated channel subunit beta 1 (CNGB1) gene. In this study, we developed and tested a novel gene supplementation therapy suitable for clinical translation. To this end, we designed a recombinant adeno-associated virus (rAAV) vector carrying a genome that features a novel human rhodopsin promoter (hRHO194) driving rod-specific expression of full-length human CNGB1 (rAAV5.hCNGB1). rAAV5.hCNGB1 was evaluated for efficacy in the Cngb1 knockout (Cngb1-/-) mouse model of RP45. In particular, increasing doses of rAAV5.hCNGB1 were delivered via single subretinal injection in 4-week-old Cngb1-/- mice and the treatment effect was assessed over a follow-up period of 9 months at the level of (i) retinal morphology, (ii) retinal function, (iii) vision-guided behavior, and (iv) transgene expression. We found that subretinal treatment with rAAV5.hCNGB1 resulted in efficient expression of the human CNGB1 protein in mouse rods and was able to normalize the expression of the endogenous mouse CNGA1 subunit, which together with CNGB1 forms the native heterotetrameric cGMP-gated cation channel in rod photoreceptors. The treatment led to a dose-dependent recovery of rod photoreceptor-driven function and preservation of retinal morphology in Cngb1-/- mice. In summary, these results demonstrate the efficacy of hCNGB1 gene supplementation therapy in the Cngb1-/- mouse model of RP45 and support the translation of this approach towards future clinical application.

Tissue block staining and domestic adhesive tape yield qualified integral sections of adult mouse orbits and eyeballs

The standard histological processing procedure, which produces excellent staining of sections for most tissues, fails to yield satisfactory results in adult mouse orbits or eyeballs. Here, we show that a protocol using tissue block staining and domestic adhesive tapes resulted in qualified integral serial cryo-sections of whole orbits or eyeballs, and the fine structures were well preserved. The histological processing protocol comprises paraformaldehyde fixation, ethylenediaminetetraacetic acid decalcification, tissue block staining with hematoxylin and eosin, embedding, adhesive tape aided sectioning, and water-soluble mounting. This protocol was proved to be the best in comparison with seven other related existing histological traditional or non-traditional processing methods, according to the staining slice quality. We observed a hundred percent success rate in sectioning, collection, and mounting with this method. The reproducibility tested on qualified section success rates and slice quality scores confirmed that the technique is reliable. The feasibility of the method to detect target molecules in orbits was verified by successful trial tests on block immunostaining and adhesive tape-aided sectioning. Application of this protocol in joints, brains, and so on,—the challenging integral sectioning tissues, also generated high-quality histological staining sections.

Brimonidine is Neuroprotective in Animal Paradigm of Retinal Ganglion Cell Damage

To investigate the neuroprotective effect of brimonidine after retinal ischemia damage on mouse eye. Glaucoma is an optic neuropathy characterized by retinal ganglion cells (RGCs) death, irreversible peripheral and central visual field loss, and high intraocular pressure. Ischemia reperfusion (I/R) injury model was used in C57BL/6J mice to mimic conditions of glaucomatous neurodegeneration. Mouse eyes were treated topically with brimonidine and pattern electroretinogram were used to assess the retinal ganglion cells (RGCs) function. A wide range of inflammatory markers, as well as anti-inflammatory and neurotrophic molecules, were investigated to figure out the potential protective effects of brimonidine in mouse retina. In particular, brain-derived neurotrophic factor (BDNF), IL-6, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its death receptor DR-5, TNF-α, GFAP, Iba-1, NOS, IL-1β and IL-10 were assessed in mouse retina that underwent to I/R insult with or without brimonidine treatment. Brimonidine provided remarkable RGCs protection in our paradigm. PERG amplitude values were significantly (p < 0.05) higher in brimonidine-treated eyes in comparison to I/R retinas. Retinal BDNF mRNA levels in the I/R group dropped significantly (p < 0.05) compared to the control group (normal mice); brimonidine treatment counteracted the downregulation of retinal BDNF mRNA in I/R eyes. Retinal inflammatory markers increased significantly (p < 0.05) in the I/R group and brimonidine treatment was able to revert that. The anti-inflammatory IL-10 decreased significantly (p < 0.05) after retinal I/R insult and increased significantly (p < 0.05) in the group treated with brimonidine. In conclusion, brimonidine was effective in preventing loss of function of RGCs and in regulating inflammatory biomarkers elicited by retinal I/R injury.

Visual stimulation with blue wavelength light drives V1 effectively eliminating stray light contamination during two-photon calcium imaging

BACKGROUND

Brain visual circuits are often studied in vivo by imaging Ca²⁺ indicators with green-shifted emission spectra. Polychromatic white visual stimuli have a spectrum that partially overlaps indicators´ emission spectra, resulting in significant contamination of calcium signals.

NEW METHOD

To overcome light contamination problems we choose blue visual stimuli, having a spectral composition not overlapping with Ca²⁺ indicator´s emission spectrum. To compare visual responsiveness to blue and white stimuli we used electrophysiology (visual evoked potentials -VEPs) and 3D acousto-optic two-photon (2P) population Ca²⁺ imaging in mouse primary visual cortex (V1).

RESULTS

VEPs in response to blue and white stimuli had comparable peak amplitudes and latencies. Ca²⁺ imaging in a Thy1 GP4.3 line revealed that the populations of neurons responding to blue and white stimuli were largely overlapping, that their responses had similar amplitudes, and that functional response properties such as orientation and direction selectivities were also comparable.

COMPARISON WITH EXISTING METHODS

Masking or shielding the microscope are often used to minimize the contamination of Ca²⁺ signal by white light, but they are time consuming, bulky and thus can limit experimental design, particularly in the more and more frequently used awake set-up. Blue stimuli not interfering with imaging allow to omit shielding.

CONCLUSIONS

Together, our results show that the selected blue light stimuli evoke responses comparable to those evoked by white stimuli in mouse V1. This will make complex designs of imaging experiments in behavioral set-ups easier, and facilitate the combination of Ca²⁺ imaging with electrophysiology and optogenetics.

Features of Retinal Neurogenesis as a Key Factor of Age-Related Neurodegeneration: Myth or Reality?

Age-related macular degeneration (AMD) is a complex multifactorial neurodegenerative disease that constitutes the most common cause of irreversible blindness in the elderly in the developed countries. Incomplete knowledge about its pathogenesis prevents the search for effective methods of prevention and treatment of AMD, primarily of its "dry" type which is by far the most common (90% of all AMD cases). In the recent years, AMD has become "younger": late stages of the disease are now detected in relatively young people. It is known that AMD pathogenesis-according to the age-related structural and functional changes in the retina-is linked with inflammation, hypoxia, oxidative stress, mitochondrial dysfunction, and an impairment of neurotrophic support, but the mechanisms that trigger the conversion of normal age-related changes to the pathological process as well as the reason for early AMD development remain unclear. In the adult mammalian retina, de novo neurogenesis is very limited. Therefore, the structural and functional features that arise during its maturation and formation can exert long-term effects on further ontogenesis of this tissue. The aim of this review was to discuss possible contributions of the changes/disturbances in retinal neurogenesis to the early development of AMD.

Photoreceptor cells and RPE contribute to the development of diabetic retinopathy

Diabetic retinopathy (DR) is a leading cause of blindness. It has long been regarded as vascular disease, but work in the past years has shown abnormalities also in the neural retina. Unfortunately, research on the vascular and neural abnormalities have remained largely separate, instead of being integrated into a comprehensive view of DR that includes both the neural and vascular components. Recent evidence suggests that the most predominant neural cell in the retina (photoreceptors) and the adjacent retinal pigment epithelium (RPE) play an important role in the development of vascular lesions characteristic of DR. This review summarizes evidence that the outer retina is altered in diabetes, and that photoreceptors and RPE contribute to retinal vascular alterations in the early stages of the retinopathy. The possible molecular mechanisms by which cells of the outer retina might contribute to retinal vascular damage in diabetes also are discussed. Diabetes-induced alterations in the outer retina represent a novel therapeutic target to inhibit DR.

OFF bipolar cell density varies by subtype, eccentricity, and along the dorsal ventral axis in the mouse retina

The neural retina is organized along central-peripheral, dorsal-ventral, and laminar planes. Cellular density and distributions vary along the central-peripheral and dorsal-ventral axis in species including primates, mice, fish, and birds. Differential distribution of cell types within the retina is associated with sensitivity to different types of damage that underpin major retinal diseases, including macular degeneration and glaucoma. Normal variation in retinal distribution remains unreported for multiple cell types in widely used research models, including mouse. Here we map the distribution of all known OFF bipolar cell (BC) populations and horizontal cells. We report significant variation in the distribution of OFF BC populations and horizontal cells along the dorsal-ventral and central-peripheral axes of the retina. Distribution patterns are much more pronounced for some populations of OFF BC cells than others and may correspond to the cell type's specialized functions.

In vivo optoretinography of phototransduction activation and energy metabolism in retinal photoreceptors

The objective of this study is to verify the anatomic correlate of the second (2nd) outer retina band in optical coherence tomography (OCT), and to demonstrate the potential of using intrinsic optical signal (IOS) imaging for concurrent optoretinography (ORG) of phototransduction activation and energy metabolism in stimulus activated retinal photoreceptors. A custom-designed OCT was employed for depth-resolved IOS imaging in mouse retina activated by a visible light flicker stimulation. The spatiotemporal properties of the IOS changes at the photoreceptor outer segment (OS) and inner segment (IS) were quantitatively evaluated. Rapid IOS change was observed at the OS almost right away, and the IOS at the IS was relatively slow. Comparative analysis indicates that the OS-IOS reflects transient OS deformation caused by the phototransduction activation, and IS-IOS might reflect the energy metabolism caused by mitochondria activation in retinal photoreceptors. The consistency of the distribution of the IS-IOS and the 2nd OCT band supports the IS ellipsoid (ISe), which has abundant mitochondria, as the signal source of the 2nd OCT band of the outer retina.

Light responses of mammalian cones

Cone photoreceptors provide the foundation of most of human visual experience, but because they are smaller and less numerous than rods in most mammalian retinas, much less is known about their physiology. We describe new techniques and approaches which are helping to provide a better understanding of cone function. We focus on several outstanding issues, including the identification of the features of the phototransduction cascade that are responsible for the more rapid kinetics and decreased sensitivity of the cone response, the roles of inner-segment voltage-gated and Ca²⁺-activated channels, the means by which cones remain responsive even in the brightest illumination, mechanisms of cone visual pigment regeneration in constant light, and energy consumption of cones in comparison to that of rods.

Retinal ganglion cell loss and gliosis in the retinofugal projection following intravitreal exposure to amyloid-beta

Pathological accumulations of amyloid-beta (Aβ) peptide are found in retina early in Alzheimer's disease, yet its effects on retinal neuronal structure remain unknown. To investigate this, we injected fibrillized Aβ1-42 protein into the eye of adult C57BL/6 J mice and analyzed the retina, optic nerve (ON), and the superior colliculus (SC), the primary retinal target in mice. We found that retinal Aβ exposure stimulated microglial activation and retinal ganglion cell (RGC) loss as early as 1-week post-injection. Pathology was not limited to the retina, but propagated into other areas of the central nervous system. Microgliosis spread throughout the retinal projection (retina, ON, and SC), with multiplex protein quantitation demonstrating an increase in endogenously produced Aβ in the ON and SC corresponding to the injected retinas. Surprisingly, this pathology spread to the opposite side, with unilateral Aβ eye injections driving increased Aβ levels, neuroinflammation, and RGC death in the opposite, un-injected retinal projection. As Aβ-mediated microglial activation has been shown to propagate Aβ pathology, we also investigated the role of the Aβ-binding microglial scavenger receptor CD36 in this pathology. Transgenic mice lacking the CD36 receptor were resistant to Aβ-induced inflammation and RGC death up to 2 weeks following exposure. These results indicate that Aβ pathology drives regional neuropathology in the retina and does not remain isolated to the affected eye, but spreads throughout the nervous system. Further, CD36 may serve as a promising target to prevent Aβ-mediated inflammatory damage.

Colour-sensitive conjugated polymer inkjet-printed pixelated artificial retina model studied via a bio-hybrid photovoltaic device

In recent years, organic electronic materials have been shown to be a promising tool, even transplanted in vivo, for transducing light stimuli to non-functioning retinas. Here we developed a bio-hybrid optoelectronic device consisting of patterned organic polymer semiconductors interfaced with an electrolyte solution in a closed sandwich architecture in order to study the photo-response of photosensitive semiconducting layers or patterns in an environment imitating biological extracellular fluids. We demonstrate an artificial retina model composed of on an array of 42,100 pixels made of three different conjugated polymers via inkjet printing with 110 pixels/mm² packing density. Photo-sensing through three-colour pixelation allows to resolve incoming light spectrally and spatially. The compact colour sensitive optoelectronic device represents an easy-to-handle photosensitive platform for the study of the photo response of artificial retina systems.

Ceramide synthase TLCD3B is a novel gene associated with human recessive retinal dystrophy

Purpose

Previous studies suggest that ceramide is a proapoptotic lipid as high levels of ceramides can lead to apoptosis of neuronal cells, including photoreceptors. However, no pathogenic variant in ceramide synthases has been identified in human patients and knockout of various ceramide synthases in mice has not led to photoreceptor degeneration.

Methods

Exome sequencing was used to identify candidate disease genes in patients with vision loss as confirmed by standard evaluation methods, including electroretinography (ERG) and optical coherence tomography. The vision loss phenotype in mice was evaluated by ERG and histological analyses.

Results

Here we have identified four patients with cone-rod dystrophy or maculopathy from three families carrying pathogenic variants in TLCD3B. Consistent with the phenotype observed in patients, the Tlcd3b^KO/KO mice exhibited a significant reduction of the cone photoreceptor light responses, thinning of the outer nuclear layer, and loss of cone photoreceptors across the retina.

Conclusion

Our results provide the first link between loss-of-function variants in a ceramide synthase gene and human retinal dystrophy. Establishment of the Tlcd3b knockout murine model, the first in vivo photoreceptor cell degeneration model due to loss of a ceramide synthase, will provide a unique opportunity in probing the role of ceramide in survival and function of photoreceptor cells.

Pigment Epithelium-Derived Factor (PEDF) Fragments Prevent Mouse Cone Photoreceptor Cell Loss Induced by Focal Phototoxicity In Vivo

Here, we evaluated the effects of PEDF (pigment epithelium-derived factor) and PEDF peptides on cone-photoreceptor cell damage in a mouse model of focal LED-induced phototoxicity (LIP) in vivo. Swiss mice were dark-adapted overnight, anesthetized, and their left eyes were exposed to a blue LED placed over the cornea. Immediately after, intravitreal injection of PEDF, PEDF-peptide fragments 17-mer, 17-mer[H105A] or 17-mer[R99A] (all at 10 pmol) were administered into the left eye of each animal. BDNF (92 pmol) and bFGF (27 pmol) injections were positive controls, and vehicle negative control. After 7 days, LIP resulted in a consistent circular lesion located in the supratemporal quadrant and the number of S-cones were counted within an area centered on the lesion. Retinas treated with effectors had significantly greater S-cone numbers (PEDF (60%), 17-mer (56%), 17-mer [H105A] (57%), BDNF (64%) or bFGF (60%)) relative to their corresponding vehicle groups (≈42%). The 17-mer[R99A] with no PEDF receptor binding and no neurotrophic activity, PEDF combined with a molar excess of the PEDF receptor blocker P1 peptide, or with a PEDF-R enzymatic inhibitor had undetectable effects in S-cone survival. The findings demonstrated that the cone survival effects were mediated via interactions between the 17-mer region of the PEDF molecule and its PEDF-R receptor.

Human Organoids for the Study of Retinal Development and Disease

Recent advances in stem cell engineering have led to an explosion in the use of organoids as model systems for studies in multiple biological disciplines. Together with breakthroughs in genome engineering and the various omics, organoid technology is making possible studies of human biology that were not previously feasible. For vision science, retinal organoids derived from human stem cells allow differentiating and mature human retinal cells to be studied in unprecedented detail. In this review, we examine the technologies employed to generate retinal organoids and how organoids are revolutionizing the fields of developmental and cellular biology as they pertain to the retina. Furthermore, we explore retinal organoids from a clinical standpoint, offering a new platform with which to study retinal diseases and degeneration, test prospective drugs and therapeutic strategies, and promote personalized medicine. Finally, we discuss the range of possibilities that organoids may bring to future retinal research and consider their ethical implications.

Functional and morphological alterations in a glaucoma model of acute ocular hypertension

To study short and long-term effects of acute ocular hypertension (AOHT) on inner and outer retinal layers, in adult Sprague-Dawley rats AOHT (87mmHg) was induced for 90min and the retinas were examined longitudinally in vivo with electroretinogram (ERG) recordings and optical coherent tomography (OCT) from 1 to 90 days (d). Ex vivo, the retinas were analyzed for rod (RBC) and cone (CBC) bipolar cells, with antibodies against protein kinase Cα and recoverin, respectively in cross sections, and for cones, horizontal (HZ) and ganglion (RGC) cells with antibodies against arrestin, calbindin and Brn3a, respectively in wholemounts. The inner retina thinned progressively up to 7d with no further changes, while the external retina had a normal thickness until 30d, with a 20% thinning between 30 and 90d. Functionally, the a-wave showed an initial reduction by 24h and a further reduction from 30 to 90d. All other main ERG waves were significantly reduced by 1d without significant recovery by 90d. Radial sections showed a normal population of RBCs but their terminals were reduced. The CBCs showed a progressive decrease with a loss of 56% by 30d. In wholemount retinas, RGCs diminished to 40% by 3d and to 16% by 30d without further loss. Cones diminished to 58% and 35% by 3 and 7d, respectively and further decreased between 30 and 90d. HZs showed normal values throughout the study. In conclusion, AOHT affects both the inner and outer retina, with a more pronounced degeneration of the cone than the rod pathway.

Validating Fluorescent Chrnb4.EGFP Mouse Models for the Study of Cone Photoreceptor Degeneration

Purpose

To validate the application of a known transgenic mouse line with green fluorescent cones (Chrnb4.EGFP) to study cone photoreceptor biology and function in health and disease.

Methods

Chrnb4.EGFP retinas containing GFP+ cones were compared with retinas without the GFP transgene via immunohistochemistry, quantitative real-time polymerase chain reaction, electroretinograms, and flow cytometry. The Chrnb4.EGFP line was backcrossed to the mouse models of cone degeneration, Pde6ccpfl1 and Gnat2cpfl3 , generating the new lines Gnat2.GFP and Pde6c.GFP, which were also studied as described.

Results

GFP expression spanned the length of the cone cell in the Chrnb4.EGFP line, as well as in the novel Gnat2.GFP and Pde6c.GFP lines. The effect of GFP expression showed no significant changes to outer nuclear layer cell death, cone-specific gene expression, and immune response activation. A temporal decrease in GFP expression over time was observed, but GFP fluorescence was still detected through flow cytometry as late as 6 months. Furthermore, a functional analysis of photopic and scotopic electroretinogram responses of the Chrnb4 mouse showed no significant difference between GFP- and GFP+ mice, whereas electroretinogram recordings for the Pde6c.GFP and Gnat2.GFP lines matched previous reports from the original lines.

Conclusions

This study demonstrates that the Chrnb4.EGFP mouse can be a powerful tool to overcome the limitations of studying cone biology, including the use of this line to study different types of cone degeneration.

Translational Relevance

This work validates research tools that could potentially offer more reliable preclinical data in the development of treatments for cone-mediated vision loss conditions, shortening the gap to clinical translation.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

Coordinated Intervention of Microglial and Müller Cells in Light-Induced Retinal Degeneration

Purpose

To analyze the role of microglial and Müller cells in the formation of rings of photoreceptor degeneration caused by phototoxicity.

Methods

Two-month-old Sprague-Dawley rats were exposed to light and processed 1, 2, or 3 months later. Retinas were dissected as whole-mounts, immunodetected for microglial cells, Müller cells, and S- and L/M-cones and analyzed using fluorescence, thunder imaging, and confocal microscopy. Cone populations were automatically counted and isodensity maps constructed to document cone topography.

Results

Phototoxicity causes a significant progressive loss of S- and L/M-cones of up to 68% and 44%, respectively, at 3 months after light exposure (ALE). One month ALE, we observed rings of cone degeneration in the photosensitive area of the superior retina. Two and 3 months ALE, these rings had extended to the central and inferior retina. Within the rings of cone degeneration, there were degenerating cones, often activated microglial cells, and numerous radially oriented processes of Müller cells that showed increased expression of intermediate filaments. Between 1 and 3 months ALE, the rings coalesced, and at the same time the microglial cells resumed a mosaic-like distribution, and there was a decrease of Müller cell gliosis at the areas devoid of cones.

Conclusions

Light-induced photoreceptor degeneration proceeds with rings of cone degeneration, as observed in inherited retinal degenerations in which cone death is secondary to rod degeneration. The spatiotemporal relationship of cone death microglial cell activation and Müller cell gliosis within the rings of cone degeneration suggests that, although both glial cells are involved in the formation of the rings, they may have coordinated actions and, while microglial cells may be more involved in photoreceptor phagocytosis, Müller cells may be more involved in cone and microglial cell migration, retinal remodeling and glial seal formation.

True S-cones are concentrated in the ventral mouse retina and wired for color detection in the upper visual field

Color, an important visual cue for survival, is encoded by comparing signals from photoreceptors with different spectral sensitivities. The mouse retina expresses a short wavelength-sensitive and a middle/long wavelength-sensitive opsin (S- and M-opsin), forming opposing, overlapping gradients along the dorsal-ventral axis. Here, we analyzed the distribution of all cone types across the entire retina for two commonly used mouse strains. We found, unexpectedly, that 'true S-cones' (S-opsin only) are highly concentrated (up to 30% of cones) in ventral retina. Moreover, S-cone bipolar cells (SCBCs) are also skewed towards ventral retina, with wiring patterns matching the distribution of true S-cones. In addition, true S-cones in the ventral retina form clusters, which may augment synaptic input to SCBCs. Such a unique true S-cone and SCBC connecting pattern forms a basis for mouse color vision, likely reflecting evolutionary adaptation to enhance color coding for the upper visual field suitable for mice's habitat and behavior.

Revisiting the role of Dcc in visual system development with a novel eye clearing method

The Deleted in Colorectal Carcinoma (Dcc) receptor plays a critical role in optic nerve development. Whilst Dcc is expressed postnatally in the eye, its function remains unknown as Dcc knockouts die at birth. To circumvent this drawback, we generated an eye-specific Dcc mutant. To study the organization of the retina and visual projections in these mice, we also established EyeDISCO, a novel tissue clearing protocol that removes melanin allowing 3D imaging of whole eyes and visual pathways. We show that in the absence of Dcc, some ganglion cell axons stalled at the optic disc, whereas others perforated the retina, separating photoreceptors from the retinal pigment epithelium. A subset of visual axons entered the CNS, but these projections are perturbed. Moreover, Dcc-deficient retinas displayed a massive postnatal loss of retinal ganglion cells and a large fraction of photoreceptors. Thus, Dcc is essential for the development and maintenance of the retina.

Single-nuclei RNA-seq on human retinal tissue provides improved transcriptome profiling

Single-cell RNA-seq is a powerful tool in decoding the heterogeneity in complex tissues by generating transcriptomic profiles of the individual cell. Here, we report a single-nuclei RNA-seq (snRNA-seq) transcriptomic study on human retinal tissue, which is composed of multiple cell types with distinct functions. Six samples from three healthy donors are profiled and high-quality RNA-seq data is obtained for 5873 single nuclei. All major retinal cell types are observed and marker genes for each cell type are identified. The gene expression of the macular and peripheral retina is compared to each other at cell-type level. Furthermore, our dataset shows an improved power for prioritizing genes associated with human retinal diseases compared to both mouse single-cell RNA-seq and human bulk RNA-seq results. In conclusion, we demonstrate that obtaining single cell transcriptomes from human frozen tissues can provide insight missed by either human bulk RNA-seq or animal models.

Topical Brimonidine or Intravitreal BDNF, CNTF, or bFGF Protect Cones Against Phototoxicity

Purpose

To develop a focal photoreceptor degeneration model by blue light-emitting diode (LED)-induced phototoxicity (LIP) and investigate the protective effects of topical brimonidine (BMD) or intravitreal brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), or basic fibroblast growth factor (bFGF).

Methods

In anesthetized, dark-adapted, adult female Swiss mice, the left eye was dilated and exposed to blue light (10 seconds, 200 lux). After LIP, full-field electroretinograms (ERG) and spectral-domain optical coherence tomography (SD-OCT) were obtained longitudinally, and reactive-Iba-1⁺monocytic cells, TUNEL⁺ cells and S-opsin⁺ cone outer segments were examined up to 7 days. Left eyes were treated topically with BMD (1%) or vehicle, before or right after LIP, or intravitreally with BDNF (2.5 μg), CNTF (0.2 μg), bFGF (0.5 μg), or corresponding vehicle right after LIP. At 7 days, S-opsin⁺ cone outer segments were counted within predetermined fixed-size areas (PFA) centered on the lesion in both flattened retinas.

Results

SD-OCT showed a circular region in the superior-temporal left retina with progressive thinning (207.9 ± 5.6 μm to 160.7 ± 6.8 μm [7 days], n = 8), increasing TUNEL⁺ cells (peak at 3 days), decreasing S-opsin⁺ cone outer segments, and strong microglia activation. ERGs were normal by 3 days. Total S-opsin⁺ cones in the PFA for LIP-treated and fellow-retinas were 2330 ± 262 and 5601 ± 583 (n = 8), respectively. All neuroprotectants (n = 7–11), including topical BMD pre- or post-LIP, or intravitreal BDNF, CNTF, and bFGF, showed significantly greater S-opsin⁺ cone survival than their corresponding vehicle-treated groups.

Conclusions

LIP is a reliable, quantifiable focal photoreceptor degeneration model. Topical BMD or intravitreal BDNF, CNTF, or bFGF protect against LIP-induced cone-photoreceptor loss.

Translational Relevance

Topical BMD or intravitreal BDNF, CNTF, or bFGF protect cones against phototoxicity.

Cellular localization of melatonin receptor Mel1b in pigeon retina

Melatonin, an important neuromodulator involved in circadian rhythms, modulates a series of physiological processes via activating its specific receptors, namely Mel1a (MT₁), Mel1b (MT₂) and Mel1c receptors. In this work, the localization of Mel1b receptor was studied in pigeon retina using double immunohistochemistry staining and confocal scanning microscopy. Our results showed that Mel1b receptor widely existed in the outer segment of photoreceptors and in the somata of dopaminergic amacrine cells, cholinergic amacrine cells, glycinergic AII amacrine cells, conventional ganglion cells and intrinsically photosensitive retinal ganglion cells, while horizontal cells, bipolar cells and Müller glial cells seemed to lack immunoreactivity of Mel1b receptor. That multiple types of retinal cells expressing Mel1b receptor suggests melatonin may directly modulate the activities of retina via activating Mel1b receptor.