Functional and neurochemical development in the normal and degenerating mouse retina

The temporal progression of retinal degeneration and early-stage idebenone treatment in the Pde6brd1/rd1 mouse model of retinal dystrophy

Photoreceptor cell death, primarily through apoptosis, related to retinal disorders like retinitis pigmentosa (RP), would result in vision loss. The pathological processes and crucial mutant conditions preceding photoreceptor cell demise are not well understood. This study aims to conduct an in-depth examination of early-stage changes in the widely utilized Pde6brd1/rd1 (rd1) mouse model, which has Pde6b gene mutations representing autosomal recessive RP disorder. We investigated the morphology and ultrastructure of retinal cells, including second-order neurons, during the initial phase of disease progression. Our findings revealed that mitochondrial alterations in rod photoreceptors were present as a predeath mutant state as early as postnatal day 3 (P3). The bipolar and horizontal cells from the rd1 mouse retina exhibited significant morphological changes in response to loss of photoreceptor cells, indicating that second-order neurons rely on these cells for their structures. Subsequent oral administration of idebenone, a mitochondria-protective agent, enhanced retinal function and promoted both photoreceptor cell survival and inner retinal second-order synaptogenesis in rd1 mice at P14. Our findings offer a mechanistic framework, suggesting that mitochondrial damage acts as an early driver for photoreceptor cell death in retinal degeneration.

Vision-Dependent and -Independent Molecular Maturation of Mouse Retinal Ganglion Cells

The development and connectivity of retinal ganglion cells (RGCs), the retina's sole output neurons, are patterned by activity-independent transcriptional programs and activity-dependent remodeling. To inventory the molecular correlates of these influences, we applied high-throughput single-cell RNA sequencing (scRNA-seq) to mouse RGCs at six embryonic and postnatal ages. We identified temporally regulated modules of genes that correlate with, and likely regulate, multiple phases of RGC development, ranging from differentiation and axon guidance to synaptic recognition and refinement. Some of these genes are expressed broadly while others, including key transcription factors and recognition molecules, are selectively expressed by one or a few of the 45 transcriptomically distinct types defined previously in adult mice. Next, we used these results as a foundation to analyze the transcriptomes of RGCs in mice lacking visual experience due to dark rearing from birth or to mutations that ablate either bipolar or photoreceptor cells. 98.5% of visually deprived (VD) RGCs could be unequivocally assigned to a single RGC type based on their transcriptional profiles, demonstrating that visual activity is dispensable for acquisition and maintenance of RGC type identity. However, visual deprivation significantly reduced the transcriptomic distinctions among RGC types, implying that activity is required for complete RGC maturation or maintenance. Consistent with this notion, transcriptomic alternations in VD RGCs significantly overlapped with gene modules found in developing RGCs. Our results provide a resource for mechanistic analyses of RGC differentiation and maturation, and for investigating the role of activity in these processes.

Progress on Designing a Chemical Retinal Prosthesis

The last major review of progress toward a chemical retinal prosthesis was a decade ago. Many important advancements have been made since then with the aim of producing an implantable device for animal testing. We review that work here discussing the potential advantages a chemical retinal prosthesis may possess, the spatial and temporal resolutions it might provide, the materials from which an implant might be constructed and its likely effectiveness in stimulating the retina in a natural fashion. Consideration is also given to implant biocompatibility, excitotoxicity of dispensed glutamate and known changes to photoreceptor degenerate retinas.

Nr2e3 is a genetic modifier that rescues retinal degeneration and promotes homeostasis in multiple models of retinitis pigmentosa

Recent advances in viral vector engineering, as well as an increased understanding of the cellular and molecular mechanism of retinal diseases, have led to the development of novel gene therapy approaches. Furthermore, ease of accessibility and ocular immune privilege makes the retina an ideal target for gene therapies. In this study, the nuclear hormone receptor gene Nr2e3 was evaluated for efficacy as broad-spectrum therapy to attenuate early to intermediate stages of retinal degeneration in five unique mouse models of retinitis pigmentosa (RP). RP is a group of heterogenic inherited retinal diseases associated with over 150 gene mutations, affecting over 1.5 million individuals worldwide. RP varies in age of onset, severity, and rate of progression. In addition, ~40% of RP patients cannot be genetically diagnosed, confounding the ability to develop personalized RP therapies. Remarkably, Nr2e3 administered therapy resulted in reduced retinal degeneration as observed by increase in photoreceptor cells, improved electroretinogram, and a dramatic molecular reset of key transcription factors and associated gene networks. These therapeutic effects improved retinal homeostasis in diseased tissue. Results of this study provide evidence that Nr2e3 can serve as a broad-spectrum therapy to treat multiple forms of RP.

Voltage-gated sodium channel-dependent retroaxonal modulation of photoreceptor function during post-natal development in mice

Juvenile (postnatal day 16) mice lacking Na_v 1.6 channels (null-mutant Scn8a^dmu ) have reduced photoreceptor function, which is unexpected given that Na_v channels have not been detected in mouse photoreceptors and do not contribute appreciably to photoreceptor function in adults. We demonstrate that acute block of Na_v channels with intravitreal TTX in juvenile (P16) wild-type mice has no effect on photoreceptor function. However, reduced light activity by prolonged dark adaptation from P8 caused significant reduction in photoreceptor function at P16. Injecting TTX into the retrobulbar space at P16 to specifically block Na_v channels in the optic nerve also caused a reduction in photoreceptor function comparable to that seen at P16 in null-mutant Scn8a mice. In both P16 null-mutant Scn8a^dmu and retrobulbar TTX-injected wild-type mice, photoreceptor function was restored following intravitreal injection of the TrkB receptor agonist 7,8-dihydroxyflavone, linking Na_v -dependent retrograde transport to TrkB-dependent neurotrophic factor production pathways as a modulatory influence of photoreceptor function at P16. We also found that in Scn8a^dmu mice, photoreceptor function recovers by P22-25 despite more precarious general health of the animal. Retrobulbar injection of TTX in the wild type still reduced the photoreceptor response at this age but to a lesser extent, suggesting that Na_v -dependent modulation of photoreceptor function is largely transient, peaking soon after eye opening. Together, these results suggest that the general photosensitivity of the retina is modulated following eye opening by retrograde transport through activity-dependent retinal ganglion cell axonal signaling targeting TrkB receptors.

Assessment of inner retinal oxygen metrics and thickness in a mouse model of inherited retinal degeneration

The retinal degeneration 1 (rd1) mouse is a well-established model of inherited retinal degeneration, displaying photoreceptor degeneration and retinal vasculature damage. The purpose of the current study was to determine alterations in the rate of oxygen delivery from retinal circulation (DO₂), the rate of oxygen extraction from the retinal circulation for metabolism (MO₂), and oxygen extraction fraction (OEF) in rd1 mice. The study was performed in a total of 18 wild type (WT) and 10 rd1 mice at both 3-weeks and 12-weeks of age. Retinal arterial and venous oxygen contents (O_2A and O_2V) were measured using phosphorescence lifetime imaging. Total retinal blood flow (TRBF) was determined by fluorescence and red-free imaging. DO₂ and MO₂ were determined as TRBF × O_2A and TRBF × (O_2A-O_2V), respectively. OEF was calculated as MO₂/DO₂. The thickness of individual retinal layers was measured from histology sections and inner retina (IR) and total retina (TR) thickness were calculated. TRBF, DO₂ and MO₂ were lower in rd1 mice compared to WT mice (P ≤ 0.001), whereas OEF was not significantly different between rd1 and WT mice (P = 0.4). TRBF and DO₂ were lower at 3-weeks of age compared to 12-weeks of age (P ≤ 0.01), while MO₂ was not significantly different between age groups (P = 0.4) and OEF was higher at 3-weeks of age compared to 12-weeks of age (P = 0.003). Additionally, the outer and inner retinal cell layer thicknesses were decreased in rd1 mice at 12-weeks of age compared to both age-matched WT mice and rd1 mice at 3-weeks of age (P ≤ 0.02). MO₂ was directly correlated with both IR and TR thickness (R ≥ 0.50; P ≤ 0.03, N = 20). The findings indicate that the rate oxygen is supplied by the retinal circulation is decreased and the reduction in oxygen extracted for metabolism is related to retinal cell layer thinning in rd1 mice.

Photoreceptor Degeneration in Pro23His Transgenic Rats (Line 3) Involves Autophagic and Necroptotic Mechanisms

Photoreceptor death contributes to 50% of irreversible vision loss in the western world. Pro23His (P23H) transgenic albino rat strains are widely used models for the most common rhodopsin gene mutation associated with the autosomal dominant form of retinitis pigmentosa. However, the mechanism(s) by which photoreceptor death occurs are not well understood and were the principal aim of this study. We first used electroretinogram recording and optical coherence tomography to confirm the time course of functional and structural loss. Electroretinogram analyses revealed significantly decreased rod photoreceptor (a-wave), bipolar cell (b-wave) and amacrine cell responses (oscillatory potentials) from P30 onward. The cone-mediated b-wave was also decreased from P30. TUNEL analysis showed extensive cell death at P18, with continued labeling detected until P30. Focused gene expression arrays indicated activation of, apoptosis, autophagy and necroptosis in whole retina from P14-18. However, analysis of mitochondrial permeability changes (ΔΨm) using JC-1 dye, combined with immunofluorescence markers for caspase-dependent (cleaved caspase-3) and caspase-independent (AIF) cell death pathways, indicated mitochondrial-mediated cell death was not a major contributor to photoreceptor death. By contrast, reverse-phase protein array data combined with RIPK3 and phospho-MLKL immunofluorescence indicated widespread necroptosis as the predominant mechanism of photoreceptor death. These findings highlight the complexity of mechanisms involved in photoreceptor death in the Pro23His rat model of degeneration and suggest therapies that target necroptosis should be considered for their potential to reduce photoreceptor death.

Targeted Blood Metabolomic Study on Retinopathy of Prematurity

Purpose

This study aims at exploring alterations of major metabolites and metabolic pathways in retinopathy of prematurity (ROP) infants and identifying biomarkers that may merit early diagnosis of ROP.

Methods

We analyzed targeted metabolites from 81 premature infants (<34 weeks of gestational age), including 40 ROP cases (15 males and 25 females, birth weight 1.263 ± 0. 345 kg, gestational age 31.20 ± 4.62 weeks) and 41 cases (30 males, 11 females, birth weight 1.220 ± 0.293 kg, gestational age 30.96 ± 4.17 weeks) of well-matched non-ROP controls. Metabolites were measured by ultra-performance liquid chromatography-tandem mass spectrometry. Standard multivariate and univariate analysis was performed to interpret metabolomic results.

Results

Glycine, glutamate, leucine, serine, piperidine, valine, tryptophan, citrulline, malonyl carnitine (C3DC), and homocysteine were identified as the top discriminant metabolites. In particular, discriminant concentrations of C3DC and glycine were also confirmed by univariate analysis as statistically significant different between ROP and non-ROP infants.

Conclusions

This study gained an insight into the metabolomic aspects of ROP development. We suggest that higher blood levels of C3DC and glycine can be promising biomarkers to predict the occurrence, but not the severity of ROP.

Systematic spatiotemporal mapping reveals divergent cell death pathways in three mouse models of hereditary retinal degeneration

Calcium (Ca²⁺ ) dysregulation has been linked to neuronal cell death, including in hereditary retinal degeneration. Ca²⁺ dysregulation is thought to cause rod and cone photoreceptor cell death. Spatial and temporal heterogeneities in retinal disease models have hampered validation of this hypothesis. We examined the role of Ca²⁺ in photoreceptor degeneration, assessing the activation pattern of Ca²⁺ -dependent calpain proteases, generating spatiotemporal maps of the entire retina in the cpfl1 mouse model for primary cone degeneration, and in the rd1 and rd10 models for primary rod degeneration. We used Gaussian process models to distinguish the temporal sequences of degenerative molecular processes from other variability sources.In the rd1 and rd10 models, spatiotemporal pattern of increased calpain activity matched the progression of primary rod degeneration. High calpain activity coincided with activation of the calpain-2 isoform but not with calpain-1, suggesting differential roles for both calpain isoforms. Primary rod loss was linked to upregulation of apoptosis-inducing factor, although only a minute fraction of cells showed activity of the apoptotic marker caspase-3. After primary rod degeneration concluded, caspase-3 activation appeared in cones, suggesting apoptosis as the dominant mechanism for secondary cone loss. Gaussian process models highlighted calpain activity as a key event during primary rod photoreceptor cell death. Our data suggest a causal link between Ca²⁺ dysregulation and primary, nonapoptotic degeneration of photoreceptors and a role for apoptosis in secondary degeneration of cones, highlighting the importance of the spatial and temporal location of key molecular events, which may guide the evaluation of new therapies.

Increased density and age-related sharing of synapses at the cone to OFF bipolar cell synapse in the mouse retina

Neural circuits in the adult nervous system are characterized by stable, cell type-specific patterns of synaptic connectivity. In many parts of the nervous system these patterns are established during development through initial over-innervation by multiple pre- or postsynaptic targets, followed by a process of refinement that takes place during development and is in many instances activity dependent. Here we report on an identified synapse in the mouse retina, the cone photoreceptor➔type 4 bipolar cell (BC4) synapse, and show that its development is distinctly different from the common motif of over-innervation followed by refinement. Indeed, the majority of cones are contacted by single BC4 throughout development, but are contacted by multiple BC4s through ongoing dendritic elaboration between 1 and 6 months of age-well into maturity. We demonstrate that cell density drives contact patterns downstream of single cones in Bax null mice and may serve to maintain constancy in both the dendritic and axonal projective field.

Spatio-temporal characterization of S- and M/L-cone degeneration in the Rd1 mouse model of retinitis pigmentosa

Background

The Pde6brd1 (Rd1) mouse is widely used as a murine model for human retinitis pigmentosa. Understanding the spatio-temporal patterns of cone degeneration is important for evaluating potential treatments. In the present study we performed a systematic characterization of the spatio-temporal patterns of S- and M/L-opsin⁺ cone outer segment and cell body degeneration in Rd1 mice, described the distribution and proportion of dual cones in Rd1 retinas, and examined the kinetics of microglial activation during the period of cone degeneration.

Results

Outer segments of S- and M/L-cones degenerated far more rapidly than their somas. Loss of both S- and M/L-opsin⁺ outer segments was fundamentally complete by P21 in the central retina, and 90% complete by P45 in the peripheral retina. In comparison, degeneration of S- and M/L-opsin⁺ cell bodies proceeded at a slower rate. There was a marked hemispheric asymmetry in the rate of S-opsin⁺ and M/L-opsin⁺ cell body degeneration. M/L-opsin⁺ cones were more resilient to degeneration in the superior retina, whilst S-opsin⁺ cones were relatively preserved in the inferior retina. In addition, cone outer segment and cell body degeneration occurred far more rapidly in the central than the peripheral retina. At P14, the superior retina comprised a minority of genuine S-cones with a much greater complement of genuine M/L-opsin cones and dual cones, whilst the other three retinal quadrants had broadly similar numbers of genuine S-cones, genuine M/L-cones and dual cones. At P60, approximately 50% of surviving cones in the superior, nasal and temporal quadrants were dual cones. In contrast, the inferior peripheral retina at P60 contained almost exclusively genuine S-cones with a tiny minority of dual cones. Microglial number and activity were stimulated during rod breakdown, remained relatively high during cone outer segment degeneration and loss of cone somas in the central retina, and decreased thereafter in the period coincident with slow degeneration of cone cell bodies in the peripheral retina.

Conclusion

The results of the present study provide valuable insights into cone degeneration in the Rd1 mouse, substantiating and extending conclusions drawn from earlier studies.

Visual responses in the dorsal lateral geniculate nucleus at early stages of retinal degeneration in rd1 PDE6β mice

Inherited retinal degenerations encompass a wide range of diseases that result in the death of rod and cone photoreceptors, eventually leading to irreversible blindness. Low vision survives at early stages of degeneration, at which point it could rely on residual populations of rod/cone photoreceptors as well as the inner retinal photoreceptor, melanopsin. To date, the impact of partial retinal degeneration on visual responses in the primary visual thalamus (dorsal lateral geniculate nucleus, dLGN) remains unknown, as does their relative reliance on surviving rod and cone photoreceptors vs. melanopsin. To answer these questions, we recorded visually evoked responses in the dLGN of anesthetized rd¹ mice using in vivo electrophysiology at an age (3-5 wk) at which cones are partially degenerate and rods are absent. We found that excitatory (ON) responses to light had lower amplitude and longer latency in rd¹ mice compared with age-matched visually intact controls; however, contrast sensitivity and spatial receptive field size were largely unaffected at this early stage of degeneration. Responses were retained when those wavelengths to which melanopsin is most sensitive were depleted, indicating that they were driven primarily by surviving cones. Inhibitory responses appeared absent in the rd¹ thalamus, as did light-evoked gamma oscillations in firing. This description of fundamental features of the dLGN visual response at this intermediate stage of retinal degeneration provides a context for emerging attempts to restore vision by introducing ectopic photoreception to the degenerate retina.NEW & NOTEWORTHY This study provides new therapeutically relevant insights to visual responses in the dorsal lateral geniculate nucleus during progressive retinal degeneration. Using in vivo electrophysiology, we demonstrate that visual responses have lower amplitude and longer latency during degeneration, but contrast sensitivity and spatial receptive fields remain unaffected. Such visual responses are driven predominantly by surviving cones rather than melanopsin photoreceptors. The functional integrity of this visual pathway is encouraging for emerging attempts at visual restoration.

Maturation of the Pupil Light Reflex Occurs Until Adulthood in Mice

With respect to photoreceptor function, it is well known that electroretinogram (ERG) amplitudes decrease with age, but to our knowledge, studies describing age-related changes in the pupil light response (PLR) of mice are lacking. This study recorded the PLR and ERG in C57BL/6 and Sv129S6 wild-type mice at three different ages during early adulthood. Dark- and light-adapted PLR and ERG measurements were performed at 1, 2, and 4 months of age. For PLR measurements, we used either a red (622 nm) or blue (463 nm) light stimulus (500 ms) to stimulate one eye. We selected various light intensities ranging across almost 4 log units and subsequently classified them as low, medium, or high intensity. From the recorded PLR, we selected parameters to quantify the early and late phases of the response such as the baseline pupil size, the maximal constriction amplitude, the maximal velocity, the early partial dilation (area under the curve of the positive peak of the first derivative of PLR tracing), and the sustained constriction amplitude. For ERG measurements, both scotopic and photopic responses were recorded following stimulation with green light (520 nm) at preselected intensities. The amplitudes and latencies of the a-wave and the b-wave were also analyzed. In both strains, 1-month-old animals presented with a smaller baseline pupil diameter compared to that in 2- and 4-month-old mice. They also exhibited greater maximal constriction amplitude in response to red stimuli of medium intensity. Further, 1-month-old Sv129S6 mice responded with greater constriction amplitude to all other red and blue stimuli. One-month-old C57BL/6 mice also demonstrated faster early partial dilation and smaller sustained response to low blue stimuli. The ERG of 1-month-old C57BL/6 mice showed a greater scotopic a-wave amplitude compared to that of 2-month-old mice, whereas no significant differences were found in Sv129S6 mice. These results suggest that the functional maturation of the neuronal pathway that mediates the PLR continues after 1 month of age. In studies that measure PLR to determine retinal integrity in adult mice, it is thus important to determine normative values in animals of 2 months of age.

Metabolic profile analysis of free amino acids in experimental autoimmune uveoretinitis rat plasma

AIM

To determine the differences of amino acid (AA) levels in experimental autoimmune uveoretinitis (EAU).

METHODS

AA analysis of the plasma samples in EAU rats induced by interphotoreceptor retinoid-binding protein emulsion were performed with high performance liquid chromatography (HPLC) and phenylisothiocyanate (PITC) pre-column derivation methods were performed. Using partial least squares discriminant analysis (PLS-DA), the potential biomarkers were identified in EAU rat plasma, and the metabolic pathways related to EAU were further analyzed.

RESULTS

The method results showed that linear (r≥0.9957), intra-day reproducible [relative standard deviation (RSD)=0.04%-1.33%], inter-day reproducible (RSD=0.06%-2.07%), repeatability (RSD=0.03%-0.89%), stability (RSD=0.05%-2.48%) and recovery (RSD=1.98%-4.39%), with detection limits of 0.853-11.4 ng/mL. The metabolic profile in EAU rats was different from that in the control groups five AAs concentrations were increased and nine AAs were reduced. Moreover, five metabolic pathways were related to the development of EAU.

CONCLUSION

The developed method is a simple, rapid and convenient for determination of AAs in EAU rat plasma, and these findings will provide a comprehensive insight on the metabolic profiling of the pathological changes in EAU.

Oxidative stress and autophagy-related changes during retinal degeneration and development

Retinitis pigmentosa (RP) is an inherited retinopathy that leads to photoreceptor loss. RP has been related to oxidative stress, autophagy, and inflammation. This study aimed to identify changes in the levels of oxidative stress and autophagy markers in the retina of control and rd10 mice during different phases of retinal development. Changes in the retinal oxidation system were investigated by measuring the levels of oxidized and reduced glutathione (GSH/GSSG), retinal avidin-positive cells, and 4-hydroxynonenal (4-HNE) staining intensity. Autophagy characterization was explored by measuring the levels of microtubule-associated protein 1 light chain 3 (LC3), beclin, autophagy-related proteins 5 and 7 (Atg5 and Atg7), and lysosomal associated membrane protein-2A (LAMP-2A). At P28 retinal GSH concentrations decreased in rd10 mice compared to the controls. No differences were found in retinal GSSG concentrations between the control and rd10 mice. There was an increase in retinal GSSG concentrations and a decrease in the GSH/GSSG ratio in the control and rd10 mice at P21 and P28 compared to P13. We observed an increase in avidin-positive cells in rd10 retinas. 4-HNE was increased in rd10 retinas at P13, and it also increased in control mice with age. We did not observe any differences in the retinal levels of LC3II/I ratio, Beclin, Atg5, or Atg7 in the rd10 mice compared to the controls. There was an increase in the LAMP-2A concentrations in the control and rd10 mice with development age (P28 concentrations vs. P13). Although only slight differences were found in the oxidative stress and autophagy markers between the control and rd10 mice, there were increases in the GSSG, 4-HNE, and LAMP-2A with age. This increase in the oxidative stress and chaperone-mediated autophagy has not been described before and occurred just after the mice opened their eyes, potentially indicating a retinal response to light exposure.

Tissue inhibitor of metalloproteinases 1 enhances rod survival in the rd1 mouse retina

Retinitis pigmentosa (RP), an inherited retinal degenerative disease, is characterized by a progressive loss of rod photoreceptors followed by loss of cone photoreceptors. Previously, when tissue inhibitor of metalloproteinase 1 (TIMP1), a key extracellular matrix (ECM) regulator that binds to and inhibits activation of Matrix metallopeptidase 9 (MMP9) was intravitreal injected into eyes of a transgenic rhodopsin rat model of RP, S334ter-line3, we discovered cone outer segments are partially protected. In parallel, we reported that a specific MMP9 and MMP2 inhibitor, SB-3CT, interferes with mechanisms leading to rod photoreceptor cell death in an MMP9 dependent manner. Here, we extend our initial rat studies to examine the potential of TIMP1 as a treatment in retinal degeneration by investigating neuroprotective effects in a classic mouse retinal degeneration model, rdPde6b-/- (rd1). The results clearly demonstrate that intravitreal injections of TIMP1 produce extended protection to delay rod photoreceptor cell death. The mean total number of rods in whole-mount retinas was significantly greater in TIMP-treated rd1 retinas (postnatal (P) 30, P35 (P<0.0001) and P45 (P<0.05) than in saline-treated rd1 retinas. In contrast, SB-3CT did not delay rod cell death, leading us to further investigate alternative pathways that do not involve MMPs. In addition to inducing phosphorylated ERK1/2, TIMP1 significantly reduces BAX activity and delays attenuation of the outer nuclear layer (ONL). Physiological responses using scotopic electroretinograms (ERG) reveal b-wave amplitudes from TIMP1-treated retinas are significantly greater than from saline-treated rd1 retinas (P<0.05). In later degenerative stages of rd1 retinas, photopic b-wave amplitudes from TIMP1-treated rd1 retinas are significantly larger than from saline-treated rd1 retinas (P<0.05). Our findings demonstrate that TIMP1 delays photoreceptor cell death. Furthermore, this study provides new insights into how TIMP1 works in the mouse animal model of RP.

The Role of the Microglial Cx3cr1 Pathway in the Postnatal Maturation of Retinal Photoreceptors

Microglia are the resident immune cells of the CNS, and their response to infection, injury and disease is well documented. More recently, microglia have been shown to play a role in normal CNS development, with the fractalkine-Cx3cr1 signaling pathway of particular importance. This work describes the interaction between the light-sensitive photoreceptors and microglia during eye opening, a time of postnatal photoreceptor maturation. Genetic removal of Cx3cr1 (Cx3cr1^GFP/GFP ) led to an early retinal dysfunction soon after eye opening [postnatal day 17 (P17)] and cone photoreceptor loss (P30 onward) in mice of either sex. This dysfunction occurred at a time when fractalkine expression was predominantly outer retinal, when there was an increased microglial presence near the photoreceptor layer and increased microglial-cone photoreceptor contacts. Photoreceptor maturation and outer segment elongation was coincident with increased opsin photopigment expression in wild-type retina, while this was aberrant in the Cx3cr1^GFP/GFP retina and outer segment length was reduced. A beadchip array highlighted Cx3cr1 regulation of genes involved in the photoreceptor cilium, a key structure that is important for outer segment elongation. This was confirmed with quantitative PCR with specific cilium-related genes, Rpgr and Rpgrip1, downregulated at eye opening (P14). While the overall cilium structure was unaffected, expression of Rpgr, Rpgrip1, and centrin were restricted to more proximal regions of the transitional zone. This study highlighted a novel role for microglia in postnatal neuronal development within the retina, with loss of fractalkine-Cx3cr1 signaling leading to an altered distribution of cilium proteins, failure of outer segment elongation and ultimately cone photoreceptor loss.SIGNIFICANCE STATEMENT Microglia are involved in CNS development and disease. This work highlights the role of microglia in postnatal development of the light-detecting photoreceptor neurons within the mouse retina. Loss of the microglial Cx3cr1 signaling pathway resulted in specific alterations in the cilium, a key structure in photoreceptor outer segment elongation. The distribution of key components of the cilium transitional zone, Rpgr, Rpgrip1, and centrin, were altered in retinae lacking Cx3cr1 with reduced outer segment length and cone photoreceptor death observed at later postnatal ages. This work identifies a novel role for microglia in the postnatal maturation of retinal photoreceptors.

Activation of liver X receptor delayed the retinal degeneration of rd1 mice through modulation of the immunological function of glia

Retinal degeneration (RD), including retinitis pigmentosa (RP), is an inherited eye disease characterized by progressive degeneration of photoreceptors. Recently, immune cells, including microglia, Müller cells and astrocytes, in degenerative retina are demonstrated to play key roles in the development of RD and can be used as potential therapeutic targets. Liver X receptors (LXRs) are important immuno-inflammatory response transcription factors that have been reported to be a new potential therapeutic drug target for neurodegenerative diseases. However, the potential therapeutic utility of LXRs for RP has not been evaluated. In the present study, Pde6β (rd1) mice received intraperitoneal injections of T0901317 (T0, 50 mg/kg/d) or vehicle (2% DMSO) for 7 days with age-matched C57/BL6 mice as controls. The effect of T0 was examined by quantitating photoreceptor apoptosis, microglial density and the expression of inflammatory mediators; the underlying mechanisms were then explored with a microarray assay. T0 markedly delayed apoptosis of the photoreceptors, partially through suppressing the activation of microglia and the gliosis of Müller cells, and decreased the expression levels of IL-6, iNOS, COX-2 and ENG in rd1 mice; as a result, the visual function of T0-treated rd1 mice measured with electroretinograms (ERG) was preserved for a longer time than that of vehicle-treated rd1 mice. The microarray assay showed that the Janus kinase/Signal Transducer and Activator of Transcription (JAK-STAT) signaling pathway was significantly affected in the retina of rd1 mice with T0 treatment. Our data suggested that T0 modulated the immunologic function of glia cells in the degenerative retina through the JAK3/STAT pathway and delayed the apoptosis of photoreceptors.

DSCAM-mediated control of dendritic and axonal arbor outgrowth enforces tiling and inhibits synaptic plasticity

Mature mammalian neurons have a limited ability to extend neurites and make new synaptic connections, but the mechanisms that inhibit such plasticity remain poorly understood. Here, we report that OFF-type retinal bipolar cells in mice are an exception to this rule, as they form new anatomical connections within their tiled dendritic fields well after retinal maturity. The Down syndrome cell-adhesion molecule (Dscam) confines these anatomical rearrangements within the normal tiled fields, as conditional deletion of the gene permits extension of dendrite and axon arbors beyond these borders. Dscam deletion in the mature retina results in expanded dendritic fields and increased cone photoreceptor contacts, demonstrating that DSCAM actively inhibits circuit-level plasticity. Electrophysiological recordings from Dscam-/- OFF bipolar cells showed enlarged visual receptive fields, demonstrating that expanded dendritic territories comprise functional synapses. Our results identify cell-adhesion molecule-mediated inhibition as a regulator of circuit-level neuronal plasticity in the adult retina.

The formation of the light-sensing compartment of cone photoreceptors coincides with a transcriptional switch

High-resolution daylight vision is mediated by cone photoreceptors. The molecular program responsible for the formation of their light sensor, the outer segment, is not well understood. We correlated daily changes in ultrastructure and gene expression in postmitotic mouse cones, between birth and eye opening, using serial block-face electron microscopy (EM) and RNA sequencing. Outer segments appeared rapidly at postnatal day six and their appearance coincided with a switch in gene expression. The switch affected over 14% of all expressed genes. Genes that switched off were rich in transcription factors and neurogenic genes. Those that switched on contained genes relevant for cone function. Chromatin rearrangements in enhancer regions occurred before the switch was completed, but not after. We provide a resource comprised of correlated EM, RNAseq, and ATACseq data, showing that the growth of a key compartment of a postmitotic cell involves an extensive switch in gene expression and chromatin accessibility.

Cerebral malaria induces electrophysiological and neurochemical impairment in mice retinal tissue: possible effect on glutathione and glutamatergic system

Background

Cerebral malaria (CM) is a severe complication resulting from Plasmodium falciparum infection. This condition has usually been associated with cognitive, behavioural and motor dysfunctions, being the retinopathy the most serious consequence resulting from the disease. The pathophysiological mechanisms underlying this complication remain incompletely understood. Several experimental models of CM have already been developed in order to clarify those mechanisms related to this syndrome. In this context, the present work has been performed to investigate which possible electrophysiological and neurochemistry alterations could be involved in the CM pathology.

Methods

Experimental CM was induced in Plasmodium berghei-infected male and female C57Bl/6 mice. The survival and neurological symptoms of CM were registered. Brains and retina were assayed for TNF levels and NOS2 expression. Electroretinography measurements were recorded to assessed a- and b-wave amplitudes and neurochemicals changes were evaluated by determination of glutamate and glutathione levels by HPLC.

Results

Susceptible C57Bl/6 mice infected with ≈ 10⁶ parasitized red blood cells (P. berghei ANKA strain), showed a low parasitaemia, with evident clinical signs as: respiratory failure, ataxia, hemiplegia, and coma followed by animal death. In parallel to the clinical characterization of CM, the retinal electrophysiological analysis showed an intense decrease of a- and-b-wave amplitude associated to cone photoreceptor response only at the 7 days post-infection. Neurochemical results demonstrated that the disease led to a decrease in the glutathione levels with 2 days post inoculation. It was also demonstrated that the increase in the glutathione levels during the infection was followed by the increase in the ³H-glutamate uptake rate (4 and 7 days post-infection), suggesting that CM condition causes an up-regulation of the transporters systems. Furthermore, these findings also highlighted that the electrophysiological and neurochemical alterations occurs in a manner independent on the establishment of an inflammatory response, once tumour necrosis factor levels and inducible nitric oxide synthase expression were altered only in the cerebral tissue but not in the retina.

Conclusions

In summary, these findings indicate for the first time that CM induces neurochemical and electrophysiological impairment in the mice retinal tissue, in a TNF-independent manner.

Digoxin-induced retinal degeneration depends on rhodopsin

Na,K-ATPases are energy consuming ion pumps that are required for maintaining ion homeostasis in most cells. In the retina, Na,K-ATPases are especially important to sustain the dark current in photoreceptor cells needed for rapid hyperpolarization of rods and cones in light. Cardiac glycosides like digoxin inhibit the activity of Na,K-ATPases by targeting their catalytic alpha subunits. This leads to a disturbed ion balance, which can affect cellular function and survival. Here we show that the treatment of wild-type mice with digoxin leads to severe retinal degeneration and loss of vision. Digoxin induced cell death specifically in photoreceptor cells with no or only minor effects in other retinal cell types. Photoreceptor-specific cytotoxicity depended on the presence of bleachable rhodopsin. Photoreceptors of Rpe65 knockouts, which have no measurable rhodopsin and photoreceptors of Rpe65^R91W mice that have <10% of the rhodopsin found in retinas of wild-type mice were not sensitive to digoxin treatment. Similarly, cones in the all-cone retina of Nrl knockout mice were also not affected. Digoxin induced expression of several genes involved in stress signaling and inflammation. It also activated proteins such as ERK1/2, AKT, STAT1, STAT3 and CASP1 during a period of up to 10 days after treatment. Activation of signaling genes and proteins, as well as the dependency on bleachable rhodopsin resembles mechanisms of light-induced photoreceptor degeneration. Digoxin-mediated photoreceptor cell death may thus be used as an inducible model system to study molecular mechanisms of retinal degeneration.

Calcium dynamics change in degenerating cone photoreceptors

Cone photoreceptors (cones) are essential for high-resolution daylight vision and colour perception. Loss of cones in hereditary retinal diseases has a dramatic impact on human vision. The mechanisms underlying cone death are poorly understood, and consequently, there are no treatments available. Previous studies suggest a central role for calcium (Ca²⁺) homeostasis deficits in photoreceptor degeneration; however, direct evidence for this is scarce and physiological measurements of Ca²⁺in degenerating mammalian cones are lacking.Here, we took advantage of the transgenic HR2.1:TN-XL mouse line that expresses a genetically encoded Ca²⁺biosensor exclusively in cones. We cross-bred this line with mouse models for primary ("cone photoreceptor function loss-1", cpfl1) and secondary ("retinal degeneration-1", rd1) cone degeneration, respectively, and assessed resting Ca²⁺levels and light-evoked Ca²⁺responses in cones using two-photon imaging. We found that Ca²⁺dynamics were altered in cpfl1 cones, showing higher noise and variable Ca²⁺levels, with significantly wider distribution than for wild-type and rd1 cones. Unexpectedly, up to 21% of cpfl1 cones still displayed light-evoked Ca²⁺responses, which were larger and slower than wild-type responses. In contrast, genetically intact rd1 cones were characterized by lower noise and complete lack of visual function.Our study demonstrates alterations in cone Ca²⁺dynamics in both primary and secondary cone degeneration. Our results are consistent with the view that higher (fluctuating) cone Ca²⁺levels are involved in photoreceptor cell death in primary (cpfl1) but not in secondary (rd1) cone degeneration. These findings may guide the future development of therapies targeting photoreceptor Ca²⁺homeostasis.

Macromolecular markers in normal human retina and applications to human retinal disease

Macromolecular cell markers are essential for the classification and characterization of the highly complex and cellularly diverse vertebrate retina. Although a plethora of markers are described in the current literature, the immunoreactivity of these markers in normal human tissue has not been fully determined. This is problematic as they are quintessential to the characterization of morphological changes associated with human retinal disease. This review provides an overview of the macromolecular markers currently available to assess human retinal cell types. We draw on immunohistochemical studies conducted in our laboratories to describe marker immunoreactivity in human retina alongside comparative descriptions in non-human tissues. Considering the growing number of eye banks services offering healthy and diseased human retinal tissue, this review provides a point of reference for future human retina studies and highlights key species specific disease applications of some macromolecular markers.

Using the rd1 mouse to understand functional and anatomical retinal remodelling and treatment implications in retinitis pigmentosa: A review

Retinitis Pigmentosa (RP) reflects a range of inherited retinal disorders which involve photoreceptor degeneration and retinal pigmented epithelium dysfunction. Despite the multitude of genetic mutations being associated with the RP phenotype, the clinical and functional manifestations of the disease remain the same: nyctalopia, visual field constriction (tunnel vision), photopsias and pigment proliferation. In this review, we describe the typical clinical phenotype of human RP and review the anatomical and functional remodelling which occurs in RP determined from studies in the rd/rd (rd1) mouse. We also review studies that report a slowing down or show an acceleration of retinal degeneration and finally we provide insights on the impact retinal remodelling may have in vision restoration strategies.

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.

Reduced photoreceptor death and improved retinal function during retinal degeneration in mice lacking innate immunity adaptor protein MyD88

The injury inflammatory response mediated by the innate immune system is an important contributor to neurodegeneration in the central nervous system (CNS) and retina. A major branch of the innate immune system is regulated by the Toll-like receptors (TLRs), which are receptors for endogenous damage associated molecules released from injured cells as well as pathogen-derived molecules, and interleukin-1 receptors (IL-1R), which are activated by IL-1α, IL-1β and IL-18 cytokines. TLRs and IL-1R are expressed on immune and non-immune cell types and act as first responders to cell damage, which results in tissue repair, or inflammation and apoptosis. Both TLR and IL-1R require the adaptor protein myeloid differentiation primary response gene 88 (MyD88) for signaling. Although inflammation is implicated in neuronal death in the retina, the role of MyD88-dependent TLR and IL-1R signaling in retinal degeneration is unknown. Therefore, the purpose of this study was to investigate the role of MyD88-mediated signaling in neuronal degeneration in the retinal degeneration 1 (rd1) mouse model, which exhibits a phenotype of rapid photoreceptor death and inflammation. To generate rd1 mice lacking the MyD88 gene, rd1 were bred with MyD88 knockout mice (MyD88(-/-)) for several generations to produce rd1/MyD88(+/+) and rd1/MyD88(-/-) genotypes. Chemokine mRNA expression levels were analyzed by qRT-PCR, and recruitment of activated microglia was quantified by immunodetection of the IBA-1 protein. Retinal outer nuclear layer cell counts were performed to quantify photoreceptor degeneration, and retinal function was assessed using electroretinograms (ERG). Our results revealed that retinal expression of Ccl2, Ccl4, Ccl7 and Cxcl10 was reduced by 2 to 8-fold in rd1/MyD88(-/-) mice compared with rd1/MyD88(+/+) mice (p<0.05), which coincided with attenuated microglial activation, higher numbers of photoreceptors and higher retina responses to photopic and scotopic stimuli. At later ages, rd1/MyD88(-/-) had reduced chemokine expression and higher photopic responses but no change in microglial recruitment compared with rd1 mice with functional MyD88. In conclusion, lack of MyD88-mediated signaling increased photoreceptor survival and retina function in rd1 mice, which implicates MyD88-mediated innate immunity pathways as an important pathogenic factor during retinal degeneration.

Vinpocetine modulates metabolic activity and function during retinal ischemia

Vinpocetine protects against a range of degenerative conditions and insults of the central nervous system via multiple modes of action. Little is known, however, of its effects on metabolism. This may be highly relevant, as vinpocetine is highly protective against ischemia, a process that inhibits normal metabolic function. This study uses the ischemic retina as a model to characterize vinpocetine's effects on metabolism. Vinpocetine reduced the metabolic demand of the retina following ex vivo hypoxia and ischemia to normal levels based on lactate dehydrogenase activity. Vinpocetine delivered similar effects in an in vivo model of retinal ischemia-reperfusion, possibly through increasing glucose availability. Vinpocetine's effects on glucose also appeared to improve glutamate homeostasis in ischemic Müller cells. Other actions of vinpocetine following ischemia-reperfusion, such as reduced cell death and improved retinal function, were possibly a combination of the drug's actions on metabolism and other retinal pathways. Vinpocetine's metabolic effects appeared independent of its other known actions in ischemia, as it recovered retinal function in a separate metabolic model where the glutamate-to-glutamine metabolic pathway was inhibited in Müller cells. The results of this study indicate that vinpocetine mediates ischemic damage partly through altered metabolism and has potential beneficial effects as a treatment for ischemia of neuronal tissues.

The effects of iodoacetic acid on the mouse retina

BACKGROUND

To characterize the effects of intravitreal injections of iodoacetic acid (IAA) in comparison to its systemic application as a measure to induce unilateral photoreceptor degeneration.

METHODS

Seven-week-old C57BL/6 J mice received either intravitreal injections of IAA or systemic treatment (intraperitoneal vs intravenous) and were observed in the following 5 weeks using ERG, OCT, and histology.

RESULTS

Systemic treatment with IAA induced high toxic effects and a high mortality in contrast to the intravitreal injection. Intraperitoneal application had no effect on the retina. Intravenous application of 2 × 30 mg/kg BW IAA (time between injections 3.5 h) resulted in an extinction of the ERG and a thinning of the retina, in particular of the outer nuclear layer (ONL) indicating photoreceptor degeneration. Animals receiving intravitreal injections developed cataracts already at low concentrations (up to 100% at 0.25 mg/kg BW). Higher intravitreal IAA doses led to extinguished ERGs. In histology, a thinning of the entire retina was observed that was most prominent in the inner part of the retina.

CONCLUSIONS

In contrast to intraperitoneal administration, intravenous application of IAA led to a selective photoreceptor degeneration. After intravitreal injection, dense cataracts were already observed at concentrations lower than those needed to induce changes in the ERG. ERG results must be interpreted carefully. A thinning of all retinal layers rather than a specific outer retinal degeneration was observed upon intravitreal injection. IAA is not a useful model to induce outer retinal degeneration in mice.

Cone photoreceptors develop normally in the absence of functional rod photoreceptors in a transgenic swine model of retinitis pigmentosa

PURPOSE

Human and swine retinas have morphological and functional similarities. In the absence of primate models, the swine is an attractive model to study retinal function and disease, with its cone-rich visual streak, our ability to manipulate their genome, and the differences in susceptibility of rod and cone photoreceptors to disease. We characterized the normal development of cone function and its subsequent decline in a P23H rhodopsin transgenic (TgP23H) miniswine model of autosomal dominant RP.

METHODS

Semen from TgP23H miniswine 53-1 inseminated domestic swine and produced TgP23H and Wt hybrid littermates. Retinal function was evaluated using ERGs between postnatal days (P) 14 and 120. Retinal ganglion cell (RGC) responses were recorded to full-field stimuli at several intensities. Retinal morphology was assessed using light and electron microscopy.

RESULTS

Scotopic retinal function matures in Wt pigs up to P60, but never develops in TgP23H pigs. Wt and TgP23H photopic vision matures similarly up to P30 and diverges at P60 where TgP23H cone vision declines. There are fewer TgP23H RGCs with visually evoked responses at all ages and their response to light is compromised. Photoreceptor morphological changes mirror these functional changes.

CONCLUSIONS

Lack of early scotopic function in TgP23H swine suggests it as a model of an aggressive form of RP. In this mammalian model of RP, normal cone function develops independent of rod function. Therefore, its retina represents a system in which therapies to rescue cones can be developed to prolong photopic visual function in RP patients.

Altered miRNA expression in canine retinas during normal development and in models of retinal degeneration

Background

Although more than 246 loci/genes are associated with inherited retinal diseases, the mechanistic events that link genetic mutations to photoreceptor cell death are poorly understood. miRNAs play a relevant role during retinal development and disease. Thus, as a first step in characterizing miRNA involvement during disease expression and progression, we examined miRNAs expression changes in normal retinal development and in four canine models of retinal degenerative disease.

Results

The initial microarray analysis showed that 50 miRNAs were differentially expressed (DE) early (3 vs. 7 wks) in normal retina development, while only 2 were DE between 7 and 16 wks, when the dog retina is fully mature. miRNA expression profiles were similar between dogs affected with xlpra2, an early-onset retinal disease caused by a microdeletion in RPGRORF15, and normal dogs early in development (3 wks) and at the peak of photoreceptor death (7 wks), when only 2 miRNAs were DE. However, the expression varied much more markedly during the chronic cell death stage at 16 wks (118 up-/55 down-regulated miRNAs). Functional analyses indicated that these DE miRNAs are associated with an increased inflammatory response, as well as cell death/survival. qRT-PCR of selected apoptosis-related miRNAs (“apoptomirs”) confirmed the microarray results in xlpra2, and extended the analysis to the early-onset retinal diseases rcd1 (PDE6B-mutation) and erd (STK38L-mutation), as well as the slowly progressing prcd (PRCD-mutation). The results showed up-regulation of anti-apoptotic (miR-9, -19a, -20, -21, -29b, -146a, -155, -221) and down-regulation of pro-apoptotic (miR-122, -129) apoptomirs in the early-onset diseases and, with few exceptions, also in the prcd-mutants.

Conclusions

Our results suggest that apoptomirs might be expressed by diseased retinas in an attempt to counteract the degenerative process. The pattern of expression in diseased retinas mirrored the morphology and cell death kinetics previously described for these diseases. This study suggests that common miRNA regulatory mechanisms may be involved in retinal degeneration processes and provides attractive opportunities for the development of novel miRNA-based therapies to delay the progression of the degenerative process.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-172) contains supplementary material, which is available to authorized users.

Dark rearing alters the normal development of spatiotemporal response properties but not of contrast detection threshold in mouse retinal ganglion cells

The mouse visual system is immature when the eyes open two weeks after birth. As in other mammals, some of the maturation that occurs in the subsequent weeks is known to depend on visual experience. Development of the retina, which as the first stage of vision provides the visual information to the brain, also depends on light-driven activity for proper development but has been less well studied than visual cortical development. The critical properties for retinal encoding of images include detection of contrast and responsiveness to the broad range of temporal stimulus frequencies present in natural stimuli. Here we show that contrast detection threshold and temporal frequency response characteristics of ON and OFF retinal ganglion cells (RGCs), which are poor at eye opening, subsequently undergo maturation, improving RGC performance. Further, we find that depriving mice of visual experience from before birth by rearing them in the dark causes ON and OFF RGCs to have smaller receptive field centers but does not affect their contrast detection threshold development. The modest developmental increase in temporal frequency responsiveness of RGCs in mice reared on a normal light cycle was inhibited by dark rearing only in ON but not OFF RGCs. Thus, these RGC response characteristics are in many ways unaffected by the experience-dependent changes to synaptic and spontaneous activity known to occur in the mouse retina in the two weeks after eye opening, but specific differences are apparent in the ON vs. OFF RGC populations.

Correlations between ERG, OCT, and Anatomical Findings in the rd10 Mouse

Background. To evaluate the correlation between ERG, OCT, and microscopic findings in the rd10 mouse. Methods. C57BL/6J wild type mice and rd10 mice were compared at the age of 2, 3, 5, 7, 9, 12, 24, and 48 weeks (each age group n = 3) using full-field electroretinography (ERG), spectral domain Optical Coherence Tomography (sd-OCT), fluorescein angiography (FA), Hematoxylin & Eosin histology (HE), and immunohistology (IH). Results. While in wild type mice, the amplitude of a- and b-wave increased with light intensity and with the age of the animals, the rd10 mice showed extinction of the ERG beginning with the age of 5 weeks. In OCT recordings, the thickness of the retina decreased up to 9 weeks of age, mainly based on the degradation of the outer nuclear layer (ONL). Afterwards, the ONL was no longer visible in the OCT. HE staining and immunohistological findings confirmed the in vivo data. Conclusion. ERG and OCT are useful methods to evaluate the retinal function and structure in vivo. The retinal changes seen in the OCT closely match those observed in histological staining.

Abnormal glutamate metabolism in the retina of aquaporin 4 (AQP4) knockout mice upon light damage

Glutamate is a major excitatory neurotransmitter in the retina. Glutamate neurotoxicity has been implicated in the pathogenesis of several ocular diseases. Aquaporin 4 (AQP4) is a water-selective membrane transport protein, and its knockout could alter retinal neuron excitability. However, the effect of AQP4 knockout on glutamate metabolism is still unclear in the retina. Here, we reported that the retinas in AQP4 knockout mice showed higher glutamate levels than that in wild-type mice upon light damage. AQP4 knockout could result in accelerated apoptosis of retinal cells, increased reactive gliosis, and attenuated survival of RGCs in response to light damage. Moreover, AQP4 knockout could affect the expression pattern of glutamate metabolism-related proteins such as GLAST and GS. Taken together, this study revealed a novel role of AQP4 in regulating glutamate metabolism. Pharmacological manipulation of AQP4 function may represent as a potent therapeutic target in the treatment of neurological ocular disorders.

Amino acid signatures in the developing mouse retina

This study characterizes the developmental patterns of seven key amino acids: glutamate, γ-amino-butyric acid (GABA), glycine, glutamine, aspartate, alanine and taurine in the mouse retina. We analyze amino acids in specific bipolar, amacrine and ganglion cell sub-populations (i.e. GABAergic vs. glycinergic amacrine cells) and anatomically distinct regions of photoreceptors and Müller cells (i.e. cell bodies vs. endfeet) by extracting data from previously described pattern recognition analysis. Pattern recognition statistically classifies all cells in the retina based on their neurochemical profile and surpasses the previous limitations of anatomical and morphological identification of cells in the immature retina. We found that the GABA and glycine cellular content reached adult-like levels in most neurons before glutamate. The metabolic amino acids glutamine, aspartate and alanine also reached maturity in most retinal cells before eye opening. When the overall amino acid profiles were considered for each cell group, ganglion cells and GABAergic amacrine cells matured first, followed by glycinergic amacrine cells and finally bipolar cells. Photoreceptor cell bodies reached adult-like amino acid profiles at P7 whilst Müller cells acquired typical amino acid profiles in their cell bodies at P7 and in their endfeet by P14. We further compared the amino acid profiles of the C57Bl/6J mouse with the transgenic X-inactivation mouse carrying the lacZ gene on the X chromosome and validated this animal model for the study of normal retinal development. This study provides valuable insight into normal retinal neurochemical maturation and metabolism and benchmark amino acid values for comparison with retinal disease, particularly those which occur during development.

Mapping cation entry in photoreceptors and inner retinal neurons during early degeneration in the P23H-3 rat retina

The proline-23-histidine line 3 (P23H-3) transgenic rat carries a human opsin gene mutation leading to progressive photoreceptor loss characteristic of human autosomal dominant retinitis pigmentosa. The aim of the present study was to evaluate neurochemical modifications in the P23H-3 retina as a function of development and degeneration. Specifically, we investigated the ion channel permeability of photoreceptors by tracking an organic cation, agmatine (1-amino-4-guanidobutane, AGB), which permeates through nonspecific cation channels. We also investigated the activity of ionotropic glutamate receptors in distinct populations of bipolar, amacrine, and ganglion cells using AGB tracking in combination with macromolecular markers. We found elevated cation channel permeation in photoreceptors as early as postnatal day 12 (P12) suggesting that AGB labeling is an early indicator of impending photoreceptor degeneration. However, bipolar, amacrine, or ganglion cells displayed normal responses secondary to ionotropic glutamate receptor activation even at P138 when about one half of the photoreceptor layer was lost and apoptosis and gliosis were observed. These results suggest that possible therapeutic windows as downstream neurons in inner retina appear to retain normal function with regard to AGB permeation when photoreceptors are significantly reduced but not lost.