Microglia Activation and Inflammation During the Death of Mammalian Photoreceptors

Müller cells are activated in response to retinal outer nuclear layer degeneration in rats subjected to simulated weightlessness conditions

JOURNAL/nrgr/04.03/01300535-202507000-00032/figure1/v/2024-09-09T124005Z/r/image-tiff A microgravity environment has been shown to cause ocular damage and affect visual acuity, but the underlying mechanisms remain unclear. Therefore, we established an animal model of weightlessness via tail suspension to examine the pathological changes and molecular mechanisms of retinal damage under microgravity. After 4 weeks of tail suspension, there were no notable alterations in retinal function and morphology, while after 8 weeks of tail suspension, significant reductions in retinal function were observed, and the outer nuclear layer was thinner, with abundant apoptotic cells. To investigate the mechanism underlying the degenerative changes that occurred in the outer nuclear layer of the retina, proteomics was used to analyze differentially expressed proteins in rat retinas after 8 weeks of tail suspension. The results showed that the expression levels of fibroblast growth factor 2 (also known as basic fibroblast growth factor) and glial fibrillary acidic protein, which are closely related to Müller cell activation, were significantly upregulated. In addition, Müller cell regeneration and Müller cell gliosis were observed after 4 and 8 weeks, respectively, of simulated weightlessness. These findings indicate that Müller cells play an important regulatory role in retinal outer nuclear layer degeneration during weightlessness.

Age-related driving mechanisms of retinal diseases and neuroprotection by transcription factor EB-targeted therapy

Retinal aging has been recognized as a significant risk factor for various retinal disorders, including diabetic retinopathy, age-related macular degeneration, and glaucoma, following a growing understanding of the molecular underpinnings of their development. This comprehensive review explores the mechanisms of retinal aging and investigates potential neuroprotective approaches, focusing on the activation of transcription factor EB. Recent meta-analyses have demonstrated promising outcomes of transcription factor EB-targeted strategies, such as exercise, calorie restriction, rapamycin, and metformin, in patients and animal models of these common retinal diseases. The review critically assesses the role of transcription factor EB in retinal biology during aging, its neuroprotective effects, and its therapeutic potential for retinal disorders. The impact of transcription factor EB on retinal aging is cell-specific, influencing metabolic reprogramming and energy homeostasis in retinal neurons through the regulation of mitochondrial quality control and nutrient-sensing pathways. In vascular endothelial cells, transcription factor EB controls important processes, including endothelial cell proliferation, endothelial tube formation, and nitric oxide levels, thereby influencing the inner blood-retinal barrier, angiogenesis, and retinal microvasculature. Additionally, transcription factor EB affects vascular smooth muscle cells, inhibiting vascular calcification and atherogenesis. In retinal pigment epithelial cells, transcription factor EB modulates functions such as autophagy, lysosomal dynamics, and clearance of the aging pigment lipofuscin, thereby promoting photoreceptor survival and regulating vascular endothelial growth factor A expression involved in neovascularization. These cell-specific functions of transcription factor EB significantly impact retinal aging mechanisms encompassing proteostasis, neuronal synapse plasticity, energy metabolism, microvasculature, and inflammation, ultimately offering protection against retinal aging and diseases. The review emphasizes transcription factor EB as a potential therapeutic target for retinal diseases. Therefore, it is imperative to obtain well-controlled direct experimental evidence to confirm the efficacy of transcription factor EB modulation in retinal diseases while minimizing its risk of adverse effects.

The unanticipated contribution of Zap70 in retinal degeneration: implications for microglial inflammatory activation

Inflammation is a major mechanism of photoreceptor cell death in the retina during macular degeneration leading to the blindness. In this study, we investigated the role of the kinase molecule Zap70, which is an inflammatory regulator of the systemic immune system, to elucidate the control mechanism of inflammation in the retina. We observed activated microglial cells migrated and populated the retinal layer following blue LED-induced photoreceptor degeneration and activated microglial cells in the LED-injured retina expressed Zap70, unlike the inactive microglial cells in the normal retina. Visual function was considerably decreased in blue-LED light-exposed mice, and animals with Zap70 mutations were adversely affected. Furthermore, extensive photoreceptor cell death was observed in the SKG mice, bearing a Zap70 mutation that induces autoimmune disease. In the blue-LED light-exposed groups, SKG retinas had significantly higher levels of inflammatory cytokines than those in wild-type mice. Furthermore, regulating Zap70 activity has a significant influence on microglial inflammatory state. We discovered that active microglial cells expressing Zap70 could modify vascular endothelial growth factor A (Vegfa) signaling in primary retinal pigment epithelial (RPE) cells. Our novel study revealed that the production of Zap70 by retinal microglial cells is responsible for inflammatory signals that promote apoptosis in photoreceptor cells. Furthermore, Zap70-positive microglial cells were capable of regulating Vegfa signaling in RPE cells, which matches the hallmark of macular degeneration. Overall, we discovered Zap70's inflammatory activity in the retina, which is necessary for upregulating multiple inflammatory cytokines and cell death. Zap70 represents a novel therapeutic target for treating retinal degeneration.

A comprehensive review of experimental models for investigating blue light-induced ocular damage: Insights into parameters, limitations, and new opportunities

Light-emitting diodes (LEDs) are widely used in modern lighting and electronic devices, including smartphones, computer monitors, tablets, televisions, and vehicle lights. Blue light (BL) hazards to eye health have received increasing attention because white LED bulbs emit higher levels of BL than traditional lighting sources. At wavelengths of 400-500 nm, BL is characterized by its high energy and risks associated with prolonged exposure, which may lead to photochemical damage and morphological alterations in the retina. Recent research has revealed that the harmful effects of BL are intricately linked to light intensity and exposure frequency, with mechanisms involving the overproduction of reactive oxygen species through photooxidative processes. This growing body of knowledge deepens our understanding of photodamage and opens avenues for exploring protective strategies for our eyes. Although current clinical trials assessing the safety of BL exposure remain limited, the development of experimental models that mimic physiological conditions has revealed BL toxicity. This review categorizes and evaluates BL-induced retinopathy in vivo, providing a comprehensive overview of the associated experimental parameters, including photosensitive fluorophores, light wavelength, illuminance, irradiance, exposure duration, animal strains, and their unique lesion patterns. Moreover, this study underscores the need for further research to evaluate photoprotective agents, which may offer valuable insights to the ongoing discussion on preserving ocular health in our increasingly illuminated digital environments.

Protective effects of fatty acid amide hydrolase inhibition in UVB-activated microglia

Neuroinflammation is a hallmark of several neurodegenerative disorders that has been extensively studied in recent years. Microglia, the primary immune cells of the central nervous system (CNS), are key players in this physiological process, demonstrating a remarkable adaptability in responding to various stimuli in the eye and the brain. Within the complex network of neuroinflammatory signals, the fatty acid N-ethanolamines, in particular N-arachidonylethanolamine (anandamide, AEA), emerged as crucial regulators of microglial activity under both physiological and pathological states. In this study, we interrogated for the first time the impact of the signaling of these bioactive lipids on microglial cell responses to a sub-lethal acute UVB radiation, a physical stressor responsible of microglia reactivity in either the retina or the brain. To this end, we developed an in vitro model using mouse microglial BV-2 cells. Upon 24 h of UVB exposure, BV-2 cells showed elevated oxidative stress markers and, cyclooxygenase (COX-2) expression, enhanced phagocytic and chemotactic activities, along with an altered immune profiling. Notably, UVB exposure led to a selective increase in expression and activity of fatty acid amide hydrolase (FAAH), the main enzyme responsible for degradation of fatty acid ethanolamides. Pharmacological FAAH inhibition via URB597 counteracted the effects of UVB exposure, decreasing tumor necrosis factor α (TNF-α) and nitric oxide (NO) release and reverting reactive oxidative species (ROS), interleukin-1β (IL-1β), and interleukin-10 (IL-10) levels to the control levels. Our findings support the potential of enhanced fatty acid amide signaling in mitigating UVB-induced cellular damage, paving the way to further exploration of these lipids in light-induced immune responses.

Prohibitin 2 deficiency in photoreceptors leads to progressive retinal degeneration and facilitated Müller glia engulfing microglia debris

Müller glia and microglia are capable of phagocytosing fragments of retinal cells in response to retinal injury or degeneration. However, the direct evidence for their mutual interactions between Müller glia and microglia in the progression of retinal degeneration (RD) remains largely unclear. This study aims to construct a progressive RD mouse model and investigate the activated pattern of Müller glia and the interplay between Müller glia and microglia in the early stage or progression of RD. A Prohibitin 2 (Phb2) photoreceptor-specific knockout (RKO) mouse model was generated by crossing Phb2flox/flox mice with Rhodopsin-Cre mice. Optical Coherence Tomography (OCT), histological staining, and Electroretinography (ERG) assessed retinal structure and function, and RKO mice exhibited progressive RD from six weeks of age. In detail, six-week-old RKO mice showed no significant retinal impairment, but severe vision dysfunction and retina thinning were shown in ten-week-old RKO mice. Furthermore, RKO mice were sensitive to Light Damage (LD) and showed severe RD at an early age after light exposure. Bulk retina RNA-seq analysis from six-week-old control (Ctrl) and RKO mice showed reactive retinal glia in RKO mice. The activated pattern of Müller glia and the interplay between Müller glia and microglia was visualized by immunohistology and 3D reconstruction. In six-week-old RKO mice or light-exposed Ctrl mice, Müller glia were initially activated at the edge of the retina. Moreover, in ten-week-old RKO mice or light-exposed six-week-old RKO mice with severe photoreceptor degeneration, abundant Müller glia were activated across the whole retinas. With the progression of RD, phagocytosis of microglia debris by activated Müller glia were remarkably increased. Altogether, our study establishes a Phb2 photoreceptor-specific knockout mouse model, which is a novel mouse model of RD and can well demonstrate the phenotype of progressive RD. We also report that Müller glia in the peripheral retina is more sensitive to the early damage of photoreceptors. Our study provides more direct evidence for Müller glia engulfing microglia debris in the progression of RD due to photoreceptor Phb2 deficiency.

Photoreceptor loss does not recruit neutrophils despite strong microglial activation

In response to central nervous system (CNS) injury, tissue resident immune cells such as microglia and circulating systemic neutrophils are often first responders. The degree to which these cells interact in response to CNS damage is poorly understood, and even less so, in the neural retina which poses a challenge for high resolution imaging in vivo. In this study, we deploy fluorescence adaptive optics scanning light ophthalmoscopy (AOSLO) to study fluorescent microglia and neutrophils in mice. We simultaneously track immune cell dynamics using label-free phase-contrast AOSLO at micron-level resolution. Retinal lesions were induced with 488 nm light focused onto photoreceptor (PR) outer segments. These lesions focally ablated PRs, with minimal collateral damage to cells above and below the plane of focus. We used in vivo (AOSLO, SLO and OCT) imaging to reveal the natural history of the microglial and neutrophil response from minutes-to-months after injury. While microglia showed dynamic and progressive immune response with cells migrating into the injury locus within 1-day after injury, neutrophils were not recruited despite close proximity to vessels carrying neutrophils only microns away. Post-mortem confocal microscopy confirmed in vivo findings. This work illustrates that microglial activation does not recruit neutrophils in response to acute, focal loss of photoreceptors, a condition encountered in many retinal diseases.

A metabotropic glutamate receptor agonist enhances visual signal fidelity in a mouse model of retinitis pigmentosa

Many inherited retinal diseases target photoreceptors, which transduce light into a neural signal that is processed by the downstream visual system. As photoreceptors degenerate, physiological and morphological changes to retinal synapses and circuitry reduce sensitivity and increase noise, degrading visual signal fidelity. Here, we pharmacologically targeted the first synapse in the retina in an effort to reduce circuit noise without sacrificing visual sensitivity. We tested a strategy to partially replace the neurotransmitter lost when photoreceptors die with an agonist of receptors that ON bipolars cells use to detect glutamate released from photoreceptors. In rd10 mice, which express a photoreceptor mutation that causes retinitis pigmentosa (RP), we found that a low dose of the mGluR6 agonist l-2-amino-4-phosphonobutyric acid (L-AP4) reduced pathological noise induced by photoreceptor degeneration. After making in vivo electroretinogram recordings in rd10 mice to characterize the developmental time course of visual signal degeneration, we examined effects of L-AP4 on sensitivity and circuit noise by recording in vitro light-evoked responses from individual retinal ganglion cells (RGCs). L-AP4 decreased circuit noise evident in RGC recordings without significantly reducing response amplitudes, an effect that persisted over the entire time course of rod photoreceptor degeneration. Subsequent in vitro recordings from rod bipolar cells (RBCs) showed that RBCs are more depolarized in rd10 retinas, likely contributing to downstream circuit noise and reduced synaptic gain, both of which appear to be ameliorated by hyperpolarizing RBCs with L-AP4. These beneficial effects may reduce pathological circuit remodeling and preserve the efficacy of therapies designed to restore vision.

Natural compounds efficacy in Ophthalmic Diseases: A new twist impacting ferroptosis

Ferroptosis, a distinct form of cell death, is characterized by the iron-mediated oxidation of lipids and is finely controlled by multiple cellular metabolic pathways. These pathways encompass redox balance, iron regulation, mitochondrial function, as well as amino acid, lipid, and sugar metabolism. Additionally, various disease-related signaling pathways also play a role in the regulation of ferroptosis. In recent years, with the introduction of the concept of ferroptosis and the deepening of research on its mechanism, ferroptosis is closely related to various biological conditions of eye diseases, including eye organ development, aging, immunity, and cancer. This article reviews the development of the concept of ferroptosis, the mechanism of ferroptosis, and its latest research progress in ophthalmic diseases and reviews the research on ferroptosis in ocular diseases within the framework of metabolism, active oxygen biology, and iron biology. Key regulators and mechanisms of ferroptosis in ocular diseases introduce important concepts and major open questions in the field of ferroptosis and related natural compounds. It is hoped that in future research, further breakthroughs will be made in the regulation mechanism of ferroptosis and the use of ferroptosis to promote the treatment of eye diseases. At the same time, natural compounds may be the direction of new drug development for the potential treatment of ferroptosis in the future. Open up a new way for clinical ophthalmologists to research and prevent diseases.

Immunology of Retinitis Pigmentosa and Gene Therapy-Associated Uveitis

The underlying immune state of inherited retinal degenerations (IRDs) and retinitis pigmentosa (RP) has been an emerging area of interest, wherein the consequences have never been greater given the widespread recognition of gene therapy-associated uveitis (GTU) in gene therapy clinical trials. Whereas some evidence suggests that the adaptive immune system may play a role, the majority of studies indicate that the innate immune system is likely the primary driver of neuroinflammation in RP. During retinal degeneration, discrete mechanisms activate resident microglia and promote infiltrating macrophages that can either be protective or detrimental to photoreceptor cell death. This persistent stimulation of innate immunity, overlaid by the introduction of viral antigens as part of gene therapy, has the potential to trigger a complex microglia/macrophage-driven proinflammatory state. A better understanding of the immune pathophysiology in IRD and GTU will be necessary to improve the success of developing novel treatments for IRDs.

Microbiome and Inherited Retinal Degenerations

Inherited retinal degenerations (IRDs) represent a genetically and clinically heterogeneous group of progressive and visually debilitating disorders that can lead to irreversible visual loss. Our understanding of IRD pathogenesis at both the genetic and cellular levels has increased tremendously over the past two decades, but the exact pathogenic mechanisms remain incompletely understood. Enhanced understanding of the pathophysiology of these diseases can result in new treatment targets. Alterations in the human gut microbiome play a key role in the pathogenesis of many ocular and nonocular diseases, such as age-related macular degeneration, neurologic and metabolic disorders, and autoimmune conditions. The gut microbiome regulates the susceptibility of mice to develop experimental autoimmune uveitis, a model for autoimmune disease of the posterior portion of the eye elicited by the systemic response to retinal antigens. Because of the mounting evidence in favor of a role for local and systemic inflammatory and autoimmune-mediated components to IRD pathogenesis, this review presents the current knowledge of gut microbiome in IRDs and discusses the association between possible changes in gut microbiome and pathogenesis of these diseases, with special attention to their possible contribution to the inflammatory underpinnings of IRDs.

Mechanism of Cone Degeneration in Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a group of genetic disorders resulting in inherited blindness due to the degeneration of rod and cone photoreceptors. The various mechanisms underlying rod degeneration primarily rely on genetic mutations, leading to night blindness initially. Cones gradually degenerate after rods are almost eliminated, resulting in varying degrees of visual disability and blindness. The mechanism of cone degeneration remains unclear. An understanding of the mechanisms underlying cone degeneration in RP, a highly heterogeneous disease, is essential to develop novel treatments of RP. Herein, we review recent advancements in the five hypotheses of cone degeneration, including oxidative stress, trophic factors, metabolic stress, light damage, and inflammation activation. We also discuss the connection among these theories to provide a better understanding of secondary cone degeneration in RP. Five current mechanisms of cone degenerations in RP Interactions among different pathways are involved in RP.

Etiological Roles of p75NTR in a Mouse Model of Wet Age-Related Macular Degeneration

Choroidal neovascularization (CNV) is a pathological angiogenesis of the choroidal plexus of the retina and is a key feature in the wet form of age-related macular degeneration. Mononuclear phagocytic cells (MPCs) are known to accumulate in the subretinal space, generating a chronic inflammatory state that promotes the growth of the choroidal neovasculature. However, how the MPCs are recruited and activated to promote CNV pathology is not fully understood. Using genetic and pharmacological tools in a mouse model of laser-induced CNV, we demonstrate a role for the p75 neurotrophin receptor (p75^NTR) in the recruitment of MPCs, in glial activation, and in vascular alterations. After laser injury, expression of p75^NTR is increased in activated Muller glial cells near the CNV area in the retina and the retinal pigmented epithelium (RPE)-choroid. In p75^NTR knockout mice (p75^NTR KO) with CNV, there is significantly reduced recruitment of MPCs, reduced glial activation, reduced CNV area, and the retinal function is preserved, as compared to wild type mice with CNV. Notably, a single intravitreal injection of a pharmacological p75^NTR antagonist in wild type mice with CNV phenocopied the results of the p75^NTR KO mice. Our results demonstrate that p75^NTR is etiological in the development of CNV.

Structural and functional distinctions of co-resident microglia and monocyte-derived macrophages after retinal degeneration

BACKGROUND

Both resident microglia and invading peripheral immune cells can respond to injury and degeneration in the central nervous system. However, after dead and dying neurons have been cleared and homeostasis is re-established, it is unknown whether resident immune cells fully resume normal functions and to what degree the peripheral immune cells take up residence.

METHODS

Using flow cytometry, in vivo retinal imaging, immunohistochemistry, and single-cell mRNA sequencing, we assess resident microglia and monocyte-derived macrophages in the retina during and after the loss of photoreceptors in the Arr1^-/- mouse model of inducible degeneration.

RESULTS

We find that photoreceptor loss results in a small, sustained increase in mononuclear phagocytes and, after degeneration wanes, these cells re-establish a spatial mosaic reminiscent of healthy retinas. Transcriptomic analysis revealed the population remained unusually heterogeneous, with several subpopulations expressing gene patterns consistent with mildly activated phenotypes. Roughly a third of "new resident" cells expressed markers traditionally associated with both microglial and monocytic lineages, making their etiology ambiguous. Using an inducible Cre-based fluorescent lineage tracing paradigm to confirm the origins of new resident immune cells, we found approximately equal numbers of microglia and monocyte-derived macrophages after degeneration had subsided. In vivo retinal imaging and immunohistochemical analysis showed that both subpopulations remained functionally responsive to sites of local damage, though on average the monocyte-derived cells had less morphological complexity, expressed higher levels of MHCII, and had less migratory activity than the native resident population.

CONCLUSIONS

Monocytic cells that infiltrate the retina during degeneration differentiate into monocyte-derived macrophages that can remain in the retina long-term. These monocyte-derived macrophages adopt ramified morphologies and microglia-like gene expression. However, they remain distinguishable in morphology and gene expression from resident microglia and appear to differ functionally, showing less responsiveness to subsequent retinal injuries. These findings support the idea that persistent changes in the local immune population that occur in response to cell loss in aging and progressive retinal diseases may include the establishment of subpopulations of bone marrow-derived cells whose ability to respond to subsequent insults wanes over time.

XLRS Rat with Rs1-/Y Exon-1-Del Shows Failure of Early Postnatal Outer Retina Development

We generated a Long Evans transgenic rat with targeted deletion of the whole Rs1 exon-1 and evaluated the pathological retinal phenotype of this Rs1-/Y rat model of X-linked retinoschisis (XLRS). The Rs1-/Y rat exhibited very early onset and rapidly progressive photoreceptor degeneration. The outer limiting membrane (OLM) was disrupted and discontinuous by post-natal day (P15) and allowed photoreceptor nuclei to dislocate from the outer nuclear layers (ONL) into the sub-retinal side of the OLM. Dark-adapted electroretinogram (ERG) a-wave and b-wave amplitudes were considerably reduced to only 20-25% of WT by P17. Microglia and Müller glial showed cell marker activation by P7. Intravitreal application of AAV8-RS1 at P5-6 induced RS1 expression by P15 and rescued the inner nuclear layer (INL) and outer plexiform layer (OPL) cavity formation otherwise present at P15, and the outer-retinal structure was less disrupted. This Rs1-/Y exon-1-del rat model displays substantially faster rod cell loss compared to the exon-1-del Rs1-KO mouse. Most unexpected was the rapid appearance of schisis cavities between P7 and P15, and then cavities rapidly disappeared by P21/P30. The rat model provides clues on the molecular and cellular mechanisms underlying XLRS pathology in this model and points to a substantial and early changes to normal retinal development.

Cellular and molecular alterations in neurons and glial cells in inherited retinal degeneration

Multiple gene mutations have been associated with inherited retinal dystrophies (IRDs). Despite the spectrum of phenotypes caused by the distinct mutations, IRDs display common physiopathology features. Cell death is accompanied by inflammation and oxidative stress. The vertebrate retina has several attributes that make this tissue vulnerable to oxidative and nitrosative imbalance. The high energy demands and active metabolism in retinal cells, as well as their continuous exposure to high oxygen levels and light-induced stress, reveal the importance of tightly regulated homeostatic processes to maintain retinal function, which are compromised in pathological conditions. In addition, the subsequent microglial activation and gliosis, which triggers the secretion of pro-inflammatory cytokines, chemokines, trophic factors, and other molecules, further worsen the degenerative process. As the disease evolves, retinal cells change their morphology and function. In disease stages where photoreceptors are lost, the remaining neurons of the retina to preserve their function seek out for new synaptic partners, which leads to a cascade of morphological alterations in retinal cells that results in a complete remodeling of the tissue. In this review, we describe important molecular and morphological changes in retinal cells that occur in response to oxidative stress and the inflammatory processes underlying IRDs.

Complement C3a receptor inactivation attenuates retinal degeneration induced by oxidative damage

Retinal degeneration causes vision loss and threatens the health of elderly individuals worldwide. Evidence indicates that the activation of the complement system is associated with retinal degeneration. However, the mechanism of complement signaling in retinal degeneration needs to be further studied. In this study, we show that the expression of C3 and C3a receptor (C3ar1) is positively associated with the inflammatory response and retinal degeneration. Genetic deletion of C3 and pharmacological inhibition of C3ar1 resulted in the alleviation of neuroinflammation, prevention of photoreceptor cell apoptosis and restoration of visual function. RNA sequencing (RNA-seq) identified a C3ar1-dependent network shown to regulate microglial activation and astrocyte gliosis formation. Mechanistically, we found that STAT3 functioned downstream of the C3-C3ar1 pathway and that the C3ar1-STAT3 pathway functionally mediated the immune response and photoreceptor cell degeneration in response to oxidative stress. These findings reveal an important role of C3ar1 in oxidative-induced retinal degeneration and suggest that intervention of the C3ar1 pathway may alleviate retinal degeneration.

A Novel Tree Shrew Model of Chronic Experimental Autoimmune Uveitis and Its Disruptive Application

Background

Previous studies have established several animal models for experimental autoimmune uveitis (EAU) in rodents without the fovea centralis in the human retina. This study aimed to develop and explore the application of a novel EAU model in tree shrews with a cone-dominated retina resembling the human fovea.

Methods

Tree shrews were clinically and pathologically evaluated for the development and characteristics of EAU immunized with six inter-photoreceptor retinoid-binding proteins (IRBPs). IRBP-specific T-cell proliferation and serum cytokine of tree shrews were evaluated to determine the immune responses. Differentially expressed genes (DEGs) were identified in the eyes of tree shrews with EAU by RNA-sequencing. The disruptive effects of the DEG RGS4 inhibitor CCG 203769 and dihydroartemisinin on the EAU were investigated to evaluate the potential application of tree shrew EAU.

Results

IRBP_1197–1211 and R14 successfully induced chronic EAU with subretinal deposits and retinal damage in the tree shrews. The immunological characteristics presented the predominant infiltration of microglia/macrophages, dendritic cells, and CD4-T-cells into the uvea and retina and pathogenic T helper (Th) 1 and Th17 responses. The subretinal deposits positively expressed amyloid β-protein (Aβ), CD8, and P2Y purinoceptor 12 (P2RY12). The crucial DEGs in R14-induced EAU, such as P2RY2 and adenylate cyclase 4 (ADCY4), were enriched for several pathways, including inflammatory mediator regulation of transient receptor potential (TRP) channels. The upregulated RGS4 in IRBP-induced EAU was associated with mitogen-activated protein kinase (MAPK) activity. RGS4 inhibition and dihydroartemisinin could significantly alleviate the retinal pathological injuries of IRBP_1197-1211-induced EAU by decreasing the expression of CD4 T-cells.

Conclusion

Our study provides a novel chronic EAU in tree shrews elicited by bovine R14 and tree shrew IRBP_1197-1211 characterized by retinal degeneration, retinal damage with subretinal Aβ deposits and microglia/macrophage infiltration, and T-cell response, probably by altering important pathways and genes related to bacterial invasion, inflammatory pain, microglial phagocytosis, and lipid and glucose metabolism. The findings advance the knowledge of the pathogenesis and therapeutics of the fovea-involved visual disturbance in human uveitis.

In depth understanding of retinitis pigmentosa pathogenesis through optical coherence tomography angiography analysis: a narrative review

Retinitis pigmentosa (RP) is the most recognized inherited retinal disorder involving progressive photoreceptors degeneration which eventually causes blindness. However, the pathogenesis of RP is still unclear, making it difficult to establish satisfying treatments. Evidence have been found to support the theory that vascular dysfunction is associated with the progression of RP. Optical coherence tomography angiography (OCTA) is a newly developed technology that enables visualization as well as quantitative assessment of retinal and choroidal vasculature non-invasively. Advances in OCTA have opened a window for in-depth understanding of RP pathogenesis. Here, we propose a hypothesis of RP pathogenesis based on the current OCTA findings in RP, which includes four stages and two important key factors, vascular dysfunction and microglia activation. Further, we discuss the future animal experiments needed and how advanced OCTA technology can help to further verity the hypothesis. The final goal is to explore potential treatment options with enhanced understanding of RP pathogenesis.

Clock Gene Nr1d1 Alleviates Retinal Inflammation Through Repression of Hmga2 in Microglia

Purpose

Retinal inflammation is involved in the pathogenesis of several retinal diseases. As one of the core clock genes, Nr1d1 has been reported to suppress inflammation in many diseases. We investigated whether pharmacological activation of Nr1d1 can inhibit retinal inflammation and delineated the mechanisms of Nr1d1 in alleviating microglia activation.

Methods

Lipopolysaccharide (LPS) induced mice models were used to examine the effects of SR9009 (agonist of NR1D1) treatment on inflammatory phenotypes in vivo. Anti-inflammatory effects of Nr1d1 and associated mechanisms were investigated in the BV2 microglia cell line, and in primary retinal microglia in vitro.

Results

SR9009 treatment alleviated LPS-induced inflammatory cell infiltration, elevated cytokine levels and morphological changes of the microglia in mice models. In LPS-stimulated BV2 cells and primary retinal microglia, SR9009 suppressed cytokine expressions by inhibiting the NF-κB signaling pathway. Moreover, SR9009 treatment increased the levels of the M2 phenotype marker (CD206) and the proportions of ramified microglia. Suppression of Nr1d1 with siRNA reversed the inhibitory effects of SR9009 on cytokine production in BV2 cells. RNA-seq analysis showed that genes that were upregulated following Nr1d1 knockdown were enriched in inflammatory-associated biological processes. Subsequently, ChIP-seq of NR1D1 in BV2 was performed, and the results were integrated with RNA-seq results using the Binding and Expression Target Analysis (BETA) tool. Luciferase assays, electrophoretic mobility shift assay (EMSA), qPCR and Western blotting assays revealed that NR1D1 binds the promoter of Hmga2 to suppress its transcription. Notably, overexpressed Hmga2 in activated microglia could partly abolish the anti-inflammatory effects of Nr1d1.

Conclusion

The clock gene Nr1d1 protects against retinal inflammation and microglia activation in part by suppressing Hmga2 transcription.

Tracking distinct microglia subpopulations with photoconvertible Dendra2 in vivo

Background

The ability to track individual immune cells within the central nervous system has revolutionized our understanding of the roles that microglia and monocytes play in synaptic maintenance, plasticity, and neurodegenerative diseases. However, distinguishing between similar subpopulations of mobile immune cells over time during episodes of neuronal death and tissue remodeling has proven to be challenging.

Methods

We recombineered a photoconvertible fluorescent protein (Dendra2; D2) downstream of the Cx3cr1 promoter commonly used to drive expression of fluorescent markers in microglia and monocytes. Like the popular Cx3cr1–GFP line (Cx3cr1^+/GFP), naïve microglia in Cx3cr1–Dendra2 mice (Cx3cr1^+/D2) fluoresce green and can be noninvasively imaged in vivo throughout the CNS. In addition, individual D2-expressing cells can be photoconverted, resulting in red fluorescence, and tracked unambiguously within a field of green non-photoconverted cells for several days in vivo.

Results

Dendra2-expressing retinal microglia were noninvasively photoconverted in both ex vivo and in vivo conditions. Local in vivo D2 photoconversion was sufficiently robust to quantify cell subpopulations by flow cytometry, and the protein was stable enough to survive tissue processing for immunohistochemistry. Simultaneous in vivo fluorescence imaging of Dendra2 and light scattering measurements (Optical Coherence Tomography, OCT) were used to assess responses of individual microglial cells to localized neuronal damage and to identify the infiltration of monocytes from the vasculature in response to large scale neurodegeneration.

Conclusions

The ability to noninvasively and unambiguously track D2-expressing microglia and monocytes in vivo through space and time makes the Cx3cr1–Dendra2 mouse model a powerful new tool for disentangling the roles of distinct immune cell subpopulations in neuroinflammation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02285-x.

New mouse for tracking microglia and all mononuclear phagocytes both ex and in vivo within the CNS over time.

Dendra2 protein is stable enough to survive tissue processing for immunohistochemistry and flow cytometry quantification.

Simultaneous fluorescence imaging of Dendra2 and light scattering measurements can be used to assess the immune response to retinal damage.

Chronic in vivo imaging reveals mixed populations of microglia and monocytes during retinal degeneration.

Myeloid Cells in the Mouse Retina and Uveal Tract Respond Differently to Systemic Inflammatory Stimuli

Purpose

In spite of clear differences in tissue function and significance to ocular disease, little is known about how immune responses differ between the retina and uveal tract. To this end we compared the effects of acute systemic inflammation on myeloid cells within the mouse retina, iris-ciliary body, and choroid.

Methods

Systemic inflammation was induced in Cx3cr1gfp/gfp and CD11c-eYFP Crb1wt/wtmice by intraperitoneal lipopolysaccharide (LPS). In vivo fundus imaging was performed at two, 24, and 48 hours after LPS, and ocular tissue wholemounts were immunostained and studied by confocal microscopy. Flow cytometry was used to investigate the expression of activation markers (MHC class II, CD80, CD86) on myeloid cell populations at 24 hours. For functional studies, retinal microglia were isolated from LPS-exposed mice and cocultured with naïve OT-II CD4+ T-cells and ovalbumin peptide. T-cell proliferation was measured by flow cytometry and cytokine assays.

Results

Systemic LPS altered the density and morphology of retinal microglia; however, retinal microglia did not upregulate antigen presentation markers and failed to stimulate naïve CD4+ T-cell proliferation in vitro. In contrast, uveal tract myeloid cells displayed a phenotype consistent with late-activated antigen-presenting cells at 24 hours. Systemic LPS induced remodeling of myeloid populations within the uveal tract, particularly in the choroid, where dendritic cells were partially displaced by macrophages at 24 hours.

Conclusions

The disparate myeloid cell responses in the retina and uveal tract after systemic LPS highlight differential regulation of innate immunity within these tissue environments, observations that underpin and advance our understanding of ocular immune privilege.

Tlr2 Gene Deletion Delays Retinal Degeneration in Two Genetically Distinct Mouse Models of Retinitis Pigmentosa

Although considered a rare retinal dystrophy, retinitis pigmentosa (RP) is the primary cause of hereditary blindness. Given its diverse genetic etiology (>3000 mutations in >60 genes), there is an urgent need for novel treatments that target common features of the disease. TLR2 is a key activator of innate immune response. To examine its role in RP progression we characterized the expression profile of Tlr2 and its adaptor molecules and the consequences of Tlr2 deletion in two genetically distinct models of RP: Pde6brd10/rd10 (rd10) and RhoP23H/+ (P23H/+) mice. In both models, expression levels of Tlr2 and its adaptor molecules increased in parallel with those of the proinflammatory cytokine Il1b. In rd10 mice, deletion of a single Tlr2 allele had no effect on visual function, as evaluated by electroretinography. However, in both RP models, complete elimination of Tlr2 attenuated the loss of visual function and mitigated the loss of photoreceptor cell numbers. In Tlr2 null rd10 mice, we observed decreases in the total number of microglial cells, assessed by flow cytometry, and in the number of microglia infiltrating the photoreceptor layers. Together, these results point to TLR2 as a mutation-independent therapeutic target for RP.

Galectins in the Pathogenesis of Common Retinal Disease

Diseases of the retina are major causes of visual impairment and blindness in developed countries and, due to an ageing population, their prevalence is continually rising. The lack of effective therapies and the limitations of those currently in use highlight the importance of continued research into the pathogenesis of these diseases. Vascular endothelial growth factor (VEGF) plays a major role in driving vascular dysfunction in retinal disease and has therefore become a key therapeutic target. Recent evidence also points to a potentially similarly important role of galectins, a family of β-galactoside-binding proteins. Indeed, they have been implicated in regulating fundamental processes, including vascular hyperpermeability, angiogenesis, neuroinflammation, and oxidative stress, all of which also play a prominent role in retinopathies. Here, we review direct evidence for pathological roles of galectins in retinal disease. In addition, we extrapolate potential roles of galectins in the retina from evidence in cancer, immune and neuro-biology. We conclude that there is value in increasing understanding of galectin function in retinal biology, in particular in the context of the retinal vasculature and microglia. With greater insight, recent clinical developments of galectin-targeting drugs could potentially also be of benefit to the clinical management of many blinding diseases.