Contribution of microglia-mediated neuroinflammation to retinal degenerative diseases

Lycium Barbarum Polysaccharide-Derived Nanoparticles Protect Visual Function by Inhibiting RGC Ferroptosis and Microglial Activation in Retinal Ischemia‒Reperfusion Mice

Retinal ischemia‒reperfusion (IR) is a major contributor to vision impairment and irreversible vision loss due to retinal ganglion cell (RGC) injury or loss. Contemporary therapeutic approaches predominantly focus on the amelioration of symptoms rather than addressing the fundamental etiological factors. Oxidative stress is a notable feature and an important mediator of IR damage. Lycium barbarum polysaccharide (LBP), the main active ingredient of Lycium barbarum, has various pharmacological effects, including antioxidation, immunoregulation, and neuroprotective effects. In this study, the ROS-consumable moiety phenylboronic acid pinacol ester (PBA) is introduced to LBP molecules, which can self-assemble into nanoparticles in aqueous solution. This nanoparticle (termed PLBP) can reduce the cellular ROS levels and enhance the antioxidant capability of RGCs by activating the NRF2 pathway, thus protecting RGCs from ferroptosis and preserving visual function in response to IR injury. PLBP also reduces neuroinflammation by inhibiting the ability of microglia to phagocytose, migrate, secrete inflammatory cytokines, and activate the NF-κB pathway. In conclusion, this approach can be used as an inspiration for the future development of neuroprotective drugs.

The Immunomodulatory Effect of Silver Nanoparticles in a Retinal Inflammatory Environment

Activation of immune response plays an important role in the development of retinal diseases. One of the main populations of immune cells contributing to the retinal homeostasis are microglia, which represent a population of residential macrophages. However, under pathological conditions, microglia become activated and rather support a harmful inflammatory reaction and retinal angiogenesis. Therefore, targeting these cells could provide protection against retinal neuroinflammation and neovascularization. In the recent study, we analyzed effects of silver nanoparticles (AgNPs) on microglia in vitro and in vivo. We showed that the AgNPs interact in vitro with stimulated mouse CD45/CD11b positive cells (microglia/macrophages), decrease their secretion of nitric oxide and vascular endothelial growth factor, and regulate the expression of genes for Iba-1 and interleukin-1β (IL-1β). In our in vivo experimental mouse model, the intravitreal application of a mixture of proinflammatory cytokines tumor necrosis factor-α, IL-1β and interferon-γ induced local inflammation and increased local expression of genes for inducible nitric oxide synthase, IL-α, IL-1β and galectin-3 in the retina. This stimulation of local inflammatory reaction was significantly inhibited by intravitreal administration of AgNPs. The application of AgNPs also decreased the presence of CD11b/Galectin-3 positive cells in neuroinflammatory retina, but did not influence viability of cells and expression of gene for rhodopsin in the retinal tissue. These data indicate that AgNPs regulate reactivity of activated microglia in the diseased retina and thus could provide a beneficial effect for the treatment of several retinal diseases.

Quantification of Photoreceptors' Changes in a Diabetic Retinopathy Model with Two-Photon Imaging Microscopy

Emerging evidence suggests that retinal neurodegeneration is an early event in the pathogenesis of diabetic retinopathy (DR), preceding the development of microvascular abnormalities. Here, we assessed the impact of neuroinflammation on the retina of diabetic-induced rats. For this aim we have used a two-photon microscope to image the photoreceptors (PRs) at different eccentricities in unstained retinas obtained from both control (N = 4) and pathological rats (N = 4). This technique provides high-resolution images where individual PRs can be identified. Within each image, every PR was located, and its transversal area was measured and used as an objective parameter of neuroinflammation. In control samples, the size of the PRs hardly changed with retinal eccentricity. On the opposite end, diabetic retinas presented larger PR transversal sections. The ratio of PRs suffering from neuroinflammation was not uniform across the retina. Moreover, the maximum anatomical resolving power (in cycles/deg) was also calculated. This presents a double-slope pattern (from the central retina towards the periphery) in both types of specimens, although the values for diabetic retinas were significantly lower across all retinal locations. The results show that chronic retinal inflammation due to diabetes leads to an increase in PR transversal size. These changes are not uniform and depend on the retinal location. Two-photon microscopy is a useful tool to accurately characterize and quantify PR inflammatory processes and retinal alterations.

Melatonin: Unveiling the functions and implications in ocular health

Melatonin, a versatile hormone produced by the pineal gland, has garnered considerable scientific interest due to its diverse functions. In the eye, melatonin regulates a variety of key processes like inhibiting angiogenesis by reducing vascular endothelial growth factor levels and protecting the blood-retinal barrier (BRB) integrity by enhancing tight junction proteins and pericyte coverage. Melatonin also maintains cell health by modulating autophagy via the Sirt1/mTOR pathways, reduces inflammation, promotes antioxidant enzyme activity, and regulates intraocular pressure fluctuations. Additionally, melatonin protects retinal ganglion cells by modulating aging and inflammatory pathways. Understanding melatonin's multifaceted functions in ocular health could expand the knowledge of ocular pathogenesis, and shed new light on therapeutic approaches in ocular diseases. In this review, we summarize the current evidence of ocular functions and therapeutic potential of melatonin and describe its roles in angiogenesis, BRB integrity maintenance, and modulation of various eye diseases, which leads to a conclusion that melatonin holds promising treatment potential for a wide range of ocular health conditions.

Deciphering perivascular macrophages and microglia in the retinal ganglion cell layers

Introduction: The classically defined two retinal microglia layers are distributed in inner and outer plexiform layers. Although there are some reports that retinal microglia are also superficially located around the ganglion cell layer (GCL) in contact with the vitreous, there has been a lack of detailed descriptions and not fully understood yet. Methods: We visualized the microglial layers by using CX3CR1-GFP (C57BL6) transgenic mice with both healthy and disease conditions including NaIO3-induced retinal degeneration models and IRBP-induced auto-immune uveitis models. Result: We found the GCL microglia has two subsets; peripheral (pph) microglia located on the retinal parenchyma and BAM (CNS Border Associated Macrophage) which have a special stretched phenotype only located on the surface of large retinal veins. First, in the pph microglia subset, but not in BAM, Galectin-3 and LYVE1 are focally expressed. However, LYVE1 is specifically expressed in the amoeboid or transition forms, except the typical dendritic morphology in the pph microglia. Second, BAM is tightly attached to the surface of the retinal veins and has similar morphology patterns in both the healthy and disease conditions. CD86+ BAM has a longer process which vertically passes the proximal retinal veins. Our data helps decipher the basic anatomy and pathophysiology of the retinal microglia in the GCL. Discussion: Our data helps decipher the basic anatomy and pathophysiology of the retinal microglia in the GCL.

Role of myeloid cells in ischemic retinopathies: recent advances and unanswered questions

Myeloid cells including microglia and macrophages play crucial roles in retinal homeostasis by clearing cellular debris and regulating inflammation. These cells are activated in several blinding ischemic retinal diseases including diabetic retinopathy, where they may exert both beneficial and detrimental effects on neurovascular function and angiogenesis. Myeloid cells impact the progression of retinal pathologies and recent studies suggest that targeting myeloid cells is a promising therapeutic strategy to mitigate diabetic retinopathy and other ischemic retinal diseases. This review summarizes the recent advances in our understanding of the role of microglia and macrophages in retinal diseases and focuses on the effects of myeloid cells on neurovascular injury and angiogenesis in ischemic retinopathies. We highlight gaps in knowledge and advocate for a more detailed understanding of the role of myeloid cells in retinal ischemic injury to fully unlock the potential of targeting myeloid cells as a therapeutic strategy for retinal ischemia.

The m6A reader YTHDC2 maintains visual function and retinal photoreceptor survival through modulating translation of PPEF2 and PDE6B

Inherited retinal dystrophies (IRDs) are major causes of visual impairment and irreversible blindness worldwide, while the precise molecular and genetic mechanisms are still elusive. N6-methyladenosine (m6A) modification is the most prevalent internal modification in eukaryotic mRNA. YTH domain containing 2 (YTHDC2), an m6A reader protein, has recently been identified as a key player in germline development and human cancer. However, its contribution to retinal function remains unknown. Here, we explore the role of YTHDC2 in the visual function of retinal rod photoreceptors by generating rod-specific Ythdc2 knockout mice. Results show that Ythdc2 deficiency in rods causes diminished scotopic ERG responses and progressive retinal degeneration. Multi-omics analysis further identifies Ppef2 and Pde6b as the potential targets of YTHDC2 in the retina. Specifically, via its YTH domain, YTHDC2 recognizes and binds m6A-modified Ppef2 mRNA at the coding sequence and Pde6b mRNA at the 5'-UTR, resulting in enhanced translation efficiency without affecting mRNA levels. Compromised translation efficiency of Ppef2 and Pde6b after YTHDC2 depletion ultimately leads to decreased protein levels in the retina, impaired retinal function, and progressive rod death. Collectively, our finding highlights the importance of YTHDC2 in visual function and photoreceptor survival, which provides an unreported elucidation of IRD pathogenesis via epitranscriptomics.

Glaucoma: from pathogenic mechanisms to retinal glial cell response to damage

Glaucoma is a neurodegenerative disease of the retina characterized by the irreversible loss of retinal ganglion cells (RGCs) leading to visual loss. Degeneration of RGCs and loss of their axons, as well as damage and remodeling of the lamina cribrosa are the main events in the pathogenesis of glaucoma. Different molecular pathways are involved in RGC death, which are triggered and exacerbated as a consequence of a number of risk factors such as elevated intraocular pressure (IOP), age, ocular biomechanics, or low ocular perfusion pressure. Increased IOP is one of the most important risk factors associated with this pathology and the only one for which treatment is currently available, nevertheless, on many cases the progression of the disease continues, despite IOP control. Thus, the IOP elevation is not the only trigger of glaucomatous damage, showing the evidence that other factors can induce RGCs death in this pathology, would be involved in the advance of glaucomatous neurodegeneration. The underlying mechanisms driving the neurodegenerative process in glaucoma include ischemia/hypoxia, mitochondrial dysfunction, oxidative stress and neuroinflammation. In glaucoma, like as other neurodegenerative disorders, the immune system is involved and immunoregulation is conducted mainly by glial cells, microglia, astrocytes, and Müller cells. The increase in IOP produces the activation of glial cells in the retinal tissue. Chronic activation of glial cells in glaucoma may provoke a proinflammatory state at the retinal level inducing blood retinal barrier disruption and RGCs death. The modulation of the immune response in glaucoma as well as the activation of glial cells constitute an interesting new approach in the treatment of glaucoma.

BAFF deficiency aggravated optic nerve crush-induced retinal ganglion cells damage by regulating apoptosis and neuroinflammation via NF-κB-IκBα signaling

Loss of retinal ganglion cells (RGCs) is a primary cause of visual impairment in glaucoma, the pathological process is closely related to neuroinflammation and apoptosis. B-cell activating factor (BAFF) is a fundamental survival factor mainly expressed in the B cell lineage. Evidence suggests its neuroprotective effect, but the expression and role in the retina have not yet been investigated. In this study, we adopt optic nerve crush (ONC) as an in vivo model and oxygen-glucose deprivation/reoxygenation (OGD/R) of RGCs as an in vitro model to investigate the expression and function of BAFF. We found that BAFF and its receptors were abundantly expressed in the retina and BAFF inhibition exacerbated the caspase 3-mediated RGCs apoptosis, glial cell activation and pro-inflammatory cytokines expression, which may be caused by the activation of the NF-κB pathway in vivo. In addition, we found that BAFF treatment could alleviate RGCs apoptosis, pro-inflammatory cytokines expression and NF-κB pathway activation, which could be reversed the effect by blockade of the NF-κB pathway in vitro. Meanwhile, we found that microglia induced to overexpress BAFF in the inflammatory microenvironment in a time-dependent manner. Taken together, our results indicated that BAFF deficiency promoted RGCs apoptosis and neuroinflammation through activation of NF-κB pathway in ONC retinas, suggesting that BAFF may serve as a promising therapeutic target for the treatment of glaucoma.

Optical coherence tomography in the management of diabetic macular oedema

Diabetic macular oedema (DMO) is the major cause of visual impairment in people with diabetes. Optical coherence tomography (OCT) is now the most widely used modality to assess presence and severity of DMO. DMO is currently broadly classified based on the involvement to the central 1 mm of the macula into non-centre or centre involved DMO (CI-DMO) and DMO can occur with or without visual acuity (VA) loss. This classification forms the basis of management strategies of DMO. Despite years of research on quantitative and qualitative DMO related features assessed by OCT, these do not fully inform physicians of the prognosis and severity of DMO relative to visual function. Having said that, recent research on novel OCT biomarkers development and re-defined classification of DMO show better correlation with visual function and treatment response. This review summarises the current evidence of the association of OCT biomarkers in DMO management and its potential clinical importance in predicting VA and anatomical treatment response. The review also discusses some future directions in this field, such as the use of artificial intelligence to quantify and monitor OCT biomarkers and retinal fluid and identify phenotypes of DMO, and the need for standardisation and classification of OCT biomarkers to use in future clinical trials and clinical practice settings as prognostic markers and secondary treatment outcome measures in the management of DMO.

Release of Pro-Inflammatory/Angiogenic Factors by Retinal Microvascular Cells Is Mediated by Extracellular Vesicles Derived from M1-Activated Microglia

The interactions between the neuronal and vascular sides of the retina during diabetic retinopathy (DR) have gained increasing attention. Microglia is responsible for the immune response to inflammation inside the retina, which could be mediated by paracrine signals carried by extracellular vesicles (EVs). We aimed to characterize EVs released from immortalized human microglial cells in inflammation and investigate their effects on the retinal microvasculature and the anti-inflammatory potential of thiamine in this context. M1 pro-inflammatory polarization in microglia was induced through a cytokine cocktail. EVs were isolated from the supernatants, characterized, and used to stimulate human retinal endothelial cells (HRECs) and pericytes (HRPs). Microvascular cell functions and their release of pro-inflammatory/angiogenic factors were assessed. M1-derived EVs showed increased content of miR-21, miR-155, CCL2, MMP2, and MMP9, and enhanced apoptosis, proliferation, migration, and ROS production in HRPs and HRECs. IL-1β, IL-6, MMP9, CCL2, and VEGF release increased in HRPs exposed to M1-derived EVs, while HRECs showed augmented IL-6, Ang2, VEGF, and PDFG-B. Addition of thiamine to M1-microglial cultures reverted most of these effects. In conclusion, M1-derived EVs stimulate functional changes and secretion of pro-inflammatory/angiogenic molecules in microvascular cells, exacerbating inflammatory damage and retinopathy features. Thiamine added to microglia exerts anti-inflammatory effects.

Glial metabolic alterations during glaucoma pathogenesis

Glaucoma is the leading cause of irreversible blindness. Current treatment options are limited and often only slow disease progression. Metabolic dysfunction has recently been recognized as a key early and persistent mechanism in glaucoma pathophysiology. Several intrinsic metabolic dysfunctions have been identified and treated in retinal ganglion cells to provide neuroprotection. Growing pre-clinical and clinical evidence has confirmed that metabolic alterations in glaucoma are widespread, occurring across visual system tissues, in ocular fluids, in blood/serum, and at the level of genomic and mitochondrial DNA. This suggests that metabolic dysfunction is not constrained to retinal ganglion cells and that metabolic alterations extrinsic to retinal ganglion cells may contribute to their metabolic compromise. Retinal ganglion cells are reliant on glial metabolic support under normal physiological conditions, but the implications of metabolic dysfunction in glia are underexplored. We highlight emerging evidence that has demonstrated metabolic alterations occurring within glia in glaucoma, and how this may affect neuro-glial metabolic coupling and the metabolic vulnerability of retinal ganglion cells. In other neurodegenerative diseases which share features with glaucoma, several other glial metabolic alterations have been identified, suggesting that similar mechanisms and therapeutic targets may exist in glaucoma.

Potential Retinal Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) represents a major diagnostic challenge, as early detection is crucial for effective intervention. This review examines the diagnostic challenges facing current AD evaluations and explores the emerging field of retinal alterations as early indicators. Recognizing the potential of the retina as a noninvasive window to the brain, we emphasize the importance of identifying retinal biomarkers in the early stages of AD. However, the examination of AD is not without its challenges, as the similarities shared with other retinal diseases introduce complexity in the search for AD-specific markers. In this review, we address the relevance of using the retina for the early diagnosis of AD and the complex challenges associated with the search for AD-specific retinal biomarkers. We provide a comprehensive overview of the current landscape and highlight avenues for progress in AD diagnosis by retinal examination.

Microglia preserve visual function loss in the aging retina by supporting retinal pigment epithelial health

BACKGROUND

Increased age is a risk factor for the development and progression of retinal diseases including age-related macular degeneration (AMD). Understanding the changes that occur in the eye due to aging is important in enhancing our understanding of AMD pathogenesis and the development of novel AMD therapies. Microglia, the resident brain and retinal immune cells are associated with both maintaining homeostasis and protection of neurons and loss of microglia homeostasis could be a significant player in age related neurodegeneration. One important characteristic of retinal aging is the migration of microglia from the inner to outer retina where they reside in the subretinal space (SRS) in contact with the retinal pigment epithelial (RPE) cells. The role of aged subretinal microglia is unknown. Here, we depleted microglia in aged C57/BL6 mice fed for 6 weeks with a chow containing PLX5622, a small molecule inhibitor of colony-stimulating factor-1 receptor (Csf1r) required for microglial survival.

RESULTS

The subretinal P2RY12 + microglia in aged mice displayed a highly amoeboid and activated morphology and were filled with autofluorescence droplets reminiscent of lipofuscin. TEM indicates that subretinal microglia actively phagocytize shed photoreceptor outer segments, one of the main functions of retinal pigmented epithelial cells. PLX5622 treatment depleted up to 90% of the retinal microglia and was associated with significant loss in visual function. Mice on the microglia depletion diet showed reduced contrast sensitivity and significantly lower electroretinogram for the c-wave, a measurement of RPE functionality, compared to age-matched controls. The loss of c-wave coincided with a loss of RPE cells and increased RPE swelling in the absence of microglia.

CONCLUSIONS

We conclude that microglia preserve visual function in aged mice and support RPE cell function, by phagocytosing shed photoreceptor outer segments and lipids, therefore compensating for the known age-related decline of RPE phagocytosis.

The effects of a highly bioavailable curcumin PhytosomeTM preparation on the retinal architecture and glial reactivity in the GFAP-IL6 mice

Uncontrolled, chronic inflammation in the retina can disturb retinal structure and function leading to impaired visual function. For the first time, in a mouse model of chronic neuroinflammation (GFAP-IL6), we investigated the impact of chronic glial activation on the retinal microglia population and structure. In addition, we tested a curcumin PhytosomeTM preparation with enhanced bioavailability to investigate the effects of a cytokine-suppressing anti-inflammatory drug on retinal architecture. Curcumin PhytosomeTM was fed to 3-month old GFAP-IL6 mice for 4 weeks and compared to their untreated GFAP-IL6 counterparts as well as wild type mice on control diet. Microglial numbers and morphology together with neuronal numbers were characterized using immunohistochemistry and cell reconstruction in the retina, using retinal wholemount and slices. GFAP-IL6 mice showed a significant increase in Iba1-labelled mononuclear phagocytes, including microglia, and displayed altered glial morphology. This resulted in a reduction in cone density and a thinning of the retinal layers compared to wild type mice. Curcumin PhytosomeTM treatment contributed to decreased microglial density, significantly decreasing both soma and cell size compared to control diet, as well as preventing the thinning of the retinal layers. This study is the first to characterize the impact of chronic retinal inflammation in the GFAP-IL6 mouse and the therapeutic benefit of enhanced bioavailable curcumin PhytosomeTM to significantly reduce microglia density and prevent neuronal loss. These data suggest that curcumin could be used as a complementary therapy alongside traditional treatments to reduce associated retinal inflammation in a variety of retinal diseases.

Neuroinflammation and gliosis in the injured and contralateral retinas after unilateral optic nerve crush

The main purpose of this study is to analyze the effects of unilateral optic nerve crush in the gene expression of pro- and anti-inflammatory mediators, and gliosis markers in injured and contralateral retinas. Retinas from intact, unilaterally optic nerve injured or sham-operated C57BL/6J mice were analyzed 1, 3, 9 and 30 days after the surgery (n = 5/group and time point) and the relative expression of TGF-β1, IL-1β, TNF-α, Iba1, AQP4, GFAP, MHCII, and TSPO was analyzed in injured and contralateral using qPCR. The results indicated that compared with intact retinas, sham-operated animals showed an early (day 1) upregulation of IL-1β, TNF-α and TSPO and a late (day 30) upregulation of TNF-α. In sham-contralateral retinas, TNF-α and TSPO mRNA expression were upregulated and day 30 while GFAP, Iba1, AQP4 and MHCII downregulated at day 9. Compared with sham-operated animals, in retinas affected by optic nerve crush GFAP and TSPO upregulated at day 1 and TNF-α, Iba1, AQP4 and MHCII at day 3. In the crushed-contralateral retinas, TGF-β1, TNF-α, Iba1 and MHCII were upregulated at day 1. TSPO was upregulated up to day 30 whereas TGF-β1 and Iba1 downregulated after day 9. In conclusion, both sham surgery and optic nerve crush changed the profile of inflammatory and gliosis markers in the injured and contralateral retinas, changes that were more pronounced for optic nerve crush when compared to sham.

Structure, Function, and Molecular Landscapes of the Aging Retina

Because the central nervous system is largely nonrenewing, neurons and their synapses must be maintained over the lifetime of an individual to ensure circuit function. Age is a dominant risk factor for neural diseases, and declines in nervous system function are a common feature of aging even in the absence of disease. These alterations extend to the visual system and, in particular, to the retina. The retina is a site of clinically relevant age-related alterations but has also proven to be a uniquely approachable system for discovering principles that govern neural aging because it is well mapped, contains diverse neuron types, and is experimentally accessible. In this article, we review the structural and molecular impacts of aging on neurons within the inner and outer retina circuits. We further discuss the contribution of non-neuronal cell types and systems to retinal aging outcomes. Understanding how and why the retina ages is critical to efforts aimed at preventing age-related neural decline and restoring neural function.

Ocular Vascular Diseases: From Retinal Immune Privilege to Inflammation

The eye is an immune privileged tissue that insulates the visual system from local and systemic immune provocation to preserve homeostatic functions of highly specialized retinal neural cells. If immune privilege is breached, immune stimuli will invade the eye and subsequently trigger acute inflammatory responses. Local resident microglia become active and release numerous immunological factors to protect the integrity of retinal neural cells. Although acute inflammatory responses are necessary to control and eradicate insults to the eye, chronic inflammation can cause retinal tissue damage and cell dysfunction, leading to ocular disease and vision loss. In this review, we summarized features of immune privilege in the retina and the key inflammatory responses, factors, and intracellular pathways activated when retinal immune privilege fails, as well as a highlight of the recent clinical and research advances in ocular immunity and ocular vascular diseases including retinopathy of prematurity, age-related macular degeneration, and diabetic retinopathy.

Elucidating glial responses to products of diabetes-associated systemic dyshomeostasis

Diabetic retinopathy (DR) is a leading cause of blindness in working age adults. DR has non-proliferative stages, characterized in part by retinal neuroinflammation and ischemia, and proliferative stages, characterized by retinal angiogenesis. Several systemic factors, including poor glycemic control, hypertension, and hyperlipidemia, increase the risk of DR progression to vision-threatening stages. Identification of cellular or molecular targets in early DR events could allow more prompt interventions pre-empting DR progression to vision-threatening stages. Glia mediate homeostasis and repair. They contribute to immune surveillance and defense, cytokine and growth factor production and secretion, ion and neurotransmitter balance, neuroprotection, and, potentially, regeneration. Therefore, it is likely that glia orchestrate events throughout the development and progression of retinopathy. Understanding glial responses to products of diabetes-associated systemic dyshomeostasis may reveal novel insights into the pathophysiology of DR and guide the development of novel therapies for this potentially blinding condition. In this article, first, we review normal glial functions and their putative roles in the development of DR. We then describe glial transcriptome alterations in response to systemic circulating factors that are upregulated in patients with diabetes and diabetes-related comorbidities; namely glucose in hyperglycemia, angiotensin II in hypertension, and the free fatty acid palmitic acid in hyperlipidemia. Finally, we discuss potential benefits and challenges associated with studying glia as targets of DR therapeutic interventions. In vitro stimulation of glia with glucose, angiotensin II and palmitic acid suggests that: 1) astrocytes may be more responsive than other glia to these products of systemic dyshomeostasis; 2) the effects of hyperglycemia on glia are likely to be largely osmotic; 3) fatty acid accumulation may compound DR pathophysiology by promoting predominantly proinflammatory and proangiogenic transcriptional alterations of macro and microglia; and 4) cell-targeted therapies may offer safer and more effective avenues for DR treatment as they may circumvent the complication of pleiotropism in retinal cell responses. Although several molecules previously implicated in DR pathophysiology are validated in this review, some less explored molecules emerge as potential therapeutic targets. Whereas much is known regarding glial cell activation, future studies characterizing the role of glia in DR and how their activation is regulated and sustained (independently or as part of retinal cell networks) may help elucidate mechanisms of DR pathogenesis and identify novel drug targets for this blinding disease.

Melatonin protects against NMDA-induced retinal ganglion cell injury by regulating the microglia-TNFα-RGC p38 MAPK pathway

Glaucoma, one of the most common ocular neurodegenerative diseases worldwide, is characterized by retinal ganglion cell (RGC) loss. There is a large body of literature that describes the neuroprotective role of melatonin against neurodegenerative diseases by regulating neuroinflammation, although the exact mechanism through which melatonin acts on RGC is still uncertain. This study assessed the protective effects of melatonin using a NMDA-induced RGC injury model, and studied the possible mechanisms involved in this process. Melatonin promoted RGC survival, improved retinal function, and inhibited the apoptosis and necrosis of retinal cells. To understand the mechanism of the neuroprotective effects of melatonin on RGC, microglia and inflammation-related pathways were assessed after melatonin administration and microglia ablation. Melatonin promoted RGC survival by suppressing microglia-derived proinflammatory cytokines, in particular TNFα, which in turn inhibited the activation of p38 MAPK pathway. Inhibiting TNFα or manipulating p38 MAPK pathway protected damaged RGC. Our results suggest that melatonin protects against NMDA-induced RGC injury by inhibiting the microglial TNFα-RGC p38 MAPK pathway. It should be considered a candidate neuroprotective therapy against retinal neurodegenerative diseases.

Attention-Deficit/Hyperactivity Disorder Animal Model Presents Retinal Alterations and Methylphenidate Has a Differential Effect in ADHD versus Control Conditions

Attention-Deficit/Hyperactivity Disorder (ADHD) is one of the most prevalent neurodevelopmental disorders. Interestingly, children with ADHD seem to experience more ophthalmologic abnormalities, and the impact of methylphenidate (MPH) use on retinal physiology remains unclear. Thus, we aimed to unravel the retina's structural, functional, and cellular alterations and the impact of MPH in ADHD versus the control conditions. For that, spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) were used as animal models of ADHD and the controls, respectively. Animals were divided into four experimental groups as follows: WKY vehicle (Veh; tap water), WKY MPH (1.5 mg/kg/day), SHR Veh, SHR MPH. Individual administration was performed by gavage between P28-P55. Retinal physiology and structure were evaluated at P56 followed by tissue collection and analysis. The ADHD animal model presents the retinal structural, functional, and neuronal deficits, as well as the microglial reactivity, astrogliosis, blood-retinal barrier (BRB) hyperpermeability and a pro-inflammatory status. In this model, MPH had a beneficial effect on reducing microgliosis, BRB dysfunction, and inflammatory response, but did not correct the neuronal and functional alterations in the retina. Curiously, in the control animals, MPH showed an opposite effect since it impaired the retinal function, neuronal cells, and BRB integrity, and also promoted both microglia reactivity and upregulation of pro-inflammatory mediators. This study unveils the retinal alterations in ADHD and the opposite effects induced by MPH in the retina of ADHD and the control animal models.

A lncRNA-encoded mitochondrial micropeptide exacerbates microglia-mediated neuroinflammation in retinal ischemia/reperfusion injury

As a common pathology of many ocular disorders such as diabetic retinopathy and glaucoma, retinal ischemia/reperfusion (IR) triggers inflammation and microglia activation that lead to irreversible retinal damage. The detailed molecular mechanism underlying retinal IR injury, however, remains poorly understood at present. Here we report the bioinformatic identification of a lncRNA 1810058I24Rik (181-Rik) that was shown to encode a mitochondrion-located micropeptide Stmp1. Its deficiency in mice protected retinal ganglion cells from retinal IR injury by attenuating the activation of microglia and the Nlrp3 inflammasome pathway. Moreover, its genetic knockout in mice or knockdown in primary microglia promoted mitochondrial fusion, impaired mitochondrial membrane potential, and reactive oxygen species (ROS) production, diminished aerobic glycolysis, and ameliorated inflammation. It appears that 181-Rik may trigger the Nlrp3 inflammasome activation by controlling mitochondrial functions through inhibiting expression of the metabolic sensor uncoupling protein 2 (Ucp2) and activating expression of the Ca²⁺ sensors S100a8/a9. Together, our findings shed new light on the molecular pathogenesis of retinal IR injury and may provide a fresh therapeutic target for IR-associated neurodegenerative diseases.

Microglia activation in the hippocampus mediates retinal degeneration-induced depressive-like behaviors via the NLRP3/IL-1β pathway

Epidemiological studies have shown that patients with glaucoma are more prone to depression, but the mechanism of comorbid depression in patients with glaucoma remains unknown. Excessive neuroinflammation has been shown to participate in glaucoma-induced retinal degeneration and hippocampal neural apoptosis in depression. However, little research has been conducted to determine whether neuroinflammation contributes to glaucoma-induced depression. Since the degeneration of retinal ganglion cells is a hallmark of glaucoma, we investigated the role of microglia-induced neuroinflammation in retinal degeneration-induced depression and its potential mechanism. An N-methyl-D-aspartate (NMDA)-induced retinal degeneration model was established, and behavioral tests were conducted at 3, 7, 14, and 21 days after retinal degeneration. After tissue collection, we used immunohistochemistry to assess the activation of microglia and real-time polymerase chain reaction to measure the levels of pro-inflammatory cytokines and the NOD-, LRR-, and pyrin-domain containing protein 3 (NLRP3) inflammasome. The mice exhibited depressive-like behaviors 14 and 21 days after retinal degeneration, based on the open field test, tail suspension test, and forced swimming test. Mice also displayed a lower body weight gain than the control group. In addition, microglial activation was observed in the hippocampus. Microglial proliferation was first observed in the dentate gyrus on day 3, while the number of microglia in cornu ammonis 1 grew the most. Moreover, not only was the expression of pro-inflammatory cytokines, including interleukin-1β, interleukin-18, and interleukin-6 promoted, but the messenger ribonucleic acid levels of the NLRP3 inflammasome were also increased. In conclusion, our research shows that NMDA-induced retinal degeneration can induce depressive-like behaviors, which may be attributed to hippocampal neuroinflammation.

Vitreous humor proteome: unraveling the molecular mechanisms underlying proliferative and neovascular vitreoretinal diseases

Proliferative diabetic retinopathy (PDR), proliferative vitreoretinopathy (PVR), and neovascular age-related macular degeneration (nAMD) are among the leading causes of blindness. Due to the multifactorial nature of these vitreoretinal diseases, omics approaches are essential for a deeper understanding of the pathophysiologic processes underlying the evolution to a proliferative or neovascular etiology, in which patients suffer from an abrupt loss of vision. For many years, it was thought that the function of the vitreous was merely structural, supporting and protecting the surrounding ocular tissues. Proteomics studies proved that vitreous is more complex and biologically active than initially thought, and its changes reflect the physiological and pathological state of the eye. The vitreous is the scenario of a complex interplay between inflammation, fibrosis, oxidative stress, neurodegeneration, and extracellular matrix remodeling. Vitreous proteome not only reflects the pathological events that occur in the retina, but the changes in the vitreous itself play a central role in the onset and progression of vitreoretinal diseases. Therefore, this review offers an overview of the studies on the vitreous proteome that could help to elucidate some of the pathological mechanisms underlying proliferative and/or neovascular vitreoretinal diseases and to find new potential pharmaceutical targets.

Caffeine protects against hippocampal alterations in type 2 diabetic rats via modulation of gliosis, inflammation and apoptosis

Type 2 diabetes (T2D) is implicated in the injury of several organs, including the brain resulting in neuronal damage, which may lead to cognitive impairment and dementia. Additionally, it is linked to inflammation, cytokine release, apoptosis and various degenerative conditions. Astrocytes and microglia might have a role in mediating these processes. Caffeine, a psychoactive beverage, has been shown to reduce the risk of cognitive and memory impairment. This study proposes anti-inflammatory and anti-apoptotic role of caffeine, which can be mediated via microglia/astrocyte activation and overexpression of pro-inflammatory molecules. T2D was induced in rats by feeding with high fat high sugar diet and injecting a single low dose streptozotocin (STZ) intraperitoneally. Other diabetic rats were given caffeine orally (in two doses) for 5 weeks, starting 1 week before STZ injection. Measurement of plasma cytokines, TNFα and IL6, was performed using enzyme-linked immunosorbent assay (ELISA) technique. After sacrificing animals, brains were obtained and processed for histological evaluation. Immunohistochemistry was also performed using the following primary antibodies, anti-astrocyte marker GFAP, anti-microglia marker CD11b and apoptotic marker (anti-cleaved caspase-3). There was upregulation of IL6 and TNF-α in diabetic rats. Additionally, histological evaluation of the hippocampus of diabetic rats revealed cellular degeneration. There was increased immunostaining of GFAP, CD11b and cleaved caspase-3 in diabetic rats. Pretreatment with caffeine to diabetic rats, resulted in improvement of structural changes and decrease in cytokine levels and immuno-markers, expression, and this was in a dose-dependent manner. In conclusion, caffeine had an ameliorative role in enhancing hippocampal degenerative changes in T2D.

Investigations into photoreceptor energy metabolism during experimental retinal detachment

Retinal detachment is a sight-threatening disorder, which occurs when the photoreceptors are separated from their vascular supply. The aim of the present study was to shed light on photoreceptor energy metabolism during experimental detachment in rats. Retinal detachment was induced in the eyes of rats via subretinal injection of sodium hyaluronate. Initially, we investigated whether detachment caused hypoxia within photoreceptors, as evaluated by the exogenous and endogenous biomarkers pimonidazole and HIF-1α, as well as by qPCR analysis of HIF target genes. The results showed no unequivocal staining for pimonidazole or HIF-1α within any detached retina, nor upregulation of HIF target genes, suggesting that any reduction in pO2 is of insufficient magnitude to produce hypoxia-induced covalent protein adducts or HIF-1α stabilisation. Subsequently, we analysed expression of cellular bioenergetic enzymes in photoreceptors during detachment. We documented loss of mitochondrial, and downregulation of glycolytic enzymes during detachment, indicating that photoreceptors have reduced energetic requirements and/or capacity. Given that detachment did not cause widespread hypoxia, but did result in downregulated expression of bioenergetic enzymes, we hypothesised that substrate insufficiency may be critical in terms of pathogenesis, and that boosting metabolic inputs may preserve photoreceptor bioenergetic production and, protect against their degeneration. Thus, we tested whether supplementation with the bioavailable energy substrate pyruvate mitigated rod and cone injury and degeneration. Despite protecting photoreceptors in culture from nutrient deprivation, pyruvate failed to protect against apoptotic death of rods, loss of cone opsins, and loss of inner segment mitochondria, in situ, when evaluated at 3 days after detachment. The regimen was also ineffective against cumulative photoreceptor deconstruction and degeneration when evaluated after 4 weeks. Retinal metabolism, particularly the bioenergetic profiles and pathological responses of the various cellular subtypes still presents a considerable knowledge gap that has important clinical consequences. While our data do not support the use of pyruvate supplementation as a means of protecting detached photoreceptors, they do provide a foundation and motivation for future research in this area.

Regulatory effect of long-stranded non-coding RNA-CRNDE on neurodegeneration during retinal ischemia-reperfusion

Ischemia/reperfusion (I/R) injury is a common pathological mechanism involved in many ocular diseases. I/R is characterized by microvascular dysfunction and neurodegeneration. However, the mechanisms of neurodegeneration induced by I/R remain largely unknown. This study showed that the expression of long non-coding RNA-CRNDE was significantly upregulated after retinal ischemia-reperfusion (RIR). LncRNA-CRNDE knockdown alleviated retinal neurodegeneration induced by RIR injury, as shown by decreased reactive gliosis and reduced retinal cells loss. Furthermore, lncRNA-CRNDE knockdown directly regulated Müller cell function and indirectly affected RGC function in vitro. In addition, lncRNA-CRNDE knockdown led to a significant reduction in the release of several cytokines after RIR. This study suggests that lncRNA-CRNDE is a promising therapeutic target for RIR.

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.

Hydrogen Sulfide Attenuates Neuroinflammation by Inhibiting the NLRP3/Caspase-1/GSDMD Pathway in Retina or Brain Neuron following Rat Ischemia/Reperfusion

Gasdermin D-executing pyroptosis mediated by NLRP3 inflammasomes has been recognized as a key pathogenesis during stroke. Hydrogen sulfide (H₂S) could protect CNS against ischemia/reperfusion (I/R)-induced neuroinflammation, while the underlying mechanism remains unclear. The study applied the middle cerebral artery occlusion/reperfusion (MCAO/R) model to investigate how the brain and the retinal injuries were alleviated in sodium hydrogen sulfide (NaHS)-treated rats. The rats were assigned to four groups and received an intraperitoneal injection of 50 μmol/kg NaHS or NaCl 15 min after surgery. Neurological deficits were evaluated using the modified neurologic severity score. The quantification of pro-inflammatory cytokines, NLRP3, caspase-1, and GSDMD were determined by ELISA and Western blot. Cortical and retinal neurodegeneration and cell pyroptosis were determined by histopathologic examination. Results showed that NaHS rescued post-stroke neurological deficits and infarct progression, improved retina injury, and attenuated neuroinflammation in the brain cortexes and the retinae. NaHS administration inhibits inflammation by blocking the NLRP3/caspase-1/GSDMD pathway and further suppressing neuronal pyroptosis. This is supported by the fact that it reversed the high-level of NLRP3, caspase-1, and GSDMD following I/R. Our findings suggest that compounds with the ability to donate H₂S could constitute a novel therapeutic strategy for ischemic stroke.

Clinical significance of metabolic quantification for retinal nonperfusion in diabetic retinopathy

Diabetic retinopathy (DR) is characterized by microvascular changes including ischemia. Degradation and metabolic changes of various retinal cells occur during ischemia. Ischemic region containing more cells will lead to greater metabolic impairment. We analyzed the non-perfusion region (NPR) by integrating histologic mapping with ultra-widefield fluorescein angiography (UWF FA) images. We also investigated the correlations of the weighted ischemic index (ISI) considering the regional distribution of retinal cells with cytokines, macular edema (ME), and neovascularization (NV). In this study, 32 patients with treatment-naïve DR and 21 age-matched control participants were included. The difference between the non-weighted and weighted ISI of NPR with leakage was greatest at the posterior region. The weighted ISI of NPR with leakage was correlated with MCP-1, IL-8, IL-6, PlGF, and VEGF-A levels, while the non-weighted ISI of NPR with leakage was correlated with IL-8 and IL-6 levels. The presence of baseline ME or NV in patients with DR was associated with the weighted ISI, with a stronger association when cones and rods were weighted. The weighted ISI reflecting both metabolic activity and cell distribution demonstrated a better correlation with clinical features and was more valuable in NPR with leakage than non-weighted ISI, which previous studies conventionally used.

Melatonin Maintains Inner Blood-Retinal Barrier by Regulating Microglia via Inhibition of PI3K/Akt/Stat3/NF-κB Signaling Pathways in Experimental Diabetic Retinopathy

Microglial activation and melatonin protection have been reported in diabetic retinopathy (DR). Whether melatonin could regulate microglia to protect the inner blood–retinal barrier (iBRB) remains unknown. In this study, the role of microglia in iBRB breakdown and the mechanisms of melatonin’s regulation on microglia were explored. In diabetic rat retinas, activated microglia proliferated and migrated from the inner retina to the outer retina, accompanied by the obvious morphological changes. Meanwhile, significant leakage of albumin was evidenced at the site of close interaction between activated microglia and the damaged pericytes and endothelial cells. In vitro, inflammation-related cytokines, such as tumor necrosis factor-α (TNF-α), inducible nitric oxide synthase (iNOS), interleukin (IL)-1β, and arginase-1 (Arg-1), were increased significantly in CoCl₂-treated BV2 cells. The supernatant derived from CoCl₂-treated BV2 cells significantly decreased the cell viability and disrupted the junctional proteins in both pericytes and endothelial cells, resulting in severe leakage. Melatonin suppressed the microglial overactivation, i.e., decreasing the cell number and promoting its anti-inflammatory properties in diabetic rat retinas. Moreover, the leakage of iBRB was alleviated and the pericyte coverage was restored after melatonin treatment. In vitro, when treated with melatonin in CoCl₂-treated BV2 cells, the inflammatory factors were decreased, while the anti-inflammatory factors were increased, further reducing the pericyte loss and increasing the tight junctions. Melatonin deactivated microglia via inhibition of PI3K/Akt/Stat3/NF-κB signaling pathways, thus maintaining the integrity of iBRB. The present data support a causal role for activated microglia in iBRB breakdown and highlight the therapeutic potential of melatonin in the treatment of DR by regulating microglia.

COVID-19 and Retinal Degenerative Diseases: Promising link “Kaempferol”

Coronavirus disease (COVID-19) outbreak has caused unprecedented global disruption since 2020. Approximately 238 million people are affected worldwide where the elderly succumb to mortality. Post-COVID syndrome and its side effects have popped up with several health hazards, such as macular degeneration and vision loss. It thus necessitates better medical care and management of our dietary practices. Natural flavonoids have been included in traditional medicine and have also been used safely against COVID-19 and several other diseases. Kaempferol is an essential flavonoid that has been demonstrated to influence several vital cellular signaling pathways involved in apoptosis, angiogenesis, inflammation, and autophagy. In this review, we emphasize the plausible regulatory effects of Kaempferol on hallmarks of COVID-19 and macular degeneration.

Microglia: Key Players in Retinal Ageing and Neurodegeneration

Microglia are the resident immune cells of the central nervous system (CNS) and play a key role in maintaining the normal function of the retina and brain. During early development, microglia migrate into the retina, transform into a highly ramified phenotype, and scan their environment constantly. Microglia can be activated by any homeostatic disturbance that may endanger neurons and threaten tissue integrity. Once activated, the young microglia exhibit a high diversity in their phenotypes as well as their functions, which relate to either beneficial or harmful consequences. Microglial activation is associated with the release of cytokines, chemokines, and growth factors that can determine pathological outcomes. As the professional phagocytes in the retina, microglia are responsible for the clearance of pathogens, dead cells, and protein aggregates. However, their phenotypic diversity and phagocytic capacity is compromised with ageing. This may result in the accumulation of protein aggregates and myelin debris leading to retinal neuroinflammation and neurodegeneration. In this review, we describe microglial phenotypes and functions in the context of the young and ageing retina, and the mechanisms underlying changes in ageing. Additionally, we review microglia-mediated retinal neuroinflammation and discuss the mechanisms of microglial involvement in retinal neurodegenerative diseases.

P2X7/P2X4 Receptors Mediate Proliferation and Migration of Retinal Microglia in Experimental Glaucoma in Mice

Microglia are involved in the inflammatory response and retinal ganglion cell damage in glaucoma. Here, we investigated how microglia proliferate and migrate in a mouse model of chronic ocular hypertension (COH). In COH retinas, the microglial proliferation that occurred was inhibited by the P2X7 receptor (P2X7R) blocker BBG or P2X7R knockout, but not by the P2X4R blocker 5-BDBD. Treatment of primary cultured microglia with BzATP, a P2X7R agonist, mimicked the effects of cell proliferation and migration in COH retinas through the intracellular MEK/ERK signaling pathway. Transwell migration assays showed that the P2X4R agonist CTP induced microglial migration, which was completely blocked by 5-BDBD. In vivo and in vitro experiments demonstrated that ATP, released from activated Müller cells through connexin43 hemichannels, acted on P2X7R to induce microglial proliferation, and acted on P2X4R/P2X7R (mainly P2X4R) to induce microglial migration. Our results suggest that inhibiting the interaction of Müller cells and microglia may attenuate microglial proliferation and migration in glaucoma.

Rapamycin Improved Retinal Function and Morphology in a Mouse Model of Retinal Degeneration

The retina is an important visual organ, which is responsible for receiving light signals and transmitting them to the optic nerve center step by step. The retina contains a variety of cells, among which photoreceptor cells receive light signals and convert them into nerve signals, and are mainly responsible for light and dark vision. Retinal degeneration is mainly the degeneration of photoreceptor cells, and retinitis pigmentosa (RP) is characterized by rod degeneration followed by cone degeneration. So far, there is still a lack of effective drugs to treat RP. Here, we established a stable RP model by tail vein injection of methyl methanesulfonate to study the mechanism of retinal photoreceptor degeneration. Mechanistic target of rapamycin (mTOR) is located in the central pathway of growth and energy metabolism and changes in a variety of diseases in response to pathological changes. We found that the mTOR was activated in this model. Therefore, the inhibitor of mTOR, rapamycin was used to suppress the expression of mTOR and interfere with photoreceptor degeneration. Electroretinogram assay showed that the function of mice retina was improved. Hematoxylin and eosin staining results displayed that retinal photoreceptor thickness and morphology were improved. Also, the autophagy in rapamycin group was activated, which revealed that rapamycin may protect the retinal photoreceptor by inhibiting mTOR and then activating autophagy.

Targeting Microglia to Treat Degenerative Eye Diseases

Microglia have been implicated in many degenerative eye disorders, including retinitis pigmentosa, age-related macular degeneration, glaucoma, diabetic retinopathy, uveitis, and retinal detachment. While the exact roles of microglia in these conditions are still being discovered, evidence from animal models suggests that they can modulate the course of disease. In this review, we highlight current strategies to target microglia in the eye and their potential as treatments for both rare and common ocular disorders. These approaches include depleting microglia with chemicals or radiation, reprogramming microglia using homeostatic signals or other small molecules, and inhibiting the downstream effects of microglia such as by blocking cytokine activity or phagocytosis. Finally, we describe areas of future research needed to fully exploit the therapeutic value of microglia in eye diseases.

Recent developments of neuroprotective agents for degenerative retinal disorders

Retinal degeneration is a debilitating ocular complication characterized by the progressive loss of photoreceptors and other retinal neurons, which are caused by a group of retinal diseases affecting various age groups, and increasingly prevalent in the elderly. Age-related macular degeneration, diabetic retinopathy and glaucoma are among the most common complex degenerative retinal disorders, posing significant public health problems worldwide largely due to the aging society and the lack of effective therapeutics. Whilst pathoetiologies vary, if left untreated, loss of retinal neurons can result in an acquired degeneration and ultimately severe visual impairment. Irrespective of underlined etiology, loss of neurons and supporting cells including retinal pigment epithelium, microvascular endothelium, and glia, converges as the common endpoint of retinal degeneration and therefore discovery or repurposing of therapies to protect retinal neurons directly or indirectly are under intensive investigation. This review overviews recent developments of potential neuroprotectants including neuropeptides, exosomes, mitochondrial-derived peptides, complement inhibitors, senolytics, autophagy enhancers and antioxidants either still experimentally or in clinical trials. Effective treatments that possess direct or indirect neuroprotective properties would significantly lift the burden of visual handicap.

Automated characterisation of microglia in ageing mice using image processing and supervised machine learning algorithms

The resident macrophages of the central nervous system, microglia, are becoming increasingly implicated as active participants in neuropathology and ageing. Their diverse and changeable morphology is tightly linked with functions they perform, enabling assessment of their activity through image analysis. To better understand the contributions of microglia in health, senescence, and disease, it is necessary to measure morphology with both speed and reliability. A machine learning approach was developed to facilitate automatic classification of images of retinal microglial cells as one of five morphotypes, using a support vector machine (SVM). The area under the receiver operating characteristic curve for this SVM was between 0.99 and 1, indicating strong performance. The densities of the different microglial morphologies were automatically assessed (using the SVM) within wholemount retinal images. Retinas used in the study were sourced from 28 healthy C57/BL6 mice split over three age points (2, 6, and 28-months). The prevalence of 'activated' microglial morphology was significantly higher at 6- and 28-months compared to 2-months (p < .05 and p < .01 respectively), and 'rod' significantly higher at 6-months than 28-months (p < 0.01). The results of the present study propose a robust cell classification SVM, and further evidence of the dynamic role microglia play in ageing.

Galectin-3 regulates microglial activation and promotes inflammation through TLR4/MyD88/NF-kB in experimental autoimmune uveitis

Galectin-3, an attractive molecule of innate immunity, has been reported to be involved in the neuroinflammatory diseases. However, the role of Galectin-3 in autoimmune uveitis is still unclear. The purpose of this study was to investigate the effect and mechanism of Galectin-3 on microglial activation and inflammation of experimental autoimmune uveitis (EAU)_. We immunized female C57BL/6 J mice with IRBP_651-670 to induce EAU and the specific inhibitor was intravitreally injected in EAU mice. Disease severity was evaluated by clinical and histopathological scores. Immunofluorescence, western blot, qRT-PCR analysis and immunoprecipitation were used to detect the functional phenotypes and mechanisms on microglia after Galectin-3 inhibition. Our results showed that the expression of Galectin-3 was conspicuously increased in microglia of EAU retinas. The specific inhibitor of Galectin-3, TD139 was found to ameliorate the clinical and histological manifestations of EAU mice. In addition, TD139 reduced the expression of proinflammatory factors in vivo and vitro, which are related to the severity of uveitis. In mechanism, TD139 down-regulated the expression of TLR4 and MyD88, and then inhibited the activation of NF-κB p65 in microglia. In conclusion, Galectin-3 may play important roles in a variety of immune related diseases including autoimmune uveitis. Additionally, the inhibition of Galectin-3 may attenuate the microglial activation and inflammatory response through TLR4/MyD88/NF-κB pathway, highlighting a potential therapeutic target of Galectin-3 for autoimmune uveitis.

Age of Rats Affects the Degree of Retinal Neuroinflammatory Response Induced by High Acute Intraocular Pressure

Purpose

To investigate whether retinal neuroinflammatory response was affected by aging in a rat model of acute glaucoma.

Methods

Young adult and aged rats were randomly assigned into normal control, 45 mmHg, 60 mmHg, and 90 mmHg groups. Intraocular pressure (IOP) of rats was acutely elevated to 45 mmHg, 60 mmHg, and 90 mmHg, respectively. Three days after high IOP treatment, loss of retinal ganglion cells (RGCs), formation of proinflammatory microglia/macrophages and neurotoxic astrocytes, and deposition of complement C3 in the retina were detected by immunofluorescence. ELISA was used to assess the protein levels of proinflammatory cytokines TNF and IL-1β in the retina.

Results

Compared with young adult retinae, (1) loss of RGCs was more severe in aged retinae under the same IOP treatment, (2) microglia/macrophages were more prone to adopt proinflammatory phenotype in aged retinae in response to elevated IOP, (3) high IOP treatment induced astrogliosis, formation of neurotoxic astrocytes, and deposition of complement C3 more easily in aged retinae, and (4) aged retinae induced higher levels of proinflammatory cytokines TNF and IL-1β under the same IOP treatment.

Conclusion

Our data indicated that aging affects the degree of retinal neuroinflammatory response initiated by ocular hypertension, which may contribute to the age-related susceptibility of RGCs to elevated IOP.

Hyperreflective Foci and Subretinal Fluid Are Potential Imaging Biomarkers to Evaluate Anti-VEGF Effect in Diabetic Macular Edema

Purpose: The aim was to investigate the effect and underlying mechanism of anti-vascular endothelial growth factor (anti-VEGF) in diabetic macular edema (DME) by optical coherence tomography angiography (OCTA). Methods: Twenty-five eyes in 18 treatment-naïve patients with DME were included. All eyes were imaged by OCTA at baseline and 1 week after monthly intravitreal aflibercept injection (IAI). Visual acuity was measured as best corrected visual acuity (BCVA). Additional parameters were evaluated by OCTA, including central macular thickness (CMT), the number of hyperreflective foci (HRF), foveal avascular zone (FAZ), vessel density (VD) in the deep capillary plexus (DCP), the en-face area of cystoid edema in DCP segmentation, and subretinal fluid (SRF) height. Results: The mean time between baseline and final follow-up by OCTA was 79.24 ± 38.15 (range, 28-163) days. Compared with baseline, BCVA was increased significantly after the 3rd IAI, while CMT was decreased significantly from the 1st IAI. SRF height and the area of cystoid edema in DCP segmentation were decreased significantly after the 2nd IAI compared with baseline. The number of HRF was decreased significantly after the 1st IAI (8.87 ± 9.38) compared with baseline (11.22 ± 10.63). However, FAZ's area and perimeter as well as VD in DCP showed no significant changes post-treatment. Conclusion: Anti-VEGF is effective in treating DME, improving visual acuity and decreasing macular edema. The decreased HRF indicates anti-inflammatory effects of aflibercept to deactivate retinal microglia/macrophages. The decreased cystoid edema and SRF height indicated improved drainage function of Müller glial cells and retinal pigment epithelium after IAI.

Low Expression of YTH Domain-Containing 1 Promotes Microglial M1 Polarization by Reducing the Stability of Sirtuin 1 mRNA

The N6-methyladenosine (m6A) modification is the most abundant posttranscriptional mRNA modification in mammalian cells and is dynamically modulated by a series of "writers," "erasers," and "readers." Studies have shown that m6A affects RNA metabolism in terms of RNA processing, nuclear export, translation, and decay. However, the role of the m6A modification in retinal microglial activation remains unclear. Here, we analyzed the single-cell RNA sequencing data of retinal cells from mice with uveitis and found that the m6A-binding protein YTH domain-containing 1 (YTHDC1) was significantly downregulated in retinal microglia in the context of uveitis. Further studies showed that YTHDC1 deficiency resulted in M1 microglial polarization, an increased inflammatory response and the promotion of microglial migration. Mechanistically, YTHDC1 maintained sirtuin 1 (SIRT1) mRNA stability, which reduced signal transducer and activator of transcription 3 (STAT3) phosphorylation, thus inhibiting microglial M1 polarization. Collectively, our data show that YTHDC1 is critical for microglial inflammatory response regulation and can serve as a target for the development of therapeutics for autogenic immune diseases.

Benefits of genetic and immunohistochemical markers in understanding abnormalities in aging retina

The aim of the study was to better understand the interplay between genetic factors and the aging process in the human retina through mapping complement factor H (CFH) and related proteins. Two human eyes, from 92- and 64-year-old donors, were genotyped for the expression of CFH-related 1 (CFHR1) and CFH-related 3 (CFHR3) genes. Deoxyribonucleic acid (DNA) was extracted and analyzed for concentration and purity with a spectrophotometer, at 260 nm. The results showed a DNA concentration of 469.17 ng∕μL in the aged retina and of 399.20 ng∕μL in the younger one. Through polymerase chain reaction (PCR) genotyping, the DNA CFHR1 and CFHR3 were visible as bands of 175 bp and 181 bp. Immunohistochemistry by immunofluorescence method was used with a panel of specific antibodies for CFH, CFHR1, CFHR3 and GFAP, a marker for Müller cells. All the samples were examined, and images captured using confocal microscopy. In the younger retina, CFH was localized in the inner plexiform layer and below the outer nuclear layer, while in the aged retina, it was found in the photoreceptors. CFH was also detected in the choriocapillaris and within the end-feet of the Müller cells. Our controls showed autofluorescence of the retinal pigment epithelium shedding light on a false positive CFH immunostaining of this layer. GFAP immunoreactivity highlighted an increased gliosis within the aged retina. CFHR3 signal was found in the microglia, while CFHR1 was detected in the choriocapillaris. In summary, underpinning the expression of these components can show the potential involvement of these modulators in implementing new treatment strategies.

Shaping the Microglia in Retinal Degenerative Diseases Using Stem Cell Therapy: Practice and Prospects

Retinal degenerative disease (RDD) refers to a group of diseases with retinal degeneration that cause vision loss and affect people's daily lives. Various therapies have been proposed, among which stem cell therapy (SCT) holds great promise for the treatment of RDDs. Microglia are immune cells in the retina that have two activation phenotypes, namely, pro-inflammatory M1 and anti-inflammatory M2 phenotypes. These cells play an important role in the pathological progression of RDDs, especially in terms of retinal inflammation. Recent studies have extensively investigated the therapeutic potential of stem cell therapy in treating RDDs, including the immunomodulatory effects targeting microglia. In this review, we substantially summarized the characteristics of RDDs and microglia, discussed the microglial changes and phenotypic transformation of M1 microglia to M2 microglia after SCT, and proposed future directions for SCT in treating RDDs.

Retinal Neurodegeneration in Different Risk Phenotypes of Diabetic Retinal Disease

Diabetic retinopathy (DR) has been considered a microvascular disease, but it has become evident that neurodegeneration also plays a key role in this complex pathology. Indeed, this complexity is reflected in its progression which occurs at different rates in different type 2 diabetic (T2D) individuals. Based on this concept, our group has identified three DR progression phenotypes that might reflect the interindividual differences: phenotype A, characterized by low microaneurysm turnover (MAT <6), phenotype B, low MAT (<6) and increased central retinal thickness (CRT); and phenotype C, with high MAT (≥6). In this study, we evaluated the progression of DR neurodegeneration, considering ganglion cell+inner plexiform layers (GCL+IPL) thinning, in 170 T2D individuals followed for a period of 5 years, to explore associations with disease progression or risk phenotypes. Ophthalmological examinations were performed at baseline, first 6 months, and annually. GCL+IPL average thickness was evaluated by optical coherence tomography (OCT). Microaneurysm turnover (MAT) was evaluated using the RetMarkerDR. ETDRS level and severity progression were assessed in seven-field color fundus photography. In the overall population there was a significant loss in GCL+IPL (−0.147 μm/year), independently of glycated hemoglobin, age, sex, and duration of diabetes. Interestingly, this progressive thinning in GCL + IPL reached higher values in phenotypes B and C (−0.249 and −0.238 μm/year, respectively), whereas phenotype A remained relatively stable. The presence of neurodegeneration in all phenotypes suggests that it is the retinal vascular response to the early neurodegenerative changes that determines the course of the retinopathy in each individual. Therefore, classification of different DR phenotypes appears to offer relevant clarification of DR disease progression and an opportunity for improved management of each T2D individual with DR, thus playing a valuable role for the implementation of personalized medicine in DR.

Neuroinflammation as a Therapeutic Target in Retinitis Pigmentosa and Quercetin as Its Potential Modulator

The retina is a multilayer neuronal tissue located in the back of the eye that transduces the environmental light into a neural impulse. Many eye diseases caused by endogenous or exogenous harm lead to retina degeneration with neuroinflammation being a major hallmark of these pathologies. One of the most prevalent retinopathies is retinitis pigmentosa (RP), a clinically and genetically heterogeneous hereditary disorder that causes a decline in vision and eventually blindness. Most RP cases are related to mutations in the rod visual receptor, rhodopsin. The mutant protein triggers inflammatory reactions resulting in the activation of microglia to clear degenerating photoreceptor cells. However, sustained insult caused by the abnormal genetic background exacerbates the inflammatory response and increases oxidative stress in the retina, leading to a decline in rod photoreceptors followed by cone photoreceptors. Thus, inhibition of inflammation in RP has received attention and has been explored as a potential therapeutic strategy. However, pharmacological modulation of the retinal inflammatory response in combination with rhodopsin small molecule chaperones would likely be a more advantageous therapeutic approach to combat RP. Flavonoids, which exhibit antioxidant and anti-inflammatory properties, and modulate the stability and folding of rod opsin, could be a valid option in developing treatment strategies against RP.

Polyphenol Metabolite Pyrogallol-O-Sulfate Decreases Microglial Activation and VEGF in Retinal Pigment Epithelium Cells and Diabetic Mouse Retina

(Poly)phenol-derived metabolites are small molecules resulting from (poly)phenol metabolization after ingestion that can be found in circulation. In the last decade, studies on the impact of (poly)phenol properties in health and cellular metabolism accumulated evidence that (poly)phenols are beneficial against human diseases. Diabetic retinopathy (DR) is characterized by inflammation and neovascularization and targeting these is of therapeutic interest. We aimed to study the effect of pyrogallol-O-sulfate (Pyr-s) metabolite in the expression of proteins involved in retinal glial activation, neovascularization, and glucose transport. The expression of PEDF, VEGF, and GLUT-1 were analyzed upon pyrogallol-O-sulfate treatment in RPE cells under high glucose and hypoxia. To test its effect on a diabetic mouse model, Ins2^Akita mice were subjected to a single intraocular injection of the metabolite and the expression of PEDF, VEGF, GLUT-1, Iba1, or GFAP measured in the neural retina and/or retinal pigment epithelium (RPE), two weeks after treatment. We observed a significant decrease in the expression of pro-angiogenic VEGF in RPE cells. Moreover, pyrogallol-O-sulfate significantly decreased the expression of microglial marker Iba1 in the diabetic retina at different stages of disease progression. These results highlight the potential pyrogallol-O-sulfate metabolite as a preventive approach towards DR progression, targeting molecules involved in both inflammation and neovascularization.

A Comparison of Hyper-Reflective Retinal Spot Counts in Optical Coherence Tomography Images from Glaucomatous and Healthy Eyes

Purpose: To compare the number of hyper-reflective retinal spots (HRS) in optical coherence tomography (OCT) images of healthy controls and patients affected with primary open angle glaucoma (POAG). Methods: Thirty patients affected with POAG and 34 healthy controls were recruited and underwent raster OCT examination of the macular region. Among the acquired B-scans, the one with the lowest foveal thickness was selected, and a central area of 3000 μm was defined (region of interest, ROI), in order to identify HRS. HRS were defined as small point-like hyper-reflective elements, detectable at the visual inspection of the OCT image. HRS were independently counted by two investigators in the ROI of each OCT scan. Results: Inter-rater agreement for HRS counting was good to excellent (ICC = 0.96, 95% CI: 0.83–0.99). More HRS were found in the OCT images from glaucoma patients, in comparison with healthy controls (average value: 90.5 ± 13.02 and 74.72 ± 11.35, for glaucoma and healthy subjects, respectively; p < 0.01). Significant correlations between the average number of HRS and visual field mean deviation (MD, p = 0.01) and pattern standard deviation (PSD, p < 0.01) were found. Conclusions: OCT images from glaucoma patients showed a higher number of HRS when compared with healthy controls. As HRS have been hypothesized to be a sign of neuroinflammation, these results may support the role of neuroinflammation in glaucoma etiopathogenesis.