Age related distribution of 4-hydroxy 2-nonenal immunoreactivity in human retina

Ferroptosis as a potential therapeutic target for age-related macular degeneration

Cell death plays a crucial part in the process of age-related macular degeneration (AMD), but its mechanisms remain elusive. Accumulating evidence suggests that ferroptosis, a novel form of regulatory cell death characterized by iron-dependent accumulation of lipid hydroperoxides, has a crucial role in the pathogenesis of AMD. Numerous studies have suggested that ferroptosis participates in the degradation of retinal cells and accelerates the progression of AMD. Furthermore, inhibitors of ferroptosis exhibit notable protective effects in AMD, underscoring the significance of ferroptosis as a pivotal mechanism in the death of retinal cells during the process of AMD. This review aims to summarize the molecular mechanisms of ferroptosis in AMD, enumerate potential inhibitors and discuss the challenges and future opportunities associated with targeting ferroptosis as a therapeutic strategy, providing important information references and insights for the prevention and treatment of AMD.

Müller cell vulnerability in aging human retina: Implications on photoreceptor cell survival

Müller glial cells (MC) support various metabolic functions of the retinal neurons, and maintain the homeostasis. Oxidative stress is intensified with aging, and in human retina, MC and photoreceptors undergo lipid peroxidation and protein nitration. Information on how MC respond to oxidative stress is vital to understand the fate of aging retinal neurons. This study examined age-related changes in MC of donor human retina (age: 35-98 years; N = 18 donors). Ultrastructural and immunohistochemical observations indicate that MC undergo gliosis and increased lipid peroxidation, and show osmotic changes with advanced aging (>80 years). Photoreceptor cells also undergo oxidative-nitrosative stress with aging, and their synapses also show clear osmotic swelling. MC respond to oxidative stress via proliferation of smooth endoplasmic reticulum in their processes, and increased expression of aquaporin-4 in endfeet and outer retina. In advanced aged retinas (81-98 years), they showed mitochondrial disorganisation, accumulation of lipids and autophagosomes, lipofuscin granules and axonal remnants in phagolysosomes in their inner processes, suggesting a reduced phagocytotic potential in them with aging. Glutamine synthetase expression does not alter until advanced aging, when the retinas show its increased expression in endfeet and Henle fiber layer. It is evident that MC are vulnerable with normal aging and this could be a reason for photoreceptor cell abnormalities reported with aging of the human retina.

Flow cytometric method for the detection and quantification of retinal cell death and oxidative stress

Retinal cell death is the major cause of vision loss in many forms of blinding retinal disease. A plethora of research is focused on understanding the mechanisms of retinal cell death to identify potential neuroprotective strategies that prevent vision loss in these diseases. Traditionally, histological techniques have been used to determine the type and extent of cell death in the retina. These techniques, such as TUNEL labeling and immunohistochemistry, are laborious and time consuming, resulting in low throughput and variable results depending on the experimenter. To increase throughput and reduce variability, we developed several flow cytometry-based assays to detect and quantify retinal cell death. The methods and accompanying data presented demonstrate that flow cytometry can readily detect both retinal cell death and oxidative stress and importantly, the efficacy of neuroprotective agents. These methods will be of interest to investigators looking to increase throughput and efficiency without compromising sensitivity as the methods herein reduce analysis time from several months to less than a week. As such, the flow cytometry methods presented have the potential to expedite research efforts focused on developing novel strategies for retinal cell neuroprotection.

Modulating antioxidant systems as a therapeutic approach to retinal degeneration

The human retina is facing a big challenge of reactive oxygen species (ROS) from endogenous and exogenous sources. Excessive ROS can cause damage to DNA, lipids, and proteins, triggering abnormal redox signaling, and ultimately lead to cell death. Thus, oxidative stress has been observed in inherited retinal diseases as a common hallmark. To counteract the detrimental effect of ROS, cells are equipped with various antioxidant defenses. In this review, we will focus on the antioxidant systems in the retina and how they can protect retina from oxidative stress. Both small antioxidants and antioxidant enzymes play a role in ROS removal. Particularly, the thioredoxin and glutaredoxin systems, as the major antioxidant systems in mammalian cells, exert functions in redox signaling regulation via modifying cysteines in proteins. In addition, the thioredoxin-like rod-derived cone viability factor (RdCVFL) and thioredoxin interacting protein (TXNIP) can modulate metabolism in photoreceptors and promote their survival. In conclusion, elevating the antioxidant capacity in retina is a promising therapy to curb the progress of inherited retinal degeneration.

Immunohistochemical features of cells in peripheral microcystoid retinal degeneration

Peripheral microcystoid retinal degeneration (PMD) is an age-related, benign condition in which the peripheral retina develops small holes and undergoes cystic degeneration. This paper demonstrates neuronal alterations in PMD, as studied by immunohistochemistry in postmortem donor eyes (age: 76-89 years; N = 6 donors). In all cases, the degeneration was located in the inferior temporal quadrant, creating holes in the far peripheral retina. There was thinning of the inner retinal layers and the outer plexiform layer (OPL) was patchy or inconspicuous. As a response, Müller cell processes showed increased vimentin immunoreactivity. None of the retinas examined expressed glial fibrillary acidic protein. Cone photoreceptor cells were significantly altered: compared to the adjoining cones that were short, those located in the cystoid retina underwent significant elongation of their inner segments, evident from calbindin immunolabeling, to maintain synaptic contacts with the remnant OPL. The latter consisted of small photoreceptor terminals and scanty processes from shrunken bipolar cells. Besides, cones and ganglion cells undergo oxidative stress, they showed immunoreactivity to 4-hydroxy 2-nonenal and nitrotyrosine. The level of superoxide dismutase-2 was relatively low in the PMD region than in adjacent area, suggesting that the former suffers from oxidative stress.

Aging of the human choriocapillaris: Evidence that early pericyte damage can trigger endothelial changes

The choriocapillaris (CC), the capillary bed in the choroid, essentially nourishes the photoreceptor cells. Its damage in aging and age-related diseases significantly influences the survival of the photoreceptor cells. Earlier reports implicated endothelial loss in aged and diseased CC; however, age-related pericyte changes and their contribution in CC death remain unknown. We examined human donor eyes (age: 56-94 years; N = 24), and found that CC pericyte damage preceded endothelial changes. With aging (>70 years), the sub-macular choroid accumulated debris in Bruch's membrane (BM). Of the debris content, the long-spaced collagens had a tendency to settle over the capillary basal lamina (BL), and this often resulted in endothelial projection into capillary lumen. Between 75 and 83 years, pericytes contained dark mitochondria, and their processes facing the BM debris showed partial loss of BL and intermediate filaments (IFs), when the endothelium remained unaltered. The endothelial changes appeared beyond 83 years, the abundance of IFs and autophagy reinforced their survival until late aging. TUNEL⁺ pericytes, and immunoreactivity to carboxymethyl lysine and 4-hydroxy 2-nonenal, but no nitro-tyrosine, was detected in aged CC walls. Iba-1⁺ dystrophic microglia were present in the vicinity of the CC. Our data indicate that (1) BM debris exerts pressure on the CC, leading to the damage of the capillary BL and pericyte processes (2) loss of IFs results in early pericyte destabilization (3) capillary wall undergoes lipid peroxidative and glycative damage, and (4) pericyte damage leads to late endothelial changes and ultimately CC loss. Future research should explore the normal ways of pericyte maintenance in the aging nervous system.

Pathogenic mechanisms contributing to the vulnerability of aging human photoreceptor cells

In human retina, photoreceptor cell death (PCD) is a slow but conspicuous event, which continues with aging. Rods die earlier than cones, the latter continue to alter in a subtle manner until advanced aging. This review summarizes the existing information on age-related changes in photoreceptor cells, especially cones and analyses the possible associated factors. Oxidative and nitrosative stress are involved in photoreceptor alterations, which may stem from light and iron toxicity and other sources. Lipid peroxidation in macular photoreceptor outer segments and mitochondrial aberrations are prominent in aging. It is important to understand how those changes ultimately trigger PCD. The redistribution of calbindin D-28K and long/middle-wavelength-sensitive opsin in the parafoveal and perifoveal cones, anomalies in their somata and axons are strong predictors of their increasing vulnerability with aging. Signs of reduced autophagy, with autophagosomes containing organelle remnants are seen in aging photoreceptor cells. Currently, mechanisms that lead to human PCD are unknown; some observations favour apoptosis as a pathway. Since cones appear to change slowly, there is an opportunity to reverse those changes before they die. Therefore, a full understanding of how cones alter and the molecular pathways they utilize for survival must be the future research goal. Recent approaches to prevent PCD in aging and diseases are highlighted.

Correcting QUEST Magnetic Resonance Imaging-Sensitive Free Radical Production in the Outer Retina In Vivo Does Not Correct Reduced Visual Performance in 24-Month-Old C57BL/6J Mice

Purpose

To test the hypothesis that acutely correcting a sustained presence of outer retina free radicals measured in vivo in 24-month-old mice corrects their reduced visual performance.

Methods

Male C57BL/6J mice two and 24 months old were noninvasively evaluated for unremitted production of paramagnetic free radicals based on whether 1/T1 in retinal laminae are reduced after acute antioxidant administration (QUEnch-assiSTed [QUEST] magnetic resonance imaging [MRI]). Superoxide production was measured in freshly excised retina (lucigenin assay). Combining acute antioxidant administration with optical coherence tomography (i.e., QUEST OCT) tested for excessive free radical–induced shrinkage of the subretinal space volume. Combining antioxidant administration with optokinetic tracking tested for a contribution of uncontrolled free radical production to cone-based visual performance declines.

Results

At two months, antioxidants had no effect on 1/T1 in vivo in any retinal layer. At 24 months, antioxidants reduced 1/T1 only in superior outer retina. No age-related change in retinal superoxide production was measured ex vivo, suggesting that free radical species other than superoxide contributed to the positive QUEST MRI signal at 24 months. Also, subretinal space volume did not show evidence for age-related shrinkage and was unresponsive to antioxidants. Finally, visual performance declined with age and was not restored by antioxidants that were effective per QUEST MRI.

Conclusions

An ongoing uncontrolled production of outer retina free radicals as measured in vivo in 24 mo C57BL/6J mice appears to be insufficient to explain reductions in visual performance.

Iron Accumulation and Lipid Peroxidation in the Aging Retina: Implication of Ferroptosis in Age-Related Macular Degeneration

Iron is an essential component in many biological processes in the human body. It is critical for the visual phototransduction cascade in the retina. However, excess iron can be toxic. Iron accumulation and reduced efficiency of intracellular antioxidative defense systems predispose the aging retina to oxidative stress-induced cell death. Age-related macular degeneration (AMD) is characterized by retinal iron accumulation and lipid peroxidation. The mechanisms underlying AMD include oxidative stress-mediated death of retinal pigment epithelium (RPE) cells and subsequent death of retinal photoreceptors. Understanding the mechanism of the disruption of iron and redox homeostasis in the aging retina and AMD is crucial to decipher these mechanisms of cell death and AMD pathogenesis. The mechanisms of retinal cell death in AMD are an area of active investigation; previous studies have proposed several types of cell death as major mechanisms. Ferroptosis, a newly discovered programmed cell death pathway, has been associated with the pathogenesis of several neurodegenerative diseases. Ferroptosis is initiated by lipid peroxidation and is characterized by iron-dependent accumulation. In this review, we provide an overview of the mechanisms of iron accumulation and lipid peroxidation in the aging retina and AMD, with an emphasis on ferroptosis.

Microtubule alterations may destabilize photoreceptor integrity: Age-related microtubule changes and pattern of expression of MAP-2, Tau and hyperphosphorylated Tau in aging human photoreceptor cells

Photoreceptor cells undergo changes with aging. It is unknown if their microtubules are stable or not with aging. This study examined photoreceptor cell ultrastructure from 18 human donor retinas (32 eyes; age: 45-94 years) and quantified the photoreceptors with altered microtubules over six to ninth decades in four defined retinal regions. In addition, immunoreactivity (IR) to microtubule-associated protein-2 (MAP-2), tau and hyperphophorylated tau was performed in retinal sections from companion eyes. In young donor retinas below 75 years of age, microtubules appeared straight in photoreceptor inner segments and axons. With age, they appeared bent or misaligned in macular and mid-peripheral photoreceptors. In addition, dense granular materials were present in photoreceptor axons and synaptic terminals in advanced ages. In all decades, rod microtubules were affected more than their cone counterparts (28% vs 15%, p < 0.005). Both rods and cones were significantly affected in mid-peripheral retina (5-8 mm outside the macular border) in eighth decade, compared to other decades or retinal regions (parafoveal, perifoveal and nasal) examined (p < 0.005). IR showed a steady expression of MAP-2 in inner segments, and tau in inner segments to axons below 75 years of age, but was absent for both markers in scattered macular and mid-peripheral photoreceptors in advanced ages (>75 years). IR to hyperphosphorylated tau was present mainly in inner retina and increased with aging. Markers of oxidative stress, e.g., lipid peroxidation (4-hydroxy 2-nonenal) and nitrosative stress (nitrotyrosine) were immunopositive in aged photoreceptors. The sporadic loss of MAP-2 and tau-IR in photoreceptors may be due to microtubule changes; all these changes may affect intracellular transport and be partly responsible for photoreceptor death in aged human retina.

Age-related changes of the human retinal vessels: Possible involvement of lipid peroxidation

BACKGROUND

Aging of the human retina is accompanied by oxidative stress that exerts profound changes in the retinal neurons. It is unknown if oxidative stress influences the cellular components of the retinal vessels in some ways.

METHODS

We examined changes in retinal vessels in human donor eyes (age: 35-94 years; N=18) by light and transmission electron microscopy, TUNEL and immunohistochemistry for biomarkers of vascular smooth muscle cells (SMC; actin), oxidative stress (4-hydroxy 2-nonenal [HNE] and nitrotyrosine), microglia (Iba-1) and vessels (isolectin B₄).

RESULTS

The earliest changes in the endothelium and pericytes of capillaries are apparent from the seventh decade. With aging, there is clear loss of organelles and cytoplasmic filaments, and a progressive thickening of the endothelial and pericyte basal lamina. Loss of filaments, accumulation of lipofuscin and autophagic vacuoles are significant events in aging pericytes and SMC. Actin immunolabelling reveals discontinuity in arterial SMC layers during eighth decade, indicating partial degeneration of SMC. This is followed by hyalinization, with degeneration of the endothelium and SMC in arteries and arterioles of the nerve fibre layer (NFL) and ganglion cell layer in ninth decade. Iba-1 positive microglia were in close contact with the damaged vessels in inner retina, and their cytoplasm was rich in lysosomes. HNE immunoreactivity, but not of nitrotyrosine, was detected in aged vessels from seventh decade onwards, suggesting that lipid peroxidation is a major problem of aged vessels. However, TUNEL positivity seen during this period was limited to few arteries and venules of NFL.

CONCLUSION

This study shows prominent age-related alterations of the pericytes and SMC of retinal vessels. These changes may limit the energy supply to the neurons and be responsible for age-related loss of neurons of the inner retina.

Localization of nitro-tyrosine immunoreactivity in human retina

Oxidative stress (OS) is associated with retinal aging and age-related macular degeneration (AMD). In both cases there are reports for the presence of markers of lipid peroxidation in retinal cells. We investigated if nitrosative stress also occurs in the human retina with aging. We examined the cellular localization of nitro-tyrosine, a biomarker of protein tyrosine nitration, in human donor retina (17-91 years; N = 15) by immunohistochemistry. Immunoreactivity (IR) to nitro-tyrosine was present in ten retinas and absent in five retinas. It was predominant in photoreceptor inner segments, cell bodies and axons. In six retinas, IR was present in abnormal, swollen axons of macular and peripheral cones. In the inner retina, weak immunoreactivity was detected in the outer and inner plexiform layer. Transmission electron microscopy revealed a variable degree of microtubule disorganization, abnormal outgrowth from the swollen macular axons (as the fibers of Henle) and few dead axons. The present study adds further evidence to the presence of aberrant photoreceptor axonal changes in the human retina and that nitro-tyrosine immunoreactivity is associated with the photoreceptor cells in select human retina.