Open access peer-reviewed chapter
Abstract
Age-related macular degeneration (AMD) is a complex, degenerative and progressive chronic disease that leads to severe visual loss. The prevalence of early AMD accounts for 18% in the population between 65 and 74 years of age and even 30% in subjects older than 74 years. The articles published in the last decade point out to a significant role of oxidative stress in the onset and development of age-related macular degeneration. Generally, reactive oxygen species (ROS) are produced in the eye during light absorption and physiological metabolic processes. The level of oxidative stress is kept under control by the action of antioxidants and reparative enzymes. Excessive synthesis of ROS leads to increased oxidative modification of lipids, proteins and DNA, causing oxidative damage of cytoplasmic and nuclear cell elements and changes of the extracellular matrix. The accumulation of oxidatively modified compounds in drusen deposits will initiate the onset and development of AMD. The objective of this review was to highlight the mechanisms of oxidative stress in order to elucidate their significance and association with the pathogenesis of AMD.
Keywords
- age-related macular degeneration
- antioxidants
- oxidative stress
- reactive oxygen species (ROS)
- retinal pigment epithelium (RPE)
1. Introduction
Age-related macular degeneration (AMD) is a complex, degenerative, progressive, multifactorial disease with multiple genetic and environmental factors contributing to its onset and progression [1].
Age-related macular degeneration (AMD) represents damage of the retinal macula and thus the central visual field and is the leading cause of blindness and visual impairment in people over 60 years of age [2, 3]. Prevalence of an early AMD (the presence of medium-sized drusen or drusen with degeneration or hyperpigmentation of the retinal pigment epithelium (RPE)) is 18% in the elderly population between 65 and 74 years of age and as much as 30% in the population older than 74 years [4].
The initial site of damage, according to most researchers, is the retinal pigment epithelium, although some authors find primary damage in the choriocapillary or extracellular matrix of the sensory retina. Regardless of the location and mechanism of initial damage, there is an opinion that oxidative stress plays an increasingly important role in the genesis of age-related macular degeneration.
The eye is a unique organ, because it is constantly exposed to radiation, atmospheric oxygen, chemicals from the environment, but also to the physical damages [5]. Therefore, oxidative stress is one of the most important mechanisms of the onset of many eye diseases such as cataract, glaucoma, uveitis, retrolental fibroplasia, age-related macular degeneration, as well as various forms of retinopathy [6]. Most free radicals are formed as bioproducts of normal cellular physiology. The most common damage to the eye by free radicals is caused by hydroxyl radical (OH
The objective of this review was to describe the mechanisms of oxidative stress in order to elucidate their significance and association with the pathogenesis of AMD.
2. The classification of AMD
Based on the Beckman AMD classification system, the disease is classified into early-stage AMD, intermediate-stage AMD and late-stage AMD [8]. Early-stage AMD encompasses the presence of medium-sized drusen (63–125 μm) without any impairment of visual function. Intermediate-stage AMD is defined by the presence of large drusen (>125 μm) or/and abnormalities in the RPE. Late-stage AMD (advanced AMD) is classified into two clinical entities: central geographic atrophy (GA, dry or nonexudative AMD) and neovascular AMD (wet or exudative AMD) [9]. Irreversible loss of vision occurs in geographic atrophy when there is an irreversible loss of RPE and photoreceptor cells, usually in the perifoveal region of the macula. In the neovascular form of AMD, there is an invasion of new choroidal blood vessels (choroidal neovascularization-CNV), followed by retinal detachment and RPE and vision loss [9].
3. Oxidative processes and the onset of AMD
Oxidative processes participate in almost all pathological processes in the eye. The presence of oxidative stress has also been registered in uveitis, diabetic retinopathy, various forms of glaucoma, cataractogenesis and other degenerative processes [10]. As highly reactive intermediates, free radicals can lead to oxidative tissue damage through a number of mechanisms such as peroxidation of unsaturated fatty acids leading to disturbances in the permeability and fluidity of biological membranes, which is accompanied by increased membrane permeability. Oxidation of thiol groups of enzymes leads to a decrease in their activity and even inactivation of enzymes. Fragmentation of fatty acid chains leads to loss of membrane integrity, while disruption of lysosomal membrane continuity leads to release of hydrolytic enzymes and cell damage [11].
The oxidation of polyunsaturated fatty acids in the phospholipids of the cell membranes could damage the cell integrity and function. In addition to negative and destructive effects, this process may have important physiological functions such as: the lipid metabolism regulation, and changes in their physicochemical properties and permeability. Under controlled conditions, ROS enables the control of synthesis of biologically active prostaglandins and leukotrienes, proliferation and initiation of cell death.
Proteins are also targeted by free radicals’ action that could change their primary, secondary and even tertiary structure. Oxidative modifications of the primary structure of proteins resulting from the modification or loss of some amino acids or aggregation and fragmentation of proteins, which are reflected in changes in solubility and charge, are described [12]. These processes affect the integrity of the cell and its function and lead to oxidative tissue damage.
Oxidation of nucleic acids leads to changes in DNA structure, gene mutations, synthesis of inadequate genes or lack of synthesis of other genes. As a result of such processes, malignant cell transformations occur. Mitochondrial DNA is particularly sensitive to such transformations.
The degree of biomolecule damage depends on their vulnerability and intensity of oxidative stress. The repair of primarily damaged molecules results in structural changes that remain at the molecular, i.e. at the cellular level. At one point, the damage becomes so great that it exceeds the critical mass. At that moment, the symptoms of illness appear [13].
Eye damages caused by these changes as well as the mechanisms of antioxidant protection show certain specifics, not only in the eye as a special organ but also in its highly differentiated and specialised structures. Oxidative stress in epithelial cells occurs mainly as a consequence of a photodynamic process or as a by-product of oxidative phosphorylation in mitochondria.
The retina is very complex in its structure, and it is one of the highest oxygen-consuming tissues that continuously transforms light into vision, generating reactive oxygen species (ROS), such as the superoxide (O2•−), the hydroxyl radical (•OH), hydrogen peroxide (H2O2) and singlet oxygen (1O2) as normal metabolic by-products [14]. Generally, ROS are produced during oxidative metabolism under physiological conditions and participate in normal cellular metabolism [15]. Retinal photoreceptor membranes are rich in polyunsaturated fatty acids.
Photooxidative retinal damage is in the function of duration of intensity and wavelength of light. Changes that occur in the pigment layer of the retina are considered to be initial in the process of genesis of age-related macular degeneration. During ageing, functions of all senses gradually weaken. Degenerative processes in the eye and especially in the lens are the first signs of ageing that are noticed [10].
4. The synthesis of free radicals during light exposure
It is well established that light exposure has the potential to cause detrimental effects in RPE and retina as well as in many other organs and tissues, such as the skin, cornea, conjunctiva and lens [16].
Large quantities of ROS are produced by exposure to ultraviolet light (λ = 100–400 nm) and to blue light (λ = 400–500 nm) [17]. The photoreceptors in the macula absorb parts of the light spectrum through rhodopsin, a photoreceptor molecule in rods [18].
Roehlecke and Schumann [19] suggested that the synthesis of ROS occurred directly in outer segments of photoreceptors in the reaction catalysed by the enzyme nicotinamide adenine dinucleotide phosphate oxidase (NOX) as well as by the mitochondrial activity of the outer segments after absorption of visible blue light (λ = 405 nm) with an output power of 1 mW/cm2 [19]. The authors found that the generation of ROS is highly increased in the photoreceptors of retinal explants after 0.5–1 h of blue light absorption, due to increased NOX activity (especially NOX2 and NOX4). Under these conditions (light exposure of 1 mW/cm2), it is possible to do the following extrapolation to the superoxide anion [20]: 1) One granule of lipofuscin can synthesise 8 x 10−19 mol of superoxide anion/min; 2) since 1 mol contains 6.02 x 1023 molecules, then 1 granule is capable of producing 4.8 x 105 molecules of superoxide/min; and 3) if we take into account that the average cell volume is 2000 μm3 and if up to 19% of that volume is occupied by lipofuscin granules 1 μm in diameter, then each RPE cell has a synthesis capacity of 3.5 x 108 superoxide anions/per cell per minute [21].
This high level of free radical synthesis may explain why RPE cells contain a high concentration of various antioxidants [22]. The spectral dependence of lipofuscin explains the so-called ‘blue light hazard’ on retina. Light with a wavelength of 550 nm or less can cause ‘actin’ or photochemical damage, but is too low to cause thermal effects [23]. These photochemical lesions are expressed at the level of RPE, where the action spectrum of ‘blue light’ is similar to the bandwidth of the absorption spectrum of melanin [24] and lipofuscin [22, 25]. Photoreactivity analysis of blue light in freshly isolated RPE cells shows a high level of oxygen uptake with increasing age of the donor and that this ‘photo-uptake’ is predominantly related to lipofuscin [26]. These observations suggest a different function of lipofuscin in cells that may explain the association between high levels of lipofuscin and AMD. RPE cells are rich in antioxidants which may be enough to detoxify any reactive oxygen species [27]. Conversely, antioxidants may be insufficient to detoxify all radicals throughout life so that oxidative damage can manifest at some point in life (for example, in old age).
It has also been observed that lipofuscin photosensitivity reactions lead to increased intragranular lipid peroxidation, measured through the accumulation of lipid peroxides and malondialdehyde in pigment granules [26, 28]. Moreover, lipofuscin can perform extracellular lipid peroxidation and enzyme inactivation. Freshly isolated lipofuscin granules incubated with visible light induced up to a 30% increase in lipid peroxidation, compared with the control. Granules incubation with catalase (antioxidant) and lysosomal enzymes (acid phosphatase), in the presence of light, causes as much as 30–50% reduction in enzyme activity. Lipid peroxidation and loss of enzyme activity can be prevented by antioxidants which indicate that lipofuscin photodamage is a product of action of free reactive oxygen species. It is generally accepted that RPE cell dysfunction is an early, crucial moment in the pathogenesis of AMD [29, 30].
RPE cells have a variety of functions from metabolic to supportive, and they are vital for photoreceptors including maintenance of the blood-retinal barrier, participation in the visual cycle (uptake, transport and release of vitamin A and its metabolites) as well as in degradation and uptake of apical phagocited parts of the photoreceptor outer segments [31]. One of the leading factors of RPE cell dysfunction is age-related phagocytic and metabolic insufficiency of postmitotic RPE cells, leading to progressive accumulation of lipofuscin granules which are mainly composed of lipids (~50%) and protein (~44%) of phagosomal, lysosomal and photoreceptor origin (including the retinoid transporter-cellular retinaldehyde binding protein/CRALBP). These substances from the lipofuscin composition can be oxidatively modified either as a result of exposure to UV light or high doses of oxygen in the eye [29, 30].
The well-known cytotoxic constituent of lipofuscin is fluorophore bisretinoid which consists of two retinoid chains derived from the pyridinium ring (A2E) which together with other photoreactive molecules is a powerful photoinducible ROS generator with a strong effect on oxidative damage of lipids, proteins and DNA [31]. N-retinyl-N-retinyldiene-ethanolamine 2-(2,6-dimethyl-8-(2,6,6- trimethyl-1-cyclohexene-1-yl) -1E, 3E, 5E, 7E-octyltetraenyl]-1-(2-hydroxyethyl)-4-[4- methyl-6-(2,6,6-trimethyl] or A2E increases the RPE sensitivity to blue light and exhibits several toxic effects on RPE cells [32, 33]. By the action of light of wavelength, λ = 430 nm, A2E is converted to A2E-epoxide by binding to oxygen. The resulting epoxide can destabilise the membranes of mitochondria and lysosomes [34] and can also inhibit cytochrome oxidase, leading to disruption of electron flow in the respiratory chain [35]. This process, in addition to producing more ROS (reactive oxygen species), reduces the efficiency of energy metabolism. An alternative A2E toxic pathway has been described by Finnemann [36], who in a study with A2E-laden RPE cells demonstrated the presence of destabilised lysosomes, resulting in incomplete digestion of phagocited photoreceptors of the outer segments during 24 h. Since phagocytosis is a circadian regulated process, this will constantly increase the non-degraded phospholipids that are a source of ROS. Mitochondrial destabilisation and incomplete digestion of lipids and proteins caused by lysosome destabilisation lead to increased free radical accumulation. In a closed circle, this mechanism destabilises RPE cells, leading to their loss and this process conditions the initiation of drusen formation [37].
Although lipid peroxidation products are considered to be the main substrates for the genesis of lipofuscin and its cytotoxic constituents, other identified lipofuscin proteins also play a significant role in cytotoxicity [30, 31].
The study of King et al. conducted on the human adult RPE cell line-19 (ARPE-19) revealed that the mitochondrial electron transport chain was an important source of ROS which played a critical role in the death of cells exposed to short-wavelength blue light (425 ± 20 nm) [38].
Except lipofuscin, several other retinal pigments, such as rhodopsin and melanin, were shown to be involved in the oxidative stress process [39]. Grimm et al. reported rhodopsin-mediated blue-light-induced damage in the retina, which occurred after short time exposure to the blue light [40].
In the RPE, lipofuscin is derived primarily from phagocytosis of shed photoreceptor outer segments and is considered a heterogeneous waste material that accumulates with age in active postmitotic cells, such as those of the RPE [41]. The RPE cells are able to phagocyte the photoreceptors of outer segments (POSs) that contain a high amounts of unsaturated fatty acids [42]. During phagocytosis, a high quantity of oxygen is consumed and a significant production of ROS occurs (generated by NOX or peroxidase) via the oxidation of fatty acids in the POSs [43]. Mitter et al. in their study have shown that autophagy plays a significant role in protection of the RPE from oxidative stress [44]. Recent evidence showed that dysfunctional autophagy/mitophagy in the RPE may lead to mitochondrial disintegration by affecting the mitochondrial fission/fusion ratio, resulting in excessive amounts of ROS [45].
5. The synthesis of oxidatively modified compounds in the eye, by the action of free radicals
Excessive synthesis of free radicals in the eye is associated with the production of oxidatively modified compounds and cytotoxic damage of ocular structures. The cell types with relatively high levels of polyunsaturated fatty acids (PUFAs), such as retinal cells, are highly sensitive to lipid peroxidation. Polyunsaturated fatty acids in phospholipids and glycolipids are the basic substrate of oxidative damage to lipids caused by free radicals.
Lipid peroxidation, a complex process involving the interaction of oxygen-derived free radicals with polyunsaturated fatty acids, finally results in a variety of primary compounds: highly reactive compounds (alkyl radicals, conjugated dienes, peroxy and alkoxy/oxyl radicals and lipid hydroperoxide). During further decomposition of primary compounds, a series of secondary products are produced such as: short-chain evaporable hydrocarbons, aldehydes and end products of lipid peroxidation (i.e. isoprostanes, MDA, 4-hydroxy-2,3, trans nonenal and 4,5-dihydroxydecenal) [46, 47].
MDA is a secondary product of peroxidation of unsaturated fatty acids, (particularly arachidonic acid) and is a physiological ketoaldehyde [48]. In a higher concentration, it reacts with free amino groups of proteins (especially with lysine cysteine or histidine residue). Such modified protein structures have immunogenic features. Some studies have shown that an increased titre of these autoantibodies directly correlates with the extent of oxidative damage and may predict the progression of some diseases. It was suggested that reduced ability to protein proteolysis after their oxidative modification with MDA and 4-HNE represents one of the main factors of lipofuscin synthesis during the development of AMD [49].
Lipid peroxidation highly reactive end products, such as 4-hydroxylnonenal (4-HNE), malondialdehyde (MDA), oxidised nucleotides and carboxyethyl pyrrole (CEP), have been demonstrated to be associated with drusen formation and RPE atrophic modifications in both human and animal eye [50].
Recently, Kim et al. [51] found that the injection of hydroperoxy-octadecadienoic acid (HpODE), (a peroxidized lipid) into the subretinal space of a murine AMD model, could initiate an early increase in the expression of markers of oxidative stress and lipid peroxidation, especially high levels of 4-HNE and MDA [51]. Zor et al. [52] documented a significantly increased MDA values (~15%) in patients with neovascular AMD compared with the controls [53].
F2-isoprostane (F2-IsoPs) is another marker of lipid peroxidation which is considered to be an important ‘in vivo’ marker of oxidative damage in AMD [54, 55]. Sabanayagam et al. [56] demonstrated that the presence of F2-IsoPs in urine was positively associated with AMD.
Oxidative DNA damage of both nuclear and mitochondrial genomes can result in strand breaks, base modifications and DNA-protein cross linkages which are all strongly implicated in ageing and age-related diseases [57, 58]. Over 20 base modifications related to ROS attack of DNA are identified, with the following oxidative DNA damage products: 8-oxo-7,8-dihydroadenine, 8-oxo-7,8-dihydroguanine, 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) and 5,6-dihydroxy-5,6-dihydrothymine as well as the ring-opened lesions of 4,6-diamino-5-formamido-pyrimidine and 2,6-diamino-4-hydroxy-5-formamido-pyrimidine [59]. The 8-oxodG, which is formed through the oxidation of guanine at the C8 position in the guanine base, serves as a reliable biomarker of oxidative stress and oxidative modification of DNA, and it is associated with ageing and ageing-related diseases [58].
Age-related increases in lipofuscin, 8-oxoguanine, CEP, 4-HNE and MDA expression have been observed in the ageing retina [60, 61, 62] which have been reported to cause inflammatory responses and AMD features [63].
6. The pathophisiology of AMD-Drusen formation
Age-related macular degeneration is characterised by degenerative changes involving the outer portion of the retina, RPE, Bruch’s membrane and choriocapillaris. Drusen are considered as a hallmark of AMD and as an amorphous deposit that accumulates extracellularly in the zone between the RPE and the inner collagen zone of the Bruch’s membrane [64]. Clinically, they are divided into two main phenotypes: ‘soft and hard’, depending on their relative size and shape. A few smaller hard drusen (<65 μm) can be found in at least 95% of the elderly population, but do not represent AMD. Only the presence of larger drusen (>125 μm), especially soft drusen (> 125–250 μm) in the macula, is considered as a major risk factor for the development of advanced forms of AMD, i.e. exudative-neovascular forms, especially if they are combined with pigmentation disorders [65].
In the later stage of AMD, neovascularization, exudative changes or disciform scars can occur. In the atrophic form of AMD, there is a loss of pigment epithelium or ‘attenuation’ of the choriocapillaris but without neovascularization [66]. Early pathological changes include basal deposits in the Bruch’s membrane which occur exclusively in pathological samples and have two types: a) basal laminar deposits consisting of basement membrane proteins and long collagen filaments located between RPE and basement membrane and b) basal linear deposits that are more specific for early AMD changes and consist mainly of membrane material located in the Bruch membrane, externally from the RPE basement membrane. The combination of these deposits with secondary changes in RPE results in the formation of drusen [67].
Many different molecules have been identified in drusen, including glycoconjugates and other compounds also found in atherosclerotic plaque (hence the link between atherosclerosis and AMD formation by some authors), including vitronectin, apoprotein B and E, α-crystalline, HtrA1 and lipids [68, 69, 70]. Macrophages found in drusen regression suggest a possible hypothesis that macrophages are involved in the process of degradation of deposits within the Bruch membrane [71]. Activated microglias have also been found in AMD degenerative lesions [72]. Discrete nodules or hard drusen deposits consisting of hyaline-like material were found between the RPE and the Bruch membrane. Soft drusen are usually large and occur with detachment of RPE cells and diffuse changes of the Bruch membrane. They can occur in deeper damages of the RPE and choroids and lead to choroidal neovascularization or cell death in the RPE as well as geographical atrophy. Autofluorescent pigments, such as lipofuscin, which are accumulated in RPE cells, reach a size that often leads to decreased cellular function, retinal ageing and degeneration, mostly in the form of geographic atrophy [73].
Lipofuscin in RPE is the most common cause of fundus autofluorescence. These are spherical particles of micrometre size with characteristic yellow fluorescence when exposed to blue light. The main component of lipofuscin is N-retinylidine-N-retinyletanol-amine (A2E), a quaternary amine and retinoid bioproduct of visual cycle [74]. Lipofuscin synthesis is a pathogenic reaction in which the resulting A2E interferes with the function of RPE cells and leads to their apoptosis. Choroidal neovascularization can occur in the macular, peripapillary and peripheral regions. Early choroidal neovascularization occurs below the RPE cells to later break through the RPE layer and develop an exudative, haemorrhagic or disciform form of AMD. In the neovascular form of AMD, lipid accumulation occurs below the RPE or neuroretin. In the haemorrhagic form of AMD, blood penetrates through the RPE into the subretinal space and sometimes through the retina to the vitreous. In the disciform form of AMD, fibrous tissue with neovascularization and changes in RPE cells proliferates and may partially or totally replace neuroretin [75]. Additional pathological lesions include serous exudation, haemorrhage, gliosis and calcification. Macrophages have been proven both morphologically and functionally in the neovascular form of AMD [76]. Activated macrophages and microglias can secrete chemokines and cytokines, causing further cell damage, degradation of the Bruch’s membrane and angiogenesis [77].
7. Oxidative stress and choroidal vascular changes in AMD
Among AMD cases, approximately 10–15% have neovascular AMD characterised by abnormal vascular morphology and growth [8]. Vascular endothelial growth factor (VEGF) upregulation plays a crucial role in the development of neovascular AMD. Yi and assoc. [78] documented an increased VEGF expression in a study using laser to induce choroidal neovascularization (CNV) in rats. This author suggests that the macrophages could be probably the most important source of VEGF in the early phase of AMD [78]. VEGF expression in subfoveal fibrovascular membranes was concentrated in cells resembling fibroblasts, implicating a significant role of fibroblasts in the progression of CNV [79]. The results showed that even temporary overexpression of VEGF in RPE cells was sufficient to induce CNV in the rat eye [80]. Wang et al. reported that IQ protein motif-containing GTPase activating protein 1 (IQGAP1), scaffold protein with a Rac1-binding domain, regulated VEGF activation by binding to Rac1GTP in choroidal endothelial cells, activating their migration [81]. IQ motif-containing GTPase-activating protein 1 (IQGAP1) is a ubiquitously expressed scaffold protein that is involved in multiple cellular functions such as cell survival and trafficking [82].
The vascular endothelial dysfunction is considered as a crucial event in development and progression of choroidal vascular dysfunction [82]. Nitric oxide and nitric oxide synthase enzymes have been shown to be involved in the upregulation of VEGF. Nitric oxide synthases (NOSs) are a family of enzymes that catalyse the conversion of L-arginine into nitric oxide (NO). They are classified into three isoforms: endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS) [83]. The eNOS maintains the physiological function of the vascular endothelium [84]. It was demonstrated that eNOS mediated endothelium-dependent vasodilation in retinal arterioles and ophthalmic arteries [85]. NO is considered not only a mediator of vasodilation, but also a regulator of various vascular functions. For example, physiologically, NO can dilate a blood vessel, inducing relaxation of vascular smooth muscle cells (VSMCs), inhibiting cell proliferation and regulating angiogenesis and vascular permeability [86]. Bhutto et al. [87] reported that eNOS and nNOS expression was significantly decreased in the eyes of AMD patients. This author suggested that the decreased expression of eNOS and nNOS might reduce the NO production that could induce hemodynamic changes in CNV [87].
It was documented that excessive amounts of NO can have detrimental effects on cells and tissues, implicated that the production of NO is not always beneficial. In that case, NO can be an important stimulator of CNV. Ando et al. suggested that blockade of nNOS and iNOS could reduce CNV formation [88]. Excessive amounts of nitric oxide can react with the superoxide anion to form peroxynitrite, a very toxic and reactive radical which compromises vascular endothelial function [89].
There is some evidence that ROS and vascular dysfunction may together contribute to the pathology of neovascular AMD. It was demonstrated that NOX was the connection between VEGF and ROS in human choroidal endothelial cells [90]. The family of NOX consists of seven isoforms such as: NOX1, NOX2, NOX3, NOX4, NOX5, dual oxidase (Duox) 1 and Duox2) which are differentially expressed in tissues and cells [91]. NOX1, NOX2 and NOX4 are expressed in choroidal vascular endothelial cells [92]. ROS generated by NOX function as signalling molecule promoting endothelial cell proliferation, migration and tube formation [92]. Some studies documented that ROS generated from NOX2 could activate the transcription factors NF-κB and activator protein 1 (AP-1) and increase the expression of intracellular adhesion molecule (ICAM)-1 and VEGF leading to vascular hyperpermeability and retinal neovascularization [93]. Moreover, NOX4-derived ROS generation is essential for the expression of hypoxia-inducible factor 1-alpha (HIF-1α) which was linked to cell proliferation and migration of vascular smooth muscle cells [94].
8. Other mechanisms involved in the development of neovascular AMD
One of the risk factors for AMD may be increased collagen synthesis in the choriocapillaris which is then incorporated into the Bruch’s membrane, creating thickenings that precede the appearance of linear deposits [95]. Chromatographic analysis of the drusen showed that they contained more than a hundred different proteins originated from retinal pigment epithelial cells, neuronal retinas and choriocapillaris. However, their composition differs depending on the existence i.e. absence of AMD. It is thought that certain ingredients can promote angiogenesis. The integrity of the RPE cellular structures in a culture that is chronically exposed to oxidative stress is impaired by the action of hydrogen peroxide due to the interruption of intercellular compounds. This is one of the possible mechanisms of breaking the blood-brain barrier in the pathogenesis of AMD [96].
Programmed cell death (apoptosis) is an essential protective mechanism of the organism against the accumulation and spread of damaged or unnecessary cells. An increased degree of apoptosis is observed in most ageing cell populations. A similar thing happens in RPE cells. There is an opinion that mitochondria play a key role in the regulation of apoptosis. Reactive oxygen metabolites that are formed in RPE cells exposed to the blue part of the spectrum originate in mitochondrial processes [97]. Oxidative stress can reduce the sensitivity of senescent cells to apoptosis through defective oxidative phosphorylation. The process of drusen formation is very similar to apoptotic process in the retina and predisposes the development of neovascularization during the progression of AMD [98].
In postmitotic tissues, during ageing, the oxidatively modified and damaged mitochondrial DNA are accumulated in mitochondria. It is believed that their genetic material is the main substrate of oxidative damage in the retinal pigment epithelium. With inefficient damage repair, redox potential of mitochondria in the human RPE retinal cells is compromised over time in photoreceptors as well [99].
It was suggested that other types of regulated cell death (e.g. pyroptosis, necroptosis and autophagy) may contribute to development of AMD [100]. Ferroptosis is a newly discovered, iron-dependent, regulated cell death pathway that is initiated by lipid peroxidation. It is implicated in neurodegeneration, ischemia–reperfusion injury and myocardial infarction [101]. It is characterised by iron-dependent accumulation [102, 103]. In contrast to apoptosis, ferroptosis is a pro-inflammatory condition that arises due to the release of intracellular content after the rupture of plasma membrane [104]. Under normal conditions, ferroptosis is a mechanism that protects cellular integrity, but leads to cell death when cellular integrity is compromised, while apoptosis represents a suicide mechanism that eliminates certain types of cells from the whole organism at specific time points [105].
It was documented that angiotensin II (Ang II) was implicated in the pathology of AMD. It was shown that Ang II can mediate various pathological processes in ocular blood vessels such as proliferation and migration of smooth muscle cells and pericytes, increase of VEGF expression and potentiation of VEGF-dependent angiogenic activity [106, 107]. Receptors for AngII have been identified in retinal and optic nerve blood vessels. Some studies have shown that blocking the renin-angiotensin system may delay the breakdown of the blood-retinal barrier and prevent retinal neovascularization and the development of AMD [108].
9. Conclusion
In this review, we tried to highlight the pathways of oxidative stress and their implication in the pathogenesis of AMD. Considering the unique structure and function of the retina in the eye, as well as the environment in which it is located, it indicates a significant synthesis of free radicals during normal physiological processes as well as during light absorption. The presence of free fatty acids and their exposure to free radicals make lipid peroxidation processes a daily occurrence in the eye. This was confirmed by many studies that found high concentrations of MDA, 4-HNE and other lipid peroxidation products in the eyes (and blood) of AMD patients. Oxidative damage of mitochondria and nuclear DNA was also observed in AMD patients, as well as increased products of oxidative damage of proteins. An impairment of autophagy and other types of cell death such as pyroptosis, necroptosis and ferroptosis were also described in AMD patients. The upregulation of VEGF and isoforms of NOX with impairment of NO synthesis have significant implications in the development of new blood vessels and the onset of choroidal neovascularization (CNV) in the pathogenesis of advanced-wet AMD. In view of all the above, further research is certainly needed in order to find adequate methods for disease prevention as well as adequate drugs for the treatment of various forms of AMD.
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