Mitochondrial alterations of retinal pigment epithelium in age-related macular degeneration
Introduction
Age-related macular degeneration (AMD) is a progressive neurodegenerative disease of the central retinal area (macula lutea), and it represents the most common cause of legal blindness in industrialized countries [26], [45]. Epidemiologic studies from several countries also showed dramatic increases in prevalence and in severity of AMD with age. For example, early retinal alterations were detected in 6–15% of patients in the sixth decade and 30–60% in their eighth decade of which 8.5% were blind [75]. Clinically, early AMD (dry form), synonymously known as age-related maculopathy, is characterized by the presence of small, yellowish deposits (drusen) under the retinal pigment epithelium (RPE), accompanied with either loss or focal accumulation of melanin pigment. Although the affected subjects usually have good visual acuity, they often complain of worsened quality of vision [51]. Progression of early AMD may lead to (i) the atrophic form in which the choriocapillaries and RPE-photoreceptor complex atrophies at the macular area or (ii) exudative or wet form in which exudation and hemorrhage into the subretinal space occurs, finally terminating with chorioretinal scar formation and neovascularisation. Both atrophic and exudative forms are associated with severe impairment of visual functions [11].
AMD is a multifactorial disease that affects primarily the RPE, a specialized glial tissue that provides metabolic support to both photoreceptor cells and the thin connective tissue layer (Bruch's membrane) interposed between the RPE and choriocapillaries [12]. The current pathophysiologic concept on AMD assigns a primary role to the age-related, cumulative oxidative damage to the RPE due to an imbalance between generation and elimination of reactive oxygen species (ROS) [7], [19], [77]. Specifically, lipofuscin, a byproduct of photoreceptor outer segment turnover, has been hypothesized to be the primary source of ROS responsible for both cellular and extracellular matrix alterations in AMD [36], [39], [78]. Lipofuscin is a heterogeneous material composed of a mixture of lipids, particularly lipid peroxides, proteins, and different fluorescent compounds derived mainly from vitamin A. It is frequently referred to as “age-pigment” because it accumulates in an age-dependent manner. Accumulation of lipofuscin, other lipid peroxides, and potentially toxic substances may dramatically influence the RPE physiology. In vitro it greatly reduces the phagocytic capacity, lysosomal enzyme activities and antioxidant potential of human RPE [63], [69]. N-Retinylidene-N-retinylethanolamine (A2E), the major autofluorescent component of lipofuscin, does not alter early steps of phagocytosis in cultured human RPE, but it inhibits the complete digestion of phospholipids [23]. Moreover, the autophagic sequestration of cytoplasmic material, i.e., turnover, is also markedly reduced after A2E loading [9]. In vivo studies support the in vitro effects of lipofuscin on RPE metabolism. A transgenic mouse model with impaired processing of phagocytized photoreceptor outer segments accumulates lipofuscin and manifests several clinical and histological features of AMD [58]. A2E also specifically targets cytochrome c oxidase [62], [70], and it causes caspase activation and RPE cell apoptosis [39], [78]. Thus lipofuscin is thought to be responsible for oxidative damage to RPE resulting in impaired metabolism and apoptosis characteristic for late AMD [19].
Mitochondria play a central role in aging and in the pathogenesis of age-related neurodegenerative diseases [47]. Earlier studies revealed several abnormalities of mitochondrial DNA (mtDNA) and subsequent disorders of respiratory enzyme complexes accompanied by reduced energy production, generation of excessive ROS, and activation of the apoptosis pathway [67]. However, recent studies suggest that mitochondrial dysfunctions may also include several other processes in which compositional and structural alterations of mitochondrial membranes (mtMEM) are primarily involved [38], [72]. Alterations in membrane lipid composition, such as decreases in cardiolipin content or changes in the omega-3:omega-6 ratio, impair the electron transport chain and energy production [55], ion channel and Ca2+ homeostasis of the mitochondria and of the cell [16], [28], [61], [68]. These alterations can also impair carnitine-mediated lipid transport, mitochondrial lipid metabolism [5], [79], cholesterol biosynthesis [6], [14], [31], [54], activity of pyruvate dehydrogenase complex [18], [60], and finally, cause activation of the apoptosis pathway [71].
Mutations of mtDNA and decreases in RPE cell number during aging and AMD have been described [3], [4], [17], [49]; however, neither alterations of mtMEM in aging nor a role in AMD have been elucidated. In the present paper, detailed electron microscopy of human RPE cells from aged and AMD specimens are described. Based on morphometric data, we found that (i) progressive deterioration of mtMEM with aging occurred in association with peroxisome proliferation and accumulation of lipofuscin in the RPE, and (ii) alterations of the RPE mtMEM and proliferation of peroxisomes were significantly more severe in AMD compared to normal aging.
Section snippets
Study population
Sixty-five human eyes, ages 2–87 years, were selected for these electron microscopic studies. Three eyes, aged 2, 7 and 27 years, were used only for qualitative analysis of age-related changes. Thirty-one of them were affected by early AMD (aged from 42 to 87 years, mean age 70.9 years, 20 female and 11 male), and 31 non-affected eyes were used for age- and sex-matched controls for both qualitative and quantitative morphomertic studies. The selection criteria for early AMD was based on the
Age-related changes of the RPE and Bruch's membrane
Comparison of young and aged RPE and Bruch's membrane by electron microscopy showed very marked differences (Fig. 1). Mitochondria in young RPE were numerous, mostly bacillus-like shaped, and oriented parallel with the apical-basal axis (Fig. 1A). They were typically rich in well preserved crista. In normal aged eyes, mitochondria of the RPE clearly decreased in number (Fig. 1B). They were variable in size, usually oval shape or rarely bacillus-like and without any preferential orientation.
Mitochondrial membrane changes are significantly different in normal aging and AMD
Our electron microscopic studies demonstrated that mitochondria of the RPE undergo significant morphological changes with age. These alterations are characterized by marked decreases in the number and area of mitochondria that were significantly more severe in AMD compared to age-matched controls. Included in these changes are partial to complete loss of cristae and decreases in the density of the mitochondrial matrix in both normal aging and AMD groups, which do not differ from one another in
Acknowledgements
The authors are very grateful to Ida Bozso for technical assistance in preparing microscopy specimens, to Ashraf Virmani Ph.D. for the critical reading of the manuscript, and to Britt Bromberg Ph.D., ELS, Xenofie Editing for the English editing. This work was supported in part by grant from the SigmaTau Health Science Inc., Pomezia, Rome, Italy, and from the Nutripharma Hungary Inc., Budapest, Hungary.
References (80)
- et al.
Interaction of carnitine with mitochondrial cardiolipin
Biochim Biophys Acta
(1992) - et al.
Interactions of lipids with peripheral-type benzodiazepine receptors
Biochem Pharmacol
(1988) - et al.
The role of oxidative stress in the pathogenesis of age-related macular degeneration
Surv Ophthalmol
(2000) - et al.
An international classification and grading system for age-related maculopathy and age-related macular degeneration. The International ARM Epidemiological Study Group
Surv Ophthalmol
(1995) - et al.
A cholesterol-binding and transporting protein from rat liver mitochondria
Biochim Biophys Acta
(2002) - et al.
Brain aging in the canine: a diet enriched in antioxidants reduces cognitive dysfunction
Neurobiol Aging
(2002) - et al.
Roles of long chain fatty acids and carnitine in mitochondrial membrane permeability transition
Biochem Pharmacol
(2001) - et al.
Effects of mitochondrial respiratory stimulation on membrane lipids and proteins: an electron paramagnetic resonance investigation
Biochim Biophys Acta
(1998) - et al.
Skeletal muscle dysfunction in chronic obstructive pulmonary disease and chronic heart failure: underlying mechanisms and therapy perspectives
Am J Clin Nutr
(2000) - et al.
Marked induction of sterol 27-hydroxylase activity and mRNA levels during differentiation of human cultured monocytes into macrophages
Biochim Biophys Acta
(2003)
Transport of fatty acids and metabolites across the peroxisomal membrane
Biochim Biophys Acta
Beta-oxidation-strategies for the metabolism of a wide variety of acyl-CoA esters
Biochem Biophys Acta
Photoreceptor degeneration and dysfunction in aging and age-related maculopathy
Ageing Res Rev
Retinoid cycle in the vertebrate retina: experimental approaches and mechanisms of isomerization
Vision Res
Oxidative stress-induced mitochondrial DNA damage in human retinal pigment epithelial cells: a possible mechanism for RPE aging and age-related macular degeneration
Exp Eye Res
Oxidized LDL regulates macrophage gene expression through ligand activation of PPARgamma
Cell
Age-dependent decline in the cytochrome c oxidase activity in rat heart mitochondria: role of cardiolipin
FEBS Lett
Progressive age-related changes similar to age-related macular degeneration in a transgenic mouse model
Am J Pathol
Mitochondrial dysfunction in lymphocytes from old mice: enhanced activation of the permeability transition
Biochem Biophys Res Commun
Phosphatidylglycerol potently protects human retinal pigment epithelial cells against apoptosis induced by A2E, a compound suspected to cause age-related macula degeneration
Exp Eye Res
A2E, a byproduct of the visual cycle
Vision Res
Metabolism of highly unsaturated n-3 and n-6 fatty acids
Biochim Biophys Acta
Age-related macular degeneration. The lipofusion component N-retinyl-N-retinylidene ethanolamine detaches proapoptotic proteins from mitochondria and induces apoptosis in mammalian retinal pigment epithelial cells
J Biol Chem
Age-related structural and functional changes of brain mitochondria
Cell Calcium
Visual field deficits in early age-related macular degeneration
Vision Res
Los Angeles Latino Eye Study Group. Prevalence of age-related macular degeneration in Latinos; The Los Angeles Latino Eye Study
Ophthalmology
Histochemical localisation of mitochondrial enzyme activity in human optic nerve and retina
Br J Ophthalmol
Delaying aging with mitochondrial micronutrients and antioxidants
Sci World J
Mitochondrial abnormalities in aging macular photoreceptors
Invest Ophthalmol Vis Sci
Accumulation of mitochondrial DNA deletions in human retina during aging
Invest Ophthalmol Vis Sci
Mitochondrial dysfunction in schizophrenia: a possible linkage to dopamine
J Neurochem
Inhibition of the ATP-driven proton pump in RPE lysosomes by the major lipofuscin fluorophore A2-E may contribute to the pathogenesis of age-related macular degeneration
FASEB J
The mitochondriotropic effects of l-carnitine and its esters in the central nervous system
Curr Med Chem Central Nervous Syst Agents
The Bowman lecture. Towards an understanding of age-related macular disease
Eye
The mitochondrial-lysosomal axis theory of aging: accumulation of damaged mitochondria as a result of imperfect autophagocytosis
Eur J Biochem
Mitochondria and aging: a role for the permeability transition
Aging Cell
Age-related changes in human RPE cell density and apoptosis proportion in situ
Invest Ophthalmol Vis Sci
Regulation of mammalian pyruvate dehydrogenase
Mol Cell Biochem
The role of apoptosis in age-related macular degeneration
Arch Ophthalmol
Photoreceptor phagocytosis selectively activates PPARgamma expression in retinal pigment epithelial cells
J Neurosci Res
Cited by (306)
Mitochondrial quality control in non-exudative age-related macular degeneration: From molecular mechanisms to structural and functional recovery
2024, Free Radical Biology and MedicineFerroptosis as a potential therapeutic target for age-related macular degeneration
2024, Drug Discovery TodayAge-associated macular degeneration: Epidemiologic features, complications, and potential therapeutic approaches
2024, Targeting Angiogenesis, Inflammation and Oxidative Stress in Chronic Diseases: Angiogenesis, Inflammation and Oxidative Stress in Chronic DiseasesCardiolipin-mediated temporal response to hydroquinone toxicity in human retinal pigmented epithelial cell line
2023, Biochimica et Biophysica Acta - Molecular Cell ResearchMitophagy in the retina: Viewing mitochondrial homeostasis through a new lens
2023, Progress in Retinal and Eye ResearchAnaphylatoxin C5a receptor signaling induces mitochondrial fusion and sensitizes retinal pigment epithelial cells to oxidative stress
2023, Biochimica et Biophysica Acta - General Subjects