Pathogenesis of age-related macular degeneration

Pathophysiology of macular neurovascular complex in AMD

The pathophysiologic process of age-related macular degeneration (AMD) primarily plays out in photoreceptor/RPE/Bruch's membrane/choriocapillaris (PR/RPE/BrM/CC) complex in the macular region. In this complex, retinal pigment epithelium (RPE), the central component, is a neuro-epithelial monolayer that acts as a metabolic interface between the choroid and the neurosensory retina. The RPE cells are connected by tight intercellular junctions, forming the outer blood–retina barrier. On the apical side, the photoreceptor cells line the RPE with interdigitated microvilli. RPE cell density consistently decreases with eccentricity from the fovea. The mean ratio of the cone-to-RPE cell decreases from 16.6 at the foveal center to less than 5 at 1 mm off the center. On the basal side, the BrM separates the basement membrane of the RPE from the choroidal microvasculature, that is, CC. CC is fenestrated and not surrounded by continuous pericytes or smooth muscle cells. The BrM consists of the inner and outer collagenous layers with an elastic layer between them. The basement membranes of the CC endothelium and the RPE epithelium are classified as part of the BrM. Embedded in the BrM are macromolecules such as proteins and proteoglycans to help in remodeling the extracellular matrix. Because all components of this complex interact with each other, the PR/RPE/BrM/CC complex has been termed the macular neurovascular complex (MNC) in Chapter 1. The central neural function of the MNC is phototransduction by photoreceptors, interneurons, glial cells, and RPE cells. To achieve phototransduction, CC provides fuel and RPE regulates the reciprocal exchange of oxygen, nutrients, biomolecules, and metabolic waste products between the circulation and retina. RPE provides critical support for both PR and the CC because RPE coupling of the activities of neurons and Muller cells to the choroidal blood flow serves as the structural and functional barrier between this part of the central nervous system, that is, the neurosensory retina, and the peripheral choroidal circulation.

Pathobiology of the MNC is associated with AMD development and progression, specifically, drusen formation, RPE hyperpigmentation, RPE atrophy, PR degeneration, BrM thickening, and CC angiogenesis. In attempting to elucidate the underlying pathobiologic mechanisms, increased oxidative stress, mitochondrial destabilization, complement dysregulation-related inflammation, and proangiogenic state have been proposed. However, because AMD development is multifactorial in origin, pinpointing the primary location of morphological and functional defects in MNC is still elusive. There is evidence that the lipid composition change of BrM is the earliest pathobiology of MNC in the context of AMD. This evidence shows that the ratio between neutral lipid and polar phospholipid of BrM is altered with aging, which may also contribute to gradually diffused inner BrM thickening observed in AMD. ^, Histological and optical coherence tomography (OCT) images show that atrophy of the CC in the macula, characterized by a decrease in the number and diameter of capillaries, is a critical factor in the AMD development. ^, Histological studies revealed that photoreceptor cells are lost early in the disease. Psychophysical testing shows that PR sensitivity losses are as high as 25% in cases of early AMD when visual acuity is normal. ^, Most evidence suggests that the primary pathology of AMD occurs in RPE cells, since no significant degenerative change in PR is apparent before overt deterioration of the RPE. Morphologic data also are consistent with the degeneration of PR and thinning of CC being secondary results of the degeneration of RPE cells. Furthermore, because focal RPE hyperpigmentation together with large drusen, the hallmark of AMD, are independent risk factors for the development of advanced AMD, the early RPE hyperpigmentation plays a pathogenic role in AMD progression. ^, In spite of numerous studies, the pathogenic sequence leading to AMD development is still in debate. There are at least two reasons explaining this disagreement. First, the variation in clinical manifestations implies that AMD does not follow the same course in all cases. Second, AMD is considered to be a disease of the neurovascular complex. Therefore, all components of this complex interplay when responding to etiologic triggers, resulting in different phenotypes. By mechanistic approach, the sequence of the events for each component may be less critical than how the defect of these components plays a pathophysiologic role in AMD development. In clinical study, the different phenotypes at the same stage of AMD, serving as different subgroups, need to be categorized.

Assuming that the pivotal event of AMD pathobiology occurs in RPE, questions may be raised. What can cause metabolic uncoupling of RPE cells for the disorders of the MNC? And how can the events of other components of MNC interact to cause the neurovascular damage in AMD?

RPE mitochondrial bioenergetic crisis and oxidative stress in AMD

Mitochondria are the fundamental source of energy in RPE, in which ATP is produced through several pathways. The main pathway for ATP production is oxidative phosphorylation (OxPhos). The citric acid cycle and β-oxidation are two additional energy-producing pathways. Unlike other cell types in the retina, RPE mitochondria can utilize fatty acids to produce β-hydroxybutyrate as an alternative energy source. Reactive oxygen species (ROS) are by-products of electron transfer and the systematic reduction of oxygen by mitochondria. ROS is important not only because it determines oxidative damage but also because it contributes to retrograde redox signaling from the mitochondria to the cytosol and nucleus. Increasing evidence shows that an accelerated oxidative stress of RPE mitochondria is apparent in postmortem eyes with AMD. The counteraction of ROS initiates compensatory upregulation of specific proteins. For instance, elevated antioxidant enzymes and heat shock proteins in RPE from donors with AMD were indirect evidence for increased oxidative stress. A global proteome analysis of RPE cells found that levels of 12 proteins are altered with AMD and, importantly, most of these proteins are localized to the mitochondria. Those mitochondrial proteins altered in early-stage AMD are proteins with regulatory functions such as stress-induced protein unfolding and aggregation, mitochondrial trafficking and refolding, as well as regulating apoptosis. The proteins related to the regulation of retinoic acid and regeneration of the rhodopsin chromophore were found to be affected in late-stage AMD. Another proteomic analysis of RPE mitochondria confirmed that energy production and mitochondrial protein import and refolding become defective in the eyes with AMD.

Bioenergetic homeostasis of RPE is essential for cellular function; whereas, impaired energy metabolism of RPE mitochondria drives overall retinal damage. In particular, RPE and PRs are typically codependent on glucose utilization. Glucose from the blood is largely unused by the RPE and is transported to PR. PR uses glucose through glycolysis to produce energy and the byproduct lactate. The latter is transported back to the RPE for OxPhos. Thus, they form a codependent ecosystem in using glucose. ^, As OxPhos alters in RPE mitochondria, ATP production is reduced, forcing RPE to rely on glycolysis to maintain the cell's energy requirement. Thus, the flow of glucose to the PR is reduced. Consequently, the decreased PR glycolysis could reduce the production of lactate for RPE as an energy source. In other words, suppression of PR glycolysis associated with decreased lactate promotes glucose overutilization by the RPE, starving the PR and leading to neuron degeneration and death. ^,

RPE cells of AMD patients exhibit abnormally high amounts of oxidatively modified proteins, lipids, and DNA. Oxidative stress is a state in which either increased levels of ROS are generated or the capacity to reduce ROS impact is insufficient. The generation of ROS has physiologic and pathologic implications for RPE cells. Mitochondrial electron transport chain (mtETC) of RPE is one source of ROS. ROS also may act as second messengers responding to a wide range of cellular functions, for instance, responses to hypoxia by HIF1α pathway. When mtETC malfunctions the amount of ROS may increase substantially. The increased ROS are mainly produced by the I and III complexes of mtETC in association with induced proton leak. ^, A small imbalance in mtETC function may lead to a transient accumulation of ROS, which could damage mtDNA, including genes encoding mtETC components. Expression of these damaged genes may lead to the synthesis of functionally deficient proteins of mtETC, further accumulation of ROS, and even more massive damage to mtDNA. As a result, the synthesis of faulty mtETC proteins and further ROS overproduction in repeated cycles may occur. This state is referred to as “mitochondrial vicious cycle,” in which an important element is contributing to premature aging and age-associated disease ( Fig. 7.1 ). ^,

The role of mtROS and damage to mtDNA of RPE in AMD development begins with the “mitochondrial vicious cycle.” As AMD risk factors, oxidative stress and/or mediated inflammation, may induce RPE mtDNA damage and mutations, resulting in an energy deficit and ultimately in cell death. Some cells may adapt to stress conditions and survive it into senescence. The senescent cells can be preferentially associated with RPE degeneration observed in AMD.