Complement activation, lipid metabolism, and mitochondrial injury: Converging pathways in age-related macular degeneration

Abstract

The retinal pigment epithelium (RPE) is the primary site of injury in non-neovascular age-related macular degeneration or dry AMD. Polymorphisms in genes that regulate complement activation and cholesterol metabolism are strongly associated with AMD, but the biology underlying disease-associated variants is not well understood. Here, we highlight recent studies that have used molecular, biochemical, and live-cell imaging methods to elucidate mechanisms by which aging-associated insults conspire with AMD genetic risk variants to tip the balance towards disease. We discuss how critical functions including lipid metabolism, autophagy, complement regulation, and mitochondrial dynamics are compromised in the RPE, and how a deeper understanding of these mechanisms has helped identify promising therapeutic targets to preserve RPE homeostasis in AMD.

Keywords: Autophagy; Ceramide; Cholesterol; Complement; Mitochondria; Retinal pigment epithelium.

Fig. 1

Lipofuscin and drusen in the RPE. A, Generation of lipofuscin bisretinoids in the RPE. Absorption of a photon of light by 11-cis-retinal results in its isomerization to all-trans-retinal. This all-trans-retinal reacts with phosphatidylethanolamine (PE) in the photoreceptor outer segment disc membrane to form a Schiff base adduct, N-retinylidene-PE (NRPE). The ATP-binding cassette transporter A4 (ABCA4) transports NRPE across the lipid bilayer to the cytoplasmic side, where it is hydrolyzed to release all-trans-retinal, which is then reduced to all-trans-retinol by retinol dehydrogenases like RDH8 in the photoreceptor cytosol. Both the all-trans and 11-cis isomers of NRPE are substrates of ABCA4, which limits the levels of reactive aldehydes in the photoreceptor disc membranes. When ABCA4 is nonfunctional, as in Stargardt disease, NRPE can react irreversibly with a second molecule of all-trans-retinal to form bisretinoids such as A2E. The precursor, A2PE, is formed from two molecules of all-trans-retinal and one molecule of phosphatidylethanolamine. B, Upon outer segment phagocytosis, phospholipase D in RPE lysosomes hydrolyzes A2PE to generate phosphatidic acid and A2E. In contrast to the well-characterized pathway that leads to lipofuscin accumulation in the RPE, mechanisms involved in the formation of sub-RPE and sub-retinal drusen remain to be understood.

Fig. 2

Mechanisms of RPE dysfunction mediated by cholesterol and ceramide. A, In the lysosomal membrane, cone-shaped lipids like A2E and cholesterol compete for space under the phospholipid umbrella to minimize interactions with the aqueous phase. A2E displaces cholesterol, which is trapped within the lysosome lumen (Lakkaraju et al., 2007). B, RPE cholesterol sequesters the anionic lipid bis(monoacyl)glycerophosphate (BMP), which is a cofactor for acid sphingomyelinase (ASM), the lysosomal enzyme that hydrolyzes sphingomyelin to ceramide (Toops et al., 2015). C, Increased ceramide in the RPE leads to the accumulation of stable, acetylated microtubules, which impairs trafficking of autophagosomes (Toops et al., 2015), recycling of the complementary regulatory protein CD59, and relocalizes lysosomes to the perinuclear region of the RPE (Tan et al., 2016). As discussed in Sections 3.2, 3.3, this impairs autophagy and makes the RPE susceptible to complementmediated mitochondrial injury (Toops et al., 2015; Tan et al., 2016). Ceramide is also a cone-shaped lipid that induces negative curvature and inward budding at the RPE plasma membrane. Increased ceramide in RPE with bisretinoids drives the formation of enlarged early endosomes that internalize the complement protein C3. Proteolysis of C3 to biologically active C3a leads to mTOR activation and associated metabolic stress (Kaur, Tan et al., 2018).

Fig. 3

Feed-forward cycles of RPE dysfunction. As discussed in Section 3, bisretinoids, lipids, oxidative stress and other genetic and environmental factors induce multiple stress pathways that culminate in loss of RPE homeostasis.

Fig. 4

Converging pathways of lipid metabolism, complement activation, and mitochondrial injury in AMD. The interconnected web summarizes studies discussed in the text regarding current knowledge about mechanisms of RPE injury. We propose that AMD-associated genetic risk variants and environmental factors act as “tipping points” or decision nodes that can drive the RPE from normal aging towards disease. In magenta are potential therapeutic interventions discussed in Section 4 to target specific nodes of this network.