Aging of the eye: Lessons from cataracts and age-related macular degeneration

Abstract

Aging is the greatest risk factor for chronic human diseases, including many eye diseases. Geroscience aims to understand the effects of the aging process on these diseases, including the genetic, molecular, and cellular mechanisms that underlie the increased risk of disease over the lifetime. Understanding of the aging eye increases general knowledge of the cellular physiology impacted by aging processes at various biological extremes. Two major diseases, age-related cataract and age-related macular degeneration (AMD) are caused by dysfunction of the lens and retina, respectively. Lens transparency and light refraction are mediated by lens fiber cells lacking nuclei and other organelles, which provides a unique opportunity to study a single aging hallmark, i.e., loss of proteostasis, within an environment of limited metabolism. In AMD, local dysfunction of the photoreceptors/retinal pigmented epithelium/Bruch's membrane/choriocapillaris complex in the macula leads to the loss of photoreceptors and eventually loss of central vision, and is driven by nearly all the hallmarks of aging and shares features with Alzheimer's disease, Parkinson's disease, cardiovascular disease, and diabetes. The aging eye can function as a model for studying basic mechanisms of aging and, vice versa, well-defined hallmarks of aging can be used as tools to understand age-related eye disease.

Keywords: Age-related macular degeneration; age-related cataract; crystallins; fovea; lens; macula; retinal pigmented epithelium.

Figure 1.. Cellular structures of retina and lens.

A) Schematic illustration of human eye anatomy. Within the macula (yellow area), note the fovea. B) Cellular organization of the lens. The hexagonal lens fiber cells elongate by a factor of ~1,000 fold and they function as highly-efficient factories to produce massive amounts of crystallin proteins (Bassnett et al., 2011). Approximatelly 90% of lens cytoplasm is represented by crystallins (Horwitz, 2003) and most remaining proteins represent lens fiber cell cytoskeleton (actin filaments, 10 nm vimentin filaments, beaded filaments, and microtubules) (Rao and Maddala, 2006; FitzGerald, 2009; Quinlan and Clark, 2022). Sustained cell proliferation of the lens epithelium generates “secondary” lens fibers that are gradually added to the outer lens surface while pushing the primary fiber cells more inside to generate that lens “nucleus” and “cortex” compartments. C) Multilayered retina and light path. The vertebrate retinal anatomy is termed “inverted” as the incoming light passes first through the retinal ganglion cell layer, next through the retinal interneurons before it reaches the outer segments of ciliated photoreceptors, rods and cones (Pearring et al., 2013). Rods express rhodopsin encoded by the OPN2 (RHO, chromosome 3q22.1) which detects light/dark contrast. Cones express three opsins: L/red-cone (OPN1LW) and M/green-cone opsin genes are located next to each other on chromosome Xq28 and S/blue-cone opsin (OPN1SW) genes are located on chromosome 7q32.1. Abbreviations: Organelle-free zone, OFZ; retinal pigmented epithelium, RPE.

Figure 2.. Human macula and fovea.

A) Visualization of human macula (61-year old) by spectral domain optical coherent tomography (SD-OCT). B) Three anatomical regions, fovea, parafovea, and perifovea as identified by tracking laser tomography. Macula lutea (yellow), note absence of blood vessels in the fovea. In the foveola, cones are tightly packed, and rods are absent (Provis et al., 2005). Even the smallest retinal capillaries do not intrude within its 0.5 mm diameter forming the “foveal avascular zone” (Provis et al., 2005). Color-coded macular thickness maps showing mean (black font) and standard deviation (red) total retinal layer thicknesses in each of the nine “Early Treatment Diabetic Retinopathy Study” subfields (each 6x6 mm) divided into three concentric circles with 1, 3 and 6 mm diameters. Central area (1 mm radius) corresponds to the foveola. For a comparison, a square image sensor in modern electronic cameras has a 6 mm diagonal. The human fovea is detected via early stages of cone differentiation between 10 to 11 weeks and is defined between weeks 15 to 16 of gestation (Provis et al., 2005).

Figure 3.. Molecular structures of α-crystallin proteins.

A) An alignment of human αA- and αB-crystallin proteins and their 3D structures. Three major domains include N-terminal, α-crystallin domain (ACD), and C-terminal domains. A conserved cluster of eight β-strands in the ACD is shown as yellow boxes. The amino acid residues 73-92 (red font) of the human αB-crystallin can prevent in vitro protein aggregation and was called “minichaperone” (Bhattacharyya et al., 2006). Examples of missense cataract-linked mutations include R49C, R54C, G98R, R116C, and R116H in αA-crystallin. Likewise, missense mutations R11H, P20S, D109H, R120G, D140N, G154S, R157H, and A171T were found in congenital cataracts (Clark et al., 2012). B) Monomeric structure of the human αB-crystallin (www.rcsb.org/structure/2KLR) (Jehle et al., 2010). C) 16-mer of the human αA-crystallin (www.rcsb.org/structure/6T1R) (Kaiser et al., 2019).

Figure 4.. Molecular structures of β/γ-crystallin proteins.

A) An alignment of representative human β/γ-crystallins and their 3D structures. Each major domain is called a double “Greek key” including “cores” that are highly hydrophobic and rich in aromatic and sulfur-containing residues (Serebryany and King, 2014). A few representative human cataract mutations are shown. For example, a cataractogenic mutation Y46D (purple font) of human γC-crystallin increases susceptibility to experimental chemical and thermal stress resulting in protein aggregation and light scattering (Vendra et al., 2023). The W42R and W42Q γD-crystallin mutants of W42 (red font) cause congenital and age-related cataracts, respectively (Fan et al., 2015; Serebryany et al., 2016). Two missense heterozygous mutations R36S and R58H in human γD-crystallin (blue font) result in spontaneous crystallization of the protein within the lens (Kmoch et al., 2000; Pande et al., 2001). The γS-crystallin (Arg/Lys ratio 13/10) is the most abundant γ-crystallin in the more hydrated cortical region of the human lens (Lampi et al., 1997). In contrast, γC- and γD-crystallins, mostly located in the denser lens core, have Arg/Lys ratios of 20/3 and 21/1, respectively (Slingsby and Wistow, 2014). B) 3D-structure of the human βB2-crystallin dimer (www.rcsb.org/structure/7K7U) (Jackson et al., unpublished data). C) 3D-structure of the human γD-crystallin monomer (www.rcsb.org/structure/2G98) (Kmoch et al., 2000).

Figure 5.. Lens and its internal microcirculation system.

A) 3D visualization of the microcirculation system of the ion and water fluxes. B) Four circulatory compartments and distribution of gap junction proteins. Ions and water/fluid enter the extracellular spaces at both the anterior and posterior poles. To exit, they cross the epithelial cell membranes at the lens equator. The key molecular components are gap junction proteins GJA3 and GJA8, lens-specific aquaporin 0 (MIP), aquaporins 1 (AQP1) and 5 (AQP5) and multiple Na⁺/K⁺ pumps and K⁺/channels (Mathias et al., 2010; Donaldson et al., 2023) that control water to protein ratio and refractive power of the lens (Lim et al., 2016). C) Na⁺ circulation system and location of Na⁺/K⁺ pumps (Donaldson et al., 2023). Epithelial cell (darker blue), differentiating fibers (blue) and mature fibers (light blue). Water transport preserves solubility of crystallins and is directly coupled with delivery of antioxidants and nutrients. Glucose delivery to the lens cortex is mediated by facultative glucose transporter GLUT1/SLC2A1 (Zahraei et al., 2022). The lens hydrostatic pressure gradient is maintained via a dual feedback system and a pair of mechano-sensitive transient receptor potential vanilloid channels TRPV1 and TRPV4 (Gao et al., 2015). D) Normal human lens compared to the advanced mixed (corticonuclear) cataract.

Figure 6.. Photoreceptors/RPE/Bruch’s membrane/choriocapillaris complex.

A) Schematic representation of multiple RPE functions and their interactions with photoreceptors and choriocapillaris including light absorption, transport across the epithelium, secretion, visual cycle, and phagocytosis (Bok, 1993; Strauss, 2005; Lehmann et al., 2014). The RPE cells are highly polarized, their side in contact with photoreceptor outer segments is comprised from apical microvilli. Bruch’s membrane is located between the RPE and the fenestrated choroidal capillaries that acts as molecular sieve to regulate bi-directional traffic of biomolecules, nutrients, O₂, CO₂, metabolites, fluids, and metabolic waste between the retina and blood circulation system. The Bruch’s membrane function also includes cell adhesion of RPE cells via integrins and support both the inner and outer blood-retina barrier functions of the RPE cells (Booij et al., 2010). The outer barrier is mediated by RPE cells individually connected by tight junctions while the inner barrier is formed by a single layer of non-fenestrated retinal vascular endothelium also connected by tight junctions (Strauss, 2005). RPE cells use exosomes for waste removal into the choriocapillaris complex (Klingeborn et al., 2018; Caceres and Rodriguez-Boulan, 2020; Manai et al., 2024). These and other cellular processes, including O₂ transport, autophagy, and cell death pathways-clearance of cell remnants are discussed in the text. For detailed models, see recent reviews (Caceres and Rodriguez-Boulan, 2020; Lakkaraju et al., 2020; Ferrington et al. 2021; Blasiak et al. 2022; Kaarniranta et al. 2023). B) Schematic description of two specific RPE roles (Letelier et al., 2017): Phagocytosis of the photoreceptor outer segments (purple) (Mazzoni et al., 2014) and visual cycle (blue) (Choi et al. 2021). The roles of Mer tyrosine kinase MERTK and its ligands Gas6 and protein S are discussed elsewhere (Nandrot, 2018). The RPE cells generate and secrete growth factors towards the choroid and/or photoreceptors. These include but are not limited to pigment epithelium-derived factor (PEDF) towards the photoreceptors (Pagan-Mercado and Becerra, 2019) and vascular endothelium growth factors (VEGFs) through the Bruch’s membrane towards the choriocapillaris (Apte et al., 2019).

Figure 7.. Aging changes towards early and intermediate AMD.

A) Schematic representation of an individual RPE cell located in the macula based on a “model” 80-years-old individual showing cumulative abnormalities leading to early AMD pathology (de Jong, 2006). Lipofuscin granules, phagosomes, and phagolysosomes (with A2E) are shown. The AMD markers include lipid/lipoprotein deposition in RPE (increased number of lipofuscin and melanolipofuscin particles) and formation of basal laminar deposits including drusen (yellow). The additional processes underlying AMD are described in the text: Metabolic stress (see 4.2.), subretinal drusenoid deposits (4.5.), oxidative stress (4.6.), mitochondrial damage (4.6.), parainflammation (4.7., 5.2, and 5.5.), impaired ECM maintenance (5.2. and 5.3.), and local changes in choroidal hemodynamics (5.6.). B) Visualization of dry AMD (age 60) marked by large drusen deposits by SD-OCT (compare with Fig. 2A).