Age-related macular degeneration (AMD) is a leading cause of central vision loss in older adults. In adults over 40, early AMD has been reported to have a prevalence of 11.64 % and late AMD has a prevalence of 1.49 % (Rein et al., 2022). Early in the development of AMD, the choriocapillaris (the microvasculature that supplies the photoreceptor cells) begins to degenerate below the retinal pigment epithelium (RPE). Choriocapillaris dropout decreases choroidal blood flow and nutrient availability to the RPE and overlying retina. Pathologic deposits called drusen, basal laminar deposits, and basal linear deposits appear between Bruch's membrane and the RPE. The RPE becomes dysfunctional and can also subsequently degenerate. Late-stage AMD exists in two manifestations: neovascular (wet) and geographic atrophic (dry), which can exist independently or in tandem. In choroidal neovascular AMD, vessels from the choroid breach Bruch's membrane and grow into the sub-RPE and/or subretinal space, disrupting the ionic composition necessary for vision. In some cases, pathologic vessels arise from the retinal circulation. In geographic atrophy, the choroid, RPE, and photoreceptors die, leaving patches of degeneration in the macula.

Risk factors for AMD include age, smoking, and multiple genetic loci, several of which are in complement regulatory genes such as Y402H in CFH (rs1061170) (Klein et al., 2005; Edwards et al., 2005; Haines et al., 2005). Homozygosity for the Y402H risk allele of CFH is associated with increased complement activation and deposition of the membrane attack complex (MAC) of complement in the choriocapillaris (Mullins et al., 2011). Abnormal complement regulation is widely appreciated as a key factor in AMD pathogenesis (Warwick et al., 2014; Geerlings et al., 2017; Tan et al., 2020; Armento et al., 2021; Rathi et al., 2023; Wilke and Apte, 2024). Recently, the complement inhibitors pegcetacoplan and avacincaptad pegol have received FDA approval for treatment of late-stage AMD (geographic atrophy), though these drugs increase the occurrence of neovascularization (Heier et al., 2023; Patel et al., 2023). Overall, there is significant progress to make in terms of treating and preventing this disease. A better understanding of the complement system and its interactions with ocular cells is warranted.

Complement inhibitors such as factor H (FH) bind cells and the extracellular matrix in part through protein-carbohydrate interactions. Cells are coated with a layer of glycoconjugates which are often capped with sialic acids: nine-carbon monosaccharides derived from neuraminic acid that have a keto acid functional group (Zhu et al., 2024). More than 50 sialic acid species exist, with N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) being the most common sialic acids found in most mammals. In humans, Neu5Ac is the most common and Neu5Gc is absent due to a mutation in the gene encoding CMP-NeuAc hydroxylase that converts Neu5Ac to Neu5Gc in other species (Irie et al., 1998; Chou et al., 1998). Sialic acids are typically found at the terminals of glycan chains, and removal of sialic acids with neuraminidase reveals the identities of penultimate carbohydrate epitopes. The process of sialylation occurs via ɑ-2,3 and ɑ-2,6 linkages of sialic acid moieties to galactose or N-acetylgalactosamine on O-linked and N-linked glycan chains. Sialic acids can also exist as polysialic acid chains through ɑ-2,8 and ɑ-2,9 linkages. A variety of sialyltransferases exist with different donor/acceptor specificities and binding conformations they can produce (Angata and Varki, 2002).

In addition to its interactions with the complement system, the glycan coating of cells informs the way cells interact with other cells and the extracellular matrix. Sialic acids are anti-adhesive due to the negative charge they confer to the cell surface. Vascular endothelial cells and erythrocytes both have a high concentration of sialylated glycans on the cell surface, which causes repulsion between the cells and allows erythrocytes to circulate unimpeded by attachment to the vessel walls or other blood cells (Eylar et al., 1962; Born and Palinski, 1985). Immune cells commonly possess sialic acid receptors including sialic acid-binding immunoglobulin-like lectins (SIGLECs) and selectins, which contribute to modulation of immune signaling and endocytosis (Crocker et al., 2007).

Despite these known essential functions of glycoconjugates and sialic acids, investigation of glycoconjugates in relation to aging and AMD is limited. Soft and hard drusen contain glycoconjugates that are bound by the lectins ConA, LCA, LFA, RCA-120, and WGA; this indicates the presence of mannose, glucosamine, galactose, and sialic acid (Mullins et al., 1997). Hard drusen can possess cores that bind to PNA after neuraminidase treatment and exclude lipids, suggesting that the drusen cores contain glycoproteins with O-linked carbohydrate chains (Mullins et al., 1999). Lectin labeling of choroidal neovascular membranes (CNVMs) showed different labeling intensities between normal and pathological vessels with SBA and sWGA (which bind ɑ-/β-linked GalNAc and GlcNAc respectively) labeling endothelial cells of CNVMs (Mullins et al., 2005). Many proteins that have specific roles in AMD pathogenesis are glycoproteins, including vascular endothelial growth factor (VEGF) (Brandner et al., 2006), the sialomucin CD34 (which shows age-related decline in the choriocapillaris (Sohn et al., 2014)), intercellular adhesion molecule 1 (ICAM1) (Regal-McDonald et al., 2020), complement factor H (FH) (Fenaille et al., 2007), factor H-like protein 1 (FHL1), and the complement factor H related proteins 1–5 (FHR1-5) (Skerka et al., 2013). Kliffen et al. (1996) investigated glycosaminoglycans in maculae with and without AMD, finding chondroitin 4-sulfate and heparan sulfate in basal laminar deposits and hyaluronic acid in nodular drusen, confluent drusen, and intercapillary pillars. Maculae with AMD also had higher amounts of total glycosaminoglycans than control maculae (Kliffen et al., 1996). AMD related pathologic basal laminar deposits contain ɑ-D-GalNAc, which is not present in other macular structures (Kliffen et al., 1994). Recently, Swan et al. (2025) performed elegant biochemical studies characterizing the sialome of the retina in relation to FH binding, and described an AMD-related loss of Bruch's membrane total sialic acids compared to aged controls.

Collectively, this evidence suggests that glycoconjugates and specifically sialic acids could play a role in AMD pathogenesis. In this study we used lectins, antibodies, and microscopy to examine sialic acids and their underlying carbohydrate moieties in healthy RPE and choroid, as well as in eyes with basal laminar deposit and choroidal neovascular membranes.