Distribution and Function of Glycosaminoglycans and Proteoglycans in the Development, Homeostasis and Pathology of the Ocular Surface

Abstract

The ocular surface, which forms the interface between the eye and the external environment, includes the cornea, corneoscleral limbus, the conjunctiva and the accessory glands that produce the tear film. Glycosaminoglycans (GAGs) and proteoglycans (PGs) have been shown to play important roles in the development, hemostasis and pathology of the ocular surface. Herein we review the current literature related to the distribution and function of GAGs and PGs within the ocular surface, with focus on the cornea. The unique organization of ECM components within the cornea is essential for the maintenance of corneal transparency and function. Many studies have described the importance of GAGs within the epithelial and stromal compartment, while very few studies have analyzed the ECM of the endothelial layer. Importantly, GAGs have been shown to be essential for maintaining corneal homeostasis, epithelial cell differentiation and wound healing, and, more recently, a role has been suggested for the ECM in regulating limbal stem cells, corneal innervation, corneal inflammation, corneal angiogenesis and lymphangiogenesis. Reports have also associated genetic defects of the ECM to corneal pathologies. Thus, we also highlight the role of different GAGs and PGs in ocular surface homeostasis, as well as in pathology.

Keywords: cornea; decorin; keratan sulfate; lumican; wound healing.

FIGURE 1

Molecular reconstruction of thermodynamic maps of water entities around a representative tetrasaccharide of the different GAGs. Molecular reconstruction of thermodynamic maps of water entities were depicted additively by combining oxygen and hydrogen against a solute. Solute-solvent interaction description was calculated by 3D-RISM (three-dimensional reference interaction site model with Universal Correlation, UC) (Luchko et al., 2010; Nguyen et al., 2019). The displacement of water molecules favorably modulates the free energy of protein-ligand complex binding. A cross section at the center of mass is represented in the x-axis for each tetrasaccharide with a licorice representation of each GAG in an overlay. The thermodynamic maps of water entities can be directly correlated with the electrostatic charges of each tetrasaccharide unit and the specificity with which they bind to their binding partners.

FIGURE 2

A cartoon representation of the SLRP lumican containing four KS side chains. The structure of lumican was modeled on the crystalized structure of decorin and presented as a cartoon representation of human lumican (green). The KS side chains at the N-glycosilation sites at N88, N127, N160 and N252 are represented as a sticks (N-acetyl glucosamine in pink, galactose in blue, oxygen in red and sulfate in yellow).

FIGURE 3

Representative image of lumican and KS imunolocalized in human corneas. Lumican (green) and KS (red) were immunolocalized in a sagittal section of a normal human cornea. Lumican was detected with a monoclonal anti-lumican antibody, clone 1F12B10, produced by our group, and KS was detected using anti-KS clone 5D4, kindly provided by Prof. Bruce Caterson. Scale bar represents 20 μm.

FIGURE 4

Spatial distribution of SLRPs, HSPGs and HA in the cornea and limbal region. Spatial distribution maps showing the distribution of the SLRPs lumican, keratocan, decorin, biglycan, and fibromodulin, the HSPGs perlecan and agrin, and HA, all represented in green, in the cornea and limbal region. A frontal view of the whole cornea and limbal region is shown on the left of each panel, with the inner circle demarcating the cornea/limbus interface. The corresponding image on the right show a cross-sectional view of the cornea with the limbal regions toward the bottom. The lines demarcate the different layers of the cornea, specifically the upper line represents the intersection between the epithelium and the stroma, with the other line representing the intersection between the stroma and the endothelium. The intensity of the green color correlates with expression levels, with the darker color representing higher expression.

FIGURE 5

A cartoon representation of the lumican core protein with indicated MT1-MMP cleavage sites. A cartoon representation of human lumican (green) modeled using the structure of decorin (pdbID 1XDC) as a scaffold. MT1-MMP cleavage sites are depicted in pink in order to represent the lumican products that would be produced upon MT1-MMP cleavage. The c-terminal 13 aas of lumican, which have been shown to promote corneal wound healing, have been shown in blue.

FIGURE 6

Representative image of keratocan and KS imunolocalized in human corneas. Immunolocalization of keratocan (green) and KS (red) in a sagittal section of normal human cornea. Keratocan was detected with a monoclonal anti-keratocan antibody and KS was detected using anti-KS kindly provided by Prof. Bruce Caterson. Scale bar represents 20 μm.

FIGURE 7

Representation of the major GAGs and PGs in the cornea and limbal region. Diagram of a cross-sectional view of the cornea and limbal region showing the distribution of major GAGs and PGs in the cornea. The different layers of the cornea and limbus are represented, specifically the epithelium (stratified squamous epithelium), stroma (ECM with sparse keratocytes) and endothelium. The basement membrane can be seen as a line (represented in green in the limbal region and red in the cornea) between the epithelium and the stroma.