TABLE 1.

Summary of studies characterizing the expression profiles of the different GAGs and PGs in the cornea.

GAGs	Gene	Expression in ocular surface	Function/deficiency	References
Keratan sulfate	Lumican	Primarily expressed in the stroma. Expressed by stromal keratocytes and corneal epithelial cells (during wound healing).	Deficiency causes corneal opacity and thinning due to abnormal collagen fibril structure, and delayed corneal wound healing.	Chakravarti et al., 1998, 2000; Saika et al., 2000
	Keratocan	Expressed throughout the corneal stroma by keratocytes only.	Deficiency causes corneal thinning and abnormal collagen fibril spacing. Mutation causes cornea plana congenital.	Pellegata et al., 2000; Kao and Liu, 2002; Carlson et al., 2005
	Fibromodulin	Expressed in the stroma of the peripheral cornea, limbus and sclera.	Deficiency causes a change in corneal curvature and the axial length of the eye.	Chen et al., 2010
	Mimecan	Expressed in the corneal stroma.	Deficiency causes an increase in collagen fiber diameter in the cornea.	Beecher et al., 2005
Heparan sulfate	Syndecan-1	Expressed in the corneal epithelium.	Deficiency causes delayed wound healing and abnormal intraepithelial nerve terminals in the cornea, as well as increased corneal neovascularization after alkali burn. Overexpression inhibits cell proliferation at the wound edge.	Götte et al., 2002; Stepp et al., 2002, 2010; Elenius et al., 2004; Pal-Ghosh et al., 2017
	Perlecan	Expressed in the entire corneal and conjunctival epithelial basement membrane and also between the endothelium and Descemet’s membrane. Expressed by corneal epithelial cells and keratocytes.	Deficiency causes a thinner corneal epithelium with decreased expression of epithelial differentiation markers.	Ljubimov et al., 1995; Inomata et al., 2012; Torricelli et al., 2015
	Agrin	High expression at the limbus decreasing toward the central cornea and conjunctiva.	Deficiency can cause anophthalmia, microphthalmia, or iris adhesion to the cornea.	Fuerst et al., 2007; Schlötzer-Schrehardt et al., 2007
	Collagen XVIII	Expressed in the corneal epithelium, corneal and conjunctival epithelial basement membrane, corneal nerve, Descemet’s membrane, and limbal and conjunctival capillaries. Increased expression in the stroma after injury.	Deficiency causes impaired corneal reinnervation after injury.	Zatterstrom et al., 2000; Fukai et al., 2002; Kato et al., 2003; Määttä et al., 2006; Sakimoto et al., 2008
	Exostosin glycosyltransferase 1 (Ext1) enzymes	Primarily expressed in the corneal epithelium.	Deletion of Ext1 in the corneal epithelium causes defective corneal stratification and delayed wound healing due to loss of tight junctions.	Coulson-Thomas et al., 2015
Chondroitin sulfate	Lubricin	Expressed in corneal and conjunctival epithelia.	Functions as a boundary lubricant. Deficiency causes the dry eye phenotype.	Cheriyan et al., 2011; Schmidt et al., 2013
	Aggrecan	Expressed in corneal and conjunctival epithelia, low expression in the anterior corneal stroma and moderate expression in the scleral stroma.	Increased expression in the stroma of sclerocornea.	Bouhenni et al., 2013
	Versican	Expressed in the subepithelial region of the anterior limbus.		Schlötzer-Schrehardt et al., 2007
Dermatan sulfate	Decorin	Expressed throughout the corneal stroma.	Plays a role in corneal fibrillogenesis. Mutation causes congenital stromal dystrophy of the cornea.	Danielson et al., 1997; Bredrup et al., 2005; Rødahl et al., 2006; Zhang et al., 2009
	Biglycan	Expressed throughout the corneal stroma.	Plays a role in corneal fibrillogenesis.	Zhang et al., 2009
Hyaluronan	Hyaluronan synthases 1, 2, and 3	Highly expressed in the limbus and to some point in the conjunctiva.	Regulates limbal stem cell differentiation. Facilitates corneal epithelial wound healing. Regulates corneal lymphangiogenesis.	Gong et al., 1994; Lerner et al., 1998; Gesteira et al., 2017; Sun et al., 2019