Small Leucine-rich repeat proteoglycans in corneal inflammation and wound healing

Abstract

The small leucine rich repeat proteoglycans are major components of the cornea. Lumican, keratocan, decorin, biglycan and osteoglycin are present throughout the adult corneal stroma and fibromodulin in the peripheral limbal area. In the cornea literature these proteoglycan have been reviewed as structural, collagen fibril-regulating proteins of the cornea. However, these proteoglycans are members of the leucine-rich–repeat superfamily, and share structural similarities with pathogen recognition toll-like receptors. Emerging studies are showing that these proteoglycans have a range of interactions with cell surface receptors, chemokines, growth factors and pathogen associated molecular patterns and are able to regulate host immune response, inflammation and wound healing. This review discusses what is known about their innate immune-related role directly in the cornea, and studies outside the field that find interesting links with innate immune and wound healing responses that are likely to be relevant to the ocular surface. In addition, the review discusses phenotypes of mice with targeted deletion of proteoglycan genes and genetic variants associated with human pathologies.

Introduction

The cornea, comprised of a stratified epithelium, basement membrane, Bowman’s layer, stroma, Descemet’s membrane and a single cell layered endothelium, is the outermost, avascular, refractive and protective barrier of the eye. Approximately 500 micrometers thick in humans, the stroma makes up 90% of the cornea. The stroma contains collagen fibrils of uniform diameter that are organized into orthogonal lamellae. The stroma is also rich in proteoglycans that interact with collagens to regulate fibril thickness, interfibrillar spacing and hydration, required to maintain the optical qualities of the cornea necessary for vision (Hassell and Birk, 2010; Meek and Knupp, 2015). This review discusses the role of the stromal proteoglycans in corneal inflammation and wound healing responses.

Proteoglycans are proteins, covalently conjugated to one or more glycosaminoglycan (GAG) side chains, chondroitin sulfate, keratan sulfate or heparan sulfate. The stromal proteoglycans belong to a group known as the small leucine-rich repeat proteoglycans (SLRPs) carrying characteristic tandem repeats of leucine-rich repeat motifs in their core proteins. Of the ~17 known SLRPs, lumican (LUM), keratocan (KERA), mimecan/osteoglycin (OGN), decorin (DCN) and biglycan (BGN) are major components of the corneal stroma. Fibromodulin (FMOD), abundant in the sclera, is also present in the peripheral limbal region of the cornea. Therefore, the review covers these six SLRPs and their known and speculated functions in the cornea.

Historically, proteoglycans were purified by dissociative extraction, ion-exchange and molecular sieve chromatography, and sedimentation-equilibrium centrifugation. The proteoglycans were characterized by gel electrophoresis before or after cleavage of the GAG side chains to visualize the core proteins (Hassell et al., 1986; Heinegard and Sommarin, 1987). The undigested samples had high “polydispersity” appearing as smears in polyacrylamide gels while the digested “monodisperse” core proteins shifted to a faster migrating sharp band - a characteristic behavior of all proteoglycans. The earliest study of SLRPs described a low buoyant density proteoglycan in cesium chloride density centrifugations of extracts from nasal cartilage that carried chondroitin sulfate side chains (Heinegard et al., 1981) and a protein with high leucine content by amino acid analysis. Later, sequencing of cDNA clones prepared from a fibroblast cell line led to the identification of decorin (Krusius and Ruoslahti, 1986). Within a decade genes encoding biglycan (Fisher et al., 1989; Fisher et al., 1991), decorin (Santra et al., 1994; Scholzen et al., 1994), lumican (Blochberger et al., 1992a; Chakravarti and Magnuson, 1995; Chakravarti et al., 1995), fibromodulin (Antonsson et al., 1993; Sztrolovics et al., 1994), keratocan (Funderburgh et al., 1998; Tasheva et al., 1999), mimecan/osteoglycin (Funderburgh et al., 1997) and other members were sequenced and localized to specific chromosomes. An explosion of molecular biological studies and development of gene-targeted mice now present an exciting and evolving picture of the breadth of functions and molecular interactions of the core proteins (Chakravarti, 2001; Chakravarti et al., 1998; Danielson et al., 1997; Liu et al., 2003; Svensson et al., 1999; Tasheva et al., 2002; Xu et al., 1998).

Core protein structure and leucine rich repeat types –

The acronym SLRP coined in the 1990s diffuses their connection to the leucine-rich repeat (LRR) superfamily of ~370 proteins which includes pathogen recognition receptors and other regulators of innate immunity. Almost the entire core proteins in SLRPs consist of tandem repeats of LRR motifs in which the minimum conserved residues are LXXLXLXXNXL with varying lengths of 20–27 amino acids (Bella et al., 2008; McEwan et al., 2006). All of the SLRPs discussed here have 12 such motifs numbered LRR 1–12. The crystal structures of decorin and biglycan core proteins have been resolved (Scott et al., 2006; Scott et al., 2004); similar to ribonuclease inhibitor (RNI), the first LRR protein to be crystallized, they have a curved solenoid shape, where each LRR motif forms a β strand and the inner concave surface forms a β sheet (Figure 1). The 4 cysteine residues at the N-terminus are disulfide bonded at alternate residues to form the N-terminal cap, while the C-terminal two-cysteine residues form disulfide bonds with each other. The difference in the spacing of the N-terminal cysteine residues is used to group the SLRPs into five classes; biglycan and decorin (Fisher et al., 1989) are Class I, lumican (Blochberger et al., 1992b), fibromodulin (Oldberg et al., 1989) and keratocan (Corpuz et al., 1996) are Class II and osteoglycin (Funderburgh et al., 1997) belongs to Class III.

Fig 1.

A crystal structure of Decorin, a Class I SLRP. Model provided by SwissModel (Kiefer et al., 2009). Decorin has a curved solenoid shape, where each LRR motif forms a β-strand and the inner concave surface forms a β-sheet (arrows).

The penultimate LRR motif, with 30–39 residues, is atypical, forms an extended loop or “ear” and found in the SLRP subfamily only. Sequence alignment studies of the “ear” motif suggest the SLRPs to have evolved from an ancestral gene by large-scale genome duplication and loss of genes (Huxley-Jones et al., 2009; Park et al., 2008). A recent study used hidden Markov modeling that uses a statistical Bayesian network-based approach to identify patterns, on all 375 LRR superfamily members to identify seven signature LRR motifs (Ng et al., 2011). Not surprisingly, the closely related SLRPs, lumican, fibromodulin and keratocan have similar distributions of 5/7 signature LRR motifs. In a hierarchical clustering of all the LRR proteins, the SLRPs form a tight cluster, and interestingly, several TLR members are placed close to the SLRPs.

Glycosaminoglycan side chains

The Class II SLRPs lumican, keratocan, fibromodulin and Class III osteoglycin, are post-translationally modified with keratan sulfate (KS) side chains covalently linked to an asparagine residue. Lumican and fibromodulin have five potential KS substitution sites, keratocan four and osteoglycin has one (Kalamajski and Oldberg, 2010). The cornea contains the type I KS form which has a long (~50) linear poly-N-acetyl lactosamine consisting of repeating units of the disaccharide[→ 3Galβ(1 → 4)GlcNAcβ(1 →], linked to an asparagine residue via a mannose containing branched oligosaccharide (Funderburgh, 2002; Quantock et al., 2010). Treatment of these SLRPs with N-glycosydase or removal of the keratan sulfate side chains with keratanase yield faster-migrating, sharper bands by gel electrophoresis. These negatively charged KS side chains regulate corneal development, wound healing, corneal hydration and transparency (Funderburgh, 2000; Quantock et al., 2010). Decorin and biglycan have one or two chondroitin/dermatan sulfate side chains, respectively, O-linked to serine residues.

Expression and distribution in the developing and adult cornea

These proteoglycans are all expressed in the human cornea. However, most of the data on expression in the developing organism is derived from studies in the mouse. Keratocan is primarily restricted to the cornea in the adult mouse, but it is more widely expressed in the developing embryo in the eye, ear, snout, tail and limb regions. The lumican transcript is detectable in the mouse embryo as early as E9.5, and in the developing cornea by E13.5 (Chakravarti, 2002; Chakravarti et al., 1998). In the adult mouse cornea, Keratocan, biglycan and decorin are present throughout, while lumican becomes more packed at the posterior stroma (Chakravarti et al., 2003). Fibromodulin is detectable throughout the developing cornea but restricted to the limbal area by P30 (Chen et al., 2010). In fungal infections of the cornea, the SLRP genes are down regulated at the early stages, whereas in mouse models of Pseudomonas keratitis lumican expression increases in the cornea within 6 hours of infection (Shao et al., 2013). Vascular endothelial cells variably express these SLRPs in response to pro-inflammatory signals. Biglycan (Moreth et al., 2010; Schaefer et al., 2005) and decorin (Koninger et al., 2006; Swirski et al., 2004) are expressed by macrophages and induced by pro-inflammatory cytokines. Lumican, on the other hand, expressed at basal levels and induced by pro-inflammatory cytokines and bacterial lipopolysaccharides (LPS) in fibroblasts, is barely detectable in macrophages and neutrophils (Lee et al., 2009; Wu and Chakravarti, 2007; Wu et al., 2007). A glycoprotein form of lumican is present in arterial blood vessel walls, and in cultured Human Umbilical Vein Endothelial Cells (HUVEC) (Funderburgh et al., 1991; Lee et al., 2009).

Regulation of collagen assembly and the cornea

A major function of SLRPs is their interactions with collagen fibrils and their ability to modulate collagen fibril structure, organization and corneal functions. These SLRP-collagen interactions were identified in a number of ways. Very early on electron microscopy of corneal sections stained with cupromeronic blue showed proteoglycans to occupy bands (a–e) along the collagen fibril (Scott and Bosworth, 1990); combined with immunoelectron microscopy, decorin (CSPG) was found primarily on the d and e bands (Pringle and Dodd, 1990). There are excellent reviews that discuss the collagen-proteoglycan associations in details (Chen and Birk, 2013; Kalamajski and Oldberg, 2010; Scott, 1991). In vitro collagen fibrillogenesis assays show that corneal proteoglycans and the core protein forms of lumican (and decorin) alter the kinetics of fibril formation and reduce overall turbidity, and by EM these fibrils appear thinner (Hassell et al., 1983; Rada et al., 1993). In solid phase binding assays, lumican and fibromodulin bind to immobilized mouse-tail collagen through LRR5–7. Fibromodulin shows stronger binding through another site in LRR11 (Kalamajski and Oldberg, 2007). Gene targeted mice deficient in selected SLRPs show altered collagen fibril structures in multiple connective tissue types as discussed later.

SLRPs in human eye diseases and phenotypes of gene targeted null-mice

Table 1 summarizes known and potential associations of these SLRPs with human disease, phenotypes of knockout mice and immune-related functions of the core proteins. Although recent studies are uncovering immune and inflammation regulatory functions, and changes in expression during infections and inflammation, there are no reported associations of SLRP gene variants with ocular wound healing, immune or autoimmune disease in humans.

Table 1.

Disease and knockout mouse phenotypes

Proteoglycan	KO mice	Mouse phenotype	Disease associations	Functions in immune responses	References
Biglycan BGN)	Bgntm1Mfy	Reduced growth rate and decreased bone mass	Spondyloepimetaphyseal dysplasia Renal disease	Binds TGF - β Promotes TLR2/4 response Upregulates renal CXCL13 & B cell infiltration,	Cho,2016 Xu, 1998 Schaefer, 2005
Decorin DCN)	Dcntm1Ioz	Thinning and fragility of the skin	Corneal dystrophy Renal disease	Elevates TLR2/4 response, binds TGF-β and attenuates signal, binds LRP-1 to promote TGF-β signals	Mellgren, 2015
Fibromodulin FMOD)	Fmodtm1Aol	Abnormal tail and Achilles tendons	B-cell chronic lymphocytic leukemia Mantle cell lymphoma	Regulates TGF-β1 bioavailability Promotes complement activation
Keratocan KERA)	Keratm1Cyl	Corneal thinning	Cornea plana congenita	Promotes neutrophil migration
Lumican LUM)	Lumtm1Chak Lumtm1Wwk	Corneal opacity & thinning, poor wound healing, fragile skin	Myopia	Bind to LPS and CD14, promotes wound healing & neutrophil migration	Chakravarti, 1998
Osteoglycin OGN)	Ogntm1Eta	Collagen fibril diameter increased in cornea and skin	Cardiac Hypertrophy & left ventricular mass	Binds TGF-β	Tasheva, 2002

Thus far, SLRP gene variants are associated with corneal structural anomalies and dystrophies. Two mutations in keratocan, a deleterious amino acid substitution and a premature stop codon were identified in patients with cornea plana, a flattening of the cornea that may be associated with other corneal malformations (Pellegata et al., 2000). The keratocan-null mice show thinning of the corneal stroma and a mild increase in collagen fibril diameter, while corneal transparency was unaffected (Liu et al., 2003). Lumican has been inconclusively associated with high myopia (Feng et al., 2013; Li et al., 2014; Park et al., 2013; Wang et al., 2006). The corneas of Lum^−/− mice have large-diameter fibrils with irregular contours, and by slit lamp biomicroscopy the cornea appears cloudy, while the skin and tendon show loss of tensile strength (Chakravarti et al., 1998). The Fmod-null mice, on the other hand have unremarkable collagen architecture in the central cornea. By contrast, in the limbal area, where fibromodulin is normally present (Chen et al., 2014), the fibrils have slightly larger diameter. Mice deficient in both lumican and fibromodulin show a severe collagen structure defect (much worse than the phenotype in mice deficient in lumican alone) in the entire cornea, suggesting functional cooperation between these two SLRPS (Chakravarti et al., 2003; Chen et al., 2010; Chen et al., 2014). The double-null mice show increased axial length reminiscent of myopia (Chakravarti et al., 2003; Song et al., 2003). A deletion on chromosome 12q21.33 encompassing KERA, LUM, DCN and EPYC (epiphycan) was reported in families with posterior amorphous corneal dystrophy (Kim et al., 2014). Congenital stromal corneal dystrophy is due to dominant negative mutations in decorin, where a truncated protein is secreted into the extracellular matrix, disrupting normal collagen architecture and corneal transparency (Bredrup et al., 2005). Gene-targeted mouse models expressing truncated decorin reflect some features of the human disease, but the phenotype is milder as the truncated protein is not secreted and thus less disruptive to collagen fibril assembly and the ECM (Chen et al., 2013; Mellgren et al., 2015). Two BGN mutations that disrupt TGF-β interactions, have been recently identified in patients with spondyloepimetaphyseal dysplasia, characterized by spine and long bone defects and osteoarthritis (Cho et al., 2016), while the Bgn-null mice display osteoporosis-like reduced bone mass (Xu et al., 1998). The corneas of Bgn-null mice have normal collagen fibril structure suggesting a minor role in collagen fibril regulation in the cornea. The collagen fibril defect in the Bgn-Dcn double null mice is exacerbated compared to the Dcn-null mice, and biglycan levels are elevated in the cornea of the latter, suggesting some biglycan-mediated rescue of the collagen fibril phenotype (Zhang et al., 2009). There are no reported ocular pathologies associated with osteoglycin, however it is implicated in the regulation of left ventricular mass cardiac pathologies in humans (Petretto et al., 2008). Ogn-null mice have thicker collagen fibrils in the skin and cornea without an overt corneal phenotype (Tasheva et al., 2002). There are no other reported connective tissue anomalies in the Ogn-nulls, except that in a pro-hypertrophic stimulus model the cardiac phenotype was attenuated, consistent with increased osteoglycin levels in human disease (Petretto et al., 2008).

SLRPs modulate corneal wound-healing responses

Recent studies suggest that SLRPs interact with multiple cell-signaling pathways. These interactions are likely to influence various aspects of corneal wound healing responses and resolution of infections as discussed here (Figure 2). The cornea is subject to traumas that include epithelial and stromal injuries, and clinical procedures such as epithelial debridement preceding UV crosslinking treatments of keratoconus patients, and refractive surgeries. Depending on the type of injury, wound-healing responses are initiated in the epithelium, stroma or both. Epithelial injuries disrupt normal epithelial exfoliation and differentiation, and promote apoptosis, loss of epithelial morphology and rapid migration of basal cells to close the wound (Mohan et al., 2003; Netto et al., 2005). Stromal keratocytes adjacent to an epithelial injury undergo apoptosis, keratocytes activated at the margin migrate into the de-cellularized region, bone marrow derived cells are recruited, and a provisional wound matrix is produced (Mohan et al., 2003; Netto et al., 2005). If the injuries are extensive or chronic, damaging myofibroblasts and scar tissues appear (Torricelli et al., 2016). These multistep processes are orchestrated by autocrine and paracrine growth factor, cytokine and chemokine signals (Klenkler and Sheardown, 2004; Ljubimov and Saghizadeh, 2015; Terai et al., 2011). These SLRPs are primarily mesenchymal connective tissue constituents, and thus major regulators of stromal wound healing. Thus far, lumican is the only known exception in that it is expressed by the epithelia during wound healing. Lumican-null mice show delayed closure of corneal epithelial wounds and recombinant lumican promotes epithelial cell migration via integrin β1, focal adhesion kinase (FAK) and ERK1/2 activation as well as through interactions of its C-terminal domain with ALK5, the TGF-β RI receptor (Saika et al., 2000; Seomun and Joo, 2008; Vij et al., 2005; Yamanaka et al., 2013). In cancer however, lumican inhibits melanoma cell migration through integrin β1 interactions, and lumican counteracts tumor progression by a combination of MMP14-inhibition and suppression of cell proliferation (Brezillon et al., 2009; Stasiak et al., 2016; Zeltz et al., 2010).

Fig 2.

A schematic representation of functions of the SLRPs in innate immune signals and corneal wound healing.

Upon injury the corneal epithelium and the stroma produce insulin, IGF-I and IGF-II that signal via IGF-IR to phosphorylate PI3K and AKT, promote cell survival and production of ECM and cell adhesive proteins in epithelial cells and stromal keratocytes (Ljubimov and Saghizadeh, 2015) (Musselmann et al., 2008; Vincent and Feldman, 2002). Studies of renal fibrosis and cartilage chondrocytes show induction of decorin by IGF-IR signaling; the renal fibrosis study further report that decorin interacts with IGF-IR, AKT/PKB to induce other ECM proteins such as fibrillin (Chubinskaya et al., 2007; Schaefer et al., 2007). This axis has not been investigated in the cornea, although increased expression and release of decorin from a remodeling ECM is likely to impact corneal wound healing.

TGF-β1 and TGF-β2, the major subtype in the cornea, promote epithelial cell growth, promote or inhibit stromal cell growth and migration during corneal wound healing reviewed by others (Saika, 2004; Saika et al., 2001). Under homeostatic conditions, the TGF-β isoforms promote ECM protein synthesis and restrict immune response to help maintain corneal immune privilege. In injury and infections, TGF-β signals cross talk with PDGF and integrins to promote myofibroblast differentiation and fibrosis (Jester et al., 2002; Xing and Bonanno, 2009). Decorin, biglycan and fibromodulin bind TGF-β (Brown et al., 2002; Hildebrand et al., 1994), to reduce its bioavailability and compete with the signaling receptors to attenuate signal transduction (Droguett et al., 2006; Hausser et al., 1994). Decorin also interacts with lipoprotein receptor-related protein 1 (LRP-1) to promote TGF-β signals. In multiple myopathies, aortic aneurysms and osteogenesis imperfecta, TGF-β signal excesses are implicated (Dietz, 2010). Interestingly, decorin levels in tissues of such patients were decreased and this was mirrored in fibroblasts of mouse models as well (Dietz, 2010; Grafe et al., 2014). In keratoconus, a stromal thinning disease of the cornea, these SLRPs were reduced and in cultured stromal fibroblasts, these SLRPs and TGF-β signals were altered (Foster et al., 2014). Thus, the SLRPs may be key modulators of TGF-β signals in multiple connective tissue and ocular pathologies.

SLRPs and complement activation

The complement pathways regulate innate immunity, inflammation and clearance of bacteria and damaged host cells (Ricklin et al., 2010) – all potentially important in protecting the cornea and maintaining its transparency. Identification of a polymorphism in the complement Factor H gene in age-related macular degeneration, some ten years ago, has led to intensive studies of the complement systems in the retina (Klein et al., 2005). In the cornea, however complement functions have not been investigated significantly. Early studies demonstrated the presence of complement pathway intermediates and activities in the central and peripheral cornea (Mondino et al., 1996; Mondino and Hoffman, 1980). In rheumatoid arthritis, SLRPs present in the cartilage have been linked to complement functions. Decorin and biglycan bind to C1q at its C-terminal domain and inhibit the classical complement activation pathway (Groeneveld et al., 2005). Fibromodulin activates the classical pathway in vitro by binding to the N-terminal domain of C1q (Sjoberg et al., 2005; Sjoberg et al., 2009). Fibromodulin also binds to C4BP (Happonen et al., 2009), an inhibitor of the classical and the lectin-mediated complement activation pathway, which could have a counter-modulatory effect on complement activation. Thus, fibromodulin, strategically located in the peripheral cornea (Chen et al., 2010), where complement components are probably higher than the central cornea, may regulate complement functions in the cornea.

SLRPs regulate innate immune signals, neutrophil chemotaxis and phagocytosis

Lumican, biglycan and, more recently decorin have been shown to interact with TLR-2 and/or −4 signaling pathways. Toll-like receptors are Type I transmembrane pathogen recognition receptors regulate a variety of host innate immune response to pathogen associated molecular patterns (PAMPs) and intracellular DNA sensing receptors. The reader is directed to several excellent reviews on TLRs for in depth discussion of these pathways (Beutler, 2004; Kawai and Akira, 2010; Medzhitov, 2001) and for their role at the ocular surface (Kumar and Yu, 2006; Pearlman et al., 2008). TLR-2 recognizes a range of pathogens from lipopeptides, peptidoglycans, lipoteichoic to fungal zymosan and viral hemagglutinin protein (Kawai and Akira, 2010). TLR-4, the first such receptor to be studied recognizes bacterial lipopolysaccharides (LPS) (Beutler et al., 2001). TLR-4 response to LPS involves initial binding of LPS with LPS-binding protein (LBP), followed by its interactions with CD14, which delivers LPS to TLR-4 complexed with MD-2 a soluble protein. CD-14 is a glycosyl phosphatidyl inositol-linked membrane protein that facilitates TLR-2 and TLR-4 signals (Jiang et al., 2005; Kim et al., 2005; Triantafilou and Triantafilou, 2002), and recently identified as a co-receptor for TLR-7 and −9 as well (Baumann et al., 2010). Lumican-null mice are hypo responsive to LPS-mediated septic shock, while Lum ^−/− peritoneal macrophages are also hypo responsive to LPS in cell culture (Lee et al., 2009), with lower induction of TNF-α that could be rescued with exogenous recombinant lumican (Wu et al., 2007). Thus, lumican cooperates with TLR-4 and increases host response to LPS. Mechanistically, this involves weak interactions of lumican with LPS and a specific high affinity binding with CD14 involving a tyrosine-containing site near its N-terminal end (Shao et al., 2012; Wu et al., 2007). In sterile LPS wounds of the cornea, the Lum ^−/− mice show lower TNF-α and reduced infiltration of neutrophils and macrophages in the cornea (Vij et al., 2005). In infectious (Pseudomonas aeruginosa) bacterial keratitis, within 24 hours post infection, the Lum ^−/− mice show poor bacterial clearance, lower neutrophil counts (Shao et al., 2013), but overall higher inflammatory cell in late disease, elevated clinical disease score and higher TNF-α levels. Consistent with poor neutrophil infiltration in injured or infected Lum ^−/− corneas, in vitro chemotaxis assays also show that lumican promotes neutrophil chemotaxis towards LPS and the chemokine CXCL1, and that this is mediated through its interactions with αM (CD11b), β2 (CD18) and β1 (CD29) integrin subunits (Lee et al., 2009). Another study showed direct binding of lumican and keratocan to CXCL1 (KC) and that this may regulate a chemokine gradient important for neutrophil migration (Carlson et al., 2007). Phagocytosis, a process by which cells take up bacteria and damaged host cells for clearance is mediated by multiple processes driven by complement, Fc receptors, dectins integrins, CD14 and lectins (Underhill and Gantner, 2004). Lumican contributes to both opsonic, integrin (CD18) mediated, and non-opsonic CD14-mediated phagocytosis; in P. aeruginosa infections lumican contributes the most towards CD14-mediated bacterial phagocytosis (Shao et al., 2012; Shao et al., 2013).

Based on studies in systems other than the eye, biglycan and decorin also regulate TLR signals. Macrophages stimulated with pro-inflammatory cytokines (IL-1β, Il-6) express biglycan. Biglycan-null mice like lumican-nulls are hypresponsive to LPS and show better survival in LPS sepsis models (Schaefer et al., 2005). However, unlike lumican, biglycan alone, without added LPS, induces pro-inflammatory cytokine production in macrophages in culture and thus is viewed as a danger associated molecular pattern. Decorin also enhances LPS response in macrophages to induce TNF-α production in a TLR-2 and −4 dependent manner (Merline et al., 2011).

Concluding remarks

The SLRPs discussed here are abundant in the cornea, sclera and other connective tissues, where they regulate collagen fibril assembly, corneal transparency and biomechanical properties. Their LRR motifs have the ability to interact with a wide range of other proteins including cell surface receptors, growth factors, ligands and microbial pathogen associated molecular patterns. While there is no concrete experimental evidence, it is tempting to suggest that collagen-matrix incorporated SLRPs are likely to be less interactive with immune signals, while their release from a remodeling matrix or de novo synthesis during injury, inflammation and infections can lead to their increased immune-modulatory functions. As the field begins to understand more about when and where these SLRPs become biologically active in host immune responses, their roles in normal healing processes and dysregulations in chronic infections and autoimmunity will become evident.

1. This review discusses small leucine rich repeat proteoglycans of the cornea.

2. The review focuses on functions of the proteoglycans in innate immune response, inflammation and wound healing of the cornea.

3. Functions of lumican, keratocan, fibromodulin, biglycan, decorin and osteoglycin are discussed.