Identification of the N-linked oligosaccharide sites in chick corneal lumican and keratocan that receive keratan sulfate

Lumican, a Multifunctional Cell Instructive Biomarker Proteoglycan Has Novel Roles as a Marker of the Hypercoagulative State of Long Covid Disease

This study has reviewed the many roles of lumican as a biomarker of tissue pathology in health and disease. Lumican is a structure regulatory proteoglycan of collagen-rich tissues, with cell instructive properties through interactions with a number of cell surface receptors in tissue repair, thereby regulating cell proliferation, differentiation, inflammation and the innate and humoral immune systems to combat infection. The exponential increase in publications in the last decade dealing with lumican testify to its role as a pleiotropic biomarker regulatory protein. Recent findings show lumican has novel roles as a biomarker of the hypercoagulative state that occurs in SARS CoV-2 infections; thus, it may also prove useful in the delineation of the complex tissue changes that characterize COVID-19 disease. Lumican may be useful as a prognostic and diagnostic biomarker of long COVID disease and its sequelae.

The Landscape of Small Leucine-Rich Proteoglycan Impact on Cancer Pathogenesis with a Focus on Biglycan and Lumican

Cancer development is a multifactorial procedure that involves changes in the cell microenvironment and specific modulations in cell functions. A tumor microenvironment contains tumor cells, non-malignant cells, blood vessels, cells of the immune system, stromal cells, and the extracellular matrix (ECM). The small leucine-rich proteoglycans (SLRPs) are a family of nineteen proteoglycans, which are ubiquitously expressed among mammalian tissues and especially abundant in the ECM. SLRPs are divided into five canonical classes (classes I-III, containing fourteen members) and non-canonical classes (classes IV-V, including five members) based on their amino-acid structural sequence, chromosomal organization, and functional properties. Variations in both the protein core structure and glycosylation status lead to SLRP-specific interactions with cell membrane receptors, cytokines, growth factors, and structural ECM molecules. SLRPs have been implicated in the regulation of cancer growth, motility, and invasion, as well as in cancer-associated inflammation and autophagy, highlighting their crucial role in the processes of carcinogenesis. Except for the class I SLRP decorin, to which an anti-tumorigenic role has been attributed, other SLPRs' roles have not been fully clarified. This review will focus on the functions of the class I and II SLRP members biglycan and lumican, which are correlated to various aspects of cancer development.

Microstructured collagen films for 3D corneal stroma modelling

PURPOSE/AIM

Corneal injury is a major cause of impaired vision around the globe. The fine structure of the corneal stroma plays a pivotal role in the phenotype and behavior of the embedded cells during homeostasis and healing after trauma or infection. In order to study healing processes in the cornea, it is important to create culture systems that functionally mimic the natural environment.

MATERIALS AND METHODS

Collagen solution was vitrified on top of a grated film to achieve thin collagen films with parallel microgrooves. Keratocytes (corneal stromal cells) were cultured on the films either as a single layer or as stacked layers of films and cells. SEM and F-actin staining were used to analyze the pattern transference onto the collagen and the cell orientation on the films. Cell viability was analyzed with MTS and live/dead staining. Keratocytes, fibroblasts, and myofibroblasts were cultured to study the pattern's effect on phenotype.

RESULTS

A microstructured collagen film-based culture system that guides keratocytes (stromal cells) to their native, layerwise perpendicular orientation in 3D and that can support fibroblasts and myofibroblasts was created. The films are thin and transparent enough to observe cells at least three layers deep. The cells maintain viability in 2D and 3D cultures and the films can support fibroblast and myofibroblast phenotypes.

CONCLUSIONS

The films provide an easily reproducible stroma model that maintains high cell viability and improves the preservation of the keratocyte phenotype in keratocytes that are differentiated to fibroblasts.

Site-specific glycosylation of proteoglycans: a revisited frontier in proteoglycan research

Proteoglycans (PGs), a class of carbohydrate-modified proteins, are present in essentially all metazoan organisms investigated to date. PGs are composed of glycosaminoglycan (GAG) chains attached to various core proteins and are important for embryogenesis and normal homeostasis. PGs exert many of their functions via their GAG chains and understanding the details of GAG-ligand interactions has been an essential part of PG research. Although PGs are also involved in many diseases, the number of GAG-related drugs used in the clinic is yet very limited, indicating a lack of detailed structure-function understanding. Structural analysis of PGs has traditionally been obtained by first separating the GAG chains from the core proteins, after which the two components are analyzed separately. While this strategy greatly facilitates the analysis, it precludes site-specific information and introduces either a "GAG" or a "core protein" perspective on the data interpretation. Mass-spectrometric (MS) glycoproteomic approaches have recently been introduced, providing site-specific information on PGs. Such methods have revealed a previously unknown structural complexity of the GAG linkage regions and resulted in identification of several novel CSPGs and HSPGs in humans and in model organisms, thereby expanding our view on PG complexity. In light of these findings, we discuss here if the use of such MS-based techniques, in combination with various functional assays, can also be used to expand our functional understanding of PGs. We have also summarized the site-specific information of all human PGs known to date, providing a theoretical framework for future studies on site-specific functional analysis of PGs in human pathophysiology.

Lumican in Carcinogenesis-Revisited

Carcinogenesis is a multifactorial process with the input and interactions of environmental, genetic, and metabolic factors. During cancer development, a significant remodeling of the extracellular matrix (ECM) is evident. Proteoglycans (PGs), such as lumican, are glycosylated proteins that participate in the formation of the ECM and are established biological mediators. Notably, lumican is involved in cellular processes associated with tumorigeneses, such as EMT (epithelial-to-mesenchymal transition), cellular proliferation, migration, invasion, and adhesion. Furthermore, lumican is expressed in various cancer tissues and is reported to have a positive or negative correlation with tumor progression. This review focuses on significant advances achieved regardingthe role of lumican in the tumor biology. Here, the effects of lumican on cancer cell growth, invasion, motility, and metastasis are discussed, as well as the repercussions on autophagy and apoptosis. Finally, in light of the available data, novel roles for lumican as a cancer prognosis marker, chemoresistance regulator, and cancer therapy target are proposed.

Communications via the Small Leucine-rich Proteoglycans: Molecular Specificity in Inflammation and Autoimmune Diseases

Inflammation is a highly regulated biological response of the immune system that is triggered by assaulting pathogens or endogenous alarmins. It is now well established that some soluble extracellular matrix constituents, such as small leucine-rich proteoglycans (SLRPs), can act as danger signals and trigger aseptic inflammation by interacting with innate immune receptors. SLRP inflammatory signaling cascade goes far beyond its canonical function. By choosing specific innate immune receptors, coreceptors, and adaptor molecules, SLRPs promote a switch between pro- and anti-inflammatory signaling, thereby determining disease resolution or chronification. Moreover, by orchestrating signaling through various receptors, SLRPs fine-tune inflammation and, despite their structural homology, regulate inflammatory processes in a molecule-specific manner. Hence, the overarching theme of this review is to highlight the molecular and functional specificity of biglycan-, decorin-, lumican-, and fibromodulin-mediated signaling in inflammatory and autoimmune diseases.

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

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.

Immunolocalization of Keratan Sulfate in Rat Spinal Tissues Using the Keratanase Generated BKS-1(+) Neoepitope: Correlation of Expression Patterns with the Class II SLRPs, Lumican and Keratocan

This study has identified keratan sulfate in fetal and adult rat spinal cord and vertebral connective tissues using the antibody BKS-1(+) which recognizes a reducing terminal N-acetyl glucosamine-6-sulfate neo-epitope exposed by keratanase-I digestion. Labeling patterns were correlated with those of lumican and keratocan using core protein antibodies to these small leucine rich proteoglycan species. BKS-1(+) was not immunolocalized in fetal spinal cord but was apparent in adult cord and was also prominently immunolocalized to the nucleus pulposus and inner annulus fibrosus of the intervertebral disc. Interestingly, BKS-1(+) was also strongly associated with vertebral body ossification centers of the fetal spine. Immunolocalization of lumican and keratocan was faint within the vertebral body rudiments of the fetus and did not correlate with the BKS-1(+) localization indicating that this reactivity was due to another KS-proteoglycan, possibly osteoadherin (osteomodulin) which has known roles in endochondral ossification. Western blotting of adult rat spinal cord and intervertebral discs to identify proteoglycan core protein species decorated with the BKS-1(+) motif confirmed the identity of 37 and 51 kDa BKS-1(+) positive core protein species. Lumican and keratocan contain low sulfation KS-I glycoforms which have neuroregulatory and matrix organizational properties through their growth factor and morphogen interactive profiles and ability to influence neural cell migration. Furthermore, KS has interactive capability with a diverse range of neuroregulatory proteins that promote neural proliferation and direct neural pathway development, illustrating key roles for keratocan and lumican in spinal cord development.

Keratan Sulphate in the Tumour Environment

Keratan sulphate (KS) is a bioactive glycosaminoglycan (GAG) of some complexity composed of the repeat disaccharide D-galactose β1→4 glycosidically linked to N-acetyl glucosamine. During the biosynthesis of KS, a family of glycosyltransferase and sulphotransferase enzymes act sequentially and in a coordinated fashion to add D-galactose (D-Gal) then N-acetyl glucosamine (GlcNAc) to a GlcNAc acceptor residue at the reducing terminus of a nascent KS chain to effect chain elongation. D-Gal and GlcNAc can both undergo sulphation at C6 but this occurs more frequently on GlcNAc than D-Gal. Sulphation along the developing KS chain is not uniform and contains regions of variable length where no sulphation occurs, regions which are monosulphated mainly on GlcNAc and further regions of high sulphation where both of the repeat disaccharides are sulphated. Each of these respective regions in the KS chain can be of variable length leading to KS complexity in terms of chain length and charge localization along the KS chain. Like other GAGs, it is these variably sulphated regions in KS which define its interactive properties with ligands such as growth factors, morphogens and cytokines and which determine the functional properties of tissues containing KS. Further adding to KS complexity is the identification of three different linkage structures in KS to asparagine (N-linked) or to threonine or serine residues (O-linked) in proteoglycan core proteins which has allowed the categorization of KS into three types, namely KS-I (corneal KS, N-linked), KS-II (skeletal KS, O-linked) or KS-III (brain KS, O-linked). KS-I to -III are also subject to variable addition of L-fucose and sialic acid groups. Furthermore, the GlcNAc residues of some members of the mucin-like glycoprotein family can also act as acceptor molecules for the addition of D-Gal and GlcNAc residues which can also be sulphated leading to small low sulphation glycoforms of KS. These differ from the more heavily sulphated KS chains found on proteoglycans. Like other GAGs, KS has evolved molecular recognition and information transfer properties over hundreds of millions of years of vertebrate and invertebrate evolution which equips them with cell mediatory properties in normal cellular processes and in aberrant pathological situations such as in tumourogenesis. Two KS-proteoglycans in particular, podocalyxin and lumican, are cell membrane, intracellular or stromal tissue-associated components with roles in the promotion or regulation of tumour development, mucin-like KS glycoproteins may also contribute to tumourogenesis. A greater understanding of the biology of KS may allow better methodology to be developed to more effectively combat tumourogenic processes.

Keratan sulfate, a complex glycosaminoglycan with unique functional capability

From an evolutionary perspective keratan sulfate (KS) is the newest glycosaminoglycan (GAG) but the least understood. KS is a sophisticated molecule with a diverse structure, and unique functional roles continue to be uncovered for this GAG. The cornea is the richest tissue source of KS in the human body but the central and peripheral nervous systems also contain significant levels of KS and a diverse range of KS-proteoglycans with essential functional roles. KS also displays important cell regulatory properties in epithelial and mesenchymal tissues and in bone and in tumor development of diagnostic and prognostic utility. Corneal KS-I displays variable degrees of sulfation along the KS chain ranging from non-sulfated polylactosamine, mono-sulfated and disulfated disaccharide regions. Skeletal KS-II is almost completely sulfated consisting of disulfated disaccharides interrupted by occasional mono-sulfated N-acetyllactosamine residues. KS-III also contains highly sulfated KS disaccharides but differs from KS-I and KS-II through 2-O-mannose linkage to serine or threonine core protein residues on proteoglycans such as phosphacan and abakan in brain tissue. Historically, the major emphasis on the biology of KS has focused on its sulfated regions for good reason. The sulfation motifs on KS convey important molecular recognition information and direct cell behavior through a number of interactive proteins. Emerging evidence also suggest functional roles for the poly-N-acetyllactosamine regions of KS requiring further investigation. Thus further research is warranted to better understand the complexities of KS.

Glycosaminoglycans and Proteoglycans

In this editorial to MDPI Pharmaceuticals special issue “Glycosaminoglycans and Proteoglycans” we describe in outline the common structural features of glycosaminoglycans and the characteristics of proteoglycans, including the intracellular proteoglycan, serglycin, cell-surface proteoglycans, like syndecans and glypicans, and the extracellular matrix proteoglycans, like aggrecan, perlecan, and small leucine-rich proteoglycans. The context in which the pharmaceutical uses of glycosaminoglycans and proteoglycans are presented in this special issue is given at the very end.

Determinants of Glycosaminoglycan (GAG) Structure

Proteoglycans (PGs) are glycosylated proteins of biological importance at cell surfaces, in the extracellular matrix, and in the circulation. PGs are produced and modified by glycosaminoglycan (GAG) chains in the secretory pathway of animal cells. The most common GAG attachment site is a serine residue followed by a glycine (-ser-gly-), from which a linker tetrasaccharide extends and may continue as a heparan sulfate, a heparin, a chondroitin sulfate, or a dermatan sulfate GAG chain. Which type of GAG chain becomes attached to the linker tetrasaccharide is influenced by the structure of the protein core, modifications occurring to the linker tetrasaccharide itself, and the biochemical environment of the Golgi apparatus, where GAG polymerization and modification by sulfation and epimerization take place. The same cell type may produce different GAG chains that vary, depending on the extent of epimerization and sulfation. However, it is not known to what extent these differences are caused by compartmental segregation of protein cores en route through the secretory pathway or by differential recruitment of modifying enzymes during synthesis of different PGs. The topic of this review is how different aspects of protein structure, cellular biochemistry, and compartmentalization may influence GAG synthesis.

Regulation of corneal stroma extracellular matrix assembly

The transparent cornea is the major refractive element of the eye. A finely controlled assembly of the stromal extracellular matrix is critical to corneal function, as well as in establishing the appropriate mechanical stability required to maintain corneal shape and curvature. In the stroma, homogeneous, small diameter collagen fibrils, regularly packed with a highly ordered hierarchical organization, are essential for function. This review focuses on corneal stroma assembly and the regulation of collagen fibrillogenesis. Corneal collagen fibrillogenesis involves multiple molecules interacting in sequential steps, as well as interactions between keratocytes and stroma matrix components. The stroma has the highest collagen V:I ratio in the body. Collagen V regulates the nucleation of protofibril assembly, thus controlling the number of fibrils and assembly of smaller diameter fibrils in the stroma. The corneal stroma is also enriched in small leucine-rich proteoglycans (SLRPs) that cooperate in a temporal and spatial manner to regulate linear and lateral collagen fibril growth. In addition, the fibril-associated collagens (FACITs) such as collagen XII and collagen XIV have roles in the regulation of fibril packing and inter-lamellar interactions. A communicating keratocyte network contributes to the overall and long-range regulation of stromal extracellular matrix assembly, by creating micro-domains where the sequential steps in stromal matrix assembly are controlled. Keratocytes control the synthesis of extracellular matrix components, which interact with the keratocytes dynamically to coordinate the regulatory steps into a cohesive process. Mutations or deficiencies in stromal regulatory molecules result in altered interactions and deficiencies in both transparency and refraction, leading to corneal stroma pathobiology such as stromal dystrophies, cornea plana and keratoconus.

Resistance of corneal RFUVA–cross-linked collagens and small leucine-rich proteoglycans to degradation by matrix metalloproteinases

PURPOSE

Extracellular matrix metalloproteinases (MMPs) are thought to play a crucial role in corneal degradation associated with the pathological progression of keratoconus. Currently, corneal cross-linking by riboflavin and ultraviolet A (RFUVA) has received significant attention for treatment of keratoconus. However, the extent to which MMPs digest cross-linked collagen and small leucine-rich proteoglycans (SLRPs) remains unknown. In this study, the resistance of RFUVA-cross-linked collagens and SLRPs to MMPs has been investigated.

METHODS

To investigate the ability of MMPs to digest cross-linked collagen and SLRPs, a model reaction system using purified collagen type I, type IV, and nonglycosylated, commercially available recombinant SLRPs, keratocan, lumican, mimecan, decorin, and biglycan in solution in vitro has been compared using reactions inside an intact bovine cornea, ex vivo.

RESULTS

Our data demonstrate that corneal cross-linked collagen type I and type IV are resistant to cleavage by MMP-1, MMP-2, MMP-9, and MMP-13, whereas non-cross-linked collagen I, IV, and natively glycosylated SLRPs are susceptible to degradation by MMPs. In addition, both cross-linked SLRPs themselves and cross-linked polymers of SLRPs and collagen appear able to resist degradation. These results suggest that the interactions between SLRPs and collagen caused by RFUVA protect both SLRPs and collagen fibrils from cleavage by MMPs.

CONCLUSIONS

A novel approach for understanding the biochemical mechanism whereby RFUVA cross-linking stops keratoconus progression has been achieved.

Effects of ultraviolet-A and riboflavin on the interaction of collagen and proteoglycans during corneal cross-linking

Corneal cross-linking using riboflavin and ultraviolet-A (RFUVA) is a clinical treatment targeting the stroma in progressive keratoconus. The stroma contains keratocan, lumican, mimecan, and decorin, core proteins of major proteoglycans (PGs) that bind collagen fibrils, playing important roles in stromal transparency. Here, a model reaction system using purified, non-glycosylated PG core proteins in solution in vitro has been compared with reactions inside an intact cornea, ex vivo, revealing effects of RFUVA on interactions between PGs and collagen cross-linking. Irradiation with UVA and riboflavin cross-links collagen α and β chains into larger polymers. In addition, RFUVA cross-links PG core proteins, forming higher molecular weight polymers. When collagen type I is mixed with individual purified, non-glycosylated PG core proteins in solution in vitro and subjected to RFUVA, both keratocan and lumican strongly inhibit collagen cross-linking. However, mimecan and decorin do not inhibit but instead form cross-links with collagen, forming new high molecular weight polymers. In contrast, corneal glycosaminoglycans, keratan sulfate and chondroitin sulfate, in isolation from their core proteins, are not cross-linked by RFUVA and do not form cross-links with collagen. Significantly, when RFUVA is conducted on intact corneas ex vivo, both keratocan and lumican, in their natively glycosylated form, do form cross-links with collagen. Thus, RFUVA causes cross-linking of collagen molecules among themselves and PG core proteins among themselves, together with limited linkages between collagen and keratocan, lumican, mimecan, and decorin. RFUVA as a diagnostic tool reveals that keratocan and lumican core proteins interact with collagen very differently than do mimecan and decorin.

The molecular basis of corneal transparency

The cornea consists primarily of three layers: an outer layer containing an epithelium, a middle stromal layer consisting of a collagen-rich extracellular matrix (ECM) interspersed with keratocytes and an inner layer of endothelial cells. The stroma consists of dense, regularly packed collagen fibrils arranged as orthogonal layers or lamellae. The corneal stroma is unique in having a homogeneous distribution of small diameter 25-30 nm fibrils that are regularly packed within lamellae and this arrangement minimizes light scattering permitting transparency. The ECM of the corneal stroma consists primarily of collagen type I with lesser amounts of collagen type V and four proteoglycans: three with keratan sulfate chains; lumican, keratocan, osteoglycin and one with a chondroitin sulfate chain; decorin. It is the core proteins of these proteoglycans and collagen type V that regulate the growth of collagen fibrils. The overall size of the proteoglycans are small enough to fit in the spaces between the collagen fibrils and regulate their spacing. The stroma is formed during development by neural crest cells that migrate into the space between the corneal epithelium and corneal endothelium and become keratoblasts. The keratoblasts proliferate and synthesize high levels of hyaluronan to form an embryonic corneal stroma ECM. The keratoblasts differentiate into keratocytes which synthesize high levels of collagens and keratan sulfate proteoglycans that replace the hyaluronan/water-rich ECM with the densely packed collagen fibril-type ECM seen in transparent adult corneas. When an incisional wound through the epithelium into stroma occurs the keratocytes become hypercellular myofibroblasts. These can later become wound fibroblasts, which provides continued transparency or become myofibroblasts that produce a disorganized ECM resulting in corneal opacity. The growth factors IGF-I/II are likely responsible for the formation of the well organized ECM associated with transparency produced by keratocytes during development and by the wound fibroblast during repair. In contrast, TGF-beta would cause the formation of the myofibroblast that produces corneal scaring. Thus, the growth factor mediated synthesis of several different collagen types and the core proteins of several different leucine-rich type proteoglycans as well as posttranslational modifications of the collagens and the proteoglycans are required to produce collagen fibrils with the size and spacing needed for corneal stromal transparency.

Expression of lumican in the articular disc of the human temporomandibular joint

Lumican belongs to the small leucine-rich repeat proteoglycan (SLRP) gene family and has been reported to exist in the cornea, intervertebral disc and tendon. Lumican plays a significant role in the assembly and regulation of collagen fibres. The human temporomandibular joint (TMJ) disc is made up of fibrocartilage with an extracellular matrix (ECM) composed of collagen and proteoglycans. The existence and behaviour of lumican have not been studied in the human TMJ disc. Therefore, we used immunohistochemical methods to detect lumican, CD34 and vascular endothelial growth factor (VEGF) and histochemical staining with toluidine blue in 13 human TMJ specimens (10 surgically removed and 3 obtained from autopsy). In both normal and deformed discs we observed staining with toluidine blue. We found that the area of metachromasia inside the deformed disc was uneven and expression of lumican was strong in the areas negative for metachromasia. Staining of VEGF and CD34 inside the deformed disc was seen. We confirmed the expression of lumican in the human TMJ disc and showed that a large number of fibroblast-like cells existed in the area of strong lumican expression. These new findings about the behaviour of lumican suggest that it may play a key role in the generation of a new collagen network by fibroblast-like cells.

Lumican, a small leucine-rich proteoglycan

Lumican belongs to the family of small leucine-rich repeat proteoglycans. Recent studies have shown that lumican participates in the maintenance of tissue homeostasis and modulates cellular functions including cell proliferation, migration, and differentiation. The expression of lumican has been correlated to the growth and metastasis of various malignancies; however, its exact role in tumorogenesis remains elusive. This review focuses upon the role of lumican in cell biology, providing insights into molecular mechanisms that lumican likely utilizes to control processes relevant to tumorogenesis.

Development of the corneal stroma, and the collagen-proteoglycan associations that help define its structure and function

The cornea of the eye is a unique, transparent connective tissue. It is comprised predominantly of collagen fibrils, remarkably uniform in diameter and regularly spaced, organized into an intricate lamellar array. Its establishment involves a precisely controlled sequence of developmental events in which the embryonic cornea undergoes major structural transformations that ultimately determine tissue form and function. In this article, we will review corneal developmental dynamics from a structural perspective, consider the roles and interrelationships of collagens and proteoglycans, and comment on contemporary concepts and current challenges pertinent to developmental processes that result in an optically clear, mature cornea.

Differential expression of the keratan sulphate proteoglycan, keratocan, during chick corneal embryogenesis

Keratan sulphate (KS) proteoglycans (PGs) are key molecules in the connective tissue matrix of the cornea of the eye, where they are believed to have functional roles in tissue organisation and transparency. Keratocan, is one of the three KS PGs expressed in cornea, and is the only one that is primarily cornea-specific. Work with the developing chick has shown that mRNA for keratocan is present in early corneal embryogenesis, but there is no evidence of protein synthesis and matrix deposition. Here, we investigate the tissue distribution of keratocan in the developing chick cornea as it becomes compacted and transparent in the later stages of development. Indirect immunofluorescence using a new monoclonal antibody (KER-1) which recognises a protein epitope on the keratocan core protein demonstrated that keratocan was present at all stages investigated (E10-E18), with distinct differences in localisation and organisation observed between early and later stages. Until E13, keratocan appeared both cell-associated and in the stromal extracellular matrix, and was particularly concentrated in superficial tissue regions. By E14 when the cornea begins to become transparent, keratocan was located in elongate arrays, presumably associated along collagen fibrils in the stroma. This fibrillar label was still concentrated in the anterior stroma, and persisted through E15-E18. Presumptive Bowman's layer was evident as an unlabelled subepithelial zone at all stages. Thus, in embryonic chick cornea, keratocan, in common with sulphated KS chains in the E12-E14 developmental period, exhibits a preferential distribution in the anterior stroma. It undergoes a striking reorganisation of structure and distribution consistent with a role in relation to stromal compaction and corneal transparency.

SLRP interaction can protect collagen fibrils from cleavage by collagenases

Decorin, fibromodulin and lumican are small leucine-rich repeat proteoglycans (SLRPs) which interact with the surface of collagen fibrils. Together with other molecules they form a coat on the fibril surface which could impede the access to collagenolytic proteinases. To address this hypothesis, fibrils of type I or type II collagen were formed in vitro and treated with either collagenase-1 (MMP1) or collagenase-3 (MMP13). The fibrils were either treated directly or following incubation in the presence of the recombinant SLRPs. The susceptibility of the uncoated and SLRP-coated fibrils to collagenase cleavage was assessed by SDS/PAGE. Interaction with either recombinant decorin, fibromodulin or lumican results in decreased collagenase cleavage of both fibril types. Thus SLRP interaction can help protect collagen fibrils from cleavage by collagenases.

Maintenance of the keratocyte phenotype during cell proliferation stimulated by insulin

Keratocytes normally express high levels of aldehyde dehydrogenase and keratocan. They proliferate and lose their keratocyte markers when they become fibroblastic during corneal wound healing. Keratocytes cultured in fetal bovine serum also become fibroblastic, proliferate, and lose these markers. In this report, we studied the effects of three serum growth factors, fibroblast growth factor-2, insulin, and platelet-derived growth factor-BB, on keratocyte proliferation and the maintenance of the keratocyte markers in 7-day cultures in cells plated at low (5,000 cells/cm2) and high (20,000 cells/cm2) density in serum-free medium. Keratocyte proliferation was measured by [3H]thymidine incorporation and by DNA content of the cultures. Cytosolic aldehyde dehydrogenase and keratocan accumulated in the medium were quantified by Western blot. The results showed that all the growth factors stimulated proliferation, but insulin stimulated proliferation more consistently. The keratocyte markers aldehyde dehydrogenase and keratocan were maintained after 7 days in culture in all growth factors, but keratocyte cell morphology was only maintained in medium containing insulin. Most of the proteoglycans were degraded in cultures of keratocytes plated at low density and cultured in the absence of growth factors. This degradation was prevented when keratocytes were cultured in the presence of the growth factors or when keratocytes were plated at high density. The results of this study show that insulin can expand keratocytes in vitro, maintain their phenotype, and prevent proteoglycan degradation.

Biochemical and biomechanical analysis of tendons of caged and penned chickens

Chickens were divided into two groups, one caged and the other penned. Superficial digital flexor tendons from penned chickens showed greater tensile strength, withstanding a greater strain before rupture than tendons from caged chickens. The tensile region of tendons from penned chickens showed more swelling in acetic acid and a higher hydroxyproline concentration compared with caged chickens, indicating the presence of large collagen amounts in the former. The tensile region of penned chickens presented higher glycosaminoglycan concentrations than the same region of caged chickens. For both groups, the predominant glycosaminoglycan in the compression regions was chondroitin sulfate, whereas dermatan sulfate was found in the tensile regions. N-terminal analysis identified the small proteoglycans fibromodulin and decorin. SDS-PAGE indicated that decorin was present in all regions and fibromodulin was mainly observed in the tensile region. These results indicate that an external condition, in this case the area available for locomotion, might influence the synthesis of extracellular matrix components and the mechanical properties of the tendon.

Monoclonal antibody 4C4-mAb specifically recognizes keratan sulphate proteoglycan on human embryonal carcinoma cells

Germ cell tumours, the most common solid cancers in young males, display pluripotentiality for embryonal and somatic differentiation. Specific surface antigens are useful in the study of cellular differentiation and for clinical diagnosis. A mouse monoclonal antibody (4C4-mAb) has been developed against a human embryonal carcinoma (EC) cell line (NCR-G3) isolated from a combined form of testicular germ cell tumour. On immunohistological and immuno-electron microscopic examination, the 4C4 antigen (4C4) was detected on the surface of NCR-G3 and gold particles were exclusively detected on the microvilli of the cells. In both formalin-fixed paraffin wax sections and touch-smear specimens, 4C4 was detected specifically in EC, while the antigen was not expressed in other types of germ cell tumour or in the other solid tumours tested. Tunicamycin diminished the antigenicity of NCR-G3 cells. In biochemical studies, 4C4 was found in a high molecular weight region ranging from 1 x 10(6) to 1 x 10(7) kD, which disappeared after periodate treatment. The density of 4C4 was 1.5 g/cm(3) after equilibrium centrifugation. These results imply that 4C4 is a proteoglycan. Furthermore, endo- and exo-glycosidase treatment revealed that 4C4 is a keratan sulphate proteoglycan that contains sialyl and fucosyl moieties. With EC-specific and formalin-resistant characteristics, 4C4 may be a specific marker for diagnosing EC among a variety of germ cell tumours.

The keratocan gene is expressed in both ocular and non-ocular tissues during early chick development

Extracellular matrix (ECM) keratan sulfate proteoglycans (KSPGs) are core proteins with sulfated polylactosamine side chains (KS). The KSPG core protein keratocan gene (Kera) is expressed almost exclusively in adult vertebrate cornea, but its embryonic expression is little known. Embryonic chick in situ hybridization reveals Kera mRNA expression in corneal endothelium from embryonic day (E) 4.5, Hamburger-Hamilton (HH) 25, in stromal keratocytes from E6.5, HH30, and in iris distal surface cells from E8, HH34. As highly sulfated, antibody I22-positive KS increases extracellularly from posterior to anterior across the stroma, nerves enter and populate only anterior stroma and epithelium. RT-PCR and in situ hybridization demonstrate that developmentally regulated Kera mRNA expression initiates in midbrain and dorsolateral mesenchyme at E1, HH7, then spreads caudally in hindbrain and cranial and trunk mesenchyme flanking the neural tube through E2, HH20. Cranial expression extends ventrally through the developing head, and concentrates in mesenchyme surrounding eye anterior regions and cranial ganglia, and in subepidermal pharyngeal arch mesenchyme by E3.5, HH22. Kera expression in the trunk at E3.5, HH22 and E4.5, HH25, is strong in dorsolateral subepidermal, sclerotomal and nephrogenic mesenchymes, but absent in neural tube, dorsal root ganglia, nerve outgrowths, notochord, heart and gut. Early limb buds express Kera mRNA throughout their mesenchyme, then in restricted proximal and distal mesenchymes. I22-positive KS appears only in notochord in E3.5, HH22 and E4.5, HH25, embryos. Results suggest the hypothesis that keratocan, or keratocan with minimally sulfated KS chains, may play a role in structuring ECM for early embryonic cell and neuronal migrations.

Keratan sulfate biosynthesis

Keratan sulfate was originally identified as the major glycosaminoglycan of cornea but is now known to modify at least a dozen different proteins in a wide variety of tissues. Despite a large body of research documenting keratan sulfate structure, and an increasing interest in the biological functions of keratan sulfate, until recently little was known of the specific enzymes involved in keratan sulfate biosynthesis or of the molecular mechanisms that control keratan sulfate expression. In the last 2 years, however, marked progress has been achieved in identification of genes involved in keratan sulfate biosynthesis and in development of experimental conditions to study keratan sulfate secretion and control in vitro. This review summarizes current understanding of keratan sulfate structure and recent developments in understanding keratan sulfate biosynthesis.

The homeobox gene Six3 is a potential regulator of anterior segment formation in the chick eye

The anterior segment of the vertebrate eye consists of highly organized and specialized ocular tissues critical for normal vision. The periocular mesenchyme, originating from the neural crest, contributes extensively to the anterior segment. During chick eye morphogenesis, the homeobox gene Six3 is expressed in a subset of periocular mesenchymal cells and in differentiating anterior segment tissues. Retrovirus-mediated misexpression of Six3 causes eye anterior segment malformation, including corneal protrusion and opacification, ciliary body and iris hypoplasia, and trabecular meshwork dysgenesis. Histological and molecular marker analyses demonstrate that Six3 misexpression disrupts the integrity of the corneal endothelium and the expression of extracellular matrix components critical for corneal transparency. Six3 misexpression also leads to a reduction of the periocular mesenchymal cell population expressing Lmx1b, Pitx2, and Pax6, transcription factors critical for eye anterior segment morphogenesis. Moreover, elevated levels of Six3 attenuate proliferation of periocular mesenchymal cells in vitro and differentiating anterior segment tissues in vivo. These results suggest that, in addition to its function in eye primordium determination, Six3 plays a role in regulating the development of the vertebrate eye anterior segment.

Expression of lumican in human colorectal cancer cells

Lumican is a member of a small leucine-rich proteoglycan family and its overexpression in human breast cancer tissues is reported to influence the growth of cancer cells. In the present study, we aimed to clarify the expression of lumican mRNA and its protein in human colorectal cancer cell lines and their localization in normal and cancerous colorectal tissues. Reverse transcription-polymerase chain reaction and western blot analysis revealed lumican mRNA and its protein expression in COLO 205, DLD-1, HCT-15, SW 480 and WiDr colorectal cancer cell lines. The lumican in colorectal cancer cells had non-sulfated or poorly sulfated polylactosamine side chains. Based on its immunoreactivity, the lumican protein was found to be localized in fibroblasts and stromal tissues of normal colorectal tissues, but not in colorectal epithelial cells. In colorectal cancer tissues, the lumican was strongly localized in cancer cells in eight of 12 cancer cases. The lumican protein was also localized in epithelial cells with mild reactive dysplasia and fibroblasts adjacent to cancer cells. Lumican mRNA was expressed in cancer cells and adjacent fibroblasts, and epithelial cells. These findings may indicate that the lumican protein synthesized by cancer cells, fibroblasts and epithelial cells with mild reactive dysplasia found adjacent to cancer cells may affect the growth of human colorectal cancer cells.

Functions of lumican and fibromodulin: lessons from knockout mice

Lumican and fibromodulin are collagen-binding leucine-rich proteoglycans widely distributed in interstitial connective tissues. The phenotypes of lumican-null (Lum(-/-)), Fibromodulin-null (Fmod(-/-)) and compound double-null (Lum(-/-)Fmod(-/-)) mice identify a broad range of tissues where these two proteoglycans have overlapping and unique roles in modulating the extracellular matrix and cellular behavior. The lumican-deficient mice have reduced corneal transparency and skin fragility. The Lum(-/-)Fmod(-/-) mice are smaller than their wildtype littermates, display gait abnormality, joint laxity and age-dependent osteoarthritis. Misaligned knee patella, severe knee dysmorphogenesis and extreme tendon weakness are the likely cause for joint-laxity. Fibromodulin deficiency alone leads to significant reduction in tendon stiffness in the Lum(+/+)Fmod(-/-) mice, with further loss in stiffness in a lumican gene dose-dependent way. At the level of ultrastructure, the Lum(-/-) cornea, skin and tendon show irregular collagen fibril contours and increased fibril diameter. The Fmod(-/-) tendon contains irregular contoured collagen fibrils, with increased frequency of small diameter fibrils. The tendons of Lum(-/-)Fmod(-/-) have an abnormally high frequency of small and large diameter fibrils indicating a de-regulation of collagen fibril formation and maturation. In tissues like the tendon, where both proteoglycans are present, fibromodulin may be required early in collagen fibrillogenesis to stabilize small-diameter fibril-intermediates and lumican may be needed at a later stage, primarily to limit lateral growth of fibrils

Lumican expression in alpha cells of islets in pancreas and pancreatic cancer cells

Lumican is a member of a small leucine-rich proteoglycan family, members of which play an important role in cell migration and proliferation during embryonic development, tissue repair, and tumour growth. Lumican is reported to be overexpressed during the wound healing process in the cornea and in human breast cancer tissues, but its expression and localization in normal pancreas and pancreatic cancer tissues are not known. The present study aimed to clarify the expression of lumican protein and its mRNA in human pancreatic cancer cell lines and their localization in normal and cancerous human pancreatic tissues. Reverse transcription-polymerase chain reaction (RT-PCR) and western blot analysis revealed lumican mRNA and its protein expression in PK-8 and MIA-PaCa-2 human pancreatic cancer cells. The tumour lumican had non- or poorly sulphated polylactosamine side-chains rather than highly sulphated keratan sulphate chains. Immunoreactivity of the lumican protein was localized in alpha cells of islets and stromal tissues of a normal pancreas. In pancreatic cancer tissues, the lumican protein was strongly localized in cancer cells, and in acinar and islet cells in chronic pancreatitis-like lesions adjacent to the cancer cells. It was also localized in fibroblasts and collagen fibres close to cancer cells. Lumican mRNA was expressed in cancer cells, in acinar and islet cells in chronic pancreatitis-like lesions, and in stromal fibroblasts in the pancreatic cancer tissues. This is the first report that lumican is synthesized in endocrine and cancer cells. Lumican protein may play a role in the maintenance of islet cell function in normal pancreas and the lumican protein synthesized by cancer cells, acinar and islet cells, and stromal fibroblasts may play a role in the growth of human pancreatic cancer cells.

Expression and Localization of Lumican in the Ischemic and Reperfused Rat Heart

Lumican belongs to the small leucine-rich proteoglycan family and has an important role in the regulation of corneal and dermal collagen fiber assembly. Recently, lumican mRNA was found in the heart and its high expression was reported during wound healing of the cornea. In the present study, the expression and role of lumican in fibrosis of an ischemic and reperfused rat heart were examined. The expression level of lumican mRNA increased in the ischemic and reperfused rat heart and was highest on the fourth week. Lumican protein existed in the forms of core protein and proteoglycan in the control rat heart. The amount of lumican in the form of proteoglycan increased, and that as the form glycoprotein was newly detected in the ischemic and reperfused rat heart on the fourth week. In the control heart, lumican was weakly expressed in the collagen fibers of the perivascular area, but it was expressed strongly in many capillary endothelial cells in the ischemic lesion on day 1. After 3 days, lumican was localized in collagen fibers and in the fibroblasts of fibrotic lesions. A few myocardial cells close to the ischemic lesion expressed lumican mRNA. Lumican is considered to play an important role in the fibrillogenesis of the ischemic and reperfused rat heart.

Analysis of the human lumican gene promoter

The human lumican gene was shown to possess one major transcription start site, resulting in exon 1 of the gene giving rise to the first 74 base pairs (bp) of the 5'-untranslated region. About 1.6 kilobase pairs of upstream promoter sequence were sequenced and analyzed to identify elements responsible for gene expression. No typical TATAA sequence was identified in the vacinity of the transcription start site, but an atypical TATCA sequence residing 41 bp upstream was shown to be necessary for transcription, although it was incapable of supporting transcription by itself. A GC box residing 74 bp upstream of the transcription start site also was essential for the initiation of transcription. Sp3 was identified as the transcriptional activator binding to the GC box. No additional elements that significantly modulated transcription were noted in the promoter sequence analyzed, when using human adult chondrocytes as the cell source for transfection in reporter assays. In contrast, reporter assays carried out in human fetal lung fibroblasts, where lumican expression is deplete, revealed the presence of a repressor element located between 384 and 598 bp upstream of the transcription start site. A GATA-binding site located between bp -386 and -391 was identified as being necessary for repression of transcription. The mouse lumican promoter does not possess an equivalent site, and this may explain why the lumican gene is expressed in fetal murine cartilage but not in fetal human cartilage.

Molecular cloning and relative tissue expression of keratocan and mimecan in embryonic quail cornea

We have cloned and sequenced the cDNAs for quail cornea keratan sulfate proteoglycan core proteins, keratocan and mimecan. The deduced quail keratocan protein contains a single conservative amino acid difference from the chick sequence, whereas quail mimecan protein contains a 58 amino acid-long avian-unique sequence that shares no homology with mammalian mimecan. Ribonuclease protection assay of Day 16 embryonic quail tissues reveals that keratocan and lumican are expressed at highest levels in cornea, whereas mimecan mRNA is expressed at a much lower level. Keratocan is expressed only in quail cornea, whereas mimecan is expressed in many different tissues as four transcripts of different sizes. Both lumican and mimecan are expressed at lowest levels in brain, liver and sternum.

Structure and sequence of the gene encoding human keratocan

Keratocan is one of the three major keratan sulfate proteoglycans characteristically expressed in cornea. We have isolated cDNA and genomic clones and determined the sequence of the entire human keratocan (Kera) gene. The gene is spread over 7.65 kb of DNA and contains three exons. An open reading frame starting at the beginning of the second exon encodes a protein of 352 aa. The amino acid sequence of keratocan shows high identity among mammalian species. This evolutionary conservation between the keratocan proteins as well as the restricted expression of Kera gene in cornea suggests that this molecule might be important in developing and maintaining corneal transparency.

Identification and characterization of conserved cis-regulatory elements in the human keratocan gene promoter

Keratocan, along with lumican and mimecan, represent the keratan sulfate-containing proteoglycans of the vertebrate cornea that play a key role in development and maintenance of corneal transparency. In this study, we cloned 4.1 kb of the human Kera 5'-flanking region and characterized the promoter structure. Using primer extension and ribonuclease protection assay, we identify two major transcriptional start sites in the first exon. Using luciferase reporter gene transfection analysis of 5'-deletion and mutation constructs, we demonstrate positive and negative regulatory elements within a 1.3 kb upstream sequence. Comparison of human and bovine 5'-flanking sequences reveals three highly conserved regions: a 450 bp region in the first exon, a 92 bp promoter proximal conserved regulatory region identified as an enhancer in the natural context, and a 223 bp promoter distal conserved regulatory region identified as a silencer both in the natural context and in a heterologous promoter system. In addition, a conserved CArG-box residing 851 bp upstream of the first transcription start site also can lead to the repression of Kera expression in cultured corneal keratocytes. DNaseI footprinting and electrophoretic mobility shift assay demonstrate that cell type-specific factors bind to regulatory elements located in the conserved regions. Competition experiments indicate that the CTC factor and a protein that binds to the CAGA motif are likely to be among the multiple factors involved in the transcriptional regulation of the human Kera gene.

Fibromodulin and lumican bind to the same region on collagen type I fibrils

Fibromodulin and lumican are closely related members of the extracellular matrix leucine-rich repeat glycoprotein/proteoglycan family. Similar to decorin, another member of this protein family, they bind to fibrillar collagens and function in the assembly of the collagen network in connective tissues. We have studied the binding of recombinant fibromodulin, lumican and decorin, expressed in mammalian cells, to collagen type I. Using a collagen fibril formation/sedimentation assay we show that fibromodulin inhibits the binding of lumican, and vice versa. Fibromodulin and lumican do not affect the binding of decorin to collagen, nor does decorin inhibit the binding of fibromodulin or lumican. Binding competition experiments and Scatchard plot analysis indicate that fibromodulin binds to collagen type I with higher affinity than lumican.

Expression of the keratan sulfate proteoglycans lumican, keratocan and osteoglycin/mimecan during chick corneal development

The corneal proteoglycans belong to the Leu-rich proteoglycan (LRP) gene family and contain chondroitin/dermatan (CS/DS) or keratan sulfate (KS) chains. These proteoglycans play a critical role in generating and maintaining a transparent matrix within the corneal stroma. Decorin which has CS/DS chains and lumican which has KS chains, were first to be identified in the cornea. Two other corneal KS proteoglycans (KSPGs), keratocan and osteoglycin/mimecan were recently identified in bovine corneas. We cloned and sequenced chick osteoglycin/mimecan and found it to contain a stretch of 60 amino acids that showed no identity to the presumed mammalian homolog. The 177 base pair DNA coding for this unique sequence shows 47% identity to an 189 base pair sequence between exons 4 and 5 of the bovine osteoglycin/mimecan gene. This indicates that this cDNA represents an alternatively spliced form of osteoglycin/mimecan containing a unique N-terminal sequence. The expression of each of the three corneal KSPGs in the developing and mature chick cornea was investigated by competitive PCR and immuno-biochemical analysis of corneal extracts. Competitive PCR was used to determine the message levels for chick lumican, keratocan and osteoglycin in embryonic day 9, 12, 15, 18 and adult corneas. Results showed that lumican mRNA fluctuated during development but remained at a relatively high level while keratocan and osteoglycin message levels declined steadily from day 9 to adult. Additionally, lumican mRNA was present at higher levels, during all stages of corneal development, than keratocan and at much higher levels than osteoglycin. Antibodies shown to be specific for each KSPG were used to characterize proteoglycans isolated from embryonic and adult chick corneas. KSPGs from embryonic corneas eluted 1-2 fractions earlier on Q-Sepharose than KSPG from adult corneas. Additionally, Western blot analysis showed that embryonic KSPGs were more keratanase-resistant, endo-beta-galactosidase sensitive than adult KSPGs. The results of this study indicate an alteration in sulfation or the fine structure of the glycosaminoglycan chains occurs during corneal maturation for the 3 KSPGs.

Cloning, chromosomal localization, and characterization of cDNA from a novel gene, SH3BP4, expressed by human corneal fibroblasts

The cornea contains, as a major element, a transparent stroma produced and maintained by keratocytes (fibroblasts). Through molecular biology studies using cultured human corneal fibroblasts, a cDNA that was shown to be novel was isolated and sequenced. This novel gene product, named SH3-domain binding protein 4 (SH3BP4), contains a 5.6-kb message that is present in normal human corneal fibroblasts and all tissues examined, with higher levels in pancreas, placenta, heart, and kidney. SH3BP4 was localized by FISH analysis to human chromosome 2q37.1-q37.2 near the telomere. The deduced sequence for SH3BP4 was found to contain a 963-amino-acid open reading frame that has homology to a 479-amino-acid protein in GenBank called EH-binding protein. Although the entire sequence of EH-binding protein aligns with SH3BP4, the alignment is not complete or contiguous. Therefore, SH3BP4 has an additional 73 amino acids at the N-terminus and an additional 411 amino acids near the C-terminus that are not present in EH-binding protein. Consensus sequence domains identified in SH3BP4 include a SH3 domain, three N-P-F motifs, a P-X-X-P motif noted for binding to SH3 domains, a bipartite nuclear targeting signal, and a tyrosine phosphorylation site. SH3BP4 homologies and consensus sequence sites indicate that it may be involved in a newly identified cascade of proteins involved in endocytosis, intracellular sorting, and the cell cycle.

Modulation of keratan sulfate synthesis on lumican by the action of cytokines on human articular chondrocytes

Adult human articular chondrocytes were used to investigate why keratan sulfate/polylactosamine chains are deficient on the lumican residing in the matrix of adult articular cartilage, whereas they are present on the lumican residing in the matrix of juvenile cartilage. Under serum-free conditions with either monolayer cultures, agarose cultures, or micromass cultures, the adult chondrocytes synthesized a form of lumican possessing keratan sulfate/polylactosamine chains. Thus, the adult chondrocytes are capable of producing a proteoglycan form of lumican and this appears to be the default synthesis preference. The micromass culture system proved useful for demonstrating that growth factors/cytokines present in the extracellular milieu are capable of influencing the structure of the keratan sulfate/polylactosamine chains on the secreted lumican. Of particular note was the ability of IL-1beta to promote the secretion of a form of lumican deficient in keratan sulfate/polylactosamine chains, whereas with bFGF, IGF-1 and TGFbeta keratan sulfate/polylactosamine chains were present, though their size or degree of substitution varied. Thus, growth factors/cytokines are able to modulate the molecular form of lumican. Furthermore, additional studies showed that this modulation was not due to the degradation of keratan sulfate/polylactosamine chains following proteoglycan secretion, but represented a direct effect on synthesis.

Corneal cell proteins and ocular surface pathology

The cornea is a transparent and avascular tissue that functions as the major refractive structure for the eye. A wide variety of growth factors, chemokines, cytokines and their receptors are synthesized by corneal epithelial and stromal cells, and are found in tears. These molecules function in corneal wound healing and in inflammatory responses. Proteoglycans and glycoproteins are essential for normal corneal function, both at the air-epithelial interface and within the extracellular matrix. The ocular MUC mucins may play roles in forming the mucus layer of the tear film, in regulating tear film spread, and in inhibiting the adhesion of pathogens to the ocular surface. Lumican, keratocan and mimecan are the major keratan sulfate proteoglycans of the corneal stroma. They are essential, along with other proteoglycans and interfibrillar proteins, including collagens type VI and XII, for the maintenance of corneal transparency. Corneal epithelial cells interact with a specialized extracellular matrix structure, the basement membrane, composed of a specific subset of collagen type IV and laminin isoforms in addition to ubiquitous extracellular matrix molecules. Matrix metalloprotein-ases have been identified in normal corneal tissue and cells and may play a role in the development of ulcerative corneal diseases. Changes in extracellular matrix molecule localization and synthesis have been noted in other types of corneal diseases as well, including bullous keratopathy and keratoconus.