Role of Lumican in the Corneal Epithelium during Wound Healing

Abstract

Lumican regulates collagenous matrix assembly as a keratan sulfate proteoglycan in the cornea and is also present in the connective tissues of other organs and embryonic corneal stroma as a glycoprotein. In normal unwounded cornea, lumican is expressed by stromal keratocytes. Our data show that injured mouse corneal epithelium ectopically and transiently expresses lumican during the early phase of wound healing, suggesting a potential lumican functionality unrelated to regulation of collagen fibrillogenesis, e.g. modulation of epithelial cell adhesion or migration. An anti-lumican antibody was found to retard corneal epithelial wound healing in cultured mouse eyes. Healing of a corneal epithelial injury in Lum ؊/؊ mice was significantly delayed compared with Lum ؉/؊ mice. These observations indicate that lumican expressed in injured epithelium may modulate cell behavior such as adhesion or migration, thus contributing to corneal epithelial wound healing.Rapid re-epithelialization is essential for restoration of homeostasis in injured tissues; impaired healing of injured epithelium increases the risks of infection and further damage underlying tissues (1, 2). The cornea provides an ideal model to evaluate interactions of migrating epithelial cells and the extracellular matrix of the underlying basement membrane during wound healing because epithelial injuries of the avascular corneal tissue heal in a bloodless wound field. Various specific proteins such as vinculin (3), keratins (4), CD44 hyaluronan receptors (5), and gelatinases and metalloproteinase inhibitors (6, 7) are up-regulated during corneal epithelial wound healing. These proteins are believed to modulate cell adhesion or migration.Lumican belongs to the family of small leucine-rich proteoglycans (SLRPs) 1 that includes keratocan, mimecan, decorin, biglycan, fibromodulin, epiphycan, and osteoadherin. In the cornea, lumican, keratocan, and mimecan are modified with keratan sulfate glycosaminoglycan chains comprising the keratan sulfate proteoglycans (KSPG) of the stromal extracellular matrix (8 -13). In normal unwounded mouse cornea, lumican mRNA is expressed in stromal keratocytes (14). Lumican KSPG is a key regulator of collagen fibrillogenesis, a process critical to corneal transparency. Mice lacking lumican show an age-dependent corneal opacity and a high proportion of abnormally thick collagen fibers in the corneal stroma (15).Lumican is also widely present as a non-or low-sulfated glycoprotein in connective tissues of many other organ systems, e.g. skeleton, heart, kidney, and lung (14, 16 -18). During mouse embryonic ocular development, lumican is synthesized by keratocytes; detected as a glycoprotein, not as a KSPG (19); and also transiently expressed by the corneal epithelium, neural retina, and epidermis (14). These observations suggest that epithelial tissues possess the capacity to express lumican under certain conditions. Several studies have demonstrated that SLRP proteins can modulate cellular behaviors, i.e. cell migration and prolifera...

Collagen fibril assembly during postnatal development and dysfunctional regulation in the lumican‐deficient murine cornea

“…We know that lumican regulates collagen fibril diameter and organization in the adult cornea (Chakravarti et al, 1998;2000;Saika et al, 2000). The spatial distribution of collagen fibrillar ultrastructural anomalies coincides with the posterior-rich distribution of lumican in the normal cornea, with transparency defects localized to the posterior stroma of the Lum Ϫ/Ϫ cornea.…”

“…Earlier structural studies were done using adult lumican knockout mice and demonstrated defects in collagen fibril structure and organization in the adult posterior stroma (Chakravarti et al, 1998Saika et al, 2000). Here, we focused on analyzing the developmental acquisition of this phenotype.…”

“…Each of these four leucine-rich proteoglycans have been "knocked out" in the mouse (Danielson et al, 1997;Chakravarti et al, 1998;Saika et al, 2000;Tasheva et al, 2002;Liu et al, 2003) and all demonstrate abnormal connective tissue phenotypes in various organs and tissues. However, only mice deficient in lumican have a significant disruption in corneal collagen fibril architecture.…”

Transparency, swelling and scarring in the corneal stroma

“…Some progress has been made, particularly with respect to our understanding of what factors govern corneal fibril size 49,50 and organisation, including the roles of the ambient ions 51 and of proteoglycans. The recent availability of gene-targeted mice with null mutations for selected proteoglycans [52][53][54] now makes it possible to correlate the structural effect of selected deletions with tissue transparency. For example, it is interesting that lumican-null mice have cloudy corneas, 52 decorin-null mice have clear corneas, 53 and keratocan-null mice have mostly clear corneas.…”

“…For example, it is interesting that lumican-null mice have cloudy corneas, 52 decorin-null mice have clear corneas, 53 and keratocan-null mice have mostly clear corneas. 54 These and similar tissues open the possibility to greatly increase our understanding of the causes of light scattering from abnormal corneas in the near future. Figure 11 The Summation of Scattered Fields method was used to predict transmission as a function of wavelength in a rabbit cornea 8 months after PTK and in its control cornea.…”