Corneal Opacity in Lumican-Null Mice: Defects in Collagen Fibril Structure and Packing in the Posterior Stroma

It has long been recognized that collagen architecture of the corneal stroma is crucially important in the ultimate transparency of the cornea.1 Collagen fibrils in the stroma are maintained in the range of 20 to 40 nm and organized into a highly ordered, latticelike configuration. The highly ordered architecture of the corneal stroma is affected by multiple factors. Recent studies of several types of hereditary corneal dystrophies elucidated abnormal collagen fibril architecture of the corneal stroma. For example, corneal opacification is a clinical feature of Scheie’s syndrome or mucopolysaccharidosis (MPS) type I, a lysosomal storage disorder with an iduronidase A deficiency.2 In addition to featuring granular deposits, transmission electron microscopy of MPS I–affected corneas revealed the presence of thicker collagen fibrils and localized disorganization of the matrix.3,4 Macular corneal dystrophy, with deficiencies in keratan sulfate (KS) biosynthesis, also causes clouding of the cornea and similar disruptions in stromal fibril structure and organization.5–8 In both cases, altered proteoglycan synthesis and composition are to be expected. Recently, a mouse model for corneal dystrophy was developed by targeted disruption of the lumican gene (lumtm1sc/lumtm1sc).9 The mutant mice had cloudy corneas and stromal collagen fibrils with increased diameter and altered structure. Lumican is a member of the leucine-rich proteoglycan (LRP) family.10 It is a major keratan sulfate proteoglycan of the corneal stroma as well as other collagenous extracellular matrices (skin, cardiac valves, cartilage, and bone).11 Other LRP members include decorin, fibromodulin, biglycan, keratocan, osteoglycin, and epiphycan.12 Decorin, a chondroitin sulfate (CS) proteoglycan widely expressed during mouse embryonic development, is also a major component of the corneal stroma.13 Previous studies have shown that the core proteins of lumican, decorin, and other LRPs from tendons can delay spontaneous collagen fibril formation and inhibit the lateral growth of fibrils in fibrillogenesis assays in vitro.14–16 Also, the abnormal lateral growth of isolated corneal fibrils stripped of their surface-associated macromolecules is prevented by the corneal proteoglycans.17 Recent gene-targeting studies of LRPs suggest a similar role for these proteoglycans in vivo. Thus, absence of lumican in our lumtm1sc/lumtm1sc mouse model of corneal dystrophy affected collagen architecture of the cornea and skin with consequent corneal opacity and reduced dermal biomechanical tensile strength. In addition to lumican, gene-targeted null mutations in decorin and fibromodulin also led to abnormal collagen fibril architecture in skin and tendons.18,19 However, to date only the lumican-deficient mice have demonstrated a corneal phenotype. The purpose of the present study was to assess corneal opacification in the lumtm1sc/lumtm1sc mice and define its source in the corneal stroma by in vivo confocal microscopy. Parallel analyses of collagen fibril structure, fibril packing, and organization in the lumican-deficient and wild-type control mice and lumican expression in the mature normal cornea indicate that lumican serves a key role in the establishment and maintenance of corneal transparency.