Mimecan, the 25-kDa Corneal Keratan Sulfate Proteoglycan, Is a Product of the Gene Producing Osteoglycin

Abstract

Bovine cornea contains three unique keratan sulfate proteoglycans (KSPGs), of which two (lumican and keratocan) have been characterized using molecular cloning. The gene for the third protein (KSPG25) has not been identified. This study examined the relationship between the KSPG25 protein and the gene for osteoglycin, a 12-kDa bone glycoprotein. The N-terminal amino acid sequence of KSPG25 occurs in osteoglycin cDNA cloned from bovine cornea. The osteoglycin amino acid sequence makes up the C-terminal 47% of the deduced sequence of the KSPG25 protein. Antibodies to osteoglycin reacted with intact corneal KSPG, with KSPG25 protein, and with a 36-kDa protein, distinct from lumican and keratocan. KSPG25-related proteins, not modified with keratan sulfate, were also detected in several connective tissues. Northern blot analysis showed mRNA transcripts of 2.4, 2.5, and 2.6 kilobases in numerous tissues with the 2.4-kilobase transcript enriched in ocular tissues. Ribonuclease protection analysis detected several protected KSPG25 mRNA fragments, suggesting alternate splicing of KSPG25 transcripts. We conclude that the full-length translation product of the gene producing osteoglycin is a corneal keratan sulfate proteoglycan, also present in many non-corneal tissues without keratan sulfate chains. The multiple size protein products of this gene appear to result from in situ proteolytic processing and/or alternative splicing of mRNA. The name mimecan is proposed for this gene and its products.The corneal stroma of vertebrate organisms contains a unique class of molecules, corneal keratan sulfate proteoglycans (KSPGs), 1 consisting of several related proteins each bearing keratan sulfate. The specialized nature of corneal proteoglycans was recognized almost 60 years ago with the initial description of keratan sulfate, the most abundant glycosaminoglycan in cornea (1). Corneal keratan sulfate is a highly sulfated, linear polymer of N-acetyllactosamine, linked to asparagine residues in the KSPG core proteins (2). The unusual abundance of keratan sulfate in the cornea and studies of heritable metabolic diseases suggest that the KSPG molecules are essential in maintenance of corneal transparency (3, 4). Understanding the role of the KSPGs in corneal transparency, their interactions with cells and other matrix molecules, and the tissue-specific nature of their biosynthesis requires complete knowledge of their structure. Although we have excellent information regarding the carbohydrate components of the KSPGs, the core proteins as representatives of the primary KSPG gene products are still not fully understood.Research from our laboratory has shown that keratan sulfate is attached to three unique proteins in bovine cornea (5, 6). The cDNAs for two of these proteins, keratocan and lumican (originally designated 37A and 37B), have been cloned and sequenced (7,8). A third 25-kDa KSPG core protein, KSPG25, was recognized as unique from lumican and keratocan, but its primary sequence is not yet known. The deduced amino acid sequence...

The bone morphogenetic protein 1/Tolloid-like metalloproteinases

“…SLRPs can be subdivided into four classes. Members of class I (decorin and biglycan) and IV (epiphican, osteoglycin and opticin) SLRPs are synthesized as precursors that are processed in vivo to produce the mature proteins (Funderburgh et al, 1997;Johnson et al, 1997;Roughley et al, 1996). Biglycan has been shown to be processed by BMP1/TLD-like proteinases in vitro and in vivo .…”

“…Class IV SLRPs osteoglycin and epiphycan N-propeptides are also proteolytically cleaved in vivo (Funderburgh et al, 1997;Johnson et al, 1997). All four BMP1/TLD-like proteinases are capable of cleaving pro-osteoglycin in vitro , and Bmp1/Tll1 doubly homozygous null mice produce pro-osteoglycin exclusively in the unprocessed form .…”

IGF-II is present in bovine corneal stroma and activates keratocytes to proliferate in vitro

“…The matrix consists primarily of collagen types I, III, IV, V, and IV (Schmut 1977;Birk et al 1986;Guerriero et al 2007), and four leucine-rich proteoglycans: decorin (Li et al 1992), which bears chondroitin sulfate chains, and lumican (Blochberger et al 1992) keratocan, (Corpuz et al 1996) and osteoglycin/mimecan (Funderburgh et al 1997), which have keratan sulfate chains. These matrix components are found in other connective tissues but the cornea stroma is unusually rich in keratan sulfate proteoglycans.…”

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

“…These collagens co-assemble into heterotypic fibrils (Birk et al, 1988(Birk et al, , 1990. The keratocytes also produce at least 4 different small leucinerich proteoglycans: decorin, lumican, keratocan, and osteoglycin (Hassell et al, 1983;Blochberger et al, 1992;Funderburgh et al, 1997;Beales et al, 1999;Dunlevy et al, 2000). Interactions of these proteoglycans with the collagen fibril can modulate fibril structures (Vogel et al, 1984;Rada et al, 1993;Svensson et al, 2000).…”