Degradation of small leucine-rich repeat proteoglycans by matrix metalloprotease-13: identification of a new biglycan cleavage site

Jordi Monfort,Peter Roughley,Ginette Tardif,Pascal Reboul,Johanne Martel-Pelletier,François Mineau,Jean-Pierre Pelletier

doi:10.1186/ar1873

Abstract

A major and early feature of cartilage degeneration is proteoglycan breakdown. Matrix metalloprotease (MMP)-13 plays an important role in cartilage degradation in osteoarthritis (OA). This MMP, in addition to initiating collagen fibre cleavage, acts on several proteoglycans. One of the proteoglycan families, termed small leucine-rich proteoglycans (SLRPs), was found to be involved in collagen fibril formation/interaction, with some members playing a role in the OA process. We investigated the ability of MMP-13 to cleave members of two classes of SLRPs: biglycan and decorin; and fibromodulin and lumican. SLRPs were isolated from human normal and OA cartilage using guanidinium chloride (4 mol/l) extraction. Digestion products were examined using Western blotting. The identities of the MMP-13 degradation products of biglycan and decorin (using specific substrates) were determined following electrophoresis and microsequencing. We found that the SLRPs studied were cleaved to differing extents by human MMP-13. Although only minimal cleavage of decorin and lumican was observed, cleavage of fibromodulin and biglycan was extensive, suggesting that both molecules are preferential substrates. In contrast to biglycan, decorin and lumican, which yielded a degradation pattern similar for both normal and OA cartilage, fibromodulin had a higher level of degradation with increased cartilage damage. Microsequencing revealed a novel major cleavage site (... G177/V178) for biglycan and a potential cleavage site for decorin upon exposure to MMP-13. We showed, for the first time, that MMP-13 can degrade members from two classes of the SLRP family, and identified the site at which biglycan is cleaved by MMP-13. MMP-13 induced SLRP degradation may represent an early critical event, which may in turn affect the collagen network by exposing the MMP-13 cleavage site in this macromolecule. Awareness of SLRP degradation products, especially those of biglycan and fibromodulin, may assist in early detection of OA cartilage degradation.

Highlights

Osteoarthritis (OA) is the most common rheumatologic disease, with high incidence and morbidity
Matrix metalloprotease (MMP)-13 degrades biglycan and decorin Biglycan in human normal and OA cartilage migrated as a doublet at 48 and 45 kDa, representing intact and amino-terminally processed biglycan
In the specimens from nonfibrillated to moderately fibrillated cartilage not treated with MMP13, a biglycan degradation product of a similar size to that generated by MMP-13 was already present, in low amounts

Summary

Introduction

Osteoarthritis (OA) is the most common rheumatologic disease, with high incidence and morbidity. Proteoglycans form a large group that can be classified into five families according to the structural properties of their core protein [2]. One group, termed the small leucine-rich proteoglycans (SLRPs), possesses a central domain of characteristic repeats that participate in protein-protein interactions [3]. The SLRPs can be divided into four classes based on gene organization and amino acid sequence homologies [1]: class I includes decorin, biglycan and asporin; class II includes fibromodulin, lumican, keratocan, PRELP (proline arginine-rich end leucine-rich repeat protein) and osteoadherin; class III includes epiphycan, mimecan and opticin; and class IV includes chondroadherin and the recently identified nyctalopin [4]. An understanding of the functions of SLRPs is only emerging, most of the members bind to other extracellular matrix constituents and contribute to the structural framework of connective tissues [3]. Some were shown to interact with various collagen types, including

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