A comparative evaluation of the small leucine-rich proteoglycans of pathological human intervertebral discs

Purpose Proteoglycans are important to the functioning of the intervertebral disc. In addition to aggrecan there are the small leucine-rich proteoglycans (SLRPs). These are less common but in other locations their functions include collagen organisation, sequestering growth factors and stimulating inflammation. We have performed a comparative analysis of the SLRP core protein species present in intervertebral discs with various pathologies. Methods Eighteen intervertebral discs from patients with scoliosis (n = 7, 19-53 years), degenerative disc disease (n = 6, 35-51 years) and herniations (n = 5, 33-58 years) were used in this study. Proteoglycans were dissociatively extracted from disc tissues and the SLRPs (biglycan, decorin, fibromodulin, keratocan and lumican) assessed by Western blotting following deglycosylation with chondroitinase ABC and keratanase. Results Intact SLRP core proteins and a number of core protein fragments were identified in most of the discs examined. Biglycan and fibromodulin were the most extensively fragmented. Keratocan generally occurred as two bands, one representing the intact core protein, the other a smaller fragment. The intact core protein of lumican was detected in all samples with fragmentation evident in only one of the older scoliotic discs. Decorin was less obvious in the disc samples and showed little fragmentation. Conclusion In this cohort of pathological intervertebral discs, fragmentation of certain SLRP core proteins was common, indicating that some SLRPs are extensively processed during the pathological process. Identification of specific SLRP fragments which correlate with disc pathology may not only help understand their aetiopathogeneses, but also provide biomarkers which can be used to monitor disease progression or to identify particular disc disorders.