Aggrecan is the prominent
proteoglycan in cartilage and is modified with approximately 100
chondroitin sulfate (CS) chains through
a tetrasaccharide linkage structure. In
osteoarthritis (OA), the viscoelastic properties of cartilage are compromised on both the quantity and integrity of
aggrecan core
protein expressed as well as reduced overall CS chain length. Herein, we postulated that chronic low-level
inflammation may also contribute to OA progression by promoting regulatory mechanisms in early CS biosynthesis that yield incomplete linkage structures on
aggrecan. To test this idea, chondrocytes extracted from human tali were cultured in
alginate beads and challenged with 5 ng/mL IL-1β as a model for chronic
inflammation leading to OA progression. Novel mass spectrometry-based methods were devised to detect and quantify partially elongated linkage structures relative to control cultures. The total mole fraction of unelongated
xylose residues per
aggrecan was significantly less (p = 0.03) after IL-1β treatment compared to control cultures, with unelongated
xylose residues constituting between 6% and 12% of the fraction of total CS measured. A portion (<1%) of the partially elongated linkage structures was found to be either phosphorylated or sulfated. These results establish quantitative mass spectrometry as a very sensitive and effective platform for evaluating truncated
proteoglycan linkage structures. Our observations using this method suggest a possible role for aberrant linkage structure elongation in OA progression.