Immunofluorescence Studies of Chondroitin Sulfate Proteoglycan Biosynthesis: The Use of Monoclonal Antibodies

Abstract

Several monoclonal antibodies which recognize different antigenic determinants of chondroitin sulfate proteoglycan were used to study chondroitin sulfate proteoglycan biosynthesis in chicken chondrocyte cultures. The intracellular sites of synthesis and processing and extracellular deposition in matrix were localized by double immunofluorescence reactions. One rat monoclonal antibody, S103L, which recognizes an antigenic determinant of the core protein of the chicken cartilage chondroitin sulfate proteoglycan monomer, was used to identify both extracellular chondroitin sulfate proteoglycan and intracellular compartments containing chondroitin sulfate proteoglycan precursors. Intracellular staining with S103L was localized to perinuclear regions, and, in some chondrocytes, to a few other cytoplasmic vesicles as well. When chondrocytes were not fed for several days, intracellular chondroitin sulfate proteoglycan precursors were accumulated in larger compartments distributed throughout the cytoplasm. Polyclonal chondroitin sulfate proteoglycan antibodies displayed similar staining characteristics. In contrast, several of the monoclonal antibodies, including the rat monoclonals S11D and P100D, and the mouse monoclonals 1-B-5, 3-B-3 and 9-A-2, did not recognize native chondroitin sulfate proteoglycan, but reacted only with chondroitinase ABC-digested (and/or hyaluronidase-digested) chondroitin sulfate proteoglycan. These antibodies were particularly useful in the demonstration of the extracellular codistribution of chondroitin sulfate proteoglycan with either type II collagen or fibronectin. In other experiments, the monoclonal antibodies to chondroitin sulfate proteoglycan served to demonstrate that the perinuclear subset of intracellular compartments is uniquely involved in the addition of chondroitin sulfate oligosaccharides to the chondroitin sulfate proteoglycan core protein. Lastly, using the mouse monoclonal 5-D-4, which recognizes keratan sulfate determinants, the perinuclear region was identified as the site for keratan sulfate addition. Results suggest heterogeneity of keratan sulfate synthesis at the level of individual chondrocytes, even for cells apparently containing equivalent amounts of intracellular chondroitin sulfate proteoglycan.