Proteoglycan 4 (PRG4) synthesis and immunolocalization in bovine meniscus

Proteoglycan 4 (PRG4) is synthesized and secreted into the synovial fluid by articular chondrocytes and synovial cells, lining the cavity of joints. A thin layer of PRG4 is also present at the articular surface, where it appears to be involved in boundary lubrication. This study investigated if PRG4 is also synthesized and secreted by the cells within meniscus, and if PRG4 is also present in, and at the surface of, meniscus. PRG4 was visualized in sections of bovine calf menisci by immunohistochemistry. PRG4 was detected in two regions: (1) at the femoral and tibial surfaces of the meniscus, and within cells below these surfaces; and (2) within and near cells along the radial tie fibers and circumferential fibers. From meniscus tissue harvested from these surfaces, PRG4 was extracted with 4 M GuHCl and quantified by ELISA. There was 0.20 ± 0.01 and 0.25 ± 0.04 μg PRG4/cm2 area of lateral and medial meniscus surface, respectively. ELISA analysis of spent medium from other samples of meniscus surface tissue incubated in medium supplemented with serum and ascorbate showed that 8.1 ± 1.1 μg PRG4/cm2 area of meniscus surface was secreted over six days. These results demonstrate that PRG4 is synthesized and secreted by certain cell populations in the meniscus, and that PRG4 is present in the meniscus at surfaces and also internal fibers where it may contribute to boundary lubrication.