MAINTENANCE OF CARTILAGE EXTRACELLULAR MATRIX: THE PARTICIPATION OF HAS-2 AND CD44

Abstract

Cartilage is a tissue whose function is highly dependent on the maintenance of its extracellular matrix. Hyaluronan (HA) provides a unique role in cartilage, serving to sequester and retain proteoglycan within the tissue. A fraction of the HA-proteoglycan rich matrix remains anchored to the chondrocytes via interactions with the HA receptor CD44. We have also determined that chondrocytes utilize the same CD44 receptor to internalize HA, thus providing one mechanism for HA catabolism in cartilage. To better understand these processes, the expression of HA synthases (HAS) and CD44 was examined. Using quantitative-competitive RT-PCR we determined that human as well as bovine articular chondrocytes express primarily HAS-2, substantially less HAS-3 and no HAS-1. Antisense oligonucleotides directed against HAS-2 inhibited chondrocyte HA production in proportion to the level of inhibition of HAS-2 mRNA. It was therefore concluded that HAS-2 is the predominant gene involved in HA synthesis by articular chondrocytes. Incubation of chondrocytes with an anabolic cellular mediator, osteogenic protein-1, an agent that stimulates collagen type II and aggrecan production, resulted in a substantial increase in HAS-2 and CD44 mRNA copy numbers and, a pronounced accumulation of pericellular HA. No change in HAS-3 mRNA was observed. Interestingly, treatment of chondrocytes with the catabolic cytokine IL-1α also resulted in an increase in HAS-2 and CD44. However in this case, less accumulation of HA within the pericellular matrix was observed. Visualization of intracellular HA suggested that the increased synthesis of HA due to IL-1 was offset by enhanced CD44-mediated HA endocytosis. Thus, chondrocytes maintain their extracellular matrix composition, in part, by coordinating the expression of two components critically necessary for retention of proteoglycan, namely HAS-2 and CD44.