Calcium pyrophosphate dihydrate (CPPD) deposition disease is a common form of degenerative arthritis that preferentially affects the elderly. The causes of CPPD crystal formation in aging articular cartilage are unknown, although many similarities exist between processes of normal cartilage mineralization in growth plate and those of pathologic mineralizing causing CPPD disease. Current evidence suggests that CPPD crystal formation results from excess elaboration of inorganic pyrophosphate (PPi) by chondrocytes, and occurs in or around articular cartilage vesicles (ACVs) and at sites of altered cartilage matrix. The processes known to promote CPPD crystal formation are strongly and uniquely enhanced by transforming growth factor beta (TGF-beta), which is stored in cartilage matrix in a latent biologically inactive form (LTGF-beta). The enzyme transglutaminase (TGase) catalyzes a unique post-translational modification of proteins, resulting in diverse biological effects in various tissues. TGase has recently been identified in mineralizing growth plate chondrocytes. Although TGase participates in processes of cell aging and LTGF-beta activation in other tissues, its role in articular cartilage remains undefined. The applicant's laboratory discovered strikingly high levels of active TGase and type II TGase protein in articular chondrocytes from old pigs compared to chondrocytes from young pigs. Inhibitors of TGase suppress PPi elaboration and reduce levels of activated TGF-beta secreted by old chondrocytes, conditions unfavorable to the formation of CPPD crystals. It is hypothesized that increased TGase activity in aging articular cartilage leads to CPPD crystal formation and the resultant degenerative arthritis. As a consequence, this application proposes to: 1) examine the function of TGase in CPPD deposition by exploring its role in LTGF-beta activation, extracellular matrix modulation, and ACV-induced mineralization in a porcine model; 2) explore the regulation of TGase activity in porcine articular cartilage by factors which modulate CPPD crystal formation; and 3) extend these findings to aging human articular cartilage and cartilage affected by CPPD disease. The goal of these studies is to understand the role and regulation of TGase in aging articular cartilage as it relates to CPPD deposition disease. This multifunctional enzyme represents a novel target for new pharmacologic agents directed against this common degenerative disease affecting our rapidly aging population.
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