Fibronectin is a multifunctional glycoprotein that regulates cell attachment, migration, differentiation, proliferation and survival. Recent studies have shown that fibronectin controls assembly of latent transforming growth factor beta binding protein-1 (LTBP1) into the extracellular matrix (ECM) of bone cells. This suggests that fibronectin may regulate availability of the Transforming Growth Factor-P (TGF-β) family of growth factors that are important modulators of bone cell function. The goal of this dissertation research was to define at the molecular level the role of fibronectin in osteoblast function and in regulation of TGFβ in bone, via its interactions with LTBP1.

In primary osteoblast cultures, critical domains in the N-terminus of LTBP1 that are required for its co-localization with fibronectin were identified. Solid phase binding assays suggested that LTBP1 does not bind directly to fibronectin and that heparan sulphate containing proteoglycans (HSPGs) may mediate LTBP1/fibronectin interactions. Treatment of primary osteoblasts with heparin, heparin-BSA or inhibitors of GAG chain assembly impaired LTBP1 and TGFP deposition into the ECM without reducing expression or secretion. These studies suggest that HSPGs may play a critical role in regulating TGFβ availability by controlling LTBP1 deposition into the ECM in association with fibronectin.

Fibroblasts derived from fibronectin-null embryos were used to further examine the role of fibronectin in osteoblast function. Although these cells could be induced to express late osteoblastic markers by treatment with bone morphogenetic protein-2 (BMP2), both BMP2 and fibronectin were required for mineralization. This effect of fibronectin on mineralization is likely due to its role as an orchestrator for ECM assembly, as the deposition of multiple bone ECM components including type I collagen, LTBP1, fibrillin-1, biglycan, decorin and perlecan was impaired in fibronectin null cells. Furthermore, mineralization was rescued by providing the cells with a pre-assembled matrix or a collagen gel. Fibronectin was also found to control matrix deposition of TGF-β, through its effects in regulating LTBP1 assembly.

Together these studies suggest that fibronectin regulates osteoblast function at multiple levels, through its effects in controlling assembly and stability of multiple bone ECM proteins, regulating mineralization and controlling the deposition and bioavailability of growth factors in the ECM.