Optimizing Chondrogenic Factors and Protein Delivery Methods for Cartilage Repair

Joint injuries are common and often result in damage to cartilage, which has a limited ability to repair itself. Tissue engineering is a promising approach for improving cartilage healing in which biomaterials and chemical factors are supplied to direct cells to create a new tissue. The objective of this thesis was to optimize cartilage-like extracellular matrix production by investigating the effects of Dexamethasone (Dex) and HB-IGF-1 (heparin-binding insulin-like growth factor-1) on cells encapsulated in the self-assembling peptide RAD and agarose hydrogels.

Dex is a synthetic corticosteroid that has been shown to improve cartilage-like tissue production by bone marrow stromal cells (BMSCs), but the mechanisms underlying BMSC response to Dex are not understood. The hypothesis that the addition of Dex to chondrogenic medium would affect matrix production and aggrecanase activity of human and bovine BMSCs in RAD and agarose hydrogels was tested. The effects of Dex were dependent on the hydrogel material and the species/age of the BMSCs. Importantly, Dex reduced aggrecanase-mediated degradation of matrix in both agarose and RAD hydrogels and for both young bovine and adult human BMSCs.

HB-IGF-1, a fusion protein of the heparin binding domain of HB-EGF and IGF-1, can be retained in cartilage matrix and stimulate proteoglycan synthesis with a single dose, whereas unmodified IGF-1 easily diffuses out of cartilage tissue. The RAD peptide was used as a scaffold for retaining growth factor to stimulate encapsulated chondrocytes and adjacent cartilage tissue. RAD was modified by adsorption of HB-IGF-1 before and after RAD assembly, as well as adsorption of heparan sulfate (HS) and IGF-1. The RAD material retained HS adsorbed pre-assembly and HB-IGF-1 delivered in both adsorption methods. Adsorbed HB-IGF-1 and IGF-1 led to increased aggrecan content regardless of the method of adsorption. A trend was found for increased proteoglycan synthesis in adjacent explants as well.

RAD self-assembling hydrogels are a promising material for culturing BMSCs undergoing chondrogenesis, retaining, and delivering HB-IGF-1. Dex decreases aggrecanase activity of differentiating BMSCs and adsorbed HB-IGF-1 appears to enhance aggrecan production by encapsulating chondrocytes and adjacent tissue. These findings show potential for improving cartilage repair in vivo. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)