GADD45ß in cartilage development and osteoarthritis

GADD45beta, a member of the growth arrest and DNA damage-induced protein family, responds to stress signals such as irradiation, hypoxia, and genotoxic drugs. Preliminary studies demonstrate previously undiscovered roles of GADD45beta in chondrocyte terminal differentiation during development and in cartilage remodeling in the adult and suggest novel mechanisms involved in its induction and action. Preliminary observations suggest that (1) low (transient) levels of GADD45beta may play a protective role in the stress responses during endochondral ossification and also in the maintenance of articular cartilage homeostasis in the adult, whereas (2) high levels, or sustained expression as observed in chondrocyte clusters and in deep zone hypertrophic-appearing cells in human or mouse OA, may result in cellular dysfunction. Thus, we hypothesize that GADD45beta may serve as a key intracellular mediator of chondrocyte hypertrophy common to terminal differentiation and OA. This hypothesis will be tested by the following Specific Aims: Specific Aim 1: Determine the role of GADD45beta during development and maturation of cartilage. The spatial and temporal expression of GADD45beta will be analyzed, by immunohistochemistry and in situ hybridization, in relation to markers of the transition from the COL2A1 (Sox9)- to COL10A1 (Runx2)- associated phenotype, skeletal remodeling (MMP-13), and apoptosis in Gadd45beta -/-, Gadd45beta +/- and wild type mice. Specific Aim 2: Determine the role of GADD45beta in mouse models that mimic OA in humans. We plan to generate compound mutant mice with type XI collagen-haploinsufficiency and GADD45beta -deficiency to determine a role for GADD45beta in the development of OA-like pathology. We will also examine cartilage maturation in the embryonic and perinatal growth plates of these compound mutant mice and transgenic mice. Specific Aim 3: Determine the signaling pathways and transcriptional events that govern GADD45beta expression and action (a) during chondrogenesis and terminal differentiation and (b) in the differentiated chondrocyte in adult articular cartilage. The ATDC5 and human chondrocyte cell lines will be used to characterize the signaling kinases and transcription factors involved. These study will provide new insights into mechanisms of chondrocyte hypertrophy that are common to terminal differentiation during development and to dysregulated chondrocyte function in osteoarthritis or aging in the adult.