Osteoarthritis is a disease of aging and is directly related to the stresses of weight bearing, mechanical overuse or injury to the joint. Both joint space cartilage and the subchondral bone are implicated in the disease process. The role of the bone in initiating and/or propagating this process is not clearly understood. More than thirty years ago Radin and other researchers proposed that the changes in bone might be the predisposing event leading to the development of osteoarthritis. Today, although it is recognized that bony changes are a prominent feature of OA, we remain uncertain of its causal relationship to this disease. In osteoarthritis, it is believed that the bone immediately below the articular cartilage (the subchondral bone) is prone to stress injury, which leads to microcracks in the bone. These microcrack may lead to the death of bone cells, and the initiation of the bone remodeling process. This bone remodeling results in the formation of bony over-growth in the form of mineralized callus at the bone injury site. These calluses increase the bone volume and stiffness, when compared to normal tissue. OSTEOPONTIN: Osteopontin (OPN) is a specialized protein like substance (phosphorylated and sulphated glycoprotein). It can be found in a variety of tissues; e.g., bone forming cells, primitive cells from the uterus, placenta, kidney, and nervous system. It has also been found in activated macrophage and lymphocytes. This protein (OPN) plays an important role in bone formation. It is believed that OPN in bone send signals to the cells in response to mechanical stimuli. In the embryonic tissue, OPN is detected in the bone-forming and cartilage cells. It is usually absent in normal healthy adult cartilage. However, in osteoarthritis, OPN is present in the cartilage and its presence increases with the severity of the disease. It can also be found in the bone forming cells (osteoblasts) of the subchondral bone. OPN production by the bone remodeling cells plays an important role in new bone formation. OPN's role may be to facilitate cell attachment to the mineral component of the bone. It also interacts with other bone forming components like type I collagen, osteocalcin, and fibronectin. Mechanical forces stimulate OPN production by acting on the cell to produce shear stress at its adhesion site. This stress transmits a signal to the cell, resulting in OPN production. The final result is a change in the shape of the cell and how it responds to (or interacts with) its environment. The objective of this proposal is to use an OPN knockout mouse model to examine the role of cartilage and bone forming cells in modulating the development of exercise-induced OA. It is our premise that the OPN-deficient (OPN -/-) mouse may be an ideal model to study chondrocyte viability in articular cartilage, and the role of the subchondral bone alterations in knee osteoarthritis. This study protocol was approved with minor revisions on 8/28/04. The mouse exercise system has been in operation for the past eight months and during this time we have generated the first set of results from the pilot study. We are currently reviewing these results. Goals during the next year: This study will use a total one hundred and fifty six animals; divided into four groups(see:Table #1). There will be seventy-eight OPN -/- and 78 wild type mice. In each of the four groups (2 exercise, and 2 non-exercise; with and without, OPN) a total of 36 animals will be tested (six animals at each of the six time points). Animals will be separated into groups immediately after weaning (~ 4 weeks) and the exercise will begin when animals are eight weeks of age and continue until the animals are 68weeks old. Knee joint CT, MRI, chondrocyte and subchondral bone cell isolation, and histological studies will be performed on 8 wk old (6 OPN-/- and 6 WT) mice to establish a baseline prior to wheel exercise. The remaining animals will be evaluated as shown in Table #1. Six mice per group are required for statistical significance in results based on similar types of studies. OPN-/- and WT exercise Groups: OPN-/- and WT non-exercise Groups* Animal age Time Exercise: Animal age Time Exercise: (Weeks) (Weeks) (Weeks) (Weeks) 8 wk 0 wk 8 0 16 8 16 8 24 16 24 16 32 24 32 24 44 36 44 36 56 48 56 48 Note: Table 1: is an overview of the proposed animal use. Thirty-six animals will be entered and studied in each of the four groups. Animals are selected for the study groups at the time of weaning (~ 4 weeks of age). At the determined time points (0, 8, 16, 24, 32, 44, and 56 weeks), six animals from each group will be euthanised and evaluated as described.