Osteoarthritis and its origins: Disease development at the cellular and molecular level
While osteoarthritis is perceived as a structural disease, the underlying pathology and chronic changes occur at a cellular and molecular level. In this article, we will discuss recent research involving the molecular mechanisms involved in osteoarthritis and newer opportunities for treatment. Key to this knowledge are research tools emerging from the scientific disciplines of functional genomics and molecular biology. Using these research tools to approach such a complex disease as osteoarthritis allows scientists to evaluate the cellular responses to the disease. Understanding the molecular events occurring within the articular chondrocyte will not only provide knowledge of the disease mechanisms, but also of new diagnostic markers and cellular targets for therapeutic and nutritional interventions.
Assessing gene-expression changes in osteoarthritis
The imbalance between anabolic and catabolic factors that leads to the breakdown and degradation of articular cartilage in osteoarthritis involves many factors at the molecular level. Many of these factors are proteins, including structural proteins of the extracellular matrix, inflammatory cytokines, catabolic and anabolic enzymes, and cell-signaling molecules. The types and amounts of proteins synthesized in the cell are specified by the expression of their respective genes. A functional genomic approach to osteoarthritis focuses on measuring changes in gene expression, allowing researchers to discover new factors involved in the disease as well as factors involved in joint tissue development or maintenance. The technologies used to measure gene expression include reverse transcription-polymerase chain reaction (RT-PCR), differential display, and microarray analysis. RT-PCR allows extremely accurate measures of specific gene transcript levels and expression differences between groups. Differential display helps visualize and isolate expression differences at a global level between two states (such as diseased vs. healthy or nutritional interventions). This gives researchers a comprehensive view of how cells respond to or are involved in a disease. Microarray technology adds the advantage of being able to quantitatively measure the expression of many genes simultaneously. However, it is limited to the genes preselected and included on the array.Many previously known genes involved in osteoarthritis have been confirmed and novel disease genes have been identified using these technologies.1-3 Nestlé Purina has employed these research technologies to construct a microarray chip containing transcripts whose expression is altered in canine osteoarthritis and to further confirm or discover novel genes involved in canine osteoarthritis.4 One of the strengths of microarray technology is its ability to detect multiple biochemical pathways in a single sample at a single time. This ability is particularly important in a complex disease, such as osteoarthritis; the technology can potentially distinguish primary responses from disease-induced, secondary responses, as well as determine which are likely intervention candidates. Results from numerous osteoarthritis gene-expression studies suggest that inflammatory pathways play a critical role in the chondrocytes' response to injury and subsequent progression to repair or to arthritis.
RT-PCR can be used to track the changes in single gene expression, elucidating the cellular changes that arthritis induces in chondrocytes. Compared with normal cartilage, osteoarthritis-affected cartilage behaves like an activated macrophage, with up-regulation of interleukin (IL)-1, IL-6, and IL-8 gene expression. Also up-regulated in arthritic chondrocytes are prostaglandin (PG) E2, tumor necrosis factor (TNF)-α, nitric oxide, and matrix metalloproteinase (MMP)-2, -3, -9, and -13.3,5-7 These data suggest a direct connection between the elevation of these inflammatory markers and the structural changes seen in the arthritic joints, as described in the next sections.
Normal articular cartilage is composed of chondrocytes imbedded in a hydrated gel of proteoglycans and a fibrous collagen framework, collectively referred to as the extracellular matrix. Chondrocytes are metabolically active cells that, among other functions, produce and maintain the extracellular matrix.8 MMPs include collagenases, stromelysins, gelatinases, and others.9,10 All these enzymes break down cartilage matrix in some manner and play an important role in the remodeling of cartilage and other connective tissues.11 In osteoarthritis, MMPs degrade glycosaminoglycans, including matrix glycoproteins, and collagen. They also reduce hyaluronic acid concentrations in the synovial fluid, leading to less viscous synovial fluid and impaired joint lubrication.10,12 Under normal conditions, the catabolic MMP processes are appropriately balanced through the inhibitory function of tissue inhibitors of metalloproteinases (TIMPs).9,11,13 However, in osteoarthritis, this balance is disrupted, with a disproportionate MMP increase.13-15