During traumatic joint injuries,
impact overloading can cause mechanical
damage to the cartilage. In the following inflammation phase, excessive
inflammatory cytokines (e.g., interleukin-1β
(IL-1β)) can act on chondrocytes, causing over-proliferation,
apoptosis, and extracellular matrix (ECM) degradation that can lead
to osteoarthritis. This study investigated the combined effects of
traumatic overloading and IL-1β challenge on the metabolic activities
of chondrocytes. Bovine cartilage explants underwent impact overloading
followed by IL-1β exposure at a physiologically relevant dosage
(1 ng/mL). New click chemistry-based methods were developed to visualize
and quantify the proliferation of in situ chondrocytes
in a nondestructive manner without the involvement of histological
sectioning or antibodies. Click chemistry-based methods were also
employed to measure the ECM synthesis and degradation in cartilage
explants. As the click reactions are copper-free and bio-orthogonal, i.e., with negligible cellular toxicity, cartilage ECM was
cultured and studied for 6 weeks. Traumatic overloading induced significant
cell death, mainly in the superficial zone. The high number of dead
cells reduced the overall proliferation of chondrocytes as well as
the synthesis of glycosaminoglycan (GAG) and collagen contents, but
overloading alone had no effects on ECM degradation. IL-1β challenge
had little effect on cell viability, proliferation, or protein synthesis
but induced over 40% GAG loss in 10 days and 61% collagen loss in
6 weeks. For the overloaded samples, IL-1β induced greater degrees
of degradation, with 68% GAG loss in 10 days and 80% collagen loss
in 6 weeks. The results imply a necessary immediate ease of inflammation
after joint injuries when trauma damage on cartilage is present. The
new click chemistry methods could benefit many cellular and tissue
engineering studies, providing convenient and sensitive assays of
metabolic activities of cells in native three-dimensional (3D) environments.