A new model to investigate osteoarthritis, combining mechanical and inflammatory factors

A new publication was released by the ENDOTARGET consortium, which characterises the collagenase-induced osteoarthritis (CIOA) model in rats and provides novel insights into the role of mechanical degradation and inflammation in this model. This scientific work was published in October 2024 in Osteoarthritis and Cartilage Open. Authors of this study are: Patrick Weber, Kajetana Bevc, David Fercher, Sami Kauppinen, Shipin Zhang, Maryam Asadikorayem, Lucia Baixauli Marin, Tanqi Zhang, Tuomas Frondelius, Gian Salzmann, Valentino Bruhin, Jakob Hax, Gonçalo Barreto, Mikko A.J. Finnilä, and Marcy Zenobi-Wong.

 

Why is the collagenase-induced osteoarthritis rat model important?

Osteoarthritis (OA) is one of the most common causes of pain and disability worldwide. It affects millions of people and involves a complex mix of factors, i.e.,  mechanical wear and tear, inflammation, genetic predisposition, and lifestyle. Despite extensive research, there are still no disease-modifying treatments that can halt or reverse OA progression. Most therapies focus only on relieving symptoms such as pain. To find better treatments, scientists rely on animal models that mimic OA in humans. However, current models have limitations. Many reproduce only one aspect of the disease (e.g., cartilage wear or inflammation) and fail to capture the full picture. The collagenase-induced osteoarthritis (CIOA) model stands out because it combines both mechanical and inflammatory factors, making it a promising tool to study OA more realistically.

 

How was the study conducted?

The team worked with skeletally mature Wistar rats, both male and female, to better reflect the diversity of the human population. The experiment was carefully designed to capture both short-term and long-term effects of collagenase on the knee joint.

Study setup: Rats received injections of either saline (as a control), 500 units or 1000 units of collagenase directly into the knee joint. Injections were given on two occasions (day 0 and day 2). Samples were collected at different timepoints to capture early, acute changes, as well as long-term effects.

Analyses performed:

• Cartilage and bone: Advanced imaging (micro-computed tomography) and histology were used to measure cartilage degeneration and bone changes.
• Synovium (joint lining): Tissue staining assessed inflammation, cell density, and fibrosis (thickening and scarring of tissue).
• Ligaments: Mechanical testing of the anterior cruciate ligament measured stiffness and strength.
• Behavioural tests: Gait analysis and sensitivity to pressure were recorded to assess pain and movement difficulties.
• Blood biomarkers: Levels of immune cells and inflammatory molecules (cytokines) were measured.

What does the analysis show us?

1. Collagenase causes dose-related cartilage degeneration: Cartilage degeneration increased with higher doses of collagenase. Damage was most severe in the medial tibia (the inner part of the shin bone), a common site of OA damage in humans. At the higher dose (1000 U), cartilage roughness and thinning were particularly pronounced.
2. Acute synovial inflammation triggers long-term changes: Within 4 days, the synovium became swollen and inflamed, regardless of collagenase dose. By day 70, swelling had subsided, but cell density and collagen deposits in the synovium remained high, showing signs of long-term fibrosis. Synovitis was more severe in the high-dose group.
3. Ligament weakening persists over time: The anterior cruciate ligament became weaker and less stiff after collagenase injections, leading to long-lasting joint instability. This effect was stronger in the high-dose group and persisted throughout the 70-day study.
4. Gait and pain-related changes return to normal: Rats treated with 1000 U collagenase showed temporary gait changes and heightened sensitivity to pressure (a sign of joint pain) during the first three weeks. By day 70, these behaviours had returned to normal, indicating that the CIOA model causes only mild long-term pain symptoms.
5. Acute increase of immune cells: Blood tests revealed a spike in immune cells (monocytes, neutrocytes) and inflammatory cytokines (interleukin-10 and interleukin-1 beta) shortly after collagenase injection, but these levels returned to normal within 8 days. However, in the joint itself, immune cell infiltration persisted, especially in the high-dose group.
6. Bone resorption rather than bone growth: By day 70, there were signs of bone resorption (loss of bone mass) in the medial tibia, which aligns with early stages of human OA. Unlike late-stage OA, no osteophytes were found in this model.
7. Minimal sex differences: The study found only minor differences between male and female rats, suggesting that the CIOA model can be applied across sexes.
8. Risks with higher doses: At the highest dose (1000 U), some rats experienced adverse effects such as joint dislocation and weight loss, underlining the need for careful dose selection.

 

What can we learn from the study?

This work provides a clearer picture of the CIOA model and its potential in osteoarthritis research:

• A realistic model: By combining mechanical and inflammatory factors, the CIOA model captures two of the most important drivers of human OA. In addition,  the CIOA model mirrors other hallmarks of human OA, such as cartilage degeneration, increased synovial lining cells, synovial fibrosis and bone remodeling.
• Dose matters: By changing the injection collagenase dose, severity of the model can be influenced. This flexibility allows researchers to choose the severity that best suits their study.
• Translational value: The similarities between CIOA changes in rats and features of human OA make it a promising tool for further use in a more hypothesis-driven and clinically relevant way in the future.
• Limitations: The study used a small number of animals, which may affect statistical strength. Moreover, the tested model still produces a relatively mild OA phenotype compared to severe cases in humans.

Read the full publication: The collagenase-induced osteoarthritis (CIOA) model: Where mechanical damage meets inflammation

 

Glossary

Anterior cruciate ligament: strong band of tissue inside the knee that connects the thigh bone (femur) to the shin bone (tibia) and helps keep the joint stable during movement.

Bone Resorption: The process by which bone is broken down and minerals are released, leading to a decrease in bone mass.

Collagenase: enzyme that breaks down collagen, the main structural protein in connective tissues such as cartilage, tendons, and ligaments. Used experimentally to induce joint damage.

Collagenase-induced osteoarthritis (CIOA) rat model: experimental setup where collagenase is injected into rat knee joints to weaken ligaments and trigger inflammation, mimicking key features of human osteoarthritis.

Cytokines: Signalling proteins released by immune cells that regulate inflammation and immune responses.

Fibrosis: Thickening or scarring of tissue due to excess collagen production.

Interleukin-1 beta (IL-1β): pro-inflammatory cytokine protein produced by activated immune cells and involved in various cellular activities, including inflammation and immune responses.

Interleukin-10: anti-inflammatory signalling protein produced by immune cells that helps regulate and limit excessive immune responses.

Medial tibia: the inner part of the shin bone that forms part of the knee joint and often bears more load, making it a common site of cartilage damage in osteoarthritis.

Micro-computed Tomography: High-resolution imaging technique used to study bone and cartilage structures in detail.

Monocytes: a type of white blood cell that circulate in the blood and can move into tissues, where they develop into macrophages or dendritic cells to help fight infection and regulate inflammation.

Neutrocytes: most common type of white blood cell and act as first responders of the immune system, quickly attacking bacteria and other pathogens during inflammation or infection.

Osteoarthritis (OA): degenerative joint disease characterised by the breakdown of cartilage, leading to pain, swelling, stiffness, and reduced mobility in affected joints.

Osteophytes: are small bony growths that form along the edges of joints, often as a result of osteoarthritis or joint degeneration.

Saline: a sterile solution of salt (sodium chloride) in water that is commonly used in science for control groups. These groups receive the saline solution instead of the active treatment being tested.

Skeletally mature Wistar rats: fully grown laboratory rats of the Wistar strain, commonly used in biomedical research once their bones have finished developing.

Synovitis: inflammation of the synovium.

Synovium: soft tissue lining the inside of joints that produces lubricating fluid.