Fibronectin: A New Player in Protecting Our Joints from Inflammation?

A new publication was released by the ENDOTARGET consortium, which investigates the presence and role of bacterial lipopolysaccharides (LPS) in the joint environment, and uncovers how the protein fibronectin may act as a novel protective mechanism. This scientific work was published in July 2025 in RMD Open. Authors of this study are: Kajetana Bevc, Shipin Zhang, Andres Pazos-Perez, Ana Alonso-Perez, David Fercher, Sami Kauppinen, Tuomas Frondelius, Valentino Bruhin, Gian Salzmann, Thomas Rauer, Hans-Christoph Pape, Mikko Arttu Jalmari Finnilä, Caroline Ospelt, Rodolfo Gomez, Kari K Eklund, Marcy Zenobi Wong, and Goncalo Barreto.

 

Why is the research topic important?

Osteoarthritis (OA) and rheumatoid arthritis (RA) are two of the most common and debilitating joint diseases, causing pain, swelling, and reduced mobility for millions of people worldwide. It is well known that inflammation plays a central role in both conditions, with elevated blood LPS known to be linked to joint inflammation and severity in OA and RA. LPS are large molecules found on the outer wall of Gram-negative bacteria and can cause strong immune reactions. Normally, the intestine keeps LPS contained, but if the gut barrier becomes weakened, LPS can be translocated from the intestine into the blood circulation. The circulation of the LPS in the blood is also known as endotoxemia. Once there, these bacterial molecules can travel to the joints and potentially trigger inflammation. In contrast to blood LPS, little is known about LPS in synovial fluid (SF), which is important for OA pathogenesis. So far, no report has been published comparing LPS concentration in SF to any control group nor investigating its biological activity.

The aim of this study was to investigate the presence and bioactivity of LPS in SF of OA patients and elucidate mechanisms modulating their inflammation potential.

 

How was the study conducted?

The study team approached this topic with a thorough experimental design:

• Human Synovial Fluid Analysis: The researchers collected SF from 56 patients with advanced OA, 7 with RA, and 39 who had suffered joint trauma but had no chronic joint disease. This allowed for a comparison of LPS in different joint conditions.
• Measuring LPS and Its Bioactivity: Two main techniques to measure LPS were used: (1) a traditional endotoxin detection kit and (2) liquid chromatography–mass spectrometry (LC-MS), which can detect both the quantity and the bacterial source of LPS. LPS bioactivity was determined by the quantification of TLR4 activation.
• Cell and Animal Models: To test whether LPS in the joint can cause inflammation, the researchers conducted experiments on human cells and in a rat model. In rats, the LPS group received continuous LPS infusion to simulate a chronic exposure to LPS, mimicking metabolic endotoxemia in the body over time. The aim was to investigate whether this would trigger joint damage or inflammation in the rats.
• Investigating fibronectin’s Role: Finally, the study examined the interaction between fibronectin, a protein found in joint fluid and tissues, and LPS. In vitro models and advanced imaging were used to analyse whether fibronectin could bind to LPS and affect its ability to cause inflammation.

 

What does the study show us?

1. LPS is commonly present in the joint, but not necessarily harmful

LPS concentration in SF is comparable between arthritis disorders (OA, RA) and trauma, suggesting that LPS is naturally present in SF. Here, concentrations of LPS were high but with low biological activity. Further, none of the included variables of the study population (disease, age, sex, body mass index) showed a statistically significant association with LPS concentration, suggesting that demographic differences between study groups did not confound the relationship between patient group and SF LPS levels. In addition, in vitro cell experiments revealed a low and similar LPS bioactivity in OA and trauma SF.

 

2. LPS does not accelerate knee joint degeneration in vivo

Rats in the LPS group received a subcutaneously implanted pump, which released LPS for 32 days to induce endotoxemia and investigate its effect on LPS concentration in SF and knee joint degeneration. Surprisingly, LPS concentration in SF did not significantly increase. Furthermore, no joint damage could be observed even after prolonged LPS exposure. These data lead to the assumption that LPS in the knee SF was neutralised and its bioactivity impaired.

 

3. Fibronectin binds LPS and blocks its bioactivity

Fibronectin is a ubiquitous molecule that is present in the synovial membrane. By performing a staining of the rat and human OA synovial membrane for LPS and fibronectin, it was revealed that LPS is present in the synovium and that it is colocalised with fibronectin in the lining layer. To confirm this finding, 3D human synovial membrane constructs from synovial-like fibroblast cells isolated from 3 OA patients were generated. Here, in the presence of fibronectin, LPS could not penetrate deeper joint tissues or trigger immune responses.

Binding affinity assays confirmed the suggestion that LPS and fibronectin can chemically bind to each other. Further experiments showed that fibronectin also blocks LPS bioactivity. This effect is counteracted by heat-inactivation of fibronectin.

 

What can we learn from the study?

Overall, the findings suggest that fibronectin can bind LPS and may contribute to limiting its inflammatory effects, which is proposed to be a potential novel protective mechanism in synovial joints. Further research will be needed to investigate the biological relationship between fibronectin and LPS more in detail. It is possible that  there is an interplay between LPS and fibronectin production, which could further elucidate the role of these factors in the exacerbation of rheumatic diseases associated with altered gut permeability and endotoxemia.

Read the full publication: Evaluating the role of lipopolysaccharides in the joint: fibronectin as a novel protective mechanism

 

Glossary

Bioactivity: How strongly a molecule (like LPS) can trigger a response in cells or tissues.

Endotoxemia: a condition characterised by the presence of endotoxins, primarily lipopolysaccharides from Gram-negative bacteria, in the bloodstream, leading to systemic inflammation and potentially contributing to various chronic disease.

Endotoxin Assay: A lab test used to detect LPS.

Fibronectin: A protein found in many tissues, including joints. It helps cells stick together and can also bind to other molecules, such as LPS.

Gram-negative bacteria: A large group of bacteria characterized by their double cell membrane and a thin cell wall. They are called “Gram-negative” because they do not retain a violet stain in the Gram staining procedure. Many types of Gram-negative bacteria have lipopolysaccharides on their outer wall.

Heat inactivation: A laboratory process in which proteins or other biological molecules are exposed to high temperatures to make them lose their activity or function. This is often used to test whether the function of a protein is dependent on its active structure.

In vitro: refers to experiments or procedures performed outside a living organism, typically in a controlled laboratory environment, such as in cell cultures, test tubes, or microfluidic systems, to study biological processes under reproducible conditions.

In vivo: refers to experiments or studies conducted within a living organism, such as humans, animals, or plants, to observe biological processes, disease progression, or drug effects in their natural physiological environment

Lipopolysaccharide (LPS): a large molecule found in the outer membrane of Gram-negative bacteria, composed of lipids and polysaccharides, that acts as an endotoxin and can trigger strong immune and inflammatory responses.

Liquid Chromatography–Mass Spectrometry (LC-MS): An advanced technique that can identify and measure tiny amounts of molecules in biological samples.

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

Rheumatoid Arthritis (RA): a chronic immune-mediated disorder that primarily affects joints, causing inflammation, pain, stiffness, and eventually joint damage and deformity.

Synovial Fluid (SF): viscous fluid within the cavities of synovial joints.

Toll-like receptor 4 (TRL4): is a protein on the surface of immune cells that recognises specific bacterial molecules and triggers an immune response to defend against infections.