Extracellular vesicles in synovial fluid: Dynamics during joint inflammation and articular development and promise for joint regeneration and restoration of joint homeostasis

This thesis addresses a potentially highly relevant, though as yet relatively untouched, avenue in extracellular vesicle (EV) research: the (possible) involvement of EVs in joint biology and joint disease. The general knowledge on EVs in joint biology is still very limited. In this thesis, we aimed at deepening our knowledge on EVs in healthy and diseased joints. This included improvement of isolation protocols that can be used as a standard, in order to perform reproducible in-depth studies of EVs in normal and pathologically altered joints. The horse was chosen as translational animal model, since this animal is both a clinical patient in its own right and a model for human joint disease. With this approach, conclusions from this thesis can be applied in equine veterinary medicine and are highly likely to be relevant to human joint biology as well. We started with developing and optimizing the urgently needed protocols for EV isolation from synovial fluid (SF) based on a literature overview of techniques used so far for this purpose (Chapter 2). This was done in samples from healthy adult horses in order to achieve baseline measurements for EVs in normal joint homeostasis. Chapter 3 investigates SF-derived EVs in a lipopolysaccharide (LPS)-induced model for transient synovitis. This well-described model mimics acute arthritis and enabled us to adequately monitor EVs during a controlled process of inflammation. More detailed investigation of the lipid composition of SF EVs from this LPS-induced synovitis revealed the presence of so far unknown phosphatidylserine species. These new lipids are described in Chapter 4. Next, in the quest for identifying possible functions of EVs in the joint, SF derived EVs were investigated for their involvement in early joint development (Chapter 5). This was based on the unique capacity of juvenile joints to orchestrate growth and to repair injuries effectively, such as for instance happens in osteochondrosis. These are all potentially EV-driven mechanisms. Finally, in Chapter 6, perspectives are sketched for the possible future use of specialised (biological or artificial) EVs to control joint inflammation and to repair injured articular tissues