Synovial Extracellular Vesicles: Structure and Role in Synovial Fluid Tribological Performances

Abstract

The quality of the lubricant between cartilaginous joint surfaces impacts the joint’s mechanistic properties. In this study, we define the biochemical, ultrastructural, and tribological signatures of synovial fluids (SF) from patients with degenerative (osteoarthritis-OA) or inflammatory (rheumatoid arthritis-RA) joint pathologies in comparison with SF from healthy subjects. Phospholipid (PL) concentration in SF increased in pathological contexts, but the proportion PL relative to the overall lipids decreased. Subtle changes in PL chain composition were attributed to the inflammatory state. Transmission electron microscopy showed the occurrence of large multilamellar synovial extracellular vesicles (EV) filled with glycoprotein gel in healthy subjects. Synovial extracellular vesicle structure was altered in SF from OA and RA patients. RA samples systematically showed lower viscosity than healthy samples under a hydrodynamic lubricating regimen whereas OA samples showed higher viscosity. In turn, under a boundary regimen, cartilage surfaces in both pathological situations showed high wear and friction coefficients. Thus, we found a difference in the biochemical, tribological, and ultrastructural properties of synovial fluid in healthy people and patients with osteoarthritis and arthritis of the joints, and that large, multilamellar vesicles are essential for good boundary lubrication by ensuring a ball-bearing effect and limiting the destruction of lipid layers at the cartilage surface.

Morphological and Mechanical Characterization of Extracellular Vesicles and Parent Human Synoviocytes under Physiological and Inflammatory Conditions

“…The fluorescence intensity of the extra-vesicular background was thus rather low, corresponding to a very low lipid quantity outside the EV ( Figure 3 C). The concentration of the total phospholipids present in these supernatants was determined by the Stewart method ( Figure 3 D) and was about 0.1 mg/mL, close to that reported for the healthy synovial fluid samples [ 44 , 49 ].…”

“…This corresponded, in the shown case, to a stack of three lipid bilayers, highlighting the multilayer structure of the vesicle shell (Arrow). Thus, in the healthy samples, we highlight the presence of multilamellar vesicles (membranes from 2 to 5 lipid bilayers) similar to those observed in synovial fluids recovered from different animal models [ 60 ] and to those reproduced ex vivo and described as “gel-in” type of vesicles [ 49 , 61 ].…”

“…The SF is discontinuous and contains microvesicular structures coated with lipid/water multilayers, called synovial vesicles. In a previous study, we analyzed the ultrastructure of vesicles present in healthy and pathological SF [ 49 ]. The phospholipid concentration in SF was found to increase in pathological contexts, but the proportion of phospholipids relative to the overall lipids decreased.…”

“…In RA and OA samples this structure was severely altered, with a predominance of small vesicles surrounded by a glycoproteic gel. Changes in phospholipid proportion and chemical composition thus induced structural alteration both in SF of the OA and RA patients, to which dysfunctions in lubricating properties of SF were in turn attributed [ 49 ].…”

“…A described above, SF is an exudate from plasma supplemented by active synthesis from FLS which secrete synovial vesicles. Are the alterations evidenced in OA and RA SF extracts [ 49 ] a consequence of plasma alterations from inflammatory or metabolic reactions or is the intrinsic synthesis of the synovial vesicles modified? In the present study, we dissociated synoviocyte synthesis from plasma lipid exchange to respond to this question.…”