Production and Secretion of Gelsolin by Both Human Macrophage- and Fibroblast-like Synoviocytes and GSN Modulation in the Synovial Fluid of Patients with Various Forms of Arthritis

Abstract

Gelsolin (GSN) is an actin-binding protein involved in cell formation, metabolism and wound closure processes. Since this protein is known to play a role in arthritis, here we investigate how the synovial membrane with its specific synoviocytes contributes to the expression of GSN and how the amount of GSN expressed is modulated by different types of arthritis. Synovial membranes from adult healthy subjects and patients with rheumatoid arthritis (RA) and osteoarthritis (OA) are analyzed by immunofluorescence, Western blot and ELISA. Macrophage-like synoviocytes (MLS) and fibroblast-like synoviocytes (FLS) were isolated, cultured and analyzed for their potential to produce and secrete GSN. In addition, the GSN concentrations in the synovial fluid of various forms of arthritis are determined by ELISA. GSN is produced by the healthy and arthritic synovial membranes. Both forms of synoviocytes (MLS and FLS) release GSN. The results show that there is a significant reduction in GSN in the synovial fluid in adult patients with OA. This reduction is also detectable in adult patients with RA but is not as evident. In juvenile arthritis, there is a slight increase in GSN concentration in the synovial fluid. This study shows that primary MLS and FLS express GSN and that these cells, in addition to articular chondrocytes, contribute to GSN levels in synovial fluid. Furthermore, GSN concentrations are modulated in different types of arthritis. Further studies are needed to fully understand how GSN is involved in joint homeostasis.

Keywords: arthritis; gelsolin; synovial fluid; synoviocytes.

Figure 1

Structure of the six homologue domains of GSN. Schematic representation of the structure of Ca-free human gelsolin (PDB ID code: 3FFN) [37].

Figure 2

MLS and FLS react positively with an antibody against gelsolin. Section through the healthy synovial membrane. The antibody against gelsolin reacts intensively positive (red) with the MLS of the synovial membrane and also shows a weak positive reaction in the FLS and with individual cells in the subsynovial connective tissue. Counterstain: hemalum. Inset magnification ×10.

Figure 3

Isolated MLS and FLS producing gelsolin. Isolated and cultured macrophage-like synoviocytes (MLS) and fibroblast-like synoviocytes (FLS) show a positive antibody response for cell-specific markers (MLS = CD68 (red); FLS = CDH11 (red)) and for gelsolin (GSN—green) by immunofluorescence.

Figure 4

ELISA analysis of MLS and FLS. Isolated and cultured MLS and FLS isolated out of the synovial membrane of osteoarthritis patients produce GSN at approximately the same concentration (0.09 µg/mL). ELISA analysis of gelsolin in MLS and FLS, t-test.

Figure 5

Gelsolin detection in synovial membranes and fluids of RA and OA patients. Immunofluorescence detection of gelsolin (GSN—green, white arrows) in the synovial membrane of patients with rheumatoid arthritis (RA) and osteoarthritis (OA). Paraffin-embedded tissue sections are stained with antibodies against CD68 (red) and CDH11 (red) to localize macrophage-like synoviocytes (MLS) and fibroblast-like synoviocytes (FLS) as well as gelsolin (GSN) to localize GSN. GSN is detectable in the subepithelial tissue of the RA synovial membrane (top row) and in the superficial cell layers of the synovial membrane of OA patients (bottom row).

Figure 6

Detection and quantification of gelsolin in synovial fluid from healthy subjects and RA and OA patients. (A) Western blot analysis of synovial fluid (SF) from a healthy control subject and patients with RA and OA. (B) Gelsolin ELISA of SF samples from healthy subjects, RA and OA patients, t-test, * p < 0.5; ** p < 0.01.