Our aim was to support the hypothesis of a specific association between gap junctions in synovial tissue and the presence of osteoarthritis, as evidenced by differences between osteoarthritis and non-osteoarthritis synovia in the number of gap junctions, the amount of gap-junction protein, and the amount of enzymatic activity produced through a pathway mediated by gap-junction intercellular communication. An average of 4.41 gap junctions were found per 100 cells counted in the osteoarthritis synovia, compared with 1.00 in the controls. The amount of the gap-junction protein connexin 43 in synovial lining cells was approximately 50% greater in patients with osteoarthritis. Synovial lining cells from patients with osteoarthritis produced matrix metalloproteinases constitutively and, at higher levels, in response to stimulation by interleukin-1␤. In both cases, intercellular communication through gap junctions was shown to be critical to the ability of the cells to secrete matrix metalloproteinases. Overall, the results indicated that gap junctions between synovial lining cells were altered significantly in patients with osteoarthritis, as a consequence of the disease process or as part of the causal chain. In either case, gap junctions seem to be a rational therapeutic target.

The role of electrophysiological events in signal transduction of interleukin-1β (IL-1β) was investigated in rabbit synovial fibroblasts using the perforated-patch method. Aggregated synovial fibroblasts occurred in two different electrophysiological states having membrane potentials ( V m) of −63 ± 4 ( n = 71) and −27 ± 10 mV ( n = 55) (high and low V m, respectively). IL-1β affected the cells with high V m; it switched the state of the cell from high to low V m. This effect was strongly dependent on the external potential applied to the cell membrane. Low V m(−30 mV) alone without IL-1β did not switch the state of the cells. Thus a synergistic effect involving the cytokine and cell V m in switching the electrophysiological state of the cell was shown, indicating that electrophysiological changes are involved in signal transduction. Gap junctions between aggregated cells were necessary for the cells to have a high V m and to respond to IL-1β. Gap junction resistance between adjacent cells was estimated as 300 ± 100 MΩ. Our findings suggest that the electrophysiological behavior of synovial fibroblasts is tightly connected to a signaling or intracellular mediator system that is triggered by IL-1β.

Synovial cells can form networks connected by gap junctions. The purpose of this study was to obtain evidence for a necessary role of gap junction intercellular communication in protein secretion by synovial cells. We developed a novel assay to measure the enzymatic activity of metalloproteinases (MMPs) produced by synovial cells in response to interleukin-1beta (IL-1beta) and employed the assay to explore the biological function of gap junctions. IL-1beta produced a dose-dependent increase in MMP activity that was blocked by exposure to the gap junction inhibitors 18alpha-glycyrrhetinic acid and octanol for as few as 50 min. The inhibitors produced an immediate and marked reduction in intercellular communication, as assessed by transient current analysis using the nystatin perforated-patch method. These observations suggest that communication through gap junctions early in IL-1beta signal transduction is critical to the process of cytokine-regulated secretion of MMPs by synovial cells.

Objectives: Whether changes in the hyaluronan moiety of synovial fluid are associated with osteoarthritis (OA) is unresolved experimentally, notwithstanding frequent statements in the literature that the disease leads to degraded hyaluronan. We evaluated this hypothesis by comparing the molecular weight and concentration of hyaluronan in synovial fluid from patients with and without OA. Methods: Synovial fluid was obtained by needle aspiration from patients with advanced OA (Kellgren-Lawrence Grade IV) and from patients with no radiological or arthroscopic evidence of OA. The distribution of the molecular weight of hyaluronan was measured using both gel electrophoresis and size-exclusion chromatography, and hyaluronan concentration was determined by immunosorbent assay and differential refractometry. Results: The distributions of molecular weight were highly variable within each group of patients; the average distributions, however, were identical in the 2 groups. The average concentration of hyaluronan also did not differ statistically between the 2 groups. Conclusions: We found no evidence of hyaluronan degradation in synovial fluid of patients with OA. Commonly expressed opinion in the literature to the contrary may have resulted from a failure to adequately consider the limitations of previous experimental studies.

The possibility that membrane depolarization of synovial fibroblasts caused by interleukin-1β (IL-1β) was mediated by protein kinase C (PKC) and Ca2+influx was studied using inhibitor and activator analysis. The effect of IL-1β was blocked by bisindolylmaleimide I, an inhibitor of PKC, and by the Ca2+ channel blockers nifedipine and verapamil. In other experiments, PKC was activated using phorbol 12-myristate 13-acetate, and Ca2+ influx was increased by means of a Ca2+ ionophore. Simultaneous application of phorbol ester and Ca2+ ionophore in the absence of IL-1β mimicked the depolarization caused by IL-1β. The results were consistent with the hypothesis that, under the conditions studied, activation of PKC and Ca2+ influx are necessary and sufficient processes in the transduction of IL-1β by synovial cells leading to membrane depolarization. The essential role of protein phosphorylation and Ca2+ influx in the early electrophysiological response of synovial fibroblasts to IL-1β was therefore established. The role of IL-1β-induced depolarization in regulating protein expression by the cells remains to be determined, but the results reported here, taken together with observations that protein phosphorylation and Ca2+influx also mediate the effect of IL-1β on protease production (1, 2), suggest that electrophysiological changes are actually part of the pathway for expression of proteases in response to IL-1β.