Biochemical alterations in inflammatory reactive chondrocytes: evidence for intercellular network communication

Abstract

Chondrocytes are effectively involved in the pathophysiological processes of inflammation in joints. They form cellular processes in the superficial layer of the articular cartilage and form gap junction coupled syncytium to facilitate cell-to-cell communication. However, very little is known about their physiological cellular identity and communication. The aim with the present work is to evaluate the physiological behavior after stimulation with the inflammatory inducers interleukin-1β and lipopolysaccharide. The cytoskeleton integrity and intracellular Ca2+ release were assessed as indicators of inflammatory state. Cytoskeleton integrity was analyzed through cartilage oligomeric matrix protein and actin labeling with an Alexa 488-conjugated phalloidin probe. Ca2+ responses were assessed through the Ca2+ sensitive fluorophore Fura-2/AM. Western blot analyses of several inflammatory markers were performed. The results show reorganization of the actin filaments. Glutamate, 5-hydoxytryptamine, and ATP evoked intracellular Ca2+ release changed from single peaks to oscillations after inflammatory induction in the chondrocytes. The expression of toll-like receptor 4, the glutamate transporters GLAST and GLT-1, and the matrix metalloproteinase-13 increased. This work demonstrates that chondrocytes are a key part in conditions that lead to inflammation in the cartilage. The inflammatory inducers modulate the cytoskeleton, the Ca2+ signaling, and several inflammatory parameters. In conclusion, our data show that the cellular responses to inflammatory insults from healthy and inflammatory chondrocytes resemble those previously observed in astrocyte and cardiac fibroblasts networks.

Gap Junction Coupled Cells, Barriers and Systemic Inflammation

“…The ratio between the polymer form of the F-actin and the monomer form of the G-actin was increased [25]. Similar changes of the actin filament structure are observed in chondrocytes [36]. Different stimuli that cause inflammation result in endocytosis of junction proteins, which causes barrier dysfunction.…”

“…The importance of the effect is seen in the chondrocytes in joints, that are connected to each other via cell-to-cell interactions and form functional gap junctions that express Cx43 [32]. They can sustain the propagation of intercellular Ca 2+ waves in rabbits, humans, and equines [33,34] and form hemichannels that exchange signals within the extracellular space [35,36]. In the adult cartilage, chondrocytes exist as individual cells embedded in the extracellular matrix, and gap junctions are mainly expressed by the flattened chondrocytes facing the outer cartilage layer where intercellular communication occurs [37].…”

Serotonin-evoked cytosolic Ca2+ release and opioid receptor expression are upregulated in articular cartilage chondrocytes from osteoarthritic joints in horses

“…The results clearly showed that OAC responded differently to 5-HT stimulation compared to HC with respect to intracellular Ca 2+ release. We have previously shown that healthy chondrocytes alter Ca 2+ -evoked signalling from single peaks to oscillating waves when exposed to inflammatory mediators (IL-1β and LPS) ( Skiöldebrand et al., 2018 ). In the current study, OAC exhibited stronger intracellular Ca 2+ release than HC in response to 5-HT and inflammation.…”

Low-grade inflammation causes gap junction-coupled cell dysfunction throughout the body, which can lead to the spread of systemic inflammation

“…Intracellular Ca 2+ release was evoked by different signaling pathways stimulated by ATP and 5-HT. These cells also express Cx43 and TLR4 [30].…”

“…TLR4 is most likely present on all cells involved in immune function [26], including gap junctioncoupled cells. Examples of these cells include chondrocytes [30], cardiac fibroblasts [31], keratinocytes [32], and tenocytes [33]. TLRs are involved in the pathogenesis of autoimmune disease, chronic inflammatory and infectious diseases, leading to overproduction of autoantibodies [34].…”