Edinburgh Napier University

  Purpose: Human OA chondrocytes are known to express several subunits of the NMDA receptor (NMDAR), a neuronal signalling molecule. This study analyses downstream activation events and interactions of NMDAR with molecules known to be involved in chondrocyte mechanotransduction.

Methods and Materials: OA chondrocytes human knee joint arthroplasty specimens were used. Cell membrane potentials of cells were measured following stimulation with NMDA (50mM, 10 min) or mechanical stimulation (MS 0.33Hz, 20 min) on the presence or absence of EGTA Nifedipine gadolinium, apamin, tetrodotoxin, NR2B9c-TAT (disrupts NMDAR-PSD95 association), anti-IL1 receptor, anti-IL4 receptor, and anti–integrin antibodies.

Results: NMDA stimulation of OA chondrocytes results in a membrane depolarisation response that is inhibited by removal of extracellular calcium by EGTA. Nifedipine, gadolinium, apamin and anti-IL4 had no effect on membrane depolarisation whilst treatment with NR2B9c-TAT completely inhibited membrane depolarisation. Whereas in the presence of tetrodotoxin NMDA and MS resulted in membrane hyperpolarisation response in OA chondrocytes; no change in membrane potential was seen when cells were stimulated in the presence of both apamin and tetrodotoxin. Anti-IL1R and b1-integrin antibodies were shown to block NMDA induced membrane depolarisation.

Conclusions: Activation of OA chondrocyte NMDAR results in a signal cascade that requires extracellular calcium and association of NMDAR with PSD95. The membrane depolarisation results from activation of a tetrodotoxin sensitive sodium channel. Blockade of the NMDA induced depolarisation by anti-IL1R and anti-b1integrin antibodies suggests activation of an autocrine/paracrine loop and association of NMDAR with integrins. MS of OA chondrocytes results in b1-integrin and IL1b dependent membrane depolarisation and NMDAR maybe involved in this mechanotransduction process.