Mechanotransduction pathways and relevance to human OA

Abstract

Purpose: We are investigating mechanoelectrical transduction (the conversion of a mechanical stimulus into an electrochemical signal) in chondrocytes. The force sensing proteins that mediate this conversion are mechanically-activated (MA) ion channels. The purpose of this study was to determine if we could directly measure MA ion channel activity in chondrocytes and to investigate how changing the mechanics of the cellular substrate impacts MA channel activity. Methods: In order to study MA channel activity a relevant mechanical stimulus must be applied to a cell whilst simultaneously monitoring the resulting flux of ions across the membrane. The gold standard for monitoring ion channel activity is to use electrophysiological techniques. We have isolated primary murine chondrocytes and used high speed pressure clamp to stretch a patch of the cell membrane. In addition, we have developed a specialized technique (using elastomeric pillar arrays) to apply deflections at contact points between the cell and the underlying substrate. Results: We found that the MA ion channels PIEZO1 and TRPV4 are both activated by substrate deflections, but only PIEZO1 is activated by membrane stretch. Given the changes in the mechanical properties of cartilage that accompany the development of osteoarthritis, we then investigated the effect of modulating substrate mechanics on TRPV4 and PIEZO1 activation. Conclusions: Our data suggest that MA channels may enable chondrocytes to integrate distinct mechanical inputs. In addition we predict that changes in tissue mechanics alone will disrupt signalling via PIEZO1 and TRPV4 in response to changing mechanical loads.