Roles of TRPV4 and piezo channels in stretch-evoked Ca²⁺ response in chondrocytes

Abstract

Chondrocyte mechanotransduction is not well understood, but recently, it has been proposed that mechanically activated ion channels such as transient receptor potential vanilloid 4 (TRPV4), Piezo1, and Piezo2 are of functional importance in chondrocyte mechanotransduction. The aim of this study was to distinguish the potential contributions of TRPV4, Piezo1, and Piezo2 in transducing different intensities of repetitive mechanical stimulus in chondrocytes. To study this, TRPV4-, Piezo1-, or Piezo2-specific siRNAs were transfected into cultured primary chondrocytes to knock down (KD) TRPV4, Piezo1, or Piezo2 expression, designated TRPV4-KD, Piezo1-KD, or Piezo2-KD cells. Then we used Flexcell® Tension System to apply cyclic tensile strains (CTS) of 3% to 18% at 0.5 Hz for 8 h to the knockdown and control siRNA-treated cells. Finally, using a Ca2+ imaging system, stretch-evoked intracellular Ca2+ ([Ca2+] i) influx in chondrocytes was examined to investigate the roles of TRPV4, Piezo1, and Piezo2 in Ca2+ signaling in response to different intensities of repetitive mechanical stretch stimulation. The characteristics of [Ca2+] i in chondrocytes evoked by stretch stimulation were stretch intensity dependent when comparing unstretched cells. In addition, stretch-evoked [Ca2+] i changes were significantly suppressed in TRPV4-KD, Piezo1-KD, or Piezo2-KD cells compared with control siRNA-treated cells, indicating that any channel essential for Ca2+ signaling induced by stretch stimulation in chondrocytes. Of note, they played different roles in calcium oscillation induced by different intensities of stretch stimulation. More specifically, TRPV4-mediated Ca2+ signaling played a central role in the response of chondrocytes to physiologic levels of strain (3% and 8% of strain), while Piezo2-mediated Ca2+ signaling played a central role in the response of chondrocytes to injurious levels of strain (18% of strain). These results provide a basis for further examination of mechanotransduction in cartilage and raise a possibility of therapeutically targeting Piezo2-mediated mechanotransduction for the treatment of cartilage disease induced by repetitive mechanical forces. Impact statement Chondrocytes in cartilage are constantly subjected to load-induced stimuli and regulate their metabolic activities in order to maintain cartilage homeostasis. Therefore, mechanotransduction is important in chondrocytes and is vital for their role in cartilage function. Our results indicate that chondrocytes might sense and distinguish the different intensities of repetitive mechanical stimulus by using different mechanosensitive ion channels. Specifically, TRPV4 is mainly responsible for sensing physiologic levels of repetitive CTS stimulus, while Piezo2 mainly contributes to chondrocyte sensing noxious levels of repetitive CTS loading. These results provide a basis for further examination of mechanotransduction in cartilage and raise the possibility of therapeutically targeting Piezo2-mediated mechanotransduction for the treatment of OA which is induced by injurious and repetitive mechanical stimulation.

K+ and Ca2+ Channels Regulate Ca2+ Signaling in Chondrocytes: An Illustrated Review

“…Some of the same investigators that have studied TRP channel function in mammalian chondrocytes have also recently reported results revealing the co-expression of the stretch-sensitive or ‘stretch-transducer ion channel’ [ 60 , 61 , 62 ]. These have been named Piezo channels [ 63 ].…”

“…Given the fact that chondrocytes in articular joints experience cyclical compression and decompression, and perhaps also stretch and shear, it is likely that physiological activation of such Piezo channels plays an important role in normal physiological function [ 61 ]. If so, activity-dependent bursts of Ca 2+ influx [ 62 ] will constitute an important early step in signal transduction. Interestingly, TRPV4 and Piezo channels exhibit different thresholds for activation by mechanical stimuli.…”

“…TRPV4 detects even very weak mechanical perturbations while Piezo channels are activated by stronger stimuli. It is plausible, therefore, that TRPV4 may be a primary physiological mechano-sensor; while Piezo channels detect stronger or more noxious stimuli [ 60 , 62 , 64 ], possibly implying that they have different functional roles in healthy vs. disease settings [ 61 ].…”

A New Hope in Spinal Degenerative Diseases: Piezo1

“…Their experiments also confirmed that mechanical stress can promote Ca 2+ influx from the extracellular matrix into the chondrocytes through the piezo1 ion channel. Similarly, the study by Du et al [ 128 ] also confirmed that Ca 2+ in chondrocytes is essential for the transduction of stretch stimulation signals. Mechanically sensitive ion channels including TRPV4, piezo1, and piezo2 play different roles in the process of calcium oscillations caused by stretch stimuli of different intensities [ 129 , 130 ].…”

“…Therefore, piezo1 is expressed in mammalian chondrocytes including humans' chondrocytes, which is a necessary condition for causing calcium influx in chondrocytes after mechanical stimulation [ 128 ]. Overload of Ca 2+ activates intracellular messengers and regulates the kinase cascade to mediate chondrocyte apoptosis and is the key mechanism of chondrocyte apoptosis [ 131 ].…”