Primary cilia mediate mechanotransduction through control of ATP‐induced Ca²⁺signaling in compressed chondrocytes

Abstract

We investigated the role of the chondrocyte primary cilium in mechanotransduction events related to cartilage extracellular matrix synthesis. We generated conditionally immortalized wild-type (WT) and IFT88(orpk) (ORPK) mutant chondrocytes that lack primary cilia and assessed intracellular Ca(2+) signaling, extracellular matrix synthesis, and ATP release in response to physiologically relevant compressive strains in a 3-dimensional chondrocyte culture system. All conditions were compared to unloaded controls. We found that cilia were required for compression-induced Ca(2+) signaling mediated by ATP release, and an associated up-regulation of aggrecan mRNA and sulfated glycosaminosglycan secretion. However, chondrocyte cilia were not the initial mechanoreceptors, since both WT and ORPK cells showed mechanically induced ATP release. Rather, we found that primary cilia were required for downstream ATP reception, since ORPK cells did not elicit a Ca(2+) response to exogenous ATP even though WT and ORPK cells express similar levels of purine receptors. We suggest that purinergic Ca(2+) signaling may be regulated by polycystin-1, since ORPK cells only expressed the C-terminal tail. This is the first study to demonstrate that primary cilia are essential organelles for cartilage mechanotransduction, as well as identifying a novel role for primary cilia not previously reported in any other cell type, namely cilia-mediated control of ATP reception.

Mechanical loading stimulates chondrogenesis via the PKA/CREB-Sox9 and PP2A pathways in chicken micromass cultures

“…Primary cilia are also present on human mesenchymal stem cells, on arthritic chondroprogenitor cells (CPCs) [26] and on chondrocytes where they are unique sensory organelles that project into the pericellular matrix and interact with their close environment (i.e. collagen types II and IV) via integrins, G proteins, and various Ca 2+ channels; many of which have been implicated as mechanoreceptors [55]. Furthermore, the PKA pathway has been recently reported to be involved in the regulation of the primary cilium length in chondrocytes [56].…”

NFATc1 and NFATc2 repress spontaneous osteoarthritis

“…Cyclic mechanical stimulation is able to evoke calcium currents and is a possible activator of NFATs (47). Mechanical stimulation may provide a link between abnormal mechanical forces that may underlie the etiology of OA and NFATs.…”

Synergy between Piezo1 and Piezo2 channels confers high-strain mechanosensitivity to articular cartilage

“…Such "injurious" loading has been modeled in vitro using explant culture systems that replicate many of the early cellular and molecular events characteristic of osteoarthritis (8). Osteoarthritis is a painful and debilitating disease of weight-bearing joints that affects over 26 million people in the United States (9) with posttraumatic arthritis being responsible for ∼12% of the incidence of osteoarthritis (10).Despite the critical importance of mechanical loading in health and disease of synovial joints, the mechanisms of mechanotransduction of chondrocytes are not fully understood and are likely to differ under physiologic and pathologic conditions (11)(12)(13)(14). Although many different mechanisms have been shown to be involved in chondrocyte mechanotransduction (13,(15)(16)(17), recent studies show that the cation channel, TRPV4, is responsible for mediating the anabolic response of chondrocytes to osmotic or mechanical stress (18)(19)(20).…”