Mechanosensing by Piezo1 regulates osteoclast differentiation via PP2A-Akt axis in periodontitis

Mechanical stimulus to the multicellular bone unit (MBU) plays a key role in normal bone remodeling, whereas disuse osteoporosis, for example, represents loss of bone owing to lack of mechanical stresses. The analogy can be applied to a variety of pathogenic bone lytic complications, including periodontitis, in which local mechanical stress appears to be diminished. The activation of mechanosensitive Piezo1 Ca ²⁺ channel expressed by osteoblasts and osteocytes in the MBU elicits the osteogenic signals in those cells. However, since osteoclast (OC)-specific Piezo1-gene knockout mice showed no skeletal phenotype, it has been assumed that Piezo1 might not play any role in OC-mediated bone remodeling. Here, however, we showed that mechanical stimulation of Piezo1 expressed on preosteoclasts (pre-OCs) downmodulates OC formation and, hence, bone resorptive activity in periodontitis, accompanied by significantly reduced expression of NFATc1, a master transcription factor for RANKL-induced OC-genesis. We know that the Ca ²⁺ /calcineurin/NFAT axis upregulates NFATc1 activation in pre-OCs. Interestingly, Piezo1-elicited Ca ²⁺ influx did not affect NFATc1 expression. Instead, PP2A-mediated dephosphorylation of Akt downregulated NFATc1 in Piezo1-activated pre-OCs. However, systemic administration with Yoda1, a Piezo1 chemical agonist, or local injection of PP2A agonist, significantly downregulated the bone resorption induced in a mouse model of periodontitis, together with reduced numbers of TRAP ⁺ /phospho-Akt ⁺ pre-OCs in local bone. These results suggest that mechanosensing by Piezo1 expressed on pre-OCs can downmodulate the RANKL-induced OC-genesis via the PP2A/Akt-dephosphorylation pathway, but that such Piezo1-mediated downregulation of bone resorption is attenuated in periodontitis.

The mechanosensitive Ca ²⁺ channel Piezo1 plays important regulatory roles in a variety of cellular activities. RANKL-mediated OC-genesis requires permissive co-stimulatory signal from ITAM receptors, such as OSCAR and TREM2, to trigger the calcineurin/calmodulin signaling axis via Ca ²⁺ oscillation, thereby upregulating NFATc1 expression. Activation of Piezo1 remarkably suppressed RANKL-induced NFATc1 activation which, in turn, reduced OC-genesis. Such mechanical activation of Piezo1 expressed on pre-OCs induced intracellular Ca ²⁺ influx. Nonetheless, PP2A-mediated dephosphorylation of Akt, not the calcineurin/calmodulin pathway, suppressed NFATc1 in RANKL-elicited OC-genesis and resultant bone resorption, both in vitro and in vivo . These results indicate that mechanostress applied to pre-OCs can downregulate pathogenic OC-genesis and that Piezo1, as the mediator, is a novel molecular target for the development of anti-osteolytic therapies.