Chewing loads alter TMJ metabolism and Zn homeostasis via PIEZO1/ TRPV4.

The biomechanical effects of bite forces on a temporomandibular joint within the craniofacial masticatory complex remain unclear. Herein, we will discuss the effect of recovering a hypofunctioning temporomandibular joint using normal bite forces. Specifically, we will correlate spatiotemporal: (1) biomolecular expressions to (2) physical characteristics of the inner and outer regions of the temporomandibular condyles. In this study, we divided twelve male Sprague-Dawley rats into three groups: hard-food (N = 3, hard pellets), soft-food (N = 3, soft-chow – hard-pellets grounded to powder), and recovery (N = 6, rats on soft-food for 9 weeks and switched abruptly to hard-food for an additional 8 weeks). Rats were euthanized at 17 weeks, and their condyles were imaged using a preclinical micro-computed tomography to estimate bone mineral density and volume fraction. The condyles were prepared for histology (proteoglycans using Safranin O counterstained with Fast Green, and matrix using Picrosirius Red). We performed immunolocalization of mechanosensory ion channels, PIEZO1 and TRPV4, hypoxia inducible factor-1α (HIF-1α), Indian hedgehog (Ihh), mitofusin 2 (MFN2), and cell senescence (p16). Analogous to in vivo conditions, the PIEZO1 and TRPV4 MS-ion channels of the ATDC5 cells were mechanically activated at 5 and 7 days in vitro. Corresponding agonists and antagonists were used for activation of the mechanosensory ion channels. Staining for zinc (Zn) alongside reverse transcription quantitative polymerase chain reaction (RT-qPCR) for quantification of PIEZO1 and TRPV4, superoxide dismutase 2 (SOD2), dynamin-related protein -1 (DRP-1), metal transcription factor-1 (MTF-1), HIF-1⍺, and metabolism (MFN2), Zn transporters1, 5, and 8 (ZnT1, ZnT5, ZIP8) were performed. The oxygen sensor HIF-1α was associated with regional metabolism, as indicated by MFN2, SOD2, and DRP expressions in proliferating (Ihh) and senescent (p16) cells across all groups. Increased Zn levels, along with MTF-1 and ZIP8 and ZnT1/5, were correlated with PIEZO1 in the proliferative zone, and TRPV4 in the inner hypertrophic and subchondral zones of the condyle. These temporal biochemical changes correlated with increased widths of the hypertrophic region and cartilage with a matrix structure loss but recovered bone mineral density and bone volume fraction in condyles of the recovery group. Results illustrate biochemical changes in the temporomandibular condyle in response to a change in bite force. A major finding with significant novelty is that the mechanically induced biochemical changes involve micronutrient Zn metabolism in relation to the activation of two mechanosensory ion channel proteins PIEZO1 and TRPV4. Hard food stimulation of cells that also are HIF-1α positive can alter Zn levels regulated by Zn transporters; a proposed mechanochemo cascade necessary for HIF-1α regulation and subsequently condylar function. In summary, though the mechanical properties are not yet known, chewing hard foods may help regain cartilage and bone thickness lost through TMJ disuse or chewing on soft chow only.