Motion has a crucial role in joints. Long-term joint immobilization causes contracture and cartilage degeneration, whereas joint motion exercise is effective to alleviate osteoarthritis symptoms. However, the molecular mechanisms of these effects have been unclear. We first established a minimized mechanical stress (MMS) model by knee joint immobilization of mice that were suspended by their tails. Histological examination showed synovitis, followed by cartilage degeneration. Bulk RNA-seq showed remarkable increases in expression of catabolic factors in synovium and decreases of matrix proteins in cartilage. ScRNA-seq showed specific subsets of myofibroblasts and activated macrophages in MMS synovium, which expressed cytokines and growth factors responsible for cartilage degeneration. Moreover, ligand-receptor analysis indicated dynamic interactions between the two MMS-specific subsets through these secreted molecules. In summary, loss of mechanical forces on joints induces specific subsets of myofibroblasts and activated macrophages in synovium, which probably cause cartilage degeneration through secretion of catabolic and inflammatory factors.

Optically pumped magnetometer (OPM) is a sensor based on electron-spin polarization of alkali-metal atoms. We have been developing super-sensitive compact OPMs as well as innovative biomagnetic neuroimaging systems. OPMs using alkali metal vapors contained in glass cells enable to measure extremely small magnetic fields. In recent years, OPMs have reached sensitivities comparable to and even surpassing those of superconducting quantum interference devices (SQUIDs). In addition, OPM has the intrinsic advantage of not requiring cryogenic cooling. After describing the principle of OPM, we introduce our recent results of MEG and NMR/MRI measurements to demonstrate its feasibility as a magnetic sensor towards innovative optical biomagnetic neuroimaging systems. Finally, we describe some future research directions of the OPM-based biomagnetic neuroimaging systems.