IntroductionDisuse-induced bone loss occurs on Earth in long-term bed-ridden patients and in astronauts during long-term spaceflight owing to microgravity1,2,3,4,5. Under these conditions reduced mechanical stress can cause significant bone loss, which subsequently leads to additional systemic disturbances in the human body6,7,8. Bone is a dynamic tissue that undergoes constant modeling and remodeling in response to mechanical loading9,10,11,12. Osteocytes were the first identified primary mechanosensors in bone, regulating the bone remodeling process by producing soluble factors such as bone morphogenetic proteins (BMPs), sclerostin, receptor activator of nuclear factor κB ligand (RANKL), and osteoprotegerin (OPG) in response to changes in mechanical loading13,14,15,16,17,18. In recent years, additional bone cell types, as well as leptin receptor (LepR) positive mesenchymal stem cells (MSCs)19, have been shown to be capable of mechanical sensing20,21,22,23,24. Moreover, mechanical unloading-induced bone loss may also be attributed to changes in MSC function25. MSCs represent one of the major sources for the initiation of osteogenic processes. The underlying molecular mechanisms by which mechanical unloading decreases the number of MSCs and whether prevention of MSC loss may attenuate disuse osteoporosis remain to be determined.Emerging evidence highlights the critical role of microRNAs in skeletal muscle systems. These small noncoding RNAs regulate bone homeostasis by fine-tuning osteogenic differentiation26,27, osteoclast activity28,29,30, and MSC fate determination31,32,33. Notably, mechanical loading dynamically modulates miRNA expression profiles in various cells. For example, fluid shear stress downregulates miR-103a to promote osteoblast mineralization34, while compressive loading induces miR-33-5p expression to enhance osteoblast differentiation35. Our previous work first identified MicroRNA-337-3p (miR-337) as a mechanical stress-sensitive regulator in tendon stem cells, where it mediates osteogenic differentiation through the direct targeting of IRS-1 under cyclic tensile stress36. This discovery identified miR-337 as a potential key mediator that translates mechanical signals into cellular responses across musculoskeletal tissues.Recent studies have established Piezo1 as a pivotal mediator of cellular mechanical stress sensing and signaling, with its ion channel activity directly linking extracellular forces to intracellular biochemical responses, and regulating osteogenesis25,37, angiogenesis38,39, and stem cell differentiation40,41. While Piezo1 activation has been shown to promote osteoblast differentiation through YAP/TAZ signaling42, its specific function in maintaining MSC proliferation under physiological loading remains unspecified. Our study reveals a novel regulatory axis in which Piezo1 governs MSC proliferation through miRNA regulation, establishing previously unrecognized crosstalk between ion channel signaling and post-transcriptional control in mechanical stress transduction.Current strategies to combat disuse osteoporosis primarily focus on suppressing bone resorption or enhancing mechanical loading43,44. However, these approaches do not comprehensively address the fundamental issue of MSC depletion in the pathological bone marrow (BM) microenvironment. The development of targeted interventions to preserve MSC populations under mechanical unloading conditions represents an urgent unmet clinical need, particularly for spaceflight applications where conventional weight-bearing countermeasures are impractical. Here we propose a novel mechanical regulatory axis centered on miR-337 that couples upstream mechanical stress sensing by the Piezo1/YAP/TEAD pathway with the downstream control of MSC proliferation through IRS-1/PI3K/Akt/mTOR signaling. The genetic ablation of miR-337 specifically rescued hindlimb unloading (HU)-induced bone loss by restoring MSC proliferation and osteogenic differentiation. Additionally, the transplantation of miR-337-deficient MSCs into wild-type mice subjected to HU robustly attenuated disuse-induced bone deterioration. Based on these findings, our data establish a foundation for the targeted inhibition of miR-337 as a novel therapeutic approach to preserve MSC populations and osteogenic potential under mechanical disuse conditions.ResultsThe number of MSCs is markedly decreased in the BM of a rat model of HUThe HU rodent model has been extensively used to study various physiological responses to certain aspects of spaceflight as well as long-term consequences of being bedridden, including bone loss2. We used an HU rat model established via tail suspension (Fig. 1a). Micro-CT imaging revealed marked bone loss (Fig. 1b, c) by day 14 after unloading. Alkaline phosphatase (ALP) staining and serum osteocalcin (Ocn) levels indicated that the bone remodeling balance shifted to favor bone resorption over bone formation as early as day 3 (Fig. 1d and Supplementary Fig. S1a, b). Bone formation activity was barely detectable seven days after unloading, whereas bone resorption activity remained high until day 14 (Fig. 1d and Supplementary Fig. S1a, b).Fig. 1: The MSC frequency was markedly reduced in BM of HU rats.a Schematic of HU models and WB controls. Representative micro-CT images (b) and quantification of three-dimensional microstructural parameters (c) of femurs from HU rats and WB controls. d Relative serum Ocn and CTX1 levels in WB and HU rats. The data are presented as the fold changes relative to those of the WB samples. e Representative images of tibia sections from rats subjected to HU for 7 days and stained with LepR. f Quantification of LepR+ cells. The data are presented as percentages of positively stained cells to total cells (DAPI-stained nuclei) *P