MicroRNAs (miRNAs) interfere with the translation of specific target mRNAs and are thought to thereby regulate many cellular processes. However, the role of miRNAs in osteoblast mechanotransduction remains to be defined. In this study, we investigated the ability of a miRNA to respond to different mechanical environments and regulate mechano-induced osteoblast differentiation. First, we demonstrated that miR-33-5p expressed by osteoblasts is sensitive to multiple mechanical environments, microgravity and fluid shear stress. We then confirmed the ability of miR-33-5p to promote osteoblast differentiation. Microgravity or fluid shear stress influences osteoblast differentiation partially via miR-33-5p. Through bioinformatics analysis and a luciferase assay, we subsequently confirmed that Hmga2 is a target gene of miR-33-5p that negatively regulates osteoblast differentiation. Moreover, miR-33-5p regulates osteoblast differentiation partially via Hmga2. In summary, our findings demonstrate that miR-33-5p is a novel mechano-sensitive miRNA that can promote osteoblast differentiation and participate in the regulation of differentiation induced by changes in the mechanical environment, suggesting this miRNA as a potential target for the treatment of pathological bone loss.

Recent studies have demonstrated that miRNAs can play important roles in osteoblast differentiation and bone formation. However, the function of miRNAs in bone loss induced by microgravity remains unclear. In this study, we investigated the differentially expressed miRNAs in both the femur tissues of hindlimb unloading rats and primary rat osteoblasts (prOB) exposed to simulated microgravity. Specifically, miR-132-3p was found up-regulated and negatively correlated with osteoblast differentiation. Overexpression of miR-132-3p significantly inhibited prOB differentiation, whereas inhibition of miR-132-3p function yielded an opposite effect. Furthermore, silencing of miR-132-3p expression effectively attenuated the negative effects of simulated microgravity on prOB differentiation. Further experiments confirmed that E1A binding protein p300 (Ep300), a type of histone acetyltransferase important for Runx2 activity and stability, was a direct target of miR-132-3p. Up-regulation of miR-132-3p by simulated microgravity could inhibit osteoblast differentiation in part by decreasing Ep300 protein expression, which, in turn, resulted in suppression of the activity and acetylation of Runx2, a key regulatory factor of osteoblast differentiation. Taken together, our findings are the first to demonstrate that miR-132-3p can inhibit osteoblast differentiation and participate in the regulation of bone loss induced by simulated microgravity, suggesting a potential target for counteracting decreases in bone formation.Numerous studies have shown that mechanical stimulations play an important role in the maintenance of bone homeostasis, skeletal morphology and strength during bone formation and development [1][2][3] . By contrast, skeletal unloading, as observed in space flight astronauts or in patients subjected to prolonged immobility or bed-rest, typically induces severe bone loss 4 . The early studies described similar phenomenon, such as cancellous osteoporosis in weight-bearing bones, decreased bone formation and abnormal bone metabolism after space flight 5,6 . During the spaceflight mission on the Soviet/Russian MIR spacecraft and the International Space Station, crew members experienced a persistently enhanced areal bone mineral density lost at an average monthly rate of 1.06% from the spine and 1.0 to 1.6% from the hip, despite adopting an intense exercise regimen to counteract mechanical unloading 7 . Decreased bone formation in both rat cortical and cancellous bones was also demonstrated by tetracycline labeling before and after space flight [8][9][10][11] . In view of spaceflight tremendous costs, more studies have been performed on the ground. The hindlimb unloading (HU) model is a well-tolerated method to mimic the cephalic fluid shift and removal of skeletal weight-bearing loads seen in spaceflight 12 . Despite the variability of data among independent studies, this model successfully replicates an osteopenia characterized by decreased bone mineral content, weakened bone resistance, and loss of femoral mass, simila...

Recent studies have confirmed that microRNAs and lncRNAs can affect bone cell differentiation and bone formation. In this study, miR-139-3p was upregulated in the femurs of hindlimb unloading mice and MC3T3-E1 cells under simulated microgravity; this effect was related to osteoblast differentiation and apoptosis. Silencing miR-139-3p attenuated the suppression of differentiation and the promotion of MC3T3-E1 cell apoptosis induced by simulated microgravity. ELK1 is a target of miR-139-3p and is essential for miR-139-3p to regulate osteoblast differentiation and apoptosis. An osteoblast differentiation-related lncRNA that could interact with miR-139-3p (lncRNA ODSM) was identified in MC3T3-E1 cells under simulated microgravity. Further investigations demonstrated that lncRNA ODSM could promote MC3T3-E1 cell differentiation. Therefore, this research was the first to reveal the critical role of the lncRNA ODSM/miR-139-3p/ELK1 pathway in osteoblasts, and these findings suggest the potential value of miR-139-3p in osteoporosis diagnosis and therapy.

Extracellular vesicles (EVs) including exosomes can serve as mediators of cell–cell communication under physiological and pathological conditions. However, cargo molecules carried by EVs to exert their functions, as well as mechanisms for their regulated release and intake, have been poorly understood. In this study, we examined the effects of endothelial cells-derived EVs on neurons suffering from oxygen-glucose deprivation (OGD), which mimics neuronal ischemia-reperfusion injury in human diseases. In a human umbilical endothelial cell (HUVEC)–neuron coculture assay, we found that HUVECs reduced apoptosis of neurons under OGD, and this effect was compromised by GW4869, a blocker of exosome release. Purified EVs could be internalized by neurons and alleviate neuronal apoptosis under OGD. A miRNA, miR-1290, was highly enriched in HUVECs-derived EVs and was responsible for EV-mediated neuronal protection under OGD. Interestingly, we found that OGD enhanced intake of EVs by neurons cultured in vitro. We examined the expression of several potential receptors for EV intake and found that caveolin-1 (Cav-1) was upregulated in OGD-treated neurons and mice suffering from middle cerebral artery occlusion (MCAO). Knock-down of Cav-1 in neurons reduced EV intake, and canceled EV-mediated neuronal protection under OGD. HUVEC-derived EVs alleviated MCAO-induced neuronal apoptosis in vivo. These findings suggested that ischemia likely upregulates Cav-1 expression in neurons to increase EV intake, which protects neurons by attenuating apoptosis via miR-1290.

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