The Role and Mechanism of Exosomes from Umbilical Cord Mesenchymal Stem Cells in Inducing Osteogenesis and Preventing Osteoporosis.

Mesenchymal stem cell (MSC) exosomes promote tissue regeneration and repair, and thus might be used to treat many diseases; however, the influence of microenvironmental conditions on exosomes remains unclear. The present study aimed to analyze the effect of osteogenic induction on the functions of human umbilical cord MSC (HucMSC)-derived exosomes. Exosomes from standardized stem cell culture (Exo1) and osteogenic differentiation-exosomes (Exo2) were co-cultured with osteoblasts, separately. Cell counting kit-8 assays, alkaline phosphatase and alizarin red staining were used to observe the exosomes' effects on osteoblast proliferation and differentiation. The levels of osteogenic differentiation-related proteins were analyzed using western blotting. Estrogen-deficient osteoporosis model mice were established, and treated with the two exosome preparations. Micro-computed tomography and hematoxylin and eosin staining were performed after 6 weeks. MicroRNAs in Exo1 and Exo2 were sequenced and analyzed using bioinformatic analyses. Compared with Exo1 group, Exo2 had a stronger osteogenic differentiation promoting effect, but a weaker proliferation promoting effect. In ovariectomy-induced osteoporosis mice, both Exo1 and Exo2 improved the tibial density and reversed osteoporosis in vivo. High-throughput microRNA sequencing identified 221 differentially expressed microRNAs in HucMSC-derived exosomes upon osteogenic induction as compared with the untreated control group. Importantly, we found that 41 of these microRNAs are potentially critical for MSC-secreted exosomes during osteogenic induction. Mechanistically, exosomal miRNAs derived from osteogenic induced-HucMSCs are involved in bone development and differentiation, such as osteoclast differentiation and the MAPK signaling pathway. The expression of hsa-mir-2110 and hsa-mir-328-3p gradually increased with prolonged osteogenic differentiation and regulated target genes associated with bone differentiation, suggesting that they are probably the most important osteogenesis regulatory microRNAs in exosomes. In conclusion, we examined the contribution of osteogenic induction to the function of exosomes secreted by HucMSCs following osteogenic differentiation in vitro and in vivo, and reveal the underlying molecular mechanisms of exosome action during osteoporosis.