Characteristics of hDPSCs
hDPSCs were extracted from the third molars of healthy people. Primary cultured hDPSCs grew around the tissue mass (Fig. 1a). Morphological observation showed that the cells had a fibroblast-like appearance (Fig. 1b). To further identify the multidirectional differentiation potential of hDPSCs, the isolated hDPSCs were induced to differentiate into osteoblasts and adipocytes. Lipid droplets were observed in the cytoplasm by oil red O staining. Matrix mineralization was increased significantly in the process of osteogenic induction compared with the undifferentiated group (Fig. 1c, d and 1e, f). Subsequently, the qRT–PCR results suggested that the expression levels of ALP, runt-related transcription factor 2 (Runx2), and osteocalcin (OCN) were upregulated (Fig. 1g). hDPSCs were identified by flow cytometry. hDPSCs exhibited high expression of CD29 (99.88%), CD44 (97.87%), and CD90 (99.37%) and were negative for CD34 (0.39%) and CD45 (0.53%) (Fig. 1h).
lncSNHG7 m6A Modification in hDPSCs
By analysing the GSE138179 and SRP214747 datasets, we found that lncSNHG7 expression was enhanced after osteogenic differentiation of hDPSCs. Through m6Avar, the WHISTLE database predicted that lncSNHG7 might have 19 m6A modification sites (Fig. 2a), among which there were three ACA modification sites with very high confidence. The m6A single base site PCR (MazF) verified that lncSNHG7 had a m6A modification on the 2081 ACA site (Fig. 2b). Then, according to the results of the StarBase database, m6A-related modifying enzymes that might bind to lncSNHG7 included METTL3/14, IGF2BP1/2/3, ALKBH5, HNRNPA2B1, FTO, YTHDC1, YTHDF1, FMR1, HNRNPC and WTAP (Fig. 2c). The expression of all m6A-related enzymes was detected in hDPSCs, and it was found that the expression levels of most of them were increased in hDPSCs after osteogenic differentiation (P<0.05). METTL3 exhibited the highest expression (Fig. 2d).
METTL3 Promoted Osteogenic Differentiation of hDPSCs
After osteogenic differentiation of hDPSCs, the protein level of METTL3 increased (Fig. 3a, b). Next, to better verify the role of METTL3 in the osteogenic differentiation of hDPSCs, we isolated hDPSCs and successfully knocked down METTL3 through siRNA in vitro functional experiments. qRT–PCR demonstrated not only the efficiency of knockdown (Fig. 3c) but also the decreased expression levels of the osteogenic genes ALP and Runx2 (Fig. 3d, e). In addition, the expression of osteogenic differentiation-related proteins was detected by western blotting, and the data were consistent with the qRT–PCR results. After silencing the expression of METTL3, expression of the osteogenic proteins ALP and Runx2 decreased. Similarly, as shown in Figure 3g, after METTL3 knockdown, ALP staining decreased in the siMETTL3 group compared with the control groups. ARS staining of mineralization showed reduced mineralization (Fig. 3g). These results indicated that METTL3 knockdown led to decreased osteogenic differentiation of hDPSCs.
lncSNHG7 Promoted Osteogenic Differentiation of hDPSCs
The ability of lncSNHG7 to regulate hDPSC osteogenesis was further validated in vitro, and siRNA-SNHG7 was constructed and transduced into hDPSCs. lncSNHG7 silencing was confirmed by qRT–PCR (Fig. 4a). qRT–PCR analysis of siRNA-SNHG7 cells showed reduced expression of ALP, OCN and Runx2 after induction for 14 days (Fig. 4b). The expression of osteogenic differentiation-related proteins was detected by western blotting. After silencing the expression of lncSNHG7, expression of the osteogenic proteins ALP and Runx2 decreased (Fig. 4c, d). After lncSNHG7 knockdown, ARS staining showed reductions in mineralized nodules (Fig. 4e) and decreased ALP staining in the knockdown compared with the control group (Fig. 4e). These results indicated that lncSNHG7 was an important regulator that could promote osteogenic differentiation of hDPSCs.
To further understand the possible roles of lncSNHG7 in functional regulation, GO enrichment and KEGG pathway analyses were performed on the predicted target mRNAs of the lncRNAs based on the StarBase database. The enriched GO functions in the three GO categories (BP, MF and CC) are shown in Fig. 4f. The GO analysis results showed that the enriched GO terms for the biological process category were regulation of transcription from the RNA polymerase II promoter, signal transduction, protein phosphorylation, etc. The molecular function structured networks indicated protein binding, transcription factor activity and sequence-specific DNA binding. Through cellular component analysis, the target genes were found to be widely involved in the cytoplasm, nucleus, plasma membrane, etc. The results of the KEGG pathway analysis showed that the target mRNAs of lncSNHG7 were enriched in many pathways. These differentially expressed genes were enriched in pathways in cancer, cytokine–cytokine receptor interactions and transcriptional misregulation in cancer. Four enriched pathways were closely related to osteogenesis: MAPK, NF-kappa B, Wnt and TGF-beta (Fig. 4g). Fig. 4h shows a map of the Wnt signaling pathway.
METTL3 Regulated the m6A Modification of lncSNHG7
METTL3 has been shown to be a m6A methyltransferase that is involved in regulating a variety of physiological processes. Therefore, we speculated that METTL3 could target and regulate the m6A modification of lncSNHG7. First, after knocking down METTL3, it was found that the m6A modification level of lncSNHG7 was reduced (Fig. 5a), and the expression level of lncSNHG7 was also reduced, indicating that METTL3 not only regulated the m6A modification of lncSNHG7 but also affected its expression (Fig. 5b). In addition, the binding between METL3 and lncSNHG7 was confirmed by RIP-qPCR (Fig. 5c).
The METTL3/lncSNHG7 axis Regulated the Wnt/β-catenin Signaling Pathway
Bioinformatics analysis predicted that the target gene of lncSNHG7 was enriched in the Wnt/β-catenin signaling pathway. We speculated that METTL3 could affect the Wnt/β-catenin signaling pathway by regulating the m6A modification of lncSNHG7 and ultimately the osteogenic differentiation of hDPSCs. First, lncSNHG7 knockdown resulted in decreased phosphorylation of the key protein GSK-3β in the Wnt/β-catenin signaling pathway, and the expression of β-catenin also decreased (Fig. 6a, b), indicating that lncSNHG7 activated the Wnt/β-catenin signaling pathway. Then, after METTL3 was knocked down, western blotting showed decreased phosphorylation of GSK-3β, and the expression of β-catenin also decreased (Fig. 6c, d). These results confirmed the presence of the METTL3/lncSNHG7 axis, which could regulate the Wnt/β-catenin signaling pathway and affect the osteogenic differentiation of hDPSCs.