piR-368 promotes odontoblastic differentiation of dental papilla cells via the Smad1/5 signaling pathway by targeting Smurf1

PURPOSE

The important role of non-coding RNAs in odontoblastic differentiation of dental tissue-derived stem cells has been widely demonstrated; however, whether piRNA (a subclass of non-coding RNA) involved in the course of odontoblastic differentiation is not yet available. This study aimed to investigate the expression profile of piRNA during odontogenic differentiation of mDPCs and the potential molecular mechanism in vitro.

MATERIALS AND METHODS

The primary mouse dental papilla cells (mDPCs) were isolated from the first molars of 1-day postnatal Kunming mice. Then, they were cultured in odontogenic medium for 9 days. The expression profile of piRNA was detected by Small RNA sequencing. RT-qPCR was used to verify the elevation of piR-368. The mRNA and protein levels of mineralization markers were examined by qRT-PCR and Western blot analysis. Alkaline phosphatase (ALP) activity and alizarin red S staining were conducted to assess the odontoblastic differentiation ability.

RESULTS

We validated piR-368 was significantly upregulated and interference with piR-368 markedly inhibited the odontogenic differentiation of mDPCs. In addition, the relationship between Smad1/5 signaling pathway and piR-368-induced odontoblastic differentiation has been discovered. Finally, we demonstrated Smurf1 as a target gene of piR-368 using dual-luciferase assays.

CONCLUSION

This study was the first to illustrate the participation of piRNA in odontoblastic differentiation. We proved that piR-368 promoted odontoblastic differentiation of mouse dental papilla cells via the Smad1/5 signaling pathway by targeting Smurf1.

Melatonin-mediated malic enzyme 2 orchestrates mitochondrial fusion and respiratory functions to promote odontoblastic differentiation during tooth development

Tooth development is a complex process that is tightly controlled by circadian rhythm. Melatonin (MT) is a major hormonal regulator of the circadian rhythm, and influences dentin formation and odontoblastic differentiation during tooth development; however, the underlying mechanism remains elusive. This study investigated how MT regulates odontoblastic differentiation, with a special focus on its regulation of mitochondrial dynamics. In rat dental papilla cells (DPCs), we found that MT promotes odontoblastic differentiation concurrently with enhanced mitochondrial fusion, while disruption of mitochondrial fusion by depleting optic atrophy 1 (OPA1) impairs MT-mediated differentiation and mitochondrial respiratory functions. Through RNA sequencing, we discovered that MT significantly upregulated malic enzyme 2 (ME2), a mitochondrial NAD(P)⁺ -dependent enzyme, and identified ME2 as a critical MT downstream effector that orchestrates odontoblastic differentiation, mitochondrial fusion, and respiration functions. By detecting the spatiotemporal expression of ME2 in developing tooth germs, and using tooth germ reconstituted organoids, we also provided in vivo and ex vivo evidence that ME2 promotes dentin formation, indicating a possible involvement of ME2 in MT-modulated tooth development. Collectively, our findings offer novel understandings regarding the molecular mechanism by which MT affects cell differentiation and organogenesis, meanwhile, the critical role of ME2 in MT-regulated mitochondrial functions is also highlighted.

RORα Regulates Odontoblastic Differentiation and Mediates the Pro-Odontogenic Effect of Melatonin on Dental Papilla Cells

Dental papilla cells (DPCs), precursors of odontoblasts, are considered promising seed cells for tissue engineering. Emerging evidence suggests that melatonin promotes odontoblastic differentiation of DPCs and affects tooth development, although the precise mechanisms remain unknown. Retinoid acid receptor-related orphan receptor α (RORα) is a nuclear receptor for melatonin that plays a critical role in cell differentiation and embryonic development. This study aimed to explore the role of RORα in odontoblastic differentiation and determine whether melatonin exerts its pro-odontogenic effect via RORα. Herein, we observed that RORα was expressed in DPCs and was significantly increased during odontoblastic differentiation in vitro and in vivo. The overexpression of RORα upregulated the expression of odontogenic markers, alkaline phosphatase (ALP) activity and mineralized nodules formation (p < 0.05). In contrast, odontoblastic differentiation of DPCs was suppressed by RORα knockdown. Moreover, we found that melatonin elevated the expression of odontogenic markers, which was accompanied by the upregulation of RORα (p < 0.001). Utilising small interfering RNA, we further demonstrated that RORα inhibition attenuated melatonin-induced odontogenic gene expression, ALP activity and matrix mineralisation (p < 0.01). Collectively, these results provide the first evidence that RORα can promote odontoblastic differentiation of DPCs and mediate the pro-odontogenic effect of melatonin.

Microenvironmental Stiffness Regulates Dental Papilla Cell Differentiation: Implications for the Importance of Fibronectin-Paxillin-β-Catenin Axis

The mechanical stiffness of substrates is recognized to be an important physical cue in the microenvironment of local cellular residents in mammalian species due to their great capacity in regulating cell behavior. Dental papilla cells (DPCs) play an important role in the field of dental tissue engineering for their stem cell-like properties. Therefore, it is essential to provide the suitable microenvironment by combining with the physical cues of biomaterials for DPCs to carry out the function of effective tissue regeneration. However, how the substrate stiffness influences the odontogenic differentiation of DPCs is still unclear. Thus, we fabricated poly(dimethylsiloxane) substrates with varied stiffness for cell behavior. Both cell morphology and focal adhesion were shown to have significant changes in response to varied stiffness. Paxillin, an important protein adapter of focal adhesion kinase protein, was shown to interact with both ectoplasmic fibronectin and cytoplasmic β-catenin by coimmunoprecipitation. The resultant changes of β-catenin by varied stiffness were confirmed by immunofluorescent stain and western blotting. Further, the higher quantity nuclear translocation of β-catenin and the less phospho-β-catenin on the stiff substrate were detected. This nuclear translocation in the stiff substrate finally led to an increased mineralization of DPCs relative to the soft substrate detected by Von Kossa and Alizarin Red stain. Taken together, this work not only points out that the substrate stiffness can regulate the odontogenic differentiation potential of DPCs via fibronectin/paxillin/β-catenin pathway but also provides significant consequence for biomechanical control of cell behavior in cell-based tooth tissue regeneration.

Expression of Oct-4, SOX-2, and MYC in dental papilla cells and dental follicle cells during in-vivo tooth development and in-vitro co-culture

During tooth development, the special structure of dental follicle and dental papilla enables dental papilla cells (DPCs) and dental follicle cells (DFCs) to make contact with each other. Octamer-binding transcription factor 4 (Oct-4), sex determining region Y box-2 (SOX-2), and cellular homologue of avian myelocytomatosis virus oncogene (MYC) (OSM) are associated with reprogramming and pluripotency. However, whether the expression of OSM could be activated through cell-cell communication is not known. In this study, the distribution of OSM in rat tooth germ was investigated by immunohistochemical staining. An in-vitro co-culture system of DPCs and DFCs was established. Cell proliferation, cell apoptosis, cell cycle stages, and expression of OSM were investigated by Cell Counting Kit 8 (CCK8) analysis, flow cytometry, real-time PCR, and immunohistochemical staining. We found that Oct-4 and SOX-2 were strongly expressed in tooth germ on days 7 and 9 after birth, whereas MYC was expressed only on day 9. Cell proliferation and apoptosis were inhibited, the cell cycle was arrested in the G0/G1 phase, and the propidium iodide (PI) value was downregulated. Expression of Oct-4 and SOX-2 was significantly elevated in both cell types after 3 d of co-culture, whereas expression of MYC was not significantly elevated until day 5. These results indicate that the optimized microenvironment with cell-cell communication enhanced the expression of reprogramming markers associated with reprogramming capacity in DPCs and DFCs, both in vivo and in vitro.