Jared A. Laney

Introduction. Bone defects are commonly seen in medical practice; 5-10% of fractures do not heal properly and approximately 100,000 fractures annually are classified as non-union in the US.^1-4 Complex bone defects/injuries, whether due to size, co-morbidities, or location can be difficult to heal.^1-3,5 Treatment of these conditions requires advanced therapy fraught with risks that has no guarantee of success.^2-3,6-7 Stem cells may provide a way to heal complex bone defects without these risks.^3,7 Dental-derived stem cells (DDSCs) are particularly promising, being safer and easier to access compared to other harvesting methods and showing superb osteogenic ability.^6,8-9 The complex signaling pathways required to promote osteogenic differentiation in DDSCs are incompletely understood, but it is known that the Notch pathway is involved.¹⁰ Experimental data to date has shown conflicting results regarding the role of Notch in DDSC differentiation; this may be due to methodology flaws that inaccurately represent the in vivo environment.¹⁰ Manokawinchoke et al. attempted to resolve some of these methodological difficulties with a novel experimental method.¹⁰ Methods. In order to mimic the tension found in the in vivo Notch receptor-ligand complex, Notch ligand Jagged1 was immobilized on tissue culture plates and subsequently exposed to human dental pulp cells (hDPs) and various experimental conditions.¹⁰ Results. Notch2 was the most common Notch receptor identified on hDPs.¹⁰ mRNA for Notch target genes HES1 and HEY1 was observed in greater amounts in the immobilized groups vs control.¹⁰  ALP was observed in higher amounts in the Jagged1 group vs control. ¹⁰ Mineral deposition was confirmed in the Jagged1 group.¹⁰ After incubating the plates in osteogenic medium, osteogenic marker genes were confirmed in greater numbers in the Jagged1 groups vs control.¹⁰  Experiments were run again with Notch2 knockout hDPs and the ALP/mineral deposition levels previously observed disappeared. A similar result occurred when the plates were exposed to DAPT.¹⁰ Conclusions. The Notch pathway seems to play an important role in stimulating osteogenesis in hDPs, with Notch2 being particularly important. Continued experimental use of immobilized Notch ligands may help to better elucidate the true in vivo impact of Notch signaling as it relates to osteogenic differentiation in hDPs. Similar methodology should be employed with other types of human dental stem cells to characterize the role of Notch signaling in those cell lines as well. Additional studies are needed to characterize the role of Notch receptors other than Notch2 in hDP osteogenesis.

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