Therapeutic potential of melatonin-pretreated human dental pulp stem cells (hDPSCs) in an animal model of spinal cord injury

Dental pulp stem cells (DPSCs) show potential for treating neurodegenerative and traumatic diseases due to their neural crest origin. Melatonin (MT), an endogenous neurohormone with well-documented anti-inflammatory and antioxidant properties, has shown promising results with MSCs in terms of engraftment, proliferation, and neuronal differentiation in animal SCI models. However, the effects of melatonin preconditioning on human dental pulp stem cells (hDPSCs) for SCI treatment remain unclear. This study investigates the impact of melatonin preconditioning on hDPSCs engraftment, neural differentiation, and neurological function in rats with SCI. Forty-two male Sprague-Dawley rats were divided into six groups: Control, Sham, Model, Vehicle, Lesion Treatment A (SCI + hDPSCs), and Lesion Treatment B (SCI + MT-hDPSCs). After obtaining hDPSCs, stem cells were evaluated using flow cytometry. Cell viability was assessed using the MTT assay. SCI was induced in the Model, Vehicle, Lesion Treatment A, and Lesion Treatment B groups. The Lesion Treatment A and B groups received hDPSCs and hDPSCs pretreated with melatonin, respectively, 1 week after SCI, while the Vehicle group received only an intravenous injection of DMEM to simulate treatment. The other groups were used for behavioral testing. Immunohistochemistry (IHC) was employed to assess hDPSCs engraftment and differentiation at the SCI site. Motor function across the six groups was evaluated using the Basso, Beattie, and Bresnahan (BBB) score. Histological studies and cell counts confirmed hDPSCs implantation at the injury site, with a significantly higher presence in the MT-hDPSCs compared to hDPSCs (p < 0.01). IHC revealed that hDPSCs and MT-hDPSCs differentiated into neurons and astrocytes, with greater differentiation observed in the MT-hDPSCs compared to the hDPSCs (p < 0.01 and p < 0.05, respectively). Functional improvement was noted in both SCI + hDPSCs and SCI + MT-hDPSCs groups compared to SCI and Vehicle groups from Week 4 onward (p < 0.001). Significant differences were also observed between the SCI + hDPSCs and SCI + MT-hDPSCs groups starting from Week 7 (p < 0.01). Preconditioning hDPSCs with melatonin enhances engraftment, neuronal differentiation, and greater performance improvement compared to hDPSCs alone in the SCI animal model.