Neurogenic maturation of human dental pulp stem cells following neurosphere generation induces morphological and electrophysiological characteristics of mature neurons

Abstract

Endogenous neural stem cells are limited in their capacity to regenerate lost neural tissue. Therefore, additional therapeutic approaches are necessary to improve the outcome in neurodegenerative diseases. Considering stem cell-based therapies,neural stem cells are the ideal stem cell source but due to isolation difficulties, alternative sources with neurogenic differentiation potential are currently being investigated. Dental pulp stem cells (DPSCs) are a promising alternative due to their neuroectodermal origin and isolation simplicity. This study hypothesizes that DPSCs contain a subset of progenitor cells that can differentiate towards mature neurons under the appropriate environmental cues. Neuronal differentiation of DPSC was evaluated using immunocytochemical, ultrastructural, RT-PCR and electrophysiological analysis. Neuronal differentiation was induced by the generation of neurospheres from explant-derived DPSCs. Subsequently, neurospheres were collected and neurogenic maturation was induced by cAMP and Neurotrophin-3 administration. Immunocytochemical analysis was performed for the neuron-specific markersNeuN and MAP-2. Ultrastructural and electrophysiological evaluation of maturated DPSCs was performed with transmission electron microscopy (TEM)and patch-clamp recordings respectively. The expression profile of neuron-related genes was determined by means of RT-PCR. DPSCs formed neurospheres after 2–3 days in culture and were monitored daily. After another 5 days in culture, neurospheres reached a maximum diameter of 250 μm and neurogenic maturation was induced for 4 weeks. On the ultrastructural level, intra-neurospheral DPSCs were characterized by a prominent Golgi-apparatus, electron-dense intracellular vesicles, and prominent nucleoli, indicating increased metabolic activity. Furthermore, intercellular vesicular transport was observed at zones of cell–cell contacts. However, cell integrity was lost in a subset of neurosphere cells and a large, dilated RER was present. Maturation of neurosphere-derived DPSCs resulted in cells displaying a large, round perikaryon with long cytoplasmatic extensions contributing to the formation of an extensive intercellular network. Ultrastructurally, these cells were characterized by a high number of intracellular organelles associated with increased cellular activity. At higher magnification, neuronal-like characteristics were observed, including multiple ‘dendro-somatic’ junctions, the presence of granular material in the intercellular cleft and long cytoplasmatic extensions packed with intracellular vesicles associated with cytoskeletal structures. In addition, immune-reactivity for NeuN and MAP-2 was present after differentiation, suggesting differentiation towards neuronal cells. RT-PCR analysis showed a differential expression pattern of neuron-related genes between neurogenic differentiated DPSCs and control DPSCs. Patch-clamp recordings demonstrated the presence of inward voltage-activated sodium currents and outward voltage-activated potassium currents in differentiated DPSCs which could be blocked with TTX and TEA respectively. The results in this study indicate that DPSCs are able to differentiate into neuron-like cells. However, additional research is necessary to further elucidate the functionality of neurogenic maturated DPSCs.

Published in doi: 10.2217/rme.13.op