Transdifferentiation of Human Dental Pulp Mesenchymal Stem Cells into Spiral Ganglion-like Neurons

Spiral ganglion neurons (SGN) carry auditory information from sensory hair cells (HCs) to the brain. These auditory neurons, which are the target neurons of cochlear implants, degenerate following sensorineural hearing loss (SNHL). Prosthetic devices such as cochlear implants function by bypassing lost HCs and stimulating the residual SGNs, allowing restoration of hearing in deaf patients. Emerging cell-replacement therapies for SNHL include replacing damaged SGNs using stem cell-derived otic neuronal progenitors (ONPs). However, the availability of renewable, accessible, and patient-matched sources of human stem cells constitutes a major prerequisite towards cell replacement for auditory nerve recovery. Human dental pulp stem cells (hDPSCs) extracted from human wisdom teeth are self-renewing stem cells that originate from the neural crest during development. In this study, we developed a stepwise in vitro guidance procedure to differentiate hDPSCs into ONPs and then to SGNs. The procedure relies on the modulation of BMP and TGF-{beta} pathways for neurosphere formation as a first step, then a differentiation step based on two culture paradigms exploiting major signaling pathways (Wnt, Shh, RA) and neurotrophic factors involved in early otic neurogenesis. Gene and protein expression analyses revealed efficient induction of a comprehensive panel of known ONP and SGN-like cell markers over the course of in vitro differentiation. The use of atomic force microscopy revealed that hDPSC-derived SGN-like cells exhibit similar nanomechanical properties compared to their in vivo SGN counterparts. Furthermore, neurites extended between hDPSC-derived ONPs and rat SGN explants 4-6 days after co-culturing, suggesting the formation of neuronal contacts. These data indicate that the in vitro differentiated cells closely replicate the phenotypic and nanomechanical characteristics of human SGNs, advancing our culture differentiation system to the level to be used in next-generation cochlear implants and/or inner ear cell-based strategies for SNHL.