Potential of embryonic stem cell-derived neurons for synapse formation with auditory hair cells

Recent studies have indicated that embryonic stem cells (ESCs) can be a source for the replacement of spiral ganglion neurons (SGNs), auditory primary neurons, and neurite projections from ESC-derived neurons to auditory sensory epithelia. However, the potential of ESC-derived neurons for synapse formation with auditory hair cells (HCs) has not been elucidated. The present study therefore aimed to examine the ability of ESC-derived neurons to form synaptic connections with HCs in vitro. Mouse ESC-derived neural progenitors expressing enhanced green fluorescence protein (EGFP) were cocultured with explants of cochlea sensory epithelia obtained from postnatal day 3 mice. After a 7-day culture, neurites of ESC-derived neurons predominantly elongated toward inner hair cells (IHCs), which play a crucial role in sound transmission to SGNs. Immunohistochemical analyses revealed the expression of synapsin 1 and synaptophysin in the nerve endings of ESC-derived neurons adjacent to IHCs, indicating the formation of synaptic connections. Transmission electron microscopy demonstrated synaptic contacts between nerve endings of ESC-derived neurons and IHCs. The present findings show that ESC-derived neurons can make synaptic connections with IHCs.