Cryopreserved Human Otic Neuronal Spheroids Self-assemble for Functional Connectivity Analysis and Long-term Ototoxicity Evaluation.

Spiral ganglion neurons (SGNs) in the inner ear are indispensable for auditory function, and their irreversible damage causes permanent sensorineural hearing loss. Although current human pluripotent stem cell (hPSC)-derived otic lineages offer a valuable resource for SGN regeneration, they face challenges in terms of reproducibility and functional maturation. Here, a robust protocol is established to generate human otic neuronal spheroids (hONS) from cryopreserved hPSC-derived pre-placodal ectoderm (PPE) cells. Post-thaw PPE cells retained high purity and differentiation efficiency comparable to fresh PPE cells. These self-assembled hONS differentiated into functionally mature SGN-like neurons, showing specific maker expression, electrophysiological activity, AMPA receptor-mediated glutamate response, and extensive neurite extension. In tripartite cocultures incorporating murine cochlear explants and human cortical organoids, hONS formed bidirectional functional synaptic connections, validated through live-cell imaging, optogenetic stimulation, and synaptic immunostaining. Notably, hONS exhibited heightened sensitivity to ototoxic insults. Short-term cisplatin exposure induced dose-dependent alterations in cellular and calcium dynamics, whereas prolonged exposure impaired glutamatergic neural functionality and triggered progressive neuronal death. Co-treatment with sodium thiosulfate attenuated cisplatin-induced damage. The hONS model also demonstrated concentration-dependent toxicity to neomycin. Collectively, this hONS model provides a reliable platform for investigating SGN regeneration and conducting preclinical evaluation of ototoxic drug.