Stem Cell Exosomes Reverse Antibiotic-Induced Inner Ear Damage in Rats

Gentamicin ototoxicity is a well-recognized clinical hazard: the antibiotic enters inner ear hair cells, triggers calcium overload, generates reactive oxygen species, and drives mitochondrial dysfunction that ultimately kills the irreplaceable sensory cells of the vestibule and cochlea. Because mammalian hair cells do not regenerate, the resulting balance disorders and high-frequency hearing loss are largely permanent. Current management is limited to antioxidants and neurotrophic factors with no established clinical protocol, making novel interventions urgently needed. This study asked whether exosomes derived from human umbilical cord mesenchymal stem cells (hucMSC-EXOs), delivered via intratympanic injection, could protect vestibular hair cells and restore function in a rat model.

Twenty male Sprague-Dawley rats (250–300 g) were divided into four groups of five: untreated controls, gentamicin injury (GEN), gentamicin plus dexamethasone (GEN+DEX), and gentamicin plus exosomes (GEN+EXO). All active treatments were delivered as a single 50 µL intratympanic injection. Exosomes were isolated from hUC-MSC conditioned medium by sequential centrifugation and magnetic-bead purification, confirmed by transmission electron microscopy, nanoparticle tracking analysis (mean diameter ~100 nm), and Western blot for canonical markers CD9, CD63, and TSG101. PKH26-labeled exosomes were traced to the utricle, saccule, and crista ampullaris seven days post-injection, confirming successful delivery to all three vestibular end organs.

Functional outcomes were striking. In the open-field test on day 6, the GEN+EXO group showed significant recovery of total movement distance and movement speed compared to GEN animals (p<0.05), with speed recovery superior to the GEN+DEX group. The beam balance test passage time was reduced by 60.5% in the exosome group versus the injury group (p<0.05). Auditory brainstem response testing at 32 kHz on day 7 showed the GEN+EXO group had thresholds 18.3 dB SPL lower than the GEN group (p<0.01), statistically equivalent to the dexamethasone group. Hair cell counts by immunofluorescence revealed exosome treatment preserved 25% more cells in the utricular striola, 44% more in the saccular striola, and 44% more in the central crista ampullaris versus GEN controls. Notably, the saccular striola showed a 109% repair rate relative to dexamethasone, indicating superior efficacy. Scanning electron microscopy confirmed reduced stereocilia loss and less structural vacuolization in exosome-treated tissues.

To understand the molecular mechanism, the team performed data-independent acquisition (DIA) proteomics on utricular maculae from GEN, GEN+DEX, and GEN+EXO groups. The GEN group showed strong activation of complement and coagulation cascades, consistent with inflammatory injury. The GEN+EXO group uniquely enriched the SNARE interaction in vesicular transport pathway, which facilitates autophagosome-lysosome fusion and thus promotes autophagic clearance of damaged organelles. Additional enriched pathways included metabolic and oxidative stress regulation. Immunofluorescence validation confirmed that exosomes significantly reduced cleaved Caspase-3 expression (apoptosis marker, p<0.01) and significantly increased LC3 puncta (autophagy marker, p<0.05) compared to the GEN group, establishing a dual anti-apoptotic and pro-autophagic mechanism.

The findings position hucMSC-EXOs as a promising cell-free therapeutic for aminoglycoside ototoxicity. The intratympanic route is already used clinically for steroid delivery, making translation feasible. The exosome approach avoids the risks of live cell transplantation while delivering a complex cargo of proteins and RNAs that modulate multiple injury pathways simultaneously. Caveats include the small sample size (n=5 per group), the acute single-injection model which may not reflect chronic clinical exposure, and the absence of long-term follow-up beyond seven days. The study is also limited to male rats, and the specific exosomal cargo molecules responsible for SNARE pathway activation remain to be identified.