Hair cells are the mechanosensory receptors of the inner ear, responsible for hearing and balance. Hair cell death and consequent hearing loss are common results of treatment with ototoxic drugs, including the widely-used aminoglycoside antibiotics. Induction of heat shock proteins (HSPs) confers protection against aminoglycoside-induced hair cell death via paracrine signaling that requires extracellular HSP70 (Heat Shock 70 kDa Protein). We investigated the mechanisms underlying this non-cell-autonomous protective signaling in the inner ear. In response to heat stress, inner ear tissue releases exosomes that carry HSP70 in addition to canonical exosome markers and other proteins. Isolated exosomes from heat-shocked utricles were sufficient to improve survival of hair cells exposed to the aminoglycoside antibiotic neomycin, while inhibition or depletion of exosomes from the extracellular environment abolished the protective effect of heat shock. Hair-cell specific expression of the known HSP70 receptor, Toll-like receptor 4 (TLR4), was required for the protective effect of exosomes, and exosomal HSP70 interacted with TLR4 on hair cells. Our results indicate that exosomes are a previously undescribed mechanism of intercellular communication in the inner ear that can mediate non-autonomous hair cell survival. Exosomes may represent a novel class of nano-carriers for delivery of therapeutics against hearing loss.
Andrew M. Breglio, Lindsey A. May, Melanie Barzik, Nora C. Welsh, Shimon P. Francis, Tucker Q. Costain, Lizhen Wang, D. Eric Anderson, Ronald S. Petralia, Ya-Xian Wang, Thomas B. Friedman, Matthew J.A. Wood, Lisa L. Cunningham