Cellular mechanisms of damage and repair in sensory hair cells

Sensory hair cells of the inner ear are susceptible to damage and death as a result of exposure to aminoglycoside antibiotics, leading to permanent sensorineural hearing loss. Here, the atypical recruitment and redistribution of membrane in hair cells exposed to aminoglycosides is characterized during two distinct cellular processes: the retrieval of membrane blebs induced by a brief exposure to a high concentration of neomycin, and the alteration of lysosomal structure in hair cells during a low-dose treatment with gentamicin-Texas Red (GTTR). When cochlear cultures from early postnatal mice are exposed to a high dose of neomycin, rapid and extensive membrane blebbing occurs at the apical pole of hair cells. Remarkably, following aminoglycoside washout hair cells rapidly repair this damage. Blebs as large as 3 µm in diameter are reinternalised by hair cells within the first 15 minutes of recovery from neomycin exposure. Repair from blebbing is robust and not blocked by inhibitors of canonical endocytic pathways. Characterisation by conventional TEM and serial TEM tomography indicates this bulk endocytic process involves the formation of large multi-laminated structures consisting of multiple concentric membrane folds. This lipid-driven endocytic event is dependent on the function of volume regulating anion channels and may provide an example of massive endocytosis (MEND) occurring in sensory hair cells. The effects of exposure to a low-dose aminoglycosides on hair cell ultrastructure were also examined. Fluorescently conjugated aminoglycosides, such as GTTR, have been used extensively to study the loading and trafficking of aminoglycosides in sensory hair cells. Here, using a lowdose, long duration, in vitro assay GTTR is demonstrated to co-localise with, and cause the formation of multi-lamellar, lipid-rich structures identifiable as membranous cytoplasmic bodies (MCBs), such as those observed in lysosomal storage disorders. The significance of these large ultrastructural aberrations for understanding the molecular mechanisms of aminoglycoside ototoxicity remains, however, unclear. Other aminoglycosides had a varying propensity to induce lipid inclusions and MCB formation. Aminoglycosides are not toxic to hair cells with nonfunctional mechanotransduction, such as those in the Myo7aSh6J/Sh6J mouse, yet it was demonstrated here that these cells are found to load with GTTR in a manner comparable to wild type cells, and to also form MCBs. Overall, these findings further our understanding of the mechanisms by which hair cells can repair from aminoglycoside induced membrane damage, explore the differing toxic effects of aminoglycosides within hair cells, and highlight fundamental considerations for future aminoglycoside/hair-cell research that utilises fluorescently conjugated aminoglycosides. Permanent sensorineural hearing loss is a severe side effect of aminoglycoside treatment, responsible for significant morbidity. This research allows us to better understand the biology of aminoglycoside ototoxicity, essential for future work in the prevention of aminoglycoside induced hearing loss.