Lysosomal augmentation during aminoglycoside uptake in cochlear hair cells

Aminoglycoside antibiotics, such as kanamycin, have ototoxic side effects, which often result in degeneration of cochlear and vestibular hair cells in the inner ear. Cytotoxic effects of aminoglycosides, however, do not appear immediately after cellular uptake of aminoglycosides. In order to understand the mechanisms responsible for the delayed emergence of aminoglycoside ototoxicity, changes in lysosomal activities in cochlear hair cells were evaluated during a repeated administration of kanamycin by two methods. Electron microscopic localization of acid phosphatase (AcPase) revealed that AcPase started to accumulate in vesicles 27 h after the start of kanamycin administration. In addition, the number and size of AcPase-filled vesicles increased with repeated kanamycin doses. Confocal microscopic localization of the LysoTracker probe, a vital lysosomal marker, showed an increase in the size of lysosomes in hair cells that were treated with kanamycin. The temporal changes in the augmentation of lysosomes paralleled those in intracellular kanamycin levels. These results suggest that the intralysosomal compartments can accumulate extensive amounts of aminoglycosides, which might lead to lysosomal swelling and subsequent rupture.