Aminoglycoside antibiotics can damage the hair cells of the inner ear, leading to hearing loss and balance disorders. Improved treatment paradigms have reduced the risk of adverse effects associated with these antibiotics, but permanent inner ear damage is still common. Hair cells are susceptible to damage because of their tendency to take up and retain aminoglycosides, whereas less vulnerable cells do not accrue aminoglycosides after exposure. To better understand the mechanisms that contribute to aminoglycoside toxicity, David Raible’s lab at the University of Washington imaged fluorescently-labeled aminoglycosides in the hair cells of live zebrafish. They found that the degree of aminoglycoside-induced toxicity was linked to how rapidly the antibiotic was distributed into lysosomes, suggesting that monitoring the kinetics of lysosomal delivery may be an effective way to evaluate different aminoglycoside treatment paradigms.
In the accompanying video, a time-lapse of zebrafish hair cells (green) shows the transit of fluorescently-labeled aminoglycosides (red) from the intracellular space into lysosomes. At the beginning of the video, aminoglycosides are visible as a diffuse pool in the cytosol. Within 20 minutes, the diffuse signal decreases as aminoglycosides merge into puncta, indicating their uptake into lysosomes.
Aminoglycosides (AGs) are broad-spectrum antibiotics that are associated with kidney damage, balance disorders, and permanent hearing loss. This damage occurs primarily by killing of proximal tubule kidney cells and mechanosensory hair cells, though the mechanisms underlying cell death are not clear. Imaging molecules of interest in living cells can elucidate how molecules enter cells, traverse intracellular compartments, and interact with sites of activity. Here, we have imaged fluorescently labeled AGs in live zebrafish mechanosensory hair cells. We determined that AGs enter hair cells via both nonendocytic and endocytic pathways. Both routes deliver AGs from the extracellular space to lysosomes, and structural differences between AGs alter the efficiency of this delivery. AGs with slower delivery to lysosomes were immediately toxic to hair cells, and impeding lysosome delivery increased AG-induced death. Therefore, pro-death cascades induced at early time points of AG exposure do not appear to derive from the lysosome. Our findings help clarify how AGs induce hair cell death and reveal properties that predict toxicity. Establishing signatures for AG toxicity may enable more efficient evaluation of AG treatment paradigms and structural modifications to reduce hair cell damage. Further, this work demonstrates how following fluorescently labeled drugs at high resolution in living cells can reveal important details about how drugs of interest behave.
Dale W. Hailey, Robert Esterberg, Tor H. Linbo, Edwin W. Rubel, David W. Raible