By Francisco Barros-Becker, Ph.D.
Mechanosensory hair cells play a pivotal role during balance and hearing. Unfortunately, many types of insult, like exposure to loud noises or certain therapeutic drugs, can cause permanent hearing loss by irreversibly damaging or killing these cells.
Aminoglycoside antibiotics are standard-of-care for many serious bacterial infections. Studies report that hearing loss in patients as a side effect from undergoing these treatments (ototoxicity) can range from 11 to 67 percent.
Understanding how these drugs are able to kill hair cells may provide new potential therapeutic avenues to make these important drugs safer.
In our publication published in Frontiers in Neurology in November 2024, we explore how different aminoglycosides, like neomycin and gentamicin (or its chemical analog G418), are able to kill hair cells in distinctive ways. Using the zebrafish model, we observed that neomycin can kill hair cells in an acute way, with most of them gone within the first hour after exposure to the drug.
On the other hand, gentamicin or G418 kills in a delayed way, needing up to 24 hours to generate a comparable amount of hair cell death.
One of the previously seen hallmarks of a neomycin-driven hair cell death is the rapid, spike-like mitochondrial calcium uptake preceding the death of the hair cell. In our study, an acute cell death generates the anticipated calcium uptake into the mitochondria, whereas hair cells undergoing a delayed cell death do not show any mitochondrial calcium spikes.
Moreover, mitochondrial calcium uptake is known to generate reactive oxygen species, which are damaging compounds for the cell. By using the mitochondria-targeted antioxidant mitoTEMPO, we can partially rescue neomycin toxicity. However, we see no protection against G418, suggesting that the mechanisms of hair cell death are different.
In the zebrafish model, lysosome-disrupting treatment (GPN) protects hair cells from slow gentamicin damage but not from rapid neomycin damage. GPN significantly increased hair cell survival when given with gentamicin (A) but had no protective effect against neomycin (B). This suggests lysosomes are crucial in gentamicin's slower cell-killing process but not in neomycin's rapid damage pathway. Credit: Wu et al./Frontiers in Neurology
By fluorescently labeling the different aminoglycosides, we were able to track them inside the cell. We show that neomycin accumulates in two main compartments, the cytoplasm and lysosomes, whereas G418 appears almost completely inside lysosomes. This could suggest that the lysosome might play a role in the delayed hair cell death process.
One way to test this idea is by altering the lysosomal function. We found that by stressing the lysosome using the lysosome-disrupting small peptide, GPN, we can induce protection against a delayed cell death, while acute death is not sensitive to lysosomal manipulations.
Overall, our research shows the complexity of the hair cell response when exposed to closely related compounds. Moreover, it introduces the lysosome as a key role player during aminoglycoside-driven ototoxicity.
Our work will continue to deepen our understanding in the molecular pathways behind the process of aminoglycoside hair cell death, with the goal of devising future tools to prevent hearing loss from these and other ototoxic drugs.
A 2023–2024 Emerging Research Grants scientist, Francisco Barros-Becker, Ph.D., is a postdoctoral fellow at the University of Washington.
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Understanding the multiple mechanisms behind how aminoglycoside antibiotics are able to kill hair cells may provide new potential therapeutic avenues to make these important drugs safer.