Mechanosensory hair cell regeneration in the zebrafish lateral line is mitotic and facilitated by innervation
Mackenzie, Scott Michael
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Mechanosensory hair cells in the inner ear are vulnerable to damage that can result in hearing loss and balance disorders. Exposure to excessive noise and ototoxic drugs are two of the leading causes of hair cell loss, which is irreversible in humans and other mammals. However, non-mammalian vertebrates including birds, amphibians, and fish are capable of significant hair cell regeneration that may provide insight into potential therapies for human patients. Zebrafish and other aquatic vertebrates also express hair cells in an external sensory structure called the lateral line system, which I used to study regeneration after treatment with several commonly used ototoxins. I observed that the mechanism of hair cell regeneration is remarkably consistent, existing entirely of dividing hair cell progenitors unlike the mix of proliferative regeneration and direct transdifferentiation observed in birds and amphibians. New hair cells expressed BrdU, a marker of recent cell division, and blocking proliferation with the microtubule-disrupting drug flubendazole also prevented regeneration. Although I observed no differences in the mechanism of regeneration, the type of ototoxin used to induce hair cell damage significantly affected the rate at which hair cells were replaced. For example, neomycin and gentamicin targeted exclusively mature, functional hair cells, which regenerated within 72 hours, but copper and cisplatin also killed non-sensory support cells. These support cells give rise to the dividing progenitors responsible for regeneration, delaying complete hair cell replacement by several days. I also examined how hair cell regeneration is affected by the removal of innervation, which has been previously shown to prevent tissue regeneration in the amputated amphibian limb. Although regeneration was intact in aneurogenic fish that develop hair cells without pre-existing innervation, regeneration was significantly delayed when the lateral line nerve was axotomized by laser ablation. This suggests that lateral line hair cell regeneration is at least partially nerve-dependent, though re-innervation was followed by a recovery in hair cell number. Researchers who have studied amphibian limb regeneration have implicated the role of trophic factors secreted by myelinating Schwann cells. Although my experiments indicate that Schwann cells are important for regeneration of the lateral line nerve, they do not appear to be directly responsible for mediating hair cell regeneration. A molecular mechanism that links innervation to hair cell regeneration has not been elucidated at this time, but the identification of its components might also contribute to our understanding of how proliferative regeneration is initiated in the lateral line neuromasts. The similar responses to several different ototoxins is encouraging in that it simplifies our understanding of regeneration in the lateral line and will contribute to new genetic and chemical screens for factors that prevent hair cell damage or facilitate regeneration. With an improved understanding the mechanisms underlying replacement of lateral line hair cells, this research can help develop new approaches to hair cell regeneration in humans.
- Neuroscience