Prolonged Dexamethasone Exposure Enhances Zebrafish Lateral-Line Regeneration But Disrupts Mitochondrial Homeostasis and Hair Cell Function

Abstract

The synthetic glucocorticoid dexamethasone is commonly used to treat inner ear disorders. Previous work in larval zebrafish has shown that dexamethasone treatment enhances hair cell regeneration, yet dexamethasone has also been shown to inhibit regeneration of peripheral nerves after lesion. We therefore used the zebrafish model to determine the impact of dexamethasone treatment on lateral-line hair cells and primary afferents. To explore dexamethasone in the context of regeneration, we used copper sulfate (CuSO4) to induce hair cell loss and retraction of nerve terminals, and then allowed animals to recover in dexamethasone for 48 h. Consistent with previous work, we observed significantly more regenerated hair cells in dexamethasone-treated larvae. Importantly, we found that the afferent processes beneath neuromasts also regenerated in the presence of dexamethasone and formed an appropriate number of synapses, indicating that innervation of hair cells was not inhibited by dexamethasone. In addition to regeneration, we also explored the effects of prolonged dexamethasone exposure on lateral-line homeostasis and function. Following dexamethasone treatment, we observed hyperpolarized mitochondrial membrane potentials (ΔΨm) in neuromast hair cells and supporting cells. Hair cells exposed to dexamethasone were also more vulnerable to neomycin-induced cell death. In response to a fluid-jet delivered saturating stimulus, calcium influx through hair cell mechanotransduction channels was significantly reduced, yet presynaptic calcium influx was unchanged. Cumulatively, these observations indicate that dexamethasone enhances hair cell regeneration in lateral-line neuromasts, yet also disrupts mitochondrial homeostasis, making hair cells more vulnerable to ototoxic insults and possibly impacting hair cell function.

Cortisol Sensitizes Cochlear Hair Cells to Gentamicin Ototoxicity Via Endogenous Apoptotic Pathway

“…The finding that acute restraint stress modulates the vulnerability of the inner ear to acoustic injury further supports the hypothesis that stress hormones may affect hair cell survival (20). Previous studies in a zebrafish model showed that prolonged dexamethasone exposure disrupted mitochondrial homeostasis in neuromast cells, which in turn affected cellular function (21). Zebrafish lateral line hair cell susceptibility to neomycin toxicity was increased by cortisol (22).…”

Cisplatin drives mitochondrial dysregulation in sensory hair cells

“…When evaluating peak changes in mitochondrial calcium levels, the median change in maximum fluorescence of hair cells was significantly greater among dying hair cells than living or control hair cells (control cells: 1.18, interquartile range = 1.03 to 1. 44 3E). To evaluate when surges in mitochondrial calcium levels occurred, we measured the time at which half the maximum change in fluorescence was observed (t1/2 maximum ∆F/Fο) relative to neuromast clearance.…”

“…To confirm heterogeneous accumulation of cisplatin in mitochondria and explore the time course of mitochondrial localization, we performed a follow-up experiment in transgenic zebrafish that do not express a reporter in mitochondria to ensure that Rho-Mito localization in mitochondria was not influenced by mitoGCaMP3. We used transgenic fish expressing GFP in neuromast supporting cells to provide a silhouette of hair cells as previously described (Tg(TNKS1bp1:EGFP); 18,43,44 ). We exposed 5 -7 dpf larvae to 1 mM cisplatin for various lengths of time, incubated them in 3 µM Rho-Mito, then acquired images (Figure 7A-C Figure 7D).…”

Hair cell toxicology: With the help of a little fish

“…Glucocorticoids have been proposed to treat hearing loss ( Meltser and Canlon, 2011 ). However, using zebrafish larvae it was demonstrated that previous exposure to glucocorticoids like cortisol or dexamethasone increased neomycin damage to hair cells ( Hayward et al, 2019 ; Saettele et al, 2022 ). On a positive note, testing the ototoxicity of new semisynthetic aminoglycoside antibiotics has shown that etimicin exerts less damage to lateral line hair cells in zebrafish larvae than gentamicin ( Shao et al, 2021 ), so this antibiotic could become an alternative for the ototoxic members of the family.…”