Mechanical overstimulation causes acute injury and synapse loss followed by fast recovery in lateral-line neuromasts of larval zebrafish

Abstract

Noise exposure damages sensory hair cells, resulting in loss of synaptic connections with auditory nerves and hair-cell death. The cellular mechanisms underlying noise-induced hair-cell damage and subsequent repair are not completely understood. Hair cells in neuromasts (NMs) of larval zebrafish are structurally and functionally comparable to mammalian hair cells but undergo robust regeneration following damage. We therefore developed a model for noise-induced hair-cell damage in this highly tractable system. Free swimming larvae exposed to strong water current for 2 hours displayed damage to NMs, including synapse loss, afferent neurite retraction, damaged hair bundles, and reduced mechanotransduction. Overstimulation also elicited an inflammatory response and macrophage recruitment. Remarkably, NM morphology and function appeared to fully recover within 2 days following exposure. Our results reveal morphological and functional changes in mechanically overstimulated lateral-line NMs that are analogous to changes observed in noise-exposed mammalian ear yet are rapidly and completely repaired.

Influence of Mpv17 on hair-cell mitochondrial homeostasis, synapse integrity, and vulnerability to damage in the zebrafish lateral line

“…It has recently been shown that Vglut3 -/null mutant mice do not lose hair-cell synapses following noise exposure, supporting a role for synaptic transmission in noise-induced synapse loss (Kim et al, 2019). Additionally, in our previous study of mechanical injury in the zebrafish lateral line, we observed significantly more severe mechanically-induced hair-cell synapse loss in fish when glutamate clearance from the synapse was pharmacologically blocked, suggesting synapse loss can be exacerbated by excess glutamate in the synaptic cleft (Holmgren et al, 2021). Our results here show that mpv17 a9/a9 hair cells have reduced FM1-43 uptake indicating reduced hair-cell transduction (Fig.…”

“…Thus, loss of Mpv17 increases sensitivity to neomycin-induced hair-cell death. We have previously reported a protocol to mechanically overstimulate zebrafish lateral line organs using strong water current (Holmgren et al, 2021). This stimulation resulted in phenotypes including mechanical disruption of neuromast morphology, loss of hair cells, neurite retraction, and loss of hair-cell synapses.…”

“…6). In our previous study, we reported intact mechanotransduction was not required for stimulus-induced displacement of neuromasts, indicating this observed phenotype is a result of mechanical injury (Holmgren et al, 2021). While it is unclear why mpv17 a9/a9 neuromasts are more susceptible to mechanically induced displacement, it is possible that mitochondrial disfunction in hair cells of disrupted mpv17 a9/a9 neuromasts sensitizes them to mechanically induced death.…”

“…Live imaging of zebrafish larvae was performed as previously described (Holmgren et al, 2021). In brief, zebrafish larvae were anesthetized with 0.01% tricaine in E3, then mounted lateral-side up on a thin layer of 1.5-2% low-melt agarose in a tissue culture dish with a coverglass bottom (World Precision Instruments) and covered in E3 media.…”

Macrophages Respond Rapidly to Ototoxic Injury of Lateral Line Hair Cells but Are Not Required for Hair Cell Regeneration

“…Hair cells of the inner ear can be damaged or lost after noise exposure, ototoxicity, or as part of normal aging. Like their counterparts in the ear, lateral line hair cells can also be damaged by exposure to ototoxic drugs or by mechanical trauma (Harris et al, 2003 ; Hernández et al, 2006 ; Uribe et al, 2018 ; Holmgren et al, 2020 ). Many of the signaling pathways that mediate hair cell death have been identified (e.g., Wagner and Shin, 2019 ).…”