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

Abstract

Noise exposure is particularly stressful to hair-cell mitochondria, which must produce enough energy to meet high metabolic demands as well as regulate local intracellular Ca2+ concentrations. Mitochondrial Inner Membrane Protein 17 (Mpv17) functions as a non-selective channel and plays a role in maintaining mitochondrial homeostasis. In zebrafish, hair cells in mpv17a9/a9 mutants displayed elevated levels of reactive oxygen species (ROS), elevated mitochondrial calcium, hyperpolarized transmembrane potential, and greater vulnerability to neomycin, indicating impaired mitochondrial function. Using a strong water current to overstimulate hair cells in the zebrafish lateral line, we observed mpv17a9/a9 mutant hair cells were more vulnerable to morphological disruption and hair-cell loss than wild type siblings simultaneously exposed to the same stimulus. To determine the role of mitochondrial homeostasis on hair-cell synapse integrity, we surveyed synapse number in mpv17a9/a9 mutants and wild type siblings as well as the sizes of presynaptic dense bodies (ribbons) and postsynaptic densities immediately following stimulus exposure. We observed mechanically injured mpv17a9/a9 neuromasts, while they lost a greater number of hair cells, lost a similar number of synapses per hair cell relative to wild type. Additionally, we quantified the size of hair cell pre- and postsynaptic structures and observed significantly enlarged wild type postsynaptic densities, yet relatively little change in the size of mpv17a9/a9 postsynaptic densities following stimulation. These results suggest impaired hair-cell mitochondrial activity influences synaptic morphology and hair-cell survival but does not exacerbate synapse loss following mechanical injury.

Prolonged dexamethasone exposure enhances zebrafish lateral-line regeneration but disrupts mitochondrial homeostasis and hair cell function

“…Numerous studies have indicated that dexamethasone can induce mitochondrial dysfunction in various tissues, including adipocytes (Luan et al, 2019), skeletal muscle (Liu et al, 2016), and neurons (Du et al, 2009). We previously reported reduced hair cell uptake of the cationic dye FM1-43 in zebrafish mutants with impaired hair cell mitochondrial function ( mpv17 -/- ), indicating that these fish possessed reduced driving force for cations through mechanotransduction channels, and elevated hair cell mitochondrial calcium (Holmgren & Sheets, 2021). We therefore evaluated whether 48-hour dexamethasone exposure affected hair cell uptake of FM1-43 and calcium homeostasis.…”

“…Elevated TMRE fluorescence was apparent not only in the mitochondria of hair cells (Fig 8 A’, B’, F; EGFP-negative cells), but also the surrounding supporting cells (Fig 8 A”, B”, G; EGFP-positive cells), suggesting prolonged dexamethasone exposure disrupts normal mitochondrial function in both hair cells and supporting cells. Given that the hair cells in dexamethasone-treated larvae had more negatively charged ΔΨm, we predicted that these hair cells would be more vulnerable to neomycin-induced hair cell death (Alassaf et al, 2019; Holmgren & Sheets, 2021). When treated with higher doses of neomycin (50-100 μM), we observed significantly more lateral-line hair cell loss in fish pre-exposed for 48 hours to dexamethasone, relative to controls (Fig 9 K; adjusted P= 0.1407 (30 μM), *P=0.0353 (50 μM), ****P<0.0001 (100 μM)).…”

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

“…Numerous studies have indicated that dexamethasone can induce mitochondrial dysfunction in various tissues, including adipocytes [32], skeletal muscle [33], and neurons [34]. We previously reported reduced hair cell uptake of the cationic dye FM1-43 in zebrafish mutants with impaired hair cell mitochondrial function (mpv17 −/− ), indicating that these fish possessed reduced driving force for cations through mechanotransduction channels, and elevated hair cell mitochondrial calcium [35]. We therefore evaluated A, B) and neomycin injured neuromasts (C-F).…”

“…8A″, B″, E; EGFP-positive cells), suggesting prolonged dexamethasone exposure disrupts normal mitochondrial function in both hair cells and supporting cells. Given that the hair cells in dexamethasone-treated larvae had more negatively charged ΔΨm, we predicted that these hair cells would be more vulnerable to neomycin-induced hair cell death [35,37]. We therefore compared neomycin-induced hair cell lesion in fish pre-treated with dexamethasone or DMSO carrier alone (Fig.…”