PI3K and Inhibitor of Apoptosis Proteins Modulate Gentamicin- Induced Hair Cell Death in the Zebrafish Lateral Line

Inner ear hair cell death leads to sensorineural hearing loss and can be a direct consequence of aminoglycoside antibiotic treatment. Aminoglycosides such as gentamicin are effective therapy for serious Gram-negative bacterial infections such as some forms of meningitis, pneumonia, and sepsis. Aminoglycosides enter hair cells through mechanotransduction channels at the apical end of hair bundles and initiate intrinsic cell death cascades, but the precise cell signaling that leads to hair cell death is incompletely understood. Here, we examine the cell death pathways involved in aminoglycoside damage using the zebrafish (Danio rerio). The zebrafish lateral line contains hair cell-bearing organs called neuromasts that are homologous to hair cells of the mammalian inner ear and represents an excellent model to study ototoxicity. Based on previous research demonstrating a role for p53, Bcl2 signaling, autophagy, and proteasomal degradation in aminoglycoside-damaged hair cells, we used the Cytoscape GeneMANIA Database to identify additional proteins that might play a role in neomycin or gentamicin ototoxicity. Our bioinformatics analysis identified the pro-survival proteins phosphoinositide-dependent kinase-1 (PDK1) and X-linked inhibitor of apoptosis protein (Xiap) as potential mediators of gentamicin-induced hair cell damage. Pharmacological inhibition of PDK1 or its downstream mediator protein kinase C facilitated gentamicin toxicity, as did Xiap mutation, suggesting that both PI3K and endogenous Xiap confer protection. Surprisingly, aminoglycoside-induced hair cell death was highly attenuated in wild type Tupfel long-fin (TL fish; the background strain for the Xiap mutant line) compared to wild type ∗AB zebrafish. Pharmacologic manipulation of p53 suggested that the strain difference might result from decreased p53 in TL hair cells, allowing for increased hair cell survival. Overall, our studies identified additional steps in the cell death cascade triggered by aminoglycoside damage, suggesting possible drug targets to combat hearing loss resulting from aminoglycoside exposure

Cutting-edge science and coffee: Auditory System Gordon Research Conference and Seminar 2012 report

At the third Gordon Research Conference and Gordon Research Seminar on the Auditory System (2012), investigators from all career stages reported on emerging research in a broad range of sub-fields. A distinguishing feature of these conferences is their attention to junior investigators, and their experience is the focus of this conference report

Saccular-Specific Hair Cell Addition Correlates with Reproductive State-Dependent Changes in the Auditory Saccular Sensitivity of a Vocal Fish

The plainfin midshipman fish, Porichthys notatus, is a seasonal breeding teleost fish for which vocal-acoustic communication is essential for its reproductive success. Female midshipman use the saccule as the primary end organ for hearing to detect and locate “singing” males that produce multiharmonic advertisement calls during the summer breeding season. Previous work showed that female auditory sensitivity changes seasonally with reproductive state; summer reproductive females become better suited than winter nonreproductive females to detect and encode the dominant higher harmonic components in the male’s advertisement call, which are potentially critical for mate selection and localization. Here, we test the hypothesis that these seasonal changes in female auditory sensitivity are concurrent with seasonal increases in saccular hair cell receptors. We show that there is increased hair cell density in reproductive females and that this increase is not dependent on body size since similar changes in hair cell density were not found in the other inner ear end organs. We also observed an increase in the number of small, potentially immature saccular hair bundles in reproductive females. The seasonal increase in saccular hair cell density and smaller hair bundles in reproductive females was paralleled by a dramatic increase in the magnitude of the evoked saccular potentials and a corresponding decrease in the auditory thresholds recorded from the saccule. This demonstration of correlated seasonal plasticity of hair cell addition and auditory sensitivity may in part facilitate the adaptive auditory plasticity of this species to enhance mate detection and localization during breeding

Sensory Hair Cell Death and Regeneration

Sensory hair cells are the specialized mechanosensory receptors found in vertebrate auditory, vestibular, and lateral line organs that transduce vibratory and acoustic stimuli into the sensations of hearing and balance. Hair cells can be damaged due to such factors as aging, ototoxic chemicals, acoustic trauma, infection, or genetic factors. Loss of these hair cells lead to deficits in hearing and balance, and in mammals, such deficits are permanent. In contrast, non-mammalian vertebrates exhibit the capability to regenerate missing hair cells. Researchers have been examining the process of hair cell death and regeneration in animal models in an attempt to find ways of either preventing hair cell loss or stimulating the production of new hair cells in mammals, with the ultimate goal of finding new therapeutics for human sensorineural hearing and balance deficits. This has led to a wide array of research on sensory hair cells- such as understanding the factors that cause hair cell loss and finding agents that protect them from damage, elucidating the cell signaling pathways activated during hair cell death, examining the genes and cellular pathways that are regulated during the process of hair cell death and regeneration, and characterizing the functional sensory loss and recovery following acoustic or ototoxic insults to the inner ear. This research has involved cell and developmental biologists, physiologists, geneticists, bioinformaticians, and otolaryngologists. In this Research Topic, we have collated reviews of the past progress of hair cell death and regeneration studies and original research articles advancing sensory hair cell death and regeneration research into the future

Saccular-specific hair cell addition correlates with reproductive state-dependent changes in the auditory saccular sensitivity of a vocal fish

The plainfin midshipman fish, Porichthys notatus, is a seasonal breeding teleost fish for which vocal-acoustic communication is essential for its reproductive success. Female midshipman use the saccule as the primary end organ for hearing to detect and locate "singing" males that produce multiharmonic advertisement calls during the summer breeding season. Previous work has shown that female auditory sensitivity changes seasonally with reproductive state; summer reproductive females become better suited than winter nonreproductive females to detect and encode the dominant higher harmonic components in the male's advertisement call, which are potentially critical for mate selection and localization. Here, we test the hypothesis that these seasonal changes in female auditory sensitivity are concurrent with seasonal increases in saccular hair cell receptors. We show that there is increased hair cell density in reproductive females and that this increase is not dependent on body size since similar changes in hair cell density were not found in the other inner ear end organs. We also observed an increase in the number of small, potentially immature saccular hair bundles in reproductive females. The seasonal increase in saccular hair cell density and smaller hair bundles in reproductive females was paralleled by a dramatic increase in the magnitude of the evoked saccular potentials and a corresponding decrease in the auditory thresholds recorded from the saccule. This demonstration of correlated seasonal plasticity of hair cell addition and auditory sensitivity may in part facilitate the adaptive auditory plasticity of this species to enhance mate detection and localization during breeding

Bax, Bcl2, and p53 Differentially Regulate Neomycin- and Gentamicin-Induced Hair Cell Death in the Zebrafish Lateral Line

Sensorineural hearing loss is a normal consequence of aging and results from a variety of extrinsic challenges such as excessive noise exposure and certain therapeutic drugs, including the aminoglycoside antibiotics. The proximal cause of hearing loss is often death of inner ear hair cells. The signaling pathways necessary for hair cell death are not fully understood and may be specific for each type of insult. In the lateral line, the closely related aminoglycoside antibiotics neomycin and gentamicin appear to kill hair cells by activating a partially overlapping suite of cell death pathways. The lateral line is a system of hair cell-containing sense organs found on the head and body of aquatic vertebrates. In the present study, we use a combination of pharmacologic and genetic manipulations to assess the contributions of p53, Bax, and Bcl2 in the death of zebrafish lateral line hair cells. Bax inhibition significantly protects hair cells from neomycin but not from gentamicin toxicity. Conversely, transgenic overexpression of Bcl2 attenuates hair cell death due to gentamicin but not neomycin, suggesting a complex interplay of pro-death and pro-survival proteins in drug-treated hair cells. p53 inhibition protects hair cells from damage due to either aminoglycoside, with more robust protection seen against gentamicin. Further experiments evaluating p53 suggest that inhibition of mitochondrial-specific p53 activity confers significant hair cell protection from either aminoglycoside. These results suggest a role for mitochondrial p53 activity in promoting hair cell death due to aminoglycosides, likely upstream of Bax and Bcl2

In ovo thyroxine exposure alters later UVS cone loss in juvenile rainbow trout

Thyroid hormones (THs) play a vital role in vertebrate neural development, and, together with the beta isoform of the thyroid hormone receptor (TRβ), the development and differentiation of cone photoreceptors in the vertebrate retina. Rainbow trout undergo a natural process of cone cell degeneration during development and this change in photoreceptor distribution is regulated by thyroxine (T4; a thyroid hormone). In an effort to further understand the role of T4 in photoreceptor ontogeny and later developmental changes in photoreceptor subtype distribution, the influence of enhanced in ovo T4 content on the onset of opsin expression and cone development was examined. Juvenile trout reared from the initial in ovo-treated embryos were challenged with exogenous T4 at the parr stage of development to determine if altered embryonic exposure to yolk THs would affect later T4-induced short-wavelength-sensitive (SWS1) opsin transcript downregulation and ultraviolet-sensitive (UVS) cone loss. In ovo TH manipulation led to upregulation of both SWS1 and long-wavelength-sensitive (LWS) opsin transcripts in the pre-hatch rainbow trout retina and to changes in the relative expression of TRβ. After 7 days of exposure to T4, juveniles that were also treated with T4 in ovo had greatly reduced SWS1 expression levels and premature loss of UVS cones relative to T4-treated juveniles raised from untreated eggs. These results suggest that changes in egg TH levels can have significant consequences much later in development, particularly in the retina