Cochlear hair cell regeneration from neonatal mouse supporting cells

Thesis (Ph. D. in Speech and hearing Bioscience and technology)--Harvard-MIT Program in Health Sciences and Technology, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 86-91).Unlike lower vertebrates, capable of spontaneous hair cell regeneration, mammals experience permanent sensorineural hearing loss following hair cell damage. Although low levels of hair cell regeneration have been demonstrated in the immature mammalian vestibular system, the cochlea has been thought to lack any spontaneous regenerative potential. Inhibition of the Notch pathway can stimulate hair cell generation in neonatal mammals, but the specific source of these new hair cells has been unclear. Here, using in vitro lineage tracing with the supporting cell markers Sox2 and Lgr5, we show that Lgr5-positive inner pillar and 3rd Deiter's cells in gentamicin-damaged organs of Corti from neonatal mice give rise to new hair cells following treatment with a Notch inhibitor. These new hair cells are generated primarily through direct transdifferentiation of supporting cells, although a small number show evidence of proliferation. Inner pillar cells show the greatest transdifferentation capability, giving rise to immature outer hair cells, and transdifferentiating in response to damage even in the absence of Notch inhibition. In vivo pharmacologic inhibition of Notch and in vivo lineage tracing with Sox2 during genetic Notch inhibition provide generally consistent results, although additional new hair cells develop in the inner hair cell region. These data suggest a spontaneous capacity for hair cell regeneration in the neonatal mammalian cochlea. In addition, the data identify Lgr5-positive supporting cells as potential hair cell progenitors, making them an attractive target for future hair cell regeneration treatments.by Naomi F. Bramhall.Ph.D.in Speech and hearing Bioscience and technolog

Lgr5-Positive Supporting Cells Generate New Hair Cells in the Postnatal Cochlea

Summary The prevalence of hearing loss after damage to the mammalian cochlea has been thought to be due to a lack of spontaneous regeneration of hair cells, the primary receptor cells for sound. Here, we show that supporting cells, which surround hair cells in the normal cochlear epithelium, differentiate into new hair cells in the neonatal mouse following ototoxic damage. Using lineage tracing, we show that new hair cells, predominantly outer hair cells, arise from Lgr5-expressing inner pillar and third Deiters cells and that new hair cell generation is increased by pharmacological inhibition of Notch. These data suggest that the neonatal mammalian cochlea has some capacity for hair cell regeneration following damage alone and that Lgr5-positive cells act as hair cell progenitors in the cochlea

A novel WFS1 mutation in a family with dominant low frequency sensorineural hearing loss with normal VEMP and EcochG findings

Background: Low frequency sensorineural hearing loss (LFSNHL) is an uncommon clinical finding. Mutations within three different identified genes (DIAPH1, MYO7A, and WFS1) are known to cause LFSNHL. The majority of hereditary LFSNHL is associated with heterozygous mutations in the WFS1 gene (wolframin protein). The goal of this study was to use genetic analysis to determine if a small American family's hereditary LFSNHL is linked to a mutation in the WFS1 gene and to use VEMP and EcochG testing to further characterize the family's audiovestibular phenotype. Methods: The clinical phenotype of the American family was characterized by audiologic testing, vestibular evoked myogenic potentials (VEMP), and electrocochleography (EcochG) evaluation. Genetic characterization was performed by microsatellite analysis and direct sequencing of WFS1 for mutation detection. Results: Sequence analysis of the WFS1 gene revealed a novel heterozygous mutation at c.2054G>C predicting a p.R685P amino acid substitution in wolframin. The c.2054G>C mutation segregates faithfully with hearing loss in the family and is absent in 230 control chromosomes. The p.R685 residue is located within the hydrophilic C-terminus of wolframin and is conserved across species. The VEMP and EcochG findings were normal in individuals segregating the WFS1 c.2054G>C mutation. Conclusion: We discovered a novel heterozygous missense mutation in exon 8 of WFS1 predicting a p.R685P amino acid substitution that is likely to underlie the LFSNHL phenotype in the American family. For the first time, we describe VEMP and EcochG findings for individuals segregating a heterozygous WFS1 mutation.NIH grants DC04945 (V.A.S), DC006901 (V.A.S.) and P30 DC04661 (V.M. Bloedel Core)

Predicting synapse counts in living humans by combining computational models with auditory physiology

Aging, noise exposure, and ototoxic medications lead to cochlear synapse loss in animal models. As cochlear function is highly conserved across mammalian species, synaptopathy likely occurs in humans as well. Synaptopathy is predicted to result in perceptual deficits including tinnitus, hyperacusis, and difficulty understanding speech-in-noise. The lack of a method for diagnosing synaptopathy in living humans hinders studies designed to determine if noise-induced synaptopathy occurs in humans, identify the perceptual consequences of synaptopathy, or test potential drug treatments. Several physiological measures are sensitive to synaptopathy in animal models includ-ing auditory brainstem response (ABR) wave I amplitude. However, it is unclear how to translate these measures to synaptopathy diagnosis in humans. This work demonstrates how a human computational model of the auditory periphery, which can predict ABR waveforms and distortion product otoacoustic emissions (DPOAEs), can be used to predict synaptic loss in individual human participants based on their measured DPOAE levels and ABR wave I amplitudes. Lower predicted synapse numbers were associated with advancing age, higher noise exposure history, increased likelihood of tinnitus, and poorer speech-in-noise perception. These findings demonstrate the utility of this modeling approach in predicting synapse counts from physiological data in individual human subjects

Auditory brainstem response demonstrates that reduced peripheral auditory input is associated with self-report of tinnitus

Tinnitus is one of the predicted perceptual consequences of cochlear synaptopathy, a type of age-, noise-, or drug-induced auditory damage that has been demonstrated in animal models to cause homeostatic changes in central auditory gain. Although synaptopathy has been observed in human temporal bones, assessment of this condition in living humans is limited to indirect non-invasive measures such as the auditory brainstem response (ABR). In animal models, synaptopathy is associated with a reduction in ABR wave I amplitude at suprathreshold stimulus levels. Several human studies have explored the relationship between wave I amplitude and tinnitus, with conflicting results. This study investigates the hypothesis that reduced peripheral auditory input due to synaptic/neuronal loss is associated with tinnitus. ABR wave I amplitude data from 193 individuals (43 with tinnitus (22%), 150 without tinnitus (78%)), who participated in up to three out of four different studies, were included in a logistic regression analysis to estimate the relationship between wave I amplitude and tinnitus at a variety of stimulus levels and frequencies. Statistical adjustment for sex and distortion product otoacoustic emissions was included in the analysis. The results suggest that smaller ABR wave I amplitudes are associated with an increased probability of reporting tinnitus