Neurotrophin-3 (Ntf3) and brain derived neurotrophic factor (Bdnf) are critical for sensory neuron survival and establishment of neuronal projections to sensory epithelia in the embryonic inner ear, but their postnatal functions remain poorly understood. Using cell-specific inducible gene recombination in mice we found that, in the postnatal inner ear, Bbnf and Ntf3 are required for the formation and maintenance of hair cell ribbon synapses in the vestibular and cochlear epithelia, respectively. We also show that supporting cells in these epithelia are the key endogenous source of the neurotrophins. Using a new hair cell CreER^T line with mosaic expression, we also found that Ntf3's effect on cochlear synaptogenesis is highly localized. Moreover, supporting cell-derived Ntf3, but not Bbnf, promoted recovery of cochlear function and ribbon synapse regeneration after acoustic trauma. These results indicate that glial-derived neurotrophins play critical roles in inner ear synapse density and synaptic regeneration after injury.

Hypotonic stress causes the activation of swelling-activated nonselective cation channels (NSCCs), which leads to Ca2+-dependent regulatory volume decrease (RVD) and adaptive maintenance of the cell volume; however, the molecular identities of the osmosensitive NSCCs remain unclear. Here, we identified TMEM63B as an osmosensitive NSCC activated by hypotonic stress. TMEM63B is enriched in the inner ear sensory hair cells. Genetic deletion of TMEM63B results in necroptosis of outer hair cells (OHCs) and progressive hearing loss. Mechanistically, the TMEM63B channel mediates hypo-osmolarity-induced Ca2+ influx, which activates Ca2+-dependent K+ channels required for the maintenance of OHC morphology. These findings demonstrate that TMEM63B is an osmosensor of the mammalian inner ear and the long-sought cation channel mediating Ca2+-dependent RVD.

MicroRNAs are small noncoding RNAs that serve as important regulators of eukaryotic gene expression and are emerging as novel diagnostic and therapeutic targets for human diseases. Robust and reliable detection of miRNAs is an essential step for understanding the functional significance of these small RNAs in both physiological and pathological processes. Existing methods for miRNA quantification rely on fluorescent probes for optimal specificity. In this study, we developed a high-performance real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay that allows specific and rapid detection of mature miRNAs using a fast thermocycling profile (10 sec per cycle). This assay exhibited a wide dynamic range (>7 logs) and was capable of detecting miRNAs from as little as 1 pg of the total RNA or as few as 10 cells. The use of modified reverse-transcription oligonucleotides with a secondary structure and hemi-nested reverse PCR primers allowed excellent discrimination of mature miRNAs from their precursors and highly homologous family members using SYBR Green I. Using a novel approach involving uracil-DNA glycosylase treatment, we showed that carryover of the reverse transcription oligonucleotide to the PCR can be successfully eliminated and discrimination between miRNA homologs could be further enhanced. These assays were further extended for multiplexed detection of miRNAs directly from cell lysates without laborious total RNA isolation. With the robust performance of these assays, we identified several miRNAs that were regulated by glial cell-line-derived neurotrophic factor in human glioblastoma cells. In summary, this method could provide a useful tool for rapid, robust, and cost-effective quantification of existing and novel miRNAs.

Significance: The auditory sensory epithelium contains two major cell types: hair cells and supporting cells. Mammalian auditory hair cells do not regenerate after damage or loss, resulting in permanent hearing impairment. How supporting cell loss affects auditory function remains to be determined. Here, we demonstrate that inner border and inner phalangeal cells, the two types of supporting cells surrounding inner hair cells, can be replenished completely after selective ablation in the neonatal cochlea, allowing hearing to be preserved. Our findings challenge the view that mammalian auditory sensory epithelium has limited intrinsic regenerative capacity and provide previously unindetified opportunities for replacement of damaged auditory cells and restoration of hearing.