Deletion of Brg1 causes abnormal hair cell planer polarity, hair cell anchorage, and scar formation in mouse cochlea

The chromatin remodeler Brg1 is essential for cochlear sensory epithelium differentiation and patterning

Human genome analyses have revealed that abnormal BAF (BRG1/BRM-associated factor) complex is highly associated with hearing loss. However, the underlying pathogenesis remains largely unknown. Disrupted structure and function of the organ of Corti is the most prevalent cause of sensorineural hearing loss in mammals. Here, we investigated the role of Brg1-based BAF complex during the differentiation and development of the auditory sensory epithelium, a crucial period for the formation of the organ of Corti. Our findings indicate that deletion of Brg1 leads to premature hair cell (HC) differentiation by inactivating Sonic hedgehog (Shh) signaling. Despite the formation of HCs, subsequent differentiation of inner hair cells (IHCs) and outer hair cells (OHCs) was impaired. Additionally, we observed that the mosaic-like arrangement of HCs and supporting cells (SCs) was disrupted resulting in abnormal sensory epithelium patterning. Furthermore, we found the planar cell polarity of the Brg1-deficient cochlea was abnormal. Our study demonstrates the pivotal role of Brg1 in the differentiation and patterning of the organ of Corti.

ATP-Dependent Chromatin Remodellers in Inner Ear Development

During transcription, DNA replication and repair, chromatin structure is constantly modified to reveal specific genetic regions and allow access to DNA-interacting enzymes. ATP-dependent chromatin remodelling complexes use the energy of ATP hydrolysis to modify chromatin architecture by repositioning and rearranging nucleosomes. These complexes are defined by a conserved SNF2-like, catalytic ATPase subunit and are divided into four families: CHD, SWI/SNF, ISWI and INO80. ATP-dependent chromatin remodellers are crucial in regulating development and stem cell biology in numerous organs, including the inner ear. In addition, mutations in genes coding for proteins that are part of chromatin remodellers have been implicated in numerous cases of neurosensory deafness. In this review, we describe the composition, structure and functional activity of these complexes and discuss how they contribute to hearing and neurosensory deafness.

Epigenetic mechanisms of inner ear development

Epigenetic factors are critically important for embryonic and postnatal development. Over the past decade, substantial technological advancements have occurred that now permit the study of epigenetic mechanisms that govern all aspects of inner ear development, from otocyst patterning to maturation and maintenance of hair cell stereocilia. In this review, we highlight how three major classes of epigenetic regulation (DNA methylation, histone modification, and chromatin remodeling) are essential for the development of the inner ear. We highlight open avenues for research and discuss how new tools enable the employment of epigenetic factors in regenerative and therapeutic approaches for hearing and balance disorders.

Loss of BAF (mSWI/SNF) chromatin-remodeling ATPase Brg1 causes multiple malformations of cortical development in mice

Mutations in genes encoding subunits of the BAF (BRG1/BRM-associated factor) complex cause various neurodevelopmental diseases. However, the underlying pathophysiology remains largely unknown. Here, we analyzed the function of Brg1, a core ATPase of BAF complexes, in the developing cerebral cortex. Loss of Brg1 causes several morphological defects resembling human malformations of cortical development (MCDs), including microcephaly, cortical dysplasia, cobblestone lissencephaly, and periventricular heterotopia. We demonstrated that neural progenitor cell (NPC) renewal, neuronal differentiation, neuronal migration, apoptotic cell death, pial basement membrane, and apical junctional complexes, which are associated with MCD formation, were impaired after Brg1 deletion. Furthermore, transcriptome profiling indicated that a large number of genes were deregulated. The deregulated genes were closely related to MCD formation, and most of these genes were bound by Brg1. Cumulatively, our study indicates an essential role of Brg1 in cortical development and provides a new possible pathogenesis underlying Brg1-based BAF complex-related neurodevelopmental disorders.

Multiple PDZ domain protein maintains patterning of the apical cytoskeleton in sensory hair cells

Sound transduction occurs in the hair bundle, the apical compartment of sensory hair cells in the inner ear. The hair bundle is formed of actin-based stereocilia aligned in rows of graded heights. It was previously shown that the GNAI-GPSM2 complex is part of a developmental blueprint that defines the polarized organization of the apical cytoskeleton in hair cells, including stereocilia distribution and elongation. Here, we report a role for multiple PDZ domain (MPDZ) protein during apical hair cell morphogenesis in mouse. We show that MPDZ is enriched at the hair cell apical membrane along with MAGUK p55 subfamily member 5 (MPP5/PALS1) and the Crumbs protein CRB3. MPDZ is required there to maintain the proper segregation of apical blueprint proteins, including GNAI-GPSM2. Loss of the blueprint coincides with misaligned stereocilia placement in Mpdz mutant hair cells, and results in permanently misshapen hair bundles. Graded molecular and structural defects along the cochlea can explain the profile of hearing loss in Mpdz mutants, where deficits are most severe at high frequencies.

Espin distribution as revealed by super-resolution microscopy of stereocilia

Auditory hair cells are the mechanical sensors of sound waves in the inner ear, and the stereocilia, which are actin-rich protrusions of different heights on the apical surfaces of hair cells, are responsible for the transduction of sound waves into electrical signals. As a crucial actin-binding and bundling protein, espin is able to cross-link actin filaments and is therefore necessary for stereocilia morphogenesis. Using advanced super-resolution stimulated emission depletion microscopy, we imaged espin expression at the sub-diffraction limit along the whole length of the stereocilia in outer hair cells and inner hair cells in order to better understand espin's function in the development of stereocilia.

Deletion of Brg1 causes stereocilia bundle fusion and cuticular plate loss in vestibular hair cells

Brg1 is an ATPase subunit of the SWI/SNF chromatin-remodeling complex, and it is indispensable for the development and homeostasis of various organs. Conditional deletion of Brg1 in cochlea hair cells (HCs) leads to multiple structural defects and profound deafness. However, the premature death of Brg1-deficient cochlea HCs hindered further study of the role of Brg1. In contrast to cochlea HCs, Brg1-deficient vestibular HCs survived for a long time. Therefore, HC apical structure and vestibular function were examined in inner HC-specific conditional Brg1 knockout mice. Vestibular HCs exhibited fused and elongated stereocilia bundles after deletion of Brg1, and the cuticular plate was absent in most HCs with fused stereocilia bundles. HC loss was observed in conditional Brg1 knockout mice at the age of 12 months. Morphological defects and HC loss were primarily restricted in the striolar region of the utricle and saccule and in the central region of ampulla. The behavioral tests revealed that Brg1 deletion in HCs caused vestibular dysfunction in older adult mice. These results suggest that Brg1 may play specific roles in the maintenance of the HC stereocilia bundle and the cuticular plate.

The endocochlear potential as an indicator of reticular lamina integrity after noise exposure in mice

The endocochlear potential (EP) provides part of the electrochemical drive for sound-driven currents through cochlear hair cells. Intense noise exposure (110 dB SPL, 2 h) differentially affects the EP in three inbred mouse strains (C57BL/6 [B6], CBA/J [CBA], BALB/cJ [BALB]) (Ohlemiller and Gagnon, 2007, Hearing Research 224:34-50; Ohlemiller et al., 2011, JARO 12:45-58). At least for mice older than 3 mos, B6 mice are unaffected, CBA mice show temporary EP reduction, and BALB mice may show temporary or permanent EP reduction. EP reduction was well correlated with histological metrics for injury to stria vascularis and spiral ligament, and little evidence was found for holes or tears in the reticular lamina that might 'short out' the EP. Thus we suggested that the genes and processes that underlie the strain EP differences primarily impact cochlear lateral wall, not the organ of Corti. Our previous work did not test the range of noise exposure conditions over which strain differences apply. It therefore remained possible that the relation between exposure severity and acute EP reduction simply has a higher exposure threshold in B6 mice compared to CBA and BALB. We also did not test for age dependence. It is well established that young adult animals are especially vulnerable to noise-induced permanent threshold shifts (NIPTS). It is unknown, however, whether heightened vulnerability of the lateral wall contributes to this condition. The present study extends our previous work to multiple noise exposure levels and durations, and explicitly compares young adult (6-7 wks) and older mice (>4 mos). We find that the exposure level-versus-acute EP relation is dramatically strain-dependent, such that B6 mice widely diverge from both CBA and BALB. For all three strains, however, acute EP reduction is greater in young mice. Above 110 dB SPL, all mice exhibited rapid and severe EP reduction that is likely related to tearing of the reticular lamina. By contrast, EP-versus-noise duration examined at 104 dB suggested that different processes contribute to EP reduction in young and older mice. The average EP falls to a constant level after ∼7.5 min in older mice, but progressively decreases with further exposure in young mice. Confocal microscopy of organ of Corti surface preparations stained for phalloidin and zonula occludens-1 (ZO-1) indicated this corresponds to rapid loss of outer hair cells (OHCs) and formation of both holes and tears in the reticular lamina of young mice. In addition, when animals exposed at 119 dB were allowed to recover for 1 mo, only young B6 mice showed collapse of the EP to ≤5 mV. Confocal analysis suggested novel persistent loss of tight junctions in the lateral organ of Corti. This may allow paracellular leakage that permanently reduces the EP. From our other findings, we propose that noise-related lateral wall pathology in young CBA and BALB mice promotes hair cell loss and opening of the reticular lamina. The heightened vulnerability of young adult animals to noise exposure may in part reflect special sensitivity of the organ of Corti to acute lateral wall dysfunction at younger ages. This feature appears genetically modifiable.

Loss of Myh14 Increases Susceptibility to Noise-Induced Hearing Loss in CBA/CaJ Mice

MYH14 is a member of the myosin family, which has been implicated in many motile processes such as ion-channel gating, organelle translocation, and the cytoskeleton rearrangement. Mutations in MYH14 lead to a DFNA4-type hearing impairment. Further evidence also shows that MYH14 is a candidate noise-induced hearing loss (NIHL) susceptible gene. However, the specific roles of MYH14 in auditory function and NIHL are not fully understood. In the present study, we used CRISPR/Cas9 technology to establish a Myh14 knockout mice line in CBA/CaJ background (now referred to as Myh14^−/− mice) and clarify the role of MYH14 in the cochlea and NIHL. We found that Myh14^−/− mice did not exhibit significant hearing loss until five months of age. In addition, Myh14^−/− mice were more vulnerable to high intensity noise compared to control mice. More significant outer hair cell loss was observed in Myh14^−/− mice than in wild type controls after acoustic trauma. Our findings suggest that Myh14 may play a beneficial role in the protection of the cochlea after acoustic overstimulation in CBA/CaJ mice.