Absence of plastin 1 causes abnormal maintenance of hair cell stereocilia and a moderate form of hearing loss in mice

Stereocilia fusion pathology in the cochlear outer hair cells at the nanoscale level

The hair bundle of cochlear hair cells comprises specialized microvilli, the stereocilia, which fulfil the role of mechanotransduction. Genetic defects and environmental noise challenge the maintenance of hair bundle structure, critically contributing to age-related hearing loss. Stereocilia fusion is a major component of the hair bundle pathology in mature hair cells, but its role in hearing loss and its molecular basis are poorly understood. Here, we utilized super-resolution expansion microscopy to examine the molecular anatomy of outer hair cell stereocilia fusion in mouse models of age-related hearing loss, heightened endoplasmic reticulum stress and prolonged noise exposure. Prominent stereocilia fusion in our model of heightened endoplasmic reticulum stress, Manf (Mesencephalic astrocyte-derived neurotrophic factor)-inactivated mice in a background with Cadherin 23 missense mutation, impaired mechanotransduction and calcium balance in stereocilia. This was indicated by reduced FM1-43 dye uptake through the mechanotransduction channels, reduced neuroplastin/PMCA2 expression and increased expression of the calcium buffer oncomodulin inside stereocilia. Sparse BAIAP2L2 and myosin 7a expression was retained in the fused stereocilia but mislocalized away from their functional sites at the tips. These hair bundle abnormalities preceded cell soma degeneration, suggesting a sequela from stereociliary molecular perturbations to cell death signalling. In the age-related hearing loss and noise-exposure models, stereocilia fusion was more restricted within the bundles, yet both models exhibited oncomodulin upregulation at the fusion sites, implying perturbed calcium homeostasis. We conclude that stereocilia fusion is linked with the failure to maintain cellular proteostasis and with disturbances in stereociliary calcium balance. KEY POINTS: Stereocilia fusion is a hair cell pathology causing hearing loss. Inactivation of Manf, a component of the endoplasmic reticulum proteostasis machinery, has a cell-intrinsic mode of action in triggering outer hair cell stereocilia fusion and the death of these cells. The genetic background with Cadherin 23 missense mutation contributes to the high susceptibility of outer hair cells to stereocilia fusion, evidenced in Manf-inactivated mice and in the mouse models of early-onset hearing loss and noise exposure. Endoplasmic reticulum stress feeds to outer hair cell stereocilia bundle pathology and impairs the molecular anatomy of calcium regulation. The maintenance of the outer hair cell stereocilia bundle cohesion is challenged by intrinsic and extrinsic stressors, and understanding the underlying mechanisms will probably benefit the development of interventions to promote hearing health.

Mutation analysis of GJB2, SLC26A4, GJB3 and mtDNA12SrRNA genes in 251 non-syndromic hearing loss patients in Fujian, China

OBJECTIVES

The molecular etiology of non-syndromic hearing loss (NSHL) in Southeastern China (Fujian) has not been precisely identified. our study selected patients with NSHL and analyzed their causative genes, which helped to improve the accuracy of the diagnosis of hereditary hearing loss (HHL) and its treatment.

METHODS

251 unrelated patients who attended the otolaryngology clinic of Fujian Maternal and Child Health Hospital with hearing loss were enrolled to our study. All patients had genetic tests and listening tests, of which 251 were diagnosed with NSHL. In addition, we used whole-exome sequencing (WES) in a patient who has a significant family history of HHL but negative for gene chip testing, as well as in his family members.

RESULT

Among of 251 patients, Nucleotide changes were found in 63 cases (25.09%), including 34 located in GJB2(13.5%, including 235delC and 299_300delAT), 13 located in SLC26A4(5.18%, including c.919-2G > A and 2168 A > G), 1 located in GJB3(0.4%,538C > T) and 16 located in mtDNA12SrRNA (6.37%,1555 A > G). In addition, we discuss the process of identifying novel PLS1 mutations from 251 patients.

CONCLUSION

Our results demonstrate the conventional deafness gene mutation in 251 NSHL patients in Fujian, China. Compared with the other area of China, we have a lower detection rate, but GJB2 235delC remains the most common mutation in Fujian. In addition, we discuss the process of discovering novel mutation locus for deafness, which provides an understanding for deafness diagnosis and genetic testing.

The actin cytoskeleton in hair bundle development and hearing loss

Inner ear hair cells assemble mechanosensitive hair bundles on their apical surface that transduce sounds and accelerations. Each hair bundle is comprised of ∼ 100 individual stereocilia that are arranged into rows of increasing height and width; their specific and precise architecture being necessary for mechanoelectrical transduction (MET). The actin cytoskeleton is fundamental to establishing this architecture, not only by forming the structural scaffold shaping each stereocilium, but also by composing rootlets and the cuticular plate that together provide a stable foundation supporting each stereocilium. In concert with the actin cytoskeleton, a large assortment of actin-binding proteins (ABPs) function to cross-link actin filaments into specific topologies, as well as control actin filament growth, severing, and capping. These processes are individually critical for sensory transduction and are all disrupted in hereditary forms of human hearing loss. In this review, we provide an overview of actin-based structures in the hair bundle and the molecules contributing to their assembly and functional properties. We also highlight recent advances in mechanisms driving stereocilia elongation and how these processes are tuned by MET.

A novel PLS1 c.981+1G>A variant causes autosomal-dominant hereditary hearing loss in a family

The fimbrin protein family contains a variety of proteins, among which Plastin1 (PLS1) is an important member. According to recent studies, variations in the coding region of the PLS1 gene are associated with the development of deafness. However, the molecular mechanism of deafness caused by PLS1 gene variants remains unknown. Whole-exome sequencing was performed on hearing-impaired family members and hearing family members to identify pathogenic variants, followed by Sanger sequencing. A minigene assay was conducted to investigate the effect of the variant on PLS1 mRNA splicing. The pathogenicity of the variant was further investigated in zebrafish. RNA-sequencing (RNA-seq) was performed to analyze the dysregulation of downstream signaling pathways caused by knockdown of PLS1 expression. We identified a novel variant, PLS1 c.981+1G>A, in a large Chinese family with hearing loss and showed that the variant is responsible for the occurrence of hearing loss by inducing exon 8 skipping. The variant caused abnormal inner ear phenotypes, characterized by decreases in the mean otolith distance, anterior otolith diameter, posterior otolith diameter, cochlear diameter, and swimming speed and distance in zebrafish. Furthermore, silencing PLS1 expression significantly upregulated the expression of genes in the PI3K-Akt signaling pathway, including Col6a3, Spp1, Itgb3 and hepatocyte growth factor (Hgf). PLS1 c.981+1G>A is a novel pathogenic variant causing hearing loss by inducing exon 8 skipping. Upregulation of the expression of genes in the PI3K-Akt signaling pathway plays an important role in the pathogenesis caused by variants in the PLS1 gene.

The morphological and functional diversity of apical microvilli

Sensory neurons use specialized apical processes to perceive external stimuli and monitor internal body conditions. The apical apparatus can include cilia, microvilli, or both, and is adapted for the functions of the particular cell type. Photoreceptors detect light through a large, modified cilium (outer segment), that is supported by a surrounding ring of microvilli-like calyceal processes (CPs). Although first reported 150 years ago, CPs remain poorly understood. As a basis for future study, we therefore conducted a review of existing literature about sensory cell microvilli, which can act either as the primary sensory detector or as support for a cilia-based detector. While all microvilli are finger-like cellular protrusions with an actin core, the processes vary across cell types in size, number, arrangement, dynamics, and function. We summarize the current state of knowledge about CPs and the characteristics of the microvilli found on inner ear hair cells (stereocilia) and cerebral spinal fluid-contacting neurons, with comparisons to the brush border of the intestinal and renal epithelia. The structure, stability, and dynamics of the actin core are regulated by a complement of actin-binding proteins, which includes both common components and unique features when compared across cell types. Further, microvilli are often supported by lateral links, a glycocalyx, and a defined extracellular matrix, each adapted to the function and environment of the cell. Our comparison of microvillar features will inform further research into how CPs support photoreceptor function, and also provide a general basis for investigations into the structure and functions of apical microvilli found on sensory neurons.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

FCHSD2 is required for stereocilia maintenance in mouse cochlear hair cells

Stereocilia are F-actin-based protrusions on the apical surface of inner-ear hair cells and are indispensable for hearing and balance perception. The stereocilia of each hair cell are organized into rows of increasing heights, forming a staircase-like pattern. The development and maintenance of stereocilia are tightly regulated, and deficits in these processes lead to stereocilia disorganization and hearing loss. Previously, we showed that the F-BAR protein FCHSD2 is localized along the stereocilia of cochlear hair cells and cooperates with CDC42 to regulate F-actin polymerization and cell protrusion formation in cultured COS-7 cells. In the present work, Fchsd2 knockout mice were established to investigate the role of FCHSD2 in hearing. Our data show that stereocilia maintenance is severely affected in cochlear hair cells of Fchsd2 knockout mice, which leads to progressive hearing loss. Moreover, Fchsd2 knockout mice show increased acoustic vulnerability. Noise exposure causes robust stereocilia degeneration as well as enhanced hearing threshold elevation in Fchsd2 knockout mice. Lastly, Fchsd2/Cdc42 double knockout mice show more severe stereocilia deficits and hearing loss, suggesting that FCHSD2 and CDC42 cooperatively regulate stereocilia maintenance.

ROS-Induced Oxidative Damage and Mitochondrial Dysfunction Mediated by Inhibition of SIRT3 in Cultured Cochlear Cells

Sensorineural hearing loss (SNHL) is one of the most common causes of disability worldwide. Previous evidence suggests that reactive oxygen species (ROS) may play an important role in the occurrence and development of SNHL, while its mechanism remains unclear. We cultured dissected organs of Corti in medium containing different concentrations (0, 0.25, 0.5, 0.75, 1, and 1.25 mM) of hydrogen peroxide (H₂O₂) and established a four-concentration model of 0, 0.5, 0.75, and 1 mM to study different degrees of damage. We examined ROS-induced mitochondrial damage and the role of sirtuin 3 (SIRT3). Our results revealed that the number of ribbon synapses and hair cells appeared significantly concentration-dependent decrease with exposure to H₂O₂. Outer hair cells (OHCs) and inner hair cells (IHCs) began to be lost, and activation of apoptosis of hair cells (HCs) was observed at 0.75 mM and 1 mM H₂O₂, respectively. In contrast with the control group, the accumulation of ROS was significantly higher, and the mitochondrial membrane potential (MMP) was lower in the H₂O₂-treated groups. Furthermore, the expression of SIRT3, FOXO3A, and SOD2 proteins declined, except for an initial elevation of SIRT3 between 0 and 0.75 mM H₂O₂. Administration of the selective SIRT3 inhibitor 3-(1H-1,2,3-triazol-4-yl) pyridine resulted in increased damage to the cochlea, including loss of ribbon synapses and hair cells, apoptosis of hair cells, more production of ROS, and reduced mitochondrial membrane potential. Thoroughly, our results highlight that ROS-induced mitochondrial oxidative damage drives hair cell degeneration and apoptosis. Furthermore, SIRT3 is crucial for preserving mitochondrial function and protecting the cochlea from oxidative damage and may represent a possible therapeutic target for SNHL.

Dimensions of a Living Cochlear Hair Bundle

The hair bundle is the mechanosensory organelle of hair cells that detects mechanical stimuli caused by sounds, head motions, and fluid flows. Each hair bundle is an assembly of cellular-protrusions called stereocilia, which differ in height to form a staircase. Stereocilia have different heights, widths, and separations in different species, sensory organs, positions within an organ, hair-cell types, and even within a single hair bundle. The dimensions of the stereociliary assembly dictate how the hair bundle responds to stimuli. These hair-bundle properties have been measured previously only to a limited degree. In particular, mammalian data are either incomplete, lack control for age or position within an organ, or have artifacts owing to fixation or dehydration. Here, we provide a complete set of measurements for postnatal day (P) 11 C57BL/6J mouse apical inner hair cells (IHCs) obtained from living tissue, tissue mildly-fixed for fluorescent imaging, or tissue strongly fixed and dehydrated for scanning electronic microscopy (SEM). We found that hair bundles mildly-fixed for fluorescence had the same dimensions as living hair bundles, whereas SEM-prepared hair bundles shrank uniformly in stereociliary heights, widths, and separations. By determining the shrinkage factors, we imputed live dimensions from SEM that were too small to observe optically. Accordingly, we created the first complete blueprint of a living IHC hair bundle. We show that SEM-prepared measurements strongly affect calculations of a bundle’s mechanical properties – overestimating stereociliary deflection stiffness and underestimating the fluid coupling between stereocilia. The methods of measurement, the data, and the consequences we describe illustrate the high levels of accuracy and precision required to understand hair-bundle mechanotransduction.

Comparative analysis of the MyTH4-FERM myosins reveals insights into the determinants of actin track selection in polarized epithelia

MyTH4-FERM (MF) myosins evolved to play a role in the creation and function of a variety of actin-based membrane protrusions that extend from cells. Here we performed an analysis of the MF myosins, Myo7A, Myo7B, and Myo10, to gain insight into how they select for their preferred actin networks. Using enterocytes that create spatially separated actin tracks in the form of apical microvilli and basal filopodia, we show that actin track selection is principally guided by the mode of oligomerization of the myosin along with the identity of the motor domain, with little influence from the specific composition of the lever arm. Chimeric variants of Myo7A and Myo7B fused to a leucine zipper parallel dimerization sequence in place of their native tails both selected apical microvilli as their tracks, while a truncated Myo10 used its native antiparallel coiled-coil to traffic to the tips of filopodia. Swapping lever arms between the Class 7 and 10 myosins did not change actin track preference. Surprisingly, fusing the motor-neck region of Myo10 to a leucine zipper or oligomerization sequences derived from the Myo7A and Myo7B cargo proteins USH1G and ANKS4B, respectively, re-encoded the actin track usage of Myo10 to apical microvilli with significant efficiency.

NAA80 bi-allelic missense variants result in high-frequency hearing loss, muscle weakness and developmental delay

The recent identification of NAA80/NAT6 as the enzyme that acetylates actins generated new insight into the process of post-translational actin modifications; however, the role of NAA80 in human physiology and pathology has not been clarified yet. We report two individuals from a single family harbouring a homozygous c.389T>C, p.(Leu130Pro) NAA80 genetic variant. Both individuals show progressive high-frequency sensorineural hearing loss, craniofacial dysmorphisms, developmental delay and mild proximal and axial muscle weakness. Based on the molecular structure, we predicted and confirmed the NAA80 c.389T>C, p.(Leu130Pro) variant to result in protein destabilization, causing severely decreased NAA80 protein availability. Concurrently, individuals exhibited a ∼50% decrease of actin acetylation. NAA80 individual derived fibroblasts and peripheral blood mononuclear cells showed increased migration, increased filopodia counts and increased levels of polymerized actin, in agreement with previous observations in NAA80 knock-out cells. Furthermore, the significant clinical overlap between NAA80 individuals and individuals with pathogenic variants in several actin subtypes reflects the general importance of controlled actin dynamics for the inner ear, brain and muscle. Taken together, we describe a new syndrome, caused by NAA80 genetic variants leading to decreased actin acetylation and disrupted associated molecular functions. Our work suggests a crucial role for NAA80-mediated actin dynamics in neuronal health, muscle health and hearing.

Inner hair cell dysfunction in Klhl18 mutant mice leads to low frequency progressive hearing loss

Age-related hearing loss in humans (presbycusis) typically involves impairment of high frequency sensitivity before becoming progressively more severe at lower frequencies. Pathologies initially affecting lower frequency regions of hearing are less common. Here we describe a progressive, predominantly low-frequency recessive hearing impairment in two mutant mouse lines carrying different mutant alleles of the Klhl18 gene: a spontaneous missense mutation (Klhl18lowf) and a targeted mutation (Klhl18tm1a(KOMP)Wtsi). Both males and females were studied, and the two mutant lines showed similar phenotypes. Threshold for auditory brainstem responses (ABR; a measure of auditory nerve and brainstem neural activity) were normal at 3 weeks old but showed progressive increases from 4 weeks onwards. In contrast, distortion product otoacoustic emission (DPOAE) sensitivity and amplitudes (a reflection of cochlear outer hair cell function) remained normal in mutants. Electrophysiological recordings from the round window of Klhl18lowf mutants at 6 weeks old revealed 1) raised compound action potential thresholds that were similar to ABR thresholds, 2) cochlear microphonic potentials that were normal compared with wildtype and heterozygous control mice and 3) summating potentials that were reduced in amplitude compared to control mice. Scanning electron microscopy showed that Klhl18lowf mutant mice had abnormally tapering of the tips of inner hair cell stereocilia in the apical half of the cochlea while their synapses appeared normal. These results suggest that Klhl18 is necessary to maintain inner hair cell stereocilia and normal inner hair cell function at low frequencies.

Synaptojanin2 Mutation Causes Progressive High-frequency Hearing Loss in Mice

Progressive hearing loss is very common in the human population but we know little about the underlying molecular mechanisms. Synaptojanin2 (Synj2) has been reported to be involved, as a mouse mutation led to a progressive increase in auditory thresholds with age. Synaptojanin2 is a phosphatidylinositol (PI) phosphatase that removes the five-position phosphates from phosphoinositides, such as PIP₂ and PIP₃, and is a key enzyme in clathrin-mediated endocytosis. To investigate the mechanisms underlying progressive hearing loss, we have studied a different mutation of mouse Synj2 to look for any evidence of involvement of vesicle trafficking particularly affecting the synapses of sensory hair cells. Auditory brainstem responses (ABR) developed normally at first but started to decline between 3 and 4 weeks of age in Synj2^tm1b mutants. At 6 weeks old, some evidence of outer hair cell (OHC) stereocilia fusion and degeneration was observed, but this was only seen in the extreme basal turn so cannot explain the raised ABR thresholds that correspond to more apical regions of the cochlear duct. We found no evidence of any defect in inner hair cell (IHC) exocytosis or endocytosis using single hair cell recordings, nor any sign of hair cell synaptic abnormalities. Endocochlear potentials (EP) were normal. The mechanism underlying progressive hearing loss in these mutants remains elusive, but our findings of raised distortion product otoacoustic emission (DPOAE) thresholds and signs of OHC degeneration both suggest an OHC origin for the hearing loss. Synaptojanin2 is not required for normal development of hearing but it is important for its maintenance.

MiR-1908/EXO1 and MiR-203a/FOS, regulated by scd1, are associated with fracture risk and bone health in postmenopausal diabetic women

BACKGROUND

Stearoyl-coenzyme A desaturase-1 (SCD1) can inhibit the development of diabetic bone disease by promoting osteogenesis. In this study, we examined whether this regulation by SCD1 is achieved by regulating the expression of related miRNAs.

METHODS

SCD1 expression levels were observed in human bone-marrow mesenchymal stem cells (BM-MSCs) of patients with type 2 diabetes mellitus (T2DM), and the effect of SCD1 on osteogenesis was observed in human adipose-derived MSCs transfected with the SCD1 lentiviral system. We designed a bioinformatics prediction model to select important differentially expressed miRNAs, and established protein-protein interaction and miRNA-mRNA networks. miRNAs and mRNAs were extracted and their differential expression was detected. The SCD1-miRNA-mRNA network was validated.

FINDINGS

SCD1 expression in bone marrow was downregulated in patients with T2DM and low-energy fracture, and SCD1 expression promotes BM-MSC osteogenic differentiation. The predictors in the nomogram were seven microRNAs, including hsa-miR-1908 and hsa-miR-203a. SCD1 inhibited the expression of CDKN1A and FOS, but promoted the expression of EXO1 and PLS1. miR-1908 was a regulator of EXO1 expression, and miR-203a was a regulator of FOS expression.

INTERPRETATION

The regulation of BM-MSCs by SCD1 is a necessary condition for osteogenesis through the miR-203a/FOS and miR-1908/EXO1 regulatory pathways.

Plastin 1 promotes osteoblast differentiation by regulating intracellular Ca2

Osteoblast differentiation is a key process in bone homeostasis. Mutations in plastin 3 have been reported to be responsible for X-linked osteoporosis. Plastin 3 and plastin 2 act synergistically to regulate osteoblast differentiation. However, the bone-related function of plastin 1, another family member of plastins, has not been assessed. In this study, we addressed the functional importance of plastin 1 in osteoblasts. We characterized the expression patterns of plastin 1 during osteoblast differentiation and revealed its important role in this process. In both HEK 293T and hFOB1.19 cells, plastin 1 was demonstrated to regulate intracellular Ca2+. Accordingly, we revealed that higher Ca2+ concentration promotes osteoblast differentiation. Finally, we found that plastin 1 may play a compensatory role in osteoporosis patients with plastin 3 deficiency. Together, our results indicate that plastin 1 promotes osteoblast differentiation by regulating intracellular Ca2+. Our work sheds new light on the role played by plastins in bone homeostasis.

D-Galactose-induced oxidative stress and mitochondrial dysfunction in the cochlear basilar membrane: an in vitro aging model

The cochlear basilar membrane (CBM) contains inner hair cells and outer hair cells that convert sound waves into electrical signals and transmit them to the central auditory system. Cochlear aging, the primary reason of age-related hearing loss, can reduce the signal transmission capacity. There is no ideal in vitro aging model of the CBM. In this study, we cultured the CBM, which was dissected from the cochlea of the C57BL/6 mice 5 days after birth, in a medium containing 20 mg/mL, 40 mg/mL, or 60 mg/mL D-galactose (D-gal). Compared with the control group, the levels of senescence-associated β-galactosidase were increased in a concentration-dependent manner in the CBM of the D-gal groups. In addition, levels of the mitochondrial superoxide and patterns of an age-related mitochondrial DNA3860-bp deletion were significantly increased. The ATP levels and the membrane potential of the mitochondrial were significantly decreased in the CBM of the D-gal groups compared with the control group. Furthermore, in comparison with the control group, damaged hair cell stereocilia and a loss of inner hair cell ribbon synapses were observed in the CBM of the D-gal groups. A loss of hair cells and activation of caspase-3-mediated outer hair cell apoptosis were also observed in the CBM of the high-dose D-gal group. These insults induced by D-gal in the CBM in vitro were similar to the ones that occur in cochlear natural aging in vivo. Thus, we believe that this is a successful in vitro aging model using cultured CBM. These results demonstrate the effects of mitochondrial oxidative damage on presbycusis and provide a reliable aging model to study the mechanisms of presbycusis in vitro.

Transcriptomic Analyses of Inner Ear Sensory Epithelia in Zebrafish

Analysis of gene expression has the potential to assist in the understanding of multiple cellular processes including proliferation, cell-fate specification, senesence, and activity in both healthy and disease states. Zebrafish model has been increasingly used to understand the process of hearing and the development of the vertebrate auditory system. Within the zebrafish inner ear, there are three otolith organs, each containing a sensory macula of hair cells. The saccular macula is primarily involved in hearing, the utricular macula is primarily involved in balance and the function of the lagenar macula is not completely understood. The goal of this study is to understand the transcriptional differences in the sensory macula associated with different otolith organs with the intention of understanding the genetic mechanisms responsible for the distinct role each organ plays in sensory perception. The sensory maculae of the saccule, utricle, and lagena were dissected out of adult Et(krt4:GFP)^sqet4 zebrafish expressing green fluorescent protein in hair cells for transcriptional analysis. The total RNAs of the maculae were isolated and analyzed by RNA GeneChip microarray. Several of the differentially expressed genes are known to be involved in deafness, otolith development and balance. Gene expression among these otolith organs was very well conserved with less than 10% of genes showing differential expression. Data from this study will help to elucidate which genes are involved in hearing and balance. Furthermore, the findings of this study will assist in the development of the zebrafish model for human hearing and balance disorders. Anat Rec, 303:527-543, 2020. © 2019 American Association for Anatomy.

Mutations in PLS1, encoding fimbrin, cause autosomal dominant nonsyndromic hearing loss

Nonsyndromic hearing loss (NSHL), a common sensory disorder, is characterized by high clinical and genetic heterogeneity (i.e., approximately 115 genes and 170 loci so far identified). Nevertheless, almost half of patients submitted for genetic testing fail to receive a conclusive molecular diagnosis. We used next-generation sequencing to identify causal variants in PLS1 (c.805G>A, p.[E269K]; c.713G>T, p.[L238R], and c.383T>C, p.[F128S]) in three unrelated families of European ancestry with autosomal dominant NSHL. PLS1 encodes Plastin 1 (also called fimbrin), one of the most abundant actin-bundling proteins of the stereocilia. In silico protein modeling suggests that all variants destabilize the structure of the actin-binding domain 1, likely reducing the protein's ability to bind F actin. The role of PLS1 gene in hearing function is further supported by the recent demonstration that Pls1^{-/ -} mice show a hearing loss phenotype similar to that of our patients. In summary, we report PLS1 as a novel gene for autosomal dominant NSHL, suggesting that this gene is required for normal hearing in humans and mice.

Genomic architecture of Shh-dependent cochlear morphogenesis

The mammalian cochlea develops from a ventral outgrowth of the otic vesicle in response to Shh signaling. Mouse embryos lacking Shh or its essential signal transduction components display cochlear agenesis; however, a detailed understanding of the transcriptional network mediating this process is unclear. Here, we describe an integrated genomic approach to identify Shh-dependent genes and associated regulatory sequences that promote cochlear duct morphogenesis. A comparative transcriptome analysis of otic vesicles from mouse mutants exhibiting loss (Smo^ecko ) and gain (Shh-P1) of Shh signaling reveal a set of Shh-responsive genes partitioned into four expression categories in the ventral half of the otic vesicle. This target gene classification scheme provides novel insight into several unanticipated roles for Shh, including priming the cochlear epithelium for subsequent sensory development. We also mapped regions of open chromatin in the inner ear by ATAC-seq that, in combination with Gli2 ChIP-seq, identified inner ear enhancers in the vicinity of Shh-responsive genes. These datasets are useful entry points for deciphering Shh-dependent regulatory mechanisms involved in cochlear duct morphogenesis and establishment of its constituent cell types.

Novel variant p.E269K confirms causative role of PLS1 mutations in autosomal dominant hearing loss

Auditory reception relies on the perception of mechanical stimuli by stereocilia and its conversion to electrochemical signal. Mechanosensory stereocilia are abundant in actin, which provides them with structural conformity necessary for perception of auditory stimuli. Out of three major classes of actin-bundling proteins, plastin 1 encoded by PLS1, is highly expressed in stereocilia and is necessary for their regular maintenance. A missense PLS1 variant associated with autosomal dominant hearing loss (HL) in a small family has recently been reported. Here, we present another PLS1 missense variant, c.805G > A (p.E269K), in a Turkish family with autosomal dominant non-syndromic HL confirming the causative role of PLS1 mutations in HL. We propose that HL due to the p.E269K variant is from the loss of a stable PLS1-ACTB interaction.

Hearing impairment locus heterogeneity and identification of PLS1 as a new autosomal dominant gene in Hungarian Roma

Roma are a socially and culturally distinct isolated population with genetically divergent subisolates, residing mainly across Central, Southern, and Eastern Europe. We evaluated the genetic etiology of hearing impairment (HI) in 15 Hungarian Roma families through exome sequencing. A family with autosomal dominant non-syndromic HI segregating a rare variant in the Calponin-homology 2 domain of PLS1, or Plastin 1 [p.(Leu363Phe)] was identified. Young adult Pls1 knockout mice have progressive HI and show morphological defects to their inner hair cells. There is evidence that PLS1 is important in the preservation of adult stereocilia and normal hearing. Four families segregated the European ancestral variant c.35delG [p.(Gly12fs)] in GJB2, and one family was homozygous for p.(Trp24*), an Indian subcontinent ancestral variant which is common amongst Roma from Slovakia, Czech Republic, and Spain. We also observed variants in known HI genes USH1G, USH2A, MYH9, MYO7A, and a splice site variant in MANBA (c.2158-2A>G) in a family with HI, intellectual disability, behavioral problems, and respiratory inflammation, which was previously reported in a Czech Roma family with similar features. Lastly, using multidimensional scaling and ADMIXTURE analyses, we delineate the degree of Asian/European admixture in the HI families understudy, and show that Roma individuals carrying the GJB2 p.(Trp24*) and MANBA c.2158-2A>G variants have a more pronounced South Asian background, whereas the other hearing-impaired Roma display an ancestral background similar to Europeans. We demonstrate a diverse genetic HI etiology in the Hungarian Roma and identify a new gene PLS1, for autosomal dominant human non-syndromic HI.

Building and repairing the stereocilia cytoskeleton in mammalian auditory hair cells

Despite all recent achievements in identification of the molecules that are essential for the structure and mechanosensory function of stereocilia bundles in the auditory hair cells of mammalian species, we still have only a rudimentary understanding of the mechanisms of stereocilia formation, maintenance, and repair. Important molecular differences distinguishing mammalian auditory hair cells from hair cells of other types and species have been recently revealed. In addition, we are beginning to solve the puzzle of the apparent life-long stability of the stereocilia bundles in these cells. New data link the stability of the cytoskeleton in the mammalian auditory stereocilia with the normal activity of mechanotransduction channels. These data suggest new ideas on how a terminally-differentiated non-regenerating hair cell in the mammalian cochlea may repair and tune its stereocilia bundle throughout the life span of the organism.

Characterization of the EF-Hand Calcium-Binding Domains of Human Plastins

The three human plastins (L-plastin, T-plastin, and I-plastin) are important regulatory Ca²⁺-binding proteins that belong to the family of actin-binding proteins. Plastins are involved in the regulation of the actin cytoskeleton as well as the cross-linking of actin filaments. In addition to four calponin-homology (CH) domains, all three plastins contain two N-terminal EF-hand Ca²⁺-binding motifs which together are homologous to a single lobe of the well-known calcium-regulatory protein calmodulin. This part of the protein allows for the regulation of the actin bundling activity in response to elevated calcium levels. In this protocol, we describe the purification of the EF-hand headpiece domains of all three plastins, as well as SPR studies, ITC studies, and NMR interaction studies with different peptides and calcium. In combination, these three experimental techniques provide detailed insights into a novel regulatory mechanism, involving the linker region between the EF-hand domain and the first CH domain of the plastins.

Sensory Hair Cells: An Introduction to Structure and Physiology

Sensory hair cells are specialized secondary sensory cells that mediate our senses of hearing, balance, linear acceleration, and angular acceleration (head rotation). In addition, hair cells in fish and amphibians mediate sensitivity to water movement through the lateral line system, and closely related electroreceptive cells mediate sensitivity to low-voltage electric fields in the aquatic environment of many fish species and several species of amphibian. Sensory hair cells share many structural and functional features across all vertebrate groups, while at the same time they are specialized for employment in a wide variety of sensory tasks. The complexity of hair cell structure is large, and the diversity of hair cell applications in sensory systems exceeds that seen for most, if not all, sensory cell types. The intent of this review is to summarize the more significant structural features and some of the more interesting and important physiological mechanisms that have been elucidated thus far. Outside vertebrates, hair cells are only known to exist in the coronal organ of tunicates. Electrical resonance, electromotility, and their exquisite mechanical sensitivity all contribute to the attractiveness of hair cells as a research subject.

The stable actin core of mechanosensory stereocilia features continuous turnover of actin cross-linkers

Stereocilia are mechanosensitive protrusions on the surfaces of sensory hair cells in the inner ear that detect sound, gravity, and head movement. Their cores are composed of parallel actin filaments that are cross-linked and stabilized by several actin-binding proteins, including fascin-2, plastin-1, espin, and XIRP2. The actin filaments are the most stable known, with actin turnover primarily occurring at the stereocilia tips. While stereocilia actin dynamics has been well studied, little is known about the behavior of the actin cross-linking proteins, which are the most abundant type of protein in stereocilia after actin and are critical for stereocilia morphogenesis and maintenance. Here, we developed a novel transgenic mouse to monitor EGFP-fascin-2 incorporation. In contrast to actin, EGFP-fascin-2 readily enters the stereocilia core. We also compared the effect of EGFP-fascin-2 expression on developing and mature stereocilia. When it was induced during hair cell development, we observed increases in both stereocilia length and width. Interestingly, stereocilia size was not affected when EGFP-fascin-2 was induced in adult stereocilia. Regardless of the time of induction, EGFP-fascin-2 displaced both espin and plastin-1 from stereocilia. Altering the actin cross-linker composition, even as the actin filaments exhibit little to no turnover, provides a mechanism for ongoing remodeling and repair important for stereocilia homeostasis.

Hypoxia-stimulated membrane trafficking requires T-plastin

AIM

Traffic between the plasma membrane and the endomembrane compartments is an essential feature of eukaryotic cells. The secretory pathway sends cargoes from biosynthetic compartments to the plasma membrane. This is counterbalanced by a retrograde endocytic route and is essential for cell homoeostasis. Cells need to adapt rapidly to environmental challenges such as the reduction of pO₂ which, however, has not been analysed in relation to membrane trafficking in detail. Therefore, we determined changes in the plasma membrane trafficking in normoxia, hypoxia, and after reoxygenation.

METHODS

Membrane trafficking was analysed by using the bulk membrane endocytosis marker FM 1-43, the newly developed membrane probe mCLING, wheat germ agglutinin as well as fluorescently labelled cholera toxin subunit B. Additionally, the uptake of specific membrane proteins was determined. In parallel, a non-biased SILAC screen was performed to analyse the abundance of membrane proteins in normoxia and hypoxia.

RESULTS

Membrane trafficking was increased in hypoxia and quickly reversed upon reoxygenation. This effect was independent of the hypoxia-inducible factor (HIF) system. Using SILAC technology, we identified that the actin-bundling protein T-plastin is recruited to the plasma membrane in hypoxia. By the use of T-plastin knockdown cells, we could show that T-plastin mediates the hypoxia-induced membrane trafficking, which was associated with an increased actin density in the cells as determined by electron microscopy.

CONCLUSION

Membrane trafficking is highly dynamic upon hypoxia. This phenotype is quickly reversible upon reoxygenation, which suggests that this mechanism participates in the cellular adaptation to hypoxia.

T-plastin is essential for basement membrane assembly and epidermal morphogenesis

The establishment of epithelial architecture is a complex process involving cross-talk between cells and the basement membrane. Basement membrane assembly requires integrin activity but the role of the associated actomyosin cytoskeleton is poorly understood. Here, we identify the actin-bundling protein T-plastin (Pls3) as a regulator of basement membrane assembly and epidermal morphogenesis. In utero depletion of Pls3 transcripts in mouse embryos caused basement membrane and polarity defects in the epidermis but had little effect on cell adhesion and differentiation. Loss-of-function experiments demonstrated that the apicobasal polarity defects were secondary to the disruption of the basement membrane. However, the basement membrane itself was profoundly sensitive to subtle perturbations in the actin cytoskeleton. We further show that Pls3 localized to the cell cortex, where it was essential for the localization and activation of myosin II. Inhibition of myosin II motor activity disrupted basement membrane organization. Our results provide insights into the regulation of cortical actomyosin and its importance for basement membrane assembly and skin morphogenesis.

Mechanotransduction current is essential for stability of the transducing stereocilia in mammalian auditory hair cells

Mechanotransducer channels at the tips of sensory stereocilia of inner ear hair cells are gated by the tension of 'tip links' interconnecting stereocilia. To ensure maximal sensitivity, tip links are tensioned at rest, resulting in a continuous influx of Ca²⁺ into the cell. Here, we show that this constitutive Ca²⁺ influx, usually considered as potentially deleterious for hair cells, is in fact essential for stereocilia stability. In the auditory hair cells of young postnatal mice and rats, a reduction in mechanotransducer current, via pharmacological channel blockers or disruption of tip links, leads to stereocilia shape changes and shortening. These effects occur only in stereocilia that harbor mechanotransducer channels, recover upon blocker washout or tip link regeneration and can be replicated by manipulations of extracellular Ca²⁺ or intracellular Ca²⁺ buffering. Thus, our data provide the first experimental evidence for the dynamic control of stereocilia morphology by the mechanotransduction current.

DOI:http://dx.doi.org/10.7554/eLife.24661.001

Our sense of hearing depends on cells known as hair cells that line the inner ear. Each hair cell has tiny projections called stereocilia, which are arranged in a bundle with rows of increasing height like a staircase and are connected to each other by tiny filaments called tip-links. When sound waves hit the stereocilia, the tension on the tip-links increases, which opens “mechanotransduction” channels on the shorter stereocilia that allow calcium ions to flow into the cells. To ensure that the ears can detect even the softest sounds, the tip-links always have a small amount of tension which allows a small, but continuous flow of calcium ions into the cell. Scientists generally consider this continuous flow of calcium ions as a potentially harmful byproduct of sensitive hearing.

Vélez-Ortega et al. isolated inner ear tissues from young mice and rats and exposed them to drugs that either block the flow of calcium ions through the mechanotransduction channels or break the tip-links on stereocilia. Surprisingly, these drugs made profound changes in the shape of individual stereocilia and the staircase architecture of the stereocilia bundle. When the drugs were rinsed out of the hair cells, the stereocilia went back to their normal shape. Sequestering of free calcium ions inside the hair cells had a similar effect on the shape of stereocilia. These findings show that the flow of calcium ions into the sterocilia via mechanotransduction channels controls the exquisite staircase-like architecture of the stereocilia bundle.

More research is needed to identify which structural proteins cause the stereocilia shape changes and to work out exactly how calcium ions are involved.

DOI:http://dx.doi.org/10.7554/eLife.24661.002

Plastins regulate ectoplasmic specialization via its actin bundling activity on microfilaments in the rat testis

Plastins are a family of actin binding proteins (ABPs) known to cross-link actin microfilaments in mammalian cells, creating actin microfilament bundles necessary to confer cell polarity and cell shape. Plastins also support cell movement in response to changes in environment, involved in cell/tissue growth and development. They also confer plasticity to cells and tissues in response to infection or other pathological conditions (e.g., inflammation). In the testis, the cell-cell anchoring junction unique to the testis that is found at the Sertoli cell-cell interface at the blood-testis barrier (BTB) and at the Sertoli-spermatid (e.g., 8–19 spermatids in the rat testis) is the basal and the apical ectoplasmic specialization (ES), respectively. The ES is an F-actin-rich anchoring junction constituted most notably by actin microfilament bundles. A recent report using RNAi that specifically knocks down plastin 3 has yielded some insightful information regarding the mechanism by which plastin 3 regulates the status of actin microfilament bundles at the ES via its intrinsic actin filament bundling activity. Herein, we provide a brief review on the role of plastins in the testis in light of this report, which together with recent findings in the field, we propose a likely model by which plastins regulate ES function during the epithelial cycle of spermatogenesis via their intrinsic activity on actin microfilament organization in the rat testis.

Plastin 1 widens stereocilia by transforming actin filament packing from hexagonal to liquid

With their essential role in inner ear function, stereocilia of sensory hair cells demonstrate the importance of cellular actin protrusions. Actin packing in stereocilia is mediated by cross-linkers of the plastin, fascin, and espin families. Although mice lacking espin (ESPN) have no vestibular or auditory function, we found that mice that either lacked plastin 1 (PLS1) or had nonfunctional fascin 2 (FSCN2) had reduced inner ear function, with double-mutant mice most strongly affected. Targeted mass spectrometry indicated that PLS1 was the most abundant cross-linker in vestibular stereocilia and the second most abundant protein overall; ESPN only accounted for ∼15% of the total cross-linkers in bundles. Mouse utricle stereocilia lacking PLS1 were shorter and thinner than wild-type stereocilia. Surprisingly, although wild-type stereocilia had random liquid packing of their actin filaments, stereocilia lacking PLS1 had orderly hexagonal packing. Although all three cross-linkers are required for stereocilia structure and function, PLS1 biases actin toward liquid packing, which allows stereocilia to grow to a greater diameter.

MyTH4-FERM myosins in the assembly and maintenance of actin-based protrusions

Unconventional myosins are actin-based molecular motors that serve a multitude of roles within the cell. One group of myosin motors, the MyTH4-FERM myosins, play an integral part in building and maintaining finger-like protrusions, which allow cells to interact with their external environment. Suggested to act primarily as transporters, these motor proteins enrich adhesion molecules, actin-regulatory proteins and other factors at the tips of filopodia, microvilli, and stereocilia. Below we review data from biophysical, biochemical, and cell biological studies, which implicate these myosins as central players in the assembly, maintenance and function of actin-based protrusions.

Stereocilia morphogenesis and maintenance through regulation of actin stability

Stereocilia are actin-based protrusions on auditory and vestibular sensory cells that are required for hearing and balance. They convert physical force from sound, head movement or gravity into an electrical signal, a process that is called mechanoelectrical transduction. This function depends on the ability of sensory cells to grow stereocilia of defined lengths. These protrusions form a bundle with a highly precise geometry that is required to detect nanoscale movements encountered in the inner ear. Congenital or progressive stereocilia degeneration causes hearing loss. Thus, understanding stereocilia hair bundle structure, development, and maintenance is pivotal to understanding the pathogenesis of deafness. Stereocilia cores are made from a tightly packed array of parallel, crosslinked actin filaments, the length and stability of which are regulated in part by myosin motors, actin crosslinkers and capping proteins. This review aims to describe stereocilia actin regulation in the context of an emerging "tip turnover" model where actin assembles and disassembles at stereocilia tips while the remainder of the core is exceptionally stable.

Impact of cordon-bleu expression on actin cytoskeleton architecture and dynamics

Cordon-bleu (COBL) is a multifunctional WASP-Homology 2 (WH2) domain-containing protein implicated in a wide variety of cellular functions ranging from dendritic arborization in neurons to the assembly of microvilli on the surface of transporting epithelial cells. In vitro biochemical studies suggest that COBL is capable of nucleating and severing actin filaments, among other activities. How the multiple activities of COBL observed in vitro contribute to its function in cells remains unclear. Here, we used live imaging to evaluate the impact of COBL expression on the actin cytoskeleton in cultured cells. We found that COBL induces the formation of dynamic linear actin structures throughout the cytosol. We also found that stabilizing these dynamic structures with the parallel actin-bundling protein espin slows down their turnover and enables the robust formation of self-supported protrusions on the dorsal cell surface. Super-resolution imaging revealed a global remodeling of the actin cytoskeleton in cells expressing these two factors. Taken together, these results provide insight as to how COBL contributes to the assembly of actin-based structures such as epithelial microvilli. © 2016 Wiley Periodicals, Inc.

The 133-kDa N-terminal domain enables myosin 15 to maintain mechanotransducing stereocilia and is essential for hearing

The precise assembly of inner ear hair cell stereocilia into rows of increasing height is critical for mechanotransduction and the sense of hearing. Yet, how the lengths of actin-based stereocilia are regulated remains poorly understood. Mutations of the molecular motor myosin 15 stunt stereocilia growth and cause deafness. We found that hair cells express two isoforms of myosin 15 that differ by inclusion of an 133-kDa N-terminal domain, and that these isoforms can selectively traffic to different stereocilia rows. Using an isoform-specific knockout mouse, we show that hair cells expressing only the small isoform remarkably develop normal stereocilia bundles. However, a critical subset of stereocilia with active mechanotransducer channels subsequently retracts. The larger isoform with the 133-kDa N-terminal domain traffics to these specialized stereocilia and prevents disassembly of their actin core. Our results show that myosin 15 isoforms can navigate between functionally distinct classes of stereocilia, and are independently required to assemble and then maintain the intricate hair bundle architecture.

Association of SNPs in LCP1 and CTIF with hearing in 11 year old children: findings from the Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort and the G-EAR consortium

BACKGROUND

The genetic basis of hearing loss in humans is relatively poorly understood. In recent years, experimental approaches including laboratory studies of early onset hearing loss in inbred mouse strains, or proteomic analyses of hair cells or hair bundles, have suggested new candidate molecules involved in hearing function. However, the relevance of these genes/gene products to hearing function in humans remains unknown. We investigated whether single nucleotide polymorphisms (SNPs) in the human orthologues of genes of interest arising from the above-mentioned studies correlate with hearing function in children.

METHODS

577 SNPs from 13 genes were each analysed by linear regression against averaged high (3, 4 and 8 kHz) or low frequency (0.5, 1 and 2 kHz) audiometry data from 4970 children in the Avon Longitudinal Study of Parents and Children (ALSPAC) birth-cohort at age eleven years. Genes found to contain SNPs with low p-values were then investigated in 3417 adults in the G-EAR study of hearing.

RESULTS

Genotypic data were available in ALSPAC for a total of 577 SNPs from 13 genes of interest. Two SNPs approached sample-wide significance (pre-specified at p = 0.00014): rs12959910 in CBP80/20-dependent translation initiation factor (CTIF) for averaged high frequency hearing (p = 0.00079, β = 0.61 dB per minor allele); and rs10492452 in L-plastin (LCP1) for averaged low frequency hearing (p = 0.00056, β = 0.45 dB). For low frequencies, rs9567638 in LCP1 also enhanced hearing in females (p = 0.0011, β = -1.76 dB; males p = 0.23, β = 0.61 dB, likelihood-ratio test p = 0.006). SNPs in LCP1 and CTIF were then examined against low and high frequency hearing data for adults in G-EAR. Although the ALSPAC results were not replicated, a SNP in LCP1, rs17601960, is in strong LD with rs9967638, and was associated with enhanced low frequency hearing in adult females in G-EAR (p = 0.00084).

CONCLUSIONS

There was evidence to suggest that multiple SNPs in CTIF may contribute a small detrimental effect to hearing, and that a sex-specific locus in LCP1 is protective of hearing. No individual SNPs reached sample-wide significance in both ALSPAC and G-EAR. This is the first report of a possible association between LCP1 and hearing function.

Developmental regulation of planar cell polarity and hair-bundle morphogenesis in auditory hair cells: lessons from human and mouse genetics

Hearing loss is the most common and costly sensory defect in humans and genetic causes underlie a significant proportion of affected individuals. In mammals, sound is detected by hair cells (HCs) housed in the cochlea of the inner ear, whose function depends on a highly specialized mechanotransduction organelle, the hair bundle. Understanding the factors that regulate the development and functional maturation of the hair bundle is crucial for understanding the pathophysiology of human deafness. Genetic analysis of deafness genes in animal models, together with complementary forward genetic screens and conditional knock-out mutations in essential genes, have provided great insights into the molecular machinery underpinning hair-bundle development and function. In this review, we highlight recent advances in our understanding of hair-bundle morphogenesis, with an emphasis on the molecular pathways governing hair-bundle polarity and orientation. We next discuss the proteins and structural elements important for hair-cell mechanotransduction as well as hair-bundle cohesion and maintenance. In addition, developmental signals thought to regulate tonotopic features of HCs are introduced. Finally, novel approaches that complement classic genetics for studying the molecular etiology of human deafness are presented. WIREs Dev Biol 2016, 5:85-101. doi: 10.1002/wdev.202 For further resources related to this article, please visit the WIREs website.

Neurog1 can partially substitute for Atoh1 function in hair cell differentiation and maintenance during organ of Corti development

Atoh1, a basic helix-loop-helix (bHLH) transcription factor (TF), is essential for the differentiation of hair cells (HCs), mechanotransducers that convert sound into auditory signals in the mammalian organ of Corti (OC). Previous work demonstrated that replacing mouse Atoh1 with the fly ortholog atonal rescues HC differentiation, indicating functional replacement by other bHLH genes. However, replacing Atoh1 with Neurog1 resulted in reduced HC differentiation compared with transient Atoh1 expression in a 'self-terminating' Atoh1 conditional null mouse (Atoh1-Cre; Atoh1(f/f)). We now show that combining Neurog1 in one allele with removal of floxed Atoh1 in a self-terminating conditional mutant (Atoh1-Cre; Atoh1(f/kiNeurog1)) mouse results in significantly more differentiated inner HCs and outer HCs that have a prolonged longevity of 9 months compared with Atoh1 self-terminating littermates. Stereocilia bundles are partially disorganized, disoriented and not HC type specific. Replacement of Atoh1 with Neurog1 maintains limited expression of Pou4f3 and Barhl1 and rescues HCs quantitatively, but not qualitatively. OC patterning and supporting cell differentiation are also partially disrupted. Diffusible factors involved in patterning are reduced (Fgf8) and factors involved in cell-cell interactions are affected (Jag1, Hes5). Despite the presence of many HCs with stereocilia these mice are deaf, possibly owing to HC and OC patterning defects. This study provides a novel approach to disrupt OC development through modulating the HC-specific intracellular TF network. The resulting disorganized OC indicates that normally differentiated HCs act as 'self-organizers' for OC development and that Atoh1 plays a crucial role to initiate HC stereocilia differentiation independently of HC viability.

Characterizing human vestibular sensory epithelia for experimental studies: new hair bundles on old tissue and implications for therapeutic interventions in ageing

Balance disequilibrium is a significant contributor to falls in the elderly. The most common cause of balance dysfunction is loss of sensory cells from the vestibular sensory epithelia of the inner ear. However, inaccessibility of inner ear tissue in humans severely restricts possibilities for experimental manipulation to develop therapies to ameliorate this loss. We provide a structural and functional analysis of human vestibular sensory epithelia harvested at trans-labyrinthine surgery. We demonstrate the viability of the tissue and labeling with specific markers of hair cell function and of ion homeostasis in the epithelium. Samples obtained from the oldest patients revealed a significant loss of hair cells across the tissue surface, but we found immature hair bundles present in epithelia harvested from patients >60 years of age. These results suggest that the environment of the human vestibular sensory epithelium could be responsive to stimulation of developmental pathways to enhance hair cell regeneration, as has been demonstrated successfully in the vestibular organs of adult mice.

Length regulation of mechanosensitive stereocilia depends on very slow actin dynamics and filament-severing proteins

Auditory sensory hair cells depend on stereocilia with precisely regulated lengths to detect sound. Since stereocilia are primarily composed of cross-linked, parallel actin filaments, regulated actin dynamics are essential for controlling stereocilia length. Here, we assessed stereocilia actin turnover by monitoring incorporation of inducibly expressed β-actin-GFP in adult mouse hair cells in vivo and by directly measuring β-actin-GFP turnover in explants. Stereocilia actin incorporation is remarkably slow and restricted to filament barbed ends in a small tip compartment, with minimal accumulation in the rest of the actin core. Shorter rows of stereocilia, which have mechanically-gated ion channels, show more variable actin turnover than the tallest stereocilia, which lack channels. Finally, the proteins ADF and AIP1, which both mediate actin filament severing, contribute to stereocilia length maintenance. Together, the data support a model whereby stereocilia actin cores are largely static, with dynamic regulation at the tips to maintain a critical length.

A short splice form of Xin-actin binding repeat containing 2 (XIRP2) lacking the Xin repeats is required for maintenance of stereocilia morphology and hearing function

Approximately one-third of known deafness genes encode proteins located in the hair bundle, the sensory hair cell's mechanoreceptive organelle. In previous studies, we used mass spectrometry to characterize the hair bundle's proteome, resulting in the discovery of novel bundle proteins. One such protein is Xin-actin binding repeat containing 2 (XIRP2), an actin-cross-linking protein previously reported to be specifically expressed in striated muscle. Because mutations in other actin-cross-linkers result in hearing loss, we investigated the role of XIRP2 in hearing function. In the inner ear, XIRP2 is specifically expressed in hair cells, colocalizing with actin-rich structures in bundles, the underlying cuticular plate, and the circumferential actin belt. Analysis using peptide mass spectrometry revealed that the bundle harbors a previously uncharacterized XIRP2 splice variant, suggesting XIRP2's role in the hair cell differs significantly from that reported in myocytes. To determine the role of XIRP2 in hearing, we applied clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated genome-editing technology to induce targeted mutations into the mouse Xirp2 gene, resulting in the elimination of XIRP2 protein expression in the inner ear. Functional analysis of hearing in the resulting Xirp2-null mice revealed high-frequency hearing loss, and ultrastructural scanning electron microscopy analyses of hair cells demonstrated stereocilia degeneration in these mice. We thus conclude that XIRP2 is required for long-term maintenance of hair cell stereocilia, and that its dysfunction causes hearing loss in the mouse.