Eps8 regulates hair bundle length and functional maturation of mammalian auditory hair cells

Pathophysiology of human hearing loss associated with variants in myosins

Deleterious variants of more than one hundred genes are associated with hearing loss including MYO3A, MYO6, MYO7A and MYO15A and two conventional myosins MYH9 and MYH14. Variants of MYO7A also manifest as Usher syndrome associated with dysfunction of the retina and vestibule as well as hearing loss. While the functions of MYH9 and MYH14 in the inner ear are debated, MYO3A, MYO6, MYO7A and MYO15A are expressed in inner ear hair cells along with class-I myosin MYO1C and are essential for developing and maintaining functional stereocilia on the apical surface of hair cells. Stereocilia are large, cylindrical, actin-rich protrusions functioning as biological mechanosensors to detect sound, acceleration and posture. The rigidity of stereocilia is sustained by highly crosslinked unidirectionally-oriented F-actin, which also provides a scaffold for various proteins including unconventional myosins and their cargo. Typical myosin molecules consist of an ATPase head motor domain to transmit forces to F-actin, a neck containing IQ-motifs that bind regulatory light chains and a tail region with motifs recognizing partners. Instead of long coiled-coil domains characterizing conventional myosins, the tails of unconventional myosins have various motifs to anchor or transport proteins and phospholipids along the F-actin core of a stereocilium. For these myosins, decades of studies have elucidated their biochemical properties, interacting partners in hair cells and variants associated with hearing loss. However, less is known about how myosins traffic in a stereocilium using their motor function, and how each variant correlates with a clinical condition including the severity and onset of hearing loss, mode of inheritance and presence of symptoms other than hearing loss. Here, we cover the domain structures and functions of myosins associated with hearing loss together with advances, open questions about trafficking of myosins in stereocilia and correlations between hundreds of variants in myosins annotated in ClinVar and the corresponding deafness phenotypes.

Comparative immunohistochemical analysis suggests a conserved role of EPS8L1 in epidermal and hair follicle barriers of mammals

The mammalian skin and its appendages depend on tightly coordinated differentiation of epithelial cells. Epidermal growth factor receptor (EGFR) pathway substrate 8 (EPS8) like 1 (EPS8L1) is enriched in the epidermis among human tissues and has also been detected in the epidermis of lizards. Here, we show by the analysis of single-cell RNA-sequencing data that EPS8L1 mRNA is co-expressed with filaggrin and loricrin in terminally differentiated human epidermal keratinocytes. Comparative genomics indicated that EPS8L1 is conserved in all main clades of mammals, whereas the orthologous gene has been lost in birds. Using a polyclonal antibody against EPS8L1, we performed an immunohistochemical screening of skin from diverse mammalian species and immuno-electron microscopy of human skin. EPS8L1 was detected predominantly in the granular layer of the epidermis in monotremes, marsupial, and placental mammals. The labeling was partly associated with cell membranes, and it was evident along the perimeter of keratinocytes at the transition with the cornified layer of the epidermis, similar to involucrin distribution. Basal, spinous, and the fully mature cornified layers lacked immunolabeling of EPS8L1. In addition to the epidermis, the hair follicle inner root sheath (IRS) was immunolabeled. Both epidermal granular layer and IRS contribute to the barrier function of the skin, suggesting that EPS8L1 is involved in the regulation of these barriers.

SUB-Immunogold-SEM reveals nanoscale distribution of submembranous epitopes

Electron microscopy paired with immunogold labeling is the most precise tool for protein localization. However, these methods are either cumbersome, resulting in small sample numbers and restricted quantification, or limited to identifying protein epitopes external to the membrane. Here, we introduce SUB-immunogold-SEM, a scanning electron microscopy technique that detects intracellular protein epitopes proximal to the membrane. We identified four critical sample preparation factors that contribute to the method's sensitivity and validate its efficacy through precise localization and high-powered quantification of cytoskeletal and transmembrane proteins. We evaluated the capabilities of SUB-immunogold-SEM on cells with highly differentiated apical surfaces: (i) auditory hair cells, revealing the presence of nanoscale Myosin rings at the tip of stereocilia; and (ii) respiratory multiciliate cells, mapping the distribution of the SARS-CoV-2 receptor ACE2 along the motile cilia. SUB-immunogold-SEM provides a novel solution for nanoscale protein localization at the exposed surface of any cell.

Differential regulation of hair cell actin cytoskeleton mediated by SRF and MRTFB

The MRTF-SRF pathway has been extensively studied for its crucial role in driving the expression of a large number of genes involved in actin cytoskeleton of various cell types. However, the specific contribution of MRTF-SRF in hair cells remains unknown. In this study, we showed that hair cell-specific deletion of Srf or Mrtfb, but not Mrtfa, leads to similar defects in the development of stereocilia dimensions and the maintenance of cuticular plate integrity. We used fluorescence-activated cell sorting-based hair cell RNA-Seq analysis to investigate the mechanistic underpinnings of the changes observed in Srf and Mrtfb mutants, respectively. Interestingly, the transcriptome analysis revealed distinct profiles of genes regulated by Srf and Mrtfb, suggesting different transcriptional regulation mechanisms of actin cytoskeleton activities mediated by Srf and Mrtfb. Exogenous delivery of calponin 2 using Adeno-associated virus transduction in Srf mutants partially rescued the impairments of stereocilia dimensions and the F-actin intensity of cuticular plate, suggesting the involvement of Cnn2, as an Srf downstream target, in regulating the hair bundle morphology and cuticular plate actin cytoskeleton organization. Our study uncovers, for the first time, the unexpected differential transcriptional regulation of actin cytoskeleton mediated by Srf and Mrtfb in hair cells, and also demonstrates the critical role of SRF-CNN2 in modulating actin dynamics of the stereocilia and cuticular plate, providing new insights into the molecular mechanism underlying hair cell development and maintenance.

Adhesion-based capture stabilizes nascent microvilli at epithelial cell junctions

To maximize solute transport, epithelial cells build an apical "brush border," where thousands of microvilli are linked to their neighbors by protocadherin-containing intermicrovillar adhesion complexes (IMACs). Previous studies established that the IMAC is needed to build a mature brush border, but how this complex contributes to the accumulation of new microvilli during differentiation remains unclear. We found that early in differentiation, mouse, human, and porcine epithelial cells exhibit a marginal accumulation of microvilli, which span junctions and interact with protrusions on neighboring cells using IMAC protocadherins. These transjunctional IMACs are highly stable and reinforced by tension across junctions. Finally, long-term live imaging showed that the accumulation of microvilli at cell margins consistently leads to accumulation in medial regions. Thus, nascent microvilli are stabilized by a marginal capture mechanism that depends on the formation of transjunctional IMACs. These results may offer insights into how apical specializations are assembled in diverse epithelial systems.

Mutations in human DNA methyltransferase DNMT1 induce specific genome-wide epigenomic and transcriptomic changes in neurodevelopment

DNA methyltransferase type 1 (DNMT1) is a major enzyme involved in maintaining the methylation pattern after DNA replication. Mutations in DNMT1 have been associated with autosomal dominant cerebellar ataxia, deafness and narcolepsy (ADCA-DN). We used fibroblasts, induced pluripotent stem cells (iPSCs) and induced neurons (iNs) generated from patients with ADCA-DN and controls, to explore the epigenomic and transcriptomic effects of mutations in DNMT1. We show cell type-specific changes in gene expression and DNA methylation patterns. DNA methylation and gene expression changes were negatively correlated in iPSCs and iNs. In addition, we identified a group of genes associated with clinical phenotypes of ADCA-DN, including PDGFB and PRDM8 for cerebellar ataxia, psychosis and dementia and NR2F1 for deafness and optic atrophy. Furthermore, ZFP57, which is required to maintain gene imprinting through DNA methylation during early development, was hypomethylated in promoters and exhibited upregulated expression in patients with ADCA-DN in both iPSC and iNs. Our results provide insight into the functions of DNMT1 and the molecular changes associated with ADCA-DN, with potential implications for genes associated with related phenotypes.

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.

BioID2 screening identifies KIAA1671 as an EPS8 proximal factor that marks sites of microvillus growth

Microvilli are defining morphological features of the apical surfaces in diverse epithelial tissues. To develop our understanding of microvillus biogenesis, we used a biotin proximity-labeling approach to uncover new molecules enriched near EPS8, a well-studied marker of the microvillus distal tip compartment. Mass spectrometry of biotinylated hits identified KIAA1671, a large (∼200 kDa), disordered, and previously uncharacterized protein. Based on immunofluorescent staining and expression of fluorescent protein-tagged constructs, we found that KIAA1671 localizes to the base of the brush border in native intestinal tissue and polarized epithelial-cell culture models, as well as dynamic actin-rich structures in unpolarized, nonepithelial cell types. Live imaging also revealed that during the early stages of microvillar growth, KIAA1671 colocalizes with EPS8 in diffraction-limited puncta. However, once elongation of the core bundle begins, these two factors separate, with EPS8 tracking the distal end and KIAA1671 remaining behind at the base of the structure. These results suggest that KIAA1671 cooperates with EPS8 and potentially other assembly factors to initiate growth of microvilli on the apical surface. These findings offer new details on how transporting epithelial cells builds the brush border and may inform our understanding of how apical specializations are assembled in other epithelial contexts.

Adhesion-based capture stabilizes nascent microvilli at epithelial cell junctions

Differentiated transporting epithelial cells present an extensive apical array of microvilli - a "brush border" - where neighboring microvilli are linked together by intermicrovillar adhesion complexes (IMACs) composed of protocadherins CDHR2 and CDHR5. Although loss-of-function studies provide strong evidence that IMAC function is needed to build a mature brush border, how the IMAC contributes to the stabilization and accumulation of nascent microvilli remains unclear. We found that, early in differentiation, the apical surface exhibits a marginal accumulation of microvilli, characterized by higher packing density relative to medial regions of the surface. While medial microvilli are highly dynamic and sample multiple orientations over time, marginal protrusions exhibit constrained motion and maintain a vertical orientation. Unexpectedly, we found that marginal microvilli span the junctional space and contact protrusions on neighboring cells, mediated by complexes of CDHR2/CDHR5. FRAP analysis indicated that these transjunctional IMACs are highly stable relative to adhesion complexes between medial microvilli, which explains the restricted motion of protrusions in the marginal zone. Finally, long-term live imaging revealed that the accumulation of microvilli at cell margins consistently leads to accumulation in medial regions of the cell. Collectively, our findings suggest that nascent microvilli are stabilized by a capture mechanism that is localized to cell margins and enabled by the transjunctional formation of IMACs. These results inform our understanding of how apical specializations are assembled in diverse epithelial systems.

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.

A critical period of prehearing spontaneous Ca2+ spiking is required for hair-bundle maintenance in inner hair cells

Sensory-independent Ca2+ spiking regulates the development of mammalian sensory systems. In the immature cochlea, inner hair cells (IHCs) fire spontaneous Ca2+ action potentials (APs) that are generated either intrinsically or by intercellular Ca2+ waves in the nonsensory cells. The extent to which either or both of these Ca2+ signalling mechansims are required for IHC maturation is unknown. We find that intrinsic Ca2+ APs in IHCs, but not those elicited by Ca2+ waves, regulate the maturation and maintenance of the stereociliary hair bundles. Using a mouse model in which the potassium channel Kir2.1 is reversibly overexpressed in IHCs (Kir2.1-OE), we find that IHC membrane hyperpolarization prevents IHCs from generating intrinsic Ca2+ APs but not APs induced by Ca2+ waves. Absence of intrinsic Ca2+ APs leads to the loss of mechanoelectrical transduction in IHCs prior to hearing onset due to progressive loss or fusion of stereocilia. RNA-sequencing data show that pathways involved in morphogenesis, actin filament-based processes, and Rho-GTPase signaling are upregulated in Kir2.1-OE mice. By manipulating in vivo expression of Kir2.1 channels, we identify a "critical time period" during which intrinsic Ca2+ APs in IHCs regulate hair-bundle function.

EPS8 variant causes deafness, autosomal recessive 102 (DFNB102) and literature review

Pathogenic variants in the EPS8 gene result in nonsyndromic hearing loss. This gene encodes the EPS8 protein in cochlear inner hair cells and performs critical roles in stimulating actin polymerization and bundling. Thus far, only four pathogenic variations in EPS8 have been described. In this study, we report the fifth pathogenic variant in the EPS8 gene in an Iranian patient with DFNB102. Furthermore, we review literature cases with EPS8 mutations.

Liquid-liquid phase separation in hair cell stereocilia development and maintenance

As an emerging concept, liquid-liquid phase separation (LLPS) in biological systems has shed light on the formation mechanisms of membrane-less compartments in cells. The process is driven by multivalent interactions of biomolecules such as proteins and/or nucleic acids, allowing them to form condensed structures. In the inner ear hair cells, LLPS-based biomolecular condensate assembly plays a vital role in the development and maintenance of stereocilia, the mechanosensing organelles located at the apical surface of hair cells. This review aims to summarize recent findings on the molecular basis governing the LLPS of Usher syndrome-related gene-encoding proteins and their binding partners, which may ultimately result in the formation of upper tip-link density and tip complex density in hair cell stereocilia, offering a better understanding of this severe inherited disease that causes deaf-blindness.

Chlamydia repurposes the actin-binding protein EPS8 to disassemble epithelial tight junctions and promote infection

Invasive microbial pathogens often disrupt epithelial barriers, yet the mechanisms used to dismantle tight junctions are poorly understood. Here, we show that the obligate pathogen Chlamydia trachomatis uses the effector protein TepP to transiently disassemble tight junctions early during infection. TepP alters the tyrosine phosphorylation status of host proteins involved in cytoskeletal regulation, including the filamentous actin-binding protein EPS8. We determined that TepP and EPS8 are necessary and sufficient to remodel tight junctions and that the ensuing disruption of epithelial barrier function promotes secondary invasion events. The genetic deletion of EPS8 renders epithelial cells and endometrial organoids resistant to TepP-mediated tight junction remodeling. Finally, TepP and EPS8 promote infection in murine models of infections, with TepP mutants displaying defects in ascension to the upper genital tract. These findings reveal a non-canonical function of EPS8 in the disassembly of epithelial junctions and an important role for Chlamydia pathogenesis.

Myosin motors in sensory hair bundle assembly

Mechanosensory hair bundles are assembled from actin-based stereocilia that project from the apical surface of hair cells in the inner ear. Stereocilia architecture is critical for the transduction of sound and accelerations, and structural defects in these mechano-sensors are a clinical cause of hearing and balance disorders in humans. Unconventional myosin motors are central to the assembly and shaping of stereocilia architecture. A sub-group of myosin motors with MyTH4-FERM domains (MYO7A, MYO15A) are particularly important in these processes, and hypothesized to act as transporters delivering structural and actin-regulatory cargos, in addition to generating force and tension. In this review, we summarize existing evidence for how MYO7A and MYO15A operate and how their dysfunction leads to stereocilia pathology. We further highlight emerging properties of the MyTH4/FERM myosin family and speculate how these new functions might contribute towards the acquisition and maintenance of mechano-sensitivity.

Sensing sound: Cellular specializations and molecular force sensors

Organisms of all phyla express mechanosensitive ion channels with a wide range of physiological functions. In recent years, several classes of mechanically gated ion channels have been identified. Some of these ion channels are intrinsically mechanosensitive. Others depend on accessory proteins to regulate their response to mechanical force. The mechanotransduction machinery of cochlear hair cells provides a particularly striking example of a complex force-sensing machine. This molecular ensemble is embedded into a specialized cellular compartment that is crucial for its function. Notably, mechanotransduction channels of cochlear hair cells are not only critical for auditory perception. They also shape their cellular environment and regulate the development of auditory circuitry. Here, we summarize recent discoveries that have shed light on the composition of the mechanotransduction machinery of cochlear hair cells and how this machinery contributes to the development and function of the auditory system.

AAV-mediated rescue of Eps8 expression in vivo restores hair-cell function in a mouse model of recessive deafness

The transduction of acoustic information by hair cells depends upon mechanosensitive stereociliary bundles that project from their apical surface. Mutations or absence of the stereociliary protein EPS8 cause deafness in humans and mice, respectively. Eps8 knockout mice (Eps8 -/- ) have hair cells with immature stereocilia and fail to become sensory receptors. Here, we show that exogenous delivery of Eps8 using Anc80L65 in P1-P2 Eps8 -/- mice in vivo rescued the hair bundle structure of apical-coil hair cells. Rescued hair bundles correctly localize EPS8, WHIRLIN, MYO15, and BAIAP2L2, and generate normal mechanoelectrical transducer currents. Inner hair cells with normal-looking stereocilia re-expressed adult-like basolateral ion channels (BK and KCNQ4) and have normal exocytosis. The number of hair cells undergoing full recovery was not sufficient to rescue hearing in Eps8 -/- mice. Adeno-associated virus (AAV)-transduction of P3 apical-coil and P1-P2 basal-coil hair cells does not rescue hair cells, nor does Anc80L65-Eps8 delivery in adult Eps8 -/- mice. We propose that AAV-induced gene-base therapy is an efficient strategy to recover the complex hair-cell defects in Eps8 -/- mice. However, this therapeutic approach may need to be performed in utero since, at postnatal ages, Eps8 -/- hair cells appear to have matured or accumulated damage beyond the point of repair.

Dync1li1 is required for the survival of mammalian cochlear hair cells by regulating the transportation of autophagosomes

Dync1li1, a subunit of cytoplasmic dynein 1, is reported to play important roles in intracellular retrograde transport in many tissues. However, the roles of Dync1li1 in the mammalian cochlea remain uninvestigated. Here we first studied the expression pattern of Dync1li1 in the mouse cochlea and found that Dync1li1 is highly expressed in hair cells (HCs) in both neonatal and adult mice cochlea. Next, we used Dync1li1 knockout (KO) mice to investigate its effects on hearing and found that deletion of Dync1li1 leads to early onset of progressive HC loss via apoptosis and to subsequent hearing loss. Further studies revealed that loss of Dync1li1 destabilizes dynein and alters the normal function of dynein. In addition, Dync1li1 KO results in a thinner Golgi apparatus and the accumulation of LC3+ autophagic vacuoles, which triggers HC apoptosis. We also knocked down Dync1li1 in the OC1 cells and found that the number of autophagosomes were significantly increased while the number of autolysosomes were decreased, which suggested that Dync1li1 knockdown leads to impaired transportation of autophagosomes to lysosomes and therefore the accumulation of autophagosomes results in HC apoptosis. Our findings demonstrate that Dync1li1 plays important roles in HC survival through the regulation of autophagosome transportation.

Hearing loss is one of the most common sensorial disorders globally. The main reason of hearing loss is the irreversible loss or malfunction of cochlear hair cells. Identifying new hearing loss-related genes and investigating their roles and mechanisms in HC survival are important for the prevention and treatment of hereditary hearing loss. Cytoplasmic dynein 1 is reported to play important roles in in ciliogenesis and protein transport in the mouse photoreceptors. Here, we described the expression pattern of Dyncili1 (a subunit of cytoplasmic dynein 1) in the mouse cochlea and used knockout mice to investigate its specific role in the hair cell of cochlea.

Promotion of row 1-specific tip complex condensates by Gpsm2-Gαi provides insights into row identity of the tallest stereocilia

The mechanosensory stereocilia in hair cells are organized into rows of graded height, a property crucial for auditory perception. Gpsm2-Gαi-Whirlin-Myo15-Eps8 complex at tips of the tallest stereocilia is proposed to define hair bundle row identity, although the underlying mechanism remains elusive. Here, we find that Gpsm2 could undergo phase separation. Moreover, row 1-specific Gpsm2-Gαi complex significantly promotes the formation of the five-component tip complex density (5xTCD) condensates. The 5xTCD condensates display much stronger actin-bundling ability than those without Gpsm2-Gαi, which may provide critical insights into how Gpsm2-Gαi specifies the tallest stereocilia. A deafness-associated mutation of Gpsm2 leads to impaired formation of the 5xTCD condensates and consequently reduced actin bundling, providing possible clues for etiology of hearing loss in patients with Chudley-McCullough syndrome.

Early onset adult deafness in the Rhodesian Ridgeback dog is associated with an in-frame deletion in the EPS8L2 gene

Domestic dogs exhibit diverse types of both congenital and non-congenital hearing losses. Rhodesian Ridgebacks can suffer from a progressive hearing loss in the early stage of their life, a condition known as early onset adult deafness (EOAD), where they lose their hearing ability within 1–2 years after birth. In order to investigate the genetic basis of this hereditary hearing disorder, we performed a genome-wide association study (GWAS) by using a sample of 23 affected and 162 control Rhodesian Ridgebacks. We identified a genomic region on canine chromosome 18 (CFA18) that is strongly associated with EOAD, and our subsequent targeted Sanger sequencing analysis identified a 12-bp inframe deletion in EPS8L2 (CFA18:25,868,739–25,868,751 in the UMICH_Zoey_3.1/canFam5 reference genome build). Additional genotyping confirmed a strong association between the 12-bp deletion and EOAD, where all affected dogs were homozygous for the deletion, while none of the control dogs was a deletion homozygote. A segregation pattern of this deletion in a 2-generation nuclear family indicated an autosomal recessive mode of inheritance. Since EPS8L2 plays a critical role in the maintenance and integrity of the inner ear hair cells in humans and other mammals, the inframe deletion found in this study represents a strong candidate causal mutation for EOAD in Rhodesian Ridgebacks. Genetic and clinical similarities between childhood deafness in humans and EOAD in Rhodesian Ridgebacks emphasizes the potential value of this dog breed in translational research in hereditary hearing disorders.

Calcium- and Integrin-Binding Protein 2 (CIB2) in Physiology and Disease: Bright and Dark Sides

Calcium- and integrin-binding protein 2 (CIB2) is a small EF-hand protein capable of binding Mg²⁺ and Ca²⁺ ions. While its biological function remains largely unclear, an increasing number of studies have shown that CIB2 is an essential component of the mechano-transduction machinery that operates in cochlear hair cells. Mutations in the gene encoding CIB2 have been associated with non-syndromic deafness. In addition to playing an important role in the physiology of hearing, CIB2 has been implicated in a multitude of very different processes, ranging from integrin signaling in platelets and skeletal muscle to autophagy, suggesting extensive functional plasticity. In this review, we summarize the current understanding of biochemical and biophysical properties of CIB2 and the biological roles that have been proposed for the protein in a variety of processes. We also highlight the many molecular aspects that remain unclarified and deserve further investigation.

Grxcr1 regulates hair bundle morphogenesis and is required for normal mechanoelectrical transduction in mouse cochlear hair cells

Tasmanian devil (tde) mice are deaf and exhibit circling behaviour. Sensory hair cells of mutants show disorganised hair bundles with abnormally thin stereocilia. The origin of this mutation is the insertion of a transgene which disrupts expression of the Grxcr1 (glutaredoxin cysteine rich 1) gene. We report here that Grxcr1 exons and transcript sequences are not affected by the transgene insertion in tde homozygous (tde/tde) mice. Furthermore, 5'RACE PCR experiments showed the presence of two different transcripts of the Grxcr1 gene, expressed in both tde/tde and in wild-type controls. However, quantitative analysis of Grxcr1 transcripts revealed a significantly decreased mRNA level in tde/tde mice. The key stereociliary proteins ESPN, MYO7A, EPS8 and PTPRQ were distributed in hair bundles of homozygous tde mutants in a similar pattern compared with control mice. We found that the abnormal morphology of the stereociliary bundle was associated with a reduction in the size and Ca2+-sensitivity of the mechanoelectrical transducer (MET) current. We propose that GRXCR1 is key for the normal growth of the stereociliary bundle prior to the onset of hearing, and in its absence hair cells are unable to mature into fully functional sensory receptors.

Ca2+ entry through mechanotransduction channels localizes BAIAP2L2 to stereocilia tips

Brain-specific angiogenesis inhibitor 1-associated protein 2-like protein 2 (BAIAP2L2), a membrane-binding protein required for the maintenance of mechanotransduction in hair cells, is selectively retained at the tips of transducing stereocilia. BAIAP2L2 trafficked to stereocilia tips in the absence of EPS8, but EPS8 increased the efficiency of localization. A tripartite complex of BAIAP2L2, EPS8, and MYO15A formed efficiently in vitro, and these three proteins robustly targeted to filopodia tips when coexpressed in cultured cells. Mice lacking functional transduction channels no longer concentrated BAIAP2L2 at row 2 stereocilia tips, a result that was phenocopied by blocking channels with tubocurarine in cochlear explants. Transduction channels permit Ca²⁺ entry into stereocilia, and we found that membrane localization of BAIAP2L2 was enhanced in the presence of Ca²⁺. Finally, reduction of intracellular Ca²⁺ in hair cells using BAPTA-AM led to a loss of BAIAP2L2 at stereocilia tips. Taken together, our results show that a MYO15A-EPS8 complex transports BAIAP2L2 to stereocilia tips, and Ca²⁺ entry through open channels at row 2 tips retains BAIAP2L2 there.

Espin overexpression causes stereocilia defects and provides an anti-capping effect on actin polymerization

Stereocilia are actin-based projections of hair cells that are arranged in a step like array, in rows of increasing height, and that constitute the mechanosensory organelle used for the senses of hearing and balance. In order to function properly, stereocilia must attain precise sizes in different hair cell types and must coordinately form distinct rows with varying lengths. Espins are actin-bundling proteins that have a well-characterized role in stereocilia formation; loss of function mutations in Espin result in shorter stereocilia and deafness in the jerker mouse. Here we describe the generation of an Espin overexpressing transgenic mouse line that results in longer first row stereocilia and discoordination of second-row stereocilia length. Furthermore, Espin overexpression results in the misregulation of other stereocilia factors including GNAI3, GPSM2, EPS8, WHRN, and MYO15A, revealing that GNAI3 and GPSM2 are dispensable for stereocilia overgrowth. Finally, using an in vitro actin polymerization assay we show that espin provides an anti-capping function that requires both the G-actin binding WH2 domain as well as either the C-terminal F-actin binding domain or the internal xAB actin-binding domain. Our results provide a novel function for Espins at the barbed ends of actin filaments distinct from its previous known function of actin bundling that may account for their effects on stereocilia growth.

Neuroplastin genetically interacts with Cadherin 23 and the encoded isoform Np55 is sufficient for cochlear hair cell function and hearing

Mammalian hearing involves the mechanoelectrical transduction (MET) of sound-induced fluid waves in the cochlea. Essential to this process are the specialised sensory cochlear cells, the inner (IHCs) and outer hair cells (OHCs). While genetic hearing loss is highly heterogeneous, understanding the requirement of each gene will lead to a better understanding of the molecular basis of hearing and also to therapeutic opportunities for deafness. The Neuroplastin (Nptn) gene, which encodes two protein isoforms Np55 and Np65, is required for hearing, and homozygous loss-of-function mutations that affect both isoforms lead to profound deafness in mice. Here we have utilised several distinct mouse models to elaborate upon the spatial, temporal, and functional requirement of Nptn for hearing. While we demonstrate that both Np55 and Np65 are present in cochlear cells, characterisation of a Np65-specific mouse knockout shows normal hearing thresholds indicating that Np65 is functionally redundant for hearing. In contrast, we find that Nptn-knockout mice have significantly reduced maximal MET currents and MET channel open probabilities in mature OHCs, with both OHCs and IHCs also failing to develop fully mature basolateral currents. Furthermore, comparing the hearing thresholds and IHC synapse structure of Nptn-knockout mice with those of mice that lack Nptn only in IHCs and OHCs shows that the majority of the auditory deficit is explained by hair cell dysfunction, with abnormal afferent synapses contributing only a small proportion of the hearing loss. Finally, we show that continued expression of Neuroplastin in OHCs of adult mice is required for membrane localisation of Plasma Membrane Ca2+ ATPase 2 (PMCA2), which is essential for hearing function. Moreover, Nptn haploinsufficiency phenocopies Atp2b2 (encodes PMCA2) mutations, with heterozygous Nptn-knockout mice exhibiting hearing loss through genetic interaction with the Cdh23ahl allele. Together, our findings provide further insight to the functional requirement of Neuroplastin for mammalian hearing.

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.

The Rho GTPase Cell Division Cycle 42 Regulates Stereocilia Development in Cochlear Hair Cells

Stereocilia are actin-based cell protrusions on the apical surface of inner ear hair cells, playing a pivotal role in hearing and balancing sensation. The development and maintenance of stereocilia is tightly regulated and deficits in this process usually lead to hearing or balancing disorders. The Rho GTPase cell division cycle 42 (CDC42) is a key regulator of the actin cytoskeleton. It has been reported to localize in the hair cell stereocilia and play important roles in stereocilia maintenance. In the present work, we utilized hair cell-specific Cdc42 knockout mice and CDC42 inhibitor ML141 to explore the role of CDC42 in stereocilia development. Our data show that stereocilia height and width as well as stereocilia resorption are affected in Cdc42-deficient cochlear hair cells when examined at postnatal day 8 (P8). Moreover, ML141 treatment leads to planar cell polarity (PCP) deficits in neonatal hair cells. We also show that overexpression of a constitutively active mutant CDC42 in cochlear hair cells leads to enhanced stereocilia developmental deficits. In conclusion, the present data suggest that CDC42 plays a pivotal role in regulating hair cell stereocilia development.

Apparent homozygosity for a novel splicing variant in EPS8 causes congenital profound hearing loss

Autosomal recessive deafness-102 (DFNB102), a new profound prelingual non-syndromic hearing loss, is caused by mutations in the EPS8 gene. To date, only three such consanguineous families with three different homozygous variants in EPS8 have been reported. Here, we report the fourth case from a non-consanguineous Chinese family, an 11-month-old male infant presented with congenital profound non-syndromic hearing loss. Trio whole-exome sequencing initially identified the patient with a novel seemingly homozygous splicing variant NM_004447.5: c.1435-2A > T in intron 14 of the EPS8 gene and was inherited from his father; further CNVs analysis identified a novel 65.9 kb intragenic deletion and was inherited from his mother. The deletion is covering intron 14 that could account for the apparent homozygosity of the patient. In vitro splicing assay showed the variant c.1435-2A > T creates a new donor site at position c.1443, which is predicted to produce a stop codon after 14 additional amino acids (p.His479Cysfs*14). Furthermore, quantitative allele-specific expression assay showed that relative EPS8 gene expression in the patient significantly decreased (0-fold for the wild-type transcript and 0.25-0.27-fold for the mutant transcript) compared to the control (P < 0.05), indicating the pathogenicity of the identified variants. Overall, our study provides additional evidence that EPS8 is a causative gene for DFNB102 and highlights the clinical utility of simultaneous analysis of CNVs and SNVs to avoid potential errors in the diagnosis and interpretation of patients with apparent homozygosity.

BAIAP2L2 is required for the maintenance of mechanotransducing stereocilia of cochlear hair cells

Stereocilia are actin-based cell protrusions of inner ear hair cells that play an essential role in mechano-electrical transduction (MET). Stereocilia are organized into several rows of increasing heights with the MET protein complex localized at the tips of shorter row stereocilia. At the tips of shorter row mechanotransducing stereocilia also resides a so-called "row 2 protein complex" whose dysfunction causes degeneration of the mechanotransducing stereocilia. In the present work, we show that BAIAP2L2 is localized at the tips of shorter row stereocilia in neonatal and adult mouse cochlear hair cells. Baiap2l2 inactivation causes degeneration of the mechanotransducing stereocilia, which eventually leads to profound hearing loss in mice of either sex. Consistently, electrophysiology and FM 1-43FX dye uptake results confirm that MET currents are compromised in Baiap2l2 knockout mice. Moreover, BAIAP2L2 binds to known row 2 complex components EPS8L2, TWF2, and CAPZB2, and the stereociliary tip localization of CAPZB2 is dependent on functional BAIAP2L2. Interestingly, BAIAP2L2 also binds to CIB2, a known MET complex component, and the stereociliary tip localization of BAIAP2L2 is abolished in Cib2 knockout mice. In conclusion, our present data suggest that BAIAP2L2 is a row 2 complex component, and is required for the maintenance of mechanotransducing stereocilia. Meanwhile, specific MET components such as CIB2 might play a direct role in stereocilia maintenance through binding to BAIAP2L2.

Direct visualization of epithelial microvilli biogenesis

Microvilli are actin-bundle-supported surface protrusions that play essential roles in diverse epithelial functions. To develop our understanding of microvilli biogenesis, we used live imaging to directly visualize protrusion growth at early stages of epithelial differentiation. Time-lapse data revealed that specific factors, including epidermal growth factor pathway substrate 8 (EPS8) and insulin-receptor tyrosine kinase substrate (IRTKS) (also known as BAIAP2L1), appear in diffraction-limited puncta at the cell surface and mark future sites of microvillus growth. New core actin bundles elongate from these puncta in parallel with the arrival of ezrin and subsequent plasma membrane encapsulation. In addition to de novo growth, we also observed that new microvilli emerge from pre-existing protrusions. Moreover, we found that nascent microvilli can also collapse, characterized first by loss of membrane wrapping and ezrin enrichment, followed by a sharp decrease in distal tip EPS8 and IRTKS levels, and ultimately disassembly of the core actin bundle itself. These studies are the first to offer a temporally resolved microvillus growth mechanism and highlight factors that participate in this process; they also provide important insights on the growth of apical specializations that will likely apply to diverse epithelial contexts.

MET currents and otoacoustic emissions from mice with a detached tectorial membrane indicate the extracellular matrix regulates Ca2+ near stereocilia

KEY POINTS

The aim was to determine whether detachment of the tectorial membrane (TM) from the organ of Corti in Tecta/Tectb-/- mice affects the biophysical properties of cochlear outer hair cells (OHCs). Tecta/Tectb-/- mice have highly elevated hearing thresholds, but OHCs mature normally. Mechanoelectrical transducer (MET) channel resting open probability (Po ) in mature OHC is ∼50% in endolymphatic [Ca2+ ], resulting in a large standing depolarizing MET current that would allow OHCs to act optimally as electromotile cochlear amplifiers. MET channel resting Po in vivo is also high in Tecta/Tectb-/- mice, indicating that the TM is unlikely to statically bias the hair bundles of OHCs. Distortion product otoacoustic emissions (DPOAEs), a readout of active, MET-dependent, non-linear cochlear amplification in OHCs, fail to exhibit long-lasting adaptation to repetitive stimulation in Tecta/Tectb-/- mice. We conclude that during prolonged, sound-induced stimulation of the cochlea the TM may determine the extracellular Ca2+ concentration near the OHC's MET channels.

ABSTRACT

The tectorial membrane (TM) is an acellular structure of the cochlea that is attached to the stereociliary bundles of the outer hair cells (OHCs), electromotile cells that amplify motion of the cochlear partition and sharpen its frequency selectivity. Although the TM is essential for hearing, its role is still not fully understood. In Tecta/Tectb-/- double knockout mice, in which the TM is not coupled to the OHC stereocilia, hearing sensitivity is considerably reduced compared with that of wild-type animals. In vivo, the OHC receptor potentials, assessed using cochlear microphonics, are symmetrical in both wild-type and Tecta/Tectb-/- mice, indicating that the TM does not bias the hair bundle resting position. The functional maturation of hair cells is also unaffected in Tecta/Tectb-/- mice, and the resting open probability of the mechanoelectrical transducer (MET) channel reaches values of ∼50% when the hair bundles of mature OHCs are bathed in an endolymphatic-like Ca2+ concentration (40 μM) in vitro. The resultant large MET current depolarizes OHCs to near -40 mV, a value that would allow optimal activation of the motor protein prestin and normal cochlear amplification. Although the set point of the OHC receptor potential transfer function in vivo may therefore be determined primarily by endolymphatic Ca2+ concentration, repetitive acoustic stimulation fails to produce adaptation of MET-dependent otoacoustic emissions in vivo in the Tecta/Tectb-/- mice. Therefore, the TM is likely to contribute to the regulation of Ca2+ levels around the stereocilia, and thus adaptation of the OHC MET channel during prolonged sound stimulation.

Actin at stereocilia tips is regulated by mechanotransduction and ADF/cofilin

Stereocilia on auditory sensory cells are actin-based protrusions that mechanotransduce sound into an electrical signal. These stereocilia are arranged into a bundle with three rows of increasing length to form a staircase-like morphology that is required for hearing. Stereocilia in the shorter rows, but not the tallest row, are mechanotransducing because they have force-sensitive channels localized at their tips. The onset of mechanotransduction during mouse postnatal development refines stereocilia length and width. However, it is unclear how actin is differentially regulated between stereocilia in the tallest row of the bundle and the shorter, mechanotransducing rows. Here, we show actin turnover is increased at the tips of mechanotransducing stereocilia during bundle maturation. Correspondingly, from birth to postnatal day 6, these stereocilia had increasing amounts of available actin barbed ends, where monomers can be added or lost readily, as compared with the non-mechanotransducing stereocilia in the tallest row. The increase in available barbed ends depended on both mechanotransduction and MYO15 or EPS8, which are required for the normal specification and elongation of the tallest row of stereocilia. We also found that loss of the F-actin-severing proteins ADF and cofilin-1 decreased barbed end availability at stereocilia tips. These proteins enriched at mechanotransducing stereocilia tips, and their localization was perturbed by the loss of mechanotransduction, MYO15, or EPS8. Finally, stereocilia lengths and widths were dysregulated in Adf and Cfl1 mutants. Together, these data show that actin is remodeled, likely by a severing mechanism, in response to mechanotransduction.

Loss of Baiap2l2 destabilizes the transducing stereocilia of cochlear hair cells and leads to deafness

KEY POINTS

Mechanoelectrical transduction at auditory hair cells requires highly specialized stereociliary bundles that project from their apical surface, forming a characteristic graded 'staircase' structure. The morphogenesis and maintenance of these stereociliary bundles is a tightly regulated process requiring the involvement of several actin-binding proteins, many of which are still unidentified. We identify a new stereociliary protein, the I-BAR protein BAIAP2L2, which localizes to the tips of the shorter transducing stereocilia in both inner and outer hair cells (IHCs and OHCs). We find that Baiap2l2 deficient mice lose their second and third rows of stereocilia, their mechanoelectrical transducer current, and develop progressive hearing loss, becoming deaf by 8 months of age. We demonstrate that BAIAP2L2 localization to stereocilia tips is dependent on the motor protein MYO15A and its cargo EPS8. We propose that BAIAP2L2 is a new key protein required for the maintenance of the transducing stereocilia in mature cochlear hair cells.

ABSTRACT

The transduction of sound waves into electrical signals depends upon mechanosensitive stereociliary bundles that project from the apical surface of hair cells within the cochlea. The height and width of these actin-based stereocilia is tightly regulated throughout life to establish and maintain their characteristic staircase-like structure, which is essential for normal mechanoelectrical transduction. Here, we show that BAIAP2L2, a member of the I-BAR protein family, is a newly identified hair bundle protein that is localized to the tips of the shorter rows of transducing stereocilia in mouse cochlear hair cells. BAIAP2L2 was detected by immunohistochemistry from postnatal day 2.5 (P2.5) throughout adulthood. In Baiap2l2 deficient mice, outer hair cells (OHCs), but not inner hair cells (IHCs), began to lose their third row of stereocilia and showed a reduction in the size of the mechanoelectrical transducer current from just after P9. Over the following post-hearing weeks, the ordered staircase structure of the bundle progressively deteriorates, such that, by 8 months of age, both OHCs and IHCs of Baiap2l2 deficient mice have lost most of the second and third rows of stereocilia and become deaf. We also found that BAIAP2L2 interacts with other key stereociliary proteins involved in normal hair bundle morphogenesis, such as CDC42, RAC1, EPS8 and ESPNL. Furthermore, we show that BAIAP2L2 localization to the stereocilia tips depends on the motor protein MYO15A and its cargo EPS8. We propose that BAIAP2L2 is key to maintenance of the normal actin structure of the transducing stereocilia in mature mouse cochlear hair cells.

Phase separation-mediated condensation of Whirlin-Myo15-Eps8 stereocilia tip complex

Stereocilia, the mechanosensory organelles on the apical surface of hair cells, are necessary to detect sound and carry out mechano-electrical transduction. An electron-dense matrix is located at the distal tips of stereocilia and plays crucial roles in the regulation of stereocilia morphology. Mutations of the components in this tip complex density (TCD) have been associated with profound deafness. However, the mechanism underlying the formation of the TCD is largely unknown. Here, we discover that the specific multivalent interactions among the Whirlin-myosin 15 (Myo15)-Eps8 complex lead to the formation of the TCD-like condensates through liquid-liquid phase separation. The reconstituted TCD-like condensates effectively promote actin bundling. A deafness-associated mutation of Myo15 interferes with the condensates formation and consequently impairs actin bundling. Therefore, our study not only suggests that the TCD in hair cell stereocilia may form via phase separation but it also provides important clues for the possible mechanism underlying hearing loss.

The ATPase mechanism of myosin 15, the molecular motor mutated in DFNB3 human deafness

Cochlear hair cells each possess an exquisite bundle of actin-based stereocilia that detect sound. Unconventional myosin 15 (MYO15) traffics and delivers critical molecules required for stereocilia development and thus is essential for building the mechanosensory hair bundle. Mutations in the human MYO15A gene interfere with stereocilia trafficking and cause hereditary hearing loss, DFNB3, but the impact of these mutations is not known, as MYO15 itself is poorly characterized. To learn more, we performed a kinetic study of the ATPase motor domain to characterize its mechanochemical cycle. Using the baculovirus-Sf9 system, we purified a recombinant minimal motor domain (S1) by coexpressing the mouse MYO15 ATPase, essential and regulatory light chains that bind its IQ domains, and UNC45 and HSP90A chaperones required for correct folding of the ATPase. MYO15 purified with either UNC45A or UNC45B coexpression had similar ATPase activities (kcat = ∼ 6 s-1 at 20 °C). Using stopped-flow and quenched-flow transient kinetic analyses, we measured the major rate constants describing the ATPase cycle, including ATP, ADP, and actin binding; hydrolysis; and phosphate release. Actin-attached ADP release was the slowest measured transition (∼12 s-1 at 20 °C), although this did not rate-limit the ATPase cycle. The kinetic analysis shows the MYO15 motor domain has a moderate duty ratio (∼0.5) and weak thermodynamic coupling between ADP and actin binding. These findings are consistent with MYO15 being kinetically adapted for processive motility when oligomerized. Our kinetic characterization enables future studies into how deafness-causing mutations affect MYO15 and disrupt stereocilia trafficking necessary for hearing.

Up-regulation of autophagy and apoptosis of cochlear hair cells in mouse models for deafness

INTRODUCTION

Hearing loss is one of the most common sensory disorders. Recent findings have shown that the apoptotic program and autophagy are related to hearing loss. The aim of the study was to explore the effects of noise and cisplatin exposure on apoptosis and autophagy in the hair cells of the cochleae.

MATERIAL AND METHODS

C57BL/6 mice were randomly divided into 3 groups (n = 10 for each): the control group, the noise model group and the cisplatin model group. Auditory brainstem response (ABR) measurements were used to detect the hearing thresholds. TUNEL assay was used to evaluate cell apoptosis. Western blot and immunofluorescence were performed to examine the apoptosis- and autophagy-related proteins.

RESULTS

The mice exhibited substantial hearing loss after noise and cisplatin exposure. Additionally, more TUNEL positive cells were observed in the mice after noise and cisplatin exposure compared with the control group. Moreover, the protein expression levels of Beclin-1, LC3-II, Bax and cleaved caspase-3 were significantly increased, while the expression of Bcl-2 was notably decreased in the cochlea after noise (p = 0.0278, 0.0075, 0.0142, 0.0158, 0.0131 respectively) and cisplatin (p = 0.0220, 0.0075, 0.0024, 0.0161, 0.0452 respectively) exposure compared with the control group. Besides, the ratio of LC3-II/LC3-I was substantially higher in the mice treated by cisplatin (p = 0.0046) and noise (p = 0.0220) compared with the control group.

CONCLUSIONS

Our findings demonstrated for the first time that noise and cisplatin exposure promoted apoptosis and autophagy in the hair cells of the cochleae. This study provides new insights into the mechanisms of noise- or cisplatin-induced hearing loss.

Myosin VII, USH1C, and ANKS4B or USH1G Together Form Condensed Molecular Assembly via Liquid-Liquid Phase Separation

Hair cell stereocilia tip-links function to sense mechanical forces generated by sound waves and maintain the structure of stereocilia by rooting the tail of cadherins to highly dense structures known as tip-link densities. Although the molecular components are largely known, the mechanisms underlying the tip-link density formation are unknown. Here, we show that Myosin VIIB (MYO7B), USH1C, and ANKS4B, which form a specific complex stabilizing tip-links in intestine microvilli, could form dense condensates via liquid-liquid phase separation in vitro and in cells. The MYO7A, USH1C, and USH1G complex also undergoes phase separation in cells. Formation of the MYO7A/USH1C/USH1G and MYO7B/USH1C/ANKS4B condensates requires strong and multivalent interactions between proteins in both tripartite complexes. Point mutations of MYO7A found in Usher syndrome patients weaken or even disrupt the multivalent interactions of the MYO7A/USH1C/USH1G complex and impair its phase separation. Thus, the stereocilia tip-link densities may form via phase separation of the MYO7A/USH1C/USH1G complex.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Cell Biology: Function Guides Form of Auditory Sensory Cells

Mechanosensory bundles on auditory sensory cells are composed of stereocilia that grow in rows of decreasing height. This pattern depends on the specification of the eventual tallest row, then the assignment of distinct molecular identities to the shorter rows. Mechanotransduction refines and maintains row identity, thus instructing the form of the bundle.

Myosin-XVa Controls Both Staircase Architecture and Diameter Gradation of Stereocilia Rows in the Auditory Hair Cell Bundles

Mammalian hair cells develop their mechanosensory bundles through consecutive phases of stereocilia elongation, thickening, and retraction of supernumerary stereocilia. Many molecules involved in stereocilia elongation have been identified, including myosin-XVa. Significantly less is known about molecular mechanisms of stereocilia thickening and retraction. Here, we used scanning electron microscopy (SEM) to quantify postnatal changes in number and diameters of the auditory hair cell stereocilia in shaker-2 mice (Myo15^sh2) that lack both "long" and "short" isoforms of myosin-XVa, and in mice lacking only the "long" myosin-XVa isoform (Myo15^∆N). Previously, we observed large mechanotransduction current in young postnatal inner (IHC) and outer (OHC) hair cells of both these strains. Stereocilia counts showed nearly identical developmental retraction of supernumerary stereocilia in control heterozygous, Myo15^sh2/sh2, and Myo15^∆N/∆N mice, suggesting that this retraction is largely unaffected by myosin-XVa deficiency. However, myosin-XVa deficiency does affect stereocilia diameters. In control, the first (tallest) and second row stereocilia grow in diameter simultaneously. However, the third row stereocilia in IHCs grow only until postnatal day 1-2 and then become thinner. In OHCs, they also grow slower than taller stereocilia, forming a stereocilia diameter gradation within a hair bundle. The sh2 mutation disrupts this gradation and makes all stereocilia nearly identical in thickness in both IHCs and OHCs, with only subtle residual diameter differences. All Myo15^sh2/sh2 stereocilia grow postnatally including the third row, which is not a part of normal development. Serial sections with focused ion beam (FIB)-SEM confirmed that diameter changes of Myo15^sh2/sh2 IHC and OHC stereocilia resulted from corresponding changes of their actin cores. In contrast to Myo15^sh2/sh2, Myo15^∆N/∆N hair cells develop prominent stereocilia diameter gradation. Thus, besides building the staircase, the short isoform of myosin-XVa is essential for controlling the diameter of the third row stereocilia and formation of the stereocilia diameter gradation in a hair bundle.

Mechanotransduction-Dependent Control of Stereocilia Dimensions and Row Identity in Inner Hair Cells

Actin-rich structures, like stereocilia and microvilli, are assembled with precise control of length, diameter, and relative spacing. By quantifying actin-core dimensions of stereocilia from phalloidin-labeled mouse cochleas, we demonstrated that inner hair cell stereocilia developed in specific stages, where a widening phase is sandwiched between two lengthening phases. Moreover, widening of the second-tallest stereocilia rank (row 2) occurred simultaneously with the appearance of mechanotransduction. Correspondingly, Tmc1^KO/KO;Tmc2^KO/KO or Tmie^KO/KO hair cells, which lack transduction, have significantly altered stereocilia lengths and diameters, including a narrowed row 2. EPS8 and the short splice isoform of MYO15A, identity markers for mature row 1 (the tallest row), lost their row exclusivity in transduction mutants. GNAI3, another member of the mature row 1 complex, accumulated at mutant row 1 tips at considerably lower levels than in wild-type bundles. Alterations in stereocilia dimensions and in EPS8 distribution seen in transduction mutants were mimicked by block of transduction channels of cochlear explants in culture. In addition, proteins normally concentrated at mature row 2 tips were also distributed differently in transduction mutants; the heterodimeric capping protein subunit CAPZB and its partner TWF2 never concentrated at row 2 tips like they do in wild-type bundles. The altered distribution of marker proteins in transduction mutants was accompanied by increased variability in stereocilia length. Transduction channels thus specify and maintain row identity, control addition of new actin filaments to increase stereocilia diameter, and coordinate stereocilia height within rows.

Elmod3 knockout leads to progressive hearing loss and abnormalities in cochlear hair cell stereocilia

ELMOD3, an ARL2 GTPase-activating protein, is implicated in causing hearing impairment in humans. However, the specific role of ELMOD3 in auditory function is still far from being elucidated. In the present study, we used the CRISPR/Cas9 technology to establish an Elmod3 knockout mice line in the C57BL/6 background (hereinafter referred to as Elmod3-/- mice) and investigated the role of Elmod3 in the cochlea and auditory function. Elmod3-/- mice started to exhibit hearing loss from 2 months of age, and the deafness progressed with aging, while the vestibular function of Elmod3-/- mice was normal. We also observed that Elmod3-/- mice showed thinning and receding hair cells in the organ of Corti and much lower expression of F-actin cytoskeleton in the cochlea compared with wild-type mice. The deafness associated with the mutation may be caused by cochlear hair cells dysfunction, which manifests with shortening and fusion of inner hair cells stereocilia and progressive degeneration of outer hair cells stereocilia. Our finding associates Elmod3 deficiencies with stereocilia dysmorphologies and reveals that they might play roles in the actin cytoskeleton dynamics in cochlear hair cells, and thus relate to hearing impairment.

New insights into regulation and function of planar polarity in the inner ear

Acquisition of cell polarity generates signaling and cytoskeletal asymmetry and thus underpins polarized cell behaviors during tissue morphogenesis. In epithelial tissues, both apical-basal polarity and planar polarity, which refers to cell polarization along an axis orthogonal to the apical-basal axis, are essential for epithelial morphogenesis and function. A prime example of epithelial planar polarity can be found in the auditory sensory epithelium (or organ of Corti, OC). Sensory hair cells, the sound receptors, acquire a planar polarized apical cytoskeleton which is uniformely oriented along an axis orthogonal to the longitudinal axis of the cochlear duct. Both cell-intrinsic and tissue-level planar polarity are necessary for proper perception of sound. Here we review recent insights into the novel roles and mechanisms of planar polarity signaling gained from genetic analysis in mice, focusing mainly on the OC but also with some discussions on the vestibular sensory epithelia.

Growth factor and receptor malfunctions associated with human genetic deafness

A variety of different signaling pathways are necessary for development and maintenance of the human auditory system. Normal hearing allows for the detection of soft sounds within the frequency range of 20 to 20 000 Hz, but more importantly to perceive the human voice frequency band of 250 to 6000 Hz. Loss of hearing is common, and is a clinically heterogeneous disorder that can be caused by environmental factors such as exposure to loud noise, infections and ototoxic drugs. In addition, variants of hundreds of genes have been reported to disrupt processes required for hearing. Noncoding regulatory variants and variants of additional genes necessary for hearing remain to be discovered as many individuals with inherited deafness are without a genetic diagnosis, despite the advent of whole exome sequencing. Here, we discuss in detail some of these deafness-causing variants of genes encoding a ligand or its receptor. Spotlighted in this review are three growth factor-receptor-pairs EDN3/EDNRB, HGF/MET and JAG/NOTCH, which individually are necessary for normal hearing. We also offer our perspective on unanswered questions, future challenges and potential opportunities for treatments emerging from molecular genetic and mechanistic studies of deafness due to these causes.

Clarin-2 is essential for hearing by maintaining stereocilia integrity and function

Hearing relies on mechanically gated ion channels present in the actin-rich stereocilia bundles at the apical surface of cochlear hair cells. Our knowledge of the mechanisms underlying the formation and maintenance of the sound-receptive structure is limited. Utilizing a large-scale forward genetic screen in mice, genome mapping and gene complementation tests, we identified Clrn2 as a new deafness gene. The Clrn2clarinet/clarinet mice (p.Trp4* mutation) exhibit a progressive, early-onset hearing loss, with no overt retinal deficits. Utilizing data from the UK Biobank study, we could show that CLRN2 is involved in human non-syndromic progressive hearing loss. Our in-depth morphological, molecular and functional investigations establish that while it is not required for initial formation of cochlear sensory hair cell stereocilia bundles, clarin-2 is critical for maintaining normal bundle integrity and functioning. In the differentiating hair bundles, lack of clarin-2 leads to loss of mechano-electrical transduction, followed by selective progressive loss of the transducing stereocilia. Together, our findings demonstrate a key role for clarin-2 in mammalian hearing, providing insights into the interplay between mechano-electrical transduction and stereocilia maintenance.

PACSIN2-dependent apical endocytosis regulates the morphology of epithelial microvilli

Apical microvilli are critical for the homeostasis of transporting epithelia, yet mechanisms that control the assembly and morphology of these protrusions remain poorly understood. Previous studies in intestinal epithelial cell lines suggested a role for the F-BAR domain protein PACSIN2 in normal microvillar assembly. Here we report the phenotype of PACSIN2 KO mice and provide evidence that through its role in promoting apical endocytosis, this molecule plays a role in controlling microvillar morphology. PACSIN2 KO enterocytes exhibit reduced numbers of microvilli and defects in the microvillar ultrastructure, with membranes lifting away from rootlets of core bundles. Dynamin2, a PACSIN2 binding partner, and other endocytic factors were also lost from their normal localization near microvillar rootlets. To determine whether loss of endocytic machinery could explain defects in microvillar morphology, we examined the impact of PACSIN2 KD and endocytosis inhibition on live intestinal epithelial cells. These assays revealed that when endocytic vesicle scission fails, tubules are pulled into the cytoplasm and this, in turn, leads to a membrane-lifting phenomenon reminiscent of that observed at PACSIN2 KO brush borders. These findings lead to a new model where inward forces generated by endocytic machinery on the plasma membrane control the membrane wrapping of cell surface protrusions.

GPSM2-GNAI Specifies the Tallest Stereocilia and Defines Hair Bundle Row Identity

The transduction compartment of inner ear hair cells, the hair bundle, is composed of stereocilia rows of graded height, a property essential for sensory function that remains poorly understood at the molecular level. We previously showed that GPSM2-GNAI is enriched at stereocilia distal tips and required for their postnatal elongation and bundle morphogenesis-two characteristics shared with MYO15A (short isoform), WHRN, and EPS8 proteins. Here we first performed a comprehensive genetic analysis of the mouse auditory epithelium to show that GPSM2, GNAI, MYO15A, and WHRN operate in series within the same pathway. To understand how these functionally disparate proteins act as an obligate complex, we then systematically analyzed their distribution in normal and mutant bundles over time. We discovered that WHRN-GPSM2-GNAI is an extra module recruited by and added to a pre-existing MYO15A-EPS8 stereocilia tip complex. This extended complex is only present in the first, tallest row, and is required to stabilize larger amounts of MYO15A-EPS8 than in shorter rows, which at tips harbor only MYO15A-EPS8. In the absence of GPSM2 or GNAI function, including in the epistatic Myo15a and Whrn mutants, bundles retain an embryonic-like organization that coincides with generic stereocilia at the molecular level. We propose that GPSM2-GNAI confers on the first row its unique tallest identity and participates in generating differential row identity across the hair bundle.

Usher syndrome and non-syndromic deafness: Functions of different whirlin isoforms in the cochlea, vestibular organs, and retina

Usher syndrome (USH) is the leading cause of inherited combined vision and hearing loss. However, mutations in most USH causative genes lead to other diseases, such as hearing loss only or vision loss only. The molecular mechanisms underlying the variable disease manifestations associated with USH gene mutations are unclear. This review focuses on an USH type 2 (USH2) gene encoding whirlin (WHRN; previously known as DFNB31), mutations in which have been found to cause either USH2 subtype USH2D or autosomal recessive non-syndromic deafness type 31 (DFNB31). This review summarizes the current knowledge about different whirlin isoforms encoded by WHRN orthologs in animal models, the interactions of different whirlin isoforms with their partners, and the function of whirlin isoforms in different cellular and subcellular locations. The recent findings regarding the function of whirlin isoforms suggest that disruption of different isoforms may be one of the mechanisms underlying the variable disease manifestations caused by USH gene mutations. This review also presents recent findings about the vestibular defects in Whrn mutant mouse models, which suggests that previous assumptions about the normal vestibular function of USH2 patients need to be re-evaluated. Finally, this review describes recent progress in developing therapeutics for diseases caused by WHRN mutations.