Gelsolin Plays a Role in the Actin Polymerization Complex of Hair Cell Stereocilia

A complex of proteins scaffolded by the PDZ protein, whirlin, reside at the stereocilia tip and are critical for stereocilia development and elongation. We have shown that in outer hair cells (OHCs) whirlin is part of a larger complex involving the MAGUK protein, p55, and protein 4.1R. Whirlin interacts with p55 which is expressed exclusively in outer hair cells (OHC) in both the long stereocilia that make up the stereocilia bundle proper as well as surrounding shorter microvilli that will eventually regress. In erythrocytes, p55 forms a tripartite complex with protein 4.1R and glycophorin C promoting the assembly of actin filaments and the interaction of whirlin with p55 indicates that it plays a similar role in OHC stereocilia. However, the components directly involved in actin filament regulation in stereocilia are unknown. We have investigated additional components of the whirlin interactome by identifying interacting partners to p55. We show that the actin capping and severing protein, gelsolin, is a part of the whirlin complex. Gelsolin is detected in OHC where it localizes to the tips of the shorter rows but not to the longest row of stereocilia and the pattern of localisation at the apical hair cell surface is strikingly similar to p55. Like p55, gelsolin is ablated in the whirler and shaker2 mutants. Moreover, in a gelsolin mutant, stereocilia in the apex of the cochlea become long and straggly indicating defects in the regulation of stereocilia elongation. The identification of gelsolin provides for the first time a link between the whirlin scaffolding protein complex involved in stereocilia elongation and a known actin regulatory molecule.     

The deaf mouse mutant whirler suggests a role for whirlin in actin filament dynamics and stereocilia development

Stereocilia, finger-like projections forming the hair bundle on the apical surface of sensory hair cells in the cochlea, are responsible for mechanosensation and ultimately the perception of sound. The actin cytoskeleton of the stereocilia contains hundreds of tightly cross-linked parallel actin filaments in a paracrystalline array and it is vital for their function. Although several genes have been identified and associated with stereocilia development, the molecular mechanisms responsible for stereocilia growth, maintenance and organisation of the hair bundle have not been fully resolved. Here we provide further characterisation of the stereocilia of the whirler mouse mutant. We found that a lack of whirlin protein in whirler mutants results in short stereocilia with larger diameters without a corresponding increase in the number of actin filaments in inner hair cells. However, a decrease in the actin filament packing density was evident in the whirler mutant. The electron-density at the tip of each stereocilium was markedly patchy and irregular in the whirler mutants compared with a uniform band in controls. The outer hair cell stereocilia of the whirler homozygote also showed an increase in diameter and variable heights within bundles. The number of outer hair cell stereocilia was significantly reduced and the centre-to-centre spacing between the stereocilia was greater than in the wildtype. Our findings suggest that whirlin plays an important role in actin filament packing and dynamics during postnatal stereocilium elongation.     

Myosin VIIa,harmonin and cadherin 23, three Usher I gene products that cooperate to shape the sensory hair cell bundle

Deaf-blindness in three distinct genetic forms of Usher type I syndrome (USH1) is caused by defects in myosin VIIa, harmonin and cadherin 23. Despite being critical for hearing, the functions of these proteins in the inner ear remain elusive. Here we show that harmonin, a PDZ domain-containing protein, and cadherin 23 are both present in the growing stereocilia and that they bind to each other. Moreover, we demonstrate that harmonin b is an F-actin-bundling protein, which is thus likely to anchor cadherin 23 to the stereocilia microfilaments, thereby identifying a novel anchorage mode of the cadherins to the actin cytoskeleton. Moreover, harmonin b interacts directly with myosin VIIa, and is absent from the disorganized hair bundles of myosin VIIa mutant mice, suggesting that myosin VIIa conveys harmonin b along the actin core of the developing stereocilia. We propose that the shaping of the hair bundle relies on a functional unit composed of myosin VIIa, harmonin b and cadherin 23 that is essential to ensure the cohesion of the stereocilia.     

Regulation of stereocilia length by myosin XVa and whirlin depends on the actin-regulatory protein Eps8

Myosin XVa (MyoXVa) and its cargo whirlin are implicated in deafness and vestibular dysfunction and have been shown to localize at stereocilia tips and to be essential for the elongation of these actin protrusions [1-4]. Given that whirlin has no known actin-regulatory activity, it remains unclear how these proteins work together to influence stereocilia length. Here we show that the actin-regulatory protein Eps8 [5] interacts with MyoXVa and that mice lacking Eps8 show short stereocilia compared to MyoXVa- and whirlin-deficient mice. We show that Eps8 fails to accumulate at the tips of stereocilia in the absence of MyoXVa, that overexpression of MyoXVa results in both elongation of stereocilia and increased accumulation of Eps8 at stereocilia tips, and that the exogenous expression of MyoXVa in MyoXVa-deficient hair cells rescues Eps8 tip localization. We find that Eps8 also interacts with whirlin and that the expression of both Eps8 and MyoXVa at stereocilia tips is reduced in whirlin-deficient mice. We conclude that MyoXVa, whirlin, and Eps8 are integral components of the stereocilia tip complex, where Eps8 is a central actin-regulatory element for elongation of the stereocilia actin core.     

An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal

We have previously shown that the seemingly static paracrystalline actin core of hair cell stereocilia undergoes continuous turnover. Here, we used the same approach of transfecting hair cells with actin-green fluorescent protein (GFP) and espin-GFP to characterize the turnover process. Actin and espin are incorporated at the paracrystal tip and flow rearwards at the same rate. The flux rates (approximately 0.002-0.04 actin subunits s(-1)) were proportional to the stereocilia length so that the entire staircase stereocilia bundle was turned over synchronously. Cytochalasin D caused stereocilia to shorten at rates matching paracrystal turnover. Myosins VI and VIIa were localized alongside the actin paracrystal, whereas myosin XVa was observed at the tips at levels proportional to stereocilia lengths. Electron microscopy analysis of the abnormally short stereocilia in the shaker 2 mice did not show the characteristic tip density. We argue that actin renewal in the paracrystal follows a treadmill mechanism, which, together with the myosins, dynamically shapes the functional architecture of the stereocilia bundle.     

Myosin-XVa is required for tip localization of whirlin and differential elongation of hair-cell stereocilia

Stereocilia are microvilli-derived mechanosensory organelles that are arranged in rows of graded heights on the apical surface of inner-ear hair cells. The 'staircase'-like architecture of stereocilia bundles is necessary to detect sound and head movement, and is achieved through differential elongation of the actin core of each stereocilium to a predetermined length. Abnormally short stereocilia bundles that have a diminished staircase are characteristic of the shaker 2 (Myo15a(sh2)) and whirler (Whrn(wi)) strains of deaf mice. We show that myosin-XVa is a motor protein that, in vivo, interacts with the third PDZ domain of whirlin through its carboxy-terminal PDZ-ligand. Myosin-XVa then delivers whirlin to the tips of stereocilia. Moreover, if green fluorescent protein (GFP)-Myo15a is transfected into hair cells of Myo15a(sh2) mice, the wild-type pattern of hair bundles is restored by recruitment of endogenous whirlin to the tips of stereocilia. The interaction of myosin-XVa and whirlin is therefore a key event in hair-bundle morphogenesis.     

A core cochlear phenotype in USH1 mouse mutants implicates fibrous links of the hair bundle in its cohesion, orientation and differential growth

The planar polarity and staircase-like pattern of the hair bundle are essential to the mechanoelectrical transduction function of inner ear sensory cells. Mutations in genes encoding myosin VIIa, harmonin, cadherin 23, protocadherin 15 or sans cause Usher syndrome type I (USH1, characterized by congenital deafness, vestibular dysfunction and retinitis pigmentosa leading to blindness) in humans and hair bundle disorganization in mice. Whether the USH1 proteins are involved in common hair bundle morphogenetic processes is unknown. Here, we show that mouse models for the five USH1 genetic forms share hair bundle morphological defects. Hair bundle fragmentation and misorientation (25-52 degrees mean kinociliary deviation, depending on the mutant) were detected as early as embryonic day 17. Abnormal differential elongation of stereocilia rows occurred in the first postnatal days. In the emerging hair bundles, myosin VIIa, the actin-binding submembrane protein harmonin-b, and the interstereocilia-kinocilium lateral link components cadherin 23 and protocadherin 15, all concentrated at stereocilia tips, in accordance with their known in vitro interactions. Soon after birth, harmonin-b switched from the tip of the stereocilia to the upper end of the tip link, which also comprises cadherin 23 and protocadherin 15. This positional change did not occur in mice deficient for cadherin 23 or protocadherin 15. We suggest that tension forces applied to the early lateral links and to the tip link, both of which can be anchored to actin filaments via harmonin-b, play a key role in hair bundle cohesion and proper orientation for the former, and in stereociliary elongation for the latter.     

Myosin VI is required for structural integrity of the apical surface of sensory hair cells in zebrafish

Unconventional myosins have been associated with hearing loss in humans, mice, and zebrafish. Mutations in myosin VI cause both recessive and dominant forms of nonsyndromic deafness in humans and deafness in Snell's waltzer mice associated with abnormal fusion of hair cell stereocilia. Although myosin VI has been implicated in diverse cellular processes such as vesicle trafficking and epithelial morphogenesis, the role of this protein in the sensory hair cells remains unclear. To investigate the function of myosin VI in zebrafish, we cloned and examined the expression pattern of myosin VI, which is duplicated in the zebrafish genome. One duplicate, myo6a, is expressed in a ubiquitous pattern during early development and at later stages, and is highly expressed in the brain, gut, and kidney. myo6b, on the other hand, is predominantly expressed in the sensory epithelium of the ear and lateral line at all developmental stages examined. Both molecules have different splice variants expressed in these tissues. Using a candidate gene approach, we show that myo6b is satellite, a gene responsible for auditory/vestibular defects in zebrafish larvae. Examination of hair cells in satellite mutants revealed that stereociliary bundles are irregular and disorganized. At the ultrastructural level, we observed that the apical surface of satellite mutant hair cells abnormally protrudes above the epithelium and the membrane near the base of the stereocilia is raised. At later stages, stereocilia fused together. We conclude that zebrafish myo6b is required for maintaining the integrity of the apical surface of hair cells, suggesting a conserved role for myosin VI in regulation of actin-based interactions with the plasma membrane.     

Correction: Talin tension sensor reveals novel features of focal adhesion force transmission and mechanosensitivity

The precise architecture of hair bundles, the arrays of mechanosensitive microvilli-like stereocilia crowning the auditory hair cells, is essential to hearing. Myosin IIIa, defective in the late-onset deafness form DFNB30, has been proposed to transport espin-1 to the tips of stereocilia, thereby promoting their elongation. We show that Myo3a^−/−Myo3b^−/− mice lacking myosin IIIa and myosin IIIb are profoundly deaf, whereas Myo3a-cKO Myo3b^−/− mice lacking myosin IIIb and losing myosin IIIa postnatally have normal hearing. Myo3a^−/−Myo3b^−/− cochlear hair bundles display robust mechanoelectrical transduction currents with normal kinetics but show severe embryonic abnormalities whose features rapidly change. These include abnormally tall and numerous microvilli or stereocilia, ungraded stereocilia bundles, and bundle rounding and closure. Surprisingly, espin-1 is properly targeted to Myo3a^−/−Myo3b^−/− stereocilia tips. Our results uncover the critical role that class III myosins play redundantly in hair-bundle morphogenesis; they unexpectedly limit the elongation of stereocilia and of subsequently regressing microvilli, thus contributing to the early hair bundle shaping.     

Role of myosin VI in the differentiation of cochlear hair cells.

The mouse mutant Snell's waltzer (sv) has an intragenic deletion of the Myo6 gene, which encodes the unconventional myosin molecule myosin VI (K. B. Avraham et al., 1995, Nat. Genet. 11, 369-375). Snell's waltzer mutants exhibit behavioural abnormalities suggestive of an inner ear defect, including lack of responsiveness to sound, hyperactivity, head tossing, and circling. We have investigated the effects of a lack of myosin VI on the development of the sensory hair cells of the cochlea in these mutants. In normal mice, the hair cells sprout microvilli on their upper surface, and some of these grow to form a crescent or V-shaped array of modified microvilli, the stereocilia. In the mutants, early stages of stereocilia development appear to proceed normally because at birth many stereocilia bundles have a normal appearance, but in places there are signs of disorganisation of the bundles. Over the next few days, the stereocilia become progressively more disorganised and fuse together. Practically all hair cells show fused stereocilia by 3 days after birth, and there is extensive stereocilia fusion by 7 days. By 20 days, giant stereocilia are observed on top of the hair cells. At 1 and 3 days after birth, hair cells of mutants and controls take up the membrane dye FM1-43, suggesting that endocytosis occurs in mutant hair cells. One possible model for the fusion is that myosin VI may be involved in anchoring the apical hair cell membrane to the underlying actin-rich cuticular plate, and in the absence of normal myosin VI this apical membrane will tend to pull up between stereocilia, leading to fusion.     

MyosinVIIa Interacts with Twinfilin-2 at the Tips of Mechanosensory Stereocilia in the Inner Ear

In vertebrates hearing is dependent upon the microvilli-like mechanosensory stereocilia and their length gradation. The staircase-like organization of the stereocilia bundle is dynamically maintained by variable actin turnover rates. Two unconventional myosins were previously implicated in stereocilia length regulation but the mechanisms of their action remain unknown. MyosinXVa is expressed in stereocilia tips at levels proportional to stereocilia length and its absence produces staircase-like bundles of very short stereocilia. MyosinVIIa localizes to the tips of the shorter stereocilia within bundles, and when absent, the stereocilia are abnormally long. We show here that myosinVIIa interacts with twinfilin-2, an actin binding protein, which inhibits actin polymerization at the barbed end of the filament, and that twinfilin localization in stereocilia overlaps with myosinVIIa. Exogenous expression of myosinVIIa in fibroblasts results in a reduced number of filopodia and promotes accumulation of twinfilin-2 at the filopodia tips. We hypothesize that the newly described interaction between myosinVIIa and twinfilin-2 is responsible for the establishment and maintenance of slower rates of actin turnover in shorter stereocilia, and that interplay between complexes of myosinVIIa/twinfilin-2 and myosinXVa/whirlin is responsible for stereocilia length gradation within the bundle staircase.     

A Myo6 mutation destroys coordination between the myosin heads, revealing new functions of myosin VI in the stereocilia of mammalian inner ear hair cells

Myosin VI, found in organisms from Caenorhabditis elegans to humans, is essential for auditory and vestibular function in mammals, since genetic mutations lead to hearing impairment and vestibular dysfunction in both humans and mice. Here, we show that a missense mutation in this molecular motor in an ENU-generated mouse model, Tailchaser, disrupts myosin VI function. Structural changes in the Tailchaser hair bundles include mislocalization of the kinocilia and branching of stereocilia. Transfection of GFP-labeled myosin VI into epithelial cells and delivery of endocytic vesicles to the early endosome revealed that the mutant phenotype displays disrupted motor function. The actin-activated ATPase rates measured for the D179Y mutation are decreased, and indicate loss of coordination of the myosin VI heads or ‘gating’ in the dimer form. Proper coordination is required for walking processively along, or anchoring to, actin filaments, and is apparently destroyed by the proximity of the mutation to the nucleotide-binding pocket. This loss of myosin VI function may not allow myosin VI to transport its cargoes appropriately at the base and within the stereocilia, or to anchor the membrane of stereocilia to actin filaments via its cargos, both of which lead to structural changes in the stereocilia of myosin VI–impaired hair cells, and ultimately leading to deafness.     

Stereocilia: the long and the short of it

Mutations in whirlin, a putative PDZ scaffold protein, have recently been shown to cause deafness and short cochlear hair cell stereocilia in whirler mice and recessive deafness (DFNB31) in humans. Through its PDZ domains, whirlin might organize a group of proteins into a functional complex required for stereocilia elongation. Identifying these protein partners will advance our understanding of the development of stereocilia and their function as mechanosensory organelles indispensable for normal hearing.     

My oh my(osin): Insights into how auditory hair cells count,measure, and shape

The mechanisms underlying mechanosensory hair bundle formation in auditory sensory cells are largely mysterious. In this issue, Lelli et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201509017) reveal that a pair of molecular motors, myosin IIIa and myosin IIIb, is involved in the hair bundle’s morphology and hearing.The mammalian cochleae are lined with hair cells, each with a precisely arranged, morphologically polar hair bundle that captures energy produced by sound stimuli. A fascinating problem in cell biology is how this elaborately structured mechanosensory hair bundle forms with precision to enable hearing. Each hair bundle is comprised of up to a few hundred actin-filled stereocilia, arrayed in a staircase-like pattern of increasing heights. Deflection of a hair bundle in the direction of the tallest stereocilium opens mechanically gated ion channels at the stereociliary tips, allowing an influx of cations from the endolymph bathing the stereocilia and thus depolarizing the cell. The staircase array of stereocilia forms by elongation of microvilli at the apical surface of the developing hair cell (Tilney et al., 1992). Over 20 years ago, Tilney et al. (1992) detailed the steps of stereociliogenesis as observed by electron microscopy. In this issue, Lelli et al. provide molecular insight into how hair cells count, measure, and shape stereocilia.Loss of hair cells is a major cause of human hearing loss, which is often the result of genetic mutations affecting the development of stereocilia (Raphael, 2002). Mutations in genes encoding several different myosin motor proteins in stereocilia have been associated with human hearing loss, including myosin VIIA (Weil et al., 1995), myosin VI (Avraham et al., 1995; Melchionda et al., 2001), and myosin XVA (Wang et al., 1998). Additionally, loss-of-function mutations in MYO3A, encoding myosin IIIA, are responsible for hereditary progressive hearing loss DFNB30 (Walsh et al., 2002).Myosins, the ATP-dependent motors that move along actin-based filaments, are typically composed of three functional domains: the head, the neck, and the tail domains (Krendel and Mooseker, 2005). Class III myosins are unconventional myosins that each have a kinase domain at their N terminus regulated by PKA phosphorylation and autophosphorylation (Kempler et al., 2007). The kinase activity of myosin III was shown to act on its own motor domain to reduce the motor activity. Myosin III proteins concentrate at actin-based cellular protrusions, such as stereocilia of inner ear hair cells (Dosé et al., 2003; Schneider et al., 2006). The two class III isoforms found in vertebrates, myosin IIIa and IIIb, differ toward their C termini: myosin IIIa is longer than myosin IIIb and has one additional actin-binding domain, myosin III tail homology domain II (3THDII). In cultured cells, whereas myosin IIIa localizes to the tips of filopodia on its own, previous work showed that myosin IIIb requires its interaction partner espin-1, an actin-binding protein, for localization to filopodia tips (Merritt et al., 2012). Mutagenesis studies performed on myosin IIIa revealed a relationship between the phosphorylation state of myosin IIIa and the length and density of filopodia (Quintero et al., 2013). How myosin IIIa and IIIb work individually or together to regulate stereociliogenesis remains to be investigated.In this issue, Lelli et al. (2016) use Myo3a (Myo3a^−/−), Myo3b (Myo3b^−/−), and double (Myo3a^−/−Myo3b^−/−) knockout mice to dissect the complex roles of myosin IIIa and IIIb in hearing. Mice null for myosin IIIa, Myo3a^−/−, were initially normal but showed defects in hearing quality from 2 to 4 mo of age, as detected by auditory brain stem response measurements, which monitor the electrical response of the auditory pathway to short sound stimuli, but not by distortion product otoacoustic emissions, which test outer hair cell (OHC) function. These results indicated that inner hair cells (IHCs) are deleteriously impacted but that OHCs are not, and that these mice had progressive hearing loss. Myo3b^−/− mutants had normal hearing, indicating that redundancy with myosin IIIa may obscure the role of this protein in single knockouts. Investigation of double knockouts, however, clarified the redundant functions of the two isoforms, as the mice were profoundly deaf at 1 mo of age.Interestingly, Lelli et al. (2016) investigated the potential requirement for myosin IIIa in the maintenance of hearing. They generated conditional knockout (cKO) mice, Myo3a-cKO, in which myosin IIIa is inactivated postnatally in a Myo3b^−/− background. These animals hear normally up to at least 6 mo of age. Therefore, myosin IIIa is required during a critical period in auditory system development but is not required to maintain hearing. The researchers further studied the relationship between myosin IIIa and myosin IIIb in the Myo3a-cKO mice and noted an increase in auditory brain stem response thresholds. This fascinating result likely indicates that inactivation of myosin IIIa elicits a pernicious effect of myosin IIIb at mature stages. Determining the mechanism by which myosin IIIb damages hearing in the absence of myosin IIIa and whether it is due to its lack of a single 3THDII domain may yield insight into the roles of class III myosins in hearing.Given that myosin III proteins localize to the tips of stereocilia and are required for hearing, Lelli et al. (2016) explored the morphology of hair bundles lacking these proteins. Although double knockout Myo3a^−/−Myo3b^−/− mice displayed normal positioning of the kinocilium, the cilium reflecting the initial position and polarity of the developing hair bundle, and of asymmetric cell division proteins Par-6 and Gαi₃, which constrain the shape and positioning of the hair bundle, many hair cells from these mice exhibited hair bundle abnormalities. These abnormalities were first described during embryonic auditory hair bundle morphogenesis, at which time 19% of IHCs and 81% of OHCs displayed misshapen bundles. Several of these misshapen IHC and OHC bundles also contained abnormally long, ungraded protrusions, which Lelli et al. (2016) referred to as “long amorphous” bundles. By birth, most IHCs displayed long amorphous bundles. Although most OHC hair bundles were abnormally shaped, they had developed normal staircase organization of their stereocilia heights, and the long amorphous protrusions were no longer present. Presumably, another molecular mechanism comes into play and “corrects” the protrusions to establish the staircase organization. Knockout of candidate proteins could be an interesting strategy to uncover this mechanism in the future. Furthermore, other stereociliary phenotypes of Myo3a^−/−Myo3b^−/− mutant mice are also intriguing. Do myosin III proteins serve different mechanistic roles in IHCs and OHCs to give rise to different phenotypes? Or, are there other key factors that drive stereocilia into various degrees of hair bundle defects in the absence of myosin III proteins?In Myo3a^−/−Myo3b^−/− mice, many of the OHC bundles had abnormal side rows of extra stereocilia, which sometimes closed the bundle off into a circular shape. Interestingly, the stereocilia in these abnormal bundles were also significantly taller than in controls. Although the height of the tallest row of stereocilia in OHCs normally decreases after birth (Sekerková et al., 2011), the height of these stereocilia did not change in the Myo3a^−/−Myo3b^−/− mice, leading to an increased height difference 9 d after birth. The increased height and number of stereocilia in Myo3a^−/−Myo3b^−/− hair bundles is suggestive of unstable actin dynamics. The authors suggest that class III myosins stabilize the F-actin cores of the stereocilia, thus controlling their selective elongation by limiting their growth (Fig. 1). Surprisingly, class III myosins, which presumably climb the full lengths of the stereocilia to perch themselves near the tips, act to restrict their growth. This result seems to contrast with previous work in which myosin IIIa had been proposed to promote elongation of the stereocilia by transporting espin-1 to the stereocilia tips (Salles et al., 2009). Interestingly, Lelli et al. (2016) found that espin-1 was still properly targeted to the tips of stereocilia in Myo3a^−/−Myo3b^−/− mutant mice. The researchers extended the study to another binding partner of myosin IIIa, retinophilin/MORN4. Surprisingly, MORN4 was normally targeted to stereocilia tips in Myo3a^−/−Myo3b^−/− mutant mice. This suggests that there may be redundancy in the mechanisms of transport of stereocilia tip proteins. Whether or not distinct myosin isoforms interchange cargoes so that these cargoes are properly localized is a relevant avenue of future investigation. Furthermore, identification and knockout of class III myosin binding partners could help define the mechanism by which these proteins control elongation of stereocilia.Open in a separate windowFigure 1.Myosin IIIa and myosin IIIb regulate stereociliary length. (top) Schematics of myosin IIIa and myosin IIIb. The stop signs indicate the role these proteins have in limiting the heights of stereocilia. (bottom) A single stereocilium with wild-type copies of myosin IIIa and myosin IIIb is shorter than a stereocilium from a Myo3a^−/−Myo3b^−/− mutant when stereocilia from equivalent positions between hair bundles are compared. In addition to characterizing the lengths of stereocilia, Lelli et al. (2016) noted transient defects in the numbers of stereocilia during development of hair cells within genetically altered mice. At birth, Myo3a^−/−Myo3b^−/− mice had an increased number and density of stereocilia in both IHCs and OHCs as compared with controls. However, 9 d after birth, the number and density of the stereocilia had decreased to levels similar to those seen in control animals. Their results indicate that class III myosins help to control the initial selection of microvilli to become stereocilia, but a different, dominant molecular mechanism overrides this step and is likely involved in refining the number of stereocilia at later, postnatal stages. Identification of this mechanism by knockout of candidate molecules in the Myo3a^−/−Myo3b^−/− mice would be of interest for future work. The defects, though transient, indicate that myosins are key molecular components that establish how cells count the number of protrusions to be produced.In this work, Lelli et al. (2016) determined that myosins IIIa and IIIb are required for normal hair bundle development and hearing. These motor proteins influence the number and lengths of stereocilia to be produced and the overarching shape of the hair bundle. Evaluating the importance of the kinase domains of myosin IIIa and IIIb in hair bundle development is an important next step. The phosphorylation states of proteins in cellular protrusions, such as filopodia and stereocilia, can be regulatory. For example, phosphorylation of the actin cross-linking protein fascin 2b reduces this protein’s capacity to lengthen filopodia (Chou et al., 2011). Moreover, phosphorylation of fascin 2b increases the exchange rate of this protein within zebrafish stereocilia (Hwang et al., 2015). Additionally, in other systems, PKA modulates a signaling cascade that activates the actin-severing protein cofilin to control actin-filament dynamics (Nadella et al., 2009). Future work will be needed to test whether signal transduction stemming from the kinase domains of class III myosins are consequential for hair bundle development.Lelli et al. (2016) observed that the constitutive absence of both class III myosins leads to deafness as a result of hair bundle developmental defects. However, mice deficient for only one of the isoforms did not display such a striking hearing phenotype, indicating that myosins IIIa and IIIb compensate for the loss of one another in the developing cochlea. A role for myosin IIIa in the maintenance of stereocilia had been suggested by the discovery of loss-of-function mutations in MYO3A in patients with hereditary progressive hearing loss DFNB30 (Walsh et al., 2002). Remarkably, the analyses of Myo3a^−/−Myo3b^−/− and Myo3a-cKO Myo3b^−/− mice showed that the redundancy between myosins IIIa and IIIb is critical during hair bundle formation, but not during later, mature stages. Furthermore, the hearing impairment seen in Myo3a-cKO mice suggests that myosin IIIb exerts deleterious effects on hearing past the developmental stages of hair bundle morphogenesis. Although the mechanisms at work are unclear, these findings suggest that the down-regulation of MYO3B expression might be an effective way of preventing late-onset hearing loss in patients with MYO3A mutations.     

Myosins and pathology: genetics and biology

This article summarizes current knowledge on the genetics and possible molecular mechanisms of Human pathologies resulted from mutations within the genes encoding several myosin isoforms. Mutations within the genes encoding some myosin isoforms have been found to be responsible for blindness (myosins III and VIIA), deafness (myosins I, IIA, IIIA, VI, VIIA and XV) and familial hypertrophic cardiomyopathy (beta cardiac myosin heavy chain and both the regulatory and essential light chains). Myosin III localizes predominantly to photoreceptor cells and is proved to be engaged in the vision process in Drosophila. In the inner ear, myosin I is postulated to play a role as an adaptive motor in the tip links of stereocilia of hair cells, myosin IIA seems to be responsible for stabilizing the contacts between adjacent inner ear hair cells, myosin VI plays a role as an intracellular motor transporting membrane structures within the hair cells while myosin VIIA most probably participates in forming links between neighbouring stereocilia and myosin XV probably stabilizes the stereocilia structure. About 30% of patients with familial hypertrophic cardiomyopathy have mutations within the genes encoding the beta cardiac myosin heavy chain and both light chains that are grouped within the regions of myosin head crucial for its functions. The alterations lead to the destabilization of sarcomeres and to a decrease of the myosin ATPase activity and its ability to move actin filaments.     

A novel gene for Usher syndrome type 2: mutations in the long isoform of whirlin are associated with retinitis pigmentosa and sensorineural hearing loss

Usher syndrome is an autosomal recessive condition characterized by sensorineural hearing loss, variable vestibular dysfunction, and visual impairment due to retinitis pigmentosa (RP). The seven proteins that have been identified for Usher syndrome type 1 (USH1) and type 2 (USH2) may interact in a large protein complex. In order to identify novel USH genes, we followed a candidate strategy, assuming that mutations in proteins interacting with this “USH network” may cause Usher syndrome as well. The DFNB31 gene encodes whirlin, a PDZ scaffold protein with expression in both hair cell stereocilia and retinal photoreceptor cells. Whirlin represents an excellent candidate for USH2 because it binds to Usherin (USH2A) and VLGR1b (USH2C). Genotyping of microsatellite markers specific for the DFNB31 gene locus on chromosome 9q32 was performed in a German USH2 family that had been excluded for all known USH loci. Patients showed common haplotypes. Sequence analysis of DFNB31 revealed compound heterozygosity for a nonsense mutation, p.Q103X, in exon 1, and a mutation in the splice donor site of exon 2, c.837+1G>A. DFNB31 mutations appear to be a rare cause of Usher syndrome, since no mutations were identified in an additional 96 USH2 patients. While mutations in the C-terminal half of whirlin have previously been reported in non-syndromic deafness (DFNB31), both alterations identified in our USH2 family affect the long protein isoform. We propose that mutations causing Usher syndrome are probably restricted to exons 1–6 that are specific for the long isoform and probably crucial for retinal function. We describe a novel genetic subtype for Usher syndrome, which we named USH2D and which is caused by mutations in whirlin. Moreover, this is the first case of USH2 that is allelic to non-syndromic deafness. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Myosin VIIA defects, which underlie the Usher 1B syndrome in humans, lead to deafness in Drosophila

In vertebrates, auditory and vestibular transduction occurs on apical projections (stereocilia) of specialized cells (hair cells). Mutations in myosin VIIA (myoVIIA), an unconventional myosin, lead to deafness and balance anomalies in humans, mice, and zebrafish; individuals are deaf, and stereocilia are disorganized. The exact mechanism through which myoVIIA mutations result in these inner-ear anomalies is unknown. Proposed inner-ear functions for myoVIIA include anchoring transduction channels to the stereocilia membrane, trafficking stereocilia linking components, and anchoring hair cells by associating with adherens junctions. The Drosophila myoVIIA homolog is crinkled (ck). The Drosophila auditory organ, Johnston's organ (JO), is developmentally and functionally related to the vertebrate inner ear. Both derive from modified epithelial cells specified by atonal and spalt homolog expression, and both transduce acoustic mechanical energy (and references therein). Here, we show that loss of ck/myoVIIA function leads to complete deafness in Drosophila by disrupting the integrity of the scolopidia that transduce auditory signals. We demonstrate that ck/myoVIIA functions to organize the auditory organ, that it is functionally required in neuronal and support cells, that it is not required for TRPV channel localization, and that it is not essential for scolopidial-cell-junction integrity.