Myosin IIIa boosts elongation of stereocilia by transporting espin 1 to the plus ends of actin filaments

Live-cell single-molecule fluorescence microscopy for protruding organelles reveals regulatory mechanisms of MYO7A-driven cargo transport in stereocilia of inner ear hair cells

Stereocilia are unidirectional F-actin-based cylindrical protrusions on the apical surface of inner ear hair cells and function as biological mechanosensors of sound and acceleration. Development of functional stereocilia requires motor activities of unconventional myosins to transport proteins necessary for elongating the F-actin cores and to assemble the mechanoelectrical transduction (MET) channel complex. However, how each myosin localizes in stereocilia using the energy from ATP hydrolysis is only partially understood. In this study, we develop a methodology for live-cell single-molecule fluorescence microscopy of organelles protruding from the apical surface using a dual-view light-sheet microscope, diSPIM. We demonstrate that MYO7A, a component of the MET machinery, traffics as a dimer in stereocilia. Movements of MYO7A are restricted when scaffolded by the plasma membrane and F-actin as mediated by MYO7A's interacting partners. Here, we discuss the technical details of our methodology and its future applications including analyses of cargo transportation in various organelles.

Live-cell single-molecule fluorescence microscopy for protruding organelles reveals regulatory mechanisms of MYO7A-driven cargo transport in stereocilia of inner ear hair cells

Stereocilia are unidirectional F-actin-based cylindrical protrusions on the apical surface of inner ear hair cells and function as biological mechanosensors of sound and acceleration. Development of functional stereocilia requires motor activities of unconventional myosins to transport proteins necessary for elongating the F-actin cores and to assemble the mechanoelectrical transduction (MET) channel complex. However, how each myosin localizes in stereocilia using the energy from ATP hydrolysis is only partially understood. In this study, we develop a methodology for live-cell single-molecule fluorescence microscopy of organelles protruding from the apical surface using a dual-view light-sheet microscope, diSPIM. We demonstrate that MYO7A, a component of the MET machinery, traffics as a dimer in stereocilia. Movements of MYO7A are restricted when scaffolded by the plasma membrane and F-actin as mediated by MYO7A's interacting partners. Here, we discuss the technical details of our methodology and its future applications including analyses of cargo transportation in various organelles.

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.

How Oratosquilla oratoria compound eye response to the polarization of light: In the perspective of vision genes and related proteins

The special rhabdom structure of the mid-band ommatidium in compound eye contributes to the mantis shrimp being the only animal species known to science that can recognize circularly polarized light (CPL). Although the number of mid-band ommatidium of Oratosquilla oratoria is reduced, the mid-band ommatidium still has orthogonal geometric interleaved rhabdom and short oval distal rhabdom, which may mean that the O. oratoria has weakened circular polarized light vision (CPLV). Here we explored the molecular mechanisms of how O. oratoria response to the polarization of light. Based on the specific expression patterns of vision-related functional genes and proteins, we suggest that the order of light response by O. oratoria compound eye was first natural light, then left-circularly polarized light (LCPL), linearly polarized light, right-circularly polarized light (RCPL) and dark. Meanwhile, we found that the expression levels of vision-related functional genes and proteins in O. oratoria compound eye under RCPL were not significantly different from those in DL, which may imply that O. oratoria cannot respond to RCPL. Furthermore, the response of LCPL is likely facilitated by the differential expression of opsin and microvilli - related functional genes and proteins (arrestin and sodium-coupled neutral amino acid transporter). In conclusion, this study systematically illustrated for the first time how O. oratoria compound eye response to the polarization of light at the genetic level, and it can improve the visual ecological theory behind polarized light vision evolution.

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.

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.

Unleashed Actin Assembly in Capping Protein-Deficient B16-F1 Cells Enables Identification of Multiple Factors Contributing to Filopodium Formation

Background: Filopodia are dynamic, finger-like actin-filament bundles that overcome membrane tension by forces generated through actin polymerization at their tips to allow extension of these structures a few microns beyond the cell periphery. Actin assembly of these protrusions is regulated by accessory proteins including heterodimeric capping protein (CP) or Ena/VASP actin polymerases to either terminate or promote filament growth. Accordingly, the depletion of CP in B16-F1 melanoma cells was previously shown to cause an explosive formation of filopodia. In Ena/VASP-deficient cells, CP depletion appeared to result in ruffling instead of inducing filopodia, implying that Ena/VASP proteins are absolutely essential for filopodia formation. However, this hypothesis was not yet experimentally confirmed. Methods: Here, we used B16-F1 cells and CRISPR/Cas9 technology to eliminate CP either alone or in combination with Ena/VASP or other factors residing at filopodia tips, followed by quantifications of filopodia length and number. Results: Unexpectedly, we find massive formations of filopodia even in the absence of CP and Ena/VASP proteins. Notably, combined inactivation of Ena/VASP, unconventional myosin-X and the formin FMNL3 was required to markedly impair filopodia formation in CP-deficient cells. Conclusions: Taken together, our results reveal that, besides Ena/VASP proteins, numerous other factors contribute to filopodia formation.

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.

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.

Studies of functional properties of espin 1: Its interaction to actin filaments

Actin is a multifunctional biomolecule that forms not only basic structural bodies such as filopodia and lamellipodia, but also large microvilli-like organelles like stereocilia. Actin consists of four sub-domains (S1, S2, S3, and S4), and the “target-binding groove” formed between S1 and S3 is the major binding site for various actin binding proteins. Actin filament dynamics are regulated by numerous actin binding proteins with different mechanisms of actin binding, assembly, and disassembly such as actin severing, branching, and bundling. Ectoplasmic specialization protein 1 (espin 1) is an actin binding and bundling protein that is specifically implicated in the elongation and stabilization of stereocilia as a binding partner with myosin III. However, little is known about the molecular structure, actin bundling, and stabilizing mechanism of espin 1; hence, we investigated the interaction between actin and espin 1 through structural data. In this study, we first purified human espin 1 in an E. coli system following a new detergent-free approach and then demonstrated the 2D structure of full-length espin 1 using transmission electron microscopy along with Nickel nitrilotriacetic acid nanogold labeling and 2D averaging using SPIDER. Furthermore, we also determined the espin 1 binding domain of actin using a co-sedimentation assay along with gelsolin and myosin S1. These findings are not only beneficial for understanding the actin binding and bundling mechanism of espin 1, but also shed light on its elongation, stabilization, and tip-localization mechanisms with myosin III. This study thus provides a basis for understanding the molecular structure of espin 1 and can contribute to various hearing-related diseases, such as hearing loss and vestibular dysfunction.

Selective binding and transport of protocadherin 15 isoforms by stereocilia unconventional myosins in a heterologous expression system

During hair cell development, the mechanoelectrical transduction (MET) apparatus is assembled at the stereocilia tips, where it coexists with the stereocilia actin regulatory machinery. While the myosin-based tipward transport of actin regulatory proteins is well studied, isoform complexity and built-in redundancies in the MET apparatus have limited our understanding of how MET components are transported. We used a heterologous expression system to elucidate the myosin selective transport of isoforms of protocadherin 15 (PCDH15), the protein that mechanically gates the MET apparatus. We show that MYO7A selectively transports the CD3 isoform while MYO3A and MYO3B transports the CD2 isoform. Furthermore, MYO15A showed an insignificant role in the transport of PCDH15, and none of the myosins tested transport PCDH15-CD1. Our data suggest an important role for MYO3A, MYO3B, and MYO7A in the MET apparatus formation and highlight the intricate nature of MET and actin regulation during development and functional maturation of the stereocilia bundle.

Deafness-related protein PDZD7 forms complex with the C-terminal tail of FCHSD2

In cochlea, deafness-related protein PDZD7 is an indispensable component of the ankle link complex, which is critical for the maturation of inner-ear hair cell for sound perception. Ankle links, connecting the different rows of cochlear stereocilia, are essential for the staircase-like development of stereocilia. However, the molecular mechanism of how PDZD7 governs stereociliary development remains unknown. Here, we reported a novel PDZD7-binding partner, FCHSD2, identified by yeast two-hybrid screening. FCHSD2 was reported to be expressed in hair cell, where it co-operated with CDC42 and N-WASP to regulate the formation of cell protrusion. The association between FCHSD2 and PDZD7 was further confirmed in COS-7 cells. More importantly, we solved the complex structure of FCHSD2 tail with PDZD7 PDZ3 domain at 2.0 Å resolution. The crystal structure shows that PDZD7 PDZ3 adopts a typical PDZ domain topology, comprising five β strands and two α helixes. The PDZ-binding motif of FCHSD2 tail stretches through the αB/βB groove of PDZD7 PDZ3. Our study not only uncovers the interaction between FCHSD2 tail and PDZD7 PDZ3 at the atomic level, but also provides clues of connecting the ankle link complex with cytoskeleton dynamics for exploiting the molecular mechanism of stereociliary development.

High-resolution immunofluorescence imaging of mouse cochlear hair bundles

High-resolution immunofluorescence imaging of cochlear hair bundles faces many challenges due to the hair bundle’s small dimensions, fragile nature, and complex organization. Here, we describe an optimized protocol for hair-bundle protein immunostaining and localization. We detail the steps and solutions for extracting and fixing the mouse inner ear and for dissecting the organ of Corti. We further emphasize the optimal permeabilization, blocking, staining, and mounting conditions as well as the parameters for high-resolution microscopy imaging.

For complete details on the use and execution of this protocol, please refer to Trouillet et al. (2021).

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Techniques for dissecting the mouse cochlea and the organ of Corti

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Dissection, permeabilization, blocking parameters to detect hair bundle proteins

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Mounting method to localize protein in the hair bundles

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Collapsin Response Mediator Protein 1 (CRMP1) Is Required for High-Frequency Hearing

Collapsin response mediator protein 1 (CRMP1), also known as dihydropyrimidinase-related protein 1, participates in cytoskeleton remodeling during axonal guidance and neuronal migration. In cochlear hair cells, the assembly and maintenance of the cytoskeleton is of great interest because it is crucial for the morphogenesis and maintenance of hair cells. Previous RNA sequencing analysis found that Crmp1 is highly expressed in cochlear hair cells. However, the expression profile and functions of CRMP1 in the inner ear remain unknown. In this study, the expression and localization of CRMP1 in hair cells was investigated using immunostaining, and was shown to be highly expressed in both outer and inner hair cells. Next, the stereocilia morphology of Crmp1-deficient mice was characterized. Abolishing CRMP1 did not affect the morphogenesis of hair cells. Interestingly, scanning electron microscopy detected hair cell loss at the basal cochlear region, an area responsible for high-frequency auditory perception, in Crmp1-deficient mice. Correspondingly, an auditory brainstem response test showed that mice lacking CRMP1 had progressive hearing loss at high frequencies. In summary, these data suggest that CRMP1 is required for high-frequency auditory perception.

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.

E3 ubiquitin ligase ASB17 is required for spermiation in mice

Background

A major goal of spermiation is to degrade the apical ectoplasmic specialization (ES) junction between Sertoli cells and elongating spermatids in preparation for the eventual disengagement of spermatids into the lumen. E3 ubiquitin ligases mediate the process of ubiquitination and the subsequent proteasomal degradation, but their specific role during spermiation remains largely unexplored.

Methods

Ankyrin repeat and SOCS box protein 17 (Asb17)-knockout mice were generated via a CRISPR/Cas9 approach. Epididymal sperm parameters were assessed by a computer-assisted sperm analysis (CASA) system, and morphological analysis of testicular tissues were performed based on histological and immunostaining staining, and transmission electron microscopy (TEM). The interactions between ASB17 and Espin (ESPN) were predicted by HawkDock server and validated through protein pull-down and immunoprecipitation assays.

Results

We report that ASB17, an E3 ligase, is required for the completion of spermiation and that mice lacking Asb17 are oligozoospermic owing to spermiation failure. ASB17-deficient mice are fertile; however, spermatids exhibit a disorganized ES junction, resulting in retention within the seminiferous epithelium. Mechanistically, ASB17 deficiency leads to excess accumulation of ESPN, an actin-binding essential structural component of the ES. We determined that ASB17 regulates the removal of the ES through ubiquitin mediated protein degradation of ESPN.

Conclusions

In summary, our study describes a role for ASB17 in the regulation of cell-cell junctions between germ cells and somatic cells in the testis. These findings establish a novel mechanism for the regulatory role of E3 ligases during spermatogenesis.

Planar polarity in primate cone photoreceptors: a potential role in Stiles Crawford effect phototropism

Human cone phototropism is a key mechanism underlying the Stiles-Crawford effect, a psychophysiological phenomenon according to which photoreceptor outer/inner segments are aligned along with the direction of incoming light. However, such photomechanical movements of photoreceptors remain elusive in mammals. We first show here that primate cone photoreceptors have a planar polarity organized radially around the optical center of the eye. This planar polarity, based on the structure of the cilium and calyceal processes, is highly reminiscent of the planar polarity of the hair cells and their kinocilium and stereocilia. Secondly, we observe under super-high resolution expansion microscopy the cytoskeleton and Usher proteins architecture in the photoreceptors, which appears to establish a mechanical continuity between the outer and inner segments. Taken together, these results suggest a comprehensive cellular mechanism consistent with an active phototropism of cones toward the optical center of the eye, and thus with the Stiles-Crawford effect.

Verschueren et al. expand our understanding of the Stiles-Crawford effect in mammals by using super-high resolution expansion microscopy of the adult macaque eye. They show that cone photoreceptors have a planar polarity organized radially around the optical center of the eye and that Usher proteins establish a mechanical continuity between the outer and inner segments, which sheds light on the Stiles-Crawford effect in this species.

Frequency and origin of the c.2090T>G p.(Leu697Trp) MYO3A variant associated with autosomal dominant hearing loss

We recently described a novel missense variant [c.2090T>G:p.(Leu697Trp)] in the MYO3A gene, found in two Brazilian families with late-onset autosomal dominant nonsyndromic hearing loss (ADNSHL). Since then, with the objective of evaluating its contribution to ADNSHL in Brazil, the variant was screened in additional 101 pedigrees with probable ADNSHL without conclusive molecular diagnosis. The variant was found in three additional families, explaining 3/101 (~3%) of cases with ADNSHL in our Brazilian pedigree collection. In order to identify the origin of the variant, 21 individuals from the five families were genotyped with a high-density SNP array (~600 K SNPs- Axiom Human Origins; ThermoFisher). The identity by descent (IBD) approach revealed that many pairs of individuals from the different families have a kinship coefficient equivalent to that of second cousins, and all share a minimum haplotype of ~607 kb which includes the c.2090T>G variant suggesting it probably arose in a common ancestor. We inferred that the mutation occurred in a chromosomal segment of European ancestry and the time since the most common ancestor was estimated in 1100 years (CI = 775-1425). This variant was also reported in a Dutch family, which shares a 87,121 bp haplotype with the Brazilian samples, suggesting that Dutch colonists may have brought it to Northeastern Brazil in the 17th century. Therefore, the present study opens new avenues to investigate this variant not only in Brazilians but also in European families with ADNSHL.

Deafness mutation in the MYO3A motor domain impairs actin protrusion elongation mechanism

Class III myosins are actin-based motors proposed to transport cargo to the distal tips of stereocilia in the inner ear hair cells and/or to participate in stereocilia length regulation, which is especially important during development. Mutations in the MYO3A gene are associated with delayed onset deafness. A previous study demonstrated that L697W, a dominant deafness mutation, disrupts MYO3A ATPase and motor properties but does not impair its ability to localize to the tips of actin protrusions. In the current study, we characterized the transient kinetic mechanism of the L697W motor ATPase cycle. Our kinetic analysis demonstrates that the mutation slows the ADP release and ATP hydrolysis steps, which results in a slight reduction in the duty ratio and slows detachment kinetics. Fluorescence recovery after photobleaching (FRAP) of filopodia tip localized L697W and WT MYO3A in COS-7 cells revealed that the mutant does not alter turnover or average intensity at the actin protrusion tips. We demonstrate that the mutation slows filopodia extension velocity in COS-7 cells which correlates with its twofold slower in vitro actin gliding velocity. Overall, this work allowed us to propose a model for how the motor properties of MYO3A are crucial for facilitating actin protrusion length regulation.

Human deafness-associated variants alter the dynamics of key molecules in hair cell stereocilia F-actin cores

Stereocilia protrude up to 100 µm from the apical surface of vertebrate inner ear hair cells and are packed with cross-linked filamentous actin (F-actin). They function as mechanical switches to convert sound vibration into electrochemical neuronal signals transmitted to the brain. Several genes encode molecular components of stereocilia including actin monomers, actin regulatory and bundling proteins, motor proteins and the proteins of the mechanotransduction complex. A stereocilium F-actin core is a dynamic system, which is continuously being remodeled while maintaining an outwardly stable architecture under the regulation of F-actin barbed-end cappers, severing proteins and crosslinkers. The F-actin cores of stereocilia also provide a pathway for motor proteins to transport cargos including components of tip-link densities, scaffolding proteins and actin regulatory proteins. Deficiencies and mutations of stereocilia components that disturb this "dynamic equilibrium" in stereocilia can induce morphological changes and disrupt mechanotransduction causing sensorineural hearing loss, best studied in mouse and zebrafish models. Currently, at least 23 genes, associated with human syndromic and nonsyndromic hearing loss, encode proteins involved in the development and maintenance of stereocilia F-actin cores. However, it is challenging to predict how variants associated with sensorineural hearing loss segregating in families affect protein function. Here, we review the functions of several molecular components of stereocilia F-actin cores and provide new data from our experimental approach to directly evaluate the pathogenicity and functional impact of reported and novel variants of DIAPH1 in autosomal-dominant DFNA1 hearing loss using single-molecule fluorescence microscopy.

The many roles of myosins in filopodia, microvilli and stereocilia

Filopodia, microvilli and stereocilia represent an important group of plasma membrane protrusions. These specialized projections are supported by parallel bundles of actin filaments and have critical roles in sensing the external environment, increasing cell surface area, and acting as mechanosensors. While actin-associated proteins are essential for actin-filament elongation and bundling in these protrusions, myosin motors have a surprising role in the formation and extension of filopodia and stereocilia and in the organization of microvilli. Actin regulators and specific myosins collaborate in controlling the length of these structures. Myosins can transport cargoes along the length of these protrusions, and, in the case of stereocilia and microvilli, interactions with adaptors and cargoes can also serve to anchor adhesion receptors to the actin-rich core via functionally conserved motor-adaptor complexes. This review highlights recent progress in understanding the diverse roles myosins play in filopodia, microvilli and stereocilia.

Functional Role of Class III Myosins in Hair Cells

Cytoskeletal motors produce force and motion using the energy from ATP hydrolysis and function in a variety of mechanical roles in cells including muscle contraction, cargo transport, and cell division. Actin-based myosin motors have been shown to play crucial roles in the development and function of the stereocilia of auditory and vestibular inner ear hair cells. Hair cells can contain hundreds of stereocilia, which rely on myosin motors to elongate, organize, and stabilize their structure. Mutations in many stereocilia-associated myosins have been shown to cause hearing loss in both humans and animal models suggesting that each myosin isoform has a specific function in these unique parallel actin bundle-based protrusions. Here we review what is known about the classes of myosins that function in the stereocilia, with a special focus on class III myosins that harbor point mutations associated with delayed onset hearing loss. Much has been learned about the role of the two class III myosin isoforms, MYO3A and MYO3B, in maintaining the precise stereocilia lengths required for normal hearing. We propose a model for how class III myosins play a key role in regulating stereocilia lengths and demonstrate how their motor and regulatory properties are particularly well suited for this function. We conclude that ongoing studies on class III myosins and other stereocilia-associated myosins are extremely important and may lead to novel therapeutic strategies for the treatment of hearing loss due to stereocilia degeneration.

Optogenetic manipulation of cellular communication using engineered myosin motors

Cells achieve highly efficient and accurate communication through cellular projections such as neurites and filopodia, yet there is a lack of genetically encoded tools that can selectively manipulate their composition and dynamics. Here, we present a versatile optogenetic toolbox of artificial multi-headed myosin motors that can move bidirectionally within long cellular extensions and allow for the selective transport of GFP-tagged cargo with light. Utilizing these engineered motors, we could transport bulky transmembrane receptors and organelles as well as actin remodellers to control the dynamics of both filopodia and neurites. Using an optimized in vivo imaging scheme, we further demonstrate that, upon limb amputation in axolotls, a complex array of filopodial extensions is formed. We selectively modulated these filopodial extensions and showed that they re-establish a Sonic Hedgehog signalling gradient during regeneration. Considering the ubiquitous existence of actin-based extensions, this toolbox shows the potential to manipulate cellular communication with unprecedented accuracy.

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.

F-actin disassembly factor MICAL1 binding to Myosin Va mediates cargo unloading during cytokinesis

Motor-mediated intracellular trafficking requires motors to position cargoes at proper locations. Myosin Va (MyoVa), an actin-based motor, is a classic model for studying cargo transport. However, the molecular basis underlying cargo unloading in MyoVa-mediated transport has remained enigmatic. We have identified MICAL1, an F-actin disassembly regulator, as a binding partner of MyoVa and shown that MICAL1-MyoVa interaction is critical for localization of MyoVa at the midbody. By binding to MICAL1, MyoVa-mediated transport is terminated, resulting in vesicle unloading at the midbody for efficient cytokinesis. The MyoVa/MICAL1 complex structure reveals that MICAL1 and F-actin assembly factors, Spires, share an overlapped binding surface on MyoVa, suggesting a regulatory role of F-actin dynamics in cargo unloading. Down-regulating F-actin disassembly by a MICAL1 mutant significantly reduces MyoVa and vesicles accumulating at the midbody. Collectively, our findings demonstrate that MyoVa binds to MICAL1 at the midbody destination and triggers F-actin disassembly to unload the vesicle cargo.

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.

Serial scanning electron microscopy of anti-PKHD1L1 immuno-gold labeled mouse hair cell stereocilia bundles

Serial electron microscopy techniques have proven to be a powerful tool in biology. Unfortunately, the data sets they generate lack robust and accurate automated segmentation algorithms. In this data descriptor publication, we introduce a serial focused ion beam scanning electron microscopy (FIB-SEM) dataset consisting of six outer hair cell (OHC) stereocilia bundles, and the supranuclear part of the hair cell bodies. Also presented are the manual segmentations of stereocilia bundles and the gold bead labeling of PKHD1L1, a coat protein of hair cell stereocilia important for hearing in mice. This depository includes all original data and several intermediate steps of the manual analysis, as well as the MATLAB algorithm used to generate a three-dimensional distribution map of gold labels. They serve as a reference dataset, and they enable reproduction of our analysis, evaluation and improvement of current methods of protein localization, and training of algorithms for accurate automated segmentation.

A novel missense variant in MYO3A is associated with autosomal dominant high-frequency hearing loss in a German family

Background

MYO3A, encoding the myosin IIIA protein, is associated with autosomal recessive and autosomal dominant nonsyndromic hearing loss. To date, only two missense variants located in the motor‐head domain of MYO3A have been described in autosomal dominant families with progressive, mild‐to‐profound sensorineural hearing loss. These variants alter the ATPase activity of myosin IIIA.

Methods

Exome sequencing of a proband from a three‐generation German family with prelingual, moderate‐to‐profound, high‐frequency hearing loss was performed. Segregation analysis confirmed a dominant inheritance pattern. Regression analysis of mean hearing level thresholds per individual and ear was performed at high‐, mid‐, and low‐frequencies.

Results

A novel heterozygous missense variant c.716T>C, p.(Leu239Pro) in the kinase domain of MYO3A was identified that is predicted in silico as disease causing. High‐frequency, progressive hearing loss was identified.

Conclusion

Correlation analysis of pure‐tone hearing thresholds revealed progressive hearing loss, especially in the high‐frequencies. In the present study, we report the first dominant likely pathogenic variant in MYO3A in a European family and further support MYO3A as an autosomal dominant hearing loss gene.

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.

Espin distribution as revealed by super-resolution microscopy of stereocilia

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

Hair-Bundle Links: Genetics as the Gateway to Function

Up to five distinct cell-surface specializations interconnect the stereocilia and the kinocilium of the mature hair bundle in some species: kinocilial links, tip links, top connectors, shaft connectors, and ankle links. In developing hair bundles, transient lateral links are prominent. Mutations in genes encoding proteins associated with these links cause Usher deafness/blindness syndrome or nonsyndromic (isolated) forms of human hereditary deafness, and mice with constitutive or conditional alleles of these genes have provided considerable insight into the molecular composition and function of the different links. We describe the structure of these links and review evidence showing CDH23 and PCDH15 are components of the tip, kinocilial, and transient-lateral links, that stereocilin (STRC) and protein tyrosine phosphatase (PTPRQ) are associated with top and shaft connectors, respectively, and that USH2A and ADGRV1 are associated with the ankle links. Whereas tip links are required for mechanoelectrical transduction, all link proteins play key roles in the normal development and/or the maintenance of hair bundle structure and function. Recent crystallographic and single-particle analyses of PCDH15 and CDH23 provide insight as to how the structure of tip link may contribute to the elastic element predicted to lie in series with the hair cell's mechanoelectrical transducer channel.

Structure of the MORN4/Myo3a Tail Complex Reveals MORN Repeats as Protein Binding Modules

Tandem repeats are basic building blocks for constructing proteins with diverse structures and functions. Compared with extensively studied α-helix-based tandem repeats such as ankyrin, tetratricopeptide, armadillo, and HEAT repeat proteins, relatively little is known about tandem repeat proteins formed by β hairpins. In this study, we discovered that the MORN repeats from MORN4 function as a protein binding module specifically recognizing a tail cargo binding region from Myo3a. The structure of the MORN4/Myo3a complex shows that MORN4 forms an extended single-layered β-sheet structure and uses a U-shaped groove to bind to the Myo3a tail with high affinity and specificity. Sequence and structural analyses further elucidated the unique sequence features for folding and target binding of MORN repeats. Our work establishes that the β-hairpin-based MORN repeats are protein-protein interaction modules.

Mechanisms of Hair Cell Damage and Repair

Sensory hair cells of the inner ear are exposed to continuous mechanical stress, causing damage over time. The maintenance of hair cells is further challenged by damage from a variety of other ototoxic factors, including loud noise, aging, genetic defects, and ototoxic drugs. This damage can manifest in many forms, from dysfunction of the hair cell mechanotransduction complex to loss of specialized ribbon synapses, and may even result in hair cell death. Given that mammalian hair cells do not regenerate, the repair of hair cell damage is important for continued auditory function throughout life. Here, we discuss how several key hair cell structures can be damaged, and what is known about how they are repaired.

Building and repairing the stereocilia cytoskeleton in mammalian auditory hair cells

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

An actin-binding protein ESPN is an independent prognosticator and regulates cell growth for esophageal squamous cell carcinoma

Background

ESPN (Espin), an actin filament-binding protein, plays an important role in regulating the organization, dimensions, dynamics, and signaling capacities of the actin filament-rich, microvillus-type specializations that mediate sensory transduction in various mechanosensory and chemosensory cells. Recent few studies show that ESPN regulates metastasis and cell proliferation in melanoma. However, the significance of ESPN in other cancers such as esophageal squamous cell carcinoma (ESCC) remains largely unknown.

Methods

Immunohistochemistry was performed in 169 patients with ESCC and correlated with clinicopathological features and survival. The functional role of ESPN in ESCC cells was determined by ESPN-mediated siRNA.

Results

Univariate analyses showed that high ESPN expression was associated with inferior overall survival (P = 0.005) and disease-free survival (P = 0.035). High ESPN expression was an independent prognosticator in multivariate analysis for overall survival (P = 0.009, hazard ratio = 1.688) and disease-free survival (P = 0.049, hazard ratio = 1.451). The 5-year overall survival rates were 30% and 54% in patients with high and low expression of ESPN, respectively. Inhibition of endogenous ESPN in ESCC cells decreased ESCC growth by reducing cell proliferating rates.

Conclusions

High ESPN expression is independently associated with poor prognosis in patients with ESCC and downregulation of ESPN inhibits ESCC cell growth. Our results suggest that ESPN may be a novel therapeutic target for patients with ESCC.

mDia1/3 generate cortical F-actin meshwork in Sertoli cells that is continuous with contractile F-actin bundles and indispensable for spermatogenesis and male fertility

Formin is one of the two major classes of actin binding proteins (ABPs) with nucleation and polymerization activity. However, despite advances in our understanding of its biochemical activity, whether and how formins generate specific architecture of the actin cytoskeleton and function in a physiological context in vivo remain largely obscure. It is also unknown how actin filaments generated by formins interact with other ABPs in the cell. Here, we combine genetic manipulation of formins mammalian diaphanous homolog1 (mDia1) and 3 (mDia3) with superresolution microscopy and single-molecule imaging, and show that the formins mDia1 and mDia3 are dominantly expressed in Sertoli cells of mouse seminiferous tubule and together generate a highly dynamic cortical filamentous actin (F-actin) meshwork that is continuous with the contractile actomyosin bundles. Loss of mDia1/3 impaired these F-actin architectures, induced ectopic noncontractile espin1-containing F-actin bundles, and disrupted Sertoli cell-germ cell interaction, resulting in impaired spermatogenesis. These results together demonstrate the previously unsuspected mDia-dependent regulatory mechanism of cortical F-actin that is indispensable for mammalian sperm development and male fertility.

Knock-In Mice with Myo3a Y137C Mutation Displayed Progressive Hearing Loss and Hair Cell Degeneration in the Inner Ear

Myo3a is expressed in cochlear hair cells and retinal cells and is responsible for human recessive hereditary nonsyndromic deafness (DFNB30). To investigate the mechanism of DFNB30-type deafness, we established a mouse model of Myo3a kinase domain Y137C mutation by using CRISPR/Cas9 system. No difference in hearing between 2-month-old Myo3a mutant mice and wild-type mice was observed. The hearing threshold of the ≥6-month-old mutant mice was significantly elevated compared with that of the wild-type mice. We observed degeneration in the inner ear hair cells of 6-month-old Myo3a mutant mice, and the degeneration became more severe at the age of 12 months. We also found structural abnormality in the cochlear hair cell stereocilia. Our results showed that Myo3a is essential for normal hearing by maintaining the intact structure of hair cell stereocilia, and the kinase domain plays a critical role in the normal functions of Myo3a. This mouse line is an excellent model for studying DFNB30-type deafness in humans.

Characterization of a novel MYO3A missense mutation associated with a dominant form of late onset hearing loss

Whole-exome sequencing of samples from affected members of two unrelated families with late-onset non-syndromic hearing loss revealed a novel mutation (c.2090 T > G; NM_017433) in MYO3A. The mutation was confirmed in 36 affected individuals, showing autosomal dominant inheritance. The mutation alters a single residue (L697W or p.Leu697Trp) in the motor domain of the stereocilia protein MYO3A, leading to a reduction in ATPase activity, motility, and an increase in actin affinity. MYO3A-L697W showed reduced filopodial actin protrusion initiation in COS7 cells, and a predominant tipward accumulation at filopodia and stereocilia when coexpressed with wild-type MYO3A and espin-1, an actin-regulatory MYO3A cargo. The combined higher actin affinity and duty ratio of the mutant myosin cause increased retention time at stereocilia tips, resulting in the displacement of the wild-type MYO3A protein, which may impact cargo transport, stereocilia length, and mechanotransduction. The dominant negative effect of the altered myosin function explains the dominant inheritance of deafness.

Myosin-Driven Intracellular Transport

The delivery of intracellular material within cells is crucial for maintaining normal function. Myosins transport a wide variety of cargo, ranging from vesicles to ribonuclear protein particles (RNPs), in plants, fungi, and metazoa. The properties of a given myosin transporter are adapted to move on different actin filament tracks, either on the disordered actin networks at the cell cortex or along highly organized actin bundles to distribute their cargo in a localized manner or move it across long distances in the cell. Transport is controlled by selective recruitment of the myosin to its cargo that also plays a role in activation of the motor.

Kinetic adaptation of human Myo19 for active mitochondrial transport to growing filopodia tips

Myosins are actin-based molecular motors which are enzymatically adapted for their cellular functions such as transportation and membrane tethering. Human Myo19 affects mitochondrial motility, and promotes their localization to stress-induced filopodia. Therefore, studying Myo19 enzymology is essential to understand how this motor may facilitate mitochondrial motility. Towards this goal, we have purified Myo19 motor domain (Myo19-3IQ) from a human-cell expression system and utilized transient kinetics to study the Myo19-3IQ ATPase cycle. We found that Myo19-3IQ exhibits noticeable conformational changes (isomerization steps) preceding both ATP and ADP binding, which may contribute to nucleotide binding regulation. Notably, the ADP isomerization step and subsequent ADP release contribute significantly to the rate-limiting step of the Myo19-3IQ ATPase cycle. Both the slow ADP isomerization and ADP release prolong the time Myo19-3IQ spend in the strong actin binding state and hence contribute to its relatively high duty ratio. However, the predicted duty ratio is lower than required to support motility as a monomer. Therefore, it may be that several Myo19 motors are required to propel mitochondria movement on actin filaments efficiently. Finally, we provide a model explaining how Myo19 translocation may be regulated by the local ATP/ADP ratio, coupled to the mitochondria presence in the filopodia.

ROS induced distribution of mitochondria to filopodia by Myo19 depends on a class specific tryptophan in the motor domain

The role of the actin cytoskeleton in relation to mitochondria function and dynamics is only recently beginning to be recognized. Myo19 is an actin-based motor that is bound to the outer mitochondrial membrane and promotes the localization of mitochondria to filopodia in response to glucose starvation. However, how glucose starvation induces mitochondria localization to filopodia, what are the dynamics of this process and which enzymatic adaptation allows the translocation of mitochondria to filopodia are not known. Here we show that reactive oxygen species (ROS) mimic and mediate the glucose starvation induced phenotype. In addition, time-lapse fluorescent microscopy reveals that ROS-induced Myo19 motility is a highly dynamic process which is coupled to filopodia elongation and retraction. Interestingly, Myo19 motility is inhibited by back-to-consensus-mutation of a unique residue of class XIX myosins in the motor domain. Kinetic analysis of the purified mutant Myo19 motor domain reveals that the duty ratio (time spent strongly bound to actin) is highly compromised in comparison to that of the WT motor domain, indicating that Myo19 unique motor properties are necessary to propel mitochondria to filopodia tips. In summary, our study demonstrates the contribution of actin-based motility to the mitochondrial localization to filopodia by specific cellular cues.

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

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

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

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

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

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

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

Generic Transport Mechanisms for Molecular Traffic in Cellular Protrusions

Transport of molecular motors along protein filaments in a half-closed geometry is a common feature of biologically relevant processes in cellular protrusions. Using a lattice-gas model we study how the interplay between active and diffusive transport and mass conservation leads to localized domain walls and tip localization of the motors. We identify a mechanism for task sharing between the active motors (maintaining a gradient) and the diffusive motion (transport to the tip), which ensures that energy consumption is low and motor exchange mostly happens at the tip. These features are attributed to strong nearest-neighbor correlations that lead to a strong reduction of active currents, which we calculate analytically using an exact moment identity, and might prove useful for the understanding of correlations and active transport also in more elaborate systems.

Myosins: Domain Organisation, Motor Properties, Physiological Roles and Cellular Functions

Myosins are cytoskeletal motor proteins that use energy derived from ATP hydrolysis to generate force and movement along actin filaments. Humans express 38 myosin genes belonging to 12 classes that participate in a diverse range of crucial activities, including muscle contraction, intracellular trafficking, cell division, motility, actin cytoskeletal organisation and cell signalling. Myosin malfunction has been implicated a variety of disorders including deafness, hypertrophic cardiomyopathy, Usher syndrome, Griscelli syndrome and cancer. In this chapter, we will first discuss the key structural and kinetic features that are conserved across the myosin family. Thereafter, we summarise for each member in turn its unique functional and structural adaptations, cellular roles and associated pathologies. Finally, we address the broad therapeutic potential for pharmacological interventions that target myosin family members.

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

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

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

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

A link between planar polarity and staircase-like bundle architecture in hair cells

Sensory perception in the inner ear relies on the hair bundle, the highly polarized brush of movement detectors that crowns hair cells. We previously showed that, in the mouse cochlea, the edge of the forming bundle is defined by the 'bare zone', a microvilli-free sub-region of apical membrane specified by the Insc-LGN-Gαi protein complex. We now report that LGN and Gαi also occupy the very tip of stereocilia that directly abut the bare zone. We demonstrate that LGN and Gαi are both essential for promoting the elongation and differential identity of stereocilia across rows. Interestingly, we also reveal that total LGN-Gαi protein amounts are actively balanced between the bare zone and stereocilia tips, suggesting that early planar asymmetry of protein enrichment at the bare zone confers adjacent stereocilia their tallest identity. We propose that LGN and Gαi participate in a long-inferred signal that originates outside the bundle to model its staircase-like architecture, a property that is essential for direction sensitivity to mechanical deflection and hearing.

Myosin-7b Promotes Distal Tip Localization of the Intermicrovillar Adhesion Complex

Transporting epithelial cells interact with the luminal environment using a tightly packed array of microvilli known as the brush border. During intestinal epithelial differentiation, microvillar packing and organization are driven by cadherin-dependent adhesion complexes that localize to the distal tips of microvilli, where they drive physical interactions between neighboring protrusions. Although enrichment of the "intermicrovillar adhesion complex" (IMAC) at distal tips is required for proper function, the mechanism driving tip accumulation of these factors remains unclear. Here, we report that the actin-based motor myosin-7b (Myo7b) promotes the accumulation of IMAC components at microvillar tips. Myo7b is highly enriched at the tips of microvilli in both kidney and intestinal brush borders, and loss of Myo7b in differentiating intestinal epithelial cells disrupts intermicrovillar adhesion and, thus, brush border assembly. Analysis of cells lacking Myo7b revealed that IMAC components and the resulting intermicrovillar adhesion links are mislocalized along the microvillar axis rather than enriched at the distal tips. We also found that Myo7b motor domains are capable of supporting tip-directed transport. However, motor activity is supplemented by other passive targeting mechanisms that together drive highly efficient IMAC accumulation at the tips. These findings illuminate the molecular basis of IMAC enrichment at microvillar tips and hold important implications for understanding apical morphogenesis in transporting and sensory epithelial tissues.

Impact of the Motor and Tail Domains of Class III Myosins on Regulating the Formation and Elongation of Actin Protrusions

Class III myosins (MYO3A and MYO3B) are proposed to function as transporters as well as length and ultrastructure regulators within stable actin-based protrusions such as stereocilia and calycal processes. MYO3A differs from MYO3B in that it contains an extended tail domain with an additional actin-binding motif. We examined how the properties of the motor and tail domains of human class III myosins impact their ability to enhance the formation and elongation of actin protrusions. Direct examination of the motor and enzymatic properties of human MYO3A and MYO3B revealed that MYO3A is a 2-fold faster motor with enhanced ATPase activity and actin affinity. A chimera in which the MYO3A tail was fused to the MYO3B motor demonstrated that motor activity correlates with formation and elongation of actin protrusions. We demonstrate that removal of individual exons (30-34) in the MYO3A tail does not prevent filopodia tip localization but abolishes the ability to enhance actin protrusion formation and elongation in COS7 cells. Interestingly, our results demonstrate that MYO3A slows filopodia dynamics and enhances filopodia lifetime in COS7 cells. We also demonstrate that MYO3A is more efficient than MYO3B at increasing formation and elongation of stable microvilli on the surface of cultured epithelial cells. We propose that the unique features of MYO3A, enhanced motor activity, and an extended tail with tail actin-binding motif, allow it to play an important role in stable actin protrusion length and ultrastructure maintenance.