Espin gene (ESPN) mutations associated with autosomal dominant hearing loss cause defects in microvillar elongation or organisation

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.

The genetic and phenotypic landscapes of Usher syndrome: from disease mechanisms to a new classification

Usher syndrome (USH) is the most common cause of deaf–blindness in humans, with a prevalence of about 1/10,000 (~ 400,000 people worldwide). Cochlear implants are currently used to reduce the burden of hearing loss in severe-to-profoundly deaf patients, but many promising treatments including gene, cell, and drug therapies to restore the native function of the inner ear and retinal sensory cells are under investigation. The traditional clinical classification of Usher syndrome defines three major subtypes—USH1, 2 and 3—according to hearing loss severity and onset, the presence or absence of vestibular dysfunction, and age at onset of retinitis pigmentosa. Pathogenic variants of nine USH genes have been initially reported: MYO7A, USH1C, PCDH15, CDH23, and USH1G for USH1, USH2A, ADGRV1, and WHRN for USH2, and CLRN1 for USH3. Based on the co-occurrence of hearing and vision deficits, the list of USH genes has been extended to few other genes, but with limited supporting information. A consensus on combined criteria for Usher syndrome is crucial for the development of accurate diagnosis and to improve patient management. In recent years, a wealth of information has been obtained concerning the properties of the Usher proteins, related molecular networks, potential genotype–phenotype correlations, and the pathogenic mechanisms underlying the impairment or loss of hearing, balance and vision. The advent of precision medicine calls for a clear and more precise diagnosis of Usher syndrome, exploiting all the existing data to develop a combined clinical/genetic/network/functional classification for Usher syndrome.

Usher Syndrome

Usher syndrome (USH) is the most common genetic condition responsible for combined loss of hearing and vision. Balance disorders and bilateral vestibular areflexia are also observed in some cases. The syndrome was first described by Albrecht von Graefe in 1858, but later named by Charles Usher, who presented a large number of cases with hearing loss and retinopathy in 1914. USH has been grouped into three main clinical types: 1, 2, and 3, which are caused by mutations in different genes and are further divided into different subtypes. To date, nine causative genes have been identified and confirmed as responsible for the syndrome when mutated: MYO7A, USH1C, CDH23, PCDH15, and USH1G (SANS) for Usher type 1; USH2A, ADGRV1, and WHRN for Usher type 2; CLRN1 for Usher type 3. USH is inherited in an autosomal recessive pattern. Digenic, bi-allelic, and polygenic forms have also been reported, in addition to dominant or nonsyndromic forms of genetic mutations. This narrative review reports the causative forms, diagnosis, prognosis, epidemiology, rehabilitation, research, and new treatments of USH.

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

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

Altered gene expression in slc4a11-/- mouse cornea highlights SLC4A11 roles

SLC4A11 is a H⁺/NH₃/water transport protein, of corneal endothelial cells. SLC4A11 mutations cause congenital hereditary endothelial dystrophy and some cases of Fuchs endothelial corneal dystrophy. To probe SLC4A11’s roles, we compared gene expression in RNA from corneas of 17-week-old slc4a11^−/− (n = 3) and slc4a11^+/+ mice (n = 3) and subjected to RNA sequencing. mRNA levels for a subset of genes were also assessed by quantitative real-time reverse transcription PCR (qRT RT-PCR). Cornea expressed 13,173 genes, which were rank-ordered for their abundance. In slc4a11^−/− corneas, 100 genes had significantly altered expression. Abundant slc14a1 expression, encoding the urea transporter UT-A, suggests a significant role in the cornea. The set of genes with altered expression was subjected to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses, revealing that alterations clustered into extracellular region, cytoskeleton, cell adhesion and plasma membrane functions. Gene expression changes further clustered into classes (with decreasing numbers of genes): cell fate and development, extracellular matrix and cell adhesion, cytoskeleton, ion homeostasis and energy metabolism. Together these gene changes confirm earlier suggestions of a role of SLC4A11 in ion homeostasis, energy metabolism, cell adhesion, and reveal an unrecognized SLC4A11 role in cytoskeletal organization.

Genetics, pathogenesis and therapeutic developments for Usher syndrome type 2

Usher syndrome (USH) is a rare, autosomal recessively inherited disorder resulting in a combination of sensorineural hearing loss and a progressive loss of vision resulting from retinitis pigmentosa (RP), occasionally accompanied by an altered vestibular function. More and more evidence is building up indicating that also sleep deprivation, olfactory dysfunction, deficits in tactile perception and reduced sperm motility are part of the disease etiology. USH can be clinically classified into three different types, of which Usher syndrome type 2 (USH2) is the most prevalent. In this review, we, therefore, assess the genetic and clinical aspects, available models and therapeutic developments for USH2. Mutations in USH2A, ADGRV1 and WHRN have been described to be responsible for USH2, with USH2A being the most frequently mutated USH-associated gene, explaining 50% of all cases. The proteins encoded by the USH2 genes together function in a dynamic protein complex that, among others, is found at the photoreceptor periciliary membrane and at the base of the hair bundles of inner ear hair cells. To unravel the pathogenic mechanisms underlying USH2, patient-derived cellular models and animal models including mouse, zebrafish and drosophila, have been generated that all in part mimic the USH phenotype. Multiple cellular and genetic therapeutic approaches are currently under development for USH2, mainly focused on preserving or partially restoring the visual function of which one is already in the clinical phase. These developments are opening a new gate towards a possible treatment for USH2 patients.

Genetic Epidemiology of Hearing Loss in the 22 Arab Countries: A Systematic Review

BACKGROUND

Hearing loss (HL) is a heterogeneous condition that causes partial or complete hearing impairment. Hundreds of variants in more than 60 genes have been reported to be associated with Hereditary HL (HHL). The HHL prevalence is thought to be high in the Arab population; however, the genetic epidemiology of HHL among Arab populations is understudied. This study aimed to systematically analyze the genetic epidemiology of HHL in Arab countries.

METHODS

We searched four literature databases (PubMed, Scopus, Science Direct, and Web of Science) from the time of inception until January 2019 using broad search terms to capture all the reported epidemiological and genetic data related to Arab patients with HHL.

FINDINGS

A total of 2,600 citations were obtained; 96 studies met our inclusion criteria. Our search strategy yielded 121,276 individuals who were tested for HL over 52 years (1966-2018), of whom 8,099 were clinically diagnosed with HL and belonged to 16 Arab countries. A total of 5,394 patients and 61 families with HHL were genotyped, of whom 336 patients and 6 families carried 104 variants in 44 genes and were from 17/22 Arab countries. Of these variants, 72 (in 41 genes) were distinctive to Arab patients. Arab patients manifested distinctive clinical phenotypes. The incidence of HHL in the captured studies ranged from 1.20 to 18 per 1,000 births per year, and the prevalence was the highest in Iraq (76.3%) and the lowest in Jordan (1.5%).

INTERPRETATION

This is the first systematic review to capture the prevalence and spectrum of variants associated with HHL in an Arab population. There appears to be a distinctive clinical picture for Arab patients with HHL, and the range and distribution of variants among Arab patients differ from those noted in other affected ethnic groups.

Improving the Management of Patients with Hearing Loss by the Implementation of an NGS Panel in Clinical Practice

A cohort of 128 patients from 118 families diagnosed with non-syndromic or syndromic hearing loss (HL) underwent an exhaustive clinical evaluation. Molecular analysis was performed using targeted next-generation sequencing (NGS) with a custom panel that included 59 genes associated with non-syndromic HL or syndromic HL. Variants were prioritized according to the minimum allele frequency and classified according to the American College of Medical Genetics and Genomics guidelines. Variant(s) responsible for the disease were detected in a 40% of families including autosomal recessive (AR), autosomal dominant (AD) and X-linked patterns of inheritance. We identified pathogenic or likely pathogenic variants in 26 different genes, 15 with AR inheritance pattern, 9 with AD and 2 that are X-linked. Fourteen of the found variants are novel. This study highlights the clinical utility of targeted NGS for sensorineural hearing loss. The optimal panel for HL must be designed according to the spectrum of the most represented genes in a given population and the laboratory capabilities considering the pressure on healthcare.

Usher syndrome: diagnostic approach, differential diagnoses and proposal of an updated function-based genetic classification

Usher syndrome (USH) manifests with congenital and apparently isolated hearing loss, followed by retinal degeneration in later life. Therefore, and because of its high prevalence in the congenitally hearing-impaired population, USH is one of the most relevant deafness syndromes. Next-generation sequencing (NGS)-based testing can now provide most analyzed USH patients with a molecular diagnosis, based on mutations in 11 genes. Given the availability of several excellent articles on the clinical and biochemical basis of USH, this short review focuses on critical assessment of new genes announced as USH genes, clinical and genetic differential diagnoses and therapeutic developments. Because obsolete loci, disproved USH genes and the inclusion of genes whose mutations cause similar phenotypes have increasingly blurred genetic classification, a revision based on phenotype restricted to genes related to the Usher protein complex is proposed.

Atypical and ultra-rare Usher syndrome: a review

Usher syndrome has classically been described as a combination of hearing loss and rod-cone dystrophy; vestibular dysfunction is present in many patients. Three distinct clinical subtypes were documented in the late 1970s. Genotyping efforts have led to the identification of several genes associated with the disease. Recent literature has seen multiple publications referring to "atypical" Usher syndrome presentations. This manuscript reviews the molecular etiology of Usher syndrome, highlighting rare presentations and molecular causes. Reports of "atypical" disease are summarized noting the wide discrepancy in the spectrum of phenotypic deviations from the classical presentation. Guidelines for establishing a clear nomenclature system are suggested.

Positive Selection and Inactivation in the Vision and Hearing Genes of Cetaceans

The transition to an aquatic lifestyle in cetaceans (whales and dolphins) resulted in a radical transformation in their sensory systems. Toothed whales acquired specialized high-frequency hearing tied to the evolution of echolocation, whereas baleen whales evolved low-frequency hearing. More generally, all cetaceans show adaptations for hearing and seeing underwater. To determine the extent to which these phenotypic changes have been driven by molecular adaptation, we performed large-scale targeted sequence capture of 179 sensory genes across the Cetacea, incorporating up to 54 cetacean species from all major clades as well as their closest relatives, the hippopotamuses. We screened for positive selection in 167 loci related to vision and hearing and found that the diversification of cetaceans has been accompanied by pervasive molecular adaptations in both sets of genes, including several loci implicated in nonsyndromic hearing loss. Despite these findings, however, we found no direct evidence of positive selection at the base of odontocetes coinciding with the origin of echolocation, as found in studies examining fewer taxa. By using contingency tables incorporating taxon- and gene-based controls, we show that, although numbers of positively selected hearing and nonsyndromic hearing loss genes are disproportionately high in cetaceans, counts of vision genes do not differ significantly from expected values. Alongside these adaptive changes, we find increased evidence of pseudogenization of genes involved in cone-mediated vision in mysticetes and deep-diving odontocetes.

Autosomal dominant Alport syndrome due to a COL4A4 mutation with an additional ESPN variant detected by whole-exome analysis

Alport syndrome (AS) is a rare hereditary disease that presents with chronic kidney disease and sensorineural hearing loss, and is diagnosed by its clinical features, pathological features on renal tissue, and mode of inheritance. We report a woman in her 20 s who exhibited persistent haematuria with normal renal function and sensorineural hearing loss. Her family members exhibited the same clinical findings among three generations and were suspected of having autosomal dominant AS (ADAS). Renal biopsy showed minor glomerular abnormalities on light microscopy and extensive thinning of the glomerular basement membrane on electron microscopy. Whole-exome analysis revealed a known COL4A4 (type IV collagen α4) mutation (c. 2510 G > C: p. Gly837Ala). Two pedigrees with the same variant have been reported previously, one as ADAS and the other as autosomal recessive AS. However, these two cases exhibited no sensorineural hearing loss. The analysis in the present case revealed another missense variant in ESPN (Espin), an actin-bundling protein, which is a causative gene for sensorineural hearing loss. Although the pathophysiological significance of this novel missense variant needs to be clarified, computational analysis predicted that the variant creates a new phosphorylation site for protein kinase C. Our case suggests a possible association of hereditary sensorineural hearing loss with ADAS. Whole-exome analysis should be considered to diagnose hereditary and multiple-organ disorders.

Identification of whirlin domains interacting with espin: A study of the mechanism of Usher syndrome type II

Usher syndrome is the most common condition of combined blindness and deafness and is classified into three types (USH1-USH3). USH2 is the most commonly diagnosed of all Usher syndrome cases. There are three identified proteins (usherin, GPR98 and whirlin) that form the USH2 complex. Defects in any of these proteins may cause failure in the formation of the USH2 complex, which is the primary cause of USH2. Whirlin is a scaffold protein and is essential for the assembly of the USH2 protein complex. It has been reported that espin is an interacting partner protein for whirlin. However, which fragment of whirlin interacts with espin remains unclear. In the present study, whirlin N- and C-terminal fragments in the pEGFP-C2 vectors were constructed. The recombinant plasmids were transfected into COS-7 cells to observe the co-localization by confocal laser scanning microscopy. The interactions between whirlin and espin were investigated by co-immunoprecipitation using the 293 cell line. It was demonstated that only the whirlin N-terminal fragment was able to interact with espin and the PR (proline-rich) region in whirlin may be important for the interaction. However, the present study did not investigate the interaction between whirlin and espin without the PR domain which warrants future research. Our findings elucidated a primary mechanism of interaction between whirlin and espin, which are crucial for further study on the USH2 complex and USH2 pathogenesis.

Regulation of the apical extension morphogenesis tunes the mechanosensory response of microvilliated neurons

Multiple types of microvilliated sensory cells exhibit an apical extension thought to be instrumental in the detection of sensory cues. The investigation of the mechanisms underlying morphogenesis of sensory apparatus is critical to understand the biology of sensation. Most of what we currently know comes from the study of the hair bundle of the inner ear sensory cells, but morphogenesis and function of other sensory microvilliated apical extensions remain poorly understood. We focused on spinal sensory neurons that contact the cerebrospinal fluid (CSF) through the projection of a microvilliated apical process in the central canal, referred to as cerebrospinal fluid-contacting neurons (CSF-cNs). CSF-cNs respond to pH and osmolarity changes as well as mechanical stimuli associated with changes of flow and tail bending. In vivo time-lapse imaging in zebrafish embryos revealed that CSF-cNs are atypical neurons that do not lose their apical attachment and form a ring of actin at the apical junctional complexes (AJCs) that they retain during differentiation. We show that the actin-based protrusions constituting the microvilliated apical extension arise and elongate from this ring of actin, and we identify candidate molecular factors underlying every step of CSF-cN morphogenesis. We demonstrate that Crumbs 1 (Crb1), Myosin 3b (Myo3b), and Espin orchestrate the morphogenesis of CSF-cN apical extension. Using calcium imaging in crb1 and espin mutants, we further show that the size of the apical extension modulates the amplitude of CSF-cN sensory response to bending of the spinal cord. Based on our results, we propose that the apical actin ring could be a common site of initiation of actin-based protrusions in microvilliated sensory cells. Furthermore, our work provides a set of actors underlying actin-based protrusion elongation shared by different sensory cell types and highlights the critical role of the apical extension shape in sensory detection.

A study of differentiating spinal sensory neurons in vivo uncovers critical factors required for the morphogenesis of sensory microvilli and reveals fine modulation of mechanosensory responses by microvillus length.

Inframe deletion of human ESPN is associated with deafness, vestibulopathy and vision impairment

BACKGROUND

Usher syndrome (USH) is a neurosensory disorder characterised by deafness, variable vestibular areflexia and vision loss. The aim of the study was to identify the genetic defect in a Pakistani family (PKDF1051) segregating USH.

METHODS

Genome-wide linkage analysis was performed by using an Illumina linkage array followed by Sanger and exome sequencing. Heterologous cells and mouse organ of Corti explant-based transfection assays were used for functional evaluations. Detailed clinical evaluations were performed to characterise the USH phenotype.

RESULTS

Through homozygosity mapping, we genetically linked the USH phenotype segregating in family PKDF1051 to markers on chromosome 1p36.32-p36.22. The locus was designated USH1M. Using a combination of Sanger sequencing and exome sequencing, we identified a novel homozygous 18 base pair inframe deletion in ESPN. Variants of ESPN, encoding the actin-bundling protein espin, have been previously associated with deafness and vestibular areflexia in humans with no apparent visual deficits. Our functional studies in heterologous cells and in mouse organ of Corti explant cultures revealed that the six deleted residues in affected individuals of family PKDF1051 are essential for the actin bundling function of espin demonstrated by ultracentrifugation actin binding and bundling assays. Funduscopic examination of the affected individuals of family PKDF1051 revealed irregular retinal contour, temporal flecks and disc pallor in both eyes. ERG revealed diminished rod photoreceptor function among affected individuals.

CONCLUSION

Our study uncovers an additional USH gene, assigns the USH1 phenotype to a variant of ESPN and provides a 12th molecular component to the USH proteome.

Overexpression of microRNA-612 Restrains the Growth, Invasion, and Tumorigenesis of Melanoma Cells by Targeting Espin

microRNA (miR)-612 shows anticancer activity in several types of cancers, yet its function in melanoma is still unclear. This study was undertaken to investigate the expression of miR-612 and its biological relevance in melanoma cell growth, invasion, and tumorigenesis. The expression and prognostic significance of miR-612 in melanoma were examined. The effects of miR-612 overexpression on cell proliferation, colony formation, tumorigenesis, and invasion were determined. Rescue experiments were conducted to identify the functional target gene(s) of miR-612. miR-612 was significantly downregulated in melanoma tissues compared to adjacent normal tissues. Low miR-612 expression was significantly associated with melanoma thickness, lymph node metastasis, and shorter overall, and disease-free survival of patients. Overexpression of miR-612 significantly decreased cell proliferation, colony formation, and invasion of SK-MEL-28 and A375 melanoma cells. In vivo tumorigenic studies confirmed that miR-612 overexpression retarded the growth of A375 xenograft tumors, which was coupled with a decline in the percentage of Ki-67-positive proliferating cells. Mechanistically, miR-612 targeted Espin in melanoma cells. Overexpression of Espin counteracted the suppressive effects of miR-612 on melanoma cell proliferation, invasion, and tumorigenesis. A significant inverse correlation (r = -0.376, P = 0.018) was observed between miR-612 and Espin protein expression in melanoma tissues. In addition, overexpression of miR-612 and knockdown of Espin significantly increased the sensitivity of melanoma cells to doxorubicin. Collectively, miR-612 suppresses the aggressive phenotype of melanoma cells through downregulation of Espin. Delivery of miR-612 may represent a novel therapeutic strategy against melanoma.

Congenital hearing loss

Congenital hearing loss (hearing loss that is present at birth) is one of the most prevalent chronic conditions in children. In the majority of developed countries, neonatal hearing screening programmes enable early detection; early intervention will prevent delays in speech and language development and has long-lasting beneficial effects on social and emotional development and quality of life. A diagnosis of hearing loss is usually followed by a search for an underlying aetiology. Congenital hearing loss might be attributed to environmental and prenatal factors, which prevail in low-income settings; congenital infections, particularly cytomegalovirus infection, are also a common risk factor for hearing loss. Genetic causes probably account for the majority of cases in developed countries; mutations can affect any component of the hearing pathway, in particular, inner ear homeostasis (endolymph production and maintenance) and mechano-electrical transduction (the conversion of a mechanical stimulus into electrochemical activity). Once the underlying cause of hearing loss is established, it might direct therapeutic decision making and guide prevention and (genetic) counselling. Management options include specific antimicrobial therapies, surgical treatment of craniofacial abnormalities and implantable or non-implantable hearing devices. An improved understanding of the pathophysiology and molecular mechanisms that underlie hearing loss and increased awareness of recent advances in genetic testing will promote the development of new treatment and screening strategies.

Two Deafness-Causing Actin Mutations (DFNA20/26) Have Allosteric Effects on the Actin Structure

Point mutations in γ-cytoplasmic actin have been shown to result in autosomal-dominant, nonsyndromic, early-onset deafness. Two mutations at the same site, K118M and K118N, provide a unique opportunity to compare the effects of two dissimilar amino acid substitutions that produce a similar phenotype in humans. K118 resides in a helix that runs from K113 to T126, and mutations that alter the position, dynamics, and/or biochemistry of this helix can result in a wide range of pathologies. Using a combination of computational and experimental studies, both employing yeast actin, we find that these mutations at K118 result in changes in the structure and dynamics of the DNase-I loop, alterations in the structure of the H73 loop as well as the side-chain orientations of W79 and W86, changes in nucleotide exchange rates, and significant shifts in the twist of the actin monomer. Interestingly, in the case of K118N, the twist of the monomer is nearly identical to that of the F-actin protomer, and in vitro polymerization assays show that this mutation results in faster polymerization. Taken together, these results indicate that mutations at this site give rise to a series of small changes that can be tolerated in vivo but result in misregulation of actin assembly and dynamics.

Effects of genetic correction on the differentiation of hair cell-like cells from iPSCs with MYO15A mutation

Deafness or hearing loss is a major issue in human health. Inner ear hair cells are the main sensory receptors responsible for hearing. Defects in hair cells are one of the major causes of deafness. A combination of induced pluripotent stem cell (iPSC) technology with genome-editing technology may provide an attractive cell-based strategy to regenerate hair cells and treat hereditary deafness in humans. Here, we report the generation of iPSCs from members of a Chinese family carrying MYO15A c.4642G>A and c.8374G>A mutations and the induction of hair cell-like cells from those iPSCs. The compound heterozygous MYO15A mutations resulted in abnormal morphology and dysfunction of the derived hair cell-like cells. We used a CRISPR/Cas9 approach to genetically correct the MYO15A mutation in the iPSCs and rescued the morphology and function of the derived hair cell-like cells. Our data demonstrate the feasibility of generating inner ear hair cells from human iPSCs and the functional rescue of gene mutation-based deafness by using genetic correction.

Myosin III-mediated cross-linking and stimulation of actin bundling activity of Espin

Class III myosins (Myo3) and actin-bundling protein Espin play critical roles in regulating the development and maintenance of stereocilia in vertebrate hair cells, and their defects cause hereditary hearing impairments. Myo3 interacts with Espin1 through its tail homology I motif (THDI), however it is not clear how Myo3 specifically acts through Espin1 to regulate the actin bundle assembly and stabilization. Here we discover that Myo3 THDI contains a pair of repeat sequences capable of independently and strongly binding to the ankyrin repeats of Espin1, revealing an unexpected Myo3-mediated cross-linking mechanism of Espin1. The structures of Myo3 in complex with Espin1 not only elucidate the mechanism of the binding, but also reveal a Myo3-induced release of Espin1 auto-inhibition mechanism. We also provide evidence that Myo3-mediated cross-linking can further promote actin fiber bundling activity of Espin1.

Identification of stage-specific markers during differentiation of hair cells from mouse inner ear stem cells or progenitor cells in vitro

The induction of inner ear hair cells from stem cells or progenitor cells in the inner ear proceeds through a committed inner ear sensory progenitor cell stage prior to hair cell differentiation. To increase the efficacy of inducing inner ear hair cell differentiation from the stem cells or progenitor cells, it is essential to identify comprehensive markers for the stem cells/progenitor cells from the inner ear, the committed inner ear sensory progenitor cells and the differentiating hair cells to optimize induction conditions. Here, we report that we efficiently isolated and expanded the stem cells or progenitor cells from postnatal mouse cochleae, and induced the generation of inner ear progenitor cells and subsequent differentiation of hair cells. We profiled the gene expression of the stem cells or progenitor cells, the inner ear progenitor cells, and hair cells using aRNA microarray analysis. The pathway and gene ontology (GO) analysis of differentially expressed genes was performed. Analysis of genes exclusively detected in one particular cellular population revealed 30, 38, and 31 genes specific for inner ear stem cells, inner ear progenitor cells, and hair cells, respectively. We further examined the expression of these genes in vivo and determined that Gdf10+Ccdc121, Tmprss9+Orm1, and Chrna9+Espnl are marker genes specific for inner ear stem cells, inner ear progenitor cells, and differentiating hair cells, respectively. The identification of these marker genes will likely help the effort to increase the efficacy of hair cell induction from the stem cells or progenitor cells.

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

Hearing relies on the mechanosensory inner and outer hair cells (OHCs) of the organ of Corti, which convert mechanical deflections of their actin-rich stereociliary bundles into electrochemical signals. Several actin-associated proteins are essential for stereocilia formation and maintenance, and their absence leads to deafness. One of the most abundant actin-bundling proteins of stereocilia is plastin 1, but its function has never been directly assessed. Here, we found that plastin 1 knock-out (Pls1 KO) mice have a moderate and progressive form of hearing loss across all frequencies. Auditory hair cells developed normally in Pls1 KO, but in young adult animals, the stereocilia of inner hair cells were reduced in width and length. The stereocilia of OHCs were comparatively less affected; however, they also showed signs of degeneration in ageing mice. The hair bundle stiffness and the acquisition of the electrophysiological properties of hair cells were unaffected by the absence of plastin 1, except for a significant change in the adaptation properties, but not the size of the mechanoelectrical transducer currents. These results show that in contrast to other actin-bundling proteins such as espin, harmonin or Eps8, plastin 1 is dispensable for the initial formation of stereocilia. However, the progressive hearing loss and morphological defects of hair cells in adult Pls1 KO mice point at a specific role for plastin 1 in the preservation of adult stereocilia and optimal hearing. Hence, mutations in the human PLS1 gene may be associated with relatively mild and progressive forms of hearing loss.

Hearing molecules, mechanism and transportation: modeled in Drosophila melanogaster

Mechanosensory transduction underlies the perception of touch, sound and acceleration. The mechanical signals exist in the environment are resensed by the specialized mechanosensory cells, which convert the external forces into the electrical signals. Hearing is a magnificent example that relies on the mechanotransduction mediated by the auditory cells, for example the inner-ear hair cells in vertebrates and the Johnston's organ (JO) in fly. Previous studies have shown the fundamental physiological processes in the fly and vertebrate auditory organs are similar, suggesting that there might be a set of similar molecules underlying these processes. The molecular studies of the fly JO have been shown to be remarkably successful in discovering the developmental and functional genes that provided further implications in vertebrates. Several evolutionarily conserved molecules and signaling pathways have been shown to govern the development of the auditory organs in both vertebrates and invertebrates. The current review describes the similarities and differences between the vertebrate and fly auditory organs at developmental, structural, molecular, and transportation levels.

Non-syndromic hereditary sensorineural hearing loss: review of the genes involved

BACKGROUND

Hereditary sensorineural hearing loss is the most frequently occurring birth defect. It has profound effects for the individual and is a substantial burden on society. Insight into disease mechanisms can help to broaden therapeutic options and considerably lower lifetime social costs. In the past few decades, the identification of genes that can cause this type of hearing loss has developed rapidly.

OBJECTIVE

This paper provides a concise overview of the currently known genes involved in non-syndromic hereditary hearing loss and their function in the inner ear.

Neuropathology of brain and spinal malformations in a case of monosomy 1p36

Monosomy 1p36 is the most common subtelomeric chromosomal deletion linked to mental retardation and seizures. Neuroimaging studies suggest that monosomy 1p36 is associated with brain malformations including polymicrogyria and nodular heterotopia, but the histopathology of these lesions is unknown. Here we present postmortem neuropathological findings from a 10 year-old girl with monosomy 1p36, who died of respiratory complications. The findings included micrencephaly, periventricular nodular heterotopia in occipitotemporal lobes, cortical dysgenesis resembling polymicrogyria in dorsolateral frontal lobes, hippocampal malrotation, callosal hypoplasia, superiorly rotated cerebellum with small vermis, and lumbosacral hydromyelia. The abnormal cortex exhibited “festooned” (undulating) supragranular layers, but no significant fusion of the molecular layer. Deletion mapping demonstrated single copy loss of a contiguous 1p36 terminal region encompassing many important neurodevelopmental genes, among them four HES genes implicated in regulating neural stem cell differentiation, and TP73, a monoallelically expressed gene. Our results suggest that brain and spinal malformations in monosomy 1p36 may be more extensive than previously recognized, and may depend on the parental origin of deleted genes. More broadly, our results suggest that specific genetic disorders may cause distinct forms of cortical dysgenesis.

Whirlin interacts with espin and modulates its actin-regulatory function: an insight into the mechanism of Usher syndrome type II

Whirlin mutations cause retinal degeneration and hearing loss in Usher syndrome type II (USH2) and non-syndromic deafness, DFNB31. Its protein recruits other USH2 causative proteins to form a complex at the periciliary membrane complex in photoreceptors and the ankle link of the stereocilia in hair cells. However, the biological function of this USH2 protein complex is largely unknown. Using a yeast two-hybrid screen, we identified espin, an actin-binding/bundling protein involved in human deafness when defective, as a whirlin-interacting protein. The interaction between these two proteins was confirmed by their coimmunoprecipitation and colocalization in cultured cells. This interaction involves multiple domains of both proteins and only occurs when espin does not bind to actin. Espin was partially colocalized with whirlin in the retina and the inner ear. In whirlin knockout mice, espin expression changed significantly in these two tissues. Further studies found that whirlin increased the mobility of espin and actin at the actin bundles cross-linked by espin and, eventually, affected the dimension of these actin bundles. In whirlin knockout mice, the stereocilia were thickened in inner hair cells. We conclude that the interaction between whirlin and espin and the balance between their expressions are required to maintain the actin bundle network in photoreceptors and hair cells. Disruption of this actin bundle network contributes to the pathogenic mechanism of hearing loss and retinal degeneration caused by whirlin and espin mutations. Espin is a component of the USH2 protein complex and could be a candidate gene for Usher syndrome.

Auditory and vestibular hair cell stereocilia: relationship between functionality and inner ear disease

The stereocilia of the inner ear are unique cellular structures which correlate anatomically with distinct cochlear functions, including mechanoelectrical transduction, cochlear amplification, adaptation, frequency selectivity and tuning. Their function is impaired by inner ear stressors, by various types of hereditary deafness, syndromic hearing loss and inner ear disease (e.g. Ménière's disease). The anatomical and physiological characteristics of stereocilia are discussed in relation to inner ear malfunctions.

Coupling of the mechanotransduction machinery and F-actin polymerization in the cochlear hair bundles

Mechanoelectrical transduction (MET), the conversion of mechanical stimuli into electrical signals operated by the sensory cells of the inner ear, enables hearing and balance perception. Crucial to this process are the tip-links, oblique fibrous filaments that interconnect the actin-filled stereocilia of different rows within the hair bundle, and mechanically gate MET channels. In a recent study, we observed a complete regression of stereocilia from the short and medium but not the tall row upon the disappearance of the tip-links caused by the loss of one of their components, cadherin-23, or of one of their anchoring proteins, sans, in the auditory organs of engineered mutant mice. This indicates the existence of a coupling between the MET and F-actin polymerization machineries at the tips of the short and medium stereocilia rows in cochlear hair bundles. Here, we first present our findings in the mutant mice, and then discuss the possible effects of the tip-link tension on stereocilia F-actin polymerization, acting either directly or through Ca(2+)-dependent mechanisms that involve the gating of MET channels.

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

Hair cells of the mammalian cochlea are specialized for the dynamic coding of sound stimuli. The transduction of sound waves into electrical signals depends upon mechanosensitive hair bundles that project from the cell's apical surface. Each stereocilium within a hair bundle is composed of uniformly polarized and tightly packed actin filaments. Several stereociliary proteins have been shown to be associated with hair bundle development and function and are known to cause deafness in mice and humans when mutated. The growth of the stereociliar actin core is dynamically regulated at the actin filament barbed ends in the stereociliary tip. We show that Eps8, a protein with actin binding, bundling, and barbed-end capping activities in other systems, is a novel component of the hair bundle. Eps8 is localized predominantly at the tip of the stereocilia and is essential for their normal elongation and function. Moreover, we have found that Eps8 knockout mice are profoundly deaf and that IHCs, but not OHCs, fail to mature into fully functional sensory receptors. We propose that Eps8 directly regulates stereocilia growth in hair cells and also plays a crucial role in the physiological maturation of mammalian cochlear IHCs. Together, our results indicate that Eps8 is critical in coordinating the development and functionality of mammalian auditory hair cells.

Roles of the espin actin-bundling proteins in the morphogenesis and stabilization of hair cell stereocilia revealed in CBA/CaJ congenic jerker mice

Hearing and vestibular function depend on mechanosensory staircase collections of hair cell stereocilia, which are produced from microvillus-like precursors as their parallel actin bundle scaffolds increase in diameter and elongate or shorten. Hair cell stereocilia contain multiple classes of actin-bundling protein, but little is known about what each class contributes. To investigate the roles of the espin class of actin-bundling protein, we used a genetic approach that benefited from a judicious selection of mouse background strain and an examination of the effects of heterozygosity. A congenic jerker mouse line was prepared by repeated backcrossing into the inbred CBA/CaJ strain, which is known for excellent hearing and minimal age-related hearing loss. We compared stereocilia in wild-type CBA/CaJ mice, jerker homozygotes that lack espin proteins owing to a frameshift mutation in the espin gene, and jerker heterozygotes that contain reduced espin levels. The lack of espins radically impaired stereociliary morphogenesis, resulting in stereocilia that were abnormally thin and short, with reduced differential elongation to form a staircase. Mean stereociliary diameter did not increase beyond ∼0.10–0.14 µm, making stereocilia ∼30%–60% thinner than wild type and suggesting that they contained ∼50%–85% fewer actin filaments. These characteristics indicate a requirement for espins in the appositional growth and differential elongation of the stereociliary parallel actin bundle and fit the known biological activities of espins in vitro and in transfected cells. The stereocilia of jerker heterozygotes showed a transient proximal-distal tapering suggestive of haploinsufficiency and a slowing of morphogenesis that revealed previously unrecognized assembly steps and intermediates. The lack of espins also led to a region-dependent degeneration of stereocilia involving shortening and collapse. We conclude that the espin actin-bundling proteins are required for the assembly and stabilization of the stereociliary parallel actin bundle.

Stereocilia are the fingerlike projections of inner ear hair cells that detect sound and motion. Stereocilia grow to specific lengths and diameters and form staircase-like arrays. The changes in size appear to be driven by matching alterations in the dimensions of an underlying molecular scaffold consisting of a bundle of actin filaments cross-linked by actin-bundling proteins. To elucidate the roles of the espin actin-bundling proteins in hair cell stereocilia, we carry out an in-depth accounting of stereociliary size and shape in the jerker mutant mouse, which lacks the espin proteins because of a mutation in the espin gene. We examine a new and improved jerker mouse with a genetic background known for high-quality lifelong hearing. We find that, in the absence of espins, stereocilia do not increase in diameter or complete their elongation, but instead bend, shorten, and disappear. Although the specifics vary according to inner ear region, the stereociliary defects are profound and can readily account for the deafness and balance problems of jerker mice and humans with certain espin gene mutations. Even reducing espin levels by one-half leads to temporary defects in stereociliary diameter. Thus, espins play crucial roles in the formation and maintenance of hair cell stereocilia.

Actin cross-linkers and the shape of stereocilia

Stereocilia are actin-based cellular protrusions essential for hearing. We propose that they are shaped by the detachment dynamics of actin cross-linkers, in particular espin. We account for experimentally observed stereocilium shapes, treadmilling velocity to length relationship, espin 1 localization profile, and microvillus length to espin level relationship. If the cross-linkers are allowed to reattach, our model yields a dynamical phase transition toward unbounded growth. Considering the simplified case of a noninteracting, one-filament system, we calculate the length probability distribution in the growing phase and its stationary form in a continuum approximation of the finite-length phase. Numerical simulations of interacting filaments suggest an anomalous power-law divergence of the protrusion length at the growth transition, which could be a universal feature of cross-linked depolymerizing systems.

Runx2 transcriptome of prostate cancer cells: insights into invasiveness and bone metastasis

Background

Prostate cancer (PCa) cells preferentially metastasize to bone at least in part by acquiring osteomimetic properties. Runx2, an osteoblast master transcription factor, is aberrantly expressed in PCa cells, and promotes their metastatic phenotype. The transcriptional programs regulated by Runx2 have been extensively studied during osteoblastogenesis, where it activates or represses target genes in a context-dependent manner. However, little is known about the gene regulatory networks influenced by Runx2 in PCa cells. We therefore investigated genome wide mRNA expression changes in PCa cells in response to Runx2.

Results

We engineered a C4-2B PCa sub-line called C4-2B/Rx2^dox, in which Doxycycline (Dox) treatment stimulates Runx2 expression from very low to levels observed in other PCa cells. Transcriptome profiling using whole genome expression array followed by in silico analysis indicated that Runx2 upregulated a multitude of genes with prominent cancer associated functions. They included secreted factors (CSF2, SDF-1), proteolytic enzymes (MMP9, CST7), cytoskeleton modulators (SDC2, Twinfilin, SH3PXD2A), intracellular signaling molecules (DUSP1, SPHK1, RASD1) and transcription factors (Sox9, SNAI2, SMAD3) functioning in epithelium to mesenchyme transition (EMT), tissue invasion, as well as homing and attachment to bone. Consistent with the gene expression data, induction of Runx2 in C4-2B cells enhanced their invasiveness. It also promoted cellular quiescence by blocking the G1/S phase transition during cell cycle progression. Furthermore, the cell cycle block was reversed as Runx2 levels declined after Dox withdrawal.

Conclusions

The effects of Runx2 in C4-2B/Rx2^dox cells, as well as similar observations made by employing LNCaP, 22RV1 and PC3 cells, highlight multiple mechanisms by which Runx2 promotes the metastatic phenotype of PCa cells, including tissue invasion, homing to bone and induction of high bone turnover. Runx2 is therefore an attractive target for the development of novel diagnostic, prognostic and therapeutic approaches to PCa management. Targeting Runx2 may prove more effective than focusing on its individual downstream genes and pathways.

Review series: The cell biology of hearing

Mammals have an astonishing ability to sense and discriminate sounds of different frequencies and intensities. Fundamental for this process are mechanosensory hair cells in the inner ear that convert sound-induced vibrations into electrical signals. The study of genes that are linked to deafness has provided insights into the cell biological mechanisms that control hair cell development and their function as mechanosensors.

Targeting of the hair cell proteins cadherin 23, harmonin, myosin XVa, espin, and prestin in an epithelial cell model

We have developed an advantageous epithelial cell transfection model for examining the targeting, interactions, and mutations of hair cell proteins. When expressed in LLC-PK1-CL4 epithelial cells (CL4 cells), the outer hair cell protein prestin showed faithful domain-specific targeting to the basolateral plasma membrane. We examined the consequences of mutations affecting prestin activity and assigned a targeting role to the cytoplasmic tail. The stereociliary link protein cadherin 23 (Cdh23) was targeted to the plasma membrane of CL4 cell microvilli, the topological equivalent of stereocilia. In cells coexpressing the Cdh23 cytoplasmic binding protein harmonin, a large fraction of harmonin became colocalized with Cdh23 in microvilli. Using this assay and in vitro protein binding assays, we formulated an alternative model for Cdh23-harmonin binding, in which the primary interaction is between the harmonin N-domain and a 35-residue internal peptide in the Cdh23 cytoplasmic tail. Contrary to a previous model, we found no role for the Cdh23 C-terminal PDZ (PSD-95/Dlg/ZO-1)-binding motif and observed that Cdh23 bound similar levels of harmonin with or without the exon 68 peptide. We also examined two proteins involved in stereocilium elongation. The stereociliary actin-bundling protein espin was targeted to CL4 cell microvilli and caused microvillar elongation, whereas espin with the c.2469delGTCA or c.1988delAGAG human deafness mutation showed defects in microvillar targeting and elongation. The unconventional myosin motor myosin XVa accumulated at the tips of espin-elongated microvilli, by analogy to its location in stereocilia, whereas myosin XVa with the c.4351G>A or c.4669A>G human deafness mutation did not, revealing functional deficits in motor activity.

Structural and viscoelastic properties of actin networks formed by espin or pathologically relevant espin mutants

The structural organization of the cytoskeleton determines its viscoelastic response which is crucial for the correct functionality of living cells. Both the mechanical response and microstructure of the cytoskeleton are regulated on a microscopic level by the local activation of different actin binding and/or bundling proteins (ABPs). Misregulations in the expression of these ABPs or mutations in their sequence can entail severe cellular dysfunctions and diseases. Here, we study the structural and viscoelastic properties of reconstituted actin networks cross-linked by the ABP espin and compare the obtained network properties to those of other bundled actin networks. Moreover, we quantify the impact of pathologically relevant espin mutations on the viscoelastic properties of these cytoskeletal networks.

Linking genes underlying deafness to hair-bundle development and function

The identification of genes underlying monogenic, early-onset forms of deafness in humans has provided unprecedented insight into the molecular mechanisms of hearing in the peripheral auditory system. The molecules involved in the development and function of the cochlea eluded characterization until recently due to the paucity of the principle cell types present in cochlear hair cells, yet a genetic approach has circumvented this problem and succeeded in identifying proteins and deciphering some of the molecular complexes that operate in these cells . In combination with mouse models, the genetic approach is now revealing some of the principles underlying the development and physiology of the cochlea. The review centers on this facet of the genetics of hearing. Focusing on the hair bundle, the mechanosensory device of the sensory hair cell, we highlight recent advances in understanding the way in which the hair bundle is formed, how it operates as a mechanotransducer and how it processes sound. In particular, we discuss how this work highlights the roles played by various hair-bundle link types.

Connections and synaptic function in the posteroventral cochlear nucleus of deaf jerker mice

Mutations in the gene that encodes espins can cause deafness and vestibular disorders; mice that are homozygous for the autosomal recessive jerker mutation in the espin gene never hear. Extracellular injections of biocytin into the anteroventral cochlear nucleus (AVCN) revealed that although the cochlear nuclei are smaller in je/je mice, the topography in its innervation resembles that in wild-type mice. Auditory nerve fibers innervate narrow, topographically organized, "isofrequency" bands in deaf animals over the ages examined, P18-P70. The projection of tuberculoventral cells was topographic in je/je as in wild-type mice. Terminals of auditory nerve fibers in the multipolar cell area included both large and small endings, whereas in the octopus cell area they were exclusively small boutons in je/je as in wild-type mice, but end bulbs near the nerve root of je/je animals were smaller than in hearing animals. In whole-cell recordings from targets of auditory nerve fibers, octopus and T stellate cells, miniature excitatory postsynaptic currents (mEPSCs) had similar shapes as in +/+, indicating that the properties of AMPA receptors were not affected by the mutation. In je/je animals the frequency of spontaneous mEPSCs was elevated, and synaptic depression in responses to trains of shocks delivered at between 100 and 333 Hz was greater than in wild-type mice, indicating that the probability of neurotransmitter release was increased. The frequency of spontaneous mEPSCs and extent of synaptic depression were greater in octopus than in T stellate cells, in both wild-type and in je/je mice.

Espin actin-cytoskeletal proteins are in rat type I spiral ganglion neurons and include splice-isoforms with a functional nuclear localization signal

The espins are Ca(2+)-resistant actin-bundling proteins that are enriched in hair cell stereocilia and sensory cell microvilli. Here, we report a novel localization of espins to a large proportion of rat type I spiral ganglion neurons (SGNs) and their projections to the cochlear nucleus (CN). Moreover, we show that a fraction of these espins is in the nucleus of SGNs owing to the presence of splice-isoforms that contain a functional nuclear localization signal (NLS). Espin antibody labeled approximately 83% of type I SGNs, and the labeling intensity increased dramatically during early postnatal development. Type II SGNs and vestibular ganglion neurons were unlabeled. In the CN, espin-positive auditory nerve fibers showed a projection pattern typical of type I SGNs, with intense labeling in the nerve root region and posteroventral CN (PVCN). The anteroventral CN (AVCN) showed moderate labeling, whereas the dorsal CN showed weak labeling that was restricted to the deep layer. Espin-positive synaptic terminals were enriched around nerve root neurons and octopus cells in the PVCN and were also found on globular bushy cells and multipolar neurons in the PVCN and AVCN. SGNs expressed multiple espin transcripts and proteins, including splice-isoforms that contain a nonapeptide, which is rich in positively charged amino acids and creates a bipartite NLS. The nonapeptide was necessary to target espin isoforms to the nucleus and was sufficient to target an unrelated protein to the nucleus when joined with the upstream di-arginine-containing octapeptide. The presence of cytoplasmic and nuclear espins in SGNs suggests additional roles for espins in auditory neuroscience.

Vestibular function in families with inherited autosomal dominant hearing loss

The inner ear contains the developmentally related cochlea and peripheral vestibular labyrinth. Given the similar physiology between these two organs, hearing loss and vestibular dysfunction may be expected to occur simultaneously in individuals segregating mutations in inner ear genes. Twenty-two different genes have been discovered that when mutated lead to non-syndromic autosomal dominant hearing loss. A review of the literature indicates that families segregating mutations in 13 of these 22 genes have undergone formal clinical vestibular testing. Formal assessment revealed vestibular dysfunction in families with mutations in ten of these 13 genes. Remarkably, only families with mutations in the COCH and MYO7A genes self-report considerable vestibular challenges. Families segregating mutations in the other eight genes do not self-report significant balance problems and appear to compensate well in everyday life for vestibular deficits discovered during formal clinical vestibular assessment. An example of a family (referred to as the HL1 family) with progressive hearing loss and clinically-detected vestibular hypofunction that does not report vestibular symptoms is described in this review. Notably, one member of the HL1 family with clinically-detected vestibular hypofunction reached the summit of Mount Kilimanjaro.

Structural polymorphism of the actin-espin system: a prototypical system of filaments and linkers in stereocilia

We examine the interaction between cytoskeletal F-actin and espin 3A, a prototypical actin bundling protein found in sensory cell microvilli, including ear cell stereocilia. Espin induces twist distortions in F-actin as well as facilitates bundle formation. Mutations in one of the two F-actin binding sites of espin, which have been implicated in deafness, can tune espin-actin interactions and radically transform the system's phase behavior. These results are compared to recent theoretical work on the general phase behavior linker-rod systems.

Espins and the actin cytoskeleton of hair cell stereocilia and sensory cell microvilli

The espins are novel actin-bundling proteins that are produced in multiple isoforms from a single gene. They are present at high concentration in the parallel actin bundle of hair cell stereocilia and are the target of deafness mutations in mice and humans. Espins are also enriched in the microvilli of taste receptor cells, solitary chemoreceptor cells, vomeronasal sensory neurons and Merkel cells, suggesting that espins play important roles in the microvillar projections of vertebrate sensory cells. Espins are potent actin-bundling proteins that are not inhibited by Ca2+. In cells, they efficiently elongate parallel actin bundles and, thereby, help determine the steadystate length of microvilli and stereocilia. Espins bind actin monomer via their WH2 domain and can assemble actin bundles in cells. Certain espin isoforms can also bind phosphatidylinositol 4,5-bisphosphate, profilins or SH3 proteins. These biological activities distinguish espins from other actin-bundling proteins and may make them well-suited to sensory cells.

Espin cytoskeletal proteins in the sensory cells of rodent taste buds

Espins are multifunctional actin-bundling proteins that are highly enriched in the microvilli of certain chemosensory and mechanosensory cells, where they are believed to regulate the integrity and/or dimensions of the parallel-actin-bundle cytoskeletal scaffold. We have determined that, in rats and mice, affinity purified espin antibody intensely labels the lingual and palatal taste buds of the oral cavity and taste buds in the pharyngo-laryngeal region. Intense immunolabeling was observed in the apical, microvillar region of taste buds, while the level of cytoplasmic labeling in taste bud cells was considerably lower. Taste buds contain tightly packed collections of sensory cells (light, or type II plus type III) and supporting cells (dark, or type I), which can be distinguished by microscopic features and cell type-specific markers. On the basis of results obtained using an antigen-retrieval method in conjunction with double immunofluorescence for espin and sensory taste cell-specific markers, we propose that espins are expressed predominantly in the sensory cells of taste buds. In confocal images of rat circumvallate taste buds, we counted 21.5 +/- 0.3 espin-positive cells/taste bud, in agreement with a previous report showing 20.7 +/- 1.3 light cells/taste bud when counted at the ultrastructural level. The espin antibody labeled spindle-shaped cells with round nuclei and showed 100% colocalization with cell-specific markers recognizing all type II [inositol 1,4,5-trisphosphate receptor type III (IP(3)R(3))(,) alpha-gustducin, protein-specific gene product 9.5 (PGP9.5)] and a subpopulation of type III (IP(3)R(3), PGP9.5) taste cells. On average, 72%, 50%, and 32% of the espin-positive taste cells were labeled with antibodies to IP(3)R(3), alpha-gustducin, and PGP9.5, respectively. Upon sectional analysis, the taste buds of rat circumvallate papillae commonly revealed a multi-tiered, espin-positive apical cytoskeletal apparatus. One espin-positive zone, a collection of approximately 3 mum-long microvilli occupying the taste pore, was separated by an espin-depleted zone from a second espin-positive zone situated lower within the taste pit. This latter zone included espin-positive rod-like structures that occasionally extended basally to a depth of 10-12 mum into the cytoplasm of taste cells. We propose that the espin-positive zone in the taste pit coincides with actin bundles in association with the microvilli of type II taste cells, whereas the espin-positive microvilli in the taste pore are the single microvilli of type III taste cells.

Targeted wild-type and jerker espins reveal a novel, WH2-domain-dependent way to make actin bundles in cells

The espin actin-bundling proteins, which are the target of deafness mutations, are present in the parallel actin bundles of stereocilia and microvilli and appear to increase their steady-state length. Here, we report a new activity of the espins, one that depends on their enigmatic WH2 domain: the ability to assemble a large actin bundle when targeted to a specific subcellular location. This activity was observed for wild-type espins targeted to the centrosome in transfected neuronal cells and for jerker espins targeted to the nucleolus in a wide variety of transfected cells as a result of the frameshifted peptide introduced into the espin C-terminus by the jerker deafness mutation. This activity, which appears specific to espins, requires two espin F-actin-binding sites and the actin-monomer-binding activity of the espin WH2 domain, but can be mimicked by adding a WH2 domain to an unrelated actin-bundling protein, villin. Espins do not activate the Arp2/3 complex in vitro, and bundle assembly is not indicative of in-vitro nucleation activity. Our results suggest a novel way to build actin bundles at specific sites in cells.

Differential expression of espin isoforms during epithelial morphogenesis, stereociliogenesis and postnatal maturation in the developing inner ear

The espins are a family of multifunctional actin cytoskeletal proteins. They are present in hair cell stereocilia and are the target of mutations that cause deafness and vestibular dysfunction. Here, we demonstrate that the different espin isoforms are expressed in complex spatiotemporal patterns during inner ear development. Espin 3 isoforms were prevalent in the epithelium of the otic pit, otocyst and membranous labyrinth as they underwent morphogenesis. This espin was down-regulated ahead of hair cell differentiation and during neuroblast delamination. Espin also accumulated in the epithelium of branchial clefts and pharyngeal pouches and during branching morphogenesis in other embryonic epithelial tissues, suggesting general roles for espins in epithelial morphogenesis. Espin reappeared later in inner ear development in differentiating hair cells. Its levels and compartmentalization to stereocilia increased during the formation and maturation of stereociliary bundles. Late in embryonic development, espin was also present in a tail-like process that emanated from the hair cell base. Increases in the levels of espin 1 and espin 4 isoforms correlated with stereocilium elongation and maturation in the vestibular system and cochlea, respectively. Our results suggest that the different espin isoforms play specific roles in actin cytoskeletal regulation during epithelial morphogenesis and hair cell differentiation.