Are MYO1C and MYO1F associated with hearing loss?

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.

Mechanoelectrical transduction-related genetic forms of hearing loss

Hair cells of the mammalian cochlea are specialized mechanosensory cells that convert mechanical stimuli into electrical signals to initiate the neuronal responses that lead to the perception of sound. The mechanoelectrical transduction (MET) machinery of cochlear hair cells is a multimeric protein complex that consists of the pore forming subunits of the MET channel and several essential accessory subunits that are crucial to regulate channel function and render the channel mechanically sensitive. Mutations have been discovered in the genes that encode all known components of the MET machinery. These mutations cause hearing loss with or without vestibular dysfunction. Some mutations also affect other tissues such as the retina. In this brief review, we will summarize gene mutations that affect the MET machinery of hair cells and how the study of the affected genes has illuminated our understanding of the physiological role of the encoded proteins.

Refining the Clinical Spectrum of the 17p13.3 Microduplication Syndrome: Case-Report of a Familial Small Microduplication

The chromosomal region 17p13.3 contains extensive repetitive sequences and is a well-recognized region of genomic instability. The 17p13.3 microduplication syndrome has been associated with a clinical spectrum of moderately non-specific phenotypes, including global developmental delay/intellectual disability, behavioral disorders, autism spectrum disorder and variable dysmorphic features. Depending on the genes involved in the microduplication, it can be categorized in two subtypes with different phenotypes. Here, we report a case of a 7-year-old boy with global developmental delay, speech impairment, hypotonia, behavioral conditions (ADHD and ODD), non-specific dysmorphic features and overgrowth. Genetic testing revealed a small 17p13.3 chromosomal duplication, which included the BHLHA9, CRK and YWHAE genes. Additionally, we observed that this was maternally inherited, and that the mother presented with a milder phenotype including mild learning disabilities, speech impairment and non-specific dysmorphic features, which did not significantly affect her. In conclusion, we present a clinical case of a 17p13.3 duplication that further delineates the clinical spectrum of this syndrome, including its intrafamilial/intergenerational variability.

The role of motor proteins in photoreceptor protein transport and visual function

BACKGROUND

Rods and cones are photoreceptor neurons in the retina that are required for visual sensation in vertebrates, wherein the perception of vision is initiated when these neurons respond to photons in the light stimuli. The photoreceptor cell is structurally studied as outer segments (OS) and inner segments (IS) where proper protein sorting, localization, and compartmentalization are critical for phototransduction, visual function, and survival. In human retinal diseases, improper protein transport to the OS or mislocalization of proteins to the IS and other cellular compartments could lead to impaired visual responses and photoreceptor cell degeneration that ultimately cause loss of visual function.

RESULTS

Therefore, studying and identifying mechanisms involved in facilitating and maintaining proper protein transport in photoreceptor cells would help our understanding of pathologies involving retinal cell degeneration in inherited retinal dystrophies, age-related macular degeneration, and Usher Syndrome.

CONCLUSIONS

Our mini-review will discuss mechanisms of protein transport within photoreceptors and introduce a novel role for an unconventional motor protein, MYO1C, in actin-based motor transport of the visual chromophore Rhodopsin to the OS, in support of phototransduction and visual function.

Ammonia exposure induces oxidative stress and inflammation by destroying the microtubule structures and the balance of solute carriers in the trachea of pigs

Ammonia (NH₃) is the most alkaline gaseous compound in the atmosphere and the primary gas pollutant in the piggery. It can cause irritation and damage to the airway after inhalation. However, the effects and toxicity mechanism of NH₃ on the trachea are still unclear. In order to evaluate the toxic effects of NH₃ inhalation on pig trachea, the changes of oxidative stress parameters (SOD, GSH, GSH-Px, and MDA), tissue structure and transcriptome in the trachea of pigs were examined after 30 days of exposure to NH₃. Our results showed SOD, GSH-Px and GSH in the trachea in the NH₃-treatment group were significantly decreased (P < 0.05) compared with the control group, on the contrary, MDA content was significantly higher (P < 0.05). The analysis of differentially expressed genes (DEGs) showed that 2542 DEGs (1109 up-regulated DEGs and 1433 down-regulated DEGs) were significantly changed under NH₃ exposure, including many DEGs associated with inflammation, oxidative stress, microtubule activity and SLC family, and the qRT-PCR verification results of these DEGs were consistent with the transcriptome results. The results indicated that NH₃ exposure could break down the mucosal barrier of the respiratory tract, induce oxidative stress and inflammation, reduce the activity of microtubules and disrupt the balance of SLC transporters. In this study, transcriptome analysis was used for the first time to explore the toxic mechanism of NH₃ on pig trachea, providing new insights for better assessing the toxicity mechanism of NH₃, as well as references for comparative medicine.

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.

Long-Tailed Unconventional Class I Myosins in Health and Disease

Long-tailed unconventional class I myosin, Myosin 1E (MYO1E) and Myosin 1F (MYO1F) are motor proteins that use chemical energy from the hydrolysis of adenosine triphosphate (ATP) to produce mechanical work along the actin cytoskeleton. On the basis of their motor properties and structural features, myosins perform a variety of essential roles in physiological processes such as endocytosis, exocytosis, cell adhesion, and migration. The long tailed unconventional class I myosins are characterized by having a conserved motor head domain, which binds actin and hydrolyzes ATP, followed by a short neck with an isoleucine-glutamine (IQ) motif, which binds calmodulin and is sensitive to calcium, and a tail that contains a pleckstrin homology domain (PH), a tail homology 1 domain (TH1), wherein these domains allow membrane binding, a tail homology 2 domain (TH2), an ATP-insensitive actin-binding site domain, and a single Src homology 3 domain (SH3) susceptible to binding proline rich regions in other proteins. Therefore, these motor proteins are able to bind actin, plasma membrane, and other molecules (adaptor, kinases, membrane proteins) that contribute to their function, ranging from increasing membrane tension to molecular trafficking and cellular adhesion. MYO1E and MYO1F function in host self-defense, with a better defined role in innate immunity in cell migration and phagocytosis. Impairments of their function have been identified in patients suffering pathologies ranging from tumoral processes to kidney diseases. In this review, we summarize our current knowledge of specific features and functions of MYO1E and MYO1F in various tissues, as well as their involvement in disease.

NCF2, MYO1F, S1PR4, and FCN1 as potential noninvasive diagnostic biomarkers in patients with obstructive coronary artery: A weighted gene co-expression network analysis

This study aims to explore the predictive noninvasive biomarker for obstructive coronary artery disease (CAD). By using the data set GSE90074, weighted gene co-expression network analysis (WGCNA), and protein-protein interactive network, construction of differentially expressed genes in peripheral blood mononuclear cells was conducted to identify the most significant gene clusters associated with obstructive CAD. Univariate and multivariate stepwise logistic regression analyses and receiver operating characteristic analysis were used to predicate the diagnostic accuracy of biomarker candidates in the detection of obstructive CAD. Furthermore, functional prediction of candidate gene biomarkers was further confirmed in ST-segment elevation myocardial infarction (STEMI) patients or stable CAD patients by using the datasets of GSE62646 and GSE59867. We found that the blue module discriminated by WGCNA contained 13 hub-genes that could be independent risk factors for obstructive CAD (P < .05). Among these 13 hub-genes, a four-gene signature including neutrophil cytosol factor 2 (NCF2, P = .025), myosin-If (MYO1F, P = .001), sphingosine-1-phosphate receptor 4 (S1PR4, P = .015), and ficolin-1 (FCN1, P = .012) alone or combined with two risk factors (male sex and hyperlipidemia) may represent potential diagnostic biomarkers in obstructive CAD. Furthermore, the messenger RNA levels of NCF2, MYO1F, S1PR4, and FCN1 were higher in STEMI patients than that in stable CAD patients, although S1PR4 showed no statistical difference (P > .05). This four-gene signature could also act as a prognostic biomarker to discriminate STEMI patients from stable CAD patients. These findings suggest a four-gene signature (NCF2, MYO1F, S1PR4, and FCN1) alone or combined with two risk factors (male sex and hyperlipidemia) as a promising prognostic biomarker in the diagnosis of STEMI. Well-designed cohort studies should be implemented to warrant the diagnostic value of these genes in clinical purpose.

The defining DNA methylation signature of Kabuki syndrome enables functional assessment of genetic variants of unknown clinical significance

Kabuki syndrome (KS) is caused by mutations in KMT2D, which is a histone methyltransferase involved in methylation of H3K4, a histone marker associated with DNA methylation. Analysis of >450,000 CpGs in 24 KS patients with pathogenic mutations in KMT2D and 216 controls, identified 24 genomic regions, along with 1,504 CpG sites with significant DNA methylation changes including a number of Hox genes and the MYO1F gene. Using the most differentiating and significant probes and regions we developed a "methylation variant pathogenicity (MVP) score," which enables 100% sensitive and specific identification of individuals with KS, which was confirmed using multiple public and internal patient DNA methylation databases. We also demonstrated the ability of the MVP score to accurately reclassify variants of unknown significance in subjects with apparent clinical features of KS, enabling its potential use in molecular diagnostics. These findings provide novel insights into the molecular etiology of KS and illustrate that DNA methylation patterns can be interpreted as 'epigenetic echoes' in certain clinical disorders.

CHARGE and Kabuki Syndromes: Gene-Specific DNA Methylation Signatures Identify Epigenetic Mechanisms Linking These Clinically Overlapping Conditions

Epigenetic dysregulation has emerged as a recurring mechanism in the etiology of neurodevelopmental disorders. Two such disorders, CHARGE and Kabuki syndromes, result from loss of function mutations in chromodomain helicase DNA-binding protein 7 (CHD7^LOF) and lysine (K) methyltransferase 2D (KMT2D^LOF), respectively. Although these two syndromes are clinically distinct, there is significant phenotypic overlap. We therefore expected that epigenetically driven developmental pathways regulated by CHD7 and KMT2D would overlap and that DNA methylation (DNAm) alterations downstream of the mutations in these genes would identify common target genes, elucidating a mechanistic link between these two conditions, as well as specific target genes for each disorder. Genome-wide DNAm profiles in individuals with CHARGE and Kabuki syndromes with CHD7^LOF or KMT2D^LOF identified distinct sets of DNAm differences in each of the disorders, which were used to generate two unique, highly specific and sensitive DNAm signatures. These DNAm signatures were able to differentiate pathogenic mutations in these two genes from controls and from each other. Analysis of the DNAm targets in each gene-specific signature identified both common gene targets, including homeobox A5 (HOXA5), which could account for some of the clinical overlap in CHARGE and Kabuki syndromes, as well as distinct gene targets. Our findings demonstrate how characterization of the epigenome can contribute to our understanding of disease pathophysiology for epigenetic disorders, paving the way for explorations of novel therapeutics.

Maternal Smoking during Pregnancy and DNA-Methylation in Children at Age 5.5 Years: Epigenome-Wide-Analysis in the European Childhood Obesity Project (CHOP)-Study

Mounting evidence links prenatal exposure to maternal tobacco smoking with disruption of DNA methylation (DNAm) profile in the blood of infants. However, data on the postnatal stability of such DNAm signatures in childhood, as assessed by Epigenome Wide Association Studies (EWAS), are scarce. Objectives of this study were to investigate DNAm signatures associated with in utero tobacco smoke exposure beyond the 12th week of gestation in whole blood of children at age 5.5 years, to replicate previous findings in young European and American children and to assess their biological role by exploring databases and enrichment analysis. DNA methylation was measured in blood of 366 children of the multicentre European Childhood Obesity Project Study using the Illumina Infinium HM450 Beadchip (HM450K). An EWAS was conducted using linear regression of methylation values at each CpG site against in utero smoke exposure, adjusted for study characteristics, biological and technical effects. Methylation levels at five HM450K probes in MYO1G (cg12803068, cg22132788, cg19089201), CNTNAP2 (cg25949550), and FRMD4A (cg11813497) showed differential methylation that reached epigenome-wide significance according to the false-discovery-rate (FDR) criteria (q-value<0.05). Whereas cg25949550 showed decreased methylation (-2% DNAm ß-value), increased methylation was observed for the other probes (9%: cg12803068; 5%: cg22132788; 4%: cg19089201 and 4%: cg11813497) in exposed relative to non-exposed subjects. This study thus replicates previous findings in children ages 3 to 5, 7 and 17 and confirms the postnatal stability of MYO1G, CNTNAP2 and FRMD4A differential methylation. The role of this differential methylation in mediating childhood phenotypes, previously associated with maternal smoking, requires further investigation.

Class I myosins have overlapping and specialized functions in left-right asymmetric development in Drosophila

The class I myosin genes are conserved in diverse organisms, and their gene products are involved in actin dynamics, endocytosis, and signal transduction. Drosophila melanogaster has three class I myosin genes, Myosin 31DF (Myo31DF), Myosin 61F (Myo61F), and Myosin 95E (Myo95E). Myo31DF, Myo61F, and Myo95E belong to the Myosin ID, Myosin IC, and Myosin IB families, respectively. Previous loss-of-function analyses of Myo31DF and Myo61F revealed important roles in left-right (LR) asymmetric development and enterocyte maintenance, respectively. However, it was difficult to elucidate their roles in vivo, because of potential redundant activities. Here we generated class I myosin double and triple mutants to address this issue. We found that the triple mutant was viable and fertile, indicating that all three class I myosins were dispensable for survival. A loss-of-function analysis revealed further that Myo31DF and Myo61F, but not Myo95E, had redundant functions in promoting the dextral LR asymmetric development of the male genitalia. Myo61F overexpression is known to antagonize the dextral activity of Myo31DF in various Drosophila organs. Thus, the LR-reversing activity of overexpressed Myo61F may not reflect its physiological function. The endogenous activity of Myo61F in promoting dextral LR asymmetric development was observed in the male genitalia, but not the embryonic gut, another LR asymmetric organ. Thus, Myo61F and Myo31DF, but not Myo95E, play tissue-specific, redundant roles in LR asymmetric development. Our studies also revealed differential colocalization of the class I myosins with filamentous (F)-actin in the brush border of intestinal enterocytes.

Crystal structure of human myosin 1c--the motor in GLUT4 exocytosis: implications for Ca2+ regulation and 14-3-3 binding

Myosin 1c (Myo1c) plays a key role in supporting motile events that underlie cell migration, vesicle trafficking, insulin-stimulated glucose uptake and hearing. Here, we present the crystal structure of the human Myo1c motor in complex with its light chain calmodulin. Our structure reveals tight interactions of the motor domain with calmodulin bound to the first IQ motif in the neck region. Several of the calmodulin residues contributing to this interaction are also involved in Ca(2+) binding. Contact residues in the motor domain are linked to the central β-sheet and the HO helix, suggesting a mechanism for communicating changes in Ca(2+) binding in the neck region to the actin and nucleotide binding regions of the motor domain. The structural context and the chemical environment of Myo1c mutations that are involved in sensorineural hearing loss in humans are described and their impact on motor function is discussed. We show that a construct consisting of the motor domain of Myo1c and the first IQ motif is sufficient to establish a tight interaction with 14-3-3β (KD=0.9 μM) and present the model of a double-headed Myo1c-14-3-3 complex. This complex has been implicated in the exocytosis of glucose transporter 4 storage vesicles during insulin-stimulated glucose uptake.

The hypertrophic cardiomyopathy myosin mutation R453C alters ATP binding and hydrolysis of human cardiac β-myosin

The human hypertrophic cardiomyopathy mutation R453C results in one of the more severe forms of the myopathy. Arg-453 is found in a conserved surface loop of the upper 50-kDa domain of the myosin motor domain and lies between the nucleotide binding pocket and the actin binding site. It connects to the cardiomyopathy loop via a long α-helix, helix O, and to Switch-2 via the fifth strand of the central β-sheet. The mutation is, therefore, in a position to perturb a wide range of myosin molecular activities. We report here the first detailed biochemical kinetic analysis of the motor domain of the human β-cardiac myosin carrying the R453C mutation. A recent report of the same mutation (Sommese, R. F., Sung, J., Nag, S., Sutton, S., Deacon, J. C., Choe, E., Leinwand, L. A., Ruppel, K., and Spudich, J. A. (2013) Proc. Natl. Acad. Sci. U.S.A. 110, 12607-12612) found reduced ATPase and in vitro motility but increased force production using an optical trap. Surprisingly, our results show that the mutation alters few biochemical kinetic parameters significantly. The exceptions are the rate constants for ATP binding to the motor domain (reduced by 35%) and the ATP hydrolysis step/recovery stroke (slowed 3-fold), which could be the rate-limiting step for the ATPase cycle. Effects of the mutation on the recovery stroke are consistent with a perturbation of Switch-2 closure, which is required for the recovery stroke and the subsequent ATP hydrolysis.

Novel myosin mutations for hereditary hearing loss revealed by targeted genomic capture and massively parallel sequencing

Hereditary hearing loss is genetically heterogeneous, with a large number of genes and mutations contributing to this sensory, often monogenic, disease. This number, as well as large size, precludes comprehensive genetic diagnosis of all known deafness genes. A combination of targeted genomic capture and massively parallel sequencing (MPS), also referred to as next-generation sequencing, was applied to determine the deafness-causing genes in hearing-impaired individuals from Israeli Jewish and Palestinian Arab families. Among the mutations detected, we identified nine novel mutations in the genes encoding myosin VI, myosin VIIA and myosin XVA, doubling the number of myosin mutations in the Middle East. Myosin VI mutations were identified in this population for the first time. Modeling of the mutations provided predicted mechanisms for the damage they inflict in the molecular motors, leading to impaired function and thus deafness. The myosin mutations span all regions of these molecular motors, leading to a wide range of hearing phenotypes, reinforcing the key role of this family of proteins in auditory function. This study demonstrates that multiple mutations responsible for hearing loss can be identified in a relatively straightforward manner by targeted-gene MPS technology and concludes that this is the optimal genetic diagnostic approach for identification of mutations responsible for hearing loss.

Expression, purification, crystallization and preliminary X-ray crystallographic analysis of human myosin 1c in complex with calmodulin

Myosin 1c (Myo1c) is implicated in several cellular processes such as vesicle transport and the mediation of adaptation in the inner ear. Consequently, mutations impairing Myo1c motor activity lead to hearing loss in humans. To understand the role of Myo1c in this process on a molecular level, its crystal structure in complex with the light chain calmodulin was determined. A human Myo1c construct encompassing the motor domain and the first IQ motif was co-expressed with calmodulin in Sf9 cells and purified to homogeneity. The protein complex crystallized readily, and the crystals belonged to space group P2(1) and diffracted to 3 Å resolution. Attempts to determine the structure by molecular replacement are currently under way.

Myo1c is an unconventional myosin required for zebrafish glomerular development

The targeting and organization of podocyte slit diaphragm proteins nephrin and neph1 is critical for development and maintenance of a functional glomerular filtration barrier. Myo1c is a non-muscle myosin motor protein that interacts directly with nephrin and neph1 and mediates their intracellular transport to the podocyte intercellular junction. Here we investigated the necessity of Myo1c in podocyte development using zebrafish as a model system. Immunofluorescence microscopy and in situ RNA hybridization analysis of zebrafish embryos showed that Myo1c is widely expressed in various tissues including the zebrafish glomerulus. Knockdown of the Myo1c gene in zebrafish using antisense morpholino derivatives resulted in an abnormal developmental phenotype that included pericardial edema and dilated renal tubules. Ultra-structural analysis of the glomerulus in Myo1c depleted zebrafish showed abnormal podocyte morphology and absence of the slit diaphragm. Consistent with these observations, the glomerular filter permeability appeared altered in zebrafish in which Myo1c expression was attenuated. The specificity of Myo1c knockdown was confirmed by a rescue experiment in which co-injection of Myo1c morpholino derivatives with orthologous Myo1c mRNA prepared from mouse cDNA lessened phenotypic abnormalities including edema in Myo1c morphants. Thus, our results demonstrate that Myo1c is necessary for podocyte morphogenesis.

Mouse nuclear myosin I knock-out shows interchangeability and redundancy of myosin isoforms in the cell nucleus

BACKGROUND

Nuclear myosin I (NM1) is a nuclear isoform of the well-known "cytoplasmic" Myosin 1c protein (Myo1c). Located on the 11(th) chromosome in mice, NM1 results from an alternative start of transcription of the Myo1c gene adding an extra 16 amino acids at the N-terminus. Previous studies revealed its roles in RNA Polymerase I and RNA Polymerase II transcription, chromatin remodeling, and chromosomal movements. Its nuclear localization signal is localized in the middle of the molecule and therefore directs both Myosin 1c isoforms to the nucleus.

METHODOLOGY/PRINCIPAL FINDINGS

In order to trace specific functions of the NM1 isoform, we generated mice lacking the NM1 start codon without affecting the cytoplasmic Myo1c protein. Mutant mice were analyzed in a comprehensive phenotypic screen in cooperation with the German Mouse Clinic. Strikingly, no obvious phenotype related to previously described functions has been observed. However, we found minor changes in bone mineral density and the number and size of red blood cells in knock-out mice, which are most probably not related to previously described functions of NM1 in the nucleus. In Myo1c/NM1 depleted U2OS cells, the level of Pol I transcription was restored by overexpression of shRNA-resistant mouse Myo1c. Moreover, we found Myo1c interacting with Pol II. The ratio between Myo1c and NM1 proteins were similar in the nucleus and deletion of NM1 did not cause any compensatory overexpression of Myo1c protein.

CONCLUSION/SIGNIFICANCE

We observed that Myo1c can replace NM1 in its nuclear functions. Amount of both proteins is nearly equal and NM1 knock-out does not cause any compensatory overexpression of Myo1c. We therefore suggest that both isoforms can substitute each other in nuclear processes.

Characterization of a novel ENU-generated myosin VI mutant mouse strain with congenital deafness and vestibular dysfunction

Myosin VI (Myo6) is known to play an important role in the mammalian auditory and vestibular systems. We have identified a novel N-ethyl-N-nitrosourea mutagenised mouse strain, charlie, carrying an intronic Myo6 splice site mutation. This mutation (IVS5+5G > A) results in skipping of exon 5, and is predicted to cause a frameshift and premature termination of the protein. We detected essentially no Myo6 transcript in tissue from charlie homozygous mutant mice (Myo6(chl/chl)). Myo6(chl/chl) mice exhibit vestibular dysfunction and profound hearing impairment when first tested at four weeks of age. Analysis of vestibular and cochlear hair cells by scanning electron microscopy and immunohistochemistry revealed highly disorganised hair bundles with irregular orientation and kinocilium position at postnatal stage P2-P3. Within a few weeks, the majority of hair cell stereocilia are missing, or fused and elongated, and degeneration of the sensory epithelium occurs. This novel mouse strain will be an important resource in elucidating the role myosin VI plays in the mammalian auditory system, as well as its non-auditory functions.

Targeted massive parallel sequencing: the effective detection of novel causative mutations associated with hearing loss in small families

Background

Hereditary hearing loss is one of the most common heterogeneous disorders, and genetic variants that can cause hearing loss have been identified in over sixty genes. Most of these hearing loss genes have been detected using classical genetic methods, typically starting with linkage analysis in large families with hereditary hearing loss. However, these classical strategies are not well suited for mutation analysis in smaller families who have insufficient genetic information.

Methods

Eighty known hearing loss genes were selected and simultaneously sequenced by targeted next-generation sequencing (NGS) in 8 Korean families with autosomal dominant non-syndromic sensorineural hearing loss.

Results

Five mutations in known hearing loss genes, including 1 nonsense and 4 missense mutations, were identified in 5 different genes (ACTG1, MYO1F, DIAPH1, POU4F3 and EYA4), and the genotypes for these mutations were consistent with the autosomal dominant inheritance pattern of hearing loss in each family. No mutational hot-spots were revealed in these Korean families.

Conclusion

Targeted NGS allowed for the detection of pathogenic mutations in affected individuals who were not candidates for classical genetic studies. This report is the first documenting the effective use of an NGS technique to detect pathogenic mutations that underlie hearing loss in an East Asian population. Using this NGS technique to establish a database of common mutations in Korean patients with hearing loss and further data accumulation will contribute to the early diagnosis and fundamental therapies for hereditary hearing loss.

Non-muscle myosins and the podocyte

Proteinuria is often accompanied by a pathological change in the glomerulus that is refereed as effacement of the podocyte foot processes. The highly dynamic podocyte foot processes contain an actin-based contractile apparatus comparable to that of pericytes, which needs to be precisely and temporally controlled to withstand high pressure in the capillaries and to maintain intact glomerular filtration properties. This review outlines the most recent concepts on the function of the podocyte contractile apparatus with a focus on the role of non-muscle myosins as they have been highlighted by studies in monogenic hereditary proteinuric diseases.

Exome sequencing identified MYO1E and NEIL1 as candidate genes for human autosomal recessive steroid-resistant nephrotic syndrome

To identify gene loci associated with steroid-resistant nephrotic syndrome (SRNS), we utilized homozygosity mapping and exome sequencing in a consanguineous pedigree with three affected siblings. High-density genotyping identified three segments of homozygosity spanning 33.6 Mb on chromosomes 5, 10, and 15 containing 296 candidate genes. Exome sequencing identified two homozygous missense variants within the chromosome 15 segment; an A159P substitution in myosin 1E (MYO1E), encoding a podocyte cytoskeletal protein; and an E181K substitution in nei endonuclease VIII-like 1 (NEIL1), encoding a base-excision DNA repair enzyme. Both variants disrupt highly conserved protein sequences and were absent in public databases, 247 healthy controls, and 286 patients with nephrotic syndrome. The MYO1E A159P variant is noteworthy, as it is expected to impair ligand binding and actin interaction in the MYO1E motor domain. The predicted loss of function is consistent with the previous demonstration that Myo1e inactivation produces nephrotic syndrome in mice. Screening 71 additional patients with SRNS, however, did not identify independent NEIL1 or MYO1E mutations, suggesting larger sequencing efforts are needed to uncover which mutation is responsible for the phenotype. Our findings demonstrate the utility of exome sequencing for rapidly identifying candidate genes for human SRNS.

Principles of unconventional myosin function and targeting

Unconventional myosins are a superfamily of actin-based motors implicated in diverse cellular processes. In recent years, much progress has been made in describing their biophysical properties, and headway has been made into analyzing their cellular functions. Here, we focus on the principles that guide in vivo motor function and targeting to specific cellular locations. Rather than describe each motor comprehensively, we outline the major themes that emerge from research across the superfamily and use specific examples to illustrate each. In presenting the data in this format, we seek to identify open questions in each field as well as to point out commonalities between them. To advance our understanding of myosins' roles in vivo, clearly we must identify their cellular cargoes and the protein complexes that regulate motor attachment to fully appreciate their functions on the cellular and developmental levels.

A hearing loss-associated myo1c mutation (R156W) decreases the myosin duty ratio and force sensitivity

myo1c is a member of the myosin superfamily that has been proposed to function as the adaptation motor in vestibular and auditory hair cells. A recent study identified a myo1c point mutation (R156W) in a person with bilateral sensorineural hearing loss. This mutated residue is located at the start of the highly conserved switch 1 region, which is a crucial element for the binding of nucleotide. We characterized the key steps on the ATPase pathway at 37 °C using recombinant wild-type (myo1c(3IQ)) and mutant myo1c (R156W-myo1c(3IQ)) constructs that consist of the motor domain and three IQ motifs. The R156W mutation only moderately affects the rates of ATP binding, ATP-induced actomyosin dissociation, and ADP release. The actin-activated ATPase rate of the mutant is inhibited >4-fold, which is likely due to a decrease in the rate of phosphate release. The rate of actin gliding, as measured by the in vitro motility assay, is unaffected by the mutation at high myosin surface densities, but the rate of actin gliding is substantially reduced at low surface densities of R156W-myo1c(3IQ). We used a frictional loading assay to measure the affect of resisting forces on the rate of actin gliding and found that R156W-myo1c(3IQ) is less force-sensitive than myo1c(3IQ). Taken together, these results indicate that myo1c with the R156W mutation has a lower duty ratio than the wild-type protein and motile properties that are less sensitive to resisting forces.

Cell-based analysis of structure-function activity of threonine aspartase 1

Taspase1 is a threonine protease responsible for cleaving intracellular substrates. As such, (de)regulated Taspase1 function is expected not only to be vital for ordered development but may also be relevant for disease. However, the full repertoires of Taspase1 targets as well as the exact biochemical requirements for its efficient and substrate-specific cleavage are not yet resolved. Also, no cellular assays for this protease are currently available, hampering the exploitation of the (patho)biological relevance of Taspase1. Here, we developed highly efficient cell-based translocation biosensor assays to probe Taspase1 trans-cleavage in vivo. These modular sensors harbor variations of Taspase1 cleavage sites and localize to the cytoplasm. Expression of Taspase1 but not of inactive Taspase1 mutants or of unrelated proteases triggers proteolytic cleavage and nuclear accumulation of the biosensors. Employing our assay combined with scanning mutagenesis, we identified the sequence and spatial requirements for efficient Taspase1 processing in liquid and solid tumor cell lines. Collectively, our results defined an improved Taspase1 consensus recognition sequence, Q(3)(F/I/L/V)(2)D(1)↓G(1)'X(2)'D(3)'D(4)', allowing the first genome-wide bioinformatic identification of the human Taspase1 degradome. Among the 27 most likely Taspase1 targets are cytoplasmic but also nuclear proteins, such as the upstream stimulatory factor 2 (USF2) or the nuclear RNA export factors 2/5 (NXF2/5). Cleavage site recognition and proteolytic processing of selected targets were verified in the context of the biosensor and for the full-length proteins. We provide novel mechanistic insights into the function and bona fide targets of Taspase1 allowing for a focused investigation of the (patho)biological relevance of this type 2 asparaginase.

Myo1c mutations associated with hearing loss cause defects in the interaction with nucleotide and actin

Three heterozygous missense mutations in the motor domain of myosin 1c (Myo1c), which mediates adaptation in the inner ear, are associated with bilateral sensorineural hearing loss in humans. With transient kinetic analyses, steady-state ATPase and motility assays, and homology modeling, we studied the interaction of these mutants with nucleotide and actin using a truncated construct, Myo1c(1IQ-SAH), which includes an artificial lever arm. Results indicate that mutation R156W, near switch 1, affects the nucleotide-binding pocket and the calcium binding by disrupting switch 1 movement. Mutation V252A, in the K helix of the upper 50 kDa domain, showed reduced actin affinity consistent with disruption of communication between the actin- and nucleotide-binding sites. T380M, in a Myo1c-specific insert in the HO linker, displayed aberrant changes in most kinetic parameters and uncoupling of the ATPase from motility. These data allow for an interpretation of how these mutations might affect adaptation.