Conditional deletion of pejvakin in adult outer hair cells causes progressive hearing loss in mice

Mutations in the Pejvakin (Pjvk) gene cause autosomal recessive hearing loss DFNB59 with audiological features of auditory neuropathy spectrum disorder (ANSD) or cochlear dysfunction. The precise mechanisms underlying the variable clinical phenotypes of DFNB59 remain unclear. Here, we demonstrate that mice with conditional ablation of the Pjvk gene in all sensory hair cells or only in outer hair cells (OHCs) show similar auditory phenotypes with early-onset profound hearing loss. By contrast, loss of Pjvk in adult OHCs causes a slowly progressive hearing loss associated with OHC degeneration and delayed loss of inner hair cells (IHCs), indicating a primary role for pejvakin in regulating OHC function and survival. Consistent with this model, synaptic transmission at the IHC ribbon synapse is largely unaffected in sirtaki mice that carry a C-terminal deletion mutation in Pjvk. Using the C-terminal domain of pejvakin as bait, we identified in a cochlear cDNA library ROCK2, an effector for the small GTPase Rho, and the scaffold protein IQGAP1, involved in modulating actin dynamics. Both ROCK2 and IQGAP1 associate via their coiled-coil domains with pejvakin. We conclude that pejvakin is required to sustain OHC activity and survival in a cell-autonomous manner likely involving regulation of Rho signaling.

Ossicular Bone Damage and Hearing Loss in Rheumatoid Arthritis: A Correlated Functional and High Resolution Morphometric Study in Collagen-Induced Arthritic Mice

Globally, a body of comparative case-control studies suggests that rheumatoid arthritis (RA) patients are more prone to developing hearing loss (HL). However, experimental evidence that supports this hypothesis is still lacking because the human auditory organ is not readily accessible. The aim of this study was to determine the association between bone damage to the ossicles of the middle ear and HL, using a widely accepted murine model of collagen-induced arthritis (RA mice). Diarthrodial joints in the middle ear were examined with microcomputer tomography (microCT), and hearing function was assessed by auditory brainstem response (ABR). RA mice exhibited significantly decreased hearing sensitivity compared to age-matched controls. Additionally, a significant narrowing of the incudostapedial joint space and an increase in the porosity of the stapes were observed. The absolute latencies of all ABR waves were prolonged, but mean interpeak latencies were not statistically different. The observed bone defects in the middle ear that were accompanied by changes in ABR responses were consistent with conductive HL. This combination suggests that conductive impairment is at least part of the etiology of RA-induced HL in a murine model. Whether the inner ear sustains bone erosion or other pathology, and whether the cochlear nerve sustains pathology await subsequent studies. Considering the fact that certain anti-inflammatories are ototoxic in high doses, monitoring RA patients’ auditory function is advisable as part of the effort to ensure their well-being.

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.

Mutations in LOXHD1, an evolutionarily conserved stereociliary protein, disrupt hair cell function in mice and cause progressive hearing loss in humans

Hearing loss is the most common form of sensory impairment in humans and is frequently progressive in nature. Here we link a previously uncharacterized gene to hearing impairment in mice and humans. We show that hearing loss in the ethylnitrosourea (ENU)-induced samba mouse line is caused by a mutation in Loxhd1. LOXHD1 consists entirely of PLAT (polycystin/lipoxygenase/alpha-toxin) domains and is expressed along the membrane of mature hair cell stereocilia. Stereociliary development is unaffected in samba mice, but hair cell function is perturbed and hair cells eventually degenerate. Based on the studies in mice, we screened DNA from human families segregating deafness and identified a mutation in LOXHD1, which causes DFNB77, a progressive form of autosomal-recessive nonsyndromic hearing loss (ARNSHL). LOXHD1, MYO3a, and PJVK are the only human genes to date linked to progressive ARNSHL. These three genes are required for hair cell function, suggesting that age-dependent hair cell failure is a common mechanism for progressive ARNSHL.

The mechanotransduction machinery of hair cells

Mechanotransduction, the conversion of mechanical force into an electrochemical signal, allows living organisms to detect touch, hear, register movement and gravity, and sense changes in cell volume and shape. Hair cells in the vertebrate inner ear are mechanoreceptor cells specialized for the detection of sound and head movement. Each hair cell contains, at the apical surface, rows of stereocilia that are connected by extracellular filaments to form an exquisitely organized bundle. Mechanotransduction channels, localized near the tips of the stereocilia, are gated by the gating spring, an elastic element that is stretched upon stereocilia deflection and mediates rapid channel opening. Components of the mechanotransduction machinery in hair cells have been identified and several are encoded by genes linked to deafness in humans, which indicates that defects in the mechanotransduction machinery are the underlying cause of some forms of hearing impairment.

Harmonin mutations cause mechanotransduction defects in cochlear hair cells

In hair cells, mechanotransduction channels are gated by tip links, the extracellular filaments that consist of cadherin 23 (CDH23) and protocadherin 15 (PCDH15) and connect the stereocilia of each hair cell. However, which molecules mediate cadherin function at tip links is not known. Here we show that the PDZ-domain protein harmonin is a component of the upper tip-link density (UTLD), where CDH23 inserts into the stereociliary membrane. Harmonin domains that mediate interactions with CDH23 and F-actin control harmonin localization in stereocilia and are necessary for normal hearing. In mice expressing a mutant harmonin protein that prevents UTLD formation, the sensitivity of hair bundles to mechanical stimulation is reduced. We conclude that harmonin is a UTLD component and contributes to establishing the sensitivity of mechanotransduction channels to displacement.

Progressive myopathy and defects in the maintenance of myotendinous junctions in mice that lack talin 1 in skeletal muscle

The development and function of skeletal muscle depend on molecules that connect the muscle fiber cytoskeleton to the extracellular matrix (ECM). beta1 integrins are ECM receptors in skeletal muscle, and mutations that affect the alpha7beta1 integrin cause myopathy in humans. In mice, beta1 integrins control myoblast fusion, the assembly of the muscle fiber cytoskeleton, and the maintenance of myotendinous junctions (MTJs). The effector molecules that mediate beta1 integrin functions in muscle are not known. Previous studies have shown that talin 1 controls the force-dependent assembly of integrin adhesion complexes and regulates the affinity of integrins for ligands. Here we show that talin 1 is essential in skeletal muscle for the maintenance of integrin attachment sites at MTJs. Mice with a skeletal muscle-specific ablation of the talin 1 gene suffer from a progressive myopathy. Surprisingly, myoblast fusion and the assembly of integrin-containing adhesion complexes at costameres and MTJs advance normally in the mutants. However, with progressive ageing, the muscle fiber cytoskeleton detaches from MTJs. Mechanical measurements on isolated muscles show defects in the ability of talin 1-deficient muscle to generate force. Collectively, our findings show that talin 1 is essential for providing mechanical stability to integrin-dependent adhesion complexes at MTJs, which is crucial for optimal force generation by skeletal muscle.

Integrin-linked kinase stabilizes myotendinous junctions and protects muscle from stress-induced damage

Skeletal muscle expresses high levels of integrin-linked kinase (ILK), predominantly at myotendinous junctions (MTJs) and costameres. ILK binds the cytoplasmic domain of β1 integrin and mediates phosphorylation of protein kinase B (PKB)/Akt, which in turn plays a central role during skeletal muscle regeneration. We show that mice with a skeletal muscle–restricted deletion of ILK develop a mild progressive muscular dystrophy mainly restricted to the MTJs with detachment of basement membranes and accumulation of extracellular matrix. Endurance exercise training enhances the defects at MTJs, leads to disturbed subsarcolemmal myofiber architecture, and abrogates phosphorylation of Ser473 as well as phosphorylation of Thr308 of PKB/Akt. The reduction in PKB/Akt activation is accompanied by an impaired insulin-like growth factor 1 receptor (IGF-1R) activation. Coimmunoprecipitation experiments reveal that the β1 integrin subunit is associated with the IGF-1R in muscle cells. Our data identify the β1 integrin–ILK complex as an important component of IGF-1R/insulin receptor substrate signaling to PKB/Akt during mechanical stress in skeletal muscle.

A forward genetics screen in mice identifies recessive deafness traits and reveals that pejvakin is essential for outer hair cell function

Deafness is the most common form of sensory impairment in the human population and is frequently caused by recessive mutations. To obtain animal models for recessive forms of deafness and to identify genes that control the development and function of the auditory sense organs, we performed a forward genetics screen in mice. We identified 13 mouse lines with defects in auditory function and six lines with auditory and vestibular defects. We mapped several of the affected genetic loci and identified point mutations in four genes. Interestingly, all identified genes are expressed in mechanosensory hair cells and required for their function. One mutation maps to the pejvakin gene, which encodes a new member of the gasdermin protein family. Previous studies have described two missense mutations in the human pejvakin gene that cause nonsyndromic recessive deafness (DFNB59) by affecting the function of auditory neurons. In contrast, the pejvakin allele described here introduces a premature stop codon, causes outer hair cell defects, and leads to progressive hearing loss. We also identified a novel allele of the human pejvakin gene in an Iranian pedigree that is afflicted with progressive hearing loss. Our findings suggest that the mechanisms of pathogenesis associated with pejvakin mutations are more diverse than previously appreciated. More generally, our findings demonstrate that recessive screens in mice are powerful tools for identifying genes that control the development and function of mechanosensory hair cells and cause deafness in humans, as well as generating animal models for disease.

Sphingosine 1-phosphate (S1P) signaling is required for maintenance of hair cells mainly via activation of S1P2

Hearing requires the transduction of vibrational forces by specialized epithelial cells in the cochlea known as hair cells. The human ear contains a finite number of terminally differentiated hair cells that, once lost by noise-induced damage or toxic insult, can never be regenerated. We report here that sphingosine 1-phosphate (S1P) signaling, mainly via activation of its cognate receptor S1P2, is required for the maintenance of vestibular and cochlear hair cells in vivo. Two S1P receptors, S1P2 and S1P3, were found to be expressed in the cochlea by reverse transcription-PCR and in situ hybridization. Mice that are null for both these receptors uniformly display progressive cochlear and vestibular defects with hair cell loss, resulting in complete deafness by 4 weeks of age and, with complete penetrance, balance defects of increasing severity. This study reveals the previously unknown role of S1P signaling in the maintenance of cochlear and vestibular integrity and suggests a means for therapeutic intervention in degenerative hearing loss.

Physical and functional interaction between protocadherin 15 and myosin VIIa in mechanosensory hair cells

Hair cells of the mammalian inner ear are the mechanoreceptors that convert sound-induced vibrations into electrical signals. The molecular mechanisms that regulate the development and function of the mechanically sensitive organelle of hair cells, the hair bundle, are poorly defined. We link here two gene products that have been associated with deafness and hair bundle defects, protocadherin 15 (PCDH15) and myosin VIIa (MYO7A), into a common pathway. We show that PCDH15 binds to MYO7A and that both proteins are expressed in an overlapping pattern in hair bundles. PCDH15 localization is perturbed in MYO7A-deficient mice, whereas MYO7A localization is perturbed in PCDH15-deficient mice. Like MYO7A, PCDH15 is critical for the development of hair bundles in cochlear and vestibular hair cells, controlling hair bundle morphogenesis and polarity. Cochlear and vestibular hair cells from PCDH15-deficient mice also show defects in mechanotransduction. Together, our findings suggest that PCDH15 and MYO7A cooperate to regulate the development and function of the mechanically sensitive hair bundle.

Beta1 integrins in muscle, but not in motor neurons, are required for skeletal muscle innervation

In vitro studies have provided evidence that beta1 integrins in motor neurons promote neurite outgrowth, whereas beta1 integrins in myotubes regulate acetylcholine receptor (AChR) clustering. Surprisingly, using genetic studies in mice, we show here that motor axon outgrowth and neuromuscular junction (NMJ) formation in large part are unaffected when the integrin beta1 gene (Itgb1) is inactivated in motor neurons. In the absence of Itgb1 expression in skeletal muscle, interactions between motor neurons and muscle are defective, preventing normal presynaptic differentiation. Motor neurons fail to terminate their growth at the muscle midline, branch excessively, and develop abnormal nerve terminals. These defects resemble the phenotype of agrin-null mice, suggesting that signaling molecules such as agrin, which coordinate presynaptic and postsynaptic differentiation, are not presented properly to nerve terminals. We conclude that Itgb1 expression in muscle, but not in motor neurons, is critical for NMJ development.

Erbb2 regulates neuromuscular synapse formation and is essential for muscle spindle development

Neuregulins and their Erbb receptors have been implicated in neuromuscular synapse formation by regulating gene expression in subsynaptic nuclei. To analyze the function of Erbb2 in this process, we have inactivated the Erbb2 gene in developing muscle fibers by Cre/Lox-mediated gene ablation. Neuromuscular synapses form in the mutant mice, but the synapses are less efficient and contain reduced levels of acetylcholine receptors. Surprisingly, the mutant mice also show proprioceptive defects caused by abnormal muscle spindle development. Sensory Ia afferent neurons establish initial contact with Erbb2-deficient myotubes. However, functional spindles never develop. Taken together, our data suggest that Erbb2 signaling regulates the formation of both neuromuscular synapses and muscle spindles.

Beta1 integrins regulate myoblast fusion and sarcomere assembly

The mechanisms that regulate the formation of multinucleated muscle fibers from mononucleated myoblasts are not well understood. We show here that extracellular matrix (ECM) receptors of the beta1 integrin family regulate myoblast fusion. beta1-deficient myoblasts adhere to each other, but plasma membrane breakdown is defective. The integrin-associated tetraspanin CD9 that regulates cell fusion is no longer expressed at the cell surface of beta1-deficient myoblasts, suggesting that beta1 integrins regulate the formation of a protein complex important for fusion. Subsequent to fusion, beta1 integrins are required for the assembly of sarcomeres. Other ECM receptors such as the dystrophin glycoprotein complex are still expressed but cannot compensate for the loss of beta1 integrins, providing evidence that different ECM receptors have nonredundant functions in skeletal muscle fibers.

Efficient expression, purification and crystallisation of two hyperthermostable enzymes of histidine biosynthesis

Enzymes from hyperthermophiles can be efficiently purified after expression in mesophilic hosts and are well-suited for crystallisation attempts. Two enzymes of histidine biosynthesis from Thermotoga maritima, N'-((5'-phosphoribosyl)-formimino)-5-aminoimidazol-4-carb oxamid ribonucleotide isomerase and the cyclase moiety of imidazoleglycerol phosphate synthase, were overexpressed in Escherichia coli, both in their native and seleno-methionine-labelled forms, purified by heat precipitation of host proteins and crystallised. N'-((5'-phosphoribosyl)-formimino)-5-aminoimidazol-4-carb oxamid ribonucleotide isomerase crystallised in four different forms, all suitable for X-ray structure solution, and the cyclase moiety of imidazoleglycerol phosphate synthase yielded one crystal form that diffracted to atomic resolution. The obtained crystals will enable the determination of the first three-dimensional structures of enzymes from the histidine biosynthetic pathway.

A histidine gene cluster of the hyperthermophile Thermotoga maritima: sequence analysis and evolutionary significance

The sequences of histidine operon genes in hyperthermophiles are informative for understanding high protein thermostability and the evolution of metabolic pathways. Therefore, a cluster of eight his genes from the hyperthermophilic and phylogenetically early bacterium Thermotoga maritima was cloned and sequenced. The cluster has the gene order hisDCBdHAFI-E, lacking only hisG and hisBp, and does not contain intercistronic regions. This compact organization of his genes resembles the his operon of enterobacteria. Sequence analysis downstream of the stop codon of hisI-E identifies a region with a significantly higher cytosine over guanosine content, which is indicative of a rho-dependent termination of transcription of the his operon. Multiple sequence alignments of N1-((5'-phosphoribosyl)-formimino)-5-aminoimidazole-4-carboxyam ide ribonucleotide isomerase (HisA) and of the cycloligase moiety of imidazoleglycerol phosphate synthase (HisF) support the previous assignment of the (beta alpha)8-barrel fold to these proteins. The alignments also reveal a second phosphate-binding motif located in the first halves of both enzymes and thereby support the hypothesis that HisA and HisF have evolved by a sequence of two gene duplication events. Comparison of the amino acid compositions of HisA and HisF from mesophiles and thermophiles shows that the thermostable variants of both enzymes contain a significantly increased number of charged amino acid residues and may therefore be stabilized by additional salt bridges.