Biomolecular condensates and disease pathogenesis

Biomolecular condensates or membraneless organelles (MLOs) formed by liquid-liquid phase separation (LLPS) divide intracellular spaces into discrete compartments for specific functions. Dysregulation of LLPS or aberrant phase transition that disturbs the formation or material states of MLOs is closely correlated with neurodegeneration, tumorigenesis, and many other pathological processes. Herein, we summarize the recent progress in development of methods to monitor phase separation and we discuss the biogenesis and function of MLOs formed through phase separation. We then present emerging proof-of-concept examples regarding the disruption of phase separation homeostasis in a diverse array of clinical conditions including neurodegenerative disorders, hearing loss, cancers, and immunological diseases. Finally, we describe the emerging discovery of chemical modulators of phase separation.

Harmony in the Molecular Orchestra of Hearing: Developmental Mechanisms from the Ear to the Brain

Auditory processing in mammals begins in the peripheral inner ear and extends to the auditory cortex. Sound is transduced from mechanical stimuli into electrochemical signals of hair cells, which relay auditory information via the primary auditory neurons to cochlear nuclei. Information is subsequently processed in the superior olivary complex, lateral lemniscus, and inferior colliculus and projects to the auditory cortex via the medial geniculate body in the thalamus. Recent advances have provided valuable insights into the development and functioning of auditory structures, complementing our understanding of the physiological mechanisms underlying auditory processing. This comprehensive review explores the genetic mechanisms required for auditory system development from the peripheral cochlea to the auditory cortex. We highlight transcription factors and other genes with key recurring and interacting roles in guiding auditory system development and organization. Understanding these gene regulatory networks holds promise for developing novel therapeutic strategies for hearing disorders, benefiting millions globally.

Natural History of KCNQ4 p.G285S Related Hearing Loss, Construction of iPSC and Mouse Model

OBJECTIVE

KCNQ4 is one of the most common disease-causing genes involved in autosomal dominant non-syndromic hearing loss. We previously found that patients with KCNQ4 p.G285S exhibited a much more rapid deterioration in hearing loss than those with other KCNQ4 variants. To determine the rate of hearing loss and assess the disease for further analysis, we performed a long-term follow-up of these patients and generated patient-derived induced pluripotent stem cells (iPSCs), and a mouse model.

METHODS

Patients with KCNQ4 p.G285S from a five-generation family with hearing loss were followed up from 2005 to 2022. iPSCs were generated by stimulating peripheral blood mononuclear cells from the proband, and their pluripotency was determined. The Kcnq4 p.G286S mouse model was generated using CRISPR/Cas9, and its genotype and phenotype were identified.

RESULTS

(1) The annual rates of hearing loss at the frequencies of speech were 0.96 dB for the proband and 0.87 dB for his father during the follow-up period, which were faster than patients with other KCNQ4 variants. (2) The patient-derived iPSC line carrying KCNQ4 p.G285S, possessed the capacity of differentiation and pluripotency capacities. (3) Mutant mice with Kcnq4 p.G286S exhibited hearing loss and outer hair cell loss at 1 month of age.

CONCLUSION

Patients with KCNQ4 p.G285S variant exhibited significantly accelerated progression of hearing loss compared to those with other reported variants. Awareness of the natural history of hearing loss associated with KCNQ4 p.G285S is beneficial for genetic counseling and prognosis. The generation of the iPSCs and mouse model can provide a valuable foundation for further in-depth analyses.

LEVEL OF EVIDENCE

4 Laryngoscope, 134:2356-2363, 2024.

Spatially resolved single-cell atlas of ascidian endostyle provides insight into the origin of vertebrate pharyngeal organs

The pharyngeal endoderm, an innovation of deuterostome ancestors, contributes to pharyngeal development by influencing the patterning and differentiation of pharyngeal structures in vertebrates; however, the evolutionary origin of the pharyngeal organs in vertebrates is largely unknown. The endostyle, a distinct pharyngeal organ exclusively present in basal chordates, represents a good model for understanding pharyngeal organ origins. Using Stereo-seq and single-cell RNA sequencing, we constructed aspatially resolved single-cell atlas for the endostyle of the ascidian Styela clava. We determined the cell composition of the hemolymphoid region, which illuminates a mixed ancestral structure for the blood and lymphoid system. In addition, we discovered a cluster of hair cell-like cells in zone 3, which has transcriptomic similarity with the hair cells of the vertebrate acoustico-lateralis system. These findings reshape our understanding of the pharynx of the basal chordate and provide insights into the evolutionary origin of multiplexed pharyngeal organs.

Molecular Mechanisms and Clinical Phenotypes of GJB2 Missense Variants

The GJB2 gene is the most common gene responsible for hearing loss (HL) worldwide, and missense variants are the most abundant type. GJB2 pathogenic missense variants cause nonsyndromic HL (autosomal recessive and dominant) and syndromic HL combined with skin diseases. However, the mechanism by which these different missense variants cause the different phenotypes is unknown. Over 2/3 of the GJB2 missense variants have yet to be functionally studied and are currently classified as variants of uncertain significance (VUS). Based on these functionally determined missense variants, we reviewed the clinical phenotypes and investigated the molecular mechanisms that affected hemichannel and gap junction functions, including connexin biosynthesis, trafficking, oligomerization into connexons, permeability, and interactions between other coexpressed connexins. We predict that all possible GJB2 missense variants will be described in the future by deep mutational scanning technology and optimizing computational models. Therefore, the mechanisms by which different missense variants cause different phenotypes will be fully elucidated.

Molecular identity of the lateral lemniscus nuclei in the adult mouse brain

Introduction

The dorsal (DLL), intermediate (ILL), and ventral (VLL) lateral lemniscus nuclei are relay centers in the central auditory pathway of the brainstem, commonly referred to as the lateral lemniscus nuclei (LLN). The LLN are situated in the prepontine and pontine hindbrain, from rhombomeres 1 to 4, extending from the more rostral DLL to the caudal VLL, with the ILL lying in between. These nuclei can be distinguished morphologically and by topological and connectivity criteria, and here, we set out to further characterize the molecular nature of each LLN.

Methods

We searched in situ hybridization studies in the Allen Mouse Brain Atlas for genes differentially expressed along the rostrocaudal axis of the brainstem, identifying 36 genes from diverse functional families expressed in the LLN.

Results

Available information in the databases indicated that 7 of these 36 genes are either associated with or potentially related to hearing disorders.

Discussion

In conclusion, the LLN are characterized by specific molecular profiles that reflect their rostrocaudal organization into the three constituent nuclei. This molecular regionalization may be involved in the etiology of some hearing disorders, in accordance with previous functional studies of these genes.

Shared and organ-specific gene-expression programs during the development of the cochlea and the superior olivary complex

The peripheral and central auditory subsystems together form a complex sensory network that allows an organism to hear. The genetic programs of the two subsystems must therefore be tightly coordinated during development. Yet, their interactions and common expression pathways have never been systematically explored. MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression and are essential for normal development of the auditory system. We performed mRNA and small-RNA sequencing of organs from both auditory subsystems at three critical developmental timepoints (E16, P0, P16) to obtain a comprehensive and unbiased insight of their expression profiles. Our analysis reveals common and organ-specific expression patterns for differentially regulated mRNAs and miRNAs, which could be clustered with a particular selection of functions such as inner ear development, Wnt signalling, K+ transport, and axon guidance, based on gene ontology. Bioinformatics detected enrichment of predicted targets of specific miRNAs in the clusters and predicted regulatory interactions by monitoring opposite trends of expression of miRNAs and their targets. This approach identified six miRNAs as strong regulatory candidates for both subsystems. Among them was miR-96, an established critical factor for proper development in both subsystems, demonstrating the strength of our approach. We suggest that other miRNAs identified by this analysis are also common effectors of proper hearing acquirement. This first combined comprehensive analysis of the developmental program of the peripheral and central auditory systems provides important data and bioinformatics insights into the shared genetic program of the two sensory subsystems and their regulation by miRNAs.

Precision Medicine Approach to Cochlear Implantation

In the early days of cochlear implantation (CI) surgery, when the types of electrodes were limited and the etiology of sensorineural hearing loss (SNHL) was not well understood, the one-size-fits-all approach to CI held true, as in all other fields. However, in the era of personalized medicine, there have been attempts to associate CI performance with the etiology of SNHL and to establish customized surgical techniques that can maximize performance according to individual cochlear dimensions. Personalized genomic-driven assessments of CI candidates and a better understanding of genotype-phenotype correlations could provide clinically applicable diagnostic and prognostic information about questions such as who, how, and when to implant. Rigorous and strategic imaging assessments also provide better insights into the anatomic etiology of SNHL and cochlear dimensions, leading to individualized surgical techniques to augment CI outcomes. Furthermore, the precision medicine approach to CI is not necessarily limited to preoperative planning, but can be extended to either intraoperative electrode positioning or even the timing of the initial switch-on. In this review, we discuss the implications of personalized diagnoses (both genetic and nongenetic) on the planning and performance of CI in patients with prelingual and postlingual SNHL.

Unexpected Motherhood-Triggered Hearing Loss in the Two-Pore Channel (TPC) Mutant Mouse

Calcium signaling is crucial for many physiological processes and can mobilize intracellular calcium stores in response to environmental sensory stimuli. The endolysosomal two-pore channel (TPC), regulated by the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP), is one of the key components in calcium signaling. However, its role in neuronal physiology remains largely unknown. Here, we investigated to what extent the acoustic thresholds differed between the WT mice and the TPC KO mice. We determined the thresholds based on the auditory brainstem responses (ABRs) at five frequencies (between 4 and 32 kHz) and found no threshold difference between the WT and KO in virgin female mice. Surprisingly, in lactating mothers (at P9–P10), the thresholds were higher from 8 to 32 kHz in the TPC KO mice compared to the WT mice. This result indicates that in the TPC KO mice, physiological events occurring during parturition altered the detection of sounds already at the brainstem level, or even earlier.

Structure of the Harmonin PDZ2 and coiled-coil domains in a complex with CDHR2 tail and its implications

Harmonin is a protein containing multiple PDZ domains and is required for the development and maintenance of hair cell stereocilia and brush border microvilli. Mutations in the USH1C gene can cause Usher syndrome type 1C, a severe inheritable disease characterized by the loss of hearing and vision. Here, by solving the high-resolution crystal structure of Harmonin PDZ2 and coiled-coil domains in a complex with the tail of cadherin-related family member 2, we demonstrated that mutations located in the Harmonin PDZ2 domain and found in patients could affect its stability, and thus, the target binding capability. The structure also implies that the coiled-coil domain could form antiparallel dimers under high concentrations, possibly when Harmonin underwent liquid-liquid phase separation in the upper tip-link density in hair cell stereocilia or microvilli of enterocytes of the intestinal epithelium. The crystal structure, together with the biochemical analysis, provided mechanistic implications for Harmonin mutations causing Usher syndrome, non-syndromic deafness, or enteropathy.

Central auditory deficits associated with genetic forms of peripheral deafness

Since the 1990s, the study of inherited hearing disorders, mostly those detected at birth, in the prelingual period or in young adults, has led to the identification of their causal genes. The genes responsible for more than 140 isolated (non-syndromic) and about 400 syndromic forms of deafness have already been discovered. Studies of mouse models of these monogenic forms of deafness have provided considerable insight into the molecular mechanisms of hearing, particularly those involved in the development and/or physiology of the auditory sensory organ, the cochlea. In parallel, studies of these models have also made it possible to decipher the pathophysiological mechanisms underlying hearing impairment. This has led a number of laboratories to investigate the potential of gene therapy for curing these forms of deafness. Proof-of-concept has now been obtained for the treatment of several forms of deafness in mouse models, paving the way for clinical trials of cochlear gene therapy in patients in the near future. Nevertheless, peripheral deafness may also be associated with central auditory dysfunctions and may extend well beyond the auditory system itself, as a consequence of alterations to the encoded sensory inputs or involvement of the causal deafness genes in the development and/or functioning of central auditory circuits. Investigating the diversity, causes and underlying mechanisms of these central dysfunctions, the ways in which they could impede the expected benefits of hearing restoration by peripheral gene therapy, and determining how these problems could be remedied is becoming a research field in its own right. Here, we provide an overview of the current knowledge about the central deficits associated with genetic forms of deafness.

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.

Impaired Hearing and Altered Subplate Circuits During the First and Second Postnatal Weeks of Otoferlin-Deficient Mice

Sensory deprivation from the periphery impacts cortical development. Otoferlin deficiency leads to impaired cochlear synaptic transmission and is associated with progressive hearing loss in adults. However, it remains elusive how sensory deprivation due to otoferlin deficiency impacts the early development of the auditory cortex (ACX) especially before the onset of low threshold hearing. To test that, we performed in vivo imaging of the ACX in awake mice lacking otoferlin (Otof-/-) during the first and second postnatal weeks and found that spontaneous and sound-driven cortical activity were progressively impaired. We then characterized the effects on developing auditory cortical circuits by performing in vitro recordings from subplate neurons (SPN), the first primary targets of thalamocortical inputs. We found that in Otof-/- pups, SPNs received exuberant connections from excitatory and inhibitory neurons. Moreover, as a population, SPNs showed higher similarity with respect to their circuit topology in the absence of otoferlin. Together, our results show that otoferlin deficiency results in impaired hearing and has a powerful influence on cortical connections and spontaneous activity in early development even before complete deafness. Therefore, peripheral activity has the potential to sculpt cortical structures from the earliest ages, even before hearing impairment is diagnosed.

Electrochemical properties of the non-excitable tissue stria vascularis of the mammalian cochlea are sensitive to sounds

Excitable cochlear hair cells convert the mechanical energy of sounds into the electrical signals necessary for neurotransmission. The key process is cellular depolarization via K⁺ entry from K⁺ -enriched endolymph through hair cells' mechanosensitive channels. Positive 80 mV potential in endolymph accelerates the K⁺ entry, thereby sensitizing hearing. This potential represents positive extracellular potential within the epithelial-like stria vascularis; the latter potential stems from K⁺ equilibrium potential (E_K ) across the strial membrane. Extra- and intracellular [K⁺ ] determining E_K are likely maintained by continuous unidirectional circulation of K⁺ through a putative K⁺ transport pathway containing hair cells and stria. Whether and how the non-excitable tissue stria vascularis responds to acoustic stimuli remains unclear. Therefore, we analysed a cochlear portion for the best frequency, 1 kHz, by theoretical and experimental approaches. We have previously developed a computational model that integrates ion channels and transporters in the stria and hair cells into a circuit and described a circulation current composed of K⁺ . Here, in this model, mimicking of hair cells' K⁺ flow induced by a 1 kHz sound modulated the circulation current and affected the strial ion transport mechanisms; the latter effect resulted in monotonically decreasing potential and increasing [K⁺ ] in the extracellular strial compartment. Similar results were obtained when the stria in acoustically stimulated animals was examined using microelectrodes detecting the potential and [K⁺ ]. Measured potential dynamics mirrored the E_K change. Collectively, because stria vascularis is electrically coupled to hair cells by the circulation current in vivo too, the strial electrochemical properties respond to sounds. KEY POINTS: A highly positive potential of +80 mV in K⁺ -enriched endolymph in the mammalian cochlea accelerates sound-induced K⁺ entry into excitable sensory hair cells, a process that triggers hearing. This unique endolymphatic potential represents an E_K -based battery for a non-excitable epithelial-like tissue, the stria vascularis. To examine whether and how the stria vascularis responds to sounds, we used our computational model, in which strial channels and transporters are serially connected to those hair cells in a closed-loop circuit, and found that mimicking hair cell excitation by acoustic stimuli resulted in increased extracellular [K⁺ ] and decreased the battery's potential within the stria. This observation was overall verified by electrophysiological experiments using live guinea pigs. The sensitivity of electrochemical properties of the stria to sounds indicates that this tissue is electrically coupled to hair cells by a radial ionic flow called a circulation current.

[The sea anemone Nematostella vectensis, an emerging model for biomedical research: Mechano-sensitivity, extreme regeneration and longevity]

Nematostella has fascinating features such as whole-body regeneration, the absence of signs of aging and importantly, the absence of age-related diseases. Easy to culture and spawn, this little sea anemone in spite of its "simple" aspect, displays interesting morphological characteristics similar to vertebrates and an unexpected similarity in gene content/genome organization. Importantly, the scientific community working on Nematostella is developing a variety of functional genomics tools that enable scientists to use this anemone in the field of regenerative medicine, longevity and mecano-sensory diseases. As a complementary research model to vertebrates, this marine invertebrate is emerging and promising to dig deeper into those fields of research in an integrative manner (entire organism) and provides new opportunities for scientists to lift specific barriers that can be encountered with other commonly used animal models.

Expression pattern of cochlear microRNAs in the mammalian auditory hindbrain

The auditory system comprises the auditory periphery, engaged in sound transduction and the central auditory system, implicated in auditory information processing and perception. Recently, evidence mounted that the mammalian peripheral and central auditory systems share a number of genes critical for proper development and function. This bears implication for auditory rehabilitation and evolution of the auditory system. To analyze to which extent microRNAs (miRNAs) belong to genes shared between both systems, we characterize the expression pattern of 12 cochlea-abundant miRNAs in the central auditory system. Quantitative real-time PCR (qRT-PCR) demonstrated expression of all 12 genes in the cochlea, the auditory hindbrain and the non-auditory prefrontal cortex (PFC) at embryonic stage (E)16 and postnatal stages (P)0 and P30. Eleven of them showed differences in expression between tissues and nine between the developmental time points. Hierarchical cluster analysis revealed that the temporal expression pattern in the auditory hindbrain was more similar to the PFC than to the cochlea. Spatiotemporal expression analysis by RNA in situ hybridization demonstrated widespread expression throughout the cochlear nucleus complex (CNC) and the superior olivary complex (SOC) during postnatal development. Altogether, our data indicate that miRNAs represent a relevant class of genetic factors functioning across the auditory system. Given the importance of gene regulatory network (GRN) components for development, physiology and evolution, the 12 miRNAs provide promising entry points to gain insights into their molecular underpinnings in the auditory system.

Phase separation-mediated condensation of Whirlin-Myo15-Eps8 stereocilia tip complex

Stereocilia, the mechanosensory organelles on the apical surface of hair cells, are necessary to detect sound and carry out mechano-electrical transduction. An electron-dense matrix is located at the distal tips of stereocilia and plays crucial roles in the regulation of stereocilia morphology. Mutations of the components in this tip complex density (TCD) have been associated with profound deafness. However, the mechanism underlying the formation of the TCD is largely unknown. Here, we discover that the specific multivalent interactions among the Whirlin-myosin 15 (Myo15)-Eps8 complex lead to the formation of the TCD-like condensates through liquid-liquid phase separation. The reconstituted TCD-like condensates effectively promote actin bundling. A deafness-associated mutation of Myo15 interferes with the condensates formation and consequently impairs actin bundling. Therefore, our study not only suggests that the TCD in hair cell stereocilia may form via phase separation but it also provides important clues for the possible mechanism underlying hearing loss.

Understanding and treating paediatric hearing impairment

Sensorineural hearing impairment is the most frequent form of hearing impairment affecting 1-2 in 1000 newborns and another 1 in 1000 adolescents. More than 50% of congenital hearing impairment is of genetic origin and some forms of monogenic deafness are likely targets for future gene therapy. Good progress has been made in clinical phenotyping, genetic diagnostics, and counselling. Disease modelling, e.g. in transgenic mice, has helped elucidate disease mechanisms underlying genetic hearing impairment and informed clinical phenotyping in recent years. Clinical management of paediatric hearing impairment involves hearing aids, cochlear or brainstem implants, signal-to-noise improvement in educational settings, speech therapy, and sign language. Cochlear implants, for example, have much improved the situation of profoundly hearing impaired and deaf children. Nonetheless there remains a major unmet clinical need for improving hearing restoration. Preclinical studies promise that we will witness clinical trials on gene therapy and a next generation of cochlear implants during the coming decade. Moreover, progress in generating sensory hair cells and neurons from stem cells spurs disease modelling, drug screening, and regenerative approaches. This review briefly summarizes the pathophysiology of paediatric hearing impairment and provides an update on the current preclinical development of innovative approaches toward improved hearing restoration.

Overloaded Adeno-Associated Virus as a Novel Gene Therapeutic Tool for Otoferlin-Related Deafness

Hearing impairment is the most common sensory disorder in humans. So far, rehabilitation of profoundly deaf subjects relies on direct stimulation of the auditory nerve through cochlear implants. However, in some forms of genetic hearing impairment, the organ of Corti is structurally intact and therapeutic replacement of the mutated gene could potentially restore near natural hearing. In the case of defects of the otoferlin gene (OTOF), such gene therapy is hindered by the size of the coding sequence (~6 kb) exceeding the cargo capacity (<5 kb) of the preferred viral vector, adeno-associated virus (AAV). Recently, a dual-AAV approach was used to partially restore hearing in deaf otoferlin knock-out (Otof-KO) mice. Here, we employed in vitro and in vivo approaches to assess the gene-therapeutic potential of naturally-occurring and newly-developed synthetic AAVs overloaded with the full-length Otof coding sequence. Upon early postnatal injection into the cochlea of Otof-KO mice, overloaded AAVs drove specific expression of otoferlin in ~30% of all IHCs, as demonstrated by immunofluorescence labeling and polymerase chain reaction. Recordings of auditory brainstem responses and a behavioral assay demonstrated partial restoration of hearing. Together, our results suggest that viral gene therapy of DFNB9—using a single overloaded AAV vector—is indeed feasible, reducing the complexity of gene transfer compared to dual-AAV approaches.

Mutations of MAP1B encoding a microtubule-associated phosphoprotein cause sensorineural hearing loss

The pathophysiology underlying spiral ganglion cell defect–induced deafness remains elusive. Using the whole exome sequencing approach, in combination with functional assays and a mouse disease model, we identified the potentially novel deafness-causative MAP1B gene encoding a highly conserved microtubule-associated protein. Three novel heterozygous MAP1B mutations (c.4198A>G, p.1400S>G; c.2768T>C, p.923I>T; c.5512T>C, p.1838F>L) were cosegregated with autosomal dominant inheritance of nonsyndromic sensorineural hearing loss in 3 unrelated Chinese families. Here, we show that MAP1B is highly expressed in the spiral ganglion neurons in the mouse cochlea. Using otic sensory neuron–like cells, generated by pluripotent stem cells from patients carrying the MAP1B mutation and control subject, we demonstrated that the p.1400S>G mutation caused the reduced levels and deficient phosphorylation of MAP1B, which are involved in the microtubule stability and dynamics. Strikingly, otic sensory neuron–like cells exhibited disturbed dynamics of microtubules, axonal elongation, and defects in electrophysiological properties. Dysfunctions of these derived otic sensory neuron–like cells were rescued by genetically correcting MAP1B mutation using CRISPR/Cas9 technology. Involvement of MAP1B in hearing was confirmed by audiometric evaluation of Map1b heterozygous KO mice. These mutant mice displayed late-onset progressive sensorineural hearing loss that was more pronounced in the high frequencies. The spiral ganglion neurons isolated from Map1b mutant mice exhibited the deficient phosphorylation and disturbed dynamics of microtubules. Map1b deficiency yielded defects in the morphology and electrophysiology of spiral ganglion neurons, but it did not affect the morphologies of cochlea in mice. Therefore, our data demonstrate that dysfunctions of spiral ganglion neurons induced by MAP1B deficiency caused hearing loss.

Dysfunctions of spiral ganglion neurons caused by Map1b deficiency leads to sensorineural hearing loss.

The molecular etiology of deafness and auditory performance in the postlingually deafened cochlear implantees

Recent advances in molecular genetic testing (MGT) have improved identification of genetic aetiology of candidates for cochlear implantation (CI). However, whether genetic information increases CI outcome predictability in post-lingual deafness remains unclear. Therefore, we evaluated the outcomes of CI with respect to genetic aetiology and clinical predictors by comparing the data of study subjects; those with an identified genetic aetiology (GD group), and those without identifiable variants (GUD group). First, we identified the genetic aetiology in 21 of 40 subjects and also observed genetic etiologic heterogeneity. The GD group demonstrated significantly greater improvement in speech perception scores over a 1-year period than did the GUD group. Further, inverse correlation between deafness duration and the 1-year improvement in speech perception scores was tighter in the GD group than in the GUD group. The weak correlation between deafness duration and CI outcomes in the GUD group might suggest the pathophysiology underlying GUD already significantly involves the cortex, leading to lesser sensitivity to further cortex issues such as deafness duration. Under our MGT protocol, the correlation between deafness duration and CI outcomes were found to rely on the presence of identifiable genetic aetiology, strongly advocating early CI in individual with proven genetic aetiologies.

Topographic map refinement and synaptic strengthening of a sound localization circuit require spontaneous peripheral activity

KEY POINTS

Loss of the calcium sensor otoferlin disrupts neurotransmission from inner hair cells. Central auditory nuclei are functionally denervated in otoferlin knockout mice (Otof KOs) via gene ablation confined to the periphery. We employed juvenile and young adult Otof KO mice (postnatal days (P)10-12 and P27-49) as a model for lacking spontaneous activity and deafness, respectively. We studied the impact of peripheral activity on synaptic refinement in the sound localization circuit from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO). MNTB in vivo recordings demonstrated drastically reduced spontaneous spiking and deafness in Otof KOs. Juvenile KOs showed impaired synapse elimination and strengthening, manifested by broader MNTB-LSO inputs, imprecise MNTB-LSO topography and weaker MNTB-LSO fibres. The impairments persisted into young adulthood. Further functional refinement after hearing onset was undetected in young adult wild-types. Collectively, activity deprivation confined to peripheral protein loss impairs functional MNTB-LSO refinement during a critical prehearing period.

ABSTRACT

Circuit refinement is critical for the developing sound localization pathways in the auditory brainstem. In prehearing mice (hearing onset around postnatal day (P)12), spontaneous activity propagates from the periphery to central auditory nuclei. At the glycinergic projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) of neonatal mice, super-numerous MNTB fibres innervate a given LSO neuron. Between P4 and P9, MNTB fibres are functionally eliminated, whereas the remaining fibres are strengthened. Little is known about MNTB-LSO circuit refinement after P20. Moreover, MNTB-LSO refinement upon activity deprivation confined to the periphery is largely unexplored. This leaves a considerable knowledge gap, as deprivation often occurs in patients with congenital deafness, e.g. upon mutations in the otoferlin gene (OTOF). Here, we analysed juvenile (P10-12) and young adult (P27-49) otoferlin knockout (Otof KO) mice with respect to MNTB-LSO refinement. MNTB in vivo recordings revealed drastically reduced spontaneous activity and deafness in knockouts (KOs), confirming deprivation. As RNA sequencing revealed Otof absence in the MNTB and LSO of wild-types, Otof loss in KOs is specific to the periphery. Functional denervation impaired MNTB-LSO synapse elimination and strengthening, which was assessed by glutamate uncaging and electrical stimulation. Impaired elimination led to imprecise MNTB-LSO topography. Impaired strengthening was associated with lower quantal content per MNTB fibre. In young adult KOs, the MNTB-LSO circuit remained unrefined. Further functional refinement after P12 appeared absent in wild-types. Collectively, we provide novel insights into functional MNTB-LSO circuit maturation governed by a cochlea-specific protein. The central malfunctions in Otof KOs may have implications for patients with sensorineuronal hearing loss.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ₁₀, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.