Distribution and development of peripheral glial cells in the human fetal cochlea

SOX2, JAGGED1, β-Catenin, and Vitamin D Receptor Expression Patterns during Early Development and Innervation of the Human Inner Ear

Sensorineural hearing loss can be caused by lesions to the inner ear during development. Understanding the events and signaling pathways that drive inner ear formation is crucial for determining the possible causes of congenital hearing loss. We have analyzed the innervation and expression of SOX2, JAGGED1, β-catenin (CTNNB1), and vitamin D receptor (VDR) in the inner ears of human conceptuses aged 5 to 10 weeks after fertilization (W) using immunohistochemistry. The prosensory domains of the human inner ear displayed SOX2 and JAGGED1 expression throughout the analyzed period, with SOX2 expression being more extensive in all the analyzed timepoints. Innervation of vestibular prosensory domains was present at 6 W and extensive at 10 W, while nerve fibers reached the base of the cochlear prosensory domain at 7-8 W. CTNNB1 and VDR expression was mostly membranous and present during all analyzed timepoints in the inner ear, being the strongest in the non-sensory epithelium. Their expression was stronger in the vestibular region compared to the cochlear duct. CTNNB1 and VDR expression displayed opposite expression trends during the analyzed period, but additional studies are needed to elucidate whether they interact during inner ear development.

Human otic progenitor cell models of congenital hearing loss reveal potential pathophysiologic mechanisms of Zika virus and cytomegalovirus infections

Congenital hearing loss is a common chronic condition affecting children in both developed and developing nations. Viruses correlated with congenital hearing loss include human cytomegalovirus (HCMV) and Zika virus (ZIKV), which causes congenital Zika syndrome. The mechanisms by which HCMV and ZIKV infections cause hearing loss are poorly understood. It is challenging to study human inner ear cells because they are encased in bone and also scarce as autopsy samples. Recent advances in culturing human stem cell-derived otic progenitor cells (OPCs) have allowed us herein to describe successful in vitro infection of OPCs with HCMV and ZIKV, and also to propose potential mechanisms by which each viral infection could affect hearing. We find that ZIKV infection rapidly and significantly induces the expression of type I interferon and interferon-stimulated genes, while OPC viability declines, at least in part, from apoptosis. In contrast, HCMV infection did not appear to upregulate interferons or cause a reduction in cell viability, and instead disrupted expression of key genes and pathways associated with inner ear development and function, including Cochlin, nerve growth factor receptor, SRY-box transcription factor 11, and transforming growth factor-beta signaling. These findings suggest that ZIKV and HCMV infections cause congenital hearing loss through distinct pathways, that is, by inducing progenitor cell death in the case of ZIKV infection, and by disruption of critical developmental pathways in the case of HCMV infection.

IMPORTANCE

Congenital virus infections inflict substantial morbidity and devastating disease in neonates worldwide, and hearing loss is a common outcome. It has been difficult to study viral infections of the human hearing apparatus because it is embedded in the temporal bone of the skull. Recent technological advances permit the differentiation of otic progenitor cells (OPCs) from human-induced pluripotent stem cells. This paper is important for demonstrating that inner ear virus infections can be modeled in vitro using OPCs. We infected OPCs with two viruses associated with congenital hearing loss: human cytomegalovirus (HCMV), a DNA virus, or Zika virus (ZIKV), an RNA virus. An important result is that the gene expression and cytokine production profiles of HCMV/ZIKV-infected OPCs are markedly dissimilar, suggesting that mechanisms of hearing loss are also distinct. The specific molecular regulatory pathways identified in this work could suggest important targets for therapeutics.

Protection from cisplatin-induced hearing loss with lentiviral vector-mediated ectopic expression of the anti-apoptotic protein BCL-XL

Cisplatin is a highly effective chemotherapeutic agent, but it can cause sensorineural hearing loss (SNHL) in patients. Cisplatin-induced ototoxicity is closely related to the accumulation of reactive oxygen species (ROS) and subsequent death of hair cells (HCs) and spiral ganglion neurons (SGNs). Despite various strategies to combat ototoxicity, only one therapeutic agent has thus far been clinically approved. Therefore, we have developed a gene therapy concept to protect cochlear cells from cisplatin-induced toxicity. Self-inactivating lentiviral (LV) vectors were used to ectopically express various antioxidant enzymes or anti-apoptotic proteins to enhance the cellular ROS scavenging or prevent apoptosis in affected cell types. In direct comparison, anti-apoptotic proteins mediated a stronger reduction in cytotoxicity than antioxidant enzymes. Importantly, overexpression of the most promising candidate, Bcl-xl, achieved an up to 2.5-fold reduction in cisplatin-induced cytotoxicity in HEI-OC1 cells, phoenix auditory neurons, and primary SGN cultures. BCL-XL protected against cisplatin-mediated tissue destruction in cochlear explants. Strikingly, in vivo application of the LV BCL-XL vector improved hearing and increased HC survival in cisplatin-treated mice. In conclusion, we have established a preclinical gene therapy approach to protect mice from cisplatin-induced ototoxicity that has the potential to be translated to clinical use in cancer patients.

Transcriptional response to mild therapeutic hypothermia in noise-induced cochlear injury

Introduction

Prevention or treatment for acoustic injury has been met with many translational challenges, resulting in the absence of FDA-approved interventions. Localized hypothermia following noise exposure mitigates acute cochlear injury and may serve as a potential avenue for therapeutic approaches. However, the mechanisms by which hypothermia results in therapeutic improvements are poorly understood.

Methods

This study performs the transcriptomic analysis of cochleae from juvenile rats that experienced noise-induced hearing loss (NIHL) followed by hypothermia or control normothermia treatment.

Results

Differential gene expression results from RNA sequencing at 24 h post-exposure to noise suggest that NIHL alone results in increased inflammatory and immune defense responses, involving complement activation and cytokine-mediated signaling. Hypothermia treatment post-noise, in turn, may mitigate the acute inflammatory response.

Discussion

This study provides a framework for future research to optimize hypothermic intervention for ameliorating hearing loss and suggests additional pathways that could be targeted for NIHL therapeutic intervention.

Human pluripotent stem cell-derived inner ear organoids recapitulate otic development in vitro

Our molecular understanding of the early stages of human inner ear development has been limited by the difficulty in accessing fetal samples at early gestational stages. As an alternative, previous studies have shown that inner ear morphogenesis can be partially recapitulated using induced pluripotent stem cells directed to differentiate into inner ear organoids (IEOs). Once validated and benchmarked, these systems could represent unique tools to complement and refine our understanding of human otic differentiation and model developmental defects. Here, we provide the first direct comparisons of the early human embryonic otocyst and fetal sensory organs with human IEOs. We use multiplexed immunostaining and single-cell RNA-sequencing to characterize IEOs at three key developmental steps, providing a new and unique signature of in vitro-derived otic placode, epithelium, neuroblasts and sensory epithelia. In parallel, we evaluate the expression and localization of crucial markers at these equivalent stages in human embryos. Together, our data indicate that the current state-of-the-art protocol enables the specification of bona fide otic tissue, supporting the further application of IEOs to inform inner ear biology and disease.

Developmental expression of high-mobility group box 1 (HMGB1) in the mouse cochlea

The expression changes of high-mobility group box 1 (HMGB1) in the mouse cochlea have recently been implicated in noise-induced hearing loss, suggesting that HMGB1 participates in regulating cochlear function. However, the precise role of HMGB1 in the auditory system remains largely unclear. This study aimed to investigate its function in the developing mouse cochlea by examining the expression pattern of HMGB1 in the mouse cochlea from embryonic day (E) 18.5 to postnatal day (P) 28 using double immunofluorescence on frozen sections. Our findings revealed that HMGB1 was extensively expressed in the cell nucleus across various regions of the mouse cochlea, including the organ of Corti. Furthermore, its expression underwent developmental regulation during mouse cochlear development. Specifically, HMGB1 was found to be localized in the tympanic border cells at each developmental stage, coinciding with the gradual anatomical in this region during development. In addition, HMGB1 was expressed in the greater epithelial ridge (GER) and supporting cells of the organ of Corti, as validated by the supporting cell marker Sox2 at P1 and P8. However, at P14, the expression of HMGB1 disappeared from the GER, coinciding with the degeneration of the GER into the inner sulcus cells. Moreover, we observed that HMGB1 co-localized with Ki-67-positive proliferating cells in several cochlear regions during late embryonic and early postnatal stages, including the GER, the tympanic border cells, cochlear lateral wall, and cochlear nerves. Furthermore, by dual-staining Ki-67 with neuronal marker TUJ1 and glial marker Sox10, we determined the expression of Ki-67 in the neonatal glial cells. Our spatial-temporal analysis demonstrated that HMGB1 exhibited distinct expression patterns during mouse cochlear development. The co-localization of HMGB1 with Ki-67-positive proliferating cells suggested that HMGB1 may play a role in cochlear development.

Complement factor B is essential for the proper function of the peripheral auditory system

Sensorineural hearing loss is associated with dysfunction of cochlear cells. Although immune cells play a critical role in maintaining the inner ear microenvironment, the precise immune-related molecular mechanisms underlying the pathophysiology of hearing loss remain unclear. The complement cascade contributes to the regulation of immune cell activity. Additionally, activation of the complement cascade can lead to the cellular opsonization of cells and pathogens, resulting in their engulfment and elimination by phagocytes. Complement factor B (fB) is an essential activator protein in the alternative complement pathway, and variations in the fB gene are associated with age-related macular degeneration. Here we show that mice of both sexes deficient in fB functional alleles (fB-/-) demonstrate progressive hearing impairment. Transcriptomic analysis of auditory nerves from adult mice detected 706 genes that were significantly differentially expressed between fB-/- and wild-type control animals, including genes related to the extracellular matrix and neural development processes. Additionally, a subset of differentially expressed genes was related to myelin function and neural crest development. Histological and immunohistochemical investigations revealed pathological alterations in auditory nerve myelin sheathes of fB-/- mice. Pathological alterations were also seen in the stria vascularis of the cochlear lateral wall in these mice. Our results implicate fB as an integral regulator of myelin maintenance and stria vascularis integrity, underscoring the importance of understanding the involvement of immune signaling pathways in sensorineural hearing loss.

Human pluripotent stem cells-derived inner ear organoids recapitulate otic development in vitro

Our molecular understanding of the early stages of human inner ear development has been limited by the difficulty in accessing fetal samples at early gestational stages. As an alternative, previous studies have shown that inner ear morphogenesis can be partially recapitulated using induced pluripotent stem cells (iPSCs) directed to differentiate into Inner Ear Organoids (IEOs). Once validated and benchmarked, these systems could represent unique tools to complement and refine our understanding of human otic differentiation and model developmental defects. Here, we provide the first direct comparisons of the early human embryonic otocyst and human iPSC-derived IEOs. We use multiplexed immunostaining, and single-cell RNA sequencing to characterize IEOs at three key developmental steps, providing a new and unique signature of in vitro derived otic -placode, -epithelium, -neuroblasts, and -sensory epithelia. In parallel, we evaluate the expression and localization of critical markers at these equivalent stages in human embryos. We show that the placode derived in vitro (days 8-12) has similar marker expression to the developing otic placode of Carnegie Stage (CS) 11 embryos and subsequently (days 20-40) this gives rise to otic epithelia and neuroblasts comparable to the CS13 embryonic stage. Differentiation of sensory epithelia, including supporting cells and hair cells starts in vitro at days 50-60 of culture. The maturity of these cells is equivalent to vestibular sensory epithelia at week 10 or cochlear tissue at week 12 of development, before functional onset. Together, our data indicate that the current state-of-the-art protocol enables the specification of bona fide otic tissue, supporting the further application of IEOs to inform inner ear biology and disease.

Generation of human otic neuronal organoids using pluripotent stem cells

Otic neurons, also known as spiral ganglion neurons (SGNs) in mammalian cochlea, transmit electrical signals from sensory hair cells to cochlear nuclei of the auditory system. SGNs are sensitive to toxic insults, vulnerable to get irreversible damaged and hardly regenerate after damage, causing persistent sensorineural hearing loss. Yet, to get authentic SGNs for research or therapeutic purpose remains challenging. Here we developed a protocol to generate human otic neuronal organoids (hONOs) from human pluripotent stem cells (hESCs), in which hESCs were step-wisely induced to SGNs of the corresponding stages according to their developmental trajectory. The hONOs were enriched for SGN-like cells at early stage, and for both neurons and astrocytes, Schwann cells or supporting cells thereafter. In these hONOs, we also determined the existence of typical Type I and Type II SGNs. Mature hONOs (at differentiation Day 60) formed neural network, featured by giant depolarizing potential (GDP)-like events and rosette-organized regions-elicited calcium traces. Electrophysiological analysis confirmed the existence of glutamate-responsive neurons in these hONOs. The otic neuronal organoids generated in this study provide an ideal model to study SGNs and related disorders, facilitating therapeutic development for sensorineural hearing loss.

Cellular distribution of the Fragile X mental retardation protein in the inner ear: a developmental and comparative study in the mouse, rat, gerbil, and chicken

The Fragile X mental retardation protein (FMRP) is an mRNA binding protein that is essential for neural circuit assembly and synaptic plasticity. Loss of functional FMRP leads to Fragile X syndrome (FXS), a neurodevelopmental disorder characterized by sensory dysfunction including abnormal auditory processing. While the central mechanisms of FMRP regulation have been studied in the brain, whether FMRP is expressed in the auditory periphery and how it develops and functions remains unknown. In this study, we characterized the spatiotemporal distribution pattern of FMRP immunoreactivity in the inner ear of mice, rats, gerbils, and chickens. Across species, FMRP was expressed in hair cells and supporting cells, with a particularly high level in immature hair cells during the prehearing period. Interestingly, the distribution of cytoplasmic FMRP displayed an age-dependent translocation in hair cells, and this feature was conserved across species. In the auditory ganglion (AG), FMRP immunoreactivity was detected in neuronal cell bodies as well as their peripheral and central processes. Distinct from hair cells, FMRP intensity in AG neurons was high both during development and after maturation. Additionally, FMRP was evident in mature glial cells surrounding AG neurons. Together, these observations demonstrate distinct developmental trajectories across cell types in the auditory periphery. Given the importance of peripheral inputs to the maturation of auditory circuits, these findings implicate involvement of FMRP in inner ear development as well as a potential contribution of periphery FMRP to the generation of auditory dysfunction in FXS.

Early development of the cochlea of the common marmoset, a non-human primate model

Background

Fine-tuned cochlear development is essential for hearing. Owing to the difficulty in using early human fetal samples, most of our knowledge regarding cochlear development has been obtained from rodents. However, several inter-species differences in cochlear development between rodents and humans have been reported. To bridge these differences, we investigated early otic development of a non-human primate model animal, the common marmoset (Callithrix jacchus).

Methods

We examined 20 genes involved in early cochlear development and described the critical developmental steps for morphogenesis, which have been reported to vary between rodents and marmosets.

Results

The results revealed that several critical genes involved in prosensory epithelium specifications showed higher inter-species differences, suggesting that the molecular process for hair cell lineage acquisition in primates differs considerably from that of rodents. We also observed that the tempo of cochlear development was three times slower in the primate than in rodents.

Conclusions

Our data provide new insights into early cochlear development in primates and humans and imply that the procedures used for manipulating rodent cochlear sensory cells cannot be directly used for the research of primate cells due to the intrinsic inter-species differences in the cell fate determination program.

Axon-glia interactions in the ascending auditory system

The auditory system detects and encodes sound information with high precision to provide a high-fidelity representation of the environment and communication. In mammals, detection occurs in the peripheral sensory organ (the cochlea) containing specialized mechanosensory cells (hair cells) that initiate the conversion of sound-generated vibrations into action potentials in the auditory nerve. Neural activity in the auditory nerve encodes information regarding the intensity and frequency of sound stimuli, which is transmitted to the auditory cortex through the ascending neural pathways. Glial cells are critical for precise control of neural conduction and synaptic transmission throughout the pathway, allowing for the precise detection of the timing, frequency, and intensity of sound signals, including the sub-millisecond temporal fidelity is necessary for tasks such as sound localization, and in humans, for processing complex sounds including speech and music. In this review, we focus on glia and glia-like cells that interact with hair cells and neurons in the ascending auditory pathway and contribute to the development, maintenance, and modulation of neural circuits and transmission in the auditory system. We also discuss the molecular mechanisms of these interactions, their impact on hearing and on auditory dysfunction associated with pathologies of each cell type.

Directed differentiation and direct reprogramming: Applying stem cell technologies to hearing research

Hearing loss is the most widely spread sensory disorder in our society. In the majority of cases, it is caused by the loss or malfunctioning of cells in the cochlea: the mechanosensory hair cells, which act as primary sound receptors, and the connecting auditory neurons of the spiral ganglion, which relay the signal to upper brain centers. In contrast to other vertebrates, where damage to the hearing organ can be repaired through the activity of resident cells, acting as tissue progenitors, in mammals, sensory cell damage or loss is irreversible. The understanding of gene and cellular functions, through analysis of different animal models, has helped to identify causes of disease and possible targets for hearing restoration. Translation of these findings to novel therapeutics is, however, hindered by the lack of cellular assays, based on human sensory cells, to evaluate the conservation of molecular pathways across species and the efficacy of novel therapeutic strategies. In the last decade, stem cell technologies enabled to generate human sensory cell types in vitro, providing novel tools to study human inner ear biology, model disease, and validate therapeutics. This review focuses specifically on two technologies: directed differentiation of pluripotent stem cells and direct reprogramming of somatic cell types to sensory hair cells and neurons. Recent development in the field are discussed as well as how these tools could be implemented to become routinely adopted experimental models for hearing research.

Migration and fate of vestibular melanocytes during the development of the human inner ear

Melanocytes are present in various parts of the inner ear, including the stria vascularis in the cochlea and the dark cell areas in the vestibular organs, where they contribute to endolymph homeostasis. Developmental studies describing the distribution of vestibular melanocytes are scarce, especially in humans. In this study, we investigated the distribution and maturation of the vestibular melanocytes in relation to the developing dark cell epithelium in inner ear specimens from week 5 to week 14 of development and in surgical specimens of the adult ampulla. Vestibular melanocytes were located around the utricle and the ampullae of the semicircular canals before week 7 and were first seen underneath the transitional zones and dark cell areas between week 8 and week 10. At week 10, melanocytes made intimate contact with epithelial cells, interrupting the local basement membrane with their dendritic processes. At week 11, most melanocytes were positioned under the dark cell epithelia. No melanocytes were seen around or in the saccule during all investigated developmental stages. The dark cell areas gradually matured and showed an adult immunohistochemical profile of the characteristic ion transporter protein Na⁺/K⁺‐ATPase α1 by week 14. Furthermore, we investigated the expression of the migration‐related proteins ECAD, PCAD, KIT, and KITLG in melanocytes and dark cell epithelium. This is the first study to describe the spatiotemporal distribution of vestibular melanocytes during the human development and thereby contributes to understanding normal vestibular function and pathophysiological mechanisms underlying vestibular disorders.

Analysis of FGF20-regulated genes in organ of Corti progenitors by translating ribosome affinity purification

BACKGROUND

Understanding the mechanisms that regulate hair cell (HC) differentiation in the organ of Corti (OC) is essential to designing genetic therapies for hearing loss due to HC loss or damage. We have previously identified Fibroblast Growth Factor 20 (FGF20) as having a key role in HC and supporting cell differentiation in the mouse OC. To investigate the genetic landscape regulated by FGF20 signaling in OC progenitors, we employ Translating Ribosome Affinity Purification combined with Next Generation RNA Sequencing (TRAPseq) in the Fgf20 lineage.

RESULTS

We show that TRAPseq targeting OC progenitors effectively enriched for RNA from this rare cell population. TRAPseq identified differentially expressed genes (DEGs) downstream of FGF20, including Etv4, Etv5, Etv1, Dusp6, Hey1, Hey2, Heyl, Tectb, Fat3, Cpxm2, Sall1, Sall3, and cell cycle regulators such as Cdc20. Analysis of Cdc20 conditional-null mice identified decreased cochlea length, while analysis of Sall1-null and Sall1-ΔZn2-10 mice, which harbor a mutation that causes Townes-Brocks syndrome, identified a decrease in outer hair cell number.

CONCLUSIONS

We present two datasets: genes with enriched expression in OC progenitors, and DEGs downstream of FGF20 in the embryonic day 14.5 cochlea. We validate select DEGs via in situ hybridization and in vivo functional studies in mice.

Novel insights into inner ear development and regeneration for targeted hearing loss therapies

Sensorineural hearing loss is the most common sensory deficit in humans. Despite the global scale of the problem, only limited treatment options are available today. The mammalian inner ear is a highly specialized postmitotic organ, which lacks proliferative or regenerative capacity. Since the discovery of hair cell regeneration in non-mammalian species however, much attention has been placed on identifying possible strategies to reactivate similar responses in humans. The development of successful regenerative approaches for hearing loss strongly depends on a detailed understanding of the mechanisms that control human inner ear cellular specification, differentiation and function, as well as on the development of robust in vitro cellular assays, based on human inner ear cells, to study these processes and optimize therapeutic interventions. We summarize here some aspects of inner ear development and strategies to induce regeneration that have been investigated in rodents. Moreover, we discuss recent findings in human inner ear development and compare the results with findings from animal models. Finally, we provide an overview of strategies for in vitro generation of human sensory cells from pluripotent and somatic progenitors that may provide a platform for drug development and validation of therapeutic strategies in vitro.

Age-dependent alterations of Kir4.1 expression in neural crest-derived cells of the mouse and human cochlea

Age-related hearing loss (or presbyacusis) is a progressive pathophysiological process. This study addressed the hypothesis that degeneration/dysfunction of multiple nonsensory cell types contributes to presbyacusis by evaluating tissues obtained from young and aged CBA/CaJ mouse ears and human temporal bones. Ultrastructural examination and transcriptomic analysis of mouse cochleas revealed age-dependent pathophysiological alterations in 3 types of neural crest-derived cells, namely intermediate cells in the stria vascularis, outer sulcus cells in the cochlear lateral wall, and satellite cells in the spiral ganglion. A significant decline in immunoreactivity for Kir4.1, an inwardly rectifying potassium channel, was seen in strial intermediate cells and outer sulcus cells in the ears of older mice. Age-dependent alterations in Kir4.1 immunostaining also were observed in satellite cells ensheathing spiral ganglion neurons. Expression alterations of Kir4.1 were observed in these same cell populations in the aged human cochlea. These results suggest that degeneration/dysfunction of neural crest-derived cells maybe an important contributing factor to both metabolic and neural forms of presbyacusis.

Molecular characterization and prospective isolation of human fetal cochlear hair cell progenitors

Sensory hair cells located in the organ of Corti are essential for cochlear mechanosensation. Their loss is irreversible in humans resulting in permanent hearing loss. The development of therapeutic interventions for hearing loss requires fundamental knowledge about similarities and potential differences between animal models and human development as well as the establishment of human cell based-assays. Here we analyze gene and protein expression of the developing human inner ear in a temporal window spanning from week 8 to 12 post conception, when cochlear hair cells become specified. Utilizing surface markers for the cochlear prosensory domain, namely EPCAM and CD271, we purify postmitotic hair cell progenitors that, when placed in culture in three-dimensional organoids, regain proliferative potential and eventually differentiate to hair cell-like cells in vitro. These results provide a foundation for comparative studies with otic cells generated from human pluripotent stem cells and for establishing novel platforms for drug validation.

Hearing requires mechanosensitive hair cells in the organ of Corti, which derive from progenitors of the cochlear duct. Here the authors examine human inner ear development by studying key developmental markers and describe organoid cultures from human cochlear duct progenitors for in vitro hair cell differentiation.

Transient auditory nerve demyelination as a new mechanism for hidden hearing loss

Hidden hearing loss (HHL) is a recently described auditory neuropathy believed to contribute to speech discrimination and intelligibility deficits in people with normal audiological tests. Animals and humans with HHL have normal auditory thresholds but defective cochlear neurotransmission, that is, reduced suprathreshold amplitude of the sound-evoked auditory nerve compound action potential. Currently, the only cellular mechanism known for HHL is loss of inner hair cell synapses (synaptopathy). Here we report that transient loss of cochlear Schwann cells results in permanent auditory deficits characteristic of HHL. This auditory neuropathy is not associated with synaptic loss, but rather with disruption of the first heminodes at the auditory nerve peripheral terminal. Thus, this study identifies a new mechanism for HHL, highlights the long-term consequences of transient Schwann cell loss on hearing and might provide insights into the causes of the auditory deficits reported in patients that recover from acute demyelinating diseases such as Guillain–Barré syndrome.

Hidden hearing loss (HHL) is an auditory neuropathy that impairs one's ability to hear, particularly in a noisy environment. Here the authors show that in mice, transient loss of cochlear Schwann cells results in permanent disruption of the cochlear heminodal structure, leading to auditory deficits characteristic of HHL.

The Glia Response after Peripheral Nerve Injury: A Comparison between Schwann Cells and Olfactory Ensheathing Cells and Their Uses for Neural Regenerative Therapies

The peripheral nervous system (PNS) exhibits a much larger capacity for regeneration than the central nervous system (CNS). One reason for this difference is the difference in glial cell types between the two systems. PNS glia respond rapidly to nerve injury by clearing debris from the injury site, supplying essential growth factors and providing structural support; all of which enhances neuronal regeneration. Thus, transplantation of glial cells from the PNS is a very promising therapy for injuries to both the PNS and the CNS. There are two key types of PNS glia: olfactory ensheathing cells (OECs), which populate the olfactory nerve, and Schwann cells (SCs), which are present in the rest of the PNS. These two glial types share many similar morphological and functional characteristics but also exhibit key differences. The olfactory nerve is constantly turning over throughout life, which means OECs are continuously stimulating neural regeneration, whilst SCs only promote regeneration after direct injury to the PNS. This review presents a comparison between these two PNS systems in respect to normal physiology, developmental anatomy, glial functions and their responses to injury. A thorough understanding of the mechanisms and differences between the two systems is crucial for the development of future therapies using transplantation of peripheral glia to treat neural injuries and/or disease.

In Vitro Models of GJB2-Related Hearing Loss Recapitulate Ca2+ Transients via a Gap Junction Characteristic of Developing Cochlea

Mutation of the Gap Junction Beta 2 gene (GJB2) encoding connexin 26 (CX26) is the most frequent cause of hereditary deafness worldwide and accounts for up to 50% of non-syndromic sensorineural hearing loss cases in some populations. Therefore, cochlear CX26-gap junction plaque (GJP)-forming cells such as cochlear supporting cells are thought to be the most important therapeutic target for the treatment of hereditary deafness. The differentiation of pluripotent stem cells into cochlear CX26-GJP-forming cells has not been reported. Here, we detail the development of a novel strategy to differentiate induced pluripotent stem cells into functional CX26-GJP-forming cells that exhibit spontaneous ATP- and hemichannel-mediated Ca²⁺ transients typical of the developing cochlea. Furthermore, these cells from CX26-deficient mice recapitulated the drastic disruption of GJPs, the primary pathology of GJB2-related hearing loss. These in vitro models should be useful for establishing inner-ear cell therapies and drug screening that target GJB2-related hearing loss.

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Mutation in GJB2 (CX26) is the most frequent cause of hereditary deafness worldwide

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Functional CX26-gap junction plaque (GJP)-forming cells were generated from iPSCs

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These cells exhibited spontaneous Ca²⁺ transients typical of the developing cochlea

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The drastic disruption of GJP was observed in in vitro disease model of GJB2 mutation

Development of the stria vascularis and potassium regulation in the human fetal cochlea: Insights into hereditary sensorineural hearing loss

Sensorineural hearing loss (SNHL) is one of the most common congenital disorders in humans, afflicting one in every thousand newborns. The majority is of heritable origin and can be divided in syndromic and nonsyndromic forms. Knowledge of the expression profile of affected genes in the human fetal cochlea is limited, and as many of the gene mutations causing SNHL likely affect the stria vascularis or cochlear potassium homeostasis (both essential to hearing), a better insight into the embryological development of this organ is needed to understand SNHL etiologies. We present an investigation on the development of the stria vascularis in the human fetal cochlea between 9 and 18 weeks of gestation (W9–W18) and show the cochlear expression dynamics of key potassium‐regulating proteins. At W12, MITF+/SOX10+/KIT+ neural‐crest‐derived melanocytes migrated into the cochlea and penetrated the basement membrane of the lateral wall epithelium, developing into the intermediate cells of the stria vascularis. These melanocytes tightly integrated with Na⁺/K⁺‐ATPase‐positive marginal cells, which started to express KCNQ1 in their apical membrane at W16. At W18, KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30 were expressed in the cells in the outer sulcus, but not in the spiral ligament. Finally, we investigated GJA1/CX43 and GJE1/CX23 expression, and suggest that GJE1 presents a potential new SNHL associated locus. Our study helps to better understand human cochlear development, provides more insight into multiple forms of hereditary SNHL, and suggests that human hearing does not commence before the third trimester of pregnancy. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 75: 1219–1240, 2015