Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function

Advances in the Study of Etiology and Molecular Mechanisms of Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL), a multifactorial progressive disorder, results from a complex interplay of genetic and environmental factors, with its underlying mechanisms remaining unclear. Several pathological factors are believed to contribute to SNHL, including genetic factors, ion homeostasis, cell apoptosis, immune inflammatory responses, oxidative stress, hormones, metabolic syndrome, human cytomegalovirus infection, mitochondrial damage, and impaired autophagy. These factors collectively interact and play significant roles in the onset and progression of SNHL. The present review offers a comprehensive overview of the various factors that contribute to SNHL, emphasizes recent developments in understanding its etiology, and explores relevant preventive and intervention measures.

Impaired tectorial membrane and ribbon synapse maturation in the cochlea of mice with congenital hypothyroidism

Thyroid hormone deficiency can lead to abnormal auditory development of varying severity. Retardation of morphological development, including delays in degeneration of Kölliker's organ and subsequent delayed formation of the inner sulcus, along with delayed opening of the tunnel of Corti and malformation of the tectorial membrane, was consistently observed in an antithyroid drug-induced congenital hypothyroidism rodent model. Abnormal morphological development could partly explain impaired adult auditory function. However, whether the development of inner hair cell ribbon synapses is influenced by hypothyroidism remains unclear. In the present study, we characterize the normal degeneration pattern of Kölliker's organ along the basal-to-apical axis. Then, we verified the retardation of morphological development in congenital hypothyroid mice. Using this model, we found that twisted collagen is present in the major tectorial membrane and delayed separation from supporting cells affects the minor tectorial membrane. Finally, we found that the number of synaptic ribbons was not significantly altered but the ribbon synapse maturation process was significantly impaired in congenital hypothyroid mice. We conclude that thyroid hormone is involved in structural development of the tectorial membrane and the ribbon synapse maturation process.

Thyroxine Regulates the Opening of the Organ of Corti through Affecting P-Cadherin and Acetylated Microtubule

Different serum thyroxine levels may influence the morphology of the inner ear during development. A well-developed organ of Corti (OC) is considered to be critical to the function of hearing. In our study, we treated mice with triiodothyronine (T3) and found that the opening of the OC occurred sooner than in control mice. We also observed an increased formation of acetylated microtubules and a decrease in the adhesion junction molecule P-cadherin the during opening of the OC. Our investigation indicates that thyroxin affects P-cadherin expression and microtubule acetylation to influence the opening of the OC.

A disease-associated mutation in thyroid hormone receptor α1 causes hearing loss and sensory hair cell patterning defects in mice

Resistance to thyroid hormone due to mutations in THRA, which encodes the thyroid hormone receptor α (TRα1), shows variable clinical presentation. Mutations affecting TRβ1 and TRβ2 cause deafness in mice and have been associated with deafness in humans. To test whether TRα1 also affects hearing function, we used mice heterozygous for a frameshift mutation in Thra that is similar to human THRA mutations (Thra^S1/+ mice) and reduces tissue sensitivity to thyroid hormone. Compared to wild-type littermates, Thra^S1/+ mice showed moderate high-frequency sensorineural hearing loss as juveniles and increased age-related hearing loss. Ultrastructural examination revealed aberrant orientation of ~20% of sensory outer hair cells (OHCs), as well as increased numbers of mitochondria with fragmented morphology and autophagic vacuoles in both OHCs and auditory nerve fibers. Molecular dissection of the OHC lateral wall components revealed that the potassium ion channel Kcnq4 was aberrantly targeted to the cytoplasm of mutant OHCs. In addition, mutant cochleae showed increased oxidative stress, autophagy, and mitophagy associated with greater age-related cochlear cell damage, demonstrating that TRα1 is required for proper development of OHCs and for maintenance of OHC function. These findings suggest that patients with THRA mutations may present underdiagnosed, mild hearing loss and may be more susceptible to age-related hearing loss.

Lineage-tracing and translatomic analysis of damage-inducible mitotic cochlear progenitors identifies candidate genes regulating regeneration

Cochlear supporting cells (SCs) are glia-like cells critical for hearing function. In the neonatal cochlea, the greater epithelial ridge (GER) is a mitotically quiescent and transient organ, which has been shown to nonmitotically regenerate SCs. Here, we ablated Lgr5+ SCs using Lgr5-DTR mice and found mitotic regeneration of SCs by GER cells in vivo. With lineage tracing, we show that the GER houses progenitor cells that robustly divide and migrate into the organ of Corti to replenish ablated SCs. Regenerated SCs display coordinated calcium transients, markers of the SC subtype inner phalangeal cells, and survive in the mature cochlea. Via RiboTag, RNA-sequencing, and gene clustering algorithms, we reveal 11 distinct gene clusters comprising markers of the quiescent and damaged GER, and damage-responsive genes driving cell migration and mitotic regeneration. Together, our study characterizes GER cells as mitotic progenitors with regenerative potential and unveils their quiescent and damaged translatomes.

The Putative Adverse Effects of Bisphenol A on Autoimmune Diseases

Bisphenol A (BPA) is a monomer that is widely used in the manufacture of polycarbonate plastics including storage plastics and baby bottles, and is considered one of the most widely used synthetic compounds in the manufacturing industry. Exposure to BPA mainly occurs after oral ingestion and results from leaks into food and water from plastic containers and according to epidemiological data exposure is widespread and estimated to occur in 90% of individuals. BPA exertspleiotropiceffects and demonstrates estrogen like effects, thus considered an endocrine disrupting chemical. Growing body of evidence highlight the role of BPA in modulating immune responses and signaling pathways resulting in a proinflammatory response by enhancing the differential polarization of immune cells and cytokine production profile to one that is consistent with proinflammation. Indeed, epidemiological studies have uncovered associations between several autoimmune diseases and BPA exposure. Data from animal models provided consistent evidence highlighting the role of BPA in the pathogenesis, exacerbation and perpetuation of various autoimmune phenomena including neuroinflammation in the context of multiple sclerosis, colitis in inflammatory bowel disease, nephritis in systemic lupus erythematosus, and insulitis in type 1 diabetes mellitus. Given the wide spread of BPA use and its effects in immune systemdysregulation, a call for careful assessment of patients' risks and for public health measures are needed to limit exposure and subsequent deleterious effects. The purpose of this paper is to explore the autoimmune triggering mechanisms and present the current literature supporting the role of BPA in the pathogenesis of autoimmune diseases.

GC-B Deficient Mice With Axon Bifurcation Loss Exhibit Compromised Auditory Processing

Sensory axon T-like branching (bifurcation) in neurons from dorsal root ganglia and cranial sensory ganglia depends on the molecular signaling cascade involving the secreted factor C-type natriuretic peptide, the natriuretic peptide receptor guanylyl cyclase B (GC-B; also known as Npr2) and cGMP-dependent protein kinase I (cGKI, also known as PKGI). The bifurcation of cranial nerves is suggested to be important for information processing by second-order neurons in the hindbrain or spinal cord. Indeed, mice with a spontaneous GC-B loss of function mutation (Npr2^cn/cn ) display an impaired bifurcation of auditory nerve (AN) fibers. However, these mice did not show any obvious sign of impaired basal hearing. Here, we demonstrate that mice with a targeted inactivation of the GC-B gene (Npr2 ^lacZ/lacZ , GC-B KO mice) show an elevation of audiometric thresholds. In the inner ear, the cochlear hair cells in GC-B KO mice were nevertheless similar to those from wild type mice, justified by the typical expression of functionally relevant marker proteins. However, efferent cholinergic feedback to inner and outer hair cells was reduced in GC-B KO mice, linked to very likely reduced rapid efferent feedback. Sound-evoked AN responses of GC-B KO mice were elevated, a feature that is known to occur when the efferent axo-dendritic feedback on AN is compromised. Furthermore, late sound-evoked brainstem responses were significantly delayed in GC-B KO mice. This delay in sound response was accompanied by a weaker sensitivity of the auditory steady state response to amplitude-modulated sound stimuli. Finally, the acoustic startle response (ASR) - one of the fastest auditory responses - and the prepulse inhibition of the ASR indicated significant changes in temporal precision of auditory processing. These findings suggest that GC-B-controlled axon bifurcation of spiral ganglion neurons is important for proper activation of second-order neurons in the hindbrain and is a prerequisite for proper temporal auditory processing likely by establishing accurate efferent top-down control circuits. These data hypothesize that the bifurcation pattern of cranial nerves is important to shape spatial and temporal information processing for sensory feedback control.

Myelin development, plasticity, and pathology in the auditory system

Myelin allows for the rapid and precise timing of action potential propagation along neuronal circuits and is essential for healthy auditory system function. In this article, we discuss what is currently known about myelin in the auditory system with a focus on the timing of myelination during auditory system development, the role of myelin in supporting peripheral and central auditory circuit function, and how various myelin pathologies compromise auditory information processing. Additionally, in keeping with the increasing recognition that myelin is dynamic and is influenced by experience throughout life, we review the growing evidence that auditory sensory deprivation alters myelin along specific segments of the brain's auditory circuit. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 80-92, 2018.

Structural and Ultrastructural Changes to Type I Spiral Ganglion Neurons and Schwann Cells in the Deafened Guinea Pig Cochlea

Sensorineural hearing loss is commonly caused by damage to cochlear sensory hair cells. Coinciding with hair cell degeneration, the peripheral fibres of type I spiral ganglion neurons (SGNs) that normally form synaptic connections with the inner hair cell gradually degenerate. We examined the time course of these degenerative changes in type I SGNs and their satellite Schwann cells at the ultrastructural level in guinea pigs at 2, 6, and 12 weeks following aminoglycoside-induced hearing loss. Degeneration of the peripheral fibres occurred prior to the degeneration of the type I SGN soma and was characterised by shrinkage of the fibre followed by retraction of the axoplasm, often leaving a normal myelin lumen devoid of axoplasmic content. A statistically significant reduction in the cross-sectional area of peripheral fibres was evident as early as 2 weeks following deafening (p < 0.001, ANOVA). This was followed by a decrease in type I SGN density within Rosenthal's canal that was statistically significant 6 weeks following deafening (p < 0.001, ANOVA). At any time point examined, few type I SGN soma were observed undergoing degeneration, implying that once initiated, soma degeneration was rapid. While there was a significant reduction in soma area as well as changes to the morphology of the soma, the ultrastructure of surviving type I SGN soma appeared relatively normal over the 12-week period following deafening. Satellite Schwann cells exhibited greater survival traits than their type I SGN; however, on loss of neural contact, they reverted to a non-myelinating phenotype, exhibiting an astrocyte-like morphology with the formation of processes that appeared to be searching for new neural targets. In 6- and 12-week deafened cochlea, we observed cellular interaction between Schwann cell processes and residual SGNs that distorted the morphology of the SGN soma. Understanding the response of SGNs, Schwann cells, and the complex relationship between them following aminoglycoside deafening is important if we are to develop effective therapeutic techniques designed to rescue SGNs.

Germain DL, Hernandez A, Forrest D. Deletion of the Thyroid Hormone-Activating Type 2 Deiodinase Rescues Cone Photoreceptor Degeneration but Not Deafness in Mice Lacking Type 3 Deiodinase

Type 2 deiodinase amplifies and type 3 deiodinase depletes levels of the active form of thyroid hormone, triiodothyronine. Given the opposing activities of these enzymes, we tested the hypothesis that they counteract each other's developmental functions by investigating whether deletion of type 2 deiodinase (encoded by Dio2) modifies sensory phenotypes in type 3 deiodinase-deficient (Dio3-/-) mice. Dio3-/- mice display degeneration of retinal cones, the photoreceptors that mediate daylight and color vision. In Dio2-/- mice, cone function was largely normal but deletion of Dio2 in Dio3-/- mice markedly recovered cone numbers and electroretinogram responses, suggesting counterbalancing roles for both enzymes in cone survival. Both Dio3-/- and Dio2-/- strains exhibit deafness with cochlear abnormalities. In Dio3-/-;Dio2-/- mice, deafness was exacerbated rather than alleviated, suggesting unevenly balanced actions by these enzymes during auditory development. Dio3-/- mice also exhibit an atrophic thyroid gland, low thyroxine, and high triiodothyronine levels, but this phenotype was ameliorated in Dio3-/-;Dio2-/- mice, indicating counterbalancing roles for the enzymes in determining the thyroid hormone status. The results suggest that the composite action of these two enzymes is a critical determinant in visual and auditory development and in setting the systemic thyroid hormone status.

Pharmacological treatment and BBB-targeted genetic therapy for MCT8-dependent hypomyelination in zebrafish

Hypomyelination is a key symptom of Allan-Herndon-Dudley syndrome (AHDS), a psychomotor retardation associated with mutations in the thyroid-hormone (TH) transporter MCT8 (monocarboxylate transporter 8). AHDS is characterized by severe intellectual deficiency, neuromuscular impairment and brain hypothyroidism. In order to understand the mechanism for TH-dependent hypomyelination, we developed an mct8 mutant (mct8^-/-) zebrafish model. The quantification of genetic markers for oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes revealed reduced differentiation of OPCs into oligodendrocytes in mct8^-/- larvae and adults. Live imaging of single glial cells showed that the number of oligodendrocytes and the length of their extensions are reduced, and the number of peripheral Schwann cells is increased, in mct8^-/- larvae compared with wild type. Pharmacological analysis showed that TH analogs and clemastine partially rescued the hypomyelination in the CNS of mct8^-/- larvae. Intriguingly, triiodothyronine (T3) treatment rescued hypomyelination in mct8^-/- embryos before the maturation of the blood-brain barrier (BBB), but did not affect hypomyelination in older larvae. Thus, we expressed Mct8-tagRFP in the endothelial cells of the vascular system and showed that even relatively weak mosaic expression completely rescued hypomyelination in mct8^-/- larvae. These results suggest potential pharmacological treatments and BBB-targeted gene therapy that can enhance myelination in AHDS and possibly in other TH-dependent brain disorders.

Thyroid hormone is required for the pruning of afferent type II spiral ganglion neurons in the mouse cochlea

Afferent connections to the sensory inner (IHCs) and outer hair cells (OHCs) in the cochlea refine and functionally mature during the thyroid hormone (TH)-critical period of inner ear development that occurs perinatally in rodents. In this study, we investigated the effects of hypothyroidism on afferent type II innervation to outer hair cells using the Snell dwarf mouse (Pit1(dw)). Using a transgenic approach to specifically label type II spiral ganglion neurons (SGNs), we found that lack of TH causes persistence of excess type II SGN connections to the OHCs, as well as continued expression of the hair cell functional marker, otoferlin (OTOF), in the OHCs beyond the maturation period. We also observed a concurrent delay in efferent attachment to the OHCs. Supplementing with TH during the early postnatal period from postnatal day (P) 3 to P4 reversed the defect in type II SGN pruning but did not alter OTOF expression. Our results show that hypothyroidism causes a defect in the large-scale pruning of afferent type II SGNs in the cochlea, and a delay in efferent attachment and the maturation of OTOF expression. Our data suggest that the state of maturation of hair cells, as determined by OTOF expression, may not regulate the pruning of their afferent innervation.

Characterization of a unique cell population marked by transgene expression in the adult cochlea of nestin-CreER(T2)/tdTomato-reporter mice

Hair cells in the adult mammalian cochlea cannot spontaneously regenerate after damage, resulting in the permanency of hearing loss. Stem cells have been found to be present in the cochlea of young rodents; however, there has been little evidence for their existence into adulthood. We used nestin-CreER(T2)/tdTomato-reporter mice to trace the lineage of putative nestin-expressing cells and their progeny in the cochleae of adult mice. Nestin, an intermediate filament found in neural progenitor cells during early development and adulthood, is regarded as a multipotent and neural stem cell marker. Other investigators have reported its presence in postnatal and young adult rodents; however, there are discrepancies among these reports. Using lineage tracing, we documented a robust population of tdTomato-expressing cells and evaluated these cells at a series of adult time points. Upon activation of the nestin promoter, tdTomato was observed just below and medial to the inner hair cell layer. All cells colocalized with the stem cell and cochlear-supporting-cell marker Sox2 as well as the supporting cell and Schwann cell marker Sox10; however, they did not colocalize with the Schwann cell marker Krox20, spiral ganglion marker NF200, nor glial fibrillary acidic acid (GFAP)-expressing supporting cell marker. The cellular identity of this unique population of tdTomato-expressing cells in the adult cochlea of nestin-CreER(T2)/tdTomato mice remains unclear; however, these cells may represent a type of supporting cell on the neural aspect of the inner hair cell layer.

Role of thyroid hormones in different aspects of nervous system regeneration in vertebrates

Spontaneous functional recovery from injury in the adult human nervous system is rare and trying to improve recovery remains a clinical challenge. Nervous system regeneration is a complicated sequence of events involving cell death or survival, cell proliferation, axon extension and remyelination, and finally reinnervation and functional recovery. Successful recovery depends on the cell-specific and time-dependent activation and repression of a wide variety of growth factors and guidance molecules. Thyroid hormones (THs), well known for their regulatory role in neurodevelopment, have recently emerged as important modulators of neuroregeneration. This review focuses on the endogenous changes in the proteins regulating TH availability and action in different cell types of the adult mammalian nervous system during regeneration as well as the impact of TH supplementation on the consecutive steps in this process. It also addresses possible differences in TH involvement between different vertebrate classes, early or late developmental stages and peripheral or central nervous system. The available data show that THs are able to stimulate many signaling pathways necessary for successful neurogeneration. They however also suggest that supplementation with T4 and/or T3 may have beneficial or detrimental influences depending on the dose and more importantly on the specific phase of the regeneration process.

Deciphering direct and indirect influence of thyroid hormone with mouse genetics

T3, the active form of thyroid hormone, binds nuclear receptors that regulate the transcription of a large number of genes in many cell types. Unraveling the direct and indirect effect of this hormonal stimulation, and establishing links between these molecular events and the developmental and physiological functions of the hormone, is a major challenge. New mouse genetics tools, notably those based on Cre/loxP technology, are suitable to perform a multiscale analysis of T3 signaling and achieve this task.

Mild iron deficiency anaemia during pregnancy and lactation in guinea pigs alters amplitudes and auditory nerve velocity, but not brainstem transmission times in the offspring's auditory brainstem response

OBJECTIVES

It is well known that postnatal/early childhood iron deficiency (ID) anaemia (IDA) adversely affects infants' cognitive development and neurophysiology. However, the effects of IDA during gestation and lactation on the offspring are largely unknown. To address this health issue, the impact of mild IDA during gestation and lactation on the offsprings' neural maturation was studied in the guinea pig, using auditory brainstem responses (ABRs) latencies and amplitudes.

METHODS

Female guinea pigs (n = 10/group) were fed an iron sufficient (ISD) or deficient diet (IDD) (144 and 11.7 mg iron/kg) during the gestation and lactation periods. From postnatal day (PNd) 9 onward, the ISD was given to both groups of weaned offspring. The offsprings' ABRs were collected on PNd24 using a broad range of stimulus intensities in response to 2, 4, 8, 16, and 32 kHz tone pips.

RESULTS

Although the IDA siblings (n = 8) did not differ in brainstem transmission times (BTTs) compared to the IS siblings (n = 8), they showed significant delayed peak I latency at 100 and 80 dB, respectively. Additionally, significantly higher ABR wave amplitudes were observed in the IDA female offspring between 35 and 50 dB (4 kHz), a phenomenon suggestive of a neural hyperactivity (hyperacusis).

DISCUSSION

In support to our previous findings, the present results indicate that a mild IDA during gestation and lactation can have detrimental effects on early development of the offsprings' hearing and nervous systems, particularly on neural synchrony and auditory nerve conduction velocity, but not on BTT.

Making sense with thyroid hormone--the role of T(3) in auditory development

The senses are our window to the world, our interface with the habitat in which we live in and the basis for our communication with each other. Although sensory systems are not generally viewed as major targets of endocrine regulation, sensory development is profoundly influenced by thyroid hormone (T(3)) signalling. In this article, we discuss this developmental role of T(3) and highlight the auditory system as the best-studied example of the interplay between systemic and local tissue mechanisms by which T(3) stimulates the onset of sensory function. Several genes that mediate the action of T(3) are known to promote sensory development in mice, including genes that encode T(3) receptors and deiodinase enzymes that amplify or deplete levels of T(3). We also discuss the current knowledge of sensory defects in human genetic disorders in which T(3) signalling is impaired. As sensory input provides the only means of acquiring information from the environment, the stimulation of sensory development is one of the most fundamental functions of T(3) signalling.

Generation of somatic electromechanical force by outer hair cells may be influenced by prestin-CASK interaction at the basal junction with the Deiter's cell

The motor protein, prestin, situated in the basolateral plasma membrane of cochlear outer hair cells (OHCs), underlies the generation of somatic, voltage-driven mechanical force, the basis for the exquisite sensitivity, frequency selectivity and dynamic range of mammalian hearing. The molecular and structural basis of the ontogenetic development of this electromechanical force has remained elusive. The present study demonstrates that this force is significantly reduced when the immature subcellular distribution of prestin found along the entire plasma membrane persists into maturity, as has been described in previous studies under hypothyroidism. This observation suggests that cochlear amplification is critically dependent on the surface expression and distribution of prestin. Searching for proteins involved in organizing the subcellular localization of prestin to the basolateral plasma membrane, we identified cochlear expression of a novel truncated prestin splice isoform named prestin 9b (Slc26A5d) that contains a putative PDZ domain-binding motif. Using prestin 9b as the bait in a yeast two-hybrid assay, we identified a calcium/calmodulin-dependent serine protein kinase (CASK) as an interaction partner of prestin. Co-immunoprecipitation assays showed that CASK and prestin 9b can interact with full-length prestin. CASK was co-localized with prestin in a membrane domain where prestin-expressing OHC membrane abuts prestin-free OHC membrane, but was absent from this area for thyroid hormone deficiency. These findings suggest that CASK and the truncated prestin splice isoform contribute to confinement of prestin to the basolateral region of the plasma membrane. By means of such an interaction, the basal junction region between the OHC and its Deiter's cell may contribute to efficient generation of somatic electromechanical force.

Irreversible damage to auditory system functions caused by perinatal hypothyroidism in rats

We examined the effect of perinatal hypothyroidism on auditory function in rats using a prepulse inhibition paradigm. Pregnant rats were treated with the antithyroid drug methimazole (1-methyl-2-mercaptoimidazole) from gestational day 15 to postnatal day 21 via drinking water at concentrations (w/v) of 0 (control), 0.002 (low dose), or 0.02% (high dose). Rats from methimazole-treated mothers were tested at ages 1, 6, and 12months using techniques to examine prepulse inhibition and startle response. The startle stimulus consisted of 40ms of white noise at 115dB, whereas the prepulse, which preceded the startle stimulus by 30ms, consisted of 20ms of white noise at 75, 85, or 95dB. When the prepulse intensity was 75 or 85dB, the high-dose group showed decreased prepulse inhibition percentages compared with the control and low-dose groups. The reduced percentages of prepulse inhibition did not return to control levels over the 12-month study period. In contrast, no differences in prepulse inhibition were observed among the three dose groups when prepulse intensity was 95dB. Moreover, the high-dose group displayed excessive reaction to auditory startle stimuli compared with the other groups. Reductions in plasma free thyroxine and body weight gain were observed in the high-dose group. We conclude that perinatal hypothyroidism results in irreversible damage to auditory function in rats.

Critical role for cochlear hair cell BK channels for coding the temporal structure and dynamic range of auditory information for central auditory processing

Large conductance, voltage- and Ca²⁺-activated K⁺ (BK) channels in inner hair cells (IHCs) of the cochlea are essential for hearing. However, germline deletion of BKα, the pore-forming subunit KCNMA1 of the BK channel, surprisingly did not affect hearing thresholds in the first postnatal weeks, even though altered IHC membrane time constants, decreased IHC receptor potential alternating current/direct current ratio, and impaired spike timing of auditory fibers were reported in these mice. To investigate the role of IHC BK channels for central auditory processing, we generated a conditional mouse model with hair cell-specific deletion of BKα from postnatal day 10 onward. This had an unexpected effect on temporal coding in the central auditory system: neuronal single and multiunit responses in the inferior colliculus showed higher excitability and greater precision of temporal coding that may be linked to the improved discrimination of temporally modulated sounds observed in behavioral training. The higher precision of temporal coding, however, was restricted to slower modulations of sound and reduced stimulus-driven activity. This suggests a diminished dynamic range of stimulus coding that is expected to impair signal detection in noise. Thus, BK channels in IHCs are crucial for central coding of the temporal fine structure of sound and for detection of signals in a noisy environment.—Kurt, S., Sausbier, M., Rüttiger, L., Brandt, N., Moeller, C. K., Kindler, J., Sausbier, U., Zimmermann, U., van Straaten, H., Neuhuber, W., Engel, J., Knipper, M., Ruth, P., Schulze, H. Critical role for cochlear hair cell BK channels for coding the temporal structure and dynamic range of auditory information for central auditory processing.

Age-related changes of myelin basic protein in mouse and human auditory nerve

Age-related hearing loss (presbyacusis) is the most common type of hearing impairment. One of the most consistent pathological changes seen in presbyacusis is the loss of spiral ganglion neurons (SGNs). Defining the cellular and molecular basis of SGN degeneration in the human inner ear is critical to gaining a better understanding of the pathophysiology of presbyacusis. However, information on age-related cellular and molecular alterations in the human spiral ganglion remains scant, owing to the very limited availably of human specimens suitable for high resolution morphological and molecular analysis. This study aimed at defining age-related alterations in the auditory nerve in human temporal bones and determining if immunostaining for myelin basic protein (MBP) can be used as an alternative approach to electron microscopy for evaluating myelin degeneration. For comparative purposes, we evaluated ultrastructural alternations and changes in MBP immunostaining in aging CBA/CaJ mice. We then examined 13 temporal bones from 10 human donors, including 4 adults aged 38-46 years (middle-aged group) and 6 adults aged 63-91 years (older group). Similar to the mouse, intense immunostaining of MBP was present throughout the auditory nerve of the middle-aged human donors. Significant declines in MBP immunoreactivity and losses of MBP(+) auditory nerve fibers were observed in the spiral ganglia of both the older human and aged mouse ears. This study demonstrates that immunostaining for MBP in combination with confocal microscopy provides a sensitive, reliable, and efficient method for assessing alterations of myelin sheaths in the auditory nerve. The results also suggest that myelin degeneration may play a critical role in the SGN loss and the subsequent decline of the auditory nerve function in presbyacusis.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Evaluation of hearing loss in patients with Graves' disease

Hearing loss has commonly been reported in association with thyroid disorders and during treatment with propylthiouracil. The relationship between hyperthyroidism and the auditory system has not been previously investigated. The aim of this cross-sectional, case-control study was to investigate hearing loss in patients with Graves' disease (GD). The study population consisted of patients with newly diagnosed GD and healthy controls. Pure tone audiometry at frequencies of 250, 500, 1000, 2000, 4000 and 8000 Hz, along with immittance measures including tympanometry and acoustic reflex tests, were performed in all participants. Twenty-two GD patients and 22 healthy controls consented to inclusion in the study. The differences between groups with regards to age and gender distribution were statistically insignificant (P = 0.567 and P = 0.757, respectively). The hearing thresholds of right and left ears were also similar in both groups (P > 0.05). When single-ear evaluations were taken into account (total of 44 ears for both groups), hearing thresholds in the GD group were significantly higher than healthy controls at all frequencies (P < 0.05). Following testing at the designated frequencies, the only significant effect of thyrotoxicosis was observed with frequencies of 4000 and 8000 Hz. The odds ratio for having hearing loss at a frequency of 8000 HZ associated with GD was 14.97 (95% confidence interval 4.03-55.64). In patients with GD, right and left pure tone audiometric findings at a frequency of 8000 Hz correlated positively with FT3, FT4 and negatively with TSH. Our results are highly suggestive of a decrease in hearing ability in patients with GD, particularly at high frequencies. Further studies are needed to help elucidate the mechanisms behind hearing loss which develops in association with GD.

Developmental and cell-specific expression of thyroid hormone transporters in the mouse cochlea

Thyroid hormone is essential for the development of the cochlea and auditory function. Cochlear response tissues, which express thyroid hormone receptor β (encoded by Thrb), include the greater epithelial ridge and sensory epithelium residing inside the bony labyrinth. However, these response tissues lack direct blood flow, implying that mechanisms exist to shuttle hormone from the circulation to target tissues. Therefore, we investigated expression of candidate thyroid hormone transporters L-type amino acid transporter 1 (Lat1), monocarboxylate transporter (Mct)8, Mct10, and organic anion transporting polypeptide 1c1 (Oatp1c1) in mouse cochlear development by in situ hybridization and immunofluorescence analysis. L-type amino acid transporter 1 localized to cochlear blood vessels and transiently to sensory hair cells. Mct8 localized to the greater epithelial ridge, tympanic border cells underlying the sensory epithelium, spiral ligament fibrocytes, and spiral ganglion neurons, partly overlapping with the Thrb expression pattern. Mct10 was detected in a highly restricted pattern in the outer sulcus epithelium and weakly in tympanic border cells and hair cells. Organic anion transporting polypeptide 1c1 localized primarily to fibrocytes in vascularized tissues of the spiral limbus and spiral ligament and to tympanic border cells. Investigation of hypothyroid Tshr(-/-) mice showed that transporter expression was delayed consistent with retardation of cochlear tissue maturation but not with compensatory responses to hypothyroidism. The results demonstrate specific expression of thyroid hormone transporters in the cochlea and suggest that a network of thyroid hormone transport underlies cochlear development.

Altered expression of securin (Pttg1) and serpina3n in the auditory system of hearing-impaired Tff3-deficient mice

INTRODUCTION

Tff3 peptide exerts important functions in cytoprotection and restitution of the gastrointestinal (GI) tract epithelia. Moreover, its presence in the rodent inner ear and involvement in the hearing process was demonstrated recently. However, its role in the auditory system still remains elusive. Our previous results showed a deterioration of hearing with age in Tff3-deficient animals.

RESULTS

Present detailed analysis of auditory brain stem response (ABR) measurements and immunohistochemical study of selected functional proteins indicated a normal function and phenotype of the cochlea in Tff3 mutants. However, a microarray-based screening of tissue derived from the auditory central nervous system revealed an alteration of securin (Pttg1) and serpina3n expression between wild-type and Tff3 knock-out animals. This was confirmed by qRT-PCR, immunostaining and western blots.

CONCLUSIONS

We found highly down-regulated Pttg1 and up-regulated serpina3n expression as a consequence of genetically deleting Tff3 in mice, indicating a potential role of these factors during the development of presbyacusis.

Non-equilibrium hyperbolic transport in transcriptional regulation

In this work we studied memory and irreversible transport phenomena in a non-equilibrium thermodynamical model for genomic transcriptional regulation. Transcriptional regulation possess an extremely complex phenomenology, and it is, of course, of foremost importance in organismal cell development and in the pathogenesis of complex diseases. A better understanding of the way in which these processes occur is mandatory to optimize the construction of gene regulatory networks, but also to connect these networks with multi-scale phenomena (e.g. metabolism, signalling pathways, etc.) under an integrative Systems Biology-like vision. In this paper we analyzed three simple mechanisms of genetic stimulation: an instant pulse, a periodic biochemical signal and a saturation process with sigmoidal kinetics and from these we derived the system's thermodynamical response, in the form of, for example, anomalous transcriptional bursts.

Thyroid hormone action in cerebellum and cerebral cortex development

Thyroid hormones (TH, including the prohormone thyroxine (T4) and its active deiodinated derivative 3,3′,5-triiodo-L-thyronine (T3)) are important regulators of vertebrates neurodevelopment. Specific transporters and deiodinases are required to ensure T3 access to the developing brain. T3 activates a number of differentiation processes in neuronal and glial cell types by binding to nuclear receptors, acting directly on transcription. Only few T3 target genes are currently known. Deeper investigations are urgently needed, considering that some chemicals present in food are believed to interfere with T3 signaling with putative neurotoxic consequences.

Identifying a window of vulnerability during fetal development in a maternal iron restriction model

It is well acknowledged from observations in humans that iron deficiency during pregnancy can be associated with a number of developmental problems in the newborn and developing child. Due to the obvious limitations of human studies, the stage during gestation at which maternal iron deficiency causes an apparent impairment in the offspring remains elusive. In order to begin to understand the time window(s) during pregnancy that is/are especially susceptible to suboptimal iron levels, which may result in negative effects on the development of the fetus, we developed a rat model in which we were able to manipulate and monitor the dietary iron intake during specific stages of pregnancy and analyzed the developing fetuses. We established four different dietary-feeding protocols that were designed to render the fetuses iron deficient at different gestational stages. Based on a functional analysis that employed Auditory Brainstem Response measurements, we found that maternal iron restriction initiated prior to conception and during the first trimester were associated with profound changes in the developing fetus compared to iron restriction initiated later in pregnancy. We also showed that the presence of iron deficiency anemia, low body weight, and changes in core body temperature were not defining factors in the establishment of neural impairment in the rodent offspring.Our data may have significant relevance for understanding the impact of suboptimal iron levels during pregnancy not only on the mother but also on the developing fetus and hence might lead to a more informed timing of iron supplementation during pregnancy.

A DNA variant within the MYO7A promoter regulates YY1 transcription factor binding and gene expression serving as a potential dominant DFNA11 auditory genetic modifier

Mutations within MYO7A can lead to recessive and dominant forms of inherited hearing loss. We previously identified a large pedigree (referred to as the HL2 family) with hearing loss that first impacts the low and mid frequencies segregating a dominant MYO7A mutation in exon 17 at DNA residue G2164C. The MYO7A(G2164C) mutation predicts a nonconservative glycine-to-arginine (G722R) amino acid substitution at a highly conserved glycine residue. The degree of low and mid frequency hearing loss varies markedly in the family, suggesting the presence of a genetic modifier that either rescues or exacerbates the primary MYO7A(G2164C) mutation. Here we describe a single nucleotide polymorphism (SNP) T/C at position -4128 in the wild-type MYO7A promoter allele that sorts with the degree of hearing loss severity in the pedigree. Electrophoretic mobility shift assay analysis indicates that the SNP differentially regulates the binding of the YY1 transcription factor with the T(-4128) allele creating an YY1 binding site. Immunocytochemistry demonstrates that Yy1 is expressed in hair cell nuclei within the cochlea. Given that Myo7a is also expressed in cochlear hair cells, Yy1 shows the appropriate localization to regulate Myo7a transcription within the inner ear. YY1 appears to be acting as a transcriptional repressor as the MYO7A promoter allele containing the T(-4128) SNP drives 41 and 46% less reporter gene expression compared with the C(-4128) SNP in the ARPE-19 and HeLa cell lines, respectively. The T(-4128) SNP may be contributing to the severe hearing loss phenotype in the HL2 pedigree by reducing expression of the wild-type MYO7A allele.

Receptors for leptin in the otic labyrinth and the cochlear-vestibular nerve of guinea pig are modified in hormone-induced anorexia

Metabolic syndromic inner ear pathology is a recognized condition in clinical practice but the possible causes remain controversial. We have previously reported that chronically-implanted estrogen implants in guinea pig results in hyperprolactinemia and hearing loss together with otic bone dysmorphology. The animals also present with anorexia. The hormone leptin has major roles in the regulation of satiety as well as bone metabolism and so we hypothesized that leptin might contribute to pathology of the otic labyrinth. We employed immunohistochemistry to investigate leptin receptor (ObR) expression. In control animals, ObR immunolabeling was not detected in the bone of the otic capsule but immunolabeling was observed in the cochlear-vestibular nerve. The labeling was associated with the astrocytic glial dome area, which marks the transition between central and peripheral parts of the nerve. In estrogen-treated animals, positive-ObR immunolabeling was observed in osteoblasts in new bone of the otic capsule and the ObR labeling was reduced in the cochlear-vestibular nerve compared to controls. The data provide evidence that leptin may target the labyrinth - affecting the bone and the nerve - and so could contribute to ongoing protection of the inner ear. Leptin disturbance might contribute to metabolic syndromes involving the audiovestibular system.

Thyroxine treatments do not correct inner ear defects in tmprss1 mutant mice

Complete deficiency of a member of the type II transmembrane serine protease family, tmprss1 (also known as hepsin), is associated with severe to profound hearing loss in mice and a gross enlargement of the tectorial membrane in the cochlea. Levels of thyroxine in these mice have been shown to be significantly lower when compared with wild-type controls. As thyroxine is critical for inner ear development, we delivered thyroxine to these mice during the prenatal or postnatal stage of development. Both the treatments could not ameliorate hearing loss or correct deformities in the tectorial membrane of these mutant mice, suggesting that a deficiency in tmprss1 affects thyroxine responsiveness in the inner ear in vivo.

Otoferlin interacts with myosin VI: implications for maintenance of the basolateral synaptic structure of the inner hair cell

Otoferlin has been proposed to be the Ca(2+) sensor in hair cell exocytosis, compensating for the classical synaptic fusion proteins synaptotagmin-1 and synaptotagmin-2. In the present study, yeast two-hybrid assays reveal myosin VI as a novel otoferlin binding partner. Co-immunoprecipitation assay and co-expression suggest an interaction of both proteins within the basolateral part of inner hair cells (IHCs). Comparison of otoferlin mutants and myosin VI mutant mice indicates non-complementary and complementary roles of myosin VI and otoferlin for synaptic maturation: (i) IHCs from otoferlin mutant mice exhibited a decoupling of CtBP2/RIBEYE and Ca(V)1.3 and severe reduction of exocytosis. (ii) Myosin VI mutant IHCs failed to transport BK channels to the membrane of the apical cell regions, and the exocytotic Ca(2+) efficiency did not mature. (iii) Otoferlin and myosin VI mutant IHCs showed a reduced basolateral synaptic surface area and altered active zone topography. Membrane infoldings in otoferlin mutant IHCs indicated disturbed transport of endocytotic membranes and link the above morphological changes to a complementary role of otoferlin and myosin VI in transport of intracellular compartments to the basolateral IHC membrane.

Multigenic control of thyroid hormone functions in the nervous system

Thyroid hormone (TH) has a remarkable range of actions in the development and function of the nervous system. A multigenic picture is emerging of the mechanisms that specify these diverse functions in target tissues. Distinct responses are mediated by alpha and beta isoforms of TH receptor which act as ligand-regulated transcription factors. Receptor activity can be regulated at several levels including that of uptake of TH ligand and the activation or inactivation of ligand by deiodinase enzymes in target tissues. Processes under the control of TH range from learning and anxiety-like behaviour to sensory function. At the cellular level, TH controls events as diverse as axonal outgrowth, hippocampal synaptic activity and the patterning of opsin photopigments necessary for colour vision. Overall, TH coordinates this variety of events in both central and sensory systems to promote the function of the nervous system as a complete entity.

Lack of Tff3 peptide results in hearing impairment and accelerated presbyacusis

Tff peptides are secreted mainly by the gastrointestinal epithelial cells and their primary role is maintaining normal structure and function of mucous epithelia. Ongoing studies on their expression pattern have disclosed other sites of their synthesis thus revealing additional physiological functions in the organism. Here we present new data about Tff3 expression in the cochlea of the rodent inner ear. On the basis of RT-PCR we describe the presence of Tff3 transcripts in both, a mouse cDNA library isolated from whole cochleae from postnatal days 3-15 (P3-P15), and also in cochlear tissue. By using a riboprobe for the fragment containing exon 1, 2 and 3 of Tff3, in situ hybridization, localized Tff3 signals in neurons of spiral ganglion and vestibular organ. We did not observe any abnormalities in the middle ear of Tff3 knock-out mice, neither did histological examination of the inner ear indicate any gross morphological changes in the cochlea. However, ABR (auditory evoked brain stem responses) audiograms revealed that the Tff3 knock-out animals show an accelerated presbyacusis and a hearing loss of about 15 dB at low frequencies increasing to 25 dB loss at higher frequencies. These findings suggest that Tff3 could play a role in neurosensory signaling. Further studies are needed to clarify this new function in the auditory system.

New insights into peripherin expression in cochlear neurons

Peripherin is an intermediate filament protein that is expressed in peripheral and enteric neurons. In the cochlear nervous system, peripherin expression has been extensively used as a differentiation marker by preferentially labeling the type II neuronal population at adulthood, but yet without knowing its function. Since the expression of peripherin has been associated in time with the process of axonal extension and during regeneration of nerve fibers in other systems, it was of interest to determine whether peripherin expression in cochlear neurons was a static phenotypic trait or rather prone to modifications following nerve injury. In the present study, we first compared the expression pattern of peripherin and beta III-tubulin from late embryonic stages to the adult in rat cochlea. The staining for both proteins was seen before birth within all cochlear neurons. By birth, and for 2 or 3 days, peripherin expression was gradually restricted to the type II neuronal population and their projections. In contrast, from postnatal day (P) 10 onwards, while the expression of beta III-tubulin was still found in projections of all cochlear neurons, only the type I population had beta III-tubulin immunoreactivity in their cell bodies. We next investigated the expression of peripherin in axotomized cochlear neurons using an organotypic explant model. Peripherin expression was surprisingly re-expressed in a vast majority of neurons after axotomy. In parallel, the expression and localization of beta III-tubulin and peripherin in dissociated cultures of cochlear neurons were studied. Both proteins were distributed along the entire neuronal length but exhibited complementary distribution, especially within the projections. Moreover, peripherin immunoreactivity was still abundant in the growth cone, whereas that of beta III-tubulin was decreasing at this compartment. Our findings are consistent with a model in which peripherin plays an important structural role in cochlear neurons and their projections during both development and regenerative processes and which is compatible with the assumption that frequently developmentally regulated factors are reactivated during neuronal regeneration.

Schwann cells revert to non-myelinating phenotypes in the deafened rat cochlea

Loss of sensory hair cells within the cochlea results in a permanent sensorineural hearing loss and initiates the gradual degeneration of spiral ganglion neurons (SGNs) - the primary afferent neurons of the cochlea. While these neurons are normally myelinated via Schwann cells, loss of myelin occurs as a precursor to neural degeneration. However, the relationship between demyelination and the status of Schwann cells in deafness is not well understood. We used a marker of peripheral myelin (myelin protein zero; P0) and a marker of Schwann cells (S100) to determine the temporal sequence of myelin and Schwann cell loss as a function of duration of deafness. Rat pups were systemically deafened for periods ranging from 2 weeks to greater than 6 months by co-administration of frusemide and gentamicin. Cochleae were cryosectioned and quantitative immunohistochemistry used to determine the extent of P0 and S100 labelling within the peripheral processes, SGN soma and their central processes within the modiolus. SGN density was also determined for each cochlear turn. P0 labelling decreased throughout the cochlea with increasing duration of deafness. The reduction in P0 labelling occurred at a faster rate than the SGN loss. In contrast, S100 labelling was not significantly reduced compared with age-matched controls in any cochlear region until 6 months post-deafening. These results suggest that Schwann cells may revert to non-myelinating phenotypes in response to deafness and exhibit greater survival traits than SGNs. The potential clinical significance of these findings for cochlear implants is discussed.

OPA1, the disease gene for optic atrophy type Kjer, is expressed in the inner ear

Autosomal dominant optic atrophy (adOA) is the most common form of hereditary optic neuropathy. The majority of cases are associated with mutations in the OPA1 gene. A few cases of adOA are known to be associated with moderate progressive hearing loss. To gain insight into the pathogenesis of this hearing loss, we performed expression analyses of OPA1 in the rat auditory and vestibular organ. In cochlear tissue, several splice variants of OPA1 were detected, which are also expressed in retinal tissue. OPA1 mRNA and protein was found in the hair cells and ganglion cells of the cochlea and vestibular organ. In ganglion cells, OPA1 mRNA and protein was already detectable at birth, whereas in the organ of Corti OPA1 mRNA and protein was up-regulated after birth and reached mature-like expression level during the onset of hearing. Comparison of an antibody directed to the mitochondrial marker protein HSP60 with antibodies directed to different amino acid stretches of OPA1 revealed a sub-cellular distribution of OPA1 in areas of significant density of mitochondria. The data suggest that defects in OPA1 cause hearing disorders due to a progressing metabolic disturbance of hair and ganglion cells in the inner ear.

Mice deficient for the type II transmembrane serine protease, TMPRSS1/hepsin, exhibit profound hearing loss

Defective proteolysis has been implicated in hearing loss through the discovery of mutations causing autosomal recessive nonsyndromic deafness in a type II transmembrane serine protease gene, TMPRSS3. To investigate their physiological function and the contribution of this family of proteases to the auditory function, we analyzed the hearing status of mice deficient for hepsin, also known as TMPRSS1. These mice exhibited profound hearing loss with elevated hearing thresholds compared with their heterozygous and wild-type littermates. Their cochleae showed abnormal tectorial membrane development, reduction in fiber compaction in the peripheral portion of the auditory nerve, and decreased expression of the myelin proteins myelin basic protein and myelin protein zero. In addition, reduced level of the large conductance voltage- and Ca(2+)-activated K(+) channel was detected in the sensory hair cells of Tmprss1-null mice. We examined thyroid hormone levels in Tmprss1-deficient mice, as similar cochlear defects have been reported in animal models of hypothyroidism, and found significantly reduced free thyroxine levels. These data show that TMPRSS1 is required for normal auditory function. Hearing impairment present in Tmprss1-null mice is characterized by a combination of various structural, cellular, and molecular abnormalities that are likely to affect different cochlear processes.

Thyroid hormone receptor alpha1 is a critical regulator for the expression of ion channels during final differentiation of outer hair cells

Cochlear outer hair cells (OHCs) terminally differentiate prior to the onset of hearing. During this time period, thyroid hormone (TH) dramatically influences inner ear development. It has been shown recently that TH enhances the expression of the motor protein prestin via liganded TH receptor beta (TRbeta) while in contrast the expression of the potassium channel KCNQ4 is repressed by unliganded TRalpha1. These different mechanisms of TH regulation by TRalpha1 or TRbeta prompted us to analyse other ion channels that are required for the final differentiation of OHCs. We analysed the onset of expression of the Ca(2+) channel Ca(V)1.3, and the K(+) channels SK2 and BK and correlated the results with the regulation via TRalpha1 or TRbeta. The data support the hypothesis that proteins expressed in rodents prior to or briefly after birth like Ca(V)1.3 and prestin are either independent of TH (e.g. Ca(V)1.3) or enhanced through TRbeta (e.g. prestin). In contrast, proteins expressed in rodents later than P6 like KCNQ4 ( approximately P6), SK2 ( approximately P9) and BK ( approximately P11) are repressed through TRalpha1. We hypothesise that the precise regulation of expression of the latter genes requires a critical local TH level to overcome the TRalpha1 repression.

Differential expression of otoferlin in brain, vestibular system, immature and mature cochlea of the rat

Mutations of the human otoferlin gene lead to an autosomal recessive nonsyndromic form of prelingual, sensorineural deafness (deafness autosomal recessive 9, DFNB9). Several studies have demonstrated expression of otoferlin in the inner ear and brain, and suggested a role of otoferlin in Ca(2+)-triggered exocytosis. So far, otoferlin expression profiles were solely based on the detection of mRNA. Here, we analysed the expression of otoferlin protein and mRNA using immunohistochemistry, in situ hybridization and RT-PCR in neonatal and mature Wistar rat tissue. In agreement with previous studies, otoferlin expression was found in the brain and in inner and vestibular hair cells. Otoferlin mRNA and protein was, however, also detected in mature outer hair cells of low-frequency processing cochlear turns and in auditory nerve fibres. In outer, inner and vestibular hair cells, otoferlin was subcellularly localized at a considerable distance from the presumed active release sites. Double-staining with the synaptic ribbon marker, C-terminal binding protein 2 (CtBP2), or the presynaptic Ca(2+)-channel, Ca(v)1.3, both assumed to mark the sites of vesicle fusion and transmitter release, did not colocalize with otoferlin expression and thus do not necessarily support a selected role of otoferlin in Ca(2+)-triggered exocytosis. The widespread distribution of otoferlin in neurons, nerve fibres and hair cells, and its subcellular distribution extending beyond the regions of synaptic vesicle fusion, i.e. coenrichment with the cytosolic Golgi matrix protein 130 (GM130) in inner hair cells or the early endosomal autoantigen 1 (EEA1) in outer hair cells support instead the idea of a more ubiquitous role of otoferlin in early/recycling endosome trans-Golgi network dynamics.

Expression of enzymes involved in thyroid hormone metabolism during the early development of Xenopus tropicalis

BACKGROUND INFORMATION

There are significant indications that amphibians require TH (thyroid hormones) prior to their involvement in the regulation of metamorphosis and before the development of a functional thyroid.

RESULTS

In order to investigate the potential role for TH in pre-metamorphic Xenopus tropicalis we have cloned cDNAs for, and analysed the expression of, TPO (thyroid peroxidase), 5'DII (type II iodothyronine deiodinase) and 5DIII (type III iodothyronine deiodinase), enzymes involved in TH metabolism. Zygotic expression of TPO was detected in neurula stage embryos. Expression was observed in the notochord and later in the thyroid. The notochord was also a common site of expression for 5'DII and 5DIII. Other sites of 5'DII expression are the otic vesicles, retina, liver, blood-forming region, branchial arches and brain. 5DIII is also expressed in the brain, retina, liver, developing pro-nephros, blood-forming region and branchial arches. Embryos exposed to the TPO inhibitor methimazole showed a distinctive dose-dependent phenotype of a crimped notochord and shortened axis, together with alterations in (125)I(-) uptake.

CONCLUSIONS

These data suggest a novel extrathyroidal role for TH during early development, and support the proposal that embryos require thyroid signalling for normal development prior to metamorphosis.

The mystery of intracellular developmental programmes and timers

There has been a revolution in understanding animal development in the last 25 years or so, but there is at least one area of development that has been relatively neglected and therefore remains largely mysterious. This is the intracellular programmes and timers that run in developing precursor cells and change the cells over time. The molecular mechanisms underlying these programmes are largely unknown. My colleagues and I have studied such programmes in two types of rodent neural precursor cells: those that give rise to oligodendrocytes, which make myelin in the CNS (central nervous system), and those that give rise to the various cell types in the retina.

Intracellular developmental timers

One of the most poorly understood aspects of animal development is how the timing of developmental events is controlled. In most vertebrate cell lineages, for example, precursor cells divide a limited number of times before they stop and terminally differentiate, but it is not known what controls when the cells stop dividing and differentiate. There is increasing evidence, however, that intracellular timers play an important part. Such cell-intrinsic timers are examples of intracellular developmental programs that change precursor cells over time. My colleagues and I have studied such intracellular timers and programs in rodent oligodendrocyte precursor cells (OPCs), as reviewed here.

Two classes of outer hair cells along the tonotopic axis of the cochlea

The molecular basis of high versus low frequency hearing loss and the differences in the sensitivity of outer hair cells depending on their cochlear localization are currently not understood. Here we demonstrate the existence of two different outer hair cell phenotypes along the cochlear axis. Outer hair cells in low frequency regions exhibit early sensitivity for loss of Ca(v)1.3 (alpha1 subunit 1.3 forming the class D L-type voltage-gated Ca(2+) channel), while high frequency regions display a progressive susceptibility for loss of the Ca(2+)-activated large conductance K(+) (BK) channel. Despite deafness, young Ca(v)1.3-deficient mice displayed distortion-product otoacoustic emissions (DPOAEs), indicating functional outer hair cells in the higher frequency range of the cochlea. Considering that DPOAEs are also found in the human deafness syndrome DFNB9 caused by mutations in the synaptic vesicle protein otoferlin, we tested the expression of otoferlin in outer hair cells. Surprisingly, otoferlin showed a distinct tonotopic expression pattern at both the mRNA and protein level. Otoferlin-expressing, Ca(v)1.3 deletion-sensitive outer hair cells in the low frequency range could be clearly separated from otoferlin-negative, BK deletion-sensitive outer hair cells in the high frequency range. In addition, BK deletion led to a higher noise vulnerability in low frequency regions, which are normally unaffected by the BK deletion alone, suggesting that BK currents are involved in survival mechanisms of outer hair cells under noise conditions. Our findings propose new mechanisms and candidate genes for explaining high and low frequency hearing loss.

A disorganized innervation of the inner ear persists in the absence of ErbB2

ErbB2 protein is essential for the development of Schwann cells and for the normal fiber growth and myelin formation of peripheral nerves. We have investigated the fate of the otocyst-derived inner ear sensory neurons in the absence of ErbB2 using ErbB2 null mutants. Afferent innervation of the ear sensory epithelia shows numerous fibers overshooting the organ of Corti, followed by a reduction of those fibers in near term embryos. This suggests that mature Schwann cells do not play a role in targeting or maintaining the inner ear innervation. Comparable to the overshooting of nerve fibers, sensory neurons migrate beyond their normal locations into unusual positions in the modiolus. They may miss a stop signal provided by the Schwann cells that are absent as revealed with detailed histology. Reduction of overshooting afferents may be enhanced by a reduction of the neurotrophin Ntf3 transcript to about 25% of wild type. Ntf3 transcript reductions are comparable to an adult model that uses a dominant negative form of ErbB4 expressed in the supporting cells and Schwann cells of the organ of Corti. ErbB2 null mice retain afferents to inner hair cells possibly because of the prominent expression of the neurotrophin Bdnf in developing hair cells. Despite the normal presence of Bdnf transcript, afferent fibers are disoriented near the organ of Corti. Efferent fibers do not form an intraganglionic spiral bundle in the absence of spiral ganglia and appear reduced and disorganized. This suggests that either ErbB2 mediated alterations in sensory neurons or the absence of Schwann cells affects efferent fiber growth to the organ of Corti.

Differential expression of genes within the cochlea as defined by a custom mouse inner ear microarray

Microarray analyses have contributed greatly to the rapid understanding of functional genomics through the identification of gene networks as well as gene discovery. To facilitate functional genomics of the inner ear, we have developed a mouse inner-ear-pertinent custom microarray chip (CMA-IE1). Nonredundant cDNA clones were obtained from two cDNA library resources: the RIKEN subtracted inner ear set and the NIH organ of Corti library. At least 2000 cDNAs unique to the inner ear were present on the chip. Comparisons were performed to examine the relative expression levels of these unique cDNAs within the organ of Corti, lateral wall, and spiral ganglion. Total RNA samples were obtained from the three cochlear-dissected fractions from adult CF-1 mice. The total RNA was linearly amplified, and a dendrimer-based system was utilized to enhance the hybridization signal. Differentially expressed genes were verified by comparison to known gene expression patterns in the cochlea or by correlation with genes and gene families deduced to be present in the three tissue types. Approximately 22-25% of the genes on the array had significant levels of expression. A number of differentially expressed genes were detected in each tissue fraction. These included genes with known functional roles, hypothetical genes, and various unknown or uncharacterized genes. Four of the differentially expressed genes found in the organ of Corti are linked to deafness loci. None of these are hypothetical or unknown genes.