Expression of potassium channels in gerbil outer hair cells during development does not require neural induction

Characterization of Hair Cell-Like Cells Converted From Supporting Cells After Notch Inhibition in Cultures of the Organ of Corti From Neonatal Gerbils

The senses of hearing and balance depend upon hair cells, the sensory receptors of the inner ear. Hair cells transduce mechanical stimuli into electrical activity. Loss of hair cells as a result of aging or exposure to noise and ototoxic drugs is the major cause of noncongenital hearing and balance deficits. In the ear of non-mammals, lost hair cells can spontaneously be replaced by production of new hair cells from conversion of supporting cells. Although supporting cells in adult mammals have lost that capability, neonatal supporting cells are able to convert to hair cells after inhibition of Notch signaling. We questioned whether Notch inhibition is sufficient to convert supporting cells to functional hair cells using electrophysiology and electron microscopy. We showed that pharmacological inhibition of the canonical Notch pathway in the cultured organ of Corti prepared from neonatal gerbils induced stereocilia formation in supporting cells (defined as hair cell-like cells or HCLCs) and supernumerary stereocilia in hair cells. The newly emerged stereocilia bundles of HCLCs were functional, i.e., able to respond to mechanical stimulation with mechanotransduction (MET) current. Transmission electron microscopy (TEM) showed that HCLCs converted from pillar cells maintained the pillar cell shape and that subsurface cisternae, normally observed underneath the cytoskeleton in outer hair cells (OHCs), was not present in Deiters’ cells-derived HCLCs. Voltage-clamp recordings showed that whole-cell currents from Deiters’ cells-derived HCLCs retained the same kinetics and magnitude seen in normal Deiters’ cells and that nonlinear capacitance (NLC), an electrical hallmark of OHC electromotility, was not detected from any HCLCs measured. Taken together, these results suggest that while Notch inhibition is sufficient for promoting stereocilia bundle formation, it is insufficient to convert neonatal supporting cells to mature hair cells. The fact that Notch inhibition led to stereocilia formation in supporting cells and supernumerary stereocilia in existing hair cells appears to suggest that Notch signaling may regulate stereocilia formation and stability during development.

Comparison of neurogenesis in the dentate gyrus between the adult and aged gerbil following transient global cerebral ischemia

In the present study, we compared differences in cell proliferation, neuroblast differentiation and neuronal maturation in the hippocampal dentate gyrus (DG) between the adult and aged gerbil induced by 5 min of transient global cerebral ischemia using Ki-67 and BrdU (markers for cell proliferation), doublecortin (DCX, a marker for neuroblast differentiation) and neuronal nuclei (NeuN, a marker for mature neuron). The number of Ki-67-immunoreactive (⁺) cells in the DG of both the groups peaked 7 days after ischemia/reperfusion (I/R). However, the number in the aged DG was 40.6 ± 1.8% of that in the adult DG. Thereafter, the number decreased with time. After ischemic damage, DCX immunoreactivity and its protein level in the adult and aged DG peaked at 10 and 15 days post-ischemia, respectively. However, DCX immunoreactivity and its protein levels in the aged DG were much lower than those in the adult. DCX immunoreactivity and its protein level in the aged DG were 11.1 ± 0.6% and 34.4 ± 2.1% of the adult DG, respectively. In addition, the number of Ki-67⁺ cells and DCX immunoreactivity in both groups were similar to those in the sham at 60 days postischemia. At 30 days post-ischemia, the number of BrdU⁺ cells and BrdU⁺/NeuN⁺ cells in the adult-group were much higher (281.2 ± 23.4% and 126.4 ± 7.4%, respectively) than the aged-group (35.6 ± 6.8% and 79.5 ± 6.1%, respectively). These results suggest that the ability of neurogenesis in the ischemic aged DG is much lower than that in the ischemic adult DG.

Comparison of newly generated doublecortin-immunoreactive neuronal progenitors in the main olfactory bulb among variously aged gerbils

In the present study, we investigated age-related differences in neuronal progenitors in the gerbil main olfactory bulb (MOB) using doublecortin (DCX), a marker for neuronal progenitors which differentiate into neurons in the brain. No difference in the number of neuronal nuclei (NeuN)-immunoreactive neurons was found in the MOB at variously aged gerbils. At postnatal month (PM) 1, DCX immunoreaction was detected in all layers of the MOB except for the olfactory nerve layer. At this time point, DCX-immunoreactive cells (neuronal progenitors) were very abundant; however, they did not have fully developed-processes. From PM 3, the number of DCX-immunoreactive neuronal progenitors was decreased with age. At PM 6, DCX-immunoreactive cells showed very well-developed processes. In western blot analysis, DCX protein level in the MOB was highest at PM 1. Thereafter, levels of DCX protein were decreased with age. In the subventricular zone of the lateral ventricle, the number of Ki-67-immunoractive cells (proliferating cells) was also significantly decreased with age. In addition, increases of α-synuclein-immunoreactive structures were observed in the MOB with age. These results suggest that decrease in DCX-immunoreactive neuronal progenitors and its protein levels in the MOB with age may be associated with reduction of cell proliferation in the SVZ and with an increase in α-synuclein in the MOB.

Age-related differentiation in newly generated DCX immunoreactive neurons in the subgranular zone of the gerbil dentate gyrus

In the present study, we investigated age-related changes of newborn neurons in the gerbil dentate gyrus using doublecortin (DCX), a marker of neuronal progenitors which differentiate into neurons in the brain. In the postnatal month 1 (PM 1) group, DCX immunoreactivity was detected in the subgranular zone of the dentate gyrus, but DCX immunoreactive neurons did not have fully developed processes. Thereafter, DCX immunoreactivity and its protein levels in the dentate gyrus were found to decrease with age. Between PM 3 and PM 18, DCX immunoreactive neuronal progenitors showed well-developed processes which projected to the granular layer of the dentate gyrus, but at PM 24, a few DCX immunoreactive neuronal progenitors were detected in the subgranular zone of the dentate gyrus. DCX protein level in the dentate gyrus at PM 1 was high, thereafter levels of DCX were decreased with time. The authors suggest that a decrease of DCX immunoreactivity and its protein level with age may be associated with aging processes in the hippocampal dentate gyrus.

Rebuilding lost hearing using cell transplantation

OBJECTIVE

The peripheral auditory nervous system (cochlea and auditory nerve) has a complex anatomy, and it has traditionally been thought that once the sensorineural structures are damaged, restoration of hearing is impossible. In the past decade, however, the potential to restore lost hearing has been intensively investigated using molecular and cell biological techniques, and we can now part with such a pessimistic view. In this review, we examine an important field in hearing restoration research: cell transplantation.

METHODS

Most efforts in this field have been directed to the replacement of hair cells by transplantation to the cochlea. Here, we focus on transplantation to the auditory nerve, from the side of the cerebellopontine angle rather than the cochlea.

RESULTS

Delivery of cells to the cochlea is potentially damaging, and nerve cells transplanted distally to the Schwann-glial transitional zone (cochlear side) may become inhibited when they reach the transitional zone. The auditory nerve is probably the most suitable route for cell transplantation.

CONCLUSION

The auditory nerve occupies an important position not only in neurosurgery but also in various diseases in other disciplines, and several lines of recent evidence indicate that it is a key target for hearing restoration. It is familiar to most neurosurgeons, and the recent advances in the molecular and cell biology of inner-ear development are of direct importance to neurorestorative medicine. In this article, we review the anatomy, development, and molecular biology of the auditory nerve and cochlea, with emphasis on the advances in cell transplantation.

Correlations between neuronal loss, decrease of memory, and decrease expression of brain-derived neurotrophic factor in the gerbil hippocampus during normal aging

It is known that the hippocampus has vital functions in learning and memory, behavioral regulation, and activity-dependent synaptic plasticity, and that the hippocampus contains high levels of brain-derived neurotrophic factor (BDNF). In the present study, we followed age-dependent changes of BDNF immunoreactivity and protein level in the gerbil hippocampus to identify the correlation between BDNF and aging. BDNF immunoreactivity and its protein level significantly increased at postnatal month (PM) 12 in the hippocampus and thereafter reduced. At PM 24, BDNF immunoreactivity in the hippocampal CA1 region and dentate gyrus was similar to that in the PM 1 group, whereas BDNF immunoreactivity in the CA2/3 region at PM 24 was higher than that at PM 1. In the PM 24 group, an age-related neuronal loss and the decrease of reference and working memory were observed. In conclusion, our results suggest that observed reduction in BDNF and reference memory may be associated with age-dependent neuronal loss in the hippocampal CA1 region.

Chronological distribution of Rip immunoreactivity in the gerbil hippocampus during normal aging

Age-dependent studies on oligodendrocytes, which are the myelinating cells in the central nervous system, have been relatively less investigated. We examined age-dependent changes in Rip immunoreactivity and its protein level in the gerbil hippocampus during normal aging using immunohistochemistry and Western blot analysis with Rip antibody, an oligodendrocyte marker. Rip immunoreactivity and its protein level in the hippocampal CA1 region significantly increased at postnatal month 3 (PM 3). Thereafter, they decreased in the hippocampal CA1 region with age. At PM 24, Rip immunoreactive processes in the hippocampal CA1 region markedly decreased in the stratum radiatum. In the hippocampal CA2/3 region and dentate gyrus, the pattern of changes in Rip immunoreactivity and its protein level was similar to those in the hippocampal CA1 region; however, no significant changes were found in the CA2/3 region and dentate gyrus at various age stages. These results indicate that Rip immunoreactivity and protein level in the hippocampal CA1 region decreases significantly at PM 24 compared to the CA2/3 region and dentate gyrus.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

Expression of alpha and beta parvalbumin is differentially regulated in the rat organ of corti during development

The expression of two calcium-binding proteins of the parvalbumin (PV) family, the alpha isoform (alphaPV) and the beta isoform known as oncomodulin (OM), was investigated in the rat cochlea during postnatal development and related to cholinergic efferent innervation. Using RT-PCR analysis, we found that OM expression begins between postnatal day 2 (P2) and P4, and peaks as early as P10, while alphaPV mRNA begins expression before birth and remains highly expressed into the adult period. Both in situ hybridization and immunoreactivity confirm that OM is uniquely expressed by the outer hair cells (OHCs) in the rat cochlea and occurs after efferent innervation along the cochlear spiral between P2 and P4. In contrast to OM expression, alphaPV immunoreactivity is expressed in both inner hair cells (IHCs) and OHCs at birth. Following olivocochlear efferent innervation, OHCs demonstrate weak OM immunoreactivity beginning at P5 and diminished alphaPV immunoreactivity after P10. In organ cultures isolated prior to the efferent innervation of OHCs, OM immunoreactivity failed to develop in OHCs, but alphaPV immunoreactivity remained present in both IHCs and OHCs. In contrast, organ cultures isolated after efferent innervation of OHCs show OHCs with low levels of OM immunoreactivity and high levels of alphaPV immunoreactivity. This study suggests that OM and alphaPV are differentially regulated in OHCs during cochlear development. Our findings further raise the possibility that the expression of PV proteins in OHCs may be influenced by efferent innervation.

Mouse outer hair cells lacking the alpha9 ACh receptor are motile

Efferent nerve fibers form chemical synapses at the bases of outer hair cells (OHC), with acetylcholine (ACh) being their principal neurotransmitter. The activation of ACh receptors on OHCs is known to influence cochlear function. These efferent effects exhibit an unusual pharmacology and are generally known to be inhibitory. Recent evidence suggests that an ACh receptor subunit, known as alpha9, plays a dominant role in mediating the olivocochlear neurotransmission to OHCs. In this investigation, we attempt to determine the possible role(s) of the alpha9 subunit in regulating OHC function by examining OHC electromotility and compound action potentials (CAP) in mice carrying a null mutation for the alpha9 gene. Results indicate that cochlear sensitivity, based on CAP thresholds, is similar for homozygous mutant and wild-type mice. Electromotility is also present in OHCs, independent of whether the alpha9 subunit is present or absent.

Lack of Bdnf and TrkB signalling in the postnatal cochlea leads to a spatial reshaping of innervation along the tonotopic axis and hearing loss

Members of the neurotrophin gene family and their high-affinity Trk receptors control innervation of the cochlea during embryonic development. Lack of neurotrophin signalling in the cochlea has been well documented for early postnatal animals, resulting in a loss of cochlear sensory neurones and a region-specific reduction of target innervation along the tonotopic axis. However, how reduced neurotrophin signalling affects the innervation of the mature cochlea is currently unknown. Here, we have analysed the consequences of a lack of the TrkB receptor and its ligand, the neurotrophin brain-derived neurotrophic factor (Bdnf), in the late postnatal or adult cochlea using mouse mutants. During early postnatal development, mutant animals show a lack of afferent innervation of outer hair cells in the apical part of the cochlea, whereas nerve fibres in the basal part are maintained. Strikingly, this phenotype is reversed during subsequent maturation of the cochlea, which results in a normal pattern of outer hair cell innervation in the apex and loss of nerve fibres at the base in adult mutants. Measurements of auditory brain stem responses of these mice revealed a significant hearing loss. The observed innervation patterns correlate with opposing gradients of Bdnf and Nt3 expression in cochlear neurones along the tonotopic axis. Thus, the reshaping of innervation may be controlled by autocrine signalling between neurotrophins and their receptors in cochlear neurones. Our results indicate a substantial potential for re-innervation processes in the mature cochlea, which may also be of relevance for treatment of hearing loss in humans.

Thyroid hormone is not necessary for the development of outer hair cell electromotility

The outer hair cell (OHC), one of two receptor cell types in the organ of Corti, plays a critical role in mammalian hearing. OHCs enhance basilar membrane motion through a local mechanical feedback process within the cochlea, termed the 'cochlear amplifier'. It is generally believed that the basis of cochlear amplification is a voltage-dependent electromotile response of OHCs. Measurements of electromotility in developing animals indicate that the onset of motility normally occurs around 7 days after birth in altricial rodents such as gerbils and rats. Thyroid hormone (TH) plays a crucial role in the development of the auditory system. Deficiency of the hormone between the late embryonic stage and the second postnatal week can cause severe hearing loss. Several studies suggest that TH deficiency might also affect the development of the cochlear amplifier. The goal of this study was therefore to examine whether TH was necessary for the development of OHC motility. The organ of Corti of gerbils was dissected out at birth and grown in culture with defined concentration of triiodothyronine (T3), the active ligand for the TH receptor. Motility was measured from OHCs isolated from 7-, 11- and 14-day-old cultures. Motility did indeed develop in OHCs deprived of normal concentration of T3. This suggests that the defective auditory function seen in TH-deficient animals is most likely due to morphological and physiological changes in the cochlea, rather than the motor function of the OHCs.

Development of the inner ear efferent system across vertebrate species

Inner ear efferent neurons are part of a descending centrifugal pathway from the hindbrain known across vertebrates as the octavolateralis efferent system. This centrifugal pathway terminates on either sensory hair cells or eighth nerve ganglion cells. Most studies of efferent development have used either avian or mammalian models. Recent studies suggest that prevailing notions of the development of efferent innervation need to be revised. In birds, efferents reside in a single, diffuse nucleus, but segregate according to vestibular or cochlear projections. In mammals, the auditory and vestibular efferents are completely separate. Cochlear efferents can be divided into at least two distinct, descending medial and lateral pathways. During development, inner ear efferents appear to be a specific motor neuron phenotype, but unlike motor neurons have contralateral projections, innervate sensory targets, and, at least in mammals, also express noncholinergic neurotransmitters. Contrary to prevailing views, newer data suggest that medial efferent neurons mature early, are mostly, if not exclusively, cholinergic, and project transiently to the inner hair cell region of the cochlea before making final synapses on outer hair cells. On the other hand, lateral efferent neurons mature later, are neurochemically heterogeneous, and project mostly, but not exclusively to the inner hair cell region. The early efferent innervation to the ear may serve an important role in the maturation of afferent responses. This review summarizes recent data on the neurogenesis, pathfinding, target selection, innervation, and onset of neurotransmitter expression in cholinergic efferent neurons.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

Firing features and potassium channel content of murine spiral ganglion neurons vary with cochlear location

Neurons from varied regions of the central nervous system can show widely divergent responses to electrical stimuli that are determined by cell-specific differences in ion channel composition. The well-ordered and highly characterized peripheral auditory system allows one to explore the significance of this diversity during the final stages of postnatal development. We examined the electrophysiological features of murine spiral ganglion neurons in vitro at a time when recordings could be made from the cell bodies before myelination. These cells carry information about sound stimuli from hair cell receptors in the basilar membrane and are arranged tonotopically. Spiral ganglion neuron responses to depolarizing current injection were assessed with whole-cell current clamp recordings from cells that were isolated separately from the apical and basal thirds of the mouse cochlea. These cells displayed systematic variation in their firing. Apex neurons (low frequency coding) showed longer latency, slowly adapting responses, whereas base neurons (high frequency coding) showed short latency, rapidly adapting responses to the same stimuli. This physiological diversity was mirrored by regional differences in ion channel content assessed immunohistochemically. Apex neurons had a preponderance of Kv4.2 subunits, whereas base neurons possessed greater levels of K(Ca), Kv1.1, and Kv3.1 subunits. Taken together, these results indicate that the distribution of a set of voltage-gated potassium channels may relate specifically to a particular range of coding frequencies. These studies also suggest that intrinsic properties of spiral ganglion neurons can contribute to the characteristic responses of the peripheral auditory system. Their potential role in development and adult function is discussed.

Development of acetylcholine receptors in cultured outer hair cells

Efferents, originating in the superior olivary complex, preferentially synapse with cochlear outer hair cells (OHCs), with acetylcholine (ACh) as their primary neurotransmitter. The OHC ACh receptors (AChRs), which have unusual pharmacology, have been cloned and identified as a new subunit (alpha9) of the nicotinic AChR family. The expression of alpha9 AChRs is first detected before birth and peaks between 6 and 10 days after birth (DAB) in developing mice and rats, while functional maturation of the receptor, as determined by measuring the ACh-induced currents, takes place between 6 and 12 DAB. In this study we attempted to examine the development of AChRs in OHCs grown in explanted cultures, deprived of efferent innervation. ACh-induced currents were used as an assay. Reverse transcription-PCR analysis was also performed to detect the expression of alpha9 subunit from cultured OHCs. PCR study indicates that mRNA of the alpha9 subunit was expressed in primary cochlear cultures, similar to that seen in the cochleae of developing animals. Measurement of whole-cell currents showed that ACh-induced outward current was first detected around 5 days in a fraction of cultured OHCs. The number of responsive cells increased between 5 and 12 days in culture. The size of ACh-induced currents also increased during this period. These results suggest that the development of AChRs in cultured OHCs is not affected by removal of efferent innervation.