Transforming growth factor-alpha-induced cellular changes in organotypic cultures of juvenile, amikacin-treated rat organ of corti

Expression of Calbindin-D28K in the Developing and Adult Mouse Cochlea

Herein, we aimed to use double-labeling immunofluorescence to describe the expression pattern of Calbindin-D28K (CaBP28K) in the mouse cochlea from late embryonic (E) stages to the adulthood. CaBP28K was expressed in the inner hair cells (IHCs) and the greater epithelial ridge (GER) at E17. In addition, its expression was observed in the interdental cells. On postnatal day 1 (P1), CaBP28K immunoreactivity was observed in the IHCs and outer hair cells (OHCs) and was also specifically expressed in the nucleus and the cytoplasm of spiral ganglion neurons (SGNs). At P8, CaBP28K labeling disappeared from the interdental cells, and the CaBP28K-positive domain within the GER shifted from the entire cytoplasm to only the apical and basal regions. At P14, CaBP28K immunoreactivity was lost from the GER; however, its expression in the IHCs and OHCs, as well as the SGNs, persisted into adulthood. The identification of CaBP28K in the hair cells (HCs) and cuticular plates, as well as SGNs, was confirmed by its colocalization with several markers for Sox2, Myosin VIIa, Phalloidin, and Tuj1. We also detected colocalization with calmodulin in the cytoplasm of both HCs and SGNs. Western blot revealed an increase in CaBP28K postnatal expression in the mouse cochlea.

Inner ear supporting cells: rethinking the silent majority

Sensory epithelia of the inner ear contain two major cell types: hair cells and supporting cells. It has been clear for a long time that hair cells play critical roles in mechanoreception and synaptic transmission. In contrast, until recently the more abundant supporting cells were viewed as serving primarily structural and homeostatic functions. In this review, we discuss the growing information about the roles that supporting cells play in the development, function and maintenance of the inner ear, their activities in pathological states, their potential for hair cell regeneration, and the mechanisms underlying these processes.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.

EGFR signaling is required for regenerative proliferation in the cochlea: conservation in birds and mammals

Proliferation and transdifferentiaton of supporting cells in the damaged auditory organ of birds lead to robust regeneration of sensory hair cells. In contrast, regeneration of lost auditory hair cells does not occur in deafened mammals, resulting in permanent hearing loss. In spite of this failure of regeneration in mammals, we have previously shown that the perinatal mouse supporting cells harbor a latent potential for cell division. Here we show that in a subset of supporting cells marked by p75, EGFR signaling is required for proliferation, and this requirement is conserved between birds and mammals. Purified p75+ mouse supporting cells express receptors and ligands for the EGF signaling pathway, and their proliferation in culture can be blocked with the EGFR inhibitor AG1478. Similarly, in cultured chicken basilar papillae, supporting cell proliferation in response to hair cell ablation requires EGFR signaling. In addition, we show that EGFR signaling in p75+ mouse supporting cells is required for the down-regulation of the cell cycle inhibitor p27(Kip1) (CDKN1b) to enable cell cycle re-entry. Taken together, our data suggest that a conserved mechanism involving EGFR signaling governs proliferation of auditory supporting cells in birds and mammals and may represent a target for future hair cell regeneration strategies.

[Regenerative medicine in the treatment of sensorineural hearing loss]

Regenerative medicine offers the prospect of causal treatment of sensorineural hearing loss. In humans, the loss of sensory hair cells is irreversible and results in chronic hearing loss. Other vertebrates, particularly birds, have the capability to spontaneously regenerate lost sensory hair cells and restore hearing. In the bird model, regeneration of hair cells is based on the proliferation of supporting cells. In mammals, supporting cells have lost their proliferative capacity and are terminally differentiated. To gain an understanding about regeneration of hair cells in mammals, cell division of supporting cells has to be controlled. Gene disruption of the cell cycle inhibitor p27(Kip1) allows supporting cell proliferation in the organ of Corti in vivo. Furthermore, in vitro studies indicate that newly generated cells may differentiate into hair cells after p27(Kip1) disruption. Other current methods to induce hair cell regeneration include the gene transfer of Math1 and transplantation of stem cells to the inner ear.

ErbB expression: the mouse inner ear and maturation of the mitogenic response to heregulin

In humans, hair cell loss often leads to hearing and balance impairments. Hair cell replacement is vigorous and spontaneous in avians and nonmammalian vertebrates. In mammals, in contrast, it occurs at a very low rate, or not at all, presumably because of a very low level of supporting cell proliferation following injury. Heregulin (HRG), a member of the epidermal growth factor (EGF) family of growth factors, is reported to be a potent mitogen for neonatal rat vestibular sensory epithelium, but its effects in adults are unknown. We report here that HRG-alpha stimulates cell proliferation in organotypic cultures of neonatal, but not adult, mouse utricular sensory epithelia. Our findings support the idea that the proliferative capabilities of the adult mammalian vestibular sensory epithelia differ significantly from that seen in neonatal animals. Immunohistochemistry reveals that HRG-binding receptors (erbBs 2-4) and erbB1 are widely expressed in vestibular and auditory sensory epithelia in neonatal and adult mouse inner ear. The distribution of erbBs in the neonatal and adult mouse ear is consistent with the EGF receptor/ligand family regulating diverse cellular processes in the inner ear, including cell proliferation and differentiation.

Expression of Prox1 defines regions of the avian otocyst that give rise to sensory or neural cells

The simple primordium of the inner ear (otocyst) differentiates into many cell types, including sensory neurons and hair cells. We examined expression of the divergent homeobox transcription factor, cProx1, during otocyst development in chickens. Nuclear cProx1 protein is not evident in the otic placode but emerges in the otic cup by stage 12. At stage 16, cProx1-positive nuclei are scattered continuously throughout the neuroepithelium, from anteroventral to posteromedial. These labeled cells are neural precursors; they express betaIII-tubulin and migrate to the cochleovestibular ganglion between stages 13 and 21. By stage 18, two areas develop a dense pattern of cProx1 expression in which every nucleus is labeled. These areas emerge at the anterior and posterior extremes of the band of scattered cProx1 expression and express the sensory markers cSerrate1 and Cath1 by stage 23. Four discrete patches of dense cProx1 expression appear by stage 23 that correspond to the future superior crista, lateral crista, saccular macula, and posterior crista, as confirmed by immunolabeling for hair cell antigen (HCA) by stage 29. The remaining sensory epithelia display a dense pattern of cProx1 expression and label for HCA by stage 29. In the basilar papilla, nuclear cProx1 expression is down-regulated in most hair cells by stage 37 and in many supporting cells by stage 40. Our findings show that regions of the otocyst that give rise to neurons or hair cells are distinguished by their relative density of cProx1-positive nuclei, and suggest a role for cProx1 in the genesis of these cell types.

Alteration in expression of p27 in auditory epithelia and neurons of mice during degeneration

The aim of this study was to examine roles of p27, a cyclin-dependent kinase inhibitor, in cochleae of adult mice. Expression of p27 was found in cochlear supporting cells and spiral ganglion neurons of normal mice. Cisplatin treatment caused progressive degeneration of cochlear supporting cells and spiral ganglion neurons, and numbers of p27-positive cells in these cells decreased. This indicates a close relationship between p27 and cell death in cochleae. However, the relationships between decrease in number of p27-positive cells and that of survival cells differed according to type of cell. For Deiters' cells, there was apparent decrease in number of p27-positive cells, although no decrease in cell numbers. The present findings indicate that p27 plays roles in degeneration of cochleae according to cell type.

Changes in MAP2 and tyrosinated alpha-tubulin expression in cochlear inner hair cells after amikacin treatment in the rat

The expression of MAP2 (microtubule-associated protein 2) and of tyrosinated alpha-tubulin was investigated immunocytochemically in the cochleas of normal and amikacin-treated rats. For MAP2, two different antibodies were used: anti-MAP2ab, against the high molecular weight forms, and anti-MAP2abc, additionally against the embryonic form c. In the cochlea of the normal rat, the outer (OHCs) and inner (IHCs) hair cells were labeled for MAP2abc. The labeling was weaker in IHCs than in OHCs. The hair cells were rarely labeled for MAPab. Both OHCs and IHCs were labeled for tyrosinated alpha-tubulin. In the cochlea of the amikacin-treated rat, aggregates of anti-MAP2abc and anti-tyrosinated alpha-tubulin antibodies were seen in the apical region of the IHCs as early as the end of the antibiotic treatment. In rats investigated during the following week, the cell body of most of the surviving IHCs were not labeled for MAP2abc and tyrosinated alpha-tubulin. Then, labeling for these two antibodies reappeared in the surviving IHCs, including their giant stereocilia. Fewer surviving IHCs were labeled for tyrosinated alpha-tubulin than for MAP2abc. The amikacin-poisoned IHCs were rarely labeled for MAP2ab. These results suggest that cochlear hair cells essentially express form c of MAP2. In the amikacin-damaged cochlea, the apical aggregation of MAP2c and tyrosinated alpha-tubulin within the poisoned IHCs could be implicated in a cell degenerative process. By contrast, the extinction and recovery of MAP2c and tyrosinated alpha-tubulin labeling in the remaining IHCs suggest the occurrence of a limited repair process. A possible role of MAP2 and tubulin in hair cell survival is discussed.

Identification of factors that maintain mammalian outer hair cells in adult organ of Corti explants

Both outer hair cells (OHCs) and inner hair cells (IHCs) survive and mature in 3 days old rat organ of Corti explants cultured for 1 month in a minimal essential medium. In contrast, under the same culture conditions, only IHCs survive in explants from adult guinea pig organ of Corti while many of the OHCs are lost within the first 48 h. Hair cell counts show OHCs loss to be greater in the lower portion (i.e. middle turn) of the cochlea than at the apex. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) indicates that there is DNA damage in adult OHCs within 8 h of explantation. Treatment of the adult organ of Corti explants with either actinomycin D (10(-7) M) or cycloheximide (10(-6) M) prevents most OHC losses. According to these results apoptosis may be the mechanism of OHC loss in adult organ of Corti explants. Stable membrane potentials recorded from the OHCs in both uncultured and actinomycin D-treated organ of Corti explants cultured for 72 h demonstrate the functional integrity of these hair cells. OHC losses in the adult guinea pig organ of Corti cultures can also be prevented by treatment with several of the growth factors tested, i.e. acidic fibroblast growth factor (aFGF), insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), transforming growth factor-beta1 (TGF-beta1), and glial cell-derived neurotrophic factor (GDNF). The results of this study suggest that growth factor therapy may be applicable to the treatment of some hearing disorders.