Identification with a recombinant antibody of an inner-ear cytokeratin, a marker for hair-cell differentiation

Adult porcine (Sus scrofa) derived inner ear cells: Characteristics in in-vitro cultures

There is a need for an animal model that closely parallels human cochlea gestational development. This study aims to document porcine inner ear anatomy, and in vitro porcine derived inner ear cell culture characteristics. Twenty-four temporal bone were harvested from 12 adult pigs (Sus scrofa). Six were formalin fixed and their maximal diameters were measured. The cochlea duct length was determined by the insertion length of a Nucleus 22 cochlear implant in two bones. Four formalin fixed bones were sectioned for histology. Cochlear and vestibular tissues were harvested from non-fixed bones, cultured and characterized at different passages (P). Gene and protein expression of multipotent stem/progenitor (Nestin and Sox2), inner ear hair (Myosin VIIa, Prestin) and supporting (Cytokeratin 18 and Vimentin) cell markers were determined. The porcine cochlea was a 3.5 turn spiral. There was a separate vestibular compartment. The cochlear mean maximal diameter and height was 7.99 and 3.77 mm, respectively. Sphere forming cells were identified on phase-contrast microscopy. The relative mRNA expression levels of KRT18, MYO7A and SLC26A5 were significantly positively correlated in cochlear cultures; and MYO7A and SLC26A5; SOX2 and KRT18; NES and SLC26A5 genes were positively correlated in vestibular cultures (p = .037, Spearman correlation [τ] = .900). Inner ear sensory and stem cell characteristics persist in passaged porcine inner ear cells. Further work is required to establish the usefulness of porcine inner ear cell cultures to the study of human inner ear disorders.

MicroRNA Signature and Cellular Characterization of Undifferentiated and Differentiated House Ear Institute-Organ of Corti 1 (HEI-OC1) Cells

MicroRNAs (miRNAs) regulate gene expressions and control a wide variety of cellular functions. House Ear Institute-Organ of Corti 1 (HEI-OC1) cells are widely used to screen ototoxic drugs and to investigate cellular and genetic alterations in response to various conditions. HEI-OC1 cells are almost exclusively studied under permissive conditions that promote cell replication at the expense of differentiation. Many researchers suggest that permissive culture condition findings are relevant to understanding human hearing disorders. The mature human cochlea however consists of differentiated cells and lacks proliferative capacity. This study therefore aimed to compare the miRNA profiles and cellular characteristics of HEI-OC1 cells cultured under permissive (P-HEI-OC1) and non-permissive (NP-HEI-OC1) conditions. A significant increase in the level of expression of tubulin β1 class VI (Tubb1), e-cadherin (Cdh1), espin (Espn), and SRY (sex determining region Y)-box2 (Sox2) mRNAs was identified in non-permissive cells compared with permissive cells (P < 0.05, Kruskal–Wallis H test, 2-sided). miR-200 family, miR-34b/c, and miR-449a/b functionally related cluster miRNAs, rodent-specific maternally imprinted gene Sfmbt2 intron 10^th cluster miRNAs (-466a/ -467a), and miR-17 family were significantly (P < 0.05, Welch’s t-test, 2-tailed) differentially expressed in non-permissive cells when compared with permissive cells. Putative target genes were significantly predominantly enriched in mitogen-activated protein kinase (MAPK), epidermal growth factor family of receptor tyrosine kinases (ErbB), and Ras signaling pathways in non-permissive cells compared with permissive cells. This distinct miRNA signature of differentiated HEI-OC1 cells could help in understanding miRNA-mediated cellular responses in the adult cochlea.

Recovery of otoacoustic emissions after high-level noise exposure in the American bullfrog

The American bullfrog (Rana catesbeiana) has an amphibian papilla (AP) that senses airborne, low-frequency sound and generates distortion product otoacoustic emissions (DPOAEs) similar to other vertebrate species. Although ranid frogs are typically found in noisy environments, the effects of noise on the AP have not been studied. First, we determined the noise levels that diminished DPOAE at 2f1-f2 using an f2 stimulus level at 80 dB SPL and that also produced morphological damage of the sensory epithelium. Second, we compared DPOAE (2f1-f2) responses with histopathologic changes occurring in bullfrogs after noise exposure. Consistent morphological damage, such as fragmented hair cells and missing bundles, as well as elimination of DPOAE responses were seen only after very high-level (>150 dB SPL) sound exposures. The morphological response of hair cells to noise differed along the mediolateral AP axis: medial hair cells were sensitive to noise and lateral hair cells were relatively insensitive to noise. Renewed or repaired hair cells were not observed until 9 days post-exposure. Following noise exposure, DPOAE responses disappeared within 24 h and then recovered to normal pre-exposure levels within 3-4 days. Our results suggest that DPOAEs in the bullfrog are sensitive to the initial period of hair cell damage. After noise-induced damage, the bullfrog AP has functional recovery mechanisms that do not depend on substantial hair cell regeneration or repair. Thus, the bullfrog auditory system might serve as an interesting model for investigation of ways to prevent noise damage.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Reinforcement of cell junctions correlates with the absence of hair cell regeneration in mammals and its occurrence in birds

Debilitating hearing and balance deficits often arise through damage to the inner ear's hair cells. For humans and other mammals, such deficits are permanent, but nonmammalian vertebrates can quickly recover hearing and balance through their innate capacity to regenerate hair cells. The biological basis for this difference has remained unknown, but recent investigations in wounded balance epithelia have shown that proliferation follows cellular spreading at sites of injury. As mammalian ears mature during the first weeks after birth, the capacity for spreading and proliferation declines sharply. In seeking the basis for those declines, we investigated the circumferential bands of F-actin that bracket the apical junctions between supporting cells in the gravity-sensitive utricle. We found that those bands grow much thicker as mice and humans mature postnatally, whereas their counterparts in chickens remain thin from hatching through adulthood. When we cultured utricular epithelia from chickens, we found that cellular spreading and proliferation both continued at high levels, even in the epithelia from adults. In contrast, the substantial reinforcement of the circumferential F-actin bands in mammals coincides with the steep declines in cell spreading and production established in earlier experiments. We propose that the presence of thin F-actin bands at the junctions between avian supporting cells may contribute to the lifelong persistence of their capacity for shape change, cell proliferation, and hair cell replacement and that the postnatal reinforcement of the F-actin bands in maturing humans and other mammals may have an important role in limiting hair cell regeneration.

Closure of supporting cell scar formations requires dynamic actin mechanisms

In many vertebrate inner ear sensory epithelia, dying sensory hair cells are extruded, and the apices of surrounding supporting cells converge to re-seal the epithelial barrier between the electrochemically-distinct endolymph and perilymph. These cellular mechanisms remain poorly understood. Dynamic microtubular mechanisms have been proposed for hair cell extrusion; while contractile actomyosin-based mechanisms are required for cellular extrusion and closure in epithelial monolayers. The hypothesis that cytoskeletal mechanisms are required for hair cell extrusion and supporting cell scar formation was tested using bullfrog saccules incubated with gentamicin (6h), and allowed to recover (18h). Explants were then fixed, labeled for actin and cytokeratins, and viewed with confocal microscopy. To block dynamic cytoskeletal processes, disruption agents for microtubules (colchicine, paclitaxel) myosin (Y-27632, ML-9) or actin (cytochalasin D, latrunculin A) were added during treatment and recovery. Microtubule disruption agents had no effect on hair cell extrusion or supporting cell scar formation. Myosin disruption agents appeared to slow down scar formation but not hair cell extrusion. Actin disruption agents blocked scar formation, and largely prevented hair cell extrusion. These data suggest that actin-based cytoskeletal processes are required for hair cell extrusion and supporting cell scar formation in bullfrog saccules.

Molecular screens for inner ear genes

Identification of the genes that encode proteins that are important for proper function of specific inner ear cell types is central to our understanding of the molecular basis of hearing and balance. Whereas the combination of electrophysiology and biophysics has resulted in an exquisite understanding of inner ear function, little is known about the proteins that confer these properties at the cellular level. Furthermore, the genes that control inner ear development, susceptibility to wear and tear, regeneration from damage, and age-related degeneration, are largely unknown. This review discusses tools that have been developed during the past few years to address this imbalance between a thorough physiologic characterization of inner ear function and a detailed understanding at a molecular level of the proteins involved in these functions. Creation of inner ear cDNA libraries has laid the foundation for the discovery of genes that are specifically expressed by cell types of the inner ear and that encode proteins that are important for molecular processes in these cells. In conjunction with expressed sequence tag database analysis, cDNA subtraction, and DNA arrays, functionally important genes, whose specific expression patterns are usually verified by gene expression analysis, can be identified. Discussion of these techniques takes into account the specific characteristics of the inner ear in relation to its study using molecular biological approaches.

Application of physiological genomics to the study of hearing disorders

Although the biophysical principles of how the ear operates are reasonably well understood, little is known about the specific genes that confer normal function to the inner ear. Nevertheless, the recent implementation of genomic tools has led to extraordinary progress in the identification of mutated genes that cause non-syndromic and syndromic forms of deafness. Part of this success is directly related to the sequencing of the human and mouse genomes and improved gene annotation methods. This review discusses how physiological genomic tools, such as genomic databases, expressed sequence tag databases and DNA arrays have been applied to find candidate genes for important molecular processes in the inner ear. It also illustrates, using the discovery of genes encoding essential components of cochlear K+ homeostasis as an example, how the combination of physiological genomic tools with physiological and morphological information has led to an in-depth understanding of cochlear ion homeostasis. Finally, it discusses how the use of applied genomic tools, such as gene arrays, will further advance our knowledge of how the inner ear works, develops, ages and regenerates.

Hair cell recovery in mitotically blocked cultures of the bullfrog saccule

Hair cells in many nonmammalian vertebrates are regenerated by the mitotic division of supporting cell progenitors and the differentiation of the resulting progeny into new hair cells and supporting cells. Recent studies have shown that nonmitotic hair cell recovery after aminoglycoside-induced damage can also occur in the vestibular organs. Using hair cell and supporting cell immunocytochemical markers, we have used confocal and electron microscopy to examine the fate of damaged hair cells and the origin of immature hair cells after gentamicin treatment in mitotically blocked cultures of the bullfrog saccule. Extruding and fragmenting hair cells, which undergo apoptotic cell death, are replaced by scar formations. After losing their bundles, sublethally damaged hair cells remain in the sensory epithelium for prolonged periods, acquiring supporting cell-like morphology and immunoreactivity. These modes of damage appear to be mutually exclusive, implying that sublethally damaged hair cells repair their bundles. Transitional cells, coexpressing hair cell and supporting cell markers, are seen near scar formations created by the expansion of neighboring supporting cells. Most of these cells have morphology and immunoreactivity similar to that of sublethally damaged hair cells. Ultrastructural analysis also reveals that most immature hair cells had autophagic vacuoles, implying that they originated from damaged hair cells rather than supporting cells. Some transitional cells are supporting cells participating in scar formations. Supporting cells also decrease in number during hair cell recovery, supporting the conclusion that some supporting cells undergo phenotypic conversion into hair cells without an intervening mitotic event.