A model for mammalian cochlear hair cell differentiation in vitro: effects of retinoic acid on cytoskeletal proteins and potassium conductances

Retinoic acid induces differentiation of cochlear neural progenitor cells into hair cells

INTRODUCTION

Inner ear progenitor cells have the potential for multi-directional differentiation. Retinoic acid is an important requirement for the development of the inner ear. Blocking the Curtyr's retinoic acid signaling pathway can significantly reduce the number of hair cells. Therefore, we believe that retinoic acid may induce the regeneration of inner ear hair cells.

OBJECTIVE

To investigate whether the cochlear neural progenitor cells maintain the characteristics of stem cells during recovery and subculture, whether retinoic acid can induce cochlear neural progenitor cells into hair cells in vitro, and whether retinoic acid promotes or inhibits the proliferation of cochlear neural progenitor cells during differentiation.

METHODS

Cochlear neural progenitor cells were cultured and induced in DMEM/F12+RA (10^-6M) and then detected the expressions of hair cell markers (Math1 and MyosinVIIa) by immunofluorescence cytochemistry and realtime-polymerase chain reaction, and the proliferation of cochlear neural progenitor cells was detected by Brdu.

RESULTS

The nestin of cochlear neural progenitor cells was positively expressed. The ratios of Math1-positive cells in the control group and experimental group were 1.5% and 63%, respectively; the ratios of MyosinVIIa-positive cells in the control group and experimental group were 0.96% and 56%, respectively (p<0.05). The ratios of Brdu⁺-labeled cells in retinoic acid group, group PBS, and group FBS were 20.6%, 29.9%, and 54.3%, respectively; however, the proliferation rate in the experimental group decreased.

CONCLUSION

Retinoic acid can promote cochlear neural progenitor cells to differentiate into the hair cells.

Crosstalk in the non-classical signal transduction of testosterone and retinol in immature rat testes

This study aimed to investigate the effects of the interaction between testosterone and retinol on the rapid responses of cultured Sertoli cells obtained from 10-day-old immature rat testes. Non-classical actions of testosterone and retinol were investigated, and the activities of L-type voltage-dependent calcium channels (L-VDCC) and voltage-dependent potassium channels (Kv) were determined by measuring ⁴⁵Ca²⁺ influx in whole testis. Additionally, the effects of testosterone and retinol on these channels were studied in primary culture of Sertoli cells using the patch-clamp technique. ⁴⁵Ca²⁺ influx was used to observe a dose-response curve on tissues treated with retinol and/or testosterone for 2 min (10^-12, 10^-9 and 10^-6 M and 10^-9 and 10^-6 M), and a concentration of 10^-6 M was selected to investigate the mechanism of action of testosterone and retinol on rapid responses. Participation of the L-VDCC and Kv channels was investigated using nifedipine and tetraethylammonium chloride (TEA) inhibitors, respectively. Both, testosterone and retinol act through non-classical mechanisms, stimulating ⁴⁵Ca²⁺ influx in immature rat testes. The response to testosterone was abolished by nifedipine and TEA, whereas the effects of retinol were partially blocked by nifedipine and completely inhibited by TEA. Retinol amplified the testosterone-induced effect on ⁴⁵Ca²⁺ influx in the testes, suggesting a crosstalk between rapid responses (calcium influx) and cell repolarization via activation of Kv channels. Whole-cell electrophysiology data demonstrated that testosterone and retinol increased voltage-dependent potassium currents (Kv) in Sertoli cells; inhibition of these responses by TEA confirmed the involvement of TEA-sensitive K⁺ channels in these effects. Taken together, we demonstrate, for the first time, crosstalk between testosterone and retinol that is mediated by a non-classical mechanism involving the L-VDCC-triggered cell depolarization and activation of repolarization by Kv currents in Sertoli cells. These ionic modulations play a physiological role in Sertoli cells and male fertility via stimulation of secretory activities.

Rho-kinase and PKCα Inhibition Induces Primary Cilia Elongation and Alters the Behavior of Undifferentiated and Differentiated Temperature-sensitive Mouse Cochlear Cells

Primary cilia, regulated via distinct signal transduction pathways, play crucial roles in various cellular behaviors. However, the full regulatory mechanism involved in primary cilia development during cellular differentiation is not fully understood, particularly for the sensory hair cells of the mammalian cochlea. In this study, we investigated the effects of the Rho-kinase inhibitor Y27632 and PKCα inhibitor GF109203X on primary cilia-related cell behavior in undifferentiated and differentiated temperature-sensitive mouse cochlear precursor hair cells (the conditionally immortalized US/VOT-E36 cell line). Our results indicate that treatment with Y27632 or GF109203X induced primary cilia elongation and tubulin acetylation in both differentiated and undifferentiated cells. Concomitant with cilia elongation, Y27632 treatment also increased Hook2 and cyclinD1 expression, while only Hook2 expression was increased after treatment with GF109203X. In the undifferentiated cells, we observed an increase in the number of S and G2/M stage cells and a decrease of G1 cells after treatment with Y27632, while the opposite was observed after treatment with GF109203X. Finally, while both treatments decreased oxidative stress, only treatment with Y27632, not GF109203X, induced cell cycle-dependent cell proliferation and cell migration.

Sphingosine 1-phosphate-mediated activation of ezrin-radixin-moesin proteins contributes to cytoskeletal remodeling and changes of membrane properties in epithelial otic vesicle progenitors

Hearing loss is among the most prevalent sensory impairments in humans. Cochlear implantable devices represent the current therapies for hearing loss but have various shortcomings. ERM (ezrin- radixin -moesin) are a family of adaptor proteins that link plasma membrane with actin cytoskeleton, playing a crucial role in cell morphology and in the formation of membrane protrusions. Recently, bioactive sphingolipids have emerged as regulators of ERM proteins. Sphingosine 1-phosphate (S1P) is a pleiotropic sphingolipid which regulates fundamental cellular functions such as proliferation, survival, migration as well as processes such as development and inflammation mainly via ligation to its specific receptors S1PR (S1P_1-5). Experimental findings demonstrate a key role for S1P signaling axis in the maintenance of auditory function. Preservation of cellular junctions is a fundamental function both for S1P and ERM proteins, crucial for the maintenance of cochlear integrity. In the present work, S1P was found to activate ERM in a S1P₂-dependent manner in murine auditory epithelial progenitors US/VOT-E36. S1P-induced ERM activation potently contributed to actin cytoskeletal remodeling and to the appearance of ionic currents and membrane passive properties changes typical of more differentiated cells. Moreover, PKC and Akt activation was found to mediate S1P-induced ERM phosphorylation. The obtained findings contribute to demonstrate the role of S1P signaling pathway in inner ear biology and to disclose potential innovative therapeutical approaches in the field of hearing loss prevention and treatment.

Transcription factor induced conversion of human fibroblasts towards the hair cell lineage

Hearing loss is the most common sensorineural disorder, affecting over 5% of the population worldwide. Its most frequent cause is the loss of hair cells (HCs), the mechanosensory receptors of the cochlea. HCs transduce incoming sounds into electrical signals that activate auditory neurons, which in turn send this information to the brain. Although some spontaneous HC regeneration has been observed in neonatal mammals, the very small pool of putative progenitor cells that have been identified in the adult mammalian cochlea is not able to replace the damaged HCs, making any hearing impairment permanent. To date, guided differentiation of human cells to HC-like cells has only been achieved using either embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs). However, use of such cell types suffers from a number of important disadvantages, such as the risk of tumourigenicity if transplanted into the host´s tissue. We have obtained cells expressing hair cell markers from cultures of human fibroblasts by overexpression of GFI1, Pou4f3 and ATOH1 (GPA), three genes that are known to play a critical role in the development of HCs. Immunocytochemical, qPCR and RNAseq analyses demonstrate the expression of genes typically expressed by HCs in the transdifferentiated cells. Our protocol represents a much faster approach than the methods applied to ESCs and iPSCs and validates the combination of GPA as a set of genes whose activation leads to the direct conversion of human somatic cells towards the hair cell lineage. Our observations are expected to contribute to the development of future therapies aimed at the regeneration of the auditory organ and the restoration of hearing.

Histone deacetylase inhibition prevents cell death induced by loss of tricellular tight junction proteins in temperature-sensitive mouse cochlear cells

Tricellular tight junctions (tTJs) are specialized structures that occur where the corners of three cells meet to seal adjacent intercellular space. The molecular components of tTJs include tricellulin (TRIC) and lipolysis-stimulated lipoprotein receptor (LSR) which recruits TRIC, are required for normal hearing. Although loss of TRIC causes hearing loss with degeneration of cochlear cells, the detailed mechanisms remains unclear. In the present study, by using temperature-sensitive mouse cochlear cells, US/VOT-E36 cell line, we investigated the changes of TRIC and LSR during cochlear cell differentiation and the effects of histone deacetylase (HDAC) inhibitors against cell degeneration induced by loss of TRIC and LSR. During cell differentiation induced by the temperature change, expression of TRIC and LSR were clearly induced. Treatment with metformin enhanced expression TRIC and LSR via AMPK during cell differentiation. Loss of TRIC and LSR by the siRNAs induced cell death in differentiated cells. Treatment with HDAC inhibitors trichostatin A and HDAC6 inhibitor prevented the cell death induced by loss of TRIC and LSR. Collectively, these findings suggest that both tTJ proteins TRIC and LSR have crucial roles for the differentiated cochlear cell survival, and that HDAC inhibitors may be potential therapeutic agents to prevent hearing loss.

The how and why of identifying the hair cell mechano-electrical transduction channel

Identification of the auditory hair cell mechano-electrical transduction (hcMET) channel has been a major focus in the hearing research field since the 1980s when direct mechanical gating of a transduction channel was proposed (Corey and Hudspeth J Neurosci 3:962-976, 1983). To this day, the molecular identity of this channel remains controversial. However, many of the hcMET channel's properties have been characterized, including pore properties, calcium-dependent ion permeability, rectification, and single channel conductance. At this point, elucidating the molecular identity of the hcMET channel will provide new tools for understanding the mechanotransduction process. This review discusses the significance of identifying the hcMET channel, the difficulties associated with that task, as well as the establishment of clear criteria for this identification. Finally, we discuss potential candidate channels in light of these criteria.

Reprogramming of single-cell-derived mesenchymal stem cells into hair cell-like cells

HYPOTHESIS

Adult mesenchymal stem cells (MSCs) can be converted into hair cell-like cells by transdetermination.

BACKGROUND

Given the fundamental role sensory hair cells play in sound detection and the irreversibility of their loss in mammals, much research has focused on developing methods to generate new hair cells as a means of treating permanent hearing loss. Although MSCs can differentiate into multiple cell lineages, no efficient means of reprogramming them into sensory hair cells exists. Earlier work has shown that the transcription factor Atoh1 is necessary for early development of hair cells, but it is not clear whether Atoh1 can be used to convert MSCs into hair cells.

METHODS

Clonal MSC cell lines were established and reprogrammed into hair cell-like cells by a combination of protein transfer, adenoviral based gene transfer, and co-culture with neurons. During transdetermination, inner ear molecular markers were analyzed using reverse transcriptase-polymerase chain reaction, and cell structures were examined using immunocytochemistry.

RESULTS

Atoh1 overexpression in MSCs failed to convert MSCs into hair cell-like cells, suggesting that the ability of Atoh1 to induce hair cell differentiation is context dependent. Because Atoh1 overexpression successfully transforms VOT-E36 cells into hair cell-like cells, we modified the cell context of MSCs by performing a total protein transfer from VOT-E36 cells before overexpressing Atoh1. The modified MSCs were transformed into hair cell-like cells and attracted contacts from spiral ganglion neurons in a co-culture model.

CONCLUSION

We established a new procedure, consisting of VOT-E36 protein transfer, Atoh1 overexpression, and co-culture with spiral ganglion neurons, which can transform MSCs into hair cell-like cells.

Activation of apoptotic pathways in the absence of cell death in an inner-ear immortomouse cell line

Aminoglycoside antibiotics and cisplatin (CDDP) are the major ototoxins of clinical medicine due to their capacity to cause significant and permanent hearing loss by targeting the mammalian sensory cells. Understanding the pathogenesis of damage is the first step in designing effective prevention of drug-induced hearing loss. In-vitro systems greatly enhance the efficiency of biochemical and molecular investigations through ease of access and manipulation. HEI-OC1, an inner ear cell line derived from the immortomouse, expresses markers for auditory sensory cells and, therefore, is a potential tool to study the ototoxic mechanisms of drugs like aminoglycoside antibiotics and CDDP. HEI-OC1 cells (and also HeLa cells) efficiently take up fluorescently tagged gentamicin and respond to drug treatment with changes in cell death and survival signaling pathways. Within hours, the c-Jun N-terminal kinase pathway and the transcription factor AP-1 were activated and at later times, the "executioner caspase", caspase-3. These responses were robust and elicited by both gentamicin and kanamycin. However, despite the initiation of apoptotic pathways and transient changes in nuclear morphology, cell death was not observed following aminoglycoside treatment, while administration of CDDP led to significant cell death as determined by flow cytometric measurements; β-galactosidase analysis ruled out senescence in gentamicin-treated cells. The ability to withstand treatment with aminoglycosides but not with CDDP suggests that this cell line might be helpful in providing some insight into the differential actions of the two ototoxic drugs.

Genomic analysis of the function of the transcription factor gata3 during development of the mammalian inner ear

We have studied the function of the zinc finger transcription factor gata3 in auditory system development by analysing temporal profiles of gene expression during differentiation of conditionally immortal cell lines derived to model specific auditory cell types and developmental stages. We tested and applied a novel probabilistic method called the gamma Model for Oligonucleotide Signals to analyse hybridization signals from Affymetrix oligonucleotide arrays. Expression levels estimated by this method correlated closely (p<0.0001) across a 10-fold range with those measured by quantitative RT-PCR for a sample of 61 different genes. In an unbiased list of 26 genes whose temporal profiles clustered most closely with that of gata3 in all cell lines, 10 were linked to Insulin-like Growth Factor signalling, including the serine/threonine kinase Akt/PKB. Knock-down of gata3 in vitro was associated with a decrease in expression of genes linked to IGF-signalling, including IGF1, IGF2 and several IGF-binding proteins. It also led to a small decrease in protein levels of the serine-threonine kinase Akt2/PKBbeta, a dramatic increase in Akt1/PKBalpha protein and relocation of Akt1/PKBalpha from the nucleus to the cytoplasm. The cyclin-dependent kinase inhibitor p27(kip1), a known target of PKB/Akt, simultaneously decreased. In heterozygous gata3 null mice the expression of gata3 correlated with high levels of activated Akt/PKB. This functional relationship could explain the diverse function of gata3 during development, the hearing loss associated with gata3 heterozygous null mice and the broader symptoms of human patients with Hearing-Deafness-Renal anomaly syndrome.

Human fetal auditory stem cells can be expanded in vitro and differentiate into functional auditory neurons and hair cell-like cells

In the quest to develop the tools necessary for a cell-based therapy for deafness, a critical step is to identify a suitable stem cell population. Moreover, the lack of a self-renovating model system for the study of cell fate determination in the human cochlea has impaired our understanding of the molecular events involved in normal human auditory development. We describe here the identification and isolation of a population of SOX2+OCT4+ human auditory stem cells from 9-week-old to 11-week-old fetal cochleae (hFASCs). These cells underwent long-term expansion in vitro and retained their capacity to differentiate into sensory hair cells and neurons, whose functional and electrophysiological properties closely resembled their in vivo counterparts during development. hFASCs, and the differentiating protocols defined here, could be used to study developing human cochlear neurons and hair cells, as models for drug screening and toxicity and may facilitate the development of cell-based therapies for deafness.

Localization of Kv2.2 protein in Xenopus laevis embryos and tadpoles

Voltage-gated potassium (Kv) channels sculpt neuronal excitability and play important developmental roles. Kv channels consist of pore-forming alpha- and auxiliary subunits. For many Kv alpha-subunits, existing mRNA probes and antibodies have allowed analysis of expression patterns, typically during adult stages. Here, we focus on the Kv2.2 alpha-subunit, for which the mRNA shows broad expression in the embryo and adult. A lack of suitable antibodies, however, has hindered detailed analysis of Kv2.2 protein localization, especially during development. We developed an antibody that specifically recognizes Kv2.2 protein in Xenopus laevis, a vertebrate well suited for study of early developmental stages. The Kv2.2 antibody recognized heterologously expressed Kv2.2 but not the closely related Kv2.1 protein. Immunodetection of the protein showed its presence at St 32 in ventrolateral regions of the hindbrain and spinal cord. At later stages, several sensory tissues (retina, otic, and olfactory epithelia) also expressed Kv2.2 protein. As development progressed in the central nervous system, Kv2.2 protein distribution expanded in close association with the cytoskeletal marker alpha-tubulin, consistent with growth of neuronal tracts. We analyzed the subcellular distribution of Kv2.2 protein within single cultured neurons. In addition to a surface membrane presence, Kv2.2 protein also resided intracellularly closely associated with alpha-tubulin, as in vivo. Furthermore, in contrast to Kv2.1, Kv2.2 protein localized to long, axonal-like processes, consistent with its in vivo location in tracts. Despite their primary sequence similarity, the contrasting localizations of Kv2.1 and Kv2.2 support different roles for the two during development and neuronal signaling.

The human fetal cochlea can be a source for auditory progenitors/stem cells isolation

The development of new stem cell-based technologies is creating new hopes in regenerative medicine. Hearing-impaired individuals should benefit greatly from the development of a cell-based regenerative strategy to treat deafness. An important achievement would be to develop a human-based system that could bring the advances made in animal models closer to clinical application. In this work, we have explored the suitability of the developing fetal cochlea to be used as a source for the extraction of auditory progenitor/stem cells. We have established cultures that express critical markers such as NESTIN, SOX2, GATA3 and PAX2. These cultures can be expanded in vitro for several months and differentiating markers such as ATOH1/HATH1 and POU4F3/BRN3C can be induced by manipulating the culture conditions using specific growth factors such as bFGF, EGF and retinoic acid.