Molecular characterization of conditionally immortalized cell lines derived from mouse early embryonic inner ear

Phoenix auditory neurons as 3R cell model for high throughput screening of neurogenic compounds

Auditory neurons connect the sensory hair cells from the inner ear to the brainstem. These bipolar neurons are relevant targets for pharmacological intervention aiming at protecting or improving the hearing function in various forms of sensorineural hearing loss. In the research laboratory, neurotrophic compounds are commonly used to improve survival and to promote regeneration of auditory neurons. One important roadblock delaying eventual clinical applications of these strategies in humans is the lack of powerful in vitro models allowing high throughput screening of otoprotective and regenerative compounds. The recently discovered auditory neuroprogenitors (ANPGs) derived from the A/J mouse with an unprecedented capacity to self-renew and to provide mature auditory neurons offer the possibility to overcome this bottleneck. In the present study, we further characterized the new phoenix ANPGs model and compared it to the current gold-standard spiral ganglion organotypic explant (SGE) model to assay neurite outgrowth, neurite length and glutamate-induced Ca²⁺ response in response to neurotrophin-3 (NT-3) and brain derived neurotrophic factor (BDNF) treatment. Whereas both, SGEs and phoenix ANPGs exhibited a robust and sensitive response to neurotrophins, the phoenix ANPGs offer a considerable range of advantages including high throughput suitability, lower experimental variability, single cell resolution and an important reduction of animal numbers. The phoenix ANPGs in vitro model therefore provides a robust high-throughput platform to screen for otoprotective and regenerative neurotrophic compounds in line with 3R principles and is of interest for the field of auditory neuroscience.

Screen for modulators of atonal homolog 1 gene expression using notch pathway-relevant gene transcription based cellular assays

Atonal homolog 1 (Atoh1) is a basic helix-loop-helix 9 (bHLH) transcription factor acting downstream of Notch and is required for the differentiation of sensory hair cells in the inner ear and the specification of secretory cells during the intestinal crypt cell regeneration. Motivated by the observations that the upregulation of Atoh1 gene expression, through genetic manipulation or pharmacological inhibition of Notch signaling (e.g. γ-secretase inhibitors, GSIs), induces ectopic hair cell growth in the cochlea of the inner ear and partially restores hearing after injuries in experimental models, we decided to identify small molecule modulators of the Notch-Atoh1 pathway, which could potentially regenerate hair cells. However, the lack of cellular models of the inner ear has precluded the screening and characterization of such modulators. Here we report using a colon cancer cell line LS-174T, which displays Notch inhibition-dependent Atoh1 expression as a surrogate cellular model to screen for inducers of Atoh1 expression. We designed an Atoh1 promoter-driven luciferase assay to screen a target-annotated library of ~6000 compounds. We further developed a medium throughput, real-time quantitative RT-PCR assay measuring the endogenous Atoh1 gene expression to confirm the hits and eliminate false positives from the reporter-based screen. This strategy allowed us to successfully recover GSIs of known chemotypes. This LS-174T cell-based assay directly measures Atoh1 gene expression induced through Notch-Hes1 inhibition, and therefore offers an opportunity to identify novel cellular modulators along the Notch-Atoh1 pathway.

Concomitant differentiation of a population of mouse embryonic stem cells into neuron-like cells and schwann cell-like cells in a slow-flow microfluidic device

BACKGROUND

To send meaningful information to the brain, an inner ear cochlear implant (CI) must become closely coupled to as large and healthy a population of remaining spiral ganglion neurons (SGN) as possible. Inner ear gangliogenesis depends on macrophage migration inhibitory factor (MIF), a directionally attractant neurotrophic cytokine made by both Schwann and supporting cells (Bank et al., 2012). MIF-induced mouse embryonic stem cell (mESC)-derived "neurons" could potentially substitute for lost or damaged SGN. mESC-derived "Schwann cells" produce MIF, as do all Schwann cells (Huang et al., a; Roth et al., 2007; Roth et al., 2008) and could attract SGN to a "cell-coated" implant.

RESULTS

Neuron- and Schwann cell-like cells were produced from a common population of mESCs in an ultra-slow-flow microfluidic device. As the populations interacted, "neurons" grew over the "Schwann cell" lawn, and early events in myelination were documented. Blocking MIF on the Schwann cell side greatly reduced directional neurite outgrowth. MIF-expressing "Schwann cells" were used to coat a CI: Mouse SGN and MIF-induced "neurons" grew directionally to the CI and to a wild-type but not MIF-knockout organ of Corti explant.

CONCLUSIONS

Two novel stem cell-based approaches for treating the problem of sensorineural hearing loss are described. Developmental Dynamics 246:7-27, 2017. © 2016 Wiley Periodicals, Inc.

Pseudo-immortalization of postnatal cochlear progenitor cells yields a scalable cell line capable of transcriptionally regulating mature hair cell genes

The mammalian cochlea is a highly specialized organ within the inner ear. Sensory hair cells (HC) in the cochlea detect and transduce sound waves into electrical impulses that are sent to the brain. Studies of the molecular pathways regulating HC formation are hindered by the very sparse nature of HCs, where only ~3300 are found within an entire mouse cochlea. Current cell lines mimic certain aspects of HCs but lack terminal HC marker expression. Here we successfully “pseudo-immortalized” cochlear progenitor cells using the “conditional reprogramming” technique. These cells, termed “Conditionally Reprogrammed Otic Stem Cells” (CR-OSC), are able to bypass the senescence inherent to cochlear progenitor cells without genetic alterations, allowing for the generation of over 15 million cells from a single cochlea. These cells can be differentiated and up-regulate both early and terminal differentiation genes associated with HCs, including the terminal HC differentiation marker prestin. CR-OSCs also respond to known HC cues, including upregulation of HC genes in response to Atoh1 overexpression, and upregulation of prestin expression after thyroid hormone application. Overall, we describe the creation of a HC line capable of regulated expression of HC genes that can easily be recreated in any laboratory from any mouse of interest.

Macrophage migration inhibitory factor acts as a neurotrophin in the developing inner ear

This study is the first to demonstrate that macrophage migration inhibitory factor (MIF), an immune system 'inflammatory' cytokine that is released by the developing otocyst, plays a role in regulating early innervation of the mouse and chick inner ear. We demonstrate that MIF is a major bioactive component of the previously uncharacterized otocyst-derived factor, which directs initial neurite outgrowth from the statoacoustic ganglion (SAG) to the developing inner ear. Recombinant MIF acts as a neurotrophin in promoting both SAG directional neurite outgrowth and neuronal survival and is expressed in both the developing and mature inner ear of chick and mouse. A MIF receptor, CD74, is found on both embryonic SAG neurons and adult mouse spiral ganglion neurons. Mif knockout mice are hearing impaired and demonstrate altered innervation to the organ of Corti, as well as fewer sensory hair cells. Furthermore, mouse embryonic stem cells become neuron-like when exposed to picomolar levels of MIF, suggesting the general importance of this cytokine in neural development.

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.

A student team in a University of Michigan biomedical engineering design course constructs a microfluidic bioreactor for studies of zebrafish development

The zebrafish is a valuable model for teaching developmental, molecular, and cell biology; aquatic sciences; comparative anatomy; physiology; and genetics. Here we demonstrate that zebrafish provide an excellent model system to teach engineering principles. A seven-member undergraduate team in a biomedical engineering class designed, built, and tested a zebrafish microfluidic bioreactor applying microfluidics, an emerging engineering technology, to study zebrafish development. During the semester, students learned engineering and biology experimental design, chip microfabrication, mathematical modeling, zebrafish husbandry, principles of developmental biology, fluid dynamics, microscopy, and basic molecular biology theory and techniques. The team worked to maximize each person's contribution and presented weekly written and oral reports. Two postdoctoral fellows, a graduate student, and three faculty instructors coordinated and directed the team in an optimal blending of engineering, molecular, and developmental biology skill sets. The students presented two posters, including one at the Zebrafish meetings in Madison, Wisconsin (June 2008).

Inner ear hair cells produced in vitro by a mesenchymal-to-epithelial transition

Sensory hair cell loss is a major contributor to disabling hearing and balance deficits that affect >250 million people worldwide. Sound exposures, infections, drug toxicity, genetic disorders, and aging all can cause hair cell loss and lead to permanent sensory deficits. Progress toward treatments for these deficits has been limited, in part because hair cells have only been obtainable via microdissection of the anatomically complex internal ear. Attempts to produce hair cells in vitro have resulted in reports of some success but have required transplantation into embryonic ears or coculturing with other tissues. Here, we show that avian inner ear cells can be cultured and passaged for months, frozen, and expanded to large numbers without other tissues. At any point from passage 6 up to at least passage 23, these cultures can be fully dissociated and then aggregated in suspension to induce a mesenchymal-to-epithelial transition that reliably yields new polarized sensory epithelia. Those epithelia develop numerous hair cells that are crowned by hair bundles, composed of a single kinocilium and an asymmetric array of stereocilia. These hair cells exhibit rapid permeance to FM1-43, a dye that passes through open mechanotransducing channels. Because a vial of frozen cells can now provide the capacity to produce bona fide hair cells completely in vitro, these discoveries should open new avenues of research that may ultimately contribute to better treatments for hearing loss and other inner ear disorders.

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.

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.

Wild-type cells rescue genotypically Math1-null hair cells in the inner ears of chimeric mice

The transcription factor Math1 has been shown to be critical in the formation of hair cells (HCs) in the inner ear. However, the influence of environmental factors in HC specification suggests that cell extrinsic factors are also crucial to their development. To test whether extrinsic factors impact development of Math1-null (Math1(beta-Gal/beta-Gal)) HCs, we examined neonatal (postnatal ages P0-P4.5) Math1-null chimeric mice in which genotypically mutant and wild-type cells intermingle to form the inner ear. We provide the first direct evidence that Math1-null HCs are able to be generated and survive in the conducive chimeric environment. beta-Galactosidase expression was used to identify genetically mutant cells while cells were phenotypically defined as HCs by morphological characteristics notably the expression of HC-specific markers. Genotypically mutant HCs were found in all sensory epithelia of the inner ear at all ages examined. Comparable results were obtained irrespective of the wild-type component of the chimeric mice. Thus, genotypically mutant cells retain the competence to differentiate into HCs. The implication is that the lack of the Math1 gene in HC precursors can be overcome by environmental influences, such as cell-cell interactions with wild-type cells, to ultimately result in the formation of HCs.

Immortalized mouse inner ear cell lines demonstrate a role for chemokines in promoting the growth of developing statoacoustic ganglion neurons

The target-derived factors necessary for promoting initial outgrowth from the statoacoustic ganglion (SAG) to the inner ear have not been fully characterized. In the present study, conditioned medium from embryonic Immortomouse inner ear cell lines that maintain many characteristics of developing inner ear sensory epithelia were screened for neurite-promoting activity. Conditioned medium found to be positive for promoting SAG neurite outgrowth and neuronal survival was then tested for the presence of chemokines, molecules that have not previously been investigated for promoting SAG outgrowth. One candidate molecule, monocyte chemotactic protein 1 (MCP-1), was detected in the conditioned medium and subsequently localized to mouse hair cells by immunocytochemistry. In vitro studies demonstrated that function-blocking MCP-1 antibodies decreased the amount of SAG neurite outgrowth induced by the conditioned medium and that subsequent addition of MCP-1 protein was able to promote outgrowth when added to the antibody-treated conditioned medium. The use of the Immortomouse cell lines proved valuable in identifying this candidate cofactor that promotes outgrowth of early-stage SAG nerve fibers and is expressed in embryonic hair cells.

Cell transplantation to the auditory nerve and cochlear duct

We have developed a technique to deliver cells to the inner ear without injuring the membranes that seal the endolymphatic and perilymphatic chambers. The integrity of these membranes is essential for normal hearing, and the technique should significantly reduce surgical trauma during cell transplantation. Embryonic stem cells transplanted at the internal auditory meatal portion of an atrophic auditory nerve migrated extensively along it. Four-five weeks after transplantation, the cells were found not only throughout the auditory nerve, but also in Rosenthal's canal and the scala media, the most distal portion of the auditory nervous system where the hair cells reside. Migration of the transplanted cells was more extensive following damage to the auditory nerve. In the undamaged nerve, migration was more limited, but the cells showed more signs of neuronal differentiation. This highlights an important balance between tissue damage and the potential for repair.

Keynote review: The auditory system, hearing loss and potential targets for drug development

There is a huge potential market for the treatment of hearing loss. Drugs are already available to ameliorate predictable, damaging effects of excessive noise and ototoxic drugs. The biggest challenge now is to develop drug-based treatments for regeneration of sensory cells following noise-induced and age-related hearing loss. This requires careful consideration of the physiological mechanisms of hearing loss and identification of key cellular and molecular targets. There are many molecular cues for the discovery of suitable drug targets and a full range of experimental resources are available for initial screening through to functional analysis in vivo. There is now an unparalleled opportunity for translational research.