A review of gene delivery and stem cell based therapies for regenerating inner ear hair cells

Self-Assessment of Cochlear Health by New Cochlear Implant Recipients: Daily Impedance, Electrically Evoked Compound Action Potential and Electrocochleography Measurements Over the First Three Postoperative Months

HYPOTHESES

In newly implanted cochlear implant (CI) users, electrically evoked compound action (eCAPs) and electrocochleography (ECochGs) will remain stable over time. Electrode impedances will increase immediately postimplantation due to the initial inflammatory response, before decreasing after CI switch-on and stabilizing thereafter.

BACKGROUND

The study of cochlear health (CH) has several applications, including explaining variation in CI outcomes, informing CI programming strategies, and evaluating the safety and efficacy of novel biological treatments for hearing loss. Very early postoperative CH patterns have not previously been intensively explored through longitudinal daily testing. Thanks to technological advances, electrode impedances, eCAPs, and ECochGs can be independently performed by CI users at home to monitor CH over time.

METHODS

A group of newly implanted CI users performed daily impedances, eCAPs, and ECochGs for 3 months at home, starting from the first day postsurgery (N = 7) using the Active Insertion Monitoring system by Advanced Bionics.

RESULTS

Measurement validity of 93.5, 93.0, and 81.6% for impedances, eCAPs, and ECochGs, respectively, revealed high participant compliance. Impedances increased postsurgery before dropping and stabilizing after switch-on. eCAPs showed good stability, though statistical analyses revealed a very small but significant increase in thresholds over time. Most ECochG thresholds did not reach the liberal signal-to-noise criterion of 2:1, with low threshold stability over time.

CONCLUSION

Newly implanted CI recipients can confidently and successfully perform CH recordings at home, highlighting the valuable role of patients in longitudinal data collection. Electrode impedances and eCAPs are promising objective measurements for evaluating CH in newly implanted CI users.

Self-assessment of cochlear health by cochlear implant recipients

Recent technological advances in cochlear implant (CI) telemetry have enabled, for the first time, CI users to perform cochlear health (CH) measurements through self-assessment for prolonged periods of time. This is important to better understand the influence of CH on CI outcomes, and to assess the safety and efficacy of future novel treatments for deafness that will be administered as adjunctive therapies to cochlear implantation. We evaluated the feasibility of using a CI to assess CH and examined patterns of electrode impedances, electrically-evoked compound action potentials (eCAPs) and electrocochleography (ECochGs), over time, in a group of adult CI recipients. Fifteen subjects were trained to use the Active Insertion Monitoring tablet by Advanced Bionics, at home for 12 weeks to independently record impedances twice daily, eCAPs once weekly and ECochGs daily in the first week, and weekly thereafter. Participants also completed behavioral hearing and speech assessments. Group level measurement compliance was 98.9% for impedances, 100% for eCAPs and 99.6% for ECochGs. Electrode impedances remained stable over time, with only minimal variation observed. Morning impedances were significantly higher than evening measurements, and impedances increased toward the base of the cochlea. eCAP thresholds were also highly repeatable, with all subjects showing 100% measurement consistency at, at least one electrode. Just over half of all subjects showed consistently absent thresholds at one or more electrodes, potentially suggesting the existence of cochlear dead regions. All subjects met UK NICE guidelines for cochlear implantation, so were expected to have little residual hearing. ECochG thresholds were, unsurprisingly, highly erratic and did not correlate with audiometric thresholds, though lower ECochG thresholds showed more repeatability over time than higher thresholds. We conclude that it is feasible for CI users to independently record CH measurements using their CI, and electrode impedances and eCAPs are promising measurements for objectively assessing CH.

Therapeutic Application of Mesenchymal Stem Cells for Cochlear Regeneration

Hearing loss is one of the major worldwide health problems that seriously affects human social and cognitive development. In the auditory system, three components outer ear, middle ear and inner ear are essential for the hearing mechanism. In the inner ear, sensory hair cells and ganglion neuronal cells are the essential supporters for hearing mechanism. Damage to these cells can be caused by long-term exposure of excessive noise, ototoxic drugs (aminoglycosides), ear tumors, infections, heredity and aging. Since mammalian cochlear hair cells do not regenerate naturally, some therapeutic interventions may be required to replace the damaged or lost cells. Cochlear implants and hearing aids are the temporary solutions for people suffering from severe hearing loss. The current discoveries in gene therapy may provide a deeper understanding in essential genes for the inner ear regeneration. Stem cell migration, survival and differentiation to supporting cells, cochlear hair cells and spiral ganglion neurons are the important foundation in understanding stem cell therapy. Moreover, mesenchymal stem cells (MSCs) from different sources (bone marrow, umbilical cord, adipose tissue and placenta) could be used in inner ear therapy. Transplanted MSCs in the inner ear can recruit homing factors at the damaged sites to induce transdifferentiation into inner hair cells and ganglion neurons or regeneration of sensory hair cells, thus enhancing the cochlear function. This review summarizes the potential application of mesenchymal stem cells in hearing restoration and combining stem cell and molecular therapeutic strategies can also be used in the recovery of cochlear function.

Differentiation capacity of dental pulp stem cell into inner ear hair cell using an in vitro assay: a preliminary step toward treating sensorineural hearing loss

PURPOSE

Sensorineural hearing loss (SNHL) is commonly caused by the death or dysfunction of cochlear cell types as a result of their lack of regenerative capacity. However, regenerative medicine, such as stem cell therapy, has become a promising tool to cure many diseases, including hearing loss. In this study, we determined whether DPSCs could differentiate into cochlear hair cell in vitro.

METHODS

DPSCs derived from human third molar dental pulp were induced into NSCs using a medium containing basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) for 7 days, and then into cochlear hair cell using a medium containing EGF and IGF-1 for the next 14 days. We used the neuroepithelial protein marker nestin and cochlear hair cell marker myosin VIIa as the markers for cells differentiation. Cells expressing the positive markers under the microscope were confirmed to have differentiated into cochlear hair cell.

RESULTS

DPSCs were successfully induced to differentiate into NSCs, with mean 24% nestin-positive cells. We found that DPSC-derived NSCs have a great capacity in differentiating into inner ear hair cell-like cells with an average of 81% cells presenting myosin VIIa. Thus, DPSCs have high potential to serve as a good resource for SNHL treatment.

CONCLUSION

We found the high potential of DPSCs to differentiate into NSC. The ability of DPSCs in differentiating into neural lineage cell made them a good candidate for regenerative therapy in neural diseases, such as SNHL.

Protein biomarkers of neural system

The utilization of biomarkers for in vivo and in vitro research is growing rapidly. This is mainly due to the enormous potential of biomarkers in evaluating molecular and cellular abnormalities in cell models and in tissue, and evaluating drug responses and the effectiveness of therapeutic intervention strategies.

An important way to analyze the development of the human body is to assess molecular markers in embryonic specialized cells, which include the ectoderm, mesoderm, and endoderm. Neuronal development is controlled through the gene networks in the neural crest and neural tube, both components of the ectoderm. The neural crest differentiates into several different tissues including, but not limited to, the peripheral nervous system, enteric nervous system, melanocyte, and the dental pulp. The neural tube eventually converts to the central nervous system.

This review provides an overview of the differentiation of the ectoderm to a fully functioning nervous system, focusing on molecular biomarkers that emerge at each stage of the cellular specialization from multipotent stem cells to completely differentiated cells. Particularly, the otic placode is the origin of most of the inner ear cell types such as neurons, sensory hair cells, and supporting cells. During the development, different auditory cell types can be distinguished by the expression of the neurogenin differentiation factor1 (Neuro D1), Brn3a, and transcription factor GATA3. However, the mature auditory neurons express other markers including βIII tubulin, the vesicular glutamate transporter (VGLUT1), the tyrosine receptor kinase B and C (Trk B, C), BDNF, neurotrophin 3 (NT3), Calretinin, etc.

Advances in translational inner ear stem cell research

Stem cell research is expanding our understanding of developmental biology as well as promising the development of new therapies for a range of different diseases. Within hearing research, the use of stem cells has focused mainly on cell replacement. Stem cells however have a broad range of other potential applications that are just beginning to be explored in the ear. Mesenchymal stem cells are an adult derived stem cell population that have been shown to produce growth factors, modulate the immune system and can differentiate into a wide variety of tissue types. Potential advantages of mesenchymal/adult stem cells are that they have no ethical constraints on their use. However, appropriate regulatory oversight seems necessary in order to protect patients from side effects. Disadvantages may be the lack of efficacy in many preclinical studies. But if proven safe and efficacious, they are easily translatable to clinical trials. The current review will focus on the potential application on mesenchymal stem cells for the treatment of inner ear disorders.

Nonviral Reprogramming of Human Wharton's Jelly Cells Reveals Differences Between ATOH1 Homologues

The transcription factor atonal homolog 1 (ATOH1) has multiple homologues that are functionally conserved across species and is responsible for the generation of sensory hair cells. To evaluate potential functional differences between homologues, human and mouse ATOH1 (HATH1 and MATH-1, respectively) were nonvirally delivered to human Wharton's jelly cells (hWJCs) for the first time. Delivery of HATH1 to hWJCs demonstrated superior expression of inner ear hair cell markers and characteristics than delivery of MATH-1. Inhibition of HES1 and HES5 signaling further increased the atonal effect. Transfection of hWJCs with HATH1 DNA, HES1 siRNA, and HES5 siRNA displayed positive identification of key hair cell and support cell markers found in the cochlea, as well as a variety of cell shapes, sizes, and features not native to hair cells, suggesting the need for further examination of other cell types induced by HATH1 expression. In the first side-by-side evaluation of HATH1 and MATH-1 in human cells, substantial differences were observed, suggesting that the two atonal homologues may not be interchangeable in human cells, and artificial expression of HATH1 in hWJCs requires further study. In the future, this line of research may lead to engineered systems that would allow for evaluation of drug ototoxicity or potentially even direct therapeutic use.