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

An in vitro model of developmental synaptogenesis using cocultures of human neural progenitors and cochlear explants

In mammals, the sensory hair cells and auditory neurons do not spontaneously regenerate and their loss results in permanent hearing impairment. Stem cell therapy is one emerging strategy that is being investigated to overcome the loss of sensory cells after hearing loss. To successfully replace auditory neurons, stem cell-derived neurons must be electrically active, capable of organized outgrowth of processes, and of making functional connections with appropriate tissues. We have developed an in vitro assay to test these parameters using cocultures of developing cochlear explants together with neural progenitors derived from human embryonic stem cells (hESCs). We found that these neural progenitors are electrically active and extend their neurites toward the sensory hair cells in cochlear explants. Importantly, this neurite extension was found to be significantly greater when neural progenitors were predifferentiated toward a neural crest-like lineage. When grown in coculture with hair cells only (denervated cochlear explants), stem cell-derived processes were capable of locating and growing along the hair cell rows in an en passant-like manner. Many presynaptic terminals (synapsin 1-positive) were observed between hair cells and stem cell-derived processes in vitro. These results suggest that differentiated hESC-derived neural progenitors may be useful for developing therapies directed at auditory nerve replacement, including complementing emerging hair cell regeneration therapies.

Otospheres derived from neonatal mouse cochleae retain the progenitor cell phenotype after ex vivo expansions

Because of their limited regenerative potential, cochlear hair cell loss is one of the major causes of permanent hearing loss in mammals. However, recent studies have shown that postnatal cochlear epithelia retain the progenitor cells that form otospheres. Otospheres are capable of self-renewing and differentiating into inner ear cell lineages, thereby suggesting a promising source for hair cell regeneration. We investigated retention of the progenitor cell phenotype in otospheres after ex vivo expansion, which is crucial for transplantation approaches. Reverse transcriptase-polymerase chain reaction and immunocytochemical analyses showed that otospheres derived from neonatal mice retained expression of stem and cochlear cell markers. After in vitro differentiation, otosphere-consisting cells differentiated into hair cell phenotypes after ex vivo expansion. However, the capacity of otospheres for self-renewal weakened with subsequent generations of ex vivo expansion. Our results indicate that ex vivo expanded-otospheres are useful experimental tools for studying hair cell regeneration in transplantation approaches and that the mechanisms for retention of the progenitor cell phenotype in otospheres should be investigated.

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.

Restoration of auditory evoked responses by human ES-cell-derived otic progenitors

Deafness is a condition with a high prevalence worldwide, produced primarily by the loss of the sensory hair cells and their associated spiral ganglion neurons (SGNs). Of all the forms of deafness, auditory neuropathy is of particular concern. This condition, defined primarily by damage to the SGNs with relative preservation of the hair cells, is responsible for a substantial proportion of patients with hearing impairment. Although the loss of hair cells can be circumvented partially by a cochlear implant, no routine treatment is available for sensory neuron loss, as poor innervation limits the prospective performance of an implant. Using stem cells to recover the damaged sensory circuitry is a potential therapeutic strategy. Here we present a protocol to induce differentiation from human embryonic stem cells (hESCs) using signals involved in the initial specification of the otic placode. We obtained two types of otic progenitors able to differentiate in vitro into hair-cell-like cells and auditory neurons that display expected electrophysiological properties. Moreover, when transplanted into an auditory neuropathy model, otic neuroprogenitors engraft, differentiate and significantly improve auditory-evoked response thresholds. These results should stimulate further research into the development of a cell-based therapy for deafness.