|Hearing restoration using human embryonic stem cells in deafened gerbils|
An estimated 250 million people worldwide have a disabling hearing impairment (WHO, 2001). This figure has risen dramatically in the last 15 years due to improvements in diagnosis, increasing longevity of the population and a combination of other factors including noise/occupational induced hearing loss and ototoxicity (damage to the Cochlea or auditory nerve), particularly from the use of aminoglycoside antibiotics.
Hearing loss can result from any perturbation of the transit of sound energy from the outer ear to the auditory cortex. The majority of age associated hearing loss is due to the loss of the mechanosensitive hair cells of the cochlea. A smaller proportion of hearing loss is caused by loss of auditory neurons that project from the cochlea to the auditory cortex.
Recent work from Dr Marcelo Rivolta, a stem cell biologist based at University of Sheffield, UK, has now shown in an online accepted publication from Nature that it is possible to produce these auditory neurons from human embryonic stem cells (hESCs) and that following transplantation into the cochleae of deafened gerbils, are able to restore functional hearing (Chen et al., 2012).
In this work, the authors present a protocol to induce auditory neuron differentiation from hESCs using signals known to be involved in the specification of the otic placode a thickening of outer embryonic ectoderm from where the ear develops. By treating hESCs with FGF3 and FGF10 they were able to produce two visually distinct types of otic progenitors; one of which developed into hair-cell-like cells and the other into auditory neurons. These were termed otic epithelia progenitors (OEPs) and otic neural progenitors (ONPs) respectively and both expressed PAX8, SOX2 and FOXG1, known otic placodal markers. The authors then used bioinformatic analyses to show that FGF treatment generated global changes in transcription compatible with otic induction.
OEPs and ONPs were then differentiated into hair cell like cells and auditory neurons under culture conditions previously published by the authors (Chen et al., 2009). Hair cell-like cells displayed typical hair cell markers including ATOH1 and MYO7a whilst auditory neuron cells expressed neuronal markers such as BRN3A and b-tubulin III in addiction to both cell types displaying characteristic electrophysiology.
In order to analyse the ability of hESC-derived auditory neurons to restore functional hearing, eighteen gerbils were deafened by oubain injection into the cochlear round window. Oubain, a glycoside known to block the plasma membrane Na+/K+ ATPase selectively kills auditory neurons whilst sparing cochlear hair cells. Oubain-treated ears then received transplants of ONPs into the cochlear modiolus and by 10 weeks transplanted ONPs had engrafted and differentiated to express typical neuronal markers including NKAa3 and GluA2. These de novo neurons contacted the basal end of the cochlear hair cells and fibres from these cells began migration from the cochlear modiolus to the brainstem. Functional performance was subsequently ascertained by measurement of auditory brainstem responses. Comparing the pre and post transplant hearing levels showed a 46% improvement in those transplanted with ONPs as compared to controls (53±.7 dB hearing level in the control animals, compared to 28.6±3.6 dB in ONP transplanted animals).
This work certainly gives great hope for those suffering from disabling hearing impairment. The authors speculate that these results could stimulate further research into the development of a cell-based therapy for deafness caused by auditory neuropathy or as an adjunct to traditional cochlear implantation. Transplantation of hESC-derived hair cell-like cells such as the OEPs generated in this study that were not tested for their integration capacity in this case, or deriving similar cell types from induced pluripotent stem cells (Oshima et al 2010) remains a tantalizing prospect with an even greater clinical potential.
WHO (2001d) Press release: WHO Calls on Private Sector to Provide Affordable Hearing Aids in Developing World,World Health Organization/34, 11 July 2001.
Chen W, Johnson SL, Marcotti W, Andrews PW, Moore HD, Rivolta MN.
Chen, W. et al.
Oshima K, Shin K, Diensthuber M, Peng AW, Ricci AJ, Heller S.
Original study from Nature.
STEM CELLS correspondent Peter Kullar reports on those studies appearing in current journals that are destined to make an impact on stem cell research and clinical studies.