There are specialized cells in the inner ear called sensory hair cells that detect sound. As most audiologists and otolaryngologists know, these hair cells are susceptible to damage by loud noise, ototoxic drugs, genetic mutations or aging, which can cause profound hearing loss. 

Since these hair cells do not regrow after being damaged, intense research efforts have been made into discovering how they develop and might be regrown once they are lost. In the past it has been almost impossible to obtain human hair cells for use in research and so researchers have relied on the use of model species such mice or chickens to investigate inner ear disorders and methods of promoting hair cell regeneration.  For researchers looking into cochlear function at the hair cell level, it is critically important to establish a way of producing and studying human hair cells in the laboratory.  One University of Indiana School of Medicine study funded by Action on Hearing Loss in the UK aims to change the methods by which human hair cells are studied.

The “Ear in a Dish” Project

The  research group is headed by Dr. Eri Hashino, the Ruth C. Holton Professor of Otolaryngology at Indiana University, School of Medicine.  She and colleague Dr. Karl R. Koehler have recently developed a method of turning mouse embryonic stem cells into inner ear cells, which has allowed them to observe the process of inner ear development outside the body. The protocol outlined in their 2014 study published in Nature, describes their culture system in which inner-ear sensory tissue is produced from mouse embryonic stem cells under chemically defined conditions. 

According to the researchers, previous attempts to “grow” inner-ear hair cells in standard cell culture systems have worked poorly in part because necessary cues to develop hair bundles—a hallmark of sensory hair cells and a structure critically important for detecting auditory or vestibular signals—are lacking in the flat cell culture dish.  According to Dr. Hashino,  the research team determined that the cells needed to be suspended as aggregates in a specialized culture medium providing an environment more like the body in its early development period. The team was able to create this early development process with a precisely timed use of several small molecules that prompted the stem cells to differentiate, from one stage to the next, into precursors of the inner ear. 

It was, however, the three dimensional suspension that also provided important mechanical cues, such as the tension from the pull of cells on each other. Dr. Koehler, then a graduate student in the medical neuroscience program at the IU School of Medicine and the paper’s first author told the Medical Press in 2014,  “The three-dimensional culture allows the cells to self-organize into complex tissues using mechanical cues that are found during embryonic development, and we were surprised to see that once stem cells are guided to become inner-ear precursors and placed in 3-D culture, these cells behave as if they knew not only how to become different cell types in the inner ear, but also how to self-organize into a pattern remarkably similar to the native inner ear.” Dr. Hashino said. “Our initial goal was to make inner-ear precursors in culture, but when we did testing we found thousands of hair cells in a culture dish.”  

The team’s electrophysiological testing further proved that hair cells generated from stem cells were 1/2 functional, and were the type that sense gravity and motion. Moreover, neurons like those that normally link the inner-ear cells to the brain had also developed in the cell culture and were connected to the hair cells.

They feel that their model is amenable to basic and translational investigations into inner ear biology and possible hair cell regeneration. In their project, now funded by Action on Hearing Loss in the UK, they will spend until 2017 adapting their animal method for use with human stem cells allowing the exploration of human inner ear development and, hopefully, develop a system that can be incorporated to discover drugs that regenerate hair cells and/or protect them from damage. The research group will further examine whether hair cells made from human stem cells are similar to normal hair cells, in terms of overall appearance, the proteins they produce and whether they can produce the same electrical signals. Additionally, their work will determine if stem cell-derived hair cells are structurally and functionally identical to the real hair cells of the human inner ear. 

Estimates are that genetic and environmental causes of damage to hair cells underlie more than 80% of all hearing loss. Thus, methods of enabling hair cell regeneration would benefit the worldwide population with sensori-neural hearing loss. Moreover, the ability to screen large numbers of potential drugs for their ability to protect hair cells from drug-induced damage will benefit those who undergo antibiotic or chemotherapy treatments. This project will produce a new human model system of inner ear hair cells that will be used to help develop these treatments.

References:

Action on Hearing Loss (2016).  An ear in a dish:  Creating a human model of the cochlea to help develop new treatments for hearing loss.  Retrieved September 6, 2016.

Fliesler, N. (2016).  An inner ear in a dish. Vector.  Retrieved September 7, 2016.

Koehler,K. & Hashino, E.  (2014). 3D mouse embryonic stem cell nature for generating inner ear organoids. Nature Protocols, Vol 9, pp. 1229-1244. Retrieved September 6, 2016.

Medical Press (2013). Researchers create the inner ear from stem cells, opening potential for new treatments.  Indiana University School of Medicine, Department of Otolaryngology.  Retrieved September 6, 2016.