The inner ear cochlea is our peripheral auditory organ. Upon the arrival of sound, the auditory receptor cells, which are also called hair cells (HCs), first transmit sound signals into electrophysiological signals. The signal then projects into the cerebrum auditory cortex through spiral ganglion neurons (SGNs).Hereditary or postnatal trauma-mediated cell death of HCs and SGNs can cause different degrees of hearing impairment or permanent deafness. According to the World Health Organization (WHO), 0.3% of newborn babies, 5% of humans before the age of 45 and 50% of the population over 70-years old suffer hearing impairment. Furthermore, hearing loss also leads to social communication problems. Therefore, our laboratory has two long-term goals: First, from the prospective of developmental neurobiology, we seek to elucidate the key mechanisms and genes underlying how inner ear neural stem cells proliferate and differentiate into mature and functional HCs and SGNs; Second, we seek to regenerate HCs or SGNs after hearing impairment. We will manipulate key developmental genes and signaling pathways in the remaining cell resources and trans-differentiate them into HCs or SGNs, with the aim of recovering hearing capacity.

  The auditory system of mice and humans share many similar characteristics in terms of development and function. Due to the ease of genome editing, the mouse is an important mammalian model in the hearing field. Our laboratory primarily uses the mouse as an experimental model. We combine gene knockout/knockin/transgenic approaches, genetics, histology, molecular biology, biochemistry, pharmacology, High-throughput RNA-Seq, and electrophysiological techniques in order to achieve our long-term goals above.

  The three main research directions: 

  1. Auditory cell subtypes identification 

  Historically, cell types in the auditory system are roughly classified based on their morphologies, locations and functions. This nomenclature standard is helpful for communication between different laboratories in the field, but it causes great gene expression heterogeneity among the same cell type populations. It is one of the main reasons why it is challenging to identify the key developmental genes that regulate HCs and SGNs development. Our laboratory has developed a pipeline protocol to perform random single-cell RNA-Seq without knowing cell subtype unique genes, through which 50% of the genome (~12,000 genes) can be sequenced. Thorough transcriptomecomparison among individual cell will identify a few genes that can subdivide all the single cells into different groups or subtypes.

  2. Identifying the key developmental genes and signaling networks of each auditory cell subtype 

  First, by taking advantage of the cell subtype unique genes identified from the above single cell RNA-Seq and the principles of Drosophila specific and permanent genetic labeling technique (Nature Neuroscience, 2014), we will establish a series of new transgenic mouse lines; these mouse lines will allow us to use genetic approaches to reproducibly label the same cell subtypes with fluorescent proteins. Second, bulk level RNA-Seq of the same cell subtype (~100 cells) will be performed at different developmental stages and 95% of the genome can be deeply sequenced. Third, bioinformatics analysis will narrow down the most promising candidate genes, with weight on relative gene expression amount and dynamic patterns through development. Lastly, gain and loss-of-function analysis of these candidate genes will be pursued in order to identify the bona-fide master genes and signal pathways that regulate maturation of HCs and SGNs.

  3. Establishing mouse disease models for human hearing impairment and regeneration  

  Cell death of either HCs or SGNs can cause human hearing impairment.Cell death of HCs or SGNs can occur independently or in a coordinated fashion. SGNs rely heavily on neurotrophic factors released from HCs; thus secondary SGN cell death occurs after loss of HCs.Our laboratory will use both genetic and pharmacological approaches to kill different subtypes of HCs or SGNs. Thenwe will test whether we can trans-differentiate the remaining cell sources into HCs or SGNs by manipulating the bona-fide master genes and signal pathways discovered in section 2. Note that regeneration of SGNs has special clinical application as the therapeutic effects of cochlear implants depend on the functions of SGNs. Given promising results, we will search for drugs that are able to activate these candidate genes.

LIU Zhiyong, Ph.D.

Senior Investigator