In a study published in Cell Reports, researchers from LIU Zhiyong’s lab at the Institute of Neuroscience, Center for Excellence in Brain Science and Intelligence Technology of the Chinese Academy of Sciences (CAS), and WEI Wu's group at the Shanghai Institute of Nutrition and Health of CAS, revealed the single-cell transcriptome characteristics of mouse otic neuronal lineage at three different embryonic stages E9.5, E11.5 and E13.5, and discovered a variety of new genes specific for otic precursor and for inner ear ganglion subtypes.

The vestibular ganglion neurons (VGNs) and spiral ganglion neurons (SGNs) of the inner ear play key roles in maintaining body balance and perceiving sound information, respectively, and their damage or degeneration leads to balance disorders and deafness. SGNs are also important for the clinical efficacy of cochlear implants. Although the mature inner ear VGNs and SGNs have distinct functions, they both originate from the otic neuroblasts, also known as the cochlear-vestibular ganglion (CVG) precursor cells.

Much remains poorly understood regarding the molecular mechanisms underlying CVG specification, and differentiation of CVGs into either VGNs or SGNs. In particular, it also unknown that when VGN and SGN subtypes emerge and if so, how many subtypes are present. Apparently, addressing these questions will not only help to uncover the molecular mechanisms underlying the development of the inner ear ganglia, but also provide potential key genes or gene networks for regeneration and repairment of adult inner ear ganglia.

Using 10x Genomics single-cell RNA-sequencing technology, this study revealed the transcriptome of mouse inner ear tissue at three early embryonic stages, and found that as neural differentiation progressed, CVG cells migrate from the otic epithelium and gradually lose their epithelial characteristics. At E9.5, a subset of Neurog1+ otic vesicle cells begin to express Insm1 and Shox2, eventually become CVG cells. VGNs and SGNs start to exhibit distinct gene profiles at E11.5.

In addition, the findings revealed that during otic neurogenesis, the production of undifferentiated CVG cells and the differentiation of VGNs or SGNs occur concurrently. According to the newly discovered genes specific for different stages of CVG precursors, VGNs and SGNs, they constructed two new transgenic mouse models of Shox2-P2A-Cre/+ and Casz1*3xHA-P2A-Tdtomato/+, which can be used for follow-up more accurate inner ear ganglion labeling and gene function analysis.

Previous studies revealed the existence of four distinct subtypes in adult SGNs. The trajectory analysis in this study suggested that the SGN subtypes have not been established at E13.5. Conversely, this study identified two subtypes of VGN at E13.5: Tlx3 +/Sall3+/Gata3- type I VGN and Tlx3+/Sall3-/Gata3+ type II VGN, therefore, the VGN subtypes appear earlier than the SGN counterparts. The specific functions of these VGN subtypes will be an important research direction in future.

The results of this study provide a new theoretical basis for the treatment of balance and auditory dysfunction caused by abnormal inner ear ganglia.