SOX2 is required for inner ear growth and cochlear nonsensory formation before sensory development.

The transcription factor sex determining region Y-box 2 (SOX2) is required for the formation of hair cells and supporting cells in the inner ear and is a widely used sensory marker. Paradoxically, we demonstrate via fate mapping that, initially, SOX2 primarily marks nonsensory progenitors in the mouse cochlea, and is not specific to all sensory regions until late otic vesicle stages. SOX2 fate mapping reveals an apical-to-basal gradient of SOX2 expression in the sensory region of the cochlea, reflecting the pattern of cell cycle exit. To understand SOX2 function, we undertook a timed-deletion approach, revealing that early loss of SOX2 severely impaired morphological development of the ear, whereas later deletions resulted in sensory disruptions. During otocyst stages, SOX2 shifted dramatically from a lateral to medial domain over 24-48 h, reflecting the nonsensory-to-sensory switch observed by fate mapping. Early loss or gain of SOX2 function led to changes in otic epithelial volume and progenitor proliferation, impacting growth and morphological development of the ear. Our study demonstrates a novel role for SOX2 in early otic morphological development, and provides insights into the temporal and spatial patterns of sensory specification in the inner ear.

Generation of inner ear sensory neurons using blastocyst complementation in a Neurog1+/--deficient mouse.

This research is the first to produce induced pluripotent stem cell-derived inner ear sensory neurons in the Neurog1+/- heterozygote mouse using blastocyst complementation. Additionally, this approach corrected non-sensory deficits associated with Neurog1 heterozygosity, indicating that complementation is specific to endogenous Neurog1 function. This work validates the use of blastocyst complementation as a tool to create novel insight into the function of developmental genes and highlights blastocyst complementation as a potential platform for generating chimeric inner ear cell types that can be transplanted into damaged inner ears to improve hearing.

SOX2 is required for inner ear neurogenesis.

Neurons of the cochleovestibular ganglion (CVG) transmit hearing and balance information to the brain. During development, a select population of early otic progenitors express NEUROG1, delaminate from the otocyst, and coalesce to form the neurons that innervate all inner ear sensory regions. At present, the selection process that determines which otic progenitors activate NEUROG1 and adopt a neuroblast fate is incompletely understood. The transcription factor SOX2 has been implicated in otic neurogenesis, but its requirement in the specification of the CVG neurons has not been established. Here we tested SOX2's requirement during inner ear neuronal specification using a conditional deletion paradigm in the mouse. SOX2 deficiency at otocyst stages caused a near-absence of NEUROG1-expressing neuroblasts, increased cell death in the neurosensory epithelium, and significantly reduced the CVG volume. Interestingly, a milder decrease in neurogenesis was observed in heterozygotes, indicating SOX2 levels are important. Moreover, fate-mapping experiments revealed that the timing of SOX2 expression did not parallel the established vestibular-then-auditory sequence. These results demonstrate that SOX2 is required for the initial events in otic neuronal specification including expression of NEUROG1, although fate-mapping results suggest SOX2 may be required as a competence factor rather than a direct initiator of the neural fate.