<i>Rasd2</i> Knockout Exaggerates the Hearing Loss Phenotype of <i>Tsc1</i> ‐Deficient Mice

Rasd2 was originally identified as a striatum-enriched transcript and has been suggested to regulate several signaling pathways including the mTOR pathway. Transcriptomic analyses revealed that Rasd2 is also enriched in inner ear hair cells, while its function in the inner ear remains elusive. Here we showed that RASD2 is localized in the stereocilia as well as the cytoplasm of the hair cells. To investigate its physiological function, we established Rasd2 knockout mice using the CRISPR/Cas9 technique. Rasd2 knockout does not affect stereocilia morphology, and Rasd2 knockout mice manifest normal auditory function. TSC1 is an important negative regulator of mTOR signaling, and Tsc1 conditional knockout mice suffer from modest hearing loss. Interestingly, Rasd2 knockout exaggerates the hearing loss phenotype of Tsc1-deficient mice. Further investigation revealed that Rasd2/Tsc1 double knockout mice show enhanced stereocilia deficits, such as extra shorter-row stereocilia, elongated tallest-row stereocilia, and decreased stereocilia numbers in OHCs as well as stereocilia fusion in IHCs. Lastly, we found that albeit the mTORC1 pathway is not affected in Rasd2 knockout cochlea, it is significantly activated in the hair cells of Rasd2/Tsc1 double knockout mice compared to those of Tsc1 conditional knockout mice. Taken together, our present data suggest that RASD2 might play an important role in stereocilia morphology and hearing transduction through regulating mTOR activity.