The Function And Underlying Mechanism Of Tmcc2 And Rasd2 In Inner Ear Hair Cells

Hearing is an important source for human to perceive the external environment.Among all kinds of sensory defects,deafness is the most common disease.Mammals’ adult cochlear hair cells can’t regenerate,so deafness caused by death of hair cells is irreversible.At present,researchers have identified several deafness genes,some of which are specifically expressed in hair cells and are essential for hair cell development and function.In addition to these genes,transcriptome studies have identified several genes that are specifically expressed in hair cells,which functions are mostly unknown.In this paper,we systematically studied the function of Tmcc2 and Rasd2 genes,which specifically expressed in hair cells,using knockout mice and other tools.Tmcc2 encodes an endoplasmic reticulum transmembrane protein.At present,there are very few studies on the function of TMCC2.In the present work,we show that Tmcc2 is specifically expressed in the auditory hair cells of mouse inner ear.About half of the Tmcc2 knockout mice died within three days of birth.The survived Tmcc2 knockout mice suffer from congenital hearing loss.Tmcc2 knockout mice show auditory hair cell loss and hair bundle loss which are begainning about 2 weeks afterbirth and increased with age.The general morphology and function of ER are unaffected in Tmcc2 knockout hair cells.The subcellular localization of transmembrane proteins such as Prestin are not affected.Suggesting that Tmcc2 knockout does not affect ER structure and membrane protein synthesis or transport functions.In vivo and in vitro experiments showed that Tmcc2 knockout leads to elevated ER stress,which may cause hair cell death.Rasd2 encodes a thyroid hormone-induced protein which regulates the physiological function of striatum.Compared with the central nervous system,RASD2 has been less studied in auditory function.We show that Rasd2 was specifically expressed in inner ear hair cells,and the expression level of Rasd2 decreased with development of mice.RASD2 is localized in the cell membrane and cytoplasm of hair cells.It has no significant effect on mice hearing function not only under normal conditions but also noise stimulation.The development and maintenance of cochlear hair bundles were also normal.These suggest that RASD2 is not essential in the auditory system.Previous researches have shown that RASD2 can regulate the mTORC1 signaling pathway.Mice knockout Tsc1,an important regulatory gene for mTORC1 pathway,showed progressive deafness,so we constructed Rasd2 and Tsc1 gene double knockout mice.More severe hearing loss and abnormal hair bundles were observed in double knockout mice than in Tsc1 conditional knockout mice.It suggests that RASD2 may co-regulate with TSC1 in cochlear hair cell development and/or function.In conclusion,we used knockout mice and other tools to explore the functions of Tmcc2 and Rasd2 genes in hair cells.We found that TMCC2 plays an important role in hair cell development and function by influencing endoplasmic reticulum stress,RASD2 plays an important role in hair cell development and function by synergistic action with the TSC1.Follow-up work will further investigate the specific mechanisms of TMCC2 and RASD2 in hair cells.