Critical Role Of Spectrin In Hearing Development And Deafness

The auditory system is the perceptual system for sound sensation,in which the hair cells(HCs)in the inner ear are the sound receptors.Inner ear HCs detect sound through the deflection of mechanosensory stereocilia.Stereocilia are inserted into the cuticular plate of HCs by parallel F-actin filaments at the center of each stereocilium,called rootlets,where they convert soundinduced mechanical vibrations into electrical signals,thereby enabling hearing.However,the molecules that allow rootlets to shear the mechanical stresses are unknown.Spectrin proteins are likely candidates.In most cells,antiparallel heterodimers formed by ?II-and ?II-spectrin subunits,together with actin filaments and other related proteins,assembled scaffold complex in the submembrane and play an important role in plasma membrane integrity.It has been reported that ?II-spectrin ? ?II-spectrin concentrated in the cuticular plate of HCs and involved in the normal function of HCs.However,the structures of these molecules have remained unknown.Using super-resolution fluorescence imaging,ring-liked spectrin is imaged around the base of stereocilia rootlets at postnatal day 14(P14).Ring-like structures of F-actin with a small F-actin point cluster at the center are observed outlining the edges of stereocilia rootlets as well,together with a previously unseen fan-shaped meshwork of F-actin in outer hair cells(OHCs).Dual STED confirms that spectrin rings are spatially aligned with the F-actin rings,and that they form two arrays of rings along the outer edge of the stereocilia rootlets with the F-actin rings on the outside.These concentric ring-like structures are closely connected to the edge of fan-shaped F-actin filaments to form a multi-protein meshwork,which plays an important role in the stability of the stereocilia.Functionally,?II-spectrin knockout mice display profound deafness,with the downregulation of ?II-spectrin,polarity loss or fusion of stereocilia,altered mophology of cuticular plate and HCs degeneration.Besides,these spectrin rings are associated with the hearing ability of mice,caused by gene depletion,noise or aging.Overall,our work has identified a novel structure of spectrin in the cuticular plate of HCs,which provides the prerequisite rigidity and flexibility to stereocilia,highlighting their critical role in mammalian hearing development.