| Abstract: | Supporting cells in the cochlea play critical roles in the development, maintenance, and function of sensory hair cells and auditory neurons. Although the loss of hair cells or auditory neurons results in sensorineural hearing loss, the consequence of supporting cell loss on auditory function is largely unknown. In this study, we specifically ablated inner border cells (IBCs) and inner phalangeal cells (IPhCs), the two types of supporting cells surrounding inner hair cells (IHCs) in mice in vivo. We demonstrate that the organ of Corti has the intrinsic capacity to replenish IBCs/IPhCs effectively during early postnatal development. Repopulation depends on the presence of hair cells and cells within the greater epithelial ridge and is independent of cell proliferation. This plastic response in the neonatal cochlea preserves neuronal survival, afferent innervation, and hearing sensitivity in adult mice. In contrast, the capacity for IBC/IPhC regeneration is lost in the mature organ of Corti, and consequently IHC survival and hearing sensitivity are impaired significantly, demonstrating that there is a critical period for the regeneration of cochlear supporting cells. Our findings indicate that the quiescent neonatal organ of Corti can replenish specific supporting cells completely after loss in vivo to guarantee mature hearing function.Inner hair cells (IHCs), the sensory cells of the mammalian auditory sensory epithelium, are surrounded by specialized supporting cells (SCs) called “inner border cells” (IBCs) and “inner phalangeal cells” (IPhCs) (Fig. S1A). IBCs and IPhCs, together with other SCs, are known to play critical roles during the development and maturation of the organ of Corti, in processes such as patterning of the epithelium, synaptogenesis, and initiation of electrical activity in auditory nerves before the onset of hearing and formation of extracellular matrices (1–7). SCs also are essential for the function of the mature organ of Corti, where they contribute to the maintenance of the reticular lamina at the apical surface of the epithelium (8), control the extracellular concentration of ions (e.g., K+) (9, 10) and neurotransmitters (e.g., glutamate) (11), and support hair cell (HC) and auditory sensory neuron survival (5, 12–15). SCs also have been proposed to regulate the effects of insults on HCs by releasing molecules that either promote (e.g., ERK1 and 2) (16) or reduce (e.g., heat shock protein 70) (17) HC death. Additionally, SCs impact the extent of damage in the auditory epithelium through scar formation and clearance of HC debris (18). Furthermore, SCs are considered a potential source of cells for HC replacement in mammals, because SCs are a documented source of new HCs in cultured neonatal cochlea (19) and in adult utricles (20). Additionally, nonmammalian vertebrates regenerate HCs and SCs after damage and recover hearing, with the SCs being the source of the regenerative response (21–23). Indeed, if SCs are damaged by insults, the regenerative response is severely compromised (1, 24). Thus, it is assumed that the presence of these cells in the postnatal cochlea is essential for hearing, but specific roles of IBCs and IPhCs in HC maintenance and cochlear function have not been established.To determine the consequences of neonatal IBC and IPhC loss on the mature organ of Corti, we ablated these cells in vivo using an inducible diphtheria toxin fragment A (DTA) transgenic approach (25). Unexpectedly, we found that when these IHC supporting cells are eliminated immediately after birth, they are replaced efficiently within days. Moreover, this regeneration preserves the structure and function of the organ of Corti, so that mice with transient IBC/IPhC loss retain normal hearing as adults. In contrast, IBCs and IPhCs do not regenerate if ablation occurs after the onset of hearing, resulting in IHC loss and severe hearing impairment. Our studies also indicate that IBC and IPhC replacement in the neonatal cochlea results from transdifferentiation of less-specified SCs within the neighboring greater epithelial ridge (GER or Kölliker’s organ), which does not require cell proliferation. The unexpected regenerative capacity of SCs in the early postnatal organ of Corti in vivo may provide new strategies to regenerate its nonsensory and sensory cells after damage. |
