How to Bury the Dead: Elimination of Apoptotic Hair Cells from the Hearing Organ of the Mouse
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Hair cell death is a major cause of hearing impairment. Preservation of surface barrier upon hair cell loss is critical to prevent leakage of potassium-rich endolymph into the organ of Corti and to prevent expansion of cellular damage. Understanding of wound healing in this cytoarchitecturally complex organ requires ultrastructural 3D visualization. Powered by the serial block-face scanning electron microscopy, we penetrate into the cell biological mechanisms in the acute response of outer hair cells and glial-like Deiters’ cells to ototoxic trauma in vivo. We show that Deiters’ cells function as phagocytes. Upon trauma, their phalangeal processes swell and the resulting close cellular contacts allow engulfment of apoptotic cell debris. Apical domains of dying hair cells are eliminated from the inner ear sensory epithelia, an event thought to depend on supporting cells’ actomyosin contractile activity. We show that in the case of apoptotic outer hair cells of the organ of Corti, elimination of their apices is preceded by strong cell body shrinkage, emphasizing the role of the dying cell itself in the cleavage. Our data reveal that the resealing of epithelial surface by junctional extensions of Deiters’ cells is dynamically reinforced by newly polymerized F-actin belts. By analyzing Cdc42-inactivated Deiters’ cells with defects in actin dynamics and surface closure, we show that compromised barrier integrity shifts hair cell death from apoptosis to necrosis and leads to expanded hair cell and nerve fiber damage. Our results have implications concerning therapeutic protective and regenerative interventions, because both interventions should maintain barrier integrity.
Keywordswound healing phagocytosis extrusion actin apoptosis Repair hearing inner ear hair cell supporting cell SBEM
This work was supported by the Academy of Finland, Jane and Aatos Erkko Foundation, Instrumentarium Foundation (U.P.), and Biocenter Finland. The authors thank Sanna Sihvo and Antti Salminen for excellent technical assistance, and R.J. Turner and K. Kaila for the generous gift of the NKCC1 antibody.
Conflict of Interest
The authors declare that they have no conflicts of interest, financial or otherwise, regarding this work.
Movie of 3D reconstructed DCs and OHCs at E18, demonstrating spatial relationship between these cell types. Nonlesioned cochlea. (MP4 5437 kb)
Movie of 3D reconstructed DCs and OHCs at P10, demonstrating spatial relationship between these cell types. Nonlesioned cochlea. (MP4 5329 kb)
Movie scanning up and down through serial sections, showing phagocytotic OHC debris within the phalangeal process of a DC. Movie starts from the reticular lamina and extends to the base of the phalangeal process. Acute lesion site. (MP4 4545 kb)
Movie of the 3D reconstructed bundleless OHC. Mitochondria, nucleus, innervated nerve fibers and the contour of the cell have been reconstructed. Late lesion site. (MP4 5234 kb)
Movie scanning up and down through serial sections from the late lesion site, showing 3D reconstruction of a DC cup, the OHC base within the cup and innervated nerve fibers. Nonlesioned cochlea. (MP4 7777 kb)
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