Reorganization of actin during repair of hair bundle mechanoreceptors

Hair bundle mechanoreceptors can be damaged by over-stimulation or by exposure to calcium-free buffers. Provided the trauma is slight, hair bundles recover, although the subcellular mechanisms for such recovery are poorly understood. Hair bundle mechanoreceptors on tentacles of sea anemones are especially resilient, recovering from severe trauma within several hours. During the recovery period, large protein complexes are secreted called "repair proteins" containing replacement linkages for those lost during trauma. In the present study, we find that recovery requires reorganization of the actin-based cytoskeleton in hair bundles. F-actin is first partially depolymerized and then repolymerized in hair bundles based on confocal microscopy. Furthermore, stereocilia show considerable motility during repair based on field emission scanning electron microscopy of hair bundles fixed at 1 min intervals after exposure to exogenously supplied repair protein complexes. Recovery of vibration sensitivity occurs at the organismal level within 8 min. Paradoxically, a full recovery of morphology of hair bundles requires approximately 45 min and a recovery of F-actin levels requires approximately 40 min. Similarly, a full recovery of mechanoelectric responses of hair cells requires approximately 45 min. Thus, it appears that the recovery of responsiveness at the organismal level precedes a full recovery of hair bundles.