On Monday, April 15, 2024 Shadan Hadi successfully defended her dissertation and earned her doctoral degree in physiology. Congratulations Dr. Hadi!
DISORGANIZATION OF ACTIN WITHIN THE SHAFTS OF STEREOCILIA IS A KEY DIFFERENCE BETWEEN TEMPORARY AND PERMANENT NOISE-INDUCED HEARING LOSS (NIHL)
During sound stimulation, mechanosensory stereocilia of the auditory hair cell pivot around their bases, where their actin cores become denser and form rootlets protruding into the cuticular plate. It is believed that actin-based cuticular plate provides a stable mechanical support for stereocilia, while rootles are responsible for their pivotal flexibility and life-long resilience to mechanical stimuli. Not surprisingly, damage to the stereocilia bundles is known as a hallmark of permanent noise-induced hearing loss (NIHL). Yet, despite decades of NIHL studies, it is still unknown which ultrastructural changes in the stereocilia bundles are evoked directly by mechanical overstimulation.
Here, we explored the changes in the actin cores of stereocilia, their rootlets, and cuticular plates, immediately after noise exposures. We compared the effects of noise that reliably generate either temporary (TTS) or permanent (PTS) shifts of hearing thresholds in the adult C57BL/6 mice in the frequency region of 16-20 kHz. Samples from this region of the organ of Corti were dissected from unexposed control and noise-exposed animals. Then, the samples were high pressure frozen, freeze-substituted, and low-temperature embedded for serial sectioning with focused ion-beam (FIB) and backscatter scanning electron microscopy (FIB-SEM).
We found that noise exposure causes global disorganization of actin within the stereocilia shafts and the cuticular plate, likely initiated by global increase in intracellular Ca2+, and expansion of the rootlet, possibly due to local mechanical breakage in the connection between the rootlet and the surrounding cuticular plate. Disorganization of actin within the cuticular plate occurred in both TTS and PTS while expansion of the rootlet started to occur in TTS and became more prominent in PTS. The only pathology that was consistently different between TTS and PTS is the disorganization of actin filaments (F-actin) in the shaft of stereocilia, the region that is known to have minimal or no turnover in mammalian auditory hair cells. We conclude that loss of F-actin integrity within stereocilia shafts is a key determinant for PTS.