Loss of BCL6 Function Leads to Increased Cochlear Hair Cell Vulnerability to Aging and Insult

Abstract

Hearing loss is the most common sensory deficit worldwide, affecting more than 30 million children and more than 430 million adults. The majority of hearing loss is caused by the loss of cochlear hair cells in the inner ear. Hair cells are critical to transducing sound waves to electrical energy, and without them hearing does not occur. This study aims to investigate the role of transcription factor (TF) BCL6 in regulating cochlear hair cell survival. The BCL6 protein can act as a DNA stabilizer in response to double-stranded breaks and as an anti-apoptotic protein that can block DNA-damage sensing proteins p53, ATM, ATR, and CHEK1. Recent hair cell-specific RNA-seq analysis shows that Bcl6 is expressed in both inner and outer hair cells (IHCs and OHCs) with upregulation in IHCs compared to OHCs. Cochlear hair cells are known to be vulnerable to exposure to noise and ototoxic drugs such as cisplatin. Since Bcl6 is expressed in hair cells and since BCL6 is anti-apoptotic, I hypothesize that deletion of Bcl6 will lead to early onset of apoptosis of hair cells. I also hypothesized that loss of function of BCL6 will accelerate age-related hearing loss and make hair cells more vulnerable to ototoxic insult. Since ubiquitous deletion of Bcl6 is lethal, the Cre-Lox system was created to generate conditional knockout of Bcl6 in Atoh1-expressing cells, including cochlear hair cells. I had three goals. First, I evaluated if loss of BCL6 function affects development and differentiation of hair cells. I found that there was no effect; hair cells developed normally and stereocilia looked normal. Measurements of auditory brainstem response and distortion product otoacoustic emission also showed the auditory function was normal in the Bcl6 knock-out mice. Second, I determined if deletion of Bcl6 accelerates age-related hearing loss. This goal aimed to evaluate necessity of Bcl6 in hair cell maintenance. I found that deletion of Bcl6 resulted in an exacerbated age-related hearing loss and hair cell degeneration. Third, I examined if loss of Bcl6 function led to an increased vulnerability to mechanical and ototoxic insult. I exposed WT and KO mice to mechanical (noise exposure) and ototoxic (cisplatin treatment) insults. Noise exposure exacerbated the hearing loss and hair cells loss in the basal turn of KO cochlea and did not abolish the differential vulnerability of IHCs and OHCs. Similarly, cisplatin treatment also exacerbated hearing loss in the KO cochlea. Cisplatin treatment increased OHC loss but did not increase IHC loss. Comparison of mice treated with cisplatin to age-matched controls showed IHCs of KO mice survived significantly more while OHCs of KO mice survived significantly less. Collectively, deletion of Bcl6 increased vulnerability of cochlear hair cells to insults. This work establishes the importance of the Bcl6 gene in cochlear hair cell apoptosis and survival. Since deletion of Bcl6 led to accelerated age-related hair cell degeneration and increased hair cell vulnerability to insults, enhancing the expression of Bcl6 using the gene therapy or drugs may preserve hearing against aging and ototoxicity.