An RNA interference-based screen of transcription factor genes identifies pathways necessary for sensory regeneration in the avian inner ear

Sensory hair cells of the inner ear are the mechano-electric transducers of sound and head motion. In mammals, damage to sensory hair cells leads to hearing or balance deficits. Non-mammalian vertebrates such as birds can regenerate hair cells after injury. In a previous study, we characterized transcription factor gene expression during chicken hair cell regeneration. In those studies, a laser micro-beam or ototoxic antibiotics were used to damage the sensory epithelia (SE). The current study focused on 27 genes that were up-regulated in regenerating SE compared to untreated SE in the previous study. Those genes were knocked down by siRNA, to determine their requirement for supporting cell proliferation and to measure resulting changes in the larger network of gene expression. We identified 11 genes necessary for proliferation and also identified novel interactive relationships between many of them. Defined components of the WNT, PAX and AP1 pathways were shown to be required for supporting cell proliferation. These pathways intersect on WNT4, which is also necessary for proliferation. Among the required genes, the CCAAT enhancer binding protein, CEBPG, acts downstream of Jun Kinase and JUND in the AP1 pathway. The WNT co-receptor LRP5 acts downstream of CEBPG as does the transcription factor BTAF1. Both of these genes are also necessary for supporting cell proliferation. This is the first large scale screen of its type and suggests an important intersection between the AP1 pathway, the PAX pathway and WNT signaling in the regulation of supporting cell proliferation during inner ear hair cell regeneration

Epigenetic Influences on Sensory Regeneration: Histone Deacetylases Regulate Supporting Cell Proliferation in the Avian Utricle

The sensory hair cells of the cochlea and vestibular organs are essential for normal hearing and balance function. The mammalian ear possesses a very limited ability to regenerate hair cells and their loss can lead to permanent sensory impairment. In contrast, hair cells in the avian ear are quickly regenerated after acoustic trauma or ototoxic injury. The very different regenerative abilities of the avian vs. mammalian ear can be attributed to differences in injury-evoked expression of genes that either promote or inhibit the production of new hair cells. Gene expression is regulated both by the binding of cis-regulatory molecules to promoter regions as well as through structural modifications of chromatin (e.g., methylation and acetylation). This study examined effects of histone deacetylases (HDACs), whose main function is to modify histone acetylation, on the regulation of regenerative proliferation in the chick utricle. Cultures of regenerating utricles and dissociated cells from the utricular sensory epithelia were treated with the HDAC inhibitors valproic acid, trichostatin A, sodium butyrate, and MS-275. All of these molecules prevent the enzymatic removal of acetyl groups from histones, thus maintaining nuclear chromatin in a “relaxed” (open) configuration. Treatment with all inhibitors resulted in comparable decreases in supporting cell proliferation. We also observed that treatment with the HDAC1-, 2-, and 3-specific inhibitor MS-275 was sufficient to reduce proliferation and that two class I HDACs—HDAC1 and HDAC2—were expressed in the sensory epithelium of the utricle. These results suggest that inhibition of specific type I HDACs is sufficient to prevent cell cycle entry in supporting cells. Notably, treatment with HDAC inhibitors did not affect the differentiation of replacement hair cells. We conclude that histone deacetylation is a positive regulator of regenerative proliferation but is not critical for avian hair cell differentiation