Thyroid hormone receptors TRα1 and TRβ differentially regulate gene expression ofKcnq4and prestin during final differentiation of outer hair cells

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Abstract
Thyroid hormone (TH or T3) and TH-receptor β (TRβ) have been reported to be relevant for cochlear development and hearing function. Mutations in the TRβ gene result in deafness associated with resistance to TH syndrome. The effect of TRα1 on neither hearing function nor cochlear T3 target genes has been described to date. It is also uncertain whether TRα1 and TRβ can act simultaneously on different target genes within a single cell. We focused on two concomitantly expressed outer hair cell genes, the potassium channel Kcnq4 and the motor protein prestin Slc26a5. In outer hair cells, TH enhanced the expression of the prestin gene through TRβ. Simultaneously Kcnq4 expression was activated in the same cells by derepression of TRα1 aporeceptors mediated by an identified THresponse element, which modulates KCNQ4 promoter activity. We show that T3 target genes can differ in their sensitivity to TH receptors having the ligand either bound (holoreceptors) or not bound (aporeceptors) within single cells, and suggest a role for TRα1 in final cell differentiation.