Deafness in TRβ Mutants Is Caused by Malformation of the Tectorial Membrane

Abstract

Thyroid hormone receptor ␤ (TR␤) dysfunction leads to deafness in humans and mice. Deafness in TR␤Ϫ/Ϫ mutant mice has been attributed to TR␤-mediated control of voltage-and Ca 2ϩ -activated K ϩ (BK) channel expression in inner hair cells (IHCs). However, normal hearing in young constitutive BK␣ Ϫ/Ϫ mutants contradicts this hypothesis. Here, we show that mice with hair cell-specific deletion of TR␤ after postnatal day 11 (P11) have a delay in BK␣ expression but normal hearing, indicating that the origin of hearing loss in TR␤ Ϫ/Ϫ mutant mice manifested before P11. Analyzing the phenotype of IHCs in constitutive TR␤ Ϫ/Ϫ mice, we found normal Ca 2ϩ current amplitudes, exocytosis, and shape of compound action potential waveforms. In contrast, reduced distortion product otoacoustic emissions and cochlear microphonics associated with an abnormal structure of the tectorial membrane and enhanced tectorin levels suggest that disturbed mechanical performance is the primary cause of deafness resulting from TR␤ deficiency.

Conditional Gene Expression in the Mouse Inner Ear Using Cre-loxP

“…These findings highlight the role of HC-SC junctions in hearing (Bahloul et al 2009). Deletion of thyroid hormone receptor beta using this Cre line clarified that malformation of the tectorial membrane (cochlear cause) and not delayed BK channel expression in HCs (retro-cochlear cause) produced hearing loss in the absence of this receptor (Winter et al 2009). …”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…Cochlear anatomy is examined on inner ear sections. Methacrylate plastic-embedded samples of paraformaldehyde/ glutaraldehyde-fixed cochlear tissue yield good preservation of cellular structure; 3-to 4-lm-thick sections can be stained with toluidine blue (448,501) or thionin (499,502) or aqueous hematoxylin (277).…”

“…A relatively rapid, noninvasive means of assessing auditory function is the measurement of the auditory-evoked brainstem response (277,448,501,502). Investigation of other features of cochlear function or cochlear nerve function may include determination of the endocochlear potential, compound action potential, and other physiological parameters (277).…”

Deficient forward transduction and enhanced reverse transduction in the alpha tectorin C1509G human hearing loss mutation

“…Sections were washed three times with PBST and then incubated with the secondary antibody at room temperature for 1 hour. The primary antibodies were rabbit anti -tectorin (1:1000) (Winter et al, 2009), rabbit anti -tectorin (1:1000) (Winter et al, 2009), rabbit anti-otogelin (1:2000; provided by Christine Petit, France) (Cohen-Salmon et al, 1997), and rabbit anti-myosin VIIa (1:200; Affinity Bioreagents, Golden, CO). The secondary antibodies were Alexa Fluor 488 or Alexa Fluor 594 donkey anti-rabbit (1:500; Invitrogen).…”