Tricellulin Expression and its Deletion Effects in the Endolymphatic Sac.

OBJECTIVES: Tricellulin is a tight junction (TJ)-forming protein that participates in the sealing function of tricellular TJs. Tricellulin-knockout (Tric-/-) mice show progressive hearing loss with degeneration of hair cells in the cochlea without physiological or physical disorders. In the present study, we investigated the tricellulin expression and its deletion effects in the endolymphatic sac (ES) using Tric-/- mice. MATERIALS AND METHODS: The ES epithelia from wild-type (WT) mice were laser-microdissected, and RT-PCR was performed. The ES sections from Tric-/- and WT mice were immunostained with an anti-tricellulin antibody. Hematoxylin and eosin staining was performed for morphological examination. The inner ear of Tric-/- mice was perfused with biotinylation reagents, and the ES sections were observed for tracer permeability assay after applying streptavidin-Alexa Fluor 488 conjugate. RESULTS: The tricellulin expression was confirmed by RT-PCR and by immunohistochemistry in the WT ES. The ES in Tric-/- mice showed normal morphology and revealed no biotin leakage from the lumen. CONCLUSION: The ES in Tric-/- mice showed no changes in morphology or disruption in macromolecular barrier function. The effects of solute leakages in the ES of Tric-/- mice may be very limited and compensatable, or that the ES epithelia may have other sealing system covering the lack of tricellulin.

Deletion of Tricellulin Causes Progressive Hearing Loss Associated with Degeneration of Cochlear Hair Cells.

Tricellulin (also known as MARVELD2) is considered as a central component of tricellular tight junctions and is distributed among various epithelial tissues. Although mutations in the gene encoding tricellulin are known to cause deafness in humans (DFNB49) and mice, the influence of its systemic deletion in vivo remains unknown. When we generated tricellulin-knockout mice (Tric(-/-)), we found an early-onset rapidly progressive hearing loss associated with the degeneration of hair cells (HCs); however, their body size and overall appearance were normal. Tric(-/-) mice did not show any morphological change pertaining to other organs such as the gastrointestinal tract, liver, kidney, thyroid gland and heart. The endocochlear potential (EP) was normal in Tric(-/-) mice, suggesting that the tight junction barrier is maintained in the stria vascularis, where EP is generated. The degeneration of HCs, which occurred after the maturation of EP, was prevented in the culture medium with an ion concentration similar to that of the perilymph. These data demonstrate the specific requirement of tricellulin for maintaining ion homeostasis around cochlear HCs to ensure their survival. The Tric(-/-) mouse provides a new model for understanding the distinct roles of tricellulin in different epithelial systems as well as in the pathogenesis of DFNB49.

Middle ear malformations in identical twins.

The majority of the congenital anomalies of middle ear are solitary and a non-hereditary. We report cases of identical twins with congenital incudo-stapedial disconnection. Case 1 was an 8-year-old girl. Hearing impairment was identified at the age of three. She was referred to our university hospital in April 2005. Pure-tone audiogram showed conductive hearing impairments. Computed tomography (CT) revealed the incudo-stapedial disconnections in both ears. The exploratory tympanotomies on the right and left ears were performed in May and July 2005, respectively. The surgical findings showed absence of the long process and presence of the lenticular process of the incus in both ears. After the reconstructions of ossicular chain, the hearing of both ears improved. Case 2 was an 11-year-old girl. The hearing impairment of the right ear was identified in May 2008. She was referred to our university hospital three months later. Pure-tone audiogram showed the conductive hearing impairment in the right ear. CT revealed the incudo-stapedial disconnection in the right ear. The surgery showed the same findings as those of case 1. Anomalies of both cases suggest that the lenticular process of the incus and the stapes originate from a common primordium.