Progression of Inner Ear Pathology in Ames Waltzer Mice and the Role of Protocadherin 15 in Hair Cell Development

Abstract

The Ames waltzer (av) mouse mutant exhibits auditory and vestibular abnormalities resulting from mutation of protocadherin 15 (Pcdh15). Ames waltzer has been identified as an animal model for inner ear pathology associated with Usher syndrome type 1F. Studies correlating anatomical phenotype with severity of genetic defect in various av alleles are providing better understanding of the role played by Pcdh15 in inner ear development and of sensorineural abnormalities associated with alterations in Pcdh15 protein structure as a result of gene mutation. In this work we present new findings on inner ear pathology in four alleles of av mice with differing mutations of Pcdh15 as well as varying alterations in inner ear morphology. Two alleles with in-frame deletion mutations (Pcdh15 av-J and Pcdh15 av-2J ) and two presumptive functional null alleles (Pcdh15 av-3J and Pcdh15 av-Tg ) were studied. Light and electron microscopic observations demonstrated that the severity of cochlear and vestibular pathology in these animals correlates positively with the extent of mutation in Pcdh15 from embryonic day 18 (E18) up to 12 months. Electron microscopic analysis of immature ears indicated early abnormalities in the arrangement of stereocilia and the inner and outer hair cell cuticular plates, stereocilia rootlets, and the actin meshwork within the cuticular plate. In severe cases, displacement of the kinocilium and alterations in the shape of the cuticular plate was also observed. Mice harboring inframe deletion mutations showed less disorganization of stereocilia and cuticular plates in the organ of Corti than the presumptive functional null alleles at P0-P10. A slower progression of pathology was also seen via light microscopy in older animals with inframe deletions, compared to the presumptive functional null mutations. In summary, our results demonstrate that mutation in Pcdh15 affects the initial formation of stereocilia bundles with associated changes in the actin meshwork within the cuticular plate; these effects are more pronounced in the presumed null mutation compared to mutations that only affect the extracellular domain. The positive correlation of severity of effects with extent of mutation can be seen well into adulthood.

Clarin-1, Encoded by the Usher Syndrome III Causative Gene, Forms a Membranous Microdomain

“…Scanning Electron Microscopy-Methods used for scanning electron microscopy have been described previously (21,25).…”

Supporting Cell Characteristics in Long-deafened Aged Mouse Ears

“…As the HCs degenerate, the support cells (SCs) expand and form scars preventing fluid mixing and maintaining the epithelium (Bohne 1976;Hawkins and Johnson 1981;Bohne and Rabbitt 1983;Forge 1985;Raphael and Altschuler 1991). The damaged epithelium can also transition to flat or simple cuboidal epithelium (Pawlowski et al 2006;Kim and Raphael 2007;Izumikawa et al 2008). Future therapeutic strategies to repair damaged mammalian auditory epithelium may involve inserting stem cells into the damaged epithelium, inducing SCs remaining in the epithelium to transdifferentiate into replacement HCs via gene therapy, and/or applying growth factors to stimulate the mitotic production of new sensory epithelial cells.…”

A new spontaneous mutation in the mouse protocadherin 15 gene

“…Reports on av mutants in the literature show that mutation in Pcdh15 affects hair bundle morphogenesis and polarity (Hampton et al, 2003;Pawlowski et al, 2006;Raphael et al, 2001;Washington et al, 2005) and mechanotransduction . More recently, a detailed study on the localization and function of Pcdh15 in hair cells by Senften et al (2006) strongly supports the role of Pcdh15 in bundle morphogenesis and polarity.…”