Expression of Myh9 in the mammalian cochlea: Localization within the stereocilia

Abstract

Mutations of non-muscle myosin Type IIA or MYH9 are linked to syndromic or nonsyndromic hearing loss. The biologic function of MYH9 in the auditory organ and the pathophysiology of its dysfunction remain to be determined. The mouse represents an excellent model for investigating the biologic role of MYH9 in the cells and tissues affected by its dysfunction. A primary step toward the understanding of the role of MYH9 in hearing and its dysfunction is the documentation of its cellular and sub-cellular localization within the cochlea, the auditory organ. We describe the localization of Myh9 within the mouse cochlea using a polyclonal anti-Myh9-antibody, generated against an 18 amino acid long peptide corresponding to the sequence at the C-terminus of mouse Myh9. The anti-Myh9 antibody identified a single, specific, immunoreactive band of 220 kDa in immunoblot analysis of homogenate from a variety of different mouse tissues. The Myh9 antibody cross-reacts with the rat but not the human orthologue. Myh9 is expressed predominantly within the spiral ligament as well as in the sensory hair cells of the organ of Corti. Confocal microscopy of cochlear surface preparations, identified Myh9 within the inner and outer hair cells and their stereocilia. Localization of Myh9 within the stereocilia raises the possibility that mutations of MYH9 may effect hearing loss though disruption of the stereocilia structure.

Nonmuscle Myosin Heavy Chain IIA Mutation Predicts Severity and Progression of Sensorineural Hearing Loss in Patients With MYH9-Related Disease

“…Studies in rodents showed that NMMHC-IIA is expressed in the hair cells of the organ of Corti, spiral ligament and spiral limbus, with only minimal expression within the spiral ganglion (Lalwani et al 2000;Mhatre et al 2006). In hair cells, NMMHC-IIA was localized along the length of the stereocilia, the cuticular plate, and the plasma membrane (Mhatre et al 2006;Lalwani et al 2008).…”

“…These cells are supposed to play a role in providing anchorage for surrounding cells or developing or reacting to tension generated in the basilar membrane-spiral ligament complex (Henson et al 1984). Mhatre et al (2006) hypothesized that dysfunction of MYH9 could contribute to SNHL by disrupting the functions mediated by the fibrocytes of the spiral ligament. Indeed, the observation that cochlear implantation has a good outcome in MYH9-RD patients is consistent with these pathogenetic hypotheses, as the cochlear implant directly stimulates the spiral ganglion and thereby bypasses hair cell stereocilia and spiral ligament.…”

“…Studies in rodents showed that NMMHC-IIA is expressed in the hair cells of the organ of Corti, spiral ligament and spiral limbus, with only minimal expression within the spiral ganglion (Lalwani et al 2000;Mhatre et al 2006). In hair cells, NMMHC-IIA was localized along the length of the stereocilia, the cuticular plate, and the plasma membrane (Mhatre et al 2006;Lalwani et al 2008). On this basis, it was hypothesized that MYH9 mutations cause SNHL by altering the structural integrity of stereocilia, similarly to what has been observed in mouse models of deafness deriving from mutations in three other members of the myosin superfamily, namely myosin VI, VIIA, and XVA ( Avraham et al 1995;Probst et al 1998;Self et al 1998;Self et al 1999;Rzadzinska et al 2004).…”

Generation and Characterization of Mice with Myh9 Deficiency

“…Alternatively, the absence of a phenotype in the other target organs may reflect relatively lower sensitivity of the assays used to uncover the differences. Myh9 within the outer and inner hair cells (Mhatre et al 2004) including their stereocilia (Mhatre et al 2006) suggests that dysfunction and/or haploinsufficiency of Myh9 can lead to lead loss of hair cell integrity and hearing. The diminished OHC function in the outliers supports this hypothesis.…”

Auditory transduction in the mouse

“…The presence of other myosin isoforms has been reported in stereocilia and the cuticular plate, in addition to myosin 7a and myosin 1c, to date myosin 15a (see below), myosin 6 [33], and myosin IIa [54] have been identified. Understanding the interactions between these different myosin molecules and their significance in transducer channel adaptation requires further investigation.…”