1. Cell Biology
  2. Neuroscience
Download icon

Hearing: It takes two

  1. Teresa Nicolson  Is a corresponding author
  1. Oregon Health and Science University, United States
Insight
  • Cited 0
  • Views 868
  • Annotations
Cite this article as: eLife 2015;4:e11399 doi: 10.7554/eLife.11399

Abstract

Two forms of an unconventional myosin motor protein have separate functions in the growth and maintenance of hair bundles in auditory hair cells.

Main text

A major challenge in hearing research is to understand how structures known as ‘hair bundles’ are formed in the cochlea. Hair bundles have a crucial role in the detection of sound and the conversion of mechanical signals (that is, sound waves) into electrical signals. The cochlea contains two types of hair cells – inner and outer – and a hair bundle protrudes from the top of every hair cell. Each hair bundle consists of a collection of smaller hair-like structures called stereocilia that line up in rows within the bundle to form a structure that resembles a staircase (Figure 1). The stereocilia are filled with filaments made of the protein actin.

The roles of the two isoforms of myosin 15 (MYO15) in hair bundles.

Left: Schematic depiction showing the three rows of stereocilia in a normal hair bundle, with the first row (dark green) being the shortest and the third row (pale purple) being the tallest. This difference in height results in a characteristic staircase-like structure. The stereocilia in the first two rows mediate the process of mechanotransduction, and the large isoform of myosin 15 localizes to the tips of these stereocilia; the small isoform is found primarily in the taller stereocilia in the third row. Right: When both isoforms are defective or absent, the stereocilia in the third row do not reach their normal height (top). If the N-terminal extension in the large isoform is absent in mice, hair bundles form normally but some of the stereocilia in the first two rows degenerate in older animals (bottom). The large isoform of myosin 15 has a large extension (shown in orange) at its N-terminus.

Through studies of deaf patients, geneticists have made remarkable progress in identifying genes that are required for hearing (see hereditaryhearingloss.org). Many of the corresponding proteins are important for the function of hair cells and more than a dozen of them have roles in the hair bundle; these proteins include several myosin motor proteins that differ from the conventional myosin motors that are found in muscle cells. Hair cells actually produce two versions (or isoforms) of one of these unconventional myosin motors, myosin 15 (Wang et al., 1998; Liang et al., 1999). One of these isoforms has a large (134kD) extension at its N-terminus, but the role played by this extension in hair cells has long been a mystery.

A clue to the importance of the extension is provided by the fact that mutations in the gene (exon 2) that encodes the additional amino acids in the extension cause deafness in humans (Nal et al., 2007). To explore the role of this extension Jonathan Bird and co-workers – including Qing Fang as first author – have compared mice in which the myosin 15 proteins have the extension (isoform 1) and mice in which they do not (isoform 2; Fang et al., 2015).

Previously our knowledge about the function of myosin 15 was based on studies of mice with a mutant shaker2 gene: this mutation leads to defective hair cells in both the cochlea and the vestibular system, which is the part of the ear that controls balance. (The name shaker was coined to describe the unsteady movements seen in these mice). The shaker2 mutation effects both isoforms of myosin 15 and prevents the stereocilia growing beyond a certain height (Probst et al., 1998). The staircase-like structure seen in normal hair bundles is not seen in the shaker2 mice.

Experiments with an antibody that recognizes both isoforms suggest that myosin 15 is located at the tips of the stereocilia (Belyantseva et al., 2003). The shaker2 phenotype suggests that myosin 15 promotes the growth of stereocilia, presumably by working as an actual motor that interacts with actin filaments (Bird et al., 2014). However, the details of how this happens are not fully understood, although it might depend on proteins that are transported to the growing tip by myosin 15 (Belyantseva et al., 2005; Zampini et al., 2011).

To examine the role played by the large extension in isoform 1, Fang, Bird and colleagues – who are based at the University of Michigan, the National Institute on Deafness and Other Communication Disorders, and the University of Kentucky – generated an antibody that is specific to this isoform and used it to investigate the effects of deleting the exon 2 gene (Fang et al., 2015). Surprisingly, they found that isoform 1 is restricted to the first two rows of stereocilia in inner hair cells (Figure 1). In outer hair cells, on the other hand, isoform 1 is also found at the tall stereocilia in the third row. As for isoform 2, it is mainly present in the third row in inner hair cells.

Finding the two isoforms in different locations came as a surprise, but it could help to explain why deletion of the N-terminus and shaker2 mutations lead to different phenotypes. Shaker2 mutations affect both isoforms and lead to short hair bundles. Deletion of the N-terminus does not affect the length of stereocilia: rather, the hair bundles develop normally at first, but the first two rows of stereocilia then wither away. This suggests that the large isoform is important for the maintenance of a subset of the stereocilia: in particular, it maintains the stereocilia are involved in converting sound energy into an electrical signal in the inner part of the cochlea.

This conversion process, which is called mechanotransduction, is largely present in both the shaker2 mutants and in the mice in which the N-terminus has been deleted, albeit with some subtle differences. This phenotype suggests that myosin 15 is not directly involved in mechanotransduction: however, it seems that the large isoform of myosin 15 can recognize and accumulate at sites where this process takes place. The localization pattern of myosin 15 observed in the outer hair cells reinforces the idea that some form of membrane tension is required for accumulation of the large isoform.

A similar result was found with another protein (called sans) that is required for growth of stereocilia: deleting sans after hair bundles had fully formed caused the first two rows of stereocilia to shrink over time (Caberlotto et al., 2011). Sans interacts with the mechanotransduction machinery in hair cells (Lefèvre et al., 2008), and the loss of sans has a more dramatic effect on mechanotransduction than the loss of myosin 15. Nevertheless, these two cases suggest that it is possible to uncouple the different roles of various proteins in development and in the subsequent maintenance of mechanically-sensitive stereocilia in hair bundles. It will be interesting to see whether other short bundle mutants may have a similar phenotype, if given the chance.

References

Article and author information

Author details

  1. Teresa Nicolson

    Oregon Hearing Research Center and the Vollum Institute, Oregon Health and Science University, Portland, United States
    For correspondence
    nicolson@ohsu.edu
    Competing interests
    The author declares that no competing interests exist.

Publication history

  1. Version of Record published: October 6, 2015 (version 1)

Copyright

© 2015, Nicolson

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 868
    Page views
  • 103
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Download citations (links to download the citations from this article in formats compatible with various reference manager tools)

Open citations (links to open the citations from this article in various online reference manager services)

Further reading

    1. Cell Biology
    2. Neuroscience
    Francois Singh et al.
    Research Article

    Parkinson’s disease (PD) is a major and progressive neurodegenerative disorder, yet the biological mechanisms involved in its aetiology are poorly understood. Evidence links this disorder with mitochondrial dysfunction and/or impaired lysosomal degradation – key features of the autophagy of mitochondria, known as mitophagy. Here, we investigated the role of LRRK2, a protein kinase frequently mutated in PD, in this process in vivo. Using mitophagy and autophagy reporter mice, bearing either knockout of LRRK2 or expressing the pathogenic kinase-activating G2019S LRRK2 mutation, we found that basal mitophagy was specifically altered in clinically relevant cells and tissues. Our data show that basal mitophagy inversely correlates with LRRK2 kinase activity in vivo. In support of this, use of distinct LRRK2 kinase inhibitors in cells increased basal mitophagy, and a CNS penetrant LRRK2 kinase inhibitor, GSK3357679A, rescued the mitophagy defects observed in LRRK2 G2019S mice. This study provides the first in vivo evidence that pathogenic LRRK2 directly impairs basal mitophagy, a process with strong links to idiopathic Parkinson’s disease, and demonstrates that pharmacological inhibition of LRRK2 is a rational mitophagy-rescue approach and potential PD therapy.

    1. Cell Biology
    Pavan Vedula et al.
    Research Article Updated

    β- and γ-cytoplasmic actins are ubiquitously expressed in every cell type and are nearly identical at the amino acid level but play vastly different roles in vivo. Their essential roles in embryogenesis and mesenchymal cell migration critically depend on the nucleotide sequences of their genes, rather than their amino acid sequences; however, it is unclear which gene elements underlie this effect. Here we address the specific role of the coding sequence in β- and γ-cytoplasmic actins’ intracellular functions, using stable polyclonal populations of immortalized mouse embryonic fibroblasts with exogenously expressed actin isoforms and their ‘codon-switched’ variants. When targeted to the cell periphery using β-actin 3′UTR; β-actin and γ-actin have differential effects on cell migration. These effects directly depend on the coding sequence. Single-molecule measurements of actin isoform translation, combined with fluorescence recovery after photobleaching, demonstrate a pronounced difference in β- and γ-actins’ translation elongation rates in cells, leading to changes in their dynamics at focal adhesions, impairments in actin bundle formation, and reduced cell anchoring to the substrate during migration. Our results demonstrate that coding sequence-mediated differences in actin translation play a key role in cell migration.