β-Actin and γ-Actin Are Each Dispensable for Auditory Hair Cell Development But Required for Stereocilia Maintenance

Abstract

Hair cell stereocilia structure depends on actin filaments composed of cytoplasmic β-actin and γ-actin isoforms. Mutations in either gene can lead to progressive hearing loss in humans. Since β-actin and γ-actin isoforms are 99% identical at the protein level, it is unclear whether each isoform has distinct cellular roles. Here, we compared the functions of β-actin and γ-actin in stereocilia formation and maintenance by generating mice conditionally knocked out for Actb or Actg1 in hair cells. We found that, although cytoplasmic actin is necessary, neither β-actin nor γ-actin is required for normal stereocilia development or auditory function in young animals. However, aging mice with β-actin– or γ-actin–deficient hair cells develop different patterns of progressive hearing loss and distinct pathogenic changes in stereocilia morphology, despite colocalization of the actin isoforms. These results demonstrate overlapping developmental roles but unique post-developmental functions for β-actin and γ-actin in maintaining hair cell stereocilia.

Insights into the effects of disease‐causing mutations in human actins

“…These results are also in line with the findings of Ervasti and colleagues who showed that knockouts of each of the two non-muscle actins separately in mouse cochlea did not interfere with development of normal hearing at birth [Perrin et al, 2010]. However, the knockouts caused deafness to develop a few weeks after birth.…”

“…However, in none of these cases did the exogenous mutant actin appear to produce substantial effects on overall cell organization or morphology. The range of in vivo effects we observed could easily result in the kinds of structural disruption that would lead to disturbed stereociliary integrity and consequent deafness, and it shows that the yeast system is valuable as an initial rapid and efficient screen for gaining insight into the types of processes that these pathogenic mutations might interfere with.These results are also in line with the findings of Ervasti and colleagues who showed that knockouts of each of the two non-muscle actins separately in mouse cochlea did not interfere with development of normal hearing at birth [Perrin et al, 2010]. However, the knockouts caused deafness to develop a few weeks after birth.…”

Cytoskeletal changes in actin and microtubules underlie the developing surface mechanical properties of sensory and supporting cells in the mouse cochlea

“…7) suggests a crucial need for actin to develop OHC surface mechanical properties. Furthermore, these surface mechanical properties describe a subset of the cytoskeleton that has previously been shown to be essential for the organization of the actin-based cuticular plate (Sobkowicz et al, 1995;Nishida et al, 1998), which contributes to formation of the stereocilia bundle (Furness et al, 2005;Perrin et al, 2010;Etournay et al, 2010). However, the finding that OHCs soften after treatment with nocodazole also implicates microtubules in cuticular plate formation.…”

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

“…Using ROSA26 LacZ reporter mice (Hebert and McConnell 2000) and Z/AP reporter mice (Pirvola et al 2002), Cre + cells were first detected at embryonic day (E)8.5 in the otic placode. By E13.5, reporter expression was found throughout the otic lines are used.…”

“…By E13.5, reporter expression was found throughout the otic lines are used. The most commonly used line is the depending on the strength of the fluorescent reporter COX ET.AL: Conditional Gene Expression in the Mouse Inner Ear Using Cre-loxP Pirvola et al 2002;Arnold et al 2006;Zelarayan et al 2007;Barrionuevo et al 2008;Jones et al 2008;Rickheit et al 2008;Grimsley-Myers et al 2009;Schultz et al 2009;Wang et al 2009;Yamamoto et al 2009;Deng et al 2010;Freyer and Morrow 2010;Haugas et al 2010;Hurd et al 2010;Hwang et al 2010;Sipe and Lu 2011 (Hebert and McConnell 2000), which has been reported to cause haploinsufficiency phenotypes that include proliferation in other organs (Shen et al 2006;Eagleson et al 2007;Siegenthaler et al 2008). However, no change in proliferation in the inner ear has been reported in several papers where proper controls of Foxg1-Cre mice (without the floxed allele) were used (Yamamoto et al 2009(Yamamoto et al , 2011Hartman et al 2010;Brown and Epstein 2011).…”