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Myosin VIIA is required for aminoglycoside accumulation in cochlear hair cells
G P Richardson1, A Forge, C J Kros
1School of Biological Sciences, University of Sussex, Brighton BN1 9QG, United Kingdom.
Abstract:
Myosin VIIA is expressed by sensory hair cells and has a primary structure predicting a role in membrane trafficking and turnover, processes that may underlie the susceptibility of hair cells to aminoglycoside antibiotics. [3H]Gentamicin accumulation and the effects of aminoglycosides were therefore examined in cochlear cultures of mice with different missense mutations in the myosin VIIA gene, Myo7a, to see whether myosin VIIA plays a role in aminoglycoside ototoxicity. Hair cells from homozygous mutant Myo7ash1 mice, with a mutation in a nonconserved region of the myosin VIIA head, respond rapidly to aminoglycoside treatment and accumulate high levels of gentamicin. Hair cells from homozygous mutant Myo7a6J mice, with a mutation at a highly conserved residue close to the ATP binding site of the myosin VIIA head, do not accumulate [3H]gentamicin and are protected from aminoglycoside ototoxicity. Hair cells from heterozygotes of both alleles accumulate [3H]gentamicin and respond to aminoglycosides. Although aminoglycoside uptake is thought to be via apical surface-associated endocytosis, coated pit numbers on the apical membrane of heterozygous and homozygous Myo7a6J hair cells are similar. Pulse-chase experiments with cationic ferritin confirm that the apical endocytotic pathway is functional in homozygous Myo7a6J hair cells. Transduction currents can be recorded from both heterozygous and homozygous Myo7a6J hair cells, suggesting it is unlikely that the drug enters via diffusion through the mechanotransducer channel. The results show that myosin VIIA is required for aminoglycoside accumulation in hair cells. Myosin VIIA may transport a putative aminoglycoside receptor to the hair cell surface, indirectly translocate it to sites of membrane retrieval, or retain it in the endocytotic pathway.
Insights
Myosin VIIA is crucial for aminoglycoside antibiotic accumulation in sensory hair cells. Mutations protecting myosin VIIA prevent gentamicin uptake and aminoglycoside ototoxicity.
Area of Science:
- Oto- and Neurobiology
- Molecular and Cellular Biology
Background:
- Sensory hair cells are susceptible to aminoglycoside antibiotics.
- Myosin VIIA, expressed in hair cells, is implicated in membrane trafficking and turnover.
- The precise role of Myosin VIIA in aminoglycoside ototoxicity is unclear.
Purpose of the Study:
- To investigate the role of Myosin VIIA in aminoglycoside accumulation and ototoxicity in mouse cochlear cultures.
- To determine if mutations in the myosin VIIA gene (Myo7a) affect hair cell susceptibility to aminoglycosides.
Main Methods:
- Utilized cochlear cultures from mice with different missense mutations in the Myo7a gene.
- Quantified [3H]gentamicin accumulation in hair cells from wild-type, heterozygous, and homozygous mutant mice.
- Assessed hair cell response to aminoglycoside treatment and examined endocytosis markers.
Main Results:
- Homozygous Myo7ash1 mutant hair cells rapidly accumulated high levels of gentamicin and were susceptible to aminoglycosides.
- Homozygous Myo7a6J mutant hair cells, with a mutation near the ATP binding site, did not accumulate gentamicin and were protected from ototoxicity.
- Apical endocytosis and transduction currents were functional in Myo7a6J mutant hair cells, ruling out diffusion via the mechanotransducer channel as the entry route.
Conclusions:
- Myosin VIIA is essential for the accumulation of aminoglycosides in sensory hair cells.
- Myosin VIIA may be involved in transporting an aminoglycoside receptor to the cell surface or retaining it within the endocytotic pathway.
- Targeting Myosin VIIA function could offer a strategy to mitigate aminoglycoside-induced hearing loss.