Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, C03
Defective functional maturation of inner hair cells in myosin VIIa mutant mice
K. M. Ranatunga1, C. J. Kros1
1. School of Life Sciences, University of Sussex, Brighton, United Kingdom.
Figure 1 Inner hair cells from Myo7a6J/Myo7a6J adult mice retain immature electrophysiological properties. Basolateral currents were elicited by 50 ms voltage-steps from -120 mV to +50 mV in 10 mV increments followed by a step to -40 mV. Selected current traces from representative cells are presented for a Myo7a6J/Myo7a6J mutant (A) and a +/Myo7a6J control (B). Voltage responses were measured in response to 250 ms current injection steps from -20 pA to +50 pA in 10 pA increments and, in addition, -200 pA to 1000 pA in 100 pA increments for +/Myo7a6J control. Selected voltage traces are presented for a Myo7a6J/Myo7a6J mutant (C) and a +/Myo7a6J control (D). Levels of voltage-step/current injection and age of animal are as indicated. All experiments were performed at ~34-36
Mutations in the Myo7a gene that encodes the unconventional, non-muscle myosin, Myosin VIIa, are associated with Usher syndrome Ib, a rare genetic disorder that leads to deaf-blindness in humans, as well as non-syndromic deafness (1). In Shaker-1 mice, the orthologous recessive gene also causes deafness with impaired cochlear function and progressively disorganized stereocilia of the auditory hair cells (1). Mechano-electrical transduction in hair cells of mutants deficient in Myosin VIIa is impaired requiring force on the bundle beyond the physiological range (2). Inner hair cell basolateral currents and voltage responses were recorded from acutely dissected organ of Corti preparations of homozygous mutant and heterozygous control Myo7a6J mutant mice using the whole-cell patch-clamp technique as previously described (3). These mice carry a missense mutation of arginine to proline in the motor domain of Myosin VIIa, resulting in an 80% reduction in expression levels of Myosin VIIa that is presumed dysfunctional (4). All data are presented as Mean±SEM (number of cells). Statistical significance was evaluated with the unpaired t-test. We demonstrate, for the first time, that adult Myo7a6J/Myo7a6J (P20-P30) inner hair cells retain immature electrical properties (3). The fast, outward potassium current IK,f, normally expressed by around P12, is absent: measured at -25 mV it was -340±37 pA (n=6) in Myo7a6J/Myo7a6J (Fig 1A) and 1330±390 pA (n=5) in +/Myo7a6J controls (Fig 1B); p<0.01. The cells display immature-like stimulated spiking behaviour (n=8) (Fig 1C) unlike the typically mature graded receptor potentials of age-matched +/Myo7a6J controls (n=4) (Fig 1D). Neonatal (P2-P4) Myo7a6J/Myo7a6J mutants (n=10) show spontaneous and evoked spiking behaviour like +/Myo7a6J controls (n=8). Also the outward potassium currents of neonatal (P2-P4) Myo7a6J/Myo7a6J and +/Myo7a6J hair cells are similar: 2470±290 pA (n=9) and 2470±370 pA (n=8) respectively measured at -25 mV; p>0.05. These data explicitly show that lack of Myosin VIIa not only impairs mechano-electrical transduction but also causes dysfunctional development of inner hair cell basolateral currents.
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