Proceedings of The Physiological Society

Physiology 2014 (London, UK) (2014) Proc Physiol Soc 31, PCB072

Poster Communications

Sensory transduction in outer hair cells does not account for the hearing loss caused by haploinsufficiency of transcription factor Gata3

T. Bardhan1, W. Marcotti1, M. Holley1

1. Department of Biomedical Science, The University of Sheffield, Sheffield, United Kingdom.

Gata3 regulates the development of mammalian auditory sensory epithelia and spiral ganglion neurons, and haploinsufficiency leads to ~30dB of hearing loss in mice (1). Haploinsufficiency accounts for hypoparathyroidism, hearing loss and renal anomaly in human HDR syndrome (2). Hearing loss in heterozygous gata3 mice has an early onset and is thought to originate primarily from functional defects in the outer hair cells (OHCs; 3), although defects in the inner hair cells (IHCs) may also be possible. We recorded the physiology of hair cells to identify potential functional deficits that could explain the observed hearing loss. Whole cell voltage clamp recordings were used to measure basolateral membrane and mechanoelectrical transducer currents in OHCs and IHCs from wild type (wt: n=37) and gata3 heterozygous (het: n=29) mice from postnatal days P6 to P30. Whole cell current clamp recordings were used to measure voltage responses and resting membrane potential. Recordings were performed at room temperature (20-25°C) using the Axopatch 200B amplifier. Data acquisition was controlled by pClamp software and analyzed using Origin. Averages are reported as Mean ± S.E.M.We found that the size of the K+ currents measured at 0mV was similar between WT and HET mice both at the prehearing stage P6 (wt 2.0 ± 0.2nA, n=8; het 2.5 ± 0.1nA, n=20) and at the mature stage P22 (wt 3.0 ± 0.3nA, n=12; het 3.0 ± 0.4nA, n=5). Resting membrane potentials of OHCs at P22 were also comparable (wt -66 ± 2mV, n=5; het -64 ± 5mV, n=6). Maximal mechanoelectrical transducer currents at -121mV in het OHCs at P6 were normal (wt 1.4 ± 0.2nA, n=6; het 1.3 ± 0.1nA, n=3). Interestingly, we observed a ~25% reduction in the number of apical OHCs in het cochleae as early as P4 (wt 62 ± 1cells/150µm, n=4; het 46 ± 1cells/150µm, n=4).In immature P6 IHCs, basolateral membrane currents elicited at 0mV were not significantly different between wt and het mice (wt 3.5 ± 0.2nA, n=21; het 4.4 ± 0.8, n=6). However, from P16, the BK current Ik,f measured at -25mV appeared to be reduced in het IHCs, and this was more pronounced at P28-30 (wt 10 ± 3nA, n=4; het 1.2 ± 0.3nA, n=6). Resting membrane potentials of IHCs at this age were comparable (wt -72 ± 1mV, n=2; het -72 ± 1mV, n=5). IHC numbers in het animals were unaffected.We conclude that the physiological differentiation of OHCs and IHCs is not influenced by gata3 haploinsufficiency and that deficits in the sensory function of OHCs do not explain deafness in gata3 heterozygous mice. Functional deficits could, however, be related to electromotility and/or innervation. The premature degeneration of hair cells is accompanied by degeneration of supporting cells and spiral ganglion neurons, which reflect a more general developmental deficit in heterozygous mice.

Where applicable, experiments conform with Society ethical requirements