Auditory brainstem responses are impaired in EphA4 and ephrin-B2 deficient mice

Abstract

The Eph receptor tyrosine kinases and their membrane-anchored ligands, ephrins, are signaling proteins that act as axon guidance molecules during chick auditory brainstem development. We recently showed that Eph proteins also affect patterns of neural activation in the mammalian brainstem. However, functional deficits in the brainstems of mutant mice have not been assessed physiologically. The present study characterizes neural activation in Eph protein deficient mice in the auditory brainstem response (ABR). We recorded the ABR of EphA4 and ephrin-B2 mutant mice, aged postnatal day 18-20, and compared them to wild type controls. The peripheral hearing threshold of EphA4(-/-) mice was 75% higher than that of controls. Waveform amplitudes of peak 1 (P1) were 54% lower in EphA4(-/-) mice than in controls. The peripheral hearing thresholds in ephrin-B2(lacZ/)(+) mice were also elevated, with a mean value 20% higher than that of controls. These ephrin-B2(lacZ/)(+) mice showed a 38% smaller P1 amplitude. Significant differences in latency to waveform peaks were also observed. These elevated thresholds and reduced peak amplitudes provide evidence for hearing deficits in both of these mutant mouse lines, and further emphasize an important role for Eph family proteins in the formation of functional auditory circuitry.

Figure 1

ABR threshold. Averaged traces from each level of click presentation are shown in the same voltage scale, stacked in increasing dB SPL order. Grey arrowheads indicate ABR threshold. (A) A representative sample of traces from an EphA4^+/+ mouse shows an ABR with threshold near 40dB. The small black arrowhead on the time axis indicates the approximate arrival of sound at the tympanic membrane (applies to all panels). (B) The EphA4^−/− mouse shows a higher threshold, in this case close to 80dB. In addition, the peak amplitudes appear smaller in the EphA4^−/− (compare 100dB traces in A and B). (C) Representative trace from an ephrin-B2^+/+ mouse. (D) Similar to the EphA4 mutant, both an elevated threshold and reduced peak amplitude is seen in ephrin-B2^lacZ/+ mice as compared to wild type (compare 100dB traces in C and D).

Figure 2

Measurement of ABR threshold in all genotypes. Both EphA4^+/− and EphA4^−/− mice show a significantly increased ABR threshold as compared to wild type (A), and the mutant value is 75% higher than control. The ephrin-B2^lacZ/+ mice also shows a 20% elevated ABR threshold as compared to wild type (B).

Figure 3

A comparison of amplitude and latency measures in all genotypes. EphA4 genotype key applies to A and B and ephrin-B2 genotype key applies to C and D. P1 and P2 showed decreased amplitude in EphA4^−/− mice (A), whereas there was no difference in latency for either peak (B). ephrin-B2^lacZ/+ mice show a decreased amplitude in P1 (C ) and a decreased latency for P2 (D).

Figure 4

The latency to P3 peak was longer in EphA4^−/− (A) and ephrin-B2^lacZ/+ (B) mice, as compared to matched wild types.