GluA4 is indispensable for driving fast neurotransmission across a high‐fidelity central synapse

Abstract

Non-technical summary Localization of sound sources in the azimuth, which makes use of interaural differences in timing and/or intensity of acoustic signals, is of vital importance for most mammals. Using the small differences in time of arrival and/or intensity at the two ears requires that propagation of electric pulses in the auditory system be temporally precise. In this study, we found that elimination of GluA4, a protein particularly abundant in auditory cells, significantly impairs their ability to faithfully transmit electric signals, leading to profound deficits in auditory responses to sound stimuli in mice. Therefore, we conclude that GluA4 is indispensable for enabling information flow with high fidelity in the auditory circuitry. Our work has identified GluA4 as a potential molecular candidate involved in human hearing deficits and disorders.Abstract Fast excitatory synaptic transmission in central synapses is mediated primarily by AMPA receptors (AMPARs), which are heteromeric assemblies of four subunits, GluA1-4. Among these subunits, rapidly gating GluA3/4 appears to be the most abundantly expressed to enable neurotransmission with submillisecond precision at fast rates in subsets of central synapses. However, neither definitive identification of the molecular substrate for native AMPARs in these neurons, nor their hypothesized functional roles in vivo has been unequivocally demonstrated, largely due to lack of specific antagonists. Using GluA3 or GluA4 knockout (KO) mice, we investigated these issues at the calyx of Held synapse, which is known as a high-fidelity synapse involved in sound localization. Patch-clamp recordings from postsynaptic neurons showed that deletion of GluA4 significantly slowed the time course of both evoked and miniature AMPAR-mediated excitatory postsynaptic currents (AMPAR-EPSCs), reduced their amplitude, and exacerbated AMPAR desensitization and short-term depression (STD). Surprisingly, presynaptic release probability was also elevated, contributing to severe STD at GluA4-KO synapses. In contrast, only marginal changes in AMPAR-EPSCs were found in GluA3-KO mice. Furthermore, independent of changes in intrinsic excitability of postsynaptic neurons, deletion of GluA4 markedly reduced synaptic drive and increased action potential failures during high-frequency activity, leading to profound deficits in specific components of the auditory brainstem responses associated with synchronized spiking in the calyx of Held synapse and other related neurons in vivo. These observations identify GluA4 as the main determinant for fast synaptic response, indispensable for driving high-fidelity neurotransmission and conveying precise temporal information.

Synaptic transmission between end bulbs of Held and bushy cells in the cochlear nucleus of mice with a mutation in Otoferlin

“…AMPA receptors in bushy cells are composed of GluA3 and 4 isoforms as flop splice variants (Gardner et al 2001;Wang et al 1998). In targets of bushy cells it has been demonstrated that GluA4, but not GluA3, is crucial and that GluA2 can substitute for GluA3 but not for GluA4 (Yang et al 2011). The finding that mEPSCs have similar shapes suggests that the subunit composition of AMPA receptors is similar in Otoferlin mutant and control mice.…”

Impaired auditory processing and altered structure of the endbulb of Held synapse in mice lacking the GluA3 subunit of AMPA receptors

“…Since the CN is the first central station that encodes the timing of auditory signaling (Trussell, 1999), it is likely that timing is deficient in the absence of GluA3. A recent study by Yang et al (2011) showed that the lack of GluA4 affects ABR amplitudes and latencies of P3 and P4. Together with our findings, these observations suggest that GluA3 and GluA4 are each necessary for normal auditory signaling within defined synapses of the auditory pathway.…”

“…Together with our findings, these observations suggest that GluA3 and GluA4 are each necessary for normal auditory signaling within defined synapses of the auditory pathway. Since in the MNTB, GluA4 is essential for fast synaptic transmission (Yang et al, 2011), it is likely that GluA3 plays a similar role in the CN. This is supported by our findings on increased latency of P2 (CN).…”

Morphological and physiological development of auditory synapses

“…For instance, AMPA receptors gate more rapidly due to increased use of the flop splice variant of GluR4 (Joshi et al, 2004; Koike-Tani et al, 2005). This shift in receptor subunit composition is critical for the rapid transmission typical of the calyx synapse, as deletion of GluR4 significantly slows the time course of AMPAR-type EPSCs, reduces the current amplitude, and exacerbates receptor desensitization (Yang et al, 2011b). Additionally, PSD-95 and Homer-1 may help to concentrate AMPA receptors within PSDs, further contributing to fast and precise neurotransmission (Hermida et al, 2010; Soria Van Hoeve et al, 2010).…”