Sex differences in glutamate AMPA receptor subunits mRNA with fast gating kinetics in the mouse cochlea

Abstract

Evidence shows that females have increased supra-threshold peripheral auditory processing compared to males. This is indicated by larger auditory brainstem responses (ABR) wave I amplitude, which measures afferent spiral ganglion neuron (SGN)-auditory nerve synchrony. However, the underlying molecular mechanisms of this sex difference are mostly unknown. We sought to elucidate sex differences in ABR wave I amplitude by examining molecular markers known to affect synaptic transmission kinetics. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) mediate fast excitatory transmission in mature SGN afferent synapses. Each AMPAR channel is a tetramer composed of GluA2, 3, and 4 subunits (Gria2, 3, and 4 genes), and those lacking GluA2 subunits have larger currents, are calcium-permeable, and have faster gating kinetics. Moreover, alternatively spliced flip and flop isoforms of each AMPAR subunit affect channel kinetics, having faster kinetics those AMPARs containing Gria3 and Gria4 flop isoforms. We hypothesized that SGNs of females have more fast-gating AMPAR subunit mRNA than males, which could contribute to more temporally precise synaptic transmission and increased SGN synchrony. Our data show that the index of Gria3 relative to Gria2 transcripts on SGN was higher in females than males (females: 48%; males: 43%), suggesting that females have more SGNs with higher Gria3 mRNA relative to Gria2. Analysis of the relative abundance of the flip and flop alternatively spliced isoforms showed that females have a 2-fold increase in fast-gating Gria3flop mRNA, while males have more slow-gating (2.5-fold) of the flip. We propose that Gria3 may in part mediate greater SGN synchrony in females.Significance Statement: Females of multiple vertebrate species, including fish and mammals, have been reported to have enhanced sound-evoked synchrony of afferents in the auditory nerve. However, the underlying molecular mediators of this physiologic sex difference are unknown. Elucidating potential molecular mechanisms related to sex differences in auditory processing is important for maintaining healthy ears and developing potential treatments for hearing loss in both sexes. This study found that females have a 2-fold increase in Gria3 flop mRNA, a fast-gating AMPA-type glutamate receptor subunit. This difference may contribute to greater neural synchrony in the auditory nerve of female mice compared to males, and this sex difference may be conserved in all vertebrates.

Variability of Auditory Brainstem Responses in Diversity Outbred Mice

“…However, there have been many investigations about the relationship between sex and hearing thresholds in mice in regards to age-related hearing loss and other noise-induced damage. Some of these studies present no differences in acoustic acuity between sexes in young pre-clinical mice [33,34,35] that align with our ABR measurements in the J:DOs, while other experiments suggest inherent variability linked to sex [36,37,38,39,40]. Whether there is a difference between males and females seems to depend on the mouse strain tested.…”

“…A comparable observation can be made in C57BL/6 mice, but only at > 100 days of age [37], at an age of 50 days the measurements of both sexes were nearly identical [41]. Young C57BL/6J mice tested at the age of 37 days display no significant difference between male and female [33]. In B6CBAF1/J mice, a cross between C57BL/6 and CBA/J, only the highest measured frequency, 32 kHz, shows a small but statistically significant lower threshold for female mice [42].…”

Interactions among insulin resistance, epigenetics, and donor sex in gene expression regulation of iPSC-derived myoblasts

“…Representative genes associated with the biological processes identified in the female-dominant clusters included TFAP2B , which showed a 2-fold increase, CNTN1 , which had a 2.9-fold increase, and the glutamate receptor GRIA2 , and doublecortin DCX genes, which showed 8-fold higher levels in female as compared with male cells ( Figure 3C ). While DCX is an X chromosome–encoded gene, GRIA2 is encoded on chromosome 4 with sex differences in expression in cochlea ( 30 ) and the brain ( 31 ). These sex-specific differences were, in general, independent of the insulin sensitivity status (compare dark- vs. light-shaded squares and circles in Figure 3C ) and occurred in vitro in the absence of sex hormones, i.e., represented cell-autonomous sex-specific changes in gene expression.…”

Sensorineural hearing loss and cognitive impairment: three hypotheses

“…Previous research has revealed that CI results in a reduction of synaptic AMPAR in the hippocampus ( Chang et al, 2006 ). Specifically, in the cochlea, AMDAR is primarily located in the nerve endings near the base of the IHCs ( Hong et al, 2018 ), Furthermore, in the cochlea, AMDAR plays a crucial role in mediating rapid excitatory transmission at mature spiral ganglion neurons SGNs afferent synapses ( Lozier et al, 2023 ). Interestingly, changes in the subunit composition of synaptic AMDAR occur as a result of HL, leading to long-term effects on synaptic integration ( Pilati et al, 2016 ).…”