Loss of Choline Agonism in the Inner Ear Hair Cell Nicotinic Acetylcholine Receptor Linked to the α10 Subunit

Abstract

The α9α10 nicotinic acetylcholine receptor (nAChR) plays a fundamental role in inner ear physiology. It mediates synaptic transmission between efferent olivocochlear fibers that descend from the brainstem and hair cells of the auditory sensory epithelium. The α9 and α10 subunits have undergone a distinct evolutionary history within the family of nAChRs. Predominantly in mammalian vertebrates, the α9α10 receptor has accumulated changes at the protein level that may ultimately relate to the evolutionary history of the mammalian hearing organ. In the present work, we investigated the responses of α9α10 nAChRs to choline, the metabolite of acetylcholine degradation at the synaptic cleft. Whereas choline is a full agonist of chicken α9α10 receptors it is a partial agonist of the rat receptor. Making use of the expression of α9α10 heterologous receptors, encompassing wild-type, heteromeric, homomeric, mutant, chimeric, and hybrid receptors, and in silico molecular docking, we establish that the mammalian (rat) α10 nAChR subunit underscores the reduced efficacy of choline. Moreover, we show that whereas the complementary face of the α10 subunit does not play an important role in the activation of the receptor by ACh, it is strictly required for choline responses. Thus, we propose that the evolutionary changes acquired in the mammalian α9α10 nAChR resulted in the loss of choline acting as a full agonist at the efferent synapse, without affecting the triggering of ACh responses. This may have accompanied the fine-tuning of hair cell post-synaptic responses to the high-frequency activity of efferent medial olivocochlear fibers that modulate the cochlear amplifier.

The Hair Cell α9α10 Nicotinic Acetylcholine Receptor: Odd Cousin in an Old Family

“…Hair cell α9α10 nAChRs are distinct from other nicotinic receptors in that a greater divergence in their coding sequence has translated into differential functional properties across clades (Lipovsek et al, 2012 , 2014 ; Boffi et al, 2017 ; Marcovich et al, 2020 ; Moglie et al, 2021a ). The only functional nAChR in inner ear hair cells is composed of α9 and α10 subunits.…”

“…Overall, when comparing functional properties, stark differences are observed between mammalian and non-mammalian α9α10 nAChRs, which may, in turn, relate to the accumulation of amino acid changes within mammalian α10 subunits. First, chicken, but not rat α10 subunits, assemble into functional homomeric receptors (Lipovsek et al, 2014 ; Moglie et al, 2021a ). Second, while the chicken α10 subunit can contribute both principal and complementary components to the ligand binding site, the accumulation of non-synonymous substitutions in mammalian α10 subunits suggests a potentially defective contribution of rat (but not chicken) α10 subunits to complementary components of the binding site of α9α10 nAChRs.…”

“…In addition, whereas the complementary face of the α10 subunit does not play an important role in the activation of the rat α9α10 receptor by ACh (Boffi et al, 2017 ), it is strictly required for receptor activation by choline (Moglie et al, 2021a ). Therefore, the evolutionary changes acquired in the mammalian α10 nAChR subunit resulted in the loss of choline acting as a full agonist in rat α9α10 nAChRs (Moglie et al, 2021a ).…”

“…In addition, whereas the complementary face of the α10 subunit does not play an important role in the activation of the rat α9α10 receptor by ACh (Boffi et al, 2017 ), it is strictly required for receptor activation by choline (Moglie et al, 2021a ). Therefore, the evolutionary changes acquired in the mammalian α10 nAChR subunit resulted in the loss of choline acting as a full agonist in rat α9α10 nAChRs (Moglie et al, 2021a ). Since choline is present at the synaptic cleft, due to ACh hydrolysis by acetylcholinesterase, this difference in the efficacy of choline on α9α10 nAChRs might result in different kinetics of efferent synapses across species.…”

“…Cholinergic responses, driven by the α9α10 receptor, have been most extensively studied in rodents (Katz et al, 2004 , 2011 ; Gomez-Casati et al, 2005 ; Lipovsek et al, 2012 ; Katz and Elgoyhen, 2014 ; Moglie et al, 2018 , 2021b ; Kearney et al, 2019 ) and avian (Fuchs and Murrow, 1992a , b ; Lipovsek et al, 2014 ; Moglie et al, 2021a ) auditory hair cells. Additionally, functional hair cell nAChRs have also been reported in reptiles and fish (Art and Fettiplace, 1984 ; Art et al, 1985 ; Holt et al, 2006 ; Parks et al, 2017 ; Carpaneto Freixas et al, 2021 ).…”