Distribution and Functional Characteristics of Voltage-Gated Sodium Channels in Immature Cochlear Hair Cells

Abstract

Voltage-gated sodium channels (VGSCs) are transiently expressed in cochlear hair cells before hearing onset and play an indispensable role in shaping spontaneous activity. In this study, we showed that Na ? currents shaped the spontaneous action potentials in developing mouse inner hair cells (IHCs) by decreasing the time required for the membrane potential to reach the actionpotential threshold. In immature IHCs, we identified 9 known VGSC subtypes (Nav1.1a-1.9a), among which Nav1.7a was the most highly expressed subtype and the main contributor to Na ? currents in developing hair cells. Electrophysiological recordings of two cochlea-specific Nav1.7 variants (CbmNav1.7a and CbmNav1.7b) revealed a novel loss-of-function mutation (C934R) at the extracellular linker between segments 5 and 6 of domain II. In addition, post-transcriptional modification events, such as alternative splicing and RNA editing, amended the gating properties and kinetic features of CbmNav1.7a (C934). These results provide molecular and functional characteristics of VGSCs in mammalian IHCs and their contributions to spontaneous physiological activity during cochlear maturation.

Expression and Physiology of Voltage-Gated Sodium Channels in Developing Human Inner Ear

“…Our data in human fetal cochlea are similar to results in developing mouse cochlea, which found SCN9A to have the highest expression of all Na + channels genes (Zhou et al, 2020). Tonotopy of SCN9A expression was evident between mouse IHCs and OHCs, with greater SCN9A relative expression in IHCs in the apex of the cochlea compared to the base, while in OHCs, the reverse was true, with greater SCN9A relative expression in the base (Zhou et al, 2020). In this study we were not able to make comparisons between the two cochlear hair cell types.…”

“…Changes in mRNA expression of four different Na + channel genes, SCN5A, SCN8A, SCN9A, and SCN10A were tracked during fetal development. These genes were chosen as changes in their expression has been reported in other species during development ( Wooltorton et al, 2007 ; Fryatt et al, 2009 ; Liu et al, 2016 ; Zhou et al, 2020 ). For each gene we first describe fold changes in gene expression over time for each inner ear region (i.e., crista, utricle, base, middle, and apex of the cochlea, respectively).…”

“…Nav1.7 and Nav1.8 expression is found in the peripheral nervous system, specifically in dorsal root ganglion neurons and are thought to have a role in nocioception ( Tan et al, 2014 ; Hameed, 2019 ). The genes investigated in this study were selected based on previous studies which showed expression in both auditory and vestibular systems in other species ( Chabbert et al, 1997 ; Wooltorton et al, 2007 ; Liu et al, 2016 ; Zhou et al, 2020 ).…”

“…In immature mice (P3–P7) genes for six TTX-sensitive Na + channels were detected with different abundance in inner and OHCs, Nav1.1, Nav1.2, Nav1.3, Nav1.4, Nav1.6, and Nav1.7, while genes for TTX-insensitive Na + channels, Nav1.5, Nav1.8, and Nav1.9 were also detected but with much lower relative expression ( Zhou et al, 2020 ). In human fetal cochlea neuroepithelium, we detected expression of genes for Nav1.5, Nav1.6, Nav1.7, and Nav1.8 from 10 WG through to 14+ WG.…”

“…The β-subunits of voltage-gated Na + channels, encoded by SCNB genes have also been tracked in the developing the inner ear ( Wooltorton et al, 2007 ; Liu et al, 2016 ). Complementing these molecular results are anatomical investigations that have documented the expression and location of various Na + channel α-subunits within hair cells, afferent terminals, and ganglia of the auditory and vestibular systems ( Wooltorton et al, 2007 ; Lysakowski et al, 2011 ; Eckrich et al, 2012 ; Kim and Rutherford et al, 2016 ; Liu et al, 2016 ; Zhou et al, 2020 ). Functional studies have characterized the presence of different types of Na + currents (attributable to α- and β-subunits) in both developing and mature vestibular and auditory hair cells and ganglion neurons of rat, mouse, and gerbil ( Santos-Sacchi, 1993 ; Rüsch and Eatock, 1996 ; Chabbert et al, 1997 ; Marcotti et al, 2003 ; Geleoc et al, 2004 ; Wooltorton et al, 2007 ; Li et al, 2010 ; Liu et al, 2016 ; Browne et al, 2017 ).…”