Neuronal heterogeneity and stereotyped connectivity in the auditory afferent system

Abstract

Spiral ganglion (SG) neurons of the cochlea convey all auditory inputs to the brain, yet the cellular and molecular complexity necessary to decode the various acoustic features in the SG has remained unresolved. Using single-cell RNA sequencing, we identify four types of SG neurons, including three novel subclasses of type I neurons and the type II neurons, and provide a comprehensive genetic framework that define their potential synaptic communication patterns. The connectivity patterns of the three subclasses of type I neurons with inner hair cells and their electrophysiological profiles suggest that they represent the intensity-coding properties of auditory afferents. Moreover, neuron type specification is already established at birth, indicating a neuronal diversification process independent of neuronal activity. Thus, this work provides a transcriptional catalog of neuron types in the cochlea, which serves as a valuable resource for dissecting cell-type-specific functions of dedicated afferents in auditory perception and in hearing disorders.

In vivo ectopic Ngn1 and Neurod1 convert neonatal cochlear glial cells into spiral ganglion neurons

“…It demonstrated that new SGNs were present in all the samples of experimental groups. In addition, we checked other SGN genes such as Calb2, Pou4f1, Lypd1, Slc17a6, Slc17a7, and Pvalb , which have recently been reported in different SGN single‐cell RNA‐Seq studies . Samples #4 and #9 showed enhanced expression of all of these SGN genes compared to other samples (red arrows in Figure D).…”

“…In addition, we checked other SGN genes such as Calb2, Pou4f1, Lypd1, Slc17a6, Slc17a7, and Pvalb, which have recently been reported in different SGN single-cell RNA-Seq studies. [3][4][5] Samples #4 and #9 showed enhanced expression of all of these SGN genes compared to other samples (red arrows in Figure 9D). In parallel to this, there was a sharp drop in the expression of glia-related genes such as Scn7a in samples #4 and #9, as compared to the levels found in the control glia cells (#1, #2, and #3).…”

“…2 Approximately 95% of spiral ganglion neuron (SGNs) are type I SGNs, which can be further subdivided into three subtypes: Type I-A, Type I-B, and Type I-C, and the remaining 5% of SGNs are type II SGNs, as supported by both morphological and single-cell transcriptome analysis. [3][4][5][6] IHCs are the primary sensory cells that are connected to myelinated type I SGNs, while OHCs are innervated by unmyelinated type II SGNs. The primary role of type I SGNs is to transmit sound-induced electrical signals from IHCs to the cochlear nucleus located in the brainstem; in contrast type II SGNs are involved in the reflexive medial olivocochlear suppression of cochlear amplification.…”

Coordinated calcium signalling in cochlear sensory and non‐sensory cells refines afferent innervation of outer hair cells

“…During the time over which OHCs become sensory competent, IHCs are still immature and fire Ca 2+ action potentials that are thought to drive the functional refinement of the auditory pathway, which mainly occurs during the first week of postnatal development (tonotopic organization in the brainstem: Snyder & Leake, 1997;Kim & Kandler, 2003; spiral ganglion neuron survival: Zhang-Hooks et al, 2016; spiral ganglion neuron subtype refinement: Shrestha et al, 2018;Sun et al, 2018). However, recent results have also indicated that the neuronal diversification process of type I SGNs is already established at birth in mice and as such it is independent of electrical activity (Petitpré et al, 2018). IHCs begin to mature towards the end of the second postnatal week (at the onset of hearing), indicating that Ca 2+ waves from the non-sensory cells are likely to drive different signals to the hair cells during early and later stages of pre-hearing development.…”

Circumvention of common labeling artifacts using secondary nanobodies

“…This was not the case with conventional antibodies, where the staining after 6 days was primarily on the surface and although at 14 days some penetration occurred, it displayed a clear disproportional staining (not homogenous), suggesting that more parvalbumin-a is at the surface that inside the spiral ganglion. Since levels of parvalbumin-a are comparable among all the different subtypes of type I spiral ganglion neurons 40 and type I spiral ganglion compose 96% of the neuronal population of the ganglion 41 , this is not expected to be true, but in turn, to be an artifact of penetration or steric hindrance of the antibodies.…”