Developmental Profiling of Spiral Ganglion Neurons Reveals Insights into Auditory Circuit Assembly

Abstract

The sense of hearing depends on the faithful transmission of sound information from the ear to the brain by spiral ganglion (SG) neurons. However, how SG neurons develop the connections and properties that underlie auditory processing is largely unknown. We catalogued gene expression in mouse SG neurons from embryonic day 12 (E12), when SG neurons first extend projections, up until postnatal day 15 (P15), after the onset of hearing. For comparison, we also analyzed the closely-related vestibular ganglion (VG). Gene ontology analysis confirmed enriched expression of genes associated with gene regulation and neurite outgrowth at early stages, with the SG and VG often expressing different members of the same gene family. At later stages, the neurons transcribe more genes related to mature function, and exhibit a dramatic increase in immune gene expression. Comparisons of the two populations revealed enhanced expression of TGFβ pathway components in SG neurons and established new markers that consistently distinguish auditory and vestibular neurons. Unexpectedly, we found that Gata3, a transcription factor commonly associated with auditory development, is also expressed in VG neurons at early stages. We therefore defined new cohorts of transcription factors and axon guidance molecules that are uniquely expressed in SG neurons and may drive auditory-specific aspects of their differentiation and wiring. We show that one of these molecules, the receptor guanylyl cyclase Npr2, is required for bifurcation of the SG central axon. Hence, our data set provides a useful resource for uncovering the molecular basis of specific auditory circuit assembly events.

Bifurcation of Axons from Cranial Sensory Neurons Is Disabled in the Absence of Npr2-Induced cGMP Signaling

“…DiI tracing experiments on the trigeminal ganglion in the absence of CNP, Npr2, or cGKI further support the notion that these three cGMP signaling components that were identified to be important for axonal branching in DRG neurons also control branching of cranial sensory neurons. In addition, a recently published study on DiI tracing of the VIII th nerve in Npr2 cn/cn mice also supports our overall conclusions (Lu et al, 2011). In contrast, collateral formation is not impaired in the absence of Npr2-induced cGMP signaling, which is consistent with our view that distinct signaling cascades already exist in a single neuron to regulate distinct branching modes.…”

“…Interestingly, at more advanced developmental stages, several other neuronal subpopulations in the brain are found to be positive for Npr2 and cGKI␣, including subpopulations in the cerebellum, hippocampus, and thalamus. However, the second messenger cGMP is implicated in several developmental and physiological processes in which cGMP signaling components are organized and function in different combinations (Lucas et al, 2000;Hofmann et al, 2006). A careful histological analysis of the localization of cGMP signaling components is therefore required before axonal branching studies are performed on these neuronal subpopulations.…”

A Comprehensive Network and Pathway Analysis of Human Deafness Genes

“…Expression of select genes was validated by summarizing their reported expression in the mouse cochlear hair cells based on RNASeq (12) or mouse spiral ganglion neurons based on GeneChips (13). Both databases catalog changes in gene expression during embryonic and early postnatal development.…”

Expression of the Olig gene family in the developing mouse inner ear

“…Although the molecular pathways that underlie the specification of cochlear vs. vestibular neurons are poorly understood, it has been reported that Gata3, the earliest known indicator of cell fate for cochlear ganglion neurons, is expressed in the developing cochlear ganglion, but not in the vestibular ganglion, as early as E13.5 (Lawoko-Kerali et al, 2004). Another study showed that the cochlear and vestibular ganglia in the CVG are molecularly distinct, as they express different combinations of cell surface receptors as early as E12.0 (Lu et al, 2011). However, an earlier study of the mouse ear at E12.5 showed that the medial and lateral populations of aggregating neuroblasts that are the precursors of the cochlear and vestibular ganglia do not separate morphologically until about E14.5 (Sher, 1971).…”