Pou4f1 defines a subgroup of Type I spiral ganglion neurons and is necessary for normal inner hair cell presynaptic Ca2+ signaling

Acoustic signals are relayed from the ear to the brain via spiral ganglion neurons (SGNs) that receive auditory information from the cochlear inner hair cells and transmit that information to the cochlear nucleus of the brainstem. Physiologically distinct classes of SGNs have been characterized by their spontaneous firing rate and responses to sound, and those physiological distinctions are thought to correspond to stereotyped synaptic positions on the inner hair cell (IHC). More recently, single cell profiling has identified multiple groups of SGNs based on transcriptional profiling, however, correlations between any of these groups and distinct neuronal physiology has not been determined. In this study, we show that expression of the POU transcription factor Pou4f1 in Type I SGNs in mice of both sexes correlates with a synaptic location on the modiolar side of IHCs. Conditional deletion of Pou4f1 in SGNs beginning in mice at embryonic day 13 rescues the early pathfinding and apoptotic phenotypes reported for germline deletion of Pou4f1 , resulting in a phenotypically normal development of SGN patterning. However, conditional deletion of Pou4f1 in SGNs alters the activation of Ca 2+ -channels in IHCs, primarily by increasing their voltage-sensitivity. Moreover, the modiolar to pillar gradient of active zone (AZ) Ca 2+- influx strength is eliminated. These results demonstrate that a subset of modiolar-targeted SGNs retain expression of Pou4f1 beyond the onset of hearing and suggest that this transcription factor plays an instructive role in presynaptic Ca 2+ signaling in IHCs. Significance: Physiologically distinct classes of Type I spiral ganglion neurons (SGNs) are necessary to encode sound intensities spanning the audible range. While anatomical studies have demonstrated structural correlates for some physiologically-defined classes of Type I SGNs, an understanding of the molecular pathways that specify each type is only emerging. Here, we demonstrate that expression of the transcription factor Pou4f1 corresponds to a distinct subgroup of Type I SGNs that synapse on the modiolar side of inner hair cells. The conditional deletion of Pou4f1 after SGN formation does not disrupt ganglion size or morphology, change the distribution of IHC synaptic locations, or impact the creation of synapses, but does influence the voltage-dependence and strength of Ca 2+ -influx at presynaptic AZs in IHCs.