SUMMARY Primary afferents are known to be inhibited by kappa opioid receptor (KOR) signaling. However, the specific types of somatosensory neurons that express KOR remain unclear. Here, using a newly developed KOR-cre knockin allele, viral tracing, single-cell RT-PCR, and ex vivo recordings, we show that KOR is expressed in several populations of primary afferents: a subset of peptidergic sensory neurons, as well as low-threshold mechanoreceptors that form lanceolate or circumferential endings around hair follicles. We find that KOR acts centrally to inhibit excitatory neurotransmission from KOR-cre afferents in laminae I and III, and this effect is likely due to KOR-mediated inhibition of Ca2+ influx, which we observed in sensory neurons from both mouse and human. In the periphery, KOR signaling inhibits neurogenic inflammation, nociceptor sensitization by inflammatory mediators, and pain and itch behaviors. These experiments provide a rationale for the therapeutic use of peripherally restricted KOR agonists.

Atoh1 function is required for the earliest stages of inner ear hair cell development, which begins during the second week of gestation. Atoh1 expression in developing hair cells continues until early postnatal ages, but the function of this late expression is unknown. To test the role of continued Atoh1 expression in hair cell maturation we conditionally deleted the gene in the inner ear at various embryonic and postnatal ages. In the organ of Corti, deletion of Atoh1 at E15.5 led to the death of all hair cells. In contrast, deletion at E16.5 caused death only in apical regions, but abnormalities of stereocilia formation were present throughout the cochlea. In the utricle, deletion at E14.5 or E16.5 did not cause cell death but led to decreased expression of myosin VIIa and failure of stereocilia formation. Furthermore, we show that maintained expression of Barhl1 and Gfi1, two transcription factors implicated in cochlear hair cell survival, depends upon continued Atoh1 expression. However, maintained expression of Pou4f3 and several hair cell-specific markers is independent of Atoh1 expression. These data reveal novel late roles for Atoh1 that are separable from its initial role in hair cell development.

A novel, unipotent progenitor population in the skin characterized by Atoh1 expression gives rise to Merkel cells both during development and adulthood.

Merkel cells are mechanosensitive skin cells whose production requires the basic helix-loop-helix transcription factor Atoh1. We induced ectopic Atoh1 expression in the skin of transgenic mice to determine whether Atoh1 was sufficient to create additional Merkel cells. In embryos, ectopic Atoh1 expression drove ectopic expression of the Merkel cell marker keratin 8 (K8) throughout the epidermis. Epidermal Atoh1 induction in adolescent mice similarly drove widespread K8 expression in glabrous skin of the paws, but in the whisker pads and body skin ectopic K8+ cells were confined to hair follicles and absent from interfollicular regions. Ectopic K8+ cells acquired several characteristics of mature Merkel cells in a time frame similar to that seen during postnatal development of normal Merkel cells. Although ectopic K8+ cell numbers decreased over time, small numbers of these cells remained in deep regions of body skin hair follicles at 3 months post-induction. In adult mice, greater numbers of ectopic K8+ cells were created by Atoh1 induction during anagen versus telogen and following disruption of Notch signaling by conditional deletion of Rbpj in the epidermis. Our data demonstrate that Atoh1 expression is sufficient to produce new Merkel cells in the epidermis, that epidermal cell competency to respond to Atoh1 varies by skin location, developmental age and hair cycle stage, and that the Notch pathway plays a key role in limiting epidermal cell competency to respond to Atoh1 expression.