Prednisolone and rapamycin reduce the plasma cell gene signature and may improve AAV gene therapy in cynomolgus macaques.

Adeno-associated virus (AAV) vector gene therapy is a promising approach to treat rare genetic diseases; however, an ongoing challenge is how to best modulate host immunity to improve transduction efficiency and therapeutic outcomes. This report presents two studies characterizing multiple prophylactic immunosuppression regimens in male cynomolgus macaques receiving an AAVrh10 gene therapy vector expressing human coagulation factor VIII (hFVIII). In study 1, no immunosuppression was compared with prednisolone, rapamycin (or sirolimus), rapamycin and cyclosporin A in combination, and cyclosporin A and azathioprine in combination. Prednisolone alone demonstrated higher mean peripheral blood hFVIII expression; however, this was not sustained upon taper. Anti-capsid and anti-hFVIII antibody responses were robust, and vector genomes and transgene mRNA levels were similar to no immunosuppression at necropsy. Study 2 compared no immunosuppression with prednisolone alone or in combination with rapamycin or methotrexate. The prednisolone/rapamycin group demonstrated an increase in mean hFVIII expression and a mean delay in anti-capsid IgG development until after rapamycin taper. Additionally, a significant reduction in the plasma cell gene signature was observed with prednisolone/rapamycin, suggesting that rapamycin's tolerogenic effects may include plasma cell differentiation blockade. Immunosuppression with prednisolone and rapamycin in combination could improve therapeutic outcomes in AAV vector gene therapy.

Notch pathway signaling in the skin antagonizes Merkel cell development.

Merkel cells are mechanosensitive skin cells derived from the epidermal lineage whose development requires expression of the basic helix-loop-helix transcription factor Atoh1. The genes and pathways involved in regulating Merkel cell development during embryogenesis are poorly understood. Notch pathway signaling antagonizes Atoh1 expression in many developing body regions, so we hypothesized that Notch signaling might inhibit Merkel cell development. We found that conditional, constitutive overexpression of the Notch intracellular domain (NICD) in mouse epidermis significantly decreased Merkel cell numbers in whisker follicles and touch domes of hairy skin. Conversely, conditional deletion of the obligate NICD binding partner RBPj in the epidermis significantly increased Merkel cell numbers in whisker follicles, led to the development of ectopic Merkel cells outside of touch domes in hairy skin epidermis, and altered the distribution of Merkel cells in touch domes. Deletion of the downstream Notch effector gene Hes1 also significantly increased Merkel cell numbers in whisker follicles. Together, these data demonstrate that Notch signaling regulates Merkel cell production and patterning.

Merkel cells are long-lived cells whose production is stimulated by skin injury.

Mechanosensitive Merkel cells are thought to have finite lifespans, but controversy surrounds the frequency of their replacement and which precursor cells maintain the population. We found by embryonic EdU administration that Merkel cells undergo terminal cell division in late embryogenesis and survive long into adulthood. We also found that new Merkel cells are produced infrequently during normal skin homeostasis and that their numbers do not change during natural or induced hair cycles. In contrast, live imaging and EdU experiments showed that mild mechanical injury produced by skin shaving dramatically increases Merkel cell production. We confirmed with genetic cell ablation and fate-mapping experiments that new touch dome Merkel cells in adult mice arise from touch dome keratinocytes. Together, these independent lines of evidence show that Merkel cells in adult mice are long-lived, are replaced rarely during normal adult skin homeostasis, and that their production can be induced by repeated shaving. These results have profound implications for understanding sensory neurobiology and human diseases such as Merkel cell carcinoma.

Ectopic Atoh1 expression drives Merkel cell production in embryonic, postnatal and adult mouse epidermis.

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.

Merkel Cell-Driven BDNF Signaling Specifies SAI Neuron Molecular and Electrophysiological Phenotypes.

The extent to which the skin instructs peripheral somatosensory neuron maturation is unknown. We studied this question in Merkel cell-neurite complexes, where slowly adapting type I (SAI) neurons innervate skin-derived Merkel cells. Transgenic mice lacking Merkel cells had normal dorsal root ganglion (DRG) neuron numbers, but fewer DRG neurons expressed the SAI markers TrkB, TrkC, and Ret. Merkel cell ablation also decreased downstream TrkB signaling in DRGs, and altered the expression of genes associated with SAI development and function. Skin- and Merkel cell-specific deletion of Bdnf during embryogenesis, but not postnatal Bdnf deletion or Ntf3 deletion, reproduced these results. Furthermore, prototypical SAI electrophysiological signatures were absent from skin regions where Bdnf was deleted in embryonic Merkel cells. We conclude that BDNF produced by Merkel cells during a precise embryonic period guides SAI neuron development, providing the first direct evidence that the skin instructs sensory neuron molecular and functional maturation. SIGNIFICANCE STATEMENT: Peripheral sensory neurons show incredible phenotypic and functional diversity that is initiated early by cell-autonomous and local environmental factors found within the DRG. However, the contribution of target tissues to subsequent sensory neuron development remains unknown. We show that Merkel cells are required for the molecular and functional maturation of the SAI neurons that innervate them. We also show that this process is controlled by BDNF signaling. These findings provide new insights into the regulation of somatosensory neuron development and reveal a novel way in which Merkel cells participate in mechanosensation.

Atoh1 directs hair cell differentiation and survival in the late embryonic mouse inner ear.

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.

MrgprdCre lineage neurons mediate optogenetic allodynia through an emergent polysynaptic circuit.

ABSTRACT: Most cutaneous C fibers, including both peptidergic and nonpeptidergic subtypes, are presumed to be nociceptors and respond to noxious input in a graded manner. However, mechanically sensitive, nonpeptidergic C fibers also respond to mechanical input in the innocuous range, so the degree to which they contribute to nociception remains unclear. To address this gap, we investigated the function of nonpeptidergic afferents using the MrgprdCre allele. In real-time place aversion studies, we found that low-frequency optogenetic activation of MrgrpdCre lineage neurons was not aversive in naive mice but became aversive after spared nerve injury (SNI). To address the underlying mechanisms of this allodynia, we recorded responses from lamina I spinoparabrachial (SPB) neurons using the semi-intact ex vivo preparation. After SNI, innocuous brushing of the skin gave rise to abnormal activity in lamina I SPB neurons, consisting of an increase in the proportion of recorded neurons that responded with excitatory postsynaptic potentials or action potentials. This increase was likely due, at least in part, to an increase in the proportion of lamina I SPB neurons that received input on optogenetic activation of MrgprdCre lineage neurons. Intriguingly, in SPB neurons, there was a significant increase in the excitatory postsynaptic current latency from MrgprdCre lineage input after SNI, consistent with the possibility that the greater activation post-SNI could be due to the recruitment of a new polysynaptic circuit. Together, our findings suggest that MrgprdCre lineage neurons can provide mechanical input to the dorsal horn that is nonnoxious before injury but becomes noxious afterwards because of the engagement of a previously silent polysynaptic circuit in the dorsal horn.

Kappa Opioid Receptor Distribution and Function in Primary Afferents.

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 and nociceptor sensitization by inflammatory mediators. Finally, peripherally restricted KOR agonists selectively reduce pain and itch behaviors, as well as mechanical hypersensitivity associated with a surgical incision. These experiments provide a rationale for the use of peripherally restricted KOR agonists for therapeutic treatment.

Unipotent, Atoh1+ progenitors maintain the Merkel cell population in embryonic and adult mice.

Resident progenitor cells in mammalian skin generate new cells as a part of tissue homeostasis. We sought to identify the progenitors of Merkel cells, a unique skin cell type that plays critical roles in mechanosensation. We found that some Atoh1-expressing cells in the hairy skin and whisker follicles are mitotically active at embryonic and postnatal ages. Genetic fate-mapping revealed that these Atoh1-expressing cells give rise solely to Merkel cells. Furthermore, selective ablation of Atoh1(+) skin cells in adult mice led to a permanent reduction in Merkel cell numbers, demonstrating that other stem cell populations are incapable of producing Merkel cells. These data identify a novel, unipotent progenitor population in the skin that gives rise to Merkel cells both during development and adulthood.