Molecular Mediators of Acute and Chronic Itch in Mouse and Human Sensory Neurons

Itch is a distinct sensation that arises from the activation of small-diameter pruriceptive nerve fibers innervating the skin. Recent strides in the field have identified several histamine-dependent and -independent pruriceptive pathways and receptors that contribute to acute and chronic itch. The work presented in this thesis further investigates the molecular mechanisms involved in the signaling, development, and sensitization of itch in mouse and human. Most pruritogen receptors are Gq-Protein Coupled Receptors (GqPCR), which canonically activate protein kinase C (PKC); however, little is known about whether specific PKC isoforms regulate itch. The first study in this thesis demonstrates that the isoform PKCδ contributes to histamine-induced scratching, but not histamine-independent itch. Our studies show that PKCδ is expressed in dorsal root ganglia (DRG), where it mediates sensory neuron responses to histamine. To investigate the mechanisms behind a common form of chronic pruritus, the second study in this thesis applied a mouse model of dry skin itch to test changes in sensory neuron structure and function. We found that dry skin was marked by a significant increase in epidermal nerve fiber innervation independent of scratching. Furthermore, dry skin was associated with a selective increase in non-peptidergic, Ret-positive fibers and a functional expansion of the proportion of chloroquine-sensitive neurons. Epidermal hyperinnervation and increased growth factor levels in the skin of patients with pruritic skin diseases suggest a potential role for neurotrophic factors (NTFs) in itch. In our third study, we tested the hypothesis that NTF signaling modulates pruritogen-evoked itch. Pretreatment with nerve growth factor (NGF) selectively potentiated histamine-induced scratching and increased the proportion of histamine-responsive sensory neurons. Artemin pretreatment, on the other hand, potentiated scratching induced by chloroquine, and increased the proportion of chloroquine-responsive neurons. Interestingly, aberrations in endogenous TrkA-NGF signaling significantly impacted normal pain sensation, but did not play a role in histamine- and chloroquine-induced itch. In the final study of this thesis, we developed a protocol to surgically extract human DRG from organ donors and culture dissociated human primary sensory neurons. Using this approach, we performed functional studies to characterize the pruritogen- and algogen- responsive sensory neuron subpopulations in humans. We found that NGF and artemin pretreatment did not change histamine and chloroquine responses in vitro, indicating a potential functional difference between mouse and human sensory neurons