MEDIATORS AND RECEPTORS OF CHRONIC ITCH IN PRIMATES AND HUMANS

Abstract

Chronic itch has a significant impact on quality of life for millions of patients worldwide, on a level comparable to that of chronic pain. Yet, although there are a host of effective drugs available for pain, there are no therapies that specifically target chronic itch. Current experimental approaches to investigate the pathogenesis of chronic pruritus and to test novel therapeutic agents are largely limited to rodent models. However, rodent models display significant dermatological, neurophysiological, and immunological differences from humans with chronic itch. The disadvantages of the current rodent paradigms call for the design of a valid primate model of chronic itch. For four years, we have monitored scratching behavior in a primate colony (n=35) of Cynomolgus macaques (Macaca fascicularis) suffering from idiopathic chronic itch. By comparing molecular and genetic analyses of the primates’ skin to their quantified scratching behavior, we attempted to characterize the underlying mechanisms of chronic itch in this model. Furthermore, the expression of itch-related proteins was examined in both the primate model and in humans with pruritic diseases. The first aim of the study was to characterize the underlying molecular and genetic basis of chronic itch in the primate model. We were able to distinguish specific peripheral targets related to pruritus by correlating the genetic and protein expression results to the primates’ scratching severity. In Aim 1a, RNA-sequencing was performed on skin biopsies from the primates to identify differentially expressed genes in pruritic, lichenified versus non-pruritic, non-lichenified skin. These results were then correlated to the quantified primate scratching behavior. This led to the identification of over 400 genes that were differentially expressed in the skin based on scratching intensity. Many of these differentially expressed transcripts were associated with sensory nerve fibers, keratinocytes, mast cells, or lymphocytes. Selected genes that were overexpressed and correlated to itch intensity were then targeted for immunohistochemical and proteomic analysis in Aim 1b. Immunohistochemical examination of the primate skin biopsies revealed that histamine levels were not elevated in primates that exhibited increased scratching behavior. However, mast cells containing tryptase were significantly increased in the skin of primates with severe scratching as compared to primates with mild scratching. The increased levels of gastrin-releasing peptide and substance P in lichenified skin were also found to be correlated to the primates’ scratching behavior. Of note, transient receptor potential channels V1, V3, and A1 were increased in the epidermis of primate skin, but the numbers of TRPV1+ and TRPA1+ nerve fibers were not significantly different between lichenified and non-lichenified skin. Transcriptome analysis of the opioid receptors and their ligands showed that primates with severe scratching behavior had a significant imbalance between the µ- and κ-opioid receptors and ligands. The µ-opioids had upregulated gene expression, while the κ-opioids were downregulated. In Aim 2, to further characterize this primate model of chronic itch, we compared immunohistochemical results from the primate studies to human findings. Lesional and non-lesional skin biopsies from patients with atopic dermatitis, psoriasis, and cutaneous T-cell lymphoma underwent immunohistochemical analysis in order to reveal the similarities and differences between the primate model and different types of chronic itch in humans. As in the primate model, substance P was found to be increased in the skin of lesional atopic and psoriasis skin. Additionally, similar to primate skin, human atopic and psoriatic skin had high levels of tryptase and its receptor in the epidermis. While IL-31 was only slightly elevated in primates, patients with cutaneous T-cell lymphoma or atopic dermatitis showed a significant correlation between itch severity and IL-31 levels. In conclusion, our primate model displayed expression patterns of many endogenous pruritogens and receptors that were similar to those of humans with atopic dermatitis or psoriasis. While the primate model did not completely mimic these specific pruritic diseases, the overlap of pruritic components suggests a commonality of signaling pathways across several different chronic itch states. The similarity of this primate model to human disease offers the combined advantages of experimental modeling and long-term behavioral follow-up.