Transcriptomic patterns of dermal Schwann cell reactivity in neuropathic itch and pain

• Applicants: Professor Dr. Frank Birklein; Professor Dr. Lucas Schirmer; Professorin Dr. Claudia Sommer
• Subject Area: Dermatology ; Clinical Neurology; Neurosurgery and Neuroradiology
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 350193106

Neuropathic pruritus leads to significant impairment of life quality. During the first funding period, we recruited 245 patients with polyneuropathies (PNP): 24 reporting itch as only symptom and 58 reporting itch and pain. We defined a PNP subgroup characterized by high pain ratings upon electrical C fiber stimulation and mechanical hyperalgesia. In skin biopsies, we noted a pronounced loss of dermal nerve fibers paralleled by a strong branching of remaining C fibers in patients with itch. By RNA-sequencing (RNA-seq) and quantitative PCR, we found increased expression of IFI44L in skin biopsy samples from patients with itch only. We showed feasibility of subepidermal Schwann cell (SC) type mapping by immunohistochemistry. Utilizing single-nucleus RNA-seq (snRNA-seq), we could characterize SC subtypes and decode their subtype-specific transcriptomic signatures. Notably, IFI44L is a direct target of the microRNA let-7d, which we previously found to be increased in skin from patients with chronic pain, and let-7 represses netrin expression in SCs. Given that let-7 family members are strongly expressed in SCs, we postulate that SC expression of let-7-IFI44L-netrin pathway members might mediate itch and pain in a subgroup of PNP patients. Thus, we will investigate SC morphology and protein expression in itch and pain, aiming at a deeper understanding of SC related pathways in the skin of well-defined subgroups of patients. PNP patients with either pain or itch, will be recruited and carefully assessed by clinical measurements. For comparison, we will include patients with brachioradial pruritus and receive skin samples of patients with psoriasis from collaborating project #5. We will then identify and characterize groups of PNP patients with itch, pain and high pain sensitivity upon C fiber stimulation aiming at understanding their psychophysical characteristics and at selecting homogenous samples to be used in high-throughput transcriptomic technologies. Next, oligonucleotide-tagged antibody multiplex snRNA-seq will allow us to analyze cell type-specific gene expression from cryo-preserved skin biopsy samples obtained from large clinical cohorts. By bioinformatic analysis, we will be able to decode the transcriptomic profile of cutaneous SCs between different clinical entities and define molecular traits of homeostatic and reactive SCs. Multiplex RNA in situ mapping will enable us to map back levels of SC subtype reactivity to the underlying skin microenvironment. In summary, work with high-quality frozen skin biopsy samples from well-characterized patients with neuropathic pain and itch in combination with single-cell and spatial transcriptomic tools will allow us to identify novel cell-type specific targets and biomarkers for future interventional studies. We hypothesize that identification of novel SC characteristics relative to itch or pain will help better understand their role in relation to axon pathology and the clinical phenotype.

• DFG Programme: Research Units
• Subproject of: FOR 2690: Translational Pruritus Research