The nerve growth factor receptor p75NTR interacts with toll like receptor 4 and the alarmins high mobility group box-1 and nucleophosmin: a novel inflammatory mechanism in psoriasis

BACKGROUND: Nerve growth factor (NGF) is up-regulated in psoriatic skin, and increasing evidence indicate that its receptor p75NTR, promotes inflammatory responses. However, the contribution of p75NTR and its ligand proNGF in psoriasis inflammation remains unclear. OBJECTIVES: Investigate the role of p75NTR in inflammatory responses of skin fibroblasts derived from plaques of psoriasis vulgaris (Pso) patients, unravelling possible novel interactors. METHODS: Plasma levels of proNGF and p75NTR extracellular domain (ECD) were measured in 54 Pso patients and 25 healthy donors (HDs). p75NTR and TrkA expression was evaluated in skin biopsies and dermal fibroblasts from Pso and HDs. Protein-Protein Interaction (PPI) analysis was used to prioritize potential p75NTR interactors. Psoriasis-related cytokine MIX (IFN-γ, TNF-α, IL-22, IL-17A) effects on receptor expression were examined in skin biopsies and fibroblasts. Pharmacological inhibition of p75NTR with LM11A-31 (p75i) or RNA interference was used to assess effects on intracellular signalling, inflammatory gene expression (IL-6, COX2, MCP-1, CCL20), and molecular interactions using qRT-PCR, Western blotting, ELISA, and proximity ligation assay (PLA). RESULTS: p75NTR expression was increased in dermal layer of psoriatic skin biopsies and plasma levels of p75NTR ECD and proNGF were enhanced in Pso patients and correlated with disease severity. Pso fibroblasts showed high basal p75NTR expression, which was inducible by MIX in HD fibroblasts and blocked by LM11A-31. In both HD and Pso skin biopsies and fibroblasts, p75NTR inhibition significantly reduced MIX-induced inflammatory gene expression and prevented NF-κB nuclear translocation. Similar effects were observed following TLR4 inhibition. PPI analysis identified the alarmins HMGB1 and NPM as potential connectors between p75NTR and TLR4. These interactions were confirmed by PLA. MIX treatment enhanced p75NTR-TLR4-alarmin interactions which were prevented by p75i. Moreover, NPM and HMGB1 inhibition decreased p75NTR-TLR4 interaction and inflammatory gene expression. Consistently in Pso fibroblasts, p75NTR inhibition reduced MIX or LPS -induced extracellular release of NPM and HMGB1. CONCLUSION: p75NTR up-regulation and signalling contribute to alarmin release and amplification of TLR4/NF-κB inflammatory pathway in psoriasis. p75NTR can be considered a sensor of inflammation required for full activation of TLR4-dependent inflammatory signalling. Thus, p75NTR targeting, may represent a novel therapeutic strategy to counteract psoriatic chronic inflammation.