Pro Nerve Growth Factor and Its Receptor p75NTR Activate Inflammatory Responses in Synovial Fibroblasts: A Novel Targetable Mechanism in Arthritis

Abstract

We have recently provided new evidence for a role of p75NTR receptor and its preferential ligand proNGF in amplifying inflammatory responses in synovial mononuclear cells of chronic arthritis patients. In the present study, to better investigate how activation of the p75NTR/proNGF axis impacts synovial inflammation, we have studied the effects of proNGF on fibroblast-like synoviocytes (FLS), which play a central role in modulating local immune responses and in activating pro-inflammatory pathways. Using single cell RNA sequencing in synovial tissues from active and treatment-naïve rheumatoid arthritis (RA) patients, we demonstrated that p75NTR and sortilin, which form a high affinity receptor complex for proNGF, are highly expressed in PRG4pos lining and THY1posCOL1A1pos sublining fibroblast clusters in RA synovia but decreased in RA patients in sustained clinical remission. In ex vivo experiments we found that FLS from rheumatoid arthritis patients (RA-FLS) retained in vitro a markedly higher expression of p75NTR and sortilin than FLS from osteoarthritis patients (OA-FLS). Inflammatory stimuli further up-regulated p75NTR expression and induced endogenous production of proNGF in RA-FLS, leading to an autocrine activation of the proNGF/p75NTR pathway that results in an increased release of pro-inflammatory cytokines. Our data on the inhibition of p75NTR receptor, which reduced the release of IL-1β, IL-6 and TNF-α, further confirmed the key role of p75NTR activation in regulating inflammatory cytokine production. In a set of ex vivo experiments, we used RA-FLS and cultured them in the presence of synovial fluids obtained from arthritis patients that, as we demonstrated, are characterized by a high concentration of proNGF. Our data show that the high levels of proNGF present in inflamed synovial fluids induced pro-inflammatory cytokine production by RA-FLS. The blocking of NGF binding to p75NTR using specific inhibitors led instead to the disruption of this pro-inflammatory loop, reducing activation of the p38 and JNK intracellular pathways and decreasing inflammatory cytokine production. Overall, our data demonstrate that an active proNGF/p75NTR axis promotes pro-inflammatory responses in synovial fibroblasts, thereby contributing to chronic synovial inflammation, and point to the possible use of p75NTR inhibitors as a novel therapeutic approach in chronic arthritis.

Acute intranasal treatment with nerve growth factor limits the onset of traumatic brain injury in young rats

“…These results correlate with the evidence that intranasal NGF delivered in adult rats after TBI, promoted an anti‐inflammatory response, decreasing brain IL‐1β and TNF‐α, and limiting the activation of NF‐κB (Lv et al, 2013). Our recent work on the link between the endogenous NGF system and the development of chronic peripheral inflammation (Farina et al, 2022), suggests that interfering with the balance between different forms of NGF (mature, as distinct from precursor) may indeed affect the development of inflammatory processes. An in‐depth analysis of endogenous NGF/proNGF activity and regulation after TBI is therefore warranted, because our therapeutic intervention may shift the balance of different forms of NGF toward the mature moiety, positively affecting both neuroinflammation and glial phenotype (Barcelona et al, 2016; Cheng et al, 2020; Rizzi et al, 2018).…”

ProBDNF and its receptors in immune-mediated inflammatory diseases: novel insights into the regulation of metabolism and mitochondria

“…A recent study using single-cell RNA sequencing has found that fibroblast-like synoviocyte (FLS) from active patients with RA overexpressed p75 NTR and sortilin compared with patients with RA in the remission stage ( 95 ). P75 NTR was significantly enriched in PRG4 pos lining FLS and THY1 pos COL1A1 pos sublining FLS, as well as remarkably expressed in FLS ( 95 ). After IL-1β stimulation in vitro , the expression level of p75 NTR on FLS of patients with RA was significantly upregulated.…”

“…After IL-1β stimulation in vitro , the expression level of p75 NTR on FLS of patients with RA was significantly upregulated. Consequently, p75 NTR signaling activated the inflammatory response in FLS, and neutralizing, or inhibiting p75 NTR could reduce the inflammatory factors IL-6, IL-8, and MCP1 in FLS, which was related to the activation of downstream JNK/p38 MAPK signaling ( 95 ). Chronic glucose metabolic changes induced by hypoxia and inflammatory mediators in FLS and synovial T cells will activate many signaling pathways, including MAPK, NF-κB, and PI3K/Akt pathways ( 96 ), which are crucial for the expression of adhesion molecules, secretion of cytokines, and inhibition of apoptosis, as well as for migration and invasion ( 96 ) ( Figure 4 ).…”

ProNGF processing in adult rat tissues and bioactivity of NGF prodomain peptides

“…Dysmetabolism, stress, and inflammation have showed to alter the NGF expression [ 29 ] and, like in brain and retina [ 30 , 31 , 32 ], an unbalance of NGF and proNGF expression has been recently proposed as a risk factor for peripheral diseases, also associated with inflammation [ 33 , 34 ]. There is currently no data on proNGF processing including the pdNGFpeps, in the central nervous system or peripheral tissues, and neither it is known whether inflammatory conditions might affect the relative expression of the pdNGFpeps.…”