Satellite Glial Cells Surrounding Primary Afferent Neurons Are Activated and Proliferate during Monoarthritis in Rats: Is There a Role for ATF3?

Abstract

Joint inflammatory diseases are debilitating and very painful conditions that still lack effective treatments. Recently, glial cells were shown to be crucial for the development and maintenance of chronic pain, constituting novel targets for therapeutic approaches. At the periphery, the satellite glial cells (SGCs) that surround the cell bodies of primary afferents neurons in the dorsal root ganglia (DRG) display hypertrophy, proliferation, and activation following injury and/or inflammation. It has been suggested that the expression of neuronal injury factors might initially trigger these SGCs-related events. We then aimed at evaluating if SGCs are involved in the establishment/maintenance of articular inflammatory pain, by using the monoarthritis (MA) model, and if the neuronal injury marker activating transcriptional factor 3 (ATF3) is associated with these SGCs' reactive changes. Western Blot (WB) analysis of the glial fibrillary acidic protein (GFAP) expression was performed in L4-L5 DRGs from control non-inflamed rats and MA animals at different time-points of disease (4, 7, and 14d, induced by complete Freund's adjuvant injection into the left hind paw ankle joint). Data indicate that SGCs activation is occurring in MA animals, particularly after day 7 of disease evolution. Additionally, double-immunostaining for ATF3 and GFAP in L5 DRG sections shows that SGCs's activation significantly increases around stressed neurons at 7d of disease, when compared with control animals. The specific labelling of GFAP in SGCs rather than in other cell types was also confirmed by immunohistochemical labeling. Finally, BrdU incorporation indicates that proliferation of SGCs is also significantly increased after 7 days of MA. Data indicate that SGCs play an important role in the mechanisms of articular inflammation, with 7 days of disease being a critical time-point in the MA model, and suggest that ATF3 might be involved in SGCs' reactive changes such as activation.

Changes in the transcriptional fingerprint of satellite glial cells following peripheral nerve injury

“…Yet we cannot fully exclude the possibility of “bleed‐through” when it comes to highly regulated neuronal genes such as for example, Gal or ATF3 (Holmes et al, ; Seijffers, Mills, & Woolf, ). These transcripts have previously been reported in SGC (Butzkueven & Gundlach, ; Hunt, Raivich, & Anderson, ; Nascimento, Castro‐Lopes, & Neto, ), so do not necessarily constitute neuronal signal. However, our current results do not allow firm conclusions on this question without further experiments, for example, using in situ hybridization or other assays allowing for spatial resolution.…”

Peripheral and central neuronal ATF3 precedes CD4+ T-cell infiltration in EAE

“…As CFA also induced ATF3 expression in MesV cell bodies, we presume that its mode of action is also via the peripheral branch of the sensory neurons. Although there are reports of ATF3-induction in some CFA-induced osteoarthritis models, this latter response is likely secondary to peripheral nerve damage (Ivanavicius et al, 2007; Nascimento et al, 2014). Importantly, we found no evidence that damage to the peripheral nerve was a prerequisite.…”

Pain pathogenesis in rheumatoid arthritis—what have we learned from animal models?

“…Like in RA patients, IL-6 is elevated in the synovial fluid of male CIA rats [133]. Intra-articular injection, and to a lesser extent systemic injection, of IL-6 inhibitors decrease pain-like behavior in female AIA rats, without affecting the joint inflammation [127]. Similar to TNF, this suggests a direct action of IL-6 on sensory neurons.…”