Study of the effect of TLR4 antagonism in experimental osteoarthritis

Abstract

Introduction: Toll-like receptor 4 (TLR4) is a pattern recognition receptor involved in the recognition of pathogen-associated molecular patterns (PAMPs) but also a "danger-sensing" receptor that recognizes host-derived endogenous molecules called damage-associated molecular patterns (DAMPs). The role of TLR4 in rheumatic diseases is becoming evident, as well as its potential role as a therapeutic option. Moreover, increasing evidence also suggests a role for TLR4 in the chronic pain states. Thus, in this study, we evaluated whether a systemic administration of an antagonist of TLR4 (TLR4-A1) could be effective on decreasing nociception and cartilage degradation in experimental osteoarthritis (OA). Furthermore, as the activation transcription factor (ATF)-3 serves as a negative regulator for TLR4-stimulated inflammatory response we also evaluated the effect of TLR4-A1 on ATF-3 expression in primary afferent neurons at the dorsal root ganglia (DRG) level. Methods: OA was induced in adult Wistar rats through an intra-articular injection of 2mg of sodium mono-iodoacetate (MIA) into the left knee. From days 14 to 28 after OA induction, animals received an intraperitoneal injection of either TLR4-A1 (1mg) or vehicle. Movement- and loading- induced nociception was evaluated in all animals, by the Knee-Bend and CatWalk tests, before and after TLR4-A1 or vehicle administration, at several time-points. Immunofluorescence for TLR4 and ATF-3 was performed in L3-L5 DRGs. Knee joints were processed for histopathological evaluation. Results: Administration of TLR4-A1 markedly reduced movement-induced nociception on OA animals, particularly in the Knee-Bend test. Moreover, the increase in ATF-3 expression observed in DRG of OA animals was significantly reduced by TLR4-A1. No effect, however, was observed in cartilage loss nor on the neuronal cytoplasmic expression of TLR4 upon antagonist administration. Conclusion: The TLR4 antagonist administration possibly interrupts the TLR4 signaling cascade, decreasing the neurotoxic environment at the joint, and as a consequence the ATF-3 expression, improving the pain symptoms.