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Damage-associated molecular patterns generated in osteoarthritis directly excite murine nociceptive neurons through
Rachel E Miller1, Abdelhak Belmadani2, Shingo Ishihara1
1Rush University Medical Center, Chicago, Illinois.
Objective:
To determine whether selected damage-associated molecular patterns (DAMPs) present in the osteoarthritic (OA) joints of mice excite nociceptors through Toll-like receptor 4 (TLR-4).
Methods:
The ability of S100A8 and α2 -macroglobulin to excite nociceptors was determined by measuring the release of monocyte chemoattractant protein 1 (MCP-1) by cultured dorsal root ganglion (DRG) cells as well as by measuring the intracellular calcium concentration ([Ca(2+) ]i ) in cultured DRG neurons from naive mice or from mice that had undergone surgical destabilization of the medial meniscus (DMM) 8 weeks previously. The role of TLR-4 was assessed using TLR-4(-/-) cells or a TLR-4 inhibitor. The [Ca(2+) ]i in neurons within ex vivo intact DRGs was measured in samples from Pirt-GCaMP3 mice. Neuronal expression of the Tlr4 gene was determined by in situ hybridization. DMM surgery was performed in wild-type and TLR-4(-/-) mice; mechanical allodynia was monitored, and joint damage was assessed histologically after 16 weeks.
Results:
DRG neurons from both naive and DMM mice expressed Tlr4. Both S100A8 and α2 -macroglobulin stimulated release of the proalgesic chemokine MCP-1 in DRG cultures, and the neurons rapidly responded to S100A8 and α2 -macroglobulin with increased [Ca(2+) ]i . Blocking TLR-4 inhibited these effects. Neurons within intact DRGs responded to the TLR-4 agonist lipopolysaccharide. In both of the calcium-imaging assays, it was primarily the nociceptor population of neurons that responded to TLR-4 ligands. TLR-4(-/-) mice were not protected from mechanical allodynia or from joint damage associated with DMM.
Conclusion:
Our experiments suggest a role of TLR-4 signaling in the excitation of nociceptors by selected DAMPs. Further research is needed to delineate the importance of this pathway in relation to OA pain.
Insights
Selected damage-associated molecular patterns (DAMPs) excite nociceptors via Toll-like receptor 4 (TLR-4) signaling. However, blocking TLR-4 did not prevent pain or joint damage in mouse models, suggesting a complex role in osteoarthritis (OA).
Area of Science:
- Neuroscience
- Immunology
- Rheumatology
Background:
- Osteoarthritis (OA) is characterized by joint pain and damage.
- Damage-associated molecular patterns (DAMPs) are implicated in OA pathogenesis.
- Nociceptors are sensory neurons that detect painful stimuli.
Purpose of the Study:
- To investigate if specific DAMPs, S100A8 and α2-macroglobulin, excite nociceptors through Toll-like receptor 4 (TLR-4).
- To assess the role of TLR-4 in mediating nociceptor activation by DAMPs in the context of OA.
- To evaluate the impact of TLR-4 signaling on pain and joint damage in an OA mouse model.
Main Methods:
- Cultured dorsal root ganglion (DRG) cells and neurons from naive and destabilization of the medial meniscus (DMM) mice were used.
- Monocyte chemoattractant protein 1 (MCP-1) release and intracellular calcium concentration ([Ca(2+)]i) were measured.
- TLR-4 knockout (TLR-4(-/-)) cells, TLR-4 inhibitor, and ex vivo DRG calcium imaging were employed.
- Mechanical allodynia and joint damage were assessed in wild-type and TLR-4(-/-) mice after DMM surgery.
Main Results:
- DRG neurons from both naive and DMM mice expressed Tlr4.
- S100A8 and α2-macroglobulin stimulated MCP-1 release and increased [Ca(2+)]i in DRG cultures, effects blocked by inhibiting TLR-4.
- Nociceptors were identified as the primary responders to TLR-4 ligands in calcium-imaging assays.
- TLR-4(-/-) mice did not show protection against mechanical allodynia or DMM-induced joint damage.
Conclusions:
- TLR-4 signaling is involved in the excitation of nociceptors by certain DAMPs.
- The precise role of this TLR-4 pathway in OA pain requires further investigation.
- These findings highlight a potential, albeit complex, therapeutic target for OA pain management.
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