Background/Purpose: Destabilization of the medial meniscus (DMM) in the mouse knee results in slowly progressive joint damage, accompanied by pain-related behaviors. Secondary mechanical allodynia develops by week 4 after surgery, but is not maintained after sham surgery while it is maintained for 16 weeks after DMM. Eight weeks after DMM but not sham surgery, mice develop activity-induced pain. Development of chronic pain is characterized by molecular changes in the dorsal root ganglia (DRG), where the cell bodies of sensory neurons reside. Therefore, we sought to perform an unbiased discovery study on DRG cells at different time points after DMM in order to uncover pathways that may be involved in generating acute vs.persistent pain associated with experimental osteoarthritis (OA).

Methods: DMM or sham surgery was performed in the right knee of 10-week old male C57BL/6 mice. Age-matched naïve mice were also included. Four, 8, or 16 weeks after surgery, ipsilateral L3-L5 DRG (innervating the knee) were collected and pooled from each mouse, RNA was extracted, and an Affymetrix Mouse Transcriptome Array 1.0 was performed following manufacturer recommendations. A total of 3 mice (3 arrays) were used for each treatment per time point. Sham 8-week samples did not amplify well and were excluded from the study. Two types of analyses were performed, one looking for genes involved in post-surgical pain (called “early pain”), and one looking for genes involved in persistent pain (called “late pain”). “Early pain” compared naïve 4-week samples to data pooled from sham and DMM 4 week-samples. “Late pain” compared data pooled from naïve 8- and 16-week and sham 16-week samples to data pooled from DMM 8- and 16-week samples. Ingenuity pathway analysis software was used to identify pathways of interest among the differentially expressed genes (p<0.01).

Results: In the “early pain” analysis, 345 genes were differentially regulated, using a p<0.01 cut-off. The top 3 networks included genes related to 1) Cell morphology/Protein synthesis/Cellular function and maintenance; 2) Hematological system development and function/Hypersensitivity response/Inflammatory response; and 3) Cell cycle/Cell death, which were enriched in the pooled DMM/sham 4-week samples relative to the naïve 4-week controls. In the “late pain” sets, 227 genes were differentially regulated. The top networks included genes related to 1) Nervous system development/Cell-to-cell signaling; 2) Cell-mediated immune response/Cell function & maintenance; 3) Amino acid metabolism/Molecular transport; 4) Humoral immune response; and 5) Cell cycle, which were enriched in the pooled DMM 8- and 16-week samples relative to sham/naïve 8- and 16-week controls. In general, there was little overlap (10 genes) between the differentially expressed genes in the “early” vs.“late” pain set, supporting our choice for separating these sets as such.

Conclusion: This molecular study represents the first unbiased analysis in the DRG in a model of chronic OA-pain. Findings support behavioral data suggesting that distinct phases exist in the pain developing after DMM surgery. Future work will seek to validate these pathways with the aim to identify new analgesic targets for treating osteoarthritis pain.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/microarray-analyses-of-dorsal-root-ganglia-for-the-study-of-pathways-contributing-to-pain-in-experimental-osteoarthritis/