Background/Purpose:

Using a surgical mouse model of osteoarthritis (OA), destabilization of the medial meniscus (DMM), we monitor pain and associated pathways over a period of 16 weeks post surgery.  We observed longitudinal changes in pain-related behaviors as joint disease progresses, with concomitant molecular changes in the innervating DRG (Miller et al, 2012, PNAS).  Specifically, mice develop progressive mechanical allodynia in the ipsilateral hindpaw over the first 4 weeks, while locomotive changes indicative of chronic pain appear 8 weeks after DMM.  It has been hypothesized that activation of microglia in the dorsal horn (DH) – a dynamic process that contributes to pain through further neuronal activation – is dependent on signaling molecules (ie MCP-1 and fractalkine (CX3CL1)) that originate in DRG neurons and are transported to the DH, where they can act on microglia (Miller et al, 2009, Handb Exp Pharmacol). Our previous work has focused on MCP-1/CCR2 signaling in DRG neurons after DMM surgery. Here, we examined fractalkine expression in the DRG and activation of microglia and neurons in the DH following DMM surgery.

Methods:

Animal procedures were approved by Rush University Medical Center.  DMM or sham surgery was performed in the right knee of 10-week old male wild-type C57BL/6 mice.  Pain-dependent behaviors were assessed at 0, 4, 8, and 16 weeks post surgery.  Mechanical allodynia in the hind paw was assessed with von Frey fibers.  Locomotion was assessed using a LABORAS platform, as described (Miller et al, 2012, PNAS).  Eight weeks post DMM, L3-L5 DRG were harvested (these DRG contain neurons that innervate the knee joint) and cells were cultured for 4 days; supernatants were collected for CX3CL1 ELISA.  For immunohistochemistry, mice were perfused transcardially with paraformaldehyde; L3-L5 DRG were dissected out or the spinal column was decalcified prior to spinal cord sectioning of the L3-L5 levels.  Anti-Iba1 or anti-cFos was used to examine microglia or neuronal activation, respectively, in the dorsal horn. 

Results:

In order to investigate whether changes in the DRG and DH of the spinal cord correlated with observed changes in locomotion, we focused on the 8-week post surgery time point.

Eight weeks post DMM, cultured DRG cells produced elevated levels of CX3CL1 protein compared to age-matched naïve DRG (p<0.05).  In addition, the DH in DMM mice showed markedly higher expression of Iba1 (marker of microglial activation) and of cFos (marker of neuronal activation) compared to age-matched naïve mice.

Conclusion:

This is the first report of increased fractalkine release by DRG neurons in a model of osteoarthritis, which may contribute to cellular activation in the DH.  Microglia and neuronal activation in the DH has been reported in murine mono-iodoacetate (MIA) arthritis (Ogbonna et al, 2013, Eur J Pain) and in non-disease-related animal models of nerve injury.  Longitudinal assessments of these dorsal horn changes are currently underway.  This will yield insight into how pain signaling spreads from the peripheral to the central nervous system in osteoarthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/pain-pathway-activation-in-dorsal-root-ganglia-and-dorsal-horn-in-a-murine-surgical-model-of-osteoarthritis/