Introduction Osteo-arthritis (OA) involves joint degradation and usually pain; with mechanisms poorly understood and few treatment options. There is evidence that the transient receptor potential canonical 5 (TRPC5) mRNA expression is reduced in OA patients’ synovia. Here we examine the profile of TRPC5 in DRG and involvement in murine models of OA. Design TRPC5 KO mice were subjected to partial meniscectomy (PMNX) or injected with monoiodoacetate (MIA) and pain-related behaviours were determined. Knee joint pathological scores were analysed and gene expression changes in ipsilateral synovium and dorsal root ganglia (DRG) determined. c-Fos protein expression in the ipsilateral dorsal horn of the spinal cord was quantified. Results TRPC5 KO mice developed a discrete enhanced pain-related phenotype. In the MIA model, the pain-related phenotype correlated with c-Fos expression in the dorsal horn and increased expression of nerve injury markers ATF3, CSF1 and galanin in the ipsilateral DRG. There were negligible differences in the joint pathology between WT and TRPC5 KO mice, however detailed gene expression analysis determined increased expression of the mast cell marker CD117 as well as extracellular matrix remodelling proteinases MMP2, MMP13 and ADAMTS4 in MIA-treated TRPC5 KO mice. TRPC5 expression was defined to sensory subpopulations in DRG. Conclusions Deletion of TRPC5 receptor signalling is associated with exacerbation of pain-like behaviour in OA which correlates with increased expression of enzymes involved in extracellular remodelling, inflammatory cells in the synovium and increased neuronal activation and injury in DRG. Together, these results identify a modulating role for TRPC5 in OA-induced pain-like behaviours.

Fibromyalgia is a debilitating widespread chronic pain syndrome that occurs in 2 to 4% of the population. The prevailing view that fibromyalgia results from central nervous system dysfunction has recently been challenged with data showing changes in peripheral nervous system activity. Using a mouse model of chronic widespread pain through hyperalgesic priming of muscle, we show that neutrophils invade sensory ganglia and confer mechanical hypersensitivity on recipient mice, while adoptive transfer of immunoglobulin, serum, lymphocytes, or monocytes has no effect on pain behavior. Neutrophil depletion abolishes the establishment of chronic widespread pain in mice. Neutrophils from patients with fibromyalgia also confer pain on mice. A link between neutrophil-derived mediators and peripheral nerve sensitization is already established. Our observations suggest approaches for targeting fibromyalgia pain via mechanisms that cause altered neutrophil activity and interactions with sensory neurons.

We showed that the chemokine receptor C-X-C Motif Chemokine Receptor 2 (CXCR2) is essential for cartilage homeostasis. Here we reveal that the CXCR2 ligand granulocyte chemotactic protein 2 (GCP-2) was expressed, during embryonic development, within the prospective permanent articular cartilage, but not in the epiphyseal cartilage destined to be replaced by bone. GCP-2 expression was retained in adult articular cartilage. GCP-2 loss-of-function inhibited extracellular matrix production. GCP-2 treatment promoted chondrogenesis in vitro and in human cartilage organoids implanted in nude mice in vivo. To exploit the chondrogenic activity of GCP-2, we disrupted its chemotactic activity, by mutagenizing a glycosaminoglycan binding sequence, which we hypothesized to be required for the formation of a GCP-2 haptotactic gradient on endothelia. This mutated version (GCP-2-T) had reduced capacity to induce transendothelial migration in vitro and in vivo, without affecting downstream receptor signaling through AKT, and chondrogenic activity. Intra-articular adenoviral overexpression of GCP-2-T, but not wild type GCP-2, reduced pain and cartilage loss in instability-induced osteoarthritis in mice. We suggest that GCP-2-T may be used for disease modification in osteoarthritis.

Fibromyalgia is a debilitating widespread chronic pain syndrome that occurs in 2-4% of the population. The prevailing view that fibromyalgia results from central nervous system dysfunction has recently been challenged with data showing changes in peripheral nervous system activity. Using a mouse model of chronic widespread pain through hyperalgesic priming of muscle, we show that neutrophils invade sensory ganglia and confer mechanical hypersensitivity on recipient mice, whilst adoptive transfer of immunoglobulin, serum, lymphocytes or monocytes have no effect on pain behaviour. Neutrophil depletion abolishes the establishment of chronic widespread pain in mice. Neutrophils from patients with fibromyalgia also confer pain on mice. A link between neutrophil released mediators and peripheral nerve sensitisation is already established. These observations suggest new approaches for targeting fibromyalgia pain through an understanding of the mechanisms that cause altered neutrophil activity and interactions with sensory neurons.

An estimated 10%–50% of patients undergoing a surgical intervention will develop persistent postsurgical pain (PPP) lasting more than 3 months despite adequate acute pain management and the availability of minimally invasive procedures. The link between early and late pain outcomes for surgical procedures remains unclear—some patients improve while others develop persistent pain. The elective nature of a surgical procedure offers a unique opportunity for prophylactic or early intervention to prevent the development of PPP and improve our understanding of its associated risk factors, such as pre-operative anxiety and the duration of severe acute postoperative pain. Current perioperative pain management strategies often include opioids, but long-term consumption can lead to tolerance, addiction, opioid-induced hyperalgesia, and death. Pre-clinical models provide the opportunity to dissect mechanisms underpinning the transition from acute to chronic, or persistent, postsurgical pain. This review highlights putative mechanisms of PPP, including sensitisation of peripheral sensory neurons, neuroplasticity in the central nervous system and nociceptive signalling along the neuro-immune axis.

Recently, we found that the deletion of TRPC5 leads to increased inflammation and pain-related behaviour in two animal models of arthritis. (-)-Englerin A (EA), an extract from the East African plant Phyllanthus engleri has been identified as a TRPC4/5 agonist. Here, we studied whether or not EA has any anti-inflammatory and analgesic properties via TRPC4/5 in the carrageenan model of inflammation. We found that EA treatment in CD1 mice inhibited thermal hyperalgesia and mechanical allodynia in a dose-dependent manner. Furthermore, EA significantly reduced the volume of carrageenan-induced paw oedema and the mass of the treated paws. Additionally, in dorsal root ganglion (DRG) neurons cultured from WT 129S1/SvIm mice, EA induced a dose-dependent cobalt uptake that was surprisingly preserved in cultured DRG neurons from 129S1/SvIm TRPC5 KO mice. Likewise, EA-induced anti-inflammatory and analgesic effects were preserved in the carrageenan model in animals lacking TRPC5 expression or in mice treated with TRPC4/5 antagonist ML204.This study demonstrates that while EA activates a sub-population of DRG neurons, it induces a novel TRPC4/5-independent analgesic and anti-inflammatory effect in vivo. Future studies are needed to elucidate the molecular and cellular mechanisms underlying EA’s anti-inflammatory and analgesic effects.

Purpose of reviewOsteoarthritis is a degenerative joint disease that features pain as a hallmark symptom. This review summarises progress and obstacles in our understanding of pain mechanisms in arthritis. Recent findingsPain phenotypes in osteoarthritis are poorly characterized in clinical studies and animal studies are largely carti-centric. Different animal models incur variable disease progression patterns and activation of distinct pain pathways, but studies reporting both structural and pain outcomes permit better translational insights. In patients, classification of osteoarthritis disease severity is only based on structural integrity of the joint, but pain outcomes do not consistently correlate with joint damage. The complexity of this relationship underlines the need for pain detection in criteria for osteoarthritis classification and patient-reported outcome measures.