The role of intra-articular neuronal CCR2 receptors in knee joint pain associated with experimental osteoarthritis in mice

Intra-articular sprouting of nociceptors accompanies progressive osteoarthritis: comparative evidence in four murine models

Objective

Knee joints are densely innervated by nociceptors. In human knees and rodent models, sprouting of nociceptors has been reported in late-stage osteoarthritis (OA). Here, we sought to describe progressive nociceptor remodeling in early and late-stage OA, using four distinct experimental mouse models.

Methods

Sham surgery, destabilization of the medial meniscus (DMM), partial meniscectomy (PMX), or non-invasive anterior cruciate ligament rupture (ACLR) was performed in the right knee of 10-12-week old male C57BL/6 NaV1.8-tdTomato mice. Mice were euthanized (1) 4, 8 or 16 weeks after DMM or sham surgery; (2) 4 or 12 weeks after PMX or sham; (3) 1 or 4 weeks after ACLR injury or sham. Additionally, a cohort of naïve male wildtype mice was evaluated at age 6 and 24 months. Mid-joint cryosections were assessed qualitatively and quantitatively for NaV1.8+ or PGP9.5+ innervation. Cartilage damage, synovitis, and osteophytes were assessed.

Results

Progressive OA developed in the medial compartment after DMM, PMX, and ACLR. Synovitis and associated neo-innervation of the synovium by nociceptors peaked in early-stage OA. In the subchondral bone, channels containing sprouting nociceptors appeared early, and progressed with worsening joint damage. Two-year old mice developed primary OA in the medial and the lateral compartment, accompanied by nociceptor sprouting in the synovium and the subchondral bone. All four models showed increased nerve signal in osteophytes.

Conclusion

These findings suggest that anatomical neuroplasticity of nociceptors is intrinsic to OA pathology. The detailed description of innervation of the OA joint and its relationship to joint damage might help in understanding OA pain.

Clearing-enabled light sheet microscopy as a novel method for three-dimensional mapping of the sensory innervation of the mouse knee

A major barrier that hampers our understanding of the precise anatomic distribution of pain sensing nerves in and around the joint is the limited view obtained from traditional two dimensional (D) histological approaches. Therefore, our objective was to develop a workflow that allows examination of the innervation of the intact mouse knee joint in 3D by employing clearing-enabled light sheet microscopy. We first surveyed existing clearing protocols (SUMIC, PEGASOS, and DISCO) to determine their ability to clear the whole mouse knee joint, and discovered that a DISCO protocol provided the most optimal transparency for light sheet microscopy imaging. We then modified the DISCO protocol to enhance binding and penetration of antibodies used for labeling nerves. Using the pan-neuronal PGP9.5 antibody, our protocol allowed 3D visualization of innervation in and around the mouse knee joint. We then implemented the workflow in mice intra-articularly injected with nerve growth factor (NGF) to determine whether changes in the nerve density can be observed. Both 3D and 2D analytical approaches of the light sheet microscopy images demonstrated quantifiable changes in midjoint nerve density following 4 weeks of NGF injection in the medial but not in the lateral joint compartment. We provide, for the first time, a comprehensive workflow that allows detailed and quantifiable examination of mouse knee joint innervation in 3D.

Molecular pathogenesis of OA pain: Past, present, and future

OBJECTIVE

To provide a historical perspective and narrative review on research into the molecular pathogenesis of osteoarthritis pain.

DESIGN

PubMed databases were searched for combinations of "osteoarthritis", "pain" and "animal models" for papers that represented key phases in the history of osteoarthritis pain discovery research including epidemiology, pathology, imaging, preclinical modeling and clinical trials.

RESULTS

The possible anatomical sources of osteoarthritis pain were identified over 50 years ago, but relatively slow progress has been made in understanding the apparent disconnect between structural changes captured by radiography and symptom severity. Translationally relevant animal models of osteoarthritis have aided in our understanding of the structural and molecular drivers of osteoarthritis pain, including molecules such as nerve growth factor and C-C motif chemokine ligand 2. Events leading to persistent osteoarthritis pain appear to involve a two-step process involving changes in joint innervation, including neo-innervation of the articular cartilage, as well as sensitization at the level of the joint, dorsal root ganglion and central nervous system.

CONCLUSIONS

There remains a great need for the development of treatments to reduce osteoarthritis pain in patients. Harnessing all that we have learned over the past several decades is helping us to appreciate the important interaction between structural disease and pain, and this is likely to facilitate development of new disease modifying therapies in the future.

Targeting an inflammation-amplifying cell population can attenuate osteoarthritis-associated pain

BACKGROUND

Understanding of pain in osteoarthritis, its genesis, and perception is still in its early stages. Identification of precise ligand-receptor pairs that transduce pain and the cells and tissues in which they reside will elucidate new therapeutic approaches for pain management. Our recent studies had identified an inflammation-amplifying (Inf-A) cell population that is expanded in human OA cartilage and is distinctive in the expression of both IL1R1 and TNF-R2 receptors and active Jnk signaling cascade.

METHODS

In this study, we have tested the function of the cartilage-resident IL1R1+TNF-R2+ Inf-A cells in OA. We have identified that the IL1R1+TNF-R2+ Inf-A cells expand in aged mice as well as after anterior cruciate ligament tear upon tibia loading and OA initiation in mice. We targeted and modulated the Jnk signaling cascade in InfA through competitive inhibition of Jnk signaling in mice and human OA explants and tested the effects on joint structure and gait in mice.

RESULTS

Modulation of Jnk signaling led to attenuation of inflammatory cytokines CCL2 and CCL7 without showing any structural improvements in the joint architecture. Interestingly, Jnk inhibition and lowered CCL2 and 7 are sufficient to significantly improve the gait parameters in treated PTOA mice demonstrating reduced OA-associated pain. Consistent with the mice data, treatment with JNK inhibitor did not improve human OA cartilage explants.

CONCLUSION

These studies demonstrate that Inf-A, an articular-cartilage resident cell population, contributes to pain in OA via secretion of CCL2 and 7 and can be targeted via inhibition of Jnk signaling.

Pathogenic Mechanisms and Therapeutic Approaches in Obesity-Related Knee Osteoarthritis

The knee is the joint most frequently involved in osteoarthritis, a common joint disorder in the adult population that is associated with significant chronic joint pain, reduced mobility and quality of life. Recent studies have established an association between obesity and the development of knee osteoarthritis that goes beyond the increased mechanical load on the knees as weight-bearing joints. This link is based on the maintenance of a chronic low-grade inflammation, altered secretion of adipokines by the adipose tissue and development of sarcopenia. Major adipokines involved in the pathogenesis of obesity-related knee osteoarthritis include adiponectin, which appears to have a protective effect, as well as leptin, resistin and visfatin, which are associated with higher pain scores and more severe structural damage. Joint pain in knee osteoarthritis may be both nociceptive and neuropathic and is the result of complex mechanisms driven by nerve growth factor, calcitonin gene-related peptide and pro-inflammatory cytokines. The role of endogenous cannabinoids and gut microbiota in common mechanisms between obesity and knee pain has recently been studied. The aim of the present review is to highlight major pathogenic mechanisms in obesity-related knee osteoarthritis with special attention on pain and to comment on possible therapeutic approaches.

Comparative Effects of Intra-Articular versus Intravenous Mesenchymal Stromal Cells Therapy in a Rat Model of Osteoarthritis by Destabilization of Medial Meniscus

Transplanted mesenchymal stromal cells (MSCs) exhibit a robust anti-inflammatory and homing capacity in response to high inflammatory signals, as observed in studies focused on rheumatic diseases that target articular cartilage (AC) health. However, AC degradation in osteoarthritis (OA) does not necessarily coincide with a highly inflammatory joint profile. Often, by the time patients seek medical attention, they already have damaged AC. In this study, we examined the therapeutic potential of a single bone marrow MSC transplant (2 × 106 cells/kgbw) through two different routes: intra-articular (MSCs-IAt) and intravenous (MSCs-IVt) in a preclinical model of low-grade inflammatory OA with an established AC degeneration. OA was induced through the destabilization of the medial meniscus (DMM) in female Wistar Kyoto rats. The animals received MSCs 9 weeks after surgery and were euthanized 4 and 12 weeks post-transplant. In vivo and ex vivo tracking of MSCs were analyzed via bioluminescence and imaging flow cytometry, respectively. Cytokine/chemokine modulation in serum and synovial fluid was measured using a multiplex panel. AC degeneration was quantified through histology, and hindlimb muscle balance was assessed with precision weighing. To our knowledge, we are the first group to show the in vivo (8 h) and ex vivo (12 h) homing of cells to the DMM-OA joint following MSCs-IVt. In the case of MSCs-IAt, the detection of cellular bioluminescence at the knee joint persisted for up to 1 week. Intriguingly, intra-articular saline injection (placebo-IAt) resulted in a worse prognosis of OA when compared to a non-invasive control (placebo-IVt) without joint injection. The systemic cytokines/chemokines profile exhibited a time-dependent variation between transplant routes, displaying a transient anti-inflammatory systemic response for both MSCs-IVt and MSCs-IAt. A single injection of MSCs, whether administered via the intra-articular or intravenous route, performed 9 weeks after DMM surgery, did not effectively inhibit AC degeneration when compared to a non-invasive control.

Age-Associated Changes in Knee Osteoarthritis, Pain-Related Behaviors, and Dorsal Root Ganglia Immunophenotyping of Male and Female Mice

OBJECTIVE

Osteoarthritis (OA) is a leading cause of chronic pain, yet OA pain management remains poor. Age is the strongest predictor of OA development, and mechanisms driving OA pain are unclear. We undertook this study to characterize age-associated changes in knee OA, pain-related behaviors, and dorsal root ganglion (DRG) molecular phenotypes in mice of both sexes.

METHODS

Male or female C57BL/6 mice 6 or 20 months of age were evaluated for histopathologic knee OA, pain-related behaviors, and L3-L5 DRG immune characterization via flow cytometry. DRG gene expression in older mice and humans was also examined.

RESULTS

Male mice at 20 months of age had worse cartilage degeneration than 6-month-old mice. Older female mouse knees showed increased cartilage degeneration but to a lesser degree than those of male mice. Older mice of both sexes had worse mechanical allodynia, knee hyperalgesia, and grip strength compared to younger mice. For both sexes, DRGs from older mice showed decreased CD45+ cells and a significant increase in F4/80+ macrophages and CD11c+ dendritic cells. Older male mouse DRGs showed increased expression of Ccl2 and Ccl5, and older female mouse DRGs showed increased Cxcr4 and Ccl3 expression compared to 6-month-old mouse DRGs, among other differentially expressed genes. Human DRG analysis from 6 individuals >80 years of age revealed elevated CCL2 in men compared to women, whereas CCL3 was higher in DRGs from women.

CONCLUSION

We found that aging in male and female mice is accompanied by mild knee OA, mechanical sensitization, and changes to immune cell populations in the DRG, suggesting novel avenues for development of OA therapies.

MyD88 dimerization inhibitor ST2825 targets the aggressiveness of synovial fibroblasts in rheumatoid arthritis patients

BACKGROUND

Dimerization of the myeloid differentiation primary response 88 protein (MyD88) plays a pivotal role in the exacerbated response to innate immunity-dependent signaling in rheumatoid arthritis (RA). ST2825 is a highly specific inhibitor of MyD88 dimerization, previously shown to inhibit the pro-inflammatory gene expression in peripheral blood mononuclear cells from RA patients (RA PBMC). In this study, we elucidated the effect of disrupting MyD88 dimerization by ST2825 on the pathological properties of synovial fibroblasts from RA patients (RA SFs).

METHODS

RA SFs were treated with varying concentrations of ST2825 in the presence or absence of bacterial lipopolysaccharides (LPS) to activate innate immunity-dependent TLR signaling. The DNA content of the RA SFs was quantified by imaging cytometry to investigate the effect of ST2825 on different phases of the cell cycle and apoptosis. RNA-seq was used to assess the global response of the RA SF toward ST2825. The invasiveness of RA SFs in Matrigel matrices was measured in organoid cultures. SFs from osteoarthritis (OA SFs) patients and healthy dermal fibroblasts were used as controls.

RESULTS

ST2825 reduced the proliferation of SFs by arresting the cells in the G0/G1 phase of the cell cycle. In support of this finding, transcriptomic analysis by RNA-seq showed that ST2825 may have induced cell cycle arrest by primarily inhibiting the expression of critical cell cycle regulators Cyclin E2 and members of the E2F family transcription factors. Concurrently, ST2825 also downregulated the genes encoding for pain, inflammation, and joint catabolism mediators while upregulating the genes required for the translocation of nuclear proteins into the mitochondria and members of the mitochondrial respiratory complex 1. Finally, we demonstrated that ST2825 inhibited the invasiveness of RA SFs, by showing decreased migration of LPS-treated RA SFs in spheroid cultures.

CONCLUSIONS

The pathological properties of the RA SFs, in terms of their aberrant proliferation, increased invasiveness, upregulation of pain and inflammation mediators, and disruption of mitochondrial homeostasis, were attenuated by ST2825 treatment. Taken together with the previously reported anti-inflammatory effects of ST2825 in RA PBMC, this study strongly suggests that targeting MyD88 dimerization could mitigate both systemic and synovial pathologies in a variety of inflammatory arthritic diseases.

CC chemokines and receptors in osteoarthritis: new insights and potential targets

Osteoarthritis (OA) is a prevalent degenerative disease accompanied by the activation of innate and adaptive immune systems-associated inflammatory responses. Due to the local inflammation, the expression of various cytokines was altered in affected joints, including CC motif chemokine ligands (CCLs) and their receptors (CCRs). As essential members of chemokines, CCLs and CCRs played an important role in the pathogenesis and treatment of OA. The bindings between CCLs and CCRs on the chondrocyte membrane promoted chondrocyte apoptosis and the release of multiple matrix-degrading enzymes, which resulted in cartilage degradation. In addition, CCLs and CCRs had chemoattractant functions to attract various immune cells to osteoarthritic joints, further leading to the aggravation of local inflammation. Furthermore, in the nerve endings of joints, CCLs and CCRs, along with several cellular factors, contributed to pain hypersensitivity by releasing neurotransmitters in the spinal cord. Given this family's diverse and complex functions, targeting the functional network of CCLs and CCRs is a promising strategy for the prognosis and treatment of OA in the future.

Piezo2 expressing nociceptors mediate mechanical sensitization in experimental osteoarthritis

Non-opioid targets are needed for addressing osteoarthritis pain, which is mechanical in nature and associated with daily activities such as walking and climbing stairs. Piezo2 has been implicated in the development of mechanical pain, but the mechanisms by which this occurs remain poorly understood, including the role of nociceptors. Here we show that nociceptor-specific Piezo2 conditional knock-out mice were protected from mechanical sensitization associated with inflammatory joint pain in female mice, joint pain associated with osteoarthritis in male mice, as well as both knee swelling and joint pain associated with repeated intra-articular injection of nerve growth factor in male mice. Single cell RNA sequencing of mouse lumbar dorsal root ganglia and in situ hybridization of mouse and human lumbar dorsal root ganglia revealed that a subset of nociceptors co-express Piezo2 and Ntrk1 (the gene that encodes the nerve growth factor receptor TrkA). These results suggest that nerve growth factor-mediated sensitization of joint nociceptors, which is critical for osteoarthritic pain, is also dependent on Piezo2, and targeting Piezo2 may represent a therapeutic option for osteoarthritis pain control.

Notch signaling is activated in knee-innervating dorsal root ganglia in experimental models of osteoarthritis joint pain

BACKGROUND

We aimed to explore activation of the Notch signaling pathway in knee-innervating lumbar dorsal root ganglia (DRG) in the course of experimental osteoarthritis (OA) in mice, and its role in knee hyperalgesia.

METHODS

Cultured DRG cells were stimulated with the TLR4 agonist, lipopolysaccharide (LPS). Notch signaling in the cells was either inhibited with the γ-secretase inhibitor, DAPT, or with soluble Jagged1, or activated through immobilized Jagged1. CCL2 production was analyzed at mRNA and protein levels. In in vivo experiments, knee hyperalgesia was induced in naïve mice through intra-articular (IA) injection of LPS. The effect of inhibiting Notch signaling was examined by pre-injecting DAPT one hour before LPS. OA was induced through surgical destabilization of the medial meniscus (DMM) in male C57BL/6 mice. Gene expression in DRG was analyzed by qRT-PCR and RNAscope in situ hybridization. Activated Notch protein (NICD) expression in DRG was evaluated by ELISA and immunofluorescence staining. DAPT was injected IA 12 weeks post DMM to inhibit Notch signaling, followed by assessing knee hyperalgesia and CCL2 expression in the DRG.

RESULTS

In DRG cell cultures, LPS increased NICD in neuronal cells. Inhibition of Notch signaling with either DAPT or soluble Jagged1 attenuated LPS-induced increases of Ccl2 mRNA and CCL2 protein. Conversely, activating Notch signaling with immobilized Jagged1 enhanced these LPS effects. In vivo, IA injection of LPS increased expression of Notch genes and NICD in the DRG. Pre-injection of DAPT prior to LPS alleviated LPS-induced knee hyperalgesia, and decreased LPS-induced CCL2 expression in the DRG. Notch signaling genes were differentially expressed in the DRG from late-stage experimental OA. Notch1, Hes1, and NICD were increased in the neuronal cell bodies in DRG after DMM surgery. IA administration of DAPT alleviated knee hyperalgesia post DMM, and decreased CCL2 expression in the DRG.

CONCLUSIONS

These findings suggest a synergistic effect of Notch signaling with TLR4 in promoting CCL2 production and mediating knee hyperalgesia. Notch signaling is activated in knee-innervating lumbar DRG in mice with experimental OA, and is involved in mediating knee hyperalgesia. The pathway may therefore be explored as a target for alleviating OA pain.

Role of Human Mesenchymal Stem Cells and Derived Extracellular Vesicles in Reducing Sensory Neuron Hyperexcitability and Pain Behaviors in Murine Osteoarthritis

OBJECTIVE

Mesenchymal stem/stromal cells (MSCs) and MSC-derived extracellular vesicles (MSC-EVs) have been reported to alleviate pain in patients with knee osteoarthritis (OA). We undertook this study to determine whether MSCs and/or MSC-EVs reduce OA pain through influencing sensory neuron excitability in OA joints.

METHODS

We induced knee OA in adult male C57BL/6J mice through destabilization of the medial meniscus (DMM) surgery. Mice were sorted into 4 experimental groups with 9 mice per group as follows: unoperated sham, untreated DMM, DMM plus MSC treatment, and DMM plus MSC-EV treatment. Treated mice received either MSCs at week 14 postsurgery or MSC-EVs at weeks 12 and 14 postsurgery. Mouse behavior was evaluated by digging and rotarod tests and the Digital Ventilated Cage system. At week 16, mouse knee joints were harvested for histology, and dorsal root ganglion (DRG) neurons were isolated for electrophysiology. Furthermore, we induced hyperexcitability in DRG neurons in vitro using nerve growth factor (NGF) then treated these neurons with or without MSC-EVs and evaluated neuron excitability.

RESULTS

MSC- and MSC-EV-treated DMM-operated mice did not display pain-related behavior changes (in locomotion, digging, and sleep) that occurred in untreated DMM-operated mice. The absence of pain-related behaviors in MSC- and MSC-EV-treated mice was not the result of reduced joint damage but rather a lack of knee-innervating sensory neuron hyperexcitability that was observed in untreated DMM-operated mice. Furthermore, we found that NGF-induced sensory neuron hyperexcitability is prevented by MSC-EV treatment (P < 0.05 versus untreated NGF-sensitized neurons when comparing action potential threshold).

CONCLUSION

MSCs and MSC-EVs may reduce pain in OA by direct action on peripheral sensory neurons.

Targeting Vascular Endothelial Growth Factor Receptors as a Therapeutic Strategy for Osteoarthritis and Associated Pain

Pain is the major reason that patients suffering from osteoarthritis (OA) seek medical care. We found that vascular endothelial growth factors (VEGFs) mediate signaling in OA pain pathways. To determine the specific contributions of VEGFs and their receptors (VEGFRs) to joint pathology and pain transmission during OA progression, we studied intra-articular (IA) injections of VEGF ligands into murine knee joints. Only VEGF ligands specific for the activation of VEGFR1, but not VEGFR2, induced allodynia within 30 min. Interventions in OA by inhibitors of VEGFRs were done in vivo using a preclinical murine OA model by IA injections of selective inhibitors of VEGFR1/VEGFR2 kinase (pazopanib) or VEGFR2 kinase (vandetanib). OA phenotypes were evaluated using pain-associated murine behavioral tests and histopathologic analyses. Alterations in VEGF/VEGFR signaling by drugs were determined in knee joints, dorsal root ganglia, and spinal cord by immunofluorescence microscopy. Pazopanib immediately relieved OA pain by interfering with pain transmission pathways. Pain reduction by vandetanib was mainly due to the inhibition of cartilage degeneration by suppressing VEGFR2 expression. In conclusion, IA administration of pazopanib, which simultaneously inhibits VEGFR1 and VEGFR2, can be developed as an ideal OA disease-modifying drug that rapidly reduces joint pain and simultaneously inhibits cartilage degeneration.

Sustained inhibition of CC-chemokine receptor-2 via intraarticular deposition of polymeric microplates in post-traumatic osteoarthritis

Posttraumatic osteoarthritis (PTOA) is mostly treated via corticosteroid administration, and total joint arthroplasty continues to be the sole effective intervention in severe conditions. To assess the therapeutic potential of CCR2 targeting in PTOA, we used biodegradable microplates (µPLs) to achieve a slow and sustained intraarticular release of the CCR2 inhibitor RS504393 into injured knees and followed joint damage during disease progression. RS504393-loaded µPLs (RS-µPLs) were fabricated via a template-replica molding technique. A mixture of poly(lactic-co-glycolic acid) (PLGA) and RS504393 was deposited into 20 × 10 μm (length × height) wells in a polyvinyl alcohol (PVA) square-patterned template. After physicochemical and toxicological characterizations, the RS504393 release profile from µPL was assessed in PBS buffer. C57BL/6 J male mice were subjected to destabilization of the medial meniscus (DMM)/sham surgery, and RS-µPLs (1 mg/kg) were administered intraarticularly 1 week postsurgery. Administrations were repeated at 4 and 7 weeks post-DMM. Drug free-µPLs (DF-µPLs) and saline injections were performed as controls. Mice were euthanized at 4 and 10 weeks post-DMM, corresponding to the early and severe PTOA stages, respectively. Knees were evaluated for cartilage structure score (ACS, H&E), matrix loss (safranin O score), osteophyte formation and maturation from cartilage to bone (cartilage quantification), and subchondral plate thickness. The RS-µPL architecture ensured the sustained release of CCR2 inhibitors over several weeks, with ~ 20% of RS504393 still available at 21 days. This prolonged release improved cartilage structure and reduced bone damage and synovial hyperplasia at both PTOA stages. Extracellular matrix loss was also attenuated, although with less efficacy. The results indicate that local sustained delivery is needed to optimize CCR2-targeted therapies.

Current understanding of osteoarthritis pathogenesis and relevant new approaches

Osteoarthritis (OA) is the most common degenerative joint disease that causes painful swelling and permanent damage to the joints in the body. The molecular mechanisms of OA are currently unknown. OA is a heterogeneous disease that affects the entire joint, and multiple tissues are altered during OA development. To better understand the pathological mechanisms of OA, new approaches, methods, and techniques need to be used to understand OA pathogenesis. In this review, we first focus on the epigenetic regulation of OA, with a particular focus on DNA methylation, histone modification, and microRNA regulation, followed by a summary of several key mediators in OA-associated pain. We then introduce several innovative techniques that have been and will continue to be used in the fields of OA and OA-associated pain, such as CRISPR, scRNA sequencing, and lineage tracing. Next, we discuss the timely updates concerning cell death regulation in OA pathology, including pyroptosis, ferroptosis, and autophagy, as well as their individual roles in OA and potential molecular targets in treating OA. Finally, our review highlights new directions on the role of the synovial lymphatic system in OA. An improved understanding of OA pathogenesis will aid in the development of more specific and effective therapeutic interventions for OA.

Biological Targets of Multimolecular Therapies in Middle-Age Osteoarthritis

Knee osteoarthritis (OA) is a common condition, prevalent in middle-agedness, associated with chronic pain and impaired quality of life. Two interrelated biological processes fuel early OA progression: inflammation and structural tissues catabolism. Procatabolic and proinflammatory mediators are interconnected and form part of a self-perpetuating loop. They leverage OA research complexity because of the impossibility to discern certain spatiotemporal tissues' changes from others. Both are shared targets of versatile regenerative multimolecular therapies. In particular, platelet-rich plasma can interfere with inflammation and inflammatory pain. The therapeutic approach is to alter the vicious inflammatory loop by modifying the molecular composition of the synovial fluid, thereby paracrine cellular cross talk. Intra-articular injections of platelet-rich plasma can provide key factors balancing proinflammatory and anti-inflammatory factors, targeting macrophage dysfunction and modulating immune mechanisms within the knee.

Targeting the tamoxifen receptor within sodium channels to block osteoarthritic pain

Voltage-gated sodium channels (NaV) in nociceptive neurons initiate action potentials required for transmission of aberrant painful stimuli observed in osteoarthritis (OA). Targeting NaV subtypes with drugs to produce analgesic effects for OA pain management is a developing therapeutic area. Previously, we determined the receptor site for the tamoxifen analog N-desmethyltamoxifen (ND-Tam) within a prokaryotic NaV. Here, we report the pharmacology of ND-Tam against eukaryotic NaVs natively expressed in nociceptive neurons. ND-Tam and analogs occupy two conserved intracellular receptor sites in domains II and IV of NaV1.7 to block ion entry using a "bind and plug" mechanism. We find that ND-Tam inhibition of the sodium current is state dependent, conferring a potent frequency- and voltage-dependent block of hyperexcitable nociceptive neuron action potentials implicated in OA pain. When evaluated using a mouse OA pain model, ND-Tam has long-lasting efficacy, which supports the potential of repurposing ND-Tam analogs as NaV antagonists for OA pain management.

Rodent models of knee osteoarthritis for pain research

Osteoarthritis (OA) is a chronic degenerative joint disease and a leading cause of disability worldwide. Pain is the main symptom, yet no current treatment can halt disease progression or effectively provide symptomatic relief. Numerous animal models have been described for studying OA and some for the associated OA pain. This review aims to update on current models used for studying OA pain, focusing on mice and rats. These models include surgical, chemical, mechanical, and spontaneous OA models. The impact of sex and age will also be addressed in the context of OA modelling. Although no single animal model has been shown ideal for studying OA pain, increased efforts to phenotype OA will likely impact the choice of models for pre-clinical and basic research studies.

Innate Immunity at the Core of Sex Differences in Osteoarthritic Pain?

Osteoarthritis (OA) is a progressive whole-joint disease; no disease-modifying drugs are currently available to stop or slow its process. Symptoms alleviation is the only treatment option. OA is the major cause of chronic pain in adults, with pain being the main symptom driving patients to seek medical help. OA pathophysiology is closely associated with the innate immune system, which is also closely linked to pain mediators leading to joint pain. Pain research has shown sex differences in the biology of pain, including sexually dimorphic responses from key cell types in the innate immune system. Not only is OA more prevalent in women than in men, but women patients also show worse OA outcomes, partially due to experiencing more pain symptoms despite having similar levels of structural damage. The cause of sex differences in OA and OA pain is poorly understood. This review provides an overview of the involvement of innate immunity in OA pain in joints and in the dorsal root ganglion. We summarize the emerging evidence of sex differences regarding innate immunity in OA pain. Our main goal with this review was to provide a scientific foundation for future research leading to alternative pain relief therapies targeting innate immunity that consider sex differences. This will ultimately lead to a more effective treatment of pain in both women and men.

The Genesis of Pain in Osteoarthritis: Inflammation as a Mediator of Osteoarthritis Pain

Chronic pain is a substantial personal and societal burden worldwide. Osteoarthritis (OA) is one of the leading causes of chronic pain and is increasing in prevalence in accordance with a global aging population. In addition to affecting patients' physical lives, chronic pain also adversely affects patients' mental wellbeing. However, there remain no pharmacologic interventions to slow down the progression of OA and pain-alleviating therapies are largely unsuccessful. The presence of low-level inflammation in OA has been recognized for many years as a major pathogenic driver of joint damage. Inflammatory mechanisms can occur locally in joint tissues, such as the synovium, within the sensory nervous system, as well as systemically, caused by modifiable and unmodifiable factors. Understanding how inflammation may contribute to, and modify pain in OA will be instrumental in identifying new druggable targets for analgesic therapies. In this narrative review, we discuss recent insights into inflammatory mechanisms in OA pain. We discuss how local inflammation in the joint can contribute to mechanical sensitization and to the structural neuroplasticity of joint nociceptors, through pro-inflammatory factors such as nerve growth factor, cytokines, and chemokines. We consider the role of synovitis, and the amplifying mechanisms of neuroimmune interactions. We then explore emerging evidence around the role of neuroinflammation in the dorsal root ganglia and dorsal horn. Finally, we discuss how systemic inflammation associated with obesity may modify OA pain and suggest future research directions.

Osteoarthritis Pain

Joint pain is the hallmark symptom of osteoarthritis (OA) and the main reason for patients to seek medical assistance. OA pain greatly contributes to functional limitations of joints and reduced quality of life. Although several pain-relieving medications are available for OA treatment, the current intervention strategy for OA pain cannot provide satisfactory pain relief, and the chronic use of the drugs for pain management is often associated with significant side effects and toxicities. These observations suggest that the mechanisms of OA-related pain remain undefined. The current review mainly focuses on the characteristics and mechanisms of OA pain. We evaluate pathways associated with OA pain, such as nerve growth factor (NGF)/tropomyosin receptor kinase A (TrkA), calcitonin gene-related peptide (CGRP), C–C motif chemokine ligands 2 (CCL2)/chemokine receptor 2 (CCR2) and tumor necrosis factor alpha (TNF-α), interleukin-1beta (IL-1β), the NOD-like receptor (NLR) family, pyrin domain-containing protein 3 (NLRP3) inflammasome, and the Wnt/β-catenin signaling pathway. In addition, animal models currently used for OA pain studies and emerging preclinical studies are discussed. Understanding the multifactorial components contributing to OA pain could provide novel insights into the development of more specific and effective drugs for OA pain management.

G Protein-Coupled Receptors in Osteoarthritis

Osteoarthritis (OA) is the most common chronic joint disease characterized, for which there are no available therapies being able to modify the progression of OA and prevent long-term disability. Critical roles of G-protein coupled receptors (GPCRs) have been established in OA cartilage degeneration, subchondral bone sclerosis and chronic pain. In this review, we describe the pathophysiological processes targeted by GPCRs in OA, along with related preclinical model and/or clinical trial data. We review examples of GPCRs which may offer attractive therapeutic strategies for OA, including receptors for cannabinoids, hormones, prostaglandins, fatty acids, adenosines, chemokines, and discuss the main challenges for developing these therapies.

Cannabinoid receptor type 2 is upregulated in synovium following joint injury and mediates anti-inflammatory effects in synovial fibroblasts and macrophages

OBJECTIVE

Joint injury-induced perturbations to the endocannabinoid system (ECS), a regulator of both inflammation and nociception, remain largely uncharacterized. We employed a mouse model of ACL rupture to assess alterations to nociception, inflammation, and the ECS while using in vitro models to determine whether CB2 agonism can mitigate inflammatory signaling in macrophages and fibroblast-like synoviocytes (FLS).

DESIGN

Mice underwent noninvasive ACL rupture (ACLR) via tibial compression-based loading. Nociception was measured longitudinally using mechanical allodynia and knee hyperalgesia testing. Synovitis was assessed using histological scoring and histomorphometry. Gene and protein markers of inflammation were characterized in whole joints and synovium. Immunohistochemistry assessed injury-induced alterations to CB1+, CB2+, and F4/80+ cells in synovium. To assess whether CB2 agonism can inhibit pro-inflammatory macrophage polarization, murine bone marrow-derived macrophages (mBMDM) were stimulated with IL-1β or conditioned medium from IL-1β-treated FLS and treated with vehicle (DMSO), the CB2 agonist HU308, or cannabidiol (CBD). Macrophage polarization was assessed as the ratio of M1-associated (IL1b, MMP1b, and IL6) to M2-associated (IL10, IL4, and CD206) gene expression. Human FLS (hFLS) isolated from synovial tissue of OA patients were treated with vehicle (DMSO) or HU308 following TNF-α or IL-1β stimulation to assess inhibition of catabolic/inflammatory gene expression.

RESULTS

ACLR induces synovitis, progressively-worsening PTOA severity, and an immediate and sustained increase in both mechanical allodynia and knee hyperalgesia, which persist beyond the resolution of molecular inflammation. Enrichment of CB2, but not CB1, was observed in ACLR synovium at 3d, 14d, and 28d, and CB2 was found to be associated with F4/80 (+) cells, which are increased in number in ACLR synovium at all time points. The CB2 agonist HU308 strongly inhibited mBMDM M1-type polarization following stimulation with either IL-1β or conditioned medium from IL-1β-treated mFLS, which was characterized by reductions in Il1b, Mmp1b, and Il6 and increases in Cd206 gene expression. Cannabidiol similarly inhibited IL-1β-induced mBMDM M1 polarization via a reduction in Il1b and an increase in Cd206 and Il4 gene expression. Lastly, in OA hFLS, HU308 treatment inhibited IL-1β-induced CCL2, MMP1, MMP3, and IL6 expression and further inhibited TNF-α-induced CCL2, MMP1, and GMCSF expression, demonstrating human OA-relevant anti-inflammatory effects by targeting CB2.

CONCLUSIONS

Joint injury perturbs the intra-articular ECS, characterized by an increase in synovial F4/80(+) cells, which express CB2, but not CB1. Targeting CB2 in murine macrophages and human FLS induced potent anti-inflammatory and anti-catabolic effects, which indicates that the CB2 receptor plays a key role in regulating inflammatory signaling in the two primary effector cells in the synovium. The intraarticular ECS is therefore a potential therapeutic target for blocking pathological inflammation in future disease-modifying PTOA treatments.