Targeting STAT6-mediated synovial macrophage activation improves pain in experimental knee osteoarthritis

BACKGROUND

Pain from osteoarthritis (OA) is one of the top causes of disability worldwide, but effective treatment is lacking. Nociceptive factors are released by activated synovial macrophages in OA, but depletion of synovial macrophages paradoxically worsens inflammation and tissue damage in previous studies. Rather than depleting macrophages, we hypothesized that inhibiting macrophage activation may improve pain without increasing tissue damage. We aimed to identify key mechanisms mediating synovial macrophage activation and test the role of STAT signaling in macrophages on pain outcomes in experimental knee OA.

METHODS

We induced experimental knee OA in rats via knee destabilization surgery, and performed RNA sequencing analysis on sorted synovial tissue macrophages to identify macrophage activation mechanisms. Liposomes laden with STAT1 or STAT6 inhibitors, vehicle (control), or clodronate (depletion control) were delivered selectively to synovial macrophages via serial intra-articular injections up to 12 weeks after OA induction. Treatment effects on knee and hindpaw mechanical pain sensitivity were measured during OA development, along with synovitis, cartilage damage, and synovial macrophage infiltration using histopathology and immunofluorescence. Lastly, crosstalk between drug-treated synovial tissue and articular chondrocytes was assessed in co-culture.

RESULTS

The majority of pathways identified by transcriptomic analyses in OA synovial macrophages involve STAT signaling. As expected, macrophage depletion reduced pain, but increased synovial tissue fibrosis and vascularization. In contrast, STAT6 inhibition in macrophages led to marked, sustained improvements in mechanical pain sensitivity and synovial inflammation without worsening synovial or cartilage pathology. During co-culture, STAT6 inhibitor-treated synovial tissue had minimal effects on healthy chondrocyte gene expression, whereas STAT1 inhibitor-treated synovium induced changes in numerous cartilage turnover-related genes.

CONCLUSION

These results suggest that STAT signaling is a major mediator of synovial macrophage activation in experimental knee OA. STAT6 may be a key mechanism mediating the release of nociceptive factors from macrophages and the development of mechanical pain sensitivity. Whereas therapeutic depletion of macrophages paradoxically increases inflammation and fibrosis, blocking STAT6-mediated synovial macrophage activation may be a novel strategy for OA-pain management without accelerating tissue damage.

Transient anabolic effects of synovium in early post-traumatic osteoarthritis: a novel ex vivo joint tissue co-culture system for investigating synovium-chondrocyte interactions

OBJECTIVE

Osteoarthritis (OA) is a serious joint disease with no disease-modifying medical treatment. To develop treatments targeting synovium, we must improve our understanding of the effects of OA-related changes in synovial physiology on joint tissue outcomes. The aim of this study was to investigate the effects of synovial pathology due to post-traumatic OA (PTOA) on articular chondrocyte physiology.

METHODS

We first developed and validated a novel joint tissue co-culture system to model the biological interactions between synovium and articular chondrocytes. Whole-joint synovial tissue from a surgical rat model of PTOA vs sham and surgical-naïve controls was placed into a co-culture system with adult primary articular chondrocytes (n = 4-5). The effects of PTOA synovium on chondrocyte anabolic, inflammatory, and catabolic gene expression and sulfated glycosaminoglycan (sGAG) secretion and aggrecan synthesis were tested, and results from early and later stages of PTOA development were compared.

RESULTS

Synovial injury by arthrotomy (sham surgery) alone decreased primary chondrocyte expression of genes including Col2a1 (0.36 ± 0.15-fold) and Acan (0.41 ± 0.28-fold). Early PTOA synovium rescued the suppression of Acan, induced increased sGAG secretion (3.94 ± 0.44 μg/mL vs surgery-naïve 2.41 ± 0.55 and sham 2.92 ± 0.73 μg/mL controls), and upregulated Mmp3 (3.73 ± 2.62-fold) and Prg4 (4.93 ± 4.29-fold). These effects were lost with later stage PTOA synovium.

CONCLUSIONS

Early PTOA synovium induces transient anabolic responses in articular chondrocytes rather than pro-inflammatory responses that would require inhibition. These results suggest that PTOA synovium plays at least a partially protective role and that loss of these protective effects may contribute to PTOA progression.

Phosphorylation-mediated structural changes within the SOAR domain of stromal interaction molecule 1 enable specific activation of distinct Orai channels

Low-conductance, highly calcium-selective channels formed by the Orai proteins exist as store-operated CRAC channels and store-independent, arachidonic acid-activated ARC channels. Both are activated by stromal interaction molecule 1 (STIM1), but CRAC channels are activated by STIM1 located in the endoplasmic reticulum membrane, whereas ARC channels are activated by the minor plasma membrane-associated pool of STIM1. Critically, maximally activated CRAC channel and ARC channel currents are completely additive within the same cell, and their selective activation results in their ability to each induce distinct cellular responses. We have previously shown that specific ARC channel activation requires a PKA-mediated phosphorylation of a single threonine residue (Thr³⁸⁹) within the cytoplasmic region of STIM1. Here, examination of the molecular basis of this phosphorylation-dependent activation revealed that phosphorylation of the Thr³⁸⁹ residue induces a significant structural change in the STIM1-Orai-activating region (SOAR) that interacts with the Orai proteins, and it is this change that determines the selective activation of the store-independent ARC channels versus the store-operated CRAC channels. In conclusion, our findings reveal the structural changes underlying the selective activation of STIM1-induced CRAC or ARC channels that determine the specific stimulation of these two functionally distinct Ca²⁺ entry pathways.