Background/Purpose:   Transient receptor potential ankyrin 1 (TRPA1) is a membrane associated cation channel, which is widely expressed in neuronal cells and known to be involved in nociception and neurogenic inflammation. More recently, TRPA1 has also been found to be expressed in some non-neuronal cells, such as keratinocytes and synoviocytes, but the functional roles of non-neuronal expression remain to be studied. TRPA1 has previously been shown to be up-regulated and activated by agents formed endogenously in inflammatory and hypoxic conditions characteristic to human and MIA-model of osteoarthritis (OA). We hypothesized that TRPA1 is expressed and functional in human OA chondrocytes and mediates cartilage destruction and joint pain in MIA-induced experimental OA.

Methods:   Expression of TRPA1 in primary human OA chondrocytes was assessed by quantitative RT-PCR and Western Blot analysis. The functionality of the TRPA1 channel was assessed by Ca²⁺-influx measurements. Production of MMP-1, MMP-3, MMP-13, IL-6 and PGE₂ subsequent to TRPA1 activation were measured by immunoassay. Wild type and TRPA1 knock-out mice were used in mouse cartilage culture experiments. To induce experimental OA, MIA was injected into mouse knee joint either at lower (37.5ug) or higher (500 ug) dose, and contralateral knee was injected with the vehicle. Joint pain was estimated by weight-bearing test at baseline and thereafter weekly. The animals were sacrificed four weeks after the MIA injections and histological changes in the knee joints were assessed according to the OARSI guidelines. The responses between wild type (WT) and TRPA1 deficient (knock-out, KO) mice were compared.

Results:   Remarkably, TRPA1 was found to be expressed and inducible by IL-1β in primary human OA chondrocytes. The TRPA1 channel was found to be functional, as stimulation with the TRPA1 agonist AITC caused an increase in Ca²⁺-influx, which was attenuated by the TRPA1 antagonist HC-030031. Pharmacological inhibition of TRPA1 with the selective antagonist HC-030031 downregulated the production of MMP-1, MMP-3, MMP-13, IL-6 and PGE₂ in primary human OA chondrocytes. Furthermore, genetic depletion of TRPA1 downregulated the production of MMP-3, IL-6 and PGE₂ in murine cartilage explants. Both doses of MIA caused spontaneous joint pain in WT mice as shown in weight-bearing test. However, in TRPA1 deficient mice no response was seen following the lower dose of MIA, and the response after the higher dose of MIA was clearly attenuated. Also, the higher dose of MIA caused cartilage changes typical for OA in the WT mice. Interestingly, the TRPA1 deficient mice did not develop such destructive joint changes following MIA injections.

Conclusion:   The TRPA1 cation channel was found to be functionally expressed in primary human OA chondrocytes and to mediate inflammatory and catabolic effects, which are both original findings. Further, in MIA-induced experimental OA TRPA1 was found to contribute to the development of cartilage changes and joint pain. The inflammatory and hypoxic environment in the OA joint is conducive to enhance the expression and activation of TRPA1. The results reveal TRPA1 as a potential mediator and drug target in osteoarthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/transient-receptor-potential-ankyrin-1-trpa1-is-functionally-expressed-in-primary-human-osteoarthritic-chondrocytes-and-mediates-cartilage-destruction-and-joint-pain-in-the-mia-model-of-osteoarthrit/