Osteoarthritis (OA) is a degenerative disease of articular cartilage that causes cartilage degradation, osteophyte formation, synovial inflammation, angiogenesis, and subchondral bone alteration. OA causes chronic disability in older people. Various factors are associated with its pathogenesis, including aging, obesity, joint instability, and joint inflammation. In healthy conditions, cartilage remodeling involves balanced interactions of synthesis and degradation to achieve homeostasis of the extracellular matrix (ECM) However, in OA this process becomes unbalanced, leading to pathologic changes in the affected joint. OA is also a highly prevalent rheumatic musculoskeletal disorder, that affected 303 million people globally in 2017. In the U.S. only, OA affects more than 32.5 million adults and is estimated to affect approximately 70 million more (i.e., 25% of the U.S. population) by 2030. Multiple OA factors might lead to stimulate the chondrocytes in the articular cartilage to produce the proteolytic enzymes which include matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinase with thrombospondin motifs
(ADAMTS) which work together to degrade joint articular cartilage leading to osteophyte formation and stiffening of joints. So far, there are no medications that can treat OA and all medicines such as analgesics, corticosteroids, and non-steroid anti-inflammatory drugs (NSAIDs) are used to reduce pain and inflammation.
Trafficking protein particle complex subunit 9 (TRAPPC9) is a protein subunit part of the Transport Protein Particle II (TRAPPII), a highly conserved trafficking pathway from yeast to human. Not only TRAPPC9 is implicated in protein trafficking, but it has been also reported that TRAPPC9 regulates/potentiates multiple cellular activities such as prefiltration, differentiation, and function for several cell types through NF-kB mediation. To emphasize the relationship between TRAPPC9 and NF-kB, a study showed that TRAPPC9 physically interacts with NF-kB-inducing kinase (NIK) and inhibitor of nuclear factor kappa-B kinase subunit beta (IKKβ) but not IKKα or IKKγ. While the role of TRAPPC9 in cartilage is unknown, our study aims for the first time to discover the role of TRAPPC9 in cartilage homeostasis in OA.
On the other hand, we also studied L-Plastin (LPL). LPL belongs to a family of plastins that exist in three isoforms: L-, I-, and T-plastin and they are actin-bundling proteins essential for actin regulation in eukaryotes. LPL, also called lymphocyte cytosolic protein 1 (LCP1), has been described as one of the 15 most abundant proteins in human monocytes and T cells. LPL phosphorylation is essential to its function and it occurs on serine 5 and serine 7, however, other plastins isoforms not phosphorylated. In recent studies, LPL was shown to have a role in inflammation, a study by Ozturk et al, showed that patients with chronic periodontitis and generalized aggressive periodontitis exhibited significantly higher tissue LPL gene expression and gingival crevicular fluid levels compared to the control group. This indicates that LPL might be involved in OA inflammation.
The current study is composed of three hypothesizes. First, TRAPPC9 has a pro-inflammatory role in osteoarthritis by inducing the catabolic genes such as MMP-3, MMP- 9, MMP-13. Second, TRAPPC9 binds physically to LPL in chondrocytes. Third, LPL has also a pro-inflammatory role in osteoarthritis by inducing the catabolic genes such as MMP-3, MMP- 9, MMP-13.
Three mouse models were used in this study to define the function, mechanism, and regulation of LPL in cartilage which involved C57BL/6, Life Act/W.T, and Life act/LPL-/-.
Our data showed that TRAPPC9 and LPL were expressed in normal articular chondrocytes, and their expression level was higher in post-IL-1β treatment. IL-1β- induced inflammatory factors involved interleukin-6 (IL-6) and cartilage degradation enzymes such as MMP-3, MMP-9, and MMP-13, were upregulated following TRAPPC9 overexpression. Conversely, MMP-3, MMP-9, and MMP-13 were downregulated following TRAPPC9 knockdown or LPL phosphorylation inhibition respectively. Furthermore, immunoprecipitation data showed that TRAPPC9 binds to LPL in chondrocytes. We also found that LPL inhibitor inhibits P-P100 a subunit of NF-kB. We also showed that LPL knockout has chondroprotective effects in post-traumatic OA in mice.
In conclusion, TRAPPC9 and LPL are highly expressed in post-IL-1β treatment compared to the control. TRAPPC9 and LPL both have an anti-inflammatory effect by inhibiting the expression of MMP-3, MMP-9, and MMP-13, which are induced by IL-1β. Also, the post-traumatic OA model (DMM) showed that LPL knockout is essential to reduce inflammation. Our results determined that LPL knockout mice have less cartilage damaged compared to the WT mice.
Overall, the TRAPPC9 and LPL inhibition may be used as a therapeutic approach in osteoarthritis and may replace the anti-inflammatory medicine that causes several side effects which include elevated blood pressure, gastrointestinal bleeding, and kidney disorders.