**3. Discussion**

It is no wonder that *matrix metallopeptidase 13* (*MMP13*), a broadly investigated critical OA-related gene [33,34], is one of the 35 DEGs that respond to OA the same trend in both cartilage and synovium, demonstrating the reliability of the current study. In addition to MMP13, some of these OA-responsive DEGs shared by cartilage and synovium have also been studied in the arena against arthritis, although their function may only be investigated solely in cartilage or synovium. For example, CHI3L2 (also known as YKL-39) has been identified as a biochemical marker for the OA progression in human cartilage since 2002 [35,36]. Functionally, CHI3HL2 has been shown to enhance the proliferation and type II collagen expression in ATDC5 mouse chondrogenic cells [37]. With the successful generation of polyclonal and monoclonal antibodies against CHI3L2, strong activation of the CHI3L2 was detected not only in human T lymphocyte cell lines and monocytes but in the synovial fluid of an OA patient [38]. More importantly, autoimmunity against CHI3L2 was also detected in OA patients [39]. However, its function on synovium tissue is still unknown.

On the other hand, as a transmembrane serine protease that is known to be associated with cell migration and cell invasiveness [40], FAP has been associated with arthritic synovium and cartilage. FAP-deficiency in hTNFtg mice led to less cartilage degradation [40], while highly expressed FAP has been detected in the rheumatoid synovium [41]. A radiolabeled anti–FAP antibody has even been used as a noninvasive strategy to monitor the course of collagen-induced arthritis in mice [41]. For the therapeutic drug development, targeted photodynamic therapy (tPDT) using the anti-FAP antibody 28H1 coupled to the photosensitizer IRDye700DX moderately delayed the collagen-induced arthritis development in mice [42]. Meanwhile, TREM-1 has also been identified as a biomarker of synovitis in RA [43] and predicting the therapeutic response to methotrexate in RA [44]. Inhibiting the expression of TREM-1 could suppress the chondrocyte injury induce by IL-1β in vitro [45], and ameliorate collagen-induced arthritis and protect bone and cartilage damage in vivo [46]. Although these published functional tests were predominantly archived in the RA scenario, especially with the RA synovial fibroblasts, our current study identified expression of FAP and TREM-1 significantly upregulated in both synovium and cartilage of OA patients, strongly suggesting that lowering FAP and TREM-1 might be beneficial for both RA and OA patients.

Besides, MSR1 is a multifunctional receptor expressed primarily on cells of the myeloid lineage [47]. It positively regulates the activation of macrophages and thus promoting the inflammation [48]. The genetic deficiency of *Msr1* decreased the incidence and severity of autoimmune arthritis in the K/BxN T cell receptor (TCR) transgenic mouse model [47]. It is well known that the immune system plays a critical role in OA pathogenesis, and previous studies have acknowledged that the application of immunomodulatory drugs is a potential avenue for OA treatments [49–51]. With the identification of MSR1 in the current study, we may further prove the immune processes' participation within the OA joint and synovium [9].

TNFAIP6 (also known as TSG-6) is an upregulated OA-responsive DEG shared by cartilage and synovium. It is a multifunctional protein with anti-inflammatory and tissue-protective biopotencies [52]. It has been shown that intra-articular delivery of TNFAIP6 could reduce cartilage damage in a rat model of OA [53]. However, in the current study, the expression of TNFAIP6 is increased in both cartilage and synovium tissue during OA. These controversial findings could be explained by the article from Chou et al. published in 2018 [54]: TNFAIP6 was highly expressed in damaged articular and meniscal cartilage and cytokine-treated chondrocytes; functionally, TNFAIP6 impairs hyaluronan-aggrecan assembly, but TNFAIP6 mediated hyaluronan-heavy chain formation reduced this adverse effect. Thus, whether TNFAIP6 could be utilized for OA treatment or if it could worsen OA prognosis still needs further investigations.

Meanwhile, our finding is in line with the previous publication that 6-phosphofructo-2 kinase/fructose2,6-bisphosphatase 3 (PFKFB3) expression was down-regulated in human OA cartilage tissues [55]. PFKFB3 is a glycolytic regulator modulating glycolytic metabolism, alleviating endoplasmic reticulum stress, reducing caspase 3 activation, and promoting aggrecan and type II collagen expressions in human OA cartilage [55]. Interestingly, PFKFB3 expression was increased in RA patients' synovial tissue but not in those of OA patients [56]. Moreover, inhibition of PFKFB3 suppresses the synovial inflammation and joint destruction in RA [56]. The distinct expression patterns and entirely opposite functions of PFKFB3 in OA and RA may set PFKFB3 as a biomarker for clinical differential diagnosis and may explain why some DMARDs are failing to improve OA.

Notably, we also identified multiple molecules that had not been correlated to OA. Nevertheless, their functions in chondrogenesis or inflammation make them as the novel targets for OA investigations in the future. For instance, APOD has been identified to be a downstream gene regulated by SRY-box transcription factor 9 (SOX9), an essential transcription factor for chondrocyte phenotype maintaining [57], in human chondrocytic cell line (SW1353) and primary human articular chondrocytes (hARCs) [58]. Particularly, *SOX9* downregulation in OA cartilage is followed by reduced expression of *APOD* [58]. Our finding is consistent with this publication that the expression of *APOD* lowered in OA cartilage tissue, and we also found that *APOD* is decreased in OA synovium. Interestingly, *SOX9* is not

one of the 35 common OA-responsive DEGs shared by cartilage and synovium, suggesting that SOX9 may not play a critical role in OA's synovium. Thus, APOD may participate in other SOX9-independent signaling pathways in the synovium tissue during OA progression. A recent study identified APOD as one of the novel molecular markers of human Th17 cells [59]. Although Th17 cells were initially investigated in RA due to their potency against autoimmune diseases [59], accumulating evidence suggests they are also increased in the OA [60]. Additionally, the high APOD protein level in the round ligament fat depot of severely obese women is associated with an improved inflammatory profile [61]. Thus, APOD may manage OA through chondrogenesis in articular cartilage and immune regulation in the synovium. Further functional investigations are necessary to verify this hypothesis.

Another example is C1QB, one of the C1Q genes which transcript component C1B predispose to RA [62]. The newly published paper demonstrated that primary hARCs express *C1QA*, *C1QB*, *C1QC*, and secret C1Q to the extracellular medium, while this expression is regulated by proinflammatory cytokines stimulations [63]. C1Q could bond to the chondrocytes in vitro, altering the expression of collagens [63]. C1Q can also bond to the cartilage matrix components, such as fibromodulin (FMOD), and activate the complement system to eliminate pathogens and damaged cells for tissue recovery and reconstruction [64]. Since the STRING database identified the biological process of "regulating the immune system process" the role of complement activation in both synovium and articular cartilage during OA might be worth more attention.

CDK1 is another commonly increased DEG in OA cartilage and synovium. As a member of the cyclin-dependent kinase family that plays a pivotal role in controlling the cell cycle [65], CDK1 is not only a hub node of the protein-protein interaction network of the 1,25-dihydroxy-vitamin D3 treated primary OA chondrocytes [66]; its expression is also related to the pathogenesis of RA [67]. It is interesting to find that CDK1 is one of the top 10 hub genes identified from the database of "osteoarthritic degenerative meniscal lesions" [65]. Thus, our current study supports the hypothesis that CDK1 plays an essential role in all three of cartilage, synovium, and meniscus. However, the "cell cycle" cluster has a broad range of effects on a diversity of biological events, and thus CDK1 may not be a unique biomarker for OA diagnosis or treatment.

FPR3 has two isoforms FPR1 and FPR2. The FPRs belong to the classical chemotactic G-protein-coupled receptor family that has recently been recognized to play critical roles in inflammation regulation in response to pathogen- or damage-associated chemotactic molecular patterns [68]. The FPRs agonists have been broadly investigated as a potential treatment strategy for various inflammation-related diseases [68]. Recently, the investigation of FPR agonists has extended to RA [69]. Thus, the anti-inflammation effect of FPR agonists might be utilized in OA targeting both the synovium and cartilage based on the finding in our research.

Our current finding is consistent with the previous studies demonstrating that the expression of KLF15 is significantly lower in chondrocytes from OA patients than from healthy subjects [70]. KLF15 is a transcriptional factor that could promote the chondrogenic differentiation of human mesenchymal stem cells by binding to the promoter of SOX9 and activating its expression [71]. Moreover, KLF15 could reduce the TNF-α-induced expression of MMP-3, a well-known cartilage-degrading enzyme [70]. On the other hand, KFL15 activation could negatively regulate inflammations in several cell types [72–75]. Thus, elevating KLF15 levels in OA may also achieve the dual effects of pro-chondrogenesis and anti-inflammation.

Last but not least, several identified genes have even not been correlated to joints so far. With the rapid development and advancement in the research fields, their OA correlations would be gradually established sooner or later. For example, amelotin (AMTN) is a novel secreted protein firstly identified in 2005 and was thought to be specific for ameloblasts [76]. Later on, most research about this protein was focused on its expression in gingival epithelial cells regulated by proinflammatory cytokines [77,78]. Since the current study distinguished *AMTN* as one of the upregulated genes in both synovium and cartilage in response to OA, its functional involvement in OA, particularly in inflammatory reactions,

may be worth assessed. Additionally, KCNJ6 (also known as GIRK2) is a potassium channel regulator that holds potential as a pain-reducing target in OA patients [79,80].

It is well known that sex is an essential factor that significantly alters the gene expression in OA pathogenesis. In humans, the prevalence of OA is knowingly higher in women than men [4,6], while women typically present with worse symptoms, including more significant complaints of pain and disability [81,82]. Meanwhile, although OA is not an inevitable consequence of growing old, older age is the most significant risk factor for OA due to the accumulation of a diversity of OA inducers, such as joint injury, obesity, genetics, and anatomical factors that affect joint mechanics [83]. It is also possible that age-related cell senescence plays a critical role in promoting OA initiation and progression. Unfortunately, currently available transcriptome datasets are not collected to reflect the influence of ages, limiting our ability to draw the "blueprint" of age-related OA gene expression. Clearly, it is an urgent task to conquer in future investigations. On the other hand, as the current study aims to identify the potential unisex markers in OA synovium and cartilage and to get the insight into novel OA therapies effective in both male and female OA patients with all ages, we believe that the identified targets mentioned above are valid for the new generation of DMOAD developing.
