HA accumulation was significantly decreased in damaged cartilage
Forty-eight pairs of cartilage samples from intact and damages areas were assessed histologically (Figure 1A) and graded using a modified Mankin score (Table S2). The mean score for intact cartilage was 4, compared with 9 for damaged cartilage (p <0.0001, Figure 1B and Table S3). Regarding IHC, positive staining was observed primarily in the nuclei of chondrocytes, although positive staining was occasionally identified in the cytoplasm (Figure 1A). The staining index was used to access the level of HA accumulation. The median staining index of intact cartilage was 6.5, compared with 3.4 for damaged cartilage (p < 0.0001, Figure 1C and Table S3). Pearson’s analyses performed using pathological and IHC data revealed a negative correlation between the Mankin score and HA expression in OA cartilage tissues (Figure 1D).
CircRNA expression profiles in intact and damaged cartilage
Based on the histologic examination, three pairs of cartilage samples were collected for further analysis (Figure 2A). Among these selected samples, the Mankin score of intact cartilage was ≤4, versus ≥9 for damaged cartilage (Table S4). According to the results of qRT-PCR, IL-1β and IL-6 expression was upregulated in damaged cartilage (Figure 2B).
Sequencing was performed to characterize the expression profiles of circRNAs in cartilage. We identified 450 differentially expressed circRNAs between paired intact and damaged cartilage (Table S5). Differentially expressed circRNAs between damaged and intact cartilages were subjected to hierarchical clustering analysis (>1-fold difference in expression, p < 0.05). Among these differentially expressed circRNAs, 200 were upregulated in damaged tissue samples, whereas 250 were downregulated (Figure 2C). The column chart revealed the frequency distribution of circRNA length in the cartilage samples (Figure 2D). The volcano map was constructed according to fold changes and p-value, illustrating the varied circRNA expression between paired intact and damaged cartilage (Figure 2E).
CircHYBID was predicted to be related to HA metabolism
To further screen candidate circRNAs, we analyzed the host genes of the differentially expressed circRNAs. The result revealed that multiple circRNA-associated host genes were involved in the pathogenesis of OA, such as those involved in ECM degradation, collagen secretion, HA metabolism, and immune inflammatory response (Table S4).
Furthermore, six circRNAs (hsa_circ_0057390 (hsa-circ-COL3A1, chr2:189859772|189861222), hsa_circ_0003893 (circHYBID), hsa_circ_0003922(FBXW2), hsa_circ_0009125 (circHABP4), hsa_circ_0002882(circAXL), and hsa_circ_0006719(circVKORC1)) potentially related to OA pathogenesis were selected to construct a network of circRNAs, mRNAs, and their commonly bound miRNAs (Figure 3A). To validate the circRNA sequencing results, qRT-PCR results confirmed that theses six circRNAs were differentially expressed between damage and intact cartilage, and the expression trends were consistent with the sequencing results (Figure 3B).The mRNA analysis revealed that multiple target genes, such as IL6, Bcl2, TGF-β1, and MMP9, are involved in HA metabolism, inflammatory response, apoptosis, and ECM degradation.
Among these circRNAs, we noticed a special cartilage-associated circRNA (circHYBID, circRNA ID hsa_circ_0003893 in circBase; http://circbase.org), and its 398-bp gene was located at chr15:81229014–81230320. The symbol of the associated gene is KIAA1199, which encodes HA-binding protein involved in HA depolymerization (HYBID) through linear expression. Because HYBID was previously identified as an important enzyme involved in HA degradation, we assumed that circHYBID might also play an important role in HA metabolism and OA pathogenesis. Therefore, circHYBID was chosen for further investigation.
To further characterize circHYBID, divergent and convergent primers were designed to amplify the circular and linear transcripts, respectively, in both cDNA and gDNA. The PCR results revealed that the circular form was amplified using the divergent primers only from cDNA, whereas convergent primers amplified both cDNA and gDNA (Figure 3C). Reverse splicing of circHYBID was successfully confirmed using Sanger sequencing (Figure 3D).
CircHYBID expression and HA accumulation were downregulated by IL-1β
To further validate the role of CircHYBID, primary chondrocytes were successfully isolated (NC group) and stimulated by IL-1β to construct the in vitro OA chondrocyte model (IL-1β group). QRT-PCR illustrated that TNF-α expression was downregulated after IL-1β stimulation, confirming that the OA chondrocyte model was successfully constructed (Figure 4A). CircHYBID expression of the chondrocytes was downregulated after IL-1β stimulation, which was consistent with the sequencing results of cartilage tissue (Figure 4B).
The mRNA expression of HA synthase 2 (HAS2) was downregulated and HYBID was upregulated in chondrocytes treated with IL-1β. However, there is no significant difference of the HAS1 expression under IL-1β stimulation (Figure 4C). WB data revealed that the protein expression of HAS2 was downregulated and HYBID was upregulated in OA chondrocytes (Figure 4D). HA level in cell culture supernatant was increased after IL-1β stimulation (Figure 4E).
CircHYBID increases HA accumulation by regulating the expression of HA-metabolizing enzymes
To investigate the relationship between circHYBID and HA metabolism, a circHYBID overexpression vector was constructed and transfected into chondrocytes. CircHYBID upregulation of circHYBID overexpression chondrocytes was verified by qRT-PCR (Figure 5A). CircHYBID-overexpressing chondrocytes (OV-circHYBID+ IL-1β group) were further incubated with IL-1β. TNF-α expression was upregulated in standard chondrocytes with IL-1β stimulation. However, there is no significant TNF-α upregulation under IL-1β stimulation in circHYBID-overexpressing chondrocytes (Figure 5B). Overexpression of circHYBID shows chondroprotective effect.
As mentioned previously, HAS2 expression was downregulated and HYBID expression was upregulated in IL-1β group. Comparing to IL-1β group, HAS2 downregulation was restored, and the upregulation of HYBID was offset in OV-circHYBID+ IL-1β group (Figure 5C). The protein expression of HAS2 and HYBID was consisted with the mRNA results (Figure 5D). HA content in the cell culture supernatant decreased after IL-1β stimulation, and HA production was recovered by circHYBID overexpression (Figure 5E). Base on the above results, circHYBID increases HA accumulation of the chondrocytes.
CircHYBID upregulates the expression of the hsa-miR-29b-3p target gene TGF-β1
Given that many circRNAs commonly function as miRNA sponges that competitively sequester and suppress miRNAs, we assumed that circHYBID may also bind to miRNAs as a sponge and regulate targets via the ceRNA mechanism. Thus, the circRNA–miRNA–mRNA network of circHYBID was constructed to explore the molecular mechanisms of circHYBID (Figure 6). Among the potential downstream targets of circHYBID, hsa-miR-29b-3p has caught our attention because it has been demonstrated to participate in the progression of OA (19). The expression of hsa-miR-29b-3p was detected using qRT-PCR, and the result show that hsa-miR-29b-3p expression was upregulated in chondrocytes treated with IL-1β. However, the expression of hsa-miR-29b-3p was inhibited by circHYBID overexpression (Figure 7A). Hsa-miR-29b-3p may be a potential binding target miRNA of circHYBID. For further confirmation, the sequence of circHYBID was separately cloned into dual-luciferase reporter vectors (circHYBID-WT), and then the potential binding sites of hsa-miR-29b-3p were mutated (circHYBID-MUT). The aforementioned plasmids were co-transfected into cells, and changes in luciferase activity were analyzed. Luciferase activity was significantly downregulated in cells co-transfected with circHYBID-WT and hsa-miR-29b-3p mimics but obviously restored in cells co-transfected with circHYBID-MUT and hsa-miR-29b-3p mimics, indicating that the sequence is the potential binding site of circHYBID and hsa-miR-29b-3p (Figure 7B).
From the circRNA–miRNA–mRNA network, we found that TGF-β1 was a potential downstream target of hsa-miR-29b-3p (Figure 6). TGF-β1 has been reported to be closely related to HA metabolism and OA pathogenesis (20). Furthermore, a previous study has described the interaction between hsa-miR-29b-3p and TGF-β1 (21). Thus, hsa-miR-29b-3p was selected as the target mRNA for further research. TGF-β1 mRNA and protein expression was detected using qRT-PCR (Figure 7C) and WB (Figure 7D), respectively. The result illustrated that IL-1β inhibits the expression of TGF-β1 in chondrocytes, whereas circHYBID overexpression restored TGF-β1 expression. TGF-β1 expression was further verified in 48 pairs of cartilage samples using IHC. The median staining index of intact cartilage was 7.3, versus 4.1 in damaged cartilage. TGF-β1 expression was downregulated in damaged cartilage samples (Figure 7E). Meanwhile, pearson’s analyses indicated a positive correlation between relative circHYBID and TGF-β1 expression in OA cartilage tissues (Figure 7F). Dual luciferase reporter assays as previous described was used to confirm the binding sites of hsa-miR-29b-3p and TGF-β1. Similarly, the luciferase activity of cells cotransfected with TGF-β1-WT and hsa-miR-29b-3p mimics was significantly downregulated but obviously restored in cells co-transfected with TGF-β1-MUT and hsa-miR-29b-3p mimics, indicating that the sequence is the potential binding site of TGF-β1 (Figure 7G). Altogether, these results reveal that circHYBID upregulates the expression of TGF-β1, which is the potential target gene of hsa-miR-29b-3p. We initially constructed the circHYBID- hsa-miR-29b-3p- TGF-β1 axis, which play regulatory function in HA metabolism of chondrocytes.