Objective:To investigate the effect and mechanism of injectable photopolymerizable porous gelatin methacrylate anhydride (Porous GelMA)/methacrylated silk fibroin (SilMA) composite hydrogel (PSE) loaded with human umbilical cord mesenchymal stem cell-derived exosomes (hUCMSC-Exos) in promoting knee joint cartilage regeneration.Methods:The porous GelMA solution (60 g/L) was mixed with SilMA solution (200 g/L) at a volume ratio of 6∶1 . The mixture was ultraviolet-irradiated for 30 seconds to form a cured Porous GelMA/SilMA hydrogel (P/S6). The hUCMSC-Exos was isolated via differential centrifugation coupled with ultrafiltration and then was incorporated into the Porous GelMA/SilMA composite solution at 200 μg/ml, followed by ultraviolet irradiation for 30 seconds to generate Exos-loaded PSE. Primary rat chondrocytes (P1) were divided into control group, P/S6 group, and PSE group to characterize the porosity, compressive strength, and sustained exosome release kinetics of PSE hydrogel. Chondrocytes were allocated to control group, interleukin-1β (IL-1β) group, P/S6 group, and PSE group, among which the last three groups were preconditioned with 10 ng/ml IL-1β for 24 hours, and then cultured in complete medium, P/S6 extract and PSE extract for 3 days, respectively, to establish in vitro cartilage defect models, while the control group remained untreated. Western blot and qRT-PCR analysis were conducted to quantify the expression levels of antibody to aggrecan core protein (ACAN), sex-determining region Y-box transcription factor 9 (SOX9), matrix metalloproteinase-13 (MMP13) and collagen type II (COL II). Murine monocyte-macrophage leukemia cells (RAW264.7) were divided into control group, P/S6 group, and PSE group, which were then cultured in complete medium, PSE extract, and PSE extract medium for 3 days, respectively. qRT-PCR was employed to detect the expression levels of recombinant arginase-1 protein (ARG1), mannose receptor (CD206), and inducible nitric oxide synthase (iNOS). Transcriptomic sequencing was used to identify differentially expressed genes during PSE-mediated chondrocyte regeneration, followed by functional enrichment analysis of key signaling pathways. Twenty-four SD rats were selected to establish cartilage defect models and assigned to injury control group, P/S6 group, and PSE group according to the random number table (8 rats per group). The right knee joints of the rats were surgically exposed, and cylindrical osteochondral defects (a diameter of 2.0 mm× a depth of 1.0 mm) were surgically created in the center of the femoral trochlear groove using a drill bit. The injury control group received phosphate-buffered saline, while the P/S6 group and PSE group were injected with corresponding hydrogels followed by photo-crosslinking. Incisions then were closed in layers. At 6 and 10 weeks after injury, specimens were harvested for HE staining and safranin O-fast green staining to evaluate cartilage regeneration and immunohistochemistry staining to quantify the positive area fractions for COL II, MMP13, ARG1, and CD206 in the defect areas. Results:PSE hydrogel exhibited compressive strength matching native cartilage (0.41 MPa), high porosity (85%), and sustained exosome release capacity (cumulative release rate of approximately 85% over 14 days). In chondrocyte repair experiments, compared to the IL-1β group, the PSE group demonstrated significantly upregulated expression of anabolic markers of cartilage (COL II expression increased by 2.1-fold, ACAN by 1.8-fold, and SOX9 by 1.5-fold) ( P<0.01) as well as significantly suppressed expression of catabolic markers (MMP13 expression decreased by 52%) ( P<0.01). In macrophage polarization assays, the PSE group exhibited ARG1 expression increased by 68% when compared to the control group ( P<0.01), thus promoting M2 polarization of macrophages. Transcriptomic analysis revealed that PSE enhanced extracellular matrix (ECM) synthesis by activating the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) pathway and ECM-receptor interaction pathway, as well as by suppressing inflammation-related gene expression. Histological evaluation in animal experiments revealed regeneration of hyaline cartilage with smooth, continuous surfaces in the defect areas in the PSE group. At 10 weeks after surgery, the neocartilage-positive area in the PSE group was (9.94±0.26)%, significantly larger than (1.67±0.11)% in the injury control group ( P<0.01). Besides, the CD206? M2 macrophage-positive area reached (14.44±0.23)% in the PSE group, significantly larger than (3.41±0.36)% in the injury control group ( P<0.01). Conclusions:The PSE hydrogel successfully engineered in the study can significantly promote regenerative repair of knee cartilage defects through a dual mechanism of enhanced ECM anabolism and remodeled inflammatory microenvironment. The core mechanisms involve specific activation of the PI3K/Akt pathway (boosting chondrocyte proliferation and survival) and ECM-receptor interaction pathway (driving ECM synthesis and assembly) by exosome-loaded PSE, while effectively polarizing macrophages toward an anti-inflammatory M2 phenotype so as to coordinately regulate cartilage ECM metabolism and suppress inflammatory responses.

Objective:To investigate the effect and mechanism of injectable photopolymerizable porous gelatin methacrylate anhydride (Porous GelMA)/methacrylated silk fibroin (SilMA) composite hydrogel (PSE) loaded with human umbilical cord mesenchymal stem cell-derived exosomes (hUCMSC-Exos) in promoting knee joint cartilage regeneration.Methods:The porous GelMA solution (60 g/L) was mixed with SilMA solution (200 g/L) at a volume ratio of 6∶1 . The mixture was ultraviolet-irradiated for 30 seconds to form a cured Porous GelMA/SilMA hydrogel (P/S6). The hUCMSC-Exos was isolated via differential centrifugation coupled with ultrafiltration and then was incorporated into the Porous GelMA/SilMA composite solution at 200 μg/ml, followed by ultraviolet irradiation for 30 seconds to generate Exos-loaded PSE. Primary rat chondrocytes (P1) were divided into control group, P/S6 group, and PSE group to characterize the porosity, compressive strength, and sustained exosome release kinetics of PSE hydrogel. Chondrocytes were allocated to control group, interleukin-1β (IL-1β) group, P/S6 group, and PSE group, among which the last three groups were preconditioned with 10 ng/ml IL-1β for 24 hours, and then cultured in complete medium, P/S6 extract and PSE extract for 3 days, respectively, to establish in vitro cartilage defect models, while the control group remained untreated. Western blot and qRT-PCR analysis were conducted to quantify the expression levels of antibody to aggrecan core protein (ACAN), sex-determining region Y-box transcription factor 9 (SOX9), matrix metalloproteinase-13 (MMP13) and collagen type II (COL II). Murine monocyte-macrophage leukemia cells (RAW264.7) were divided into control group, P/S6 group, and PSE group, which were then cultured in complete medium, PSE extract, and PSE extract medium for 3 days, respectively. qRT-PCR was employed to detect the expression levels of recombinant arginase-1 protein (ARG1), mannose receptor (CD206), and inducible nitric oxide synthase (iNOS). Transcriptomic sequencing was used to identify differentially expressed genes during PSE-mediated chondrocyte regeneration, followed by functional enrichment analysis of key signaling pathways. Twenty-four SD rats were selected to establish cartilage defect models and assigned to injury control group, P/S6 group, and PSE group according to the random number table (8 rats per group). The right knee joints of the rats were surgically exposed, and cylindrical osteochondral defects (a diameter of 2.0 mm× a depth of 1.0 mm) were surgically created in the center of the femoral trochlear groove using a drill bit. The injury control group received phosphate-buffered saline, while the P/S6 group and PSE group were injected with corresponding hydrogels followed by photo-crosslinking. Incisions then were closed in layers. At 6 and 10 weeks after injury, specimens were harvested for HE staining and safranin O-fast green staining to evaluate cartilage regeneration and immunohistochemistry staining to quantify the positive area fractions for COL II, MMP13, ARG1, and CD206 in the defect areas. Results:PSE hydrogel exhibited compressive strength matching native cartilage (0.41 MPa), high porosity (85%), and sustained exosome release capacity (cumulative release rate of approximately 85% over 14 days). In chondrocyte repair experiments, compared to the IL-1β group, the PSE group demonstrated significantly upregulated expression of anabolic markers of cartilage (COL II expression increased by 2.1-fold, ACAN by 1.8-fold, and SOX9 by 1.5-fold) ( P<0.01) as well as significantly suppressed expression of catabolic markers (MMP13 expression decreased by 52%) ( P<0.01). In macrophage polarization assays, the PSE group exhibited ARG1 expression increased by 68% when compared to the control group ( P<0.01), thus promoting M2 polarization of macrophages. Transcriptomic analysis revealed that PSE enhanced extracellular matrix (ECM) synthesis by activating the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) pathway and ECM-receptor interaction pathway, as well as by suppressing inflammation-related gene expression. Histological evaluation in animal experiments revealed regeneration of hyaline cartilage with smooth, continuous surfaces in the defect areas in the PSE group. At 10 weeks after surgery, the neocartilage-positive area in the PSE group was (9.94±0.26)%, significantly larger than (1.67±0.11)% in the injury control group ( P<0.01). Besides, the CD206? M2 macrophage-positive area reached (14.44±0.23)% in the PSE group, significantly larger than (3.41±0.36)% in the injury control group ( P<0.01). Conclusions:The PSE hydrogel successfully engineered in the study can significantly promote regenerative repair of knee cartilage defects through a dual mechanism of enhanced ECM anabolism and remodeled inflammatory microenvironment. The core mechanisms involve specific activation of the PI3K/Akt pathway (boosting chondrocyte proliferation and survival) and ECM-receptor interaction pathway (driving ECM synthesis and assembly) by exosome-loaded PSE, while effectively polarizing macrophages toward an anti-inflammatory M2 phenotype so as to coordinately regulate cartilage ECM metabolism and suppress inflammatory responses.