Fig 2: ER stress-induced osteoarthritic changes in the knee joint. Knee joints of aged ERp57 cKO animals display severe OA cartilage degeneration with enhanced MMP3, MMP9, ADAMTS4, and ADAMTS5 expression and osteophyte formation. These changes are preceded by reduced compressive stiffness of the cartilage ECM and increased apoptotic cell death. Both effects are likely induced by ER stress that cannot be adequately compensated by the UPR.

Fig 3: ERp57 cKO mice show an accelerated development of osteoarthritis during aging. (A) Histological analysis and OARSI scoring of toluidine blue-stained knee sections of 18-month-old mice reveals an increase in cartilage degradation, the occurrence of hypertrophic chondrocytes (*), and osteophyte formation (scattered box) in ERp57 cKO mice, but not in WT animals (p = 0.0039). No significant difference was detected between younger WT and ERp57 cKO mice at the age of 9 months (p = 0.7924) and 12 months (p = 0.2503). Scale Bars = 100 µm. (B) TUNEL staining indicates a higher percentage of apoptotic cells in the articular cartilage of ERp57 cKO mice at 9 (p = 0.0271), 12 (p = 0.0490) and 18 months (p = 0,000000028) compared to WT animals. Scale bars = 50 µm (C) Immunofluorescence of 18-month-old ERp57 cKO cartilage reveals increased staining intensities of matrix degrading proteases MMP3 (p = 0.0004), MMP9 (p = 0.0097), ADAMTS4 (p = 0.0428), and ADAMTS5 (p = 0.0288) compared to WT controls. M = months. n = 4. *** p < 0.005, ** p < 0.01, * p < 0.05.

Fig 4: Treadmill exercise and clodronate liposomes affect cartilage degeneration. (A) Histological images of articular cartilage stained with Safranin-O Fast Green and immunohistochemical staining for Gremlin-1, MMP-13, and ADAMTs4 (Scale bars: 50 ?µm). (B) Scoring results by OARSI score to assess cartilage degeneration and positive cell rates for Gremlin-1, MMP-13, and ADAMTs4 (n ?= ?8, in each group. One-Way ANOVA, post-hoc Tukey). Data are presented as means and 95% confidence interval. Ex and MF-de group showed lower OARSI scores and lower MMP-13 positive cell rates. Gremlin-1 positive cell rate increased in the exercise group.

Fig 5: Treadmill exercise affects cartilage degeneration. (A) Histological images of articular cartilage stained with Safranin-O Fast Green and immunohistochemical staining for Gremlin-1, MMP-13, and ADAMTs4 (Scale bars: 50 ?µm). (B) Scoring results by OARSI score to assess cartilage degeneration and positive cell rates for Gremlin-1, MMP-13, and ADAMTs4 (n ?= ?8, in each group. One-Way ANOVA, post-hoc Tukey). Data are presented as means and 95% confidence interval. In mild-ex group, OARSI score, MMP-13, and ADAMTS4 positive cell rate were suppressed. Positive cell rate for gremlin-1 increased in an exercise intensity-dependent manner.