Investigating the role of nuclear receptor proliferator-activated receptor delta (PPARδ) in aging and metabolic models of osteoarthritis

Abstract

nuclear genome but different mtDNA variants (named conplastic mice) during aging and forced exercise. Methods: Conplastic mice (BL/6 NZB) strain was developed with the C57BL/6JOlaHsd nuclear genome and the NZB/OlaHsd mtDNA to compare with the original C57BL/6JOlaHsd strain (BL/6 C57 ). Knee joints from BL/6 NZB mice as well as from BL/6 C57 mice were processed and cut into coronal sections. The mice were sacrificed at 25, 75 and 90 weeks of age and knee joints were collected for histological analysis. All sections were stained with Hematoxylin-Eosine and Safranin O-fast green and graded using a Mankin scoring system. All quadrants of the joint (medial tibial plateau, medial femoral condyle, lateral tibial plateau, and lateral femoral condyle) were scored separately and averaged. Three criteria were selected for histological assessment of each quadrant: structure, cellularity and matrix staining. Another group of mice from both BL/6 NZB and BL/6 C57 strains were subjected to exercise by running in a treadmill 400m/day three times a week. After 75 and 90 weeks of age, mice were sacrificed and knee joints were processed for histological analysis. Cartilage expression of markers of autophagy like LC3 and metalloproteinases like MMP-13 were also analyzed by immunohistochemistry in both strains. The results are given as mean ± SEM and statistical analysis was performed using non parametric unpaired t-test (Graph Pad Prism v 6.0). Results: In response to aging, conplastic mice BL/6 NZB presented reduced cartilage Mankin score at 25 (p¼0.0079), 75 (p¼0.0087) and 90 (p¼0.064) weeks when compared with mice of the original strain BL/ 6 C57 at the same age. Specifically, we showed a reduced score in both femoral condyle (FC) and tibial plateau (TP) of BL/6 NZB mice that reached the statistical significance at 25 (FC: p¼0.0317; TP: p¼0.0079), 75 (FC: p¼0.0411; TP: p¼0.0238) and borderline the statistical significance at 90 (FC: p¼0.0649; TP: p¼0.0628) weeks of age. These results were accompanied with more expression of LC3 in cartilage from BL/6 NZB mice at 75 weeks when compared with cartilage from BL/ 6 C57 at the same age (p¼0.0152). We also reported a significant decrease of LC3 expression in cartilage from mice at 75 weeks when compared with mice at 25 weeks in both strains confirming the decrease of autophagy with aging. Difference in MMP13 cartilage expression between the two mice strains were also found. In the mice subjected to exercise, BL/6 C57 presented an increased cartilage score in the medial compartment (p¼0.0286) and lateral compartment (p¼0.057) of the joint at 90 weeks when compared with BL/6 NZB mice at the same age.Conclusions: This study demonstrated that aging and forced exercise in conplastic mice BL/6 NZB are associated with a reduced joint deterioration compared with the original strain BL/6 C57 . Moreover, we showed that mtDNA variants can improve the aging process at joint level through the modulation of autophagy. These results support the hypothesis that mtDNA background has a role in the process...