Mitochondrial Transfer from Bone-Marrow-Derived Mesenchymal Stromal Cells to Chondrocytes Protects Against Cartilage Degenerative Mitochondrial Dysfunction in Rat Chondrocytes
Osteoarthritis (OA) is a prevalent degenerative joint disease characterized by cartilage degradation, chronic pain, and functional impairment. Chondrocytes, the primary functional cells in cartilage, regulate extracellular matrix synthesis and maintenance. However, mitochondrial dysfunction in chondrocytes has emerged as a critical contributor to OA pathology, affecting processes such as apoptosis, oxidative stress, and inflammatory responses. This study investigates the therapeutic potential of mitochondrial transfer from bone-marrow-derived mesenchymal stromal cells (BM-MSCs) to OA chondrocytes, aiming to restore mitochondrial function and mitigate cartilage degeneration.
Mitochondrial Dysfunction in Osteoarthritis
Mitochondrial abnormalities in OA chondrocytes include reduced activity of mitochondrial respiratory chain (MRC) complexes, diminished adenosine triphosphate (ATP) production, and disrupted mitochondrial membrane potential (ΔΨm). These deficits impair energy metabolism, exacerbate oxidative stress, and accelerate chondrocyte apoptosis, further degrading cartilage. Prior studies highlight the role of mitochondrial dysfunction in OA progression, but strategies to restore mitochondrial integrity remain underexplored.
Experimental Design and Methodology
Cell Isolation and Culture
BM-MSCs were isolated from femurs and tibias of 2-week-old rats and cultured in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum (FBS). OA chondrocytes were harvested from an OA rat model induced by surgical destabilization of the knee joint (anterior cruciate ligament transection and medial meniscus resection). Cartilage from femoral and tibial plateaus was digested with collagenase to isolate chondrocytes, which were then cultured under serum-starved conditions to synchronize cell cycles.
Mitochondrial Labeling and Co-Culture
BM-MSCs were transfected with lentiviral mitochondrial-targeted green fluorescent protein (LV-Mito-GFP) to visualize mitochondria. OA chondrocytes were labeled with CellTrace Violet, a cytoplasmic dye. Labeled BM-MSCs and chondrocytes were co-cultured at a 1:1 ratio for 24 hours. Chondrocytes that acquired GFP-labeled mitochondria were sorted via fluorescence-activated cell sorting (FACS) for subsequent analyses.
Assessment of Mitochondrial Transfer and Function
- Transmission Electron Microscopy (TEM): Mitochondrial morphology in chondrocytes was evaluated, focusing on cristae structure, membrane integrity, and organelle quantity.
- Mitochondrial Membrane Potential (ΔΨm): JC-1 dye was used to measure ΔΨm. Healthy mitochondria (high ΔΨm) accumulate JC-1 aggregates emitting red fluorescence, while depolarized mitochondria (low ΔΨm) show green monomeric fluorescence.
- MRC Enzyme Activity and ATP Content: Activities of MRC complexes I, II, III, and citrate synthase were quantified spectrophotometrically. ATP levels were measured using a bioluminescence assay.
Cellular Viability and Matrix Secretion
- Proliferation: Cell Counting Kit-8 (CCK-8) assessed proliferation over 7 days.
- Apoptosis: Annexin V/propidium iodide staining and flow cytometry determined apoptotic rates.
- Extracellular Matrix Proteins: Western blotting quantified type II collagen and proteoglycan levels, normalized to glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
Key Findings
1. Successful Mitochondrial Transfer from BM-MSCs to OA Chondrocytes
Confocal microscopy confirmed GFP-labeled mitochondria from BM-MSCs within CellTrace Violet-labeled OA chondrocytes after 24-hour co-culture (Figure 1C). FACS-sorted chondrocytes with dual fluorescence (Mito-GFP+ and CellTrace Violet+) validated mitochondrial uptake. TEM revealed structural improvements in recipient chondrocytes: mitochondria were elongated with intact cristae and continuous outer membranes, contrasting with swollen, fragmented mitochondria in untreated OA chondrocytes (Figures 2A, 2B).
2. Restoration of Mitochondrial Function
- ΔΨm Enhancement: The red/green fluorescence ratio, indicative of ΔΨm, was 1.79 ± 0.19 in chondrocytes receiving BM-MSC mitochondria (MSCs + OA group) versus 0.71 ± 0.12 in OA controls (t = 10.42, P < 0.0001) (Figures 2C–2E).
- MRC Complex Activity: MSCs + OA chondrocytes exhibited significantly higher MRC enzyme activities:
- Complex I: 69.11 ± 16.69 vs. 30.61 ± 6.24 nmol·min⁻¹·mg⁻¹ (t = 3.174, P = 0.013)
- Complex II: 36.67 ± 5.65 vs. 12.52 ± 2.96 nmol·min⁻¹·mg⁻¹ (t = 6.323, P = 0.0002)
- Complex III: 118.74 ± 9.50 vs. 47.61 ± 8.94 nmol·min⁻¹·mg⁻¹ (t = 6.523, P = 0.0002)
- Citrate Synthase: 196.40 ± 26.18 vs. 115.00 ± 15.37 nmol·min⁻¹·mg⁻¹ (t = 5.362, P = 0.0007)
- ATP Production: ATP content in the MSCs + OA group (161.90 ± 13.49 nmol/mg) doubled that of OA controls (87.62 ± 11.07 nmol/mg; t = 8.515, P < 0.0001).
3. Improved Chondrocyte Viability and Matrix Synthesis
- Proliferation: CCK-8 assays showed a 98% increase in proliferation by day 7 in MSCs + OA chondrocytes compared to OA controls (P < 0.05) (Figure 3A).
- Apoptosis Reduction: Total apoptosis decreased from 15.89% ± 1.30% in OA chondrocytes to 7.09% ± 0.68% in the MSCs + OA group (t = 13.39, P < 0.0001) (Figures 3B–3D).
- Matrix Secretion: Type II collagen and proteoglycan levels in the MSCs + OA group were twice those of OA controls:
- Collagen II: 2.01 ± 0.14 vs. 1.06 ± 0.11 (t = 9.141, P = 0.0008)
- Proteoglycan: 2.08 ± 0.20 vs. 0.97 ± 0.12 (t = 8.227, P = 0.0012) (Figure 4).
Mechanistic Implications and Therapeutic Potential
Mitochondrial transfer from BM-MSCs rescued OA chondrocytes by replenishing functional mitochondria, thereby restoring energy production, reducing apoptosis, and enhancing matrix synthesis. This process likely involves intercellular tunneling nanotubes or vesicular transport, though the exact mechanisms require further study. The findings align with prior reports of MSC-mediated mitochondrial rescue in lung, cardiac, and neural injury models, underscoring its broad therapeutic applicability.
Limitations and Future Directions
While paracrine signaling and mitochondrial transfer are both implicated in MSC therapy, their relative contributions in OA remain unclear. Additionally, the study did not address in vivo efficacy or long-term safety. Future research should explore optimal delivery methods, such as intra-articular injections of mitochondria-loaded MSC exosomes, and validate outcomes in preclinical OA models.
Conclusion
This study demonstrates that mitochondrial transfer from BM-MSCs to OA chondrocytes reverses mitochondrial dysfunction, enhances cell survival, and restores cartilage matrix production. By addressing the root cause of OA pathology—mitochondrial failure—this approach offers a novel, cell-free strategy for OA treatment. Further development could revolutionize regenerative therapies for degenerative joint diseases.
doi.org/10.1097/CM9.0000000000001057
Was this helpful?
0 / 0