A single-atom nanozyme mimicking cytochrome c oxidase restores mitochondrial energy production in stem cells, shifting their metabolism to accelerate bone regeneration in critical-sized defects.
(Nanowerk Spotlight) Cells that build and repair tissue need fuel. That fuel is adenosine triphosphate, or ATP, and the machinery that produces it sits inside mitochondria. Bone is among the most energy-intensive tissues to regenerate, because forming new mineralized matrix demands sustained ATP output over weeks. Stem cells at a fracture site must dramatically increase their mitochondrial activity to meet this demand.
Bone injuries, however, create a local environment that undermines the very energy supply needed for repair. Reactive oxygen species accumulate at the defect site, damaging mitochondrial membranes and proteins. This triggers a vicious cycle: the injury impairs mitochondrial function, which starves cells of energy, which slows regeneration, which prolongs the hostile conditions.
Most therapeutic materials designed for bone defects try to break this cycle by scavenging reactive oxygen species. Bioinspired nanomaterials designed to protect stem cells during bone repair, for example, can neutralize oxidative damage effectively. But clearing harmful byproducts does not restore the underlying energy deficit. The mitochondria remain dysfunctional, and stem cells remain underpowered.
Schematic diagram illustrating the ability of TPP-DMSN-Fe/Cu nanozymes to improve mitochondrial function, regulate cellular energy metabolism, and promote osteogenic differentiation of stem cells. (Image: Reproduced from DOI:10.1002/adma.202522108, CC BY) (click on image to enlarge)
The enzyme in question is cytochrome c oxidase, also called Complex IV. It occupies the final position in the mitochondrial electron transport chain, where it accepts electrons from a carrier protein and uses them to reduce oxygen to water. This step generates the electrochemical gradient that drives ATP synthesis. When Complex IV fails, the entire energy production line stalls.
The team constructed the nanozyme from dendritic mesoporous silica nanoparticles, which feature branching pores and a large internal surface area. They anchored isolated iron and copper atoms throughout the silica scaffold to mimic the bimetallic catalytic center of natural cytochrome c oxidase.
A surface coating of triphenylphosphonium, a positively charged molecule attracted to the strongly negative mitochondrial membrane potential, guided the particles to their intracellular target. The nanozyme oxidized ferrous cytochrome c in solution, replicating the key catalytic step of its natural counterpart. Silica nanoparticles without iron and copper failed to do so.
Inside stem cells, the TPP-coated nanozymes accumulated at mitochondria and made direct contact with mitochondrial surfaces, as electron microscopy confirmed. Uncoated versions showed only moderate overlap with mitochondria. These results established both the catalytic function and the targeting precision of the nanozyme.
The central question was whether mimicking Complex IV could reprogram stem cell metabolism. Treated cells shifted away from glycolysis and toward oxidative phosphorylation. Maximum mitochondrial respiration nearly doubled, and ATP levels rose by about 64% compared to untreated controls. Fatty acid oxidation also increased, mirroring the metabolic transition that stem cells naturally undergo as they mature into bone-forming osteoblasts.
The molecular basis of this shift emerged from gene expression profiling. The nanozyme activated genes governing mitochondrial biogenesis, fatty acid transport, and TCA cycle substrate supply. It also upregulated proteins associated with autophagy, indicating enhanced clearance of damaged mitochondria. The CAMKK-AMPK signaling cascade, which connects calcium sensing to mitochondrial renewal, showed strong activation in treated cells.
These molecular changes translated into measurable gains in bone-forming activity. The treated cells, like other nanoparticles that drive osteogenic stem cell differentiation, expressed higher levels of key osteogenic markers, including RUNX2, osteocalcin, and collagen type I. Alkaline phosphatase activity and mineral deposition both increased, confirming enhanced early and late stages of bone formation. The study used C3H/10T1/2 cells, a murine mesenchymal stem cell line, so validation in primary human cells remains a next step.
The decisive test came in a rat model of critical-sized femoral bone defects. The researchers loaded the nanozymes into gelatin-based hydrogel scaffolds and implanted them into 3 mm defects. After four weeks, the nanozyme group showed a 177% increase in bone volume and a 12% increase in mineral density compared to empty defects. Scaffolds with unmodified nanoparticles produced smaller gains.
By eight weeks, the advantages widened, with the nanozyme group reaching 4.1 times the bone volume of empty controls. The regenerated bone appeared denser and more mature, and markers of mitochondrial biogenesis and ATP production remained elevated in the healing tissue. Comprehensive blood analysis and organ examination revealed no signs of toxicity.
Beyond restoring energy metabolism, the nanozyme also neutralized hydrogen peroxide, hydroxyl radicals, and DPPH radicals more effectively than bare silica particles. This dual capacity to power mitochondria and scavenge oxidative species sets the platform apart from materials that address only one side of the problem.
The work demonstrates that targeting a specific bottleneck in the electron transport chain and delivering the catalytic fix to mitochondria can create conditions that favor the metabolic reprogramming stem cells need to build bone. The same principle could apply to osteoporotic bone, cartilage, or other tissues where mitochondrial dysfunction limits repair, provided future studies confirm long-term safety and efficacy in larger animal models.
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