Uracil-DNA glycosylase 1 mitigates acute kidney injury inflammation by maintaining mtDNA homeostasis

The human body is a complex machine where every cell relies on tiny power plants called mitochondria to generate the energy needed for life. These power plants contain their own small set of genetic instructions, known as mitochondrial DNA, which is distinct from the main DNA found in the cell's nucleus. Because this mitochondrial DNA sits so close to the machinery that produces energy, it is constantly under attack from the very chemical reactions it helps create. When these power plants are damaged, the genetic material inside can break apart and leak out into the rest of the cell. The cell interprets this leaked genetic material as a sign of a viral invasion, triggering a powerful alarm system that causes inflammation. In the kidneys, which filter blood and are packed with these energy-hungry cells, such inflammation can lead to sudden and severe organ failure, a condition known as acute kidney injury. While doctors can support patients through this crisis, there is currently no cure that stops the underlying damage from happening in the first place.

Researchers at the Second Affiliated Hospital of Chongqing Medical University have identified a specific protein that acts as a guardian for this fragile genetic material, potentially offering a new way to prevent this cascade of damage. The team focused on a protein called UNG1, which resides inside the mitochondria and functions as a repair crew for the DNA. In healthy cells, UNG1 constantly scans the mitochondrial genome, finding and fixing small errors before they can cause the DNA to break. However, the researchers discovered that during acute kidney injury, the levels of this protective protein drop significantly, leaving the DNA vulnerable. By studying both human kidney cells in the lab and mice with induced kidney injury, the team found that when UNG1 is missing, the mitochondrial DNA breaks, leaks out, and sets off the inflammatory alarm that damages the kidney. Conversely, when they artificially increased the amount of UNG1 in the cells, the DNA stayed intact, the alarm was never triggered, and the kidney tissue remained healthy.

To understand exactly how this protection works, the scientists looked deeper into the molecular interactions inside the cell. They found that UNG1 does not work alone; it partners with another protein called SSBP1, which helps organize the mitochondrial DNA into tight, stable bundles. The researchers demonstrated that UNG1 helps SSBP1 clump together into larger, more effective groups, which in turn allows SSBP1 to hold the DNA more securely. When UNG1 is present, these bundles stay tight, preventing the DNA from breaking or escaping. When UNG1 is absent, the bundles fall apart, the DNA leaks, and the immune system attacks the kidney. This mechanism was confirmed through a series of experiments where the researchers blocked the inflammatory pathway in mice. Even when the inflammatory alarm system was turned off, the presence of extra UNG1 still provided additional protection to the kidney, suggesting that UNG1 works by stabilizing the DNA to suppress the inflammatory pathway that causes the damage.

The study tested these findings in two different ways to ensure the results were robust. First, they used human kidney cells in a dish, subjecting them to a cycle of low oxygen and re-oxygenation to mimic the stress of a kidney injury. In these cells, increasing UNG1 levels reduced the amount of broken DNA and lowered the production of inflammatory chemicals. Second, they applied this to living mice. They created mice that had their kidneys temporarily cut off from blood flow and then restored, a standard model for studying kidney injury. In these mice, those that received an extra dose of UNG1 showed significantly better kidney function, with lower levels of waste products in their blood and less visible damage to the kidney tissue under a microscope. The same protective effect was observed in mice treated with a drug known to cause kidney damage, proving that this mechanism works across different types of injury.

Crucially, the researchers also investigated whether this protection relied entirely on the inflammatory alarm system. They repeated the experiments using mice that were genetically engineered to lack the ability to trigger this alarm. Even in these mice, where the inflammatory response could not occur, the extra UNG1 still improved kidney function and reduced tissue damage. This finding suggests that UNG1 works by stabilizing the mitochondrial DNA, which in turn suppresses the inflammatory pathway; the fact that it still offers benefits in the absence of the alarm system indicates that it provides an additive protective effect by maintaining cellular integrity upstream of the inflammation. The study also identified a specific spot on the SSBP1 protein where UNG1 attaches, and when the researchers altered this spot, the protective effect disappeared, confirming the precise nature of their partnership.

The implications of these findings point toward a new strategy for treating acute kidney injury. Currently, medical care for this condition is largely supportive, focusing on keeping the patient alive while the kidneys attempt to heal on their own. This research suggests that boosting the levels of UNG1 could be a way to actively prevent the initial damage that leads to organ failure. By ensuring that the mitochondrial DNA remains stable and does not leak out, the cell avoids the inflammatory response that causes so much harm. While the study was conducted in mice and cells, and further research is needed to see if this approach works in humans, the results provide a clear and concrete path forward. The work highlights that maintaining the stability of the cell's internal power plants is just as important as the power plants themselves, and that a single protein, UNG1, plays a pivotal role in keeping the system running smoothly.