Downregulation of circRNA_29625 in the Hippocampus Drives Perioperative Neurocognitive Disorder by Reducing SIRT1 and Exacerbating Anesthesia/Surgery-Induced Neuroinflammation

Imagine your brain as a bustling, high-tech city. Inside this city, there are tiny construction crews called "microglia" that act as the sanitation workers and security guards. Usually, they keep things clean and safe. But sometimes, when the body goes through a major event like surgery or heavy anesthesia, these guards get overworked and start panicking. They turn into a chaotic mob, shouting inflammatory signals that damage the delicate wiring of the city's memory centers. This chaos leads to a condition called Perioperative Neurocognitive Disorder (PND), where older patients wake up from surgery feeling foggy, confused, or unable to remember things they used to know easily.

To keep the city running smoothly, the brain has a special "brake pedal" protein called SIRT1. Think of SIRT1 as the chief traffic controller who tells the panicked security guards to calm down and stops the oxidative stress (which is like rust forming on the wires) from eating away at the brain's connections. Scientists have long known that in PND, this brake pedal gets stuck in the "off" position, letting the chaos take over. But the big mystery was: what was jamming the brake? Is it a broken part, a missing instruction manual, or something else entirely? This is the question a team of researchers set out to solve, looking for the hidden switch that turns off the brain's safety system during surgery.

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The Hidden Switch: A Story of a Lost Messenger

In this study, the researchers, led by Wei Dong and Qian Liu, decided to investigate what happens inside the hippocampus—the brain's memory library—of elderly mice after they undergo a simulated surgery (a broken leg fixed under anesthesia). They wanted to find the specific molecule that gets turned off, causing the SIRT1 brake to fail.

The Discovery of the "Missing Link"
The team started by scanning the brain cells of these mice for circular RNAs (circRNAs). If you imagine regular RNA as a straight string of instructions, circRNAs are like those strings tied into a perfect, unbreakable loop. They are stable little messengers that can hang out and talk directly to proteins.

Using a high-tech scanner (microarray analysis), they found that out of hundreds of these circular loops, one specific one, named circRNA_29625, went missing in the brains of the mice that had surgery. It was significantly downregulated, meaning its levels dropped drastically compared to healthy mice. The researchers suspected this tiny loop was the key to the whole problem.

The Detective Work: Connecting the Dots
To prove their hunch, the team played detective in two ways:

1. The "What If" Test: They used a virus to inject extra copies of circRNA_29625 directly into the memory center (the CA1 region) of the mice. When they did this, the levels of the SIRT1 brake protein went back up! The mice's memory improved, and the brain's "rust" (oxidative stress) decreased.
2. The "Cut the Wire" Test: They did the opposite, using a virus to silence circRNA_29625. When they cut this link, SIRT1 levels crashed, even without surgery. The brain's defenses collapsed, and the mice showed signs of cognitive decline.

The Direct Handshake
But how does a tiny loop of RNA talk to a protein? The researchers used a technique called RNA Immunoprecipitation (RIP), which is like fishing for specific pairs of molecules. They found that circRNA_29625 physically grabs onto SIRT1. It's not just a distant signal; it's a direct handshake. The paper suggests that when the brain is under the stress of surgery, the inflammation (the panicked security guards) causes the levels of circRNA_29625 to drop. Without this circRNA interacting with it, SIRT1 expression is reduced, leaving the brain vulnerable to damage.

The Vicious Cycle
The study reveals a scary but clear chain reaction:

1. Surgery/Anesthesia causes inflammation.
2. Inflammation reduces the levels of circRNA_29625.
3. Without circRNA_29625, SIRT1 levels drop.
4. Without SIRT1, the brain can't stop the inflammation or the oxidative stress.
5. The brain's wiring (synapses) gets damaged, and the mouse (or patient) loses memory.

The "Rescue" Mission
The most exciting part of the story is the rescue. The researchers showed that if you give the mice a drug called minocycline (which calms the panicked security guards), the levels of circRNA_29625 bounce back, and SIRT1 is saved. Furthermore, if they artificially boost circRNA_29625 or SIRT1 directly, they can alleviate the memory problems caused by the surgery.

However, there is a catch. When they tried to boost circRNA_29625 but simultaneously knocked out SIRT1, the rescue didn't work. This proved that circRNA_29625 works through SIRT1. It's the upstream manager, but SIRT1 is the actual worker doing the heavy lifting. You can't fix the problem just by sending more managers if the workers are missing.

What This Means (and What It Doesn't)
The paper concludes that the circRNA_29625–SIRT1 axis is a major reason why surgery can mess up memory in older people. It suggests that this specific loop of RNA is a new target for treatment. If doctors could find a way to keep circRNA_29625 levels high, or boost SIRT1 directly, they might be able to prevent this brain fog.

The authors are careful to note that while this works beautifully in mice, we don't know yet if it works exactly the same way in humans. They also admit that the brain is complex, and there might be other "loops" or signals involved that they haven't found yet. But for now, they have identified a very specific, broken link in the chain that leads to post-surgery confusion.

In simple terms: The surgery stress reduces a tiny, circular messenger (circRNA_29625). This messenger usually helps regulate a vital safety protein (SIRT1). When the messenger drops, the safety protein fails, the brain gets damaged, and memory fades. Fixing the messenger or the protein could be the key to keeping our memories safe after surgery.