Unresolved excess accumulation of myelin-derived cholesterol contributes to scar formation after spinal cord injury
Abstract: Background: Spinal cord injury triggers complex pathological cascades, resulting in destructive tissue damage and incomplete tissue repair. Scar formation is generally considered as a barrier for regeneration in central nervous system (CNS), while the intrinsic mechanism of scar-forming after spinal cord injury has not been completed deciphered. Methods: We assessed cholesterol hemostasis in spinal cord lesions and injured peripheral nerves using confocal reflection microscopy and real-time PCR analyses. The involvement of the proteins, which were predicted to promote cholesterol efflux in spinal cord lesions, were assessed with Liver X receptor (LXR) agonist and Apolipoprotein E (APOE) deficiency. The role of reverse cholesterol transport (RCT) in cholesterol clearance was examined in APOE KO mice injured sciatic nerves and myelin-overloaded macrophages in vitro. Finally, we determined the consequence of excess cholesterol accumulation in CNS by transplantation of myelin into neonatal spinal cord lesions. Results: We found that excess cholesterol accumulates in phagocytes and is inefficiently removed in spinal cord lesions in young-adult mice. Interestingly, we observed that excessive cholesterol also accumulates in injured peripheral nerves, but is subsequently removed by RCT. Meanwhile, preventing RCT led to macrophage accumulation and fibrosis in injured peripheral nerves. Furthermore, the neonatal mouse spinal cord lesions are devoid of myelin-derived lipids, and able to heal without excess cholesterol accumulation. We found that transplantation of myelin into neonatal lesions disrupts healing with excessive cholesterol accumulation, persistent macrophage activation and fibrosis, indicating myelin-derived cholesterol plays a critical role in impaired wound healing.
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What is this paper about?
This paper looks at why scars form after a spinal cord injury and focuses on one hidden culprit: extra cholesterol from damaged myelin (the “insulation” around nerve fibers). The authors show that when this myelin-derived cholesterol piles up and isn’t cleared away, it sets off long-lasting inflammation and leads to scar tissue that blocks healing.
The big questions the researchers asked
- Does cholesterol from broken myelin build up after spinal cord injury?
- If it does, why doesn’t the body remove it efficiently in the spinal cord?
- Is the situation different in peripheral nerves (like the sciatic nerve in the leg), which often heal better than the spinal cord?
- What happens if the normal cholesterol “trash pickup” system is blocked?
- Is myelin-derived cholesterol itself enough to cause scarring?
- Can giving cells good “cholesterol acceptors” help them get rid of the extra cholesterol?
What did they do? (Methods in simple terms)
The team used mice to compare injuries in two places:
- The spinal cord (part of the central nervous system, CNS), which usually heals poorly and forms scars.
- The sciatic nerve in the leg (part of the peripheral nervous system, PNS), which usually heals better.
They used several tools and tricks:
- Seeing crystals of cholesterol: They used special microscopes (confocal reflection microscopy and electron microscopy) that can “see” tiny shiny crystals—like sugar crystals—inside cells. Crystals are a sign of too much cholesterol.
- Staining fats and proteins: Dyes like Oil Red O highlight fat droplets, and antibodies highlight specific proteins to identify cell types (for example, macrophages) and scar components.
- Checking gene and protein activity: Real-time PCR and western blots measured how cells changed their cholesterol-handling machinery and inflammation signals.
- Turning knobs in the system:
- Gave an “LXR agonist” (a drug that normally turns on cholesterol-export genes) to see if it boosted cholesterol removal.
- Used mice lacking ApoE (a protein that helps move cholesterol in the body) to test how cholesterol clearing works in living tissue.
- Simple cell culture tests: They fed bone marrow–derived macrophages (“garbage collector” immune cells) with myelin and watched whether adding serum or HDL (“good cholesterol” particles) helped those cells spit out the extra cholesterol.
- Neonatal experiment: Newborn mice don’t have much myelin yet. The team injured newborn spinal cords (which normally heal without scarring) and then added extra myelin or cholesterol-loaded macrophages to see if that would create scarring.
Think of it like this: nerves are electrical wires, myelin is their insulation, cholesterol is a key ingredient in that insulation, macrophages are the cleanup crew, HDL is the garbage truck, and ApoE helps load the trucks. The team watched what happens when the cleanup and trucking systems work—or fail.
What did they find, and why is it important?
1) Cholesterol piles up and crystallizes in spinal cord injuries—and sticks around
- In adult mouse spinal cord injuries, macrophages filled up with cholesterol and even formed needle-like cholesterol crystals within a week. These crystals persisted for at least six weeks.
- This buildup damaged cell compartments (lysosomes) and switched on a “fire alarm” system inside cells called the NLRP3 inflammasome, which drives inflammation. In simple terms, too much cholesterol made the cleanup crew sick and angry, keeping inflammation going.
Why it matters: Long-lasting inflammation and sick macrophages are linked to scar formation that blocks nerve regrowth.
2) The spinal cord can’t clear extra cholesterol efficiently—even when “export” genes are turned on
- A drug that normally boosts cholesterol export (an LXR agonist) didn’t reduce crystals or fat droplets in the spinal cord.
- ApoE, a key cholesterol carrier protein in the brain, was increased after injury, but knocking it out didn’t make spinal cord cholesterol buildup worse.
- Conclusion: The problem isn’t that spinal cord macrophages can’t push cholesterol out; it’s that there aren’t effective “acceptors” or transport routes to carry it away. The spinal cord is behind a strict barrier (the blood–brain barrier), and it lacks a good exit pathway.
Why it matters: This explains why cleanup stalls in the spinal cord, fueling chronic inflammation and scarring.
3) Peripheral nerves do better because they can ship cholesterol out
- After sciatic nerve injury, cholesterol and crystals also appeared—but then they disappeared by six weeks.
- This cleanup depended on reverse cholesterol transport (RCT), where HDL in the blood carries cholesterol to the liver for disposal. In mice lacking ApoE (which RCT needs), cholesterol, inflammation, and macrophage buildup lingered, and scar-like molecules (CSPGs) increased.
Why it matters: The PNS has a working “cholesterol garbage truck” system; the CNS mostly doesn’t. That may be a big reason the PNS heals better.
4) Serum and HDL rescue overloaded cells in a dish
- Macrophages stuffed with myelin shed their cholesterol when serum or purified HDL was added. Without these acceptors, fat droplets and crystals piled up.
- Excess cholesterol switched on specific inflammatory genes, including parts of the NLRP3 inflammasome, but this calmed down once HDL/serum helped remove the cholesterol.
Why it matters: Give the cells the right “trucks,” and they can clean themselves up and cool down the inflammation.
5) Myelin-derived cholesterol is enough to cause scarring
- Newborn spinal cords (which normally heal without scars) had no cholesterol buildup and healed cleanly.
- But when researchers added myelin debris—or transplanted cholesterol-loaded macrophages—into newborn injuries, cholesterol crystals formed, macrophages stayed activated, and scar tissue (including fibrous proteins like CSPG, fibronectin, and laminin) appeared.
Why it matters: This proves myelin-derived cholesterol itself can drive scarring and block scar-free healing.
Why this research matters
- It identifies a key, underappreciated reason for poor healing after spinal cord injury: the spinal cord’s lack of an efficient system to remove extra myelin cholesterol.
- It connects the dots between cholesterol buildup, ongoing inflammation, and scar formation that stops nerve regrowth.
- It suggests new treatment ideas:
- Help macrophages get rid of cholesterol by supplying effective “acceptors” (like HDL-like particles) or by creating a route for reverse cholesterol transport in the CNS.
- Develop therapies that prevent cholesterol crystallization or detoxify overloaded macrophages.
- Target specific inflammatory pathways (like the NLRP3 inflammasome) that are switched on by cholesterol crystals.
In short, improving “cholesterol trash pickup” in the injured spinal cord could reduce scarring and open the door to better recovery after spinal cord injuries. While these results are in mice and cells, they point toward a practical, testable path for future treatments.
