Dual Role of Microglial TREM2 in Neuronal Degeneration and Regeneration After Axotomy

This study reveals that microglial TREM2 plays a dual role in spinal cord injury by promoting the clearance of degenerating motoneurons while simultaneously facilitating the regeneration of others, with these effects varying by activation profile and sex.

The Gist

Imagine your body's nervous system as a vast, intricate city of roads (nerves) connecting a central command center (the brain and spinal cord) to the workers in the field (your muscles). When a major road is cut (a nerve injury), the workers (motoneurons) in the spinal cord get a shock. Some workers panic and try to rebuild, while others give up and collapse.

Enter the Microglia. Think of these as the city's emergency response teams and sanitation crews. They rush to the scene of the accident. For decades, scientists have been confused about what these crews are actually doing. Are they helping the workers rebuild, or are they cleaning up the mess by removing the collapsed ones?

This paper reveals that the answer is "both," and it all depends on a specific tool the crews carry called TREM2.

Here is the story of what happens, broken down simply:

1. The Two Types of Emergency Scenes

When a nerve is cut, the microglia rush to the injured nerve cells (motoneurons). They form two very different groups:

• The "Renovation Crew" (Regenerating Neurons): These microglia surround the injured cells that are trying to heal. They stay as individuals, gently wrapping around the cell like a supportive hug. These cells are swelling up, getting bigger, and working hard to rebuild their connections.
• The "Demolition Crew" (Degenerating Neurons): These microglia form tight, crowded clusters around cells that are dying. They look like a swarm of bees or a tight knot. Their job is to recognize that the cell is beyond saving and prepare to remove it.

2. The Magic Tool: TREM2

The researchers discovered that the microglia in both groups use a special tool called TREM2. You can think of TREM2 as a universal remote control or a smart sensor on the microglia's surface.

• In the Renovation Crew: TREM2 acts like a green light. It helps the microglia support the injured cell, encouraging it to swell up (a sign of metabolic activity) and start the complex process of regrowing its nerve connection to the muscle.
• In the Demolition Crew: TREM2 acts like a red light or a cleanup trigger. It tells the microglia, "This cell is damaged beyond repair; activate the cleanup mode." It supercharges their ability to eat up (phagocytose) the dying cell.

3. What Happens When You Remove the Tool?

To prove this, the scientists created mice that didn't have this TREM2 tool. They cut the nerves and watched what happened.

• The Renovation Crew Stalls: Without TREM2, the microglia surrounding the regenerating cells couldn't do their job. The injured nerve cells didn't swell up properly (they stayed small and weak), and the muscles took much longer to get reconnected. It was like trying to rebuild a house without the right blueprints; the workers were confused and slow.
• The Demolition Crew Changes: Without TREM2, the microglia still formed their tight clusters around the dying cells, but they were less efficient at the actual "cleaning." Interestingly, because the cleanup was less aggressive, some small nerve cells that usually get removed actually survived.

4. The Gender Twist

The study also found a fascinating difference between male and female mice.

• Female microglia relied heavily on TREM2 to do their cleanup work. Without it, their cleanup ability dropped significantly.
• Male microglia seemed to have a backup plan. Even without TREM2, they could still clean up, though perhaps not as efficiently. It's like the female crew had one specific key to the door, while the male crew had a master key and a spare.

The Big Takeaway

This paper teaches us that the immune system isn't just "good" or "bad." It's a dual-purpose system.

• TREM2 is the switch that tells the microglia whether to nurse a patient back to health or remove a patient who is beyond saving.
• If we want to help nerves heal after an injury, we need to make sure TREM2 is working correctly to support the "Renovation Crew."
• However, if we want to stop the loss of certain nerve cells, we might need to be careful about how we tweak TREM2, because it's also the signal that tells the "Demolition Crew" to clear out the dying cells.

In short: The body's cleanup crew uses the same tool (TREM2) to both save the survivors and remove the victims. Understanding how to control this tool could lead to better treatments for nerve injuries, helping more people walk and move again after an accident.

Technical Summary

1. Problem Statement

Following peripheral nerve injury (PNI), such as axotomy, ventral horn microglia in the spinal cord proliferate and migrate toward injured motoneurons (MNs). Historically, this interaction was viewed as protective ("synaptic stripping"), but recent findings suggest a dual outcome: some injured MNs regenerate, while others degenerate and die.

• The Gap: The mechanisms determining whether microglia facilitate regeneration or promote degeneration are unclear. Specifically, two distinct microglial phenotypes have been observed:
  1. "On MN" microglia: Individualized cells surrounding regenerating MNs.
  2. "Death Cluster" (DC) microglia: Tight, ameboid clusters fully enveloping degenerating MNs.
• The Question: What molecular signals regulate the switch between these phenotypes, and how do they influence MN fate? The authors hypothesized that TREM2 (Triggering Receptor Expressed on Myeloid Cells 2), a receptor known for promoting phagocytosis and metabolic support, plays a critical but nuanced role in this process.

2. Methodology

The study utilized a combination of genetic mouse models, surgical nerve injury models, and advanced imaging/quantitative analysis.

• Animal Models:
  • Wildtype (WT): Tmem119-eGFP mice (microglia-specific GFP labeling).
  • Global Knockout (GKO): Trem2⁻/⁻ mice crossed with Tmem119-eGFP.
  • Conditional Knockout (CKO): Tmem119-CreERT2 crossed with Trem2-flox/flox and Rosa26-tdTomato. Tamoxifen was administered at P28 to delete Trem2 specifically in microglia before injury.
• Injury Models:
  • Sciatic Nerve Cut-Ligation: To induce axotomy and degeneration.
  • Sciatic Nerve Cut-Repair: To induce regeneration.
  • Retrograde Labeling: Fast Blue or AAV1-tdTomato injected into the lateral gastrocnemius (LG) muscle to label specific MN pools.
• Time Course: Analyses performed at 3, 7, 14, 21, and 28 days post-injury (dpi), with long-term reinnervation assessed at 8 weeks.
• Key Techniques:
  • Immunohistochemistry (IHC): Staining for TREM2, CD68 (phagocytosis marker), p-SYK (downstream signaling), Ki-67 (proliferation), ChAT, NeuN, and NFH/VAChT.
  • RNAscope: In situ hybridization to quantify trem2 mRNA levels.
  • Confocal Microscopy & 3D Reconstruction: Used for Sholl analysis (morphology), cell counting, and volumetric analysis of CD68 granules.
  • Statistical Analysis: ANOVA, Kolmogorov-Smirnov (K-S) tests for cell size distributions, and estimation statistics (Hedge's g, bootstrapping).

3. Key Contributions

1. Characterization of Microglial Phenotypes: The study rigorously defines and quantifies the morphological and molecular differences between "On MN" (regenerative) and "Death Cluster" (degenerative) microglia.
2. Sex-Specific Dimorphism: The paper highlights significant sex differences in microglial activation, particularly in females, who exhibit higher TREM2 expression, p-SYK signaling, and CD68 content.
3. Dual Function of TREM2: The central discovery is that TREM2 has opposing roles depending on the health state of the motoneuron:
   • It facilitates regeneration in healthy MNs.
   • It promotes phagocytosis in degenerating MNs.
4. Mechanistic Insight: The study demonstrates that TREM2 is required for the retrograde chromatolytic reaction (cell body swelling) in regenerating MNs and for the efficient removal of degenerating MNs via CD68 upregulation.

4. Key Results

A. Microglial Morphology and TREM2 Expression

• Morphology: "On MN" microglia are ramified but less complex than surveying microglia. "Death Cluster" microglia are ameboid with retracted processes.
• TREM2 Upregulation: TREM2 protein and mRNA are upregulated in both "On MN" and "Death Cluster" microglia after injury. However, Death Cluster microglia express the highest levels of TREM2.
• Signaling: Downstream p-SYK signaling follows TREM2 expression patterns, being highest in Death Clusters and significantly reduced in GKO/CKO mice.

B. Effects of TREM2 Deletion on Microglial Behavior

• Proliferation & Migration: TREM2 deletion (GKO or CKO) did not significantly alter microglial proliferation (Ki-67), migration toward MNs, or the formation of Death Clusters.
• Phagocytic Markers (CD68):
  • TREM2 deletion significantly reduced CD68 expression (granule volume and size) in Death Cluster microglia.
  • Sex Difference: This reduction was more pronounced in females. In males, TREM2 deletion led to an increase in the number of small CD68 granules in non-cluster microglia, suggesting compensatory or alternative pathways.

C. Impact on Motoneuron Fate

• Regeneration (Cell Body Swelling):
  • In WT mice, axotomized MNs undergo a chromatolytic reaction characterized by cell body swelling (increased cross-sectional area) peaking at 14 dpi.
  • TREM2 Deficiency: GKO and CKO mice failed to show this normal swelling. The cell size distribution remained static, indicating a failure in the metabolic upregulation required for regeneration.
• Degeneration (Loss of Small MNs):
  • In WT mice, 8 weeks post-injury, there was a partial loss of small $\gamma$-MNs (identified by low NeuN expression and small cell size).
  • TREM2 Deficiency: In CKO mice, the $\gamma$-MN population was preserved. The loss of small MNs seen in WT was absent, suggesting TREM2 is necessary for the phagocytic removal of a specific subpopulation of degenerating MNs.
• Functional Outcome (Reinnervation):
  • Muscle reinnervation (measured by VAChT coverage of NMJs at 8 weeks) was significantly delayed in GKO and CKO mice compared to WT.
  • CKO mice showed a 56% reduction in fully innervated NMJs compared to WT, confirming that the lack of TREM2 impairs functional recovery.

5. Significance

• Paradigm Shift: The study challenges the view of microglia as uniformly protective or destructive. Instead, it proposes a context-dependent dual role for TREM2: it is pro-regenerative for surviving neurons (facilitating metabolic changes) and pro-phagocytic for dying neurons.
• Therapeutic Implications:
  • Simply blocking TREM2 to prevent neurodegeneration might inadvertently hinder regeneration and delay functional recovery after nerve injury.
  • Conversely, enhancing TREM2 might be beneficial for clearing debris but could risk eliminating neurons that might otherwise recover.
• Sexual Dimorphism: The findings underscore that microglial responses to injury are sexually dimorphic, with females showing a stronger reliance on TREM2 for phagocytic activity. This is crucial for designing sex-specific therapies for nerve injuries and neurodegenerative diseases.
• Mechanism of Regeneration: The study links TREM2 signaling directly to the retrograde chromatolytic reaction (cell swelling), a hallmark of neuronal metabolic adaptation to injury, suggesting TREM2 is a key regulator of neuronal bioenergetics during regeneration.

In conclusion, the paper establishes that microglial TREM2 is a critical switch that determines the fate of axotomized motoneurons, promoting the clearance of irreversibly damaged cells while simultaneously supporting the metabolic machinery required for the regeneration of surviving cells.