The Detrimental Role of Arginase-1 in Macrophages After Ischemic Stroke

Academic Background

Ischemic stroke remains one of the leading causes of long-term disability worldwide. Although arginase-1 (Arg1) is typically associated with anti-inflammatory responses and tissue repair, its specific role in post-stroke recovery remains unclear. Arg1 is predominantly expressed in macrophages, especially under inflammatory conditions, where it is thought to influence the inflammatory microenvironment by regulating L-arginine metabolism. However, the functional impact of Arg1 on post-stroke recovery has not been thoroughly investigated. This study aims to elucidate the role of Arg1 in post-stroke recovery, particularly how its expression in infiltrating macrophages affects the inflammatory milieu and functional outcomes.

Source of the Paper

The research was conducted by Hyung Soon Kim, Seung Ah Jee, and others from Ajou University School of Medicine, with contributions from Won-Suk Chung of Korea Advanced Institute of Science and Technology. The paper was published on February 14, 2025, in PNAS (Proceedings of the National Academy of Sciences), titled “Detrimental influence of arginase-1 in infiltrating macrophages on poststroke functional recovery and inflammatory milieu.”

Research Workflow

1. Expression of Arg1 Following Photothrombotic Stroke

The researchers first induced photothrombotic stroke in a mouse model and assessed Arg1 protein expression using immunohistochemistry. Results showed that Arg1 expression began at 3 days post-stroke, peaked at 7 days, and nearly returned to baseline levels by 14 days. Arg1 was primarily expressed in macrophages around the infarct core, showing significant interactions with microglia.

2. Construction of Arg1 Conditional Knockout Mouse Model

To investigate the function of Arg1 in macrophages, the researchers developed an Arg1 conditional knockout (CKO) mouse model by crossing Arg1flox/flox mice with Lysm-Cre mice, successfully deleting the Arg1 gene in Lysm-positive macrophages. Experimental results demonstrated that Arg1 CKO mice exhibited significantly better motor function recovery after stroke compared to controls.

3. Assessment of Motor Function Recovery

The researchers evaluated post-stroke motor function recovery in Arg1 CKO mice using a series of behavioral tests, including the pellet retrieval test, cylinder test, and ladder rung test. Results indicated that Arg1 CKO mice began to show faster motor function recovery starting at 2 weeks post-stroke, with a notably lower error rate in the ladder rung test.

4. Effects on Fibrotic Scar Formation and Myelin Regeneration

The researchers further explored the impact of Arg1 CKO on fibrotic scar formation and myelin regeneration after stroke. Results showed that fibrotic scars were significantly reduced in Arg1 CKO mice, with enhanced myelin regeneration in the peri-infarct area. This suggests that Arg1 expression in macrophages may influence post-stroke functional recovery by modulating the fibrotic microenvironment.

5. Regulation of Microglial Synaptic Phagocytosis

Through in vitro co-culture experiments, the researchers found that Arg1 activity in macrophages regulated the synaptic phagocytosis capacity of microglia. Arg1 CKO significantly reduced microglial synaptic phagocytosis, thereby protecting synaptic structures in the peri-infarct region.

Key Results

1. Time-dependent expression of Arg1: Arg1 peaked at 7 days post-stroke and was primarily expressed in infiltrating macrophages.
2. Improved motor function recovery in Arg1 CKO: Arg1 CKO mice showed significantly better motor function recovery after stroke compared to controls.
3. Reduced fibrotic scarring and enhanced myelin regeneration: Arg1 CKO mice exhibited significantly reduced fibrotic scarring and enhanced myelin regeneration in the peri-infarct area.
4. Reduced microglial synaptic phagocytosis: Arg1 CKO significantly decreased microglial synaptic phagocytosis, protecting synaptic structures.

Conclusion

This study reveals the detrimental role of Arg1 in post-stroke recovery, particularly how its expression in infiltrating macrophages influences functional recovery by modulating the inflammatory microenvironment and microglial function. Arg1 CKO improved motor function recovery after stroke by reducing fibrotic scar formation and enhancing myelin regeneration. Additionally, Arg1 influenced the integrity of synaptic structures by regulating microglial synaptic phagocytosis. These findings provide new potential therapeutic targets for post-stroke treatment, suggesting that inhibiting Arg1 may be an effective strategy.

Highlights of the Study

1. Detrimental effects of Arg1: The study is the first to reveal the harmful effects of Arg1 in post-stroke recovery, challenging its traditional perception as an anti-inflammatory marker.
2. Innovative experimental model: By constructing an Arg1 CKO mouse model, the researchers were able to precisely study the function of Arg1 in macrophages.
3. Multi-level mechanistic investigation: The study comprehensively explored the multiple mechanisms of Arg1’s role in post-stroke recovery, ranging from cellular to behavioral levels.

Research Value

This study not only provides new insights into understanding the complex mechanisms of the post-stroke inflammatory microenvironment but also offers theoretical support for developing therapeutic strategies targeting Arg1. Modulating Arg1 expression may improve functional recovery in stroke patients and reduce the incidence of long-term disabilities.