# SIX2‐Mediated Microglial M2 Polarization and Exosomal miR‐3470b Delivery Protect Dopaminergic Neurons in Parkinson's Disease

## TL;DR

This study shows that the SIX2 protein helps protect brain cells in Parkinson's disease by shifting immune cells to a protective state and delivering a key molecule via exosomes.

## Contribution

The study reveals a novel SIX2-mediated pathway involving microglial polarization and exosomal miR-3470b delivery for neuroprotection in Parkinson's disease.

## Key findings

- SIX2 activates DDIT4, leading to mTOR inhibition and autophagy, which promotes microglial M2 polarization.
- Exosomal miR-3470b from M2 microglia suppresses GREM1 and enhances TGF-β signaling in dopaminergic neurons.
- This pathway rescues neuronal apoptosis and restores motor function in PD models.

## Abstract

Neuroinflammation driven by dysregulated microglial activation exacerbates dopaminergic neuron loss in Parkinson's disease (PD). This study investigated whether the transcription factor SIX2 mitigates neuroinflammation and provides neuroprotection by promoting microglial M2 polarization and exosome‐mediated communication.

Using LPS‐stimulated BV2 microglia and MPTP‐induced mouse models of PD, we systematically investigated the role of SIX2. Gain‐ and loss‐of‐function approaches for SIX2, DDIT4, miR‐3470b, and GREM1 were combined with ChIP, RNA‐seq, exosome isolation/transfer, and behavioral tests to analyze the SIX2‐DDIT4‐autophagy axis and the exosomal miR‐3470b/GREM1/TGF‐β pathway.

RNA‐seq and ChIP‐qPCR revealed that SIX2 transcriptionally activated DDIT4. This led to mTOR inhibition and autophagy induction, driving a shift in microglial phenotype from pro‐inflammatory M1 to protective M2. Consequently, M2‐polarized microglia released exosomes highly enriched in miR‐3470b, as identified by miRNA sequencing. Upon internalization by dopaminergic neurons, miR‐3470b directly bound to and suppressed GREM1, which in turn potentiated TGF‐β signaling activity. Ultimately, this SIX2‐initiated cascade rescued neuronal apoptosis and restored motor coordination in both cellular and animal models of PD.

SIX2 promotes microglial M2 polarization via the DDIT4/mTOR/autophagy axis and mediates neuroprotection through exosomal miR‐3470b targeting of GREM1/TGF‐β signaling, revealing novel therapeutic targets for PD immunotherapy.

We found that SIX2 directly upregulates DDIT4 expression by binding to its promoter region, leading to the suppression of mTOR signaling and subsequent activation of autophagy. This autophagy induction facilitates the transition of microglial cells from a pro‐inflammatory M1 phenotype to the neuroprotective M2 phenotype. Additionally, we discovered that M2‐polarized microglial cells secrete exosomes containing miR‐3470b, which are taken up by dopaminergic cells. Within these cells, miR‐3470b targets the GREM1/TGF‐β signaling pathway, thereby exerting significant neuroprotective effects.