Hypothesis

Senescent microglia drive age‑dependent loss of striatal D2 receptors in cholinergic interneurons through mitochondria‑derived DNA activation of the cGAS‑STING pathway, which triggers type I interferon signaling and HDAC2‑mediated epigenetic silencing of the Drd2 gene.

Mechanistic Rationale

1. Mitochondrial dysfunction in aged microglia releases oxidized mtDNA into the cytosol.
2. Cytosolic mtDNA activates cGAS, producing STING‑dependent IFN‑β release.
3. IFN‑β acts on neighboring cholinergic interneurons, stimulating STAT1 phosphorylation and up‑regulating HDAC2 expression.
4. Increased HDAC2 deacetylates histone H3K27 at the Drd2 promoter, compacting chromatin and reducing transcription.
5. Selective D2 loss disrupts the dopamine‑acetylcholine balance, favoring D1‑mediated signaling and impairing habit formation.

This mechanism places immune senescence upstream of receptor imbalance, converting the seed idea into a concrete epigenetic cascade.

Testable Predictions

• Prediction 1: Pharmacological inhibition of cGAS or STING in microglia of aged mice will prevent D2 receptor loss in striatal cholinergic interneurons without altering overall microglial number.
• Prediction 2: Genetic deletion of HDAC2 specifically in cholinergic interneurons will rescue D2 expression and motor automaticity in aged mice, even if microglial senescence persists.
• Prediction 3: Adoptive transfer of senescent microglia (identified by p16^INK4a^+ and mtDNA release) into young mice will recapitulate D2 downregulation and motor deficits, an effect blocked by STING inhibition.

Experimental Approaches

• Use CX3CR1‑CreER; STING^fl/fl^ mice to delete STING in microglia; assess Drd2 mRNA by RNAscope and D2 protein by autoradiography in 24‑month‑old animals.
• Generate ChAT‑Cre; HDAC2^fl/fl^ mice; perform rotarod and lever‑press automation tests to measure habit formation.
• Isolate p16^INK4a^+ microglia from old donors, label with CFSE, inject into striatum of 2‑month‑old recipients; track Drd2 levels over 4 weeks.
• Measure cytosolic mtDNA, cGAMP, IFN‑β, and p‑STAT1 in FACS‑sorted cholinergic interneurons via flow cytometry and western blot.

Potential Implications

If validated, this hypothesis reframes immunomodulation as a disease‑modifying strategy for Parkinson’s risk: targeting microglial mtDNA release or downstream STING‑HDAC2 signaling could preserve D2‑dependent circuit function before neurodegeneration emerges. It also offers a biomarker avenue—circulating mtDNA or IFN‑β levels predicting imminent D2 loss.