Hypothesis

Chronic type‑I interferon (IFN‑I) signaling in the aging brain is maintained not by excess ligand production but by failure of intracellular negative feedback. In aged microglia, the genes encoding the suppressors of cytokine signaling SOCS1 and SOCS3 become epigenetically silenced through increased promoter DNA methylation and repressive histone marks. This silencing prevents JAK‑STAT pathway termination, leading to persistent STAT1/2 phosphorylation, sustained transcription of IFN‑stimulated genes, and downstream suppression of MEF2C‑dependent synaptic plasticity programs. The result is a locked‑in transcriptional state that manifests as reduced neuroplasticity, accumulated lipofuscin, and cognitive rigidity.

Predictions

1. Aged microglia will show significantly higher CpG methylation at the SOCS1 and SOCS3 promoters compared with young microglia, accompanied by reduced SOCS1/3 mRNA and protein levels despite unchanged IFN‑β ligand concentrations.
2. Pharmacological inhibition of DNA methyltransferases (e.g., 5‑aza‑2´‑deoxycytidine) or targeted demethylation via CRISPR‑dCas9‑TET1 in microglia will restore SOCS1/3 expression, decrease phospho‑STAT levels, and normalize IFN‑stimulated gene signatures.
3. Restoring SOCS1/3 function in aged microglia will reduce lipofuscin accumulation in both microglia and neurons, reactivate MEF2C‑driven transcriptional networks, and improve performance on hippocampus‑dependent learning tasks.
4. Conversely, overexpression of SOCS1/3 in young microglia will not impair baseline IFN‑I responsiveness but will protect against experimentally induced chronic IFN‑I exposure, preventing the development of rigidity phenotypes.

Experimental Approach

• Cell isolation: Fluorescence‑activated cell sorting (FACS) of CD11b^+CD45^low microglia from young (3 mo) and aged (24 mo) C57BL/6J mice.
• Epigenetic profiling: Bisulfite sequencing of SOCS1/3 promoters; ChIP‑qPCR for H3K27me3 and H3K9me3.
• Functional rescue: Treat aged microglial cultures with 5‑aza‑dC or transduce with AAV‑Syn‑dCas9‑TET1 targeting SOCS promoters; measure SOCS expression, pSTAT1/2, and ISG transcripts by qPCR and western blot.
• In vivo validation: Stereotactic delivery of AAV‑Cx3cr1‑SOCS1 (or SOCS3) to the hippocampus of aged mice; assess lipofuscin via autofluorescence, synaptic protein levels (PSD‑95, Synapsin‑1), and behavior (Morris water maze, novel object recognition).
• Control experiments: Include IFN‑β neutralizing antibody groups to distinguish effects of ligand reduction versus feedback restoration.

Potential Impact

Confirming that epigenetic silencing of SOCS feedback regulators underlies persistent IFN‑I signaling would shift the therapeutic focus from broad IFN blockade—which risks compromising host defense—to precision epigenome editing or small‑molecule demethylation agents that restore endogenous feedback. This approach aligns with the seed idea that aging brains become over‑confident in their predictive models; reinstating the ability to terminate inflammatory signals would re‑introduce the "controlled uncertainty" needed for adaptive plasticity without compromising essential immune vigilance.