Hypothesis

Aging-related cognitive rigidity stems not from synaptic loss but from excessive stabilization of hippocampal networks by perineuronal nets (PNNs). Mitochondrial-derived peptides (MDPs) such as MOTS-c and humanin counteract this over‑consolidation by modulating astrocyte‑derived chondroitin sulfate proteoglycan (CSPG) secretion, thereby loosening PNNs and reinstating experience‑dependent plasticity.

Mechanistic Rationale

• MDPs exert retrograde signaling from mitochondria to the cytosol and nucleus, influencing transcriptional programs in glia. MOTS-c activates AMPK in astrocytes, shifting metabolism toward glycolysis and increasing lactate release, which has been shown to suppress CSPG synthesis via reduced HIF‑1α activity {1}.
• Humanin engages the GP130/IL6ST receptor complex, triggering STAT3 phosphorylation that can downregulate the expression of aggrecan and link protein, core components of PNNs {2}.
• Elevated circulating humanin in offspring of centenarians {3} and its depletion in Alzheimer’s patients {4} suggest a lifelong tone that sets the threshold for PNN formation.
• When MDPs are low, astrocytes overproduce CSPGs, leading to dense PNNs that limit dendritic spine turnover and impede the encoding of novel information, manifesting as behavioral rigidity.

Testable Predictions

1. Chronic MOTS-c or humanin treatment in aged mice will reduce PNN density in the CA1 stratum radiatum, measured by wheat germ agglutinin (WFA) staining.
2. Reduced PNN density will correlate with enhanced long‑term potentiation (LTP) and decreased paired‑pulse facilitation, indicating restored synaptic plasticity.
3. Behavioral assays requiring cognitive flexibility—such as reversal learning in the Morris water maze or attentional set‑shifting—will show significant improvement after MDP treatment, whereas reference memory (e.g., platform location acquisition) will remain unchanged.
4. Genetic knockdown of astrocytic AMPK or STAT3 will abolish the MDP‑induced PNN remodeling and plasticity rescue, confirming glial mediation.

Experimental Design

• Subjects: 20‑month‑old C57BL/6J mice (n=10 per group).
• Groups: (1) Vehicle control, (2) MOTS-c (5 mg/kg/day, subcutaneous), (3) Humanin (3 mg/kg/day, intraperitoneal), (4) MOTS-c + humanin, (5) MOTS-c + astrocyte‑specific AMPK shRNA, (6) Humanin + astrocyte‑specific STAT3 shRNA.
• Duration: 8 weeks of treatment.
• Outcomes:
  • WFA immunoreactivity quantified as % area occupied in hippocampal sections.
  • Ex vivo electrophysiology: LTP magnitude at Schaffer collateral‑CA1 synapses.
  • Behavioral: reversal learning trials to criterion, attentional set‑shifting errors.
  • Molecular: ELISA for hippocampal CSPG levels; western blot for p‑AMPK, p‑STAT3.

Potential Outcomes and Falsifiability

If MDPs genuinely loosen PNNs to restore plasticity, we expect a significant decrease in WFA staining, increased LTP, and improved reversal learning in treatment groups, with shRNA controls blocking these effects. Failure to observe any of these changes—specifically, no reduction in PNN density despite confirmed MDP delivery—would falsify the hypothesis and suggest that MDPs act through alternative mechanisms unrelated to extracellular matrix remodeling.