Hypothesis

Age-related cognitive rigidity arises from a bidirectional loop in which mitochondrial dysfunction in parvalbumin‑positive (PV) interneurons drives excessive perineuronal net (PNN) deposition, and the resulting PNN‑mediated inhibition further impairs mitochondrial activity. This circuit locks cortical networks into high‑precision, low‑entropy states that resist updating, mimicking decay while the underlying synaptic machinery remains largely intact. Restoring mitochondrial health in PV interneurons will break the loop, reduce pathological PNN accumulation, and reinstate plasticity without enzymatically removing PNNs.

Mechanistic Rationale

1. PV interneuron mitochondria gate GABA output – PV cells rely on oxidative metabolism to sustain fast‑spiking firing; compromised ATP production lowers vesicular GABA release, disinhibiting pyramidal cells and elevating intracellular calcium via voltage‑gated L‑type channels (LTCC).
2. Calcium‑calcineurin signaling upregulates PNN components – Elevated calcium activates calcineurin, which dephosphorylates NFAT transcription factors, increasing expression of chondroitin sulfate synthases (e.g., Chsy1) and link proteins that scaffold PNNs. This links mitochondrial‑derived calcium stress directly to the molecular machinery that builds PNNs (see calcium dyshomeostasis via calcineurin/LTCC impairs NMDA‑dependent LTP)[5].
3. PNNs exacerbate mitochondrial stress – Dense PNNs restrict extracellular ion diffusion and limit glial‑derived lactate shuttling to PV axons, forcing greater reliance on mitochondrial oxidative phosphorylation and increasing reactive oxygen species (ROS). ROS further damage mitochondrial DNA, creating a vicious feed‑forward loop.
4. Outcome: Entropy‑biased network states – The combined effect is heightened PV‑mediated inhibition variance, sharpening neuronal tuning curves and reducing network entropy. Behaviorally, this manifests as over‑consolidation: reliable predictions but poor surprise tolerance, consistent with observations that PNN degradation reactivates juvenile‑like plasticity[2] while synaptic proteins remain preserved in high‑performing aged rats[5].

Testable Predictions

• Prediction 1: In aged mice, PV‑specific overexpression of PGC‑1α (to boost mitochondrial biogenesis) will normalize calcium transients in PV cells, reduce NFAT nuclear translocation, and lower hippocampal PNN density (measured by WFA staining) without chondroitinase treatment.
• Prediction 2: Despite unchanged PNN levels, mitochondrial rescue will restore NMDA‑independent LTP thresholds and improve performance on reversal learning tasks (e.g., water‑maze platform shift) compared to aged controls.
• Prediction 3: Pharmacological inhibition of calcineurin (e.g., FK506 at low dose) in aged mice will mimic the mitochondrial rescue effect, decreasing PNN deposition and rescuing behavioral flexibility, confirming the calcium‑calcineurin‑PNN axis.
• Prediction 4: Conversely, inducing mitochondrial dysfunction in PV interneurons of young mice (via PV‑Cre‑driven dominant‑negative Mfn2) will prematurely increase PNN density and impair reversal learning, reproducing an aged‑like rigidity phenotype.

Experimental Approach

Use Cre‑dependent AAV vectors to manipulate PGC‑1α, Mfn2, or calcineurin activity specifically in PV interneurons of young (3 mo) and aged (24 mo) mice. Assess:

• Mitochondrial function (Seahorse OCR, ROS) in isolated PV cells.
• Calcium imaging (GCaMP6f) during spontaneous activity.
• PNN density and composition (WFA, lectin hybridization, qPCR for Acan, Chsy1).
• Electrophysiological LTP/LTD ratios in hippocampal slices.
• Cognitive flexibility assays (reversal learning, attentional set‑shifting).

Falsification

If mitochondrial enhancement in PV interneurons fails to alter PNN markers or behavioral flexibility, or if PNN reduction does not rescue plasticity despite mitochondrial rescue, the hypothesis would be refuted, indicating that rigidity and decay are independent rather than coupled mechanisms.

This framework synthesizes consolidation and dysfunction perspectives, proposing that targeting mitochondrial health in inhibitory interneurons offers a lever to re‑engage adaptive plasticity without dismantling the structural scaffolds that preserve neural circuits.