Hypothesis

Mitochondrial DNA (mtDNA) variants that increase reactive oxygen species (ROS) production in layer II entorhinal cortex (EC) grid cells activate calcium‑dependent kinases (CaMKII, GSK‑3β) that phosphorylate tau at disease‑relevant sites, initiating oligomeric tau formation and its trans‑synaptic spread. Restoring mtDNA integrity in these neurons prevents tau pathology and preserves spatial navigation.

Mechanistic rationale

• Grid cells rely on high‑frequency firing and precise calcium buffering to maintain phase‑precession; this demands robust ATP supply and tight ROS control [1].
• mtDNA encodes core subunits of Complex I (ND1‑ND6, ND4L) and Complex IV; heteroplasmic mutations in these genes raise electron leak, boosting superoxide without triggering nuclear‑encoded antioxidant responses because mtDNA lacks histones and has limited repair [3].
• Elevated ROS oxidizes ryanodine receptors, causing calcium leak from the endoplasmic reticulum; sustained cytosolic calcium activates CaMKII, which in turn primes GSK‑3β‑mediated tau phosphorylation at Ser202/Thr205 (AT8 epitope) [5].
• Phosphorylated tau loses microtubule affinity, forms oligomers, and spreads along the EC’s afferent/efferent pathways, matching the observed trans‑entorhinal progression [2].

This model places mtDNA dysfunction upstream of both oxidative damage and kinase activation, directly linking the mitochondrial genome to the anatomical epicenter of Alzheimer’s tauopathy.

Testable predictions

1. Inducing mtDNA heteroplasmy in EC layer II of adult mice (using mitoTALENs or DdCBE to introduce pathogenic ND4 or ND5 mutations) will:

   • Increase mitochondrial ROS and calcium transient amplitude in grid cells (measured with MitoSOX and GCaMP).
   • Elevate AT8‑positive tau oligomers within 4‑6 weeks, preceding detectable nuclear tau aggregation.
   • Degrade grid‑cell firing periodicity and impair performance on the Morris water maze or virtual‑reality navigation tasks.

2. Rescuing mtDNA integrity by delivering a mitochondrially targeted base editor that corrects the introduced mutation will:

   • Normalize ROS and calcium handling.
   • Prevent AT8‑tau accumulation and preserve grid‑cell firing.
   • Rescue spatial navigation deficits, even when exogenous human tau is overexpressed.

3. Pharmacological inhibition of CaMKII (e.g., with KN‑93) or GSK‑3β (e.g., with tideglusib) in heteroplasmic mice will attenuate tau phosphorylation without correcting mtDNA, supporting the kinase‑mediated link.

Falsifiability

If mtDNA manipulation in EC layer II fails to alter ROS, calcium signaling, tau phosphorylation, or navigation behavior—or if rescuing mtDNA does not mitigate tau pathology despite confirmed correction—the hypothesis is falsified. Conversely, consistent support across these readouts would substantiate the claim that the mitochondrial genome, not the nuclear genome, is the primary driver of early tauopathy in the brain’s navigation system.