Hypothesis

Episodic bursts of grid‑cell firing in entorhinal cortex layer II (ECII) stellate cells generate localized calcium microdomains that exceed buffering capacity, activating calpain‑2 and causing N‑terminal truncation of tau. This truncated tau seeds aggregation preferentially in reelin‑positive stellate cells and Wfs1‑expressing pyramidal neurons, linking spatial navigation activity to the earliest tau pathology in Alzheimer’s disease.

Mechanistic Basis

• Grid‑cell stellate cells fire in theta‑band bursts during spatial navigation, producing transient calcium influx through L‑type voltage‑gated channels and NMDA receptors.[4]
• Calbindin, a high‑affinity calcium buffer, is markedly low in ECII compared with layer III, leaving stellate cells vulnerable to calcium overload.[2]
• Elevated intracellular calcium activates calpain‑2, which cleaves tau at Asp421, generating a fragmentation‑prone species that promotes seeding.[6]
• Truncated tau interferes with reelin signaling by sequestering Dab1, further suppressing GSK3β inhibition and creating a feed‑forward loop of tau phosphorylation.[5]
• Soluble truncated tau released from perforant path terminals increases presynaptic release probability via synaptogyrin‑3 dysregulation, facilitating trans‑synaptic spread to hippocampal CA1.[6][7]

Predictions and Experimental Tests

1. Calcium buffering rescue – Viral overexpression of calbindin in ECII stellate cells of APP/PS1 mice will reduce calpain‑2 activity, lower tau truncation (detected by tau‑Asp421 immunoblot), and decrease seeded tau pathology in hippocampal CA1 after spatial navigation training.
2. Calpain inhibition – Pharmacological blockade of calpain‑2 with MDL‑28170 administered during daily navigation tasks will attenuate tau seeding in ECII without affecting baseline firing rates, measured via in vivo calcium imaging and PET tau tracer.
3. Optogenetic firing modulation – Selective suppression of grid‑cell bursting (using ArchT) during navigation will lower calcium microdomain amplitude, reduce tau truncation, and preserve spatial memory performance, whereas enhancing bursting (using ChR2) will exacerbate tau pathology.
4. Sex‑specific modulation – Female mice will show greater complement‑C3 deposition downstream of tau truncation; complement inhibition will have a larger protective effect in females than males, aligning with observed neuroimmune signatures.[3]

Falsifiability

If calbindin overexpression fails to reduce tau truncation or seeding despite verified calcium buffering, or if calpain inhibition does not alter tau pathology despite target engagement, the hypothesis would be falsified. Likewise, if optogenetic manipulation of grid‑cell bursting does not correlate with changes in tau truncation or downstream spread, the causal link between activity‑dependent calcium dynamics and tau seeding would be refuted.

Implications

This model integrates network activity, subcellular calcium handling, and proteolytic tau processing to explain why ECII is the epicenter of tauopathy. It suggests that therapies targeting calcium buffering or calpain activity—potentially timed with cognitive engagement—could delay the initiation of tau spread, offering a mechanistic bridge between spatial navigation deficits and early Alzheimer’s pathology.