Hypothesis

Grid cell hyperactivity triggers calcium‑dependent activation of calpain, which cleaves WIPI2 and blocks autophagosome elongation. This activity‑dependent suppression of autophagy is an adaptive response to metabolic stress but creates a permissive environment for tau oligomer accumulation, explaining the selective vulnerability of excitatory grid cells in early Alzheimer’s disease.

Rationale

• Excitatory grid cells in the medial entorhinal cortex (EC) fire at high rates during spatial navigation, generating sustained calcium influx [2].
• Chronic elevation of intracellular calcium activates the calcium‑dependent protease calpain, which has been shown to cleave autophagy‑related proteins in other contexts (e.g., ATG5, LC3) [3] but has not been examined for WIPI2 in neurons.
• WIPI2 is essential for autophagosome elongation; its downregulation stalls autophagy without affecting initiation [3]4].
• In aged neurons, WIPI2 loss is observed, but the upstream trigger remains unclear [3]
• Suppressing autophagy can be beneficial when damaged organelles risk overwhelming a compromised lysosomal system, as seen in C. elegans where neuronal Beclin‑1 inhibition extends lifespan [4]
• However, in grid cells, chronic autophagy suppression prevents clearance of tau oligomers that are generated by heightened neuronal activity [2], leading to their seeding and spread.

Novel Mechanistic Insight

We propose that activity‑dependent calpain activation directly cleaves WIPI2 at a conserved hydrophobic linker region, generating a C‑terminal fragment that lacks the PI3P‑binding domain required for phagophore expansion. This cleavage would:

1. Occur preferentially in neurons with high firing rates (i.e., grid cells) because calpain activation correlates with calcium load.
2. Produce a dominant‑negative WIPI2 fragment that competes with any remaining full‑length WIPI2, further suppressing elongation.
3. Be reversible: calpain inhibition or expression of a cleavage‑resistant WIPI2 mutant should restore autophagic flux even under high activity.

If autophagy is suppressed as a protective adaptation, then restoring flux in hyperactive grid cells should exacerbate metabolic stress (e.g., increase ROS, reduce ATP) unless lysosomal capacity is concurrently boosted. This predicts a biphasic outcome: moderate autophagy enhancement rescues tau clearance without harming cell viability, whereas excessive flux triggers energetic crisis.

Testable Predictions

1. Biochemical: In EC slices from young mice, optogenetic stimulation of grid cells will increase calpain activity and reduce full‑length WIPI2 levels, detectable by Western blot with an N‑terminal WIPI2 antibody. Calpain inhibitor (MDL‑28170) will prevent this loss [3]
2. Imaging: Expressing a FRET‑based WIPI2 cleavage sensor in EC excitatory neurons will show heightened FRET signal during spatial navigation tasks, correlating with local calcium rises measured by GCaMP.
3. Functional: Chemogenetic activation (hM3Dq) of grid cells for 2 weeks will decrease autophagic flux (LC3‑II/I ratio, p62 accumulation) and increase tau oligomer staining (HT7) specifically in excitatory, not inhibitory, EC neurons. Co‑expression of a calpain‑resistant WIPI2 (WIPI2ΔC) will rescue flux and reduce tau accumulation.
4. Behavioral: Mice expressing WIPI2ΔC in EC excitatory neurons will retain spatial navigation performance in the Morris water maze despite chronic grid cell activation, whereas control mice will show early spatial deficits.
5. Rescue via Lysosomal Boost: Overexpressing TFEB alongside WIPI2ΔC will further improve tau clearance without inducing metabolic stress, indicating that the adaptive suppression is specifically aimed at avoiding lysosomal overload.

Falsifiability

If optogenetic activation of grid cells fails to alter WIPI2 levels or autophagic flux, or if calpain inhibition does not prevent WIPI2 loss under high activity, the hypothesis would be refuted. Similarly, if expressing cleavage‑resistant WIPI2 does not reduce tau accumulation despite restored autophagy, the proposed link between WIPI2 cleavage and tau seeding would be unsupported.

Implications

This model reframes early EC vulnerability not as a passive failure of cleanup systems but as an activity‑driven trade‑off: grid cells temporarily shut down a costly autophagy program to survive metabolic demands, inadvertently allowing tau to gain a foothold. Therapeutically, it suggests that timely, neuron‑specific modulation of calpain‑WIPI2 signaling—or complementary lysosomal enhancement—could decouple adaptive autophagy suppression from pathogenic tau accumulation, preserving spatial cognition in preclinical Alzheimer’s disease.