Neurons are born quiescent and die quiescent they permanently exit cell cycle at differentiation. Stress tricks them into attempting division, but lacking cytokinesis machinery, they suicide instead.

The Mechanism:

Mitotic Triggers: Oxidative stress, amyloid exposure, or tau pathology aberrantly activate cyclins and CDKs in post-mitotic neurons reawakening cell cycle machinery.

DNA Replication: Neurons synthesize DNA, replicating genomes partially. Flow cytometry shows 4N DNA content in vulnerable neurons aneuploidy.

Checkpoint Failure: Without functional mitosis machinery, neurons cannot complete division. They arrest at G2/M checkpoint indefinitely.

Apoptosis Activation: Prolonged checkpoint activation triggers caspase cascade. Neurons die attempting division mitotic catastrophe.

Tau Phosphorylation: CDK5 activated during re-entry hyperphosphorylates tau, destabilizing microtubules and impairing transport.

AD Specificity: Cell cycle markers appear in MCI precede tangles. Vulnerable neurons (hippocampus, entorhinal) show re-entry; resistant neurons don't.

Therapeutic Implications:

CDK inhibitors (flavopiridol) blocking re-entry

Cell cycle checkpoint modulators preventing apoptosis

E2F-1 suppressors silencing transcription factors

Senescence inducers as alternative to apoptosis

This reframes neurodegeneration as failed regeneration neurons die trying to divide.