Mitochondrial dynamics imbalance — excessive fission (DRP1) and insufficient fusion (MFN2) — is consistently observed in Parkinson's and Alzheimer's disease neurons. The hypothesis proposes a vicious cycle where CDK5-mediated DRP1 phosphorylation and PRKCD-mediated MFN2 downregulation reinforce each other. This challenge requires demonstrating: (1) the causal directionality of the cycle, (2) CDK5 inhibition as a viable intervention point, (3) downstream rescue of bioenergetics. Falsifiable prediction: CDK5 inhibitor roscovitine should reduce DRP1-S616 phosphorylation by ≥50% and restore mitochondrial membrane potential in A53T-SNCA neurons within 48h.