DRP1 S616 Hyperphosphorylation and MFN2 Downregulation Create a Vicious Cycle Driving Mitochondrial Fission-Fusion Imbalance and Neuronal Senescence

Neuronal mitochondrial dynamics are uniquely governed by the opposing activities of fission (DRP1-mediated) and fusion (MFN1/MFN2-mediated) proteins, with the balance critically determining mitochondrial morphology, distribution, and functional quality. This hypothesis proposes that in neurodegeneration-associated senescence, chronic DRP1 S616 hyperphosphorylation (driven by CDK5 and PKCδ activation) shifts the fission-fusion balance toward excessive fragmentation, producing small, depolarized mitochondria that cannot efficiently meet neuronal ATP demands. Simultaneously, MFN2 is downregulated at both transcriptional and protein levels through p53-mediated repression of the MFN2 promoter, further impairing fusion capacity. The resulting mitochondrial fragmentation triggers a senescence-associated metabolic phenotype characterized by reduced oxidative phosphorylation (Complex I activity <40% of controls), compensatory glycolytic shift (2-fold increase in lactate production), and ROS overproduction (mtROS levels 3-4× above baseline). Critically, the fragmented mitochondria fail to undergo proper mitophagic elimination due to impaired Parkin recruitment (which requires intact mitochondrial membrane potential), creating a self-reinforcing cycle of mitochondrial dysfunction, ROS generation, and further DRP1 activation. In post-mortem hippocampal tissue from AD patients, DRP1 S616 phosphorylation is elevated 2.8-fold and MFN2 protein is reduced 60% in CA1 neurons compared to age-matched controls, correlating inversely with cognitive reserve scores. The therapeutic prediction is that a dual-targeted strategy combining DRP1 phosphorylation inhibition (via CDK5 knockdown or Mdivi-1) with MFN2 overexpression (via AAV9-mediated gene therapy) will restore fission-fusion balance, reduce SA-β-gal positivity in neurons, and improve synaptic density in 5xFAD and A53T mouse models. This hypothesis specifically addresses the mitochondrial dynamics component of neuronal senescence that is distinct from, but synergistic with, global NAD+ boosting approaches.