Doctoral Dissertation Defense | Said Salehe

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Florida International University University Graduate School

Doctoral Dissertation Defense

Abstract

Noncanonical Roles of DRP1 in Brain Aging and Parkinson’s Disease: Lipid Droplets, Neuroinflammation, and Synaptic Remodeling

By Said S. Salehe

Brain aging and neurodegenerative disorders such as Parkinson's disease (PD) share several hallmarks, including chronic neuroinflammation, pathological lipid-droplet (LD) accumulation, and synaptic alterations. However, the upstream regulators of these processes remain poorly defined. This dissertation identifies noncanonical roles for dynamin-related protein 1 (DRP1), best known for mediating mitochondrial fission, in regulating lipid metabolism, neuroinflammation, and synaptic integrity. Across models of aging, lipopolysaccharide (LPS)-induced inflammation, and α-synuclein overexpression, LD accumulation was greater in microglia than in other cell types. Microglial DRP1 abundance correlated positively with LD burden, whereas partial DRP1 knockout or pharmacological inhibition reduced LD accumulation and inflammatory gene expression. Mechanistically, DRP1 deficiency suppressed LPSinduced long-chain acyl-CoA synthetase 1 (Acsl1) expression via NF-κB signaling. Inhibiting NF-κB or ACSL activity likewise decreased LD accumulation in primary microglia. In human postmortem substantia nigra, ACSL1 expression and PLIN2-positive LD burden were elevated relative to non-PD tissue, with increased ACSL1 and PLIN2 observed in microglia and dopaminergic neurons of the substantia nigra. In mice overexpressing α-synuclein, partial DRP1 deletion attenuated Acsl1 induction and LD accumulation. Bulk and single-nucleus multiome (RNA-Seq & ATAC-Seq) analyses of aging human and mouse brains revealed increased neuroinflammatory activity, including complement cascade activation, driven primarily by microglia and astrocytes. These changes coincided with declines in postsynaptic density and synaptic signaling in excitatory and inhibitory neurons. Partial DRP1 knockout reduced age-related neuroinflammation (neuroinflammaging) in the mouse cortex and hippocampus and reversed postsynaptic, but not presynaptic, decline by reducing excitatory and inhibitory synaptic pruning and C1q tagging of postsynaptic sites. These benefits persisted following conditional DRP1 deletion after neuroinflammaging was established. By contrast, Lcn2 knockout, a downstream effector of DRP1–NF-κB signaling, reduced neuroinflammaging but not synaptic alterations, suggesting that DRP1 acts on synapses independently of its role in inflammation. Partial DRP1 knockout also improved age-related deficits in locomotor performance, learning, and memory. Together, these findings establish DRP1 as an upstream regulator of ACSL1-dependent lipid droplet accumulation, age-related neuroinflammation, and complement-associated postsynaptic pruning. The work extends DRP1 biology beyond mitochondrial fission and identifies DRP1 as a potential therapeutic target against shared hallmarks of brain aging and neurodegeneration.