O3-03-03: Apolipoprotein E4 and its fragment impair mitochondrial dynamics in neuronal cultures

Abstract

Apolipoprotein (apo) E4 has a gene-dose effect on the risk and age of onset of Alzheimer's disease (AD), but the molecular mechanisms are unclear. ApoE undergoes proteolytic cleavage into C-terminal-truncated fragments found in AD brains. The fragments, which are neurotoxic in vitro and cause neurodegeneration and behavioral deficits in transgenic mice, escape the secretory pathway and locate to mitochondria in neuronal cells, leading to mitochondrial dysfunction and loss of synaptic spines. The regulation of mitochondrial distribution is essential to maintaining distinct axonal and dendritic domains in response to local metabolic demand, morphologic plasticity, and synapse formation. We studied the effects of apoE4 and its predominant fragment on mitochondrial dynamics in neurites of differentiated PC12 cells. Time-lapse recordings demonstrated that apoE (7.5 μg/ml, 24 h) alters mitochondrial motility in an isoform-specific manner (apoE4 fragment > apoE4 > apoE3 > apoE2). Exogenous apoE4 and its fragment significantly decreased the percentage of motile mitochondria by 28±7% and 37±5%, respectively, their average speed by 37±10% and 57±6%, and the distance traveled by 60±7% and 73±4%. Similar effects were seen in PC12 cells stably expressing much lower levels of apoE. The apoE4 fragment reduced the activity-dependent response of mitochondria to KCl depolarization to only 45% of that in control cells. In contrast, when neuronal activity was blocked with 1 μM tetrodotoxin, the apoE isoforms had similar effects, suggesting that the detrimental effect of apoE4 and its fragment on mitochondrial dynamics involves an activity-dependent mechanism. The impaired mitochondrial motility was rescued by an R61T mutation in apoE4, which abolishes domain interaction and decreases susceptibility to proteolysis. These results demonstrate that apolipoprotein E4 and its fragment impair mitochondrial dynamics in neuronal cultures and reveal domain interaction as a potentially important drug target in apoE4-associated AD.