[P4–431]: MITOCHONDRION HYPERFUSION AND HYPERMOTILITY IN APOE4 ASTROCYTES

Apolipoproteins (ApoE) are cholesterol carriers that in humans exist in three alleles: E2, E3 and E4. ApoE4 is the most important genetic risk factor in late-onset Alzheimer's disease (LOAD)—particularly affecting women—but its pathogenic mechanism remains unclear. The novelty of our approach is: 1) we focus on the pathological actions of endogenous ApoE4 in astrocytes, the cells that produce most of ApoE in the brain; and 2) we aim to link mitochondrial dysfunction, a hallmark and biomarker of LOAD, with ApoE4-elicited astrocyte dysfunction. Specifically, a decrease in mitochondrial DNA (mitDNA) contents has been detected in cerebrospinal fluid (CSF) of patients at early stages of LOAD, particularly in ApoE4 carriers (Podlesniy et al. Annals of Neurology 74:655–668, 2013), suggesting alteration of cycles of fusion, fission and mitophagy, which are essential to keep mitochondria healthy. We seek to determine the whether impaired mitochondrial dynamics in ApoE4 carriers originate in astrocytes. For proof-of-concept, we used immortalized astrocytes expressing human ApoE isoforms. Mitochondria dynamics were assessed by computerized morphometry (Image J), and by measurements of squared displacement (D2, Imaris) of mitochondria stained with MitoTracker. Molecules that regulate mitochondrial fusion/fission, kinesis and mitophagy were examined by qPCR and Western blots. ATP and lactate were assessed as bioenergetics markers. As compared to ApoE3 astrocytes, ApoE4 astrocytes showed: 1) defective fission, as evidenced by increased network contents in cells challenged by stimuli that fragment mitochondria such as oligomycin; 2) greater D2, suggesting increased motility; 3) reduced ATP and increased lactate production, suggesting impairment of mitochondrial respiration and activation of compensatory glycolysis, and 4) decreased PARKIN expression, a protein key to mitophagy; and 5) increased expression of Miro mRNA—which encodes for a protein that regulates mitochondrial motility. ApoE4-harboring astrocytes may present hyperfusion and hypermotility associated with impaired energetics and altered mitophagy. Ongoing work is aimed to: 1) confirm these observations in ApoE4 mouse and human brains, and 2) assess whether the decrease in mitDNA concentration correlates with altered contents of proteins related to mitophagy and motility in CSF of LOAD patients, in order to generate a mitochondrion-based biomarker signature of early AD. Funded by Marato-TV3 Foundation, Barcelona, Catalunya.