Pharmacologic inhibition of astrocyte ApoE release

Abstract

AbstractBackgroundThe ApoE4 allele is the strongest known genetic risk factor for late onset Alzheimer’s disease. Despite this knowledge, therapeutic strategies seeking to modulate brain ApoE homeostasis remain largely unexplored. Several lines of evidence demonstrate that astrocytes function as the primary producers of ApoE within the brain. In this study, we examine how targeted disruption of astrocyte lipid metabolism and protein prenylation effect the secretion of ApoE from primary astrocyte cultures.MethodPrimary cell cultures of mouse astrocytes were grown from 1‐3 day old pups. Drugs were applied to cells in serum free conditions over a range of timepoints and doses. Astrocyte conditioned media was collected from cultures and ApoE and other proteins of interest were measured by western blot.ResultWe find that inhibiting HMG‐CoA reductase, a rate‐limiting enzyme in the cholesterol synthesis pathway, with simvastatin reduces astrocyte ApoE levels in conditioned media in a dose dependent manner, while secretion of other proteins remain preserved. Inhibition of downstream cholesterol synthesis through blockade of the enzyme FDFT1 with zaragozic acid, however, has no effect on total ApoE secretion. Mice with genetic disruption of brain cholesterol metabolism also have similar levels of ApoE in CSF samples compared to control. These data suggest the effect of HMGCR inhibition on ApoE is not mediated through cholesterol. It is known that HMGCR activity is also required for maintenance of cellular protein prenylation. We tested the prenylation inhibitor FGTI‐2734 and found that it is sufficient to block astrocyte ApoE secretion, suggesting that HMGCR inhibitors may exert their effect on ApoE through this mechanism. Recent work suggests that inflammatory astrocytes exert toxic effects via neurotoxic lipids carried by ApoE and another carrier protein, CLU. We find that inhibiting HMGCR in astrocytes does not prevent cytokine‐induced conversion to an inflammatory phenotype, but remains able to block ApoE releaseConclusionWe conclude that targeted inhibition of prenylation enzymes may provide a therapeutic strategy to modulate ApoE levels in the brain.