Prion diseases are transmissible neurodegenerative disorders of prion protein (PrP) conformation. Prion replication by conversion of benign PrP C isoforms into disease-specific PrP Sc isoforms is intimately involved in prion disease pathogenesis and may be initiated in cholesterol-rich caveolae-like domains (CLD). Concentrations of the cholesterol transporter ATP-binding cassette A1 protein (ABCA1) are elevated in pre-clinical scrapie prion-infected mice and in prion-infected cells in vitro. Elevation of ABCA1 in prion-infected brain is not a direct consequence of local PrP Sc accumulation, indeed levels of ABCA1 are comparable in brain regions that differ dramatically in the amount of PrP Sc . Similarly, ABCA1 concentrations are identical in normal mice, transgenic mice overexpressing PrP and PrP knockout mice. In contrast, PrP C and PrP Sc levels, but not Prnp mRNA, were increased by overexpression of ABCA1 in N2a neuroblastoma cells and scrapie prion-infected N2a cells (ScN2a). Conversely, RNAi-mediated knock down of Abca1 expression decreased the concentrations of PrP C in N2a cells and of PrP Sc in ScN2a cells. These results suggest that ABCA1's effects on PrP C levels are post-translational and may reflect an increase in of PrP C stability, mediated either indirectly by increasing membrane cholesterol and CLD formation or by other functions of ABCA1. The increased supply of PrP C available for conversion would lead to increased PrP Sc formation. INTRODUCTIONCreutzfeldt-Jakob disease and Kuru in humans, bovine spongiform encephalopathy, scrapie in sheep and chronic wasting disease in cervids are members of the group of neurodegenerative protein folding disorders caused by prions (Prusiner, 1998). Prion replication involves conversion of benign cellular isoforms, designated PrP C , into disease-specific isoforms, designated PrP Sc . The cellular site for prion replication is not well defined; in vitro and in vivo studies suggest that it takes place at the plasma membrane, where the initial interaction between endogenous glycosylphosphatidylinositol (GPI)-anchored PrP C and exogenous PrP Sc occurs, and in endosomal compartments (Campana et al., 2005;Harris, 1999). Substantial evidence suggests that PrP Sc generation takes place in cholesterol-rich, detergent insoluble membrane domains that are identified by various names, including caveolae-like domains (CLD), detergent resistant membranes and lipid rafts (Caughey & Raymond, 1991;Kaneko et al., 1997; Sarnataro et al., 2004;Vey et al., 1996). Cholesterol is required for PrP C cellsurface expression and stability (Caughey & Raymond, 1991;Gilch et al., 2006;Kaneko et al., 1997; Sarnataro et al., 2004) and pharmacological treatments that lower cellular cholesterol reduce PrP Sc generation and prolong prion incubation time (Bate et al., 2004;Gilch et al., 2006;Marella et al., 2002; Taraboulos et al., 1995).Genes involved in cholesterol metabolism and transport are consistently found among differentially expressed genes (DEGs) in prion-infected mice and cell line...