Expression of human ABCG1 in mice modulates cholesterol metabolite levels and does not influence guanidine-extractable abeta levels in brain

Growing evidence suggests that cholesterol plays a key role in Alzheimer's Disease (AD). Importantly, membrane cholesterol levels influence generation of the amyloid-beta (Aß) peptide from amyloid precursor protein (APP). The amyloid cascade hypothesis of AD posits that increased accumulation of (Aß) peptides and their deposition as amyloid in the brain, promote neurodegeneration in AD. The ATP-binding-cassette-G1 (ABCG1) transporter is highly expressed in the brain and periphery and is believed to regulate cholesterol levels in peripheral cells and tissues. This suggests that the ABCG1-mediated effects on cholesterol homeostasis may influence Aß production and AD. To determine the effect of ABCG1 overexpression in brain and to evaluate whether ABCG1 could influence Aß production or amyloid deposition, ABCG1 overexpression was studied both in vitro and using a novel ABCG1 transgenic mouse. We generated an ABCG1 overexpressing mouse containing the entire ABCG1 genomic locus and regulatory sequence, to promote "physiological" expression patterns of overexpression. We found that the ABCG1 transgene is physiologically regulated in vivo and overexpressed 3-6-fold at the protein level. Using a LacZ reporter driven by endogenous ABCG1 regulatory sequences, we showed that ABCG1 is highly expressed in hippocampal neurons. Importantly, ABCG1 overexpressing mice express the ABCG1 transgene in primary neurons and astrocytes. Unexpectedly, the level of cholesterol biosynthetic precursors and 24S-hydroxycholesterol levels were reduced in adult ABCG1 overexpressing brain, suggesting a novel function of ABCG1. Total levels of cholesterol and other lipids were unaffected. In contrast to numerous published studies that studied constitutive overexpression of ABCG1 in vitro, overexpression of physiologically regulated ABCG1 in primary astrocytes did not facilitate cholesterol efflux to HDL. These experiments suggest that the role of ABCG1 in vivo is not entirely understood and that ABCG1 may influence cholesterol biosynthesis or cholesterol precursor transport. ABCG1 overexpression increased APP processing by ß-secretase and the production of Aß in transfected HEK cells. However, in vivo ABCG1 overexpression or deficiency did not significantly affect murine Aß levels nor did overexpression affect amyloid formation or ß-secretase mediated APP cleavage. These experiments suggest that ABCG1 overexpression does not impact amyloid formation as hypothesized.Medicine, Faculty ofPathology and Laboratory Medicine, Department ofGraduat