ATP-binding cassette transporter A1 (ABCA1) mediates cholesterol efflux to lipid-free apolipoproteins such as apolipoprotein A-I (apoA-I) and apolipoprotein E (apoE). ABCA1 is essential regulator of high density lipoprotein generation (HDL) a role that defines its significance for cardiovascular disease. ABCA1 and APOE are transcriptionally regulated by Liver X Receptors (LXR) and Retinoic X Receptors (RXR).
The inheritance of 4 allele of APOE is the major genetic risk factor for late-onset Alzheimers disease (AD). Patients carrying APOE4 allele, compared to those with the other two isoforms, have more amyloid plaques a finding replicated in human Amyloid precursor protein (APP) transgenic mice. Recent data suggest that ABCA1 via its control over apoE lipidation may have a role in AD. Studies from our and other groups have demonstrated that lack of ABCA1 increases amyloid deposition and cognitive decline in different AD model mice accompanied by significant decrease in the levels of apoE and apoA-I. In contrast, treatment with LXR or RXR ligands increases ABCA1 and apoE levels and significantly ameliorates amyloid pathology. In a recent study using APP mice we demonstrated that ABCA1 haplo deficiency had a differential effect on the phenotype of APOE3 or APOE4 expressing mice. The lack of one copy of Abca1 significantly aggravated memory deficits, Aβ plaques and Aβ clearance in APP/APOE4 but not in APP/APOE3 mice. Interestingly, we found a correlation between HDL in plasma and amyloid load in brain, suggesting a causative connection between peripheral lipoproteins and Aβ load in the CNS.
In conclusion, these studies demonstrate that future therapies targeting ABCA1 and APOE expression such as LXR/RXR agonists could have a favorable effect on the outcome in AD patients including APO4 carriers.