Genetic variants of ABCA1 (ATP-binding cassette A1), an ATP-driven transporter that pumps cholesterol out of cells, recently joined the ranks of potential risk factors for late-onset Alzheimer’s disease (see Wollmer et al., 2003 and Katzov et al., 2004). Why these variants may predispose carriers to AD is uncertain, but the role of the transporter in the periphery is to mobilize cholesterol out of cells and onto lipid-poor apolipoproteins, and it may work similarly in the central nervous system (CNS). If so, ABCA1 could provide a link between two major AD risk factors, cholesterol and apolipoprotein E (ApoE), the major lipoprotein of the CNS (see ARF related news story on the link among cholesterol, ApoE and AD). Ironically, just as two papers from independent labs report that ABCA1 keeps ApoE levels high and saturated with cholesterol, a third paper casts doubt on the link between the transporter and Alzheimer’s.
The first two papers are currently in press in the Journal of Biological Chemistry and are already available online. In the first, Dave Holtzman and colleagues, at Washington University, St. Louis, and the Carnegie Mellon University, Pittsburgh, used ABCA1 knockout mice to test the role of the transporter in the CNS. When first author Suzanne Wahrle and colleagues measured ApoE in these animals, they found that levels in the cortex and cerebrospinal fluid (CSF) were 80 and 98 percent lower than in normal mice, while animals missing only one copy of the gene had intermediate levels of ApoE (13 and 46 percent, for cortex and CSF, respectively).
To determine why levels may be so ablated, the authors looked in the CSF where the majority of ApoE exists as lipoprotein particles around 10-17 nanometers in diameter. When Wahrle fractionated CSF from ABCA1 knockout mice, she found that some particles were much smaller than normal, about 7 nanometers wide. Given the role of ABCA1 as a cholesterol transporter, this suggests that the particles may be poorly bound with the lipid. To investigate this, Wahrle examined lipoproteins secreted from cultured astrocytes, cells that are the major source of ApoE in the brain. The authors found that about 75 percent of ApoE secreted from ABCA1-negative astrocytes ends up in the smaller lipoproteins, and these particles were indeed cholesterol poor (0.69 mg cholesterol/mg ApoE, compared to 2.3 mg cholesterol/mg ApoE in particles from normal astrocytes). The authors conclude that “ABCA1 plays a major role in maintaining normal ApoE levels in vivo,” and suggest that “modulation of ABCA1 function and levels may be a novel therapeutic target for AD.”
Also reporting in the Journal of Biochemistry, Cheryl Wellington and collaborators from the University of British Columbia, Vancouver, and the Clinical Research Institute of Montreal, came to very similar conclusions. First author Veronica Hirsch-Reinshagen and colleagues also found that ApoE is depleted in the brains of ABCA1-negative mice, being 65 percent lower overall than normal, and 76 and 79 percent lower in the hippocampus and striatum, respectively.
Hirsch-Reinshagen also looked at secretion of cholesterol by cultured astrocytes and microglia, adding exogenous ApoA1 or ApoE isoforms to the culture medium to provide an apolipoprotein “sink.” The ABCA1-negative astrocytes secreted poorly in comparison to their wild-type counterparts, releasing less than five percent of their total cholesterol in comparison to 7-10 percent released from normal cells. The microglia fared slightly better, secreting about 30-40 percent less than normal cells. ApoE3 and ApoE4 provided more “pulling power” than the other lipoproteins because when these were added to the medium, more cholesterol was secreted by the cells. However, this difference was only significant in the case of astrocytes.
As for secretion of ApoE, Hirsch-Reinshagen’s data were again in agreement with the work from Holtzman’s lab. The authors found that ApoE secretion was reduced in cells lacking the cholesterol transporter. It was down by 30 percent in astrocytes and by about 90 percent in microglia.
Taken together, these papers provide a clear link between ABCA1 and ApoE metabolism in the brain. However, as Hirsch-Reinshagen and colleagues write, “the mechanisms by which ABCA1 affect ApoE metabolism in glial cells are not yet understood.” It is interesting, for example, that neither group found any alteration in ApoJ levels in the CNS of the knockout mice, suggesting that the transporter may selectively impinge on ApoE metabolism.
As for ABCA1 variants as risk factors for AD, Andrew Grupe and colleagues from Celera Diagnostic, Alameda, California, and elsewhere, report the results of a case control study in the August 19 Neuroscience Letters (currently available online).
First author Yonghong Li and colleagues genotyped ABCA1 polymorphisms in DNA samples from a total of 2,146 individuals, 980 diagnosed with AD and 1,166 controls. They identified nine single nucleotide polymorphisms (SNPs) in the ABCA1 gene, three of which were identical to those previously predicted to confer risk for AD (see Katzov et al., 2004). Li, however, found that none of the SNPs showed any significant association with late-onset Alzheimer’s, even when they conducted pairwise linkage disequilibrium analysis. The discrepancy may be due to the larger sample size analyzed in the newest study.—Tom Fagan
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