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ABCA1 belongs to the A class of ABC transporter, which is absent in yeast. ABCA1 elicits lipid translocation at the plasma membrane through yet elusive processes. We successfully expressed the mouse Abca1 gene in Saccharomyces cerevisiae. The cloned ABCA1 distributed at the yeast plasma membrane in stable discrete domains that we name MCA (membrane cluster containing ABCA1) and that do not overlap with the previously identified punctate structures MCC (membrane cluster containing Can1p) and MCP (membrane cluster containing Pma1p). By comparison with a nonfunctional mutant, we demonstrated that ABCA1 elicits specific phenotypes in response to compounds known to interact with membrane lipids, such as papuamide B, amphotericin B and pimaricin. The sensitivity of these novel phenotypes to the genetic modification of the membrane lipid composition was studied by the introduction of the cho1 and lcb1-100 mutations involved respectively in phosphatidylserine or sphingolipid biosynthesis in yeast cells. The results, corroborated by the analysis of equivalent mammalian mutant cell lines, demonstrate that membrane composition, in particular its phosphatidylserine content, influences the function of the transporter. We thus have reconstituted in yeast the essential functions associated to the expression of ABCA1 in mammals and characterized new physiological phenotypes prone to genetic analysis. This article is a part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010)., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Macrophages are central players in both lipid metabolism and innate immunity. Their determinant role in the pathogenesis of atherosclerosis is under the control of the ATP-binding cassette transporter (ABCA1), which by minimizing cellular lipid content, limits development of pro-inflammatory foam cells. Considering the differential contribution of monocyte subsets to the generation of vascular lesions we analyzed the immunophenotype of ABCA1-expressing cells in the myeloid lineage, by the combined use of flow cytometry and real-time quantitative RT-PCR. ABCA1 expression is limited to "non-inflammatory" Ly6C(lo) circulating monocytes and tissue-resident macrophages expressing markers of alternative activation. In ABCA1(-/-) peritoneal macrophages the transcriptional programs induced by LPS/IFN-gamma or IL-4 cytokines are altered and deviated phosphorylation patterns of STAT transcriptional regulators in response to stimuli are observed.

The ABCA1 transporter orchestrates cellular lipid homeostasis by promoting the release of cholesterol to plasmatic acceptors. The molecular mechanism is, however, unknown. We report here on the biophysical analysis in living HeLa cells of the ABCA1 lipid microenvironment at the plasma membrane. The modifications of membrane attributes induced by ABCA1 were assessed at both the outer and inner leaflet by monitoring either the lifetime of membrane inserted fluorescent lipid analogues by fluorescence lifetime imaging microscopy (FLIM) or, respectively, the membrane translocation of cationic sensors. Analysis of the partitioning of dedicated probes in plasma membrane blebs vesiculated from these cells allowed visualization of ABCA1 partitioning into the liquid disordered-like phase and corroborated the idea that ABCA1 destabilizes the lipid arrangement at the membrane. Specificity was demonstrated by comparison with cells expressing an inactive transporter. The physiological relevance of these modifications was finally demonstrated by the reduced membrane mobility and function of transferrin receptors under the influence of an active ABCA1. Collectively, these data assess that the control of both transversal and lateral lipid distribution at the membrane is the primary function of ABCA1 and positions the effluxes of cholesterol from cell membranes downstream to the redistribution of the sterol into readily extractable membrane pools.

The ABCA1 transporter is the prototype of the A class of mammalian adenosine triphosphate binding cassette transporters and one of the largest members of this family. ABCA1 has been originally identified as an engulfment receptor on macrophages and, more recently, it has been shown to play an essential role in the handling of cellular lipids. Indeed by promoting the effluxes of membrane phospholipids and cholesterol to lipid-poor apoprotein acceptors, ABCA1 controls the formation of high-density lipoproteins and thus the whole process of reverse cholesterol transport. A number of additional phenotypes have been found in the mouse model of invalidation of the ABCA1 gene. In spite of their clinical diversity, they all are extremely sensitive to variations in the physicochemical properties of the cell membrane, which ABCA1 controls as a lipid translocator.