Influence of ABCA1 and ABCA7 on the lipid microenvironment of the plasma membrane

The ABCA1 transporter organizes cellular lipid homeostasis by promoting the release of cholesterol to plasmatic acceptors such as ApoA-I. Despite intensive investigation, the molecular mechanism of such a process has not yet been clarified. In the present study we report on the analysis of the ABCA1 lipid microenvironment at the plasma membrane of living cells, by a novel approach based on fluorescence lifetime imaging microscopy (FLIM). In the plasma membrane of mammalian cells, a broad fluorescence lifetime distribution sensitive to treatments interfering with the membrane lateral and transbilayer organization was found. In agreement with investigations in giant unilamellar vesicles and giant plasma membrane vesicles, our results are consistent with the existence of a large variety of submicroscopic lipid domains. Based on that, FLIM in HeLa cells expressing ABCA1 revealed the destabilizing function of the transporter on the lipid arrangement at the membrane, indicating that lipid packing was a primary target of ABCA1 activity. This was corroborated by the analysis of plasma membrane fractions isolated by density fractionation. On the basis of our analysis and previous data, we speculate that the exposure of phosphatidylserine on the cell surface is a central event for ABCA1-dependent modifications. However, a comparative study of cells expressing ABCA7, the member of the ABCA subfamily with the highest homology to ABCA1, revealed that exposure of PS alone cannot account for the detected effects. Collectively, our data suggest that the ability of ABCA1 to promote cholesterol efflux is independent and precedes its actual binding to ApoA-I. Rather, ABCA1-induced plasma membrane modifications are necessary for the lipidation of ApoA-I and the generation of high density lipoprotein particles.