LAUSR

High-density lipoproteins (HDLs) are highly heterogeneous complexes of hundreds of lipids and 100 proteins that can be classified according to their composition, intrinsic density, size, and functions. They are often defined as lipoproteins containing apolipoprotein (apo) A-I, the most abundant protein in HDL. HDL from healthy people exerts multiple antiatherogenic effects including cellular cholesterol efflux, anti-inflammatory, antithrombotic, and vasculoprotective activities. These normal functions of HDL may explain the established inverse relationship of HDL-cholesterol levels and incident cardiovascular disease. However, simply raising HDL-cholesterol levels is not a successful strategy to prevent cardiovascular events: disappointing results of phase III studies conducted with niacin or cholesterylester transfer protein (CETP) inhibitors in patients with high cardiovascular risk suggest that enhancing HDL functionality is rather the path to explore. Indeed, beneficial effects of HDL could not exist without HDL biogenesis, the molecular events that leads to lipidation of apo A-I with cellular membrane lipids by the key transporter ABCA1 and the subsequent esterification of cholesterol by lecithin:cholesterol acyltransferase (LCAT). In the arterial wall, the majority of apo A-I can be found in the lipid-poor state rather than in mature particles. In atherosclerotic plaques, lipid-poor apo A-I molecules undergo oxidative modifications, lose the capability to interact with the ATP-binding cassette A1 (ABCA1) transporter, and accumulate through mechanisms that remain to be defined. These observations support the concept that HDL biogenesis is impaired in the context of atherosclerotic plaques. In this issue of the journal, Choi and colleagues provide a seminal contribution to the understanding of how HDL biogenesis is prevented by the interaction of apo A-I with a subtype of cholesteroland sphingomyelin-rich membrane microdomain, enriched in desmocollin-1 (Dsc1) and other desmosomal cadherins such as desmoglein 1 and 3. They isolated and characterized these microdomains from human fibroblasts with a new method involving: (i) detergent-free isolation of membranes on sucrose gradients in the presence of apo A-I; (ii) immunoprecipitation of apo A-I-associated microdomains after sonication in the absence of cross-linking agents; and (iii) lipidomic and proteomic analyses of the composition of microdomains. Co-immunoprecipitation of apo A-I with Dsc1 was also obtained in the absence of ABCA1 (in fibroblasts from Tangier disease patients) and was confirmed in human embryonic kidney (HEK) cells overexpressing human Dsc1. In both cell types, apo A-I co-localized with Dsc1 as shown by confocal microscopy. Using transient overexpression of Dsc1 or ABCA1 in HEK cells, the authors concluded that both proteins are able to increase apo A-I binding independently, but that cholesterol efflux was uniquely stimulated through ABCA1 expression and not by Dsc1 overexpression. While Dsc1 overexpression per se did not reduce ABCA1-mediated cholesterol efflux to apo A-I, interference with endogenous Dsc1 expression by stable transfection of short hairpin RNA or CRISPR/Cas9 constructs increased ABCA1-mediated cholesterol efflux. This was paralleled by an increase in ABCA1 protein, which was apparently stabilized by the increased availability of apo A-I for binding to ABCA1 or increased availability of membrane cholesterol following reduced Dsc1 expression. Both mechanisms have been shown to extend the half-life of this rapid turnover protein. The concept that Dsc1-containing microdomains sequester apo A-I and/or membrane lipids efficiently and impact ACBA1-mediated biogenesis is novel (Figure 1). As the authors justly point out, the balance between Dsc1 and ACBA1 expression at the cell surface could regulate the intensity of HDL biogenesis. One limitation of this study is that HDL biogenesis was described essentially by cellular cholesterol efflux assays, but the process of apo A-I lipidation also involves