Pathways and Mechanisms of Cellular Cholesterol Efflux—Insight From Imaging

Abstract

Cholesterol is an essential molecule in cellular membranes, but too much cholesterol can be toxic. Therefore, mammalian cells have developed complex mechanisms to remove excess cholesterol. In this review article, we discuss what is known about such efflux pathways including a discussion of reverse cholesterol transport and formation of high-density lipoprotein, the function of ABC transporters and other sterol efflux proteins, and we highlight their role in human diseases. Attention is paid to the biophysical principles governing efflux of sterols from cells. We also discuss recent evidence for cholesterol efflux by the release of exosomes, microvesicles, and migrasomes. The role of the endo-lysosomal network, lipophagy, and selected lysosomal transporters, such as Niemann Pick type C proteins in cholesterol export from cells is elucidated. Since oxysterols are important regulators of cellular cholesterol efflux, their formation, trafficking, and secretion are described briefly. In addition to discussing results obtained with traditional biochemical methods, focus is on studies that use established and novel bioimaging approaches to obtain insight into cholesterol efflux pathways, including fluorescence and electron microscopy, atomic force microscopy, X-ray tomography as well as mass spectrometry imaging.

SMN Deficiency Destabilizes ABCA1 Expression in Human Fibroblasts. Novel Insights in Pathophysiology of Spinal Muscular Atrophy

“…Accordingly, ABCA1 protein, whose expression levels were increased in an activity-dependent context, appeared downregulated in SMN-depleted stimulated cells. One of downstream effects of the reduced expression levels of ABCA1 is the cytoplasmatic accumulation of cholesterol due to its efflux impairment [21]. Accordingly, in our system we demonstrated an aberrant distribution of cholesterol in siSMN-transfected fibroblasts and, most importantly, in SMA type I patient-derived fibroblasts.…”

“…Dysregulated cholesterol homeostasis is implicated in several human pathologies, including cancer, cardiovascular and neurodegenerative disease [19,20]. Mammalian cells have developed sophisticated mechanisms to prevent abnormal accumulation of cholesterol in cellular membranes [21].…”

“…Energy-dependent efflux of cholesterol from cells is mediated by ATP-binding cassette (ABC) transporters, primarily ABCA1. ABCA1 is a membrane-associated protein that acts to transfer intracellular cholesterol to apoliproteins (ApoA-I) promoting the formation of nascent high-density lipoproteins/HDLs [21][22][23]. Mutations in ABCA1 have been associated with Tangier disease, a rare genetic disorder characterized by extremely low levels of HDL and apoA-I; excessive deposition of cholesteryl esters in macrophages, and lipid deposits in cells including fibroblasts, Schwann cells, and myofibers [24][25][26][27].…”

Kinetic modelling of sterol transport between plasma membrane and endo-lysosomes based on quantitative fluorescence and X-ray imaging data

“…We showed recently that human fibroblasts release sterol-rich vesicles of varying sizes during efflux and observed that at least a portion of such vesicles were shed directly from the PM (Juhl et al, 2021b). Such so-called ectosomes have been observed in a variety of cell systems, and they have been associated with cholesterol efflux in several cell types, for example in macrophages and in migrating cancer cells (Ma et al, 2015;He et al, 2018;Juhl and Wüstner, 2022). We found that the abundance of vesicles released from cells correlates with the activity of ABCA1 at the PM, which might work in tandem with NPC2 to export cholesterol from cells (Juhl et al, 2021b).…”

“…Regulation of this trafficking pathway is complex and ensures sufficient cholesterol supply but also prevention of cholesterol accumulation (Goldstein et al, 2006;Juhl and Wüstner, 2022).…”